8 th World Congress of Chemical Engineering Process Design Symposium

1. Compartmental modeling of an industrial bubble columnChristophe Wylock, Aurélie Larcy, Thierry Cartage, Benoît HautULB – Transfers, Interfaces and ProcessesAugust, 27th,2009 8th World Congress of Chemical Engineering Process Design Symposium Canada - Québec - Montréal 2009

2. Presentation plan • Introduction • Modeling • Division into a set of compartments • Gas-gas exchange • Gas-liquid exchange • Liquid-solid exchange • Chemical reactions • Simulation • Results and discussion • Conclusion Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles

3. Introduction Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles • Refined sodium bicarbonate (NaHCO3) • One of the oldest production process of the Solvay group • Has numerous applications • Produced in bubble columns, called the BIR columns • Dispersion of an air-CO2 mixture under the form of bubbles in an aqueous solution of Na2CO3 and NaHCO3 • CO2 is absorbed and chemical reactions occur in liquid phase (CO2)g (CO2)l CO2 + OH- HCO3- CO3= + H2O  HCO3- + OH- • Create a supersaturation of NaHCO3 precipitation of NaHCO3 crystals: Na+ + HCO3- (NaHCO3)s

4. Introduction Gas outlet (air –residual CO2 ) z Liquid inlet Htot Htr,n Htr,i Trays Htr,2 Cylindrical core of the column Htr,1 Hcr Degassing loops Hl Circulation loops Suspension 0 Suspension outlet (liquid – refined NaHCO3) Gas inlet (air – CO2) Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles Schematic view of a BIR column

5. Introduction Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles • Limiting step : CO2 absorption rate (only 50%) • Moreover, new applications of NaHCO3 require more control on precipitation process • Past optimizations: empirical approach, that has reached its limit  more fundamental approach is needed, but complex • 3 phases system • Many phenomena, on different space and time scale • Coupling of the phenomena • Goal of this work: development of an operational model of BIR column, using compartmental modeling approach (Cholette&Cloutier)

6. Introduction The Cholette & Cloutier model for a CSTR Real CSTR Model bypass bypass perfectly mixed zone perfectly mixed zone diffusion dead zone dead zone

7. Presentation plan • Introduction • Modeling • Division into a set of compartments • Gas-gas exchange • Gas-liquid exchange • Liquid-solid exchange • Chemical reactions • Simulation • Results and discussion • Conclusion Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles

8. Modeling Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles • Compartmental modeling at steady-state • Schematic view

9. Modeling Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles • Compartmental modeling at steady-state • The column is divided into a set of compartments • Each compartment contains: • Perfectly mixed liquid phase • Gaseous phase  2 bubble populations, modeled by plug flow • In the lowest: perfectly mixed solid phase, modeled by PBE • Mass transfers occur: • Between compartments • Global gaseous flow rate • Global and back-mixing liquid flow rates • Inside each compartment • Gas-liquid (CO2 absorption) • Gas-gas (between the 2 bubble populations) • Liquid-solid (NaHCO3 precipitation) • Chemical reactions (in the liquid phase)

10. Modeling Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles All equations and jump conditions: see «Compartmental modeling of an industrial bubble column» in the special issue of Chemical Product and Process Modeling

11. Presentation plan • Introduction • Modeling • Division into a set of compartments • Gas-gas exchange • Gas-liquid exchange • Liquid-solid exchange • Chemical reactions • Simulation • Results and discussion • Conclusion Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles

12. Modeling Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles • Gas-gas exchange (between small and large bubbles) • 2 bubble populations • large bubbles : 5 - 8 cm • small bubbles : 2 - 5 mm • Flow modeled by a plug flow • Gas-liquid CO2 transfer only from the small bubble population • Exchange between the 2 populations by the break-up and coalescence phenomena proportional to the gas hold-up • Equations function of the liquid and solid characteristics

13. Presentation plan • Introduction • Modeling • Division into a set of compartments • Gas-gas exchange • Gas-liquid exchange • Liquid-solid exchange • Chemical reactions • Simulation • Results and discussion • Conclusion Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles

