Crystallization equipment

The type of equipment to be used in a crystallization process depends primarily upon the solubility characteristic of the solute. Solutions from fermentation processes can be classified as follows  

Chemicals where a change in solution temperature has little effect on the solubility. An example is hexamethylenetetramine as shown in Fig. 3. The supersaturated solution is produced by evaporation of the solvent. The equipment needed here is called an evaporative crystallizer (see Fig. 4). 

Chemicals, e.g., fumaric acid, which show only a moderate increase in solubility with increasing temperature. A combination of evaporation and cooling may be used to produee the supersaturated solution. Depending upon the )deld required, this operation may be carried out in either a vacuum cooling crystallizer or an evaporative crystallizer (see Fig. 5). 

Chemicals, e.g., adipic acid, which show a large increase in solubility with increasing temperature. Cooling the solution can be an effective way to produce the supersaturated solution, although a combination of evaporation and cooling can also be employed. In addition to the two types of crystallizers mentioned above, a cooling crystallizer may be used (see Fig. 6). 

The requirements for particle off-bottom suspension are also discussed in many literature references including Zwieterling (1958), Chowdhury et al. (1995), and Paul et al. (2003, Chapter 10). Methods of calculating various degrees of homogeneity in solids are presented. These calculations are an important part of crystallization scale-up studies. 

A stirred vessel crystallizer is shown in Fig. 6-4. Included are a dual-impeller pitched-blade turbine with a tickler blade (see Section 6.6.1.6), a subsurface addition line, baffles, and a ram-type bottom outlet valve to aid in discharge of slurries. 

The workhorse impeller is the pitched-blade mrbine because of its ability to create good circulation at relatively low shear. These attributes help reduce secondary nucleation and crystal breakage while achieving good suspension and circulation. The flat-blade turbine is less versatile because of high shear and less overall circulation. The Ekato Intermig has proven to have superior performance in some crystaUization operations because of its combination of excellent circulation and low shear. 

Computational fluid dynamics representations of flow in these vessels may be helpful in visualizing flow patterns and particle paths. 

The versatility of the glass-lined vessel in a large variety of chemical environments has made it the most common in the industry. For these reasons, a crystaUization step may be carried out in an equipment train in a glass-lined vessel whether or not this is required to prevent corrosion. These reactors range in size from 80 to 20,000 liters or more. One limitation in the use of glass-lined vessels related to mixing and heat transfer is that the limit of the temperamre difference between jacket and batch is 125 C. However, such temperature extremes 

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Manufacture and Processing. Isophthahc acid is synthesized commercially by the Hquid-phase oxidation of / -xylene [108-38-3]. The chemistry of the oxidation is almost identical to that of -xylene oxidation to terephthahc acid, and production facihties can be used interchangeably for these two dicarboxyhc acids. However, because isophthahc acid is more soluble than terephthahc acid in reaction solvents as can be seen by comparing data in Tables 16 and 25, crystallization equipment is more important in isophthahc acid facihties. 

Classified removal of course material also can be used, as shown in Figure 16. In a crystallizer equipped with idealized classified-product removal, crystals above some size ate removed at a rate Z times the removal rate expected for a perfecdy mixed crystallizer, and crystals smaller than are not removed at all. Larger crystals can be removed selectively through the use of an elutriation leg, hydrocyclones, or screens. Using the analysis of classified-fines removal systems as a guide, it can be shown that the crystal population density within the crystallizer magma is given by the equations... 

Coefficient of Variation One of the problems confronting any user or designer of crystallization equipment is the expected particle-size distribution of the solids leaving the system and how this distribution may be adequately described. Most crystalline-product distributions plotted on arithmetic-probability paper will exhibit a straight line for a considerable portion of the plotted distribution. In this type of plot the particle diameter should be plotted as the ordinate and the cumulative percent on the log-probability scale as the abscissa. 

It is emphasized that the delta L law does not apply when similar crystals are given preferential treatment based on size. It fails also when surface defects or dislocations significantly alter the growth rate of a crystal face. Nevertheless, it is a reasonably accurate generahzation for a surprising number of industrial cases. When it is, it is important because it simphfies the mathematical treatment in modeling real crystallizers and is useful in predicting crystal-size distribution in many types of industrial crystallization equipment. 

