Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 93 of 707

results in operational cost savings. A typical recovery rate is 98% or better of the captured solvent-laden air stream in a properly designed and installed carbon adsorption system. This solvent recovery procedure will be described later in this article. The recovered solvent is returned to the degreasing system for reuse. Typically one can reduce gross annual solvent purchases by 50% when properly operating a degreaser with a carbon adsorption system installed. When a carbon adsorption system is properly sized and installed, the ambient air around the degreaser is pulled into the suction duct along with fugitive solvent emissions. Figure 1 shows a typical adsorption schemat- ic. This action continually removes ambient air from the area as long as the carbon unit is operational. Thus, any fugitive emissions will be continually removed from the workplace to lower the ppm levels in the immediate area. Eventually the activated carbon in the vessel will adsorb enough solvent to reach a point of near saturation. At this point in time, the solvent-laden vessel must be desorbed to recover the solvent. Figure 2 shows a typical desorption schematic . CARBON ADSORPTION Carbon adsorption is a time-proven technology that has been success- fully used for years regarding MC, TCE, PCE, Freon™, and other solvents. Carbon adsorption is included in the NESHAP guidelines as a control device. It is primary used with degreaser systems that have large tanks (vapor/air interface) or high throughput and specialized conveyor sys- tems such as monorails/cross-rods/elevator degreasers. Carbon adsorption systems are also used to ensure exposure limits in certain applications regardless of the size or type of process. Carbon is the preferred material used in adsorption systems because of the unique surface tension properties it exhibits. Due to its non-polar surface, activated carbon will preferentially attract other non-polar materials, such as organic solvents, rather than polar materials like water. Carbon's granular multifaceted geometry also possesses tremen- dous surface area (with one lb. having an area greater than 750,000 sq. ft.). This characteristic allows carbon to adsorb up to 30% of its own weight in solvent. In the early stages of alternative solvent recognition by the solvent degreasing equipment manufactures, nPB was acknowledged as a viable replacement for chlorinated solvents in a degreasing cleaning applica- tion. But lab tests revealed that the normal method of solvent recla- mation using a traditional carbon adsorber with this solvent would 92

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