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Fluid Phase Equilibria
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xperiments and theory for the surface tensions of carbonated MDEA–PZqueous solutions
ong Fu ∗, Yifei Xu, Xueying Huachool of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, PR China
r t i c l e i n f o
rticle history:eceived 30 July 2011eceived in revised form 13 October 2011
a b s t r a c t
Surface tensions of carbonated N-methyldiethanolamine (MDEA)–piperazine (PZ) aqueous solutionswere measured at the temperatures ranging from 293.15 to 323.15 K. A theoretical model was proposed tocorrelate the surface tensions and the calculated results agreed well with the experiments. The residual
ccepted 9 November 2011vailable online 18 November 2011
eywords:urface tensionDEA–PZ aqueous solution
MDEA and PZ in the carbonated aqueous solutions were determined on the basic of ‘shuttle’ mecha-nism. The temperature, amine concentration and CO2 loading (˛, mole number of CO2/mole number ofMDEA + PZ) dependences of the surface tensions were demonstrated.
In recent decades, atmospheric levels of CO2 have increasedapidly due to the utilization of grand amount of fossil fuel. Theeduction of CO2 emission became a global issue [1,2]. Currently,queous solutions of alkanol amines are popularly used for theemoval of CO2 from a variety of gas streams [3–8].
Among the alkanol amine series, N-methyldiethanolamineMDEA) takes the advantages of high resistance to thermal andhemical degradation, low solution vapor pressure, and lownthalpy of absorption. However, compared with other amines likeonoethanolamine (MEA) or piperazine (PZ), MDEA has a lower
bsorption rate. Adding small amount of MEA or PZ to an aqueousolution of MDEA has found widespread application in the removalf CO2 [9–20]. PZ has greater capacity and higher reaction rates thanEA, e.g., 4.6 mol/L PZ aqueous solution has about 75% greater CO2
apacity than 4.91 mol/L MEA aqueous solution, and CO2 reactionates for PZ are shown to be 2–3 times faster than MEA [19,20].ence PZ is considered as the most promising additive to MDEqueous solution. The mixtures of PZ and MDEA preserve the high
ate of the reaction of PZ with CO2, and the low enthalpy of theeaction of MDEA with CO2, hence lead to higher absorption rates inhe absorber column, yet lower heat of regeneration in the stripperection.
Surface tensions of aqueous solutions are required whendesigning or simulating an absorption column for CO2 absorp-tion associated with chemical reactions [21–27]. There are someexperiments concerning the surface tensions of aqueous solu-tions containing PZ, MDEA and their mixtures [9,10], in particular,Ayyaz et al. [9] measured the surface tensions of MDEA–PZ aque-ous solutions at the temperatures ranging from 303.1 to 333.3 K.However, the surface tensions of carbonated MDEA–PZ aqueoussolutions have been rarely reported. Moreover, due to the lack ofthe experiments and theoretical models, the influence of amineconcentrations, temperatures and CO2 loading on the surface ten-sions of carbonated MDEA–PZ aqueous solutions have not been welldocumented by far.
In this work, the surface tensions of carbonated MDEA–PZ aque-ous solutions were measured at the temperatures ranging from293.15 to 323.15 K. A theoretical model was proposed to correlatethe surface tensions for both the unloaded and loaded MDEA–PZaqueous solutions. The amine concentration, temperature and CO2loading dependence of the surface tensions were demonstrated onthe basic of experiments and calculations.
2. Experimental
2.1. Materials
Both MDEA and PZ were purchased from HuaXin chemical Co.,with purity ≥99%. The sample description is shown in Table 1.They were used without further purification. Aqueous solutions ofMDEA–PZ were prepared by adding doubly distilled water.
Aqueous solution of MDEA–PZ was put into a volumetric flaskmmersed in the thermostatic bath (CH-1006) with a built-in stirreror uniform temperature distribution. CO2 from a high-pressureank was inlet into the volumetric flask with a flow rate 300 ml/min.nce the carbonated solution was prepared, varying proportionsf the unloaded and loaded solutions were then mixed together toroduce a set of samples having a fixed amine-to-water ratio butith varying CO2 loading.
