Clathrate Hydrate Equilibrium in Methane–Water Systems with the

Article pubs.acs.org/jced

Clathrate Hydrate Equilibrium in Methane−Water Systems with the Addition of Monosaccharide and Sugar Alcohol Yusuke Jin,* Masato Kida, Yoshihiro Konno, and Jiro Nagao Methane Hydrate Production Technology Research Group, Research Institute of Energy Frontier, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan ABSTRACT: Phase equilibrium pressure−temperature (pT) conditions in methane clathrate hydrate (CH4 hydrate) systems with miscible additives (monosaccharide and sugar alcohol) were measured by an isochoric method using xylose (monosaccharide) and xylitol (sugar alcohol) as additive molecules. The equilibrium pT data were collected in the pressure range of 3−7 MPa and an additive mole fraction range in aqueous solution, xadditive, of 0.006−0.03. The equilibrium pT conditions in both systems existed in higher-pressure regions than that for a additivefree CH4−water system. Crystals formed in both the systems were identified as structure I CH4 hydrates by powder X-ray diffraction and Raman spectroscopy measurements. Considering the phase boundary and crystallography results, xylose and xylitol acted as thermodynamic inhibitors for clathrate hydrate systems.

1. INTRODUCTION Gas clathrate hydrates (gas hydrates) are crystalline compounds that comprise H2O and gas molecules,1,2 in which guest molecules are trapped in cages formed by the hydrogen-bonded H2O framework. In general, gas hydrates can be formed in guest−water systems under appropriate pressure−temperature (pT) conditions. The phase equilibrium pT conditions vary according to the guest molecules.2 The addition of certain chemicals to a hydrate system can shift the phase equilibrium pT conditions from those of an additive-free hydrate system to higher-pressure and lower-temperature regions. These chemicals are called thermodynamic hydrate inhibitors (THIs). Therefore, THI chemicals are used to prevent hydrate formation in oil/gas pipelines. Methane (CH4) hydrates, with guest CH4 molecules, are present in nature in permafrost and oceanic sediments.2 Therefore, natural CH4 hydrates are attractive as newly unconventional energy resources. The Research Consortium for Methane Hydrate Resources in Japan (MH21),3 which initiated Japan’s Methane Hydrate R&D Program (managed by the Ministry of Economy, Trade, and Industry), is developing methods for methane production from natural hydrate reservoirs to establish commercial gas production. Depressurization, in which hydrostatic pressure in the sediment layer is decreased to a value lower than the equilibrium pressure conditions for CH4 hydrates, is considered to be an efficient gas production method. Gas production at an offshore field in the eastern Nankai area (Japan) was performed by the depressurization method.4 However, ice formation5 and hydrate reformation6 in pore spaces around gas production wells could be frequently observed with decreasing temperatures during depressurization. In particular, hydrate reformation not only causes a decrease in gas/water permeability in sediment layers but also a trap of CH4 molecules produced © XXXX American Chemical Society

from methane hydrates. Because hydrate reformation is considered to occur around gas production wells, THI injection into production wells is being developed to resolve this problem, with methanol and sodium chloride (NaCl) being common THIs for hydrate systems. These additive chemicals can shift the equilibrium conditions to a temperature approximately 4 K lower than that of the additive-free system at 4 MPa with 10 wt % additive concentration. The mechanism of inhibition is different for methanol and NaCl, but these two THIs exhibit similar temperature shift values.7,8 In this study, we focus on two nonionic molecules with OH groups, similar to methanol, and assessed the degree of inhibition through the measurement of phase equilibrium pT conditions in CH4− additive−water systems. Xylose (monosaccharide) and xylitol (sugar alcohol) were used as nonionic additives with lowly toxic nature for the environment. Molecular structures of xylose and xylitol are shown in Figure 1. Furthermore, using powder X-ray

Figure 1. Molecular structures of hydrate inhibitors used in this study: (a) xylose as a monosaccharide and (b) xylitol as a sugar alcohol. Green, red, and yellow indicates carbon, oxygen, and hydrogen atoms, respectively. Molecular structures were energy optimized using MOPAC 6. Received: August 25, 2016 Accepted: October 27, 2016

A

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ments). Details of our spectrometer have been described previously in the report.10

diffraction (PXRD) and Raman spectroscopy measurements, we investigated whether the hydrate samples were crystallographically changed by the additives.

