THEORIES OF DISTILLATION Essential Oils

I. THEORIES OF DISTILLATION


Essential, volatile or ethereal oils are mixtures composed of volatile, liquid and solid compounds which vary widely in regard to their composition and boiling points. Every substance with a determinable boiling point is volatile and possesses a definite vapor pressure, which depends upon the prevailing temperature, and which is very low in the case of very high boiling substances. Hence, the intensity of an odor may be considered, to a certain extent and with many exceptions, as a manifestation of the volatility (boiling point and vapor pressure) of the substance which emits the odor.
Distillation may be defined as "the separation of the components of a mixture of two or more liquids by virtue of the difference in their vapor pressure" (Stephen Miall, "A New Dictionary of Chemistry/' London, Longmans Green, 1940). The process of distillation is obviously of considerable importance to the essential oil producer. There are two general types to be considered :
1. Distillation of mixtures of liquids which are not miscible, and hence form two phases. Practically, this applies to the rectification and fractionation of essential oils with steam, and, what is much more important, to the isolation of volatile oils from aromatic plants with steam. Distillation with steam may also be called hydrodistillation, which general term implies that distillation may be carried out either by boiling the plant material or the essential oil with water, and creating the necessary steam within the still, or by introducing into the retort live steam generated hi a separate steam boiler.
2. Distillation of liquids which are completely miscible in each/ other, and therefore form only one phase. Practically, this applies to the rectification and separation of an essential oil into several fractions (fractionation)," without the use of steam.
The difference between the behavior of single-phase mixtures and twophase mixtures can best be understood by considering what happens when a liquid vaporizes, especially on boiling. Let us consider first the case of a pure liquid in a closed container. At a given, fixed temperature, the average energy of the molecules is fixed. The molecules are in constant and completely random motion. Any molecule in the main body of the liquid can travel only a short distance before it comes under the influence of other molecules at which moment its direction of motion is changed. Any molecule in the surface layer, however, which happens to be moving in a direction away from the main body of the liquid can escape into the space above the liquid, thus becoming a vapor molecule. Now, the vapor molecules, too, are in constant motion, the speed of the molecules of any kind being determined solely by the prevailing temperature. Any vapor molecule hitting the liquid surface has a chance of being captured by the liquid in other words of being reliquefied (condensed). As the temperature is raised the number of vapor molecules increases. Obviously the chances of a molecule returning into the liquid also increase, so that after a short time the number of molecules vaporizing in a unit of time exactly equals the number condensing (being reliquefied) in the same time. Thus, there arises a condition of dynamic equilibrium, with the total number of molecules in the vapor state remaining constant. If the space filled with saturated vapors is opened, vapor escapes and will be replaced by the same number of molecules, i.e., by the same quantity of vapor newly developed from the liquid mass. This applies not only to liquids but to solids, because, as pointed out above, every substance with a determinable boiling point is volatile.
Let us now suppose that, still at constant temperature, a second liquid, completely miscible with the first one, is added. Since the two liquids form a single phase, the surface of the liquid mixture consists only partially of molecules of the first kind. The number of molecules of the first kind escaping into the vapor space per unit time must certainly depend on the number present in the surface layer, and will, therefore, be smaller now than it was for the pure liquid. However, the molecules being completely miscible, the total number returning from the vapor to the liquid will not immediately be changed. Since the total amount of surface is unchanged and since now more molecules of the first kind are condensing than are being vaporized, temporarily the equilibrium originally established will be disturbed. This process continues until a new equilibrium is established, when these rates again become equal, and this in turn causes a decrease in the number of molecules of the first kind present in the vapor phase at any one time. Exactly the same law applies to the second component of the mixture. In general, the number of molecules of any component of a homogeneous mixture present in the vapor phase will thus be smaller than the number present in the same vapor space if the pure liquid is involved. The fraction of the surface occupied by either liquid is, of course, proportional to its relative amount, and consequently the extent to which the rate of vaporization decreases will depend on the composition of the liquid. The vapor composition of a one phase mixture will, therefore, be determined at any fixed temperature by the composition of the liquid.
