Weiguo Liu,Yanghui Li*,Xiaohu Xu

Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education,Dalian University of Technology,Dalian 116024,China

Keywords:Methane hydrate Ice Heating Secondary pressurization Surfactant

A B S T R A C T A series of experiments of forming hydrate from ice powders in different conditions have been carried out with constantvolumemethodtoevaluatetheinfluencefactorssuchas pressure,temperature,andSDSsurfactant.The change of temperature and pressure were collected as a function of elapsed time,which were used to calculate thegasconsumptionandhydratesaturationduringhydrateformation(pVTmethod).Basedontheexperimental results and theanalysis,itisconcluded that:(1)Both initial pressureand temperaturehaveeffect onthe hydrate formationandtemperatureplaysa moreimportantroleintheprocess;(2)heating andsecondary pressurization will promote the gas hydrate formation and enhance the hydrate saturation as a result.Meanwhile,the promotion of heating seems to be more obvious than that of secondary pressurization;(3)different concentrations of SDS surfactant have clearly influence on the saturation of gas hydrate and there is an optimal concentration to promote the hydrate formation.

1.Introduction

Natural gas hydrate has been recognized as an important resource thatcanreplacetraditionalfossilenergybecauseofitswidedistribution,abundant reserves,high energy density and clean combustion[1].They may occur in two distinctly different geographic settings,in the permafrost and in deep ocean sediments,where the necessary conditions of low temperature and high pressure exist for its formation and stability[2-5].The reserve of fossil fuel is commonly cited as 500 billion carbon tons,but the reserve of gas hydrate is estimated as twice the amount of fossil fuel[6-8].Under standard atmospheric condition,a maximum of 172 m3of methane gas can be stored in 1 m3of pure methane hydrate and its energy efficiency can be as high as 9.5[9,10].Thus,methane hydrate has been regarded as an ideal material to store up or transport energy and studied by the researchers both at home and abroad[11-13].According to the literatures,the transportationcostofgashydrateisexpectedtobe18%-24%lowerthanthatofliquefied natural gas[14,15].Besides,gas hydrate also can be used in the desalination of seawater[16]and refrigeratingindustry[17,18]because of its specialties.However,the gas hydrate reservoir is very sensitive to the environmental condition,any inappropriate human intervention to it will probably cause terrible consequences[19,20].Therefore,it is necessary to study the physicochemical properties of gas hydrate during formation and dissociation.

Whereas,it is really uneconomical and uneasy to obtain natural hydrate-bearing cores from the seabed,and synthesizing gas hydrate by simulating in-situ conditions has become a necessary way in laboratory.According to the literatures,ice was much easier to generate the hydrate with methane gas than water[21,22].In fact,gas hydrate formed from ice and gas mixtures is one of the primary methods to simulate the gas hydrate formation in the cryolithozone and have comprehensive understanding of hydrate formation kinetics.A strong dependence of the methane hydrate reaction on the area of the gas-ice contact surface was demonstrated by Barrer and Edge[23].Hwang et al.studied the methane hydrate growth on ice as an interfacial(heterogeneous)phenomenon and measured the hydrate forming rates duringicemeltingat differentgas pressures[24].SloanandFleyfel discussed molecular mechanisms of the hydrate-crystal nucleation on theicesurface,emphasizingtheroleofthequasi-liquidlayer[25].Rafiei et al.studied the formation kinetics of CO2hydrate below the ice point,and the result indicated that the higher temperature would lead to higher generating rate[26].Rivera and Janda studied the effect of ice particle size and temperature on the formation of propane hydrate,and firstly obtained the activation energy of propane hydrate formed from ice[27].Chen et al.studied the effect of pressurization rate and ethyl alcohol on methane hydrate formed from ice,and they draw a conclusion that lower pressurization rate and ethyl alcohol were good forthehydrateformation[28].ThestudyofVerrettandServioindicated that surfactants would clearly promote the generating of hydrate[29].Besides,many researchers were reported to have studied the effect of surfactants such as SDS and THF,and expounded the influence mechanism[30,31].

