Exploitation of methane in the hydrate by use of carbon dioxide in the presence of sodium chloride

Petroleum Science, Nov 2009

The replacement process of CH4 from CH4 hydrate formed in NaCl solution by using pressurized CO2 was investigated with a self-designed device at temperatures of 271.05, 273.15 and 275.05 K and a constant pressure of 3.30 MPa. The mass fraction of the NaCl solution was either 0.5 wt% or 1.0 wt%. The effects of temperature and concentration of NaCl solution on the replacement process were investigated. Experimental results showed that high temperature was favorable to the replacement reaction but high NaCl concentration had a negative effect on the replacement process. Based on the experimental data, kinetic models of CH4 hydrate decomposition and CO2 hydrate formation in NaCl solution were established. The calculated activation energies suggested that both CH4 hydrate decomposition and CO2 hydrate formation are dominated by diffusion in the hydrate phase.

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Exploitation of methane in the hydrate by use of carbon dioxide in the presence of sodium chloride

Pet.Sci. Exploitation of methane in the hydrate by use of carbon dioxide in the presence of sodium chloride Li Zunzhao 0 Guo Xuqiang 0 Yang Lanying 0 Ma Xiaona 0 0 State Key Laboratory of Heavy Oil Processing, China University of Petroleum , Beijing 102249 , China The replacement process of CH4 from CH4 hydrate formed in NaCl solution by using pressurized CO2 was investigated with a self-designed device at temperatures of 271.05, 273.15 and 275.05 K and a constant pressure of 3.30 MPa. The mass fraction of the NaCl solution was either 0.5 wt% or 1.0 wt%. The effects of temperature and concentration of NaCl solution on the replacement process were investigated. Experimental results showed that high temperature was favorable to the replacement reaction but high NaCl concentration had a negative effect on the replacement process. Based on the experimental data, kinetic models of CH4 hydrate decomposition and CO2 hydrate formation in NaCl solution were established. The calculated activation energies suggested that both CH4 hydrate decomposition and CO2 hydrate formation are dominated by diffusion in the hydrate phase. CO2 hydrate; CH4 hydrate; guest molecule replacement; kinetic model 1 Introduction CH4 hydrates are found naturally in regions of permafrost and beneath the sea in outer continental margins. The total amount of methane in CH4 hydrates worldwide is estimated at 0.9×109-1.02×1010 tons of oil equivalent (Makogon et al, 2007) . There are three methods available for CH4 hydrate recovery including thermal treatment, depressurization and inhibitor addition (Sloan, 1998) . All these methods are aimed at promoting CH4 hydrate decomposition by external stimulation. There are some shortcomings in using the above methods for CH4 hydrate exploitation. For example, the problem with the thermal treatment is the heat lost to reservoir rock and water. For the depressurization method, the decomposition processes for CH4 recovery could lead to weakening the ocean floor (Lee and Holder, 2001) . Adding inhibitor into the gas hydrates reservoir would pollute the sea environment. Another method proposed for gas hydrate exploitation is the injection of CO2. The idea of exchanging CO2 for CH4 in gas hydrates was first advanced by Ohgaki et al (1996 ). With this method the recovery of natural gas from a gas hydrate is combined with CO2 sequestration. It offers two advantages: reducing CO2 emissions into atmosphere and assisting gas hydrate exploitation, thereby helping overcome the economic impediments of CO2 sequestration and CH4 hydrate dissociation. Due to the consumption of fossil fuels, the world CO2 emissions are expected to increase from 24 billion metric tons in 2001 to 39 billion metric tons in 2025 (Goel, 2006) . CO2 sequestration has become an urgent problem deserving more attention. The phase equilibrium of the CO2-CH4 binary system forming hydrates was investigated by Adisasmito et al (1991 ) and Ohgaki et al (1996 ). Experiment results show that the formation pressure of CO2 hydrate is lower than that of CH4 hydrate. Seo et al (2001) determined the two-phase equilibria of vapor and hydrate at the three different pressures 20, 26 and 35 bar. They reported that under lower pressure, the empty cavities of the hydrates are preferentially occupied by CO2 rather than CH4 and