Dense metal membrane

Composite metal membranes are most often the structure of choice when a reactive group 3-5 metal or alloy is the principle constituent of the membrane. The relative chemical reactivity of these metals dictates that an inert coating must be applied to at least the feed surface of the membrane. Palladium, or better yet a palladium alloy, customarily serves as the coating layer. If it can be guaranteed that the permeate side of the membrane will never be exposed to reactive gases (e.g., water, carbon oxides, and hydrocarbons), then a two-layer composite membrane is a satisfactory choice. However, normal operating procedures and the potential for process upsets typically favors the selection of a three-layer composite structure. 

The first commercial metal membranes for hydrogen separation and purification were made of palladium alloyed with 23-25 wt % silver. These membrane were of the unsupported type and tubular in shape. Nevertheless, the wall thickness was substantial by current standards—typically at least 100- an thick. Advances in drawing thin-walled metal tubes has allowed for palladium-silver tubular membranes to be made with much thinner walls, about 20- an thick. Composite membranes are also usually at least 25-/an thick. REB Research and Consulting (Oak Park, MI) provides tubular composite metal membranes consisting of a palladium coating over a tantalum base metal, although other group 4 or 5 base metals may be used. 

If a thinner membrane is required, then one must choose a supported membrane. The permselective metal layer may be palladium or, more commonly, palladium-silver alloy, palladium-copper alloy, or other alloy of palladium. The permselective layer ranges in thickness from about 2-25 /an thinner than 2/rm is very difficult to achieve without introducing pin holes and other adverse defects into the permselective layer. The support layer is porous and is composed of either metal (such as sintered stainless steel or tightly woven wire cloth) or an inert ceramic alumina is very common. Since all of the mechanical strength is derived from the support layer, consideration must be given to its shape and thickness. 

the shape of the membrane (flat or tubular) and dimensions are dictated by limits on commercially available supports, or by limits of commercial metal rolling and drawing machines. It is very difficult (and consequently expensive) to roll a flat foil to 20-/on thickness if the width of the rolled foil is greater than 10 cm. Also, drawing thin-walled ( 100 /an) tubing is difficult and expensive if the diameter of the tube is 10 times or more than the wall thickness. These general rules apply irrespective of the composition of the metal alloy. 

The manufacture of dense metal membranes or thin films can be effected by a number of processes casting/rolling, vapor deposition by physical and chemical means, electroplating (or electroforming) and electroless plating. By far, casting in combination with rolling is the predominant preparation and fabrication technique. It is noted that many of these processes have been demonstrated with palladium and its alloys because of their low oxidation propensity. Preparation of dense metal membranes is summarized in some detail as follows. 

Traces of certain specific elements (such as C, S, Si, Cl and O) and inclusion of foreign particles or gases may connect both sides of the membrane (with a thickness of 10 pm or less) and thus render it unsuitable for separation purposes. Aluminum foils have b made down to a thickness of 10 pm and special fabrication methods can be used to produce palladium (or its alloys) foils with a thickness under 1 pm [Shu et al., 1991]. Commercially available Pd alloy foils, however, are in the 10 to 100 pm range. Cold rolling often generates lattice dislocation and it can enhance hydrogen solubility in palladium and some of its alloys due to the accumulation of excess hydrogen around the dislocation. 

Vapor deposition. Both physical and chemical vapor deposition methods can be used to prq)are dense inorganic membranes. In either process, vaporization of the membrane material to be deposited is effected by physical means (such as thermal evaporation and sputtering) or chemical reactions. 

In the chemical vapor deposition (CVD) process, heat is supplied through resistive heating, infrared heating, laser beam or plasma to effect a gas-phase chemical reaction involving a metal complex. The metal produced from the reaction deposits by nucleation and growth on the hot substrate which is placed in the CVD reactor. Effective reactants 

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. 

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Ceramic, Metal, and Liquid Membranes. The discussion so far implies that membrane materials are organic polymers and, in fact, the vast majority of membranes used commercially are polymer based. However, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafHtration and microfiltration appHcations, for which solvent resistance and thermal stabHity are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported or emulsified Hquid films are being developed for coupled and facHitated transport processes. 

