Heat of Absorption of CO2 and Heat Capacity Measurements in

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Cite This: J. Chem. Eng. Data 2019, 64, 3392−3406

Heat of Absorption of CO2 and Heat Capacity Measurements in Aqueous Solutions of Benzylamine, N‑(2-Aminoethyl)-ethanolamine, and Their Blends Using a Reaction Calorimeter Satyajit Mukherjee and Amar N. Samanta*

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Chemical Engineering Department, Indian Institute of Technology Kharagpur, Kharagpur 721302, India ABSTRACT: In this study, the heats of absorption of CO2 in aqueous benzylamine (BZA), N-(2-aminoethanolamine) (AEEA), and their blends were measured using an automated reaction calorimeter in the temperature range of 303.15−343.15 K. The experimental concentrations of aqueous amine solutions were in the range of (0.49−4.0) mol BZA·kg−1 water, (0.51−4.11) mol AEEA·kg−1 water, (2.03 mol BZA + 2.09 mol AEEA)·kg−1 water, and (2.71 mol BZA + 1.39 mol AEEA)·kg−1 water. The heat capacities of the unloaded aqueous solutions of BZA, AEEA, and (BZA + AEEA) were also measured in similar temperature and concentration ranges to those for heat of absorption. The experimental heat of absorption of CO2 was presented as a function of CO2 loading, temperature, and amine concentration.

calculated with ±(2−3)% accuracy.2 Therefore, the direct calorimetric measurement of the enthalpy of acid gas dissolution in the chemical solvent is needed to be determined for an economical design of the absorber and stripper section in the acid gas removal plant. The calorimetric measurements are used to determine two important thermodynamic values, which are the gas solubility limit and heats of absorption of acid gas in the chemical solvent.3 The heat of absorption of CO2 in aqueous amine solutions through calorimetric measurements presents both the heat effects due to the physical absorption and chemical reaction of CO2 in the amine solvent with good measurement accuracy, which is a persistent parameter to avoid overestimation in plant design. Different kinds of calorimeters, e.g., isothermal displacement calorimeter, flow calorimeter (e.g., Setaram C80), and reaction calorimeter (e.g., CPA reactor series, RC1 reactor series) are used to measure absorption enthalpy of CO2 in aqueous amine solvents.1 A reaction calorimeter is a fully automated computer-controlled stirred reactor system, which is also facilitated for simulation according to plant conditions (temperature and pressure) and also capable to measure thermal power generated by the process. Therefore, it is the best-opted laboratory-scale measurement technique for measuring the heat and mass transfer characteristics.4 Although the CO2 solubility and the rate kinetics data in aqueous benzylamine (BZA) as a function of temperature, amine concentration, and partial pressure of CO2 were presented in our previous work,5,6 no experimental data on direct calorimetric measurements of enthalpy of CO2 absorption studies are available for the system (CO2 + BZA + H2O) in the open literature.

1. INTRODUCTION Undoubtedly, the increase of CO2 concentration in the atmosphere is the most critical problem for the mankind in the 21st century. The CO2 level has increased from 280 ppm in the pre-industrial era to 405 ppm at present. Various point sources (e.g., power plants) are the major contributor to this high rise of CO2 concentration through postcombustion CO2 release into the atmosphere. The regenerative amine scrubbing process is a widely used technology for postcombustion CO2 capture, which is extensively used in many industrial applications, e.g., CO2 removal from the power plant flue gases, natural gas purification, etc. The amine absorption process for the treatment of industrial effluent is a combination of chemical absorption and physical dissolution. The cost of the solvent regeneration process in the stripper unit to separate CO2 from the spent aqueous amine solvent is nearly 80% of the total effluent removal cost.1 The accounting of amine regeneration energy in terms of the steam requirement in the design of a stripper unit is directly related to the knowledge of the heat of reaction of CO2 in the aqueous amine solvent. As the steam cost in the amine regeneration unit estimates as the half of the total running cost of an effluent treatment plant, an amine solvent with low heat of absorption is desirable for the economical design of a stripper unit. The data of experimental heat of absorption of CO2 in aqueous amine solvents is relatively scarce in the open literature. The heat of absorption (ΔHabs) data are commonly derived from the CO2 solubility results in most of the reported work in the literature. The Gibbs−Helmholtz equation is very often used to calculate the heats of absorption of CO2 in amine solvents from the data of partial pressure of CO2 as a function of temperature and loading. The accuracy of calculated ΔHabs normally falls in the range of ±(20−30)% if the CO2 solubility data are © 2019 American Chemical Society

Received: March 3, 2019 Accepted: June 27, 2019 Published: July 10, 2019 3392

DOI: 10.1021/acs.jced.9b00205 J. Chem. Eng. Data 2019, 64, 3392−3406

Journal of Chemical & Engineering Data

Article

Table 1. Specification and Source of Chemicals Used in This Work chemical

chemical formula/Alias

molar mass (g·mol−1)

purity

source

benzylamine (CAS: 100-46-9) N-(2-aminoethyl) ethanolamine (CAS: 111-41-1) carbon dioxide (CAS:124-38-9) nitrogen (CAS: 7727-37-9) double-distilled water

C6H5CH2NH2/BZA NH2CH2CH2NHCH2CH2OH/AEEA CO2 N2 H2O

107.15 104.15 44.01 28.01 18.01

0.99 (mass fraction) 0.99 (mass fraction) 0.999 (mole fraction) 0.999 (mole fraction) 0.99 (mass fraction)

Sigma-Aldrich India Sigma-Aldrich India Linde India Limited Linde India Limited Water distillation system, Borosil

Figure 1. Heat of absorption of CO2 measurement setup: (1) double-jacketed glass reactor (reaction calorimeter model: RC1e); (2) anchor-type impeller; (3) temperature sensor of the reactor; (4) calibrated heater (25 W); (5) pressure transmitter (Rosemount, 3051TA); (6) magnetically coupled stirrer; (7) inlet for liquid; (8a) N2 cylinder, (8b) CO2 cylinder; (9a) N2 gas regulator, (9b) CO2 gas regulator; (10) thermostated water bath; (11) temperature sensor to control water bath temperature; (12) coil-type CO2 feeding arrangement.

Ma’mun and co-researchers7−9 have performed a screening test with a large number of amine solvents, and based on their study, they selected (2-aminoethyl)ethanolamine (AEEA) as a potential diamine for CO2 removal from postcombustion flue gases with high absorption rate and high absorption capacity. Kim and Svendsen2 have reported the experimental data on the heat of absorption of CO2 in 30 wt % AEEA solution in a reaction calorimeter, which is the only available information in the literature on the direct calorimetric measurements of the enthalpy of absorption of CO2 for the system (CO2 + AEEA + H2O). The results on CO2 solubility and physical properties in our previous study10 concluded the use of optimum blends of aqueous BZA and AEEA as a potential solvent for CO2 capture. Therefore, the direct calorimetric measurements of the heats of absorption of CO2 in aqueous (BZA + AEEA) is an important parameter to determine the design accuracy and the calculation of steam economy in the stripper section, which are not available till date in the open literature. Moreover, information about the heat capacity of BZA, AEEA, and its mixtures is also not available in the literature. In this work, heats of absorption of CO2 with aqueous solutions of BZA, AEEA, and their mixtures were measured

using an isothermal reaction calorimeter as a function of CO2 loading, temperature, and solvent composition. The heat capacities of the unloaded solutions of BZA, AEEA, and their blends were also measured in the temperature range of 303.15− 343.15 K.

