Ind. Eng. Chem. Res. XXXX, xxx, 000
A
Thermodynamic Modeling for CO2 Absorption in Aqueous MDEA Solution with
Electrolyte NRTL Model
Ying Zhang
AspenTech, Limited, Pudong, Shanghai 201203, People’s Republic of China
Chau-Chyun Chen*
Aspen Technology, Inc., Burlington, Massachusetts 01803
Accurate modeling of thermodynamic properties for CO2 absorption in aqueous alkanolamine solutions is
essential for the simulation and design of such CO2 capture processes. In this study, we use the Electrolyte
Nonrandom Two-Liquid activity coefficient model to develop a rigorous and thermodynamically consistent
representation for the MDEA-H2O-CO2 system. The vapor-liquid equilibrium (VLE), heat capacity, and
excess enthalpy data for the binary aqueous amine system are used to determine the NRTL interaction
parameters for the MDEA-H2O binary. The VLE, heat of absorption, heat capacity, and NMR spectroscopic
data for the MDEA-H2O-CO2 ternary system are used to identify the NRTL interaction parameters for the
molecule-electrolyte binaries and the previously unavailable standard-state properties of the amine ion, MDEA
protonate. The calculated VLE, heat of absorption, heat capacity, and the species concentrations for the
MDEA-H2O-CO2 system are compared favorably to experimental data.
1. Introduction
CO2 capture by absorption with aqueous alkanolamines is
considered an important technology to reduce CO2 emissions
from fossil-fuel-fired power plants and to help alleviate global
climate change.1 Methyldiethanolamine (MDEA), which is an
alternative to monoethanolamine (MEA) for bulk CO2 removal,
has the advantage of relatively low heat of reaction of CO2 with
MDEA.2 To properly simulate and design the absorption/
stripping processes with MDEA-based aqueous solvents, it is
essential to develop a sound process understanding of the
transfer phenomena3 and accurate thermodynamic models4 to
calculate the driving forces for heat and mass transfer. In other
words, scalable simulation, design, and optimization of the CO2
capture processes start with modeling of the thermodynamic
properties, specifically vapor-liquid equilibrium (VLE) and
chemical reaction equilibrium, as well as calorimetric properties.
Accurate modeling of thermodynamic properties requires
availability of reliable experimental data. Earlier literature
reviews5,6 suggested that, while there are extensive sets of
experimental data available for the MDEA system, some of the
published CO2 solubility data for the aqueous MDEA system
may be questionable. The use of a thermodynamically consistent
framework makes it possible to correlate available experimental
data, to screen out questionable data, and to morph these diverse
and disparate data into a useful and thermodynamically consistent form for process modeling and simulation.
Excess Gibbs energy-based activity coefficient models provide a practical and rigorous thermodynamic framework to
model thermodynamic properties of aqueous electrolyte systems,
including aqueous alkanolamine systems for CO2 capture.4,7 For
example, Austgen et al.8 and Posey9 applied the electrolyte
NRTL model10-12 to correlate CO2 solubility in aqueous MDEA
solution and other aqueous alkanolamines. Kuranov et al.,5
Kamps et al.,6 and Ermatchkov et al.13 used Pitzer’s equation14
to correlate the VLE data of the MDEA-H2O-CO2 system.
* To whom correspondence should be addressed. Tel.: 781-221-6420.
Fax: 781-221-6410. E-mail: chauchyun.chen@aspentech.com.
10.1021/ie1006855
Arcis et al.15 also fitted the VLE data with Pitzer’s equation
and used the thermodynamic model to estimate the enthalpy of
solution of CO2 in aqueous MDEA. Faramarzi et al.16 used the
extended UNIQUAC model17 to represent VLE for CO2
absorption in aqueous MDEA, MEA, and mixtures of the two
alkanolamines. Furthermore, they predicted the concentrations
of the species in both MDEA and MEA solutions containing
CO2 and in the case of MEA, compared to NMR spectroscopic
measurements.18,19
In the present work, we expand the scope of the work of
Austgen et al.8 and Posey9 to cover all thermodynamic properties. We use the 2009 version10 of the electrolyte NRTL
model10-12 as the thermodynamic framework to correlate
recently available experimental data for CO2 absorption in
aqueous MDEA solution. Much new data for thermodynamic
properties and calorimetric properties have become available
in recent years, and they cover wider ranges of temperature,
pressure, MDEA concentration, and CO2 loading. The binary
NRTL parameters for MDEA-water binary are regressed from
the binary VLE, excess enthalpy, and heat capacity data. The
binary NRTL parameters for water-electrolyte pairs and
MDEA-electrolyte pairs and the standard-state properties of
protonated MDEA ion are obtained by fitting to the ternary VLE,
heat of absorption, heat capacity, and NMR spectroscopic data.
This expanded model should provide a comprehensive thermodynamic representation for the MDEA-H2O-CO2 system over
a broader range of conditions and give more reliable predictions
over previous works.
In conjunction with the use of the electrolyte NRTL model
for the liquid-phase activity coefficients, we use the PC-SAFT20,21
equation of state (EOS) for the vapor-phase fugacity coefficients.
While both PC-SAFT EOS and typical cubic EOS would give
reliable fugacity calculations at low to medium pressures, we
choose PC-SAFT for its ability to model vapor-phase fugacity
coefficients at high pressures, which is an important consideration for modeling CO2 compression. The PC-SAFT parameters
used in this model are given in Table 1. The parameters for
water and CO2 are taken from the literature21 and the Aspen
 XXXX American Chemical Society
B
Ind. Eng. Chem. Res., Vol. xxx, No. xx, XXXX
Table 1. Parameters for PC-SAFT Equation of State
MDEA
Table 2. Parameters of the Characteristic Volume for the
Brelvi-O’Connell Modela
H2O
CO2
Aspen
Databank22
2.5692
source
this work
segment number
parameter, m
segment energy
parameter, ε
segment size
parameter, σ
association energy
parameter, εAB
AB
k
3.3044
Gross and
Sadowski 21
1.0656
237.44 K
366.51 K
152.10 K
3.5975 Å
3.0007 Å
2.5637 Å
3709.9 K
2500.7 K
0K
0.066454 Å3
0.034868 Å3
0 Å3
2. Thermodynamic Framework
2.1. Chemical and Phase Equilibrium. CO2 solubility in
aqueous amine solutions is determined by both its physical
solubility and the chemical equilibrium for the aqueous phase
reactions among CO2, water, and amines.
