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Binary Ethanol-Methane Clathrate Hydrate Formation in the System CH4-C2H5OH-H2O: Phase Equilibria and Compositional Analyses Ross Anderson, Antonin Chapoy, Hooman Haghighi, and Bahman Tohidi* Centre for Gas Hydrate Research, Institute of Petroleum Engineering, Heriot-Watt UniVersity, Edinburgh, EH14 4AS, United Kingdom ReceiVed: March 10, 2009; ReVised Manuscript ReceiVed: May 5, 2009
Ethanol (EtOH) is commonly used as a gas hydrate inhibitor in hydrocarbon production operations. However, a number of stable and metastable hydrate phases have been reported for the binary EtOH-H2O system at temperatures of 0.056, aqueous ethanol forms binary EtOH-CH4 clathrate hydrates stable over a wide PT range. In the HEtOH-CH4+L+G region, this results in significantly less hydrate inhibition than would be expected from ice melting point depression. In the ice region, ethanol enclathration actually increases hydrate stability relative to the methane-water system; the HEtOH-CH4+L+G region being bounded by a univariant HEtOH-CH4+L+I+G quadruple point locus line at temperatures much higher than the typical HCH4+I+G boundary (or HCH4+I+L+G quadruple point univariant locus in the presence of an aqueous hydrate inhibitor). Compositional analyses of the clathrate phase yields the formula 2.30CH4 · 0.66EtOH · 17H2O at 246.7 K and 3.68 MPa, which is consistent with structure II. Independent powder X-ray diffraction and Raman spectroscopic studies presented in an accompanying article in this journal issue confirm ethanol-methane clathrates to be of structure II type. I. Introduction Ethanol (C2H5OH, EtOH) is commonly utilized as a gas hydrate inhibitor in hydrocarbon production operations, particularly in regions of high industrial production (e.g., South America). Like methanol (MeOH)swhich is one of the most popular hydrate inhibitorssethanol is polar and hygroscopic, showing complete mutual miscibility with water. Thus, it might be expected that it should similarly depress the activity of water and offer a comparable degree of thermodynamic hydrate inhibition. Previously, however, we have demonstrated that alcohols of suitable molecular diameter can readily form clathrate hydrates: both 1- and 2-propanol (POH) forming simple (single guest) clathrate hydrates at temperatures close to the ice point of water (e.g., -10.1 °C for 1-propanol), and binary (or greater) propanol-gaseous guest(s) (e.g., methane) hydrates at ambient temperatures and elevated pressures.1,2 Modeling studies,1,2 independent Raman spectroscopic measurements (for 2-propanol),3 and phase equilibrium data4 suggest that POH hydrates are of structure II, with propanol occupying the large hexakaidecahedral 51264 cavities, as is typical for other water-miscible organic clathrate hydrate formers such as tetrahydrofuran (THF) and 1,4-dioxane.5 While there is no evidence to suggest that methanol can form gas hydrates at conditions relevant to oil/gas operations (although it can apparently form structure II hydrates at very low temperatures),6 a number of stable and metastable ethanol hydrates have been reported for the binary EtOH-H2O system at temperatures below 223 K,7-12 raising the question as to whether ethanol, like 1- and 2-propanol, can be stabilized in * To whom correspondence should be addressed. E-mail: bahman.tohidi@ pet.hw.ac.uk, Tel: +44-131-451-3127, Fax: +44-131-451-3672.
