H2 Purge

Voltage situation during startup of a fuel cell with both anode and cathode chambers filled with H2. 

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Figure 10.9. Equilibrium-controlled desorption characteristics of C02 by H2 purge.

The key impurities present in a typical ROG for the recovery of H2 by a PSA process are bulk Ci and C2 and dilute C3 and C4 hydrocarbons. Figures 10.11 and 10.12 describe the pure gas adsorption isotherms of the components of ROG at 30 °C on the BPL activated carbon and a silica gel sample (Sorbead H produced by Engelhard Corp.), respectively.31 These data were also measured in Air Products and Chemicals, Inc. laboratories. It may be seen that the carbon adsorbs C3+ hydrocarbons very strongly. Consequently, desorption of these hydrocarbons from the carbon by H2 purge becomes rather impractical requiring a large volume of purge gas. 

Figure 10.13 compares the isothermal desorption characteristics of pure (a) C2H6 and (b) C3H8 by H2 purge from the BPL carbon and the silica gel samples at 1.0 atm and 30 °C.31 These data were also generated in the same fashion as those for the components of the SMROG. They show that desorption of C3H8 from the carbon by... 

Fig. 5 shows typical results of coke formation continuously measured with the TEOM. Upon introducing the 12DCP feed into the N2/H2-purged microbalance, one immediately... 

Glass vials in a 11 autoclave, 500 pi, RT, 40 bar H2 Library screened under H2 purge,... 

A drawback of the above described H2 purging is that a H2 purging loop must be provided to the cathode, which slightly increases the system cost and the operational complexity. In order to overcome this drawback, we can rely on H2 diffusion through the PEM from the anode to the cathode as shown in Figure 2.20. After the fuel cell is shut down, the anode outlet, the cathode inlet and outlet are closed but the anode inlet is kept open. The initial stage... 

Despite the larger capacity and selectivity of adsorption of CO2 on the zeolite, the activated carbon is chosen as the preferred adsorbent for bulk CO2 removal from SMROG because it is easier to desorb CO2 from the carbon by H2 purge as shown by Figure 6 [4]. It shows the fractional amount of CO2 desorbed from a BPL carbon or 5A zeolite column, which was initially equilibrated with CO2 at 1 bar and 30 C, as a function of the specific amount of H2 leaving the column during the isobaric and isothermal purge process. Clearly, much less H2 is consumed to remove CO2 from the carbon column. This property makes the activated carbon the material of choice for removal of bulk CO2 by a PSA process. 

Figure 9 shows a schematic flow diagram and an example of the hybrid H2 PSA-SSF membrane concept. The fresh feed to the PSA process is SMROG. The PSA process cycle is an abridged version of the Poly-bed process with only two co-current depressurization steps, having a H2 recovery of 77.6%. The countercurrent depressurization effluent gas is fractionated. The initial part of this gas, which is richer in H2, is directly fed to a SSF membrane at a pressure of 3 bar. The H2 purge effluent gas is compressed to 3 bar and fed to the same membrane. The H2 enriched high pressure effluent gas from the membrane is recompressed and recycled as feed gas to the PSA process. This increased the overall H2 recovery of the hybrid process to 84.0% [23]. 

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