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International Journal of Greenhouse GasControl 19 (2013) 312
Contents lists available at ScienceDirect
InternationalJournal ofGreenhouse Gas Control
j ournal homepage: www.elsevier .com/ locate / i jggc
Analysis and predictive correlation ofmass transfer coefficient KGavofblended MDEA-MEA for use in post-combustion CO2capture
Abdulaziz Naamia,b, Teerawat Semaa, Mohamed Edalib, Zhiwu Lianga,,Raphael Idema,b, Paitoon Tontiwachwuthikula,b
aJoint International Center for CO2 Capture andStorage (iCCS), College of Chemistry andChemical Engineering, Hunan University, Changsha 410082, PR
Chinab International Test Centre for CO2 Capture (ITC), Faculty of Engineering andApplied Science, University of Regina, Regina,Saskatchewan S4S0A2, Canada
a r t i c l e i n f o
Article history:
Received 22March2013
Received in revised form 7 August 2013
Accepted12 August 2013
Available online 18 September 2013
Keywords:
CO2 absorption
Packed column
Structuredpacking
Mass transfer coefficient
Cyclic capacity
a b s t r a c t
The mass transfer performance ofthe absorption ofCO2 in aqueous blended MDEA-MEA solutions was
evaluatedexperimentally in a lab-scaleabsorber packedwith high efficiencyDX structured packing over
MDEA-MEA concentrations of 27/3, 25/5, and 23/7%wt under atmospheric pressure using a premixed
feed gas containing 15% CO2 balanced with N2. The absorption performance was presented in terms
of overall mass transfer coefficient (KGav) and CO2 concentration profile. The results showed that themass transfer performance increased as ratio ofMEA in the blended solution, temperature, and liquid
flow rate increased but decreased as CO2 loading increased. In addition, it was found that the cyclic
capacity and relative solvent regeneration ability decreased as the ratio ofMEA in the blended solution
increased. Based onmass transfer performance, cyclic capacity, and relative solvent regeneration ability,
23/7%wtMDEA-MEAwas found to be the most effective blend ratio among the three ratios investigated
in the present work. Also, the correlation for predicting KGavfor CO2 absorption into aqueous blendedMDEA-MEAwas successfully developedwith an AAD of21.8%.
2013 Elsevier Ltd. All rights reserved.
1. Introduction
One of the options for reducing carbon dioxide (CO2) emis-
sions is absorption of CO2 from gas streams using reactive amine
solvents, which has recently been considered as one of the
most mature and reliable CO2 reduction technologies (Kohl and
Nielsen, 1997; Rao and Rubin, 2002; Liang et al., 2011). The
use of an effective solvent is considered to be one of the key
parameters of this technology. Charkravarty et al. (1985) firstly
introduced blended solvent systems by mixing primary (or sec-
ondary) amines with tertiary amines in order to capitalize on
the advantages of each amine and counter the disadvantages
of one amine with another amine. Presently, several blendedconventional amines have been introduced for absorbing CO2suchasblendedmonoethanolamine(MEA)-methyldiethanolamine
(MDEA), diethanolamine (DEA)-MEA, triethanolamine (TEA)-MEA,
MEA-2-amino-2-methyl-1-propanol (AMP), AMP-MDEA, AMP-
piperazine (PZ), and MDEA-PZ (Horng and Li, 2002; Mandal et al.,
2003; Aroonwilas and Veawab, 2004; Ramachandran et al., 2006;
Setameteekul et al., 2008; Huang et al., 2011; Samanta and
Corresponding author. Tel.: +8613618481627; fax: +8673188573033.
E-mail address: [email protected](Z. Liang).
Bandyopadhyay, 2011). Blended MDEA-MEA solutions have been
widely investigated for several years. This is because, firstly, both
the single amine solvents ofMEA andMDEAare mature since they
have been used for decades in fossil fuel-fired power generation,
natural gasprocessing, and chemical production industries. Exam-
ples of thecommercial facilities that have employed these solvents
are the Warrior Run Power Plant in Maryland, USA, Shady Point
PowerPlant inOklahoma,USA, PlatteRiverPowerAuthorityin Col-
orado, USA, Searles Valley Minerals Soda Ash Plant in California,
USA, Schwarze Pumpe Pilot Plant in Germany, Salah Natural Gas
Production Facility in Algeria, Sumitomo Chemicals Plant in Japan,
andProsint Methanol Production Unit in Brazil (Barchas andDavis,
1992; Dooley et al., 2009; Eswaran et al., 2010). Secondly, blendedMDEA-MEA canbe easily used in commercial absorbers since both
MEA and MDEA have generally been used at the industrial scale.
Blended MDEA-MEA has been commercially used in large-scale
processes at Yokosuka Power Plant in Japan and, Tokyo Electric
Power Corporation in Japan (Eswaran et al., 2010). This blended
amine system capitalizes on the advantages of high absorption
capacity, high solvent stability, low corrosiveness, and low energy
requirement for regenerationofMDEA andfast reaction kinetics of
MEA.
