Density of Liquefied Natural Gas

Abstract

An experimentally derived tabular correlation for LNG density as a function of temperature, pressure, and gas gravity has been designed for use in the field. The advantages of this correlation are that a gas gravity analysis is more easily obtained than a composition analysis and that density is determined by direct interpolation of the gas gravity tables. Introduction Liquefied natural gas, LNG, is being handled in increasing quantities, and a reliable and convenient method of determining LNG density is desired. If a composition analysis is available, the correlations of Harmens and of Lyckman, Eckert, and Prausnitz may be used to calculate LNG density. Harmens' correlation was developed for the density of pure light hydrocarbon liquids at saturation and for the density of mixtures of such hydrocarbons, particularly of LNG. In the correlation, Eq. 1 is used particularly of LNG. In the correlation, Eq. 1 is used to calculate liquid density: (1) p = C * F ( Tr ) C and F(Tr) were tabulated by Harmens. C is an empirical density constant, and F(Tr) is Harmens generalized density function. Tr is reduced temperature, T/Tc. To apply the correlation to mixtures, mixing rules were given to compute C and Tc from mixture composition and pure component values. In the correlation of Lyckman, Eckert, and Prausnitz, reduced saturated volume Vr is given as a Prausnitz, reduced saturated volume Vr is given as a quadratic function of acentric factor: (2) (1) 2 (2)Vr = V/Vc = Vr(0) + wVr + w Vr (3) p = M / V , where V is molar volume, Vc is critical molar volume, and M is molecular weight. The generalized functions of reduced temperature, Vr(0), Vr(1), and Vr(2), were tabulated by Lyckman, et al., in the range Tr = 0.560 to Tr = 0.990. Chueh and Prausnitz fitted the tabulated values to Eq. 4 and provided the coefficients a (j) to f(j). (4) ( j ) ( j ) ( j ) ( j ) 2 ( j ) 3Vr = a + b Tr + c Tr + d Tr ( j ) ( j ) + e / Tr + f ln ( 1 − Tr ) Eq. 4 is valid for reduced temperatures from 0.560 to 0.995. Mixing rules were suggested by Chueh and Prausnitz to calculate pseudocritical volume and Prausnitz to calculate pseudocritical volume and temperature, and mixture acentric factor, for application of the correlation to mixtures of known composition. In using the foregoing correlations, a composition analysis is required, and mixture pseudo constants must be calculated. To provide an easier and quicker procedure for the man in the field, a study was procedure for the man in the field, a study was undertaken to correlate LNG density as a function of three measurable properties: temperature, pressure, and the gas gravity of gasified LNG. A technician can make these measurements routinely with commercially available equipment. Theory To derive the LNG density correlation, equations are required for computing ternary mixture compositions, gas gravities, and liquid densities at fixed temperatures and pressures. JPT P. 683