A numerical study of pellets having both catalytic- and capture properties for SE-SMR process: Kinetic- and product layer diffusion controlled regimes

• Published in: Fuel processing technology Vol. 106; pp. 231 - 246
• Main Authors: Rout, K.R., Jakobsen, H.A.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01-02-2013; Elsevier
• Subjects: active sites; adsorbents; Applied sciences; calcium carbonate; calcium oxide; carbon dioxide; carbonation; catalysts; Diffusion model; diffusivity; Energy; Energy. Thermal use of fuels; Exact sciences and technology; Fuels; Gas–solid kinetics; hydrogen; hydrogen production; Intra-particle mass diffusion; mass transfer; mathematical models; methane; pellets; Product layer diffusion; sorption; Spherical grain model; steam; temperature; Spherical grain model; Product layer diffusion; Gas–solid kinetics; Diffusion model; Intra-particle mass diffusion; Methane; Gaseous diffusion; Gas-solid kinetics; Gas solid reaction; In situ; Active site; Carbon dioxide; Modeling; Convection; Reaction mechanism; Adsorbent; Diffusion coefficient; Mathematical model; Void fraction; Reaction rate; Diffusivity; Mass transfer; Sorption; Numerical analysis; Carbonation; Steam reforming; Kinetics; Performance; Sorbent; Catalyst
• ISSN: 0378-3820; 1873-7188
• DOI: 10.1016/j.fuproc.2012.07.029

Sorption enhanced steam methane reforming (SE-SMR) is a promising process for the production of H2. The in situ removal of CO2 in the steam methane reforming (SMR) process shifts the reaction equilibrium according to the Le Chatelier's principle towards higher H2 production. Most of the earlier studies worked on the SE-SMR process using two different pellets (2-P): one for the reforming catalyst and the other one for the CO2 adsorbent. In this study, we propose a novel pellet design where both the catalyst and sorbent are put together in a support (1-P). In the 1-P system, the produced CO2 molecules due to the reforming reactions will be captured by the sorbent inside the pellet. Whereas in the 2-P system, the CO2 molecules have to transported in between the pellets to be captured by the sorbent. Hence, 1-P design has less resistance for mass transfer between the catalyst and sorbent active sites. In this study, the internal mass transport limitations of the 1-P design for the SE-SMR process have been investigated.
The dusty gas multicomponent mass diffusion pellet model formulated for the SE-SMR process describes the evolution of species mole fractions, pressure, total concentration, temperature, fluxes, and convection within the voids of the porous pellet. The internal effectiveness factors of the SE-SMR process have been calculated for the SE-SMR process. The effective diffusivities are described according to the parallel pore- and random pore models. The carbonation reaction in the SE-SMR process is a gas–solid reaction system, which generally follows one out of two controlled regimes, namely the kinetic- and product layer diffusion (PLD) controlled regimes. A mathematical model based on the grain model was applied modelling the carbonation reaction. The change in pellet void-fraction due to the formation of the solid CaCO3, the diffusion of gaseous phase through the product layer, the structural changes of the spherical grains by the inclusion of variable diffusion coefficient and multiple carbonation/calcination cycles were considered in the pellet model. The effect of pellet diameter on the carbonation reaction rate is investigated to observe the complex mechanism of gas–solid reaction system.
The 1-P performance is promising compared to the conventional 2-P design. Further, the reduction in void-fraction with the CaO conversion, product layer diffusion, pellet diameter and multiple cycles are all important effects in the SE-SMR process because these mechanisms influence on the reaction rate, capture capacity, and the effectiveness factors.
► Grain model was derived modelling the carbonation reaction. ► Grain model considers both kinetic- and product layer controlled regimes. ► Diffusional limitation of the pellet has been studied using grain model.