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Investigation of Factors Affecting Crystallization of Cyclopentane Clathrate Hydrate Catherine A

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Investigation of Factors Affecting Crystallization of Cyclopentane Clathrate Hydrate Catherine A Investigation of factors affecting crystallization of cyclopentane clathrate hydrate Catherine A. Whitman, Roman Mysyk, and Mary Anne White Citation: The Journal of Chemical Physics 129, 174502 (2008); doi: 10.1063/1.3005379 View online: http://dx.doi.org/10.1063/1.3005379 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/129/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Clathrate structure-type recognition: Application to hydrate nucleation and crystallisation J. Chem. Phys. 142, 244503 (2015); 10.1063/1.4922696 Order parameters for the multistep crystallization of clathrate hydrates J. Chem. Phys. 135, 074501 (2011); 10.1063/1.3613667 Heat capacity of tetrahydrofuran clathrate hydrate and of its components, and the clathrate formation from supercooled melt J. Chem. Phys. 124, 154507 (2006); 10.1063/1.2188944 Evidence for phase separation during the crystallization of hyperquenched glassy clathrate hydrate forming solutions J. Chem. Phys. 110, 6475 (1999); 10.1063/1.478550 Differential scanning calorimetry, x-ray diffraction and 19 F nuclear magnetic resonance investigations of the crystallization of InF 3 -based glasses J. Chem. Phys. 109, 2432 (1998); 10.1063/1.476812 Reuse of AIP Publishing content is subject to the terms: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to IP: 129.173.74.49 On: Wed, 15 Jun 2016 12:02:33 THE JOURNAL OF CHEMICAL PHYSICS 129, 174502 ͑2008͒ Investigation of factors affecting crystallization of cyclopentane clathrate hydrate ͒ Catherine A. Whitman, Roman Mysyk, and Mary Anne Whitea Department of Chemistry and Institute for Research in Materials, Dalhousie University, Nova Scotia B3H 4J3, Canada ͑Received 23 May 2008; accepted 29 September 2008; published online 3 November 2008͒ We report the results of systematic investigations of the influence of thermal history and other factors on crystallization of a model clathrate hydrate ͑cyclopentane hydrate͒ studied as water-in-oil and oil-in-water emulsions to remove the nucleation influence of substrates other than ice and hydrates. Hydrate and ice seem to form simultaneously under the conditions of these experiments, with ice forming preferentially. Thermal treatment, melting the ice, and leaving only the hydrate, promotes further hydrate formation. Not all the hydrate formed can be accounted for by the recrystallization of water freed by melting ice. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.3005379͔ I. INTRODUCTION ambient pressure, are often used as analogs for laboratory studies. Both THF and CP form sII cubic hydrates.7 Their Clathrate hydrates are crystalline icelike compounds that structure consists of 16 small cavities ͑called 512 to indicate consist of cages composed of hydrogen-bonded water mol- 12 pentagonal faces͒ and 8 large cavities ͑called 51264 to ͑ ͒ ecules the host lattice stabilized by small molecules indicate 12 pentagonal faces and 4 hexagonal faces͒ per unit ͑ ͒ guests that reside in the cages. These materials are fasci- cell containing 136 water molecules.7 The THF and CP mol- nating from the scientific and technological perspectives. ecules reside in the large 51264 cavities at an ideal stoichi- Naturally occurring clathrate hydrates are of importance to ometry of 1 guest:17 H O.10 ͑ 2 the petroleum industry where gas hydrate i.e., clathrate hy- Previously we have shown that the thermodynamic driv- ͒ drate with gaseous guests formation in off-shore pipelines ing force for clathrate formation is enthalpy stabilization has caused delays in productivity, and also has led to safety relative to the separated components, whereas entropic fac- and environmental concerns. A common solution to pipeline tors work against formation.11 However, formation and blockages has been to depress the hydrate formation thermo- growth of clathrate hydrates are a complicated mix of ther- dynamically, for example, by flushing pipelines with metha- modynamic and kinetic factors. For example, in detailed mo- nol or glycols. The high concentrations of these hydrate in- lecular dynamic simulations, the rate of growth of methane hibitors can cost oil and gas companies hundreds of millions 1 hydrate has been found to be considerably higher than that of of dollars per year and can be detrimental to the environ- ice Ih in similar circumstances.12 In recent experiments, ice ment. Recent efforts in hydrate prevention have included de- was found to be a good nucleating agent in the formation of velopment of kinetic inhibitors that prevent hydrate 13 1,2 clathrate hydrate, whereas