Water-gas shift in membrane reactors

Barhieri et al. developed a membrane reactor for water-gas shift 544]. A palladium/ silver film containing 23 wt.% silver, which was between 1- and 1.5-pm thick was produced hy sputtering. This film was coated onto a porous stainless steel support. This patented production method allowed a much higher ratio of pore size to film thickness compared with conventional methods. Tubular membranes of 13-mm outer diameter, 10-20-mm length and 1.1-1.5-pm thickness, respectively, were prepared. A commercial Cu based catalyst supplied by Haldor-Topsoe was used for the water-gas shift reaction. At 210 °C a permeating flux of 4.5 L (m s) was determined for pure hydrogen at 0.2-bar pressure drop. At a reaction temperature of 260-300 °C, and 2085 h gas hourly space velocity, the thermodynamic equilibrium conversion could be exceeded by 5-10% with this new technology. 

Both the preferential oxidation of carbon monoxide and the hydrogen oxidation, which occurs in parallel as an undesired side reaction, are highly exothermic. Preferential oxidation 

Formation of carbon monoxide over the catalyst by the reverse water-gas shift reaction (RWGS) in an oxygen-deficient atmosphere is frequently observed especially under conditions of partial load, because most catalysts for preferential oxidation of carbon monoxide have some activity for WGS and its reverse reaction. Therefore oversizing the reactor bears the danger of impaired conversion and the same applies for partial load of the reactor unfortunately. Because the concentration of carbon monoxide that is tolerated by low-temperature fuel cells is usually in the range below 100 ppm or less, even low catalytic activity for reverse shift becomes an issue. 

Selective methanation of carbon monoxide does not require air addition to the reformate but suffers not only from competing CO2 methanation [148], 

The most critical issue is that because the concentration of carbon dioxide is much higher in the reformate compared to that of the carbon monoxide, the catalyst has to be very selective for carbon monoxide methanation. The operating window of methanation catalysts is relatively small, in the temperature range around 250-300°C, because a trade-off is required between sufficient activity and selectivity. Well above 250°C 

---

Marigliano et al. performed further modelling work for methane steam reforming and water-gas shift in membrane reactors [415]. They defined the sweep factor I as the ratio of flow rates of inert gas on the permeate side to the flow rate of methane on the reaction side of the membrane ... 

---