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The Modeling of Hydrogen Purification and Carbon Dioxide Capture by two Dimensional Porous Membranes Using Molecular Dynamicssimulations

Rezaee, Parham | 2018

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 51272 (03)
  4. University: Sharif University of Technology
  5. Department: Chemistry Science
  6. Advisor(s): Tafazzoli, Mohsen
  7. Abstract:
  8. Carbon dioxide is one of the most significant greenhouse gases in biocycle. This gas makes the equilibrium temperature by absorbing IR radiations on the earth. It is the reason that prevent the earth from freezing. Increasing CO2 emissions in atmosphere, makes problems such as global warming. According to the Kyoto Protocol, all the countries attended to this protocol, must decrease greenhouse gases emissions. Although H2 fuel is one of the best alternative fuel against fossil fuels, the most important industrial process (steam-methane reforming) produce CO2 as by-product. Thus, gas separation has a prominent role in industrial processes such as CO2 capture and H2 purification.Today, gas separation by using membrane is one of the best economical ways as low energy consumption, lower investment cost, possibility for continuous operation and its uncomplicated operation. In customary membranes like polymeric ones, gas separation is accomplished by employing selective absorption, difference between gas permeance and mechanism of gas separation. Recently, numerous theoretical and experimental studies have been done on two-dimensional graphene and graphene-like structures such as hexagonal boron nitride, polyphenylene and etc. to separate gas mixtures efficiently by controlling the size of the pores. In this project we concentrate on two-dimensional hexagonal boron nitride structures that possess regular periodic defect distribution.
    First, we studied the potential energy well for the gas molecules, which permeated through the membrane with 1B3N, 1N3B, 3B6N, 3N6B and 3B3N defect by “ab initio” methods. Then we simulated a box that the defected hexagonal boron nitride, optimized by DFT methods, was placed on simulation box. The periodic boundary condition in the xyz directions were employed. The pressure on the feed side was constant. Selectivity, permeance and flux for light gases had calculated in the range of temperature and pressure to H2 purification and CO2 capture.The results of simulation shown that membrane with 1B3N defects was suitable for helium purification against H2 molecules. Selectivity for helium molecules was near 20 times more than selectivity for hydrogen molecules and flux for helium molecules was near 558 times more than flux for hydrogen molecules. Selectivity was decreased by increasing temperature. Also membrane with 1N3B defects was suitable for hydrogen purification and results shown that this membrane had infinite selectivity for hydrogen molecules against nitrogen and methane molecules. Eventually, according to nitrogen atoms in h-BN membrane, this membrane had suitable absorption for CO2 gases in the room temperature. To show this phenomenon, we calculated the average of CO2 molecules positions in the z direction of simulation box during the time. These calculations shown that the maximum probability of CO2 displacement in the simulation box, is near 1-2 angstrom from membrane
  9. Keywords:
  10. Carbon Dioxide Capture ; Molecular Dynamics ; Hydrogen Purification ; Helium Purification ; Two Dimensional Porous Membranes

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