he separation of droplet dispersions remains an important T problem in liquid extraction processes and in the treatment of liquid effluent streams. The design of liquid/liquid separators is still largely based on empirical data, as relatively little research has becn carried out on detailed investigation of coalescence within droplet dispersions and the inter-relation of this on separation efficiency. This is undoubtedly due to the complexity of the process. Extensive studies on the coalescence of single drops at a plane interface (the simplest practical case) have revealed many of the complex stages involved. Recently, by considering coalescence within a dispersion band in a gravity settler, Ilavies and Jeffreys(*s2) have developed mathematical niodels from which the dimensions of a dispersion zone can be calculated as a function of flow rate. The results of this work show that, contrary to the conclusions of Ryan et ul. (3) interdroplet coalescence does take place within a dispersion and that this contributes considerably to the separation. This is confirmed in the work of Lee and Lewis(4) using a batch system.In all extraction processes it is necessary to minimize droplet entrainment in the bulk phases from the settling zone. T o overconie this the axial velocity through the cquipment must be controlled and since the probability of entrainment is higher for small droplets, conditions in the mixing zone where the dispersion is generated, need to be carefully determined to minimize the formation of secondary emulsions.It has long been established that coalescence of a single droplet at a plane interface may proceed by way of a stepwise process giving successively sinall drops. As many as eight discrete stages have been reported ( 5 ) . In separating droplet dispersions an important question that arises is, are secondary drops formed by coalescence present in a dispersion and can thesc be adequately removed?A program of work has been carried out in these laboratories to investigate coalescence within a droplet dispersion. A glass cell 3-in. diameter, 24-in. long was used, (Figure 1). The dispersed phase was fed under gravity from a constant head tank to a perforated distributor positioned in the cell. The phase boundary between the two liquids in the cell could be controlled accurately at any position. Small side arms were fitted into the cell to allow the interface between the phases to be removed. The cell was fitted with a T' junction at one end.This was sealed with an optical glass surface. A mirror inclined at 45" was fitted into the cell which enabled the phase boundary between the two liquids to be viewed and photographed. Photographs of the droplet dispersion at the phase boundary were MWOR Figure 1-Apparatus.simultaneously taken from side elevation through a travelling microscope.In carrying out experiments to determine the factors affecting the depth of the droplet dispersion bed at the phase boundary thc presence of small droplets in the bed was detected. (Figure 2.) Exhaustive tests confirmed that the...
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