Experiments and model for the surface tension of (MDEA + [Bmim][BF4]) and (MDEA + [Bmim][Br]) aqueous solutions

J. Chem. Thermodynamics 71 (2014) 1–5

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Experiments and model for the surface tension of (MDEA + [Bmim][BF4]) and (MDEA + [Bmim][Br]) aqueous solutions Dong Fu ⇑, HongMei Wang, LeiXia Du School of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 16 October 2013 Received in revised form 15 November 2013 Accepted 16 November 2013 Available online 23 November 2013 Keywords: Surface tension MDEA [Bmim][BF4] [Bmim][Br] Aqueous solution

a b s t r a c t The surface tension (c) of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]), 1-butyl-3methylimidazolium bromide ([Bmim][Br]), (N-methyldiethanolamine(MDEA) + [Bmim][BF4]) and (MDEA + [Bmim][Br]) aqueous solutions were measured by using the BZY-1 surface tension meter. The temperature ranged from (293.2 to 323.2) K. The mass fraction of MDEA ranged from 0.35 to 0.45. A thermodynamic equation was proposed to model the surface tension of (MDEA + ionic liquids) (ILS) aqueous solutions and the calculated results agreed well with the experiments. The effects of temperature, mass fractions of MDEA and ILS on the surface tension were demonstrated on the basis of experiments and calculations. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction In recent decades, atmospheric levels of CO2 have increased rapidly due to the utilization of large quantities of fossil fuels. Development of affordable yet technically feasible separation technologies for reducing CO2 emission has attracted global attention. Among the available separation technologies including absorption [1–10], adsorption [11,12], membrane [13–15] and hydration [16–18], chemical absorption is one of the most effective approaches because CO2 can be satisfactorily removed and the absorbents can be well regenerated by heating. Currently, aqueous solutions of alkanol amines are widely used for the removal of CO2 from a variety of gas streams. Among the alkanol amine series, N-methyldiethanolamine (MDEA) has the advantage of high resistance to thermal and chemical degradation, low solution vapor pressure, low corrosivity and low enthalpy of absorption. However, compared with other amines like monoethanolamine (MEA) or piperazine (PZ), MDEA has a lower absorption rate. Using the mixtures of MDEA and some promoters as the absorbent is considered to be a good approach to enhance the CO2 absorption rate. For example, adding small amount of MEA or PZ to an aqueous solution of MDEA has found widespread application in the removal of CO2 [19–31]. The mixtures of the promoter and MDEA preserve the high rate of the reaction of promoter with CO2, and the low enthalpy of the reaction of MDEA with CO2, hence

⇑ Corresponding author. Tel.: +86 312 7522037; fax: +86 312 7523127. E-mail address: [email protected] (D. Fu). 0021-9614/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jct.2013.11.024

lead to higher absorption rates in the absorber column, yet lower heat of regeneration in the stripper section. Besides MEA and PZ, the ionic liquids (ILS) are also considered to be potential promoters for the absorption of CO2 using MDEA aqueous solution due to their unique characteristics including wide liquid range, thermal stability, negligible vapor pressure, tuneable physicochemical character and high CO2 solubility [32–36]. In particular, Jacquemin et al. [34] showed that CO2 is highly soluble in 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) at pressures close to atmospheric. Ahmady et al. [35,36] showed that under certain conditions the presence of ionic liquid increased the initial rate of CO2 absorption in MDEA aqueous solution. Moreover, the mixtures of ILS and MDEA preserve the desired property of ILS for CO2 capture, but without many of their inherent drawbacks such as high viscosity, and intractable tars because of their corresponding CO2 adducts. Bidart et al. [37] showed that although 1-butyl-3-methylimidazolium bromide ([Bmim][Br]) did not present a greater absorption capacity than the amine solutions, the mixture of [Bmim][Br] and amine solution was explored as an option to enhance CO2 absorption due to the advantages of low volatility, chemical stability and stability of the complex formed when CO2 saturation is achieved. A knowledge of surface tension is required when designing or simulating an absorption column for CO2 capture. It can significantly affect the absorption efficiency because both the penetration of CO2 molecules from gas phase to the liquid phase and the enhancement of the absorption closely relate to the surface tension. In recent years, there are some experimental and theoretical work concerning the surface tension of aqueous solutions

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D. Fu et al. / J. Chem. Thermodynamics 71 (2014) 1–5

TABLE 1 Sample provenance and mass fraction purity. Chemical

[Bmim]BF4

Supplier

Shanghai Chengjie Chem. Co., Ltd [China]

[Bmim]Br MDEA

Sinopharm Chemical Reagent Co., Ltd [China]

CAS No.

