Refining Destruction Distillation Products

When the total distillate is condensed together the tar does not commonly settle to the bottom as in the case of hardwood distillation, but on account of the low gravity oils produced from the resin the total oily distillate is lighter than the pyroligneous acid and floats. On distilling a pine wood with very little pitch in it the tar would probably sink, but in anjf commercial plant distilling wood that is pitchy enough to make a profitable operation the total oils always float. Figure 19 gives the boiling point and specific gravity curves for a typical total distillate from a wood containing about 23 per cetit resin. There is no standard method for the separation of such a mixture into commercial products since different commercial plants have developed special products for their customers or vary the products to suit the demands of the trade. Sometimes the total oil as separated from the pyroligneous acid is sold without further refining. A typical refining operation will be described, however, as an example of a simple process with a small number of products.

The Oil

The oil whose composition is indicated in Figure 20 is distilled with steam for the separation of the oils with boiling points up to about 190° C. or the corresponding gravity of about 0.92. Since the principles underlying distillation of an oil with steam are not generally understood and since these principles are frequently applied in resinous wood distillation, a short explanation will be given here. When a single liquid like water or a mixture of two miscible liquids like water and alcohol is heated it begins to boil when the vapor pressure of the liquid equals the atmospheric pressure. The physico-chemical law governing steam distillation states that when two non-miscible (mutually insoluble) liquids like oil and water are heated while kept in equilibrium (well stirred) the mixture will begin to boil when the sum of the vapor pressures of the two constituents is equal to atmospheric pressure. This means that the mixture will boil at a temperature lower than the ordinary boiling point of either the oil or the water. Advantage is pftçn fàkçn of this principle to distil oils with boiling points abovç

those readily reached by pressure steam. Since the distillation of a mixture of oil and water is not convenient on account of still capacity and tendency to bump and froth, the same thing is accomplished by blowing live steam into the oil.

100 140 iso zeo 260 300 540 500 boiling point-degrees centigrade

Fig. 20.—Distillation Curve of Crude Oil from Destructive Distillation of Resinous Wood,

100 140 iso zeo 260 300 540 500 boiling point-degrees centigrade

Fig. 20.—Distillation Curve of Crude Oil from Destructive Distillation of Resinous Wood,

There is another invariable law in connection with steam distillation which regulates the relative amounts of oil and water which distil at the same time. If at the time when the sum of the vapor pressures of the water and the oil reaches atmospheric pressure the partial pressure exerted by the water is twice as great as that exerted by the oil, then twicç as much water as oil will pistil (njeasuring the amounts by molecules, not by actual weight). That is, there is an invariable relation between the amounts of oil and water distilling depending only on the vapor pressures and the molecular weights of the constituents. This is the reason for the larger amounts of steam required for distilling-the oils with high boiling point.

Steam distillation is convenient and is fairly efficient with low-boiling oils which require only small proportions of water to make the equilibrium mixture. It is sometimes necessary in order to distil an oil from a mixture when some component of the mixture decomposes at the normal boiling point of the oil. It is very inefficient, however, for the distillation of high boiling oils, since not only must the steam furnish practically the same amount of heat required for the volatilization of the oil by straight distillation but also the heat is lost which is contained in the large quantity of steam which goes over with the oil vapors.

Another kind of steam distillation is sometimes used which may seem to be the same kind of an operation and subject to the same laws, but which works on a different principle. To cut down condensation of steam in the still in which steam distillation is carried out, dosed steam coils are frequently used. If these closed coils are used only enough to keep down condensation but not enough to heat the oil above its boiling point with steam, then the operation is like the steam distillation just described. If, however, the oil is heated by the closed coils to a temperature above the boiling point of water there is no liquid water in contact with the oil and the principle of the distillation of immiscible liquids does not hold. There is no fixed boiling temperature nor fixed proportion between water and oil in the distillate. The laws which govern here are exactly the same as those in the evaporation of water below its boiling point in a current of air. The temperature of the evaporation increases with the amount of heat furnished by the closed coils and decreases with the amount of live steam blown into the still. The proportion of oil to water in the distillate depends on the vapor pressure which in turn depends on the temperature of the oil. If the vapor pressure of the oil is say, one-fourth the total pressure in the still, then the volume of oil vapor is one-fourth and the water vapor three-fourths'. By heating the oil to a higher temperature a larger ratio of oil to water can be obtained in the distillate. The efficiency of this method as determined by steam consumption is greater than that of the regular saturated steam distillation and the efficiency increases as the temperature of the distilling oil is increased.

This discussion of steam distillation has been confined to a single oil of constant boiling point, but it can now be applied to a complex mixture, such as the total oils obtained by the destructive distillation of pine wood. As in distillation without steam the lower boiling oils, or the ones with the highest vapor pressure at the temperature of the steam distillation, distil first and fractionation according to boiling points is obtained. Many people seem to have the opinion that steam distillation is a "mechanical process" and that the specific gravity of the

oil is the factor which controls the fractionation, the lighter oils being carried in the current of steam more readily than the heavier oils. Since the lighter oils usually have the higher vapor pressure this opinion is perhaps excusable.

On distillation of the oil whose boiling curves are shown in Figure 20 a first fraction may be obtained which has a composition indicated in Figure 21. This is a typical crude turpentine obtained from a commercial plant1 and from it the refined wood turpentine can be obtained by a refining process consisting of chemical treatments and redistillations. Figure 21 shows that the boiling points of this crude turpentine

-legend-• temperature o specific gravity q index of refraction

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