Compressibility pseudocritical factor

The pseudocritical compressibility factor is obtained directly from the i acentric factor using the expression ... 

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. 

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... 

Pseudocritical properties obtained by either of the methods described previously are adjusted in this manner. These adjustments bring the accuracy of calculated compressibility factors to within the limits expressed above. 

The volume of the gas in the reservoir may be calculated using the compressibility equation of state. This calculation is based on 1 lb mole of gas using Equation 3-39. The composition of the gas in the reservoir calculated by the recombination method can be used to compute the pseudocritical properties so that the compressibility factor may be obtained in the same manner as illustrated in Examples 3-10 and 3-12. 

The units of the critical and pseudocritical properties are psia and °R. Figures, 3-7 and 3-8, pages 112-413, Compressibility Factors of... 

Calculate the volume using Kay s method. In this method, V is found from the equation V = ZRT/P, where Z, the compressibility factor, is calculated on the basis of pseudocritical constants that are computed as mole-fraction-weighted averages of the critical constants of the pure compounds. Thus, T = Z K, 71, and similarly for Pc and Z, where the subscript c denotes critical, the prime denotes pseudo, the subscript i pertains to the ith component, and Y is mole fraction. Pure-component critical properties can be obtained from handbooks. The calculations can then be set out as a matrix ... 

For a quick estimate one may compute the pseudocritical parameters for the mixture using Kay s mole-fraction-averaging mixing rule and obtain the compressibility factor from the generalized corresponding-state correlation as shown in step 2. 

To perform PVT calculations for nonideal gas mixtures, you may use Kay s rule. Determine pseudocritical constants (temperature and pressure) by weighting the critical constants for each mixture component by the mole fraction of that component in the mixture then calculate the reduced temperature and pressure and the compressibility factor as before. 

Use Kay s method of pseudocritical values to calculate the pseudo-reduced values and predict p, V, T, and n via the compressibility factor. 

Kay s method is known as a two-parameter rule since only pc and Tc for each component are involved in the calculation of z. If a third parameter such as the Pitzer acentric fector, or Vc is included in the determination of the mean compressibility factor, then we would have a three-parameter rule. All the pseudocritical methods do not provide equal accuracy in predicting p V-T properties, but most suffice for engineering work. Stewart et al. reviewed 21 different methods of determining the pseudoreduced parameters by three-parameter rules (see Table 3.6). Although Kay s method was not the most accurate, it was easy to use and not consid-... 

Once this function is determined, it could be applied to any substance, provided its critical constants Pc, T, and V are known. One way of applying this principle is to choose a reference substance for which accurate PVT data are available. The properties of other substances are then related to it, based on the assumption of comparable reduced properties. This straightforward application of the principle is valid for components having similar chemical structure. In order to broaden its applicability to disparate substances, additional characterizing parameters have been introduced, such as shape factors, the acentric factor, and the critical compressibility factor. Another difficulty that must be overcome before the principle of corresponding states can successfully be applied to real fluids is the handling of mixtures. The problem concerns the definitions of Pq P(> and Vc for a mixture. It is evident that mixing rules of some sort need to be formulated. One method that is commonly used follows the Kay s rules (Kay, 1936), which define mixture pseudocritical constants in terms of constituent component critical constants ... 

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