Hydrogen purification using palladium

Deshpande, K., Meldon, J.H., Schmidt, M.A. and Jensen, K.F. (2010) SOI-Supported microdevice for hydrogen purification using palladium-silver membranes. Journal of Microelectromechanical Systems, 19,402 09. 

Deshpande K, Meldon J H, Schmidt M A and Jensen K F (2010), SOI-supported microdevice for hydrogen purification using palladium-silver membranes , J... 

In Figure 5.11, the procedure for hydrogen purification using a palladium thin sheet is illustrated. 

Kirchner, A., Brown, I.W.M., Bowden, M.E., Kemmitt, T. (2007b). Hydrogen Purification using Ultra-thin Palladium Films supported on Porous Anodic Alumina Membranes, in Functional Nanoscale Ceramics for Energy Systems, ed. E. Ivers-Tiffee and S. Barnett (Mater. Res. Soc. Symp. Proc. 1023E, Warrendale, PA, 2007), Paper 1023-JJ09-02. 

The separation factors are relatively low and consequently the MR is not able to approach full conversion. With a molecular sieve silica (MSS) or a supported palladium film membrane, an (almost) absolute separation can be obtained (Table 10.1). The MSS membranes however, suffer from a flux/selectivity trade-off meaning that a high separation factor is combined with a relative low flux. Pd membranes do not suffer from this trade-off and can combine an absolute separation factor with very high fluxes. A favorable aspect for zeoHte membranes is their thermal and chemical stability. Pd membranes can become unstable due to impurities like CO, H2S, and carbonaceous deposits, and for the MSS membrane, hydrothermal stability is a major concern [62]. But the performance of the currently used zeolite membranes is insufficient to compete with other inorganic membranes, as was also concluded by Caro et al. [63] for the use of zeolite membranes for hydrogen purification. 

Palladium-based dense metallic membranes have been known to be completely selective for hydrogen permeation and are used in commercially available small-scale hydrogen purification units (e.g., Johnson Matthey, 2007 REB Research, 2007 Power + Energy, 2007 ATI Wah Chang, 2007). These hydrogen purification units typically use palladium-alloy... 

Researchers at Lehigh University are developing a methanol reforming silicon reactor with a palladium membrane for a hydrogen purification system built using semiconductor fabrication techniques. The device is designed to produce hydrogen for fuel cells for portable electronic devices, such as laptop computers and cell phones. 

Ethylene. During World War II the Germans manufactured more than 60,000 t/yr of ethylene [74-85-1], C2H4, by hydrogenation of acetylene, using palladium on silica gel as catalyst. Subsequendy, cracking of hydrocarbons displaced this process. However, it is still utilized for purification of ethylene containing small amounts of acetylene as contaminant (5) (see Ethylene). 

Palladium is an expensive metal and this imposes limits on the thickness of material that can be used for hydrogen purification in competition with other industrial methods. Emonts et al. estimated that films less than about 5 p,m in thickness need to be used in a fuel-cell methanol reformer [7], while Criscuoli et al. [8] concluded that 20 p,m is an upper limit for membranes to be economically competitive. These economic estimates overlook the possibility of recycling the palladium or palladium alloy. This becomes a very real possibility in the use of free-standing membranes rather than composite structures with other metals or ceramics. Recycling prospects probably increase the thickness constraint to something between 5 jxm and 8 p.m, a value that is also consistent with factors such as limitations on the volume of space occupied by a multiple membrane assembly. 

The first scientific study on palladium-based membranes, available in the Elsevier Scopus database [1], where more than 6,000 scientific journals are taken into account, is dated 1955, when Juenker et al. [2] analyzed the use of palladium membranes for hydrogen purification. Today, it is well known that the palladium membranes are, mainly, applied in the field of gas separation and, particularly, in the issue of the hydrogen rich-stream purification [3], As reflected by the data of Fig. 2.1, the scientific interest towards palladium-based membranes is increased... 

The condition of steady state diffusion means that concentrations and fluxes at every point on the x-axis are constant with time. It then follows from eq. (5-29) for the case of a constant diffusion coefficient that the concentration gradient is locally constant everywhere, i. e. independent of x. In other words, there will be a linear concentration profile. Examples of such a steady state diffusional process would be the diffusion of numerous gases through metal foils when constant but different partial gas pressures are maintained on either side of the foils. In the system palladium-hydrogen, because of the high diffusivity of hydrogen, direct use is made of the diffusion of the gas through the metal for purposes of gas purification. 

Monomers. m-Phenylene diamine (MPD) can be prepared by the continuous liquid phase hydrogenation of m-dinitrobenzene at moderate temperatiu-es (7). One process employs a dispersion of the m-dinitrobenzene in water (8) another uses a solvent, such as DMF, which dissolves both the reactant and the MPD product (9). Catalysts for the hydrogenation include platinum, palladium, and nickel that must be recovered by filtration. Purification steps include vacuum distillation. 

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