Supercritical Fluids - Fundamentals and Applications




The aim of this work was to perform supercritical CO 2extraction of essential oil from lavender on a laboratory apparatus as well as in a pilot plant. A separation procedure was used to indicate fractional separation of the extracts. GC-MS analysis of the products was performed to assess the best extraction and the best separation conditions. A semicontinuous-flow extractor was used. Extraction rate was measured as a function of pressure and temperature. The extraction rate increased with an increase in CO 2 flow rate. The effect of pressure and temperature on Lavender extraction suggested the use of higher pressure and lower temperature at which solvent density is greater and thus the solubility of the oil in the solvent is measured in the range of 40 ºC- 45 ºC and 15-20 MPa. The major components of the extracted oil were identified as camphor, fenchone, eucalyptol, camphol, fenchol, camphene, thymol, myrtenol, furfural alcohol and many other volatile constituents.

Lavender is the name for any of several aromatic shrubs. Lavandula stoechas is an endemic, widely distributed taxon of the whole Mediterranean region. The reported life zone of lavender is 7 ºC to 211 ºC; with an annual precipitation of 0,3 to 1,3 meters and a soil pH of 5,8 to 8,3. Lavender grows in well drained, dry, calcareous soils located in full sun. Plants can be directly seeded, but are usually transplanted from vegetative cuttings and growth is slow, and it takes a few years to the crop to develop fully. The lesves and flowers of lavender are used in regions where the plant is grown as a flavoring in salads, dressings, fruit desserts, jellies and wines. Flowers and the leaves are sometimes used in sachets, potpourris, and dried bouquets. The plant and oil are used in herbal teas and as a flavoring mixed with black teas. The plant material is used to perfume linen and scent tobacco. The oil is used in perfumes, toilet water, and cosmetics. Also, lavender plants are attractive to bees.

As a medical plant, the lavender have traditionally been considered antispasmodics, carminatives, diuretics, nervines, stimulants, and tonics. It’s also used for antibacterial and panel cytotoxic activities. They have been used as a folk remedy aganist colic and headaches. The essential oil of lavender is reported to have antiseptic, carminative, and spasmolytic activity. In addition, we can find the insect attractant properties of fenchone and myrtenyl acetate on one side, and insect repellent properties of 1,8- cineole and camphor on the other side indicating the possible ecological role of these substances. Hence, L. stoechas is a well known plant species used all over the Mediterranean region for several medicinal purposes, attributed mainly to its essential oil content. It is also cultivated for parfumery and ornamental purposes.

Lavender oil is obtained by the immediate steam distillation or solvent extraction ( such as supercritical CO 2 extraction) of flowers harvested at full bloom. The essential oil contains camphor, fenchone, eucalyptol, camphol, fenchol, camphene, thymol, myrtenol, furfural alcohol and many other volatile constituents.

The extraction of essential oil from flowers and leaves represents an attemt to isolate this mixture while preserving the original composition that produces the natural fragrance. Unfortunately, the techniques usually adapted, such as steam distillation and solvent extraction, suffer several limitations in extracting and preserving the composition of natural fragrances. Thay can produce the degradation of thermolabile compounds, and solvent contamination. Moreover, steam distillation can produce an inapproprate collection of compounds responsible for the fragrance. Indeed, this technique is based on evaporation of volatile compounds induced by steam. Therefore, compounds with low vapour pressure can not be completely extracted by this technique.

So, supercritical fluid extraction (SFE) is an attractive alternate to conventional liquid extraction due to its use of environmentally compatible fluids, oxygen free extraction environment, property of extracting high boiling components at relatively low temperatures by minimizing thermal degradation of thermelly labile compounds, and shorter extraction times. Carbondioxide used in supercritical extraction has many advantages such as being inexpensive, contaminant free and less costly to dispose, safely than organic solvents. Also, it is non-toxic, non-flammable + without chemical residue problem being operated at low critical temperature (31,2 ºC) (Table 1). For these reasons, supercritical fluid CO 2 is the reagent used to extract plenty of materials.

