sep purif tech 1

ARTICLE IN PRESSG ModelSEPPUR-9227; No. of Pages 9

Separation and Purification Technology xxx (2008) xxx–xxx

Contents lists available at ScienceDirect

Separation and Purification Technology

journa l homepage: www.e lsev ier .com/ locate /seppur

Advanced modelling in performance optimization for reactive separation inindustrial CO2 removal

2006,

has btion wtionsg theion fof nonThe m

rationt to t

M. Ahmadia, V.G. Gomesa,∗, K. Ngianb

a School of Chemical and Biomolecular Engineering, University of Sydney, Sydney, NSWb Huntsman Corporation Australia, Botany, NSW, Australia

a r t i c l e i n f o

Article history:Received 29 November 2007Received in revised form 27 March 2008Accepted 5 April 2008

Keywords:Carbon dioxideAbsorptionPotassium carbonateMathematical modellingReactive separationEnvironmental remediation

a b s t r a c t

A comprehensive modelpotassium carbonate soluand considers the interaccient was calculated usinbased on the bicarbonatecomprising coupled sets opackage for the absorber.temperature and concentconcentration with respec

1. Introduction

Gas absorption processes are concerned with the use of a suit-

Please cite this article in press as: M. Ahmadi, et al., Advanced modelling iremoval, Sep. Purif. Technol. (2008), doi:10.1016/j.seppur.2008.04.016

able liquid solvent to remove one or more gaseous contaminantsfrom a gas mixture. The removal of carbon dioxide (CO2) is an essen-tial step in many industrial processing operations such as synthesisof ammonia, natural gas purification, and oil refining. In these pro-cesses, CO2 is considered to be an impurity that must be removedfrom industrial gases (e.g., ethylene oxide as product in this study)in order to enhance the quality of the gas products and to avoidprocess problems such as catalyst poisoning.

The process of absorption with chemical reaction in hot potas-sium carbonate is recommended for bulk CO2 removal. The useof carbonate solutions provides an economical method for CO2separation from gas mixtures. Such solutions are relatively easyto process, regenerate and it offers sufficient capacity for acidgases.

During the past several years, various additives have beensuggested for increasing the efficiency of potassium carbonatescrubbing processes. One group of such additives which hasthe effect of improving both the reaction rate coefficient andthe equilibria characteristic of the solution is the alkanolaminesolution such as monoethanolamine (MEA) and ethanolamines

∗ Corresponding author. Tel.: +61 2 9351 4868; fax: +61 2 9351 2854.E-mail address: [email protected] (V.G. Gomes).

1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.seppur.2008.04.016

Australia

een developed for investigating the absorption of carbon dioxide in hotith boric acid catalyst. The model consists of the kinetics of key reactionsbetween mass-transfer and chemical kinetics. The mass-transfer coeffi-surface renewal theory and the chemical reaction in the liquid phase isrmation from CO2 as the rate-determining step. The mathematical models-linear differential and algebraic equations were solved using our software

odel was validated using plant data and was used to compute the flow,profiles in the absorber for sensitivity analysis. The variation in exit CO2

he operating conditions was also examined.© 2008 Elsevier B.V. All rights reserved.

[1,3,4,12,15,20]. In addition to the organic additives mentionedabove, a number of inorganic salts have been suggested as addi-tives with potassium carbonate solutions to improve absorptionefficiency, for example, arsenic trioxide (As2O3), alkali metal salts ofselenious or tellurous acid, and alkali metal salts of weak inorganicacids such as potassium and sodium salts of boric acid, vanadic acid,

n performance optimization for reactive separation in industrial CO2

and arsenious acid [11].While some organic additives, particularly the ethanolamines,

provide substantial improvements in overall reaction efficiency,they have the limitation of possible oxidative degradation whenexposed to oxidisers during the scrubbing operation. For exam-ple, during the removal of CO2 from recycle gas generated whilemanufacturing ethylene oxide from ethylene and oxygen, the CO2containing recycle gas contains substantial amounts of oxygen.The oxygen contained in such gases reacts with and degrades theorganic activators, particularly when the absorption is carried out atelevated temperatures. Thereby causing the absorption solution tolose efficiency and possibly enabling the production of undesirableby-products in the solution [11,14].

