Densities and Surface Tensions of CO2 Loaded Aqueous

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Densities and Surface Tensions of CO2 Loaded Aqueous Monoethanolamine Solutions with r = (0.2 to 0.7) at T = (303.15 to 333.15) K Sanoja A. Jayarathna,† Achini Weerasooriya,† Sithara Dayarathna,† Dag A. Eimer,†,‡ and Morten C. Melaaen*,†,‡ †

Telemark University College, Postboks 203, N-3901 Porsgrunn, Norway Tel-Tek, Kjølnes ring 30, 3918 Porsgrunn, Norway

‡

S Supporting Information *

ABSTRACT: Densities and surface tensions of carbon dioxide (CO2) loaded (partially carbonated), aqueous monoethanolamine (MEA) solutions with amine mass ratios from (0.2 to 0.7) are measured within a temperature range from (303.15 to 333.15) K. The series of aqueous MEA solutions covers a range of CO2 loading from (0 to 0.5). All the density data points are compared with the predictions of the model from Weiland et al., and data regression is performed to fit the parameters of the model. Predictions from the fitted model are compared with the data reported in this work. The model of Connors and Wright is selected to represent the surface tension data of the H2O−MEA−CO2 tertiary system. Data reported in this work are used to fit the parameters of the Connors and Wright model, and the model predictions are compared with the same set of data.

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INTRODUCTION

This work presents a set of density and surface tension measurements from aqueous MEA solutions with amine mass ratio (MAmine/MAmine+Water), r = (0.2 to 0.7), and CO2 loading, n(CO2)/n(MEA), α = (0 to 0.5), in the range of temperature, T = (303.15 to 333.15) K. Here “M” stands for mass, and “n” stands for moles. A widely used correlation exists for prediction of the densities of the aqueous amine solutions either single or mixed and CO2 loaded or unloaded.5,11 Since the data fit presented in the literature for the correlation does not cover the temperature and mole fraction ranges used in this work, the predictions from the correlation are not in good agreement with the measurements made during this work. Estimation of the parameters of the correlation is performed as a part of this work to obtain good representation of the density of the partially carbonated MEA solutions considered in this work. Density and surface tension measurements of partially carbonated MEA solutions expand the research frontier further as these measurements cover a composition and temperature range that has not been reported earlier. Surface tension data from this work are given as a custom fit equation as well as a fit to a literature model.17,18

Chemical absorption is an efficient method for CO2 capture, especially from the gas streams with low CO2 concentrations.1−3 Aqueous alkanolamine solutions are widely used in the absorption−desorption process for the removal of CO2 from the gas streams. Industrially important alkanolamines for the absorption of CO2 are monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA). Each of the amines has at least one hydroxyl group and one amino group. The hydroxyl group serves to reduce the vapor pressure and increase the water solubility, while the amino group provides the necessary alkalinity in water solutions to cause the absorption of acidic gases. The reactivity of the amines with CO2 and the ability to reproduce a relatively clean CO2 stream by elevating the temperature are the main attractions of amines for CO2 capture. One of the main concerns of the amine absorption of CO2 is the cost of regeneration of the solvent stream. High concentration amine solutions are interesting since amine circulation (amount of amine flow inside and between the absorption and stripping towers) can be reduced and energy can be saved. Physical properties like density, surface tension, viscosity, and solubility of CO2 in such solutions are of great importance for further analysis of the use for CO2 capture as well as for the equipment design and related analysis via modeling and simulation. Density and surface tension measurements of single alkanolamine solutions, mixed amine solutions, and activated amine solutions have been presented in the literature.4−16 Published measurements of partially carbonated MEA solutions are scarce though. © 2013 American Chemical Society

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EXPERIMENTAL SECTION This section provides an insight into the procedures of sample preparation, sample analysis, and measurement performance. CO2, received from “AGA” with an initial mole fraction of Received: December 4, 2012 Accepted: February 19, 2013 Published: March 6, 2013 986

