Hydrogen permeation driving force

Defining Concentration Polarization Coefificient According to Hydrogen Permeation Driving Force... 

By comparing the expressions in eqn (14.7), this definition can be read in two ways when concentration polarization is negligible, i.e. CPC 0, membrane permeance is, in fact, coincident with that of bulk, or, dually, the permeation driving force between the bulks is the same as that between the membrane surfaces. It should also be noticed that the condition of maximum polarization, i.e. CPC= 1, is an asymptotic conditions where permeating flux tends to zero because the hydrogen partial pressure at membrane surface approaches to zero. This situation is summarized in Table 14.2. 

The results of the DaPe analysis for the above example are reported in the Fig. 16.3. Because the example only includes DaPe values greater than 1, the hydrogen permeated is much less than that produced. The DaPe decreased when the overpotential increased, that is, a higher overpotential involves higher which increases the permeation driving force. 

In the MREF, the methane reacts forming CO, COj, H2O and H2, where the latter can permeate across the membrane and feed the fuel cell. The injection of a sweep gas (in this case steam) decreases the partial pressure of hydrogen on the permeate side of the membrane enhancing the permeation driving force (see Equation [14.34]). The permeation flow is counter-current to the reacting mix in order to have the highest partial pressure difference along the membrane. The assumed inlet velocity profiles are parabolic for sweep gas and hot gas streams, and flat for the feed section. 

Temperature profiles, methane conversion, HRF and permeated flow are shown in Fig. 14.6. Figure 14.7 shows the membrane temperature profile and the permeation driving force along the reactor. Table 14.4 shows the permeation results and the product outcome, outlining that total hydrogen permeated is lower (26%) for the counter-current configuration. The calculated HRF and methane conversion ( CH4 ) in the counter-current flow... 

Much earlier in this book (Section 7.10), the point was made that the path and rds for hydrogen evolution divide themselves into those (e.g., H30+-> Hads+H20 2Hads- H2) in which 9H is 1 and those (e.g., H30++ e H20 + H H30+ + Hads + e —> H20 + H2) in which 6 —> 1. The distinction between these mechanisms is important for Fe because of the effect of the value of the coverage with 9 on embrittlement and the related stress corrosion cracking (Section 12.6.5). This is more likely if 9 is larger than when it is small because a large 9 increases the driving force for the permeation of H into the metal. 

The syngas in the C02 Absorber overhead stream is water-washed and fed to a MEDAL membrane unit. The membrane feed gas is sent to a coalescing filter to remove liquids and is preheated before is enters the permeator. In the permeator, syngas is separated into a hydrogen-rich permeate and the syngas product. The operation of the membrane unit is very simple. The driving force for separation is the difference in partial pressure between the hydrogen in the feed gas and that of the permeate. 

Subsequent studies of these proton-transport membranes will be conducted over a wide range of hydrogen partial pressures on the permeate side. This will allow assessment of whether the driving force for hydrogen permeance is more accurately represented as the natural logarithm of the ratio of the hydrogen partial pressures on the retentate and permeate sides, as observed in ion transport oxygen membranes. 

In the case of dense membranes, where only hydrogen can permeate (permselectivity for H2 is infinite), the permeation rate is generally much lower than the reaction rate (especially when a fixed bed is added to the membrane). Experimental conditions and/or a reactor design which diminishes this gap will have positive effects on the yield. An increase of the sweep gas flow rate (increase of the driving force for H2 permeation) leads to an increase in conversion and, if low reactant flow rates are used (to limit the H2 production), conversions of up to 100% can be predicted [55]. These models of dense membrane reactors explain why large membrane surfaces are needed and why research is directed towards decreasing the thickness of Pd membranes (subsection 9.3.2.2.A.a). 

The 5-fold change in the partial pressure of hydrogen on the high-pressure side of the membrane and the high partial pressure in the permeate makes use of an average driving force invalid. 

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