Chemistry Everday for Everyone
Past, Present, and Possible Future Applications of Supercritical Fluid Extraction Technology Cindy L. Phelps, Neil G. Smart,1 and C. M. Wai Department of Chemistry, University of Idaho, Moscow, ID 83843 In recent years there has been a tremendous amount of attention paid to the area of supercritical fluid extraction (SFE) and supercritical fluid chromatography (SFC). This interest has been fueled by changes in environmental regulations; conventionally used solvents are being replaced by “cleaner” processes such as those which utilize a supercritical fluid (SF). The number of research papers published in the fields of SFE and SFC has greatly increased since the early 1980s and symposia focusing entirely on one or the other of these topics have become commonplace. While the research and industrial communities are embracing the technologies wholeheartedly, thorough coverage of supercritical fluids and their applications is lacking in most undergraduate textbooks. Most often the topic of supercritical fluids is mentioned in passing as part of the larger topic of phase equilibria and one or two applications, such as their use in the decaffeination of coffee, are usually presented. In October 1988 a two-part series outlining the principles, instrumentation, and application of SFC appeared in this Journal (1). Since that time no articles covering the principles and applications of SFE have been published here. The purpose of this article is to present a general overview of the principles of SFE, to update the reader on some of the fundamental research currently being conducted within this field of study, and to make the reader aware of the wide variety of chemical and industrial applications of SFE.
stance reaches the supercritical state the physical properties (density, viscosity, diffusivity) of the fluid become intermediate between those of the liquid and gas phases of the substance. The fluid’s solvating powers are most like a liquid’s, whereas its diffusivity and viscosity are gas-like (2). An excellent overview of the physical chemistry of supercritical fluids can be found in an article by Lira (3). The properties of SFs give them the ability to dissolve nonpolar solids, making them extremely useful for chemistry, especially when chemical separations are required. Although SFs are not the super solvents they were originally believed to be (many organic compounds are more soluble in liquid solvents than in SFs), they possess other characteristics that make them, in many cases, more “workable” than liquid solvents or solvent systems. The properties of an SF can easily be changed by changing the temperature and pressure applied to the fluid. Changing the density of a supercritical fluid allows the fluid’s solvating power to mimic a wide variety of liquid solvents. This is illustrated in Table 1. As can be seen, CO2, by variation of temperature and pressure, can assume the equivalent solvent properties of a range
Table 1. Solubility Parameters of Some Common Solvents vs. Those of Some Substances Used as Supercritical Fluids (4 )
Supercritical Fluids: The Ideal Solvent for Today’s Extractions?
Solubility Parametera
Liquids
Creating an SF is a fairly simple process: use heat and pressure to move a substance beyond its critical point and you’ve got a supercritical “fluid”. This is illustrated schematically in Figure 1, where the four phases of a substance (solid, liquid, gas, and SF) are shown at varying temperatures (T) and pressures (P). When a sub-
Methanol
Supercritical Fluids
14-15 13-14
NH3
Ethanol
12-13
NO2
2-Propanol
11-12
H2S, HBr, HCl
Pyridine, Dioxane
10-11
N2O/CH3SH, Cl2, CH3Cl
Benzene, Chloroform, Ethyl Acetate, Acetone
9-10
Cyclohexane, Carbon Tetrachloride, Toluene
8-9
Ethyl Ether, Pentane
7-8
b
CH2CHF/(CH3)2NH
c
CH3CHF2, CHF3, Freon, C2H4
d
CCl2F2, CClF3, SF6, CO
a
The solubility parameter is calculated based on the following equation:
ρ(∆ Hv – RT ) ∆ Ev = V M where ∆ Ev is the vaporization energy, V the molar volume, ρ the density, ∆ Hv the heat of vaporization, M the molecular weight of the solute, R the gas constant, and T temperature. b The solubility parameter of CO2 at a density of 1.23 g/cm3 corresponds to that of the liquid solvents in this range. c CO2 at a density of 0.9 g/cm3 has a solubility parameter corresponding to liquids in this range. d CO 2 at a density of 0.6 g/cm3 can replace solvents in this range. δ=
Figure 1. Phase diagram for a pure substance. Solid (S), liquid (L), and vapor (V) phases are shown. Supercritical region is denoted by SF. Critical temperature (T c) and pressure (Pc ) are represented by dotted line.
