Apparatus of Extraction-EXTRACTION WITH VOLATILE SOLVENTS

(b) Apparatus of Extraction.


General Arrangement. The extraction buildings of the Grasse region are usually of light masonry, one-storied, painted in light colors. The flat roof serves as a shallow water tank, insulating the building against sunheat and providing the condensers in the building below with water. The steam boilers are housed separately, at a safe distance from the main building, in order to exclude any danger of fire. The employment of volatile, highly inflammable petroleum ether or benzene also necessitates that all electric motors and switches be explosion proof, or be housed outside of the extraction building. The reserve solvent which is not in circulation must be stored in fireproof cellars, separate from the buildings.
The extraction building is equipped with one or two stills or columns for fractionating the solvent, a few batteries for extracting the flowers, and stills for concentrating the flower oil solutions. The batteries are of different size, so that they can be used according to the quantity of available flower material, which varies according to weather conditions and with the progressing harvest.

Construction of Apparatus.

Until some years ago the extractors and stills were constructed of copper, because this metal retains its value after scrapping the apparatus, and the pliable copper can be hammered and repairs are easily made. Lately, however, the extractors are constructed more often of heavily tinned sheet iron which is much cheaper and, therefore, offers the advantage of lower investment and quicker amortization.
The apparatus must be of solid construction to stand wear and tear; all extractor pipes and valves should be within easy reach to save time and exclude mistakes on the part of the operators. Pipes and valves must be sufficiently wide to prevent formation of pressure by air and petroleum ether vapors, one of the principal causes for loss of the solvent in vapor phase. Large diameters also permit quicker flow of solvent and solutions to and from the apparatus, and considerably speed up operation. Pumping of solvents and solutes is done by air pressure created by air compressors.
The extractors are mounted on elevated metal platforms along the inside walls of the building. The platform runs even with the ground outside of the building, so that the arriving flower material may be charged directly into the extractors and the exhausted flowers discharged with equal ease.
Loss of solvent during the operation presents one of the most serious problems. These losses arc usually caused by incomplete distillation of the solvent from the exhausted flowers, by insufficient condensation in the condensers, or by too narrow pipes and valves creating pressure and blowing off a mixture of air and solvent vapor. To avoid this as much as possible the whole system of extractors, evaporators and solvent tanks is arranged as a closed circle, with only one outlet where escaping solvent vapors can be condensed. The Société Carbonisation et Charbons Actifs ome years ago developed an efficient, rather small sized apparatus in which solvent vapors are absorbed by activated carbon and recovered by blowing live steam through the saturated carbon. A current of hot air then reactivates the carbon. The apparatus is simple and permits considerable economy in factories where large amounts of solvent are in circulation.

Description of Extraction Batteries.

A battery usually consists of three or four extractors, four or five metal tanks holding solvent and solutions, and an evaporator for concentrating the flower oil solutions. There exist two types of extractors viz., the stationary and the rotatory types. Some factories employ both, but only one type can be used in the same battery.
The stationary extractors usually have a capacity of 1,200 liters, holding about 135 kg. of jasmine flowers, or 180 kg. of rose flowers. From 400 to 450 liters of solvent are required for extracting 100 kg. of flowers. Losses of petroleum ether may range from 12 to 14 liters for 100 kg. of flowers treated, but the loss can be considerably reduced with the solvent recovery apparatus described above. 
The extraction of jasmine flowers with volatile solvents

 PLATE 9. The extraction of jasmine flowers with volatile solvents.
The stationary extractors are cylindrical, standing vertical. In the interior they should be provided with several perforated metal grids arranged horizontally around a vertical central support shaft. The flowers are charged upon these grids, thus spreading loosely over a larger surface and preventing lumping. The solvent can thus penetrate the mass freely and uniformly. After the flowers have been charged into the extractor, the metal cover on top is closed tightly with clamps.
Extraction is carried out methodically by successive washings whereby each batch of flowers is treated three times with solvent. A third washing is used as second washing for the next flower batch, then as first washing, and is finally pumped into the evaporator for concentration. The solvent distilled off is used as fresh solvent for a third washing which serves again as second of the next flower batch, etc. A fourth extraction, in most cases, yields at best only small quantities of waxes and other inert substances. The actual flower oil is contained in the first and second washings, while the third one serves merely to wash down parts of the second washing still adhering to the flower material. There exist, however, cases of emergency, especially during the height of the harvest, when great quantities of flowers arrive and must be worked up quickljr in order to avoid fermentation. In such cases the third washing is often eliminated altogether, in order to save time.
Schematic diagram of an extraction system.  (Extraction with volatile solvents.)
  Fio. 3.24. Schematic diagram of an extraction system.
(Extraction with volatile solvents.)
Just how to proceed requires experience and good judgment on the part of the factory manager.
After the third washing the flowers are practically exhausted, and can be discharged. However, they still contain a considerable amount of adhering solvent which, before discharging the flowers from the extractor, must be recovered by steam distillation, i.e., by simply blowing live steam through the mass. Water and solvent, after condensation, separate automatically in a specially constructed Florentine flask.
The first washing requires about 45 inin., the second 35 min., the third 
25 min. For drawing off the solutions and pumping in the next washing, 5 to 10 min. must be allowed for each operation. Including 90 min. of steam distillation for recovering the solvent still adhering to the exhausted flowers, complete extraction of one batch of flowers requires about 41/2 to 5 hr. However, no strict rules can be laid down, as every flower type requires a different method of working, and every manufacturer follows his own ideas.
Fig. 3.24 shows a schematic diagram of a system for extraction of flower material with volatile solvents.

