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Journal of the Chinese Institute of Chemical Engineers 39 (2008) 645–651

www.elsevier.com/locate/jcice

Ternary diffusion coefficients of monoethanolamine

and N-methyldiethanolamine in aqueous solutions
Chih-Chiang Ko, Wen-Haur Chang, Meng-Hui Li *
R&D Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan Christian University, Chung Li 32023, Taiwan
Received 19 February 2008; received in revised form 13 April 2008; accepted 14 April 2008

Abstract
Ternary diffusion coefficients of monoethanolamine (MEA) and N-methyldiethanolamine (MDEA) in aqueous solutions have been measured at
303.2, 313.2, and 323.2 K. The systems studied are aqueous solutions containing total amine concentrations of 2.5 and 4.0 kmol/m3 and each
solution prepared with four different amine molar ratios. The main diffusion coefficients (D11 and D22) and cross-diffusion coefficients (D12 and
D21) and the density and viscosity of the aqueous amine solutions are discussed and analyzed as a function of temperature and their concentration.
# 2008 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Ternary diffusion coefficients; Taylor dispersion technique; Monoethanolamine; N-Methyldiethanolamine

1. Introduction 2-PE (Chang et al., 2005); N,N-dimethylethanolamine

(DMEA), N,N-diethylethanolamine (DEEA), MIPA, piperazine
A wide variety of alkanolamines, such as monoethanolamine (PZ) and sulfolane (Kao and Li, 2006).
(MEA), diglycolamine (DGA), diethanolamine, di-isopropa- The Taylor dispersion technique has been used frequently
nolamine (DIPA), N-methyldiethanolamine (MDEA), trietha- for measuring binary diffusion coefficients of various solutions
nolamine (TEA), 2-amino-2-methyl-l-propanol (AMP), and 2- (Alizadeh et al., 1980; Baldauf and Knapp, 1983; Leaist et al.,
piperidineethanol (2-PE), can be used as absorbents for the 1998; Taylor, 1953) as well as three- and four-component
removal of CO2 and H2S from gas streams in the natural gas, systems (Deng and Leaist, 1991; Leaist and Abdu, 2001). The
petroleum chemical plants and ammonia industries (Kohl and study of Cussler (1976) classified the ternary diffusion
Nielsen, 1997). coefficients in liquids into four classes of systems, namely:
The rate of molecular diffusion in liquid is normally the rate- (a) electrolytes (e.g., LiCl–NaCl–water), (b) electrolyte–non-
determining factor in unit operations, such as the absorption of electrolyte mixtures (e.g., KCl–glycine–water), (c) non-
acid gases in alkanolamine solutions and heterogeneous gas– electrolytes in water (e.g., glycine–sucrose–water), and (d)
liquid chemical reactions. The mass transport properties are organic liquids (e.g., acetone–benzene–cyclohexane).
usually determined from density, viscosity and diffusion For the diffusion in a three-component system, the diffusion
coefficient and based on the study of Tyrrell and Harris of solutes can be described by the Fick’s law as follows (Leaist
(1984), the latter are also useful for investigating the structure et al., 1998):
of liquids and developing theories of liquid states. The aqueous
alkanolamine solutions, for which the literature values of J 1 ðsolute 1Þ ¼ D11 rC 1  D12 rC2 (1)
mutual diffusion coefficients have been reported are: MEA,
monoisopropanolamine (MIPA), DEA, DIPA, DGA, ethylene- J 2 ðsolute 2Þ ¼ D21 rC 1  D22 rC2 (2)
diamine (EDA) and TEA (Hikita et al., 1980, 1981); MEA,
DEA, MDEA, and DIPA (Snijder et al., 1993); MDEA (Rowley where Ji is the molar flux of solute i with respect to the volume-
et al., 1997); TEA (Leaist et al., 1998); DGA, TEA, AMP, and fixed plane, Dij is the ternary diffusion coefficient, and Dij5Cj
denotes the molar flux of solute i produced by the gradient in the
concentration of solute j. Thus, a gradient of concentration of
* Corresponding author. Tel.: +886 3 2654109; fax: +886 3 2654199. solute j will produce a flux of solute i. In Eqs. (1) and (2), D11
E-mail address: mhli@cycu.edu.tw (M.-H. Li). and D22 are the main diffusion coefficients and D12 and D21 are
0368-1653/$ – see front matter # 2008 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.jcice.2008.04.007
646 C.-C. Ko et al. / Journal of the Chinese Institute of Chemical Engineers 39 (2008) 645–651

