Hydrodistillation of Plant Material at High and at Reduced Pressure, and with Superheated Steam

(e) Hydrodistillation of Plant Material at High and at Reduced Pressure, and with Superheated Steam.

Steam Distillation of Plant Material at High Pressure. Certain plant materials orris root, sandalwood, cloves, caraway seed, pine needles, for example are occasionally distilled with steam of a pressure higher than atmospheric, in order to obtain a more favorable ratio of oil to water in the distillate, i.e., to shorten the length of distillation and to increase the total yield of oil. Purely physical considerations, a discussion of which would lead too far, show that a substantial gai can be achieved only with a pressure of several atmospheres within the retort.
This, however, usually causes such profound decomposition of the plant material and of the volatile oil that the method cannot generally be applied in practice. The actual pressure within the retort, when using high pressure steam of 4 atmospheres as measured in the steam boiler, is certainly less than one atmosphere above normal atmospheric pressure. If, notwithstanding, such modest excess pressure leads to favorable results primarily to a shortening of the distillation process the explanation must be sought in other, perhaps purely mechanical factors. If the steam were throttled by a valve in the gooseneck and the pressure thus increased, a manometer would indicate continuous pressure fluctuations within the still. These fluctuations prevent the steam from stagnating in the too densely packed portions of the plant charge, and seem to loosen all parts of the charge. This is particularly true of direct steam distillation and, to a certain extent, also of water and steam distillation, if the plant material has been packed high, and not sufficiently uniformly or tightly. Water distillation, on the other hand, does not seem to be affected by excess pressure. The effect of high pressure appears to be more pronounced when the plant material has been charged improperly into the still, and when a less efficient distillation, at atmospheric pressure, has been carried out previously.
The use of high-pressure steam for the rectification of volatile oils per se is not advisable, nor is it necessary, because for this purpose superheated steam gives better results. Xor should it be made a general practice to distill plant material with high-pressure steam, as this will increase the quantity of decomposition products in the plant material and in the oil, the degree of decomposition being influenced by the height of the pressure applied, the resulting rise of the temperature, and the length of distillation. Ordinary steam distillation, even at atmospheric pressure, affects some of the constituents of the essential oil and of the plant material itself (the latter being even more sensitive to high pressure steam than the oils). The nonvolatile plant matter may thus undergo more or less profound decomposition, with accompanying formation of undesirable volatile substances, which may considerably impair the color and odor of the oil. The distillate may become so much contaminated with foreign matter that even rectification no longer yields a normal oil. (von Rechenberg).
For all these reasons distillation with high pressure steam is not recommended, if the operation aims at obtaining a volatile oil containing delicate constituents. It may, however, be advantageous in some cases, where the distillate is to be further processed as with oil of camphor and steam distilled pine (stump) oil.

Water Distillation of Plant Material at High Pressure.

It is not advisable to employ this method, because the resulting higher temperature gives rise to decomposition products which impart a disagreeable "burnt" odor to the oil. Neither is there any appreciable gain in the ratio of oil to water in the distillate, except perhaps in cases where previous distillation under atmospheric pressure has been carried out inefficiently.

Steam Distillation of Plant Material at Reduced Pressure.

