CO 2 capture using monoethanolamine solutions: Development and validation of a process model based on the SAFT-VR equation of state Charles Victor Brand A thesis submitted for the degree of Doctor of Philosophy and the Diploma of Imperial College London Centre for Process Systems Engineering Department of Chemical Engineering Imperial College London United Kingdom May 2013 Declaration I hereby declare that all the material in this dissertation is own and has been otherwise appropriately referenced. Charles Brand May 2013 The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work. 3 Abstract The development of a predictive model for an absorber-desorber process for the separa- tion of carbon dioxide (CO 2) from a gas stream using an aqueous alkanolamine solution as a solvent is presented. Post-combustion carbon dioxide capture by absorption with aqueous amine solvents is likely to play an important role in climate change mitigation, by helping to reduce a significant fraction of CO 2 emissions from fossil fuel power plants. There are, however, a number of concerns with the large scale deployment of this tech- nology, including energy requirements, solvent degradation and the environmental and health impact resulting from a potential loss of solvent and solvent degradation products. Modelling studies can play an invaluable and complementary role in addressing some of these issues, including the choice of solvent and operating conditions that yield optimal performance. The model presented here incorporates state-of-the-art SAFT-VR thermo- dynamics into a rate-based process model. A characteristic of the proposed approach is that all the reactions are treated within a thermodynamic description, assuming chemical equilibrium throughout. This greatly reduces the amount of experimental data required to model the behaviour of the absorber. Furthermore, in contrast with many treatments of reactive systems of this type, no enhancement factor is used in the process model. The absorber-desorber process model is implemented in the gPROMS software platform and validated using published pilot plant experimental data for the removal of CO 2 from an air and CO 2 stream using monoethanolamine (MEA) solutions. A scaling of the diffusivity in the liquid phase, that is found to be transferable to different operating conditions, is proposed. Reliable predictions are obtained for the temperature and composition profiles in the gas and liquid phases, including a good description of the temperature bulge which sometimes appears along the height of the absorber column. The same transferable model is used to describe both the absorber and the desorber columns. The influence of key pa- rameters of the model for different operating conditions is assessed through a sensitivity analysis. The model developed in this study is applied to simulate a complete amine-based carbon capture absorber-desorber process. Given the relatively simple modelling of the solvent/CO 2 interactions, in which the reactions are treated implicitly through a physical approach, the proposed model lends itself well to the investigation of other solvents. 5 Acknowledgements The work presented in this thesis would not have been possible hadn’t it been for the contribution of a number of people. First and foremost, I would like to express my sincer gratitude to my supervisors Professors Claire Adjiman, Amparo Galindo and George Jackson for their academic support and personal involvment during this PhD. Thank you for sharing with me your knowledge in process modelling and thermodynamic. Thanks to the members of the MSE group for the fun and the help during these years, in particular Niall, Frances, Tom, Carlos, Javier, Alex, Vassilis, Jens, Andrew, Nina, Roochi, Zara, Lara, Olga, Hendrick. Thanks to my Father, Mother, Brother and Sisters for the regular support and the wise guidance. I am grateful to Nicolas, Quentin, Pierre, Philippe, Frank, Benjamin, and Paul for sharing their advice and experience. Thanks to you Sophie for your presence, you are of great importance to me. 6 Contents 1 Introduction 24 1.1 Motivation for CO 2 capture .......................... 24 1.2 Objectivesofthiswork . 26 1.3 Outlineofthesis................................. 27 2 Techniques for CO 2 capture 29 2.1 CO 2 capturetechnologies. 30 2.1.1 Calciumlooping............................. 30 2.1.2 Oxy-fuelcombustion . 31 2.1.3 Integratedgasification combinedcycle . .... 32 2.1.4 Pressureswingadsorption . 32 2.1.5 Chemicalabsorption . 34 2.2 Amine-basedprocesses . 36 2.2.1 Importance of the thermodynamic and the transport models . 