Supercritical method

A comparison of the duration of the impregnation by the traditional method and by the procedure here proposed is favourable to the latter. This is related to mass transfer easier in a supercritical medium than in liquid phase because of higher diffiisivity of the species. The advantage of the supercritical method is all the more marked since further steps of filtration and elimination of the solvent are necessary in the classical technique. 

Particle formation by supercritical methods is emerging as a viable platform technology for pharmaceuticals and drug delivery systems. Several requirements should be considered for an ideal particle-formation process ... 

El Rassy H, Buisson P, BouaU B, Perraid A, Pierre AC (2(X)3) Surface Characterization of Silica aerogels with Different Proportions of Hydrophobic Groups, dried by the CO2 Supercritical Method Langmuir, 19 358-363 Harreld JH, Ebina T, Tsubo N, Stucky G (2(X)2) Manipulation of pore size distributions in silica and ormosil gels dried under ambient pressure crmditions. J Non-Cryst Solids 298 241 251... 

El Rassy, H, Buisson, P, Bouali, B, Perrard, A, Pierre, A C (2003) Surface characterization of silica aerogels with different proportions of hydrophobic groups, dried by the CO2 supercritical method. Langmuir 19(2) ... 

Navarro-Diaz, H. J., S. L. Gonzalez, B. Irigaray, 1. Vieitez, 1. Jachmanian, H. Hense, and J. V. Oliveira. 2014. Macauba Oil as an Alternative Feedstock for Biodiesel Characterization and Ester Conversion by the Supercritical Method. Paper presented at III Iberoamerican Conference on Supercritical Fluids—PROSCIBA 2013, Cartagena,... 

An alternative route to the production of bioactive glasses is to use low temperature processing methods including Sol-Gel Technology. Materials can be prepared from colloidal powders and simple molecular alkox-ide precursors. The glass phase originally forms as a gel that can subsequently be dried by supercritical methods or conventional heating to moderate temperatures of... 

Devaraju MK, Rangappa D, Honma I (2012) Controlled synthesis of plate-like LiCoP04 nanoparticles via supercritical method and their electrode property. Electrochim Acta 85 548-553... 

In some cases, the temperature of the system may be larger than the critical temperature of one (or more) of the components, i.e., system temperature T may exceed T. . In that event, component i is a supercritical component, one that cannot exist as a pure liquid at temperature T. For this component, it is still possible to use symmetric normalization of the activity coefficient (y - 1 as x - 1) provided that some method of extrapolation is used to evaluate the standard-state fugacity which, in this case, is the fugacity of pure liquid i at system temperature T. For highly supercritical components (T Tj,.), such extrapolation is extremely arbitrary as a result, we have no assurance that when experimental data are reduced, the activity coefficient tends to obey the necessary boundary condition 1... 

To illustrate calculations for a binary system containing a supercritical, condensable component. Figure 12 shows isobaric equilibria for ethane-n-heptane. Using the virial equation for vapor-phase fugacity coefficients, and the UNIQUAC equation for liquid-phase activity coefficients, calculated results give an excellent representation of the data of Kay (1938). In this case,the total pressure is not large and therefore, the mixture is at all times remote from critical conditions. For this binary system, the particular method of calculation used here would not be successful at appreciably higher pressures. 

Several standard methods for the quantitative analysis of food samples are based on measuring the sample s mass following a selective solvent extraction. For example, the crude fat content in chocolate can be determined by extracting with ether for 16 h in a Soxhlet extractor. After the extraction is complete, the ether is allowed to evaporate, and the residue is weighed after drying at 100 °C. This analysis has also been accomplished indirectly by weighing a sample before and after extracting with supercritical GO2. 

The concern by consumers about cholesterol has stimulated the development of methods for its removal. Three principal approaches are in the pilot-plant stages use of enzymes, supercritical fluid extraction, and steam distillation. Using known techniques, it is not possible to remove all cholesterol from milk. Therefore, FDA guidelines identify cholesterol-free foods as containing less than 2 mg cholesterol per serving, and low cholesterol foods as containing from 2 to 20 mg (37). 

