The effect of surface active additives on bubble coalescence and gas holdup in viscous aerated batches

Abstract

An experimental study has been aimed at examining the possibility of modifying the behaviour of viscous liquids in bubble column reactors by addition of surface active compounds (aliphatic alcohols). Results of coalescence measurements showed that bubble coalescence in viscous saccharose solutions could be fully suppressed by addition of alcohols. The transition concentrations of individual alcohols were within the whole set ethanol–octanol successfully correlated as a function of the number of carbon atoms in the alcohol molecule. The bubble column experiments proved that the properties of viscous aerated liquids could be significantly modified by the addition of relatively small amount of alcohol ( c A ≃1×10 -5–1×10 -2 kmol m -3). The results of gas holdup measurements indicated significant increase of bubble bed voidage with increasing alcohol concentration. Fitting of gas holdup data with the Zuber and Findlay drift-flux model also implied favourable effect of the surface active additives on the radial uniformity of voidage profiles, indicated by decreasing values of the distribution parameter, C o. The effect of alcohol addition increased with increasing viscosity of the original medium and was most pronounced in highly viscous saccharose solutions ( μ L ⩾100 mPa s). In full agreement with the results of the coalescence studies, the favourable effect of alcohols on bubble bed voidage increased with increasing length of the carbon chain. Comparison of gas holdup data obtained with different distributor plates indicated significant synergistic effect of alcohol concentration and the mode of primary gas dispersion on the properties of resulting bubble bed and on the values of its voidage. Addition of alcohols thus compensated even for the negative effect of liquid viscosity on the formation and stability of the homogeneous bubbling regime.