Although many researchers have studied the hydrate formation formed from ice in kinds of respects,there is no research which couples several factors and analyzes them systematically.In this paper,a series of experiments of forming hydrate from ice in different conditions have been carried out with constant volume method to evaluate the effects of pressure,temperature,and SDS surfactant on the formation of methane hydrate.

2.Experiment

2.1.Experimental apparatus

The schematic of the experimental apparatus is showed in Fig.1.It includes high pressure reactor,cold storage,CH4/N2gas cylinders,thermostatic bath control system,pressurization system and data acquisition system.The capacity of pressure reactor is 30 MPa and its effective volume is 460 ml.A pressure sensor with an accuracy of±0.01 MPa is used to measure the pressure.Five thermocouples with an accuracy of±0.05Kareplacedintheaxialdirectionofthereactortomeasuretheinsidetemperature.Thethermostaticbathcontrolsystemisusedtocontrol the temperature inside the reactor precisely.The cold storage with an rangeof?15-20°Cisusedtoprovidethelowenvironmenttemperature.

Fig.1.Schematic of gas hydrate formation apparatus.(1)High pressure reactor;(2)Thermocouples;(3)Glycol-water bath;(4)Pressure sensor;(5)Cold storage;(6)A/D module;(7)Computer;(8)Vacuum pump;(9)Reducing valve;(10)Shut-off valve;(11)Reducing valve;(12)Waste bag;(13)CH4cylinder;(14)N2cylinder;(15)Flow meter;(16)Check valve.

2.2.Sample preparation

Icepowdersweremadebyfreezingdeionizedwatermixedwithdifferent concentrations of sodium dodecyl sulfate(SDS).The ice was smashed by ice crusher and then sieved by the standard 20,40 and 80-mesh sieves to obtain the particles with grain size of 380-830 μm and 180-380 μm respectively.In this study,180 μm and 380 μm were used to describe the grain size of 180-380 μm and 380-830 μm in the experiments for convenience.Thebatchof icepowdersused in eachexperimenthadsubstantiallyidenticalmassof(155±5)g.Allabovework was carried out in the cold storage.

2.3.Experimental procedures

Firstly,the high pressure reactor was washed two or three times by freezing deionized water.Then,the nitrogen and methane gas was charged into the reactor to make it dry.Next,the pressure reactor was precooled in the glycol-water bath before the ice powders were placed.After that,the prepared ice powders were put into the pressure reactor inch by inch to make sure that the ice powders in the reactor were relativelyloose.Afterthat,thepressurereactorwascarefullyclosedupand thewholesystemwasdischargedbyavacuumpump.Finally,themethane gas precooled by the cold storage(272.15 K)was injected into the reactor and the reaction started.Experimental parameters such as temperature,pressure and gas flow were respectively acquired by the thermocouples,pressure sensor and the gas flow meter.

2.4.Calculation method

Generally,the formation of methane hydrate can be illuminated by an equation[32,33]:

The hydrate formation rate and the pressure drop rate have positive correlation.In order to accurately reflect the actual relations among temperature T,pressure P and volume V in the vessel,the compressibility factor Z is introduced to the ideal gas equation of state:

where n(mol)is the number of moles of methanegas,P(Pa)is the present pressure of the gas in the reactor,T(K)is the temperature,which is assumed to be a constant,R(8.3144 J·mol?1·K?1)is the universal gas constant,Z is the compressibility factor of methane gas obtained from general compression factor chart,and V(m3)is the present volume of the methane gas in vessel.

The moles of methane gas remained in the vessel is determined by the equation:

where n0is the initial moles of methane gas,ncis the moles of methane gas consumption.

Considering the hydration number in the Eq.(1),the volume of ice consumed is determined by the equation:

where ρiis the density of ice,and Vicis the volume of ice consumed.Thevolumesofspeciesinthevesselaredeterminedbythefollowing equations:

where V is the present volume of methane gas,V0is the volume of vessel,Viis thepresent volume of ice,Vhis thevolume of methanehydrate,Vi0is theinitialvolumeof ice,whichcanbecalculatedbythemassof ice and its density,and ρhis the density of methane hydrate.

