Supercritical CO2 Extraction
Professor Jeff Sczechowski
Civil and Environmental Engineering Department
Cal Poly, San Luis Obispo, CA
March 7, 1997
ENVE 436-01 Hazardous Waste Management
A fluid becomes supercritical when compressed to a pressure and elevated to a temperature greater than that of its critical point (see figure 1). Although a supercritical fluid (SCF) is at single phase, it exhibits properties of both a liquid and a gas. A SCF has a relatively high liquid-like density. Solubility increases with density and pressure, thus, SCFs have a high absorption capacity (at high pressures, solubility increases with temperature as well). The gas-like properties of high diffusivity and low viscosity allow for high mass transfer rates between a solute and a SCF. Fluids in their supercritical states, having a high rate of absorbtion, are widely used in organic contaminant extraction from soil, water and precipitates.
The process of supercritical fluid extraction, as depicted in Figure 2, is fairly simple. When a fluid is released from its storage vessel, it travels through a compressor and heat exchanger, where the pressure and temperature are increased to the compound's supercritical conditions. A continuous stream of the SCF is supplied to the cleaning vessel where it absorbs the contaminant. The solvent and solute stream travel to the expansion vessel. Here, the reduced pressure decreases the solubility of the solute and the two components separate. The contaminant is collected and the extracting fluid is recycled back to the storage tank to begin the process again. Alternatively, the expansion vessel can be replaced with a heat exchanger where a temperature reduction would reduce the solubility and separate the components.
Replacing conventional organic solvents with SCFs in extraction procedures is a major advancement in today's pollution prevention programs. Supercritical fluid extraction allows for waste separation and minimization, as well as solvent recycling. Other advantages of supercritical extraction include high efficiency, high extraction rates and more selectivity.
Carbon dioxide (CO2) is the most beneficial SCF used in extraction. Its non-toxic and non-combustible properties make it environmentally friendly. Supercritical CO2 has a higher density (thus solubility) and lower critical parameters than most of the other SCFs. With a critical temperature of 31°C and critical pressure of 73 atm, supercritical CO2 extraction energy costs are lower than those of other fluids (water's critical temperature and pressure are 374°C and 218.4 atm). CO2 is readily available in high purity and is therefore, inexpensive to purchase. Supercritical CO2 is the most popular and inexpensive solvent used in industry today.
As discussed below, the application of supercritical CO2 in the food industry is widely developed for extraction of organics. Recent new technologies are emerging for the use of supercritical CO2 in the extraction of metals and non-organics. Supercritical CO2's uses continue to be explored and expanded due to its many benefits.
Food Industry Applications
The food and beverage industry was the first to make commercial use of supercritical carbon dioxide extraction. Three examples of this are as follows:
Research continues to be done in the extraction of such things as cholesterol from butter and the extraction of aroma and flavor oils from various spices.
- Caffeine extraction from coffee beans - As early a 1974, European companies were extracting caffeine using SCF-CO2.
- Fat extraction from processed snack foods
- Extraction of flavor oils from hops for the brewing industry
Coffee decaffeination stands out as the most successful use thus far of SCF-CO2 extraction. "…(I)t is the first example of a supercritical fluid process that has reached the commercial processing level and whose primary step is, indeed, supercritical extraction (McHugh (1986) 185)." The use of SCF-CO2 replaced dichloromethane, a solvent that has been proven to cause cancer, and was previously used to remove caffeine from coffee beans. In November 1991, OSHA "proposed lowering from 500 ppm to 25 ppm the permitted hourly industrial exposure to methylene chloride (dichloromethane) … (Chemical & Engineering News vol. 69 8)." The three advantage to using SCF-CO2 instead of the traditional solvent methods are:
Dichloromethane extraction is a carcinogenic process.
SCF-CO2 extracts caffeine only in green coffee beans and does not remove the
precious aroma oils.
No post-decaffeination treatment is needed to remove the solvent from the beans.