Boiling point may be defined as "the temperature at which, under atmospheric or any other specified pressure, a liquid is transformed into a vapor; i.e., the temperature at which the vapor pressure of the liquid equals the pressure of the surrounding gas or vapor" ("Hackh's Chemical Dictionary/' Philadelphia, 1944). When distilling at atmospheric pressure, this vapor pressure corresponds to the weight of a mercury column of 760 mm.2 in height. Any reduction of the pressure above a liquid causes a lowering of the boiling point, any increase of pressure results in a higher boiling point. A liquid consisting of several constituents, completely miscible in one another and possessing different boiling points, in most cases (except the so-called "constant boiling mixtures") does not have a uniform boiling point but a boiling range. As the lower boiling constituents vaporize or distill off, the boiling temperature of the liquid rises and finally approaches that of the highest boiling constituent.
Next, let us consider the effect of adding to a pure liquid in equilibrium with its vapor a second liquid which is completely immiscible with the first one. This brings us to a discussion of the distillation of heterogeneous liquids, as in the case of essential oil distillation with steam or boiling water (hydrodistillation). To facilitate visualization, imagine that the two media are kept well stirred, so that the percentage of each liquid present remains the same in all parts of the mixture, including the surface. Such mixing has little effect on the ultimate result. Again, the rate of vaporization decreases, because the number of molecules of the first liquid in the surface layer is decreased. In this case, however, the liquids are not miscible, and the vapor molecules can only be condensed when they strike a molecule of their own kind, so that the rate of condensation will also be decreased. Now, the rate of vaporization and the rate of condensation both depend upon the percentage of molecules of the first kind present on the surface. These rates will be affected equally, and there will be no change in the number of vapor molecules of the first component present. Applying the same reasoning to the case of the other component leads to the same conclusion. We thus arrive at the important law that the total number of molecules present in the vapor space above a two-phase liquid mixture at ary gwtn temperature is equal to the sum of the numbers of molecules so present if either liquid were dealt with alone. Furthermore, since the relative amounts of the two liquids present have not in any way entered our reasoning, this conclusion must be true regardless of the relative amounts so long as both liquids are present.
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2 Equals 29.922 in.; or a pressure of 14.6974 Ib. per sq. in. 1.0333 kg. per sq. cm.

In other words, in the case of a two-phase (heterogeneous) liquid the composition of the mixed vapor, at a given temperature, does not depend upon the composition of the liquid.
A system of water and essential oil forms a two-phase liquid; therefore, this type of distillation is of primary importance to the essential oil producer. Let us then consider further the results of the above reasoning for our case. The pressure exerted by a vapor, whether it consists of one or several kinds of molecules, is a manifestation of the constant bombardment by the rapidly moving vapor molecules hitting the walls enclosing the vapor. Pressure measures a force acting on a unit area, and this force, in the case of a vapor, results from the vapor molecules striking the wall and rebounding. The total pressure exerted will be equal to the pressure expended by one molecule multiplied by the number of molecules hitting a unit area of the wall in a unit of time. The kinetic energy expended by one molecule will depend on the temperature, but the number of collisions with the wall will depend on the number of molecules, of whatever kind, present in the vapor space. In other words, the pressure will depend on the concentration of the molecules or, stated differently, on the concentration of the vapor.
Now, it has been shown that in the case of a two-phase liquid the total number of molecules present in the vapor phase in equilibrium with it is greater than the number which would be present if either pure liquid were present alone at the same temperature. Hence, the pressure exerted by the vapor mixture will be greater than that exerted by either pure vapor alone. In the distillation of volatile oils with steam or boiling water (hydrodistillation), the pressure in the vapor space is maintained constant, either by connecting the vapor space with the atmosphere or by suitable controls to maintain a reduced or elevated pressure. For definiteness we shall consider an operation at atmospheric pressure. If pure water is heated in a still, it will begin to boil (or in other words, the pressure of its vapor will equal that of the atmosphere), when its temperature has reached 1000C. (212 0F.). Let us suppose that an oil insoluble in water is introduced into the still along with the water. If permitted to do so, the pressure in the vapor space would increase as previously shown. But in our case the vapor space is connected to the atmosphere; therefore, the pressure will be reduced to atmospheric pressure, which can be accomplished only by automatic lowering of the temperature. When the temperature of a liquid is lowered, the tendency of the liquid molecules to go into the vapor phase also decreases, thus decreasing the concentration of the molecules in the vapor, and consequently the vapor pressure. Hence, the temperature will be lowered to a value such that the total pressure exerted by the vapor mixture is again equal to the operating pressure (atmospheric pressure in our case). Thus the boiling temperature for any two-phase liquid will always be lower than the boiling point of either of the pure liquids at ike same total pressure. For example, water (boiling at 1000) and benzene (boiling at 800) present two such insoluble liquids : when a mixture of the two is brought to a boil at atmospheric pressure (760 mm.), it vaporizes (distills) constantly at 690 so long as both constituents remain present in the liquid mixture. The moment either of the two constituents is completely vaporized (distilled off), the temperature rises to the boiling point of the remaining constituent. Such conditions prevail with all volatile substances, provided they are insoluble in water or only very slightly soluble, and are not chemically reacted upon by water. When brought to boiling together with water, they vaporize at a temperature below that of boiling water and also below those of the boiling points of the pure compounds insoluble in water.