Scientific model 202) having an accuracy of 0.01 K. A

Nomenclature
metering pump was used to provide a constant laminar flow at a
ci concentration of solute i rate of 0.08–0.12 mL/min. By switching a 6-way injection
c̄i concentration of solute i in the carrier stream valve (Rheodyne, Model 7725i) located upstream of the
Di binary diffusion coefficients diffusion coil, a d function pulse of 20 mL was introduced into
Dii main diffusion coefficient the carrier fluid. The injected pulse of the solute usually
Dij cross-diffusion coefficient consists of a solution with a small concentration (Dc1 and Dc2)
f1 relative molar ratio of amine greater than that in the carrier fluid (c̄1 and c̄2 ). Dispersion of the
Ji the molar flux of solute i with respect to the pulse was observed through the laminar flow of the carrier fluid.
volume-fixed plane The initial concentration difference (Dc1 and Dc2) was typically
0.05 kmol/m3. A few runs were made with concentration
differences one-half as large, as suggested by Deng and Leaist
(1991). Since the results did not differ significantly for varying
the cross-diffusion coefficients. Deng and Leaist (1991) devel- concentration differences, the measured diffusion coefficients
oped a simple least-squares procedure to calculate the ternary can be interpreted as being measured at the composition in the
diffusion coefficients from the refractive index profiles using carrier stream (c̄1 and c̄2 ). The concentration gradient at the end
the Taylor dispersion techniques. of capillary was determined by a differential refractometer
Employing the absorption advantages of each amine, the (Precision Instruments, IOTA 2) with a cell volume of 8 mL.
aqueous blended amines such as MEA + MDEA + H2O, The analog output was transferred to a computer system by an
MEA + TEA + H2O, and MEA + AMP + H2O have been integrating converter INT5 (DataApex, Chromatography
suggested for acid gases removal (Chakravarty et al., 1985; Station, CSW 1.7). In the data analysis system, the detector
Horng and Li, 2002; Kohl and Nielsen, 1997; Liao and Li, signals were collected and plotted as a function of time. A least
2002; Xiao et al., 2000). Some thermophysical properties such squared procedure (Deng and Leaist, 1991) was applied to
as density, viscosity, Henry’s constant, diffusivity of a gas in determine the ternary mutual diffusion coefficients. The
liquid, vapor–liquid equilibria, and reaction kinetics data for standard deviation of the Dik values, 0.020  109 m2/s,
the CO2 absorption into MEA + MDEA + H2O have been was estimated by doing 4–6 sets of replicate runs.
reported in the literature (Austgen et al., 1991; Chen et al.,
2001; Li and Lai, 1995; Liao and Li, 2002). Since the ternary 2.3. Density and viscosity measurements
diffusion coefficient of MEA + MDEA + H2O has not yet been
reported it is the objective of this research to measure this An Anton Paar Stabinger viscometer (model SVM 3000)
coefficient in MEA + MDEA + H2O systems. was used to measure the viscosity (kinematic and dynamic) and
density of aqueous solutions. The repeatabilities of the
2. Experimental viscometer were: viscosity, 0.2% of the measured value;
density, 0.0002 g/cm3; estimated uncertainties of the density
2.1. Chemicals and viscosity measurements at 0.0004 g/cm3 and 0.5%,
respectively. The estimated uncertainty of temperature was
Reagent grade MEA (99 wt% pure) and MDEA (98 wt% 0.002 K.
pure) were obtained from Riedel-de Haën. A water purification
system (the Barnstead EASYpure LF) was used to provide Type 3. Results and discussion
I reagent-grade water with a resistivity of up to 18.3 MV cm
and with a total organic carbon content of less than 2 ppb. The To test the accuracy of the apparatus, the mutual diffusion
mass concentration of the aqueous solutions was prepared using coefficients of MEA (1) + water were measured at 2.0 kmol/m3
weighing scale with the accuracy of 1  104 g. The and atmospheric pressure. The results are 1.029  0.002, 1.276
estimated uncertainty of the mole fraction of the aqueous  0.002, 1.648  0.008, 2.051  0.008 and 2.487 
solutions was 1.5  104. The prepared aqueous alkanola- 0.004  109 m2/s for temperatures of 303.2, 313.2, 323.2,
mines solutions were degassed using an ultrasonic cleaner 333.2, and 343.2 K, respectively. The standard deviation was
(Branson, Model 3510). obtained from 4 to 6 replicated runs. A comparison between the
measured mutual diffusion coefficients and the literature values
2.2. Diffusion coefficient measurement (Hikita et al., 1980; Snijder et al., 1993) for this system is
shown in Fig. 1. The line in Fig. 1 was calculated using the
The ternary diffusion coefficients of MEA (1) + MDEA equation of Snijder et al. (1993). As shown in this figure, the
(2) + H2O were measured in a Taylor dispersion apparatus as obtained mutual diffusion coefficients of MEA + water are in
described in the previous work by Kao and Li (2006). good agreement with the literature values.
The length of the diffusion tube was 50.218 m with an The ternary diffusion coefficients determined from the
internal radius of 0.268 mm and was horizontally tempered into refractive index profiles using Taylor dispersion techniques, the
a 20 cm coil radius. The diffusion tube was maintained in a working equations and analytical procedures in order to obtain
constant temperature water bath with a thermometer (Hart the ternary mutual diffusion coefficients were based on the
 C.-C. Ko et al. / Journal of the Chinese Institute of Chemical Engineers 39 (2008) 645–651 647