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0.9999, and MEA, received from “Merck” with an initial mole fraction of 0.995, are used as received. Sample Preparation. Aqueous solutions of MEA were prepared using degassed, deionized water and MEA. Water deionization and degassing were done using a Milli-Q integral water purification system and a rotary evaporator, respectively. Amines are used as received. Water and amines are measured using an analytical balance with an accuracy of ± 1·10−7 kg and mixed with the appropriate proportion to achieve the required amine mass ratio (MAmine/MAmine+Water), r = (0.2 to 0.7). The loaded amine samples were prepared by diluting an aqueous amine solution of selected mass ratio and high CO2 loading (> 0.5) with an unloaded amine solution in proper proportions to get the required CO2 loading values (n(CO2)/ n(MEA)), α = (0 to 0.5) in the prepared samples. The high loaded amine solutions were prepared by bubbling CO2 gas through an unloaded amine solution of required mass ratio. The CO2 gas was used as received. Sample Analysis. All the CO2 loaded aqueous amine solutions were analyzed using a titration method to check the loading value and the amine mass ratio. The mass ratio analysis was done via titration of the prepared samples with a 1 mol·L−1 HCl solution to find the equilibrium point. The samples used for titration were prepared by mixing a sample of 2 g from each prepared amine solution with deionized water until each sample became 100 cm3 in total. The amount of HCl used in the titration was used to calculate the amount of amine present in each sample and subsequently the mass ratio of the corresponding aqueous amine solution. The sample preparation for the loading analysis was performed by mixing about (0.5 to 1.0) g of the loaded amine solution with 50 cm3 each from 0.3 mol·L−1 BaCl2 and 0.1 mol·L−1 NaOH. Those samples were boiled for 5 min to allow the CO2 in the samples to react with BaCl2 and precipitate as BaCO3 and then cooled in a water bath and filtered to collect the precipitate. Each collected precipitate was added into 100 cm3 deionized water and then titrated with 0.1 mol·L−1 HCl solution until the mixture reached the equilibrium point. The mixture was then heated to remove all the dissolved CO2 and back-titrated with 0.1 mol·L−1 NaOH solution to calculate the amount of excess HCl. Finally, the moles of HCl reacted with BaCO3 precipitate was used to find the amount of CO2 in the corresponding partially carbonated aqueous amine sample and subsequently the CO2 loading value of the solution. Density Measurements. Densities of the partially carbonated aqueous MEA solutions were measured using an Anton Paar densimeter, with a high-pressure cell (model DMA HP). The DMA HP can be used at higher pressures and temperatures beyond atmospheric pressure and 363.15 K of temperature. Calibration of DMA HP was done using nitrogen and degassed water every time before the use of the machine. The measuring cell is based on an oscillatory U-tube method, and the accuracy of the measurements is very much dependent on the calibration of the equipment. Density measurements of the aqueous MEA solutions in the range of T = (303.15 to 333.15) K were taken using the highpressure cell, DMA HP. Bubble formation due to the evaporation of amines and water and desorption of CO2 was a challenge for using the low-pressure cell (model DMA 4500) for the aqueous MEA solutions even below the temperature limit of the equipment. The DMA HP was used under a pressure of 8·105 Pa to retain the evaporation and desorption in the capillary U-tube.

Surface Tension Measurements. Surface tension values of the partially carbonated aqueous MEA solutions were measured under atmospheric pressure using a Ramé−Hart advanced goniometer (model 500 with DROPimage Advanced v2.4 software). The goniometer was calibrated using the calibration tool provided with the instrument occasionally, but surface tension measurements of water were performed more frequently to check the previous calibration. The measured surface tension values for water were compared with literature data to make sure that the error is within ± 0.0004 N·m−1, which is about ± (0.5 to 0.6) %. This error is considered as the instrument accuracy for uncertainty calculations of the surface tension measurements reported in this work. The measuring procedure of the goniometer is based on a calculation of the droplet/bubble geometry size, which is obtained by digitizing the image from the camera, and the accuracy of the measurements is very much dependent on the calibration of the equipment. The traditional pendant drop method was not adopted due to possible concentration changes and issues related to temperature monitoring and control as explained by Han et al.11 The same measurement procedure as explained by Han et al.11 is adopted, and the CO2 loading of the used samples was analyzed via titration to observe losses of CO2 from the samples. Each surface tension value reported is an average of the measurements for at least 10 different bubbles with about 10 measurements per bubble, with a maximum deviation less than ± 0.001 N·m−1 from the average value. A set of surface tension measurements are taken for pure H2O and MEA, and those are compared with the data from the literature as an attempt of demonstrating the measurement accuracy. Data from this work show a good agreement with the literature data, and the comparison is given in Table 1. Table 1. Surface Tension, σ, of Pure H2O and MEAa,b,c 293.15 K this work Han et al.11 Vázquez et al.13

