(630e) Multiobjective Two-Stage Methodology for Process and Product Design: Optimization of Hydrogen Storage Systems Based on Lohcs

The reduction of Greenhouse Gas (GHG) emissions is regarded as mandatory to reach the climatic targets. To accomplish it, the introduction of renewable energies is required. However, its fluctuating nature hinders the deployment of an energy system with a high penetration of these resources. Therefore, the use of energy storage technologies is demanded to support the renewable generation (Kebede et al., 2022). Among them, hydrogen is proposed as a useful energy carrier, specifically in those difficult to electrify applications (industrial heating, heavy transport, long-term energy storage, ...) (Griffiths et al., 2021). However, the transport and storage of this molecule is difficulted mainly due to its low volumetric density, the highly demanding storage conditions or the very specific material compatibility (Moradi and Groth, 2019). Beyond the traditional storage options (compression and liquefaction), new emerging alternatives such as Liquid Organic Hydrogen Carriers (LOHCs) are proposed as an easy and safe storage and transport option (Reus et al., 2017).

To implement LOHCs technology, the selection of the most suitable organic liquids is paramount presenting a process and product selection design problem. Hence, in this work, a two-stage methodology is developed to select the LOHC system and optimize the hydrogen storage and its release. The first stage consists of a systematic screening that considers a multi objective metric covering: economic, safety and sustainability. The process is analyzed at pre conceptual stage to assess these areas. Then, in a process synthesis stage, the best candidates are considered for a more detailed process analysis where storage costs are optimized for different capacities. A mathematical optimization problem is formulated. The hydrogenation and dehydrogenation reactors are the central units of this technology. The combination of slurry and trickle bed reactors units is assessed for this process using surrogate models developed from a more detailed previous reactor design analysis. The final results are compared with previous works considering them as isolate units (Prieto et al., 2023).

The methodology identified N-ethylcarbazole and an indole system as the preferred options considering the three proposed indicators. Moreover, this holistic approach showed that some of the most desirable properties for LOHCs selection are complied. For instance, moderate reaction conditions, non-toxic, easy purification, among other characteristics (Niermann et al., 2019). The process optimization resulted in a hydrogen storage cost between 1.00-2.20 $/kg H₂, where dehydrogenation stage was the highest contributor. The steam used in this operation and the dehydrogenation reactors (30% of the total equipment costs) have a clear impact on it. Therefore, the optimization of this section has a great importance, although hydrogenation reactors along with secondary equipment also play a relevant role on the final costs. In spite of the importance of the reactors, this optimization-based process design showed some differences from reactor optimization alone. Furthermore, slurry reactors were preferred over trickle bed units due to a better catalyst use. This holistic evaluation is essential towards an effective implementation of LOHCs in a system with a high penetration of renewable resources.

References

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Prieto C., Sánchez A., Martín M. A three-phase reactor assessment for the deployment of Liquid Organic Hydrogen Carriers (LOHCs): dybenzyltoluene and indoles mixture systems as case studies. Energy Conversion and Management. 2023. 294. 117548.

Reus M., Grube T., Robinius M., Preuster P., Wasserscheid P., Stolten D. Seasonal storage and alternative carriers: A flexible hydrogen supply chain model. Applied Energy. 2017. 200. 290-302.

Moradi R., Groth K.M. Hydrogen storage and delivery: review of the state of the art technologies and risk and reliability analysis. International Jorunal of Hydrogen Energy. 2019. 44. 12254-12269.

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Acknowledgements

The authors acknowledge the support from the Regional Government of Castilla y León (Junta de Castilla y León) and by the Ministry of Science and Innovation MICIN and the European Union NextGenerationEU/PRTR (H2MetAmo project-C17.I01.P01.S21) and the FPU, Spain grant (FPU21 /02413) to C.P.