clathrate cages at higher temperatures in the presence of smaller secondary guest gases (e.g., methane). Previously, in Anderson et al.,12 we presented phase equilibrium and thermodynamic modeling results which supported the formation of binary ethanol-methane clathrate hydrates at ambient temperatures and elevated pressures. Here, we report experimentalDTA(differentialthermalanalysis),ethanol-methane clathrate PVTX equilibrium, and hydrate compositional data for the binary ethanol-water and ternary ethanol-water-methane systems, respectively. Data clearly demonstrate the formation of binary EtOH-CH4 clathrate hydrates, with compositional analyses of the hydrate phase yielding a stoichiometry consistent with structure II. Independent powder X-ray diffraction (PXRD) and Raman spectroscopic studies reported by Yasuda et al.13sin an accompanying article within this journal issuessupport phase equilibrium results, confirming ethanol-methane clathrates to be of structure II type. II. Experimental Materials and Methods Ethanol and methanol, supplied by Sigma-Aldrich, were both 99.5 mass %+ (anhydrous) pure. Methane, supplied by BOC, was 99.995 mass % pure. Deionized water was used in all tests. Three different sets of experimental equipment were used: (1) an in-house DTA setup, (2) a high-pressure autoclave cell, and (3) a small volume static cell. For all experiments, temperature was measured using platinum resistance thermometers (PRTs, precision/accuracy (0.01 K/(0.1 K for DTA and (0.05 K/(0.1 K for PVT studies) calibrated against a Prema 3040 precision thermometer, with pressure being determined using strain gauge transducers (precision (0.0007 MPa, accuracy (0.003 MPa) calibrated regularly using a dead weight tester. A. Differential Thermal Analyses. DTA studies on the binary EtOH-H2O system were conducted using an in-house
10.1021/jp9021536 CCC: $40.75 2009 American Chemical Society Published on Web 06/18/2009
Ethanol-Methane Clathrate Hydrate Equilibria
J. Phys. Chem. C, Vol. 113, No. 28, 2009 12603 + ethanol) was added to a 0.300 mol fraction aqueous ethanol solution. Binary EtOH-CH4 hydrates were then formed under methane pressure and equilibrium achieved. Small (>0.5 mL) samples of the liquid phase were subsequently drawn off at isobaric/isothermal conditions (methane injected to maintain pressure) for analysis by standard gas chromatographic (GC) methods. PT conditions of sampling for both tests are shown in Figure 3. To determine methane content in the second analyses, the empty cell volume was first determined precisely by injecting a known mass of nitrogen at fixed temperature. A known mass of aqueous phase was then added to the cell, meaning gas volumesand thus total moles of all components in the systemscould be established. This information, combined with methanol tracer results, allowed reliable calculation of the complete hydrate stoichiometry.
Figure 1. Binary phase diagram for the system EtOH-H2O compiled using experimental data from various sources (literature and this work). Solid and dashed lines are interpolations. Compositions and dissociation conditions for the two most commonly observed clathrate hydrates are shown.
designed apparatus described previously elsewhere.2 A lowtemperature (minimum 183 K) Lauda cryostat with pure methanol as the coolant was utilized for temperature control. Sample volumes were 1 mL, with typical heating rates for measurement runs being 1-2 K min-1. B. Equilibrium PVT and Hydrate Compositional Analyses. Phase equilibrium and hydrate compositional data for ternary CH4-EtOH-H2O systems were generated using a standard high-pressure autoclaves, as described previously elsewhere.2,12 For very low temperature (below 283 K) work on systems with methane pressure, a small volume (2 mL) cylindrical cell was prepared from Sitec tubing and fittings.12 This was immersed directly in the Lauda cryostat, with a single outlet line for the pressure transducer. In the absence of mixing, to increase liquid/gas interfacial areasthus the time required to achieve equilibriumsaqueous ethanol samples were dispersed on 0.1 mm inert silica spheres. Hydrate dissociation and/or ice melting conditions for all ternary systems were determined using reliable isochoric equilibrium step-heating techniques, as described by Tohidi et al.14 Two compositional tests were undertaken: one to determine the ratio of ethanol to water in the hydrate structure and a second to establish both ethanol and methane contents. In each case, a small volume of methanol (0.005 mol fraction relative to water