2. MATERIALS AND METHODS 2.1. Chemicals. Benzylamine (BZA, mass fraction purity > 0.99), N-(2-aminoethanolamine) (AEEA, mass fraction purity > 0.99), double-distilled water, nitrogen (N2, volume fraction purity > 0.999), and carbon dioxide (CO2, volume fraction purity > 0.999) were used in this work without any further purification. BZA and AEEA were procured from Sigma-Aldrich, India, and N2 and CO2 were purchased from Linde India Limited. Specifications of the chemicals are mentioned in Table 1. For the preparation of aqueous amine solution, doubledistilled degassed water was used. The degasification was performed by prolonged boiling followed by cooling to ambient temperature in airtight conditions. A precision analytical balance (CITIZEN, CX- 301 model, accuracy: ±0.001 g) was used to weigh the chemicals. 3393



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2.2. Experimental Setup. The heats of absorption of CO2 in aqueous amine solutions were measured using a fully automated, computer-controlled reaction calorimeter (Mettler Toledo, Model: RC1e). The double-walled jacketed glass reactor (volume: 1200 cm3, ID: 8.2 cm) was equipped with a magnetically coupled anchor-type impeller through a stirrer shaft, a temperature sensor (Pt 100, accuracy: ±0.01 K), a calibrated heater (25 W), and a digital pressure transducer (Rosemount 3051TA; accuracy: ±0.04% of the range). The schematic of the experimental setup is given in Figure 1. The reaction calorimeter can be used in the temperature range of 223.15−473.15 K and up to the pressure of 6 bar. A thermostat and heat circulating fluid in the jacket were used to control the reactor temperature within ±0.01 K. An isothermal buffer vessel (1.39 dm3) equipped with a temperature sensor (K-type thermocouple) and a digital pressure transducer (Rosemount 3051TA; accuracy: ±0.04% of the range) were used to feed CO2 into the reactor. The operating parameters and output variables were recorded by the data acquisition system using iControl software. 2.3. Heat of Absorption Measurement. The reaction calorimetric system was used to measure the evolved heat, reactor temperature, jacket temperature, stirrer speed, and pressures in the reactor and buffer vessels as a function of time. The reactor was first purged with nitrogen and then filled with an aqueous amine solution. Then, the evacuation process was accomplished to maintain the amine solution in the reactor at its own vapor pressure. The heat of absorption of CO2 measurement process was followed by the calibration of the reactor system. The calibration for the measurement of the heat transfer coefficient (U) before each experimental run was performed by estimation of the heat transfer coefficient of the reactor wall using the expression in eq 1. The calibration procedure in detail was mentioned elsewhere.1

Table 2. Comparison of the Vapor−Liquid Equilibrium Dataa of This Work Using a Reaction Calorimeter with the Previously Published Data Using the VLE Setupb 4.0 mol BZA· kg−1 water, 313.15 K, Mukherjee et al.5

a b

4.0 mol BZA· kg−1 water, 313.15 K, this work

PCO2/kPa

αCO2

PCO2/kPa

α CO2

2.4431 3.9026 40.133 83.062 112.98 131.56 174.47

0.3367 0.4500 0.5368 0.5725 0.5918 0.6193 0.6249

4.0748 5.5089 8.6322 51.890 97.588 168.44

0.1509 0.3053 0.4567 0.5618 0.5984 0.6166

Standard uncertainty: u(T): 1 K; u(PT): 1 kPa; u(CT): 0.0001 mol·kg−1. Relative standard uncertainty: ur(αCO2): 0.03.

t

U=

∫t qc dt 0

Figure 2. Comparison of VLE data of CO2 in 30 wt % aqueous BZA (4.0 mol BZA·kg−1 water) solvent of this work using a reaction calorimeter with previously published data using the VLE setup.5

t

A ∫ (Tr−Tj) dt t0

(1)

where A is the wetted area of heat transfer, qc is the heat generation rate of the calibration heater, t is the time of calibration heating, Tr and Tj are the temperatures of the reactor and jacket, respectively, as a function of time. The prefilled buffer vessel with CO2 was allowed to reach the desired temperature before the batch feeding of CO2 into the calibrated reactor (RC1e) at constant solution vapor pressure (Pv) and constant (Tr − Tj). The respective pressures of the reactor and the buffer vessel were recorded for each pre- and post-CO2 feeding. Isothermal heat generation of CO2 absorption as a function of time was recorded continuously using the iControl software system. The system was assumed to be at equilibrium as the heat generation curve touched the baseline, satisfying the condition of no change in total pressure (PT = Pv + PCO2) and heat transfer across the reactor wall. The process was repeated to obtain heat of CO2 absorption data at each CO2 loading (mol CO2·mol−1 total amine) point. The reactions of CO2 with aqueous BZA, AEEA, or (BZA + AEEA) are presented as follows.

Figure 3. Comparison of differential and integral enthalpy of CO2 absorption in 30 wt % aqueous AEEA (4.11 mol AEEA·kg−1 water) solvent at 313.15 K with literature data.9

CO2 (gas) ↔ CO2 (aqueous)

(2a)

H 2O ↔ H+ + OH−

(2b)

HCO−3 ↔ H+ + CO32 −

(2d)

H 2O + CO2 ↔ H+ + HCO3−

(2c)

BZA + H+ ↔ BZAH+

(2e)

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Table 3. Integral (ΔHint) and Differential (ΔHdiff) Heats of Absorption of CO2 in (4.0 mol BZA·kg−1 Water) Solutiona−c T/K

PCO2/kPa

αCO2/mol CO2·mol−1 total amine

XCO2/mol·mol−1

−ΔHdiff/kJ·mol−1 CO2

−ΔHint/kJ·mol−1 CO2

−ΔHint/kJ·mol−1 total amine

313.15

4.07 5.51 8.63 51.89 97.59 168.44 3.03 4.27 4.94 6.08 13.37 43.00 82.63 118.65 146.73 3.97 6.21 5.94 8.57 23.48 70.64 132.52 171.36 1.19 1.50 2.36 8.95 35.60 90.30 142.52 177.91

0.151 0.305 0.457 0.562 0.598 0.617 0.096 0.194 0.292 0.388 0.480 0.542 0.576 0.596 0.612 0.097 0.197 0.297 0.393 0.480 0.542 0.579 0.598 0.099 0.197 0.297 0.391 0.473 0.527 0.555 0.579

0.0102 0.0204 0.0302 0.0369 0.0392 0.0403 0.0065 0.0131 0.0195 0.0258 0.0317 0.0356 0.0377 0.0390 0.0400 0.0066 0.0132 0.0198 0.0261 0.0317 0.0356 0.0379 0.0392 0.0067 0.0133 0.0198 0.0260 0.0312 0.0346 0.0364 0.0379

