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  • Cooney et al examined the permeation

    2018-11-05

    Cooney et al. (2014) examined the permeation stability of hydrogen through a Palladium Vanadium composite membrane at 500 °C for 120 h. The permeation stability of hydrogen was reported to have varied significantly. The membrane failed within the first 20 h due to poor mechanical integrity to a combination of Pd–V inter-diffusion and high susceptibility to oxidation. Okazaki et al. (2011) investigated the thermal stability of Pd80–Ag20 supported by porous α-Al2O3 at temperatures between 380 and 830 °C. At temperatures above 600 °C the permeation rate of hydrogen declined due to intermetallic rho inhibitor between Pd and the support. Stable permeation was observed for 100 h at 550 °C. A study by Chotirah et al (Chotirach et al., 2012) confirms the findings of Okazaki et al. (2011) that intermetallic diffusion starts occurring at the Tamman temperature of the support material. (Tamman Temperature is generally half the melting temperature and is considered the point at which sintering begins in ceramics materials). Mardilovich et al. (1998) tested a palladium membrane-based reactor for 1100 h at 350 °C. The membrane suffered from recrystallization texture (cluster of grains due to nucleation) and aggregation of Pd grains due to long-term hydrogen exposure. The membrane could not withstand high temperatures of 550 °C due to intermetallic diffusion. Augustine et al. (2012) studied the durability of Pd membranes supported by oxidized porous stainless steel with an aluminium oxide inter metallic diffusion barrier for mixed gas under WGS reaction conditions for 1000 h. The membrane suffered from coke formation after 65 h of operation. The membrane also suffered from leaks, some of which were due to welding. Furthermore, work by Augustine et al. (2011) showed a decline in the permeation of hydrogen due to coke formation. The selectivity also declined to an unknown leak growth mechanism. Abdollahi et al. (2012) utilized a Pd-based membrane to produce ultra-pure hydrogen. The membrane showed complete conversion of carbon monoxide and almost 100% hydrogen recovery.
    Experimental procedure
    Results and discussion
    Conclusions The Pd–Ag membrane reactor diffused hydrogen through the membrane. The diffusion of hydrogen atoms from surface into the bulk membrane and surface contamination was found to be the rate limiting step for hydrogen permeability. The membrane showed good thermal stability at 320 °C for 170 h, but failed at 180 h. The membrane failed earlier at 430 °C (after 150 h) with cracks being formed due to hydrogen diffusion through the membrane. From SEM images, it was seen that cracks were formed on the surface of the membrane film after hydrogen exposure. Micro surface defects were observed due to hydrogen adsorption and desorption. The XRD data showed lattice expansion. Therefore, it can be suggested that the phase transition α- to β-phase caused lattice expansion which may have resulted in the membrane failure. For the WGS reaction, the membrane reactor successfully achieved 84% hydrogen recovery and 88% carbon monoxide conversion in the presence of iron oxide catalyst. High purity hydrogen was produced before the membrane failed. Membrane failure was indicated by the passage of non-permeating gas (N2).
    Recommendations
    Acknowledgements The financial support from the South African Department of Science and Technology (DST) towards HySA Infrastructure KP4 (grant no: HTC004X) is gratefully acknowledged.
    Introduction Industrial bioprocesses (environmental biotechnology, biocatalysis, bioremediation or similar) are becoming increasingly important for the production of chemical and energy products over conventional chemical synthesis, owing to the emphasis on the use of renewable raw materials, the specificity and complexity of biologically catalysed reactions, or both (Dorsch and Miller, 2003; Finlay, 2003; Hermann and Patel, 2007; Lynd et al., 1999; Lynd, 2008; McLaughlin et al., 2002). These biological processes are frequently claimed to provide benefit over conventional chemical processes from an environmental or sustainable process perspective (Botha and von Blottnitz, 2006; Gavrilescu and Chisti, 2005; Heller et al., 2003, 2004; Organisation for Economic Co-operation and Development, 2001; Sheehan et al., 2003; von Blottnitz and Curran, 2007; Harding et al., 2007, 2008), owing largely to their mild operating conditions, aqueous systems and the nature of the bio-system used.