Near-critical fluid

Erequenfly, the term compressed fluid, a more general expression than supercritical fluid, is used. A compressed fluid can be either a supercritical fluid, a near-critical fluid, an expanded Hquid, or a highly compressed gas, depending on temperature, pressure, and composition. 

Polymers and Supercritical Fluids. Prior to the mid-1980s, Httie information was pubhshed regarding polymer processing with supercritical and near-critical fluids (1). In 1985, the solubiUties of many polymers in near- and supercritical CO2 were reported. These polymers were examined for thek abiUty to increase viscosity in C02-enhanced oil recovery (24). Since then, a number of studies have examined solubiUties of polymers in... 

Catchpole-Kinp examined binaiy diffusion data of near-critical fluids in the reduced density range of 1 to 2.5 and found that their data correlated with average deviations of 10 percent and a maximum deviation of 60 percent. They observed two classes of behavior. For the first, no correction fac tor was required R = 1). That class was comprised of alcohols as solvents with aromatic or ahphatic solutes, or carbon dioxide as a solvent with ahphatics except ketones as solutes, or... 

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. 

The benefits from tuning the solvent system can be tremendous. Again, remarkable opportunities exist for the fruitful exploitation of the special properties of supercritical and near-critical fluids as solvents for chemical reactions. Solution properties may be tuned, with thermodynamic conditions or cosolvents, to modify rates, yields, and selectivities, and supercritical fluids offer greatly enhanced mass transfer for heterogeneous reactions. Also, both supercritical fluids and near-critical water can often replace environmentally undesirable solvents or catalysts, or avoid undesirable byproducts. Furthermore, rational design of solvent systems can also modify reactions to facilitate process separations (Eckert and Chandler, 1998). 

As regards the dynamics of the fluid composition, the experimental results are very difficult to understand [66,67]. We expect that, if the pore size b is very large, the diffusion constant should first behave as in bulk near-critical fluids, but it will cross over to a value of order kBTZb/Gntis 2, being the correlation length (see Eq. (6.67) below). It would also be interesting to find whether the time correlation function of c would be influenced by structural relaxation of network (see Sect 6.2). 

If to is of the order of the concentration decay rate, the inequality co < (rjs/p)q2 holds for most q in the integration as in the case of near-critical fluids. This allows us to set... 

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

Composition of Original Near Critical Fluid, and Liquid Compositions and Vapor Phase Amounts from High and Medium Pressure Separators... 

Hydrate Dissociation Conditions for Near Critical Fluid with Distilled Water, Salt Formation Water, and Methanol... 

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

The density of the liquid phase of the system oleic acid/carbon dioxide increases with increasing pressure, whereas the density of the liquid phase of the system oleic acid/ethane decreases with increasing pressure. Therefore the hydrodynamics in a countercurrent column and mass transfer may dramatically depend on the activity of the near-critical fluid. 

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. 

Single Sphere Model II (Equations 4, 5, 8, 9 and 10 in reference 6) In this model allowance is made for the resistance to mass transfer offered by the surface film surrounding the herb particles. The mass transfer coefficient kf was obtained from correlations proposed by Catchpole et al (8, 9) for mass transfer and diffusion into near-critical fluids. An average of the binary diffusivities of the major essential oil components present was used in calculating kf (these diffusivities were all rather similar because of their similar structures). 

Today most chemists are familiar with the critical point of a substance as defined by the critical temperature and pressure, and Pc respectively, but often their knowledge goes no further. The behavior of gases close to the critical point is so far removed from ideal that, the topic is usually ignored. Hence, many scientists are unaware that near-critical fluids display unusual and intriguing properties which can lead to new chemistry with exciting applications. 

Table 3. Examples of Polar Solutes Solubilized in Supercritical and Near-critical Fluid Microemulsions...

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