88.7 84.5 80.1 53.9 24.0 8.2 84.3 85.0 84.7 82.4 78.4 60.2 42.9 22.5 13.1 86.1 85.0 82.9 82.6 75.6 62.8 36.0 31.2 86.3 85.6 83.0 85.5 78.1 60.4 52.4 24.7

88.7 86.6 84.4 78.7 75.4 73.4 84.3 84.7 84.7 84.1 83.0 80.4 78.2 76.3 74.7 86.1 85.5 84.6 84.1 82.6 80.3 77.5 76.0 86.3 85.9 84.9 85.1 83.9 81.5 80.0 77.7

13.4 26.4 38.6 44.2 45.1 45.3 8.1 16.5 24.7 32.6 39.9 43.6 45.0 45.5 45.7 8.4 16.8 25.1 33.1 39.7 43.6 44.9 45.5 8.6 17.0 25.2 33.3 39.6 42.9 44.4 45.0

323.15

333.15

343.15

T denotes temperature, αCO2 denotes equilibrium CO2 loading, XCO2 denotes liquid phase CO2 mole fraction, and PCO2 denotes equilibrium partial pressure of CO2 in the reactor at each equilibrium point. bStandard uncertainty: u(T): 1 K; u(PT): 1 kPa; u(CT): 0.0001. cRelative standard uncertainty: ur(αCO2): 0.03; ur(XCO2): 0.03; ur(ΔHint): 0.06; ur(ΔHdiff): 0.05. a

BZA + HCO−3 ↔ BZACOO− + H 2O

(2f)

AEEA + H+ ↔ AEEAH+

(2g)

AEEA +

HCO−3

−

↔ AEEACOO + H 2O

AEEACOO− + H+ ↔ + HAEEACOO−

corresponding compressibility factor at reaction temperature, Pv and PT are the vapor pressure of the solvent and the equilibrium total pressure, respectively, and related as in eq 4. The heat generation (Qr) in the reactor during the exothermic absorption was measured as a function of time, which was proportional (proportionality constant = U × A) to the temperature difference between the reactor (Tr) and jacket (Tj) as in eq 5. The value of proportionality constant was calculated through the reactor calibration process. The heat of absorption was calculated by eq 6.

(2h) (2i)

In this work, the measured enthalpy of absorption was a combination of the exothermic effect of physical and chemical absorption, and it was presented as a function of CO2 loading (αCO2) as calculated by eq 3. ÄÅ ÉÑ l nCO Pb2 yzz V cg PCO2 ÑÑÑÑ 1 ÅÅÅÅ Vb ijj Pb1 2 ÅÅ ÑÑ αCO2 = = − j z− namine namine ÅÅÅ RT jjk Z1 Z 2 zz{ RT ZCO2 ÑÑÑ Ç Ö

Q r = UA

(3)

PCO2 = PT − Pv

ΔΗabs =

(4)

∫t

tf

(Tr − Tj) dt

i

(5)

Qr l nCO 2

(6)

where A is the wetted area of heat exchange and U is the heat transfer coefficient of the reactor. Differential and integral heats of enthalpy were calculated using eq 6 and were based on the differential dosing of CO2 of each feed and total mole of CO2 present in the liquid phase at the final equilibrium point, respectively. 2.4. Heat Capacity Measurement. The heat capacity (Cp) was measured using the jacketed reaction calorimeter. The

where namine is the moles of amine in the solution in the reactor, nlCO2 is the moles of CO2 in the liquid phase, Vb is the volume of the buffer vessel, T is the reaction temperature, Pb1 and Pb2 are the CO2 buffer pressures before and after feeding, Z1 and Z2 are the compressibility factors corresponding to Pb1 and Pb2, respectively, Vgc is the gas phase volume in the reactor, PCO2 and ZCO2 are equilibrium partial pressure of CO2 and the 3395



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PCO2/kPa


XCO2/mol·mol−1




313.15

4.25 5.64 7.10 25.71 80.45 130.55 175.45 5.27 6.86 9.76 37.20 103.93 158.28 202.46 5.35 7.42 13.76 51.54 131.83 202.94 230.02 3.90 7.01 30.11 111.24 189.74

0.151 0.304 0.457 0.573 0.654 0.702 0.743 0.149 0.296 0.440 0.560 0.634 0.673 0.703 0.148 0.296 0.440 0.550 0.626 0.676 0.724 0.156 0.310 0.475 0.577 0.634

0.0061 0.0123 0.0183 0.0228 0.0260 0.0279 0.0295 0.0060 0.0119 0.0177 0.0223 0.0252 0.0267 0.0279 0.0060 0.0120 0.0176 0.0220 0.0249 0.0269 0.0287 0.0063 0.0125 0.0190 0.0230 0.0252

85.4 84.5 80.2 57.8 34.2 26.0 21.3 84.9 83.8 79.6 63.7 37.3 25.1 23.2 85.3 86.5 84.6 76.6 65.7 19.1 14.2 87.3 90.2 89.9 83.6 56.0

85.4 84.3 82.8 77.5 72.1 69.5 66.1 84.9 84.6 84.1 77.4 71.5 68.2 66.8 85.3 84.3 82.1 79.7 75.2 70.9 67.3 87.3 87.8 84.6 79.4 77.3

12.9 25.6 37.8 44.4 47.1 48.8 49.1 12.6 25.0 37.0 43.3 45.4 45.9 47.0 12.6 25.0 36.1 43.8 47.1 48.0 48.7 13.7 27.2 40.2 45.8 49.0

323.15

333.15

343.15



PCO2/kPa


XCO2/mol·mol−1




313.15

4.27 9.68 60.46 129.98 208.15 273.67 4.54 15.57 76.93 155.01 226.80 3.38 12.05 59.32 135.19 204.40 6.85 18.04 62.56 123.40 190.68

0.271 0.547 0.743 0.857 0.944 1.015 0.281 0.553 0.721 0.830 0.914 0.257 0.494 0.651 0.762 0.849 0.235 0.453 0.602 0.704 0.783

0.0050 0.0100 0.0136 0.0156 0.0172 0.0185 0.0052 0.0102 0.0132 0.0152 0.0167 0.0047 0.0091 0.0119 0.0139 0.0155 0.0043 0.0083 0.0110 0.0129 0.0143

84.5 73.4 42.2 25.6 19.0 12.0 86.7 74.7 46.9 23.1 14.8 86.8 83.2 83.3 35.2 20.2 91.5 88.7 87.7 36.2 23.7

84.5 78.5 66.7 61.2 60.6 57.6 86.4 80.7 69.3 61.3 59.0 86.9 85.1 86.0 78.5 72.6 91.5 90.2 89.5 81.8 76.0

22.9 43.0 49.6 52.4 57.2 58.4 24.3 44.6 49.9 50.9 54.0 22.3 42.0 55.9 59.9 61.6 21.5 40.8 53.9 57.6 59.5

323.15

333.15

343.15


temperature of the thermostatic fluid (Tj) in the jacket was automatically adjusted by heating to maintain the temperature of the amine solution in the reactor (Tr). A temperature ramp

