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Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom, July 17-21, 2011. MOLECULAR DYNAMICS STUDIES OF THE STABILITY OF CO2 AND CH4 HYDRATES IN THE PRESENCE OF UNDERSATURATED FLUIDS Srinath Velaga1,2, Venkata Vedam1,2, Brian J. Anderson1,2* 1 National Energy Technology Laboratory Morgantown, WV-26507, USA 2 Department of Chemical Engineering West Virginia University Morgantown, WV-26505, USA ABSTRACT Methane hydrates are globally distributed in sediments along the continental margins and potentially contain more energy than all fossil fuel reserves. However, methane is also a potential greenhouse gas which could play a major role in global climate change. Accurately characterizing the stability of methane and CO2 hydrates in water can help us understand their effects on earth’s environment and also fesibilty of long term CO2 sequestration in the sediments under the ocean floor. Hydrate stability can be better predicted by understanding the phenomena related to hydrate dissolution in water. Under hydrate stability conditions, the concentration difference between the hydrate and water phases of hydrate-forming gases should be an important factor affecting hydrate stability as oceanic hydrates are exposed to under-saturated sea water. In this work, the dissolution of methane and CO2 hydrates have been studied and compared to one another in the presence of water using molecular dynamics simulations. The average lattice constant for structure I and II hydrate was calculated for two different potentials of methane OPLS and GROMACS and compare to the experimental value for validation. Methane hydrate dissociation in the presence of water was also studied using these two potentials in addition to the Anderson et al. model. The Harris and Yung model for CO2 as well as our own ab initio-derived CO2 potential were used in the simulations of CO2 hydrate in the study. In order to provide comparison of the CO2 potentials, the reference chemical potentials, densities of pure CO2 liquid and vapor, and solubility of CO2 in water were calculated using each of the potential models. Keywords: gas hydrates, stability, molecular dynamics, dissolution NOMENCLATURE ε k σ Characteristic energy [kcal/mol] Boltzmans constant [J/K] Collision diameter [Å] ∆ Reference chemical potential [J/mol] ∆ Reference enthalpy [J/mol] INTRODUCTION Large deposits of CH4 hydrates are stored in the permafrost and on the continental margins; these could be potentially a source of energy if recovered in an efficient way[1]. However, methane is also a potent green house gas so the destabilization of methane gas hydrates can affect the ocean environment and geologic column along * with the atmosphere[2]. Additionally, the idea of sequestrating the carbon dioxide in the oceanic sediments to hold the increase in green house gas in the atmosphere has been proposed[3]. Therefore knowing the stability of methane and CO2 hydrates in presence of water will help in predicting their effects on earth’s environment and the feasibility of long term storage of CO2 as hydrates in the oceanic sediments. The stability of gas hydrates depends not only on the temperature and pressure but also, the composition of both water and gas. Stability of gas hydrates in sea water is largely influenced by two phenomena dissociation and dissolution[4]. The process of decomposition of hydrate caused due to the change in temperature and pressure outside the hydrate stability field Corresponding author: Phone: +1 304 293 9334 Fax: +1 304 293 4139 E-mail: Brian. [email protected] (HSF) is called dissociation[4]. Alternately, gas hydrates can be decomposed when there exists an under-saturated liquid water phase surrounding the hydrates within the HSF pressure and temperature. This process can be called dissolution, as the hydrate dissolves in to water and gas and thus the saturated concentrations are reestablished[4-6]. Rehder et al.[6] first performed in situ experimental studies to measure the dissolution rates of pure synthetic samples of CH4 and CO2 hydrates directly on the sea floor at pressure and temperature conditions which are well within hydrate stability field (HSF). They proposed a diffusive boundary layer model for the dissolution process and the kinetics of which is a mass transfer controlled. They concluded this because of the finding that the ratio of the dissolution rates of CH4 and CO2 are similar to their solubilities ratio. Hester et al.[7] conducted in situ experiment on two distinct natural samples for the dissolution studies, unlike those done by Rehder et al.