Among the inorganic additives, it has been found that theoverall efficiency of potassium carbonate solutions for the absorp-tion of CO2 can be improved by the presence of small amountsof a mixture of sodium or potassium borate in the scrub-bing solution. Although a number of authors have investigatedthe modelling and simulation of hot potassium carbonate pro-cess [6,21–23], no systematic work is available on the impactof potassium borate on reaction rate of carbonate solution

dx.doi.org/10.1016/j.seppur.2008.04.016

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ARTICLE ING ModelSEPPUR-9227; No. of Pages 9

2 M. Ahmadi et al. / Separation and Purificat

Nomenclature

a specific surface area (m2/m3)Ck concentration of kth component (kmol m−3)Cke equilibrium concentration of kth component in the

bulk of liquid (kmol m−3)Cki

concentration of the kth component at interface(kmol m−3)

CPG molar specific heat of gas (kJ kmol−1 K−1)CPk

specific heat of kth component (kJ kmol−1 K−1)CPL molar specific heat of liquid (kJ kmol−1 K−1)dP packing nominal size (m)DCO2 diffusivity of CO2 in K2CO3 solution (m2 h−1)Dg CO2 diffusivity of CO2 in gas (m2 h−1)E enhancement factorG molar velocity of gas (kmol m−2 h−1)G mass velocity of gas (kg m−2 h−1)hg heat transfer coefficient in gas phase

(kJ m−2 h−1 K−1)H solubility of carbon dioxide in solution

(kmol atm−1 m−3)�HH2O heat of vapourization of water (kJ kmol−1)�HCO2 heat of reaction and absorption of CO2 (kJ kmol−1)I ionic strength of solution (kg ion m−3)k pseudo-first-order rate constant (h−1)kg gas side mass-transfer coefficient

(kmol h−1 atm−1 m−2)kL liquid side mass-transfer coefficient (m h−1)kOH forward rate constant of reaction (m3 kmol−1 s−1)k−OH backward rate constant of reaction (s−1)Kg overall mass-transfer coefficient

(kmol h−1 atm−1 m−2)L molar velocity of liquid (kmol h−1 m−2)L liquid mass flow rate (kg h−1)Nk mass-transfer flux of kth component

(kmol h−1 m−2)Pk partial pressure of the kth component in gas phase

(atm)

The unit consists of (i) an absorption section where CO2 isremoved from a gas stream by a liquid solvent and (ii) a regen-eration section where the absorption capability of the used solventis restored. In the absorption section, the gas stream containing CO2is passed upward through the absorption column, counter-currentto the liquid solvent entering the column at the top. In this stage,CO2 is transferred from the gas stream to the liquid solvent. Thisprovides a treated gas, with low CO2 content, passing out of thecolumn top and a rich solvent, with high CO2 content, leaving thecolumn at the bottom.

The rich solvent is then heated in a rich-lean heat exchanger,and enters the regeneration column at a point near the topof the column. In the regeneration section, the rich solvent isheated to its boiling point in a hot stream reboiler locatedat the bottom of the regeneration column, which strips outthe CO2. Finally, the lean solvent, with low CO2 content, fromthe regeneration column is pumped through the rich-lean heatexchanger and a cooler prior to re-introduction to the absorp-tion column. Table 1 shows the characteristics of the absorptioncolumn.

Pke equilibrium vapour pressure of kth component inthe gas phase (atm)

Pkipartial pressure of kth component in the liquid–gasinterface (atm)

R universal gas constant (m3 atm kmol−1 K−1)S cross-sectional area (m3)


TG gas temperature (K)TL liquid temperature (K)x mole fraction of kth component in the liquid phaseY mole fraction of kth component in gas phasez spatial variable along the height of the column (m)

Greek symbols˛ carbonation ration�g gas phase viscosity (kg m−1 h−1)�L liquid phase viscosity (kg m−1 h−1)�g gas phase density (kg m−3)�L liquid phase density (kg m−3)

buffer and the separation process. Furthermore, no previouswork has examined the effect of potassium borate as a cat-alyst in the absorber for the development of an improvedprocess, though limited experimental data have been published[11,22].

