Pitzer correlation

Although the Pitzer correlations are based on data for pure materials, they may also be used for the calculation of mixture properties. A set of recipes is required relating the parameters T, Pc, and (0 for a mixture to the pure-species values and to composition. One such set is given by Eqs. (2-80) through (2-82) in Sec. 2, which define pseudopa-rameters, so called because the defined values of T, Pc, and (0 have no physical significance for the mixture. 

The Pitzer correlation provides reliable results for gases which are nonpolar or only slightly polar for these, errors of no more than 2 or 3 percent are indicated. When applied to highly polar gases or to gases that associate, larger errors can be expected. 

The truncated virial equation (331), with a value of B from the generalized Pitzer correlation,... 

The generalized correlations of Pitzer provide an alternative to the use of a cubic equation of state for the calculation of thermodynamic properties. However, no adequate general method is yet known for the extension of the Pitzer correlations based on the compressibility factor to mixtures. Nevertheless, Z, as given by... 

Compute the heat of vaporization using the Pitzer correlation. Thus,... 

Pitzer correlation discussed in Example 1.12. If a heat-of-vaporization value at any other temperature is available, use the Watson correlation (Example 1.12) to obtain the value at the normal boiling point. 

Phase rule, 37-39 362-363, 529-532 Pitzer correlations (see Generalized correlations)... 

The subscript 0 denotes simple fluids with acentric factor to = 0, and subscript r denotes a reference fluid with acentric factor to,. = 0.3978 from its origin in n-octane. The Lee-Kesler equation constants for Zq and Zy are presented in Table 4.4. The compressibihty factors in Equation (4.239)— Zq, Zy—are at the same [T,.,pJ. This correlation is a three-parameter generahzed correlation that improves the Pitzer correlation to a wider range of states. The lower temperamre bound of the correlation is extended from = 0.8 to 0.3. 

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... 

We then obtain values of the parameters a, P, and y by a least squares fit to B(T) data for gases composed of small rigid nonpolar molecules. Using the numerical values in (P4.31.1) as initial guesses in the fit, the result is the expression for Bg given by the Pitzer correlation in Problem 4.22. 

Assume air is a binary mixture of nitrogen and oxygen. Use the following models to estimate the fugadties of each component at 25°C and 40 bar. (a) Ideal gas. (b) Lewis-Randall ideal solution with components obeying the simple vir-ial equation Z = 1 + BP/RT and values of the second virial coefficients provided by the Pitzer correlation in Problem 4.22. 

Use the Pitzer correlation with the Lee-Kesler graphs to estimate the density of carbon dioxide at 35 bar, 75 °C. Solution First we collect the critical properties of CO2 ... 

Calculate the molar volume of ethylene at 40 °C, 90 bar, using the (a) ideal-gas law, (b) the truncated virial equation, and (c) the Pitzer correlation with the Lee-Kesler values for Zc°>, Z ). 

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