Coalescence, effect fine solids

However, as described in Chapter 5, because the aggregation status of liquid is different from that of solid, both re-atomization and coalescence are possible in gas-liquid impinging streams with liquid as the dispersed phase. This introduces some complicated uncertainty factors. According to the results obtained, it is uncertain whether droplet re-atomization increases the interface area for transfer, because coalescence of fine droplets decreasing the interface area counteracts or even exceeds the positive effect of re-atomization. More possibly, a negative effect may be obtained, i.e., the interface area may be reduced to a certain degree. 

Other Considerations. The presence of a third phase can affect liquid-liquid dispersion and coalescence. Fine solids have little effect on drop dispersion but often affect coalescence. Gas bubbles affect dispersion by reducing the effective continnons phase viscosity and lead to a loss in momentum transport, hence dispersion capability. Tiny gas bubbles reduce probability of coalescence by interfering with film drainage rates between colliding drops. This subject is complex and is best stndied experimentally at different scales. 

Very finely disperse solids, which are adsorbed at the liquid/liquid interfaces, forming films of particles around the disperse globules. Certain powders can very effectively stabilize against coalescence. The solid s particle size must be very small compared with the emulsion droplet size and must exhibit an appropriate angle of contact at the three-phase (oil/water/solid) boundary [141]. 

Very fine dispersions can be separated effectively with disk-type centrifuges. Commercial units have capacities of 5-500 gpm and are capable of removing water from hydrocarbons down to the ppm range. A mild centrifugal action is achieved in hydrocyclones. They have been studied for liquid-liquid separation by Sheng, Welker, and Sliepcevich (1974), but their effectiveness was found only modest. The use of hydrocyclones primarily for the recovery of solid particles from liquids is described in the book of Bradley (1965). A symposium on coalescence has papers by Belk (1965), Jordan (1965), Landis (1965), and Waterman (1965). 

Yet another mode of foam stabilisation is by finely divided, hydrophobic solid particles such as coal-dust or particles of metal soaps. Their effectiveness arises from their aggregation at. the film surfaces a closely packed layer of solid particles prevents coalescence. However, the phenomenon is very dependent on the surface properties of the solid, since it is necessary for the contact angles to be such that the particles are held in the film. The details of the mechanism have not been fully established, but Figure 12.15(a) suggests a possible configuration leading to stabilisation. 

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