Clathrate-Based CO₂ Capture from Co₂‑Rich Natural Gas and Biogas

In this study, clathrate-based CO₂ capture was investigated in the presence of thermodynamic promoters such as tetrahydrofuran (THF) and tetra-<i>n</i>-butyl ammonium chloride (TBAC) for upgrading CO₂-rich natural gas and biogas. The phase equilibria, gas uptakes, gas composition measurements, and spectroscopic analyses of CH₄ (50%), CO₂ (50%), and promoter clathrates were examined with a primary focus on the effects of thermodynamic promoters on clathrate stability and cage filling behavior. The addition of THF and TBAC significantly enhanced the thermodynamic stability of CH₄ (50%) and CO₂ (50%) clathrates. ¹³C NMR and Raman spectroscopy clearly revealed that CO₂ and CH₄ are enclathrated in the clathrate cages. THF solutions demonstrated a faster growth rate of clathrates, but CO₂ was less selective than CH₄ in the THF clathrate phase due to the lower thermodynamic stability of the CO₂ and THF clathrate compared to the CH₄ and THF clathrate. TBAC solutions produced higher CO₂ selectivity in the semiclathrate phase due to the presence of distorted small cages, which have a strong preference for CO₂ molecules. The experimental results demonstrated that CO₂ selectivity in the clathrate phase can be influenced by the thermodynamic stability, cage shape and dimension, and cage filling behavior in the presence of thermodynamic promoters, and thus, a suitable promoter and their optimum concentration should be carefully determined in designing and operating clathrate-based CO₂ capture from natural gas or biogas