Gas−Liquid Mass Transfer in Benchscale Stirred TanksFluid Properties and Critical Impeller Speed for Gas Induction

Abstract

This work is concerned with the gas−liquid mass transfer in stirred tanks with gas-inducing impellers. Experiments were performed to determine the critical impeller speed for the onset of gas induction and the mass-transfer group kLa, using water, n-hexadecane, toluene, acetone, and sunflower oil at various temperatures as liquids and hydrogen and nitrogen as gases. The use of different liquids and gases allowed the influence of fluid properties on the mass transfer to be studied. Special emphasis is on reducing the experimental effort in downscaling, i.e., the use of benchscale autoclaves to mimic industrial conditions. In benchscale tanks, the capillary pressure is significant, with respect to the static liquid pressure at the impeller, and must be taken into account to describe the critical impeller speed for gas induction. Downscaling implies relatively low mass-transfer rates, which can be achieved by operating close to the critical impeller speed. The critical impeller speed must be included in models for the mass-transfer rate. Our data set of kLa values was best described by a dimensionless correlation, in which a Froude group (based on the stirrer speed N and the critical stirrer speed Ncr) was used to account for the gas-induction rate, together with the Reynolds number and Schmidt number, to account for turbulence intensity and fluid properties.