Methane hydrate stability in the presence of water-soluble hydroxyalkyl cellulose

Available online at www.sciencedirect.comJOURNWLOF: ScienceDirectNATURAL GASd CHEMISTRYELSEVIERJoumal of Natural Gas Chemistry 21(2012)119-125www. elsevier.com/locate jngcMethane hydrate stability in the presence ofwater- soluble hydroxyalkyl celluloseM. Mohammad-Taheri'，A. Zarringhalam Moghaddaml*，K. Nazari?，N. Gholipour Zanjanil1. Chemical Engineering Department, Tarbiat Modares University, 14115-143. Tehran, Iran;2. Chemistry and Petrochemical Division, Research Institute of Petroleum Industry, 14665.137, Tehran lran[Manuscript received July 12. 2011; revised October 17, 2011]Abstractpolymer group on methane hydrate stability was investigated by monitoring hydrate dissociation at pressures greater than atmospheric pressurein a closed vessel. In particular, the influence of molecular weight and mass concentration of hydroxyethyl cellulose (HEC) was studiedwith respect to hydrate formation and dissociation. Methane hydrate formation was performed at 2°C and at a pressure greater than 100 bar.Afterwards, hydrate dissociation was initiated by step heating from - 10°C at a mild pressure of 13 bar to - 3 °C, 0。C and 2 °C. With respectto the results obtained for methane hydrate formation/dissociation and the amount of gas uptake, we concluded that HEC 90.000 at 5000 ppmis suitable for long-term gas storage and transportation under a mild pressure of 13 bar and at temperatures below the freczing point.Key wordshydrate stabilit; dissociation kinetic; low-dose polymer; gas storage1. Introductionreported an enhancement of hydrate formation in the presenceof polymers that contain OH bonds in their chemical struc-Solid-phase, crystalline gas hydrates encage gastures [6]. However, a low dosage of polyvinyl pyrrolidonemolecules in a hydrogen-bonded framework of water(PVP) also showed the promotion effect on hydrate formationmolecules at high pressures and low temperatures above the[7]. Sodium dodecyl sulfate (SDS) has also been shown tofreezing point [1]. The application of hydrate technology inremarkably improve the methane hydrate formation rate andthe field of gas storage and transportation has become fa-the storage capacity, and several researchers have stated thatvorable, because extensive research has been performned onSDS is the best methane hydrate promoter [8- 10].the enhancement of hydrate formation and gas storage ca-In addition to the potential of promoters with respect topacity, especially through the use of chemical additives ingas hydrate formation, their effects on gas hydrate dissocia-low doses [2-6]. The effect of chemical additives on thetion are important for the economics of hydrate technology inenhancement of gas hydrate formation was first observedgas transportation. Lin et al. [11] studied methane hydratefor some surfactants in 1993 [2]. The promotion effect ofdissociation in the presence of SDS (650 ppm) at atmosphericsurfactants was investigated with respect to natural gas hy-pressure and at temperatures below the freezing point (in thedrate formation as functions of the surfactant type (as anionic,range of 264 -270 K). They reported that hydrate dissocia-cationic and nonionic) and initial surfactant concentrationtion rates increased in the presence of SDS relative to that in(range of 0.005-1 wt%) by Karaaslan and Parlaktuna [3].pure water. This increased dissociation rate was attributed toThey observed that the presence of surfactants changed thethe effect of SDS on the metastable characteristic of hydratesmorphology of gas hydrates from a dense film to a porousknown as“self-preservation" below the freezing point.structure, irrespective of the initial concentration and hydrateIn the early 1990s, researchers used "frozen hydrates"species, which resulted in an improvement in the hydrate for-to demonstrate the possibility of storing hydrates below