Study of CO2 Absorption into Phase Change Solvents MAPA and DEEA

Article pubs.acs.org/jced

Study of CO2 Absorption into Phase Change Solvents MAPA and DEEA Mikael Man̈ nistö, Petri Uusi-Kyyny,* Dominique Richon, and Ville Alopaeus Department of Chemical and Metallurgical Engineering, Aalto University, PL 16100, 00076 Aalto, Finland

ABSTRACT: This work provides new data for carbon dioxide (CO2) absorption into phase change solvent systems consisting of an aqueous solution of 2-(diethylamino)ethanol (DEEA) and 3-(methylamino)propylamine (MAPA). The variables related to CO2 absorption are studied and a Lewis cell type apparatus is taken into use. The latter was first validated with experiments of CO2 absorption into aqueous methyl diethanolamine (MDEA) solutions. Experiments are carried out for the determination of apparent kinetic absorption constant and absorption rate of CO2 for aqueous DEEA/MAPA mixtures. The determined kinetic constants (0.008−0.007 mol·(m3·s)−1 for MAPA/DEEA between 308 and 318 K and 0.0184−0.0022 m3·(mol·s)−1 for MDEA between 296 and 318 K) are compared to the literature where available. The phenomena related to phase change solvent absorption are carefully studied. A relation between total CO2 loading and absorption rate is observed, and reasons behind the dependency and the phenomena are analyzed.

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equilibrium pressures,9 and heat of absorption10 have also received interest in the past. To develop processes with such novel solvents, considerable amount of data, such as phase equilibria, physical properties, absorption kinetics, and reaction kinetics, is required. In our work, we focus on the study of the latter two, absorption kinetics and reaction kinetics, while discussing the other properties (physical properties and phase equilibria) from the viewpoints of amine capacity and biphasic behavior.

INTRODUCTION Global greenhouse gas emissions drastically increase as more countries reach industrial state.1 Sources of such emissions are, among other things, power plants, industrial processes, and natural gas deposits.2 To tackle the increase, technologies like absorption of carbon dioxide (CO2) have been developed along with the changing focus to biobased renewable energy sources. A typical process for absorption (see Figure 1) consists of an absorber and a desorber or stripper. This process was initially patented by R. R. Bottoms3 in 1930 and has been traditionally used with solvents like monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), or methyldiethanolamine. However, these solvents have limitations related to low absorption capacity and are expensive to regenerate due to high energy demands. Therefore, novel solvents have been a target of substantial research. One solvent that has recently received a large amount of attention is an aqueous mixture of 2(diethylamino)ethanol and 3-(methylamino)propylamine, which has the property of forming a liquid−liquid phase split when a critical amount of carbon dioxide is loaded into the solution.4,5 This solvent mixture has been studied for its absorption kinetics toward CO2 in a reaction calorimeter similar to the kinetic cell used in this work.6 The same group had also studied other activators for DEEA in addition for MAPA and conclude MAPA to have the best enhancement of absorption of their studied candidates.6 Pinto et al.7 studied the solvent in a pilot plant study for CO2 capture. VLE,8 © 2017 American Chemical Society

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EXPERIMENTAL SECTION Materials. Alkanolamines MAPA (CAS: 6291-84-5, Aldrich), DEEA (CAS: 100-37-8, Fluka), and MDEA (CAS: 10559-9, Aldrich) were used as purchased without additional purification. CO2 (CAS: 124-38-9) and N2O (CAS: 10024-972) were purchased from AGA. Their reported purities are 99 mol % and 99.99 mol % respectively. Water was treated inhouse with a Millipore Milli-Q reverse osmosis system. Refractive indices were measured with a Dr. Kernchen Abbemat digital automatic refractometer. Densities were measured with an Anton Paar DMA 5000 M densimeter. It was calibrated using 99.999 wt % N2 and deionized water. Data about chemicals used and their measured densities and refractive Received: December 15, 2016 Accepted: July 3, 2017 Published: July 17, 2017 2261

DOI: 10.1021/acs.jced.6b01038 J. Chem. Eng. Data 2017, 62, 2261−2271

Journal of Chemical & Engineering Data

Article

Figure 1. Typical absorption process with absorber and stripper units.

used to calculate the amount of gas transferred and the change in pressure to determine the volume of the gas tank. The measurement was repeated 8 times at 298 K. The calculated volume of the gas tank was found to be (161.6 ± 0.6) cm3. The gas feed tank was then filled with CO2 to a pressure of approximately 200 kPa, and subsequently the valve to the cell was opened. This allowed for the determination of the volume of the cell through volume/pressure relation. The experiment was repeated 9 times at 300 K. The volume of the cell according to the aforementioned process was (317.49 ± 0.25) cm3. The feed lines for the amine result in a small dead volume when amine is transferred to the cell. This volume was calculated to be 0.39 ± 0.05 cm3 based on internal diameter (1.00 mm) and length of the transfer lines (50.0 cm). Lewis Cell. The principal part of the apparatus consists of a Lewis cell,13,14 designed and constructed in-house. Similar apparatus has been utilized by Pani et al.15,16 for H2S and CO2 absorption measurement data into aqueous solutions of MDEA, and their measurements were used as a reference when validating our cell performance. The cell consists of a glass casing with metallic flanges on both ends. Liquid is fed through a connection in the bottom flange and gas through a connection in the top flange. The bottom part of the cell has four baffles on the sides that prevent vortex formation within the liquid phase during mixing. The mixers are held in place by sapphire bearings in the flanges and in a metal plate held at the center of the cell. The center piece is also used to stabilize the liquid interface area during mixing to prevent wave formation on the surface as it would change the surface area. The cell is presented in detail in Figure 3. The mixers in the cell are of the Rushton type and have a diameter of 5.00 cm. They are magnetically driven using frequency controller driven motors. The diameter of the interface area inside the outer ring was 4.50 cm, and an inner ring, which supports the mixers, had a diameter of 1.40 cm. The interfacial area was calculated from these geometrically to be (14.28 ± 0.01) cm2. Solution was fed to the cell until it reached the “lower” liquid level as seen in Figure 4, which also illustrates the interfacial area. As gas was absorbed, the liquid level slowly rose and eventually reached the “top” level allowed; in such cases, experiments were stopped as the interfacial area loses its assumed characteristics (stable surface, known area) above this top level. The cell was encased inside a thermostated jacket with water flowing through it and placed inside the oven as seen in Figure

