Simulating large-scale biotech bubble columns: challenges and insights from the parcel approach.

The growing industrial demand for economically attractive large-scale bioprocesses has increased the need for reliable and computationally feasible modelling approaches. Gas fermentation plays a particular role as the targeted commodities ask for large-scale solutions using bubble-column type bioreactors that allow them to benefit from the economy-of-scale. To reduce development efforts, large-scale designs increasingly build on computational fluid dynamics (CFD) simulations. State-of-the-art approaches employ finite-volume Reynolds-averaged Navier-Stokes (FV-RANS) methods that require enormous computational efforts. In contrast, Euler-Lagrange (E-L) approaches offer detailed local resolution as a prerequisite for optimum bioreactor design. By tracking individual bubbles, they predict bubble population dynamics while computations are parallelized. However, to cope with realistic bubble numbers, large-scale simulations typically make use of the so-called 'parcel size' approach, i.e. multiple bubbles are represented by a single computational particle. This study applies Lattice Boltzmann large eddy simulations (LB-LES) to illustrate the observed consequences: Swarm-like bubble behavior is found that results in inhomogeneous bubble distributions. As a consequence, the volumetric mass transfer coefficient k