Hydrogen recovery and utilization from water splitting processes
- Wasserstoffrückgewinnung und -nutzung aus Wasserspaltungsprozessen
Alsayegh, Sari; Weßling, Matthias (Thesis advisor); Favre, Eric (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2018
Renewable energy sources must be adopted in order to satisfy the increase in global energy demand all the while minimizing the carbon footprint. One obvious energy source is solar. Aside from traditional electricity generation technologies, solar energy can be utilized to produce H2 (as an energy carrier) via photocatalytic water splitting. In a typical process, both H2 and O2 are produced in the same reactor environment, thereby creating a potentially hazardous scenario. This obstacle can be avoided by utilizing flammability suppressants to recover the product outside the flammability regime. The scope of this thesis is to identify membrane-based processes to recover and utilize H2 generated from photocatalytic water splitting while maintaining safety and flammability constraints throughout the separation process. Two flammability suppressants were investigated, namely: N2 and CO2. Detailed information about H2 flammability in these two diluents and the impact of the operating conditions were described in order to identify the parametric range that ensures a safe separation process. To be more genuine in designing the membrane-based process, an optimization study for the whole process economic was implemented using commercially available membrane materials. These membrane units were incorporated in different process layouts to achieve high purity and recovery values while applying flammability constraints in all pertinent streams. The results for both suppressants revealed the advantage of CO2 over N2 as a suppressant, where the H2 product was recovered at a higher purity with lower specific cost and O2 concentration. However, both diluent systems revealed imposing recovery costs due to the low H2 concentration in the feed. Further studies were conducted to show the impact of varying feed compositions, high performance polymeric membrane materials (not commercialized), and alternative membrane configurations (hollow fiber / spiral wound) on the process economics. As a conceptual culmination of the initial process design work, a renewable methanol production route was proposed to integrate technologies into a complete petrochemical facility. Through integration, the aim was to improve the overall process economics through the production of a more value-added product. This approach utilized H2 from photocatalytic water splitting and captured CO2 (e.g. flue gas). Contrary to the previous approach, the membrane-based separation process was optimized to produce a 3:1 H2 and CO2 mixture. This binary mixture was used as the feedstock for a conceptual direct CO2 hydrogenation methanol synthesis plant. Based on a detailed economic analysis, the break-even value of the methanol produced using this approach is higher than the current market of methanol. However, it is very comparable to other renewable methanol routes proposed in the literature. Sensitivity analysis was carried out on different economic and energy parameters to show their impact on both economic and energy efficiencies of the proposed process. The sensitivity analysis revealed the strong influence of CO2 market price on the process economics over other considered parameters. The design approach and the optimization models developed in this study are not limited to H2 recovery from photocatalytic water splitting. Other gas separation applications, that involve flammability constraints, can be easily implemented. Hence, these models provide a strong tool for similar future works.