Dense Pd-based membranes

Let us now consider a simple reaction in a membrane reactor where only a product is permeating through the membrane (and where the pressure in the permeate side is dilferent than zero). For this calculation consider the typical steam reforming of methane which is an equilibrium reaction producing hydrogen, carried out in a Pd-based dense membrane which allows only the product hydrogen to permeate and leave the reaction zone. 

The flux term depends on the membrane used. For hydrogen production and purification, dense hydrogen perm-selective membranes are often used, which often exhibit (virtually) infinite selectivity toward hydrogen (in the case of Pd-based dense membranes). In this particular case, the flux term reduces to the flux of hydrogen though the membrane, and it is equal to zero for all the other components. To compute... 

Catalytic bed reactors are usually in turbulent conditions, making the ID model absolutely reliable for a sufficiently deep assessment of reactor performance. On the other hand, the 2D models add information on radial temperature and concentration profiles and, in some cases, are necessary for reactor design. Particularly in membrane reactors, there is the problem of membrane overheating, for example the hydrogen-selective Pd-based dense membrane, which can be integrated in many hydrogen production processes (Dittmeyer et al, 2001 Howard et al, 2004 Peachey, et al. 1996 ... 

Mathematical model of the Pd-based dense membrane MSR reaction... 

Among the hydrogen-selective membranes that can be assembled in a steam reforming membrane reactor, the Pd-based dense membranes are surely the most interesting. 

In the catalytic partial oxidation of methane to produce syngas the use of permselective dense perovskite membranes avoids (or minimizes) the need of air separation, the most costly step in the process. Although both these O2- and H2-permeoselective membranes (based on perovskites or thin supported Pd-based dense films, respectively) have still to be further developed for commercial applications the outlook appears quite interesting for intensifying various large chemical processes. 

Currently, the membranes incorporated in MMRs are mainly zeolite and Pd-based dense metal ones. Incorporation of these membranes in microreactors can be achieved using one of the preparation methods described in previous chapters. 

Dense Pd-based membranes have been first used for CMRs applications [4]. They are indeed highly selective for H2 permeation but are expensive, sensitive to ageing and poisoning and are strongly limited by their low permeabilities. 

As explained in Chapter 5, the transport mechanism in dense crystalline materials is generally made up of incessant displacements of mobile atoms because of the so-called vacancy or interstitial mechanisms. In this sense, the solution-diffusion mechanism is the most commonly used physical model to describe gas transport through dense membranes. The solution-diffusion separation mechanism is based on both solubility and mobility of one species in an effective solid barrier [23-25], This mechanism can be described as follows first, a gas molecule is adsorbed, and in some cases dissociated, on the surface of one side of the membrane, it then dissolves in the membrane material, and thereafter diffuses through the membrane. Finally, in some cases it is associated and desorbs, and in other cases, it only desorbs on the other side of the membrane. For example, for hydrogen transport through a dense metal such as Pd, the H2 molecule has to split up after adsorption, and, thereafter, recombine after diffusing through the membrane on the other side (see Section 5.6.1). 

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],... 

When dense Pd-based membranes are used, permeabilities are low. To increase the membrane surface per unit volume of reactor the use of spiral tubes or helix-shaped Pd membranes has been proposed [39]. 

Electroplating. Basically in electroplating, a substrate is coated with a metal or its alloy in a plating bath where the substrate is the cathode and the temperature is maintained constant Membranes from a few microns to a few millimeters thick can be deposited by carefully controlling the plating time, temperature, current density and the bath composition. Dense membranes made of palladium and its various alloys such as Pd-Cu have been prepared. Porous palladium-based membranes have also been made by deposition on porous support materials such as glass, ceramics, etc. 

Transport and therefore separation mechanisms in porous inorganic membranes are distinctively different from and more varying than those prevailing in dense membranes. Because of the variety of the mechanisms that can be operative, these membranes in principle are capable of separating more varieties of compound mixtures. Compared to dense membranes, these porous membranes generally exhibit permeabilities of one or two orders of magnitude higher. For example, Pd-based membranes typically have a... 

As fabricated, a Pd or its alloy membrane suffers from the relatively low catalytic activity of the attached catalyst due to the typical low surface area to volume ratio of the membrane geometry. The catalyst in a dense Pd-based membrane can be the Pd itself or its alloy or some other materials attached to the membrane. Pretreatments to the Pd-based membranes can help alleviate this problem. This and other membrane material and catalysis issues will be further covered in Chapter 9. 

In contrast to the studies on gas- and vapor-phase hydrogenation reactions utilizing dense Pd-based membrane reactors, dehydrogenation reactions have been consistently observed to benefit from the concept of a membrane reactor. In almost all cases the reaction conversion is increased. This is attributed to the well known favorable effect of equilibrium displacement applied to dehydrogenation reactions which are mostly limited by the equilibrium barrier. 

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]. 

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