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Crystallisation

By Maria Willis,2014-03-24 17:03
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Crystallisation

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    Crystallisation

Crystallisation 结晶ranks列队 high in the list of industrial processes devoted to the production of pure

    chemicals. Apart from the fact that its final product has an attractive appearance, crystallization frequently proves to be the cheapest and sometimes the easiest way in which a pure substance can be produced from an impure solution. Conventional 传统的distillation 蒸馏techniques cannot separate

    efficiently close-boiling liquids or those that from azeotropes, 共沸点 混合物yet

    crystallization may often lead to their complete separation. There is evidence that the petroleum 石油

    industry is now turning its attention to crystallization techniques to deal with difficult separations. Apart from 除了

    The methods available for crystallization are many and varied. Crystals can be grown from the liquid or the vapor phase, but in all cases the state of supersaturation超饱和 has first to be

    achieved. The way in which supersaturation is produced depends on the characteristics of the crystallizing system; some solutes are readily deposited沉积 from their solutions merely仅仅 by

    cooling, while others have to be evaporated蒸发 to a more concentrated form. In cases of very

    high solubility, or for heat-labile易变化的; 不稳定的 solute in the solvent. Again,

    supersaturation of the liquid or gaseous phase may be caused by the chemical reaction of two substances; one of the reaction products is then precipitated沉淀.

    Cooling and Evaporation

    One of the most common ways in which the supersaturation of a liquid can be achieved is

    by means of a cooling process, If the solubility of the solute in the solvent decreases with a decrease in temperature, some of the solute will be deposited on cooling; a slow controlled rate of cooling in an agitated 搅拌system can result in the production of crystals of regular size. The

    crystal yield may be slightly increased if some of the solvent evaporates during the cooling

process.

    If the solubility characteristics of the solute in the solvent are such that there is little change with a reduction in temperature, some of the solvent may have to be deliberately慎重地!谨慎地!

    有意的!故意的evaporated使蒸发 from the system in order to effect the necessary

    supersaturation and crystal deposition. Cooling and evaporative techniques are widely used in

    industrial crystallization; the majority of the solute solvent systems of commercial importance can be processed by one or other of these methods. Descriptions of many of the cooling and evaporating crystallisers commonly encountered are given later.

    The yield from a cooler or evaporator can be calculated from the general equation:

     Y= WR[C1-C2(1-V)] /1-C2(R-1) (17.1)

    where Y=crystal yield (kg);W=weight of solvent present initially (kg); V=weight of solvent lost, either deliberately or unavoidably, by evaporation(kg per kg of original solvent); R=ratio of the molecular weights of solvated (e.g. hydrate) and unsolvated (e.g. anhydrous) solute; and C1,

    C2=initial and final solution concentrations, respectively (kg of unsolvated solute per kg of solvent). They yield calculated from the above equation is the theoretical 理论的maximum on

    the assumptions 假设(a) that C2 refers to the equilibrium saturation at the final temperature, and (b) that no solute is lost when the crystals are washed after being separated from the mother liquor.

    Controlled Seeding

    During a crystallization operation the accidental production of nuclei (false grain) must be

    avoided at all costs; the solution must never be allowed to become labile. The deliberate addition of carefully selected seeds, however, is permitted so long as the deposition of crystalline结晶质的

    matter takes place on these nuclei only. The seeds should be dispersed uniformly throughout the solution by means of gentle agitation; and if temperature is carefully regulated, considerable control is possible over the final product size. Deliberate seeding is frequently employed in industrial crystallisations; the actual weight of seed material to be added depends on the solute deposition, the size of the seeds and the seeds and the product:

    WS=WP(LS/LP) (17.2)

    where WS and WP are the weights and LS and LP are the mean particle sizes of the seed and

    product, respectively. The product weight, WP, is the crystal yield, Y, e.g. as calculated from solubility data using equation (1), plus the weight of added seeds, i.e. Wp=Y+WS, and

    Ws=YLs/LP-LS (17.2a)

    Seeds 晶种as small as 5µm have been used in sugar boiling practice; these tiny particles are produced by prolonged 长时期的ball-milling球磨研磨in an inert[]性的; 不活泼的;

