Equipment for Distillation of Aromatic Plants

(c) Equipment for Distillation of Aromatic Plants.

The equipment required for carrying on distillation of plant materials depends upon the size of the operation and the type of distillation to be used. There are, however, three main parts which, in varying size, form the base for all three types of hydrodistillation. A fourth part is necessary for any method of heating the still other than by direct fire. The three universally employed parts are :

1. The retort, or still proper;

2. The condenser;

3. The receiver for the condensate.

The fourth part consists of a boiler for generating steam. The latter is necessary for the process which, in the preceding discussion, we have called steam distillation, since direct live steam, often slightly superheated, is required, and this can be produced only in a separate steam boiler. In the case of water distillation, or water ami steam distillation, the still may be heated by direct fire but even here heating is frequently, and indeed preferably, accomplished by steam jacketing the retort, or by means of closed (or occasionally open) steam coils. A separate steam boiler becomes indispensable, also, if any one of the latter heating methods, or a combination of them, is used. These four parts of the distillation equipment will be considered in order.

The Retort. 

The retort, or still proper, commonly also called "tank,” serves primarily as a container for the plant material, and as a vessel in which the water and/or steam contacts the plant material and vaporizes its essential oil. In its simplest form the retort may consist merely of a cylindrical container or tank, Avith a diameter equal to or slightly less than its height, and equipped with a removable cover which can be clamped upon the cylindrical section. On or near the top of the cylindrical section a pipe (gooseneck) is attached for leading the vapors to the condenser. For water 

its proper functioning is assured at all times. Steam traps are apt to lose their efficiency after a certain time and permit unutilized steam to escape; or they do not separate the condensed water effectively and more and more water accumulates within the still. This means an ever-increasing wetting of the plant charge in the still. Such a condition is recognized by a crackling noise and by trembling of the still. If there is no steam trap, a funnel should be attached beneath the outlet at the still bottom, and the water thus conducted away. Here, too, the water faucet is regulated in such a way that no unused steam escapes, and at the same time, no condensed water accumulates within the still.

This arrangement completes the still proper. Needless to say, all joints must be soldered steam tight, as any steam leak represents loss of essential oil and fuel.

Brief comment should be made about the top of the still and the gooseneck, i.e., the tube connecting the retort with the condenser. The oldfashioned convex or crane-like still heads are becoming obsolete and rare.

The top of a modern retort is simply pierced, and a pipe inserted to serve as a gooseneck. The perfect still head is short and well insulated ; if convex, it curves gradually and tapers, so that it fits into the gooseneck. Any fancy designs, sudden turns, bends, or too narrow tubing must be avoided, as these would result in a throttling effect and in back pressure within the still.

The gooseneck also is only slightly curved, and, gradually descending, leads from the retort directly into the condenser. It should not ascend, as this would give rise to considerable vapor condensation, the resultant liquid refluxing into the retort. A semicircular gooseneck, such as is sometimes found on old stills, has a purpose only if high boiling and resinous constituents of an essential oil can, by its means, be condensed and returned into the retort. A gooseneck of this type, therefore, may be useful in the rectification of essential oils, but not in the distillation of plant material. In fact, these two operations must never be confused. Ascending and high goosenecks are excusable only if the distillation waters are purposely made to flow automatically back into the retort from the higher placed Florentine flask ; but in such a case the gooseneck must be well insulated. It usually is preferable to return the distillation waters into the retort by an injector, which measure makes high goosenecks superfluous. Furthermore, a high gooseneck produces a slight back pressure within the retort ; it must, therefore, be amply wide.

The retort cover shown in Figs. 3.6 and 3.7 may be made of sheet metal similar to that used on the retort. It should be strengthened and ringed with strap metal to coincide with the horizontal face of the top angle iron supporting ring on the retort. 

wires attached to three or four equally spaced points around the circumference of the grid may serve as handles so that the plant charge can be easily removed after distillation simply by lifting the grid. If charges in excess of 200 or 300 Ib. are to be distilled, it will be convenient to use more than one such section, placing a new one on top of the first layer and continuing the charge above this section. This arrangement prevents excessive packing, assures better steam distribution, and facilitates discharging the spent material, inasmuch as only a fraction of the total charge need be removed at one time. Coarser and specifically lighter material can be packed higher, whereas finer and heavier material should not exceed a certain height. 