other experiments suggest in- formation. Furthermore, there is considerable interest in stead that hydrates form first on freezing.14 With an aim to gas hydrates as potential fuel sources since massive natural understand clathrate hydrate nucleation and stability, we gas deposits in the form of hydrates are found in arctic per- 3 present a systematic study of the influence of conditions and mafrost and along the continental margins. Several recent thermal history on hydrate formation. We have concentrated studies have explored the potential of using clathrate hy- 4,5 6 on the CP hydrate system, prepared as water-in-oil or oil-in- drates for CO2 sequestration and H2 gas storage. water emulsions to remove the nucleation influence of sub- Clathrate hydrates can crystallize in several different strates other than ice and hydrates. structures depending on the guest molecules in the cages. Structures identified to date include cubic structures I and II II. EXPERIMENTAL SECTION ͑sI and sII, respectively͒7 and a hexagonal structure ͑sH͒.8 All three are naturally occurring with sH most recently dis- The type of emulsion formed in the CP/water system covered in gas hydrate samples collected off the coast of depends on the type of surfactant used for stabilization. Sur- Vancouver Island.9 Methane forms sI and sII hydrates while factants can be characterized by the hydrophilic/lipophilic propane crystallizes as sII hydrate.7 balance ͑HLB͒ value, where a higher number indicates a Gas hydrates typically require high pressure to achieve greater hydrophilic portion for that surfactant and greater wa- stability, so clathrate hydrates such as tetrahydrofuran ͑THF͒ ter solubility. As in the studies of Zhang et al.,13 we used hydrate and cyclopentane ͑CP͒ hydrate, which are stable at Span 65 ͑sorbitane tristearate͒, which has a low HLB value ͑2.1͒ to stabilize water-in-oil emulsions. To stabilize oil-in- a͒ ͑ Author to whom correspondence should be addressed. Tel.: 1-902-494- water emulsions, Tween 65 polyoxyethylenesorbitan 3894. FAX: 1-902-494-8016. Electronic mail: [email protected]. tristearate͒, which has an HLB value of 10.5, was used. Span 0021-9606/2008/129͑17͒/174502/6/$23.00129, 174502-1 © 2008 American Institute of Physics Reuse of AIP Publishing content is subject to the terms: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to IP: 129.173.74.49 On: Wed, 15 Jun 2016 12:02:33 174502-2 Whitman, Mysyk, and White J. Chem. Phys. 129, 174502 ͑2008͒ 65 was obtained from Sigma-Aldrich and Tween 65 was ob- 30 15.32 tained from Fluka. CP at 95% purity was obtained from 25 Sigma-Aldrich. De-ionized water was obtained from a nano- T = 6.9 °C 15.27 T =-0.2°C ⍀ 20 -32 712 ∆H =0.3J/g pure filtration system and had a resistivity of ca. 18 M cm. ∆H = 96.0 J/g 13 Stage 1 The emulsions were prepared as per Zhang et al. Span 65 mW 15 or Tween 65 was dissolved in the continuous phase at 4% )/ T = -38.0 °C T=-0.2°C 10 ∆H = 65.1 J/g w/w. The dispersed phase was added and the resultant emul- ∆H = 95.2 J/g sion was hand shaken for 30 s. The emulsion was placed in endo up 5 an ice bath ͑to ensure that CP did not evaporate͒ and was ( Stage 2 homogenized for 10 min using a 4710 ultrasonic homog- 0 T = -38.0 °C T =6.6°C ∆ enizer from Cole Palmer. The output power was ca. 8 W. The -5 H = 65.2 J/g ∆H =26.1J/g average diameter of the droplets determined by optical mi- Heat Flow T =-0.2°C croscopy was 3Ϯ1 ␮m, with typical diameters ranging -10 Stage 3 ∆H = 73.9 J/g ␮ from 2–7 m. Water-in-oil emulsions with water:CP v/v ra- -15 T = -38.0 °C tios of 1:5, 1:10, and 1:20 ͑i.e., CP rich compared to 3.3:1 ∆H =58.0J/g v/v for the stoichiometric hydrate͒ were investigated in this -20 study. Oil-in-water emulsions with water:CP v/v ratios of 5:1 -50 -30 -10 10 Temperature / °C also were studied. Clathrates are known to be poor thermal conductors, so FIG. 1. DSC thermograph of 1:5 water-in-CP emulsion stabilized with 4% special care must be taken to ensure good heat flow for re- Span 65. Cooling curves are below heating curves. Thermographs for a ͑ duplicate trial of 1:5 and of 1:10 and 1:20 were qualitatively similar ͑same producible results. We melted small samples 5–10 mg of ⌬ ͒ ⌬ ͑ onset temperatures of features, but different trsH . Values of trsH are J/ g emulsion͒ in Perkin–Elmer volatile aluminum pans. The re- ͒ of emulsion and temperatures are Tonset. producibility of our results at different scan rates ͑0.25 to 1 K min−1͒ indicates efficient heat flow. Differential scan- ⌬ ͑ ning calorimetry ͑DSC͒ was carried out using a Q200 instru- The trsH values are reported in the figures in units of J/ gof ment from TA Instruments with a helium gas atmosphere in emulsion͒ and their experimental uncertainties are Ϯ10%, the sample chamber ͑flow rate=25 mL min−1͒.
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