Nucleation The mechanism of crystal nucleation from solution has been studied by many scientists, and recent work suggests that—in commercial crystallization equipment, at least—the nucleation rate is the sum of contributions by (1) homogeneous nucleation and (2) nucleation due to contaci between crystals and a) other crystals, h) the walls of the container, and (c) the pump impeller. If is the net number of new crystals formed in a unit volume of solution per unit of time. 

Column crystallizers of the end-fed type can be used for purification of many eutectic-type systems and for aqueous as well as organic systems (McKay, loc. cit.). Column crystallizers have been used for xylene isomer separation, but recently other separation technologies including more efficient melt crystallization equipment have tended to supplant the Phillips style crystallizer. 

For instance, the Swenson Evaporator Company maintains a pilot plant for sizing crystallization equipment. They will rent it to prospective customers, who can then... 

Because the handling of solids is difficult, particularly that of soft organic crystals, several crystallization processes have been developed in which solids do not appear outside the crystallizing equipment, and the product leaves the equipment in molten form. For organic substances, crystalline form and size usually are not of great importance as for products of crystallization from aqueous solutions. If needed, the molten products can be converted into flakes or sprayed powder, or in extreme cases they can be recrystallized out of a solvent. 

Gitsov presented a series of poly(benzyl ether) monodendrimers capable of cation complexation lacking alkyloxy side chains which were non-mesomorphic [93], Laschat followed a different route to obtain disk-like liquid crystals equipped with crown ether moieties. The crown ether was not attached to the mesogenic... 

Figure 9.4 Experimental set-up Bench-scale multi-purpose batch crystallizer equipped with an in situ ATR FTIR probe. Reprinted from Fevotte (2002)43 with permission from Elsevier.

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. 

The basic requirements of a crystallization system are (1) a vessel to provide sufficient residence time for crystals to grow to a desired size, (2) mixing to provide a uniform environment for crystal growth, and (3) a means of generating supersaturation. Crystallization equipment is manufactured and sold by several vendors, but some chemical companies design their own crystallizers based on expertise developed within their organizations. Rather than attempt to describe the variety of special crystallizers that can be found in the marketplace, this section will provide a brief general survey of types of crystallizers that utilize the modes outlined above. 

The liquid, together with the crystals, is run into a crystallizer equipped with a lead cooling coil and a stirrer rotating at a low speed. Here the rest of the picric acid is precipitated and then separated from the spent acid on a vacuum filter. 

Both batch and continuous crystallization equipment are available at industrial scale, although batch operation is normally favored for pharmaceutical products where batch integrity must be maintained for quality control reasons. Continuous crystallization is suited to higher throughputs and enables more energy efficient operation. 

Crystallization equipment is simple, easy to procure and operationally trouble free... 

Crystallization can occur from melts, solutions, or vapors. Since crystallization from aqueous solutions is most pertinent to chemical engineering, this aspect of the general topic is stressed in the following presentation. Historical and descriptive material are minimized. The fundamental principles underlying solubility, nucleation, and crystal growth are presented first, followed by a brief discussion of their application in modem practice, so that the reader may be apprised of recent significant advances in the design and operation of crystallization equipment. 

A detailed historical development of crystallization equipment is not the purpose of this presentation, but a few general remarks are appropriate. Crystallization has been practiced for centuries and its beginnings, like those of other classical unit operations, are lost in antiquity (Cl, S10). The earliest crystallizers were shallow open tanks, operated batch-wise and cooled by surface evaporation. Simple, inexpensive equipment... 

In the discussion which follows, crystallization equipment has been classified according to the means of suspending the growing product. This technique reduces the number of major classifications and segregates those to which Eq. (18-31) applies. 

Krystal Crystallization Equipment , Bulletin 50a, Warren, Pa (19 4S) 3)Kirk Othmer 4 (1948), 619-36, A.R.Thompson, Crystalliza-tinn 4 A - i/ 5 "Pbvs csl Wsthods of... 

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