Gang Yue ,Aixian Liu ,Qiang Sun ,Xingxun Li,Wenjie Lan ,Lanying Yang ,Xuqiang Guo ,*

1 State Key laboratory of Heavy Oil Processing,China University of Petroleum,Beijing 102249,China

2 China University of Petroleum-Beijing,Beijing 102249,China

3 China University of Petroleum-Beijing At Karamay,Xinjiang 834000,China

Keywords:Biogas Gas separation Hydrate Ionic liquid

A B S T R A C T[C8min]BF4 was used in this work to combine with TBAB or THF for the investigation about thermodynamic and kinetic additives on CO2 and CH4/CO2 hydrates.The results show that[C8min]BF4 has the inhibition effect on the equilibrium of hydrate formation.About the kinetic study,[C8min]BF4 could improve the rate of CO2 hydrate formation and increase the gas uptake in hydrate phase.At the same time,the combination of TBAB and[C8min]BF4 could increase the mole friction of CH4 in residual gas comparing with the data in THF solution.CH4 separation efficiency was strongly enhanced.Since that the size of CO2 and CH4 molecules are similar,CH4 and CO2 could form the similar hydrate,so the recovery of CH4 from biog as decreases lightly.The CH4 content in biog as can purified from 67 mol%to 77 mol%after one-stage hydrate formation.In addition,the combination of THF and[C8min]BF4 do not have obvious promoting effect on CH4 separation comparing with the gas separation results in pure THF solution.

1.Introduction

CO2is one of the greenhouse gases,which has great influence on the global temperature.With the increase of CO2emitting into the atmosphere,the people's life has been affected greatly.However,the gases,such as CO2and CH4(biogas),CO2and N2(fuel gas),CO2and H2,do not have value in use because of the larger percentage of CO2in the mixed gas[1–3].In order to separate CO2from the mixed gases,the researchers have developed many ways to accomplish the task.There exist three main conventional methods for CO2separation.The separation in cold and deep condition,the membrane separation and the pressure swing adsorption separation.But results show that they need great energy and material costs[4,5].With the development of technology,hydrate separation is one innovative method which has developed quickly in recent years.The hydrate is non-stoichiometric crystalline which are formed by water molecule in the function of hydrogen bond,and hydrate formation must be under the condition of high pressure and lower temperature.The molecule of the gases would be engaged in the crystal such as CO2,N2,CH4and so on[6–8].However,the smaller gas molecule,such as H2,would be hardly to form the gas hydrate.The structures of the hydrate are like the cage and they are classified into three kinds which are named structure I,structure II and structure H respectively[9].The method of gas hydrate separation is based on various formation conditions,such as the temperature,pressure,the size of molecule and so on.For CH4/CO2gas separation,since the hydrate equilibrium condition for CH4and CO2is very close,so it would be fairly difficult for CO2separation from biog as efficiently.However,recent studies have shown that the addition of some chemicals to liquid would promote one of gases selectivity by hydrate formation[10].If the proper condition can be controlled for CO2formation,so CO2hydrate formation rate would be higher than CH4hydrate formation rate[11].After the hydrate formation,the gas constitution in residual gas and hydrate phase changed greatly and the needed gas was purified to attain the commercial specifications.Nowadays,many researchers focus on additives in hydrate formation to effectively separate CH4from gas mixtures[12,13].The hydrate formation additives can be classified into two categories.It contains the thermodynamic additives and kinetic additives.The thermodynamics additive is related to the temperature and the pressure of hydrate formation.The common thermodynamic additives are tetrahydrofuran(THF),Tetrabutylammonium bromide(TBAB),cyclopentane(CP),1,3-Dioxolane(DIOX)and so on[14–18].The kinetic additives are mainly related to the formation rate of gas hydrate and the gas storage of hydrate.The common kinetics additives are ionic liquids,sodium dodecyl sulfate(SDS),dodecyl trimethyl ammonium chloride(DATCl),sodiumdodecylbenzene sulfonate(SDBS).There existed many researches about hydrate formation additives,the additives can be optimized from relative hydrate formation experiments.From the related experiments and the literature,it can be find that TBAB and THF are favorable and common hydrate additives and they can greatly decrease the pressure of hydrate formation under the same temperature[19–21].But from the view of environmental protection,the ionic liquids are green solvents which synthesized in the organic chemical processes[22].From the related experiments,the introduction time of hydrate formation was shortened and the final gas consumption of in hydrate was improved when ionic liquids were put into the solution[23,24].The common ionic liquids contain positive ions liquid and anion ions liquid[25].The positive ions liquid contains quaternary ammonium salt,imidazole salt,Pyrrole salt and so on.The anion ions liquid contain tetra fluoroborate,halogeno salt and so on.The tetra fluoroborate and its related chemical compounds have been used in the hydrate formation.As for CO2/CH4separation experiments,ionic liquid,especially the category of imidazolium has remarkable solubility for CO2.[BMIm]BF4is one of common ionic liquid and has been used in the gas hydrate formation.The previous result indicated that it not only promoted CO2absorption,but also shortened the hydrate formation time greatly.The CO2/CH4mixed gas has different solubility in ionic liquid.The solubility of CO2was higher than CH4and other gases.The imidazolium-based ionic liquid has preferable effect on CO2selectivity in the solution absorption and hydrate formation because of effect interaction between two molecules.The 1-octyl-3-methylimidazolium tetra fluorborate([C8min]BF4)was homologous compound of omidazolium-based ionic liquid and has not been taken experiments in hydrate formation and gas separation.So the research about the thermodynamics and kinetics[C8min]BF4aqueous solution were studied.The simulative biogas(67 mol%CH4/37 mol%CO2)was prepared in this work.Because the heat value of biogas is not high because of the existing of CO2and the heat value of biogas is mainly concerned with the percentage of CH4in biogas.So in order to remove CO2from biogas and promote the CH4recovery;various chemical additives are added to water phase in the process of hydrate separation[26,27].From the literature,we all know that TBAB and THF are excellent hydrate promoters.Interestingly,a small quantity of TBAB or THF added to the aqueous phase would dramatically decrease hydrate thermodynamic formation pressure at the given temperature and increase hydrate kinetic formation rate.So the combination of TBAB or THF with[C8min]BF4have not been applied to gas mixtures about hydrate formation.So TBAB and THF were combined with[C8min]BF4to added to the solution and study the gas hydrate separation process[28,29].

