Reaction Kinetics of Carbon Dioxide (CO2) with ... - ACS Publications

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Reaction Kinetics of Carbon Dioxide (CO2) with Diethylenetriamine and 1‑Amino-2-propanol in Nonaqueous Solvents Using StoppedFlow Technique Nan Zhong,†,⊥ Helei Liu,†,‡,⊥ Xiao Luo,*,† Mohammed J. AL-Marri,§ Abdelbaki Benamor,§ Raphael Idem,†,‡ Paitoon Tontiwachwuthikul,†,‡,§ and Zhiwu Liang*,†,‡,§ †

Joint International Center for CO2 Capture and Storage (iCCS), Hunan Provincial Key Laboratory for Cost-Effective Utilization of Fossil Fuel Aimed at Reducing CO2 Emissions, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, People’s Republic of China ‡ Clean Energy Technologies Research Institute (CETRI), University of Regina, Regina, Saskatchewan S4S0A2, Canada § Gas Processing Center, Qatar University, Doha 2731, Qatar S Supporting Information *

ABSTRACT: In this work, the reaction kinetics of carbon dioxide (CO2) with diethylenetriamine (DETA) and 1-amino-2-propanol (1-AP) in methanol and ethanol systems were measured using the stopped flow technique over a temperature range of 293−313 K in terms of pseudo-first-order rate constant (k0). Concentration in the range of 10 to 50 mol/m3 for diethylenetriamine, and 20 to 100 mol/m3 for 1-amino2-propanol were studied. The experimental data show that the pseudo-first-order rate constants (k0) increase with the increase of both amine concentration and temperature. The zwitterion mechanism and the termolecular mechanism were used to represent the data for DETA in methanol and ethanol systems with excellent ADDs of 3.5% and 2.4%, respectively, and 1-AP in methanol and ethanol systems with excellent ADDs of 2.4% and 2.6%, respectively. In comparison with EDA and AEEA in terms of k2, DETA exhibits a better reaction kinetics performance for capturing CO2. Those results will be useful in finding an efficient method for the removal of CO2 from industrial gases.

1. INTRODUCTION In recent decades, the greenhouse effect has been the most notable global climate issue, and has caused serious negative effects on society and the economy around the world.1 It is well-known that the massive emission of carbon dioxide(CO2), mainly produced by fuel-fired power plants, aggravates the greenhouse effect. CO2 capture and storage technology (CCS), especially the postcombustion capture (PCC) of CO2, has been regarded as the most promising method to reduce these CO2 emissions.2 Various methods have been developed for PCC of CO2, such as absorption, cryogenics distillation and membrane separation. Among these different methods, the amine-based chemical absorption method is deemed to be a highly cost efficient technology due to its well-developed commercial maturity and potential for wide-scale industry application.3,4 To date, several alkanolamines with fast reaction rates and high capacities have been developed and commonly applied in CO2 removal from flue gas.5 However, there are still some shortcomings, such as high corrosiveness and high energy cost which are serious hindrances to the development and further adoption of this technique. Compared to aqueous systems, the use of nonaqueous systems (i.e., methanol system or ethanol system) is considered to be a promising alternative method for CO2 capture, due to © 2016 American Chemical Society

high solubility and capacity, low corrosiveness and low energy consumption for the regeneration of used solvents. Nonaqueous systems consist of amines and different alcohols. Recently, nonaqueous systems comprising methanol solutions of alkanolamines have been commercially employed for the absorption of carbon dioxide, hydrogen sulfide, etc., but they must be operated in recycle loops, which seem to be more applicable for the removal of acid gases.6 In recent years, studies of the reaction kinetics of CO2 with amines in nonaqueous systems using the stopped-flow technique have examined systems such as aniline/cyclohexamine/hexamine− ethanol,7ethylenediamine(EDA)/−methanol/ethanol8 and 3amino-1-propanol(3-AP)−methanol/ethanol, 8 2-((2aminoethyl)amino)ethanol(AEEA)−ethanol/methanol.9 There are still other experimental methods like stirred cell, stirred semibatch bank, and wetted column for measuring the kinetics of the reaction between CO2 and different amines in various nonaqueous solvents like ethanol, methanol, 2-propanol, nbutanol, ethylene glycol and propylene glycol.10−15 Therefore, Received: Revised: Accepted: Published: 7307

March 15, 2016 June 5, 2016 June 17, 2016 June 17, 2016 DOI: 10.1021/acs.iecr.6b00981 Ind. Eng. Chem. Res. 2016, 55, 7307−7317

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Industrial & Engineering Chemistry Research

The main goal of this study is to supplement the reaction kinetic data for 1-AP and DETA in nonaqueous systems using the stopped-flow technique over a temperature range of 293− 313 K, with the concentration range of 10 to 50 mol/m3 for diethylenetriamine, and 20 to 100 mol/m3 for 1-amino-2propanol and to compare the reaction rate of different amines in nonaqueous systems as it relates to molecular structure. The molecular structures of the amines compared in this study (1AP, EDA, AEEA, DETA) are displayed in Figure 1.

