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CO2
Fluid Phase Equilibria 394 (2015) 118–128
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Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d
CO2 solubility measurement and thermodynamic modeling for 1-methylpiperazine/water/CO2 Han Li a , Yann Le Moullec b , Jiahui Lu c, Jian Chen a, * , Jose Carlos Valle Marcos c, Guofei Chen c, Fabrice Chopin c a b c
State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Fluid Dynamics, Power Generation and Environment Department, EDF R&D, 6 Quai Watier, 78401 Chatou Cedex, France EDF China R&D Center, EDF Asia Pacific Direction, China Division, Beijing 100005, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 8 October 2014 Received in revised form 3 March 2015 Accepted 10 March 2015 Available online 14 March 2015
An accurate thermodynamic model is the primary element needed for the process simulation and optimization for CO2 absorption in aqueous amine solutions. In this work, the thermodynamic model was built in Aspen Plus, using the electrolyte nonrandom two-liquid (ENRTL) activity coefficient model to represent vapor pressure and heat capacity data, simultaneously, for amine, vapor–liquid equilibrium (VLE), excess enthalpy (HE), and pKa data for amine/H2O, and CO2 solubility data for amine/CO2/H2O. The cyclic diamine 1-methylpiperazine (1MPZ) is a promising amine for CO2 capture. CO2 solubility was measured for 1MPZ aqueous solutions at three concentrations – 10 wt%, 30 wt%, and 40 wt% and four temperatures – 313.15 K, 343.15 K, 373.15 K, and 393.15 K. The excess enthalpy for 1MPZ + H2O was obtained by the Setaram C80 calorimeter at 303.15 K and 323.15 K, within a whole mole-fraction range. The interaction parameters of nonrandom two-liquid model (NRTL) and ENRTL, along with the standard state properties of amine ions – protonated 1MPZ (1MPZH+, 1MPZH2+), 1MPZ carbamate (1MPZCOO), and protonated 1MPZ carbamate (H1MPZCOO) – were regressed from data obtained from this work as well as literature, which agreed with the model calculation. ã 2015 Elsevier B.V. All rights reserved.
Keywords: CO2 solubility Thermodynamic model 1-Methylpiperazine Excess enthalpy
1. Introduction Amine scrubbing – promising technology for the reduction of CO2 emissions from conventional coal-fired power plants – has been successfully applied in ammonia production and natural gas process [1]. The challenges of applying amine scrubbing on a large scale center on the unacceptably high energy consumption, and hence cost, which owes to the large flow rate of the flue gas and its low CO2 partial pressure. The bulk of the required energy is devoted to desorbing CO2 from the rich amine solutions in the reboiler at the bottom of the stripper – that is, to regenerating the amine, which is directly related to the CO2 absorption heat of amine. High cyclic CO2 capacity reduces the circulating rate of amine solution, thus reducing the size of the absorber, pump, and heat exchanger, all of which mainly determine the equipment cost. Furthermore, the use of fast kinetics saves the amount of packing, thereby decreasing the size of absorber. Developing new amines
* Corresponding author. Tel.: +86 10 62782748; fax: +86 10 62770304. E-mail address: [email protected] (J. Chen). http://dx.doi.org/10.1016/j.fluid.2015.03.021 0378-3812/ ã 2015 Elsevier B.V. All rights reserved.
with the favorable properties represents one way to lower regeneration energy and cost. At the first stage of searching for the best amine for CO2 capture, aqueous monoethanolamine (MEA) attracted a wide range of research [2–5]; this is usually used as a reference. Rochelle’s group at the University of Texas at Austin claimed that concentrated piperazine (PZ) has a significant advantage over MEA in consideration to the reaction rate, volatility, and degradation [6,7]. They focused on solving the solids-solubility problem of concentrated PZ [6] by adding additives [8] or varying the substituent group on PZ molecular [9]. The screening results, although not preferred in Chen and Rochelle’s paper [9], show that 1-methylpiperazine (1MPZ) has lower heat-of-CO2 absorption than, a similar absorption rate to, and higher cyclic capacity than PZ. Thermal degradation results at 150 C from Freeman’s dissertation [10] show that 1MPZ is thermally stable up to 148 C. Freeman [10] also found out that the main degradation products of 1MPZ are PZ and 1,4-dimethyl piperazine (DMPZ). Both 1MPZ and DMPZ have greater volatility than MEA [11]. High volatility will be a problem in commercial use and must be managed. Our previous work [12] shows that, among the derivatives of PZ, 1MPZ is an outstanding solvent in terms of cyclic capacity and
H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128 Table 1 Suppliers and purity of chemicals used.
Nomenclature Excess enthalpy in J/mol Equilibrium partial pressure in kPa Coefficients as defined in Eq. (12) Mole fraction in the liquid phase Mole fraction in the gas phase Universal gas constant in J/mol/K Temperature in K Chemical equilibrium constant Gibbs energy change Enthalpy change Heat capacity Total pressure in kPa Henry’s constant Vapor pressure in kPa Model constant Differential heat of absorption in kJ/mol Fugacity Apparent number of moles of components other than CO2
Greek letters a Nonrandomness factor t ij Binary parameter in ENRTL or NRTL model F Fugacity coefficient g Activity coefficient s Bubble point Superscripts 0 Reference state 1 Infinite dilution aq Aqueous phase ig Ideal gas * Asymmetric Subscripts f Formation j Number of reaction
rough energy requirement. Therefore, this work aims at developing a rigorous thermodynamic model for the 1MPZ/CO2/H2O system to give an accurate calculation of the energy consumption in the capture process. The process optimization and reaction-rate-based process simulation also require a rigorous thermodynamic model to represent all of the relevant thermodynamic properties, especially the vapor–liquid equilibrium (VLE), chemical reaction equilibrium, and heat capacity. The electrolyte nonrandom two-liquid (ENRTL) activity coefficient model is used to represent the excess Gibbs energy of aqueous multicomponent electrolyte systems [13]. Having already been successfully applied to MEA/CO2/H2O system [14–16], PZ/ CO2/H2O system [15,17], and AMP/PZ/CO2/H2O system [18], it was applied to 1MPZ/CO2/H2O in this work. By use of the Redlich–Kwong equation of state, the vapor phase fugacity coefficients were calculated. The model was developed in three steps. First, the vapor pressure and liquid heat capacity of 1MPZ were regressed to get the Antoine’s constants as well as the constants for ideal gas-heat capacity. Second, the binary NRTL parameters were regressed from the VLE and excess enthalpy data for 1MPZ/H2O. In the third and final step, the ENRTL parameters for molecular-ion pair and the standard state properties of protonated 1MPZ, 1MPZ carbamate, and protonated 1MPZ carbamate were
Chemicals
Suppliers
Purity
1-Methyl piperazine Carbon dioxide Nitrogen
J&K Scientific Beijing QX Gas Beijing QX Gas
99 mass% 99.9 mol% 99.9 mol%
obtained by fitting to CO2 solubility data. Since literature concerning the excess enthalpy and CO2 solubility data is limited, these factors were measured in this work to cover a wide range of temperature, 1MPZ concentration, and CO2 loading. 2. Experimental methods 1MPZ with a purity of 99 mass% from J&K Scientific; it was used without further purification. The aqueous solutions were prepared with deionized water. CO2 and nitrogen (N2) with a purity of 99.9 mol% were obtained from Beijing QX Gas. The suppliers and purity of chemicals used are given in Table 1. 2.1. CO2 solubility The CO2 solubility was measured in a 400-cm3 stainless steel reactor. Temperature at the gas and liquid phase was controlled by a heating jacket and measured by two temperature transducers (PT100, Kunlunhaian Co.), with an accuracy of 0.1 K. The total pressure was measured by a pressure transducer (JYB-KO-HAA, Kunlunhaian Co.) with an accuracy of 0.5%. The same transducers were used for a 500-cm3 CO2 gas container, whose temperature and pressure, before and after each injection, gave the total amount of CO2 introduced into the reactor. At the low and high temperature, respectively, twenty-four hours and ten hours were given for absorption equilibrium after each CO2 injection. Gas chromatography (GC) was applied to the gas samples at low temperature and low CO2 partial pressure to obtain the molar ratio of CO2 to N2. The initial total pressure of the unloaded solution at the studied temperature was recorded. The initial partial pressure of H2O in the unloaded solution was calculated by Raoult’s law, and the partial pressure of amine was neglected. Then the initial partial pressure of N2 (PN2 ) was obtained by subtracting the initial partial pressure of H2O from the initial total pressure. PN2 was assumed to
[(Fig._1)TD$IG] 100000 10000 1000 100 PCO2, kPa
HE PCO2 aij, bij x y R T K DG DH Cp P H Ps A,B,C,D DHabs f {n}
119
10 1 0.1
Ref [2] Ref [3] Ref [21] This work
0.01 0.001 0
0.2
0.4
0.6 0.8 1 Loading, mol CO2/mol MEA
Fig. 1. CO2 solubility for 30 wt% MEA + H2O at 313.15 K.
1.2
1.4
120
H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128
[(Fig._2)TD$IG]
[(Fig._4)TD$IG] 100000
1000
10000 1000
100
PCO2, kPa
PCO2, kPa
100 10
10
1 Ref [2]
0.1
Ref [2]
1
Ref [3]
Ref [22]
Ref [21]
0.01
Ref [3]
This work 0.001 0
0.2
0.4 0.6 0.8 Loading, mol CO2/mol MEA
1
this work
0.1
1.2
0
Fig. 2. CO2 solubility for 30 wt% MEA + H2O at 333.15 K.