In the preceding discussion we emphasized repeatedly that the vapor in equilibrium with a two-phase liquid consists of two kinds of molecules. The total pressure exerted by such a mixture is due, therefore, to the sum of the pressures of each kind of molecule alone. The pressure exerted by either of the pure vapors at the same temperature would be the vapor pressure of that pure component, while the total vapor pressure of the mixture is thus equal to the sum of the partial vapor pressures. By partial pressure we mean the vapor pressure of any one component in a mixed vapor. Obviously for such two-phase liquid systems the partial pressure and vapor pressure of any component are the same. This simple rule of the additivity of partial pressures affords a ready means of estimating the temperature at which any particular steam distillation (hydrodistillation) will occur. The vapor pressures of the two pure components are simply tabulated at a series of temperatures. The operating' temperature will then be that temperature at which the sum of the two vapor pressures equals the operating pressure, in the above cited example the atmospheric pressure. In that case, the vapor pressure of water at 690 is 225 mm., the vapor pressure of benzene 535 mm., added together 760 mm. This condition permits the combined vapors of the constituents to overcome the (normal) atmospheric pressure; in other words, the mixture starts to boil at 690 under normal atmospheric pressure. In order to effect the boiling of a volatile compound insoluble in water, it remains immaterial whether the substance in question is brought to a boil with water or whether live steam is injected into the liquid or finely powdered substance. It is the steam (water vapors whence the term hydrodistillation) that causes the boiling (distillation, in our case) of the compound insoluble in water, at a 'emperature below the boiling point of the compound itself and below that of water.
The composition of the vapor formed from a two-phase liquid mixture depends on the partial vapor pressures of the pure constituents. Thus, if the vapor pressure of component A is high and that of B low, the mixed vapor will consist very largely of component A . The ratio between the weights of component A and B will be given by the ratio of their vapor pressures multiplied by the ratio of their molecular weights. As pointed out, boiling will take place only when the sum of the partial pressures exerted by the components is equal to the pressure maintained in the vapor space ; therefore, a heterogeneous (two-phase) liquid boils or distills at a temperature which, at the same total pressure, always lies below the boiling point of the lowest boiling constituent, so long as the latter remains in the mixture, pt is for this reason primarily that hydrodistillation has been used for such a long time and so generally in the isolation of essential oils from aromatic plants. By vaporizing (boiling) mixtures of water and essential oils (also from plant material), the temperature will always be maintained lower than the boiling point of water at the same total pressure and, in this way, damage and decomposition of the essential oils by overheating are prevented. The fact that the vapor pressures of most essential oils are low relative to the vapor pressures of water at corresponding temperatures accounts for the fact that the ratio of water to essential oil in the condensate is relatively high. It will make no fundamental difference in the behavior of the mixture whether or not a steam distillation is carried out in the presence of a liquid water phase, but it does influence certain practical aspects of the process, as will be indicated in the second part of this chapter.
In order to isolate an essential oil from an aromatic plant, the material, in actual practice, is packed into a still, a sufficient quantity of water added and brought to a boil, or live steam is injected into the plant charge. Due to the influence of hot water and steam, the essential oil will be freed from the oil glands in the plant tissue. The still, therefore, will contain a mixture of two liquids, viz.., hot water and volatile oil which are not mutually soluble, or only very slightly so. Gradually the liquid in the still is brought to a boil, the vapor mixture then consisting of water vapors (steam) and oil vapors. This vapor mixture passes through a connecting tube into a condenser, where it is reliquefied (condensed) by external cooling, usually with cold water. From the condenser the distillate flows into a receiver (separator), where the oil separates automatically from the distillation water. In the course of distillation it is necessary continuously to replace the water evaporating from the still, or to inject a sufficient quantity of live steam to vaporize all the volatile oil contained in the plant material or present in the still. When the last traces of volatile oil have been recovered, only pure water will distill over, and distillation is completed.