0.07256

this work Han et al.11 Vázquez et al.13

0.04848

303.15 K

313.15 K

σw/N·m−1 0.07090 0.06981 0.07130 0.06960 0.07121 0.06952 σa/N·m−1 0.04764 0.04688 0.04810 0.04670 0.04814 0.04643

323.15 K 0.06772 0.06800 0.06792

0.04560 0.04481

σw = surface tension of H2O; σa = surface tension of MEA. bStandard uncertainties u are u(T) = ± 0.5 K. Instrument accuracy = ± 0.0004 N·m−1. cThe combined expanded uncertainties Uc are Uc(σw) = ± 0.0004 N·m−1 and Uc(σa) = ± 0.0004 N·m−1 (level of confidence = 0.95, where k = 2). a

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RESULTS AND DISCUSSION Results obtained from this work are 2-fold: 1. Density measurements of partially carbonated aqueous MEA solutions from T = (303.15 to 333.15) K for r = (0.2 to 0.7) and α in the range from (0.0 to 0.5). 2. Surface tension measurements of partially carbonated aqueous MEA solutions from T = (303.15 to 333.15) K for r = (0.2 to 0.7) and α in the range from (0.0 to 0.5). Density measurements and surface tension measurements of the MEA solutions with r = (0.2 to 0.7) are given in Tables 2 and 3, respectively. Both the density and the surface tension of the partially carbonated aqueous MEA solutions with constant 987

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Table 2. Densities, ρ, of Partially Carbonated MEA Solutions from T = (303.15 to 333.15) K and CO2 Loading, α, from (0.00 to 0.50) with MEA Mass Ratio, r = (0.2 to 0.7), at P = (0.8) MPaa,b 303.15 K xMEA

xCO2

α

0.0687 0.0682 0.0677 0.0673 0.0668 0.0664

0.0000 0.0068 0.0135 0.0202 0.0267 0.0332

0 0.1 0.2 0.3 0.4 0.5

xMEA

xCO2

α

0.1122 0.1110 0.1097 0.1085 0.1074 0.1062

0.0000 0.0111 0.0219 0.0326 0.0430 0.0531

0 0.1 0.2 0.3 0.4 0.5

xMEA

xCO2

α

0.1643 0.1616 0.1591 0.1566 0.1542 0.1518

0.0000 0.0162 0.0318 0.0470 0.0617 0.0759

0 0.1 0.2 0.3 0.4 0.5

xMEA

xCO2

α

0.2277 0.2227 0.2178 0.2132 0.2087 0.2045

0.0000 0.0223 0.0436 0.0640 0.0835 0.1022

0 0.1 0.2 0.3 0.4 0.5

xMEA

xCO2

α

0.3067 0.2976 0.2890 0.2808 0.2732 0.2659

0.0000 0.0298 0.0578 0.0843 0.1093 0.1330

0 0.1 0.2 0.3 0.4 0.5

xMEXA

xCO2

α

0.4076 0.3916 0.3769 0.3632 0.3505 0.3386

0.0000 0.0392 0.0754 0.1090 0.1402 0.1693

0 0.1 0.2 0.3 0.4 0.5

313.15 K ρ/kg·m

1003.6 1019.1 1032.4 1046.7 1061.1 1074.4 303.15 K

−3

999.4 1014.8 1028.1 1042.4 1056.7 1069.8 313.15 K

323.15 K

Table 3. Surface Tension, σ, of Partially Carbonated MEA Solutions from T = (303.15 to 333.15) K and CO2 Loading, α, from (0.00 to 0.50) with MEA Mass Ratio, r = (0.2 to 0.7), at P = (0.1) MPaa,b

