Investigation of Thermodynamic Properties on CO2 Absorbents

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

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Investigation of Thermodynamic Properties on CO2 Absorbents Blended with Ammonia, Amino Acids, and Corrosion Inhibitors at 313.15, 333.15, and 353.15 K Yunsung Yoo, Dongwoo Kang, Injun Kim, and Jinwon Park* Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea

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S Supporting Information *

ABSTRACT: The wet absorption method is currently one of the key technologies employed for the capture of carbon dioxide. In this method, the selection of an optimal absorbent is of particular importance. As such, we herein propose a series of novel and efficient absorbents based on a 7 wt % NH3 solution containing 3 wt % amino acid (β-alanine or L-arginine) and 1 wt % corrosion inhibitor (imidazole or 1,2,3-benzotriazole). To investigate the thermodynamic properties including density, viscosity, diffusivity of CO2, kinetic constant, rate equation, and physical and chemical CO2 solubility data on capturing carbon dioxide in these absorbents, various experiments were conducted on each absorbent at 313.15, 333.15, and 353.15 K. All of the proposed absorbents showed better capacity than monoethanolamine which has been most widely used as a wet absorbent. Among them, the absorbents with added L-arginine were found to be superior to absorbents with added β-alanine.

1. INTRODUCTION The optimal terrestrial temperature that allows living organisms to survive on Earth is maintained by a mild greenhouse effect, without which the average surface temperature of the Earth would be ∼253.15 K.1 However, this same greenhouse effect can lead to problems when the concentrations of greenhouse gases exceed a key level, as this can lead to global warming. Such effects can promote phenomena such as glacier retreat, seasonal changes, etc.2−4 Thus, to prevent further increases in global temperatures, the concentrations of greenhouse gases in the atmosphere should be reduced. One of the most prominent greenhouse gases is carbon dioxide, which mainly accumulates due to anthropogenic emissions. As such, significant efforts have focused on the global reduction in carbon dioxide emissions, thus leading to the development of carbon capture and storage (CCS) technologies. More specifically, CCS technologies permit the capture of carbon dioxide followed by its subsequent transport to an adequate site where the CO2 can be stored deep underground or in the ocean crust. In this sight, CCS technology can be said to be comprised of capture technology and storage technology.5 One of the key methods reported to date for CO2 capture is the © XXXX American Chemical Society

wet absorption method, which exhibits potential for successful application in large and commercial scale CO2 capture. In this method, CO2 is captured using aqueous absorbents or liquidtype absorbents. However, due to differences in plant design, the efficiency of the total process and its resulting economic feasibility depend on the characteristics of the absorbents, and as such, the selection of an optimal absorbent is of particular importance. Conventionally, amines like monoethanolamine (MEA) and diethanolamine (DEA) have been employed as absorbents. In a typical amine-based process, an absorber and a desorber are employed, where both absorption and desorption reactions take place. These various drawbacks make it hard for amine based processes to be commercialized or applied in real industrial plants. Even though it is commercialized, there are many problems to be overcome. For example, due to high costs and the large amounts of energy required to regenerate the absorbents, this process is economically unfeasible compared to systems which employed other types of absorbent. In addition, Received: March 5, 2018 Accepted: July 11, 2018

A

DOI: 10.1021/acs.jced.8b00175 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

amine-type absorbents are vulnerable to degradation, and so their durability is comparably lower than those of other absorbents. As such, in order to make up for some drawbacks of amine-type absorbents in large-scale CO2 disposal, improved absorbent development would be needed.6 To overcome these issues, alternative absorbents based on ammonia have been examined, and so far, these have exhibited a number of advantages over amine systems. For example, compared with energy consumption in the desorption process (3.1 GJ/tCO2) of 30 wt % aqueous MEA solution, which is wellknown for the wet absorbent that is widely used in CO2 capture, that of the 7 wt % NH3 absorbent has been reported less than 2 GJ/tCO2.7,8 In addition, while 30 wt % aqueous MEA solution required 100% of regeneration energy, 7 wt % NH3 absorbent in H2O required only 45% of regeneration energy. Whereas, higher CO2 absorption performances have also been reported.9,10 Also, ammonia has a low cost, suitability for the removal of other pollutants such as SO2 and NOx, and a lack of corrosion issues, with the exception of Cu-containing materials. Moreover, ammonia does not degrade in the presence of O2.11−13 However, due to the high volatility of ammonia, adequate condensation and reaction rates must be considered. To improve reaction rates in the ammonia system, the addition of rate promoters such as sodium hydroxide, 1aminocyclohexanecarboxylic acid, and piperazine has been considered.14,15 Among the various rate promoter candidates, amino acids have received particular attention, as they play a key role as an activating agent for the CO2 absorption reaction. In addition, amino acids are considered environmental friendly materials, and so they do not contribute to environmental pollution. Moreover, amino acids exhibit high resistance to oxidative degradation, as well as a good absorption capacity for CO2 capture due to the presence of amine functional groups.16,17 Reaction between aqueous ammonia solution and CO2 can be described as in reactions R1−R5.18 NH3 + H 2O ↔ NH+4 + OH−

(R1)

CO2 + H 2O ↔ HCO−3 + H+

(R2)

HCO−3 ↔ CO32 − + H+

(R3)

NH3 + HCO−3 ↔ NH 2COO− + H 2O

(R4)

+

H 2O ↔ H + OH

−

Thus, we herein report our investigations into the solubility of carbon dioxide in blended solutions prepared by mixing ammonium hydroxide solution (28% NH3 in H2O), an amino acid (β-alanine or L-arginine), and a corrosion inhibitor (imidazole or 1,2,3-benzotriazole) based on vapor−liquid equilibrium. Although many different categories of amino acids exist, we selected representative amino acids from the neutral (β-alanine) and basic (L-arginine) amino acid groups to determine their effect on CO2 solubility. Furthermore, in the context of industrial applications, the corrosive properties of ammonia must also be considered, as ammonia is known to corrode copper-containing alloys. Thus, to prevent the corrosion of instruments composed of copper-containing materials, imidazole and 1,2,3-benzotriazole were employed as corrosion inhibitors.21 Indeed, Lee et al.22 reported that the corrosion prevention activity of imidazole and 1,2,3-benzotriazole resulted from the formation of a passivation film, which constituted a physical interaction rather than a chemical interaction. Based on these reports, we conducted research only for thermodynamic data measurement, not for corrosion performance measurement. Four different blended solutions will therefore be examined, which contain 7 wt % NH3 + 3 wt % βalanine + 1 wt % imidazole, 7 wt % NH3 + 3 wt % β-alanine + 1 wt % 1,2,3-benzotriazole, 7 wt % NH3 + 3 wt % L-arginine + 1 wt % imidazole, and 7 wt % NH3 + 3 wt % L-arginine + 1 wt % 1,2,3benzotriazole. Considering the specifications of the existing industrial facilities, absorbent solutions based on 7 wt % NH3 were blended to have similar physical properties with 30 wt % aqueous monoethanolamine (MEA) solution. To enhance the CO2 capture performances, amino acids were mixed with these. The concentration of the amino acid was decided considering the chemical properties of the amino acids. When amino acids with high concentration are exposed to the CO2, precipitation reaction may occur, and this may interrupt absorbent recycle. Considering these properties and the concentration of NH3, 3 wt % of amino acids were blended. Throughout the manuscript, u(X) refers to a standard uncertainty of variable X, ur,c(X) refers to a relative combined standard uncertainty, and uc(X) refers to the combined standard uncertainty of variable X. The combined uncertainty was calculated by following equations, ci =

