Modelling the evaporation of cryogenic liquids in storage tanks

Abstract

Cryogenic liquids are substances with a normal boiling point below -150°C. Recently, the interest in cryogenic liquids has skyrocketed because of their role in the energy transition, particularly for LNG and liquid hydrogen. Cryogenic liquids are stored in highly insulated tanks, which are nevertheless subject to heat ingress from the surroundings. The heat ingress drives thermal stratification, natural convection, pressure build-up and evaporation. Managing the evaporated cryogen, denominated boil-off gas (BOG), pose techno-economic, safety and environmental challenges. To facilitate the design and operation of cryogenic storage tanks, new models for cryogenic liquids evaporation have been developed. For isobaric storage, a 1-D model has been developed. The model includes wall heating, heat conduction and advection in the vapour phase. The model shows that advection dominates vapour heat transfer. A 2-D CFD model has been developed to validate the assumptions of the 1-D model. The CFD model validates the 1-D model assumption of one-dimensional advective flow. Additionally, the CFD model shows that thermal stratification dampens natural convection in the vapour. Analytical solutions of the 1-D model valid for the pseudo-steady state have been developed. The analytical solutions constitute an easy-to-use tool for practitioners to improve BOG management. For non-isobaric storage, a 1-D model that considers wall heating, heat conduction and wall boiling has been developed. The 1-D model demonstrates that wall boiling is relevant even for low heat fluxes. The 1-D model predictions were in good agreement with experimental pressure and vapour temperature profiles. The assumptions of the 1-D model have been validated by developing a new single-phase CFD model. A multiphase model has been developed to investigate interfacial transport phenomena. It shows that interfacial momentum transfer slightly enhances liquid heat transfer, and that vapour heating dominates pressure build-up at the beginning of the storage period.