0.1
0.2 0.3 0.4 Loading, mol CO2/mol MEA
0.5
0.6
Fig. 4. CO2 solubility for 30 wt% MEA + H2O at 393.15 K.
be constant by neglecting the volume change of the solution caused by dissolved CO2. CO2 partial pressure was calculated from the molar ratio of CO2 to N2 from GC results after each CO2 injection. At high temperature and high CO2 partial pressure, CO2 partial pressure at high temperature was read directly from the total pressure increase, with the assumption that the partial pressure of N2 and H2O was constant during each experiment. The precise amount of CO2 introduced into the reactor and in the gas phase was then determined through its volume, pressure, and temperature with the Peng–Robinson (PR) cubic equation [19]. This dissolved CO2 concentration in the liquid phase is expressed in terms of CO2 loading with the unit of mole CO2/mole amine. The uncertainty of loading is 8%, determined from the uncertainties of pressure, temperature and volume, which are 0.5%, 0.1, and 0.5%. The estimated uncertainty of CO2 partial pressure is 2%. A detailed introduction to this can be found in Dong et al. [20].
[(Fig._3)TD$IG]
2.2. Excess enthalpy The measurement for excess enthalpy HE was performed in the C80 calorimeter (Setaram Instrumentation, France), which works on the Tian-Calvet heat-flow principle. The calorimeter works from room temperature to 573.15 K with an accuracy of 0.05 K. Two identical membrane-mixing cells were used, one as the reference cell and the other as sample cell. This type of cell consists of a movable rod with a compeller on the bottom, a chamber that is separated into two parts by a polytetrafluoroethylene (PTFE) membrane, and several stoppers above the chamber along the rod to insulate the heat. The upper part of the cell has a volume of approximately 2.9 mL, and lower part 2.5 mL. Based on their weight, 1MPZ and H2O were put separately in the lower and upper part of the sample chamber, the lighter solvent going in the lower part. In the reference chamber, the premixed 1MPZ + H2O solution with the desired mole fraction was also put in two parts, with the
[(Fig._5)TD$IG]
100000
0 this work
10000
Ref [23]
-500
Ref [24] -1000 HE, J/mol
PCO2, kPa
1000
100
-1500
10 Ref [2] Ref [3]
1
-2000
Ref [21] this work 0.1 0
0.2
0.4
0.6
0.8
Loading, mol CO2/mol MEA Fig. 3. CO2 solubility for 30 wt% MEA + H2O at 373.15 K.
1
-2500 0
0.2
0.4
0.6 xDEA
0.8
Fig. 5. Excess enthalpy for DEA + H2O at 298.15 K.
1
H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128
121
[(Fig._6)TD$IG]
Table 2 Data used for regression. T (K)
Cp (1MPZ)
298– 353 356–410
Ps1MPZ
P (kPa)
17–100 373– 101.3 400 HE (isothermal) 308, 323 pKa 298– 323 313–393 0.01– CO2 solubility 500 VLE (isobaric)
x1MPZ
Data points Reference
221
1
12
[28]
220
1 0.2–0.9
9 13
[29] [30]
0.05–0.9
23 8
This work [25]
0.02– 0.12
144
This work, [9]
Aspen model results Ref [28]
219 Cp, J/mol/K
Data type
222
218 217 216
exactly same weight as in the sample chamber. The uncertainty of the balance used is 0.0001 g. Once the heat flow between two cells was stable at around zero and the desired temperature had been obtained, the membranes were punctured at the same time, and the stirrer was turned on. The heat flow was recorded until it again reached stability at around zero. The uncertainty of the excess enthalpy measurement is 2%.
215 214 213 290
300
310
320
330 T, K
340
350
360
Fig. 6. Heat capacity of 1MPZ.
3. Validation of experimental methods To validate the experimental methods, CO2 solubility for 30 wt% MEA was measured at 313.15 K, 333.15 K, 373.15 K, and 393.15 K, and excess enthalpy for diethanolamine (DEA) + H2O at 298.15 K. The detailed data for 30 wt% MEA and DEA + H2O are presented in Tables A.1 and A.2 in the Appendix. These data were then compared with the data from literature, shown in Figs. 1–5 . For CO2 solubility, CO2 equilibrium partial pressure – from Shen and Li [21] – at 333.15 K and 373.15 K are higher than others’ when it is higher than 1000 kPa, which was not measured in this work. The only CO2 pressure considered was that lower than 1000 kPa. Numbers from Jou et al. [3] are about 20% smaller than those from Lee et al. [2] and Shen and Li [21], especially at 313.15 K and low pressure range. Data from this work, which falls between data from Jou et al. [3] and from Lee et al. [2] – from 313.15 K to 373.15 K – validates the experimental method. Also, data at 393.15 K, from this work, agrees with data from Ma’mum et al. [22]. Excess enthalpy data from Maham et al. [23] are in agreement with the findings of Mundhwa and Henni [24]. The largest relative deviation of this work from the literature happens at the maximum heat released area, which is 2%.
CO2(aqueous) + 2H2O $ H3O+ + HCO3
(2)
HCO3 + H2O $ H3O+ + CO32
(3)
1MPZH+ + H2O $ 1MPZ + H3O+
(4)
1MPZH2+ + H2O $ 1MPZH+ + H3O+
(5)
1MPZCOO + H2O $ 1MPZ + HCO3
(6)
H1MPZCOO + H2O $ 1MPZCOO + H3O+
(7)
The chemical-equilibrium constants were calculated by the standard-state thermodynamic properties of species: RTlnK j ¼ DG0j
4. Thermodynamic model
(8)
where Kj is the chemical equilibrium constant of reaction j,R is the 4.1. Chemical equilibrium
universal gas constant, T is the temperature, and DG0j is the reference state Gibbs energy change of reaction j. The Benson group contribution was used to estimate the
Reactions in the aqueous phase, when CO2 is absorbed into aqueous 1MPZ solution, are expressed as: 2H2O $ H3O+ + OH
standard-state enthalpy of formation (Df Hig 298:15 ) and Gibbs of
(1)
Table 3 Parameters regressed for 1MPZ and their deviations. Parameter
Species
Value
Standard deviation
Parameter
Species
Value
Standard deviation
C ig p /1
1MPZ
25840
Fixed
Antoine/1
1MPZ
64.04
5.10
C ig p /2
1MPZ
3016
315
Antoine/2
1MPZ
16558
2470
C ig p /3
1MPZ
13.35
1.35
Antoine/3
1MPZ
78.03
23.04
C ig p /4
1MPZ
0.014
0.002
Antoine/4
1MPZ
0.045
0.004
2
The temperature dependence of Antoine’s constants is in the form of A þ þ ClnT þ DT , while for B T
the letters from A to D.
C ig p,
2
3
it is A +BT + CT + DT . The numbers from 1 to 4 correspond to
122
[(Fig._7)TD$IG]
H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128
[(Fig._8)TD$IG]
100
0
Ref [29] Aspen model results
303.15K,this work 323.15K,this work
-1000
303.15K,Aspen results
Vapor pressure, kPa
323.15K,Aspen results
HE, J/mol
-2000
-3000
-4000
-5000 10 340
360
380 T, K
400
-6000
420
0
Fig. 7. Vapor pressure of 1MPZ.
0.2
0.4
x1MPZ
0.6
0.8
1
Fig. 8. Excess enthalpy of 1MPZ + H2O.
4.2. Physical equilibrium formation (D for ideal gas at 298.15 K, since 1MPZ is absent in Aspen Plus. The values are 108.06 kJ/mol and 394.63 kJ/mol, respectively. The aqueous phase free energy of formation 1;aq (Df G1;aq 298:15 ) and heat of formation (Df H298:15 ), at infinite dilution and 298.15 K, in addition to the aqueous phase heat capacity, at infinite dilution (C 1;aq ) for 1MPZH+ and 1MPZH2+, were manually p adjusted to fit the pKa values from Khalili et al. [25] For 1;aq 1;aq 1MPZCOOand H1MPZCOO, Df G298:15 , Df H1;aq are 298:15 , and C p regressed from CO2 solubility data, the initial values of which were calculated from the chemical equilibrium constants of Eqs. (6) and (7), as reported by Fernandes et al. [26]. ig f G298:15 )
In an equilibrium system, the fugacity of CO2 in the gas phase is equal that in the liquid phase: yCO2 fCO2 P ¼ xCO2 g CO2 HCO2
(9)
where yCO2 is the mole fraction of CO2 in the gas phase, fCO2 is the fugacity coefficient of CO2 in the gas phase, P is the total pressure, xCO2 is the mole fraction of CO2 in the liquid phase, g CO2 is the asymmetric activity coefficient of CO2 in water, and HCO2 is the Henry’s constant of CO2 in water. The Henry’s constant of CO2 in water was obtained from Chen et al. [27]
[(Fig._9)TD$IG] Table 4 Parameters regressed for 1MPZ/H2O and their deviations (a = 0.3). Parameter
Species 1MPZH+
Value
Parameter Species
310.67 C 1;aq p
1MPZH2+
284.87 aij
1 MPZ/ H2O
Value
410 Ref [30] Standard deviation
Aspen results
15.02
400
Df G1;aq 298:15
Df G1;aq 298:15 23.5
bij
1 MPZ/ H2O
1MPZH2 50.5
aij
H2O/ 1MPZ
1MPZH
D
+
1;aq f H298:15
+
Df H1;aq 298:15
0.63
-1015
Aspen results
395
0.12
T, K
1MPZH2 +
Ref [30]
405
390
48
385
7.06
0.55
380 375
1MPZH+
10.58 bij
C 1;aq p
H2O/ 1MPZ
1358
1;aq 1;aq J/mol/K. The unit of Df G1;aq 298:15 and Df H298:15 is kJ/mol, C p
aij and bij are coefficients defined in Eq. (12).
210
370 0
0.2
0.4
0.6
x1-MPZ (y1-MPZ)
0.8
Fig. 9. VLE of 1MPZ + H2O (upper line: y1MPZ; lower line: x1MPZ).
1
H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128
[(Fig._10)TD$IG]
Table 5 Parameters used to fit 1MPZ/CO2/H2O VLE data (a = 0.2 between ion pair and molecular).
