Viscosity Measurement and Correlation of Unloaded and CO2-Loaded

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Viscosity Measurement and Correlation of Unloaded and CO2‑Loaded Aqueous Solutions of N‑MethyldiethanolaminePiperazine Nithin B. Kummamuru,†,‡ Zulkifli Idris,† and Dag A. Eimer*,† †

Downloaded by UNIV AUTONOMA DE COAHUILA at 22:24:08:673 on May 28, 2019 from https://pubs.acs.org/doi/10.1021/acs.jced.9b00021.

Faculty of Technology, Natural Science and Maritime Sciences, University of South-Eastern Norway, Kjølnes Ring 56, Porsgrunn 3918, Norway ‡ Department of Chemical Engineering, Birla Institute of Technology and Science, Hyderabad Campus, Pilani, Telangana 500078, India S Supporting Information *

ABSTRACT: New and complementary viscosity data for aqueous N-methyldiethanolamine-piperazine (MDEA-PZ) solutions with and without CO2 are presented in this paper for different temperatures and mass fractions. The first part of this work represents viscosity of pure and aqueous MDEA to validate the measurement system. The second part represents viscosity of unloaded aqueous MDEA-PZ solutions with total mass fractions of MDEA ranging from 0.20 to 0.40 and PZ mass fractions at 0.05 and 0.1. Two different techniques for representing viscosity of the unloaded systems are discussed. The third part of this paper reports viscosities of CO2-loaded aqueous MDEA-PZ at different CO2 loadings. These data were correlated using a modified Setschenow equation.

using a reaction rate activator such as piperazine (PZ),12 where PZ favors rapid carbamate formation (theoretically absorbs 2 mol CO2 per 1 mol PZ) and together with aqueous amine solution can act as effective solvents for CO2 capture.12,13 Recently, there has been an increase in attention to PZ as a promising additive to MDEA aqueous solutions due to its ability to enhance the rate of absorption of CO2 along with satisfactory stripping characteristics.9,14−16 Good and reliable physicochemical data of unloaded and CO2-loaded aqueous alkanolamine solution such as viscosity, surface tension, density, thermal conductivity, heat capacity, and thermodynamic equilibrium are essential for designing both the absorber and the stripper with auxiliary equipment. Previously reported experimental data on the viscosity of aqueous MDEA-PZ solutions are tabulated in Table S1.8−11,14,16−56 The availability of literature for viscosity of aqueous MDEA-PZ solutions is limited in compositions and temperatures. The present paper reports new viscosity data for aqueous MDEA-PZ solutions in unloaded and CO2-loaded solutions. Alongside measuring new viscosity data of aqueous amine solutions, it is also good to have reliable correlations. In this

1. INTRODUCTION Separation of acid gases like H2S and CO2 from gas mixtures by chemical absorption using aqueous alkanolamines is an effective technology and a vital operation in the gas processing industries.1 The commercial regenerative chemical absorption of CO2 capture technology adopts typically a 30 wt % of aqueous monoethanolamine (MEA) solution because of its advantages such as high absorption rates and chemical kinetics.2,3 However, utilization of MEA has shown high regenerative energy demand, amine loss, degradation rates, and equipment corrosion rates.4−6 Apart from MEA, other alkanolamines such as diethanolamine (DEA), diisopropanolamine (DIPA), and 2-amino-2methyl-1-propanol (AMP) are also used to capture CO2. Over different classes of alkanolamines, tertiary amine such as Nmethyldiethanolamine (MDEA) has shown its importance as an alternative absorbent7 because of its properties like high equilibrium loading capacity (1 mol CO2/1 mol amine),8 high resistance to thermal and chemical degradation,9 and low heat of reaction with CO2, which leads to low energy requirements for amine regeneration.10 Blended MDEA-based alkanolamines are widely used as further improvements to single solvent MDEA, where the blended mixture consists of either a mixture of primary or secondary amine (having high CO2 reaction rates) with MDEA (having high CO2 loading capacity).11 Nowadays, there is a growing interest in promoting alkanolamine solvents in chemical absorption technology by © XXXX American Chemical Society