333.15 K

303.15 K

(r = 0.2) 994.6 1010.2 1023.3 1037.6 1051.8 1064.9 323.15 K

989.1 1004.8 1018.0 1032.3 1046.4 1059.4 333.15 K

ρ/kg·m−3 (r = 0.3) 1008.2 1027.8 1047.5 1066.1 1086.3 1106.0 303.15 K

1003.3 1022.9 1042.7 1061.4 1081.5 1101.0 313.15 K

998.1 1017.8 1037.6 1056.3 1076.3 1095.8 323.15 K

992.3 1012.0 1032.0 1050.7 1070.7 1090.1 333.15 K

ρ/kg·m−3 (r = 0.4) 1013.3 1037.8 1062.8 1087.6 1113.9 1139.6 303.15 K

1007.8 1032.5 1057.6 1082.5 1108.6 1134.4 313.15 K

1002.1 1026.9 1052.1 1077.1 1103.2 1129.0 323.15 K

995.7 1020.8 1046.2 1071.3 1097.5 1123.2 333.15 K

ρ/kg·m−3 (r = 0.5) 1017.8 1047.2 1075.1 1106.7 1136.4 1167.7 303.15 K

1011.8 1040.6 1069.4 1099.9 1131.0 1162.1 313.15 K

1005.4 1034.4 1063.6 1093.9 1125.4 1156.5 323.15 K

998.7 1027.9 1057.4 1088.0 1119.6 1150.7 333.15 K

ρ/kg·m−3 (r = 0.6) 1021.3 1058.6 1091.6 1126.2 1162.1 1197.6 303.15 K

1014.6 1052.3 1085.6 1120.4 1156.5 1192.0 313.15 K

1007.8 1045.9 1079.5 1114.6 1150.8 1186.3 323.15 K

1000.6 1039.2 1073.1 1108.5 1144.9 1180.4 333.15 K

ρ/kg·m−3 (r = 0.7) 1022.6 1063.9 1101.1 1143.7 1185.4 1226.2

1015.5 1057.3 1094.8 1137.8 1179.7 1220.4

1008.2 1050.6 1088.4 1131.8 1174.0 1214.7

1000.6 1043.6 1081.9 1125.6 1168.1 1208.7

xMEA

xCO2

α

0.0687 0.0682 0.0677 0.0673 0.0668 0.0664

0.0000 0.0068 0.0135 0.0202 0.0267 0.0332

0 0.1 0.2 0.3 0.4 0.5

xMEA

xCO2

α

0.1122 0.1110 0.1097 0.1085 0.1074 0.1062

0.0000 0.0111 0.0219 0.0326 0.0430 0.0531

0 0.1 0.2 0.3 0.4 0.5

xMEA

xCO2

α

0.1643 0.1616 0.1591 0.1566 0.1542 0.1518

0.0000 0.0162 0.0318 0.0470 0.0617 0.0759

0 0.1 0.2 0.3 0.4 0.5

xMEA

xCO2

α

0.2277 0.2227 0.2178 0.2132 0.2087 0.2045

0.0000 0.0223 0.0436 0.0640 0.0835 0.1022

0 0.1 0.2 0.3 0.4 0.5

xMEA

xCO2

α

0.3067 0.2976 0.2890 0.2808 0.2732 0.2659

0.0000 0.0298 0.0578 0.0843 0.1093 0.1330

0 0.1 0.2 0.3 0.4 0.5

xMEA

xCO2

α

0.4076 0.3916 0.3769 0.3632 0.3505 0.3386

0.0000 0.0392 0.0754 0.1090 0.1402 0.1693

0 0.1 0.2 0.3 0.4 0.5

313.15 K σ/N·m

0.0667 0.0676 0.0684 0.0697 0.0714 0.0736 303.15 K

−1

0.0652 0.0663 0.0670 0.0679 0.0700 0.0718 313.15 K

323.15 K

333.15 K

(r = 0.2) 0.0636 0.0647 0.0654 0.0664 0.0684 0.0702 323.15 K

0.0616 0.0631 0.0638 0.0649 0.0664 0.0689 333.15 K

σ/N·m−1 (r = 0.3) 0.0637 0.0650 0.0664 0.0678 0.0698 0.0728 303.15 K

0.0624 0.0636 0.0650 0.0662 0.0683 0.0714 313.15 K

0.0610 0.0620 0.0632 0.0647 0.0669 0.0698 323.15 K

0.0595 0.0605 0.0615 0.0628 0.0652 0.0685 333.15 K

σ/N·m−1 (r = 0.4) 0.0615 0.0630 0.0646 0.0663 0.0693 0.0724 303.15 K

0.0598 0.0619 0.0634 0.0651 0.0681 0.0711 313.15 K

0.0584 0.0603 0.0620 0.0638 0.0667 0.0698 323.15 K

0.0570 0.0587 0.0602 0.0624 0.0649 0.0681 333.15 K

σ/N·m−1 (r = 0.5) 0.0596 0.0609 0.0627 0.0644 0.0671 0.0705 303.15 K

0.0581 0.0594 0.0613 0.0631 0.0660 0.0695 313.15 K

0.0565 0.0582 0.0596 0.0618 0.0648 0.0682 323.15 K

0.0551 0.0567 0.0579 0.0604 0.0634 0.0670 333.15 K

σ/N·m−1 (r = 0.6) 0.0574 0.0589 0.0606 0.0633 0.0663 0.0707 303.15 K

