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Thermal oxidizers handling VOC materials that contain chlorine, fluorine, or bromine atoms generate HCl, Cl2, HF, and HBr as additional reaction products during oxidation. A gaseous absorber (scrubber) is used as part of the air pollution control system to collect these contaminants prior to gas stream release to the atmosphere.
Catalytic oxidizers operate at substantially lower temperatures than thermal oxidizers. Due to the presence of the catalyst, oxidation reactions can be performed at temperatures in the range of 500°F to 1000°F. Common types of catalysts include noble metals (i.e. platinum and palladium) and ceramic materials. VOC destruction by catalytic oxidizers usually exceeds 95% and often exceeds 99%.
Catalytic oxidizers are also applicable to a wide range of VOC-laden streams; however, they cannot be used on sources that also generate small quantities of catalyst poisons. As with thermal oxidizers, catalytic oxidizers should not exceed 25 percent of the LEL, a value that is often equivalent to a VOC concentration of 10,000 to 20,000 ppm.
Adsorption systems beds are generally used in the following two quite different situations:
- When the VOC-laden gas stream only contains one to three organic solvent compounds, and it is economical to recover and reuse these compounds, or
- When the VOC-laden gas stream contains a large number of organic compounds at low concentration, and it is necessary to pre-concentrate these organics prior to thermal or catalytic oxidation.
Adsorption systems can be used for a wide range of VOC concentrations from less than 10 ppm to approximately 10,000 ppm.
The adsorption removal efficiency usually exceeds 95% and is often in the 98% to 99% range for both solvent recovery and pre-concentrator type systems. In both types of units, the removal efficiency increases with reduced gas temperatures.
Condensation, refrigeration, and cryogenics
Condensation, refrigeration, and cryogenic systems remove organic vapor by making them condense on cold surfaces. These cold conditions can be created by passing cold water through an indirect heat exchanger, by spraying cold liquid into an open chamber with the gas stream, by using a freon-based refrigerant to create very cold coils, or by injecting cryogenic gases such as liquid nitrogen into the gas stream.
The concentration of VOCs is reduced to the level equivalent to the vapor pressures of the compounds at the operating temperature. Condensation and refrigeration systems are usually used on high concentration, low gas flow rate sources. Typical applications include gasoline loading terminals and chemical reaction vessels.
Biological systems are a relatively new control device in the air pollution control field. VOCs can be removed by forcing them to absorb into an aqueous liquid or moist media inoculated with microorganisms that consume the dissolved and/or adsorbed organic compounds. The control systems usually consist of an irrigated packed bed that hosts the microorganisms (biofilters). A presaturator is often placed ahead of the biological system to increase the gas stream relative humidity to more than 95%.
Biological oxidation systems are used primarily for very low concentration VOC-laden streams. The VOC inlet concentrations are often less than 500 ppm and sometimes less than 100 ppm. The overall VOC destruction efficiencies are often above 95%.
Biological oxidation systems are used for a wide variety of organic compounds; however, there are certain materials that are toxic to the organisms. In these cases, an alternative type of VOC control system is needed.
Continuous emissions monitoring techniques are constantly being developed. Among them hydrochloric acid (HCl) continuous monitors and Ammonia (NH3) monitoring systems, as well as systems capable of measuring multiple compounds such as Fourrier transform infrared (FTIR) spectroscopy and ultraviolet differential optical absorption spectroscopy (DOA).
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