CO2Enclathration in Hydrates of Peralkyl-(Ammonium/Phosphonium) Salts: Stability Conditions and Dissociation Enthalpies

Abstract

The present work investigates equilibrium conditions and dissociation enthalpies of semiclathrate hydrates formed from CO2 + tetra-n-butylammonium chloride (TBACl) + water, CO2 + tetra-n-butylammonium nitrate (TBANO3) + water, and CO2 + tetra-n-butylphosphonium bromide (TBPB) + water mixtures. Differential scanning calorimetry (DSC) was used for the determination of hydrate−liquid−vapor (H−L−V) equilibrium conditions in the presence of TBACl, TBANO3, and TBPB solutions at ammonium salt mass fractions of 0.3618, 0.3941, and 0.3707, respectively, and at CO2 pressure in the range of (0.5 to 2.0) MPa. Results reveal that semiclathrate hydrates of TBACl, TBANO3, and TBPB are able to incorporate carbon dioxide in their structure and that the resulting mixed hydrates have significantly lower formation pressures than those of pure CO2 hydrate. The dissociation enthalpies of semiclathrate hydrates of TBACl, TBANO3, and TBPB with CO2 were determined by both DSC and the Clausius−Clapeyron equation. The DSC experiments demonstrate that mixed hydrates of TBANO3, TBACl, and TBPB with CO2 have higher melting enthalpies than single hydrates. From our measurements, it appears that mixed TBPB + CO2 hydrate has appropriate stability conditions (p, T) and latent heat content for secondary refrigeration applications. DSC measurements combined with the Clausius−Clapeyron equation show that mixed TBPB + CO2 hydrate can store large amounts of CO2 and thus could be attractive for gas capture and storage applications.