Supercritical components, liquids

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. 

In some cases, the temperature of the system may be larger than the critical temperature of one (or more) of the components, i.e., system temperature T may exceed T. . In that event, component i is a supercritical component, one that cannot exist as a pure liquid at temperature T. For this component, it is still possible to use symmetric normalization of the activity coefficient (y - 1 as x - 1) provided that some method of extrapolation is used to evaluate the standard-state fugacity which, in this case, is the fugacity of pure liquid i at system temperature T. For highly supercritical components (T Tj,.), such extrapolation is extremely arbitrary as a result, we have no assurance that when experimental data are reduced, the activity coefficient tends to obey the necessary boundary condition 1... 

In a binary liquid solution, we differentiate between the subcritical component 1, called the solvent, (for which T TCi). For component 1, we normalize 7 in the usual way ... 

If we use the symmetric convention for normalization,/ 0 is the fugacity of pure liquid / at the temperature of the mixture and at some specified pressure, usually taken to be the total pressure of the system. Equation (69) presents no problem for subcritical components, where the pure liquid can exist at the system temperature. However, for supercritical components in the symmetric convention,/,0 is a fictitious quantity which must be evaluated by some arbitrary extrapolation. 

For components near or above their critical temperatures, the liquid volume t>j was evaluated by extrapolation with respect to temperature. For supercritical components, the fugacity f° was also evaluated by extrapolation the effect of pressure was found from the Poynting relation using the previously extrapolated liquid molar volumes. 

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

Table 3 shows the 19 chemical species considered in the model, that is, 5 inert gases, C02 to be absorbed, solvent water, MDEA, and PZ amines, and 10 ionic species. All ionic species, zwitterion included, exist only in the liquid phase. The vapor phase components are mainly inert, that is, supercritical components that dissolve sparingly into the liquid. 

To estimate the pure component parameters, we used the technique of Panagiotopoulos and Kumar (11). The technique provides parameters that exactly reproduce the vapor pressure and liquid density of a subcritical component. Table II presents the pure component parameters that were used. For the supercritical components, the usual acentric factor correlation was utilized. 

In addition, several equations of state have been developed to predict the VLE behavior of a subcritical liquid mixture with a supercritical component. These theoretical models are of current research interest. In addition, several approaches have been formulated to extend the analysis to multicomponent systems utilizing concepts of continuous thermodynamics(9. 101. 

According to Eq. (42), equating the chemical potentials of the gaseous and the liquid phase of the supercritical component SO2... 

Both the mutual solubility of the coexisting liquid and supercritical gas phases and the density of carbon dioxide are the most important parameters in influencing interfacial tension in systems with both a non-volatile liquid and a supercritical component. 

The critical region becomes very interesting when one considers the behavior of mixtures. At its most basic level, the critical point is the point at which the vapor and the liquid become indistinguishable at fixed overall composition. The path to the critical point follows an indirect route when the composition is rich in a supercritical component like CO2 and leans in a... 

One complication with this description is that a species can be present in a liquid mixture, though at the temperature and pressure of the mixture the substance would be a vapor or a solid as a pure component. This is especially troublesome if the compound is below its melting point, so that it is the solid sublimation pressure rather than the vapor pressure that is known, or if the compound is above its critical temperature, so that the vapor pressure is undefined. In the first case one frequently ignores the phase change and extrapolates the liquid vapor pressure from higher temperatures down to the temperature of interest using, for example, the Antoine equation, eqn. (2.3.11). For supercritical components it is best to use an EOS and compute the fugacity of a species in a mixture, as described in Section 2.5. 

Earlier chapters use simplified and binary models to analyze in a very informative manner some fundamentals such as the effect of reflux ratio and feed tray location, and to delineate the differences between absorption/stripping and distillation. Following chapters concentrate on specific areas such as complex distillation, with detailed analyses of various features such as pumparounds and side-strippers, and when they should be used. Also discussed are azeotropic, extractive, and three-phase distillation operations, multi-component liquid-liquid and supercritical extraction, and reactive multistage separation. The applications are clearly explained with many practical examples. 

An equation of state, applicable to all fluid phases, is paitiodariy useful for phase-equilibrium calculations where a liquid phase and a vapor phase coexist at high pressures. At such conditions, conventional activity coefficients are not useful because, with rare exceptions, at least one of the mixture s components is supercritical that is, (he system temperature is above (hat component s critical temperature. In that event, one must employ special standard states for the activity coefficients of the supercritical components (see Section 1.5-2). That complication is avoided when ail fugacities are calculated front en equation of state. 

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. 

Vapor and liquid phases coexist in virtually all areas of petroleum production operations, including reservoirs, wellbores, surface-production units, and gas-processing plants. Knowledge of fluid properties and phase behavior is required to calculate the fluid in place, fluid recovery by primary depletion, and fluid recovery by enhanced oil recovery techniques such as gas cycling, hydrocarbon solvent injection, and C02 displacement. Because of the complex nature of petroleum reservoir fluids and the often complicated phase behavior observed at elevated temperature and pressure conditions, the fluid properties and phase behavior historically have been measured experimentally. The complex nature of the fluids arises because of the supercritical components which are dissolved in the mixture of paraffinic, naphthenic, and... 

The phase-split block can be a single flash, a series of flashes, or a combination of flash and absorption/stripping columns. Flash temperature and pressure are design variable that may be optimised to fulfil a separation objective, as sharp gas/liquid split or recovery of some components. For water-driven condensers the recommended condensation temperature is of about 35 °C. Vapour components can be condensed and sent to the liquid separation system. The supercritical components carried in the liquid phase can be recovered in a stabiliser column (see later in this section). Further, these can be sent to the gas separation system, used as fuel, or purged. 

Separation processes with supercritical gases, called supercritical fluid extraction (SFE), is a group of separation processes that applies supercritical fluids (SCFs) as separating agents in the same way as other separation processes, such as liquid-liquid extraction or absorption, make use of liquid solvents. In these processes, the solvent is a supercritical component or a supercritical mixture of components [1-3]. 

Compare the behavior of the ethylene-n-propanol system to a type I system, CO2-hexane, shown in Figure 13. Note that in both cases, solubility of the supercritical component in the liquid phase increases rapidly as the pressure increases. This phenomenon can lead to substantial swelling of the liquid phase. The solubility of the heavy component in the lighter phase does not increase rapidly with an increase in pressure until the mixture critical point is approached or an additional phase is formed. The conditions of these occurrences may be substantially removed from the pure supercritical fluid critical conditions. 

Interesting behavior occurs in ternary systems when two immiscible liquids are contacted with a supercritical fluid. The supercritical fluid is preferentially dissolved in the most chemically similar phase, effectively increasing the degree of liquid-liquid immiscibility ( ). In addition, it is possible to create immiscibility from a homogeneous liquid mixture by contacting the liquid phase with a high pressure gas if one liquid component is sufficiently chemically different than the supercritical component. 

Stability of phase boundaries depends on the surface tension. Surface tension in a supercritical fluid system is of major importance for drying, surfactant eflicacy, and extraction. The surface tension of a gas increases with pressure and approaches zero at the critical point while the surface tension of liquid decreases with pressure resulting in dissolution of supercritical components in the liquid phase. The mefliods useful in correlating surface tension include Macleod-Sugden correlation and corresponding states theory. ... 

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),... 

---