Dynamic gas disengagement: A new technique for assessing the behaviour of bubble columns

Abstract

A precise measure of the liquid motion and distribution of bubble sizes is essential for designing gas-liquid reactors for complex reactions in which product distribution may be a function of bubble size. A theoretical approach is presented which shows how the interaction of bubble size distribution and bubble rise velocity functions leads to predictions of the overall steady state hold-up in a bubble column within which the liquid flow is understood. Since this approach is based on a physical understanding of how bubble flow at a given superficial velocity must relate to the static hold-up, the theory can be immediately extended to describe the disengagement of gas bubbles if the gas feed is cut off. Thus the dynamic gas hold-up during gas disengagement can be used to provide new insights into the fundamentals of bubble column behaviour. In this way it becomes possible in principle to inter-relate macroscopic properties such as surface area, gas phase residence time distribution and intensity of mixing in the liquid phase. A new experimental technique is described which measures the dynamic gas hold-up during gas disengagement Experimental results at a nominal 20 mm/s gas superficial velocity are compared with various approaches based upon the theory. The effects of the accompanying induced liquid movements are represented by a simplified core-annulus circulation model. Bubble size distributions and liquid circulation can then be related to both the static and dynamic hold-up behaviour. It is shown that ambiguity due to uncertainty about the relative differences in bubble size distribution in upflow and downflow regions can be resolved from a knowledge of the surface area.