Thermodynamic Stability Conditions of Clathrate Hydrates in Methane/Carbon Dioxide plus Tetrahydrofuran plus Cyclopentane plus Water Systems: Experimental Measurement and Modeling

In this communication, experimental hydrate dissociation data for methane + tetrahydrofuran (THF) (0.03 and 0.0555 mole fractions in aqueous solution) + cyclopentane (CP) + water systems are reported in the temperature range from 294.8 to 301.3 K and pressure range from 1.91 to 4.95 MPa, which are measured using an isochoric pressure-search method. Other experimental data were collected from the literature. Then a thermodynamic model for four-phase equilibria, hydrate (H)-aqueous liquid (Lw)-organic liquid (La)-vapor/gas (V), systems composed of water, methane (CH4), carbon dioxide (CO2), and/or CP was developed. The thermodynamic model is based on the solid solution theory of van der Waals-Platteeuw (vdW-P) for the hydrate phase and Peng-Robinson equation of state (P-R EOS) for vapor/gas phase. For calculating the activity coefficient of water and THF in the aqueous phase, the universal quasichemical (UNIQUAC) functional-group activity coefficient (UNIFAC) model is also used. The Langmuir constants of THF and CP are calculated by using the Parrish and Prausnitz equations under this assumption that THF and CP occupy large cavity and CH4 occupies small cavity of structure II hydrate. The calculated Langmuir constants are extended to other systems. Using the vdW-P model, PR EOS, and calculated Langmuir constants, the binary interaction parameters for van Laar, nonrandom two-liquid, and UNIFAC models are calculated. Results show that THF and CP decrease the equilibrium pressure, and by adding both THF and CP into the pure water; these mixtures show better promotion effects in comparison to the individual ones.