III. Results and Discussion Binary EtOH-H2O System. As earlier noted, previous literature thermal, dielectric, and X-ray diffraction studies of the binary EtOH-H2O system have revealed the existence of a number of stable and metastable ethanol hydrates below ∼223 K.7-12 While there is some uncertainty concerning the exact structure and composition of hydrates, there is a general consensus that clathrate hydrates include a stable structure II type, possibly semiclathrate in nature, with a composition close to EtOH · 17H2O,7-9 and a potentially modified cubic structure I hydrate with a composition in the range EtOH · 4.757.66H2O.8,9,11 In addition, Zelenin11 reported two nonclathrate hydratessan apparently stable EtOH · 2H2O and a metastable EtOH · 3H2Osalthough these were not observed by other authors. Figure 1 shows the binary phase relations for the system EtOH-H2O at atmospheric pressure, as compiled from selected literature sources.7-12 Data have been interpolated to delineate the various phase regions, including compositions and measured dissociation conditions for the two reported clathrate hydrates. Also shown are experimental DTA generated as part of this work (given in Table 1). While we were unable to detect the peritectic transition(s) observed by others at around 198 K, temperatures of 188 K were achieved, but this was apparently insufficient subcooling to induce growth of the clathrate(s) responsibly, as can be seen, data generated for I+L liquidus curve are in good agreement with existing literature data. Furthermore, a metastable hydrate phase was observed in a number of runs for ethanol mole
TABLE 1: Experimental Liquidus (Ice Melting) and Metastable EtOH Hydrate Dissociation Data for the Binary System EtOH-H2O at Atmospheric Pressure liquidus (ice melting)
metastable hydrate
XEtOH
T (K) (0.5
XEtOH
T (K) (0.5
XEtOH
T (K) (0.5
XEtOH
T (K) (0.5
XEtOH
T (K) (0.5
XEtOH
T (K) (0.5
0.079 0.084 0.086 0.089 0.090 0.096 0.100 0.101 0.103 0.105 0.110 0.115 0.117 0.120 0.125
263.8 263.1 263.0 262.3 262.2 261.2 260.7 260.6 260.3 259.8 258.7 258.0 257.5 256.9 255.8
0.127 0.130 0.135 0.136 0.139 0.144 0.146 0.149 0.155 0.162 0.164 0.165 0.169 0.170 0.175
255.6 254.7 254.0 253.8 253.4 252.7 252.1 251.7 250.7 249.6 249.4 249.1 248.7 248.8 247.8
0.180 0.189 0.193 0.195 0.200 0.200 0.200 0.205 0.210 0.215 0.220 0.225 0.230 0.235 0.240
247.2 246.1 245.4 245.2 244.8 244.7 244.7 244.0 243.8 243.3 242.7 242.1 241.7 241.2 240.7
0.245 0.250 0.250 0.255 0.260 0.265 0.270 0.274 0.280 0.285 0.290 0.296 0.306 0.310 0.315
240.1 239.8 239.8 239.4 238.8 238.6 238.0 237.7 237.1 236.8 236.4 236.0 235.3 235.0 234.8
0.320 0.325 0.330 0.340 0.350 0.362 0.370 0.382 0.390 0.401
234.5 234.0 233.8 233.2 232.5 232.1 231.7 231.0 230.7 230.2
0.205 0.220 0.230 0.285 0.290 0.296 0.300 0.306 0.310 0.315 0.319 0.325 0.340 0.350 0.400
226.5 225.7 225.2 222.7 222.4 222.1 221.8 221.6 221.3 221.2 221.2 220.9 220.7 220.4 218.7
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Figure 2. Example DTA thermogram showing metastable hydrate dissociation (Td,h) and ice melting (Tm,i) endotherms. Ts is sample temperature, ∆Tsr is difference between sample and reference temperatures.
fractions of >0.020. As shown in Figure 2, the dissociation of this phase was characterized by a large endothermic peak well below the ice melting point, but at a notably higher temperature than the established peritectic transition. Metastability was confirmed by a lack of repeatability across different runs on the same sample; while the hydrate peak was commonly observed, it was often subdued or disappeared at the expense of the stable ice melting peak following low-temperature annealing. Boutron and Kaufmann9 also observed a metastable hydrate phase in this temperature range. The melting temperature of this hydrate was found to be higher for faster heating rates, which could be attributed to growing disequilibrium as the transition lagged increasingly further behind temperature. Furthermore, authors found that this solid disappeared at the expense of the stable ice phase if samples were left to anneal before heating, which is in agreement with our findings. While exact composition and structure of all stable and metastable hydrates formed in the binary ethanol-water system remain somewhat ambiguous, there is clear evidence thatsas
Figure 3. PTX diagram for the ternary system CH4-EtOH-H2O showing clathrate hydrate dissociation conditions as a function of aqueous mole fraction ethanol. Solid lines are interpolations. CH4-H2O data from various literature sources.15-17 Filled squares numbered 1(A-C) and 2 are conditions of sampling for hydrate-phase compositional analyses.
