Palladium, hydrogen permeation into

A commercial nickel catalyst was used for methane steam reforming performed at a 500 °C reaction temperature, a S/C ratio of 3.0 and atmospheric pressure, while the permeate side was evacuated. The performance of the vapour deposited platinum membrane was similar to the plated dense palladium membrane. In the permeate of the deposited ruthenium and palladium membranes, small amounts of carbon oxides and also methane were observed. While it was expected that all these species had passed through the membranes by diffusion, in addition some methane was converted into carbon dioxide over the noble metals of the membranes. Kikuchi et al. demonstrated by simulations that conversion and hydrogen permeation in a membrane reactor is higher, where the first portion of the catalyst bed is not coupled to the membrane. Such an arrangement as shown in Figure 7.16 would clearly save expensive membrane surface area. Experimental work by Itoh et al. performed for methanol steam reforming [521] confirmed the assumptions of Kikuchi et al. 

Ferreira-Aparicio et al. reported the development of a laboratory-scale membrane reactor for the partial dehydrogenation of methylcyclohexane into toluene in a membrane reactor [527]. A platinum/alumina catalyst containing 0.83 wt% platinum was put into a porous stainless steel tube, which had been prior coated with a palladium membrane by electroless plating. At 350 °C reaction temperature and a pressure of 1.4 bar at the reaction side, 99% of the hydrogen product could be separated through the membrane, which had a thickness of 11 pm. However, the sweep stream required on the permeate side was more than 20 times higher than the hydrogen permeate flow rate that could be achieved. 

Hydrogen is to be recovered from a gas mixture by permeation through a thin film (thickness S ) of palladium (Pd) membrane into an aqueous solution prior to chemical reaction in the aqueous solution. The diatomic gas H2 is present in the feed gas at a partial pressure Pnzg/ - hydrogen concentration in the bulk water is the mass-transfer coefficient of H2 in the aqueous film next to the palladium membrane is kn w The permeability coefficient of H2 through the Pd membrane via Sievert s law is... 

Volumetric steady-state gas permeation tests at elevated temperatures are typically used to characterize the performance of paUadium membranes. Electrochemical methods are also effective, even at high temperatures [90, 166]. Permeation through palladium depends on a solution-diffusion mechanism, including the steps of chemisorption and dissociation into atoms, absorption into the metal, diffusion through the metal lattice, transfer from the bulk metal to the opposite side, and recombination into molecules for desorption [167, 168]. Difiusion of molecular hydrogen through boundary layers adjacent to the surface is also necessary. 

The main thermodynamic properties of the methanol and ethanol steam reforming reactions are plotted in Fig. 18.3 for comparison. Both are endothermic. However, they are spontaneous, due to large entropy changes associated with the dissociation of the alcohols. Spontaneous methanol dissociation is obtained at a lower temperature (AG < 0 at - 325 K) compared to ethanol (AG < 0 at 475 K), which requires practical dissociation temperatures of 600 K. It is therefore possible to incorporate a palladium permeation membrane into a methanol steam reformer (in such cases, both processes are operating at the same temperature) whereas for ethanol, reforming and extraction of hydrogen are usually performed in two separate... 

D Eha Camacho et al. (2011) proposed a novel concept using an assisted electrochemical reaction to produce atomic hydrogen from water electrolysis for different heterorganic compounds conversion. The electrochemical reactor is divided into two compartments by a palladium membrane in which atomic hydrogen is absorbed and permeated. Organic sulfur in the oil can be desulfurized and transformed to H2S in the electrochemical compartment. In addition, Lam et al. (2012) recently presented a review of electrochemical desulfurization technologies for fossil fuels. Various electrodes and electrolytes that have been used for desulfurization accomphshed by oxidation, reduction, or both were summarized by Lam et al. in their paper. Some electrochemical desulfurization processes for transportation fuels were chosen for listing in Table 14.2. 

Kikuchi [111] described a natural gas MR, which had been developed and operated by Tokyo Gas and Mitsubishi Heavy Industries to supply PEM fuel cells with hydrogen. It was composed of a central burner surrounded by a catalyst bed filled with commercial nickel catalyst. Into the catalyst bed 24 supported palladium membrane tubes were inserted. The membranes had been prepared by electroless plating and were 20 pm thick. Steam was used as sweep gas for the permeate. The reactor carried 14.5 kg catalyst. It was operated at 6.2 bar pressure, S/C ratio of 2.4, and 550°C reaction temperature. The conversion of the natural gas was close to 100%, wdiile the equilibrium conversion was only 30% under the operating conditions. The retentate composition was 6 vol.% hydrogen, 1 vol.% carbon monoxide, 91 vol.% carbon dioxide, and 2 vol.% methane. 

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