Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 243 of 331

aromatic naphtha 100). Because activated carbon has a wide range of pore sizes it does not exhibit this type of selectivity. The two absorbents can be viewed as complimentary rather than competing technologies. One can take advantage of their different adsorption characteris- tics and use carbon and zeolite together, both as separate phase and mix media, to control complex VOC streams at coating and other manufacturing facilities. In many cases the most advantageous type of media can be selected based on general guide- lines; however specific performance guarantees must be developed from laboratory analysis of individual process conditions. In many cases, one or more concentra- tor types may be practical and a detailed economic analysis based upon your spe- cific costs of fuel and electricity will be required to determine the best selection. ALTERNATIVE STRATEGIES Alternative technologies have been developed to oxidize solvents without the use of high temperatures. Ultraviolet Light, Ozone Oxidation (UV/OX) Systems This technology has been used in a limited number of paint finishing appli- cations. Solvent-laden air is fed into a chamber and exposed to high-intensity ultraviolet (UV) light. High-intensity UV light prepares the solvent molecule for oxidation. The air is then scrubbed with a high-intensity water-wash scrub- ber. Much of the solvent is transferred to the scrubber water. The water contains a strong oxidant (ozone), which converts the solvent to carbon dioxide and water. Solvent that is not removed in the scrubber passes through a two-bed car- bon system. One bed adsorbs solvent while the other bed is in a solvent de- struction mode. Ozone is injected into this bed and the solvent is oxidized right on the carbon. No nitrogen oxides or carbon monoxide are formed in this process, and high destruction efficiencies are possible. Wet scrubbing can remove particulate as well as VOC. UV/OX systems are complicated, with many dampers, valves, and motors. Systems are large, and operating costs to pro- duce ozone can be high. These systems have not been proven on very large airstreams. Another disadvantage is that a wastewater stream is produced. Biofiltration Biofiltration units have been successful in abating odors and some VOC streams. Large chambers charged with bacteria are used to convert VOC to carbon diox- ide and other compounds. No nitrogen oxides are created with biofiltration and energy consumption is very low. However, bacteria need a relatively constant supply of solvents to remain active. Very large amounts of space are required and very little past experience in paint applications is available. CONCLUSION Applying the Right Solution It is quite clear that no one solution can be applied universally to all VOC abate- ment scenarios. The ideology of "One Size Fits All" is false and potentially costly. In choosing the right technology, it is important to examine both the process and the airstream constituents to be abated. A careful review of current and future regulations, along with local site considerations, i.e. utility costs, space constraints 242

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