50
Parameter
Species
Df G1;aq 298:15 Df H1;aq 298:15
1MPZCOO
+
300
310
320
330
340
350
aij aij
T, K Fig. 10. Volatility for 1.05 m 1MPZ + H2O.
aij
8477:711 21:957lnTðKÞ lnHCO2 ðPaÞ ¼ 170:7126 TðKÞ þ 0:005781TðKÞ
aij
(10)
(1MPZH ,HCO3 )/H2O H2O/(1MPZH+,HCO3) (1MPZH+,1MPZCOO)/ H2O H2O/(1MPZH+,1MPZCOO) H2O/(1MPZH+,CO32) (1MPZH+,HCO3)/1MPZ (1MPZH+,1MPZCOO)/ 1MPZ (1MPZH+,CO32)/1MPZ (1MPZH+,HCO3)/ H1MPZCOO (1MPZH+,1MPZCOO)/ H1MPZCOO (1MPZH+,CO32)/ H1MPZCOO (1MPZH+,1MPZCOO)/CO2
aij aij aij aij
Ref [31]
-5.27 458.7
H1MPZCOO
aij aij aij
0.5
0.225
H1MPZCOO
C ig p
Aspen calculation
9.59 406.4
H1MPZCOO
Df Gig 298:15 Df Hig 298:15
5
Value
1MPZCOO 1MPZCOO
C 1;aq p
P1MPZ, Pa
123
aij
Standard deviation
Unit
1.23
kJ/mol
10.1
kJ/mol
0.337
kJ/ mol/K kJ/mol
0.81 10.5
0.833
0.219
4.975 8.930 3.095
0.771 1.754 0.855
kJ/mol kJ/ mol/K
4.361 2.886 3.574 22.641 2.050 5.815 0.581 1.485 1.060 4.490
18.311 3.134
10.748
26.148
7.714
1.795
14.573
8.714
For 1MPZ, the equilibrium between vapor and liquid phase is y1MPZ f1MPZ P ¼ x1MPZ g 1MPZ Ps1MPZ
(11)
where y1MPZ and F1MPZ respectively represent the mole fraction and fugacity coefficient of 1MPZ in vapor phase, x1MPZ is 1MPZ mole fraction in liquid phase, g1MPZ is the symmetric activity coefficient of 1MPZ in liquid phase, and Ps1MPZ is the vapor pressure of 1MPZ. The gas phase was described by Redlich–Kwong equation of state model.
dilution in water – i.e., asymmetric reference state – which was also applied to all the ionic species. The reference state for H2O and 1MPZ is pure component, i.e., symmetric reference state. The ENRTL model was applied to calculate the activity coefficients for molecular-molecular binary, molecular-ion pair binary, and ion pair–ion pair binary. The ion pair refers to the pair of cation and anion. The temperature dependence of the binary parameters was expressed by the following:
t ij ¼ aij þ
4.3. Activity coefficient In this work, the Henry’s components are set at H1MPZCOO and CO2. Thus the reference state for H1MPZCOO and CO2 is infinite
[(Fig._1)TD$IG]
bij T
(12)
[(Fig._12)TD$IG]
Unless specified otherwise, the default values for all the molecularmolecular binary parameters and ion pair–ion pair binary
10000
9.5 1000
8.5 100 PCO2, kPa
pKa1 (pKa2)
7.5 Ref [25] Aspen calculation
6.5
10
1
5.5 0.1
4.5 0.01 0
3.5 290
320
T,K
Fig. 11. pKa for 1MPZ.
350
380
0.2
0.4 0.6 0.8 CO2 loading, mol CO2/mol Amine
1
1.2
Fig. 12. CO2 solubility into 10 wt% 1MPZ + H2O (lines, model results; points, this work: [TD$INLE], 313.15 K; [TD$INLE], 343.15 K; [TD$INLE], 373.15 K; [TD$INLE], 393.15 K).
124
H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128
[(Fig._13)TD$IG]
[(Fig._15)TD$IG]
10000
1000
1000
100
PCO2, kPa
PCO2, kPa
100
10
10
1
1
0.1
0.01 0
0.2
0.4 0.6 0.8 CO2 loading, mol CO2/mol Amine
1
0.1
Fig. 13. CO2 solubility into 30 wt% 1MPZ + H2O (lines, model results; points, this work: [TD$INLE], 313.15 K; [TD$INLE], 343.15 K; [TD$INLE], 373.15 K; [TD$INLE], 393.15 K).
parameters are zero. The molecular-ion pair binary parameters aij are defaulted to (8, 4) when the molecular is H2O. When the molecular is 1MPZ, CO2, or H1MPZCOO, the default values for aij are (8, 15). The default values for bij are zero. 5. Modeling results All data used for regression is summarized in Table 2. The same table lists data range and their resource. 5.1. 1MPZ Heat capacity data from Poozesh et al. [28] and vapor pressure data from Chen et al. [29] of 1MPZ were regressed to obtain the ideal gas heat capacity constants (C ig p ) and the Antoine's constants, the values and standard deviations of which are listed in Table 3. The first parameter of C ig p was fixed at 25840, based on several
[(Fig._14)TD$IG]
1000
100
PCO2, kPa
0.1
10
1
0.1
0.2
0.3 0.4 0.5 CO2 loading, mol CO2/mol Amine
0.6
Fig. 15. CO2 solubility into 8 m (44.5 wt%) 1MPZ + H2O (lines, model results; points, Ref. [9]: [TD$INLE], 313.15 K; [TD$INLE], 333.15 K; [TD$INLE], 353.15 K; [TD$INLE], 373.15 K.
runnings of the Aspen regression. The standard deviation for all the parameters listed in Table 3 is much smaller than the values contained therein. Figs. 6 and 7 show good agreement between the data and model results. The average relative deviations are 0.03% for heat capacity and 0.19% for vapor pressure. 5.2. 1MPZ–H2O In this section, excess enthalpy from this work and VLE data from Gu and Zhang [30] were incorporated into the model. Nguyen [31] measured the volatility data for 1.05 m 1MPZ at various temperatures. These data were also regressed in this model. pKa values were regressed by manually adjusting the standard-state 1;aq 1;aq properties of 1MPZH+ and 1MPZH2+, Df G298:15 , Df H1;aq . 298:15 and C p The nonrandomness factor a was fixed at 0.3. Table 4 shows the regressed parameters along with their standard deviations. Figs. 8– 10 demonstrate the comparison with the Aspen calculation. The detailed excess enthalpy data for 1MPZ + H2O are shown in Table A.2, in Appendix A. The largest relative deviation for excess enthalpy regression happens when the mole fraction of 1MPZ is 0.8 at 303.15 K, which comes to 5.7%. The average relative deviation for excess enthalpy regression is 2.6%. In Fig. 9, the upper red line represents for the mole fraction of 1MPZ in the gas phase (y1MPZ), while the lower blue line stands for the mole fraction of 1MPZ in the liquid phase (x1MPZ). When y1MPZ is around 0.65, the model result seems more reliable than the experimental data, since the boiling temperature at 100 kPa is around 410 K, as shown in Fig. 7. The average relative deviation of the mole fraction of 1MPZ in vapor phase y1MPZ is 10.6%. In Fig. 10, volatility data from Nguyen [31] were compared with Aspen calculation. The biggest deviation is 20.2%, which the average is 13.4%. Fig. 11 shows good correlation of pKa values calculated by Aspen with the experimental data from Khalili et al. [25]. 5.3. 1MPZ–H2O–CO2
0.01 0
0.2
0.4 0.6 0.8 CO2 loading, mol CO2/mol Amine
1
Fig. 14. CO2 solubility into 40 wt% 1MPZ + H2O (lines, model results; points, this work: [TD$INLE], 313.15 K; [TD$INLE], 343.15 K; [TD$INLE], 373.15 K; [TD$INLE], 393.15 K).
CO2 solubility data, obtained in this work and from Chen and Rochelle [9], were regressed to get the ENRTL parameters and 1;aq 1;aq for standard-state properties Df G1;aq 298:15 , Df H298:15 , and C p ig ig 1MPZCOO and Df Hig 298:15 , Df G298:15 , and C p for H1MPZCOO. The
H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128
[(Fig._16)TD$IG] 90
90
a
80
b
80
70
70
60
60 -ΔHabs/ kJ/mol
-ΔHabs/ kJ/mol
125
50
50
40
40
30
30
20
20
10
10
0
0 0.1
0.2
0.3
0.4 0.5 0.6 0.7 Ldg, mol CO2/mol 1MPZ
0.8
0.9
1
0.1
0.2
0.3
0.4 0.5 0.6 0.7 Ldg, mol CO2/mol 1MPZ
0.8
0.9
1
90
c
80 70
-ΔHabs/ kJ/mol
60 50 40 30 20 10 0 0.1
0.2
0.3
0.4 0.5 0.6 0.7 Ldg, mol CO2/mol 1MPZ
0.8
0.9
1
Fig. 16. Heat of absorption for 10 wt% (a), 30 wt% (b), 40 wt% (c) 1MPZ + H2O solutions at various temperatures (blue, 313.15 K; red 333.15 K; green, 353.15 K; purple, 373.15 K; black, 393.15 K). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
nonrandomness factor a was fixed at 0.2. The parameters, together with their standard deviations, are presented in Table 5. Figs. 12–15 show the experimental data and the calculation by the model. The first experimental point from Chen and Rochelle [9] at 333.15 K, in Fig. 15, is apparently inconsistent with other data; therefore, it was not included in the regression. The experimental data were correlated by the model. The average relative deviation of the regression for 10 wt% 1MPZ in Fig. 12 is 9.6%, while it is 12.5% for 30 wt% 1MPZ in Fig. 13, 10.0% for 40 wt% 1MPZ in Fig. 14, and 14.2% for 8 m (44.5 wt%) 1MPZ from Chen and Rochelle [9] in Fig. 15. Detailed data from the measurement in this work are attached in Tables A.3–5, in Appendix A.
