Ind. Eng. Chem. Res. 2004, 43, 2753-2758
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Separation of r-Tocopherol and Squalene by Pressure Swing Adsorption in Supercritical Carbon Dioxide Hongtao Wang, Motonobu Goto,* Mitsuru Sasaki, and Tsutomu Hirose Department of Applied Chemistry and Biochemistry, Kumamoto University, Kumamoto 860-8555, Japan
Tocopherols are the major components of deodorizer distillate, and one of the most important byproducts of soybean oil from the deodorization step of the soybean refining process. In the past, tocopherols were removed conventionally from the soybean deodorizer distillate using solvent extraction and molecular and vacuum distillation. Recently, supercritical carbon dioxide was tested as an alternative solvent for the separation processing of tocopherols. A separation process for a binary-component model mixture composed of R-tocopherol and squalene was developed using the pressure swing adsorption concept in supercritical carbon dioxide. In the present study, the pressure swing operation between the adsorption step at a lower pressure, and the desorption step at a higher pressure was performed with octadecylsilica (ODS) as adsorbent. The objective of the current work was to experimentally measure the effect of operating parameters such as half-cycle time, pressure ratio, and flow-rate ratio on the product purity, yield, and recovery. Alpha-tocopherol was satisfactorily concentrated from 20 wt % of tocopherol in feed mixture to 60 wt % in product in the desorption step, and squalene was also concentrated from 80 wt % in feed mixture up to 98 wt % in product in the adsorption step. Optimal values for product purity, yield, and recovery were determined from these operating parameters. Model calculations were in rough agreement with experimental results. Introduction Tocopherols are the most common commercially exploited natural antioxidants and the most important fatsoluble antioxidants for humans. They consist of four isomer structures (R, β, γ, and δ) varying according to the position and extent of methyl substitution. Alphatocopherol has the highest activity of the four isomers. The structural formula of tocopherol and the differences between the four isomers are shown in Figure 1(A). Tocopherols are commercially produced from soybean deodorizer distillate. Another high-value compound present in soybean deodorizer distillate is squalene. In its natural form, squalene is normally used in cosmetic preparations as a moisturizing or emollient agent. The structural formula of squalene is presented in Figure 1(B). Tocopherols are recovered conventionally from the soybean deodorizer distillate which is obtained during the deodorization step of the soybean oil refining process. Deodorizer distillate is a complex mixture of more than 200 components, including many high-value compounds such as tocopherols, squalene, and sterols. The composition of the soybean deodorizer distillate varies according to soybean growing and harvesting conditions and the applied process steps. Brunner et al. evaluated a typical composition of a soybean deodorizer distillate from the water vapor distillation process, which resulted in 13-14% tocopherols, 3.5% squalene, 26% sterols and free fatty acid, and tri-, di-, and monoglycerides.1 To concentrate tocopherols from the deodorizer distillate of the soybean oil, pretreatment of the raw material, including the esterification of free fatty acids and the * To whom correspondence should be addressed. Phone: +81-96-342-3664. Fax: +81-96-342-3679. E-mail: mgoto@ kumamoto-u.ac.jp.
Figure 1. Structural formulas of tocopherol and squalene.