Special Issue: Celebrating Our High Impact Authors Received: January 9, 2019 Accepted: May 16, 2019

A

DOI: 10.1021/acs.jced.9b00021 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Chemicals Used in This Worka chemical name

CAS number

mole fraction puritya

source

purification

N-methyldiethanolamine (MDEA) piperazine (PZ) carbon dioxide (CO2) nitrogen (N2) sodium hydroxide (NaOH) hydrochloric acid (HCl) barium chloride dihydrate (BaCl2·2H2O)

105-59-9 110-85-0 124-38-9 7727-37-9 1310-73-2 7647-01-0 10326-27-9

≥0.99 ≥0.99 0.99999 0.99999 N/A N/A ≥0.99

Sigma-Aldrich Sigma-Aldrich AGA Norge AS AGA Norge AS Merck KGaA Merck KGaA Merck KGaA

degas no no no no no no

a

As stated by the supplier. N/A: not available.

used. As given by the manufacturer, the rheometer has standard uncertainties of 0.03 K and 0.0002 N m for temperature and torque, respectively. The combined standard uncertainty was calculated using root-sum of square formula. Combined standard uncertainties for the unloaded and CO2loaded systems in this work were calculated at 0.10 and 0.15 mPa·s, respectively, which is in line with our recent publication.60 Based on the observations of viscosity measured over time in cells (see Figure S1 of Supporting Information), it is concluded that solvent and CO2 losses are negligible during the measurement period.

work, the measured viscosities of unloaded aqueous MDEA-PZ solutions were correlated as a function of temperature using two different models available in the literature.48,57 Concurrently, the viscosities of CO2-loaded aqueous MDEA-PZ systems were correlated using a modified version of the Setschenow equation.58,59

2. MATERIALS AND METHODS The chemicals used in this work are listed in Table 1. All of the chemicals were of analytical grade and used as received. In the case of MDEA, it was subjected to degassing before sample preparation. Degassed Milli-Q water (resistivity 18.2 MΩ·m) was used as a diluter in sample preparation. All of the aqueous solutions in this work were prepared gravimetrically using an analytical balance from Mettler Toledo with a resolution of 1 mg. As explained in an earlier publication,60 titration with aqueous 1 M HCl using an automated Metrohm 905 Titrando was also performed to confirm the amine content in the solutions, and we found that the compositions of amine content agree within ±0.1%. The CO2-loaded aqueous solutions were prepared by bubbling high purity CO2 gas into the unloaded amine solutions at a flow rate of 0.15 L·min−1. For each unloaded aqueous MDEA-PZ solution, five different CO2-loaded samples with loading values between (0.2 and 0.8) mole of CO2/mol of MDEA-PZ were prepared. The actual values of CO2 loading in the solutions were determined using an acid− base titration method, as explained in a previous publication.61 Dynamic viscosities of unloaded and CO2-loaded systems in this work were measured using an Anton-Paar Physica MCR 101 rheometer (part number 16101) with a double-gap pressure cell XL (part number 24076) at temperatures between (298.15 and 373.15) K. The cell was filled with recommended 6 mL amine solution per run. Our earlier publication provides a thorough description on the viscosity measurement in our laboratory.60 The reported viscosity values in this work are the average values from 30 different readings at each temperature with each set of experiments repeated three times. The parallels agreed well with a few exceptions where points were discarded if deviation was more than two times the standard deviation. The standard viscosity solution S3S from Paragon Scientific Ltd. was used to calibrate the rheometer at temperatures between (273.15 and 373.15) K, and all the experimental values were corrected against the calibrated values.

