A fast calculation method of thermodynamic properties of variable-composition natural gas mixtures in the supercritical pressure region based on implicit curve-fitting

Vaporizers are widely used to re-gasify the liquefied natural gas which consists of variable compositions at supercritical pressures. The duration of vaporizer design based on simulations severely depends on the calculation speed of thermodynamic properties of natural gas due to millions of thermodynamic property iterations. This paper presents a fast calculation method of thermodynamic properties of natural gas in supercritical pressure region. In this method, a calculation model for a reference composition is established at first, by dividing the supercritical pressure region into three subsections and regressing each subsection separately based on implicit curve-fitting; and then composition based implicit curve-fitting equations are developed to extend thermodynamic property calculation from the reference composition to any variable compositions. With this method, the equations for typical compositions of natural gas including C1, C2, C3, n-C4, n-C5 and N2 are presented, covering the temperature range from −170 °C to 60 °C and the pressure range from 5 MPa to 10 MPa. The present method is about 104 times faster than GERG-2008, and the mean deviation is less than 0.5%.