Pd-Ag foils

Sheintuch and Dessau [15] reported significant improvement in butane and propene yield in a PBMR equipped with Pd-Ru and Pd-Ag foils (250 and 76 pm. 

Based on this paper and on various other papers dealing with both theoretical and experimental works, recently the same group presented the experimental findings of pre-commercial MR prototypes for both methane reforming and ATR (with air addition) [49, 50]. The authors used optimized plane membranes, where a single two-side planar membrane panel consists of two Pd-Ag foils mounted on a porous stainless steel base using proprietary MRT sealing and protection techniques. 

In the past, dense selective Pd-Ag thin-waU tubes have been produced via cold rolling and diffusion welding procedures. According to this technique, commercial Pd-Ag foils have been cold rolled in order to reduce... 

The limbs of the annealed thin Pd-Ag foil are then welded to form the permeator tube. Several joining techniques have been considered ... 

This technique consists of pressing the metal parts during heat treatment carried out at high temperatures, usually at 50-75% of the metal s melting point. The procedure uses a thermo-mechanical press consisting of two stainless steel plates tightened by screws made of Invar, a nickel-steel alloy with a very low coefficient of thermal expansion. The thin Pd-Ag foil is wrapped around an alumina bar and its Umbs are kept close by the thermomechanical press shown in Fig. 16.4. 

The heat treatment is carried out under vacuum or inert gas, and the temperature increases so that all materials (Pd-Ag foil, steel plates, etc.) expand except for the Invar screws, which apply pressure to the metal parts being joined. As shown in detail in Rg. 16.5, the resulting Pd-Ag thin-wall membrane tube is joined to two more rigid steel tube ends (100 xm wall thickness), in order to give the permeator the necessary stiffness for a tight connection to the hydrogen recovery system. 

Figure 7.4 shows the structure of an FBMR with plate-type Pd-Ag dense metal membranes for hydrogen production [8, 9]. Two-sided planar membrane panels are suspended vertically in the reactor. Each side of the panels consists of 25 pm thick Pd-Ag foil mounted on a porous stainless steel base with a barrier layer to prevent interdiffusion... 

Through Equation 13.30, the diffusion length of silver is calculated versus time for different temperatures, as presented in Figure 13.16. For instance, Pd-Ag foils of thickness 50 m can be joined through diffusion welding when the diffusion length is about 100 pm the thickness of two metal foils overlapped. For such a case, it is observed that less than 1 h at 1000°C is needed. 

The cross section of the welded zone is shown in Figure 13.18a, whereas the scheme of the overlapped limbs of the Pd-Ag foils is shown in Figure 13.18b. 

In an alternative method, a thermomechanical press applies the pressure needed to weld the metal limbs of the Pd-Ag foil under the heat treatment. In fact, this press. 

Metal supports used for these membranes consisted of both stainless steel grids and nickel-perforated sheets. A Pd-Ag composite membrane fabricated using a stainless steel grid is shown in Figure 13.22. The cross section of this membrane presented in Figure 13.23 shows detail of the joint between the Pd-Ag foil (50 pm thick) and the stainless steel grid. 

A nickel-perforated metal support of thickness 210 pm and hole diameter 2.5 mm used for producing a tubular supported membrane with a Pd-Ag foil of thickness 42 pm is presented in Figure 13.24. First, diffusion welding is applied for joining the Pd-Ag foil to the flat nickel support and then for welding the limbs of the supported manbrane wrapped around an alumina bar, as described in the scheme of Figure 13.25. 

FIG U RE 13.21 Schematic representation of the press used for joining the Pd-Ag foils to flat metal supports. (Reprinted fromlnl. J Hydrogen Energy, 28, S. Tosti, Supported and laminated Pd-based metallic membranes, 1455-1464, Copyright 2003, with pamission from Elsevio-.)... 

The cross section of a flat laminated membrane consisting of an Ni sheet covered over its surfaces by two Pd-Ag foils of thickness 28 pm is shown in the microphotography image of Figure 13.28. 

Another laminated membrane prepared by covering an Nb sheet (1 nun) with two Pd-Ag foils (25 pm) has been cold-roUed to obtain a composite membrane of overall thickness 128 pm, with about 122 pm of Nb bulk with two very thin Pd-Ag layers of 3 pm. Permeator tube produced with this laminated membrane was tested with hydrogen at 180°C and 200 kPa and exhibited very poor durability. Embrittlanent due to the high hydrogen uploading produced a quick failure of this membrane, as shown in Figure 13.30. 

The diffusion welding and cold-rolling procedure previously described for the synthesis of the Ni-based membrane has been also applied to the production of a Pd-Ag/Nb/Pd-Ag composite membrane. In this application, two Pd-Ag foils with a thickness of 25 pm were used to cover a Nb plate with a thickness of 1 mm. This composite membrane was rolled down to a thickness of 128 pm (of which about 122 pm was Nb bulk, between two very thin Pd-Ag layers of 3 pm). The permeation tests carried out at 180°C... 

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