Comparison of the CO2 Absorption Characteristics of Aqueous

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Comparison of the CO2 Absorption Characteristics of Aqueous Solutions of Diamines: Absorption Capacity, Specific Heat Capacity, and Heat of Absorption Young Eun Kim, Soung Hee Yun, Jeong Ho Choi, Sung Chan Nam, Sung Youl Park, Soon Kwan Jeong, and Yeo Il Yoon* Green Energy Process Laboratory, Climate Change Research Division, Korea Institute of Energy Research, 152, Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea S Supporting Information *

ABSTRACT: Aqueous amine solutions have been widely used for the absorption of carbon dioxide (CO2) from the gas mixtures. An understanding of the physical and chemical properties of aqueous amine solutions is important for the successful design and operation of CO2 absorption processes. Particularly, the absorption capacity, absorption rate, and heat of absorption of CO2 are major factors that affect the CO2 absorption and stripping performance. A comparison study of the aqueous piperazine (PZ), 2-methylpiperazine (2-MPZ), homopiperazine (HomoPZ), and hexamethylenediamine (HMDA) solutions was conducted in this study. Absorption capacities and heats of absorption of these diamine solutions were measured using a semibatch-type reactor and a differential reaction calorimeter (DRC). The species distributions of the absorbents were investigated using a nuclear magnetic resonance spectroscopy (NMR), and the CO2 absorption mechanism was also discussed. Aqueous PZ and PZ derivative solutions (2-MPZ and HomoPZ) displayed excellent characteristics as CO2 absorbents. Aqueous 10 wt % PZ and PZ derivative solutions had higher absorption capacities and lower heats of absorption than that of aqueous 10 wt % monoethanolamine (MEA) at 313 K (−ΔHabs of the CO2-saturated PZ, 2-MPZ, HomoPZ, and MEA solutions: 62, 58, 68, and 80 kJ/mol CO2).

1. INTRODUCTION Carbon dioxide (CO2) capture and sequestration (CCS) technologies have the potential to reduce greenhouse gas emissions from industries and coal-fired power plants. CO2 capture technologies fall into three categories: precombustion, postcombustion, and oxyfuel CO2 capture. In precombustion CO2 capture, carbon fuel is separated prior to combustion. In oxyfuel CO2 capture, combustion is performed using pure oxygen, leading to a flue gas consisting of only CO2 and steam.1,2 In postcombustion CO2 capture, CO2 is selectively separated from flue gas components. Processes that rely on chemical absorption using aqueous amine solutions are widely accepted for use in postcombustion CO2 capture. The absorbent performance is the most important factor in the absorption process. An absorbent should have a high absorption capacity, a fast absorption rate, a low heat of absorption, a low degree of degradation, and a low degree of corrosion upon reaction with CO2. These characteristics are related to the physical properties (vapor pressure, viscosity, density, heat capacity, boiling point, etc.) and chemical properties of the absorbent. Aqueous alkanolamine solutions such as monoethanolamine (MEA), methyldiethanolamine (MDEA), and 2-amino-2methyl-1-propanol (AMP) have been used as typical absorbents in the absorption process. CO2 absorption rates and heats of absorption (−ΔHabs) of the amine solutions are following the order: MEA > AMP > MDEA,3,4 whereas CO2 loadings (mol CO2/mol amine) of the amine solutions show the opposite tendency.5 In the typical conditions where there are no © 2015 American Chemical Society

thermodynamic limitations, kinetics of CO2 absorption is the crucial factor on the overall CO2 removal efficiency.3 However, in the specific conditions where thermodynamic limitations are reached (e.g., high CO2 loading of the feed absorbent in the absorber, low liquid load), thermodynamics (CO2 loading) is the important factor.3 Therefore, a comparative study among the absorbents should be conducted, and theoretical and experimental studies are needed to determine the efficiency of the process. Development of new absorbents is one of the most effective ways to achieve the cost-effective process with high CO2 removal efficiency. Furthermore, blended absorbents (e.g., aqueous solutions: amine + amine, alkaline salt + amine, alkaline salt + amino acid, amine + amino acid, etc.) show the potential to improve disadvantages of the single amine process. Several studies have reported the advantages of using amines containing multiple amino groups. Freeman et al.6 and Rochelle et al.7 studied the kinetics and thermodynamics of CO2 absorption in the aqueous piperazine (PZ) solutions. PZ, one of the cyclic diamines, can be used up to 150 °C without significant thermal degradation, is resistant to oxidative degradation, and has less volatility than MEA.6,7 PZ provides a high CO2 loading and a rapid absorption rate; therefore, PZ and its derivatives were also used as absorption rate activators in aqueous MDEA,8,9 AMP,10 and potassium carbonate (K2CO3)11−16 solutions. Robinson et al.17 studied the effects Received: March 12, 2014 Revised: March 10, 2015 Published: March 13, 2015 2582

