Supersaturation agglomeration

Nucleation of particles in a very short time foUowed by growth without supersaturation yields monodispersed coUoidal oxide particles that resist agglomeration (9,10). A large range of coUoidal powders having controUed size and morphologies have been produced using these concepts (3,14). 

A secondary particle formation process, which can increase crystal size dramatically, is crystal agglomeration. This process is particularly prevalent in systems exhibiting high levels of supersaturation, such as from precipitation reactions, and is considered along with its opposite viz. particle disruption in Chapter 6. Such high levels of supersaturation can markedly accentuate the effects of spatial variations due to imperfect mixing within a crystallizer. This aspect is considered further in Chapter 8. 

Thus in a mixed system, as e.g. in a stirred tank, the rate of agglomeration additionally depends on the shear field and therefore on the energy dissipation e in the vessel. Furthermore, in precipitation systems solution supersaturation plays an important role, as the higher the supersaturation, the stickier the particles and the easier they agglomerate (Mullin, 2001). This leads to a general formulation of the agglomeration rate... 

Agglomeration rates also depend on the level of supersaturation in the reaetor and on the power input. Wojeik and Jones (1997) found a linear inerease of the agglomeration kernel with the growth rate. Therefore, the level of supersaturation was aeeounted for by Zauner and Jones (2000a) using the relation... 

In all such laboratory studies, plant conditions and compositions should be employed as far as possible. Agglomeration rates tend to increase with the level of supersaturation, suspension density and particle size (each of which will, of course, be related but the effects may exhibit maxima). Thus, agglomeration may often be reduced by operation at low levels of supersaturation e.g. by controlled operation of a batch crystallization or precipitation, and the prudent use of seeding. Agglomeration is generally more predominant in precipitation in which supersaturation levels are often very high rather than in crystallization in which the supersaturation levels are comparatively low. 

To reduce or avoid agglomeration removal of certain impurities such as charged polymers could be effective - once identified, while to enhance agglomeration certain additives could be used as are commonly employed as flocculating agents to enhance solid-liquid separation in the water industry for example. These effects have to be determined empirically with care, however, since they can be pH and supersaturation dependent. 

Furthermore, the agglomeration and disruption kernels are also assumed to depend on the supersaturation in power law form (Zauner and Jones, 2000a)... 

Each stage of particle formation is controlled variously by the type of reactor, i.e. gas-liquid contacting apparatus. Gas-liquid mass transfer phenomena determine the level of solute supersaturation and its spatial distribution in the liquid phase the counterpart role in liquid-liquid reaction systems may be played by micromixing phenomena. The agglomeration and subsequent ageing processes are likely to be affected by the flow dynamics such as motion of the suspension of solids and the fluid shear stress distribution. Thus, the choice of reactor is of substantial importance for the tailoring of product quality as well as for production efficiency. 

The same applies to crystallizers, in which particle sizes and particle number concentrations not only depend on nucleation and growth from supersaturated mother liquid, but are also affected by shear-dominated agglomeration and by secondary nucleation as a result of particle particle and particle-impeller collisions. Some of the subprocesses involved may be limited to specific and different parts of the vessel e.g., nucleation may be restricted to a flame-like region around the outlet of a feed pipe (Van Leeuwen, 1998). In addition, in... 

Condensation is generally a transient operation in which, as discussed by Ueda and Takashima(106), simultaneous heat and mass transfer are further complicated by the effects of spontaneous condensation in the bulk gaseous phase. After the creation of supersaturation in the vapour phase, nucleation normally occurs which may be homogeneous in special circumstances, but more usually heterogeneous. This process is followed by both crystal growth and agglomeration which lead to the formation of the final crystal product. As a rate process, the condensation of solids from vapours is less well understood than vaporisation(98). Strickland-Constable(107) has described a simple laboratory technique... 

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