Supercritical fluid carbon dioxide

The same types of catalyst have been employed in 1-octene hydroformylation, but with the substrates and products being transported to and from the reaction zone dissolved in a supercritical fluid (carbon dioxide) [9], The activity of the catalyst is increased compared with liquid phase operation, probably because of the better mass transport properties of scC02 than of the liquid. This type of approach may well reduce heavies formation because of the low concentration of aldehyde in the system, but the heavies that do form are likely to be insoluble in scC02, so may precipitate on and foul the catalyst. The main problem with this process, however, is likely to be the use of high pressure, which is common to all processes where supercritical fluids are used (see Section 9.8). 

Ranalli A, Contento S, Lucera L, Pavone G, Di Giacomo G, Aloisio L, Di Gregorio C, Mucci A and Kourtikakis I. 2004. Characterization of carrot root oil arising from supercritical fluid carbon dioxide extraction. J Agric Food Chem 52(15) 4795-4801. 

Hopper M, King JW. 1991. Enhanced supercritical fluid carbon dioxide extraction of pesticides from foods using pelletized diatomaceous earth. J Assoc Off Anal Chem 74(4) 661-666. 

The process employs the supercritical fluid carbon dioxide as a solvent. When a compound (in this case carbon dioxide) is subjected to temperatures and pressures above its critical point (31°C, 7.4 MPa, respectively), it exhibits properties that differ from both the liquid and vapor phases. Polar bonding between molecules essentially stops. Some organic compounds that are normally insoluble become completely soluble (miscible in all proportions) in supercritical fluids. Supercritical carbon dioxide sustains combustion and oxidation reactions because it mixes well with oxygen and with nonpolar organic compounds. 

The supercritical fluid carbon dioxide, C02, is of particular interest This compound has a mild (31°C) critical temperature (Table 1) it is nonflammable, nontoxic, and, especially when used to replace freons and certain organic solvents, environmentally friendly. Moreover, it can be obtained from existing industrial processes without further contribution to the greenhouse effect (see Air pollution). Carbon dioxide is fairly miscible with a variety of organic solvents, and is readily recovered after processing owing to its high volatility. It is a small linear molecule and thus diffuses more quickly than... 

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. 

Analytical methods for monitoring the compounds were developed or modified to permit the quantification of all 23 compounds of interest. As noted earlier, the compounds were initially studied in small-scale extractions by groups. This approach assured minimal interferences in the analyses conducted during the initial supercritical fluid carbon dioxide extractions. Table II summarizes the data on the recovery of organics from aqueous samples containing the compounds of interest at concentration levels listed in Table I when the sample preparation techniques and analytical methods described were used. For each experimental run, blank and spiked aqueous samples were carried through the sample prepration and analytical finish steps to ensure accurate and reproducible results. Analyses of sodium, calcium, and lead content were also conducted on selected samples by using standard atomic ab-... 

By using data from the small-scale extractions of dichlorophenol as an example, the maximum theoretical amount that can be collected at —76 °C can be calculated to be 77. Actual experimental values show recovery to be about 62 for the three small-scale supercritical fluid carbon dioxide extractions of dichlorophenol. These data support the suggestion that the vapor pressure of the compound being trapped is an extremely critical physical constant when large volumes of C02 relative to the aqueous sample volume are being used for the extraction process. 

Additional treatment with supercritical fluid carbon dioxide would likely increase the extraction efficiency of the process but might present additional trapping (recovery) problems. 

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. 

Lemert, R.M., Johnston, K.P., "Chemical Complexing Agents for Enhanced Solubilities in Supercritical Fluid Carbon Dioxide," submitted to I EC Res., 1990. 

Figure 11 illustrates the parameter space defined by the equilateral triangle. The initial pressure and conditions for the 3 vertices of the pressure gradient/ temperature triangle were determined arbitrarily from the critical conditions of the supercritical fluid (carbon dioxide), the retention characteristics of nitroaromatic compounds, and the following criteria (i) the first analyte should not co-elute with the sample solvent and (ii) the retention factor of the last analyte should not exceed 30. 

Wang and Wai [43] showed that bioaccumulated mercury in plants can be recovered using a methanol-modified supercritical-fluid carbon dioxide containing a dichromate liquid. 

Berger and Wilson also reported the separation of 10 antidepressants (amitriptyline, imipramine, nortriptyline, desipramine, protripyline, bucliz-ine, benactyzine, hydroxyzine, perphenazine, and thioridazine) using a packed-column SFC with a tertiary mobile phase [37], A Lichrosphere cyanopropyl column with a mobile phase consisting of supercritical fluid carbon dioxide with 10% modifier (methanol with 0.5% isopropylamine) was used for the separation. It was noted that solutes did not elute without the addition of isopropylamine. Detection limits obtained were as low as 88 ppb for a 5-jul injection. 

Solvatochromic shift data have been obtained for phenol blue in supercritical fluid carbon dioxide both with and without a co-solvent over a wide range in temperature and pressure. At 45°C, SF CO2 must be compressed to a pressure of over 2 kbar in order to obtain a transition energy, E, and likewise a polarizability per unit volume which is comparable to that of liquid n-hexane. The E,j, data can be used to predict that the solvent effect on rate constants of certain reactions is extremely pronounced in the near critical region where the magnitude of the activation volume approaches several liters/mole. 

Nguyen, LJ., Anstee, M. and Evans, D.A. (1 998) Extraction and fractionation of spices using supercritical fluid carbon dioxide. Presented at The 5th International Symposium on Supercritical Fluids, Nice, France. 

Chandra, A. Nair, M.G. 1997. Supercritical fluid carbon dioxide extraction of a-and P-carotene from carrot (Daucus carota L.). Phytochem. Anal. 8 244-246. 

Motyl, K. M., Cleaning Studies of Metal Substrates Using Liquid/ Supercritical Fluid Carbon Dioxide, Rockwell International, Rocky Flats Plant, Golden, Colorado (1988)... 

---