17450165-6 8510077-2 105-599

Mass fraction purity

Molecular mass/ (Dalton)

99.0

226.02

99.0

219.20

99.5

119.16

containing amines and their blends [29–31,38–43] and ILS [44–48], However, experimental and theoretical work concerning the surface tension of (ILS + MDEA) aqueous solutions are relatively rare so far. The main purpose of this work is to investigate the surface tension of (MDEA + [Bmim][BF4]) and (MDEA + [Bmim][Br]) aqueous solutions experimentally and theoretically, so as to demonstrate the effects of temperature, mass fractions of MDEA and ILS on the surface tension. To this end, the surface tension was measured at the temperatures from (293.2 to 323.2) K. The mass fraction of MDEA and ILS respectively ranged from 0.35 to 0.45 and 0.05 to 0.15. Besides experimental work, a thermodynamic equation is also proposed in this work to model the surface tension.

TABLE 2 Surface tension ca of [Bmim]BF4 aqueous solutions under different values of mass fraction of [Bmim]BF4 (w[Bmim]BF4).

a

The surface tension was measured by using the BZY-1 surface tension meter produced by Shanghai Hengping Instrument Factory. The BZY-1 meter employs the Wilhemy plate principle, i.e., the maximum tensile force competing with the surface tension is measured when the bottom edge of platinum parallel to the interface and just touches the interface. The measurement ranges for temperature and surface tension are respectively (268.15 to 383.15) K and (0.1 to 400.0) mN  m1, respectively. The sensitivity, repeatability and uncertainty are respectively 0.1 mN  m1, 0.1 mN  m1 and 0.1 mN  m1. The size-volume of the different samples used in BZY-1 meter is 20 mL. During the experiments, the copper pan in the host of the BZY-1 meter is connected with the thermostatic bath (uncertainty is T = ±0.1 K). Via the circulation of the water, the temperature of the water in the copper pan is kept the same as that in the thermostatic bath, hence the temperature readings in the surface tension measuring cell may be regulated within 0.1 K. The aqueous solution is put into the solution container immersed in the copper pan and its temperature can be measured by a thermocouple. The scale reading of the thermocouple has been well calibrated by a mercury thermometer (uncertainty is T = ±0.05 K). 3. Results and discussion

293.2 K

303.2 K

313.2 K

323.2 K

0.05 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

53.5 48.6 45.3 45.5 45.7 45.8 46.0 46.1 46.0 45.7 44.8

52.9 48.3 45.2 45.2 45.5 45.5 45.6 45.5 45.2 44.6 44.1

50.3 47.5 44.6 44.9 45.3 45.1 45.1 44.2 44.3 43.8 43.5

48.5 46.7 42.6 44.1 44.8 44.5 44.3 43.5 43.4 43.0 42.6

Standard uncertainties r are r(T) = ±0.1 K; r(w) = ±0.0001; r(c) = ±0.5 mN  m1.

c/(mN  m1)

wMDEA/w[Bmim]BF4

0.35/0.05 0.35/0.10 0.35/0.15 0.40/0.05 0.40/0.10 0.40/0.15 0.45/0.05 0.45/0.10 0.45/0.15

2.1. Materials

2.2. Apparatus and procedure

c/(mN  m1)

TABLE 3 Surface tension cb of (MDEA + [Bmim]BF4) aqueous solutions in different mass fractions of [Bmim]BF4 (w[Bmim]BF4) and MDEA (wMDEA).

2. Experimental

MDEA and ILS were purchased from HuaXin chemical Co. The sample description is shown in table 1. They were used without further purification. Aqueous solutions of MDEA, ILS and (MDEA + ILS) were prepared by adding doubly distilled water. The uncertainty of the electronic balance is T = ±0.1 mg.

w[Bmim]BF4

b

293.2 K

303.2 K

313.2 K

323.2 K

51.4 49.1 47.6 50.8 49.0 47.5 50.2 48.4 47.2

49.8 48.4 47.0 49.5 48.0 46.8 48.7 47.3 46.3

48.1 46.7 46.0 47.7 46.5 45.8 46.8 45.7 44.6

46.4 45.4 44.8 45.8 45.1 44.4 44.7 44.1 43.2

Standard uncertainties r are r(T) = ±0.1 K; r (w) = ±0.0001; r (c) = ±0.5 mN  m1.