for propanol-water systemssclathrate hydrate formation does occur at lower temperatures. If the clathrate stable below 198 K is an s-II type as suggested by various authors,7-9 then it should similarly have empty dodecahedral (512) cages available (assuming ethanol occupies the large 51264 cavities), thus could potentially be stabilized at higher temperatures in the presence of smaller molecular diameter gas molecules such as methane. Ternary CH4-EtOH-H2O System. To assess whether ethanol could form binary clathrate hydrates at temperatures close to ambient, hydrate dissociation data were generated for a range of aqueous ethanol concentrations up to an EtOH aqueous molar fraction of 0.400 and methane pressures of 40 MPa. All data are reported in Table 2, with results for selected concentrations plotted in Figure 3. As can be seen in Figure 3, the addition of ethanol to the aqueous phase reduces hydrate stability in the H+L+G (hydrate+liquid+gas) region, as might be expected. However, as shown in Figure 4, EtOH offers significantly less hydrate inhibition than methanol for comparable aqueous molar con-
TABLE 2: Experimental Clathrate Hydrate Dissociation (H+L+G) and Univariant Quadruple Point Locus (H+L+I+G) Data for the System CH4-EtOH-H2Oa H+L+G
H+L+I+G
XEtOH
T (K) (0.5
P (MPa) (0.02
XEtOH
T (K) (0.5
P (MPa) (0.02
T (K) (0.5
P (MPa) (0.02
T (K) (0.5
P (MPa) (0.02
0.030 0.056 0.056 0.056 0.056 0.056 0.065 0.065 0.065 0.085 0.089 0.089 0.089 0.089 0.100 0.108 0.115 0.125 0.140
290.0 267.3 281.3 287.2 287.8 290.5 272.0 279.8 287.3 279.0 276.6 282.5 285.3 287.7 278.2 277.4 276.1 275.5 274.9
20.24 2.14 9.06 18.48 19.93 30.19 3.72 8.36 21.04 9.38 7.21 14.34 20.20 28.13 9.65 9.53 9.25 9.16 9.53
0.144 0.144 0.144 0.144 0.155 0.170 0.180 0.190 0.207 0.230 0.251 0.300 0.300 0.300 0.300 0.400 0.400 0.400 0.400
269.0 277.5 277.9 281.7 273.3 273.1 272.0 271.5 270.5 269.8 269.2 257.6 265.7 273.9 279.3 259.5 265.3 271.7 277.1
4.86 12.69 13.20 20.81 8.74 9.45 9.09 9.22 8.80 9.17 9.36 3.52 7.80 20.29 39.75 5.85 10.99 23.61 46.53
203.2 208.2 213.2 215.2 221.2 227.2 233.2 238.3 242.5 244.1 246.1 248.8 250.3 251.7 253.3 253.7 254.6 255.1 256.8
0.12 0.14 0.17 0.19 0.27 0.35 0.43 0.54 0.65 0.71 0.76 0.86 0.91 0.98 1.06 1.09 1.14 1.16 1.25
257.0 258.9 259.6 259.7 259.7 262.3 263.2 264.2 264.3 265.0 265.0 265.4 265.8 266.2 266.3 266.7 266.8 267.0
1.27 1.39 1.43 1.45 1.45 1.65 1.73 1.82 1.82 1.88 1.89 1.91 1.96 1.98 2.00 2.04 2.05 2.07
a
XEtOH is aqueous mole fraction ethanol.
Ethanol-Methane Clathrate Hydrate Equilibria
Figure 4. Plot of clathrate hydrate dissociation temperature depression (∆Th,d, from phase boundary for methane with distilled water) as a function of pressure for selected ethanol and methanol aqueous molar fractions in the presence of methane. Methanol data from literature sources.18,19 Solid lines are to guide the eye.
Figure 5. Plot of (methane) hydrate dissociation temperature depression (∆Td,h) versus ice melting point depression (∆Tm,i) for CH4EtOH-H2O, CH4-MeOH-H2O, and CH4-MEG-H2O systems. MeOH and MEG data are from various literature sources.18,19,21 Ethanol data are from this work (average values for ∆Td,h).