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Chemistry Everyday for Everyone
of conventional solvents, from pentane to pyridine. This range notably includes solvents such as benzene, toluene, carbon tetrachloride, and other chlorinated liquids that are now finding limited use in chemical processing due to disposal and emission problems. This table also indicates the existence of the great variety of supercritical fluids available to replace conventional liquid solvents. The solubility parameter used in the table is a numerical measure of a solvent’s ability to dissolve a given solute (4). The solvent strength of supercritical fluids has most often been quantified by studying the solvatochromic shifts in the UV-vis absorption bands of organic dyes. These absorption bands shift with changing density of the SF. The precise location of these absorption bands in an SF at a particular density are compared to the position of the absorption bands of the dye in a conventional solvent. When a match is made, the SF is said to have the same solvent strength as the conventional solvent (5). In addition to being able to change the solvent strength of an SF through temperature and pressure programming there are additional benefits to using SFs as solvents. Extractions carried out in SFs are fast compared to those done using more conventional liquid systems. Liquid solvents have solute diffusivities an order of magnitude lower (10-5 vs. 10 -4 cm2/s) and viscosities an order of magnitude higher (10-3 vs. 10-4 N s/m2) than SFs. This causes liquids to have poor mass transfer qualities compared to SFs and results in longer extraction times when using liquids than when using SFs (6). This is particularly relevant to extractions from solid materials where solutes are within the porous matrix of the sample. In such circumstances, mass transport and low viscosity are critical to efficient extractions. Because the substances used in SF applications are generally inert gases at reduced temperatures and pressures, their use is considered to be more environmentally friendly than the use of organic solvents. As we move into the next century where clean processes will be both desired and mandated, the use of SFE technology will undoubtedly become more commonplace. Much of the research currently being conducted using SFE centers on the reduction of organic- and aqueous-based waste streams from industrial processes. Which Substances Make the Best Supercritical Fluids? Although supercritical CO2 has been the choice for most SFE studies, there are many substances which are used as supercritical solvents. This list includes ammonia, argon, freon, propane, xenon, and water. Table 2 shows the critical temperatures and pressures of some selected substances (7). The choice of substance to be used as a supercritical solvent is influenced by the polarity of the target analyte. Practical considerations, such as the temperature and pressure required to push a substance into its critical region, are also important in the choice of an SF. The relatively low critical temperature and pressure of CO2, coupled with its wide availability and low cost, toxicity, and reactivity, have made it the substance of choice for a variety of extractive processes. Supercritical CO2 is used for extracting nonpolar and slightly polar species such as alkanes, terpenes, aldehydes, esters, alcohols, and fats. Utilizing changes in temperature and pressure, the specific solvation properties of supercritical CO2 can
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Table 2. Supercritical Parameters for Selected Substances (2 ) Critical Temperature Critical Pressure Substance (°C) (atm) CO2
31.3
72.9
N2O
36.5
72.5
NH3
132.5
112.5
Xe
16.6
58.4
Ar
150.9
48.0
H2O
374.1
217.7
CCl2F2
111.8
40.7
be tuned to particular solutes, as illustrated in Table 1. It is not effective for polar compounds. However, the addition of modifiers to increase the polarity of supercritical CO2 has resulted in its ability to solvate species that are slightly polar. The addition of other compounds that are themselves soluble in the supercritical CO2 has led to some very interesting reaction chemistry and related applications. The Role of Modifiers in Supercritical CO2 The solubility of both polar and nonpolar solids in an SF may be enhanced through the use of a modifier. Modifiers are added to the SF in low concentrations (5% or less on a v/v basis) and are themselves either polar (acetone, methanol) or nonpolar (propane, octane). CO2 has a small polarizability and no dipole moment, so additives increase the polarizability of the supercritical CO2. Modifiers have been shown to increase the solubility of a solute in the SF by an order of magnitude (8). While it is known that the polarizability of CO2 is affected by the modifier, its interaction with the solute is still a matter of investigation. Methanol and acetone have been the modifiers most often studied. Methanol can act as either a Lewis acid or Lewis base. During SFE the methanol may interact with functional groups on the solute or it may only be involved in solvation sphere formation. Solvent sphere formation seems to be more a function of methanol concentration rather than of its ability to gain or lose electron density (9). In addition to the above considerations, acid–base interactions may occur between the CO2 and aqueous systems in contact with the solute. The pH of water in contact with supercritical CO2 may have a crucial effect upon extraction processes. In the past, its actual value has been estimated using Henry’s law calculations. Recent work has shown that when supercritical CO2 is in contact with water the pH falls to a fairly low value: 2.8– 2.9. This fact, coupled with the fact that most samples will contain some amount of water, would indicate that acid–base chemistry in supercritical CO2 is a very important consideration (10). Instrumentation Used in SFE This section will cover basic instrumentation used for bench-top SFE. The instrumentation used in industrial processing will be discussed in the methodology section. There is actually very little instrumentation involved in carrying out an extraction with an SF. A simple schematic of such a system is shown in Figure 2. The most expensive piece of required extractor equipment is the pump. The pump is initially filled with the gas of
Journal of Chemical Education • Vol. 73 No. 12 December 1996
Chemistry Everday for Everyone
Figure 2. Schematic of the instrumentation required for SFE.
