Palladium alloy membranes module

For a well designed membrane module of reasonably large scale, the initial cost should be dominated by the cost of the palladium content of the thin metal membrane (assuming a palladium alloy comprises the permselective layer). The module itself will be made of steel, most likely a nickel alloy, with or without significant chromium addition. The cost of the steel and the assembly labor should not exceed the cost of the palladium alloy membrane. 

It is often found that the sintered metal and porous ceramic supports that have been used for many academic membrane studies are very expensive, sometimes even exceeding the cost of the palladium alloy membrane by several fold. This situation cannot be accepted for commercial membrane modules, especially when a large membrane area of up to several hundred to several thousand square meters is required. Some of the least expensive membrane supports include tension springs for tubular membranes and woven wire mesh for planar membranes, but even these supports can be costly, and further development of exceptionally low cost membrane supports is needed. 

The data presented in Fig. 5.6 were derived from membrane modules using Pd- Cu foil (planar) membranes with the membrane area sized to deliver 0.78 Nm h i of product hydrogen. Even though palladium alloy membranes are often criticized as being too expensive for commercial applications due to the cost of palladium, it is clear that as membrane thickness is reduced to 10 pm and less, the cost associated with the value of palladium in the membrane can be very reasonable. 

Membrane failure modes have been discussed above, and the connection to module design has also been discussed. Poor design for cyclic durability will be measured by the customer - the operating costs will be adversely affected by the requirement to replace membranes prematurely. Although the cost of membrane replacement should be offset by a recycle credit, the cost of materials and labor, and potential lost productivity, is stQl likely to be significant. The credit for recy-cUng palladium alloy membranes may be as great as 95% of the market value of the palladium (for foil membranes, perhaps only 85% of the market value if the palladium alloy is deposited onto a porous substrate). Environmentally, recycle also offers benefits versus recovery and purification of palladium from ore. 

Tokyo Gas Co., Ltd. (TGC) has developed a 40 Nm /h-class membrane reformer system with the world s highest efficiency (a value of 81.4%). The company has demonstrated the use of the hydrogen produced to refuel fuel cell vehicles (FCV), together with CO2 capture at the hydrogen station. An advanced hydrogen separation membrane module consisting of a palladium alloy membrane on a structured porous catalyst, which can be used to produce a membrane reformer that is more compact and less expensive, has also been developed. This chapter introduces the development of these two membrane reformer technologies. 

In parallel with the development of the membrane reformer system, a new concept membrane module, which has a palladium alloy membrane coated on the porous support tube with catalytic activity has been developed (Nishii, 2009). This membrane module is expected to provide a more compact reactor because the reactor does not require a separate catalyst. It is also expected that this module can be manufactured at low cost by applying the industrially-established mass production process used to make oxygen sensors for combustion control in vehicles with internal combustion engines. 

An integrated proof-of-concept (POC) size fluidized-bed methane reformer with embedded palladium membrane modules for simultaneous hydrogen separation is being developed for demonstration (Tamhankar et al., 2007). The membrane modules will use two 6 in. X 11 in. Pd-alloy membrane foils, 25-pm thick, supported on a porous support. The developmental fluidized-bed reactor will house a total of five (5) membrane modules with a total membrane area of about 0.43 m2 and is scheduled for demonstration by September 2007. 

The MRT Purifier, developed for Tecnimont-KT in the framework of FISR project, has been designed to house five membrane modules and operate at 450°C and up to 25 barg. Each module consists of two double sided, planar 30 cm X 12 cm membrane panels welded in series. Each panel has a palladium (Pd) alloy active membrane area of 0.03 m per side for a total installed membrane area of 0.6 m in the purifier. The modules are housed in a rectangular core which, along with the inlet distributor, promotes uniform reformate flow across the membrane modules. The core assembly (Fig. 3.11) is housed inside a pressure vessel. 

The membrane module has a plate-type structure 40 mmW x 460 mmL x 8 mmT in size. Figure 12.2 illustrates the configuration of the membrane module, and Fig. 12.3 shows a view of the membrane modules. The membrane modules consist of palladium-rare earth alloy thin film with thickness of less than 20 pm and a porous structural support. The hydrogen permeability of the membrane is several times higher than that of the widely used conventional palladium-silver alloy membrane (Sakamoto, 1992). 

Palladium or its alloys are the most practical membrane materials, due to their high hydrogen permeability and stability at high temperatures. The membrane reformer is composed of a steam reformer equipped with palladium-based alloy modules in its catalyst bed, and can perform steam reforming reaction and hydrogen separation processes concurrently with no help from shift converter and PSA, as shown in Fig. 12.1. 

---