 FIG. 3.9. Sketches of two types of multi-tray retorts.

Retorts serving for water distillation should be wider than they are high, so that the plant charge can be kept shallow, avoiding the pressure caused by the weight of a high charge. This will permit the comminuted plant particles to move freely in the boiling water, and assure quicker distillation and a better yield of oil. Retorts serving for water and steam distillation may be of approximately equal height and diameter. Retorts for direct steam distillation should be somewhat higher than they are wide so that the rising steam passes as much plant material as possible. As a rule, the diameter should be 6 to 8 ft. at the most; if larger-scale operation requires a larger still capacity it is preferable to increase the height rather than the diameter of the retort. In this case it will be necessary to guard against excessive packing of the charge, which would cause uneven distribution of steam and excessive pressures near the bottom. When calculating the dimensions of a still one should keep in mind not only that some plant materials are very voluminous but also that during distillation the mass often swells and expands by one-third of its original volume. The height of the retort in relation to its width depends upon the porosity of the plant material.

A greater height is chosen for voluminous material, and shorter stills are preferred for more compact material. Excessive pressure can be avoided by a construction similar to that shown in Fig. 3.9.

The screen or grid trays may be permanently installed at intervals of 2 to 3, or 3 to 4 ft., according to the size of the retort, and each tray must then be filled or emptied individually through the 2-ft. or 3-ft. manholes. By supporting each section of the charge separately, excessive pressures ir any one section are avoided and packing is kept at a minimum. Care must be exercised to fill each tray with only a. relatively shallow layer, to insure a uniform distribution of material and, therefore, of the steam. This is particularly true of seed distillation, which requires much more experience and attention than distillation of herbs or leaves.

 FIG. 3.10. Use of baskets (perforated on bottom) for holding still charge.

As pointed out above, the trays may also be movable, so that they can be lifted from the retort with chains or strong wire. For best results, the trays should not lie directly on top of the charge of the next lower tray but be separated by a space of 2 ft. or more, depending upon the size of the retort. This may be effected in several ways e.g., by supporting legs, or attaching all of the trays to a central vertical shaft on which the trays may be hoisted from the retort after completion of the operation. The principal precaution is to be sure that the steam actually penetrates the plant charge and does not find an easy passage along the side of the still wall. This may be prevented by coiling ropes around the outer edges of the various trays where they touch the wall of the retort. For the same reason, baskets are not generally to be recommended, particularly those with perforated sides, 

but should be used always for distillation of only one type of plant material. Wood has a tendency to absorb a little essential oil, which cannot be removed even by the most thorough washing and boiling with lye. Hence, a certain odor always adheres to wooden retorts which might easily spoil the odor of another type of oil, if the latter were distilled in the same wooden retort.

Insulation of the Retort. 

In all cases the retort, including the top, should be well insulated to conserve heat. This holds true particularly of stills exposed to cold air, wind and draft. If insulation is neglected, excessive condensation of steam within the retort will occu as n, result of heat losses from its surface. This causes undue wetting of the charge, lumping and agglutinating of the plant particles, excessive steam consumption, prolonged distillation, and, usually, an inferior yield of oil. For small portable units, considerable insulation can be afforded by surrounding the retort with a jacket made of wooden planks and held in place by wire. The interspace may be filled with powdered cork or sawdust. Much better insulators are asbestos and magnesia. Either of these can be applied directly to the retort in the form of a very thick paste in water, which dries to a hard adherent layer. Three to six inches of this material will suffice for most economic operation.

A high grade of insulation of this sort appears particularly important in large installations, where much steam is required. There, all heated sections and steam lines should be well insulated to prevent escape of heat, which represents an unnecessary expense. Probably the most effective insulation material is asbestos, which, in the form of bricks or pipe covering, can be suitably fastened to the still and pipes, or, in the form of powder, can be made into a thick paste with water. This paste may be applied with a trowel to the parts to be insulated. A paste made from ground kieselguhr, water and animal hair, if available, also serves as insulation. In any case, such an insulating layer should be about 2 in. thick. Von Rcchenberg¹⁴ suggested the following method of insulating stills and steam pipes :

"Fifty liters of calcined kieselguhr, ten liters of gritty ground cork waste, and three handfuls of clear pulled pigs* or calves' hair are thoroughly mixed. A thin, hot, stirred soup of rye, wheat, or corn flour is added, to make a viscous, stiff mash. Stones of brick size and strength are then formed and dried on the steam boiler or elsewhere. These bricks serve to cover the stills and steam armatures after they have been covered with a

viscous flour soup. If necessary, the bricks are held in place by iron straps. The whole cover is smoothed, and the joints and grooves are filled with a mash of calcined kieselguhr. Finally, cheap, thin fabric is pasted on top and painted over twice with oil paint."

