The Carbon Dioxide-n-Hexane System

Parameter Value Standard Deviation Objective Function 

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If incorrect phase behavior is predicted by the EOS then constrained least squares (CLS) estimation should be performed and new parameter estimates be obtained. Subsequently, the phase behavior should be computed again and if the fit is found to be acceptable for the intended applications, then the CLS estimates should suffice. This was found to be the case for the carbon dioxide-n-hexane system presented later in this chapter. 

Table 14.3 Parameter Estimates for the Carbon Dioxide-n-Hexane System...

Figure 14.3 Vapor-liquid equilibrium data and calculated values for the carbon dioxide-n-hexane system. Calculations were done using interaction parameters from implicit and constrained least squares (LS) estimation, x and y are the mote fractions in the liquid and vapor phase respectively [reprinted from the Canadian Journal of Chemical Engineering with permission]...

Figure 3.3 Pressure-composition behavior of (a) the carbon dioxide-n-hexane system (Li, Dillard, and Robinson, 1981) and (b) the carbon dioxide-toluene system (Ng and Robinson, 1978). Both systems exhibit type-1 phase behavior.

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. 

This system illustrates the use of simplified constrained least squares (CLS) estimation. In Figure 14.3, the experimental data by Li et al. (1981) together with the calculated phase diagram for the system carbon dioxide-n-hexane are shown. The calculations were done by using the best set of interaction parameter values obtained by implicit LS estimation. These parameter values together with standard deviations are given in Table 14.3. The values of the other parameters (k, kj) were equal to zero. As seen from Figure... 

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. 

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