nonaqueous systems are well demonstrated to have significant advantages over the corresponding aqueous systems, including high solubilities and capacity, low corrosiveness and low energy consumption during the regeneration of used solvents. Indeed, nonaqueous systems have had a significant impact on the development of CCS techniques. In addition, the reaction kinetics is a critical parameter to simulate the optimized absorption process and design the appropriate absorption column. This is because the reaction kinetics of CO2 and amines definitely have a significant effect on the height of absorption column.16,17 For a fixed quantity of CO2, a faster reaction will necessitate a shorter absorption column, and result in a lower cost of CO2 capture.18 There are several alternative techniques for the investigation of reaction kinetics, including stirred cell, stirred semibatch, laminar jet absorber, wetted sphere and stopped-flow.19 Among the several techniques, stopped-flow, a direct method, has been widely used due to its characteristics, such as the large coverage of reaction rates and reproduceable experiment data.20 Furthermore, the stopped-flow technique is very suitable for screening novel solvents due to its requirement for very small solvent quantities and its simple experimental procedure.21 Therefore, in this study, the reaction kinetics of DETA and 1AP were measured with the stopped-flow technique. As well as choosing experimental tool and method, it is also very important to find effective amines for the study of reaction kinetics. Generally, to be effective, amines need to have a lot of advantages, such as fast reaction rates, high mass transfer coefficients, high absorption capacity, low heat duty for regeneration, be not easily degraded, and have low corrosiveness.22 Recently, many studies have turned to focus on developing novel amines as substitutions for the traditional and well-industrialized amines, such as monoethanolamine (MEA) and diethanolamine (DEA). Compared with traditional amines, some novel amines, such as diethylenetriamine (DETA),23 4-(diethylamino)-2-butanol (DEAB)24,25 and 2-(1piperazinyl)ethylamine,26 have been considered to have great performance in capturing CO2. Diethylenetriamine (DETA) is a linear triamine, including two primary amine groups and one secondary amine group. The kinetics of DETA and CO2 in an aqueous system was studied by Ardi Hartono et al. using a contactor that employed a string of discs.23 In addition, a lot of effort has been spent on studying properties of this efficient and effective amine (DETA), such as the solubility of CO2 in aqueous 2.5 M of DETA solution, qualitative determination of species in a DETA−H2O−CO2 system using the 13C NMR technique and density, viscosity, and other properties of aqueous solution of DETA.27−30 Furthermore, the mass transfers of aqueous DETA in structured packed columns and randomly packed columns were both investigated by Fu et al.31,32 These studies all indicated that DETA is a promising solvent for CO2 capture, with its high capacity and high CO2 absorption rate compared with the traditional amines (e.g., MEA, DEA, MDEA and AMP). 1-Amino-2-propanol (1-AP) is a nonlinear primary amine, a constitutional isomer of 3-amino-1-propanol (3-AP), which is a linear primary amine. Penny and Ritter,33 Bavbek and Alper34 both studied the kinetics of 1-AP in aqueous solutions at 303 K. Li et al.35 comprehensively studied the same aqueous 1-AP system in the temperature range of 298−313 K using the stopped-flow technique.

Figure 1. Molecular structure of correlation amines.

2. EXPERIMENTAL SECTION 2.1. Materials. Reagent grade diethylenetriamine (DETA) with a mass purity of ≥98% was obtained from Adams Reagent Co., Ltd. and reagentgrade1-amino-2-propanol (1-AP) was obtained from Aladdin Industrial Inc. A series of concentrations of nonaqueous amine solutions were prepared by adding a measured volume of methanol or ethanol into weighed quantities of reagent. All chemicals used in the whole experiment were prepared without further purification. For every experiment, freshly saturated nonaqueous CO2 solution was obtained by mixing the gas through methanol or ethanol in a jacketed glass-stirred reactor. The freshly saturated nonaqueous CO2 solution was diluted with methanol or ethanol to ensure the molar ratio of CO2 to amine was kept greater than 10.7,36 2.2. Apparatus. In this study, the pseudo-first-order rate constant (k0, s−1) for different amines in nonaqueous systems 7308

DOI: 10.1021/acs.iecr.6b00981 Ind. Eng. Chem. Res. 2016, 55, 7307−7317

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Figure 2. Schematic drawing of experimental stopped flow equipment.

Figure 3. Sample data of DETA by using the stopped-flow technique in the concentration of 80 mol/m3 at 298 K.

was directly measured by the standard SF-61DX stopped flow unit from Hi-Tech Scientific, Ltd. (U.K.). The equipment is assembled with four main parts: a sample handling unit, a conductivity detection cell, A/D converter, and a microprocessor of an HP computer. The schematics of this equipment is presented in detail in Figure 2. To maintain a thermostable environment for the enclosed sample flow circuit, the sample-handling unit was made of stainless steel. The frontpanel of the sample-handling unit consisted of a temperature display, an air pressure indicator, and pneumatic push plate on the bottom of four syringes. The temperature shift in this equipment was maintained within ±0.1 K by an external water bath circulation, which was controlled by a thermostatic equipment by the side of the stopped-flow equipment. Two syringes powered by a pneumatic air system and common push plate were used to inject CO2 solution and amine solution. In every experimental run, equal volumes of CO2 solution and amine solution were separately injected into the conductivity detection cell, where the intrinsic rate of the rapid homogeneous reaction between amine and CO2 was measured by monitoring the voltage change caused by rapid ion formation.

