The Effect of Operating Conditions on Gas Holdup and Mixing in Tower Fermenters

Abstract

The design and operation of bubble columns is a subject which has recently received a great deal of attention, since applications of such equipment are increasing. Recent work has shown that there is a real possibility of utilizing bubble columns in aerobic systems used for fermentations. At present there is a considerable body of knowledge about the various parameters affecting design and operation of bubble columns. Unfortunately much of the research work carried out has been concerned with small diameter columns and operating conditions not applicable to those suitable for fermentation purposes. The discrepancies in the data published by different researchers also suggest the need for further investigations. The object of this research was to attempt to coordinate previous knowledge about flow patterns in bubble columns with fresh data obtained in a wider range of column geometries and using operating conditions applicable to fermentation processes. Gas holdup was investigated thoroughly under different operating conditions in air-water systems. A number of fermentation media were also used as the liquid phase. It was found that apart from superficial gas velocity which is the governing factor in air-water systems, other liquid phase properties have a marked effect on gas holdup within the range of operating conditions used. Indeed it was noticed that these effects could, in some cases, dominate the effects of other operating parameters. Mixing studies in the Liquid phase have also been carried out. A steady—state tracer injection method was used and concentration profiles were measured over the length of the columns. Although the axially dispersed plug-flow model was used to calculate the dispersion coefficients, attempts have been made to look for better models. It is suggested that a series of stirred tanks with back-mixed flow provides an appropriate alternative. Variation of dispersion coefficients with superficial gas velocity showed a similar trend to that of gas holdup in the case of air—-water systems. It is concluded that the research has opened up a new approach to the behaviour of such systems with the possibility of accounting for mixing effects in the axial as well as the radial direction.