Even though blended MDEA-MEA was introduced in 1985,
most of the investigations have focusing on the reaction kinetics
1750-5836/$ seefrontmatter 2013 Elsevier Ltd. All rightsreserved.
http://dx.doi.org/10.1016/j.ijggc.2013.08.008
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4 A. Naami et al. / International Journal of Greenhouse Gas Control 19 (2013) 312
Nomenclature
AAD absolute average deviation
AMP 2-amino-2-methyl-1-propanol
Cs concentration for amine in the solution, kmol/m3
CO2 carbon dioxide
DEA diethanolamine
GI inert gas flow rate, kmol/m2 h
kL liquid phase mass transfer coefficient in the case ofchemical reaction,m/h
KG overall interfacial areagasphasemasstransfercoef-
ficient, kmol/m2 hkPa
KGav overall volumetric mass transfer coefficients,kmol/m3 hkPa
L1 liquid flow rate, m3/m2 h
MDEA methyldiethanolamine
MEA monoethanolamine
N2 nitrogen
P total pressure of the system, kPa
PZ piperazine
T absolute temperature, K
TEA triethanolamine
xCO2 mole fraction of CO2yA
mole fraction of solute A at interface
yA,G mol fraction of solute A at bulk gas phase
YA,G mole ratio of soluteA in gas phase
Z height of the absorption column, m
Greek letters
CO2 loading,mol CO2/mol amine
(Critchfield and Rochelle, 1987; Versteeg et al., 1990; Glasscock
etal., 1991;Rangwalaet al., 1992;Hagewiescheetal., 1995;Mandal
etal.,2001; Liao andLi,2002;Ramachandranet al., 2006;Edaliet al.,
2009), solubility (Shen and Li, 1992; Li and Shen, 1993; Dawodu
and Meisen, 1994; Li and Mather, 1994; Kaewsichan et al., 2001;Mamun et al., 2005), physiochemical and thermodynamics prop-
erties (Austgen et al., 1991; Li and Shen, 1992; Jou et al., 1994;
Li and Lai, 1995; Hsu and Li, 1997; Weiland et al., 1998; Bensetiti
et al., 1999; Mandal et al., 2005; Vrachnos et al., 2006), and sol-
vent degradation (Lawal et al., 2005; Lawal and Idem, 2005, 2006;
Dawodu and Meisen, 2009). Only a few studies have investigated
the mass transfer of CO2 absorption into blends of MDEA-MEA in
packed columns.
Aroonwilas andVeawab (2004)investigatedmass transfer per-
formance forCO2 absorption intoblendedMDEA-MEA,MDEA-DEA,
and AMP-MEA in structured DX packing using 10% CO2 balanced
with nitrogen (N2) as feed gas. The mass transfer performance
was reported in terms of the overall mass transfer coefficient,
removal efficiency, and relative columnheight requirement.How-ever, the experiments for the blended amines were conducted at
only 1/1 mole ratio (with total amine concentration of 3M). Itwas
found that the mass transfer performance of blended AMP-MEA
is higher than those of MDEA-MEA and MDEA-DEA, respectively.
Since the reaction kinetics of CO2 absorption can be ranked as:
AMP>DEA>MDEA, (i) themass transfer performanceofAMP-MEA
then exceeds that of MDEA-MEA and (ii) that of MDEA-MEA then
exceeds that of MDEA-DEA. However, reboiler heat duty should
also be taken into consideration since it contributes about 70% of
the operating cost of the CO2 capture process (Kohl and Nielsen,
1997). Sakwattanapong et al. (2005) investigated the reboiler heat
duty for CO2 capture with blends of AMP-MEA, MDEA-MEA, and
MDEA-DEA. They reported that the reboiler heat duty of MDEA-
DEAis lower than thoseofMDEA-MEAandAMP-MEA, respectively.
By taking into consideration both mass transfer performance and
heat duty requirement for solvent regeneration, blended MDEA-
MEA seems to be the best solvent combination among the three
because it provides good performance in both mass transfer and
reboiler heat duty requirement.
Later, Setameteekul et al. (2008) studied themass transfer per-
formance of blended MDEA-MEA at various MDEA-MEA blending
ratios of 1/3, 1/1, and 3/1 molar. They concluded that (i) the blend-
ing ratio of the blended MDEA-MEA has a significant effect on
themass transfer performance, (ii) themass transfer performance
increases asmolar ratio ofMEA increases, and (iii) theMDEA-MEA
of 1/3molar ratio provided the best mass transfer performance.
Additionally, the CO2 absorption not only depends on using
an effective solvent, but the contact between gas and liquid
solvent also plays an important role in the CO2 absorption per-
formance in a packed column. It has been over forty years since
the tray columns have been replaced by the more effective packed
columns (Aroonwilas and Tontiwachwuthikul, 1998; Aroonwilas
and Veawab, 2004). In the packed columns, the packing cre-
ates a gasliquid contact area by generating liquid droplets. The
most promising packing should provide large surface area per
volume ratio, low pressure drop across the column, and uniform
gasliquid distribution throughout the column. Aroonwilas and
Tontiwachwuthikul (1998)and deMontignyet al. (2001)compared
the CO2 absorption performance using randomly (IMTP#15, 0.63,
3 in. Pall Ring, 0.5in. Berl Saddles) and structured (Gempack 4A,
Sulzer EX) packings. They concluded that the structured pack-
ing provide higher mass transfer performance than the random
packing, which is in good agreement with the results observed by
Fernandes et al. (2009).
