Modifications Of The Still-Head

It has been pointed out that the inefficiency of the plain vertical still-head is due to the want of thorough contact between the vapour and the condensed liquid, owing, firstly, to the central portion of the vapour passing rapidly up the tube and possibly never meeting with the liquid at all, and, secondly, to the condensed liquid flowing very rapidly back to the still. Consequently, any modification that brings about better admixture of the ascending vapour or that retards the down-flow of condensed liquid should increase its efficiency.

Sloping Still-Head

The simplest and most obvious alteration that can be made is to change the slope of the tube so as to retard the down-flow of liquid. This may be done by bending the tube near the top and bottom so that its ends remain vertical, while the middle part slopes very gently upwards.

It will be seen from Table 50 that, by altering a tube in the manner described, a notable improvement is effected, especially as regards the separation of the benzene, and it is remarkable that, although the down-flow of liquid is retarded, the quantity of liquid in the still-head is slightly diminished.

Spiral Still-Head

The efficiency is further improved by bending the sloping portion of the tube into the form of a spiral probably because a better admixture of the vapour is thus produced ; by this device the amount of liquid in the still-head is still further reduced.1

"Rod and Disc" Still-head. - In the still-head shown in Fig. 42a the down-flow of part of the condensed liquid is greatly retarded by the discs on the central glass rod, and this liquid is protected from the cooling action of the air ; at the same time eddies and cross currents are produced in the ascending vapour. The increase in efficiency with this apparatus is very marked, as will be seen from the second part of Table 50 ; it is easily constructed and is very convenient to handle. The quantity of liquid and vapour in the still-head is the same as that in the plain tube of 19.4 mm. diameter, but the efficiency is very much greater. On removing the source of heat, the liquid returns almost completely to the still, but when it is of special importance to avoid all loss, it is advisable with this and many other forms of still-head to disconnect the condenser and to tilt the still and still-head, while hot, from side to side so as to facilitate the back-flow of liquid.

1 G. Berlomont,," New Tube for Fractional Distillations, modified by Lebel," Bull. Soc. Chim., 1895, [III], 13, 674; J. Soc. Chem. Ind., 1895, 14, 821.

Table 50

If that is done, the whole of the liquid usually returns to the still. A slight further improvement, both in efficiency and as regards the quantity of liquid in the still-head, is effected by constricting the outer tube between the discs (Fig. 356). Better contact between the vapour and the condensed liquid on the outer tube is thus ensured.

Fig. 42. - The "rod and disc " still-heads ; (a) without, (b) with constrictions in outer tube.