According to Eqs.(2),(3),(5)-(7),the moles of methane gas consumption can be calculated:

The conversion rate of ice can be calculated:

3.Results and Discussion

3.1.Effect of pressure on methane hydrate formation

3.1.1.Effect of initial pressure

Fig.2 shows the pressure drop during methane hydrate formation under different initial pressures with the temperatures of 269.35 K and 264.55 K.It can be observed that experiment No.E1 has a sharp pressure drop in initial stage and a relatively slow drop in the subsequent stage.Besides,the experiment No.E1 also has a longer reaction time than that of experiment No.E3.When it comes to experiments No.E4 and No.E5,there is no clear difference between the pressure drop trends of these two experiments,but cost much more time to achieve the equilibrium state than that of the experiments No.E1 and No.E3.Accordingto theTable1,the initialconversion rates of ice for experimentNo.E1 andNo.E3are6.74%and5.39%,and thefinalconversion rates are 23.58%and 17.92%respectively.However,when the experiment temperature changes to 264.55 K,there is slight difference between the initial conversion rates of ice for experiment No.E4 and No.E5,which are 3.28%and 3.34%respectively.

Fig.2.Pressure drop during methane hydrate formation under different initial pressures with the temperatures of 269.35 K and 264.55 K.

Table 1 Parameters of methane hydrate formation under different conditions

Based on the experimental results,an obvious conclusion can be drew that initial pressure has an apparent effect on the formation of gas hydrate,and higher initial pressure will promote the formation of gas hydrate.Wu et al.and Wang et al.found the same phenomenon in the experiments of hydrate formation in porous media[34,35].In the physical sense,thepressure can be regarded asthe fugacity,andthe excess fugacity(excess pressure)is a form of driving force of hydrate formation.Therefore,thehighertheinitialpressureis,thesharperpressure drop in the initial stage,which means higher initial conversion rates of ice.In addition,the hydrate generated in the initial stage will form an“armor”on the surface of ice particles,which will hinder the formation of hydrate and lengthen the reaction time[36].

AsshowninFig.2,theeffectofinitialpressureonthehydrateformationisnotapparentforexperimentsNo.E4andNo.E5,whichisassumed to be related to the temperature.In order to clarify this phenomenon,the experiments No.E6,No.E7 and No.E8 are conducted.Fig.3 shows the pressure drops during methane hydrate formation under different initial pressures with the temperature of 270.15 K.From Fig.3 and Table 1,it is easy to draw that even a small increase of initial pressure will significantly increase the initial and final conversion rate of ice at the temperature of 270.15 K.It can be deduced that the effect of initial pressure on the hydrate formation from ice powders is obvious when thetemperatureisbelowandclosetofreezingpoint,theinitialpressure leads to more positive effect on the hydrate formation and higher conversion rate of ice under higher temperature.Whereas,when the temperature is below but far away from freezing point,there is no obvious effectofinitialpressureonpromotinghydrateformation.Thesameconclusion is also drew by Staykova's study of kinetic of methane hydrate formation from ice[37],which also indicates the temperature plays an important role in the hydrate formation.

3.1.2.Effect of secondary pressurization

Fig.3.Pressure drop during methane hydrate formation under different initial pressures with the temperature of 270.15 K.

Fig.4.Pressure and temperature changes during methane hydrate formation under different initial pressures with secondary pressurization.

Table 2 Parameters of methane hydrate formation with secondary pressurization

Fig.4 shows the pressure and temperature changes during hydrate formationunderdifferentinitialpressureswithsecondarypressurization.From Fig.4,it can be observed that the secondary pressurization always havepositiveeffectonthehydrateformationandcanimprovethegashydratesaturation,becausethereisalwaysagaspressuredropaftersecondary pressurization.Table 2 presents the experimental parameters of methane hydrate formation with secondary pressurization.In the experiment No.S2 of low initial pressure,it takes 420 min to reach the equilibrium state while the pressure drops from 4.2 MPa to 2.43 MPa,and the conversion rate of ice powders is 20.14%.After secondary pressurization,the pressure drops slowly,and eventually stops at 4.35 MPa,the conversion rate of ice powders is 24.07%during this period.The same trend occurs in the experiment No.S1 of high initial pressure,the conversion rate oficepowdersinthefirstandsecondperiodis26.50%and23.02%respectively.Compare the two sets of data,it can be found that the increase of saturation due to secondary pressurization under low initial pressure is more than that under high pressure,which indicates that the secondary pressurization is more effective under the lower initial pressure.

