RTO care

오존

관련기술2016.06.27 13:48

오존이란?

   

6. 미반응 오존의 처리장치

6-1) 대기중의 미반응 오존처리 시스템

오존이 접촉조에서 완전용해가 되기 어려우므로 미반응 오존이 배출될 수 밖에 없다.

미반응 오존은 국내의 경우 0.1mg/ 이하로 처리하여 배출해야 한다.

   

산업안전보건법과 환경기준에 명시되어있다. 미반응 오존 파괴장치는 주로 활성탄 흡착법, 열분해법, 촉매법, 약액세정법 등이 있으나 국내에서는 주로 열분해법, 활성탄 흡착법, 촉매법이 많이 이용되고 있다.

   

<미반응 오존처리 시스템 비교>

   

항 목

활성탄 흡착법

열 분해법

촉매법

분 해

방 법

활성탄과 오존의 직접반응에

의해 활성탄 표면에서 촉매

적 분해가 평형 하게 일어난다.

2O3 + 2C 2CO2 + O2

2O3 + C CO2 + 2O3

2O3 + C 3CO2 + C

활성탄 0.27g은 오존 1g을 분해

한다.

오존 1mol은 대기 중에서 분해

할 때 34kcal의 열을 발생하므로

고농도 오존처리 및 대용량은

설계에 반영치 않는다.

기체 속의 오존은 비교적 안정 되어있지만 고온을 가하게 되 면 불안정화 되어 급속히 분해 되어 산소가 되는 성질을 이용 하는 것이다. 300℃이상에서 13초 접촉 시 미 반응 오존 100% 파괴

TiO2 MnO2

오존가스를 50℃150℃가

온하여 SV10.000/h 운전

하여 O2로 환원시키는 원리.

배기가스의 수분제거 장치

와 공존가스에 영향을 심각

하게 받는다.

장·

단 점

·소용량에 적합하다.

·설비가 간단하다.

·수처리용에 적합하다

·화재위험이 있다.

·감시가 어렵다.

·설치 면적이 크다.

·O3농도가 고농도 일 때

·활성탄 처리비용보다 싸다

·공존가스에 영향이 없다.

·동력이외에 유지관리비가 없

다.

·대용량 처리가 가능하다.

·폐열 이용이 가능하다.

·촉매 교체비용이 과다하다.

·수분에 약하다.

·오존의 파괴효율이 오존농도 에 따라서 변할 수 있다.

·설치비용이 적다.

6-2) 폐오존화 물의 용존 오존의 분해장치

자외선 조사법과 활성탄 여과에 의한 분해 처리법이 있다.

6-2-1) 자외선 조사법

오존에는 Harley Band대의 광파장 흡수가 있다. 그림 1-1에 나타낸다. 수중에 있어서도 다음과 같은 광화학 반응에 따라 분해가 진행된다고 여겨지고 있다.

분해 경과는 그림 1-2를 참조했으면 한다.

O3 + hv --- O + O2

O + H2O --- 2OH

OH + O3 --- HO2 + O2

HO2 + O3 --- OH + 2O2

OH + HO2 --- H2O + O2

   

<그림 1-2 오존의 광파장 흡수 특성> <그림 1-2 용해오존의 자외선에 의한 분해>

   

   

   

   

광화학 반응에 의한 것이기 때문에 상온에서는 분해반응에 장시간을 요하는 단점이 있으나 단축시키기 위해서는 물 온도를 상승시키거나, pH를 조절하거나 자외선의 조사량을 증가 시킨다면, 가능하다. 이 방법은 사용이 끝나고 오존의 수중잔류 오존을 분해해 버리기 때문에, 물을 버리는 일이 없이 이용할 수 있는 것이 특징이다.

6-2-2) 활성탄 흡착법

폐오존수를 그대로 701000mmH 높이의 활성탄 여과장치를 통과시켜 오존을 흡착시키는 방법이 있다. 높은 농도에서 소량, 저농도로 양이 많은 경우의 배치처리에 적당하다.

다만 활성탄 g당의 처리능력 파악은 필요하다. 사용할 활성탄은 콜베이스, 야시가라베이스 어떤 쪽도 좋고 0.6mm정도의 파쇄탄이 좋다.

6-3) 오존의 노출에 위한 인체에의 영향

   

오존/PPM

작 용

0.01 0.02

다소의 냄새를 느낀다.

0.1

확실히 냄새가 나고, 코나 목에 자극을 느낀다.

0.2 0.5

36시간 노출되면 시각이 떨어진다.

0.5

확실히 에 자극을 느낀다.

1 2

2시간 노출로 두통, 흉부통, 상부기도건조와 기침이 나며, 계속 노출되면 만성중독증에 걸린다.

5 10

맥박증가, 몸이 아프며, 마비증세가 올 수 있다.

15 20

작은 동물은 2시간 이내에 사망한다.

50

인간은 1시간으로 생명이 위험하다.

   

출처: <http://kozone.com/d4-5.htm>

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Comment +0

Ozonolysis

상태와 변화2016.06.27 13:46

Ozonolysis

From Wikipedia, the free encyclopedia

Jump to: navigation, search

Ozonolysis is an organic reaction where the unsaturated bonds of alkenes, alkynes, or azo compounds are cleaved with ozone. Alkenes and alkynes form organic compounds in which the multiple carbon–carbon bond has been replaced by a carbonyl group[1][2][3] while azo compounds form nitrosamines.[4] The outcome of the reaction depends on the type of multiple bond being oxidized and the work-up conditions.

Contents

 [hide

Ozonolysis of alkenes[edit]

Alkenes can be oxidized with ozone to form alcohols, aldehydes or ketones, or carboxylic acids. In a typical procedure, ozone is bubbled through a solution of the alkene in methanol at 78 °C until the solution takes on a characteristic blue color, which is due to unreacted ozone. This indicates complete consumption of the alkene. Alternatively, various other chemicals can be used as indicators of this endpoint by detecting the presence of ozone. If ozonolysis is performed by bubbling a stream of ozone-enriched oxygen through the reaction mixture, the gas that bubbles out can be directed through a potassium iodide solution. When the solution has stopped absorbing ozone, the ozone in the bubbles oxidizes the iodide to iodine, which can easily be observed by its violet color.[5] For closer control of the reaction itself, an indicator such as Sudan Red III can be added to the reaction mixture. Ozone reacts with this indicator more slowly than with the intended ozonolysis target. The ozonolysis of the indicator, which causes a noticeable color change, only occurs once the desired target has been consumed. If the substrate has two alkenes that react with ozone at different rates, one can choose an indicator whose own oxidation rate is intermediate between them, and therefore stop the reaction when only the most susceptible alkene in the substrate has reacted.[6] Otherwise, the presence of unreacted ozone in solution (seeing its blue color) or in the bubbles (via iodide detection) only indicates when all alkenes have reacted.

After completing the addition a reagent is then added to convert the intermediate ozonide to a carbonyl derivative. Reductive work-up conditions are far more commonly used than oxidative conditions. The use of triphenylphosphine, thiourea, zinc dust, or dimethyl sulfide produces aldehydes or ketones while the use of sodium borohydride produces alcohols. The use of hydrogen peroxide produces carboxylic acids. Recently, the use of amine N-oxides has been reported to produce aldehydes directly.[7] Other functional groups, such as benzyl ethers, can also be oxidized by ozone. It has been proposed that small amounts of acid may be generated during the reaction from oxidation of the solvent, so pyridine is sometimes used to buffer the reaction. Dichloromethane is often used as a 1:1 cosolvent to facilitate timely cleavage of the ozonide. Azelaic acid and pelargonic acids are produced from ozonolysis of oleic acid on an industrial scale.

An example is the ozonolysis of eugenol converting the terminal alkene to an aldehyde:[8]

By carefully controlling the reaction/workup conditions, unsymmetrical products can be generated from symmetrical alkenes:[9]

   

Reaction mechanism[edit]

Carbonyl oxide (Criegee zwitterion)

In the generally accepted mechanism proposed by Rudolf Criegee in 1953,[10][11][12] the alkene and ozone form an intermediate molozonide in a 1,3-dipolar cycloaddition. Next, the molozonide reverts to its corresponding carbonyl oxide (also called the Criegee intermediate or Criegee zwitterion) and aldehyde or ketone in a retro-1,3-dipolar cycloaddition. The oxide and aldehyde or ketone react again in a 1,3-dipolar cycloaddition or produce a relatively stable ozonide intermediate (a trioxolane).

Evidence for this mechanism is found in isotopic labeling. When 17O-labelled benzaldehyde reacts with carbonyl oxides, the label ends up exclusively in the ether linkage of the ozonide.[13] There is still dispute over whether the molozonide collapses via a concerted or radical process; this may also exhibit a substrate dependence.

History[edit]

Ozonolysis was invented by Christian Friedrich Schönbein in 1840. Before the advent of modern spectroscopic techniques, it was an important method for determining the structure of organic molecules. Chemists would ozonize an unknown alkene to yield smaller and more readily identifiable fragments. The ozonolysis of alkenes is sometimes referred to as "Harries ozonolysis", because some attribute this reaction to Carl Dietrich Harries.[14]

Ozonolysis of alkynes[edit]

Ozonolysis of alkynes generally gives an acid anhydride or diketone product,[15] not complete fragmentation as for alkenes. A reducing agent is not needed for these reactions. The exact mechanism is not completely known.[16] If the reaction is performed in the presence of water, the anhydride hydrolyzes to give two carboxylic acids.

Ozonolysis of elastomers[edit]

The method was used to confirm the structural repeat unit in natural rubber as isoprene. It is also a serious problem, known as "ozone cracking" where traces of the gas in an atmosphere will cut double bonds in susceptible elastomers, including natural rubber, polybutadiene, Styrene-butadiene and Nitrile rubber. Ozone cracking creates small cracks at right angles to the load in the surfaces exposed to the gas, the cracks growing steadily as attack continues. The rubber product must be under tension for crack growth to occur.

Ozone cracking in Natural rubber tubing

Ozone cracking is a form of stress corrosion cracking where active chemical species attack products of a susceptible material. Ozone cracking was once commonly seen in the sidewalls of tires but is now rare owing to the use of antiozonants. Other means of prevention include replacing susceptible rubbers with resistant elastomers such as polychloroprene, EPDM or Viton.

See also[edit]

References[edit]

   

출처: <https://en.wikipedia.org/wiki/Ozonolysis>

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Comment +0

Ozonolysis

상태와 변화2016.06.27 13:45

Ozonolysis

Table of contents

  • Introduction
  • Reaction Mechanism
  • References
  • Problems
  • Ozonolysis is a method of oxidatively cleaving alkenes or alkynes using ozone (O 3  O3 ), a reactive allotrope of oxygen. The process allows for carbon-carbon double or triple bonds to be replaced by double bonds with oxygen. This reaction is often used to identify the structure of unknown alkenes. by breaking them down into smaller, more easily identifiable pieces. Ozonolysis also occurs naturally and would break down repeated units used in rubber and other polymers. On an industrial scale, azelaic acid and pelargonic acids are produced from ozonolysis.

    Introduction

    The gaseous ozone is first passed through the desired alkene solution in either methanol or dichloromethane. The first intermediate product is an ozonide molecule which is then further reduced to carbonyl products. This results in the breaking of the Carbon-Carbon double bond and is replaced by a Carbon-Oxygen double bond instead.

    Reaction Mechanism

    Step 1:

    The first step in the mechanism of ozonolysis is the initial electrophilic addition of ozone to the Carbon-Carbon double bond, which then form the molozonide intermediate.  Due to the unstable molozonide molecule, it continues further with the reaction and breaks apart to form a carbonyl and a carbonyl oxide molecule.

    Step 2:

       

    The carbonyl and the carbonyl oxide rearranges itself and reforms to create the stable ozonide intermediate. A reductive workup could then be performed to convert convert the ozonide molecule into the desired carbonyl products.

    References

  • Vollhardt, K., Schore, N. Organic Chemistry: Structure and Function. 5th ed. New York, NY: W. H. Freeman and Company, 2007.
  • Shore, N. Study Guide and Solutions Manual for Organic Chemistry. 5th ed. New York, NY: W.H. Freeman and Company, 2007.

    Problems

    Answers

       

    출처: <http://chemwiki.ucdavis.edu/Core/Organic_Chemistry/Hydrocarbons/Alkenes/Reactivity_of_Alkenes/Ozonolysis>

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Pollutant Degradation in Gas Streams by means of Non-Thermal Plasmas

Milko Schiorlin1, Cristina Paradisi2, Ronny Brandenburg1, Michael Schmidt1, Ester Marotta2, Agata Giardina2 and Ralf Basner1

[1] Leibniz Institute for Plasma Science and Technology (INP Greifswald), Greifswald, Germany

[2] Department of Chemical Sciences, University of Padova, Padova, Italy

1. Introduction

In the industrialized society air pollution has become a major concern for the environment and public health. Pollutants such as particulate matter and harmful chemicals are causing disease or death to humans and other living organisms and strongly affect ecosystems (e.g. acidification, eutrophication). Although significant progress has been made in recent years in tackling the emissions of some air pollutants (e.g. sulphur dioxide), a large fraction of the population is still exposed to excessive concentrations of certain air pollutants, in particular volatile organic compounds, particulate matter, and ammonia. Therefore, environmental standards are continuously being raised and more effective technologies for depollution of gas streams are needed.

Air non-thermal plasmas (NTPs) are strongly oxidizing environments and thus useful means for the activation of advanced oxidation processes for air and water remediation. NTPs are used for the generation of ozone as an important oxidant for water or air cleaning and the removal of dust from flue gases in electrostatic precipitators [1, 2, 3]. Within the last years deodorization by means of NTP or NTP-supported methods has reached the level of commercialisation.

The term plasma denotes an ionised gas containing free electrons, ions and neutral species (atoms and molecules) characterized by collective behaviour. Often referred to as the "4th state of matter", plasmas have unique physical and chemical properties distinct from solids, liquids and gases. They are electrically conductive, respond to electromagnetic fields, contain chemically reactive species as well as excited species and emit electromagnetic radiation in various wavelength regions. Plasmas are generated artificially by supplying energy to gases, liquids or solids.

In principle, gaseous plasma depollution can be done by an increase of the gas enthalpy in so-called "translational" plasmas, which are non-equilibrium plasmas (electrons and heavy particles have different mean kinetic energies) but with gas temperatures reaching several thousand K. Plasma torches, arcs, arc jets or gliding arcs are examples for such plasma-based incineration sources [4]. However in the following we will focus on non-thermal plasmas which stays at moderate gas temperatures, also referred to as "cold" non-thermal plasmas. Another approach, which is also excluded in this chapter, is the electron beam injection, which is under development for very large combustion facilities (coal fired power plants) [5].

The most common method of producing "cold" non-thermal plasmas for technological applications is based on the application of an electric field to a gas. If the applied field exceeds a certain threshold value (breakdown field strength) a gas discharge and thus plasma are generated. The specific feature of so-called non-thermal plasmas is that most of the coupled energy is primarily released to the free electrons which exceed in temperature that of the heavy plasma components (ions, neutrals) by orders of magnitude. Thus, strong non-equilibrium conditions are achieved in which the gas temperature remains nearly at or slightly above room temperature ("cold") while "hot" electrons initiate chemical processes resulting eventually in the oxidation of pollutants.

This chapter is meant to provide some insight into the application of NTP for air remediation. After a brief introduction to summarize the principles of NTP generation, the main aspects of discharge physics and plasma chemistry involved in air treatment are described and discussed. Special focus is on the removal of volatile organic compounds (VOCs). The two major types of plasma sources for such applications, namely dielectric barrier discharges and corona discharges, are described in the next section. Various aspects of plasma generation and the ensuing chemical processes will be discussed in two separate sections based largely on work carried out by the Authors in Padova and in Greifswald. Finally, some conclusions and perspective outlook on the field are given.

2. Overview non-thermal plasma-reactors and processes for depollution of gases

The most common principles for the generation of "cold" non-thermal plasmas are dielectric barrier discharges (DBDs) and corona discharges. They are most suitable for treating exhaust gases from manufacturing processes and mobile emission sources, as they offer a compact design and good scalability.

DBD based devices consist of at least two electrodes enclosing a gas space which is filled with, or bound by, an insulating material [6]. Typically, dielectric materials such as glass, quartz, ceramics, enamel, plastics, silicon rubber or Teflon are used as barrier materials. There are many possible DBD arrangements. Traditionally, DBDs were generated in parallel plate reactor geometries or in coaxial cylindrical reactor geometries, as shown in Figure 1 (left). The dielectric barrier(s) can cover one or both electrodes entirely, but they can also be separated from both electrodes, forming two discharge gaps. The gap widths are typically in the range of 0.1 – 5 mm. When a sufficiently high voltage is applied between the electrodes, an electrical breakdown occurs and plasma is formed. Furthermore, both electrodes can be arranged in such a way that they are in direct contact with the barrier. In this case, the gas discharge is formed in the gas at the exposed electrode and propagates along the dielectric surface, and is therefore called 'surface discharge' or 'surface DBD'. In 'co-planar DBDs' both electrodes are embedded in the insulator. The so-called 'sliding DBD' is based on surface DBDs but with a third electrode which is placed opposite to the top electrode on the dielectric surface [7]. This arrangement allows the discharge 'to slide' over the dielectric. The so-called 'packed bed reactor' is also often classified as DBD-type plasma. Dielectric or ferroelectric pellets are packed in between the two electrodes. Due to polarization of the pellet material, regions with high electrical fields are generated, leading to gas discharges in the void spaces between the pellets and on their surfaces [8]. Porous ceramic foams can also be used instead of pellets beds [9]. Packed-bed reactors are relevant for depollution since the filling can feature catalytic properties.

The insulator suppresses large currents on the electrodes and thus keeps the plasma in the non-thermal regime [6]. Because of the capacitive coupling of the insulating material to the gas gap, DBD generation requires alternating or pulsed operating voltages. For the treatment of gas streams the gas is injected into the device flowing along the electrode arrangement. In industry DBDs are used for the generation of ozone, deodorization, surface treatment and many more.

Corona discharges are characterized by non-uniform electrical field geometries, e.g. needle-to-plate or wire-in-cylinder electrode arrangements, as shown in Figure 1 (right). DC and low frequency AC operated corona discharges expand from the needle or wire electrode in the outer regions towards the plate or cylinder electrode. The energy is mainly dissipated in the high-ohmic region of non-ionized gas in the outer drift region, where the electrical field is lower than in the plasma region around the wire/needle electrode. In this region the discharge is not supported anymore and is thus kept in the non-thermal regime [10]. DC-operated corona discharges are used in electrostatic precipitators. For environmental applications (e.g. VOC removal, water purification) corona discharges operated by pulsed high voltage are proposed since higher densities of reactive species can be achieved [11]. Pulsed corona discharges are characterized by plasma regions which fill a much larger fraction of the discharge gap than DC or low frequency corona discharges.

Figure 1.

Top pictures: Cross sectional view of coaxial DBD (left) and DC corona discharge (right) arrangement; Bottom pictures: Coaxial DBD (left) and corona discharge (right) reactor for treatment of air streams.

In molecular gases at atmospheric pressure, corona discharges and DBDs are typical examples of non-uniform, filamentary plasmas, consisting of many individual microdischarges or discharge channels. Each volume element of the flowing gas is repeatedly subjected to the action of these filaments as it passes through the reactor. Non-thermal plasma based remediation of air is due to chemical reactions with photons and active species created in the plasma, namely radicals or ions. The different physical and chemical processes associated with and induced by non-thermalizing discharges span a time range of about 12 orders of magnitude. The equilibrium of the electrons with the local electrical field is usually approached within picoseconds [6, 12, 13]. Ionization and electrical breakdown typically proceed at the nanosecond time scale via electron collisions. For example, DBD microdischarges in atmospheric air have duration of about 20 - 50 ns. The development of discharge channels and microdischarges is dominated by the build-up and spatio-temporal enhancement of volume space charges, resulting in propagating perturbation of the electric field, which has been investigated as a cathode directed streamer or ionization wave [12, 13]. In the filaments the highest electron density and electron temperature are achieved and electron-induced dissociation and thus formation of radicals occur. The pollutant degradation is initiated by secondary reactions with these free radicals and ions, mainly on a micro- to millisecond time scale, i.e. after the active discharge filament has faded [14]. Ion-molecule reactions occur on an intermediate time scale, typically in the range of 10 ns up to 1 µs. The pollutant molecules react with oxidizing atoms and radicals (e.g. O, OH) or with plasma-generated ozone (O3). Water vapor may play an important role, as it acts as the precursor for hydroxyl radicals (OH) and hydroperoxyl radicals (HO2). When hydrocarbons or other VOCs are present in the gas, other radicals are also produced and radical chain reactions occur. Beside electrons, photons or collisions with metastable or excited species can lead to ionization.