Natural Products. Various methods have been and continue to be employed to obtain useful materials from various parts of plants. Essences from plants are obtained by distillation (often with steam), direct expression (pressing), collection of exudates, enfleurage (extraction with fats or oils), and solvent extraction. Solvents used include typical chemical solvents such as alcohols and hydrocarbons. Liquid (supercritical) carbon dioxide has come into commercial use in the 1990s as an extractant to produce perfume materials. The principal forms of natural perfume ingredients are defined as follows the methods used to prepare them are described in somewhat general terms because they vary for each product and suppHer. This is a part of the industry that is governed as much by art as by science. 

The WAG process has been used extensively in the field, particularly in supercritical CO2 injection, with considerable success (22,157,158). However, a method to further reduce the viscosity of injected gas or supercritical fluid is desired. One means of increasing the viscosity of CO2 is through the use of supercritical C02-soluble polymers and other additives (159). The use of surfactants to form low mobihty foams or supercritical CO2 dispersions within the formation has received more attention (160—162). Foam has also been used to reduce mobihty of hydrocarbon gases and nitrogen. The behavior of foam in porous media has been the subject of extensive study (4). X-ray computerized tomographic analysis of core floods indicate that addition of 500 ppm of an alcohol ethoxyglycerylsulfonate increased volumetric sweep efficiency substantially over that obtained in a WAG process (156). 

Overview. Three approaches are used to make most sol—gel products method 1 involves gelation of a dispersion of colloidal particles method 2 employs hydrolysis and polycondensation of alkoxide or metal salts precursors followed by supercritical drying of gels and method 3 involves hydrolysis and polycondensation of alkoxide precursors followed by aging and drying under ambient atmospheres. 

Production of net-shape siUca (qv) components serves as an example of sol—gel processing methods. A siUca gel may be formed by network growth from an array of discrete coUoidal particles (method 1) or by formation of an intercoimected three-dimensional network by the simultaneous hydrolysis and polycondensation of a chemical precursor (methods 2 and 3). When the pore Hquid is removed as a gas phase from the intercoimected soHd gel network under supercritical conditions (critical-point drying, method 2), the soHd network does not coUapse and a low density aerogel is produced. Aerogels can have pore volumes as large as 98% and densities as low as 80 kg/m (12,19). 

Mixtures can be identified with the help of computer software that subtracts the spectra of pure compounds from that of the sample. For complex mixtures, fractionation may be needed as part of the analysis. Commercial instmments are available that combine ftir, as a detector, with a separation technique such as gas chromatography (gc), high performance Hquid chromatography (hplc), or supercritical fluid chromatography (96,97). Instmments such as gc/ftir are often termed hyphenated instmments (98). Pyrolyzer (99) and thermogravimetric analysis (tga) instmmentation can also be combined with ftir for monitoring pyrolysis and oxidation processes (100) (see Analytical methods, hyphenated instruments). 

The development of methods of analysis of tria2ines and thek hydroxy metabohtes in humic soil samples with combined chromatographic and ms techniques has been described (78). A two-way approach was used for separating interfering humic substances and for performing stmctural elucidation of the herbicide traces. Humic samples were extracted by supercritical fluid extraction and analy2ed by both hplc/particle beam ms and a new ms/ms method. The new ms /ms unit was of the tandem sector field-time-of-flight/ms type. 

Numerous high pressure Hquid chromatographic techniques have been reported for specific sample forms vegetable oHs (55,56), animal feeds (57,58), seta (59,60), plasma (61,62), foods (63,64), and tissues (63). Some of the methods requite a saponification step to remove fats, to release tocopherols from ceHs, and/or to free tocopherols from their esters. AH requite an extraction step to remove the tocopherols from the sample matrix. The methods include both normal and reverse-phase hplc with either uv absorbance or fluorescence detection. AppHcation of supercritical fluid (qv) chromatography has been reported for analysis of tocopherols in marine oHs (65). 

Separation Techniques. Current methods for separating fatty acids are by solvent crystaUi2ation or by the hydrophili2ation process. Other methods that have been used in the past, or perhaps could be used in the future, are panning and pressing, solvent extraction, supercritical fluid extraction, the use of metal salts in assisting in separation, separations using urea complexes, and adsorption/desorption. 

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