Separation of CO2 and CH4 from biogas by formation of clathrate hydrates: importance of the driving force and kinetic promoters

This paper relates to the upgrading of model biogas mixtures, typically 60/40 CH4/CO2, by clathrate (gas) hydrates, which have recently been considered as a safe alternative to high-pressure or liquefied gas storage, and as an economic, chemical-free process for the separation of gas mixtures. Several factors affecting the driving force to hydrate formation are considered, such as the degree of over pressurization and the presence of chemical promoters. Promoters used were several anionic and zwitterionic surfactants which are demonstrated to affect the hydrate-forming ability of water. Some lignin derivatives were also tested. Promoted hydrates were also compared to hydrate-based separation starting from nonpromoted water. Separation experiments were conducted under pressures of 4 and 2.5 MPa at 274 K, under either pressure-dropping or constant pressure conditions. Results show that the separation ability of clathrate hydrates as determined by the separation factor S is highest when no promoters are added to the water phase; the well-known promoter sodium dodecyl sulfate (SDS) shows a value of S which is approximately half the value of that in pure water while higher separations were obtained with some lignin derivatives and a non-surface-active naphthalenesulfonate derivative. We also show that the contribution of CO2 solubility in water to S is a main player in the overall process. Finally, the separation ability of hydrates seems to be inversely proportional to the amount of gas mixture enclathrated, i.e., the occupancy.