Table 54. Eleventh Fractionation

D = temperature of vapour on leaving the bubbling still-head.

B = temperature of bath.

T = temperature of vapour before entering the condenser.

The bath was kept at such a temperature, and the source of heat was so regulated, that the drops of distillate fell as nearly as possible at the rate of 60 per minute, and it will be noticed that the temperature of the vapour before condensation was in all cases nearly the same as that of the bath, generally very slightly higher. It will also be seen that the fall in temperature of the vapour, during its passage through the regulated temperature still-head (D - T), is greatest for the middle and least pure fractions and smallest for the highest and purest; indeed, for the last fraction the three temperatures are almost identical, while for the fourth there is a difference of 1 .2° between D and T.

Very careful regulation of the temperature of the bath was necessary when the liquid distilled was nearly pure.

Suitability For Very Volatile Liquids

For very volatile liquids there can be no question that the regulated temperature still-head is the most suitable and the principle has been applied by Ramsay, Dewar and others to the purification of gases. Thus, if a mixture of helium with any other gases is passed at the ordinary pressure through a spiral tube cooled by liquid hydrogen boiling under reduced pressure, all other known gases would be condensed in the tube and the helium alone would pass through.

Separation Of Oxygen And Nitrogen From The Air

Large quantities of pure oxygen and also of nitrogen are now prepared by the fractional distillation of liquid air or by the combined fractional condensation of air and distillation of the resulting liquids.

Claude's apparatus may be regarded as a combination of a constant temperature differential liquefier with a rectifying column ; by means of it not only pure oxygen but also pure or nearly pure nitrogen are obtained. The air, under a pressure of five atmospheres, enters the chamber A (Fig. 54) through the tube b. From a it passes upwards through the vertical tubes, immersed in liquid oxygen, into the small chamber c. Partial liquefaction and fractionation takes place in these vertical tubes, the condensed liquid which collects in a containing about 47 per cent of oxygen. The remaining air, rich in nitrogen, which escaped condensation in the upward passage through the vertical tubes, passes downwards through the outer vertical tubes and is there finally condensed. The crude liquid nitrogen collects in a sort of hydraulic main D, flows into the vessel e, and is forced up through the tube f into the top of the rectifying column G. The liquefied air, rich in oxygen, which collects in a is forced up through the other vertical tube and enters the column at a lower level. In its passage down-wards the liquid from e and A undergoes fractionation, so that pure liquid oxygen leaves the bottom of the column and fills the vessel in which the vertical tubes are placed. Pure oxygen gas, formed by evaporation of the liquid, passes out by the tube h, and pure nitrogen leaves the top of the rectifying column through the tube J.

Fig 54.

Fig. 55.

In Linde's apparatus (Fig. 55) there is nothing in the nature of a differential condenser, the two baths of liquid oxygen causing the complete condensation of the compressed air. The air entering the apparatus through the tube A is liquefied in b and b', and the liquid air is forced up the tubes c and c' and is delivered at the top of the rectifying column d. In its descent through the column fractionation takes place and the liquid which reaches the bottom and flows into b is pure oxygen : the excess, together with any oxygen gas, passes into B', and the pure oxygen gas formed by evaporation of the liquid is carried off through the tube e. The gas which passes away from the top of the column through the tube f contains about 7 per cent of oxygen.

Helium

It has been found that helium is present in nearly all natural gases, those of the Western States of America containing from 1 to 2 per cent of the element, whilst the Bow Island gas in Alberta, supplied to Calgary, contains about 0.35 per cent. On account of its lightness and non-inflammability helium would be an ideal gas for aeronautical purposes if it could be obtained in sufficient quantity at not too great a cost. From preliminary calculations as to cost, etc., carried out by Sir Richard Threlfall, he was led to believe that helium might be successfully obtained from natural gas. The necessary investigations were entrusted to and were successfully carried out by J. C. M'Lellan, who has recently given a full description1 of the plant actually used at Calgary for the production of helium of at least 99 per cent purity, and of a commercial plant designed to deal with the whole of the natural gas supplied from Bow Island. This plant should yield more than 10,000,000 cubic feet of 97 per cent helium annually, assuming that the supply of natural gas showed no diminution.

In order to isolate the helium it is necessary to condense all the less volatile gases, chiefly methane and nitrogen. The principle of the method is similar to that on which the production of pure oxygen and nitrogen from the air is based, but the difficulties are much greater inasmuch as it is the most volatile component which has to be separated and as this component is present in relatively very small quantity.

1 J. C. M'Lellan, "Helium: its Production and Uses," Trans. Chem. Soc, 1920, 117, 923.