Pseudocritical properties for

The constants Ci and Cj are both obtained from Fig. 2-40 Ci, usually from the saturated liquid fine and Cj, at the higher pressure. Errors should be less than 1 percent for pure hydrocarbons except at reduced temperatures above 0.95 where errors of up to 10 percent may occur. The method can be used for defined mixtures substituting pseudocritical properties for critical properties. For mixtures, the Technical Data Book—Petroleum Befining gives a more complex and accurate mixing rule than merely using the pseudocritical properties. The saturated low pressure value should be obtained from experiment or from prediction procedures discussed in this section for both pure and mixed liquids. 

For hydrocarbon and nonpolar gas mixtures, the Pitzer pure component method can be used to predict vapor density by replacing the true critical properties with pseudocritical properties defined in... 

No specific mixing rules have been tested for predicting compressibility factors for denned organie mixtures. However, the Lydersen method using pseudocritical properties as defined in Eqs. (2-80), (2-81), and (2-82) in place of true critical properties will give a reasonable estimate of the compressibihty faclor and hence the vapor density. 

For mixtures, the pseudocritical properties of the mixture should be used to obtain the compressibility factor. 

These pseudocritical properties were devised simply for use in correlating physical properties. Pseudocritical properties are not equal to the actual critical properties of a gas mixture. Equations 3-42 are often called Kay s mixture rules.5 A somewhat more accurate method of calculating pseudocritical properties is given in Appendix B. 

Often the composition of a natural gas is unknown. However, specific gravity, being more easily measured, is usually available for a gas of interest. In this situation the correlation presented in Figure 3-11 can be used to determine the pseudocritical properties of the gas.8... 

The accuracy of the compressibility equation bf state is not any better than the accuracy of the values of the z-factors used in the calculations. The accuracy of Figures 3-7 and 3-8 was tested with data from 634 natural gas samples of known composition.8 Experimentally determined z-factors of these gases were compared with z-factors obtained from the charts using Kay s rules for calculating the pseudocritical properties and Figure 3-10 for properties of heptanes plus. 

However, the presence of hydrogen sulfide and carbon dioxide causes large errors in compressibility factors obtained by the methods previously discussed. The remedy to this problem is to adjust the pseudocritical properties to account for the unusual behavior of these acid gases.10 The equations used for this adjustment are... 

An alternative to Kay s method for calculating pseudocritical properties when gas composition is known is given below. This method provides pseudocritical properties which give more accurate values of z-factors when used with Figure 3-7 or 3-8. 

Sutton and Whitson evaluated several methods for estimating the pseudocritical properties of heptanes plus.2,5 The Kessler Lee equations are recommended.6... 

Sutton derived equations for pseudocritical properties of natural gases based on measured z-factors for 264 natural gas samples.2 He used the Dranchuk Abou-Kassem equation for z-factors and the Wichert Aziz adjustment for nonhydrocarbon components.4,7,8 Sutton s equations are... 

Figure 3-12, page 122, Nonhydrocarbon Adjustment Factors for Pseudocritical Properties of Natural Gases... 

For mixtures, pseudocritical properties are used for the evaluation of the reduced properties. For use with Figure 7.28, Kay s rules are applicable, namely,... 

Here is the mole fraction of species i in a mixture. We may use Figure 2.4 for fluid mixtures, with pseudocritical properties instead of critical properties. This procedure yields reasonable accuracy for mixtures of chemically similar substances. 

Reduced properties are then calculated for the mixture based on the pseudocritical properties. 

Mixtures Application of CS methods and analytical equations of state (EoS) to mixtures typically requires pseudocritical temperatures and pressures. These values are obtained from the pure-component critical properties by applying mixing rules specific to the method under consideration, mile pseudocritical values should be used for mixture CS and EoS applications, the values are usually quite different from the mixtures true critical properties. A variety of methods are available for estimating true critical properties for mixtures (see PGL4), but only modest accuracy can be expected. 

You should note that when employing corresponding-states correlations of PVT and thermodynamic properties, it is appropriate to employ pseudocritical values for hydrogen. 

For mixtures of dense gases the pseudocritical method is recommended. Here the critical properties for the mixture are given by... 

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