Experimental study on mixing time and hydrodynamics in mixing element of an air-lift mixer settler

Abstract

Gas-liquid hydrodynamics and mixing characteristics in batch vessel equipped with mixing element of air-lift solvent extraction contactor (CALMSU) is reported. Photographic technique involving a coloured tracer is used. Mixing dynamics of tracer is tracked using high speed camera in a 5 L glass beaker with mixing element. The resultant video is analysed using three different algorithms for mixing time determination. Measurements are taken at three different axial locations (top, bottom, middle) to ensure throughout mixing. A systematic study on the effects of air flow rate, mixing element submergence, and tracer addition point on mixing time is reported. The mixing time decreases with an increase in air flow rate, reaches a minimum, and then increases. Mixing time is seen to monotonically decrease with increase in submergence of the mixer. Mixing is also dependent on the location of tracer release. Successive experiments were performed to determine effect of air flow rate and submergence on bubble size distribution and estimation of Saunter mean bubble diameter. Bubble size is seen to be rather independent of air flow rate while it decreases with increase in liquid level. Ejection length is also found to be increasing with increase in air flow rate up to a level before plateauing out.