5.4. Heat of absorption The Gibbs–Helmholtz equation gives a good approximation of the differential heat of CO2 absorption, as verified by Mathias and O’Connell [32]: @lnf CO2 DHabs R (13) @ð1=TÞ s;fng where DHabs is the differential heat of absorption, f CO2 is the fugacity of CO2, s means the bubble point, {n} is the apparent number of moles of components other than CO2.
Table 6 Cyclic capacity for 10 wt%, 30 wt%, and 40 wt% 1MPZ solutions. Concentration of 1MPZ wt%
Lean loading mol CO2/mol 1MPZ
Rich loading
Loading difference
Cyclic capacity g CO2/kg solution
10 30 40
0.409 0.392 0.402
0.689 0.595 0.582
0.28 0.203 0.18
12.3 26.8 31.6
126
H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128
[(Fig._17)TD$IG] 1.5
1MPZ 1MPZH+2 HCO3H1MPZCOO
1.2
8
1MPZH+ 1MPZCOOCO3-2 CO2
1MPZ 1MPZH+2 HCO3H1MPZCOO
a 7
1MPZH+ 1MPZCOOCO3-2 CO2
b
Activity coefficient
Activity coefficient
6
0.9
0.6
5 4 3 2
0.3 1 0
0 0.1
0.2
0.3
0.4 0.5 0.6 0.7 Loading, mol CO2/mol amine
30
0.8
0.9
1MPZ 1MPZH+2 HCO3H1MPZCOO
25
1
0.1
0.2
0.3
0.4 0.5 0.6 0.7 Loading, mol CO2/mol amine
1MPZH+ 1MPZCOOCO3-2 CO2
0.8
0.9
1
c
Activity coefficient
20
15
10
5
0 0.1
0.2
0.3
0.4 0.5 0.6 0.7 0.8 Loading, mol CO2/mol amine
0.9
1
Fig. 17. Activity coefficient of species when CO2 is absorbed into 10 wt% (a), 30 wt% (b), 40 wt% (c) 1MPZ + H2O solutions at 313.15 K.
Heat of absorption for 10 wt%, 30 wt%, and 40 wt% 1MPZ solutions at various temperatures are shown in Fig. 16. At 313.15 K, the heat of absorption is less dependent on loading when the loading is lower than 0.8. When the loading is higher, the heat of absorption decreases dramatically. As the temperature goes higher, the loading dependence becomes stronger, while the values are higher when the loading is lower than some point, after which, the heat of absorption decreases as the temperature increases. However, heat of absorption turns to go lower as the temperature goes higher than 373.15 K at the whole studied loading range. The concentration of 1MPZ shows a positive effect on the heat of absorption at low loadings and a negative effect at high loadings. Also, the trend is different at 393.15 K. The concentration of 1MPZ shows a positive effect on the heat of absorption at the whole studied loading range.
5.5. Cyclic capacity Cyclic capacity is an important property for characterizing amine performance. Considering 90% CO2 removal rate in the absorber, lean loading was defined as the CO2 loading when the partial pressure of CO2 is 1 kPa at 313.15 K, and rich loading as 10 kPa of CO2 partial pressure at 313.15 K. Cyclic capacity is the difference between rich and lean loading at the unit of g of CO2/ kg of solvent. Calculated results for 10 wt%, 30 wt%, and 40 wt% 1MPZ solutions are shown in Table 6. Cyclic capacity for 30 wt% 1MPZ is one time higher than 10 wt%, even though the concentration two times higher. The reason is that when 1MPZ concentration is higher, the loading difference between rich and lean loading is smaller. The same result can be seen from the comparison of 10 wt% with 40 wt% in Table 6. The 1MPZ
H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128
127
[(Fig._18)TD$IG] 0.02
x1MPZ x1MPZH+2 xHCO3xH1MPZCOO
0.08
x1MPZH+ x1MPZCOOxCO3-2
0.07
0.015
x1MPZH+ x1MPZCOOxCO3-2
b
0.06 Mole fraction
Mole fraction
x1MPZ x1MPZH+2 xHCO3xH1MPZCOO
a
0.01
0.05
0.04 0.03 0.02
0.005
0.01 0
0 0
0.2
0.4 0.6 Loading, mol CO2/mol amine 0.12
0.8
1
x1MPZ x1MPZH+2 xHCO3xH1MPZCOO
0.1
0
0.2
x1MPZH+ x1MPZCOOxCO3-2
0.4 0.6 Loading, mol CO2/mol amine
0.8
1
c
Mole fraction
0.08
0.06
0.04
0.02
0 0
0.2
0.4 0.6 Loading, mol CO2/mol amine
0.8
1
Fig. 18. Speciation for CO2 absorbed into 10 wt% (a), 30 wt% (b), 40 wt% (c) 1MPZ + H2O solutions at 313.15 K.
concentration is three times higher, while the cyclic capacity is only 1.5 times higher.
5.6. Activity coefficient and speciation Activity coefficient and speciation data for 10 wt%, 30 wt%, and 40 wt% 1MPZ solutions at 313.15 K were predicted by the model and calculated in Figs. 17 and 18. As the concentration of 1MPZ goes higher, the activity coefficient of all the species becomes larger, especially that of CO32 and HCO3. From Fig. 18, it is seen that when the loading is lower than 0.5, most of the CO2 absorbed is converted into MPZCOO. At greater loading, 1MPZCOO is consumed and converted to H1MPZCOO and HCO3. CO32 is never an important product during the whole studied loading range. As the concentration of 1MPZ increases from 10 wt% to 40 wt%, the proportion of 1MPZH+ and HCO3 decreases, while the proportion of H1MPZCOO increases.
6. Conclusions CO2 solubility for 10 wt%, 30 wt% and 40 wt% 1MPZ – with the CO2 loading up to 1 and CO2 partial pressure up to 500 kPa – have been measured in this work. The temperature ranges from 313.15 to 393.15 K. Excess enthalpy data over the whole mole fraction range, at 303.15 K and 323.15 K, have also been obtained. Using the ENRTL model, and calling on data from literature, a rigorous thermodynamic model for 1MPZ/CO2/H2O system has been built. The model works effectively at various 1MPZ concentrations and temperatures. Successfully represented by the model were various kinds of data, including vapor pressure and heat capacity data for 1MPZ, binary vapor–liquid equilibrium, excess enthalpy, pKa data for 1MPZ/H2O, and CO2 solubility data for 1MPZ/CO2/H2O. The model can be used for the modeling and simulation of the CO2-capture process. The calculated heat of absorption results show a plateau at 313.15 K, and are more dependent on loading as the temperature is
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higher. As 1MPZ concentration goes higher, heat of absorption is higher at low temperatures and lower at high temperatures, while cyclic capacity goes higher, activity coefficient of all the species becomes larger, especially that of CO32 and HCO3. At greater loading, the main products are H1MPZCOO and HCO3. CO32 is never an important product during the whole studied loading range. Acknowledgments This work was supported by EDF research foundation, the National Natural Science Foundation of China (No. 51134017) and State Key Laboratory of Chemical Engineering of China (SKL-ChE-12Z01). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fluid.2015.03.021. References [1] G.T. Rochelle, Science 325 (2009) 1652–1654. [2] J.I. Lee, D.O. Frederick, A.E. Mather, J. Appl. Chem. Biotechnol. 26 (1976) 541– 546. [3] F.-Y. Jou, A.E. Mather, F.D. Otto, Can. J. Chem. Eng. 73 (1995) 140–147. [4] J.P. Jakobsen, J. Krane, H.F. Svendsen, Ind. Eng. Chem. Res. 44 (2005) 9894–9903. [5] R. Idem, M. Wilson, P. Tontiwachwuthikul, A. Chakma, A. Veawab, A. Aroonwilas, D. Gelowitz, Ind. Eng. Chem. Res. 45 (2006) 2414–2420. [6] S.A. Freeman, R. Dugas, D.H. Van Wagener, T. Nguyen, G.T. Rochelle, Int. J. Greenhouse Gas Control 4 (2010) 119–124.
[7] G. Rochelle, E. Chen, S. Freeman, D. Van Wagener, Q. Xu, A. Voice, Chem. Eng. J. 171 (2011) 725–733. [8] L. Li, A.K. Voice, H. Li, O. Namjoshi, T. Nguyen, Y. Du, Energy Procedia 37 (2013) 353–369. [9] X. Chen, G.T. Rochelle, Chem. Eng. Res. Des. 89 (2011) 1693–1710. [10] S.A. Freeman, Thermal degradation and oxidation of aqueous piperazine for carbon dioxide capture, Ph.D. Dissertation, The University of Texas at Austin, Austin, TX, 2011 [11] S.A. Freeman, X. Chen, T. Nguyen, H. Rafique, Q. Xu, G.T. Rochelle, Oil Gas Sci. Technol. 69 (2014) 903–914. [12] H. Li, Y. Le Moullec, J.-H. Lu, J. Chen, J.C. Valle Marcos, G.-F. Chen, Int. J. Greenhouse Gas Control 31 (2014) 25–32. [13] C.-C. Chen, L.B. Evans, AICheE J. 32 (1986) 444–454. [14] Y. Zhang, H. Que, C.-C. Chen, Fluid Phase Equilib. 311 (2011) 67–75. [15] M.A. Hilliard, A predictive thermodynamic model for an aqueous blend of potassium carbonate, piperzine, and monoethanolamine for carbon dioxide, Ph.D. Dissertation, The University of Texas at Austin, Austin, TX, 2008 [16] E.T. Hessen, T. Haug-Warberg, H.F. Svendsen, Chem. Eng. Sci. 65 (2010) 3638– 3648. [17] P.T. Frailie, J.M. Plaza, D.H. Van Wagener, G.T. Rochelle, Energy Procedia 4 (2011) 35–42. [18] H. Li, P.T. Frailie, G.T. Rochelle, J. Chen, Chem. Eng. Sci. 117 (2014) 331–341. [19] D.Y. Peng, D.B. Robinson, Ind. Eng. Chem. Fundam. 15 (1976) 59–64. [20] L.-H. Dong, J. Chen, G.-H. Gao, J. Chem. Eng. Data 55 (2010) 1030–1034. [21] K.-P. Shen, M.-H. Li, J. Chem. Eng. Data 37 (1992) 96–100. [22] S. Ma'mum, R. Nilsen, H.F. Svendsen, J. Chem. Eng. Data. 50 (2005) 630–634. [23] Y. Maham, A.E. Mather, C. Mathonat, J. Chem. Thermodyn. 32 (2000) 229–236. [24] M. Mundhwa, A. Henni, J. Chem. Thermodyn. 39 (2007) 1439–1451. [25] F. Khalili, A. Henni, A.L.L. East, J. Chem. Eng. Data 54 (2009) 2914–2917. [26] D. Fernandes, W. Conway, R. Burns, G. Lawrance, M. Maeder, G. Puxty, J. Chem. Thermodyn. 54 (2012) 183–191. [27] C.-C. Chen, H.I. Britt, J.F. Boston, L.B. Evans, AIChE J. 25 (1979) 820–831. [28] S. Poozesh, A.V. Rayer, A. Henni, J. Chem. Eng. Data 58 (2013) 1989–2000. [29] S. Chen, W.-J. Fang, J. Yao, H.-X. Zong, J. Chem. Eng. Data 46 (2001) 596–600. [30] F.-Y. Gu, Z.-M. Zhang, CIESC J. 49 (1998) 372–376. [31] T. Nguyen, Amine volatility in CO2 capture, Ph.D. Dissertation, The University of Texas at Austin, Austin, TX, 2013 [32] P.M. Mathias, J.P. O’Connell, Ind. Eng. Chem. Res. 21 (2012) 5090–5097.