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(Sigma-Aldrich, 99%), were used without further purification. The aqueous amine solutions were prepared using deionized water. A semibatch-type reactor was used to investigate the CO2 absorption capacities of the aqueous 30 wt % amine solutions. Aqueous 20−30 wt % MEA solutions have been used in the industrial processes that are the most commercial and mature technologies;7,26 therefore, 30 wt % diamine solutions were used for comparison in this study. The internal volume of the reactor was 2 L, and the reactor was filled with 1 L of an aqueous amine solution. Impurities were removed from the reactor by injecting N2 gas (99.999 vol %) over 30 min. CO2 gas was then injected at a flow rate of 1 L/min using a mass flow controller (MFC) from Horiba, Japan (N2 gas: SEC-E40 model, max. range 1500 mL/min; CO2 gas: SEC-40 model, max. range 500 mL/ min) once the temperature and pressure in the reactor reached the experimental conditions. Flue gases containing high CO2 concentrations (20−30%) are emitted from the cement and iron industries, whereas relatively low CO2 concentration (8−15%) gases are generated from the natural gas (NG) boiler and coal-fired power plants. In this study, the inlet gas, 30% CO2, was prepared using a gas mixer (99.99 vol % CO2 + 99.999 vol % N2) in order to extend the application area. Gas chromatography (GC) (7890A from Agilent Co.) was used to analyze the CO2 concentration at the reactor outlet. The emitted gas from the reactor was transferred to the GC at a flow rate of 35 mL/min using a MFC from Brooks, Japan (5850E model, max. range 100 mL/min). It assumed that the absorbents chemically absorbed CO2 and N2 gas did not react with the absorbents. All experiments were conducted at 313, 333, and 353 K and at a pressure of 115 kPa. A vapor−liquid equilibrium (VLE) apparatus was used to prepare the CO2-loaded aqueous 7.5 wt % 2-MPZ solutions for the NMR measurements. Information about CO2-loaded solution samples for the NMR measurements is summarized in Tables S1 and S2 in the Supporting Information. The specific experimental conditions and basic techniques for the VLE are listed here. Detailed descriptions and the procedure have been provided previously.14,16 In the VLE experiment, 0.1 L of the aqueous solution under investigation was fed into the reactor and residual gas was removed using a vacuum pump. All experiments were conducted at 333 K and at a gas injection pressure of 882 kPa from the gas reservoir. The inlet gas (99.99 vol % CO2) was heated in the gas reservoir at 313 K prior to entering the reactor. In all experiments, the stirring speed in the reactor was 170 rpm. Once the desired temperatures of the gas reservoir and reactor had been reached, the valve was opened to inject CO2 gas into the reactor. In this experiment, the CO2 was injected into the reactor again once equilibrium had been reached. 1H NMR and 13C NMR measurements were performed using a Bruker Avance 500 MHz instrument. The liquid samples for NMR measurement were prepared in NMR sample tubes by adding the solvent deuterium oxide (D2O). D2O was also used as the internal standard reference for 1H NMR measurements. The samples were prepared by mixing the absorbents and D2O (absorbent/D2O; 1H NMR: 30 μL/500 μL; 13CNMR: 300 μL/200 μL). The 1H NMR and 13C NMR spectra were obtained with a delay time (D1) of 1 and 120 s and the number of scans (NS) of 32 and 64, respectively. 1,4-Dioxane was used as an external standard reference for the 13C NMR measurements. The D2O and 1,4-dioxane peaks were observed at 4.8 ppm in the 1H NMR spectra and at 67.0 ppm in the 13C NMR spectra, respectively. The protons in the amino group (primary amine: RNH2; secondary amine: RR′NH) exchanged with deuterium upon the addition of D2O. Therefore, the peaks of these protons were absent from the 1H NMR spectra. The specific heat capacity (Cp) data obtained from a differential scanning calorimeter (DSC) were used to calculate accurate heats of the reaction using a differential reaction calorimeter (DRC) software.27,28 The specific heat capacity data can also be used to design the heat exchanger in the absorption process. The specific heat capacities of aqueous 10 wt % amine solutions (fresh solution) were measured using DSC techniques (using a μDSC3 Evo from Setaram Co.). The DSC device was operable in the temperature range from 273.15 to 393.15 K. The DSC measurements involved two runs using continuous methods with two standard batch-type cells: a reference

