A STUDY OF SOLID CO2 FORMATION FROM RAPID FLUID EXPANSION USING CFD AND MATHEMATICAL MODELLING

Rapid carbon dioxide expansion from an accidental pipeline leakage is an adiabatic
process that forms solid CO2 micro-particles entrained in CO2 vapor. While the vapor
is subsequently dispersed as vapor cloud, the micro-particles – at sizes larger than 100
μm – can rain out to form a solid pool. The pool will then sublimate to the atmosphere
and contribute significantly to the concentration of vapor cloud. Ultimately, the effect
of solid rainout pool on vapor cloud concentration and dispersion has to be taken into
consideration when calculating safety distance. In order to investigate the sizes of solid
micro-particles formed under varying discharge scenarios, the process of rapid fluid
expansion through an orifice (leakage) is emulated using a simulation model. It
involves an integration of two sub-models: (1) a 3-D Computational Fluid Dynamics
(CFD) model using FLUENT 14.0, and (2) a mathematical model published by authors
Hulsbosch-Dam, Spruijt, Necci & Cozzani (2012). The CFD model employs the
FLUENT software to obtain temperature and velocity profiles of rapid fluid expansion.
The mathematical model calculates the droplet size distribution from the point of
release and size of final solid particles formed. The combination of the two models
generates results and parametric trends (mainly the effect of leakage size on the size
of particles formed). They are then compared with experimental data available in
literatures, and validation is achieved. Finally, the model is used to simulate rapid
carbon dioxide expansion from pipeline leakage at supercritical storage conditions.
Conclusive evidence shows that at supercritical storage conditions (specifically at 310
K and 150 bar), a pipeline leakage will not produce solid CO2 micro-particles big
enough to form a solid rainout pool.