EFFECTS OF POISONING ON THE DYNAMICS OF FIXED-BED REACTORS .1. ISOTHERMAL IN A CYCLIC POLICY OF OPERATION

The dynamic behavior of a fixed bed reactor has been examined with benzene hydrogenation on Ni/kieselguhr, poisoned by thiophene, as a model exothermic reaction. The reactor was operated isothermally, with exit conversion declining with time to a preset lower limit, and then temperature increased to restore the initial conversion level. This procedure was repeated over several cycles. The overall results are interpreted in terms of wave propagation in the fixed bed, combined with the complex nature of thiophene chemisorption on Ni. It is found that successive temperature cycling increases the effective thiophene capacity of the catalyst (increasing temperature). A geometric blockage variation, suggested before, seems to be a reasonable interpretation. A simulation model based on this variable poison adsorption capacity is in excellent accord with experimental results. It is found that an integral guardbed, growing with time, is formed at the entrance to the reactor entrance, and this to a large extent governs the cyclic behavior. © 1990, American Chemical Society. All rights reserved.