A comparative study of CPA and PC-SAFT equations of state to calculate the asphaltene onset pressure and phase envelope

Abstract

The Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) equation of state (EoS) and the Cubic-Plus-Association (CPA) EoS have been widely applied to describe the asphaltene precipitation onset in crude oils because the association term of these models can appropriately account for the polar-polar interactions between asphaltene and resin molecules. In this study we have compared the PC-SAFT and CPA EoS capabilities in the calculation of the Asphaltene Precipitation Envelope (APE) of eight oils from the literature and discuss the physical meaning of the cross-association energy parameter between asphaltene and resin. For all oils the PC-SAFT EoS predicts an increase in the stability of the oil at higher temperatures with the consequent closure of the APE while the CPA model predicts a decrease in the stability at higher temperatures. For both models the fitted values of the cross-association energy parameter between asphaltene and resin show a dependency on temperature, but for PC-SAFT model this parameter has a clearer physical meaning. Results indicate that the PC-SAFT EoS is more accurate for APE calculations outside the experimental temperature range.