Supercritical components, liquids containing

A Predictive Method for PVT and Phase Behavior of Liquids Containing Supercritical Components... 

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. 

Mathias, P. M. and J. P. O Connell. 1979. A predictive method for PVT and phase behavior of liquids containing supercritical components. In Equations of State in Engineering and Research. Washington, DC American Chemical Society. 

When the system temperature T is greater than die critical temperature T of component i, then pure c cannot exist as a liquid. The procedure of Section 1.5-1, which incorporates the vapor-liquid saturation pressure P7 . ts therefore inappropriate for representing VLE for mixtures containing supercritical component i. Several methods are available for ffie quantitative description of such cases the most powerful of them is that using an equation of state as discussed briefly in S tion 1.7. Alternatively, one may use Eq. (1.5-2),... 

The only possible satisfactory procedure for proper use of activity coefficients of supercritical components is to use Henry s constants as the standard-state fugacity. Henry s constants are not hypothetical but are experimentally accessible also, at least in principle, they can be calculated from an equation of state. Remarkably little attention has been given to the formal thermodynamics of liquid mixtures containing supercritical components. 

However, if the liquid solution contains a noncondensable component, the normalization shown in Equation (13) cannot be applied to that component since a pure, supercritical liquid is a physical impossibility. Sometimes it is convenient to introduce the concept of a pure, hypothetical supercritical liquid and to evaluate its properties by extrapolation provided that the component in question is not excessively above its critical temperature, this concept is useful, as discussed later. We refer to those hypothetical liquids as condensable components whenever they follow the convention of Equation (13). However, for a highly supercritical component (e.g., H2 or N2 at room temperature) the concept of a hypothetical liquid is of little use since the extrapolation of pure-liquid properties in this case is so excessive as to lose physical significance. 

IX. Liquid-Liquid Equilibria in Ternary Systems Containing One Supercritical Component... 

An efficient and convenient methodology for the aerobic oxidation of alcohols catalysed by sol-gel trapped perruthenate and promoted by an encapsulated ionic liquid in supercritical carbon dioxide solution has been reported. The reaction is highly selective and useful for substrates otherwise difficult to oxidize.263 A four-component system consisting of acetamido-TEMPO-Cu(C104)2-TMDP-DABCO has been developed for aerobic alcohol oxidation at room temperature. The catalytic system shows excellent selectivity towards the oxidation of benzylic and allylic alcohols and is not deactivated by heteroatom-containing (S, N) compounds. The use of DMSO as the reaction medium allows the catalysts to be recycled and reused for three runs with no significant loss of catalytic activity.264... 

The homopolymerization of ethylene is often carried out with a mixture containing four components monomer (M), polymer (P), diluent (D) and initiator (I). The phases which may co-exist are solids (c), liquid (1), vapor (g) and a supercritical fluid phase (s) which may upon a change of temperature, pressure or composition separate into a liquid and a vapor phase. The components coexisting in a given phase will be written within a parenthesis, the phase will be labeled with a superscript and coexistence of two or more phases will be indicated by a slash (/) for example, the notation (MD) /(MPDI) indicates that a vapor and... 

To illustrate calculations for a binary system containing a supercritical, condensable component. Figure 12 shows isobaric equilibria for ethane-n-heptane. Using the virial equation for vapor-phase fugacity coefficients, and the UNIQUAC equation for liquid-phase activity coefficients, calculated results give an excellent representation of the data of Kay (1938). In this case,the total pressure is not large and therefore, the mixture is at all times remote from critical conditions. For this binary system, the particular method of calculation used here would not be successful at appreciably higher pressures. 

The method of analysis involves extraction of 1 L of aqueous sample (liquid-liquid extraction) or 25 g of soil (sonication or Soxhlett extraction or supercritical fluid extraction) or an appropriate amount of the sample with methylene chloride. The extract is dried, concentrated to a volume of 1 mL, and injected into a capillary GC column for separation and detection by FID. For quantitation, the area or height response of all peaks eluting between C-10 and C-28 are summed and compared against the chromatographic response of the same peaks in a 2 Fuel or Diesel Oil standard. A 10-component n-alkanes mixture containing even numbered alkanes ranging between 10 and 28 C atoms has been recommended as an alternative calibration standard. These alkanes occur in all types of diesel oils, and each compound constitutes approximately a 1% total mass of diesel fuel, i.e., 1 g of diesel fuel contains about 10 mg each of any of the above alkanes. Therefore, when using the latter as a calibration standard, the result must be multiplied appropriately by 100. 

This study is by no means comprehensive and covers only a narrow range of variables. However, it does demonstrate the influence of entrainer in the improvement of separation over a single supercritical solvent. The increase in selectivity (1.4 to 1.8) for butene/butadiene mixtures is compared with the value of 1.63 obtained with liquid ammonia for the same binary system(l). Moreover, it has been demonstrated that a mixture of a pure solvent and an entrainer permits an improvement in the separation at temperatures and pressures lower than would have been otherwise predicted with a single gas solvent(20). For mixtures containing a highly polar component, such as ammonia, molecular size alone cannot account for the large selectivities observed in these experiments. At present, all theories are inadequate in explaining the chemical interactions between the entrainer and the mixture. The state of the art is comparable to liquid phase solvent extraction. 

The separation of the liquid components in the presence of a supercritical solvent occurs much as it does in liquid extraction with the entrainer, ammonia, concentrating in the liquid to increase the relative volatility of the butene to butadiene. The butadiene migrates to the ammonia-rich phase while the solvent gas phase or "vapor" will contain the butene. 