Even though CO2 absorption technology has been industrially
used for over half a century, themost promising solvents have still
to be discovered. Recently, a number of novel solvents (e.g., 2-N-
methylamino-2-methyl-1-propanol,2-N-ethylamino-2-methyl-1-
propanol, 2-(isopropylamino)ethanol, 2-(isobutylamino)ethanol,
2-(secondarybutyamino)ethanol, 2-(isopropyl)diethanolamine, 1-
methyl-2-piperidineethanol, 4-diethylamino-2-butanol, RITE-A
and RITE-B) has been introduced (Chowdhury et al., 2011; Gotoet al., 2011; Sema et al., 2013). The CO2absorption performance of
these novel amines was found to exceed the conventional MEA,
AMP, DEA, and MDEA. However, the major drawbacks of these
novel solvents are (i) expense, (ii) difficult synthesis, (iii) difficult
mass production, (iv) phase separation or turning into solidsunder
certain conditions, and (v) undiscovered disadvantages.
Another approach for improving the CO2 absorption perfor-
mance rather than working with novel solvents is to enhance and
optimize the performance of conventional solvents. In order to
achieve this, the conventional solvents have to be comprehen-
sively investigated. Even though blended MDEA-MEA has been
widely investigated, the predictive correlation for overall mass
transfer performance of CO2 absorption into aqueous solutions of
blended MDEA-MEA has not yet been established. In the presentwork, the blended MDEA-MEA ratio was selected at 27/3, 25/5,
and 23/7%wt (equivalent to MDEA-MEA molar ratio of 2.3/0.5,
2.1/0.8, and 1.95/1.16, respectively). These blend concentrations
were selected in order to capitalize on the great advantages of
both MDEA and MEA in that MDEA requires low energy for sol-
vent regeneration and has high absorption capacity while MEA
provides fast CO2 absorption rate. Since the heat requirement for
solvent regeneration contributes about 70% of the cost of captur-
ing CO2 (Kohl and Nielsen, 1997), MDEA was, therefore, selected
as the major component of the blended solutions in order tomin-
imize the heat requirement for solvent regeneration. The addition
ofMEA,which has fast reactionkinetics,can then enhance the CO2absorption rate of the blended solutions. However, the total con-
centration of the blended solution cannot be high because MDEA
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A. Naami et al. / International Journal of Greenhouse Gas Control 19 (2013) 312 5
itself is considered to be a slightly viscous solvent already. Thus,
the total concentration of the blended MDEA-MEA used in the
present work was then limited at 30% wt. In addition, the CO2absorption experiments were done over ranges of temperatures
(298, 303, and 318K), CO2loadings at the absorber top (0.05, 0.17,
and 0.25mol CO2/mol amine), and liquid flow rates (2.8, 3.8, and
5m3/m2 h). Theexperimentswere performedin a laboratory-scale
absorption column with DX structured packing (27.5mm ID from
Sulzer Chemtech Canada, Inc.) at atmospheric pressure. The over-
allvolumetricmass transfer coefficients(KGav) atvarious operating
conditions were then used to establish thepredictive correlation.
The data obtained from the present workwill be very crucial to
further pilot plant-scale test at ITCs technology pilot plant (diam-
eter of 12 incheswith capture capacity of 1 ton of CO2/day). Itwas
mentioned by Idem et al. (2006,2011a) that only one molar ratio
of 1/4 of blended MDEA-MEA was previously tested in the pilot
plant so the results from the present work will be very useful to
(i) determine the scope of the pilot plants operating conditions
with blendedMDEA-MEAand(ii) expandthe pilotplant-scale test-
ing to test various blended ratios other than 1/4 molar ratio of
blended MDEA-MEA. In addition, the results obtained from this
study can also lead to improvement of CO2 absorption perfor-
mance of blended MDEA-MEA by addition of a third component
into the system. Up to the present, several promising amines
(e.g., AMP, PZ, 1-methyl-2-piperidineethanol, 4-diethylamino-2-
butanol)have beenintroducedfor capturingCO2 (Chowdhuryet al.,
2011; Goto et al., 2011; Sema et al., 2013). These solvents can
potentially enhance the performance of the well-known blended
MDEA-MEA. In order to effectively achieve this goal, a complete
understanding of blended MDEA-MEA over the range of relevant
operatingconditionsis required.Additionally,the thirdcomponent
canalso be ionic liquids, physical solvents, or even solid/liquidcat-
alyst that canreducethe heat requirement forsolvent regeneration
andimprovethe performanceofCO2removal as suggestedby Idem
et al. (2011b) and Shannon andBara (2012).