the replacement process may have good selectivity for CO2. Another mechanism for replacement process is that the exothermic heat of CO2 hydrate formation might induce the decomposition of CH4 hydrate. This mechanism based on the fact that the exothermic heat of CO2 hydrate formation is higher than that required for CH4 hydrate decomposition. Several researchers have investigated the replacement process for CH4 recovery from CH4 hydrate using pressurized CO2 on a laboratory scale. Ota et al (2005a) conducted a CO2 replacement process of CH4 gas from CH4 hydrate at a temperature range of 271.05-275.05 K and a pressure of 3.10-3.34 MPa. Jadhawar et al (2005 ) conducted replacement experiments in porous medium at laboratory scale and the results obtained indicated that the CH4 recovery rate in porous medium was higher than that in bulk conditions obtained by Hirohama et al (1996 ). The previous work concentrated on the replacement process with methane hydrates formed in pure water. Experiments of methane recovery from methane hydrates formed in NaCl solution have rarely been reported so far. As we all know, the natural gas hydrates stored in the seabed were formed in sea water. Although a salt-removing effect exists in the hydrate formation process, some NaCl is still attached to the hydrates and the porous media. So investigation of the kinetic process and the effect of NaCl on the replacement process are necessary for consideration of any future industrial production of methane from methane hydrates using CO . 2 For the first time, we conducted a series of CH4 replacement experiments of CH4 hydrate formed in NaCl solution by using pressurized CO2 at laboratory scale at isothermal conditions (271.05 K, 273.15 K and 275.05 K) under pressures just above the CH4 hydrate dissociation pressure (3.30 MPa). Based on the experimental data, a kinetic model was developed for CH4 hydrate decomposition and CO2 hydrate formation. Our objective was to investigate the effect of different factors on CH4 replacement from CH4 hydrates and provide a guide for possible future industrial exploitation of methane hydrates with CO2 sequestration. 2 Experimental 2.1 Experimental apparatus Fig. 1 is the schematic diagram of the experimental apparatus which consists of four major sections: an equilibrium cell equipped with a magnetic agitator, a sample supply system, a pressure-temperature measurement system and a composition analyzing system. 1 2 3 4 5 6 7 8 9 10 11 12 13 18 21 22 23 24 14 17 TT PT 15 16 19 20 25 1-sample connection; 2-pressure pump; 3, 5, 8, 9, 13, 14, 17, 18, 19, 21, 22, 24-valve; 4-inlet/outlet for liquid; 6-piston; 7-glass window; 10-equilibrium cell; 11-magnetic stirrer; 12-air bath; 15-temperature sensor; 16-pressure sensor; 17-data acquisition system; 20-CH4 gas cylinder; 23-vacuum pump; 25-CO2 gas cylinder Equilibrium cell The equilibrium cell, with an internal volume of 420 cm3, consists of a visual cell and a blind cell whose volume can be changed by a piston in it using a pressure pump. The pressure of the equilibrium cell can be maintained by changing the volume of the blind cell. The equilibrium cell was made of stainless steel, which allowed a maximum pressure of 20 MPa, and had a pair of glass windows on opposite side for observing gas hydrate and phase behavior. A magnetic stirrer was used to agitate fluids in the cell. Water or solution was fed into the cell by a plunger pump. Sample supply system The gas-sample charging system consists of CH4 and CO2 gas cylinders and a vacuum pump which was used to evacuate the cell. Pressure-temperature measurement system The equilibrium pressure was measured by using an online pressure sensor (JYB-KH) with an accuracy of about 0.01MPa. The temperature was controlled by a programmed t h e r m o - c o n t r o l l e r ( C W Y F - 1 ) a n d m e a s u r e d w i t h a temperature sensor (PT 100) with an accuracy of ±0.1K. Analyzing system for gas composition For a mixed hydrate system, the composition of the gas-phase in equilibrium at a given temperature and pressure was measured directly by a gas chromatograph (HP6890). As the amount of water in the gas phase is negligibly small in the present experimental conditions, the gas-phase composition was only analyzed for CO2 and CH4. The composition was determined by comparing the peak area ratio of the unknown sample with that of a known standard sample of similar concentration. Materials CH4 and CO2 gases used in this study were purchased from AP Beifen Gases Industry Co., Ltd, Beijing, China, with a certified purity of 99.99 vol%. NaCl was purchased from Beijing Shuanghuan Chemical Reagent Factory, China, with a certified purity 99.5 wt%. Distilled water was produced with a water distillation apparatus (SZ-93, Shanghai Yarong Biochemistry Instrument Factory, China). 