" Regulated " or "Constant Temperature " Still-heads

By surrounding the still-head with water or any other liquid, the temperature of which is kept as little above the boiling point of the more volatile component as will allow of vapour passing through, a considerable improvement in the separation is effected. The temperature of the bath, however, requires very careful regulation if the boiling points of the components are near together, or if one component is present in large excess, for, in either case, a fall of a fraction of a degree would cause complete condensation of the vapour, while a rise of temperature to a similar extent would prevent any condensation from taking place and there would be no fractionation at all.

When a regulated temperature still-head is employed, it is better, for two reasons, to bend the tube into the form of a spiral; in the first place, the effective length of the still-head may be thereby greatly increased without unduly adding to the height of the bath, and in the second place, as has been already pointed out, the spiral form is more efficient than the vertical. Rosanoff and his co-workers have found, however, that a double - walled metallic cylinder (Fig. 31, p. 75), open at both ends and immersed in liquid kept at a constant temperature and vigorously stirred, gives better results than the ordinary spiral form of still-head.

Warren's Still-Head

The employment of an elongated spiral still-head, kept at a constant or slowly rising temperature, was first recommended by Warren.1 The spiral tube was heated in a bath of water or oil; its length varied from 1 1/2 to 10 feet and its internal diameter from 1/4 to 1/2 inch.

Warren carried out fractional distillations of petroleum and other complex mixtures, and observed that, as the fractions became purer, the temperature of the bath had to be brought nearer to the boiling point of the liquid in the still and required more careful regulation.

Brown's Still-Head

A modification of this apparatus, devised with a view to the better control of the temperature of the still-head, is described by Brown.1 The liquid to be distilled is boiled in the vessel A (Fig. 52) ; the vapour rises through the coil c, the temperature of which is that of a liquid boiling in the vessel e. The vapour from the liquid in e passes through the tube d to the worm condenser f, and the condensed liquid returns by the tube k to the bottom of the vessel e. The pressure under which the jacketing liquid boils is regulated and measured by a pump and gauge connected with the tube M. The liquid in E is heated by the ring burner B.

1 Warren, "On the Employment of Fractional Condensation," Liebigs Annalen, 1865, Suppl., 4, 51.

The vapour of the liquid which is being distilled passes through the side delivery tube G and is condensed and collected in the usual manner. The temperature of the vapour as it leaves the still-head is registered by a thermometer at a; that of the jacketing liquid is regulated by the pressure and may be read from the vapour pressure curve, or a second thermometer may be placed in the central tube.

Brown obtained very good results with his apparatus, and arrived at the important conclusion (p. 73) that "in distillations with a still-head maintained at a constant temperature, the composition of the distillate is constant, and is identical with that of the vapour evolved by a mixture whose boiling point equals the temperature of the still-head." As already stated (p. 75) this conclusion has been confirmed by Rosanoff and Bacon.

In Brown's apparatus the temperature of the still-head can be kept constant for any length of time or it can be altered rapidly and easily by altering the pressure under which the jacketing liquid is boiling, but, owing perhaps to the fact that the number of observations and the amount of attention required during a distillation are greater than when the still-head is merely exposed to the cooling action of the air; it has not come into general use.

Separation of Pentanes from Petroleum, - A regulated temperature still-head, combined with a bubbling still-head (Fig. 53), has been found very useful for the separation of the lower paraffins from the use of a chimney. The temperature of the bath was registered by the thermometer b.

1 Brown, " The Comparative Value of Different Methods of Fractional Distillation," Trans. Chem. Soc, 1880, 37, 49; " Fractional Distillation with a Still-head of Uniform Temperature," ibid., 1881, 39, 517. A still-head similar in principle to Brown's has been described by Hahn, Ber., 1910, 43, 419 ; J. Soc. Chem. Ind., 1910, 29, 300.

Fig. 52. - Brown's "regulated temperature" still-head.

American petroleum.1 The vapour from the boiling "petroleum ether "passed first through a six column Young and Thomas still -head, on leaving which its temperature was read on a thermometer, a It then passed upwards through a spiral tube in a large bath, the water in which was either cooled by adding ice, or warmed by a ring burner below, and was kept constantly stirred by an arrangement similar to that employed by Oswald, in which a propeller, with four blades of thin sheet copper, was kept rotating by a windmill actuated by a ring of gas jets, the efficiency of the windmill being greatly increased by still-head.

Fig. 53. - Combined " regulated temperature " still-head and " Young and Thomas ".

After leaving this part of the apparatus the vapour passed through a vertical tube, where its temperature was read on the thermometer c, then into a spiral condenser cooled by ice ; the condensed liquid was collected in ice-cooled flasks.

The general course of this separation has already been referred to (p. 113), and is indicated by the curves in Fig. 36. Details of a single fractionation, the eleventh, are given in Table 54; the temperatures are the final ones for the fractions and they are all corrected to 760 mm., and for the thermometric errors. Since the temperature of the vapour is affected by changes of the barometric pressure, that of the water bath has been altered in each case to the same extent. The lowest fractions consist chiefly of isopentane; the middle ones are mixtures of iso- and normal pentane; the highest consist of nearly pure normal pentane.

1 Young and Thomas, " Some Hydrocarbons from American Petroleum. I. Normal and Isopentane," Trans. Chem. Soc., 1897, 71, 440. Cf. F. M. Washburn, " Constant Temperature Still-head for Light Oil Fractionation," J. Ind. and Eng. Chem., 1920, 12, 73.