0.0561 0.0576 0.0596 0.0622 0.0655 0.0698 313.15 K

0.0550 0.0564 0.0586 0.0608 0.0643 0.0686 323.15 K

0.0536 0.0549 0.0573 0.0595 0.0631 0.0672 333.15 K

σ/N·m−1 (r = 0.7) 0.0545 0.0570 0.0590 0.0612 0.0655 0.0692

0.0536 0.0559 0.0580 0.0606 0.0648 0.0684

0.0525 0.0545 0.0568 0.0594 0.0638 0.0673

0.0512 0.0530 0.0555 0.0582 0.0627 0.0662

a

Standard uncertainties u are u(α) = ± 0.005, u(T) = ± 0.05 K, U(r) = ± 0.05. Instrument accuracy = ± 0.1 kg·m−3. bThe combined expanded uncertainty Uc is Uc(ρ) = ± 4.42 kg·m−3 (level of confidence = 0.95, where k = 2).

Standard uncertainties u are u(α) = ± 0.01, u(T) = ± 0.1 K, U(r) = ± 0.001. Instrument accuracy = ± 0.0004 N·m−1. bThe combined expanded uncertainty Uc is Uc(σ) = ± 0.0012 N·m−1 (level of confidence = 0.95, where k = 2).

MEA mass ratio show an increase with increasing CO2 loading and a decrease with increasing temperature. The molecular interactions increase with the increasing CO2 loading, which in

return increases the density and the surface tension of the solutions. The decrease in the surface tension is because when the temperature increases the thermal motion of molecules

a

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increases; molecules at the surface stretch more; intermolecular attraction decreases; and then the surface tension decreases. As the motion of the molecules increases, the molecular interactions decrease allowing the volume to increase, and then the density decreases. A decrease in the surface tension and an increase in the density are observed with an increase of the MEA mass ratio. Comparison of the density data and the surface tension data from this work with literature data4,5,11,13,15,16 is given as Supporting Information. Data from this work show a good agreement with the literature data with some deviation at some data ranges which can be neglected for engineering calculations. Density Model. A model to predict the density of partially carbonated amine solutions (for MEA, DEA, and MDEA) has been published by Weiland et al.5 This model predicts the density as a ratio of the average molar weight (M̅ ) to the mean molar volume of the solution (Vs), eq 1. Weiland’s model has been fitted for partially carbonated MEA by Han et al.11 for data from r = 0.3 up to r = 0.6. Predictions from the Weiland model with the originally fitted parameters and with the parameters fitted by Han et al.11 were found to be unsatisfactory for partially carbonated MEA solutions in the range of r, T, and α used in this work, especially for high mass ratios. The reason for this observation is that the data they have used to fit the parameters are not in the range of r, T, and α used in this work. The average absolute deviation (AAD) of the predictions from the Weiland model with the parameters from Weiland et al.5 and Han et al.11 from the measurements reported in this work are given in Table 4.

