The Steam Distillation and Solvent Extraction Process

This process has been in use for so short a time that it is not yet well standardized in details and very little literature on the subject is available. The steam distillation part of the process has, however, been studied experimentally.1

Steam Distillation

The theory of steam distillation of volatile oils outlined in Chapter IV applies in die removal of crude turpentine (turpentine and pine oil) from resinous wood by means of steam. There are, however, two additions that must be made to the principles previously developed to cover the effects of the presence of the wood and the rosin. The wood prevents free contact between the steam and the oils to be distilled, so that there is not a true equilibrium and the ratio of water to oil in the distillate is greater than that which would be required to distil the oil by itself. The rosin also has a similar effect, due to the fact that it lowers the vapor pressure of the oil in which it is dissolved and thus decreases the ratio of oil to water in the distillate.

Since the wood prevents free contact between the steam and the oil, the larger the pieces of wood the greater is this effect. It has been stated that the heat conductivity of wood and the penetration of wood by liquids or vapors are greatest in the direction of the grain of the wood and, therefore, the variation in size of the piece of wood to be distilled has the greatest effect in directions at right angles to the grain. ^ Since the establishment of equilibrium between the oil and the steam is difficult on account of the protective effect of the wood, the ratio between oil and water will be varied by the speed at which the steam is blown through the wood. With a very slow current of steam almost the theoretical proportion of oil might be carried over, but with increasing speed of steam the ratio of oil to water would decrease rapidly.

It has also been shown that the steam pressure has a considerable effect on the results of steam distillation of wood, both on the total

1 The pistiUatiop of Re^nous Wood with Saturated Steam, For. gerv, ?ull. 109.

yield of oil and on the ratio between oil and water. This is not because there is any noticeable effect of steam pressure on the distillation of oil by itself but because the increased steam pressure by further penetration of steam into the wood brings better contact between oil and steam, thus increasing the total yield of oil and the oil-water ratio.

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