Estimating the viscosity of pressurized gases

In this dossier, we develop methods for calculating the viscosity of pure gases as a function of temperature and pressure or density. We remind you that the gaseous domain comprises the region of the phase diagram below the liquid-vapor transition curve and the region above the critical temperature. Figures 2 and 3 in the dossier have shown that the variation in viscosity is small as a function of pressure or density. For example, for carbon dioxide above the critical temperature, it varies as a function of pressure between 0.7 µPa·s/MPa at 320 K and 0.2 µPa·s/MPa at 1,100 K. For nitrogen, around critical density, the variation is of the order of 0.05 µPa·s/(kg·m- ³ ).

The theories that have been developed to predict transport coefficients in dense gases are so complex that it has been necessary to make a series of approximations, in order to obtain expressions that can be evaluated numerically. In many cases, these approximations are dictated by mathematical rather than physical convenience.

Of these theories, only one has been widely used to represent viscosity variation as a function of pressure: Enskog's theory. Gas molecules are assimilated to hard spheres, in purely repulsive interaction. It applies to gases at high pressures, generally in excess of 100 MPa.

The viscosity of gases at moderate pressures has been determined on the basis of relationships deduced from the modified Enskog theory, which takes into account an additional attractive component in addition to the purely repulsive one considered previously. The correlations thus defined are in fact expressions of the viscosity virial, they propose a method for calculating the first coefficient of the virial and they are in good agreement with experimental results up to critical pressure.

To represent viscosity above the critical pressure, numerous empirical correlations have been proposed, based on the corresponding states. The variables viscosity, temperature, pressure and density are reduced by their respective values at the critical point, and correlations are calculated from an analysis of experimental data on selected gases.

Tables showing viscosity as a function of temperature and pressure, for several inorganic and organic compounds, in the liquid and gaseous domains, are the subject of a separate dossier.