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Carbon Dioxide Absorption into Aqueous Blends of Methyldiethanolamine (MDEA) and Alkyl Amines Containing Multiple Amino Groups Song Yi Choi,†,‡ Sung Chan Nam,† Yeo Il Yoon,† Ki Tae Park,*,† and So-Jin Park*,‡ †

Greenhouse Gas Laboratory, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea Department of Chemical Engineering, Chungnam National University, 99 Deahak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea

‡

ABSTRACT: Alkyl amines, which have multiple amino groups, are used as activators to improve the CO2 absorption performances of aqueous methyldiethanolamine (MDEA) solutions. The aqueous MDEA blends consisted of 20% (w/w) of MDEA and 10% (w/w) of activators, which are 3-methylamino propylamine (MAPA), diethylenetriamine (DETA), triethylene tetramine (TETA), and tetraethylenepentamine (TEPA). Aqueous solutions of monoethanolamine (MEA; 30% (w/w)) and MDEA (30% (w/w)) are used as reference absorbents for comparison. The CO2 absorption performances of aqueous MDEA blends were investigated by measurements of absorption capacities, absorption rates, and heats of absorption. The MDEA blends have higher CO2 absorption capacities than MEA and MDEA. MDEA/TEPA shows the highest CO2 loading amount of 0.753 mol-CO2·mol-absorbent−1 at 313 K. In addition, the MDEA blends show high cyclic capacities (0.241−0.330 mol-CO2·molabsorbent−1), the values of which are about 3 times higher than that of MEA. All MDEA blends show higher absorption fluxes than MDEA. The MDEA/MAPA showed the highest overall mass transfer coefficient of 3.351 × 103 mol·m−2·s−1·kPa−1, 8 times higher than that of MDEA (0.451 × 103 mol·m−2·s−1·kPa−1) and even higher than that of MEA (3.014 × 103 mol·m−2·s−1·kPa−1). The heats of absorption of the MDEA blends (57.21−59.53 kJ·mol-CO2−1) are about 30% higher than that of MDEA and about 30% lower than that of MEA.

1. INTRODUCTION Carbon dioxide (CO2) emission from combustion of fossil fuels is regarded as a major contribution to global warming. The main source of CO2 emission is flue gases from industrial and power plants. Thus, CO2 removal from these sources has become a great challenge in recent years. At present, one effective method for the removal of CO2 usually employs chemical absorption technology using aqueous solutions of alkanolamines or their mixtures. A wide variety of alkanolamines have been used for this purpose. Among them, the aqueous solutions of monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA), and N-methyldiethanolamine (MDEA) are the most popular solvents. Typically, alkanolamines are classified into three amine groups of primary, secondary, and tertiary by the numbers of hydrogen atoms substituted with other functional groups, which are attached to the nitrogen atom of the amino group. The absorption capacity of amines for CO2 decreases from the primary to the tertiary type. However, the absorption capacity is not the most important characteristic of absorbents, because the energy requirement for solvent regeneration increases in the same direction. Among the different types of alkanolamines, MDEA, the tertiary amine, has found great interest for CO2 absorption, because MDEA has lower volatility, thermal stability, less alkalinity, higher CO2 loading capacity (up to 1.0 mol-CO2·mol-amine−1), and less regeneration cost.1 One of its drawbacks is that because it is a tertiary amine, it does not react with CO2 directly but just promotes CO2 hydrolysis to form bicarbonate; thus, the absorption of CO2 into MDEA is too slow to use as absorbent alone.1,2 © 2014 American Chemical Society

Many efforts have been focused on the enhancement of absorption rate of CO2 into MDEA solution. The technology of adding activators to utilize the advantages of each amine has been proposed3 and has found widespread application in the selective removal of CO2 from process gas streams.4 The blended amine system is based on the combination of the higher equilibrium capacity and lower reaction enthalpy of the tertiary amine and the higher reaction rate of the primary or secondary amine. This leads to considerable improvement in absorption rate while maintaining a low energy penalty caused by regeneration of the used solvent in the stripping column. Astarita et al.5 proposed a “shuttle mechanism”, which describes the fast-reacting component as being regenerated by means of the interaction of the equilibrium reactions in the liquid bulk. In the same vein, Critchfield and Rochelle6 and Chakravarty et al.3 have proposed the mechanism of improvement of the absorption rate of MDEA by addition of the primary and secondary amines to the MDEA aqueous solution. The shuttle mechanism is shown in Figure 1. The primary and secondary amines can rapidly react with CO2 molecules near the gas−liquid interface. The reaction product is a carbamate that penetrates into the bulk liquid phase and reacts with excess MDEA molecules. In this process, the activator transfers the absorbed CO2 to the MDEA molecule, and it is regenerated to the original amine molecule, which can react with CO2 at the Received: Revised: Accepted: Published: 14451

June 17, 2014 August 20, 2014 August 27, 2014 August 27, 2014 dx.doi.org/10.1021/ie502434m | Ind. Eng. Chem. Res. 2014, 53, 14451−14461

