Prediction of the viscosity of natural gas at high temperature and high pressure using free-volume theory and entropy scaling

Eighteen models based on two equations of state (EoS), three viscosity models, and four mixing rules were constructed to predict the viscosities of natural gases at high temperature and high pressure (HTHP) conditions. For pure substances, the parameters of free volume (FV) and entropy scaling (ES) models were found to scale with molecular weight, which indicates that the ordered behavior of parameters of Peng-Robinson (PR) and Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) propagates to the behavior of parameters of viscosity model. Predicting the viscosities of natural gases showed that the FV and ES models respectively combined with MIX4 and MIX2 mixing rules produced the best accuracy. Moreover, the FV models were more accurate for predicting the viscosities of natural gases than ES models at HTHP conditions, while the ES models were superior to PRFT models. The average absolute relative deviations of the best accurate three models, i.e., PC-SAFT-FV-MIX4, tPR-FV-MIX4, and PC-SAFT-ES-MIX2, were 5.66%, 6.27%, and 6.50%, respectively, which was available for industrial production. Compared with the existing industrial models (corresponding states theory and LBC), the proposed three models were more accurate for modeling the viscosity of natural gas, including gas condensate.