4. RESULTS AND DISCUSSION This section is divided into three parts for easy reading. The first part reports viscosity of pure and aqueous MDEA to validate the measurement system. The second part reports viscosities of aqueous MDEA-PZ solutions and two methods to correlate these data. In the third part, viscosities of CO2loaded aqueous MDEA-PZ solutions and one correlation method are discussed. To evaluate the performance of the models used for representing the experimental viscosity data, absolute average deviation (Ω) values were determined using eq 1, Ω=

1 N

i=1

∑ |ηi E − ηiC | N

(1)

where N, η i, and η i refer to number of data points and experimental and calculated viscosities, respectively. 4.1. Viscosity of Pure and Aqueous MDEA. To validate the experimental procedure, the viscosities of pure MDEA were measured at (298.15 to 373.15) K. Table S2 tabulates the viscosities of pure MDEA obtained from this work and the corresponding values from literature.9,10,17,18,21,25,26,28−34,38,40−51,54,56 A graphical comparison between experimental values from this work and literature is shown in Figure 1, and calculated Ω values are shown in Table S3. As can be seen, there is a good agreement between the data obtained from this work and literature. The low deviations also suggest that our instrument is functioning well and would be expected to give reliable experimental data. The observed minimal deviations could be due to several factors such as purity of chemicals, uncertainty of measurements, and usage of different apparatus for measurement. Overall, these deviations can be considered acceptable. Figure 1 also shows the linear correlation between log of viscosity and inverse temperature, as expected. Table 2 shows viscosities of unloaded aqueous MDEA solutions to complement data available in the literature and also as an extension to validation of our experimental technique. The overall change of viscosity against mole E

3. EXPERIMENTAL UNCERTAINTY Experimental uncertainty in this work is reported as the combined standard uncertainty with a k-factor equals one (confidence level 67%). A standard uncertainty of 0.01 is used for the mass fraction based on the purity of MDEA and PZ B

C



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Figure 1. Viscosity of pure MDEA at different temperatures. Symbols: this work (■), Khan et al.21 (□), Gao et al.26 (○), Pinto et al.28 (△), García et al.31 (▽), Paul et al.10 (◊), Zhao et al.32 (◁), Haghtalab et al.34 (▷), Li et al.38 (□), Zhang et al.42 (☆), DiGuilio et al.44 (□), Beak et al.45 (●), Chowdhury et al.46 (▲), Paul et al.47 (▼), Foo et al.18 (⧫), Paul et al.49 (◀), Arachchige et al.56 (▶).

Figure 2. Plots of viscosity of unloaded aqueous MDEA solutions against MDEA mole fractions. Symbols: 298.15 K (■), 303.15 K (□), 308.15 K (●), 313.15 K (○), 318.15 K (▲), 323.15 K (△), 328.15 K (▼), 333.15 K (▽), 338.15 K (◊), 343.15 K (⧫), 348.15 K (◀), 353.15 K (◁), 358.15 K (▶), 363.15 K (▷), 368.15 K (★), 373.15 K (☆). Dotted lines refer to the calculated viscosity values.

4.2. Viscosity of unloaded aqueous MDEA-PZ solutions. Viscosity of unloaded aqueous MDEA-PZ solutions was measured at different mass fractions of MDEA (wMDEA) and PZ (wPZ) from (303.15 to 373.15) K with five degree temperature increments. The experimental results for unloaded aqueous MDEA-PZ solutions are shown in Table 3. Figure 3 shows the influence of temperature on viscosities of unloaded MDEA-PZ aqueous solutions. It is seen that at given temperature and wPZ, the viscosity increases with increasing wMDEA and decreases with increase in temperature. Theoretical models that can correlate experimental viscosities are also desirable. Among semiempirical methods available in the literature,10,13,17,18,20,38,48,53 we choose two

fraction of MDEA is presented in Figure 2. The viscosities were measured at different mass fractions ranging from 0.3 to 0.95 with 5 degree temperature increments from (298.15 to 373.15) K. Table S4 tabulates the absolute average deviations between this work and literature for different mass fractions of MDEA. It is observed from the experimental results that viscosity is dependent on MDEA concentration and temperature, as expected. Similar viscosity behavior is observed in previously studied unloaded alkanolamine solutions such as MEA and MDEA.30,60 For temperature below 313.15 K, viscosity appears to go through a maximum as mole fraction of MDEA increases. This must be due to intermolecular forces that in turn are affected by temperature. Table 2. Viscosity (η) of Aqueous MDEA Solutionsa w2

