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Title: Breakage of drops in two-liquid phase dispersions in mechanically agitated vessels
Author: Boye, Andrew Martey
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 1997
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Measurements of drop size distributions were made for a range of xylene-water and sunflower oil-water dispersions in a 0.15 m diameter unbaffled cylindrical vessel, mechanically agitated with a range of standard Rushton turbine impellers. The effects of dispersed phase concentration, viscosity of the two-phase liquid mixture, impeller speed and diameter on drop size distribution were investigated. All dispersions contained 0.3%w/w of the surfactant. Sodium Dodecyl Sulphate (SDS) used to minimise the effect of drop coalescence during the experiments. Experimental evidence for xylene-water dispersions showed that as the dispersed phase concentration increased beyond 50% by volume, the physical properties of the dispersion changed, notably the rheological properties of the dispersion deviated from Newtonian behaviour and became shear thinning; the apparent viscosity of the two-phase liquid mixture increased exponentially with dispersed phase concentration. Measurements of the electrical conductivity and visual assessment of the dispersion using a photomicrography technique showed no evidence for the occurrence of global phase inversion during the experiments. The evidence however indicated that the structure of the dispersion changed progressively as the dispersed phase concentration was increased. For xylene phase concentration greater than 50% by volume, the bulk flow in the vessel was non-turbulent (Re < 10000) and the average drop diameter was successfully described using a model developed in this thesis based on shear breakage in the boundary layer flow next to the rotating disk of the impeller. For xylene phase concentration less than 50% by volume, the rheology of the dispersion was primarily determined by the viscosity of the continuous phase (water) and drop size distributions were described well by the established equations based on Kolmogoroff's (1949) theory of isotropic turbulence. The drop size distribution data obtained for sunflower oil-water dispersions were more erratic and less reproducible compared with the xylene-water system, especially for sunflower oil concentrations greater than 50% by volume. However, the general trend in data was similar for both dispersions. The lack of reproducibility and conformity of drop size distribution data for sunflower oil-water dispersions with the model was largely attributed to the flow regime remaining predominantly turbulent and the possible presence of small amounts of contaminants present in the low-grade food oil samples purchased locally for these experiments.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available