Direct modeling of voidage at layer inversion in binary liquid-fluidized bed

摘要

In liquid-fluidized bed chemical and biochemical reactors composed of binary mixtures, peculiar segregation effects can determine layers rich in one or the other component to revert their position in the bed (layer inversion phenomenon), with significant influence on the process performance. In contrast to previous modeling formulation that focused on the simultaneous prediction of the layer inversion velocity and voidage, in the present work two approaches for the direct prediction of the critical voidage are presented, validated and discussed. One is a straightforward extension of a similar model developed for binary gas-fluidized beds (named PSM) and the other one includes more appropriate treatment of the expanded bed conditions typical of liquid-fluidized beds (named ePSM). Both are tested in their prediction capability in comparison with three previously available models, all against experimental values of the inversion voidage, and for float/sink experimental observations available in the literature. Using the simplified PSM formulation the average discrepancy from inversion experiments results considerable (0.097 in voidage units), while with the ePSM it is much smaller (0.052), good in comparison with the others (0.051, 0.086 and 0.033). With dense float/sink experiments the agreement is very good for both PSM and ePSM. Analysis of the effects of the relevant variables on the inversion voidage is carried out, showing how it is possible to derive an inversion map, a universal one for the PSM and one at each composition with ePSM. Finally, the addition to ePSM of the separate calculation of the inversion velocity is shown by comparing the predicted and experimentally observed expansion characteristics of the two solids with velocity. Qualitative matching of trends is found with reasonable quantitative agreement. (C) 2015 Elsevier B.V. All rights reserved.