Phase behavior of supercritical fluids

Lam, D.H., JangkamoUculchai, A. and Luks, K.D. (1990) Liquid-liquid-vapor phase equilibrium behavior of certain binary carbon dioxide + n-alkanol mixtures. Fluid Phase Equilibria, 60,131-141. Gurdial, G.S., Foster, N.R., Jimmy Yun, S.L. and Tilly, KJ3. (1993) Phase behavior of supercritical fluid-entrainer systems, in Siqtercritical Fluid Engineering Science, Fundamentals and Applications, E. Kiran and JJ. Brennecke (Eds.), ACS Symposium Series No. 514, pp. 34-45. 

For a pure supercritical fluid, the relationships between pressure, temperature and density are easily estimated (except very near the critical point) with reasonable precision from equations of state and conform quite closely to that given in Figure 1. The phase behavior of binary fluid systems is highly varied and much more complex than in single-component systems and has been well-described for selected binary systems (see, for example, reference 13 and references therein). A detailed discussion of the different types of binary fluid mixtures and the phase behavior of these systems can be found elsewhere (X2). Cubic ecjuations of state have been used successfully to describe the properties and phase behavior of multicomponent systems, particularly fot hydrocarbon mixtures (14.) The use of conventional ecjuations of state to describe properties of surfactant-supercritical fluid mixtures is not appropriate since they do not account for the formation of aggregates (the micellar pseudophase) or their solubilization in a supercritical fluid phase. A complete thermodynamic description of micelle and microemulsion formation in liquids remains a challenging problem, and no attempts have been made to extend these models to supercritical fluid phases. 

The reverse micelle phase behavior in supercritical fluids is markedly different than in liquids. By increasing fluid pressure, the maximum amount of solubilized water increases, indicating that these higher molecular weight structures are better solvated by the denser fluid phase. The phase behavior of these systems is in part due to packing constraints of the surfactant molecules and the solubility of large micellar aggregates in the supercritical fluid phase. 

Sen, Y. L., and E. Kiran. 1990. A new experimental system to study the temperature and pressure dependence of viscosity, density, and phase behavior of pure fluids and solutions. J. Supercritical Fluids 3 91-99. 

The solute-solvent interactions independent of the stationary phase are obviously of major importance to understanding the retention mechanism in SFC. The solvatochromic behavior of supercritical fluid probes has been used to investigate the specific and non-specific solute-solvent interactions occurring in a fluid as a function of density at different temperatures. Yonker et al. (12.,... 

KLE Kleintjens, L.A., van der Haegen, R., van Opstal, L., and Koningsveld, R., Mean-field lattice-gas modelling of supercritical phase behavior, J. Supercrit. Fluids, 1, 23, 1988. 

HAR Haruki, M., Matsuura, K., Kaida, Y., Kihara, S.-L, and Takishima, S., Microscopic phase behavior of supercritical carbon dioxide + non-ionic surfactant + water systems at elevated pressures. Fluid Phase Equil, 289, 1, 2010. 

Although modeling of supercritical phase behavior can sometimes be done using relatively simple thermodynamics, this is not the norm. Especially in the region of the critical point, extreme nonideahties occur and high compressibilities must be addressed. Several review papers and books discuss modeling of systems comprised of supercritical fluids and soHd orHquid solutes (rl,i4—r7,r9,i49,r50). 

A number of theoretical models have been proposed to describe the phase behavior of polymer—supercritical fluid systems, eg, the SAET and LEHB equations of state, and mean-field lattice gas models (67—69). Many examples of polymer—supercritical fluid systems are discussed ia the Hterature (1,3). 

A third motivation for studying gas solubilities in ILs is the potential to use compressed gases or supercritical fluids to separate species from an IL mixture. As an example, we have shown that it is possible to recover a wide variety of solutes from ILs by supercritical CO2 extraction [9]. An advantage of this technology is that the solutes can be removed quantitatively without any cross-contamination of the CO2 with the IL. Such separations should be possible with a wide variety of other compressed gases, such as C2LL6, C2LL4, and SF. Clearly, the phase behavior of the gas in question with the IL is important for this application. 

Polymer-supercritical fluid systems, phase behavior of, 24 11... 

Shariati, A. and Peters, C. J., High-pressure phase behavior of systems with ionic liquids Measurements and modeling of the binary system fluoroform + l-ethyl-3-methylimidazolium hexafluorophosphate, /. Supercrit. Fluids, 25, 109, 2003. 

