Rectification and Fractionation of Essential Oils

(g) Rectification and Fractionation of Essential Oils.

Many essential oils, when distilled from the plant material, are contaminated with volatile products arising from the decomposition of complex plant substances, under the influence of hot water or steam. This takes place especially in the case of water distillation in directly fired stills if, through carelessness, the plant charge "burns" on contact with the retort walls touched by the fire. Some of these decomposition products are gaseous e.g., hydrogen sulfide and ammonia ; others such as methylalcohol, acetaldehyde, acetone, and acetic acid are very soluble in water. Therefore, they occur mainly in the distillation water, and accumulate in the water oil when cohobating the distillation waters. For this reason the water oil usually possesses a rather disagreeable odor and should not be mixed with the main oil without previous careful purification.

Occasionally the main oil, too, contains as normal constituents substances of somewhat objectionable odor e.g., certain aldehydes or sulfur compounds. In order to improve the odor of such oils, they must be freed from these undesirable compounds by redistillation. This applies also to crude oils possessing too dark a color, which is often due to the presence of metals, or to fine plant dust carried over by the steam, especially when the live steam enters the still too forcefully or too rapidly. When the steam is injected more slowly, the plant charge becomes somewhat wet by steam condensation, and the dust particles are retained by the plant material.

Redistillation of a volatile oil does not necessarily bring about an improvement in its quality ; in fact, in some cases the contrary may be true. This is particularly so with oils possessing easily saponifiable esters, such as bergamot or lavender oil. Linalyl acetate, the main constituent of these oils, is hydrolized by boiling with water, or by rectification with live steam, the freed acetic acid causing further hydrolysis.

For the redistillation of a volatile oil two general methods are employed, viz., rectification and fractionation, both of which will be described in more detail.

Rectification aims at the separation of volatile and nonvolatile compounds if a lighter colored oil is desired ; the coloring matter remains as residue in the still. This may be achieved by dry distillation in vacuo or by hydrodistillation (with live steam or by boiling with water). Hydrodistillation can also be carried out at reduced pressure.

Fractionation or fractional distillation aims at separating the volatile oil into various fractions, according to their boiling points and odor. In most cases this is achieved by dry distillation in vacuo. A volatile oil should never be fractionated at atmospheric pressure, because the high temperatures involved cause decomposition and resinification, the distillate then possessing an odor and physicochemical properties quite different from those of the original oil. The boiling temperature can be considerably lowered by distilling the volatile oil at greatly reduced pressure, a process also referred to as dry distillation in vacuo. Decomposition of the oil is thus reduced to a minimum.

Rectification of Essential Oils. 

Rectification with water vapors (steam) is the older of the two methods. Retorts employed for this purpose are usually spherical, made of copper, heavily lined with tin, and heated with a steam jacket. To prevent coloring of the oil by contact with metal, the gooseneck and condenser should be made of pure tin or of heavily tinned copper. Condenser and oil separator should be installed at such a height that, if it seems desirable, the distillation water can return automatically into the retort during distillation. Water is poured into the retort to a level of about 4 or 5 in. above the steam jacket and the oil added. Some oils peppermint and caraway seed oil, etc. easily assume a disagreeable by-odor when coining in contact with the hot still walls. This by-odor, known as "still odor," may be partly avoided by covering the steam jacket or the steam coils with sufficient water before starting the operation. The water level must be retained throughout the distillation. Flat-bottomed steam jackets are, therefore, preferable for the rectification of volatile oils. A steam coil, provided with many small holes and inserted close to the bottom of the retort, serves for direct heating with live steam (if this modification is preferred) and also for steaming out (cleaning) the still after completion of the operation. Steaming out is usually preceded by a washing with hot water, soap or alkali solution or with volatile solvents.

The speed of rectification is influenced by several factors. If the distillation waters should return automatically into the retort, the speed might be limited by excess pressure developing within the retort ; in fact, this might altogether prevent the distillation water from returning automatically into the retort. If the distillate should be absolutely colorless, rectification must be carried out very slowly; otherwise very fine droplets, often invisible in the vapors, are carried into the condenser and oil separator, and color the distillate.

As has been said, some crude volatile oils contain compounds of objectionable odor, which are often more soluble in water than the main constituents. This fact can be taken advantage of by rectifying the volatile oils through hydrodistillation : the distillation water containing most of these objectionable compounds is not returned to the retort, but the water distilling off must be replaced by fresh water ; or, instead of heating indirectly, direct live steam may be injected into the oil charge. In the latter case only sufficient water to cover the steam coil need be charged into the retort. However, the danger of oil droplets being carried over mechanically becomes somewhat greater as the live steam entering the retort has a tendency to whirl the oil upward. A short rectification column may be of service in this respect. When rectifying a volatile oil with direct live steam at atmospheric pressure (in other words, with low-pressure steam), some steam will be condensed to water continuously within the retort. The distillation water, in this case, cannot be returned into the retort, but must be cohobated in another apparatus or extracted with volatile solvents. Actually, rectification of a volatile oil with direct steam of low pressure has all the characteristics of a water distillation, because steam continuously separates water as condensate within the retort. If, however, high-pressure live steam (10 atmospheres for instance) is injected into a well-insulated still, condensation of water may be prevented, provided the steam has been carefully dried prior to its entering the still. The distillation then becomes a superheated steam process, because saturated, high-pressure steam, on expansion, changes into superheated steam. In other words, distillation of a volatile oil purely by live steam is not practicable. It turns either into a distillation with superheated steam or into a water distillation, the latter with the modification that there will be only a little water within tho rotort.

