2013 ijep paper

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Int. J. Environment and Pollution, Vol. 51, Nos. 1/2, 2013 121

Copyright 2013 Inderscience Enterprises Ltd.

Modelling and simulation of reaction kinetics ofcarbon dioxide absorption into aqueous ammonia ina wetted wall column

Ujjal K. Ghosh* and Su Ching Yee

Department of Chemical Engineering,

School of Engineering and Science,

Curtin University,

CDT 250, 98009 Miri, Sarawak, Malaysia

Fax: +60 85 443837

E-mail: [email protected]: [email protected]

E-mail: [email protected]

*Corresponding author

Abstract: Carbon dioxide accounts for about 80% of all greenhouse gases(GHG) and thus becomes the major source responsible for global warmingwhich is considered as the greatest environmental challenge the world is facing.The post-combustion capture is the main way to lower the emission of existingpower plants and future power plants where CO2 is produced during thecombustion. Solvent-based CO2 capture technology a proven technology forCO2 capture. Carbon dioxide absorption process using an ammonia solutionprovides many advantages including higher absorption capacity, no degradationand lower regeneration energy requirement. In this paper, a mathematical

model for the reaction kinetics of carbon dioxide absorption into aqueousammonia in a wetted wall column is developed. Simulation of the mathematicalmodel is performed and the simulated results are compared with literature data.

Keywords: absorption; carbon dioxide; aqueous ammonia; wetted wallcolumn; carbon capture.

Referenceto this paper should be made as follows: Ghosh, U.K. and Yee, S.C.(2013) Modelling and simulation of reaction kinetics of carbon dioxideabsorption into aqueous ammonia in a wetted wall column,Int. J. Environmentand Pollution, Vol. 51, Nos. 1/2, pp.121138.

Biographical notes: Ujjal K. Ghosh is currently a Senior Lecturer at theDepartment of Chemical Engineering, Curtin University, Malaysia campus. Hefinished his PhD from Indian Institute of Technology, Kharagpur, India before

joining the Department of Chemical and Biomolecular Engineering, Universityof Melbourne Australia as a Postdoctoral Research Fellow to work on carbonsequestration project.

Su Ching Yee finished her Bachelor degree in Chemical Engineering fromCurtin University, Malaysia campus. She is currently working as a ProductionEngineer in a margarine and frozen dough manufacturing company based inSingapore.

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122 U.K. Ghosh and S.C. Yee

1 Introduction

The main sources of carbon dioxide emissions are mainly from the combustion of fossil

fuels. Among all fossil fuels, coal is the main energy source and coal-fired plant is

distributed widely across the Earths surface (Oexmann et al., 2008). All of these coal-

fired plants discharge a huge amount of carbon dioxide into the atmosphere and amount

of carbon dioxide rises dramatically every year. Due to their low cost, availability,

existing reliable technology for energy production, and energy density, fossil fuels

currently supply over 85% of the energy needs of the USA and a similar percentage of

the energy used worldwide (EIA, 2006a, 2006b).

Many development and research activities have been done in the field of carbon

capture and storage (CCS) due to rise of awareness of the greenhouse effect or climate

change. The implementation of CCS allows the use of fossil fuels and coals as a source of

energy while in the mean time reduce the emissions of greenhouse gases (GHG) into theenvironment. In CCS, carbon dioxide is removed from plant and stored securely in

reservoir. CCS is an integrated process which made up of three parts; carbon capture,

transport and storage. Carbon dioxide in flue gases is removed by capture technology and

transport to storage location via pipelines. The first step of CCS is capturing carbon

dioxide from flue gases. Currently, the main strategies for the carbon dioxide capture in a

fossil fuel combustion process are post-combustion capture, pre-combustion capture and

oxy-fuel combustion (Davidson and Metz, 2005). For each combustion process, a

different method of capturing CO2produced as a by-product of energy production.