This method may be subdivided into two types, viz., (a) steam distillation at slightly reduced pressure, and (b) vacuum steam distillation at such a low pressure that the temperature remains just enough above that of the cooling water to permit sufficient condensation of the steam oil vapors.
(a) It is a known fact that a pressure reduction within the still often shortens the length of distillation. Even a slight reduction may shorten the duration to only one-half the time required for steam distilling at atmospheric pressure. Von Rechenberg24 demonstrated that this effect is caused by fluctuations in the steam/oil vapor pressure which, as in the case of distillation at high pressure, exert a continuous loosening effect upon the plant charge.
(b) The principal advantage of steam distillation of plant material in vacuo consists in the pure odor of the volatile oil thereby obtained. It will be free from any off-odor caused by decomposition, which accompanies most oils distilled above 70.
If the hydrodistillation in vacuo is not carried out with steam generated by boiling the water within the still (water distillation, or water and steam distillation) but by steam generated in a separate steam boiler, a distillation with superheated steam at reduced pressure will result. Even high boiling constituents of the volatile oil will then readily distill over; a previously air-dried plant charge, under these circumstances, may, however, gradually dry out until the volatile oil enclosed within the oil glands can no longer be vaporized, because the forces of hydrodiffusion no longer play their important role. It will then become necessary to apply saturated steam at atmospheric pressure, so that steam condensation within the plant charge again forms (liquid) water, which will permit the forces of hydrodiffusion to act anew.
When hydrodistilling at reduced pressure it is preferable to employ spiral condensers rather than tubular ones, because the former can be tightened better. The surface of condensation should be about five times larger than when distilling at atmospheric pressure. This increase is necessary for several reasons: (1) The differential in the temperature of steam and cooling water is much smaller at reduced pressure. The rate of condensation, therefore, decreases. (2) The volume of a given quantity (weight) of steam is much larger at reduced, than at atmospheric, pressure. For instance the volume of 1 kg. of steam at the following
Millimeter Pressure                Cubic Kfefers
      760                            1.650
      380 .                                                                3.150
      150 7                             650
      76 14                             530

The velocity at which the steam enters the condenser will affect the transfer of heat to the cooling surface. Therefore, depending on other variables, an appropriately designed condenser (as to type, length, etc.) will have to be employed. Too long a condenser being impractical, several spiral condensers connected with a T tube may be installed side by side. Since an efficient vacuum pump creates a higher vacuum than is actually required for the distillation of plant material, the pressure within the still should be regulated by a valve permitting enough air to enter the still to sustain the desired pressure. The pressure should be measured by two manometers, one reaching into the receiver and one directly into the retort.
Steam distillation of plant material in vacuo is limited in application by the fact that cooling and condensation of the vapors become increasingly difficult as the pressure and temperature of distillation are lowered. The general application of hydrodistillation in vacuo to plant material is restricted by another factor. With
24 "Theorie der Gewinnung und Trennung der atherischen Ole," Leipzig (1010), 392,
lowered pressure in the still, the partial pressure of the oil vapors decreases relatively more than that of the water vapors (steam) ; hence, the ratio of the volatile oil in the distillate is smaller than when distilling at atmospheric pressure. In other words, more steam will be consumed when hydrodistilling a certain quantity of oil in vacuo than at atmospheric pressure. This lower rate of vaporization of the volatile oil is particularly pronounced in the case of water distillation of plant material containing high boiling and partly water soluble constituents. (See below.) In this case, a multiple volume of steam (as compared with distillation at atmospheric pressure) is often required to attain the same yield of oil. Any increased steam consumption also results in higher working cost, since much more distillation water must be redistilled or extracted.
When processing of plant material by water and steam distillation is practiced at reduced pressure, pressure variations in the still may cause loosening of the plant charge, so the rising steam is better saturated with oil vapors. This factor occasionally results in a lower consumption of steam than when working at atmospheric pressure. The most suitable method of distilling plant material at reduced pressure is u ith water and steam, provided the nature of the plant material permits its application. In general, it can be said that steam distillation of aromatic plants, under reduced pressure, remains very limited in practice.

Water Distillation of Plant Material at Reduced Pressure.

