STUDY FOR NATURAL GAS HYDRATE CONVERSED FROM ICE

第54卷增刊学报Vol 54 Suppl2003年12月Journal of Chemical Industry and Engineering ( China)December 2003STUDY FOR NATURAL GAS HYDRATECONVERSED FROM ICEWANG Shengjie, shen Jiandong, HAO Miaoli and LIU Furong(School of Environmental and Chemical Engineering, Xi'an Jiaotong University, Xi'an 710049, Shaanxi, China)Abstract Natural gas hydrates are crystalline clathrate compounds composed of water and gases of smallmolecular diameters that can be used for storage and transport of natural gas as a novel method. In thepaper a series of experiments of aspects and kinetics for hydrate formed from natural gas and ice werecarried out on the industrial small scale production apparatus. The experimental results show thatformation conditions of hydrate conversed from ice are independent of induction time and bigger degrees ofsupersaturation and supercooling improved the driving force and advanced the hydrate formation.Superpressure is also favorable for ice particle conversion to hydrate. In addition, it was found there havean optimal reaction time during hydrate formation.Keywords natural gas hydrate, formation, ice, supersuturation, supercooling, superpressure1 INTRODUCTIONtransport large volumes of natural gas overdistance, Gudmunsson proposed a methodNatural gas hydrates are crystalline clathrate hydrate formed from the mixture consisting of icecompounds composed of water and gases of small and water and natural gas, the results show thatmolecular diameters in the solid phase. The gases, the cost of natural gas transported by the form ofsuch as methane, ethane, propane, n-butane and hydrate reduced 26%4, 5. In addition, a fewcarbon dioxide, hydrogen sulfide, are trapped in scientists suggested the feasibility of hydratethe microcavities of a crystal lattice provided by formed by frozen ground containing ice onhost water moleculesdiscussing geological mode of hydrate formation.It has already been found that the volume of a In China, the investigation of hydrate is just on theiven gas hydrate is much less than the normal beginning and most of the experimental apparatusvolume of the gas from which the hydrate was focus on the hydrate formation upon oC6.Soformed. This fact clearly suggested the possibility the experiments of aspects and kinetics for hydratethat natural gas could be stored and transported at formed from natural gas and ice were carried out ornormal pressure and low temperature in the form the industrial small scale production apparatusof the hydrate. In China natural gas resources designed by ourselves, the results were analyzedare abundant(the known reserve of natural gas is fully, too1.17X10 2m), but about one third of the reserveis the small or medium scale gas fields. The2 EXPERIMENTAItransportation of natural gas in the form of hydrateAs showed in fig. 1, the procedure of gas flow isprovides a novel approach for these gas fields and that中国煤化工eservoir bottle wasthe natural gas under sea sediments, and it will releash a reduction valveafford investigative foundation for natural gas andYHCNMHhe gas was injectedhydrates car, too[2.3.nto the reactor through the pressure accumulator,In the study of frozen hydrate being used to which could stabilize the experimental pressure. The·24·化工学报December 2003experimental ice was added to the reactor before the guangdong Kangyu Measuring and. Controllinggas injected, then the reactor temperature was reduced Apparatus Co, Ltd). Its range is 0--5MPa, andto experimental temperature to start reaction.the accuracy is 0. 1%.The experimental data were recorded by theThe platinum resistance thermometer thedata acquisition system.ype is WzPK-103) was used to test the reactorrotate speedtemperature, and the results are changed tofilter mass flowcontrollerelectrical signals by a transmitter. Thethermometer can check the temperature from -50Ccompressorto+50C, and its accuracy reaches to 0. 5%.eractionAll the data are recorded by the computerbuffer tankgas supplycomputerhrough an acquisition board eventuallycompressorcondenser evaporato2. 3 Materials and preparation of samplesFig 1 Schematic figure of experimental apparatusFor the sake of approaching to the reallyindustrial process, the reactant is simple. Natural2. 1 Experimental apparatusThe hydrate formation is a reaction at the gas is purchased from Xi'an Natural Gas Corp,and the mass composition of experimental gas isconditions of high pressure and low temperature, 90. 54%CH,, 0. 74%C2 H6, 0.19%C3 Hg,0.068%and the hydrate formed from ice need agitation, i-C H1o, 0. 034%n-C, H1o,8. 29%CO2, 0.0085%thus the reactor must correspond with theseconditions. In this paper, the available capacity ofH2,0.13%N2( analyzed using HP4890 gaschromatography)reactor is 1 liter, the working pressure is belowThe water is the tap water. Firstly it is frozen9. 8MPa, and the working temperature rangesand then comminuted before added to the reactor43. 