Separation processes crystallization

Separation Processes for PX. There are essentially two methods that are currendy used commercially to separate and produce high purity PX (/) crystallization and (2) adsorption. A third method, a hybrid crystallization /adsorption process, has been successfiiUy field-demonstrated and the first commercial unit is expected in the near future. 

Separation Processes. The product of ore digestion contains the rare earths in the same ratio as that in which they were originally present in the ore, with few exceptions, because of the similarity in chemical properties. The various processes for separating individual rare earth from naturally occurring rare-earth mixtures essentially utilize small differences in acidity resulting from the decrease in ionic radius from lanthanum to lutetium. The acidity differences influence the solubiUties of salts, the hydrolysis of cations, and the formation of complex species so as to allow separation by fractional crystallization, fractional precipitation, ion exchange, and solvent extraction. In addition, the existence of tetravalent and divalent species for cerium and europium, respectively, is useful because the chemical behavior of these ions is markedly different from that of the trivalent species. 

Zone refining is one of a class of techniques known as fractional solidification in which a separation is brought about by crystallization of a melt without solvent being added (see also Crystallization) (1 8). SoHd—Hquid phase equiUbria are utilized, but the phenomena are much more complex than in separation processes utilizing vapor—Hquid equiHbria. In most of the fractional-solidification techniques described in the article on crystallization, small separate crystals are formed rapidly in a relatively isothermal melt. In zone refining, on the other hand, a massive soHd is formed slowly and a sizable temperature gradient is imposed at the soHd—Hquid interface. 

Recovery Process. Boron values are recovered from brine of Seades Lake by North American Chemicals Corp. In one process the brine is heated to remove some water and burkeite. The remaining brine is cooled to remove potassium chloride. This cooled brine is then transferred to another crystallizer where borax pentahydrate, Na2B40y 5H20, precipitates (18). In a separate process, boron is removed by Hquid—Hquid extraction followed by stripping with dilute sulfuric acid (19). Evaporator-crystallizers are used to recover boric acid [10043-35-3] H BO. In a third process, borax is recovered by refrigerating a carbonated brine. 

Mineral Matter in Goal. The mineral matter (7,38) in coal results from several separate processes. Some comes from the material inherent in all living matter some from the detrital minerals deposited during the time of peat formation and a third type from secondary minerals that crystallized from water which has percolated through the coal seams. 

Products. In all of the instances in which crystallization is used to carry out a specific function, product requirements are a central component in determining the ultimate success of the process. These requirements grow out of how the product is to be used and the processing steps between crystallization and recovery of the final product. Key determinants of product quaHty are the size distribution (including mean and spread), the morphology (including habit or shape and form), and purity. Of these, only the last is important with other separation processes. 

Process. In each of the systems discussed above there is a need to form crystals, to cause the crystals to grow, and to separate the crystals from residual Hquid. There are various ways to accomplish these objectives lea ding to a multitude of processes that are designed to meet requirements of product yield, purity, and, uniquely, crystal size distribution. 

Deviations from Raonlt s law in solution behavior have been attributed to many charac teristics such as molecular size and shape, but the strongest deviations appear to be due to hydrogen bonding and electron donor-acceptor interac tions. Robbins [Chem. Eng. Prog., 76(10), 58 (1980)] presented a table of these interactions. Table 15-4, that provides a qualitative guide to solvent selection for hqnid-hqnid extraction, extractive distillation, azeotropic distillation, or even solvent crystallization. The ac tivity coefficient in the liquid phase is common to all these separation processes. 

Dyes are synthesized in a reactor, then filtered, dried, and blended with other additives to produce the final product. The synthesis step involves reactions such as sulfonation, halogenation, amination, diazotization, and coupling, followed by separation processes that may include distillation, precipitation, and crystallization. 

Figure 9.12 Summary of the unified procedure for synthesizing crystallization-based separation processes (Wihowo and Ng, 2000)...

Jones, A.G., 1984. The Design of Well-Mixed Batch Crystallizers. SPS DR17. (Harwell/ Warren Spring Separation Processes Service), 40pp. 

Jones, A.G., 1988. Agglomeration during crystallization and precipitation. In Crystallization Manual (Harwell Separation Processes Service, AEA Technology), Volume CR II, Part 2, 60pp. 

Shock, R.A.W., 1983. Encrustation of crystallizers. Journal of separation process technology, 4, 1-13. 

Wibowo, C. and Ng, K.M., 2000. Unified approach for synthesising crystallization-based separation processes. American Institution of Chemical Engineers Journal, 46, 1400-1421. 

Witkowski, W.R., Miller, S.M. and Rawlings, J.B., 1990. Light scattering measurements to estimate kinetic parameters of crystallization. In Crystallization as a separation process, ACS Symposium Series, 438, 102. 

Labb recommends a slight modiflcation of the above process, by which he claims that the separation of crystals at the junction between the unabsorbed oil and the sulphite liquor is prevented, and greater accuracy in reading off the. percentage therefore attained. He employs a stoppered bulb, prolonged at the bottom into a graduated cylindrical closed tube, and into this are introduced 5 c.c. of the oil, together with... 

That benzene hexachloride isomer mixture is then the raw material for lindane production. The production of lindane per se is not a chemical synthesis operation but a physical separation process. It is possible to influence the gamma isomer content of benzene hexachloride to an extent during the synthesis process. Basically, however, one is faced with the problem of separating a 99%-plus purity gamma isomer from a crude product containing perhaps 12 to 15% of the gamma isomer. The separation and concentration process is done by a carefully controlled solvent extraction and crystallization process. One such process is described by R.D. Donaldson et al. Another description of hexachlorocyclohexane isomer separation is given by R.H. Kimball. 

Because p-xylene is the most valuable isomer for producing synthetic fibers, it is usually recovered from the xylene mixture. Fractional crystallization used to be the method for separating the isomers, but the yield was only 60%. Currently, industry uses continuous liquid-phase adsorption separation processes.The overall yield of p-xylene is increased... 

It may finally be pointed out that certain separation processes in addition to packed-bed gas absorption are gas-liquid-particle operations. Examples are flotation and a special type of fluidized crystallization process (Z2). 

Most natural solids, including minerals, are formed by one of two very different processes crystallization, which takes place when a hot melt cools down slowly, or precipitation, which occurs mainly when a solid separates from a water solution. 

Polymer crystallization is usually divided into two separate processes primary nucleation and crystal growth [1]. The primary nucleation typically occurs in three-dimensional (3D) homogeneous disordered phases such as the melt or solution. The elementary process involved is a molecular transformation from a random-coil to a compact chain-folded crystallite induced by the changes in ambient temperature, pH, etc. Many uncertainties (the presence of various contaminations) and experimental difficulties have long hindered quantitative investigation of the primary nucleation. However, there are many works in the literature on the early events of crystallization by var-... 

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