Palladium membrane fluxes

Measurement of palladium membrane permeability. The permeation rate of hydrogen gas through the palladium membrane, Q , was assumed to obey the half-power pressure law(20). The permeation flux of hydrogen through the membrane is proportional to the difference between the souare roots of the hydrogen partial pressure on the high and low pressure sides of membrane. 

Measurement of the steady state permeation flux for both the crystalline V-Ti-Ni alloys (see microstracture of V51-Ti28-Ni21 and V54-Ti28-Nil8 alloys in Figure 6) and the amorphous metalhc glass materials was conducted and compared to hterature data for palladium membranes. 

The permeation flux of oxygen through a mixed oxide membrane described above depends on the oxygen partial pressures across the membrane, membrane thickness and temperature. The dependence, however, is embedded in a complicated implicit equation [Lin et al., 1994]. Only in special cases the permeation Oux shows a pressure dependence similar to that for palladium membranes as given in Eq. (4-10). For example, when electronic conductivity predominates, the value of the exponent, n, is equal to 0.5 for thin membranes and 0.25 [Dou et al., 1985 Itoh et al., 1993] for thick oxide films. If the oxide membrane is essentially an ionic conductor and the surface reaction is the rate-limiting step, n takes on a value of 0.5. 

Alloys help alleviate the problems associated with flux reduction from impurity adsorption on the surface of palladium membranes [74, 122]. Steam, CO, and H2S inhibit the hydrogen flux through palladium membranes by chemisorption on the surface. There is some indication that palladium alloys are affected less by CO poisoning [111, 117, 123-125]. Recently, work on Pd-Cu alloys has increased... 

According to Eq. (3.1), flux should be inversely proportional to metal thickness, hence the incentive (besides cost) to fabricate composite membranes with very thin palladium layers. Flux is generally not inversely proportional to thickness for composite membranes with very thin palladium layers [179]. Moreover, because flux depends on the difference in hydrogen partial pressure, a sweep gas may be employed on the permeate side or it may be kept in vacuo. However, evacuation is... 

From Eqs. 4.23 and 4.24, the following equation can be derived for the hydrogen flux at constant temperature through a dense palladium membrane ... 

A two-step membrane manufacturing process has been reported where a defect free Pd-alloy membrane is first prepared by sputtering deposition onto the perfect surface of a silicon wafer, for example. In a second step the membrane is removed from the wafer and transferred to a porous stainless steel support (see Figure 11.1). This allows the preparation of very thin ( 1-2 pm) defect-free membranes supported on macroporous substrates (pore size equals 2 pm). By this technique, the ratio of the membrane thickness over the pore size of the support may become less than 1, which is two orders of magnitude smaller than obtained by more conventional membrane preparation techniques. Tubular-supported palladium membranes prepared by the two-step method show a H2/N2 permselectivity equal to 2600 at 26 bars and hydrogen flux of 2477 mL(STP) min cm . Since the method enables the combination of macro-porous stainless steel supports and thin membrane layers, the support resistance is negligible. ... 

Keurentjes J T F, Gielens F C, Tong H D, Rijn C J M and Vorstman MAG (2004), High-flux palladium membranes based on microsystem technology , Ind Eng Chem Res, 43,4768-4772. 

Under galvanostatic charging conditions, the steady-state diffusion flux through a palladium membrane, as noted in Section IX. 5 (ii), was found to be proportional to the cathodic current at low current densities " ... 

The permeation rate parameter used in this study is shown in Table 13.1. The composite palladium membrane, which was prepared by the chemical vapour deposition (CVD) technique in this study, was found to have a very large selectivity for hydrogen (ca. 10 000 of ideal selectivity for Hj/Nj at 300°C), so that the permeabilities of the other components were assumed to be zero (Itoh et al, 2007). Heat transfer through the membrane takes place in two ways that is, heat conduction, and heat exchange by the permeation. Heat conduction through the membrane is modelled by the CFD code function. Thermal conductivity of 5 W/(m K) is applied to the membrane and support material. Heat exchange through the membrane is calculated as the sum of the permeation flux and enthalpy for each species. 

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