As a function of time, the conductivity change was measured by a circuit as described by Knipe et al.37 The conductivity is in proportion to the output voltage given by the Hi-Tech Scientific data acquisition and analysis software suite, IS-2. The schematic drawing of the stopped flow equipment is displayed in Figure 2 and a sample experimental run is shown in Figure 3, where the conductivity is plotted against time. The observed experimental data of each experimental run for the amine−CO2 reaction was generated based on the output conductivity values. For all amine concentrations, every experiment run in the same temperature was repeated at least 7 times to ensure accurate results. The conductivity change of each experimental run with respect to time is fitted based on the relation G(t),37 which was shown as G(t ) = −A × exp( −k 0 × t ) + C

(1)

where k0 is the pseudo-first-order reaction rate constant, G(t) is the value of conductivity obtained from the conductivity meter, A is the amplitude of the signal, C is the constant conductance value at the end of observed reaction and t is the time (s). 7309


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Industrial & Engineering Chemistry Research This method was deemed to not involve a gas-phase absorption, so the experiment results were able to be directly correlated with reaction rate of an intrinsic homogeneous reaction between amine and CO2 in liquid phase. As well, the stopped-flow technique did not need to consider the reversibility of the reaction nor the depletion of the amine at the liquid interface, thus eliminating these potential experimental errors. It is important to note that this work using the stopped-flow technique is based on the validation by our previous work on the kinetics of aqueous DEA solutions ranging from 10 to 50 mol m−3 in the temperature of 298−313 K.38−40 In addition, when the results in our previous work are compared with the works of Ali et al.7,41 in terms of pseudo-first-order constant (k0), they show good agreement. Therefore, the experimental data obtained in this study was regarded as credible and reliable.

Then, the observed pseudo-first-order rate constants (k0) becomes k0 =

k −1k 2

Base

eq 9 becomes k 0 = k 2[Am]

k −BasekBase

Base

9 becomes k0 =

k 2[CO2 ][Am] k −1 + ∑ kBase[B]

[CO2 ][Am] (1/k 2) + (k −1/(k 2(∑ kBase[B])))

(3)

Am + B + CO2 ↔ AmCOO− ··· BH+ (4)

k 2[CO2 ][Am] 1 + (k −1/(∑ kBase[B]))

(5)

rCO2 = k 0[CO2 ] = [CO2 ][Am]{∑ kB[B]} = [CO2 ][Am]{ka[Am] + kBase[Base]}

(6)

(14)

the case of aqueous system: kBase = kw = (kT2 kwater)/k−1. the case of methanol system: kBase = km = (kT2 kmethanol)/k−1. the case of ethanol system: kBase = ke = (kT2 kethanol)/k−1.

In In In If amine is assumed to be the main contributor in the termolecular mechanism, the rate expression can then be simplified to

(7)

Because of the pseudo-first-order conditions, the concentration of the amine [Am] was always kept much higher than the concentration of CO2, eq 7 could be written as rCO2 = k 0[CO2 ]

(13)

When the zwitterion deprotonation is the rate-determining step, the deprotonation of zwitterion is slower than the reverse reaction. A termolecular mechanism is equivalent to the asymptotic limit of the zwitterion mechanism. The reaction rate expression for the termolecular mechanism is shown as below:

Then, eq 6 can be simplified to rCO2 =

(12)

3.2. Termolcular Mechanism. This mechanism was originally proposed by Crooksand Donellan45 and recently revisited by da Silva and Svendsen.46 It was assumed that an amine directly bonds to one molecule of CO2 and a proton is transferred to one molecule of a base at the same time. This reaction proceeds in a single step via a loosely bound encounter complex as the intermediate, which breaks up to form reagent molecules. Just a fraction of the complexes react with a second molecule of the amine or water to form ionic products. This mechanism can be represented as

(2)

With the application of pseudo-steady-state principle to the zwitterion, the overall forward reaction rate of CO2 can be derived as rCO2 =

(11)

CO2 + 2AmH ↔ AmHCOO− + AmH 2+

The overall CO2 reaction rate was equal to −rCO2 = k 2[CO2 ][Am] − k −1[zwitterion]

k 2 ∑ kBase[B][Am] k −1

In this case, if amine is the major contribution of eq 3, as suggested by Versteeg and Van Swaaij14 and Laddha and Danckwerts,44 the overall reaction between CO2 and primary or secondary amine can be represented as

The suffix B represents amine, methanol or ethanol in nonaqueous system. The zwitterion concentration can be presented as [zwitterion] =

(10)

On the other hand, when the zwitterion deprotonation step in eq 3 is nearly instantaneous in comparison with the reverse reaction rate in eq 1, the deprotonation step is considered to be k the rate-determining step with the case of ∑ k −1 [B] ≫ 1. So, eq

In the second step, the zwitterion was then deprotonated by a base B, and carbamate was formed, as shown below: AmH+COO− + B ←⎯⎯⎯⎯⎯⎯→ AmCOO− + BH+

(9)

There are two limits that exist for eq 9. If the zwitterion formation step is relatively fast in comparison with the reverse reaction in eq 2, the zwitterion formation is regarded as the k rate-determining step with the case of ∑ k −1 [B] ≪ 1. Therefore,

3. REACTION MECHANISM Generally, the reaction kinetics of primary and secondary amines are interpreted with the zwitterion mechanism and termolecular mechanism. 3.1. Zwitterion Mechanism. Commonly, the reaction of primary and secondary amines with CO2 is explained by the two-step zwitterion mechanism, which was proposed by Caplow et al.,42 and later reintroduced by Danckwerts et al.43 It was suggested that a zwitterion was formed as an intermediate as the first step (the amine is denoted here as AmH): CO2 + AmH ←→ ⎯ AmH+COO−

k 2[Am] 1 + (k −1/(∑ kBase[B]))

rCO2 = [CO2 ]ka[Am]2

(15)

In the case of eq 15, the rate expression for the termolecular mechanism is the third-order reaction rate, which can be