(ΔT) of 3 K was applied to the reactor temperature, the resulted heat addition (Q) was measured using eq 7, and the heat capacity was calculated using eq 8. The calibration factor 3396



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PCO2/kPa


XCO2/mol·mol−1




313.15

3.98 8.46 33.34 84.25 180.66 254.64 0.70 13.90 59.96 127.98 194.58 246.68 0.49 25.70 106.51 184.81

0.295 0.574 0.771 0.955 1.131 1.243 0.315 0.644 0.817 0.985 1.163 1.314 0.288 0.664 0.930 1.143

0.0026 0.0051 0.0068 0.0084 0.0099 0.0109 0.0028 0.0057 0.0072 0.0087 0.0102 0.0115 0.0025 0.0059 0.0082 0.0100

84.6 66.9 38.7 24.0 18.0 7.5 81.4 66.3 40.5 22.7 9.5 4.7 82.9 73.0 62.5 20.5

84.6 76.8 66.4 58.2 52.0 49.0 81.4 74.2 63.7 55.1 48.6 44.2 82.7 77.2 73.1 63.2

24.9 44.1 51.2 55.6 58.8 61.0 25.6 47.8 52.1 54.3 56.5 58.0 23.8 51.3 67.9 72.3

323.15

333.15


Figure 4. Integral heat of absorption of CO2 in aqueous BZA solutions.

(U × A) was determined by pre- and postcalibration of applied temperature ramp to the reactant mass (m) in the calorimeter. Q = UA

Cp =

∫ (Tr − Tj)·dt

loading and heat of absorption in this work. The standard uncertainty u(y) of calculated output “y” was functionally related to the measured uncertainty of input variable u(xi), as mentioned in eq 9. The relative standard uncertainty associated with CO2 loading and heat of absorption was calculated using eqs 10 and 11, respectively.

(7)

ÄÅ ÉÑ2 ÅÅ ∂f ÑÑ Å Å u (y) = ∑ ÅÅ u(xi)ÑÑÑÑ ; ÅÇ ∂xi ÑÑÖ i=1 Å

UA ∫ (Tr − Tj)· dt m·ΔT

N

2

(8)

2.5. Experimental Uncertainty. The law of propagation of uncertainty11 was used to determine the related uncertainty of CO2

with, y = f (xi); i = 1, 2, ......, N 3397

(9) DOI: 10.1021/acs.jced.9b00205 J. Chem. Eng. Data 2019, 64, 3392−3406


Article

Figure 5. Differential heat of absorption of CO2 in aqueous BZA solutions.

Figure 6. Heat of absorption of CO2 in (4.0 mol BZA·kg−1 water) solution at 313.15 K and in (4.11 mol AEEA·kg−1 water) solution at 313.15 K (saturation loading points).

ur(αCO2) =

u(αCO2) αCO2

l ij u(namine) yz ji u(nCO2) zyz jj zz + jjj zz jj n zz jjj l z nCO2 zz k amine { k { 2

=

u(ΔΗabs) ur(ΔΗabs) = = ΔΗabs

Figure 7. Integral heat of absorption of CO2 in (4.0 mol BZA·kg−1 water) solution at different temperatures presented in kJ·mol−1 BZA.

2

2 l ij u(nCO ij u(Q ) yz ) yz jj 2 z r z jj zz z + j jj zz jjj l z nCO2 zz k Qr { k {

temperature (T), reactor volume (V), etc. u(αCO2) and u(ΔHabs) were the standard uncertainties calculated for the CO2 loading and enthalpy of CO2 absorption, respectively. The relative standard uncertainties associated with the loading and enthalpy of CO2 absorption were estimated using the standard and relative standard uncertainties of the different inputs: u(P): 1 kPa; u(T): 1 K; u(V): 2 × 10−6 m3; ur(namine): 0.02; ur(Qr): 0.03. The standard uncertainty of the concentration in molality u(CT) and relative standard uncertainties associated with loading, integral and differential heats of absorption of CO2, and heat capacity were calculated and are presented as footnote of associated data tables. Some of the experimental runs were repeated to check the reproducibility of the

(10) 2

(11)

The uncertainty associated with liquid phase CO2 content u(nlCO2) was a function of estimated errors in inputs such as pressure (P), 3398



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Table 7. Integral (ΔHint) and Differential (ΔHdiff) Heats of Absorption of CO2 in (4.11 mol AEEA·kg−1 Water) Solutiona−c T/K

PCO2/kPa


XCO2/mol·mol−1




303.15

6.39 8.28 9.08 9.49 9.67 9.58 9.88 10.02 10.37 15.77 60.66 126.62 3.24 4.11 4.39 4.53 4.47 4.34 3.96 5.87 21.53 91.89 166.79 0.70 0.77 6.90 6.97 7.65 12.87 13.41 16.44 23.92 99.80 184.00 0.97 1.24 1.57 2.26 2.81 3.39 3.57 27.06 99.47 188.04 245.96

0.094 0.187 0.277 0.375 0.463 0.563 0.664 0.755 0.856 0.968 1.059 1.105 0.102 0.207 0.313 0.425 0.536 0.650 0.767 0.888 0.999 1.074 1.110 0.112 0.225 0.337 0.448 0.546 0.657 0.763 0.869 0.982 1.053 1.092 0.117 0.225 0.342 0.467 0.579 0.706 0.832 0.946 1.021 1.060 1.080

0.0065 0.0129 0.0190 0.0256 0.0314 0.0379 0.0444 0.0502 0.0565 0.0634 0.0690 0.0718 0.0071 0.0143 0.0214 0.0288 0.0361 0.0435 0.0509 0.0585 0.0653 0.0699 0.0721 0.0078 0.0155 0.0230 0.0304 0.0368 0.0439 0.0507 0.0573 0.0643 0.0686 0.0710 0.0081 0.0155 0.0233 0.0316 0.0389 0.0470 0.0550 0.0621 0.0667 0.0690 0.0702

77.6 78.2 77.3 76.8 76.2 76.2 74.2 76.8 70.8 63.0 49.0 34.4 76.5 76.1 75.3 75.5 74.6 72.9 71.9 68.3 61.9 45.3 32.2 77.9 76.2 77.2 74.6 74.3 74.4 70.4 66.0 62.5 55.8 31.6 77.5 75.4 74.6 73.6 73.5 70.7 67.1 70.2 64.2 33.6 15.0

77.6 78.0 77.2 76.8 76.4 76.3 75.3 75.9 75.0 73.9 71.3 69.3 76.7 77.3 76.4 77.1 76.2 75.1 75.3 73.8 72.9 71.0 70.1 77.9 76.7 76.5 75.5 75.4 74.9 74.0 72.9 71.4 70.5 69.6 77.1 78.2 78.8 77.2 75.8 74.5 74.0 71.3 72.5 71.1 70.0

7.3 14.6 21.3 28.8 35.4 43.0 50.0 57.3 64.2 71.6 75.5 76.6 7.8 16.0 23.9 32.7 40.8 48.8 57.8 65.6 72.9 76.2 77.8 8.7 17.2 25.8 33.8 41.2 49.2 56.5 63.4 70.1 74.2 76.0 9.0 17.6 26.9 36.0 43.9 52.6 61.5 67.5 74.0 75.3 75.6

313.15

323.15

333.15


(4.11 mol AEEA·kg−1 water) aqueous AEEA respectively were measured at 313.15 K and compared with the literature data,5,9 which are presented in Table 2 and shown in Figures 2 and 3. It can be observed from Figures 2 and 3 that the experimental data are in good agreement with the data available in the literature except at low CO2 partial pressure. At low partial pressure of CO2, close to the vapor pressure of amine solution in the reactor, the uncertainty in the CO2 partial pressure measurement is high. Moreover, the experimental setup used to measure VLE data in the literature is not similar the reaction calorimeter used in this work. In Figure 2, the partial pressure of

experimental results. The average repeatability of CO2 solubility and heat of CO2 absorption are ±0.8 and ±1.3%, respectively.