Industrial & Engineering Chemistry Research

Article

spheric pressures. They found that the addition of DEA increases the CO2 absorption rate. They also determined the reaction rate constant for the reaction between CO2 and DEA considering a pseudo-first-order reaction. Zoghi et al.12 investigated the kinetics of absorption of CO2 into aqueous solutions of MDEA with different activators of PZ, DIPA, 2-amino-2-methyl-1-propanol (AMP), diglycolamine (DGA), and aminoethylethanolamine (AEEA) at a temperature range of 23.1−27.7 °C and 10 atm. They reported that the fastest absorption rate is achieved for the aqueous solution of AEEA/MDEA with a molar ratio of activator to MDEA equal to 0.125. Amann and Bouallou19 selected triethylene tetramine (TETA) to activate an aqueous solution of MDEA. It has four amino groups, two primary groups, and two secondary groups, making it very reactive with CO2. They investigated CO2 absorption into aqueous MDEA/TETA solutions with different concentrations of MDEA and TETA at 298, 313, and 333 K for several CO2 loadings in the solvent. They reported that the addition of a small amount of TETA leads to a significant enhancement of the absorption rates compared to an aqueous MDEA solution and, moreover, the absorption capacity of the solvent is increased. Amann and Bouallou20 investigated the performance of CO2 capture processes assessed for two types of power plants of natural gas combined cycle power plant (NGCC) and pulverized coal power plant (PC) with aqueous blend of MDEA/TETA using the software Aspen Plus. The new solvent containing the blend of MDEA and TETA showed 15−25% reduced energy requirement for the solvent regeneration compare to the results obtained with MEA and MDEA. All the previous works show that the addition of a small amount of reactive amine results in a significant improvement of the CO2 absorption rate. Results of reaction kinetics study on CO2 absorption with alkyl amines containing multiple amino groups such as 3methylamino propylamine (MAPA), diethylenetriamine (DETA), TETA, and tetraethylenepentamine (TEPA) have been reported in the literature.19,21−26 A summary of a literature review for forward second-order reaction rate constants (k2) and observed reaction rate constants (kobs) for 1 kmol·m−3 amine solutions are given in Table 1.19,21−23,26−29 As shown in Table 1, MAPA, DETA, and TETA have at least hundreds of times higher rate constants than that of MDEA. Aronu et al.24 investigated the absorption rate of TEPA using

Figure 1. Shuttle mechanism of CO2 absorption by activator.

gas−liquid interface again. This cycle is repeated and, as a result, the absorption rate increases. Many kinds of activators of fast reactivity have been investigated and reported in the literature.6−13 Xu et al.7 measured the absorption rate of pure CO2 gas at atmospheric pressure into MDEA/piperazine (PZ) using a disk column in the temperature range of 303−343 K. The concentration range of PZ was 0.014−0.21 M, which was too low to be used commercially for CO2 removal processes. Zhang et al.8 have found the second-order carbon dioxide absorption rate constant in the MDEA/PZ solution. The molar ratio of PZ/MDEA has been fixed, and the experiments have been carried out at atmospheric pressure. They also suggested the mechanism of CO2 absorption into an aqueous MDEA/PZ solution using a disk column with PZ concentration range of 0.2−0.6 M. Bishnoi and Rochelle9 reported CO2 absorption data in 0.6 M PZ in 4.0 M aqueous MDEA solutions at 313 and 343 K using a wetted wall column reactor. A rate model based on eddy diffusivity theory was used for prediction of the absorption rate and enhancement factor. Mandal et al.14 investigated the aqueous blends of MDEA/ MEA solution. They reported that the addition of small amounts of MEA to an aqueous solution of MDEA can increase the rate of CO2 absorption significantly. Liao and Li15 investigated CO2 absorption into aqueous solutions of MEA/ MDEA with different concentrations of MDEA and MEA over a wide range of temperatures. Their results coincide well with the results obtained by Mandal et al.14 Although they used different concentrations of MEA/MDEA solutions, they both found that the addition of small amounts of MEA increase the absorption rate of CO2. Ramachandran et al.16 reported that the absorption rate of CO2 into MEA/MDEA solutions is governed by the zwitterion mechanism. Zhang et al.17 found the absorption rate constant of CO2 into aqueous DEA/MDEA solutions at different temperatures and concentrations. Lin et al.18 used aqueous DEA/MDEA solutions at different temperatures and a range of subatmo-

Table 1. Summary of Literature Review for Reaction Rate Constants absorbent MEA

MDEA

MAPA

DETA

TETA

temperature (K) 298 298.15 303.15 298

kobs (at 1 kmol·m−3) (103 s−1)

k2 (m3·kmol−1·s−1) 4.89

6.0 0.0084 0.0051

311.5 298.15

96.5

298 297.9

26.5

313.5 14452

ref 27 26 28 29

68−74

22 21

21.048

23 23

3.2

19

dx.doi.org/10.1021/ie502434m | Ind. Eng. Chem. Res. 2014, 53, 14451−14461


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Table 2. Summary of Literature Review for CO2 Solubility amine concentration

temperature (K)

PCO2 (kPa)

loading (mol-CO2·mol-absorbent−1)

ref

MEA

30% (w/w)

313 333 353

12.8 15.1 14.7

0.512 0.495 0.421

36 36 36

MDEA

2 kmol·m−3 30% (w/w)

313 313 333 353

15.4 6.5 7.7 8.9

0.478 0.401 0.312 0.220

4 36 36 36

MAPA

2 kmol·m−3

313 353

14.4 17.2

1.184 1.033

31 31

DETA

30% (w/w)

313 333 353

33.8 33.8 33.8

1.23 1.14 0.97

35 35 35

TETA

18% (w/w)

313

15

1.71

30

TEPA

1 kmol·m−3

313

1.916

24

absorbent

9.5

Table 3. Concentrations and Compositions of the Tested Absorbents absorbent MEA MDEA MDEA/MAPA MDEA/DETA MDEA/TETA MDEA/TEPA a

concn (composition) (% (w/w)) 30 30 30 30 30 30

(20/10) (20/10) (20/10) (20/10)

molar concn of absorbent (mol-absorbent·L−1)

molar concn of amino group (mol-amino group·L−1)

4.91 2.52 2.81 2.65 2.36 2.21

4.91 2.52 3.95 4.59 4.41 4.32

no. of amino groupsa 1P 1T 1T/1P, 1T/2P, 1T/2P, 1T/2P,

1S 1S 2S 3S

P, S, and T mean primary, secondary, and tertiary amino groups, respectively.