choice that has been cooled to its liquid state; it then increases the pressure on the liquid so that the gas is above its critical pressure (which for CO2 is 72.9 atm). The substance, now in a high-pressure liquid state, is pumped through tubing to an oven where the substance is heated above its critical temperature (this is 31.3 °C for CO2). Inside the oven is a cell of known volume, which contains the solute. The solute and the SF are either allowed to interact for a given amount of time (called a static extraction) or the SF is allowed to flow continuously through the extractor cell and over the solute (a dynamic extraction). Extraction methods sometimes call for a combination of both modes of extraction. Once the SF and its solubilized solute have passed out of the extractor cell, a means of separating the solute from the SF is required. This is usually achieved by reducing the pressure, which decreases the SF density and returns the supercritical substance to the gas phase. The solvating properties of the fluid/gas are greatly decreased and the solute precipitates. In practice, this step is achieved by the restrictor, as illustrated in Figure 2. Physically, the restrictor is a piece of stainless steel or fused silica capillary with a very small-bore i.d. (something on the order of 30–50 µm). Because the restrictor is essentially open to atmospheric pressure on its free end, it allows for depressurization of the SF with a concomitant release of the dissolved solute. The solute is often trapped in a solvent in which the restrictor has been submerged, or a method of adsorbing or layering of the trapped solute into a thin film can be used. The trapping method chosen will depend on the application under study.
Hag AG Corporation, which began to build the first industrial scale decaffeination plant in 1976. The plant began production in 1978. This was quickly followed by construction of a hops extraction plant in Munchester in 1982 and then a tea decaffeination plant a few years later (both built by the SKW company) (13). The development of industrial processes using SFE in Europe greatly outpaced such development in the United States. From the mid 1970s until 1985, U.S. companies were studying the use of the technique but didn’t act on it until 1988, when Maxwell House opened its decaffeination plant in Houston, Texas. John Haas, Inc., installed an SF CO2 extraction plant for hops in Yakima, Washington, in l990. Once again, the motivation for turning to an extraction process other than one based on organic solvents had come from changes in government regulations. The U.S. Environmental Protection Agency (EPA) was funding analytical methodology development on environmental sample extraction and workup using supercritical CO2. The increasingly stringent scrutiny of many common industrial solvents coupled with stronger pollution controls and greater demands on the purity of consumer materials have all combined to move SFE from the laboratory to very large-scale production within the past eight years in the United States (14).