3. RESULTS AND DISCUSSION The heat of absorption of CO2 in aqueous solutions of single amines (BZA, AEEA) and amine mixtures (BZA + AEEA) were studied as a function of CO2 loading and temperature using a reaction calorimeter (model: RC1e, Mettler Toledo) system. For the validation of the experimental setup, the vapor−liquid equilibrium (VLE) and the heat of absorption of CO2 in 30 wt % (4.0 mol BZA·kg−1 water) aqueous BZA and 30 wt % 3399



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PCO2/kPa


XCO2/mol·mol−1




303.15

4.59 6.68 7.41 7.57 7.90 12.05 54.29 139.83 0.85 1.21 1.23 1.28 5.36 61.85 161.88 0.90 0.68 1.35 1.92 6.18 67.21 163.23 0.02 0.72 0.74 3.00 28.40 139.01 244.30

0.163 0.321 0.491 0.659 0.826 0.980 1.090 1.155 0.190 0.379 0.567 0.756 0.940 1.073 1.138 0.185 0.370 0.554 0.737 0.918 1.046 1.106 0.183 0.353 0.560 0.766 0.951 1.055 1.101

0.0068 0.0133 0.0202 0.0269 0.0335 0.0395 0.0437 0.0462 0.0079 0.0156 0.0233 0.0307 0.0379 0.0431 0.0456 0.0077 0.0153 0.0227 0.0300 0.0371 0.0420 0.0443 0.0076 0.0146 0.0229 0.0311 0.0384 0.0424 0.0442

75.9 75.3 73.7 71.6 67.9 62.5 37.9 33.8 76.8 75.1 72.5 70.4 62.5 49.0 32.2 75.5 73.6 71.3 70.1 63.4 54.3 37.4 77.2 75.9 75.1 73.8 70.0 63.2 52.4

75.9 75.7 75.1 74.1 72.6 71.1 67.7 65.8 76.3 76.1 74.3 73.2 70.8 68.1 66.0 75.8 74.7 73.6 73.4 71.7 69.4 67.7 76.9 76.6 75.6 75.1 73.6 71.7 70.8

12.3 24.3 36.9 48.8 59.9 69.6 73.8 76.0 14.5 28.8 42.1 55.3 66.5 73.1 75.1 14.0 27.7 40.8 54.1 65.9 72.6 74.8 14.0 27.1 42.3 57.6 70.0 75.6 78.0

313.15

323.15

333.15

a

Standard uncertainty: u(T): 1 K; u(PT): 1 kPa; u(CT): 0.0001. bRelative standard uncertainty: ur(αCO2): 0.03; ur(XCO2): 0.03; ur(ΔHint): 0.06; ur(ΔHdiff): 0.05. cT denotes temperature, αCO2 denotes equilibrium CO2 loading, XCO2 denotes liquid phase CO2 mole fraction, and PCO2 denotes equilibrium partial pressure of CO2 in the reactor at each equilibrium point.

concentration (Figures 4 and 5). The heat of reaction is the total amount of heat produced in the reactor per mole of CO2 absorbed in the aqueous amine solvent. The CO2 absorption reaction is exothermic in nature; therefore, according to Le Chatellier’s principle, the heat of CO2 absorption is increased with the increase in temperature. In Figure 6, the three types of heats of absorption of CO2 (i.e., differential and integral heats of absorption per mole of CO2 and integral enthalpy per mole of BZA) in (4.0 mol BZA·kg−1 water) solution at 313.15 K are presented. It can be observed in Figure 6 that the heat of absorption of CO2 (ΔHdiff,CO2, ΔHint,CO2, and ΔHint,BZA) in aqueous BZA is almost constant up to a certain limit of CO2 loading (saturation point ∼ 0.55 mol CO2·mol−1 BZA) and then reduces sharply with the increase of CO2 loading at constant temperature. Many researchers observed the similar behavior of the heat of absorption of CO2 in a single amine system.2,12,13 The heat of absorption of CO2 till the saturation loading point is mostly contributed by the heat generated due to the exothermic carbamate formation reaction, as indicated by the reaction of BZA (eq 2f). After the saturation loading point, a sharp drop in the heat of absorption of CO2 was observed due to the dominance of the highly endothermic carbamate reversal reaction over the exothermic physical absorption. In Figure 7, the integral heat of absorption of CO2 per mole of amine in the aqueous BZA solvent (4.0 mol BZA·kg−1 water) is presented as a function of loading (mol CO2·mol−1 amine) in the temperature

CO2 increases with CO2 loading at constant temperature. The heat of CO2 absorption (ΔHdiff, ΔHint) in aqueous AEEA is almost constant up to a certain limit of CO2 loading (saturation point ∼ 0.8 mol CO2·mol−1 AEEA) and reduces sharply with the increase of CO2 loading at constant temperature. Normally, the exothermic carbamate and bicarbamate formation reactions contribute to the generation of heat till the saturation point and the endothermic carbamate reversion reaction is responsible for the sharp drop of magnitude in heat of CO2 absorption after the saturation point. 3.1. Heat of Absorption of CO2 in Aqueous BZA. The heat of absorption of CO2 in aqueous BZA was measured in the temperature range of 313.15−343.15 K and the concentration range of (0.49−4.0) mol BZA·kg−1 water. Both integral and differential heats of absorption of CO2 at equilibrium CO2 loading in aqueous BZA solutions are presented in Tables 3−6 and plotted as a function of temperature and solvent composition in Figures 4 and 5, respectively. In the figures, a constant plateau of the differential and integral heats of absorption of CO2 was observed in all solvent compositions up to a certain loading point, and after that, it decreases sharply. The variation of amine concentration has a negligible effect on integral and differential heats of absorption of CO2 in the lower temperature (313.15− 323.15 K) range at all concentrations of BZA. However, slightly increasing tendency of both differential and integral heats of absorption of CO2 was observed in the higher temperature (333.15−343.15 K) range with the decrease in amine 3400



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PCO2/kPa


XCO2/mol·mol−1




303.15

2.32 4.03 111.57 241.28 303.69 2.90 4.49 15.92 131.19 233.17 1.12 1.22 6.41 114.16 233.21 2.57 3.56 26.32 157.68 263.92

