Hydrodynamics and mixing in airlift contactors: Experimental work and CFD modelling

Abstract

Fluid flow was examined in an airlift contactor equipped with a central baffle operated at superficial air velocities between 0.07 and 0.34m/s. Measurements were made of the Bubble Size Distribution (BSD), profiles of the average local hold-up and upwards liquid velocity, the overall hold-up and the mixing time. Overall hold-up was similar to that for a bubble column while profiles of the average local hold-up and average upwards liquid velocity were markedly asymmetric (unlike those measured when this pilot-plant was operated as a bubble column). Mixing times ranged from 7 to 27s, decreasing as the superficial velocity increased. Neither the tracer addition point nor the measurement location had a major impact on the observed mixing time, this contrasts with work in bubble columns where the tracer addition and measurement locations had a large impact on the mixing time while the influence of the superficial velocity was relatively small. Associated CFD modelling generally offered good agreement with our experimental measurements of the average local hold-up and upwards liquid velocity profiles. However, it was found that the model underestimated the extent to which air is entrained into the downcomer section and the mixing time at low superficial velocities. This study examined an airlift system operated at high superficial gas velocities for which there are very limited data available in the open literature; comparing experimental and computational results is instructive in defining the strengths and limitations of employing a single bubble size CFD model.