PERIODIC OPERATION OF ASYMMETRIC BIDIRECTIONAL FIXED-BED REACTORS - ENERGY EFFICIENCY

Evaluation of the energy efficiency of a fixed-bed reactor in cyclic o peration, subject to alternate swings of an exothermic and an endother mic reaction, is the basis of this study. During an exothermic semi-cy cle an exothermic reaction takes place to heat the bed while during an endothermic semi-cycle that follows an endothermic reaction takes pla ce in the bed, with reactants fed into the bed from the opposite direc tion. A fixed-bed operated periodically in the above-described mode is asymmetric as the dynamic nature of the bed changes during each cycle . It is shown that to operate such an asymmetric fixed-bed reactor in a periodic steady state, two conditions need to be satisfied - the ene rgy liberated during the exothermic semi-cycle should be equal to or g reater than the energy consumed during the endothermic semi-cycle, and the product of the front velocity and semi-cycle period for the endot hermic semi-cycle must be equal to or greater than that for the exothe rmic semi-cycle. The energy efficiency and conversions for both reacti ons increase with increasing bed length and decreasing semi-cycle peri ods for a fixed ratio of the exothermic and endothermic semi-cycle per iods. The effect on operability and energy efficiency of the heats of reactions, inlet mass fluxes, front velocities during both semi-cycles , and of the ratio as well as the magnitude of individual semi-cycle p eriods is discussed. (C) 1997 Elsevier Science Ltd.