2. EXPERIMENTAL SECTION 2.1. Materials. The CO2 and N2 gases with volume fractions of 0.9999 and 0.99999, respectively, were supplied by Special Gas, Korea. MDEA (≥99%), MEA (≥99%), and DETA (≥99%) were obtained from Sigma-Aldrich. MAPA (≥97%), TETA (≥97%), and TEPA (technical grade) were obtained from Fluka, Junsei, and Aldrich, respectively. All chemicals used in this work were used as received without further purification. The aqueous solutions of 30% (w/w) MEA and 30% (w/w) MDEA were used as reference absorbents to compare with MDEA blends about the CO2 absorption performances such as absorption capacity, absorption rate, and heat of absorption. The total amine concentration of all aqueous blends of MDEA was fixed at 30% (w/w) including 20% (w/w) of MDEA with 10% (w/w) of activators. All of the solutions were prepared with deionized water. The concentrations and compositions of the aqueous solutions and the number of amino groups in each absorbent used in this work are given in Table 3. 2.2. Measurement of CO2 Absorption Capacity. A semibatch absorption system was used to measure the CO2 absorption capacities of the absorbents. Figure 2 shows a schematic diagram of the experimental apparatus used in this work. The apparatus consisted of a gas mixer for mixing the CO2 and N2 and for supplying the mixture at a certain concentration, a reactor, and a water bath for maintaining the reactor at a constant temperature. The 30% (v/v) CO2 gas mixture was prepared by mixing 99.99% (v/v) CO2 and 99.999% (v/v) N2 in the gas mixer. The gas mixture was

combined absorption−desorption analysis, and 1 M TEPA showed an absorption rate higher than that of 5 M MEA. In addition, many researchers have reported the CO2 solubility into alkyl amines containing multiple amino groups such as MAPA, DETA, TETA, TEPA, and their mixture.30−35 A summary of a literature review for CO2 solubility is given in Table 2.4,24,30,31,35,36 The CO2 solubilities into MAPA, DETA, TETA, and TEPA were higher than those into MEA and MDEA. From these literature results, it is found that alkyl amines containing multiple amino groups have high reactivity with CO2. The objective of the work described here is to propose new types of activators and evaluate the enhancement of CO2 absorption rate and absorption capacity, resulting from the addition of different activators to aqueous MDEA solution. The compounds that have been considered as activators are MAPA, DETA, TETA, and TEPA. Those have multiple amino groups including primary and secondary amino groups. Because these are very reactive with CO2, it was proposed to use these amines as activators in an aqueous MDEA solution. In addition, the heats of absorption of the aqueous blends of MDEA solutions were also measured at 298 K using a differential reaction calorimeter (DRC). From the results, we discuss the effects of the activators on the CO2 absorption performances of aqueous MDEA solution. 14453



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Figure 2. Schematic diagram of the semibatch CO2 absorption system.

supplied continuously to the bottom of the reactor through a sparger to form fine bubbles at a flow rate of 1 L·min−1. The internal volume of the stainless steel reactor was 2 L. The measurements were carried out at temperatures of 313, 333, and 353 K under atmospheric pressure. The temperature and pressure in the reactor were measured by using a K-type thermocouple (±1.2 K) and a pressure transmitter (PSCH0030K, Sensys, ±7.5 kPa), respectively. For all experiments, the volume of absorbents was 1 L, and the solution was stirred at 500 rpm. A gas chromatograph (GC; 6890N, Agilent) analyzer was used to measure quantitatively the total amount of CO2 dissolved in the amine solutions. The concentrations of CO2 discharged after reaction with the absorbents were measured by the GC analyzer. A packed column (Porapak-Q, 0.32 m × 6 ft, Supelco) was used as the GC column, along with a thermal conductivity detector (TCD). The experiments were carried out by 4.3 min intervals, retention time of GC, until equilibrium, when the CO2 concentration of the outlet is the same as that of the inlet. At each time, the molar amount of CO2 absorbed was calculated by eq 1 from the difference of the number of CO2 molecules between inlet and outlet gas streams. The reduced amount of CO2 in the outlet gas stream by gas− liquid reaction corresponds to the absorbed amount of CO2 into absorbent. The use of the ideal gas law in eq 1 is allowed, as the maximum pressure in the apparatus never exceeded 4 bar. nCO2 =

Figure 3. Schematic diagram of the (a) wetted-wall column system and (b) wetted-wall column reactor.

and a 12.6 mm outside diameter. The column is enclosed in a cylindrical thick-wall glass, and the whole chamber is surrounded by a second glass wall with water flowing between as heat transfer medium. The liquid absorbent was pumped up at a flow rate of 150 mL·min−1 into the vertical tube and then flowed downward under gravity, forming a thin film along the outside surface of the tube. Feed gas supplied to the bottom of the chamber at a flow rate of 5 L·min−1 counter-currently contacts the liquid and then exits from the top of the chamber. Both gas and liquid streams were heated to a reaction temperature of 313 K by a preheater, and the inside of the chamber was controlled to a constant temperature at 313 K. The concentration of CO2 of the feed gas stream was varied from 3 to 9% (v/v) by a mass flow controller (Brooks, 5850E). The gas concentrations were analyzed using the nondispersive infrared detector (NDIR; Fuji, ZKJ) continuously. Measurement of CO2 concentrations in the inlet and outlet gas streams provides CO2 partial pressures as well as CO2 flux between gas and liquid. Overall mass transfer coefficient (KG) was calculated according to the procedure below. Mass transfer of CO2 took place from the gas phase to a liquid layer flowing countercurrently over the surface of the tube with chemical reaction. The amount of CO2 absorbed at the gas−liquid interface and mass flux (NCO2) were determined by multiplying the driving force with the mass transfer coefficient (eq 3). Because it is difficult to acquire information on the CO2 concentration at the

Δ(pCO Vgas) 2

(1) RT Consequently, the CO2 loading can be obtained from eq 2 t

αCO2 =

∫0 eq nCO2 dt nabsorbent

(2)

where αCO2, nabsorbent, and teq indicate the CO2 loading (molCO2·mol-absorbent−1), the number of moles of absorbent molecules in the solution, and the reaction time until equilibrium. 2.3. Measurements of CO2 Absorption Rate. Overall mass transfer coefficients of the absorbents were measured by a short wetted-wall column (WWC) apparatus at 313 K. The construction of the WWC apparatus is shown in Figure 3. The WWC is consisted of a stainless steel tube with a 90 mm height 14454



