Kinetic study of semiclathrate hydrates formed with CO2 in the presence of tetra-n-butyl ammonium bromide and tetra-n-butyl phosphonium bromide

The kinetics of semiclathrate hydrates formation with CO 2 in the presence of tetra- n -butyl ammonium bromide (TBAB), tetra- n -butyl phosphonium bromide (TBPB), and TBAB + TBPB were investigated. The experiments were conducted at the stoichiometric concentration of TBAB hydrate (2.57 mol%) and three subcoolings (Δ T  = 6 K, 9 K, and 12 K) with the initial pressure fixed at 2.8 MPa. It was found that adding TBPB into the TBAB solution can promote hydrate nucleation, and this promotion effect at a lower subcooling (Δ T  = 6 K) was stronger compared to that at higher subcoolings (Δ T  = 9 K and 12 K). The gas consumption obtained in TBAB, TBPB, and TBAB + TBPB solutions decreased with the increase of subcooling, despite the increase of the rate of hydrate growth. At a given subcooling, gas consumption (CO 2 uptake) in TBAB + TBPB solutions was greater than that in TBAB and TBPB solutions, indicating that hydrate formation kinetics and the CO 2 storage capacity were improved in the TBAB + TBPB solution. The largest gas consumption was obtained at Δ T  = 6 K in the TBAB + TBPB solution, which was 28.7% and 17.2% larger than those obtained in TBAB and TBPB solutions, respectively. Therefore, a mixture of TBAB + TBPB is a promising option to improve the kinetics of semiclathrate hydrate formation with CO 2 .