The chemical equilibrium including heat and mass transfer is commonly settled within milliseconds to seconds.

The application of NTP for pollutant degradation in gases in industry must comply with the demands on removal efficiency, energy efficiency and selectivity.

The removal efficiency is defined as the removed molar fraction of the pollutant related to the initial molar fraction Cin: η=(Cin-Cout)/Cin (Cout is the molar fraction of the pollutant after the plasma treatment) [2, 14]. Energy efficiency relates to the energy needed to achieve a given removal efficiency and can be expressed in several ways. For example the energy yield is the decomposed pollutant mass per dissipated energy. A high energy yield is not necessarily assuring sufficient removal efficiency as this is determined by the initial molar fractions of the pollutant [2]. The removal efficiency and the energy efficiency depend on the specific energy density of the plasma and are also determined by a number of conditions (gas composition, humidity and temperature; level of initial contamination) [2, 14].

Selectivity is defined as the fraction of the desired product of the plasma-chemical conversion to the total amount of products of the conversion process. The chemistry and fraction of desired products and undesired by-products can also be characterized by mass balances (e.g. carbon balance in the case of conversion of hydrocarbons). A high selectivity is required to achieve a reasonable performance in terms of energy efficiency and by-products [2]. High reactivity of radicals usually results in a poor selectivity, since competing reactions which result in the formation of undesired by-products happen simultaneously. One important reaction which consumes oxygen atoms alongside the reactions with pollutant molecules is the generation of O3. For some pollutants (e.g. NOx, alkenes and other unsaturated VOCs) O3 is an efficient oxidizer, but in other cases it constitutes an additional pollutant by-product.

In the following paragraphs the main aspects of pollutant degradation by means of DBDs and corona discharges are discussed using selected examples. The discussion is focussed on VOCs as a class of contaminants present in many different industries (e.g. semiconductor manufacturing, chemical processing, painting and coating) as well as in indoor air (outgassing of paint, carpets etc.). VOCs contribute to the generation of photochemical smog and to certain health diseases like nausea and skin irritation. Some are associated with high cancer risk [15, 16]. Conventional methods for VOCs removal are thermal oxidation, condensation, absorption and biofiltration. The thermal oxidation and condensation are economic only for situations in which VOCs are present in moderate to high concentrations. The absorption process does not destroy VOCs but only transfers them to another medium. In addition, this technology suffers from problems arising by deposits of dirt or clog on filters. Biofilters are useful only for VOCs that have some solubility in water and they are cost-effective if the volume of air to be treated is in the range of 104 – 105 m3/h. In NTP energy of about 10-30 eV are needed to produce an O-atom or an OH-radical in (humid) air, which makes the decomposition also energy consuming [14]. However, the total energy consumption can be low in case of small concentrations of pollutants. Thus NTP-based processes are feasible for low contamination levels. For VOCs, this level is about 100 mg/mN3 (N refers to standard conditions for pressure and temperature) [2, 14].

Among the different VOCs which are being routinely monitored for air quality, toluene (methylbenzene, C7H8) is one of the most important ones. Toluene is widely used as feedstock in the chemical industry for the synthesis, among others, of drugs, dyes, explosives, and as a solvent (e.g. thinner, paints, adhesives). Exposure to toluene is known to affect the central nervous system and may cause tiredness, confusion, weakness, memory loss, and nausea. Toluene is water-insoluble and thus cannot be scrubbed. For its wide use, diffusion and well known properties and reactivity, toluene has become sort of a standard for testing and comparing non-thermal plasma based air treatment for VOCs removal. Thus, the discussion in the following sections will focus largely on experiments with toluene. The conversion of toluene in hybrid systems in which NTP is combined with a catalyst is also being extensively studied and has been reviewed [17]. Such hybrid processes will not be covered in this chapter.

3. VOC removal by means of corona discharges

Air plasma produced by corona discharges and its performance in the oxidation of VOCs are being investigated in Padova using a prototypal large corona reactor [18–20]. The reactor and the auxiliary apparatus were designed in order to achieve stable and reproducible plasma regimes and experimental conditions, which are necessary for quantitative kinetic and product studies. Reproducibility and stability of experimental conditions in our set-up allow to test and compare the performance of different corona regimes, notably dc+, dc– and pulsed+ within the same apparatus and under otherwise identical experimental conditions. The experimental set-up, comprising the corona reactor, the gas flow line and instrumentation for in line and off line analysis of the treated gas, is schematically reproduced in Figure 2.

Figure 2.

Schematics of corona reactor, gas flow line and instrumentation for in line and off line analysis of the treated gas.

The corona reactor has a wire/cylinder electrode configuration. The wire electrode (stainless steel, outer diameter 1 mm) is electrically connected to the high voltage supply and fixed along the axis of a stainless steel cylinder (38.5 mm i.d. x 600 mm) which is electrically grounded.

The reactor can be energized by dc or pulsed high-voltage power. The dc power supply has an output voltage of ± 25 kV and an output current of 0 – 5 mA. For generating pulsed corona, a pulsed high voltage with dc bias (PHVDC) was used, based on a spark gap switch with air blowing, with the following specifications: dc bias of 0 – 14 kV, peak voltage of 25 – 35 kV (with dc bias), peak current up to 100 A, maximum frequency 300 Hz, rise-time of the pulses less than 50 ns. To measure the power input two homemade current probes (shunt), one of 1.1 Ω for pulsed current, the other of 52 Ω for dc current, were used. The experimental apparatus was described in detail previously [18].

The reactor is connected to a gas flow line made of Teflon tubing (inner diameter 4 mm). The air/VOC mixture is prepared by bubbling synthetic air (80% nitrogen: 20% oxygen from AirLiquide) through a sample of liquid VOC and by diluting the outcoming flow with a second flow of synthetic air to achieve the desired gas composition (VOC mixing ratio in the 100 - 1000 ppm range) and flow rate (usually kept constant at 450 mLNmin-1). The gas flow line is equipped with a loop for humidification and with a probe to measure the humidity. The treated gas exiting the reactor goes through a small glass reservoir equipped with a sampling port from which aliquots are withdrawn with a gastight syringe for off-line chemical analysis by GC-MS (Agilent Technologies 5973) and GC-TCD/FID (Agilent Technologies 7890). In line IR analysis is performed with an FTIR Nicolet 5700 spectrophotometer using a 10 cm long gas cell with windows of NaCl (for experiments with dry air) or of CaF2 (for experiments with humidified air). The determination of ozone, CO, and CO2 were performed by integration of characteristic IR bands as described previously [18]. The determined conversion data, i.e. [VOC]/[VOC]0 as a function of the specific input energy (SIE, also referred to as specific energy density SED) usually follow a first order exponential decay profile. The SIE was determined as described previously for dc [18, 21] and pulsed [20, 21] corona, respectively. The data are thus interpolated with the equation (1) to obtain the energy constant kE, which is a measure of the process energy efficiency.

[VOC]=[VOC] 0 e (k E SIE)  [VOC]=[VOC]0e(kESIE)

(1)

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Current/voltage characteristics of dc corona, both of positive and negative polarity, were monitored in synthetic air with and without VOC admixture (500 ppm VOC concentration). For each applied voltage, the mean current intensity was measured, after a stabilization time of 5 minutes, using a multimeter. The ions present in the air plasma produced by +dc and –dc corona were investigated using an APCI (Atmospheric Pressure Chemical Ionization) interfaced to a quadrupole mass analyzer (TRIO 1000 II, Fisons Instruments) [22, 23]. A schematic drawing of the arrangement and the gas inlet systems is shown in Figure 3.

Figure 3.

Schematics of APCI ion source and gas inlet system (1) quadrupole analyzer, (2) rotary pump, (3) diaphragm pump.

The corona discharge is kept at atmospheric pressure by a flow of synthetic air (4–5 Lmin-1) introduced through the nebulizer line, a capillary of ca. 2 mm (inner diameter). Vapors of the desired VOC, stripped by an auxiliary flow of synthetic air (typically 5–50 mLmin-1) from a liquid sample contained in a reservoir, enter the APCI source through another capillary (inner diameter 0.3 mm) placed coaxially inside the nebulizer line. A second line allows for the introduction of water vapors as desired. The needle electrode for corona discharge was kept at 3 kV. Ions leave the source through an orifice (50 μm in diameter) in the counter electrode, called the "sampling cone" and held at 0150 V relative to ground. The ions then cross a low pressure region (down to ca. 10-2 Torr) and, through the orifice in a second conical electrode, called the "skimmer cone" and kept at ground potential, reach the low pressure region hosting the focusing lenses and the quadrupole analyser. Prior to running the experiments with the VOC, a preliminary analysis is routinely conducted to acquire the "background" spectra with only synthetic air and humidified synthetic air a).

The efficiency, products and mechanisms of VOC oxidation were studied systematically under variation of the corona type (dc or pulsed), the corona polarity (negative or positive), the VOC (a few hydrocarbons, halogenated and oxygenated organic compounds have been investigated), the VOC inlet concentration and the level of humidity. These studies have provided a large body of experimental results which give insights into corona induced chemical oxidation and useful hints for its application.

The type of corona has major impact on the process efficiency. In Figure 4 an example is shown, which is reporting a comparison of the decay profile of toluene concentration as a function of SIE under three different corona regimes: dc+, dc– and pulsed+ [21]. The much better efficiency of pulsed+ corona with respect to dc corona of either polarity is evident. Also evident is the better performance of dc– with respect to dc+ corona. Analogous results were obtained in similar experiments with other VOCs, including n-hexane [18, 20] and dibromomethane [24].

Figure 4.

Decay profile of toluene (500 ppm in synthetic air) as a function of SIE in corona induced oxidation under the following regimes: pulsed+, dc– and dc+ [21].

The better efficiency of pulsed+ corona with respect to dc corona of either polarity is consistent with the results of an emission spectroscopy study which showed that at any specific input energy significantly greater average electron energy is obtained with pulsed corona than with dc coronas [25]. Correspondingly, a higher density of reactive O atoms is observed in pulsed+ corona than with dc coronas [25]. Due to the filamentary nature of the plasma, not only the energy but also the spatial distribution of electrons and other short-lived reactive species is very different from that in glow dc coronas: the plasma is affecting a relatively larger volume thus accounting for a more efficient process.

A second important variable is the VOC initial concentration. Usually, the corona induced oxidation efficiency decreases as the VOC initial concentration [X]0 is increased and often a linear correlation is observed between kE and 1/[X]0 within a significant range of concentrations. An example is shown in Figure 5.

Figure 5.

Dependence of process efficiency (kE) on the reciprocal of VOC initial concentration for dc+ corona induced oxidation of acetone in dry synthetic air.

Other similar cases are reported in the literature [14, 24, 2629] and have been interpreted based on a simple scheme of inhibition by the intermediates formed in the VOC reaction [30]. Finally, the VOC chemical composition and structure also matters and different VOCs are oxidized with different efficiencies under the same experimental conditions. A few representative data are reported in Table 1.

VOCb)

dc–

  

dc+

  

  

dry air

humid air

dry air

humid air

n-hexane c)

7.710-1

1.1

2.010-1

1.810-1

i-octane

4.210-1

7.510-1

1.310-1

1.210-1

toluene

4.110-1

8.110-1

1.410-1

1.310-1

CH2Br2

2.110-1

2.610-1

6.410-2

5.110-2

CF2Br2

1.410-1

1.110-1

4.510-2

3.910-2

Table 1.

Reaction efficiency data, expressed as kE in L kJ-1 units, for corona processing of different VOCs in dry and in humid (40% RH) synthetic air.

[i] - a) Data are from ref. [27] unless otherwise specified. b) VOC initial concentration was 500 ppm. c) Data from ref. [25].

This outcome might not have been anticipated a priori since the generally accepted notion is that plasma chemical processes proceed via radical reactions which are usually very fast and poorly selective. This is the case, for example, for the reaction of OH radicals with organic compounds which is viewed as a major contributor to VOC oxidation in humid air plasmas.

The data in Table 1 show instead that air plasmas are somewhat selective. This selectivity might originate from either of two circumstances (or possibly a combination of the two): within a given type of plasma, say that produced by dc–, different VOCs either react along different paths or react with the same species but at different rates. The vast available bibliography on rate constants for reactions of many VOCs with air plasma reactive species (atoms, radicals, ions) and on ionization energies and electron affinities provides tools to exclude some possibilities and sort out which reactions are most likely paths. Chemical knowledge and intuition help in providing model VOCs to be used as reactivity probes.

A most intriguing and informative response is found in studying the effect of humidity on the efficiency of VOCs oxidation. The data in Table 1 show that for all VOCs considered, except CF2Br2, the presence of humidity in the air produces an increase in efficiency with dc– corona and no effect or a slight decrease in efficiency with dc+ corona. The increase in efficiency observed with dc– is rather straightforwardly attributed to the OH radicals formed by corona discharges in humid air. OH radicals are among the strongest known oxidants of VOCs. Compare for example the rate constants for reaction of toluene with atomic oxygen (2) [31] and with OH radical (3) [32], a channel becoming more available in humid air plasma:

C 7 H 8  + O  Products                    k 298  = 7.610 14  cm 3 molecule 1  C7H8 + O  Products                    k298 = 7.61014 cm3molecule1s1

(2)

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C 7 H 8  + OH  Products                 k 298  = 5.710 12  cm 3 molecule 1  C7H8 + OH  Products                 k298 = 5.71012 cm3molecule1s1

(3)

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Support for the conclusion that reaction with OH radicals is important in dc– corona induced oxidation of hydrocarbons and of CH2Br2 (Table 1) came from experiments with CF2Br2 (halon 1020) [27]. Like other perhalogenated saturated hydrocarbon, CF2Br2 is not attacked by OH and other atmospheric radicals: the reaction of CF2Br2 with OH radicals is more than 220 times slower [33] than that of CH2Br2. And indeed there was no increase in efficiency for dc– processing of CF2Br2 in humid air, but rather a slight decrease with respect to dry air. This slight decrease in efficiency was attributed to reaction (4) which contributes to reduce the average electron energy while producing OH radical, which is unable to attack this specific VOC.

H 2 O + e    OH + H + e   H2O + e  OH + H + e

(4)

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Less straightforward was to explain the decrease in efficiency observed with dc+. To make sure that OH radicals also form in dc+ corona regime and to compare their relative densities in dc+ and dc– air plasmas the well known reaction of OH with CO to form CO2 (eq. (5)) was used [34].

CO + OH  CO 2  + H                   k 298  = 2.4110 13  cm 3 molecule 1  CO + OH  CO2 + H                   k298 = 2.411013 cm3molecule1s1

(5)

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Indeed, in a control experiment CO did not react at all in dry air under the effect of either dc+ or dc– corona. In contrast, in humid air (40% RH) reaction (5) occurs both with dc–, more efficiently, but also with dc+ corona, thus proving the presence of OH radicals in such plasmas. Since with dc+ oxidation of hydrocarbons is less efficient in the presence of OH radicals than it is in dry air (Table 1), it was concluded that reaction with OH radicals is not the dominant initiation channel for their oxidation in dc+ corona. Thus, it appears clearly that VOC oxidation induced by dc+ and dc– corona in air occurs by different mechanisms. For the investigated hydrocarbons (see Table 1) oxidation induced by dc+ corona is believed to be initiated by ion-molecule reactions. Support for this hypothesis comes from direct observation of the ions within the plasma achieved by APCI-mass spectrometry analysis and from comparison of current/voltage (I/V) profiles measured with only synthetic air and with VOC-containing synthetic air.

Figure 6 (a) reports I/V data monitored in experiments with toluene (500 ppm initial concentration). It is seen that for dc– the profiles determined with and without toluene are nearly superimposed, whereas in the presence of toluene for dc+ the current intensity measured is significantly lower, at any applied voltage, than found in pure synthetic air. Since corona current is due to ion transport across the drift region of the interelectrode gap, the results suggest that with dc+ corona different ions are present in pure air and in toluene containing air. Accordingly, different average ion mobilities are derived from the current/voltage characteristics of Figure 6 (a) [35–37]: 2.35 cm2V-1s-1 for pure air and 1.79 cm2V-1s-1 for toluene-containing air, respectively.

APCI-mass spectrometry is a powerful tool for monitoring and characterizing the ions formed by corona discharges and their reactions. The APCI-mass spectra reported in Figure 6 (b) show the ions present in the plasma produced in synthetic air by dc– and dc+ corona discharge, respectively. These ions are water clustered O2 and O3 ions (O2(H2O)n (n = 0-2: m/z 32, 50, 68), O3(H2O)n (n = 0-1: m/z 48, 66)) as well as O2(O2) (m/z 64) for dc– corona and H3O+(H2O)n (n = 2 - 3: m/z 55, 73) and NO+(H2O)n (n = 1 - 2; m/z 48, 66) for dc+ corona, respectively.

Figure 6.

(a) Current/voltage profiles measured with dc– and with dc+ corona in pure air (open symbols) and in toluene (500 ppm) containing air (closed symbols). (b) APCI mass spectra recorded with dc– and dc+ corona in pure air. (c) APCI mass spectra recorded with dc– and dc+ corona in air containing toluene (500 ppm).

The APCI mass spectra recorded under the same experimental conditions except for the presence of a small amount of toluene (500 ppm) in the air are shown in panel (c) of Figure 6. The effects are significantly different for dc– and dc+ corona. Thus, with dc– corona the major ions observed in the plasma are the same regardless of whether toluene is present or not in the gas (compare the mass spectra on the left-hand side in panels (b) and (c) of Figure 6). These observations are fully consistent with and provide a rationale for the nearly identical I/V curves determined for dc– in pure air and in toluene containing air (panel (a) of Figure 6).

In contrast, in the case of dc+ the mass spectrum of toluene containing air is completely different from that of pure air (compare the mass spectra on the right-hand side in panels (b) and (c) of Figure 6). Thus, in air contaminated with toluene (500 ppm) the prevailing charged species are T+ (m/z 92) and [T+H]+ (m/z 93) (T stands for the toluene molecule), along with their ion-molecule complexes T+(T) (m/z 184) and [T+H]+(T) (m/z 185). These ions form [38-40] via exothermic charge- and proton transfer ion-molecule reactions (eq. 6 and 7), characterized by rate constants of 1.8x10-9 and 2.2x10-9 cm3 molecule-1 s-1, respectively [38], followed by ion-molecule complex formation (eq. 6a and 7a).

T + O + 2   T +  + O 2 T +  + T + M  T + (T)+ M  (a)  T + O2+  T+ + O2T+ + T + M  T+(T)+ M  (a)

(6)

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T + H 3 O +   [T+H] +  + H 2 O[T+H] +  + T + M  [T+H] + (T)+ M   (a)  T + H3O+  [T+H]+ + H2O[T+H]+ + T + M  [T+H]+(T)+ M   (a)

(7)

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Finally, the NO+(T) (m/z 122) ion-molecule complex is also observed. Thus, mass spectroscopic ion analysis provides a rationale, at the molecular level, for interpreting I/V curves observed with dc+ (Figure 6 panel (a) right hand side). In addition, these results suggest that ionic reactions might be responsible for the initial stages of toluene decomposition induced by dc+ corona. This hypothesis is consistent with the observed insensitivity of the dc+ process efficiency to the presence of humidity, which rules out a significant role of the OH radical.

Analogous results were obtained with other hydrocarbons leading to the conclusion that the initial step of oxidation depends on the plasma regime applied: ion-molecule reactions are favored with dc+ whereas reactions with O atoms and OH radicals prevail in the case of dc– corona discharges.

The yield of the final oxidation product, CO2, as a function of SIE has been determined and compared with the profile of VOC conversion (Figure 7). CO2 production is clearly less energy efficient than VOC conversion as is reasonable to expect for a process which involves many steps and oxidation intermediates.

Figure 7.

Profiles of VOC decay and CO2 production as a function of SIE for treatment of toluene (500 ppm) with dc– corona in dry synthetic air.

In comparing pulsed and dc coronas, at any given value of VOC conversion the yield of CO2 increases in the order pulsed+ < dc– < dc+. This is evident from the data shown in Figure 8a concerning experiments with n-hexane. Comparing the results corresponding to a given decomposition fraction of the VOC, for example 0.7 (70% conversion), the corresponding amount of CO2 released with dc+ is about 4 times larger than with pulsed+ and about 1.6 times larger than with dc–. Thus, among the different types of corona tested, dc+ has the poorest efficiency for VOC conversion but the best selectivity for CO2 production. On the other hand, a consistently lower CO2/CO ratio is found with dc+ than with dc– and pulsed+ (Figure 8b).

Figure 8.

CO2 production and CO2/CO ratio for treatment of n-hexane (500 ppm) with dc+, dc– and pulsed+ corona in dry synthetic air. The data are displayed as a function of the fraction of decomposed n-hexane.

Figure 9.