The effect of initial pressure has been discussed in Section 3.1.1,the excess pressure is regarded as the driving force of hydrate formation.When the pressure drops to the equilibrium state,there is no excess pressure for hydrate formation,at this time,the gas is charged into the vessel,which provide extra gas for excess pressure to drive the hydrate formation.However,this promotion of hydrate formation is related to the initial pressure.According to the study on the generation mechanism of methane hydrate formed from ice powders,it indicated that theformationofhydrateistheprocessofadsorption,catalysisandcomplexation for ice crystals and gas molecules[25].As the reaction progress,the hydrate will cover the surface of ice particles,and a hydrate cladding will be formed on the surface[36].Under higher initial pressure,the hydrate cladding will be thicker than that under lower initial pressure,which will slow down and prevent the reaction.

3.2.Effect of temperature on methane hydrate formation

3.2.1.Effect of initial temperature

Fig.5.Pressure and temperature changes during methane hydrate formation at different initial temperatures.

Fig.5 shows pressure and temperature changes during methanehydrate formation under the pressure of 4.3 MPa with the initial temperature of 269.35 K and 270.35 K.From Fig.5,it can be seen that the pressure in experiment No.E1 with the temperature 269.35 K drops faster than that in experiment No.E9 with the temperature of 270.35 K in the initial stage.During the initial stage,the conversion rates of ice for experiment No.E1 and No.E9 are 6.47%and 5.15%respectively(Table1).Meanwhile,No.E1takesmuchmoretimetoreachtheequilibriumstatethanNo.E9.Andfinally,thetotalconversionrateoficeforexperiment No.E1 and No.E9 is 23.58%and 18.83%respectively.It can be concluded that temperature plays an important role in the hydrate formation,andthehydrateiseasytobeformedunderlowerinitialtemperature.In the experiment,the difference between the actual operating temperature and equilibrium temperature under certain pressure is called supercoolingdegree[38],which has a critical impacton the reactionofhydrateformationasaformofdrivingforce.Thehydrateiseasier to be formed under higher supercooling degree(lower temperature).

3.2.2.Effect of heating

Fig.6 shows the pressure and temperature changes during methane hydrateformationwithheatingatdifferentinitialpressures.FromFig.6,it can be seen that the pressure keeps almost constant before heating to about 0°C(melting point),which indicating that the reaction is almostfinish.However,the pressure drops when the temperature rises to melting point,which means that the reaction is restarted.More direct results can be seen in Table 3,the conversion rates of ice for the experimentNo.S7andNo.S8afterheatingare45.75%and20.81%respectively.Theheatingsignificantlypromotetheformationofhydrateandenhance thesaturationofhydrate.Moudrakovskietal.observedthegas-solidreaction by NMR imaging and found that the ice covered by hydrate can melt under normal melting temperature[39],which indicates that whenthetemperaturerisestomorethan0°C,theiceunderthehydrate“armor”will be superheated and melt,and the methane gas will restart to diffuse through the “armor”(the gas diffusion is more fierce under higher temperature),then the hydrates will continue to be generated[36].

Fig.6.Pressureandtemperature changesduringmethanehydrate formationwithheating at different initial pressures.

As shown in Table 3,the conversion rates of ice in the experiment No.S8 by secondary pressurization and heating are 9.7%and 20.81%respectively.It can be seen that superheated ice has a more apparent effect on improving the methane hydrate saturation than that of secondarypressurization.Intermsofinfluence mechanism,the secondary pressurization is only related to the excess fugacity which affect the gas diffusion to the inside of hydrate cladding,however,the risingtemperature affects the gas diffusion as well as the reaction inside the hydrate cladding,which indicates that heating has a double effect on promoting the hydrate formation.

