Hydrogen permeation through Pd–Ag membranes: Surface effects and Sieverts' law

Metal membranes mainly made of Pd alloys can be applied in membrane reformers for ultrapure hydrogen production from hydrocarbons and alcohols.Knowledge of the hydrogen mass transfer mechanisms through metals is very important to the purpose of properly designing and operating the membrane reactors. With the aim of understanding the deviations from the ideal behavior of the transport mechanisms predicted by the Sieverts' law, a permeation model which takes into account the surface effects has been applied to the permeability measurements that have been carried out on dense Pd–Ag permeator tubes from 473 to 623 K. The model has been validated by the results of permeation tests in a wide range of pressure (200–800 kPa) and membrane thickness (84–200 μm).The new model modifies the Sieverts' law by introducing the mass transfer resistances due to surface effects. The values of the surface resistance and permeability calculated by the model have been compared with the literature.Finally, the new hydrogen permeation expression has been applied in order to analyze the cost of a separation system which would consist of tubular Pd-based membranes.

► Dense Pd–Ag tubes of thickness 84, 150 and 200 μm have been tested.
► Deviation from Sieverts' law has been observed in the range 200–800 kPa.
► A permeation model taking into account the surface resistance has been developed.