Fabrication of Ultrathin Palladium Composite Membranes by a New Technique and Their Application in the Ethanol Steam Reforming for H₂ Production
This thesis describes a new technique for the preparation of ultrathin Pd based membranes supported on a hollow-fiber α-alumina substrate for H₂ separation. The effectiveness of the membranes is demonstrated in the ethanol steam reforming (EtOH SR) reaction in a membrane reactor (MR) for H₂ production.
The membrane preparation technique uses an electric-field to uniformly deposit Pd nanoparticle seeds on a substrate followed by deposition of Pd or Pd-Cu layers on the activated surface by electroless plating (ELP). The well distributed Pd nanoparticles allow for enhanced bonding between the selective layer and the substrate and the formation of gas tight and thermally stable Pd or Pd-Cu layers as thin as 1 µm, which is a record in the field. The best Pd membrane showed H₂ permeance as high as 5.0 × 10⁶ mol m²s⁻¹Pa⁻¹ and stable H²/N₂ selectivity of 9000 - 7000 at 733 K for 5 days. The Pd-Cu alloy membrane showed H₂ permeance of 2.5 × 10⁶ mol m⁻²s⁻¹Pa⁻¹ and H₂/N₂ selectivity of 970 at the same conditions.
The reaction studies were carried out with a Co-Na/ZnO catalyst both in a packed bed reactor (PBR) and in a MR equipped with the Pd or Pd-Cu membrane to evaluate the benefits of employing membranes. For all studies, ethanol conversion and hydrogen product yields were significantly higher in the MRs compared to the PBR. Average ethanol conversion enhancement and hydrogen molar flow enhancement were measured to be 12 % and 11 % in the Pd MR and 22 % and 19 % in the Pd-Cu MR, respectively. These enhancements of the conversion and product yield can be attributed to the shift in reaction equilibria by continuous hydrogen removal by the Pd based membranes. The comparative low enhancement in the Pd MR was found to be the result of significant contamination of Pd layer by CO or carbon compounds deposition during the reaction.
A one-dimensional modeling of the MR and the PBR was conducted using identical conditions and their performances were compared with the values obtained from the experimental study. The model was developed using a simplified power law and the predicted values matched experimental data with only minor deviations indicating that the model was capturing the essential physicochemical behavior of the system. Enhancements of ethanol conversion and hydrogen yield were observed to increase with rise in space velocity (SV), which could be explained by the increase in H₂ flux through the membranes with SV in the MRs.