The last ATR cell described here in detail was designed for the study of catalytic reactions at high pressures and in particular in supercritical fluids. A schematic representation of the design is shown in Fig. 17 (76). An important issue in this type of reaction is the phase behavior of the system, which can have a large influence on the catalytic reaction 77,IS). The cell consists of a horizontal stainless-steel cylinder. It is designed to allow monitoring of the phase behavior via a video camera. For this purpose, one end of the cylinder is sealed with a sapphire window, behind... 

Chapter 14 describes the phase behavior of binary mixtures. It begins with a discussion of (vapor -l- liquid) phase equilibria, followed by a description of (liquid + liquid) phase equilibria. (Fluid + fluid) phase equilibria extends this description into the supercritical region, where the five fundamental types of (fluid + fluid) phase diagrams are described. Examples of (solid + liquid) phase diagrams are presented that demonstrate the wide variety of systems that are observed. Of interest is the combination of (liquid + liquid) and (solid 4- liquid) equilibria into a single phase diagram, where a quadruple point is described. 

Stradi, B. A. Kohn, J. P. Stadtherr, M. A. Brennecke, J. F. Phase Behavior of the Reactants, Products and Catalysts Involved in the Allylic Epoxidation of trans-2-Hexen-l-ol to (2R,3R)-( + )-3-Propyloxiramethanol in High Pressure Carbon Dioxide. J. Supercrit. Fluids 1998, 12, 109-122. 

Sample introduction is a major hardware problem for SFC. The sample solvent composition and the injection pressure and temperature can all affect sample introduction. The high solute diffusion and lower viscosity which favor supercritical fluids over liquid mobile phases can cause problems in injection. Back-diffusion can occur, causing broad solvent peaks and poor solute peak shape. There can also be a complex phase behavior as well as a solubility phenomenon taking place due to the fact that one may have combinations of supercritical fluid (neat or mixed with sample solvent), a subcritical liquified gas, sample solvents, and solute present simultaneously in the injector and column head [2]. All of these can contribute individually to reproducibility problems in SFC. Both dynamic and timed split modes are used for sample introduction in capillary SFC. Dynamic split injectors have a microvalve and splitter assembly. The amount of injection is based on the size of a fused silica restrictor. In the timed split mode, the SFC column is directly connected to the injection valve. Highspeed pneumatics and electronics are used along with a standard injection valve and actuator. Rapid actuation of the valve from the load to the inject position and back occurs in milliseconds. In this mode, one can program the time of injection on a computer and thus control the amount of injection. In packed-column SFC, an injector similar to HPLC is used and whole loop is injected on the column. The valve is switched either manually or automatically through a remote injector port. The injection is done under pressure. 

In order to increase the sustainability of chemical processes, environmentally friendly solvents such as supercritical fluids (SCFs) are widely investigated. Han and coworkers studied the ethenolysis of ethyl oleate in SC C02 in relation with the phase behavior of the reaction mixture [62]. They carried out the ethenolysis reaction at 35°C in the absence of C02 and in the presence of C02 at three different pressures (50, 82, and 120 bar). The reaction in the absence of C02 reached equilibrium in 1 h at 80% conversion. The reaction rate in the presence of 50 bar of C02 was higher than without C02 and, at 82 bar, again increased with respect to 50 bar. However, when the pressure was increased to 120 bar, the reaction rate decreased. This effect was explained according to the variations on the phase behavior with the pressure an increase in the C02 pressure carried an increase of solubility of reactants, products, and C02, which produced a decrease of the viscosity of the reaction mixture. This positive effect was enhanced at 82 bar and was accompanied by an increase of selective solubility of the products in the vapor phase that further increased both reaction rate and conversion. The decrease of efficiency at 120 bar was related to an increase of the solubility of the reactants in the C02 phase. 

Mixtures of C02 and methanol were selected for the initial investigation of the solvatochromic behavior in supercritical fluid systems. This combination is of interest as it combines the low critical temperature and pressure of carbon dioxide with a polar, less volatile modifier. This system exhibits relatively simple Type I phase behavior and several groups have published measurements of mixture critical points (19-21). At intermediate compositions the critical pressure for this fluid is much higher than that of either pure C02 or pure methanol, reaching a maximum of approximately 2400 psi (20). 

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