Separation Of Pure Benzene

When the liquid collected below 80.45° in the seventh fractionation was distilled at the beginning of the eighth, the temperature remained at the boiling point of benzene, 80.2°, for a short time, and a considerable amount of distillate came over below 80.25°. As the range of this fraction was reduced to 0.15°, it was concluded that the first portion of the first distillate in the ninth fractionation would consist of pure benzene; it was therefore collected separately as there was no necessity to redistil it, and in each of the subsequent fractionations the first portion of the first distillate was taken to be pure benzene.

Elimination Of Fractions

In the tenth fractionation the weights of the 11th and 12th fractions had become very small, and their temperature ranges were reduced to 0.2° and 0.05° respectively. In the next fractionation, therefore, after No. 12 distillate had been added to the residue in the still, the 11th receiver was left in position until the temperature reached 110.6° ; the distillation was then stopped, and the residue taken as pure toluene. In this way the number of fractions was reduced to eleven by the elimination of No. 12. In the 12th and 13th fractionations the last fraction was in each case similarly eliminated, and, in the latter, the number of fractions was further reduced by the exclusion of No. 1, and in the 14th by that of No. 2.

In the 14th fractionation, after No. 6 had been added to the residue in the still, it was found that the temperature rose at once above 81°. so that nothing was collected in No. 5, and also that when the distillation had been carried as far as possible, the temperature had not risen higher than 91.4°. The distillation was therefore stopped at this point, and the residue in the still, after cooling, was placed in a separate receiver r. Fraction No. 7 was then placed in the still, and the distillate, collected in r until the temperature reached 100°, and then in No. 7. The rest of the fractionation was carried on as before.

Final Fractionations

In the remaining distillations the fractions below and above the middle point were treated separately, the residue from the last of the lower fractions and the first portion of distillate from the first of the higher ones being collected each time in r.

Four additional operations were required for the separation of the benzene and two for that of the toluene. The total weight of pure benzene recovered by pushing the series of fractionations to their extreme limit was 81.4 grams out of 100 taken ; the weight of toluene was 88.8 grams. The amount of liquid collected in r, and rejected, was 7.8 grams, and the loss by evaporation, and by transference from flasks to still, was 22.0 grams. The amount of time required for the whole operation was a little over thirty hours.

Mixtures separable into Three Components

We have seen that when a mixture tends to separate on distillation into two components, it is the less volatile component which is the easier to obtain in a pure state. When a mixture tends to separate into three components the least volatile of them can, as a rule, be most readily isolated and, of the other two, the more volatile is the easier to separate.

Methyl, Ethyl and Propyl Acetates. - An illustration of this is afforded by the fractionation of a mixture of 200 c.c. of methyl acetate (b.p. 57.1°), 250 c.c. of ethyl acetate (b.p. 77.15°) and 200 c.c. of propyl acetate (b.p. 101.55o) carried out with a plain vertical still-head one metre in length. Full details of this fractionation have been published in the Philosophical Magazine1 and in Table 40 the results of the first eight fractionations and of 12th, 16th, 20th and 24th are given.2 It will be noticed that the method adopted differed from that already described in so far that the number of fractions was small at first and was gradually increased. For convenience, the fractions are so numbered that they fall into their proper places in the later distillations. The data given are as follows : - F, the number of the fraction ; t, the final temperature, corrected and reduced to 760 mm. pressure, for each fraction, but in the first four fractionations the lowest temperature (in brackets) is that at which the distillate began to come over ; Aw, the weight of each fraction, and the ratio Aw/At.

First And Second Fractionations

The first fractionation, I., requires no comment; the second, II., was carried out in the following manner : - the first fraction from I. (No. 5) was distilled and the distillate was collected in receiver No. 4 until the temperature rose to 63.8°, when receiver No. 5 was substituted for it and the distillation was continued until the temperature reached 71.0°. The gas was then turned out and the second fraction from I. (No. 8) was added to the residue in the still. Heat was again applied and the distillate was collected in No. 5 until the temperature rose to 71.0°, when this receiver was replaced by No. 8, and the distillation was continued until the temperature reached 77.1°. The third fraction from I. (No. 11) was then added to the residue and the distillate was collected in No. 8 until the

1 Barrell, Thomas and Young, " On the Separation of Three Liquids by Fractional Distillation," Phil. Mag., 1894, [V.], 37, 8.

2 For the results obtained with an improved still-head, see Table 57, p. 160.

Table 39