Date post: | 16-Dec-2015 |
Category: |
Documents |
Upload: | maleah-nunn |
View: | 222 times |
Download: | 9 times |
Coupling between mass transfer and chemical reactions Coupling between mass transfer and chemical reactions during the absorption of COduring the absorption of CO22 in a NaHCO in a NaHCO33-Na-Na22COCO33 brine : brine :
experimental and theoretical studyexperimental and theoretical study
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
CRE XI, August 2007, Bilbao
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
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
Inletgaseous mixture
air – CO2
Outlet gaseous mixtureair – residual CO2
Suspension withrefined NaHCO3 (solid)
Input of brineCO3
= rich
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
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
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
Introduction
Goal : create a complete model, taking into account all the phenomena taking places in BIR columnOptimization 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
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
Presentation plan
Scale of the diffusion boundary layer1. Model of the coupling between mass transfer and
chemical reactions in a quiescent liquid2. Mach-Zehnder interferometer
Scale of the bubble1. Model of the bubble-liquid mass transfer2. Isothermal stirred tank reactor
Conclusion and perspectives
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
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
Gaseousphase
Liquid phaseNaHCO3/Na2CO3 brine
(pH~10)
x=0x
- -2 3
- - 2-3 3 2
CO +OH HCO
HCO +OH CO +H O
2 2g lCO CO
Axis pointed toward the liquid phase in normal direction of the interface
2CO
Gas-liquidequilibrium
Diffusion Chemical reactions
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
Mass balance on an infinitesimal element of quiescent liquid transfer-reactions PDEs:
2
-
-3
2-3
2 2
1CO
- -
1 2OH
- -3 3
1 2HCO
2- 2-3 3
2CO
CO CO
OH OH
HCO HCO
CO CO
D rt x x
D r rt x x
D r rt x x
D rt x x
Modelling of the coupling between mass transfer and chemical reactions in a quiescent liquid
-3-
1 11 21
=3- -
2 21 32
HCOCO OH
COHCO OH
r kK
r kK
with :
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
Numerical solving (COMSOL Multiphysics) : dimensionless concentration profiles :
Modelling of the coupling between mass transfer and chemical reactions in a quiescent liquid
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
Presentation plan
Scale of the diffusion boundary layer1. Model of the coupling between mass transfer and
chemical reactions in a quiescent liquid2. Mach-Zehnder interferometer
Scale of the bubble1. Model of the bubble-liquid mass transfer2. Isothermal stirred tank reactor
Conclusion and perspectives
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
Experimental cellMach-Zehnder block diagram
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
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
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/kgpCO2=1bar
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
Presentation plan
Scale of the diffusion boundary layer1. Model of the coupling between mass transfer and
chemical reactions in a quiescent liquid2. Mach-Zehnder interferometer
Scale of the bubble1. Model of the bubble-liquid mass transfer2. Isothermal stirred tank reactor
Conclusion and perspectives
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
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
2 2
2
CO CO
CO( ) (0, )t D t
x
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
Bubble-liquid mass transfer model
Liquid seen as a mosaïc of semi-infinite liquid elements continuously renewed
Parameter : contact time tC
tC ≈ 0,04s for dbubble= 5mm(Haut&Cartage, 2005)
Mean CO2 flux density :
Diameter of bubble : 1mm<db<1cmDiffusion boundary layer : ~10-5mInterface supposed to be a plan
2
2
2COCO ,
0
0,
COCt
Higbie
Cx t
Ddt
t x
2
mol
m s
Liquid element
Bubble
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
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)
2CO ,Higbie
2CO , 2
mol in
m sHigbie
Example in representative conditions of the process
Useful to find composition optimizing transfer taking into account the constraint of the process
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
Presentation plan
Scale of the diffusion boundary layer1. Model of the coupling between mass transfer and
chemical reactions in a quiescent liquid2. Mach-Zehnder interferometer
Scale of the bubble1. Model of the bubble-liquid mass transfer2. Isothermal stirred tank reactor
Conclusion and perspectives
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
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 DepartmentApplied Sciences Faculty, ULB
Isothermal stirred tank reactor
• pH versus time : • Cumulated CO2 versus time :
T=25°C - pCO2=1bar - [NaHCO3]t=0= 10 g/kg - [Na2CO3]t=0= 30 g/kgGas flow 1dm3/min - Stirrer speed 180 rpm
Se adjusted to 0.031 m2
Experimental vs simulation : example
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
Presentation plan
Scale of the diffusion boundary layer1. Model of the coupling between mass transfer and
chemical reactions in a quiescent liquid2. Mach-Zehnder interferometer
Scale of the bubble1. Model of the bubble-liquid mass transfer2. Isothermal stirred tank reactor
Conclusion and perspectives
Chemical Engineering DepartmentApplied Sciences Faculty, ULB
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 DepartmentApplied Sciences Faculty, ULB
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 DepartmentApplied Sciences Faculty, ULB
Thank you for your attention