On Equation of State

Abstract

Summary To predict the phase and volumetric behavior of hydrocarbon mixtures by using an Equation of State; e.g. the Peng and Robinson Equation of State "PREOS", the critical properties in terms of the critical pressure "pc" and critical temperature "Tc" as well as the acentric factor "ω" must be given for each component present in the mixture including the plus-fraction. For pure compounds, the required properties are well-defined, but nearly all naturally occurring gas and crude oil fluids contain some heavy fractions that are not well defined and are not mixtures of discretely identified components. These heavy fractions often are lumped and called the "plus-fraction" (e.g. C7+ fraction). Adequately characterizing these undefined plus fractions in terms of their critical properties and acentric factors has long been a problem. Changing the characterization of the plus fraction can have a significant effect on the volumetric and phase behavior of a mixture predicted by the PREOS. The inaccuracy of any the cubic equation of state results from the following two apparent limitations: improper procedure of determining coefficients a, b, and α for the plus fraction Equations of state treatment of hydrocarbon components with critical temperatures less than the system temperature (i.e. methane and nitrogen). Numerous authors have suggested that the EOS is generally not predictive and extensive splitting of the C7+ fraction is often required when matching laboratory data. This paper presents a practical approach for calculating the coefficients a, b, and α of the plus-fraction from its readily available measured physical properties in terms of molecular weight "M" and specific gravity "γ" with the objective of improving the predictive capability of equation of state. The predictive capability of the relationship is displayed by matching a set of laboratory data on several crude oil and gas-condensate systems. In addition; the performance of the proposed method was also compared with predictive PVT results as generated by using PVTSimTM software of Calsep. Additional comparisons are made by comparing the proposed modified PR EOS results with those of Coats and Smart4 regression methodology with PR EOS. This study concludes that when the coefficients of the plus-fraction, i.e. a, b, and α, are determined based on the proposed methodology; splitting of the C7+ into a number of pseudo-components is essentially unnecessary.