2.2 Experimental procedures Formation of CH4 hydrates NaCl solution (0.5 wt% or 1.0 wt%) was added into the equilibrium cell at 273.2 K. CH4 gas was introduced into the cell from the CH4 cylinder. The cell was then pressurized up to 8.0 MPa, which is much higher than the three-phase equilibrium pressure of CH4 hydrate at the operating temperature which was listed in Table 2. Agitation in the cell was started to promote the CH4 hydrate forming. The gas phase pressure (8.0 MPa) was kept constant during the CH4 hydrate formation. R e p l a c e m e n t o f C H 4 f ro m C H 4 h y d r a t e u s i n g pressurized CO2 First, the piston in the blind cell was moved to its bottom to make the volume of the equilibrium cell minimum. The temperature of the air bath was set to 267.8 K, at which CH4 hydrate is very stable (Stern et al, 2001) . Then in order to purge the cell by CO2 gas, the CH4 gas in the cell was released gradually to 1.60 MPa which was slightly higher than the equilibrium pressure of CH4 hydrate at 267.8 K. Second, CO2 gas was introduced into the cell gradually to 3.00 MPa which was slightly lower than the saturation pressure of CO2 at 267.8 K and at the same time the volume of the blind cell was expanded to the maximum value (200 mL) by moving the piston to the top of the blind cell. The above two steps were repeated for three times to make sure that only a little CH4 gas (less than 0.075 mol%) remained in the gas phase. The temperature of the cell was gradually raised to the desired operating value (Table 1) and the pressure was also increased to the desired value (Table 1) by driving the piston in the blind cell. A small amount of gas phase was sampled with an injector for analysis of the initial composition of the gas phase by gas chromatography. The time for temperature rising was usually less than 60 minutes. The gas phase was sampled and analyzed as a function of time. Finally, after 100-150 hours passed, the hydrate mixture was decomposed by increasing the cell’s temperature to 298.15 K and the gas composition was analyzed. Table 1 lists the experimental conditions. The total amount (nendTotal) of the gas mixture in the gas phase was determined with PT equation of state (Patel and Teja, 1982) by using the experimental temperature, and the gas phase volume and the gas composition were measured after the CamHo4 uanntd(CniOCH24,hHy)dorafteCsHm4 itxrtauprpeewdaisndtehceomCHpo4shedy.drTahtee pinhiatisael was determined by Eq. 1. The initial amount (niCH4, H) of CO2 and the total amount (ni Total) of the gas mixture were also calculated with PT equation of state by using the experimental temperature, the gas phase volume and the gas composition which were measured at the beginning of the replacement reaction. The results showed that not all the H O molecules 2 can react with CH4 molecules to form CH4 hydrate and the average gas storage of CH4 hydrate is about 45.2 Sm3 of methane per m3 of pure hydrate. i nCH4 ,H i nCO2 ,G nend Total nCH4 % i n Total nCO2 % (1) (2) Where, nCH4% and nCO2% mean the mole fraction of CH4 and CO2, respectively. The volume of gas phase at the beginning of the replacement reaction, which was used to calculate the n Total in Eq. 2, was calculated from the volume of equilibrium cell, the dilatation of structure I hydrate, and the initial volume of the liquid solution or water. During the replacement process, the moles of each components in the gas phase and in the hydrate phase were calculated with the following equations. Where nTotal, G refers the total moles of CH4 and CO2 in gas phase. While nCH4, H and nco2 mean the moles of CH4 and CO2 respectively trapped in the hydrate phase. (3) (4) (5) (6) 3 Results and discussion Fig. 2 and Fig. 3 show the relationship of CH4 amount (nCH4, G) from CH4 hydrate decomposition, and CO2 amount (nCo2, H) trapped in CO2 hydrates, with time, respectively, at different temperatures. The amount of CH4 recovered from CH4 hydrate and that of CO2 formed to CO2 hydrate increased with time. The effect of temperature on the replacement efficiency is significant. Increasing the