Hydrogen is the only energy carrier which can cater to the rising power demands across the world without compromising with the environmental impact. Its high efficiency to generate power comes in conjunction with fuel cells that can produce a desired output with absolutely no carbon emissions. However, the hydrogen fed to fuel cells must be of high purity to ensure stable performance of Pt-based catalysts of fuel cell anode. Therefore, production of hydrogen with high purity is considered crucial. In the current work, hydrogen generation is targeted with methanol steam reforming using synthesized Cu-based catalysts. To achieve high hydrogen purity, Pd-based dense supported membranes were employed after their synthesis, characterization and testing. The target of this work is to suitably integrate the catalysts with the membranes in one single unit. Such integration will cause continuous permeation of hydrogen through a H
2 selective Pd membrane. Disturbed reaction equilibrium will therefore cause an increase in the forward reaction rate according to Le Chatelier’s rule and hence improve methanol conversion. This system of simultaneous generation and separation of hydrogen in one single unit is termed as a membrane reformer (MR). Catalyst placing relative to the membrane determines the reaction residence time in a membrane reformer and thereby plays a key role. Further, this factor is inter-related with reaction kinetics, hydrogen mass transfer rate from reaction zone to membrane, and lastly membrane permeation flux and perm-selectivity. In order to observe the effect of these parameters on total system performance, the membrane reformer study was paralleled between a packed bed and a fluidized bed MR. Packed bed consists of a catalyst bed (~bed ht. 2 cm) in the shell of the reactor with membrane in its lumen separated by a distance of approximately 15 cm. In contrast to that, fluidized bed gives the advantage of catalyst being fluidized in the same shell that brings it in close contact with the membrane.
The current study would determine performance of packed bed MR with varying catalyst positions and parameters such as temperature, flow rate and weight hour space velocity (WHSV). Based on these operating conditions, fluidization regime was characterized by u/umf in the range of 1.4–3.6 (where umf corresponds to the minimum fluidization velocity). All testing was performed in an in-house designed test rig. Dense Pd-based membranes were prepared using modified electroless deposition approach on porous SS (PSS) with yttria stabilized zirconia (YSZ) as the intermediate layer. YSZ minimizes the inter-metallic diffusion between Pd and SS. Further, catalysts with varying bi-metallic compositions were synthesized by incipient wetness impregnation. Optimization of standalone membrane as well as catalyst performance was performed prior to the integration.
Once integrated, the packed bed membrane reformer (MR) showed >95% methanol conversion and 45% H2 recovery at 723 K and 3 bar. Further, a comparison of packed bed MR was made with a fluidized bed MR. It was observed that with increasing u/umf greater contact between the catalyst and the membrane enhances the hydrogen yield and thereby leads to better separation in comparison to packed bed. Moreover, a complete description of these results will be elaborated in terms of effectiveness factor, reaction kinetics and hydrogen permeation flux through the membrane.