centrations (e.g., a 0.232 aqueous molar fraction of methanol offers comparable hydrate inhibition to a 0.400 molar fraction of ethanol). As water-miscible organic inhibitors and soluble electrolytes reduce hydrate stability primarily by depression of the activity of water,5,20 this behavior suggests that either ethanol has much less of an effect on the water activity than methanol or it is taking part in clathrate hydrate formation. Mohammadi and Tohidi20 previously demonstrated that, for a wide range of aqueous concentrations, the ice melting point depression (∆Tm,i) of an aqueous electrolyte and/or organic inhibitor solution can be linearly related to the degree of hydrate inhibition (∆Th,d); both being dependent on the activity of water. Figure 5 shows all experimental hydrate dissociation point data for ethanol-methane-water systems generated as part of this work plotted as ∆Tm,i versus ∆Th,d. Also shown are literature data for methanol-methane-water and monoethylene glycol (MEG)-methane-water systems. As can be seen, both MeOH and MEG data closely follow the linear ∆Tm,i ) 1.5∆Td,h relationship. However, ethanol data only follow this trend at low aqueous concentrations (low ∆Tm,i/∆Td) and deviate increasingly at higher aqueous molar fractions (higher ∆Tm,i/ ∆Td). This demonstrates that the effect of ethanol on the activity of water cannot alone be responsible for the observed low degree
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Figure 6. PTX diagram for the ternary system CH4-EtOH-H2O showing clathrate hydrate dissociation and ice melting conditions as a function of aqueous mole fraction ethanol. Solid and dashed lines are interpolations. CH4-H2O and CH4-MeOH-H2O data from various literature sources.15-17,19,22
of hydrate inhibition offered. The observed pattern does not itself prove ethanol enclathration, at least for the H+L+G region. It does, however, have important implications for phase relations at lower pressures in the ice region. As discussed by Anderson et al.,12 in the presence of an aqueous organic hydrate inhibitor (e.g., MeOH, MEG) or electrolyte (e.g., NaCl, KCl), the H+I+G (hydrate+ice+gas) univariant line for simple methane-water systems becomes the locus for inhibitor-depressed quadruple H+L+I+G points as a function of aqueous concentration (see Figure 6, data for 0.360 mol fraction MeOH), i.e., ice phase boundaries (low pressure I+L+G region) meet appropriate (in terms of aqueous molar inhibitor fraction) hydrate phase boundaries (higher pressure H+L+G region) at their common quadruple point. As both a new component (the inhibitor) and a new phase (hydrate can coexist with ice, liquid and gas, rather than only ice and gas) are added to the system, the locus line remains univariant. As discussed, hydrate dissociation data for CH4-EtOH-H2O systems do not follow the ∆Tm,i ) 1.5∆Td,h relationship typical for thermodynamic hydrate inhibitors. This means that H+L+I+G quadruple points for aqueous ethanol-methane systems cannot lie on the common (to thermodynamic inhibitors) univariant locus line, but rather, I+L+G and H+L+G boundaries must converge at lower pressures. To test this possibility, we have closely examined phase relations in the ice region for a range of aqueous ethanol concentrations. As shown in Figure 6, H+L+I+G quadruple points fall on an alternative univariant line at lower pressures/higher temperatures (greater than 10 K at 0.5 MPa) than the typical locus. This means that ethanol is actually increasing hydrate stability in this region relative to simple methane-water systems. The only logical explanation for this behavior is that ethanol is lending stability to the hydrate structure through enclathration; i.e., it is forming binary clathrate hydrates with methane. With respect to the composition of these EtOH-CH4 clathrate hydrates, two features of the HEtOH-CH4+I+L+G quadruple point locus line provide clues: (1) it apparently originates at the atmospheric binary structure II peritectic at ∼198 K, and (2) it terminates at the quadruple point for 0.056 aqueous molar fraction ethanol, i.e., maximum thermal clathrate stability is achieved at this aqueous concentration. This behavior is almost identical to that observed for binary 1- and 2-propanol-methane clathrate hydrates,1,2 0.056 molar fraction aqueous being the stoichiometric ratio for complete occupancy of large structure