The quantity of oil to be charged into a rectifying still depends upon the final purpose of the rectification. If the oil is only to be decolorized, very little oil need be let into the retort, the vaporizing oil being replaced continuously as new oil is pumped in. This method offers the advantage that the contact of oil and steam is shortened to a minimum, only a small quantity of oil being in the retort at one time. A prolonged contact of volatile oil with boiling water or steam at atmospheric pressure is likely to cause considerable decomposition, resinification, and chemical action, such as hydrolysis of esters, etc.

As has been explained, rectification aims also at freeing the oil from disagreeable by-odors. If these impurities possess a low boiling point in other words, if they boil below the main portion of the oil they can be removed in the foreruns of the distillate. Foreruns are then separated so long as they exhibit the objectionable odor. Since a forerun usually amounts to only a small percentage of the total oil, it should be distilled off very slowly. The total amount of oil charged in the still, however, must always be so measured that it can be processed within one day. In order to utilize the capacity of a small still to the fullest, rectification is best carried out with direct live steam ; otherwise a part of the retort must be occupied by the water necessary for distillation. After the forerun has distilled over, the speed or rate of distillation may be increased to whatever degree condenser capacity and purity of the distillate will permit.

If the volatile oil to be rectified contains impurities boiling higher than the main part of the oil, the main run should be distilled off slowly, as this will permit better separation and a diminution of the last runs. The speed of distillation may be increased when the last runs containing the impurities start to distill over. To achieve a more complete separation of the foreruns and last runs, a fractionation column may be used and, if necessary, a dephlegmator above the column. Such a dephlegmator causes partial condensation, which affects the higher boiling constituents more than the lower. It thus becomes possible to reduce the quantity of the forerun and to increase the quantity of the main run. As was explained under "Theories of Distillation” rectification columns are equipped with perforated trays, often with Raschig rings or porcelain balls. Columns filled with rings or balls have a practical advantage over columns equipped with bell or sieve plates, in that the former retain less condensed liquid and, therefore, exert less pressure upon the vapors in the still.

The composition of the condensate, i.e., the average oil content of the steam and vapor mixture, depends primarily upon the boiling point or the vapor pressure of the oil constituents. The lower the normal boiling point in other words, the higher the vapor pressure of the oil constituents at the prevailing temperature of distillation the greater will be the ratio of oil in the condensate. The average oil content of the steam and vapor mixture in the distillation of oil is much larger than it is in the distillation of plant material. (See section on "Steam Consumption in Plant Distillation/')

Fractionation of Essential Oils. 

We shall now proceed to a description of the fractionation, which is carried out at reduced pressure (partial vacuum) and usually by distilling the oil alone, without leading water into the retort or injecting live steam into the oil. This process of dry distillation in vacuo is widely applied in the essential oil industry today. By its means pressure can be so far lowered that temperature has no longer any marked influence upon quality. The pressure should not be higher than 5 to 10 mm. Hg as measured in the still above the boiling liquid. How far the temperature of some oil constituents can be reduced is shown by this example: linalool, the main constituent of linaloe oil, boils at a temperature of 198^o at atmospheric pressure (760 mm.), and at:

105.4^o at 30 mm. pressure

97.2^o at 20 mm. pressure

84.4^o at 10 mm. pressure

72.8 ^o at 5 mm. pressure.

In practice, any further lowering of the pressure requires that distillation be carried out very slowly; it also necessitates an efficient vacuum still, absolutely airtight joints, and an effective condenser, so that the low boiling constituents of the volatile oil may be recovered, and not lost in the vacuum pump. In the case of almost every vacuum distillation, small quantities of vapors escape into the pumps, especially if the vacuum still is not absolutely tight. The air leaking into the still has a tendency to carry along some volatile oil vapors that may not always be condensed in the condensers. It is advisable, therefore, to insert an absorption vessel, filled with neutral substance which absorbs the vapors, between the oil receiver and the vacuum pump.

