Mass transfer and mixing characteristics in an airlift‐driven fibrous‐bed bioreactor

An internal loop airlift‐driven fibrous bed bioreactor (ILALFBB) was designed and developed with a high degree of flexibility to handle genetically engineered and fragile shear‐sensitive cells. The mixing and oxygen mass transfer characteristics have been investigated. A cotton fibre was set in the downcomer of the ILALB to represent the fibrous packed bed and the outcome results were compared with those of the polyurethane foam (PUF) packed and unpacked ILALB systems. The effects of fibre, packing height, bed top and bottom clearances, spacing between adjacent fibre surfaces, and superficial gas velocity were investigated. The liquid phase mixing output variables included the liquid circulation velocity (U_Lc), circulation time (t_Lc), mixing time (t_Lm), Bodenstein number (Bo_L), and axial dispersion coefficient (E_L), whereas the mass transfer out variable was the K_La. Bo_L and E_L in the riser and downcomer regions of all packed systems increased with increasing in packing height, packing top clearance, and superficial gas velocity, except the overall Bo_L was independent of gas velocity at low gas velocities. The Bo_L was found highest in the riser of the large cotton and small PUF packed system with large spacing and the E_L in the downcomer of PUF packed systems with smaller spacing between fibre surfaces. Increased amounts of packing in the ILALB, whether in the form of cotton or PUF decreased the U_Lc in the bioreactor because of the increased frictional resistance and tortuosity. The reduction in U_Lc was significant for large packing with smaller spacing between fibre surfaces and increased bottom clearances of the cotton packed system. High circulation times (t_Lc) and shorter mixing times (t_Lm) were achieved using small PUF packing with large top clearance. Relatively high K_La values were obtained using large packing with large top clearances and spacing between fibre surfaces. The boost in K_La was associated with increased gas holdup and/or interfacial area, due to bubble breakage by the shearing action of the fibrous‐bed. Empirical correlation proposed for E_L, Bo_L, and K_La gave a good fit of the experimental data.