Transient reaction phenomena of sorption-enhanced steam methane reforming in a fixed-bed reactor

Abstract

The transient chemical reaction phenomena of the sorption-enhanced steam methane reforming (SE-SMR) by using Ni/Al2O3 catalyst and CaO sorbent in a tubular fixed-bed reactor were numerically investigated by an experimentally verified unsteady 2D model. Four chemical reactions are involved in SE-SMR including steam reforming (SR), water gas shift (WGS), global steam reforming (GSR), and CO2 sorption. The reaction process in time is divided into period 1, transient period, and period 2. The high-purity H2 is produced in period 1 which is defined as the outlet molar fractions of H2 ≥ 90% and CO ≤ 1% (dry basis) in this work. In the first half of period 1, the endothermic reaction rates of SR and GSR are dominant in the entrance region of catalyst/sorbent bed. The WGS and CO2 sorption reactions are triggered by SR and GSR reactions. The heat transfer from the wall plays an important role. Higher CaO conversion, temperature, and reaction rates appear first near the wall region, then they gradually expand to the central region. In the second half of period 1, a sharp wave-shaped curve of strong CO2 sorption reaction occurred in downstream becomes dominant and it moves to downstream almost at a constant speed, as time progresses. The peak value of the CO2 sorption reaction is more than twice larger than that of SR or WGS. The SR and WGS reaction rates are significantly enhanced by CO2 sorption reaction. The great sorption, WGS, and SR reactions result in a high-purity H2 production with the outlet molar fractions of 95.8% H2, 0.998% CO, and 0.73% CO2 at the end of period 1, based on the parameters used in this work such as reactor temperature of 600 °C. The maximum CaO conversion is about 76% in end of period 1 and the average CaO conversion in the reactor is 51%. The 2D distributions of CaO conversion, temperature, and reaction rates are also presented and discussed.

Topics & Concepts