theirmation rate and an increase in the amount of water convertedfreezing point at atmospheric pressure [12]. The dominantto hydrate [4,5]. In adition, Karaaslan and Parlaktuna alsophenomenon 1中国煤化工pciation rate below. Coresponding author. Tel: +2-1-8283337; Fax: +2-2-8288-3381; E-mail: zrrin@modYHCNMHGThis work was spprted by the Resarch Insiute of Peroleum ICopyrightO2012, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.doi:10.1016/S1003-9953(1 1)60343-5120M. Mohanmad-Taheri et al./ Joumal of Natural Gas Chemistry VoL 21 No.22012the freezing point was based on the effect of a thickening icewith the molecular weights of 90,000 and 250,000 were rep-layer that prevented methane dispersion from the hydrate sur-resented as HEC9 and HEC25, respectively.face to the gas phase [12- 17].After observing a minimum amount of dissociation atCH2OHpressures between 1 and 4 bar and at low temperatures withconstant volume, Lin et al. concluded that a temperature de-crease (i.e., to less than 266 K) was not always an efficientK OHmethod to increase the stability of methane hydrate for stor-age purposes [17]. Hence, they suggested that the storage of1 OHhydrates at higher pressures (approximately 4 bar instead ofFigure 1. Molecular structure of HBCatmospheric pressure) resulted in a greater stability of the hy-drate at reasonable temperatures (approximately ~266 K).Moreover, Ganji et al. [18] studied the stability o2.2. Apparatusmethane hydrate in the presence of different types of surfac-tants in a closed system by a step heating method at tem-Figure 2 shows the schematic of the experimental appa-peratures below the freezing point of a system initially at at-ratus. A high-pressure vessel (Parr Instruments, HC-276) wasmospheric pressure. As the temperature was increased fromused for methane hydrate investigations. The volume of this-5°C to -3 °C, a higher initial pressure at -3。C comparedcylindrical vessel was 450 cm'. It was equipped with a mag-with that at - -5。C prevented hydrate dissociation consider-netically driven stirer. A programmable thermostatic bathably. Hence, initial pressure is as important as temperature(Lauda RP 855 C) was used to control the temperature withfor hydrate dissociation in closed system. The authors postu-an accuracy of士0.01 °C.lated that, compared with other surfactants, SDS provided aThe pressure and temperature of the vessel were measuredhigher storage capacity for methane hydrate and a satisfactoryby a pressure transducer (Indumart) with a precision of +0.1%stability at 268.2 K at atmospheric pressure.In another work, Ganji et al. [19] studied the effectand a PT- 100 platinum resistance thermometer with a preci-of water-soluble biological polymers with high molecularsion of土0.1 K, respectively. Experimental data (vessel tem-perature, bath temperature and vessel pressure) were recordedweights on the stability of methane hydrate in the presence ofon a PC during the experiments.SDS. They showed that although these additives decreased thepromotion effect of methane hydrate formation rate to someextent when mixed with SDS, they increased the stability ofmethane hydrate during hydrate dissociation.回rAs mentioned above, there are few reports on hydratedissociation in the presence of chemical additives, especiallyhigh-molecular-weight polymers. The main purpose of the母present work is to investigate the effect of hydroxyethyl cel-1|lulose polymer (HEC) on the isochoric formation and disso-ciation of methane bydrate at pressures slightly above atmo-spheric pressure. The influence of molecular weight and massconcentration of HEC on methane hydrate formation and dis-sociation was studied and compared with the results for purewater. Hydrate dissociation was monitored below and abovethe freezing point during step heating from -10°C, -3°C,0 °C and 2 °C, respectively.Figure 2. Scherme of the high-pressure setup. 