PRESSion Technology xxx (2008) xxx–xxx

In this work, we investigate the effects of the process oper-ating conditions on carbon dioxide absorption with potassiumcarbonate. For this purpose, we developed a model for themass-transfer and chemical reaction in an industrial absorberusing activated potassium carbonate. The current work is basedon the surface-renewal theory. The effect of various operatingparameters on the performance of absorber has been investi-gated and the design consequences of different options have beendiscussed.

2. Process description

CO2 absorption occurs through both reaction and mass-transfermechanisms when a gas mixture containing CO2 is brought intodirect contact with a liquid phase under appropriate conditions.This takes place in a CO2 absorption unit, for which a typical processflow diagram is given in Fig. 1.


Fig. 1. Schematic diagram of absorber–regenerator arrangement.


INrificat

tion, which is always first-order with respect to carbon dioxide, canbe written in the following way [6,22]:

rcat = kcat[CO2] (5)

kcat = KKBO2 [BO2−] (6)

Table 2 gives the catalytic constants of typical catalysts obtainedfrom the data of Kiese and Roughton [16,22].

As shown in the table, the catalytic effect of borate is significantand can be considered as a catalyst in carbonate and bicarbonate

ARTICLEG ModelSEPPUR-9227; No. of Pages 9

M. Ahmadi et al. / Separation and Pu

Table 1Characteristics of packing for industrial CO2 absorption column

Parameters Absorber packing

Height of packing 18.29 mDiameter of packed bed 1.219 mPacking size (dp) 50 mmPacking type Hy-paca, steelSpecific surface area of packing (a) 95 m2/m3

Packing void fraction 97 m2/m3

a Hy-pac: column internals from Norton, Inc., USA.

3. Reaction system

When carbon dioxide is absorbed in potassium carbonate andbicarbonate solution, it reacts according to the following overallreaction [3,7–9,21,22,24]:

CO2 + H2O + K2CO3 ⇔ 2KHCO3 (R1)

Since potassium carbonate and bicarbonate are both strong elec-trolytes, we can assume that the metal is present only in the formof K+ ions, so reaction (R1) may be written as

CO2 + CO32− + H2O ⇔ 2HCO3

− (R2)

The overall reaction is made up of a sequence of elementary stepswhich as follows:

CO2 + H2O ⇔ HCO3 + H+ (R3)

CO2 + OH− ⇔ HCO3− (R4)

H2O ⇔ OH− + H+ (R5)

Reactions (R3) and (R4) are both followed by subsequent instanta-neous reactions (finally leading to the overall result represented byreaction (R2)):

CO32− + H+ ⇔ HCO3

− (R6)

CO32− + H2O ⇔ HCO3

− + H− (R7)

The sequence (R3), (R5) and (R6) is known as the “acidic” mecha-nism [2]. The contribution of the acidic mechanism to the overallreaction rate is negligible unless the pH of the liquid solution isvery low. Almost all cases of industrial absorption is held at high pH(pH > 8). Hence, the acidic mechanism is neglected in the presentstudy. Since reactions (R5) and (R7) are instantaneous reactions,


reaction (R4) is the rate-controlling step for absorption of CO2 in hotpotassium carbonate solution. Thus, the rate equation for the reac-tion of carbon dioxide with unpromoted hot potassium carbonate[5,21] is

rOH = kOH(COH− )(CCO2 ) − kOH− (CHCO3− ) (1)

where kOH and kOH− are forward and backward rate constants ofreaction (R4).