2. Determination of theoverall mass transfer coefficient
It hasbeen widelyunderstood that oneof theprimary concerns
in removing CO2 using absorption processes is the concentration
of CO2 in the gas phase. Also, it is easier and more convenient
to measure the concentration of CO2 in gas phase than in liq-
uid phase. Thus, for the CO2 absorption process, the gas phase
mass transfer coefficient is generally accepted to be more suit-
able than the liquid phase coefficient. However, it is very difficult
to measure the gasliquid interfacial area in an absorption col-
umn. Therefore, the mass transfer coefficient in a packed column
is generally represented by overall volumetric gas phase mass
transfer coefficient (KGav; kmol/m3 hkPa) instead of that based
on interfacial area (KG; kmol/m2 hkPa) (Astarita et al., 1983; Kohl
and Nielsen, 1997; Aroonwilas and Tontiwachwuthikul, 1998;
deMontigny et al., 2001; Aroonwilas and Veawab, 2004; Fu et al.,
2012). The overall volumetric gas phase mass transfer coefficient
KGavcan be calculated as follow:
KGav=
GI
P(yA,G yA)
dYA,GdZ
(1)
whereGIis inert gasflow rate (kmol/m2 h), Pis totalpressureof the
system (kPa), Zis height of the absorption column (m),yA,Gis mol
fractionof soluteA atbulkgas phase,yAis mole fractionof soluteA
at interface,YA,Gis the concentration (in terms ofmole ratio) in gas
phaseofsoluteA,which isCO2 inthepresentwork,and(dYA,G/dZ) is
thesolutemoleratio concentrationgradient,whichcanbeobtained
by taking a slope ofYA,Gversus Zplot.
3. Experimental
3.1. Chemicals
MDEAand MEA were purchased from Fisher Scientific, Canada,
with purities of99%. The premixed 15% CO2 (balanced with N2)
wassuppliedby Praxair Inc.,Canada.Allmaterialsin thisstudywere
used as receivedwithout further purification.Aqueous solutionsof
blendedMDEA-MEA of desired concentrationswere prepared by a
known amount of deionizedwater andpredetermined amounts of
MDEA and MEA.
3.2. CO2absorption in packed column
ThemasstransferinapackedcolumnoccurswhenCO2 inthegas
phasetransfersacrossthegasliquidinterface intothe liquidphase.
In thepresent work, themass transfer performancewasevaluated
in termsof theoverall volumetricmass transfercoefficientKGavandthe CO2 concentration profile along the height of the column. The
KGavcan be calculated using Eq. (1), in which several parameterscan be obtained from the experiment. Generally, the experiments
were done at atmospheric pressureP. The inert gas flow rateGIcan
bedetermined from thegas flowratemeasurementusingelectron-ics Aalborg GFM-17mass flow meter (ranging from 5 to 50L/min
with a 0.15%/C accuracy). The mole fraction of CO2 in the gas
phase yA,G can be determined from an infrared CO2 gas analyzer
(model 301D, Nova Analytical System Inc., Hamilton, ON,Canada),
which is capable of measuring CO2 concentration up to 20% with
0.5% accuracy. Themeasurement of CO2concentration was done
along the height of the column through the sampling ports, which
are connected to the CO2gas analyzer. The mole fraction of CO2at
interface (yA) canbe calculated using theHenrys law relationship.
TheHenrys lawconstant of blendedMDEA-MEA canbe calculated
bythecorrelationestablishedbyWangetal.(1992). Themoleratios
ofCO2 inthegasphase(YA,G) atvariousheights of thecolumncanbe
obtained from the CO2analyze, and the, plotted against theheight
of the column to get dYA,G/dZ.The glass laboratory absorption column (diameter of
27.5102m and height of 2.15m) was packed with 37 ele-
ments of stainless steel Sulzer DX structured packing (with
900m2/m3 packing surface area). The schematic diagram of the
experimental setup of CO2 absorption in the packed column is
presented in Fig. 1. The operational procedure of the absorption
columncan be found in our previousworks (Fu et al., 2012; Naami
et al., 2012). Inorder to validate the absorption columnused in the
present work, 2M MEA solution was tested and compared with
the results from deMontigny (2004). It was found that the results
obtained in the present work are in good agreement with those
obtained in deMontigny (2004) as shown inFig. 2.
4. Results and discussion
Themass transfer performance of CO2 absorption into aqueous
solutions of blended MDEA-MEA was experimentally determined
in the laboratory-scale absorption column packed with DX struc-
turedpacking andreportedin termsofKGavandCO2concentrationprofile along the height of the column. The values ofKGavare pro-
portional to the mass transfer performance in that the higher the
KGav, the higher mass transfer performance. For the CO2 concen-tration profile, a lower CO2 concentration profile indicates a larger
amountof CO2 thathasbeenremovedfromthegasstreamresulting
in highermass transfer performance.
The experiments were done at various MDEA-MEA con-
centrations of 27/3, 25/5, and 23/7%wt (which are equivalent
to MDEA-MEA molar ratios of 2.3/0.5, 2.1/0.8, and 1.95/1.16,
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6 A. Naami et al. / International Journal of Greenhouse Gas Control 19 (2013) 312
Fig. 1. Schematicdiagram of theexperimental setup of CO2 absorption in packed column.
respectively) over a range of temperatures (298, 303, and 318K),
CO2 loadings at the absorber top (0.05, 0.17, and 0.25molCO2/mol
amine), and liquid flow rates (2.8, 3.8, and 5m3/m2 h). A mass
balance error,which canbecalculated using Eq. (2), was taken into
consideration in order to confirm the validity of each absorption
experimental run. The mass balance error compares the amount
of CO2 removed from the gas phase (which can be measured via
an infrared CO2 gas analyzer) with that added into liquid phase
(which can be measured by titration with standard 1.0M HCl
until methyl orange end point as mentioned in Association of
Official Analytical Chemists (AOAC) methods by Horwitz (1975)).
