Phase equilibrium and characterization of ionic clathrate hydrates formed with tetra-n-butylammonium bromide and nitrogen gas

We report the three-phase (gas-hydrate-aqueous) equilibrium measurements and phase characterization of ionic clathrate hydrates formed in a nitrogen + tetra-n-butylammonium bromide (TBAB) + water system. The measurement conditions used were 0.0029 ≤ xTBAB ≤ 0.0257 (0.05 ≤ wTBAB ≤ 0.32), 280 K < T < 291 K, and 1 MPa < P < 12 MPa, where xTBAB, wTBAB, T, and P denote the mole fraction of TBAB in the aqueous phase, mass fraction of TBAB in the aqueous phase, temperature, and pressure, respectively. We also performed single-crystal structure analyses of the crystals formed at 2.1 and 5.8 MPa, which indicated that they possessed the same orthorhombic Pmma hydrate phase but significantly different gas capacities. We also showed that the low nitrogen gas pressure supply extends the melting point of the simple TBAB hydrate formed in the absence of gas. CO2 may be efficiently captured from flue gas when the partial pressure of N2 is maintained below 4 MPa.