2.Experimental

2.1.Materials

TBAB was supplied by Tianjin Nankai share compounds co.,ltd.,and the purity of TBAB is 0.99(mass friction).THF was supplied by Tianjin Nankai share compounds co.,ltd.,and the purity of THF is 0.98(mass friction).The gas of CO2with a purity of 0.999(mass friction)was supplied by Beijing BEI Temperature gas company,and CH4/CO267.0 mol%/33.0 mol%was supplied by Beijing BEI Temperature gas company.[C8min]BF4with the purity of 98%was supplied by Guangdong Wengjiang chemical reagent co.,ltd.The deionized water was supplied by our own laboratory.The uncertainty of these additives in mass friction is 0.0001.

2.2.Apparatus

Fig.1.Schematic diagram of the hydrate formation and gas separation apparatus.

The operated equipment of experiment was shown in the Fig.1.The reactor was fabricated by Jiangsu Hai An Oil Research Instrument Co.,LTD.The maximal pressure of reactor is 25 MPa.There exists a rabble that has three paddles at the bottom,and the rabble was motivated by magnetic force in the function of electrical machine.The total volume of the reactor is 465 ml.There exist two visual windows in the front and back of the reactor.There existed one compressed air chamber in the left of reactor and its maximum volume was 200 ml.The piston in compressed air chamber has the function of sealing.The hand pump was made by Nantong Hua Xing Petroleum Instrument Co.LTD.;its type was TB-50;the span of it was 0-50 MPa.The hand pump was connected with piston by ethylene glycol whose mass friction was 0.2 in aqueous solution.The hand pump rotated to control volume of reactor so as to change the pressure of reactor.The air bath was made by the Shanghai experimental instrument factory co.LTD.,and its type was WD2050,the range of temperature was-20 °C-130 °C.The pressure sensor was supplied by Fujian Shun Chang Hong Run Precision Instruments Co.,LTD.,and its type is HR-M21KF2AA3;the range of pressure measurement was 0-25 MPa,and its accuracy was 0.5 MPa.In the experiments,the composition of the mixed gas was analyzed by gas chromatograph.The type of the gas chromatograph(GC)7890B which was made by the Agilent Technologies of America,and its accuracy was 0.01 mol%.