0.458 0.642 0.901 1.002 1.052 0.352 0.687 1.000 1.154 1.227 0.308 0.597 0.923 1.122 1.200 0.302 0.635 0.973 1.115 1.180

0.0087 0.0121 0.0169 0.0188 0.0197 0.0067 0.0129 0.0187 0.0215 0.0228 0.0058 0.0113 0.0173 0.0209 0.0224 0.0057 0.0120 0.0182 0.0208 0.0220

73.0 68.5 50.0 27.7 14.5 75.5 72.0 63.8 41.6 29.9 78.6 70.8 66.8 49.8 37.8 83.1 78.6 72.6 48.8 17.6

72.8 71.6 69.9 65.9 63.4 75.5 73.8 70.8 66.7 64.1 78.6 74.3 73.4 68.3 66.0 83.3 80.5 82.0 77.7 74.4

33.3 46.0 63.0 66.0 66.7 26.5 50.7 70.8 77.0 78.7 24.2 44.3 67.8 76.7 79.2 25.1 51.1 79.7 86.7 87.8

313.15

323.15

333.15



PCO2/kPa


XCO2/mol·mol−1




303.15

2.96 32.87 163.79 266.97 1.45 38.23 148.11 237.54 2.36 34.27 164.32 247.74 1.81 66.02 177.86

0.595 1.101 1.316 1.454 0.587 1.062 1.226 1.338 0.542 1.035 1.202 1.299 0.680 1.049 1.172

0.0054 0.0099 0.0119 0.0131 0.0053 0.0096 0.0110 0.0121 0.0049 0.0093 0.0108 0.0117 0.0062 0.0095 0.0106

74.3 57.8 27.7 20.0 76.3 62.7 31.6 16.7 75.9 66.9 34.1 16.8 84.3 85.8 83.1

74.3 68.7 61.4 58.0 77.0 71.9 64.2 62.4 76.3 70.3 63.8 60.3 84.3 84.8 84.7

44.2 75.6 80.8 84.3 45.2 76.3 78.7 83.5 41.4 72.7 76.7 78.3 57.4 89.0 99.2

313.15

323.15

333.15


3.2. Heat of Absorption of CO2 in Aqueous AEEA. In this work, the heat of absorption of CO2 in aqueous AEEA was measured in the temperature range of 303.15−333.15 K and the concentration range of (0.51−4.11) mol AEEA·kg−1 water. In Figure 3, the experimental data of the differential and integral heats of absorption of CO2 in aqueous AEEA (4.11 mol AEEA·kg−1 water) solution at 313.15 K were in good agreement with the literature data.14 Both the integral and differential heats of absorption of CO2 at equilibrium CO2 loading are presented in Tables 7−10 and plotted as a function of temperature and solvent composition in Figures 8 and 9, respectively. In the figures, both differential and integral heats of absorption of CO2 show a constant plateau in all solvent compositions up to a certain loading point and then fall sharply. It can be observed

range of 313.15−343.15 K. Two distinct regions can be observed in this plot. Initially, the heat of absorption of CO2 in the plot increased linearly with CO2 loading passing through origin up to the saturation point and then became flat with a constant value. The value of the initial slopes of the lines passing through the origin in the plot (Figure 7) slightly decreases with an increase in temperature, which indicated a trivial or an insignificant temperature effect on the heat of absorption of CO2. A similar trend was also observed for the other BZA concentrations. The calculated values of the slopes in Figure 7 were 85.35, 83.77, 82.47, and 81.39 kJ·mol−1 CO2 at 313.15, 323.15, 333.15, and 343.15 K, respectively, which were in close proximity to the actual integral heat of absorption of CO2 values at the corresponding temperature and composition. 3401



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Figure 8. Integral heat of absorption of CO2 in aqueous AEEA solutions.

Figure 9. Differential heat of absorption of CO2 in aqueous AEEA solutions. 3402



Article

reaction of AEEA (eq 2h) and then a sharp drop in the heat of absorption of CO2 was observed with the increase of CO2 loading at constant temperature due to the dominance of the highly endothermic carbamate reversal reaction over the exothermic physical absorption. In Figure 10, the integral heat of absorption of CO2 per mole of amine in aqueous AEEA (4.11 mol AEEA·kg−1 water) solution is presented as a function of loading (mol CO2·mol−1 amine) in the temperature range of 303.15−333.15 K. It can be observed in the plot that the integral heat of CO2 absorption per mole of amine increased linearly from origin up to the saturation loading point2 and beyond that it became flat with a constant value. The similar trends were also observed for the other aqueous AEEA solutions. The calculated value of the slopes in Figure 10 were 76.78, 75.98, 74.29, and 73.58 kJ·mol−1 CO2 at 303.15, 313.15, 323.15, and 333.15 K, respectively, which were very close to the actual integral heat of absorption values at the corresponding temperature and composition. The experimental heat of absorption of CO2 (ΔHabs ∼ 75 kJ·mol−1 CO2) in aqueous AEEA was lower than that of in aqueous MEA (∼80 kJ·mol−1 CO22). The lower value of the equilibrium constant of the carbamate formation reaction of the (AEEA + H2O + CO2) system than that of the (MEA + H2O + CO2) system could be the possible reason for the lower value of heat of absorption of CO2 in the aqueous AEEA solvent than that in the aqueous MEA solvent.15 3.3. Heat of Absorption of CO2 in Aqueous Mixture of BZA and AEEA. The heats of absorption of CO2 in the aqueous

Figure 10. Integral heat of absorption of CO2 in (4.11 mol AEEA·kg−1 water) solution at different temperatures presented in kJ·mol−1 AEEA.

that the variation of amine concentration and temperature has negligible effect on integral and differential heats of absorption of CO2. In Figure 6, the heat of absorption of CO2 in aqueous AEEA (4.11 mol AEEA·kg−1 water) solution at 313.15 K was observed to remain constant up to a certain loading point (the saturation loading point ∼1.0 mol CO2·mol−1 AEEA) due to the exothermic carbamate formation reaction as indicated by the

Table 11. Integral (ΔHint) and Differential (ΔHdiff) Heats of Absorption of CO2 in Aqueous (2.03 mol BZA + 2.09 mol AEEA)· kg−1 Water Solutiona−c T/K

PCO2/kPa


XCO2/mol·mol−1




313.15

16.93 21.39 22.64 23.30 23.52 24.07 29.68 60.74 119.15 3.73 4.67 4.68 4.22 4.32 9.97 46.35 114.06 166.72 3.45 3.72 3.06 2.23 3.18 10.58 42.73 104.90 163.46 210.50

0.112 0.217 0.322 0.430 0.536 0.642 0.744 0.827 0.885 0.115 0.229 0.346 0.465 0.583 0.699 0.793 0.853 0.890 0.115 0.221 0.329 0.436 0.548 0.656 0.745 0.801 0.836 0.856

0.0077 0.0148 0.0218 0.0288 0.0357 0.0425 0.0488 0.0540 0.0575 0.0078 0.0155 0.0233 0.0311 0.0387 0.0460 0.0519 0.0556 0.0579 0.0079 0.0151 0.0222 0.0292 0.0365 0.0433 0.0489 0.0524 0.0546 0.0558

