Hydrodynamic and oxygen mass transfer in an external loop airlift bioreactor with a packed bed

Abstract

A stainless steel mesh packing with 99.0% porosity has been inserted in the riser section of an external loop airlift bioreactor (ELAB). The hydrodynamic characteristics and oxygen mass transfer rates of the ELAB, both with and without packing, were compared. The packing increased the overall volumetric oxygen mass transfer coefficient by an average factor of 2.45 compared to the unpacked column. The packing increased gas holdup, decreased bubble size, and decreased liquid circulation rates in the bioreactor, all of which contributed to the dramatic improvement in the oxygen mass transfer rates. A dynamic, spatial model was used to predict the transient, oxygen concentration distribution in the ELAB with and without a packed bed. This model was compared to simulating the ELAB as a completely stirred reactor and demonstrated improved prediction of the cyclical changes in liquid oxygen concentrations. The oxygen mass transfer coefficient was determined as a best fitting parameter of the model and at higher gas superficial velocities was found to increase to values approaching 0.021 s −1 using the small amount of packing. Finally, simplified correlations were developed to predict the oxygen mass transfer coefficient in the ELAB with and without the packed bed.