Phase equilibrium data for the semiclathrate hydrates formed in three three-component systems, the CO 2 + tetra-n-butyl ammonium bromide (TBAB) + water system, the CO 2 + tetra-n-butyl ammonium chloride (TBAC) + water system, and the CO 2 + tetra-n-butyl ammonium fluoride (TBAF) + water system, were measured in the pressure range of (0.40 to 3.77 MPa) and temperature range of (280.2 to 293.5 K) at (2.93 • 10 -3 and 6.17 • 10 -3 ) mole fraction of tetra-n-butyl ammonium halide. The experimental data were generated using an isochoric pressure-search method. The equilibrium data for the CO 2 + TBAB + water system were compared with some experimental data from the literature. The effects of tetra-n-butyl ammonium halide concentration on the stability zone of the semiclathrate hydrates were studied. It was shown that TBAB, TBAC, and TBAF all can enlarge the hydrate stability zone, and as the tetra-n-butyl ammonium halide concentration increases, so does the hydrate stability zone. The three-phase equilibrium pressure of the CO 2 + TBAF + water system is lower than others at the same temperature.

Capturing CO 2 by forming hydrate is an attractive technology for reducing the greenhouse effect. The most primary challenges are high energy consumption, low hydrate formation rate, and separation efficiency. This work presents efficient capture of CO 2 from simulated flue gas (CO 2 (16.60 mol %)/N 2 binary mixtures) by formation of semiclathrate hydrates at 4.5 and 7.1 °C and feed pressures ranging from 2.19 to 7.31 MPa. The effect of 0.293 mol % tetra-n-butyl ammonium bromide (TBAB) and tetra-n-butyl ammonium fluoride (TBAF) on the hydrate formation rate, reactor space velocity, and CO 2 separation efficiency was studied in a 1 L stirred reactor. The results showed the hydrate formation rate constant increased with increasing feed pressure and reached the maximum at 2.82 × 10 -7 mol 2 /(s • J) with TBAB and 8.26 × 10 -7 mol 2 /(s • J) with TBAF. The space velocity of the hydrate reactor increased with increasing feed pressure and reached a maximum of 13.46 h -1 with TBAB and 25.96 h -1 with TBAF. CO 2 recovery was about 50%, and the optimum CO 2 separation factor with TBAF was 36.98, which was about 4 times higher than that with TBAB in the range of feed pressure. CO 2 could be enriched to 90.40 mol % from simulated flue gas under low feed pressure by two stages of hydrate separation with TBAF. The results demonstrated that TBAB, especially TBAF, could accelerate hydrate formation. The space velocity of the hydrate reactor with TBAB or TBAF was higher than that with THF. CO 2 could be easily enriched in the hydrate phase by two stages of hydrate separation under gentle conditions.

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