Mass transfer to viscous liquids in bubble columns and air-lift reactors: influence of baffles

Abstract

Gas—liquid mass transfer has been studied in highly viscous Newtonian and non-Newtonian liquids contained in (i) a bubble column of 0.14 m diameter and (ii) an air-lift reactor formed by introducing a draft tube, 0.09 m in diameter, into the bubble column. The mass transfer coefficient, k L a, has been calculated from the rate of mass transfer determined experimentally by noting continuously the weight of pure CO 2 gas absorbed in these liquids. Effects of liquid viscosity, liquid height and gas sparger have been investigated in the bubble column. In addition, the influence of baffles such as baffle caps and sieve plates, introduced into (i) the bubble column and (ii) the draft tube of the air-lift reactor, has also been studied. The effect of sparger (number and diameter of holes) on gas hold-up and k L a was found to be strong at low liquid height: at a given gas velocity, gas hold-up and k L a decreased with increasing liquid height up to 0.8 m; above this height, they remained unaffected. Liquid viscosity strongly influenced k L a but hardly affected the gas hold-up: k L a decreased as viscosity increased; this decrease was marked up to about 0.3 Pa s, above which value, the decrease was gradual. For viscous Newtonian liquids, k L a values in the air-lift reactor were lower than those in the bubble column; this is due to the lower gas hold-up in air-lift reactor caused by the higher liquid circulation velocity and the bubble coalescence in the draft tube. Introduction of baffles in the bubble column and air-lift reactor enhanced the interfacial area and hence k L a. For shear thinning liquids contained in the air-lift reactor, the increased liquid circulation velocity also resulted in a lower value of apparent viscosity, this had a favourable effect on mass transfer. Following a similar argument, the introduction of baffles in the bubble column and air-lift reactor reduced liquid circulation velocity and increased the apparent liquid viscosity. Hence for the shear thinning liquids, the benefit of bubble break-up caused by the baffles is partially offset by the effect of higher liquid apparent viscosity.