of molecular structural variations on the CO2 absorption characteristics of seven heterocyclic amines. Aqueous solutions of PZ and methyl substituted derivatives, hexahydropyrimidine (HHPY) and methyl substituted derivatives, and hexahydropyridazine (HHPZ) have been tested in absorption experiments involving ATR-FTIR analysis. Among these amines, HHPY was the best compromise with regards to high CO2 loading, carbamate formation, hydrolysis to bicarbonate, and water solubility for the pulverized coal combustion (PCC) system. Singh et al.18,19 measured the CO2 loading and the relative rate of the various amines to determine the structure−activity relationship of the absorbents. They investigated the effect of chain length on the performance of alkanolamines, alkylamines, and diamines and the effect of the number of functional groups on the CO2 loading and absorption rate using a screening apparatus. A decrease in the initial absorption rate and an increase in the CO2 loading resulted from an increase in chain length in alkylamines except for six carbon chain length amine based absorbents. Hexamenthylenediamine (HMDA) had a high CO2 loading and an absorption rate with a high solubility in water. In this study, aqueous solutions of four aliphatic diamines, PZ, 2-methylpiperazine (2-MPZ), homopiperazine (HomoPZ), and HMDA, were tested for their utility in CO2 capture. It was found that these amines showed excellent characteristics of CO2 absorption;14−17,20−23 however, experimental data (CO2 loading, specific heat capacity, and heat of absorption) and considerations of the absorption mechanism were insufficient. Nuclear magnetic resonance spectroscopy (NMR) has been powerfully used to analyze qualitative and quantitative results of CO2 absorbent samples. CO2 absorption mechanisms of aqueous PZ and HomoPZ solutions have been determined by investigation of the species distribution study using NMR in the literature.14,16 CO2 absorption mechanisms of aqueous 2MPZ solutions were investigated by a speciation study using NMR in this report. Oexmann et al. suggested that the calculation of the overall reboiler heat duty in the process consisted of the sum of three terms:24,25 Q reb = Q sens + Q vap,H O + Q abs,CO 2

2

(1)

where Qsens is the heat required to raise the temperature of the absorbent downstream of the heat exchanger to the reboiler temperature (sensible heat), Qvap,H2O is the heat of vaporization required to produce stripping steam in the stripper, and Qabs,CO2 is the heat of CO2 absorption. Qsens and Qvap,H2O were determined by cycling through the absorption/stripping process and these values changed, depending on the operating conditions (the ratio of absorbent flow rate and flue gas flow rate, L/G).25 Qabs,CO2 is the heat required to desorb the CO2 from the absorbent. The same amount of heat released from the CO2 absorption should be provided for the reverse reaction. The heat of absorption data of this study can be used to calculate the overall reboiler heat duty in the process. Furthermore, the results can be used to develop new absorbents and processes.

2. EXPERIMENTAL SECTION The regents, PZ (Samchun Chemicals, 99%), 2-MPZ (Acros, 98%), HomoPZ (Alfa, 98%), HMDA (Sigma-Aldrich, 98%), and MEA 2583

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Figure 1. CO2 absorption capacities of 30 wt % diamine solutions: (a) PZ, (b) 2-MPZ, (c) HomoPZ, and (d) HMDA. The emitted gas from the reactor was transferred to the GC at a flow rate of 55 mL/min using a MFC from Brooks, Japan (5850E model, max. range 100 mL/min). It assumed that the absorbents chemically absorbed CO2 and N2 did not react with the absorbents. The principles underlying the DRC technique were described in a previous study.27,28 Experimental procedures and setup parameters for the CO2 loading and heat of absorption measurements were described in detail in the literature.5,29

cell and a measuring cell. The cells were made of Hastelloy C276 and were composed of a cylinder with a 6.4 mm internal diameter and a 19.5 mm height. The sample mass was 350−400 mg for every measurement. A constant stream of dry N2 gas was used to avoid steam concentration in the calorimetric wall. The first run was performed using two reference cells (two empty cells, blank test). The second run was performed using a reference cell and a measuring cell (an empty cell and a sample cell, sample test). Each run involved a ramp between an initial temperature Ti (300.65 K) and a final temperature Tf (355.65 K) at a constant heating rate β (0.2 K min−1). Cp (kJ kg−1 K−1) was calculated according to eq 2:

