Carbon dioxide, super critical

Lecture 15-18 solvents - water, liquid carbon dioxide, super-critical water, super-critical carbon dioxide, fluorous solvents, deep eutectics, ionic liquids Lecture 19 renewable feedstocks... 

A recent development in liquid-liquid extraction has been the use of supercritical fluids as the extraction-solvent. Carbon dioxide at high pressure is the most commonly used fluid. It is used in processes for the decaffeination of coffee and tea. The solvent can be recovered from the extract solution as a gas, by reducing the pressure. Super critical extraction processes are discussed by Humphrey and Keller (1997). 

A numerical example of the carbon dioxide supercritical cycle has been made by Feher (Feher, E.G., The super-critical thermodynamic power cycle. Energy Conversion, vol. 8, pp. 85-90, 1968). The reasons for the neglect of the supercritical cycle until now are not known. 

The most important extraction technique nowadays is simple solvent extraction. The traditional solvent for extraction was benzene, but this has been superseded by other solvents because of concern over the possible toxic effects of benzene on those working with it. Petroleum ether, acetone, hexane and ethyl acetate, together with various combinations of these, are typical solvents used for extraction. Recently, there has been a great deal of interest in the use of carbon dioxide as an extraction solvent. The process is normally referred to as super-critical carbon dioxide extraction but, in fact, the pressures employed are usually below the critical pressure and the extraction medium is sub-critical, liquid carbon dioxide. The pressure required to liquefy carbon dioxide at ambient temperature is still considerable and thus the necessary equipment is expensive. This is reflected in the cost of the oils produced, but carbon dioxide has the advantage that it is easily removed and there are no concerns about residual solvent levels. 

Super critical fluid extraction enlarges the range of solvents. Using super critical carbon dioxide has the advantage that the final extract is automatically concentrated to dryness. The polarity of supercritical carbon dioxide is rather low therefore, small amounts of methanol are added as modifier to increase the polarity of the extractant. The solvents also have a large penetrating power because of their low viscosity. The invention of ASE has reduced the interest in SFE due to the more versatile applicability and robustness of ASE. 

Bioconversion of Marine Phospholipid in Super Critical Carbon Dioxide to Produce Functional DHA-Enriched Lysophospholipid... 

Supercritical fluid chromatography, SFC a super critical fluid is produced when a gas, e.g. carbon dioxide is maintained above its critical pressure (73 atm) and temperature (3rC). The super critical fluid is used as the mobile phase with GC like capillary columns or HPLC reverse phase columns and a flame ionisation or flame photometric detector. 

One active area is in using super-critical fluids (SCFs) as a reaction medium. While the usefulness of SCFs as an extraction solvent, a cleaning solvent or in analytical methodologies has been well-established, the use of super-critical carbon dioxide as well as other SCF s is a far less explored area of research. There have been recent successes documented in the use of SCFs as a reaction medium for polymerization reactions (25), free-radical transformations (26), and in certain cat ytic transformations (27). 

Rinki, K., Tripathi, S., Dutta, P. K. etal., 2009. Direct chitosan scaffold formation via chitin whiskers by super-critical carbon dioxide method A green approach. Journal of Materials Chemistry, 19 8651-8655. 

Alternative mechanisms, including the combination of two molar equivalents of phenoxide with one mole of carbon dioxide, have been proposed, however spectroscopic and empirical observations along with ab initio calculations are consistent with the activation of CO2 via a phenylcarbonate salt such as 2. Recently developed modifications using super-critical 002 and ionic liquids, however, may involve alternative mechanistic pathways as such reaction conditions have not yet been carefully examined. 

Other interesting polymerizations include the use of metal-activated hydrogen peroxide to deliver low molecular weight pol5uners (83,84), continuous polymerization of water-soluble monomers in extruders (85), dry polymerization of acrylic acid in super critical carbon dioxide (86,87) and on a powder bed (88), and the use of sodium nitrate mediated aqueous pol5unerization to allow high solids (89). 

VDF polymerization technology based upon super critical or liquid carbon dioxide as polymerization media has been reported (57-62). This technology offers an advantage in the polymer isolation step where a clean dry polymer is produced simply by depressimzation. The residual monomeifs) and CO2 can be recycled back to the reactor. PVDF is not soluble in CO2 (58-60) and as a result, additional pol5uneric stabilizers are required to produce stable particles. Adequate CO2 density for pol5unerization requires pressure significantly higher (typically >100 bar) than a conventional emulsion polymerization. 

One variant, close to LC, that has been successfully interfaced to infrared spectroscopy, is supercritical fluid chromatography (SFC). In this case, one of the key mobile phases, super critical carbon dioxide, is virtually transparent for most of the mid-IR region. This may be operated directly on-line with a high-pressure liquid flow cell. 

All oil-contaminated solids are treated as hazardous and sent to sanitary landfills. Recently, super-critical extraction with carbon dioxide has been used with great success to remove oil from contaminated dirt. 

Postigo et al. developed novel UV-induced iodoperfluoroallg lation reactions of electron-rich and -deficient alkenes with C4F9I in water.Allyl benzoate derivatives in THF, 1,4-dioxane, or 2-propanol upon 305 nm irradiation afforded the solvent-adducts. A chiral saccharide ester of 1,4-naphthalenedicarboxylic acid acted as a sensitizer for the enantio-differentiating photocyclization of 5,5-diphenyl-4-penten-l-ol (174) to (175) in solvents and solvent mixtures, including near-critical/super-critical carbon dioxide. ... 

Ngo, T. T. V., Duchet-Rumeau, J., Whittaker, A. K., and Gerard, J. F. 2010. Processing of nanocomposite foams in super critical carbon dioxide. Part I Effect of surfactant. Polymer 51 3436-3444. 

There are known experiments on separation in super critical fluids. Beckman reported separation of PVC and PET, using carbon dioxide and sidphur esofluoride. The conditions under which experiment was conducted are objectively difficult and, at present, an apphcation of this method is not practical. 

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