indices are presented in Table 1 and compared to literature values. Table 1. Used Chemicals and Their Reported and Measured Densities (ρ25) and Refractive Indices (n25 D ) Measured at 101.3 kPaa ρ25 [kg/m3]

chemical MAPA DEEA MDEA CO2 N2O deionized water

reported GC purity 97 99.5 99+ 99 99.99

n25 D

measd

refs 11 and 12b

measd

ref 11

848 ± 2.5 880 ± 2.6 1037 ± 1.1

847.6 882.0 1036.5

1.44564a,c 1.43833a,d 1.46532a,e

1.4475 1.4389 1.4674

1.33249

1.3325

a

Refractive indices measured at atmospheric pressure (101 kPa) and room temperature, standard uncertainty (u) reported by the refractometer manufacturer u(nD) = 0.0005 and u(T/K) = 0.03. Determined uncertainty for the pressure was u(P/kPa) = 2.7. Reported standard uncertainties by the densimeter manufacturer were u(ρ) = 0.001 kg/m3 and u(T/K) = 0.02. Estimated uncertainties for the densities were ur(ρMAPA) = 0.003, ur(ρDEEA) = 0.001, ur(ρMDEA) = 0.001 kg/m3. bLiterature value for MAPA density was obtained from Pinto et al.;12 other densities were obtained from DIPPR 801.11 Densities were measured at 298.15 K and literature values taken at same temperatures. cCalculated AARD for MAPA was 0.00028 (4 measurement points). dCalculated AARD for DEEA was 0.00009 (8 measurement points). eCalculated AARD for MDEA was 0.00013 (6 measurement points).

Mixtures were prepared gravimetrically using a Mettler Toledo XP2004S analytical balance with a reported uncertainty of ±0.1 mg. Apparatus and Procedure. Our experimental setup (see Figure 2) consists of a thermostated oven, a Lewis type cell, a gas feed tank, two PT100 temperature sensors, two UNIK 5000 pressure sensors, and an Agilent 34972A LXI Data Acquisition system with an Agilent 34901A 20 channel multiplexer card. The volumes of the gas tank and of the cell were first determined using a gas vessel of known volume and a calibrated pressure transducer. The gas was released from the vessel into the gas feed tank, and the pressure change from the vessel was logged. Due to the low pressure of the system, ideal gas law was 2262



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Figure 2. Experimental setup.

Figure 4. Interfacial area (top view on the right side, shadowed area is the interfacial area) and the liquid fill levels of the cell (side view on the left side). Figure 3. Measurement cell.

Measurements. The cell was initially carefully evacuated, and then the amine was fed into the cell through the bottom connection. The cell was then evacuated again, and the amine was heated to the desired temperature. Also, the gas feed tank was evacuated and the pressure of the tank logged at the lowest pressure obtained, after which the gas was filled into the gas feed tank and the valves were closed. The pressure was logged again to calculate the molar amount of gas within the tank. When the cell had reached the desired temperature, the valve between the tank and the cell was opened and the gas was fed quickly into the cell. After the pressure had increased to approximately 200 kPa within the cell, the valve was closed. The pressure decrease in the gas feed tank was used to calculate the amount of gas transferred to the cell, and the pressure

2. Temperature of the water was controlled with a Lauda E200 water bath. The absorption kinetics themselves are measured by calculating the change in molar amount of gas in the cell as a function of measured pressure. Pressure within the cell was measured using a UNIK 5000 (0−200 kPa abs, reported uncertainty 1% of full span, i.e., 2 kPa) pressure transducer and pressure within the gas feed tank with a similar UNIK 5000 transducer with 0−600 kPa abs and a reported uncertainty of 6 kPa. Both transducers were calibrated against a BEAMEX MC2 calibration unit, certified by the Finnish Centre for Metrology and Accreditation to have an uncertainty of 0.05% of the full span of 0 to 200 kPa, i.e., u(P) = 0.1 kPa. 2263



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decrease in the cell was logged to calculate the rate of absorption. The initial stages of the absorption are not logged as the gas starts to absorb as soon as it becomes in contact with the liquid, however the total loading can be calculated from the pressure drop in the gas feed tank. The concentrations for the various amine solutions were calculated with eq 1 and their uncertainties with the partial derivative of that equation for each variable, Camine =

∑

m ∑ Mi i mi m water + ρi ρwater

Sh = 0.34·Re 2/3·Sc1/3

Re =

Sc =

where i refers to a component (amine or water) in the mixture.

