The Methane-n-Hexane System

The proposed methodology was also followed and the best parameter estimates for the various types of data are shown in Table 14.7 for the methane-n-hexane system. As seen, the parameter set (ka, kd) was found to be the best to correlate the VL2E, the LiL2E and the VL2LtE data and another (k , kb) for the VL E data. 

Parameter Values Standard Deviation Used Data Objective Function 

12 VLE data and calculated phase diagram for the hydrogen sulfide-water system [reprinted from Industrial Engineering Chemistiy Research with permission from the American Chemical SocietyJ. 

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Data for the hydrogen sulfide-water and the methane-n-hexane binary systems were considered. The first is a type III system in the binary phase diagram classification scheme of van Konynenburg and Scott. Experimental data from Selleck et al. (1952) were used. Carroll and Mather (1989a b) presented a new interpretation of these data and also new three phase data. In this work, only those VLE data from Selleck et al. (1952) that are consistent with the new data were used. Data for the methane-n-hexane system are available from Poston and McKetta (1966) and Lin et al. (1977). This is a type V system. 

Table 14.7 Parameter Estimates for the Methane-n-Hexane System...

Using the estimated interaction parameters phase equilibrium computations were performed. It was found that the EoS is able to represent the VL2E behavior of the methane-n-hexane system in the temperature range of 198.05 to 444.25 K reasonably well. Typical results together with the experimental data at 273.16 and 444.25 K are shown in Figures 14.14 and 14.15 respectively. However, the EoS was found to be unable to correlate the entire phase behavior in the temperature range of 195.91 K (Upper Critical Solution Temperature) and 182.46K (Lower Critical Solution Temperature). 

Five critical points for the methane-n-hexane system in the temperature range of 198 to 273 K measured by Lin et al. (1977) are available. By employing the Trebble-Bishnoi EoS in our critical point regression least squares estimation method, the parameter set (k , kb) was found to be the optimal one. Convergence from an initial guess of (ka,kb=0.001, -0.001) was achieved in six iterations. The estimated values are given in Table 14.8. 

Poston, R.S. McKetta, J. "Vapor-Liquid Equilibrium in the Methane-n-Hexane System", J. Chem. Eng. Data, 11,362-363 (1966). 

Using the estimated interaction parameters phase equilibrium computations were performed. It was found that the EoS is able to represent the VL2E behavior of the methane-n-hexane system in the temperature range of 198.05 to... 

Merrill, R. C., K. D. Luks, and J. P. Kohn. 1983. Three phase liquid-liquid-vapor equilibria in the methane -f n-pentane + M-octane, methane + n-hexane + n-octane, and methane + n-hexane + carbon dioxide systems. J. Chem. Eng. Data 28 210. 

Type IV systems have three critical curves, two of which are VLL. If the hydrocarbon mixtures differ significantly in their critical properties, they conform to type IV or V. The primary difference between Type IV and V is that type IV exhibits UCST and LCST while type V has LCST only. One important elass of systems that exhibit type IV behavior is solvent polymer mixtures such as cyclohexane -i- polystyrene. Other examples of type IV include carbon dioxide 4- nitrobenzene and methane + n-hexane while ethane with ethanol or 1-propanol or 1-butanol exhibit type V behavior. 

Class C Consists of systems which show partial miscibility over a limited range of temperatures spanning the critical temperature of the more volatile component but which become completely miscible at higher and lower temperatures (though partial miscibility may reappear at substantially subcritical temperature). This category includes types IV and V in Rowlinson s treatment. The systems ethane/ethanol [20] ethane/propanol, ethane/butanol, carbon dioxide/nitrobezene, carbon dioxide/2-nitrophenol, methane/n-hexane and methane/1-hexene [16] belong to this class. 

Fig. 3. Temperature dependence of predicted hj by means of Eq. (35). Solid line system methane/ propane. Short dashed line system methane/n-hexane. Long dashed line system methane/ n-decane. In abscissa, T, is the reduced temperature of the heavy n-alkane (respectively propane, n-hexane, n-decane).

Although comparisons for the steam-methane system have been presented, similar trends were noted for the other binary systems previously published by Wormald, namely mixtures of steam with nitrogen, carbon dioxide, n-hexane, and benzene. 

The interaction parameters for binary systems containing water with methane, ethane, propane, n-butane, n-pentane, n-hexane, n-octane, and benzene have been determined using data from the literature. The phase behavior of the paraffin - water systems can be represented very well using the modified procedure. However, the aromatic - water system can not be correlated satisfactorily. Possibly a differetn type of mixing rule will be required for the aromatic - water systems, although this has not as yet been explored. 

As discussed in Sec. 4, the K value of a species is a complex function of temperature, pressure, and equilibrium vapor- and hquid-phase compositions. However, for mixtures of compounds of similar molecular structure and size, the K value depends mainly on temperature and pressure. For example, several major graphical K-value correlations are available for light-hydrocarbon systems. The easiest to use are the DePriester charts [Chem. Eng. Prog. Symp. Ser. 7, 49, 1 (1953)], which cover 12 hydrocarbons (methane, ethylene, ethane, propylene, propane, isobutane, isobutylene, n-butane, isopentane, n-pentane, n-hexane, and n-heptane). These charts are a simplification of the Kellogg charts [Liquid-Vapor Equilibria in Mixtures of Light Hydrocarbons, MWK Equilibrium Constants, Polyco Data, (1950)] and include additional experimental data. The Kellogg charts, and hence the DePriester charts, are based primarily on the Benedict-Webb-Rubin equation of state [Chem. Eng. Prog., 47,419 (1951) 47, 449 (1951)], which can represent both the liquid and the vapor phases and can predict K values quite accurately when the equation constants are available for the components in question. 

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