(386f) A Modeling Study of Different Membrane Process Schemes and Spiral-Wound Module Flow Configurations for Hydrogen Purification

Hydrogen obtained from syngas needs to be purified to ppm and ppb levels of CO and H₂S, respectively, for use in platinum-based proton-exchange membrane (PEM) fuel cells.  We have shown the feasibility of using an amine-containing facilitated transport polymer membrane-based process for the purification.  A membrane separator to simultaneously remove H₂S and CO₂ is at the heart of this process.  Air or steam can be used as a sweep gas to carry the permeated gases and provide greater driving force for separation.  This step is followed by a water-gas-shift (WGS) reactor to reduce the CO amount in a CO₂ lean syngas.  Alternatively, the WGS reactor can be replaced by a membrane reactor housing the WGS catalyst on the feed side.  The membrane reactor can help reach lower levels of CO due to effective shifting of the equilibrium but is more difficult to operate and model due to catalyst integration and heat management.

The membrane mentioned previously, which is a thin-film composite membrane can potentially be manufactured as a flat-sheet membrane and spirally wound over a central perforated tube.  Such a spiral-wound module configuration has been traditionally used for reverse osmosis and pervaporation applications in a cross-current mode without a sweep gas.  However, a spiral-wound module capable of handling a sweep gas can be found in the patent literature [1].  Such a module can be used in both cross-current as well as countercurrent modes.

This study involves the modeling of the aforementioned spiral-wound module in both the membrane separator as well as the membrane reactor modes.  The modeling has taken into account material and energy balances.  1-D model countercurrent flow calculations were performed in MATLAB using the bvp4c solver while the 2-D cross-current calculations were performed in COMSOL Multiphysics, a simulation software which utilizes the finite element technique.  Although it is expected that countercurrent flow between feed and sweep streams would provide a greater average driving force for separation and reduce the membrane area, the calculations have shown that the effect of flow mode becomes more important for the membrane separator under lower driving force across the membrane or lower sweep to feed molar ratios.  In case of the membrane reactor, the flow mode affects the separation and reaction more intricately due to its simultaneous effects on both the heat and mass transfer driving forces.  Temperature and concentration profiles and membrane areas have been quantified and compared for the two flow modes under various operating conditions.

Reference:

[1] D. Reddy, S. Ramon, T.Y. Moon, C.E. Reineke, Counter current dual-flow spiral wound dual-pipe membrane separation, U.S. Patent, 5,034,126 (1991).