Partition Ratio Evaluation for Safety Risk Management of Liquid Ammonia Spills to Seawater

Refrigerated liquid ammonia at atmospheric pressure is considered the safest transport and storage mode. In recent years, ammonia has gained attention as an alternative fuel to abate carbon emissions from the shipping fleet. Its implementation and use on the waterways imply some careful adjustments of technical systems and handling procedures on board to ensure, first and foremost, the safety of the public and the crew in case of accidental spill. Ammonia is toxic for humans, but it is also a threat to the marine environment, potentially killing fish by direct contact and by stimulating algae production leading to localized oxygen scarcity. On-board systems and procedures should therefore be thoughtfully engineered to minimize the risks to humans and the environment due to accidental ammonia releases. A complete and detailed understanding of the physical behavior of ammonia releases is therefore of paramount importance to ensure adequate design. This work presents a case study considering an emergency situation on a ship vessel caused by a large release of liquid ammonia to the sea. Accurate thermodynamics of the ammonia-water system are used to calculate the ratio of dissolution and evaporation (partition ratio) following a spill on water. The partition ratio calculation is based on the best available models in literature. The work discusses the sources of uncertainties in the modelling of accurate thermophysical behaviors of ammonia-water interactions. Finally, we discuss how accurate thermodynamic models might serve the design of safety systems and procedures to safeguard both crews and marine life and prevent unnecessary over-conservatism.

Keywords: Ammonia, Alternative fuels, Risk management, Maritime safety, Source term modelling.