High pressure viscosity measurements of ternary (methane plus propane plus heptane) mixtures

Prediction of thermophysical properties for natural gas mixtures close to the mixture's critical point and cricondenbar (the maximum pressure at which two fluid phases can co-exist) is challenging but important as often natural gas processes operate close to these conditions. Here we present a comprehensive study of the viscosity of (CH4 + C3H8 + C7H16) mixtures with heptane fractions up to 15 mol%. Accurate measurements of saturated phase and compressed-fluid (single phase) viscosities, at temperatures from 200 K to 423 K and pressures up to 35 MPa, are presented. The collected data were compared with the prediction of the extended corresponding states (ECS) model, with relative deviations spanning between (-20 to 11) % from the model. Additionally, pure component parameters needed for the residual entropy scaling (RES) model were fit to literature data and then applied predictively to mixtures viscosity without any additional fit parameters. The relative deviation of the measured viscosities from values calculated with the RES model is generally within 15%. This work demonstrates that no current model satisfactorily predict mixtures viscosity, and that high quality experimental data are needed to anchor the methods used to design and optimize natural gas processing.

Automated summary

This study investigated the viscosity of (CH4 + C3H8 + C7H16) mixtures with varying heptane fractions, presenting accurate measurements and comparing them with the extended corresponding states (ECS) model. The residual entropy scaling (RES) model was also used to predict mixture viscosities, with relative deviations generally within 15% of measured values. The results highlight the need for high quality experimental data to improve current models for predicting mixture viscosities in natural gas processing.