Palladium-based membranes reactions

The unusual interaction of hydrogen with palladium-based membrane materials opens up the possibility of oxidative hydrogen pump for tritium recovery from breeder blankets. The feasibility for this potential commercial application hinges on the hot-fusion and cold-fusion technology under development [Saracco and Specchia, 1994]. At first, Yoshida et al. [1983] suggested membrane separation of this radioactive isotope of hydrogen followed by its oxidation to form water. Subsequently, Hsu and Bauxbaum [1986] and Drioli et al. [1990] successfully tested the concept of combining the separation and reaction steps into a membrane reactor operation. 

As mentioned earlier, two compatible reactions may be coupled or conjugated properly by a shared membrane through which the species (as a product on one side of the membrane and a reactant on the other) common to both reactions selectively passes. Summarized in Table 8.5 are some documented studies of reaction coupling using dense palladium-based membranes with the alloying component ranging from nickel, ruthenium, rhodium to silver. 

While most of the membrane reactor studies on ethylbenzene dehydrogenation employ fixed-bed membrane reactors, Abdalla and Elnashaie [1995] evaluated the concept of a fluidized-bed membrane reactor through modeling. Since hydrogen is released from the reaction, a palladium-based membrane can be used for this application. 

Both theoretical and experimental studies have been performed on palladium-based membrane reactors for the water-gas shift reaction. Ma and Lund simulated the performance achievable in a high temperature water-gas shift membrane reactor using both ideal membranes and catalysts [18]. By comparing the results obtained with those related to the existing palladium membrane reactors, they concluded that better membrane materials are not needed, and that higher performances mainly depend on the development of a water-gas shift catalyst not inhibited by CO2. Marigliano et al. pointed out how the equilibrium shift conversion in membrane reactors is an increasing function of the sweep factor (defined as the ratio between the flow rate of the sweep at the permeate side and the flow rate of CO at the reaction side) [19]. The ratio is an index of the extractive capacity of the system. 

Moreover, Fig. 2.2 points out further statistics data on palladium membranes applied in the field of membrane reactors (MRs), devices combining the separation properties of the membranes with the typical characteristics of catalytic reaction steps in only one unit. In particular, this figure reports the number of publications on palladium-based membranes reactors with respect to the total number of publications in the membrane reactors area. 

As a main scope, the present chapter will give an overview on the general classification of the membranes, paying particular attention to the palladium-based membranes and their applications, pointing out the most important benefits and the drawbacks due to their use. Finally, the application of palladium-based membranes in the area of the membrane reactors will be illustrated and such reaction processes in the issue of hydrogen production will be discussed. 

Reaction Processes Using Palladium-based Membranes... 

The applications and the research studies performed on this kind of reaction were realized by many scientists. In particular, a great literature is present on this issue concerning the use of palladium-based membrane reactors, as resumed briefly in Table 2.7, where CO conversion values obtained in MR and compared with the thermodynamic equilibrium ones of some scientific works are reported. In particular, among these works, Kikuchi et al. [115] demonstrated that, using a 20 pm layer of palladium-coated onto a porous glass tube produced by the electroless plating method, allows to obtain almost complete CO conversion. 

BasUe et aL [116] studied the WGS reaction using a MR consisting of a composite palladium-based membrane realized with an ultrathin palladium film ( 0.1 pm) coated on the inner surface of a porous ceramic support (y-Al203) by the co-condensation technique. The authors pointed out the benefit of applying a palladium MR, taking into account that, at 320°C and 1.1 bar, the thermodynamic equilibrium of CO conversion is around 70%, while the authors obtained with the MR CO conversion of around 100%. Moreover, the same authors illustrated that a complete CO conversion could be reached by using a composite membrane with a thinner palladium layer (10 pm Pd film coated on a ceramic support) [117]. 

Membrane Reactor Technologies Ltd (MRT) has experimentally verified the permeative-stage membrane reactor concept. With the membranes outside the reaetor, operation at more favorable conditions for both reaction (750 °C) and membrane separation (450 °C or lower) is possible. A decrease in the metal cost of palladium-based membranes by 86.5% and membrane area by >70% to aehieve equal hydrogen production capacity was reported. The volume of reformer decreases accordingly, thus, the costs of both the reactor and membrane module are reduced. 

As mentioned, generally the use of Pd-based membranes impUes, as a constraint, that the stream fed to the reactor does not contain sulfur. Palladium can also interact with other species involved in the water-gas shift reaction. For example, a reduction of hydrogen permeation can occur because of the adsorption of CO and steam onto the palladium surface, but generally the phenomena are limited to operating temperatures below 250 °C [17]. 

S. Chaturabul, K. Wongkaew, U. Pancharoen, Selective transport of palladium through a hollow fiber supported liquid membrane and prediction model based on reaction flux, Sep. Sci. Technol. 48 (2013) 93-104. 

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