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Kinetic Reaction Characteristics of Quasi-aqueous/Non-aqueous Sorbents for CO2 Absorption Using MEA/H2O/Ethylene Glycol Min-Kyoung Kang, Joon-Hyung Cho, Jae-Hwa Lee, Sang-Sup Lee, and Kwang-Joong Oh Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 23 Jun 2017 Downloaded from http://pubs.acs.org on June 23, 2017
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Kinetic Reaction Characteristics of Quasi-aqueous/Non-aqueous
2
Sorbents for CO2 Absorption Using MEA/H2O/Ethylene Glycol
3
4
Min-Kyoung Kang+, Joon-Hyung Cho†, Jae-Hwa Lee‡, Sang-Sup Lee§, and Kwang-Joong
5
Oh*,‡
6 7
+
8
Geumjeong-Gu, Busan 609-735, Republic of Korea
9
†
Institute of Environmental Studies, Pusan National University, San 30, Jangjeon-dong,
Division of Creative Low Impact Development and Management for Ocean Port City
10
Infrastructures, Pusan National University, San 30, Jangjeon-dong, Geumjeong-Gu, Busan
11
609-735, Republic of Korea
12
‡
13
Dong, Geumjeong-Gu, Busan 609-735, Republic of Korea
14
§
15
763, Republic of Korea
Department of Environmental Engineering, Pusan National University, San 30, Jangjeon-
Department of Environmental Engineering, Chungbuk National University, Cheongju 361-
16 17 18 19
-----------------------------------------------------------------------* Corresponding author. Tel: 82-51-510-1707. Fax: 82-51-583-0559.
20
E-mail address:
[email protected] 1
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ABSTRACT: We lowered the amount of water used in the solvent to 50% or less and
2
examined the physical characteristics of quasi-aqueous and non-aqueous sorbents using an
3
organic solvent with a high specific heat, instead of water. The physical solubility,
4
diffusivity, and CO2 absorption rate were measured as a function of temperature (293,
5
303, 313, and 323 K) and sorbent (monoethanolamine, MEA/H2O; quasi-aqueous
6
MEA/H2O/ethylene glycol; non-aqueous MEA/ethylene glycol). Results show that the
7
rate of CO2 absorption by the non-aqueous sorbent increases with temperature and
8
ethylene glycol content. Rate constants for CO2 absorption by the sorbents were
9
evaluated based on the physical data and absorption rate. The reaction rate constants of
10
the water-based, quasi-aqueous, and non-aqueous sorbents were determined to be ~698,
11
~1518, and ~1780 m3 kmol-1 s-1, respectively. Further, the CO2 absorption/regeneration
12
efficiency and CO2 loading amount were evaluated during continuous CO2 absorption
13
and sorbent regeneration.
14 15
KEYWORDS: carbon dioxide, quasi-aqueous, non-aqueous, kinetic reaction, ethylene glycol
16 17 18 19 20 21 2
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1. INTRODUCTION
2
Greenhouse gas concentrations in the atmosphere have increased sharply since the industrial
3
revolution due to industrial development and increased use of fossil fuels. The average
4
concentration of carbon dioxide has exceeded 480 ppm, which has caused an average
5
annual increase of 2.3 ppm CO2 over the last 10 years.1 The greenhouse effect causes
6
global warming. Global warming is a problem that is inevitable due to daily life and
7
economic activities; it is expected to become even more serious considering the continual
8
increase in energy consumption and changes in lifestyle.2,3 Gases causing global warming
9
include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and freon gas (CFCs).4
10
Carbon dioxide is a regulated gas with a low global warming index, accounting for more than
11
80% of the total greenhouse gas emissions; it is classified as the most important greenhouse
12
gas of the six greenhouse gases. Large amounts of carbon dioxide are emitted worldwide due
13
to the combustion of carbon-based fuels and industrial processes.5
14
Technologies that can capture CO2 from flue gases of industrial facilities include absorption,
15
adsorption, cryogenic separation, and membrane separation. At present, the most practical
16
method that can be applied to combustion flue gas in a short period of time is absorption. 6
17
Absorption processes using alkanolamine are the most widely known; many alkanolamines
18
(e.g., monoethanolamine, MEA; N-methyldiethanolamine, MDEA; diethanolamine, DEA;
19
and 2-amino-2-methyl-mineral based absorbents, AMP) are widely used.7,8
20
Sorbents based on amine aqueous solutions have high absorption efficiencies and rapidly
21
absorb carbon dioxide. Moreover, the associated technology can be commercialized because
22
it can be scaled up. Among the sorbents, MEA is a low-cost sorbent that has a high reaction
3
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rate with carbon dioxide; commercially, it is more widely used than other alkanolamine
2
sorbents.9
3
However, the process using MEA as a sorbent involves strong chemical bonding of CO2 to
4
the amine functional group of MEA. High temperatures (120°C or higher) are necessary for
5
the regeneration process to break this bond; the required high-energy consumption is a
6
problem. Furthermore, the decomposition of MEA occurs during the regeneration of the
7
amine and the performance of the sorbent sharply deteriorates. It is necessary to continuously
8
supply the sorbent to replace the consumed amount.10-12
9
The use of water as a solvent is also a problem. Water has high specific and latent heat; thus,
10
the energy consumption is high enough to break the chemical bond between carbon dioxide
11
and the sorbent.13 The energy required to reach these high temperatures is ~40%–60% of the
12
total energy required in the carbon dioxide absorption process.14,15 Because the latent heat
13
consumed during the heating of water is not likely to reduce, the sorbent must be regenerated
14
at < 80°C–100°C to reduce the energy of the overall absorption process.16-18
15
In this study, we aimed to solve problems with existing amine-based sorbents used in water,
16
while taking advantage of the wet CO2 capture process. The reaction characteristics of several
17
new quasi-aqueous/non-aqueous sorbents that might replace the existing water sorbents were
18
examined. The objectives of the investigation were to reduce the amount of water used as
19
solvent to 50% or less and to reduce the energy consumption by sensible heat and latent heat.
