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

Abstract The effects of inorganic electrolytes (NaCl, MgCl 2 , CaCl 2 ) 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 via the reduction of bubble size and the consequent increment of a and gas holdup ( ɛ g ). Finally, regarding the effects of v sg on mass transfer, transition behaviors between those expected for isolated bubbles and bubble swarms were observed.