Xiaoya Zang,Lihua Wan,Deqing Liang*

CAS Key Laboratory of Gas Hydrate,Guangzhou 510640,China

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development,Guangzhou 510640,China

Guangzhou Institute of Energy Conversion,Guangzhou Center for Gas Hydrate Research,Chinese Academy of Sciences,Guangzhou 510640,China

Keywords:Gas hydrate Formation Sediments CO2/N2mixture

ABSTRACT To obtain the fundamental data of CO2/N2gas mixture hydrate formation kinetics and CO2separation and sequestration mechanisms,the gas hydrate formation process by a binary CO2/N2gas mixture(50:50)in fine sediments(150-250 μm)was investigated in a semibatch vessel at variable temperatures(273,275,and 277 K)and pressures(5.8-7.8 MPa).During the gas hydrate reaction process,the changes in the gaseous phase composition were determined by gas chromatography.The results indicate that the gas hydrate formation process of the binary CO2/N2gas mixture in fine sediments can be reduced to two stages.Firstly,the dissolved gas containing a large amount of CO2formed gas hydrates,and then gaseous N2participated in the gas hydrate formation.In the second stage,all the dissolved gas was consumed.Thus,both gaseous CO2and N2diffused into sediment.The first stage in different experiments lasted for 5-15 h,and ＞60%of the gas was consumed in this period.The gas consumption rate was greater in the first stage than in the second stage.After the completion of gas hydrate formation,the CO2content in the gas hydrate was more than that in the gas phase.This indicates that CO2formed hydrate easily than N2in the binary mixture.Higher operating pressures and lower temperatures increased the gas consumption rate of the binary gas mixture in gas hydrate formation.

1.Introduction

Fossil fuels such as coal and oil have been the primary energy source for human society since the 18th century.Meanwhile,they are believed to remain the main source of energy up to the middle of the 21st century[1-3].Correspondingly,with the continuous combustion of fossil fuels,a large amount of CO2was released into the atmosphere.The carbon dioxide can cause greenhouse effect and climate change.The main gases released from the power plants flue gas are the CO2/N2gas mixture[4,5].CO2capture and storage(CCS)[6,7]can effectively reduce CO2emissions in the atmosphere and reduce the global warming.In first step,CO2was separated by the conventional CO2separation methods including absorption methods,membrane separation techniques,electrochemical methods,and cryogenic distillation.After that,the captured CO2were sealed in the seabed in various ways,such as dissolution in seawater or in the form of mineral carbonates[8-10].

Gas hydrates are nonstoichiometric crystalline compounds.Hydrates are formed by water and molecules of suitable size under conditions of low temperature and high pressure.Water molecules form hydrogen bonds and hydrate-cage structures,and guest molecules occupy the cage structures to form stable compounds[11].Hydrates were found in undersea sediments in nature.Both CO2and N2can form gas hydrates under appropriate pressure and temperature conditions.In the past decades,many studies about CO2/N2gas injection into methane hydrate-bearing sediments for methane recovery and for direct CO2capture and sequestration as hydrate were reported[12-20].However,the results of the study show that the methane recovery efficiency from hydrates by this method is low.Due to the limitations of replacing methane hydrates with CO2or CO2/N2mixtures,some reports have proposed that CO2or CO2/N2can be permanently sequestered in seabed sediments in the form of hydrates in recent years[6[21-23]].The main component of power plant flue gas are CO2and N2mixture.Considering the physical characteristics of CO2/N2mixture gas hydrate and the characteristics of CO2pure hydrate,the power plant flue gas can be directly sequestrated under seabed.Accordingly,the CO2separation process can be eliminated to save energy.In 2005,Seo[24]has investigated the formation process of CO2/N2mixture in porous media of silica gel,and the results indicated that the CO2concentration can be more than 96 mol%in the gas product after three cycles of hydrate formation and dissociation.The results also indicated that the feasibility of hydrate method for removing CO2from flue gas.Combined with the investigation results of CO2sequestration in sediments and porous media [25,26],CO2capture and storage from CO2/N2gas mixture by hydrate formation in sediments directly also can provide a potential solution of carbon emission and CO2sequestration.

