Retenate partial pressure

Figure 5.34 Hydrogen partial pressure along the length axis of a membrane tubular reactor operated in parallel (a) and counter-flow (b) arrangements solid lines, retenate partial pressure dashed lines, permeate partial pressure reformate flow rate, 162cm min reformate pressure, 1.36 bar sweep gas flow rate, 40cm min permeate pressure, 1.01 bar left, reaction temperature 300°C right, reaction temperature 500°C [411].

Another measure to increase the hydrogen partial pressure difference between permeate and retenate is to use sweep gas on the retenate side. Because the hydrogen requires humidification for low temperature PEM fuel cells to prevent membrane dry-out, steam is the preferred sweep gas [405]. OHany et al. highlighted the effect of steam as the sweep gas for the permeate in a methane steam reforming membrane reactor [406]. Higher methane conversion was observed, which originated from back-diffusion of steam from the permeate to the reaction side of the membrane, which increased the S/C ratio and consequently the conversion. 

A membrane separation device was prepared by Wilhite et al. by micro-electromechanical techniques [526]. The palladium/silver membrane deposited onto a silicon oxide support was only 20-nm thick, which was possibly the lowest membrane thickness ever reported for hydrogen separative purposes. A lanfhanum/nickel/cobalt oxide catalyst (LaNi0.95Co0.05O3) catalyst for partial oxidation of methanol was deposited onto the membrane. At a O/C ratio of 0.86 and 475 °C reaction temperature, up to 64% methanol conversion and more than 90% hydrogen selectivity could be achieved. These workers claimed a deviation from Sievert s law (see Section 5.2.4) for their membrane. N amely, the hydrogen flux did not depend by a power of 0.5 of the retenate and permeate pressure but rather by a power of 0.97, which they attributed to the absence of internal solid-state diffusion limitations in their ultra-thin membrane. 

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