Permeation of hydrogen

Fig. 19.43 Apparatus for studying the permeation of hydrogen through thin metal foils...

This equilibrium is of importance in providing diagnostic criteria for the mechanism of the h.e.r., since the rate of permeation of hydrogen through a thin iron membrane can provide information on the coverage of the surface with adsorbed hydrogen. 

The permeation of hydrogen through an iron diaphragm is illustrated in Fig. 20.20 for the sequence of events... 

Fig. 20.20 Permeation of hydrogen through a thin iron membrane...

Bockris and Subramanyan during studies of the permeation of hydrogen through pure Fe and Fe-SNi alloy found that a normal permeation transient was obtained (Fig. 20.21), providing the overpotential was less than a critical value, and when the overpotential was less than it was possible to reproduce the normal permeation curve, i.e. apply the polarising current at a constant rj < t , allow J to attain a steady value, switch off, reapply... 

Fig. 20.21 J-l transients for the permeation of hydrogen through ferrous alloys. The normal transient enables the diffusion coefficient ) to be evaluated from the relationship /, = L /6D, where /, is the time at which J attains a value of 0-63 of the steady-state permeation J...

Kameyama, T., M. Dokiya, K. Fukuda and Y. Kotera. 1979. Differential permeation of hydrogen sulfide through a microporous vycor-type glass membrane in the separation system of hydrogen and hydrogen sulfide. Separ. Sci. Technol. 14(10) 953-957. 

In [105], a reduction procedure involving the permeation of hydrogen through palladized Pd sheet electrodes is described. The electrocatalytic activity of various metal (Ti, Zr, Nb, No, Fe, Ni,... 

The content of this chapter is closely related to permeation, which is the transport of a solute across a layer of solvent (or membrane) under the action of a difference in activity. For example, the permeation of hydrogen through a metal foil has been studied, particularly for palladium [F.A. Lewis (1967)] and iron [J. P. Hirth (1980) H. H. Johnson (1988)]. One reason for studying the permeation of hydrogen through iron is to understand the hydrogen embrittlement of steel. 

Figure 8.7 Mechanism of permeation of hydrogen through metal membranes...

The permeation of hydrogen and methane through the membrane was investigated, revealing increased permeability for hydrogen on raising the temperature, while the methane permeability remained at a low level. Between 100 and 525 °C, the separation factor (see Section 2.6.3) increased from 7.5 to 31. The hydrothermal stability of the membrane was verified at 525 °C for 8 h for a feed composed of 18% hydrogen, 18% methane and 74% steam. It revealed a decrease of the separation factor from 31 to 26 [51]. 

Palladium, which shows an FCC structure, is a good absorbent of hydrogen. In this regard, permeation of hydrogen through Pd membranes, for its purification, is a well-known process. 

Recent results on isobutane dehydrogenation have been reported, and a conventional reactor has been compared with membrane reactors consisting of a fixed-bed Pt-based catalyst and different types of membrane [51]. In the case of a mesoporous y-AKOi membrane (similar to those used in several studies reported in the literature), the observed increase in conversion could be fully accounted for simply by the decrease in the partial pressures due to the complete mixing of reactants, products and sweep gas. When a permselective ultramicroporous zeolite membrane is used, this mixing is prevented the increase in conversion (% 70%) can be attributed to the selective permeation of hydrogen shifting the equilibrium. 

Hoshi, T., Saiki, H., Kuwazawa, S., Tsuchiya, C., Chen, Q., Anzai, J. (2001). Selective permeation of hydrogen peroxide through polyelectrolyte multilayer films and its use for amperometric biosensors, Chem. 73 5310-15. 

However, when membrane tubes are inserted in the fluidized-bed reactor, hydrogen is continuously removed from the reaction mixture thus, the main reaction of ethylbenzene dehydrogenation continues to move in the direction of forward reaction. The ethylbenzene conversion and the yield of styrene increase as a result of the selective permeation of hydrogen through the membrane. Both the conversion and the yield exceed those of the industrial fixed-bed reactors and fluidized-bed reactors without membranes. When 16 membrane tubes are used, the selectivity to styrene is expected to be almost 100% due to suppression of by-products such as toluene [Abdalla and Elnashaie, 1995]. A high ethylbenzene conversion (96.5%) along with a high styrene yield (92.4%) is possible under properly selected realistic conditions. 

Itoh, N., Kaneko, Y., Igarashi, A. (2002). Efficient hydrogen production via methenol steam reforming by preventing back-permeation of hydrogen in a palladium membrane reactor. Industrial Eng. Chem. Res. 41,4702-4706. 

Shindo Y, Obata K., Hakuta T., Yoshitome H., Todo N. and Kato J., Permeation of hydrogen through a porous vycor glass membrane, Adv. Hydrogen Energy Progr, 2 325 (1981). 

Research on other types of materials for H2 separation has been motivated by relatively high cost of Pd and possible membrane degradation by acidic gases and carbon as summarized in Tsuru et al.76 These authors examined microporous silica membranes together with an Ni catalyst layer for SMR reaction. However, this type of membrane allows the permeation of hydrogen as well as other gases in reactants and products, which markedly reduces hydrogen selectivity and limits methane... 

Figure 8.2. Commonly accepted mechanism for the permeation of hydrogen through dense metal membranes.

In order to explain this, the influence of the kinetics of the main reaction on the performance of the membrane reactor has been studied, for microporous membranes implemented in the second reactor. The reaction rate of the main reaction is successively multiplied by a factor 2 and 10, and as a consequence the reaction equilibrium is reached much faster. Under these circumstances increases are found in both yield and selectivity for the conventional dehydrogenation reactor without membranes. The results of the calculations are presented in Table 14.9 in which the differences in yield and conversion are given in percentage points with respect to the conventional case. The higher yields and conversions for the PBMR compared to the conventional reactor are due to the fact that the conversion is no longer limited by the kinetics, as in the previous cases, but by the permeation of hydrogen. 

As for the tritium penneation, the pemieability on Hastelloy XR was obtained and the numerical equation was introduced to predict the behavior on the counter-permeation of hydrogen and tritium at the chemical reactor. 

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