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MODELING The cell and its use were validated using measurements, as a function of time, of CO2 absorption into aqueous solutions of MDEA. The results were analyzed with a method proposed by Pani et al.15 Rate of absorption is related in their work as in eq 4, dPCO2 R·T =− ·kL·A ·E ·CCO2, i dt VG (4)

b=

(10)

μsolution ρsolution ·DCO2

(11)

(12)

R·T · kL · A · E VG·HCO2

(13)

and pressure is taken in the beginning of the experiment and at approximately 10 kPa decrease and t is the time taken to reach the 10 kPa decrease. The liquid side mass transfer coefficient was calculated as suggested by Pani et al.15 with correlations from Al-Ghawas et al.17 for the Henry’s law coefficient of CO2 in MDEA as well as for the physical properties of MDEA. For MAPA and DEEA the Henry’s law coefficients were calculated with the nitrous dioxide correlation as measured by Monteiro et al. 18 at various temperatures and MAPA and DEEA concentrations. Pani et al.15 also suggested that the enhancement factor is defined by the chemical kinetics at the interface as shown in eq 14,

(5)

(6)

The physical properties and Henry’s law coefficients for MDEA were estimated using the correlations by Al-Ghawas et al.17 Monteiro et al.18 have measured the same properties and constants for MAPA and DEEA at various temperatures and for various compositions. Diffusion coefficients were calculated with the method proposed by Versteeg and van Swaaij19 and shown in eq 7. = DCO2,water ·μwater

μsolution

where b is given in eq 13,

The interfacial CO2 concentration is obtained with Henry’s law through eq 6,

DCO2,amine ·μamine

ρsolution ·N ·Dag 2

⎛ P − Pinitial ⎞ ⎟⎟ = −b·t ln⎜⎜ measured ⎝ Pmeasured,0 − Pinitial ⎠

where R refers to the gas constant, T to temperature, Vg to the gas volume in the cell, kL to the gas phase mass transfer coefficient, A to the mass transfer area, E to enhancement factor, and CCO2,i to the interfacial CO2 concentration. The pressure of CO2 is calculated as in eq 5,

0.8

(9)

where ρi refers to the density of a component, N to the rotation speed of the mixer, and Dag to the agitator diameter. Reynolds numbers were found to be in the range of 3000 to 6000 for the validation experiments, proving that the mixing takes place below turbulent region in these experiments. For the phasechange solvent the initial Reynolds number was ∼700 and ∼450 for the first and second mixtures, respectively. Due to the changes in the viscosity of the liquid, the Reynolds number during the various absorption steps was not calculated. The slope of the pressure against time plot of the absorption experiments was used to calculate the enhancement factor (E) of absorption. The slope was plotted as in eq 12, using initial pressure measured before gas feed (Pinitial), pressure measured at the beginning of experiment (Pmeasured,0), and the pressure measured at desired time t (Pmeasured),

where i refers to a component in the mixture (amine or water). The mass fractions were calculated with eq 3 and their uncertainties with the partial derivative of the equation for each variable, m wi = i i ∑k = 1 mk (3)

0.8

(8)

and Reynolds (Re) and Schmidt (Sc) numbers as in eqs 10 and 11,

where i is an index of an amine component (MDEA, MAPA, DEEA). The liquid volume in the cell was calculated using eq 2 and its uncertainty with the partial derivative of the equation for each variable, m − mafterfeed Vliq = beforefeed ∑ wiρi (2)

PCO2 = HCO2·CCO2, i

Dcell

with Sherwood number (Sh) proposed to be calculated as in eq 9 and Dcell referring to the cell diameter,

(1)

PCO2 = Pmeasured − Pinitial

Sh·DCO2

kL =

E=

1 ·(kov ·DCO2)1/2 kL

(14)

Absorption was assumed to be in the fast regime as the enhancement factor in all cases was over 3. Using this definition and the mass transfer and diffusion coefficients, the overall reaction rate constant (kov) can be calculated. By defining kov as a function of only MDEA interfacial concentration and the actual kinetic reaction rate constant (k), with the assumption that CO2 would only react with MDEA at the interface, it is

(7)

The liquid side mass transfer coefficient (kL) for a Lewis type cell has also been suggested by Pani et al.15 as in eq 8, and our cell was verified with N2O measurements to follow this correlation. 2264



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acid gas loading. As there was no method of sampling available for the liquid phases in this apparatus, the accurate concentrations of the amines in the two phases could not be determined.

possible to determine the reaction kinetic coefficient as seen in eq 15,

k=

kov CMDEA, i

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(15)

RESULTS AND DISCUSSION The measured CO2 absorption data are presented for both the validation measurements with MDEA and the new measurements with the phase change solvent MAPA/DEEA. With MDEA the data are analyzed further to provide the kinetic constants of absorption, and they are compared against those found in the literature.15,23−26 The data measurements were done using a synthetic method and calculating the absorption of CO2 through the decrease of pressure within the cell. The different mixtures used in measurements with MDEA are presented in Tables 2 and 3 along with calculated enhancement factors and reaction coefficients.

