Metal Finishing Guide Book

2012-2013

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ADSORPTION TECHNOLOGY Concentrators Rotary concentrators are a continuous adsorption technology commonly applied to very dilute airstreams with relatively low hydrocarbon concentrations. Classified as a capture device, Rotary adsorbers can be used to concentrate the emissions into smaller airstreams with much higher concentrations (typically by a factor of 10 or higher) that can be handled by a smaller oxidation or destruction device much more economically. Continuous adsorption is achieved through the use of rotating media, a section of which is simultaneously desorbed. This design eliminates the need for dual running and stand-by fixed adsorption beds. The hydrocarbon-laden air passes through the rotary adsorption unit where the hydrocarbons are adsorbed onto an adsorbent media such as activated carbon or hydrophobic zeolite. The large volume of incoming air, now purified by the adsorption process, is exhausted to atmosphere. The hydrocarbons which were adsorbed are then continuously removed from the media by desorption with a higher-temperature, low-volume airstream. This high concentration desorption air is delivered to an oxidation device for destruction. Concentration of hydrocarbons into a smaller airstream is a significant benefit to operating costs to a destruction device. By decreasing the airflow, the device is inherently smaller and less costly to purchase. By increasing the concentration, the auxiliary fuel benefit of the hydrocarbons is increased, in many cases, almost to the level of self-sustaining operation, where the customer���s natural gas requirements are virtually eliminated. Traditionally, concentrators were applied and justified on very large airstream volumes, but recent commercial applications have been on airstreams of 30,000 scfm and smaller. Media Choices The key to effective adsorption is the medium that is used. The most widely used medium is activated carbon because it is very effective, readily available and long lasting. Zeolite has also found a niche due to higher removal efficiencies for low molecular weight, polar, solvents. Activated Carbon Being relatively inexpensive and lightweight, with pores ranging from 1 to 50 ��ngstroms (��), carbon can adsorb most paint solvents and even semiVOCs (SVOCs) such as plasticizers. Though widely used and preferred, activated carbon is not without disadvantages. The three primary drawbacks are: 1. Its combustibility, with the potential to promote a fire when heated above 600��F. 2. Its hydrophobic structure, which requires relative humidity control. Carbon���s adsorption capacity drops significantly at 50 to 60% relative humidity. Reheat coils are often required, especially when controlling a wet venture paint spray booth. 3. Impurities that naturally occur in carbon. These impurities can act as catalysts and promote polymerization or oxidation of solvents such as methyl ethyl ketone (MEK) and cyclohexanone, resulting in byproducts that cannot be desorbed or that might be hazardous. 698

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