temperature can enhance the replacement rate. 0.009 4 Development of kinetic model In order to further study the replacement process, the hydrate decomposition and formation models suggested by Bishnoi and Natarajan (1996) were applied to correlate the obtained experimental data. The models were also used for describing the CH4 replacement process in a pure water system by Ota et al (2005a). In this work, the driving force for the CH4-CO2 replacement process was assumed to be proportional to the fugacity difference between the gas phase and the hydrate phase. The model for CH4 hydrate decomposition is given as follows: dnCH4 ,H dt 1 Fig. 4 and Fig. 5 show the effect of NaCl concentration on the replacement process. It can be seen that NaCl concentration did not influence the replacement rate nearly as much as the temperature did. And that at the same temperature, the lower the NaCl concentration, the higher the replacement rate, indicating that NaCl concentration is an influencing factor for the replacement process. Fig. 4 and Fig. 5 also show that the CH4 released from CH4 hydrate decomposition by replacement with CO2 is not equal to the amount of CO2 trapped in the hydrate phase, the former is larger than the later. Table 1 shows that the gas storage of CH4 hydrate did not reach the ideal storage (150-180 Sm3 of methane per m3 of pure hydrate) (Hao et al, 2008) and some absorbed water existed in the CH4 hydrate phase. One reason for the difference in CH4 and CO2 amounts might be that some CO2 molecules formed hydrate with the absorbed water in the CH4 hydrate phase or dissolved in the water phase. Another reason might be that the CH4 molecules encaged in the small cages are more stable (Ota et al, 2005b) than those in the large cages at low CH4 gas storage, hence the CO2 can replace the most of CH4 trapped in the large cages of methane hydrate and only a small part of CH4 in the small cages. The (7) (8) above two reasons may result in the difference between the CH4 amount from CH4 hydrate decomposition and the CO2 amount trapped in the hydrate phase. 0 20 40 60 80 100 120 140 Time, h Because the CH4 amount increased in the gas phase was equal to that of CH4 hydrate decomposition, Eq. 7 can be expressed as the following form: dnCH4 ,H dnCH4 ,G kDec A( fCH4 ,H dt dt Where t is the reaction time (s), f is the fugacity (MPa), kDec is the overall rate constant of the CH4 hydrate decomposition (mol-1m-2MPa-1), A is the surface areas (m2) between the gas phase and the hydrate phase, which can be calculated from the diameter of the equilibrium cell. kDec consists of kDec, R and kDec, D. The former (kDec, R) is the rate constant of CH4 hydrate decomposition reaction and the latter (kDec, D) is the rate constant of mass transfer of CH4 in the hydrate phase. The model for CO2 hydrate formation during the replacement was similarly given by: (9) dnCO2 ,H dt 1 kForm kForm A( fCO2 ,G fCO2 ,H ) 1 kForm,D 1 kForm,R (10) (11) Where kForm is the overall rate constant of the CO2 hydrate formation (mol·s-1·m-2·MPa-1), which also consists of two rate constants. In these models, fCH4, G and fCO2, G are calculated with PT equation of state (Patel and Teja, 1982) for a certain elapsed time using the experimental temperature, pressure and gas composition of the gas phase. The fugacity in the hydrate phase ( fCH4, H and fCO2, H) was calculated with the Chen-Guo hydrate model (Chen and Guo, 1998) using the experimental conditions. The kDec and kForm were determined by minimizing the sum of squares of differences between the experimental values and those calculated from the proposed models. The calculated results and deviations are listed in Table 3. Fig. 6 shows the relationship of lnkDec and lnkForm with temperature. The activation energies for CH4 hydrate decomposition and CO2 hydrate formation were calculated from the slope of the Arrhenius plot shown in Fig. 6. The measured activation energies for CH4 hydrate decomposition and CO2 hydrate formation were 52.01 kJ/mol and 308.20 kJ/ mol, respectively. In this work, the activation energy of CH4 hydrate decomposition was calculated to be 52.01kJ/mol. Ota et al (2005a ) investigated the kinetics of methane recovery from (a) (b) methane hydrate formed in pure water system by use of CO2 and obtained the activation energy for the CH4 hydrate decomposition as 14.5 kJ/mol. Li et al (2007 ) conducted the replacement process of methane from methane hydrate formed in sodium dodecyl