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14 "Theorie der Gewinnung und Trennung der atherischen Ole," Leipzig (1910), 599.

Very hot steam pipes are more advantageously covered with asbestos fiber. The joints of the steam armatures, which are best made with flanges, should not be insulated.

Charging of the Still. 

The problems of charging a retort with plant material, and of discharging' it, are more important than is usually realized, and should be attacked by considering the labor involved. Any labor saving device might mean considerable economy in the final calculation. As a rule the plant material should be transported (trucked, hauled, etc.) as near as possible to the still. If the material has to be comminuted, the machines should be located near-by, if possible on a floor or platform above the stills, so that the comminuted material falls or slides by gravity into the retort. The old-fashioned way of charging and discharging with pitchforks and shovels is costly, and, although the initial cost is high, a conveyor belt, or a small crane, will soon pay for itself and in general speed up the operation.

The Condenser. 

We shall now proceed to a description of the condenser, the second major part of the distillation equipment. Here again the size and design are variable, and several typical cases will be considered. The condenser serves to convert all of the steam and the accompanying oil vapors into liquid. This requires the removal of an amount of heat equivalent to the heat of vaporization of the vapors plus steam, and a small additional amount of heat to cool the condensed material (condensate) to a convenient temperature below its boiling point. The rate at which heat will be removed from the vapors is expressed by

q = UA∆t

              in which : q = heat removed per unit time ;

U = a constant depending on operating conditions ;

A = the area available for removal of heat ;

∆t = the temperature difference between the hot vapors and the cooling medium.

The scope of this work does not permit a full discussion of all factors that affect the value of U. Several of them will be considered in the discussion of condenser operation. Probably the most important ones are the rate of flow of the cooling medium (cold water) past the heating surface, the rate of flow of the vapors, and the material of which the condenser is constructed. The value of U increases as these factors increase, and this fact should always be borne in mind when constructing a condenser. The area available can be made as large or as small as desired, but it is evident from the above relation that the total capacity of a condenser, and therefore of a still, will be directly determined by the area used. The temperature difference can be controlled by the temperature of the cooling medium (hereinafter eferred to as water, since water is by far the most commonly used cooling medium) because the temperature of the vapors is fixed within rather narrow limits by the distillation itself. Fig. 3.12 shows the simplest type of condenser, now seldom used, and described here chiefly for its historic interest. 

 FIG. 3.12. Sketch of an old-fashioned zigzag condenser.

Water is fed to the overhead reservoir from which it flows to a distributor trough which consists simply of a shallow pan with a perforated bottom. This permits the water to trickle over the entire length of the condenser tubes. The water may be caught in an intermediate catch pan, as shown, and a second distributor installed to insure efficient condensation. It will be noted that the condenser tubes are all sloped downward slightly, to insure proper drainage of the condensed oil and steam. Also the size of the condenser tubes becomes smaller as the cold end is approached. In order to avoid excessive back pressures being built up in the still, it is necessary to use fairly large tubes to accommodate the vapors immediately after they leave the retort. Since the volume of the vapors, and, therefore their velocity, decreases rapidly on cooling, as a result of condensation, the size of the condenser pipes can be reduced proportionately. In Fig. 3.12, for example, the first two tubes may be 4-in. pipe, the next two 3-in. and the remainder 2-in. A 4-in. pipe coming from the still will accommodate up to 700 Ib. per hr. of condensate (about 85 gal.), in so far as the development of back pressure is concerned. The length and number of tubes to be used will be determined by the amount of vapor to be condensed. An estimate of the pipe area required can be made by using a value of 40 for the factor U in the above equation. The temperature difference will be equal to the average value of the difference between 212^oF. (100^oC.) (the temperatuer of saturated steam at ordinary pressure) and the temperature of the water in the first and second troughs. For example, if the fresh water in the top trough is 60^o F. (15.56^o C.) and the water in the first catch trough is 90^o F. (32.22^o C.), the temperature difference to be used would be the mean of (212 - 60) and (212 - 90) or 137^o F. The value of q, the amount of heat to be removed, can be calculated approximately by multiplying the number of pounds of condensate per hour by 1,000. The pipe area required will then be given in square feet. By connecting two or more such zigzag sections in parallel, the same cooling water system can be used for all of them, thus increasing their capacity, conserving height and permitting the use of shorter tubes for a given amount of condensation.