(R5)

uc(X ) =

Also, reaction between amino acid and CO2 is shown in the following reactions R6−R7, where B is base.19 CO2 + H 2N − CHR′ − COO

−

(R6)

COO+H 2N − CHR′ − COO− + B → −COOHN − CHR′ − COO− + BH+

(1)

∑ (ci × u(xi))2

(2)

where ci is the sensitivity coefficient and xi is the estimated value. We investigated the thermodynamic properties including density, viscosity, diffusivity of CO2, kinetic constant, rate equation, and physical and chemical CO2 solubility data for these four solutions at 313.15, 333.15, and 353.15 K. We derived thermodynamic properties of these absorbents by considering them as a single product. As such, we expect that the addition of a corrosion inhibitor will extend the lifetime of any instrument employed during this process, and we hope that the data obtained in this study will aid in process design for novel CO2 capture systems.

−

↔ −COO+H 2N − CHR′ − COO−

∂X ∂xi

(R7)

Since it is reported that the reaction with carbon dioxide is related to the basicity of the substance, amino acids whose basicity is higher than that of ammonium ion were selected and utilized as rate promoters. Among various types of amino acids, β-alanine and L-arginine, which are neutral and basic amino acids, respectively, were selected considering the basicity and CO2 absorption capacity. The order of basicity was L-arginine > β-alanine > ammonia.20

2. EXPERIMENTAL SECTION 2.1. Materials. The ammonium hydroxide solution (28 wt % NH3 in H2O, ≥99.99 wt %, CAS No. 1336-21-6), β-alanine (≥99 wt %, CAS No. 107-95-9), L-arginine (≥99 wt %, CAS No. B



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Table 1. Information for the Samples Used in This Paper purity (mass or mole fraction)a

chemical name

CAS no.

source

monoethanolamine 28 wt % NH3 in H2O β-alanine L-arginine imidazole 1,2,3-benzotriazole carbon dioxide nitrogen nitrogen (high purity) nitrous oxide

141-43-5 1336-21-6 107-95-9 74-79-3 288-32-4 95-14-7 124-38-9 7727-37-9 7727-37-9 10024-97-2

Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Samheung Specialty Gas Samheung Specialty Gas Samheung Specialty Gas Samheung Specialty Gas

mass fraction

0.99 0.9999 0.99 0.99 0.99 0.99 0.999 0.999 0.99999 0.99

mole fraction

analysis methodb GC titration NMR FT-IR titration GC provided by supplier provided by supplier provided by supplier provided by supplier

a The purity in mass or mole fraction was provided by the supplier. bThe analysis method for mass fraction was provided by the suppliers. NMR, nuclear magnetic resonance; FT-IR, Fourier transform infrared; and GC, gas chromatography.

Figure 1. Molecular structures of ammonia, β-alanine, L-arginine, imidazole, and 1,2,3-benzotriazole.

and lower temperature of the coolant can prevent changes in the concentration of the absorbent. 2.2.2. Physical Solubility of CO2 and Diffusivity. As the absorbents employed herein react with carbon dioxide, their physical solubilities and diffusivities cannot be measured directly. Hence, CO2/N2O analogy was used in the calculation of physical solubilities and diffusivities since the molecular structure of N2O is similar to CO2 and they do not react with the absorbent.23 As such, the Henry constant of CO2 in the absorbents was initially determined as outlined in eq 3:

74-79-3), imidazole (≥99 wt %, CAS No. 288-32-4), and 1,2,3benzotriazole (≥99 wt %, CAS No. 95-14-7) were purchased from Sigma-Aldrich. The concentration of the ammonium hydroxide solution was specified by titration using HCl, and the relative uncertainty of the initial NH3 concentration was 0.001. Table 1 presents information on the sample used in this paper. The chemical structures of these materials are provided in Figure 1. Absorbent blends containing 7 wt % NH3, 3 wt % amino acid (β-alanine or L-arginine), and 1 wt % corrosion inhibitor (imidazole or 1,2,3-benzotriazole) in distilled water were then prepared. These blends will herein be referred to as follows: NH3 + β-alanine + imidazole = BALA-IMID, NH3 + β-alanine + 1,2,3-benzotriazole = BALA-BENZ, NH3 + L-arginine + imidazole = LARG-IMID, and NH3 + L-arginine + 1,2,3benzotriazole = LARG-BENZ. 2.2. Methods. 2.2.1. Viscosity. The viscosities of each absorbent blend (i.e., BALA-IMID, BALA-BENZ, LARG-IMID, and LARG-BENZ) were measured at 313.15, 333.15, and 353.15 K using a SV-10 viscometer (AMD). The SV-10 viscometer was equipped with two sensor plates for viscosity measurements. Each sensor plate is rotated by an electromagnetic force at the equal frequency. In order to keep a constant amplitude, the electromagnetic drive regulated the vibration of each plate. The viscosity of the sample fluid can be deduced based on the electric current produced by viscidity between the sensor plates and the sample. To prevent the evaporation of NH3 from the absorbent blends and resulting changes in the blend concentrations, a cool-water condenser (i.e., at 258.15 K) was installed on the reactor. This temperature is lower than the temperature of the coolant when amine-type absorbent is used. This is due to the high volatility of the NH3,

HCO2,absorbent HN2O,absorbent

=

HCO2 ,water HN2O,water

(3)

where HCO2,absorbent is the Henry constant of CO2 in the absorbent, HN2O,absorbent is the Henry constant of N2O in the absorbent, HCO2,water is the Henry constant of CO2 in pure water, and HN2O,water is the Henry constant of N2O in pure water. HCO2,water and HN2O,water were proposed as functions of temperature as indicated in the following equations obtained from the literature.24 HCO2,water (kPa ·m 3/kmol)

i −1984.8 yz zz × P (kPa) = 2.3314 × 104 expjjjj z k T (K) {

C

(4)



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Pvapor(T ) = x H2OPHSat2O(T )

HN2O,water (kPa ·m 3/kmol)

i −2178.1 yz zz × P (kPa) = 6.0456 × 104 expjjjj z (5) k T (K) { To derive HN2O,absorbent, experiments using N2O and the various absorbent blends were conducted. Figure 2 shows a

