Ind. Eng. Chem. Res. 1995,34, 3941-3946
3941
Fractional Extraction with Supercritical Carbon Dioxide for the Removal of Terpenes from Citrus Oil Masaki Sato,) Motonobu Goto,*'*and Tsutomu Hirose8 Department of Industrial Science, Graduate School of Science a n d Technology, a n d Department of Applied Chemistry, Kumamoto University, Kumamoto 860, J a p a n
Fractional extraction with supercritical carbon dioxide was studied for the preparation. of terpeneless citrus oil using a 9 mm i.d. and 1.0 m long packed column with a n axial temperature gradient of 0-20 K. The separation of citrus oil model mixture, which consists of limonene, linalool, and citral, was investigated a t various column temperature distributions of 313-333 K, pressures of 8.8-11.8 MPa, and C02 flow rates of 0.091-0.512 g/s. The selective separation was performed due to hhe internal reflux in the column induced by an axial temperature gradient. A little increase in pressure or C02 flow rate accelerated the extraction rate without decreasing the selectivity. Raw orange oil was processed successfully a t a temperature gradient of 20 K, from 313 K at the bottom to 333 K a t the top of the column, and a pressure of 8.8 MPa.
Introduction Fractionation processes of the products prepared from various natural materials, such as essential oil from citrus peels or herb leaves, are important in varieties of chemical industries. The fractionation of citrus oil to produce essential oils is one of the most important subjects in perfume and food industries (e.g., Bruno and Ely (19911, Gerard (1984), Stahl and Gerard (19851, Temelli et al. (1988)).Citrus oil consists of terpenes and oxygenated compounds. Terpenes must be removed to stabilize the product, because they are unstable t o heat and light, rapidly degrade, and produce undesirable offflavor compounds (e.g., Brandani et al. (19901, Gerard (1984), Giacomo et al. (19891, Matos et al. (19891, Shibuya et al. (19931, Stahl and Gerard (19851,Temelli et al. (1988)). The conventional process of deterpenation of citrus oil involves distillation so that heat degradation of products is an inevitable problem. Fractionation with supercritical carbon dioxide (SC-COB)is a prominent candidate for the terpene removal, since the separation can be performed at low temperature. Significant change in solubility with slight changes in temperature or pressure is one of the most important features of using supercritical fluids. This property can provide highly selective fractionation. The temperature dependence of solubility can be effectively utilized in supercritical fractional extraction processes using a rectification column. The process consists of an extraction part and a fractionation part. An extractor is followed by a fractionation column in a semibatch mode. The temperature at the top of the column is usually held higher than that at the bottom and the extractor. When the supercritical fluid containing dissolved solutes flows into the higher temperature zone in the column, less volatile components condense and drop back because of a decrease in the solubility of solutes. The internally refluxed drops contact counter-
* To whom correspondence should be addressed. E-mail:
[email protected]. FAX: +81-96-342-3679. ' Department of Industrial Science, Graduate School of Science and Technology. Department of Applied Chemistry. 0888-5885/95/2634-3941$09.00/0
currently with the fluid flowing up in the column, resulting in the rectification and the selective separation. Pioneering works were done by Eisenbach (1984), who used an extractor and a fractionation column with an axial temperature gradient in a semibatch mode. He reported the supercritical fluid fractionation of fatty acid ethyl esters using a rectification column with an internal hot finger. In his apparatus, the temperature at the top of the column was kept higher than that of the rest of the column, inducing separation of esters by carbon number and concentration of C-20 esters fraction. Nilsson et al. (1988) and Suzuki et al. (1989) investigated the fractionation of urea-crystallized fish oil esters by supercritical carbon dioxide using a rectification column with axial temperature profiles. Nilsson et al. (1988) used a 4-stage column with temperature zones, which increased from the bottom to the top, while Suzuki et al. (1989) used a column with a cosine curve temperature profile. By using a rectification column with axial temperature distribution, concentrated EPA and PHA with purities higher than 90%were obtained. Shibuya et al. (1993) studied supercritical fluid fractionation of model orange oil mixtures consisting of limonene and linalool with a column having a linear temperature gradient from 313 K at the bottom t o 340 K at the top and at a pressure of 8.0 MPa. The concentration of limonene in the extract obtained by the column was higher than that obtained with a column at 313 K without a temperature gradient. On the other hand, Gerard (1984) suggested a continuous countercurrent extraction process for the production of the terpeneless essential oil. An extraction takes place in the fractionation column in a continuous mode, and the terpene-containing oil is continuously fed into the middle of the column. He suggested that an internal reflux induced by a temperature gradient is useful for the enriching section. In this work, we applied supercritical fluid extraction with a rectification column t o the separation of citrus oil. Raw orange oil and a model mixture of citrus oil, limonene, linalool, and citral were used. Our objective is to show the effect of an internal reflux induced by a temperature gradient on the separation behavior. We studied the effect of temperature distribution, pressure, and C 0 2 flow rate on the separation of a model citrus
0 1995 American Chemical Society
3942 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995
Limonene (CIOHM)
Linalool ( CloHiaO)
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Geranial (trans)
Citral ( CLOHis 0 ) Figure 1. Molecular structures of major components in citrus oil.
