Measurement of Thermodynamics and Kinetics of Carbon Dioxide Hydrate in the Presence of 2-Methoxyethyl ether, Tetrabutylammonium hydroxide and 1,1,3,3-Tetramethylguanidine

In this study, phase equilibrium conditions for carbon dioxide hydrates in the presence of 2-methoxyethyl ether, tetrabutylammonium hydroxide and 1,1,3,3-tetramethylguanidine were experimentally measured. The three-phase (H-Lw-V) equilibrium pressures and temperatures were determined by isochoric method in the pressure range from 1.68 to 3.44 MPa with various concentrations of the additives. Also, the kinetic behaviors were investigated in the presence of 2-methoxyethyl ether at 0.2 and 0.3 mass fraction. Compared with pure water system, addition of 2-methoxyethyl ether and 1,1,3,3-tetramethylguanidine in the system cause inhibition effect on carbon dioxide hydrate formation and the maximum decrease of dissociation temperature is about 5.7 K and 11.1 K, respectively. On the other hand, addition of tetrabutylammonium hydroxide in the system gives rise to promotion effect on carbon dioxide hydrate formation and the maximum increase temperature is about 11 K, compared with pure water system. To simulate the seawater environment, this study also measured the additives in brine system with 0.035 mass fraction of NaCl. Moreover, the structure and dissociation enthalpy of hydrates are estimated by using Clausius-Clapeyron equation. The structures of carbon dioxide hydrates with addition of 2-methoxyethyl ether and 1,1,3,3-tetramethylguanidine are both classified as structure I, whereas those with addition of tetrabutylammonium hydroxide are classified as structure TS-I. In this study, the kinetics of carbon dioxide hydrate with 2-methoxyethyl ether as the additive at 0.2 and 0.3 mass fraction were also investgated. With an increase in initial operating pressure, the driving force increased. That is due to the fact that higher initial pressure created higher supersaturation, which induced stronger driving force. At 0.3 mass fraction of 2-methoxyethyl ether in the system, the induction time was shortened as the driving force increased. In addition, the carbon dioxide consumption was increased almost linearly with increasing the driving force. However, the average hydrate formation rate stayed almost constant with the increased driving force. Also, addition of 2-methoxyethyl ether at 0.2 mass fraction in the system was studied. The results showed the induction time was shortened in comparison to that of 0.3 mass fraction. However, the average hydrate formation rate and carbon dioxide consumption were not effectively influenced.