Hydrogen purification through water-gas shift (WGS) is a favored option in fuel processing for hydrogen fuel cells. A three-dimensional single channel model is developed to simulate the behavior of a water-gas shift micro reactor. The flow regime is assumed to be steady and laminar; furthermore, it is presumed that the walls are isothermal. A water-gas shift reaction rate model is utilized to simulate the surface reaction on Pt/TiO2 catalyst. The gas feed composition is taken as the efflux of a typical auto-thermal reforming (ATR) reactor. A parametric study is conducted to investigate the effect of gas feed temperature, gas space velocity and channel length on water-gas shift micro reactor performance. The study resulted in an optimum water-gas shift micro reactor design. It should be noted that a water-gas shift micro reactor is an essential part of a reactor train to remove carbon monoxide from a hydrogen rich mixture. Such a mixture can be used as fuel for a PEM fuel cell in portable devices. The results of these simulations revealed that the optimum reactor consists of a square cross section channel with 100 μm hydraulic diameter, 20 mm length, space velocity of 1000 h−1 and gas feed temperature of 270°C. The carbon monoxide mole fraction in the efflux is in a range suitable for a typical preferential oxidation (PROX) reactor.

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