1 Post-combustion capture:In post-combustion capture, CO2is separated from the flue

gas emitted after the combustion of fossil fuels from a standard gas turbine combined

cycle, or a coal-fired steam power plant. CO2is being separated at relatively low

temperature, from a gaseous stream at atmospheric pressure and with low CO2concentration (ca. 525% if air is used during combustion). Minor amounts of SO2,

NO2and O2may also be present. This possibility is by far the most challenging since

a diluted, low pressure, hot and wet CO2/N2mixture has to be treated. Nevertheless,

it also corresponds to the most widely applicable option in terms of industrial sectors

(power, kiln and steel production for instance). Moreover, it shows the essential

advantage of being compatible to a retrofit strategy (i.e., an already existing

installation can be, in principle, subject to this type of adaptation) (Favre, 2007).

2 Pre-combustion capture:In pre-combustion capture the conversion of the fuel is

performed to a mixture of H2and CO (syngas mixture) through partial oxidation,

steam reforming or auto-thermal reforming of hydrocarbons, followed by water-gas

shift reaction. The separation of CO2(at 3035%) from the H2that is then fed as

clean fuel to turbines. In these cases, the CO2separation could happen at very highpressures (up to 80 bar of pressure difference) and high temperatures (300700

oC).

3 Oxy-fuel combustion:First step of oxy-fuel combustion is separation of

oxygen/nitrogen on the oxidant stream, so that a CO2/H2O mixture is produced

through the combustion process. The advantage of feeding an oxygen-enriched gas

mixture (95% oxygen) instead of air, is the achievement of a purge stream rich in

CO2and water with very low N2content, therefore the CO2can be easily recovered

after the condensation of the water vapour.

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Modelling and simulation of reaction kinetics of carbon dioxide absorption 123

However, CCS is still facing some challenges towards a full-scale demonstration and

commercialisation. Besides the technical and economic issues, CCS also faces to safety

and regulatory issues when dealing with the way of transport and storage of carbon

dioxide after the capturing process (Oexmann et al., 2008). These challenges need to be

overcame before CCS can be fully commercialised and minimise the concentration of

carbon dioxide within the atmosphere.

As mention before, in post-combustion capture process, carbon dioxide is separated

from the flue gas by chemical solvents absorption. In the absorption process, carbon

dioxide contained gas stream is dissolved in the chemical solvent, while at the mean time,

other gases remain in the gas stream and are not absorbed by the solvent. Typical

industrial process of carbon dioxide absorption and regeneration system can be

represented in Figure 1. The absorption and regeneration system consists of an absorber

and regenerator. In the absorber, carbon dioxide is removed from the gas stream by

contacting counter-currently with chemical solvents. The chemical solvents are thenrecovered in the regenerator and recycled back and fed to the absorber. High

concentration of carbon dioxide is separated from the chemical solvents and directed to

next storage location.

Figure 1 Schematic diagram of carbon dioxide absorption and regeneration system

The use of monoethanolamine (MEA) in the carbon dioxide is limited due to some issues,

such as high heat of absorption and corrosion issues. On the other hand, the use of

potassium carbonate is also limited by the rate of absorption. Somehow, introducing of

promoters in the potassium carbonate solvent will increase the operation cost. The

advantages of using ammonia solution include high carbon dioxide absorption rates, high

carbon dioxide removal rates and also high carbon dioxide loading. The loss of ammonia

solution in the adsorption and desorption process is considered acceptable in terms of

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environment as well as operating perspective (Dave et al., 2006). Yet, more researches

are needed because the absorption rate of carbon dioxide in ammonia solution isinfrequently researched. Thus, the paper will focus reaction kinetics of absorption and

equilibrium of carbon dioxide into aqueous ammonia.

In this research, the absorption of carbon dioxide into aqueous ammonia is studied. A

mathematical model is developed based on the mass balance of carbon dioxide

absorption. Besides that, the model is simulated in order to observe the absorption rate

with operational parameters. Finally, the simulated result is compared with literature data.

2 Solvents for carbon dioxide capture

The following are the criteria for selection of chemical solvents for carbon dioxide

absorption (Dutta, 2007):

a Solubility The ideal chemical solution will only dissolve desired components, in

this case carbon dioxide, but should not dissolve other undesired components.

