Jun Chen ,Yo-Song Zeng ,Xing-Yu Yu ,Qing Yun ,To Wng ,Bin Deng ,Ke-Le Yn ,Jin-Hong Jing ,Li-Ming To ,Chng-Zhong Chen

a Hunan Provincial Key Laboratory of Xiangnan Rare-Precious Metals Compounds and Applications,College of Chemistry and Environmental Science,Xiangnan University,Chenzhou,Hunan Province,423000,China

b School of Chemistry and Chemical Engineering,Liaocheng University,Liaocheng,252059,China

c SINOPEC Research Institute of Safety Engineering,339 Songling Road,Qingdao,266000,China

Keywords:

ABSTRACT

1.Introduction

Gas(such as methane)can be inserted into a water-constituted cage in the form of gas hydrate at elevated pressure and low temperature.Natural gas hydrate energy sources(Sun et al.,2019;Feng et al.,2017;Yang et al.,2019;Liu et al.,2018),plugs caused by gas hydrates(Ding et al.,2019;Park et al.,2017;Yan et al.,2014;Song et al.,2019;Xu et al.,2016 Saberi et al.,2020),and technologies derived from gas hydrates(Sa and Sum 2019;Shi et al.,2019;Wu et al.,2019;Zheng et al.,2019;Chen et al.,2019;Chazallon and Pirim,2018;Rasoolzadeh et al.,2019;Xiao et al.,2018,2019)were paid much attention for gas hydrate research in recent decades.

Methane(CH4)storage and transportation by forming CH4hydrate is one of the technologies derived from the gas-concentrated properties of gas hydrate.One volume of CH4hydrate contained a 150-180 volume of CH4(Fan et al.,2014;Lim et al.,2014;Wang et al.,2014).The pressure for natural gas storage by forming natural gas hydrate was much lower than that of compressed natural gas(CNG)(20-30 MPa),which suggested that natural gas hydrate is a potential safe method for natural gas storage and transportation.

Both thermodynamic stable and unstable system of gas hydrate can be used for gas storage by hydrate-based technology.For thermodynamic stable system,gas was concentrated steadily as the form of the hydrate under the equilibrium conditions theoretically.Thermodynamic promoters,such as THF(Kumar et al.,2018),quaternary ammonium salt(Babu et al.,2014;Fan et al.,2016;Fukumoto et al.,2015),cyclohexane,and CP(Delroisse et al.,2018)can be added into hydrate formation systems to moderate the equilibrium conditions and accordingly reduce the pressure of gas hydrate for gas storage.However,thermodynamic promoters also formed hydrates or semi-hydrates,which occupied hydrate cages and accordingly decreased the gas storage capacity.Therefore,as little as possible of the thermodynamic promoters was added in the consideration of gas storage capacity.From the thermodynamic unstable system,the self-preservation effect made it possible to be used in the gas storage and transportation at low pressures below 273.2 K(freezing point of water)(Stern et al.,2001a,b).When the temperature is below 273.2 K,gas hydrate decomposes slower than the common gas hydrate decomposition rate when the pressure and temperature are out of gas equilibrium conditions(Zhong et al.,2016;Xie et al.,2020,2021).Zhang and Rogers(2008)found that the presence of Al tubes,Cu tubes,or solid copper can help the ultrastability of gas hydrates at ambient pressure and 268.2 K.Natural gas hydrate pellets can also be used for the long-term storage of natural gas hydrate at atmospheric pressure(Mimachi et al.,2015).From above two examples,additional methods are needed to enhance the self-preservation effect,and enhancement of self-preservation effect makes it possible for gas hydrate storage at lower and safer pressure,even 1 atm pressure.Therefore,intensification of self-preservation effect is still important and deserved further investigation.In addition,Takeya et al.(2001)investigated the self-preservation effect of CH4hydrate in situ by X-ray diffraction and proposed an ice-shielding model for the selfpreservation effect.Although the accurate mechanism of selfpreservation effect is still unknown(Stern et al.,2003),iceshielding model was considered an acceptable mechanism to explain self-preservation effect(Falenty et al.,2014;Falenty and Kuhs,2009).