Contents lists available at ScienceDirect
Fluid Phase Equilibria j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u i d
CO2 solubility measurement and thermodynamic modeling for 1-methylpiperazine/water/CO2 Han Li a , Yann Le Moullec b , Jiahui Lu c, Jian Chen a, * , Jose Carlos Valle Marcos c, Guofei Chen c, Fabrice Chopin c a b c
State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Fluid Dynamics, Power Generation and Environment Department, EDF R&D, 6 Quai Watier, 78401 Chatou Cedex, France EDF China R&D Center, EDF Asia Pacific Direction, China Division, Beijing 100005, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 8 October 2014 Received in revised form 3 March 2015 Accepted 10 March 2015 Available online 14 March 2015
An accurate thermodynamic model is the primary element needed for the process simulation and optimization for CO2 absorption in aqueous amine solutions. In this work, the thermodynamic model was built in Aspen Plus, using the electrolyte nonrandom two-liquid (ENRTL) activity coefficient model to represent vapor pressure and heat capacity data, simultaneously, for amine, vapor–liquid equilibrium (VLE), excess enthalpy (HE), and pKa data for amine/H2O, and CO2 solubility data for amine/CO2/H2O. The cyclic diamine 1-methylpiperazine (1MPZ) is a promising amine for CO2 capture. CO2 solubility was measured for 1MPZ aqueous solutions at three concentrations – 10 wt%, 30 wt%, and 40 wt% and four temperatures – 313.15 K, 343.15 K, 373.15 K, and 393.15 K. The excess enthalpy for 1MPZ + H2O was obtained by the Setaram C80 calorimeter at 303.15 K and 323.15 K, within a whole mole-fraction range. The interaction parameters of nonrandom two-liquid model (NRTL) and ENRTL, along with the standard state properties of amine ions – protonated 1MPZ (1MPZH+, 1MPZH2+), 1MPZ carbamate (1MPZCOO), and protonated 1MPZ carbamate (H1MPZCOO) – were regressed from data obtained from this work as well as literature, which agreed with the model calculation. ã 2015 Elsevier B.V. All rights reserved.
Keywords: CO2 solubility Thermodynamic model 1-Methylpiperazine Excess enthalpy
1. Introduction Amine scrubbing – promising technology for the reduction of CO2 emissions from conventional coal-fired power plants – has been successfully applied in ammonia production and natural gas process [1]. The challenges of applying amine scrubbing on a large scale center on the unacceptably high energy consumption, and hence cost, which owes to the large flow rate of the flue gas and its low CO2 partial pressure. The bulk of the required energy is devoted to desorbing CO2 from the rich amine solutions in the reboiler at the bottom of the stripper – that is, to regenerating the amine, which is directly related to the CO2 absorption heat of amine. High cyclic CO2 capacity reduces the circulating rate of amine solution, thus reducing the size of the absorber, pump, and heat exchanger, all of which mainly determine the equipment cost. Furthermore, the use of fast kinetics saves the amount of packing, thereby decreasing the size of absorber. Developing new amines
* Corresponding author. Tel.: +86 10 62782748; fax: +86 10 62770304. E-mail address: [email protected] (J. Chen). http://dx.doi.org/10.1016/j.fluid.2015.03.021 0378-3812/ ã 2015 Elsevier B.V. All rights reserved.
with the favorable properties represents one way to lower regeneration energy and cost. At the first stage of searching for the best amine for CO2 capture, aqueous monoethanolamine (MEA) attracted a wide range of research [2–5]; this is usually used as a reference. Rochelle’s group at the University of Texas at Austin claimed that concentrated piperazine (PZ) has a significant advantage over MEA in consideration to the reaction rate, volatility, and degradation [6,7]. They focused on solving the solids-solubility problem of concentrated PZ [6] by adding additives [8] or varying the substituent group on PZ molecular [9]. The screening results, although not preferred in Chen and Rochelle’s paper [9], show that 1-methylpiperazine (1MPZ) has lower heat-of-CO2 absorption than, a similar absorption rate to, and higher cyclic capacity than PZ. Thermal degradation results at 150 C from Freeman’s dissertation [10] show that 1MPZ is thermally stable up to 148 C. Freeman [10] also found out that the main degradation products of 1MPZ are PZ and 1,4-dimethyl piperazine (DMPZ). Both 1MPZ and DMPZ have greater volatility than MEA [11]. High volatility will be a problem in commercial use and must be managed. Our previous work [12] shows that, among the derivatives of PZ, 1MPZ is an outstanding solvent in terms of cyclic capacity and
H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128 Table 1 Suppliers and purity of chemicals used.
Nomenclature Excess enthalpy in J/mol Equilibrium partial pressure in kPa Coefficients as defined in Eq. (12) Mole fraction in the liquid phase Mole fraction in the gas phase Universal gas constant in J/mol/K Temperature in K Chemical equilibrium constant Gibbs energy change Enthalpy change Heat capacity Total pressure in kPa Henry’s constant Vapor pressure in kPa Model constant Differential heat of absorption in kJ/mol Fugacity Apparent number of moles of components other than CO2
Greek letters a Nonrandomness factor t ij Binary parameter in ENRTL or NRTL model F Fugacity coefficient g Activity coefficient s Bubble point Superscripts 0 Reference state 1 Infinite dilution aq Aqueous phase ig Ideal gas * Asymmetric Subscripts f Formation j Number of reaction
rough energy requirement. Therefore, this work aims at developing a rigorous thermodynamic model for the 1MPZ/CO2/H2O system to give an accurate calculation of the energy consumption in the capture process. The process optimization and reaction-rate-based process simulation also require a rigorous thermodynamic model to represent all of the relevant thermodynamic properties, especially the vapor–liquid equilibrium (VLE), chemical reaction equilibrium, and heat capacity. The electrolyte nonrandom two-liquid (ENRTL) activity coefficient model is used to represent the excess Gibbs energy of aqueous multicomponent electrolyte systems [13]. Having already been successfully applied to MEA/CO2/H2O system [14–16], PZ/ CO2/H2O system [15,17], and AMP/PZ/CO2/H2O system [18], it was applied to 1MPZ/CO2/H2O in this work. By use of the Redlich–Kwong equation of state, the vapor phase fugacity coefficients were calculated. The model was developed in three steps. First, the vapor pressure and liquid heat capacity of 1MPZ were regressed to get the Antoine’s constants as well as the constants for ideal gas-heat capacity. Second, the binary NRTL parameters were regressed from the VLE and excess enthalpy data for 1MPZ/H2O. In the third and final step, the ENRTL parameters for molecular-ion pair and the standard state properties of protonated 1MPZ, 1MPZ carbamate, and protonated 1MPZ carbamate were
Chemicals
Suppliers
Purity
1-Methyl piperazine Carbon dioxide Nitrogen
J&K Scientific Beijing QX Gas Beijing QX Gas
99 mass% 99.9 mol% 99.9 mol%
obtained by fitting to CO2 solubility data. Since literature concerning the excess enthalpy and CO2 solubility data is limited, these factors were measured in this work to cover a wide range of temperature, 1MPZ concentration, and CO2 loading. 2. Experimental methods 1MPZ with a purity of 99 mass% from J&K Scientific; it was used without further purification. The aqueous solutions were prepared with deionized water. CO2 and nitrogen (N2) with a purity of 99.9 mol% were obtained from Beijing QX Gas. The suppliers and purity of chemicals used are given in Table 1. 2.1. CO2 solubility The CO2 solubility was measured in a 400-cm3 stainless steel reactor. Temperature at the gas and liquid phase was controlled by a heating jacket and measured by two temperature transducers (PT100, Kunlunhaian Co.), with an accuracy of 0.1 K. The total pressure was measured by a pressure transducer (JYB-KO-HAA, Kunlunhaian Co.) with an accuracy of 0.5%. The same transducers were used for a 500-cm3 CO2 gas container, whose temperature and pressure, before and after each injection, gave the total amount of CO2 introduced into the reactor. At the low and high temperature, respectively, twenty-four hours and ten hours were given for absorption equilibrium after each CO2 injection. Gas chromatography (GC) was applied to the gas samples at low temperature and low CO2 partial pressure to obtain the molar ratio of CO2 to N2. The initial total pressure of the unloaded solution at the studied temperature was recorded. The initial partial pressure of H2O in the unloaded solution was calculated by Raoult’s law, and the partial pressure of amine was neglected. Then the initial partial pressure of N2 (PN2 ) was obtained by subtracting the initial partial pressure of H2O from the initial total pressure. PN2 was assumed to
[(Fig._1)TD$IG] 100000 10000 1000 100 PCO2, kPa
HE PCO2 aij, bij x y R T K DG DH Cp P H Ps A,B,C,D DHabs f {n}
119
10 1 0.1
Ref [2] Ref [3] Ref [21] This work
0.01 0.001 0
0.2
0.4
0.6 0.8 1 Loading, mol CO2/mol MEA
Fig. 1. CO2 solubility for 30 wt% MEA + H2O at 313.15 K.
