Comparative Results With Plain And Improved Still-Heads

The great improvement effected by employing an efficient apparatus is clearly shown in Table 56.

Table 56

The chief gain was in the time occupied, which was reduced nearly to one-fifth, but the recovery also was somewhat better, and in many cases would be much better; there was also much less actual loss of material, though the amount left undistilled was greater.

Methyl, Ethyl and Propyl Acetate. - The separation of a mixture of 100 grams of methyl acetate, 120 grams of ethyl acetate and 100 grams of propyl acetate was carried out by distillation with a modified evaporator still-head of 8 sections. Details of the fractionations are given in Table 57 (p. 160).

[Table

Table 57

In the second fractionation, when fraction No. 7 from I. had been distilled over as completely as possible, the temperature had risen only to 81.5°. Fraction 8 from I. was, however, added to the residue in the still as usual and, on distillation, a considerable amount was collected below 89.35°, but the quantity that came over between this temperature and 100° was very small, and it was clearly not worth while to attempt to recover any more ethyl acetate in the next fractionation from the eighth fraction of II. This last fraction was therefore merely redistilled to recover as much propyl acetate as possible, and all that came over below 101.55 (5.15 grams) was rejected.

Similarly, in the third fractionation, when fraction 3 from II. had been distilled as completely as possible, the temperature had only risen to 65°, but fraction 4 from II. was added, and the distillation continued as usual. In fractionation IV., after fraction 3 from III. had been added to the residue and the distillation continued, the quantity of liquid left in the still was extremely small when the temperature reached 58° ; the distillation was therefore stopped, and the residue was rejected. The fractionations numbered III. to VI. were therefore not continuous, but in each case consisted of two parts. Thus in III. the distillation was stopped when the temperature had reached 77.7° ; the still and still-head were dried, and the last fraction from II. was separately distilled, when the recovery of propyl acetate was completed.

In IV. the first part of the fractionation ended when the temperature reached 58°, and was recommenced by the distillation of fraction 4 from III. In this case the temperature did not rise above 7745°, even when the highest fraction from III. was distilled ; fraction 6 was therefore taken to be pure ethyl acetate.