From the inspiration of the ice-shielding model referenced above,a novel method was proposed to intensify self-preservation effect(Zeng et al.,2020).CH4is the main content in natural gas,and our previous publication has focused on CH4hydrate formation in sodium dodecyl sulfonate(SDS)-dry solution hydrate formation systems(Zeng et al.,2020).As a continuous work,the covering method focused on the hydrate samples formed with SDS solution by covering with THF,CP,cyclohexane,and n-tetradecane respectively in this work.In addition,comprehensive insight into the covering liquid method was also discussed in this work.

2.Experimental section

2.1.Experimental materials and apparatus

n-Tetradecane was purchased from Tianjin Weiyi Chemical Technology Co.,Ltd.The details of CH4,SDS,THF,CP,cyclohexane,and hydrophobic silica nanoparticles(HB630)can be found in a previous publication(Zeng et al.,2020).Four covering liquids of THF,CP,cyclohexane,and n-tetradecane have its own feature.THF,CP,and cyclohexane can form(THF+CH4),(CP+CH4),and(cyclohexane+CH4)binary hydrate.THF is hydrophilic,while CP and cyclohexane are lipophilic.The solidifying point of cyclohexane is 279.7 K.Although n-tetradecane cannot form binary hydrate,the solidifying point of n-tetradecane is 279.2 K.Therefore,all above four chosen covering liquid may solidify as the form of hydrate or solid.HB630 were used to prepare SDS-dry solutions with SDS solution in the blender.The details of SDS-dry solution preparation can refer to previous work(Fan et al.,2014).An electronic balance with an accuracy of±0.1 mg was used to weigh SDS.Double distilled water was prepared in our laboratory.

As shown in Fig.1,the experimental apparatus is composed of two autoclaves,a temperature controlling system,and a data collection system.Each autoclave has an effective volume of 100 mL.The temperature can be adjusted by the temperature controlling system.The data of pressure and temperature can be automatically collected by the collected system.More details about the apparatus can be found in our previous publication(Zeng et al.,2020).

2.2.Experimental procedure

The experimental process included three experimental sections that were selection of the CH4hydrate formation system,CH4hydrate formation,and CH4hydrate decomposition.

2.2.1.Selection of the CH4hydrate formation system and CH4hydrate formed process

In this section,a CH4hydrate formation system was chosen to study the effect of the covering liquid method on self-preservation effect.Two autoclaves and relative connections were cleaned three times.Then,THF-SDS solution,CP-SDS solution,or cyclohexane-SDS solution was prepared by adding 12 mL of THF,CP,and cyclohexane into 30 mL of SDS solution with an SDS concentration of 500 ppm.After the solutions and autoclaves were prepared,the solution(SDS solution,THF-SDS solution,CP-SDS solution,or cyclohexane-SDS solution)was injected into autoclave 6(as shown in Fig.1).Then,autoclave 6,autoclave 7,and the connections were placed under vacuum to remove the air in both the gas and solution phases.The temperature of the glycol bath was set to the experimental value,and CH4was injected into autoclave 7.When the temperature reached the setting experimental value and then maintained for approximately 30 min,the CH4was injected from autoclave 7 into autoclave 6 until the pressure reached the desired value,and the CH4hydrate formation process started.After that,the whole process maintained for 5 h for each CH4hydrate formation system.

The SDS solution was finally chosen as the CH4hydrate formation system that was used for CH4hydrate decomposition in this work.Therefore,SDS solutions or SDS-dry solutions were used to form CH4.The CH4hydrate can be considered finished when the pressure changes very little.