Fig. 1. Mutual diffusion coefficients of aqueous MEA (2.00 kmol/m3) solution: Fig. 2. Ternary diffusion coefficients of Na2SO4 (1) + MgSO4 (2) + H2O at a
(*) data of Hikita et al. (1980); (&) data of Snijder et al. (1993); (~) this total salt concentration of 0.40 kmol/m3 at 298.2 K; lines, smoothed values.
study; line, calculated using equation of Snijder et al. (1993).

maintain electroneutrality, the field decreases the speed of the

Table 1 Na+ ions and increases the speed of the SO42 ions. In the
Ternary diffusion coefficients of Na2SO4 (1) + MgSO4 (2) + H2O for a total salt presence of MgSO4, the electric field will also drive a coupled
concentration of 0.40 kmol/m3 at 298.2 K
flow of Mg2+ ions, countercurrent to the flow of Na2SO4, which
Concentration Dij (109 m2/s) explains why the limiting cross-coefficient D021 have negative
(kmol/m3)
values. The D0ik value can provide a qualitative guide to the
c̄1 c̄2 D11 D12 D21 D22 diffusion behavior of solute or even moderately concentrated
0.00 0.40 0.461 univalent mixed salt solutions (Deng and Leaist, 1991).
0.10 0.30 0.906 0.004 0.156 0.475 The ternary mutual diffusion coefficients of MEA
0.20 0.20 0.857 0.021 0.116 0.465 (1) + MDEA (2) + H2O for a total amine concentration of
0.30 0.10 0.865 0.070 0.085 0.523 2.5 kmol/m3 for temperatures of 303.2, 313.2, and 323.2 K are
0.0 0.40 0.807

Table 2
method developed by Deng and Leaist (1991). For ternary Ternary diffusion coefficient of MEA (1) + MDEA (2) + H2O for a total amine
mutual diffusion coefficient measurement, the ternary mutual concentration of 2.50 kmol/m3
diffusion coefficients of Na2SO4 (1) + MgSO4 (2) + H2O for a T (K) Concentration Dij (109 m2/s)
total salt concentration of 0.4 kmol/m3 at 298.2 K were (kmol/m3)
measured. The results are presented in Table 1 and a c̄1 c̄2 D11 D12 D21 D22
comparison of the results is shown in Fig. 2. The x-axis
303.2 0.00 2.50 0.449
coordinate in Fig. 2 is the relative molar ratio of amines and was 0.50 2.00 0.506 0.004 0.070 0.464
defined as 1.00 1.50 0.604 0.032 0.061 0.477
1.50 1.00 0.696 0.073 0.029 0.511
c̄1
f1 ¼ (3) 2.00 0.50 0.795 0.113 0.014 0.535
c̄1 þ c̄2 2.50 0.00 0.975

