Decompression wave speed in CO2 mixtures: CFD modelling with the GERG-2008 equation of state

Abstract

The development of CO2 pipelines for Carbon Capture and Storage (CCS) raises new questions regarding the control of ductile fracture propagation and fracture arrest toughness criteria. The decompression behaviour in the fluid must be determined accurately in order to estimate the proper pipe toughness. However, anthropogenic CO2 may contain impurities that can modify the fluid decompression characteristics quite significantly. To determine the decompression wave speed in CO2 mixtures, the thermodynamic properties of these mixtures must be determined by using an accurate equation of state. In this paper we present a new decompression model developed using the Computational Fluid Dynamics (CFD) package ANSYS Fluent. The GERG-2008 Equation of State (EOS) was implemented into this model through User Defined Functions (UDF) to predict the thermodynamic properties of CO2 mixtures. The model predictions were in good agreement with the experimental data of two 'shock tube' tests. A range of representative CO2 mixtures was examined in terms of the changes in fluid properties from the initial conditions, with time and distance, immediately after a sudden pipeline opening at one end. Phase changes that may occur within the fluid due to condensation of 'impurities' in the fluid were also investigated. Simulations were also conducted to examine how the initial temperature and impurities would affect the decompression wave speed.

Multi‐phase decompression modeling of CO2 pipelines

“…It is a constitutive equation which provides a mathematical relationship between three or more relevant 'state variables' , such as its temperature, pressure, volume, or internal energy. As the GERG-2008 EOS is recommended as the reference EOS for gas pipelines, 8 it is employed in this work. To simulate the real behavior of liquid and vapor flow, the thermodynamic properties must be predicted using an accurate EOS.…”

“…The CCS technique involves transporting CO 2 from the points of capture to the storage sites where it will not enter the atmosphere. Fracture propagation in fluid pipelines is commonly treated using the semi-empirical Battelle Two-Curve Model (BTCM) 8,9 where the aim is to estimate the resistance of the material to rapid crack propagation. Pipelines are preferred when transporting large quantities of CO 2 over long distances.…”

“…5 When using pipelines to transport CO 2 , safety issues must be considered. 8,9 Clearly, for two-phase decompression, the curve of decompression wave speed vs pressure contains a plateau caused by phase change. To facilitate this, and to reduce costly experimental procedures, reliable and accurate numerical models have to be developed.…”

“…8 Following initiation of pipeline rupture, the pressure at the exit declines to the 'choke' pressure. 8 Following initiation of pipeline rupture, the pressure at the exit declines to the 'choke' pressure.…”

“…One aspect of CO 2 pipeline design is to avoid running-ductile fracture, which necessitates accurate models predicting the depressurization behavior when the fracture happens. Fracture propagation in fluid pipelines is commonly treated using the semi-empirical Battelle Two-Curve Model (BTCM) 8,9 where the aim is to estimate the resistance of the material to rapid crack propagation. This method involves the superposition of two independently determined curves: the fluid decompression wave speed and the fracture propagation speed (the 'J curve'), each expressed as a function of pressure.…”