Cp =

60(A s − A b) msβ

3. RESULTS AND DISCUSSION 3.1. CO2 Absorption Capacity. The CO2 absorption capacities of the aqueous 30 wt % diamine solutions were measured at three temperatures: 313, 333, and 353 K. The experimental CO2 absorption capacities of the aqueous diamine solutions are shown in Figure 1. The graphs show the CO2 absorption capacity with two different units of mol CO2/kg solution and mol CO2/mol amine according to changes in Co/ Ci. Ci and Co values are the CO2 concentrations of the inlet gas (30%) and the outlet gas emitted after passage through the absorbent, respectively. Co/Ci is the ratio between the two gas concentrations. The absorption rate of the aqueous 2-MPZ solution was slower than that of PZ.15 Aqueous 2-MPZ solution had the highest difference of the amount of CO2 absorbed between the temperatures of 313 and 353 K. The result showed that the 2-MPZ solution had the possibility of better regeneration characteristics at the relatively low temperature. These characteristics of the aqueous 2-MPZ solution are related to steric hindrance caused by the methyl group. The CO2 loadings of the absorbents are presented in Table 1. CO2 loading is defined as the moles of absorbed CO2 per moles of amine. The moles of absorbed CO2 were calculated using GC data. The CO2 loadings were calculated as the average experimentally determined values from three independent runs.

(2)

where As and Ab are the heat flows (mW) of the sample test and blank test and ms is the sample mass (mg). The heats of CO2 absorption (−ΔHabs) of the aqueous 10 wt % amine solutions were measured using a DRC (Setaram Co.). Among diamines tested in this study, some diamines have low solubility in water (PZ: 150 g/1 L; HomoPZ: unknown). Solid or slurry formations cause problems such as clogging pipes and shutdown of the process. For these reasons, a lower amine concentration (10 wt %) was used in this experiment. This restricted condition of no solid formation will be useful for another absorbent study such as blending with methyldiethanolamine (MDEA) or potassium carbonate (K2CO3). The DRC experiments involved two glass vessels: a reference reactor and a measuring reactor. The temperatures of the two reactors were measured and recorded over time. The signal of temperature difference (ΔT) between the two reactors was automatically calculated and used to calculate the heat of absorption. The mass of fresh absorbent used in this measurement was 150 g for each reactor. Impurities were removed from the reactor by injecting N2 gas (99.999 vol %) over 40 min. Subsequently, 30 vol % CO2 (CO2 + N2) was injected at a rate of 150 mL/min using a MFC from Brooks, Japan (5850E model, max. range 500 mL/min). In the DRC experiment, the gases after contact with absorbent were emitted from the reactor; therefore, CO2 concentration in the gas phase was measured by GC. 2584

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Standard uncertainty of the mean (SU) and the relative standard uncertainty percent (%RSU) were calculated by eqs 5 and 6 (A-type method).

Table 1. CO2 Absorption Capacities in Aqueous Diamine Solutions CO2 loading (mol CO2/mol amine) absorbent (wt %)

temperature (K)

average value

SD

%RSDa

313 333 353 313 333 353 313 333 353 313 333 353

0.775 0.700 0.598 0.770 0.672 0.532 0.839 0.814 0.744 1.108 0.997 0.890

0.002 0.009 0.007 0.009 0.001 0.015 0.014 0.004 0.008 0.014 0.002 0.019

0.197 1.216 1.206 1.132 0.086 2.876 1.669 0.511 1.044 1.265 0.232 2.173

PZ 30

2-MPZ 30

HomoPZ 30

HMDA 30

a

SU =

%RSU =

The standard deviation (SD) of probability distribution was calculated using the following equation: n

∑i = 1 (Xi − X̅ )2 n−1

(3)

where Xi and X̅ are the individual and mean values of the samples and n is the number of samples tested. The relative standard deviation percent (%RSD) was calculated as follows:

%RSD =

SD × 100 X̅

SU × 100 X̅

(5)

(6)

The SD, %RSD, and %RSU of the CO2 loadings are also listed in Table 1. The %RSD of CO2 loadings of the aqueous 30 wt % diamine solutions were 0.086−2.876%. CO2 loadings of the aqueous 30 wt % diamine solutions at 313 K for amine (CO2 loading) followed the order: 2-MPZ (0.770), PZ (0.775) < HomoPZ (0.839) < HMDA (1.108). In the case of monoamine, the proton transfer takes place from the zwitterions (RNH2+COO−, R2NH+COO−) which are generated from the reaction of the amine and CO2 to other amine molecules to form a protonated structure (RNH3+, R2NH2+). The theoretical CO2 loadings of the primary and secondary monoamines are, therefore, limited to approximately 0.5 mol CO2/mol amine.30 Aqueous PZ, 2-MPZ, HomoPZ, and HMDA solutions can theoretically reach CO2 loading of 1.0 mol CO2/mol amine because two amino groups are present in each molecule; however, CO2 loading can increase or decrease because it is affected by the experimental conditions (e.g., temperature, pressure, and amine concentration) and molecular structures of the amines. Singh et al. reported that 2.6 M MEA had higher CO2 loading than that of 5 M MEA at 313 K.18 2.5 M primary diamines had high CO2 loading (ethylenediamine: 1.08; 1,3-diaminopropane: 1.30; 1,4-diaminobutane: 1.26; hexamethylenediamine: 1.48; 1,7-diaminoheptane: 1.34).18 The CO2 loadings of the aqueous diamine solutions decreased

%RSU of CO2 loading in the semibatch-type reactor: ± 0.1−2.2%.

SD =

SD n

(4)

Figure 2. Molecular structures of the species in the 2-MPZ-H2O-CO2 system. 2585

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Energy & Fuels with increasing temperature over 313−353 K. The reaction between CO2 and an amine can easily occur at lower temperatures because it is an exothermic reaction. The physical solubility and diffusivity of CO2 into absorbents increase as the temperature decreases or amine concentrations are decreased.31−33 3.2. Species Distribution. NMR spectroscopy was used to determine the species present in the aqueous PZ and HomoPZ solutions after CO2 absorption.11,14,16 No previous studies have examined the species or discussed CO2 absorption mechanisms in aqueous 2-MPZ solutions using 1H NMR and 13 C NMR. Two-dimensional (2D) and one-dimensional (1D) NMR techniques were used to determine the relationships between the protons and carbons in the species present in the aqueous 2-MPZ solution. Information about the coupling between protons can be obtained using the correlation spectroscopy (COSY) 2D NMR technique. Information about direct coupling, i.e., the connectivity between the protons and the carbons, can be obtained using a heteronuclear multiple quantum coherence (HMQC) 2D NMR technique. The species present in the 2-MPZ-H2O-CO2 system are illustrated in Figure 2, and the COSY and HMQC spectra of 2MPZ are shown in Figure 3. Three protons in the methyl group of 2-MPZ, as shown in Figure 3a, gave rise to a peak group (7H in Figure 2), that appeared as a doublet through coupling to 6H. The peak area of this group of protons (7H) was three times that of 5Hax. 5Hax was coupled with 6H and 5Heq, so the 5Hax peak appeared as a quartet. The subscripts ax and eq refer to axial and equatorial, respectively. 2Hax was coupled with 3Hax, 3Heq, and 2Heq, so the 2Hax peak appeared as a sextet. The peak area of the 5Hax was similar to that of 2Hax. It was difficult to determine the exact positions of other protons due to peak overlap, but the rough locations could be estimated using 2D NMR results and coupling constants (J) (Figure S1, Supporting Information). Typical 1H NMR and 13C NMR spectra of the aqueous 2MPZ solution after CO2 absorption are shown in Figure 4. The results of the 2-MPZ carbamate (2-MPZCOO−, H +2MPZCOO−) were similar to those obtained for 2-MPZ. Figure 4a shows that three protons in the methyl group of 2-MPZ appeared in a peak group (7H) as a doublet at 1.157−1.144 ppm. Furthermore, three protons in the methyl group of 2MPZ carbamate appeared in a peak group (7′H) at 1.249− 1.236 ppm. In the 1H NMR spectrum, the peaks of the protons of 2-MPZ carbamate appeared at a lower field strength than those of 2-MPZ. Interestingly, the 3′Heq and 5′Heq peaks, which were close to those of CO2, appeared at a low field due to deshielding by oxygen atoms. Most of the peaks in the 1H NMR spectra overlapped; however, the peaks of the methyl groups of 2-MPZ and 2-MPZ carbamate were clearly distinct in the 1H NMR spectra. The species distribution of the aqueous 2MPZ solution could, therefore, be investigated by qualitatively analyzing the peaks. Unlike the aqueous PZ and HomoPZ solutions,14,16 the aqueous 2-MPZ solution did not form a dicarbamate (2-MPZ(COO−)2), even at high alkalinities. Figure 4b shows that the carbon peaks related to CO2 (8′C and 10C) appeared at low field strengths (above 160 ppm). It was not possible to distinguish between the protonated and unprotonated forms, such as 2-MPZ (2-MPZ and 2-MPZH+), 2-MPZ carbamate (2-MPZCOO− and H+2-MPZCOO−), or bicarbonate/carbonate (HCO3−/CO32−) due to the rapid proton exchange of these species with water.34−36 The 1H NMR and 13C NMR spectra of the aqueous 2-MPZ solutions at

