General Methods of Distillation Essential Oils

(b) General Methods of Distillation. 


No investigation has yet been undertaken of the process by which steam actually isolates the essential oil from aromatic plants. It is commonly assumed that the steam penetrates the plant tissue and vaporizes all volatile substances. If this were true, the isolation of oil from plants by hydrodistillation would appear to be a rather simple process, merely requiring a sufficient quantity of steam. However, such is not the case. In fact, hydrodistillation of plants involves several physicochemical processes which will be discussed later.
There has developed in the essential oil industry a terminology which distinguishes three types of hydrodistillation. These are referred to respectively as :
1. Water distillation ;
2. Water and steam distillation ;
3. Direct steam distillation.
Originally introduced by von Rechenberg, the above terms have become established in the essential oil industry and will, therefore, be retained in our discussion. In order to avoid needless repetition, their significance will be indicated at this point. All three methods are subject to the same
A typical old-fashioned lavender still as used years ago by the lavender oil producers in Southern France. Only a few of these stills are being employed today. It is a typical water distillation, the still being heated by a fire beneath
PLATE 1. A typical old-fashioned lavender still as used years ago by the lavender oil
producers in Southern France. Only a few of these stills are being employed today. It is
a typical water distillation, the still being heated by a fire beneath.

general theoretical considerations presented in the first part of this chapter which dealt with distillation of two-phase systems. The differences lie mainly in the method of handling the plant material. Water Distillation. When this method is employed, the material to be distilled comes in direct contact with boiling water. It may float on the water or be completely immersed, depending upon its specific gravity and the quantity of material handled per charge. The water is boiled by application of heat by any of the usual methods i.e., direct fire, steam jacket closed steam coil, or, in a few cases, open or perforated steam coil. The characteristic feature of this method lies in the direct contact it affords between boiling water and plant material. Some plant materials (e.g., powdered almonds, rose petals, and orange blossoms) must be distilled while fully immersed and moving freely in boiling water, because on distillation with injected live steam (direct steam distillation) these materials agglutinate and form large compact lumps, through which the steam cannot penetrate.

Water and Steam Distillation. 

When this second common method ofdistillation is used, the plant material is supported on a perforated gridor screen inserted some distance above the bottom of fhe stiil. The lower part of the still is filled with water, to a level somewhat below this grid. The water may be heated by any of the methods previously mentioned. Saturated, in this case, wet, steam of low pressure rises through the plant material. The typical features of this method are : first, that the steam is always fully saturated, wet and never superheated ; second, that the plant material is in contact with steam only, and not with boiling water.
A field distillery of lavender in Southern France. A typical case of water and steam distillation. Many of these stills are in use today in the lavender regions of Southern France. For discharging of the spent plant material the stills can be tilted.

 PLATE 2. A field distillery of lavender in Southern France. A typical case of water
and steam distillation. Many of these stills are in use today in the lavender regions of
Southern France. For discharging of the spent plant material the stills can be tilted.

Steam Distillation. The third method, known as steam distillation or direct steam distillation, resembles the preceding one except that no water is kept in the bottom of the still. Live steam, saturated or superheated, and frequently at pressures higher than atmospheric, is introduced through open or perforated steam coils below the charge, and proceeds upward through the charge above the supporting grid.
In so far as the distillation process itself is concerned, and from the purely theoretical point of view, there should be no fundamental difference between these three methods. There exist, however, certain variations in practice, and in the practical results obtained, which in some cases are considerable; they depend on the method employed, because of certain reactions which occur during distillation. 
Field distillation of lavender flowers in Southern France. The steam is generated in a separate steam boiler
 PLATE 3. Field distillation of lavender flowers in Southern France. The steam is generated
in a separate steam boiler.
The principal effects accompanying hydrodistillation are :
1. Diffusion of essential oils and hot water through the plant membranes, whence the term hydrodiffusion ;
2. Hydrolysis of certain components of the essential oils ;
3. Decomposition occasioned by heat.

These effects will be considered in order. 

