Hydrodynamic and Mass Transfer Studies on Rheologically Complex Fluids in Airlift Reactors

Abstract

Airlift reactors are widely used as multiphase contactors in various chemical, biochemical and petrochemical industries due to simple design, low cost, low shear rate, perfect mixing, high heat and mass transfer rate. Hydrodynamic and mass transfers are important parameters for the design and scale-up of airlift reactors. The hydrodynamic parameters include gas-phase holdup, flow regime transition, bubble diameter and liquid velocity. Whereas, the mass transfer parameters include overall mass transfer coefficient, liquid side mass transfer coefficient and interfacial area. The factors that affect the hydrodynamic and mass transfer parameters are superficial gas velocity and liquid-phase properties. The fluids handled in biochemical processes show complex rhelogical behavior due to the presence of organic chemicals and microorganism suspensions and affect the performance of the airlift reactors by affecting the hydrodynamic and mass transfer parameters. Newtonian and non-Newtonian fluids behave differentaly as the apparent viscosity of non-Newtonian fluids is dependent on the shear rate. Further in biochemical processes, the low shear rate is required as microbial cells are highly sensitive to high shear rate. However, the certain level of shearing is required to obtain sufficient heat and mass transfer rates. For non-Newtonian fluids, many researchers have assessed the gas-phase holdup and mass transfer coefficient in terms of apparent viscosity at a fixed shear rate. However, the average shear rate is dependent on the superficial gas velocity and rheology of the fluid. Thus, there is a need to find average shear rate for non-Newtonian fluids at operating range of superficial gas velocity and by considering the fluid rheology (K and n) in the airlift reactors. Therefore, in present study, the average shear rate for non-Newtonian fluids was analyzed using power due to aeration and fluid rheology and was further us