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Effect of membranes in methane dehydroaromatization on a bifunctional Mo/H-ZSM-5 catalyst in a packed bed reactor, The
Kee, Benjamin Lyons
Kee, Benjamin Lyons
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2016
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Methane is becoming an increasingly attractive resource. Conversion to liquids and higher-value fuels add to the opportunities available for natural gas processes. Conventional methods for converting methane to larger hydrocarbons are through Fischer-Tropsch (F-T), which require multiple unit steps that can add inefficiencies. Methane dehydroaromatization (MDA) is a direct catalytic route to from methane to benzene. Unfortunately, the shortcomings of MDA include likelihood to coke and poor yield. Conversion can be increased by removing product hydrogen via a membrane to shift the thermodynamic equilibrium towards the products. A model is developed to study the coupling of hydrogen-selective membranes with packed-bed methane dehydroaromatization (MDA) reactors using bifunctional Mo/H-ZSM-5 catalysts. The computational predictions are supported by previously published literature. The effect of hydrogen removal is evaluated as a function of reactor temperature, gas-hourly space velocity (GHSV), and hydrogen removal rate. Consistent with Le Chåtelier's principle, the results reveal that membrane integration significantly increases methane conversion. However, the desired benzene yield can be diminished by the yield of undesired naphthalene and higher hydrocarbons. The benzene-to-naphthalene ratio depends strongly and nonlinearly on the membrane hydrogen removal. The results suggest that hydrogen membranes are most beneficial when the GHSV is relatively high and the catalyst temperature is relatively low. Although the single-pass benzene yield remains below 10%, the hydrogen membrane can increase benzene production rates. Hydrogen removal was also explored for MDA with steam addition. Water can preferentially react with naphthalene to hinder the catalyst deactivation process. Naphthalene can crack to form lighter gases such as hydrogen, ethylene, and benzene. However, condensation reactions may occur to form larger polyaromatic hydrocarbons (PAH) such as anthracene. Two percent steam premixed into the feed of an MDA reactor was simulated. Results suggest that water addition slightly decreases conversion, but instead improves the selectivity toward smaller hydrocarbons. Water reacts with the naphthalene to hinder PAH growth, which can lead to longer catalyst lifetime. High flow rates, low temperatures, and low hydrogen removal rates maximize non-coking benzene production.
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