Theoretically, these twovalues shouldbe equal.However, because
of (i) experimental errors from the CO2 gas analyzer and CO2
loading measurement apparatus and (ii) an elongated liquid trap
in the packed column, the average mass balance error obtained
from the present work was found to be 4%, which is considered
to be in an acceptable range of less than 10%. Therefore, it can
be inferred from this observation that the experimental results
obtained from the present work are correct and reliable.
Mass balance error=
absorbed CO2 removed CO2absorbed CO2
100% (2)The total experimental data points obtained from the present
work are 600 from30 experimental runs, which include 300 mea-
sured points of CO2 concentration and another 300 points of
temperature. From these data, the effects of MDEA-MEA blended
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A. Naami et al. / International Journal of Greenhouse Gas Control 19 (2013) 312 7
Fig. 2. Validation of packed column using 2.0M MEAat temperatureof 294K, CO2loading at absorber topof 0.2molCO2/molamine,liquidflowrate of5 m
3/m2, inert
gas flow rate of 17.85kmol/m2 h, and YCO2 of 0.09.
ratio, temperature, CO2 loading at the absorber top, and liquid
flow rate onmass transfer performance (in terms ofKGavand CO2concentration profile along the height of the column) were then
examined.
4.1. Effect ofMDEA-MEA blended ratio onmass transfer
performance
In order to examine the effect of MDEA-MEA blended ratio
on the mass transfer performance, three blended ratios of 27/3,
25/5, and 23/7%wt were tested in the packed column. The results
showed that the mass transfer performance in terms ofKGavandCO2 concentration profile increased as the weight ratio of MEA in
theblended solutions increasedas presented in Figs. 3 and 4. It can
bereasoned thatMEAhasfaster reactionkinetics ofCO2absorption
thanMDEA (Kohl and Nielsen, 1997; Semaet al., 2012). Thehigherthe ratio of MEA in the blended solutions, the higher amounts of
the more reactive MEA molecules that can absorb CO2; thus, a
highermass transfer performance was observed. Therefore, it can
be seen from Figs. 3 and 4 that 23/7%wt MDEA-MEA (molar ratio
of 1.95/1.16) provides the best mass transfer performance among
the three investigated ratios. However, by increasing the ratio of
MEA, theratio ofMDEA in theblended solutionswouldbereduced,
Fig. 3. Effect of MDEA-MEA blended ratio on CO2 concentration profile at temper-
ature of 294K, CO2 loading at absorber topof 0.25mol CO2/mol amine, liquidflow
rate of 5m3
/m2
, and inert gas flow rate of 15.99kmol/m2
h.
Fig. 4. Effect of MDEA-MEA blended ratio on KGav at temperature of 294K, CO2loading at absorber top of 0.25mol CO2/mol amine, liquid flow rate of 5m
3/m2,
inert gas flowrate of 15.99kmol/m2 h, and YCO2 of 0.09.
which can lead to (i) a reduction of CO2 absorption capacity and
(ii) an increment of heat requirement of solvent regeneration. This
is because MDEA has higher CO2absorption capacity and requires
much lower heat forsolvent regeneration(Kohl andNielsen, 1997;
Sakwattanapong et al., 2005; Lianget al., 2011). Itwasdiscussedby
Kohl and Nielsen (1997) that the energy requirement for solvent
regeneration contributes about 70% of the cost of capturing CO2.
Therefore, in order to maintain low heat requirement for solvent
regeneration andhighabsorption capacity characteristicsofMDEA
in theblended solutions,much higherblended ratiosofMDEA over
MEAwere selected in this work.
4.2. Effect of temperature onmass transfer performance
In the present work, the liquid inlet temperature was varied
from294 to 318K. It was found that the CO2concentration profile
became lower as temperature increased as presented in Fig. 5. The
mass transfer performance in terms ofKGavwas also found to cor-respond well with that of the CO2concentrationprofile in that the
KGavincreased as temperature increasedas shown in Fig. 6. There-fore, it can be inferred from Figs. 5 and 6 that the mass transfer
performance of the blended MDEA-MEA increases as temperature
increases over a temperature range of 294318K. This is because
the reaction kinetics of blended MDEA-MEA increase as tempera-
ture increases as discussed in the works ofMandal et al. (2001),
Liao and Li (2002), and Edali et al. (2009).
Fig.5. Effect of temperatureon CO2 concentrationprofileof 23/7%wtMDEA-MEAat
CO2loading at absorbertopof 0.04mol CO2/mol amine, liquidflow rateof 5m3/m2,
and inert gas flowrate of 15.99kmol/m3
h.
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8 A. Naami et al. / International Journal of Greenhouse Gas Control 19 (2013) 312
Fig. 6. Effect of temperature on KGav of blended MDEA-MEA at blended ratios of
27/3, 25/5, and 23/7%wt, CO2 loading at absorber top of 0.04mol CO2/mol amine,
liquidflow rateof 5m3/m2, inert gasflow rate of 15.99kmol/m2 h,andYCO2 of 0.09.