2.2.2.The covering liquid method

After CH4hydrate formed completely in the SDS solutions or SDS-dry solutions,CH4hydrate decomposition started.In order to prevent initial fast decomposition of CH4hydrate,initial temperature of CH4hydrate were 266.0 K.When the decomposition temperature was 266.0 K,the whole decomposition time was maintained for 12 h.When the decomposition temperature was 272.2 K and 274.2 K,the whole CH4hydrate formation system was maintained at 266.0 K for 1 h,and then set the temperature to the decomposition temperature.For the covering liquid,THF,CP,cyclohexane,and n-tetradecane were precooled to reduce heat exchange during covering process.Following processes were the details of the covering liquid method.For CH4hydrate decomposition at 266.0 K,valve 3(as shown in Fig.1)was open,to decrease CH4to 0.1 MPa within 1 min,and then,valve 3 was rapidly closed.The CH4hydrate decomposed under the situation without a covering liquid.The decomposition time was maintained for 12 h,and the pressure data were recorded.After 12 h,the experimental temperature was set to 293.2 K to make the CH4hydrate decomposed completely.If a covering liquid was used during the CH4hydrate decomposition process at 266.0 K,precooled liquid was injected into autoclave 6(as shown in Fig.1)by a hand pump after the CH4pressure in autoclave 6 was vented to 0.1 MPa.Then,all connections and valves were closed to allow CH4hydrate decomposition.The time for CH4hydrate decomposition processes was also maintained at 12 h.After 12 h,the experimental temperature was set to 293.2 K to make the CH4hydrate decomposes completely.The CH4hydrate decomposition percentage can be calculated by the following equation(Zeng et al.,2020):

Fig.1.Schematic diagram of the experimental apparatus.1,Valve;2,Vacuum pump;3,4,and 5,Valve;6 and 7,Autoclave;8,Glycol bath;9 and 10,Valve;11 and 12,Pressure transducer;13 and 14,PT-100;15，Valve;16,Computer;17,Gas cylinder;18,Hand pump;19,Valve.

Where ydecomp.was the CH4hydrate decomposition percentage,Pfinal.was the final pressure,and Ptwas the pressure at time t in the process of hydrate decomposition.When CH4hydrate decomposed at 272.2 K and 274.2 K,the experimental temperature was first set and maintained at 266.0 K to inhibit fast CH4hydrate decomposition.For CH4hydrate decomposition without a covering liquid,the pressure was decreased to 0.1 MPa by valve 3 for CH4hydrate decomposition.Afterwards,the experimental temperature was maintained at 266.0 K for 1 h and then set to 272.0 K or 274.2 K to decompose CH4hydrate for another 12 h.Then,the experimental temperature was heated to 293.2 K to make CH4hydrate decomposes completely.For CH4hydrate decomposition with a covering liquid,the precooled covering liquid was injected into the autoclave by the hand pump after the pressure decreased to 0.1 MPa.After the temperature was maintained at 266.0 K for 1 h,the experimental temperature was set to 272.0 K or 274.2 K to decompose CH4hydrate for 12 h.After that,the experimental temperature was heated to 293.2 K to decompose CH4hydrate completely.The CH4hydrate decomposition percentage can also be calculated by equation(1).

3.Results and discussion

3.1.Selection of the CH4 hydrates formation system

SDS is the kinetic promoter for CH4hydrate formation.THF,CP,cyclohexane are thermodynamic promoters for CH4hydrate formation.Thekineticpromoteror(kinetic promoter+thermodynamic promoter)mixtures can accelerate gas hydrate process.Therefore,promoters of SDS,(THF+SDS),(CP+SDS),and(cyclohexane+SDS)were added to accelerate CH4hydrate formation at 274.2 K.The variation in pressure with time during the CH4hydrate formed process is shown in Fig.2.