coiumn
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Figure 2. Experimental apparatus.
oil mixture and discussed the rectification behavior in the supercritical fluid fractionation column.
Experimental Section Materials. A model mixture of limonene, linalool (Kanto Chem. Ltd.), and citral (Aldrich Ltd.), which are respectively principal constituents of the terpenes, alcohols, and aldehydes in citrus oil, was used as the feed. The composition of the feed was 33 wt % of limonene, 33 wt % of linalool and 33 wt % of citral. Citral has cis-trans isomers, geranial and neral, which are present in the ratio of about 2 to 1. These molecular structures are shown in Figure 1. Liquefied carbon dioxide from a cylinder with a siphon attachment (Uchimura Sanso Ltd.) was used as the extraction solvent. Experimental Apparatus. A schematic diagram of the experimental apparatus is illustrated in Figure 2a. The apparatus consisted of an extractor and a fractionation column in series. The extractor, with volume 8.0 x m3, was immersed in a thermostated water bath. The fractionation column consisted of four 250 mm length tubes of 9 mm i.d. connected with tees. The total column length was 1000 mm. The temperature of the fractionation column was controlled by five PID controllers. Thermocouples were placed at the center of the column inserted perpendicularly from the column wall at tees. Four liquid distributors were placed in the column to prevent channeling of the liquid. The column was packed with 2 x 2 mm stainless steel Dixon packing. The extractor was operated in semihatch mode with continuous C02 flow. Carbon dioxide from a cylinder was passed through a cooled line and compressed to operating pressure by
a high-pressure pump. The C02 flow rate was adjusted by the stroke of a piston pump. The compressed COz was brought to the desired temperature in a water bath. The fluid exiting from the top of the fractionation column was expanded to ambient pressure through a back-pressure regulator (Tescom Co.) which was heated by a rihhon heater to prevent freezing. The extracts were collected in the separator, and the COz flow rate was measured by a dry gas meter. The pressure in the extractor and in the column was controlled by the backpressure regulator located at the exit of the column. The extractor cell is shown in detail in Figure 2b. A feed mixture was charged in an inner cylinder whose capacity was 7.0 x m3 in volume. The inner cylinder was placed in the high-pressure extractor cell. The feed mixture was extracted by bubbling SC-COz as illustrated in Figure 2b. For each run, about 3 g of raffinate remained unextracted, since the liquid phase remaining under the bubbling nozzle could not be provided sufficient contact with SC-COz. The extracts trapped in the separator were collected in sampling tubes over a certain time intervals, weighed, and analyzed by a capillary gas chromatograph equipped with an FID detector (Shimadzu GC-l4A, Shimadzu HiCap-CBP20-M25-025). Experimental Procedure. A 24-g mixture of limonene, linalool, and citral was charged into the extractor. At the beginning, valve 2 was closed and valve 1 was opened, so that the carbon dioxide flowed to the fractionation column bypassing the extractor cell. After the temperature, pressure, and flow rate reached the desired values, valve 2 was opened and valve 1 was closed so that carbon dioxide huhhled in the extractor cell to start the extraction. The column temperature was controlled t o achieve a linear distribution from 313 K a t the bottom to 333 K at the top. The operating pressure was in the range of 8.8-11.8 MPa. The effect of temperature distribution on the separation behavior was evaluated in comparison with the column controlled at a uniform temperature of 313 or 333 K. The COz flow rate ranged from 0.091 to 0.512 g/s. To evaluate the dispersion behavior of the column, an empty column without any packing material was compared with the column packed with Dixon packing. Results and Discussion The influence of the packing on the separation was previously investigated by comparing extractions performed with an empty column and a packed column. The separation was slightly better for the packed column than the empty column. The packing in the column may have been expected to improve the rectification performance in two ways; the improvement of the heat transfer and the contact area between the supercritical fluid phase and the liquid phase. Since the difference of separation behavior between the packed column and the empty column was small, these factors may not have played an important role. The following experiments were carried out using a fractionation column packed with Dixon packing. Effect of Temperature Distribution and Pressure. Figure 3 shows the changes in composition of the extracts at 8.8 MPa. In these experiments the feed was an equiponderate mixture of limonene, linalool, and citral (nerd + geranial). Extraction for the column with a uniform temperature distribution may be similar to simple extraction without a fractionation column. The
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Figure 3. Changes in composition of extracts for a SC-CO2 fractionation of model citrus oil at 8.8 MPa and 0.302 g/s.
