Institute of Petroleum Engineering

Centre for Gas Hydrate Research Publications

Abstract 153

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

Ross Anderson, Antonin Chapoy, Jeerachada Tanchawanich, Hooman Haghighi, Joanna Lachwa-Langa and Bahman Tohidi
6th International Conference on Gas Hydrates, Vancouver, Canada, July 6-10 (2008)

Ethanol (EtOH) - a well-known anti-freeze - is commonly used as a hydrate inhibitor in oil and gas production operations, particularly in areas of high industrial production (e.g. South America). As a polar, hygroscopic, highly water soluble (complete mutual miscibility) alcohol, intuition might suggest that, like methanol, ethanol should depress the activity of water significantly, offering comparable hydrate inhibition. However, literature data for the binary EtOH−H2O system show that ethanol can form a number of stable and metastable solid hydrate phases at low temperatures (<−50 °C). The precise structure and composition of these hydrates remains somewhat ambiguous, although a stable phase found consistently below −74 °C is understood to be a structure-II type clathrate similar to those formed by other water-soluble organic liquid hydrate formers (e.g. THF, 1,4-dioxane and 2‑propanol). Here, we present experimental DTA and PVTX data for the binary ethanol−water and ternary ethanol−methane−water systems respectively. Binary liquidus data are in good agreement with literature data and confirm the appearance of metastable EtOH hydrates above the established stable clathrate peritectic transition at −74 °C. In the ternary system with methane, at aqueous concentrations >~5.6 mole%, ethanol forms binary CH4−EtOH clathrates hydrates stable over a wide PT range. At higher pressures, in the HCH4-EtOH+L+G region, this behaviour results in significantly less hydrate inhibition than would be expected from ice melting point depression, and much less than that offered by methanol for comparable aqueous molar concentrations. In the ice region, ethanol actually increases hydrate stability relative to the methane−water system; the HCH4-EtOH+L+G region extending to pressures lower than the normal HCH4+I+G boundary, where it is delimited by an alternate univariant HCH4-EtOH+L+I+G line. PTVX data, combined with preliminary thermodynamic modelling studies, suggest EtOH−CH4 clathrates are most likely of structure-II, although this requires further confirmation.

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