Oxygen Mass-Transfer in Tower Fermenters

Abstract

The research described in this thesis is concerned with tower fermenters and the measurement of the rate of oxygen mass—transfer in such systems using fast-response oxygen electrodes. Consideration has been given to ways of modelling the behaviour of fast-response probes and to the evaluation of mass-transfer coefficients from the results of unsteady-state experiments. The effects of gas flow-rate, nutrient concentration, antifoams and microbial concentration on mass—transfer have been studied in detail: superficial air velocity and the presence of anti-foams are shown to be the most important factors. Measurements with the oxygen electrode have also been used to estimate microbial respiration rates during work on the batch growth of the filamentous fungus Aspergillus niger. Gas holdup in tower fermenters has been investigated since this provides valuable information about the behaviour of the gas phase. A manometric method of measurement was used except when the fermenters were run aseptically: in such cases a light-transmittance technique was employed. The factors found to have the greatest effect on gas hold—up were the superficial gas velocity and the presence of nutrients and anti-foams. It was not possible to make meaningful estimates of average bubble size using photographic methods: nevertheless, valuable qualitative information was obtained and this has been used to interpret both the mass-transfer and gas holdup measurements.