A bidirectional fixed-bed reactor for coupling of exothermic and endothermic reactions



Performance of a bidirectional fixed-bed reactor subject to both flow reversal and switching between exothermic and endothermic reactions is simulated. During odd semicycles (blows) an exothermic reaction heats the bed, during even semicycles an endothermic reaction cools the bed and produces the desired product in the hot zone. Such an operation is possible and efficient when the inlet gas temperature is lower than the initial bed temperature, leading to the wrong-way behavior when the temperature front moves with a finite velocity (creep velocity) from the feed end to the exit during a semicycle. Since the dynamic nature of the fixed-bed reactor changes with each semicycle, the front velocity during an exothermic semicycle differs from the front velocity during an endothermic semicycle and an asymptotic expression is developed for the differential creep (front) velocity that quantifies this difference. Due to a nonzero differential creep velocity, the front exhibits an effective displacement after each cycle. This asymptotic expression for the creep velocity works very well except in the inlet and outlet region of the fixed-bed reactor. An expression developed for 100% energy efficiency shows that it can be reached only if the differential creep velocity is zero. A relation for the balanced operation of a reactor-regenerator is discussed as well as differences in reactor performance caused by reactions occurring in the gas or solid phase.