Hydrodistillation of Essential Oils at High and at Reduced Pressure, and With Superheated Steam

(h) Hydrodistillation of Essential Oils at High and at Reduced Pressure, and With Superheated Steam.

Water Distillation of Essential Oils at Reduced Pressure. 

This type of distillation is used to prevent decomposition of the volatile oil, because by its use even easily hydrolyzed esters are retained intact. With certain oils the method gives most favorable results.

On the other hand, it should be kept in mind that the rate of vaporization of water-soluble and high boiling constituents decreases as their boiling point and degree of water solubility increase. Stated differently, in the water distillation of essential oils at reduced pressure, the ratio of oil to water in the distillate is even more unfavorable than when water distillation of the same products at atmospheric pressure is practiced, because any lowering in the external pressure reduces the vapor pressure of all high boiling compounds relatively much more than that of water (steam). Also, the differential between the temperature of distillation and that of the cooling water in this case is slight; therefore, considerable oil losses may be caused by evaporation, particularly when the temperature differential is still further reduced by any excessive and unnecessary lowering of the distillation pressure. The same conditions prevail here as with hydrodistillation of plant materials at reduced pressure.

To achieve a high rate of distillation when hydrodistilling volatile oils at reduced pressure, the empty space above the liquid in the vacuum still should be kept sufficiently large to permit the still content to boil without foaming into the condenser. In addition, the condenser surface must be larger (about five times larger than that required for distillation at atmospheric pressure). In the case of vacuum distillation, the efficiency of the condenser is considerably reduced by the high speed at which the steam and oil vapors rush through the tubes, and also by the fact that with lower temperatures of distillation the capacity of heat absorption by the cooling water diminishes.

In general, it can be stated that hydrodistillation at reduced pressure is especially suitable for the rectification of liquids that possess medium volatility and do not withstand heating, as well as for the purification of high boiling mixtures which are to be freed from lower boiling impurities. The method can also be used for removing traces of a solvent from an extract. Hydrodistillation can be conducted at as low a temperature and pressure as the temperature and the efficiency of the condenser permit.

Water Distillation of Essential Oils at High Pressure.

Pressure within the retort can be increased by inserting a throttling valve into the tube (gooseneck) connecting the retort with the condenser. When operating at a pressure above atmospheric, the unfilled space in the retort above the charge should be sufficient to prevent foaming over of the still content. The use of live steam is preferable, because refilling the still with water during the operation offers some difficulties. When heat is first applied to the retort, no excess pressure must be applied until all air has escaped from the still.

Water distillation of volatile oils at high pressure is useful for certain purposes for instance, for the hydrolysis of esters, if so desired. This modification, however, by no means represents a general method of rectification. Relative to the steam pressure, the vapor pressure of higher boiling oil constituents increases more as the temperature rises; thus the ratio of oil in the distillate will be more favorable. However, from this angle, and from the practical point of view, water distillation at high pressure is not as effective as distillation with superheated steam, because the latter method vaporizes more oil without necessitating the high pressure of the former method.

Distillation of Essential Oils with Superheated Steam.

This occurs when the steam in the steam/vapor mixture rising from the oil is superheated. As was stated previously, this condition of the water component in the steam/vapor mixture is of great importance for the vaporization of oil. The same unit space occupied by a mixture of oil vapors and steam will contain relatively a much smaller quantity of steam, in a superheated state, than it would contain of saturated steam.

In actual practice, steam can be superheated by two methods:

1. By superheating within the retort.

The volatile oil is poured into the retort (without addition of water) and through a steam jacket or closed steam coil or oil bath, heated above the boiling point of water at the corresponding pressure. If saturated but dry steam is injected into the oil and thoroughly distributed, the steam will be superheated in the hot oil layer.

2. By superheating outside of the retort.

The steam is superheated in a special oven before it enters the retort and, as such, is injected into the oil, which does not have to be specially heated.

A combination of the two methods increases effectiveness of each. The stills serving for distillation of volatile oils with superheated steam should be constructed high with a small diameter; they should be well insulated, and provided with a steam jacket and a many-coiled perforated steam pipe. These precautions permit the injected steam to assume the temperature of the heated oil and to become thoroughly saturated with its vapors. When a distillate of high purity is desired, the force of distillation should be moderate in other words, the quantity of the injected steam should be reduced. This is especially important in the case of vacuum distillation with superheated steam. A reduction in the rate of the injected steam also permits a more thorough saturation of the steam with oil vapors.

