Mathematical Modeling of High Temperature and High Pressure Dense Membrane for Separation of Hydrogen from Gasification

• Main Author: Iwuchukwu, Ifeyinwa J
• Published: Trace: Tennessee Research and Creative Exchange 2006
• Subjects: Chemical Engineering
• Online Access: http://trace.tennessee.edu/utk_gradthes/248

There is an increasing interest in the use of inorganic membranes as a means of separating gas mixtures at high temperatures and pressures. The most important membrane properties are high permeability and selectivity, and good mechanical, thermal and chemical stability. Dense Pd-based composite membranes are suitable for hydrogen separation and use in catalytic membrane reactors because of their high permeability, good surface properties and high selectivity for hydrogen transport. At UTSI, Pd/AlO23 membranes were prepared by a special method of laser based thermal deposition of the thin film Pd on a ceramic substrate by Nd-YAG laser irradiation of PdCl2 coating on a γ-alumina substrate. This work reports a mechanistic model for the hydrogen permeation process in the Pd/Al2O3 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 11000F) 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 (90000F and 1300F). 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/Al2O3 membrane. A simplified resistance model was also employed to analyze the permeation behavior of hydrogen through the Pd/Al2Omembrane 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 > 60 scfh/ft2 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 it’s porosity to about 50%. v