The results of previous studies indicate CO2capture and storage in binary CO2/N2gas mixture by hydrate in sediments and sequestrated beneath the ocean floor is feasible.However,the fundamental experimental data on the formation kinetics of the binary CH4/N2gas hydrate in sediments is scarce.The basic reaction mechanism of the gas mixtures formation kinetics and CO2/N2separation process are not clear.In this work,CO2/N2mixed hydrate formation process in fine sediments with NaCl solution will be investigated in detail based on the research achievements.CO2and N2mixture gas hydrate formation rate in sediment pore water and the composition change in both vapor phase and hydrate phase will be the key issues to solve in this research.The results in this work can effectively improve the greenhouse effect caused by CO2emission and helps to avoid geological disasters such as submarine landslides.

2.Experimental

2.1.Apparatus and materials

Fig.1 illustrated a schematic representation of the experimental setup.The main components of the setup are two reaction vessels.The large reaction vessel is made of 316#stainless steel.The effective volume is 668 cm3.The other small reaction vessel is made of polyfluortetraethylene (PTFE)and the effective volume is 170 cm3.In addition to the above,it made up of a gas reservoir,a supercharging device,a vacuum pump,gas pipeline,and a temperature-controlled water bath.Meanwhile,a smart pressure transducer was employed to monitoring gas pressure data with a maximum range of 10 MPa.Its measurement error is ±0.025 MPa.The temperature of the gas phase in the crystallizer was measured using a T-type temperature sensor with a measurement accuracy of±0.1 K.Pressure and temperature data were monitored in real time via Agilent data acquisition(34970A).

The binary gas mixtures of high-purity CO2and N2were purchased by Guangzhou Puyuan Gas Plant.The initial gas molar ratio of CO2and N2was 50:50 and analyzed using a gas chromatograph for confirmation.

2.2.Procedures

Firstly,fine sediments collected from seabed of the northern South China Sea were cleaned,dried and sieved into different fractions.The particle diameters of the sediments used in these experiments were 150-250 μm,and the porosity of the sediment sample was 36.7%.Then,the sediments were soaked in the prepared 3.5 wt% NaCl salt water solution until the sediment pore spaces were fully saturated.The sediment and salt water solution were added to the PTFE vessel.The PTFE vessel with a total volume of～338 cm3was covered by a sieve(mesh opening 58 μm)in the top.Then,it was placed at the bottom of the large reaction vessel.Thus,gas can enter into the sediment pore spaces easily.Considering that the volume of the large reaction vessel was 668 cm3,and～330cm3volume was available for free gas to occupy.After that,the entire system was evacuated using the vacuum pump to exhaust extra air.Then,high pressure CO2/N2gas mixtures were injected until the desired pressure was achieved.The large reaction vessel was disconnected from the gas pipeline and isolated.The system was left for 1 h to maintain the equilibrium and the gas enter valve was closed.During this time,the temperature and pressure of the gas phase were recorded.Thus,the total amount of the gas mixture injected into the vessel can be calculated by the pressure(P),temperature(T)and gas compressibility factor(Z).Then,the temperature was maintained at 298 K for 48 h to dissolve a fraction of the gas mixture.The gas-phase temperature and pressure were recorded again.Thus,the molar ratio of the dissolved gas can be determined by the P-T condition at this moment.Simultaneously,the gas phase was sampled and analyzed by gas chromatography(GC).Then,the temperature of the water bath was set at a desired value and remained.Hydrate reaction started when the temperature achieved the set value.Over the whole hydrate formation process,the temperature of the reaction vessel was maintained constant at the set value.The compositions of the gas phase were analyzed every several hours by GC.The gas consumption amount of the binary gas mixtures can be calculated by the P-T value at each time point during the hydrate formation process.

3.Results and Discussions

Fig.1.Experimental setup for hydrate formation by the binary gas mixture of CO2/N2in fine sediments.V1-6:valves,T:temperature sensor,P:pressure sensor.