sulfate (SDS) system and obtained an activation energy for the CH4 hydrate decomposition as 28.8 kJ/mol. The reason for great difference between the activation energies of CH4 hydrate decomposition reported by the above two group researchers and that obtained in this study might be that the mass transfer of CH4 gas in hydrate phase formed in NaCl system was more difficult than that in pure water system or SDS system. As mentioned above, the absorbed water existed in CH4 hydrate formed in NaCl system, making it difficult for CO2 diffusion in the hydrate phase. The activation energy for molecular diffusion in the solid phase has been reported often, for instance, the activation energy for H2O molecular diffusion in ice was 70 kJ/mol (Livingston et al, 1997) . In addition, Wang et al (2002 ) measured the kinetics of CH4 hydrate formation from neutron diffraction and reported that the activation energy for CH4 diffusion in the hydrate phase was 61.5 kJ/mol. From the above results, kDec, D in Eq.8 seem to be the controlling factor in the CH4-CO2 replacement process. The activation energy for CO2 hydrate formation was determined to be 308.20 kJ/mol in this work, which was larger than that for CH4 hydrate decomposition (52.01kJ/ mol). Ota et al (2005a ) obtained the activation energy for CO2 hydrate formation as 73.3kJ/mol and concluded that the CO2 diffusion in hydrate controlled the process of replacement. The activation energy for CO2 hydrate formation obtained in this study was very much larger than the value measured by Ota et al (2005a). We suggested that the CO2 hydrate formation may involve the following processes: As lots of absorbed water surrounded the CH4 hydrates, CO2 had to dissolve in the water phase and some CO2 contact with the CH4 hydrates to replace CH4 from CH4 hydrate. This process may need more reaction energy. So the diffusion for CO2 in hydrate (kFrom, D) in Eq.11 seems to be the controlling factor in the replacement process. 5 Conclusion The replacement of CH4 from CH4 hydrate formed in NaCl solution by using pressurized CO2 was investigated. It was found that the amount of CH4 released from CH4 hydrate and that of CO2 trapped in the hydrate increased with time. The higher the temperature, the more rapid the replacement reaction . The experimental data showed that NaCl has a negative effect on the replacement process. Kinetic models for CH4 hydrate decomposition and CO2 hydrate formation were developed by hypothesizing that the fugacity difference between the gas phase and the hydrate phase was the driving force for the CH4-CO2 replacement process. The experimental data and activation energies indicated that both CH4 hydrate decomposition and the CO2 hydrate formation were dominated by diffusion in the hydrate phase. Acknowledgements The financial support received from the National N a t u r a l S c i e n c e F o u n d a t i o n o f C h i n a ( 2 0 4 7 6 0 5 8 , 20676146) and Ministry of Science and Technology of China (2006AA09A208, 2009CB219504) and Specialized Research Fund for the Doctoral Program of Higher Education (20070425001) are gratefully acknowledged. Nomenclatures A fCH4, H fCH4, G fCO2, G fCO2, H kDec kDec, D kDec, R kForm kForm, R Surface area between gas phase and hydrate phase, m2 Fugacity of CH4 in hydrate phase, MPa Fugacity of CH4 in gas phase, MPa Fugacity of CO2 in gas phase, MPa Fugacity of CO2 in hydrate phase, MPa Overall rate constant of the decomposition, mol·s-1·m-2·MPa-1 Rate constant of mass transfer of CH4 in hydrate phase, mol·s-1·m-2·MPa-1 Rate constant of decomposition reaction of CH4 in hydrate phase, mol·s-1·m-2·MPa-1 Overall rate constant of formation, mol·s-1·m-2·MPa-1 Rate constant of formation of CO2 in hydrate phase, mol·s-1·m-2·MPa-1 Initial amount of CH4 trapped in hydrate phase, mol Initial amount of CO2 in gas phase, mol Mole fraction of CH4 in gas phase Mole fraction of CO2 in gas phase Total amount of CH4 and CO2 in gas phase, mol Temperature, K Reaction time, hour Superscripts i Initial stage of the reaction Subscripts D Dec Form G H R Diffusion of molecule Decomposition reaction Formation reaction Gas phase Hydrate phase Reaction Adisasmito S , Frank R J and Sloan E D. 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Zunzhao Li, Xuqiang Guo, Lanying Yang, Xiaona Ma. Exploitation of methane in the hydrate by use of carbon dioxide in the presence of sodium chloride, Petroleum Science, 2009, 426-432, DOI: 10.1007/s12182-009-0065-1