At equilibrium, Eq. (1) gives

kOH− (CHCO3− ) = kOH(COH− )(CCO2 )e (2)

where (CO2)e is the equilibrium concentration of CO2. The expres-sion for reverse part of reaction (R4) has been evaluated byconsidering conditions at equilibrium in Eq. (2). This is a reasonableapproximation even when the system is not exactly at equilibrium[10,21]. Substituting Eq. (2) into Eq. (1) gives

rOH = (kOH(COH− ))((CCO2 ) − (CCO2 )e) (3)

The carbonate–bicarbonate system is a buffer solution, hence theconcentration of OH− ion in the solution near the surface of theliquid is not significantly affected by the absorption of CO2. In this

PRESSion Technology xxx (2008) xxx–xxx 3

case, carbon dioxide undergoes a pseudo-first-order reaction andEq. (3) can be rewritten as [5,10]:

rOH = k1((CCO2 ) − (CCO2 )e)

when H3BO3 is added to the carbonate-bicarbonate buffer solution,the absorption process occurs via several mechanisms. A conve-nient procedure for adding potassium borate to the potassiumcarbonate scrubbing solution is by adding boric acid, whereuponthe boric acid is converted to potassium borate KBO2 in accordancewith the following reaction:

2H3BO3 + K2CO3 → 2KBO2 + CO2 + 3H2O (R8)

In this event, the potassium carbonate content of the solution mustbe properly adjusted to compensate for that consumed by reac-tion with the added boric acid. It is noted that the small amount ofvanadium salts as additives for potassium carbonate solutions hasbeen used for corrosion inhibition [11,21]. The vanadium salt canbe formed by adding V2O5 to potassium carbonate as the followingequation:

K2CO3 + V2O5 → 2KVO3 + CO2 (R9)

The amount of catalyst present in the solution depends on the dis-sociation level of potassium borate. The percentage of dissociation˛ is given by [6]:

˛ = K

K + [H+](4)

where K is the dissociation equilibrium constant of the boric acidwhich depends on the ionic strength of the solution. The main roleplayed by the buffer solution, when KBO2 is present, is that of creat-ing a medium in which the salt can dissociate and then the catalyticactivity may be exploited. The rate equation for the catalysed reac-


buffer solution. Using the same approach for deriving Eq. (3), givesthe following pseudo-first-order rate equation for rKBO2 [5,6]:

rKBO2 = (KKBO2 [KBO2])([CO2] − [CO2]e) (7)

Eqs. (3) and (7) lead to the overall pseudo-first-order rate equa-tion of carbon dioxide with activated potassium carbonate in liquidphase:

r = (KOH[OH−] + KKBO2 [KBO2])([CO2] − [CO2]e)

= k([CO2] − [CO2]e)

Table 2Data on pK and kinetic coefficients for catalysts with buffers of appropriate species

Catalyst pK k (l/mol s)

Phosphate 7.1 0.017Arsenate 6.8 0.013Sulphite 7.0 1.89Selenite 8.0 3.57Borate 9.4 ∼1.5Tellurate 7.8 1.26


INrificat


4 M. Ahmadi et al. / Separation and Pu

where k is the overall apparent first-order rate constant and isdefined as [6,21]:

k = (KOH[OH−] + KKBO2 [BO2−]) (8)


4. Process model

The material and energy balance equations were developedaround a differential height of the packed absorber shown in Fig. 2b.Envelope III is an elemental volume in the differential packedheight, dz, of the absorber, consisting of the gas and liquid filmsdenoted by envelopes I and II, respectively. The main assumptionsare (1) steady-state conditions exist, (2) pressure drop across thepacked bed is negligible and (3) CO2 and H2O are the only compo-nents transported across the interface.