(7)

where xH2O is the mole fraction of water in solution, and Psat H2O(T) is vapor pressure of pure water. To induce diffusivity, the CO2/N2O analogy was used. For a single gas, it has a constant ratio of CO2 diffusivity into an electrolyte solution or water, and the relationship between the diffusivities was expressed by eq 8: DN2O,absorbent

=

DN2O,water

DCO2,absorbent DCO2,water

(8)

where DN2O,absorbent is the diffusivity of N2O into the absorbent, DN2O,water is the diffusivity of N2O into pure water, DCO2,absorbent is the diffusivity of CO2 into the absorbent, and DCO2,water is the diffusivity of CO2 into pure water. DCO2,water and DN2O,water were proposed as functions of temperature according to eqs 9 and 10:24 i −2196.1 yz zz DCO2,water (m 2/s) = 3.0651 × 10−6 expjjjj z T (K) k {

i −2288.4 yz zz DN2O,water (m 2/s) = 4.009 × 10−6 expjjjj z (10) k T (K) { Furthermore, due to the difficulty in measuring the diffusivity of a gas into a solution, a number of studies regarding the relationship between physical properties and diffusivity have been conducted. For example, the relationship between diffusivity and viscosity was demonstrated according to the modified Stokes−Einstein equation:

Figure 2. Schematic representation of the setup employed for the vapor−liquid equilibrium experiments.

schematic representation of the experimental apparatus employed, where P and T represent the pressure and temperature sensors, and the interfacial area of reactor (A) is 50.27 mL. As shown, the apparatus was divided into two main parts, namely, the gas reservoir and the reactor. In the gas reservoir, only the gas used for injection into the reactor was present. In contrast, the inner part of the reactor contained a gas phase and a liquid phase, both of which were equipped with magnetic stirrers. To maintain a constant reaction area, the stirrer speed was adjusted to ensure that no vortex existed at the interface between the gaseous and liquid phases. The reactor volume was 986 mL, VL was 450 mL, and VG was 536 mL. Initially, the absorbent blend (450 mL) was injected into the reactor, and any gas within the system was removed by means of a vacuum pump connected to the top of the reactor. The temperature inside the reactor was then maintained at the desired temperature (i.e., 313.15, 333.15, or 353.15 K) using a water bath. The initial injection pressure of N2O, the equilibrium pressure, and the temperature within the reactor were recorded, and the quantity of absorbed gas was derived using the ideal gas law. Subsequently, HN2O,absorbent was calculated using eq 6:25,26 HN2O,absorbent(T ) =

Peq(T ) − Pvapor(T ) Pinit − Pvapor(Tinit) Tinit

−

Peq(T ) − Pvapor(T ) T

(9)

Dηα = constant

(11)

where η is the solution viscosity, D is the diffusivity, and α is a constant determined by the type of diffusion gas and solution employed. In this case, a value of 0.8 was used for α.27 Furthermore, DN2O,absorbent was calculated using eq 11 and DCO2,absorbent was calculated using eq 8. 2.2.3. Measurement of the CO2 Absorption Kinetics. The molar flow of CO2 into the absorbent was calculated using eq 12:28 nCO ̇ 2 = E · kL

PCO2 HCO2

A (12)

where ṅCO2 is the molar flow, E is the enhancement factor, kL is the physical mass transfer coefficient (m/s), HCO2 is the Henry constant for CO2 in the absorbent (kPa m3 kmol−1), and A is the reaction area between CO2 and the absorbent (m2). More specifically, E was defined as the contribution of the chemical reaction to enhancement of the mass transfer rate in the absorption of a gas by an absorbent. In other words, as the rate of the chemical reaction increases, E increases. Therefore, E was expressed according to eq 13:29

ij RVL yz jj z jj V zzz k G {

(6)

E=−

where, VG and VL are the volume of the gaseous and liquid phases, respectively, and P is expressed as a function of T, where Pvapor is the vapor pressure of the absorbents, and Peq and Teq are the pressure and temperature at which the inner part of reactor reached equilibrium. In addition, R is the universal gas constant (8.314 J K−1 mol−1), Pinit is the initial injection pressure of N2O in the gas reservoir, and Pvapor(T) was calculated using Raoult’s law:

Ha 2 + 2(E∞ − 1)

E ·Ha 2 Ha 4 + ∞ +1 2 E∞ − 1 4(E∞ − 1) (13)

where E∞ is the infinite enhancement factor and Ha is the Hatta number, which is defined as outlined in eq 14: Ha = D

kovDCO2 kL

(14) DOI: 10.1021/acs.jced.8b00175 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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where kov is the overall reaction kinetic constant (s−1) and DCO2 is the diffusivity of CO2 into the absorbent. Based on the penetration model, E∞ can be expressed as follows:30 i yz zz DCO2 jjjj D C RT zz Ab Ab jj1 + × z j P CO2 z DAb jjj DCO2 vCO2 × H zzz CO2 { k

E∞ =

the reaction was heated to the desired temperature (i.e., 313.15, 333.15, and 353.15 K) using a temperature-controlled air bath, after which CO2 gas (100−1500 kPa final pressure) was injected into the CO2 reservoir, and the injection temperature (Ti) and pressure (Pi) in the CO2 reservoir were recorded. Upon reaching the desired temperature, the vapor pressure (Pvapor) was measured, and the desired volume of CO2 gas was supplied to the reactor by opening the valve located between the reactor and the CO2 reservoir. To rapidly achieve equilibrium, the magnetic stirrer was set at 60 rpm, although stirring was temporarily stopped prior to CO2 injection to prevent the excess injection of CO2. Once equilibrium was established, the pressure (Pf) in the CO2 reservoir was recorded along with the temperature (Treactor) and pressure (Preactor) in the reactor. For the purpose of this study, the CO2 solubility was expressed as α (mol CO2/mol absorbent). These steps were repeated until 10 points of PCO2 (kPa, the partial pressure of carbon dioxide in the gas phase) versus α were obtained. This experimental process was repeated for all absorbent preparations (BALA-IMID, BALA-BENZ, LARG-IMID, and LARG-BENZ) at the desired temperatures. The densities for the exact calculations of the α values for each absorbent were measured using a DMA4500 M density meter (Anton-Paar). The standard uncertainty of the DMA 4500 M density (ρ) meter was 0.001 g·cm3. To confirm the accuracy of this system, a blank experiment was also carried out using a 15.3 wt % monoethanolamine solution (MEA, in H2O) alone, and the results were compared with that of the reference literature at 313.15 K.31−34 In addition, to modify the nonideality of carbon dioxide, the recorded pressures and temperatures were treated with the Pitzer correlation model for the second virial coefficient, as indicated in eqs 20−26 below:

(15)

where DAb is the diffusivity of the absorbent, νCO2 is the stoichiometric coefficient, and CAb is the absorbent concentration. When the ratio of E∞ to Ha is large (3 < Ha ≪ E∞), the reaction of CO2 with the absorbent could be considered as a pseudo-first order reaction, and so Ha is equal to E. As such, eq 16 could be expressed as follows:28 nCO ̇ 2=

kovDCO2

PCO2 HCO2

A (16)