Surface tensions of the carbonated MDEA–PZ aqueous solutionsere measured from 293.15 to 323.15 K by using the BZY surface
ension meter produced by Shanghai Hengping Instrument Factory.ZY meter employs the Wilhemy plate principle, and its measure-ent ranges for temperature and surface tensions are respectively
68.15–383.15 K and 0.1–400.0 mN m−1. The schematic diagram ofZY meter is shown in Fig. 1.
During the experiments, the copper pan in the host of the BZYeter is connected with the thermostatic bath. Via the circulation
f the water, the temperature of the water in the copper pan is kepthe same as that in the thermostatic bath. The aqueous solution isut into the solution container immersed in the copper pan and itsemperature can be measured by a thermocouple.
. Results and discussion
Surface tensions of the unloaded and loaded MDEA–PZ aque-us solutions at different conditions are respectively shown inables 2 and 3.
To well describe the influence of amine concentrations, tem-eratures and CO2 loading on the surface tensions of carbonatedDEA–PZ aqueous solutions, one needs to establish a thermo-
ynamic equation. By far, some equations have been proposedor modeling the surface tensions of amine aqueous solutions, inarticular, Alvarez et al. [26] correlated the experimental data of
DEA–MEA aqueous solutions with high accuracy. However, due
o the complexity of the amine aqueous solutions, equations thatimultaneously take the concentration and temperature depen-ence into account have been rarely reported. Moreover, for the
ig. 1. Schematic diagram for the automatic surface tension meter. I Host of the tensioneter: (A) Platinum hanging film; (B) thermostatic copper pan; (C) solution container; (D
The uncertainties of temperature and surface tensions are respectively ±0.05 K and±0.1 mN m−1.
carbonated systems, due to the great variety of ions, the applicationof advanced approaches like density gradient theory (DGT) [28–33]to such systems is very difficult.
In this work, the surface tension of unloaded MDEA–PZ aqueoussolutions is formulated as following:
�aq = �0 + � ′ (1)
in which �0 and� ′ are expressed as:
�0 = x1�1 + x2�2 + x3�3 (2)
� ′ = x1x2G12 + x1x3G13 + x2x3G23 (3)
where the subscripts 1, 2 and 3 stand for MDEA, PZ and waterrespectively. xi is the mole fraction of component i in the aque-ous solution, � i is the surface tension of pure component i. �1 and
�3 can be expressed as the functions of temperatures by fitting tothe experimental data of pure MDEA and pure water. �2 can betreated as an adjustable parameter because PZ appears solid state
meter; II CH1006 thermostatic bath; III Inner structure of the host of the tension) thermocouple; (E) water pipe; (F) auto-lift specimen stage.
D.
Fu et
al. /
Fluid Phase
Equilibria 314 (2012) 121– 127
123
Table 3Surface tensions of carbonated MDEA–PZ aqueous solutions.
t room temperatures. Gij is expressed as the function of T and massraction w:
13 = a13
w1+ b13T (4)
23 = a23
w2+ b23T (5)
12 = a12
(w1 + w2)/2+ b12T (6)
he model parameters aij and bij, and the surface tension of PZ,2, can be regressed by fitting to the experimental data. The object
unction (the average relative deviation, ARD) is defined as:
RD =n∑
i=1
[1 − �cal
�exp
]× 100% (7)
here the superscripts ‘exp’ and ‘cal’ respectively stand for thexperimental and calculated data, n is the data numbers. The opti-ized results are: a12 = 31.42, b12 = 0.83, a13 = −23.05, b13 = −0.22,
23 = −29.70, b23 = −0.009, �2 = 31.94 mN m−1. The ARD is 0.58%.he comparison of the calculated and experimental results is shownn Figs. 2–4.
From Figs. 2–4, one may find that at given MDEA mass frac-ion, the surface tensions of unloaded MDEA–PZ aqueous solutionsecrease with the increase of both temperature and PZ mass frac-ion. At given temperature, the surface tensions decrease withhe increase of total amine mass fraction. The proposed equa-ion correctly captured both the temperature and concentrationependence of the surface tensions of unloaded MDEA–PZ aqueousolutions and the calculation agreed well with the experiments.
