Article pubs.acs.org/EF
“Self-Preservation” of CH4 Hydrates for Gas Transport Technology: Pressure−Temperature Dependence and Ice Microstructures Andrzej Falenty,*,†,‡ Werner F. Kuhs,‡ Michael Glockzin,† and Gregor Rehder† †
Leibniz Institute for Baltic Sea Research, Seestrasse 15, 18119 Warnemünde, Germany GZG, Abt. Kristallographie, Universität Göttingen, Goldschmidtstrasse 1, 37077 Göttingen, Germany
‡
S Supporting Information *
ABSTRACT: “Self-preservation” is a kinetic anomaly that allows for storing a substantial amount of gas locked in gas hydrate far outside its thermodynamic stability field for a period of days, weeks, or even months under very mild pressure−temperature (p−T) conditions, by merely maintaining temperatures below the melting point of ice. Utilizing this phenomenon for low-cost storage and transportation of natural gas is not yet sufficiently developed to be competitive with already existing, well-established methods (e.g., liquefied natural gas (LNG), gas to liquid (GTL), compressed natural gas (CNG), or pipeline (PL)). Aside from the refinement of numerous engineering and safety aspects, a deeper understanding of the “self-preservation” phenomenon is needed in order to promote these technologies. We address some of these outstanding issues in a series of isothermal−isobaric pressure−volume−temperature (pVT) experiments exploring the kinetics of the dissociation of pure sI methane hydrate to ice and CH4 gas in a wide p−T field applicable to gas-hydrate-based technologies. By means of ex situ cryo-SEM, we correlate the kinetic data with the morphology of initially formed ice coatings recovered at various stages of the transformation. 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. Moreover, we recognize a progressive sintering of ice coatings of individual particles when close to the melting point of ice. The optimal conditions for the transport and storage at ambient pressure, where this issue is minimized and the preservation strength is still very high, have been found at ∼250 K. Further fine-tuning of the storage capacity may involve elevating the storage pressure and active temperature control.
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INTRODUCTION Gas hydrates (GH, also called clathrate hydrates or clathrates) are renowned for their ability to trap a substantial volume of gas within a crystalline framework of cages made from hydrogenbonded water molecules.1 Depending on the gas composition, most hydrocarbon-containing clathrate hydrates crystallize to cubic crystallographic structures (sI or, less frequently, sII; while the first structure is typically formed in the presence of gas compositions close to pure methane, more-complex mixtures tend to form the latter).1 The GH lattice of both types can store up to one molecule per cage, which gives a maximum of 174.6 m3 for sI and 174.9 m3 for sII (STP) of gas in 1 m3 of clathrate. The cage occupancy of industrially and laboratory-formed compounds is nevertheless slightly lower than the maximum value, with additional variability of a few percent correlated with the gas fugacity under the formation conditions. In the case of large molecules such as CH4 or CO2, gas/water ratios exceeding one molecule per cage have only been found for high-pressure (several GPa) polymorphs (see, e.g., refs 2 and 3) and remain beyond industrial application. The high gas capacity and simple composition (i.e., gas and water) have made gas hydrates a potential candidate for costefficient gas storage and transportation; however, early attempts to develop such technologies (see, e.g., refs 4−9) have proven otherwise. Economically speaking, GH compounds are not yet competitive with existing gas transport methods due to their comparably low energetic values, unfavorable formation/ dissociation kinetics, and the intrinsic fragility of the GH © 2014 American Chemical Society
structure, which requires elevated pressures for formation and/or low temperatures to remain stable. More-recent approaches attempt to circumvent some of these issues by utilizing surfactants10−13 or silica14 in the formation process. Other studies focus on a kinetic anomaly exhibited by some gas hydrates upon dissociation; known as “self-preservation” or “anomalous preservation”.15−17 In the “self-preserved” state, a substantial volume of metastable gas hydrates can persist far outside the thermodynamic stability field for an extended period of time under very mild conditions: ambient to elevated pressures and temperatures from ∼240 K to the ice point. Under these conditions, unstable gas hydrates predominantly dissociate to the hexagonal form of water ice (Ih).18,19 Particularly strong preservation has been found for larger particles that could survive for a few weeks or even months.15,20−23 The anomaly is typically associated with ice enveloping the metastable clathrates that acts as a diffusion barrier to escaping gas molecules (see, e.g., refs 16 and 23). The dissociation does not come to a complete halt, and the ice layer gradually thickens during the anomalously slow conversion. The protective ice layer is initially composed of numerous small crystals that undergo a gradual annealing, thus improving the shielding effect.18,23 The technologies based on this phenomenon are particularly focused on methane-dominated smaller conventional deposits or Received: June 25, 2014 Revised: September 17, 2014 Published: September 18, 2014 6275
dx.doi.org/10.1021/ef501409g | Energy Fuels 2014, 28, 6275−6283
Energy & Fuels
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(2) undisturbed, consolidated clathrate pieces of cylindrical shape with