The actual decaffeination process using SCF-CO2 is a simple one. Basically, green coffee beans are saturated with water and exposed to humidified SCF-CO2. "Saturating the raw beans with water and humidifying the supercritical CO2 are essential if high extraction rates are to be realized (AIChE Journal 762)." "Extraction rates increase at higher temperatures and pressures due to the increased value of the partition coefficient for caffeine between supercritical carbon dioxide and water (AIChE Journal 763)." An efficiency of up to 95% can be expected with the use of SCF-CO2. It should be noted that once the SCF-CO2 has passed through the bed of beans and stripped the caffeine, it is depressurized. However, the carbon dioxide must be either water stripped or passed through a carbon bed to remove all of the caffeine. This is the only disadvantage to the SCF-CO2 process as opposed to the traditional solvent method.
Soon after recognizing that supercritical carbon dioxide had a future as a solvent, scientists began seeking ways to get a larger variety of molecules (contaminants) to mingle with the CO2. However, since SCF-CO2 is nonpolar, many contaminants will not stick to it. Thus, to find a CO2-loving surfactant (a surface active solution) with one end adhering to CO2 and the other end to the contaminant of concern, creative chemical research was needed.
Researchers looking for solutions to the surfactant solution have recently found that by using a co-solvent with the CO2, contaminants can be easily extracted. Currently, fluorine has been found to be the most effective co-solvent. One research team (DeSimone's group at Oak Ridge National Laboratory in Tennessee) has linked a fluorinated CO2-loving compound to the removal of polystyrene. The team tested the fluorocarbon compound by dumping it into a CO2 filled vat containing a plate coated with polystyrene pieces. Alone, polystyrene won't dissolve in SF-CO2, but with the help of fluorine, researchers expected the plate to be cleaned. They found that the surfactant's polystyrene loving ends of the molecules picked up the polystyrene from the plate. The polystyrene ends then slipped into a ball configuration (a micelle) with the CO2 loving ends sticking out and polystyrene was trapped in the middle: the plate became clean. The flourocarbon surfactant connected the polystyrene with the CO2 and when later depressurized, the solvent and contaminant separate. The CO2 is recycled for other uses. An advantage of dissolving fluorocarbon polymers in the fluid is that the critical point is lowered, making it supercritical at lower economically viable pressures. The team is currently creating other surfactants for compounds other than polystyrene.
A group of researchers at the University of Idaho, Moscow are seeing supercritical CO2 as an ideal way to solvate toxic heavy metals. Toxic metals such as mercury and plutonium are a major environmental problem and are currently treated with complicated procedures that generate voluminous, sometimes toxic, waste. By itself, the nonpolar supercritical CO2 is almost useless for solvating positively charged heavy-metal ions. However, researchers have discovered that metals can be solvated if they are neutralized by chelating agents, and furthermore, that the solvency increases dramatically when the chelating agents are fluorinated. Researchers have found that the solubility of metal complexes in fluorinated mixtures "is always two to three orders of magnitude higher than nonflourinated complexes."
The mechanisms by which the highly electronegative fluorinated groups combine with supercritical CO2 are leading researchers to believe radioactive elements, thornium and uranimum, can also be extracted from nitric acid solutions or from solid materials. Previously, radioactive wastes had to be dissolved in acid, extracted into an organic solvent, and then it became a waste. In addition to producing a waste, solvents left traces in the treated solids.
Industry is still using many hazardous organic solvents each year, because nonpolar SCF-CO2 can not dissolve many substances on its own. Once researchers develop co-solvents to cure this problem, SCF-CO2 will become the ideal environmentally and economically friendly solvent. But in general, while fluorine has been found to expand the use of supercritical carbon dioxide for extraction, it is also costly. Though fluorinated compounds may likely be too expensive to be used on a large scale, researchers are hoping that once developed, the surfactants can be mass produced much more inexpensively.
Engineering practices that require traditional solvents are no longer the only option. Through new focuses, such as pollution prevention, solvent use is being steadily replaced with more environmentally friendly methods. Supercritical carbon dioxide offers many advantages over traditional solvents. Extraction processes can achieve the same results without the negative side effects of large waste streams and work place hazards from working with carcinogenic materials. These advantages have been encompassed by the food and beverage industry and are making their way into more technical fields.
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