Aldehydes and ketones detected as intermediates in the oxidation of n-hexane (500 ppm in dry synthetic air) induced by pulsed+, dc– and dc+. The data are displayed as a function of the fraction of decomposed n-hexane [18].

In search for the missing fraction of organic carbon several oxidation intermediates were identified and quantified by means of GC/MS and GC/FID analysis and proper standards. In the case of n-hexane the major detected intermediates were a few aldehydes and ketones, as shown in Figure 9. It is seen that the concentration of most of these intermediates reaches a maximum and then decays, showing that they are in turn oxidized in air non-thermal plasmas. It is also seen that, with the exception of acetaldehyde, the concentrations of these organic intermediates are very small under any of the applied conditions. The experimental data have been fitted according to a simple kinetic model for consecutive reactions [41] to obtain relative reactivity data of the intermediates with respect to that of the precursor, n-hexane [18].

Figure 10.

FT-IR spectra recorded in experiments with dc+ corona (+19 kV) in (a) pure synthetic air, used as reference spectrum, and in synthetic air containing 500 ppm of the following VOCs: (b) n-hexane; (c) toluene; (d) CH2Br2; (e) CF2Br2.

In line FT-IR spectroscopy gives a comprehensive overview of the composition of the treated gas analyzed at the outlet of the corona reactor. Besides CO2 and CO, other species can be conveniently determined, including ozone, various nitrogen oxides and derivatives (N2O, HNO3, etc), and, depending on the specific VOC, also other volatile organic oxidation intermediates. A few examples, reported in Figure 10, show characteristic bands for VOC specific products such as formic acid in the case of toluene and CF2O in the case of CF2Br2.

The production of HNO3 is high and ozone is almost completely absent in corona discharge treatment of air containing bromo derivatives CH2Br2 and CF2Br2, (Figure 10 d and e). These observations have been explained considering reactions and catalytic cycles involving BrOx (x = 0, 1) and NOx (x = 1, 2) species [24], which have been extensively investigated as major contributors to the depletion of stratospheric ozone [42].

4. VOC removal by means of barrier discharges

The experimental results obtained with a surface DBD reactor developed at INP Greifswald are reported. The discharge arrangement consists of two metal woven meshes and a dielectric plate (mica) in between the two electrodes [43]. As shown in Figure 11 the surface DBD arrangement (110 x 80 x 3 mm) was installed in a gastight chamber made of poly(methyl methacrylate) (PMMA) where the gas mixture to be treated (mixed from gas cylinders by means of mass flow controllers) was conveyed with a total gas flow of 75 LN/h. In order to vary the humidity of the gas mixture the partial gas flow from oxygen gas cylinder was directed through a water containing bubbler. Water content of around 0.5% was realized in this way. Most of the gas flows along the electrodes configuration instead of entering the active plasma region between the electrodes and the dielectric as depicted by the arrows in Figure 11 (right). An advantage of this configuration is the very small back pressure, which is desired for the treatment of large gas flows.

The plasma reactor was energized with a programmable high-voltage power source and a high-voltage transformer. The frequency of the applied voltage was ranged from 400 Hz to 1 kHz. The power dissipated into the plasma was analyzed by recording the high voltage operating the reactor via a high voltage probe. Additionally, the voltage drop over a capacitor (capacitance 100 nF), connected in series with the reactor between the grounded electrode and protected earth, was recorded. By multiplying the voltage drop over the capacitor by the capacitance the transferred charge was obtained.

Samples of the gas mixture were analyzed by Flame Ionization Detector (FID) (Testa FID-2010T), measuring the total amount of organic carbon present in the exhaust gas. Additionally, a Fourier Transform InfraRed spectrometer (FTIR) (Bruker Alpha, spectral resolution 1 cm-1, optical path length 5 m) was used to monitor the processed gas mixtures. The gas cell of the spectrometer was heated up to 40°C in order to avoid water condensation. Higher temperatures would be desirable to avoid water condensation but cannot be used otherwise ozone will decompose and give rise to sort of a "post-plasma" contribution to toluene oxidation which should be avoided.

Figure 11.

Experimental setup for toluene removal studies (left) and detailed horizontal cut view of the Surface DBD reactor (right).

In order to obtain comparable results of the measurements under the selected conditions the operating voltage was chosen as the electrical parameter to be set for every measurement. The electrical data recorded during the experiments were investigated during the analysis procedure. The power input into the plasma reactor was calculated by integrating the area of the charge-voltage plot (Q-V plot) and multiplying the resulting value with the frequency of the applied voltage [44]. By this, it was found that the Q-V plots for different frequencies at the same driving voltage were almost identical.

Examples are given in the left part of Figure 12. The black curves show the Q-V plots recorded at a frequency of 400 Hz, whereas the red curves display that one recorded at 1 kHz. The operating voltage was 8.3 kV and 6.9 kV, respectively. The equality of the Q-V plots implies that the energy transferred per cycle into the plasma is almost identical. Thus, the power input for a fixed operating voltage should depend only on the frequency in a linear relation [44]. Using the frequency ratio 2.5 (1000 Hz divided by 400 Hz) and multiplying it with the power input measured at 400 Hz, one gets the calculated power at 1 kHz. In the right part of Figure 12 the power measured at 400 Hz (black boxes), the power calculated for 1 kHz (blue triangles) and the power measured at 1 kHz (red circles) are shown. The values calculated and measured at 1 kHz are in good agreement and can be taken as another evidence of the proportionality of frequency and power, as already mentioned by Kogelschatz [44] and Manley [45].

Further investigations showed that the slope from the bottom right corner to the upper right one of the charge-voltage plot, which gives the capacitance of the plasma reactor during the discharge period, increases with increasing the operating voltage (Figure 13, left). The reason is suggested to be the increase in the active area of the electrode, which means the surface of the electrode covered with plasma. Photographs of the plasma were taken (Nikon D5100, aperture 5.6, exposure time 30 s) and reworked with an image manipulation program (Gimp 2.8, color correction) to make the plasma visible (Figure 13, right). An analysis of the extension of the visible plasma was performed. The obtained values were normalized as well as the values of the measured capacitance. The result is given in the left part of Figure 13. The normalized capacitance (red circles) increases linearly with the increasing driving voltage. The normalized active area of the plasma (black boxes) increases almost linearly, except the value at 7.3 kV. The linear relation between the dielectric capacitance and the amplitude of the applied voltage is further confirmed by the fact that the bottom right corner as well as the left top corner of the Q-V-plot (which both correspond to the inset of the discharge in every half period of the applied voltage) are not sharp. This would be the case for a uniform breakdown of the gas discharge.

Figure 12.

Left: Q-V plots recorded at 8.3 kV and 6.9 kV at 400 Hz and 1 kHz under dry conditions. Right: Power input under dry conditions at 400Hz and 1 kHz and at 1 kHz calculated based on the power of 400 Hz.

Figure 13.

Left: Increase in active plasma area and increase in capacitance with respect to the operating voltage. Right: Photographs of the plasma operated at different voltages.

In a first step the overall plasma chemistry of toluene removal was investigated by FTIR. In Figure 14 samples of selected spectra are presented showing the so-called fingerprint region (i.e. wavenumber 700–1500 cm-1). The left graph shows infrared-spectra taken at an applied voltage amplitude of 8.3 kVPP and a frequency of 400 Hz under dry conditions. In the untreated gas mixture (black curve) the absorption band related to toluene is the only detectable band. With plasma (red curve) the strong absorption band of ozone appears at 1053 cm-1, which is the main stable by-product of an NTP operated under ambient air conditions. Additionally, nitric acid HNO3 is detected. It is assumed that HNO3 is formed by the reaction of intermediate NOx with hydroxyl radicals produced by the toluene decomposition. The toluene absorption band is replaced by a broad absorption band whose origin could not be identified. Because no infrared absorption spectrum of the known products or intermediates (formaldehyde, benzaldehyde, benzoic acid, benzene, nitrobenzene, phenol, formic acid, and acetic acid) of the toluene removal process fits to the measured spectra it is assumed that it is a compound emitted from the material of the reactor housing (PMMA). According to the results given by the FID this analyzed gas mixture does not contain any hydrocarbons at all.

Figure 14.

FTIR absorption spectra without plasma (black lines) and with plasma (frequency 400 Hz, operating voltage 8.3 kV, red lines) under dry (left) and wet conditions (right) of 50 ppm toluene in synthetic air.

Under wet conditions (right graph, same electrical parameters) there is no nitric acid detectable. Moreover, the absorption of ozone is much smaller than under dry conditions. Both phenomena are attributed to the consumption of energetic electrons, which under dry conditions are used to produce NOx as a necessary intermediate for the production of nitric acid. These changes result in a considerable production of formic acid. The lower energy efficiency in toluene removal under wet conditions is also assumed to be due to the consumption of high energy electrons for the vibrational excitation of water.

Figure 15.

Molar fraction of toluene under dry (black boxes) and wet (red circles) conditions at 400 Hz (circle) and 1 kHz (square).

FID is used to study the toluene removal since it is not sensitive to the main by-products of toluene removal that were identified by FTIR. With the concentration of toluene at about 50 ppm in the untreated gas mixture the molar fraction was calculated and plotted against the SIE. The results are shown in Figure 15. The removal efficiency increases with the SIE up to total removal at around 55 JL-1 under dry conditions (black boxes). The same efficiency is achieved for 400 Hz and 1 kHz. Under wet conditions (red circles) the removal efficiency is smaller and about twice as much energy is needed to achieve complete removal of toluene which is only obtained at 1 kHz for this conditions. This dependency on the frequency is only found under wet conditions.

In order to discuss the energy efficiency the energy constant parameter kE was evaluated according to the eq. (1). As reported in Table 2 the energy constants obtained under dry conditions are very similar. Those obtained under wet conditions are significantly smaller but also differ significantly.

Frequency

kE LJ-1

  

  

dry

wet

400 Hz

5.8510-2

3.0310-2

1 kHz

5.6310-2

2.0610-2

Table 2.

Reaction energy efficiency data, expressed as kE in L J-1 units, for DBD processing of toluene.

Quantitative analysis data for CO2 and CO produced under different experimental conditions are shown in Figure 16 as a function of the SIE. At dry conditions the production of CO is favored compared to CO2, while in humid conditions the amounts of both compounds are almost the same. The increased selectivity towards CO2 in humid conditions could be explained as follows. In the presence of water vapor in the plasma area the production of OH radicals is higher. These radicals can react with the CO molecules to produce CO2, according to the eq. (5).

Usually the formation of OH radicals in the plasma area is also accompanied by an enhancement of the energy efficiency in the VOC removal process (e.g. see Section 3 of this Chapter). In the case of this setup, as discussed above, the presence of water vapor in the process gas is obviously responsible of a decrease in the energy efficiency of toluene removal.

Figure 16.

Left: CO (empty symbols) and CO2 (full symbols) production during toluene decomposition experiments. Comparison at 400 Hz (black) and 1 kHz (red) and between dry conditions (full line) and wet conditions (dashed line). Right: CO2/CO ratio as a function of SIE at 400 Hz (black) and 1 kHz (red). Comparison between dry condition (full symbols and straight lines) and humid conditions (empty circle symbols).

Figure 17.

Carbon balance for the different experimental conditions being tested. 400 Hz (black symbols) and 1 kHz (red symbols) under dry (full square symbols) and wet (empty symbols and dashed lines) conditions.

In Figure 17 the selectivity to CO2 of the plasma treatment for pollutant degradation is reported. The black vertical line is referred to a value of SIE of 55 JL-1, the energy value at which the toluene is completely decomposed, but as reported in Figure 17 there is selectivity of 40% in dry conditions and of 60% in humid conditions. The complete oxidation of the toluene is achieved at a value of SIE of 150 JL-1, where all the VOC is decomposed to CO and CO2. The carbon fraction which is missing is mainly formic acid that could be easily removed from the effluent gas by means of water scrubbing. Despite the reduction of the toluene removal efficiency made by the presence of water vapor, it is clear how the selectivity to CO2 production is improved (Figure 16, right), but also the carbon balance is clearly improved at least until the energy value of 100 JL-1. Above the value of 150 JL-1 the carbon balance is exceeding 100% (marked by the red horizontal line). This is due to some additional degradation of the acrylic housing. This effect was also noted in the IR spectra where an additional band around 700 cm-1 was recorded (see Figure 14).

Because of the construction of the NTP-reactor with the electrode configuration in the middle of the discharge reactor and, therefore, a huge gas volume not in direct contact with plasma, the toluene to be removed hardly comes in contact with plasma. Thus, electron dissociation cannot be the main process. As ozone is generated in the air plasma it is a possible oxidizer of toluene. The effect of ozone on the toluene removal has been studied in a separate experiment. Therefore the reactor was operated with a pure oxygen gas flow which was mixed with the toluene polluted air in a separate reaction chamber (volume about 250 mL; not shown in Figure 11). The results of this experiment are presented in Figure 18.

Figure 18.

Ozone and toluene concentrations as a function of SIE (frequency 1 kHz) at the surface DBD arrangement. Comparison of the direct plasma treatment (red symbols) with the ozone injection experiment (black).

Ozone concentration increases and toluene concentration decreases with increasing SIE. The ozone production is similar in both direct discharge and ozonation treatments, but the toluene removal varies significantly. In the case of indirect treatment with ozone a small reduction of toluene is obtained. In additional experiments a similar reduction was obtained even without plasma operation. Thus it must be that toluene is adsorbed somewhere in the system (reaction chamber, pipes etc). In case of direct treatment at 157 JL-1 toluene is completely removed. These results show that reactions with ozone are not dominant, which is in agreement with the fact that the rate coefficient of the reaction of toluene with ozone is small (eq. (8) [46]) compared to the reaction with atomic oxygen (eq. (2)).

C 7 H 8  + O 3   Products              k 298  = 1.510 22  cm 6 molecule 1  C7H8 + O3  Products              k298 = 1.51022 cm6molecule1s1

(8)

Options

The reaction with atomic oxygen (eq. (2)) is considered to be the most important removal process.

Since for the production of these species energetic electrons are needed, the lower efficiency under wet conditions can be explained by their lower production efficiency due to the consumption of these electrons as well as a reduction of the electron temperature by water dissociation and vibrational excitation. More detailed investigation on the toluene by-products are needed to clarify the carbon balance and the selectivity of the process which remains to future investigations.

5. Conclusions and outlook

The feasibility of NTP for the removal of VOCs has been demonstrated by means of two different gas discharge concepts, namely DBD and corona discharge. There are evident peculiarities in the two different approaches and, notably, also among different types of coronas. Different discharge regimes create NTPs with distinct properties and compositions (type and density of reactive species), which reflect on the process chemical outcome. This knowledge is important for mastering the process product selectivity, i.e. the chemical composition of the treated gas. Likewise important is the process efficiency. Unfortunately, up to now it is possible to make a comparison between the two reactor arrangements only for the specific case of toluene. There are reviews in the literature in which the efficiency of different NTP processes is compared [17].

Regarding the main chemical reaction involved in the decomposition process of toluene a similar mechanism for DBD and dc– corona can be proposed. Here, the most important reactive species are radicals. The main difference in the overall efficiency of the decomposition process is when water vapour is present in the treated gas, but this opposite effect is totally in agreement with the physical properties of the DBD. In the case of the DBD configuration the volume of gas which is directly affected by plasma is small compared to dc– corona. Accordingly, in the case of the corona a much higher probability of collisions between reactive species and substrate molecules (toluene) exists, but also a high probability to generate OH radicals. In the case of DBD the presence of water vapour is quenching the high energetic electrons and reducing the chance to generate more reactive species with an overall result of a decrease in energy efficiency. Despite this, if we calculate the Energy Yield (EY) according to the equation reported in [47]:

EY=C in ηM0.15SIE  EY=CinηM0.15SIE

(9)

Options

where Cin is the starting concentration of the pollutant to be treated (ppm), η is the removal efficiency and M is the molar mass of the pollutants. For the DBD configuration EY value is 13.6 g/kWh while for the corona reactor, in pulsed+ mode, the value is 6.9 g/kWh.

Plasma type

Concentration range (ppm)

Maximum removal efficiency (%)

Energy Yield

(g/kWh)

DBD (this work, sec. 4)

50

>99

13.6

DBD packed with glass pellets [47]

1100

80

11.5

Pulsed Corona (this work, sec. 3)

500

>99

6.9

DBD packed with glass beads [47]

240

36

6.8

DBD [47]

400

23

5.2

DC corona [47]

5 – 200

93

0.4

Table 3.

Selected results on toluene removal with NTP in dry air.

In Table 3 are summarized selected results on toluene removal with NTP taken from [47] to better evaluate the EY of the two setups reported above. In general, the DBD setups present higher values compared to the corona ones. What is really interesting to note from this table is that not only the values of EY obtained with the setups developed by the authors are among the highest ones, but also these are the only cases in which the complete removal of toluene is reached.

In contrast to many other established technologies of air cleaning, NTPs can be controlled more or less instantaneously by their electrical operation parameters, and they can thus be adjusted to fluctuating gas flow volumes and/or contamination levels. However, nearly all practical processes of pollutant degradation in gases by means of NTPs are hybrid processes or a combination of NTPs with other technologies. In such combinations the NTP acts as an oxidation stage. However, the combination is not only a processing by means of subsequent methods but also offer multiple process synergies. Therefore NTP can be coupled with catalysts, adsorbing agents or scrubbing. For example, the oxidation of non-soluble VOCs results in soluble by-products such as formaldehyde or formic acid. This can also be used for the removal of NOx [48]. The so-called Plasma Enhanced Selective Catalytic Reduction (PE-SCR) of NOx offers many synergies between plasma and catalyst.

In this context, the combination of plasma treatment with adsorption methods has also been proposed for VOC abatement and deodorization [49]. Several manufacturers offer devices for deodorization which sometimes combine a NTP with an active carbon or molecular sieve stage. The odour reduction of so-called indirect plasma treatment was also demonstrated. Indirect treatment means that the plasma processed air is injected in the VOC containing off-gas. In such case short lived radicals and ions may be less involved in the decomposition processes but the operation lifetime of such system is much longer. During the direct plasma decomposition aerosols can be formed by nucleation of intermediate products and deposit as layers on the electrodes which interfere with the plasma generation. This is avoided by indirect treatment. Many of such DBD-based installations are worldwide used for deodorization in several factories for producing food for fattening, fish meal and flavouring substances. The installations are low-maintenance and need about one third to one fifth of the space as conventional technologies. In [50] the investment- and running cost of numerous waste air purification processes for a gas flow of 50,000 mN³/h and for <100 mg VOC/m³ in the flavour processing industry were determined and compared. NTP installation had the lowest investment costs (about 400,000 € compared to at least 700,000 € for combustive methods, biological filter or molecular sieve filtration) and second lowest operating cost (about 8 €/h, compared to 70 €/h for combustion and biological filtration with 35 €/h). Although the applicability of NTPs is devoted to low-contaminated gas streams, these examples show the high economic relevance and potential of such technologies.

Furthermore, the combination of NTP with absorbers offers the possibility to establish cyclic processes for the removal of low-concentrated pollutants [49]. In such processes, the low-concentrated pollutants are adsorbed and thus concentrated on solid matter in a storage phase. In the subsequent plasma phase, the adsorbed molecules are desorbed and decomposed by plasma activity. Since the retention time of the pollutants in the plasma phase and their concentration are increased, less energy is consumed in such a plasma-enhanced adsorption process. Such processes have been established to decompose different VOCs and NOx, as summarized in [51]. The decomposition of adsorbed ethanol on active carbon samples by means of ozone generated in the plasma has been investigated in [52]. The regeneration of clinoptilolite (a natural zeolite) loaded with NH3 has recently been shown by means of a packed-bed DBD reactor. The adsorbed NH3 is released at a relatively low temperature and low energy consumption [53]. A cycled adsorption and plasma process using mineral granulates consisting of 80 % halloysite in a packed-bed DBD reactor for the removal of formaldehyde CH2O was investigated in [51]. Here, the adsorbed CH2O molecules were decomposed into COX and hydrocarbons in N2 plasma. The total amount of decomposed CH2O and the selectivity towards CO2 increased with N2 gas space-times (i.e. the time required to process one packed bed volume of adsorbing material with gas) and with oxygen fraction in the carrier gas. The above examples demonstrate the high potential of plasma-enhanced techniques, which can increase efficiency and lower operational costs. However, more research and development are necessary in order to establish a wider industrial breakthrough.

6. Acknowledgements

The authors would like to thank Dr. Tomáš Hoder, Mr. Wolfgang Reich and Mr. Alexander Schwock for their support on this work.

The work is partly supported by the European Regional Development Fund, Baltic Sea Region programme 2007-2013 (project No 033, "Dissemination and Fostering of Plasma Based Technological Innovation for Environment Protection in The Baltic Sea Region", PlasTEP). The research leading to these results has received further funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n°316216.

