Dynamics of asymmetric fixed-bed reactors: Coupling of exothermic and endothermic reactions.

摘要

Performance of a bi-directional fixed-bed reactor subject to both flow reversal and switching between exothermic and endothermic reactions is studied. During odd semi-cycles (blows) an exothermic reaction heats the bed, during even semi-cycles an endothermic reaction cools the bed and produces the desired product in the hot zone. It is shown that such 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 semi-cycle. A fixed-bed operated periodically in the above described mode is asymmetric as the dynamic nature of the bed changes during each cycle. The front velocity during an exothermic semi-cycle is different from the front velocity during an endothermic semi-cycle and an asymptotic expression is developed for the differential creep (front) velocity that quantifies this difference. The asymptotic expression for the creep velocity works very well except in the inlet and outlet region of the fixed-bed reactor. Due to a non zero differential creep velocity the front exhibits an effective displacement after each cycle. A developed expression for energy efficiency indicates that 100% efficiency can be reached only if the differential creep velocity is zero. A relation for the balanced operation of a reactor-regenerator is developed. Differences in reactor performance caused by reactions occurring in the gas or solid phase are also discussed.; It is shown that to operate such an asymmetric fixed-bed reactor in a periodic steady state, two conditions need to be satisfied--the energy liberated during the exothermic semi-cycle should be equal to or greater than the energy consumed during the endothermic semi-cycle, and the product of the front velocity and semi-cycle period for the endothermic semi-cycle must be equal to or greater than that for the exothermic semi-cycle. The energy efficiency and conversions for both reactions increase with increasing bed length and decreasing semi-cycle periods for a fixed ratio of the exothermic and endothermic semi-cycle periods. 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 periods is discussed.; An approach based on the asymmetric bi-directional fixed-bed reactor operated in a cyclic mode to produce synthesis gas by coupling endothermic steam reforming reaction with exothermic methane combustion is simulated and discussed. The processes simulated above exhibit very steep moving fronts.; A robust numerical algorithm is required to simulate the physics of the problem. A spatially and temporally adaptive completely implicit finite difference algorithm, which can accurately capture the moving temperature and reaction fronts in fixed-bed reactors with highly exothermic reactions, is developed for that purpose and presented. The spatial grid adaptation based on the magnitude of the second derivative and manipulation of the time step size based on the characteristic time of the process are the two key features of the developed algorithm. Robustness of the algorithm is demonstrated by simulating stiff problems.