20
For this purpose, we identified the best non-aqueous sorbent for the use with an organic
21
solvent with high specific heat, instead of water.
4
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The solubility, diffusion coefficient, and absorption rate were investigated to determine the
2
physical and chemical properties of the sorbents that most affect the process design and
3
operating parameters of the quasi-aqueous/non-aqueous sorbents. The reaction rate constant
4
was determined to understand the characteristics of the reaction between the quasi-
5
aqueous/non-aqueous sorbents and CO2. The adsorption/regeneration efficiency of the quasi-
6
aqueous/non-aqueous sorbents was also investigated in terms of reaction time and CO2
7
loading in a continuous absorption/regeneration process.
8 9
2. THEORETICAL BASIS
10
2.1 Reactions of CO2 with the sorbent
11
The following reactions with CO2 occur in aqueous solutions of primary alkanolamines.19
12
The zwitterion mechanism originally proposed by Caplow20 and reintroduced by
13
Danckwerts12 is generally accepted as the reaction mechanism for Eq. (1).
14
(1)
15
This mechanism comprises two steps, namely, the formation of the CO2-amine zwitterions,
16
that is, Eq. (2) and subsequent base-catalyzed deprotonation of these zwitterions, that is, Eq.
17
(3).
18
(2)
19
(3) is a base such as amine, OH−, or H2O [21]. The equilibrium-loading capacities of
20
where
21
the primary and secondary alkanolamines are limited to 0.5 mol CO2/mol amine by the 5
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stoichiometry of Eq. (1). For normal primary amines, such as MEA, the carbamates formed in
2
Eq. (1) are quite stable.
3
Organic alcohols cannot directly react with CO2 to form carbamates because the molecule has
4
no N–H bond. Instead, the organic alcohol is converted to ammonium hydroxide
5
([R1R2R3NH]OH) by reacting with water as shown in Eq. (4) and the anion OH– reacts with
6
CO2 to form bicarbonate.
7
⇔
+
(4)
8
The reaction temperature and reaction rate of the above-mentioned reaction are lower and
9
slower, respectively, than that of the amine sorbent during carbamate formation. Furthermore,
10
because the rate of CO2 absorption is greatly influenced by the pH, higher basicity, in terms
11
of the reaction rate, is of advantage. The reaction has a low CO2 absorption rate; however, the
12
regeneration of the sorbent is relatively easy and the degradation of the absorbability of
13
repeated absorption/regeneration experiments is low. The reaction is influenced by the
14
functional group of the organic alcohol and amine-absorbing agent. Therefore, the CO2
15
absorbing ability, absorption rate, and sorbent regeneration of the reaction depend on the
16
performance of the amine as CO2 absorbing agent because the functional group can change
17
the basicity.
18
The acid dissociation constant, pKa, of the sorbent is a measure of the basicity; it indicates
19
how well the sorbent can react with CO2, an acid gas. If the basicity of the sorbent is low, the
20
removal of H+ from the amphoteric ionic salt formed by amine and CO2 is difficult. As a
21
result, 1 mole of CO2 per mole of amine remains bonded. The molecule of organic alcohol
22
contains a hydroxyl group, which destabilizes the N–H bond of the amphoteric compound 6
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that is produced by the reaction between amine and CO2. In addition, the basicity of the
2
amine can be increased by increasing the electron density.
