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Description
Sandrine Ricote1

1, Colorado School of Mines, Golden, Colorado, United States

A large fraction of the natural gas reserves is considered stranded. Because transportation of natural gas is impractical and expensive, interest has increased in the field of gas-to-liquids technology. Two routes are considered in this work: methane dehydroaromatization (MDA) and steam methane reforming (SMR). Methane is converted into benzene and hydrogen in the MDA reaction (equation 1) and into CO, CO2 and H2 in the SMR reaction (equation 2) together with the water gas shift reaction (equation 3).
6 CH4 ↔ C6H6 + 9 H2 (1)
CH4 + H2O ↔ CO + 3 H2 (2)
CO + H2O ↔ CO2 + H2 (3)
The MDA conversion rate is limited to 7-8% at 700°C [1] using molybdenum supported on H-ZSM-5 zeolite. The SMR methane conversion rate increases with the amount of steam. For a steam to carbon ratio of 3, the methane conversion rate varies within 20-45% in the temperature range 500 to 700 °C with Ru or Ni based catalysts and various flow rates [2]. These conversion rates can be increased using a membrane reactor that combines chemical conversion process with a hydrogen separation membrane that removes the hydrogen from the products and shifts the equilibrium to higher yields [3-4].
High temperature proton conductors, such as yttrium doped barium zirconate (BaZr1-xYxO3-d, BZY), were discovered in 1981 [5] and have been widely investigated since. A hydrogen flux of about 5 mL.min-1.cm-2 can be achieved through a 25 micron thick BZY membrane at a current density of 0.7 A.cm-2 at 700°C. Details about the membrane preparation using the unique and cost-effective solid-state reactive sintering technique will be presented. Shortly, dense and defect free BZY membranes can be fabricated on a BZY/Ni support with only one high-temperature sintering step. Tailored electrodes were designed for the MDA and SMR applications. Very encouraging results using membrane reactors have been obtained so far: enhanced conversion rates and longer catalyst lifetime. Nevertheless some limitations, such as upscaling and decrease of the total area specific resistance, still need to be addressed for commercialized applications.

References
[1] L. Wang, L. Tao, M. Xie, G. Xu, J. Huang, Y. Xu, Catal. Lett., 21 (1993) 35-41.
[2] P. Hacarlioglu, Y. Gu, S. T. Oyama, Journal of Natural Gas Chemistry 15 (2006) 73-81.
[3] S.H. Morejudo, R. Zanon, S. Escolastico, I. Yuste-Tirados, H. Malerød-Fjeld, P.K. Vestre, W.G. Coors, A. Martínez, T. Norby, J. M. Serra, C. Kjølseth, Science 353 (2016) 563-566.
[4] V. Kyriakou, I. Garagounis, A. Vourros, E. Vasileiou, A. Manerbino, W.G. Coors, M. Stoukides, Applied Catalysis B Environmental 186 (2015) DOI: 10.1016/j.apcatb.2015.12.039.
[5] H. Iwahara, T. Eska, H. Uchida, N. Maeda, Solid State Ionics 3/4 (1981) 359-363.

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