Study of gas hold-up in aqueous solutions of surface-active solutes

Abstract

The gas hold-up is an important hydrodynamic parameter that affects the interfacial area of gas-liquid dispersions. The ability of liquid phase to support or hinder bubble coaJescence significantly influences the gas hold-up. Presence of surface-active solutes in liquid phases decreases the bubble coalescence ability; thus increases the gas holdup in comparison to the pure solvent. The electrolytes and aliphatic alcohols are two important groups that behave as surface-active solutes in aqueous solutions. In this paper a systematic study was made to investigate the effect of aqueous solutions electrolytes on gas hold-up. A 50 mm diameter bubble column was used for the experimental study. Comprehensive data of gas hold-up with respect to concentrations in four electrolyte-water solutions were generated. The enhancement of gas hold-up in these types of solutions was observed at low concentration. The coalescence hindering nature of electrolytes was explained by the dynamic surface effect i.e. the surface tension gradient that. exists between bubble surface and bulk of the liquid and quantified by the dimensionless parameter (crk2/cr). A gas hold-up model was proposed which included the parameter (crk2/cr) to incorporate the coalescencehindering phenomenon of liquids with gas hold-ups. The model predicts the trend and magnitude of gas hold-ups in four electrolyte solutions adequately. The corresponding concentrations of the maximum gas hold-up values were predicted satisfactorily. The predicted values of maximum gas hold-ups deviated from the measured values by less than 6%.