Here eq 1 comes from the basic definitions of the density of a solution. The symbols ρ, x, M, and Vs are the density, mole fraction, molar weight, and mean molar volume of the solution. The symbol Vj represents the molar volume of pure components, and the species CO2, H2O, and amine are represented by j = 1, 2, and 3, respectively. Note that the molar volume of CO2, VCO2, is used to represent the dissolved CO2, and it is different from the component’s pure component value.5 Molar volumes associated with the interactions between the amine and CO2 and the amine and H2O are given by V** and V*. The V** is found as a function of mole fraction of amine and given by eq 3 V ** = e + f ·xj = 3

(3)

where e and f are constants and x3 is the mole fraction of MEA. The VCO2, V** and V* values are estimated with the data from this work using linear data regression (with the method of least-squares) and the fitted parameters are given in Table 5. Molar volumes of H2O and MEA are predicted using the correlations that have been developed via fitting the density data presented in Han et al.11 The fitted density correlations for MEA and H2O are in the form of eq 4 ρj = a ·T 3 + b ·T 2 + c·T + d

(4)

−3

where ρj is the density/kg·m of species j and T is the temperature/K. The fitted parameters are given in Table 5. The predictions from the newly fitted model for MEA at r = 0.4 and r = 0.6 are given in Figures 1 and 2 along with the data

Table 4. AAD of the Density Predictions at Each MEA Mass Ratio for CO2 Loaded Aqueous MEA Solutions source Weiland et al.5 Han et al.11 this work

r = 0.2 0.59 3.98 1.56

r = 0.3

r = 0.4

4.62 1.09 1.10

r = 0.5

r = 0.6

r = 0.7

AAD/kg·m−3 7.19 12.74 2.80 9.21 0.91 1.65

15.04 14.02 1.72

19.62 22.86 2.03

Figure 1. Measured density of MEA from T = (303.15 to 313.15) K at r = 0.4. ○, α = 0.0; ●, α = 0.1; □, α = 0.2; ■, α = 0.3; Δ, α = 0.4; ▲, α = 0.5. , Model Density Prediction.

Parameters of the Weiland model are fitted against the measurements from this work with the expectation of expanding the usability of the model for CO2 loaded aqueous MEA solutions with high mass ratios, up to r = 0.7. The structure of the model is given below by eq 1 and eq 2. ρ=

M̅ = Vs

from this work, respectively. The average absolute deviations (AADs) between the predictions and our data in kg·m−3 are given in Table 4. The deviation of the predictions from the experimental data is within acceptable error. Surface Tension Model. Surface tension data presented in this work are correlated with a polynomial function of CO2 loading value, α, and temperature, T, which has the format of eq 5, at each amine mass ratio.

n ∑ j = 1 (xj·Mj)

Vs

(1)

3

Vs =

∑ (Vj·xj) + xj = 2·xj = 3·V * + xj = 1·xj = 3·V ** (2)

j=1

Table 5. Parameters of the Weiland Density Model5 Fitted for Partially Carbonated MEA and the Density Correlation (Equation 4) for Pure H2O and MEA a

b

c

d

e

MEA H2O

0 4.007(−6) f

−5.327(−4) −6.875(−3) VCO2

−4.566(−1) 2.740(−4) V*

1.195(3) 6.852(2) M

3.780(−5)

MEA H2O

−6.066(−5)

6.109(−2) 1.802(−2)

4.401(−2)

6.516(−7)

2.396(−6)

989

MCO2

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Figure 3. Measured surface tension of MEA from α = (0.0 to 0.5) at r = 0.6. ■, T = 303.15 K; □, T = 313.15 K; ●, T = 323.15 K; ○, T = 333.15 K; , Model Surface Tension Prediction. Figure 2. Measured density of MEA from α = (0.0 to 0.5) at r = 0.6. Δ, T = 303.15 K; ●, T = 313.15 K; *, T = 323.15 K; ■, T = 333.15 K. , Model Density Prediction.

σmix

2

σmix = p00 + p10·α + p01·T + p20·α + p11·α ·T + p02·T 2 + p30·α 3 + p21·α 2·T + p12·α ·T 2 + p40α 4 + p31α 3T + p22α 2T 2

⎛ ⎞ ⎜ ⎟ bixi ⎟· = σ2 + ∑ ⎜1 + aj ⎛ ⎞⎟ i = 1,3 ⎜ ⎜ ⎟ ⎜ (1 − ai) · 1 + ∑j = 1,3 (1 − a ) ·xj ⎟ ⎝ ⎠⎠ j ⎝ (xi·(σi − σ2))

(5)

(6)

Here, σ1, σ2, and σ3 represent the surface tension of CO2, H2O, and MEA, respectively, and a1, a3, b1, and b3 are fitting parameters of the model. The surface tension of pure H2O and MEA is computed using a correlation presented by Asprion.18 The surface tension of pure CO2 was considered as a fitting parameter as CO2, σ1, does not exist as a liquid above its critical point temperature. A linear function of temperature is used to represent the surface tension of CO2, which has the form of eq 7.