(8) 7310


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(0.91−0.95) for DETA in ethanol system. All experimental data was interpreted by eq 10 in consideration of the obtained unity order and the low DETA concentrations (10−50 mol m−3) to get the second-order rate constants. As is well-known from the Arrhenius equation (eq 16), k2 is a strong function of temperature as shown below:

considered to be the same with that case of the zwitterion k mechanism with ∑ k −1 [B] ≫ 1. Base

4. RESULTS AND DISCUSSION 4.1. DETA in Ethanol System. Anhydrous solutions of DETA with concentration ranging from 10.00 to 50.06 mol m−3 was studied over a temperature range of 293−313 K, with the stopped-flow technique in ethanol system. In this experiment, the observed pseudo-first-order rate constant (k0, s−1) as a function of DETA concentration (mol m−3) and temperature was plotted against the DETA concentration (mol m−3) in Figure 4. The values of k0 listed in Table 1 were

⎛ −E ⎞ k 2 = A exp⎜ a ⎟ ⎝ RT ⎠

(16)

where A is Arrhenius constant (m3 mol−1 s−1)), Ea is the activation energy (kJ/mol) and R is the universal gas constant (0.008 315 kJ mol−1 K−1). The obtained values of k2 were plotted as a function of temperature as presented in Figure 5. The temperature dependency of k2 through the Arrhenius relationship of the DETA ethanol system can be expressed as ⎛ −2165 ⎞ ⎟, k 2z (m 3 mol−1 s−1) = 1.04 × 104exp⎜ ⎝ T ⎠ [R2 = 0.99]

(17)

Figure 4. Plot of pseudo-first-order reaction rate constants k0 verus DETA concentration at different temperatures studied in ethanol.

Table 1. Pseudo-First-Order Reaction Rate Constants for DETA in Ethanol concentration (mol·m−3)

pseudo-first-order rate constant (k0) at varying temp.

DETA

ethanol

293 K

298 K

303 K

308 K

313 K

10.005 19.993 29.995 39.907 45.004 50.060

17.107 17.085 17.063 17.040 17.029 17.018

70.9 132.9 192.1 258.0 287.8 337.9

80.0 140.6 203.7 279.1 321.8 379.2

94.0 165.8 222.1 323.5 356.5 415.9

105.0 185.5 245.7 369.0 412.0 465.1

121.4 210.7 309.4 401.7 478.2 518.9

Figure 5. Second-order rate constant k2 plotted by Arrhenius equation for the reaction between DETA and CO2 in ethanol system.

From the slope of the plot, the activation energy (Ea) was found to be 18.00 kJ/mol. The plot of correlated k0 calculated using eq 17 against experimental results is displayed in Figure 6, and the AAD was found to be within an acceptable 4%, which shows a great parity for the ethanol system of DETA. 4.2. DETA in Methanol System. Anhydrous solutions of DETA in methanol ranging from 9.98 to 50.04 mol m−3 were studied over a temperature range of 293−313 K with the stopped-flow technique. In this experiment, the observed pseudo-first-order rate constant (k0, s−1) as a function of DETA concentration (mol m−3) and temperature was plotted against the DETA concentration (mol m−3) in Figure 7. The values of k0 listed in Table 2 were directly extracted from the stopped-flow conductivity raw data. Figure 7 shows that the pseudo-first-order rate constants (k0, s−1) increase with increase of the DETA concentration and temperature as is generally known. On the basis of the work of Couchaux et al.,20 the

directly extracted from the stopped-flow conductivity raw data. Figure 4 shows that the pseudo-first-order rate constants (k0, s−1) increase with the increase of DETA concentration and temperature as is generally known. On the basis of the work of Couchaux et al.,20 the concentrations of ethanol were calculated, and the values are also listed in Table 1. According to the study of Kierzkowska-Pawlak et al.,47 the reaction between CO2 and OH− is so fast that the concentration of OH− ions under the pseudo-first-order reaction conditions can be considered to be negligible. Therefore, in this kind of nonaqueous system of ethanol, the consideration of OH− ions in the calculations can be ignored.8,9 The reaction orders with respect to different temperatures were given by the measurement of empirical curve fitting, and found to be virtually unity 7311


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Figure 8. Second-order rate constant k2 plotted by Arrhenius equation for the reaction between DETA and CO2 in methanol system.

Figure 6. Parity plot for DETA in ethanol.

Table 2. Pseudo-First-Order Reaction Rate Constants for DETA in Methanol concentration (mol·m−3) methanol