2.1.1. Physical Solubility. Physical solubility is the equilibrium between gaseous CO2 molecules and CO2 molecules in
the aqueous amine solutions:
(1)
It can be expressed by Henry’s law:
PyCO2φCO2 ) HCO2xCO2γ*CO2
(2)
where P is the system pressure, yCO2 the mole fraction of CO2
in the vapor phase, φCO2 the CO2 fugacity coefficient in the vapor
phase, HCO2 the Henry’s law constant of CO2 in the mixed
solvent of water and amine, xCO2 the equilibrium CO2 mole
fraction in the liquid phase, and γ*CO2 the unsymmetric activity
coefficient of CO2 in the mixed solvent of water and amine.
The Henry’s constant in the mixed solvent can be calculated
from those in the pure solvents:23
()
ln
Hi
γ∞i
)
∑x
A
ln
A
( )
HiA
∞
γiA
(3)
where Hi is the Henry’s constant of supercritical component i
in the mixed solvent, HiA the Henry’s constant of supercritical
component i in pure solvent A, γi∞ the infinite dilution activity
coefficient of supercritical component i in the mixed solvent,
∞
γiA
the infinite dilution activity coefficient of supercritical
component i in pure solvent A, and xA the mole fraction of
solvent A.
We use wA in lieu of xA in eq 3 to weigh the contributions
from different solvents.22 The parameter wA is calculated using
eq 4:
wA )
∞ 2/3
xA(ViA
)
∑ x (V
B
∞ 2/3
iB)
(4)
B
∞
parameter
source
V1,i
V2,i
MDEA
this work
0.369b
0
CO2
H2O
Brelvi and O’Connell
0.0464
0
24
Yan and Chen25
0.175
-3.38 × 10-4
a
The Brelvi-O’Connell model has been described in ref 24. The
correlation of the characteristic volume for the Brelvi-O’Connell model
BO
(Vi ) is given as follows: ViBO ) V1,i + V2,iT, where T is the
temperature (given in Kelvin). b Here, the critical volume was used as
the characteristic volume for MDEA.
Databank.22 The parameters for MDEA are obtained from the
regression of experimental data on vapor pressure, liquid density,
and liquid heat capacity.
CO2(V) T CO2(l)
Characteristic Volume (m3/kmol)
Here, ViA represents the partial molar volume of supercritical
∞
component i at infinite dilution in pure solvent A. ViA
is
24
calculated from the Brelvi-O’Connell model with the charBO
acteristic volume for the solute (VBO
CO2) and solvent (Vs ), which
are listed in Table 2.
Table 3. Parameters for Henry’s Constant (Expressed in Units of Pa)a
solute i
solvent j
source
aij
bij
cij
dij
CO2
H2O
Yan and Chen25
91.344
-5876.0
-8.598
-0.012
CO2
MDEA
this work
19.8933
-1072.7
0.0
0.0
a
The correlation for Henry’s constant is given as follows: ln Hij ) aij
+ bij/T + cij ln T + dijT, where T is the temperature (given in Kelvin).
Henry’s law constants for CO2 with water and for CO2 with
MDEA are required. The former has been extensively studied,25
although knowledge about the latter is relatively limited.
Because it is not feasible to directly measure CO2 physical
solubility in pure amines, because of the reactions between them,
the common practice is to derive the CO2 physical solubility in
amines from that of N2O for their analogy in molecular structure
and, thus, in physical solubility as believed:26
HCO2,MDEA
HN2O,MDEA
)
HCO2,water
(5)
HN2O,water
In 1992, Wang et al.27 reported the solubility of N2O in pure
MDEA solvent as follows:
-1312.7
HN2O,MDEA (kPa m3 kmol-1) ) (1.524 × 105) exp
T
(6)
(
)
Based on the work of Versteef and van Swaaij,28 we obtained
the following two equations for the solubilities of N2O and CO2
in water:
-2284
HN2O,water (kPa m3 kmol-1) ) (8.5470 × 106) exp
T
(
)
(7)
-2044
HCO2,water (kPa m3 kmol-1) ) (2.8249 × 106) exp
T
(
)
(8)
We use eqs 5-8 to determine HCO2,MDEA and the parameters
are summarized in Table 3.
The Henry’s constant of CO2 in pure solvent A is corrected
with the Poynting term for pressure:25
( RT1 ∫
HCO2,A(T, P) ) HCO2,A(T, po,l
A ) exp
P
p◦,l
A
)
∞
VCO
dp
2,A
(9)
where HCO2,A(T,P) is the Henry’s constant of CO2 in pure solvent
A at system temperature and pressure, HCO2,A(T,p°,l
A ) the Henry’s
constant of CO2 in pure solvent A at system temperature and
∞
the solvent vapor pressure, and VCO
2,A the partial molar volume
Ind. Eng. Chem. Res., Vol. xxx, No. xx, XXXX
C
Table 4. Parameters for the Reference States Properties
property
∆fGig
298.15 (J/kmol)
ig
∆fH298.15
(J/kmol)
MDEA
H2O
CO2
H3O+
OHHCO3CO23
MDEAH+
-1.6900 × 10
-2.2877 × 108
-3.9437 × 108
-3.8000 × 10
-2.4200 × 108
-3.9351 × 108
8
∆fG∞,aq
298.15 (J/kmol)
∞,aq
∆fH298.15
(J/kmol)
source
-2.8583 × 108
-2.2999 × 108
-6.9199 × 108
-6.7714 × 108
-5.1422 × 108 a
Aspen Databank22
Aspen Databank22
Aspen Databank22
Aspen Databank22
Wagman et al.29
Wagman et al.29
Wagman et al.29
this work
8
-2.3713 × 108
-1.5724 × 108
-5.8677 × 108
-5.2781 × 108
-2.5989 × 108 a
a
The values of MDEAH+ are calculated from the chemical equilibrium constant in Kamps and Maurer,30 which are used as the initial guess to fit
experimental data.
of CO2 at infinite dilution in pure solvent A calculated from
the Brelvi-O’Connell model.
At low pressures, the Poynting correction is almost unity and
can be ignored.
2.1.2. Aqueous-Phase Chemical Equilibrium. The aqueousphase chemical reactions involved in the MDEA-water-CO2
system can be expressed as
2H2O T H3O+ + OH-
(10)
CO2 + 2H2O T H3O+ + HCO3
(11)
2+
HCO3 + H2O T H3O + CO3
(12)
MDEAH+ + H2O T H3O+ + MDEA
(13)
We calculate the equilibrium constants of the reaction from
the reference-state Gibbs free energies of the participating
components:
-RT ln Kj ) ∆Goj (T)
(14)
where Kj is the equilibrium constant of reaction j, ∆G°j (T) the
reference-state Gibbs free energy change for reaction j at
temperature T, R the universal gas constant, and T the system
temperature.
For the aqueous phase reactions, the reference states chosen
are pure liquid for the solvents (water and MDEA), and aqueous
phase infinite dilution for the solutes (ionic and molecular).
