Biogas Upgrading and Solvent Regeneration in Monoethanolamine and Ionic Liquids for Carbon Dioxide Utilisation

Biogas is a renewable energy source, consisting primarily of methane and carbon dioxide (CO2), which can be upgraded by stripping the CO2 to produce biomethane. One of the standard industrial solvents used in biogas upgrading is monoethanolamine (MEA). Despite the effective CO2 capture capability of MEA, it is energy intensive to regenerate since it requires heating the solvent to approximately 120 oC. Additional issues with MEA include material losses due to degradation and evaporation. This work investigates methods for improving the biogas upgrading process by utilising microbubbles and comparing ionic liquid solvents to MEA. It is also examined whether CO2 can be released from a solvent with high enough purity to be utilised directly in conjunction with carbon dioxide utilisation (CDU) processes.
The study experimentally investigates factors influencing bubble size and absorption of CO2 by ionic liquids with a view to biogas upgrading.
It was found that bubble size in the majority of ionic liquids tested was far greater than that in aqueous glycerol mixtures of similar viscosity; however, three of the ionic liquids tested had a similar bubble size. Further analysis showed that rather than viscosity, the surface tension of the ionic liquid has the most significant influence on bubble size due to its stabilising effect on the homogeneous regime in bubble columns.
30 wt.% aqueous MEA solution was shown to have a significantly greater rate of mass transfer than the ionic liquids tested due to its greater absorption capacity, greater diffusivity and tendency to form smaller bubbles. Of the ionic liquids tested, the greatest absorption capacity was exhibited by [C2mim][NTf2] and the greatest volumetric mass transfer coefficient by [C2mim][EtSO4].