(652d) Modeling of CO2-Selective Membrane Processes for Hydrogen Purification

Acid gas separation is integral to hydrogen purification and carbon sequestration. Syngas produced from any carbonaceous fuel mainly contains carbon dioxide, carbon monoxide, hydrogen and water vapor among other gases. Industrially, absorption using amines is typically used to remove acid gases, while a combination of two water gas shift reactors can bring down the CO to about 0.5 ? 1%. For use of hydrogen in platinum-based proton-exchange membrane (PEM) fuel cells, the CO level needs to be reduced to below 10 ? 100 ppm depending upon the operating temperature of the fuel cell. For a high temperature fuel cell, the limit is about 100 ppm. H2S should go down to 10 ppb to protect both the fuel cell and the low temperature water-gas-shift catalyst.

Using experimentally obtained performance parameters of our highly CO2-selective and permeable facilitated transport membranes containing amines in polymer networks, modeling calculations have shown that hydrogen can be successfully purified to these impurity levels with different water gas shift reactor/membrane separator/membrane reactor series configurations. The modeling has taken into account material and energy balances as well as pressure drop characteristics of the membrane module. Two types of membrane modules were considered for the calculations: Hollow fiber and Spiral wound modules. 1-D hollow fiber model calculations were performed in MATLAB using the bvp4c solver while the 2-D spiral wound model calculations were performed in COMSOL Multiphysics, a simulation software which utilizes the finite element technique. Different process configurations using different types of modules were simulated to calculate the various stream conditions according to the afore-mentioned outlet concentration constraints and thus show their technical feasibility.

The three process configurations studied include: (a) Isothermal membrane separator/Low temperature water gas shift reactor, (b) High temperature shift reactor/Membrane reactor, and (c) High temperature shift reactor/Isothermal membrane separator + Low temperature shift reactor. A high temperature shift reactor was found to be desirable for the membrane reactor case (Case (b) above) to reduce the temperature rise taking place in an adiabatic membrane reactor. This is necessary to keep the temperature within the optimum temperature range for membrane performance. Common constraints of 100 ppm CO and 10 ppb H2S were applied in all the cases. In the case of the membrane reactor, H2S was reduced to below 10 ppb before the water gas shift reaction started. The study has successfully shown that the membrane provides a high degree of flexibility in the design of the process for a hydrogen purification system.