[6] Their analysis gave similar results as those of Rehder et al. and concluded that the dissolution is mass transfer controlled. Nihous and Masutani[8] proposed an elementary model on the dissolution of pure hydrate in under-saturated sea water. They applied this model to the experimental data of Rehder et al. on dissolution rates and suggested that the concentration of the hydrate guest species at the interface between the desorption film and diffusive boundary layer may be much lower than ambient solubility. Lapham et al.[9] measured the in situ pore water methane concentrations and calculated the dissolution rates and these dissolution rates are significantly less than dissolution rates predicted for methane undersaturated pore-water in direct contact with pure methane gas hydrate if equilibrium methane concentrations exist immediately adjacent to the hydrate surface. Sea floor methane hydrate outcrops have been found to show considerably longer life times than the theoretical predictions[10]. An understanding of the dissolution of gas hydrates in contact with under-saturated water is important to determine the long-term stability of gas hydrate reservoirs. The concentration difference of the hydrate forming gas between the hydrate and water phases have a key role in the stability of the hydrate. Molecular dynamics (MD) computer simulations can be used for studying the stability of hydrates. Baez and Clancy[11] carried out MD simulations to study the dissolution of spherical methane hydrate crystal in presence of water using non-polarizable SPC/E[12] model for water at 270 K and 40 bar, where the hydrate crystallite of radius 12 Å had around 250 molecules of water and 32 molecules of methane in the hydrate phase. They found that the melting occurred in a stepwise manner and the hydrate decay was governed by the breakup of partial cages at the interface. They also noted that the size of the crystallite, initially or during the dissociation, did not affect the rate of hydrate break up. English et al.[13] also studied dissolution of spherical hydrate crystallites in water at 277 K and 68 bar using TIP4P-FQ[14] water model. They found that the crystallites dissociate within 400 ps. The authors observed that the dissolution rates were not affected by the size of the crystallite and by the methane composition of the hydrate phase between 80100%, but in contrast the empty hydrate clusters were found to dissolve rapidly. Although the liquid phase considered is unsaturated with respect to methane, hydrate dissolution is supposed to be a slow process and cannot occur in very short timescales as those observed by these authors. So, it is more likely that the dissolution occurred because the crystallites considered were subcritical in size. The critical size of a hydrate cluster for nucleation has been estimated by Radhakrishnan et al.[15] to be in between 10-14 Å in a linear dimension. So, the size of the hydrate phase has to be more than the critical size of nucleation to remain stable and to have a continuous growth. Myshakin et al.[16] performed molecular dynamics studies on methane hydrate dissociation at 277 K and 68 bar. They observed that the dissociation rate depends on the hydration number and found that a decrease in cage occupancy from 100% to 95% causes a greater change in the dissociation rate than for a decrease from 95% to 85%, and also noted that the presence of empty cages destabilizes the hydrate lattice and stimulates the dissociation process. There have been very few studies on CO2 hydrate using MD simulations. Hirai et al.[17] studied the stability of CO2 hydrate using MD simulations using an inter atomic potential function based on a model given by Kumagai et al.[18] for both CO2 and water. They found that it is unstable compared to both empty and Argon clathrate hydrates. This was attributed to the destabilizing effect caused by the repulsive force acting between the O atoms of CO2 and O atom in H2O on the lattice structure. Kvamme et al.[19] al. performed MD simulations to study the CO2 hydrate-water water interface at conditions within the hydrate stability zone using the SPC/E[12] SPC/E water model. They evaluated the steady state interface thickness using different analyses like the hydrogen density profiles and radial distribution functions. The objective of this work is to study and compare the dissolution behavior of methane and CO2 hydrates and understand the importance of gas concentration in driving the hydrate dissolution process. Also, the simulations were performed to have the comparison of the CO2 potentials. SIMULATION DETAILS All the simulations in this work were done in an NPT ensemble. In this study, the very popular TIP4P[20] force field has been used for water in both methane hydrate/water and CO2 hydrate/water simulations. Three different potentials namely OPLS[21],, GROMACS GROMACS[22] and the Anderson et al.