removal of sterols with alcohol recrystallization, is needed to obtain the primary tocopherols concentrate.2 For further enrichment of tocopherols, many conventional processes such as solvent extraction and molecular and vacuum distillation3 are used, but these processes involve inherent drawbacks such as residual solvents, high temperature, and large amounts of energy consumption. The separation and enrichment of tocopherols from soybean deodorizer distillate have been studied using supercritical carbon dioxide.1,2,4-6 Carbon dioxide is the most desirable supercritical fluid solvent for the separation of natural products used in foods and medicines because of its inertness, nontoxicity, low cost, and high
10.1021/ie0308339 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/30/2004
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volatility. In supercritical carbon dioxide processing, carbon dioxide is suitable for thermolabile natural products because of its near-ambient critical temperature. In addition, the resultant extract does not contain solvent residue and hence natural-quality extracts can be obtained. The feasibility of concentrating tocopherol from soybean sludge by supercritical carbon dioxide was investigated at temperatures ranging from 308 to 343 K and pressures ranging from 20 to 40 MPa.6 A simple batch process was utilized to recover tocopherols at 40% concentration from the esterified soybean sludge which initially contained 13-14% tocopherols. The solubility of the esterified soybean sludge in supercritical carbon dioxide was more than 4 to 6 times higher than that of the sterols. A higher enrichment of tocopherols from soybean deodorizer distillate using supercritical carbon dioxide as a solvent has been obtained compared to results obtained by Lee et al.1 A German research group recovered tocopherols from a model mixture of squalene, tocopherols, and sterols using two continuous countercurrent fractionation columns. Squalene was separated from the model mixture in the first column. Sterols were removed from the bottom of the second column, resulting in 85-95% concentration of tocopherol being obtained at the top of the second column. The phase equilibrium for recovering R-tocopherol from a mixture of squalene, tocopherol, and campesterol was also studied,7 where it was concluded that the separation factor for squalene-R-tocopherol varied between a value of 4 at low squalene concentrations (0.5 wt %), to a value of 1 at high squalene concentrations (85 wt %), at pressures ranging from 20 to 30 MPa and temperatures ranging from 343 to 373 K. The combination of supercritical fluid extraction and the other methods, such as adding an entrainer1 or adsorption methods2, has been widely studied. A system utilizing supercritical fluid extraction in tandem with supercritical fluid chromatography for enriching and fractionating tocopherols from soybean flakes has also been developed.8 In the supercritical fluid extraction step, enrichment factors of 1.83-4.33 for the four tocopherol isomers in the extract were obtained relative to the starting concentrations in soybean flakes. Additional enrichment factors of 30.8 for δ-tocopherol, and 12.1 for R-tocopherol were realized in the supercritical fluid chromatography stage with a silica gel column. Therefore, the trend of using adsorption coupled with supercritical fluid extraction is a promising development for the separation and enrichment of various components from natural materials. Pressure swing adsorption (PSA) is a widely used process in the separation of gas mixtures for air-drying, oxygen and nitrogen separation of air, hydrogen purification, and various other separations. The PSA process is based on the regeneration of adsorber by the difference in adsorbed amounts of gas solute as a function of pressure. In the case of a two-bed process, one bed is in the adsorption step, while the other is simultaneously in the desorption step. We have developed a PSA process using supercritical carbon dioxide for the deterpenation of orange oil.9 High concentration factors, up to 10, and a 60% recovery of oxygenated compounds were obtained at a half-cycle time of 120 min for the desorption step. Moreover, we tested the PSA process in supercritical carbon dioxide for deterpenation of bergamot oil.10
Higher purity and recovery of oxygenated compounds were obtained in the desorption step, and model calculations agreed well with the experimental results. The objective of the present work was to apply the PSA process using supercritical carbon dioxide for the separation of R-tocopherol from a model mixture composed of 20 wt % R-tocopherol and 80 wt % squalene. This study began with the measurement of the adsorption behavior of R-tocopherol and squalene in supercritical carbon dioxide. The influence of important operating parameters, such as half-cycle time, pressure ratio, and flow-rate ratio on R-tocopherol product purity, recovery, and yield in the desorption step was investigated. A simple mass transfer model, with a multicomponent Langmuir isotherm, was used to simulate the process for comparison with the experimental observation. Experimental Section 1. Adsorption Behavior. Chemicals used in this work were R-tocopherol and squalene (both from Wako Pure Chemical Industries, Ltd., Osaka, Japan). The adsorbent used was octadecylsilica (ODS) (20/40 µm diam, 120 A, GL Sciences Inc. Tokyo, Japan). Adsorption equilibrium is an important fundamental property in the design and development of the adsorption process. In the present work, we measured the adsorption equilibrium constants of R-tocopherol and squalene in supercritical carbon dioxide, using an impulse response technique.11 In addition, we measured the breakthrough curves for R-tocopherol and squalene using a column (120 mm long and 9 mm i.d.) packed with adsorbent ODS and an online UV-visible spectrophotometer (870-UV, Jasco, Japan). Carbon dioxide was pressurized to the desired pressure and supplied to an extractor equipped for dissolving the solute ahead of the adsorption column. The extractor and the adsorption column were kept at the required temperatures. The fluid-dissolved solute flowed through the adsorption column and the effluent passed through the UV-visible spectrophotometer to measure the concentration of solute in the supercritical carbon dioxide. The effluent was then collected in a separator, and expanded to ambient pressure through a back-pressure regulator that followed the UV detector. 