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interface, KG and equilibrium CO2 partial pressure of the liquid phase (p∗CO2) were used. ∗ NCO2 = K G(pCO − pCO ) 2

respectively. The temperature probes and Joule effect calibration probe were additionally installed in the measurement reactor. One hundred milliliters of absorbent was supplied to both reactors, and a gas mixture of 30% (v/v) CO2 (N2 based) was supplied at a flow rate of 100 mL·min−1 into the measurement reactor, whereas there was no gas stream in the reference reactor. The inlet gas was fed to the measurement reactor in bubble form, and the absorbent was stirred at 200 rpm to increase the contact area between the gas and liquid phases. The 7890N model GC from Agilent and Porapak-Q (0.32 m × 6 ft, Supelco) column and TCD detector were used to measure the CO2 concentration in the inlet and outlet gas streams. The experimental temperature was controlled to 298 K. The measurement of reaction heat was conducted according to the following three steps:38 (i) calibration before reaction; (ii) reaction; (iii) calibration after reaction. The principle of the reaction heat determination is that the exchanged heat between the reaction mixture and the reactor wall is proportional to the area under the ΔT versus time curve. The temperature difference between the two reactors was recorded as a function of time. While thermal phenomenon occurs, the heat flow is

(3)

2

The term of pressure difference, the driving force, can be represented using the log mean average pressure difference. pCO ,in − pCO ,out 2 2 ΔPCO2, lm = ∗ ∗ ln((pCO ,in − pCO )/(pCO ,out − pCO )) (4) 2

2

2

2

p∗CO2

pCO2,in and pCO2,out are experimental data, and can be negligible in the case of CO2 unloaded absorbent. Then KG can be represented as below: KG =

NCO2 ΔPCO2,lm

(5)

Mass flux (NCO2) can be calculated using the exposure area of the liquid film and the absorption rate (q), as q NCO2 = π d hh (6) where dh is the hydraulic diameter, including the tube diameter (d) and liquid film thickness ( f), and h is the column height. The hydraulic diameter (dh) was calculated by using the following equation: dh = d + f

Q flow = UA ×

t

ΔT dt

(9)

UA is the overall heat transfer coefficient of the reactor in W· K−1. The electrical calibration realized by Joule effect allows us to determine the heat transfer value (UA), but the UA value depends on the mixture and its composition, so UA is unknown along the reaction. Thus, an average value (UAav) is calculated from two calibrations.

(7)

The liquid film thickness ( f) can be calculated by using the following equation:37 ⎛ 3μυ ⎞1/3 f=⎜ ⎟ ⎝ πgdρ ⎠

∫0

UA av =

(8)

1 (UA1 + UA 2) 2

(10)

The density and viscosity of the absorbent were measured at 313 K by a densimeter (Mettler-Toledo RM40) and viscometer (Brookfield DV-II+PRO), respectively. Obtained densities and viscosities of the MDEA blends are described in Table 4. Then the KG can be determined by analyzing the linear regression of the ΔPCO2,lm versus NCO2 plot on the several log mean driving force of CO2 partial pressure.

UA1 and UA2 are heat transfer values before and after reaction, respectively. After measurement, the calculated UA av was used to calculate the reaction heat (QR) with the integration of the ΔT versus time curve recorded during the reaction step.

Table 4. Densities and Viscosities of Aqueous Blends of MDEA at 313 K

3. RESULTS AND DISCUSSION 3.1. CO2 Absorption Capacities. CO2 loading amounts of the aqueous blends of MDEA at 313, 333, and 353 K are shown in Figures 5, 6, and 7, respectively. The CO2 loading amounts of absorbents were almost saturated after 200 min of absorption time at all three different temperature conditions, except for 30% (w/w) MDEA at 313 K. The CO2 loading amount of 30% (w/w) MDEA was saturated after 1000 min with a saturated loading amount of 0.482 (mol-CO2·mol-absorbent−1) at 313 K. The CO2 absorption capacities, the saturated CO2 loading amounts, of the tested absorbents are given in Table 5. When the results of absorption capacities of MEA and MDEA are compared with literature data of CO2 solubility given in Table 2, results in this work are slightly lower than those of the literature at all tested temperature conditions. This is because more reaction time was needed to reach the gas−liquid equilibrium even after the outlet CO2 concentration is the same as that of the inlet gas stream. The MDEA blends showed higher CO2 loading amounts than MEA and MDEA at all tested conditions, and those values were increased with the

absorbent

density (g·cm−3)

viscosity (cP)

MEA MDEA MDEA/MAPA MDEA/DETA MDEA/TETA MDEA/TEPA

1.004 1.019 1.005 1.016 1.019 1.020

1.69 1.89 2.12 2.24 2.36 2.46

Q R = UA av ×

2.4. Measurements of Heat of Absorption. The heat of absorption of the absorbents was measured using a DRC (SERARAM). Figure 4 shows a schematic diagram of the apparatus. It consisted of two mechanically agitated glass reactors (250 mL volume) of measurement reactor and reference reactor. The cooling water was circulated between the reactor and jacket to maintain the temperature of each reactor wall. A digital agitator and a thermocouple (±0.01 °C) were used to stir the solution and measure the temperature, 14455

∫0

t

ΔT dt

(11)



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Figure 4. Schematic diagram of the differential reaction calorimeter (DRC).

Figure 5. CO2 loading amount of aqueous blends of MDEA with absorption time at 313 K.


The CO2 loading amount decreased with increasing temperature as shown in Figures 5−7 due to the change of gas−liquid equilibrium. In addition, the differences between the CO2 loading amount (mol-CO2·mol-absorbent−1) of 30% (w/ w) MEA and the MDEA blends were narrowed with increasing temperature. The average CO2 loading amounts of MDEA blends were about 57% higher than that of MEA at 313 K; however, the difference was reduced to about 13% at 353 K. These results can be explained by the stability of carbamate ions (RNHCOO−), products of CO2 absorption reaction of primary and secondary amines. The absorption reactions of CO2 into primary (RNH2) and secondary (RR′NH) amines are as follows:

number of amino groups in the activators. The MDEA/TEPA showed the highest loading amount of 0.753 mol-CO2·molabsorbent−1. This is a predictable result, because they have multiple amino groups (more than two), and TEPA has five amino groups as described in Table 1. Nevertheless, the CO2 loading amount of the MDEA blends, based on the moles of amino groups in the absorbents (values in parentheses in Table 5), were comparable with those of MEA and MDEA. These results show that the amino groups in the activators are reactive and can participate in the CO2 absorption reaction. Thus, the activators are effective in increasing the absorption capacity of MDEA. The results of mass basis CO2 absorption capacities (gCO 2·kg-solution −1 ) also support this. The mass basis absorption capacities of MDEA blends were about 1.5 times higher than that of 30% (w/w) MDEA at 313 K, the absorber temperature in the CO2 capture processes. The MDEA blend of MDEA/DETA shows the highest CO2 loading amount of 84 g-CO2·kg-solution−1 among the tested MDEA blends.