1- Gas cylinder, 2-Regulator pressure, 3- -High-pressure vessel, 4- -Thermostatic bath,Magnetic drive, 6- -Motor drive, 7- -Pressure transducer, 8- -Thermocouple,2. Experimental9- Safety valve, 10- Sampling valve, 11- -Data logger, 12- -PC2.1. Materials2.3. MethodsMethane gas with the purity of 99.99% was supplied byRoham Gas Company. Hydroxyethyl cellulose (HEC) wasFirst, the vessel was loaded with 50 cm3 water. The vesselpurchased from Aldrich in different molecular weights of ap-was sealed, and the air in the vessel was exhausted by pressur-proximately Mw~ 90,000 and 250,000 and with purity greaterizing and pur中国煤化Ie gas. The initialthan 99%. The molecular structure is shown in Figure 1pressure was tY片CN MH Gy 125 bar at40°CDeionized distilled water was used in all experiments. Aque-The stirring spccuwassctlu 2UU rpu urnlg the experiments.ous solutions of HEC were prepared and homogenized in massThe temperature program used during all experiments is ilus-concentrations of 1000 and 5000 ppm. In this work, HECtrated schematically in Figure 3.Jourmal of Natural Gas Chemistry Vol. 21 No. 22012121resent the pressure driving-force changes during dissociation7-40Cand was calculated according to Equation 3.Hydrate dissociation ratio =Pexp(t)- P3)Pequi-Pwhere, Pexp(t) and P represent the experimental and initialpressures of the vessel at each temperature step and Pequi isthe equilibrium pressure of methane hydrate calculated usingthe HWHYD software [20] (more details are provided in sec-7=2 C7∞c r I-2Ction 3.2).7=3CT-10C3. Results and discussion51001520Time ()In the present work, methane hydrate was formed at highFigure 3. Temperature program of thermostatic bath during bydrate forma-pressures, whereas the dissociation was monitored during hy-tion and dissociationdrate storage at mild pressures and at temperatures below andHydrate formation was initiated at a constant tempera-above the freezing point (near the phase boundary of Lw-H-Vor I-H-V). As evident in the results for pure water in Figure 4,ture of 2°C by decreasing the bath temperature at the ratewhen hydrate growth ended at 2 °C, the temperature was de-of 4 °C/min from 40°C to2°C. When the rate of the pres-creased to - 10°C, and the system was allowed to reach asure decrease related to hydrate formation became negligiblesteady state at that temperature. The pressure was then gently(less than 0.1 bar/h) at a constant temperature of 2 °C, the bathdecreased to 13 bar by gas purging out of the hydrate stabil-temperature was decreased to - 10°C.ity zone, and hydrate dissociation was monitored using a stepTo investigate methane hydrate stability at mild pressures,heating process in a closed system at temperatures below andthe system was purged at a rate of 4 bar/min. The valve ofabove the freezing point.the vessel was closed when the system pressure decreasedto approximately 13 bar, whereas the equilibrium pressure at .1410 °C was calculated as 19.8 bar for methane hydrate usingthe HWHYD (Heriot-Watt Hydrate) software program [20].120SuboolingHydrate stability was evaluated by monitoring the pressure100[ Hydrate frmaionincrease relevant to hydrate dissociation at constant tempera-包ture for approximately 40 h. Afterwards, hydrate dissociationg 80was continued by step heating at temperatures of- 3 °C, 0°Cand 2 °C, respectively. The experiments were repeated for theHEC solutions.40。