A differential mole balance in the gas phase around �z yields

dG

dz= −(NCO2 + NH2O)a (9)

where G is the superficial molar velocity of gas; NCO2 and NH2O aremass-transfer fluxes of CO2 and water, and ‘a’ is the specific surfacearea of packing. A differential mole balance in gas phase for CO2 andwater gives the following differential equations for mole fraction of

Fig. 2. (a) Schematic diagram of packed column and (b) differential section of packedcolumn.


carbon dioxide and water in the gas phase [21,22,13,18]:

dyCO2

dz= [NH2OyCO2 − NCO2 (1 − yCO2 )]a

G(10)

dyH2O

dz= [NCO2 yH2O − NH2O(1 − yH2O)]a

G(11)

Also a differential mole balance in liquid phase around �z gives

dL

dz= (NCO2 − NH2O)a (12)

where L is the superficial molar velocity of liquid.A differential mole balance in liquid phase for K2CO3, KHCO3

and water gives the following differential equations for the molefraction of carbon dioxide and water in the liquid phase:

dxH2O

dz= [NCO2 (1 + xH2O) − NH2O(1 − xH2O)]a

L(13)

dxK2CO3

dz= [xK2CO3 (NH2O + NCO2 ) + NCO2 ]a

L(14)

dxKHCO3

dz= [xKHCO3 (NH2O + NCO2 ) − 2NCO2 ]a

L(15)

Differential energy balances for the gas and liquid phases aroundthe differential height of packed bed, give the following differen-tial equations for temperature of gas and liquid phase [21,22,13,18],respectively:

dTG

dz= NCO2 + NH2Oa

GTG −

[NCO2 CPCO2+ NH2OCPH2O ]a

GCPG

TG

−hga(TG − TL)GCPG

(16)

dTL

dz= (NH2O + NCO2 )a

LTL −

[NCO2 CPCO2+ NH2OCPH2O ]a

LCPL

TG

−hga(TG − TL)LCPL

− (NCO2 �HCO2 + NH2O�HH2O)aLCPL

(17)

The heat effects due to mass flux, phase change, reaction and con-vective heat transfer between phases are taken into account. Thereference temperature used for the sensible heat is 0 ◦C.

The rate of absorption of carbon dioxide in the liquid phase,


according to the surface-renewal theory and homogenous catalysismechanism, can be expressed as follows [10]:

NCO2 = EkL(CCO2,i− CCO2,e ) (18)

where CCO2,iis the carbon dioxide concentration at the interface and

CCO2,e is the equilibrium concentration of unreacted carbon dioxidein the bulk liquid when the reverse reaction of carbon dioxide isconsiderable.

The enhancement factor, E, includes the mass-transfer coeffi-cient coupled with the chemical reaction rate coefficients, and isgiven as follows [10]:

E =√

1 + DCO2 k

k2L

(19)

where k is the overall apparent first-order rate constant.Eq. (18) can be rewritten in terms of the physical solubility, H, of

carbon dioxide in the reactive K2CO3 solution as [21,22]:

NCO2 = kLEH(PCO2,i− PCO2,e ) (20)


IN PRESSrification Technology xxx (2008) xxx–xxx 5

elling and simulation of the absorber unit are shown in Table 4. Thepseudo-first-order rate constant for the reaction of carbon dioxidewith carbonate solution in the presence of potassium borate as acatalyst is about 8234.5 h−1, which is consistent with data given in


M. Ahmadi et al. / Separation and Pu

Table 3Contributions from CO2 and other participating ions

Species h (l/g-ion)

Na+ 0.094K+ 0.071NH4

+ 0.031CO3

2− 0.021SO4

2− 0.021OH− 0.061HCO3

− 0.021CO2 −0.015

where kL is liquid side mass-transfer coefficient, E is enhancementfactor and H is physical solubility. The physical solubility of carbondioxide, H, can be estimated from [3,7,1,22,19]:

log10

(H

Ho

)= hI (21)

where I is solution ionic strength; h is the sum of contributions fromthe positive and negative ions in the solution and input from thegas species [3,21]:

h = h+ + h− + hG (22)

The value of h for CO2 absorption with reference to different ionsis given in Table 3.

