(608d) Non-Isothermal CFD Study of Ethanol Steam Reforming in a Catalytic Membrane Reactor

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).