Hydrogen production membrane types

The high-cost of materials and efficiency limitations that chemical fuel cells currently have is a topic of primaiy concern. For a fuel cell to be effective, strong acidic or alkaline solutions, high temperatures and pressures are needed. Most fuel cells use platinum as catalyst, which is expensive, limited in availability, and easily poisoned by carbon monoxide (CO), a by-product of many hydrogen production reactions in the fuel cell anode chamber. In proton exchange membrane (PEM) fuel cells, the type of fuel used dictates the appropriate type of catalyst needed. Within this context, tolerance to CO is an important issue. It has been shown that the PEM fuel cell performance drops significantly with a CO con-... 

Metal- and alloy-containing membranes are currently applied mainly in ultrapure hydrogen production. Pilot plants with palladium alloy tubular membrane catalyst were used in Moscow for hydrogenation of acetylenic alcohols into ethylenic ones. In the Topchiev Institute of Petrochemical Synthesis, a laboratory-scale reactor of the same type was tested... 

In the conceptual design, the nuclear plant is a type of SFR, mixed oxide fuel, sodium cooled with power output of 240 MWt for producing 200 000 Nm /h. The schematic diagram of nuclear-heated recirculation-type membrane reformer is shown in Figure 15. The hydrogen production cost of this process is assessed to be competitive with those of the conventional, natural gas burning, steam methane reformer plants. 

Membrane separators offer the possibility of compact systems that can achieve fuel conversions in excess of equilibrium values by continuously removing the product hydrogen. Many different types of membrane material are available and a choice between them has to be made on the basis of their compatibility with the operational environment, their performance and their cost. Separators may be classified as (i) non-porous membranes, e.g., membranes based on metals, alloys, metal oxides or metal—ceramic composites, and (ii) ordered microporous membranes, e.g., dense silica, zeolites and polymers. For the separation of hot gases, the most promising are ceramic membranes. 

Palladium membranes have been known for years, and the concept of a membrane reactor that would allow the continuous removal of a reaction product from an equilibrium controlled reactor has been discussed and to some degree practiced for several decades. Hydrogen production, via some type of hydrocarbon reforming or catalytic partial oxidation, coupled with the WGS reaction CO -I- H2O H2 -I- CO2 (AG° = -41 kJ/mole) is one application for such a membrane reactor. 

Different types of membrane reactors for hydrogen production have been proposed in the literature. Most of the previous work has been performed in packed bed membrane reactors (PBMRs) however, there is an increasing interest in novel configurations such as fluidized bed membrane reactors (FBMRs) and membrane micro-reactors (MMRs), especially because better heat management and decreased mass transfer limitations can be obtained in these novel reactor configurations. 

A number of studies have verified the beneficial potential of Pd-based membrane reactors for hydrogen production by investigating critical parameters such as pressure, temperature, catalyst, gas composition and reactor design on the overall performance, as will be discussed below. As the thermodynamics, kinetics and by-product formation vary with the type of fuel/feedstock applied, both the Pd-based membrane and operating conditions need to be tailored in order to guarantee a cost-effective process and sufficient membrane stability. 

Palladium membranes have been proposed for both production and separa-tion/purilication of hydrogen. Pure and CO-free H2 produced might be used to feed the PEM-FC. This membrane type is not discussed here since other chapters in this book deal deeply with hydrogen and palladium-based membranes. 

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