1.2
1.4
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[(Fig._2)TD$IG]
[(Fig._4)TD$IG] 100000
1000
10000 1000
100
PCO2, kPa
PCO2, kPa
100 10
10
1 Ref [2]
0.1
Ref [2]
1
Ref [3]
Ref [22]
Ref [21]
0.01
Ref [3]
This work 0.001 0
0.2
0.4 0.6 0.8 Loading, mol CO2/mol MEA
1
this work
0.1
1.2
0
Fig. 2. CO2 solubility for 30 wt% MEA + H2O at 333.15 K.
0.1
0.2 0.3 0.4 Loading, mol CO2/mol MEA
0.5
0.6
Fig. 4. CO2 solubility for 30 wt% MEA + H2O at 393.15 K.
be constant by neglecting the volume change of the solution caused by dissolved CO2. CO2 partial pressure was calculated from the molar ratio of CO2 to N2 from GC results after each CO2 injection. At high temperature and high CO2 partial pressure, CO2 partial pressure at high temperature was read directly from the total pressure increase, with the assumption that the partial pressure of N2 and H2O was constant during each experiment. The precise amount of CO2 introduced into the reactor and in the gas phase was then determined through its volume, pressure, and temperature with the Peng–Robinson (PR) cubic equation [19]. This dissolved CO2 concentration in the liquid phase is expressed in terms of CO2 loading with the unit of mole CO2/mole amine. The uncertainty of loading is 8%, determined from the uncertainties of pressure, temperature and volume, which are 0.5%, 0.1, and 0.5%. The estimated uncertainty of CO2 partial pressure is 2%. A detailed introduction to this can be found in Dong et al. [20].
[(Fig._3)TD$IG]
2.2. Excess enthalpy The measurement for excess enthalpy HE was performed in the C80 calorimeter (Setaram Instrumentation, France), which works on the Tian-Calvet heat-flow principle. The calorimeter works from room temperature to 573.15 K with an accuracy of 0.05 K. Two identical membrane-mixing cells were used, one as the reference cell and the other as sample cell. This type of cell consists of a movable rod with a compeller on the bottom, a chamber that is separated into two parts by a polytetrafluoroethylene (PTFE) membrane, and several stoppers above the chamber along the rod to insulate the heat. The upper part of the cell has a volume of approximately 2.9 mL, and lower part 2.5 mL. Based on their weight, 1MPZ and H2O were put separately in the lower and upper part of the sample chamber, the lighter solvent going in the lower part. In the reference chamber, the premixed 1MPZ + H2O solution with the desired mole fraction was also put in two parts, with the
[(Fig._5)TD$IG]
100000
0 this work
10000
Ref [23]
-500
Ref [24] -1000 HE, J/mol
PCO2, kPa
1000
100
-1500
10 Ref [2] Ref [3]
1
-2000
Ref [21] this work 0.1 0
0.2
0.4
0.6
0.8
Loading, mol CO2/mol MEA Fig. 3. CO2 solubility for 30 wt% MEA + H2O at 373.15 K.
1
-2500 0
0.2
0.4
0.6 xDEA
0.8
Fig. 5. Excess enthalpy for DEA + H2O at 298.15 K.
1
H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128
121
[(Fig._6)TD$IG]
Table 2 Data used for regression. T (K)
Cp (1MPZ)
298– 353 356–410
Ps1MPZ
P (kPa)
17–100 373– 101.3 400 HE (isothermal) 308, 323 pKa 298– 323 313–393 0.01– CO2 solubility 500 VLE (isobaric)
x1MPZ
Data points Reference
221
1
12
[28]
220
1 0.2–0.9
9 13
[29] [30]
0.05–0.9
23 8
This work [25]
0.02– 0.12
144
This work, [9]
Aspen model results Ref [28]
219 Cp, J/mol/K
Data type
222
218 217 216
exactly same weight as in the sample chamber. The uncertainty of the balance used is 0.0001 g. Once the heat flow between two cells was stable at around zero and the desired temperature had been obtained, the membranes were punctured at the same time, and the stirrer was turned on. The heat flow was recorded until it again reached stability at around zero. The uncertainty of the excess enthalpy measurement is 2%.
215 214 213 290
300
310
320
330 T, K
340
350
360
Fig. 6. Heat capacity of 1MPZ.
3. Validation of experimental methods To validate the experimental methods, CO2 solubility for 30 wt% MEA was measured at 313.15 K, 333.15 K, 373.15 K, and 393.15 K, and excess enthalpy for diethanolamine (DEA) + H2O at 298.15 K. The detailed data for 30 wt% MEA and DEA + H2O are presented in Tables A.1 and A.2 in the Appendix. These data were then compared with the data from literature, shown in Figs. 1–5 . For CO2 solubility, CO2 equilibrium partial pressure – from Shen and Li [21] – at 333.15 K and 373.15 K are higher than others’ when it is higher than 1000 kPa, which was not measured in this work. The only CO2 pressure considered was that lower than 1000 kPa. Numbers from Jou et al. [3] are about 20% smaller than those from Lee et al. [2] and Shen and Li [21], especially at 313.15 K and low pressure range. Data from this work, which falls between data from Jou et al. [3] and from Lee et al. [2] – from 313.15 K to 373.15 K – validates the experimental method. Also, data at 393.15 K, from this work, agrees with data from Ma’mum et al. [22]. Excess enthalpy data from Maham et al. [23] are in agreement with the findings of Mundhwa and Henni [24]. The largest relative deviation of this work from the literature happens at the maximum heat released area, which is 2%.
CO2(aqueous) + 2H2O $ H3O+ + HCO3
(2)
HCO3 + H2O $ H3O+ + CO32
(3)
1MPZH+ + H2O $ 1MPZ + H3O+
(4)
1MPZH2+ + H2O $ 1MPZH+ + H3O+
(5)
1MPZCOO + H2O $ 1MPZ + HCO3
(6)
H1MPZCOO + H2O $ 1MPZCOO + H3O+
(7)
The chemical-equilibrium constants were calculated by the standard-state thermodynamic properties of species: RTlnK j ¼ DG0j
4. Thermodynamic model
(8)
where Kj is the chemical equilibrium constant of reaction j,R is the 4.1. Chemical equilibrium
universal gas constant, T is the temperature, and DG0j is the reference state Gibbs energy change of reaction j. The Benson group contribution was used to estimate the
Reactions in the aqueous phase, when CO2 is absorbed into aqueous 1MPZ solution, are expressed as: 2H2O $ H3O+ + OH
standard-state enthalpy of formation (Df Hig 298:15 ) and Gibbs of
(1)
Table 3 Parameters regressed for 1MPZ and their deviations. Parameter
Species
Value
Standard deviation
Parameter
Species
Value
Standard deviation
C ig p /1
1MPZ
25840
Fixed
Antoine/1
1MPZ
64.04
5.10
C ig p /2
1MPZ
3016
315
Antoine/2
1MPZ
16558
2470
C ig p /3
1MPZ
13.35
1.35
Antoine/3
1MPZ
78.03
23.04
C ig p /4
1MPZ
0.014
0.002
Antoine/4
1MPZ
0.045
0.004
2
The temperature dependence of Antoine’s constants is in the form of A þ þ ClnT þ DT , while for B T
the letters from A to D.
C ig p,
2
3
it is A +BT + CT + DT . The numbers from 1 to 4 correspond to
122
[(Fig._7)TD$IG]
H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128
[(Fig._8)TD$IG]
100
0
Ref [29] Aspen model results
303.15K,this work 323.15K,this work
-1000
303.15K,Aspen results
Vapor pressure, kPa
323.15K,Aspen results
HE, J/mol
-2000
-3000
-4000
-5000 10 340
360
380 T, K
400
-6000
420
0
Fig. 7. Vapor pressure of 1MPZ.
0.2
0.4
x1MPZ
0.6
0.8
1
Fig. 8. Excess enthalpy of 1MPZ + H2O.
