Dense metal membrane transport mechanism

The inorganic membranes had until the late nineties received fairly little attention for applications in gas separation. This has mainly been due to their porous stmcmre, and therefore lack of ability to separate gas molecules. Within the group of inorganic membranes there are however the dense metallic membranes and the solid oxide electrolytes these are discussed separately in Section 4.3.5. With reference to Section 4.2, the possible transport mechanisms taking place in a porous membrane may be summarized as in Table 4.4 below, as well as the ability to separate gases (+) or not (—). Recent findings [29] have however documented that activated Knudsen diffusion may take place also in smaller pores than indicated in the table. 

Dense Metal Membrane Materials, Configurations, Mechanisms of Transport, and Permeability... 

Transport Mechanism of Hydrogen Through Dense Metal Membranes and Governing Permeation Equations... 

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...

Defect free dense metal membranes are infinitely selective to hydrogen due to the atomic transport mechanism that can be described through derivation of Pick s Law (defining the transient mass transfer of hydrogen in the direction of decreasing concentration). 

Researchers have reported the influence of various chemical species on the performance of palladium-based membranes. Observed reductions in hydrogen flux through dense metal membranes are typically attributed to the blocking of the adsorption of molecular hydrogen, which is a necessary step in the atomic transport mechanism [13, 80,112,114,120-123],... 

Hydrogen can permeate selectively dense metal membranes, behaviour that permits the separation of hydrogen from gas mixtures. The mass transfer mechanism consists of several steps dissociation of hydrogen molecules into atoms, interaction of hydrogen atoms with the metal surface and their adsorption, diffusion of hydrogen into the metal lattice, and desorption of hydrogen atoms from the other metal surface and their recombination into molecules.The overall transport process through the metal wall is called permeation and is ruled by the expression ... 

Hydrogen can permeate selectively dense metal layers, and, accordingly, this phenomenon is exploited when metal membranes are applied for separating ultrapure hydrogen from gas mixtures [24-27]. The hydrogen mass transfer through dense metal membranes includes several transport mechanisms, which are discussed in the following sections. 

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). 

A widely accepted transport mechanism for certain gases through dense metal, solid electrolyte or ceramic membranes, similar to that for organic counterparts, is the soludon-difhision type. 

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