The relevant role of hydrogen in the new and clean energy systems motivates the interest in membrane technologies able to separate and purify such element. Dense Pd-based membranes are particularly attractive for the high values of hydrogen selectivity and good permeance provided. An established approach to model the mass transfer of hydrogen through dense metal membranes describes permeation flux as proportional to the difference of the square root of the hydrogen partial pressure in the feed and permeated side divided by the inverse of the membrane thickness. However, deviations from this law are reported in literature especially when large range of pressure and membrane thickness are considered.This work presents the results of an experimental activity carried out on three dense, defect-free, self-supported Pd-Ag membranes having a thickness of 84, 150 and 200μm, respectively. During testing, the measurements of the hydrogen flux permeated through the membranes were collected in the pressure and temperature ranges of 200-800. kPa and 473-623. K, respectively. The observed permeability pre-exponential factors and activation energies exhibited a dependence on the pressure and the membrane thickness, in agreement with the square root formula mentioned above.This dependence has been discussed by investigating the impact of two effects on the considered process: the relationship between the hydrogen diffusivity from the H/Pd ratio and the surface reactions. The analysis reported in this work shows that Sieverts' law does not account for these two effects. In fact, when applying Sieverts law at the experimental data, a specific pattern is observed in the activation energy (Ea) of the hydrogen permeability. This fact is in contrast with the definition of permeability which is an intrinsic property of the material. © 2013 Elsevier B.V.
Testing of dense Pd-Ag tubes: Effect of pressure and membrane thickness on the hydrogen permeability
Tosti, S.;Vadrucci, M.;Borgognoni, F.;Santucci, A.
2013-01-01
Abstract
The relevant role of hydrogen in the new and clean energy systems motivates the interest in membrane technologies able to separate and purify such element. Dense Pd-based membranes are particularly attractive for the high values of hydrogen selectivity and good permeance provided. An established approach to model the mass transfer of hydrogen through dense metal membranes describes permeation flux as proportional to the difference of the square root of the hydrogen partial pressure in the feed and permeated side divided by the inverse of the membrane thickness. However, deviations from this law are reported in literature especially when large range of pressure and membrane thickness are considered.This work presents the results of an experimental activity carried out on three dense, defect-free, self-supported Pd-Ag membranes having a thickness of 84, 150 and 200μm, respectively. During testing, the measurements of the hydrogen flux permeated through the membranes were collected in the pressure and temperature ranges of 200-800. kPa and 473-623. K, respectively. The observed permeability pre-exponential factors and activation energies exhibited a dependence on the pressure and the membrane thickness, in agreement with the square root formula mentioned above.This dependence has been discussed by investigating the impact of two effects on the considered process: the relationship between the hydrogen diffusivity from the H/Pd ratio and the surface reactions. The analysis reported in this work shows that Sieverts' law does not account for these two effects. In fact, when applying Sieverts law at the experimental data, a specific pattern is observed in the activation energy (Ea) of the hydrogen permeability. This fact is in contrast with the definition of permeability which is an intrinsic property of the material. © 2013 Elsevier B.V.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
