A Molecular Dynamics Simulation Study on Separation Selectivity of ...

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ScienceDirect Energy Procedia 86 (2016) 144 – 149

The 8th Trondheim Conference on CO2 Capture, Transport and Storage

A molecular dynamics simulation study on separation selectivity of CO2/CH4 mixture in mesoporous carbons Thuat T. Trinh,a* Khanh-Quang Tranb, Quang-Vu Bach,b Dao Q. Trinhc b

a Department of Chemistry, Norwegian University of Science and Technology, Trondheim, Norway Department of Energy and Process Energineering, Norwegian University of Science and Technology, Trondheim, Norway c Department of Chemistry, University La Rochelle, France

Abstract The effect of surface charged defects in carbon mesoporous materials on adsorption selectivity for a CO2/CH4 gas mixture relevant to natural gas was studied by means of classical molecular dynamics (MD) simulation. The mole fraction of CO2 was 0.1, T = 300K and the partial pressure of CO2 was up to 40 bars. A graphite slit pore of 3.0 nm was taken as a model for mesoporous carbon. The localized charged defects within an electroneutral pore surface were found to increase the CO2/CH4 separation selectivity. In addition, such surface charges interact with CO2 molecules only. The selectivity reduces with increasing pressures. Our model with a defect charge of 0.2 electron/atom can reproduce the selectivity CO2/CH4 of the experimental data reported in the literature. A perfect charge graphite model underestimates the adsorption selectivity of CO2/CH4. The results show that a very high selectivity around 25 can be achieved with a charged defect of 0.45 electron/atom. This suggests that controlling the charged defects on surface of carbon mesoporous can be used to enhance the CO2/CH4 separation for natural gas.

© © 2016 2015 The The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Programme Chair of The 8th Trondheim Conference on CO2 Capture, Transport and Storage. Peer-review under responsibility of the Programme Chair of the 8th Trondheim Conference on CO2 Capture, Transport and Storage Keywords: CO2/CH4 separation, carbon mesopores, molecular dynamics, simulations

1. Introduction The separation of CO2 from CH4 is of primary importance in many industrial processes such as natural gas sweetening, biogas upgrading, and landfill gas purification [1]. Industrial technologies conventionally employed for this separation purpose include chemical absorption, pressure swing adsorption (PSA) and cryogenic separation [2]. Despite the maturity, these technologies usually involve substantially sophisticated equipment, energy-intensive operation and high investment cost. As an alternative, membrane separation technology has emerged and attracted increasing attention in CO2 separation and capture research during the last decades, due to its high energy efficiency, simplicity in design and module construction and environmental compatibility [3]. Various membrane types for CO2/CH4 separation are available. Based on the membrane materials, they can be categorized in three major groups, which include (organic) polymeric membranes, inorganic membranes, and mixed matrix membranes. Inorganic membranes are more suitable for CO2 separation under more severe conditions (higher temperatures and pressures) relevant for natural sweetening, when organic based membranes are not functional. However, a

* Corresponding author. Tel.: +0-000-000-0000 ; fax: +0-000-000-0000 . E-mail address: [email protected]

1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Programme Chair of the 8th Trondheim Conference on CO2 Capture, Transport and Storage doi:10.1016/j.egypro.2016.01.015

Thuat T. Trinh et al. / Energy Procedia 86 (2016) 144 – 149

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major drawback of these membranes is their substantially higher cost. Therefore, research and production of inexpensive membranes for CO2/CH4 separation is of great importance for the realization of carbon capture and sequestration technologies [4-7]. Recently, an experimental study on using inexpensive mesoporous carbon material for separation of CO2 from CH4 and N2 was reported [8]. The tested material displayed very high adsorption capacity and selectivity for the gas mixtures. In another work, the importance of charged defects and charge distribution of carbon pore surfaces on the adsorption of CO2 in microporous carbon was theoretically studied by means of Monte Carlo simulation [9]. It was reported that the surface heterogeneity changes the adsorbates’ accumulation configuration by changing the geometry of the pore surface and the charge distribution of the surface. As the pore width decreases, the surface functionalities dictate the adsorption, and thus the surface functionalities play a more important role in increasing the CO2 adsorption capacity. However, this information is currently not available in the open literature for mesoporous carbon material and therefore the study reported in this paper was carried out using molecular dynamics simulation method. The objective was to study the effect of localized surface charges on the adsorption and separation of the CO2 and CH4 gas mixture, relevant for natural gas. For this purpose, a gas mixture with the mole fractions of 10% CO2 and 90% CH4 was employed to simulate the chemical composition of natural gas. Molecular dynamics (MD) simulations are well suited to determine such properties surface charges can be varied in a controlled way. In the next section, we will introduce briefly the method and models used in MD simulations. We will show how important the surface charge is for CO2 adsorption. The CO2/CH4 separation selectivity will also be demonstrated with varied pressure. 2. Method 2.1. Simulation method and model In this work, LAMMPS package was employed for classical molecular dynamics (MD) simulations [10]. The simulated model was consisted of a graphite crystal and a mixture of CO2 and CH4 molecules. The mole fraction of CO2 was 0.1 to mimic the composition of natural gas. The graphite structure was hexagonal, without any defect. The model system and was constructed from 10 sheets of graphene, containing 32000 carbon atoms. It is important to use a flexible and high number of graphene sheets to model graphite layers[11, 12]. Periodic boundary conditions in all directions were applied. The surface of graphite was 98x85 Å2. In the z direction, there were two graphite regions, and the distance between them was equal to the pore width W=3 nm (Figure 1). The total surface charge was always kept neutral. One carbon atom in the graphite layer was positively charged, and surrounded by 3 negatively charged atoms. The distribution of charges around the positive atom was thus [C3δ+ - 3Cδ-], as pictured in Figure 1. A random amount, 30%, of the total atoms in the surface layer was charged positively or negatively in this manner. An average charged defect value was determined as the total absolute charge divided by the total number of atom (electron/atom). This simulation model can represent the charged defects inside carbon material. At each value of charged defects, ten systems with different total number N of molecules were established to simulate pressure of CO2 up to 40 bars. All simulation was performed at temperature 300 K. The initial configuration was constructed by randomly distributing the CO2 and CH4 molecules over the graphite surface. The system was stabilized during 2 ns by NVT runs with the Nosé-Hoover thermostat [13]. When a thermal equilibrium was established in the system, another 2 ns run were performed with micro-canonical ensemble conditions (NVE) to study the adsorption properties. The average values of temperature and pressure in the NVE simulations were within 1% of the expected values. In total, 4x106 MD steps with a time step of 1fs was performed and this is sufficiently long to obtain good statistics and consistent trajectories.