Figure 3. 2D NMR spectra of 2-MPZ at 295 K; (a) COSY spectrum and (b) HMQC spectrum.

various CO2 loadings are shown in Figures S2 and S3, Supporting Information. These spectra revealed the variations in the chemical shifts as a function of the CO2 loading. The pH changed as a function of CO2 absorption causing a change in the chemical shifts of the protonated species (2-MPZH+, H+2MPZCOO−) in the NMR spectra. Bicarbamate (2-MPZ(COO−)2) peaks are not present in the 1H NMR and 13C NMR spectra. 2-MPZ carbamate is adjacent to the methyl group and can be easily hydrolyzed and form bicarbonate. The chemical shifts of the unprotonated species (2-MPZ(COO−)2) would not be changed if the species existed. The amount of bicarbonate/carbonate increased according to the increase in CO2 loading (see Figure S4, Supporting Information). A hindered amine yielded an unstable carbamate, the hydrolysis of which formed bicarbonate. Therefore, two carbon peaks in the low field region were related to the 2-MPZ carbamate and bicarbonate/carbonate. 2586

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Figure 5. DSC curve of water.

Figure 4. NMR spectra of CO2-loaded 7.5 wt % 2-MPZ solution (2MPZ-CO2-H2O system) at 295 K, CO2 loading = 0.928; (a) 1H NMR spectrum and (b) 13C NMR spectrum.

3.3. Specific Heat Capacity. The heat flow curves and Cp values of water and pure MEA are shown in Figures 5 and 6, respectively. The plots shown in Figures 5−7 are the average values for each temperature. The Cp values were measured on the basis of four DSC experiments. The %RSD values of Cp measurements of the water were 0.13−0.40%. The Cp values were slightly higher than literature values; however, the difference was very small, so that the Cp values measured in this study were within the margin of error range of the literature values.37−40 The deviations were similar to those reported by Harris et al. (0.04−0.26%)39 but were smaller than those reported by Chiu et al. (1.46−1.62%).38 The %RSDs of Cp measurements of the MEA were 0.25−2.42%. The Cp values of the 10 wt % fresh solutions are shown in Figure 7 and Tables S3−S7, Supporting Information. Figure 7 shows that the Cp values increased according to PZ < MEA, HomoPZ < 2-MPZ < HMDA. The %RSD of each measurement was: MEA 10 wt % (0.09−0.40%), PZ 10 wt % (0.09− 0.32%), 2-MPZ 10 wt % (0.09−0.49%), HomoPZ 10 wt %

Figure 6. Comparison with the specific heat capacities in the literature: (a) water and (b) MEA.

