Hydrodynamic Characteristics in an External Loop Airlift Bioreactor Containing a Spinning Sparger and a Packed Bed

An external loop airlift bioreactor (ELAB) and the packed bed bioreactor (PB) design strategies have been combined into one vessel. The hydrodynamic behavior of the combined system has been investigated. Woven nylon packing was placed in the riser section of the ELAB to represent the packed bed. A novel spinning sparger was employed to generate air bubbles. The controlled input variables were packing porosity, packing height, superficial gas velocity, and sparger rotational speed. The hydrodynamic output variables included gas holdup, liquid circulation velocity, axial dispersion, and bubble-size distribution. Gas holdup continuously increased with increases in both the packing height up to 0.8 m and the porosity up to 0.99, but at a porosity of 1.0 (no packing), there was a significant drop in the gas holdup. Increased amounts of packing in the ELAB, whether in the form of packing height or packing density, decreased the liquid circulation rate in the bioreactor because of increased frictional resistance and tortuosity. Packing also decreased the Bodenstein number, indicating greater axial dispersion and enhanced mixing. Bubble sizes were more uniform and had smaller diameters after passing through the packing material. Empirical models are presented which accurately predict gas holdup and liquid circulation velocities as functions of all four independent variables (packing height, packing porosity, gas flow rate, and sparger spinning speed). The optimum hydrodynamic conditions were observed with packing at the highest porosity (0.99) used in this study.