[23] model have been used for methane in order to perform a comparison among them with respect to the dissociation properties of hydrates. The Harris and Yung[24] potential as well as our own ab initio-derived derived CO2 potential were used for CO2 in the simulations. The charges and the Lennard-Jones nes parameters of methane or CO2 for each of the potentials have been taken from the corresponding references provided. Periodic boundary conditions were used in all the directions for all the simulations. The LorentzLorentz Berthelot[25] combination rules were used to calculate the Lennard-Jones[25] parameters between the water and gas molecules. The time step of 1fs was used and the cutoff radius for the LJ interactions was chosen to be 9 Å for all the systems. Ewald summation[26] was used to account for the long range electrostatic interactions. The temperature and pressure was controlled by Berendsen thermostat and barostat respectively for dissociation and dissolution simulations. The GROMACS package was used to perform the MD simulations and Visual molecular dynamics (VMD) was used to view the simulations. METHODOLOGY Hydrate crystals for both CO2 and CH4 hydrates were constructed using the programs in GROMACS package based on thee lattice structure of unit cell of the hydrate for structure I (sI) and structure II (sII). To study the dissociation of hydrate in the presence of water, a 32 unit cell (4×4×2) simulation box of sI methane hydrate was constructed and a cell of 6917 water molecules have been appended on one side of z-direction z making the simulation box of 4.7×4.7×7.2 nm as shown in Figure 1. The simulations were performed at a pressure of 1 bar and different temperatures from 250 K to a time where it dissociates completely. CH4 hydrate dissociation in the presence of water was studied using the OPLS, GROMACS and Anderson et al. potential models. The dissociation process was studied based on the number of gas molecules that moved out of hydrate in to the water phase. To validate the methane OPLS and GROMACS potentials used for dissociation, simulations were performed to calculate the lattice constants on an 8 unit cell (2×2×2) simulation box each of sI and sII for 200 ps at a constant pressure of 1 bar and different temperatures res ranging from 250 K K-400 K. For the dissolution studies, a 64-unit 64 cell (4×4×4) simulation box of sI hydrate was constructed for both methane and CO2 and a cell of 6917 water molecules has been appended to it to make it a simulation cell of dimensions 4.7×4.7×14 4. nm as shown in the Figure 2. The hydrate phase has 512 gas molecules (CO2 or CH4 in the corresponding hydrates) and 2944 water molecules. It was observed by the initial simulations that a 32 unit cell hydrate phase dissolved in 5ns compared to a 64 unit cell hydrate which was stable even after 20 ns in same amount of water. This might be because of the sub critical size of 32 unit cell hydrate which has 24 Å in the z-direction, direction, compare to the critical size of nucleation estimated by classical classica theory as 32 Å[27]. Figure 1. 2-D D view of a box with 32 unit cell (4×4×2) of sI CH4 hydrate appended to 6917 water molecules in z-direction. Figure 2. Initial configuration of a simulation cell with 64 unit cell box of sI CO2 hydrate appended with 6917 water molecules in z-direction. Different models of the simulation cells have been constructed varying the concentration of guest gas in the hydrate phase se (cage occupancy) and in liquid phase (under-saturation). saturation). The effect of cage occupancy was studied by varying the small cage occupancy at 100%, 75%, 50%, 25% and 0% for both CO2 and CH4 hydrates. The effect of water phase undersaturation was studied by varying arying the gas saturation in the liquid phase at 100%, 75%, 50% and 0% for both 0% and 100% cage occupancy of small cages. The details of all different models constructed are listed in Table 1. Simulations were carried out for longer timescales of 36 ns. The Harris and Yung[24] potentials were used for CO2 hydrate simulations. Table 1.. Summary of the concentration of guest gas in hydrate