The phase behavior of a system consisting CO2 hydrate is of great importance for many industrial and natural processes. On the one hand, carbon dioxide and water are part of natural gas streams and also they are found in oil reservoirs during enhanced oil recovery. In these cases, formation of hydrate may cause problems during production and processing. On the other hand, carbon dioxide hydrate formation may be desirable because it can facilitate separation processes, freezing and refrigeration processes. These technological interests explain the need for phase equilibrium data of systems containing CO2. Among these systems are systems containing CO2 in electrolyte solutions. It is generally accepted that the hydrate formation is inhibited by the presence of electrolytes in liquid water. While electrolytes cannot enter the lattice of hydrates, they act by lowering the activity of water in the coexisting liquid phase, causing hydrates to form at lower temperatures and higher pressures compared to their formation in pure water1. The increase of pressures and lowering of temperatures may reduce the attractiveness of gas hydrate technology as alternative for processes such as separation of highly soluble salts from aqueous solution by eutectic freeze crystallization or desalination of water due to the increase in operational cost. The presence of some specific organic components such as tetrahydrofuran, tetrahydropyran and 1-3-dioxolane are known to be able to reduce these pressures and might enable gas hydrate processes as a practical option.

Hydrate equilibrium data on CO2 in electrolytes solution have been reported by several researchers2, 3,4. However, no data have been reported on equilibrium conditions of mixtures of CO2 and a promoter in an aqueous electrolyte solution. In the present work, data for systems containing CO2 and a promoter in aqeous electrolyte solutions are presented. The organic component chosen as promoter is tetrahydrofuran (THF). Seven different electrolytes are used in this work namely potassium chloride (KCl), sodium chloride (NaCl), calcium chloride (CaCl2), magnesium chloride (MgCl2), potassium bromide (KBr), sodium fluoride (NaF) and sodium sulphate (Na2SO4). Experimental data on the equilibrium conditions of mixed CO2 and THF hydrates in single and mixed electrolytes are reported. All equilibrium data of this work are measured by using Cailletet Equipment. The concentrations of THF and electrolytes are varied while the concentration of CO2 is kept constant at 0.3 mol fraction of the overall concentration throughout the work. The experimental temperature ranged from 275 to 295K and pressures up 10 to 75 bars have been applied.

From the experimental results it is concluded that THF, which is soluble in water, shows a hydrate promoting effect in the range of concentrations studied. The promoting effect is concentration-dependent up to 7 mol% of THF in the aqueous solution. At higher concentration, THF shows an inhibition effect of hydrate formation. In contrast, all electrolytes show inhibition effects at all concentrations, although the inhibition effect also is concentration-dependent. Adding electrolyte such as NaCl causes the equilibrium pressure to increase significantly, though the competing effect of THF is large enough to overrule this effect at lower concentrations of electrolytes. At higher electrolyte concentrations, a salting-out effect has been observed in the system, reducing the solubility of THF in water and forcing a liquid-liquid phase split in the system. The strength of inhibition effect among the electrolytes is also compared.

References

[1] Duan, Z. and Sun, R. American Mineralogist. 2006, 91, 1346-1354. [2] Tohidi, B., Danesh, A., Todd, A.C. and Burgrass, R.W. Chemical Engineering Science. 1997, 52, 3257-3263. [3] Englezos, P. and Hall, S. Canadian Journal of Chemical Engineering. 1994, 72, 887-893. [4] Dholabhai, P.D., Kalogerakis, N., and Bishnoi, P.R. Journal of Chemical Engineering Data. 1993, 38, 650.654.