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Comment +0

Removal of VOCs Using Nonthermal Plasma Technology

Tao Zhu1

[1] School of Chemical and Environmental Engineering, China University of Mining& Technology, Beijing 100083, China

1. Introduction

Volatile organic compounds (VOCs) are liquids or solids that contain organic carbon (carbon bonded to carbon, hydrogen, nitrogen, or sulfur, but not carbonate carbon as in CaCO3 nor carbide carbon as in CaC2 or CO2), which vaporize at significant rates. VOCs are probably the second-most widespread and diverse class of emissions after particulates.

VOCs are a large family of compounds. Some (e.g., benzene) are toxic and carcinogenic, and are regulated individually as hazardous pollutants. The control of VOCs in the atmosphere is a major environmental problem now. Toluene and benzene are two of the typical VOCs. They effluents in some industries, such as paints, paint thinners, fingernail polish, lacquers, adhesives, rubber, and some printing and leather tanning processes, have attracted more and more attention of researchers. The traditional methods of VOCs removal such as absorption, adsorption, and incineration and so on, which are referred to the new environmental condition have many technical and economic disadvantages. In these years, some new technologies, such as biologic process, photo-catalysis process, and plasma technology, were paid more and more attention.

As an emerging technology for environmental protection, there have been extensive researches on using non-thermal plasma (NTP) over the past 20 years. The major advantages of NTP technology include the moderate operation conditions (normal temperature and atmospheric pressure), moderate capital cost, compact system, easy operations and short residence times, Etc., compare to the conventional technologies). In the field of air pollution control, the NTP technology has been tested for the abatement of various types of hazardous air pollutants such as volatile organic compounds (VOCs), SO2, NOx, CFCs, odors, mercury, etc.

In this chapter, we will introduce a new synergy technology basing on non-thermal plasma for VOCs decomposition.

2. Experimental setup

The reaction system was a tube-wire packed-bed reaction system at ambient temperature and atmospheric pressure. The schematic diagram of the NTP system is shown in Fig.1. Dry air (78.5% N2, 21.5% O2) was used as a balance gas for VOCs decomposition. Air supplied from an air compressor was divided into two airflows and each flow rate was controlled with a mass flow meter. One airflow was introduced into a VOCs liquid bottle (3), which contained liquid VOCs. The air with a mass of saturated VOCs vapor was mixed with the other airflow in a blender (4) and the gaseous phase VOCs was diluted to a prescribed concentration. A wire-tube DBD reactor with packed materials as shown in Fig.2 was used.

Figure 1.

Schematic diagram of the NTP system - 1.air compressor 2.buffer 3.toluene liquid bottle 4.attemperator 5.blender 6.NTP reactor 7.mass flow meter 8.needle valve 9.high voltage 10.oscukkograph 11.gas chromatograph

Figure 2.

The NTP reactor - Reactor: organic-glass tube (i.d.50mm, length 150mm)Internal electrode: tungsten filament (i.d.0.5mm)External electrode: dense steel mesh

An AC power supply of 150 Hz was employed in the NTP reactor. The AC voltage was applied to the reactor in the radial direction and the voltage extension changed from 0 kV to 50 kV. The voltage and current of the discharge process were detected by an oscillograph (manufactured by American Tektronix Co., TDS2014). Primary power values were measured with the voltage-charge (V-Q)Lissajous method in the plasma reactors. The circuit diagram on power measurement with Lissajous developed and used in this study is shown in figure 3.

To investigate the electric characteristics of dielectric barrier discharge (DBD), the voltage applied to the reactor was sampled by a voltage divider with a ratio of 12500:1. Also, the current was determined from the voltage drop across a shunt resistor (R3 =10kΩ) connected in series with the grounded electrode. In order to obtained the total charge and discharge power simultaneously, a capacitor (Cm =2μF) was inserted between the reactor and the ground. The electrical power provided to the discharge was measured using the Q–V

Figure 3.

Circuit diagram on power measurement with Lissajous - R1=250MΩ, R2=10KΩ, R3=10KΩ, R4=20KΩ, C=0.33μF,K-Switch, CH1-Voltage sampling of oscillograph,CH2-Lissajous sampling of oscillograph,CH3-Current sampling of oscillograph

Lissajous diagram. Typical Lissajous diagram represents to be a parallelogram, and we could calculate power though calculated the area of parallelogram.

VOCs analysis was carried out by gas chromatography (manufactured by Aglient Co., HP6890N) with a flame ionization detector (FID). The byproducts were detected by GC-MS (manufactured by American Thermo Finnegan Co.) using EI mode, 70 eV and full scan. Ozone concentration produced in the NTP reactor was measured by an iodine-titration method. The plasma reactor employed an AC power supply of 50-500Hz scanning from 0 kV to 100 kV was applied to the reactor in the radial direction.

As evaluation criteria, the VOCs removal efficiency, reactor energy density, energy efficiency, the selectivity of CO2 and the selectivity of CO (eg., benzene) in the gas phase were calculated as follows:

VOCs removal efficiency (η):

η(%)=[VOCsinlet [VOCsoutlet [VOCsinlet  ×100% η(%)=[VOCs]inlet[VOCs]outlet[VOCs]inlet×100%

(1)

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Reactor energy density (RED):

RED(kJ/L)=inputpower(W)gasflowrate(L/min) ×60×10 3  RED(kJ/L)=inputpower(W)gasflowrate(L/min)×60×103

(2)

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Energy efficiency (ζ):

ς(g/kWh)=[VOCsinlet ×ηRED ×3.6×10 3  ς(g/kWh)=[VOCs]inlet×ηRED×3.6×103

(3)

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Selectivity of CO2 (ζ ζ ):

ζ(%)=[CO 2 ]6([Benzeneinlet [Benzeneoutlet ) ×100%  ζ(%)=[CO2]6([Benzene]inlet[Benzene]outlet)×100%

(4)

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Selectivity of CO (ξ ξ ):

ξ(%)=[CO]6([Benzeneinlet [Benzeneoutlet ) ×100%  ξ(%)=[CO]6([Benzene]inlet[Benzene]outlet)×100%

(5)

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2.1. Adsorption-enhanced Non-thermal Plasma

We previously reported the oxidative decomposition of formaldehyde, benzene, VOCs and odorin air using a plasma reactor packed with various materials. The frequency of the applied high voltage alternating current (AC) had an important influence on energy efficiency and intermediate frequency of the applied AC power was beneficial for VOCs removal, especially 150 Hz.

Urashitnal et al. used NTP packed with active carbon to decompose VOCs and trichoroethylene (TCE) with the discharge energy efficiencies of 26 g/(kW h) and 13 g/(kW h), respectively. The TCE removal efficiency was 40% using NTP technology alone and it was up to 90% using NTP packed with active carbon. Ogata et al. found MS-4A molecular griddle played a special role in VOCs removal using NTP technology and reported that the VOCs removal efficiency in the dielectric barrier discharge (DBD) reactor packed with MS-4A molecular griddle and BaTiO3 particles was 1.4~2.1 times higher than that in DBD reactor packed with BaTiO3 particles.

In this experiment, the mechanism of adsorption-enhanced NTP for volatile organic compounds (Toluene) removal was discussed. A sorbent was packed into the space of discharge plasma so that reaction time was prolonged between VOCs molecules and NTP and removal efficiency of VOCs was improved without increasing the size of NTP reactor. The sorbent was helpful for enhancing discharge energy efficiency due to VOCs molecules enrichment on the surface of the sorbent.

2.2. Adsorption kinetics

The pellets of γ-Al2O3, 5~7mm in diameters, was used as the sorbent to be packed into the NTP reactor. Figure 4 shows the VOCs concentration profile as a function of time on γ-Al2O3 pellets in the NTP reactor before plasma was applied (VOCs: 800 mg/m3; flow rate: 2 mL/min; dry air). VOCs was adsorbed on the surface of γ-Al2O3 and its concentration was gradually increased with time. After 150 min, the VOCs concentration reached the adsorption-desorption equilibrium. So the decomposition tests were initiated after the adsorption-desorption equilibrium.

The collision frequency between adsorption rate (Ra) and adsorbate molecules on the adsorbent surface is proportional to fraction of the vacant active sites on the adsorbent surface (1-θ):

R a =k aT 1/2 (1q)y 0  Ra=ka0T1/2(1q)y0

(6)

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where ka0=α/(R/2M)1/2.

The desorption rate (Rd) is proportional to the fraction of the occupied active sites (θ):

R d =k a0 qexp(E d /RT) Rd=ka0qexp(Ed/RT)

(7)

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And the net-desorption rate of adsorbate molecules should be the difference between the desorption rate and the adsorption rate:

Figure 4.

VOCs concentration on the surface of γ-Al2O3 pellets

dq/dt=R d R a =k d qk a (1q)y 0  dq/dt=RdRa=kdqka(1q)y0

(8)

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where ka=ka0T-0.5 and kd=kd0exp(-Ed/RT)

According to the reaction status on the surface of catalyst in the NTP reactor, adsorbate molecules are in the slow diffusion in the adsorbent channels and the balance between adsorbent surface molecules and gas phase would be established for a long time. In this case, the net adsorption rate can be expressed as:

dq/dt=k(y 0 ydq/dt=k(y0y)

(9)

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During a certain period of time dt, the adsorption process mass balance between the gas and solid phase adsorbent is:

F y M=m s q m (dq/dt) FyM=msqm(dq/dt)

(10)

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The adsorbate molecules in the pores of adsorbent diffuse slowly and the mass transfer rate constant k is smaller. As we known, the desorption temperature is a linear function of the frequency of high voltage and the reaction time:

T=T 0 +α H f+β H t T=T0+αHf+βHt

(11)

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By combining Eq.(9), (10) and (11), the coverage rate of adsorbate on the surface of adsorbent as a function of temperature can be obtained:

dq/dt=( k d q/α H β H )/{[k a (1q) /k]+[k a (1q)b1]} dq/dt=( kdq/αHβH)/{[ka(1q) /k]+[ka(1q)b1]}

(12)

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Where b=-ms qm /FM.

As mentioned earlier, assuming that the mass transfer coefficient is small enough, the contaminants adsorbed on the surface would be desorped and degraded immediately on the surface of catalyst, that is, ka(1-) /k >> [ka(1-)b-1]. Then eq. (12) becomes:

dq/dt=k k d q/ [α H β H k a (1q)] dq/dt=k kdq/ [αHβHka(1q)]

(13)

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When the desorption rate is maximum and –d2 /dT2=0, eq. (13) can be obtained:

dq/dT   T=Ti = q i (1q i ) (1/2T i +E d /RT i  2 ) dq/dT|T=Ti=qi(1qi) (1/2Ti+Ed/RTi2)

(14)

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In the situation of γ-Al2O3 adsorption interaction with the plasma, the VOCs concentration in the surface of γ-Al2O3 reached adsorption equilibrium. The removal amount of VOCs equaled to 25% of the inlet total concentration of VOCs by adsorption ofγ-Al2O3.

2.3. Effect of packed materials on removal efficiency

Figure 5 shows the relationship between reactor energy density (RED) and VOCs removal efficiency with different packed materials in the NTP reactor. The VOCs removal efficiency increased with RED and was in an order of no padding < common packed materials < γ-Al2O3. On one hand, γ-Al2O3, as a sorbent, could adsorb short-living free redicals. In gas discharge process, these free redicals would accelerate decomposition reactions on the surface of microhole structure of the sorbent. The surface of the cellularity particles could also become active sites with electrons stricking. On the other hand, due to their higher permittivity of 11, γ-Al2O3 pellets gained more electric charges and enhanced the local discharge and the discharge current. As a result, γ-Al2O3 pellets were helpful for the VOCs removal reaction.

Figure 5.

Relationship between RED and removal efficiency of VOCs with different packed materials

2.4. Effect of packed materials on ozone concentration

Figure 6 shows the relationship between the packed materials and ozone concentration. The effect on the ozone concentration was in an order of γ-Al2O3 < no padding < common packed materials at various REDs. The ozone concentration reached the peak at RED of 0.7 kJ/L with the same packed materials in the NTP reactor as shown in figure 6.

High energy electrons produced by gas discharge ionized the VOCs molecules which were adsorbed on the γ-Al2O3 surface to produce more positive ions including N+, N2+, O2+ and H3O+. These positive ions activated VOCs molecules for VOCs decomposition though electric discharge transferring reactions. In the NTP reactor, the VOCs molecules were decomposed by the radicals of O, OH and N, etc. Evans et al. believed that O played a key role for VOCs decomposition in the NTP process. Ozone as the main long-living radical was transported to the surface of γ-Al2O3 and could take part in oxidation reaction. The pathways of reaction were stated as follows:

e+O 2 2O+e e+O22O+e

(15)

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O+O 2 +MO 3 +M O+O2+MO3+M

(16)

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O+O 3 2O 2  O+O32O2

(17)

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e+O 3 O+O 2 +e e+O3O+O2+e

(18)

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The active oxygen species formed during the O3 decomposition would also be helpful for the VOCs removal.

Figure 6.

Relationship between the packed materials and ozone concentration

2.5. Effect of packed materials on energy efficiency

Figure 7 shows the relationship between RED and electric energy efficiency with different packed materials in the NTP reactor. The energy efficiency for VOCs removal decreased with increasing RED and was in an order of no padding < common packed materials < γ-Al2O3. Ions, electrons, excited neutral molecules and metastable free radicals in the plasma process attacked the surface of γ-Al2O3 pellets and made many kinds of reciprocity possible:

  1. The plasma brought sub-electrons launching.
  2. The plasma induced chemical reactions on the surface of the γ-Al2O3 pellets and produced active atoms, molecules and free radicals, which would take part in chemical reactions for the further VOCs decomposition.
  3. Many active oxidation groups in plasma were adsorbed on the surface of the γ-Al2O3 pellets. The plasma induced the desorption of VOCs molecules and active oxidation groups so that further reactions took place. As a result, the whole reaction process was accelerated and more reaction routes were induced.
  4. VOCs removal efficiency and electric energy efficiency were improved.

Figure 7.

Relationship between RED and electric energy efficiency with different packed materials

3. Effect of modified ferroelectric on nonthermal plasma process

The major bottleneck of developing NTP with catalysis technology is the reduction of energy consumption. If this requirement is not satisfied, the non-thermal plasma process may lose its potential for commercial applications. In order to resolve this problem, Ayrault et al. used platinum (Pt)-based catalyst supported on an alumina wash-coated honeycomb monolith by means of a high voltage bi-polar pulsed excitation. The energy efficiency was 0.14 mol/kWh at an energy density of 200 J/L for 2-Heptanone decomposition. For a comparison, the energy efficiency decreased to 0.029 mol/kWh using an uncoated monolith even at a higher energy density of 500 J/L.

In this investigation, we developed a new ferroelectric packed bed NTP reactor and prepared a sample of Ba0.8Sr0.2Zr0.1Ti0.9O3 to serve as modified ferroelectrics. The permittivity of Ba0.8Sr0.2Zr0.1Ti0.9O3 was 104, 12 times higher than that of the pure phase of BaTiO3, while dielectric loss was 1/6 in room temperature. The experimental results show that this type of modified ferroelectrics packed into the NTP reactor could both reduce the energy consumption and raise energy efficiency significantly. Compared with BaTiO3, Ba0.8Sr0.2Zr0.1Ti0.9O3 had better ferroelectric physical properties to improve NTP process for VOCs control.

3.1. Materials and methods

In the experiment, three kinds of packed materials, including ceramic rings, BaTiO3 rings and Ba0.8Sr0.2Zr0.1Ti0.9O3 rings (hollow cylinder shape, 5 mm i.d., 1 mm wall thick, and 10 mm length), were used to pack into the NTP reactor.

Nano-size Ba0.8Sr0.2Zr0.1 Ti0.9O3 powder was prepared using the method of water-thermal composite action at atmospheric pressure. Inorganic salts, including TiCl4, Ba(OH)2 8H2O and Sr(OH)2 8H2O, were the precursors for Ba0.8Sr0.2Zr0.1 Ti0.9O3 formation. Firstly, a proper quantity of TiCl4 was added to 100mL water as the precursor solution and a ammonia used to adjust pH to 7. By strictly controlling the reaction conditions in a ventilation cabinet, the precursor solution hydrolyzed to α-H2TiO3. And then, Cl- was removed by hot water washing and filtrated by decompression and boiled at 100 ºC for 4 hours. Certain amounts of Ba(OH)2 8H2O and Sr(OH)2 8H2O dropped into H2TiO3 and ammonia adjusted pH to 6~6.5 and the solution was shielded from air and agitated for hours. During the preparation, if needed, water was added to keep the balance of the liquid quantity. Whereafter, gained solid (nano-Ba0.8Sr0.2Zr0.1Ti0.9O3) was ground to powders and dried at 100 ºC in a crucible. The powder was made into rings (5 mm i.d., 1 mm wall thick, and 10 mm length) which were placed in a muffle furnace to calcine at 1200 ºC for two hours. The calcined product was cooled to ambient temperature and served as the packed materials in the NTP reactor. At the same time, a BaTiO3 (powders made in Beijing Research Institute of Chemical Engineering & Metallurgy) ring was also made with the same weight as the Ba0.8Sr0.2Zr0.1Ti0.9O3 ring.

The crystal structure and the surface shape of the Ba0.8Sr0.2Zr0.1Ti0.9O3 sample were detected by XRD (manufactured by Germany Bruker Co., D8 ADVANCE) and SEM (manufactured by Japan, JEOL-JSM-6500F) and the BET surface area determined by Micromeritics (manufactured by American Quantachrome Co., NOVA 1000). The relative permittivity of the Ba0.8Sr0.2Zr0.1Ti0.9O3 sample was measured using an LCR automatism test instrument (manufactured by China, 4210).

3.2. Characteristic of modified ferroelectric

The crystal structure of Ba0.8Sr0.2Zr0.1Ti0.9O3 detected by XRD as shown in Figure 8 should be similar to cube crystal structure of calcium-titanium oxide. Ba0.8Sr0.2Zr0.1Ti0.9O3 was a type of ferroelectric like BaTiO3. The average diameter of sample particulates was of 59 nm. Fig.9 shows that crystal shape of Ba0.8Sr0.2Zr0.1Ti0.9O3 was spherical. The BET surface area of the

Figure 8.

XRD testing results of Ba0.8Sr0.2Zr0.1Ti0.9O3

Figure 9.

SEM testing results of Ba0.8Sr0.2Zr0.1Ti0.9O3

Ba0.8Sr0.2Zr0.1Ti0.9O3 powders are 8.8 m2/g, and Longmuir surface area detected by Micromeritics are 12.3 m2/g respectively. The relative permittivity of Ba0.8Sr0.2 Zr0.1Ti0.9O3 detected by LCR is about 12000.

3.3. Effect of Ba0.8Sr0.2Zr0.1Ti0.9O3 on removal efficiency of toluene

Fig.10 shows the effect of different packed materials in the NTP reactor on the removal efficiency (η) of toluene. The removal efficiency of toluene increases with reactor energy density (RED) and is in the order of without packed materials < with BaTiO3 < with Ba0.8Sr0.2Zr0.1Ti0.9O3, at the same RED.

Figure 10.

Effect of packed materials on removal efficiency (toluene concentration: 1000mg/m3 or so; gas flow rate: 2L/min; AC frequency: 150Hz)

The influence on toluene removal efficiency of some other parameters than could be taking into account such as the permittivity of packed materials, surface specific area, adsorption properties or catalytic effect:

  1. It is well known that the presence of solid material in the electrode gap enhances the NTP efficiency likely by favoring the formation of homogeneous plasma rather than a filamentous one. Eliasson et al. reported that the packed materials in NTP reactors played a key role for the proper functioning of DBD and generating more high energy electrons. As the removal efficiency was proportional to the numbers of high energy electrons, because these high energy electrons could destroy the molecular structure of toluene and decompose toluene molecules into CO2, CO and H2O by effective collisions taking place between the high energy electrons and the toluene molecules.
  2. Ba0.8Sr0.2Zr0.1Ti0.9O3 has higher permittivity than BaTiO3. The electric field strength is positive to the permittivity of packed materials in the NTP reactor.
  3. It is well known that the surface specific area is positive to the adsorption properties of the adsorbent. The rings of BaTiO3 and Ba0.8Sr0.2Zr0.1Ti0.9O3 have bigger surface specific area of 59 and 57 or so after the rings are calcined at 1200℃. The adsorption properties can adsorb radicals to accelerate decomposition reaction on the surface of the sorbent and the surface of the cellular rings can also become active sites with electrons striking. So the NTP reactor with packed materials can obtain higher toluene removal efficiency than that without packed materials.

Fig.11 shows the voltage and current waveforms detected by oscillograph with and without packed materials (Ba0.8Sr0.2Zr0.1Ti0.9O3 rings) at electric field strength of 10 kV/cm. Ricketts et al.[16] believed that the pulse peak numbers of gas discharge were directly proportion to the removal efficiency of VOCs. As shown in figure 6, the pulse peak numbers are higher with packed materials than those without in the NTP reactor. So packed materials increase the pulse peak numbers of DBD and help for increasing the removal efficiency.