In general, it can be stated that distillation with superheated steam is particularly valuable in the case of those volatile oils or oil constituents which are partly soluble in water, because only a small quantity of water (steam) is required, and this stays in contact with the oil to be vaporized. The vaporizing liquid, therefore, acts like a water-insoluble compound. The method is well adapted to the distillation and purification of benzyl alcohol, cinnamic alcohol, phenyl ethyl alcohol, etc. in other words, to all high boiling and chemically stable compounds which contain higher boiling impurities.

Distillation of Essential Oils with Superheated Steam at Reduced Pressure. 

In the above described process, the steam can be superheated inside or outside of the still. An important modification, however, consists in connecting the retort and the closed oil/water separator (receiver) with a vacuum pump so that the oil vaporizes in the retort at reduced pressure. By this means it is possible to regulate the temperature of the oil vapors at will. According to the chosen temperature, the vapors will be more or less superheated which means a more favorable ratio of oil in the distillate than is the case when the oil is merely steam distilled without superheating. For example, by heating the oil charge in the retort indirectly with steam of 10 atmospheres pressure, by injecting dry live steam of high pressure very slowly into the oil at the same time, and by carefully adjusting the vacuum pump and the direct steam inlet to a distillation pressure of 30 to 40 mm. at a temperature of about 160 within the retort, even high boiling compounds such as glycerin, palmitic and oleic acid will distill over in ample quantities. For the vaporization of high boiling substances, this method therefore exceeds even dry vacuum distillation in efficiency. As for every type of hydrodistillation in vacuo, it is necessary to provide for sufficiently large condensers, and to inject the direct steam very slowly into the oil charge, so that no foaming takes place, and the distillate will not be contaminated with impurities mechanically carried over. (von Rechenberg).

As was stated in the section on "Distillation," most essential oils are today isolated from the respective plants, or parts of plants in which they occur, by the process of distillation. A few essential oils i.e., those present in the peels of citrus fruit can be, and in large part are, obtained by cold pressing, which yields products of superior quality.

In our discussion of distillation it was emphasized that the process of distillation suffers from several inadequacies: the relatively long action of steam or boiling water on the plant material affects some of the more delicate constituents of the oil deleteriously ; hydrolysis, polymerization and resinification may and do take place; high boiling constituents, especially if somewhat soluble in water, are not carried over by steam, and are therefore lacking in the distilled oil. Other constituents dissolve partly in the distillation water, and cannot readily be recovered. As a result of all these factors, a distilled oil does not always represent the natural oil as it originally occurred in the plant.

A few types of flowers and this is the case with some very delicate ones yield no direct oil at all on distillation. The oil is either destroyed by the action of steam, or the minute quantities of oil actually distilling over are "lost" in the large volume of distillation water from which the oil cannot be recovered. This applies to jasmine, tuberose, violet, jonquil, narcissus, mimosa, acacia, gardenia, hyacinth and a few others. When hydrodistilled, these flowers yield either practically no oil, or in such low yield, or of such inferior quality, that for all purposes it is useless. Therefore, flowers of this type must be processed by methods other than distillation. This fact was recognized empirically hundreds of years ago when such flowers were treated by maceration in cold or hot fat, which process yielded fragrant pomades. From this primitive beginning there developed in the Grasse region of Southern France, in the course of many years, a highly specialized industry, employing the processes of maceration and of enfleurage and, for the last forty years, the modern process of cold extraction with volatile solvents. Despite similar, but much less important developments in other parts of the world (Bulgaria, Egypt, Algeria, Sicily, Calabria, Madagascar, etc.), Grasse has remained the center of this picturesque and charming industry, which today supplies the perfume manufacturers with a great variety of highly prized so-called "natural flower oils." Representing the authentic scents as exhaled by the flowers, these flower oils are the finest and most delicate ingredients at the disposal of the modern perfumer, enabling him to create masterpieces of his art by skillful application and blending.

The term "natural flower oil," as used today commercially, does not include the distilled essential oils; it applies only to flower oils obtained by the methods of enfleurage, maceration and extraction with volatile solvents, which will be described later in detail. A few oils--e.g., those derived from rose petals and from the blossoms of the sour (bitter) orange tree can be isolated either by distillation or by extraction. The oils are then called essential oils and natural flower oils, respectively, the latter reproducing and representing the original scent of the flowers in a more complete way. It is principally the elaborate apparatus required and the higher cost of manufacturing which prevent a more general adaptation of the process of extraction.