coupling between mass transfer and chemical reactions during the absorption of co 2 in a nahco 3 -na...

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


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


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


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


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


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


-3-

1 11 21

=3- -

2 21 32

HCOCO OH

COHCO OH

r kK

r kK

with :


Numerical solving (COMSOL Multiphysics) : dimensionless concentration profiles :



Presentation plan






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


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


Presentation plan






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



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



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


Presentation plan






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


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


Presentation plan







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



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


Thank you for your attention

coupling between mass transfer and chemical reactions during the absorption of co 2 in a nahco 3 -na...

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