A comparative Study Between Helmholtz and Entropy Scaling Approaches for Viscosity of Natural Gas Mixtures Based on the GERG2008 Equation of State

THERAN BECERRA, OSCAR I.; BENELLI, FEDERICO E.; CISMONDI-DUARTE, MARTÍN

Campinas

Congreso; XII IBEROAMERICAN CONFERENCE ON PHASE EQUILIBRIA AND FLUID PROPERTIES FOR PROCESS DESING; 2022

Unicamp

In the natural gas industry, furthermore to an accurate knowledge of thermodynamic properties, it is of great importance to have a good approximation of transport properties such as viscosity over a wide range of pressure and temperature conditions. The study of these properties has different industrial applications, ranging from processing to storage and transport of natural gas. In the last decades, different approaches have been proposed to correlate fluid viscosities with different properties obtained from an equation of state. In parallel, Kunz and Wagner developed a very accurate equation of state for the calculation of thermodynamic properties of natural gas and related mixtures, known as GERG 2008. This equation of state calculates Helmholtz’s free energy as a function of density, temperature, and composition. In recent work, Mairhofer presented a viscosity model with a residual entropy scaling approach based on the GERG 2008 equation of state. On the other hand, Gonçalves et al. recently proposed a new Helmholtz energy scaling approach for viscosity calculation using the PCP-SAFT as the equation of state, whose fundamental idea we adopt to implement it with the GERG 2008 and compare with other approaches.In this work, a simple model based on the Helmholtz free energy is proposed to calculate the viscosity of natural gases and related mixtures. This Helmholtz scaling approach will apply the Chapman-Enskog relation as the reference viscosity and the GERG 2008 as the equation of state to calculate the viscosity of the fluid of interest.This study aims to present the predictions of natural gas viscosities, evaluating the accuracy of the Helmholtz scaling approach against other approaches such as residual entropy scaling. It is assumed that the deviations presented will have their origin in the approach used to calculate the viscosity and not in the equation of state, since the GERG 2008 has been adopted as an ISO standard, presenting very precise results in the calculation of thermodynamic properties for natural gases and its mixtures. This will allow comparing such approaches on a more objective basis than in most works using engineering equations of state with a certain level of error in the thermodynamic properties, which are highly dependent on pressure and temperature conditions.The calculation of the thermodynamic properties will be performed using PyForFluids (Python-Fortran-Fluids), a Python package focused on the calculation of multicomponent fluids properties and phase equilibrium based on Equations of State (EoS). PyForFluids, developed in our Fluids Thermodynamics Group (GTF-IPQA), will be presented in a separate work at EQUIFASE 2022 and includes the implementation of the GERG 2008 equation to calculate both the ideal and residual Helmholtz free energy, used to calculate viscosities.