The Gibbs free energy of solvents is calculated from that of
ideal gas and the departure function:
Gs(T) ) Gsig(T) + ∆Gsigfl(T)
(15)
where Gs(T) is the Gibbs free energy of solvent s at temperature
T, Gsig(T) the ideal gas Gibbs free energy of solvent s at
temperature T, and ∆Gsigfl(T) the Gibbs free energy departure
from ideal gas to liquid at temperature T.
The Gibbs free energy of an ideal gas is calculated from the
Gibbs free energy of formation of an ideal gas at 298.15 K, the
enthalpy of formation of an ideal gas at 298.15 K, and the ideal
gas heat capacity.
ig
Gsig(T) ) ∆fHs,298.15
+
(
∫
T
298.15
ig
Cp,s
dT - T ×
ig
ig
∆fHs,298.15
- ∆fGs,298.15
+
298.15
)
ig
Cp,s
dT
298.15 T
∫
T
(16)
where Gsig(T) is the ideal gas Gibbs free energy of solvent s at
ig
temperature T, ∆fGs,298.15
the ideal gas Gibbs free energy of
ig
formation of solvent s at 298.15 K, ∆fHs,298.15
the ideal gas
ig
enthalpy of formation of solvent s at 298.15 K, and Cp,s
the
ideal gas heat capacity of solvent s.
Table 5. Parameters for Ideal Gas Heat Capacity
Heat Capacity (J/(kmol K))
parameter
source
C1i
C2i
C3i
C4i
C5i
C6i
C7i
C8i
MDEA
this work
2.7303 × 104
5.4087 × 102
0
0
0
0
278
397
CO2
H2O
22
Aspen Databank
3.3738 × 104
-7.0176
2.7296 × 10-2
-1.6647 × 10-5
4.2976 × 10-9
-4.1696 × 10-13
200
3000
Aspen Databank22
1.9795 × 104
7.3437 × 10
-5.6019 × 10-2
1.7153 × 10-5
0
0
300
1088.6
a
The correlation for the ideal gas heat capacity is given as follows:
2
3
4
5
Cig
p ) C1i + C2iT + C3iT + C4iT + C5iT + C6iT , C7i < T < C8i,
where T is the temperature (given in Kelvin).
ig
ig
The reference-state properties, ∆fGs,298.15
and ∆fHs,298.15
, are
shown in Table 4. The ideal gas heat capacities are shown in
Table 5. For water, the Gibbs free energy departure function is
obtained from the ASME steam tables. For MDEA, the
departure function is calculated from the PC-SAFT equation
of state.
For molecular solute CO2, the Gibbs free energy in aqueous
phase infinite dilution is calculated from Henry’s law:
G∞,aq
(T) ) ∆fGig
i
i (T) + RT ln
( )
Hi,w
Pref
(17)
where G∞,aq
(T) is the mole fraction scale aqueous-phase infinite
i
dilution Gibbs free energy of solute i at temperature T, ∆fGig
i (T)
the ideal gas Gibbs free energy of formation of solute i at
temperature T, Hi,w the Henry’s constant of solute i in water,
and Pref the reference pressure.
For ionic species, the Gibbs free energy in aqueous-phase
infinite dilution is calculated from the Gibbs free energy of
formation in aqueous-phase infinite dilution at 298.15 K, the
enthalpy of formation in aqueous-phase infinite dilution at
298.15 K, and the heat capacity in aqueous-phase infinite
dilution:
G∞,aq
(T) ) ∆fH∞,aq
i
i,298.15 +
(
∫
T
298.15
∞,aq
∆fH∞,aq
i,298.15 - ∆fGi,298.15
+
298.15
C∞,aq
p,i dT - T ×
)
( )
C∞,aq
p,i
1000
dT + RT ln
298.15 T
Mw
(18)
∫
T
Here, Gi∞,aq(T) is the mole fraction scale aqueous-phase infinite
dilution Gibbs free energy of solute i at temperature T, ∆fG∞,aq
i,298.15
the molality scale aqueous-phase infinite dilution Gibbs free
energy of formation of solute i at 298.15 K, ∆fH∞,aq
i,298.15 the
aqueous phase infinite dilution enthalpy of formation of solute
i at 298.15 K, and C∞,aq
p,i the aqueous-phase infinite dilution heat
capacity of solute i. The term RT ln (1000/Mw) is added because
D
Ind. Eng. Chem. Res., Vol. xxx, No. xx, XXXX
Table 6. Parameters for Aqueous-Phase Infinite Dilution Heat Capacitya
Heat Capacity (J/(kmol K))
parameter
H3O+
OH-
HCO3
CO23
MDEAH+
source
C1
Aspen Databank22
7.5291 × 104
Aspen Databank22
-1.4845 × 105
Criss and Cobble31
-2.9260 × 104 b
Criss and Cobble31
-3.9710 × 105 b
this work
2.9900 × 105 b
∞,aq
∞,aq
The aqueous-phase infinite dilution heat capacity is assumed to be constant (Cp,i
) C1). b The Cp,i
value of MDEAH+ is calculated from the
30
∞,aq
chemical equilibrium constant in Kamps and Maurer, which is used as the initial guess to fit experimental data. The Cp,i
values of HCO3- and CO23
are the average values of heat capacity between 298 K and 473 K (taken from Criss and Cobble31).
a
∞,aq
∆fGi,298.15
, as reported in the literature, is based on molality
concentration scale while G∞,aq
is based on mole fraction scale.
i
∞,aq
∞,aq
The standard-state properties ∆fG∞,aq
i,298.15, ∆fHi,298.15, and Cp,i
are available in the literature for most ionic species, except those
of MDEAH+. (See Tables 4 and 6.) We calculate the referencestate properties of the MDEAH+ ion from the experimental
equilibrium constant of eq 13, as reported in 1996 by Kamps
∞,aq
and Maurer.30 The calculated ∆fG∞,aq
i,298.15 and ∆fHi,298.15 values
∞,aq
are given in Table 4, and the calculated Cp,i values are given
in Table 6. As will be shown later, we use these calculated
reference-state properties for MDEAH+ as part of the adjustable
parameters in the fitting experimental data of thermodynamic
properties, including VLE, heat of solution, heat capacity, and
species concentration from NMR spectra. Also given in Table
26 are estimated values of C∞,aq
p,i for HCO3 and CO3 . They have
been taken from the 1964 work of Criss and Cobble.31
2.2. Heat of Absorption and Heat Capacity. The CO2 heat
of absorption in aqueous MDEA solutions can be derived from
an enthalpy balance of the absorption process:
∆Habs )
l
l
g
- nInitialHInitial
- nCO2HCO
nFinalHFinal
2
nCO2
(19)
l
where ∆Habs is the heat of absorption per mole of CO2, HFinal
the molar enthalpy of the final solution, HlInitial the molar enthalpy
g
of the initial solution, HCO2 the molar enthalpy of gaseous CO2
absorbed, nFinal the number of moles of the final solution, nInitial
the number of moles of the initial solution, and nCO2 the number
of moles of CO2 absorbed.