36 2.2.2 Columndescription. 38 2.2.3 Theconceptofastage . 38 2.2.4 Rate-basedvsEquilibriummodels. 39 2.2.5 Twofilmtheoryandenhancementfactor . 40 2.2.6 Masstransfercorrelations . 42 7 2.2.7 Phase equilibrium and chemical reactions . ..... 42 2.2.8 IntegrationofSAFTintoprocessmodels . 44 2.2.9 Summaryofexistingprocessmodels . 45 2.2.10 Overviewofavailablepilotplant. .... 45 2.2.11 Concludingremarks. 48 3 Thermodynamictheoryandmolecularmodel 49 3.1 Thermodynamictheory. 50 3.1.1 Chemicalapproach . 51 3.1.2 Quasichemicalapproach . 51 3.1.3 Molecularperturbationapproach,SAFT . 52 3.1.4 SAFT-VR................................ 54 3.2 Molecularmodel ................................ 57 3.3 Validationofthethermodynamicmodel . .... 62 3.4 Enthalpyofabsorption . 65 3.4.1 Methodology .............................. 65 3.4.2 Results.................................. 66 3.5 Concludingremarks............................... 68 4 DevelopmentofanabsorbermodelusingSAFT 69 4.1 Introduction................................... 69 4.2 Non-equilibriumabsorbermodel. 70 4.3 Modelequations................................. 73 4.4 Conludingremarks ............................... 86 5 Absorber model results 87 5.1 Validationofthemodel. 87 5.2 Modelpredictions................................ 94 5.3 Sensitivityanalysis . 108 5.3.1 Masstransfer ..............................108 8 5.3.2 Heattransfer ..............................110 5.3.3 Extentofwaterinthefluegas. .118 5.4 Conludingremarks ...............................121 6 Model of the regeneration module 122 6.1 Introduction...................................122 6.2 Modelofthedesorbercolumn . 125 6.3 Reboiler .....................................128 6.4 Condenser....................................130 6.5 Concludingremarks............................... 132 7 Regeneration module results 133 7.1 Modelpredictions................................ 133 7.2 Sensitivityanalysis . 139 7.2.1 Masstransfer ..............................139 7.2.2 Heattransfer ..............................142 7.3 Energyrequirementsinthereboiler . 149 7.4 Effectofthepressureinthereboiler. 152 7.5 Concludingremarks............................... 155 8 Application of the model 156 8.1 Closedflowsheet.................................156 8.1.1 AdditionofMEA ............................161 8.2 Applicationtoanotherpilotplant . 166 8.2.1 Absorber.................................166 8.3 Concludingremarks............................... 168 9 Concluding remarks 170 9.1 Summaryofthesis ...............................170 9.2 Keycontributions................................ 172 9.3 Directionsforfuturework . 172 9 9.3.1 Increasethemodelaccuracy . 173 9.3.2 Validatefurther.............................173 9.3.3 Processdesign..............................174 9.3.4 Extending the model to account for impurities . 174 9.3.5 Potentialuseofthismodel. 175 9.4 Publicationsandpresentations. 175 9.4.1 Publications...............................175 9.4.2 Conferencepresentationsandposters . 176 10 List of Figures 2.1 Flowsheet of an amine-based carbon capture process . ........ 35 2.2 Diagram of a non-equilibrium stage. This stage represents a section of packinginapackedcolumn. 39 2.3 A schematic of the two-film model of a column stage. δV and δL represent the thickness of the gas and liquid films, respectively. P V , P I , and P L are the pressure in the bulk vapour phase, at the gas-liquid interface and in the bulk liquid, respectively. As can be seen an isobaric profile is assumed. The temperatures of the bulk vapour phase, at the gas-liquid interface, and V I L of the bulk liquid are denoted by T , T , and T , respectively. Finally, yi and xi are the mole fractions of component i in the bulk vapour and liquid I I phases, respectively, and yi and xi are the mole fractions of component i at the vapour-liquid interface in the vapour and the liquid phases, respectively. 41 3.1 The square-well potentials for a monomer, Φ mono , and association between the sites, Φ assoc , employed in the SAFT-VR approach. (a) Φ mono is defined by a hard-core of diameter σ, range of attraction λσ and depth ε. (b) − Φassoc is defined by an off-centre potential of depth ε and of range r . − ab c;ab (c) The centre of the site is at a distance rd from the centre of the segment. 54 3.2 Schematic representation of the association scheme between MEA and CO 2 showingtworeactionproducts. 56 11 3.3 A schematic of the molecular model used in the SAFT-VR method. (a) H2O, (b) MEA, (c) CO 2 and (d) N 2. The sites e represent the electron lone pairs on the oxygen atom, the sites