Table 49. Plain Vertical Still-heads

Vertical height in cm.

12

62

120

From 62 to 70 cm.

Internal diameter in mm.

From 14 to 15.5 mm.

51

8.0

140

19.4

25.7

Final temperature.

Percentage weight of distillate.

83.2°.......................................

86.2.....

....

0.1

1.0

0.5

0.4

0.1

1.3

0.2

89.2.....

2.1

12.8

19.4

22.2

16.2

12..8

15.2

20.0

92.3.....

28.4

21.5

16.6

16.5

18.3

21.5

19.4

18.8

95.4.....

18.0

11.0

10.6

9.2

11.7

11.0

11.2

9.2

98.5.....

10.9

8.9

7.2

7.2

8.7

8.9

8.6

7.2

101.6........................................

8.6

7.4

6.5

5.8

7.5

7.4

6.1

4.7

104.6........................................

7.2

6.2

6.0

5.5

5.8

6.2

5.6

5.9

107.6........................................

7.2

7.3

6.1

5.9

6.9

7.3

6.6

6.4

110.0.....

6.8

8.4

7.8

7.1

8.2

8.4

9.5

10.0

110.6 .......................................

5.9

9.0

8.1

8.9

9.5

9.0

8.0

7.2

Pure toluene by difference.

4.9

7.4

10.7

11.2

6.8

7.4

8.5

10.4

100.0

100.0

100. 0

100.0

100 0

100 0

100.0

100.0

Weight of liquid and } vapour in still-head . }

0.3

1.55

3.55

1.05

1.15

1.55

2.35

315

Influence Of Length

In the first three distillations the diameter of the still-head was nearly the same, but the third tube was ten times as long as the first. As might be expected, the efficiency is improved by this alteration, but the weight of liquid and vapour also increases and is, roughly, proportional to the length.

Influence Of Width

For tubes of approximately equal length the efficiency is smallest when the diameter is rather less than 14 mm., and rises when it is either increased or diminished, as will be seen from the results of the last five distillations. On the other hand, the weight of liquid and vapour in the still-head increases with the diameter of the tube, and it is therefore clearly more advantageous, in making a plain still-head, to use very narrow rather than very wide tubing. It will be noticed also that the narrowest tube gives a better result than the tube of medium diameter nearly twice as long, though the weight of liquid and vapour is only 1.05 as compared with 3.55 grams.

The diameter of the tubing cannot, however, be diminished beyond a certain amount, depending on the nature of the liquid distilled, especially on its boiling point, for if the tube is too narrow, the condensed liquid will unite into columns which will be driven bodily upwards. This blocking is much less liable to occur if a short piece of wider tube is sealed to the bottom of the narrow one. There must also be a wider piece at the top to admit the thermometer.

Condensation in the still-head may be diminished by covering it with cotton wool or any other non-conducting material, but while the amount of liquid in the tube is thus diminished, so also is the efficiency. The device is, however, occasionally useful, for if the liquid in the tube is just on the point of joining up into columns, the blocking may be prevented by slightly diminishing the amount of condensation.