Table 3 Parameters of methane hydrate formation with heating

3.3.Effect of SDS surfactant on methane hydrate formation

Fig.7showsthepressureandtemperaturechangeswithandwithout SDS surfactant.As shown in Fig.7,the time to reach the equilibrium state of No.S3 experiment with 300 μg·g?1SDS is shorter than that without SDS,and the pressure drop is larger,which indicates that the SDS does can promote the formation of hydrate.Fig.8 shows the pressure and temperature changes with different concentration of SDS surfactant.Compare the three set of data,it can be seen that different concentrations of SDS clearly influence the formation of hydrate.The pressure drops most quickly with 300 μg·g?1of SDS.In addition,the Table4alsoshowsthattheconversionrateoficewith300μg·g?1isbigger than that with 100 μg·g?1and 500 μg·g?1of SDS,which indicates that it is not a positive correlation between the hydrate saturation and theconcentrationofSDS,andthereisprobablyanoptimalconcentration ofSDS topromotethehydrateformation.This phenomenonis similarto that SDS acts in the formation of methane hydrate from liquid phase[40].

Fig.7.Pressure and temperature changes during methane hydrate formation with and without SDS surfactant.

Fig.8.Pressure and temperature changes during methane hydrate formation with different concentration of SDS surfactant.

Table 4 Parameters of methane hydrate formation with different concentration of SDS

SDS is a surfactant which may change the interfacial tension and the relative supersaturation degree.The presence of SDS will reduce the interfacialtension,whiletheexistenceofmicelleinevitablyaffectstherelative supersaturation degree.Yoslim has given his measured results that there is an increase in contact angle in the surface of ice attached with the surfactant as the concentration of surfactant decreases[41].Theeffectofmicellarsolubilizationwillinfluencethechemicalpotential of species,which may increase the speed of crystal growth.The surfactant does not get into the crystal lattice,but can change the rate of crystal growth.If the surfactant on the surface of the particles forms special adsorption,it can lead to a change of crystal shape in the endmember crystal structure.

In fact,Pizadeh and Kusalik have studied the effect of ice-gas interface on the hydrate formation in the view of molecular and found that the methane molecules coupled with defective ice particles on the surface and promoted the formation of hydrate cage structure on the surface of defective ice particles[42].The existence of surfactant changes the surface free energy of solution,which may affect the structure of ice powders formed by this solution and creates the defect conductive to formation of hydrate on the surface,meanwhile,surfactant adsorbs onthesurfaceofthecrystalnucleusasthecentreoftheinducedcrystallization.Wooldridgeetal.suggestedthat“Bjerrumdefects”areessential to the growth of hydrates[43].The propagation of a growing ice-like(crystal)structure is dependenton the presence of mobile orientational(Bjerrum L type)defects within the structure.The mobile point defects within a growing ice-like phase,whether crystalline or amorphous,are instrumental in the propagation of the new phase.

4.Conclusions

Based on the experimental results and analysis above,the conclusions are drawn as follow:

(1)Initialpressurehasadirecteffectontheformationofgashydrate,and this effect is more obvious when the temperature is close to the freezing point.

(2)Secondary pressurization can improve the saturation of gas hydratetoacertainextent.Whentheinitialpressureislow,thesecondary pressurization has an obvious effect on improving the saturation of gas hydrate,and relatively shortens the time to achieve the same saturation in the condition of high initial pressures.

(3)Temperature is very important to the formation of gas hydrate,and the hydrate is easier to be formed under lower temperature(higher supercooling degree).

(4)Heating leads to the formation of superheated ice during gas hydrate formation,which will obviously increase the hydrate saturation.Also,heating has a more obvious effect on improving the gas hydrate saturation than that of secondary pressurization.

(5)Different concentrations of SDS surfactant have clearly influence on the saturation of gas hydrate and there is an optimal concentration to promote the hydrate formation.

Nomenclature

d average particle size of ice powders,μm

Peequalized pressure,MPa

Pe1first equalized pressure,MPa

Pe2second equalized pressure,MPa

Pe3third equalized pressure,MPa

Pmmaximum pressure,MPa

Pm1initial pressure,MPa

Pm2second filling pressure,MPa

Pssteady pressure,MPa

α0initial conversion rate of ice powders,%

α final conversion rate of ice powders,%

α1first conversion rate of ice powders,%

α2second conversion rate of ice powders,%

α3third conversion rate of ice powders,%