However, some undesired components also dissolve in the solvents. Thus, selectivity

of the right solvent is important in order to dissolve the desired product and also to

minimise the absorption of undesired products.

b Volatility The selected solvent should have low volatility in order to prevent losses

of solvent during the process.

c Viscosity The selected solvent should have low viscosity so that it can flow easily

during the process; especially operate in a packed column.

d Corrosiveness The selected solvent should have no or limited corrosiveness, thuslower cost of maintenance to the equipment is needed.

2.1 MEA as solvent

Mores et al. (2011) has investigated the modelling and optimisation of the chemical

absorption process in removing carbon dioxide by using MEA solutions. They had

considered the ratio between total absorbed carbon dioxide and the total cooling and

heating utilities, and the ratio between total absorbed carbon dioxide and the total amine

flow rates. Optimisation variables such as temperature, composition and flow rates of the

aqueous solution as well as the reboiler and condenser duties are considered in the

investigation.

The chemical reactions involved in the process are listed as below (Mores et al.,2011):

2 2 3 32H O CO H O HCO+ + + (1)

22 3 3 3H O HCO H O CO

+ + + (2)

2 3H O MEAH H O MEA+ ++ + (3)

3 2EAH HCO H O MEACOO + + (4)

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2 2 3EA CO H O MEACOO H O ++ + + (5)

2 3CO OH HCO + (6)

MEA shows high capacity of carbon dioxide absorption; however, concerns such as high

operating condition, high heat of generation as well as corrosive properties of MEA

limited its application in practical plants.

2.2 Potassium carbonate (K2CO3) as solvent

As compare to MEA, potassium carbonate has lower solvent costs and requires lower

heat of regeneration of solvent. Conversely, the absorption of carbon dioxide by

potassium carbonate is relatively slow and thus, promoters are needed in the absorption

process in order to increase the rate of carbon dioxide absorption.

Cullinane and Rochelle (2004) had developed a model in order to investigate the

effect of aqueous potassium carbonate in the carbon dioxide absorption promoted by

piperazine in wetted-wall column. In the investigation, 0.6M of piperazine was added to

2030% of potassium carbonate at the temperature range of 4080C. A significant

decrease of intermediate carbon dioxide loading, e.g., 85%, was shown by the addition of

0.6M promoter. The rate of absorption has been increased by 200% as the temperature

increases from 4080C. At high loading, a significant amount of piperazine dicarbamete

was formed while the piperazine carbamate remains unchanged. However, the absorption

did occur at low loading.

On the other hand, Ghosh et al. (2009) had done an experiment to study the

absorption rate of carbon dioxide into aqueous potassium carbonate promoted by boric

acid. In the experiment, the absorption rates of carbon dioxide in a wetted-wall column

were measured with and without 30% potassium carbonate and 15% of boric acid at thetemperature of 4080C. The addition of increasing of small amount of boric acid

resulted a significant increase in the rate of absorption by factor of 2. As the temperature

increased from 4080C in 30% potassium carbonate and 1% of boric acid, the

absorption rate has increased by a factor of approximately 2.

The chemical reactions involved in the process are listed as below (Cullinane and

Rochelle, 2004):

2( ) 2( )gas aqCO CO (7)

2( ) 2 3 32aqCO H O HCO H O ++ + (8)

23 2 33HCO H O CO H O ++ + (9)

2 32H O H O OH+ + (10)

2 2( ) 3aqPZ H O CO PZCOO H O ++ + + (11)

2 3PZH H O PZ H O+ ++ + (12)

2 2( ) 2 3( )aqPZCOO H O CO PZ COO H O ++ + + (13)

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2 3H PZCOO H O PZCOO H O+ ++ + (14)

2.3 Aqueous ammonia as solvent

Aqueous ammonia can be used to capture carbon dioxide by absorption process. Qin

et al. (2010) measured the kinetics of CO2 absorption in unloaded aqueous ammonia

solution by using a string of discs contactor. They found out that the kinetics rate constant

for CO2absorption in ammonia solution was around 10% compared to other amines.

