Crystallization from solution operation

Melt Crystallization. The use of a solvent can be avoided in some systems. In such cases, the system operates with heat as a separating agent, as do several processes involving crystallization from solution, but formation of crystalline material is from a melt of the crystallizing species rather than a solution. 

All freeze separation processes depend on the formation of pure solvent crystals from solution, as described for eutectic systems in Section 15.2.1. which allows single-stage operation. Solid-solution systems, requiring multistage-operation, are not usually economic. Several types of freeze crystallisation processes may be designated according to the kind of refrigeration system used as follows . 

Various equipment and process improvements have been introduced in the industrial practice of crystallization from solution (S15, S16). Saeman (S14) has published a comprehensive discussion of crystallizer design principles, extending some of his earlier work cited above (Si, S4). A companion paper by Garrett (G9) considers the application of theory to the selection and operation of commercial equipment, Design and operation of draft tube and baffle type crystallizers is reviewed by Caldwell (C6),... 

Crystallization may be carried out from a vapor, from a melt, or from a solution. Most of the industrial applications of the operation involve crystallization from solutions. Nevertheless, crystal solidification of metals is basically a crystallization process, and much theory has been developed in relation to metal crystallization. This topic is so specialized, however, that it is outside the scope of this subsection, which is limited to crystallization from solution. 

Liquid-liquid extraction also can be an attractive alternative to separation methods, other than distillation, e.g., as an alternative to crystallization from solution to remove dissolved salts from a crude organic feed, since extraction of the salt content into water ehminates the need to filter solids from the mother liquor, often a difficult or ejq)ensive operation. Extraction also may compete with process-scale chromatography, an example being the recovery of hydroxytyrosol (3,4-dihydrojq -phenylethanol), an antioxidant food additive, from olive-processing wastewaters [Guzman et al., U.S. Patent 6,849,770 (2005)]. 

Not only is timely seeding an excellent way to do away with batch-to-batch vagaries in the width and sensitivity ofthe metastable, supersaturated region, but the avoidance of nucleation is often crucial for achieving crystallization elegance—a requisite for enhanced rejection of impurities. To trade away such a powerful tool for the sake of trivially lessened inconveniences in record keeping in a GMP plant is most unsound as an operating principle. For those frequent and difficult purification tasks that crystallization from solution does so well, seed early, seed often and, above all, seed always. 

Anhydrous sodium sulphate can be manufactured by spray crystallization. Below 32.4 °C sodium sulphate crystallizes from solution in the form of the decahydrate above this temperature the anhydrous salt is formed. However, anhydrous sodium sulphate has an inverted temperature-solubility characteristic (see Figure 3.1b), and trouble is encountered with scale formation on the heat transfer surfaces of conventional evaporating crystallizers when operating temperatures in excess of 32.4 °C are used. In a plant described by Holland (1951) a concentrated solution of sodium sulphate was sprayed, or splashed, in the form of tiny droplets into a chamber through which hot gases flowed. The gases entered at about 900 °C. The continuously operated unit produced a powdered anhydrous product. 

Transport of solute from a fluid phase to a spherical or nearly spherical shape is important in a vari of separation operations such as liquid-liquid extraction, crystallization from solution, and ion exchange. The situation depicted in Fig. 2.3-12 assumes that there is no forced or natural convection in the fluid about the particle so that transport is governed entirely by molecular diffusion. A steady-state solution can be obtained for the case of a sphere of fixed radius with a constant concentration at the interface as well as in the bulk fluid. Such a model will be useful for crystallization from vaqxtrs and dilute solutions (slow-moving boundary) or for ion exchange with rapid irreversible reaction. Bankoff has reviewed moving-boundary problems and Chapters 11 and 12 deal with adsorption and ion exchange. 

The first industrial crystallizers and crystallization processes came up about 150 years ago. The crystallization process became entirely independent of locations (e.g., solar ponds) or stationary energies and all product purities became educible. The further development led to different crystallizers adjusted to the respective crystallization processes and concentrated on the product quality aspect demanded by the market. Applying vacuum technology opened the possibility to choose operation apart from the atmospheric pressure and herewith the opportunity to precisely design the crystallization processes to the respective phase systems and the material properties. Today, these vacuum crystallization processes have become the common standard in the industrial continuous single mass crystallization from solutions. 

While the unit operation evaporation, that is, the mass transfer from the liquid phase to the vapor phase, still possesses a direct connection with vacuum techniques, the connection of today s single mass crystallization from solution with vacuum techniques is only indirect. The techniques of vacuum cooling and vacuum evaporation are only the mostly used means for inducing the crystallization process. The reason for the dominant position of vacuum crystallization over classical surface cooling crystallization is the considerably reduced inclination to form incrustations. Vacuum crystallization is used in the low vacuum field down to 1 mbar. There are also applications in the overpressure field, although with increasing pressure the number of applications is reduced. In vacuum crystallization, one can find all the classical process control options used in the more familiar vacuum evaporation processes. However, an important difference to evaporation is the fact that the separation process is not concluded with the crystallization step. The suspension formed still has to be separated into crystal mass and mother liquor. Crystallization is therefore always associated with a mechanical separation process. The better this separation, the greater the purity of the crystallized masses. 

In more recent times, the fast expansion of the chemical industry has required a thorough study of the dynamics of crystallization, and this unit operation is now used in many industrial manufacturing areas table salt, sugar, sodium sulfate, urea, just to name a few, are produced by crystallization from solutions. 

Evaporative crystalli rs generate supersaturation by removing solvent, thereby increasing solute concentration. These crystallizers may be operated under vacuum, and, ia such circumstances, it is necessary to have a vacuum pump or ejector as a part of the unit. If the boiling poiat elevation of the system is low (that is, the difference between the boiling poiat of a solution ia the crystallizer and the condensation temperature of pure solvent at the system pressure), mechanical recompression of the vapor obtained from solvent evaporation can be used to produce a heat source to drive the operation. 

The reaction mixture is filtered. The soHds containing K MnO are leached, filtered, and the filtrate composition adjusted for electrolysis. The soHds are gangue. The Cams Chemical Co. electrolyzes a solution containing 120—150 g/L KOH and 50—60 g/L K MnO. The cells are bipolar (68). The anode side is monel and the cathode mild steel. The cathode consists of small protmsions from the bipolar unit. The base of the cathode is coated with a corrosion-resistant plastic such that the ratio of active cathode area to anode area is about 1 to 140. Cells operate at 1.2—1.4 kA. Anode and cathode current densities are about 85—100 A/m and 13—15 kA/m, respectively. The small cathode areas and large anode areas are used to minimize the reduction of permanganate at the cathode (69). Potassium permanganate is continuously crystallized from cell Hquors. The caustic mother Hquors are evaporated and returned to the cell feed preparation system. 

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or crystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binary mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirectly cooled, and/or solvent is evaporated to effect crystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt crystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining section of the apparatus. 

The kinetic behaviour of fructose crystallization from aqueous ethanolIc solutions, typical in composition to those operated on an industrial scale, is strongly dependent on supersaturation, solvent composition and temperature. Provided the supersaturation is kept below 35 C of subcooling, nucleation does not occur. 

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