Local structure effects on hydrodynamics in slender fixed bed reactors: Spheres and rings

Abstract

Fixed bed reactors play a crucial role in the chemical industry, and their performance is influenced by the unique structural effects observed in the small tube-to-particle diameter ratio range (1.5<D/dp<9.3). Experimental void fraction data for beds made of spherical and ring-shaped particles reveal sudden changes, deviating significantly from theoretical calculations. These effects, categorized into four zones for spherical particles, i.e., single particle string, central channel, annular gap, and central channel + annular gaps, exhibit varying impacts on pressure drop. To describe this, the factors of the Ergun equation are modified accordingly. Furthermore, tortuosity is introduced as an additional parameter to describe the structural effects on fixed bed behavior. Classic correlations prove inadequate, leading to the adaptation of the Millington correlation for random beds, as well as those with a central channel and/or annular gaps. With particle-resolved Computational Fluid Dynamics (PRCFD) simulations, the residence time behavior is quantified of differently structured beds of spheres and rings, revealing deviations from plug flow and the presence of stagnation zones in beds containing a central channel. Notably, beds with an annular gap displays residence time behavior akin to plug flow, with lower pressure drop and an ordered, reproducible structure. These results highlight the importance of the D/dp ratio as an additional descriptor to characterize transport phenomena in slender fixed bed reactors.