3 4
2.2 Physical Properties
5
Determining the physical solubility (HA) and diffusivity (DA) of A in the solution is very
6
important for the calculation of the reaction kinetics and to design the process. To determine
7
the physical properties, the “N2O analogy” was used to estimate the solubility and diffusivity
8
of such systems. This was necessary because carbon dioxide chemically reacts with amine
9
solutions. The mass, molecular structure, and molecular interactions of dinitrogen monoxide
10
are similar to those of carbon dioxide.21-23 The “N2O analogy” for the measurement of the
11
physical solubility and diffusivity of carbon dioxide in amine solutions is as follows:
12
=
(5)
13
=
(6)
14
The physical solubility and diffusivity were calculated based on Henry’s law and Higbie’s
15
penetration theory, respectively. The calculation details are presented elsewhere.24
16 17
2.3 Determination of the reaction rate constant
18
Several models have been proposed to describe the reaction of carbon dioxide with amines. It
19
is generally accepted that the zwitterion mechanism governs the formation of carbamate with
20
respect to primary and secondary amines.25 The zwitterion mechanism of the carbon dioxide
21
reaction with secondary amines and the reaction rate based on quasi-steady-state conditions
22
can be summarized as follows:26
7
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↔
1
(7)
2
(8) [
3
][
]
∑
∑
[ ]
4
When
5
following equation can be used:
the analysis is simplified to second order kinetics and the
[
6
(9)
[ ]
][
]
(10)
7
The reaction order and reaction rate constant can be determined using the following specific
8
absorption equation:19
9
(11)
10
where n and m are reaction orders that are functions of the amine concentration and partial
11
pressure of carbon dioxide (pA), respectively. For a fast chemical reaction between the
12
dissolved gas and reactant, the specific absorption rate is as follows:
13 14 15
√
(12)
Similarly, using Eq. (13), the overall reaction rate constant (kov) can be calculated as follows: 2
(13)
16
The chemical absorption of carbon dioxide in a two-phase sorbent is a pseudo-first order
17
reaction.27,28 When the equilibrium pressure of carbon dioxide in solution is small compared
18
with the absorption pressure and the gas-phase resistance is negligible during fast pseudo-mn8
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order reactions, the absorption rate is given by Eq. (14).25,26 After integrating Eq. (15), the
2
reaction rate constant (k2) can be obtained using Eq. (17). (
3
)
( )
4
5
(
6
(
√
√
√
(14)
√
(15)
)
(16)
)
(17)
7
3. EXPERIMENTAL SECTION
8
3.1 Materials
9
Analytical grade MEA (99%) was supplied by Sigma-Aldrich (USA). The organic alcohol
10
sorbent (ethylene glycol, 99.0%) was supplied by Samchun Pure Chemicals (Korea).
11
Aqueous solutions were prepared using distilled water. The CO2 and N2 gases were of
12
commercial grade, with purities of 99.99%. High-purity N2O (99.9%) gas was also used. The
13
chemicals and gases used in this study are listed in Table 1.
14 15
3.2 Experimental apparatus and procedure
16
3.2.1 Physical solubility
17
The experimental apparatus for physical solubility measurements is shown in Figure 1. The
18
reactor (160 mm height and 95 mm inner diameter) is located in a temperature-controlled
19
vessel; four 5-mm-wide glass plates are adhered to the inner wall of the reactor as baffles.
20
The total volume of the reactor is ~1134 cm3, with an active interface area (As) of 70.88 cm2. 9
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A two-blade impeller (70 mm × 20 mm) was installed in the middle of the liquid level. The
2
reactor temperature was measured with a K-type thermocouple (0.1 K accuracy). A pressure
3
transducer (MGI/MGAMP series, ±0.1 kPa accuracy) was installed in the reactor; a feeder
4
was used to measure the pressure. The gas flow rates were controlled using mass flow
5
controllers (5850E, Brooks Instruments). After the temperature of the reactor stabilized, the
6
reactor was purged with pure N2O for one hour to remove the remaining air from the reactor.
7
After the reactor reached atmospheric pressure, 200 mL amine solution were injected with a
8
syringe and then agitated. The revolution per minute (rpm) and the absorption solution
9
suitable for the size of the reactor used in this experiment were tested with the condition
10
that the gas–liquid contact interface was not broken. Therefore, experiments were
11
carried out at 50 rpm and with 200 mL of absorbing solution. The reactor pressure
12
decreased when the amine solution absorbed N2O; therefore, N2O was continuously injected
13
to maintain the reactor pressure at 1 atm. When the reactor pressure stabilized and further
14
N2O injection was not required, the solubility was calculated based on the difference of the
15
feeder pressure before and after absorption. The experimental uncertainty of the measured
16
solubility was estimated to be approximately ± 2%; N2O was injected into the feed tank
17
with a needle valve when the pressure became low as the absorbing solution started to
18
absorb N2O gas. It was assumed that the absorption equilibrium was reached when
19
there no change in pressure occurred in more than 30 minutes. The physical solubility
20
was estimated using the pressure difference in the feed tank before and after absorption.