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0.95

1.00

x2

0.061

0.091

0.13

0.18

0.26

0.37

0.57

0.74

1.00

T/K 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15 368.15 373.15

3.02 2.64 2.28 1.97 1.75 1.54 1.38 1.24 1.15 1.06 0.97 0.89 0.84 0.81 0.79 0.76

5.21 4.41 3.71 3.15 2.73 2.36 2.07 1.83 1.65 1.49 1.35 1.22 1.13 1.05 0.97 0.91

9.13 7.51 6.17 5.13 4.34 3.68 3.18 2.76 2.43 2.16 1.92 1.72 1.58 1.45 1.33 1.23

16.91 13.40 10.73 8.68 7.15 5.92 4.99 4.23 3.66 3.18 2.77 2.43 2.18 1.97 1.77 1.61

η/mPa·s 30.09 23.11 18.01 14.22 11.42 9.26 7.64 6.36 5.38 4.59 3.94 3.41 3.00 2.65 2.37 2.12

53.42 40.03 30.42 23.47 18.44 14.67 11.84 9.67 8.02 6.71 5.66 4.81 4.16 3.63 3.18 2.82

75.12 56.05 42.37 32.56 25.45 20.13 16.18 13.16 10.81 8.98 7.52 6.35 5.44 4.69 4.09 3.58

76.07 57.55 44.18 34.30 27.04 21.55 17.38 14.14 11.64 9.66 8.09 6.82 5.83 5.01 4.33 3.77

73.103 55.89 43.45 34.15 27.15 21.82 17.79 14.63 12.20 10.21 8.60 7.31 6.29 5.43 4.72 4.12

a

Standard uncertainties u are u(w2) = 0.01, u(T) = 0.03 K, u(P) = 0.2 kPa. The combined standard uncertainty for viscosity measurement uc(η) is 0.10 mPa·s. Experiments were performed at different MDEA mass fractions w2 and temperatures T. The operating pressure was maintained by N2 gas throughout the temperature range (p = 400 kPa). C



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Table 3. Viscosities of Unloaded MDEA-PZ Aqueous Solutions at Different xMDEA and xPZ (wMDEA and wPZ)a η/mPa·s wPZ: 0.05, xPZ: 0.013

wPZ: 0.05, xPZ: 0.015

wPZ: 0.05, xPZ: 0.017

wPZ: 0.10, xPZ: 0.027

wPZ: 0.10, xPZ: 0.031

wPZ: 0.10, xPZ: 0.036

T/K

wMDEA: 0.20, xMDEA: 0.038






303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15 368.15 373.15

2.30 1.98 1.71 1.51 1.33 1.19 1.06 0.97 0.89 0.82 0.74 0.73 0.72 0.71 0.67

3.69 3.12 2.64 2.29 1.98 1.75 1.54 1.40 1.27 1.16 1.05 0.99 0.95 0.89 0.84

6.33 5.26 4.43 3.77 3.20 2.78 2.44 2.19 1.96 1.74 1.63 1.62 1.49 1.26 1.18

3.28 2.81 2.39 2.09 1.81 1.62 1.43 1.30 1.17 1.06 0.95 0.89 0.82 0.77 0.76

5.49 4.62 3.87 3.32 2.84 2.50 2.19 1.96 1.76 1.58 1.42 1.30 1.18 1.13 1.01

9.54 7.69 6.28 5.26 4.39 3.75 3.24 2.83 2.51 2.23 2.01 1.85 1.60 1.50 1.38

a

Standard uncertainties u are u(wMDEA+PZ) = 0.01, u(T) = 0.03 K, u(P) = 0.2 kPa. The combined standard uncertainty for viscosity measurement uc(η) is 0.10 mPa·s. The pressure during experiments was maintained by N2 gas (p = 400 kPa).