Figure 4. Changes in composition of the extracts for a SC-CO2 fractionation of model citrus oil at 9.8 MPa and 0.302 g/s.
components of the feed mixture were not separated for the extraction carried out at a uniform temperature of 313 K without a temperature gradient in the column, as shown in Figure 3a. Because the solubility of these pure components in SC-CO2 is large, almost infinite, the difference among these solubilities is small at this condition. When the column was controlled at 333 K uniformly without a temperature gradient at a pressure of 8.8 MPa, limonene was extracted selectively at the beginning of the extraction and the oxygenated compounds (linalool and citral) followed as shown in Figure 3b. This separation was due to the difference of solubility among limonene, linalool, and citral. In this condition the solubilities of pure limonene and linalool are reported to be 5.2 and 3.9 mg/g, respectively (Giacomo et al. (19891, Shibuya et al. (1993)). The solubility of citral is not available at this condition in the literature. However, it may be smaller than the others, since its solubility is 0.9 mg/g at a temperature of 323 K and a pressure of 8.8 MPa (Brandani et al. (1990), Giacomo et al. (1989)). Although limonene and oxygenated compounds were able to be separated at this condition, the selectivity for each component was small and a long time was required for complete extraction due to the low solubility compared with the solubility at 313 K. The result for the column with a temperature gradient from 313 K at the bottom to 333 K at the top is shown in Figure 3c. Selectivity of the separation was improved by the temperature gradient along the column. Limonene, linalool, and citral were extracted sequentially more selectively than in Figure 3b. Comparison of these results indicates that the temperature
gradient in the column affects the fractional separation. This fractionation was caused by an internal reflux, where less soluble components condensed and dropped back when the supercritical fluid containing dissolved solutes at the extractor condition of 313 K flowed into the higher temperature zone in the column,because the solubility of solutes decreased with the increase in temperature near the critical point. These internally refluxed drops countercurrently contact with the fluid flowing up in the column, resulting in the rectification. Amounts of refluxed solutes induced by an axial temperature gradient cannot be calculated since the solubility data of the mixture are unknown. However, it is expected that a considerable amount of solutes are refluxed, because the solubility of pure solutes a t the condition of the top of the column is much smaller than that of the bottom of the column. However, the separation between limonene and linalool in the initial part was not as good as that in Figure 3b. Poor separation between limonene and linalool in the initial part in Figure 3c may be due t o the transient phenomena. The solute concentration dissolved in the supercritical fluid phase is lower than the solubility a t the exit condition of the column because the amount of solute extracted is lower than the solubility and no liquid phase exists in the column. The solubility at 313 K is much larger than that at 333 K, so that extraction at the extractor is less selective at 313 K as shown in parts a and b of Figure 3. Consequently, efficient reflux does not occur at the beginning, resulting in the poor separation. Parts a-c of Figure 4 show the changes in composition at 9.8 MPa. The extraction rate at 9.8 MPa was larger than 8.8 MPa because of larger solubility. For
3944 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 I
