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Multistage compression and transient flow in CO₂ pipelines with line packing

Daud, NKB; (2018) Multistage compression and transient flow in CO₂ pipelines with line packing. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

The main purpose of this thesis is to develop rigorous analytical and CFD models followed by their applications to real case studies in order to: i) identify the optimum multistage compression strategies for minimising the compression and intercooler power requirements for real CO2 feed streams containing various types and amounts of impurities associated with the various types of CO2 capture technologies; and ii) investigate the buffering efficacy of realistic CO2 transmission pipelines as a line packing strategy for smoothing out temporal fluctuations in feed loading and maintaining the desired dense-phase flow for both pure CO2 and its various realistic mixtures representative of the most common types of capture technologies. An analytical model based on thermodynamics principles is developed employing Plato Silverfrost FTN95 software and applied to determine the power requirements for various compression strategies and inter-stage cooling duties for typical pre-combustion (98.07 % v/v of CO2) and oxy-fuel CO2 mixtures of 85 and 96.7 % v/v CO2 purity compressed from a gaseous state at 15 bar and 38 oC to the dense-phase fluid at 151 bar. Compression options examined include conventional multistage integrally geared centrifugal compressors, advanced supersonic shockwave compressors and multistage compression combined with subcritical and supercritical liquefaction and pumping. In each case, the compression power requirement is calculated numerically using a 15-point Gauss-Kronrod quadrature rule in QUADPACK library, and employing the Peng-Robinson Equation of State (PR EOS) implemented in REFPROP v.9.1 to predict the pertinent thermodynamic properties of the CO2 and its mixtures. In the case of determining the power demand for inter-stage cooling and liquefaction, a thermodynamic model based on Carnot refrigeration cycle is applied. The study shows that a decrease in the impurity content from 15 to 1.9 % v/v in the CO2 streams reduces the total compression power requirement by ca. 1.5 % to as much as 30 %, while for all cases, inter-stage cooling duty is predicted to be significantly higher than the compression power demand. It is found that multistage compression combined with subcritical liquefaction using utility streams and subsequent pumping can offer a higher efficiency than conventional integrally geared centrifugal compression for high purity (> 96.7 % v/v) CO2 streams. In the case of a raw/dehumidified oxy-fuel mixture, that carries a relatively large amount of impurities (85 % v/v CO2), subcritical liquefaction at 62.53 bar is shown to increase the cooling duty by as much 50 % as compared to that for pure CO2. The second part of this study focuses on the development and testing of a numerical CFD model employing Plato Silverfrost FTN95 software for simulating the transient fluid flow behaviour in CO2 pipelines with line packing. The model is based on the numerical solution of the conservation equations using the Method of Characteristics, incorporating PR EOS to deal with CO2 and its various mixtures. Following its verification, the numerical model is employed to conduct a systematic study on the impact of operational flexibility involving a temporal reduction in the upstream CO2 feed flow rate on the transient flow behaviour in the pipe over a period of 8 hours. A particular focus of attention is determining the optimum pipeline design and operating line packing conditions required in order to maximise the delay in the transition from dense phase flow to the highly undesirable two-phase flow following the ramping down of the CO2 feed flow rate. The investigations were conducted for both pure CO2 and its various realistic mixtures. For the case studies examined, the results show that the efficacy of line packing can be increased by increasing the pipeline length from 50 to 150 km for the same pipe inner diameter of 437 mm. However, as the pipelines length increased to 150 km, the increase in the pipe inner diameter beyond 486 mm was found to have no further impact on the line drafting time. While, in the case of inlet feed temperature, the line drafting time increases following an increase in the inlet feed temperature of transported fluid from 283.15 K up to 303.15 K. Beyond the operating inlet feed temperature of 311.15 K, the line drafting time only marginally increased. It is also shown that the presence of impurities reduces the transition time to two-phase flow following the ramping down of the feed flow rate.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Multistage compression and transient flow in CO₂ pipelines with line packing
Event: UCL (University College London
Open access status: An open access version is available from UCL Discovery
Language: English
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Chemical Engineering
URI: https://discovery.ucl.ac.uk/id/eprint/10044828
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