CO2 + 2RNH 2 ⇔ RNHCOO− + RNH+3 −

RNHCOO + H 2O ⇔ RNH 2 +

HCO−3

HCO−3 + H 2O ⇔ HCO−3 + H3O+ 14456

(12) (13) (14)



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primary−secondary dicarbamate ions were formed as the secondary amino groups participate in the absorption reaction as the CO2 loading increased. The stabilities of the carbamate ions were higher in the order primary carbamate > primary− primary dicarbamate > primary−secondary dicarbamate.35 The activators used in this study, which have primary and secondary amino groups, react with CO2 to form primary carbamate, primary−primary dicarbamate, primary−secondary dicarbamate, and secondary carbamate ions.35,40 The CO2 loading amounts of the MDEA blends became similar to that of MEA with reaction temperature increasing from 313 to 353 K, because the secondary and primary−secondary carbamate ions became relatively unstable compared to primary and primary−primary carbamate ions. Therefore, it is considered that the main species of the absorption reaction of MDEA blends were primary carbamate ions and a few primary− primary dicarbamate ions at 353 K, although the loading amounts of MDEA blends were slightly higher than that of MEA due to some presence of primary−primary dicarbamate ions. The differences of CO2 loading amounts of the absorbents between 313 and 353 K are given in Table 6. Typically, in the


Satori and Savage39 reported the stability of carbamate ions in aqueous solutions of amine. On the basis of eq 12, the maximum CO2 loading of primary and secondary amines is limited to 0.5 mol-CO2·mol-absorbent−1; however, the CO2 loading increases when the partial pressure of CO2 increases or the carbamate ions are unstable. If the carbamate ions are unstable, free amines are generated as the carbamate ions are converted to bicarbonate (HCO−3 ) or carbonate (CO2− 3 ) ions by hydrolysis as shown in eqs 13 and 14. Carbamate ions have more stable bonding with CO2 than bicarbonate and carbonate ions. In addition, Kim et al.35 investigated the species distribution in the 30% (w/w) aqueous solutions of MAPA, DETA, TETA, and TEPA after CO2 absorption reaction. They confirmed formations of primary carbamate, primary−primary dicarbamate, and primary−secondary dicarbamate ions from 13C NMR studies. Initially, primary carbamate ions were formed at low CO2 loading, and the primary−primary dicarbamate and

Table 6. Difference of CO2 Loading Amount of MDEA Blends between 313 and 353 K Δ (CO2 loading amount) absorbent

mol-CO2·mol-absorbent−1

g-CO2·kg-solution−1


0.077 0.356 0.241 0.268 0.306 0.330

16.66 39.47 29.65 31.15 31.63 31.85

CO2 capture processes, the absorptions and regeneration temperatures of chemical CO2 absorbents are about 313 and

Table 5. CO2 Absorption Capacities of Aqueous Blends of MDEA CO2 loading amount absorbent

temperature (K)


313


333


353

mol-CO2·mol-absorbent

−1

(mol-CO2·mol-amino group−1)

0.459 0.482 0.651 0.727 0.743 0.753 0.448 0.204 0.523 0.605 0.598 0.593 0.382 0.126 0.411 0.459 0.438 0.424 14457

g-CO2·kg-solution−1

(0.463) (0.420) (0.398) (0.385)

99.19 53.44 80.47 84.67 77.11 73.08

(0.372) (0.349) (0.320) (0.303)

96.79 22.62 64.66 70.54 62.10 57.66

(0.292) (0.265) (0.234) (0.217)

82.53 13.97 50.82 53.52 45.48 41.23



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393 K, respectively. The difference between rich- and lean-CO2 loadings of absorbent, so-called cyclic capacity, is one of the most important parameters in the estimation of the absorption performance of absorbents and to design CO2 capture process. In this study, the gaps of CO2 absorption capacities between 313 and 353 K were used to estimate the cyclic capacities of the absorbents. The aqueous solution of MEA showed the lowest cyclic capacity of 0.77 mol-CO2·mol-absorbent−1, whereas that of MDEA showed the highest cyclic capacity of 0.356 mol-CO2· mol-absorbent−1. The mass basis cyclic capacities of the aqueous solutions of MEA and MDEA also showed the same tendency with the results of mole basis. The aqueous solution of MDEA has the highest cyclic capacity of 39.47 g-CO2·kgsolution−1. This value is 2.4 times higher than that of MEA (16.66 g-CO2·kg-solution−1). MEA is the typical primary amine, which produces carbamate ions when it reacts with CO2. Thus, relatively stable carbamate ions of MEA could exist; however, the bicarbonate ions and MDEAH+, the CO2 absorption products of MDEA (eq 15), are converted to H2O and free MDEA easily by the release of CO2 molecules at high temperature. CO2 + H 2O + MDEA ⇔ HCO−3 + MDEAH+

Figure 8. CO2 absorption fluxes of aqueous blends of MDEA (CO2 unloaded) with different CO2 partial pressures at 313 K.

