Particle formation supersaturation

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. 

Secondary nucleation is an important particle formation process in industrial crystallizers. Secondary nucleation occurs because of the presence of existing crystals. In industrial crystallizers, existing crystals in suspension induce the formation of attrition-like smaller particles and effectively enhance the nucleation rate. This process has some similarity with attrition but differs in one important respect it occurs in the presence of a supersaturated solution. 

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. 

If one chooses a chelating agent with a relatively high stability constant, such as EDTA, one may find some ideal separation of nucleation and growth stages as a decisive prerequisite for monodispersed particles formation, since a drastic change of supersaturation can be expected in a system with metal ions around the stoichio-... 

Figure 12.2. Particle-formation criteria L is the nozzle length, D is the nozzle internal diameter, S is the supersaturation, J is the nucleation rate, iRESS is the characteristic time of the process (the residence time of the solution in the nozzle), and Tlife is the lifetime of the supersaturated solution in a homogeneous state.

In precipitation, particle formation is extremely fast due to high supersaturations which in turn lead to fast nucleation. At least in the beginning, size distributions are narrow with particle sizes around one 1 nm. Nanomilling in stirred media mills is characterized by relatively slow particle formation kinetics, particle sizes ranging from several microns down to 10 nm and high sohds volume concentrations of up to 40%. Large particles may scavenge the fine fractions. The evolution of the particle size distribution can be described for both cases by population balance equations (Eq. (7)),... 

If particles (or ions) are already present in a supersaturated vapor, nucleation will take place preferentially on these particles at supersaturations far smaller than for the homogeneous vapor. In this case, nucleation takes place heterogeneously on the existing nuclei at a rate dependent on the free energy of a condensate cap forming on or around the nucleus. Heterogeneous nuclei always occur in the earth s atmosphere. They are crucial to the formation of water clouds and to the formation of ice particles in supercooled clouds. 

Another process involves molecular aggregation by means of direct chemical reactions akin to polymerization. The best known example of this is the process of carbon particles in a premixed acetylene-oxygen flame. Evidently particle formation in this case does not involve condensation from a supersaturated vapor, but proceeds directly through the pyrolysis of the acetylene, forming in the process unstable polyacetylenes as intermediates in the flame. 

In contrast to more or less well defined kinetics of the crystal growth (5,6,12-16), various nucleation mechanisms have been proposed as zeolite particles forming processes. Most authors explained the formation of primary zeolite particles by nucleation in the liquid phase supersaturated with soluble silicate, aluminate and/or aluminosilicate species (1,3,5,7,16-22), with homogeneous nucleation (1,5,7,17,22), heterogeneous nucleation (5,2 1), cell walls nucleation (16) and secondary nucleation (5) as dominant processes of zeolite particles formation, but the concepts dealing with the nucleation in the gel phase are also presented in the literature (2,6,11,12,1 1,23-25). 

Crystallization is the formation of solid particles from supersaturated liquid solution. Supersaturation is produced by the following ways. 

Particle formation events from gaseous precursors are observed frequently almost everywhere in the troposphere, both in polluted cities and remote clean areas [4]. It is likely that different nucleation mechanisms are at work in different conditions, but no formation mechanism has been identified so far. It is, however, clear that particles are formed by nucleation of a multicomponent vapor mixture. Water vapor is the most abundant condensable gas in the atmosphere, but it can not form particles on its own homogeneous nucleation requires such a high supersaturation, that heterogeneous nucleation on omnipresent pre-existing particles always starts first and consumes the vapor. However, vapor that is un-... 

The RESS and PGSS techniques involve two steps continuous dissolution of the solid solute in the SCF solvent or vice versa, and rapid depressurization of the solution through a nozzle and particle formation by nucleation induced by a uniform and a very high degree of supersaturation. However GAS is operated on a semibatch mode in which the solution is taken in a high-pressure crystallizer through which CO2 is passed continuously for some time,... 

The optical and scanning electron micrographs presented in this chapter show that the particle size of solid materials, such as polymers, monomers, and intermediate chemicals, can be altered by precipitation from a supercritical fluid solution. The only requirement for carrying out the SCF particle reduction process is that the compound must exhibit some solubility in a supercritical fluid. Because the pressure reduction rates are so rapid during the expansion of the solution, supersaturation ratios can be achieved that are much, much greater than can be achieved by thermal, chemical, or antisolvent precipitation processes. Furthermore, it is conjectured that such rapid nucleation rates can result in the particle formation of some materials with a size distribution or morphology that cannot now be achieved by any other process. 

La Mer s qualitative interpretation of sulfur particle formation mechanism can be understood from the diagram shown in Fig. IV-13. The concentration of the molecularly dispersed sulfur formed in the above reaction slowly increases until critical supersaturation is reached. At that point nucleation and the formation of solid phase embryos take place. 

With high solubilities of SCFs in organic solvent, a volume expansion occurs when the two fluids make contact, leading to a reduction in solvent density and parallel fall in solvent capacity. Such reductions cause increased levels of supersaturation, solute nucleation, and particle formation. This process, generally termed gas antisolvent recrystallization, thus crystallizes solutes that are insoluble in SCFs from liquid solutions, with the SCF, typically SCF CO2, acting as an antisolvent for the solute. The GAS process was initially developed for crystallizing explosive materialsP. ... 

It is also possible to consider particle formation as the result of the aggregation of ohgomers of different chain lengths. In this case, the total supersaturation of the ohgomers forming the aggregate can be expressed as ... 

Once nucleation takes place, remaining supersaturation can be used for the condensation, or reaction of vapor-phase molecules, resulting in the reaction of initiating the particle formation toward growth phase having high chemical purity... 

When creating supersaturation levels sufficient to induce particle formation, precipitation of sparingly soluble salts and sol-gel processes are viewed differently. Precipitation normally involves mixing a cation solution with a precipitant solution. For example, consider preparation of an oxalate precursor to a CoO- and MnO-doped ZnO powder. In this process, the Zn, Mn, and Co are coprecipitated with controlled stoichiometry and the precipitate is calcined to the oxide. To form the oxalate, a state of supersaturation is created by mixing an aqueous solution of the metal nitrates or chlorides with an oxalate precipitant solution. The system is supersaturated with respect to the different metal oxalate phases and a crystalline coprecipitate forms. Depending on precipitation conditions (pH, concentrations, temperature, etc.), different metal complexes are present in solution. The form and concentration of these complexes determine the phase, morphology, and particle size distribution of the resulting precipitate. 

When the supercritical fluid and drug solution make contact, a volume expansion occurs leading to a reduction in solvent capacity, increase in solute saturation, and then supersaturation with associated nucleation and particle formation. A number of advantages are claimed by using this platform technology (6), such as particle formation from nanometers to tens of micrometers, low residual solvent levels in products, preparation of polymorphic forms of drug, etc. 

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