Figure 11.

Voltage and current waveforms

Fig.12 shows V-Q Lissajous diagram with or without packed materials (Ba0.8Sr0.2Zr0.1Ti0.9O3 rings) at electric field strength of 10 kV/cm. BC and AD shown in fig.12 represent DBD courses. The packed materials enhance the discharge intensity of BC and AD courses and produce higher the pulse peak numbers. Pulse peak numbers are directly proportion to RED and increase the removal efficiency of toluene.

Figure 12.

V-Q Lissajous diagram

Therefore, the removal efficiency was higher with packed materials than that without packed materials, in agreement with the results shown in fig.10. During the preparation of Ba0.8Sr0.2Zr0.1Ti0.9O3 sample, strontium (Sr) and zirconium (Zr) ions were adulterated into the powder particles and crystal boundary. These metal ions enter crystal lattices of BaTiO3 equably and lower the Curie temperature (Tc). As a result, the permittivity of Ba0.8Sr0.2Zr0.1Ti0.9O3 is 12000, 8 times higher than that of pure phase of BaTiO3 (1500) in room temperature. According to Yamamoto et al., the dielectric constant had a significant influence on the discharge energy of the NTP reactor. The electric field strength is calculated as follows:

E r =3εε+2 E 0 cosθ Er=3εε+2E0cosθ

(19)

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Where Er is the local electric field strength after dielectric polarization, E0 the local electric field strength before dielectric polarization, and ε the relatively permittivity. As shown in formula (19), Er is in direct ratio with E0. Er equals to 3 times of E0 (θ=0) with ε close to infinity. So, the electric field strength is positive to the relative permittivity of packed materials in the NTP reactor. RED increases with the electric field strength, improving the removal efficiency of toluene. Therefore, Ba0.8Sr0.2Zr0.1Ti0.9O3 leads to better the removal efficiency of 97% for toluene decomposition.

3.4. Effect of Ba0.8Sr0.2Zr0.1Ti0.9O3 on energy efficiency

Fig.13 shows the change of energy efficiency (ζ) for toluene removal with and without the packed materials. At the identical RED, the energy efficiency is in the order of without packed materials < with BaTiO3 < with Ba0.8Sr0.2Zr0.1Ti0.9O3. The energy efficiency is 15 g/kWh with Ba0.8Sr0.2Zr0.1Ti0.9O3, 11 g/kWh with BaTiO3, and 6 g/kWh without packed materials at RED of 0.23 kJ/L in the NTP reactor. The results show that Ba0.8Sr0.2Zr0.1Ti0.9O3 has a better ferroelectric property to improve energy efficiency and reduce energy consumption in the NTP process for VOCs control, compared with BaTiO3.

Figure 13.

Effect of packed materials on energy efficiency (toluene concentration: 1000mg/m3 or so; gas flow rate: 2L/min; AC frequency: 150Hz)

3.5. Effect of Ba0.8Sr0.2Zr0.1Ti0.9O3 on ozone formation

Fig.14 shows the ozone (O3) concentration with and without the packed materials. O3 concentration is the highest with Ba0.8Sr0.2Zr0.1Ti0.9O3 and is in the order of without the packed materials < with BaTiO3 < with Ba0.8Sr0.2Zr0.1Ti0.9O3 at the identical RED.

O3 as the main long-living radical was generated and transported to the packed materials and could take part in oxidation reaction on the packed materials' surface. The pathways of reaction were stated as follows:

e+O 2 2O+e e+O22O+e

(20)

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O+O 2 +MO 3 +M O+O2+MO3+M

(21)

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O+O 3 2O 2  O+O32O2

(22)

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e+O 3 O+O 2 +e e+O3O+O2+e

(23)

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In figure 9, it also shows that O3 concentration increases with the REDs at the first stage from 0 to 0.7 kJ/L and reaches the maximum at the RED of 7 kJ/L or so. This pattern of ozone production had also been reported by Yamamoto et al. In this experiment, because Ba0.8Sr0.2Zr0.1Ti0.9O3 has higher relative permittivity than BaTiO3, the electric field strength and RED are enhanced significantly in the NTP process with Ba0.8Sr0.2Zr0.1Ti0.9O3 as the

Figure 14.

Effect of packed materials on O3 concentration (toluene concentration: 1000mg/m3 or so; gas flow rate: 2L/min; AC frequency: 150Hz)

packed materials. As a result, O3 concentration increases according to Equation (21) (RED 0.7 kJ/L). While RED 0.7 kJ/L, the superfluous high-energy electrons accelerate the decomposition of O3 to O2 according to Equation (22) and Equation (23). The active oxygen species formed during the O3 decomposition would also be helpful for the toluene removal on the surface of packed materials.

4. Decomposition of benzene in dry air by super-imposed barrier Discharge NonThermal plasma–photocatalytic system

In this section, NTP coupled with nano-titania (TiO2) photo-catalyst for benzene decomposition to further reducing the energy consumption and harmful byproducts in plasma process.

O2 and H2O are adsorbed on the surface of TiO2 to form adsorption oxygen and adsorption water:

O 2 (g)O 2 (ads) O2(g)O2(ads)

(24)

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H 2 O(g)H 2 O(ads) H2O(g)H2O(ads)

(25)

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The molecules of VOC are also adsorbed on the surface of TiO2 to form adsorption matter:

VOCs(VOCsads  VOCs(VOCs)ads

(26)

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Discharge plasma as a driving force of photocatalyst furnished a mess of UV light. According to Kim's report, hole-electron pairs are produced by supplying energy larger than the band-gap energy of TiO2 (3.2 eV for anatase crystal type).

TiO 2 +TiO 2 (e  +h + ) TiO2+hνTiO2(e+h+)

(27)

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And then, OH radicals come into being:

h + +H 2 O(ads)O  H+H +  h++H2O(ads)O•H+H+

(28)

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h + +OH  O  H h++OH−→O•H

(29)

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High-energy particles, such as electrons, excited molecules, and radicals may transfer their energy to TiO2 by bombardment when TiO2 is placed in a NTP reactor. Various chemical reactions are induced on the excited TiO2 surface through the following reactions:

e  +O 2 (ads)O    2 (ads) e+O2(ads)O2(ads)

(30)

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O     2  (ads)+H + HO 2    O•2(ads)+H+HO2

(31)

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2HO 2   O 2 +H 2 O 2  2HO2•O2+H2O2

(32)

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H 2 O 2 +O     2  (ads)  OH+OH  +O 2  H2O2+O•2(ads)→•OH+OH+O2

(33)

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At last, the molecules of VOC are decomposed as follow:

 H ads (HO  2ads ,O ads or  h + ,etc.)+(VOCs) ads (active intermediate  products)→→→CO 2 +H 2 O+CO O•Hads(HO2ads•,Oadsorh+,etc.)+(VOCs)ads(activeintermediateproducts)→→→CO2+H2O+CO

(34)

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This section illuminates the experimental results of the effect of packed materials on benzene decomposition using NTP generated by dielectric barrier discharge (DBD) coupled with nano-TiO2 catalyst. The effects of A and B packed materials on benzene removal were compared in the paper. The results show that removal effect was visible by B packed materials in NTP reactor. At the same time, we got higher removal efficiency and a better selectivity of carbon dioxide or carbon monoxide with B packed materials coated with nano-TiO2. Above all that means this technology of self-support ray polarization would have a great potential for application in the future.

4.1. Materials and methods

There are two kinds of packed materials of A and B (i.d.2, 4, 5, 7 mm, thickness 3 mm, length 10 mm, raschig ceramic ring) in NTP reactor. The structure characteristics of packed materials A and B were detected by XRD as shown in fig.15 (manufactured by Japan, D/MAX-RA). According to fig.15, the results of physics characteristic of two packed materials were indicated in tab.1. Non-crystal content of A packed materials was up to 70% and non-crystal content of B packed materials was 50%. It means B packed materials interstitial rate was higher than A packed materials, and adsorption capability was bigger than A packed materials.

The packed materials coated with nano-TiO2 catalyst were packed into the reactor. The reactor was made of ceramic tube (i.d.50 mm, reaction length 500 mm), inner axes electrode (i.d.0.8 mm, stainless steel wire), and outer electrode (80 circles stainless steel wire). The characteristic of packed materials includes volume density is 21.7 g/cm3, hole rate is 12.7 % and bibulous is 5.9 %.

Packed materials

Component(%)

  

  

Interstitial rate(%)

Hygroscopic coefficient (%)

  

quartz

Al2O3

non-crystal

  

  

A

15

15

70

1.8

0.8

B

15

35

50

19.5

9.4

Table 1.

Physics characteristic of two packed materials

Figure 15.

XRD pattern of packed materials

Nano-TiO2 films were prepared by the Sol-Gel method in the experiment. Flow chart on preparing nano-TiO2 thin film by Sol-Gel method referred to Fig.16.

Figure 16.

Flow chart on preparing nano-TiO2 thin film by Sol-Gel method

The nanometer TiO2 thin film was inspected and analyzed by Scan Electric Mirror (SEM, Made in Japan, S-2700). The results of SEM micrograph show that average particulate diameters of TiO2 were less than 100 nm. SEM micrograph of the samples referred to Fig.17.

Figure 17.

SEM micrograph of TiO2

The plasma reactor employed an alternating current (AC) power supply of 60 Hz (designed by ourselves). The AC voltage was applied to the reactor in the radial direction, and the AC voltage extension lied from 0 kV to 30 kV. The benzene concentration was determined on a gas chromatography (manufactured by American Thermo Finnegan Co., TRACE-GC ULTRA) with a flame ionization detector (FID) and a capillary column of DB-1. Separately, another GC (SC—1001) equipped with an FID detector and a methane converter was used to analyze concentration of CO2 and CO. Reaction gas samples were taken by a syringe from the sampling ports of the reactor. The byproducts were identified by GC-MS with a 30-m-long wide–bore capillary column (DB-1). The experimental condition was in atmospheric pressure (760 mmHg) and temperature (20℃).

4.2. Relationship packed materials between and removal efficiency

Fig.18 & Fig.19 showed the effect of A & B packed materials on removal efficiency of benzene (benzene concentration of 600 mg/m3, gas flux is 100 L/h, dry air, A & B packed materials coated with nano-TiO2). With increasing electric field strength, the removal efficiency of benzene increased. During an impulse cycle in NTP reactor, a mass of high-energy electrons were produced in discharge space. When effective collisions between high energy electrons and benzene molecules took place in NTP reactor, electron energy would destruct molecular structure of benzene and benzene molecules could be converted into inorganic little molecules like carbon dioxide (CO2), carbon monoxide (CO) and water (H2O) at last. Thus, removal efficiency of benzene was proportional to the number of electrons, while the electrons' number was positive to electric field strength. It had come to light that TiO2 was helpful for generating higher concentrations of different types of active oxygen species in non-thermal plasma. So the hybrid system would have an effective utilization of active oxygen species in benzene removal.

As shown in fig.18, removal efficiency with B packed materials (i.d.2 mm) was higher than that with A packed materials (i.d.2 mm). Firstly, compared with the component of A & B packed materials, non-crystal type being in existence had influence on dielectric polarization. Secondly, B packed materials possessed higher interstitial rate and hygroscopic coefficient and bigger surface area. These factors were helpful for benzene molecule adsorption so that they prolonged reaction time between benzene molecule and high electrons or free radicals. Thirdly, the surface of B packed materials was rough and could assemble more polarization electric charge to form more local electric field, so that electrons in NTP gained higher energy to improve reaction efficiency.

As shown in fig.19, the size of B packed materials (i.d.2, 4, 5, 7 mm) had effect on removal efficiency of benzene. Removal efficiency increased with the size of raschig ceramic ring decreasing. The removal efficiency was an order of i.d.2 mm > i.d.4 mm > i.d.5 mm > i.d.7 mm. The removal efficiency with B packed materials of i.d.2 mm was 81% with electric field

Figure 18.

Effect of different packed materials on removal efficiency

Figure 19.

Effect of different diameters of B packed materials on removal efficiency

strength of 12 kV/cm. With the size of raschig ceramic ring decreasing, packed materials density increased and interspace between packed materials reduced. These factors were helpful for dielectric polarization and enhanced electric field strength. The same result was gained by Ogata et al.

4.3. Relationship between packed materials and ozone concentration

Fig.20 and fig.21 showed ozone concentration increased with increasing electric field strength (benzene concentration of 600 mg/m3, gas flux of 100 L/h, dry air, A & B packed materials coated with nano-TiO2). When electric field strength increased, more high-energy electrons and radicals were generated in the early discharge phase. They possessed high energy compared to the dissociation energy of O2 so that a series of reaction took place in NTP reactor. The oxygen dissociation was the most important radical formation reaction.

e+O 2 e+O+O e+O2e+O+O

(35)

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Electronically excited atomic oxygen (O(1D)) was a very short-lived radical, whereas ground state atomic oxygen (O) and hydroxyl (OH) had a longer lifetime. O(1D) reacted with H2O resulting in formation of OH radicals. O- and OH- radicals were consumed by O3 formation. O3 as the main long-living radical was transported to packed materials and could take part in oxidation reaction on packed materials' surface. The pathways of reaction were stated as follows:

O+O 2 +MO 3 +M O+O2+MO3+M

(36)

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O+O 3 2O 2  O+O32O2

(37)

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e+O 3 O+O 2 +e e+O3O+O2+e

(38)

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.

As shown in Fig.20, ozone concentration with A packed materials (i.d.2 mm) was higher than that with B packed materials (i.d.2 mm) in NTP reactor. Because the adsorption capability of B packed materials in NTP reactor was higher than that of A packed materials. It meant B packed materials would be helpful for decreasing ozone concentration though prolonging reaction time on the surface of B packed materials followed equation (37) to (38).

As shown in fig.21, ozone concentration was an order of B packed materials of i.d.2 mm > i.d.3 mm > i.d.5 mm > no padding. It was obvious that B packed materials was helpful for increasing ozone concentration. The reaction took place just like equation (36).

Figure 20.

Effect of different packed materials on ozone concentration

Figure 21.

Effect of different diameters of B packed materials on ozone concentration

4.4. Relationship between power consumption and removal efficiency

Fig.22 showed the biggest electric power consumption was up to 120 W with electric field strength changing from 0 to 12 kV/cm with B packed materials coated with nano-TiO3. Kuniko Urashima et al. got the similar conclusion at the same experimental conditions.

Fig.23 showed that power consumption was 13.5 W if removal efficiency was up to 85% and benzene concentration was 600 mg/m3. Through calculating, electric energy consumption was 2.25×10-4 kWh to treat benzene quality of 1 mg. If benzene concentration was 1500 mg/m3, power consumption was 20 W for the same removal efficiency. Through calculating, electric energy consumption was 1.33×10-4 kWh to treat benzene quality of 1 mg.

Figure 22.

Relationship between electric field strength and power (B packed materials coated with nano-TiO2)

Figure 23.

Relationship between power consumption and removal efficiency (B packed materials coated with nano-TiO2)

4.5. Photocatalyst and ozone effect

In the next experiment, we chose B packed materials. The packed materials(5 mm i.d., 1 mm wall thick, 10 mm length) were divided into two groups, coated with photocatalyst or without photocatalyst.

Fig.24 shows the relationship between removal efficiency of benzene and electrostatic field strength in the plasma reactor with or without packed materials.

Figure 24.

The effect of removal efficiency with or without packed materials

Figure 25.

The effect of ozone concentration with or without packed materials

Fig. 25 shows the relationship between ozone concentration and electrostatic field strength in the plasma reactor with or without packed materials.

With increasing electrostatic field strength, the removal efficiency of benzene increases. When initial concentration of benzene is 1300 mg/m3, the average electrostatic field strength is 13.6 kV/cm and gas flux is 100 L/h, the removal efficiency of benzene arrives at 80% in the reactor with packed materials as shown in Fig.24.

In the reactor, the space occupied by contamination air is always full of high energy electrons. When effective collisions between high energy electrons and benzene molecules take place in the reactor, electron energy will destruct molecular structure of benzene and benzene molecules will be converted into inorganic little molecules like carbon dioxide (CO2), carbon monoxide (CO) and water (H2O). Thus, removal efficiency of benzene are proportional to the electrons. In Fig.3, when packed materials placed in the plasma reactor, with electrostatic field strength increasing, more and more high energy electrons are produced due to mediums polarization of packed materials. So packed materials in the reactor increases the removal efficiency of benzene.

Fig.25 shows when initial concentration of benzene is 1300 mg/m3, the average electrostatic field strength is 12 kV/cm and gas flux is 100 L/h, ozone concentration is about 3.04 mg/L with packed materials and ozone concentration is about 2.16 mg/L without packed materials. Ozone concentration with packed materials heightens 1 mg/L than that without packed materials in the plasma reactor. It is obvious that packed materials in the reactor is helpful of increasing ozone concentration. The reason is high energy electrons and radicals are generated in the early discharge phase. They possess high energy compared to the dissociation energy of O2 so that a series of reaction takes place in the plasma. The oxygen dissociation is the most important radical formation reaction.

e+O 2 e+O+O e+O2e+O+O

(39)

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Electronically excited atomic oxygen (O(1D)) are very short-lived radicals, whereas ground state atomic oxygen (O) and hydroxyl (OH) have a longer lifetime. O(1D) reacts with H2O resulting in formation of OH radicals. O- and OH- radicals are removed by formation of O3.

In Fig.24 and Fig.25, the test results also indicate ozone is helpful for benzene removal, at least, ozone acts as the oxidant precursor. With ozone concentration increasing, the removal efficiency of benzene increases. Because ozone as a kind of oxidative species produced by the initial oxidation just like OH radical, has an effect on further reaction of benzene.

The humidities of contaminated air in the reactor have influence on ozone concentration as shown in Fig.26a. Ozone concentration without vapor is higher 35% than that with relative humidity 67%, and ozone concentration decreases with humidity increasing. Because H2O molecule have electronegative, it will consume the electrons in the plasma. At the same time, H2O will react with O (1D) which is the origin of formation of O3.

H 2 O+O(D 1 )2OH H2O+O(D1)2OH

(40)

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So humidity counteracts the formation of ozone.

In Fig.26b, the findings show the removal efficiency reduces with humidity increasing. Probably, the active sites for benzene removal are reduced by water vapor occurring competitive adsorption on the surface of nano-TiO2.

Thus, humidity affects the benzene removal in three ways: deactivation of high energy electrons, inhibition of ozone formation, and suppression of the catalyst activity of nano-TiO2 for benzene oxidation with ozone in the plasma reactor.

Figure 26.

The effect of humidity on ozone concentration without photocatalyst

Figure 27.

The effect of humidity on removal efficiency with photocatalyst

Fig.27 shows the relationship between ozone concentration and gas flux with catalyst or without catalyst in the plasma reactor under three initial concentration of benzene. When gas passes the reactor, and electrostatic field strength is 10kV/cm, ozone concentration increases with gas flux increasing as shown in Fig.27a, 27b and 27c, regardless of with or without photocatalyst.

Fig.27d and 27e show benzene concentration reduces with initial concentration of benzene increasing, regardless of with or without photocatalyst.

The relationship between benzene degradation and electrostatic field strength with or without photocatalyst is shown in Fig.28 where benzene initial concentrations changes from 600 mg/m3 to 1500 mg/m3.

When initial concentration is 600 mg/m3, the average electrostatic field strength is 10 kV/cm, and gas flow rate is 14 mm/s, the removal efficiency of benzene attains 98% in the reactor with photocatalyst, but the removal efficiency of benzene attains 78% in the reactor

Figure 28.

Relationship between ozone concentration and flux with or without catalyst when benzene concentration is 0mg/m3

Figure 29.

Relationship between ozone concentration and flux with or without catalyst when benzene concentration is 700mg/m3

Figure 30.

Relationship between ozone concentration and flux with or without catalyst when benzene concentration is 2000mg/m3

Figure 31.

Relationship between ozone concentration and flux with catalyst

Figure 32.

Relationship between ozone concentration and flux without catalyst

without photocatalyst as shown in Fig.28a. When initial concentration is 1500 mg/m3, the average electrostatic field strength is 12 kV/cm and gas flow rate is 14 mm/s, the removal efficiency is higher 19% with photocatalyst than without photocatalyst in the plasma reactor as shown in Fig.28b. The results indicate photocatalyst enhanced the benzene removal efficiency obviously with ozone. When both photocatalyst and ozone coexist, there will be an improved removal efficiency of benzene in the plasma reactor. As you know, TiO2 is a photocatalyst material of 3.2 eV band gap. If it absorbs bigger energy than band gap, it makes photo-excited electron–hole pairs that could oxidize benzene. At same time, the surface hydroxyl groups are oxidized to form composition of benzene in the photocatalytic reactions. So we have thought that it was advantageous to use photocatalyst in plasma system to control of oxidation step of benzene.