21 22
3.2.2 Diffusivity
23
The diffusivity was studied using a wetted wall column (WWC). The setup of the WWC
24
apparatus is shown in Figure 2. The gas–liquid contact in the center was constructed with a 10
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stainless steel tube (length: 91 mm; outer diameter: 12.6 mm). The reactor consists of a
2
double jacket to maintain a constant temperature. The actual temperature inside the
3
reactor was measured using a K-type thermocouple (±0.1 K) with an accuracy of ±0.1 K.
4
The dual jacket for temperature compensation is made of glass and the paraffin oil is
5
designed to pass through the double jacket and thermostat. The water and sorbent moved
6
upward inside the column and flowed out of the top, forming a liquid film outside the column.
7
The liquid flow rate was controlled with a peristaltic pump (Masterflex L/S, Cole-Palmer
8
Instruments). The gases made contact with the liquid against the current and then exited on
9
the top. The operating pressure of the reactor was maintained at normal pressure. The
10
pressure inside the reactor was measured using pressure transducers (MGI/MGAMP series,
11
accuracy ±0.1 kPa) installed in the reactor and feeder. The flow rate of dinitrogen monoxide
12
and carbon dioxide gas was controlled using mass flow controllers (5850; Brooks Instrument,
13
Hatfield, PA, USA).The flow rate of the outlet gas was analyzed using a wet-gas meter (W-
14
NK-1, SIBATA, Japan).
15 16
3.2.3 Continuous absorption/regeneration process
17
A
18
absorption/regeneration is shown in Figure 3. The apparatus consists of a gas injector,
19
absorber, regenerator, and CO2 analyzer. The absorber and regenerator are made of glass with
20
an internal diameter of 50 mm and a height of 600 mm. The experiments to study the
21
absorption characteristics were carried out by operating both the absorber and regenerator.
22
The aqueous amine solution (500 g) was injected into a storage tank and circulated through
23
the regenerator. The temperatures in the absorber and regenerator were kept constant to
schematic
diagram
of
the
experimental
apparatus
used
to
study
CO2
11
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prevent regeneration. The output CO2 concentration of the absorber was continuously
2
measured with the CO2 gas analyzer. The prepared solvent was then pumped to the top of the
3
column at a given flow rate. A needle valve was installed on top of the column to control the
4
liquid flow rate and create counter-current contact between the gas and liquid. After
5
absorbing CO2 and traveling through the column, the CO2-rich solution was continuously
6
collected in the liquid-receiving tank. This operation was continued for at least 30 min to
7
allow the system to reach steady-state conditions. At the same time, liquid samples were
8
taken from the bottom of the column; the concentrations and CO2 loading were analyzed.
9 10
3.2.4 Lean/rich amine solution sample analysis
11
Rich and lean amines were extracted from the absorber and regenerator, respectively. To
12
determine the characteristics of CO2 absorption and regeneration, the samples were analyzed
13
using the titrimetric method: A liquid amine solution sample of known volume was placed in
14
the reaction flask. Acid titrant was introduced to the reaction flask using a graduated titration
15
burette. As CO2 vapor evolved from the reaction, the fluid in the reservoir was displaced,
16
allowing the measurement of the evolved gas. The amine solution concentration can be
17
determined based on the titration and the following relationship:
18
(18)
19
where C1 is the amine solution concentration (M = mol/L), V1 is the amine solution sample
20
volume (mL), C2 is the acid concentration (M), and V2 is the acid volume from the titration
21
(mL). First, 100 mL distilled water was injected into a 250 mL beaker and stirred with a
22
magnetic stirrer. The pH of the distilled water was increased to 11.4–11.6 by adding 0.5 M
23
NaOH solution. The amine solutions were added to the beaker using a 10 mL pipette for the 12
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rich amine solution and a 25 mL pipette for the lean amine solution. Subsequently, 0.5 M
2
NaOH was added to the amine solution using the titrimetry until the solution pH reached 11.5.
3
The amount of CO2 absorbed by the amine solution can be obtained from the concentration
4
derivation. The CO2 captured during the titration can be determined with the following
5
equation; the CO2 loading is defined as moles of CO2 per mole of amine:
6
Calculate L (R):
7
L (R) (eq/L) = 0.02 (0.05) × mL NaOH
8
Calculate the CO2 loading in mol/mol:
9
(
)
(19)
10 11
4. RESULTS AND DISCUSSION
12
4.1 Physical properties (solubility and diffusivity)
13
The solubility of CO2 in an amine solution is not directly measurable. The solubility of CO2
14
was estimated based on a N2O analogy experiment using N2O. Solubility experiments were
15
performed for 30 wt% MEA/H2O sorbent, 30 wt% quasi-aqueous MEA/H2O/ethylene
16
glycol sorbent, and 30 wt% non-aqueous MEA/ethylene glycol sorbent at varying
17
temperature (293, 303, 313, and 323 K).