As said, the composition of the distillate from a mixture of two insoluble liquids in other words, the weight quantities of the two substances depends primarily upon their boiling points, or upon their vapor pressures at the temperature of distillation. If, for example, we distill a water insoluble compound with a boiling point of only 500, the distillate will consist of a certain volume of water and a larger volume of the water insoluble compound. If, on the other hand, a water insoluble compound with a boiling point of 300o is hydrodistilled, the distillate will contain mostly water and very little of the high boiling substance. Thus, in the distillation of a water insoluble volatile compound, the percentage of the latter in the distillate decreases with rising boiling point of the compound. This decrease, however, is not uniform with all substances. Some substances with similar boiling points will occur in the distillate in different proportions ; others with a marked differential in their boiling points may accumulate in the distillate in almost the same proportions. Deviations of this sort are caused primarily by the chemical constitutions and reactivity of the various essential oil components. As explained above, the quantitative composition of the distillate (condensate) can be calculated in advance when hydr:xlistilling chemically uniform, water insoluble substances. The rule underlying hydrodistillation of essential oils or volatile substances in general may be expressed as follows :
The ratio between the weights of the two vapor components, and therefore of the two liquids in the distillate (condensate), is expressed by the ratio of their partial vapor pressures multiplied by the ratio of their molecular weight. 
The ratio between the weights of the two vapor components
Essential oils are not chemically pure substances but consist of several, often many, compounds possessing different chemical and physical properties. The boiling points of the volatile oil components range in most cases from 150o to 300o at 760 mm. pressure. According to the preponderance of lower or higher boiling constituents we speak of a low boiling or of a high boiling oil. Distillation of an essential oil reveals its higher or lower volatility to a very marked degree if the oil is in free, direct contact with the boiling water or with the passing steam : in the early stages of distillation the lower boiling components distill over; the higher boilitg ones pass over later.
Let us now study hydrodistillation of a volatile oil with a very simple example : peppermint oil is placed into a glass flask and live steam is introduced into the oil. The external pressure and temperature, in this case, remain immaterial, so long as at least a portion of water remains in steam form. The steam then causes the peppermint oil to form vapors, to vaporize, each steam bubble presenting to the vaporized oil an empty space into which the oil immediately sends vapor molecules. Every volume unit of steam will be filled with an equal volume of oil vapors, rise to the top of the flask and enter the condenser, where steam and oil vapors are condensed. The hydrodistillation of any essential oil is based upon this simple principle which, however, does not fully apply to the oils when they are still enclosed within the plant tissue. There the steam must exert yet another action of considerable influence, i.e., it must transmit heat. Unlike a liquid, the rigid plant matter is not able to conduct the heat from the still walls to all parts of the plant charge. The heat is actually transmitted by water, either as boiling water when distilling immersed plant material or as water vapors when distilling plants by blowing live steam into the charge. Also, the volatile oils occur in special oil glands, sacks or intracellular spaces of the plant tissue ; hence the oils must be freed, prior to distillation, by breaking down the plant tissue, and by opening the oil glands as much as possible, so that their volatile content can be readily attacked and vaporized by the passing steam. In unreduced, whole plant material, the oil must be freed during distillation by the force of hydrodiffusion, a very important feature which will be discussed later in more detail.
Let us now return to the more theoretical aspects of hydrodistillation. In steam distillation it is frequently possible to change materially the ratio of water to oil in the condcnsate by changing the operating pressure. As pointed out earlier, this ratio is determined by the relationship
the ratio of water to oil in the condcnsate by changing the operating pressure
In any hydrodistillation using saturated steam, the sum of PH2O and Poil will equal the operating pressure and the still temperature will automatically adjust itself until this condition is met. As the operating pressure is lowered below atmospheric pressure, the temperature of the operation will decrease. In general, the vapor pressure of water decreases much more slowly with the temperature than does the vapor pressure of an essential oil, so that the weight ratio of water to oil increases. Conversely, this ratio decreases with increasing temperature. Data for a typical case are given in Table 3.1.
EFFECT OF OPERATING PRESSURE ON WATER TO OIL RATIO IN STEAM DISTILLATION OF CITRONELLAL
These data demonstrate that operation at reduced pressure results in a lower operating temperature, but also requires the use of more steam per weight unit of Citronellal recovered. Operation at elevated pressure (use of high-pressure steam in the still), on the other hand, permits a consid' rable saving in the amount of steam required per weight unit of oil, but also involves a higher operating temperature. Provided that the higher temperature does not damage the oil, there is evidently some advantage to be gained by the use of high-pressure steam. Details will be discussed in the second part of the chapter on distillation.
