Effect of surface tension gradient on gas hold-up enhancement in aqueous solutions of electrolytes

The effect of addition of electrolytes on gas hold-up of air/water system was investigated experimentally in a laboratory scale bubble column. The experiments were carried out with four electrolytes, namely, NaCl, MgSO4·7H2O, Na2SO4 and CaCl2·2H2O and the concentrations of the solutions were varied from 0 to 0.3 mol/l. Enhancement of gas hold-up was observed for all four electrolytes at concentrations less than 0.1 mol/l. With the increase in concentration, the gas hold-up showed two different trends; in Na2SO4 and CaCl2·2H2O solutions, gas hold-up formed a sharp peak after the enhancement and leveled off at a value somewhat higher than that in water, whereas in NaCl and MgSO4·7H2O solutions, gas hold-up leveled off immediately after the enhancement without forming any peak. Experiments were also conducted to measure the surface tensions of the solutions with special focus in the low concentration region. A strong relation between the gas hold-up enhancement and the change of surface tension with the addition of electrolyte was found. It was also observed that the concentration at which maximum value of C(dσ/dC)2 i.e. (concentration × surface tension gradient with respect to concentration2) is obtained corresponds to the concentration at which maximum gas hold-up enhancement occurs.

► Addition of NaCl, MgSO4.7H2O, Na2SO4 or CaCl2.2H2O to water enhances the gas hold-up of air/water system in bubble columns.
► Maximum enhancement of gas hold-up occurs at concentrations less than 0.1 mol/l.
► The enhancement patterns in NaCl and MgSO4.7H2O solutions are distinct from those in Na2SO4 or CaCl2.2H2O solutions.
► The gas hold-up enhancement is strongly related to the surface tension change of the solution with the addition of the electrolyte.
► The electrolyte concentration at which maximum value of C(dσ/dC)2 is obtained corresponds to the concentration at which maximum gas hold-up occurs