There are two types of heat of absorption: integral heat of
absorption and differential heat of absorption. The integral heat
of absorption for a certain amine-H2O-CO2 system refers to
the heat effect per mole of CO2 during the CO2 loading of the
amine solution increasing from zero to the final CO2 loading
value of that amine-H2O-CO2 system. The differential heat
of absorption for an amine-H2O-CO2 system refers to the heat
effect per mole of CO2 if a very small amount of CO2 is added
into this amine-H2O-CO2 system.
For calculation of both types of heat of absorption, enthalpy
calculations for the initial and final amine-H2O-CO2 systems
and for gaseous CO2 are required. The heat capacity of the
MDEA-H2O-CO2 system can be calculated from the temperature derivative of enthalpy.
We use the following equation for liquid enthalpy:
Hl ) xwHwl + xsHsl +
∑xH
i
∞,aq
i
+ Hex
(20)
Table 7. Parameters for Heat of Vaporization (Expressed in Units
of J/kmol)a
component i
source
C1i
C2i
C3i
C4i
C5i
Tci
MDEA
this work
9.7381 × 107
4.6391 × 10-1
0
0
0
741.9b
a
The DIPPR equation for the heat of vaporization is given as follows:
∆vapHi ) C1i(1 - Tri)Z, where Z ) C2i + C3iTri + C4iTri2 + C5iTri3 and
Tri ) T/Tci (here, Tci is the critical temperature of component i). The
temperatures are given in Kelvin. b The Tci value for MDEA is obtained
from Von Niederhausern et al.32
The liquid enthalpy of pure water is calculated from the ideal
gas model and the ASME Steam Tables EOS for enthalpy
departure:
ig
Hwl (T) ) ∆fHw,298.15
+
∫
T
298.15
ig
Cp,w
dT + ∆Hwigfl(T, p)
(21)
where Hwl (T) is the liquid enthalpy of water at temperature T,
ig
∆fHw,298.15
the ideal gas enthalpy of formation of water at 298.15
ig
K, Cp,w
the ideal-gas heat capacity of water, and ∆Hwigfl(T,p)
the enthalpy departure calculated from the ASME Steam Tables
EOS.
Liquid enthalpy of the nonaqueous solvent s is calculated
from the ideal-gas enthalpy of formation at 298.15 K, the idealgas heat capacity, the vapor enthalpy departure, and the heat of
vaporization:
ig
Hls(T) ) ∆fHs,298.15
+
∫
T
298.15
ig
Cp,s
dT + ∆HVs (T, p) - ∆vapHs(T)
(22)
Here, Hsl (T) is the liquid enthalpy of solvent s at temperature T,
ig
∆fHs,298.15
the ideal-gas enthalpy of formation of solvent s at
V
298.15 K, Cig
p,s the ideal-gas heat capacity of solvent s, ∆Hs (T,p)
the vapor enthalpy departure of solvent s, and ∆vapHs(T) the
heat of vaporization of solvent s.
The PC-SAFT EOS is used for the vapor enthalpy departure
and the DIPPR heat of vaporization correlation is used for the
heat of vaporization. Table 7 shows the DIPPR equation and
the correlation parameters for the heat of vaporization.
The enthalpies of ionic solutes in aqueous phase infinite
dilution are calculated from the enthalpy of formation at 298.15
K in aqueous-phase infinite dilution and the heat capacity in
aqueous-phase infinite dilution:
i
Here, Hl is the molar enthalpy of the liquid mixture, Hwl the
molar enthalpy of liquid water, Hsl the molar enthalpy of liquid
nonaqueous solvent s, Hi∞,aq the molar enthalpy of solute i
(molecular or ionic) in aqueous-phase infinite dilution, and Hex
the molar excess enthalpy. The terms xw, xs, and xi represent
the mole fractions of water, nonaqueous solvent s, and solute i,
respectively.
H∞,aq
(T) ) ∆fH∞,aq
i
i,298.15 +
∫
T
298.15
C∞,aq
p,i dT
(23)
where Hi∞,aq(T) is the enthalpy of solute i in aqueous-phase
infinite dilution at temperature T, ∆fH∞,aq
i,298.15 the enthalpy of
formation of solute i in aqueous-phase infinite dilution at 298.15
K, and C∞,aq
the heat capacity of solute i in aqueous-phase
p,i
infinite dilution.
Ind. Eng. Chem. Res., Vol. xxx, No. xx, XXXX
a
phase chemical equilibrium calculations, we choose the aqueousphase infinite dilution reference state for molecular solute CO2
and all ionic species.
In applying the electrolyte NRTL model for liquid-phase
activity coefficient calculations, the binary NRTL interaction
parameters for molecule-molecule binary, molecule-electrolyte
binary, and electrolyte-electrolyte binary systems are required.
Here, electrolytes are defined as cation and anion pairs. In
addition, solvent dielectric constants are needed to facilitate
calculations of long-range ion-ion interaction contribution to
activity coefficients. Table 8 shows the dielectric constant
correlation used in this work for MDEA.
Unless specified otherwise, all molecule-molecule binary
parameters and electrolyte-electrolyte binary parameters are
defaulted to zero. All molecule-electrolyte binary parameters are
defaulted to (8,-4), average values of the parameters as reported
for the electrolyte NRTL model.12 The nonrandomness factor (R)
is fixed at 0.2. The calculated thermodynamic properties of the
electrolyte solution are dominated by the binary NRTL parameters
associated with the major species in the system. In other
words, the binary parameters for the water-MDEA binary, the
+
2water-(MDEAH+, HCO3 ) binary, the water-(MDEAH , CO3 )
+
binary, and the MDEA-(MDEAH , HCO3 ) binary systems
determine the calculated thermodynamic properties. These binary
parameters, in turn, are identified from fitting to available experimental data.
Table 8. Parameters for Dielectric Constant
component i
source
Ai
Bi
Ci
MDEA
Aspen Databank22
21.9957
8992.68
298.15
a
The correlation for the dielectric constant is given as follows: εi(T)
) Ai + Bi[(1/T) - (1/Ci)], where T is the temperature (given in Kelvin).
∞,aq
Both ∆fH∞,aq
i,298.15 and Cp,i are also used in the calculation of
Gibbs free energy of the solutes, thus impacting chemical
∞,aq
equilibrium calculations. In this study, ∆fH∞,aq
i,298.15 and Cp,i for
+
MDEAH are determined by fitting to the experimental phase
equilibrium data, the heat of solution data, and the speciation
data, together with molality scale Gibbs free energy of formation
at 298.15 K, ∆fG∞,aq
i,298.15, and NRTL interaction parameters.