TABLE 4 Surface tension cc of [Bmim]Br aqueous solutions at different values of mass fraction of [Bmim]Br (w[Bmim]Br).

c

w[Bmim]Br

c/(mN  m1) 293.2 K

303.2 K

313.2 K

323.2 K

0.05 0.10 0.15 0.20 0.30 0.40 0.50 0.60 0.70

65.5 64.4 62.6 61.2 58.6 54.9 53.1 50.8 49.2

64.4 62.3 59.5 57.3 54.2 52.7 51.5 48.9 47.4

60.1 58.9 57.8 56.1 52.4 51.2 50.2 46.8 45.5

58.5 57.4 56.1 52.5 50.1 47.4 46.7 45.7 44.0

Standard uncertainties r are r(T) = ±0.1 K; r(w) = ±0.0001; r(c) = ±0.5 mN  m1.

because [Bmim][Br] appears in the solid state. From an examination of the variation in the experimental values, the accuracy of the measurement is reported as 0.5 mN  m1 instead of the nominal accuracy of the measuring apparatus as 0.1 mN  m1. Besides experiments, theoretical work is also presented in this work. The surface tension of (MDEA + ILS) aqueous solutions is formulated as following:

caq ¼ c0 þ c0 ; 0

ð1Þ 0

in which c and c are expressed as: The experimental results of the surface tension of [Bmim][BF4], (MDEA + [Bmim][BF4]), [Bmim][Br] and (MDEA + [Bmim][Br]) aqueous solutions are respectively shown in tables 2 to 5. In table 4, the mass fraction of [Bmim][Br] only ranges from 0.05 to 0.7

c0 ¼ x1 c1 þ x2 c2 þ x3 c3 ;

ð2Þ

c0 ¼ x1 x2 G12 þ x1 x3 G13 þ x2 x3 G23 ;

ð3Þ

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D. Fu et al. / J. Chem. Thermodynamics 71 (2014) 1–5

0.35/0.05 0.35/0.10 0.35/0.15 0.40/0.05 0.40/0.10 0.40/0.15 0.45/0.05 0.45/0.10 0.45/0.15 d

c/(mN  m1) 293.2 K

303.2 K

313.2 K

323.2 K

52.9 52.0 51.4 51.9 51.1 49.9 50.8 49.8 48.6

51.1 50.0 48.8 50.0 49.2 47.7 48.7 47.6 46.8

48.6 47.8 47.3 48.1 47.1 46.2 46.6 45.9 44.9

46.4 46.0 45.2 45.9 45.4 44.6 44.3 43.9 43.1

Standard uncertainties r are r(T) = ±0.1 K; r(w) = ±0.0001; r(c) = ±0.5 mN  m1.

where the subscripts 1, 2 and 3 stand for MDEA, ILS and water, respectively; xi is the mole fraction of component i in the aqueous solution, ci is the surface tension of the pure component i, which can be expressed as a function of the temperature by fitting to the experimental results. The Gij is expressed as the function of temperature and mass fraction of amines:

G13 ¼ ða13 þ b13 w1 þ c13 w21 ÞT;

ð4Þ

G23 ¼ ða23 þ b23 w2 þ c23 w22 ÞT;

ð5Þ

G12 ¼ ða12 þ b12 ½ðw1 þ w2 Þ=2 þ c12 ½ðw1 þ w2 Þ=22 ÞT:

ð6Þ

The model parameters aij, bij and cij can be obtained by fitting to the experimental values. The objective function (the average relative deviation, ARD) is defined as:

ARD ¼

n X ½1  ccal =cexp   100%=n;

