Process crystallization

The appearance and disappearance of embryos of the ether-soluble (ES) fraction of stereoblock PP were observed by Schonherr et al. [73]. Features 1, 2, and 3, which are clearly visible in Fig. 5a, disappear in Fig. 5b. A stable nucleus, for example, Feature B in Fig. 5a, develops into a lamella. These studies have clearly demonstrated that the primary nuclei of polymers can be observed using AFM. 

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The cleanup of this oil is exactly like that which was done in Method 1. The oil is dissolved in about SOOmL of 3N HCl and the solution extracted with TOOmL of DCM. The chemist remembers that in this particular case the MDMA or meth is going to stay in the HCl/water but that unreacted, valuable MD-P2P or P2P is going to be in that DCM so it, of course, is saved. The HCl/MDMA solution is then basified with concentrated NaOH so that at around pH 9 the happy little beads of final, freebase product will appear in the solution. As usual, the oil is extracted with DCM, dried through Na2S04 and the DCM removed by distillation. The final product here is usually a little darker in color than the product achieved in Method 1, but it is still remarkably clean and may be crystallized as is with the crystallization process removing most of the color impurities. Of course the chemist may wish to vacuum distill to afford clear product. The average yield with this method is 60-70%. 

In spite of these obstacles, crystallization does occur and the rate at which it develops can be measured. The following derivation will illustrate how the rates of nucleation and growth combine to give the net rate of crystallization. The theory we shall develop assumes a specific picture of the crystallization process. The assumptions of the model and some comments on their applicability follow ... 

While there are several instances of redundancy among the Avrami exponents arising from different pictures of the crystallization process, there is also enough variety to make the experimental value of this exponent a valuable way of characterizing the crystallization process. In the next section we shall examine the experimental side of crystallization kinetics. 

Currendy, about 40% of the PX produced woddwide uses crystallization technology. A number of crystallization processes have been commercialized over the years. The more common ones are those developed by Chevron, Kmpp, Amoco, ARCO (LyondeU), and Phillips. Some of the features of these processes are discussed herein. 

Xylene Isomerization. After separation of the preferred xylenes, ie, PX or OX, using the adsorption or crystallization processes discussed herein, the remaining raffinate stream, which tends to be rich in MX, is typically fed to a xylenes isomerization unit in order to further produce the preferred xylenes. Isomerization units are fixed-bed catalytic processes that are used to produce a close-to-equiUbrium mixture of the xylenes. To prevent the buildup of EB in the recycle loop, the catalysts are also designed to convert EB to either xylenes, benzene and lights, or benzene and diethylbenzene. 

Fractional crystallization processes are also used commercially to produce high purity metal from lower grade alurninum. These processes rely on the... 

Graining, flaking, and spraying have all been used to make soHd ammonium nitrate particles. Most plants have adopted various prilling or granulation processes. Crystallized ammonium nitrate has been produced occasionally in small quantities for use in specialty explosives. The Tennessee Valley Authority developed and operated a vacuum crystallization process (25), but the comparatively small crystals were not well received as a fertilizer. 

Gas AntisolventRecrystallizations. A limitation to the RESS process can be the low solubihty in the supercritical fluid. This is especially evident in polymer—supercritical fluid systems. In a novel process, sometimes termed gas antisolvent (GAS), a compressed fluid such as CO2 can be rapidly added to a solution of a crystalline soHd dissolved in an organic solvent (114). Carbon dioxide and most organic solvents exhibit full miscibility, whereas in this case the soHd solutes had limited solubihty in CO2. Thus, CO2 acts as an antisolvent to precipitate soHd crystals. Using C02 s adjustable solvent strength, the particle size and size distribution of final crystals may be finely controlled. Examples of GAS studies include the formation of monodisperse particles (<1 fiva) of a difficult-to-comminute explosive (114) recrystallization of -carotene and acetaminophen (86) salt nucleation and growth in supercritical water (115) and a study of the molecular thermodynamics of the GAS crystallization process (21). 

