Comparison of a general model with a simplified approach for the transformation of solid-gas media used in chemical heat transformers

Abstract

The influence of mass transfer has drawn great attention in the study of gas-solid reversible reactions. Experiments have been conducted for the IMPEX blocks (Mauran et al., 1991, patent no. 91 0303) in reaction with ammonia at low pressure when the influence of mass transfer is significant (Prades, 1992, Thèse de doctorat, Université de Perpignan; Lu et al., 1996b, A.I.Ch.E. J., to be submitted). A general gas-solid reaction model has been developed (Lu et al., 1996a, Chemical Engineering Science 51, 3829–3845) to quantify the non-negligible influence of mass transfer on the global transformation of the reactive media (especially for the cases of less-permeable material). Two levels of coupling have been considered. Firstly, at the grain level, mass transfer and chemical kinetics are coupled, and secondly, at the pellet level, heat transfer and mass transfer are coupled. The simulation results of the general model have revealed that in the process of reaction, there exist two progressive reactive fronts in the pellet, one is the ‘heat front’ moving from the heat exchanger to the gas diffuser and the other is the ‘mass front’, moving from the gas diffuser to the heat exchanger. In this paper, a simplified approach has been made by simplifying the progressive reactive fronts shown in the general model as ‘sharp’ reactive fronts. This simplified approach does not enter into the details at the grain level and does not have the accumulative terms in the model equations. Therefore, it has significantly simplified the numerical calculation. The comparison has shown similarities between the results of these two models, regarding the global advancement and local profiles. If the physical characteristics of the reactive block are constant in the process of the reaction, the simplified model has confirmed to be basically reliable to predict the gas-solid reversible reaction and therefore will serve as an efficient tool for the optimal sizing of a chemical heat transformer. In any other cases, the general model should be used.