2.3.The procedure of experiment

In this work,the first section was CO2hydrate experiments about thermodynamics and kinetics studies.The mass fraction of TBAB was 0.05,and the different concentration of the[C8min]BF4was added to the aqueous solution;they were 500 μg·g-1,1000 μg·g-1and 1500 μg·g-1.In the preparation progress,the reactor was cleaned three times by demonized water,and then the air of reactor was pulled out by vacuum pump.In the thermodynamics experiments,50-ml solution was purged into the reactor through the vessel.And then the reactor was sweep one time by the reaction gases.When the reactor reached the desired temperature,then the agitator blade was started to stir,the speed of rotation was controlled at 700 r·min-1.In the experiment,the method of“pressure seeks”was applied to identify the phase equilibrium under the condition of different temperature.The procedures of kinetic and separation were operated in the similar way with thermodynamic process.Different concentration of additives was added into the aqueous phase under the certain temperature and pressure.However,the pressure of hydrate formation and resolve was controlled by hand pump.

The second section of experiment was about hydrate phase equilibrium and gas separation of CH4/CO2.The main process of biogas separation was similar with process of CO2hydrate formation.Results were analyzed by gas chromatograph to compare the composition of the feed gas.When CH4/CO2hydrate formation reached balance,the temperature of reactor was cooled down to-5°C.So when the sample of residual gas was taken out to analysis by gas chromatograph,the hydrate cannot dissolve because of the pressure drop.The method fully used the self-preservation property of gas hydrate.And then the remaining gas above the hydrate of reactor was pulled out by vacuum pump.The temperature of reactor was heated up to 20°C.When the hydrate dissociated completely,the gas consumed in hydrate was analyzed by gas chromatograph.When all of the experiments were accomplished,the related calculation and results would be discussed and analyzed.

2.4.The calculation method of the experiment

2.4.1.The moles calculation of the gas consumed in the hydrate

First,the moles of gas contained in the hydrate were calculated.The Peng-Robinson Equation of state(PR-EOS)was used to calculate the volume of biogas at experimental conditions.The formula of calculation was listed as follows.

In the equation,n0stands for the moles of feed gas;ntstands for the gas that cannot form gas hydrate;t stands for the time when the reaction arrives at equilibrium.P stands for pressure.The Z stands for the compression factor of gas which was calculated by the SRK equation.

2.4.2.The recovery of the CH4and the separator factor of the CO2

The biogas was separated by one-stage hydrate formation,and the percentage of CH4in residual gas was increased.So the recovery of CH4was calculated by the equation as follows:

3.Results and Discussion

3.1.The thermodynamic phase equilibrium study of CO2 hydrate and CH4/CO2 hydrate

The result of thermodynamics phase equilibrium was analyzed at first.By comparing the result,the additives have effect on the pressure of hydrate formation under the certain temperature.And the section contains the experiments of CO2and CH4/CO2hydrate formation.Through the experiments,the main analysis of result can provide the guidance for hydrate separation study.In the Fig.2,the six photos showed the progress of hydrate formation in the reactor and depicted the shape of the hydrate.The solid-like hydrate layer formed on the vapor–liquid interface and grow up quickly through the reactor window.