4.2. Physical equilibrium formation (D for ideal gas at 298.15 K, since 1MPZ is absent in Aspen Plus. The values are 108.06 kJ/mol and 394.63 kJ/mol, respectively. The aqueous phase free energy of formation 1;aq (Df G1;aq 298:15 ) and heat of formation (Df H298:15 ), at infinite dilution and 298.15 K, in addition to the aqueous phase heat capacity, at infinite dilution (C 1;aq ) for 1MPZH+ and 1MPZH2+, were manually p adjusted to fit the pKa values from Khalili et al. [25] For 1;aq 1;aq 1MPZCOOand H1MPZCOO, Df G298:15 , Df H1;aq are 298:15 , and C p regressed from CO2 solubility data, the initial values of which were calculated from the chemical equilibrium constants of Eqs. (6) and (7), as reported by Fernandes et al. [26]. ig f G298:15 )
In an equilibrium system, the fugacity of CO2 in the gas phase is equal that in the liquid phase: yCO2 fCO2 P ¼ xCO2 g CO2 HCO2
(9)
where yCO2 is the mole fraction of CO2 in the gas phase, fCO2 is the fugacity coefficient of CO2 in the gas phase, P is the total pressure, xCO2 is the mole fraction of CO2 in the liquid phase, g CO2 is the asymmetric activity coefficient of CO2 in water, and HCO2 is the Henry’s constant of CO2 in water. The Henry’s constant of CO2 in water was obtained from Chen et al. [27]
[(Fig._9)TD$IG] Table 4 Parameters regressed for 1MPZ/H2O and their deviations (a = 0.3). Parameter
Species 1MPZH+
Value
Parameter Species
310.67 C 1;aq p
1MPZH2+
284.87 aij
1 MPZ/ H2O
Value
410 Ref [30] Standard deviation
Aspen results
15.02
400
Df G1;aq 298:15
Df G1;aq 298:15 23.5
bij
1 MPZ/ H2O
1MPZH2 50.5
aij
H2O/ 1MPZ
1MPZH
D
+
1;aq f H298:15
+
Df H1;aq 298:15
0.63
-1015
Aspen results
395
0.12
T, K
1MPZH2 +
Ref [30]
405
390
48
385
7.06
0.55
380 375
1MPZH+
10.58 bij
C 1;aq p
H2O/ 1MPZ
1358
1;aq 1;aq J/mol/K. The unit of Df G1;aq 298:15 and Df H298:15 is kJ/mol, C p
aij and bij are coefficients defined in Eq. (12).
210
370 0
0.2
0.4
0.6
x1-MPZ (y1-MPZ)
0.8
Fig. 9. VLE of 1MPZ + H2O (upper line: y1MPZ; lower line: x1MPZ).
1
H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128
[(Fig._10)TD$IG]
Table 5 Parameters used to fit 1MPZ/CO2/H2O VLE data (a = 0.2 between ion pair and molecular).
50
Parameter
Species
Df G1;aq 298:15 Df H1;aq 298:15
1MPZCOO
+
300
310
320
330
340
350
aij aij
T, K Fig. 10. Volatility for 1.05 m 1MPZ + H2O.
aij
8477:711 21:957lnTðKÞ lnHCO2 ðPaÞ ¼ 170:7126 TðKÞ þ 0:005781TðKÞ
aij
(10)
(1MPZH ,HCO3 )/H2O H2O/(1MPZH+,HCO3) (1MPZH+,1MPZCOO)/ H2O H2O/(1MPZH+,1MPZCOO) H2O/(1MPZH+,CO32) (1MPZH+,HCO3)/1MPZ (1MPZH+,1MPZCOO)/ 1MPZ (1MPZH+,CO32)/1MPZ (1MPZH+,HCO3)/ H1MPZCOO (1MPZH+,1MPZCOO)/ H1MPZCOO (1MPZH+,CO32)/ H1MPZCOO (1MPZH+,1MPZCOO)/CO2
aij aij aij aij
Ref [31]
-5.27 458.7
H1MPZCOO
aij aij aij
0.5
0.225
H1MPZCOO
C ig p
Aspen calculation
9.59 406.4
H1MPZCOO
Df Gig 298:15 Df Hig 298:15
5
Value
1MPZCOO 1MPZCOO
C 1;aq p
P1MPZ, Pa
123
aij
Standard deviation
Unit
1.23
kJ/mol
10.1
kJ/mol
0.337
kJ/ mol/K kJ/mol
0.81 10.5
0.833
0.219
4.975 8.930 3.095
0.771 1.754 0.855
kJ/mol kJ/ mol/K
4.361 2.886 3.574 22.641 2.050 5.815 0.581 1.485 1.060 4.490
18.311 3.134
10.748
26.148
7.714
1.795
14.573
8.714
For 1MPZ, the equilibrium between vapor and liquid phase is y1MPZ f1MPZ P ¼ x1MPZ g 1MPZ Ps1MPZ
(11)
where y1MPZ and F1MPZ respectively represent the mole fraction and fugacity coefficient of 1MPZ in vapor phase, x1MPZ is 1MPZ mole fraction in liquid phase, g1MPZ is the symmetric activity coefficient of 1MPZ in liquid phase, and Ps1MPZ is the vapor pressure of 1MPZ. The gas phase was described by Redlich–Kwong equation of state model.
dilution in water – i.e., asymmetric reference state – which was also applied to all the ionic species. The reference state for H2O and 1MPZ is pure component, i.e., symmetric reference state. The ENRTL model was applied to calculate the activity coefficients for molecular-molecular binary, molecular-ion pair binary, and ion pair–ion pair binary. The ion pair refers to the pair of cation and anion. The temperature dependence of the binary parameters was expressed by the following:
t ij ¼ aij þ
4.3. Activity coefficient In this work, the Henry’s components are set at H1MPZCOO and CO2. Thus the reference state for H1MPZCOO and CO2 is infinite
[(Fig._1)TD$IG]
bij T
(12)
[(Fig._12)TD$IG]
Unless specified otherwise, the default values for all the molecularmolecular binary parameters and ion pair–ion pair binary
10000
9.5 1000
8.5 100 PCO2, kPa
pKa1 (pKa2)
7.5 Ref [25] Aspen calculation
6.5
10
1
5.5 0.1
4.5 0.01 0
3.5 290
320
T,K
Fig. 11. pKa for 1MPZ.
350
380
0.2
0.4 0.6 0.8 CO2 loading, mol CO2/mol Amine
1
1.2
Fig. 12. CO2 solubility into 10 wt% 1MPZ + H2O (lines, model results; points, this work: [TD$INLE], 313.15 K; [TD$INLE], 343.15 K; [TD$INLE], 373.15 K; [TD$INLE], 393.15 K).
124
H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128
[(Fig._13)TD$IG]
[(Fig._15)TD$IG]
10000
1000
1000
100
PCO2, kPa
PCO2, kPa
100
10
10
1
1
0.1
0.01 0
0.2
0.4 0.6 0.8 CO2 loading, mol CO2/mol Amine
1
0.1
Fig. 13. CO2 solubility into 30 wt% 1MPZ + H2O (lines, model results; points, this work: [TD$INLE], 313.15 K; [TD$INLE], 343.15 K; [TD$INLE], 373.15 K; [TD$INLE], 393.15 K).
parameters are zero. The molecular-ion pair binary parameters aij are defaulted to (8, 4) when the molecular is H2O. When the molecular is 1MPZ, CO2, or H1MPZCOO, the default values for aij are (8, 15). The default values for bij are zero. 5. Modeling results All data used for regression is summarized in Table 2. The same table lists data range and their resource. 5.1. 1MPZ Heat capacity data from Poozesh et al. [28] and vapor pressure data from Chen et al. [29] of 1MPZ were regressed to obtain the ideal gas heat capacity constants (C ig p ) and the Antoine's constants, the values and standard deviations of which are listed in Table 3. The first parameter of C ig p was fixed at 25840, based on several
[(Fig._14)TD$IG]
1000
100
PCO2, kPa
0.1
10
1
0.1
0.2
0.3 0.4 0.5 CO2 loading, mol CO2/mol Amine
0.6
Fig. 15. CO2 solubility into 8 m (44.5 wt%) 1MPZ + H2O (lines, model results; points, Ref. [9]: [TD$INLE], 313.15 K; [TD$INLE], 333.15 K; [TD$INLE], 353.15 K; [TD$INLE], 373.15 K.
runnings of the Aspen regression. The standard deviation for all the parameters listed in Table 3 is much smaller than the values contained therein. Figs. 6 and 7 show good agreement between the data and model results. The average relative deviations are 0.03% for heat capacity and 0.19% for vapor pressure. 5.2. 1MPZ–H2O In this section, excess enthalpy from this work and VLE data from Gu and Zhang [30] were incorporated into the model. Nguyen [31] measured the volatility data for 1.05 m 1MPZ at various temperatures. These data were also regressed in this model. pKa values were regressed by manually adjusting the standard-state 1;aq 1;aq properties of 1MPZH+ and 1MPZH2+, Df G298:15 , Df H1;aq . 298:15 and C p The nonrandomness factor a was fixed at 0.3. Table 4 shows the regressed parameters along with their standard deviations. Figs. 8– 10 demonstrate the comparison with the Aspen calculation. The detailed excess enthalpy data for 1MPZ + H2O are shown in Table A.2, in Appendix A. The largest relative deviation for excess enthalpy regression happens when the mole fraction of 1MPZ is 0.8 at 303.15 K, which comes to 5.7%. The average relative deviation for excess enthalpy regression is 2.6%. In Fig. 9, the upper red line represents for the mole fraction of 1MPZ in the gas phase (y1MPZ), while the lower blue line stands for the mole fraction of 1MPZ in the liquid phase (x1MPZ). When y1MPZ is around 0.65, the model result seems more reliable than the experimental data, since the boiling temperature at 100 kPa is around 410 K, as shown in Fig. 7. The average relative deviation of the mole fraction of 1MPZ in vapor phase y1MPZ is 10.6%. In Fig. 10, volatility data from Nguyen [31] were compared with Aspen calculation. The biggest deviation is 20.2%, which the average is 13.4%. Fig. 11 shows good correlation of pKa values calculated by Aspen with the experimental data from Khalili et al. [25]. 5.3. 1MPZ–H2O–CO2
0.01 0
0.2
0.4 0.6 0.8 CO2 loading, mol CO2/mol Amine
1
Fig. 14. CO2 solubility into 40 wt% 1MPZ + H2O (lines, model results; points, this work: [TD$INLE], 313.15 K; [TD$INLE], 343.15 K; [TD$INLE], 373.15 K; [TD$INLE], 393.15 K).