Industrial Uses of SFE from 1988 to the Present Table 3, adapted from Krukonis, Brunner, and Perrut, outlines some of the current industrial uses of SFE in the United States, Europe, and Asia. All of these processing plants utilize supercritical CO2 as the solvent and all of them have some means of recovering and recycling the CO 2. The Hag AG decaffeination plant in Germany processes over 50,000,000 kg/yr of coffee, while the Maxwell House plant in Texas has a throughput of over 25,000,000 kg/yr. The Maxwell House plant employs a semicontinuous flow of coffee beans through its extraction vessels without ever having to depressurize. Because the flow of carbon dioxide is never stopped and the cof-
Table 3. Industrial Uses of SFE Year
Operator
1982
SKW/Trotsberg
Materials Processed Hops
Extraction Methodologies Based on Supercritical Fluids
1984
Fuji Flavor Co. Barth and Co. Natural Care Byproducts
Tobacco Hops Hops, Red pepper
History of the Technique Although the first literature report on supercritical phenomena in fluids appeared in 1822 (11), it was not until the 1950s that application of the phenomena to industrial processes began to be considered. Some of the first investigations into applying the concept of SFE to industry were carried out by workers at the Max Planck Institute for Kohlenforschung. These investigators studied the feasibility of using SFE in the foods, petroleum, and chemical industries. It was Zosel and coworkers from the Max Planck Institute who were the first to characterize the use of supercritical CO2 as a solvent for caffeine (12). The prior use of methylene chloride had been prohibited by the Ministry of Commerce, providing the motivation for finding an alternative solvent. The business division of the Max Planck Institute licensed the rights for supercritical CO2 decaffeination of coffee to
1986
SKW/Trotsberg Fuji Flavor Co. CEA
Hops Tobacco Aromas, Pharmaceuticals
1987
Barth and Co. Messer Griesheim
Hops Various
1988
Nippon Takeda CAL–Pfizer
Tobacco Acetone residue from antibiotics Aromas
1989
Clean Harbors Ensco, Inc.
Waste waters Solid wastes
1990
Jacobs Suchard Raps and Co. Pitt–Des Moines
Coffee Spices Hops
1991
Texaco
Refinery wastes
1993
Agrisana Bioland U.S. Air Force
Pharmaceuticals from botanicals Bone Aircraft gyrosopic components
1994
AT&T
Fiber optics rods
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Chemistry Everyday for Everyone
fee containing the lowest percentage of caffeine is always in contact with the freshest CO2, extraction efficiency for this system is very high (15). The Bioland Company, as listed in Table 3, uses supercritical CO2 to remove the lipids from bones (femur condyles and femoral heads from the meat processing industry), which are then further processed to remove protein. The fat- and protein-free bone is then used in orthopedic surgeries as grafting material (16). Another interesting use of supercritical CO2 as a solvent is its use in the cleaning of the intricate components of guidance systems by the U.S. Air Force. The parts had previously been cleaned by a number of methods including the use of chlorofluorocarbons (CFCs), but with the phase-out of these compounds another solvent system had to be found. Again, the increasing concern for limiting environmental releases of toxic compounds has driven the development of another supercritical process (17). AT&T has recently installed a system for removing traces of oil from long quartz rods that are to be used in the production of fibers for fiber optic transmissions. After the rods are formed their optical properties are measured using an index matching oil, which must then be removed before the rods are drawn into fibers. Again, CFCs were originally used for this extraction but are currently being phased out (18). The Phillip Morris tobacco denicotinization plant utilizes extraction vessels 5 m in length for its batch process. Moistened tobacco is dropped into the vessel, then the vessel is sealed and heated to above the critical temperature of CO2. The flow of CO2 over the tobacco removes the nicotine, which is then recovered by isobarically passing the SF and its contents over an activated carbon bed contained in a downstream vessel similar to the one that contains the tobacco. The CO2 is recycled back to the extractor and continues this cycle until the nicotine has been quantitatively removed. Upon depressurization, the CO2 is recycled again through another batch (Phillip Morris Co., personal communication, 1995). In addition to the applications outlined in Table 3, other processes that seem to be both technically and economically viable include the recrystallization of pharmaceuticals, the fractionation of oils and polymers, the extraction of residual solvents and monomers from polymers, polymerization in SFs, treatment of liquid and solid wastes, enzyme reactions in SFs, concentration of fish oils, extraction of cholesterol from butter, lard, tallow and eggs, and the removal of fats from snack foods such as potato chips. There are also several other “not-solely extractive” processes being developed by various sectors of industry or whose resultant products are being sought by industry. These include the use of so-called “antisolvents” in which a solid polymer is dissolved in a conventional solvent and then subsequently precipitated by injecting the solution into an SF. The precipitation occurs because the solvent is miscible with the supercritical CO2 while the polymer is insoluble in it. Furthermore, the morphology of the polymer can be controlled by controlling the kinetics of the precipitation through alteration of the pressure of the CO2 (19).
