RTO 비교

Comparison of a Regenerative Thermal Oxidizer to a Rotary Concentrator

By Stephen W. Blocki, P.E

Courtesy of Dürr Systems, Inc. - Environmental and Energy Systems

Originally published 2003

INTRODUCTION

The regenerative thermal oxidizer (RTO) is a very durable, well-proven technology suitable for treating a wide variety of emission streams. However, The RTO may not be the most economical technology available. Two very common systems for treating emissions from coating and finishing processes are the RTO and the concentrator/ oxidizer system. To determine which system is best suited for a given application, the capabilities and life-cycle costs of each must be evaluated and compared.

Below is a summary of the RTO and the concentrator/ oxidizer system, how each operates, how their characteristics affect both plant operation and life-cycle costs, and details on actual life cycle costs and performance data.

ADD-ON CONTROL OPTIONS- CATALYTIC, RECUPERATIVE, AND REGENERATIVE OXIDIZERS, ROTARY CONCENTRATOR

Emissions limits established by the Clean Air Act forced the finishing industry to greatly reduce the solvents exhausted from coating and finishing facilities. Production and quality demands eliminate any chance of reducing emissions through reduced production or alternate coatings. Thus finishers are forced to install an add-on solvent emission control system. MACT standards require that any add-on solvent control system exceed 95% DRE; future requirement of 99% DRE is also possible.

The evaluation by most industry plants to determine the most economical control system takes into account the following factors.

• Capital and operating costs
• Expandability- Can the system be expanded when production increases?
• System down time- Will the entire system shut down if one component requires maintenance?
• Ability to increase system efficiency- Can the system meet possible future requirements of 99% DRE
• Ability to handle widely varying solvent loads- The efficiency must remain essentially constant over the wide range of inlet solvent loadings
• Ability to handle a solvent stream containing 100% ketones- The hazards of readily-oxidizing ketones must be adequately addressed
• Footprint- Site constraints
• Noise Levels- Surrounding offices and residential areas may mean noise is a concern

Several solvent control technologies can be considered, including solvent recovery, recuperative, catalytic, and regenerative oxidation, and rotary solvent concentration. The recuperative and catalytic oxidizers are quickly eliminated because at large air volumes and relatively low solvent levels the operating cost were prohibitive.

The evaluation for the most economical and efficient solvent emission control system therefore focuses on: a) a large stand-alone regenerative thermal oxidizer (RTO); and, b) a rotary solvent concentrator followed by a small RTO.

SUMMARY OF THE RTO

An RTO consists of a combustion chamber located above several energy recovery chambers (see Figure I). The energy recovery chambers are filled with ceramic heat exchange media. The solvent-laden air enters the inlet header and is directed to one of the energy recovery chambers through an inlet control valve. The air passes through the heat exchange media, adsorbing heat from the media. It then enters the combustion chamber at a temperature very close to the oxidation temperature. The oxidation process is completed in the combustion chamber. A gas burner maintains a preset oxidation temperature. If the incoming air contains enough solvents, the solvent combustion energy provides the necessary heat to raise the temperature to the combustion set-point.

The clean air leaves the RTO through the heat exchange media of an adjacent chamber. The energy in the clean, hot exhaust air is transferred for storage to the heat exchange media. The clean air then passes through the exhaust manifold and is discharged through a stack to atmosphere. The temperature of the air as it leaves the RTO is close to the temperature of the incoming air. At least one chamber is always on inlet mode and one chamber on outlet mode to allow the RTO to continuously process a solvent-laden air stream.

The RTO is often equipped with a purge system, which clears out solvent-laden air trapped below the heat exchange media. The automatic purge cycle forces the trapped solvent-laden air into the combustion chamber where the solvents are destroyed. The purge system ensures a continuous high destruction efficiency. RTOs are typically designed for heat recovery of up to 95% and up to 99% DRE.

SUMMARY OF THE ROTARY CONCENTRATOR

For air streams with relatively low solvent concentration, rotary concentrators can be used to concentrate the emissions into smaller air streams that can be handled much more economically.

The rotary concentration system is designed to continuously adsorb solvents from an air stream onto an adsorbent media and discharge clean air. This is achieved through the use of a moving adsorbing wheel, a section of which is simultaneously desorbed. This design eliminates the need for dual running and stand-by adsorption beds. Solvents are adsorbed onto the adsorbent material and the clean air exits through the center of the cylinder. A portion of the rotating cylinder is simultaneously desorbed by passing hot air through a section of the cylinder. This desorption section is sealed off from the remaining adsorption section of the rotor by seals, so that very high efficiencies can be obtained in the system. Capture efficiencies for this type of system may be as high as 99%. Rotary concentration systems can be designed using zeolite or activated carbon as adsorption media.

