Viscosity correlation for n-butane in the fluid region
Eckhard Vogel, Cornelia Küchenmeister, Eckard Bich
A new representation of the viscosity of n-butane based on the residual viscosity concept has been developed. The representative equations are formed by a combination of a zero-density correlation, an initial-density dependence correlation, double polynomials in density and reciprocal temperature, as well as a free-volume term with a temperature-dependent close-packed density. The critical enhancement has not been considered, because no reliable data are available in the critical region. The zero-density and initial-density viscosity correlations were deduced using recent results of high-precision measurements on n-butane by Küchenmeister and Vogel and a new way of predicting the data in the vapour phase. The higher density contributions to the residual viscosity are based on primary experimental data sets from the literature critically assessed. The surface correlation is valid from the triple point to 500 K and 70 MPa in accordance with the modified Benedict – Webb – Rubin equation of state by Younglove and Ely. The uncertainty of the representative equations varies from ±0.4% in the dilute gas phase between room temperature and 600 K to ±3% in the thermodynamic range, in which the equation of state is valid and primary experimental data are available.