Crystallization Primary

Air is typically used as the heating medium in both beds, whereby the gas temperature generally does not exceed 185 °C. Higher temperatures can be employed, but nitrogen rather than air is used to prevent oxidation and the yellowing of pellets. 

The spouting bed temperature is generally in the range of 150-170 °C, which is close to the maximum spherulite growth rate, and therefore ensures quick completion of the primary crystallization. The material temperature at the outlet of the pulsed fluid bed is usually 180°C. 

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Note In isothermal crystallization, primary crystallization is often described by the Avrami equation. 

Fig 3. In a cubic crystal, primary and secondary arms are normal to each other... 

In this theory it is assumed that a chain stem, one fold period long, is laid down on the lateral growth face of the crystal. This is the slowest step because the stem has only one surface face on which to sit. Once this stem is in place, however, an adjacent stem is more easily laid down (i.e., has a lower free energy barrier to cross), because it can now contact two surfaces, the crystal substrate and the side face of the first stem. The row therefore quickly fills up once the first stem is deposited. The growth rate of the crystal (primary crystallization) is thus largely determined by secondary nucleation (Figure 10-31). 

In the industrial crystallizer, primary homogeneous nucleation is not possible due to the low levels of supersaturation employed and the presence of impurities. Primary heterogeneous nucleation is possible at places where high supersaturation is being created— for example, in heat transfer equipment. This could lead to crystal deposits on these surfaces. Undesirable nucleation can be reduced by choosing proper production rates and by allowing adequate mixing. 

Crystal Primary glide plane Polarizability (10 /m ) Lattice constant (nm)... 

Particle size is an important physical quality property of raw materials. With regards to particles in this section a distinction is made between firstly loose crystals (primary particles) and secondly agglomerates coagulated small particles (secondary particles) that have such a strong cohesion that they resist normal dispersion techniques and therefore are not easy to disperse in the production process. 

Consequently, Chapter 1 written by R. W. Thompson gives a modern account of our present understanding of zeolite synthesis. The fundamental mechanisms of zeolite crystallization (primary and secondary nucleation and growth) in hydrothermal systems are highlighted. 

Compared with run 52, the passage from domain 2 to domain 4 in run 54 was much slower. This slowness was due to the induction time of the co-crystal primary nucleation and to the great quantity of co-crystals produced. Run 61 differed from run 54 by the amount of CBZ crystals in the initial slurry (0.63 wt% for run 61) and also by the amount of NCT used to trigger the SMPT from domains 2 to 4. The differences in the pathways could be translated in terms of kinetics. First, the induction time of co-crystal primary nucleation at point G was reduced to 15 minutes due to a higher CBZ/NCT supersaturation ratio. Second, no plateau of CBZ dissolution o-crystal growth was detected and the pathway was parallel to the bisecting line up until the final equilibrium (point H ). An important desupersaturation rate was also observed. 

These observations are explained by a less numerous population of CBZ crystals and a higher co-crystal primary nucleation frequency. In this situation the consumption of CBZ by the co-crystal growth was not balanced by the CBZ crystals dissolution. From a process point of view, the introduction of an important quantity of co-crystal agent brings a net gain of operating time due to a faster SMTP rate and an increased co-crystallization yield because of lower final co-crystal solubility. 

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