Institute of Petroleum Engineering

Centre for Gas Hydrate Research Publications

Abstract 170.

Binary Ethanol-Methane Clathrate Hydrate Formation in the System CH4-C2H5OH-H2O: Experimental Data and Thermodynamic Modelling

Anderson, R., Chapoy, A., Haghighi, H., and Tohidi, B.
Journal of Physical Chemistry C, 113, 12602-12607 (2009).

Ethanol (EtOH) is commonly used as a gas hydrate inhibitor in hydrocarbon production operations. However, a number of stable and metastable hydrate phases have been reported for the binary EtOH−HO2 system at temperatures of <223 K, including a structure II type clathrate hydrate stable below 198 K. Here, we present experimental DTA and PVT phase equilibrium data for the binary ethanol−water and ternary ethanol−methane−water systems, respectively. Binary DTA data confirm the appearance of metastable EtOH hydrates above the established structure II clathrate peritectic transition. In the ternary system with methane, at XEtOH > 0.056, aqueous ethanol forms binary EtOH−CH4 clathrate hydrates stable over a wide PT range. In the HEtOH−CH4+L+G region, this results in significantly less hydrate inhibition than would be expected from ice melting point depression. In the ice region, ethanol enclathration actually increases hydrate stability relative to the methane−water system; the HEtOH−CH4+L+G region being bounded by a univariant HEtOH−CH4+L+I+G quadruple point locus line at temperatures much higher than the typical HCH4+I+G boundary (or HCH4+I+L+G quadruple point univariant locus in the presence of an aqueous hydrate inhibitor). Compositional analyses of the clathrate phase yields the formula 2.30CH4·0.66EtOH·17H2O at 246.7 K and 3.68 MPa, which is consistent with structure II. Independent powder X-ray diffraction and Raman spectroscopic studies presented in an accompanying article in this journal issue confirm ethanol−methane clathrates to be of structure II type.

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