Axial Variation of Mass Transfer Volumetric Coefficients in Bubble Column Bioreactors

Abstract

Bubble column bioreactors used to perform aerobic fermentations consist of a liquid medium containing microorganisms that uptake oxygen for metabolic reactions and gas bubbles that supply this oxygen. Mass transfer rate from gas to liquid phase is a crucial factor for the performance of bioreactors because microorganisms’ life and metabolic reactions depend directly upon it. The maximum transfer rate of oxygen from gas bubbles to the liquid medium is a function of two important parameters: the specific interfacial area and the mass transfer coefficient. These two parameters are lumped into the volumetric specific transfer coefficient. Since size of gas bubbles is not constant along the bioreactor, gas-liquid mass transfer rate changes continuously. In order to optimize mass transfer rates, it is essential to know the bubble size distribution and the interfacial phenomena in each particular system at different operating conditions. Due to the complexity of hydrodynamics and bubble interphase characteristics, the current state of the problem does not consider a universal model to evaluate mass transfer rates in gas-liquid systems; moreover, information about bubble size and its distribution is often neglected. This work presents a model to evaluate axial distribution of values of volumetric mass transfer coefficients considering changes in bubble size and its influence on bubble area along the reactor in homogeneous regime. Simultaneously, this model evaluates changes of volumetric mass transfer coefficient and its effect on the fermentation kinetics, which influences performance of the bioreactor.