Superheating Clathrate Hydrates for Anomalous Preservation

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C: Energy Conversion and Storage; Energy and Charge Transport

Superheating Clathrate Hydrates for Anomalous Preservation Hassan Sharifi, Akio Yoneyama, Satoshi Takeya, John A Ripmeester, and Peter Englezos J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02930 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 18, 2018

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The Journal of Physical Chemistry

Superheating Clathrate Hydrates for Anomalous Preservation Hassan Sharifi§, Akio Yoneyama†, Satoshi Takeya‡, John Ripmeester¥, and Peter Englezos§* §

Chemical and Biological Engineering Department, The University of British Columbia, 2360 East Mall, Vancouver, BC, Canada †

‡

SAGA Light Source, 8-7 Yayoigaoka Tosu, Saga 841-0005, Japan

National Institute of Advanced Industrial Science and Technology (AIST), Central5,1-1-1 Higashi, Tsukuba 305-8565, Japan ¥

National Research Council, Ottawa, Ontario, Canada

Corresponding Author *

Dr. Peter Englezos, Chemical and Biological Engineering Department, The University of British

Columbia, 2360 East Mall, Vancouver, BC, Canada, E-mail: [email protected]

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Abstract: Tetrahydrofuran (THF) hydrate crystal conglomerates were kept intact above their melting point by immersing them in cyclopentane (CP) as observed visually and by diffractionenhanced X-Ray Imaging. THF and CP form isostructural sII hydrates with melting points of 277.5 and 280.8 K, respectively. Placing the THF-hydrate crystal in CP converted the outer layer of THF-hydrate to a thin layer of CP-hydrate which then prevented the THF-hydrate from melting at its usual melting point. The coated THF-hydrate melted at a temperature about 2 K higher than the usual melting temperature for THF-hydrate. Therefore, we can postulate that superheating less stable clathrate hydrates by coating them with hydrates of greater stability may be an approach suitable for the anomalous preservation of less stable clathrate hydrates, e.g. of fuel gases, for a considerable length of time for transport or storage.


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Introduction Clathrate or gas hydrates are crystalline inclusion compounds. Properly sized molecules such as methane, carbon dioxide, tetrahydrofuran, cyclopentane and others can be trapped in cavities of hydrogen bonded water molecules under suitable thermodynamic conditions.1,2 They have a large capacity for gas storage. One type, known as Structure I clathrate hydrate, can store gas molecules in its cavities thus reducing the storage volume by a factor of about 164 as compared to the gas under standard conditions.3 This property makes them candidates for consideration as natural gas storage media (solid natural gas storage) and for transportation purposes.4–7 However, keeping hydrates stable at ambient temperatures during transportation requires high pressures. Cost analysis has shown that natural gas transportation as hydrate can be competitive for some specific applications, e.g. where pipeline transport isn’t feasible and quantities of gas are insufficient for LNG transport.8 Nevertheless, if gas hydrate can be kept intact outside its normal hydrate stability region, for instance at a pressure below its equilibrium value at ambient temperature, it might be attractive for energy transportation reasons. Early experiments noted the delayed decomposition pattern of methane hydrate where gas release from decomposing hydrate was not completed until at or above the ice melting point. Ice at the hydrate particle surface evidently plays a key role, as this self-preservation effect disappears when the surface ice melts at the ice point9,10 and mechanisms were suggested that involved an ice barrier to diffusion on the surface of the hydrate.11–14 Stern et al.15–17 studied the anomalous preservation effect in detail, noting the effect of various decomposition protocols on methane hydrate decomposition.



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Although thoroughly investigated over a period of for more than ten years13,14,18,19, a review published in 201420, concluded that “The P–T dependence of the “self-preservation” strength is seen as a complex interplay between (1) ice microstructures (shape, arrangement, and size of ice crystals) and (2) annealing rate of the ice coating that acts as a diffusion barrier for escaping gas.” Without knowing the exact mechanistic details, the anomalous preservation phenomenon has been firmly established as methane existing in a metastable or superheated state by establishing appropriate interactions at the hydrate surface with ice.20,21 Anomalous preservation of methane hydrate in many ways was a surprise and the description of the preserved state as a metastable solid phase allows a connection to a field of general interest in chemistry and physics, that of superheated solids.22 An early observation in this area was the superheating of silver crystals by applying a thin coating of gold.23 This procedure was deemed to be successful as silver and gold form an isomorphous alloy system so that there is essentially a continuous defect-free boundary between the silver and gold. The silver does not have a free surface where defects can initiate a melting process. Note that this is markedly different from the case of methane hydrate superheating, as the surface ice is not structurally commensurate with the underlying hydrate. In this work, we apply the concept illustrated by the silver–gold experiment to clathrate hydrates. When the outermost layers of a low melting hydrate are decomposed and converted to a hydrate that is higher melting can we produce a metastable hydrate phase that remains intact above its usual melting point? If we choose isostructural hydrate phases, we may expect the interface to be continuous and defect free so that the outer, higher melting hydrate will cover the lower melting hydrate and prevent it from melting. We have studied this by converting the outer layer of tetrahydrofuran (THF) hydrate crystals with cyclopentane (CP) hydrate. Each form a cubic sII hydrate and has near-identical lattice 4 ACS Paragon Plus Environment

