Thermodynamic modeling of phase equilibria of semi-clathrate hydrates of CO2, CH4, or N2+tetra-n-butylammonium bromide aqueous solution

Abnormal structural transformation of tetra-n-butyl ammonium chloride + Xe clathrates and its significance for clathrate-based Xe capture and storage

Tetra-n-butyl ammonium chloride (TBAC) is a semi-clathrate former that can be used for clathrate-based gas capture and storage since TBAC semi-clathrate has vacant small cages available for entrapping gas molecules under mild conditions. In this study, the phase equilibria and structural information of TBAC + Xe + water systems were experimentally investigated at two different TBAC concentrations (1.0 and 3.3 mol%). The slopes of the three-phase (clathrate [H] - liquid [L] - vapor [V]) equilibrium lines for the TBAC + Xe + water systems altered at one or two points as the pressure and temperature changed, which indicates that this slope change might be caused by the structural transformation of the clathrates that were formed. The powder X-ray diffraction (PXRD) patterns, in situ Raman spectra, and ¹²⁹Xe nuclear magnetic resonance (NMR) spectra demonstrated that the clathrate structure of the TBAC + Xe + water systems changed from tetragonal (P4₂/m) or orthorhombic (Pmma) to cubic (Pm3̄n) as the pressure increased. Surprisingly, in the higher-pressure region, TBAC acted as a thermodynamic inhibitor without being enclathrated in the clathrate lattices. The thermodynamic and structural information of the TBAC + Xe clathrates will be helpful for conceptualizing and designing the clathrate-based noble gas or radioactive gas capture and storage process.

Thermodynamic investigation of hydrate-based CO2 capture from simulated flue gas with new mixed promoters

CO2 hydrate-based desalination (CHBD) has been developing for decades to meet the global demands of decreasing carbon dioxide (CO2) emissions. In this work, the CO2 was captured from the simulated flue gas which consists of 18.30 mol% carbon dioxide and 81.70 mol% nitrogen in the presence of tetra-n-butyl ammonium bromide (TBAB) + cyclopentane (CP) + glucoamylase. Then the phase equilibrium data of CO2 hydrate were measured by the method of isochoric pressure-search. Among the seven cases with same concentration of TBAB (0.29 mol%) and CP (5.00 vol%) and different glucoamylase proportions (ranging from 0.00 to 20.00 wt%), the optimum concentration of glucoamylase in the mixed promoters was 3.00 wt%. The phase equilibrium data was calculated by the modified van der Waals–Platteeuw (vdW–P) model with a modification of vapor pressure of water in the empty hydrate lattice. The Peng–Robinson equation of state was used to calculate the fugacity of gas. The maximum average absolute deviation was 4.09% between the calculated results and the experimental results. It revealed that the calculated results were in good agreement with the experimental results.

Semiclathrate Hydrates in Tri-n-butylphosphine Oxide (TBPO)-Water and TBPO-Water-Methane Systems

In the present work, we studied semiclathrate hydrates in the TBPO-H₂O and TBPO-H₂O-CH₄ systems. The stoichiometry, temperature, and enthalpy of dissociation of TBPO semiclathrate hydrate crystals formed in the TBPO-H₂O binary system were found to be TBPO·33.6 ± 0.9H₂O, 280.0 K, and 253.1 ± 4.7 J g^-1, respectively. The crystal structure determined by single crystal XRD analysis (150 K) was the orthorhombic with space group Pbam and unit cell parameters a = 19.9313(8) Å, b = 23.4660(7) Å, and c = 12.1127(5) Å. The structural stoichiometry is TBPO·34.5H₂O. The TBPO guest molecules arrangement within the host water framework has been refined for the first time. The discrepancy between the analytically measured and structural stoichiometry is likely to be attributed to the structure defects, which cannot be revealed by the routine single-crystal XRD analysis. The methane capacity of TBPO + CH₄ double hydrate was measured by the thermovolumetric method in the range 14.9-55.8 wt % TBPO aqueous solution at a methane pressure of 8.5 ± 0.5 MPa and temperature of 274 ± 1 and 286 ± 1 K. The maximum included methane volumes of 61.6-74.6 mL/g were observed for the TBPO + CH₄ double hydrates synthesized from ∼26-30 wt % TBPO aqueous solutions. Powder X-ray diffraction measurements of the hydrate samples used in the thermovolumetric experiments revealed that the TBPO + CH₄ double hydrate has the same structural characteristics as the simple TBPO hydrate. The study of the Raman spectra of the TBPO + CH₄ double hydrate and TBPO simple hydrate showed that in the TBPO + CH₄ double hydrate CH₄ molecules selectively occupy the small D cages. The results of the present study did not confirm the earlier suggestion of the formation of several structural types of hydrates in the TBPO-H₂O system. The obtained results indicate that the TBPO-H₂O binary system has a potential for application in gas separation and as cold storage and transportation media.