Table 1.

Critical conditions for various supercritical solvents

Fluid

Critical Temperature (K)

Critical Pressure (bar)

Carbon dioxide

304.1

 73.8

Ethane

305.4

48.8

Ethylene

282.4

50.4

Propane

369.8

42.5

Propylene

364.9

46.0

Trifluoromethane (Fluoroform)

299.3

48.6

Chlorotrifluoromethane

302.0

38.7

Trichlorofluoromethane

471.2

44.1

Ammonia

405.5

113.5

Water

647.3

221.2

Cyclohexane

553.5

40.7

n-Pentane

469.7

33.7

Toluene

591.8

41.0

The special properties of supercritical fluids bring certain advantages to chemical separation processes. Several applications have been fully developped and commercialized. Some of them are food and flavouring, pharmaceutical industry, environmental protection for volatile and lipid soluble compounds, extraction of high value oils, isolation of lipid soluble compounds, extractiom of high grade natural aromas, purification of raw materials, extraction of hop resins, extraction of spices, reduction of nicotine in tobacco, coffee and tea decaffeination, recovery of aromas from fruits, meat, fish, etc.

In this study, a process for supercritical CO 2 extraction of volatile oils from lavender and supercritical fluid chromatography has been applied. Also, the examination of experimental conditions (pressure and temperature) effects on the extraction yield and essential oil composition were aimed.

After dried Lavandula stoechas plant material was acquaried, it was stored at room temperature until they are grounded using knife mill. We cut the leaves without the use of any solvent in order to obtain small leaf particles. Sieving was not needed to adjust the minimum size of the leave particles. The application of supercritical CO 2 extraction was applied to obtain lavender volatile oil content. The extraction yield was also measured at various extraction lengths under various experimental conditions.

 For the supercritical extraction of Lavandula stoechas, CO 2 is used as supercritical fluid. Other solvent types will be used in analytical determinations.

  Extraction measurements were carried out using semi-batch supercritical fluid extractor set up by AKICO company. Supercritical carbondioxide (SCCO 2) was used as solvent. The extraction experiments were performed with a tubular extractor of 500 ml capacity. A schematic flow diagram of the extraction apparatus is shown in Figure 1.

Liquid CO 2 from the supply cylinder passed through a cold bath (about-6 ºC) and then was pumped with an air driven liquid pump and heated in a heat exchanger to the extraction temperature. Pressure was controlled with two back pressure regulation valves.

Fig. 1. Flow diagram of the SFE apparatus

The extractor, containing the raw material to be extracted, was placed in a controlled temperature water bath, the temperature inside the extractor being controlled by a digital controller with an accuracy of ± 0,02 ºC. The pressure at the exit of the extractor was measured by using a monometer with an accuracy of 0,1-0,2 Mpa. After leaving the extractor, the stream of CO 2 loaded with extract, flowed through an on/off valve and a sequence of pressure expansion valves. The stream pressure was in this way reduced down to atmospheric pressure, and the oily extract was recovered in a glass collector. Water and volatile compounds were deposited in a second collector. A dry test meter (Model DC-I) was used to measure the delivered volume of CO 2, being measured the pressure and the temperature conditions.

Using this experimental apparatus, the mass of essential oil extract (and hence the yield of extract) was determined as a function of extraction time and pressure passed through the ground Lavandula stoechas bed, at each of the experimental conditions studied.

Before each set of yield determinations at given extraction conditions, the extractor was filled with 25,0 grams of ground Lavandula stoechas and CO 2 was pumped into the extractor until the desired extraction pressure value. After ensuring that there is no leak in the equipment, the expansion valves were opened and solvent was allowed to pass through the bed of ground particles, at predetermined pressure and temperature values. The exit fluid from the extractor was expended to ambient pressure by a back pressure regulator. After a given extraction period these valves were closed and the weight of the extract deposited in the glass collector was determined.

