Super-critical solvent

The gas concentration (partial pressure) at temperatures above critical can act as a super critical solvent. Rubbers in this environment are subjected to high swells leading to subsequent extraction of plasticizers, low molecular weight polymers etc. 

The ROSE process is a solvent deasphalting process with minimum energy consumption using a super-critical solvent recovery system and the process is of value in obtaining oils for further processing. (Gearhart, 1980 Low et al., 1995, Hydrocarbon Processing, 1996 Northrup and Sloan, 1996). 

ROSE process a solvent deasphalting process (q.v.) that uses a super-critical solvent recovery system to obtain high-quality oils from heavy feedstocks (q.v.) for further processing. 

A discussion of energy costs and capital costs for possible extraction processes using a marginally subcritical (liquid) solvent and for a super-critical solvent is given below. 

The binodal and spinodals as a function of the solvent weight fraction are presented in panel (a) of Fig. 9, where we have used the ratio of the molar masses of hexadecane and carbon dioxide. This representation is more convenient for large molecular masses, because the molar faction x of the coexisting phases is very close to unity. For long polymers and pressures far below the triple pressure, the polymer-rich phase at saturation contains only a few weight-percent of the super-critical solvent, and this amount decreases upon increasing molecular mass. Moreover, both binodals and spinodals depend only on mass fraction, but become fairly independent from pressure for large N. 

If the mother phase is a dense polymer-rich liquid, its compressibility will be small and the term 4> PpI I p will be large compared to derivatives of the chemical potential ps of the super-critical solvent with respect to the densities. Hence, 4>Soai Ps 4>Poatdpp. Using the Gibbs-Duhem relation, we then find ... 

This example illustrates the similarity and difference of using an accurate equation of state instead of the simpler Flory— Huggins theory. The latter almost completely ignores the solvent, apart from its effect on the solute. Therefore, one does not know what the pressure of the system is, and there is no distinction between sub-or super-critical solvents. Only the requirement that it should behave as an incompressible fluid is built in. By using an equation of state of the kind that allows to include the solvent explicitely, one is able to accurately locate the behavior observed in the context of the phase diagram of the solvent. This has allowed us to suggest the existence of a 0 point at unexpected conditions. 

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). 

Yao et al. (1994) extracted and quantified capsaicin and dihydrocapsaicin using SCF-C02 and organic solvent extraction from Scotch Bonnet (C. annuum L.). The super critical C02 extract afforded 3.2 and 0.58% capsaicin and dihydrocapsaicin, while the combined organic extract yielded 0.5 and 0.09%, respectively, per g (DW). 

Several methods have been proposed to produce polyunsaturated fatty acid (PUFA) concentrates particularly high in eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Most PUFA enrichment methods are based upon a combination of techniques such as saponification, solvent extraction, urea fractionation, molecular distillation, fractionation distillation, liquid chromatography, and super critical carbondioxide extraction. Current evidence suggests that the physiological effects of omega-3 fatty acids are such that the annual world supply of fish oils will be grossly inadequate as a source of these materials, and alternative sources will be needed (Belarbi et al, 2000). 

The comparison of six commercial extracts (Figure 6.3) showed that C02 extraction results in the highest proportion of carnosic acid among the phenolic diterpenes (Schwarz and Temes, 1992b). Extraction solvents used for the extracts shown in Figure 6.3 were ethanol (extract 1) hexane and acetone (extract 2) ethanol (extract 3) methanol (extract 4) hexane, ethanol and methanol (extract 5) and super critical C02 (extract 6). 

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. 

Franck, E. U. Super-critical water as electrolytic solvent. Angewandte Chemie, 73 309, 1961. 

Several modifications of the water-soluble catalysts using co-solvents (cf. Section 4.3 and [14]), micelle forming reagents (Section 4.5 and [15]), super-critical C02-water biphasic system (cf. Section 7.4 and [16]), SAPC (Section 4.7 and [17]), and catalyst binding ligands (interfacial catalysis) [18, 24] have been proposed to overcome the lower rates observed in biphasic catalysis due to poor solubilites of reactants in water. So far endeavors were centered on innovating novel catalyst and development of the existing systems. However, limited information is available on the kinetics of biphasic hydroformylation. 

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. 

The reaction rate constant k is not truly a constant it is merely independent of the concentrations of the species involved in the reaction. The quantity k is referred to as either the specific reaction rale or the rale constant. It is almost always strongly dependent on temperature. It depends on whether or not a catalyst is present, and in gas-phase reactions, it may be a function of total pressure, in liquid systems it can also be a function of other parameters, such as ionic strength and choice of solvent. These other variables normally exhibit much less effect on the specific reaction rate than temperature does with the exception of supercritical solvents, such as super critical water. 

Current trends have been to accelerate the extraction of pesticides from the sample matrix by accelerated solvent extraction (ASE) or pressurized solvent extraction (PSE), microwave-assisted solvent extraction (MASE), and super critical fluid extraction (SEE). 

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). 

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... 

Its use as a reaction has been reviewed very recently [42]. Just a few examples will be mentioned here. Diels-Alder reactions occur normally and show no significant discontinuities in rate of going from sub- to super-critical [43]. A recent publication [44] describes the application of C02/water as a combined acid and solvent for the diazotisation and coupling of a weakly basic amine (equation 12.19). 

Liquid-phase catalytic hydrogenation involves dissolving H2 gas in a solvent such as propane or dimethyl ether at a super critical pressure to achieve a single phase (see Fig. 6.28), thereby reducing the number of resistances (see Fig. 6.29). 

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. ... 

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