and liquid phasess for initial simulation box. Cage CO2 molecules CH4 molecules occupancy Hydrate Liquid Hydrate Liquid (small phase Phase phase Phase cages) 100% 512 0 512 0 75% 480 0 480 0 50% 448 0 449 0 25% 416 0 416 0 0% 384 0 384 0 Varying the concentration in liquid phase 100% small cage occupancy levels of saturation 100% 512 52 512 9 75% 512 39 512 7 50% 512 26 512 5 0% 512 0 512 0 0% small cage occupancy 100% 384 52 384 9 75% 384 39 384 7 50% 384 26 384 5 0% 384 0 384 0 In order to have comparison of the CO2 potentials, Harris and Yung and ab initio potentials as given in Table 2, simulations imulations were performed at constant temperature and pressure to obtain the density of pure CO2 and solubility in water at different temperatures. temperat Density simulations were performed on 1000 pure CO2 (4.5Å×4.5Å×4.5Å) molecules and the solubility calculations with 512 CO2 molecules appended with 6917 H2O molecules. The temperature was controlled by Nose-Hoover Hoover thermostat thermostat[28-29] and the pressure by Parrinello-Rahman Rahman[30] barostat. Table 2. CO2 Intermolecular potentials used in simulations. Bold atoms indicate specific atom-atom atom interactions. L-J 6-12 This model & Yung Harris& ε/k (K) ε/k (K) σ(Å) Charge σ(Å) O2C – CO2 76.765 3.595 28.129 2.757 OCO –OCO 56.414 2.975 80.507 3.033 CO2 0.652 CO2 -0.326 RESULTS AND DISCUSSIONS A time averaged value of lattice constant was calculated after the equilibrium is reached for different temperatures. The he plot of average lattice constant with respect to temperature for both the potentials of methane is shown in the Figure 3 for sI and sII. The lattice constant increases with temperature and this effect was observed till the hydrate dissociated completely.. Lattice constant of sI methane hydrate at 273 K and 1 bar was noted from Figure 3aa to be 1.187 nm for OPLS and 1.189 nm for GROMACS potential and was found to be comparable to the experimental value[1] value of 1.2 nm. Lattice constant of structure II methane hydrate at the same conditions were noted to be 1.68 nm for OPLS and 1.73 nm for GROMACS potential, which were also comparable to the experimental value[1] of 1.73 nm. This validates the potentials considered for methane. Dissociation At 250 K and 1 bar, the hydrate was found to be stable for the entire simulation run for 20 ns. To see the effect of pressure, simulations were also performed at temperature of 250 K and different pressures between 1 bar and 50. It was found that the dissociation ociation rates of the hydrate were not affected by a change in the pressure. At 255 K and 1 bar, the hydrate did not completely dissociate till 20 ns, but was found to be unstable and had a higher dissociation rate compared to that of 250 K. Number density of CH4 molecules Lattice constant (nm) 1.210 opls gmx 1.205 1.200 1.195 1.190 1.185 (a) 270 290 310 330 2ns 3ns 4ns 5ns 0 1.180 250 16 14 12 10 8 6 4 2 0 350 2 4 z- parameter of the cell (nm) 6 370 Temperature(K) Lattice constant (nm) 1.740 opls gmx 1.735 1.730 1.725 1.720 1.715 Figure 4. Density plot of CH4 molecules and a snapshot of sI CH4-H2O system dissociated in 5 ns at 270 K. It is clear from the snap shot that the hydrate is completely dissociated. 1.710 (b) 1.705 1.700 250 270 290 310 330 Temperature (K) 350 370 Figure 3. Effect of temperature on lattice constant on (a) sI and (b) sII for CH4 hydrate. gmx-Gromacs potential, opls –OPLS potential. A further increase in temperature to 260 K at same pressure caused the hydrate to completely dissociate within 20 ns. Hydrate at 265 K dissociated in 15 ns, while it dissociated in 5 ns at 270 K. Increase in temperature has fastened the dissociation rate. The density plot and corresponding snapshot of the system of the simulation at 270 K and 1 bar is shown in Figure 4. The dissociation temperature of methane hydrate at 1 bar was noted to be 260 K. This was for the Anderson et al. model of methane. These simulations were also done using the other two potentials for methane. Similar trends of dissociation rates of hydrates were observed for all the three potentials with changes in temperature and pressure. The dissociation temperature was noted to be 260 K for OPLS potential and 285 K for GROMACS potential. Hydrate with GROMACS potential for methane was found to withstand higher temperatures compared to OPLS and Anderson et al. model. The GROMACS intermolecular potential for methane seemed to be attractive for molecules in a hydrate compared to other two potentials. Anderson et al.