18
Table 2 shows the experimentally determined N2O solubility and the CO2 solubility obtained
19
from Eq. (5) based on the N2O analogy method. Figure 4 shows the CO2 solubility in terms of
20
sorbent and temperature. The Henry’s constant tends to increase with increasing ethylene
21
glycol, which means that the CO2 solubility decreases with increasing amount of ethylene 13
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1
glycol. In addition, because the temperature increases with physical absorption characteristics,
2
the Henry's constant increases and the solubility decreases. The addition of ethylene glycol to
3
30 wt% MEA leads to a higher Henry constant compared with that for 30 wt% MEA solution
4
only because the difference between the gas partial pressure and vapor pressure in the feed
5
tank increases in the solubility equation due to the decreasing gas movement in the feed tank
6
affected by the vapor pressure.
7
The diffusion coefficient of CO2 in the quasi-aqueous/non-aqueous solution was measured.
8
Diffusivity experiments were performed for 30 wt% MEA/H2O sorbent, 30 wt% quasi-
9
aqueous MEA/H2O/ethylene glycol sorbent, and non-aqueous 30 wt% MEA/ethylene
10
glycol sorbent at varying temperature (293, 303, 313, and 323 K).
11
Table 3 shows the diffusion coefficient of N2O measured directly during the experiment and
12
the diffusion coefficient of CO2 obtained from the N2O analogy. Figure 5 shows the diffusion
13
coefficients of N2O and CO2 for quasi-aqueous and non-aqueous solutions based on sorbent
14
and temperature. When the solute of a small molecule is diffused in a solution of a large
15
molecule, the diffusion coefficient of the solute decreases as the viscosity increases. The
16
diffusion coefficient decreases with the addition of ethylene glycol.
17 18
4.2 CO2 absorption rate
19
Absorption rate experiments were performed for 30 wt% MEA/H2O sorbent, 30 wt%
20
quasi-aqueous
21
MEA/ethylene glycol sorbent at varying temperature (293, 303, 313, or 323 K). The
22
partial pressure of carbon dioxide is 15 kPa.
MEA/H2O/ethylene glycol sorbent; and
30 wt%
non-aqueous
14
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Figure 6 shows the absorption rate of the sorbent according to the reaction temperature at the
2
same CO2 partial pressure (15 kPa). The absorption rates of the MEA/H2O aqueous sorbent,
3
quasi-aqueous MEA/H2O/ethylene glycol sorbent, and non-aqueous MEA/ethylene glycol
4
sorbent are 6.88–9.63×10−6, 9.15×10−6, and 10.86–14.21×10−6 kmol m−2 s−1, respectively. As
5
the temperature increases, the absorption rate also increases because the CO2 uptake rate is
6
higher at higher temperatures. The absorption rate increases linearly with increasing amounts
7
of solvent and solute capable of absorbing CO2 and diffusion and collision at the gas–liquid
8
interface.
9
In addition, the absorption rates of the water-based sorbent and the quasi-aqueous/non-
10
aqueous sorbents were compared. The differences in absorption rate between the water-based
11
sorbent and quasi-aqueous sorbent, between the quasi-aqueous sorbent and non-aqueous
12
sorbent, and between the water-based sorbent and non-aqueous sorbent are 2.27–3.24×10-6,
13
1.34–1.71×10-6, and 3.98–4.58×10-6 kmol m−2 s−1, respectively. The absorption rate of the
14
non-aqueous sorbent mixed with MEA and ethylene glycol was the highest.
15
The results vary depending on the basicity, which in turn depends on the functional
16
group of ethylene glycol. The presence of a hydroxyl group in the molecule destabilizes
17
the bonds of the amphoteric compound that is produced by the reaction of amine with
18
CO2. Increasing the electron density of the amine increases the basicity of the amine.
19
The higher the basicity is, the higher is the reactivity with CO2, which is an acid gas,
20
and the higher is the absorption rate of CO2. Table 4 shows the absorption rate of
21
previously studied sorbents. The absorption rate of non-aqueous sorbent is higher than
22
that of the existing water-based sorbent under the same conditions as that for the quasi-
15
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Page 16 of 34
1
aqueous/non-aqueous sorbents developed in this study. This indicates that the non-
2
aqueous sorbent removes more CO2 than the other sorbents and is effective.
3 4
4.3 Reaction kinetics
5
In this study, reaction rate constants were determined for the 30 wt% MEA/H2O
6
sorbent, 30 wt% quasi-aqueous MEA/H2O/ethylene glycol sorbent, and 30 wt% non-
7
aqueous MEA/ethylene glycol sorbent. For this purpose, Henry's constant, diffusion
8
coefficient, and absorption rate (as described in Sections 4.1 and 4.2) were applied to Eq.
9
(16). The change of the
√
value based on each sorbent and temperature is
10
shown in Figure 7.
