Effects of an additive mixture (THF + TBAB) on CO2 hydrate phase equilibrium

• The effects of additive mixture (THF + TBAB) on CO2 hydrate phase equilibrium were measured.
• When THF is 5% mass fraction, hydrates phase equilibrium pressure decreases with TBAB concentration increase (3–8% mass fraction).
• THF + TBAB is superior than TBAB for reducing CO2 hydrate phase equilibrium pressure.
• The induction time of hydrate formation from the mass fraction of 5% THF + 5% TBAB solution is shortest in this study.

A decrease in hydrate formation pressure is crucial for the development of hydrate-based gas separations. Both tetrahydrofuran (THF) and tetra-n-butyl ammonium bromide (TBAB) are thermodynamic additives for hydrate-based CO2 capture, which can effectively decrease the CO2 hydrate formation pressure. To obtain fundamental data, the effects of an additive mixture (THF + TBAB) on CO2 hydrate phase equilibrium were investigated. The experiments were conducted using an isochoric method to study the effects of additive mixtures with different compositions. A mass fraction of 1% THF with the mass fraction of 0.5%, 1%, 2%, 3% and 5% TBAB, and 5% THF with 3%, 5% and 8% TBAB were used. The experiments were conducted at 276.35–291.05 K and 0.9–4.4 MPa. For all mixtures with the mass fraction of 1% THF, the effect was subtle (with the exception of the 1% THF + 5% TBAB mixture). On the other hand, the hydrate equilibrium pressure decreased dramatically when mixtures containing 5% THF were added. Compared with the addition of only THF, only TBAB or a THF + SDS additive mixture, a THF + TBAB additive mixture had numerous advantages, such as drastically decreasing the hydrates phase equilibrium pressure. In addition, the influence of the additive mixture on the induction time of hydrate formation was examined and discussed. In the present study, a mass fraction of 5% THF + 5% TBAB greatly reduced the induction time of hydrate formation compared with the other concentrations tested.