The solubility of CO2 in pure water Ho is given by:

log(Ho) = 1140TL

− 5.30 (23)


where TL is the liquid temperature.The overall mass-transfer coefficient includes the combination

of the two individual mass-transfer coefficients in the gas and liquidphases [6,2,7,21]:

1KG

= 1kg

+ 1EHkL

(24)

Thus, the following equation is obtained for CO2 absorption rate:

NCO2 =(

kgCO2 kLEHkgCO2 + kLEH

)(PCO2 − PCO2e ) = KgCO2 (PCO2 − PCO2e )

(25)

The gas and liquid film mass-transfer coefficients were estimatedfrom the following equations [17,21,22]:

kgkRT

aDgk

= 5.32

(G

a�g

)0.7(�g

�gDgk

)1/3

(adp)−2 (26)

kL(�2L /�2

L g)1/3

DCO2

= 0.015

(L

a�L

)(�L

�LDCO2

)1/3

I (27)

Table 4Sorption–reaction parameter values after simulation (at T = 340 K, P = 20.4 atm)

Parameter Typical value

KL 0.057 m h−1

KOH 127 × 104 m3 kmol−1 s−1

KgCO20.03 kmol h−1 atm−1 m−2

KgH2 O0.004 kmol h−1 atm−1 m−2

Dg 0.007 m2 h−1

DCO2 10−5 m2 h−1

H 0.0065 kmol atm−1 m−3

˛ 0.47I 1.6 kg ion m−3

k 8234.5 h−1

Fig. 3. Comparison of outlet gas temperature model prediction against plant dataduring 30 days.

I = (�2L /�2

L g)1/3

(kOHCOH/DL,k)1/2

tanh((�2L /�2

L g)1/3

(kOHCOH/DL,k)1/2)(28)

where I is the ionic strength of the solution.The value of sorption–reaction parameters used in the mod-


the literature [10,17]. We also observed similar consistency for otherparameters such as the solubility of carbon dioxide in the mixture(H) and the diffusivity of CO2 in the gas and carbonate solutions(Dg and DCO2 ).

5. Results and discussions

5.1. Model validation

We validated our model predictions validated against data fromindustrial plant under steady-state operating conditions for the CO2removal unit. Figs. 3 and 4 show the liquid and gas outlet temper-atures measured in the plant data over 30 days of plant operation.The model predictions were obtained for each day separately basedon steady-state simulation for the input plant data available foreach day. As shown in Figs. 3 and 4, the model predictions are ingood agreement with plant data and closely follow the transitions

Fig. 4. Comparison of the outlet liquid temperature model prediction against plantdata during 30 days.



6 M. Ahmadi et al. / Separation and Purification Technology xxx (2008) xxx–xxx

Fig. 7. Liquid flow rate profile along bed height.

Fig. 5. Concentration profiles of CO2 and water in gas stream along the column.

during the plant operation. The maximum differences between themodel predictions and plant data for gas and liquid temperaturesare ±1.1 ◦C and ±1.5 ◦C, respectively. These are within the expectedexperimental error.

Please cite this article in press as: M. Ahmadi, et al., Advanced modelling in performance optimization for reactive separation in industrial CO2

removal, Sep. Purif. Technol. (2008), doi:10.1016/j.seppur.2008.04.016

The concentration profiles for the CO2 and H2O species in the gasphase are shown in Fig. 5. As the gas stream moves up the column,the relevant components interact with and are absorbed by theliquid solution. Thus, the gas species concentrations decrease withincrease in the bed height. We observe that the concentration ofwater vapour reaches equilibrium much earlier compared to CO2due to the additional kinetic and diffusion resistances encounteredby CO2 as indicated by the model equations.

The concentration profiles for K2CO3, KHCO3 and H2O in theliquid phase are shown in Fig. 6. These profiles clearly demonstratea decrease in potassium carbonate and water mole fractions due tothe reaction, as liquid moves down the column. Since KHCO3 is thereaction product, its concentration increases, as the liquid streamflows from the top to the bottom of the column.

The liquid and gas components undergo significant changes inflow rates, as the two counter-current streams move along theabsorber, shown in Figs. 7 and 8, respectively. The gas flow ratedecreases along the packed bed height from the bottom to the topbecause of absorption of CO2 and water vapour, while the liquidflow rate increases correspondingly, testifying to the overall con-servation of mass.