11
The reaction rate constant increases linearly with increasing ethylene glycol content and
12
temperature. This is due to the difference in the physical properties of HA and DA under each
13
condition. The increase of the absorption rate with increasing collision and diffusion at the
14
vapor–liquid interface and increasing amount of solvent and temperature is large. The kinetic
15
data for the derivation of the reaction rate constant between the sorbent and CO2 are
16
summarized in Table 5. Based on the slope of each sorbent and the temperature, the reaction
17
rate constant of the water-based, quasi-aqueous, and non-aqueous sorbents was determined to
18
be ~698 m3 kmol-1 s-1, ~1518 m3 kmol-1 s-1, and ~ 1780 m3 kmol-1 s-1. The difference of the
19
reaction rate constant depending on the sorbent is due to the reactor type and the reaction
20
conditions and factors.
21
In addition, the (NA1/HA1/DA10.5/pA1)2 value of the … increases compared with the water-
22
based sorbent; the increase is caused by interactions with ethylene glycol. This suggests that
23
the reaction rate can be improved. Compared with the water-based sorbent, the reaction 16
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rate constant of the quasi-aqueous sorbent exhibits a difference of ~820 m3 kmol-1 s-1. In
2
the case of the non-aqueous sorbent, the reaction rate constant is ~1082 m3 kmol-1 s-1
3
higher than that of the water-based sorbent. Based on this, the reactivity of the sorbent
4
can be enhanced by the interaction of 30 wt% aqueous MEA sorbent with quasi-
5
aqueous/non-aqueous sorbents containing ethylene glycol.
6 7
4.4 Absorption/regeneration based on the reaction time
8
Regeneration accounts for the largest portion of the operating costs of the carbon dioxide
9
absorption/regeneration process. Reducing the renewable energy in the regenerator is
10
important to ensure economic efficiency. In this experiment, 30 wt% MEA/H2O sorbent,
11
30 wt% quasi-aqueous MEA/H2O/ethylene glycol sorbent, and 30 wt% non-aqueous
12
MEA/ethylene glycol sorbent were used in continuous absorption/regeneration
13
experiments. Figure 8 shows the variation of the exhaust gas concentration emitted by the
14
reactor at a fixed absorption tower temperature of 313 K, regeneration tower temperature of
15
353 K, gas flow rate of 500 L/h, and sorbent circulation flow rate of 40 mL/min.
16
The aqueous MEA/H2O sorbent initially maintains a high CO2 removal efficiency (0 to 0.16
17
vol%) but rapidly increases thereafter.
18
On the other hand, the quasi-aqueous MEA/H2O/ethylene glycol sorbent and non-aqueous
19
MEA/ethylene glycol sorbent are ~0–1.74 and 0–1.24 vol%, respectively. The CO2 removal
20
efficiency of the water-based sorbent is somewhat lower than that but effective in
21
absorption/regeneration of CO2 by maintaining high removal efficiency for a long time. In
22
case of the water-based sorbent, the stoichiometric reaction ratio with CO2 was low. It is
23
believed that the CO2 emission rapidly increases because CO2 is not regenerated well in the
24
regeneration tower but produces a stable compound. In addition, because the water-based 17
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Page 18 of 34
1
sorbent is generally regenerated at a high temperature (≥ 373 K), significant energy is used
2
when separating CO2. At the lower temperature of 353 K, this type of regeneration does not
3
perform well.
4
On the other hand, quasi-aqueous/non-aqueous sorbents reversibly react with CO2 in the
5
presence of ethylene glycol at a molar ratio of 1:1. The formation of carbonate of amphoteric
6
ions suggests that it is relatively well regenerated in repeated absorption/regeneration
7
experiments. It is also believed that the regeneration is effective at lower temperature,
8
compared with that of the water-based sorbent, because it is thermally stable. Therefore, it
9
can be expected that quasi-aqueous/non-aqueous sorbents more easily liberate CO2 from the
10
sorbent and use less energy than water-based sorbents.
11 12
4.5 CO2 loading based on absorption/regeneration
13
In this experiment, the initial CO2 removal efficiency and the rich and lean amines in the
14
absorber and regeneration tower, respectively, during continuous absorption/regeneration
15
were analyzed. Here, CO2 loading means the number of moles of carbon dioxide
16
absorbed per mole of sorbent. The CO2 loading value of the rich amine is the loading
17
value of the sorbent after reacting with carbon dioxide in the absorption tower. The
18
loading value of lean amine is the loading value of the sorbent after separation of
19
carbon dioxide and sorbent from the stripping column. Thus, the absorber column
20
(40°C) and regenerator column (80°C) show the amount of CO2 absorbed in the sorbent
21
after performing the experiment (Section 4.4).
