Kinetic CO 2 selectivity in clathrate-based CO 2 capture for upgrading CO 2 -rich natural gas and biogas

Abstract

Upgrading CO 2 -rich natural gas or biogas through CO 2 capture is essential to reduce greenhouse gas emissions and to increase its energy density. In this study, clathrate-based CO 2 capture from CO 2 -rich natural gas or biogas was investigated with a primary focus on kinetic CO 2 selectivity. The time-dependent CO 2 selectivity during clathrate formation for pure water, tetrahydrofuran (THF, 5.6 mol%) solution, and tetra-n-butylammonium chloride (TBAC, 3.3 mol%) solution was examined through direct composition analysis and in situ Raman spectroscopy. In pure water, the CO 2 composition in the clathrate phase was much higher at the early stage of clathrate formation than that at equilibrium, indicating that CO 2 is kinetically and thermodynamically selective. For both the THF (5.6 mol%) and TBAC (3.3 mol%) solutions, the CO 2 composition in the clathrate phase was almost constant during clathrate formation. However, the TBAC (3.3 mol%) solution showed significantly higher CO 2 composition (∼74%) throughout the reaction, whereas the THF (5.6 mol%) solution exhibited enrichment of CH 4 in the clathrate phase. The experimental results clearly demonstrate that CO 2 selectivity is dependent on both kinetics and equilibrium of clathrate hydrates and that the addition of thermodynamic promoters, such as THF and TBAC, can affect kinetic CO 2 selectivity as well as equilibrium CO 2 selectivity in the clathrate phase.