‌应用化学专业英语unit17

‌应用化学专业英语unit17 Unit 17 Crystallization 结晶 Crystallisation ranks 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, srystallisation frequently proves to be the cheapest and sometimes the easiest way in which a pure substance can be produced from an impure solution. Conventional distillation techniques cannot separate efficiently close-boiling liquids or those that from azeotropes, yet crystallization may often lead to their complete separation. There is evidence that the petroleum industry if now turning its attention to crystallisation techniques to deal with difficult separations. 结晶在化工过程中占据重要地位是由于生产纯化学品的需要。除了制得的最 终产品在外观上更具有吸引力的事实之外，结晶常被 证明 住所证明下载场所使用证明下载诊断证明下载住所证明下载爱问住所证明下载爱问 是最廉价、有时也是做 简便的从不纯溶液中准备纯物质的方法。传统的蒸馏技术不能有效的分离沸点接 近的液体或共沸混合物，而结晶常常可以使它们完全分开。有证据表明石油工业 已经将注意力转向结晶技术以堆肥困难的分离。 The methods available for crystallization are many and varied. Crystals can be grown from the liquid or thee vapor phase, but in all cases the state of supersaturation 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 from their solutions merely by cooling, while others have to be evaporated to a more concentrated form. In cases of very high solubility, or for heat-labile solutions, another substance may have to be added to the system to reduce the solubility of the 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. 适用于分离的方法很多也各不相同。晶体可以在液体或气态中生成，但在所 有的情况下都必须首先达到过饱和，达到过饱和的方法取决于晶体系的性质;有 些溶液仅通过冷却就很容易从它们的溶液中沉淀，而另一些需要经过蒸发达到更 高浓度，在溶解度很高的情况下，或热不稳定溶液，必须向系统中加入另一种物 质以降低溶液中溶质的溶解度。而且，可以通过两种物质的反应生成过饱和液体 或气体，其中一种反应产物沉淀出来。 Cooling and Evaporation冷却和蒸发 One of the most common ways in which the supersaturation of 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 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. 最常见的获得过饱和溶液的方法之一使通过冷却过程。如果溶液的溶解度随 温度的降低而降低，刘强时一部分溶剂会沉淀出来，对搅拌充分的系统的缓慢的 控制的冷却速度能够得到有规则尺寸的晶体。如果在冷却过程中蒸发部分溶剂， 晶体的产率将增加。 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 mey have to be deliberately evaporated 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. 如果溶液中溶剂的溶解性质随温度变化很小，则必须审慎的蒸发部分溶剂以 达到必要的过饱和以及晶体沉淀。冷却和蒸发技术广泛用于工业结晶。多数商业 上重要的溶质溶液系统可以使用这些方法的一种或另一种。常用的冷却和蒸发结 晶器将在后面给出。 The yield from a cooler or evaporator can be calculated from the general equation: WR[C-C(1-V)]12Y=1-C(R-1)2 冷却和蒸发的产率可由下式计算: 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 solvate (e.g. hydrate) and unsolvated (e.g. anhydrous) solute; and C,C=initial and final solution concentrations, 12 reapectively (kg of unsolvated solute per kg of solvent). The yield calculated from the above equation is the theoretical maximum on the assumptions (a0 that C refers to 2 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. 其中Y,结晶产率(kg);W,初始溶剂质量(kg);V,失去溶剂的质量，蓄 意的或不可避免的，通过蒸发(kg每kg初始溶剂);R,溶剂化(如水合物)与 非溶剂化(如无水的)溶质的分子量比值;C1，C2,相应的初始和最终溶液浓度 (kg非溶剂化溶质每kg 溶剂)。由上式计算的产率是理论上的最大值，假设(a) C2表示最终温度下的平衡浓度，和(b)当从母液中分离出来的晶体洗涤时没有 溶质损失。 Controlled Seeding控制加入晶种 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 matter takes place on these nuclei only. The seeds should be dispersed uniformly throughout the solution by means of gentle agitation; and if the temperature is carefully regulated, considerable control is possible over the final product size. Deliberate seeding is frequently employed in industrial crystallizations; the actual weight of seed material to be added depends on the solute deposition, the size of the seeds and the product: 33Ws=Wp(L/L) sp 在结晶过程中必须不惜任何代价避免副产物(细糖晶粒)的产生;决不能让 溶液变得不稳定。蓄意添加物，即精心选择的晶种，只要结晶沉淀仅在这些晶核 上发生就是允许的。通过缓缓搅拌是晶种在溶液中均匀分散，如果仔细控制温度， 则对最终产品尺寸进行相当程度的控制是可能的。精选的晶种经常用于工业结晶 中;加入的晶种的精确重量取决于溶质沉淀、晶种粒度和产品粒度: Where Ws and Wp are the weights and L and L are the mean particle sizes of sp 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 3LsYWs=33L-Lps 其中Ws和Wp是重量。而Ls和Lp相应的为晶粒和产品的主要粒子尺寸。产 ，Ws和Ws, ;晶体产率，品重量Wp为晶体产率Y加上晶种的重量，如Wp,Y 是通过式(1)的溶解度数据计算而得到的。 