Experimental and theoretical investigation of phase inversion in liquid-liquid dispersions
This thesis describes an experimental and theoretical investigation of phase inversion in concentrated liquid-liquid dispersions, as part of a joint project between University College London (UCL) and Imperial College London (ICL). Experimental studies of phase inversion behaviour and associated phenomena in pipeline flows were carried out on the Water-Oil Liquid Flow (WOLF) facility at the Department of Chemical Engineering at UCL. Two inversion routes (w/o to o/w and o/w to w/o) were followed to elucidate the hysteresis effect in pipeline flows in both upward and downward flows at either constant or increased mixture velocity. System parameters, such as frictional pressure gradient, in-situ holdup, velocity ratio, drop velocity and drop size distribution were studied for flows before and after phase inversion. The velocity ratio of two liquid phases was shown to play a key role in the phase inversion process. A hot-film anemometer (HFA) was also employed in this work to measure the mean and turbulent fluctuation velocities of the continuous phase at different dispersed phase input fraction. Enhancement or attenuation of turbulence level of the continuous phase was found to depend on a number of parameters such as local concentration, drop size, flow direction and velocity. It is evident that high concentrations and large drops of the dispersed phase are likely to increase local turbulence. An improved analytical method was also developed to derive stable drop size distributions (DSD) from the distributions of chord lengths (CLD), measured by an impedance probe. The effect of biased sampling towards larger drops was included while smoothing equations were introduced to eliminate the negative DSD values that can arise from direct backward transformation of CLD. Two PBEs models were developed for liquid-liquid dispersions formed in stirred vessels and pipeline flows, respectively. A novel combination of population balance equations (PBEs) model with studies of phase inversion was presented in this work, which provided further understanding of the influence of breakage and coalescence of dispersed drops on the process of phase inversion. PBEs model indicated that there is a difference in the distance required to achieve the fully-developed state for different inversion routes, which suggests the existence of an ambivalent region in terms of location rather than input oil fraction in pipeline flow this distance from the inlet where inversion occurs depends on the initial conditions, mixture velocity and fluid physical properties. Also, modelling of phase inversion and the ambivalent region in stirred vessels with heterogeneous and homogeneous distribution on turbulent energy were presented. To achieve better predictions for stirred vessels, a 'two-region' model was postulated which assumed that drop breakup and coalescence take place preferentially in the vicinity of the impeller and away from that region, respectively. The predictions from the two-region model were found to be in good agreement with experimental data. Finally, a framework of studying the behaviour of secondary dispersions was developed and incorporated into a PBEs model, by taking into account the inclusion and escape of secondary droplets.