Enhanced Oxygen Mass Transfer in an External Loop Airlift Bioreactor Using a Packed Bed

A small quantity of nylon mesh packing inserted in the riser section of an external loop airlift bioreactor (ELAB) was found to increase the overall volumetric oxygen mass transfer coefficient by a factor of 3.73 compared to an unpacked riser. 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 oxygen mass transfer. A dynamic, spatial model was developed to predict the mass transfer behavior between air bubbles and the continuous liquid phase in the ELAB with and without a packed bed. The model demonstrated superior accuracy compared to simulating the ELAB as a well-mixed vessel and also correctly predicted the cyclical behavior in liquid oxygen concentrations. The oxygen mass transfer coefficient was determined as a best fitting parameter of the model and was found to increase to 4.2 × 10^-3 s^-1 using a small amount of packing (96.3% porosity) compared to the unpacked ELAB. This is similar to values measured in well-mixed bioreactors operating at the same aeration rates. The ELAB containing a packed bed is a novel bioreactor with much higher mass transfer and increased surface area for cell immobilization, and therefore it has potential to greatly enhance gas-liquid fermentations and other gas-liquid biochemical operations.