from universities, institutes, and other government organizations. Thus, it should come as no surprise that a great deal of research is currently being conducted within the field of SFE. While some of the research is devoted to discovering more about the physical behavior of supercritical fluids during the extraction process, a good deal of it focuses on further applications of the process to real-world problems. Much of this research deals with waste management and environmental cleanup. Extraction of Metals Using Supercritical Fluids Most of the processes utilizing supercritical CO2 mentioned so far in this report have had as their goal the removal of some unwanted organic compound of fairly low polarity from a sample matrix. But researchers in the field of SFE are trying to go a step further in their thinking by utilizing the same strategies employed in the SFE of nonpolar organic compounds to either dissolve a metal ion from its matrix or to selectively separate metal ions one from another in a mixture. Such a technique would require that the SF contain some type of complexing agent that would not only be soluble in the SF but would also provide charge neutralization for the metal species in question to allow its dissolution into the SF. Finding such an agent was originally the thorn in the side of the researchers attempting this type of extraction. When researchers were beginning to question the possibility of using an SF for metal extraction, supercritical CO2 seemed to be the best candidate for the SF because of those properties mentioned above. Many of the commonly used ligands for metal ions are barely soluble in supercritical CO2. It was found that the fluorination of ligands can enhance their solubilities in supercritical CO2 (20). The solubilities of some fluorinated metal chelates in supercritical CO2 are 2–3 orders of magnitude higher than the nonfluorinated analogues (Table 4). With the ability to dissolve a ligand in an SF has come the increased effort by researchers to optimize the removal of metal ions from a sample matrix. One of the leading investigators in this field has documented the extraction of transition metals (including toxic metals such as mercury), lanthanides, and actinides from both Table 4. Solubilities of Fluorinated (FDDC) and NonFluorinated (DDC) Metal Diethyldithiocarbamates in Supercritical CO2 at 100 atm and 50 °C (20 )
Na(FDDC)
Solubility (mol/L) (4.7 ± 0.3) × 10-4
Na(DDC)
(1.5 ± 0.1) × 10-4
Cu(FDDC)2
(9.1 ± 0.3) × 10-4
Cu(DDC)2
(1.1 ± 0.2) × 10-4
Ni(FDDC)2
(7.2 ± 1.0) × 10-4
Metal Chelate
Ni(DDC)2
(8.5 ± 1.0) × 10-7
Co(FDDC)3
(8.0 ± 0.6) × 10-4
Co(DDC)3
(2.4 ± 0.4) × 10-6
Current Research Using SFE
Bi(FCCD)3
1/2 compared to the signals observed in conventional solvents (36). Using an SF as a solvent for NMR will also allow investigators to study some compounds that are otherwise too reactive with conventional NMR solvents (37). The Future What does the future hold for SFE and its applications? The companies that have manufactured supercritical processing plants, especially the smaller, skidmounted models, are receiving inquiries from industries worldwide. Much of the demand for the use of SFE will come from consumers who may be willing to pay a little extra for products that were produced without the use of organic solvents. A most recent addition to consumer fare is the NuEgg produced by the Nutrasweet Company. Extraction of egg yolk has resulted in a product in which 80% of the fat and 95% of the cholesterol have been removed (38). The future of SFE in research looks bright. With so many new applications under consideration and funding available for any technology that promises to be clean, there will be increased research activity for many years into the next century. Applications such as the use of SFE in the remediation of nuclear waste sites and its use in the separation and cleanup of toxic metals seems feasible. The development of techniques for directly coupling SFE with methods of analysis such as GC, SFC, and IR will continue to be a focus for analytical chemists. Materials scientists will continue to look into the use of SFE for synthesis of new materials and production of fine powders for catalysis and thin films for the electronics industry. Engineering research will continue to work on better pump designs and methods for utiliz-
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ing the process in a more continuous mode. Batch processing is deemed to be less efficient than a continuous feed process. It seems that SFE is now over the hump and that it is rapidly developing into the extraction method of choice for the 21st century. Note 1. NGS is on leave from Company Research Lab, British Nuclear Fuels plc (BNFL), Salwick Preston PR4 OXJ, U.K.
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