Hydrogen permeation concentration polarization

In another work [58], the authors studied the influence of CO and CO2 (components always present in reforming reactions) on the permeation of hydrogen. Although concentration polarization is not a problem, at low temperatures CO preferentially absorbs on the Pd surface (as found for all kind of configurations) depleting the hydrogen permeation rate. 

The effect of a promoter varies with its concentration. Figure 1 shows the rate of hydrogen permeation through a steel membrane under cathodic polarization in the presence of Group VB and VIB elements [12]. In most cases, the promoters achieve their maximum effect at relatively low concentrations. Because of hydrolysis and other secondary reactions, higher concentrations often lead to the deposition of insoluble products that can inhibit hydrogen entry. 

Both forward and backward permeation are considered in order to analyze both cases where the membrane behaves as hydrogen purifier (forward permeation) or hydrogen supplier (backward permeation). The permeation process is considered to be divided into several elementary steps, each of which is characterized by its own model equations. The details of such a model can be found elsewhere where the model introduced in was modified in some steps to take into account the presence of an inhibitor species (CO). The same approach was followed by Catalano et al., who analyzed the inhibiting effect of CO without considering the concentration polarization. ... 

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

All of the considerations made in the previous section lead to the necessity of defining a concentration polarization coefficient that could be directly connected to specific permeation quantities.Here, this approach will be direct to hydrogen permeation through Pd-based membranes. 

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. 

For this aim, an overall permeance reduction coelScient , PRCP was introduced, to take into account the permeation reduction owing to both concentration polarization and inhibition at the same time. Its definition, reported in eqn (14.19), arises from the simple consideration that the lowest permeance is evaluated at bulk conditions when both polarization and inhibition affect the membrane system, whereas, on the other hand, the ideal membrane behavior and the maximum permeance can be found under pure hydrogen conditions ... 

From these plots it is possible to see that the CPC coeiScient is a decreasing function of the upstream hydrogen molar fraction. Furthermore, high upstream total pressures favor concentration polarization because of a higher permeating flux. 

In particular, by comparing the performance of the same membrane in different configurations, the authors showed that in tubular configuration the extent of concentration polarization is the limiting step for hydrogen permeation (as also indicated above), while with the same membrane used in micro-channel configuration the concentration polarization effect can be completely neglected [27, 57]. 

Kara, S., Sakaki, K. and Itoh, N. (1999) Decline in Hydrogen Permeation Due to Concentration Polarization and CO Hindrance in a Palladium Membrane Reactor. Industrial and Engineering Chemistry Research, 38,4913 918. 

Chen, W. H., Syu, W. Z., Hung, C. I. (2012). Numerical characterization on concentration polarization of hydrogen permeation in a Pd-based membrane tube. International Journal of Hydrogen Energy, 36, 14734—14744. 

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