where c̄1 and c̄2 are concentrations in the carrier stream. In this 313.2 0.00 2.50 0.576
figure, the four mutual diffusion coefficients (D11, D22, D12, and 0.50 2.00 0.681 0.151 0.057 0.555
1.00 1.50 0.807 0.154 0.088 0.614
D21), obtained from this study are generally in good agreement 1.50 1.00 0.908 0.165 0.061 0.642
with the data of Deng and Leaist (1991). 2.00 0.50 1.073 0.194 0.010 0.672
In Table 1 and Fig. 2, both D12 and D21 have negative values. 2.50 0.00 1.227
Deng and Leaist (1991) discussed these coupled phenomena for 323.2 0.00 2.50 0.727
MgCl2 + MgSO4 + H2O system using the limiting diffusion 0.50 2.00 0.956 0.009 0.121 0.729
coefficients of ions at infinite dilution in H2O, D0ik , and extended 1.00 1.50 1.137 0.097 0.029 0.729
it for Na2SO4 + MgSO4 + H2O system. The diffusion coeffi- 1.50 1.00 1.248 0.117 0.044 0.817
2.00 0.50 1.451 0.229 0.025 0.876
cient of Na+ ions is larger than that of SO42 ions. As a result,
2.50 0.00 1.536
an electric field is generated by the gradient in Na2SO4. To
648 C.-C. Ko et al. / Journal of the Chinese Institute of Chemical Engineers 39 (2008) 645–651

Fig. 3. Ternary diffusion coefficients of MEA (1) + MDEA (2) + H2O solutions Fig. 5. Main diffusion coefficients D22 of MEA (1) + MDEA (2) + H2O solu-
at a total amine concentration of 2.50 kmol/m3 at 303.2 K: (&) D11; (b) D12; tions at a total amine concentration of 2.50 kmol/m3: (&) 303.2 K; (b)
(") D21; (*) D22; lines, smoothed values. 313.2 K; (") 323.2 K; lines, smoothed values.

presented in Table 2. The corresponding mutual diffusion Also, at this limit, D21, the diffusion of MDEA due to the
coefficients in binary systems were also measured and are also concentration gradient of MEA, becomes zero since the
presented in Table 2. A plot of ternary mutual diffusion concentration gradient in MEA cannot produce a coupled flow
coefficients of MEA (1) + MDEA (2) + H2O versus the relative of MDEA, i.e., the solution is free of MDEA. Similarly, as
concentration of solutes at 303.2 K is shown in Fig. 3. The zero f 1 ! 0 (i.e., c̄1 ! 0), D22 is approaching the binary diffusion
values of Dik were also shown for easy observation of the coefficient in MDEA + H2O and D12 becomes zero since there
limiting case as c̄i ! 0. Fig. 3 shows that both D11 and D22 is no MEA in the solution.
increase as f 1 increases. The ratio of D12–D11 varies from 0.01 to 0.285 while the
In Fig. 3, it shows that at the limiting condition of f 1 ! 1 ratio of D21–D22 varies from 0.05 to 0.16. It can be concluded
(i.e., c̄2 ! 0), the main mutual diffusion coefficient, D11 that for the diffusion of MEA, the cross-diffusion effect due to
approaches the binary diffusion coefficient for MEA + H2O. the concentration gradient of MDEA is larger than that for the
diffusion of MDEA due to the concentration gradient of MEA.
In Fig. 4, the main diffusion coefficients, D11 of MEA
(1) + MDEA (2) + H2O solutions are shown as a function of
temperature at a total amine concentration of 2.5 kmol/m3. This
figure shows that D11 increases as the temperature increases at a
constant molar ratio and it also increases as the molar ratio
increases at a constant temperature. The main diffusion
coefficients, D22 of MEA (1) + MDEA (2) + H2O solutions
are shown in Fig. 5 as a function of temperature at a total
alkanolamine concentration of 2.5 kmol/m3. Similarly, D22
increases as the temperature increases at the same molar ratio
and also increases as the molar ratio increases at a constant
temperature. Fig. 6 shows the cross-diffusion coefficients D12
of MEA (1) + MDEA (2) + H2O solutions as a function of
temperature at a total amine concentration of 2.5 kmol/m3. In
this figure, as f 1 ! 0 (i.e., c̄1 ! 0), D12 becomes zero since
there is no MEA in the solution. In Fig. 6, D12 increases as the
temperature increases at the same molar ratio and also increases
as the molar ratio increases. Thus, at a constant temperature,
D12 increases when the amount of surrounding MDEA (a larger
Fig. 4. Main diffusion coefficients D11 of MEA (1) + MDEA (2) + H2O at a
molecule compared to MEA) molecules decreases.
total amine concentration of 2.50 kmol/m3: (&) 303.2 K; (b) 313.2 K; (") The ternary diffusion coefficients of MEA (1) + MDEA
323.2 K; lines, smoothed values. (2) + H2O for a total amine concentration of 4.00 kmol/m3 are
 C.-C. Ko et al. / Journal of the Chinese Institute of Chemical Engineers 39 (2008) 645–651 649