From Fig.2,CH4pressure in gas phase decreased quickly during the first 30 min when CH4hydrate formed in SDS solution.The CH4pressure for CH4hydrate formation in SDS solution decreased to 2.94 MPa after 30 min,which was much lower than the pressures of 5.43 MPa,5.95 MPa,and 5.83 MPa when CH4hydrate formed in the THF+SDS solution,CP+SDS solution,and cyclohexane+SDS solution systems after 30 min,respectively.No other guest molecule(CP,cyclohexane,or THF)occupied the hydrate cages,which resulted in the highest pressure reduction in 30 min when CH4hydrate formed in SDS solution.In addition,water-insoluble CP and cyclohexane can inhibit the transfer of CH4,which results in a lower rate of CH4hydrate formation in the(CP+SDS solution)and(cyclohexane+SDS solution)systems than in the water-soluble THF-SDS solution.Fig.2 also shows that the final pressure of CH4hydrates in SDS solution decreased to 2.80 MPa after 300 min of CH4hydrate formation,which was equal to the CH4hydrate formation pressure in bulk water(2.80 MPa at 274.2 K)calculated by the Chen-Guo hydrate model(Chen and Guo,1998).However,when CH4hydrate formed in THF-SDS solution,CP-SDS solution,and cyclohexane-SDS solution,the final pressures after 300 min of CH4hydrate formation were 5.31 MPa,5.64 MPa,and 5.50 MPa,respectively,which were much higher than the final pressure of CH4hydrate formation in SDS solution.The lowest final pressure suggested the highest amount of CH4consumption when CH4hydrate formed in SDS solution.CP,cyclohexane,and THF can occupy hydrate cages,which reduces CH4consumption for CH4hydrate formation in the(THF+SDS solution),(CP+SDS solution),and(cyclohexane+SDS solution)systems,respectively.In addition,gas-water ratio was not suitable for forming ideal CH4hydrate.Initial gas-water ratio for CH4hydrate formation in SDS solution was 168:1,which was lower than ideal gas-water ratio of 205:1(1 volume of CH4hydrate contained 0.8 volume of water and 164 volume of CH4).And the gas-liquid ratio for(THF+SDS solution),(CP+SDS solution),and(cyclohexane+SDS solution)decreased to 138:1,139:1,and 140:1,respectively.Therefore,initial gas-water ratio also limited the formation was CH4hydrate.If the gas-water ratio was suitable,the CH4pressure would be close to the final pressure of CH4hydrate formation in SDS solution.In consideration of CH4consumption for CH4storage during methane hydrate formation process in this work,SDS solution was chosen as the CH4hydrate formation system for testing the covering method in the following experiments.

Fig.2.Variation in pressure with time for CH4 hydrates formation in different hydrate formation systems at 274.2 K.

Fig.3.Variation in CH4 hydrate decomposition percentages with time for CH4 hydrate decomposition in SDS-dry solution hydrate formation systems at 274.2 K.

3.2.The reliability of CH4 hydrates decomposition

To test the reliability of CH4hydrate decomposition,two runs of CH4hydrate decomposition by covering with n-tetradecane were conducted in SDS-dry solution hydrate formation systems.The results are shown in Fig.3.

From Fig.3,the final pressure of CH4hydrate decomposition for Run 1 and Run 2 were 2.03 MPa and 2.33 MPa,respectively.And the final CH4hydrate decomposition percentages for Run 1 and Run 2 were 96.2% and 97.5%,respectively.The results suggested that the initial volume fraction of methane hydrate was different for methane hydrate formation process was a kinetic process.However,difference between the two runs was only 1.3%,which also suggested that the experiments can be repeated.

3.3.The results of the covering liquid method

3.3.1.Effect of covering liquid method on CH4hydrate decomposition at 266.0 K

CH4hydrate decomposition percentage with time was used to investigate the effect of the covering liquid(THF,CP,cyclohexane,and n-tetradecane)on CH4hydrate decomposition at 266.0 K.The reason to choose THF,CP,cyclohexane,and n-tetradecane as covering liquid is that all above four liquid may solidified when covering on the surface of CH4hydrate.THF,CP,and cyclohexane can form binary hydrate with CH4respectively(Sun et al.,2002;Lee et al.,2012).The equilibrium conditions of(CH4+THF)hydrate,(CH4+CP)hydrate,(CH4+cyclohexane)hydrate,and CH4hydrate were shown in Fig.4.Cyclohexane and n-tetradecane can solidified at the experimental temperature during covering process.The variation in the CH4hydrates decomposition percentage with time is shown in Fig.5.

From Fig.5,CH4hydrate decomposed quickly,and the CH4hydrate decomposition percentage reached 11% in 1 h without covering with any liquid.The CH4hydrate decomposition process tended to slow down with continuously increasing time.The final CH4hydrate decomposition percentage was 17.6% after 12 h of decomposition with the pressure of 0.61 MPa,which suggested that a self-preservation phenomenon appeared at 266.0 K during the decomposed process.

Fig.4.Equilibrium conditions of CH4 hydrate with/without addition of covering liquid.