22
Figure 9 shows that the absorption/regeneration efficiency of the 30 wt% water-based MEA
23
sorbent is ~76.8% when the sorbent circulation flow rate is 40 mL/min and the gas flow rate
24
is 500 L/h. The efficiency of the quasi-aqueous MEA/H2O/ethylene glycol sorbent is ~88.6% 18
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and that of the non-aqueous MEA/ethylene glycol sorbent is ~95.3%. The rich amine CO2
2
loading is approximately 0.43, 0.81, and 0.86 mol CO2/mol sorbent; the lean amine CO2
3
loading is ~ 0.26, 0.19, and 0.11 mol CO2/mol sorbent. The difference in the loading capacity
4
of the rich and lean amines related to the sorbent used is ~0.17 mol CO2/mol sorbent. The
5
difference for the quasi-aqueous and non-aqueous sorbents is ~0.62 mol CO2/mol and ~0.75
6
mol CO2/mol sorbent, respectively.
7
This is because the OH- of ethylene glycol reacts with CO2 to form bicarbonate. The more
8
bicarbonate is produced, the more favorable is the regeneration. The difference of the CO2
9
loading capacity between rich amine and lean amine is significantly higher in quasi-
10
aqueous/non-aqueous sorbents than in water-based sorbents. These results indicate that their
11
absorption performance is excellent. This allows us to determine the appropriate replacement
12
time to maintain sorbent efficiency over time and to consistently achieve high absorption
13
efficiency during the actual continuous absorption/regeneration. It is believed that the quasi-
14
aqueous/non-aqueous sorbents can remove more CO2 than the water-based sorbent.
15 16
5. Conclusions
17
To overcome problems of conventional amine-based sorbent in water, the amount of
18
water used in the solvent was reduced to 50% or less and quasi-aqueous/non-aqueous
19
sorbents were developed using an organic solvent with high specific heat instead of
20
water. The solubility of water-based/quasi-aqueous/non-aqueous sorbents indicates a
21
high Henry's constant for non-aqueous sorbents and decreases at diffusion coefficients.
22
The absorption rate of quasi-aqueous MEA/H2O/ethylene glycol and water-based 19
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Page 20 of 34
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MEA/H2O sorbents is ~6.88–9.63×10-6 kmol m-2 s-1 and ~9.15–12.87×10-6 kmol m-2 s-1,
2
respectively; the non-aqueous MEA/ethylene glycol sorbent has the highest absorption
3
rate of 10.86–14.21×10-6 kmol m-2 s-1. Based on these reaction characteristics, the overall
4
reaction rate constant based on kinetic data was estimated to be two times higher than
5
that of water-based absorbers. This suggests that the interaction between MEA and
6
quasi-aqueous/non-aqueous ethylene glycol sorbents might improve the CO2 absorption
7
performance and reactivity.
8
As a result of applying quasi-aqueous/non-aqueous sorbents to conventional CO2
9
absorption/regeneration
processes,
the
absorption
efficiencies
of
water/quasi-
10
aqueous/non- aqueous sorbents are 76.8%, 88.6%, and 95.3%, respectively. The
11
difference in CO2 loading is 0.17, 0.62, and 0.75 mol CO2/mol sorbent, respectively. The
12
difference in the CO2 loading capacity of the non-aqueous sorbent was the largest; the
13
absorption/regeneration capacity was therefore superior to that of the water-based
14
sorbent. These results show that the reaction rate can be improved, CO2 can be
15
effectively absorbed, and the process efficiency can be expected to increase.
16 17
Acknowledgments
18
This work was supported by the Brain Korea 21 Plus Project of the Division of Creative Low
19
Impact Development and Management for Ocean Port City Infrastructures. Moreover, we
20
acknowledge a two-year research grant from Pusan National University.
21 22 20
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and AMP/MEA solutions. Chem. Eng. Sci. 2001, 56, 6217.
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Monoethanolamine in a CO2 Capture Facility. Energy Fuels. 2003, 17, 1034.
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amine/alcohol/water blends. Greenhouse Gas Control. 2014, 26, 69.
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(14) Singh, P.; Niederer, J. P. M.; Versteeg, G. F. Structure and activity relationships for
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amine-based CO2 absorbents- II. Chem. Eng. Res. Design, 2009, 87, 135.
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(15) Peeters, A. N. M.; Faaij, A. P. C.; Trukenburg, W. C. Techno-economic analysis of
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evaluation of the development potential. Int. J. Greenh. Gas Control 2007, 1, 396.
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(16) Barbarossa, V.; Barzagli, F.; Mani, F.; Lai, S.; Stoppioni, P.; Vanga, G. Efficient
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solvent regeneration. RSC Adv. 2013, 3, 12349.
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(17) Barzagli, F.; Mani, F.; Peruzzini, M. Efficient CO2 absorption and lowtemperature
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(22) Li, M. H.; Lai, M. D. Solubility and diffusivity of N2O and CO2 in (monoethanolamine +
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N-methyldiethanolamine + water) and in (monoethanolamine + 2-amino-2-methyl-1-
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propanol + water). J. Chem. Eng. Data 1995, 40, 486.