Fig. 6. Comparison of cross-diffusion coefficients D12 of MEA (1) + MDEA Fig. 7. Comparison of main diffusion coefficients D11 of MEA (1) + MDEA
(2) + H2O for a total amine concentration of 2.50 kmol/m3: (&) 303.2 K; (v) (2) + H2O solutions at 303.2 K: (&) 2.50 kmol/m3; (*) 4.00 kmol/m3.
313.2 K; (") 323.2 K.

presented in Table 3. The dependence of the ternary diffusion to MEA + H2O, and the binary diffusion coefficient, D1, at
coefficients on the temperature and molar ratio of amine is 2.5 kmol/m3 is 0.975  109 m2/s and D1 at 4.0 kmol/m3 is
similar as in the total amine concentration of 2.5 kmol/m3 0.878  109 m2/s. This corresponds to the general behavior of
solution. the mutual diffusion coefficient in binary system, i.e., the
At 303.2 K, a plot of D11 as a function of various total amine diffusion coefficient increases as the concentration of solute
concentrations of 2.5 and 4.0 kmol/m3 is shown in Fig. 7. The decreases. Due to the presence of MDEA in solution, D11
main diffusion coefficient, D11 increases as f 1 increases for both becomes smaller than D1. This effect increases as the
total amine concentrations and D11 in a total amine concentration of MDEA increases. Thus, at a constant molar
concentration of 2.5 kmol/m3 is higher than that in a total ratio, D11 in a total amine concentration of 2.5 kmol/m3 is
amine concentration of 4.0 kmol/m3. At f 1 = 1, system reduces higher than that in a total amine concentration of 4.0 kmol/m3.
The plot of D22 as a function of different total amine
Table 3 concentrations at 303.2 K is shown in Fig. 8. This figure shows
Ternary diffusion coefficient of MEA (1) + MDEA (2) + H2O for a total amine
concentration of 4.00 kmol/m3
T (K) Concentration Dij (109 m2/s)
(kmol/m3)
c̄1 c̄2 D11 D12 D21 D22
303.2 0.00 4.00 0.309
1.00 3.00 0.311 0.026 0.092 0.346
1.50 2.50 0.361 0.065 0.054 0.344
2.50 1.50 0.514 0.108 0.064 0.368
3.00 1.00 0.593 0.117 0.033 0.424
4.00 0.00 0.878
313.2 0.00 4.00 0.398
1.00 3.00 0.432 0.017 0.150 0.449
1.50 2.50 0.565 0.043 0.099 0.464
2.50 1.50 0.711 0.112 0.069 0.496
3.00 1.00 0.828 0.130 0.044 0.542
4.00 0.00 1.108
323.2 0.00 4.00 0.509
1.00 3.00 0.694 0.051 0.199 0.566
1.50 2.50 0.727 0.072 0.101 0.553
2.50 1.50 0.921 0.083 0.065 0.618
3.00 1.00 1.038 0.113 0.059 0.721
Fig. 8. Main diffusion coefficients D22 of MEA (1) + MDEA (2) + H2O at
4.00 0.00 1.412
303.2 K: (&) 2.50 kmol/m3; (*) 4.00 kmol/m3.
650 C.-C. Ko et al. / Journal of the Chinese Institute of Chemical Engineers 39 (2008) 645–651

Fig. 10. Main diffusion coefficients of MEA (1) + MDEA (2) + H2O as func-
Fig. 9. Comparison of cross-diffusion coefficients D21 of MEA (1) + MDEA tion of viscosity of solution at a total amine concentration of 2.50 kmol/m3 and
(2) + H2O solutions at 303.2 K: (&) 2.50 kmol/m3; (5) 4.00 kmol/m3.
303.2 K: (4) D11; (&) D22; lines, smoothed values.

