Vapor-liquid equilibria supercritical

We will begin our discussion by describing (vapor + liquid) equilibrium, which we will extend into the supercritical fluid region as (fluid + fluid) equilibrium. (Liquid + liquid) equilibrium will then be described and combined with (vapor + liquid) equilibrium in the (fluid + fluid) equilibrium region. Finally, we will describe some examples of (solid + liquid) equilibrium. 

Bamberger, A.G. Sieder, and G. Maurer. 2000. High-pressure (vapor + liquid) equilibrium in binary mixtures of (carbon dioxide + water or acetic acid) at temperatures from 313 to 353 K. /. Supercrit. Fluids 17 97-110. 

There has been criticism directed toward the oversimplicity of the cubic equation form, especially in the modeling of supercritical vapor-liquid equilibriiun. Nevertheless, this representation does describe at least qualitatively all the important characteristics of vapor-liquid equilibrium behavior. Alternative equations of state have been suggested, but none have been widely used and tested. Also, other EOS are significantly more complex and bring with them additional parameters which must be evaluated by regression from experimental data. 

Experimental measurements of heat transfer coefficients are reported for three binary mixtures near their lower consolute points. Two of these, respectively n-pentane and n-decane in solution with supercritical CO2, involve vapor--liquid equilibrium whereas the third, triethylamine--water, involves liquid--liquid equilibrium. Anomalously high heat transfer coefficients were found for the supercritical mixtures at compositions which condense on heating (retrograde condensation). 

Prausnitz (1,2) has discussed this problem extensively, but the most successful techniques, which are based on either closed equations of state, such as discussed in this symposium, or on dilute liquid solution reference states such as in Prausnitz and Chueh (3), are limited to systems containing nonpolar species or dilute quantities of weakly polar substances. The purpose of this chapter is to describe a novel method for calculating the properties of liquids containing supercritical components which requires relatively few data and is of general applicability. Used with a vapor equation of state, the vapor-liquid equilibrium for these systems can be predicted to a high degree of accuracy even though the liquid may be 30 mol % or more of the supercritical species and the pressure more than 1000 bar. 

To date little or no thermodynamic modeling of the phase behavior of the ligand/C02 or metal chelate/C02 systems has been conducted. However, in order for supercritical fluid extraction to be considered as a possible replacement for organic solvent extraction, accurate models must be developed to predict the phase behavior of these systems to allow for both equipment and process design. Equation of state (EOS) modeling was chosen here to model the vapor-liquid equilibrium of the P-diketone/C02 systems studied. Cubic EOSs are the most widely used in modeling high pressure and supercritical fluid systems. This is... 

HAT Hattori, K., Wang, N., Takishima, S., and Masuoka, H., Chromatographic measurement of vapor-liquid equilibrium ratios of organic solvents in supercritical CO2 + poly(vinyl acetate) system, m. Solvent Extraction 1990, Ed. T. Sekine, Elsevier Sci. Publ., 1992, 1671. 

The Handbook is divided into seven chapters (1) Introduction, (2) Vapor-Liquid Equilibrium (VLB) Data of Binary Copolymer Solutions, (3) Liquid-Liquid Equilibrium (LLE) Data of Quasibinary or Quasitemary Copolymer Solutions, (4) High-Pressure Phase Equilibrium (HPPE) Data of Quasibinary or Quasitemary Copolymer Solutions in Supercritical Fluids, (5) Enthalpy Changes for Binary Copolymer Solutions, (6) PVT Data of Molten Copolymers, and (7) Second Virial Coefficients ( 2) of Copolymer Solutions. Finally, four appendices quickly route the user to the desired data sets. 

Critical Point This marks the end of the vapor-liquid equilibrium line. Above this temperature and pressure, the fluid will pass from properties that are liquid-like to properties that are gas-like without undergoing a phase change. A fluid above its critical point is said to be supercritical. 

Supercritical fluid extraction (SFE), a newly developed technique, is used for laboratorial and industrial purposes because it presents a series of advantages compared to the conventional extraction processes, especially for the extraction of thermolabile components [21,22]. It was first presented as a patent for decaffeination of coffee [23]. Since then, SFE has been used for many years as an alternative extraction method, which causes less pollution to the environment. The concept of the critical point was defined in 1822 as the highest pressure and temperature at which a pure substance could exist in vapor-liquid equilibrium. Above this point, supercritical fluid (SCF) is formed. These qualities make SCFs have higher diffiisivities and less degradation of solutes than ordinary solvents to extract active components. 

