Measuring and Modelling of the Thermodynamic Equilibrium Conditions for the Formation of TBAB and TBAC Semi-Clathrates formed in the Presence of Xenon and Argon
Creators: name: Garcia Mendoza, Marlon Ilich
Publisher: name: Graduate Studies
Year: 2015
Subjects: subject: Engineering--Chemical
subject: Semi-clathrates
subject: Thermodynamic modelling
subject: Hydrates
subject: TBAB
subject: TBAC
subject: argon
subject: xenon

description: Semiclathrates are crystalline compounds similar in nature to gas hydrates. Like gas hydrates, semiclathrates can trap small gas molecules inside a molecular framework of water molecules. Quaternary ammonium salts (QAS) semiclathrates hydrates, such as tetra-n-butyl ammonium bromide (TBAB) and tetra-n-butyl ammonium chloride (TBAC), are ionic compounds that have a stabilizing effect on the framework of water molecules. TBAB and TBAC semiclathrates formed in the presence of a gas can form at much milder conditions than gas hydrates. Thus, there has been much interest, in recent years, on the possible use of TBAB and TBAC semiclathrates in the storage and separation of gases. To date, the majority of research in the area has been directed towards experimental studies and only a handful of studies have attempted to model the equilibrium conditions of semi-clathrate formation. The present study aims to measure and correlate equilibrium dissociation conditions for semiclathrates formed from aqueous solutions of TBAB and TBAC, in combination with argon and xenon. In the experimental part of this study, a constant-volume reactor was used for measuring the solid–vapor–liquid equilibrium conditions of semiclathrates formed in aqueous solutions of TBAB and TBAC. The TBAB and TBAC semiclathrates were formed from pure argon and pure xenon. The experimental temperatures ranged from (284 to 303) K, the experimental pressures ranged from (266 to 6114) kPa, the weight fraction of TBAB ranged from wTBAB = (0.05 to 0.20), and the weight fraction of TBAC ranged from wTBAC = (0.05 to 0.20). As expected, at a given temperature, the pressure required to form TBAB and TBAC semiclathrates with argon and with xenon was much lower than the pressure that are required to form pure gas hydrates. From the equilibrium data, the enthalpy of formation was estimated to be between (133 and 188) kJ•mol–1 for semiclathrates formed from argon and (55 and 127) kJ•mol–1 for semiclathrates formed from xenon. For modeling the experimental data obtained in the present study, the PSRK equation of state is used to describe the vapour phase, the LIFAC activity coefficient model is used to describe the aqueous phase and the van der Waals and Platteeuw theory combined with the model of Paricaud was employed to describe the solid semiclathrate phase. The new model differs from previous modeling efforts in that it does not neglect the solubility of the gas in the aqueous phase or the presence of water in the vapour phase. Rather, the solubility of the gas and molar fraction of water in vapour phase are computed from a flash calculation. The new model also computes the Langmuir constants from the Kihara potential rather than from an empirical correlation. The model is capable of describing the solid-liquid equilibrium for the semiclathrate in the absence of gas molecules. The new modeling approach is applied to TBAB and TBAC semiclathrates that are formed from xenon and argon. For both gases, new Kihara potential parameters were regressed from the experimental data. Further testing of the new modelling approach was conducted by correlating available data for TBAB/TBAC semiclathrates formed in the presence of pure methane (CH4), carbon dioxide (CO2), nitrogen (N2), and hydrogen (H2). It was found that the new approach was able to correlate the experimental data to a high degree of accuracy with fewer adjustable parameters for all but one of the existing modelling attempts.