2. PSA Process. A mixture of 20 wt % R-tocopherol and 80 wt % squalene was used as the feed in the present work. The experimental apparatus is shown in Figure 2. The extraction column (600 mm long and 9 mm i.d.) was placed before the adsorption column to dissolve the feed mixture completely in supercritical carbon dioxide. A pair of adsorption columns (120 mm long and 9 mm i.d.) packed with 6.3 g of adsorbent ODS in each column were used as adsorbers. The columns and pipes were wrapped with a ribbon heater to keep a constant system temperature. Sets of 9 air-regulated valves were used for switching flows in the adsorptiondesorption cycle.9 The operational behavior of the system during a cycle is shown in Figure 3. The feed mixture dissolved in CO2 is supplied continuously to column 1 in the adsorption step, while column 2 is pressurized, desorbed, and depressurized in the reverse flow direction. The two columns are switched, in turn, between adsorption and the other steps during the cyclic continuous operation. Pressure was 16.0 MPa in the adsorption step and 30.0 MPa in the desorption step, unless otherwise stated.
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tween adsorption and the other steps, for a cyclic continuous operation. The CO2 flow rate was measured with a wet gas meter placed after the separators. A detailed description of the experimental apparatus and operational procedures were reported previously.9 Because R-tocopherol is selectively adsorbed over squalene, R-tocopherol is eluted in the desorption and blowdown steps. However, under our experimental conditions, the sample amount collected in the blowdown step was negligible. Thus, the process performance was evaluated by analyzing the product in the desorption step only. To evaluate the PSA process performance, we defined the following factors:
Purity [-] ) fraction of R-tocopherol in the product in desorption step (1) Figure 2. Experimental apparatus for the pressure swing adsorption process in supercritical carbon dioxide.
Recovery [-] ) flow rate of R-tocopherol in desorption step [kg/s] flow rate of R-tocopherol in feed [kg/s] (2) Yield [-] ) flow rate of product in desorption step [kg/s] (3) feed flow rate [kg/s] Throughput ) feed flow rate [kg/s] (4) total flow rate of carbon dioxide [kg/s]
Figure 3. Pressure changes in the column during cyclic operation.
Typically, a CO2 flow rate of 8.60 × 10-5 kg/s was used for the adsorption step and 7.80 × 10-5 kg/s for the desorption step. The temperature in the system was kept at 313 K. The effluents were collected into sampling tubes and weighed with an analytical balance, and then analyzed by a GC FID (GC-14A, Shimadzu Co., Kyoto, Japan) using a capillary column (J&W Scientific, DB-5, 15 m × 0.25 mm × 0.25 µm). The temperature program ran from 200 to 300 °C at a rate of 5 °C/min. Helium was used as the carrier gas. The feed mixture, supplied by a feed pump (880-PU, HPLC Pump, Jasco, Japan), and CO2 at 16.0 MPa were countercurrently contacted with the mixture in the mixing column. Most parts of a-tocopherol and squalene were dissolved in supercritical CO2, whereas waxes and pigments were recovered from the bottom of the mixing column because of their lower solubility. The fluiddissolved R-tocopherol and squalene in the mixing column were in homogeneous phase, and flowed from the top to the bottom of the adsorber during the adsorption step, in which R-tocopherol was adsorbed on the ODS selectively and squalene was obtained by depressurization in a separator. In contrast, pure CO2 compressed to desorption pressure flowed from the bottom to the top of the other adsorber to recover adsorbed R-tocopherol in the desorption step. The two columns switched in turn be-
3. Mathematical Model. An isothermal model was developed to simulate the two-bed cycle pressure swing adsorption in supercritical fluid. To develop a mathematical model for this system the following assumptions were made: (1) the system is isothermal; (2) pressure drop in the bed is negligible; (3) axial and radial dispersions are negligible; (4) equilibrium relationships for both R-tocopherol and squalene are represented by the competitive Langmuir isotherms; (5) mass transfer rates are expressed by the linear driving force approximation and the rate coefficients R-tocopherol and squalene are the same for both high-pressure and low-pressure cycles; and, (6) during pressurization and de-pressurization, the profiles of adsorbed amounts in the beds are fixed. On the basis of these assumptions, the dynamic behavior of the system for the adsorption, rinse, and desorption steps can be described by material balance and the mass transfer rate. Moreover, the equilibrium relationship is given by the multicomponent Langmuir isotherm equation. Mathematical equations that describe the dynamic behavior based on these assumptions were reported in detail previously.9 Material balance for adsorption, rinse, and desorption steps is as follows:
∂Ci,j ∂qi,j ∂Ci,j + vj + γj )0 ∂t ∂z ∂t
(5)
where the subscripts i and j refer to the component and the bed number, respectively. The mass transfer rate is given by
∂qi,j ) (Ksa)i,j(q*i,j - qi,j) ∂t
(6)
2756 Ind. Eng. Chem. Res., Vol. 43, No. 11, 2004
Figure 4. Relationship between adsorption equilibrium constant and solvent density.
