Table 52

When there are many sections, the inverted tube should be made somewhat shorter in the lower sections, so as to increase the vertical distance from the bottom of that tube to the bottom of the inner tube ; in the modified still-head the vertical distance from the depression to the flattened top of the inverted tube should be somewhat increased in the lower sections. It is important that the holes near the top of the inner tubes should be as large as possible.

1 The specified dimensions were unfortunately not adhered to in constructing the still-head of thirteen sections, with the result that there was a tendency for two or three of the sections to become blocked with condensed liquid. The efficiency of the apparatus was thus increased, but its usefulness was greatly diminished and the quantity of liquid in the still-head was much greater than it should have been.

The Kubierschky Column

The Kubierschky column1 belongs to this type of still-head, but has only been used for large-scale work. See p. 307 and Fig. 115, p. 308.

The "Hempel" Still-head. - The great advantages of the "Hem-pel" apparatus 2 are simplicity and efficiency; on the other hand the amount of liquid in the still-head is excessive, and it is therefore unsuitable for the distillation of small quantities of liquid.

The still-head consists simply of a wide vertical tube, filled with glass beads of special construction, and constricted below to prevent the beads from falling out. A short, narrower, vertical tube with side delivery tube is fitted by means of an ordinary cork into the wide tube.

From the following table it will be seen that in efficiency the Hempel tube, which contained 200 of the large beads now used, came about midway between the original "evaporator" of 3 sections, the length of which was nearly the same, and the modified "evaporator' of 5 sections, which was a little longer. But the weight of liquid in the 3-section "evaporator" still-head was considerably less than half, and in the 5-section "evaporator " not much more than half as great as in the Hempel tube.

Length for length, the modified "evaporator " still-head would be more efficient, and would contain only about half the amount of condensed liquid.

Many other fillings for the still-head to replace Hempel's heavy glass beads have been suggested. Partington and Parker3 employ four pointed stars. Raschig4 recommends thin sheet-iron rings one inch long and one inch in diameter for large still-heads, smaller rings being employed for laboratory purposes. Goodwin5 uses rings each of which consists of two hollow truncated cones united at their narrower ends. Lessing6 has modified the Raschig rings in such a manner that they are easier to manufacture and give a larger surface of contact between liquid and vapour. Fig. 47 shows the form of the Lessing ring, which may be made of any suitable metal or of earthenware. For laboratory purposes Lessing recommends rings of 1/4-inch diameter, 1000 of which would occupy 400 c.c. The larger rings have been found very suitable for the distillation of large quantities of liquid ; for example, a ring still of 18 inches diameter was found to give a better separation than a column still of the Coffey type of the same height but 2 feet in diameter.

I am indebted to Dr. Lessing for 1500 of the 1/4-inch rings, and have carried out distillations of 200 grams of the mixture of equal weights of benzene and toluene at the usual rate of 60 drops of distillate per minute. The tube was 3.7 cm. in diameter and the height of the column of rings 60 cm.

1 E. Graefe, "Use of the Kubierschky Column in the Distillation of Mineral Oils," Petroleum, 1913, 9, 303 ; J. Soc. Chem. Ind., 1914, 33,1146 ; Borrmann, Zeilschr. ang. Chem., 1915, 28, 377, 381 ; J. Soc. Chem. Ind., 1915, 34, 1232.

2 Hempel, " Apparatus for Fractional Distillation," Fersenius' Zeitschr. filr Anal. Chem., 1882, 20, 502.

3 J. Soc. Chem. Ind., 1919, 38, 75 T. 4 Eng. Patent, 1914, No. 6288.

5 Ibid., 1917, No. 110,260. e Ibid., 1920, No. 139,880.

Fig. 47.

The separation of benzene was distinctly better than that obtained with an evaporator still-head of 8 sections but not nearly so good as with an evaporator of 13 sections. It would be about equal to that given by a Young and Thomas still-head of 10 sections. The separation of toluene was about equal to that with the evaporator of 8 sections, but the amount of toluene actually recovered was far less owing to the large quantity left adhering to the rings, in spite of the fact that as soon as the distillation was over the still-head was disconnected from the condenser and was tilted from side to side and was then left standing for an hour to allow the liquid to flow back to the flask as completely as possible. The average weight of toluene lost in this way was 12.5 grams as against 0.2 gram in the case of the evaporator still-head. The amount of liquid in the still-head during distillation was more than 28 grams as great as with the evaporator still-head.

On account of its ease of construction and convenience the Lessing ring still-head may be strongly recommended for the distillation of large quantities of liquid, especially if the residual liquid is not of value ; but for the distillation of small quantities and for the recovery of the least volatile component it is not to be compared with the evaporator still-head.

Lessing states that the principal point is the adjustment of the amount of reflux obtainable over the whole tube. For a still-head of 40 cm. length he recommends a reflux condenser, but for the longer still-heads he states that the lower part of the column - up to about 10 cm. from the top - should be lagged with asbestos. The reflux condenser would, of course, increase the amount of liquid in the still-head and would make it still less suitable for the distillation of small quantities of liquid. The lagging would have the reverse effect, but the amount of liquid in the still-head and the loss of material would remain excessive.

. In the experiments referred to above the still-head was not lagged, nor was a reflux condenser used.

A simple and efficient still-head has been described by Foucar.1 It consists essentially of a helical septum which traverses the annular space between two concentric cylinders. The vapour from the still passes up the long spiral passage before reaching the condenser. If desired, the inner cylinder can be used as a thermostat, the column then acting as a regulated temperature still-head (Chapter XII (Modifications Of The Still-Head (Continued)).). The Foucar apparatus may be described as a greatly improved spiral still-head.

An easily constructed spiral still-head, suitable for laboratory work, has been described by Dutton.2 A spiral of copper wire is wound round an inner closed tube which fits quite loosely into an outer tube, thus leaving a spiral passage up which the vapour ascends. The amount of condensed liquid in the still-head increases from top to bottom and, to allow for this, tubes of increasing size may be used - either the diameter of the outer tube increasing downwards or that of the inner tube upwards, the thickness of the wire increasing downwards in either case. It was found necessary to protect the still-head from too rapid cooling by packing it in a thickness of about two inches of cotton-wool wadding covered with asbestos yarn. Very good results are claimed with a still-head 120 cm. in height, but as the distillation was carried out with extreme slowness - 10 drops of distillate a minute - comparison with other still-heads is hardly possible with the data at present available.

1 Eng. Patent, 1908, No. 19,999. 2 J. Soc. Chem. Ind., 1919, 38, 45 T.

M. Robert1 has obtained very good results with a vacuum jacketed column, with a reflux condenser above it to act as a dephlegmator.

1 M. Robert, "Colonne a distiller pour Laboratoire," Ann. de Chim. analytique, 1919, Series II. 1, 372.