| Natural gas can be converted directly to high value-added aromatics and hydrogen in a single step via nonoxidative methane dehydroaromatization?MDA?.This reaction has great application prospects in natural gas chemical industry.However,MDA reaction is thermodynamically unfavorable,and thus results in low methane conversion and benzene yield,which impede its practical application in chemical industry.In order to enhance methane conversion and aromatics yield,a silica membrane reactor was used for selective removal of hydrogen to overcome thermodynamic limitation.In this thesis,MDA reaction in a silica membrane was studied theoretically and experimentally.A mathematical model of silica membrane reactor was built to investigate the effects of temperature,pressure,catalyst activity,and membrane selectivity on the performance of catalytic membrane reaction for MDA.The simulation study shows that methane conversion,benzene yield,and hydrogen yield increase with catalyst activity and membrane selectivity,and initially increase and then decrease gradually with the increase of permeation of silica membrane.Methane conversion can approach 30%at 700?and atmospheric pressure,provided that hydrogen permeance and H2/CH4 selectivity of silica membrane have reached 1×10-66 mol·m-2·s-1·Pa-1 and 100,respectively.Microporous silica membrane was fabricated by sol-gel method.The surface of membrane is uniform and defect-free and the thickness of membrane is about 300400 nm.Gas permeation tests were conducted with various pure gases such as He,H2,N2,CH4,SF6 at 200500?.The silica membrane shows high H2 permeance of 1×10-63×10-6mol·m-2·s-1·Pa-1,with H2/CH4selectivity and H2/SF6 selectivity of 1020 and 40135,respectively.The permeance of small gases such as He,H2 increase with temperature,indicative of activated diffusion mechanism.On the contrary,the permeance of large gases such as N2,CH4,SF6 decrease with temperature,indicative of Knudsen diffusion mechanism.After heat treatment at 700?,thermal stability test shows that a drastic decrease in He permeance can be ascribed to the densification of amorphous silica membrane,and a slow increase in N2 permeation is expected due to the emergence of defects in membrane.The performance of silica membrane reactor for MDA was investigated at 700?and atmospheric pressure.Using Mo/HZSM-5 as catalyst,methane conversion reaches a maximum of 15%and aromatics yield attains a maximum of 7.5%in silica membrane reactor.In comparison with fixed bed reactor,methane conversion and aromatics yield in silica membrane reactor are 8%47%and 4%16.5%higher than that in fixed bed reactor,respectively.Using Mo/HMCM-22 as catalyst,methane conversion reaches a maximum of 16.7%and aromatics yield attains a maximum of 8.7%in silica membrane reactor.In comparison with fixed bed reactor,methane conversion and aromatics yield in silica membrane reactor are 12%30%and5%40%higher than that in fixed bed reactor,respectively.In conclusion,membrane reactor enhances the methane conversion and aromatics yield in comparison with fixed bed reactor. |
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