On the Consistent Modeling of Porous Catalyst Pellets: Mass and Molar Formulations

Accurate modeling of intraparticle heat and mass transport is a prerequisite for the design of many industrial processes and the interpretation of experiments. In the recent literature, deviations are noticed between simulation results of mole and mass formulated pellet model equations. For this reason, in this study, a comprehensive mathematical and numerical analysis has been performed. Several pellet model complexities are simulated to elucidate the effects of modeling assumptions on the behavior of the mass and mole formulated pellet model equations. Noted findings are as follows: (i) The simulation results show that the Wilke and Wilke–Bosanquet models with respect to the mass and mole average velocity definitions are not consistent. The Maxwell–Stefan and dusty gas closures are recommended; identical simulation results can be obtained with the mass and mole formulated pellet equations where the species mole and mass diffusion fluxes are defined relative to the respective average mixture velocities. (ii) Considering simplified models in which the convective terms are neglected, the species mole balance should be manipulated with the continuity equation to enforce the continuity balance of the mole number change. For the rigorous models retaining the convective terms, model manipulation by use of the continuity equation is still beneficial simplifying the model complexity and thus the implementation. Moreover, it is important to identify the velocity definitions used in the pellet equations and apply them consistently.

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