Effect of adding glycerol and Tween 80 on gas holdup and bubble size distribution in an aerated stirred tank

This work presents the results of an experimental parametric study of air-water dispersions in a laboratory scale stirred tank. The quantities monitored are total gas holdup and bubble size distribution. The physical parameters varied, in order to study their effect on the dispersion properties, are liquid viscosity as well as surface tension (static and dynamic) and surface viscoelasticity. Viscosity is varied by adding glycerol whereas surface properties are varied by adding Tween 80. In addition, the effect of the gas flowrate and the stirring rate are studied. Measurements of gas holdup are taken primarily by a non-invasive electrical technique which utilizes ring electrodes flush mounted to the wall. These measurements are verified against simultaneous differential pressure measurements and video images of the instantaneous height of the liquid free surface. Bubble size distributions are estimated analyzing still photographs. In the examined range of parameters, as the concentration of glycerol increases the gas holdup first increases and then decreases going through a peak at 41.6% v/v glycerol (viscosity: 5 mPa s). On the contrary, as the concentration of Tween 80 increases, the gas holdup increases monotonically up to a plateau value at and above 50 mg/l of Tween 80 (static surface tension: 38.3 mN/m). Further analysis of the results indicate no influence of coalescence/breakage phenomena in the observed bubble size distributions.