3. RESULTS AND DISCUSSION Phase Equilibrium pT Conditions. Figure 2 shows phase equilibrium pT conditions in the CH4−xylose−water systems

2. EXPERIMENTAL SECTION Materials. Xylose with >98% purity (Tokyo Chemical Industry Co., Ltd.), xylitol with >99% purity (Aldrich-Sigma Co., Inc.), and CH4 gas with >99.99% purity (Sumitomo Seika Chemicals Co.) were used. Water was purified by ultrafiltration, reverse osmosis, deionization, and distillation (>18 MΩ·cm). All of these materials were used without further purification. Equilibrium Pressure−Temperature Measurement. We measured the phase equilibrium pT data for the CH4− additive (xylose or xylitol)−water systems by an isochoric method using a pressure vessel (TVS-N2, Taiatsu Techno Co.). The pressure vessel used in this study was equipped with a stirring fin. The uncertainty of the sample temperature (T) was estimated to be ±0.02 K with a confidence level of approximately 95%. The uncertainty of the pressure (p) measurements was ±0.025 MPa with a confidence level of approximately 95%. The experimental setups are provided in detail in a previous report.9 Xylose or xylitol solutions of a certain concentration were poured into the pressure vessel. Before the phase equilibrium pT measurement, air removal by a vacuum pump and gas pressurization by CH4 at 1 MPa were performed thrice at 293 K. Then, the vessel was pressurized with CH4 to a designated pressure condition and refrigerated to initiate hydrate formation. After hydrate crystal formation was confirmed by a distinct pressure decrease and temperature increase, the system temperature was increased in 0.1−0.5 K steps. We increased the system temperature in increments of 0.5 K in the region with lower pressure than the initial pressure. When the pressure increase due to hydrate dissociation during temperature ramping was approximately equal to the initial pressure, the system temperature was increased in increments of 0.1 K. The sample temperature was maintained for 12−24 h at each step. When the hydrate crystals in the system were completely dissociated, the increase in the system pressure becomes a gentle one. The phase equilibrium pT conditions were determined from the slope of the increase in pressure. The reliability of our experimental setups and procedure was demonstrated by measuring phase equilibrium pT conditions of CH4−water systems, as shown in Figure S1 in the Supporting Information of our previous report.9 Crystallographic Analysis. To confirm hydrate formation in the CH4−additive (xylose or xylitol)−water systems, the PXRD patterns of crystal samples obtained were collected using an X-ray diffractometer (RINT-2500; Rigaku Co.) with CuKα radiation. The X-ray source voltage and current were 40 kV and 249 mA, respectively, during the PXRD measurement. The powdered samples were introduced into a quartz glass capillary cell (2.0 mm diameter, 0.01 mm thickness, and 10 mm length), and the sample temperature was maintained approximately below 153 K by blowing with cooled, dry nitrogen gas. The PXRD profiles were acquired in 0.02° steps with a counting time of 1.2 s/step and 100 data acquisitions. In addition, Raman spectra of the samples were collected using a Raman spectrometer (LabRAM HR-800, Horiba Ltd., Japan). The Raman shifts of the samples were calibrated using Si emission lines (520.6 cm−1). To avoid sample dissociation, spectral measurements were performed at approximately 84 K using a cooling stage (HFS600E-P, Linkam Scientific Instru-

Figure 2. L−H−V three-phase equilibrium pressure−temperature conditions in the CH4−xylose−water system. Blue: xxylose = 0.00627; red: xxylose = 0.01316; green: xxylose = 0.02913; “+” symbol: CH4−water system.11−15

(molar fraction of xylose in aqueous solution, xxylose = 0.00627, 0.01316, and 0.02913). The pT data are listed in Table 1. As Table 1. L−H−V Three-Phase Equilibrium Pressure− Temperature Conditions in the CH4−Xylose−Water System xxylose = 0.00627

xxylose = 0.01316

a

a

b

b

xxylose = 0.02913

T /K

p /MPa

T /K

p /MPa

Ta/K

pb/MPa

274.45 276.67 279.03 280.64 282.09

3.135 3.920 4.938 5.772 6.849

273.51 276.43 278.77 280.23 281.44

3.024 4.012 5.074 5.889 6.729

274.95 277.35 279.04 280.33

3.944 5.003 5.955 6.826

a

Uncertainties in temperature measurements were estimated to be ±0.02 K. bUncertainties in the pressure measurements were ±0.025 MPa.

shown in Figure 2, the phase equilibrium pT conditions with xylose shift to higher-pressure and lower-temperature regions than those with the CH4−water system. The shift of the equilibrium pT curve increases with xxylose. At 4 MPa, the equilibrium T (xxylose = 0:277.4 K) changes to approximately 276.9, 276.3, and 275.1 K when xxylose = 0.00627, 0.01316, and 0.02913, respectively. Figure 3 shows phase equilibrium pT conditions in the CH4−xylitol−water systems (molar fraction of xylitol in aqueous solution, xxylitol = 0.00298, 0.01298, and 0.02935). The pT data are summarized in Table 2. Similar to the CH4− xylose−water system, the phase equilibrium pT conditions in the CH4−xylitol−water systems shift to higher-pressure and lower-temperature regions than those of the CH4−water B

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Figure 4. Powder X-ray diffraction profiles of samples obtained from (a) CH4−water,15 (b) CH4−xylose−water (xxylose = 0.02913), and (c) CH4−xylitol−water (xxylitol = 0.02935) systems. Circles: observed data. Dashed line: background. Red line: sI hydrate. Blue line: ice Ih. All PXRD profiles were collected at 153 K.