4.3. Effect of CO2loading on mass transfer performance
For the effectof CO2loadingat theabsorbertoponmass transferperformanceofblendedMDEA-MEA,it canbe found inFigs.7and8
that the mass transfer performance (in terms of CO2 concentra-
tion profileandKGav) is significantlyaffected byCO2loading at the
Fig.7. Effectof CO2loadingatabsorber topon CO2 concentrationprofileof 23/7%wt
MDEA-MEA at temperature of 294K, liquidflow rate of 5m3/m2, and inert gas flow
rate of 18.65kmol/m2 h.
Fig. 8. Effect of CO2 loading at absorber top on KGav of blended MDEA-MEA at
blended ratios of 27/3, 25/5, and 23/7%wt, temperature of 294K, liquid flow rate
of5m
3
/m
2
, inert gas flowrate of 18.65kmol/m
2
h, and YCO2 of 0.09.
Fig.9. Effectof liquidflowrateonCO2concentrationprofile of 23/7%wtMDEA-MEA
at temperatureof 294K, CO2loading at absorbertop of 0.2molCO2/molamine, and
inert gas flowrate of 18.65kmol/m2 h.
Fig. 10. Effect of liquid flow rate on KGavof blended MDEA-MEA at blended ratiosof27/3, 25/5,and 23/7%wt, at temperatureof 294K, CO2 loading at absorber topof
0.2mol CO2/mol amine, inert gasflow rate of 18.65kmol/m2 h, and YCO2 of 0.09.
absorber top in that the mass transfer performance decreased as
CO2 loading increased over a CO2 loading range of 0.050.25mol
CO2/mol amine. This is because the amounts of active free amines
(MEA and MDEA) decreases as CO2loading increases.
4.4. Effect of liquid flow rate onmass transfer performance
It can be seen from Fig. 9 that the CO2 concentration profile
became lower as liquid flow rate increased over the range of
2.85.0m3/m2 h. This observation correspondswell with themasstransfer performance in terms ofKGav. As presented in Fig. 10,the KGav increased as liquid flow rate increased. Regard to the
results observed in the present work (from Figs. 9 and 10), it
can be inferred that the liquid flow rate has a very significant
effect on the mass transfer performance for CO2 absorption into
aqueous solutionsof blendedMDEA-MEA in that themass transfer
performance increases as liquidflow rate increases over the liquid
flow rate range of 2.85.0m3/m2 h. Thus, it can be reasoned that
increase of liquid flow rate results in an increase of liquid side
mass transfer coefficient (kL), which significantly increases the
KGavin the case of liquid-phase controlled mass transfer (Astaritaet al., 1983). Additionally, by increasing the liquid flow rate, the
gasliquidsurfaceareaisgreatly increased,resultinginmoreliquid
spreading on the packing surface. The increase of liquid flow rate
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A. Naami et al. / International Journal of Greenhouse Gas Control 19 (2013) 312 9
not only increases the mass transfer performance in the packed
column, but also leads to higher circulation and regeneration
costs; thus, it might not improve the overall system efficiency. In
addition, the range of liquid flow rate used in the present work
(2.85.0m3/m2 h) is in the effective range of using DX structured
packing, which is 0.15.0m3/m2 h, as provided by the packing
supplier (Sulzer Chemtech Canada, Inc.). The use of a liquid flow
rate higher than 5.0m3/m2 h can lead to a decline ofmass transfer
performance.
4.5. Effect ofMDEA-MEA blending ratio on cyclic capacity
Astarita et al. (1983) and Maneeintr et al. (2009) described the
cyclic capacity concept in which cyclic capacity is defined as the
differenceof molesof CO2absorbedin thesolutionper unit volume
of solution in theabsorption step andthat in theregenerationstep,
which is, then, crucial to the CO2 absorption process. With the
same volume of solvent, the solvent with higher cyclic capacity
can carry a higher amount of absorbed CO2 than that with lower
cyclic capacity. Therefore, it can be inferred from this concept that
for a solvent that has high cyclic capacity, a low liquid circulation
rate can be applied, which results in (i) a lower operation cost
for pumping liquid and (ii) a smaller volume of solvent can beused in the packed column. In order to investigate the effect of
MDEA-MEA blended ratio on the cyclic capacity, three blended
ratios of 27/3, 25/5, and 23/7%wt were tested in the present work.
The experimental and calculation procedures for cyclic capacity
can be found in our previous work (Maneeintr et al., 2009). The
results showed that the cyclic capacity of the blended MDEA-MEA
increased as the ratio of MDEA in the blended solution increased
and can be ranked as: 27/3%wt MDEA-MEA> 25/5%wt MDEA-
MEA>23/7%wt MDEA-MEA, as presented in Fig. 11. Based on this
observation, this result occurred because MDEA has higher cyclic
capacity than MEA (Chowdhury et al., 2011); thus, by increasing
the ratio of MDEA (in another words, decreasing the ratio of MEA)
in theblended solution, the cyclic capacity of the blended solution
was then found to be increased.
Fig. 11. Effectof MDEA-MEA blended ratio on cyclic capacity.
4.6. Relative solvent regeneration ability
One of the key points for the CO2 absorption process usingchemical solvent is the ability of solvent to be generated. It has
beengenerallyacceptedthatgood/promisingsolventsfor thistech-
nology should be easy to regenerate; in other words, they should
require lowheatfor solvent regeneration.Thisis becausethemajor-
ity (about 70%) of the cost of CO2 capture comes from the solvent
regeneration unit.