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3. Results and discussions. 3.1. Adsorption of mixtures

Figure 2. Concentration profile of CO2 and CH4 as a function of distance to the surface. There are two layers of CH4 and CO2 on the surface. The second layer of CO2 is very thin due to a small number of molecules in the initial gas mixture.

Figure 2 depicted the concentration profile of CO2 and CH4 as the function of distance to the graphite surface. CH4 concentration is much larger than CO2 in proportion with the initial gas mixture (90% CH4, 10% CO2). It is interesting to see that there are two layers of CH4 in the pore developed up to position α [11,14]. The position α is the Gibbs dividing surface, where the concentration is different than the gas layers by at least 5%. We counted the total number of molecules in the adsorbed layer and in the gas layer to construct the isotherm and calculate the adsorption selectivity. Figure 3 shows the isotherm adsorption of CO2 and CH4 with differently surface charged defects. The adsorption of CO2 is strongly enhanced by charging the surface. The effect of the charge distribution can be explained by the large quadruple moment of CO2 molecule (-14.27x10-40 Cm2) [20]. This moment favours interaction between the molecule and charged carbon atoms on surface [9]. The CH4 molecule has no dipole or quadruple moment, so charged defects on the surface has no effect on its adsorption.

Figure 3. The adsorption of CO2 and CH4 as a function of partial pressure of CO2 (left) and CH4 (right) at different surface charged defect.

3.2. Adsorption Selectivity The adsorption selectivity is defined by the separation ratio of CO2 versus CH4 in the mixture: CO2/CH4 SAds =

n CO2 (adsorbed) n CO2 (gas)

×

n CH4 (gas) n CH2 (adsorbed)

(4)

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This parameter gives the most common way to characterize the separation properties of a membrane for the gas components in a gas mixture. Figure 4 shows the pressure effect on the adsorption selectivity of CO2 over CH4. The perfect model of surface without charged defects has a lower CO2/CH4 selectivity. We can see clearly that the adsorption on the perfect graphite surface of CO2 is slightly more favourable than CH4. In general, CO2 molecules have stronger Van der Waals interactions with carbon atoms of the graphite and CH4 molecules. Hence it is difficult to separate CO2 from CH4 if the material has no charge particle on the surface. The electrostatic can improve the separation selectivity of CO2/CH4 from 2 to 6 with more charged defects. In addition, the selectivity is observed to decrease with increasing total pressure especially at higher pressures. This observation is similar to that found earlier with a graphite surface [14]. It is important to note that in our simulations the pressure is much higher than that reported in the experimental work by Yuan et al. (maximum 1 bar) [8]. At a lower pressure range, the selectivity was kept constant. The CO2/CH4 adsorption selectivity for the experimentally tested mesoporous carbon is corresponding to a charge defect of 0.225 electron/atom in our model. This is important to highlight that the perfect graphite underestimate the selectivity and hence not a good model to study adsorption for a real material. We extended our simulations at 1 bar as the partial pressure of CO2 and found that a high selectivity can be obtained with defected charge of 0.45 electron/atom. This suggests that mesoporous carbons with high charged defects are promising for CO2/CH4 separation.

Figure 4. Adsorption selectivity of CO2 relative to CH4 vs the total pressure of the system with various charged defects on the surface at T=300 K (left). The selectivity increases with surface charge defect value (right).

4. Conclusions Equilibrium molecular dynamics simulations have been used to study the effect of surface charged defects on the adsorption and selectivity of CO2 and CH4 in mesoporous carbon. The adsorption of CO2 is strongly affected by charged defects because of the electrostatic interaction between CO2 molecules and surface charged atoms. On the other hand, CH4 is inert to the charged modification. However, the CO2/CH4 selectivity is relatively low with a small number of charged defects. In order to have good adsorption selectivity for natural gas sweetening, a strong localized charge as 0.5 electron/atom is required. This suggests a strategy for experimental synthesis of highly charged defects for mesoporous carbon to improve the CO2/CH4 separation selectivity. Acknowledgements The authors acknowledge The Research Council of Norway NFR project no 209337 and The Faculty of Natural Science and Technology, Norwegian University of Science and Technology (NTNU) for financial support. The Norwegian Metacenter for Computational Science (NOTUR) granted the calculation power. References 1.

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