Konzeption von Membranmodulen zur effizienten Abtrennung von Kohlendioxid aus Gasgemischen
Jülich / Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag (2016, 2017) [Book, Dissertation / PhD Thesis]
Page(s): VI, 231 Seiten : Illustrationen, Diagramme
Membrane-based CO2-separation technology attached to large emission sources is consid-ered a possible bridging solution towards a carbon dioxide-neutral energy system. To make this technology competitive with the standard separation process for post-combustion cap-ture – chemical absorption – necessitates the development of a membrane module that uses the installed membrane area effectively, in addition to research on high-performance mem-brane materials and energy-efficient separation processes. Therefore, based on a concept developed by the Helmholtz Center Geesthacht, in this work a novel module for flat sheet membranes operated in countercurrent is fluid-dynamically and procedurally analyzed and designed to enable CO2-separation from power plant flue gas in two steps. In the first step, a membrane module with a membrane area of 5.66 m² was investigated re-garding dead zones, bypass currents and inhomogeneous flow distributions using computa-tional fluid dynamic (CFD) simulations. Therefore, an overall methodology was developed that enabled simulation of the entire membrane module within a reasonable time. The spa-cers between two membrane envelopes of a membrane stack are approximated by a validated substitutional model by means of a porous body with equal pressure drop characteris-tics. Permeation through the membrane was enabled by a user-defined source-and-sink function. The degree of separation calculated in the CFD simulations is compared with that for ideal incident flow and complete utilization of the installed membrane area. It was found that 27 % of the incoming feed gas bypasses the active membrane area through the tapering at the edge of the membrane envelopes. By sealing this bypass, the degree of separation could be increased by 12.8 %-points to 81.3 %. Although a variation in the inlet geometry of the membrane module by installing baffle plates resulted in a uniform flow distribution at the very beginning of the membrane stack, the degree of separation for the chosen geometry could not be improved, since the backpressure of the spacers installed between the feed channels already causes homogenization of the flow distribution. The second step in the design was to find a module geometry that is optimized with regard to the CO2 avoidance costs via process simulations. Therefore, a two-stage separation process was taken as a basis and the module geometry of both separation stages was simultaneous-ly optimized by means of a systematic variation of single dimensions. For this, a quasi-one-dimensional substitutional model for the simulation of the membrane module was developed that takes into account fluid mechanical effects. To provide a standard of comparison, a re-ference case was defined and simulated. This has a membrane area of 3.1 million m² and an efficiency loss of 6.6 %-points. The CO2 avoidance costs amount to 68.0 €·tCO2-1 and this could be reduced to 57.2 €·tCO2-1 with the choice of optimized channel heights in the feed and permeate of 1 mm and 2 mm, respectively. At the same time, the inlet velocity into the mem-brane stack was optimized to 0.5 m·s-1 in the first separation stage and 1.1 m·s-1 in the sec-ond. Furthermore, it was shown that in the second separation stage the division of the mem-brane module into a first part that is operated in a co-current and a subsequent part operated in a counter-current is useful because a sweep gas effect is exploited.
Wirsum, Manfred Christian