(15)

Therefore, the aqueous solution of MEA could maintain a relatively high CO2 loading at high temperature; on the other hand, the CO2 loading of aqueous MDEA decreased drastically with increasing reaction temperature. The MDEA blends showed high cyclic capacities in the range from 0.241 to 0.330 mol-CO2·mol-absorbent−1, these values being about 3 times higher than that of MEA and corresponding to 68−93% of MDEA’s. In addition, activators that have many more amino groups showed higher cyclic capacities. The mass basis cyclic capacities of MDEA blends (29.65−31.85 g-CO2·kg-solution−1) were 2 times higher than that of MEA and corresponded to 73% of MDEA’s. As a result, MDEA/TEPA showed the highest cyclic capacity among the tested MDEA blends. 3.2. CO2 Absorption Rates. Although the aqueous solution of MDEA showed the highest CO2 loading amount and cyclic capacity among the tested absorbents, the CO2 absorption rate was too slow to use as absorbent alone. The CO2 loadings of the MEA and MDEA blends were saturated after 200 min; however, that of MDEA was about 0.382 after 300 min and saturated after 1000 min. The absorption rate of absorbent is a very important parameter to determine the size of the CO2 capture process. The CO2 absorption fluxes of the MDEA blends were measured using a short wetted-wall column apparatus at 313 K with different CO2 concentrations (3, 6, and 9% (v/v)). CO2 unloaded absorbents were used for this experiment. Figure 8 shows the results of CO2 absorption fluxes of the absorbents as a function of CO2 partial pressure. All MDEA blends showed higher absorption fluxes than MDEA, and higher fluxes were obtained from activators, which have fewer amino groups. The MDEA/MAPA showed the highest flux among the tested absorbents. The overall mass transfer coefficients of the absorbents were obtained from the slope of the plot of flux versus log-mean partial pressure change. The overall mass transfer coefficients (KG) of the absorbents at 313 K are shown in Figure 9, and compared to results of 5 M MEA reported by Hartono et al.41 and Dugas et al.,42 Figure 9 shows that results obtained in this work correspond to literature data and can be considered to be reliable. The MDEA/MAPA showed the highest KG of 3.351 × 103 mol·m−2·s−1·kPa−1 among the tested absorbents. This value

Figure 9. Overall mass transfer coefficients (KG) of aqueous MDEA blends (CO2 unloaded) at 313 K.

is 8 times higher than that of MDEA (0.415 × 103 mol·m−2·s−1· kPa−1) and even higher than that of MEA (3.014 × 103 mol· m−2·s−1·kPa−1). This result can be explained by the shuttle mechanism, and activators, such as the shuttle, come and go between the liquid film and the liquid phase bulk as shown in Figure 1. The action of the activator is as a carrier of CO2, essentially to enhance the mass transfer of CO2. The reaction mechanism of amines used in this study is more complex because these amines have multiple amino groups; however, the effects of these amines are obvious in that these amines act as activators by the shuttle mechanism. Amann and Bouallou19 also reported that the addition of a small amount of TETA into an aqueous MDEA solution results in a significant enhancement of the absorption rate at the gas−liquid interface. The overall mass transfer coefficients of the MDEA blends were decreased from 3.351 × 103 to 1.926 × 103 mol·m−2·s−1· kPa−1 gradually with the increase in the number of amino groups in the activators. It is considered that this is caused by the molecular structures of activators. An activator of smaller molecule with high reactivity such as MAPA will be better as a CO2 carrier because it can easily come and go between the liquid film and the liquid phase bulk. MAPA is the smallest molecule among the tested activators, and MDEA/MAPA had the highest CO2 absorption capacity (0.463 mol-CO2·molamino group−1) based on the molar concentration of amino groups among the MDEA blends. Although MDEA/TEPA had 14458



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the lowest KG value among the MDEA blends, the value is 4.6 times higher than that of MDEA. Consequently, the activators used in this study are evidently effective in enhancing the apparent absorption rate of the aqueous solution of MDEA. 3.3. Heat of Absorption. In the case of CO2 capture process using chemical absorbents, the regeneration of absorbents is required for continuous processing. The heat of absorption is an important factor, directly associated with the energy requirement of absorbent regeneration. The heats of absorption of MDEA blends were measured using a DRC at 298 K, and the results are shown in Figure 10; compared to

values are considerably low when compared with the value of MEA.

4. CONCLUSIONS Alkyl amines containing multiple amino groups (MAPA, DETA, TETA, and TEPA) were used as activators to improve the CO2 absorption performances of aqueous MDEA solutions. The CO2 absorption performances of aqueous MDEA blends were investigated by measurements of absorption capacity, absorption rate, and heat of absorption. The MDEA blends showed higher CO2 loading amounts than MEA and MDEA, and those values increased with the number of amino groups in the activators. MDEA/TEPA shows the highest loading amount of 0.753 mol-CO2·mol-absorbent−1. That is, the amino groups in the activators are reactive and can participate in the CO2 absorption reaction. Thus, the activators are effective in increasing the absorption capacity of MDEA. In addition, the MDEA blends showed high cyclic capacities, the values of which are 3 times higher than that of MEA and correspond to 68−93% of MDEA’s. The cyclic capacities of the MDEA blends were increased with the number of amino groups in the activators. All MDEA blends showed higher absorption fluxes than MDEA. In addition, the MDEA/MAPA showed the highest overall mass transfer coefficient of 3.351 × 103 mol· m−2·s−1·kPa−1 among the tested absorbents. This value is 8 times higher than that of MDEA and even higher than that of MEA. The overall mass transfer coefficients of the MDEA blends decreased gradually with increase in the number of amino groups in the activators. An activator of a smaller molecule with high reactivity such as MAPA was better for activation. Although MDEA/TEPA had the lowest KG value among the MDEA blends, the value is 4.6 times higher than that of MDEA. The heats of reaction of the MDEA blends are about 30% higher than that of MDEA and about 30% lower than that of MEA. The highest reaction heat was obtained from MDEA/DETA. However, the values are considerably low when compared with the value of MEA. The activators used in this study are clearly effective in enhancing the absorption capacity and apparent absorption rate of aqueous solution of MDEA with considerably low heats of absorption. The MDEA/MAPA and MDEA/TEPA blends showed high absorption rate and high absorption capacity (also high cyclic capacity), respectively, with reasonably low heats of absorption among the tested MDEA blends. Considering these results, MDEA blends tested in this work can be seen as alternatives to MEA, achieving high CO2 loading and high reaction kinetics with low heat of reaction. These would allow reducing the solvent flow rate and solvent regeneration energy in the CO2 capture process. However, the high viscosity and high corrosiveness of amines with four and five amino groups such as TETA and TEPA have to be considered.

