Summary.

The objective of this study is to present a review of eight equations of state (EOS's) and compare their ability to predict the volumetric and phase equilibria of gas-condensate systems. Included in the study are the Peng-Robinson (PR), the Soave-Redlich-Kwong (SRK), the Schmidt-Wenzel (SW), the Usdin-McAuliffe (UM), the Heyen, the Kubic, the Adachi-Lu (AL), and the Patel-Teja (PT) EOS'S. The SW equation exhibits a superior predictive capability for volumetric properties of condensate systems. The PR equation is found to represent the phase equilibrium behavior of condensate systems accurately. In terms of compressibility factors, the SW and PT equations give better predictions than other equations.

Introduction

Since the proposal of the van der Waals equation. Many EOS's have been proposed for the representation of fluid volumetric, thermodynamics, and phase equilibrium behavior. These equations, many of them a modification of the van der Waals EOS, range in complexity from simple expressions containing two or three constants to complicated forms containing more than 30, Although the complexity of any EOS presents no computational problem, most authors prefer to retain the simplicity found in the van der Waals cubic equation while improving its accuracy through modifications. Semiempirical EOS's set forth in recent years have retained the van der Waals thermal repulsive term. Differences exist in the expression of the attractive pressure term. The most significant milestone in the development of cubic EOS's is the publication by Soave of a modification in the evaluation of the parameter a in the attractive term of the Redlich-Kwong equation. This suggestion by Soave has prompted an enormous increase in activity on the part of scientists and engineers who are interested in the use of EOS's to calculate fluid properties. The objective of this study is to present a review of developments in cubic EOS's and to compare their predictive capabilities for volumetric and vapor/liquid equilibrium (VLE) predictions.

Review of Recent Developments in Cubic EOS's
SRK EOS and Modifications.

The SRK EOS has the form

.....................(1)

where the dimensionless factor a is a function of temperature:

.................(2)

Soave correlated the slope, m, against acentric factor, w, by the generalized relationship

.............(3)

Graboski used a regression program to re-evaluate the term m on the basis of a detailed set of hydrocarbon vapor pressure data with the purpose of improving pure-component vapor pressure predictions by the SRK EOS. They proposed the following predictions by the SRK EOS. They proposed the following expression for m:

........(4)

For any pure component, the constants a and b are found from the critical properties:

................(5)

and

............(6)

where, and, are the SRK dimensionless pure-component parameters. For the unmodified SRK equation, =0.42747 and parameters. For the unmodified SRK equation, =0.42747 and, =0.08664 are independent of temperature, pressure. composition, or particular component. Eq. 1 can be expressed in terms of the compressibility factor as

..............(7)

where Z is the compressibility factor and A and B are defined by

.............(8)

and

..............(9)

In the VLE calculations for mixtures, Soave suggested the following mixing rules:

...............(10)

and

................(11)

where zi is the mole fraction of the phase, gas or liquid, and the values of Kij are binary interaction coefficients that are usually set equal to zero for regular solutions. The fugacity coefficient, of a component in a mixture is given by

.....................(12)

where

.....................(13)

SPERE

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