The three-phase hydrate-liquid-vapor (HLV)equilibrium of the binary CO2-N2gas mixture and water was investigated at different molar ratios of CO2and N2[27,28].Under the same temperature condition,the hydrate phase equilibrium pressure required to form pure CO2gas hydrate was much smaller than that required to form pure N2gas hydrate.The mixture gas hydrates formed over wide pressure and temperature ranges of 1.5-30 MPa and 272-282 K largely depend on the gas molar ratios and compositions.Under the same temperature condition,a binary CO2/N2gas mixture can form gas hydrates in moderate pressure,which is higher than that required for CO2gas hydrate and lower than that required for N2gas hydrate.The sand used in the experiment was from the South China Sea,and the particle size distribution was used to represent the natural condition of the seabed hydrate deposit area.There are also sands with clay and coarse particles in natural sediments.However,the effect of coarse-grained sand on hydrate formation can be negligible,while it is difficult to form hydrates in clay systems.Moreover,the sediment particle size almost did not affect the equilibrium condition when the diameter was ＞1 μm[29].The grain diameter of sediments used in these experiments was approximately 150-250 μm,and the effect of particle size on gas hydrate equilibrium can be ignored.Therefore,experiments were carried out at three temperatures(273,275,and 277 K)in this study.The reaction pressures were different under the same temperature condition according to gas compositions.To enhance gas hydrate formation rate,the experimental pressures were higher than the phase equilibria pressures of mixed gas hydrates at the three temperature conditions.

Pure N2is known to form structure II hydrate and it can be stabilized in the small cages(512)of structure II hydrate.However,for binary gas mixtures of CO2/N2,the hydrate structure can be structure I or II.CO2easily forms structure I hydrate,and N2also can enter into the large cages[30].The structure of gas mixture hydrate depending on the relative molar ratio of CO2and N2in the large and small cavities[31,32].Seo and Lee[33]studied the guest distribution and structure of the mixture gas hydrates of CO2and N2using X-ray diffraction and13C NMR spectroscopy.The results showed that for mixed(CO2+N2)hydrates with 3 mol%-20 mol% CO2,the hydrate structure formed was structure I.The hydrate structure of the binary mixture gas of CO2/N2was transformed to structure II when the molar ratio of CO2was reduced to 1 mol%.Based on these results,we concluded that the gas hydrate structure formed in our experiments was structure I.The conversion of water to hydrate could be calculated using the hydration number and gas consumption.

3.1.The conversion rate of water to hydrate

After the binary gas mixture was injected into the reaction vessel,is the initial molar ratio of CO2,andis the initial molar ratio of N2.The volume of free gas phase is 330 cm3.The pressure and temperature data were recorded by the data acquisition system.According to the GC analyze results and the molar ratios of CO2and N2at this time,the total mole of gas ngin the gas phase could be calculated using the improved Starling-Han-Benedict-Webb-Rubin(SHBWR)equation of gas state.

where P and T are the pressure and temperature of the gas phase,respectively；V is the effective volume of the gas phase,V=330 cm3；R is the universal gas constant,here R=8.314 J·mol-1·K-1；and Z is the compression factor.In these experiments,the compression factor of the binary gas mixture can be obtained using the improved SHBWR equation of state.

The initial molar quantities of CO2and N2(nCO2and nN2)could be calculated using the initial molar ratio.

After 48 h,a part of the dissolved gas entered into the sediment pore water.Therefore,the pressure and temperature of the gas phase decreased to P0and T0.The total mole of gas ng0at this moment also could be obtained using the improved SHBWR equation.Then,the integral molar quantity of CO2and N2in the gas phase can be estimated using the following equations:

The gas dissolved in sediment pore watercould be obtained using Eqs.(2)-(5).

When the gas hydrate formation started,the free gas phase was sampled and analyzed by GC every few hours.The molar fraction of CO2and N2at different times were defined asrespectively.Similarly,the molar quantity of CO2and N2could be calculated using the same procedure.The consumption amount of gas during hydrate formation of CO2and N2at any time could be calculated through molar quantity of gas at the initial time subtract to the molar quantity of gas at this time.We assumed that all the dissolved gases in sediment pore water participated in hydrate formation,and there was no free gas in the sediment pore water.

Table 1 shows the summary of experimental results under various pressure and temperature conditions.The gas hydrate conversion rate increased with increasing pressure under the same temperature condition for the multigas samples.In other words,a higher pressure or lower temperature can enhance gas hydrate formation.The gas hydrate conversion rate reached the maximum value under 7.8 MPa and 273 K conditions in our experiments.The correspondingly driving force was maximize and about 5.3 MPa.The increasing of driving force or subcooling degree can promote the formation of mixed gas hydrates in the pores of the sediment,increases the reaction rate,and enhance the hydrate conversion ratio.Pure CO2gas easily formed gas hydrates,and the gas hydrate conversion rate was higher than that of the binary gas mixture.Mixing N2in pure CO2gas made the gas hydrate formation conditions harsher and reduced the conversion rate of water to hydrate.

