Dehydrogenation units used to extract hydrogen from liquid organic hydrogen carriers (LOHCs) on ships are critical components for enabling efficient maritime hydrogen transport. These systems must balance compactness, safety, and energy efficiency while operating under the constraints of a marine environment. The process involves releasing hydrogen from the carrier molecules through catalytic reactions, often leveraging waste heat recovery to improve overall efficiency.

LOHCs such as dibenzyltoluene (DBT), toluene, or methylcyclohexane (MCH) are commonly used due to their stability, high hydrogen storage density, and compatibility with existing fuel infrastructure. The dehydrogenation process requires specific catalysts and reaction conditions to ensure high conversion rates and minimal degradation of the carrier material.

Catalysts for LOHC dehydrogenation typically include platinum (Pt), palladium (Pd), or ruthenium (Ru) supported on materials like alumina (Al₂O₃) or activated carbon. These metals facilitate the breaking of C-H bonds in the carrier molecule, releasing hydrogen gas. For example, Pt/Al₂O₃ is widely used for dehydrogenating perhydro-dibenzyltoluene (H18-DBT) due to its high activity and selectivity. The reaction typically occurs at temperatures between 250°C and 350°C, with pressures maintained near atmospheric to slightly elevated (1–5 bar) to optimize hydrogen release while minimizing side reactions.

Reaction conditions on ships are carefully controlled to account for vessel motion, limited space, and safety regulations. The dehydrogenation unit must be robust against vibrations and tilting, requiring reinforced reactor designs and stable catalyst beds. Heat management is critical, as the reaction is endothermic, requiring continuous energy input. Ships often integrate waste heat recovery systems, capturing excess heat from engine exhaust or other onboard processes to preheat the LOHC feed or sustain the reaction temperature. This improves energy efficiency and reduces reliance on additional fuel combustion.

Land-based dehydrogenation systems share similarities but benefit from more stable operating conditions and larger-scale infrastructure. Industrial units often operate at higher capacities, allowing for better heat integration and lower specific energy consumption. Fixed facilities can employ multi-stage reactors or advanced heat exchangers to maximize efficiency, whereas shipboard systems prioritize compactness and modularity. Land-based systems may also use alternative catalysts, such as nickel (Ni) or iron (Fe) with promoters, to reduce costs, though these often require higher temperatures or exhibit lower activity compared to noble metals.

Waste heat recovery in marine applications is more challenging due to space constraints and variable heat sources. Ships may use compact heat exchangers or thermal oil loops to transfer heat efficiently. In contrast, land-based plants often employ steam generation or direct heat recuperation into other process steps, achieving higher overall thermal efficiency.

A key difference between marine and land-based systems is the handling of byproducts. Dehydrogenation produces spent LOHCs, which must be stored until rehydrogenation. On ships, storage space is limited, necessitating efficient logistics for offloading and reloading carriers at ports. Land-based facilities can integrate rehydrogenation units on-site, creating closed-loop systems that minimize transport needs.

Safety considerations are also more stringent for marine applications. Dehydrogenation units on ships must comply with international maritime safety standards, including explosion-proof enclosures, advanced leak detection, and redundant shutdown systems. The confined environment increases risks, requiring robust ventilation and hydrogen dispersion mechanisms. Land-based systems, while still safety-critical, can rely on larger separation distances and fixed gas monitoring networks.

Maintenance and catalyst replacement present additional challenges at sea. Shipboard systems must minimize downtime, often using modular reactor designs that allow for quick catalyst changes during port calls. Land-based facilities can schedule maintenance more flexibly and may employ continuous regeneration systems to extend catalyst life.

In summary, dehydrogenation units for LOHCs on ships are specialized systems optimized for compactness, safety, and integration with marine heat sources. They rely on noble metal catalysts and carefully controlled reaction conditions to ensure efficient hydrogen release. While sharing fundamental principles with land-based systems, marine units face unique constraints that influence their design and operation. Waste heat recovery is critical in both settings but is implemented differently due to scale and environmental factors. The development of these technologies is essential for enabling global hydrogen trade via maritime transport.

The future of ship-based LOHC dehydrogenation will likely focus on improving catalyst durability, reducing energy requirements, and enhancing integration with vessel power systems. Advances in materials science and thermal engineering could further bridge the efficiency gap between marine and land-based systems, supporting the growth of a sustainable hydrogen economy.