Influence Of Pressure On Composition Of Distillate

In a technical steam distillation it is generally desired that the amount of the sub-stance of higher boiling point than water should be a maximum, or in other words, the conditions should be so chosen that the vapour pressure of the substance of high boiling point should be a maximum relatively to that of water.

The change of the vapour pressure of a substance with change of temperature is of a logarithmic nature and is expressed by the equation where L is the latent heat, R the gas constant in calories, and T the absolute temperature. This equation may be rewritten and has been simplified by Sidgwick1 to

Influence Of Pressure On Composition Of Distillate 538Influence Of Pressure On Composition Of Distillate 539Influence Of Pressure On Composition Of Distillate 540

A and B being constants determined empirically for each substance. Sidgwick shows that if log P be plotted against 1/T, for any number of substances, a series of nearly straight lines is obtained radiating from points on the log P axis (where 1/T = 0 and hence log P = A); that is to say, the lines approach one another as the temperature rises. The distance between two lines is the logarithm of the ratio of their vapour p1 pressures. Hence as the temperature rises the value of log P1/P2 approaches zero and the ratio P1 : P2 approaches unity. In other words, by increasing the temperature at which the distillation is carried out a larger proportion of the substance of high boiling point is obtained. The temperature of distillation can be increased by increasing the total pressure of the system. This is effected technically by working the still under pressure, a pressure valve being interposed between the vapour outlet of the still and the condenser.

A mixture of benzaldehyde and water at normal pressure boils at 97.9° C. and the distillate contains 31.4 per cent of benzaldehyde, but if the pressure be increased to four atmospheres the temperature rises to 140-7° C. and the percentage of benzaldehyde to 38.2. Conversely, by reducing the pressure the temperature and the relative amount of benzaldehyde are lowered ; thus at 76 mm. the temperature is 45.3° C. and the distillate contains 22.5 per cent of benzaldehyde.

1 Trans. Chem. Soc, 1920, 117, 396.

Distillation With High Pressure Steam

As an alternative to working the still under increased pressure, high pressure steam may be employed in a still worked at any pressure lower than that of the steam itself. When high pressure steam is injected into a still under a lower pressure the temperature of the steam diminishes, due to the work done by the steam in expanding against the pressure in the still. The following table indicates the drop in temperature over a range of one to ten atmospheres initial pressure: Table 121

Initial pressure, lbs. per sq. inch.

Initial temperature.

Temperature after expansion

to 760 mm.

to 76 mm.

14

10000° C.

1000° C.

83.3° C.

21

111.74

108.1

91.4

28

120.60

113.4

96.7

42

1.33.91

121.9

105.2

56

144.00

128.2

111.5

70

152.22

133.3

116.6

84

159.22

137.7

1210

98

165.34

141.4

124.7

112

170.81

144.8

128.1

126

175.77

147.9

131.2

140

180.31

150.7

133.9

This drop in temperature may be calculated with the aid of Zeuner's equation,1

pv = BT-Cpn, where B, C, and n are constants, and

C/B = 38.11, n = 0.25, p being the pressure in atmospheres, v the volume in cubic metres per kilogram, and T the temperature in degrees centigrade.

If the steam have the initial temperature of T1 at a pressure p1 and a volume v1v and it be released into a still at T2, p2, and v2 respectively, then p1v1=p2v2 = Bt1 - Cp1n = Bt2 - Cp2n, whence T1 - T2 = C/B (p1n-p2n).

For example, if we consider steam at 70 lb. per square inch (5 atmospheres) introduced into a still at 1 atmosphere we have, substituting figures taken from the table,

152.22 - T2 = 38.11(50.25 -10.25), whence T2 = 133.3° C.

»

The use of superheated steam in a still worked at ordinary pressure has the advantage that less condensation, due to the radiation of heat, takes place, the "superheat" of the steam causing the condensed water to evaporate, thus obviating the use of supplementary heat; but it follows that beyond this no gain over steam used under ordinary pressure will result so long as condensed water is present in the still. If all condensation of the steam is prevented, then the higher temperature of the superheated steam will cause an increase of the proportion of the oil in the distillate.

1. Zcuner, Technische Thermodynamik, 1900, 2, p. 221.

Thus, if we employ steam at 5 atmospheres (56 lb. on the boiler gauge) to distil benzaldehyde under atmospheric pressure and allow no condensation to take place in the still, the temperature, as calculated above, will be 133.3° C. At this temperature the vapour pressure of benzaldehyde is 220 mm.1 The total pressure in the still is 760 mm., hence, using previous formulae,

PA + PB = 760,

PA = 540 (water), PB = 220 (benzaldehyde); whence

Distillation With High Pressure Steam 541

The total distillate is 583 + 243 parts and contains 583 parts of benzaldehyde, i.e. 70.6 per cent, whereas with steam at 100° C. and one atmosphere the distillate contained only 314 per cent of benzaldehyde.

If the benzaldehyde be heated above the temperature of the ingoing steam, the tendency is similarly to increase the proportion of benzalde-hyde in the distillate.