The moles of methane encaged in hydrate (nexp) were es-timated from the experimental pressure drop at constant vol-2(> Hydrate dssciationume using Equation 1.V(P1_. P0nexp=瓦(动- z2t2)(1)Temperature (C)Figure 4. Sequential cooling/heating step to investigate methane hydrate sta-where P, V and T are the pressure, volume and temperaturebility for pure water (Solid line: hydrate three-phase equilibriumof the vessel, respectively, and R is the gas constant. Thecluding Lw-H-V and I-H-V [20])compressibility factor, z, was estimated by the Peng-Robinsonequation of state for pure methane.The gas uptake ratio is defined as the ratio between the3.1. Hydrate formationmoles of methane encaged in hydrate (nexp) and its maximumvalue, assuming that all hydrate cavities have been filled withFigure 5 depicts the pressure changes in the system dur-methane molecules (nstochi), as given by Equation 2.ing gas compression (due to a decrease of temperature from40°C to 2 °C) and methane hydrate formation (from initiationGas uptake ratio :nstochito completion at 2 °C) in the presence of HECs with differentmolecular weights and concentrations. The dotted verticalThe stoichiometric ratio of methane/water is 1/5.75 in hy-lines in Figure中国煤化工rease of the coolingdrate stucture type I (sI). The amount of water in all experi-bath from 40CNMH(-ely 0.3 h. Becausements was 2.77 mol (50 cm3). Thu, nstochi was 0.48 mol.The methane hydrate dissociation ratio was determinedcorresponding pressure drop was relevant to methane solubil-to evaluate hydrate stability. This ratio was considered to rep-ity. The presence of HEC25 at a concentration of 1000 ppm122M. Mohammad Taheri et al./ Journal of Natural Gas Chemistry Vol. 21 No. 22012obtained for pure water. In contrast, the presence of HEC25 at followed by a slower rate that approached a plateau in Fig-5000 ppm induced the largest pressure drop, subsequently theure 6. The presence of HECs affected second step of hydratehighest methane solubility. The solubility presumably im-growth, subsequently on the maximum gas uptake, whereasproved, because more OH bonds were supplied by higher con-the initial rate of hydrate growth was similar for all samples.centration of HEC.Table 1 summarizes the values of induction time, hydrate gasWhen the temperature of cooling bath was set to 2°C,consumption and gas uptake ratio in the presence of HECs.the pressure of the system remained constant, which was at-As seen from Table 1, the presence of HECs significantlytributed to hydrate nucleation; the corresponding period was decreased the induction time from 10h to less than 1 h, .referred to as the induction time. As shown in Figure 5, thewhereas it increased the gas consumption in some cases. Al-presence of HECs decreased the induction time considerably.though the induction time in the presence of HEC25 at a con-After nucleation, the growth of hydrate crystals was initiatedcentration of 1000 ppm approached zero, less gas uptake af-with respect to a sharp pressure decrease in the system. Whenter 40 h was unexpectedly observed compared with the up-the pressure approached to a plateau, the growth rate was neg-take when pure water was used. The lower gas solubilityligible.and instantaneous hydrate crystal growth apparently resultedin more occluded water, and this may prevent further conver-14sion of water to hydrate. To achieve additional gas uptakeafter 40 h, the temperature was increased to 10°C (i.e., in the-0一 Pure waterhydrate stability zone) and then decreased to a constant tem-- 0- HEC25 1000 ppm130T≈2C- o- HEC255000 ppmperature of 2 °C again. Hence, the gas uptake increased from- V HEC9 5000 ppm0.13 to 0.20 mol.Table 1. Results of methane hydrate formation inthe presence of various HECsInductionGasRunconcentrationtimeconsumption uptake(Ppm)h)_(mol)10Pure water00.200.41HEC2510000.130.2750000.60.210.44HEC9).20.5625.1015.2025303540Time (h)Figure 5. Effect of dfferent concentration and molecular weight of HECsonThe presence of HEC9 at 5000 ppm resulted in the great-gas compression and hydrate formation during 40hest gas consumption, whereas HEC25 did not change the gasuptake. These results were consistent with those obtained byFigure 6 represents the methane uptake versus time, asKaraaslan and Parlaktuna, in which in the presence of addi-calculated using Equation 1. Preliminary gas uptake was duetives with OH-bonds, additives with smaller molecular sizesto solubility, whereas a constant number of moles in the