Near-critical liquids

Microemulsions have the ability to partition polar species into the aqueous core or nonpolar solutes into the continuous phase (See Fig. 1). They can therefore greatly increase the solvation of polar species in essentially a nonpolar medium. The surfactant interfacial region provides a dramatic transition from the highly polar aqueous core to the nonpolar continuous-phase solvent. This region represents a third type of solvent environment where amphiphilic solutes can reside. Such amphiphilic species will be strongly oriented in the interfacial film so that their polar ends are in the core of the microemulsion droplet and the nonpolar end is pointed towards or dissolved in the continuous phase solvent. When the continuous phase is a near-critical liquid (7)j = r/7 > 0.75) or supercritical fluid, additional parameters such as transport properties, and pressure (or density) manipulation become important aids in applying this technology to chemical processes. 

The question arises as to whether the microemulsion phase retains the high transport rates for which supercritical fluids are well know.f lf Various spectroscopic studies that are described in the following section have been used to directly measure the transport properties in supercritical fluid or near-critical liquid microemulsions. 

The first chiral separation using pSFC was published by Caude and co-workers in 1985 [3]. pSFC resembles HPLC. Selectivity in a chromatographic system stems from different interactions of the components of a mixture with the mobile phase and the stationary phase. Characteristics and choice of the stationary phase are described in the method development section. In pSFC, the composition of the mobile phase, especially for chiral separations, is almost always more important than its density for controlling retention and selectivity. Chiral separations are often carried out at T < T-using liquid-modified carbon dioxide. However, a high linear velocity and a low pressure drop typically associated with supercritical fluids are retained with near-critical liquids. Adjusting pressure and temperature can control the density of the subcritical/supercritical mobile phase. Binary or ternary mobile phases are commonly used. Modifiers, such as alcohols, and additives, such as adds and bases, extend the polarity range available to the practitioner. 

Table IL Effect of Phase on TMP Regeneration Using C4 and C5 Alkanes at Near-critical Liquid and Supercritical Conditions...

The last decade has given clear evidence for the existence of a second critical or near-critical liquid-liquid (L-L) transition in a one component fluid. " This suggests that the Kohnstamm-Gibbs phase rule has to be supplemented since it suggests the existence of a single gas - liquid critical... 

Liquid carbon dioxide at hquid-hke densities can be used as an effective extraction solvent. In fact, the viscosities and diffusivities of a near-critical liquid (i.e., one that is within 30-40°C of the critical temperature) are nearly as advantageous as those of the fluid in the supercritical region. What is lost with a compressible near-critical liquid is the adjustable solvent power. Furthermore, as temperature is increased in order to support increased solvation of compounds with higher melting points and/or molecular weights, the user moves into the supercritical region an3rway. 

As is by now evident to the reader, the phenomenon of solubility in supercritical fluids is not new. Since 1879 (or 1861, if we include the high-pressure, near-critical liquid carbon dioxide studies of Gore), solubility, phase, and spectroscopic studies have been performed on a large number of solute-SCF mixtures. They were made for their inherent scientific and technical interest and value. And they received a resurgence of interest with the work of Diepen, Scheffer, and coworkers in the late 1940s and early 1950s. 

A fourth alternative for carrying out the naphthalene extraction utilizes the dissolving capacity of near-critical liquid CO2. This operating mode is illustrated in figure 6.1 by the LV tie line. Liquid CO2 is employed to dissolve and extract the naphthalene from the mixture, and the liquid solution leaving the extractor is heated to vaporize the CO2 and recover the naphthalene. The CO2 is then condensed and recycled to the extractor. 

If an intermediate resin fraction is desired, another separator and stripper system would be installed right after the asphaltene separator in figure 7.5. To recover a resin fraction, the overhead solution from the asphaltene separator, which now consists of butane, resins, and light oils, is heated to near the critical temperature of butane. At this elevated, near-critical temperature the solvent power of compressed liquid butane decreases and the resins precipitate from solution. The overhead stream from this separator consists of light oils dissolved in near-critical liquid butane. 

Warren K. Lewis and J. M. Whitely in an earlier patent, U.S. 2,202,389, which is not reviewed in this appendix, described a method for changing the solvent power of a solvent in the near critical liquid region by varying the pressure on the liquid. In this patent, varying the solvent power by varying pressure of a solvent above its critical temperature is the invention, and specifically a process for the separation of hydrocarbon oils using 2 to 6 carbon aliphatic solvents is described. 

By now we assume that the general similarities among many of the patents are becoming obvious, but we see also that the inventions are different in some slight way. So, of course, are the specific limits in the claims. For example, a near critical liquid at its vapor pressure is different from a high pressure liquid above the critical pressure, but still below its critical temperature, which in turn is different from a supercritical fluid. We continue to read that the prior art in many of the patents describe the limitations of all the other processes and point out the advantages of the instant process... 

Would there be any change in Mg. 5.25 when a near-critical gas or a near-critical liquid mixture is considered for wax precipitation ... 

It has been shown that the principle of isomorphic critical behaviour accounts not only for the thermodynamic behaviour of mixtures near vapour-liquid critical points and near critical liquid-liquid mixing critical points, but also near special critical points, like azeotropic critical points, critical points where the critical temperature exhibits a maximum or a minimum as a function of temperature, re-entrant critical points and critical double points, depending on the values of the coefficients a,-, bi, and c,- in the expressions for the scaling fields. In this chapter we restrict ourselves to some more common cases of critical phase behaviour in mixtures. 

An interesting variation of the sink / float separation utilizing super- or near-critical liquid has been proposed [12], [13]. At the critical conditions of temperature and pressure the difference between liquid and gas phase disappears. Near - and super - critical liquids are compressible and their density can be varied by adjusting pressure. The advantages of some of the near - and super - critical fluids ( for example COj) are... 

Altland, B. L., Cox, D., Beckerman, E. J. (1995) Optimization of the high pressure, near-critical liquid - based microsortation of recyclable post - consumer plastics. Resources, Conservation and Recycling, 15, pp. 203-217... 

The marginally subcritical part of the near-critical region is shown with slanted hatching. A lower limit (here taken to be about 0.9) to the reduced temperature in this area is provided by the gradual onset of normal liquid properties as Tr is lowered below unity. As discussed above and as may be seen from Figure 1.1, the liquid phase remains appreciably compressible at reduced temperatures down to 0.9 and a little below. A consequence of this appreciable compressibility is that the solvent power of near-critical liquids is appreciably pressure dependent. 

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