Example crystallization evaporative

The crystallization tendency of polycarbonate is enhanced by the action of solvents. For example, crystallization may be accomplished by slow evaporation of solvent from cast film (14, 17) or by treatment with swelling agents such as ethyl acetate or acetone. 

During electrolysis, per mole of potassium permanganate one mole of potassium hydroxide is produced, which has to be recovered. This can be achieved, for example, by evaporating the mother liquor to 750 g KOH per L, whereupon the dissolved potassium manganate(Vl) and calcium hydroxide crystallize out and are removed. The potassium hydroxide can be returned to the dissolution step. Other dissolved impurities from the ores, such as silicates or aluminates, have to be removed from the alkali cycle. 

The second step is to select an operating mode and to calculate the heat and material balances. For this example, an evaporative crystallizer will be used. Complete evaporation and crystallization of the feed will be assumed. 

Particles consisting of multiple salts exhibit a more complicated behavior (Figure 10.7). For example, the evaporation of a KCl-NaCl particle is characterized by two step changes the first at 65% with the formation of KC1 crystals and the second at 62% with the... 

There seems to be even less structural similarity for many other metal halides when the crystalline systems are compared with the molecules in the vapor phase. Aluminum trichloride, for example, crystallizes in a hexagonal layer structure. Upon melting and then evaporation at relatively low temperatures, dimeric molecules are formed. At higher temperatures, they dissociate into monomers (Figure 9-60) [9-60]. The coordination number decreases from six to four and then to three in this process. 

The process of fractional crystallization is a technique by which chemists separate and purify many substances. This process takes advantage of differing variations with temperature of the solubilities of several components in a given solvent. If one of the components is insoluble at all temperatures whereas other components are soluble at some temperature, the insoluble component can be separated easily by filtration. If the solubility of one of the soluble components in the mixture increases rapidly with temperature whereas the solubility of the other soluble component increases only slightly with temperature, an appropriate temperature can be chosen such that one of the components will be only slightly soluble whereas the other will be almost entirely in solution. With one component present primarily in the solid phase and the other in solution, a separation can again be achieved easily by filtration. The solvent used may dissociate components that are salts, but must not react further chemically with the components. The solvent must also allow the components to separate as well formed crystals and must be easily removed from the components, for example, by evaporation. Water serves as an appropriate solvent in this experiment. In many industrial operations mixed solvent systems must be used. 

Figure 7-2 shows, for example, crystallization of ammonium sulfate from a crude solution, obtained from a caprolactam process, in a three stage evaporation crystallization unit [7.16], The unsaturated crude solution is fed to a crystal washer (1), in which... 

Typical examples of evaporation are concentration of aqueous solutions of sugar, sodium chloride, sodium hydroxide, glycerol, glue, milk, and orange juice. In these cases the concentrated solution is the desired product and the evaporated water is normally discarded. In a few cases, water, which has a small amount of minerals, is evaporated tp give a solids-free water which is used as boiler feed, for special chemical processes, or for other purposes. Evaporation processes to evaporate seawater to provide drinking water have been developed and used. In some cases, the primary purpose of evaporation is to concentrate the solution so that upon cooling, salt crystals will form and be separated. This special evaporation process, termed crystallization, is discussed in Chapter 12. 

Most fiindamental surface science investigations employ single-crystal samples cut along a low-index plane. The single-crystal surface is prepared to be nearly atomically flat. The surface may also be modified in vacuum. For example, it may be exposed to a gas that adsorbs (sticks) to the surface, or a film can be grown onto a sample by evaporation of material. In addition to single-crystal surfaces, many researchers have investigated vicinal, i.e. stepped, surfaces as well as the surfaces of polycrystalline and disordered materials. 

Anhydrous zinc chloride can be made from the reaction of the metal with chlorine or hydrogen chloride. It is usually made commercially by the reaction of aqueous hydrochloric acid with scrap zinc materials or roasted ore, ie, cmde zinc oxide. The solution is purified in various ways depending upon the impurities present. For example, iron and manganese precipitate after partial neutralization with zinc oxide or other alkah and oxidation with chlorine or sodium hypochlorite. Heavy metals are removed with zinc powder. The solution is concentrated by boiling, and hydrochloric acid is added to prevent the formation of basic chlorides. Zinc chloride is usually sold as a 47.4 wt % (sp gr 1.53) solution, but is also produced in soHd form by further evaporation until, upon cooling, an almost anhydrous salt crystallizes. The soHd is sometimes sold in fused form. 

Enei y. In recent years the concern for energy conservation has resulted in many innovative process improvements to make the manufacture of citric acid more efficient. Eor example, heat produced by the exotherm of the neutralization of citric acid with lime is used in another part of the process where heat is requited, such as the evaporation/crystallization step. 

The dependence of solubility on temperature affects the mode of crystallization. For example, Figure 4 shows that the solubiUty of potassium nitrate is strongly influenced by the system temperature but that temperature has Httle influence on the solubiUty of sodium chloride. As a consequence, a reasonable yield of KNO crystals can be obtained by cooling a saturated feed solution on the other hand, cooling a saturated sodium chloride solution accomplishes Httle crystallization, and evaporation is required to increase the yield of sodium chloride crystals. 

Batch Crystallization. Crystal size distributions obtained from batch crystallizers are affected by the mode used to generate supersaturation and the rate at which supersaturation is generated. For example, in a cooling mode there are several avenues that can be followed in reducing the temperature of the batch system, and the same can be said for the generation of supersaturation by evaporation or by addition of a nonsolvent or precipitant. The complexity of a batch operation can be ihustrated by considering the summaries of seeded and unseeded operations shown in Figure 19. 

Example 1 Yield from a Crystallization Process A 10,000-lh batch of a 32,5 percent MgS04 solution at 120°F is cooled without appreciable evaporation to 70°F, What weight of MgS04-7H20 crystals will be formed (if it is assumed that the mother liquor leaving is saturated) ... 

In addition to diamond and amorphous films, nanostructural forms of carbon may also be formed from the vapour phase. Here, stabilisation is achieved by the formation of closed shell structures that obviate the need for surface heteroatoms to stabilise danghng bonds, as is the case for bulk crystals of diamond and graphite. The now-classical example of closed-shell stabilisation of carbon nanostructures is the formation of C o molecules and other Fullerenes by electric arc evaporation of graphite [38] (Section 2.4). 

Batch crystallizers are widely used in the chemical and allied industries, solar saltpans of ancient China being perhaps the earliest recorded examples. Nowadays, they still comprise relatively simple vessels, but are usually (though not always) provided with some means of agitation and often have artificial aids to heat exchange or evaporation. Batch crystallizers are generally quite labour intensive so are preferred for production rates of up to say 10 000 tonnes per year, above which continuous operation often becomes more favourable. Nevertheless, batch crystallizers are very commonly the vessel of choice or availability in such duties as the manufacture of fine chemicals, pharmaceutical components and speciality products. 

Crystallization takes place when the solute slowly comes out of solution as crystals, perhaps as the solvent evaporates. For example, salt crystals that form when water evaporates line the shores of the Great Salt Lake in Utah. In precipitation, a solute comes out of solution so rapidly that a single crystal does not have time to form. Instead, the solute forms a finely divided powder called a precipitate. Precipitation is often almost instantaneous (Fig. G.4). 

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