CO2 absorption kinetics experiments were then conducted using a similar process to that of the N2O solubility experiments. Thus, the absorbent (450 mL) was injected into the reactor, and any gases present in the reactor were removed by means of a vacuum pump. The temperature of the reactor was maintained at the desired temperature (i.e., 313.15, 333.15, and 353.15 K) using a water bath. The initial injection pressure of CO2, the equilibrium pressure, and the temperature were recorded, and the amount of absorbed gas was derived using the ideal gas law. In addition, the reactor pressure, the decrease in pressure during absorption, and the temperature were recorded using the CIMOM X program (KDT Systems Co.). As the reaction taking place in this process was considered a pseudo-first order reaction, the overall reaction constant (kov) could be expressed according to eq 17:

Z=1+

ln(PCO2,time) = ln(Pt − Pvapor) = ln(P0 − Pvapor) −

kovDCO2

RT A·t VGHCO2

(21)

B̂ = B0 + ωB1

(22)

(17)

S=

kov =

kovDCO2

RT A VGHCO2

B0 = 0.083 − B1 = 0.139 −

(18)

(SVGHCO2)2 DCO2(RTA)2

Tr =

T Tc

Pr =

P Pc

(19)

2.2.4. Chemical Solubility of CO2. In this section, the experimental apparatus shown in Figure 2 was also used. The experimental procedure was as follows. First, the absorbent blend (250 g) was prepared using distilled water 177.5 g), ammonium hydroxide solution (28 wt % NH3 in H2O, 62.5 g), the desired amino acid (7.5 g), and the corrosion inhibitor (2.5 g), prior to injection into the reactor vessel using a vacuum pump. The standard uncertainty in the weight (m) measurement was 0.0001 g. Any impurities in the reactor, such as air or other contaminant gases, were removed using a brief application of a vacuum pump to prevent loss of ammonia. Also, to make composition of the vapor phase be pure CO2 in the CO2 reservoir, CO2 purging was conducted for 10 min. Subsequently,

(20)

BPc RTc

B̂ =

where PCO2,time was the partial pressure of CO2 at a specific time. When the above equation was considered as a function of PCO2 and time, the slope of the function (S) and kov could be determined as outlined below:

P BP = 1 + B̂ r RT Tr

0.422 Tr1.6

(23)

0.172 Tr4.2

(24)

(25)

(26) 0

1

where Z is a compressibility factor, B and B are parameters for the Pitzer correlation, ω is the acentric factor (0.228 for CO2), Tc is the critical temperature, Pc is the critical pressure, Tr is the reduced temperature, Pr is the reduced pressure, and R is the gas constant. In addition, for carbon dioxide Tc = 304.25 K and Pc = 7,383 kPa. Based on this Pitzer correlation, the α values were derived for 7 wt % NH3 solution (in H2O) and for each absorbent at the desired temperature in the reactor and the CO2 reservoir, respectively. The pressures inside the cylinder and the reactor were measured using a Wallace & Tiernan precision digital barometer with an accuracy of ±0.1 kPa, while the E



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various absorbent blends are plotted against 1000/T (K), as shown in Figure 3. The combined standard uncertainty of the Henry constants uc(H) = 6.5 kPa m3 kmol−1.

temperature was measured using a Pt-100 temperature probe with an accuracy of ±0.1 K. The pressure and temperature of the reactor were continuously recorded using a data recorder connected to the apparatus. The relative combined standard uncertainty of the CO2 solubility (α) measurement was 0.025. The experimental apparatus and measurement methods employed were described in our previous study.35

3. RESULTS AND DISCUSSION 3.1. Viscosity. The viscosities of the various absorbent blends measured at 303.15, 313.15, 323.15, and 333.15 K are presented in Table 2, where the viscosity of water was obtained from the literature.36 Table 2. Viscosities, ηa × 103 (kg m−1 s−1), of the Various Absorbents at Different Temperature, Tb (K) = 313.15, 333.15, and 353.15, and at Ambient Pressure, Pc (kPa) = 101.3 ηa × 103 (kg m−1 s−1) Tb (K) absorbent

313.15

333.15

353.15

water36 BALA-IMIDd BALA-BENZd LARG-IMIDd LARG-BENZd

0.6535 0.9481 1.1319 1.0291 1.0773

0.4666 0.6832 0.8533 0.7384 0.8016

0.3551 0.4593 0.4931 0.6027 0.5788

Figure 3. Henry constants of CO2 in the four absorbent blends at 313.15, 333.15, and 353.15 K.

The diffusivity coefficients of CO2 and N2O in water were derived according to eqs 9 and 10, while the diffusivity coefficients of CO2 and N2O in the four absorbent blends were derived using eqs 8 and 11 (see Table 4 and Figure 4). The relative combined standard uncertainty of diffusivity coefficient ur,c(D) = 0.08 3.3. Kinetic Measurements. As the reaction between CO2 and the various absorbents occurs at the gas−liquid interface, the reaction involves both a chemical reaction (i.e., absorption) and physical diffusion. In this case, the chemical reaction occurs only at the gas−liquid interface and the physical diffusion of the reactant to the interface is slower than the chemical reaction. As such, the chemical reaction reaches equilibrium prior to completion of the physical diffusion process. The chemical reaction was therefore considered to be the rate-determining step, as indicated in eq 27:

a

The relative uncertainty of viscosity ur(η) = 0.031. bThe standard uncertainty of temperature u(T) = 0.1 K. cThe standard uncertainty of pressure u(P) = 1 kPa. dBALA-IMID: 7 wt % NH3 + 3 wt % βalanine + 1 wt % imidazole + 89 wt % H2O. BALA-BENZ: 7 wt % NH3 + 3 wt % β-alanine + 1 wt % 1,2,3-benzotriazole + 89 wt % H2O. LARG-IMID: 7 wt % NH3 + 3 wt % L-arginine + 1 wt % imidazole + 89 wt % H2O. LARG-BENZ: 7 wt % NH3 + 3 wt % L-arginine + 1 wt % 1,2,3-benzotriazole + 89 wt % H2O. The standard uncertainty of these mixture compositions is u(ξ) = 0.001.