15K to room temperature. The reactor waagitated by agitator blade and cooled by the 3 EXPERIMENTAL RESULTScircular coolant in the jacket.AND DISCUSSINGThe refrigeration cycle consists ofaThe hydrate formation is analogous to theeration compressor the type is CAJ4461A, crystallization, and could be sub-divided intomade in France ), a throttle valve and annucleation and growth processes. Induction isevaporator. The cryogen compressed by the found during nucleation process. The period ofcompressor expands into the throttle valve, and induction is named induction time (or inductionthen cools the coolant in the evaporator byperiod), which is the time elapsed for nucleation.vaporization. Finally the cooled coolant is pumped The induction time is an important parameter forto the reactor jacket by a recycle pump. The hydrate formation [mreactor temperature is stabilized by the aid ofThe formation rate of natural gas hydrate iscontrolling the evaporator temperature and governed by a multitude of factors, including theadjusting the flux of the circular coolant.pressure, temperature and gas composition, alsoThe buffer tank is used to stabilize the called PVT-effects. Also, the rate of hydratepressure and the flow in the pipe, its effective formation is determined by the combined effects ofvolume is 40 liters and its working pressure is heat and mass transfer. Cooling is required tobelow 5. OMParemo中国煤化工 formation.Mass2.2 Measuring instrumentsCNMHG the natural gas inThe pressure of the reactor is measured bywater,and bring the dissolved gas molecules inpressure pickup(the type is KYB600G, made by contact with a growing hydrate crystal. Finally theVol 54 Suppl.Journal of Chemical Industry and Engineering (China)·25·rate of hydrate formation depends on the nature ofcrystal growth, also referred to as chemicalreaction kinetics. Therefore, the overall rate ofhydrate formation depends on PVT--effectstransport-effects and reaction-effects [7.8JIn the paper the influence of induction timeduring hydrate formation and the effects ofsupersaturation, supercooling, superpressure andreaction time on the formation kinetics wereinvestigated.3.1 Influence of induction time on formationconditionsFig 2 Relationship between formationInduction time is the period needed fortemperature and induction timenucleation, the growing clusters of gas and water hydrate formation have made many difficulties formolecules could be regarded as precursors to experimental study. And meanwhile, both thehydrate nuclei formation. So the induction time is degree of the concentration of reactant reducesalso a measure of the ability of a supersaturated along with the time and the degree of the additionsystem to remain in the state of metastability.of the concentration of reaction resultantSloan [9] investigated the induction of hydrate corresponding with time can estimate the grade offormed from ice, and introduced the ratio of the reaction. So the degree of the addition of resultanthydrate former molecular diameter to cavity size as concentration is selected to evaluate the degree ofa nucleation parameter to explain induction time reaction.consideration of the practidata. Furthermore, he proposed a hypothesis for conditions of experiments in the paper, thethe nucleation of gas hydrates from ice. The concentration of resultant is defined as the ratio ofypothesis is that the agitation must supply gas content of the formed hydrate, which is thenergy to form a mobile, value of the gas volume in 1g hydrate at standardlocalized water layer on the ice/hydrate surface, conditions.even at low temperatures. Then the localized liquid 3.2. 1 Influence of supersaturaion on hydratelayer enables dissolution of gas in the water at gas formation Supersaturation o! is defined as f/agsurface of the ice/water surface. At last the nuclei1, where is the fugacity of the gas at theare formed in the liquid layer. But induction time experimental temperature and at the equilibriunshould not change the phase equilibrium hydrate pressure, and the fugacity of the gas, fgconditions, the experimental result showed in is at the experimental temperature and pressurefig2, which is the profile of formation In figure 3 the ratios of gas content at varioustemperature T to induction time t at 1. 6MPa.supersaturation and 263 15K are displayedIt can be seen that the formation temperaturear thatthe ratio of gas contendoesn't vary approximately at various induction increases along with the addition of the degree oftime. So we can say that the induction time does supersaturation, which coincides with the generalnot affect the equilibrium condition of natural gasconclusion. Another conclusion is that thehydrate中国煤化工gas content Is3.2 Influencing factors on kinetics ofCNMHGhydrate formation3. 2.2 Influence of supercooling on hydrateWe know that the special conditions of formation The degree of supercooling is defined26化工学报December 2003the initial pressure of the reactor, and the Pexpthe experimental pressure. The procedure is thatfirstly the reactor pressure is stabilized at theinitial pressure for sometimes, then was reducedthe experimental pressure, the difference betweennitial pressure and the experimental prissure issuperpressure and the stabilized period at initialpressure Is superpressure time tsupaaaa.a.a。