The enthalpies of molecular solutes in aqueous phase infinite
dilution are calculated from Henry’s law:
(
2
(T) ) ∆fHig
H∞,aq
i
i (T) - RT
∂ ln Hi,w
∂T
)
E
(24)
where ∆fHiig(T) is the ideal gas enthalpy of formation of solute
i at temperature T, and Hi,w Henry’s constant of solute i in water.
Excess enthalpy (Hex) is calculated from the activity coefficient model (i.e., the electrolyte NRTL model).
2.3. Activity Coefficients. Activity coefficients are required
in phase equilibrium calculations, aqueous-phase chemical
equilibrium calculations, heat of absorption, liquid heat capacity,
and liquid enthalpy calculations. The activity coefficient of a
component in a liquid mixture is a function of temperature,
pressure, mixture composition, and choice of reference state.
In VLE calculations, we use the asymmetric mixed-solvent
reference state for the molecular solute CO2, and in aqueous-
3. Modelling Results
Table 9 summarizes the model parameters and sources of
the parameters used in the thermodynamic model. Most of the
parameters can be obtained from the literature. The remaining
parameters are determined by fitting to the experimental data.
Table 9. Parameters Estimated in Modeling
parameter
component
source
Antoine equation
∆vapH
MDEA
MDEA
regression
regression
dielectric constant
Henry’s constant
MDEA
CO2 in H2O
CO2 in MDEA
CO2-H2O binary
MDEA-H2O binary
molecule-electrolyte binaries
Aspen Databank22
Yan and Chen25
this work
Yan and Chen25
regression
regression
H2O, MDEA, CO2
H2O, MDEA, CO2
H2O, CO2
MDEA
2H3O+, OH-, HCO3 , CO3
MDEAH+
Aspen Databank22
Aspen Databank22
Aspen Databank22
regression
Aspen Databank22
regression
∞,aq
∆fH298.15
2H3O+, OH-, HCO3 , CO3
MDEAH+
Aspen Databank22
regression
C∞,aq
p
H3O+, OH2HCO3 , CO3
MDEAH+
Aspen Databank22
Criss and Cobble31
regression
NRTL binary parameters
ig
∆fG298.15
ig
∆fH298.15
Cig
p
∞,aq
∆fG298.15
data used for regression
vapor pressure of MDEA
heat of vaporization of MDEA, calculated from the vapor
pressure using the Clausius-Clapeyron equation
VLE, excess enthalpy, and heat capacity for the MDEA-H2O binary
VLE, excess enthalpy, heat capacity, and species concentration from
NMR spectra for the MDEA-H2O-CO2 system
liquid heat capacity of MDEA
VLE, excess enthalpy, heat capacity, and species concentration from
NMR spectra for the MDEA-H2O-CO2 system
VLE, excess enthalpy, heat capacity, and species concentration from
NMR spectra for the MDEA-H2O-CO2 system
VLE, excess enthalpy, heat capacity, and species concentration from
NMR spectra for the MDEA-H2O-CO2 system
Table 10. Experimental Data Used in the Regression for Pure MDEA
data type
temperature, T (K)
pressure, P (kPa)
data points
average relative deviation, |∆Y/Y| (%)
reference
vapor pressure
vapor pressure
vapor pressure
liquid heat capacity
liquid heat capacity
liquid heat capacity
293-401
420-513
420-738
299-397
303-353
278-368
0.0006-1.48
3.69-90.4
3.69-3985
26
14
23
5
11
19
1.5
4.0
2.9
0.5
0.4
0.3
Daubert et al.33
Noll et al.34
VonNiederhausern et al.32
Maham et al.35
Chen et al.36
Zhang et al.37
F Ind. Eng. Chem. Res., Vol. xxx, No. xx, XXXX
Table 11. Antoine Equation Parameters for Pure MDEAa
parameter
component i
value
C1i
C2i
C3i
C4i
MDEA
MDEA
MDEA
MDEA
1.2276 × 102
-1.3253 × 104
-1.3839 × 10
3.20 × 10-6
a
The correlation for the Antoine equation is given as follows: ln Pi*,l
) C1i + C2i/T + C3i ln T + C4iT2, where T is the temperature (given in
Kelvin).
3.1. MDEA. Extensive experimental vapor pressure data and
liquid heat capacity data are available for MDEA. The data used
in the regression for MDEA and the correlation results are
summarized in Table 10.
Table 11 shows the Antoine equation parameters regressed from
the recently available vapor pressure data.32-34 The heat of
vaporization (from 293 K to 473 K) generated with the regressed
Antoine equation parameters through the Clausius-Clapeyron
equation are used to determine the DIPPR heat of vaporization
equation parameters (shown in Table 7). The ideal-gas heat capacity
correlation parameters are obtained by fitting to the liquid heat
capacity data35-37 (shown in Table 5). Table 10 shows the excellent
correlation of the experimental data for vapor pressure, with an
average relative deviation of <4%, and liquid heat capacity, with
an average relative deviation of <0.5%.
The PC-SAFT parameters of MDEA (shown in Table 1) are
regressed from the vapor pressure data32-34 (with an average
relative deviation of 13.1%), the liquid heat capacity data35-37
(with an average relative deviation of 22.6%), and the liquid
density data38,39 (with an average relative deviation of 2.7%).
3.2. MDEA-H2O System. Extensive literature data on
VLE,40-42 excess enthalpy,9,35,43 and heat capacity36,37,44 of the
MDEA-H2O binary system are available (see Table 12). These
data cover the complete MDEA-H2O binary concentration
range from room temperature to 458 K. Together, all of these
data are used to identify the NRTL binary parameters, including
their temperature dependencies for the MDEA-H2O binary
system.
The regressed NRTL parameters are summarized in Table
13. The experimental data for the binary MDEA-H2O system
are well-represented. The average relative deviations between
the calculated values and the experimental data are summarized
in Table 12. Figure 1 shows the parity plot, while Figure 2 shows
the comparison for the experimental total pressure data and the
calculated results from the model. Figure 3 shows the comparison results for the MDEA vapor composition. The excess
enthalpy fit is given in Figure 4. Both the experimental excess
enthalpy data from Posey9 and those of Maham et al.35,43 are
represented very well. Figure 5 shows the model also provides
satisfactory representation of the heat capacity data.
