Liquids supercritical fluids

Table 1. Typical liquid, supercritical fluid (SCF), and gas properties.

Solvent a liquid in which certain kinds of molecules dissolve. Although they typically are liquids with low boiling points, they may include high-boihng liquids, supercritical fluids, or gases. 

The possible alternatives to ozone-depleting or otherwise environmentally challenging organic solvents for chemical processes are abundant. We can begin a review by looking at five broad categories aqueous, ionic liquids, supercritical fluids, fluori-nated solvents, or solventless processes (llhnan, 1993). 

Koppenhoefer, B., Nothdurft, A., Pierrot-Sanders, J., Piras, P.,Popescu, P, Roussel, C., Stiebler, M., and Trettin, U. 1993. CHIRBASE A graphical molecular database on the separation of enantiomers by liquid, supercritical fluid, and gas chromatography. Chirality 5 213-219. 

In these systems, the interface between two phases is located at the high-throughput membrane porous matrix level. Physicochemical, structural and geometrical properties of porous meso- and microporous membranes are exploited to facilitate mass transfer between two contacting immiscible phases, e.g., gas-liquid, vapor-liquid, liquid-liquid, liquid-supercritical fluid, etc., without dispersing one phase in the other (except for membrane emulsification, where two phases are contacted and then dispersed drop by drop one into another under precise controlled conditions). Separation depends primarily on phase equilibrium. Membrane-based absorbers and strippers, extractors and back extractors, supported gas membrane-based processes and osmotic distillation are examples of such processes that have already been in some cases commercialized. Membrane distillation, membrane... 

To understand any extraction technique it is first necessary to discuss some underlying principles that govern all extraction procedures. The chemical properties of the analyte are important to an extraction, as are the properties of the liquid medium in which it is dissolved and the gaseous, liquid, supercritical fluid, or solid extractant used to effect a separation. Of all the relevant solute properties, five chemical properties are fundamental to understanding extraction theory vapor pressure, solubility, molecular weight, hydrophobicity, and acid dissociation. These essential properties determine the transport of chemicals in the human body, the transport of chemicals in the air water-soil environmental compartments, and the transport between immiscible phases during analytical extraction. 

Many of the same properties that make supercritical fluids advantageous in chromatography also enhance their ability to extract compounds from within a sample matrix.(4) Also, since the solubility of most compounds is dependent on the density of the supercritical fluid, selective extraction is possible. (J5) These properties are well known, and have been exploited in some cases where the extraction was formerly done with a liquid. In many cases, the quality of extract is higher, and extractions are of higher efficiency than with liquids. Supercritical fluids, especially CO, are also often less expensive, less toxic and less flammable than their organic liquid phase counterparts. 

Although C02 is the most common solvent for supercritical extraction processes because of it s abundance, non-toxicity and non-flammability, other compounds may prove to be better solvents in certain instances. In choosing a solvent, a balance between solubility and selectivity has to be struck. In the case of solutes with a melting point well below the decomposition temperature, it is usually desirable to perform a liquid-supercritical fluid extraction to circumvent the problems associated with handling solids at high pressures. In... 

Table 1 groups together differing orders of magnitude of physical parameters for the three states of the same fluid. It should be noted that in spite of high densities (similar to liquids), supercritical fluids are only slightly viscous and, from this point of view, have similar properties to gases. 

In addition to fluorous solvents and ionie liquids, supercritical fluids sc-fluids, scf s), sueh as supercritical carbon dioxide (se-C02), constitute a third class of neoteric solvents that can be used as reaction media. Although sc-fluids have been known for a long time and have been advantageously used as eluants in extraction and chromatography processes (see Sections A.6 and A.7 in the Appendix), their application as reaction media for chemical processes has become more popular only during the last decade. Some of their physical properties and the supercritical conditions necessary for their existence have already been described in Section 3.2 (see Figure 3-2 and Table 3-4) see also references [209, 211-220, 224-230] to Chapter 3 for reviews on sc-fluids and their applications (particularly for SC-CO2 and SC-H2O). 

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

Thermodynamic constraints to the SAS process can be summarized in the required miscibility between the liquid solvent and the supercritical antisolvent and the insolubility of the solute in the antisolvent and in the solvent-antisolvent mixture. Data are available for various binary mixtures liquid-supercritical fluid and can be described as type I using the classification of van-Konynenburg and Scott. If jet break-up is obtained and mass transfer is very fast, high-pressure VLEs of the ternary system liquid-I-solute-I-supercritical fluid can control the precipitation process. 

The solubility of solids in supercritical fluids is a very sensitive function of temperature and pressure. Unlike liquids, supercritical fluids are highly compressible and minor temperature or pressure changes lead to large changes in density and, therefore, solvent power QJ. The expansion of supercritical solutions, therefore, results in a substantial solubility decrease. The solubility of Naphthalene in carbon dioxide at 45 C, for example, decreases by about two orders of magnitude upon reducing the pressure from 127 to 62 bars (21. If this... 

Supercritical fluid extraction is used to recover small organic solutes with molecular weight below 1500 daltons. In a state of continuity between vapour and liquid, supercritical fluids exhibit intermediate transport properties with lower viscosities than liquid and higher diffusivities than gcises. Because of its... 

Garcia, S. Lourenco, N.M.T. Lousa, D. Sequeira, A.F. Mimoso, P. Cabral, J.M.S. Afonso, C.A.M. Barreiros, S. A comparative study of biocatalysis in non-conventional solvents ionic liquids, supercritical fluids and organic media. Green Chem. 2004, 6 (9), 466-470. 

Supercritical fluid. The physical state of a substance above its critical temperature (i.e., the temperature above which a gas cannot be condensed into a liquid). Supercritical fluids are unique in their properties, differing from liquids and gases but having characteristics of both. 

Phase Equilibrium in Solid-Liquid-Supercritical Fluid Systems... 

One of the benefits of using supercritical fluids as the solvent is the strong dependence of the solubility of the solute on the solvent density. This is a property that could be exploited for facilitating the separation of the solute from the solvent as it leaves the column, by dropping its pressure or raising its temperature, thereby lowering its density and the solubility of the solute. As a result, the extract separates into a liquid solute and a vapor solvent. Another favorable property of supercritical fluids as solvents is the high diffusivity of the solute in these fluids compared to that in liquids. Supercritical fluids also have a substantially lower viscosity than liquids. Because of these properties the mass transfer rate of the solute... 

Canton S, Hanefeld U, Basso A (2007) Biocatalysis in non-conventional media-ionic liquids, supercritical fluids and the gas phase. Green Chem 9 954-971... 

Table 6.1 Comparison of densities, viscosities and diffusivities for liquid, supercritical fluid and gas...

S. (2004) A comparative study of biocatalysis in non-conventional solvents ionic liquids, supercritical fluids and organic media. Green Chem., 6 (9), 466-470. 

Indeed, flow processes are already combined with functionalized solid phases, with ionic liquids, supercritical fluids such as carbon dioxide, and microwave irradiation. Nevertheless, numerous examples of above mentioned processes with standard and more and less innovative laboratory equipment take advantage of this new approach. Whatever chemists and chemical engineers require - synthesis of few milligrams of compounds in drug chemistry, synthesis of building blocks on a multigram scale for parallel synthesis, or even the kilogram production of fine chemicals - flow processes are a helpful tool and a crucial link between differently scaled reactions. 

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