Figure 5. Breakthrough curve of R-tocopherol and squalene at 313 K and 16.0 MPa.
Boundary conditions are as follows:
ci,j (t,0) ) ci0,j, F ) FL for adsorption step
(7)
ci,j (t,L) ) 0, F ) FL for rinse step
(8)
ci,j (t,L) ) 0, F ) FH for desorption step
(9)
Initial conditions are
ci,j (0,z) ) qi,j (0,z) ) 0, F ) FL
(10)
The cyclic operation forms are as follows:
qi,1(0) ) qi,2(tc)
(11)
qi,1(tc) ) qi,2(0)
(12)
The above equations were written in the dimensionless form and solved numerically by the improved Euler method. Results and Discussion 1. Adsorption Equilibrium Constant. The adsorption equilibrium constant at temperatures ranging from 313 to 333 K, and pressures ranging from 10 to 30 MPa, is shown in Figure 4. The adsorption equilibrium constants for R-tocopherol and squalene were correlated linearly in a log-log plot as a function of the density of supercritical carbon dioxide, and showed that R-tocopherol was selectively adsorbed on the adsorbent ODS over squalene. An increase in the density of carbon dioxide decreased the adsorbed amounts of both R-tocopherol and squalene. On the basis of this condition, a PSA process using supercritical carbon dioxide can be realized in which R-tocopherol is adsorbed selectively on the adsorbent at a lower pressure and then desorbed at a higher pressure. 2. Breakthrough Curve. Figure 5 shows the breakthrough curves for R-tocopherol and squalene at 313 K and 16.0 MPa. Squalene was slightly adsorbed on the adsorbent and immediately showed a breakthrough in the column. The strongly adsorbed R-tocopherol was concentrated on the adsorbent. This indicates that R-tocopherol and squalene can be separated using the adsorbent ODS in supercritical carbon dioxide. 3. Effect of Half-Cycle Time. A continuous cyclic operation was initiated with clean columns. Thirteen half-cycles were required to reach the cyclic steady state. Figure 6 shows the effect of half-cycle time on product
Figure 6. Effect of half-cycle time on product purity, yield, and recovery (PA ) 16.0 MPa, PD ) 30.0 MPa, QA ) 8.6 × 10-5 kg/s, QD ) 7.8 × 10-5 kg/s).
