Short-Contact-Time Reactor for Catalytic Partial Oxidation of Methane

Abstract

An internally circulating fluidized-bed (ICFB) reactor (i.d.eff = 1.9 cm, Hriser = 10 and 20 cm, respectively) was applied to investigate the catalytic partial oxidation of methane to synthesis gas over Ni/α-Al2O3 (Ni loading, 1 and 5 wt %; particle diameter, 71−160 and 250−355 μm). The experiments were performed at 800 °C by applying a methane to oxygen ratio of 2:1. The contact times in the draft tube (≪ 0.1 g·s·mL-1) were sufficient to obtain nearly thermodynamic equilibrium values for the methane conversion (XCH4,eq = 92%) and selectivities to hydrogen (SH2,eq = 97%) and carbon dioxide (SCO,eq = 97%). The achieved results indicate the general suitability of an ICFB reactor for the partial oxidation of methane to synthesis gas. However, stable and controllable reactor operation with sufficient solids circulation through the draft tube was possible only over a narrow range of gas velocities (0.75−1.50 m·s-1). Furthermore, the conversion and selectivities were strongly influenced by the temperature distribution in the ICFB reactor. Especially, the decrease of temperature in the top of the draft tube and in the fountain region promoted backreactions reducing the conversion of methane. Furthermore, catalyst deactivation due to carbon depositions occurred in the ICFB reactor. This effect caused for the 1 wt % Ni catalyst a significant drop of the activity during 150 h of time on stream. The methane conversion decreased in the range of 12% for a catalyst particle fraction of 71−160 μm. It was shown by TEM that two different types of carbon (encapsulating and whisker carbon) were deposited on the surface. The first species resulted in a reversible deactivation (activation of the catalyst by carbon dioxide in the reverse Boudouard reaction was partially possible), whereas the second species resulted in an irreversible deactivation due to a Ni loss by removing the carbon fibers which had Ni sites on top of the filament.