Vacuum Evaporation Crystallization

Unlike vacuum cooling crystallization, this method does not depend on the concentration and temperature of the feed solution. Additional heat can be introduced by means of a heat exchanger and thus undersaturated solutions treated in a crystallizer. Furthermore, the concentration factor of the mother liquor can be selected that is, the quantity of solvent to be evaporated is adjustable to the requirements of the mass balance. The mass balance is based on the corresponding 

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A Kryslai type of vacuum evaporator-crystallizer B Forced-circulation pump C Slurry-redrculation pump D Mother-liquor recycle pump E Mother-liquor tank F Continuous or batch centrifuge G Slurry concentrator H Drier conveyor I Rotary drier J Dried-product conveyor K Vacuum condenser-ejector unit... 

Figure 11.12 shows the different designs of the simple FC group depending on the crystallization process such FC crystallizers can be used for (vacuum cooling crystallization or vacuum evaporative crystallization), one can find the typical FC , and with and without a tube and shell heat exchanger. These are the simple stirred tank 0, the draft tube (DT) crystallizer (2), and the FC crystallizer ((J), ). 

As mentioned above, the crystallization process is not yet complete with the crystallization itself. The suspension produced first has to be separated, while the crystals still have to be dried and packaged. The vapors released have to be condensed and the noncondensable gases extracted from the system by means of a vacuum pump. Figure 11.22 shows a simplified flowchart of the principle, using vacuum evaporation crystallization as an example. 

Both anatase and mtile are broad band gap semiconductors iu which a fiUed valence band, derived from the O 2p orbitals, is separated from an empty conduction band, derived from the Ti >d orbitals, by a band gap of ca 3 eV. Consequendy the electrical conductivity depends critically on the presence of impurities and defects such as oxygen vacancies (7). For very pure thin films, prepared by vacuum evaporation of titanium metal and then oxidation, conductivities of 10 S/cm have been reported. For both siugle-crystal and ceramic samples, the electrical conductivity depends on both the state of reduction of the and on dopant levels. At 300 K, a maximum conductivity of 1 S/cm has been reported at an oxygen deficiency of... 

Evaporative crystalli rs generate supersaturation by removing solvent, thereby increasing solute concentration. These crystallizers may be operated under vacuum, and, ia such circumstances, it is necessary to have a vacuum pump or ejector as a part of the unit. If the boiling poiat elevation of the system is low (that is, the difference between the boiling poiat of a solution ia the crystallizer and the condensation temperature of pure solvent at the system pressure), mechanical recompression of the vapor obtained from solvent evaporation can be used to produce a heat source to drive the operation. 

The reaction mixture is diluted with 250 ml of water, the mixture is transferred to a 2 liter flask using methanol as a wash liquid, and the organic solvents are distilled at 20-25 mm using a rotary vacuum evaporator. The product separates as a solid and distillation is continued until most of the residual toluene has been removed. The solid is collected on a 90 cm, medium porosity, fritted glass Buchner funnel and washed well with cold water. After the material has been sucked dry, it is covered with a little cold methanol, the mixture is stirred to break up lumps, and the slurry is kept for 5 min. The vacuum is reapplied, the solid is rinsed with a little methanol followed by ether, and the material is air-dried to give 9.1 g (85%), mp 207-213° after sintering at ca. 198°. Reported mp 212-213°. The crude material contains 1.0-1.5% of unreduced starting material as shown by the UV spectrum. Further purification may be effected by crystallization from methanol. 

Lead sulfide films have been prepared by various deposition processes like vacuum evaporation and chemical bath deposition. Electrochemical preparation techniques have been used in a few instances. Pourbaix diagrams for all three aqueous lead-chalcogen Pb-S, Pb-Se, and Pb-Te systems, along with experimental results and cited discussion on the chemical etching and electrolytic polishing of lead chalcogenide crystals and films, have been presented by Robozerov et al. [201]. 

From the manipulative standpoint, the critical step lies in the vacuum evaporation of the solvent from the solution of Compound 118 and the two drops of phenyl azide. Care must be observed that no foaming or undue chilling occurs during the evaporation undue chilling may cause some Compound 118 to crystallize out of contact with the phenyl azide and prevent quantitative formation of the dihydrotriazole. 

Offringa, J.C.A., de Kruif, C.G., Van Ekeren, P.J., Jacobs, M.H.G. (1983) Measurement of the evaporation coefficient and saturation vapor pressure of fraras -diphenylethene using a temperature-controlled vacuum quartz-crystal microbalance. J. Chem. Ther-modyn. 15, 681-690. 

Property measurements of fullerenes are made either on powder samples, films or single crystals. Microcrystalline C6o powder containing small amounts of residual solvent is obtained by vacuum evaporation of the solvent from the solution used in the extraction and separation steps. Pristine Cgo films used for property measurements are typically deposited onto a variety of substrates (< . , a clean silicon (100) surface to achieve lattice matching between the crystalline C60 and the substrate) by sublimation of the Cr,o powder in an inert atmosphere (e.g., Ar) or in vacuum. Single crystals can be grown either from solution using solvents such as CS and toluene, or by vacuum sublimation [16, 17, 18], The sublimation method yields solvent-free crystals, and is the method of choice. 

Keep the solution overnight at 0 °C and allow the nicotinic acid crystals to settle down to the bottom of the container. Separate the crystals and dissolve in distilled water, adjust the pH to 4—5 and cool to 0-4 °C. Decant the liquid and dry the crystals in vacuum evaporator or at 50 °C in an oven (90% yield). 

Anhydrous a-dextrose is manufactured by dissolving dextrose monohydrate in water and crystallizing at 60—65°C in a vacuum pan. Evaporative crystallization is necessary to avoid color formation at high temperatures and hydrate formation at low temperatures. The product is separated by centrifugation, washed, dried to a moisture level of ca 0.1%, and marketed as a very pure grade of sugar for special applications. 

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