Dense cermet membranes for hydrogen separation

https://doi.org/10.1016/j.seppur.2013.10.001Get rights and content

Highlights

  • Cermet (ceramic–metal) membranes have been developed for hydrogen separation.

  • Flux is limited by the bulk diffusion of hydrogen through metal phase.

  • Long term stability for a period of 4 months showed relatively stable flux.

  • Flux showed no degradation in synthesis gas mixture at high pressures (≈200 psig).

  • Membrane is stable in atmospheres containing up to 400 ppm H2S and 80% H2.

Abstract

Dense cermet (i.e., ceramic–metal composite) membranes have been developed for separating hydrogen from mixed gases, particularly product streams generated during coal gasification and/or steam methane reforming. Hydrogen separation with these membranes yields high-purity hydrogen, thereby eliminating the need for post-separation purification steps. Extensive tests have been conducted with cermet membranes made by mixing ≈50–60 vol.% Pd with Y2O3-stabilized ZrO2. Using several feed gas mixtures, the hydrogen permeation rate, or flux, for the membranes was measured in the temperature range 400–900 °C. With pure hydrogen at ambient pressure as feed gas, an ≈18-μm-thick membrane on a porous substrate gave a measured flux of ≈26 cm3[STP]/min-cm2 at 400 °C and ≈52 cm3[STP]/min-cm2 at 900 °C. We also measured the hydrogen flux through a thicker (≈150 μm) membrane at 400 °C using a mixture of H2, CO, CO2, H2O, and He at ≈200 psig as feed gas. Hydrogen flux measurements in H2S-containing atmospheres showed that the cermet membranes are stable at 900 °C in gases that contain ≈80% H2/400 ppm H2S. Because formation of palladium sulfide (Pd4S) can seriously degrade hydrogen permeation through Pd-containing cermet membranes, the Pd/Pd4S stability phase boundary of the cermet membrane was determined in the temperature range 450–650 °C using various feed gases that contained 10–73% H2 and 8–400 ppm H2S. Given these promising results, longer studies using real-world coal gasification conditions should be pursued.

Introduction

The Office of Fossil Energy (FE) at the U.S. Department of Energy (DOE) sponsors a wide range of research, development, and demonstration programs to maximize the use of the vast domestic fossil resources and to ensure a fuel-diverse energy sector while responding to global environmental concerns. Cost-effective, membrane-based reactor and separation technologies are of considerable interest to DOE’s clean coal program to develop advanced coal-based power and fuel technologies. Argonne National Laboratory is developing dense hydrogen transport membranes (HTMs) for separating hydrogen from mixed gases, particularly product streams generated during coal gasification and/or methane reforming. Hydrogen separation with these membranes is nongalvanic (i.e., it does not use electrodes or an external power supply to drive the separation), and hydrogen separated from the feed stream is of high purity, so post-separation purification steps are unnecessary.

Materials development for the HTM at Argonne has followed a three-pronged approach. In one approach, we investigated single-phase mixed (ionic/electronic) conducting perovskite ceramics [1], [2]. The single-phase membranes gave a low hydrogen flux due to their poor electronic conductivity [3], [4]. In our second approach, we developed cermet (i.e., ceramic–metal composite) membranes that contained mixed-conducting perovskite ceramics combined with a metallic component [5], [6], [7], [8], [9]. In these cermets, the metal enhanced the hydrogen flux of the ceramic phase by increasing the electronic conductivity of the cermet. In our third approach, we dispersed a hydrogen transport metal, i.e., metal with high hydrogen permeability (Pd, Pd–Ag), in a thermodynamically and mechanically stable ceramic matrix (Al2O3 or yttria-stabilized ZrO2, YSZ) [10]. The cermets, made by our third approach, exhibit the highest hydrogen flux [11], [12].

In this paper, we report hydrogen flux measurements for Pd/YSZ cermet membranes as a function of temperature. Good chemical stability is a critical requirement for HTMs due to the corrosive nature of product streams from coal gasification and/or methane reforming. Hydrogen sulfide (H2S) is a particularly corrosive contaminant that HTMs are expected to encounter. When H2S reacts with a Pd/YSZ cermet membrane, palladium sulfide (Pd4S) forms on the membrane surface. Because Pd4S impedes hydrogen permeation through the membrane, the chemical stability of HTMs was evaluated by determining the conditions under which Pd4S forms. The Pd/Pd4S phase boundary was determined in the temperature range ≈450–650 °C using various feed gases that contained 10–73% H2 and ≈8–400 ppm H2S. To assess the effect of water vapor on hydrogen permeation through HTMs, the chemical stability of the cermet membrane was tested in the presence of steam by measuring its hydrogen flux in feed gas that contained 0.03–0.49 atm H2O. Finally, we report here the hydrogen flux of the cermet membrane versus time (up to 120 days) during exposure to simulated syngas containing H2, CO, CO2, and H2O. In addition, it is anticipated that the cermet-type membranes can overcome some drawbacks of pure Pd or Pd alloy membranes, i.e., reduce the cost and improve the mechanical strength of the membrane at high temperature (>700 °C).

Section snippets

Experimental

The powder mixture for fabricating the cermet membranes was prepared by mechanically mixing ≈50 vol.% Pd (average particle size ≈1.5 μm) with Y2O3-stabilized ZrO2 (average particle size, ≈1.0 μm). This powder mixture was pressed into disks and sintered at ≈1400 °C for ≈5 h in ambient air. For hydrogen permeation tests, both sides of the disks were polished with 600-grit SiC paper to obtain the desired thickness and produce faces that were flat and parallel to one another. Thin films of cermet

Results and discussion

Fig. 1 shows the temperature dependence of hydrogen flux through an ≈18-μm-thick Pd/YSZ cermet membrane on a porous alumina substrate disk made by the paste-painting method [8]. The feed gas was 90% H2/He and the sweep gas was N2. The thickness of the porous alumina substrate was ≈1.65 mm. The flow rate of both feed and sweep gas was adjusted to ≈500 mL/min to minimize concentration polarization. The feed and sweep gases were at ambient pressure. This membrane gave record high flux values of ≈26 cm

Conclusions

We have developed dense Pd/YSZ cermet membranes for separating hydrogen from mixed gases, particularly product streams generated during coal gasification and/or steam methane reforming. The highest measured hydrogen flux was ≈52 cm3[STP]/min-cm2 for a ≈20-μm-thick membrane on a porous substrate at 900 °C using 100% H2 at ambient pressure as the feed gas. The effect of hydrogen partial pressure on hydrogen flux indicates that the flux is limited by the bulk diffusion of hydrogen through the Pd

Acknowledgment

Work supported by the U.S. Department of Energy, Office of Fossil Energy, National Energy Technology Laboratory’s Advanced Fuels Program, under Contract DE-AC02-06CH11357.

References (18)

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The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

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