Palladium-based dense metallic membranes have been known to be completely selective for hydrogen permeation and are used in commercially available small-scale hydrogen purification units (e.g., Johnson Matthey, 2007 REB Research, 2007 Power + Energy, 2007 ATI Wah Chang, 2007). These hydrogen purification units typically use palladium-alloy... 

Dense metal membranes, 15 800 Dense nonaqueous phase liquids... 

Dense metallic membranes have the advantage of very high selectivities since only certain species can be dissolved in their structural lattice. However, the permeabilities are lower by a factor of 100 than those of porous membranes (Ilias and Govind 1989, van Vuren et al. 1987, Itoh 1987, Suzuki, Onozato and Kurokawa 1987). For example, the permeability of... 

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],... 

The first membrane reactor studies made use of dense metallic membranes, but due to certain limitations of these dense materials (sec below) and due to the rapid progress in the development of (micro)porous... 

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. 

Dense metallic membranes, in particular those based on palladium alloys, have been extensively studied for the selective transport of In the case of O2,... 

Based on matenal considerations, membrane reactors can be classified into (1) organic-membrane reactors, and (2) inorgamc-membrane reactors, with the latter class subdivided into dense (metals) membrane reactors and porous-membrane reactors Based on membrane type and mode of operation, Tsotsis et al. [15] classified membrane reactors as shown in Table 3. A CMR is a reactor whose permselective membrane is the catalytic type or has a catalyst deposited in or on it. A CNMR contains a catalytic membrane that reactants penetrate from both sides. PBMR and FBMR contain a permselective membrane that is not catalytic the catalyst is present in the form of a packed or a fluidized bed PBCMR and FBCMR differ from the foregoing reactors in that membranes are catalytic. 

The inorganic membranes had until the late nineties received fairly little attention for applications in gas separation. This has mainly been due to their porous stmcmre, and therefore lack of ability to separate gas molecules. Within the group of inorganic membranes there are however the dense metallic membranes and the solid oxide electrolytes these are discussed separately in Section 4.3.5. With reference to Section 4.2, the possible transport mechanisms taking place in a porous membrane may be summarized as in Table 4.4 below, as well as the ability to separate gases (+) or not (—). Recent findings [29] have however documented that activated Knudsen diffusion may take place also in smaller pores than indicated in the table. 

There are essentially four different types of membranes, or semipermeable barriers, which have either been commercialized for hydrogen separations or are being proposed for development and commercialization. They are polymeric membranes, porous (ceramic, carbon, metal) membranes, dense metal membranes, and ion-conductive membranes (see Table 8.1). Of these, only the polymeric membranes have seen significant commercialization, although dense metal membranes have been used for commercial applications in selected niche markets. Commercial polymeric membranes may be further classified as either asymmetric (a single polymer composition in which the thin, dense permselective layer covers a porous, but thick, layer) or composite (a thick, porous layer covered by a thin, dense permselective layer composed of a different polymer composition).2... 

Figure 8.2. Commonly accepted mechanism for the permeation of hydrogen through dense metal membranes.

In cases where high purity hydrogen is valued, dense metal membranes are an attractive option over polymeric membranes and porous membranes that exhibit much lower selectivities. Two examples where this is true are low-temperature fuel cells (e.g., proton exchange membrane fuel cells [PEMFCs] and alkaline fuel cells [AFCs]) and hydrogen-generating sites where the product hydrogen is to be compressed and stored for future use. 

Figure 8.5. Example purification process using a dense metal membrane and 70% hydrogen...

In contrast, if the membrane is an inorganic composition (e.g., a dense metal membrane or a nanoporous ceramic membrane), the membrane module may be operated at the elevated temperature of 450 °C. In this case, there is no need for optional HEX 2 as the fuel gas stream will exit the membrane module at 450 °C and pass to the burner without further cooling. In addition to a net increase in overall process energy efficiency, the elimination of HEX 2 also represents a reduction in capital cost for the system. 

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