The Effects of Hydrodiffusion in Plant Distillation. Even after the plant material has been carefully prepared by proper comminution, only part of the essential oil is present on the surfaces of the material and immediately available for vaporization by steam. The remainder of the oil arrives at the surface only after diffusing through at least a thin layer of plant tissue.
The term diffusion, as used in this connection, implies the mutual penetration of different substances until an equilibrium is established within the system. Such diffusion is caused by the live force of molecules. Where two substances are not separated by a wall (diaphragm), the term “free diffusion" is applied, whereas diffusion through a permeable membrane is called osmosis. The diaphragm may be permeable by only one sui stance, or by all.
The distillation of plant material is connected with processes of di (fusion, and principally of osmosis. In the steam distillation of plant material the steam does not actually penetrate the dry cell membranes. This can easily be proved by distilling plants with superheated (dry) steam. The plant charge, in this case, finally dries out completely, and yields the retained volatile oil only when saturated (moist) steam is applied, after superheated (dry) steam no longer vaporizes the oil. Thus, dry plant material can be exhausted with dry steam only when all of the volatile oil has first been freed from the oil bearing cells by previous very thorough comminution of the plants.
Entirely different conditions obtain if the plant tissue is soaked with water. The exchange of vapors within the tissue of living plants is based primarily upon their permeability while in swollen condition. Microscopic studies have led some to believe that the walls of normal plant cells are almost impermeable for volatile oils. According to von Rechenberg, only limited osmosis of volatile oil can take place at ordinary temperatures. This may easily be proved by soaking uncomminuted dried spices (such as cinnamon or cloves) in cold water for a day or two, then pouring off and distilling the water. The yield of oil, if any, will be negligible, all the oil being retained within the plant tissue. If, on the other hand, the spices (or other plant material) are first sufficiently powdered so that the cell walls are broken and the oil liberated, the water poured off contains considerable quantities of essential oil.
Distillation offers better conditions for the osmosis of oil, because the higher temperature and the movement of water, caused by temperature and pressure fluctuations within the still, accelerate the forces of diffusion to such a point that all the volatile oil contained within the plant tissue can be collected. The effect of a higher temperature may easily be demonstrated by repeating the above described experiments, but by soaking the spices in hot, instead of cold water. The hot water will extract much larger quantities of oil.
Von Rechenberg describes the process of hydrodiffusion, in the case of plant distillation, as follows : At the temperature of boiling water a part of the volatile oil dissolves in the water present within the glands. This oilin-water solution permeates, by osmosis, through" the swollen membranes, and finally reaches the outer surface, where the oil is vaporized by passing steam. Replacing this vaporized oil, additional quantities of oil go into solution and, as such, permeate the cell membranes while water enters. This process continues until all volatile substances are diffused from the oil glands and are vaporized by the passing steam.
The speed of oil vaporization in hydrodistillation of plant material is influenced not so much by the volatility of the oil components (or in other words by the differential in their boiling points), as by their degree of solubility in water. If von Rechenberg's assumption is correct, the higher boiling, but more water-soluble, constituents of an oil enclosed within the plant tissue should distill before the lower boiling, but less water-soluble, constituents. That this actually takes place can be demonstrated by steam distilling comminuted and uncomminuted caraway seed. Uncomminuted (whole) caraway seed will first yield the higher boiling, but more watersoluble, carvone and only later the lower boiling, but less water-soluble, limonene. With crushed seed the opposite is true: the first fraction consists of limonene, the following of carvone. The fact that occasionally the final fraction may contain some limonene only goes to show that, as a result of incomplete comminution, the forces of hydrodiffusion come into play anew. Distillation of uncrushed caraway seed requires almost twice as much time as that of crushed. This well-known fact applies to distillation of all seed material. The explanation is simply that hydrodiffusion acts only slowly, and requires time: in the distillation of uncrushed seeds, all volatile oil enclosed within the plant tissue must first be brought to the surface of the seeds by hydrodiffusion.
It is a well known fact, borne out by experience, that comminution (crushing) of seed material increases the yield of oil. This, however, does not imply that uncomminuted plant material always gives a veiy low oil yield. Von Rechenberg12 soaked whole (uncrushed) caraway seed in tepid water until it became swollen, and distilled it with direct, saturated steam at pressure of 5 atmospheres in a well-insulated still. He thus obtained a very slightly lower yield of oil than by distilling crushed caraway seed. This small loss consisted exclusively of carvone, which had been resinified during the longer hours of distillation required for uncrushed, thoroughly wetted seed. Such soaking, steeping, or macerating of plant material was frequently resorted to in the old days of small-scale distillation, when saturated steam of high pressure, generated in a separate steam boiler, was not yet available. In fact, steeping in water as a preliminary process should not be condemned in the case of seed material containing relatively low boiling volatile oils caraway, fennel, coriander seed, for example. Obviously this process requires more steam, fuel, time and equipment, but the oil yield will be about normal, provided distillation has been carefully carried through. It should be borne in mind, however, that saturated steam of low pressure, if not properly employed, may easily result in a thorough wetting of the plant charge, and that this factor becomes much more troublesome with a comminuted charge than with an uncomminuted. Von Rechenberg performed experiments in point with dill, ajowan and fennel seed, as well as with cloves and clove stems. His results again prove that, in the case of uncomminuted material, the oil constituents vaporize according to the degree of their solubility in water, and not iu the sequence of their boiling points: carvone distills before limonene in the case of dill seed; thymol before pinene, dipentene and p-cymene in the case of ajowan seed; anethole before fenchone in the case of fennel seed; methyl amyl ketone before eugenol and caryophyllcne in the case of cloves ; eugenol before caryophyllene in the case of clove stems. In von Rechenberg's experiments the distillation of uncomminuted material required twice as many hours as that of comminuted material, and the yield of oil was slightly, and in some cases considerably, lower.
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12 "Theorie der Gewinnung und Trennung der atherischen Ole," Leipzig (1910), 430.


