Du kanske gillar. Lifespan David Sinclair Inbunden. Inbunden Engelska, Spara som favorit. Skickas inom vardagar. Gas-expanded liquids and near-critical fluids are tunable solvent media for conducting chemical reactions, separations, and materials processing. In recent years, there has been an expanding body of fundamental and applied research in the use of these media, particularly for applications to sustainable processes based on the principles of green chemistry and engineering.
This volume spans research and development activities ranging from theoretical and experimental investigations of the fundamental science underlying the field to developmental aspects of the technology for commercial applications. 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 CO 2 were focused.
The relatively low critical temperature and low reactivity of CO 2 allow extraction without altering or damaging the product. Decaffeination of coffee was one of the first processes commercialized using supercritical CO 2. Prior to the use of supercritical CO 2 , 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 CO 2 is used. The extraction process. Extraction of food and natural products with supercritical CO 2 consists of two steps: first, the extraction of supercritical CO 2 soluble components from the feed; and, second, the separation of the components from supercritical CO 2.
The separation of supercritical CO 2 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 CO 2 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 CO 2 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.
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Supercritical CO 2 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 CO 2 extraction offer toll processing for these smaller volume products. The most important driving force for using supercritical CO 2 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. Food and Drug Admin. 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.
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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.
In fact, extraction of food and natural products using supercritical or liquid CO 2 can be considered a relatively mature CO 2 technology. A wide range of other applications for supercritical CO 2 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.
Solvents and sustainable chemistry
Supercritical CO 2 has been tested in a variety of industrially important reactions, such as alkylations, hydroformylations, and hydrogenation, as an alternative reaction medium. Relatively high rates of molecular diffusion and heat transfer are possible with a homogenous, supercritical-CO 2 reaction-medium. Limitations to the use of supercritical CO 2 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 CO 2 medium.
Polymer production and processing. Applications of supercritical CO 2 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.
A wide variety of applications
At moderate pressure, very few polymers, except for certain amorphous fluoropolymers and silicones, show any significant solubility in CO 2. Very high pressure is typically needed to dissolve polymers in supercritical CO 2. Its solvent power is weaker than that of n -alkanes. However, high degrees of swelling of the polymer by CO 2 can occur at significantly lower pressure.
Although many polymers have very low solubility in CO 2 , the solubility of CO 2 in polymers is typically high. This has led to the use of CO 2 as a plasticizer. One example of this application area is a process to produce fluoropolymers using supercritical CO 2 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 The pilot plant is capable of producing 1, metric tons per year m. Several grades of melt-processable fluoropolymers produced from this process became commercially available in 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 CO 2 in wafer processing.