Efficiency, fractionating column

Reflux ratio. This is defined as the ratio between the number of moles of vapour returned as refluxed liquid to the fractionating column and the number of moles of final product (collected as distillate), both per unit time. The reflux ratio should be varied according to the difficulty of fractionation, rather than be maintained constant a high efficiency of separation requires a liigh reflux ratio. ... 

The efficient fractionating columns in general use may be divided into two main groups —... 

Attention is directed to the fact that ether is highly inflammable and also extremely volatile (b.p. 35°), and great care should be taken that there is no naked flame in the vicinity of the liquid (see Section 11,14). Under no circumstances should ether be distilled over a bare flame, but always from a steam bath or an electrically-heated water bath (Fig.//, 5,1), and with a highly efficient double surface condenser. In the author s laboratory a special lead-covered bench is set aside for distillations with ether and other inflammable solvents. The author s ether still consists of an electrically-heated water bath (Fig. 11, 5, 1), fitted with the usual concentric copper rings two 10-inch double surface condensers (Davies type) are suitably supported on stands with heavy iron bases, and a bent adaptor is fitted to the second condenser furthermost from the water bath. The flask containing the ethereal solution is supported on the water bath, a short fractionating column or a simple bent still head is fitted into the neck of the flask, and the stUl head is connected to the condensers by a cork the recovered ether is collected in a vessel of appropriate size. 

Pure pyridine may be prepared from technical coal-tar pyridine in the following manner. The technical pyridine is first dried over solid sodium hydroxide, distilled through an efficient fractionating column, and the fraction, b.p. 114 116° collected. Four hundred ml. of the redistilled p)rridine are added to a reagent prepared by dissolving 340 g. of anhydrous zinc chloride in a mixture of 210 ml. of concentrated hydrochloric acid and 1 litre of absolute ethyl alcohol. A crystalline precipitate of an addition compound (probable composition 2C5H5N,ZnCl2,HCl ) separates and some heat is evolved. When cold, this is collected by suction filtration and washed with a little absolute ethyl alcohol. The yield is about 680 g. It is recrystaUised from absolute ethyl alcohol to a constant m.p. (151-8°). The base is liberated by the addition of excess of concentrated... 

The 40-45 per cent, hydrazine solution may be concentrated as follows. A mixture of 150 g. (144 ml.) of the solution and 230 ml. of xylene is distilled from a 500 ml. round-bottomed flask through a well-lagged Hempel (or other efficient fractionating) column fitted into a cork covered with tin foil. All the xylene passes over with about 85 ml. of water. Upon distillation of the residue, about 50 g. of 90-95 per cent, hydrazine hydrate (5) are obtained. 

Any other efficient fractionating column may be used, e.g., an all-glaas Dufton column—see Sections 11,15 and 11,17. 

Freshly distilled ethyl formate must be used. Commercial ethyl formate may be purified as follows. Allow the ethyl formate to stand for 1 hour with 16 per cent, of its weight of anhydrous potassium carbonate with occasional shaking. Decant the ester into a dry flask containing a little fresh anhydrous potassium carbonate and allow to stand for a further hour. Filter into a di flask and distil through an efficient fractionating column, and collect the fraction, b.p. 53-54° protect the receiver from atmospheric moisture. 

The lower pyridine layer contains most of the excess of thionyl chloride it may be recovered by distillation through an efficient fractionating column. 

Any fractionating column of moderate efficiency is satisfactory, e.g., a Dufton column (20 cm. long containing a spiral 10 cm. in length, 2 cm. in diameter with 8 turns of the helix) or a Vigreux column (20-25 cm. long). 

The approximate dimensions of the packing are 25 cm. X 18-20 mm. Any other form of efficient fractionating column may bo used. 

The best results are obtained with a fractionating column surrounded by an electrically-heated jacket (compare Figs. II, 17. 2. and II. 17, 3), but this is not essential for n-caproic anhydride. For the preparation of propionic or n-biityric anhydride, a highly efficient fiactionating column must be used in order to obtain satisfactory results. 

Ethyl acetate. Use 58 g. (73-5 ml.) of absolute ethyl alcohol, 225 g. of glacial acetic acid and 3 g. of concentrated sulphuric acid. Reflux for 6-12 hours. Work up as for n-propyl acetate. B.p. 76- 77°. Yield 32 g. Much ethyl acetate is lost in the washing process. A better yield may be obtained, and most of the excess of acetic acid may be recovered, by distilhng the reaction mixture through an efficient fractionating column and proceeding as for methyl acetate. 

Ethyl n-butyrate. Use a mixture of 88 g. (92 ml.) of n-butyric acid, 23 g. (29 ml.) of ethanol and 9 g. (5 ml.) of concentrated sulphuric acid. Reflux for 14 hours. Pour into excess of water, wash several times with water, followed by saturated sodium bicarbonate solution until all the acid is removed, and finally with water. Dry with anhydrous magnesium sulphate, and distU. The ethyl n-but3rrate passes over at 119 5-120-5°, Yield 40 g. An improved yield can be obtained by distilhng the reaction mixture through an efficient fractionating column until the temperature rises to 125°, and purifying the crude ester as detailed above under methyl acetate. 

The benzyl chloride may also be isolated by distillation under atmospheric pressure. The material boiling between 165° and 185° is collected and redistilled the final product is collected at 178-182° (pure benzyl chloride has b.p. 179°). The resulting benzyl chloride is, however, of lower purity unless an efficient fractionating column is used. 

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