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the column with a uniform temperature of 313 K, limonene and oxygenated compounds could not be separated as compared with Figure 3a at 8.8 MPa, since carbon dioxide and solutes formed a completely homogeneous phase where the solubility exceeded the solute amount. The separation at 333 K and 9.8 MPa in Figure 4b was worse than that at 333 K and 8.8 MPa in Figure 3b, although the extraction rate was larger at higher pressure. However, when the column had temperature gradients of 20 K, the selective separation was realized as similar to Figure 3c a t 8.8 MPa. This indicates that the rectification was performed by a slight temperature gradient. The extraction time was reduced to one fourth of the time required at 8.8 MPa because of the solubility increase. To compare the extraction rates, the cumulative extraction curves a t 8.8 and 9.8 MPa are shown in parts a and b of Figure 5 , respectively. At a pressure of 8.8 MPa, the curve for the column with a temperature gradient of 20 K almost agreed with the curve for the uniform temperature column at 333 K. On the other hand, the curve for the column with the temperature gradient at 9.8 MPa was between the curves at uniform temperature of 313 and 333 K without a temperature gradient. The fluid leaving the column was in equilibrium at the top of the column at 8.8 MPa, while it was not in equilibrium at 9.8 MPa. The difference between the conditions at 8.8 and 9.8 MPa might be caused by the solubility difference or density difference. Since the density of the supercritical fluid phase is closer to the density of the liquid phase at 9.8 MPa, entrainment of the liquid droplets may take place. Therefore, the internal reflux at 9.8 MPa for the column with temperature gradients of 20 K was less efficient. At pressures greater than 11.8 MPa terpenes and oxygenated compounds were not able to be separated because of the formation of a homogeneous one-phase solution of solutes and SC-CO2 as shown in Figure 6 . Effect of COZ Flow Rate. The effect of COZ flow rate on the separation behavior was investigated at constant feed amounts of 24 g. Figures 7 and 8 show the changes in composition of the extracts at 9.8 MPa
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and the extraction curves at various COz flow rates as a function of the amount of COZ consumed. The extraction rate increased for higher COz flow rates without reduction of separation selectivity at these conditions. The increased extraction rate at higher flow
Ind. Eng. Chem. Res., Vol. 34, No. 11,1995 3945 rate. In this case the packing of Dixon packing provided effective contact between the refluxed liquid oil phase and the SC-COZphase. Raw Orange Oil Processing. A total of 50 g of raw orange oil (Kishida Chem. Co., Ltd.) was charged into the extractor and extracted a t a pressure of 8.8 MPa and a temperature gradient of 20 K from 313 K a t the bottom to 333 K at the top of the column by the same procedure for a model mixture. Figure 9A shows the gas chromatogram of the orange oil. The raw orange oil used as a feed contained 98.7% of terpenes. Parts B and C of Figure 9 show the gas chromatograms of extract obtained in the time fraction ranging from 60 to 120 min and from 240 to 300 min, respectively. The extraction of oxygenated compounds was prevented, and terpenes were extracted selectively a t the time ranging from 0 to 200 min as shown in the chromatogram of Figure 9B. The content of terpenes in a B fraction was 99.4%. The concentration of terpenes in the extractor decreased with the progress of the extraction. Consequently, concentrated oxygenated compounds were extracted a t the end of the extraction run as shown in Figure 9C. The content of terpenes in a C fraction was 49.7%. Therefore, the deterpenated citrus oil was obtained by SC-CO2 extraction in the semibatch mode operation, where the internal reflux induced by a temperature gradient was effectively utilized in the enriching section.
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Figure 9. Gas chromatograms of extracts and the extraction curve from orange oil a t a pressure of 8.8 MPa and a temperature of 313 K a t the bottom and 333 K a t the top of the column. The temperature of the GC oven was programmed linearly from 343 to 413 K a t a rate of 3 Wmin.
