T h e coalescence rates of equisized drops of organic liquids fluidized by water under conditions of mutual saturation were studied. A drop-size separation technique permitted the direct count of the coalesced drops. Six organic liquids (benzene, cyclohexane, n-butylacetate, methyl isobutyl ketone, diisobutyl carbinol, and tetrahydrobenzaldehyde) were used, and the coalescence was in a l l cases total, that is, without formation of small satellite drops.A new hydrodynamic model i s presented to calculate the parameters governing the drainage of liquid between two deformable drops in head-on collision. A parameter from this model permits correlation of the coalescence rates with only two arbitrary constants, provided the ratio of dispersed to continuous liquid viscosities lies between 0.5 and 2. High viscosity ratios or the presence of trace quantities of surfactant greatly depress the coalescence rate, while continued reuse of the dispersed liquid leads to a statistically significant reduction. The presence of a fog of very small droplets increases the coalescence rate.In recent years evidence has been accumulating that the coalescence of drops in a liquid-liquid dispersion has a strong influence on the performance of liquid-extraction devices. For example, coalescence has been shown to be detrimental to the performance of spray towers (1 ). On the other hand, coalescence and redispersion of the drops in an agitated vessel are suspected of being responsible for enhanced mass transfer rates (20).extensive work on the time of coalescence of single drops at a flat interface, recently reviewed ( 4 , 14), has resulted in only one proposed correlation (13), which i s totally empirical and involves many arbitrary constants. Quantitative studies for multiple-drop systems have been confined to agitated vessels (8,10,13, 17,18). The techniques used usually involved some type of drop tracer study requiring an indirect method of observation and contamination of the liquid system with a tracer. This makes interpretation of the results difficult, since the coalescence phenomenon i s extremely sensitive to contaminants. The only correlation relating coalescence rate to all physical properties is that of Hillestad and Rushton ( l o ) , which uses many arbitrary constants. Only qualitative information i s available for spray towers (7, 15).In the present study, the coalescence rates for a swarm of uniformly sized drops maintained in a fluidized bed were studied. A drop-size segregation technique was used, so that newly formed coalesced drops were quickly removed from the bed, at a rate determined by direct counting. It i s believed that the conditions of measurement exerted a minimal influence on the phenomenon.The coalescence phenomenon i s poorly understood. The APPARATUS AND PROCEDUREThe vertical glass tube of Figure 1 consists of two tubes of uniform but different diameters joined by a tapered section. The continuous liquid enters from the top and flows downward. The dispersed phase, consisting of uniform, small drops, enters...