Non-isothermal CFD Study of Ethanol Steam Reforming in a Catalytic Membrane Reactor
Rui Ma1, Bernardo Castro-Dominguez2, Anthony G. Dixon1, Yi Hua Ma1
1.Center for Inorganic Membrane Studies, Department of Chemical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609, USA.
2. University of Limerick, Sreelane, Castletroy, Co. Limerick, Ireland
Single unit hydrogen production and separation achieved by conducting ethanol steam reforming (ESR) in a catalytic membrane reactor (CMR) has attracted more interest due to the carbon neutral property of bio-ethanol, and the high reactant conversion due to process intensification.1 In ESR process, temperature control helps to achieve higher conversions. Compared with traditional packed bed reactors (PBR), heat supply for CMR needs to compensate for the enthalpy that hydrogen carried across the membrane during the permeation process, in addition to the heat loss from endothermic reactions. Furthermore, hydrogen flux through the membrane is temperature dependent as the hydrogen permeability follows Arrhenius law. In this work, a comprehensive heat and mass transfer study of an ESR process in a membrane reactor is carried out using an experimentally-validated non-isothermal CFD model. Retentate and permeate domains were modeled by introducing a negative and positive hydrogen flux term respectively, using Sieverts’ law. In order to increase the hydrogen permeation driving force, a purge gas was applied to the permeate side. Effective thermal conductivity and diffusivity were introduced to the simulation in order to take into account the catalyst bed.
A “cold spot” adjacent to the membrane was observed (Fig 1) since this highly endothermic process benefits from hydrogen removal adjacent to the membrane. From the reaction rate study of the ESR process, the process takes place very rapidly, and a slight reverse reaction for methane steam reforming was observed due to the decrease in equilibrium conversion at lower temperature. This phenomenon is most noticeable within the “cold spot”. A clear hydrogen generation and depletion along the reactor on the retentate side was observed. In addition, the reactions reach equilibrium shortly after reactants enter the reactor which leads to an inefficient use of the reactor and the membrane surface area; thus, counter-current flow of the purge gas is studied, which helps with shifting the reaction equilibrium throughout the entire reactor. Using this model, the effect of temperature and pressure on hydrogen recovery and yield are studied in both co-current and counter-current cases, and a process optimization study was carried out.
Fig 1. Temperature distribution in the reactor at LHSV=3.77h-1, Pret =6 bar, S/E=5 and Tin=723K on both permeate and retentate side.
1) Ma, R., Castro-Dominguez, B., Mardilovich, I.P., Dixon, A.G. and Ma, Y.H., "Experimental and Simulation Studies of the Production of Renewable Hydrogen through Ethanol Steam Reforming in a Pilot-Scale Catalytic Membrane Reactor," Chem. Eng. J., 303, 302-313 (2016).