3.2. Physical Solubility of CO2 and Diffusivity Coefficients. The physical solubilities of CO2 and N2O (i.e., the Henry constants) in water were derived according to eqs 4 and 5, while the physical solubilities of CO2 and N2O in the absorbent were derived using eqs 3 and 6. The derived Henry constants for N2O and CO2 in water and in the absorbents are outlined in Table 3, and the Henry constants for CO2 in the

−rCO2 = kovCCO2

(27)

To obtain the above CO2 absorption rate equation (i.e., the rate equation for the chemical reaction), the value of the overall

Table 3. Henry Constants, Ha (kPa m3 kmol−1), of N2O and CO2 in Water and in the Different Absorbent Blends at Different Temperature, Tb (K) = 313.15, 333.15, and 353.15, and at Ambient Pressure, Pc (kPa) = 101.3 Ha (kPa m3 kmol−1) Tb (K) 313.15 absorbent water24 BALA-IMIDd BALA-BENZd LARG-IMIDd LARG-BENZd

N2Oe

333.15

CO2e 5764 5703.3 5996.8 5584.4 6276.7

N2Oe 4126 4077.4 4287.2 3992.5 4487.3

353.15

CO2e 8751.1 9013.0 9694.3 9217.6 9857.2

N2Oe 6036 6209.2 6678.5 6350.1 6790.7

CO2e 12672.7 12134.7 12500.7 12874.4 13885.1

8457.7 8089.5 8333.5 8582.6 9256.4

a The combined standard uncertainty of Henry constants uc(H) = 6.5 kPa m3 kmol−1. bThe standard uncertainty of temperature u(T) = 0.1 K. cThe standard uncertainty of pressure u(P) = 1 kPa dBALA-IMID: 7 wt % NH3 + 3 wt % β-alanine + 1 wt % imidazole + 89 wt % H2O. BALA-BENZ: 7 wt % NH3 + 3 wt % β-alanine + 1 wt % 1,2,3-benzotriazole + 89 wt % H2O. LARG-IMID: 7 wt % NH3 + 3 wt % L-arginine + 1 wt % imidazole + 89 wt % H2O. LARG-BENZ: 7 wt % NH3 + 3 wt % L-arginine + 1 wt % 1,2,3-benzotriazole + 89 wt % H2O. The standard uncertainty of these mixture compositions u(ξ) = 0.001. eN2O was derived from experimental data, and CO2 was calculated by analogy.

F



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Table 4. Diffusivity Coefficient, Da × 109 (m2 s−1), of N2O and CO2 in Water and in the Different Absorbent Blends at Different Temperatures, Tb (K) = 313.15, 333.15, and 353.15, and at Ambient Pressure, Pc (kPa) = 101.3

Table 5. Overall Kinetic Constants, kova (s−1), for the Reactions between the Four Absorbent Blends and CO2 at Different Temperatures, Tb (K) = 313.15, 333.15, and 353.15, and at Ambient Pressure, Pc (kPa) = 101.3

D × 109 (m2 s−1)

kova (s−1)

b

Tb (K)

T (K) 313.15 absorbent water24 BALA-IMIDd BALA-BENZd LARG-IMIDd LARG-BENZd

333.15

353.15

N2Oe

CO2e

N2Oe

CO2e

N2Oe

CO2e

2.69 2.00 1.73 1.87 1.80

2.76 2.05 1.78 1.92 1.85

4.17 3.07 2.57 2.89 2.70

4.2 3.10 2.59 2.91 2.73

6.15 5.01 4.73 4.03 4.17

6.11 4.98 4.7 4.01 4.14

absorbent

313.15 K

333.15 K

353.15 K

BALA-IMIDd BALA-BENZd LARG-IMIDd LARG-BENZd

13764 18882 30649 35833

50631 74591 97146 114535

87161 142710 115736 209973

a

The relative combined standard uncertainty of overall kinetic constants ur,c(kov) = 0.015. bThe standard uncertainty of temperature u(T) = 0.1 K. cThe standard uncertainty of pressure u(P) = 1 kPa. d BALA-IMID: 7 wt % NH3 + 3 wt % β-alanine + 1 wt % imidazole + 89 wt % H2O. BALA-BENZ: 7 wt % NH3 + 3 wt % β-alanine + 1 wt % 1,2,3-benzotriazole + 89 wt % H2O. LARG-IMID: 7 wt % NH3 + 3 wt % L-arginine + 1 wt % imidazole + 89 wt % H2O. LARG-BENZ: 7 wt % NH3 + 3 wt % L-arginine + 1 wt % 1,2,3-benzotriazole + 89 wt % H2O. The standard uncertainty of these mixture compositions u(ξ) = 0.001.

a The relative combined standard uncertainty of the diffusivity coefficient ur,c(D) = 0.08. bThe standard uncertainty of temperature u(T) = 0.1 K. cThe standard uncertainty of pressure u(P) = 1 kPa. d BALA-IMID: 7 wt % NH3 + 3 wt % β-alanine + 1 wt % imidazole + 89 wt % H2O. BALA-BENZ: 7 wt % NH3 + 3 wt % β-alanine + 1 wt % 1,2,3-benzotriazole + 89 wt % H2O. LARG-IMID: 7 wt % NH3 + 3 wt % L-arginine + 1 wt % imidazole + 89 wt % H2O. LARG-BENZ: 7 wt % NH3 + 3 wt % L-arginine + 1 wt % 1,2,3-benzotriazole + 89 wt % H2O. The standard uncertainty of these mixture compositions u(ξ) = 0.001. eN2O was derived from experimental data, and CO2 was calculated by analogy.

kov for the BALA-IMID, BALA-BENZ, LARG-IMID, and LARG-BENZ systems were obtained as follows: i −6112 yz zz kov(BALA‐IMID) = 3.684 × 1012 expjjj k T {

i −6084 yz zz kov(BALA‐BENZ) = 4.768 × 1012 expjjj k T {

i −5436 yz zz kov(LARG‐IMID) = 9.148 × 1011 expjjj k T {

(29)

(30)

(31)

i −6080 yz zz kov(LARG‐BENZ) = 8.150 × 1012 expjjj (32) k T { The relative combined standard uncertainty of kov ur,c(kov) = 0.015 Finally, Figure 5 shows the relationship between the kov values (s−1) of eqs 29−32 and 1000/T (K). As such, the final rate equations could be derived from eqs 29−32 as follows:

Figure 4. Diffusivity of CO2 in each absorbent at 313.15, 333.15, and 353.15 K.

reaction constant (kov) was required. The calculated values for kov obtained using eq 19 are presented in Table 5. As shown, an increase in temperature resulted in increased values of kov. Thus, to obtain an equation reflecting k as function of temperature, the following equation derived from the Arrhenius equation was employed: ln(k) = ln(A p.e.) −

Ea ij 1 yz jj zz R kT {

(28)

where k is the kinetic constant, Ap.e. is the pre-exponential factor, Ea is the activation energy, and R is the gas constant. In general, an Arrhenius plot is defined as a function of the logarithm of the kinetic constant (ln(k), ordinate) against inverse temperature (1/T, abscissa). In the linear function, the y-intercept represents ln(Ap.e.), while the slope of the line represents −Ea/R. Using the Arrhenius plot, the equations for the overall reaction constants

Figure 5. Overall reaction constants (kov) for the reaction between CO2 and the four absorbent blends at 313.15, 333.15, and 353.15 K. G



H

a

The relative combined standard uncertainty of CO2 solubility ur,c(α) = 0.025. byCO2 means the mole fraction of CO2 in the liquid absorbent and the relative combined standard uncertainty of mole fraction of CO2 ur,c(yCO2) = 0.025. cAbsorbent composition: 15.3 wt % MEA + 84.7 wt % H2O. dThe standard uncertainty of temperature u(T) = 0.1 K. eThe standard uncertainty of pressure u(P) = 0.1 kPa.