Figure 10. Heats of absorption (ΔHabs) of aqueous blends of MDEA at 298 K.

literature results of MEA and MDEA reported by Carson et al.,43 Figure 10 shows the experimental data coincide well with the data of heats of absorption in the literature, and therefore the results obtained with the current experimental setup and using the experimental procedure can be considered to be reliable. The highest and the lowest values of the reaction heats were obtained for MEA (82.39 kJ·mol-CO2−1) and MDEA (44.65 kJ·mol-CO2−1), respectively. The MDEA blends showed similar heats of reaction in the narrow range from 57.21 to 59.53 kJ·mol-CO2−1. It is expected that an activator containing many more amino groups has a higher heat of absorption. Because, in this study, the MDEA blends have the same mass concentration of activators of 10% (w/w) as described in Table 3, the molar concentrations of activators decrease as the number of amino groups in the activator increase. However, the MDEA blends have similar molar concentrations of amino groups in the solutions in the range of 3.95−4.59 mol-aminogroup·L−1 (Table 3). Therefore, the MDEA blends showed similar results of absorption heats. The contents of primary amino group in the activators MAPA, DETA, TETA, and TEPA are 50, 67, 50, and 40%, respectively. The highest absorption heat was obtained from MDEA/DETA, which has the highest content of primary amino groups, whereas MDEA/TEPA, which has the lowest content of primary amino groups, showed the lowest absorption heat. The heats of absorption of the MDEA blends were about 30% higher than that of MDEA and about 30% lower than that of MEA. The activators increase the absorption heat of MDEA because the activators have primary and secondary amino groups, which produce stable carbamate ions. On the other hand, aqueous MDEA solution produces bicarbonate and carbonate ions, which could be regenerated more easily when compared with carbamate ions. However, the

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AUTHOR INFORMATION

Corresponding Authors

*(K.T.P.) Tel.: +82-42-860-3257. E-mail: [email protected]. *(S.-J.P.) Tel.: +82-42-821-5684. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 14459



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(20) Amann, J.-M. G.; Bouallou, C. CO2 Capture from power stations running with natural gas (NGCC) and pulverized coal (PC): assessment of a new chemical solvent based on aqueous solutions of N-methyldiethanolamine + triethylene tetramine. Energy Procedia 2009, 1, 909−916. (21) Monteiro, J. G. M.-S.; Hussain, S.; Majeed, H.; Mba, E. O.; Hartono, A.; Knuutila, H.; Svendsen, H. F. Kinetics of CO2 absorption by aqueous 3-(methylamino)propylamine solutions: experimental results and modeling. AIChE J. 2014, DOI: 10.1002/aic.14546. (22) Monteiro, J. G. M.-S.; Pinto, D. D. D.; Luo, X.; Knuutila, H.; Hussain, S.; Mba, E. O.; Hartono, A.; Svendsen, H. F. Activity-based kinetics of the reaction of carbon dioxide with aqueous amine systems. Case studies: MAPA and MEA. Energy Procedia 2013, 37, 1888−1896. (23) Zhu, B.; Liu, Q.; Zhou, Q.; Yang, J.; Ding, J.; Wen, J. Absorption of carbon dioxide from flue gas using blended amine solutions. Chem. Eng. Technol. 2014, 37, 635−642. (24) Aronu, U. E.; Hoff, K. A.; Svendsen, H. F. CO2 capture solvent selection by combined absorption−desorption analysis. Chem. Eng. Res. Des. 2011, 89, 1197−1203. (25) Hartono, A.; da Silva, E. F.; Svendsen, H. F. Kinetics of carbon dioxide absorption in aqueous solution of diethylenetriamine (DETA). Chem. Eng. Sci. 2009, 64, 3205−3213. (26) Versteeg, G. F.; Van Dijck, L. A. J.; Van Swaaij, W. P. M. On the kinetics between CO2 and alkanolamines both in aqueous and nonaqueous solutions. Chem. Eng. Commun. 1996, 144, 113−158. (27) Penny, D. E.; Ritter, T. J. Kinetic study of the reaction between carbon dioxide and primary amines. J. Chem. Soc., Faraday Trans. 1 1983, 79, 2103−2109. (28) Ko, J.-J.; Li, M.-H. Kinetics of absorption of carbon dioxide into solutions of N-methyldiethanolamine + water. Chem. Eng. Sci. 2000, 55, 4139−4147. (29) Little, R. J.; van Swaaij, W. P. M.; Versteeg, G. F. Kinetics of carbon dioxide with tertiary amines in aqueous solution. AIChE J. 1990, 36, 1633−1640. (30) Schäffer, A.; Brechtel, K.; Scheffknecht, G. Comparative study on differently concentrated aqueous solutions of MEA and TETA for CO2 capture from flue gases. Fuel 2012, 101, 148−153. (31) Arshad, M. W.; Svendsen, H. F.; Fosbøl, P. L.; Solms, N.; Thomsen, K. Equilibrium total pressure and CO2 solubility in binary and ternary aqueous solutions of 2-(diethylamino)ethanol (DEEA) and 3-(methylamino)propylamine (MAPA). J. Chem. Eng. Data 2014, 59, 764−774. (32) Brúder, P.; Lauritsen, K. G.; Mejdell, T.; Svendsen, H. F. CO2 capture into aqueous solutions of 3-methylaminopropylamine activated dimethyl-monoethanolamine. Chem. Eng. Sci. 2012, 75, 28−37. (33) Kim, I.; Svendsen, H. F.; Børresen, E. Ebulliometric determination of vapor-liquid equilibria for pure water, monoethanolamine, N-methyldiethanolamine, 3-(methylamino)-propylamine, and their binary and ternary solutions. J. Chem. Eng. Data 2008, 53, 2521− 2531. (34) Aronu, U. E.; Svendsen, H. F.; Hoff, K. A.; Juliussen, O. Solvent selection for carbon dioxide absorption. Energy Procedia 2009, 1, 1051−1057. (35) Kim, Y. E.; Moon, S. J.; Yoon, Y. I.; Jeong, S. K.; Park, K. T.; Bae, S. T.; Nam, S. C. Heat of absorption and absorption capacity of CO2 in aqueous solutions of amine containing multiple amino groups. Sep. Purif. Technol. 2014, 122, 112−118. (36) Shen, K. P.; Li, M.-H. Solubility of carbon dioxide in aqueous mixtures of monoethanolamine with methyldiethanolamine. J. Chem. Eng. Data 1992, 37, 96−100. (37) Danckwerts, P. V. Gas-Liquid Reactions; McGraw-Hill Book: New York, 1970. (38) Nogent, H.; Tacon, X. L. The differential reaction calorimeter: a simple apparatus to determine reaction heat, heat transfer value and heat capacity. J. Loss Prevent. Proc. 2002, 15, 445−448. (39) Satori, G.; Savage, D. W. Sterically hindered amines for carbon dioxide removal from gases. Ind. Eng. Chem. Fundam. 1983, 22, 239− 249.