−1

Here σmix is the surface tension of the solution/N·m . The values of the parameters p00, p10, p01, p20, p11, p02, p30, p21, p12, p40, p31, and p22 at each mass ratio and the AAD between the predictions and the data reported in this work are given in Table 6. Predictions of the fitted correlation and the experimental data of the CO2 loaded MEA solutions with r = 0.6 are given in Figure 3. Deviation between the predictions and the experimental data is small and negligible for engineering calculations. In applications like modeling and simulation, it is always interesting to present data with relation to existing models describing the physical properties, rather than a polynomial function fitted to a specific set of data. To achieve such a general presentation of the surface tension data for partially carbonated MEA solutions, extension of an existing model is considered. Followed by the good predictability expressed for aqueous MEA and the ease of extension, the model presented by Connors and Wright17,18 is selected to be fitted in this work, after comparing several models presented in the literature.17−20 The Connors and Wright model is given by eq 6.

σCO2 = S1 + S 2·T

(7)

Parameters of the model proposed by Connors and Wright,17,18 fitted using the data reported in this work, and S1 and S2 are given in Table 7 with the AAD between the predictions and the measurements. Predictions from the fitted Connors and Wright model show higher AADs compared to that of the polynomial function (eq 5) for the data from this work, but both models can be useful for engineering calculations depending on the focus. Model predictions together with the experimental data from this work for loaded MEA with r = 0.3 and r = 0.4 are presented in Figures 4 and 5, respectively. As can be seen by the figures, the surface tension of the amine solutions has increased with the increasing CO2 loading. Increased molecular interactions in the solutions due

Table 6. Parameters and Prediction Accuracy of the Surface Tension Correlation (Equation 5) for CO2 Loaded MEA Solutions

p00 p10 p01 p20 p11 p02 p30 p21 p12 p40 p31 p22 AAD/N·m−1

r = 0.2

r = 0.3

r = 0.4

r = 0.5

r = 0.6

r = 0.7

−2.302(−2) 5.022(−1) 7.186(−4) −1.169(−1) −3.322(−3) −1.395(−6) −1.948(−1) 1.422(−3) 5.666(−6) −2.990(−1) 1.687(−3) −4.375(−6) 8.207(−5)

8.788(−2) −4.072(−1) −2.419(−5) 8.929(−1) 2.868(−3) −1.830(−7) 1.583(−1) −6.328(−3) −4.862(−6) 2.552(−2) −3.935(−4) 1.094(−5) 5.840(−5)

1.414(−1) −9.994(−1) −3.687(−4) 1.365(0) 6.443(−3) 3.455(−7) 4.284(−1) −9.374(−3) −1.012(−5) −2.578(−1) 3.935(−4) 1.504(−5) 1.796(−4)

1.234(−1) −5.149(−1) −2.690(−4) 6.830(−1) 3.423(−3) 1.920(−7) 3.015(−1) −5.015(−3) −5.494(−6) −1.432(−1) −4.046(−4) 8.616(−6) 7.878(−5)

7.010(−2) −1.150(−1) 3.400(−5) −1.239(−1) 8.264(−4) −2.509(−7) 1.412(−1) 6.161(−4) −1.333(−6) 3.385(−2) −4.565(−4) −4.911(−7) 8.214(−5)

−8.932(−3) 8.303(−2) 4.996(−4) −7.022(−1) 5.750(−5) −9.580(−7) 1.454(0) 1.309(−3) −6.759(−7) −5.891(−1) −2.712(−3) 1.295(−6) 1.167(−4)

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Table 7. Fitted Parameters of the Connors and Wright Model17,18 (Equation 6) and the Prediction Accuracy of the Fitted Model a1 a3 b1 b3 S1 S2 AAD/N·m−1

r = 0.2

r = 0.3

r = 0.4

r = 0.5

r = 0.6

r = 0.7

3.073(−1) 1.067 −8.574(−1) 1.701(−1) 8.286(−2) 4.309(−4) 2.344(−4)

9.409(−2) 1.114(0) −7.392(−1) 1.757(−1) 1.605(−1) 1.316(−4) 2.143(−4)

1.478(−1) 1.157(0) −8.982(−1) 3.062(−1) 1.184(−1) 1.954(−4) 2.611(−4)

1.964(0) 3.544(0) −3.329(0) −9.698(0) 6.791(−2) 2.141(−4) 3.070(−4)

1.654(0) 5.228(0) −2.154(0) −7.825(0) 9.374(−2) 8.033(−5) 3.237(−4)

3.377(−1) 9.872(−1) 2.673(2) 6.857(−1) 9.568(−2) −2.074(−5) 4.232(−4)

the higher value assigned as the instrument accuracy compared to the value used by Han et al.11 also has a considerable contribution.

