(449ak) A Computational Fluid Dynamics Study of a Pilot-Scale Multitube Membrane Module for Hydrogen Purification

A Computational
Fluid Dynamics Study of a Pilot-scale Multitube Membrane
Module for Hydrogen Purification

Rui Ma, Bernardo Castro-Dominguez, Ivan P.
Mardilovich, Nikolaos K. Kazantzis, Anthony G. Dixon, Yi Hua Ma

Center
for Inorganic Membrane Studies, Department of Chemical Engineering, Worcester
Polytechnic Institute, 100 Institute Road, Worcester, MA 01609, USA.

      Palladium alloy
membranes play an important role in the separation of hydrogen, which is an
essential material in industrial manufacturing and also a clean energy carrier with
an expanding market. A pilot-scale multitube membrane
module has been built and tested previously; however, a thorough understanding
of the flux distribution, species profiles and mass transfer limitations is
needed to further optimize the model performance. For this purpose, a 3-D
computational fluid dynamics (CFD) simulation model was built utilizing COMSOL Multiphysics and validated with the experimental data.

      The module
contains seven Pd/Au/Pd membrane
tubes with one at the central axis and six placed symmetrically around the axis,
giving a total permeable area of 1050 cm². Coal-derived syngas was fed
into the module from the shell side (retentate side) and hydrogen permeated through
the membrane was collected from the tube side (permeate side) while the
unwanted gas was depleted from the shell side. The species continuity equations
and the equation of motion were solved simultaneously by treating the fluids on
both sides of the membrane as concentrated mixtures. In this model no species
is considered as solvent or solute, thus, while hydrogen is being removed from
the retentate side, the gas mixture properties such as density varied along
with the composition change. A flux term was defined at the tube surface to
describe the process of hydrogen transport through the membrane using SievertsÕ
law (Eq. 1):

in which PH2 is
hydrogen permeance [Nm³h^-1m^-2bar^-0.5]
and PH2shell, PH2tube are the
hydrogen partial pressure on the shell side and tube side [bar], respectively.

     A fine mesh with
2,000,000 elements and a stabilization method with a tuning parameter of d=0.25 were
applied in this model in order to achieve better convergence. The simulation
showed a mass balance error of 2%, which could be further reduced by applying a
finer mesh. The module showed a high accuracy compared with experimental data (permeate
flow rate [L/min], retentate H₂ percentage [%], and H₂
recovery [%]). From the simulation results of the module, concentration
polarization can be visualized and characterized, and the influence of operating
conditions on membrane performance including hydrogen recovery and membrane
usage can be analyzed to gain insight.

      It was also observed that the performance
of the seven membrane tubes is differentiated based on their relative position due
to the following reasons: 1) The central tube displays a competing role when
its performance is comparatively assessed against the 6 surrounding tubes which
leads to a lower H₂ flux of the central tube in comparison to the
other six. 2) The retentate outlet is on the reactor side as shown in Figure 1,
which makes the flow pattern and H₂ profile in the reactor
asymmetric and causes the driving force of each tube to vary. Finally, within
the context of the present study, it is demonstrated that the results derived
and the data generated could reliably inform efforts for further model scale-up.

Figure 1. Representative sketch of the module