An Evaluation of Cubic Equations of State for Phase Behavior Calculations Near Miscibility Conditions

Abstract

ABSTRACT The paper presents results from vapour-liquid equilibrium experiments simulating gas injection processes, using 25-component fluids as black and volatile oils. Gases were added to the oil to approach the system to the critical point (miscibility) and the volume, density and compositions of equilibrated phases were measured at various levels of pressure and added gases. The measured near miscibility data as well as pertinent literature data have been used to evaluate the reliability of EOS modelling. It is shown that whilst certain EOS are capable of predicting the phase behaviour at these conditions, others, including some widely used equations, can give unreliable results. The predicted volumes of equilibrated phases are shown to be particularly inaccurate with deviations over 100%. A volume translation method, based on equilibrium ratios, has been proposed to improve the predicted phase density for mixtures near critical point. The significance of input compositional and volumetric data for accurate phase behaviour prediction near miscibility is demonstrated.