如,Fig 3 Relationship between ratio ofgas content and supas AT=Ten-Tex 1o], where T is the equilibrium Rhydrate formation temperature at a given pressureand Texp is the experimental temperature. Figure 4gives the measured results of the relationship ofthe ratio of gas content in natural gas hydrate andthe degree of supercooling, the results show thathigher degree of supercooling is useful for hydrateFig 5 Relationship between ratio offormation. At the same time, the phenomena ofgas content and superpressuremore little amount of freeze at higher degree ofPrimarily, the results of relationship betweensupercooling were found.the ratio of gas content and the degree ofsuperpressure are gained, shown in fig. 5. It wasfound that superpressure heightened the ratio ofgas content in hydrate, and higher degree ofsuperpressure brought higher ratio ofgas content.The effects of superpressure time to hydrateformation were obtained, too. As shown in fig. 6we can conclude that longer superpressure timeincreases the ratio of gas content.3. 2.4 Relationship between ratio of gas content i1-TKhydrate and reaction time The experimentalFig. 4 Relationship between ratio of gasresults are showed in figure 7, from which we cancontent and degree of supercoolingconclude that the ratio of gas content improved as3.2.3 Influence of superpressure on hydrate the addition of reaction time. Yet when theHydrate formation is crystalline reaction time is greater than 342 minutes, the ratioreaction. Then the disturbance could increase the varies weakly along with the extending of reactionnumber of crystal nuclei. In the nucleation process中国煤化工 ptimal reaction timeof hydrate formation, superpressure may be one oftart yhCNMH Gion, and the gain ofthe disturbances. The superperpressure in the optical reaction time is useful for commercialpaper is defined as Psp=Pint-Pexp, where Pint is process.VoL 54 SuppL.ournal of Chemical Industry and Engineering (China)·27·experimental temperature. From the equation itcan be found that bigger degrees of supersaturationand supercooling improved the driving force andadvanced the hydrate formation. Superpressureelevated the driving force during nucleation, whichis favorable for ice particle conversion to hydrate.As the gas consumption in the batch reactor in thepaper, the driving force decreases gradually, thenthe formation rate slowed. When the driving forceapproaches to zero, the reaction will stop for thesystem has achieved balance. Allese arecoincident with experimental resultsig. 6 Relationship between ratio ofgas content and superpressure time4 CONCLUSIONSThe hydrate formation is analogous to thecrystallization process and gas-solid reactionprocess with solid reactant layer formed. Theformation rate of natural gas hydrate is governedPVT-effects, transport-effects and reaction-effects. From the experimental results it was foundthat(1) Hydrate formation is independent ofinduction time and induction didn't change thet/minequilibrium conditionsFig 7 Ratio of gas content-reaction time curve(2)The ratio of gas content in hydrate3. 3 Discussingncreases along with the addition of the degrees ofHydrate formation from ice could also be supersaturation and supercooling, and alsoconsidered as a gas-solid reaction process with solid improves corresponding with higher value ofreactant layer formed, so it has the characteristics superpressure and longer superpressure time;of reaction and crystallization. Induction will affect(3) There exists an optimal reaction timethe nucleation rate, yet not influence the phase during the period of hydrate formation and theequilibrium of hydrate.extending of reaction time is useless for theOn the other way, if we assumed that all reaction.particles have the same diameter, the hydratecrystals begin to grow at the same time and the Acknowledgements The financial assistance provided by thedifferent of frugalities is defined as the reaction Natural Science Found of China for this project with thedriving force, the kinetics model of hydrate contact No. 40272064 is gratefully acknowledgedformation can be obtained in the followingReferer-KA,(f-fa)中国煤化工MWhere K is the reaction coefficient, A is theYYHCNMHinterface, f is the fugacity of gas phase and fa is2 Shi Dou, Zheng JuThe Status and Prospects of Researchand Exploitation of Natural Gas Hydrate in the Word. Aduancefugacity related to the equilibrium pressure ofin earth sciences, 1999, 14(4): 330-338化工学December 2003Sun Zhigao, Ma rongsheng, Guo Kaihua, et al. ExperimentInvestigation of the Effect of Surfactants on Storage Properties 7 Bishnoi PR, Natarajan V. Formation and Decomposition of Gasof Methane Hydrates, Journal of Xi'an Jiaotong UniversityHydrates. Fluid Phase Equilibria, 1996, 117: 168--1772003,37(7);724-7308 Wang Shengjie, Han Xiaohui, Liu Furong, et al. Kinetics of4 Gudmundsson J S. Gas Storage and Transport Using HydratesFormation of Natural Gas Hydrate Below Ice Point. Journal of[C]. Offshore Mediterranean Conference, Ravenna, 1997.Xi'an jiaotong University, 2002, 36 (1): 99--1025 Zhang Wenling, Li Haiguo, Wang Shengjie etc. Progress9 Sloan E D, Fleyfel F. A Molecular Mechanism for GasResearching Natural Gas Storage and Transportation in HydrateHydrates Nucleation from Ice. AIChE Journal, 1991, 37Form. Natural Gas Industry, 2000, 20(3): 95-98(9):1281-1292Fan Shuanshi, Guo Tianmin. Progress in Study of Clathrate.0 Englezos P. Clathrate Hydrate. Ind Eng. Chem. Res, 1993Chemical Industry and Engineering Progress, 1999, 132:1252-1274中国煤化工CNMHG