Oxidative Coupling of Methane on Na2WO4-Mn/SiO2: Impact of Reactor Configuration
Aseem, Michael P. Harold
University of Houston, Department of Chemical and Biomolecular Engineering, Houston, TX 77204
The oxidative coupling of methane (OCM) to higher molecular weight hydrocarbons has great appeal as an alternative route to chemicals and polymers. The reaction system is challenging because of large differences in the reactivity of the reactant methane and desired products (e.g. Ethylene, ethane etc.) at the elevated temperatures (> 700oC) needed to activate methane. Moreover, the reaction system is highly exothermic and is noted for significant parametric sensitivity. The goal is to achieve a C2+ yield in excess of 30% for the process to be potentially viable.
In spite of the large number of catalysts and reactor configurations have been studied over the past few decades, a catalyst and reactor combination has yet to be found out that gives sufficiently high C2+ yield. The most studied catalysts to date for the OCM reaction system have been alkali-promoted alkaline earth metal oxides, transition metal oxides and rare earth metal oxides. Na2WO4-Mn/SiO2 catalyst has attracted attention because of its comparatively favorable catalytic performance [1]. To date there have been no systematic studies that have compared the performance of different rector types for the Na2WO4-Mn/SiO2 catalyst.
In this study a systematic study is underway to evaluate different reactor types using the Na2WO4-Mn/SiO2 catalyst. We have evaluated the fixed bed reactor and monolith reactor systems and obtained C2+ hydrocarbon yields between ~20% for the former to ~6% for later. The data show that as the reaction system is oxygen limited beyond a particular temperature, with the methane conversion limited to ~32%. Thus distributed oxygen feed might be helpful to enable further reaction with the goal of achieving higher selectivity for desired products. Previous studies have shown favorable results through distributed oxygen addition [2].
In our current work we are using a modified porous asymmetric γ-alumina tubular membrane to contain the catalyst and provide distributed oxygen to a continuous stream of methane. The permeability of membrane is modified to achieve an optimum flux of oxygen through membrane [3]. Membranes are impregnated by silica sol and characterized by permeability measurements with nitrogen gas. The fresh membrane has a permeability of 28cm3/cm2minbar. After impregnation with silica the permeability reduces 4-fold to 6cm3/cm2minbar. The permeability reduction requires a relatively high imposed transmembrane pressure gradient which facilitates homogeneous distribution of oxygen along the reactor.
Figure 1 shows that on increasing the pressure gradient across the membrane nitrogen flow to the tube side increases which correspondingly decreases the back diffusion of methane to shell side. By distributing the feed the aim is to maintain a low but uniform local concentration of oxygen along the catalyst bed length in order to avoid complete oxidation of desired products which in turn will increase the selectivity of C2+ hydrocarbons. Experiments are underway for a range of residence times and temperature. The performance of this fixed bed membrane reactor will be compared with conventional fixed bed reactor at same conversion under identical conditions.
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References
[1] Arndt, S., Otremba, T., Simon, U., Yildiz, M., Schubert, H., Schomäcker, R, Mn-Na2WO4/SiO2 as catalyst for oxidative coupling of methane. What is really known? (2012) Applied Catalysis A: General, 425-426, pp. 53-61
[2] Lu Y., Dixon A.G., Moser W.R., Ma Y.H. ,Oxidative coupling of methane in a modified γ-alumina membrane reactor (2000) Chem. Eng. Sci., 55 (21), pp. 4901–4912
[3]
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