Advantages Of Efficient Still-Head

With an evaporator still-head of eight sections the following results were obtained (p. 159).

Weight of methyl acetate taken . . . .

100

,, ethyl ,, ,,....

120

propyl „ „ . . . .

100

320

Weight of distillate below first middle point, 97.95; corrected, 98.45, Weight of distillate between first and second middle points.....120.7; „ 120.8

Percentage composition of mixture.

Found.

Taken.

Methyl acetate . . . .

30.77

3125

Ethyl „ ...

37.75

37 50

Propyl „ . . ..

31.48

3125

-

100 00

100 00

The distillation was continued until the temperature reached the boiling point of propyl acetate. The residue in the still was weighed when cold, and the loss, due partly to evaporation and partly to the minute amount of liquid left in the still-head, was found to be 0.8 gram. It was assumed that of this loss 0.7 gram was due to evaporation, and that 0.5 gram was lost below the first middle point, 0.1 between the two middle points, and 0.1 above the second middle point.

The fractionation of the mixture was continued, and the results obtained in the second complete distillation are given below

Weight below first middle point . . . . .

99.5

Weight between first and second middle points . . . . . .

118.25

Weight above second middle point . . . . . . . .

99.55

317.3

Total loss ......

2.7

320 0

Here, again, it is impossible to distribute the loss correctly between the different fractions, and it is best to calculate the percentages on the total amount of material left at the end of the distillation.

Percentage composition of mixture.

Found.

Taken.

Methyl acetate

3136

3125

Ethyl „ . .

37.27

37.50

Propyl „ . .

31.37

3125

100 00

100 00

The agreement is better than after a single distillation, and is very satisfactory.

Complex Mixtures

The separation of isopentane and normal pentane (b.p. 27.95° and 36.3° ; middle point 32.15°) from a mixture containing also butanes, hexanes, and a very little pentamethylene has been already referred to (p. 113). Taking the weight of distillate between 27.95° and 36.3° as 100 in each case, the percentage coming over between 27.95° and 32.15° became roughly constant after the first distillation. The variation in this case was greater than with the esters (40 - 46, mean 42 per cent in 12 fractionations), but the difference between the boiling points of the components is only 8.35° against 20.15° and 22.4°.

It would appear, then, that in American petroleum,. of the two paraffins, about 42 per cent consists of isopentane and 58 per cent of normal pentane. At the end of the fractionations 101 grams of pure isopentane and 175 grams of pure normal pentane were obtained, or 36.6 per cent of the isoparaffin ; but the loss by evaporation must have been greater, and there.is never so good a recovery of the more volatile component.

Mixtures Of Constant Boiling Point

For the sake of brevity, an azeotropic mixture containing two components will be referred to in this chapter simply as a "binary" mixture, and an azeotropic mixture containing three components as a " ternary " mixture.

The quantity of a mixture of constant boiling point may be estimated by the distillation method in exactly the same way as that of a single substance. The methods of experiment and of calculation are similar in all respects.

Two examples may be given to prove this point; binary mixtures of isopropyl alcohol and tertiary butyl alcohol respectively with water were mixed with excess of water and distilled with the following results:Table 64

Isopropyl Alcohol and Water Boiling points : Binary mixture, 80.37° ; water, 100.0° ; middle point, 90.2°.

Mixture taken.

Weight below middle point.

Percentage composition of mixture.

Found.

Taken.

Binary mixture ....

57.7

Observed 57.3

Binary mixture ...

74.05

74.15

Water .................

20.1

Corrected 57.6

Water .........

25.95

25.85

77.8

100.00

100.00

[Table