Another very simple and inexpensive type of condenser consists merely of a series of long pipes, usually 2 in. in diameter, laid horizontally in a trough through which water flows. Four 2-in. pipes will have the same vapor capacity as one 4-in pipe as given above, but will offer considerably more cooling surface. Since the value of the factor U in both of these cases is somewhat lower, the length of the pipes must be proportionately greater. Again, the pipes should have a definite slope toward the cool end, to insure adequate drainage of the condensate.

The above described methods of condensing vapors, although cheap and entirely satisfactory, lead to rather awkward and bulky construction.

The most commonly used condenser is that in which coils are inserted into a tank supplied with running cold water, which enters from below and flows against the steam and oil vapors. In order to utilize the cooling water more effectively, it is advisable to insert two adjoining coils into one condenser tank. Fig. 3.13 shows a coil condenser.

In an even more satisfactory condenser arrangement, advantage is taken of the fact that a more rapid flow of cooling water results in more efficient cooling. The condenser tubes are assembled in a single vertical bundle, the number and length depending on the amount of condensation to be accomplished, in such a way that the vapors to be condensed enter the tubes, and cooling water circulates around the tubes. Fig. 3.14 shows a typical construction.

Condensers of this type are available ready built from any equipment supply house, and should be purchased from such a specialist. The construction of a satisfactory leak-proof tubular condenser presents an exceedingly difficult problem for an unskilled workman. The factor U for such a condenser will usually be about 200 ; thus, for a given amount of condensation and a given cooling water temperature, only one-fifth of the area required in a zigzag condenser will be required. Tubular condensers should be used in a vertical position with vapors entering the top and condensate leaving the bottom. Connection with the retort must again be of adequate size to avoid excessive back pressure in the still. Tubular condensers not only are more efficient and require much less space than spiral condensers, but they also permit easier and more thorough cleaning. If possible they should be fed with soft water to prevent the formation of scale (incrustation), which reduces the exchange of heat, and necessitates frequent cleaning.

It is always better to construct the condenser a little too large rather than too small. Longer tubes or coils require less cooling water, as the contact with the vapors and with the flowing condensate lasts longer and permits the absorption of more heat, so that the temperature of the condensate at the end more closely approaches that of the inflowing cooling water. At any rate, the condenser surface must be large enough to cool the distillate sufficiently, even at a very high rate (speed) of distillation. Slow distillation has many disadvantages, such as hydrolysis of esters, wetting, agglutination and conglomeration of the plant charge, frequently with a concomitantly low yield of oil.

The cooling water in the condenser tank does not need to be cold from top to bottom ; such a condition, on the contrary, is rather a disadvantage, because too rapid and excessive cooling of the steam/vapor mixture causes the distillate to run off the condenser unevenly or jerkily. For this reason, the condenser tank should be fed with only as much cold water as is necessary to condense the vapor mixture and to cool the condensate sufficiently a factor depending also upon the type of oil produced. The maximum efficiency of a condenser is attained when the condensate has been cooled to a sufficiently low temperature by heat transfer to the cooling water, which then flows out at a temperature approaching that of the incoming vapors. This effect, however, is rarely achieved. Usually it suffices if the cooling water flows out at a temperature of 80^o C. (about 175^o F.) and if the distillate has a temperature of 25^o to 30^o C. (77^o to 86^oF.).