From the Fig.3,the experiments about additives on thermodynamic phase equilibrium of CO2and CH4/CO2hydrate formation.In the Fig.3(a),it shows 500 μg·g-1/1000 μg·g-1/1500 μg·g-1[C8min]BF4concentration have inhabitation effect on of CO2hydrate phase equilibrium comparing with data in pure water phase.It is obvious that the pressure of phase equilibrium increases with the addition of[C8min]BF4,and CO2hydrate formation pressure in[C8min]BF4solution was about 0.2-0.5 MPa higher than in the water[30].When the concentration of[C8min]BF4increases,the prohibitive effect of[C8min]BF4on CO2hydrate formation was strengthened.In the experiments,in order to study the influence of different combined additives on gas hydrate formation,[C8min]BF4was combined with the TBAB or THF respectively in the process of CH4/CO2hydrate formation,and this was the foundation research for the next biogas hydrate separation.The concentration of TBAB in water phase was 5 wt%;it was determined from the reference of relevant experiments,and the preferable concentration was optimized[31].The concentration of THF was 4 wt%in water.The selection of concentration was referred to the previous literature,and it would have an efficient water-to-hydrate conversion in this condition[32].From the Fig.3(b),the certain concentration of[C8min]BF4has inhibitive effect on CH4/CO2hydrate formation.Comparing with the CH4/CO2thermodynamic phase equilibrium in pure water,the hydrate formation pressure was 1.2 times higher in[C8min]BF4aqueous liquid.When TBAB and THF were added into the[C8min]BF4solution,the CH4/CO2hydrate formation pressure declined dramatically.Because TBAB and THF are excellent kinetic and thermodynamic promoters respectively on gas hydrate formation.The hydrate formation pressure in TBAB+[C8min]BF4was 1.59 times higher than the pressure in pure TBAB solution.Meantime,the hydrate formation pressure in THF+[C8min]BF4was 1.46 times higher than the pressure in pure THF solution.In addition,the thermodynamic promotion effect of THF was better than TBAB and the hydrate formation pressure in TBAB solution was about 1.15 times higher than the data in THF solution under the same temperature.In the whole,the addition of[C8min]BF4in the random aqueous solution indicated the thermodynamic inhabitation effect.

Fig.2.The snapshots taken in the process of hydrate formation.

Fig.3.The thermodynamic phase equilibrium of CO2 in the[C8min]BF4(a)solution and CH4/CO2 hydrate formation in TBAB+[C8min]BF4,THF+[C8min]BF4,pure TBAB and pure THF(b)solution.

3.2.The kinetic study of CO2 hydrate in the[C8min]BF4 solution

In the experiment,the[C8min]BF4was added to water explore the effect on CO2hydrate formation.The conditions were controlled at 3.25 MPa,277.15 K.From the Fig.4,the introduced time of CO2hydrate formation was shorten,and the rate of hydrate formation was accelerated dramatically.With the increase of[C8min]BF4concentration,the final gas consumption in the hydrate phase increased slightly.So[C8min]BF4could increase the CO2hydrate formation rate greatly on kinetic study,but the influence on the hydrate gas uptake in hydrate was not obvious.

3.3.The kinetic of CH4/CO2 hydrate in[C8min]BF4 solution

From the precious Fig.3,it described that different concentration of[C8min]BF4on kinetic study of CO2hydrate formation.The concentration of[C8min]BF4was selected at 500 μg·g-1,1000 μg·g-1and 1500 μg·g-1respectively.The introduced time of hydrate formation and hydrate formation rate were compared with CO2hydrate formation in pure water.The moles of CO2that were consumed in hydrate were calculated by the relevant formulas.

Fig.4.The kinetic study of CO2 hydrate formation in the[C8min]BF4 solution.

Fig.5.The hydrate formation of CO2/CH4 in the pure TBAB solution and the TBAB+[C8min]BF4 solution.

