Measurement and Theoretical Calculation of CO₂ Solubility Data in Liquid CH₄ + C₂H₆ Mixtures at Cryogenic Temperatures

In the liquefied natural gas (LNG) industry, CO₂ needs to be removed to 50 ppm to avoid CO₂ freeze-out under cryogenic conditions. However, in LNG produced from unconventional natural gas that is rich in ethane, the allowance of CO₂ may be much higher due to the much higher CO₂ solubility than that in conventional LNG. Considering that CO₂ solubility data in liquid CH₄ + C₂H₆ mixtures are still quite lacking, a static solid–liquid equilibrium experimental setup is built to measure the CO₂ solubility data. The experiments are carried out at the temperature range of 148–203 K, covering an ethane content of 0–100%. The experimental results are compared with theoretical calculated results based on equations of state and Gibbs–Helmholtz relation. CO₂ solubility in the CH₄ + C₂H₆ mixture increases exponentially with increasing temperature. The addition of ethane significantly increases the CO₂ solubility in the entire temperature zone, indicating that the purification specification of CO₂ in natural gas with high ethane content can be significantly enlarged, and it is easier to achieve carbon removal by cryogenic distillation. Furthermore, the increase in CO₂ solubility in the CH₄ + C₂H₆ mixture approximates a logarithmic relationship to the C₂H₆ content.