Gas Permeation in Dense Metallic Membranes

Gas solubility (and thus permeation rate) in dense metal membranes typically decreases with increasing temperatures. Therefore, dense metal membrane reactors have the inherent advantage of avoiding runaway reactions. 

The effect of reactant loss on membrane reactor performance was explained nicely in a study by Harold et al [5.25], who compared conversion during the cyclohexane dehydrogenation reaction in a PBMR equipped with different types of membranes. The results are shown in Fig. 5.4, which shows the cyclohexane conversion in the reactor as a function of the ratio of permeation to reaction rates (proportional to the ratio of a characteristic time for reaction in the packed bed to a characteristic time for transport through the membrane). Curves 1 and 2 correspond to mesoporous membranes with a Knudsen (H2/cyclohexane) separation factor. Curves 3 and 4 are for microporous membranes with a separation factor of 100, and curves 5 and 6 correspond to dense metal membranes with an infinite separation factor. The odd numbered curves correspond to using an inert sweep gas flow rate equal to the cyclohexane flow, whereas for the even numbered curves the sweep to cyclohexane flow ratio is 10. 

This chapter focuses mainly on Pd-based MRs with respect to the gas permeation mechanism, membrane preparation, MR construction and operation, as well as applications in a variety of chemical reactions. In addition to a general description of Pd membranes and MRs, recent progress and critical issues in the dense metal membrane area will also be presented at the end of this chapter. 

Hydrogen can permeate selectively dense metal layers, and, accordingly, this phenomenon is exploited when metal membranes are applied for separating ultrapure hydrogen from gas mixtures [24-27]. The hydrogen mass transfer through dense metal membranes includes several transport mechanisms, which are discussed in the following sections. 

Some materials, capable of being selectively permeated by only one gas, are present in nature. Generally speaking, almost all metals are permeable only to Hj, while other materials are capable of being selectively permeated only by oxygen. In particular, some dense materials (Pd and its alloys, Pt, V, Ta, Nb and SiOj ceramic membrane) exhibit signihcant H2 permeability... 

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