Sundara, V; (2015) Transient flow dynamics in high pressure carbon dioxide pipelines. Doctoral thesis , UCL (University College London).

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Abstract

The purpose of this thesis is to model, investigate and where possible validate the impact of Emergency Shutdown Valve (ESDV) closure on mitigating the fugitive releases from failed CO2 pipelines employed as part of the Carbon Capture and Storage (CCS) chain. Additional mathematical modelling work is also presented for simulating steady-state fluid flow and mixing in CO2 pipeline networks containing the various types of impurities representative of the different capture technologies, including pre-combustion, post-combustion and oxyfuel. The pipeline rupture transient flow model, based on the numerical solution of the conservation equations using the Method of Characteristics, incorporates Wu’s Modified Peng-Robinson equation of state to deal with pipelines containing pressurised CO2. It utilises the homogeneous equilibrium flow (HEM) assumption, where the constituent phases in a two-phase mixture are assumed to be in thermal and mechanical equilibrium. The first part of this study focuses on the development and experimental validation of the CFD model for simulating the dynamic response of inline ESDV’s in limiting outflow following the rupture of pressurised pipelines. The model accounts for the pertinent valve characteristics including the activation and closure times as well as its proximity to the rupture location. The validation of the model involves comparison of its predictions against measurements taken following the controlled Full Bore Rupture (FBR) of a 113 m long, 0.15 m i.d. pipeline containing CO2 at 151 bara and 27 oC incorporating a ball valve along its length. The data recorded and simulated include the transient fluid temperatures and pressures immediately upstream and downstream of the closing valve following FBR. Excellent agreement between the two sets of data is obtained throughout the depressurisation process. The above is followed by the linking of the publically available SLAB dispersion model for heavy gas clouds to the validated outflow model. The combined model is then tested against existing experimental data from the CO2Pipetrans research project involving the blowdown of a 30 m long, 0.6 m i.d. of a CO2 pipeline from initial temperatures and pressures ranging 278 to 284 K and 104 to 156 bara respectively. The combined outflow and dispersion model is next used to determine the optimal spacing of ESDVs for CO2 pipelines. This is done by solving an optimisation problem involving trading off the 7 % (vol./vol.) CO2 concentration contour area (concentrations above this are considered fatal) against the cost for valve installation. Level diagrams are then used to determine the optimal separation distance for ESDVs. Finally, the problem of steady-state flow in pipeline networks is analysed. A flow model is developed to determine the required inlet pressure at CO2 source locations to obtain a specific delivery pressure for given source CO2 mixture compositions and flowrates. The model is then used in a realistic case study with two inlet sources and one delivery location. The required inlet pressures at the source locations are determined for given initial feed flowrates and compositions, to attain a desired delivery pressure. In addition, the downstream fluid temperature and fluid compositions are also determined.

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