Table 4
that at the same amine molar ratio, D22 decreases as the total
Densities and viscosities of MEA (1) + MDEA (2) + H2O solutions
amine concentration increases—a similar behavior as D11. The
T (K) c̄1 (kmol/m3) c̄2 (kmol/m3) r (g/cm3) m (mPa s) plot of D21 as a function of different total amine concentrations
303.2 0.00 2.50 1.0249 2.6959 at 303.2 K is shown in Fig. 9. In this figure, the values of D21
0.50 2.00 1.0214 2.3469 show only slight difference between the total amine concentra-
1.00 1.50 1.0173 2.0050 tions of 2.5 and 4.0 kmol/m3; D21 in a total amine concentration
1.50 1.00 1.0132 1.7424
2.00 0.50 1.0093 1.5725
of 4.0 kmol/m3 is slightly higher than that in a total amine
2.50 0.00 1.0048 1.3291 concentration of 2.5 kmol/m3. Usually, D21, the diffusion
313.2 0.00 2.50 1.0204 2.0370 coefficient of MDEA due to the concentration gradient of MEA
0.50 2.00 1.0169 1.8037 (a smaller molecule compared to MDEA), is normally small
1.00 1.50 1.0131 1.5557 because of the size difference and do not vary appreciably with
1.50 1.00 1.0091 1.3884 the total concentration of amines.
2.00 0.50 1.0052 1.2636
In the Stokes–Einstein equation, the diffusion coefficient is
2.50 0.00 1.0014 1.1409
related to the viscosity of the solvent. In order to find out the
323.2 0.00 2.50 1.0210 1.6283
0.50 2.00 1.0119 1.4187
dependence of the diffusion coefficient on the viscosity of
1.00 1.50 1.0079 1.2782 solution, the viscosity and density of solutions have also been
1.50 1.00 1.0042 1.1654 measured, and the results are presented in Table 4. At a constant
2.00 0.50 1.0007 1.1245 temperature and total concentration of amines, both the density
2.50 0.00 0.9974 1.0058 and viscosity of the solution decrease as the concentration of
303.2 0.00 4.00 1.0405 6.6817 MEA increases. In Fig. 10, the main diffusion coefficients, D11
1.00 3.00 1.0335 4.8453
and D22, are plotted as function of viscosity of solutions. As
1.50 2.50 1.0300 4.1669
2.50 1.50 1.0222 3.0756 shown in this figure, both D11 and D22 decrease as the viscosity
3.00 1.00 1.0179 2.6597 of solutions increases; also D11 shows stronger dependence on
4.00 0.00 1.0095 2.0928 the viscosity than that of D22.
313.2 0.00 4.00 1.0344 4.6996
1.00 3.00 1.0278 3.4950 4. Conclusion
1.50 2.50 1.0244 3.0946
2.50 1.50 1.0169 2.3346
3.00 1.00 1.0130 2.0722
Ternary diffusion coefficients of MEA and MDEA in
4.00 0.00 1.0052 1.7321 aqueous solutions at a total amine concentrations of 2.5 and
323.2 0.00 4.00 1.0280 3.5042 4.0 kmol/m3 with four different molar amine ratios have been
1.00 3.00 1.0218 2.6434 measured at 303.2, 313.2, and 323.2 K using the Taylor
1.50 2.50 1.0186 2.3507 dispersion technique. For the total amine concentration, both
2.50 1.50 1.0120 1.8960 D11 and D22 increase as the temperature increases at a constant
3.00 1.00 1.0086 1.6192
amine molar ratio and also increases as the amine molar ratio
4.00 0.00 1.0005 1.3513
increases at a constant temperature. At a constant temperature
 C.-C. Ko et al. / Journal of the Chinese Institute of Chemical Engineers 39 (2008) 645–651 651

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