Continuous hydrate-based CO₂ separation from H₂ + CO₂ gas mixture using cyclopentane as co-guest

CO₂ separation from H₂ + CO₂ gas mixture is a necessary technology for a stable supply of blue hydrogen. Among CO₂ separation technology, hydrate-based CO₂ separation is an environmentally and economically superior technology. Cyclopentane (CP) was selected as guest compound to enable hydrate-based CO₂ separation under thermodynamic conditions closer to normal pressure and temperature. Hydrate-based continuous CO₂ separation experiments are conducted in the H₂ + CO₂ + CP + H₂O system at 284 K. The results of these experiments showed that mole fraction of H₂ in the gas phase reached 0.9 at the steady state in the H₂ + CO₂ + CP + H₂O system after the CO₂ was extracted via hydrate formation. At the same time, more H₂ was incorporated into the mixed hydrate during this process as the concentration of H₂ in the gas phase increases post hydrate formation. Therefore, it was demonstrated that hydrate-based CO₂ separation was successful in H₂ + CO₂ + CP + H₂O system at 284 K but there is also increase of energy costs caused by H₂ loss. The PXRD results identified H₂ + CO₂ + CP hydrate as structure II hydrate. Although H₂ + CO₂ + Tetrahydropyran (THP) hydrate also forms structure II hydrate, more H₂ is encapsulated in the H₂ + CO₂ + CP mixed hydrate than in the H₂ + CO₂ + THP mixed hydrate. This difference may be caused by the difference in the lattice constants of CP and THP hydrates, which arises from the difference in molecular size of CP and THP, from the differences between the large guest–host interactions, and from differences arising from hydrogen bonding of THP with the host water framework which can decrease the H₂ holding capacity. In this study, it was revealed that the molecular diameter of the guest molecules need also to be considered in addition to the phase equilibrium conditions of the hydrates when selecting guest compound of hydrates.