293 K

298 K

303 K

308 K

313 K

9.9885 19.992 30.004 39.989 44.986 50.039

24.681 24.648 24.616 24.584 24.568 24.552

49.2 112.9 164.8 220.8 247.4 280.1

62.0 124.5 178.2 250.0 271.8 315.7

74.9 135.5 205.7 288.6 312.8 349.2

83.0 154.7 244.1 315.3 342.4 382.8

91.8 171.9 260.9 362.7 381.6 432.5

From the slope of the plot, the activation energy (Ea) was found to be 17.32 kJ/mol. The plot of correlated k0 calculated using eq 18 against experimental results is displayed in Figure 9, and the AAD was found to be within an acceptable 2.4%, which shows a great parity for the methanol system of DETA. 4.3. 1-AP in Ethanol System. The reaction kinetics between CO2 and nonaqueous 1-AP in ethanol system was studied ranging from 20.4 to 100.4 mol/m3 at 293, 298, 303, 308 and 313 K. Figure 10 shows that the pseudo-first-order rate constants (k0) increase with the concentration and temperature increase of 1-AP, and displays a function relationship between the pseudo-first-order rate constants k0 and the concentration of 1-AP in an ethanol system, which was proved by the experimental values in Table 3. On the basis of the work of Couchaux,20 the values of the concentrations of ethanol were calculated, and the results are listed in Table 3. According to the study of Kierzkowska-Pawlak et al.,47 the reaction between CO2 and OH− is so fast that the concentration of OH− ions under the pseudo-first-order reaction conditions can be considered to be negligible. Therefore, in this kind of nonaqueous system of ethanol, the consideration of OH− ions in the calculations can be ignored.8,9 Determined by the empirical power law in Figure 10, the reaction orders between the pseudo-first-order rate constants (k0) and 1-AP concentration were calculated to be fractional orders, which were 1.60−1.68 over a temperature range of 293−313 K. In addition,

Figure 7. Plot of pseudo-first-order reaction rate constants k0 verus DETA concentration at different temperatures in methanol.

concentrations of methanol were calculated, and the values are also listed in Table 2. In accordance with the ethanol system, the consideration of OH− ions in the calculations can be ignored in the methanol system. The reaction orders with respect to different temperature were given by the measurement of empirical curve fitting, and found to be virtually unity (0.97−1.03) for DETA in methanol system. All experimental data was interpreted by eq 10 in consideration of the obtained unity order and the low DETA concentrations (10−50 mol m−3). The obtained values of k2 were plotted as a function of temperature as presented in Figure 8. The temperature dependency of k2 through Arrhenius relationship of the DETA methanol system can be expressed as ⎛ −2083 ⎞ ⎟, k 2z (m 3 mol−1 s−1) = 6.74 × 103exp⎜ ⎝ T ⎠ [R2 = 0.99]


DETA

(18) 7312


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Table 4. Pseudo-First-Order Reaction Rate Constants for 1AP in Methanol concentration (mol·m−3)


1-AP

methanol

293 K

298 K

303 K

308 K

313 K

20.4 40.02 59.69 80.45 90.48 100.19

24.579 24.531 24.486 24.438 24.415 24.391

6.49 15.61 26.02 35.05 38.84 44.61

7.42 17.32 27.84 38.50 43.13 52.56

8.52 18.28 29.90 44.26 51.25 57.73

9.44 20.86 33.19 48.02 56.40 66.61

10.16 22.11 35.88 52.35 59.97 70.95

increase with the increase of temperature in an ethanol system. At each temperature, it is found that kethanol is too small to be comparable with k1−AP, so 1-AP is considered to be the major contribution in each system, and it is demonstrated to be the same condition with eq 12 in the zwitterion mechanism, which the deprotonation step is considered to be the rate-determining step. For the (1-AP+CO2+ethanol) system, the Arrhenius expression showing the temperature dependency of the kinetics rate constants k1−AP and kethanol were presented by

Figure 9. Parity plot for DETA in methanol.

⎛ −1483 ⎞ ⎟, k1T− AP(m 6/mol2 s) = 3.89 × 105exp⎜ ⎝ T ⎠ [R2 = 0.99]

(19)

⎛ −3579 ⎞ T ⎟, kethanol (m 6/mol2 s) = 5.57 × 105exp⎜ ⎝ T ⎠ [R2 = 0.99]

(20)

Figure 11 is parity plots for the experimental k0 values and calculated k0 values at different temperatures in 1-AP ethanol

Figure 10. Plot of pseudo-first-order reaction rate constants k0 verus 1AP concentration at different temperatures in ethanol.

Table 3. Pseudo-First-Order Reaction Rate Constants for 1AP in Ethanol concentration (mol·m−3)


DETA

ethanol

293 K

298 K

303 K

308 K

313 K

20.437 40.121 59.819 80.795 90.68 100.439

17.093 17.061 17.029 16,994 16.978 16.962

2.09 5.64 11.47 20.25 24.53 29.48

2.46 6.14 12.73 22.42 27.17 32.58

2.60 7.58 14.80 25.88 29.47 36.48

3.10 8.25 16.54 28.47 33.14 39.60

3.56 9.03 19.50 29.76 36.74 44.98

Figure 11. Parity plot for 1-AP in ethanol.

the rate constants k1−AP and kethanol were determined by fitting the pseudo-first-order rate constants (k0) to the termolecular mechanism using the Matlab software. All fitted rate constants k1−AP and kethanol are presented in Table 4 at each temperature. From data in Table 4 for the termolecular mechanism, it can be seen that the two parameters are well determined, and the obtained results of k1−AP and kethanol were both found to

system, and the AAD of these experimental data in ethanol system was calculated to be an acceptable 2.4%, which shows a great parity for ethanol system of 1-AP. 4.4. 1-AP in Methanol System. The reaction between CO2 and nonaqueous 1-AP in methanol system was studied ranging from 20.4 to 100.19 mol/m3 at 293, 298, 303, 308 and 7313


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methanol system. The same result was found in the ethanol system. At each temperature, it was found that kethanol is much smaller than k1−AP, so 1-AP is considered to be the major contributor in each system, and it is demonstrated to be the same condition with eq 12 in the zwitterion mechanism, where the deprotonation step is considered to be rate-determining step. For the (1-AP+CO2+methanol) system, the Arrhenius expressions showing the temperature dependency of the kinetics rate constants k1−AP and kmethanol were presented by

