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

The effects of inorganic electrolytes (NaCl, MgCl2, 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 (ctc), and six superficial gas velocities (vsg), were evaluated. The volumetric mass transfer (kLa) and the mass transfer (kL) coefficients were experimentally determined. It was found that the concentration of electrolytes reduced the kL, but the interfacial area (a) increased enough to result in a net increase of kLa. Using as independent variable a normalizing variable (cr = c/ctc), and maintaining fixed vsg, similar values of kLa were observed regardless the kind of electrolyte; the same happened for kL. This suggests that cr quantifies the structural effects that these solutes exert on mass transfer. Also, once cr = 1 was reached, no significant variations were found in kLa and kL for constant vsg. It is concluded that the gradual inhibition of bubble coalescence (cr < 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 (ɛg). Finally, regarding the effects of vsg on mass transfer, transition behaviors between those expected for isolated bubbles and bubble swarms were observed.