Near-critical fluids, extraction

Sanders N. 1993. Food legislation and the scope for increased use of near-critical fluid extraction operations in food, flavoring and pharmaceutical industries. In King MB and Bott TR, editors. Extraction of Natural Products Using Near-Critical Solvents. Glasgow Blackie Academic, pp. 34 19. 

Heidlas, J. De-oiling of Lecithins by Near-Critical Fluid Extraction. Agro-Food-Industry Hi-Tech. 1997, 1,9- 11. 

King, M.B. and O.J. Catchpole (1993) Physicochemical Data Required for the Design of Near-Critical Fluid Extraction Process in EIxtraction of Natural Products Using Near-Critical Solvents, M.B. King and Botl, T.R., Eds., Blackie, Glasgow. 

Food legislation and the scope for increased use of near-critical fluid extraction operations in the food, flavouring and pharmaceutical industries... 

Physico-chemical data required for the design of near-critical fluid extraction process... 

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. 

Interaction of Density, Viscosity and Interfacial Tension in Countercurrent Extraction with Near-Critical Fluids... 

POROCRITICAL FLUID EXTRACTION A NEW TECHNIQUE FOR CONTINUOUS EXTRACTION OF LIQUIDS WITH NEAR-CRITICAL FLUIDS. Marc Sims, James R. Robinson and Anthony J. Dennis, (Marc Sims SFE, Setec Inc., Tastemaker), 1012 Grayson... 

A new supercritical fluid process has been developed for the continuous extraction of liquids. The most useful solvent employed in the recently patented process is supercritical or near-critical carbon dioxide(l). At the heart of the process are porous membranes. Their porosity combined with a near-critical fluid s high diffusivity create a dynamic non-dispersive contact between solvent and feed liquid. The technique is dubbed porocritical fluid extraction and will be commercialized as the Porocrit Process. 

Sims M, McGovern WE, and Robinson JR. Porocritical fluid extraction from liquids using near-critical fluids. Membr. Technol. 1998 97 11-12. 

Supercritical fluid extraction - also referred to as dense gas extraction or near critical solvent extraction - means that the operational temperature of the process is in the vicinity of the critical temperature of the solvent. Since the extraction of herbal raw materials requires non-drastic gentle process temperatures the choice of suitable near critical solvents is limited to pure or partly halogenated C,-Cj hydrocarbons, dinitrogen monoxide and carbon dioxide. All these solvents, especially carbon dioxide, exhibit favourable properties in view of the afore-mentioned aspects. 

Supercritical or near-critical fluids can be used both for extraction and chromatography. Many chemicals, primarily organic species, can be separated and analyzed using this approach [6], which is particularly useful in the food industry. Substances that are useful as supercritical fluids include carbon dioxide, water, ethane, ethene, propane, xenon, ammonia, nitrous oxide, and a fluoroform. Carbon dioxide is most commonly used, typically at a pressure near 100 bar. The required operating pressure ranges from about 43 bar for propane to 221 bar for water. Sometimes a solvent modifier is added (also called an entrainer or cosolvent), particularly when carbon dioxide is used. 

Solubility calculations are merely phase-equilibrium calculations applied to supercritical gases in liquids, solids in liquids, and solutes in near-critical fluids. The last application has drawn substantial attention, for near-critical extraction processes are being applied, not only in the chemical and energy industries, but also in food processing, purification of biological products, and clean-up of hazardous wastes. 

In addition to supercritical fluid extraction and supercritical fluid chromatography, supercritical and near-critical fluids are of increasing interest for other applications. These include their use as processing fluids for dyeing of fibers, production of finely dispersed particles (RESS, PGSS, SAS, GAS, etc.), as promising solvents for syntheses and kinetic studies, for the destructive oxidation of wastes with supercritical water (SCWO), for the purification of filters, catalysts, contaminated soils, for drying and sterilization processes, and others [13,19,20]. 

A situation in which extraction with a near-critical fluid may have a substantial advantage is one in which a high pressure stream of a gas suitable for extraction purposes is required for other aspects of the process. This is the case for example, in a Japanese process for the production of M.E.K. [26], probably with secondary butyl alcohol as intermediate [27]. In this process, which has a throughput of 40 000 tonnes of product per annum, the supercritical extraction step is integrated into the overall process thus, it is claimed, halving production costs [27]. 

Recent findings on the extraction of beds of particulate solids with near-critical fluids [3-6] may be summarised as follows (carbon dioxide is the solvent considered in all these studies) ... 