Fig.5.Variation in CH4 hydrate decomposition percentage with time by covering with different liquids at 266.0 K.

In addition,the final percentage of 17.6% was less than final percentage of 25.5% for CH4hydrate decomposition in SDS-dry solution after 12 h of decomposition(Zeng et al.,2020).Water droplets in SDS-dry solution were smaller than water in SDS solution,which provided more surfaces for CH4hydrate formation.Therefore,there are more surfaces for CH4hydrate decomposition,which accordingly resulted in faster CH4hydrate decomposition in SDS-dry solution than in SDS solution.When the surface of CH4hydrate was covered by THF,CP,cyclohexane,and n-tetradecane,the same trends were observed respectively.CH4hydrate first decomposed quickly and then slowed down after a certain time.The final CH4hydrate decomposition percentages were 12.4%,13.8%,13.0%,and 8.3% with the pressures of 0.26 MPa,0.33 MPa,0.51 MPa,and 0.37 MPa by covering with THF,CP,cyclohexane,and n-tetradecane,respectively.The results suggested that covering with THF,CP,cyclohexane,and n-tetradecane effectively inhibited CH4hydrate decomposition,which accordingly enhanced the selfpreservation effect.THF,CP,cyclohexane,and n-tetradecane have its own characteristics,which may affect the covering method to inhibit CH4hydrate decomposition.Covering with THF,CP,and cyclohexane can form(CH4+THF)hydrate,(CH4+CP)hydrate,and(CH4+cyclohexane)hydrate,respectively,which may provide a hydrate layer to inhibit CH4hydrate decomposition.CP,cyclohexane,and n-tetradecane are lipophilic,while THF is hydrophilic.Therefore,more THF than CP and cyclohexane may permeate between hydrates.In addition,the solidifying points of cyclohexane and n-tetradecane are 279.7 K and 279.0 K,respectively,and the cyclohexane and n-tetradecane coverings may solidify and adhere on CH4hydrate surface.From the discussion above,the hydrate layer and solidified layer formed a barrier that inhibited CH4transfer and accordingly inhibited CH4hydrate decomposition.In fact,the amount of covering liquid also affects CH4hydrate decomposition.40 mL of THF was used to cover on the surface of CH4hydrate at 266.0 K.Fig.6 showed variation of CH4hydrate decomposition percentage with time with 12 mL and 40 mL of covering THF,respectively.

Fig.6.Variation of CH4 hydrate decomposition percentage with time at different covering THF quantity at 266.0 K.

From Figs.6 and 40 mL of THF can effectively prevent CH4hydrate decomposition at 266.0 K CH4hydrate decomposition percentage was only 9.8% with the pressure of 0.25 MPa after 12 h when 40 mL of THF was covered on the CH4hydrate surface,which was smaller than 12.4%with the pressure of 0.26 MPa when 12 mL of THF was covered on the CH4hydrate surface.The results suggested more covering liquid was more effective to prevent CH4hydrate decomposition.More covering liquid increased the height on the CH4hydrate surface,which increased the resistance for CH4transfer,and according prevented CH4hydrate decomposition.If the covering liquid was sufficient,the covering method was similar to the immersion method proposed by Sharifi et al.(2018).In addition,the final CH4hydrate decomposition percentages in SDS solutions are also greater than the final CH4hydrate decomposition percentages in SDS-dry solutions(Zeng et al.,2020)by covering THF,CP,and cyclohexane,respectively.

3.3.2.Effect of the covering liquid method on CH4hydrate decomposition at 274.2 K

CH4hydrate decomposition with/without covering with THF,CP,cyclohexane,and n-tetradecane was also conducted at 274.2 K.The variation in the CH4hydrates decomposition percentage with time is shown in Fig.7.

Fig.7.Variation in CH4 hydrate decomposition percentage with time at 274.2 K with different covering liquids.

