A Helmholtz energy model for liquefied natural gas systems

In this work a Helmholtz energy model is applied to the prediction of saturated-liquid densities of liquefied natural gas systems. The Helmholtz energy of the mixture is represented as two contributions: one from a proven accurate extended corresponding states model and the other is a correction term. The corresponding states model relies on pure-component shape factors relative to methane and extension to mixtures with the van der Waals one-fluid mixture model with ordinary combining rules. The correction term is temperature-, density- and composition-dependent with the use of a theoretically consistent local composition model with a coordination number model derived from lattice gas theory. For 261 state points corresponding to 61 different mixtures of liquefied natural gas an overall AAD of 0.085% in orthobaric densities was obtained while the corresponding requirement set by the natural gas industry is 0.1%.