1. Coupling between mass transfer and chemical reactions during the absorption of CO2 in a NaHCO3-Na2CO3 brine : experimental and theoretical study
CRE XI, August 2007, Bilbao
C. WYLOCK (F.N.R.S. Research fellow)(1), P. COLINET(1), T. CARTAGE(2), B. HAUT(1)
(1) Chemical Engineering Department, Applied Sciences Faculty, ULB (2) Solvay SA

2. Introduction
Key step of the Solvay process to produce refined sodium bicarbonate (BIR®) : gaseous CO2 transfer from bubbles to Na2CO3/NaHCO3 brine
Takes place in large bubble columns (BIR column) BUT leaving gas contains important quantity of CO2
Input of brine
CO3= rich
Outlet
gaseous mixture
air – residual CO2
SOIT introduction plus large avec contexte et enjeu par rapport à la diminution de CO2 mais réduire la fin sur la cuve agité
SOIT comme à la présentation au GLS 7
Inlet
gaseous mixture
air – CO2
Suspension with
refined NaHCO3 (solid)
Chemical Engineering Department
Applied Sciences Faculty, ULB

3. Introduction
Key step of the Solvay process to produce refined sodium bicarbonate (BIR®) : gaseous CO2 transfer from bubbles to Na2CO3/NaHCO3 brine
Takes place in large bubble columns (BIR column) BUT leaving gas contains important quantity of CO2
CO2 produced by lime calcination  requires large amount of energy
Past optimization of the process : by empiric approach
A more fundamental approach is seeked
SOIT introduction plus large avec contexte et enjeu par rapport à la diminution de CO2 mais réduire la fin sur la cuve agité
SOIT comme à la présentation au GLS 7
Chemical Engineering Department
Applied Sciences Faculty, ULB

4. Introduction
Goal : create a complete model, taking into account all the phenomena taking places in BIR column
Optimization of the process will be looked for in order to increase mass transfer
Gaseous CO2 transfer is coupled with chemical reactions in the liquid phase
This work : modelling of the coupling between gas-liquid CO2 transfer and the chemical reactions in the thin layer of liquid near the bubble interface
Multiscale approach is followed :
diffusion boundary layer (quiescent liquid)
gas bubble rising up in the liquid
SOIT introduction plus large avec contexte et enjeu par rapport à la diminution de CO2 mais réduire la fin sur la cuve agité
SOIT comme à la présentation au GLS 7
Chemical Engineering Department
Applied Sciences Faculty, ULB

5. Presentation plan Scale of the diffusion boundary layer
Model of the coupling between mass transfer and chemical reactions in a quiescent liquid
Mach-Zehnder interferometer
Scale of the bubble
Model of the bubble-liquid mass transfer
Isothermal stirred tank reactor
Conclusion and perspectives
Chemical Engineering Department
Applied Sciences Faculty, ULB

6. Modelling of the coupling between mass transfer and chemical reactions in a quiescent liquid
CO2 gas-liquid absorption to a NaHCO3-Na2CO3 brine: view of the system
Gas-liquid
interface
Gaseous
phase
Liquid phase
NaHCO3/Na2CO3 brine
(pH~10)
Gas-liquid
equilibrium
Diffusion
Chemical reactions x
x=0
Axis pointed toward the liquid phase in normal direction of the interface
Chemical Engineering Department
Applied Sciences Faculty, ULB

7. Modelling of the coupling between mass transfer and chemical reactions in a quiescent liquid
Mass balance on an infinitesimal element of quiescent liquid  transfer-reactions PDEs:
with :
Chemical Engineering Department
Applied Sciences Faculty, ULB

8. Modelling of the coupling between mass transfer and chemical reactions in a quiescent liquid
Numerical solving (COMSOL Multiphysics) : dimensionless concentration profiles :
Chemical Engineering Department
Applied Sciences Faculty, ULB

9. Presentation plan Scale of the diffusion boundary layer
Model of the coupling between mass transfer and chemical reactions in a quiescent liquid
Mach-Zehnder interferometer
Scale of the bubble
Model of the bubble-liquid mass transfer
Isothermal stirred tank reactor
Conclusion and perspectives
Chemical Engineering Department
Applied Sciences Faculty, ULB

10. Mach-Zehnder interferometer
Aim : validation of the model of the coupling between mass transfer and chemical reactions
Brine in a Hele-Shaw cell in contact with gaseous CO2 set in a Mach-Zehnder interferometer
Refractive index variation profiles near the interface, caused by the CO2 transfer, can be observed  interference fringes bend near the interface
Experimental cell
Mach-Zehnder block diagram
Chemical Engineering Department
Applied Sciences Faculty, ULB