Fig. 6. Concentration profiles of various components in liquid stream along bedheight.

Fig. 8. Gas flow rate profile along bed height.

Fig. 9. Gas temperature profile along bed height.



M. Ahmadi et al. / Separation and Purification Technology xxx (2008) xxx–xxx 7

Fig. 10. Liquid temperature profile along bed height.

The temperature profiles were predicted as a function of packedbed height with the help of our model, as shown in Figs. 9 and 10.


The gas temperature (Fig. 9) decreases as the gas flows up theabsorber since heat is transferred to the liquid due to reaction,absorption, and convective heat transfer. Conversely, the liquidtemperature (Fig. 10) increases as the liquid flows down theabsorber on interacting with the gas stream, demonstrating theoverall conservation of energy.

A verification of our model was carried out by comparingthe model predictions with operating data for the hot carbonateprocess in conjunction with the design specification for CO2removal column [14]. The predicted results and the inlet/outletdata from the plant are given in Table 5. The table shows that thecalculated total flow rates and temperatures for both the liquid andgas streams agree with the observed values. The predicted liquidand gas compositions also compare well with plant data havingdeviations within 0.1–10%. These are well within the measurementerror of 15%, caused by the usual measurement devices deployed inplants for relatively narrow concentration ranges. The key indicatorof CO2 removal efficiency of 95.1 mol% predicted compares wellwith the value of 94.9 mol% measured. Further, the decrease inwater vapour concentration (species with maximum deviation

Table 5Comparison of model predictions with corresponding measured plant data

Parameters Inlet Outlet %Deviation

Measured Model

Liquid temperature (K) 377.1 386.5 385.3 0.3Liquid flow rate (kmol h−1) 5196.89 5260.53 5266.04 −0.1

Liquid composition (mol%)K2CO3 3.09 1.53 1.48 3.26KHCO3 2.85 6.04 5.9 2.32Water 91.28 89.62 88.33 1.44KVO3 0.28 0.28 0.28 0KBO2 1.25 1.25 1.25 0

Gas temperature (K) 321.2 309.07 308 0.3Gas flow rate (kmol h−1) 833.14 750.65 763.99 −1.77

Gas composition (mol%)CO2 10.320 0.530 0.504 4.9C2H4 15.09 16.68 16.75 −0.4N2 63.81 70.80 70.67 0.1Ar 6.27 6.96 6.87 0.4H2O 3 0.4 0.44 −10

Fig. 11. CO2 concentration profiles along the packed bed height with only water,with carbonate buffer, and with carbonate buffer plus catalyst.

from predictions) in the gas phase also compares reasonably wellwith the predictions.


5.2. Effect of potassium borate

The overall efficiency of potassium carbonate solution for theabsorption of CO2 can be improved by the incorporation of smallamounts of potassium borate (∼4% equivalent in 30% equivalentcarbonate solution) in the mixture. We investigated the effects ofthe addition of potassium borate as a catalyst on the absorptionefficiency.

Fig. 11 shows the role of potassium carbonate buffer solution andactivated potassium carbonate with potassium borate as a catalyston the CO2 removal unit compared to absorption without using anybuffer solution and catalyst. The figure shows that the activatedpotassium carbonate solution provides greater removal capacitythan potassium carbonate buffer solution, while there is no signif-icant change in the outlet CO2 concentration for operation withoutbuffer solution and catalyst.

In the case of using no buffer or catalyst the amount of outletCO2 is 0.08, i.e., 80% of CO2 inlet flow. The carbonate buffer solution(K2CO3 + KHCO3) provides greater absorption capacity compared touse of water without any buffer solution.

Fig. 12. Exit carbon dioxide concentration versus potassium borate concentration.