1 3.2 10.0 31.6 100 316 1000 3160 0.0216 0.0241 0.0265 0.0292 0.0329 0.0376 0.0436 0.0522 0.4370 0.4880 0.5380 0.5950 0.6730 0.7720 0.9020 1.0900 2.0 3.9 19.9 132.3 245 365 929. 1639 2873 0.0227 0.0247 0.0276 0.0334 0.0364 0.0389 0.0434 0.0476 0.0514 0.4600 0.5020 0.5620 0.6840 0.7480 0.8000 0.8970 0.9890 1.0720 0.3 2.0 2.3 22.0 22.5 75.1 120.7 0.0204 0.0239 0.0244 0.0294 0.0295 0.0335 0.0348 0.4120 0.4850 0.4950 0.5990 0.6020 0.6850 0.7130

PCO2e (kPa) PCO2e (kPa) PCO2e (kPa)

15.7 24.1 35.3 55.6 89.4 120.7 139.9 563 1000 1370 0.0276 0.0299 0.0303 0.0314 0.0335 0.0352 0.0358 0.0423 0.0435 0.0454 0.5610 0.6090 0.6190 0.6410 0.6850 0.7220 0.7340 0.8730 0.9000 0.9420 1.3 3.0 6.5 25.1 46.7 134.5 343.3 701.6 1554.5

ref 34

yCO2b (mol CO2 mol−1 total component) αa (mol CO2 mol−1 MEA)

ref 33

yCO2b (mol CO2 mol−1 total component) αa (mol CO2 mol−1 MEA)

Jones et al.32

yCO2b (mol CO2 mol−1 total component)

0.0222 0.0239 0.0255 0.0281 0.0305 0.0337 0.0372 0.0417 0.0458

We then compared the results obtained for the blank experiment with the reference data on CO2 solubility in 15.3 wt % MEA solution (in H2O) at 313.15 K, as outlined in Table 7 and Figure 6. This comparison indicated a clear correlation between the experimental and reference data,31−34 and so the system employed herein could be considered accurate. It should also be noted here that, for all data, the relative combined standard uncertainty of the solubility (α) measurements is 0.025, while that of PCO2 is 0.1 kPa. We then moved on to examine the absorption capabilities of the various blend systems. We first investigated the solubility of CO2 in the BALA-IMID and BALA-BENZ systems at 313.15,

0.4483 0.4842 0.5186 0.5722 0.6217 0.6906 0.7635 0.8612 0.9492

The relative uncertainty of density ur(ρ) = 0.011. bThe standard uncertainty of temperature u(T) = 0.1 K. cThe standard uncertainty of pressure u(P) = 1 kPa. dBALA-IMID: 7 wt % NH3 + 3 wt % βalanine + 1 wt % imidazole + 89 wt % H2O. BALA-BENZ: 7 wt % NH3 + 3 wt % β-alanine + 1 wt % 1,2,3-benzotriazole + 89 wt % H2O. LARG-IMID: 7 wt % NH3 + 3 wt % L-arginine + 1 wt % imidazole + 89 wt % H2O. LARG-BENZ: 7 wt % NH3 + 3 wt % L-arginine + 1 wt % 1,2,3-benzotriazole + 89 wt % H2O. The standard uncertainty of these mixture compositions u(ξ) = 0.001.

PCO2e (kPa)

a

α (mol CO2 mol−1 MEA)

353.15 − 0.894 0.9269 0.9154 0.9025

(mol CO2 mol−1 total component)

333.15 0.9544 0.938 0.9539 0.9534 0.9445


313.15 − 0.9704 0.9723 0.9702 0.9719

yCO2 (mol CO2 mol−1 total component)

absorbent 7 wt % NH3 BALA-IMIDd BALA-BENZd LARG-IMIDd LARG-BENZd


Tb (K)

a

ρa (g cm−3)

yCO2b

Table 6. Densities, ρa (g cm−3), of the Absorbent Blends at Different Temperature, Tb (K) = 313.15, 333.15, and 353.15, and at Ambient Pressure, Pc (kPa) = 101.3

Shen et al.31

where −rCO2 is the reaction rate of CO2 absorption and CCO2 is the CO2 concentration. In this research, kov values for the absorbents containing different types of amino acids and corrosion inhibitors were investigated. As a result, the absorbent containing L-arginine showed the highest kov value compared to that containing β-alanine regardless of the corrosion inhibitor. This result coincided with our expectation that reactivity of Larginine toward carbon dioxide will be higher than β-alanine since L-arginine has two amino groups in its molecule. 3.4. Chemical Solubility of CO2. We first examined the densities of each absorbent at the desired temperature as outlined in Table 6. As expected, the densities of the absorbents decreased upon increasing the temperature.

a

(36)

b

i −6080 yz zzCCO2 −rCO2(LARG‐BENZ) = 8.150 × 1012 expjjj k T {

(35)

experimental data

i −5436 zy zzCCO2 −rCO2(LARG‐IMID) = 9.148 × 1011 expjjj k T {

(34)

a

i −6084 yz zzCCO2 −rCO2(BALA‐BENZ) = 4.768 × 1012 expjjj k T {

(33)

Lee et al.33,34

Table 7. Experimental and Reference Data for CO2 Solubility, αa (mol CO2 mol−1 MEA), and Mole Fraction of CO2, yCO2b (mol CO2 mol−1 total component) for 15.3 wt % MEA solution in H2O (in CO2−water−MEA system)c at Temperature Td (K) = 313.1528−31

i −6112 zy zzCCO2 −rCO2(BALA‐IMID) = 3.684 × 1012 expjjj k T {

PCO2e (kPa)

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Figure 6. CO2 solubility graph indicating the correlation between PCO2 and α in a 15.3 wt % MEA solution (in H2O) at 313.15 K and comparison with literature data.28−31

Figure 7. CO2 solubility graph for the BALA-IMID blend at 313.15, 333.15, and 353.15 K.