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II hexakaidecahedral (51264) cavities (e.g., POH · 17H2O, THF · 17H2O). Thus, a preliminary conclusion would be that the binary EtOH-CH4 clathrates observed may represent the structure II type (EtOH · 17H2O) formed in the binary system below 198 K stabilized to ambient temperatures by the inclusion of methane into available small dodecahedral (512) cavities. Hydrate Compositional Analyses. To determine the stoichiometry (and so infrastructure) of binary EtOH-CH4 clathrates hydrates, hydrate compositional analyses were carried out on two CH4-EtOH-H2O systems using a small fraction of methanol (MeOH) as a liquid-phase tracer as detailed (MeOH being excluded from the hydrate structure). On the basis of standard mass balance calculations, samples 1A-C (conditions shown in Figure 3) yielded the formulas (excluding methane) 0.73, 0.67, and 0.80EtOH · 17H2O, respectively, confirming ethanol enclathration. These formulas are based on the assumption of structure II; complete filling of the large s-II 51264 cavities by the same species of guest (G) molecule yielding the stoichiometric formula G · 17H2O as noted. Thus, if ethanol clathrates are of s-II type, then EtOH large cavity fractional occupancy is 0.67-0.80 for the conditions tested, implying that methane must also occupy some of the large 51264 cavities in addition to small 512 cages to ensure thermodynamic stability. Following confirmation of ethanol enclathration, a further analysis was undertaken to determine both EtOH and methane content of clathrates. Using precise volumetric/gas consumption (pressure drop) data, liquid densities from the HWHYD thermodynamic model,2,12,23 and assuming an empty s-II hydrate lattice density of 0.786 gcm-3,5 moles of methane in the hydrate were calculated in addition to the ethanol content (from liquidphase GC analysis). This yielded the formula 2.30CH4 · 0.66EtOH · 17H2O for conditions tested (246.7 K, 3.680 MPa), which is consistent with complete filling of remaining large and almost all available small (512) cavities by methane, assuming structure II. If structure I is assumed (empty hydrate lattice density of 0.796 gcm-3),5 this yields the formula 6.3CH4 · 1.8EtOH · 46H2O. This would equate to full occupancy of small cavities and 0.7 fractional occupancy of large 51262 cavities by methane, with the remaining 0.3 fraction of large cavities occupied by ethanol. However, this gives an excess of methane and would require complete small cavity occupation, which would be unusual for structure I.5 Thus, compositional analyses favor an s-II type as the most likely structure for binary ethanol-methane clathrate hydrates. Subsequent to our initial report of binary CH4-EtOH hydrate formation,12 Yasuda et al.13 (this journal issue) have analyzed the structure and composition of clathrates formed in the CH4-EtOH-H2O system using PXRD and Raman spectroscopic techniques. Results confirm EtOH-CH4 hydrates to be of structure II type, in agreement with phase equilibrium data reported here and previous modeling studies.12 Authors analyzed the composition of a CH4-EtOH hydrate formed at 256.7 K/1.4 MPa at 100 K and atmospheric pressure, calculating the formula 2.17CH4 · 0.33EtOH · 17H2O, which is in good agreement with our findings that methane occupies a significant proportion of large 51264 cavities (0.67 fractional occupancy in this case) in addition to small 512 cages (0.75 fractional occupancy). The lower total cage filling for both ethanol and methane could be attributed to the actual analyses being carried out at PT conditions very different from formation/equilibrium conditions, i.e., the clathrate may have partially decomposed or reequilibrated.
Anderson et al.
Figure 7. Plot of calculated (from compositional analyses) fractional ethanol s-II large cage (51264) occupancy versus temperature depression (∆Th,d) from the H+L+G phase boundary of maximum binary clathrate thermal stability assuming complete occupancy at that condition (stoichiometric ethanol to water ratio, XEtOH aqueous ) 0.056, XCH4 · EtOH · 17H2O).
