A two-step methodology for the selection and process optimization of Liquid Organic Hydrogen Carriers (LOHCs)

Abstract

The reduction of GHG emissions is clearly bounded to an increase in the use of renewable resources. Hence, the use of energy carriers such as hydrogen seems crucial to reduce the fluctuations in the power generation. However, the physico-chemical properties of hydrogen hinder its transportation and storage. Alternatives such as LOHCs are regarded as a promising option to improve them. The selection among the available LOHCs systems and the process optimization are some of the key points to enhance the deployment of this storage technology. In this work, a systematic two-step design methodology is proposed. First, a prescreening stage is developed to select the best LOHCs candidates considering the economy, safety and environmental of hydrogenation and dehydrogenation processes. In a second step, the optimization of the most promising LOHCs systems from the first stage is carried out. N-ethylcarbazole and indoles mixture systems resulted to be the most promising options from the first stage of the methodology to be used for hydrogen storage. Hence, both processes are optimized. Surrogate models are developed to represent the hydrogenation and dehydrogenation reactors. Although trickle bed and slurry reactor designs were considered for both tasks, the optimization results suggest the use of slurry reactors over trickle beds due to better catalyst use. However, the trickle bed technology presents numerous advantages and it cannot be discarded yet for LOHCs applications. The hydrogen storage costs ranged between 1.30–2.34 $/kg H 2 for different plant capacities. The dehydrogenation stage, and specifically the dehydrogenation reactors and the heating agent used, resulted to have a major impact on it. Thus, this methodology allows to identify the most promising LOHCs options and to design its process, analyzing the challenges for its deployment. • A two-step methodology is developed for LOHCs selection and optimization. • N-ethylcarbazole and an indoles mixture were selected for hydrogen storage. • Around 75%–85% of the utility costs is attributed to dehydrogenation heating. • The dehydrogenation reactors have a great impact in the total equipment costs. • The costs of storing hydrogen with LOHCs technology range from 1.30–2.34 USD/kg H2 .

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