Mixture Adsorption in Metal-Organic Frameworks for Biogas and Light Hydrocarbon Separation
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Li, Chunyi
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Abstract
As the global energy demand continues to increase, developing technologies to use renewable energy sources has become increasingly urgent. The development of biogas separation for biomethane production opens possibilities for renewable energy utilization. Biogas is versatile in terms of both production and utilization. It can be used for heat, power, and electricity generation in place of fossil fuels. Additionally, it can be integrated into areas where natural gas is used for vehicular fuel and chemical production. Along with the various biogas sources, numerous challenges are present with effective biogas separation. Biogas contains mainly carbon dioxide and methane, with low concentrations of contaminants such as hydrogen sulfide (H2S) and ammonia (NH3). In municipal landfill biogas, mercaptans and siloxanes are also present in trace amounts. In addition, biogas is saturated with water from the natural fermentation processes. Typical biogas utilization requires pre-drying and desulfurization. Specifically, the H2S concentration needs to be reduced to below 20 parts per million (ppm) for pipeline transportation from starting concentrations of above 3000 ppm.
The desulfurization step is typically achieved by biologically oxidizing H2S into elemental sulfur. This process requires air injection and sometimes bacterial introduction. Despite the advantage of desulfurization inside the fermentation chamber, air injection lowers the heating value of biogas by introducing large quantities of nitrogen. Aqueous scrubbing using water or polyethylene glycol solutions faces the problem of producing large amounts of contaminated solutions and high-cost regeneration. Membrane separation is not ideal for desulfurization due to the low driving force from the low concentrations of contaminants. After drying and desulfurization, most of the CO2 is removed to produce higher-heating value biomethane. Adsorption using activated carbon requires low-cost operation, but the relatively low affinity to H2S and CO2 results in low adsorption selectivities over CH4.
Much effort has been focused on developing sorbent materials for selective H2S and CO2 separation. There is a need to study the adsorption performance in realistic gas mixtures to understand the competitive adsorption behavior of gases. In this thesis, the mixture adsorption equilibrium of H2S and CO2 was studied using simulated biogas mixtures containing H2S, CO2, methane, and water vapor. The approach of this thesis is summarized in four objectives.
The first objective investigates the chemical stability of metal-organic frameworks (MOFs) in H2S and CO2-containing environments. In Chapter 4, the chemical stability and structural stability of MIL-125-NH2 in the presence of dry H2S up is studied using in situ and ex situ characterization methods. In Chapter 6 and Chapter 7, the H2S adsorption performance in diamine-impregnated MIL-101(Cr) is studied in adsorption-desorption cycles using dry and humid simulated biogas mixtures. The effect of the type of diamine on the cyclic adsorption stability of H2S and CO2 is studied using diamines containing 1°,1° amines, 1°,2° amines, and 1°,3° amines. The study concludes that the impregnation of diamine in MIL-101(Cr) improves the cyclic adsorption stability of H2S.
The second objective studies the effect of metal substitution on mixture adsorption equilibrium of ethylene and acetylene using isostructural MOFs. The production of ethylene from biomethane may contain trace amounts of acetylene and ethane. Acetylene is also a problematic contaminant in ethylene produced from naphtha cracking. Chapter 5 uses four isostructural MOFs based on rare-earth metals to study the acetylene/ethylene adsorption selectivity. The experimental results show that the pore size decrease across the rare-earth MOFs played a role in determining the acetylene/ethylene selectivity and acetylene adsorption capacity.
The third objective studies the mixture adsorption equilibrium in simulated biogas mixtures containing CO2, H2S, CH4, and water vapor. In Chapter 4, the binary adsorption equilibrium of CO2/CH4 and H2S/CO2 mixtures using a custom-built chromatographic method. The study concludes that MIL-125-NH2 is H2S-selective in H2S/CO2 mixtures when the H2S is only present in trace amounts. In Chapter 6, the effect of diamine type (1°1°, 1°2°, 1°3°) is studied in terms of the CO2 and H2S adsorption capacity and regeneration temperature. The results suggest that the 1°1°and 1°2° diamine-impregnated MIL-101(Cr) has the highest CO2 adsorption capacity among the diamines studied. The 1°3° diamine-impregnated MIL-101(Cr) showed the highest H2S adsorption capacity and the highest H2S/CO2 adsorption selectivity among the diamines studied under the simulated biogas compositions in this work. In Chapter 7, the effect of humidity on the adsorption capacity is studied using 1°3° diamine-impregnated MIL-101(Cr) powder and fiber sorbent modules. The results show that the 1°3° diamine-impregnated MIL-101(Cr) retained the H2S adsorption capacity in the presence of water vapor.
The fourth objective focuses on developing a fiber sorbent module for streamlined biogas separation. The fiber sorbent consists of a cellulose acetate polymer matrix and 1°3° diamine-impregnated MIL-101(Cr) as filler. In the simulated dry biogas mixture, the fiber sorbent showed enhanced mass transfer rates compared to the powder fixed-bed module. In addition, experimental results show that the gas flow rates can be increased over 3-fold that of the powder fixed-bed module without inducing significant pressure drop.
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2024-08-02
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