Metal Finishing Guide Book


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environmental controls AIR POLLUTION CONTROL IN THE FINISHING INDUSTRY BY GORDON HARBISON D���RR ENVIRONMENTAL INC., PLYMOUTH, MICH. Being responsible for reducing volatile organic compound (VOC) emissions in paint and coating operations seems to be akin to a quest to circumnavigate the globe. At the end of your quest, you are right back where you started. If custom coaters are not able to convert to ���environmentally friendly��� coating alternatives, such as waterbornes, UV-cure or powder coatings, they must deal with ever-increasing emission regulations through some kind of VOC control technology. Choosing the right equipment for VOC control applications depends primarily on the exhaust air volume and the average concentration of VOCs. VOC CONTROL PRIMER VOC Destruction Thermal oxidation is a process whereby most of the VOCs are broken down and recombined with oxygen to produce water vapor and carbon dioxide. The water vapor and carbon dioxide are naturally occurring and environmentally friendly, therefore safe for venting into the atmosphere. Thermal oxidation occurs by heating the polluted air to an elevated temperature (typically 1,300��F to 1,800��F). At such temperatures, the pollutant molecules spontaneously disassociate and recombine with available oxygen to create the carbon dioxide and water vapor. The efficiency of oxidation and the design of most oxidizers is governed by the residence time, the combustion chamber temperature and the amount of turbulence the air stream sees. Catalyst Improves Fuel Efficiency A Catalyst is a substance that promotes oxidation without being consumed by the process. VOC catalyst can be added to the combustion chamber of almost any oxidizer to promote VOC destruction at lower operating temperatures (typically 600��F to 900��F), lowering fuel usage. Note: Catalytic oxidizers are only suitable for processes whose constituents will not adversely affect the life of the catalyst. VOC Capture Concentrators take advantage of a chemical surface phenomenon and the tendency of VOCs and other pollutants to adhere to certain types of materials such as activated carbon and zeolites. Adsorbent media are selected for their tendency to attract pollutants as well as their high surface area ��� qualities that allow them to trap and hold more pollutants. When emission gases pass through the adsorbent media in a concentrator the pollutants stay behind, trapped in the media. The pollutants can then be removed from the media by desorption ��� passing a much smaller quantity of very hot air through the media. The smaller volume of desorption air contains a very high concentration of pollutants that can be destroyed efficiently by oxidation. 690

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