gasexhibited a stronger promotion effect [6].phase macroscopically indicated hydrate nucleation. The sec-The ratio of gas consumption in the presence of additivesond gas consumption was attributed to hydrate growth, whichto that corresponding to pure water was suggested as an ade-quate criterion to compare the performance of additives with0.30respect to gas uptake, thereby eliminating the different operat-ing parameters, such as initial water content or initial pressure.. HEC25 1000 ppmFor example, the gas consumptions in the presenceHEC25 5000 ppm. HEC9 5000 ppmof HEC9 (5000 ppm) and pure water were 0.27 mol and0.20 mol, respectively (Table 1). Thus, the ratio of gas uptakein the presence of HEC9 at 5000 ppm to that corresponding to.15∞booopure water was 1.37.For purposes of comparison, Ganji et al. [18] reported a0.10gas uptake ratio of 1.30 in the presence of SDS (anionic sur-0.05factant) at a concentration of 500 ppm. In addition, this ratiowas 1.43 in th中国煤化工4 ammonium bro-0.00mide (CTAB)concentration of1230401000 ppm. ThCNMH G'9 with respect toTime (1)gas uptake enhancement was comparable to the performancesFigure 6. Gas uptake in the presence of various HECsof CTAB and SDS..Journal of Natural Gas Chemitry VoL. 21 No. 220121233.2. Hydrate stability30 r-0- Pure waterBased on Figure 4, the dissociation was investigated af--0 HEC25 1000 ppmter the temperature was decreased to - 10 °C and the gas was25[- 心HEC25 5000 ppmgently exhausted to 13 bar (i.e. out of the hydrate stability下- HEC9 5000 ppmzone) in a closed system. Utilizing a step heating procedure,the dissociation was monitored for approximately 40 h at each0ftemperature step. Figure 7 shows methane hydrate dissocia-tion in the presence of various HECs at -10°C, - -3°C, 0°Cand 2 °C. The corresponding results are summarized in Ta-s[le 2.The pressure of the closed system during hydrate dissoci-ation is expected to approach its equilibrium value at a con-stant temperature. The equilibrium pressures of methane hy-60drate were 19.8, 24.4, 27.2 and 32.9barat -10°C, -3°C,Time (1)0°C and 2 °C, respectively, as calculated using the HWHYDFigure 7. Methane hydrate dissociation utilizing step beating method in thesoftware.presence of various HECsTable 2. Hydrate dissociation results in the presence of various HECs at a mild pressureMass concentrationHydrate dissociation (mol)Maximum hydrate dissociation ratioRun(ppm)-10°C-3°C0c2°C-10°C-3oC0CC00.0280.0310.2240.1290.030.020.780.50HEC2510000.2010.1330.060.050.810.5650000.0260.0390.2190.040.260.87HEC90.0420.200.110.83Based on Equation 3, the dimensionless methane hy-sociation for pure water at 0°C (solid line). The initialdrate dissociation ratio was defined to evaluate pressureand equilibrium pressures of the corresponding system werechanges during dissociation with respect to the equilib-14.5 and 26.8 bar, respectively. Subsequently, Figure 8(b)rium pressure (details were given in section 2.3). Fig-shows the corresponding methane hydrate dissociation ratioure 8(a) shows the typical pressure increase of bydrate dis-calculated using Equation 3.30tP(bPJ_Q0-PJ]PwrP)6-易20>.(0-P。0.4 t150.2P。1020Time (h)rime ()Figure& (a) Hydrate dissociation at 0°C for pure water (solid line) at the initial value of 14.5 bar (dotted line), (b) corresponding methane bydrate dissociationFigure 9 shows the effects of HECs on the methane hy- (Figure 9a).中国煤化工te dssociation ratiodrate dissociation ratio parameter below the freezing point.in the presenc:C.HcNMHGnof100ppmwasThe presence of HEC9 (5000 ppm) increased the dissocia-calculated as 0.vo, ncezs al wie cuicuiration of 5000 ppmtion ratio to its highest value of approximately 0.12at-10°Cdid not show any significant effect with respect to pure water.124M. Mohammad Taheri et al./ Journal of Natural Gas Chemisoy Vol. 21 No.2 2012(approximately ~0.03).0.26 was considerably higher than that at - 10 °C (Table 2).As shown in Figure 9(b), HEC9 and HEC25 at concen- However, HEC25 at a concentration of 1000 ppm preservedtrations of 5000 ppm exhibited the same ascending trend atits ascending rate of hydrate dissociation ratio when the tem--3 °C, whereas the maximum hydrate dissociation ratio ofperature was increased from -10°Cto-3 °C.0.300.25田).25 t(b! 0.20 E一。