Process of coalescence

Particles attract whenever they approach to within a minimum distance. Whatever the magnitude of the interparticle attraction, energetic molecules will separate and continue moving after their encounter but, conversely, molecules of lower energy do not separate after the collision because the attraction force is enough to overwhelm the momentum that would cause the particles to bounce apart. The process of coalescence has begun. 

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. 

Coalescence. The process of coalescence in water-treating systems is more time dependent than dispersion. In dispersion of two immiscible liquids, immediate coalescence seldom occurs when two droplets collide. If the droplet pair is exposed to turbulent pressure fluctuations, and the kinetic energy of oscillations induced in the coalescing droplet pair is larger than the energy of adhesion between them, contact will be broken before coalescence is completed. 

The processes of coalescence and diffusion gas transfer accelerate drainage initiation. 

Bubble flow-slug flow transition. Transition from dispersed bubbles to slugs requires a process of coalescence. As the gas flow rate is increased, the bubble density increases. This closer bubble spacing results in an increase in agglomeration. However, increased liquid flow rate can cause a breakup of larger bubbles, and this might be sufficient to prevent the transition. The maximum bubble void fraction at which the transition happens is around 0.25 (see Refs. 5 and 3). 

Film formation of PUDs involves coalescence of the particles and the formation of a continuous film. A PUD particle has a hydrophilic shell and a hydrophobic core. Upon film formation, there is a very slow process of coalescence on a molecular level whereby there is a physical barrier to polymer chain mixing. The resultant coatings have hydrophilic and hydrophobic regions, which create a more water-permeable film (Chainey etaL, 1985). 

The exact mechanism of inversion remains unclear, although obviously some processes of coalescence and dispersion are involved. In the region of the inversion point multiple emulsions may be encountered. The process is also not always exactly reversible. That is, hysteresis may occur if the inversion point is approached from different sides of the composition scale. Figure 18 shows the irreversible inversion of a diluted bitumen-in-water emulsion brought about by the application of shear (60). 

The process of coalescing PEI from its inclusion compound with y-CD has resulted in the extension and parallelization of PEI chains during the formation of the inclusion compound, which do not completely disappear after the removal of... 

When two drops first come into contact in the process of coalescing, a film of continuous phase becomes trapped between them. The film is compressed at the point of encounter until it drains away and the two drops can merge. Decreasing the viscosity of the continuous phase, by heating or by addition of a low-viscosity diluent, may promote drop coalescence by increasing the rate of film drainage. Surface-active impurities or surfactants, when present, also can affect the coalescence rate, by accumulating at the surface of the drop. Surfactants tend to stabihze the film and reduce coalescence rates. Fine... 

Fig. 6 Basic PHP structures (A) primary pores with large interconnecting holes (B) primary pores with nanosized interconnecting holes (C) large coalescence pores (three such pores are partially shown) dispersed into the primary pores in the process of coalescence and (D) detail of the coalescence pores. Note that these pore structures can be prepared over a wide size range.

An interesting implication of a colloidal model as suggested by Pratt for UF resins lies in the possible structures that may result from the hardened coalesced material. If colloidal particles do form oligomeric UF condensates, the process of coalescing should be ordered in a systematic way. 

If the polydispersity of bubbles generated in air-dissolved flotation or electroflotation is high, there is no need for additional introduction of centimicron bubbles. Optimal flow of two-stage flotation corresponds to the maximum attainable degree of monodispersity of bubbles. In this case the ratio between volume fractions of micro- and macrobubbles and collision efficiencies of the processes of particle capture by small bubbles and bubble coagulation must be such that the particle capture process outweighs the process of coalescence. 

Since a reduction in the bubble spectrum is connected to additional difficulties and polydispersity of bubbles in electroflotation and air-dissolved flotation cannot be therefore avoided, it is important to know the effect of polydispersity. This effect should not be too high since the process of particle capture should be ahead of the process of coalescence. It should also not be too low so large bubbles can capture small bubbles in a moderate time. Unfortunately, such a scheme which appears convincing at a first glance means a slow rate of coalescence of small bubbles. The coalescence can proceed faster than particle capture, so that the intensification of capture becomes very important. Hence it is necessary to combine introduction of small bubbles with aggregation of particles. 

Mechanisms of Single-Foam Film Stability. Soap bubbles and soap films have been the focus of scientific interest since the days of Hooke and Newton (2—9). The stability and structure of foams are determined primarily by the relative rate of coalescence of the dispersed gas bubbles (10). The process of coalescence in foams is controlled by the thinning and rupture of the foam films separating the air bubbles. Experimental observations suggest that the lifetime (stability) of foam films is determined primarily by the thinning time rather than by the rupture time. Hence, if the approaching bubbles have equal size, the process of coalescence can be split into three stages ... 

The Ru/HDA particles are prepared by decomposition of the mthenium precursor Ru(COD)(COT) with 3 bar of dihydrogen at 293 K in the presence of 0.2eq HDA in THF. The particles are monocrystalline, as evidenced by HREM analysis (Fig. 18.14) and a careful observation of some electron micrographs reveals the presence of spherical nanoparticles which are in the process of coalescing. The size could be determined as about 1.9 nm. 

A large disparity exists between knowledge concerning kinetic stability and thermodynamic stability. The main attention has been paid to kinetic stability for both macroemulsions (16-22) and miniemulsions (23-30). As a result, the droplet-droplet interaction and the collective processes in dilute emulsions are quantified (38, 39) and important experimental investigations are made (27, 28, 40). Some models are elaborated for the entire process of coalescence in concentrated emulsions as well (41, 42). Given thermodynamic stability, a thin interdroplet film can be metastable. 

Figure 22 The total number of constituent drops in a flocculating emulsion, decreases with time, t, because of a parallel process of coalescence. The curves are calcualted for the following parameter values initial number of constituent drops iiq = lO cm coalescence rate constant F = 10 s h Curve 1 is a numbeiical solution to Eq. (121) Curves 2 and 3 are the results predicted by the models of Bor-wankar et al. (194) and van den Tempel (193), respectively. The values of the flocculation rate constant are (a) ar= 10 " cmVs (b) ar=...

Next, Professor Dukhin et al. contribute a ehapter dealing with fundamental processes in dilute 0/W emulsions. A basic problem is to couple the processes of coalescence and flocculation by introducing a reversible flocculation, i.e., a process whereby the floe is disintegrated into individual droplets. The authors have utilized video-enhanced microscopy (VEM) to study the emulsified systems and to determine critical time eonstants for a stepwise flocculation/deflocculation and coalescence. 

---