Collision-coalescence mechanism

The collision-coalescence mechanism of particle growth discussed in this chapter is thought to control primary particle size in Hame reactors. The emphasis is on the synthesis of transition metal oxide particles, which are important in the manufacture of pigments, addili ve.s, and ceramic powders. Also discussed are the factors that determine the formation of necks between particles and particle crystallinity. As demands on product quality become more stringent, more research will be needed on particle size, unifonnity. crystallinity, and aggregate formation. 

THE COLLISION-COALESCENCE MECHANISM OF PRIMARY PARTICLE FORMATION... 

Industrial flame reactors are operated at high particle concentrations and high gas temperatures. As a result, particle collision rates are high primary particle size is determined by the relative rales of particle collision and coalescence (Ulrich, 1971). The collision/coalescence mechanism for particle formation is based on a series of steps assumed to proceed as follows ... 

Aerosol Reactors Commercial and Pilot Scale 332 Flame Reactors 332 Pyrolysis Reactors 334 Electron-Beam Dry Scrithhirif 335 Evaporation-Condensation Generators 336 The Collision-Coalescence Mechanism of Primary Particle Formation 338... 

Kapur and Fuerstenau (K6) have presented a discrete size model for the growth of the agglomerates by the random coalescence mechanism, which invariably predominates in the nuclei and transition growth regions. The basic postulates of their model are that the granules are well mixed and the collision frequency and the probability of coalescence are independent of size. The concentration of the pellets is more or less fixed by the packing... 

The effects of secondary flow on droplet collision and coalescence mechanisms have not been considered in the literature currently reviewed. The scale of secondary flow is much larger than the Kolmogorov microscale q and it will, therefore, only affect droplets of diameter d > q (those subject to turbulent inertia). Secondary flow will be most prevalent following changes in duct geometry, particularly where there is some form of duct divergence. 

Modeling and Simulation subsection.) It is necessary to determine both the mechanism and kernels which describe growth. For fine powders within the noninertial regime of growth, all collisions result in successful coalescence provided binder is present. Coalescence occurs via a random, size-independent kernel which is only a func tion of liquid loading, or... 

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,... 

Droplet collision is a phenomenon inherent in the dense region of a spray. Droplet collisions may lead to local agglomeration that affects the droplet size distribution. There have been considerable efforts in modeling droplet-droplet collisions and coalescence,12291 but the models are still not generally applicable. 1576] Moreover, the calculations in the dense region of a metal spray is much more complicated than in a diesel spray because the physical phenomena and mechanisms in the dense region are not well understood. 

Besides the already mentioned techniques, a low-temperature plasma has been adopted to enhance the reaction in CVC. Through the synthesis of AIN UFPs by an RF-plasma-enhanced CVC using trimethylaluminum [A1(CH3)3] and NH3 as reactants, the effect of experimental parameters on the rate of powder formation, particle size, and structure was examined (60). A high RF current was primarily connected to a high electron density, which activated the gas-phase reaction to promote the powder formation rate. The increase of both susceptor temperature and A1(CH3)3 concentration also increased the powder formation rate and enhanced the grain growth, where both mechanisms—coalescence by particle collision and vapor deposition on to particle surfaces—were believed to occur. 

Coalescence is the key to particle size. Any process which gathers the small droplets together to form larger ones c.m he considered forced coalescence. This can Ik- accomplished bv electrostatic fields or by conventional mechanical means such as excelsior packs or collision bailies. 

What is necessary for a collision to be followed by a coalescence What is the mechanism of coalescing These are questions that have fascinated many scientists and engineers both in the field of the physical chemistry of emulsions and foams and in the engineering field of agitated dispersions. 

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