As show in the Fig.5,the Fig.5(a)shows the separation of CH4/CO2in TBAB solution,at the same time,the pressure was controlled at4 MPa and temperature was 276.15 K,277.15 K,278.15 K and 279.15 K respectively.In the Fig.5(a),the final gas consumption in hydrate increased when the temperature was lowered in TBAB solution.TBAB could form the semi-clathrate hydrate with the small gas molecules(CH4or CO2).The formation of gas hydrate and TBAB semi-clathrate were the two process of hydrate formation,so they could enhanced the gas uptake in the hydrate[33].The second figure depicts that CH4/CO2formation in TBAB solution and different concentration of[C8min]BF4were added into it.Comparing with hydrate formation in the TBAB solution,the[C8min]BF4would enhanced the gas consumption in the hydrate formation.When 5000 μg·g-1/1000 μg·g-1/1500 μg·g-1[C8min]BF4were added two the 0.05 mass friction TBAB solution,the change of volume of residual gas in[C8min]BF4solution was 2.6 times more than in pure TBAB solution.The hydrate introduced time in TBAB+[C8min]BF4solution was within 50 min.From the Fig.5(b),with the increase of[C8min]BF4concentration,the time of biogas absorb in liquid became shorter,mainly the CO2.The combination of TBAB and[C8min]BF4not only improve the final gas uptake in the hydrate,but also shorten the time to reach the kinetic hydrate formation for biogas.In particular,[C8min]BF4has the amino group which can promote more CO2dissolved in the TBAB+[C8min]BF4solution.So the mass transfer process was strengthened for hydrate formation[34].

In the above Fig.6,it describes that temperature and concentration of[C8min]BF4have the influence on the kinetics of hydrate formation.In Fig.6(a),it described that THF could greatly increase the hydrate formation rate;the hydrate introduced time could be greatly shortened to 25 min compared with the hydrate introduced time in TBAB solution.Under the same pressure,it can be seen that more gas was consumed to form hydrate in THF solution when the temperature decreased.However,there existed one obvious pressure drop in 263.16 K.Because it provided a high enough driving force to form the hydrate crystallization in this condition,the consumption reached the maximum in 263.15 K.When different concentration of 500 μg·g-1/1000 μg·g-1/1500 μg·g-1[C8min]BF4were added to THF solution at 4 MPa,277.15 K,the time to reach equilibrium seems 50 min longer than in the THF solution.THF and[C8min]BF4could not have favorable synergy effects each other.The decrease ofCH4/CO2volume in pure THF solution was 1.2 times more than in THF+[C8min]BF4solution.So[C8min]BF4could promote the CH4/CO2consumption in hydrate greatly.From Fig.6(b),the promotion effect was not enhanced when the concentration of[C8min]BF4increased.In all different concentrations experiments,1000 μg·g-1[C8min]BF4was preferable concentration to increase biogas consumption in hydrate formation,and 500 μg·g-1was the preferable concentration to shorten the equilibrium time.Through the combination of the THF and the[C8min]BF4,the introduced time of hydrate formation was shortened.It was about 10 min in THF+[C8min]BF4solution and it would be 25 min in pure THF solution.Biogas consumption in hydrate are improved widely.

Fig.6.The hydrate formation of CO2/CH4 in THF solution and THF+[C8min]BF4 solution.

3.4.CH4 recovery by gas hydrate separation from the biogas

In Table 1,it shows the influence of different parameters on the CH4/CO2hydrate separation.From the Fig.7(a),it showed that the study of biogas hydrate separation in TBAB solution and the temperature was different.When gas hydrate formation gets to the equilibrium,the constitution of residual gas is analyzed by gas chromatography(GC).When the gas hydrate was heated to 20°C and the hydrate dissolved entirely,the gas consumed in hydrate was analyzed by GC as well.When TBAB concentration in solution was 0.05(mass friction)and the pressure was 4 MPa,the range of temperature was 276.15 K,277.15 K,278.15 K and 279.15 K,the CH4content in residual gas was about 75 mol%,and the recovery of CH4was between 0.923-0.932.When the temperature was 276.15 K,the recovery of CH4was higher than the hydrate separation in other temperature.When different concentration of[C8min]BF4was added to TBAB solution.The mole fraction of CH4in residual gas is enhanced about 2 mol%.However,when[C8min]BF4was added to solution,the percentage of CO2in the hydrate decreased from 40.14 mol%-47.85 mol%to 35.45 mol%-37.40 mol%,and the recovery of CH4decreased.From the first equation,it can be seen that much more CH4and CO2are consumed to form hydrate in[C8min]BF4solution,and the mole fraction in residual gas decreased slightly.In the passage,the t50was the other standard to judge the effect of[C8min]BF4on the hydrate formation and CH4separation.It can be seen that the combination of TBAB and[C8min]BF4would prolong the time to reach hydrate formation equilibrium.

