The behaviour of polytetrafluoroethylene at high pressure

Imperial Users onl

CO2 pipelines material and safety considerations

This paper presents an overview of some of the most important factors and areas of uncertainty affecting integrity and accurate hazard assessment of CO2 pipelines employed as part of the Carbon Capture and Sequestration (CCS) chain. These include corrosion, hydrate formation, hydrogen embrittlement and propensity to fast running ductile and brittle factures. Special consideration is given to the impact of impurities within the CO2 feed from the various capture technologies on these possible hazards. Knowledge gaps in the modelling of outflow and subsequent dispersion of CO2 following the accidental rupture of pressurised CO2 pipelines, central to their safety assessment, are also presented

Modelling brittle fracture propagation in gas and dense-phase CO2 transportation pipelines

The development and application of a fluid–structure interaction model for simulating the transition of a through-wall defect in pressurised dense (150 bar, 283.15 K) and gas phase (34 bar, 283.15 K) CO2 pipelines into a running brittle fracture is presented. Given the economic incentives, the fracture model is employed to test the suitability of the existing stock of natural gas pipelines with the relatively high ductile to brittle transition temperatures of 0 and −10 °C for transporting CO2 in the terms of their resistance to brittle fracture propagation. The hypothetical but nevertheless realistic scenarios simulated involve both buried and above ground 10 km long, 0.6 m i.d. pipelines. Based on the assumption of no blowout of the surrounding soil upon the formation of the initial leak, the results show that the transition of the leak into a running brittle fracture in buried CO2 pipelines is far more likely as compared to above ground pipelines. In addition, gas phase pipelines are more prone to undergoing a propagating brittle fracture as compared to dense phase pipelines despite the lower operating pressures of the former. Furthermore, counter-intuitively, isolation of the feed flow following the discovery of a leak is shown to facilitate brittle fracture failure. The initial defect geometry on the other hand is shown to have a profound impact on the pipeline's resistance to propagating brittle fractures

Hybrid fluid–structure interaction modelling of dynamic brittle fracture in steel pipelines transporting CO2 streams

Pressurised steel pipelines are considered for long-distance transportation of dense-phase CO2 captured from fossil fuel power plants for its subsequent sequestration in a Carbon Capture and Storage (CCS) chain. The present study develops a hybrid fluid–structure methodology to model the dynamic brittle fracture of buried pressurised CO2 pipeline. The proposed model couples the fluid dynamics and the fracture mechanics of the deforming pipeline exposed to internal and back-fill pressures. To simulate the state of the flow in the rupturing pipeline a compressible one-dimensional Computational Fluid Dynamics (CFD) model is applied, where the fluid properties are evaluated using rigorous thermodynamic model. In terms of the fracture model, an eXtended Finite Element Method (XFEM)-based cohesive segment technique is used to model the dynamic brittle fracture behaviour of pipeline steel. Using the proposed model, a study is performed to evaluate the rate of brittle fracture propagation in a real-scale 48 in. diameter API X70 steel pipeline. The model was verified by comparing the obtained numerical results against available semi-empirical approaches from the literature. The simulated results are found to be in good correlation with the simulations using a simple semi-empirical model accounting for the fracture toughness, indicating the capability of the proposed approach to predict running brittle fracture in a CO2 pipeline

Novel process design and techno-economic simulation of methanol synthesis from blast furnace gas in an integrated steelworks CCUS system

