Modelling of LNG Pool Spreading on Land with Included Vapour-Liquid Equilibrium and Different Boiling Regimes

Abstract

This paper presents a source term model for estimating the rate of unconfined LNG pool spreading on land. The model takes into account the composition changes of a boiling mixture, the variation of thermodynamic properties due to preferential boiling in the liquid mixture and the effect of boiling regime on conductive heat transfer. The heat, mass and momentum balance equations were solved for continuous and instantaneous spills. A sensitivity analysis was conducted to determine the relative effect of each of these phenomena on pool spreading. The model was compared to a commonly used gravityinertia integral pool-spreading model with one-dimensional conductive heat transfer. 1. Introduction Although some pool spreading models have been developed as outlined by Webber et al. (2010), most of them are integral models and based on work carried out in the early 1970’s on non-volatile liquid spills. These early models tend to overlook the complexity associated with cryogenic pool spreading. Cryogens (e.g. LNG: boiling point = -162 °C) exhibit vigorous boiling upon being released on land or water at ambient temperature. In the case of an LNG release, some of the LNG will flash while rest will spread very quickly. The conditions of heat transfer from the surroundings and the composition of the LNG mixture may affect the spread rate and the vaporization rate. Most of the current modelling work on LNG spillage estimates the LNG properties either using fixed thermo-physical properties of a mixture or using those of pure methane as an analogue. This is questionable since the vaporization behaviour of a mixture is different from that of a pure fluid due to preferential boil-off of the lighter hydrocarbons, which in turn results to time varying properties of the spilled mixture. The complex phenomena governing the spread and vaporization of a LNG pool have to be accounted for and their relative importance must be understood to develop a comprehensive model that is both adaptable and applicable to a variety of scenarios. In this paper, a pool-spreading model that takes into account the composition changes of a boiling LNG mixture, the effect of different boiling modes on conductive heat transfer as well as the varying mixture thermodynamics and vapour liquid equilibrium effects is proposed. A sensitivity analysis is conducted to identify the relative importance of each of the governing parameters on the pool spreading process. The model incorporates the multicomponent nature of LNG and its effect on the behaviour of the spill with the aim of developing the most accurate and comprehensive representation of the pool spreading process.