From Fig.7,it can be seen that CH4hydrate decomposed quickly without a covering liquid at 274.2 K,and the CH4hydrate decomposition percentage increased to 70.5% after 3 h.The final decomposition percentage increased to 80.1% with the pressure of 2.64 MPa after 13 h of CH4hydrate decomposition,which suggested that self-preservation effect vanished at 274.2 K without a covering liquid.There was no self-preservation effect observed above 273.2 K for gas hydrate decomposition previously.Therefore,CH4hydrate decomposed quickly when the conditions were out of equilibrium conditions.When CP,cyclohexane,and THF was used to cover the surface of CH4hydrate,CH4hydrate also decomposed quickly,and the maximum decomposition percentages reached 91.1%,66.0%,and 77.4%with the pressure of 1.95 MPa,1.86 MPa,and 1.72 MPa after 2.6 h,1.8 h,and 1.6 h,respectively.The quickly increasing CH4hydrate decomposition percentage also suggested that the self-preservation effect also disappeared at the temperature of 274.2 K,even by covering CP,cyclohexane,and THF.Afterwards,the CH4hydrate decomposition percentage started to decrease,and the final decomposition percentage achieved by covering with CP,cyclohexane,and THF decreased to 45.8%,23.9%,and 17.4% with the pressure of 0.93 MPa,0.61 MPa,and 0.31 MPa after 13 h of CH4hydrate decomposition,respectively.Initial quick increase of CH4hydrate decomposition percentage from 1 h to 3 h by covering CP,cyclohexane,and THF suggested that the covering liquid method failed to inhibit quick decomposition of CH4hydrate and simultaneously failed to prevent fast transfer of CH4.Combined the results with section 3.3.1,it revealed that covering with THF,CP,cyclohexane can only enhance self-preservation but cannot generate self-preservation.In addition,CH4hydrate decomposition percentage decreased with continuous increasing of time by covering THF,CP,and cyclohexane,respectively.The decrease in decomposition percentage suggested that binary hydrates of(CH4+CP)hydrate,(CH4+cyclohexane)hydrate,and(CH4+THF)hydrate formed.The reason for the formation of binary hydrate ascribed to that the conditions at the time of CH4hydrate decomposition percentage decreased was higher than the conditions for binary hydrate formation conditions.However,the decomposition percentage started to decrease at different times by covering with different liquids.The equilibrium pressures for(CH4+CP)hydrate,(CH4+cyclohexane)hydrate and(CH4+THF)hydrate at 282.15 K,282.02 K,and 282.1 K were 0.165 MPa,2.212 MPa,and 0.44 MPa respectively as shown in Fig.4(Sun et al.,2002;Lee et al.,2012).The time for forming(CH4+CP)hydrate should be shorter than that for forming(CH4+THF)hydrate,and the longest time should be for the formation of(CH4+cyclohexane)hydrate.However,hydrate formation was stochastic because the induction time for hydrate formation was stochastic(Chen et al.,2019),which resulted in the stochastic formation times of(CH4+CP)hydrate,(CH4+THF)hydrate,and(CH4+cyclohexane)hydrate.Therefore,the time for the reduction in the CH4hydrate decomposition percentage was also stochastic,as shown in Fig.7.To cover with n-tetradecane at 274.2 K,CH4hydrate decomposed quickly after the temperature increased,and the CH4hydrate decomposition percentage reached 80% in the first 2 h of decomposition.The quick decomposition of CH4hydrate also suggested the disappearance of the selfpreservation effect at 274.2 K,and covering n-tetradecane was ineffective to inhibit CH4hydrate decomposition at 274.2 K.After 13 h,the final CH4hydrate decomposition percentage increased to 89.5% with the pressure of 2.64 MPa.Although n-tetradecane solidified at 274.2 K,covering with n-tetradecane cannot inhibit fast CH4transfer during CH4hydrate decomposition.From the results of covering with THF,CP,cyclohexane,and n-tetradecane,all covering liquids failed to inhibit CH4hydrate decomposition at 274.2 K.THF,CP and cyclohexane can maintain relatively low CH4hydrate decomposition percentages at later stages by forming binary hydrates,while n-tetradecane cannot maintain relatively low CH4hydrate decomposition percentages,even though n-tetradecane cannot solidify at 274.2 K.