313 K. Figure 12 shows that the pseudo-first-order rate constants (k0) increase with the concentration and temperature

⎛ −2695 ⎞ ⎟, k1T− AP(m 6/mol2 s) = 1.42 × 107exp⎜ ⎝ T ⎠ [R2 = 0.98]

(21)

⎛ −1598 ⎞ T ⎟, k methanol (m 6/mol2 s) = 3.06 × 103exp⎜ ⎝ T ⎠ [R2 = 0.99]

(22)

Figure 13 is parity plots for the experimental k0 values and calculated k0 values at different temperatures in methanol

Figure 12. Plot of pseudo-first-order reaction rate constants k0 verus 1AP concentration at different temperatures in methanol.

increase of 1-AP, and displays a function relationship between the pseudo-first-order rate constants k0 and the concentration of 1-AP in methanol system, which was proved by the experimental values in Table 5. On the basis of the work of Table 5. Reaction Rate Constants of 1-AP in Ethanol T (K)

ka (m6·kmol−2·s−1)

ke (m6·kmol−2·s−1)

293 298 303 308 313

2461.9 2680.1 2909.5 3150.1 3402.0

2.8 3.4 4.1 5.0 6.0

Couchaux,20 the values of the concentrations of methanol were calculated, and listed in Table 5. In accordance with the methanol system, the consideration of OH− ions in the calculations can be ignored in methanol system. Determined by the empirical power law in Figure 12, the reaction orders between the pseudo-first-order rate constants (k0) and 1-AP concentration were calculated to be fractional orders, which were given to 1.20−1.21 relating to methanol system over a temperature range of 293−313 K. The reaction orders in the methanol system are found to have a little fluctuation. In addition, the rate constants k1−AP and kmethanol were determined by fitting the pseudo-first-order rate constants (k0) to the termolecular mehanism using the Matlab software. All fitted rate constants k1−AP and kmethanol are presented in Table 6 at each temperature, and the obtained k1−AP and kmethanol were both found to increase with the increase in temperature in

Figure 13. Parity plot for 1-AP in methanol.

system, and the AAD of these experimental data in methanol system was calculated to be within 2.6%, which shows a great parity for the methanol system of 1-AP. 4.5. Comparison of Reaction Kinetics of Two Amines (DETA and 1-AP) in Aqueous System and Nonaqueous Systems. Generally, the pseudo-first-order rate constants in aqueous and nonaqueous systems obey a similar sequence: aqueous > ethanol > methanol.8 However, it was found that the pseudo-first-order rate constants of 1-AP in methanol were higher than those in ethanol system in this study. According to the pH test shown in Table 7, the basicity rank of the 1-AP

Table 6. Reaction Rate Constants of 1-AP in Methanol T (K)

ka (m6·kmol−2·s−1)

km (m6·kmol−2·s−1)

293 298 303 308 313

1414 1632 2028 2306 2538

12.9 14.5 15.7 17.4 18.3

Table 7. pH Values of Three 1-AP Systems at the Temperature of 298 K

7314

system

1-AP+H2O

1-AP+CH3OH

1-AP+C2H5OH

concentration (mol/L) pH value

0.097 11.22

0.0995 10.9

0.1014 10.75


Article


Table 8. Second-Order Rate Constants (k2) at 298.15 K in Ethanol and Methanol Systems and the Activation Energy of DETA, EDA, AEEA (Zwitterion Mechanism) amine/system

kz2 (m3 kmol−1 s−1)

activation energy (kJ/mol)

amine concentration (kmol/m3)

DETA/ethanol DETA/methanol EDA/ethanol EDA/methanol AEEA/ethanol AEEA/methanol

8437 7275 3960 3722 2720 2440

18.00 17.32 14.36 11.42 19.82 17.28

0.01−0.05 0.01−0.05 0.021−0.0995 0.0315−0.1 0.03−0.088 0.044−0.099

source this work Kadiwala et al. (2012) Rayer et al. (2012)

law that the higher the value of k2 the faster the reaction kinetics would be, which indicates that DETA has a higher reaction rate than EDA and AEEA in nonaqueous systems. Although DETA in aqueous system has a higher reaction rate than those in ethanol and methanol systems, the DETA combined with ethanol and methanol will be a good choice for designing or optimizing the processes of capturing CO2 due to low energy consumption.