Table 4. Parameters G12, G23, and G13 of Model 1 for Unloaded Aqueous MDEA-PZ Solutions parameter G12

G23

G13

ternary mixture: MDEA + PZ + H2O a b c a b c a b c

−8244.3 45.567 −0.0601 1077.1 −5.812 0.00766 294.1 −1.429 0.0017

The weakness in this model is that it will not represent the viscosity of the pure component as either xi or xj approaches 1. Figure 3. Temperature dependence of the viscosities of unloaded MDEA-PZ aqueous solutions at wPZ = 0.05 and 0.1. Symbols: wMDEA/ wPZ, 0.20/0.05 (■); wMDEA/wPZ, 0.30/0.05 (●); wMDEA/wPZ, 0.40/ 0.05 (▲); wMDEA/wPZ, 0.20/0.10 (□); wMDEA/wPZ, 0.30/0.10 (○); wMDEA/wPZ, 0.40/0.10 (△).

ln(ηmix /mPa·s) = x1x 2G12 + x 2x3G23 + x1x3G13

Model 2: As viscosity values for unloaded aqueous MDEAPZ solutions show a nonlinear decreasing tendency with an increase in temperature, a logarithmic function as shown in eq 5 can be used to fit the viscosity data.48,53 B ln(ηmix /mPa·s) = A + (5) T Here, ηmix refers to viscosity of the mixture, A and B are empirical parameters, and T is temperature in K. The values of A and B are listed in Table 5. Figures 4 and 5 show viscosity of unloaded MDEA-PZ aqueous solutions calculated from models 1 and 2, respectively. For model 1, the calculated viscosities show a good agreement with experimental viscosities. The Ω between experimental and calculated viscosities for the unloaded aqueous MDEA-PZ solution is 0.15 mPa·s. Model 2 also shows a good agreement between calculated and experimental viscosities. The corresponding Ω between calculated and experimental viscosities of unloaded aqueous MDEA-PZ solution is 0.09 mPa·s. The deviation plots between experimental and calculated data from model 1 and model 2 are illustrated in Figures 6A and B,

different methods for correlating our viscosity data for the unloaded ternary mixtures. Model 1: The viscosity of the ternary liquid mixture is calculated using the following eq 2.10 ln(ηmix /mPa·s) =

∑ ∑ xixjGij

(2)

where ηmix refers to viscosity of the mixture and xi and xj refer to mole fraction of ith and jth components in the liquid mixture. Parameter Gij is temperature dependent and may be represented by eq 3. Gij = a + b(T /K ) + c(T /K )2

(4)

(3)

Parameters a, b, and c in eq 3 are obtained by regression analysis of the experimental data and are presented in Table 4. The viscosity of pure water was taken from Kestin et al.62 The final form of model 1 for ternary components is shown in eq 4. D



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Table 5. Fitting Parameters A and B for Model 2 for Unloaded Aqueous MDEA-PZ Solutions parameter wMDEA/wPZ

A

B

0.20/0.05 0.30/0.05 0.40/0.05 0.20/0.10 0.30/0.10 0.40/0.10

−6.63 −7.42 −7.79 −7.39 −7.86 −9.01

2250.64 2634.20 2910.45 2593.04 2892.59 3405.61

Figure 6. Panel A: Average deviations[ηexp − ηcal] between experimental and calculated values from model 1 for various wMDEA/wPZ at different temperatures. Panel B: Average deviations[ηexp − ηcal] between experimental and calculated values from model 2 for various wMDEA/wPZ at different temperatures. Symbols: wMDEA/wPZ, 0.20/0.05 (■); wMDEA/wPZ, 0.30/0.05 (●); wMDEA/wPZ, 0.40/0.05 (▲); wMDEA/wPZ, 0.20/0.10 (□); wMDEA/wPZ, 0.30/0.10 (○); wMDEA/ wPZ, 0.40/0.10 (△).

