Distillation heat source

The suitabiHty and economics of a distillation separation depend on such factors as favorable vapor—Hquid equiHbria, feed composition, number of components to be separated, product purity requirements, the absolute pressure of the distillation, heat sensitivity, corrosivity, and continuous vs batch requirements. Distillation is somewhat energy-inefficient because in the usual case heat added at the base of the column is largely rejected overhead to an ambient sink. However, the source of energy for distillations is often low pressure steam which characteristically is in long supply and thus relatively inexpensive. Also, schemes have been devised for lowering the energy requirements of distillation and are described in many pubHcations (87). 

There are a number of ways to provide the heating or cooling medium at temperatures closer to the optimum level. One is by use of double-effect distillation, which uses the overhead vapor from one column as the heat source for another column such that the second column s reboiler becomes the first column s condenser. This basically cuts the temperature differential in half, and shows up as an energy saving because external heat is suppHed to only one of the units. 

Interlock cold liquid feeds with heat source (e.g., distillation column)... 

Heat sources for distillation must be closely controlled to prevent overheating or too rapid distillation. The best heat sources are electrically heated liquid baths. Mineral oil or wax is a satisfactory medium for heat exchange up to about 240°. The medium may be... 

The residue from vacuum distillation at 92-135°C/2.6 mbar darkened, thickened, then exploded after removal of the heat source. 

The major portion of wet solid is removed from the flask and transferred to a 1-1. Erlenmeyer flask. The remaining solid is washed out with a little distilled water, and the washings are transferred to the flask. The solid is dissolved in the smallest amount of boiling water required, the flask removed from the heat source, and 50 ml. of ethyl alcohol added (Note 5). The flask is allowed to cool slowly, and, after the major portion of the product has separated, the flask is cooled to 5° overnight. The solid is removed by filtration and washed with cold alcohol, then acetone, and finally ether. The white, crystalline, air-dried product weighs 82-93 g. (73-82%), m.p. 291-293° dec. (rapid heating) (be compound starts to turn brown at about 280-285° (Note 6). 

Owing to the possibly explosive nature of the ester, the distillation was conducted behind a safety screen, using a water bath for the heat source and keeping the pressure as low as convenient. 

A water bath should be used as a heat source to avoid overheating, which leads to lowered yields. This distillation should be carried out carefully to prevent loss of product by co-distillation with the ethanol. 

Remarkable thermal stability was attributed to 3,5-diphenyloxa-diazole by the first workers in the field in 1884. Tiemann and Kriiger7 observed that this compound could be distilled without decomposition at normal pressure (b.p. 296°). Ainsworth157 modified that position when he found slow decomposition occurred at 250° with an open flame as heat source. In contrast, 2,4-diphenyl-1,3,4-oxadiazole decomposed only at 500°.156 With substitution on aryl groups, Ainsworth157 found that 3-... 

Vapor compression uses the reverse principle of multieffecl distillation. As a vapor is compressed, its temperature and pressure increase. The compressed vapor can be used instead of fresh steam as a heat source at the high-temperature end of the distillation process. 

B. Azetidine. A stirred mixture of 38 g. (0.68 mole) of potassium hydroxide pellets in 100 ml. of white mineral oil (Note 8) is heated to 140-150° in a four-necked 500-ml. round-bottomed flask, fitted with an air-driven Hershberg stirrer, a thermometer, a dropping funnel, and a 6-in. Vigreux column fitted with a vacuum-distillation head. The flask is removed from the heat source, and 50 g. (0.32 mole) of purified l-(2-carbethoxyethyl)azetidine is added dropwise at a rate sufficient to maintain the reaction temperature at 150° (Note 9). After addition is complete, the reaction mixture is heated to 200° at 50 mm. to remove all traces of ethanol (Note 10). The flask is fitted with a distillation head and a nitrogen bubbler, and the distillation is resumed at atmospheric pressure until azetidine distills (210° maximum pot temperature) (Note 11). The resulting product (19.6 g., 85% purity) is dried over potassium hydroxide and redistilled through a short Vigreux column to furnish 14.5-15.8 g. (80-87%) of purified azetidine, b.p. 62-63° (Note 12). 

As stated earlier, the separation of mixtures into their constituents requires energy. In distillation, this energy is supplied as heat. To this end, it is useful to recall that Carnot (see Chapter 3 for details) showed that the maximum amount of work that can be extracted from a heat source Q at T > T0 with respect to the surroundings at T0 is given by... 

Replace the condenser with a flash distillation apparatus fitted with a 500 mL receiver cooled in a dry ice/IPA bath. Flash distill all of the volatile material from the reaction flask under vacuum at room temperature. When transfer of volatile materials is no longer observed, add the TG to the reaction mixture via syringe and heat the mixture with an oil-bath to 70°C. When the TG begins to flash distill, remove the heat source but allow the mixture to remain under full vacuum for 2.5 h at room temperature. 

Vapor recompression eliminates the need for a conventional heat source, such as steam, to drive the reboiler. There is, however, an electrical energy requirement to drive the compressor which is not present in conventional distillation. The key advantage of vapor recompression is that the cost of running the compressor is often lower than the cost of driving a conventional reboiler. Under ideal conditions, the operating cost of a vapor recompression... 

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