The interfacial concentration was calculated at 5 kPa decrease from initial pressure using the equation derived from Brian et al.’s20 film theory by Pani et al.15 (eq 16). ⎛ ⎞ ⎛ DCO2 ⎞1/2 PCO2 ·⎜ Camine, i = Camine,total· ⎜⎜1 − ⎟ ·(E − 1)⎟⎟ HCO2·Camine,total ⎝ Damine ⎠ ⎝ ⎠ (16)

The diffusion coefficient of amine was calculated using correlation by Versteeg and van Swaaji19 as seen in eq 17, ⎛ μ ⎞0.2 ⎛ DCO2 ⎞ ⎛ DCO2 ⎞ =⎜ ·⎜⎜ water ⎟⎟ ⎜ ⎟ ⎟ ⎝ Damine ⎠aq.solution ⎝ Damine ⎠ water ⎝ μamine ⎠

(17)

Table 2. Amine Concentrations and Weighed Masses for the Measured Mixtures with MDEAa

where the ratio of diffusion coefficients in water is related to the molar volumes as suggested by Bosch et al.21 (eq 18). ⎛ DCO2 ⎞ ⎛v ⎞0.6 = ⎜ water ⎟ ⎜ ⎟ ⎝ Damine ⎠ water ⎝ vamine ⎠

(18)

Molar volumes for the amine solutions were calculated using amine concentrations, their specific molar masses, solution density, and the total volume of the solution at the defined temperature. Total loading of the amine was calculated at the end of each absorption by calculating the total moles of gas that had transferred from the gas feed tank to the cell and assuming total loading of the amine after pressure had dropped to starting levels. Henry’s law estimation has been used by various groups6,15,18 studying the same systems, and agreement with their data proves that it is fit for the task. While this method for kinetics is sound for a mixture of a single amine and water, the modeling of the biphasic solvent is a different case. The same method can be applied to determine the kinetic constant of the 5D2M solution; however, this kinetic constant reflects, in reality, three different constants. The first one takes into account the impact of MAPA, the second constant is for DEEA, and the third one is for H2O. The effect of water in such mixtures is small due to the slow reaction rate;18 however, the other two do play a role in the kinetic constant. Monteiro et al.18 suggest that, in such a mixture at these temperatures, the kinetic term for DEEA would represent up to 95% of the total kinetic constant. The method by Pani et al.15 was used in the determination of the overall kinetic constant for the MAPA/DEEA absorption experiment. In MAPA/DEEA runs, the mass transfer coefficient from eq 5 was calculated with the assumption that all the runs were made with a stable solution with constant concentrations of 5 mol/L DEEA and 2 mol/L MAPA. This assumption holds well in the beginning of the absorption runs, but as the loading of the amine continues to grow, the free concentration of both of these components decreases. As the used diffusion coefficients (eqs 14 and 15) and physical properties used in eqs 7 and 8 depend highly on amine concentration as shown by Monteiro et al.,18,22 we can assume that the kinetic constant calculated here does not describe the system perfectly. In addition, certain physical properties are also affected heavily by

run

CMDEA/mol·m−3

T/K

mMDEA/g

mwater/g

Vliq/mL

1 2 3 4 5 6 7 8 9

843.8 837.7 842.5 841.7 840.8 845.3 842.2 841.4 843.3

296.0 301.1 300.8 295.9 296.2 317.9 318.3 318.2 296.3

20.02 20.01 19.99 19.99 20.07 20.02 20.18 20.03 19.99

179.11 180.47 179.18 179.27 180.21 178.71 180.91 179.69 178.99

167.20 167.48 167.46 167.20 167.22 168.57 168.60 168.59 167.22

a

The uncertainties for the values were calculated as u(CMDEA) = 22.7 mol/m3, u(T) = 0.2 K, u(m) = 0.01 g, u(Vliq) = 0.12 mL.

Pani et al.15 have suggested an Arrhenius type of an equation for the reaction rate constant of CO2 absorption in MDEA (eq 19) against which our validation measurements were compared. k = 4.68· 108·e−5461/ T

(19)

Table 3. Conditions of Validation Runs with MDEA and CO2a run

T/K

CMDEA/ mol· m−3

1 2 3 4 5 6 7 8 9

296.0 301.1 300.8 295.9 296.2 317.9 318.3 318.2 296.3

843.8 837.7 842.5 841.7 840.8 845.3 842.2 841.4 843.3

P0/ kPa

Pi/ kPa

PCO2,i/ kPa

3 3 4 3 3 5 9 10 3

60 67 96 181 198 186 186 180 181

57 63 92 178 196 181 177 171 178

Vg/mL

kL/ 10−5 m·s−1

b/ 10−4 s−1

153.46 153.99 150.48 153.08 152.61 142.59 145.74 144.57 144.81

2.025 2.32 2.097 1.718 1.74 3.417 2.945 3.216 1.757

5.91 3.67 5.83 4.37 4.13 5.25 8.22 8.89 3.95

a

T is temperature, CMDEA is the concentration of MDEA, and P0, Pi, and PCO2,i are the pressures before gas feed, at the total pressure, and initial CO2 pressure at beginning of the absorption, respectively. Vg is the gas space volume calculated from the fed amine in NTP. kL signifies the mass transfer coefficient calculated from eq 8 and b the slope as in eq 12. Uncertainties were u(T) = 0.2 K and u(P) = 2 kPa, u(Vgas) = 0.12 mL. 2265



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Similar Arrhenius equations were proposed by Rinker et al.24 (eq 20), Ko and Li26 (eq 21), and Versteeg and Van Swaaij25 (eq 22) (presented correspondingly below).