In the present work, the relative solvent regeneration ability
of the blended MDEA-MEA was determined and compared with
those of 2M MDEA and 5M MEA. The experiment was conducted
at 80 C in a temperature controlled bath (Cole-Parmer; the tem-
perature can range from -20 to 200 C with 0.01 C accuracy).
3000ml of the solutions, which include 27/3%wt (or 2.3/0.5 molar
ratio) MDEA-MEA, 2M MDEA, and 5M MEA, at the same initial
CO2 loading of 0.5 were introduced into round-bottom flasks
Fig. 12. Experimental set up for determining relative solvent regenerationability.
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10 A. Naami et al. / International Journal of Greenhouse Gas Control 19 (2013) 312
Fig. 13. Relative solvent regeneration ability of 5M MEA, 2M MDEA, and2.3/0.5M
(27/3%wt) MDEA-MEA.
connected with condensers as shown in Fig. 12. Within the same
operating conditions, the CO2loadings of the three testedsolvents
were then measured as the experiment proceeded by titrating
with standard 1.0M HCl using methyl orange as an indicator.
The results obtained from this experiment suggest thesolvents
relative regeneration ability in that the CO2 loading of a solu-
tion that has high relative solvent regeneration ability (in other
words, one that is relatively easy to regenerate) would decrease
more rapidly than one that has lower relative solvent regeneration
ability. Itwas found that the CO2 loadings of the three testing solu-
tions decreased as the operational time increased as presented in
Fig. 13. Even though the results observed from Fig. 13 can gener-
ally beunderstood as showing that therelativeregenerationability
of blended MDEA-MEAwould fall in between those of MDEA and
MEA, the results obtained from the present work clearly show
how close the relative regeneration ability of 27/3%wt (or 2.3/0.5
molar ratio) blended MDEA-MEA is to that of 2M MDEA. This newobservation is considered to be very useful in order to (i) use the
blended MDEA-MEAmore effectively in the pilot plant-scale test-
ing and (ii) enhance the performance of the blended MDEA-MEA
by addition of the third component (e.g., amine or additive). In
addition, at the end of the experiment, the total amine concentra-
tions of the three testing solutions were determined (by titration
with standard 1.0M HCl (Horwitz, 1975)) and compared with the
initial concentrations. It was found that the total amine concen-
trations were very close to the initial concentrations. Thus, it can
be said that the experimental set up as shown in Fig. 12 is effec-
tive. More importantly, the results obtained from this study are
reliable.
In the present work, the mass transfer performance of CO2
absorption intoaqueoussolutionsof blendedMDEA-MEAwas com-prehensively investigated in a DX structured packed column at
various operating conditions of MDEA-MEA blend ratio, temper-
ature, CO2 loading at the absorber top, and liquid flow rate. The
mass transfer performancewasevaluated in termsofKGavandCO2concentration profile along the height of the column. It was found
that themass transfer performanceincreasedas theratioof MEAin
the blended solution, temperature, and liquid flow rate increased
butdecreased asCO2loading increased(aspresentedin Figs.310)
over the testing conditions. In addition, the mass transfer perfor-
mance (in terms of both KGav and CO2 concentration profile) of
blended MDEA-MEA increased as the ratio of MEA in the blended
solution increasedas shown inFigs. 3,4, 6,8 and 10. This is because
MEAhasfasterreactionkineticsofCO2 absorption thanMDEA(Kohl
andNielsen,1997;Semaetal., 2012). The higherthe ratio ofMEA in
theblended solutions, thehigher theamounts of themore reactive
MEA molecules that can absorb CO2; thus, a higher mass transfer
performance was observed. By comparing the three MDEA-MEA
blendratios(27/3, 25/5, and23/7%wtMDEA-MEA), itwasobserved
that the23/7%wtMDEA-MEA provided the best mass transfer per-
formance among the three as shown in Figs. 3, 4, 6, 8 and 10.
However, in order to effectively use blended MDEA-MEA, not only
mass transfer performance,but also thecycliccapacityandrelative
regeneration ability shouldbe taken into consideration.
In summary, by increasing theratio ofMEAin theblended solu-
tion, the mass transfer performance would be increased but the
cyclic capacity and the relative solvent regeneration ability would
be decreased. As shown in Figs. 3, 4, 6, 8 and10, it can be observed
that themass transfer of 23/7%wtMDEA-MEA ismuchhigher than
those of 25/5 and 27/3%wt MDEA-MEA, respectively. On the other
hand, the cyclic capacities of the three blend ratios are consider-
ably close to each other as presented in Fig. 11. Therefore, it can be
reported that the effect of ratio ofMEA in the blendedMDEA-MEA
solutions on mass transfer performance is more significant than
that on cyclic capacity. As a result, based on above discussion, it
is reasonable to conclude that the 23/7%wt MDEA-MEA provides
the best CO2 absorption performance among the three blend-
ing ratios over the operating conditions conducted in the present
work.
4.7. Empirical predictive correlation for KGavof blended
MDEA-MEA
Liang et al. (2011) mentioned that in order to effectively
design an absorption column, the KGav is required. However, itis an expensive and time consuming process to experimentally
determine the KGav in a packed column. Thus, the predictive cor-relation for KGav i s then found to be important since it can be
used to determine the KGav from operating conditions withoutexperimental work. Dey and Aroonwilas (2009) proposed a pre-
dictive correlation for blended AMP-MEA. The model was tested
with the experimental mass transfer data of blended AMP-MEA
in a laboratory-scale absorption column packed with DX struc-
tural packing. They concluded that the predictive results were
found to be in good agreement with the experimental results.
