Superpermeability

Mundschau, M.V., X. Xie, and C. Evenson, Superpermeable hydrogen transport membranes, Non-porous Inorganic Membranes, Chapter 4, eds. A.F. Sammells and M.V. Mundschau, Wiley VCH Verlag GmBH, Weinheim, 2006. 

When the superpermeability of the measuring gas through the material of the SE takes place, the big value of W < 0.1 1 can be achieved by different ways, including realization of diffuse rate in the SE if the conditions D (N, x, T) H g(ccoPo are fulfilled. In this situation, the establishment of the time for gas flow diffusing through the SE can differ sigiuficandy from the typical time for the diffusion processes value 8 /(6D (A, x, 7)). 

Mundschau, Michael V, Xie, Xiaobing, Evenson IV Carl R. Superpermeable hydrogen transport membranes. In SammeUs, A. F, Mundschau, M. V, editors. Nonporous inorganic membranes. Weinheim, Germany WUey-VCH 2006. pp. 107-38, and references therein. 

For a superpermeable membrane, the flux of hydrogen through the membrane cannot exceed the number of gas phase hydrogen molecules incident upon the feed side surface of the membrane. This ultimate limit holds for all types of membrane, whether porous or dense. It is instructive, therefore, to calculate this limit for the hydrogen flux of superpermeable membranes, which, by definition, transport every atom of hydrogen incident upon the feed side surface of the membranes [1-3]. 

Under true superpermeable hydrogen flux conditions, the large numbers of molecules predicted to impact upon a membrane surface follow, in part, as a consequence of the very high molecular speeds of gas phase hydrogen relative to the size of reactor vessels. For example, the mean velocity of a hydrogen molecule, H2, in the gas phase at 273 K (0 °C) is 1.7 km s [8]. Mean molecular velocity increases in proportion to the square root of the absolute temperature. In a chemical reactor at 673 K (400 °C), for example, the mean velocity of H2 will increase by a factor of (673 K/273 K) / from 1.7 km s at 273 K to 2.7 km s at 673 K. Mean molecular velocity decreases inversely with the square root of the molecular mass. For deuterium molecules, D2, with a molecular mass approximately twice that of H2, the mean molecular velocity is less than that of H2 by a factor of 2 /, approximately 1.2 km s at 273 K (0 °C) [8]. 

Although the membranes used to extract hydrogen isotopes from plasmas meet the criterion for the definition of superpermeability, in that every atom of hydrogen incident upon the feed side surface of the membrane is transported through the dense membrane, it must be noted that superpermeability is achieved, in very large part, because of the relatively low flux of hydrogen incident upon the membranes. The plasma density in the quoted experiments was 5 x 10 cm-, and the total gas pressure was relatively low, 0.002-0.004 torr (0.3-0.6 Pa) [2]. 

In the plasma experiments, the sticking coefficient of hydrogen on niobium must be equal to one if the membrane is to be classified as truly superpermeable. For highly energetic and ionized plasma species, including H2+, H3+, D2+ and D3+,... 

Because diffusion is simply rapid, random, translational motion of atoms and molecules, hydrogen in superpermeable membranes will travel just as quickly from the permeate side of the membranes to the retentate side, as it does in diffusing from the retentate to the permeate side. From the fundamental laws of diffusion, diffusion will tend to eliminate concentration gradients across a membrane, and at equiUbrium the net flux across a membrane will be zero [14]. For superpermeable membranes, equilibrium will be very quickly approached, unless hydrogen is removed extremely rapidly from the permeate side of the membrane. 

For hydrogen partial pressures in feeds of above a few torr, and for thicker membranes (> 250 pm) of Nb, Ta, Ti, V and Zr, hydrogen flux can become limited by bulk diffusion, and, in practice, superpermeability is not achieved. Nevertheless, these elements do possess the highest known permeabilities for hydrogen of any of the common elements, far superior to those of palladium and its alloys, the standards in the hydrogen purification membrane industry. 

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