Methane enrichment from CH4 + CO2 gas mixture via structure II clathrate hydrate: A continuous separation experiment for biogas upgrading

• Hydrate-based semi-batch and continuous biogas purification experiments. • Tetrahydropyran promotes preferential capture of CH 4 in the hydrate structure. • Mole fraction of CH 4 from 0.60 to 0.81 through semi-batch operation. • Long-term investigation into mole fraction changes through continuous operation for engineering practice. Toward the widespread adoption of biomass energy for carbon neutral age, demand for biogas separation technologies that enables easier operation and maintenance is increasing. CH 4 enrichment in biogas in solid structure II clathrate hydrate has recently attracted attention, yet the knowledge is limited to the batch-type operation, which is far from engineering practice. In the present study, we have conducted hydrate-based CH 4 enrichment experiments with semi-batch and continuous separation methods in CH 4 + CO 2 + tetrahydropyran (THP) + water system, which are closer to applied-side operations. Semi-batch CH 4 /CO 2 separation experiment with powder X-ray diffraction (PXRD) measurement revealed that CH 4 is enriched in the hydrate phase with its mole fraction from 0.60 to 0.81 via the formation of structure II hydrate with THP. The continuous CH 4 /CO 2 separation experiment showed that CH 4 is concentrated in the hydrate slurry during the continuous formation process. The concentration of CH 4 in the hydrate slurry reached 0.65 in mole fraction at the steady state at t = 24 h from 0.45 at t = 1h. The experimental results of time-dependent changes in the compositions in the gas phase as well as in the hydrate slurry exhibited that the long-term operation of hydrate-based CH 4 enrichment was achieved and therefore could be implemented on a continuous industrial scale. The overall results demonstrated that the continuous CH 4 enrichment in biogas is an applicable scheme utilizing structure II hydrate.