Thermodynamic Equilibrium Conditions for Simulated Landfill Gas Hydrate Formation in Aqueous Solutions of Additives

Abstract

This work presents the thermodynamic study of separating CH 4 and CO 2 from the simulated landfill gas (LFG) [CO 2 (0.45) + CH 4 (0.55)] based on hydrate crystallization in the presence of tetra-n-butyl ammonium bromide (TBAB), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), and their mixtures. The mole fractions of TBAB, THF, and DMSO aqueous solutions were fixed at 0.0234, 0.0556, and 0.0165, respectively. The equilibrium hydrate formation conditions were measured by T-cycle method in the temperature range of (274.15 to 294.95) K and the pressure ranges up to 6.72 MPa. The gas phase in the crystallizer at the equilibrium points was also sampled and analyzed. For the additives with the fixed concentrations studied in this work, it was found that both TBAB and THF can remarkably reduce the equilibrium hydrate formation pressure of LFG mixture gas, but the effect of THF is better than that of TBAB in the high temperature region, while DMSO have no obvious pressure drop effect on the equilibrium hydrate formation conditions but can promote the solubility of CO 2 in the solution. However, the mixture additives of TBAB + DMSO and THF + DMSO can not only remarkably promote the solubility of CO 2 but also remarkably reduce the equilibrium hydrate formation pressure of CO 2 + CH 4 + H 2 O hydrate. Moreover, the pressure drop effect of THF + DMSO is better than that of TBAB + DMSO on the CO 2 + CH 4 + H 2 O equilibrium hydrate formation in the high temperature region.

Hydrate-based CO2 capture and CH4 purification from simulated biogas with synergic additives based on gas solvent

“…2, both the two synergic additives (THF-DMSO and TBAB-DMSO) have prominent thermodynamic effect on the CH 4 /CO 2 equilibrium hydrate formation conditions, although DMSO has slightly influence on the thermodynamic effect of the two synergic additives may be due to that the addition of DMSO lowers the activity of water and simultaneously lowers the chemical potential of aqueous water [47]. Furthermore, for CH 4 /CO 2 equilibrium hydrate formation in the high temperature region, the thermodynamic effect of THF-DMSO is better than that of TBAB-DMSO [17]. It provides the thermodynamic validity of the hydrate-based biogas purification process.…”

“…Coincidentally, the thermodynamic conditions of simulated landfill gas hydrates formation with the synergic additives have been studied in our previous research [17]. For convenience, some representative data were selected and plotted in Fig.…”

“…It is known that CO 2 can form hydrate under the milder condition, for instance, which is at higher temperature or lower pressure than CH 4 [36]. In fact, the precious researches have proved that CH 4 can be separated with high selectivity from the mixtures containing CO 2 [17,[37][38][39].…”

“…Despite these advantages, a commercially viable hydrate-based CO 2 separation process has been challenging due to it must satisfy the following factors [41][42][43][44]: (1) Moderate operating condition; (2) high selectivity of CO 2 ; (3) rapid hydrate formation rate; and (4) excellent gas storage capability. For the first factor, some outstanding thermodynamic additives such as tetrahydrofuran (THF) [27], Tetra-b-utyl ammonium/phosphonium salts [17,20,26] and cyclopentane (CP) [28,45] have been proved. For instance, when the simulated landfill gas hydrate formed with 0.0234 mol fraction tetra-n-butyl ammonium bromide (TBAB), the equilibrium hydrate formation pressure could reduce by approximately 90% [17], it is very close to the realistic temperature and pressure condition for separation operating.…”

“…For the first factor, some outstanding thermodynamic additives such as tetrahydrofuran (THF) [27], Tetra-b-utyl ammonium/phosphonium salts [17,20,26] and cyclopentane (CP) [28,45] have been proved. For instance, when the simulated landfill gas hydrate formed with 0.0234 mol fraction tetra-n-butyl ammonium bromide (TBAB), the equilibrium hydrate formation pressure could reduce by approximately 90% [17], it is very close to the realistic temperature and pressure condition for separation operating. However, it has been reported that even though the quaternary ammonium salts hydrate could offer greatly enhanced thermal stability, the hydrate formation rate and gas storage capability were very low [21].…”