Modeling and Simulation of the Transient Reaction Behavior in Reactive Sorption Enhanced Reforming of Methane with Nano CaO-Based Sorbents for Hydrogen Production

Modeling and Simulation of the Transient Reaction Behavior in Reactive Sorption Enhanced Reforming of Methane with Nano CaO-based Sorbents for Hydrogen Production

Haoliang Ping, Sufang Wu^*

College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, Zhejiang, P. R. China

* Corresponding author E-mail: wsf@zju.edu.cn

Abstract

Reactive sorption enhanced reforming (ReSER) process employing the nano CaO-based sorbents for in situ CO₂ sorption offered a promising high purity hydrogen production technique with a single-reactor.

However, the coupling of steam methane reforming reaction, water shift reaction and CO₂ sorption reaction in one reactor makes it much more difficult to simulate the concentrations of components and temperature distribution along the axial of the reactor exactly. ^[1-2] The key issue is the synergy effect between steam methane reforming (SMR) reaction rate and CaO carbonation rate in the reactor. In order to investigate the above mentioned synergy effect, a two-dimensional axisymmetric pseudo-homogeneous model was set up to describe the transient performance of the reactions within a fixed bed. The mathematic model was numerically solved by COMSOL software with the finite element methods (FEM) to obtain transient methane conversion, concentration of each outlet gas compositions and temperature profile change with time regards the variation of the rate constant of CaO-based sorbents carbonation (k_carb), CO₂ sorption rate and SMR reactions rate.

The simulation results demonstrated that a methane conversion of 96% and a purity of 98% of hydrogen yield (on dry basis) could be obtained by ReSER process, while the methane conversion was 76% and the hydrogen purity was only 74% via steam methane reforming (SMR) process without sorption enhancement under the same reaction conditions. A comparison between the predicted and the experimental data was made and the average relative deviation was 3.86%, which means the model accurately depicted the ReSER process and the simulation results were reliable.

Moreover, the simulation results demonstrated that the sorption rate of the CaO-based sorbents played a decisive role in the overall efficiency of ReSER process because an enhanced CO₂ sorption rate would promote the rate of SMR reactions and then resulted in a higher methane conversion as well as the produced hydrogen purity. The methane conversion with nano-CaO sorbent (with k_carb=0.35 s^-1) was 5% higher than the one with limestone (with k_carb = 0.5 s^-1) and the yield hydrogen purity was enhanced by 3%. Therefore, sorbents with faster CO₂ sorption rate resulted in an enhanced CH₄ conversion and a higher hydrogen purity. An optimum reaction temperature of 600 ¡ãC could assure a hydrogen purity of 98%. Alternatively, the purity of produced hydrogen declines form 96% to 89% with the increase of the reaction pressure from 1bar to 5bar due to the suppressed CH₄ conversion.

Keywords: reactive sorption enhanced reforming, hydrogen, CO₂, CaO-based sorbent, simulation

References:

[1]   J. Xu and G. F. Froment, AIChE Journal, 1989, 35, 97-103.

[2]   D. K. Lee, I. H. Baek and W. L. Yoon, Chemical Engineering Science, 2004, 59, 931-942.