3.3.3.CH4hydrate decomposition from the SDS-dry solution hydrate formation system by covering with n-tetradecane

SDS-dry solution was produced to increase CH4hydrate formation by changing the water to many water droplets with small diameters(Fan et al.,2014;Wang et al.,2008).Previous publications have studied covering with THF,CP,and cyclohexane to inhibit CH4hydrate formation in SDS-dry solution hydrate formation systems.In this section,CH4hydrate decomposition by covering with n-tetradecane is conducted at different temperatures.The results are shown in Fig.8.For convenient comparison,CH4hydrate decomposed without covering n-tetradecane was also shown in Fig.8.

From Fig.8,when the CH4hydrate decomposition temperature was 266.0 K without covering with n-tetradecane,the CH4hydrate decomposition percentage was 11.7% after 1 h of decomposition,and the CH4hydrate decomposition percentage reached 25.5%with the pressure of 0.61 MPa after 12 h of decomposition.When ntetradecane was used,the CH4hydrate decomposition percentage decreased to 5.8%after 1 h of decomposition.The results indicated that covering with n-tetradecane can effectively inhibit initial CH4hydrate decomposition.Interestingly,the CH4hydrate decomposition percentage reached 25.0% after 12 h of decomposition with the pressure of 0.48 MPa,which was similar to 25.5% after 12 h of decomposition without covering with n-tetradecane at 266.0 K.The results suggested that covering with n-tetradecane may have little effect on CH4hydrate decomposition in SDS-dry solution hydrate formation systems.When the CH4hydrate decomposition temperature increased to 272.2 K,the CH4hydrate decomposition percentage was 48.5%after 12 h of decomposition,and the final CH4hydrate decomposition percentage increased to 49.4% with the pressure of 1.14 MPa by covering with n-tetradecane after 13 h of decomposition.The results demonstrated that the selfpreservation effect was weakening at 272.2 K than that at 266.0 K.The CH4hydrate decomposition percentage without covering with n-tetradecane at 272.2 K reached 71.1% with the pressure of 1.33 MPa after 12 h of decomposition,as reported in our previous publication(Zeng et al.,2020).The results also suggested that covering with n-tetradecane can effectively inhibit CH4hydrate decomposition at 272.2 K and accordingly intensify the selfpreservation effect.When the temperature increased to 274.2 K,the CH4hydrate decomposition percentage reached 76.5%after 2 h,and the final CH4hydrate decomposition percentage reached 96.8%with the pressure of 2.18 MPa after 13 h of decomposition.CH4hydrate decomposed at 274.2 K suggested that the selfpreservation effect vanished when the CH4hydrate decomposition temperature was above 273.2 K(freezing point of water),even when the covering n-tetradecane method was used.CH4hydrate decomposed from the SDS-dry solution hydrate formation system by covering with CP,cyclohexane,and THF,as can be found in our previous publication(Zeng et al.,2020).The results of covering with CP,cyclohexane,THF,and n-tetradecane in the SDS-dry solution hydrate formation system also show that the covering liquid can only enhance the self-preservation effect but cannot generate a self-preservation effect.In addition,hydrate particle size may affect the covering method to inhibit CH4hydrate decomposition.From the results of section 3.3.1,section 3.3.2,section 3.3.3,and our previous publication(Zeng et al.,2020),the covering liquid method was more effective in the SDS solution hydrate formation system(larger hydrate particles)than in the SDS-dry solution hydrate(smaller hydrate particles)formation systems.

Fig.8.Variation in CH4 hydrate decomposition percentages with time for CH4 hydrate decomposition by covering n-tetradecane in SDS-dry solution hydrate formation systems at different temperatures.(a),(c),and(e)was no covering process during methane hydrate decomposition at 266.0 K,272.2 K,and 274.2 K,respectively(Zeng et al.,2020);(b),(d),and(f)was CH4 hydrate decomposition by covering n-tetradecane at 266.0 K,272.2 K,and 274.2 K,respectively.

Fig.9.Schematic diagram of the covering liquid method to inhibit CH4 hydrate decomposition.(a)CH4 hydrate formation in SDS solution;(b)CH4 hydrate formation in SDS-dry solution.

3.4.Insights into the covering liquid method to inhibit CH4 hydrate decomposition

From the results and discussion in section 3.3 and a previous publication(Zeng et al.,2020),a comprehensive schematic diagram of CH4hydrate decomposition by a covering liquid is shown in Fig.9.

