Intensified gas-liquid mixing in bioreactors equipped with a dual coaxial mixer containing biopolymer solutions

Abstract

One of the significant challenges in the efficient operation of bioreactors is uniform gas dispersion in very viscous non-Newtonian fluids because of the complex rheology exhibited by these fluids. The uneven gas distribution is more pronounced when large-scale aerated bioreactors with aspect ratios of higher than one are utilized. Prior studies have shown that using a close clearance anchor and two centrally positioned impellers in gas-liquid coaxial mixers can improve gas dispersion in pseudoplastic fluids. Nonetheless, despite the extensive applications of yield-pseudoplastic fluids in many industries, no research has been conducted on the gas dispersion in yield-pseudoplastic fluids within a dual coaxial bioreactor with a greater-than-one aspect ratio. In this regard, electrical resistance tomography (ERT) and computational fluid dynamics (CFD) were employed to assess the effect of operating parameters, namely central impeller speed, anchor speed, aeration rate, rotating mode, and pumping mode of central impeller on the aeration efficiency in yield-pseudoplastic fluids. The experimental measurement of the overall gas hold-up affirmed the profound impact of using a close clearance anchor. Even though the down-pumping and co-rotational mode at Na = 30 rpm provided the highest aeration efficiency, more uniform gas dispersion was observed at Na = 10 rpm owing to a greater axial velocity in the downward direction and more effective circulation loops. Compared to the downward pumping direction, lower gas hold-up values were obtained for the up-pumping configurations. Finally, the superiority of the down-pumping and co-rotational mode was demonstrated by comparing the aeration efficiency of various mixing configurations.