Measurement and modelling of drop charge effects on coalescence in agitated liquid-liquid dispersions

Thomas G Tobin, Purdue University

Abstract

As a means of integrating detailed theoretical considerations of turbulent drop coalescence with experimental studies, this work puts forth a three-fold methodology of analysis which has at its core the measurement of drop size distribution changes in turbulent dispersions under conditions in which drop coalescence predominates drop breakup. As an aid to this analysis process, a new scheme for discretizing the population balance equation for a coalescing system is formulated in this work. This new method, which is a generalization of the binary method first proposed by Batterham in 1981, circumvents the problem of an excessive number of solution equations associated with the common linear-discretization method, while retaining full accuracy of the solution result. A primary result of this thesis is the demonstration of the essential need to consider drop charge effects on turbulent drop coalescence whenever system ionic strengths lie below $\sim$0.1 M. Experimental studies presented here clearly reveal the strength and nature of such effects on the coalescence process however, and suggest promising ways in which pH and electrolyte concentration might be manipulated in order to control drop size distributions. Mechanistic models of drop coalescence based upon a stochastic film-drainage analysis are developed to describe the dual effects of drop surface potential and system electrolyte concentration on coalescence, as well as the effects of previously-known factors such as turbulence intensity and continuous phase viscosity. These analyses account for all size-dependent effects, including that of drop deformability. From the equations describing the stochastic film drainage process, an approximate expression is proposed as an explicit equation for drop coalescence efficiency for dispersions containing either rigid or deformable drops, or some combination of the two. Results indicate that drop deformation is an extremely important factor to consider when interfacial tensions are low ($\sim$17 dynes/cm), but that its effects are less substantial in systems having larger interfacial tensions. In all cases studied, the effect of drop charge can be seen either through a reduction of large-drop coalescence rates, a reduction of very small drop coalescence rates, or both. The effect of large ionic strengths is to markedly promote the coalescence rate between the largest drops in a system, with the result that such dispersions typically exhibit a rapid skewing of the distribution under transient (coalescing) conditions. (Abstract shortened by UMI.)

Degree

Ph.D.

Advisors

Ramkrishna, Purdue University.

Subject Area

Chemical engineering

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