Figure 6 shows the model predictions for water and MDEA
activity coefficients at 313, 353, and 393 K. While the water
Table 13. Regressed NRTL Parameters for the MDEA-H2O Binary
System with r ) 0.2a
parameter
component i
component j
value
standard deviation
aij
aij
bij
bij
H2O
MDEA
H2O
MDEA
MDEA
H2O
MDEA
H2O
8.5092
-1.7141
-1573.9
-261.85
0.1641
0.0566
45.70
22.97
a
Correlation for the NRTL parameters: τij ) aij + bij/T, where T is
the temperature (given in Kelvin).
Figure 1. Parity plot for the MDEA-H2O system total pressure, experiment
versus model: (4) Xu et al.,40 (O) Voutsas et al.,41 and (0) Kim et al.42
Figure 2. Comparison of the experimental data from Kim et al.42
(represented by symbols: (O) T ) 373 K, (4) T ) 353 K, (0) T ) 333 K,
and (×) T ) 313 K) for total pressure of the MDEA-H2O binary solution
and the model results (represented by lines).
activity coefficient remains relatively constant, the model
suggests that the MDEA activity coefficient varies strongly with
MDEA concentration and temperature, especially in dilute
aqueous MDEA solutions.
Table 12. Experimental Data Used in the Regression for the MDEA-H2O System
data type
VLE (isoconcentration), TP
VLE (isobaric), Tx
VLE (isothermal), TPxy
excess enthalpy (isothermal)
excess enthalpy (isothermal)
excess enthalpy (isothermal)
heat capacity (isobaric)
heat capacity (isobaric)
heat capacity (isobaric)
a
temperature, T (K)
pressure, P (kPa)
326-381
349-458
313-373
298, 342
298, 303
338
303-353
303-353
278-368
The average relative deviation is that of total pressure.
13-101
40.0-66.7
6-100
100
100
100
MDEA mole fraction
0.016-0.26
0-0.93
0-0.36
0.02-0.74
0.03-0.93
0.10-0.90
0.13-0.80
0.20-0.80
0-1.0
data
points
34
30
61
19
26
9
44
44
228
average relative
deviation, |∆Y/Y| (%)
a
3.4
3.3a
3.0a
6.3
5.4
11.2
2.2
2.2
2.8
reference
Xu et al.40
Voutsas et al.41
Kim et al.42
Posey9
Maham et al.35
Maham et al.43
Chiu and Li44
Chen et al.36
Zhang et al.37
Ind. Eng. Chem. Res., Vol. xxx, No. xx, XXXX
Figure 3. Comparison of the experimental data from Kim et al.42
(represented by symbols: (O) T ) 373 K, (4) T ) 353 K, (0) T ) 333 K,
and (×) T ) 313 K) for the vapor composition of the MDEA-H2O binary
solution and the model results (represented by lines).
Figure 4. Comparison of the experimental data from Posey9 (represented
by full symbols: (b) T ) 298 K, ([) T ) 342 K) and Maham et al.35,43
(represented by empty symbols: (O) T ) 298 K, (4) T ) 313 K, (0) T )
338 K) for excess enthalpy of the MDEA-H2O binary solution and the
model results (represented by lines).
Figure 5. Comparison of the experimental data from Chen et al.36
(represented by symbols: (O) MDEA mole fraction ) 0.8, (4) MDEA mole
fraction ) 0.6, (0) MDEA mole fraction ) 0.4, and (×) MDEA mole
fraction ) 0.2) for heat capacity of the MDEA-H2O binary solution and
the model results (represented by lines).
3.3. MDEA-H2O-CO2 System. Extensive VLE,5,6,8,13,45-64
heat of absorption,65,66 heat capacity,67 and NMR spectroscopic68 data of the ternary MDEA-H2O-CO2 system are
available.
G
Figure 6. Model predictions of water and MDEA activity coefficients at
313, 353, and 393 K over the entire mole fraction range; solid lines represent
water activity coefficients and dashed lines represent MDEA activity
coefficients.
∞,aq
∞,aq
The terms ∆fG∞,aq
of MDEAH+ and
298.15, ∆fH298.15, and Cp
the binary NRTL parameters for major molecule-electrolyte
pairs are regressed from selected experimental data of the
MDEA-H2O-CO2 system. Table 14 summarizes the VLE,5,6,13
heat of absorption,65,66 heat capacity,67 and species concentration68 data used to obtain these parameters.
Species concentration data from NMR spectra are very useful
to validate the model predictions for the species distribution in
the ternary system. Calculated heat of absorption of CO2 by
the MDEA solution also strongly depends on the species
distribution.
For VLE data, we choose the total pressure data of Kuranov
et al.,5 Kamps et al.,6 and the CO2 partial pressure data of
Ermatchkov et al.13 in the regression. Together, these data cover
temperatures from 313 K to 413 K, pressures from 0.1 kPa to
6000 kPa, MDEA mole fractions from 0.03 to 0.13, and CO2
loadings from 0.003 to 1.32. The CO2 partial pressure data of
Jou et al.45 also cover wide ranges for temperature, pressure,
MDEA concentration, and CO2 loading. However, considering
the reported inconsistency5,6,13 between these data45 and those
of Kuranov et al.,5 Kamps et al.,6 and Ermatchkov et al.,13 we
choose to exclude the data of Jou et al.45 from the regression.
The Jou et al. data45 and all other available literature VLE
data8,46-64 are used only for model validation.
The average relative deviations between the correlation results
and the various experimental data are shown in Table 14. The
regressed parameters for the MDEA-H2O-CO2 system are
summarized in Table 15. As expected, the regressed values of
∞,aq
∞,aq
∆fG∞,aq
for MDEAH+ in Table 15 are
298.15, ∆fH298.15, and Cp
comparably close to the estimated values reported in Tables 4
and 6.
Figures 7 and 8 show that most of the total pressure data of
Kuranov et al.5 and Kamps et al.6 are fitted within (20%.