Combining section 3.2 and our previous publication(Zeng et al.,2020)with Fig.9,the covering liquid method can only enhance the self-preservation effect below the freezing point of water,which means that the covering liquid method proposed in this work may not generate a self-preservation effect.When the covering liquid method was used for CH4hydrate decomposition from the SDS solution CH4hydrate formation system below the freezing point of water,as shown in Fig.9(a),a solid layer(binary hydrate or solidified liquid)formed on top of the CH4hydrate to inhibit CH4transfer and accordingly intensify the self-preservation effect.In addition,some liquid may penetrate into CH4hydrates,which may affect the efficiency of the covering method.The amount of penetrating liquid was affected by the density of the covering liquid,the hydrophilicity of the covering liquid,and other possible factors.

For the SDS-dry solution CH4hydrate formation system,as shown in Fig.9(b),the covering method can also only enhance the self-preservation effect below the freezing point of water but cannot generate a self-preservation effect.Except for the factors of the solid layer(binary hydrate or solidified liquid),the density of the covering liquid,equilibrium conditions of binary hydrate,and the hydrophilicity of the covering liquid can affect the covering method.The hydrate particle size may also affect the penetrating liquid and accordingly affect the efficiency of the covering method.

When the temperature was above the freezing point of water,the self-preservation effect vanished for CH4hydrate decomposition in both SDS solution and SDS-dry solution.The solid layer formed from the covering liquid may not inhibit so much CH4gas obtained from CH4hydrate decomposition and finally fails to inhibit CH4hydrate decomposition.The possible reason that the covering liquid method failed to inhibit CH4hydrate decomposition was the fast decomposition of CH4hydrate,which obtained a large amount of CH4,and the formed solid layer could not support enough force to resist the pressure from CH4.Therefore,covering with CP,cyclohexane,THF,and n-tetradecane can only enhance the self-preservation effect but cannot generate a self-preservation effect.

4.Conclusions

CH4hydrate decomposition by covering with THF,CP,cyclohexane,and n-tetradecane was conducted at temperatures both below the freezing point of water and above the freezing point of water.The following conclusions and insights were obtained.

(1)Covering with THF,CP,cyclohexane,and n-tetradecane can inhibit CH4hydrate decomposition and accordingly enhance the self-preservation effect for CH4hydrate formation in the SDS solution at 266.0 K.Covering with THF,CP,cyclohexane,and n-tetradecane can decrease the CH4hydrate decomposition percentage from 17.6% with the pressure of 0.61 MPa without covering with any liquid to 12.4%,13.8%,13.0%,and 8.3%with the pressures of 0.26 MPa,0.33 MPa,0.51 MPa,and 0.37 MPa by covering with THF,CP,cyclohexane,and n-tetradecane,respectively.

(2)Covering with THF,CP,cyclohexane,and n-tetradecane cannot inhibit CH4hydrate decomposition at 274.2 K,which suggested that the covering liquid method may not generate a self-preservation effect.However,binary hydrate formation can decrease the CH4hydrate decomposition percentage by covering with THF,CP,and cyclohexane at later stages.

(3)CH4hydrate particles may affect the covering method to inhibit CH4hydrate decomposition.Smaller hydrate particles tend to increase CH4hydrate decomposition by using the covering liquid method.

(4)The hydrophilicity,density,solidifying point,and equilibrium conditions of binary hydrate may affect the covering method to inhibit CH4hydrate decomposition.

(5)Only solid layer cannot inhibit fast decomposition of CH4hydrate,which results in the failure of covering with THF,CP,cyclohexane,and n-tetradecane to inhibit CH4hydrate decomposition at 274.2 K.

Acknowledgments

This work is supported by the Hunan Provincial Natural Science Foundation of China(Nos.2020JJ3030,2019JJ50567),the National Natural Science Foundation of China(Nos.21506065,21978126,and 51904330),the Projects of Scientific Research Fund of Hunan Provincial Education Department(No.17A199),and the Scientific Research Foundation of Xiangnan University for High-Level Talents.