Supercritical fluid carbon dioxide water

Isolation of Organic Compounds Present in Water at Low Concentrations Using Supercritical Fluid Carbon Dioxide... 

The use of supercritical fluid carbon dioxide to extract low levels of organic substances from water was investigated for 23 different compounds. In general, compounds that were volatile and/or not highly soluble in water were readily extracted under the conditions used. Compounds of higher water solubility did not show evidence of extraction. In addition, those materials that tended to precipitate or form more soluble species under acidic conditions were not extracted. 

On the basis of this work, it appears that the supercritical fluid carbon dioxide extraction of organic compounds present in water at low levels may be useful only in the case of volatile organic species. Even for these compounds, however, if efficient recovery of the compounds is desired and appropriate, certain limitations of existing trapping techniques remain to be overcome. 

In the patent of the Krupp company [60], ground paprika and oleoresin may be used as the raw material in the SFE process. The solvent can be supercritical fluid carbon dioxide, ethane, ethylene, or a mixture of the last two. As modifiers ethanol, acetone, water, and mixtures of these solvents were proposed. 

Other important physical chemical properties are polarity and dielectric constant. Water has a high dielectric constant (78.5 at STP), which would effectively mask ionic charges and lead to high solubility of ionic compounds. The dielectric constant of CO2 at 200 bar and 40°C is approximately 1.5, and CO2 is considered a very non polar solvent. As would be expected, polarity influences solubility for supercritical fluids. Carbon dioxide has a dipole moment of 0.0 Debye, while the value for NH3 is approximately 1.5. Therefore, C02 by itself is poorly suited for dissolving polar compounds. 

Supercritical fluid extraction (SFE) has been used mostly in environmental analysis [129,205] for the extraction of nonpolar organic pollutants from solid samples, for example, sediments. The technique has abo been used together with SPE dbks (silica Cis, polymeric, and ion exchanger disks) for the extraction of phenols from water samples followed by GC-MS [206]. After adsorbing the phenols onto the SPE disk, they are eluted with a supercritical fluid (carbon dioxide). 

Oil-in-oil emulsion systems display a relatively strong ER effect. Examples of such ER active emulsions are chlorinated paraffin/polydimethylsiloxane [11], castor oil/polydimethylsiloxane [13], urethane-modified polypropylene glycol/dimethylsiloxane [12] etc.. The ER effect in emulsions is attributed to the stretched droplets that Ibrm fibrillation chains along the direction of the electric field. This is a typical feature for any emulsion system in which the two liquids have a quite different dielectric constant and conductivity. Figure 17 shows the water droplet chains formed in a supercritical fluid carbon dioxide medium under a 60 Hz ac field of a very low field strength, Emax=IO V/mm [115]. A synergetic effect is observed in an system composing of polyanilines dispersed in a chlorinated paraffin/silicone oil emulsion [107],... 

Supercritical fluids Carbon dioxide forms a supercritical phase under relatively low temperatures and pressures and it therefore has been explored as a solvent for tbe Heck reaction. Following the reaction, the gas can be collected and recycled, allowing the design of true waste-free technology. The dielectric constant of supercritical CO2 is similar to pentane and therefore modified catalytic systems are often required to aid solubility.Unusually, one of the best catalytic precursors in supercritical CO2 is Pd(OCOF3)2, a strong electrophile and oxidant. Heck reactions in supercritical water have also been investigated. ... 

There have been a number of subsitutions of chemicals in recent years, many of them driven by environmental concerns and regulations resulting from those concerns. One of the greater of these has been the substitution of hydrochloro-fluorocarbons (HCFCs) and hydrofluorocarbons (MFCs) for chlorofluorocarbons (Freons or CFCs) driven by concerns over stratospheric ozone depletion. Substitutions of nonhalogenated solvents, supercritical fluid carbon dioxide, and even water with appropriate additives for chlorinated hydrocarbon solvents will continue as environmental concerns over these solvents increase. 

Bamberger, A.G. Sieder, and G. Maurer. 2000. High-pressure (vapor + liquid) equilibrium in binary mixtures of (carbon dioxide + water or acetic acid) at temperatures from 313 to 353 K. /. Supercrit. Fluids 17 97-110. 

Supercritical fluid extraction processes are particularly appropriate for the separation and isolation of biochemicals where thermal decomposition, chemical modification, and physiologically-active solvents are undesirable. Examples of these bioseparations include the extraction of oils from seeds using carbon dioxide (1), of nicotine from tobacco using carbon dioxide-water mixtures (2), and of caffeine from coffee beans again using carbon dioxide-water mixtures (3). 

Supercritical or near-critical fluids can be used both for extraction and chromatography. Many chemicals, primarily organic species, can be separated and analyzed using this approach [6], which is particularly useful in the food industry. Substances that are useful as supercritical fluids include carbon dioxide, water, ethane, ethene, propane, xenon, ammonia, nitrous oxide, and a fluoroform. Carbon dioxide is most commonly used, typically at a pressure near 100 bar. The required operating pressure ranges from about 43 bar for propane to 221 bar for water. Sometimes a solvent modifier is added (also called an entrainer or cosolvent), particularly when carbon dioxide is used. 

Because oxygen, carbon dioxide, methane, and other alkanes are completely miscible with dense supercritical water, combustion can occur in this fluid phase. Both flameless oxidation and flaming combustion can take place. This leads to an important application in the treatment of organic hazardous wastes. Nonpolar organic wastes such as polychlorinated biphenyls (PCBs) are miscible in all proportions in supercritical water and, in the presence of an oxidizer, react to produce primarily carbon dioxide, water, chloride salts, and other small molecules. The products can be selectively removed from solution by dropping the pressure or by cooling. Oxidation in supercritical water can transform more than 99.9 percent of hazardous organic materials into environmentally acceptable forms in just a few minutes. A supercritical water reactor is a closed system that has no emissions into the atmosphere, which is different from an incinerator. 

Wendland, M., Hasse, H., and Maurer, G., (1993) Multiphase high-pressure equilibria of carbon dioxide-water-isopropanol, J. Supercrit. Fluids 6, 211-222. 

Fluid-solid extraction, also called leaching, is used to separate a substance from a solid matrix by solution into a suitable solvent. The solvent is either a liquid, for example, hot water or an organic solvent, or a supercritical fluid, most notably water or carbon dioxide. 

For any substance in the supercritical state, that is, at a temperature above its critical temperature, the distinction between liquid and vapor (gas) is not relevant. The properties of a supercritical fluid, especially its solvent power, can be varied over a considerable range by adjusting the pressure. A number of substances whose critical temperatures or pressures are not inconveniently high have been used as solvents in this way. Typically, a desired product of a reaction, or a substance to be obtained from a natural source material, is extracted by the fluid at high pressure, and then cansed to precipitate simply by lowering the pressure. The solvent is recompressed, to reenter the cycle. Sahu (2003) lists 13 substanees that are in use as supercritical solvents. They include carbon dioxide, water, ammonia, and a number of lower hydrocarbons and chlorofluorocarbons. Water, at 647.3 K and 221.2 bar, has both the highest critical temperature and the highest critical pressure in the list, but is stiU in frequent use. 

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