Comparison of computational fluid dynamics (CFD) and pressure drop correlations in laminar flow regime for packed bed reactors and columns

Abstract

Empirical correlation (EC) equations are still of a great designing importance for industrial plant construction. They are an indispensable modeling tool for engineers, reducing the time to find the optimal operating conditions. Nonetheless, numerical method complexity and product yield optimisation are advancing. Computational fluid dynamics (CFD) is thus nowadays applicable for optimizing chemical reactors. In contrast to EC, CFD acknowledges specific vessel geometry, where local physical and chemical phenomena, contributing to apparent catalytic turnover, prevail. Presently, EC and CFD were compared considering the pressure drop predictions within the packed bed columns for spherical, cylindrical, trilobe and quadrilobe particle packing, in order to determine the limits of EC accuracy. 52 configurations were simulated and the estimations within EC validity range margins were in agreement with CFD (<15%), while in extremes (non-negligible entrance and exit patterns), a 70% deviation could be exceeded. Furthermore, boundary wall effects were found to be dependent on the stacked pellet shape and orientation, and did not necessarily lead to an increase of viscous friction loss, relative to the infinitely wide systems, for the column-to-particle diameter ratios, lower than 10, which is contradictory to non-mechanistic relationship models. While the induced pressure difference within realistic fixed beds is elevated due to gas or liquid surface interaction and back-mixing, it can also be decreased by channeling/tunneling, which is why the effective net influence should be analyzed with CFD simulations, particularly in novel intensified and micronized processes, in which momentum, mass and heat transfer resistances are far from bulk medium continuity.