Performance of iterative equation solvers for mass transfer problems in three-dimensional sphere packings in COMSOL

Packed bed chromatography is commonly applied for the separation of large molecules in biopharmaceutical industry. A technical chromatography system is typically composed of a cylindrical column that is filled with porous spheres. Particularly in small columns, which are increasingly applied for parallel experiments on lab robotic platforms, the impacts of inhomogeneous packing and wall effects on separation performance can be quite significant. We hence study mass transfer by convection, diffusion and adsorption in three-dimensional sphere packings. Random packings are externally generated and imported into COMSOL where the model equations are easy to implement. However, the COMSOL functions for automatic meshing and for iteratively solving the resulting equation systems fail to work with default settings. We have previously established a semi-automated and half-manual meshing procedure for rather small packings with less than 150 spheres that works with the direct PARDISO solver. The present contribution addresses the evaluation and optimization of the iterative equation solvers that are provided by COMSOL for the given spatial geometries with up to ten million degrees of freedom. The presented results illustrate that we can iteratively solve systems with up to 750 spheres using less memory and less computational time.