Coalescence, process colliding drops

Drops coalesce because of coUisions and drainage of Hquid trapped between colliding drops. Therefore, coalescence frequency can be defined as the product of coUision frequency and efficiency per coUision. The coUision frequency depends on number of drops and flow parameters such as shear rate and fluid forces. The coUision efficiency is a function of Hquid drainage rate, surface forces, and attractive forces such as van der Waal s. Because dispersed phase drop size depends on physical properties which are sometimes difficult to measure, it becomes necessary to carry out laboratory experiments to define the process mixing requirements. A suitable mixing system can then be designed based on satisfying these requirements. 

Coalescence is the combining of drops either by collision or at a surface. It is desirable in some applications, while undesirable in others. It aids mass transfer and separation processes, but it is harmful to suspension and emulsion polymerization. Coalescence can occur when drops collide with one another or come to rest on surfaces and interfaces. Collisions between drops can result in either coalescence or rebounding. The impact creates transient forces that act on the colliding drops to thin the film separating them. As this film gets thinner, rupture (coalescence) occurs when the thickness reaches a critical value. If the film thickness does not reach this critical thickness during contact, the drops will depart without coalescing. 

Flow-induced coalescence is accelerated by the same factors that favor drop breakup, e.g., higher shear rates, reduced dispersed-phase viscosity, etc. Most theories start with calculation of probabilities for the drops to collide, for the liquid separating them to be squeezed out, and for the new enlarged drop to survive the parallel process of drop breakup. As a result, at dynamic equilibrium, the relations between drop diameter and independent variables can be derived. 

There are three scenarios for the behavior of two colliding particles in a dispersion (e.g., emulsion) depending on the properties of the films (Fig. 1) (1) When the film formed upon particle collision is stable, floes of attached particles can appear. (2) When the attractive interaction across the film is predominant, the film is unstable and ruptures this leads to a coalescence of the drops in emulsions or of the bubbles in foams. (3) K the repulsive forces are predominant, the two colliding particles will rebound and the colloidal dispersion will be stable. In some cases, by var3ring the electrolyte concentration or pH, it is possible to increase the repulsion between the particles in a flocculated dispersion and to cause the inverse process of peptization [1]. 

Fundamental studies have focused on the more complex film drainage step, by precisely monitoring the coalescence of a single drop at a plane interface or the interaction between two colliding drops under precisely controlled conditions. These studies elucidate the complexities of the coalescence process. 

The function of all water-treating equipment is to cause oil droplets, which exist in the water continuous phase, to float to the surface of the water. These droplets are subjected to continuous dispersion and coalescence during the trip up the wellbore through surface chokes, flowlines, control valves and process equipment. When energy is put into the system at a high rate, drops are dispersed to smaller sizes. When the energy input rate is low, small droplets collide and join together in the process of coalescence. 

There are many factors that determine whether a collision results in a coalescence. The processes by which two drops coalesce are those of film thinning and final rupture of the intervening film. These processes are determined by factors such as surfactants, mass transfer, surface tension gradients, physical properties, Van der Waals forces, and double-layer forces. In a turbulent flow field the situation is more involved.The droplets must first collide and remain in contact for a sufficient time for the coalescence to take place. A realistic coalescence efficiency will account for these factors. 

The coalescence theories are either based on equilibrium thermodynamics, or on hydrodynamics. The first ones consider quiescent systems in which the coalescence originates from minimization of the total energy, thus reduction of the interface area. These have been successful in predicting drop size in emulsions, but not so in polymer blends. The second ones consider flowing systems, in which the drops collide, compress and coalesce. Both the collision and coalescence are probabilistic processes, leading to polydispersity of drop sizes. Experimentally, the log-normal distribution of drop sizes was observed [Bordereau et al., 1992]. 

Basic Principles. Coalescence is the process of combining two or more drops to form one or more larger drops. It occurs when drops, suspended in a moving fluid, collide with one another as shown in Figure 12-14. Coalescence also occurs when drops rise or settle due to gravity to a condensed layer, as in a... 

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