Figures 9-11 show the excellent correlation results for the total
pressure data for MDEA concentration from 2 m to 8 m, CO2
loading from 0.11 to 1.32, temperature from 313 K to 413 K,
and pressure up to 6000 kPa. Figures 12 and 13 show that most
of the CO2 partial pressure data of Ermatchkov et al.13 are fitted
within (30%. Figure 12 suggests that there is a slight systematic
deviation that changes from negative to positive as the CO2
loading increases. Figures 14-16 show the satisfactory correlation results for the CO2 partial pressure data for MDEA
concentration from 2 m to 8 m, CO2 loading from 0.003 to 0.78,
temperature from 313 K to 393 K, and pressure from 0.1 kPa
H
Ind. Eng. Chem. Res., Vol. xxx, No. xx, XXXX
Table 14. Experimental Data Used in the Regression for the MDEA-H2O-CO2 System
data type
temperature,
T (K)
pressure,
P (kPa)
MDEA
mole fraction
CO2 loading
data
points
average relative
deviation, |∆Y/Y| (%)
reference
VLE, TPx, total pressure
VLE, TPx, total pressure
VLE, TPx, CO2 pressure
heat of solution
heat of solution
heat capacity (isobaric)
species concentration
313-413
313-393
313-393
313-393
298
298
293-313
70-5000
200-6000
0.1-70
0.035-0.067
0.126
0.033-0.132
0.06
0.017-0.061
0.061-0.185
0.04
0-1.32
0.13-1.15
0.003-0.78
0.1-1.4
0.02-0.25
0-0.64
0.1-0.7
82
23
101
112
40
39
8
6.8
10.5
17.7
6.8
2.1
3.0
47.5
Kuranov et al.5
Kamps et al.6
Ermatchkov et al.13
Mathonat65
Carson et al.66
Weiland et al.67
Jakobsen et al.68
Table 15. Regressed Parameters for the MDEA-H2O-CO2 System with r ) 0.2
parameter
∞,aq
∆fG298.15 (J/kmol)
∆fH∞,aq
298.15 (J/kmol)
Cp∞,aq (J/(kmol K))
τij
τij
τij
τij
τij
τij
component i
component j
value
(MDEAH+, HCO3)
H2O
(MDEAH+, CO23 )
H2O
(MDEAH+, HCO3)
MDEA
-2.5951 × 10
-5.1093 × 108
3.3206 × 105
8.7170
-4.2995
10.4032
-4.9252
5.2964
-0.8253
+
MDEAH
MDEAH+
MDEAH+
H2O
(MDEAH+, HCO3)
H2O
+
2(MDEAH , CO3 )
MDEA
(MDEAH+, HCO3)
standard deviation
8
to 70 kPa. The correlation results match the experimental data
well, except that the calculated CO2 pressure at 313 K is slighter
higher than the measured values for the 8 m MDEA solution at
high CO2 loading (see Figure 16). Upon further examination,
we find that, under the same conditions, the predicted total
pressure matches the experimental data of Sidi-Boumedine et
al.62 well.
2.1986 × 105
5.8718 × 105
1.2799 × 104
0.2246
0.0836
0.3676
0.1248
0.2746
0.0685
Table 16 shows a comparison of model predictions and
experimental VLE data from numerous other sources not
included in the regression. The results highlight the fact that
we cannot match all the VLE data because the experimental
Figure 9. Comparison of the experimental data from Kuranov et al.5
(represented by symbols: (O) T ) 413 K, (4) T ) 393 K, (×) T ) 373 K,
(0) T ) 353 K, and (]) T ) 313 K) for total pressure of the
MDEA-H2O-CO2 system and the model results (represented by lines);
the MDEA concentration is ∼2 m.
Figure 7. Ratio of experimental total pressure to calculated total pressure,
as a function of CO2 loading ((4) data from Kuranov et al.5 and (O) data
from Kamps et al.6).
Figure 8. Parity plot for the MDEA-H2O-CO2 system total pressure:
experiment versus model ((4) Kuranov et al.5 and (O) Kamps et al.6).
Figure 10. Comparison of the experimental data from Kuranov et al.5
(represented by symbols: (O) T ) 413 K, (4) T ) 393 K, (×) T ) 373 K,
(0) T ) 353 K, and (]) T ) 313 K) for total pressure of the
MDEA-H2O-CO2 system and the model results (represented by lines);
the MDEA concentration is ∼4 m.
Ind. Eng. Chem. Res., Vol. xxx, No. xx, XXXX
I
Figure 11. Comparison of the experimental data from Kamps et al.6
(represented by symbols: (O) T ) 393 K, (0) T ) 353 K, and (]) T ) 313
K) for total pressure of the MDEA-H2O-CO2 system and the model results
(represented by lines); the MDEA concentration is ∼8 m.
Figure 14. Comparison of the experimental data for CO2 partial pressure
of the MDEA-H2O-CO2 system and the model results; the MDEA
concentration is ∼2 m. Empty symbols are data from Ermatchkov et al.13
((O) T ) 393 K, (0) T ) 353 K, (]) T ) 313 K), solid lines represent the
2009 eNRTL model results, and dashed lines represent the 1986 eNRTL
model results.
Figure 12. Rato of experimental CO2 partial pressure ((O) Ermatchkov et
al.13) to calculated CO2 partial pressure (line), as a function of CO2 loading.
Figure 15. Comparison of the experimental data for CO2 partial pressure
of the MDEA-H2O-CO2 system and the model results; the MDEA
concentration is ∼4 m. Empty symbols are data from Ermatchkov et al.13
((O) T ) 393 K, (0) T ) 353 K, (]) T ) 313 K), solid lines represent the
2009 eNRTL model results, and dashed lines represent the 1986 eNRTL
model results.
Figure 13. Parity plot for CO2 partial pressure of the MDEA-H2O-CO2
system: experiment ((O) Ermatchkov et al.13) versus model (line).
data from different sources can be inconsistent. With the
exception of the data from Jou et al.,45 Silkenbaeumer et al.,56
and Ali and Aroua,61 the model predictions are very satisfactory,
with the average relative deviation on pressure (either total
pressure or CO2 partial pressure) in the range of 7%-80%. It
is particularly significant that the model predictions give an
excellent match with the recent data of Kamps et al.,60 SidiBoumedine et al.,62 and Ma’mum et al.63
Figure 16. Comparison of the experimental data for CO2 partial pressure
of the MDEA-H2O-CO2 system and the model results; the MDEA
concentration is ∼8 m. Empty symbols are data from Ermatchkov et al.13
((O) T ) 393 K, (0) T ) 353 K, (]) T ) 313 K), solid lines represent the
2009 eNRTL model results, and dashed lines represent the 1986 eNRTL
model results.