333.15, and 353.15 K, as outlined in Table 8 and Figure 7, and Table 9 and Figure 8, respectively. As the CO2 concentration in flue gas emitted from coal-fired power plants was reported to be 15 vol %, we examined each blend system at a PCO2 of 15 kPa, since flue gas is vented at atmospheric pressure (i.e., 1 atm, 100 kPa). Thus, the α values obtained at 15 kPa were investigated in the context of potential industrial application. Comparison of the data shown in Figures 4 and 5 and Tables 4 and 5 indicated that when β-alanine was employed as the amino acid (i.e., in the BALA-IMID absorbent system) superior CO2 solubilities were observed compared to the BALA-BENZ system at all temperatures examined. Subsequently, we examined the solubility of CO2 in the LARG-IMID and LARG-BENZ systems at 313.15, 333.15, and 353.15 K, as shown in Table 10 and Figure 9, and Table 11 and Figure 10, respectively. In addition, the use of L-arginine as the amino acid additive instead of β-alanine resulted in enhanced CO2 solubilities, indicating that the basicity of L-arginine through the presence of two amino groups may have an effect on CO2 solubility. The pKb values of the two amino acids should

also be considered in this context. More specifically, the pKb values of L-arginine and β-alanine are 1.509 and 10.37, respectively. As pKb values are inversely proportional to the basicity of a substance, the experimental results correlate with an increased basicity of L-arginine. Similar results were also observed for the corrosion inhibitors when the pKa values were considered (a lower pKa corresponds to a higher acidity). For example, the pKa value of 1,2,3benzotriazole is 8.2, while for imidazole, the corresponding values are 14.5 (for imidazole itself) and 7.05 (for the conjugate acid). When β-alanine was employed as the rate promoter, the absorbent containing imidazole as the corrosion inhibitor exhibited higher performances than that containing 1,2,3benzotriazole. However, when L-arginine was used as the rate promoter, little difference was observed between the CO2 solubilities obtained using the different corrosion inhibitors. These observations may be due to the higher possibility of imidazole forming a conjugate acid when exposed to basic conditions. As L-arginine exhibits a higher basicity than βalanine, the absorbent blend containing L-arginine and imidazole

Table 8. CO2 Solubility Data, αa (mol CO2 mol−1 absorbent), and Mole Fraction of CO2, yCO2b (mol CO2 mol−1 total component), for the BALA-IMID (in the CO2−water−NH3 - β-alanine−imidazole system)c Blend at Different Temperatures, Td (K) = 313.15, 333.15, and 353.15 313.15 Kd α (mol CO2 mol−1 absorbent) a

0.6090 0.6889 0.7194 0.7495 0.7789 0.8087 0.8308 0.8611 0.8893 0.9443

333.15 Kd −1

b

yCO2 (mol CO2 mol total component) 0.0478 0.0537 0.0559 0.0581 0.0603 0.0625 0.0641 0.0662 0.0683 0.0722

PCO2e

(kPa)

α (mol CO2 mol−1 absorbent)

7 22.1 41.6 63.8 93.7 135.9 196.3 286.2 407.5 706.9

0.5146 0.5469 0.5802 0.6176 0.6561 0.7005 0.7495 0.7874 0.8222 0.8521

a

yCO2b

(mol CO2 mol total component) 0.0407 0.0431 0.0456 0.0484 0.0513 0.0546 0.0582 0.0609 0.0634 0.0656

353.15 Kd −1

PCO2e

(kPa)



PCO2e (kPa)

9.8 23 47.7 78.6 133.3 209.8 323.5 489.5 672.9 901.0

0.3398 0.3726 0.4101 0.4538 0.5015 0.5517 0.6028 0.6346 0.6689 0.7033

0.0272 0.0298 0.0327 0.0360 0.0397 0.0435 0.0473 0.0497 0.0522 0.0548

6.3 13 28.4 52.4 100.7 184.4 325.2 476.7 604.2 768.3

a

The relative combined standard uncertainty of CO2 solubility ur,c(α) = 0.025 NH3 and β-alanine were considered as absorbents in calculations on α. byCO2 means the mole fraction of CO2 in the liquid absorbent and the relative combined standard uncertainty of mole fraction of CO2 ur,c(yCO2) = 0.025. cBALA-IMID: 7 wt % NH3 + 3 wt % β-alanine + 1 wt % imidazole + 89 wt % H2O. The standard uncertainty of the mixture composition u(ξ) = 0.001. dThe standard uncertainty of temperature u(T) = 0.1 K. eThe standard uncertainty of pressure u(P) = 0.1 kPa. a

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Table 9. CO2 Solubility Data, αa (mol CO2 mol−1 absorbent), and Mole Fraction of CO2, yCO2b (mol CO2 mol−1 total component), for the BALA-BENZ (in CO2−water−NH3−β-alanine−1,2,3-benzotriazole system)c Blend at Different Temperatures, Td (K) = 313.15, 333.15, and 353.15 313.15 Kd

333.15 Kd

353.15 Kd

αa (mol CO2 mol−1 absorbent)


PCO2e (kPa)



PCO2e (kPa)



PCO2e (kPa)

0.5444 0.5828 0.6250 0.6704 0.7167 0.7558 0.7976 0.8355 0.8601 0.9003

0.0430 0.0459 0.0490 0.0524 0.0558 0.0587 0.0617 0.0645 0.0662 0.0691

5.1 16 32.8 53.7 88.4 152.3 240.7 379.6 555.5 780.1

0.4974 0.5251 0.5634 0.6098 0.6608 0.7065 0.7494 0.7923 0.8264 0.8528

0.0394 0.0415 0.0444 0.0479 0.0517 0.0551 0.0582 0.0613 0.0638 0.0657

9.2 24.1 44.9 82.5 134.1 210.5 342.8 468.2 619.5 841.4

0.3203 0.3559 0.3961 0.4519 0.5092 0.5641 0.6183 0.6734 0.7241 0.7790

0.0257 0.0285 0.0316 0.0359 0.0403 0.0445 0.0485 0.0526 0.0564 0.0604

7 18.5 38.8 73.7 148.5 266.7 407.8 554.9 676.0 849.2

a The relative combined standard uncertainty of CO2 solubility ur,c(α) = 0.025 NH3 and β-alanine were considered as absorbents in calculations on α. byCO2 means the mole fraction of CO2 in the liquid absorbent and the relative combined standard uncertainty of mole fraction of CO2 ur,c(yCO2) = 0.025. cBALA-BENZ: 7 wt % NH3 + 3 wt % β-alanine + 1 wt % 1,2,3-benzotriazole + 89 wt % H2O. The standard uncertainty of the mixture composition u(ξ) = 0.001. dThe standard uncertainty of temperature u(T) = 0.1 K. eThe standard uncertainty of pressure u(P) = 0.1 kPa.

Figure 8. CO2 solubility graph for the BALA-BENZ blend at 313.15, 333.15, and 353.15 K.

Figure 9. CO2 solubility graph for the LARG-IMID blend at 313.15, 333.15, and 353.15 K.