Rotatory Extractors.

Years ago, Charles Gamier invented a rotatory extraction apparatus which was adopted by many factories. In modified and improved form39 the apparatus represents a simple, solid and moderately priced piece of machinery.
Rotary extractor, Gamier type


 FIG. 3.25. Rotary extractor, Gamier type.
The latest model (Fig. 3.25) consists of a heavily tinned iron drum rotating around a horizontal axle. Four perforated metal partitions, rectangularly and horizontally arranged around the central axle, divide the interior into four compartments into which the flowers are charged through four manholes. While the whole system rotates slowly, the flower material moves, tumbles and dips into the solvent lying on the bottom of the extractor. The liquid seeps through the perforations and drips back to the bottom when one compartment is lifted out of the solvent in the continuous movement of the whole drum. Thus, the solvent does not fill the extractor but only the lowest part, the flowers dipping slowly and continuously into the solvent and rising again in the rotatory movement.
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39 French Patent No. 585199, October 30 (1923). See also Apparatus of Hugues, French Patent No. 508085, December 12 (1919).

Three successive washings are usually made, carried out similarly to the systematic extraction method described under stationary apparatus. The first, i.e., the most saturated washing, is then pumped into the evaporator and concentrated by distilling off the solvent. After the third washing has been drawn off the exhausted flower material, live steam is blown into the extractor to distill over and recover the petroleum ether still adhering to the extracted flowers.
The advantages of the rotatory extractors as compared with stationary apparatus are evident. Through the movement of the flower material in the solvent, its action becomes more penetrating and more effective, resulting in a somewhat higher yield. Figured as concrete, it is about 8 per cent higher in the case of the rotatory apparatus. Since the solvent covers only the bottom of the rotating drum and not the whole flower material, as in the stationary apparatus, much less solvent is in circulation and, therefore, evaporation losses are reduced. Only 160 to 170 liters are required in the rotatory apparatus to extract 100 kg. of flowers. The loss of solvent for 100 kg. of flowers is less than in stationary extractors; it may be 8 to 12 liters but can be reduced by using the previously described solvent recovery apparatus.
One rotatory extractor does the work of three or four stationary extractors arranged in one battery. Although superior in many ways the rotatory extractors suffer from several disadvantages and cannot altogether replace the stationary type ; for example, the latter is better adapted to voluminous plant material, such as lavender, which cannot be so easily charged and discharged through the manholes of the rotating drum.

Concentration of Solutions.

The first, i.e., the most concentrated, washing is filtered through a fine screen and pumped into the so-called evaporator in which the greater part of the solvent is driven (distilled) off. These evaporators are of varying construction, representing basically a modified water or steam bath. In other words, the heating is done by indirect steam blown into a steam jacket beneath the still. The solvent, however, should not be completely driven off in this operation. Most manufacturers stop operation when the temperature in the still (evaporator) reaches about 60o, because any higher temperature at atmospheric pressure would be harmful to the delicate flower perfume. The first washing contains, of course, only a small percentage of flower oil. Therefore, concentrating of this washing hi the evaporator means driving off 90-95 per cent of the solvent. (The recovered solvent serves as fresh solvent for a third washing.) The concentrated solution remaining in the evaporators is permitted to cool, is filtered, then transferred to a special, smaller vacuum still, and there completely concentrated in vacuo.

Final Concentration. 

Vacuum stills (Fig. 3.26) of small capacity (50 to 100 liters) serve for this purpose. The final concentration represents a most delicate operation and requires much experience and constant attention on the part of the operator. The concentrating has to be done at as low a temperature as possible, yet any trace of solvent must be eliminated. Every manufacturer has his own, often secret, methods of purification. The completely concentrated and purified products represent the so-called floral concretes, which contain the odoriferous principles of the natural flower perfume, plus a considerable amount of plant waxes, albuminous material and color pigments. 
Vacuum still for the final concentration of natural flower oils  (removal of last traces of solvent)

 FIG. 3.26. Vacuum still for the final concentration of natural flower oils
(removal of last traces of solvent).
The concretes are, therefore, usually of solid consistency and only partly soluble in 95 per cent alcohol.

