Abstract
Hydrogen is considered as a real alternative for improving the current energy scenario in the near future and separation processes are a crucial step for the economy of the process in both centralized and distributed production systems. In this context, Pd-based composite membranes appear as an attractive technology trying to reduce the Pd thickness by modifying the commercial supports, mainly formed by metals to fit properly in conventional industrial devices. In most cases, a final calcination step is required and hence, the metallic support can be oxidized. This work analyzes in detail the properties of intermediate layers generated by in-situ oxidation of tubular PSS supports as a crucial step for the preparation of Pd/PSS membranes. The oxidation temperature determines the modification of original morphology and permeability by increasing the presence of mixed iron-chromium oxides as temperature rises. A compromise solution need to be adopted in order to reduce the average pore mouth size and the external roughness, while maintaining a high permeation capacity. Temperature of 600 °C lets to reduce the average pore size from 3.5 to 2.1 μm or from 4.5 to 2.3 μm in case of using PSS supports with 0.1 or 0.2 μm porous media grades, respectively but maintaining a hydrogen permeation beyond targets of United States of America Department of Energy (US DOE). Lower temperatures provoke an insufficient surface modification, while greater values derive in a drastic reduction of permeability. In these conditions, two composite membranes were prepared by ELP-PP, obtaining 14.7 and 18.0 μm thick palladium layers in case of modifying PSS tubes of 0.1 or 0.2 μm media grades, respectively. In both cases, the composite Pd membranes exhibited a hydrogen perm-selectivity greater than 2000 with permeances ranged from 2.83 to 5.84·10−4 mol m−2 s−1 Pa−0.5 and activation energies of around 13–14 kJ mol−1.
Highlights
Hydrogen is considered as a real alternative to improve the energy scenario currently based on fossil fuels, promoting the use of renewable energies in the near future [1,2]
The properties of intermediate layers generated by in-situ oxidation of tubular 316L porous stainless-steel supports (PSS) supports with two different commercial media grades have been studied as crucial parameters for the preparation of composite Pd membranes
The presence of mixed iron-chromium oxides increases as the temperature rises, independently of media grade considered in the PSS support
Summary
Hydrogen is considered as a real alternative to improve the energy scenario currently based on fossil fuels, promoting the use of renewable energies in the near future [1,2]. New routes from waste materials are being widely studied in the last years, considering both solid matter such as biomass [11], plastics [11], tea [12] or industrial municipal wastes [13] and liquids, in example olive mill wastewater [14] In most of these cases, hydrogen purification is required for particular applications, i.e., combustion engines, turbines or fuel cell systems and Pd-based membranes are suggested as an attractive technology [15,16,17] to be used as independent separator [18,19] or in a membrane reactor configuration [17,20]. Ma et al [51] proposed and patented a controlled in-situ oxidation of porous stainless-steel supports (PSS) for fabricating Pd-based composite membranes They affirmed that temperatures higher than 600 ◦C generated enough oxides to prevent the undesirable intermetallic diffusion and to act as an effective intermediate barrier. The support was oxidized for 12 h at a relatively low temperature (500 ◦C) and modified with pre-activated alumina particles
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