Coalescence preferential

Holdup and Flooding At this point it is useful to introduce the concepts of holdup and flooding in column contactors. It is normal practice to select the phase which preferentially wets the internals of the column as the continuous phase. This then allows the dispersed phase to exist as discrete droplets within the column. If the dispersed phase were to preferentially wet the internals, this could cause the dispersion to prematurely coalesce and pass through the column as rivulets or streams which would decrease interfacial area and therefore column efficiency. 

If a hydrophobic solid is suspended in water and a small amount of oil is added to the system, the oil will preferentially wet out the solid. If a sufficient amount of oil is added, the oil films are brought together and coalescence (a first-order reaction) and agglomeration (a second-order reaction) take place. 

Finely divided solid particles that are wetted to some degree by both oil and water can also act as emulsifying agents. This results from the fact that they can form a particulate film around dispersed droplets, preventing coalescence. Powders that are wetted preferentially by water form O/W emulsions, whereas those more easily wetted by oil form W/O emulsions. The compounds most frequently used in pharmacy are colloidal clays, such as bentonite (aluminum silicate) and veegum (magnesium aluminum silicate). These compounds tend to be adsorbed at the interface and also increase the viscosity of the aqueous phase. They are frequently used in conjunction with a surfactant for external purposes, such as lotions or creams. 

An empty vessel may be employed, but horizontal baffles can be used to reduce turbulence and assist the coalescence through preferential wetting of the solid surface by the disperse phase. More elaborate methods to assist the coalescence include the use of mesh pads in the vessel or the use of an electric field to promote coalescence. Chemical additives can also be used to promote coalescence. 

Thus far it has not been possible to derive from the first principles the coalescence rate function for preferential combination of pellet species of different sizes. Kapur (K4) has proposed an ad hoc rate function in continuous sample space as follows ... 

This means that the coalescence occurs preferentially between small granules at least for the limestone system under scrutiny. Equation (83) now becomes... 

Demulsihcation by induced coalescence requires the rupture of the protecting him as the emulsion hows through the small passages in the hbrous media. Coalescence of the dispersed phase is then possible because of the preferential oil-wetting characteristics of the media surface. 

Coalescence on a solid wetted preferentially by the organic phase... 

Two suggested ways to explain the continuous rapid supply of the more active or less noble metal give rise to two models of dissolution. One of them proposes that the less noble element is preferentially dissolved. The remaining more noble element is now in a highly disordered state and begins to reorder by surface diffusion and nucleation of islands of almost pure noble metal. The coalescence of these islands continues to expose fresh alloy surface where further dissolution will occur, leading to the formation of tunnels and pits. (Corcoran)5... 

An aging treatment results in a partial coalescence of particles and a strengthening of the network occurs. At the neck joining the particles there is a negative radius of curvature. Thus, local solubility at the neck is less than near the particle surface. Therefore, transport and deposition of silica occur preferentially to the neck region and neck thickening results. This results in a strengthening of the particulate network, Fig.7.5 (Zarzycki etal, 1982). 

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