Modelling the increased gas capacity of self-inducing impellers

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In many gas—liquid mixing processes occuring in stirred vessels, the rate of reaction is limited by mass transfer between the gas and liquid phases, e.g. acetic acid fermentation. An important aspect in the design of such devices is to increase the rate of reaction by improving the mass transfer performance. This study examines the effect of increasing the number of gas outlet orifices on each blade on the gas induction rate and mass transfer performance of a concave-bladed self-inducing impeller. Increasing the number of orifices is found to significantly increase the rate of gas induction, however, this effect diminishes as the number of orifices increases. Volumes of liquid per unit volume of gas per minute (VVMs) of up to 0.6 can be achieved using all four orifices on each blade at an impeller speed of 10 rps; this may be compared to a value of 0.25 obtained under similar conditions for a single orifice. Furthermore, this method of increasing the gas capacity is not at the expense of generating bigger bubbles, and thus large gas—liquid interfacial areas are maintained. This is reflected in experimental measurements of the mass transfer coefficient; the value of k L a for a given specific power input approximately doubles in moving from one to four orifices per blade. Mass transfer coefficients of up to 0.055s −1 (at a specific power input of 1.7 kW/m 3) are attainable for the coalescing air—water system. An existing model for gas-inducing impeller design (Evans., 1991a) is extended to include the effect of multiple orifices. The revised model gives acceptable predictions of the induced gas rate, to within 20% of the experimental data, over the full range of measurements.