rates may he due t o the entrainment of liquified solutes with COz flow. As discussed in Figure 5h, the fluid leaving the column was not in equilibrium a t the top of the column a t 9.8 MPa. Consequently, the dissolved solutes in the fluid at the extractor was not completely dropped hack in the higher temperature zone because the difference of density between the supercritical fluid phase and the liquid phase is close or the short residence time in the column at a higher flow rate is not sufficient to form the refluxed droplets. The selectivity of the separation did not decrease because of the improvement of the dispersion between the supercritical fluid and liquid oil phase at higher flow
A model mixture of citrus oil was fractionated using an extractor followed by a rectification column with an axial distribution of temperature. The results were compared with those obtained for the column without temperature distribution. The slight temperature gradient of 20 K improved fractional separation due to the considerahle change of soluhilities with temperature. A little increase in pressure accelerated the extraction rate. However, separation was impossible at pressures above 11.8 MPa, because a homogeneous mixture of solutes and SC-COZwas formed. The increase of COz flow rate caused an increase in the extraction rate. The result indicated the possibility of the removal of terpene from citrus oil. Raw orange oil was successfully deterpenated at a pressure of 8.8 MPa and a temperature gradient from 313 to 333 K. Selection and optimization of the process condition for SC-COZare important from the process point of view. However, a simple extraction process was not successful since high solubility and high selectivity was not compatible. Namely, a lower pressure gave a higher selectivity but a lower extraction yield, whereas a higher pressure gave higher yield but a lower selectivity. In spite of this, the fractionation column with temperature gradient provided the higher selectivity and higher extraction rate a t 9.8 MPa. Although equilibrium was not achieved in the fractionation column a t this condition, we obtained the high selectivity at higher flow rate because hydraulic properties in the column, such as drop formation rate, density and size of drops, and dispersion phenomena between the SC-COZphase and the liquid phase, may improve the fractionation.
Acknowledgment This work was supported by a Grant-in-Aid for Scientific Research (No. 04238106) from the Ministry of Education, Science and Culture, Japan.
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Literature Cited Brandani, V.; Re, G. D.; Giacomo, G. D.; Mucciante, V. Phase Equilibria of Essential Oil Components and Supercritical Carbon Dioxide. Fluid Phase Equilib. 1990, 59, 135-145. Bruno, T. J.; Ely, J. F. Supercritical Fluid Technology; CRC Press: Boca Raton, FL 1991. Eisenbach, W. Supercritical Fluid Extraction: A Film Demonstration. Ber. Bunsen-Ges. Phys. Chem. 1984,88,882-887. Gerard, D. Kontinuier Deterpenierung atherischer Ole durch Gegenstromextraktion mit verdichtetem Kohlendioxide. Chem. Zni Tech. 1984, 56, 794-795. Giacomo. G. D.: Brandani., V.:, Re., G. D.: Mucciante. V. Solubilitv of Essential Oil Components in Compressed ’ Supercriticil Carbon Dioxide. Fluid Phase Equilib. 1989, 52, 405-411. Matos, H. A.; Azevedo, E, G. D.; Simoes, P. T.; Carrondo, M. T.; Ponte, M. N. D. Phase Equilibria of Natural Flavours and Supercritical Solvents. Fluid Phase Equilib. 1989,52,357-364. Nilsson, W. B.; Gauglitz, E. J. Jr.; Hudson, J. K.; Stout, V. F.; Spinelli, J . Fractionation of Menhaden Oil Ethyl Esters Using Supercritical Fluid COz J.Am. Oil Chem. Soc. 1988, 65, 109117.
Shibuya, Y.; Ohinata, H.; Yonei, Y.; Ono, T. Extraction and Purification of Natural Food Additives in a Packed Column Using Supercritical C02 Proc. Znt. Soh. Extr. Conf.: York 1993, 684-691. Stahl, E.; Gerard, D. Solubility Behavior and Fractionation of Essential Oils in Dense Carbon Dioxide. Perfum. Flavor. 1985, 10,29-37. Suzuki, Y.; Konno, M.; Arai, K.; Saito, S. Fractionation of MonoEsters Derived from Fish Oil Using Supercritical Fluid Extraction Tower. Kagaku Kogaku Ronbunshu 1989,15,439-445. Temelli, F.; Chen, C. S.; Braddock, R. J. Supercritical Fluid Extraction in Citrus Oil Processing. Food Technol. 1988, 42, 145-150. Received for review January 3, 1995 Revised manuscript received J u n e 27, 1995 Accepted July 12, 1995@ IE950003Y Abstract published in Advance A C S Abstracts, October 1, 1995. @