To get an idea about the relative volatilities of components we proceed with a simple flash of the outlet reactor mixture at 33 °C and 9 bar. The selection of the thermodynamic method is important since the mixture contains both supercritical and condensable components, some highly polar. From the gas-separation viewpoint an equation of state with capabilities for polar species should be the first choice, as SR-Polar in Aspen Plus [16]. From the liquid-separation viewpoint liquid-activity models are recommended, such as Wilson, NRTL or Uniquac, with the Hayden O Connell option for handling the vapor-phase dimerization of the acetic acid [3]. Note that SR-Polar makes use of interaction parameters for C2H4, C2H6 and C02, but neglects the others, while the liquid-activity models account only for the interactions among vinyl acetate, acetic acid and water. To overcome this problem a mixed manner is selected, in which the condensable components are treated by a liquid-activity model and the gaseous species by the Henry law. 

Besides these thermodynamic criteria, the most common approach used in the literature is based on the operation at pressures above the binary (liquid - SC-CO2) mixture critical point, completely neglecting the influence of solute on VLEs of the system. But, the solubility behavior of a binary supercritical COj-containing system is frequently changed by the addition of a low volatile third component as the solute to be precipitated. In particular, the so-called cosolvency effect can occur when a mixture of two components solvent+solute is better soluble in a supercritical solvent than each of the pure components alone. In contrast to this behavior, a ternary system can show poorer solubility compared with the binary systems antisolvent+solvent and antisol-vent+solute a system with these characteristics is called a non-cosolvency (antisolvent) system. hi particular, in the case of the SAS process, they hypothesize that the solute does not induce cosolvency effects, because the scope of this process lies in the use of COj as an antisolvent for the solute, inducing its precipitation. 

In hydrocarbon processing, residual streams and asphaltenes contain oils and resins mixed with high molecular weight hydrocarbons and undesirable metals and carbon residues. The oils and resins may be recovered more effectively by supercritical extraction than by other processes. One possible extraction fluid is supercritical pentane, which selectively dissolves the oils and resins. The extracted components are then conveniently separated from the pentane by raising the extract temperature. The extract separates into a liquid phase containing the oils and resins, and a vapor phase containing the pentane. 

This is the first of the coffee decaffeination patents that describe a continuous, counter-current liquid-liquid extraction. A brief description of the process is provided here. A water extract of roasted coffee beans, called coffee liquor, which contains aromas and caffeine and other water soluble components such as carbohydrate and protein materials is fed to a vacuum suipper. The extract is concentrated to about 30-50% in an evaporator-condenser and is fed to a sieve tray tower. The liquor passes across the hays in the tower downward through downspouts countercurrent to supercritical CO2 which enters the tower at the bottom and passes upward through the holes in the sieve trays. CO2 extracts caffeine from the liquor, and the decaffeinated liquor leaves the near the bottom of tower. The condensate water from the vacuum stripper prior to the tray tower is fed to the sieve trays in the top section of the tower. The water washes the caffeine from the supercritical CO2 passing upward. The caffeine-free CO2 is recycled to the bottom of the column. 

Solubility is an oft ill-defined term, used rather indiscriminately to refer to small amounts of a solute of one phase dissolved in a solvent of another phase. Invariably, the solvent is a liquid or dense fluid, though it may contain any number of components, while the solute may be gas, liquid, or solid. Solubility problems are really phase-equilibrium problems and are attacked using the general strategies presented in Chapter 10. In this section we describe the three common solubility problems gas solubility, which refers to supercritical gases dissolved in liquids ( 12.2.1) solid solubility, which refers to solids dissolved in liquids ( 12.2.2) and solubilities in near-critical... 

This brief survey begins in Sec. II with studies of the aggregation behavior of the anionic surfactant AOT (sodium bis-2-ethylhexyI sulfosuccinate) and of nonionic pol-y(ethylene oxide) alkyl ethers in supercritical fluid ethane and compressed liquid propane. One- and two-phase reverse micelle systems are formed in which the volume of the oil component greatly exceeds the volume of water. In Sec. Ill we continue with investigations into three-component systems of AOT, compressed liquid propane, and water. These microemulsion systems are of the classical Winsor type that contain water and oil in relatively equal amounts. We next examine the effect of the alkane carbon number of the oil on surfactant phase behavior in Sec. IV. Unusual reversals of phase behavior occur in alkanes lighter than hexane in both reverse micelle and Winsor systems. Unusual phase behavior, together with pressure-driven phase transitions, can be explained and modeled by a modest extension of existing theories of surfactant phase behavior. Finally, Sec. V describes efforts to create surfactants suitable for use in supercritical CO2, and applications of surfactants in supercritical fluids are covered in Sec. VI. 

Supercritical carbon dioxide extraction Supercritical carbon dioxide extraction is a relatively recent process and yields products of extremely high quality. The process is, however, relatively expensive due to high cost of liquid carbon dioxide. The critical temperature of carbon dioxide is 31°C and the critical pressure just over 1000 psi. Critical carbon dioxide is an excellent solvent for essential oils. Due to the relatively low temperature of the extraction, it can easily handle thermally labile oils without degradation and in addition it is chemically very inert and so does not react with any of the essential oil components. The essential oil is easily recovered from the extract by reducing the pressure in a controlled manner and allowing the carbon dioxide to evaporate. The extraction is carried out in a pressurized container constructed from heavy duty stainless steel at 35°C and 1000 psi. The equipment can also be very expensive. 

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