Hydrogen production via Pd-Ag permeators: Membrane reactor design and process study

Thin wall Pd-Ag permeator tubes have been produced by a diffusion welding procedure: these low-cost membranes are proposed for separating and producing highpure hydrogen in membrane reactors. The reliability of these dense metallic permeators is strongly related to the design configuration of the membrane modules. In fact, as a consequence of hydrogen and thermal cycling, the dense metallic tubes vary their length: therefore, in case of constrains between the membrane and the module, cyclic axial stresses on the tube can rise and involve the rupture of the permeator. In our application, a fingerlike assembly of the membrane tubes inside the reactor has been designed: it permits the free elongation and contraction of the palladium alloy tube avoiding any mechanical cycling stress. A process in which a membrane reactor produces ultra pure hydrogen needed to feed a polymeric fuel cell of power 500 W has been designed: the process foresees the use of both traditional and membrane reactors. The membrane reactor consists of a bundle of Pd-Ag thin wall tubes operating in parallel where the water gas shift reaction takes place at 350°C and 200 kPa: the hydrogen stream of 4,46·10-3 mol s-1 (about 6 liter/min) permeating through the membrane is recovered in the shell side by means of nitrogen sweep gas at 100 kPa.