Selectivity and CO2 capture efficiency in CO2-N2 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 CO 2 and N 2 . Clathrates forming conditions and dissociation pressures are provided between 270.5 K and 278.3 K (−2.5 °C and 5.3 °C). Specific CO 2 Raman signatures show structure sII to be thermodynamically stable at feed gas composition of 1% and kinetically favored at ∼2%. Structure sI is stable at feed gas CO 2 ranging from ∼2% up to 70%. Our Raman quantitative analysis demonstrates the removal of CO 2 from a typical flue gas mixture (1–20% CO 2 ) and from samples of 47% and 70% CO 2 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’ (CO 2 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 CO 2 molecules preferentially occupy sI large cages (5 12 6 2 ) for a CO 2 feed gas concentration in the range of 2–20% (i.e. ∼1–16% equilibrium gas compositions), whereas CO 2 molecules severely compete with N 2 to fill sI small cages (5 12 ) at equilibrium CO 2 concentrations greater than 30%. As a consequence, the derived selectivity shows a significant decrease from ∼7.2 (at 20% CO 2 ) to ∼3.8 for CO 2 -rich feed gas compositions (i.e. above 20%). In addition, the selectivity is reduced in structure sII formed at 1% CO 2 feed gas composition. Then, the influence of thermodynamic parameters on hydrate selectivity and recovery fraction is described qualitatively at typical CO 2 flue gas concentrations found in conventional power plants. [ABSTRACT FROM AUTHOR]