Local gas distribution and mass transfer characteristics in an annulus-rising airlift reactor with non-Newtonian fluid

Many chemical and biochemical processes operated in airlift reactors are limited by the gas-liquid mass transfer of the reactants. Therefore, it is essential to have good understanding of the mass transfer characteristics and gas distribution in airlift reactors to be able to design effective reactors and to optimize aeration conditions. To accomplish this goal, experimental studies on the mass transfer characteristics, i.e. gas holdup, bubble size, interfacial area, volumetric mass transfer coefficient and liquid-side mass transfer coefficient were performed in an annulus-rising airlift reactor with Newtonian and non-Newtonian fluids. A 3D electrical impedance tomography (EIT) technique was used to measure local gas holdup and visualize gas distribution. The influences of aeration rate and liquid viscosity on local gas holdup and interfacial area were investigated at three axial positions along the riser. The EIT results show that the uniformity of gas distribution reduces with increasing liquid viscosity. Bubble size was observed to have a bimodal distribution in highly viscous liquids at high gas superficial velocities due to the intensified coalescence and breakup of bubbles. The maximum value of the local interfacial area is found at the fully developed flow zone in the riser. The liquid-side mass transfer coefficient was found to depend on liquid viscosity and bubble size for non-Newtonian liquids. Empirical correlations based on the experimental data obtained in this work predict well the local gas holdup and volumetric mass transfer coefficient.