Two‐ and three‐phase mixing in a concentric draft tube gas‐lift fermentor

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Abstract
Two‐ and three‐phase mixing studies were carried out in a 44‐L concentric draft tube gas‐lift fermentor. It was proposed to use the fermentor for the production of solvents using immobilized bacteria. Bubble size, gas holdup, liquid velocities, circulation, and mixing times were determined for various superficial gas velocities in distilled water, starch, carboxymethyl cellulose, and ethanol solutions. The observed trends for two phase mixing were similar to other studies but the results were found to be more sensitive to liquid properties. This was possibly due to the large value of downcomer to riser area used in this study. Mixing in three phases highlighted the difficulty in predicting the effect of adding solids to the gas‐liquid system. Results showed that the gas‐lift fermentor was ideally suited to dealing with three phases but more work is necessary before accurate models can be developed to account for the effect of solids.