Selection of a Crystallizer

A crystallizer is an apparatus in which an environment can be created suitable for the formation and growth of crystalline materials. Paramount in the design of such equipment is the means that are chosen to create supersaturation at a temperature level that produces a desired or proper hydrate or composition of the product crystals. 

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Table 4-3 is a guide for the selection of a crystal for a given analysis.. Iost of the commonly available crystals have been included and rated with respect to their relative effectiveness for the analysis of the different elements. The table suggests that most laboratories would need an LiF and an ADP crystal as a minimum. 

Crystallization equipment can vary in sophistication from a simple stirred tank to a complicated multiphase column, and the protocol can range in complexity from simply allowing a vat of liquor to cool to the careful manipulation required of batch cyclic operations. In principle, the objectives of these systems are the same to produce a product meeting specifications on quality at an economical yield. This section will examine some of the considerations that go into the selection of a crystallizer so as to meet these objectives. 

Selection of a crystallizer that has both classified-fines and classified-product removal is done to combine the best features of each increased dominant size and narrower distribution. Figure 20 illustrates the effects of both removal functions on population density. Note that this plot of population density results from sampling the magma within a crystallizer, not from sampling the product stream, which for the ideal classification devices considered here can only have crystals larger than Lq. As discussed earlier for the classified-product crystallizer, the population densities shown in Fig. 20 represent those found in the crystallizer. 

To a certain extent, industrial crystallization is still more an art than science, which can be seen in the selection of a crystallizer. Often existing crystallization methods that have been previously proven snccessful for certain materials are used for these same materials in new plants, instead of new untried crystallizers. If exotic and unusual crystallization... 

Additional considerations for the selection of a crystallizer are given in Fig. 7-26. Figure 7-27 shows the specific crystal product rate of CSCPR crystallizers, with fluid-bed suspension. Modeling of crystallizers is described in [7-66]. 

An enlarged view of a crystal is shown in Fig. VII-11 assume for simplicity that the crystal is two-dimensional. Assuming equilibrium shape, calculate 711 if 710 is 275 dyn/cm. Crystal habit may be changed by selective adsorption. What percentage of reduction in the value of 710 must be effected (by, say, dye adsorption selective to the face) in order that the equilibrium crystal exhibit only (10) faces Show your calculation. 

In order to obtain appreciable conductivities, semiconductors must be doped witli small amounts of selected impurities. It is possible to switch tire doping type from n to p type, or vice versa, eitlier during tire growtli of a crystal or by tire selective introduction of impurities after tire growtli. The boundary region between tire p type and n type regions is... 

Crystallization is one of the oldest unit operations in the portfoho of industrial and/or laboratory separations. Almost all separation techniques involve formation of a second phase from a feed, and processing conditions must be selected that allow relatively easy segregation of the two or more resulting phases. This is a requirement for crystallization also, and there are a variety of other properties of the sohd product that must be considered in the design and operation of a crystallizer. Interactions among process, function, product, and phenomena important in crystallization ate iRustrated in Figure 1. 

Although many commercial crystallizers operate with some form of selective crystal removal, such devices can be difficult to operate because of fouling of heat exchanger surfaces or blinding of screens. In addition, several investigations identify interactions between classified fines and course product removal as causes of cycling of a crystal size distribution (7). Often such behavior can be rninirnized or even eliminated by increasing the fines removal rate (63,64). 

The most critical element in melt-growth of single crystals is the container, or crucible. The first requirement for selection of a suitable crucible is that the crucible does not react with the melt. The second is... 

Selectivity at formation of a respective inclusion compound and its thermal stability behavior might differ (cf. Tables 1 and 2), since for both representations different processes should be taken into consideration. Formation of a crystal inclusion compound is normally controlled by kinetics, whereas the thermal stability (decomposition property) is a result of thermodynamics. Thus, we speak of formation selectivity , on the one hand, and of binding selectivity , on the other. 

For convenience, only one-dimensional random movement will be considered. In this case, an atom is constrained to jump from one stable site to the next in the x direction, the choice of +x or -x being selected in a random way.6 For example, imagine a diffusion experiment starting with a thin layer of N atoms on the surface of a crystal. 

The selection of a potential list of crystallization solvents is made against the following criteria ... 

The birth of a crystal and its growth provide an impressive example of nature s selectivity. In qualitative analytical chemistry inorganic solutes are distinguished from each other by a separation scheme based on the selectivity of precipitation reactions. In natural waters certain minerals are being dissolved, while others are being formed. Under suitable conditions a cluster of ions or molecules selects from a great variety of species the appropriate constituents required to form particular crystals. 

The temperature-solubility relationship for solute and solvent is of prime importance in the selection of a crystalliser and, for solutions that yield appreciable amounts of crystals on cooling, either a simple cooling or a vacuum cooling unit is appropriate. An evaporating crystalliser would be used for solutions that change little in composition on cooling and... 

The use of tailor made additives holds great promise in the area of crystal growth and morphology control. The routine selection and use of these type of additives will require a fundamental understanding of the mechanism which the additives work on a molecular basis. At the same time, the effect of solvent molecules on the crystal growth process is another related and important problem. In both instances, the relationship between internal aystal structure, aystal growth rate, solvent and impurities are needed to predict the habit of a crystal and thus allow seleaion of the proper conditions and components required to obtain a desired habit... 

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