Analysis of membrane reactor integration in hydrogen production process

Abstract

In the H2 production field, the membrane reactor (MR) technology is considered a promising and interesting technology. In this thesis work the integration in a small scale hydrogen generator of an MR, to carry out the water gas shift reaction (WGS), has been studied. In particular, the effect of MR integration from a systems perspective, i.e. specifically assessing the impact of MR on the whole process, has been investigated. A preliminary design of a pilot scale MR to produced 5 Nm3/h of H2 by reformate stream upgrading has been performed. A CO conversion of 95% and an hydrogen recovery yield of 90% have been fixed as minimum performance target of the WGS-MR. Depending on the system considered to promote the driving force for the permeation, three scenarios have been proposed: base, vacuum and sweep scenario. On the basis of results from a preliminary scenario screening, the required membrane area (ca. 0.179 m2), for vacuum and sweep scenarios, has been estimated by means of an MR modelling and simulation. The results obtained from the pilot scale have been used for the scale-up of the WGS-MR integrated in the 100 Nm3/h hydrogen production unit. The plant for the integrated process (reformer and WGS-MR) has been simulated by using the commercial simulation tool Aspen Plus®. The MR integration, actually, implies a re-design of the process downstream the WGS reactor. Since more than 90% of the produced H2 is directly recovered in the permeate stream, the PSA unit can be removed, leading to a more compact system. For the retentate stream post processing, the possibility to recover the CO2, by means of membrane gas separation technology has been proposed. The results for a two stages membrane separation unit confirmed the technological feasibility of the CO2 capture, achieving the CO2 purity target. Pursuing the logic of process intensification, the comparison with the reference technology (reformer, high temperature shift, PSA) showed as the WGS-MR integrated system results in a more “intensified” process since a higher H2 productivity, a smaller plant and an enhanced exploitation of raw materials are obtained. In addition, since the MR delivers a high-pressure CO2-rich stream, it provides an opportunity for small-scale CO2 capture and thus possible emission reduction. The possibility to extend the spectrum of MR application in reactions of industrial interest, where hydrogen is produced as by-product, has been also studied. In particular, as case study, the direct conversion of n-butane to isobutene has been analysed showing as, from a thermodynamic point of view, better performance (equilibrium conversion up to seven times higher than the one of a traditional reactor) can be obtained.