[23] model was used for methane in all the simulations done for dissolution of hydrates in water in this work. Dissolution Simulations on methane and CO2 hydrates to study their dissolution phenomenon were performed at a chosen temperature and pressure conditions 275 K and 50 bar which are within the hydrate stability zone for both the hydrates. Different combinations of temperature and pressure conditions were tried before choosing these conditions. The temperature was chosen so that it is within the hydrate stability zone but not too far from the hydrate dissociation point at the chosen pressure, so that considerable amount of dissolution is observed with both methane and CO2 hydrates. Hydrate dissolution was not found to be sensitive to change in pressure between 2050 bar at 275 K. A pressure of 50 bar was chosen because it is theoretically more stable pressure for 275 K. Effect of cage occupancy The effect of cage occupancies on the dissolution rates were studied by varying the percentage occupancy of small cages as given in Table 1. A high amount of dissolution was found to occur within the first 10 ns in the case of CO2 hydrate and a slow decrease in the overall rate of dissolution was observed thereafter and for methane hydrate it increases very slowly as given in Figure 5. From Figure 6, it is evident that the amount of dissolution in the case of methane hydrate is lower compared to that of CO2 hydrates, Number of gas molecules dissolved in the water phase 200 150 100 CH4 CO2 50 0 0 20 40 Time (ns) 60 80 Figure 5. Dissolution rate of CO2 and CH4 with all cavities completely occupied and water phase completely unsaturated at the start of the simulation. On the other hand, CO2 hydrate was found to dissolve faster at 100% cage occupancy compared to hydrate at other occupancies. The dissolution rate was found to decrease with decrease in cage occupancy till 0%. So, these results imply a higher stability of CO2 hydrate at lower cage occupancies. The symmetrical structure of the CO2 hydrate at 0% cage occupancy could be an added advantage for the hydrate to be much stable compared to that at other occupancies. This shows that CO2 hydrate is stable at lower cage occupancies close to 0%. Water phase undersaturation The solubility of CH4 and CO2 in water was estimated at 275 K and 50 bar for 20 ns to construct the under-saturation of the liquid phase. 180 Number of gas molceules dissolved in the liquid phase in 35 ns by a factor of 10 or greater. It was observed that the amount of dissolution of methane hydrate is lower at 100 % cage occupancy and increased with decrease in occupancy till 25%. But methane hydrate was found to be very stable at 0% occupancy. A relatively higher stability or lesser dissolution of hydrate at 100% or 0% cage occupancy over hydrate at other occupancies can be explained by their lack of defects in the periodic crystal structure. At these occupancies, the hydrate has a more symmetrical crystal structure compared to other occupancies that introduce crystal defects. This effect is observed more in the case of methane hydrate than CO2 hydrate as the amount of methane hydrate dissolution is also very low unlike the dissolution in the case of CO2 hydrates. The regular increase in the amount of methane hydrate dissolution with decrease in cage occupancy shows that methane hydrate is more stable at cage occupancy close to 100%. 160 CO2 140 CH4 120 100 80 60 40 20 0 100% 75% 50% 25% 0% Percentage of cage occupancy Figure 6. Comparing the effect of small cage occupancy on the dissolution of CH4 and CO2 hydrates in water. It was found from the density plots that 9 molecules of methane completely dissolved in to water phase and 52 molecules of CO2 in 6917 water molecules. The corresponding molefractions are 0.001322 and 0.007461, where as the experimental values are 0.002334[16] and 0.01739[31] respectively. Methane hydrate with 100% cage occupancy in hydrate and 100% gas saturation in liquid phase was found to be very stable. As, it has already been seen that the methane hydrate is very stable at 100% cage occupancy and 0% gas saturation, it is expected that it should be much more stable at 100% gas saturation. But, it was interesting to observe that the number of methane molecules in the hydrate phase was found to increase past the number that was present initially. In fact, this was observed even with the simulation at 75% gas saturation. This can be seen from the plot in Figure 7 where there are times at which the curve goes above the black solid line in the case of 100% and 75% gas saturation. This might have