k = 2.91· 107 ·e−4579/ T

(20)

k = 4.01· 108·e−5400/ T

(21)

k = 1.19· 108·e−5100/ T

(22)

available literature data when compared to any of the Arrhenius equations devised. The measured data corresponds to our and other literature Arrhenius equations adequately to validate the cell. The Arrhenius equation presented in this work has a relative absolute average deviation (RAAD) of 0.0902 and a maximum relative deviation of about 0.5 toward all the available measured data for MDEA + CO2. For MAPA/DEEA the accurate analysis proved more challenging due to the lack of the concentrations of each phase. The kinetic constant was however calculated as a function of amine loading. The different mixtures used in measurements with MAPA/DEEA runs are presented in Tables 4 and 5. The MAPA/DEEA runs are numbered as RiAi, where

All these models report the rate constant as m3·(kmol·s)−1 and are presented in Figure 5 along with our measured data and literature data from the model developers and Versteeg and Van Swaaij.25

Table 4. Amine Concentrations and Weighed Masses for the Measured Mixtures with DEEA and MAPAa run 5D2MR1 5D2MR2

Camine/mol· m−3

T/K

mDEEA/g

mMAPA/g

mwater/g

Vliq/mL

6988.0

318.9

117.28

35.25

25.74

166.71

6918.8

307.9

117.20

35.14

28.06

168.53

a

The uncertainties for the values were calculated as u(Camine) = 30.5 mol/m3, u(T) = 0.2 K, u(m) = 0.01 g, u(Vliq) = 0.94 mL.

R signifies the run ID and Ai states the absorption run to the solution used in the run. The runs were made sequentially by letting all the CO2 absorb to the amine solution, waiting for the pressure to stabilize, and feeding a new batch of CO2. This allowed us to follow how the kinetic behavior develops as a function of amine loading. The calculated carbon dioxide flux into the liquid was plotted as a function of amine loading and is presented in Figure 7. The trend of three staged absorption (fast, planar, and fast again) observed in some of the curves of Figure 7 could be explained with the lack of adequate replenishing of the interface/film layer between the bulk liquid and the gas phase. This would cause the film to expand slowly until it reaches a point where the rate of absorption for CO2 from the gas phase is the same as the rate it transfers from the interface layer to the bulk from below the film. At this stage, the absorption rate would essentially be constant, as is visible in the third curve in Figure 8. The behavior is illustrated in Figure 9. The dense CO2 phase forms on the top of the interface and begins to slowly dissolve into the bulk phase. As the droplet of the rich phase moves down from the interface, a wake type phenomenon takes place as a wake follows the droplet down as it absorbs and finally disappears into the bulk as presented in Figure 10. The general behavior over the different runs is presented in Figure 11 using an earlier design of the cell. The design had the same dimensions as the current one; however, the materials were different. Lower flange, the baffles, and supports were made out of PVC in that design. It is evident in the final picture of the series in Figure 11 that, as the acid gas loading reaches high enough levels, the heavy phase seen on the top of the interface begins to form another layer at the bottom of the cell. It can also be observed that at the temperature of 318 K the planar absorption phenomenon does not become dominant, which further would suggest that viscosity might have a considerable effect. As at the lower temperature the viscosity of the CO2 loaded phase is higher, the mixing is not as efficient, whereas at the higher temperature the replenishing of the

Figure 5. Measured and literature15,23−26 reaction kinetic coefficients presented along with models from the literature: red ●, this work; ×, Camacho et al.;23 □, Pani et al.;15 ◇, Rinker et al.;24 △, Versteeg and van Swaaij;25 × with vertical bar, Ko and Li;26  , Pani et al.15 model; ···, Rinker et al.24 model; − · −, Versteeg and van Swaaij model;25 − − −, Ko and Li26 model.

The available literature data along with our measured data was used to regress the parameters of an Arrhenius equation as seen in eq 23. Figure 6 presents the data and the Arrhenius equation. ln(k) = 1.20·109 ·e−5755.2/(T / K )

(23)

There is some deviation in our measurements and the models. However, similar scatter can be found in all the

Figure 6. Calculated kinetic reaction coefficients from measured and literature data15,23−26 presented along with an Arrhenius equation derived from all the used literature data: red ●, this work; ×, Camacho et al.;23 □, Pani et al.;15 ◇, Rinker et al.;24 △, Versteeg and van Swaaij;25 × with vertical bar, Ko and Li;26  , Arrhenius equation. 2266



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Table 5. Conditions of Measurements Using MAPA and DEEA with CO2a run

α0/mol CO2·mol amine−1

P0/ kPa

Pi/ kPa

PCO2,i/ kPa

b/10−4 s−1

R1A1b R1A2b R1A3b R1A4b R1A5b R1A6b R1A7b R1A8b R1A9b R1A10b R1A11b R1A12b R1A13b R1A14b R1A15b R2A1c R2A2c R2A3c R2A4c R2A5c R2A6c R2A7c R2A8c R2A9c R2A10c R2A11c R2A12c R2A13c R2A14c R2A15c R2A16c R2A17c

0 0.01 0.018 0.026 0.034 0.042 0.051 0.059 0.068 0.076 0.084 0.093 0.101 0.109 0.117 0 0.009 0.018 0.027 0.036 0.044 0.053 0.061 0.071 0.08 0.088 0.097 0.105 0.113 0.122 0.13 0.139

