Crystallization , from solutions

Many of these classes are self-explanatory, but some require definition. For example, the term controlled refers to supersaturation control. The term classifying refers to the production of a selected product size by classification in a fluidized bed of crystals. In a circulating-liquor crystallizer the crystals remain in the crystallization zone only the clear mother liquor is circulated, e.g., through a heat exchanger. In the circulating-magma crystallizer the crystals 

The simplest type of cooling crystallizer is the unstirred tank a hot feedstock solution is charged to the open vessel where it is allowed to cool, often for several days, predominantly by natural convection. Metallic rods may be suspended in the solution so that large crystals can grow on them and reduce the amount of product that sinks to the bottom of the crystallizer. The product is removed by hand. 

Because cooling is slow, large interlocked crystals are usually obtained and retention of mother liquor is unavoidable. As a result, the dried crystals are generally impure. Because of the uncontrolled nature of the process, product crystals range from a fine dust to large agglomerates. 

Labour costs are generally high, but the method may be economical for small batches because capital, operating, and maintenance costs are low. However, the productivity of this type of equipment is low and space requirements are high. 

The installation of an agitator in an open-tank crystallizer generally results in smaller, more uniform crystals and reduced batch time. The final product tends to have a higher purity because less mother liquor is retained by the crystals after filtration and more efficient washing is possible. Vertieal baffles may be fitted inside the vessel to induce better mixing, but they should terminate below the liquor level to avoid excessive encrustation. For the same reason, water jackets are usually preferred to coils for cooling purposes and, where possible, the internal surfaces of the crystallizer should be smooth and crevice-free (section 9.5). 

For polymers that crystallize from the melt, an important parameter in the characterization of the two-phase systems, is the weight fraction of the crystalline regions. The degree of crystallinity that can be reached is dependent on the temperature at which crystallizatiOTi takes place. At low temperatures one attains a much lower degree of crystallization than at higher temperatures. This implies that crystallization remains incomplete for kinetic reasons [7]. 

Normal conditions of cooling of the molten polymer establish the crystalline texture of the polymer and usually result in formation of very tiny crystals. These crystals are part of a closely spaced cluster called spherulite. The formation of a single nucleus in a polymer cooled below its melting point favors the formation of another nucleus in its vicinity due to creation of local stresses. 

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Metaldehyde [9002-91-9] a cycHc tetramer of acetaldehyde, is formed at temperatures below 0°C in the presence of dry hydrogen chloride or pyridine—hydrogen bromide. The metaldehyde crystallizes from solution and is separated from the paraldehyde by filtration (48). Metaldehyde melts in a sealed tube at 246.2°C and sublimes at 115°C with partial depolymerization. 

Aqueous solutions of 50% acrylamide should be kept between 15.5 and 38°C with a maximum of 49°C. Below 14.5°C acrylamide crystallizes from solution and separates from the inhibitor. Above 50°C the rate of polymer buildup becomes significant. Suitable materials of constmction for containers include stainless steel (304 and 316) and steel lined with plastic resin (polypropylene, phenoHc, or epoxy). Avoid contact with copper, aluminum, their alloys, or ordinary iron and steel. 

Peroxohydrates are usually made by simple crystallization from solutions of salts or other compounds in aqueous hydrogen peroxide. They are fairly stable under ambient conditions, but traces of transition metals catalyze the Hberation of oxygen from the hydrogen peroxide. Early work on peroxohydrates has been reviewed (92). 

The general manufacturing scheme for phosphate salts is shown in Figure 11. Condensed phosphates are prepared from the appropriate orthophosphate or mixture of orthophosphates, so the preparation of orthophosphates must be considered first for the manufacture of any phosphate salt. Phosphoric acid is neutralized to form a solution or slurry with a carefully adjusted acid/base ratio according to the desired orthophosphate product. The orthophosphate may be recovered either by crystallization from solution, or the entire solution or slurry may be evaporated to dryness. The dewatering (qv) method is determined by the solubihty properties of the product and by its desired physical properties such as crystal size and shape, bulk density, and surface area. Acid orthophosphate salts may be converted to condensed phosphates by thermal dehydration (calcination). 

Similady, hquid-crystal polymers exhibit considerable order in the hquid state, either in solution (lyotropic) or melt (thermotropic). When crystallized from solution or melt, they have a high degree of extended-chain crystallinity, and thus have superior mechanical properties. Kevlar (Du Pont) is an aromatic polyamide (atamid) with the repeating unit designated as (2). It is spun into... 

