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Comment PDF Separation Processes

Supercritical CO2: A Green Solvent

By Chemical Engineering |

Many reactions, extractions, separations and other operations in the chemical process industries (CPI) involve the use of organic solvents. In addition to handling and disposal issues, organic solvents can pose a number of environmental concerns, such as atmospheric and land toxicity. In many cases, conventional organic solvents are regulated as volatile organic compounds (VOCs). In addition, certain organic solvents are under restriction due to their ozone-layer-depletion potential.

Supercritical carbon dioxide is an attractive alternative in place of traditional organic solvents. CO2 is not considered a VOC. Although CO2 is a greenhouse gas, if it is withdrawn from the environment, used in a process, and then returned to the environment, it does not contribute to the greenhouse effect. There have been an increasing number of commercialized and potential applications for supercritical fluids. This article summarizes the fundamentals of supercritical CO2 properties and processing, and presents a number of current and potential applications.

Supercritical fluids

Above its critical values, a compound’s liquid-vapor phase boundary no longer exists and its fluid properties can be tuned by adjusting the pressure or temperature. Although supercritical fluid has liquid-like density, it exhibits gas-like diffusivity, surface tension and viscosity. Its gas-like viscosity results in high mass transfer. Its low surface tension and viscosity lead to greater penetration into porous solids. Because of its liquid-like density, a supercritical fluid’s solvent strength is comparable to that of a liquid.

The critical temperatures and pressures of materials vary quite significantly (Table 1). Generally, substances that are very polar at room temperature will have high critical temperatures since a large amount of energy is needed to overcome the polar attractive energy.

Table 1. Critical conditions for various materials
Critical temperature (C) Critical pressure (bar)
Ammonia 132.5 112.8
Benzene 289.0 48.9
Carbon dioxide 32.1 73.8
Cyclohexane 280.3 40.7
Ethane 32.2 48.8
Ethylene 9.3 50.4
Isopropanol 235.2 47.6
Propane 96.7 42.5
Propylene 91.9 46.2
Toluene 318.6 41.1
Water 374.2 220.5

At critical conditions, the molecular attraction in a supercritical fluid is counterbalanced by the kinetic energy. In this region, the fluid density and density-dependent properties are very sensitive to pressure and temperature changes. The solvent power of a supercritical fluid is approximately proportional to its density. Thus, solvent power can be modified by varying the temperature and pressure. Because their properties are a strong function of temperature and pressure, supercritical fluids are considered tunable solvents. In contrast, conventional liquid solvents require relatively large pressure changes to affect the density.

Supercritical CO2 properties

Unlike many organic solvents, supercritical CO2 is non-flammable. It is inert, non-toxic, has a relatively low cost and has moderate critical constants. Its solvation strength can be fine-tuned by adjusting the density of the fluid. CO2 leaves a lower amount of residue in products compared to conventional solvents, and it is available in relatively pure form and in large quantities.

CO2’s critical temperature (Tc; 32.1°C) is near ambient, making it an attractive solvent for temperature-sensitive materials. CO2’s critical pressure is 73.8 bar (Pc; 1,070 psi), as shown in its phase diagram (Figure 1).

Figure 1. The phase diagram for carbon dioxide shows
its supercritical region

CO2 as a solvent. Supercritical CO2 is a good solvent for many nonpolar, and a few polar, low-molecular-weight compounds. It is not a very good solvent for high-molecular-weight compounds and the majority of polar compounds. Uneconomically high process pressure may be required to solvate polar, inorganic or high-molecular-weight material in CO2. To increase the solubility of such compounds in supercritical CO2, small amounts of polar or non-polar co-solvents may be added. Highly CO2-soluble surfactants and CO2-phillic ligands have also been developed to improve the solubility of compounds in CO2.

Extractions using CO2

Currently, the widest application of supercritical CO2 is in extraction. Total CO2 consumption in supercritical extraction processes is estimated to have been approximately 15,000 to 25,000 ton/yr in 1994 and between 30,000 and 35,000 tons in 2002. Worldwide, over 100 facilities are estimated to use dense CO2 for extraction and purification. Large-scale commercial plants using supercritical CO2 extraction are found in the food industries (Table 2).