Measurement of Residual Stress and Strain. The displacement of the 2 -value of a particular line in a diffraction pattern from its nominal, nonstressed position gives a measure of the amount of stress retained in the crystaUites during the crystallization process. Thus metals prepared in certain ways (eg, cold rolling) have stress in their polycrystalline form. Strain is a function of peak width, but the peak shape is different than that due to crystaUite size. Usually the two properties, crystaUite size and strain, are deterrnined together by a computer program. 

The USP/FCC grade of benzoic acid is usually produced by extraction and crystalliza tion, although distillation has also been used. In the extraction—crystallization process, toluene, water, and methanol have all been used and each is capable of producing a high quaUty benzoic acid product. 

Potassium and ammonium dichromates are generally made from sodium dichromate by a crystallization process involving equivalent amounts of potassium chloride or ammonium sulfate. In each case the solubiHty relationships are favorable so that the desired dichromate can be separated on cooling, whereas the sodium chloride or sulfate crystallizes out on boiling. For certain uses, ammonium dichromate, which is low in alkaH salts, is required. This special salt may be prepared by the addition of ammonia to an aqueous solution of chromic acid. Ammonium dichromate must be dried with care, because decomposition starts at 185°C and becomes violent and self-sustaining at slightly higher temperatures. 

Solidification. Production of a product in a form suitable for use and acceptable to the consumer also may be an objective of a crystallization process. For example, the appearance of sucrose (sugar) varies with local customs, and deviations from that custom could lead to an unacceptable product. A final crystallization may thus be called for to bring the product appearance into compliance with expectations. 

It is often important to control the CSD of pharmaceutical compounds, eg, in the synthesis of human insulin, which is made by recombinant DNA techniques (1). The most favored size distribution is one that is monodisperse, ie, all crystals are of the same size, so that the rate at which the crystals dissolve and are taken up by the body is known and reproducible. Such uniformity can be achieved by screening or otherwise separating the desired size from a broader distribution or by devising a crystallization process that will produce insulin in the desired form. The latter of these options is preferable, and considerable effort has been expended in that regard. 

Nucleation. Crystal nucleation is the formation of an ordered soHd phase from a Hquid or amorphous phase. Nucleation sets the character of the crystallization process, and it is, therefore, the most critical component ia relating crystallizer design and operation to crystal size distributions. 

Example 1 Yield from a Crystallization Process A 10,000-lh batch of a 32,5 percent MgS04 solution at 120°F is cooled without appreciable evaporation to 70°F, What weight of MgS04-7H20 crystals will be formed (if it is assumed that the mother liquor leaving is saturated) ... 

FIG. 22-16 Siilzer MWB-crystallization process, a) Stepwise operation of the process, (h) System flow sheet. (Sulzer Chemtecli)... 

The results of determination of the form of presence of As, Se, Nb, Mo, Ni, Cu in different solid compounds ai e given. The application of RII LEL for the study of stmctural transformations in chalkogenid glasses is shown. The X-ray spectral determination of crystal water, the possibility of studying of dissolution-crystallization processes and kinetics of some chemical reactions ai e discussed. 

In some cases, whole parts of the protein are missing from the experimentally determined structure. At times, these omissions reflect flexible parts of the molecule that do not have a well-defined structure (such as loops). At other times, they reflect parts of the molecule (e.g., terminal sequences) that were intentionally removed to facilitate the crystallization process. In both cases, structural models may be used to fill in the gaps. 

Crystalline polyesters are highly important as adhesive raw materials. They are normally crystalline waxes and are highly symmetrical in nature, which can aid the crystallization process [26]. Poly(hexamethylene adipate) and poly(caprolactone), shown in Table 2, are only two of the many crystallizable backbones. Poly(ethylene adipate) and poly(letramethylene adipate) are also commonly used in urethane adhesives. The crystalline polyesters are used in curing hot melts, waterborne polyurethanes, thermoplastic polyurethanes, and solvent-borne urethane adhesives. The adipates are available mostly as diols. The poly(caprolactones) are available as diols and triols. 

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