The chemical reactions involved in the process are listed as below:

2 3 2 42CO NH NH COONH + (15)

2 3 2 42CO NH NH COO NH ++ + (16)

According to zwitterion mechanism, carbon dioxide reacts with ammonia to formzwitterions. The zwitterions form in reaction [equation (17)] is involved in the second

reaction (18) in the presence of B, which is ammonia and water (NH3 and H2O) (Derks

and Versteeg, 2009):

2,2 3 2 42

BK kCO NH NH COO NH ++ + (17)

23NH COO B NH COO BH+ ++ + (18)

The overall rate equation is represented as below (Qin et al., 2010):

2 3

2 3

1

2 2

[ ][ ]

1 1

[ ]

CO NH

B

CO NH R

k

k k k B

=

+

(19)

Ammonia and water are considered the dominant based in aqueous ammonia. Therefore,

the overall rate equation is modified into equation (20) (Qin et al., 2010).

2 3

3 2

2 3

2 3 2

[ ][ ]

1 1

[ ] [ ]

CO NH

Z ZNH H O

CO NH R

k k NH k H O

=+

+

(20)

3

3

2

1

NHZNH

k kk

k= (21)

2

2

2

1

H OZH O

k k

k k= (22)

where

k2= forward reaction constant (m3.kmol.s

1)

[CO2] = concentration of CO2(mol.m3)

[NH3] = concentration ofNH3(mol.m3

)

[H2O] = concentration ofH2O(mol.m3)

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2 3CO NH

R

= reaction rate (kmol.m3s1)

k1= backward reaction rate constant.

For zwitterion mechanism,

232

9062.01.7809 10 expk x

T

=

(23)

3

84451.4

3.9376 10 expZNH

k xT

=

(24)

2

303.0847.002expZ

H Ok

T

=

(25)

where

T= operating temperature (K).

3 Theory of wetted wall column (WWC)

Wetted wall column is a classical experimental model set up for the investigation of the

absorption rate of carbon dioxide. The wetted wall column is made up of two vertical

cylinders, where the smaller cylinder is installed inside the other cylinder. The gas stream

containing carbon dioxide is flowing upwards through the annular space while the

solvent, aqueous ammonia is flowing in the opposite direction forming a film on the outer

wall of the inner tube. Absorption of carbon dioxide happens when the gas and liquid

streams contact with each others. The details of WWC can be found elsewhere (Ghosh

et al., 2009).

Figure 2 Schematic diagram of a wetted wall column (see online version for colours)

Liquid stream

Gas stream

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3.1 Film theory for absorption accompanied by reaction

The motion of liquid ammonia can be assumed to be laminar when the liquid flows near

the wall. The concentration of dissolved carbon dioxide will decrease from2 ,CO i

C at the

CO2-liquid ammonia interface to2 ,CO b

C at the liquid bulk. The film theory is based on

the following assumptions (Dutta, 2007): The carbon dioxide transfer through the film is

at steady state conditions. No carbon dioxide will be accumulated in the system. Carbon

dioxide transfers through the stagnant fluid layer at the phase boundary.

In the mass balance, the rate of consumption of carbon dioxide by reaction must be

taken into consideration. Since the film theory occurs in a steady state condition, the mass

balance is written as below:

2 2 2 3z z z| | ( )( )CO CO CO NH N N z R + = (26)

where 2 z|CON is CO2flux at pointz. The flux is written in (27):

2

2 2

COCO CO

dCN D

dz= (27)

Dividing the mass balance by z and putting the limit z0, and substituting the flux,

2

2 32

2

2

COCO NH CO

d CD R

dz = (28)

where

2 2 ,CO CO iC C= atz= 0, (concentration of CO2at gas-liquid interface)

2 2 ,CO CO bC C= atz= , (concentration of CO2at the bulk liquid).

4 Mathematical model development

Material balance in term of CO2:

In Out Generation Consumption Accumulation + = (29)

Assumption:

No CO2is generated in the system (generation = 0)

Steady-state system. In a absorption system, no accumulation happens in the system

(Accumulation = 0).

