Drop size distributions in high holdup fraction dispersion systems: Effect of the degree of hydrolysis of PVA stabilizer

Although turbulence-stabilized dispersions are of considerable industrial importance, their behaviour is generally not well understood, especially for the high dispersed phase volume fractions encountered in suspension polymerization reactors. The dispersion is formed by two dynamic processes, namely, drop breakage and coalescence. Breakage mainly occurs in regions of high shear stress near the impeller or as a result of turbulent velocity and pressure fluctuations along the surface of a drop. Droplet coalescence is prevented by the combined action of turbulent agitation and the addition of protective colloids such as poly(vinyl alcohol) (PVA). Depending on the combination of the degree of agitation and the concentration and type of surface active agent, the average droplet size can exhibit a U-shaped variation with respect to the impeller speed or the degree of hydrolysis of the PVA stabilizer. This U-type behaviour has been confirmed both experimentally and theoretically and has been attributed to a manifestation of the balance between breakage and coalescence rates of the monomer drops. Both processes are related to the interfacial tension and its variation with time as a result of the varying polymer concentration and conformation at the monomer/water interface. In the present study, a series of experiments were carried out with a model system of 50% n-butyl chloride in water in the presence of PVA with varying degrees of hydrolysis at different agitation rates. A generalized numerical algorithm incorporating the most comprehensive models describing the breakage and coalescence processes has been used for simulating the steady-state drop size distributions in a high holdup liquid-liquid dispersion system in order to elucidate the breakage and coalescence mechanisms as a function of the droplet interfacial characteristics. It is shown that the model simulations are in satisfactory agreement with measurements of the drop size distributions over the whole range of the experimental conditions investigated.