Nucleation streams, effect

Jenne (1992) suggests that in the presence of a few mg/L of inhibiting DOC, which is common in many groundwaters, SI, values may have to exceed +0.3 or more for calcite nucleation and precipitation. (See also Appelo and Postma 1993.) Jacobson et al. (1971) reported no carbonate precipitation in a stream in which SI values averaged 0.74 diurnally and were as high as 1.5 (see below). The.se experiences, presumably reflect both nucleation-inhibition effects and the relatively slow rate of calcite precipitation compared to the re.sidence times of streams. 

Achieving steady-state operation in a continuous tank reactor system can be difficult. Particle nucleation phenomena and the decrease in termination rate caused by high viscosity within the particles (gel effect) can contribute to significant reactor instabilities. Variation in the level of inhibitors in the feed streams can also cause reactor control problems. Conversion oscillations have been observed with many different monomers. These oscillations often result from a limit cycle behavior of the particle nucleation mechanism. Such oscillations are difficult to tolerate in commercial systems. They can cause uneven heat loads and significant transients in free emulsifier concentration thus potentially causing flocculation and the formation of wall polymer. This problem may be one of the most difficult to handle in the development of commercial continuous processes. 

When a process is continuous, nucleation frequently occurs in the presence of a seeded solution by the combined effects of mechanical stimulus and nucleation caused by supersaturation (heterogeneous nucleation). If such a system is completely and uniformly mixed (i.e., the product stream represents the typical magma circulated within the system) and if the system is operating at steady state, the particle-size distribution has definite hmits which can be predicted mathematically with a high degree of accuracy, as will be shown later in this section. 

If maximum product size is desired, the process stream flow is parallel, with a fresh feed and slurry discharge to product recovery equipment from each crystallizer. Cascaded product flow is avoided because flow through a series of tanks narrows the residence time distribution and the CSD and reduces the mass mean size by reducing the number of larger crystals in the distribution tail. This tail represents the major mass-weighted fraction. Population models have been solved to verify this effect, assuming an equal nucleation rate in each stage (Randolph and Larson 1988). The actual mean in industrial practice is lower because transfer of... 

The processes of growth and nucleation interact in a crystallizer, and both contribute to the crystal size distribution (CSD) of the product (see section 9.1). Kinetic data needed for crystallizer design purposes (effective growth and nucleation rates) can be conveniently measured on the laboratory scale in a mixed-suspension, mixed-product removal (MSMPR) crystallizer operated continuously in the steady state Figure 9.3). The assumptions made are that no crystals are present in the feed stream, that all crystals are of the same shape, that crystals do not break down by attrition, and that crystal growth rate is independent of crystal size. 

The rate of nucleation, I, has been estimated by Turnbull and Fisher [25] from the shape of AG and the influence of local viscosity, governed by a free enthalpy AG,. In Fig. 3.61 this equation is listed. The rate I applies to the case that nucleation is unhindered. The first exponent of the equation expresses retardation of nucleation due to viscosity effects with the given parameters (see Sect. 5.6). It stops nucleation as the glass transition temperature is approached. The nucleation described is homogeneous nucleation and creates a continuous stream of new crystals in the remaining melt or solution. For polymers it takes a supercooling of about 50 K to overcome the... 

Avoid "shock nucleation, which can occur by a sudden temperature drop at a mix point or by a salting out effect if streams of different compositions are combined. 

Some pigments act as nucleation agents and significantly accelerate the crystallization of the polymer stream below the spinneret, e.g. phthalo-cyanine pigments. Others, such as carbon black and Ti02, have less effect on the kinetics of crystallization. 

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