Exhaust streams, cleanup

Fi. ed-bed react.us ar.- . ominonly employed by the peirochemicals industries to -> n-plish a broad spectruni of reactions, including steps to clean up air streams prior to exhausting them into the atmosphere. For example, fixed-bed reactors arc used to pr. icess air. streams containing NOj by reaction with N llj over solid catalysts to give N2 and H2O. An interesting application of fixed-bed reactors for environmental cleanup is from workers... 

Spent acids are used where feasible to neutralize alkaline waste streams, or are neutralized with purchased lime, or caustic, and are then routed to the normal effluent treatment system for further cleanup before discharge [61]. Waste solids from refinery operations include such materials as spent clay from decolorization of lubricating oils or waxes, sand used as a catalyst support, filter aid, or filter base, and exhausted Claus catalysts (primarily ferric oxide on alumina). These are disposed of by landfilling by the majority of Canadian refineries. However, some use landfarming for disposal of these materials. One refinery recovers spent Claus catalysts for regeneration into new catalyst. 

Limitations to active carbon usage include compounds that have low MWs and high polarity (EPA, 1991). In addition, active carbons should not be used in streams with suspended solids >50 mg/dm and oil and grease content >10 mg/dm, which may cause fouling. High levels of organics such as 1,(XX) mg/dm may result in rapid exhaustion of the bed, but this is not determined for chemical spill cleanup applications. Compounds not well adsorbed require substantial amounts of activated carbon, and this increases the treatment costs significantly. Some examples of activated carbon performance data are presented in Table 15.2 (O Brien and Fisher, 1983). 

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