Comparative CFD simulation studies on monolith and packed bed reactors for oxidation of Cu in unmixed combustion (UMC) process

Monolith reactors (MR) have structured packing of catalyst and are commonly used for catalytic converters in automobile applications. Compared to a randomly packed bed reactor (PBR), MR has advantages like lower pressure drop, enhanced mass transfer, etc. Recent investigations have revealed that MR performs better than PBR for certain applications including highly exothermic reactions, mainly because of better conduction in the former. Hence, the use of MR for the process of unmixed combustion (UMC)/chemical looping combustion (CLC) which is typically carried out in PBR and involves oxidation of copper (Cu), an exothermic gas-solid reaction, was investigated using modeling and simulation studies. A Pseudo-homogeneous model was developed and two cases were considered, viz. Case I: insulated wall, and Case II: convective heat transfer from the wall to coolant air. COMSOL Multiphysics (TM) 5.4 was used for this purpose. The heat transfer behavior of MR was compared with PBR based systems having spherical particles. The simulation results showed that for Case I, it is possible to achieve a high temperature outlet stream (>1200 K) for a longer duration of time in MR as compared to that obtained in the existing CLC based PBR. In Case II, the MR system showed a better rate of radial heat transfer, while maintaining the bed temperatures within the desired range, than that in the PBR system for similar operating conditions. Moreover, in the MR system, there is scope for increasing the heat transfer rate, whereas PBR has process limitations to do so. Effects of inlet concentration and inlet velocity on the performance of reactors have also been investigated. MR was found to have a feasible operation at relatively higher concentrations and velocities. Thus MR proves to be a potential alternative to PBR, for the oxidation of Cu in UMC/CLC processes. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.