The effect of non-uniform temperature on the sorption-enhanced steam methane reforming in a tubular fixed-bed reactor

The effect of non-uniform temperature on the sorption-enhanced steam methane reforming (SE-SMR) in a tubular fixed-bed reactor with a constant wall temperature of 600 °C is investigated numerically by an experimentally verified unsteady two-dimensional model. The reactor uses Ni/Al ₂ O ₃ as the reforming catalyst and CaO as the sorbent. The reaction of SMR is enhanced by removing the CO ₂ through the reaction of CaO + CO ₂ → CaCO ₃ based on the Le Chatelier's principle. A non-uniform temperature distribution instead of a uniform temperature in the reactor appears due to the rapid endothermic reaction of SMR followed by an exothermic reaction of CO ₂ sorption. For a small weight hourly space velocity (WHSV) of 0.67 h ⁻¹ before the CO ₂ breakthrough, both a low and a high temperature regions exist simultaneously in the catalyst/sorbent bed, and their sizes are enlarged and the temperature distribution is more non-uniform for a larger tube diameter (D). Both the CH ₄ conversion and the H ₂ molar fraction are slightly increased with the increase of D. Based on the parameters adopted in this work, the CH ₄ conversion, the H ₂ and CO molar fractions at D = 60 mm are 84.6%, 94.4%, and 0.63%, respectively. After CO ₂ breakthrough, the reaction of SMR dominates, and the reactor performance is remarkably reduced due to low reactor temperature. For a higher value of WHSV (4.03 h ⁻¹ ) before CO ₂ breakthrough, both the reaction times for SMR and CO ₂ sorption become much shorter. The size of low temperature region becomes larger, and the high temperature region inside the catalyst/sorbent bed doesn't exist for D ≥ 30 mm. The maximum temperature difference inside the catalyst/sorbent bed is greater than 67 °C. Both the CH ₄ conversion and H ₂ molar fraction are slightly decreased with the increase of D. However, this phenomenon is qualitatively opposite to that for small WHSV of 0.67 h ⁻¹ . The CH ₄ conversion and H ₂ molar fraction at D = 60 mm are 52.6% and 78.7%, respectively, which are much lower than those for WHSV = 0.67 h ⁻¹ .