Nucleation suppression

Growth rate kinetics for most of the stereoisomer resolution processes run at Merck have been essentially first-order with respect to supersaturation, although there have been exceptions (see Chapter 11). For one of the stereoisomer separation systems, the (first-order) growth rate constant was measured for both the stirred tank (CSTR) and fluidized bed flow systems, with and without a nucleation suppressant. The data are shown in Table 7-5. 

In this case at least, the fluidized bed system showed considerably faster growth rate kinetics than the stirred tank system, presumably by convective enhancement in the boundary layer on the surface of the crystals. Additionally, since nucleation suppressants nearly always also inhibit the growth rate, the slower growth kinetics in this case are not unexpected. 

No nucleation suppressant Nucleation suppressant added Nucleation suppressant added... 

A schematic diagram of the jet structure is shown in Fig. 10.9. As discussed in the last chapter, processes occurring in the shear layer of a jet strongly affect particle formation by homogeneous nucleation. Lesniewski and Friedlander (1998) hypothesized that there is a range of operation for which particle formation occurs in the shear layer of the jet but is quenched by dilution, depletion, or nucleation suppression as the particles move down the axis. If nucleation is confined to the shear layer, useful scaling laws can be derived for correlating particle concentration data. 

For N x 1 the free energy barrier hence is very large and homogeneous nucleation is suppressed. Very close to T, where Eq. (10) does not hold, Eq. (15) is replaced by fm/kaTc... 

It is also of significance that the agitation suppresses nucleation in a fluid which might otherwise deposit crystals. 

Kosterin, S. I., 1949, Study of Influence of Tube Diameter and Position upon Hydraulic Resistance and Flow Structure of Gas-Liquid Mixtures, Izvestiza Akademii Nauk SSSR, Otdelema Tekhni-cheskikh No. 12, 1824, Translation 3085, Henry Brutcher Tech. Translation, Altadena, CA. (3) Kottowski, H., and G. Grass, 1970, Influence on Superheating by Suppression of Nucleation Cavities and Effect of Surface Microstructure on Nucleation Sites, Proc. Symp. LIM Heat Transfer and Fluid Dynamics, p. 108, ASME, New York. (2)... 

When the flow through the CNC was exhausted outside of the laboratory, we observed particle formation at higher SOp concentrations as expected (Table II). To prove that the radical scavenger effect is reproducible, another radical scavenger (92 ppb nitric oxide) was used in the presence of 110 ppb SOp concentration and 2% humidity, and the supression in particle formation was observed. Another possible mechanism that supressed the particle formation is that more neutralization of polonium ions occurred at the higher humidities and thus ion-induced nucleation would be suppressed. 

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