Membranes hydrogen transport, metallic

Preparation of BC Y powders for ANL-1 and -2 membranes is described in detail elsewhere. BCY and metal powders were mixed together to prepare powders for ANL-la and -2a membranes. Powders for ANL-3 membranes were prepared by mixing one of two hydrogen transport metals with ceramic powders that are reported to be poor proton conductors. All membranes contained 40 vol% metal, except where otherwise noted. The powder mixtures were pressed uniaxially to prepare disks ( 22 mm in diameter and 2 mm thick) for sintering. Cermet membranes were sintered in either air or 4% H2/balance He, N2, or Ar in the temperature range of 1,350 to 1,420°C. 

A 40-pm-thick ANL-3a membrane containing 50 vol% of a hydrogen transport metal attained the highest hydrogen flux (20 cm (STP)/min-cm ) to date for an ANL membrane however, these membranes sometimes contain interconnected porosity... 

Equations (11.1)—(11.10) provide a basis for rationalizing the principal features of coupled transport membranes. It follows from Equation (11.8) that coupled transport membranes can move metal ions from a dilute to a concentrated solution against the metal ion concentration gradient, provided the gradient in the second coupled ion concentration is sufficient. A typical experimental result demonstrating this unique feature of coupled transport is shown in Figure 11.7. The process is counter-transport of copper driven by hydrogen ions, as described in Equation (11.1). In this particular experiment, a pH difference of 1.5 units is... 

As explained in Chapter 5, the transport mechanism in dense crystalline materials is generally made up of incessant displacements of mobile atoms because of the so-called vacancy or interstitial mechanisms. In this sense, the solution-diffusion mechanism is the most commonly used physical model to describe gas transport through dense membranes. The solution-diffusion separation mechanism is based on both solubility and mobility of one species in an effective solid barrier [23-25], This mechanism can be described as follows first, a gas molecule is adsorbed, and in some cases dissociated, on the surface of one side of the membrane, it then dissolves in the membrane material, and thereafter diffuses through the membrane. Finally, in some cases it is associated and desorbs, and in other cases, it only desorbs on the other side of the membrane. For example, for hydrogen transport through a dense metal such as Pd, the H2 molecule has to split up after adsorption, and, thereafter, recombine after diffusing through the membrane on the other side (see Section 5.6.1). 

Hydrogen transport through Pd and Pd-based alloys comprises the next steps [30,31]. The H2 molecules during adsorption are dissociated on top of the metal surface, giving a proton to the interstitial sites and an electron to the metal conduction band (see Section 2.4.2). The second step is the diffusion of atomic H, since the proton will be surrounded by an electron cloud [32], through the bulk of the metal. Finally, an associative desorption process of H2 molecules occurs from the metal surface at the other end of the membrane. 

In yet another variation, composite membranes are fabricated by sintering together powders of highly hydrogen permeable metals, Pd, Nb, Ta, Ti, V, Zr and their alloys, with powders of a second metal or alloy that is non-permeable to hydrogen [12]. The function of the non-permeable metal is to provide mechanical support for the hydrogen transport materials, especially if the latter are to be... 

Hydrogen transport through a dense metal membrane is usually envisioned as following an atomic transport mechanism. The atomic transport process, assuming transport from the high hydrogen pressure surface to the low pressure surface. 

Fig. 10.6 Illustration of the accepted hydrogen transport mechanism through dense metal membranes...

For all hydrogen transport membranes, the partial pressure of hydrogen in the permeate must always be maintained at a partial pressure lower than that of the hydrogen in the source in order to maintain a net hydrogen flux from the source to the permeate [14]. For membrane applications with molten metal cooling fluids, in which upstream partial pressures in the retentate can be equivalent to only 2.7 x 10 Pa to 6.7 x lO" Pa (2.0 x lO" torr to 5.0 x 10 torr) [15], the partial pressure of hydrogen in the permeate must be kept exceptionally low. 

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