Crystallization from solution equipment

Crystallization from solution is a widely utilized separation and purification technique in chemical industry. It is characterized by the formation of a spectrum of differently sized crystals. This spectrum, called the Crystal Size Distribution or CSD, is highly important for the performance of the crystallizer, the crystal handling equipment like centrifuges and dryers, and the marketability of the produced crystals. However, in many industrial crystallizers, the observed CSD s show large transients due to disturbances or are unstable because of the internal feedback mechanisms of the crystallization process ). The main limitation for effective CSD control was the lack of a good on-line CSD measurement device, but recent developments show that this hurdle is taken (2). 

Since crystallization has developed largely as an art, orderly classification of equipment is difficult. Several systems have been proposed by various authors, but disagreement and confusion persists in the literature. The broadest classification is based on the nature of the phases involved (a) crystallization from solution, (b) crystallization from the melt, and (c) crystallization from the vapor phase. 

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),... 

Thiosulfates also are sometimes used as reducing agents. They result from the reaction of sulfur or a polysulfide with a sulfite. Sodium thiosulfate is widely available in solid form, both anhydrous and as the pentahydrate (the photographer s hypo ). The pentahydrate crystallizes from solutions with concentrations between 30% and 60% Na2S203. The compound is quite soluble in water (33.3% at 0°C from the anhydrous form), and hypo dissolves in its water of hydration at 48°C. All forms are nontoxic under normal conditions and are used as food additives. In addition, Na2S203 forms neutral solutions. While correspondingly easier to handle than NaHS03, it is normally stored in similar 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. 

Another major type of equipment among the family of crystallizers is the flaker. The flaker is a piece of equipment that is used for the production by chilling of flakes, chips, large crumbs, or crystals from a hot, concentrated solution or melted crystals. 

Hydroxy-B-homo-5a-cholestan-7-one acetate (54b) A solution of 3/3-hydroxy-5a-cholestan-7-one acetate (51b 5 g mp 146-148°) in dioxane-ethanol (100 ml, 1 1) is placed in a 250 ml three-necked flask equipped with a mechanical stirrer and thermometer and is cooled to 0° (iee-salt bath). Powdered potassium cyanide (7.3 g) is added portionwise with stirring. Acetic acid (8 ml) is then added dropwise with constant stirring over 30 min. The resultant mixture is stirred for 1 hr at 0° C and for an additional 2 hr at room temperature. It is then poured into ice water (200 g ice, 100 ml water) and after standing for 1 hr the precipitate is collected by filtration. The product is dissolved in ether (100 ml), the ether solution is washed with 5% sodium bicarbonate, water and dried over anhydrous sodium sulfate. The filtrate is evaporated at reduced pressure and the solid residue (5.1 g) is crystallized from ethyl acetate (30 ml) to yield 2.8 g of cyanohydrin (52b) mp 160-164° repeated crystallization from the same solvent gives a product mp 164-167°. An alternative method of isolation of the cyanohydrin is used when 100 g or larger quantities are worked up. The reaction mixture is poured directly into a mixture of ice water and sodium bicarbonate, the precipitate (mp 155-156°) is washed well with water, dried and used directly for the next step. 

Crystallization from benzene. For crystallization wet, freshly washed, deacidified tetryl is used. It is introduced into container (1) (Fig. 10) equipped with a mixer, a heating jacket and a reflux condenser (2). Container (1) is filled with benzene. The whole is heated, while stirring, until the tetryl is dissolved. The solution is then passed through the warmed filter (5). The products insoluble in benzene are retained... 

In a i-l. three-neck flask equipped with a mechanical stirrer, a reflux condenser, and a thermometer (with bulb immersed in the liquid), are placed ioo g. (0.75 mole) of anhydrous aluminum chloride and 300 g. of carbon disulfide (Note 1). The suspension is cooled to 15-25° and a solution of 60 g. (0.71 mole) of thiophene (p. 72) and 105 g. (0.75 mole) of benzoyl chloride in 225 g. of carbon disulfide is added through the condenser, with stirring, over a period of three and one-half hours (Note 2). The solution is allowed to warm up to room temperature, and stirring is continued for three more hours the reaction mixture is then allowed to stand overnight. The mixture is refluxed on the water bath for three and one-half hours, cooled, poured on ice, and extracted with ether. The ether extract is washed successively with sodium carbonate solution and water, and then dried over calcium chloride. The ether is removed by distillation on the water bath, and the residue is distilled under reduced pressure. The yield of product boiling at 200-209 V30-40 mm. is ir7-r20 g. (88-90 per cent of the theoretical amount). On crystallization from 1 I. of petroleum ether (b.p. 65-1x0°) there is obtained 110 112 g. of product melting at 52°. Another crystallization... 

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. 

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