Crystallization general practices

It is also of practical interests to control hydrate decomposition. The early studies were intended to understand the mechanism of hydrate decomposition when plugging in pipeline was encountered. The hydrate crystals generally decompose by de-pressurization (Kelkar et al., 1998, Peters et al., 2000 Hong et al., 2006). Thermal stimulation has also been considered in order to provide strategies for methane recovery from the natural hydrate by thermal stimulation (Selim and Sloan, 1989 Ji et al., 2001 Hong et al., 2003 Hong and Pooladi-Darvish, 2005). Hydrate decomposition studies may also be applicable to gas storage in hydrates. 

Liquid polymers (at ambient temperature) are in general macromolecules with a relatively low molecular weight, many of them being in fact oligomers. Some liquid polymers are utilized as synthetic oils. Certain polymers can form liquid crystals in other words they can have an ordered structure while being in liquid state (either melted or in a solution). The orientation of certain polymeric molecules in liquid state such that the properties of the material are anisotropic is possible. Polymer liquid crystals have practical applications, and solution of liquid crystal polymers can be used for extruding fibers that have a highly crystalline structure after solvent elimination. 

Unfortunately, many precipitates cannot be formed as crystals under practical laboratory conditions. A colloidal solid is generally encountered when a precipitate has such a low solubility that S in Equation 12-1 always remains negligible relative to Q. The relative supersaturation thus remains enormous throughout precipitate formation, and a colloidal suspension results. For example, under conditions feasible for an analysis, the hydrous oxides of iron(lll), aluminum, and chromium(III) and the sulfides of most heavy-metal ions form only as colloids because of their very low solubilities.- ... 

The theory and experiment of direct crystallization of enantiomers is quite well understood at present [10]. There are a number of variables which affect the resolution by direct crystallization in practice. Several technological schemes based on this principle are realized on the commercial scale. These are, for example, the Merck process used for the production of antihypertensive drug methyldopa [11], a process developed by Harman and Reimer for (-)-menthol, which is separated as an ester [12], the process patented by Industria Chimica Profarmaco for the resolution of naproxen enantiomers as the ethylamine salt [13], the production of L-glutamic acid by the Japanese company Ajinomoto on a scale in excess of 10000 tons annually as early as the 1960s [14], etc. In general, it seems that spontaneous crystallization is a very useful technique for the enantioseparation of the naturally occurring a-amino acids. All of them may be resolved either directly or as derivatives [10]. 

The presence of seed crystals generally reduces the induction period, but does not necessarily eliminate it. Even if the system is seeded at time t = 0, a measurable induction period tmd may elapse before new crystals are detected. By definition, these are secondary nuclei and they may appear in several bursts throughout the latent period, making it difficult to attach any real significance to the induction time itself For these reasons it may be preferable to record the latent period as the more practical characteristic of the system. Factors that can influence the induction and latent periods and the rate of desupersaturation are temperature, agitation, heat effects during crystallization, seed size, seed surface area and the presence of impurities. 

Most of the structure of the product is formed in a (tubular) B-unit. The residence time should be related to the rate of crystallization of the fat blend so that at the end of the tube, crystallization is practically complete. In general, a residence time of some minutes is sufficient. To obtain a smooth, plastic and packable product, some working is usually carried out by placing a metal perforated plate or screen at the end of the unit. Without working there, a brittle and too hard structure may result. 

The kind of single crystals discussed above are all made starting from solution. In industrial practice, bulk polymeric products are generally made from the melt, and... 

Anhydrous caustic soda (NaOH) is available but its use is generally not considered practical in water and wastewater treatment applications. Consequently, only liquid caustic soda is discussed here. Liquid caustic soda is generally shipped at two concentrations, 50 percent and 73 percent NaOH. The densities of the solutions as shipped are 12.76 Ib/gal for the 50 percent solution and 14.18 Ib/gal for the 73 percent solution. These solutions contain 6.38 Ib/gal NaOH and 10.34 Ib/gal NaOH, respectively. The crystallization temperature is 53 F for the 50 percent solution and 165 F for the 73 percent solution. The molecular weight of NaOH is 40. The pH of a 1 percent solution of caustic soda is 12.9. 

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