According to established thermodynamic principles and to the explanation given in the preceding pages, hydrodistillation at reduced pressure has the effect that, with equal quantities of conderisato, the steam volume in the distilling space, and therefore the steam velocity, will increase enormously as the pressure in the still is reduced. For example, a given quantity (weight) of totally saturated steam and benzaldehyde vapor fills a certain volume at atmospheric pressure (760 mm.); at 76 mm. pressure the volume will be approximately ten times larger, at 31 mm. approximately twenty-four times larger than that occupied under atmospheric pressure. The velocity under which the vapor mixture rushes through the condenser increases in the same ratio. Hence water distillation of plants at reduced pressure is connected with certain inconveniences with which the operator should be familiar.
Any increase in the speed of distillation affects the purity of the distillate because minute plant particles are carried over mechanically. As a precaution against this, speed must be moderated as much as possible; flat, wide, rather than tall, stills should be selected for this purpose. It should also be borne in mind that the steam is to some extent throttled in the gooseneck (the narrowest part of the still). This may result in a slight back pressure within the retort, relative to the pressure in the receiver, which differential might easily amount to 10 mm.
It is, therefore, advisable to adjust the speed of distillation to the temperature prevailing within the retort. This will prevent a rise in the distillation temperature above a desired point.
The great advantage of water distillation of plant material at reduced pressure lies in the fact that it can be carried out at relatively low temperatures e.g., at 50 which reduces decomposition of the essential oil. It is not advisable to operate at lower temperatures, because the oil vapors can then no longer be sufficiently condensed, and considerable losses of oil might occur. Furthermore, higher boiling, slightly water-soluble compounds are retained partially in the plant material and in the water, and the oil will be deficient in these constituents. This phenomenon, already discussed under water distillation of plants at atmospheric pressure, is even more pronounced in its effects when reduced pressure is employed. This very factor limits the application of water distillation in vacua to only a few plant materials.
Temperatures of only 30 to 50, and the presence of water offer favorable conditions for fermentation of the plant material, for which reason distillation of this type should not last longer than a few hours. The oils obtained by this method will never possess a "still" or "burnt" odor, but rather a slight "fermented" one.

Superheated Vapors.

As was pointed out in the theoretical part of this chapter, a vapor is saturated so long as it remains in contact with the liquid from which it originates. Saturated vapors possess characteristic properties by which they differ sharply from vapors separated from their liquid sources. The slightest cooling of a saturated vapor causes partial condensation, the slightest heating results in increased vaporization. So long as it remains in contact with its liquid, a saturated vapor is seldom absolutely dry ; usually it contains admixed particles of the liquid in the form of spray. Moderate vaporizing, even evaporating of the liquid phase, carries microscopically small droplets upward into the vapor space. Vigorous boiling ejects larger quantities from the turbulent liquid; these are kept suspended by the flow of the vapors, or they drop back into the boiling liquid, to be replaced by new ones. Very wet vapors are more or less hazy. The transparency of a vapor, however, merely proves that it does not contain larger quantities of the liquid phase; it does not prove that the vapor is absolutely free of liquid, since minute droplets floating in the clear vapors are invisible to the eye. Their actual presence in the vapors is proved by the fact that the condensate of plant materials or of volatile oils is usually contaminated with dust or with nonvolatile colored substances.
Let us assume that we continue to heat and vaporize a liquid at constant external pressure to the point where the last molecule of the liquid phase is transformed into vapor. At this very moment the vapors are still saturated, dry saturated. Further heating no longer induces the formation of vapors, it only increases the temperature of the formed vapors, with a resulting expansion of their volume. The vapors then become superheated. Thus, superheated vapors possess a higher temperature, a larger volume and a lower density than saturated vapors at the same pressure. Superheating of a vapor may also be interpreted as a heating beyond the point of saturation. Saturated vapors, as compared with superheated vapors at the same pressure, therefore, contain a maximum of mass, as well as the highest specific gravity and the lowest specific volume (the specific volume being the reciprocal value of the specific gravity). When comparing the two types of vapors at the same temperature, superheated vapors possess a lower pressure than saturated vapors. A saturated vapor exerts the maximum pressure at the given temperature. Cooling merely lowers the temperature of superheated vapors, without causing condensation (as would be the case with saturated vapors). Only by further cooling, to and below the point of saturation, will a portion of the vapor be condensed. The moment a superheated vapor is brought into contact with the liquid phase from which it originated, vaporizing will take place, until the saturation point is reached once more. The superheated vapor thus passes into a saturated vapor, (von Rechenberg).

Distillation of Plant Material with Superheated Steam.

