A Modified Equation of State for Gas-Condensate Systems

Accurate prediction of thermophysical properties is an essential requirement for optimum design and operation of most process equipment involved in petroleum production, transportation, and processing. Equipment failure is often directly attributed to lack of accurate design data. Methods of fluid property prediction can be divided into three main categories, namely the Corresponding States Theory (CST), Equation of State (EOS) model, and Activity Coefficient Model (ACM). The EOS approach is most popularly used for natural gas systems due to its applicability at high pressures for both liquid and vapor phases. Probably the most successful cubic equation of state for natural gas property calculation is the one proposed by Peng and Robinson (PR)¹. However, Peng-Robinson EOS assumes a fixed value of critical compressibility factor and, as a result, the predicted densities of the saturated liquids and the predicted critical volumes differ considerably from experimental values. Patel and Teja (PT)² introduced a substance dependent critical compressibility factor that allowed them to more accurately reproduce the experimental saturated liquid volume at a particular temperature. The present work is an extension of that of Patel and Teja. The modified Patel and Teja's EOS improves the prediction of liquid phase density and vapor liquid equilibrium. A comprehensive assessment of the modified equation-of-state is conducted through performance comparison with the more popular equations of state. For saturated liquid volume predictions, the improved equation of state yields 33% and 11% improvement over PR and PT EOS, respectively. For vaporization equilibrium ratio predictions, 36% improvement over PR and 67% improvement over PT EOS are observed.