BINARY ETHANOL−METHANE CLATHRATE HYDRATE
FORMATION IN THE SYSTEM CH4−C2H5OH−H2O:
EXPERIMENTAL DATA AND THERMODYNAMIC MODELLING
Ross Anderson1,2, Antonin Chapoy1,2, Jeerachada Tanchawanich1, Hooman
Haghighi1, Joanna Lachwa-Langa1 and Bahman Tohidi1,2
Centre for Gas Hydrate Research1 & Hydrafact Ltd2, Institute of Petroleum
Engineering
Heriot-Watt University, Edinburgh, EH14 4AS
UNITED KINGDOM
ABSTRACT
Ethanol (EtOH) 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, water soluble alcohol, intuition might
suggest that like methanol, ethanol should similarly depress the activity of
water, 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 below ~ −75 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). Here, we present experimental DTA and PVT 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 −75 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±L+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 clathrates are
most likely of structure-II type, although this requires further confirmation.
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Corresponding author: Phone: +44 (0)131 451 3798 Fax +44 (0)131 451 3127 E-mail: ross.anderson@pet.hw.ac.uk