In order to distill over the highest boiling oil constituents in vacuo, temperatures of 150^o to 200^o are often necessary. Such temperatures can be obtained by the use of an oil bath which surrounds the lower part of the retort; in a corresponding steam jacket very high pressure or superheated steam would be required. The oil bath offers the advantage that the heat transmission between the two liquids is more gradual than that between superheated steam and volatile oil. Under these circumstances hydrocarbons possessing boiling points up to 300^o (at atmospheric pressure) can be distilled off, provided any condensation of oil vapors along the upper walls of the retort is prevented by good insulation.

Between the pressure in the receiver and that within the retort there exists always a differential of a few millimeters. If the pressure in the closed receiver is 1 to 2 mm., the pressure in the vacuum still itself (retort) will be about 5 mm., provided the vapor development remains moderate. The faster the distillation, the lower will be the performance of the vacuum pump; the narrower the condenser tubes, the greater will be this pressure differential.

The stills serving for vacuum distillation of volatile oils are spherical, sufficiently strong to withstand at least atmospheric pressure, made of copper and heavily tinned on the inside, with gooseneck, condenser tubes and oil receivers also tinned. A small and strong glass window permits watching of the boiling liquid within the retort. All joints must be absolutely airtight. A jacket around the lower half of the retort forms an oil bath or a steam bath for heating with high-pressure steam (for at least 75 Ib. jacket working pressure). A column directly above the still, equipped with plates, or filled with Raschig rings, or with other packing materials, provides for better fractionation of the boiling liquid (see "Theories of Distillation"). The oil receiver consists of two closed, strong vessels with vertical glass tubes, through which the level of the liquid within each receiver can be gaged. 

 PLATE C. Vacuum stills at Fritzschc Brothers, Inc., Clifton Factory, Clifton, N. J.

These receivers arc tightly connected with the condenser outlet through a three-way stopcock, which permits one receiver to remain under vacua and to collect the fraction distilling over at a given temperature, while the other receiver may be opened to draw off the previous fraction. Pressure manometers on the retort and on the oil receivers indicate the pressure within the retort and within the oil receivers. One thermometer held by a nipple reaches within the retort and ends above the boiling liquid, whereas another thermometer registers the temperature on top of and inside of the fractionation column. An airtight suction line connects the receivers with the vacuum pump which should be of high efficiency.

The still shown in Fig. 3.22 serves for the dry (without direct steam) vacuum distillation (fractionation or rectification) of essential oils. Heating is achieved by a steam jacket (or oil bath if so desired). The rectification column can be by-passed. Provision is also made for the rectification or fractionation of essential oils by the use of direct steam at atmospheric pressure.

 FIG. 3.22. Dual-purpose essential oil still.

In this case, the distillation waters may be automatically returned to the still (cohobated). The same still may be used for the preparation of terpeneless oils.

Inadequacies of Hydrodistillation. 

A comparative summary of the advantages ind disadvantages associated with hydrodistillation of volatile oils and with dry listillation in vacuo would reveal, according to von Rechenberg, an almost general superiority of hydrodistillation over the latter method. Depending upon the nature of the compound to be vaporized, it is possible to adjust the temperature of hydrodistillation to any desired level. The use of dry vacuum distillation remains limited, because high boiling compounds decompose below their boiling points, even in vacuo. Vacuum distillation with superheated steam is more advantageous in this respect. On the other hand, the use of hydrodistillation is restricted for several reasons:

1. As in the case of dry vacuum distillation, the compound to be vaporized should be distilled in liquid form, or should at least melt below the temperature of distillation. However, solid compounds, and even those with very high boiling points, can be vaporized by steam, provided they are reduced to a moderately small size. Comminuted particles should be properly packed on perforated grids within the retort, so that the rising steam penetrates the mass uniformly, just as with plant material.

2. Hydrodistillation cannot be applied to substances which, even at low temperatures, react with water, or are hydrolyzed by water (esters, etc.).

3. Solubility in water, as well as decomposition by water, may, under certain circumstances, present an insurmountable obstacle to the use of hydrodistillation. This is particularly so if the compound to be distilled is high boiling (aside from being water soluble) or, in the case of plant distillation, if the plant material contains only very small quantities of the water-soluble constituent. Solubility in water lowers the vapor pressure of the compound and reduces its capability for vaporization; in other words, relatively much more steam will be required to vaporize the same quantity of oil. Since this lowering of the vapor pressure depends upon the quantity of water present, water soluble and high boiling compounds or corresponding plant matter should be distilled with steam, and not with boiling water. For instance, if it were practically possible to distill rose floweis with steam, the phenyl ethyl alcohol would probably not be retained by the flowers or by the residual still waters.

Solubility in water not only reduces the rate of evaporation, it also impedes the separation of the oil from the distillate. For this very reason the aroma of many flowers cannot be isolated by distillation. Any odoriferous compound is also volatile ; any compound which, of itself, dissipates vapors into the air should yield the same, if not a larger quantity to steam, and particularly at a temperature of distillation higher than that of the air. The difficulty is only that the small quantity of volatile substances cannot be isolated from the large volume of distillation waters.