Pure phase thermal property results: sII Tetrahydrofuran Hydrate

Tetrahydrofuran (THF) is not a naturally occurring hydrate. THF hydrate has been used in the laboratory as a proxy for methane hydrate, but questions persist about THF hydrate's suitability in this role [National Research Council, 2004]. Methane and THF form different hydrate structures. Methane is incorporated into structure I hydrate with as few as 5.75 water molecules per methane molecule [Sloan, 1998]. THF forms structure II hydrate with 17 water molecules per THF guest [Davidson, 1973]. Methane and THF also have large differences in molecular weight, but despite these differences, THF hydrate is an enticing potential analog material because it forms at atmospheric pressure and moderate temperatures (below 4.4°C) and, like most naturally occurring methane hydrate, forms from solution [Sloan, 1998]. To assess the viability of THF hydrate as a thermal property analog for methane hydrate, we have made thermal property measurements in each. Here we compare thermal conductivity results between the two hydrates, and discuss how thermal properties measured only in THF can be applied to our understanding of methane hydrate properties.

Thermal conductivity, λ, of sI hydrates (open symbols), sII hydrates (closed symbols and heavy line) and water (crosses). The nonlinear increase for THF hydrate above -7.5°C, suggests a phase change occurs over this temperature range, likely the endothermic break down of THF hydrate. (Click for larger image).

Sample Preparation:

THF hydrate is formed by cooling a mixture of liquid THF and water below 4.4°C. Ideally, THF hydrate forms in the molar ratio 1 THF:17 H₂0. By weight, this is a mixture of 19.1% THF and 80.9% water [Sloan, 1998]. To account for THF evaporating during sample preparation, we increase the THF fraction to 22.6%, mixed with 77.4% deionized water that is degassed using the same technique described in the procedure for Ice Ih. This mixture produces THF hydrate and a slight excess of THF. Completely using up both reactants when forming THF hydrate is difficult, but excess THF is preferable to an excess of ice because unlike ice, THF has thermal properties comparable to THF hydrate. Sample preparation then follows the process described for Ice Ih.

Measurement Results:

Relative to methane hydrate thermal properties, thermal properties in THF hydrate display a more complicated dependence on temperature. Below ‑7.5°C, our THF hydrate thermal property measurements are fairly linear with temperature, in agreement with the published THF hydrate results. The thermal properties of THF hydrate are lower than those of methane hydrate over this linear temperature region. THF hydrate thermal properties appear to change dramatically above ‑7.5°C however, limiting the utility of THF hydrate as a thermal property analog for methane hydrate at temperatures above 0°C, which are relevant for naturally occurring methane hydrate [Waite et al., 2005].

Thermal Conductivity, λ: The nonlinear rise in our measured thermal conductivity above -7.5°C is consistent with endothermic breakdown of THF hydrate to THF + water. Even though the THF stability temperature is 4.4°C [Sloan, 1998], decomposition of THF hydrate was observed by Leaist et al. [1982], beginning below ‑3°C during heat capacity measurements. Above 4.4°C, the conductivity in THF + water drops to the expected value as calculated from the THF properties provided by BASF [1998].

Hydrate dissociation absorbs heat generated by the conductivity probe, thus making the sample appear to conduct heat more efficiently than in the absence of a phase change. Since our measurement technique assumes there is no change in phase, results above ‑7.5°C should be termed “apparent” thermal properties and discussed as an indication of phase change rather than as an intrinsic thermal property.

Thermal diffusivity of sII THF hydrate (red) relative to that of sI methane hydrate (blue). THF hydrate displays a highly nonlinear decrease in κ above 7.5°C that is not observed in methane hydrate. Above the THF hydrate stability temperature of 4.4°C, the diffusivity returns to that expected for a mixture of THF and water. (Click for larger image).

Thermal Diffusivity, κ: The nonlinear behavior seen in thermal conductivity for THF hydrate is also apparent in the thermal diffusivity. In the linear behavior range below ‑7.5°C, THF hydrate has a measurably lower thermal diffusivity than that of methane hydrate.

Specific heat of sII THF hydrate (red) relative to that of sI methane hydrate (blue). Uncertainties are approximately equal to the symbol size. THF hydrate displays a highly nonlinear increase in c_p above -7.5°C that is not observed in methane hydrate. Above the THF hydrate stability temperature of 4.4°C, the specific heat returns to that expected for a mixture of THF and water. (Click for larger image).

Specific Heat, c_p: The nonlinear behavior seen in λ and κ for THF hydrate is also apparent in the specific heat. A material that can absorb heat without significantly increasing in temperature has a high specific heat, and if the THF hydrate is indeed breaking down at temperatures below its accepted stability temperature of 4.4°C, heat added to the system goes toward breaking the hydrate into liquid THF and water, a process occurring at essentially constant temperature.

Implications:

Because the nonlinear behavior is likely due to a phase change, and our thermal property measurement technique assumes no phase change occurs, the results measured above -7.5°C are to be considered an indication of changes occurring in the sample, not intrinsic thermal properties of THF hydrate. Based on our results, THF does not appear to be a reasonable proxy for methane hydrate in terms of thermal properties, particularly at temperatures above -7.5°C.

Publications:

The thermal properties of THF hydrate below -7.5°C, our measurement range over which the properties vary nearly linearly with temperature, are published in: Waite, W.F., Gilbert, L.Y., Winters, W.J., and Mason, D.H., 2006, Estimating thermal diffusivity and specific heat from needle probe thermal conductivity data, Review of Scientific Instruments, 77, 044904, doi:10.1063/1.2194481.

The complete thermal property results, including the nonlinear behavior above -7.5°C, are published in: Waite, W.F., Gilbert, L.Y., Winters, W.J. and Mason, D.H., 2005, Thermal property measurements in Tetrahydrofuran (THF) hydrate and hydrate-bearing sediment between -25 and +4°C, and their application to methane hydrate, Paper 5042 in: Fifth International Conference on Gas Hydrates, Trondheim, Norway, Tapir Academic Press, vol. 5, 1724-1733.

Measurement Data: