Chemical and Biomolecular Engineering - Theses
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ItemA narrow pore zeolite: ZSM-25 for natural gas purification( 2019)Due to both increased greenhouse gas emissions and increased natural gas demand, the development of separating CO2 and N2 from methane-rich streams (e.g. natural gas, biogas and landfill gas) has arisen worldwide research interest. Greenhouse gas emissions can be mitigated by post-combustion technology and switching the energy structure to CH4-based energy sources. Natural gas is the most significant source of CH4, which typically contains around 80%-95% CH4, less than 10% CO2 and N2, and small amounts of hydrocarbons. Hence, removing the CO2 and N2 is critical for purifying natural gas, with the respect of both increasing the energy density and preventing the corrosion of the pipeline caused by acid CO2 for transporting CH4. Adsorption-based capture of CO2 and N2 from natural gas has attracted tremendous interest owing to its economic advantages. Porous materials play very important roles in the adsorption process where the material is exposed to the gas mixture at high pressure and then desorbs at low pressure or vacuum. The significant index of evaluating a porous material is the selectivity, capacity, adsorption kinetics and regenerability. Narrow-pore zeolite (8MR zeolite) has significant potential in natural gas purification via pressure-swing adsorption (PSA), which is attributed to its pore size fitting between CO2 and CH4, and close to N2. Hence, the selectivity is relatively much higher than other zeolites (e.g. FAU, ZSM-5). However, the slow adsorption kinetics are limiting its application in the natural gas industry, and no zeolites have been found with preferential adsorbing N2 from CH4 at equilibrium, unable to effectively separate N2 from natural gas. This thesis describes the development of small-pore ZSM-25 based zeolites, and their applications in membrane separations. The study provides a rational strategy of designing ZSM-25 zeolite for effective CO2/CH4 and N2/CH4 separation in the natural gas purification industry. In this thesis, an extensive literature on 8MR zeolite for natural gas purification and their modification approaches has been sourced and analyzed in Chapter 1. Chapter 2 a Li+/ZSM-25 zeolite (LZZ) was developed via partial ion exchange of the Na+ with Li+. This exchange enabled higher CO2 capacity and adsorption kinetics due to higher pore volume and stronger affinity of CO2 with Li+, and the ultra-high CO2/CH4 selectivity remained. The CO2 isotherms showed deviation from typical Type I isotherm and 'breathing' behavior. This observation was explained by synchrotron in situ X-ray powder diffraction, demonstrating a gradual structural expansion induced by CO2. This expansion resulted in the increased CH4 admission in binary gas adsorption. This work enables the possibility of applying small-pore zeolites in natural gas purification which are kinetically-limited. Chapter 3 The Li+/ZSM-25 zeolite (LZZ) was incorporated into a commercial polymer Matrimid 5218 yielding a mixed-matrix-membrane (MMM). Li+/ZSM-25 was chosen as filler because of its fitting pore diameter between CO2 and CH4, which merely adsorbed CH4 while allowing considerable CO2 transport. The CO2/CH4 separation performance of the optimal MMMs at 5 wt% filler loading, showed higher CO2/ CH4 selectivity than that of the pristine Matrimid in both single- and mixed-gas separation. The dominant molecular sieving effect contributed to the increasing selectivity with increased pressure, showing unusual plasticization-resistance behavior. The optimized membrane (M-5) achieved ideal CO2/CH4 selectivity of 169, which surpassed the latest CO2/CH4 upper bound. Chapter 4 A new 'trapdoor' material K-ZSM-25 was designed for N2/CH4 separation by incorporating K+ as a 'door-keeping' cation. The extent of the temperature-dependent oscillations of the K+ cation regulated the accessibility of the cage, controlling the adsorption capacity of the material. There were distinguishable gate-opening temperatures (Ts) between N2 and CH4 molecules. Within this temperature range, N2 molecules had full access to the pathway into the cage, while CH4 molecules were hindered due to the blockage of K+. Both the experimental results and simulations demonstrated that K-ZSM-25 can achieve effective N2/CH4 separation at around ambient temperature with outstanding selectivity of over 30 in single gas adsorption and 5.7 in dynamic breakthrough simulation. The large N2 capacity, outstanding N2/CH4 selectivity, fast kinetics of K-ZSM-25, and it is readily regenerated ataround room temperature, all of which makes this adsorbent ideally suited to PSA-based industrial separations.