Concrete Flower Oils.

Although these insoluble concretes are more difficult to work with, some perfumers prefer them to the alcohol soluble absolutes, which are obtained by precipitating and eliminating the insoluble waxes with strong alcohol, and concentrating the filtered alcoholic solutions. Distilling off the alcohol from the solutions when making these absolutes undoubtedly entails the loss of some of the most volatile and delicate constituents of the natural flower oil. It is often claimed that an alcoholic washing of a concrete is superior and more true to nature than a simple alcoholic solution of the corresponding absolute. On the other hand, the processing of concretes requires special equipment and considerable time; therefore, the absolutes represent a more convenient form of flower oils than the concretes. 
Vacuum still for the concentration of the alcoholic washings in the preparation  of flower oil absolutes

 PLATE 10. Vacuum still for the concentration of the alcoholic washings in the preparation
of flower oil absolutes.
Conversion of Concretes intp Absolutes. The alcohol soluble absolutes are prepared from the concretes by the following general method: The concrete is either thoroughly rubbed down in a large mortar with a quantity of high-proof alcohol or, as some ma .ufacturers prefer, melted and dissolved in warm alcohol. Subsequently eight to ten times the amount of alcohol is added and the mass stirred for a prolonged period in batteuses, as described under enfleurage. Usually five to six washings of the concrete are made in a systematic way, i.e., a third washing serves as a second one for a following batch of concrete ; the second is used as first for a batch of new concrete, the first washing consequently representing the most concentrated solution. After standing and drawing off the clear solution from the alcohol insoluble waxes, the first washing is then thoroughly cooled in a refrigerator or in a special room, at temperatures ranging from 20 to 25, when more wax precipitates and is filtered off in the cold. The resulting clear solution can be used as such in alcoholic perfumes.
Most perfume houses have neither the time nor the facilities to carry out their own washing of concretes, and prefer using alcohol soluble floral oils. For those, the manufacturers in Grasse offer the so-called liquid absolutes as the most concentrated and convenient form of floral oils. These liquid absolutes of extraction are obtained by carefully concentrating the first alcoholic washing of the corresponding concrete at low temperature in a good vacuum still. This process of concentrating involves a loss of several liters of alcohol per kilogram of absolute.
The absolutes are usually viscous oils with a more or less pronounced color, according to the degree of final purification (for which each manufacturer employs his own process). The absolutes are soluble in high-proof alcohol, and represent the most concentrated form of natural flower oils used in practical perfume work. However, they must not be contused with the actual volatile flower oil in the scientific sense. The absolutes usually contain from 50 to 80 per cent of alcohol soluble waxes, and only 20 to 25 per cent volatile oil, which can be isolated from the absolute by steam distillation. However, these volatile oils from the absolutes are not offered on the market because of their excessively high price, and because they completely lack the high fixation value of an absolute, which is due to the presence of alcohol soluble natural waxes, etc., in the absolute.

CONCLUSIONS


It might be worthwhile to review briefly the advantages and disadvantages of the various methods of manufacturing natural flower oils.
1. Steam Distillation of flowers yields volatile oils for example oil of neroli bigarade, rose, ylang ylang. Not all types of flowers, however, can be processed by hydrodistillation, because boiling wr,ter and steam have a deteriorating influence upon the rather delicate odoriferous constituents. The flowers of certain plants yield no oil at all when distilled, and hence must be processed by methods other than distillation.
2. Enfleurage (extraction with cold fat). This method is carried out only in France, where it is still practiced, but on a much smaller scale than in former years. The method is restricted to those flowers (jasmine, tuberose, and a few others) which, after picking, continue their plant physiological activities in forming and emitting perfume. Enfleurage, in these cases, gives a much greater yield of flower oil than other' methods. Despite this advantage, enfleurage has lately been replaced by extraction with volatile solvents because enfleurage is a very delicate and lengthy process, requiring much experience and labor.
3. Maceration (extraction with hot fat). This process used to be applied to those flowers which gave a very small yield by distillation or by enfleurage. Maceration, however, has lately been almost entirely superseded by the modern process of extraction with volatile solvents.
4. Volatile Solvent Process. Of general application, this process is today applied to many types of flowers, and carried out in several countries. It is technically the most advanced process, yielding concretes and alcohol soluble absolutes, the odor of which truly represents the natural flower oil as it occurs in the living flowers, or in the plants.

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