Table 2. Commercial-Scale Supercritical- CO2 Extraction Plants and Processes
Process Location Manufacturer
Coffee decaffeination Houston, Tex., U.S. Maximus Coffee Group LP (formerly General Foods)
Bremen, Germany Kaffe HAG AG
Bremen, Germany Hermsen
Poszillo, Italy SKW-Trostberg AG
Tea decaffeination Munchmuenster, Germany SKW-Trostberg AG
Fatty acids from spent barley Dusseldorf, Germany Marbery, GmbH
Vitamin E oil, phytosterol, fatty acid methyl ester, ginger oil Wuhan, Hubei, China Wuhan Kaidi Fine Chemical Industrial Co.
Nicotine extraction Hopewell, Va., U.S. Philip Morris
Natural insecticide/pesticide (Pyrethrum extract) High Wycombe, U.K. Agropharm
Hops extraction Wolnzach, Germany Hopfenextraktion, HVG
Yakina, Washington, U.S. Hops Extraction Corp. of America
Melbourne, Australia Carlton & United Beverages Ltd.
West Midlands, U.K. Botanix
Spices/flavors/aromas/ natural products/colors Munchmuenster, Germany SKW-Trostberg AG
Rehlingen, Germany Flavex Gmbh
Edmonton, Canada Norac Technologies
Tsukaba, Japan Ogawa Flavours and Fragrances
Milwaukee, Wisc., U.S. Sensient Technologies
Japan Kirin Food-Tech Co.

Conventional processes for extracting various components from food products have limitations regarding the solvent toxicity, flammability and wastefulness. This area is where early commercial applications of supercritical CO2 were focused. The relatively low critical temperature and low reactivity of CO2 allow extraction without altering or damaging the product. Decaffeination of coffee was one of the first processes commercialized using supercritical CO2. Prior to the use of supercritical CO2, several different solvents including methylene chloride, ethyl acetate, methyl acetate, ethylmethylketone and trichloroethane have been used for decaffeination. Extraction of hops during the beer brewing process is another area where CO2 is used.

The extraction process. Extraction of food and natural products with supercritical CO2 consists of two steps: first, the extraction of supercritical CO2 soluble components from the feed; and, second, the separation of the components from supercritical CO2. The separation of supercritical CO2 from the extract can be done by either modifying the thermodynamic conditions or by using an external agent. By modifying the thermodynamic conditions via changing the pressure or temperature, the solvent power of CO2 is changed. If an external agent is used, separation is carried out by adsorption or absorption. If separation occurs with an external agent, no significant pressure change occurs. Therefore, the operating cost that is associated with pressure requirement is lower. But, an additional step is required, the recovery of the extract from the external agent. In addition, higher losses of the extract can occur during the recovery step.

The feed material is typically ground solid material, which is fed to the extractor. Most commercial operations for supercritical fluid extraction are batch or semi-batch operation especially when the feed material is solid. For liquid feed material, the extraction occurs in a countercurrent column filled with random or structured packing material. However, for highly viscous liquid feed, the viscous liquid and supercritical fluid may be mixed and sprayed through a nozzle into the extractor vessel.

Recent extraction applications. There has been a great deal of interest in supercritical CO2 extraction beyond caffeine extraction, particularly in the preparation of high value products, such as flavors and fragrances, food supplements and nutraceuticals.

Specialty oils, for example, are high in value and typically low in volume. They have high concentrations of bioactive lipid components that are valued because of various possible health benefits. Herbal extracts from a wide range of botanical raw materials are used as ingredients to the food-and-flavor, nutraceuticals, pharmaceuticals and the cosmetics industries. Supercritical CO2 extraction can also be used to purify materials that are used for the production of medical devices.

These high-value-product applications typically involve small volumes. Flexible, medium-capacity plants for supercritical CO2 extraction offer toll processing for these smaller volume products. The most important driving force for using supercritical CO2 in this application area is that it is a generally recognized as safe (GRAS) solvent that leaves no traces in the product. GRAS is the U.S. Food and Drug Admin. (FDA) designation that a chemical or substance added to food is considered safe by experts, and, therefore, is exempt from the usual Federal Food, Drug, and Cosmetic Act (FFDCA) food additive requirements.

Multi-product plants. The high capital cost of building and operating a production plant utilizing supercritical extraction promotes expanding the use of the plant to a multi-product platform. Selective extraction of multiple products can be accomplished by modifying the solvent power of the supercritical fluid. The solvent power is modified by varying the extraction pressure or by adding a co-solvent.