Thus, the material balance of CO2is simplified into:

In Out Consumption = (30)

2 2 2 3, ,2 2

/ / / /, ,( ) ( )

g CO g S CO Sg CO g S CO S CO NHF C F C F C F C R V+ + = (31)

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where

Fg= flow rate of gas stream entering the system (m3.s1)

FS= flow rate of solvent stream entering the system (m3.s

1)

/

gF = flow rate of gas stream leaving the system (m

3.s

1)

/

SF = flow rate of solvent stream leaving the system (m3.s1)

2 ,CO gC = concentration of CO2in the gas stream entering the system (mol.m

3)

2 ,CO SC = concentration of CO2in the solvent stream entering the system (mol.m

3)

,2

/

CO gC = concentration of CO2in the gas stream leaving the system (mol.m

3)

,2

/

CO SC = concentration of CO2in the solvent stream leaving the system (mol.m

3)

2 3CO NH R = overall rate of reaction between ammonia and CO2 (mol.m

3.s1)

V= volume of the solvent in the system (m3).

In the absorption system, ammonia solution is chosen as the solvent used to absorb CO2

in the gas stream. Initially, the ammonia solution stream contains no CO2, which the

concentration of CO2 in the solvent stream entering the system is equal to zero

( )2 ,

0 .CO SC = The reaction between CO2 and ammonia is assumed to follow the

zwitterion mechanism. In the reaction, ammonia and CO2 are reacted and form

zwitterion, where zwitterion is further deprotonated by a base (B) (Qin et al., 2010). The

overall rate of equation between NH3and CO2is represented by equation (20).

Since the CO2 is reacted and forms ammonium compound when leaving the system,

therefore, the concentration of CO2leaving the system will be zero ( )2 ,

0 .CO SC = Thus,

the material balance of CO2is further simplified into:

2 2 3,2

/ /,

g CO gg CO g CO NHF C F C R V = (32)

In this system, the effect of temperature on the rate of absorption is tested at the operating

pressure of 1.4 bar. Two different operating temperatures, 293 K and 313 K (20C and

40C) were chosen in order to observe the effects. The overall rate of reaction for twotemperatures is calculated by using equations (23) to (25) (Qin et al., 2010). Both gas and

solvent flow rates are 5 SLPM and 2 106

m3/s respectively. The gas flow rate of

5 SLPM is converted into gas flow rates at 293 K and 313 K at pressure of 1.4 bar. At

standard condition, the temperature and pressure for gas are 273 K and 1.01325 bar or

101325 Pa, respectively. The gas flow rate is calculated from:

1 1 2 2

1 1 2 2

PV P V

n RT n RT = (33)

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Since n1= n2, the equation is simplified into:

1 1 2 2

1 2

PV P V

T T= (34)

Therefore, the gas flow rate at 313 K is 6.915 10S ms1.

The outlet partial pressure for CO2in the gas stream is calculated from the material

balance. The temperature and the flow rates of inlet and outlet of gas stream is assumed

to be the same.

2 2

/, 2 3,

( )g CO g CO NHCO gF C C R V = (35)

Since,

n PCV RT

= = (36)

Equation (36) is modified into:

2 2 32

/, ,

( )g

CO g CO NH CO g

FP P R V

RT = (37)

where

2 ,CO gP = partial pressure of CO2in the inlet of gas stream (kPa)

2

/,CO gP = partial pressure of CO2in the outlet of gas stream (kPa).

In order to calculate the overall gas transfer coefficient,KG, the log mean pressure need to

be determined as the overall gas transfer coefficient,KG, is calculated as the slope of flux

versus log mean pressure (Liu et al., 2009).

2 2

2

2

2

/

,

/

,CO CO

CO b

CO

CO

P PLog mean pressure P

PLn

P

=

(38)

where

2 ,CO bP = log mean pressure (kPa)

2COP = partial pressure of CO2in the inlet of gas stream (kPa)

2

/COP = partial pressure of CO2in the outlet of gas stream (kPa).

The flux of CO2is the number of moles of CO2travel into and out of the solution. Thus,

the flux is calculated as below (Liu et al., 2009):

2 2 2

/, ,

( )g

CO CO g CO g

FN P P

RT= (39)

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where

2CON = CO2flux, number of moles of CO2absorbed per second per unit area, moles

m2s1.

The overall gas transfer coefficient,KG, is calculated as the slope of flux to the log mean

pressure (Liu et al., 2009).

2

2 ,

COG

CO b

NK

P= (40)

where

KG= overall gas transfer coefficient (mol.pa1.cm2.s1).

