Simulation of adsorption, separation, and permeability of CO2, CH4, and N2 in Zeolites

1. Adsorption and separation of gaseous compounds is a special issue in industry. Zeolites are the common material used for gaseous compound separation. Separation and adsorption features of zeolites in gaseous compounds has been investigated in terms of temperature, pressure, pore size, and structure. Many of studies used PCFF force field and Monte-Carlo method for investigating carbon dioxide adsorption behavior of zeolites. Rahmati and Modarress, and Lim et al. studied zeolites with MSE, IHW, UFI, SIV, IWV, and JSR structures. They showed that PCFF force field is a good choice for simulation. Lennard-Jones (L-J) potential function was also applied in most of the papers. Shu-Mei Wang, and Rahmati and Modarress used this potential function to model unlinked interactions. The results illustrated good coherence with experimental data. Furthermore, zeolite with NaY structure was modeled with GCMC and experimental method by A. Ghoufi et al. Yue et al applied L-J and Colon function and also used molecular dynamics and Monte-carlo method for modeling unlinked interaction in zeolite with MFI structure. Adsorption and separation of benzene and carbon dioxide compound were investigated. The effects of temperature, pressure, mixture of feed elements and adsorption heat were studied and results of simulation correspond well with experimental data. There are many structures for zeolite and grow annually. Therefore, their performance features of them in terms of separation and adsorption of gaseous compounds must be investigated. Hence, in this study we use molecular simulation by Monte-Carlo and molecular dynamic methods to investigate and compare separation and adsorption capacity of MFI and FAU zeolites.
2. We have to simulate 2 kinds of interaction: gas-gas and gas-zeolites. We learned from previous study that L-J potential function is appropriate for interactions of gas molecules and atoms in nanotube. We set PCFF force field for inter-molecular interactions and consider that zeolites have rigid and nonflexible structure. We used periodic boundary layer for creating a limitless system. In simulation by MonteCarlo method we used metropolis algorithm. We also applied GCMC condition. Chemical potential is a function of gas fugacity. We assumed Ideal gas assumption instead of fugacity in all gases simulated. By using metropolis algorithm and according to generation, elimination, movement, rotation, and angular bending of molecules, alterations are applied to system and rejected and accepted by accepting terms of that transformation.
3. First of all we need to investigate the accuracy of simulation’s results which include: 1. the results of gas adsorption in zeolite, 2. Adsorption of pure gas, 3. Adsorption of the mixture of two gasses, 4. Selectivity, and 5. Permeability. Figure 1 shows constant temperature adsorption of carbon dioxide in zeolite with MFI structure. We compare our result with other studies to illustrate that our method in acceptable and better simulate the behavior of adsorption in zeolite. We set temperature and pressure on 298K and 0-1000KPA respectively. We also applied PCFF force field.
Figure 2 illustrates the results of pure gas adsorption of nitrogen. Figure 2 demonstrates that with the increase of pressure, adsorption of nitrogen is increased but, this trend is converse about temperature.
In the temperature of 298K, pressure of 100, 1000, and 5000 KPa, and different feed concentration of 10 to 90 percent, competitive adsorption of CO2 with N2 and also CO2 with CH4 are investigated by Monte-Carlo method. The results are shown in figure 3. Dash lines stand for CO2 and solid lines account for N2 and CH4 Figure 3 proves that in zeolite with FAU structure, CH4 is more adsorbed in comparison with N2 with different feed concentration.
Figure 4 illustrates that with increasing the total pressure of feeding, selectivity is reduced. As we expected, FAU adsorbs N2 less than CH4. Therefore, selectivity of carbon dioxide versus N2 is more than versus CH4
Figure 5 shows that with the increasing of feed concentration, permeability coefficient is reduced and this reduction is bigger when CO2 is adjacent to CH4. Moreover, results illustrate that permeability coefficient is bigger when CO2 is mixed with CH4 in comparison to N2.
4. The results of this study demonstrate that with increasing temperature and reducing pressure, adsorption is decreased. In terms of selectivity, the selectivity of CO2 is dependent on pressure, feed concentration, and molecular structure. Finally, the results of permeability coefficient of CO2 in 2 structures shows that MFI is dominant.