The presence of some water is distinctly beneficial in that it increases the rate of removal of essential oils by distillation, and it would appear, from this fact alone, that water distillation or water and steam distillation should be preferred to steam distillation. However, the maximum temperature that can be obtained with water distillation, and water and steam distillation, is limited entirely by the operating pressure in the still, which in ordinary operation equals atmospheric pressure. A complete summary of the advantages and disadvantages of the three methods of distillation will be given after the other factors affecting distillation have been discussed. It should be remembered, too, that all essential oils are soluble in hot water to at least a slight degree; therefore, the amount of water present will determine the extent to which the yield of oil will be decreased as a result of the retention (by water in the still) of oil, or certain constituents of the oil.
This factor is of special importance in water distillation, since all of the essential oil must first go through the water solution stage, and the water in the still will always be very nearly saturated with oil, especially with the more water-soluble constituents of an oil with phenylethyl alcohol for example, in the case of rose distillation. The situation is not quite so serious in the case of water and steam distillation because a little of the oil dissolves in the still water only as a result of drainage from the still charge which is mechanically separated from the still water. The extent of this drainage will depend upon the amount of condensation taking place within the plant charge, and especially along the still walls, but it can be kept at a minimum by suitable insulation of the still.

The Effect of Hydrolysis in Plant Distillation. 

The second effect accompanying distillation of plant material is hydrolysis. Hydrolysis in our case can be defined as a chemical reaction between water and certain constituents of the essential oils. These natural products consist partly, and in some instances largely, of esters, which are compounds of organic acids and alcohols. In the presence of water, and particularly at elevated temperatures, the esters tend to react with the water to form the parent acids and alcohols. Two characteristic features are important in determining the effect of these reactions during distillation. In the first place, the reactions are not complete in either direction. Starting with the ester and hot water, only a part of the ester will react, so that when equilibrium is reached there will be present in the system esters, water, alcohols and acids. Similarly, if only alcohols and acids had been present at the start, all four constituents would be present when equilibrium is established. The relationship between the concentrations of the various constituents at equilibrium may be written as
K = X (alcohol) x (acid)/(ester) x (water)
in which K = a constant value at any fixed temperature ;
(alcohol) = molal concentration of alcohol at equilibrium;
(acid) = molai concentration of acid at equilibrium;
(ester) = molal concentration of ester at equilibrium;
(water) = molal concentration of water at equilibrium.
Consequently, if the amount of water, and hence its concentration, is large, the amounts of alcohol and acid will also be large and hydrolysis will proceed to a considerable extent. As a result, the yield of essential oil will be correspondingly decreased. This result is one of the principle disadvantages of water distillation, since the amount of water present is always large, and hydrolysis relatively extensive. In the case of water and steam distillation, the degree of hydrolysis is much less ; it is even less with steam distillation, particularly with slightly superheated (dry) steam.
As second important characteristic of hydrolysis reactions in the disdillation of essential oils, it should be noted that hydrolysis proceeds at a measurable rate. The fact that these reactions are not infinitely rapid means that the extent to which they proceed will depend upon the time of contact between oil and water; this holds particularly true for short periods of contact. This is another obvious disadvantage of water distillation, since the oil and water have a maximum time of contact under the conditions there employed.

The Effect of Heat in Plant Distillation. 

The third important effect accompanying distillation is the influence of temperature on essential oils.
The pressure of distillation (atmospheric, excess or reduced) can be selected at will, but the temperature of the steam/vapor mixture rising through the charge in the still varies and fluctuates in the course of the operation. It is lowest at the beginning because the lowest boiling constituents of the volatile substances, freed by comminution of the plant material, vaporize first. As the higher boiling constituents begin to predominate in the vapors, and as the quantity of oil vapors per se in the steam/vapor mixture decreases, the temperature gradually rises, until it reaches that of saturated steam at the given pressure. Practically all constituents of essential oils are somewhat unstable at high temperatures. In order to obtain the best quality of oil, it is therefore necessary to insure that during distillation the essential oils (or the plant material) are maintained at low temperature or, at worst, that they be kept at a high temperature for as short a time as possible. So far as operating temperature is concerned, there is really little choice between the three commonly used methods of distillation. In the case of water distillation, or water and steam distillation, the temperature is determined entirely by the operating pressure. If the still is open to the atmosphere the usual procedure the temperature will be at, or slightly below, 100o C (212 F.). If a valve is inserted between the still and condenser, and if the apparatus is sufficiently strong to withstand the pressure, the still can be operated at pressures above atmospheric, and at temperatures correspondingly above 100o. In the case of steam distillation, the operating temperature will be at, slightly below, or above 100o, even at atmospheric pressure, depending on whether low pressure saturated or superheated steam is used. Any of the methods may be operated at temperatures below 100o by use of suitable pressures below atmospheric.

Conclusions.

Although the three processes of diffusion, hydrolysis and thermal decomposition have been considered independently, it must be remembered that in practice all three occur simultaneously, and hence they will frequently affect one another. This holds particularly true of the effect of temperature. The rate of diffusion usually will be increased by higher temperatures. The solubility of the essential oils in water an important factor, as indicated above in most cases also increases with higher temperatures. The same holds true of both the rate and extent of hydrolysis. Since the products of hydrolysis are in general more water soluble, they will also affect the diffusion process. Hence, a complete analysis of the various processes incidental to distillation offers a difficult problem. In general, observance of the following principles leads to the best yields, and to a high quality of essential oil : (1) maintenance of as low a temperature as is feasible, not forgetting, however, that the rate of production will be determined by the temperature; (2) in the case of steam distillation, use of as little water as possible in direct contact with plant material, but keeping in mind that some water should be present in order to promote diffusion; (3) thorough comminution of plant material before distillation, and very careful, uniform packing of the still charge, remembering, however, that in all but water distillation excessive comminution will result in channeling of steam through the mass of plant material, thus reducing efficiency because of poor contact between steam and charge.
A brief résumé of the advantages and disadvantages of the three distillation methods in the light of the above discussion will be helpful, and is presented below.
For small-scale installations, particularly in portable units, water distillation or water and steam distillation offers the advantage of simplicity of equipment. The latter method is rapidly superseding water distillation (except in a few special cases) because of the better quality and yield of oil, and higher rate of vaporization, i.e., speedier distillation.
For larger and fixed installations, steam distillation unquestionably offers the most advantages. In such plants the necessary control can be readily installed, and under these conditions the quality, yield and rate of oil aro superior. Also, as a result of possibility of temperature control, the method is more adaptable. Plant materials containing either low or high boiling oils can be handled in the same equipment with equal ease. Because of the auxiliary equipment required steam distillation cannot be recommended for all distillation. It is especially impracticable for the small producer in the field. Whenever conditions permit the construction of a suitably located, modern plant to. process raw material from a large area, such distillery should be equipped to carry on direct steam distillation.
Before closing the general discussion of the three principal distillation methods, it should be mentioned briefly that each method can be modified by changing the pressure in the still. Accordingly, distillation can be carried out:
(a) At reduced pressure ;
(6) At atmospheric pressure ;
(c) At excess pressure.
The effect of these variations may be observed in the ratio of distillation (condensed) water to volatile oil.
Any type of distillation carried out below the prevailing atmospheric pressure (usually with the aid of a vacuum pump) falls into class (a). Characteristic of distillation at reduced pressure is a low distillation temperature which has its limit only in the temperature of the cooling water and the efficiency of the condenser. The outstanding advantage of this form of distillation consists in the absence of the decomposition products resulting from heat. On the other hand, the vaporization capacity of high 
PRACTICE OF DISTILLATION
THE PRODUCTION OF ESSENTIAL OILS

boiling substances, especially of those somewhat soluble in water, is considerably reduced.
By inserting a valve into the gooseneck of the retort and by partly closing this valve during distillation, it is possible to throttle the outflow of the steam/oil vapors and to increase the pressure within the still. 13 Such distillation at excess pressure (0.5 to 1.0 atmospheres excess pressure, or 1.5 to 2.0 atmospheres absolute pressure) is occasionally resorted to in the essential oil industry, but its use remains very limited, because of the Resulting decomposition of many oil constituents.
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13 For the sake of clarity, it should be mentioned that the injection of high pressure steam per se does not, to any marked degree, increase the pressure in a still, which, through its gooseneck and condenser, has a free outlet to the atmosphere. Unless throttled by a valve, or by too narrow a gooseneck, or by the heavy mass of tightly packed plant material, any excess pressure of injected steam is reduced almost at once to the atmospheric pressure.

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