ð7Þ

i¼1

where the superscripts ‘exp’ and ‘cal’ respectively stand for the experimental and calculated values; n is the number of data points. For example, with the experimental surface tension of MDEA aqueous solutions as input, the model parameters for MDEA aqueous solutions were optimized as a13 = 1.74, b13 = 3.19 and c13 = 1.58 [29]. Similarly, a23, b23 and c23 can be regressed from the experimental data of ILS aqueous solutions. Once a13, b13, c13, a23, b23 and c23 are obtained, a12, b12 and c12, can be regressed from the experimental data of (MDEA + ILS) aqueous solutions. However, it is worth noting that the mass fraction dependence of the surface tension of ILS aqueous solutions is similar to that of the surfactant aqueous solutions, i.e., when the values of the mass fraction of ILS are small, the surface tension decreases monotonically and rapidly with increasing mass fraction of ILS. Exceeding a certain value, the surface tension tends to change slightly. The present model is unable to describe accurately such a tendency of surface tension. For example, we correlated the surface tension of [Bmim][BF4] aqueous solutions under low w[Bmim][BF4] (from 0.05 to 0.2). The optimized model parameters are a23 = 9.33, b23 = 16.10 and c23 = 22.79. The ARD is 9.22%. The agreement between the calculations and experiments is not satisfactory. Hence, the parameters a23, b23, c23, a12, b12 and c12 are simultaneously regressed from the experimental results of (MDEA + ILS) aqueous solutions. Using the experimental values presented in table 3, we optimized the residual 6 model parameters of (MDEA + [Bmim][BF4]) aqueous solutions as: a23 = 0.078, b23 = 2.890, c23 = 1.563, a12 = 5.232, b12 = 2.839 and c12 = 1.309. The ARD is 1.67%. Figure 1 shows the temperature dependence of the surface tension of (MDEA + [Bmim][BF4]) aqueous solutions. One finds that at given mass fractions of MDEA and [Bmim][BF4], the surface tension decrease with the increase of temperature. The agreement between

54

52

50

52

γ/(mN⋅m-1)

wMDEA/w[Bmim]Br

the experiments and calculations is good, despite that the surface tension at low temperature were overestimated. Figure 2 shows the influence of the mass fraction of [Bmim][BF4] on the surface tension of (MDEA + [Bmim][BF4]) aqueous solutions. One finds that at given temperature and given mass fraction of MDEA, the surface tension decreases monotonically with the increase of the mass fraction of [Bmim][BF4]. When the mass fraction of [Bmim][BF4] is low, the calculation matches the experiments well, however, the deviation between calculation and experiment gets larger with the increase of the mass fraction of [Bmim][BF4]. One may also find form figures 1 and 2 that the predicted surface tension decreases almost linearly with the increase of temperature, however, the relationship between the surface tension and the mass fraction of [Bmim][BF4] cannot be described using linear functions. When regressing the model parameters for (MDEA + [Bmim][Br]) aqueous solutions, the surface tension of [Bmim][Br] (c[Bmim][Br]) was treated as an adjustable parameter because [Bmim][Br] appears solid state. Using the experimental values presented in table 5, we obtained a23 = 0.561, b23 = 1.500, c23 = 1.360, a12 = 1.850, b12 = 0.279, c12 = 2.430 and c[Bmim][Br] = 7.590. The ARD is 0.54%. Figure 3 shows the temperature dependence of the surface tension of (MDEA + [Bmim][Br]) aqueous solutions. One finds that at given mass fractions of MDEA and [Bmim][Br], the surface tension decrease almost linearly with the increase of temperature. From (293.2 to 323.2) K, the calculations match the experiments satisfactorily. By adding an adjustable parameter, c[Bmim][Br], the agreement between the experiments and calculations is much better than that of (MDEA + [Bmim][BF4]) aqueous solutions. Figure 4 shows the influence of the mass fraction of [Bmim][Br] on the surface tension of (MDEA + [Bmim][Br]) aqueous solutions. One finds that at given temperature and given mass fraction of MDEA, the surface tension decreases monotonically with the increase of the mass fraction of [Bmim][Br]. The relationship between the surface tension and the mass fraction of [Bmim][Br] cannot be described using linear functions, which is similar to that of (MDEA + [Bmim][BF4]) aqueous solutions.

48

46

44

50

γ /(mN⋅m-1)

TABLE 5 Surface tension cd of (MDEA + [Bmim]Br) aqueous solutions at different values of mass fraction of [Bmim]Br (w[Bmim]Br) and MDEA (wMDEA).

42 290

300

310

320

330

T/K

48

46

44 290

300

310

320

330

T/K FIGURE 1. Temperature dependence of the surface tension of (MDEA + [Bmim][BF4]) aqueous solutions. Main plot: the mass fraction of [Bmim][BF4] (w[Bmim][BF4]) = 0.05, d the mass fraction of MDEA (wMDEA) = 0.35; s wMDEA = 0.40; j wMDEA = 0.45. Insert plot: wMDEA = 0.45; d w[Bmim][BF4] = 0.05; s w[Bmim][BF4] = 0.10; j w[Bmim][BF4] = 0.15. Symbols: experimental results from this work, Lines: calculated results.

