Distillation in Vacuum

N. A. Shishakov, Moscow

As is known, distillation in vacuum is used to lower the boiling points of substances which, at atmospheric pressure, cannot be distilled without decomposition. It is clear that in this respect we may expect all the greater success, the lower the pressure maintained in the apparatus. Therefore Burch1, in his new and very interesting experiments, used a powerful condensation pump of a modern type to obtain the vacuum. However, beginning already with the comparatively large value of \(10^{-3}\) mm, merely reducing the gas pressure brings little benefit, since a further decrease of temperature while maintaining a sufficiently high rate of distillation could be achieved only by reducing the resistance of the column of the saturated vapor itself in the apparatus. For this there is only one way, namely to reduce the height of this column and, of course, to increase the evaporating surface and the condensing surface.

It is obvious that the maximum rate of distillation, for given values of these surfaces, can be obtained when the distance between them is so short that the vapor molecules fly to the condensing surface without undergoing any collisions along their path.

An apparatus of this kind was first used by Brønsted and Hevesy (Bronsted und Hevesy) for the separation

preparation of mercury. Berthelot applied this idea to the distillation of organic compounds, in particular to the distillation of petroleum and its derivatives. In his apparatus, shown in Fig. 1, \(A\) denotes a copper trough into which the liquid to be distilled was poured; its surface had dimensions of \(16 \times 2.5\), and its height was \(1.1\) cm. Heating was effected by an electrical resistance located in the hollow bottom. The latter communicated with the atmosphere by means of tube \(C\), which kept the trough in a horizontal position. This same tube also served as one of the current leads; the other lead was located inside it. The temperature was measured by thermocouple \(E\), which, like

Fig. 1

the resistance, was soldered to the trough with silver. The leads from the thermocouple are not shown in the drawing. It was impossible to place the thermocouple, as well as the electric heater, in the trough itself with the liquid, since in that case one would inevitably have to deal with boiling of the liquid and its splashing onto the condensation surface. In the receiver, to which tube \(B\) was soldered, there were six graduated test tubes for collecting fractions; their rotation was effected by means of the ground joint \(F\). The bottom of the receiver was sealed with wax; the metal plate for fastening it to the receiver, or for removing it from the receiver, could be cooled with water or heated with steam. The end of the nozzle was arranged in such a way that, as may be seen from the drawing, its temperature was to differ little from the temperature of the condensation surface. At point \(I\) one electrode was soldered in; the condensation

the pump, the copper trough, and plate \(G\) were connected to ground. To reduce the temperature gradient, the upper bottom of the trough was made sufficiently thick.

When the apparatus was evacuated, the mercury vapors did not freeze out. It is not easy to give an exact theory of the distillation process, but simplified considerations show that, when the distance between the surface of the liquid and the condensation surface is \(2\ \mathrm{cm}\), and when the vapor pressure of mercury is about \(10^{-3}\ \mathrm{mm}\), the rate of distillation decreases, owing to collisions of the molecules of the substance being distilled with mercury molecules, by only \(20\%\) compared with the rate of distillation in a perfect vacuum.

All organic liquids, including petroleum, contain considerable quantities of dissolved gases, because of which, when the pressure in the apparatus is reduced, foaming of the liquid and the formation of large bubbles inevitably occur. To avoid splashing of the liquid as a result of the bursting of bubbles and its reaching the condensation surface, Burch reduced the pressure very gradually, each time waiting, before the next lowering of the pressure, for the disappearance of the small gas bubbles that arose. The entire pumping process usually required several hours, even when the trough was heated in order to reduce the surface tension and accelerate the bursting of the bubbles. The condensation pump was set in operation at \(0.05\ \mathrm{mm}\). The approach of the end of degassing could be ascertained by the cessation of the discharge from the induction coil.

The essence of the distillation process itself, as developed by Burch, is as follows. The ordinary method of separating organic substances, both in vacuum and at atmospheric pressure, is based on the fact that here the vapor located directly above the surface of the liquid in the apparatus, at a given density and temperature, can exist in a state of equilibrium with the upper layer of the liquid. In this case the composition of the distillate will depend, generally speaking, only on the temperature, which therefore can serve as a criterion of composition during repeated distillation. At—

conversely, in the distillation process described here by slow evaporation, the temperature by itself cannot serve as such a criterion, and consideration of it can have meaning only together with consideration of the rate of distillation. If we are dealing with a chemically pure liquid, then the rate of its evaporation from each sq. cm will be equal to \(\rho c/4\) g per second, where \(\rho\) is the density of the saturated vapor at the given temperature, and \(c\) is the mean molecular velocity. It is obvious that in the case of a mixture of liquids the rate of evaporation of the \(n\)-th component will be \(\rho_n c_n/4\), where the subscript \(n\) refers, of course, to this component. Thus, the success of separation by the second method will depend on \(\rho_n c_n\), and not on \(\rho_n\), as in the first method.