Another method to extract multiple products is by sequential depressurization, in which all products are extracted simultaneously. The separation step is performed sequentially through a series of separator vessels. This process is referred to as fractional separation.

A wide variety of applications

Supercritical CO2’s use in extraction processes has grown fairly quickly. In fact, extraction of food and natural products using supercritical or liquid CO2 can be considered a relatively mature CO2 technology. A wide range of other applications for supercritical CO2 has been investigated, including chemical reactions, polymer production and processing, semiconductor processing, powder production, environmental and soil remediation and dry cleaning. Commercialization for these applications has, however, proceeded at a slower pace than for extraction. Several of these applications are highlighted here.

Chemical reactions. Supercritical CO2 has been tested in a variety of industrially important reactions, such as alkylations, hydroformylations, and hydrogenation, as an alternative reaction medium. The incentives to use supercritical CO2 as reaction medium can include (a) replacement of the conventional organic solvent with a “green” solvent, (b) improved chemistry such as reactivity and selectivity, (c) new chemistry, and (d) improved separation and recovery of products and catalysts. Relatively high rates of molecular diffusion and heat transfer are possible with a homogenous, supercritical-CO2 reaction-medium.

Limitations to the use of supercritical CO2 as a reaction medium include (a) poor solubility of polar and high-molecular-weight species, (b) no observed improvement in reaction chemistry in some cases, and (c) higher capital investment cost due to higher operating pressures. For reactions not limited by reactant-gas concentrations or other mass-transfer limitations, there is no improvement in reactivity observed when using a homogeneous, supercritical CO2 medium.

Polymer production and processing. Applications of supercritical CO2 in polymers include polymerization, polymer composite production, polymer blending, particle production, and microcellular foaming. Several applications, particularly those involving low pressures, have been successfully commercialized.

At moderate pressure, very few polymers, except for certain amorphous fluoropolymers and silicones, show any significant solubility in CO2. Very high pressure is typically needed to dissolve polymers in supercritical CO2. Its solvent power is weaker than that of n -alkanes. However, high degrees of swelling of the polymer by CO2 can occur at significantly lower pressure. Although many polymers have very low solubility in CO2, the solubility of CO2 in polymers is typically high. This has led to the use of CO2 as a plasticizer.

One example of this application area is a process to produce fluoropolymers using supercritical CO2 as the reaction medium that was developed by scientists at the University of North Carolina (Chapel Hill). DuPont has an exclusive license for this process until 2015. A $40-million pilot plant was built in 2000 to produce fluoropolymers using this process technology. The pilot plant is capable of producing 1,100 metric tons per year (m.t./yr) of fluoropolymers. Several grades of melt-processable fluoropolymers produced from this process became commercially available in 2002. However, no further progress to develop the process beyond the pilot plant phase to a large-scale industrial process has occurred.

Semiconductor processing. Currently, chip manufacturing involves many wet-chemical processes that use hydroxyl amines, mineral acids, elemental gases, organic solvents and large amounts of high purity water during chip fabrication. One potential application is the use of supercritical CO2 in wafer processing. The low viscosity and surface tension of supercritical CO2 allow for efficient cleaning of small feature sizes, which is of great importance with the continued miniaturization of integrated circuits. However, the main obstacle to the use of supercritical CO2 in semiconductor cleaning is the high cost.

Powder production. One promising application for supercritical CO2 is the production of micro- and nano-scale particles. The pharmaceutical industry currently uses supercritical CO2 mainly to control the powder particle size of products during synthesis. In the 1990s, a U.K.-based company, Bradford Particle Design (now Nektar), developed the Solution Enhanced Dispersion by Supercritical Fluids (SEDS) system to control powder formation from a diverse range of chemicals, including inorganic and organic substances, polymers, peptides and proteins. The use of supercritical CO2 for micronization of pharmaceutical compounds has several potential advantages over conventional techniques such as spray drying, jet milling and grinding. These advantages include minimum product contamination, reduced waste streams, suitability for the processing of thermally, shock or chemically sensitive compounds and the possibility of producing particles with narrow size distribution in a single-step operation.

Edited by Dorothy Lozowski

Acknowledgement

This article has been excerpted by Dorothy Lozowski from a 223-page report by Susan Bell of SRI Consulting. For details on how to order the full report, contact the author using the contact information on p. 15.




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