Extraction, advantages with reflux

Fractionation of liquid mixtures with supercritical carbon dioxide in counter-cur-rent columns can be operated continuously, because liquids can be easily pumped into and out of a column. This represents a big advantage over extrachon from solid materials, as it allows real process intensification - large quantities of feed can be processed with only a small volume under high pressure at any given time. Frac-tionahon, mostly of natural products or extracts, has been extensively studied at the laboratory and pilot-plant scale. The design principles of this type of column have been established, and scale-up procedures devised [1,6]. They can be operated with reflux, as in distillation, and frachonahon can therefore become an extremely se-lechve process. Difficult separahons can be effechvely carried out. 

Place 45 g. (43 ml.) of benzal chloride (Section IV,22), 250 ml. of water and 75 g. of precipitated calcium carbonate (1) in a 500 ml. round-bottomed flask fltted with a reflux condenser, and heat the mixture for 4 hours in an oil bath maintained at 130°. It is advantageous to pass a current of carbon dioxide through the apparatus. Filter off the calcium salts, and distil the filtrate in steam (Fig. II, 40, 1) until no more oil passes over (2). Separate the benzaldehyde from the steam distillate by two extractions with small volumes of ether, distil off most of the ether on a water bath, and transfer the residual benzaldehyde to a wide-mouthed bottle or flask. Add excess of a concentrated solution of sodium bisulphite in portions with stirring or shaking stopper the vessel and shake vigorously until the odour of benzaldehyde can no longer be detected. Filter the paste of the benzaldehyde bisulphite compound at the pump... 

Preparation of benzyl cyanide. Place 100 g. of powdered, technical sodium cyanide (97-98 per cent. NaCN) (CAUTION) and 90 ml. of water in a 1 litre round-bottomed flask provided with a reflux condenser. Warm on a water bath until the sodium cyanide dissolves. Add, by means of a separatory funnel fitted into the top of the condenser with a grooved cork, a solution of 200 g. (181-5 ml.) of benzyl chloride (Section IV.22) in 200 g. of rectified spirit during 30-45 minutes. Heat the mixture in a water bath for 4 hours, cool, and filter off the precipitated sodium chloride with suction wash with a little alcohol. Distil off as much as possible of the alcohol on a water bath (wrap the flask in a cloth) (Fig. II, 13, 3). Cool the residual liquid, filter if necessary, and separate the layer of crude benzyl cyanide. (Sometimes it is advantageous to extract the nitrile with ether or benzene.) Dry over a little anhydrous magnesium sulphate, and distil under diminished pressure from a Claisen flask, preferably with a fractionating side arm (Figs. II, 24, 2-5). Collect the benzyl cyanide at 102-103°/10 mm. The yield is 160 g. 

If the nitration is caiiied out in accordance with this outline, the product will be crystalline and pale yellow in color. The color is due to traces of dinitrothiophene and the other impurities. Mononitrothiophene has been crystallized by earlier workers from ether, alcohol, benzene, and other solvents. As a rule these solvents fail to yield a snow-white product. It has been found in this work that petroleum ether (b.p. 20-40 ) possesses decided advantages in that by prolonged refluxing it extracts mononitrothiophene but does not readily dissolve the impurities. With petroleum ether, snow-white crystals have been obtained in needles 10 to 20 cm. in length. 

The apparatus for Soxhiet extraction is shown in Fig. 14.5 and comprises a flask containing the solvent, a Soxhiet extractor and a reflux condenser (p. 116). The sohd to be extracted is placed in a porous thimble, made from hardened filter paper, and the solvent is heated so that its vapour flows past the thimble, condenses and fills the extractor with hot solvent to extract the solid. When the extractor is full, the solvent (together with the extracted material) siphons back into the solvent flask and the process is repeated automatically. The advantage of this procedure is that fresh solvent continually extracts the solid, which is concentrated in the flask. The disadvantage is that the compound extracted is kept at the boiling point of the solvent for a prolonged period. Soxhiet extractors come in sizes of lOmL to 5000 mL, based on the volume of solvent contained in the extractor. The procedure for using a Soxhiet extraction system is described in Box 14.2. 

Soxhlet extraction with solvent refluxing. Both manual and automated versions are available. The advantage is in reduced time of extraction over conventional extraction. 

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