Tables 6−11 for different CO2 loadings and amine mass fractions. In the tables, x4 represents a hypothetical mole Figure 4. Viscosities of unloaded aqueous MDEA-PZ solutions calculated from model 1 and comparison with experimental values. Panel A: wMDEA/wPZ, 0.20/0.10 (□); wMDEA/wPZ, 0.30/0.10 (○); wMDEA/wPZ, 0.40/0.10 (△). Panel B: wMDEA/wPZ, 0.20/0.05 (▲); wMDEA/wPZ, 0.30/0.05 (●); wMDEA/wPZ, 0.40/0.05 (■). Dotted lines refer to calculated viscosity values from using model 1.

Table 6. Measured Viscosity (η) of CO2-Loaded Aqueous MDEA-PZ Solutions at wMDEA/wPZ 0.20/0.05 Mass Fraction at Different Temperatures T, CO2 Loading Values α, and CO2 Mole Fraction x4a

Figure 5. Viscosities of unloaded aqueous MDEA-PZ solutions calculated from model 2 and comparison with experiments. Panel A: wMDEA/wPZ, 0.20/0.10 (□); wMDEA/wPZ, 0.30/0.10 (○); wMDEA/wPZ, 0.40/0.10 (△). Panel B: wMDEA/wPZ, 0.20/0.05 (▲); wMDEA/wPZ, 0.30/0.05 (●); wMDEA/wPZ, 0.40/0.05 (■). Dotted lines refer to calculated viscosity values from using model 2.

α/(mol CO2/mol amine)

0.32

0.52

0.74

x4

0.016

0.026

0.036

T/K 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15 368.15 373.15

2.39 2.09 1.82 1.63 1.45 1.31 1.17 1.08 0.99 0.92 0.83 0.79 0.76 0.73 0.68

η/mPa·s 2.43 2.13 1.86 1.66 1.46 1.33 1.19 1.10 1.02 0.94 0.88 0.84 0.84 0.73 0.73

2.49 2.20 1.94 1.76 1.57 1.43 1.30 1.24 1.14 1.05 0.97 0.92 0.89 0.86 0.84

a


respectively. These low deviation values convey that both models 1 and 2 are able to correlate viscosity of unloaded aqueous MDEA-PZ solutions satisfactorily. 4.3. Viscosity of CO2-Loaded Aqueous MDEA-PZ Solutions. Measured viscosities of carbonated MDEA-PZ solutions at temperatures (303.15 to 373.15) K are listed in

fraction of CO2 accounting for all forms of CO2 bound in the solution. Viscosity increased with CO2 loading as expected and showed its dependency on overall amine concentration as well. A possible explanation for increase in viscosity upon CO2 loading is because of an increase in intermolecular forces between CO2, amines, water, and reaction products. E



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0.29

0.45

0.56

0.73


0.60

0.68

0.81

x4

0.022

0.034

0.042

0.055

x4

0.04

0.044

0.052

T/K 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15 368.15 373.15

4.06 3.47 2.98 2.61 2.29 2.04 1.82 1.65 1.50 1.37 1.24 1.16 1.08 1.01 0.95

η/mPa·s 4.09 4.16 3.50 3.60 3.02 3.12 2.66 2.76 2.33 2.42 2.14 2.17 1.93 1.94 1.76 1.77 1.59 1.61 1.44 1.46 1.31 1.33 1.22 1.23 1.13 1.16 1.06 1.08 1.00 1.01