When absorbing CO2, there exist many kinds of ions in the solu-ions. Hence the description of the surface tensions of carbonatedolutions is more complex than that of unloaded solutions.
The surface tension of carbonated MDEA–PZ aqueous solutionss expressed as:
= �aq + � ion (8)
here � ion stands for the contribution from ions.To determine the surface tensions corresponding to each CO2
oading ˛, one needs the information of the composition of the
wPZ
Fig. 3. The same as in Fig. 2, except that wMDEA equals 0.236.
solution, which is closely related to the chemical reactions in thesolutions.
In the case of MDEA, the kinetic equation suggests that CO2 mayundergo a reaction of the following type:
R1R2R3N + CO2 → R1R2R3NCOO (9)
R1R2R3NCOO + H2O ⇔ R1R2R3NH+ + HCO3− (10)
KMDEA = [R1R2R3NH+][HCO3−]
[R1R2R3NCOO](11)
In the case of PZ, the reaction may be written as:
PZ + 2CO2 → PZ(COO)2 (12)
0.02 0.04 0.06 0.08
46
wPZ
Fig. 4. The same as in Fig. 2, except that wMDEA equals 0.354.
D. Fu et al. / Fluid Phase Equilibria 314 (2012) 121– 127 125
0.0 0.2 0.4 0.6 0.8 1.0
60
62
64
66
68
70
α
0.0 0.2 0.4 0.6 0.8
52
56
60
64
α
γ /(m
N.m
-1) γ /
(mN
.m-1
)
Fig. 5. Surface tensions of carbonated MDEA–PZ aqueous solution at T = 293.15 K.Symbols: experimental data, the mass fraction for MDEA and PZ are respectively:�
00
F‘PhT
P
K
C
E
(
waoft
n
wrMa
Ptovat
0.0 0.2 0.4 0.6 0.8
50
52
54
56
0.0 0.2 0.4 0.6 0.8 1.052
56
60
64
α
γ /(m
N.m
-1)
γ /(m
N.m
-1)
the surface tensions of carbonated MDEA–PZ aqueous solutionsincrease with the increase of CO2 loading ˛, but decrease with theincrease of temperature. At given temperature and ˛, the surfacetensions decrease with the increase of total amine mass fraction.
or MDEA–PZ aqueous solutions, the absorption of CO2 obeys theshuttle’ mechanism [34,35], that is, PZ absorbs CO2 and formsZ(COO)2 very quickly, and then PZ(COO)2 transfers CO2 to MDEA,ence the small amount of PZ activates the absorption significantly.he reaction may be expressed as:
Z(COO)2 + 2R1R2R3N ⇔ 2R1R2R3NCOO + PZ (15)
C = [R1R2R3NCOO]2[PZ]
[PZ(COO)2][R1R2R3N]2(16)
ombining Eqs. (9)–(16), one may obtain:
[R1R2R3N]2
[R1R2R3NH+]2(KCK2
MDEA) = [PZ]
[PZH2+2]
(KPZ) (17)
q. (17) can be rewritten as:
1 − ˛MDEA
˛MDEA
)2 ̆ = 1 − ˛PZ
˛PZ, with ̆ = KCK2
MDEAKPZ
(18)
here ˛MDEA and ˛PZ are respectively the conversion rates of MDEAnd PZ. It is worth noting that Eq. (18) is only applicable when wPZ isf small value compared with wMDEA, otherwise, the contributionsrom PZH+, (C4H8)2(NCOO)2− and (C4H8)2NH(NCOO)− should beaken into account in Eqs. (16)–(18).
In the equilibrium state, there exists:
CO2 = n0MDEA˛MDEA + 2n0
PZ˛PZ (19)
here nCO2 is defined as nCO2 = (n0MDEA + n0
PZ)˛, n0MDEA and n0
PZ areespectively the mole number of MDEA and PZ in the unloadedDEA–PZ aqueous solutions. ̆ = KCK2
MDEA/KPZ can be treated asn adjustable parameter.
