The extent to which dispersed-phase viscosity influences equilibrium mean drop size and drop size distribution at constant interfacial tension is determined for dilute suspensions by dispersing silicone oils of various viscosity grades in water. A mechanistic model for mean drop size is developed which predicts the moderate-viscosity data and whose parameters correlate the high-viscosity results. Trends in the mean size data coincide with those for the drop size distribution, which broadens considerably as viscosity increases and suggests a dependency on breakage mechanism.Drops are stabilized in dilute agitated liquid-liquid systems by surface and dispersed-phase viscous forces and are broken up by forces associated with the continuous-phase turbulence. Most studies have been limited to surface force stabilized dispersions, so the extent to which dispersed-phase viscosity influences mean drop size and drop size distribution is not well understood.For inviscid dispersed phases, equilibrium mean drop sizes produced by turbulent stirred-tank contacting processes are correlated by the well-known Weber number theory. This semiempirical theory, as applied to dilute suspensions, has been extended via mechanistic arguments to account for the effect of dispersedphase viscosity. A viscosity group, N,, is introduced which accounts for the importance of dispersed-phase viscosity relative to interfacial tension. Numerous experiments were conducted in four, baffled cylindrical tanks of standard geometry, equipped with six-blade Rushton turbines, by photographically examining dilute suspensions of silicone oils in water. Five grades of oil, ranging in viscosity from about 0.1 to 10 Pa -s and exhibiting the same interfacial tension with water ([0.0378 N / m), were employed. The range of variables studied includes 13,000 < Re < 101.000. 44 < We < 1.137, and 0.065 < < 0.50 m2/s3. The objectives of the experimental program were to examine the extent to which dispersed-phase viscosity influences equilibrium mean drop size and drop size distribution at constant interfacial tension, and to determine the relevance of the predicted correlating parameters and the range of applicability of the semiempirical theory.Results are compared to the data of Chen and Middleman (1967) for inviscid dispersed phases and the limited data of Arai et al. (1977) for viscous drops. Trends in the data are interpreted in light of the relevant correlating variables, the functional form of the drop size distribution, and the mechanism of drop breakup. AIChE JournalApril 1986 Vol. 32, No. 4 CONCLUSIONS AND SIGNIFICANCEThe experimental data have been analyzed and compared to the derived correlations and to literature data for inviscid dispered phases. The following conclusions apply to the range of variables studied.1. At constant conditions of agitation, the equilibrium drop size distribution broadens considerably as dispersed-phase viscosity, pd, increases. The size of the smallest drops decreases, while their number increases. The size of the largest drops ...

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