The pipes leading from the extractor to the collector were cleaned using ethanol to remove any extract trapped in this region. After washing, the valves of the extractor were opened and the extraction apparatus was unloaded and cleaned with ethanol in order to resume further periods.

 The carbondioxide extracted lavender oil was analysed using GC-MS, analysis of the more volatile fractions were obtained. The identification of compounds was based on the comparison of retention times and mass spectra with pure compouns whenever possible. But, it should be considered that the list of compouns resulting from GC-MS analysis can be different depending on several experimental conditions such as temperature and pressure composition used and even extraction time. The extraction time can play an important role in determining the exact compositions because various families of compounds constituting the oil are characterized by different diffusion times. For this reason, the characterization of essential oil was made using total quality of lavender oil obtained during an exhaustive experiment. By considering that fact, the traces are shown in Figure 2 with compound identifications expressed in Table 2.Main components of Lavandula stoechas essential oil are observed as camphor, fenchone, eucalyptol, camphol, fenchol, camphene, thymol, myrtenol, furfural alcohol.  

Fig. 2. GC chromatogram of Lavandula stoechas extract collected from the separator

 Table 2.

Compounts identified in Lavandula stoechas oil and their concentration (% peak area) in extracts obtained in different extraction times

Number of Retention

components Component time (min) Area (%)

1 Camphene 3.543 0.171

2 Eucalyptol 5.751 4.940

3 Furfural alcohol 7.389 1.081

4 Fenchone 8.051 19.359

5 Fenchol 9.229 0.657

6 Camphor 10.744 40.979

7 Camphol 11.762 1.240

8 Thymol 13.108 0.401

9 Myrtenol 13.480 1.057

 The effect of various process parameters, namely extraction pressure and temperature on the extraction rate was examined. The operational conditions for each experiment are given in Table 3, while the experimental results were expressed and the yield of extract (kg extract/kg feed) versus experimental pressure and temperature graphs were plotted as shown in Figure 3.

 Fig. 3. Effects of pressure and temperature on extraction yield

  The yield was was defined as the weight extracted divided by the weight of the original. The extraction yield was increased with decreasing temperature at constant pressure and decreased with increasingpressure at constant temperature.

Table. 3. Experimental conditions

Exp. Number

T ( ºC )

P ( Mpa )

exp 1

40

15

exp 2

40

20

exp 3

45

20

exp 4

45

15

Several extractions were carried out at different pressures to find out the relation with extraction efficiency at 40 ºC and 45 ºC. From these curves, it was apparent that the extraction yield decreased with increasing pressure. To determine the influence of temperature in the efficiency of extraction for lavender essential oil, temperature was increased from 40 ºC to 45 ºC at constant pressure of 20 Mpa. It has been seen that the extraction rate and temperature are in an inversely proportional relationship. So, lower temperature suggests the increased solubility of essential oil in the solvent used.

To conclude, supercritical fluid extraction of lavender oil was examined in a semi-batch operating supercritical fluid extractor unit and the effects of various process parameters on the extraction rate of the process were tried to be found out. The supercritical corbondioxide can be used as solvent to obtain extracts from Lavandula stoechas.

It was shown that the increase of pressure results decrease of the extraction rate. On the other hand, the increase of temperature decreases the extraction yield due to the decrease of solubility in the first case and the increase of diffusion resistance. But, if a comparison is done with referances in order to check the reliability of the experimental results, it can be seen that normally, for various SFE applications to obtain essential oil from different materials, the general approach to the results obtained considering the effect of temperature were same; but the higher the pressure, the higher the extraction yields due to the solubility of the oil components.

In conclusion, the experimental results indicated that the essential oil fraction of Lavandula stoechas could be efficiently extracted by SCCO 2 extraction.

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