79.5 80.1 79.5 78.8 78.0 74.6 69.8 52.5 38.7 79.2 78.4 78.6 78.3 76.6 72.4 59.0 42.0 27.3 82.5 79.6 79.8 80.3 78.4 76.3 69.1 61.4 48.0 20.0

79.6 79.7 79.4 79.1 79.2 78.3 77.0 74.0 72.1 79.3 79.2 79.7 78.4 78.4 76.8 74.2 72.4 70.5 82.4 81.7 81.5 80.9 80.8 80.3 77.7 75.5 73.7 72.4

8.9 17.3 25.6 34.0 42.5 50.3 57.3 61.2 63.8 9.1 18.1 27.6 36.5 45.7 53.7 58.9 61.8 62.8 9.5 18.1 26.8 35.3 44.3 52.7 57.9 60.5 61.6 62.0

323.15

333.15


3403



Article

Table 12. Integral (ΔHint) and Differential (ΔHdiff) Heats of Absorption of CO2 in Aqueous (2.71 mol BZA + 1.39 mol AEEA)· kg−1 Water Solutiona−c T/K

PCO2/kPa


XCO2/mol·mol−1




313.15

2.98 3.64 3.85 5.28 29.45 122.09 216.83 5.84 7.34 7.78 9.52 36.52 130.92 219.44 9.20 10.45 10.22 10.38 13.22 43.26 114.70 186.09

0.141 0.285 0.432 0.578 0.709 0.789 0.832 0.141 0.281 0.421 0.561 0.686 0.761 0.800 0.114 0.224 0.339 0.451 0.560 0.656 0.717 0.750

0.0096 0.0192 0.0288 0.0382 0.0465 0.0514 0.0541 0.0096 0.0190 0.0281 0.0371 0.0451 0.0497 0.0521 0.0078 0.0152 0.0228 0.0300 0.0371 0.0431 0.0470 0.0490

81.5 80.3 80.4 78.0 64.3 42.2 34.5 81.5 81.6 80.5 77.7 68.2 47.5 41.6 84.2 85.8 81.8 85.1 84.4 78.7 68.0 36.4

81.5 81.1 80.4 80.5 77.1 73.3 71.9 81.5 81.9 80.9 79.6 77.1 74.4 72.8 84.2 82.1 83.1 83.3 84.6 83.7 82.4 80.3

11.5 23.1 34.7 46.5 54.7 57.8 59.8 11.5 23.1 34.1 44.6 52.9 56.6 58.2 9.6 18.4 28.2 37.6 47.4 54.9 59.1 60.3

323.15

333.15


Figure 11. Integral heat of absorption of CO2 in (2.03 mol BZA + 2.09 mol AEEA)·kg−1 water.

Figure 12. Integral heat of absorption of CO2 in (2.71 mol BZA + 1.39 mol AEEA)·kg−1 water.

mixture of (2.03 mol BZA + 2.09 mol AEEA)·kg−1 water and (2.71 mol BZA + 1.39 mol AEEA)·kg−1 water in the temperature range of 313.15−333.15 K are presented in Tables 11 and 12 and plotted in Figures 11 and 12 as a function of CO2 loading and temperature. It can be observed from Figures 11 and 12 that the heat of absorption of CO2 increases with the increase in temperature and BZA concentration in the mixture. In Figure 13, the heats of absorption of CO2 in aqueous (BZA + AEEA) solvents were compared with aqueous BZA (4.0 mol BZA·kg−1 water) and AEEA (4.11 mol AEEA·kg−1 water) at 313.15 K. The heat of absorption of CO2 in the aqueous mixture of [(2.71 mol BZA + 1.39 mol AEEA)·kg−1 water; 87 kJ·mol−1 CO2] was lower than that of in the aqueous BZA [(4.0 mol BZA·kg−1 water);

81 kJ·mol−1 CO2] solvent at a constant CO2 loading (αCO2 = 0.3). However, further increase of AEEA concentration in the aqueous mixture of (BZA + AEEA) resulted to be nonbeneficial in terms of the heat of CO2 absorption value (81−79 kJ·mol−1 CO2). The CO2 loading (αCO2) in aqueous (2.71 mol BZA + 1.39 mol AEEA)·kg−1 water solution increased by 0.2 mol CO2·mol−1 amine than that in the aqueous BZA solvent, which was also better than the performance (by 0.05 mol CO2·mol−1 amine) shown in the aqueous (2.03 mol BZA + 2.09 mol AEEA)·kg−1 water solution. Therefore, considering the physical properties of AEEA (higher viscosity and density than in aqueous BZA10), the aqueous (2.71 mol BZA + 1.39 mol AEEA)·kg−1 3404



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Table 14. Heat Capacity of Aqueous AEEA and (BZA + AEEA) Solvents at 0.1 MPa; T Denotes Temperature and Cp Denotes Heat Capacitya,b solvent −1

0.51 mol AEEA·kg

water

1.07 mol AEEA·kg−1 water



Figure 13. Comparison of integral heat of absorption of CO2 in experimental solvents at 313.15 K.

Table 13. Heat Capacity of Pure Water and Aqueous BZA Solvents at 0.1 MPa; T Denotes Temperature, Cp Denotes Heat Capacity, and XH2O Denotes Mole Fraction of Water in the Unloaded Solventa,b T/K

solvent water (NIST)

water (XH2O = 1)

0.49 mol BZA·kg−1 water



−1

4.00 mol BZA·kg

water

−1

Cp/J·K ·g

303.15 313.15 323.15 333.15 343.15 304.15

4.180 4.179 4.181 4.185 4.190 4.168

314.15 324.15 334.15 344.15 314.15 324.15 334.15 344.15 314.15 324.15 334.15 344.15 314.15 324.15 334.15 344.15 314.15 324.15 334.15 344.15

4.173 4.178 4.181 4.185 3.888 3.931 4.040 4.201 3.835 3.902 4.002 4.179 3.641 3.730 3.819 4.052 3.468 3.555 3.653 3.824

(2.03 mol BZA + 2.09 mol AEEA)·kg−1 water

(2.71 mol BZA + 1.39 mol AEEA)·kg−1 water

−1

a b

T/K

Cp/J·K−1·g−1

304.15 314.15 324.15 334.15 304.15 314.15 324.15 334.15 304.15 314.15 324.15 334.15 304.15 314.15 324.15 334.15 314.15 324.15 334.15 314.15 324.15 334.15

3.818 3.867 3.948 4.108 3.758 3.810 3.862 4.018 3.655 3.715 3.787 3.919 3.534 3.588 3.645 3.819 3.532 3.604 3.742 3.502 3.584 3.702

Standard uncertainty: u(CT): 0.0001; u(PT): 0.93 kPa; u(T): 1 K. Relative standard uncertainty: ur(Cp): 0.03.