5 Simulation of mathematical model

The developed model is simulated using six cases:

Case 1 10% CO2contact with 1%, 5%, 10% and 15% NH3at 293 K

Case 2 10% CO2contact with 1%, 5% and10% NH3at 313 K

Case 3 10% NH3contact with 5%, 10%, 15% and 20% CO2at 293 K

Case 4 10% NH3contact with 5%, 10%, 15% and 20% CO2at 313 K

Case 5 Effect of different height to 10% CO2contact with 1%, 5% and 10% NH3at293 K

Case 6 Effect of different height to 10% CO2contact with 1%, 5% and 10% NH3at

313 K.

The results obtained from simulation based on Case 1 to Case 4 will be compared with

the experimental results obtained from (Liu et al., 2009; Qin et al., 2010). Further results

discussion will be presented on results and discussion.

6 Results and discussion

For Case 1, 10% of inlet concentration of CO2 is contacted with 1%, 5% and 10%

ammonia solution at operating temperature of 293 K. For Case 2, 10% of inlet

concentration of CO2 is contacted with 1%, 5%, 10% and 15% ammonia solution at

operating temperature of 313 K. Higher ammonia solution concentrations show better

absorption rate in both case (Figure 3). 15% ammonia solution shows better absorption

rate than other ammonia solution concentration at operating temperature of 293 K. 10%

ammonia solution shows better absorption rate than other ammonia solution

concentration at operating temperature of 313 K. The reaction rate between CO2 and

ammonia solution is highly depended on the concentration of ammonia solution.

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However, experiment conducted by Liu et al. (2009) shows that the concentration of

ammonia solution should be selected in the range from 5%10%. The use of higherconcentration of ammonia solution in practical may faced issues such as volatility of

solution (Qin et al., 2010). As mention by Cullinane and Rochelle (2004), increase of

temperature from 40C to 80C increase the rate of CO2absorption by promoted K2CO3

solution by a factor of two at a constant CO 2 vapour pressure. Yet, the effect of

temperature was not obvious from the result (Figure 3). The ammonia solution is believed

to be evaporated at a higher temperature.

Figure 3 Flux for absorption of CO2into different concentration of ammonia solution (see onlineversion for colours)

For both Case 3 and Case 4, 10% concentration of ammonia solution is contacted with

5%, 10%, 15% and 20% CO2at operating temperature of 293 K and 313 K respectively

(Figure 4). Higher concentration of CO2in the inlet gas stream shows better absorption

rate in both operating temperatures. Higher concentration of CO2in the inlet gas stream,

increase the concentration gradient between two streams which leads to higher absorption

rate of CO2. However, the effect of temperature was not obvious from the result

(Figure 4). This phenomenon is similar to the results shown in Figure 3.

The overall gas transfer coefficient,KG

is calculated from equations (38) and (39).

TheKGlines at 293 K in 1%, 5%, 10% and 15% ammonia solution is plotted and shown

in Figure 5, while the KGlines at 313 K in 1%, 5% and 10% ammonia solution is plotted

and shown in Figure 6. The KG lines at 293 K in 5%, 10%, 15% and 20% CO2 inlet

concentration is plotted and shown in Figure 7, while the KGlines at 313 K in 5%, 10%,

15% and 20% CO2 inlet concentration is plotted and shown in Figure 8. By assuming KGlines are linear, the plotted lines are used to predict the CO2flux at different temperatures

and concentration of ammonia solutions as well as different CO2 inlet concentration. At

the same temperature, the CO2flux increases with the concentration of ammonia solution.

