Computational Fluid Dynamics for Bioreactor Design

Abstract

Mixing is a critical phenomenon in bioreactors and its importance increases with the size of the reactor. For proper microbial growth, it is desirable to have uniform distribution of oxygen, carbon dioxide, cells, nutrients, and other metabolites across the reactor volume. Achieving uniform mixing in large reactors is nontrivial. Ideal flows such as “plug flow” and “perfectly mixed” do not exist in reality. Nonidealities in flow originate from imperfect mixing that results in stagnant zones, channeling or bypassing, recycling or cross-flow streams. Improvements in hydrodynamics result in higher mass transfer coefficient and specific interfacial area between the phases. But, this is also likely to increase shear rate and shear stress, both of which need to be monitored to avoid cell lysis. Process parameters such as rotational speed of the impeller, inlet gas flow rate, feed characteristics (viscosity and density), and design of bioreactor (number of baffles and type of impeller) affect mass transfer coefficient of O2 and limit productivity. This chapter addresses the topic of use of computational fluid dynamics (CFD) to model hydrodynamics of aerated stirred bioreactors and to estimate the volume-averaged mass transfer coefficient, k L a. Effects of process parameters such as impeller speed and gas flow rate have been illustrated. Multiphase modeling approaches such as the k–ϵ turbulence model and population balance model are discussed. We hope that the chapter will be of value to those involved in performing CFD of mixing in bioreactors.