Figure 3. L−H−V three phase equilibrium pressure−temperature conditions in the CH4−xylitol−water system. Blue: xxylitol = 0.00298; red: xxylitol = 0.01298; green: xxylitol = 0.02935; “+” symbol: CH4−water system.11−15

0.02913) and CH4−xylitol−water (xxylitol = 0.02935) systems. The measured PXRD samples from these systems were formed in the pressure vessel used for phase equilibrium measurement. After sample crystals were formed in the higher p region of the phase equilibrium pT conditions, the samples in the vessel were immediately quenched by liquid nitrogen. In Figure 4b and c, seven diffraction peaks (red line) can be observed in the 2θ range of 25−35°. The PXRD profile in Figure 4a indicate a structure I (sI) CH4 hydrate. It was observed that both the CH4−xylose−water and the CH4− xylitol−water systems form an sI hydrate. The lattice parameters a for the CH4−xylose−water (xxylose = 0.02913) and CH4−xylitol−water (xxylitol = 0.02935) systems are calculated as 1.1879(10) and 1.1897(4) nm, respectively, by the PXRD analysis software PDXL (Rigaku Corp.). In contrast, a of the additive-free CH4 hydrate is 1.1875(10) nm. Thus, the additives (xylose and xylitol) do not change the lattice parameters. The lattice parameters are summarized in Table 4. Because both of measurement samples show the PRXD

Table 2. L−H−V Three-Phase Equilibrium Pressure− Temperature Conditions in the CH4−Xylitol−Water System xxylitol = 0.00298

xxylitol = 0.01298

a

a

b

xxylitol = 0.02935

T /K

b

p /MPa

T /K

p /MPa

Ta/K

pb/MPa

274.44 276.72 279.25 280.86 282.51

3.077 3.865 4.972 5.836 6.909

273.56 276.52 278.75 279.91 281.88

3.021 4.061 5.073 5.744 7.093

275.14 277.38 278.88 280.27

4.011 5.005 5.882 6.821

a

Uncertainties in temperature measurements were estimated to be ±0.02 K. bUncertainties in the pressure measurements were ±0.025 MPa.

system. At 4 MPa, the equilibrium T (xxylitol = 0:277.4 K) changes to approximately 277.1, 276.4, and 275.1 K with xxylitol = 0.00298, 0.01298, and 0.02935, respectively. As shown in Figures 2 and 3, the addition of xylose and xylitol shifts the phase equilibrium conditions to higher p or lower T than those of the CH4−water system (i.e., the additivefree system).11−15 Crystal Characterization. Table 3 lists the molecular properties of xylose and xylitol. The geometries of xylose and

Table 4. Lattice Parameters and Raman Peak Results of Samples Obtained in the CH4-Inhibitor (Xylose or Xylitol)− Water Systems xylose (xxylose = 0.02913) lattice parameter, a/nm L Raman peak (cm−1) S θL/θS

Table 3. Molecular Properties of Xylose and Xylitol −1

molecular weight (g mol ) volumea (nm3) longest lengtha (nm) a

xylose (monosaccharide)

xylitol (sorbitol)

150.13 0.1272 0.85

152.15 0.1369 1.04

Parameter was estimated by Winmostar.

17

1.1879(10)

xylitol (xxylitol = 0.02935) 1.1897(4)

pure CH4 hydrate 1.1875(10)

2901.34

2901.47

2901.27

2913.53 1.19

2913.59 1.14

2913.30 1.26

profile of sI hydrate structure, xylose and xylitol molecules can act as THIs in the sI CH4 hydrate system. The equilibrium conditions determined in this study are xylose/xylitol solution− CH4 hydrate−methane-rich vapor (i.e., L−H−V) three-phase equilibrium pT conditions. Figure 5 shows the Raman spectra of the C−H stretching vibration mode of the crystal samples obtained from the CH4−

xylitol are estimated to be 0.85 and 1.04 nm in length and 0.127 and 0.137 nm3 in molecular volume, respectively, using the structural energy minimization program Winmostar.16,17 Considering these geometries, xylose may form an sH-hydrate structure as an sH-hydrate promoter (large-molecule guest substance).15,18 Figure 4 shows typical PXRD profiles of samples obtained from the CH4−xylose−water (xxylose = C

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Figure 5. Raman spectra of the C−H stretching vibration of CH4 molecules in sI clathrate cages. (a) CH4−xylose−water (xxylose = 0.02913), (b) CH4−xylitol−water (xxylitol = 0.02935), and (c) CH4− water system. Red and blue lines are the C−H stretching vibration in 51262 and 512 cages, respectively. All spectra were measured at 84 K.

