(268e) Performance of Pd-Alloy Membranes for Hydrogen Separation from Mixed Feed Streams Containing 1000 Ppm H2s

Introduction

There is growing interest in the integration of membrane technology into commercial processes such as coal gasification for simultaneous hydrogen production and separation. Consequently, membrane materials will be exposed to varying concentrations of impurities in the feed gas stream. H₂S is a major concern since it is known to adversely affect the performance of metal-based membranes with respect to hydrogen transport and membrane stability.

Pd-Cu alloys have gained widespread interest and are regarded as promising candidates for membrane reactor applications as a result of their catalytic activity towards the water-gas shift reaction (Bustamante et al., 2005), in addition to potential H₂S tolerance at certain operating conditions and H₂S concentrations (Morreale et al., 2004 ). As a result, NETL is conducting extensive tests of these materials as part of a program to develop H₂S tolerant, post-gasifier water-gas shift membrane reactors to enhance hydrogen production via coal gasification (Howard et al., 2004).

Results and Discussion

Effect of H₂S

The performance of various Pd-Cu alloy membranes as well as pure Pd membranes have been studied over a range of temperatures in the presence of 1000 ppm H₂S in H₂. Preliminary analysis shows dramatic losses of H₂ flux for 100 µm Pd membranes at moderate temperatures in addition to the growth of a stable sulfide layer on the membrane surface. Figure 1 depicts the decrease in H₂ transport of a pure Pd membrane on exposure to the H₂-H₂S feed stream at 623K and 0.6 MPa. SEM elemental mapping of the membrane cross section revealed a Pd-S layer approximately 18 µm-thick on the membrane surface after a 120 hour exposure. XRD analysis determined the sulfide layer to be Pd₄S. Similar performance decreases were observed for Pd-Cu alloy membranes at moderate temperatures.  By contrast, at higher temperatures, preliminary results for some Pd-Cu alloys suggest minimal change in membrane performance.

Figure 1. Flux results for a 100 mm Pd membrane exposed to 1000 ppm H₂S?H₂ at 623K as a function of exposure time.

Figure 2. Pd and S elemental maps of the cross section of a 100 µm Pd membrane exposed to 1000 ppm H₂S-H₂ at 623K for 120 hrs.

Effect of CO, CO₂ and H₂O

Experiments to investigate the influence of high CO, CO₂ and H₂O concentrations on 80wt%Pd-Cu alloy membranes were conducted using 1 mm thick membrane samples. Although these samples were too thick to accurately demonstrate the influence of these gases on the H₂ transport properties of the membrane, the experiments revealed valuable information regarding the influence of these components on membrane surface morphology. For example, SEM analysis of the membrane surface revealed significant surface modification after exposure to a 50%H₂-CO feed at 1038K and 1.11 MPa (Figure 3). Similar catalytic surface etching was observed in the H₂-H₂O environment at temperatures between 908 and 1038K. Figure 4 shows the image of an 80wt%Pd-Cu membrane after exposure to the H₂-H₂O environment at 908K and 1.55 MPa.

Figure 3. SEM micrograph of the feed-side surface of the 80wt%Pd-Cu membrane after exposure to a 50%H₂-CO feed at 1038K and 1.11 MPa. The membrane was exposed to the feed stream for 24 hrs. CO₂, CH₄ and H₂O were also present as a result of side reactions.

Figure 4. SEM micrograph of 80wt%Pd-Cu membrane after exposure to a 50%H₂-H₂O feed stream at 908K and total unit pressure of 1.55 MPa for 24hrs.

In contrast to the 80wt%Pd-Cu membranes exposed to binary mixtures of H₂ with H₂O, CO or CO₂ feed streams at temperatures between 873 and 1038K which appeared significantly roughened, the membranes exposed to similar feed stream at the higher temperature of 1173K remained relatively smooth (Figure 5).

Figure 5. SEM micrograph of the feed-side surface of the 80wt%Pd-Cu membrane after exposure to a 50%H₂-CO feed at 1173K at 1.11 MPa. The membrane was exposed to the feed stream for 24hrs.

Conclusion

Preliminary data for 100 µm thick membranes suggest that in addition to markedly reducing H₂ flux, H₂S impurity levels of 1000 ppm result in corrosion of Pd and some Pd-Cu alloy membranes at low to moderate temperatures. Also, exposing the 80wt%Pd-Cu membranes to CO, CO₂ and H₂O at temperatures between 873 and 1173K resulted in dramatic surface modification. However, the membranes showed minimal adverse surface modification in the presence of the binary mixed feed streams at 1173K. Together, these preliminary results suggest that at the high temperature conditions of post-gasifier water-gas shift membrane reactors, some Pd-Cu compositions may tolerate similar H₂S impurity levels.  Future experiments are planned to test this hypothesis.

References

F. Bustamante, R. Enick, R. Killmeyer, B. Howard, K. Rothenberger, A. Cugini, B. Morreale, M. Ciocco. Uncatalyzed and Wall-catalyzed forward water-gas shift reaction kinetics, AIChE J. 51 (2005) 1440

B. Howard, R. Killmeyer, K. Rothenberger, A. Cugini, B. Morreale, R. Enick, F. Bustamante. Hydrogen permeance of palladium-copper alloy membranes over a wide range of temperatures and pressures, Journal of Membrane Science 241 (2004) 207

B. Morreale, M Ciocco, B. Howard, R. Killmeyer, A. Cugini, R. Enick. Effect of hydrogen-sulfide on the hydrogen permeance of Palladium-copper alloys at elevated temperatures, Journal of Membrane Science 241 (2004) 219