systems was: aqueous > methanol > ethanol, under the same temperature and concentration, which demonstrates that the higher basicity of the 1-AP methanol system has the higher k0 values than those in 1-AP ethanol system. In Figure 5 and Figure 8, the second-order rate constants of DETA for the zwitterion mechanism show that these values of kz2 for ethanol and methanol systems both increase with the increase of temperature. The second-order rate constant of DETA in ethanol and methanol systems are displayed in the form of corresponding Arrhenius plot (ln k2 versus 1/T) in Figure 5 and Figure 8, respectively. The activation energy obtained from the slope of the plot in Figure 5 and Figure 8 were found to be equal to 18.00 kJ/mol for the DETA+ethanol system, and 17.32 kJ/mol for the DETA+methanol system. In Table S1 shown in the Supporting Information, though the values of parameter (kz2) of DETA in aqueous system were much higher than those in ethanol and methanol systems, DETA still exhibited a high CO2 absorption rate in ethanol and methanol systems. Different from the fractional order (n ∼ 1.73) of DETA in a water system, the reaction orders were all found to be unity in ethanol and methanol systems in this work. The reaction orders of 1-AP in ethanol and methanol systems were all found to be fractional. In Table S2 shown in the Supporting Information, the values of ka of 1-AP in ethanol and methanol are compared to be 10% of ka in water and it shows that the values of methanol (km) are higher than the values of ethanol (ke), but lower than the values of ethanol (kw). Compared with the values of the term of ka of 1-AP at each temperature in aqueous system and nonaqueous systems, it is found that kw(ke/km) is too small to be comparable with ka of 1-AP, so 1-AP is considered to be the major contributor in each system, and it is demonstrated to be the same condition with eq 12 in zwitterion mechanism, which the deprotonation step is considered to be rate-determining step. 4.6. Comparison of Reaction Kinetics of Three Amines (DETA, EDA, AEEA) in Nonaqueous Systems. In this work, the reaction orders with respect to DETA in methanol and ethanol systems at the temperature range of 293−313 K were all found to be unity. In related work, the unity order is also seen from Kadiwala et al.8 and Rayer et al.,9 which studied kinetics of ethylene diamine (EDA) and 2-((2-aminoethyl)amino)ethanol (AEEA) in methanol and ethanol, respectively. In accordance with DETA in methanol and ethanol systems, the zwitterion mechanism were also used to fit the data for ethylene diamine (EDA) and 2-((2-aminoethyl)amino)ethanol (AEEA) in methanol and ethanol systems respectively by taking the zwitterion formation step as the rate-determining step (eq 10) with an acceptable AAD. The second-order reaction rate constants of DETA, EDA, AEEA at 298.15 K in ethanol and methanol systems are displayed in Table 8, and it can be seen that the kz2 of DETA is about 3.5 times higher than that of AEEA, 2.5 times higher than that of EDA. According to general

5. CONCLUSION In this study, the kinetics of the reaction of carbon dioxide (CO2) in ethanol and methanol systems of diethylenetriamine (DETA) and 1-amino-2-propanol (1-AP) were measured using the stopped flow technique over a temperature range of 293− 313 K. The reaction orders were found to be essentially unity with respect to DETA, and fractional for 1-AP in ethanol and methanol systems. For DETA, the reaction rates are still at a much higher level than those of other amines in nonaqueous systems with the reaction rate constants higher in ethanol systems than those in methanol systems. For 1-AP, the reaction rates in nonaqueous systems are much lower than those in aqueous systems with the reaction rate constants higher in the methanol system than those in the ethanol system, which is different from the universal law. The zwitterion mechanism was used to fit the data for the DETA nonaqueous system by taking the zwitterion formation step as the rate-determining step (eq 10) with an acceptable AAD of 3.5% and 2.4%, respectively. The termolecular mechanism was used to fit the data for 1-AP nonaqueous systems with an acceptable AAD of 2.4% and 2.6%, respectively. Analysis using the termolecular mechanism suggested that the contribution of methanol/ethanol to deprotonation was too small to compare with that of 1-AP in ethanol and methanol systems, which is demonstrated to be the same condition with eq 11 in the zwitterion mechanism by considering the deprotonation step as the rate-determining step. A number of amines studied with the stopped-flow technique in nonaqueous systems are compared in terms of the second-order reaction rates, and the rank of different amines was DETA > EDA > AEEA. This study further shows that DETA has a higher reaction rate than other amines in nonaqueous systems. The results of the fundamental kinetics study on nonaqueous solutions in this work will help efforts to find efficient methods for application in the removal of CO2 from industrial gases.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00981. Comparsion of second-order rate constants for DETA in aqueous systems and non-aqueous systems and compar7315