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CONCLUSIONS

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ASSOCIATED CONTENT

Densities and surface tensions of the CO2 loaded aqueous MEA solutions with mass ratio, r = (0.2 to 0.7), and CO2 loading, α = (0 to 0.5), at temperature T = (303.15 to 333.15) K have been measured. An increase in the density and surface tension with increasing CO2 loading and a decrease in the density and surface tension with increasing temperature were observed. The model presented by Weiland et al.5 was used to correlate the density data. The average absolute deviation between the predictions and measurements in kg·m−3 are 1.56, 1.10, 0.91, 1.65, 1.72, and 2.03 for r = (0.2 to 0.7), respectively. Surface tension data were correlated by a polynomial function, which has shown a predictability with 0.00008, 0.00006, 0.0002, 0.00008, 0.00008, and 0.0001 of AAD/N·m−1 with the present data at each amine mass ratio, r = (0.2 to 0.7). The model presented by Connors and Wright17,18 was fitted to predict the surface tension of MEA solutions with r = (0.2 to 0.7) and absorbed CO2. The AAD between the predictions and measurements in N·m−1 are 0.0002, 0.0002, 0.0003, 0.0003, 0.0003, and 0.0004 at r = (0.2 to 0.7), respectively. The fitted models for estimating the density and surface tension values of partially carbonated aqueous MEA solutions show a satisfactory representation with errors that would be negligible for engineering applications.

Figure 4. Surface tension of MEA from α = (0.0 to 0.5) at r = 0.3. ■, T = 303.15 K; □, T = 313.15 K; ●, T = 323.15 K; ○, T = 333.15 K; , Model Surface Tension Prediction from the Connors and Wright model.17,18

Figure 5. Surface tension of MEA from T = (303.15 to 333.15) K at r = 0.4. ○, α = 0.0; ●, α = 0.1; □, α = 0.2; ■, α = 0.3; Δ, α = 0.4; ▲, α = 0.5; , Model Surface Tension Prediction from the Connors and Wright model.17,18

to the reactivity of aqueous MEA with CO2 can be given as the reason for this observation.

S Supporting Information *

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Additional experimental details and tables. This material is available free of charge via the Internet at http://pubs.acs.org.

EXPERIMENTAL UNCERTAINTIES Uncertainty of the density and surface tension measurements of CO2 loaded aqueous MEA solutions arises as a combination of the uncertainties of temperature measurements, CO2 loading, amine mass ratio, and the measuring instrument itself. All the uncertainty values related to the measurements reported in this work are given under the corresponding data tables. Detailed calculations of the experimental uncertainties are given as Supporting Information. The combined expanded uncertainties of the CO2 loaded aqueous MEA solutions are about three times higher than that of the unloaded aqueous MEA solution reported by Han et al.11 This can be justified by relating with the difficulties in the experimental determination of the surface tension when CO2 is involved. The tendency to release CO2 from the MEA solution during the experiments (when the samples are heated up to the required temperature) is considered as the main cause for the comparatively large uncertainties of the measurements. Further,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +47 3557 5286. Fax: +47 3557 5001. Funding

The authors would like to thank the Norwegian Research Council and Statoil for financial support. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The technical assistance from Anita Elverøy and Chameera K. Jayarathna is gratefully acknowledged. 991

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(19) Tahery, R.; Modarress, H. A new and a simple model for surface tension prediction of water and organic liquid mixtures. Iran. J. Sci. Technol., Trans. B, Eng. 2005, 29, 501−509. (20) Chunxi, L.; Wenchuan, W.; Zihao, W. A surface tension model for liquid mixtures based on the Wilson equation. Fluid Phase Equilib. 2000, 175, 185−196.

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