When absorbing CO2, the mass fractions of residual MDEA andZ, w1 and w2, decrease with the increase of ˛. In the calculation ofhe surface tensions of carbonated MDEA–PZ aqueous solutions,
ne firstly needs to determine w1 and w2. At given ˛, the con-ersion rates ˛MDEA and ˛PZ can be obtained by solving Eqs. (18)nd (19), hence the residual MDEA and PZ in the carbonated solu-ions may be determined. Once w1 and w2 are available, �aq can be
α
Fig. 6. The same as in Fig. 5, except that T equals 303.15 K.
calculated using Eqs. (1)–(6). The contribution from ions, � ion, maybe empirically formulated as:
� ion = c1 ̨ + c2˛2
T− c3
[(w1 + w2)/2]2/T(20)
with ˛, w1 and w2 as input, the adjustable parameters c1, c2, c3and ̆ can be regressed by fitting to the experimental data. Thefitting procedure and the object function are the same as thosefor CO2-unloaded solutions. The optimized values are: c1 = 717.61,c2 = 1439.40, c3 = −0.032, ̆ = 1.481, and the corresponding ARD is0.61%. The comparison of the calculated and experimental resultsis shown in Figs. 5–8.
From Figs. 5–8, one may find that at given amine mass fractions,
0.0 0.2 0.4 0.6 0.8 1.0
56
α
Fig. 7. The same as in Fig. 5, except that T equals 313.15 K.
126 D. Fu et al. / Fluid Phase Equilib
0.0 0.1 0.2 0.3 0.4 0.5
46
48
50
52
α
0.0 0.2 0.4 0.6 0.8
50
52
54
56
58
60
α
γ /(m
N.m
-1)
γ /(m
N.m
-1)
IttPdugftat
FnT
Fig. 8. The same as in Fig. 5, except that T equals 323.15 K.
t is worth noting that in the carbonated MDEA–PZ aqueous solu-ions, there exist residual MDEA and PZ, as shown in Fig. 9, withhe increase of CO2 loading, the conversion rates of both MDEA andZ increase, yet the residual mole numbers of both MDEA and PZecrease. However, even in the case of high CO2 loading, the resid-al mole number of both MDEA and PZ do not tend to be zero. Ineneral, the increase of ion concentration tends to increase the sur-ace tensions, but the interfacial activity of MDEA and PZ decreaseshe surface tensions much more significantly, hence in this work,ll the surface tensions of the carbonated solutions are lower than
hose of pure water.
0.0 0.2 0.4 0.6 0.8 1.0
0.0
0.2
0.4
0.6
0.8
1.0
α
αM
DEA
and
αPZ
n MD
EAan
d n P
Z
α
0E+0
2E-4
4E-4
6E-4
8E-4
1E-3
0.0 0.2 0.4 0.6 0.8 1.0
ig. 9. Conversion rates ˛MDEA and ˛PZ, and the residual mole numbers (insert)MDEA and nPZ for MDEA (—) and PZ (—-) in carbonated MDEA–PZ aqueous solutions.
= 293.15 K, wMDEA = 0.118, wPZ = 0.021.
[
[
[
[
ria 314 (2012) 121– 127
4. Summary
In this study, the surface tensions of carbonated MDEA–PZ aque-ous solutions were measured and a theoretical model was proposedto correlate the surface tensions. The temperature, amine concen-tration and CO2 loading dependence of the surface tensions weredemonstrated. Our results showed that:
(1) For both the unloaded and loaded MDEA–PZ aqueous solutions,the increase of mass fraction of amines and the temperaturetends to decrease the surface tensions;
(2) For the loaded MDEA–PZ aqueous solutions, the increase of CO2loading tends to increase the surface tensions;
(3) With the increase of CO2 loading, the residual mole numbersof both MDEA and PZ decrease but they do not tend to be zeroeven in the case of high CO2 loading, hence the surface tensionsof the carbonated solutions are lower than those of pure water.
Greek letters˛ CO2 loading� surface tension, mN m−1
Superscripts1 MDEA2 PZ3 water
Subscriptsaq aqueous solutionscal calculated resultsexp experimental resultsion contribution from ions
Acknowledgments
The authors appreciate the financial support from the NationalNatural Science Foundation of China (No. 21076070), and the Fun-damental Research Funds for the Central Universities (Nos. 09MG13and 11ZG10).
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