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D. Fu et al. / J. Chem. Thermodynamics 71 (2014) 1–5

65

65

50

50

60

48

46

44

55

42 0.00

0.04

0.08

w[Bmim][BF4]

0.12

0.16

50

45

40 0.00

γ /(mN⋅m-1)

γ /(mN⋅m-1)

52

γ /(mN⋅m-1)

γ /(mN⋅m-1)

60

52

48

46

44

55

42 0.00

0.04

0.08

w[Bmim][Br]

0.12

0.16

50

45

0.04

0.08

0.12

40 0.00

0.16

0.04

w[Bmim][BF4]

0.08

0.12

0.16

w[Bmim][Br]

FIGURE 2. w[Bmim][BF4] dependence of the surface tension of (MDEA + [Bmim][BF4]) aqueous solutions. Main plot: wMDEA = 0.40; Insert plot: wMDEA = 0.45. Symbols: experimental results from this work, d T = 293.2 K; s T = 303.2 K; j T = 313.2 K, h T = 323.2 K. Lines: calculated results.

FIGURE 4. w[Bmim][Br] dependence of the surface tension of (MDEA + [Bmim][Br]) aqueous solutions. Main plot: wMDEA = 0.40; Insert plot: wMDEA = 0.45. Symbols: experimental results from this work, d T = 293.2 K; s T = 303.2 K; j T = 313.2 K, h T = 323.2 K. Lines: calculated results.

65

56

52

52

52

50

γ /(mN⋅m-1)

-1

γ /(mN⋅m )

50

60

48

46

48

46

42 290

48

55 300

310

320

330

T/K

44

40 290

γ /(mN⋅m-1)

γ /(mN⋅m-1)

44

44 290

300

310

320

330

T/K

50

45

300

310

320

330

40 0.00

Figure 5 shows the comparison of the effects of [Bmim][Br] and [Bmim][BF4] on the surface tension of (MDEA + ILS) aqueous solutions. One finds from figure 5 that at a given mass fraction of MDEA, [Bmim][BF4] tends to decrease the surface tension of MDEA aqueous solutions more rapidly than [Bmim][Br] because the surface tension of [Bmim][BF4] aqueous solution is smaller than that of [Bmim][Br] aqueous solution at given temperature (as shown

0.08

0.12

0.16

w ILS

T/K FIGURE 3. Temperature dependence of the surface tension of (MDEA + [Bmim][Br]) aqueous solutions. Main plot: the mass fraction of [Bmim][Br] (w[Bmim][Br]) = 0.15, d wMDEA = 0.35; s wMDEA = 0.40; j wMDEA = 0.45. Insert plot: wMDEA = 0.45; d w[Bmim][Br] = 0.05; s w[Bmim][Br] = 0.10; j w[Bmim][Br] = 0.15. Symbols: experimental results from this work, Lines: calculated results.

0.04

FIGURE 5. Comparison of the effects of [Bmim][Br] (— and solid symbols) and [Bmim][BF4] (- - - and hollow symbols) on the surface tension of (MDEA + ILS) aqueous solutions. wMDEA = 0.40. Main plot: d s T = 293.2 K; j h T = 313.2 K; Insert plot: d s the mass fraction of ILS(wILS) = 0.05; j h wILS = 0.10. Symbols: experimental data from this work, Lines: calculated results.

in tables 2 and 4). With the increasing of the mass fraction of ILS, such effect becomes more significant. 4. Conclusions In this work, the surface tension of [Bmim][BF4], [Bmim][Br], (MDE + [BF4]) and (MDEA + [Bmim][Br]) aqueous solutions were

D. Fu et al. / J. Chem. Thermodynamics 71 (2014) 1–5

measured by using the BZY-1 surface tension meter and modeled by using a thermodynamic equation. The effects of the temperature, the mass fractions of MDEA and ILS on the surface tension were demonstrated. Our results show that: (1) The increase of the temperature and the mass fractions of ILS tends to decrease the surface tension of (MDEA + ILS) aqueous solutions; (2) Over the ranges of temperature and mass fraction of MDEA and ILS studied, the surface tension of (MDEA + ILS) aqueous solutions decreases almost linearly with increasing temperature; however, the relationship between the surface tension and the mass fraction of ILS cannot be described using linear functions; (3) At a given mass fraction of MDEA, [Bmim][BF4] tends to decrease the surface tension of MDEA aqueous solutions more rapidly than [Bmim][Br]; (4) The proposed model correctly captures the effects of temperature and mass fraction of ILS on the surface tension, and the calculated results agree well with the experimental values.

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JCT 13-612