purity, recovery, and yield in the desorption step. Product purity increased to approximately 0.60 in the desorption step, with an increase in half-cycle time, and was almost constant when the half-cycle time exceeded 3600 s. Desorption is achieved by the flow of pure carbon dioxide at the constant desorption pressure and a longer half-cycle time, creating an adsorbent bed that is saturated with R-tocopherol in the adsorption step. A product recovery of 0.90 in the desorption step was obtained when the half-cycle time was 1800 s, and then decreased with an increase in half-cycle time, due to the R-tocopherol that broke through the column and flowed into the effluent in the adsorption step, when the halfcycle time was too large. Thus, higher product recovery could be obtained in the desorption step at an optimal half-cycle time that is shorter than the time when the column was saturated with R-tocopherol in the adsorption step. The same tendencies were observed for the yield of product. Therefore, to get both high purity and high recovery, half-cycle time must be optimized. 4. Effect of Pressure Ratio. The effect of the pressure ratio of the desorption step to the adsorption step (PD/PA), for a constant adsorption pressure of 16.0 MPa, is shown in Figure 7. Product recovery and purity in the desorption step increased with the pressure ratio. When the pressure ratio increased to 1.80, the product purity was almost constant at 0.60 because R-tocopherol adsorbed onto the column in the adsorption step, but was almost completely desorbed at a pressure ratio of up to 1.80 in the desorption step. The product recovery had the same tendencies as the purity. A product recovery of up to 0.60 was obtained at a pressure ratio of 1.80. The product yield was near 0.20 and was not influenced by the pressure ratio. Thus, higher performances for purity and recovery were
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Figure 7. Effect of pressure ratio on product purity, yield, and recovery (PA ) 16.0 MPa, QA ) 8.6 × 10-5 kg/s, QD ) 7.26 × 10-5 kg/s, half-cycle time ) 60 min).
where Langmuir parameters were qs ) 0.40, K1 ) 2.719 × 1015 F-5.293 for squalene, and K2 ) 4.111 × 1011 F -3.652 for R-tocopherol. The overall mass transfer coefficient used was 0.001 L/s for both squalene and R-tocopherol as used in our previous paper.10 The curves in Figures 6-8 indicate the calculated results. The product purity, yield, and recovery in the desorption step were well predicted in Figure 6. A large deviation was observed between the calculated and experimental values for product recovery within the region of the low-pressure ratio shown in Figure 7. In Figure 8, the calculated values of recovery and purity were larger than the experimental values at a low flowrate ratio. These deviations are due to insufficient desorption of adsorbed R-tocopherol at low pressures during the desorption step, and insufficient CO2 flow rate to recover adsorbed R-tocopherol during the adsorption step. Furthermore, while more complicated phenomena may occur in the columns, and all parameters used in the present calculations were the same as in our previous paper, these factors may have caused and increased the deviations between the calculations and experiments. Conclusions
Figure 8. Effect of flow-rate ratio on product purity, yield, and recovery (PA ) 8.8 MPa, PD ) 30.0 MPa, QA ) 8.6 × 10-5 kg/s, half-cycle time ) 60 min).
obtained for the operation at higher pressure ratios. The effect of the pressure ratio is also associated with halfcycle time. When the pressure ratio is high, half-cycle time can be shortened due to the improved desorption performance at higher pressure. 5. Effect of Flow-Rate Ratio. Flow rates of carbon dioxide for the adsorption step and the desorption step were changed. Figure 8 shows the effect of the flow-rate ratio of the desorption step to the adsorption step (QD/ QA) with a constant adsorption flow rate of 8.60 × 10-5 kg/s. The product purity had maximum values in the desorption step in the flow-rate ratio range between 0.80 and 1.50. When the flow-rate ratio was below these values, a steep increase occurred until the maximum value for product purity when the flow-rate ratio increased. Thus, the flow rate of carbon dioxide in the adsorption step was too low, and the purity decreased because the low flow rate of carbon dioxide was insufficient to totally elute R-tocopherol adsorbed on the adsorbent during the adsorption step. However, product recovery increased with increases in the flow-rate ratio, and a recovery of 0.9 was obtained at a flow-rate ratio of 1.50. The product yield was around 0.20-0.30 and was not influenced by the flow-rate ratio. Similar results were obtained when the effect of pressure ratio on the product yield above was evaluated. 6. Model Calculation. The performance of PSA was simulated using a mathematical model.9 The mathematical model described the dynamic behavior involving material balance and the mass transfer rate, as well as the adsorption equilibrium given by the multicomponent Langmuir equation. The experimental data were correlated with the multicomponent Langmuir equation.