14. Modeling Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles • Gas-liquid exchange (CO2 absorption) • Small bubble-liquid mass transfer modeled using Higbie approach (depending on concentrations and liquid flow) • Coupling with chemical reactions: only 1 pseudo 1st order reversible reaction: CO2+OH- HCO3- Page 14

15. Presentation plan • Introduction • Modeling • Division into a set of compartments • Gas-gas exchange • Gas-liquid exchange • Liquid-solid exchange • Chemical reactions • Simulation • Results and discussion • Conclusion Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles

16. Modeling Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles • Liquid-solid exchange (NaHCO3 precipitation) • Solid phase: modeled by Population Balance Equation (PBE)  Crystal Size Distribution (CSD) : • Solid mass fraction and liquid-solid NaHCO3 transfer rate can be deduced from the CSD (function of concentrations and liquid flow rate)

17. Presentation plan • Introduction • Modeling • Division into a set of compartments • Gas-gas exchange • Gas-liquid exchange • Liquid-solid exchange • Chemical reactions • Simulation • Results and discussion • Conclusion Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles

18. Modeling Chemical conversion rates Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles • Chemical reactions in liquid phase • Species concentrations in a compartment are affected by: • flow rates from and to neighbor compartments • gas-liquid CO2 transfer • liquid-solid NaHCO3 transfer  Chemical conversions to reach back the equilibrium • Set of equations come from mass balances and chemical equilibria

19. Presentation plan • Introduction • Modeling • Division into a set of compartments • Gas-gas exchange • Gas-liquid exchange • Liquid-solid exchange • Chemical reactions • Simulation • Results and discussion • Conclusion Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles

20. Simulation Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles • All modeling parameters can be estimated (at least roughly) : • Experimental measurements • Literature review of theoretical and experimental work • Theoretical analysis • Computational Fluid Dynamics simulation  The model can be simulated

21. Simulation Gas flow rates with height CO2 transfer rate with height Concentration evolution with height Concentration evolution following compartment Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles Simulation results

22. Presentation plan • Introduction • Modeling • Division into a set of compartments • Gas-gas exchange • Gas-liquid exchange • Liquid-solid exchange • Chemical reactions • Simulation • Results and discussion • Conclusion Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles

23. Results and discussion Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles • Simulation results comparison with experimental measurements • For the initial set of estimated modeling parameters fsim=0.18 (fmeas~ 0.2) Global simulated CO2 absorption: 50% (measured~ 50%)

24. Results and discussion Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles • Simulation results comparison with experimental measurements • With another set of modeling parameter values fsim=0.19 (fmeas~ 0.2) Global simulated CO2 absorption: 55% (measured~ 50%)

25. Presentation plan • Introduction • Modeling • Division into a set of compartments • Gas-gas exchange • Gas-liquid exchange • Liquid-solid exchange • Chemical reactions • Simulation • Results and discussion • Conclusion Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles

26. Conclusions Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles • An "operational" BIR column model is developed • Reasonable agreement with experimental measurement (despite the roughly determination of some parameters) • Important differences with observed results remain • Further studies has to be performed • Development of accurate correlation to estimate or adjust modeling parameters • Experimental validation  Already a tool to develop new design of BIR bubble column reactors (number of trays/compartment, height, liquid or gas flow rates, diameter of core or loops,...)

27. Thanks for your attention.

28. Appendix : Modeling • Gas-gas exchange (between small and large bubbles) • Bubble population gas hold-up • Net exchange • Exchange superimposed (break-up - coalescence) • Pressure Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles Page 28

29. Appendix : Modeling • Gas-gas exchange (between small and large bubbles) • Molar gas flow rate • CO2 molar flow rate Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles Page 29

30. Appendix : Modeling • Gas-liquid exchange (CO2 absorption) • Bubble-liquid CO2 mean flux density • Gas-liquid CO2 transfer function • Transferred CO2 amount inside the ith compartment Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles Page 30

31. Appendix : Modeling • Liquid-solid exchange (NaHCO3 precipitation) • Solid phase: modeled by Population Balance Equation (PBE) • Mean slurry residence time : with • Solid mass fraction: • Liquid-solid NaHCO3 transfer rate: Transfers, Interfaces and Processes Applied Science Faculty, Université Libre de Bruxelles Page 31

32. Appendix : Modeling Chemical reactions in liquid phase