8 9 9 8 8 8 8 8 8 8 8 8 8 8 8 4 4 5 5 4 5 5 4 5 5 5 5 5 5 5 5 5

156 168 164 163 161 165 173 167 177 167 170 173 177 174 177 149 174 181 186 178 178 180 188 181 180 180 182 181 180 181 206 184

148 159 155 155 152 157 166 159 168 159 162 164 169 166 169 145 170 176 182 174 173 175 184 176 175 175 177 176 176 176 201 180

10.57 7.67 7.95 7.96 7.54 6.81 6.38 5.81 5.65 5.61 5.52 5.35 4.77 4.71 4.96 7.46 4.76 3.16 3.04 3.95 4.4 3.66 4.39 4 3.71 3.17 2.86 2.49 2.3 2.39 1.92 2.51

Figure 8. Extract for the first runs of absorption experiments for the calculated flux of carbon dioxide as a function of amine loading: , 308 K measurement; ···, 318 K measurement.

its change as a function of temperature and loading may prove to be an important process design consideration. Kinetic and thermodynamic studies by others4,6 suggest that MAPA reacts with CO2 first and would act as the activator for DEEA. MAPA and CO2 are suggested to form the heavy phase of the system, which could also explain the planar behavior. If MAPA is consumed on the interface and is not replenished quickly enough, DEEA would take over the kinetics, causing the stagnant absorption seen in higher loadings. This would mean that, in the initial absorption experiment on fresh solution, the interface is saturated with MAPA and all of it reacts with the CO2 as there is no planar behavior seen. In later stages of the 308 K measurement, the viscosity of the liquid is lower and MAPA is not replenished quickly enough on the interface, leading to planar region in absorption curve. The overall kinetic constant (k), taking into account all three species reacting in the solution (calculated as suggested by Pani et al.15 at 10 kPa pressure decrease for each gas addition), is plotted as a function of amine loading in Figure 12. When the slope (b from eq 13) is plotted as a function of amine loading for the MAPA/DEEA experiments, a trend of first decreasing and then increasing slope is seen. This could signify that the absorption is heavily mass transfer limited and, as the partial pressure of CO2 decreases, the interface clears and the amine refreshes faster, allowing more efficient mass transfer. The calculated slopes are presented in Figure 13. Accurate analysis of the kinetic constant and the reactions affecting it requires data from the liquid phase concentration profiles, which were not measured during the experiments. Further work is needed for the online determination of the concentrations and the development of a loading dependent model for the kinetic constant. Current measurement apparatus does not allow for sampling of the two liquid phases that form during the absorption phenomena. Recently Kierzkowska-Pawlak and Kruszczak6 measured kinetics of CO2 absorption in the same solvent system with lower MAPA and DEEA concentrations. They report the flux of CO2 into the amine against CO2 pressure in pure amine solutions, and their values were compared to the first absorption runs in both temperatures. The comparison is presented in Figure 14. The DEEA/MAPA mixture of 5 mol/dm3 DEEA and 2 mol/ dm3 MAPA forms a phase split at approximately 6% loading of the solution, i.e., 0.06 mol of CO2 for a mole of total amine. This however has no clear effect on the slope behavior seen in Figure 13. The kinetic constant behavior however changes drastically for both the 318 and 308 K measurements at phase

α0 is the loading of the amine with CO2, P0 the pressure before gas feed, Pi the initial total pressure at feed, PCO2,i the initial CO2 pressure at feed, and b the slope as in eq 12. Uncertainties were u(T) = 0.2 K, u(α) = 0.0002, and u(P) = 2 × 103 Pa. bTemperature 318 K, amount of gas space 151.37 cm3, kL calculated to be 2.9584 × 10−5, and total amine concentration was 6.988 mol·m−3. cTemperature 308 K, amount of gas space 149.55 cm3, kL calculated to be 1.9564 × 10−5, and total amine concentration was 6.919 mol·m−3. a

Figure 7. Calculated flux of carbon dioxide as a function of amine loading: , 308 K measurement; ···, 318 K measurement.

interface layer would occur faster due to lower viscosity. This observation implies that the viscosity as a physical property and 2267



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Figure 9. Behavior of the surface in the interface area of the cell as the absorption proceeds over time from initial high pressure to the final low pressure. The phenomena described in the illustration takes place at low acid gas loading, the surface becomes saturated, and droplets start to detach from the surface.

Figure 10. Behavior of a liquid droplet as it moves from the interface into the bulk layer and absorbs completely.

Figure 11. Behavior of the absorbed gas, the liquid droplets, and the interface over one of the measurement runs done with an older cell design. The vortex is caused by the surface being pulled down by the current and then dispersed into the bulk liquid.

split loading as seen in Figure 12. Various authors4,6,7,27 suggest that the CO2 rich second phase initially consists mostly of the absorbed CO2 bound to MAPA. As more gas is absorbed by the solution, DEEA moves to the MAPA rich phase, increasing the

volume of that phase. It was visible that as the absorption continued, the lower phase increased in volume, which would suggest such behavior. This is consistent with Pinto et al.27 measurements for 5D2M mixture. As the second phase forms, 2268



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related to kinetics of absorption in similar studies,6 it is not possible to accurately say if the observed phenomena are present in other work, and if it is caused by the earlier discussed phenomenon of saturation of the interfacial layer. The observed phenomena could cause various challenges in industrial absorption columns, as the packing surface would be wetted by the rich phase, the lean phase, or a mixture of both. Each of these would have its own absorption rate, which could present problems in consistent modeling of such systems.

