Poisoning and Carbon Deposition Problems

11 — CURRENT DEVELOPMENTS AND FUTURE RESEARCH IN CATALYTIC MEMBRANE REACTORS 

H2S can have significant effects on dense Pd membranes resulting in some instances in complete membrane failure in short periods of time. Short of replacing Pd with other metals, it is imclear at this point how one overcomes the problem of Pd sensitivity to sulphur and chloride impurities, which are unfortunately present in many industrial hydrocarbon streams. 

Less understood is the effect of carbon deposition. Ali et al. [27], for example, showed that after an initial pretreatment, membrane exposure to MCH + H2 or TOL + H2 mixtures did not significantly influence its permeability. Surface carbon on the other hand, which often results from exposure to hydrocarbons, has a tendency to dissolve in the bulk of the metal. In a study on the topic [28] this resulted eventually in membrane mechanical failure. 

The only other metal that has received some serious attention for membrane reactor applications is Ag [14,38] which is permeable to oxygen. Ag has similar thermal/mechanical stability problem as Pd and in addition its oxygen permeability is orders of magnitude lower. 

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To conclude this section, it is necessary to state that Pd and Pd-based membranes are currently the membranes with the highest hydrogen permeability and selectivity. However, the cost, availability, their mechanical and thermal stabilities, poisoning, and carbon deposition problems have made the large-scale industrial application of these dense metal membranes difficult, even when prepared in a composite configuration [26,29,33-37],... 

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