Significant amounts of comonomer also reduce the abiUty of the polymer to form lamellar crystals from solution. In some cases, the polymer merely gels the solution as it precipitates rather than forming distinct crystals. At somewhat higher VDC content, it may precipitate in the form of aggregated, ill-defined particles and clusters. 

Cu(N03 )26H2 0, is produced by crystallization from solutions below the transition poiat of 26.4°C. A basic copper nitrate [12158-75-7] Cu2(N02)(0H)2, rather than the anhydrous product is produced on dehydration of the hydrated salts. The most common commercial forms for copper nitrate ate the ttihydtate and solutions containing about 14% copper. Copper nitrate can be prepared by dissolution of the carbonate, hydroxide, or oxides ia nitric acid. Nitric acid vigorously attacks copper metal to give the nitrate and evolution of nitrogen oxides. 

Crystallization from Solution. Crystallization techniques are related to the methods used to iaduce a driving force for soflds formation and to the medium from which crystals are obtained. Several approaches are defined ia the foUowiag discussion. 

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. 

Figure 9.20 Potential environmental impacts and resource usage associated with the formation of solid products by crystallization from solution after Sharratt, 1996)...

Cisternas, L.A. and Rudd, D.F., 1993. Process designs for fractional crystallization from solution. Industrial and Engineering Chemistry Research, 32, 1993. 

Larson, M.A. and Klekar, S. A., 1973. In-situ measurement of supersaturation in crystallization from solution. Presented at American Institute of Chemical Engineers 66th Annual Meeting, Philadelphia, November. 

D. Maynes, J. Klewicki, P. McMurty, H. Robey. Hydrodynamic scalings in the rapid growth of crystals from solution. J Cryst Growth 178 545, 1997. 

The pentahydrate Bi(N03)3.5H20 can be crystallized from solutions of Bi oxide or carbonate in cone HNO3. Dilution causes the basic salt BiO(N03) to precipitate. Attempts at thermal dehydration yield complex oxocations by reactions which have been formulated as follows ... 

The propensity for iodine to catenate is well illustrated by the numerous polyiodides which crystallize from solutions containing iodide ions and iodine. The symmetrical and unsymmetrical 13 ions (Table 17.15) have already been mentioned as have the I5- and anions and the extended networks of stoichiometry (Fig. 17.12). The stoichiometry of the crystals and the detailed geometry of the polyhalide depend sensitively on the relative concentrations of the components and the nature of the cation. For example, the linear ion may have the following dimensions ... 

The most stable solid hypochlorites are those of Li, Ca, Sr and Ba (see below). NaOCl has only poor stability and cannot be isolated pure KOCl is known only in solution, Mg yields a basic hypochlorite and impure Ag and Zn hypochlorites have been reported. Hydrated salts are also known. Solid, yellow, hydrated hypobromites Na0Br.xH20 (x = 5, 7) and K0Br.3H20 can be crystallized from solutions obtained by adding Br2 to cold cone solutions of MOH but the compounds decompose above 0°C. No solid metal hypoiodites have yet been isolated. 

A mixture of 3.4 parts of 7-chloro-4-fluorobutyrophenone, 4 parts of 1-(4-piperidyl)-2-benzimidazolinone hydrochloride, 6 parts of sodium carbonate and 0.1 part of potassium iodide in 176 parts of 4-methyl-2-pentanone is stirred and refluxed for 48 hours. The reaction mixture is cooled and 120 parts of water is added. The separated organic layer is dried over magnesium sulfate and the solvent is evaporated to leave an oily residue which is dissolved in dilute hydrochloric acid and boiled. The acidic solution is filtered and cooled at room temperature whereupon there crystallizes from solution l-<1-[ y-(4-fluorobenzoyl)-propyl]-4-piperidvl>-2-benzimidazolinone hydrochloride hydrate melting at about 134°-142°C. 

Random and structured packings are susceptible to surface fouling due to process conditions and/or the presence of oxygen as may be related to bacterial growth. Some systems will precipitate solids or crystals from solution usually due to the temperature and concentration effects. Bravo [135] discusses air-water stripping and illus-... 

A characteristic feature of the structure of samples obtained under the conditions of molecular orientation is the presence of folded-chain crystals in addition to ECC. Kawai22 has emphasized that the process of crystallization from the melt under the conditions of molecular orientation can be regarded as a bicomponent crystallization in which, just as in the case of fibrous structures in the crystallization from solutions, the formation of crystals of the packet type (ECC) occurs in the initial stage followed by the crystallization with folding . 

The activity of a solvent can be determined in a solution from the change in the freezing temperature or the boiling temperature due to the addition of a solute. Consider a process in which a solution is cooled until solid (component 1) crystallizes from solution. An equilibrium is established so that... 

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