Seeds as well as 5μm have been used in sugar boiling practice; these tiny particles are produced by prolonged ball-milling in an inert medium, e.g. isopropyl 3alcohol or mineral oil, and 500g of such seeds may be quite sufficient for 50m of massecuite. 在糖的沸腾试样中常加入5μm大小的晶种;这些微小的晶体是在惰性介质 中经长期球磨研磨而得到的，惰性介质如异丙醇或矿物油，500g这种晶体足以3用于50m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 substance may be used to introduce crystallization. For example, phosphates will often nucleate solutions of arsenates. Small quantities of sodium tertraborate decahydrate can induce the crystallization of sodium sulphate decahydrate. Crystalline organic homologues, derivatives and isomers are frequently used for inducing crystallization; phenol can nucleate m-cresol, and ethyl acetanilide can nucleate methyl acetanilide. 晶种不必含有待结晶物，除非需要绝对纯的产品。使用同晶化合物的微小晶 体来引发结晶。例如，磷酸盐常作为砷酸盐溶液的晶种。一定量的四硼酸钠十水 合物可以引发硫酸钠十水合物的结晶。结晶有机同系物、衍生物和同分异构体常 使用诱导结晶;苯酚可诱导m,甲酚，乙基乙酰苯胺可诱导甲基乙酰苯胺。 During the operation if the temperature is curbed, then the crystallization could be called “controlled crystallization”. 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 a regular and predetermined size can be grown. Many large-scale crystallization operations are carried out in this manner. 在控温的反应中，结晶可称为“控制的结晶”，因为温度是控制的，所以在 整个反应过程中，体系保持在亚稳定状态，微晶生长的速度完全由冷却速度控制， 没有纯晶体的突然沉积，因为体系没有进入不稳定带，可长出规则的，预定尺寸 的晶体。工业规模的结晶可以用这种方法实现。 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 seed can be exceptionally large. If, for simplicity, spheres of diameter d and density ρ are 3considered, the mass of one seed is π/6ρd. Thus 100g of 0.1mm (~150mesh) seeds of 3a substance of density of 2g/cm will contain about 100 million separate particles, and every seed is a potential crystal. For controlled growth, the liquor in the crystallizer 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; high magma densities, up to 20~30 per cent suspended solids, help in this respect. If, despite these precautions, unwanted nucleation still occurs, some system of false grain removal must be operated; continuous cieculation of the spent liquor through a fines trap is one such method. 在工业结晶中伴随产生的晶种尺寸实际上非常小，而考虑到它们的极小的尺 寸，晶粒数目将非常巨大。如果，简单化地，考虑到球直径d和密度ρ，一个晶33粒的尺寸为π/6ρd。因此，100g0.1mm(,150目)的密度为2g/cm的晶种，大 约包含1亿个独立粒子，每个离子都是潜在晶体。在控制生长中，结晶器中的液 体不允许产生晶核，剧烈的搅动以及动力学和热力学震动应该避免。如果，即使 采取了预防措施，晶核仍然出现，有些系统需要启动临时的晶粒转移，通过一个 精细汽水阀是废液连续循环是这样的方法之一。 Salting-out Crystallisation盐析结晶 Another way in which the supersaturation of a solution can be effected is by the addition to the system of some substance that reduced the solubility of the solute in the solvent. The added substancewhich may be a liquid, solid, or gas, is often refered 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. 过饱和溶液能够有效的实现过饱和溶液结晶的方法是在体系中加入一些能 够降低溶质在溶剂中溶解度的物质，加入的物质可以是液体、固体或气体，常称 为稀释剂或沉淀剂，常用的是液体稀释剂，这种过程称为盐析、沉淀或盐析结晶。 稀释剂要求能和原始溶液的溶剂混溶，至少在涉及到的温度范围内是可溶的，而 溶质在其中相对不溶，而且，这种溶液,稀释剂混合物易于分离，如通过精馏。 Although salting-out is widely employed industrially, relatively few published data are available regarding its use in crystallization operations. The process is commonly encountered, for instance, in the crystallization of organic substances from water-miscible orgabic solvents by the controlled addition of water to the solution; the term “water-out” is used in this connection. Some of the advantages of salting-out or dilution crystallization are as follows. Very concentrated initial solutions can be prepared, often with great ease, by dissolving the impure crystalline mass in a suitable 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 crystallization 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 solventdiluent 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. 尽管盐析在工业上应用很广，但关于它在结晶过程的应用方面发表的数据很 少。这种过程是经常遇到的，例如，通过控制水的加入量从水混溶有机溶剂中结晶有机物质;在这种联系中常用到“水析”这个词。盐析或盐析结晶的好处如下:可以制备非常浓的原始溶液，常常非常简单，通过蒸馏在合适溶剂中的不纯结晶物得到。在盐析的同时冷却可以提高溶质的回收率。如果溶质在最处的溶液中溶解度很大，高的溶解温度是不合适的，在结晶过程中每批次的温度都控制得很低;这对于热不稳定物质是很有利得。当由于不需要的不纯物在溶剂,稀释剂混合物中溶解度很大，而使得母液中保留了不必要的不纯物时，提纯变得非常简单。大概盐析结晶的最大缺点就是需要分离单元来处理相对大量的母液，以分离溶剂和稀释剂，其中得一种或两种时贵重的。