Figure 14. Heat capacity of aqueous BZA solvent and pure water.

and plotted in Figures 4−6. The heat capacity of pure water was compared with corresponding NIST16 data for validation of the instrumental setup and was found to be in good agreement (AAD = 0.1%) with experimental measurements. In this study, it can be observed (Figures 14 and 15) that Cp of aqueous BZA and AEEA solvents increases with temperature but decreases with the increase in amine concentration. In Figure 16, Cp for the aqueous (BZA + AEEA) mixtures increases with the increase in temperature as well as with the increase in the AEEA concentration in the mixed solvents keeping the total amine concentration constant.

a

Standard uncertainty: u(CT): 0.0001; u(PT): 0.93 kPa; u(T): 1 K. Relative standard uncertainty: ur(Cp): 0.03.

b

water solution among all of the studied aqueous amine solutions might be used as an effective and economical solvent in postcombustion CO2 capture applications with better CO2 loading capacity and lower amine regeneration and pumping cost. 3.4. Heat Capacity. Heat capacities (Cp) of aqueous BZA, AEEA, and (BZA + AEEA) mixed solvents along with pure water were measured in the temperature range of 304.15−344.15 K. Experimental heat capacity data are presented in Tables 13 and 14

4. CONCLUSIONS A reaction calorimeter was used to measure the heat of absorption of CO2 and heat capacity of aqueous BZA, AEEA, 3405



Article

the stripper section to raise the solvent temperature for solvent regeneration.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Amar N. Samanta: 0000-0002-0290-7461 Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Mondal, B. K.; Bandyopadhyay, S. S.; Samanta, A. N. Measurement of CO2 absorption enthalpy and heat capacity of aqueous hexamethylenediamine and its aqueous mixture with Nmethyldiethanolamine. J. Chem. Thermodyn. 2017, 113, 276−290. (2) Kim, I.; Svendsen, H. F. Heat of absorption of carbon dioxide (CO 2 ) in monoethanolamine (MEA) and 2-(aminoethyl)ethanolamine (AEEA)solutions. Ind. Eng. Chem. Res. 2007, 46, 5803−5809. (3) Mathonat, C.; Majer, V.; Mather, A. E.; Grolier, J.-P. E. Enthalpies of absorption and solubility of CO2 in aqueous solutions of methyldiethanolamine. Fluid Phase Equilib. 1997, 140, 171−182. (4) Kierzkowska-pawlak, H.; Zarzycki, R. Calorimetric measurements of CO2 absorption into aqueous N-Methyldiethanolamine solutions. Chem. Pap. 2002, 56, 219−227. (5) Mukherjee, S.; Bandyopadhyay, S. S.; Samanta, A. N. VaporLiquid Equilibrium (VLE) of CO2 in Aqueous Solutions of Benzylamine: New Data and Modeling Using ENRTL-Equation. Int. J. Greenhouse Gas Control 2017, 56, 12−21. (6) Mukherjee, S.; Bandyopadhyay, S. S.; Samanta, A. N. Kinetic Study of CO2 Absorption in Aqueous Benzylamine Solvent Using a Stirred Cell Reaction Calorimeter. Energy Fuels 2018, 32, 3668−3680. (7) Ma’mun, S. Selection and characterization of new absorbents for carbon dioxide capture. Ph.D. Thesis; Norwegian University of Science and Technology: Trondheim, Norway, 2005. (8) Ma’mun, S.; Jakobsen, J. P.; Svendsen, H. F.; Juliussen, O. Experimental and modeling study of the solubility of carbon dioxide in aqueous 30 mass% 2-((2-aminoethyl) amino) ethanol solution. Ind. Eng. Chem. Res. 2006, 45, 2505−2512. (9) Ma’mun, S.; Svendsen, H. F.; Hoff, K.; Juliussen, O. Selection of new absorbents for carbon dioxide capture. Energy Convers. Manage. 2007, 48, 251−258. (10) Mukherjee, S.; Bandyopadhyay, S. S.; Samanta, A. N. Experimental measurements and modelling of CO2 solubility in aqueous mixtures of benzylamine and N-(2-aminoethyl) ethanolamine. Asia-Pac. J. Chem. Eng. 2018, 13, No. e2264. (11) Haghtalab, A.; Izadi, A. Simultaneous measurement solubility of carbon dioxide + hydrogen sulfide into aqueous blends of alkanolamines at high pressure. Fluid Phase Equilib. 2014, 375, 181−190. (12) Arcis, H.; Rodier, L.; Ballerat-Busserolles, K.; Coxam, J.-Y. Enthalpy of solution of CO2 in aqueous solutions of methyldiethanolamine at T = 322.5 K and pressure up to 5MPa. J. Chem. Thermodyn. 2008, 40, 1022−1029. (13) Arcis, H.; Rodier, L.; Coxam, J. Y. Enthalpy of solution of CO2 in aqueous solutions of 2-amino-2-methyl-1-propanol. J. Chem. Thermodyn. 2007, 39, 878−887. (14) Kim, I.; Hoff, K. A.; Hessen, E. T.; Haug-Warberg, T.; Svendsen, H. F. Enthalpy of absorption of CO2 with alkanolamine solutions predicted from reaction equilibrium constants. Chem. Eng. Sci. 2009, 64, 2027−2038. (15) Da Silva, E. F.; Svendsen, H. F. Study of the carbamate stability of amines using ab initio methods and free-energy perturbations. Ind. Eng. Chem. Res. 2006, 45, 2497. (16) National Institute of Standard and Technology. Thermophysical Properties of Fluid Systems. http://webbook.nist.gov/chemistry/ fluid/ (accessed April 05, 2016).

Figure 15. Heat capacity of aqueous AEEA solvent and pure water.

Figure 16. Heat capacity of aqueous BZA, AEEA, and (BZA + AEEA) solvents.

and (BZA + AEEA) systems over the temperature range of 303.15−343.15 K. For the solvent systems, the amount of heat released due to isothermal absorption of CO2 was presented as a function of solvent temperature, composition, and CO2 loading. Temperature and amine concentration did not significantly affect the heat of absorption of CO2 within the range of this study for single amine systems. For the aqueous BZA and AEEA systems, a plateau of absorption enthalpy was observed up to CO2 loadings of 0.65 and 1.0 mol CO2·mol−1 amine (saturation loading point). Heat of absorption of CO2 in the aqueous AEEA solvent (∼75 kJ·mol−1 CO2) was found to be lower than that of aqueous BZA (∼83 kJ·mol−1 CO2) and MEA (∼84 kJ·mol−1 CO22). For the aqueous (BZA + AEEA) system, the heat of absorption of CO2 is a function of temperature and solvent composition and increases with the increase in temperature and BZA concentration in the mixture. The value of heat of absorption of CO2 was reduced due to the presence of AEEA compared to that of only BZA system. This mixed system demonstrated higher loading capacity as well as lower heat of absorption of CO2 compared to that of BZA and MEA solvents. Although the heat of absorption of CO2 in the aqueous (BZA + AEEA) system was higher than that of aqueous AEEA, the heat capacity and viscosity of aqueous (BZA + AEEA) were comparatively lower than those of the aqueous AEEA system, which could be an advantage in the solvent recirculation cost and heat economy in 3406


Heat of Absorption of CO2 and Heat Capacity Measurements in

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