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CONCLUSIONS A new measurement apparatus based on the Lewis cell13,14 design presented by Pani et al.15,16 was taken into use in the laboratory and validated for measuring absorption kinetics using a known absorption experiment of CO2 absorption into MDEA. A new Arrhenius equation was derived, where all available literature data for MDEA is taken into account. New data on the absorption kinetics of CO2 into the phase changing solvent mixture of aqueous MAPA/DEEA was measured with the cell. Data was further analyzed to provide the kinetic constant of the absorption as a function of amine loading. The results for CO2 show that up to the loading observed in these measurements (up to 0.15) it would seem that all the CO2 absorbed into the liquid, which suggests the combination of MAPA/DEEA to be a very efficient solvent, as has been suggested by others7,27 as well. The behavior for the phase split seemed consistent with the results presented in Pinto et al.27 work. The effect of the phase separation on kinetics requires further study, as it is apparent that the second phase, at least in this apparatus, forms on the interface and slows down the absorption. This behavior adds a further challenge in modeling of absorption columns with this mixture. Based on the measured results, the temperature and loading dependency of the viscosity may prove to be an important parameter for the design of the process. The observed phase behavior also suggests that a single device may not be sufficient to analyze mass transfer, phase equilibria, and fluid flow phenomena in industrial columns. Further measurements with phase analysis for both liquid phases would be beneficial in determining how the absorption kinetics are affected by the phase transfer.

Figure 12. Calculated overall kinetic constant of carbon dioxide absorption as a function of amine loading: □, 308 K measurement; ○, 318 K measurement.

Figure 13. Calculated slope of eq 9 as a function of amine loading: , 308 K measurement; ···, 318 K measurement.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: petri.uusi-kyyny@aalto.fi.

Figure 14. Flux of CO2 into the amine. Our measurements (5 M DEEA + 2 M MAPA): , 308 K; ···, 318 K. Kierzkowska-Pawlak at 303.15 K: □, 1.8 M DEEA + 0.2 M MAPA; ○, 1.9 M DEEA + 0.1 M MAPA; △, 1.95 M DEEA + 0.05 M MAPA.

ORCID

Petri Uusi-Kyyny: 0000-0002-8339-4601 Funding

Tekes - the Finnish Funding Agency for Innovation, FiDiPro Program (www.fidipro.fi) is acknowledged for funding the FiDiPro position of D.R. and the work by M.M. P.U.-K. acknowledges the Academy of Finland (Grant 252664) for the financial support. M.M. would also like to acknowledge Fortum Foundation for their support.

the calculated overall kinetic constant levels out. This would also suggest that the behavior could be as suggested, i.e., the kinetics of DEEA take over on the interface as the second phase forms and the majority of MAPA is tied to it. Additionally, as seen in Figures 7 and 8, our experiments show that the absorption rate is dependent on both temperature of the system and the total loading of CO2 within the system. A change in the absorption behavior is observed in higher loadings, where the rate has three distinct regimes, initial fast absorption and the subsequent plateauing slower rate followed by a second fast absorption regime. Similar dependency is seen in Figure 12 and in Figure 13 for the kinetic constant and the slope of the pCO2 vs t curve. As the data for pressure against time is not reported in other recent work

Notes

The authors declare no competing financial interest.

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LIST OF SYMBOLS A = interfacial area [m2] Cj,i = interfacial concentration of component j in liquid film [mol·m−3] Cj,bulk = bulk concentration of component j in liquid film [mol·m−3] DOI: 10.1021/acs.jced.6b01038 J. Chem. Eng. Data 2017, 62, 2261−2271