For the MDEA-MEA system, the predictive correlation can be seen
in Eq. (3).
KGav= K eA(MDEA/MEA)
eB eCxCO2 LD1 eECs eF/T (3)
where KGav is the overall volumetric mass transfer coefficient(kmol/m3 hkPa), (MDEA/MEA) is themolar ratioofMDEAandMEA,
xCO2 is the mole fraction of CO2, is theCO2loading (mol CO2/molamine), Csis theconcentration of amine in thesolution (kmol/m
3),
L1 is the liquid flow rate (m3/m2 h), Tis the absolute temperature
(K), and K, A, B, C, D, E, and Fare the coefficients for the respective
parameters modeled in theproposed equation.Theexperimental values ofKGavobtainedin thepresentwork at
various operating conditions of MDEA-MEA blending ratios (27/3,
25/5, and23/7%wt), temperatures (298, 303, and318K), CO2load-
ings at the absorber top (0.05, 0.17, and 0.25mol CO2/mol amine),
and liquid flow rates (2.8, 3.8, and 5m3/m2 h) were then used to
correlate the predictive correlation in Eq. (3) using the nonlinear
regression analysis package NLREG (with a minimum confidence
level of 97%). The coefficients (K, A, B, C, D, E, and F) obtained
from the NLREG program are presented in Table 1. By compar-
ing the predicted and experimental results in the parity chart,
as shown in Fig. 14, it can be seen that the predicted values of
KGav calculated from Eq. (3) are in fairly good agreement withthe experimental valueswith an absolute average deviation (AAD)
of 21.8%.
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A. Naami et al. / International Journal of Greenhouse Gas Control 19 (2013) 312 11
Table 1
Summary of parameters for predictive equationpresented in Eq. (3).
Concentrationof
blended
MDEA-MEA (%wt)
Confidential level A B C D E F K AAD
27/3 0.97 0.0032 4 10.325 0.8531 0.1438 595.211 0.9254 20.9%
25/5 0.97 0 1.1 14.3 0.559 0.3665 511.5 1 21.7%
23/7 0.99 0.04 3.65 11.4 0.91 0.2 443 1 22.8%
Fig. 14. Parity chart compares predicted and experimental values ofKGav for CO2absorption intoblendedMDEA-MEA solutions.
5. Conclusions
Themass transfer performance of CO2 absorption into aqueous
solutions of blended MDEA-MEA was experimentally determined
(in terms ofKGavand CO2 concentration profile) in a laboratory-scale absorption column packed with DX structured packing. The
experiments were conducted at various operating conditions ofMDEA-MEA blending ratios (27/3, 25/5, and 23/7%wt), temper-
atures (298, 303, and 318K), CO2 loadings at the absorber top
(0.05, 0.17, and 0.25mol CO2/mol amine), and liquid flow rates
(2.8, 3.8, and 5m3/m2 h). The results show that the mass transfer
performance increasesas ratio ofMEAintheblendedsolution, tem-
perature,and liquidflow rate increasebutdecreases asCO2loading
increaseswithin the range of conditions using in thepresent work.
On the other hand, the cyclic capacity and the relative solvent
regeneration ability decrease as the ratio of MEA in the blended
solution increases. However, after taking into consideration all
three parameters (i.e., mass transfer performance, cyclic capac-
ity, and relative solvent regeneration ability), it can be concluded
that 23/7%wtMDEA-MEAprovides thebest CO2absorption perfor-
mance among the three blend ratiosover the operating conditionsconducted in thepresent work.
Acknowledgments
The first author (A. Naami) would like to acknowledge and
appreciate the scholarship support from the Libyan Higher Edu-
cational studies through thecultural section of theLibyanEmbassy
inOttawa, Canada. Thefinancial support from theNationalNatural
Science Foundation of China (NSFC No. 21276068, 21250110514,
and 21376067), Ministry of Science and Technology of the Peo-
ples of Republic of China (MOST No. 2012BAC26B01), Ministry
of Education of the Peoples of Republic of China-Supported Pro-
gram for Innovative Research Team in University (No. IRT1238),
Shaanxi Yanchang Petroleum (Group) Co., LTD, Chinas State
Project985inHunanUniversityNovelTechnologyResearchand
Development for CO2 Capture as well as Hunan University to
the Joint International Center for CO2 Capture and Storage (iCCS)
is gratefully acknowledged. In addition, we would also like to
acknowledge the research supports over the past many years of
the Industrial Research Consortium Future Cap Phase II of the
International Test Center for CO2Capture (ITC) at theUniversity of
Regina. We would also like to acknowledge the research support
from the following organizations: Natural Sciences and Engineer-
ing Research Council of Canada (NSERC), Canada Foundation for
Innovation (CFI), Saskatchewan Ministry of Energy & Resources,
Western Economic Diversification, Saskatchewan Power Corpo-
ration, Alberta Energy Research Institute (AERI), and Research
Institute of Innovative and Technology for the Earth (RITE).
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