If the ratio between condenser surface and heating surface (in the still) is correctly maintained the condenser will permit rapid distillation. But if the condenser surface is too small and in many of the small field distilleries this is true the rate of distillation must be adjusted to the efficiency of the condenser. Distillation must then be slow, and this, as pointed out, involves many disadvantages and inadequacies. Otherwise, the vapors blow at high speed through the condenser coils or tubes, which are too short for complete condensation of the vapors or for sufficient cooling of the condensate. Considerable oil may then be lost by evaporation.

The condenser tubes or coils must be made of heavily tinned copper, of pure tin, aluminum, or stainless steel, if discoloration of the oil by iron or copper is to be prevented. Aluminum, however, cannot be used with oils containing phenols.

If distillation is to be carried out at reduced pressure, the tubes or coils must be made strong enough to support a pressure differential of one atmosphere without letting water seep from the condenser tank into the condenser. This is particularly important in the case of oil distillation (rectification, fractionation) in vacuo. Condensers serving for distillation at reduced pressures should also be sufficiently wide to permit an unhindered flow of steam and vapors, as any throttling by too small a diameter increases the pressure within the still in other words, creates back pressure. As a general principle in the construction of distilling equipment it should be kept in mind that the steam and oil vapors should flow easily and smoothly through the system, without encountering any sharp bends or curves in the tubes.

A wire screen, inserted between condenser and gooseneck, prevents plant particles lifted up by live steam from entering the condenser tubes or coils. As the wire screen may become clogged, and would then cause an explosion in the still, the retort should be provided with one or two efficient safety values.

The Oil Separator. 

The third essential part of the distillation equipment consists of the condensate receiver, decanter or oil separator. Its function is to achieve a quick and complete separation of the oil from the condensed water. Since the total volume of water condense i will always be much greater than the quantity of oil, it is necessary to remove th^ ur ator continuously. The condensate flows from the condenser into the oil sepaiator, where distillation water and volatile oil separate automatically. Many separators are constructed according to the principle of the ancient 1 lorentine flask, hence, are often called Florentine flasks. Volatile oil and water are mutually insoluble; because of the difference in their specific gravities, the two liquids form two separate layers, the usually specifically lighter oil floating on top of the water. Whenever the specific gravity of the oil is greater than 1.0, the oil sinks to the bottom of the separator. The design of the receiver should permit the removal of water whether the oil being distilled is heavier or lighter than water. 

FIG. 3.15. Florentine flasks.

Smaller Florentine flasks arc made of glass, larger separators (about 15 liters and more) of metal usually tin, tinned copper, aluminum or galvanized iron. For all-around use, heavily tinned copper vessels are most practical. Lead must not be employed, as oils containing free fatty acids would form lead salts, which might cause poisoning if the oil were used internally. Rubber tubing or rubber stoppers cannot be used because rubber, being partly soluble in essential oils, gives to them an objectionable odor. Fig. 3.15 shows two oil separators, one for oil lighter than water, and one for oil heavier than water.

Another and quite satisfactory type of receiver operates according to the following principle: A cylindrical or rectangular vessel is divided into two chambers by a partition which ends a few inches above the bottom of the vessel. The two

should run as cold as possible. Any increase in the temperature, in this case, would further decrease the already small differential between the specific gravity of the oil and that of the water, and separation of the two layers would become even more difficult, if not impossible.

This, however, is the exception. As a general rule, and in the case of most essential oils, the temperature of the condensers should be kept as low as possible in order to prevent evaporation and loss of oil.

The separated oil is finally set aside until suspended water droplets and solid or mucilaginous impurities have separated, when it is filtered clear and stored in well-filled, airtight containers in a cool, dark cellar, or in an air-conditioned room.

It should be remembered that the condensed water will always be saturated with oil. Discarding this water means a loss in yield of oil. In the case of water distillation or water and steam distillation this condensed water may be used again as the water supply for the next charge of the same type of plant material, or the distillation water may be returned into the still and redistilled (cohobated) during distillation. For this purpose the oil separator (Florentine flask) must be installed sufficiently high above the still so that the pressure of the flowing distillation water may overcome the slight pressure usually prevailing within the still. In order to avoid excessive height of the gooseneck, the condenser can be set up side by side with the still, the distillation water then being pumped or injected into the still with a steam injector. This procedure prevents loss of oil, since the oil in the water simply means an additional volatile oil charge to the still. It has been suggested that the condensed water be returned to the steam generating equipment (boiler), but this idea cannot be recommended because of the difficulties encountered with the boiler, and also because of the heat in the steam boilers, which would have a deteriorative effect upon the quality of the dissolved oil. In the case of direct steam distillation, the dissolved oil is recovered through redistillation (cohobation) of the distillation water, or through extraction with volatile solvents, both of which will be discussed later in more detail.