The influence of the catalyst on ozone formation is presented in Fig.27a, 27b and 27c. It shows the catalyst could reduce the ozone formation to a certain extent. This is because ozone as the main long-living radical can capture free electrons which are produced by photocatalysis and produce OH radical. It not only avoids hole-electron pairs compounding but increases photons efficiency. Further more, OH radical is a kind of good oxidant and it can transform organism into mineral.

O 3 +e  O  3  O3+e−→O3−•

(41)

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H + +O  3 ∙→HO 3  H++O3−•→HO3•

(42)

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HO 3 ∙→O 2 +OH HO3•O2+•OH

(43)

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O 3 +hv→∙O+O 2  O3+hv→•O+O2

(44)

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O+H 2 O2OH •O+H2O2•OH

(45)

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From (41)~(45), we can arrive at conclusions. On the one hand, ozone increases photons efficiency of photocatalusis so that is helpful of benzene removal. On the other hand, photocatalyst promotes ozone to separate into OH radical and reduces ozone output.Complete oxidation of benzene to CO2 is the final goal of the experiment, and the catalytic effect on the product distribution had been investigated. Photo-oxidation activity should be expressed as selectivity to CO2 because other byproducts except CO2 were emitted in plasma process.

4.6 The compare of photocatalyzed characteristic by different heat treatment

The packed materials with phot ocatalyst were to calcine at 450ºC, 600ºC and 700ºC in the muffle. Then, they were packed into the plasma reactor. The samples of packed materials were detected by X-Ray and testified that nano-TiO2 was anatase at 450 ºC and nano-TiO2 was mixture of anatase and rutileat at 600ºC and nano-TiO2 was rutile mostly at 700ºC. The experimental results are shown in Fig.28.

Figure 33.

Compare with catalyst characteristic of different treatment temperature

The decomposition efficiency of benzene is best with anatase photocatalyst in the plasma reactor. Next, the decomposition efficiency is better with the mixture photocatalyst of anatase and rutileat, and the last to rutileat photocatalyst. The above all are shown in Fig.28(a), Fig.28(b) shows the decomposition efficiency of benzene reduces gradually when the packed materials with photocatalyst by heat treatment from 450ºC to 700 ºC. On one hand, when sinter temperature is raised, the surface areas of catalyst reduce, and the surface adsorption capacity decrease. On the other hand, nano-TiO2 catalyst will transform from anatase to rutileat. In fig.28(a), the test shows the reaction activity of anatase catalyst is higher than that of rutileat catalyst. There are four reasons.

  1. Because of structural difference, two type of catalyst have different quality densities and different structure of energy gap of electron. The quality density of anatase of 3.894g/cm3 is less than that of rutileat of 4.250g/cm3. The energy of energy gap of anatase who is 3.2 eV is higher than that of rutileat who is 3.1 eV. The higher energy of energy gap leads to the higher reaction activity for catalyst.
  2. The surfaces of anatase possess symmetrical structure with the molecular structure of benzene, so it can adsorb benzene effectively.
  3. The hydroxyl of surface of rutileat is not more than that of anatase. Because the hydroxyl of surface is helpful for benzene removal, anatase is better than rutileat on benzene degradation.
  4. The surface area of rutileat catalyst declines sharply because a large number of particals converge under high temperature. The adsorption capacity of rutileat of TiO2 is bad for O2, so catalyst activity is low.

So nano-TiO2 photocatalyst of anatase crystal was employed for next experiment.

4.7. Analysis of reaction products

Though GC-MS, the main products in the plasma reactor were CO2, H2O, and a small quantity of CO. Ozone was the only byproduct, and no other byproducts could be detected in the tail gas. In addition, certain brown-yellow products that were observed in the plasma reactor regardless of with or without catalyst appeared. The composition of the brown-yellow products was indistinct, and maybe it was aromatic polymer detected by GC-MS.

The minimum of CO/CO2 is 0.286 and CO/CO2 decreases with electrostatic field strength increasing as shown in Fig.29a&b. There are no products except CO2 and H2O at 11 kV/cm and 12 kV/cm. CO/CO2 of byproducts is lower 8.2% with catalyst than that without catalyst. These findings show the plasma reactor packed with materials with catalyst has a better selectivity of CO2 than that without catalyst.

Figure 34.

Results of byproducts detection when benzene concentration is 750mg/m3 with or without catalyst

Figure 35.

Results of byproducts detection when benzene concentration is 1500mg/m3 with or without catalyst

Figure 36.

CO2 selectivity when benzene concentration is 750mg/m3 with or without catalyst

From Fig.29c, it is found the selectivity of CO2 ranges from 65% to 69% in the plasma reactor without catalyst, while the selectivity of CO2 ranges from 68% to 73% in the reactor with catalyst. The selectivity of CO2 is independent of electrostatic field strength. The selectivity of CO2 is enhanced due to the benzene oxidation near or on the photocatalyst surface. For that, it could be thought that intermediates and secondary products are more oxidized to CO2 on photocatalyst surface. With benzene concentration increasing, the total output of CO2 increases.

Fig.30 showed the change of CO2 and CO selectivity in NTP reactor with nano-TiO2 catalyst. The CO2 and CO selectivity were 61% and 30%, while removal efficiency was 94% at electric field strength of 12 kV/cm, benzene concentration of 1500 mg/m3 and gas flux of 100 L/h. According to calculating, the total carbon was up to 91%, close to removal efficiency of 94%. It is obvious that NTP coupled with nano-TiO2 catalyst resulted in a higher CO2 selectivity and a more thorough removal effect in NTP processing, i.e. the final reaction products were almost CO2, CO and H2O.

Fig.31 showed the reaction products detected by GC-MS. According to GC-MS patterns, the main reaction products were CO2, CO and H2O, including a very little mass of aldehyde, ketone, acylamide and acetic acid, etc.

Figure 37.

Relationship between CO2 and CO selectivity and removal efficiency (B packed materials coupled with nano-TiO2)

Figure 38.

GC-MS patterns of reaction products (electric field strength of 10 kV/cm, benzene concentration of 1500 mg/m3, gas flux of 100 L/h, B packed materials of i.d.2 mm coated with nano-TiO2)

As shown in fig.29-31, the reaction mechanism could be speculated as follows.

Free radicles formation in the surface of nano-TiO2:

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The reaction between free radicles and benzene molecules:

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At last, the final reaction products should be CO2, CO and H2O if there was enough energy in NTP reactor from AC high voltage.

5. Synergistic effect of catalyst for oxidation removal of toluene

In our opinion, the best way for removing VOCs is by a combination of plasma and catalytic treatments. Ayrault et al. used platinum (Pt)-based catalyst supported on an alumina wash-coated honeycomb monolith by means of a high voltage bipolar pulsed excitation. The energy removal efficiency of 2-Heptanone was of 0.14 mol/kWh at an energy density of 200 J/L. They also found that energy efficiency decreases to 0.029 mol/kWh when using an uncoated monolith even at an energy density of 500 J/L. Sekiguchi studied degradation of VOCs with an ozone-decomposition catalyst under conditions of UV irradiation. They found that TiO2 has a lower VOCs removal ratio without UV irradiation and MnO2-based catalyst has higher VOCs removal ratio at a higher H2O humidity. MnO2 is a catalyst for the decomposition of ozone. The researches on the synergistic action of NTP and catalyst have been carried out for more than 10 years, but there are only a few researches, which have been done adequately, involving byproducts (e.g. O3) and showing energy efficiency, especially those using MnO2/γ-Al2O3 in AC of 50~500Hz.

In the present work, the synergistic effect of NTP and catalyst for VOCs removal has been studied. It was found that the catalyst could solve the problem of O3 formation and improve energy efficiency and at the same time increase significantly the removal efficiency of NTP decomposition.

5.1. Materials and methods

VOCs decomposition was studied by using a manganese–alumina catalyst. The manganese catalyst was prepared by intrusion of pellet type alumina, with granule diameter of being 5–7 mm and BET (Brunauer Emmett Teller) surface of 228 m2/g (Detected by Micromeritics, Amercian Quantachrome Co., NOVA 1000).

The specific surface of the catalysts (as determined by the BET method), is given in table 2. BET surface area has not changed too much with catalysts onγ-Al2O3.

Catalyst

BET Surface Area(m2/g)

γ-Al2O3

228

TiO2/γ-Al2O3

203

5wt%MnO2/γ-Al2O3

218

10wt%MnO2/γ-Al2O3

202

Table 2.

BET surface areas of the catalysts

5.2. Effect of mass percentage of MnO2 onγ-Al2O3

Fig.32 shows the effect of mass percentage of MnO2 on removal efficiency of VOCs (VOCs concentration: 1000mg/m3 or so; gas flow rate: 2L/min; AC frequency: 150Hz). The removal efficiency of MnO2/γ-Al2O3 increased with increasing RED and was of about 10 wt% or 15 wt% >5 wt% at the same RED. However, the removal efficiency of 10 wt% was practically equal to that of 15 wt% at the same RED.

Fig.33 shows the effect of the mass percentage of MnO2 catalyst in NTP reactor on the concentration of VOCs and ozone in the gas exhaust (RED: 0.5 kJ/L). As the mass percentage of MnO2 catalyst increased, ozone and VOCs concentrations were diminished, especially in the case of 10 wt% of MnO2 onγ-Al2O3. It is clear that manganese oxides accelerated the decomposition of O3 to O2 in gas phase. The active oxygen species formed during the O3 decomposition must be helpful for VOCs removal by MnO2/γ-Al2O3.

5.3. Effect of catalyst on removal efficiency and energy efficiency

VOCs removal efficiency is shown in fig.34 as a function of RED with or without TiO2 or MnO2 on γ-Al2O3. The removal efficiency increased with increasing RED and was in the order of MnO2/γ-Al2O3 > TiO2/γ-Al2O3 > γ-Al2O3 at the same RED. It was obvious that MnO2 and TiO2 played a role in VOCs oxidation to a certain extent. γ-Al2O3 possessed sorbent characteristic, so it could improve VOCs concentration on the catalyst surface and increase the reaction time. MnO2 is known as metal oxide catalyst and was found to possess a potential activity in redox reactions. MnO2 surface has been found to expose metal (Mnn+), oxide (O2) and defect sites of various oxidation states, degrees of coordination unsaturation, and acid and base properties. Furthermore, the dd electron exchange

Figure 39.

Effect of mass percentage of MnO2 on removal efficiency of VOCs

Figure 40.

The changes in the concentration of VOCs and ozone with the amount of MnO2 on Al2O3

Figure 41.

Effect of RED on removal efficiency with NTP and catalyst combined reactor (10wt% MnO2/ γ -Al2O3; VOCs concentration: 1000mg/m3 or so; gas flow rate: 2L/min; AC frequency: 150Hz)

Figure 42.

Relationship between RED and energy efficiency with NTP and catalyst combined reactor (10wt% MnO2/γ -Al2O3; VOCs concentration: 1000mg/m3 or so; gas flow rate: 2L/min; AC frequency: 150Hz)

interactions between intimately coupled manganese ions of different oxidation states [Mnn+–O–Mn(n+1)+] furnish the electron-mobile environment necessary for the surface redox activity. These factors would be helpful for VOCs removal.

At the same experimental conditions, the change of energy efficiency is shown in fig.35. The energy efficiency increased with increasing RED and was in the order of MnO2/γ-Al2O3>TiO2/γ-Al2O3>γ-Al2O3 at the same RED. The result indicate that NTP coupled with MnO2/γ-Al2O3 catalyst saved more energy to decompose the same amount of VOCs.

6. Synergistic effect of a combination of catalysts with nonthermal plasma

Many researcher found that for VOCs control, ferroelectric could improve energy efficiency significantly, but ozone concentration increased due to ferroelectric presence. Ogata et al. investigated the effects of alumina and metal ions in plasma discharge using NTP reactors packed with a mixture of BaTiO3 and porous Al2O3 pellets. The results indicated that the oxidative decomposition of benzene was enhanced by concentrating benzene on the Al2O3 pellets. The selected catalyst of MnO2 was well known for high potentials to decompose ozone. Futamura et al. tested catalytic effects of TiO2 and MnO2 with NTP. The results showed that the ozone generated from gaseous oxygen is decomposed by MnO2, but not by TiO2.

A series of experiments were performed for toluene decomposition from a gaseous influent at normal temperature and atmospheric pressure. In this section, the prepared nano-Ba0.8Sr0.2Zr0.1Ti0.9O3 catalyst was used in the plasma reactor. Doped some ions (Sr & Zr) into the powder particles and crystal boundary in the experiment. The metal ions such as strontium, zinc and zirconium entered into crystal lattices of BaTiO3 equably and the Curie temperature (Tc) fell. As a result, the permittivity of nano-Ba0.8Sr0.2Zr0.1Ti0.9O3 was up to 104 which were 12 times higher than that of pure BaTiO3, while dielectric loss reduced to 1/6 in normal temperature. This study found that this nano-material could reduce the energy consumption and increase energy efficiency significantly.

The oxidative decomposition of toluene was enhanced by concentrating toluene on the Al2O3 pellets. The selected catalyst of MnO2 was well known for high potential to decompose ozone. In the experiment, the prepared MnO2/γ-Al2O3 was used as catalyst to reduce the byproducts and toluene concentrations --- also justify about 10 wt%. The objective of this study was to use a combination of catalysts (MnO2/γ-Al2O3 coupled with modified ferroelectric of nano-Ba0.8Sr0.2Zr0.1Ti0.9O3) in the NTP process for toluene decomposition in order to enhance toluene decomposition efficiency and increase energy efficiency and reduce byproducts for commercial applications.

6.1. Materials and methods

An alternating current (AC) of 150 Hz was supplied to the NTP reactor in the radial direction, and the voltage extension changed from 0 kV to 50 kV. The experimental parameters of the process of discharge were detected by an oscillograph (model TDS2014, manufactured by American Tektronix Co.). The primary power values were measured with the voltage-charge(V-Q)Lissajous method in the plasma reactor.

Toluene decomposition was studied with a combination of catalysts including MnO2/γ-Al2O3 and nano-Ba0.8Sr0.2Zr0.1Ti0.9O3 catalysts (volume ratio of 1:1). The manganese oxide catalysts (5wt%, 10wt%, 15wt%) were prepared by impregnation of pellet typeγ-alumina with the granules diameter of 5~7 mm and BET surface area of 228 m2/g detected by Micromeritics (model NOVA 1000, manufactured by American Quantachrome Co.).

Nanometer-sized Ba0.8Sr0.2Zr0.1Ti0.9O3 powders were prepared with inorganic salts, such as TiCl4 and Ba(OH)2, as the raw materials by a water-thermal method at normal pressure. Particulate diameters of Ba0.8Sr0.2Zr0.1Ti0.9O3 was 59 nm which was detected by XRD (model D8 ADVANCE, manufactured by Germany Bruker Co.) and BET surface area was 8.8 m2/g. The relative permittivity of nano-Ba0.8Sr0.2Zr0.1Ti0.9O3 was about 104 (detected by LCR automatism test instrument 4210).

The toluene concentration was determined using a gas chromatography (model HP6890N, manufactured by Agilent Co.) with a flame ionization detector (FID) and a capillary column of HP-5 (internal diameter of 0.32 mm, length 30 m). The byproducts such as aldehyde, alcohols, amide, hydroxybenzene and polymerization products, etc, were identified by GC-MS (manufactured by American Thermo Finnegan Co.) and FT-IR (model Vertex 70, manufactured by Germany). Ozone concentration was measured by a chemical titration method of iodine.

6.2. Effect of combined catalysts on toluene removal efficiency

As the MnO2/γ-Al2O3 catalyst has the best effect for ozone decomposition but not for toluene decomposition, and nano-Ba0.8Sr0.2Zr0.1Ti0.9O3, a type of developmental material on base of pure BaTiO3 (typical ferroelectric), enhances energy efficiency because of its higher relative permittivity of 104, a combination of nano-Ba0.8Sr0.2Zr0.1Ti0.9O3 with MnO2/γ-Al2O3 as a combined catalyst was tested in this study.

The effect of various catalysts such as multiple catalyst, nano- Ba0.8Sr0.2Zr0.1Ti0.9O3, MnO2/γ-Al2O3 and no padding on removal efficiency is shown in Fig.36. The removal efficiency increased significantly with the catalysts than that without. The removal efficiency increased in the order of: combined catalyst >nano-Ba0.8Sr0.2Zr0.1Ti0.9O3 >MnO2/γ-Al2O3 >no padding.

The best removal efficiency of 98.7% was achieved in the NTP process. It indicated that the combination of catalysts exhibited a synergistic effect for toluene decomposition.

Figure 43.

The change of removal efficiency with various padding (toluene concentration: 800-1000 mg/m3; gas flow rate: 2 L/min; AC frequency: 150 Hz)

6.3. Effect of combined catalysts on ozone formation

Fig.37 shows the influence of various catalysts on ozone formation with the order of: combined catalyst > MnO2/γ-Al2O3 > no padding > nano-Ba0.8Sr0.2Zr0.1Ti0.9O3 at RED of 0.5 kJ/L. This result suggested that MnO2/γ-Al2O3 in the combination of catalysts should have a main effect on ozone decomposition.

Figure 44.

The change of O3 concentration with various padding

6.4 Effect of the combination of catalysts on energy efficiency

Fig.38 shows the influence of various catalysts on energy efficiency with the order of: combined catalyst > nano-Ba0.8Sr0.2Zr0.1Ti0.9O3 > MnO2/γ-Al2O3 > no padding at the same SED. These results indicated that the nano-Ba0.8Sr0.2Zr0.1Ti0.9O3 in the combination of catalysts should play an important role for improving energy efficiency.

As a result, the combination of catalysts shows the best removal efficiency of toluene, the best decomposition effect of ozone and the best energy efficiency for toluene removal.

Figure 45.

The energy efficiency with various padding

6.5. Byproducts and decomposition pathways of toluene

Non-thermal plasma has high potential in air cleaning technology, but in some cases unwanted byproducts are formed which could be more harmful than the original VOCs. Fig.39 shows the FT-IR spectrum of the byproducts of toluene decomposition and Fig.40 shows the FT-IR spectrum of the byproducts on the surface of the combination of catalysts.

As shown in Fig.40(a), the -NH- and -NH2 peak appeared at 3350 cm-1 while the peak of 2730 cm-1 N=C-N was absent. The peak -NH- with benzene ring appeared at 3450 cm-1, –OH at 3400 cm-1, -CH3/-CH2 at 2900 cm-1, benzene derivative (hydroxybenzene, polymerization products, etc) at 1700~1100 cm-1, and CO2 and CO separately at the rang of 2300~2100 cm-1 and 700~500 cm-1. So the byproducts on the surface of the combination of catalysts involved aldehyde, alcohols, amide, and benzene derivative. However, when the combination of catalysts were packed into the NTP reactor, the byproducts on the surface of the packed materials in the NTP reactor reduced greatly as shown in Fig.40(b). Except of amine, CO2 and CO, no other byproducts were detected on the surface of catalysts. It illuminated that the synergic effect of the NTP with the combination of catalysts could control byproducts effectively.

In Fig.39, the products of toluene decomposition included CO2, CO and H2O. At the same time, there are a mass of ozone (strong peak at 1000 cm-1), and several amide and benzene derivatives. Compared spectrum 'a' with 'b' in Fig.39, the benzene derivatives and ozone concentration reduce while the amounts of CO2 and H2O increase with the increase of the electric field strength.

A large number of high-energy electrons, ions and free radicals were produced in the NTP reaction process. Firstly, the high-energy electrons could take part in reaction with oxygen in air as follow:

 +O 2  e +O  2 (A 3  + u )  e + O( 3 P) + O( 3 P) e+O2 e +O2•(A3u+)  e + O(3P) + O(3P)

(46)

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 + O 2  e +(B 3   u )  e + O( 1 D) + O( 3 P) e+ O2 e +(B3u e + O(1D) + O(3P)

(47)

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Figure 46.

FT-IR spectrum of the products from toluene decomposition (a. electric field strength of 10 kV/cm; b. electric field strength of 13kV/cm)

Figure 47.

FT-IR spectrum of the byproducts on the surface of the combination of catalysts

The oxygen free radical groups react with oxygen and other molecules to form ozone:

O + O 2 + M  3 + M  O 3 + M O + O2+ M O3•+ M  O3+ M

(48)

Options

At the same time, the high-energy electrons react with H2O and N2 in gaseous phase:

 +H 2 OOH  +H   e+H2OOH+H

(49)

Options

 +H 2 O2H  +O   e+H2O2H+O

(50)

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H 2 O + O( 1 D) 2OH   H2O + O(1D) 2OH·

(51)

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 +N 2 2N   e+N22N

(52)

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Toluene bond energy between the carbon of benzene ring and the carbon of the substituent radical is 3.6 eV, which is lower than that of carbon-carbon bond or hydrocarbon bond. As a hydrogen atom in a benzene ring is replaced by a methyl radical to form toluene, the newly formed bond is less stable and the most vulnerable. Of course, the other bonds are also likely to be destroyed by high energy electrons. Formulas 71 to 76 are the possible reaction equations of the process of toluene removal.

