Supercritical gas extraction

Various models of SFE have been published, which aim at understanding the kinetics of the processes. For many dynamic extractions of compounds from solid matrices, e.g. for additives in polymers, the analytes are present in small amounts in the matrix and during extraction their concentration in the SCF is well below the solubility limit. The rate of extraction is then not determined principally by solubility, but by the rate of mass transfer out of the matrix. Supercritical gas extraction usually falls very clearly into the class of purely diffusional operations. Gere et al. [285] have reported the physico-chemical principles that are the foundation of theory and practice of SCF analytical techniques. The authors stress in particular the use of intrinsic solubility parameters (such as the Hildebrand solubility parameter 5), in relation to the solubility of analytes in SCFs and optimisation of SFE conditions. 

Supercritical gas extraction depends on the fact that the vapour pressure of a solid or liquid can be greatly Increased by contacting It with a compressed gas, and that this enhancement Is the more pronounced the greater the gas density ). In principle, It Is consequently possible to transfer Into the vapour phase substances that are otherwise substantially non-volatlle. 

Supercritical gas extractions Method A. Extractions were carried out for 1 h at temperature in a 1 l semi-continuous reactor (6). 

Supercritical gas extractions Method B. Extractions were carried out in a 500 ml rocking autoclave fitted with a stainless steel liner. The Internal volume of the autoclave with liner was 420 ml. The autoclave was charged with coal and solvent, heated to and maintained at temperature for 1 h. The residue was washed out of the cooled reactor with acetone, filtered, washed with pyridine and then acetone and dried under vacuum. In some cases, g.c. analyses of the gases in the cooled reactor were carried out. 

The asphaltene and pre-asphaltene contents of the extracts are high (see Figure 2), as is the case with supercritical gas extraction using hydrocarbon solvents. 

Whitehead, J. C., "Development of a Process for the Supercritical Gas Extraction of Coal", paper presented at the 88th AIChE National Meeting, June 1980. 

A process pioneered by the National Coal Board in England that has not reached the fully developed stage but that has considerable potential is supercritical gas extraction. In this process the coal is pyrolized at a relatively low temperature, around 400°C, in the presence of a compressed supercritical gas, that is, a gas whose temperature is above the critical temperature at which it can be liquefied. Suitable gases are, for example, a number of petroleum fractions. Under these conditions at high pressures, around 10 MPa, the gas density is like that of a liquid, and the gas acts like a strong solvent that causes the liquids to volatilize and be taken up by the vapor. By transferring the gas to a vessel at atmospheric pressure, the density of the solvent gas is reduced and the extracted... 

Design, Construction, and Operation of a Multipurpose Plant for Commercial Supercritical Gas Extraction... 

Gangoli, N., and G. Thodos. 1977. Liquid fuels and chemical feedstocks from coal by supercritical gas extraction. Ind. Eng. Chem. Process Des. Dev. 16 208. 

Randall, L. G. 1982. The present status of dense (supercritical) gas extraction and dense gas chromatography Impetus for DGC/MS development. Sep. Sci. Tech. 17 1. 

Attempts to isolate low-molecular-weight materials from coal have centered on the use of flash heating and supercritical gas extraction. In the former case, the products indicated that the low-energy flash heating brought about the production of high yields of aliphatic hydrocarbons. Of course, it is necessary to assume that the low-molecular-weight species are the primary products of the process insofar as they are r idly removed from the reaction zone with little, if any, further molecular alteration. 

Application of supercritical gas extraction (Schneider et al., 1980 Bright and McNally, 1992 Kiran and Brennecke, 1993) has lately received considerable attention when applied to coal. For example, studies have been reported relating to the chemical nature of extracts of coal (volatile matter content in excess of 36%) obtained using toluene (under pressure) at 350°C (660°F). The extracts contained aromatics (benzene derivatives, naphthalene derivatives, and phenanthrene derivatives) as well as n-paraffins, sterane, and materials such as phytane, pristane, and farnesane (Table 10.2) (Bartle et al., 1975 Smith and Smoot, 1990). There was a predominance of n-alkanes. 

Low-Molecular-Weight Constituents in a Supercritical Gas Extract of Coal... 

FIGURE 10.2 Average molecular structures of supercritical gas extracts of coal. (From Bartle, K.D. et al.. Fuel, 54, 226, 1975.)... 

National Coal Board supercritical extraction In separate hydrogenation reactor 350-450 100-200 H2 in separate reactor Supercritical gas extraction of portion of coal with PhMe as solvent... 

Supercritical gas extraction Depends upon solubility of a component in a gas variable solubility with pressure permits recovery of component and re-use of gas De-caffeination of coffee using CO2... 

Blood, C.T. (1981) Supercritical gas extraction with carbon dioxide. Report to the National Research Development Co., UK. 

Komer, J-P. (1984) Supercritical gas extraction. Food Engineering International, November Issue, 40. 

Weidner, E. and Peter, S. (1987) Separation of Lecithin and Soya Oil by near Critical Fluid Extraction, in Int. Symp. on Supercritical Gas Extraction, NTS INC. Tokyo. 

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