MASS-TRANSFER TO VISCOUS-LIQUIDS IN BUBBLE-COLUMNS AND AIRLIFT REACTORS - INFLUENCE OF BAFFLES

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 d iameter and (ii) an air-lift reactor formed by introducing a draft tub e, 0.09 m in diameter, into the bubble column. The mass transfer coeff icient, k(L)a, has been calculated from the rate of mass transfer dete rmined experimentally by noting continuously the weight of pure CO2 ga s absorbed in these liquids. Effects of liquid viscosity, liquid heigh t and gas sparger have been investigated in the bubble column. In addi tion, the influence of baffles such as baffle caps and sieve plates, i ntroduced into (i) the bubble column and (ii) the draft tube of the ai r-lift reactor, has also been studied. The effect of sparger (number a nd 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 d ecreased 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 incr eased; this decrease was marked up to about 0.3 Pas, above which value , the decrease was gradual. For viscous Newtonian liquids, ka values i n the air-lift reactor were lower than those in the bubble column; thi s is due to the lower gas hold-up in air-lift reactor caused by the hi gher liquid circulation velocity and the bubble coalescence in the dra ft tube. Introduction of baffles in the bubble column and air-lift rea ctor enhanced the interfacial area and hence k(L)a. For shear thinning liquids contained in the air-lift reactor, the increased liquid circu lation velocity also resulted in a lower value of apparent viscosity, this had a favourable effect on mass transfer. Following a similar arg ument, the introduction of baffles in the bubble column and air-lift r eactor 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 effe ct of higher liquid apparent viscosity.