    钝的; 不起化学作用的; 中和()medium, e.g. isopropyl alcohol or mineral oil, and 500g of

    3 such seeds may be quite sufficient for 50mof massecuite. 糖膏

    The seeds do not necessarily have to consist of the material being crystallized, unless absolute purity of the final product is required. A few tiny crystals of some isomorphous同晶型的substance may be used to

    induce引起; 导致crystallization. For example, phosphates will often nucleate (使)成核 solutions of

    arsenates. Small quantities of sodium tetraborate decahydrate can induce the crystallisation of sodium

    sulphate decahydrate. Crystalline organic homologues同系物; 同系化合物, derivatives and isomers are

    frequently used for inducing crystallisation; phenol can nucleate m-cresol, and ethyl acetanilide can

    nucleate methyl acetanilide.

During the operation if the temperature is curbed, 勒马索!控制; 约束 then the crystallisation could be

    called controlled crystallisation”. The temperature is controlled so that the system is kept in the

    metastable state throughout the operation, and the rate of growth of the small crystal is governed solely

    by the rate of cooling. There is no sudden deposition of fine crystals, because the system does not enter

    the labile zone, then crystals of f regular and predetermined size can be grown. Many large-scale

    crystallization operations are carried out in this manner.

    The mass of crystal seeds accidentally produced in large crystallisers may be very small indeed, yet on account of their minute size the number of seeds can be exceptionally large. If, for simplicity, spheres of diameter d and density ρ are considered, the mass of one seed is π/6ρd3.

    Thus 100g of 0.1mm (150mesh) seeds of a substance of density 2g/cm3 will contain about 100 million separate particles, and every seed is a potential crystal. For controlled growth, the liquor in the crystalliser must not be allowed to nucleate. Vigorous agitation and mechanical and thermal shock should be avoided, and the supersaturation should be kept to the absolute working minimum.

    Salting-out Crystallisation

    Another way in which the supersaturation of a solution can be effected is by the addition to the system of some substance that reduces the solubility of the solute in the solvent. The added substance which may be a liquid, solid, or gas, is often referred to as a diluent or precipitant. Liquid diluents are most frequently used. Such a process is known as salting-out, precipitation or crystallisation by dilution. The properties required of the diluent are that it be miscible with the solvent of the original solution, at least over the ranges of concentration encountered, that the solute be relatively insoluble in it, and also that the final solvent diluent mixture be capable of

    easy separation, e.g. by distillation.

    Although salting-out is widely employed industrially, relatively few published data are available regarding its use in crystallisation operations. The process is commonly encountered, for instance, in the crystallisation of organic substances from water-miscible organic solvents by the controlled addition of water to the solution; the term watering-out is used in this connection.

    Some of the advantages of salting-out or dilution crystallisation are as follows. Very concentrated initial solutions can be prepared, often with great ease, by dissolving the impure crystalline mass in a suitable can solvent. A high solute recovery can be made by cooling the solution as well as salting it out. If the solute is very soluble in the initial solvent, high dissolution temperatures are not necessary, and the temperature of the batch during the crystallisation operation can be kept low; this is advantageous when heat-labile substances are being processed. Purification is sometimes greatly simplified when the mother liquor retains undesirable impurities owing to their greater solubility in the solvent diluent mixture. Probably the biggest disadvantage of dilution crystallisation is the need for a recovery unit to handle fairly large quantities of the mother liquor in order to separate solvent and diluent, one or both of which may be valuable.

    thFrom Unit Operation of Chemical Engineering 4 edition,

    by Warren L. Mccabe, et.al,1993

    New Words and Expressions

acetanilide n.乙酰苯胺!退热冰

    coexist vi. 同时?同地,存在!共存 coexistence n. 共存!共处 coexistent a.同存的!共处的 deposition n.沉淀!沉淀物 depositor n. 存放者!淀积器 evaporator n. ?食物干燥,蒸发器 homologue n. 同系物!同源染色体!相应物 isopropanol n. 异丙醇 mesh n. 筛眼!每平方英寸的网孔?筛眼,数 metastable a. 亚稳的!准稳的 methyl acetanilide 甲基乙酰苯胺

    mother liquor 母液

    precipitator n. 除尘器!沉淀器 supersaturate vt.使过饱和 supersaturation n.过饱和?现象, nuclei (false grain) 晶核

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