The morphology of hydrate formation in the sediments pores is influenced by the water and gas distribution and displacement during reaction process.Hydrate crystals is easily formed on the gas-liquid surface in the interior of the sediment porous structures when the pore spaces are filled with a mixture of gas and water.When the sediment pores are filled by liquid with dissolved gas,the hydrate is easily formed on the surface of the sediment grains[34-36].After reaction finished,hydrate surrounded and cemented sediment particles,as shown in our previous work[37].

The gas hydrate conversion curves over time are shown in Fig.2.The conversion of water into hydrates can be divided into two-stages.In first stage,the conversion rate is fast.Then,the conversion rate decreased because the hydrate formation process was limited by gas diffusion rate in the second stage [38,39].The general shapes of these curves agree with that behavior.The first stage involved a fast reaction period,and the gas hydrate conversion rate was greater in this stage.In thesecond stage,a slight amount of gas was consumed,and the gas hydrate formation rate decreased until the process was completed.The first stage lasted for several hours in experiments.It was determined by the temperature and pressure conditions together.A lower temperature and higher operating pressure indicate a shorter duration of the first stage.

Table 1 Gas hydrate conversion rate of binary CO2+N2mixtures in sediments at different pressures and temperatures

Fig.2.Gas hydrate conversion vs.time for different pressures and temperatures.

3.2.Changes in gas ratio in vapor and hydrate phase s

The equilibrium pressure of gas mixtures required for gas hydrate formation was higher than that required for pure CO2and lower than that required for pure N2under the same temperature condition.To complete the reaction process,the experiment period was extended to 50 h in every reaction.The gas volume sampled for every time was about 10 ml at atmosphere pressure.A total of 100 ml gas (about 0.004 mol)was sampled after reaction completed.The initial gas amount under different experimental conditions was more than 0.4 mol.The percentage of sampled gas was below 1%.Therefore,compared with the total gas amount in react cell,the amounts of sampled gas can be ignored.Furthermore,the total gas consumption amount of CO2and N2calculated in manuscript was more than 0.1 mol,the proportion of sampled gas amount was below 4%in gas consumption amount,and the amount of sample gas was already eliminated.

The typical change in CO2and N2molar fractions with time during the gas hydrate formation is shown in Fig.3.The initial ratio of CO2/N2feed gas was 50:50.The reaction cell was maintained at 298 K for 48 h after the gas mixture was injected.Therefore,a portion of the binary gas mixture was dissolved into the pore water in sediments.The solubility of N2in salt water was lower than that of CO2at a high pressure.When the solubilization process was completed,the molar quantity of CO2dissolved in sediment pore water was much higher than that of N2[40-42].Before the start of the gas hydrate formation,the molar fraction of CO2in the gas phase decreased because of the dissolution of CO2in sediment pore water,and that of the N2in the gas phase increased accordingly compared to the initial molar ratio of the two gases.

When the temperature reached the set value,the gas hydrate formation started.CO2easily formed gas hydrate than N2at the same experimental conditions.First,the sediment pore water formed gas hydrate crystal structure,and then the dissolved CO2in sediment pore water entered into the hydrate cage.The amount of CO2dissolved in sediment pore water was much more than that of N2.During the initial stage of gas hydrate formation,CO2gas hydrate was formed,and a large amount of dissolved CO2gas was consumed.Then,the N2in the gas phase entered into the sediments by diffusion and reacted with the sediment pore water to form gas hydrate.Therefore,the molar fraction of N2in the gas phase decreased,whereas that of CO2increased relatively compared to the molar ratio of the two gases in the beginning.

After the gas hydrate formation for 10 h,a large amount of dissolved CO2in the sediment pore water was consumed.Then,the CO2in the free gas phase diffused into sediments and entered into the gas hydrate cages to form CO2gas hydrates continually.At this stage,the amount of CO2in the gas hydrates was more than that of N2.Therefore,the molar fraction of CO2in the gas phase decreased,whereas that of N2increased until the reaction was completed.

