Abstract
The development, testing and validation of a two-fluid transient flow model for simulating outflow following the failure of high pressure CO2 pipelines is presented. Thermal and mechanical non-equilibrium effects during depressurisation are accounted for by utilising simple constitutive relations describing inter-phase mass, heat and momentum transfer in terms of relaxation to equilibrium. Pipe wall/fluid heat exchange on the other hand is modelled by coupling the fluid model with a finite difference transient heat conduction model. The two-fluid transient flow model's performance is tested by comparison of the predicted transient pressure and temperature profiles along the pipeline against those based on the simplified homogeneous equilibrium model (HEM) as well as real data captured during the full bore rupture of a 260m long, 233mm internal diameter pipeline containing CO2 at 36bara and 273°C. The two-fluid model is found to produce a reasonably good degree of agreement with the experimental data throughout the depressurisation process. The HEM based flow model on the other hand performs well only near the rupture plane and during the early stages of the depressurisation process.
Highlights
The internationally agreed objective of limiting the increase in global average temperatures to less than 2 ◦C above pre-industrial levels requires a 50–80% reduction in CO2 emissions by 2050 (Edenhofer et al, 2011)
= 5 ×10−5 s diverge visibly from the saturation line, while those using = 5 ×10−6 s as well as the homogeneous equilibrium model (HEM) model both remain on the saturation line throughout. This comparison shows that the HEM is incapable of producing reasonable predictions for this scenario, as the departure from thermodynamic equilibrium is too great for the flow to be approximated accurately
In this paper a non-equilibrium two-phase model describing fully compressible transient vapour-liquid flow was developed for simulating the depressurisation of high pressure CO2 pipelines
Summary
The internationally agreed objective of limiting the increase in global average temperatures to less than 2 ◦C above pre-industrial levels requires a 50–80% reduction in CO2 emissions by 2050 (Edenhofer et al, 2011). The accurate modelling of the decompression process during pipeline rupture requires accounting for a number of complex and interacting phenomena. In the case of CO2 the limited work reported on this topic (for interphase friction see Cheng et al, 2008) is restricted to steadystate two-phase CO2 flows in small diameter tubes (≤8 mm i.d.) It is uncertain whether these empirically driven correlations are applicable to transient releases from large diameter pipelines (Thome and Ribatski, 2005) of interest in CCS. In the case of pipeline punctures the switch between the two flow regimes was found to be far more rapid
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