ACKNOWLEDGMENTS This work was conducted under the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER) (B4-2481-11)

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REFERENCES

(1) Feng, Z.; Cheng-Gang, F.; You-Ting, W.; Yuan-Tao, W.; Ai-Min, L.; Zhi-Bing, Z. Absorption of CO2 in the aqueous solutions of functionalized ionic liquids and MDEA. Chem. Eng. J. 2010, 160, 691− 697. (2) Kohl, A.; Nielsen, R. Gas Purification, 5th ed.; Gulf Publishing: Houston, TX, USA, 1997. (3) Chakravarty, T.; Phukan, U. K.; Weiland, R. H. Reaction of acid gases with mixtures of amines. Chem. Eng. Prog. 1985, 81 (4), 32−36. (4) Derks, P. W. J.; Hogendoorn, J. A.; Versteeg, G. F. Experimental and theoretical study of the solubility of carbon dioxide in aqueous blends of piperazine and N-methyldiethanolamine. J. Chem. Thermodyn. 2010, 42, 151−163. (5) Astarita, G.; Savage, D. W.; Longo, J. M. Promotion of CO2 mass transfer in carbonate solutions. Chem. Eng. Sci. 1981, 36, 581−588. (6) Critchfield, J.; Rochelle, G.T. CO2 Absorption into Aqueous MDEA and MDEA/MEA Solutions, presented at the AIChE National Meeting, Houston, TX, USA, 1987; Paper 43e. (7) Xu, G.-W.; Zhang, C.-F.; Qin, S.-J.; Wang, Y.-W. Kinetics study on absorption of carbon dioxide into solutions of activated methyldiethanolamine. Ind. Eng. Chem. Res. 1992, 31, 921−927. (8) Zhang, X.; Zhang, C.-F.; Qin, S.-J.; Zheng, Z.-S. A kinetics study on the absorption of carbon dioxide into a mixed aqueous solution of methyldiethanolamine and piperazine. Ind. Eng. Chem. Res. 2001, 40, 3785−3791. (9) Bishnoi, S.; Rochelle, G. T. Absorption of carbon dioxide in aqueous piperazine/methyldiethanolamine. AIChE J. 2002, 48, 2788− 2799. (10) Kamps, P.-S.; Xia, A. J.; Maurer, G. Solubility of CO2 in (H2O + piperazine) and in (H2O + MDEA + piperazine). AIChE J. 2003, 49, 2662−2670. (11) Ali, B. S.; Aroua, M. K. Effect of piperazine on CO2 loading in aqueous solutions of MDEA at low pressure. Int. J. Thermophys. 2004, 25, 1863−1870. (12) Zoghi, A. T.; Feyzi, F.; Zarrinpashneh, S. Experimental investigation on the effect of addition of amine activators to aqueous solutions of N-methyldiethanolamine on the rate of carbon dioxide absorption. Int. J. Greenhouse Gas Control 2012, 7, 12−19. (13) Samanta, A.; Bandyopadhyay, S. S. Absorption of carbon dioxide into piperazine activated aqueous N-methyldiethanolamine. Chem. Eng. J. 2011, 171, 734−741. (14) Mandal, B. P.; Guha, M.; Biswas, A. K.; Bandyopadhyay, S. S. Removal of carbon dioxide by absorption in mixed amines: modelling of absorption in aqueous MDEA/MEA and AMP/MEA solutions. Chem. Eng. Sci. 2001, 56, 6217−6224. (15) Liao, C.; Li, M. H. Kinetics of absorption of carbon dioxide into aqueous solutions of monoethanolamine + N-methyldiethanolamine. Chem. Eng. Sci. 2002, 57, 4569−4582. (16) Ramachandran, N.; Aboudheir, A.; Idem, R.; Tontiwachwuthikul, P. Kinetics of the absorption of CO2 into mixed aqueous loaded solutions of monoethanolamine and methyldiethanolamine. Ind. Eng. Chem. Res. 2006, 45, 2608−2616. (17) Zhang, X.; Zhang, C. F.; Liu, Y. Kinetics of absorption of CO2 into aqueous solution of MDEA blended with DEA. Ind. Eng. Chem. Res. 2002, 41, 1135−1142. (18) Lin, C. Y.; Soriano, A. N.; Li, M. H. Kinetics study of carbon dioxide absorption into aqueous solutions containing N-methyldiethanolamine + diethanolamine. J. Taiwan Inst. Chem. Eng. 2009, 40, 403−412. (19) Amann, J.-M. G.; Bouallou, C. Kinetics of the absorption of CO2 in aqueous solutions of N-methyldiethanolamine + triethylene tetramine. Ind. Eng. Chem. Res. 2009, 48, 3761−3770. 14460



Article

(40) Hartono, A.; da Silva, E. F.; Grasdalen, H.; Svendsen, H. F. Qualitative determination of species in DETA-H2O-CO2 system using 13 C NMR spectra. Ind. Eng. Chem. Res. 2007, 46, 249−254. (41) Hartono, A. Characterization of diethylenetriamine (DETA) as absorbent for CO2. Chemical Engineering; Norwegian University of Science and Technology: Trondheim, Norway, 2009. (42) Dugas, R.; Rochelle, G. Absorption and desorption rates of carbon dioxide with monoethanolamine and piperazine. Energy Procedia 2009, 1, 1163−1169. (43) Carson, J. K.; Marsh, K. N.; Mather, A. E. Enthalpy of solution of carbon dioxide in (water + monoethanolamine, or diethanolamine, or N-methyldiethanolamine) and (water + monoethanolamine + Nmethyldiethanolamine) at T = 298.15 K. J. Chem. Thermodyn. 2000, 32, 1285−1296.

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