Process design and optimization for recovery of biohydrogen

  • Prozessentwicklung und -optimierung der Biowasserstoffaufbereitung

Ohs, Burkhard Eike Ludger; Wessling, Matthias (Thesis advisor); Bathen, Dieter (Thesis advisor)

Aachen : RWTH Aachen University (2020, 2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020

Abstract

H2 is regarded as a promising energy carrier and serves as an important raw material for the chemical industry. Up to now, H2 is mainly produced by steam reforming of fossil resources. Besides their limited availability, tremendous CO2 emissions are associated therewith. This demands for alternative production routes. For example, H2 can be produced by fermentation processes using waste streams, which combines waste treatment with energy production. However, H2 streams from biogenic origin also contain other by-products, e.g. CO2, which must be removed before utilization. The separation of the gaseous by-products is a major cost driver for the H2 production, but possible separation processes have only been analyzed insufficiently. In comparison to the conventionalH2 production, the feed for the gas treatment is present at ambient conditions and thus needs particular attention. Therefore, different separation processes were compared under technical and economic aspects. Membrane-based processes are attractive for biohydrogen upgrading due to their high energy efficiency. They can reach high recoveries, but only a limited purity. In comparison, pressure swing adsorption (PSA) allow producing highly pure H2 but suffer from low recoveries for bulk separation. Luckily, hybrid membrane-PSA processes allow combining both their advantages. However, the development of hybrid processes is very challenging due to the infinite number of process variants. Thus, an optimization framework for hybrid membrane-PSA separation is presented, which can identify optimal process designs with relatively little effort. In addition to optimizing the overall process, it is also possible to increase the efficiency of adsorption processes using hollow fiber adsorbents. Compared to conventional adsorption beds, they are characterized by a reduced mass transfer, a low pressure loss and defined flow conditions. In this work, an optimization model for hollow fiber adsorbers not only identified the optimal process but also the design of the hollow fiber itself. Furthermore, the H2 stream is often diluted by inert components, e.g. by N2 for sparging of the fermentation. For diluted H2 streams, compression of the feed gas is usually uneconomical. Thus, in the second part of the thesis, a process consisting of an electrochemicalH2 separation and temperature swing adsorption for the separation of N2and CO2 is presented. The purified N2 can then be recycled for sparging. Aminefunctionalizedsorbents are particularly suitable for the separation of the co-producedCO2, which is present at low partial pressures. Yet, the adsorption kinetics for CO2 for these materials have not been adequately described. This makes a detailed process analysis difficult. Therefore, a kinetics model for CO2 adsorption was developed. Finally, this work shows the influence of the H2 fraction on the separation costs. The costs of the separation of diluted H2 streams are more than four times higher than for high-concentrated H2 streams. Thus, it is crucial that the biohydrogen production processes target designs with high H2 fractions. With this thesis at hand, overall process analysis of the biogenic H2 production is possible.

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