Benzene And Toluene

That much time is saved by the use of an improved still-head is seen by comparing the results of a fractional distillation of 200 grams of a mixture of equal weights of benzene and toluene with a modified "evaporator' still-head of 5 sections with those obtained with a plain vertical tube 30 cm. in height.

Table 55

When the "evaporator " still-head was used it was not necessary to divide the distillate into more than five fractions. In the first distillation (Table 55), more than half the total quantity of toluene was recovered in a pure state, and it will be noticed that the values of for the middle fractions are relatively very low. In the second fractionation, a little liquid came over at the boiling point of benzene, but it was not considered advisable to collect this separately. The value of for the first fraction was more than 300 times as great as for the fourth. More than 82 per cent of the toluene was now obtained in a pure state.

In the third fractionation 21.9 grams of pure benzene was obtained, and when fraction No. 3, collected from 81.4° to 95.4°, was added to the residue in the still, and the distillation was continued, the temperature did not rise above 81.7° ; the whole of the distillate was therefore collected in receiver No. 2, and the small residue (4.95 grams) was rejected. The apparatus was then dried, and the higher fractions were distilled, when only 7.25 grams came over below 110.6°, the residue (9.3 grams) consisting of pure toluene. The distillate was too small to allow of further fractionation.

Two more distillations of the lower fractions sufficed to complete the recovery of the benzene.