Solid-Liquid-Vapor Equilibrium Prediction for Typical Helium-Bearing Natural Gas Mixtures

Industrial, large-scale helium recovery from natural gas is typically performed though cryogenic distillation. These technologies need a deep knowledge of the thermodynamics of the treated mixture: in the case of natural gas to a pipeline, CO2 present in the feed stream might freeze at the process operating temperatures. The aim of this work is to analyze the thermodynamic behavior of the four-component mixture CH4-N2-He-CO2 to predict its triphasic solid-liquid-vapor equilibrium (SLVE). Through a developed computational method based on the classical approach, the nitrogen and helium effect on CO2 solidification has been assessed. The investigated conditions are consistent with typical cryogenic procesthesing temperatures (i.e., 100-200 K) and natural gas compositions. Pressure-temperature and temperature-composition equilibrium loci are provided for each analyzed case, varying the N2 and He content in mixture. Helium behavior as a quantum gas has been considered by introducing temperature-dependent critical parameters, as suggested by Prausnitz and co-workers, valid for an acentric factor equal to zero. Referring to the proposed thermodynamic modeling, the risk of CO2 freezing within a cryogenic helium recovery plant can be avoided by carefully managing the process operating conditions.