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 art endothermic reaction cools the be d 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 d ie temperature front moves with a finite velocity (creep velocity) fro m the feed end to the exit during a semicycle. Since the dynamic natur e of the fixed-bed reactor changes with each semicycle, the front velo city during an exothermic semicycle differs front the front velocity d uring an endothermic semicycle and an asymptotic expression is develop ed for the differential creep (front) velocity that quantifies this di fference. Due to a nonzero differential creep velocity, the front exhi bits an effective displacement after each cycle. This asymptotic expre ssion for the creep velocity works very well except in the inlet and o utlet region of the fixed-bed reactor. An expression developed for 100 % energy efficiency shows that it can be reached only if the different ial creep velocity is zero. A relation for the balanced operation of a reactor-regenerator is discussed as well as differences in reactor pe rformance caused by reactions occurring in the gas or solid phase.