Precipitation without primary nucleation

Let us consider the situation that reaction (6.1) is relatively slow, or that the solublility of the reaction product P is moderate. The reaction is carried out in a continuous stirred reactor, filled with a slurry of F-particles. The relative supersaturation is so low, that there is no primary nucleation and all product is deposited on the existing F-surface. To maintain a steady state, small P-particles (nuclei) have to be added continuously, or they have to be formed by secondary nucleation, i.e. by rupture of formed particles. The number of small particles that are added or generated, per unit time, must be equal to the number of product particles that are removed from the reactor with the product flow. Let us further assume that the conversion in the reactor is practically complete. The reaction rate, which is given by the feed rate of the reactants, must equal the mass transfer rate to the particles  

When the specific reaction rate and the number of particles per unit volume of the slurry are given, the volume fraction and the particle diameter follow. Since the mass transfer coefficient is determined by the particle size, the physical properties of the system and the process conditions (see section 45,13)y the supersaturation follows. This shows how conditions can be chosen so that primary nucleation is excluded. 

Such a precipitation process can be controlled accurately when also secondary nucleation is in fact excluded, and when a calculated amount of nuclei is fed to the reactor. When the diameter of these nuclei is much larger than the Kolmogorov microscale, aggregation is likely to be excluded too, so that particles may grow by crystal growth only. This would result in massive particles. Particle size control could be very effective. 

Sometimes a fluidized bed of particles is used for this type of precipitation process see sections 4.5,13 and 5.4.4. 

The chemical desposition of solid reactor products on existing walls is rather uncommon, except in electrodeposition. When a metal is precipitated on a flat electrode, the current density is usually chosen so that die deposition rate is completely determined by mass transfer limiting current density), see sections 43.2.2 and 11.2.4. 

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