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Air Purification in an Adiabatic Reverse-Flow Reactor by Catalytic Combustion

Volatile organic compounds (VOCs) emitted mostly by the chemical process industries are hydrocarbon compounds that react with nitrogen oxides and other airborne chemicals to form photochemical ozone which is the primary component of smog. Polluted air streams, eg. from tank farms, dye chambers, dry cleanings or soil cleanings, are contaminated with VOCs in a wide range of concentration and composition. In the case of valuable or high concentrated contaminants, separation methods with recovery of the organic compounds might be considered. Another group of cleaning methods is composed of non-selective destructive processes, like thermal or catalytic combustion. Comparing catalytic combustion to thermal combustion, heterogeneous, catalyzed reactions usually proceed at much lower temperatures, consume less additional fuel and form no NO,-emissions. Under commercial conditions a catalytic combustion set-up should run under minimal energy consumption and should be either insensitive to input fluctuations or well controllable. For practical purposes the catalytic oxidation preferably is carried out in an adiabatic packed bed reactor with periodic flow reversal (RFR). The catalyst bed serves as reactor as well as regenerative heat exchanger. After preheating the bed for start-up, cold inlet air is heated by the hot solid phase and a reaction heat front travels through the reactor, after some time the direction of the flow is reversed and the heat front moves in the opposite direction. This process is repeated continuously by periodic reversal of the flow The reaction zone is moving between two boundaries correspondingly to the temperature profile. The so-called pseudo-steady-state is reached when the same shape of the temperature profile is observed periodically At this stabile dynamic performance of the RFR the heat of reaction is removed from the reactor by the slightly heated outlet flow An experimental reactor has been constructed, which almost completely reached the goal of an adiabatically operating system. The influence of several operating parameters like gas throughput, cycle period, chemical character and composition of the contaminants and reactor pressure is discussed. The reactor has been operated autothermally, when the inlet concentrations were sufficiently high. Not only the reactor behavior at fixed operating conditions, but also the response of the reactor towards variations in inlet conditions is reported. To respond rapidly to feed composition fluctuations an independent set-up is implanted to receive the full kinetic information of the feed fast. In conclusion, the catalytic, oxidative combustion of VOCS in a RFR under autothermal operation is a very efficient method in air purification. To ensure the autothermal conditions a minimum adiabatic temperature rise is needed. Only a very small amount of external energy supply for start-up as well as for at very low contaminant concentrations is required. A RFR can handle fluctuations in inlet conditions well; complete conversion is still maintained for a long period of time after blow-out conditions have been established. Of course, the complete installation is automated and operated continuously by computer assistance.