In this study, hydrate based CO2 separation with thermodynamic promotors (THF and TBAC) and dual functional diamine inhibitors (PZ and HZ) were investigated for the safe and efficient natural gas production and transport. With regard to the CH4/CO2 mixed gas separation by clathrate formation, thermodynamic stability of mixed gas hydrate was improved by adding thermodynamic promotors. The phase equilibrium temperature was increased by 16K with 5.6mol% THF and 13.5K with 3.3mol% of TBAC at 3 MPa. The stoichiometric THF and TBAC solution form sII hydrate and semi-clathrate respectively, and gas molecules occupy the empty small cage in both systems. 5.6 mol% of THF + CO2
+ CH4 showed faster hydrate growth but the system is not CO2 selective in the hydrate phase. 3.3 mol%
of TBAC + CO2 + CH4 semi-clathrate showed the highest separation efficiency in that the CO2
composition in feed gas increased from 50% to 76% in the hydrate phase. Thermodynamic stability of CH4 and CO2 is the significant factor to separate mixed gas by using hydrate. Also, the kinetic selectivity was implied by the faster CO2 growth rate, and this would improve the CO2 selectivity in the early stage of hydrate formation.
Regarding the diamine inhibitors, it is found that the thermodynamic inhibition of PZ and HZ was dependent on the diamine concentrations, and HZ showed more significant thermodynamic inhibition, slower hydrate growth, and longer induction time than the addition of PZ did. Neither PZ nor HZ is captured in the hydrate cages and that they just inhibit CH4 hydrate formation outside of hydrate cages.
The interaction energy between sI small cage and each diamine supported the strong inhibition effects of HZ as the strongest attractive interaction was found in the HZ system. The thermodynamic, spectroscopic, and computational results demonstrated that the PZ and HZ can function as both thermodynamic and kinetic inhibitors for CH4 hydrate and they have a potential for application to flow assurance in natural gas production and transportation.
In conclusion, this study using the thermodynamic, kinetic features of clathrate hydrate and diverse methods can be applied in the large range of the energy process; especially gas separation and hydrate inhibition. Hydrate based gas separation for the gas production and diamine inhibitor for the gas transport can be applied to the combined process to improve the energy efficiency of the natural gas production and transportation. Furthermore, the biodegradable additives for the environment and kinetic surfactant for the efficient process can be suggested for the future works.
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