Catalyst deactivation during deep oxidation of chlorohydrocarbons

Abstract

A commercial chromia-alumina catalyst was investigated for an extended period of time for the oxidation of two gas streams containing volatile organic compounds (VOCs) with compositions similar to those expected from groundwater air strippers. The first gas stream ( chlorinated stream ) contained 500 ppm of C 1 to C 2 chlorohydrocarbons, while the second stream (mixed stream) contained 450 ppm of C 5 to C 9 hydrocarbons plus 50 ppm of trichloroethylene. Catalyst deactivation was studied at constant conversion of the main reaction, achieved by increasing the temperature, when needed, to compensate for the loss of intrinsic catalyst activity. The catalytic activity and selectivity were found to be a function of the type of feed stream and the reactor configuration. Although the catalyst used for the oxidation of the chlorinated stream did not require any temperature increase when used fore 153 days-on-stream in a fixed bed reactor, the results suggest progressive loss in catalyst activity down the catalyst bed. A fluid bed reactor was found to be more effective than a fixed bed reactor in maintaining the catalyst activity for the oxidation of the chlorinated stream. The physical attrition of catalyst particles in the fluid bed reactor accompanied with the loss of chromium, via oxychloride formation, appears to be beneficial in maintaining the catalyst activity by constantly exposing the fresh catalyst surface. The catalyst used for mixed stream oxidation required a temperature increase of 33°C over 210 days-on-stream to maintain constant conversion. This decrease in catalytic activity was related to a decrease in BET surface area.