Effect of electrolytes in aqueous solutions on oxygen transfer in gas-liquid bubble columns

The effects of inorganic electrolytes (NaCl, MgCl, CaCl2) in aqueous solutions on oxygen transfer in a bubble column were studied. Electrolyte concentrations (c) below and above the critical concentrations for bubble coalescence (c(tc), and six superficial gas velocities (v(sg)), were evaluated. The volumetric mass transfer (k(L)a) and the mass transfer (k(L)) coefficients were experimentally determined. It was found that the concentration of electrolytes reduced the k(L), but the interfacial area (a) increased enough to result in a net increase of k(L)a. Using as independent variable a normalizing variable (c(r) = c/c(tc)), and maintaining fixed v(sg), similar values of k(L)a were observed regardless the kind of electrolyte; the same happened for k(L). This suggests that c(r) quantifies the structural effects that these solutes exert on mass transfer. Also, once c(r) = 1 was reached, no significant variations were found in k(L)a and k(L) for constant v(sg). It is concluded that the gradual inhibition of bubble coalescence (c(r) < 1) governs the significant changes in hydrodynamics and mass transfer via the reduction of bubble size and the consequent increment of a and gas holdup (epsilon(g)). Finally, regarding the effects of v(sg) on mass transfer, transition behaviors between those expected for isolated bubbles and bubble swarms were observed. (C) 2014 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers.