(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
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.
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.
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.
----------
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
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.
---------
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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