Pitzer-Curl

For nonpolar compounds and mixtures, the correlation is a modified form of the Pitzer-Curl relationship (see Ref. 1) ... 

Guggenheim Equations 1 and 2 are in corresponding-states form that is, the reduced-form equations pr(Tr) explicitly contain no material constants and are formally applicable to all substances (in practice, to substances having small nearly spherical molecules, with modest accuracy). The desirable corresponding-states formulation will be retained in the equation presented here in a modified form, by including (as is now usual) the Pitzer-Curl acentric factor w as an explicit material constant ... 

The values for the enthalpies of the streams in the database were based on the Curl-Pitzer congelations (Green, 1997). The enthalpies are calculated from correlations at zero pressure (functions of temperature and composition only) and then corrected via the enthalpy deviation ... 

Pitzer, K.S., Lippmann, D.Z., Curl, R.F., Huggins, C.M. and Peterson, D.E. (1955b). The Volumetric and Thermodynamic Properties of Fluids. II. Compressibility Factor, Vapor Pressure and Entropy of Vaporization. JAm.Chem.Soc., 77, 3433-3440. 

The Kesler-Lee correlations for liquid and vapour phase heat capacities of petroleum fluids are used for estimating the respective enthalpies at temperatures of interest. The Lee-Kesler corresponding-states method is used for obtaining estimates of the heats of vaporization and for developing the saturation envelope enthalpies. This method uses the Curl and Pitzer approach and calculates various thermodynamic properties by representing the compressibility factor of any fluid in terms of a simple fluid and a reference fluid as follows ... 

The liquid-phase fugacity coefficient = /f/P may be calculated from a generalized correlation in terms of reduced temperature and pressure such as those of Lydersen et al.42 and Curl and Pitzer.15 Chao and Seader used a modified form of the Curl and Pitzer correlation. The correlation was modified by use of experimental data such that appropriate values of could be computed for the case where a component does not exist as a liquid and for the case of low temperatures. The following expression was proposed for the calculation of the fugacity coefficient for any component / in the liquid phase... 

Equation 25 was developed from an empirical representation of thg second virial coefficient correlation of Pitzer and Curl (I) parameter b was left unchanged at its classical value of 0.0866. Because of the substantial improvement in the prediction of and its temperature derivatives for nonsimple fluids, the Barner modification of the RK equation gave improved estimates of enthalpy deviations for nonpolar vapors and for vapor-phase mixtures of hydrocarbons. However, the new equation was unsuitable for fugacity calculations. 

It should be mentioned that the difference between Equations 37 and 27 is very small. The second virial coefficient calculated from Equation 25 using the new expression of f0(T) still agrees very well with the correlation of Pitzer and Curl (Equation 26). 

Table 3.2 presents a selection of the most used thermodynamic options for phase equilibrium with suitable enthalpy and entropy methods. The accuracy of both phase equilibrium and enthalpy/entropy computation must be examined when using EOS models. For example, often a cubic EOS underestimates the enthalpy of vaporisation. In this case other methods are more accurate, as those based on three-parameters corresponding states law (Lee-Kesler, Curl-Pitzer, etc.). Mixtures rich in components with particular behaviour, as or CH, need special methods for accurate simulation. When binary interaction parameters for liquid activity models are absent, the UNI FAC predictive method may be employed. It is worth to note that UNIFAC is suitable only for exploratory purposes, but not for final design. When high non-ideal mixtures are involved at higher pressure then the combination of EOS with liquid activity models is recommended (see Chapter 6). 

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