Mass Transfer Rates from Oscillating Bubbles in Bubble Columns Operating with Viscous Fluids

Abstract

In spite of the work on bubble columns, their design and scale-up is still a difficult task due to the lack of understanding of bubble dispersions and mass transfer mechanisms. Even less known are viscous or non-Newtonian fluids. Therefore, a theoretical model for predicting the volumetric mass transfer coefficient, kLa, in bubble columns operating with viscous fluids has been proposed. The model consists of a population balance coupled with a theoretical equation for the Sherwood number for oscillating bubbles, considering the effect of liquid viscosity on both. Experimental results for Newtonian and non-Newtonian viscous liquids from the literature are used to validate the model. Bubble dispersions have been simulated with good agreement using the Weber critical number, Wec, as a parameter to account for the effect of liquid viscosity, which increases bubble stability. A correlation between the liquid viscosity and Wec has also been proposed. The mass transfer resistance is calculated taking into account the hydrodynamic processes involving bubbles (collisions, breakup, coalescence, detachment) because they provide initial oscillation amplitudes. However, bubble oscillation decays in viscous liquids because the oscillating energy is absorbed as viscous dissipation. Good agreement is found between the experimental and the predicted kLa when considering that bubble oscillations do not decay completely by viscous dissipation due to the continuous bubble collisions, breakup, and coalescence.