A novel process design and techno-economic performance assessment for methanol synthesis from Blast Furnace Gas (BFG) is presented. Methanol synthesis using BFG as a feedstock, based on direct CO2 hydrogenation at commercial scale was simulated using Aspen Plus software to evaluate its technical performance and economic viability. The applied process steps involve first conditioning BFG using adsorption based desulfurisation, water-gas shift, dehydration, then separation of components into N2, CO2 and H2 rich streams using pressure swing adsorption. The H2 stream and a fraction of the CO2 stream are fed to a methanol synthesis system, while the remaining CO2 may be considered for geological storage in a Carbon Capture, Utilization and Storage (CCUS) case, or not in a Carbon Capture Utilization (CCU) case. Techno-economic analysis confirms methanol production from BFG is economically attractive under certain conditions, with Levelized Cost of Methanol production (LCOMeOH) calculated to be 344.61 £/tonne-methanol, and costs of CO2 avoided of - 20.08 £/tonne-CO2 for the CCU process and 9.01 £/tonne-CO2 for the CCUS process when using a set of baseline engineering assumptions. Sensitivity analysis of the process simulation explores opportunities for optimising the methanol synthesis system in terms of the impact of reactor size and/or recycle ratio on LCOMeOH. Economic viability of the CCU(S) processes is also found to be highly dependent on the cost of the feedstock BFG. Future cost savings as compared to business-as-usual steel production by 2030 in consideration of expected increases in the carbon price are estimated to be 10.59 £/tonne-steel for CCU and 24.61 £/tonne-steel for CCUS

An Aspen Plus Kinetic Model for the Gasification of Biomass in a Downdraft Gasifier

Gasification is a useful technology to recover energy from renewable biomass by producing a versatile syngas which can be converted into useful chemicals or fuels, or used directly for energy generation. The quality and composition of the syngas is highly dependent on the biomass feedstock, design parameters and process conditions, such as temperature, gasifying agent and Equivalence Ratio (ER). Downdraft gasifiers are considered to be a good option for low tar syngas production. In this work, a kinetic model for a downdraft gasifier is assembled and incorporated into a flowsheet using Aspen Plus with the aim of performing detailed process analysis. The model is organised according to the assumption that in a downdraft gasifier pyrolysis, oxidation and reduction occur almost as separate consecutive processes, with the pyrolysis considered as an instantaneously occurring process while oxidation and reduction are governed by chemical kinetics. The model has been validated against experimental data for different conditions of ER ranging from 0.2 to 0.35. The results show an overall agreement of the main species, with slight discrepancies in the prediction of CH4, which is over-predicted at lower ERs and under predicted at ER 0.345. This has an effect on the calculated Lower Heating Value (LHV) of the syngas which is generally higher than the experimental value. A set of sensitivity analyses were performed to investigate the impact of the value of the Char Reactivity Factor (CRF) on the composition of the producer gas and the kinetic parameters used in the model on the production of CH4. Sensitivity analyses show that a CRF of 14 gives the best prediction of the syngas composition and that the kinetics of the reactions in the reduction zone do not have a large impact on the final levels of methane in the syngas. More important is the sensitivity to variation of the kinetic parameters in the oxidation stage. By doubling the rate of oxidation of CH4 in the oxidation zone, the final levels of CH4 in the syngas are reduced by almost 20%

Hybrid fluid–structure interaction modelling of dynamic brittle fracture in steel pipelines transporting CO2 streams

Pressurised steel pipelines are considered for long-distance transportation of dense-phase CO2 captured from fossil fuel power plants for its subsequent sequestration in a Carbon Capture and Storage (CCS) chain. The present study develops a hybrid fluid–structure methodology to model the dynamic brittle fracture of buried pressurised CO2 pipeline. The proposed model couples the fluid dynamics and the fracture mechanics of the deforming pipeline exposed to internal and back-fill pressures. To simulate the state of the flow in the rupturing pipeline a compressible one-dimensional Computational Fluid Dynamics (CFD) model is applied, where the fluid properties are evaluated using rigorous thermodynamic model. In terms of the fracture model, an eXtended Finite Element Method (XFEM)-based cohesive segment technique is used to model the dynamic brittle fracture behaviour of pipeline steel. Using the proposed model, a study is performed to evaluate the rate of brittle fracture propagation in a real-scale 48 in. diameter API X70 steel pipeline. The model was verified by comparing the obtained numerical results against available semi-empirical approaches from the literature. The simulated results are found to be in good correlation with the simulations using a simple semi-empirical model accounting for the fracture toughness, indicating the capability of the proposed approach to predict running brittle fracture in a CO2 pipeline