11. Mach-Zehnder interferometer
Example of experimental result
Time evolution of the refractive index variation profiles
Promising experimental results BUT a calibration curve is now required
A model correlating the refractive index variations in function of the transferred CO2 is in development
Validation of the model of the coupling between mass transfer and chemical reactions will be performed in the continuation of this work.
T=20°C
[NaHCO3]t=0= 60 g/kg
[Na2CO3]t=0= 60 g/kg
pCO2=1bar
Chemical Engineering Department
Applied Sciences Faculty, ULB

12. Presentation plan Scale of the diffusion boundary layer
Model of the coupling between mass transfer and chemical reactions in a quiescent liquid
Mach-Zehnder interferometer
Scale of the bubble
Model of the bubble-liquid mass transfer
Isothermal stirred tank reactor
Conclusion and perspectives
Chemical Engineering Department
Applied Sciences Faculty, ULB

13. Bubble-liquid mass transfer model
[CO2] profile  interfacial CO2 transfer rate :
Used to estimate interfacial CO2 transfer rate from rising bubble in a bubble column
Required : representation of the liquid flow around the bubble  Higbie model
Chemical Engineering Department
Applied Sciences Faculty, ULB

14. Bubble-liquid mass transfer model
Liquid seen as a mosaïc of semi-infinite liquid elements continuously renewed
Bubble
Liquid element
Diameter of bubble : 1mm<db<1cm
Diffusion boundary layer : ~10-5m
Interface supposed to be a plan
Parameter : contact time tC
tC ≈ 0,04s for dbubble= 5mm
(Haut&Cartage, 2005)
=> Faut se donner tau ou s = paramètre de résolution lié à l’hydrodynamique
Flux moyen dépend bien sur du choix de la fonction de distribution
Mean CO2 flux density :
Chemical Engineering Department
Applied Sciences Faculty, ULB

15. Bubble-liquid mass transfer model
Contours of the mean CO2 flux density by unit of time and interfacial area as a function of the brine composition (solving:MATLAB coupled with COMSOL)
Example in representative conditions of the process
Useful to find composition optimizing transfer taking into account the constraint of the process
Chemical Engineering Department
Applied Sciences Faculty, ULB

16. Presentation plan Scale of the diffusion boundary layer
Model of the coupling between mass transfer and chemical reactions in a quiescent liquid
Mach-Zehnder interferometer
Scale of the bubble
Model of the bubble-liquid mass transfer
Isothermal stirred tank reactor
Conclusion and perspectives
Chemical Engineering Department
Applied Sciences Faculty, ULB

17. Isothermal stirred tank reactor
Aim : validation of the bubble-liquid mass transfer model
Pure CO2 bubbles pass through a brine
Continuous pH and [CO2]bulk measurement
Bubble-liquid mass transfer model coupled with mass balances in the stirred tank reactor
- Se (total interfacial area) adjustable parameter
- contact time tc posed to 0.02 s
(tc for bubble of diameter 1.5 mm)
Able to reproduce time evolution of the pH and transferred CO2 for different brine compositions
Chemical Engineering Department
Applied Sciences Faculty, ULB

18. Isothermal stirred tank reactor
Experimental vs simulation : example
pH versus time :
Cumulated CO2 versus time :
T=25°C - pCO2=1bar - [NaHCO3]t=0= 10 g/kg - [Na2CO3]t=0= 30 g/kg
Gas flow 1dm3/min - Stirrer speed 180 rpm
Se adjusted to m2
Chemical Engineering Department
Applied Sciences Faculty, ULB

19. Presentation plan Scale of the diffusion boundary layer
Model of the coupling between mass transfer and chemical reactions in a quiescent liquid
Mach-Zehnder interferometer
Scale of the bubble
Model of the bubble-liquid mass transfer
Isothermal stirred tank reactor
Conclusion and perspectives
Chemical Engineering Department
Applied Sciences Faculty, ULB

20. Conclusion and perspectives
Multiscale approach :
Scale of the diffusion boundary layer
Mathematical model of the coupling between mass transfer and chemical reactions in a quiescent liquid
Scale of the bubble
Model of the coupling completed by the Higbie representation of the liquid flow around a gas bubble
Model of the bubble-liquid mass transfer to calculate the mean CO2 flux density by unit of time and interfacial area in a BIR bubble column
Chemical Engineering Department
Applied Sciences Faculty, ULB

21. Conclusion and perspectives
An experimental device proposed for each scale
Mach-Zehnder interferometer
Allow to visualize refractive index variation profiles caused by the CO2 transfer
Correlation between transferred CO2 and refractive index variations in development
Validation of the model of the coupling : to be continued
Isothermal stirred tank reactor
Able to reproduce time evolution of the pH and transferred CO2 for different brine composition
Cross validation has to be performed
After this step : validation of the bubble-liquid mass transfer model
Chemical Engineering Department
Applied Sciences Faculty, ULB

22. Thank you for your attention
Chemical Engineering Department
Applied Sciences Faculty, ULB