Figure 17 shows the species distribution as a function of CO2
loading for a 23 wt % MDEA solution at 293 K. The calculated
J
Ind. Eng. Chem. Res., Vol. xxx, No. xx, XXXX
Table 16. Comparison between Experimental Data and Model Predictions for Total Pressure or CO2 Partial Pressure of the MDEA-H2O-CO2
System
source
data points
temperature, T (K)
pressure, P (kPa)
MDEA concentration
CO2 loading
|∆P/P|a (%)
Jou et al.45
Chakma and Meisen46
Maddox et al.47
Austgen et al.8
MacGregor and Mather48
Jou et al.49
Dawodu and Meisen50
Liu et al.51
Mathonat et al.52
Rho et al.53
Baek and Yoon54
Rogers et al.55
Silkenbaeumer et al.56
Xu et al.57
Lemoine et al.58
Bishnoi and Rochelle59
Kamps et al.60
Ali and Aroua61
Sidi-Boumedine et al.62
Ma’mun et al.63
Dicko et al.64
118
76
99
14
5
37
12
16
9
103
12
34
11
65
13
3
5
15
103
34
5
298-393
373-473
310-388
313
313
313-373
373-393
303-363
313-393
323-373
313
313-323
313
328-363
298
313
313
313-353
298-348
328-358
323
0.001-6000
100-5000
20-6000
0.005-100
1-4000
0.004-260
160-4000
20-350
2000-10000
0.1-268
1-2000
0.00007-1
12-4000
4-800
0.02-1.64
0.1-0.7
80-5000
0.08-100
2.7-4500
66-813
6-434
0.044-0.128
0.03-0.12
0.02-0.04
0.045-0.13
0.04
0.07
0.12
0.09
0.06
0.008-0.31
0.06
0.04-0.13
0.07
0.07-0.13
0.04
0.13
0.03
0.04
0.05-0.11
0.12
0.12
0.0004-1.68
0.01-0.95
0.17-1.51
0.003-0.67
0.12-1.2
0.002-0.80
0.09-0.8
0.09-0.85
0.5-1.3
0.006-0.68
0.12-1.13
0.0002-0.12
0.2-1.3
0.04-0.9
0.02-0.26
0.01-0.03
1.06-1.41
0.05-0.8
0.008-1.30
0.17-0.81
0.1-0.9
204
31.5
21.1
32.2
22.1
37.5
13.3
28.5
57.4
78.7
53.6
27.1
135
20.2
11.9
17.9
8.9
495
7.7
6.5
48.5
a
Experimental pressure expressed either as total pressure or CO2 partial pressure.
Figure 17. Comparison of the experimental data for species concentration
in MDEA-H2O-CO2 and the model results at T ) 293 K. MDEA
concentration is 23 wt %. Symbols represent experimental data from
+
2Jakobsen et al.68 ((O) MDEA, (4) HCO3 , (0) MDEAH , (×) CO3 , (])
CO2); lines represent model results ((s) MDEA, (- - -) HCO3-, (- · -)
MDEAH+, (- · · -) CO23 , and ( · · · ) CO2.
Figure 18. Integral CO2 heat of absorption in 30 wt % MDEA aqueous
solution at 313 K. Symbols (0) represent experimental data from Mathonat;65 lines represent model results ((s) integral heat of absorption,
(- · · -) differential heat of absorption, ( · · · ) contribution of reactions,
(- · -) contribution of CO2 dissolution, (- - -) contribution of excess
enthalpies).
concentrations of the species are consistent with the experimental
NMR measurements from Jakobsen et al.68
Figures 18-20 show comparisons of the model correlations
and the experimental data of Mathonat65 for the integral heat
of CO2 absorption in aqueous MDEA solution at 313, 353, and
393 K, respectively. The calculated values are in reasonable
agreement with the experimental data. Also shown in Figures
18-20 are the predicted differential heats of CO2 absorption.
The integral heat and the differential heat overlap at low CO2
loadings, and then diverge much at high CO2 loadings (i.e.,
>0.8), where the differential heat decreases by >50%. We further
show the computed integral heat of absorption as the sum of
the various contributions from reactions 10-13, CO2 dissolution,
and excess enthalpy.
key component reacted, and ∆ni the reaction extent of the
reaction key component for reaction i when 1 mol CO2 is
absorbed.
The heat of CO2 dissolution (∆Hdissolution) is calculated as the
enthalpy difference between 1 mol of CO2 in the vapor phase
and 1 mol of CO2 in aqueous-phase infinite dilution. The
contribution of excess enthalpies (∆Hex) is computed as the
excess enthalpy difference between the final and initial compositions of the solution per mole of CO2 absorbed.
The results in Figures 18-20 show that the heat of absorption
is dominated by MDEAH+ dissociation and excess enthalpy.
In addition, CO2 dissolution is important near room temperature,
whereas CO2 dissociation becomes more important at higher
temperatures.
Figure 21 shows a comparison of the model correlations and
the experimental data of Weiland et al.67 for heat capacity of
the MDEA-H2O-CO2 system. The model results are consistent
with the data.
To show the impact of the different versions of the
electrolyte NRTL model to the model results, we perform
k
∆Habs )
∑ ∆n ∆H° + ∆H
i
i
dissolution
+ ∆Hex
(25)
i)1
where ∆Habs is the integral heat of absorption per mole of CO2,
∆H°i the standard heat of reaction for reaction i per mole of
Ind. Eng. Chem. Res., Vol. xxx, No. xx, XXXX
K
11
model. Figures 14-16 show that the two versions of the
model yield practically identical results at low MDEA
concentrations. The difference increases slightly with increasing MDEA concentration.
4. Conclusion
Figure 19. Integral CO2 heat of absorption in 30 wt % MDEA aqueous
solution at 353 K. Symbols (4) represent experimental data from Mathonat;65 lines represent model results ((s) integral heat of absorption,
(- · · -) differential overall absorption heat, ( · · · ) contribution of
reactions, (- · -) contribution of CO2 dissolution, (- - -) contribution
of excess enthalpies).
To support process modeling and simulation of the CO2
capture process with MDEA, the electrolyte NRTL model has
been successfully applied to correlate the available experimental
data on thermodynamic properties of the MDEA-H2O-CO2
system. The model has been validated for predictions of vaporliquid equilibrium (VLE), heat capacity, and CO2 heat of
absorption of the MDEA-H2O-CO2 system with temperatures
from 313 K to 393 K, MDEA concentrations up to 8 m (∼50
wt %), and CO2 loadings up to 1.32. This model should provide
a comprehensive thermodynamic property representation for the
MDEA-H2O-CO2 system over a broader range of conditions
and give more-reliable predictions than those from previous
works.
Acknowledgment
The authors thank Huiling Que and Joseph DeVincentis for
their support in preparing the manuscript.
Literature Cited
Figure 20. Integral CO2 heat of absorption in 30 wt % MDEA aqueous
solution at 393 K. Symbols (O) represent experimental data from Mathonat;65 lines represent model results ((s) integral heat of absorption,
(- · · -) differential heat of absorption, ( · · · ) contribution of reactions,
(- · -) contribution of CO2 dissolution, (- - -) contribution of excess
enthalpies).
Figure 21. Comparison of the experimental data for heat capacity of the
MDEA-H2O-CO2 system and the model results at T ) 298 K. Symbols
represent experimental data from Weiland et al.67 ((O) 60 wt % MDEA,
(4) 50 wt % MDEA, (0) 40 wt % MDEA, and (]) 30 wt % MDEA); lines
represent model results.
VLE predictions with the same model parameter values given
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ReceiVed for reView March 19, 2010
ReVised manuscript receiVed June 25, 2010
Accepted July 12, 2010
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