occurred because there is a possibility of the methane molecules already present in the water phase moving in to the hydrate phase, which again is an indication that a hydrate growth is possible. At 0% cage occupancy, the amount of dissolution was found to show a similar trend with respect to percentage gas undersaturation. But, the amount of dissolution of methane molecules lowered by a small amount when compared to that at 100% cage occupancy. So, it seems that the dissolution is driven by a difference between methane concentration and its solubility in water phase. The concentration of the guest species definitely plays an important role in providing the necessary concentration gradient as noted by Nihous and Masutani[8] based on their model. This can be Table 3. Effect of gas saturation on hydrate dissolution Number of gas molecules that moved in to the water phase averaged over last 4 ns gas saturation Number of gas molecules initially present in the liquid phase 100% small cage occupancy 0% small cage occupancy CH4 CO2 CH4 CO2 CH4 100% 1 145 -2* 88 75% 4 149 4 50% 11 154 0% 10 162 Total number of gas molecules in the water phase averaged over last 4 ns of simulation 100% small cage occupancy 0% small cage occupancy CO2 CH4 CO2 CH4 CO2 9 52 10 197 7 140 96 7 39 11 188 11 135 7 101 5 26 15 180 12 127 8 111 0 0 11 162 8 111 * seen from the Table 3. In the case of CO2 hydrate, a high amount of dissolution was found to occur within the first 10 ns followed by a slow increase in the number of CO2 molecules moving in to the water phase and a steady decrease in the dissolution rate. As the level of gas saturation decreased from 100% to 0%, the number of CO2 molecules moving in to the liquid phase increased, but the increase was not proportional to the decrease in the level of undersaturation. At 100% cage occupancy, the number of CO2 molecules in the water phase was found to be more than 3 times the number of molecules required to saturate the water phase as per the value of its solubility that has been obtained. Number of CH4 molecules in hydrate phase 516 514 512 510 508 506 504 100% 75% 50% 0% 502 500 498 0 10000 20000 Time (ps) 30000 Figure 7. Number of methane molecules in the hydrate phase during the simulation, the initial number of molecules is marked by a black solid line. Number of CO2 molecules moving in to the liquid phase negative sign indicates the opposite direction of the movement. 180 160 140 120 100 80 60 40 20 0 100% 75% 50% 0% 0 10000 20000 Time (ps) 30000 Figure 8. Number of CO2 molecules moving in to liquid phase during the simulation at 100% cage occupancy It has already been seen that CO2 hydrate is stable at 0% cage occupancy. So, the effect of water phase under-saturation has also been studied at 0% small cage occupancy and the results are tabulated as shown in Table 3. At 0% small cage occupancy, the number of CO2 molecules moving in to the liquid phase was found to display a similar trend with the change in percentage gas saturation in the water phase. But, the amount of dissolution was found to be much lower at 0% cage occupancy than at 100% occupancy in all different cases of water phase under-saturation. Even in the case of 0% small cage occupancy, the total number of CO2 molecules in the water phase at the end of each simulation was found to be much higher than that required to saturate the water phase. However, the reason for the increase in solubility of CO2 in water in the presence of a hydrate is not clearly understood. This exceptional behavior of high CO2 hydrate dissolution might be because of the weak intermolecular potentials given by Harris and Yung for CO2 in hydrate. Density (Kg/m3) 1100 bar the ab initio potential predicted as in liquid phase, where as it should be in vapor phase. 1200 1000 Density (kg/m3) Comparison of CO2 potentials Using the Harris and Yung potentials, reference chemical potentials were calculated with the van der Waals and Platteeuw[32] model frame work. The reference parameters obtained were ∆ = 178.4 3 J/mol and ∆ = 95.8 12 J/mol.[33] for CO2 hydrates, where as the reference parameters obtained using our own ab initio-derived CO2 potentials were ∆ = 1204 3 J/mol and ∆ = 1190 12 J/mol. The reference parameters obtained using Harris and Yung potentials are well outside the range obtained by earlier researchers either numerically or experimentally given in Table 1 of Anderson et al.