The effects of exothermic catalytic reactions upon combined transport of heat and mass in porous microreactors

• Published in: International journal of heat and mass transfer Vol. 134; pp. 1227 - 1249
• Main Authors: Hunt, Graeme, Karimi, Nader, Yadollahi, Bijan, Torabi, Mohsen
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.05.2019; Elsevier BV
• Subjects: Catalysis; Catalytic activity; Chemical synthesis; Computational fluid dynamics; Double-diffusive; Entropy generation; Exact solutions; Exothermic reactions; Fluid flow; Heat flux; Heat transfer; Kinetic energy; Local thermal non-equilibrium; Mass transfer; Mass transport; Mathematical models; Microchannels; Microreactors; Organic chemistry; Porosity; Porous materials; Porous media; Soret effect; Surface heat release; Thermal analysis; Two dimensional analysis; Two dimensional models; Wall thickness; Local thermal non-equilibrium; Soret effect; Surface heat release; Double-diffusive; Microreactors; Entropy generation
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2019.02.015

•2D double-diffusive analytical model developed using extension of model A.•Microstructure asymmetry strongly affects temperature and concentration fields.•Thermal conductivity ratio and wall thickness have greatest impact overall.•Microchannel thickness more important than asymmetry in Sherwood number.•Nusselt number yielded a singular point for wall thickness for any heat flux ratio. Microreactors for chemical synthesis and combustion have already attracted significant attention. Exothermic catalytic activity features heavily in these devices and thus advective-diffusive transport is of key importance in their analyses. Yet, thermal modelling of the heat generated by catalytic reactions on the internal surfaces of porous microreactors has remained as an important unresolved issue. To address this, the diffusion of heat of catalytic reactions into three phases including fluid, porous solid and solid walls is investigated by extending an existing interface model of porous media under local thermal non-equilibrium. This is applied to a microchannel fully filled with a porous material, subject to a heat flux generated by a catalytic layer coated on the porous-wall boundary. The finite wall thickness and viscous dissipation of the flow kinetic energy are considered, and a two-dimensional analytical model is developed, examining the combined heat and mass transfer and thermodynamic irreversibilities of the system. The analytical solution is validated against the existing theoretical studies on simpler configurations as well as a computational model of the microreactor in the limit of very large porosity. In keeping with the recent findings, the wall thickness is shown to strongly influence the heat and mass transport within the system. This remains unchanged when the symmetricity of the microchannel is broken through placing walls of unequal thicknesses, while deviation from local thermal equilibrium is significantly intensified in this case. Importantly, the Nusselt number is shown to have a singular point, which remains fixed under various conditions.