| Gas energy is a clean and low-carbon fuel, and plays a more and more important role in the word energy consumption. The main composition of biogas are CO2and CH4, which is a kind of precious green gas energy. CO2present in biogas can reduce its heating value as well as cause equipment and pipeline corrosion, and impact on its use and transport, so the separation of CO2will inevitably be needed. Low concentration coal bed methane is a kind of unconventional natural gas and in the process of mining with a large of air is directly discharged into the atmosphere, which not only causes the waste of resources but also results in pollution of the environment. The removal of N2is purpose of methane enrichment in the low concentration coal bed methane. Pressure swing adsorption (PSA) has gained much attention because of its simple operation, separation efficiency, low energy requirements and so on, absorbent is the key. So looking for adsorbent with high selectivity is a topic worth studying. Microporous materials as adsorbent has showed advantages in gas adsorption separation. Accordingly, in this paper, we choose materials with different aperature as the research object, the aperture size from0.38nm to1.4nm, including small pore zeolites and metal-organic frameworks (MOFs). In order to make the experimental separation is close to industrial separation process, adsorbent needs to be tabletting. We tested gas adsorption in porous materials under high pressure, and study dynamic separation performance of the pure gas and the mixture in microporous materials. From the aspects of pore size and pore structure, we analyzed the influence of gas separation. The main contents and conclusions are listed as follows:(1) We prepared three kinds of small pore zeolites by hydrothermal synthesis closed to the dynamic diameter of CO2, CH4, and N2, which were characterized by XRD to ensure the structure of the samples not destroyed under the pressure of10MPa. The adsorption properties of the sample particles were reduced at high pressure, which was caused by granulation resulting in the decrease of the outer specific surface area. The breakthrough curves of pure gas and the mixture were tested at the same experimental condition, we found that separation efficiency of KFI was best, CHA was second and zeolite-4A was worst for CO2/CH4and CH4/N2. From the perspective of pore structure, aperture of KFI was more advantageous to the separation of the mixture, and gas only entered the hole cavity of zeolite-4A through faster, because the hole cavity of zeolite-4A is composed of the same diameter which is linked together, so the existence of a large number of microporous was not conductive to the separation of mixture.(2) The high-silica zeolites are not afraid to trace the influence of water molecules in the filed of adsorption separation. Silicalite-1(10-membered and Si/Al=1555) was synthesized and pelletized at the pressure of10MPa, which were characterized by XRD to ensure the structure of the samples not destroyed. By N2adsorption test, we found sample particles had lower surface area and pore volume, which caused due to the outer surface area to reduce by pelletizing. The adsorption properties of Silicalite-1and zeolite-5A were tested at high pressure, we found zeolite-5A had higher adsorption capacity of CO2, because CO2can be into micropore which is linked together to the hole cavity under static condition. By performing breakthrough experiments of the single-component and the mixture, results consistently showed that Silicalite-1separated CO2/CH4and CH4/N2better than did zeolite-5A. Because CO2can not be into micropore which is linked together to the hole cavity for dynamic test, and the existence of a large number of microporous was not conductive to the separation of mixture. Silicalite-1has the appropriate aperture (5.1X5.6Aå'Œ5.3×5.6A), so the mixture separation efficiency was better.(3) Structure and chemical composition of Metal-organic frameworks (MOFs) can be easily tuned to enhance their selectivity and adsorption capacity of CO2, such as increasing their unsaturated site. MOF-5and Cu3(BTC)2were synthesized and pelletized, the latter containing unsaturated metal sites and former containing none. which were characterized by XRD, to observe the impact on the structure of the samples at the difference pressure of granulation. We found that the skeleton becomes increasingly damaged as the pelletization pressure increases. By performing breakthrough experiments, we found that Cu3(BTC)2separated CO2/CH4slightly better than did MOF-5. Because unsaturated accessible metal sites predominantly interacted with CO2molecules and had almost no effect on the separation of CH4/N2, the CH4molecules suffered a stronger steric effect in the window (3.5A) of skeletal structure and was not conductive to larger size of CH4free diffusion. Conversely, MOF-5with a suitable pore size separates CH4/N2more efficiently in our breakthrough test. Compared with similar aperture of zeolite-13X, we found that the experimental synthesis of MOFs was better than commercial zeolite-13X for separation efficiency of CO2/CH4and CH4/N2. |
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