International Journal of Greenhouse Gas Control 17 (2013) 349–356 Contents lists available at ScienceDirect International Journal of Greenhouse Gas Control j ourna l h o mepage: www.elsevier.com/locate/ijggc A homogeneous relaxation flow model for the full bore rupture of dense phase CO 2 pipelines S. Brown a,∗ , S. Martynov a , H. Mahgerefteh a , C. Proust b a Department of Chemical Engineering, University College London, London WC1E 7JE, United Kingdom b INERIS, Dept. PHDS, Parc Technologique ALATA, BP 2, 60550 Verneuil-en-Hallate, France a r t i c l e i n f o Article history: Received 8 February 2013 Received in revised form 24 April 2013 Accepted 16 May 2013 Available online 20 June 2013 Keywords: Multi-phase flow Carbon capture and storage CO2 pipeline safety a b s t r a c t The development of an homogeneous relaxation flow model for simulating the discharge behaviour fol- lowing the full bore rupture of dense phase CO 2 pipelines is presented. Delayed liquid–vapour transition during the decompression process is accounted for using an empirically derived equation for the relax- ation time to thermodynamic equilibrium. The flow model’s robustness is successfully demonstrated based on a series of hypothetical shock tube tests. Model validation on the other hand is performed by comparison of the predictions against experimental data obtained for the full bore rupture of realistic scale CO 2 pipelines. Within the ranges investigated, it is found that although delayed phase transition effects have negligible impact on the pipeline decompression rate, ignoring such phenomena results in underestimating the transient discharge rate. This is important since the latter governs the minimum safety distances to CO 2 pipelines and emergency response planning in the unlikely event of pipeline failure. © 2013 Elsevier Ltd. All rights reserved. 1. Introduction As part of the carbon capture and sequestration (CCS) chain, pressurised pipelines are considered to be the most practical and efficient means for the transportation of the large amounts of CO 2 captured from fossil fuel power plants for subsequent sequestra- tion (IPCC, 2005). It is widely expected that such pipelines will cover distances of several hundreds of kilometres, some passing near populated areas, at times at line pressures above 100 bar. Given that CO 2 is increasingly toxic at concentrations higher than 7% (Kruse and Tekiela, 1996), the safety of CO 2 pipelines is of great importance and indeed pivotal to the public acceptability of CCS as a viable means for tackling the impact of global warming (IPCC, 2005). As such, as part of their safety assessment, much the same as for hydrocarbon transportation pipelines, all the major hazards asso- ciated with the accidental CO 2 pipeline failure must be quantified and, where necessary, appropriate mitigating steps undertaken to bring their consequences to acceptable levels. Central to this is the determination of the highly transient outflow parameters includ- ing the discharge rate, pressure, temperature and fluid phase at the rupture plane in the event of pipeline failure. Such data serves as the input for predicting the subsequent atmospheric dispersion ∗ Corresponding author. Tel.: +44 2076793809. E-mail address: solomon.brown@ucl.ac.uk (S. Brown). of the escaping CO 2 , and hence the basis for determining the mini- mum safe distances to populated areas. Depending on the initiating mechanism, pipeline failure may be in the form of the more likely puncture, or the less frequent but far more catastrophic, full bore rupture (FBR). The modelling of outflow following pipeline failure is especially challenging given large number of complex and often interac- ting process governing the discharge process. The rupture of the pipeline results in a series of expansion waves that propagate into the undisturbed fluid towards the intact end of the pipeline. These waves result in the acceleration of the fluid particles in the oppo- site direction and hence outflow. The precise tracking of these expansion waves and their propagation as a function of time and distance along the pipeline is necessary for the prediction of out- flow, as well as propagating fractures (Mahgerefteh et al., 2012). The above involves detailed consideration of several processes including heat and mass transfer, unsteady fluid flow and thermo- dynamics. In addition, for most fluids, such as dense phase CO 2 being cur- rently considered for pipeline transportation, the near isentropic expansion following rupture will likely result in two-phase flow. As such the flow model must also incorporate an accurate equation of state. Due consideration must also be given to the effects of friction and heat transfer which are both flow and phase dependent. Given that the constituent conservation equations governing the flow can only be solved numerically (see for example Mahgerefteh et al., 1999), the choice of a robust, stable and computationally efficient solution technique is essential. 1750-5836/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijggc.2013.05.020