The gas consumption amount of CO2and N2are shown in Fig.4.In the first 10 h,the CO2gas dissolved in sediment pore water solution was consumed significantly,and it was difficult for the N2gas to participate in the gas hydrate reaction.Therefore,CO2was the main gas hydrate formation gas,and the N2gas consumption can be ignored in this stage.After 10 h,the N2in the gas phase participated in the gas hydrate formation process along with CO2.After the completion of hydrate formation,the CO2consumption amount was much more than the N2consumption amount.

As shown in Fig.4,both the CO2and N2gas consumption amounts increased under lower temperature and higher pressure conditions.The increase in experimental pressure indicates a large reaction driving force in the gas hydrate formation process and much more gas consumption amount.Because the initial molar ratio of the binary CO2/N2gas mixture was 50:50,the CO2gas participated more in the gas hydrate formation phase than N2.Furthermore,this also indicates that CO2gas hydrate was easy to form under the same experimental conditions.

3.3.CO2and N2molar ratio in hydrate phase

Fig.3.Changes in the molar fractions of CO2and N2in the gas phase during gas hydrate formation.Sediment grain size:150-250 μm.

Fig.4.Gas consumption amount in gas hydrate formation.

Table 2 The molar ratios of CO2and N2in hydrate phase after the completion of reaction

The molar fractions of CO2and N2in the hydrate phase can be calculated using the gas consumption amount in gas hydrate formation reaction.Table 2 summarizes the CO2and N2gas amounts in the hydrate phase after the reaction completed.The molar fraction of CO2in the hydrate phase was～0.730 under 273 K and 7.8Mpa conditions.The content of CO2in the hydrate was greater than that of N2with the initial CO2/N2molar ratio of 50:50 at the corresponding P-T conditions.This is probably because CO2formed gas hydrate under much milder temperature and pressure conditions than N2.Under the same temperature condition,the increase in pressure made more CO2enter the hydrate phase and occupy more hydrate cages than N2.The experimental results proved the presence of a higher concentration of CO2gas in the hydrate phase and that CO2molecules plays a key role in stabilizing both the large and small cages[43].CO2was the critical factor in CO2/N2mixture gas hydrate formation in sediments.

Fig.5.CO2and N2gas consumption rates in the gas hydrate formation.(a)CO2gas consumption rate in hydrate formation.(b)N2gas consumption rate in hydrate formation.

3.4.Gas consumption rate

A large amount of gas participated in the hydrate cage structures in the first stage of hydrate formation process.The gas consumption rate was greater at this time.Gas hydrates were formed and accumulated.Then,the reaction gradually went into the second stage that was determined by gas diffusion ability.Then,the gas consumption rate reduced greatly,as shown in Fig.5.Notably,higher operating pressures and lower temperatures increased the gas consumption rate of the binary gas in our experiments.

4.Conclusions

In this work,the gas hydrate reaction process in fine sediments by a binary CO2/N2gas mixture(50:50 molar ratio)was investigated at different temperatures and pressures.In general,the gas hydrate formation of the binary CO2/N2gas mixture in sediments can be divided into two steps.In the first stage,the dissolved gas containing a large amount of CO2formed gas hydrates,and then gaseous N2participated in the gas hydrate reaction.Therefore,the molar fraction of N2in the gas phase decreased,whereas that of CO2increased.In the second stage,all the dissolved gas was consumed,and the gas in the free gas phase diffused into the sediment.The gas hydrate formation rate was evaluated by the diffusion of the gas to sediments and hydrates.The molar fraction of CO2in the gas phase decreased and that of N2increased until the reaction was completed.Therefore,the gas consumption rate was greater in the first stage than the second stage.Moreover,when comparing the final gas composition with the initial gas composition,the CO2concentration decreased.This indicates that CO2easily formed gas hydrates than N2under the same P-T conditions.The CO2content in the hydrate phase exceeded the initial CO2content in the vapor phase.

The investigation results indicated that it was feasible to capture CO2from CO2/N2gas mixtures.The research also can provide a theoretical basis for the utilization CO2/N2capture and sequestrated by hydrate in ocean sea sediments.