Mass transfer between single bubbles and Newtonian and non-Newtonian liquids

Abstract

Instantaneous values of bubble volume, shape, rise velocity, overall mass transfer product, surface area and overall mass transfer coefficient for single carbon dioxide bubbles in free rise through liquids contained in a 10.16 cm I.D. column were experimentally evaluated.In the experimental investigations, a novel data processing technique was used which allowed accurate measurement of the instantaneous rate of change of pressure of the constant volume system, consisting of a liquid contained in the mass transfer column and isolated from the atmosphere, as a bubble rose through the liquid and dissolved in it.A wide range of bubble sizes (0.2 - 6.0 cm diameter) was investigated in three Newtonian liquids - distilled water and two aqueous Glycerol solutions - and a non-Newtonian viscoelastic aqueous Polyox solution, which exhibited pseudoplastic behaviour in steady shear. Detailed comparisons of the collected data with existing theoretical models relating to bubble rise velocity and overall mass transfer coefficient were carried out.In the carbon dioxide-distilled water system, bubble shape was seen to go from spherical to oblate spheroidal to spherical cap with increasing bubble size. Bubble shapes observed in the two aqueous Glycerol solutions were similar to those observed in water, with an additional > hemisphere shape and the occurrence of spherical cap bubble skirting in the more concentrated Glycerol solution.The shapes and terminal rise velocities of large bubbles in the Newtonian liquids were found to be influenced by the proximity of the column walls. The terminal rise velocity data for bubbles in the non-Newtonian Polyox solution was lower than for distilled water; and exhibited an abrupt increase by a factor in excess of 4 at a critical bubble size. Bubbles in the Polyox solution exhibited tailing over the size range 0.27 cm ≤ Dₑ < 2.2 cm. Bubble tailing is known to be peculiar to non -Newtonian fluid fields. In the Polyox solution suitable bubble sizes adopted spherical, prolate spheroidal, oblate spheroidal and spherical cap shapes. A previously unreported spherical cap bubble rear configuration was observed for large bubbles in the Polyox solution.Instantaneous overall mass transfer coefficients based on actual bubble surface areas were evaluated. The transfer coefficients of small bubbles in all four solutions investigated were closely predicted by theoretical equations for circulating spheres and for rigid spheres in creeping flow. The transfer coefficient data for carbon dioxide bubbles in the Polyox solution confirmed the suggestion (A7) that at the critical bubble size in viscoelastic pseudoplastic liquids a transfer from rigid to circulating body behaviour in creeping flow occurred. Certain bubble shape transitions were found to result in maxima in the transfer coefficient versus diameter plots for the solutions investigated.