Pure water0.20-0- HEC251000 ppm△HEC255000 ppm0,15-一HEC95000 ppm0.15- 0- Pure water .- o- HEC251000 ppm意0.100.10- HEC255000 ppm←HEC9 5000 ppmi 0.05 E0.050.0014030Time (h)Figure 9. Methane hydrate dsciation in the presence of HEC below the frezing point at -10°C (a) and -3°C 6)A mechanism of self-preservation has been proposed to case of higher concentration of HEC25 (5000 ppm). Consid-describe the much lower hydrate dissociation below the freez-ering the same maximum gas uptakes of 0.2 mol (previouslying point, which results in a metastable zone [15- 17]. Thismentioned in section 3.1) and the same initial pressures ofmechanism was based on the agglomeration of ice particles13 bar for HEC25 at concentrations of 1000 and 5000 ppm,with hexagonal (|h) crystalline structures in the range of0 tothe extent of hydrogen bonding between water molecules was- -33 °C, which prevented the gas molecules from leaving theprobably decreased at the higher concentration of 5000 ppm.hydrate structure [21]. The effects of water-soluble additivesGanji et al. [19] reported the results for xanthan, a water-on the kinetic behavior of hydrate dissociation can potentiallysoluble biological polymer with high molecular weights,be explained by their effects on self-preservation.mixed with SDS. The mixture showed a weaker promotionLin et al. [11] stated that compared with pure water, theeffect than SDS on the methane hydrate formation rate. Inpresence of SDS lowered the extent of self-preservation andaddition, xanthan improved methane hydrate stability at tem-increased methane hydrate dissociation at temperatures belowperatures below the freezing point by decreasing the mole per-the freezing point. They claimed that the porous structure ofcent of dissociation by approximately 10%. The authors sug-hydrate crystals formed in the presence of surfactants such asgested that HEC, which has a structure similar to that of xan-SDS supplied more surface area to decrease self- preservationthan, might show similar behavior with respect to hydrate for-and accelerate hydrate dissociation at temperatures below themation/dissociation. Nazari et al. [21] also introduced HECfreezing point [4.5].Moreover, methane hydrate dissociation using step heat-polymers to prevent hydrate dissociation at temperatures be-ing showed that, when the temperature was increased fromlow the freezing point.-5°C to -3。C, the higher initial pressure at - 3°C com-Figure 10 demonstrates the methane hydrate dissociationpared with that at - 5。C prevented hydrate dissociation con-ratio in the presence of various HECs at temperatures abovesiderably [18]. Hence, the initial pressure was as impor-the freezing point. The hydrate dissociation ratio was re-tant as the temperature and the presence of additives on self-markably increased to that achieved at temperatures below thepreservation during hydrate dissociation at temperatures be-freezing point, suggesting that the presence of HECs facili-tated the regasification of methane hydrate. HEC25 at a con-In the present work, HECs appeared to relatively de-centration of 5000 ppm exhibited the same behavior as purecreased the self-preservation and subsequently increased thewater on methane hydrate dissociation, whereas the lowerdissociation rate compared with pure water. However,concentration (1000 ppm) decreased the dissociation rate toHEC9 at a concentration of 5000 ppm showed the maximum some extent at 0°C. HEC9 at a concentration of 5000 ppminitial dissociation hydrate rate below the freezing point be-presented the highest dissociation hydrate rate (Figure 10a).cause of the attained storage capacity compared with that ofIn Figurecontinued for purepure water [18,19].water and HE中国煤化工The presence ofAs shown in Figure 9(b), the lower concentration of HEC25 showedPYHCNMHGeffectonhydrateHEC25 (1000 ppm) intensified self-preservation and de-dissociation than that of pure water in contrast to the resultscreased hydrate dissociation at - -3 °C in comparison with the obtained at a temperature of 0 °C (Figure 10a)..Jourmal of Natural Cas Chemistry VoL. 21 No.2 2012125.0 r1..8 f(a.owooooooooo。