Sulfur-tolerant membranes

In the case of sulfur-tolerant membranes, the behavior of H2S should also be carefully considered when selecting the process integration strategy. In the case that H2/H2S selectivity is not sufficiently high, H2S content in the gas turbine fuel leading to unacceptable levels of sulfur dioxide (SO2) emissions might result. In this case, H2S should be separated before the H2... 

Kamakoti, P., B.D. Morreale, M.V. Ciocco, B.H. Howard, R.P. Killmeyer, A.V. Cugini, and D.S. Sholl, Prediction of hydrogen flux through sulfur-tolerant binary alloy membranes, Science, 307, 569-573,2005. 

IimovaTek s S-Tolerant Membrane as a Fnnction of Temperature and Sulfur Content... 

It is worth noting that both Pd-aUoy and sUica-based membranes present some problem about material instability in the WGS environment. The Pd-aUoy membranes can be negatively affected by surface carbonization, sulfur poisoning, and hydrothermal embrittlement, whereas the amorphous silica-based membranes can show some degradation caused by the condensation reaction of sUanol in hydrothermal conditions (Tang et al., 2010). In particular, the siliceous MFI-type zeolite membranes, constituted by a crystalline microporous zeolite membrane, in recent years have been seen as attractive candidates for the WGS reaction because of the high-temperature hydrogen separation and for their intrinsic sulfur tolerance and hydrothermal stability. 

At 635 °C and in the presence of 1000 ppm H2S, all three alloys exhibit H2 flux that approaches the flux in the absence of H2S - true sulfur tolerance. However, the absence of a scale layer does not guarantee high rates of H2 transport. At lower temperatures, H2S suppresses H2 transport, even in cases where no macroscopic scale layer forms. For example, as illustrated in Figure 7, in l(X)0ppm H2S at 350 C, H2 flux through a Pd47Cu53 membrane H2 drops to zero immediately upon addition of H2S [26]. In cases such as this, H2S suppresses transport by reducing rates of H2 dissociation at the membrane surface. 

Coulter KE, Way JD, Cade SK, Chaudhari S, Alptekin GO, Paglieri SN, et al. Sulfur tolerant PdAu and PdAuPt alloy hydrogen separation membranes. J Membr Sci 2012 405 06 ll-9. 

Burke A, Li SW, Winnick J et al (2004) Sulfur-tolerant cathode materials in electrochemical membrane system for H2S removal liom hot fuel gas. J Electrochem Soc 151 D55-D60. doi 10.1149/ 1.1758815... 

The steady-state permeability results obtained for the 1000 pm thick 80, 60 and 53wt% alloys under flowing 1000 ppm H2S/H2 were compared to the transient 1000 ppm H2S/H2 permeability reported previously. Nearly identical trends in permeability were found for these three alloys using these two measurement methods. This agreement reinforces the suggested correlation between the alloy crystal structure and H2S tolerance. The results of these steady-state permeability experiments indicated that when the Pd-Cu alloys had an fee structure, H2S had little impact on permeability but when die structure was bcc, H2S had a moderate to severe impact However, because of the extreme 1000 pm thickness of the membranes used in this test series which could potentially mask effects arising from sulfur interactions, methods were sought to enable continuous H2S exposure of 100 pm and thinner membranes. 

Reformate obtained from low to moderate sulfur liquid fuels (< 15-30 ppm S by weight) would contain about 1-2 ppm sulfin species by volume. This sulfur concentration is low enough to be tolerated by the 60wt.% Pd-40wt.% Cu system according to the literature. Kulprathipanja et al. give an excellent discussion of the impact of sulfur [23] and other components of reformate [24], CO, CO2 and H2O on self supporting Pd-Cu foil and supported thin film Pd-Cu membranes. 

Phosphoric acid is less corrosive and hazardous than is concentrated sulfuric or nitric acid. Its concentrated solutions are irritants to the skin and mucous membranes. The vapors (P2O5 fumes) can cause irritation to the throat and coughing but could be tolerated at <10 mg/m (see also Section 52.4). The acute oral toxicity in rats is reported to be low, the LD50 value being 1530 mg/kg (NIOSH 1986). 

In general, the concentration of sulfur and its compounds would likely need to be reduced to the part per billion level to protect unalloyed or palladium-silver-based membranes. Removal of sulfur to tolerable levels from feeds originating from natural gas is much more straightforward relative to removal of sulfur and other impurities from coal according to Riesenfeld and Kohl [73]. 

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