Up to this point our discussion has dealt entirely with the use of saturated steam. It is also possible indeed, in some cases advantageous to distill essential oils by using superheated steam. Pressure and temperature of superheated steam are no longer mutually dependent. Thus, it is feasible to use superheated steam at a fixed pressure and at any desired temperature above the boiling point at that pressure. The temperature at which such a distillation is carried out can thus be raised without increasing the concentration (partial pressure) of the steam. Since the temperature alone determines the vapor pressure, and consequently the partial pressure of the volatile oil, distillation with superheated steam results in a lower ratio of water to oil, accomplishing a further saving in the amount of steam used. In the above cited case of water and citronellal mixtures the steam would normally be saturated at 90o. If superheated to 100o at a pressure of 526 mm., and then used in the distillation, the molal ratio of water to citronellal is reduced to 23.3 (weight ratio = 1.72), the total operating pressure then being 548.5 mm. By increasing the pressure of the superheated steam any ratio between this and 33.8 (corresponding to the use of saturated steam at 100o) can be obtained.
Two features affecting the use of superheated steam should be pointed out. First, in order to obtain the above cited advantage of superheated steam the still must be completely free of water. When superheated steam comes into contact with water it immediately vaporizes some of the water, being itself cooled in the process and being reconverted into saturated steam. If the quantity of water present is small, it will be vaporized quickly and the process will continue as with superheated steam after the water has been evaporated. Second, the temperature of superheated steam is independent of the pressure ; hence the characteristic safeguard against overheating common with saturated steam operation no longer remains operative. The temperature of the charge will reach that of the superheated steam; therefore, the latter temperature must be controlled carefully in order to avoid damage to the essential oil. Also, since there is no water present in the still, the plant charge tends to dry out during distillation with superheated steam, and the forces of hydrodiffusion can no longer play their part. This causes a slowing down in the rate of recovery of essential oil, and in extreme cases may stop it entirely, long before the recovery is complete; in other words, the yield of essential oil will be subnormal. For all these reasons superheated steam distillation may be undertaken only with caution.
It should be mentioned in this connection that for distillation any hot gas (air, flue gas, etc.) could be used in place of steam but, since these gases are not condensable, the size of the cooler required would be so great as to be impracticable.
Let us now again study the behavior of mixtures of liquids which form a single liquid phase. These considerations apply particularly to the fractionation of essential oils after they have been isolated from the plant material. As has already been pointed out, all liquids have a tendency to change to vapors, the extent of this tendency depending on the temperature at which the liquid is maintained. This tendency to vaporize may be gaged by the vapor pressure of the liquid. In general, the components of the liquid mixture will have different vapor pressures at any particular temperature. When such a mixture is vaporized, the component with the greater vapor pressure (the more volatile component) consequently tends to concentrate in the vapor phase, while the less volatile component will be correspondingly concentrated in the liquid phase. This condition holds for all mixtures of liquids which are soluble in one another, and which do not form constant boiling mixtures. Liquid mixtures which form 'Constant boiling mixtures behave somewhat differently and will not be discussed here. The tendency of the more volatile liquid to concentrate in the vapor phase can be observed very readily by reference to the accompanying Diagram 3.1.
Typical boiling point and vapor-liquid equilibrium diagram for a single-phase binary mixture at constant pressure
 DIAGRAM 3.1. Typical boiling point and vapor-liquid equilibrium diagram for a
single-phase binary mixture at constant pressure.
In this diagram the composition of the liquid mixture and its boiling temperature have been plotted. The lower of the two curves represents the relationship between the boiling point of any mixture of these two components and its composition. The upper curve represents the composition of the vapor which is formed from any liquid mixture at its boiling point. Proceeding along a vertical line in the region below the lower curve may be said to correspond to heating a mixture of fixed composition without vaporization. At the temperature corresponding to the point at which this vertical path intersects the lower curve, this particular mixture will begin to vaporize, and the vapors arising first will have a composition represented by the intersection of a horizontal line through the boiling point of this mixture with the upper curve. In the particular case illustrated, a liquid containing A per cent of the more volatile constituent would produce an initial vapor containing a percentage of the more volatile constituent represented by point B. The vapor produced is thereby enriched with the more volatile constituents. If the distillation is continued without adding liquid to the still, the liquid in the still will become progressively poorer in the more volatile constituents. Furthermore, on condensing and then redistilling the vapor produced, a further enrichment in the more volatile constituents will be achieved. Theoretically, then, it appears possible to obtain a vapor consisting entirely of the more volatile components by a suitable number of redistillations. An effect corresponding to a series of redistillations can be produced in a fractionating column such as that shown in Fig. 3.1.