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Figure 4 Flux for absorption of CO2into different concentration of CO2inlet (see online version

for colours)

Figure 5 Flux CO2partial pressure lines by calculatedKGat 293 K promoted by differentconcentration of NH3(aq) (see online version for colours)

KG = KG 1010

(mol/pa.cm2.s)

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Figure 6 Flux CO2partial pressure lines by calculatedKGat 313 K promoted by different

concentration of NH3 (aq) (see online version for colours)

Figure 7 Flux CO2partial pressure lines by calculatedKGat 293 K promoted by different inletconcentration of CO2(see online version for colours)

KG = KG 1010

(mol/pa.cm2.s)

KG = KG 10 (mol/pa.cm .s)

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Figure 8 Flux CO2partial pressure lines by calculatedKGat 313 K promoted by different inlet

concentration of CO2(see online version for colours)

For both Case 5 and Case 6, the effect of column heights is tested on 10% concentration

of ammonia solution contacted with 5%, 10%, 15% and 20% CO 2 at operating

temperature of 293 K and 313 K. The absorption rates of CO 2are highly dependent on

the contact area during the process. However, increase of contact area or column height

may increase the pressure drops in the column, which caused large effects on the CO 2

flux across two streams (Mores et al., 2011). As the pressure drop increases the driving

force for CO2separation decreases and hence the CO2flux decreases.

Figure 9 Effect of column heights to the CO2flux with different concentration of ammoniasolution at 293K (see online version for colours)

KG = KG 1010(mol/pa.cm2.s)

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Figure 10 Effect of column heights to the CO2flux with different concentration of ammonia

solution at 313 K (see online version for colours)

The modelling results are compared with experimental results by Liu et al. (2009) and

tabulated in Table 1 to Table 4. However, the modelling results are different from the

experiment results obtained by Liu et al. (2009). There are some factors that will cause

the difference of both results. During the experiment, random errors are encountered

during the measurement as disturbances may occur when the readings are taken. The

supply of gas stream may be not consistent, which leads to fluctuation of CO2 partial

pressure readings. Besides that, the operating temperature may vary as heat is transferred

to the environment during practical.

In term of modelling, assumptions which have been made previously may cause

errors of modelling results. During modelling, the inlet and outlet gas flow rates are

assumed to be the same and this may cause the deviation between the experimental and

modelling results. On the other hand, the modelling methodology is considered ideal and

may be impractical to implement into real world.

Table 1 Comparing simulated results with literature data for Case 1

T (K) NH3Concentrations (%)KG(mol/pa.cm

2.s)experiment results

KG(mol/pa.cm2.s)

modelling results

293 1 0.243 1010 0.022 1010

293 5 0.698 1010 0.111 1010

293 10 0.978 1010 0.225 1010

293 15 1.392 1010 0.341 1010

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T (K) NH3Concentrations (%)KG(mol/pa.cm


KG(mol/pa.cm2.s)

modelling results

313 1 0.351 1010 0.022 1010

313 5 1.191 1010 0.111 1010

313 10 1.644 1010 0.228 1010


T (K) CO2Concentrations (%)KG(mol/pa.cm


KG(mol/pa.cm2.s)

modelling results

293 5 1.054 1010 0.280 1010

293 10 0.978 1010 0.225 1010

293 15 0.971 1010 0.215 1010

293 20 1.103 1010 0.209 1010


T (K) CO2Concentrations (%)KG(mol/pa.cm


KG(mol/pa.cm2.s)

modelling results

313 5 1.743 1010 0.282 1010

313 10 1.644 1010 0.228 1010

313 15 1.507 1010 0.215 1010

313 20 1.607 1010 0.209 1010

7 Conclusions

In this paper, a mathematical model for the reaction kinetics of carbon dioxide absorption

into aqueous ammonia in a wetted wall column is developed. Simulation of the

mathematical model is performed and the simulated results are compared with literature

data. Higher ammonia solution concentrations show better absorption rate, and yet, the

use of higher concentration of ammonia solution in practical may faced issues such as

volatility of solution. Liu et al. (2009) suggested that the concentration of ammonia

solution should be selected among 5%10%. Higher concentrations of CO2 in the inlet

gas stream show better absorption rate in both operating temperature. However, the effect

of temperature was not obvious from the modelling results obtained. By assuming KGlines are linear, the plotted lines are used to predict the CO2flux at different temperatures

and concentration of ammonia solutions as well as different CO2 inlet concentration. At

the same temperature, the CO2flux increases with the concentration of ammonia solution.

The absorption rates of CO2are highly dependent on the contact area during the process.

However, increase of contact area or column height may increase the pressure drop in the

column, which causes large effects on the CO2flux across two streams.

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2013 ijep paper

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