Correlations Chueh-Prausnitz

The Chao-Seader correlation and its many modifications, and the Chueh-Prausnitz correlation are examples of this approach. In the Chao-Seader correlation f and ir are combined as a pure liquid fugacity coefficient, v, so that Eq. (5) has three distinct parts —each requiring a unique equation, as just described. 

Rather, an equation of state and semiemplrical correlations are used e.g. the Chueh-Prausnltz correlation scheme (10). In this approach the critical pressure of the mixture is obtained indirectly from a modified version of the Redlich-Kwong-Chudi equation of state after Tcm and have been obtained directly from quadratic mixing rules which employ the respective Chueh-Prausnitz Interaction parameters, I12 and V3 2 An example based on this method is given in Figure 5 for the ethane-n-heptane binary mixture. Agreement is rather good except in the immediate vicinity of the maximum critical pressure of the mixture. 

At pressures above a few atmospheres, the deviations from ideal behaviour in the gas phase will be significant and must be taken into account in process design. The effect of pressure on the liquid-phase activity coefficientmustalso be considered. A discussion of the methods used to correlate and estimate vapour-liquid equilibrium data at high pressures is beyond the scope of this book. The reader should refer to the texts by Null (1970) or Prausnitz and Chueh (1968). 

One of the most surprising early successes was with cryogenic HL-hydrocarbon systems. Here what was so surprising was not just that the method could be made to work, but that it worked so easily once the procedure for incorporating H2 developed by Chueh and Prausnitz (7) was adopted. Results for H2-hydrocarbon systems have been explored in more detail in a recent study (22) for the present discussion it is sufficient to note that a single Ct, value correlates H2 and hydrocarbon K-values over substantial ranges of temperature and pressure for cryogenic systems. 

The COSTALD correlation is quite accurate even at high reduced temperatures and pressures. Predicted liquid densities generally agree with measured values within 1-2% provided the errors in the critical property predictions are low. A potential problem can occur if the reduced temperature is greater than 1. There can be discontinuity from the Spencer-Danner equation in the density prediction which may cause some process models to fad. However, at a reduced temperature greater than 1, the equation of state becomes more accurate and can be used directly. Aspen HYSYS includes a smoothing approach (using the Chueh and Prausnitz correlation [16]) to ensure a smooth transition from the COSTALD densities to equation-of-state-based densities. 

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