Mass transfer rates from bubbles in stirred tanks operating with viscous fluids

In this paper, the role of liquid viscosity on the mass transfer rates in stirred tank reactors has been theoretically studied. Liquid viscosity affects liquid diffusivity and bubble size distribution by defining bubble stability in the flow. A population balance, taking into account the effect of liquid viscosity on the coalescence and break-up closures, has been combined with Higbie–Kolmogorov's theory to predict the effect of liquid viscosity on the mass transfer rates. Experimental results from the literature for stirred tanks operating with one single Rushton turbine have been used as comparison. Different moderately viscous aqueous solutions (glucose, glycerol and millet-jelly) have been considered. Bubble break-up depends on the critical deformation of the bubbles in the continuum phase. A correlation between the Weber critical number and the liquid viscosity has been found. Once the bubble distribution is accurately determined, the volumetric mass transfer rate in viscous solutions can be predicted theoretically.