Design of an integrated membrane system for high level hydrogen purification

Aim of this work is the theoretical analysis of the potential of integrated membrane systems to recover hydrogen at a very high purity level (with CO content lower than 10 ppm), suitable for fuel cell applications. Both polymeric and palladium (Pd) separators as well as a Pd-based membrane reactor have been investigated in order to identify the appropriate operation conditions for each unit taking advantage of the synergic effects of their combination in the design of the whole integrated system. A feed stream containing hydrogen, carbon monoxide and carbon dioxide has been considered. By means of a screening of different materials it has been possible to identify a membrane able to achieve high hydrogen purity and recovery. In order to improve the single membrane stage performance, a two stage system has been analysed taking into account both compression and membrane surface requirements. At fixed stage cut values, higher purity levels are reached in a two membrane stage arrangement with an increase of membrane surface and compression requirements. High hydrogen purities can also be achieved by operating at high feed pressure values. The presence of a membrane reactor to further enhance the hydrogen amount by converting the carbon monoxide has also been investigated. As a low driving force is available (e.g. 5 atm), the combination that seems to be the most convenient assumes that the syngas mixture is first fed to the two-stage polymeric membrane unit, then the permeate stream is further treated in a palladium separator (Pd-Sep) while the retentate streams are processed in two palladium membrane reactors (Pd-MRs). On the contrary, at 10 atm a single polymeric stage followed by a Pd-Sep and a Pd-MR represents the most adequate solution because a comparable membrane surface is combined to lower compression power and H(2) losses.