CO2 solubility data, obtained in this work and from Chen and Rochelle [9], were regressed to get the ENRTL parameters and 1;aq 1;aq for standard-state properties Df G1;aq 298:15 , Df H298:15 , and C p ig ig 1MPZCOO and Df Hig 298:15 , Df G298:15 , and C p for H1MPZCOO. The
H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128
[(Fig._16)TD$IG] 90
90
a
80
b
80
70
70
60
60 -ΔHabs/ kJ/mol
-ΔHabs/ kJ/mol
125
50
50
40
40
30
30
20
20
10
10
0
0 0.1
0.2
0.3
0.4 0.5 0.6 0.7 Ldg, mol CO2/mol 1MPZ
0.8
0.9
1
0.1
0.2
0.3
0.4 0.5 0.6 0.7 Ldg, mol CO2/mol 1MPZ
0.8
0.9
1
90
c
80 70
-ΔHabs/ kJ/mol
60 50 40 30 20 10 0 0.1
0.2
0.3
0.4 0.5 0.6 0.7 Ldg, mol CO2/mol 1MPZ
0.8
0.9
1
Fig. 16. Heat of absorption for 10 wt% (a), 30 wt% (b), 40 wt% (c) 1MPZ + H2O solutions at various temperatures (blue, 313.15 K; red 333.15 K; green, 353.15 K; purple, 373.15 K; black, 393.15 K). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
nonrandomness factor a was fixed at 0.2. The parameters, together with their standard deviations, are presented in Table 5. Figs. 12–15 show the experimental data and the calculation by the model. The first experimental point from Chen and Rochelle [9] at 333.15 K, in Fig. 15, is apparently inconsistent with other data; therefore, it was not included in the regression. The experimental data were correlated by the model. The average relative deviation of the regression for 10 wt% 1MPZ in Fig. 12 is 9.6%, while it is 12.5% for 30 wt% 1MPZ in Fig. 13, 10.0% for 40 wt% 1MPZ in Fig. 14, and 14.2% for 8 m (44.5 wt%) 1MPZ from Chen and Rochelle [9] in Fig. 15. Detailed data from the measurement in this work are attached in Tables A.3–5, in Appendix A.
5.4. Heat of absorption The Gibbs–Helmholtz equation gives a good approximation of the differential heat of CO2 absorption, as verified by Mathias and O’Connell [32]: @lnf CO2 DHabs R (13) @ð1=TÞ s;fng where DHabs is the differential heat of absorption, f CO2 is the fugacity of CO2, s means the bubble point, {n} is the apparent number of moles of components other than CO2.
Table 6 Cyclic capacity for 10 wt%, 30 wt%, and 40 wt% 1MPZ solutions. Concentration of 1MPZ wt%
Lean loading mol CO2/mol 1MPZ
Rich loading
Loading difference
Cyclic capacity g CO2/kg solution
10 30 40
0.409 0.392 0.402
0.689 0.595 0.582
0.28 0.203 0.18
12.3 26.8 31.6
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H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128
[(Fig._17)TD$IG] 1.5
1MPZ 1MPZH+2 HCO3H1MPZCOO
1.2
8
1MPZH+ 1MPZCOOCO3-2 CO2
1MPZ 1MPZH+2 HCO3H1MPZCOO
a 7
1MPZH+ 1MPZCOOCO3-2 CO2
b
Activity coefficient
Activity coefficient
6
0.9
0.6
5 4 3 2
0.3 1 0
0 0.1
0.2
0.3
0.4 0.5 0.6 0.7 Loading, mol CO2/mol amine
30
0.8
0.9
1MPZ 1MPZH+2 HCO3H1MPZCOO
25
1
0.1
0.2
0.3
0.4 0.5 0.6 0.7 Loading, mol CO2/mol amine
1MPZH+ 1MPZCOOCO3-2 CO2
0.8
0.9
1
c
Activity coefficient
20
15
10
5
0 0.1
0.2
0.3
0.4 0.5 0.6 0.7 0.8 Loading, mol CO2/mol amine
0.9
1
Fig. 17. Activity coefficient of species when CO2 is absorbed into 10 wt% (a), 30 wt% (b), 40 wt% (c) 1MPZ + H2O solutions at 313.15 K.
Heat of absorption for 10 wt%, 30 wt%, and 40 wt% 1MPZ solutions at various temperatures are shown in Fig. 16. At 313.15 K, the heat of absorption is less dependent on loading when the loading is lower than 0.8. When the loading is higher, the heat of absorption decreases dramatically. As the temperature goes higher, the loading dependence becomes stronger, while the values are higher when the loading is lower than some point, after which, the heat of absorption decreases as the temperature increases. However, heat of absorption turns to go lower as the temperature goes higher than 373.15 K at the whole studied loading range. The concentration of 1MPZ shows a positive effect on the heat of absorption at low loadings and a negative effect at high loadings. Also, the trend is different at 393.15 K. The concentration of 1MPZ shows a positive effect on the heat of absorption at the whole studied loading range.
5.5. Cyclic capacity Cyclic capacity is an important property for characterizing amine performance. Considering 90% CO2 removal rate in the absorber, lean loading was defined as the CO2 loading when the partial pressure of CO2 is 1 kPa at 313.15 K, and rich loading as 10 kPa of CO2 partial pressure at 313.15 K. Cyclic capacity is the difference between rich and lean loading at the unit of g of CO2/ kg of solvent. Calculated results for 10 wt%, 30 wt%, and 40 wt% 1MPZ solutions are shown in Table 6. Cyclic capacity for 30 wt% 1MPZ is one time higher than 10 wt%, even though the concentration two times higher. The reason is that when 1MPZ concentration is higher, the loading difference between rich and lean loading is smaller. The same result can be seen from the comparison of 10 wt% with 40 wt% in Table 6. The 1MPZ
H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128
127
[(Fig._18)TD$IG] 0.02
x1MPZ x1MPZH+2 xHCO3xH1MPZCOO
0.08
x1MPZH+ x1MPZCOOxCO3-2
0.07
0.015
x1MPZH+ x1MPZCOOxCO3-2
b
0.06 Mole fraction
Mole fraction
x1MPZ x1MPZH+2 xHCO3xH1MPZCOO
a
0.01
0.05
0.04 0.03 0.02
0.005
0.01 0
0 0
0.2
0.4 0.6 Loading, mol CO2/mol amine 0.12
0.8
1
x1MPZ x1MPZH+2 xHCO3xH1MPZCOO
0.1
0
0.2
x1MPZH+ x1MPZCOOxCO3-2
0.4 0.6 Loading, mol CO2/mol amine
0.8
1
c
Mole fraction
0.08
0.06
0.04
0.02
0 0
0.2
0.4 0.6 Loading, mol CO2/mol amine
0.8
1
Fig. 18. Speciation for CO2 absorbed into 10 wt% (a), 30 wt% (b), 40 wt% (c) 1MPZ + H2O solutions at 313.15 K.
concentration is three times higher, while the cyclic capacity is only 1.5 times higher.
5.6. Activity coefficient and speciation Activity coefficient and speciation data for 10 wt%, 30 wt%, and 40 wt% 1MPZ solutions at 313.15 K were predicted by the model and calculated in Figs. 17 and 18. As the concentration of 1MPZ goes higher, the activity coefficient of all the species becomes larger, especially that of CO32 and HCO3. From Fig. 18, it is seen that when the loading is lower than 0.5, most of the CO2 absorbed is converted into MPZCOO. At greater loading, 1MPZCOO is consumed and converted to H1MPZCOO and HCO3. CO32 is never an important product during the whole studied loading range. As the concentration of 1MPZ increases from 10 wt% to 40 wt%, the proportion of 1MPZH+ and HCO3 decreases, while the proportion of H1MPZCOO increases.
6. Conclusions CO2 solubility for 10 wt%, 30 wt% and 40 wt% 1MPZ – with the CO2 loading up to 1 and CO2 partial pressure up to 500 kPa – have been measured in this work. The temperature ranges from 313.15 to 393.15 K. Excess enthalpy data over the whole mole fraction range, at 303.15 K and 323.15 K, have also been obtained. Using the ENRTL model, and calling on data from literature, a rigorous thermodynamic model for 1MPZ/CO2/H2O system has been built. The model works effectively at various 1MPZ concentrations and temperatures. Successfully represented by the model were various kinds of data, including vapor pressure and heat capacity data for 1MPZ, binary vapor–liquid equilibrium, excess enthalpy, pKa data for 1MPZ/H2O, and CO2 solubility data for 1MPZ/CO2/H2O. The model can be used for the modeling and simulation of the CO2-capture process. The calculated heat of absorption results show a plateau at 313.15 K, and are more dependent on loading as the temperature is
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H. Li et al. / Fluid Phase Equilibria 394 (2015) 118–128
higher. As 1MPZ concentration goes higher, heat of absorption is higher at low temperatures and lower at high temperatures, while cyclic capacity goes higher, activity coefficient of all the species becomes larger, especially that of CO32 and HCO3. At greater loading, the main products are H1MPZCOO and HCO3. CO32 is never an important product during the whole studied loading range. Acknowledgments This work was supported by EDF research foundation, the National Natural Science Foundation of China (No. 51134017) and State Key Laboratory of Chemical Engineering of China (SKL-ChE-12Z01). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fluid.2015.03.021. References [1] G.T. Rochelle, Science 325 (2009) 1652–1654. [2] J.I. Lee, D.O. Frederick, A.E. Mather, J. Appl. Chem. Biotechnol. 26 (1976) 541– 546. [3] F.-Y. Jou, A.E. Mather, F.D. Otto, Can. J. Chem. Eng. 73 (1995) 140–147. [4] J.P. Jakobsen, J. Krane, H.F. Svendsen, Ind. Eng. Chem. Res. 44 (2005) 9894–9903. [5] R. Idem, M. Wilson, P. Tontiwachwuthikul, A. Chakma, A. Veawab, A. Aroonwilas, D. Gelowitz, Ind. Eng. Chem. Res. 45 (2006) 2414–2420. [6] S.A. Freeman, R. Dugas, D.H. Van Wagener, T. Nguyen, G.T. Rochelle, Int. J. Greenhouse Gas Control 4 (2010) 119–124.
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