[23] for structure I hydrate. This shows the inability of the Harris and Yung potentials to accurately model carbon dioxide hydrates using the van der Waals and Platteeuw[32] model frame work. When used with the ab initio derived potentials the reference potentials were well in the range and were also able to accurately calculate the phase equilibrium and cage occupancy of CO2 hydrate. The average density was calculated after the equilibrium was reached. The densities obtained are given in Figure 9 and Figure 10 for ab initio potential and Harris and Yung potentials respectively. When compared to the experimental 800 50 bar H&Y 50 bar 100 bar-H&Y 100 bar200 bar H&Y 200 bar500 bar H&Y 600 400 200 0 255 265 275 285 Temperature(K) 295 305 Figure 10. Densities obtained using Harris and Yung potential for pure CO2. Experimental-solid lines, Harris and Yung[24]-points Simulations were carried out to calculate the solubility of the carbon dioxide at a temperature and pressure. The simulation box contains 512 CO2 molecules and 6917 H2O molecules appended to it. The simulation box was allowed to equilibrate in NVT ensemble for 15 ps and then in NPT ensemble for 12 ns. The solubilites were obtained by averaging over the last 4ns using density plots. The solubilities obtained using ab initio CO2 potentials are compared to experimental data[35] and shown in Table 4. The snapshot and density plot of CO2-water system at 275K and 20 bar is given in Figure 11. 900 50 bar 50 bar100 bar 100 bar200 bar 200 bar500 bar 500 bar- 700 500 300 100 255 265 275 285 295 Temperature (K) Pressure (bar) 20 Simulation. (g CO2/kg H2O) 51.24 Exp[35] (g CO2/kg H2O) 55.410 274 275 20 54.084 53.533 275 50 59.789 81.678 280 50 73.78 76.188 305 Figure 9. Densities obtained using ab initio potentials for pure CO2 . Experimental-solid lines, ab initio-points. Table 4. Solubilites obtained using ab initio-derived CO2 potentials. data obtained from equation of state[34], the Harris and Yung potentials under predicted the densities in liquid phase. For 275 K and 50 bar, the Harris and Yung model predicted as vapor, 90 kg/m3, compare to the experimental value of 928 kg/m3 which is in liquid phase. The densities obtained using ab initio potentials are in good agreement to experimental values compare to Harris and Yung potential. But, for 300K and 50 The Harris and Yung potentials predicts the critical point and VLE for CO2 accurately[24], but it fails when applied to CO2 hydrates. The ab initio-derived potentials well predicted the cage occupancies and reference properties of CO2 hydrate, but it fails to predict the vapor density. So there is a need of intermolecular potential which can work in all three phases. laboratory’s on-going research in methane hydrates under the RES contract DEFE0004000. Number density of CO2 molecules across the box 40 35 30 25 20 15 10 5 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 z-parameter of the simulation box (nm) Figure 11. Density plot of CO2 molecules along the length of the box and snap shot of CO2-water system at 275 K and 20 bar. CONCLUSIONS Dissolution of CH4 and CO2 was studied at 275 K and 50 bar and it was observed that the CO2 hydrate dissolves at much higher rate in comparison to CH4 hydrate. For CH4 hydrates the amount of dissolution was found to be lower at 100% cage occupancy and increase with decrease in cage occupancy, but for CO2 hydrate the dissolution was higher at 100% occupancy and decreases with decrease in cage occupancy. The effect of gas under-saturation on hydrate dissolution of CH4 and CO2 hydrates was studied at 100% and 0% cage occupancies. The amount of dissolution was found to be proportional to the level of under-saturation in the water phase in the case of CH4 hydrate. In the case of CO2 hydrate, the amount of dissolution of CO2 molecules in to the water phase was found to be higher than the saturation value. The higher dissolution rates in CO2 hydrate might be because of the repulsive nature of the potentials in the hydrate phase. Density and solubility calculations were performed for CO2 using Harris and Yung potentials and ab initio-derived intermolecular potentials. The ab initio-derived potentials predicted the densities and solubilities at higher pressures well compare to the Harris and Yung intermolecular potentials. ACKNOWLEDGEMENTS This technical effort was performed in support of the National Energy Technology REFERENCES 1. Sloan, E. and C. Koh, Clathrate hydrates of natural gases. 2007: CRC Press. 2. Archer, D., Methane hydrate stability and anthropogenic climate change. Biogeosciences Discussions, 2007. 4(2): p. 993-1057. 3. Ohgaki, K. and Y. 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