In this type of system the vapors rising from the still, as always partially enriched with the more volatile component, are essentially condensed and redistilled on the first section above the still. The vapors rising from this section are again condensed and redistilled in the next higher section, this process continuing to the top of the fractionation tower. Such equipment, 
Still with fractionating column. Schematic diagram showing essential parts and typical arrangement

 FIG 3.1. Still with fractionating column. Schematic diagram showing essential
parts and typical arrangement.
then, permits obtaining a final distillate which contains a higher percentage of the more volatile components of the mixture than the original material this, too, in a single piece of equipment. Heat is supplied to such a fractionating system in the still only. On the plates in the tower above the still the heat liberated by condensation of the vapors furnishes in turn the heat necessary to revaporize the material. Of course, the entire system must be insulated thoroughly in order to prevent excessive condensation of vapors due to the heat losses from the tower. In actual operation, such a tower would ordinarily be run by returning part of the condensate at the top to the top plate as reflux. The greater the ratio of reflux to product, the more complete will be the separation of the more volatile from the less volatile components. A system of this kind can be operated at any desired pressure either above or below normal atmospheric pressure. In the final purification of many essential oils (not hydrodistillation), the operation must proceed at very low pressures in order to avoid overheating and consequent destruction of the material. The number of plates required in the fractionation tower is determined largely by two factors :
1. The relative volatility of the components of the mixture.
2. The extent of separation required or desired.
Whenever one component is much more volatile than the other, only a few plates will be necessary to give a high degree of separation, but when the volatilities are more nearly equal, the number of plates must be greatly increased. A rough estimate of the relative volatilities can be drawn from the boiling points at atmospheric pressure of the components of the mixture. There exist quite satisfactory methods for calculating the number of plates required for any particular separation. Details of these methods go beyond the scope of this work and those interested should consult references. 3,4,5
The above considerations show that sorne separation of the components of a mixture of mutually soluble constituents (such as essential oils) can be achieved simply by vaporizing the mixture and condensing the vapors. Usually, however, this separation will be relatively small, and it will be necessary to resort either to redistillation of the condensate or to the use of  fractionating towers as indicated.
In order to consider in more detail the behavior of mixtures of soluble liquids, let us take the case of a mixture of only two constituents. The same principles apply to more complex mixtures, but will be easier to follow in the simpler case. In single-phase mixtures the tendency of either component to vaporize will depend on the temperature of the mixture, and on its composition. In the simplest case, the partial pressure of one constituent will be given by the expression
P1 = P1 X N1       (1)

3 Robinson and Gilleland, "Elements of Fractional Distillation," McGraw-Hill, New York, 1939.
4 Badger and McCabe, "Elements of Chemical Engineering," McGraw-Hill, New York, 1936.
5 Walker, Lewis, MeAdams and Gilleland, "Principles of Chemical Engineering," McGraw-Hill, New York, 1937.

in which
P1 = partial pressure of constituent 1;
P1= vapor pressure of pure constituent 1 at the temperature of the liquid;
N1= mol fraction of constituent 1.
THEORIES OF DISTILLATION
THE PRODUCTION OF ESSENTIAL OILS
terized by low density (weight per unit volume of the packing), relatively large amount of open space and a large surface area. For example, crushed rock can be used as packing, but because of its high density and low percentage of open space would not be very efficient. Several typical packin materials are shown in Fig. 3.2. 
Raschig rings

 FIG. 3.2. Raschig rings.
In the distillation of single-phase mixtures, it should be kept in mind that changing the pressure in the still has only a minor effect on the overall operation. Since in the distillation of essential oils the principal reason for ever operating at pressures other than atmospheric is to lower the distillation temperature, the pressure will usually vary between atmospheric and some lower pressure, thus limiting the possible variations in pressure. The efficiency of any particular piece of equipment may be changed slightly by operating at different pressures, but the net result will be practically unaffected. This holds true particularly in the case of mixtures such as those encountered in the purification and fractionation of essential oils.
PRESSURE EQUIVALENTS

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