COMPARISON OF FEATURES

RTO	Rotary Concentrator
Expandability
Additional chambers can be added to the RTO for increased capacity though this is very difficult in practice.	Additional concentrator units in parallel can be easily added for increased capacity; however the desorption equipment and oxidizer must be initially sized for the future capacity.
On-line Efficiency
Very high, but if any one component requires the RTO to shut down, the entire abatement system must shut down.	The only moving part of the concentrator is a 0.5 hp motor, which means concentrator down time is very rare; when it does occur, the motor can be change very quickly. If the desorption section or oxidizer shuts down for any reason, the concentrator can remain on line for an extended period of time, controlling emissions. Overall system on-line time is therefore extremely high.
Ability to Increase the Destruction and Removal Efficiency
The RTO must be designed with a purge system to consistently achieve very high DRE. The DRE can be increased beyond 98% by increasing the combustion temperature but is still limited by the cycling of the valves.	The concentrator can achieve very high efficiencies but the solvent blend must be identified. The oxidizer has a very high DRE because the inlet solvent concentration has been increased 10 times. The overall system can achieve 99.0-99.9% DRE though an exhaust polishing bed may be needed.
Ability to Handle Widely Varying Solvent Loads
The RTO can readily handle varying solvent loads, though the DRE will decrease at lower concentrations.	The concentrator can handle varying solvent levels to a point; beyond approximately 1000 ppmC3, design changes may be required (the actual limit is case-specific); The DRE of the concentrator increases at lower concentrations.
Ability to Handle a Solvent Stream Containing 100% Ketones
Ketone emissions are no problem for the RTO	Readily oxidizing solvents such as ketones can present flammability problems with adsorption systems. These problems occur most often with fixed bed, batch systems. With continuous systems such as rotary concentrators, the problem is eliminated because the continuous air flow guarantees continuous media cooling.
Footprint
A standard layout for an RTO sized for example for 143,500 scfm will not fit in the available area of 65' x 100'.	The concentrator system layout is very flexible and can be located in the 65' x 100' area with room remaining for an additional concentrator to be added in the future.
Noise Levels
Significant noise is expected with the large fan capacity required by this size of an RTO; a sound deadener will probably be required.	The noise from a smaller capacity fan will be quite a bit lower.

COMPARISON OF CAPITAL AND OPERATING COSTS

Table I lists order-of-magnitude costs for each system. It is important to note that many site-specific factors will affect the capital cost of this or any abatement system. Though these costs indicate approximate costs for similarly sized systems, case specifics will change the final cost.

Table I includes fuel and electricity consumption for each option. Key variables such as pressure drop and temperature rise are shown to demonstrate how the technology performance affects operating cost. For example, if the temperature rise across an oxidizer were increased, the impact on fuel consumption, and the impact on operating cost, can be determined.

Table I also includes the amortized costs for components requiring future replacement, such as adsorbent media for the concentrator.

Finally, the local utility rates have been included to show the bottom line operating cost for each technology. These can be readily changed to determine how different utility rates will affect final system economics.

Table I shows that the annual operating cost of the concentrator system, including the amortized cost of periodic replacements, is >80% less than the RTO operating cost.

SYSTEM SELECTION

Both the RTO and the rotary concentrator/RTO are viable control systems. Both systems have limitations that can be overcome with proper design. The RTO is more difficult to fit into a small area, will generate much more noise, and is more likely to shut an entire facility down; the concentrator system involves more components, and will require more routine maintenance.

Factors that favor the concentrator system over the RTO include: the annual operating cost savings per year, and the fact that if part of the concentrator system requires maintenance, most or all of the exhaust air can still be treated- with the RTO, if one component in the system requires maintenance the entire abatement system, and therefore the facility, is down.

CONCLUSION

Many different technologies can efficiently control solvent emissions. Many finishers have found that the RTO and the rotary concentrator followed by a small RTO were both viable options to control their widely-fluctuating ketone emissions. Due to the ability of the concentrator/ oxidizer to control solvent emissions even while portions of the system were undergoing maintenance and because the concentrator system costs much less to install, operate and maintain, the rotary concentrator/ small RTO system is chosen as the preferred system.

The system can also be designed with built-in expansion capabilities. Performance testing on various installations has verified that the concentrator system can achieve the 95% DRE required today and as well as much higher efficiencies possibly mandated in the future.

TABLE I - EXAMPLE OF CAPITAL AND OPERATING COST SAVINGS

	Direct Regenerative Thermal Oxidizer	Rotary Concentrator/ Regenerative Thermal Oxidizer
BASIS
Process Exhaust (SCFM)	143,500 240 13,500 $3.30 $0.055 6,000
Total VOCs (#/Hr)
Estimated Heat of Combust (BTU/#)
Fuel Cost ($/106 BTU)
Electrical Cost ($/KWH)
Operation (Hr/Yr)
FUEL CONSUMPTION
Oxidizer Capacity (SCFM)	143,500 92 - 93 100 1500 200 100 15,498,000 (3,240,000) 12,258,000	16,500 84 250 1500 450 200 3,564,000 (3,240,000) 324,000
Thermal Efficiency (%)
Oxidizer Temperatures (°F)
Inlet
Combustion
Outlet
Temperature Difference
Fuel Requirements (BTU/Hr.)
Gross Energy Required
Energy From Solvents
Net Purchased Fuel
ELECTRICAL CONSUMPTION
Main Fan (Kw)	649 ---- 6 10 3 668	137 72 6 ---- ---- 215
Desorption Fan (Kw)
Control Panel (Kw)
Hydraulic Power (Kw)
Combustion Blower (Kw)
Total Kw
ADSORBER BLOCKS
Life (Years)	----	8
Replacement Cost ($/Yr)	----	$10,041
TOTAL UTILITY COST	$463,100/yr	$ 87,400/yr
SYSTEM CAPITAL COST	$2,500,000	$2,700,000

The most valuable RTO vendor to the plant will be the one that engages in the knowledge of the operating characteristics of the RTO from a wide variety of vendors. Additionally, They will understand the exhaust characteristics of different wood species within each of the wood products processes so that they may optimize any RTO design based upon plant specific, real time criteria.

   

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