Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 242 of 331

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 efficien- cies for low molecular weight, polar, solvents. Activated Carbon Being relatively inexpensive and lightweight, with pores ranging from 1 to 50 Ångstroms (Å), carbon can adsorb most paint solvents and even semiVOCs (SVOCs) such as plasticizers. Though widely used and preferred, activated car- bon 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. Car- bon'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 ke- tone (MEK) and cyclohexanone, resulting in byproducts that cannot be des- orbed or that might be hazardous. In certain applications, a granular activated carbon (GAC) pre-filter is installed upstream of the carbon adsorption media. A GAC prefilter, often termed a sac- rificial bed, adsorbs high boiling VOCs or SVOCs. GAC protects the activat- ed carbon media from being saturated with compounds that can not be com- pletely desorbed by the limited desorption temperature (250°F) typically used with carbon media. A GAC bed also dampens fluctuations in VOC con- tent, 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 interconnect- ing 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 mole- cules can be achieved. Synthetic zeolite has a much greater adsorp- tion 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 with- standing 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 al- lows 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 241

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