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Thermodynamic models for CO 2 -alkanolamine-water systems

Thermodynamic models for CO 2 -alkanolamine-water systems. Erik T. Hessen and Hallvard F. Svendsen Norwegian University of Science and Technology (NTNU) Department of Chemical Engineering Trondheim, NORWAY. Erik Troøien Hessen, Research Review Meeting - Austin , 10-11.01.2008. Outline.

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Thermodynamic models for CO 2 -alkanolamine-water systems

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  1. Thermodynamic modelsfor CO2-alkanolamine-water systems Erik T. Hessen and Hallvard F. Svendsen Norwegian University of Science and Technology (NTNU) Department of Chemical Engineering Trondheim, NORWAY Erik Troøien Hessen, Research Review Meeting - Austin, 10-11.01.2008

  2. Outline • Problem description and challenges • Electrolyte theory • Debye-Hückel • Frameworks for electrolyte thermodynamics • Model selection and development • Recapitulation • eNRTL • Clegg-Pitzer model Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  3. Thermodynamic modelling - challenges Vapour – liquid equilibria (VLE) Chemical equilibria (liquid phase) Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  4. Thermodynamic modelling - challenges Many species – complex reactions System changes character as CO2 is absorbed into the liquid phase Molecular components  Electrolyte components Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  5. Thermodynamic modelling - challenges General model structure (short range and long range terms): Helmholtz energy formulation (EOS) : A= ASR + ALR e.g. A = Aclassical eos + ADH/MSA +(ABorn) Excess Gibbs energy formulation GE = GE,SR + GE,LR e.g. GE = GE,NRTL + GE,DH/MSA +(GE,Born) Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  6. Thermodynamic modelling - challenges A = ASR + ALR GE = GE,SR + GE,LR Typically: NRTL, UNIQUAC, UNIFAC, Pitzer Many components many interaction parameters Difficult parameter estimation Model equations are empirical Typically: Debye-Hückel, MSA, (Born term) Electrolyte theory: Theoretical basis not fully explored Problems with mixed solvents (varyingε) Different frameworks: McMillan-Mayer, Lewis-Randall Long range interactions Short range interactions Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  7. Electrolyte theory • Electrolytes dissociate into ions • Long range coloumbic forces  non-idealities • Important at high loadings • Field not entirely explored • Ion pairing, association, dielectricum effects, mixed solvents Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  8. Electrolyte theory – Debye-Hückel • Debye, P., Hückel, E., Zur Theorie der Elektrolyte,1923 • Presented a model for the ”additional electrostatic energy” (Zusatzenergie) Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  9. Electrolyte theory – Debye-Hückel Assumptions made (partly by Debye and Hückel): Correct assumption? Legendre transformation In reality: Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  10. Electrolyte theory – Debye-Hückel The correct Debye-Hückel activity coefficient may then be found as: Which corresponds to a McMillan-Mayer – Lewis-Randall framework conversion. Thus the different frameworks of electrolyte theory should not be understood as something being on the side of ordinary thermodynamics. Literature: Breil, Mollerup (2006) Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  11. Electrolyte theory – shortcomings Mixed solvents: • Dielectricum assumption is probably an oversimplification • Inconsistencies arise when differentiating ε (mixed solvents) • Inconsistent use of the model equations (e.g. A vs. GE) Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  12. Model selection and development General model requirements: • Thermodynamic consistency • Accurate predictions for variable loadings and process conditions • Must be able to deal with mixed solvents and molecular solutes • Reasonable computational effort • Reasonable number of parameters to be estimated • Versatility (applied to different systems, and properties) Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  13. EOS based models Few models found – weak basis. Fürst - Renon (Cubic EOS + MSA) CPA models (cubic + association) Versatile models (+) Complex models (-) Mixing rules (-) Less experience and know-how (-) Many parameters to be estimated (-) GE models (γ-φ) Rigorous models: eNRTL, Pitzer, UNIFAC, UNIQUAC, etc. More complex (-) Often more parameters (-) Better predictability (+) Parameter databases (+) Much experience and know-how (+) Especially eNRTL is much used Most work is done in this field. Model selection and development Numerous models found in literature. Most are GE based. Non-rigorous models • Kent-Eisenberg, Desmukh-Mather • Narrow validity ranges (-) • Simple structure (+) • Few parameters (+) • Limited usefulness (-)  Not speciation, γH2O, γamine Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  14. Model selection and development Electrolyte-NRTL model: • Ya and Yc kept constant in differentiation to yield activity coefficients • Inconsistency error: • With this approximation the eNRTL model will not fulfill the Gibbs-Duhem equation for multi-cation/anion systems Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  15. Model selection and developmentThe Clegg-Pitzer model • Clegg, S. L. , Pitzer, K. S., J.Phys. Chem., 92, 1992 • Based on a Margules expansion term (SR) and a Debye-Hückel based term (LR) • gE = gE, short range + gE,long range • Two models – symmetrical and unsymmetrical electrolytes • Mole fraction based • Complex mathematical structure • Few sources in literature • Not applied to mixed solvents Unsymmetrical model (short range term) Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  16. Model selection and developmentThe Clegg-Pitzer model Full activity coefficient sheet For the unsymmetrical model Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  17. Model selection and developmentThe Clegg-Pitzer model Symmetrical matrix Zeros on diagonal Asymmetrical matrix Aji = -Aij Zeros on diagonal Revised structure Possible to differentiate Implemented in automatic differentiation package. Collaboration with associate professor Tore Haug-Warberg Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  18. Clegg-Pitzer based models • Li, Y., Mather, A. E., Ind. Eng. Chem. Res., 1994 (++) • Simplified model with easier mathematical structure. • Theoretical basis not treated well (single solvent  mixed solvent) • Based on CP for symmetrical electrolytes. • Free CO2 and carbonate neglected! • Fewer parameters than e.g. eNRTL • Applied to mixed solvents • Kundu, M., Bandyopadhay, S., J. Chem. Eng. Data, 2006 (++) • Based on the Li-Mather model • Free CO2 and carbamate neglected Symmetrical Introduced by Li & Mather Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  19. Speciation plot, 30 wt% MEA Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

  20. Conclusion • Many existing models found in literature • Hard to conclude upon which model to use • Difficult parameter estimations • eNRTL and Clegg-Pitzer models are explored • Theoretical basis is weak in some fields (especially for mixed solvents) • Ongoing investigations, especially related to Debye-Hückel terms Erik Troøien Hessen, Research Review Meeting, 10-11.01.2008

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