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(4) Arshad, M. W.; von Solms, N.; Thomsen, K. Thermodynamic modeling of liquid−liquid phase change solvents for CO2 capture. Int. J. Greenhouse Gas Control 2016, 53, 401−424. (5) Knuutila, H. K.; Nannestad, Å Effect of the concentration of MAPA on the heat of absorption of CO2 and on the cyclic capacity in DEEA-MAPA blends. Int. J. Greenhouse Gas Control 2017, 61, 94−103. (6) Kierzkowska-Pawlak, H.; Kruszczak, E. Revised kinetics of CO2 absorption in aqueous N,N-diethylethanolamine (DEEA) and its blend with N-methyl-1,3-propane-diamine (MAPA). Int. J. Greenhouse Gas Control 2017, 57, 134−142. (7) Pinto, D. D. D.; Knuutila, H.; Fytianos, G.; Haugen, G.; Mejdell, T.; Svendsen, H. F. CO2 post combustion capture with a phase change solvent. Pilot plant campaign. Int. J. Greenhouse Gas Control 2014, 31, 153−164. (8) Hartono, A.; Saleem, F.; Arshad, M. W.; Usman, M.; Svendsen, H. F. Binary and ternary VLE of the 2-(diethylamino)-ethanol (DEEA)/3-(methylamino)-propylamine (MAPA)/water system. Chem. Eng. Sci. 2013, 101, 401−411. (9) Arshad, M. W.; Svendsen, H. F.; Fosbøl, P. L.; von Solms, N.; Thomsen, K. Equilibrium Total Pressure and CO2 Solubility in Binary and Ternary Aqueous Solutions of 2-(Diethylamino)ethanol (DEEA) and 3-(Methylamino)propylamine (MAPA). J. Chem. Eng. Data 2014, 59, 764−772. (10) Arshad, M. W.; von Solms, N.; Thomsen, K.; Svendsen, H. F. Heat of Absorption of CO2 in Aqueous Solutions of DEEA, MAPA and their Mixture. Energy Procedia 2013, 37, 1532−1542. (11) Design Institute for Physical Properties DIPPR Project 801 (Full Version), 2014. https://www.aiche.org/dippr/projects/801 (accessed February 2016). (12) Pinto, D. D. D.; Monteiro, J. G. M.-S.; Johnsen, B.; Svendsen, H. F.; Knuutila, H. Density measurements and modelling of loaded and unloadedaqueous solutions of MDEA (N-methyldiethanolamine), DMEA(N,N-dimethylethanolamine), DEEA (diethylethanolamine) and MAPA(N-methyl-1,3-diaminopropane). Int. J. Greenhouse Gas Control 2014, 25, 173−185. (13) Lewis, J. B. The mechanism of mass transfer of solutes across liquid-liquid interfaces: Part I: the determination of individual transfer coefficients for binary systems. Chem. Eng. Sci. 1954, 3, 248−259. (14) Lewis, J. B. The mechanism of mass transfer of solutes across liquid-liquid interfaces: Part II.The transfer of organic solutes between solvent and aqueous phases. Chem. Eng. Sci. 1954, 3, 260− 278. (15) Pani, F.; Gaunand, A.; Cadours, R.; Bouallou, C.; Richon, D. Kinetics of Absorption of CO2 in Concentrated Aqueous Methyldiethanolamine Solutions in the Range 296 to 343 K. J. Chem. Eng. Data 1997, 42, 353−359. (16) Pani, F.; Gaunand, A.; Richon, D.; Cadours, R.; Bouallou, C. Absorption of H2S by an Aqueous Methyldiethanolamine Solution at 296 and 343K. J. Chem. Eng. Data 1997, 42, 865−870. (17) Al-Ghawas, H. A.; Hagewiesche, D. P.; Ruiz-Ibanez, G.; Sandall, O. C. Physicochemical properties important for carbon dioxide absorption in aqueous methyldiethanolamine. J. Chem. Eng. Data 1989, 34, 385−391. (18) Monteiro, J. G. M.-S.; Majeed, H.; Knuutila, H.; Svendsen, H. F. Kinetics of CO2 absorption in aqueous blends of N,N-diethylethanolamine (DEEA) and N-methyl-1,3-propane-diamine (MAPA). Chem. Eng. Sci. 2015, 129, 145−155. (19) Versteeg, G. F.; van Swaaij, W. P. M. Solubility and diffusivity of acid gases (carbon dioxide, nitrous oxide) in aqueous alkanolamine solutions. J. Chem. Eng. Data 1988, 33, 29−34. (20) Brian, P. L. T.; Hurley, J. F.; Hasseltine, E. H. Penetration theory for gas absorption accompanied by a second order chemical reaction. AIChE J. 1961, 7, 226−231. (21) Bosch, H.; Versteeg, G. F.; Van Swaaij, W. P. M. Gasliquid mass transfer with parallel reversible reactionsI. Absorption of CO2 into solutions of sterically hindered amines. Chem. Eng. Sci. 1989, 44, 2723−2734. (22) Monteiro, J. G. M.-S.; Knuutila, H.; Penders-van Elk, N. J. M. C.; Versteeg, G.; Svendsen, H. F. Kinetics of CO2 absorption by

Ci = the concentration of component i in the liquid phase of the studied system [mol·m−3] Ci,total = the total concentration of component i in the liquid phase of the studied system [mol·m−3] CO2 = carbon dioxide DEEA = 2-(diethylamino)ethanol DEA = diethanolamine Dag = diameter of the agitator/mixer [m] Dcell = diameter of the cell [m] Di = diffusion coefficient of component i [m2·s−1] E = enhancement factor Hi = Henry’s law coefficient for component i [Pa·m3·mol−1] kL = liquid mass transfer coefficient [m·s−1] kov = the overall reaction coefficient for the system [s−1] ki = the kinetic reaction coefficient for reaction i [m3·(mol· s)−1] Ki = the equilibrium constant for reaction i MAPA = 3-(methylamino)propylamine MDEA = methyldiethanolamine MEA = monoethanolamine Mi = molar weight of the component i [g·mol−1] mi = mass of component i [g] N = rotation speed of the mixer [s−1] N2O = nitrous oxide ΔPCO2 = the change in the partial pressure of component i [Pa] Ptotal = total pressure of the system [Pa] Pmeasured = pressure of the system measured at given time [Pa] Pinitial = pressure of the system measured prior to gas injection [Pa] Pi = partial pressure of component i [Pa] R = molar gas constant [J·(mol·K)−1] ri = reaction rate of component i [mol·s−1] Re = Reynolds Number Sc = Schmidt number Sh = Sherwood number t = time [s] T = temperature [K] TEA = triethanolamine Vg = gas space volume in the system [m3] wi = the mass fraction of component i xi = molar fraction of component i in liquid phase yi = molar fraction of component i in gas phase ρi = density of the component or mixture i [kg·m−3] μi = the viscosity of the solution i [kg·(m·s)−1] αi = the loading of the amine [mol(sour gas)·mol(amine)−1]

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REFERENCES

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Study of CO2 Absorption into Phase Change Solvents MAPA and DEEA

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