NUMERICAL-SIMULATION OF A PERIODIC-FLOW REVERSAL REACTOR FOR SULFUR-DIOXIDE OXIDATION

A one-dimensional, heterogeneous model was employed to simulate the performance of an adiabatic, periodic flow reversal reactor for the reversible catalytic oxidation of sulfur dioxide to sulfur trioxide. For a given set of operating conditions, variation of the feed gas temperature over a sufficiently wide range allows one to understand the qualitative effects of changes in the reactor operating variables (inlet gas velocity, flow reversal frequency and catalyst pellet size) on reactor performance. In general, operating conditions that cause reaction rate limitations give rise to bell-shaped temperature profiles, which facilitate better enthalpy trapping in the reactor. The temperature profiles are plateau-shaped when operating conditions are such that thermodynamic limitations to sulfur dioxide conversion prevail in the central portion of the reactor. In such a case, sulfur dioxide oxidation is primarily confined to the cooler entrance and exit regions of the reactor. For typical operating conditions, overall sulfur dioxide conversion remains high (> 80%) even when feed temperatures are within 10 K of reaction extinction. While the maximum catalyst temperature, the average catalyst temperature and the sulfur dioxide conversion decrease consistently as an extinction point is approached, these reactor performance measures vary non-monotonically away from an extinction point. Because the catalyst bed acts as a heat reservoir, the reaction extinction process is relatively slow. Hence, if detected sufficiently early, reaction extinction can be thwarted by suitable changes in operating conditions. These qualitative results are applicable in general to reversible exothermic reactions occurring in adiabatic flow reversal reactors.