Since \(c_n\) is inversely proportional to the square root of the molecular weight, while \(\rho_n\), generally speaking, will be greatest for the component with the smallest molecular weight, then in slow distillation one should expect the best separation of liquids. For example, when heating is carried out to a temperature corresponding to a saturated-vapor pressure of the distillate not higher than \(1.5 \cdot 10^{-3}\) mm, only 20% of the molecules return to the surface of the liquid, and moreover chiefly those with the smallest molecular weight. And since such a method is, in addition, the only method by means of which distillation of substances with very high molecular weight can be carried out without danger of decomposition, its outstanding significance for organic chemistry appears quite indisputable. It goes without saying that distillation cannot be carried out at such temperatures when decomposition of the substances begins. Since, with such decomposition, intense formation of bubbles is observed on the surface of the liquid, it was not difficult to determine these decomposition temperatures. For the oils tested, decomposition begins suddenly at temperatures lying within the range from \(307^\circ\) to \(340^\circ\).

Let us name some of the products obtained by Berczem. In the distillation of a residue that had been obtained from Pennsylvania petroleum during its distillation under atmospheric

pressure with the aid of superheated steam, and which had previously been freed of gasoline at \(100^\circ\) C; the onset of condensation was observed at \(112^\circ\) C, and the first drops of distillate appeared at \(165^\circ\) C, the distillation itself proceeding at a rate of 6 drops per minute; thereafter, at \(250^\circ\), from 5 to 3 drops per minute distilled over, and finally at \(314^\circ\) decomposition began. Here is the result:

In approximately the same way, distillation was also carried out on certain other petroleum residues, for example lubricating oil, cylinder oil, and heavy machine oil, ordinary paraffin, and others. As a result of all this, a whole series of new oils and fat-like substances was obtained. It goes without saying that, when collecting such greases, the temperature of the condenser had to be maintained at a sufficient height, which in other cases was even associated with the danger of spoiling the vacuum and required constant observation of the discharge in the receiver. Among the many new organic substances thus obtained in pure form there proved to be substances possessing remarkable physical properties.

The substance remaining in the retort as a result of the distillation of vaseline at \(320^\circ\) C and constituting approximately 13% of the starting material proves to be similar to the ordinary grease used in vacuum work. The vapor pressure of this substance, when determined by the evaporation method with the use of liquid air to cool the condensation surface, is, at \(70^\circ\) C, less than \(7 \cdot 10^{-7}\) mm. It is evident that this grease can be used for lubricating ground joints and stopcocks in systems with a very high vacuum, a purpose for which none of the substances known up to now was suitable, owing to too high a vapor pressure.

Another remarkable substance is obtained upon re-

distillation at \(118^\circ\) of the oil used for rotary pumps. The vapor pressure of the oil obtained in this way is approximately \(7 \cdot 10^{-4}\) mm at \(100^\circ\) C. Obviously, at room temperature it must already be quite negligible, whence arises the idea of using it for condensation pumps instead of mercury, whose vapor pressure at room temperature is of the order of \(10^{-3}\) mm. Experiments carried out with metallic condensation pumps filled with such oil and operating at a fore-vacuum of \(0.01\)—\(0.02\) mm showed the following. The limiting pressure that such pumps make it possible to reach depends chiefly on the degree of degassing of the glass and metals. A more rigorous test of the working capacity of an oil condensation pump in exhausting a large cathode lamp, which before this had lain open to the air for two years, showed that after bombardment of the anode a pressure of less than \(1.5 \cdot 10^{-6}\) mm is obtained. Of course, here one has to deal with the deterioration of the vacuum due to gases liberated by the glass. How well such a pump works can also be seen from the fact that at \(10^{-3}\) mm its speed was \(2000\) cm/sec; at higher pressures it was limited by the speed of the preliminary pump. Further, with the aid of such an oil pump a cathode tube with a metallic window was exhausted, and such a rarefaction was obtained that as much as 300 kilovolts could be applied to the tube. Finally, perhaps the most instructive result was obtained when exhausting a cathode lamp while heating the glass and bombarding the electrodes: in this case the pressure obtained was so small that it could not be detected with an ionization manometer, whose sensitivity was at least \(10^{-4}\) dyn/cm\(^2\), i.e. \(7 \cdot 10^{-8}\) mm.

It is thus obvious that these experiments are not only of great importance for organic chemistry, but they also promise to produce a significant revolution in vacuum technology, where perhaps in the near future not only liquid air and solid carbon dioxide for freezing out mercury and other vapors will lose their importance, but also the very

mercury, without which work on high vacuum had until now been almost impossible.

There can be no doubt that this work of Berch will very soon give rise to a whole series of repeated and further investigations. For the time being, i.e. a year and more after the appearance of this article, we have found no new information in the literature on these questions; but somehow the article by Fritz Friedrichs (Chem. Ztg., 54, 144, Febr. 1930), in which he describes an apparatus patented by him for distillation in vacuum, involuntarily caught our eye. The basis of this apparatus is, in general, the same idea as in Berch’s, but its construction is entirely different. The apparatus is shown in Fig. 2. Here the height of the vapor column is only 3 cm. The tube for the exit of vapor begins almost directly above the surface of the liquid. To the condensation surface the vapor travels a distance of only about 10 cm. The condensed liquid cannot fall back into the original liquid. The large cross-section of the condenser (minimum 20 mm) also facilitates the distillation process. The entire apparatus is assembled on ground joints. The receiver is very conveniently arranged; it can be removed without interrupting the operation of the apparatus. Friedrichs says nothing about the results of work with such an apparatus, but it seems that this apparatus, like Berch’s apparatus, should in the future play no small role, at least in the development of organic chemistry.

Fig. 2

1. C. R. Burch. Some Experiments on Vacuum Distillation, Proc. Roy. Soc., Ser. A. 123, 271, March 1929. ↩