Selectivity and CO2 capture efficiency in CO2-N-2 clathrate hydrates investigated by in-situ Raman spectroscopy

Thermodynamic measurements and Raman spectroscopy are performed to investigate the structure, selectivity and capture efficiency in mixed gas hydrates containing CO2 and N-2. Clathrates forming conditions and dissociation pressures are provided between 270.5 K and 278.3 K (-2.5 degrees C and 5.3 degrees C). Specific CO2 Raman signatures show structure sII to be thermodynamically stable at feed gas composition of 1% and kinetically favored at similar to 2%. Structure sI is stable at feed gas CO2 ranging from similar to 2% up to 70%. Our Raman quantitative analysis demonstrates the removal of CO2 from a typical flue gas mixture (1-20% CO2) and from samples of 47% and 70% CO2 during hydrate crystallization. For the first time, quantitative bulk guest compositions in hydrates are directly obtained by Raman using relative cross sections of the guests' (CO2 and N-2) vibrational modes. The equilibrium data thereby generated satisfactorily compare with predictions based on thermodynamic models (CSMGem) and thus remove any ambiguity from previous literature data. It appears that CO2 molecules preferentially occupy sI large cages (51262) for a CO2 feed gas concentration in the range of 2-20% (i.e. similar to 1-16% equilibrium gas compositions), whereas CO2 molecules severely compete with N-2 to fill sI small cages (512) at equilibrium CO2 concentrations greater than 30%. As a consequence, the derived selectivity shows a significant decrease from similar to 7.2 (at 20% CO2) to similar to 3.8 for CO2-rich feed gas compositions (i.e. above 20%). In addition, the selectivity is reduced in structure sII formed at 1% CO2 feed gas composition. Then, the influence of thermodynamic parameters on hydrate selectivity and recovery fraction is described qualitatively at typical CO2 flue gas concentrations found in conventional power plants.