Permeation, hydrogen dense membranes

FIGURE 10.3 (a) Hydrogen and (b) oxygen permeation through dense oxide ceramic membranes. 

Instead of using a sweep gas for removing the permeating hydrogen, several authors who employed dense Pd membranes proposed the application of a second reaction in which H2 is consumed. This second reaction, which has to take place on the permeate side can be cither a hydrogenation [52] or the formation of water by reaction with O2 [53], both being catalyzed... 

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). 

There has been a large volume of data showing the benefit of having thin dense membranes (mostly Pd-based) on the hydrogen permeation rate and therefore the reaction conversion. An example is catalytic dehydrogenation of propane using a ZSM-5 based zeolite as the catalyst and a Pd-based membrane. Clayson et al. [1987] selected a membrane thickness of 100 m and achieved a yield of aromatics of 38% compared to approximately 80% when a 8.6 pm thick membrane is used [Uemiya et al., 1990]. 

In dense, non-porous membranes, surface limitations to oxygen permeation are a common phenomenon as can be understood from the very low adsorption levels and large activation energies on the dense membrane materials (see Chapter 10). For hydrogen permeation in dense metal membranes estimates have been made by Govind [105]. 

Inorganic membrane development is still in progress [57] (see also Section 14.2.2). Microporous silica membranes have been developed at several universities and research institutes. Membrane selectivities of 15 and 20 for the separation of H2 from CO2 have been reported. Even higher selectivities for H2 arid CO, CH4 and N2 have been measured [20,57]. Most measurements reported in the literature have been performed on a laboratory scale. However, it has been shown that it is possible to upscale these microporous ceramic membranes to, at least, bench scale [31,57]. With other membranes such as noble (Pd) metal membranes and dense ceramic membranes very high and almost infinite selectivities for hydrogen are possible [58]. The permeation of these membranes is generally smaller than the permeation of microporous membranes. 

Light alkane (C2-C4) dehydrogenation was the reaction studied by Gryaznov and coworkers in their pioneering studies [2.1, 2.2]. In their dehydrogenation reaction studies, they used Pd or Pd-alloy dense membranes, which were 100 % selective towards hydrogen permeation. The choice of these membranes in many of the early studies is because they were commercially available at that time in a variety of compositions, and their metallic nature allows the construction of multitubular and other complex-shaped membrane reactor systems. Comprehensive review papers on Pd membrane reactors have been published by the same group [2.1, 2.2], and also by Shu et al [2.3]. 

The materials of membrane construction can be classified as either dense or porous. Dense metal materials include palladium membranes that are semiperme-able to hydrogen, and silver membranes that are semipermeable to oxygen. The low permeation rates for silver membranes have led to the more recent use of solid oxide electrolyte dense membranes such as modified zirconias and perovs-kites, which have higher O2 permeation rates at high temperatures. ... 

Dense membranes have been used to feed hydrogen for hydrogenation reactions.Improved yields have been observed, due not to the above kinetics reasons, but attributed to the better availability of the active available on the membrane surface. Most dense membrane use has been to feed oxygen. Early studies considered silver membranes, but cost and low permeation rates did not favor these. More recent work has used solid oxide electrolytes as membranes. Initially, investigators used yittria- and calcia-stabilized zirconias (YSZ or CSZ), which had reasonable oxygen anion conductivity. Their low electron conductivity dictated the use of an external circuit, as shown in Figure 4(a). 

By the term dense, it is implied that there are no intentional interconnected pores in the membranes other than atomic interstices, atomic vacancies and dislocations. Such void spaces are too small to accommodate even molecular hydrogen, and dense membranes, of the type reviewed, transport hydrogen only in a dissociated form. Dense membranes block transport even of helium, and the absence of larger pores gives dense membranes hydrogen selectivity approaching 100%. Transport of hydrogen in a dissociated form implies that dense membranes must possess adequate catalytic activity for the adsorption and dissociation of H2 on the feed-side surface (retentate) as well as for the subsequent recombination and desorption from the permeate-side surface. 

This chapter aims to keep these challenges in mind as we review the defect chemistry, transport theory and aspects of characterization of hydrogen permeation in dense ceramics. We will first look at some applications and simple schemes of operation of hydrogen-permeable membranes and then, briefly, at the literature and status of hydrogen-permeable dense ceramics. 

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