While we have not examined cage occupancy as a function of PTX conditions in great detail, compositional analyses were carried out over a small temperature range (246.7 to 264.5 K). If, like other water miscible polar liquid hydrate formers (e.g., THF, 1- and 2-propanol), there is complete large 51264 occupancy by ethanol at the stoichiometric composition of maximum thermal stability (0.056 aqueous mole fraction EtOH), then compositional analyses suggest that ethanol fractional filling of large cavities does vary as a function of temperature (or temperature depression, ∆T, from that of maximum thermal stability at a given pressure), as illustrated in Figure 7. However, this would require a more in-depth analysis for confirmation. IV. Conclusions Experimental phase equilibrium and compositional data provide conclusive evidence for the formation of binary ethanol-methane clathrate hydrates at ambient temperatures and elevated pressures. Phase behavior is typified by the formation of EtOH-CH4 hydrates at aqueous molar ethanol fractions of >0.056, which are stable over a wide PT range. At higher pressures, in the HEtOH-CH4+L+G region, ethanol enclathration results in significantly less hydrate inhibition than would be expected when compared to other water-miscible organic hydrates inhibitors (e.g., methanol, monoethylene glycol). In the ice region, ethanol enclathration actually increases hydrate stability relative to the methane-water system; the HEtOH-CH4+L+G region extending to pressures lower/temperatures higher than the normal HCH4+I+G boundary (or HCH4+I+L+G quadruple point univariant locus in the presence of an aqueous hydrate inhibitor), where it is delimited by an alternate univariant HEtOH-CH4+L+I+G quadruple point locus line apparently extending to the binary EtOH-H2O atmospheric pressure s-II clathrate peritectic. Experimental phase behavior and compositional analyses suggest that EtOH-CH4 clathrates are most likely of the structure II type, with varying large 51264 cage occupancy by both methane and ethanol and small 512 cavities being occupied by solely by methane. Recent PXRD and Raman spectroscopic analyses reported by Yasuda et al.13 in this journal issue support these findings, confirming binary EtOH-CH4 clathrates to be of structure II type. Acknowledgment. This work was funded by Clariant Oil Services, Petrobras, StatoilHydro, TOTAL, and the UK Depart-
Ethanol-Methane Clathrate Hydrate Equilibria ment for Energy and Climate Change (DECC), whose support is gratefully acknowledged. The authors thank Colin Flockhart, Thomas McGravie, and Jim Allison for manufacture and maintenance of experimental equipment. References and Notes (1) Østergaard, K. K.; Tohidi, B.; Anderson, R.; Todd, A. C.; Danesh, A. Ind. Eng. Chem. Res. 2002, 41, 2064. (2) Chapoy, A.; Anderson, R.; Haghighi, H.; Edwards, T.; Tohidi, B. Ind. Eng. Chem. Res. 2008, 47, 1689. (3) Ohmura, R.; Takeya, S.; Uchida, T.; Ebinuma, T. Ind. Eng. Chem. Res. 2004, 43, 4964. (4) Maekawa, T. Fluid Phase Equilib. 2008, 267, 1. (5) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2008. (6) Blake, D.; Allamandola, L.; Sandford, S.; Hudgins, D. Science 1991, 254, 548. (7) Potts, A. D.; Davidson, D. W. J. Phys. Chem. 1965, 69, 996. (8) Calvert, D.; Srivastava, P. Acta Crystallogr. A 1969, 25, S131. (9) Boutron, P.; Kaufmann, A. J. Chem. Phys. 1978, 68, 5032.
J. Phys. Chem. C, Vol. 113, No. 28, 2009 12607 (10) Ott, J. B.; Goates, J. R.; Waite, B. A. J. Chem. Thermodyn. 1979, 11, 739. (11) Zelenin, Y. M. J. Struct. Chem. 2003, 44, 130. (12) Anderson, R.; Chapoy, A.; Tanchawanich, J.; Haghighi, H.; LachwaLanga, J.; Tohidi, B. Proc. 6th Int. Conf. Gas Hydrates 2008, 5502. (13) Yasuda, K.; Takeya, S.; Sakashita, M.; Yamawaki, H.; Ohmura, R. J. Phys. Chem. C 2009, XX, XXXX. (14) Tohidi, B.; Burgass, R. W.; Østergaard, K. K.; Todd, A. C. Ann. N.Y. Acad. Sci. 2000, 912, 924. (15) Deaton, W. M.; Frost, E. M. Oil Gas. J. 1946, 45, 170. (16) McLeod, H. O.; Campbell, J. M. J. Petrol. Technol. 1961, 13, 590. (17) Jhaveri, J.; Robinson, D. B. Can. J. Chem. Eng. 1965, 43, 75. (18) Robinson, D. B.; Ng, H. J. J. Can. Petrol. Technol. 1986, 25, 26. (19) Ng, H. J.; Chen, C. J.; Saeterstad, T. Fluid Phase Equilib. 1987, 36, 99. (20) Mohammadi, A. H.; Tohidi, B. Can. J. Chem. Eng. 2005, 83, 951. (21) CRC Handbook of Chemistry and Physics; CRC Press: Boca Ranton, FL, 1989. (22) Makogon, T. Y.; Sloan, E. D. J. Chem. Eng. Data 1994, 39, 351. (23) Heriot-Watt Hydrate V2.1 CPA; Heriot-Watt University: Edinburgh, U.K., 2008.
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