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(2) Abu-Zahra, M. R.; Schneiders, L. H.; Niederer, J. P.; Feron, P. H.; Versteeg, G. F. CO2 capture from power plants: Part I. A parametric study of the technical performance based on monoethanolamine. Int. J. Greenhouse Gas Control 2007, 1 (1), 37−46. (3) Kohl, A. L.; Nielsen, R. Gas purification; Gulf Professional Publishing: Houston, TX, 1997. (4) Rao, A. B.; Rubin, E. S. A technical, economic, and environmental assessment of amine-based CO2 capture technology for power plant greenhouse gas control. Environ. Sci. Technol. 2002, 36 (20), 4467− 4475. (5) Wall, J. Gas processing handbook. Hydrocarbon Process. 1979, 58 (4), 110. (6) Bratzler, K.; Doerges, A. Amisol process purifies gases. Hydrocarbon Process. 1974, 53 (4), 78−80. (7) Ali, S. H.; Merchant, S. Q.; Fahim, M. A. Kinetic study of reactive absorption of some primary amines with carbon dioxide in ethanol solution. Sep. Purif. Technol. 2000, 18 (3), 163−175. (8) Kadiwala, S.; Rayer, A. V.; Henni, A. Kinetics of carbon dioxide (CO2) with ethylenediamine, 3-amino-1-propanol in methanol and ethanol, and with 1-dimethylamino-2-propanol and 3-dimethylamino1-propanol in water using stopped-flow technique. Chem. Eng. J. 2012, 179, 262−271. (9) Rayer, A. V.; Henni, A.; Li, J. Reaction kinetics of 2-((2aminoethyl) amino) ethanol in aqueous and non-aqueous solutions using the stopped-flow technique. Can. J. Chem. Eng. 2013, 91 (3), 490−498. (10) Crooks, J. E.; Donnellan, J. P. Kinetics of the formation of N, Ndialkylcarbamate from diethanolamine and carbon dioxide in anhydrous ethanol. J. Chem. Soc., Perkin Trans. 2 1988, No. 2, 191− 194. (11) Sada, E.; Kumazawa, H.; Han, Z.; Matsuyama, H. Chemical kinetics of the reaction of carbon dioxide with ethanolamines in nonaqueous solvents. AIChE J. 1985, 31 (8), 1297−1303. (12) Sada, E.; Kumazawa, H.; Ikehara, Y.; Han, Z. Chemical kinetics of the reaction of carbon dioxide with triethanolamine in non-aqueous solvents. Chemical Engineering Journal 1989, 40 (1), 7−12. (13) Sada, E.; Kumazawa, H.; Osawa, Y.; Matsuura, M.; Han, Z. Reaction kinetics of carbon dioxide with amines in non-aqueous solvents. chemical engineering journal 1986, 33 (2), 87−95. (14) Versteeg, G.; Van Swaaij, W. On the kinetics between CO2 and alkanolamines both in aqueous and non-aqueous solutionsI. Chem. Eng. Sci. 1988, 43 (3), 573−585. (15) Yu, C.-H.; Wu, T.-W.; Tan, C.-S. CO2 capture by piperazine mixed with non-aqueous solvent diethylene glycol in a rotating packed bed. Int. J. Greenhouse Gas Control 2013, 19, 503−509. (16) Liang, Z. H.; Sanpasertparnich, T.; Tontiwachwuthikul, P. P.; Gelowitz, D.; Idem, R. Part 1: Design, modeling and simulation of post-combustion CO2 capture systems using reactive solvents. Carbon Manage. 2011, 2 (3), 265−288. (17) Sema, T.; Naami, A.; Liang, Z.; Idem, R.; Ibrahim, H.; Tontiwachwuthikul, P. 1D absorption kinetics modeling of CO2DEAB-H2O system. Int. J. Greenhouse Gas Control 2013, 12, 390−398. (18) Van Loo, S.; Van Elk, E.; Versteeg, G. The removal of carbon dioxide with activated solutions of methyl-diethanol-amine. J. Pet. Sci. Eng. 2007, 55 (1), 135−145. (19) Sema, T.; Naami, A.; Liang, Z.; Shi, H.; Rayer, A. V.; Sumon, K. Z.; Wattanaphan, P.; Henni, A.; Idem, R.; Saiwan, C.; Tontiwachwuthikul, P. Part 5b: Solvent chemistry: reaction kinetics of CO2 absorption into reactive amine solutions. Carbon Manage. 2012, 3 (2), 201−220. (20) Couchaux, G.; Barth, D.; Jacquin, M.; Faraj, A.; Grandjean, J. Kinetics of carbon dioxide with amines. I. Stopped-flow studies in aqueous solutions. A review. Oil Gas Sci. Technol. 2014, 69 (5), 865− 884. (21) Vaidya, P. D.; Kenig, E. Y. Gas−Liquid reaction kinetics: a review of determination methods. Chem. Eng. Commun. 2007, 194 (12), 1543−1565. (22) Yamada, H.; Chowdhury, F. A.; Goto, K.; Higashii, T. CO2 solubility and species distribution in aqueous solutions of 2-

sion of kinetics rate constants for 1-AP in different systems (PDF).

AUTHOR INFORMATION

Corresponding Authors

*Dr. Zhiwu Liang. Tel.: +86-13618481627. Fax: +86-73188573033. E-mail: [email protected] (Z. Liang). *Dr. Xiao Luo. E-mail: [email protected] (Xiao Luo). Author Contributions ⊥

Nan Zhong and Helei Liu contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the National Natural Science Foundation of China (NSFC Nos. 21536003, 21476064, U1362112, 21376067 and 51521006), National Key Technology R&D Program (MOST Nos. 2012BAC26B01 and 2014BAC18B04), Innovative Research Team Development Plan (MOE-No. IRT1238), Specialized Research Fund for the Doctoral Program of Higher Education (MOE No. 20130161110025), China’s State “Project 985” in Hunan University-Novel Technology Research & Development for CO2 Capture, Key Project of International & Regional Cooperation of Hunan Provincial Science and Technology plan (2014WK2037), and China Outstanding Engineer Training Plan for Students of Chemical Engineering & Technology in Hunan University (MOE No. 2011-40), NPRP Grant # 71154-2-433 from the Qatar National Research Fund (a member of Qatar Foundation) and China Scholarship Council (CSC) are gratefully acknowledged.

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NOMENCLATURE [] = concentration (mol m−3) A = Arrhenius constant (m3 mol−1 s−1) AAD = absolute average deviation B = amine, ethanol or methanol C(S) = constant conductance value at at the end of observed reaction (μs/m) Ea = activation energy (kJ mol−1) G = value of conductivity obtained from the conductivity meter (μs/m) k0 = observed pseudo-first-order reaction rate constant (s−1) k−1 = backward first-order reaction rate constant (s−1) k2 = forward second-order reaction rate constant (m3 kmol−1 s−1) k2z = forward second-order reaction rate constant in zwitterion mechanism (m3 kmol−1 s−1) ka = k2kamine/k−1 (m6 kmol−2 s−1) kBase = zwitterion deprotonation rate constant by Base (amine or ethanol/methanol) (m3 mol−1 s−1) ke = k2kethanol/k−1 (m6 kmol−2 s−1) km = k2kmethanol/k−1 (m6 kmol−2 s−1) R = universal gas constant (0.008 315 kJ mol−1 K−1) T = temperature (K) t = time(s)

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