Steam Boilers. 

Before leaving the subject of equipment, we must make brief mention of the use of auxiliary boilers when water and steam distillation, or steam distillation is used. The size of the boiler will depend on the amount of steam required ; no generalization can be made. Because of the danger involved in the operation of a steam boiler, it is recommended tha such equipment be purchased from an established dealer in power generation equipment. Briefly, besides the usual fire box and tube heater, the system should include gages for determining w'ater level and pressure, safety valves to guard against operation at too high pressure, a pump or injector for circulating the water, and all necessary piping for the particular operation at hand. The supplier should be consulted before ordering any equipment. All reputable suppliers maintain well-trained engineering staffs for the purpose of analyzing customers' requirements, and advantage should be taken of this service.

There are two types of boiler, viz., the so-called low-press j re boiler, developing 40 to 45 Ib. of pressure, as measured at the boiler gage, and the high-pressure boiler, which develops a steam pressure of approximated 100 Ib. and more. High-pressure steam is used to attain higher temper it ares rather than merely to force the steam through the plant material contained in the retort. Theoretically the temperature of saturated steam is a function of the steam pressure. Steam, as developing from boiling water (pressure at the gage = 0), has a temperature of 212^o F. (100^o C.) ; at 40 Ib. it has a temperature of 287^o F. (141.7^o C.) and at 100 Ib., 338 F. (170^o C.). Steam of low pressure and, therefore, of comparatively low temperature, is likely to be recondensed to water in the lower part of the plant charge, whereas steam of higher pressure and temperature penetrates the plant material more effectively and with less condensation in the still. High-pressure boilers are, therefore, more efficient in regard to distillation, shortening its length. On the other hand, it is claimed that low-pressure steam, as a rule, yields more alcohol soluble oils, free of bitter resinous matter.

In actual operation low-pressure boilers produce little pressure but a large volume of steam. They are constructed of appropriate gage sheet metal with cast-iron beads. Even the flues are made of galvanized sheet metal. All of the other boilers are "high pressure." It is true that some distillers use 30 to 100 Ib. of pressure, but that depends on the steam requirements. Data collected by experts of Purdue University, Lafayette, Indiana, on retort temperatures in the distillation of peppermint oil, show that there exists little difference between the temperature of the trays at 20 Ib. and at 80 Ib., but the speed with which the distillation takes place is an important factor economically. The explanation is obvious if one considers that the steam is released into a large retort, not under pressure. There the steam temperature will be reduced to the still temperature immediately without pressure. In some cases, of course, the steam is "pushed" in so fast that a slight back pressure results, but this will seldom cause more than a 10^oF. (about 5^oC.) rise above 212^o F. (100 ^o C.) in the still. If superheated steam is to be used, a superheater of one form or another must be installed. One method16 of superheating steam consists of permitting high-pressure, dry saturated steam to expand suddenly to a lower pressure through a well-insulated valve. This will result in a moderate amount of superheating, at least theoretically speaking. A well-designed boiler should produce very nearly saturated steam and the above method will, therefore, result in slight superheating. If the steam as generated is very wet, it will be necessary to do one of two things in order to accomplish superheating. One method consists of installing in the high-pressure line a water separator, which will remove most of the liquid water from the steam. This dried steam may then be expanded as described above to produce superheated steam. An alternative method is to expose the line carrying wet or saturated steam to a temperature sufficiently above the boiling point of water at the steam pressure to permit the extent of superheating desired. This can be accomplished by running the steam line through a region in which the waste gases from the boiler can transfer part of their heat to the steam. The amount of exposure must be carefully controlled, to avoid excessive superheating. If desired, this heating may also be done in an entirely separate unit, and since the stack gases always contain waste heat, this might just as well be recovered. In the installation of superheating equipment, the boiler supplier can again be of great assistance.

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15 For theoretical explanation, see von Rechenberg, "Theorie der Gcwinnung und Trennung der atherischen Ole," Leipzig (1910), 400.