(0.06−0.29%), and HMDA 10 wt % (0.06−0.49%). The sensible heat (Qsens) related to the overall reboiler heat duty in the absorption/desorption process could be reduced for small absorbent Cp values.24,25 Chiu et al.,38 Harris et al.,39 Chen and Li,41 and Song et al.42 reported a simple relationship between the Cp values and the temperature: 2587

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MEA solution was 86 kJ/mol at CO2 loading = 0.573 (CO2saturated solution). Kim and Svendsen gave the −ΔHabs integral value of aqueous 30 wt % MEA solution of 81.603 kJ/mol CO2 at CO2 loading = 0.507, T = 313 K, and P = ∼300 kPa.43 A differential reaction calorimeter (DRC) consisted of two reactors (reference and measurement) to obtain the integral heats of absorption data in this study, whereas Kim and Svendsen used a single reactor calorimeter. The aqueous 30 wt % MEA solution reached a CO2 loading of 0.573 in the DRC experiment in this study. Our partial pressure of CO2 was slightly higher than Park et al.’s44 data at 0.52 loading because of differences in the experimental conditions. However, the solubility data at 0.57 loading was in good agreement with literature data of Park et al. (this study: partial pressures of CO2 of 34.5 kPa at 0.573 loading; Park et al.:44 partial pressure of CO2 of 34.5 kPa at 0.57 loading). The −ΔHabs integral values of aqueous 10 wt % diamine solutions at 313 K are shown in Figure 8 in comparison with

Figure 7. Specific heat capacities of aqueous diamine solutions at various temperatures.

Cp (kJ·kg −1·K−1) = a + bT

(7)

where a and b are correlation constants and T is the temperature (K). The results of 10 wt % amine solutions did not fit well to eq 5. Another equation (eq 6) was used in this study, and the regression results are listed in Table 2. The temperature dependence of the Cp values of these aqueous amine solutions could be described as follows: Cp (kJ·kg −1·K−1) = a + bT + cT 2 + dT 3

(8)

The parameters of eq 8 and %RSD of the measurements were also presented in Table 2. 3.4. Heat of Absorption. In general, the heat of reaction between CO2 and an absorbent is represented in terms of the enthalpy change (−ΔHabs, kJ/mol CO2). Researchers presented the integral and/or the differential heat of absorption data using different methods. These data can be obtained from the direct measurements for enthalpy values or from experimental data on partial pressure of CO2 over absorbents (VLE data) using the Gibbs−Helmholtz equation.43 Direct calorimetric measurement provides accurate heat of absorption data with enthalpy values; however, there are limited direct calorimetric measurement data. Kim and Svendsen measured differential enthalpies of absorption of CO2 with 30 wt % MEA and converted them to integral values by integration.43 In this study, the −ΔHabs values are integral values that represent the total heat released from zero loading to the chosen loading point. We obtained integral values from DRC software (SETARAM Co.). The −ΔHabs integral value of aqueous 30 wt % MEA solution was 78 kJ/mol at CO2 loading = 0.506, T = 313 K, and P = 115 kPa. Furthermore, the −ΔHabs integral value of aqueous 30 wt %

Figure 8. Comparison of the heats of CO2 absorption of aqueous 10 wt % MEA, PZ, 2-MPZ, HomoPZ, and HMDA solutions at 313 K.

that of aqueous MEA solution. Primary amines (MEA and HMDA) maintained high −ΔHabs integral values because these amines reacted with CO2 at a rapid rate to form carbamates (RNHCOO−). Secondary diamines (PZ, 2-MPZ, and HomoPZ) yielded relatively lower −ΔHabs integral values compared to the values obtained from the primary diamine, HMDA. Heats of absorption data at various CO2 loadings were presented in Table 3. Calories (−Q) (kJ), which are accumulated heat, were linearly increased when CO2 loading increased; however, the −ΔHabs (kJ/mol CO2) integral values did not follow this tendency.45 At a low CO2 loading (CO2 loading = 0.2), the −ΔHabs integral values of amine solutions

Table 2. Regression Parameters of Specific Heat Capacity for Aqueous Amine Solutions Cpa measurement

a

a

a

−3 a

−6 a

absorbent (wt %)

a

b

10 c

10 d

R2

SD

%RSD

MEA 10 PZ 10 2-MPZ 10 HomoPZ 10 HMDA 10

−36.5422 −37.2869 −35.6884 −25.8524 −70.1763

0.3741 0.3781 0.3648 0.2726 0.6869

−1.1514 −1.1540 −1.1154 −0.8287 −2.1134

1.1857 1.1776 1.1391 0.8430 2.1663

0.9682 0.9897 0.9879 0.9754 0.9398