Metal Finishing Guide Book


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installed upstream of the carbon adsorption media. A GAC prefilter, often termed a sacrificial bed, adsorbs high boiling VOCs or SVOCs. GAC protects the activated carbon media from being saturated with compounds that can not be completely desorbed by the limited desorption temperature (250°F) typically used with carbon media. A GAC bed also dampens fluctuations in VOC content, typical of paint spray booth applications, providing a relatively steady VOC concentration to the downstream media. • Hydrophobic Zeolite: Zeolites are sometimes called molecular sieves because of their crystalline framework with pores and interconnecting voids. The resulting homogeneous pore size prevents molecules larger than a certain size from entering the lattice. By varying the structure and pore size, the selectivity for various size solvent molecules can be achieved. Synthetic zeolite has a much greater adsorption capacity than carbon at low solvent concentrations, but carbon has a higher capacity at high concentrations. Hydrophobic zeolite, a synthetic porous silicate, is non combustible and capable of withstanding temperatures as high as 1,100°F when coated on a ceramic, honeycomb structure. It can be desorbed at 400° F, the working limit of the desorption section seals. A higher operating temperature allows the removal of solvents with boiling points above 175°C (350° F). Often, versatility is sacrificed for selectivity. Synthetic zeolite has a lower capacity for some common solvents (e.g., xylene and high flash 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 characteristics 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 guidelines; however specific performance guarantees must be developed from laboratory analysis of individual process conditions. In many cases, one or more concentrator types may be practical and a detailed economic analysis based upon your specific 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 applications. 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 scrubber. 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 twobed carbon system. One bed adsorbs solvent while the other bed is in a solvent destruction mode. Ozone is injected into this bed and the solvent is oxidized 656

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