Modelling Dry Ice Formation Following Rapid Decompression of CO₂ Pipelines

A fundamentally important issue regarding the safety assessment of CO2 pipelines is the possibility of solid or ‘dry ice’ discharge during an accidental release. This is particularly relevant given the near-adaibatic decompression process and the unusually high Joule Thomson coefficient of expansion of CO2. Solids discharge will affect many aspects of the ensuing hazard spanning the erosion of surrounding equipment, modification of the toxic dose duration, atmospheric dispersion and possibly, the pipeline’s propensity to fracture propagation. This paper describes the development of a Cubic Equation of State capable handling solid CO2 as a third phase. Pipeline rupture outflow data are reported based on the coupling of this new equation of state into a rigorous transient outflow model in order to investigate the impact of the pipeline design and operating conditions as well as the presence of the typical impurities on solid CO2 discharge

Entropy – stagnation enthalpy interpolation tables for calculation of the critical flow properties of compressible fluids

High-pressure pipelines provide the most cost-effective and established method for long-distance transportation of large quantities of compressible fluids, such as natural gas, hydrogen and carbon dioxide. Given significant safety hazards associated with these pipelines, their design and operation requires using mathematical modelling tools quantifying consequences of accidental pipeline failure. Central to this is the accurate and robust prediction of the critical discharge flow from the pipeline, accounting for the real fluid thermodynamic behaviour, including the phase transition induced by the decompression process. In this work, a method of inverse interpolation tables is developed to calculate physical properties of compressible fluid, for use in a computational model of transient outflow from an accidently ruptured pipeline. In particular, the density – energy interpolation tables are applied to calculate the fluid pressure, temperature and phase composition as required for solving the mass, momentum and energy conservation equations describing the decompression flow inside pipeline, while the entropy – stagnation enthalpy interpolation tables are introduced to obtain the critical (choked) flow properties at the rupture section of the pipe. To construct the latter, the choked flow properties are calculated by solving simultaneously the total enthalpy conservation equation along with the constant entropy condition. The interpolation is performed using Akima splines fitted to the thermodynamic properties data predicted using highly-accurate Perturbed Chain-SAFT (PC-SAFT) equation of state. The interpolation tables are constructed for ethylene and carbon dioxide, covering pressures from 0.1 to 10 MPa and temperatures ranging from the triple point to 350 K. The study provides recommendations for the optimal resolution of the interpolation tables to achieve a balance between the accuracy and computational efficiency of the calculated physical properties. Practical implementation of the interpolation method in a pipeline decompression flow model is discussed. Acknowledgement. This research has received funding from the European Union’s Horizon 2020 Research & Innovation Programme under the Grant Agreement No 884418, and Qatar National Research Fund (a member of the Qatar Foundation) NPRP award 8-1339-2-569

Biomass gasification in a downdraft gasifier with in-situ CO2 capture: A pyrolysis, oxidation and reduction staged model

Biomass gasification with in-situ CO2 capture, using calcium oxide as sorbent, has attracted increasing interest as a renewable source of high value products through the production of H2 rich syngas, while simultaneously presenting considerable potential for mitigating global warming by reducing CO2 emissions. Many factors influence the final composition of the syngas, such as type and amount of gasifying agent and residence time. Kinetic models play an important role in identifying the specific conditions for controlling the yield and composition of the product gas. When in-situ CO2 capture is used, accurate characterisation of the adsorption reactions in the kinetic scheme is essential for accurate prediction of the H2 rich syngas composition and the overall assessment of the technology. In this work, a kinetic model for biomass gasification with in-situ CO2 capture in a downdraft gasifier is developed. The model is divided into thermochemical stages of pyrolysis, oxidation and reduction in which gasification in a downdraft gasifier occurs, characterised by different compositions and temperature gradients. The model extends the kinetics to the oxidation zone and includes a mechanism for tar oxidation. Given downdraft gasifier designs, a simplification is made where the kinetic behaviour in each of the different stages is modelled separately and in series by a unique set of reactions. The model is validated against two sets of experimental data and different scenarios of equivalence ratio, steam-to-biomass ratio and sorbent-to-biomass ratio are analysed. Sensitivity analysis show that, employing carbon capture, H2 yields can increase of up to 50% under selected conditions. The study aims to provide a better understanding of biomass gasification kinetics and to aid the design and operation of downdraft gasifiers