Activated gaseous diffusion

The second contribution, i.e., permeation through the microporous zeolitic-channel network, can be due to activated gaseous diffusion or surface diffusion of adsorbed species. As a general rule, the smaller the pores size the greater the interaction of the adsorbed molecule with the pore walls. 

The relative importance of surface diffusion and activated gaseous diffusion depends strongly on temperamre [63-65] because adsorption and diffusion phenomena are molecular processes of opposite temperature dependency. Figure 10.14 is a simplified diagram showing the evolution of permeance as a function of temperamre [64]. 

Figure 17.5 shows the typical curve of the permeance as a function of the temperature of a defect-free zeolitic membrane. This trend is a combination of two mechanisms the surface diffusion in the inner surface of the pores and the activated gaseous diffusion. 

At high temperatures, activated gaseous diffusion or translational diffusion becomes the dominating transport mechanism. Then the permeation flux can be expressed by... 

At high temperatures, the adsorption phenomena can become negligible and molecules can be considered as being in a quasi-gaseous state even within the constrained environment of the zeolite framework. This state is referred to as gas translational diffusion , activated gaseous diffusion or activated-Knudsen diffusion . 

When activated gaseous diffusion occurs, the flux is described by the next... 

Uranium oxide [1344-57-6] from mills is converted into uranium hexafluoride [7783-81-5] FJF, for use in gaseous diffusion isotope separation plants (see Diffusion separation methods). The wastes from these operations are only slightly radioactive. Both uranium-235 and uranium-238 have long half-Hves, 7.08 x 10 and 4.46 x 10 yr, respectively. Uranium enriched to around 3 wt % is shipped to a reactor fuel fabrication plant (see Nuclear REACTORS, NUCLEAR FUEL reserves). There conversion to uranium dioxide is foUowed by peUet formation, sintering, and placement in tubes to form fuel rods. The rods are put in bundles to form fuel assembHes. Despite active recycling (qv), some low activity wastes are produced. 

Fig. 7.80 A schematic thermodynamic phase stability diagram for the A-C-O system, showing three reaction paths. Paths 2 and 3 are only possible if gaseous diffusion in pores in the oxide product results in a carbon activity increase through the scale, as shown in Fig. 7.81 (after...

The above results indicate that the general characteristics of gaseous diffusion in glassy polymer membranes can be represented reasonably well in terms of the dualmode concept. The basic reason for the observed increasing D(C) function is seen to be the concurrent increasing proportion of less strongly sorbed (and hence more easily activated) penetrant molecules. The model is, no doubt, highly idealized, but is nevertheless shown to be physically reasonable and consistent with the correspond-... 

The separation of chemical isotopes is based on small differences in their physical and chemical properties. For the lower-mass isotopes, chemical exchange, distillation, and electrolysis have been used. For the higher-mass isotopes, techniques based on mass have been used, including gaseous diffusion, centrifugation, thermal diffusion, and ion activation.29,30 A newer method uses lasers that produce coherent light tuned to the specific wavelength of a vibration bond related to the desired isotope in an atom or molecule. This technique is still under development but... 

A particular advantage of using immersion microcalorimetry for the study of ultramicroporous materials is that the molecular entry into very fine pores takes place much more rapidly from the liquid phase than from a gas. There are two reasons for this difference gaseous diffusion may be slow (thermally activated) -especially at 77 K - and the higher liquid density also favours a more rapid molecular penetration. 

Gas flow method. The above methods have a common characteristic. That is, they do not discriminate between the active pores and the "passive" pores. For separation applications, only the "active" pores arc most relevant. A nondestructive technique for inferring the pore size distribution of a membrane is the gas flow method. Having been developed and tested at Oak Ridge Gaseous Diffusion Plant [Fain. 1989], it departs greatly from the above methods in that the pore size distribution is based on gas flow... 

Dq = gaseous-diffusion coefficient, ft /h B = activation energy, units so that E/RT is dimensionless Eg = overall column efficiency, percent A =... 

Diffusion in gas space between catalyst affecting experimental activation energy. In the case of gaseous diffusion across a finite volume of space before molecules reach the catalyst, the consequences of diffusion effects on such an interpretation is seen from inspecting formula (14), keeping in mind that k increases exponentially as the temperature is raised and thus kL/D increases in the same manner, since the variation of D with temperature (%-power) can be neglected compared to the exponential temperature dependence of k. 

P. B, Weisz (Socony Mobil Oil Co.) In connection with the question of how the two types of activity centers collaborate, Dr. Mills has already stated that physical mixtures of considerable particle size will successfully catalyze the reaction. We have recently shown that successful cooperation of the two types of centers can be had through the medium of ordinary gaseous diffusion as a transport mechanism for the molecules of intermediates (1). For example, we find that a typical, useful reaction rate can be supported through an intermediate species existing at a vapor pressure of 10 atm. if the two catalytic functions are as far as 100 A. apart, or through an intermediate at a partial pressure of 10 atm. for distances as large as about 50y. The latter partial pressure represents, in fact, the magnitude of the thermodynamically attainable concentration of some of the olefins in hydroisomerization reactions. 

Uranium hexafluoride has a molecular formula of UF. It is a colorless, volatile crystal that sublimes and reacts vigorously with water. It is highly corrosive and is a radiation risk. The four-digit UN identification number for fissile material containing more than 1% of uranium 235 is 2977 for lower specific activity, the number is 2978. Uranium hexafluoride is used in a gaseous diffusion process for separating isotopes of uranium. 

The temperature coefficient is low, corresponding to an apparent activation energy of 10-15 kJ mol" gaseous diffusion processes do not in fact obey the Arrhenius equation, their rates being proportional to... 

If helium-3 (or another inert impurity) accompanies tritium gas that is absorbed onto a tritide former, helium blanketing may occur. The absorption rate slows as the concentration of helium in the metal crevices leading toward active sites becomes high. Normal gaseous diffusion is often not sufficient to overcome this effect. Forced diffusion by recirculating the gas supply can be used to overcome blanketing. 

The report that accompanied Biish s letter stated that five to ten pounds of active material would be Tairly certain to explode with a force equivalent to 2,000 tons of TNT, up from 600 tons in the third National Academy of Sciences report of the previous November 6. It recommended building a centrifuge plant at a cost of 20 million that could produce enough U235 for one bomb a month and estimated that such a plant could be completed by December 1943. A gaseous diffusion plant, its... 

The values of the specific activity include the activity of U-234, which is concentrated during the enrichment process these values do not include any daughter product contribution. The values are for the material originating from natural uranium enriched by a gaseous diffusion method. 

Fig. 7.81 A sketch illustrating how gaseous diffusion processes in pores within an oxide layer can result in an increase in the CO/CO2 ratio, and hence the carbon activity, through the layer...

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