Extraction capability of supercritical

Finally, supercritical fluid chromatography, in which a supercritical fluid is used as the mobile phase, was introduced by Klesper [164-166]. SFE directly coupled to SFC provides an extremely powerful analytical tool. The efficient, fast and selective extraction capabilities of supercritical fluids allows quantitative extraction and direct transfer of the selected solutes of interest to be accomplished to the column, often without the need for further sample treatment or cleanup. Extraction selectivity is usually achieved by adjusting the pressure of the supercritical fluid at constant temperature or, less often, by changing the temperature of the supercritical fluid at constant pressure. SFE coupled with packed column SFC has found... 

Table 6 compares the properties of supercritical CO2 and typical gas and liquid values.34 High diffusivity and low viscosity lend to the efficient extraction capabilities of supercritical fluids. The solvating power of... 

Another widely-used extraction technique is supercritical fluid extraction (SFE) with supercritical CO2. The extraction capability of supercritical CO2 is dependent upon the applied pressure. For pressures in the range of 40 bar, CO2 has an extraction polarity similar to that of hexane. For pressures near 400 bar, the extraction behavior is similar to that of dichloromethane. Thus, by using a pressure gradient, effective extraction of lipophilic compounds can be performed. Figure 7-10 shows a feasible arrangement for online SFE-NMR coupling [13], [14]. 

K Yokota, Y Hanakata, K Fujimoto. Supercritical phase Fischer-Tropsch synthesis reaction. 3. Extraction capability of supercritical fluids. Fuel 70 989-994, 1991. 

In closing, this paper was not intended to represent an exhaustive process development effort in flavors extraction from natural materials nor a development of the quantitative analytical capabilities of supercritical carbon dioxide. However, even though the examples and the conditions of extraction were somewhat arbitrary, they point out some of the interesting features of the pressure dependent dissolving power properties of supercritical fluids. They can be further refined by virtue of more narrow ranges and ratios of pressure and temperature to accomplish still more narrow separations. 

Barnabas et al. [51] have discussed an experimental design approach for the extraction of polyaromatic hydrocarbons from soil using supercritical carbon dioxide. They studied 16 different polyaromatic hydrocarbons using pure carbon dioxide and methanol modified carbon dioxide. The technique is capable of determining down to lOOmg kgy1 polyaromatic hydrocarbons in soils. 

This is true even if it is capable of dissolving the solutes. Supercritical solvents such as N2O and CHC1F2 are more efficient in extracting polar compounds, but their routine use is uncommon due to environmental concerns. The extraction efficiency of polar compounds by C02 can be improved by the addition of small quantities (1 to 10%) of polar organic solvents, referred to as modifiers. This is a common practice in SFE. Table 3.4 lists some common modifiers for supercritical CO 2. 

With the demonstration of supercritical fluid extraction, an obvious extension would be to extract or dissolve the compounds of interest into the supercritical fluid before analysis with SFC.(6) This would be analogous to the case in HPLC, where the mobile phase solvent is commonly used for dissolving the sample. The work described here will employ a system capable of extracting materials with a supercritical fluid and introducing a known volume of this extract onto the column for analysis via SFC. Detection of the separated materials will be by on-line UV spectroscopy and infrared spectrometry. The optimized SFE/SFC system has been used to study selected nonvolatile coal-derived products. The work reported here involved the aliphatic and aromatic hydrocarbon fractions from this residuum material. Residua at several times were taken from the reactor and examined which provided some insight into the effects of catalyst decay on the products produced in a pilot plant operation. 

The main conclusion to be drawn from the experimental data presented here is that fractionation of residuum through the use of a supercritical fluid system incorporating internal reflux produced by retrograde condensation results in sharper fractions than those obtained by ordinary supercritical extraction. The capability of the FDU to process coal-derived residuum in the... 

Some properties of supercritical fluids endow them with improved extraction capabilities relative to liquids. Their individual influence is discussed below. 

It can be argued that the first supercritical fluid extractions (SFE) were performed in 1879 when Flannay and Hogarth investigated the solvating capabilities of ethanol.28 However, it took roughly 100 years before supercritical fluids made any significant impact on industrial processes. The removal of caffeine from coffee beans was reported in the 1970s29 and led to... 

Methylation of 2,4-dichlorophenoxyacetic acid and 3,6-dichloro-2-methoxybenzoic acid in soil samples was examined using trimeth-ylphenylammonium hydroxide prior to supercritical extraction with C02 (Hawthorne et al. 1992). Although recoveries of these compounds were apparently limited by competition for the reagent of components of the soil matrix, this procedure possesses obvious advantages for polar analytes such as carboxylic acids and is capable of extension to other analytes. 

Supercritical fluids exhibit gas-like mass transfer rates and yet have liquid-like solvating capability. The high diffusivity and low viscosity of supercritical fluids enable them to penetrate and transport solutes from porous solid matrices. From this point of view, SFE is an ideal method to extract uranium and lanthanides from solid wastes. Carbon dioxide (CO2) is most frequently used in SFE because of its moderate critical pressure (Pc) nd temperature (Jc), inertness, low cost, and availability in pure form. Figure 1 illustrates moderate values of Pc and Tc compared with those of water. 

In the early stages of process development in the U.S., many "promises" were made about the capabilities of, and applications for, supercritical fluid extraction. When the widely-touted applications did not materialize, as exemplified by commercialized processes, for example, the "luster" of supercritical fluid solvents rather quickly faded. It is informative to review some of the trade and association journal articles that were published during this period because of their influence on many researchers and companies in their future investigations of supercritical fluid extraction. 

---