Mathematical modeling of high temperature and high-pressure dense membrane separation of hydrogen from gasification

Abstract

At UTSI, Pd/Al 2O 3 membranes were prepared by a special method of laser based thermal deposition of a thin film Pd on a ceramic substrate by Nd–YAG laser irradiation of PdCl 2 coating on a γ-alumina substrate. This paper describes a mechanistic model for the hydrogen permeation process in such Pd/Al 2O 3 composite membrane developed at UTSI. The model takes into account the well-known kinetics of hydrogen adsorption/desorption in the palladium surface and hydrogen permeation in the porous alumina layer. Reasonable values for all mass transfer rate parameters were estimated based on the available surface science and membrane permeation literature. One set of experimental data (at 866.48 K) was used to determine the best values of the necessary rate parameters. These values of rate parameters were then used to predict and compare the experimental hydrogen flux data at two other temperatures (755.37 and 977.59 K). The results demonstrated that the atomic hydrogen diffusion through the palladium layer and pore diffusion in the porous alumina support both played important roles in the permeation of hydrogen through the composite Pd/Al 2O 3 membrane. A simplified resistance model was also employed to analyze the permeation behavior of hydrogen through the Pd/Al 2O 3 membrane to identify the major resistances to the mass transfer. The results indicated that the mass transfer in the Pd layer contributed about 90% of the total mass transfer resistance. Our model calculations also indicated that by reducing the thickness of the Pd layer to about 18 μm, the DOE goal of >5 × 10 −3 m 3/m 2 s (60 scfh/ft 2) for hydrogen gas flux can be achieved. This can also be achieved by reducing the thickness of the Pd layer to about 20 μm and reducing the thickness of the alumina support layer to about 2 mm or by increasing its porosity to about 50%.