Vacuum Crystallisation

In modern equipment for crystallisation, on the other hand, any crystalline product can be produced with purities of about 99.9% and with almost any desired crystal size distribution (CSD). The specific rate of production in industrial crystaUisers is several thousand times larger than in solar ponds [1-4]. 

The result of any crystallisation process is a crystalline matter with a certain CSD, certain crystal habit and purity. These properties very often are quality requirements and well defined by market demands. Certainly, also the crystallisation process itself requires a minimum CSD, as the resulting suspension still has to be separated. This separation can be effected the more perfectly (purity) as more compact (crystal habit) and coarser (CSD) the crystals are. Quality and economy of the separation process crystallisation are therefore strongly dependent from crystal size, CSD and crystal habit. These properties also take influence to the storage ability, the dust-freeness and also the bulk density of the product. 

Vacuum Technology in the Chemical Industry, First Edition. Edited by Wolfgang Jorisch. 

As higher the supersaturations are, as faster the crystal growth is and as more effective the crystalliser. Certainly, not any supersaturation can be chosen, because also the nucleation processes are supersaturation dependent. There is the spontaneous or primary nucleation which is caused by a critical height of supersaturation 

On the left hand side, it is shown a sketch of a (forced circulated) FC-type crystalliser, a crystalliser with external circulation loop. On the right hand side, one can find a simplified solubility system in which the metastable limit is marked with a dashed line. What happens within one circulation loop is indicated by numbers in the solubility diagram as well as in the FC-crystalliser sketch. 

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Bis-(tetrabutylammonium) dichromate [56660-19-6] M 700.9, m 139-142°. Wash with water and dry in a vacuum. Crystallises from hexane (m 79-80°). [Synth Commun 10 75 1980.] (Possible CARCINOGEN). 

The formation of solids in evaporators is not always undesirable and, indeed, this is precisely what is required in the evaporator-crystalliser discussed in Chapter 15. The evaporator-crystalliser is a unit in which crystallisation takes place largely as a result of the removal of solvent by evaporation. Cooling of the liquor may, in some cases, produce further crystallisation thus establishing conditions similar to those in vacuum crystallisation. The true evaporator-crystalliser is distinguished, however, by its use of an external heat source. Crystallisation by evaporation is practised on salt solutions having... 

What is the yield of sodium acetate crystals (CH3C00Na.3H20) obtainable from a vacuum crystalliser operating at 1.33 kN/m2 when it is supplied with 0.56 kg/s of a 40 per cent aqueous solution of the salt at 353 K The boiling point elevation of the solution is 11.5 deg K. 

A vacuum crystalliser operates on a slightly different principle from the reduced-pressure unit since supersaturation is achieved by simultaneous evaporation and adiabatic cooling of the feedstock. A hot, saturated solution is fed into an insulated vessel maintained under reduced pressure. If the feed liquor temperature is higher than the boiling point of the solution under the low pressure existing in the vessel, the liquor cools adiabatically to this temperature and the sensible heat and any heat of crystallisation liberated by the solution evaporate solvent and concentrate the solution. 

Methyl 2-naphthyl ether (2-methoxynaphthalene, Nerolin) [93-04-9] M 158.2, m 73.0-73.6°, b 138°/10mm 273°/760mm. Fractionally distil the ether under vacuum. Crystallise it from absolute EtOH, aqueous EtOH, C6H6, pet ether or -heptane, and dry it under vacuum in an Abderhalden pistol or distil it in vacuo. The picrate has m 118° (from EtOH or CHCI3). [Kikuchi et al. J Phys Chem 91 574 1987, Beilstein 6 III 2969, 6 IV 4257.]... 

HCl (16mL/g), the crystals are separated by centrifugation, washed with chilled EtOH and diethyl ether, and dried under vacuum. Crystallise it from 50% aqueous EtOH, wash it with absolute EtOH, and dry it at 50-55° for 24hours. It has also been crystallised from benzene/MeOH (3 1). It has been salted out with NaCl from a commercial concentrated aqueous solution, then crystallised from water, and dried at 100° in an oven for 8-lOhours. [Beilstein 27 IIEIV 5152.]... 

Boilers are operated in many industries for generating steam, which is required for process heating of reactors, evaporators, distillation columns, etc. Generally, these units need steam at a pressure of 4-6 kg/cm. Only certain equipment like vacuum crystallisers, steam ejectors, etc. need higher steam pressure for operating them to produce the necessary vacuum.)... 

Vacuum drying Vacuum distillation Thin film evaporation Molecular distillation Sublimation Vacuum crystallisation Degassing and mixing Filtration... 

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