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According to the FT-IR spectrums (Fig.40), the author speculated the reaction pathways for toluene decomposition with the NTP and the combination of catalysts (Fig.41). The oxygen and hydroxyl free radicals of should be the inducement during the process of toluene oxidation. The oxidation process of toluene may involve many reactions and these reactions cooperate and interact with each other for toluene decomposition. Firstly, a series of chain reactions take place between OH radicals and toluene molecules due to the higher oxidation ability of OH radicals than that of oxygen radicals:

()

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()

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()

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Then, the idiographic reactions occur because of oxygen free radicals during the subsequent oxidation reaction as follows:

()

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()

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The binding bonds inside the benzene ring break down after the bonds outside the benzene ring break as follows:

()

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()

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()

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()

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At last, the byproducts were oxidized to CO2 and H2O with increasing RED and the help of catalysis.

()

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()

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()

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The byproducts of toluene decomposition were detected using GC-MS at electric field strength of 8 kV/cm and the results show the peak of these products in Fig.42 (a). Products including aldehyde, alcohols, amide, and benzene derivative have been identified.

Fig.42 (b) shows a minor amount of toluene and trace amounts of the products exist at electric field strength of 14 kV/cm. Chang et al. claimed that VOC removal depended on two main mechanisms: direct electrons attack on VOC molecules and indirect reaction between VOC molecules and radicals. These radicals involved oxygen plasma, free radical groups, ozone, etc., which were reactive and could react with toluene molecules to form less hazardous products. If the electric field strength was strong enough or RED was high enough, the toluene molecules would be oxidized to form CO2, CO and H2O as the final products.

Figure 47.

Mass spectrum of byproducts of toluene decomposition

Atkinson et al. (1977) reported that aromatic compounds react with OH radicals by two pathways: hydrogen atom abstraction and OH addition to the aromatic ring. Reaction control pathways I–XII were illustrated in Fig.43. The results showed in a complex oxidation

Figure 49.

Abatement pathways of toluene by NTP with the combination of catalysts

mechanism of toluene via several pathways, producing either ring-retaining or ring-opening products. The final products were CO2 and H2O.

The synergistic effect of the combination of catalysts with the NTP reactor is presented in Fig.44. The catalyst carrier of -Al2O3 possesses sorbent characteristic, so it could improve toluene concentration on the catalyst surface and increase the reaction time. MnO2 is known as a metal oxide catalyst and has been reported to possess a potential activity in redox reactions. MnO2 surface has been found to expose metal (Mnn+), oxide (O2) and defect sites of various oxidation states, present degrees of coordination instauration, and exhibit acid and base properties. Furthermore, the dd electrons exchange interactions between intimately coupled manganese ions of different oxidation states [Mnn+OMn(n+1)+] furnish the electron-mobile environment necessary for the surface redox activity:

()

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()

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These factors would be helpful for toluene decomposition. Radhakrishnan reported that ozone decomposed to O2- and O22- in the surface of MnO2. Naydenov et al. believed that O- existed in the surface of MnO2 according to the oxidation of benzene in the surface of MnO2. As a modified ferroelectric, nano-Ba0.8Sr0.2Zr0.1Ti0.9O3 has a higher dielectric constant than BaTiO3 and is polarized at lower electric field strength. More high energy electrons and active radicals are generated to accelerate the reaction between NTP and toluene molecules.

Figure 50.

Catalysis chart of the combination of catalysts in the process of gas discharge

7. Conclusions

In section secend, the adsorption kinetics was studied and the removal amount of VOCs was 25% or so through adsorption of γ-Al2O3. In the experiment, with the non-thermal plasma reactor size fixed, the immediate advantage of adsorption of the packed materials into the space of air discharge is the longer reaction time of VOCs with plasma and higher removal efficiency. The functions of γ-Al2O3 in plasma reactor were to adsorb free radicals and VOCs molecules and to provide reaction surface for VOCs decomposition and to release reaction products. Plasma decomposed air molecules and provided free radicals for catalysis reactions on the surface of the γ-Al2O3 pellets. So adsorbent γ-Al2O3 enhanced NTP technology and resulted in higher VOCs removal efficiency and energy efficiency and a better inhibition for O3 formation in the gas exhaust. For the study in the future, some catalysts should be considered to add into the NTP reactor.

In section third, a series of experiments for the effect of NTP technology were performed to abate toluene from a gaseous influent at room temperature and atmospheric pressure. Three types of NTP reactors were used in the NTP process for toluene removal with and without packed materials. A new modified ferroelectric material of Ba0.8Sr0.2Zr0.1Ti0.9O3 as the packed materials was prepared by us in laboratory. Compared with the two packed materials in terms of removal efficiency of toluene, RED, energy efficiency and ozone concentration, the experimental results were obtained as follows: Packed materials with Ba0.8Sr0.2Zr0.1Ti0.9O3 enhanced removal efficiency of toluene and energy efficiency than those with BaTiO3. Ba0.8Sr0.2Zr0.1Ti0.9O3 had better ferroelectric than BaTiO3. By operating at the RED of 0.76 kJ/L, removal efficiency was up to 97% and the energy efficiency was 6.48 g/kWh when the packed materials of Ba0.8Sr0.2Zr0.1Ti0.9O3 are used. O3 concentration had a maximum value at the RED of 0.7 kJ/L or so. The hybrid NTP technology should be more effective to improve energy efficiency for VOCs removal than the simple technology of NTP. Based on the above results, we would consider how to control the formation of ozone in the further experiments. Our research would provide a reference to improve energy efficiency for the commercial applications of the NTP technology.

In section fourth, the laboratory-scale plasma reactor was used for benzene removal in an air stream and the following conclusions are obtained. B packed materials was better than A packed materials for benzene removal. Compared different size of packed materials, B packed materials of i.d.2 mm was better than the others' size for benzene decomposition.

In section fifth, nano-TiO2 packed bed reactor is used to decompose benzene. The experimental results show as follows: With ozone concentration increasing, the removal efficiency of benzene increases. Ozone concentration with packed materials is heigher than that without packed materials in the plasma reactor. Water vapor reduces ozone concentration, and occurring competitive adsorption on the surface of TiO2 Ozone concentration increases with gas flux increasing, and the removal efficiency of benzene reduces with initial concentration of benzene increasing. When both photocatalyst and ozone coexist, there will be an improved removal efficiency of benzene in the plasma reactor. Effective utilization of active oxygen species is essential in VOCs removal, and TiO2 can generate higher concentrations of different types of active oxygen species in non-thermal plasma. It is facile and promising to simultaneously hybridize plasma with TiO2 based on the data presented. The plasma reactor packed with catalyst (B packed materials coated nano-TiO2) showed a better selectivity of CO2. Detected by GC-MS, the main products in the plasma reactor are CO2, H2O and a small quantity of CO. The plasma reactor packed with catalyst shows a better selectivity of CO2 than that without catalyst. The selectivity of CO2 is independent of electrostatic field strength. The selectivity of CO2 is enhanced due to the benzene oxidation near or on the photocatalyst surface. With benzene concentration increasing, the total output of CO2 increases. The hybrid system comprising a non-thermal plasma reactor and nanometer TiO2 catalyst, not only in the gas phase but on the catalyst surface, resulted in the higher energy efficiency and enhanced performance for the oxidative removal of benzene with lower medium reactivities and higher CO2 selectivity in non-thermal plasma.

In section sixth, the synergistic effect of NTP and catalyst for VOCs removal is tested in the experiment. The results show that removal efficiency increased with increasing RED and was in the order of 10wt% MnO2/γ-Al2O3 15wt% MnO2/γ-Al2O3 >5wt% MnO2/γ-Al2O3 at the same RED. As the mass percentage of MnO2 catalyst increased, ozone and VOCs concentrations were decreased, especially for 10 wt% MnO2/γ-Al2O3. The removal efficiency and energy efficiency increased with increasing RED and was in the order of MnO2/γ-Al2O3>TiO2/γ-Al2O3>γ-Al2O3 at the same RED. So we could draw a conclusion that MnO2/γ-Al2O3 has a better potential than the other catalysts in the experiment to improve the energy efficiency and reduce O3 formation.

In section seventh, a series of experiments basing on above all researches, were performed for removal of toluene gaseous influent at room temperature and atmospheric pressure. The self-prepared combined catalyst was used to improve the NTP process and to take the catalytic advantages of both MnO2/γ-Al2O3 and nano-Ba0.8Sr0.2Zr0.1Ti0.9O3. From the view of materials application, the authors adopted NTP coupled with the combination of catalysts technology to decompose VOCs in there. The catalyst materials could be prepared easily and cheap, and at the same time, this combined technology resolved the key bottlenecks effectively, i.e. saving energy consumption and reducing byproducts what we don't want. Therefore, the combination of catalysts technology could advance to the NTP technology and improve applications in the industry in the future.

8. Acknowledgements

This work was supported by the Youth Research Funding of China University of Mining & Technology (Beijing), and China College of innovative pilot projects (No.101141309), and the Fundamental Research Funds for the Central Universities (No.2009QH03), and Environmental Protection Commonweal Industry Research Special Projects (No.201009052-02),and the Doctoral Program of Higher Education of China (20040005009).

   

출처: <http://www.intechopen.com/books/chemistry-emission-control-radioactive-pollution-and-indoor-air-quality/vocs-removal-using-the-synergy-technology-basing-on-nonthermal-plasma-technology>

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Performance Catalytic Ozonation over the Carbosieve in the Removal of Toluene from Waste Air Stream

   

Mohammad Reza Samarghandi (PhD)a, Seyed Alireza Babaee (MSc)a, Mohammad Ahmadian (MSc)b, Ghorban Asgari (PhD)a, Farshid Ghorbani Shahna (PhD)c, and Ali Poormohammadi (MSc)d*

a Research Center for Health Sciences and Department of Environmental Engineering, School of Public Health, Hamadan University of Medical Sciences, Hamadan, Iran

b School of Public Health, Kermanshah University of Medical Sciences, Kermanshah, Iran

c Department of Occupational Health Engineering, School of Public Health and Research Center for Health Sciences, Hamadan University of Medical Sciences, Hamadan, Iran

d Social Development & Health Promotion Research Center, Kermanshah University of Medical Sciences, Kermanshah, Iran

* Correspondence: Ali Poormohammadi (MSc), E-mail: apoormohammadi000@yahoo.com

Received: 13 October 2013, Revised: 17 February 2014, Accepted: 17 March 2014, Available online: 06 April 2014

   

Abstract

Background: Toluene is a volatile organic compound, one of 189 hazardous air pollutants (HAPs) and the most important pollutant found in most industries and indoor environments; owing to its adverse health, toluene must be treated before being released into the environment.

Methods: In this research study, a continuous-flow system (including an air compressor, silica gel filters and activated charcoal, impinger, an ozone generation and a fixed bed reactor packed with the carbosieve in size 1.8-2.3 mm, specific surface: 972 m2/g,) was used. This glass reactor was 0.7 m in height; at a distance of 0.2 m from its bottom, a mesh plane was installed so as to hold the adsorbent. Moreover, 3 l/min oxygen passed through this system, 0.43 g/h ozone was prepared. The flow rate of waste airstream was 300 ml/min. The efficiency of this system for removal of toluene was compared under the same experimental conditions.

Results: Under similar conditions, performance of catalytic ozonation was better in toluene removal than that of ozonation and carbosieve alone. On average, increasing the removal efficiency was 45% at all concentrations. When carbosieve and ozone come together, their synergistic effects increased on toluene degradation.

Conclusions: Catalytic ozonation is a suitable, high-efficient and available method for removing toluene from various concentrations of waste air stream. This process due to the short contact time, low energy consuming and making use of cheap catalysts can be used as a novel process for removing various concentrations of volatile organic compounds.

Keywords: atalytic Ozonation, Volatile Organic Compounds, Carbosieve, Toluene

   

Introduction

Volatile Organic Compounds (VOCs) are a major group of air pollutants emitted from various sources and pollute the air. They contain more than 300 compounds such as oxygenated hydrocarbons, aromatic hydrocarbons and halogenated hydrocarbons1-3. Benzene, toluene, ethyl benzene, xylene (BTEX) compounds are the most important and common chemical compounds of aromatic hydrocarbons and are considered as indices of VOCs. They are abundantly found in cities and industrial areas and have been classified as toxic priority pollutants. Their major problem in the atmosphere is the possibility of cancer and production of "Air Toxic" 4,5. Toluene or methyl benzene is one of volatile aromatic compounds which is very similar to benzene, but less volatile. This compound has a methyl branch on a benzene ring. Toluene is transparent, colorless and volatile liquid with obnoxious odor. As a solvent, toluene is highly used in oils, paints, resins, detergents and glues and pastes. It is also used in gasoline, petrol and other aviation fuels to raise octane number.

According to ACGIH, toluene is one of 189 hazardous air pollutants and is a primacy and prioritized pollutant which has been classified in A4 group of carcinogenic substances6-8. Due to various environmental and health effects of this compound, different methods (including adsorption, thermal and catalytic oxidation and advanced oxidation processes) have been examined to remove it from waste air stream9,10. In comparison with conventional treatment methods, Advanced Oxidation Processes (AOPs) are suitable methods for degradation of toxic air pollutants. Degrading pollutants by AOPs is based on production of free active radicals that have high oxidative power and are capable of changing most organic compounds to minerals. The radicals by attaching molecules of organic substances and taking their hydrogen ion, convert these compounds to minerals, during one or some steps.

Catalytic ozonation process (COP) is a new method of single ozonation process (SOP) that has recently been taken into consideration in air pollution control industry. In this process, to improve SOP efficiency, various substances are used as catalysts to accelerate degradation process, increase reaction time and be sure of non-presence of ozone residues in exhaust air11-15. So far some catalysts such as activated carbon, silica gel, metal oxides and various resins have been used in catalytic ozonation process to remove toluene from polluted air16-18.

According to capabilities of various carbon adsorbents in treating air pollutants, carbosieve, a kind of carbon-based adsorbent which is produced as a result of pyrolysis of synthetic polymers or some of petroleum compounds with high porosity and specific surface area (BET) and is mostly used to adsorb volatile hydrocarbons19,20.

In this study, the carbosieve was used as a non-polar catalyst and adsorbent in catalytic ozonation process.

   

Methods

Experimental Pilot

This empirical laboratory study was carried out as a pilot scale from December 2012 to May 2013. A laboratory-scale pilot was designed and launched at Hamadan Environmental Chemistry Laboratory (Figure 1), western Iran. Toluene liquid with purity of 99.5% was prepared from German Merck Company. Air containing polluted gas and ozone gas entered bottom of a reactor with a continuous flow. Considering the aim of this study the reaction conditions were closer to the actual conditions thus a dynamic system with continuous flow was used. Contaminant gas in the first of the reactor enters into the bottom of the reactor and exits from the other side (Due to the absence of gas leaks in the system). To prevent condensation of contaminants in the reactor, all parts of the reactor was placed in isothermal chamber. Input air before entering to the bed enter to the converging chamber and after passing through reticulated plate and finally enter to the catalyst bed.

This glass reactor was 0.7 m in height; at a distance of 0.2 m from its bottom, a mesh plane was installed so as to hold the adsorbent. Required air was supplied by a compressor equipped with oil trap. After the pressure was adjusted, the air passed through a combined column of silica gel and activated charcoal in order to dry and remove any organic contamination. To prepare toluene-containing air in given concentrations, Standard Atmosphere Production method was used in a dynamic way with the aid of an impinger21. In this system, the air, produced by a compressor and dried as well as cleaned before, was divided into two branches; calibrated rotameter and needle valve were used to control carefully in the air stream in each branch so as to achieve certain concentrations. In one branch, toluene vapors entered air stream through impinger-bubbler (by keeping temperature of toluene constant) and in the other branch, there was pure stream of air. These two branches lead to a single branch in which a standard stream of air with the given concentration of toluene was created. The reactor was placed in the isothermal chamber for controlling temperature in the reactor. Our concept from constant pressure was atmospheric pressure in the laboratory environment. Dynamic method was used to create standard atmosphere. Inert and contaminant gas at determined concentration enter to the system with a regulated and specific ratio. This works was performed by maintain impinger temperature (Containing liquid toluene due to its vapor pressure) by a hot water bath. Besides for keeping a steady flow and to avoid concentration fluctuations a baffled mixing chamber was used before entering flow to the main reactor. However, in practice, only minor changes in concentration levels (a few ppm) was existed. Therefore, input and output load in the system was reported as removal efficiency.

Ozone was made using an ozone-making machine, ARDA, model COG-OM. In this generator, ozone is produced through creating electrical discharge with alternating current in a sluice in presence of oxygen. Oxygen required for the ozone generator was supplied by an oxygen-making machine, model PORSA VF-3 with high purity level and the ability to adjust oxygen injection level. The amount of ozone produced by an ozone generator was measured according to Iodometry method in the presence of 2% potassium iodide solution after 10 min of contact22,23. As 3 l/min oxygen passed through this system, 0.43 g/h ozone was prepared. In ozonation unit, ozone gas entered toluene-containing waste air stream; to prevent and reduce the probable fluctuations and to equalize and fix equilibrium, a baffled mixing chamber is used before the reactor which contains adsorbent bed. After current exits this mixing chamber, it enters carbosieve-containing reactor as a catalyst and non-polar adsorbent bed. Prior to experiments, carbosieve was placed in a in an oven for 2 h at 400 °C and was dried completely in order to remove all probable pollution. Carbosieve used in this study was purchased from Air Tools Co. in Tehran. Then the surface area was measured by BET method. Ten g of Carbosieve were used at each stage of the experiment. Although changes in pressure drop has not been measured before and after bed but the system was designed in such a way do not create the pressure drop in the input stream.

   

Sample Analysis

Toluene concentration was measured before and after experiments in an air stream using 1501 NMAM (NIOSH Manual Analytic Method) and gas-chromatography device, model SHIMADZU 2010, equipped with flame ionization detector (FID) with a column length of 60 m, column internal diameter of 0.25 mm, film thickness of 0.25 mm and temperature range of 50 to 180 °C according to temperature programming. Optimum analysis conditions of gas chromatography was prepared with injection site temperature of 200 °C, detector temperature of 250 °C, carrier gas flow of approximately 30 ml/min, hydrogen gas flow of 30 ml/min and air stream of about 300 ml/min 24,25. Toluene vapors collected with activated charcoal tube using sampling pump accordance with Method 150124. After extracting the samples in carbon disulfide 1 μL of extracted sample was injected into the GC in quick time. Experiments were performed in normal ambient temperature (27 ±2 °C) and desirable atmospheric pressure and ventilation. In order to assess the accuracy of the analysis method, was injected three times into the capillary column of gas chromatograph and their mean was reported as the amount of that parameter. The efficiency of carbosieve bed, single ozonation and catalytic ozonation were examined and compared separately.

   

   

Performance Catalytic Ozonation over the Carbosieve in the Removal of Toluene from Waste Air Stream | Samarghandi | Journal of Research in Health Sciences

http://jrhs.umsha.ac.ir/index.php/JRHS/article/view/1182/html

화면 캡처: 2016-05-13 오전 8:58

   

   

Figure 1: Schematic diagram of the experimental setup

Results

Efficiency of carbosieve bed

Parameters affecting system efficiency including air flow discharge entering the reactor, adsorbent bed size, ozone level and relative humidity were kept constant at 3 l/min, 15g, 0.43 g/h and 5% respectively; only system efficiency was examined on various concentrations of toluene in range of 20 to 200 ppm. As toluene concentration increased, bed saturation time and absorbent breakpoint decreased. Results of carbosieve efficiency of toluene removal are shown in Figure 2 (A).

Ozone efficiency

Figure 2(B) shows the removal efficiency of toluene from waste air stream by the single ozonation process separately and without the presence of carbosieve. In this step, experiments conducted in various concentrations of toluene in range of 20-200 ppm and only ozone gas with 3 1/min input of pure oxygen to ozone-making machine and production of 0.43 g/h ozone was used, also other variables affecting system efficiency were kept constant as before. In this step, as concentration increased from 20 to 200 ppm, toluene removal efficiency decreased. In 20 ppm of toluene in the reactor inlet, the removal efficiency reached to 31%. On the other hand, in 200 ppm of toluene in the reactor inlet, ozone efficiency was about 16%. Therefore, the highest efficiency of process was observed in toluene concentration equal to 20 ppm and lowest removal efficiency was in toluene concentration equal to 200 ppm.

