Hydrogen permeation inhibition

D.H. Coleman, B.N. Popov, R.E. White, Hydrogen permeation inhibition by thin Layer Zn-Ni alloy electrodeposition, J. Appl. Electrochem. 28 (1998) 889—894. 

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. 

The effect of inhibition by CO on hydrogen permeation has recently become the object of several investigations, since CO is typically present in significant amounts in the hydrogen streams to be separated. In particular, a macroscopic model equation was developed to describe the influence of CO on permeation. This equation modifies Sieverts law to take into account the permeance reduction due to CO as reported in eqn (14.15), where the quantities a, and 6co represent a coefficient accounting for other additional effects of CO on adsorbed hydrogen and the membrane surface coverage by CO, respectively ... 

Based on thermodynamic data, temperatures above 673 K are needed to reach complete ammonia decomposition (>99%). Coupling NHj decomposition with Pd-membrane separation can be an ideal way to provide high-purity CO -free H2 with improved yield at low temperatures [62]. Although NH3 may inhibit hydrogen permeation slightly, the permeability of the membrane used can be recovered by air and subsequent H2 treatments. However, the combination of an NH3 cracker with a Pd-membrane separator is more preferable for on-site H2 generation [63]. 

An additional boundary between primary and secondary circuit would be provided by an intermediate circuit purified by a sweep gas flow. Adding oxygen or steam to the sweep gas, a fraction of the tritium will be bound as tritiated water HTO and no longer be available for permeation plus it would form a protective oxide layer. An inhibition effect was also found if hydrogen was injected (factor of 2 -3). This effect, however, needs further understanding [47]. 

This observation is surprising given literature reports that steam adsorbs strongly on Pd membranes [17] and also causes inhibition of the Ha flux when present in the feed gas stream at lower temperatures. The enhancement due to the steam sweep stream could be due to a cleaning effect where the steam reacts with CO or carbon present on the permeate side of the membrane to form CO2 as we have observed that the Pd-Cu membrane is a catalyst for the WGS reaction. Carbon dioxide is less strongly bound to the surface of the membrane than CO that would free additional sites for the formation of molecular hydrogen. The He sweep could have a similar... 

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