0.8b).6-0.6 f000000000000pmoo0000p000.4导0.4_。_HECYSOO-Pure water0.2- AHEC25 500pmf 0.2-o- HEC25 1000 ppm一HEC9 5000 p0.01012Time ()Time (6)Figure 10. Methane hydrate dissociation in the prescnce of HEC with different concentrations and molecular weights above the freezing point at 0°C (a) and2°C (b)4. ConclusionsOrleans, 1993. 375[3] Karaaslan U, Parlaktuna M. Energy Fuels, 2000, 14: 1103The effect of water- soluble hydroxyethyl cellulose with[4] Gayet P, Dicharry C, Marion G, Graciaa A, Lachaise J, NesterovA. Chem Eng Sci, 2005, 60: 5751molecular weights of 90,000 and 250,000 at concentrations of[5] Okutani K, Kuwabara Y, Mori Y H. Chem Eng Sci, 2008, 63:1000 and 5000 ppm on methane hydrate formation and dis-183sociation are studied. The presence of HECs decreases the[6] Karaaslan U, Parlaktuna M. Energy Fuels, 2002, 16: 1413induction time considerably compared with the results ob-[7] Khokhar A A, Gudmundsson J s, Sloan E D. Fluid Phase Equi-tained for pure water. Depending on the molecular weightlib, 1998, 150-151: 383and concentration, HECs are effective in enhancing the hy-[8] Zhong Y, Rogers R E. Chem Eng Sci, 2000, 55: 4175drate growth, subsequently the maximum gas uptake, which[9] Link D D, Ladner E P, Elsen H A, Taylor C E. Fluid Phaseis observed in the presence of HEC9 at 5000 ppm.Equilib, 2003,211: 1According to the increase in the hydrate dissociation rate10] Daimaru T, Yamasaki A, Yanagisawa Y. J Pet Sci Eng, 2007, 56:in the presence of HECs at temperatures below the freezingpoint, HECs lower the self-preservation effect compared with[1]LinW,ChenGJ,SunCY,GuoXQ,WuZK,LiangMY,ChenL T, YangL Y. Chem Eng Sci, 2004, 59: 4449pure water.A comparison of the formation rate, gas uptake and disso-12] Gudmundsson J s, Parlaktuna M, Khokhar A A. In: SPE Pro-duction & Facilities, 1994, SPE 24924: 69ciation rate of methane hydrate indicates that HEC9 at a con-[13] Ershov E D, Yakushev V s. Cold Reg Sci Technol, 1992, 20: 147centration of 5000 ppm is a candidate for use in gas hydrate[14] Gudmundsson J s, Borrehaug A. In: Proceedings of the 2ndstorage and transportation at -10°C under a mild pressureInternational Conference on Natural Gas Hydrates. Toulouse,of 13 bar and at temperatures below the freezing point. InFrance, 1996.415addition, its performance on hydrate dissociation at tempera- [15] Sterm L A, Circone S, Kirby S H, Durham W B. Energy Fuels,tures above the feezing point can be considered for hydrate2001, 15: 499regasification.[16] Takeya s, Ebinuma T, Uchida T, Nagao J, Narita H. J CrystGrowth, 2002, 237-239: 379Acknowledgements[17] Shirota H, Aya 1I, Namic s. In: Procedings of the Fourth In-The financial support of the Research Institute of Petroleum In-ternational Conference on Gas Hydrates. Yokohama Symposia,Yokohama, 2002.972dustry is gratefully acknowledged.[18] Ganji H, Manteghian M, Zadeh K s, Omidkhah M R, Mofrad HR. Fuel, 2007. 86: 434References[19] Ganji H, Manteghian M, Mofrad H R. Fuel Process Technol,2007, 88: 891[1] Sloan E D, Koh C A. Clathrate Hydrates of Natural Gases. 3th [20] HWHYD Software, Gas hydrate research center, Heriot WattEds. New York: CRC Rress, 2007University, UK, 2000[2] Kalogerakis N, Jamaluddin A K M, Dholabhai P D, Bishnoi PR.[21] Brjanian H, Kameli M, Khodafarin R, Nazari K, Rahimi H. usIn: SPE Intemnational Symposium on Oilfield Chemistry. NewPatent 2009062579.2009中国煤化工MYHCNMH G.