Efficiency of catalytic ozonation

Results of efficiency of catalytic ozonation are shown in Figure 3, 4. In this step, experiment was performed under the same conditions and efficiency of catalytic ozonation was examined in terms of toluene output concentration and adsorbent break point. In this step, efficiency of catalytic ozonation was examined on various concentrations of toluene in range of 20-200 ppm. In 20 ppm of toluene, the efficiency of catalytic ozonation in removal of this compound reached about 16 ppm (80%). On the other hand, in 200 ppm of toluene was observed 155 ppm (77.5%). Therefore, by increasing toluene concentration from 20 to 200 ppm, toluene removal efficiency decreased. Moreover, carbosieve saturation and break point were occurred significantly later than single carbosieve system.

   

Figure 2: (A) Efficiency of carbosieve bed in removing toluene from waste air stream (B) Ozone efficiency (single ozonation) on removal of toluene from waste air stream

   

   

Performance Catalytic Ozonation over the Carbosieve in the Removal of Toluene from Waste Air Stream | Samarghandi | Journal of Research in Health Sciences

http://jrhs.umsha.ac.ir/index.php/JRHS/article/view/1182/html

화면 캡처: 2016-05-13 오전 9:01

   

   

   

Figure 3: Efficiency of catalytic ozonation in removal of toluene from waste air stream

   

   

Performance Catalytic Ozonation over the Carbosieve in the Removal of Toluene from Waste Air Stream | Samarghandi | Journal of Research in Health Sciences

http://jrhs.umsha.ac.ir/index.php/JRHS/article/view/1182/html

화면 캡처: 2016-05-13 오전 9:02

   

   

Figure 4: Comparison between times of toluene emergence in carbosieve system with COP process in the different concentrations

   

   

Performance Catalytic Ozonation over the Carbosieve in the Removal of Toluene from Waste Air Stream | Samarghandi | Journal of Research in Health Sciences

http://jrhs.umsha.ac.ir/index.php/JRHS/article/view/1182/html

화면 캡처: 2016-05-13 오전 9:03

   

   

Figure 5: A schematic of toluene decomposition in the catalytic ozonation over the carbosieve

   

   

Performance Catalytic Ozonation over the Carbosieve in the Removal of Toluene from Waste Air Stream | Samarghandi | Journal of Research in Health Sciences

http://jrhs.umsha.ac.ir/index.php/JRHS/article/view/1182/html

화면 캡처: 2016-05-13 오전 9:03

   

   

Discussion

This study investigated the efficiency of catalytic ozonation process in treating toluene-containing air as one of hazardous air pollutants. Toluene removal was examined separately on carbosieve bed, single ozonation and catalytic ozonation.

Results revealed that removal efficiency by SOP process was, on average, less than 23% at given concentrations; that is, as the concentration increased from 20 ppm at the beginning of the system to 200 ppm, removal efficiency by SOP process decreased from 31% to 16%. In a similar study by Mousavi et al. on removal of xylene from waste air stream, xylene removal efficiency by ozone gas was about 10% 26. On removal of volatile organic compounds (VOCs) using catalytic ozonation that removal efficiency of volatile organic compounds using only ozone was low and thus would result in production of harmful byproducts27. Besides, obtained results are in accordance with the findings of Alvarez et al. on comparison between catalytic ozonation and activated carbon adsorption/ozone- regeneration processes for wastewater treatment28.

Decreased efficiency by SOP process is due to ozone low ability to oxidize aromatic compounds and alkenes and low retention time necessary for complete reaction between ozone and toluene as a result of continuity of the system29. According to obtained results while using carbosieve alone as concentration increased from 20 ppm to 200 ppm, adsorbent break point in the reactor outlet decreased from 10 h to 3 h (Figure 5). Accordingly, adsorbent beds including carbosieve remove the pollutants available in the air passing through absorbent due to early saturation, especially at high concentrations, necessity to reclamation and most lack of change in the nature of pollutants that have be(A) Efficiency of carbosieve bed in removing toluene from waste air stream (B) Ozone efficiency (single ozonation) on removal of toluene from waste air streams hazardous to the environment, making use of absorbents alone may be impractical.

In the present study, to remove restrictions of SOP process and to adsorb over carbosieve, the combinations of these two methods were used. Results of this study indicate that catalytic ozonation has a significant efficiency compared with separate adsorption methods over carbosieve and SOP process. This advantage is show in Figure 4. Accordingly, as concentration increased from 20 to 200 ppm, absorption break point and removal of toluene from hybrid reactor decreased from 12 to 6 h, while this time was between 3 and 10 h for carbosieve. ThusEfficiency of catalytic ozonation in removal of toluene from waste air stream was higher in hybrid reactor output and COP process under similar laboratory conditions. For example, for concentration of 200 ppm, toluene discharge time increased by 50% compared to carbosieve bed; on average, efficiency in COP process increased by 45% at all concentrations compared with SOP. In carbosieve system, after the bed was saturated at reactor outlet, output toluene concentration was equal to input concentration, while in COP process concentration of output toluene was less than input concentration after saturation; it reflects that combining carbosieve and ozone systems strengthens the effect of them and thus results in more molecular degradation of toluene passing over bed. Xylene removal level increased 22% in COP process compared to activated charcoal and ozone26. Similarly, Wung et al. revealed that toluene removal level increased 20% and 40% in COP process in comparison with zeolite adsorption and MCM-41 respectively. Furthermore, efficiency of ozonation alone in removal nitrobenzene from aqueous solution was about 25% and while was used of COP, the process efficiency reached to about 70%; these results are consistent with results of this seA schematic of toluene decomposition in the catalytic ozonation over the carbosieve">27,30.

Since ozone is adsorbed over adsorbent, carbosieve adsorbs ozone which rapidly decomposes to activated oxygen atoms over adsorbent bed due to its instability. In other words, carbosieve acts as a catalyst and decomposes ozone into activated oxygen atoms. Having specific surface area and high porosity, carbosieve creates a surrounding bed; in addition to increasing contact surface and collision of toluene and ozone molecules, carbosieve increases retention time for reaction and thus improves removal efficiency.

In reaction between ozone and positions of acid Luis of adsorbent, ozone is decomposed into activated oxygen atoms which decompose toluene into CO2 and H2O through attacking single connective bonds between carbon atoms (C-C) or connective bonds between carbon-hydrogen atoms (C-H). Since, in practice, reaction between toluene and ozone is not done completely and may result in production of intermediate compounds, these compounds are again adsorbed by carbosieve (due to its high specific surface) and are decomposed by ozone or by the produced activated radicals (Figure 5).

No intermediate compound was observed in COP process output26,27. In positions of acid Luis of adsorbents, toluene is decomposed into O2 and O• according to the following reactions:

(1)  O3 + CMS – s CMS – 3SO        

(2)  CMS-3SºCMS-3Sº-sO•+O2               

(3)  CMS- sO•+O32CMS- sO•+O2           

(4)   CMS- SO2 CMS- s+O2               

CMS-S and CMS-sO3, respectively, reflect positions of acid Luis and ozone molecules adsorbed over carbosieve. CMS-sO3 and CMS-sO• show types of carbosieve surface oxides. CMS-sO• can also react with water vapor available in the air entering the system and can produce activated hydroxyl radicals (OHº) which indirectly take part in decomposition and oxidation of toluene and thus improve removal efficiency.

(5)  CMS-S O•+H2O CMS-S(OH)2•

When toluene- and ozone-containing air passes over carbosieve, catalysed reactions of 1-5 occur and toluene is directly and indirectly removed from waste air stream by ozone and radicals resulted from ozone decomposition. In the following equations, R represents intermediate compounds produced during direct and indirect oxidation reactions.

Direct oxidation:        

(6) CMS- SO3+ C6H5CH3 CO2 + H2O + R    

(7) CMS- SC6H5CH3+O3 CO2 + H2O + R               

(8)  C6H5CH3 +O3 CO2 + H2O + R                                 

Indirect oxidation:

(9) CMS-S O•+ C6H5CH3 CO2 + H2O + R                     

     (10) CMS-S(OH•)2+ C6H5CH3 CO2 + H2O + R               

Since the system used in this study was operated in dry mode, hydroxyl radicals are less likely to be formed so they cannot have an important and vital role in toluene decomposition. In COP process, in addition to ozone and radicals resulted from its decomposition, oxygen molecules play a significant role in toluene decomposition. In the presence of oxygen, ozone decomposition rate increases for more toluene oxidation. In autoxidation processes, this molecule oxides radical intermediate compounds (R*) which are one of the reasons for deactivation of catalyst26,27,29,31,32.

(11) R• + O2RO•2→→→CO2, CO    

Conclusions

Removal of toluene from waste air stream, especially in higher concentrations, is more efficient by COP process than by adsorption system over carbosieve and SOP. This system can be one of promising alternatives for removing various concentrations of volatile organic compounds, especially toxic and hazardous volatile compounds, from waster air stream due to  low energy consuming, making use of cheap catalysts and applying adsorbents proportional to pollutant. COP process adsorbs and destroys pollutant simultaneously; in addition to ozone O3 (E= 2.08 v), this process uses activated hydroxyl radicals (OHº) (E= 2.80 v) and atomic oxygen O• (E= 2.42 v) resulted from catalyzed reactions over bed. This phenomenon lead to bed chemical reclamation, increased system operating time and decreased the cost of bed reclamation. Therefore, the main benefit of catalytic ozonation is that reaction time is shortened, causing decomposition and purification of volatile organic compounds from air stream in high concentrations of ozone gas and pollutant without removal of excess ozone concentrations at the reactor outlet. However, additional studies are required to develop techniques to enhance the oxidation activity and industrial application in greater scales.

   

Acknowledgments

This article is extracted from the Master's thesis. The authors would like to thank Hamadan University of Medical Sciences for the financially supporting of this research.

Conflict of interest statement

This study did not have any conflict of interest statement.

   

References

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    출처: <http://jrhs.umsha.ac.ir/index.php/JRHS/article/view/1182/html>

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FIGURE 1. Overall removal efficiencies and the corresponding toluene decomposition fractions at various ozone inlet concentrations with NaX, NaY, and MCM-41 adsorbents. Experimental conditions: toluene, 1.5 ppmv; ozone, 0 - 80 ppmv; adsorbent mass, 2 g; residence time, 0.13 s; temperature, 23 - 25 ° C; error bars are 1 σ . 

   

출처: <https://www.researchgate.net/figure/23649745_fig1_FIGURE-1-Overall-removal-efficiencies-and-the-corresponding-toluene-decomposition>

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Carbon monoxide, CO

Carbon monoxide has a small linear molecule with a relatively short C-O bond, earlier described as a double bond but nowadays mostly as a triple bond. The valence electrons are slightly drawn towards oxygen, which makes the molecule polar.

Carbon monoxide molecule shown in two ways, from Wikipedia. The C-O triple bond has the unit pm (picometer) which means 10-12 m.

   

CO is an uncolored and non-smelling gas at room temperature. Carbon monoxide is highly toxic since it replaces O2 molecules in hemoglobin, causing suffocation after a short while, and the state continues even after a poisoned person is taken into fresh air. Carbon monoxide is flammable and easily forming CO2 in this reaction: 2CO(g) + O2(g) 2CO2(g) The gas is found in all types of burning, for example car exhaust gases or smoke from a fire. It is also found in cigarette smoke. Moreover, it has been used in wartime for killing people.

   

   

IR spectrum of carbon monoxide, CO

The IR spectrum of carbon monoxide has a major absorption band at 2100 cm-1 or 4.8 µm, due to unsymmetrical vibration modes.

   

Theoretical IR spectrum of carbon monoxide, CO. The spectrum was simulated at: http://spectralcalc.com/info/about.php.

   

   

The IR spectrum of CO below shows an exhaust gas mixture from an oldtime car without catalytical cleaning. Nowadays, modern car exhaust contains mostly CO2 and H2O - the CO concentration is very low.

IR spectrum of 1942 Packard exhaust, showing a mixture of hydrocarbons, water, carbon monoxide and carbon dioxide. The spectrum was found on Internet in a paper by Jane A. Ganske: http://chemeducator.org/sbibs/s0008006/spapers/860353jg.htm

   

출처: <http://www.senseair.com/senseair/gases-applications/carbon-monoxide/>

   

Carbon dioxide, CO2

Carbon dioxide has a perfectly linear molecule due to the C=O double bonds.

   

Carbon dioxide molecule and C=O bond distance, from Wikipedia. The unit pm (picometer) is 10-12 m.

CO2 is an uncolored and non-smelling gas at room temperature which, directly freezes into solid state below -78°C ("dry ice") at normal pressure. The concentration of CO2 in atmosphere is around 0.039 %, or 390 ppm (parts per million). The gas is active in the large carbon cycle since plants use CO2 for photosynthesis and living beings evaporate CO2 when they breath.

   

CO2 is regarded as non-toxic at low concentrations. At higher levels it may displace oxygen in a closed space and cause a risk for suffocation. Also it directly influences on human performance and capacity when the levels are >1 % (10 000 ppm). Carbon dioxide is a major greenhouse gas and due to its steadily increase mainly caused by fossile fuel combustion, it is believed to bring on global heating. All greenhouse gases are compared to each other using the unit CO2 equivalents expressed as the Global Warming Potential (GWP), which is the heat rise a certain amount of CO2 will cause during a period of 100 years. CO2 has by convention a GWP of 1. More powerful greenhouse gases are for example methane or dinitrogen oxide, which have a GWP of 25 and 298, respectively. The GWP value for a gas molecule depends on how stable it is, how strongly it absorbs IR radiation and where in the spectrum the absorption peaks are placed.

   

IR spectrum of CO2

The IR spectrum of carbon dioxide has a strong absorption band consisting of many overlapping peaks. This band, caused by unsymmetrical C=O stretching, is placed at 2300 cm-1 corresponding to a wavelength of 4.3 μm.

CO2 molecules in the atmosphere absorb emitted heat radiation from the earth at this wavelength and in that way decrease cooling especially during clear nights. The mechanism of the increasing amount of carbon dioxide and other more powerful greenhouse gases is that they block IR radiation from exiting from earth back to space.

   

Theoretical IR spectrum of CO2 calculated with the program Spectracalc found at: http://spectralcalc.com.

   

출처: <http://www.senseair.com/senseair/gases-applications/carbon-dioxide/>

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Detector

기기류2016.06.27 12:41

INSTRUMENTAL ANALYSIS AND SEPARATIONS

   

Detectors for Gas chromatography - LECTURE 24

   

   

There are a number of detectors used in GC. Generally the detector produces a signal in proportion to the amount of analyte present. Some types are based on concentration and others are mass dependent. The detectors produce an electrical signal that is sent to a recording and integrating device. The signal strength and retention time are recorded for further analysis by the chemist. For analysis, the analyte must be thermally stable and volatile. However, if the analytes meets this criteria, GC is preferable to HPLC because of its much higher resolution. GC detectors also typically have lower limits of detection and higher linear dynamic range.

   

Flame Ionization Detector

   

The most commonly used detector is the flame ionization detector (FID) it is a general carbon detector. It does not detect compounds that do not contain carbon such as nitrogen(N2), oxygen(O2), or water. The presence of N, O, or S in a carbon compound will tend to decrease the response of the FID.

The Carbon atoms (C-C bonds) are burned in a hydrogen flame. The hydrogen can be supplied ether from a cylinder or from an electrolytic hydrogen generator. The hydrogen must be pure to avoid background noise. A charcoal filter is often placed in the hydrogen supply line to remove any organic contaminants.

The response of the detector depends on the flow of the hydrogen, air and the makeup gas (if it is used). A certain amount of inert gas is needed for optimum response of the detector. Generally the flow from a capillary is too low so a makeup gas is used to provided the inert gas flow. The makeup gas has other beneficial effects such as stabilizing the detector, prolonging the lifetime of the jet, and purging any unswept areas of the detector. It is also very important to adjust the air and hydrogen gas flows for optimum response.

The FID must be heated. There are two main reasons for this. First, the burning of hydrogen in air produces water, which can reduce the detector response and even put out the flame. The second reason for heating the detector is to avoid condensation and deposition of compounds in the detector.

The detector response depends on the ionization of carbon atoms. Only a small portion are actually ionized (about 1 in 10,000), but since there is such a low background signal with the FID, this is enough. The ions carry a charge from the flame to the walls of the detector which surrounds the flame. The charge is electronically amplified and sent to a recording device. The FID is very sensitive down to 10-12 g. It has a high linear dynamic range 107 and is very robust and reliable.

   

Nitrogen Phosphorous Detector

   

Another common type of detector is the nitrogen/phosphorous detector (NPD). It is sometimes called the alkali flame detector or flame thermionic detector. It uses a bead of a compound such as rubidium silicate above a jet of H2. The bead is heated by an electric current to 600-800oC forming a plasma in the region of the bead. Nitrogen and phosphorous react in this boundry layer of plasma forming specific ions that carry a small current from the plasma to the charged collector (The mechanism of NPD response is still not well understood). The NPD is electronically similar to the FID, but since a flame is not sustained in the detector, hydrocarbon ionization does not occur resulting in a high selectivity. The response to N over C is about 103 – 105 greater and response to phosphorous 104 – 10 5.5 higher than response to C. The linear dynamic range is 104. However, the detector is only fairly reliable since the bead burns up over time causing drift in the signal.

   

   

   

Flame Photometric Detector

   

The flame photometric detector (FPD) is an element specific detector which can be used for analyzing many specific compounds However, commercially available instruments are generally limited to the detection of P and S containing molecules. The analytes are burned in a H2 flame causing electrons to move to an excited unstable state. When the electrons return to the ground state, they emit a specific wavelength of light (526 nm for P and 394 nm for S). These wavelengths are monitored by a photomultiplier, amplified, and turned into an electrical signal. This detector is sensitive to 10-9g and has a linear dynamic range for P of only 103 – 104. For S, the response is non-linear. This is because the response is due to the formation of the S2 radical and therefore the response is proportional to the square of the sulfur content. Commercial instruments often have a square root adjustment function built in, but since the response can deviate from the theoretical square root relationship, the output can still be non-linear.

   

Electron Capture Detector

   

Another useful detector is the electron capture detector (ECD) It is an excellent detector for molecules containing an electronegative group such as Cl or F etc. (or derivitized molecules) It is probably the second most common detector after the FID. It is most often used for the trace measurement of halogen compounds in environmental applications for detecting insecticide and herbicide residues.

The ECD uses a radioactive source such as Ni63 which produces Beta particles which react with the carrier gas producing free electrons. These electrons flow to the anode producing an electrical signal . When electrophillic molecules are present, they capture the free electrons, lowering the signal. The amount of lowering is proportional to the amount of analyte present. It is sensitive down to 10-15 but the dynamic range is only about 104.

   

   

Atomic Emission Detector

   

One of the newest gas chromatography detectors is the atomic emission detector (AED). The AED is quite expensive compared to other commercially available GC detectors, but can be a powerful alternative. The strength of the AED lies in the detector's ability to simultaneously determine many of the elements in analytes that elute from the column. It uses microwave energy to excite helium molecules (carrier gas) which emit radiation which breaks down molecules to atoms such as S, N, P, Hg, As, etc. These excited molecules emit distinctive wavelengths which can be separated by a grating and sent to the detector (typically a photodiode array) which produces the electrical signal. The atomic emission detector is very sensitive (10-15) and has a dynamic range of 104.

   

Photoionization Detector

   

The photoionization detector (PID) uses a UV lamp (xenon, krypton or argon lamp, depending on the ionization potential of the analytes) to ionize compounds. The ionization produces a current between the two electrodes in the detector. The detector is non-destructive and can be more sensitive than an FID for certain compounds (substituted aromatics and cyclic compounds for example).

   

Thermal Conductivity Detector

   

The thermal conductivity detector (TCD) consists of an electrically-heated wire or thermistor. The temperature of the sensing element depends on the thermal conductivity of the gas flowing around it. Changes in thermal conductivity, such as when organic molecules displace some of the carrier gas, cause a temperature rise in the element which is sensed as a change in resistance. Low molecular weight gases have high conductivities so hydrogen and helium are often used as carrier gases. Nitrogen and argon have similar conductivities to many organic volatiles and are often not used. However if nitrogen is used as a carrier gas, the detector can be used to measure hydrogen or helium. TCD's are often used to measure lightweight gases or water (compounds for which the FID does not respond). The TCD is not as sensitive as other detectors but it is a universal detector and is non-destructive. However, modern detectors called micro-TCD's have very small cell volumes, and new electronics that produce much higher sensitivities and wider linear ranges. Due to its increased sensitivity, and the fact that it is a universal non-destructive detector, it is again becoming more popular for certain applications.

   

   

출처: <http://fshn.ifas.ufl.edu/faculty/MRMarshall/fos6355/handout/lec24.gc_detector.doc>

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