qi ) qsKiCi/(1 +
∑ KiCi)
(13)
The PSA process using supercritical carbon dioxide for the separation of R- tocopherol and squalene was demonstrated in the current work. The effects of operating parameters such as half-cycle time, pressure ratio, and flow-rate ratio on the product purity, yield, and recovery were studied. An increase in the pressure ratio produced high purity, recovery, and yield in the desorption step. Product purity up to 0.60 and recovery of 0.9 were obtained in the desorption step at a pressure ratio of 1.87. Other parameters had optimal values for higher performance of the process. Model calculations agreed roughly with the experimental results. Acknowledgment This work was supported by 21st COE program “Pulsed Power Science”. Nomenclature C ) concentration of each component [kg/kg] Co ) concentration of each component in the feed mixture [kg/kg] PA ) pressure in adsorption step [MPa] PD ) pressure in desorption step [MPa] QA ) flow rate in adsorption step [kg/s] QD ) flow rate in desorption step [kg/s] K ) Langmuir parameter [kg/kg] Ks ) overall mass transfer coefficient [1/s] L ) column length [m] q ) adsorbed amount [kg/kg] qs ) saturated adsorbed amount [kg/kg] t ) time [s] v ) interstitial velocity [m/s] z ) axial distance [m] γ ) mass ratio of adsorbent to fluid [kg/kg] F ) fluid density [kg/m3]
Literature Cited (1) Brunner, G.; Malchow, Th.; Stu¨rken, K.; Gottschau, Th. Separation of Tocopherols from Deodorizer Condensates by Countercurrent Extraction with Carbon Dioxide. J. Supercrit. Fluids 1991, 4, 72-80.
2758 Ind. Eng. Chem. Res., Vol. 43, No. 11, 2004 (2) Shishikura, A.; Fujimoto, K.; Kaneda, T.; Arai, K.; Saito, S. Concentration of Tocopherols from Soybean Sludge by Supercritical Fluid Extraction. J. Jpn. Oil Chem. Soc. 1988, 37, 8-12. (3) Chun, B. S.; Lee, H. G.; Han, J. H.; Wilkinson, G. Fatty Oil and Tocopherol Extraction from Korean Garlic Using Supercritical Carbon Dioxide. Proc. 4th Int. Symp. on Supercritical Fluids, Sendai, Japan, 1997; pp 179-182. (4) Mendes, M. F.; Pessoa, F. L. P.; Uller, A. M. C. An Economic Evaluation Based on An Experimental Study of the Vitamin E Concentration Present in Deodorizer Distillate of Soybean Oil Using Supercritical CO2. J. Supercrit. Fluids 2002, 23, 257-265. (5) Chang, C. J.; Chang, Y. F.; Lee, H. Z.; Lin, J. Q.; Yang, P. W. Supercritical Carbon Dioxide Extraction of High-Value Substances from Soybean Oil Deodorizer Distillate. Proc. 5th Int. Symp. on Supercritical Fluids, Atlanta, GA, 8-12 April 2000. (6) Lee, H.; Chung, B. H.; Park, Y. H. Concentration of Tocopherols from Soybean Sludge by Supercritical Carbon Dioxide. J. Am. Oil Chem. Soc. 1991, 68 (8), 571-573. (7) Brunner, G. Gas Extraction; Springer: New York, 1994.
(8) King, J. W.; Favati, F.; Taylor, S. L. Production of Tocopherol Concentrates by Supercritical Fluid Extraction and Chromatography. Sep. Sci. Technol. 1996, 31, 1843-1857. (9) Sato, M.; Goto, M.; Kodama, A.; Hirose, T. New Fractionation Process for Citrus Oil by Pressure Swing Adsorption in Supercritical Carbon Dioxide. Chem. Eng. Sci. 1998, 53, 40954104. (10) Goto, M.; Fukui, G.; Wang, H. T.; Kodama, A.; Hirose, T. Deterpenation of Bergamot Oil by Pressure Swing Adsorption in Supercritical Carbon Dioxide. J. Chem. Eng. Jpn. 2002, 35, 372376. (11) Goto, M.; Sato, M.; Kawajiri, S.; Hirose, T. Impulse Response Analysis for Adsorption of Ethyl Acetate on Activated Carbon in Supercritical Carbon Dioxide. Sep. Sci. Technol. 1996, 31, 1647-1661.
Received for review November 12, 2003 Revised manuscript received March 23, 2004 Accepted March 26, 2004 IE0308339