In these systems, the interface between two phases is located at the high-throughput membrane porous matrix level. Physicochemical, structural and geometrical properties of porous meso- and microporous membranes are exploited to facilitate mass transfer between two contacting immiscible phases, e.g., gas-liquid, vapor-liquid, liquid-liquid, liquid-supercritical fluid, etc., without dispersing one phase in the other (except for membrane emulsification, where two phases are contacted and then dispersed drop by drop one into another under precise controlled conditions). Separation depends primarily on phase equilibrium. Membrane-based absorbers and strippers, extractors and back extractors, supported gas membrane-based processes and osmotic distillation are examples of such processes that have already been in some cases commercialized. Membrane distillation, membrane... 

The above conclusions have resulted from an analysis of computer simulation data carried out on pure liquids and supercritical fluids, and on liquids in equilibrium with their vapor. One immediate question one should ask concerns thus a more general validity of the reached conclusions. Particularly important problem is to what extent they may remain valid for mixtures. Due to polarizability and other possible effects brought about by electrostatic interactions between unlike species, the pair interaction, and hence the local and, particularly, orientational arrangement may be changed considerably. With respect to a wide variety of mixtures this problem will require rather an extensive investigation. The most difficult mixtures will evidently be solutions of charged objects as e.g. electrolytes. 

The pressure-temperature phase diagrams also serve to highlight the fact that the polymorphic transition temperature varies with pressure, which is an important consideration in the supercritical fluid processing of materials in which crystallization occurs invariably at elevated pressures. Qualitative prediction of various phase changes (liquid/vapor, solid/vapor, solid/liquid, solid/liquid/vapor) at equilibrium under supercritical fluid conditions can be made by reference to the well-known Le Chatelier s principle. Accordingly, an increase in pressure will result in a decrease in the volume of the system. For most materials (with water being the most notable exception), the specific volume of the liquid and gas phase is less than that of the solid phase, so that... 

Consider first the schematic P-T and P-x diagrams for the naphthalene-ethylene system. Figure 3.18b depicts the solubility behavior of naphthalene in supercritical ethylene at a temperature greater than the UCEP temperature. Solid-gas equilibria exist at low pressures until the three-phase SLV line is intersected. The equilibrium vapor, liquid, and solid phases are depicted as points on the horizontal tie line at pressure Pj. As the pressure is further increased a vapor-liquid envelope is observed for overall mixture concentrations less than Xl- A mixture critical point is observed for this vapor-liquid envelope, as described earlier. If the overall mixture composition is greater than Xl, then solid-gas equilibria are observed as the pressure is increased above Pj. 

Various modeling procedures have been proposed in the literature to predict the phase behavior of vapor-liquid systems at high pressures. (The designation vapor will be used synonomously with supercritical fluid in this chapter.) Regardless of the modeling procedure, the following thermodynamic relationships, or their equivalent relationships in terms of chemical potentials, must be satisfied for two phases to be in equilibrium. 

Since then. Dr. Woldfarth s main researeh has been related to polymer systems. Currently, his research topics are molecular thermodynamics, continuous thermodynamics, phase equilibria in polymer mixtures and solutions, polymers in supercritical fluids, PVT behavior and equations of state, and sorption properties of polymers, about which he has published approximately 100 original papers. He has written the following books Vapor-Liquid Equilibria of Binary Polymer Solutions, CRC Handbook of Thermodynamic Data of Copolymer Solutions, CRC Handbook of Thermodynamic Data of Aqueous Polymer Solutions, CRC Handbook of Thermodynamic Data of Polymer Solutions at Elevated Pressures, CRC Handbook of Enthalpy Data of Polymer-Solvent Systems, and CRC Handbook of Liquid-Liquid Equilibrium Data of Polymer Solutions. 

Both adsorption from a supercritical fluid to an adsorbent and desorption from an adsorbent find applications in supercritical fluid processing. The extrapolation of classical sorption theory to supercritical conditions has merits. The supercritical conditions are believed to necessitate monolayer coverage and density dependent isotherms. Considerable success has been observed by the authors in working with an equation of state based upon the Toth isoterm. It is also important to note that the retrograde behavior observed for vapor-liquid phase equilibrium is experimentally observed and predicted for sorptive systems. 

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