Relative to its weight, superheated steam can vaporize and entrain more volatile substances than saturated steam. In practice, steam may be superheated by passing it through fire tubes in a boiler in other words, through a superheater. This superheated steam, mixed with high-pressure and saturated steam, is then injected into the still beneath the grid which supports the plant material. The mixing of superheated steam with saturated steam serves as a precaution against "burning" and decomposition of the plant material. Thus dry, slightly superheated steam is obtained. However, there remains the danger of decomposition, at least to a certain degree. The distillation will be shortened, but the oil yield may suffer, because the plant charge easily dries out as the forces of hydrodiffusion no longer play their important role.
Although rectification of certain essential oils with superheated steam at atmospheric and especially at reduced pressure has been found valuable, its use in the distillation of plant material is limited, and often connected with more disadvantages than advantages.

Advantages and Disadvantages of High-Pressure and Superheated Steam in Plant Distillation.

When high-pressure or superheated steam is employed in distillation with direct, live steam (but not in water distillation, or water and steam distillation), the condensation of water vapors in the plant charge may be greatly reduced, if not prevented altogether, except in the part of the charge along the walls of the retort, which usually becomes moist despite good insulation. This feature permits a more complete exhaustion of the plant charge. Furthermore, the use of high-pressure steam with its elevated temperature increases the partial pressure of the volatile oil, and the ratio of oil to water in the condensate becomes more favorable. In other words, distillation will be shortened. To exploit this effect of high-pressure steam, any condensed water accumulating beneath the steam coil in the retort must be prevented from rising above the coil, since highpressure or superheated steam would then be transformed into low-pressure, saturated steam, of 100 direct steam distillation thus being transformed into a water and steam distillation. Therefore, any condensed water must be drawn off from time to time. Such condensed water always contains extractive matter dissolved and dripping down from the plant charge, and this matter has a tendency to undergo decomposition. Some of the resultant products are volatile and of disagreeable odor, and when carried over into the receiver will contaminate the volatile oil. As mentioned previously, not only the volatile oils themselves, but also the plant materials, are very sensitive to the influence of heat and easily decompose. This takes place even at a temperature of 100, but the effect is much more pronounced at a higher distillation temperature. High-pressure steam or superheated steam gives rise also to resinification and to the formation of insoluble compounds, parts of which vaporize and distill over. Such oils are less soluble in dilute alcohol or, when soluble, cause turbidity on further addition of dilute alcohol. Hence, the use of high-temperature steam in the distillation of aromatic plants cannot be recommended generally.
As was explained in our discussion of hydrodiffusion, the volatile oil enclosed in the cell membranes of aromatic plants must first be dissolved by hot water; and then, by forces of diffusion, be brought to the surface of the plants or plant particles, where the volatile oil may be vaporized and entrained by the passing steam. The exuded water must be replaced, so that the process of hydrodiffusion is not interrupted. The water necessary for this purpose comes partly from the moisture contained originally in the plant material itself, partly from steam condensation (which takes place particularly at the beginning of distillation). When highpressure, or dry, superheated steam is used, only that part of the volatile oil is vaporized which has been freed by comminution ; at the same time the moisture present in the plant material vaporizes, and the plant charge dries out. Then any oil remaining within the plant tissue can no longer reach the outside by hydrodiffusion, as there is no longer any water present or available; distillation will therefore be incomplete, and the yield of oil subnormal, unless saturated steam of low pressure is injected after the application of high-pressure or superheated steam.
There are a few cases in which distillation with superheated steam becomes advantageous e.g., with plants that contain much moisture (60 to 80 per cent) and are difficult to dry. If such material is distilled with low-pressure saturated steam, the high moisture content of the charge will cause much steam condensation: the plant charge lumps and is difficult to exhaust. This can be prevented by applying superheated steam, a smaller or larger portion of the water within the plant charge then vaporizing while hydrodiffusion still functions.
In general, it can be said that plant material containing low boiling essential oils is preferably distilled with low-pressure steam, whereas high-temperature steam recommends itself for the distillation of high boiling oils.

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