The Fig.7(c)and(d)shows the results of addition of 4 wt%THF in solution.In the study,the pressure was 4 MPa,and the temperature was 276.15 K,277.15 K,278.15 K and 279.15 K.It describes that the CH4mole fraction in residual gas is about 35.62 mol%–38.35 mol%,and the recovery of CH4from biogas was about 0.912–0.918.When[C8min]BF4was added to the THF solution,there did not exist evident difference with the gas separation in THF solution,it explains that the combination of[C8min]BF4and THF cannot increase CH4separation efficiency by hydrate method.

4.Conclusions

In the study,the objective is to combine of ionic liquids with thermodynamic and kinetic additives for CO2separation and CH4recovery from simulative biogas.At first,thermodynamic phase equilibrium and kinetic of CO2hydrate formation were studied in[C8min]BF4solution.[C8min]BF4has inhibition effect on thermodynamics compared with hydrate formation in deionized water.The combination of[C8min]BF4and TBAB or THF on CO2/CH4hydrate thermodynamics was studiedand the addition of TBAB or THF could greatly decrease the CO2/CH4hydrate formation pressure,and they could expand the gas hydrate separation operation region.However,the addition of[C8min]BF4to THF solution can prolong the time to reach hydrate phase equilibrium.The cooperative effect of TBAB with[C8min]BF4is better than THF with[C8min]BF4on CH4recovery from the biogas.As for the phenomenon,it would be possible that THF does not coordinate with[C8min]BF4effectively,so there is still a need for more experiments to study hydrate formation process in microcosmic method about the interaction between molecules.On the other hand,[C8min]BF4could increase CO2consumption in hydrate.The[C8min]BF4has the inhibition effect on CH4/CO2hydrate formation equilibrium;it plays a thermodynamic inhibition role on biogas hydrate formation.When[C8min]BF4was applied into the gas hydrate formation and separation,it should combine with the thermodynamic promoters,such as TBAB or THF and so on.As for hydrate kinetic study,the combination of TBAB with[C8min]BF4could increase CH4mole fraction of in residual gas;the amount of hydrocarbon loss is increased by additional of[C8min]BF4.It should be noticed that the amount of produced hydrate increased in the presence of[C8min]BF4,and the CO2content in hydrate decreased by almost 7 mol%.Therefore,although the concentration of CH4increased in residual gas by 2 mol%,but the concentration of CH4in hydrate phase increased by almost 9 mol%;[C8min]BF4showed that imidazolium-base ionic chemical liquid was remarkable CO2solvent.So we concluded that because CH4and CO2are small molecules;it was easy for CH4to enter the cavity of hydrate and accompany with CO2molecule by means of one-stage separation at 277.15 K and 4 MPa.More CO2entered into the hydrate phase when[C8min]BF4was in the TBAB solution and[C8min]BF4promoted more water converted into hydrate.The combination of THF and[C8min]BF4does not have obvious promotion effect on CH4separation.Although the[C8min]BF4concentration changed,The CH4in the residual gas was about 75 mol%in the THF solution.But the CO2in hydrate phase increased by 2 mol%.Through these experiments,the combination of[C8min]BF4with kinetic additives and thermodynamic additives would be one promising method to apply on the hydrate formation.As for the mechanism,there needs further investigation to comprehend and spectroscopic measurements should be used,such as Roman analysis.

Table 1The CH4 separation from biogas in different chemical additives solution

Fig.7.The percentage of CH4 in residual gas and the recovery of CH4 in TBAB(a),TBAB+[C8min]BF4(b),THF(c),THF+[C8min]BF4(d)solution.