Extraction from Aqueous Solutions Critical Fluid Technologies, Inc. has developed a continuous countercurrent extraction process based on a 0.5-oy 10-m column to extract residual organic solvents such as trichloroethylene, methylene chloride, benzene, and chloroform from industrial wastewater streams. Typical solvents include supercritical CO9 and near-critical propane. The economics of these processes are largely driven by the hydrophihcity of the product, which has a large influence on the distribution coefficient. For example, at 16°C, the partition coefficient between liquid CO9 and water is 0.4 for methanol, 1.8 for /i-butanol, and 31 for /i-heptanol. 

Sovova, H., Near-critical extraction of pigments and oleoresin from stinging nettle leaves, J. Supercrit. Fluids, 30, 213, 2004. 

Heterogeneously catalyzed hydrogenation reactions can be run in batch, semibatch, or continous reactors. Our catalytic studies, which were carried out in liquid, near-critical, or supercritical C02 and/or propane mixtures, were run continuously in oil-heated (200 °C, 20.0 MPa) or electrically heated flow reactors (400 °C, 40.0 MPa) using supported precious-metal fixed-bed catalysts. The laboratory-scale apparatus for catalytic reactions in supercritical fluids is shown in Figure 14.2. This laboratory-scale apparatus can perform in situ countercurrent extraction prior to the hydrogenation step in order to purify the raw materials employed in our experiments. Typically, the following reaction conditions were used in our supercritical fluid hydrogenation experiments catalyst volume, 2-30 mL total pressure, 2.5-20.0 MPa reactor temperature, 40-190 °C carbon dioxide flow, 50-200 L/h ... 

Supercritical fluid extraction is an attractive process primarily because the density and solvent power of a fluid changes dramatically with pressure at temperatures near the critical. In complex... 

Fluids are highly compressible along near-critical isotherms (L01-1.2 Tc) and display properties ranging from gas-like to Liquid-Like with relatively small pressure variations around the critical pressure. The liquid-like densities and better-than-liquid transport properties of supercritical fluids (SCFs) have been exploited for the in situ extraction of coke-forming compounds from porous catalysts [1-6], For i-hexene reaction on a low activity, macroporous a catalyst, Tiltschcr el al. [1] demonstrated that reactor operation at supercritical... 

Employing 1-hexene isomerization on a Pt/y-ALOj reforming catalyst as a model reaction system, we showed that isomerization rates are maximized and deactivation rates are minimized when operating with near-critical reaction mixtures [2]. The isomerization was carried out at 281°C, which is about 1.1 times the critical temperature of 1-hexene. Since hexene isomers are the main reaction products, the critical temperature and pressure of the reaction mixture remain virtually unaffected by conversion. Thus, an optimum combination of gas-like transport properties and liquid-like densities can be achieved with relatively small changes in reactor pressure around the critical pressure (31.7 bars). Such an optimum combination of fluid properties was found to be better than either gas-phase or dense supercritical (i.e., liquid-like) reaction media for the in situ extraction of coke-forming compounds. 

The simulated and experimental variations of the end-of-run (i.e., 8 hr.) isomerization rates with density are compared in Figure 1. Details of the experiments are provided elsewhere [2, 3]. At subcritical densities, the extraction of coke precursors is insignificant. Hence, an increase in the concentration of the hexene and coke precursors (i.e., oligomers) leads to lower isomerization rates. At near-critical densities, the extraction of coke precursors becomes significant. Hence, the isomerization rate increases. Both the experimental and simulated rates show a decreasing trend when the density is increased from near-critical to supercritical values. This is attributed to pore-diffusion limitations as the fluid changes from gas-like to liquid-like. Above 2.0 pc, the isomerization rate increases with density as the ability of the reaction mixture to extract the coke precursors increases. 

Plants and plant extracts have been used as medicine, culinary spice, dye and general cosmetic since ancient times. Plant extracts are seen as a way of meeting the demanding requirements of the modem industry. In the past two decades, much attention has been directed to the use of near critical and supercritical carbon dioxide solvent, particularly in the food pharmaceutical and perfume industries. CO2 is an ideal solvent because it is non-toxic, non-explosive, readily available and easily removed from the extracted products. At present the major industrial-scale applications of supercritical fluid extraction (SFE) are hop extraction, decaffeination of coffee and tea, and isolation of flavours, fragrances and other components from spices, herbs and medicinal plants [1-4]. 

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