ING Model

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d (b) e

ARTICLESEPPUR-9227; No. of Pages 9

8 M. Ahmadi et al. / Separation and Pu

Fig. 13. (a) Exit CO2 concentration versus pressure an

By using carbonate buffer solution the mole fraction of exit CO2is decreased to 0.007 (i.e., 7% of inlet); while on using the carbonatebuffer plus potassium borate as catalyst this value reduces to 0.005(i.e., 5% of inlet). Thus, the potassium borate catalyst improves theoverall CO2 removal efficiency by 40%. The difference between theCO2 removal efficiency by the buffer solution and by the activatedcarbonate solution is primarily influenced by the enhancement ofthe reaction rate as described in the previous section.

We also tested cases with increases and decreases in potassiumborate concentration in the absorbent medium. Fig. 12 indicatesthat increases in potassium borate concentration enhance CO2removal initially. However, beyond a certain concentration the exitCO2 concentration does not change significantly. This is becausesubsequent increases in potassium borate concentration occurs atclose to the high end of the enhancement factor in the liquid phase,E, which is proportional to the overall Kg in case of liquid-phase con-trolled mass-transfer. With greater increase in potassium borateconcentration, the gas phase mass-transfer acts as a major factorin controlling the absorption process. Thus, CO2 removal remainsunaffected by increasing the catalyst concentration beyond a cer-tain value.

5.3. Effect of pressure and temperature

From the perspective of plant operation, it is important toknow the sensitivity of the absorption and reaction processes withrespect to the key operating variables in the plant. In the present


case, where the objective is to remove CO2 from the gas for recyclingunconverted reactants, one would like to find how the exit CO2 con-centration would vary if the operating pressure of the system wasto change from its design value. Fig. 13a demonstrates the resultsfor changes in pressure using the carbonate buffer solution pluspotassium borate. We observe that as the pressure is reduced, theabsorption performance deteriorates. This is because with lowerlevels of solute partial pressure, the driving force for mass-transferdecreases. Thus, the current operating pressure of 20.4 atm is closeto the optimum value. However, beyond about 25 atm pressure, nosignificant gain in CO2 removal efficiency is indicated.

The inlet temperature of the lean solution also affects theabsorption performance as shown in Fig. 13b. The figure indicatesthat a higher mass-transfer performance is achievable at lower tem-peratures, due to a decrease in the equilibrium vapour pressureof CO2 over the portion of the solution last contacted by the gas.It is observed from the curve that a substantial reduction in inletliquid temperature is required to achieve a decrease in outlet car-bon dioxide concentration. This is because significant decreases intemperature will eventually reduce the rate of reaction, leading toreductions in overall mass transport.


xit CO2 concentration versus lean liquid temperature.

6. Conclusions

A comprehensive model for CO2 absorption was developed tosimulate the removal of carbon dioxide in an industrial absorber.The model includes the effects due to potassium borate in thehot carbonate solution and incorporates other key process condi-tions such as temperature and pressure. Potassium borate plays animportant role as a catalyst in improving the performance of thehot potassium carbonate absorption process. Theoretically basedon the surface-renewal formalism, a model for the simultaneousmass-transfer and chemical reaction in an absorber with acti-vated potassium carbonate solution was employed. The model wasused to estimate the liquid and gas stream flow rate, temperature,and concentration profiles in the absorber. The model predictionscompared with the plant data. The computed exit carbon diox-ide concentration, the temperature profiles, and the liquid andgas flow rates agreed with plant data. Thus the model was vali-dated.

The effects of process variables and operating parameters on theperformance of absorber were investigated and the design conse-quences of various options were tested. Potassium carbonate andpotassium borate were found to have significant effects on the reac-tive absorption process. The CO2 removal performance was foundto improve by 93% with use of potassium carbonate and by 95%with potassium borate compared for absorption with water alone.Therefore, potassium borate as a catalyst improves the CO2 removalefficiency by 40% compared to carbonate buffer solution. The result-


ing exit CO2 concentration with variable operating pressures andtemperatures were also examined. The optimal lean liquid temper-ature was found to be about 97 ◦C and the operating pressure about20.4 atm. The model provides an efficient means to investigate theeffects of new promoters and process conditions for the industrialCO2 removal unit.

Acknowledgements

The authors gratefully acknowledge the support of HuntsmanCorporation and the Australian Research Council for this work.

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