Table 10. CO2 Solubility Data, αa (mol CO2 mol−1 absorbent), and Mole Fraction of CO2, yCO2b (mol CO2 mol−1 total component), for the LARG-IMID (in CO2−water−NH3−L-arginine−imidazole system)c Blend at Different Temperatures, Td (K) = 313.15, 333.15, and 353.15 313.15 Kd α (mol CO2 mol−1 absorbent) a

0.6375 0.6747 0.7159 0.7633 0.8185 0.8798 0.9440 1.0131 1.0769 1.1252

333.15 Kd −1

b

yCO2 (mol CO2 mol total component) 0.0483 0.0510 0.0539 0.0573 0.0611 0.0654 0.0699 0.0746 0.0789 0.0822

PCO2e

(kPa)


3.6 10.0 23.5 43.6 79.2 133.7 221.4 338.9 497.4 706.2

0.4660 0.4954 0.5313 0.5731 0.6198 0.6707 0.7227 0.7729 0.8301 0.8980

a

yCO2b

(mol CO2 mol total component) 0.0358 0.0379 0.0406 0.0436 0.0470 0.0507 0.0544 0.0579 0.0620 0.0667

353.15 Kd −1

PCO2e

(kPa)



PCO2e (kPa)

4.6 14.6 23.8 40.5 71.6 123.6 204.6 336.5 562.3 777.4

0.4106 0.4348 0.4646 0.5019 0.5459 0.5952 0.6473 0.6987 0.7480 0.7862

0.0316 0.0334 0.0357 0.0384 0.0416 0.0452 0.0490 0.0527 0.0562 0.0589

6.3 12.4 22.8 41.8 75.1 135.6 239.5 396.9 629.2 866.3

a

a

The relative combined standard uncertainty of CO2 solubility ur,c(α) = 0.025 NH3 and L-arginine were considered as absorbents in calculations on α. byCO2 means the mole fraction of CO2 in the liquid absorbent and the relative combined standard uncertainty of mole fraction of CO2 ur,c(yCO2) = 0.025. cLARG-IMID: 7 wt % NH3 + 3 wt % L-arginine + 1 wt % imidazole + 89 wt % H2O. The standard uncertainty of the mixture composition u(ξ) = 0.001. dThe standard uncertainty of temperature u(T) = 0.1 K. eThe standard uncertainty of pressure u(P) = 0.1 kPa. J



Article

Table 11. CO2 Solubility Data, αa (mol CO2 mol−1 absorbent), and Mole Fraction of CO2, yCO2b (mol CO2 mol−1 total component), for the LARG-BENZ (in CO2−water−NH3−L-arginine−1,2,3-benzotriazole system)c Blend at Different Temperatures, Td (K) = 313.15, 333.15, and 353.15 313.15 Kd

333.15 Kd

353.15 Kd


yCO2d (mol CO2 mol−1 total component)

PCO2e (kPa)



PCO2e (kPa)



PCO2e (kPa)

0.6582 0.6938 0.7332 0.7778 0.8260 0.8765 0.9288 0.9784 1.0153 1.0403

0.0498 0.0524 0.0552 0.0583 0.0617 0.0653 0.0689 0.0723 0.0748 0.0765

6.9 16.2 34.1 77.3 131.7 220.6 299.0 455.1 697.8 947.2

0.5516 0.5797 0.6115 0.6492 0.6896 0.7337 0.7790 0.8210 0.8585 0.8923

0.0421 0.0441 0.0465 0.0492 0.0521 0.0552 0.0584 0.0614 0.0640 0.0664

2.7 11.5 32.6 60.4 98.0 158.6 252.5 389.0 571.8 781.0

0.3940 0.4199 0.4505 0.4854 0.5240 0.5668 0.6119 0.6555 0.6966 0.7359

0.0304 0.0324 0.0346 0.0372 0.0401 0.0432 0.0465 0.0496 0.0526 0.0552

3.1 11.6 27.0 48.9 90.9 155.8 253.4 406.7 596.3 817.4

a The relative combined standard uncertainty of CO2 solubility ur,c(α) = 0.025 NH3 and L-arginine were considered as absorbents in calculations on α. byCO2 means the mole fraction of CO2 in the liquid absorbent and the relative combined standard uncertainty of mole fraction of CO2 ur,c(yCO2) = 0.025. cLARG-BENZ: 7 wt % NH3 + 3 wt % L-arginine + 1 wt % 1,2,3-benzotriazole + 89 wt % H2O. The standard uncertainty of the mixture composition u(ξ) = 0.001 dThe standard uncertainty of temperature u(T) = 0.1 K. eThe standard uncertainty of pressure u(P) = 0.1 kPa.

More specifically, when L-arginine was employed as the amino acid additive, most of the performances were superior to that of β-alanine, likely due to the basicity of the basic L-arginine compared to the neutral β-alanine. In addition, these results mean that the L-arginine-based absorbents exhibit good potential for replacing conventional alkanolamine-based absorbents. Indeed, comparison of the absorbent systems employed herein with the widely used 30 wt % aqueous monoethanolamine (MEA) solution showed that our blends exhibited enhanced CO2 solubility compared to MEA. When PCO2 was 15 kPa, theoretically, 2 mol of MEA can hold 1 mol of CO2 resulting in the CO2 solubility (α) of 0.5 (mol of CO2/mol of absorbent) at ambient temperature.11 Whereas, the absorbents in this study showed that when PCO2 was 15 kPa and temperature was 313.15 K, 1 mol of LARG-IMID and LARG-BENZ could hold 0.6964 and 0.6925 mol of CO2, respectively. We could therefore conclude that the application of a novel and improved absorbent to replace conventional carbon capture and storage absorbents may be possible. In the future, we will examine and compare the effects of other basic amino acids (e.g., lysine and histidine) on the solubility of CO2 in similar systems.

Figure 10. CO2 solubility graph for the LARG-BENZ blend at 313.15, 333.15, and 353.15 K.

resulted in a significantly higher CO2 solubility than the other blend combinations. Finally, for all absorbents, an increase in temperature resulted in a decrease in CO2 solubility, likely due to the inversely proportional relationship of gas solubility and temperature.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00175. Table of original experimental data used to derive Henry constants (PDF)

4. CONCLUSION We herein aimed to develop an aqueous CO2 absorbent that can be used in various industrial processes where carbon dioxide is produced. For our aqueous absorbent blend system, we selected NH3, amino acids, and corrosion inhibitors as the base materials. In terms of the amino acid and corrosion inhibitor additives selected for examination, the various constituents of the absorbents were chosen based on their merits regarding cost and degradation inhibition. Through various experiments, the thermodynamic properties including density, viscosity, diffusivity of CO2, kinetic constant, rate equation, and physical and chemical CO2 solubility (expressed as α, mol CO2/mol absorbent) in the absorbent mixtures containing 7 wt % NH3, 3 wt % amino acid, and 1 wt % corrosion inhibitor were investigated at 313.15, 333.15, and 353.15 K.

■

AUTHOR INFORMATION

Corresponding Author

*(J.P.) E-mail: [email protected]. Tel: +82 2-364-1807. ORCID

Jinwon Park: 0000-0001-7729-5490 Funding

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20152010201850) and the Human Resources Program in Energy Technology of KETEP, which granted K



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financial resources from the MOTIE, Republic of Korea (No. 20174010201640).

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Notes

The authors declare no competing financial interest.

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