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parameters. Air-free THF hydrate melts at 277.5 K,24 whereas air-free CP-hydrate melts at 280.8 K at ambient pressure.25 If our experiment is successful, application of similar concepts to methane and natural gas hydrates may be useful strategies for extending conditions under which solid gas transport may be feasible under mild storage conditions. Experimental Section A solution of 0.2 mass fraction of tetrahydrofuran in deionized water was used to form THF-hydrate crystal conglomerates. Tetrahydrofuran (THF, purity of 99+ %; from Alfa Aesar), and cylcopentane (CP, 99 % pure; from Acros Organics) were used to form hydrates. The experimental set-up consisted of a 20 mL-test tube submerged in a circulating water bath and connected to a vacuum aspiration pump through a valve, was used to form air-free THF-hydrate crystals. An external refrigerating/heating programmable circulator (VWR Scientific) was used to regulate the temperature of the circulating bath. A copperconstantan thermocouple (uncertainty of 0.1 K; Omega Engineering) was used to measure the water bath temperature. A data acquisition system (National Instruments) was connected to receive transmitted data from the thermocouple. LabVIEW full development system software (National Instruments) was employed to convert the receiving signals for recording into Microsoft Excel. Initially, 5 mL of THF aqueous solution was loaded into the test tube. Afterward, the test tube was connected to the vacuum aspiration pump through the installed valve. The presence of dissolved air in the aqueous solution may increase the hydrate decomposition temperature by about 0.6 to 1 K.24 Therefore, in order to remove dissolved air, the sample was subjected to a sequence of freeze-pump-thaw (freeze the solution using liquid nitrogen, start the aspiration pump to remove air, and subsequently thaw the system while the vacuum pump was off). Three cycles of this procedure were repeated to remove most of the dissolved air. During the thawing step, air bubbles were observed to escape from the solution to be pumped away. It is important to note that the sample must be kept solid during the vacuum step; else one would lose THF to the gas phase, due to its high volatility. The test tube was finally placed in the liquid



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nitrogen such that only the tip of the tube was submerged. This allowed the solution to nucleate THFhydrate crystals. Once crystals have started to form, the test tube was submerged in the water bath set at about 275.1 K to provide adequate sub-cooling to keep THF-hydrate crystals forming (equilibrium THFhydrate temperature under atmospheric pressure is reported as 277.5 K). The system was left under this condition overnight to form as much as possible THF-hydrate. Subsequently, the test tube was opened, and the formed THF-hydrate crystals were crushed (using a cooled stainless-steel spoon) in the test tube. During the crushing of hydrate crystals, the test tube was kept at the experimental temperature to prevent the decomposition of the THF-hydrate crystals formed. A piece of crushed THF-hydrate crystal was put in a vial containing 10 mL of CP previously cooled and kept in a refrigerator at a temperature about 275.1 K. The vial was submerged in the water bath with the temperature set at 275.1 K. A digital camera equipped with a high zoom lens was used to observe hydrate crystals during CP-hydrate formation and hydrate dissociation events. The specification of the digital camera is described elsewhere.26 The digital camera was used to take pictures to show the entire crystal, and the pictures were taken regularly to observe any morphological changes to the crystals during the reaction with CP and the consequent dissociation event. When THF-hydrate is placed in CP below the THF-hydrate melting point, the outermost THF-hydrate layer will decompose as the hydrate must be in contact with a THF solution for stability. The liberated water and CP will form a CP hydrate layer which is continuous with the THF-hydrate layer as the temperature is below the equilibrium melting temperature for CP hydrate. Once the crystal appears stable (formation of a white layer of CP-hydrate covering THFhydrate crystals) at the experimental temperature for five hours, the temperature of the system was increased in small temperature increments (0.5 K in 5 hours) to observe at what temperature the coated hydrate crystals start to deform which is attributed to the hydrate dissociation event. As the temperature of the water bath was measured during the experiments, such a low heating rate would keep the difference between solution and bath temperature quite small. The experiment was repeated three times to determine the data reproducibility.


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For a comparison, a blank test was performed in which a piece of THF-hydrate crystal was placed in an empty vial submerged in the water bath at temperature of 275.1 K. The THF-hydrate crystals then were also subjected to a temperature increase to compare the appearance and decomposition during the dissociation stage. The Diffraction Enhanced X-Ray Imaging (DEI) method can visualize gas hydrates coexisting with ice, to a density resolution of ~ 0.01 g/cm3. X-ray images were collected using 35 keV monochromatic synchrotron X-ray radiation, supplied by the vertical wiggler beam line (BL-14C) at the Photon Factory (Tsukuba, Japan). The DEI system had a field of view of 25 × 35 mm, and spatial resolution of 40 µm. A conglomerate crystal of THF-hydrate, which was formed at 274.1 K, was introduced into a cylindrical polypropylene container with a diameter of 8 mm filled with CP at a temperature below 253.1 K. The sample container was placed in a cryochamber equipped with an X-ray imaging system, and the sample container was immersed in liquid methyl acetate (99.5%, Kishida Chemical Co., Japan) maintained at 253.1 K. Then the temperature was raised slowly up to the measurement temperature. To obtain threedimensional (3D) X-ray CT images, samples with a diameter of

Superheating Clathrate Hydrates for Anomalous Preservation

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