4.19 3.64 3.17 2.82 2.48 2.23 1.99 1.81 1.65 1.50 1.34 1.26 1.20 1.10 1.02

T/K 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15 368.15 373.15

2.84 2.45 2.17 1.90 1.71 1.53 1.41 1.30 1.19 1.09 1.03 0.97 0.90 0.83

η/mPa·s 3.06 2.67 2.37 2.10 1.89 1.70 1.56 1.43 1.31 1.19 1.12 1.05 1.01 0.97

3.09 2.70 2.41 2.14 1.93 1.73 1.60 1.47 1.37 1.26 1.22 1.15 1.06 1.02

a Standard uncertainties u are u(wMDEA+PZ) = 0.01, u(T) = 0.03 K, u(P) = 0.2 kPa. The combined standard uncertainty for viscosity measurement uc(η) is 0.15 mPa·s. The pressure during experiments was maintained by N2 gas (p = 400 kPa).

a





0.34

0.44


0.28

0.38

0.44

0.69

x4

0.033

x4

0.031

0.042

0.048

0.073

T/K 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15 368.15 373.15

6.98 5.82 4.91 4.22 3.64 3.18 2.78 2.48 2.22 2.00 1.79 1.64 1.50 1.38 1.27

η/mPa·s 7.38 7.74 6.20 6.49 5.24 5.48 4.53 4.70 3.94 4.04 3.42 3.54 3.03 3.09 2.73 2.77 2.44 2.48 2.23 2.24 1.96 2.01 1.75 1.85 1.61 1.69 1.47 1.55 1.36 1.44

8.83 7.43 6.34 5.49 4.75 4.17 3.66 3.29 2.94 2.63 2.41 2.20 2.07 1.87 1.69

T/K 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15 368.15 373.15

5.80 4.90 4.18 3.64 3.16 2.79 2.46 2.22 2.02 1.83 1.65 1.53 1.41 1.31 1.21

0.53

0.58

0.71

0.042

0.050

0.055

0.066

5.96 5.04 4.30 3.73 3.24 2.85 2.52 2.26 2.04 1.84 1.66 1.53 1.42 1.32 1.22

η/mPa·s 6.19 5.23 4.40 3.79 3.26 2.86 2.62 2.43 2.17 1.96 1.76 1.62 1.50 1.39 1.31

6.43 5.48 4.71 4.12 3.59 3.17 2.81 2.53 2.29 2.08 1.89 1.75 1.62 1.51 1.41

6.69 5.74 4.93 4.32 3.79 3.36 2.99 2.70 2.45 2.22 1.96 1.78 1.70 1.57 1.43

a


a


solution.61 The modified Setschenow equation is presented in eq 6, ij η yz lnjjjj zzzz = k ηr {

In this work, viscosities of CO2-loaded aqueous MDEA-PZ solutions were correlated using a modified Setschenow equation.59 This equation has been used earlier to represent physical properties of aqueous amine solutions;8,63,64 albeit, it was originally developed to explain the salting effect in liquid mixtures.58 We used this equation with good results in a recent publication to represent viscosity of CO2-loaded 3A1P

n

∑ kjα j j=1

(6)

where η/ηr represents ratio between viscosities of CO2-loaded and -unloaded amine systems at equal temperatures, and kj F



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Table 11. Measured Viscosity (η) of CO2-Loaded Aqueous MDEA-PZ Solutions at wMDEA/wPZ 0.40/0.10 Mass Fraction at Different Temperatures T, CO2 Loading Values α, and CO2 Mole Fraction x4a α/(mol CO2/mol amine)

0.25

0.31

0.42

0.54

0.61

x4

0.034

0.042

0.055

0.070

0.079

T/K 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15 368.15 373.15

11.01 9.04 7.48 6.31 5.34 4.62 4.03 3.57 3.13 2.83 2.47 2.28 2.09 1.91 1.77

11.09 9.11 7.57 6.38 5.41 4.65 4.05 3.61 3.16 2.84 2.53 2.31 2.11 1.94 1.80

η/mPa·s 12.79 10.40 8.66 7.32 6.24 5.40 4.69 4.14 3.69 3.30 2.95 2.65 2.40 2.18 1.87

13.60 11.24 9.37 7.93 6.77 5.86 5.08 4.45 3.96 3.50 3.07 2.71 2.47 2.23 1.99

14.20 11.68 9.76 8.28 7.06 6.09 5.29 4.66 4.13 3.67 3.28 2.93 2.65 2.38 2.11

Figure 7. Comparison between experimental and calculated viscosity data for CO2-loaded aqueous MDEA-PZ solutions.

experimental and calculated viscosities using a modified Setschenow equation.

a


5. CONCLUSION This paper reports experimental data for viscosities of both unloaded and CO2-loaded aqueous MDEA-PZ solutions at different temperatures (298.15 to 373.15 K) and amine mass fractions. For unloaded aqueous MDEA-PZ solutions, viscosity increased with increase in amine concentration and decreased with increase in temperature. Viscosities for unloaded ternary systems were correlated as a function of temperature using two different models available in the literature, and both of these models fit the experimental data satisfactorily. For CO2-loaded aqueous MDEA-PZ solutions, viscosity increased with increase in CO2 loading and amine concentration. A modified Setschenow equation was able to predict the effects of CO2 loading, amine mass fractions, and temperatures on viscosities of CO2-loaded MDEA-PZ solutions and fits the experimental data satisfactorily.

values are used as parameters α represents CO2 loading value, i.e. mole of CO2/mol of amine. As previously shown by Shokouhi et al.,63 the parameter k is linearly dependent on temperature. A second order modified Setschenow equation was used to correlate viscosity of CO2-loaded aqueous MDEAPZ solutions. The regressed parameters at various MDEA-PZ concentrations studied in this work are shown in Table 12. As Table 12. Regressed Parameters k and Ω Values Based on Setschenow Equation for CO2-Loaded MDEA-PZ Solutions for Different Mass Fractions of MDEA-PZ, wMDEA/wPZ wMDEA/wPZ: 0.20/0.05

wMDEA/wPZ: 0.30/0.05

k1,1 = k1,0 = 1.125 −0.003 Ω = 0.035 mPa·s k2,1 = k2,0 = −2.395 0.007 wMDEA/wPZ: 0.40/0.05

k1,0 = k1,1 = −0.297 0.003 Ω = 0.055 mPa·s k2,0 = k2,1 = 0.209 −0.002 wMDEA/wPZ: 0.20/0.10

k1,1 = k1,0 = 1.849 −0.005 Ω = 0.095 mPa·s k2,0 = k2,1 = −1.925 0.007 wMDEA/wPZ: 0.30/0.10

k1,0 = k1,1 = −0.334 0.006 Ω = 0.038 mPa·s k2,0 = k2,1 = −1.121 0.005 wMDEA/wPZ: 0.40/0.10

k1,0 = −1.764 k2,0 = 1.715

k1,0 = −1.739 k2,0 = 3.888

k1,1 = 0.006 k2,1 = −0.004

Ω = 0.052 mPa·s

k1,1 = 0.008 k2,1 = −0.012

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.9b00021. Overview of previous work on viscosity of MDEA, pure and in solution; average absolute deviation values of viscosity between this work and each work from the literature; observed trend of viscosity with respect to time for selected measurements; further figures showing viscosity versus temperature for CO2-loaded aqueous MDEA-PZ solutions (PDF)

Ω = 0.134 mPa·s

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can be seen from the table, the Ω values calculated for all systems studied in this work are within our expected experimental uncertainty of 0.15 mPa·s. Viscosity dependency on temperature for the CO2-loaded MDEA-PZ systems are shown in Figures S2 and S3. Figure 7 depicts a parity plot comparing experimental and calculated viscosity data for CO2-loaded MDEA-PZ solutions studied in this work. There is a good agreement between our

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel.: +47 3557 4000. ORCID

Dag A. Eimer: 0000-0001-5514-2864 Funding

This work was supported by The Research Council of Norway (Grant 199890). G



Article

Notes

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The authors declare no competing financial interest.

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Viscosity Measurement and Correlation of Unloaded and CO2-Loaded

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