Amount Of Heat Required

In order to calculate the amount of heat required to steam-distil a given quantity of an essential oil it is necessary to consider each component separately. If we have to deal with an oil of comparatively simple composition such as that of almonds, cloves, caraway, or wintergreen, the calculation is within the region of practical application. We have to know, not only the proportion of each constituent, but also the amount of steam that is required to distil each of them separately. This latter can be calculated from a knowledge of the molecular weight of the substance and its vapour pressure at 100° C, as has already been indicated. These figures having been obtained we can calculate the amount of heat required to vaporise at constant pressure each component of the system - e.g. water, carvacrol, and cymene in the case of origanum oil - the total heat required being the sum of the heats necessary for the constituents. The heat of vaporisation may be calculated from the change of vapour pressure which accompanies a change of temperature.

If H be the heat of vaporisation in calories per molecule, T be the absolute temperature at pressure P, and dP the small rise in pressure which results from a small rise in temperature dT, then

1 This figure may be obtained by the interpolation of a boiling point-pressure curve or by actual experiment.

As an example, consider cymene. This boils at 174° C. under a pressure of 760 mm. For every 1 mm. increase of pressure (dP) the boiling point increases by 0.057° {dT), thus at normal pressure the heat of vaporisation is given by

= 9156 calories per molecule.

By experiment H is found to be about 2 per cent lower, viz. 8978.

If we divide the value for H by the molecular weight of cymene, 134, we find the heat of vaporisation of 1 kilogram to be 68 (calculated).

The value of dP: dT can be estimated from a boiling point-pressure curve taken over a small range in the neighbourhood of the pressure employed, either by evaluating the tangent drawn to the curve at that pressure or by averaging the difference between two points one on each side and equidistant from the point of which the value dP : dT is required. Alternatively, H may be estimated experimentally by well-known methods. We may also employ the formula previously quoted, since

ML =H and R = 1.985.

Very few experimental determinations have been published of the heats of vaporisation of the many bodies which occur in essential oils ; the following are available: Table 122