Factors affecting coalescence frequency of droplets in a stirred liquid‐liquid dispersion

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Abstract
Liquid‐liquid dispersions are commonly used for separations and reaction systems, because the added interfacial area due to dispersion aids in mass transfer and chemical reaction rates. Transient coalescence experiments have been performed for a wide range of dispersed‐phase fractions and impeller speeds. A similarity transformation of the experimental drop‐size distribution is possible for most of the experiments. The drop‐size‐dependent coalescence frequency is obtained from an inverse problem procedure previously developed. Over the range of drop sizes encountered in our experiments, the coalescence frequency increases with increases in the size of the coalescing drops, the dispersed‐phase fraction, and the impeller speed. For a dispersephase system of neutrally buoyant droplets of benzene/carbon tetrachloride in water, the coalescence frequency is K(v, v′)= 3.72 (10−3)ϕ1.42N(v1/2+v′1/2) as a function of dispersed‐phase fraction, ϕ, impeller speed, N2, and volumes of the coalescing droplets, v and v′. The results are compared with previous models for the coalescence frequency. Models predicting an eventual decrease in coalescence rates with drop size are disqualified unless this decrease occurs beyond the sizes observed in our experiments.