Lo-Yim, Mei Yee Aideo; (1998) Theoretical and experimental studies of drop breakage in two-liquid phase dispersions in mechanically agitated vessels. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Two-liquid phase dispersion in mechanically stirred vessels is a key step in many operations in the chemical, biochemical, food and pharmaceutical industries. Examples of such operations include emulsification, solvent extraction, biotransformation and fermentation. In all these cases, the size and size distribution of drops are amongst the most important parameters determining the stability of the dispersion and the efficiency of the contacting operation. The stable size and size distribution of drops and the rate at which they are achieved is a delicate balance between drop breakage and coalescence. Prediction of the mean drop size and size distribution therefore requires knowledge of these two rate processes. In this study models are developed for breakage rate based on fluid dynamics considerations. These are incorporated into a general population balance equation which is then solved numerically in order to simulate both the transient and steady state size distributions. Measurements of drop size distributions as a function of agitation time are carried for the dispersion of xylene in water in the presence of a surfactant to minimise drop coalescence, in a 0.15m diameter vessel agitated by a standard Rushton turbine. The volume concentration of xylene in these experiments are kept to less than 50%, conditions for which the mixture viscosity remains relatively low and Newtonian and agitation in the vessel remains turbulent. Limited comparative experiments are also reported in turbulent flow by using an up-and-down moving impeller. These experiments are aimed primarily towards establishing the influence of operating conditions and physical properties of the dispersion on drop breakage. The experimental drop size distributions are analysed and compared with the simulations and good agreement is obtained between the two. Additionally, experimental data are reported for xylene-in water dispersions in the mechanically stirred vessel under high xylene phase concentration, conditions for which the rheology of the dispersion is Newtonian and flow in the vessel is laminar and/or transitional. The population balance equations are modified to account for these effects and solved using a two-zone model. Good agreement is found between experimental data and simulations. The Sauter mean drop diameter of the steady-state drop size distributions for both the up-and-down moving impellers and the Rushton turbine are analysed using established models of drop breakage under turbulent and laminar flow regime and new method is presented for the analysis of power input data for up-and-down moving impellers. Recommendations were made to model coalescence and breakage simultaneously, to study vibromixer at high dispersed phase volume and to improve the discretization techniques so as to manipulate the grid size during simulation.

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