Coalescence between particles

The level of coalescence between particles, the size of the particles, and the packing arrangement dictate the size of air cavities and, thus, the size of the bubble initially formed in the melt. Once formed, the bubbles remain stationary in the melt. A relatively small bubble diameter, combined with the high viscosity of the melt, prevents the movement of the bubbles into the melt. The bubble removal is known to be a diffusion-controlled process. The identification of key parameters in the dissolution of bubbles formed in the melt has been done using a theoretical model that describes this process. The disappearance of the air bubble formed into the melt was modeled based on... 

Inspite of the obvious, good conditions for agglomeration, a high probability of coalescence between particles, it took several decades and the beginning of interdisciplinary evaluation of processes before, by design, fluidized and spouted beds were utilized for agglomeration [B.42, B.57]. 

Figure 4.22. Mechanism of aggregation of silica particles into sheets. Particles can approach the edges of a sheet of particles where the repulsion energy barrier is less than on the face. Coalescence between particles in sheet is shown in cross-section.

Wet scrubbers can have a high efficiency for small particle sizes, especially the venturi t3rpe scrubbers, where the flow is subjected to strong acceleration—a condition that encourages efficient coalescence between particles and droplets. In wet scrubbers, the pressure drop remains constant (except in some relatively rare cases where calcium or other deposits may accumulate in the throat area of the scrubber). Degradation of the separation medium is normally not a problem. However, most scrubbers recycle most of the scrubbing water and the efficiency of the scrubber will suffer if the solids or other... 

Figure 2 illustrates a proposed growth process[3] of a polyhedral nanoparticle, along with a nanotube. First, carbon neutrals (C and C2) and ions (C )[16] deposit, and then coagulate with each other to form small clusters on the surface of the cathode. Through an accretion of carbon atoms and coalescence between clusters, clusters grow up to particles with the size fi-... 

Vaux (1978), Ulerich et al. (1980) and Vaux and Schruben (1983) proposed a mechanical model of bubble-induced attrition based on the kinetic energy of particles agitated by the bubble motion. Since the bubble velocity increases with bed height due to bubble coalescence, the collision force between particles increases with bed height as well. The authors conclude that the rate of bubble-induced attrition, Rbub, is then proportional to the product of excess gas velocity and bed mass or bed height, respectively,... 

In addition, DNS of turbulent flow in a periodic box offer interesting opportunities for studying in a fully resolved mode the intimate details of the flow field, its interaction with particles and the mutual interaction between particles (including particle-particle collisions and coalescence). Such simulations may yield new insights see, e.g., Ten Cate et al. (2004) and Derksen (2006b). The same can be said about our understanding of particle-turbulence interactions in wall-bounded flows this has increased due to Portela and Oliemans (2003) exploiting both DNS and LES and due to Ten Cate et al. (2004). 

Figure 5.21. Possible scenarios for partial coalescence, (a) Crystallization induced by contact between soUd particle and undercooled droplet, (b) Partial coalescence between two semiUquid droplets.

On the assumption of a somewhat mobile double layer, electrostatic attraction between particles may occur, due to this displacement effect, even if the total net charge be not zero, i.e. coagulation may take place before the isoelectric point is reached. The data of Zsigmondy on gold particles, and of Powis Zeit. Phys. Ohem. Lxxxix 186, 1915) on oil particles, have indeed shown that the optimum point for precipitation is not actually at the isoelectric point, although in the case of gold, practically complete discharge of the double layer had to take place before coalescence. 

During the subsequent growth step each nuclei gives rise to one nonporous final particle without any coalescence between the nuclei or the growing particles. 

From the data listed in Table 13.5 it can be seen that the Sauter mean diameter of the dried product, di2, is larger than that of the wet precipitate obtained under the same reaction conditions by about 10%, or by 0.15 pm. An obvious fact is that no matter whether at the bottom of the dryer or in the cyclone or in the bag filter, the recovery of the finer particles must be lower than that of the larger particles. These differences between the recoveries of particles of different sizes must lead to an increased mean diameter of the product. If this fact is taken into account, the sizes of the particles can be considered to be stable enough during the final treatment of the precipitate, without coalescence of particles occurring. 

At the critical point, sterically stabilized particles are formed, and coalescence between similar-sized particles is terminated. At this point one has R=... 

According to Paine [129], computer simulations using the multibin kinetic model for the coalescence between the unstable moieties indicate that the particle number (N) at the critical point is given by... 

Figure 6. Mixing in a flow reactor with two unmixed feedstreams (60). Left, Maximum age, maximum species mixedness middle, minimum age, maximum species mixedness (mixing in this case can also he assumed to occur by molecular diffusion between two tubes having the same and (T9.)) and right, mixing between particles of the same life expectancy by a random-coalescence process (77).

Products comprising hydrophilic polymers dissolved in water are well-known and used widely as adhesives but are of little general significance for bonding plastics. The present chapter is concerned only with products based on polymer dispersions, which consist of small discrete particles of diameter about one micron (1 pm, or 10-3 mm) suspended in a continuous water phase. In most instances a protective colloid is present at the interface between the particles of polymer and the water and this helps to stabilize the dispersion and prevent premature coalescence of particles. Dispersions such as these are known as oil-in-water types. With them, the molar mass of the polymer species comprising the dispersed particles does not affect the viscosity and so polymers of high molecular weight can be applied in this way. 

Two modes of melt agglomeration based on the elementary mechanisms have been proposed—distribution and immersion. In agglomeration by the distribution mode, a distribution of molten binding liquid on the surfaces of primary particles will occur, and agglomerates are formed via coalescence between the wetted nuclei (Fig. 3). In agglomeration by the immersion mode, nuclei are formed by immersion of the primary particles onto the surface of a droplet of molten binding liquid (Fig. 3). The distribution of molten binding liquid to surfaces of... 

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