Figure 6. Correlation between molar fraction of inhibitor and equilibrium temperature shift at 4 MPa. Blue circles: xylose. Red circles: xylitol. Black opened circles: methanol.26−28 Black opened squares: ethanol.2,28,29 Red opened squares: ethylene glycol.28,30 Blue opened squares: glucose.31 Green opened circles: urea.32 Red opened diamonds: DE acetate.33 All plot data were predicted using the equation fitted by each equilibrium pT data point.

xylose−water (xxylose = 0.02913) and the CH4−xylitol−water (xxylitol = 0.02935) systems. The C−H stretching vibration mode of CH4 molecules encaged in the clathrate spaces can be distinguished.15,19−21 In Figure 5, the Raman peaks at approximately 2901 and 2913 cm−1 are identified as the C− H stretching vibration of CH4 molecules in L (51262) and S (512) cages, respectively. In addition to the PXRD result, crystal samples formed in the systems can be identified as sI clathrate hydrate structures by the peak positions of the C−H stretching vibration. As listed in Table 4, no peak position shift in each C−H stretching vibration mode is observed compared with the spectrum of the additive-free CH4 hydrate. The cage occupancy ratio θL/θS can be obtained from the peak intensity I of the C− H stretching vibration mode of the L- and S-cages. Considering the cage numbers of the L- and S-cages, θL/θS can be estimated from IL/3IS. The estimated θL/θS of the CH4−xylose−water (xxylose = 0.02913) and the CH4−xylitol−water (xxylitol = 0.02935) systems are 1.19 and 1.14, respectively. These values are approximately equal to those of artificially synthesized CH4 hydrates (θL/θS = 1.1−1.3).20,22−25 Considering the lattice parameters obtained from the PXRD results, inhibitors used in this study do not significantly affect the sI CH4 hydrate crystal structure. As shown in Figures 2 and 3, xylose and xylitol additions shift the stable CH4 hydrate pT conditions to higher-pressure and lower-temperature regions with increasing additive concentrations. We compared the magnitudes of inhibition upon adding xylose and xylitol with those of other THI molecules. Figure 6 shows the temperature shift ΔT from the equilibrium of the additive-free system upon the addition of THIs at 4 MPa against THI concentration (molar fraction) in the aqueous phase. The compared THIs are nonionic molecules with an OH group (methanol, ethanol, and ethylene glycol).2,26−30 Additionally, we used glucose with OH and ether groups, urea with carbonyl group, and diethylene glycol monoethyl ether acetate (DE acetate) with an ester group for comparison.31−33 Here, DE acetate is announced to use for second gas production test at an offshore Nankai field by MH21.34 Every data point was predicted using the equation fitted by each pT data. All THIs show an increase in ΔT with increasing THI molar fraction. Figure 6 shows that there is no molar fraction dependence of ΔT on THI species, except for urea with only the carbonyl group. OH groups in molecules can affect hydrogen-bonding

networks in an aqueous solution. Considering the measurement uncertainties in the experiments, there is apparently slight dependence of ΔT on the number of OH groups in the molecules. In the nonionic THIs having OH, ether, or esther groups, the magnitudes of inhibition can be explained by THI molar fractions and have little relation to molecular species, as shown in Figure 6.

4. CONCLUSIONS We measured phase equilibrium pT conditions of CH4− xylose−water and CH4−xylitol−water systems by an isochoric method. The observed phase boundaries of both systems were shifted to a higher-pressure region than that observed for an additive-free CH4−water system. The magnitude of the higherpressure shift increased with the mole fractions of xylose and xylitol in the aqueous solutions. The CH4−xylose−water and CH4−xylitol−water systems possibly form structure-H hydrate crystals, considering the shape and volume of xylose and xylitol. In the PXRD profiles and Raman spectra, crystals formed in both the systems were identified as structure-I CH4 hydrates. Considering the phase boundary and crystallography results, xylose and xylitol act as thermodynamic inhibitors for clathrate hydrate systems.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Address: 2-17-2-1, Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan. Telephone number: +81-11-857-8526. Fax number: +81-11-857-8417. Funding

This study was funded by the Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium) planned by the Ministry of Economy, Trade and Industry (METI), Japan. Notes

The authors declare no competing financial interest. D

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ACKNOWLEDGMENTS The authors thank Dr. S. Kimura, Dr. M. Oshima, and Dr. A. Kato of AIST for their valuable input.

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