| An in-depth study of the mass-transfer, mixing and hydrodynamic characteristics of airlift contactors was carried out in four sizes of the external-loop type of airlift contactor, having downcomer-to-riser cross-sectional area ratios (A(,d)/A(,r)) ranging from 0.11 to 0.69 and in three sizes of the concentric-tube type (0.13 (LESSTHEQ) A(,d)/A(,r) (LESSTHEQ) 0.56) as well as in a bubble column (A(,d)/A(,r) = 0) for comparison. The dispersion volumes ranged from 32.7L for the bubble column to 55.0L for the largest airlift contactor.; Experimentation was done in batch charges of water and 0.15M NaCl solution, and at a constant height of the contactors (1.80 (+OR-) 0.05m). The overall volumetric mass transfer coefficient, (K(,L)a(,D))(,T), was estimated from the transient response, to a step input of air (absorption) or nitrogen (desorption), of the liquid-phase oxygen tension, measured with a dissolved-oxygen probe. The riser liquid velocity, the mixing time (t(,m)), the circulation time (t(,c)) and the Bodenstein number in the contactors were obtained from tracer studies.; The liquid circulation generated in the airlift contactor, without mechanical agitation, distinctly distinguishes it from other types of contactors (bubble column, stirred-tank) and was found to be a very influential parameter on its characteristics. Both the riser gas hold-up, ((epsilon)(,G))(,r), (K(,L)a(,D))(,T), and the mixing time (t(,m)) decreased as the riser superficial liquid circulation velocity, (u(,L))(,r), increased. The liquid velocity is shown to be easily predicted with the knowledge of the geometry, the riser superficial gas velocity, (u(,G))(,r), and the physical properties of the liquid and is greatly dependent on A(,d)/A(,r) more than on any other physical parameter ((u(,G))(,r), contactor height or diameter).; Both the mass transfer economy {lcub}(K(,L)a(,D))(,T) C*/(P(,G)/V(,D))(,T){rcub} and the mixing capabilities of the airlift contactors were found to be intermediate between those of the bubble column and those calculated for a stirred tank with an equivalent volume.; Generalized correlations are given for (K(,L)a(,D))(,T), ((epsilon)(,G))(,r) and t(,m). |
