Five-stage crystallization

Figure 9.14 Optimal flow profiles for a five stage KNO3 crystallization process Sheikh and Jones, 1997)...

As a result of their researches and those of others in Britain and the U.S.S.R. in particular, Bowden and Yoffe [3] separated the response of pseudostable crystals into five stages and developed a qualitative theory of fast reactions in condensed phases. Azides were of inestimable value as model substances in delineating the theory. 

Illustration of five stages In crystallization showing spherulite growth. At time fo—the supercooled melt. At later times t, to fg—the growth of spherulites. Finally, at time f4— the specimen Is composed completely of spherulites. 

Fig. 2.7. Illustration of five stages in crystallization showing spherulite growth. At time...

In the last two decades, a new process has been developed, very similar to one used for carbon fibers [YAJ 76]. It consists of a thermal decomposition or pyrolysis of organometallic polymer fibers containing silicon or boron. This method is used to manufacture long weavable fibers with low diameter (8-15 pm), controlled composition and state of crystallization. This process generally includes five stages summarized below for silicon carbide fiber, which is currently the most developed. 

Timetable for work plan. The work plan spans five years and involves eight related projects (Figure 3). Enzyme preparation, syntheses of I4/T- and D/T-labeled NADPI4, and preparation of high resolution crystals have been started, or will be started, in the early stages of the project. Studies related to KIEs, the temperature dependence of KIEs, and DI4FR dynamics will take place in years 1-5 modeling and interpretation of the results will take place in years 2-5. 

The crystalline fractions are now systematically recrystallized from methanol. The head crop is recrystallized from about 600 cc. of this solvent, and the remaining crops are crystallized in order from the successive mother liquors. At each stage the volume of the solution is adjusted by addition or distillation of solvent so that from one-third to one-half of the salt crystallizes. A series of large Erlenmeyer flasks, fitted with a reflux condenser or distilling head as required, is convenient for the purpose. The recrystallization is continued, using fresh or recovered solvent, until the head fraction has [a]i> — 2.8° or less (c = 4, methanol). It is unnecessary to take rotation values until the crystalline appearance and solubility behavior of the head fraction become approximately constant. Usually five to seven recrystallizations are necessary for purification. When pure, the head fraction is removed from the series and the recrystallization of the remaining crops is continued as long as it appears profitable. The final mother liquor from each series is reserved. 

Since the formation of five-ring-rich zeolites like ZSM-5 can be easily envisaged starting from D5R silicates only, we have considered in this work the possibility of the D5R being a likely precursor species in either the nucleation and/or the crystallization stage. The results can be summarized as follows. 

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