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and diffusivity of N2O and CO2 into aqueous solutions of (2-amino-2-methyl-1-propanol+
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diethanolamine) and (N-methyldiethanolamine+ diethanolamine). J. Chem. Eng. Data 2004,
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(24) Choi, W. J.; Min, B. M.; Seo, J. B.; Park, S. W.; Oh, K. J. Effect of ammonia on the
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(25) Xu, S.; Wang, Y. W.; Otto, F. D.; Mather, A. Kinetics of the reaction of carbon dioxide
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with 2-amino-2-methyl-1-propanol solutions. Chem. Eng. Sci. 1996, 51, 841.
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(26) Yih, S. M.; Shen, K. P. Kinetics of carbon dioxide reaction with sterically hindered 2-
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amino-2-methyl-1-propanol aqueous solutions. Ind. Eng. Chem. 1988, 27, 2237.
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(27) Park, S. W.; Cho, H. B.; Sohn, I. J.; Kumazawa, H. CO2 absorption into w/o emulsion
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(28) Park, S. W.; Kumazawa, H.; Sohn, I. J. Unsteady-state absorption of CO2 into W/O
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emulsion with aqueous alkaline liquid droplets. Korean J. Chem. Eng. 2002, 19, 75.
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Figure captions
2
Figure 1. Schematic diagram of the experimental apparatus for the measurement of the
3
physical solubility: (1) N2O cylinder; (2) CO2 cylinder; (3) mass flow controller; (4) feeder;
4
(5) magnetic drive; (6) controller for temperature and agitation speed; (7) reactor (agitated
5
vessel); (8) pressure transducer; (9) computer.
6
Figure 2. Experimental apparatus used for wetted wall column experiment: (1) N2 cylinder;
7
(2) CO2 cylinder; (3) N2O cylinder; (4) mass flow controller; (5) mixing chamber; (6)
8
saturator; (7) wetted wall column; (8) water bath; (9) absorbent inflow; (10) absorbent
9
outflow; (11) paraffin oil inflow; (12) paraffin oil outflow; (13) gas inflow; (14) gas outflow;
10
(15) condenser; (16) GC/TCD; (17) wet-gas meter; (18) thermocouple; and (19) pressure
11
transducer.
12
Figure 3. Schematic diagram of the absorption and regeneration experimental system: (1) air
13
compressor; (2) filter; (3) CO2 cylinder; (4) mass flow controller; (5) analyzer; (6) mixing
14
chamber; (7) absorber; (8) thermocouple; (9) liquid pump; (10) heat exchanger and storage
15
tank; (11) regenerator; (12) condenser; and (13) gas meter.
16
Figure 4. Henry's constant of CO2 in aqueous solution as a function of temperature.
17
Figure 5. Diffusivities of CO2 in aqueous solution as a function of temperature.
18
Figure 6. Absorption rate of CO2 in aqueous solution as a function of reaction temperature.
19
Figure 7. The pseudo-first order overall reaction rate constant for the reaction of CO2 in
20
aqueous solution as a function of absorbent concentration at different temperatures.
21
Figure 8. Effect of the reaction time on the CO2 outlet concentration of the absorbent in a
22
continuous process of CO2 absorption/regeneration.
25
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1
Figure 9. CO2 absorption/regeneration efficiency of CO2 loading (rich/lean amine) with
2
respect to the absorbent solution.
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
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Table 1
2
The sorbent type and gas characteristics used in this study Type
Name
Purity
Manufacturer
CAS number
MEA(monoethanolamine)
99%
OCI Company Ltd.
141-43-5
Ethylene glycol
99%
SAMCHUN
107-21-1
N2
99.9%
N2O
99.5%
CO2
99.9%
CO2 sorbent
Gas
KOSEM
-
3 4 5
Table 2
6
Henry's constant of CO2 in aqueous solution as a function of temperature Sorbents [wt%]
293 K
303 K
313 K
323 K
30 wt% MEA/H2O
3405
4029
4761
5383
3518
4175
4941
5615
3637
4295
5045
5777
30 wt% MEA/H2O/ ethylene glycol 30 wt% MEA/ ethylene glycol 7 8 9 10 11 12
27
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Table 3
2
Diffusivity of CO2 in aqueous solution as a function of temperature Sorbents [wt%]
293 K
303 K
313 K
323 K
30 wt% MEA/H2O
1.33
1.59
1.94
2.27
1.11
1.40
1.73
2.00
0.92
1.19
1.58
1.84
30 wt% MEA/H2O/ ethylene glycol 30 wt% MEA/ ethylene glycol 3 4 5
Table 4
6
Absorption rate (NA) of CO2 in aqueous solutions at various temperatures NA of CO2 (kmol/m2·sec) 106 CO2 sorbent (wt%) 30°C
40°C
50°C
AMP(30)/MEA(1)
6.08
7.08
8.02
AMP(30)/MEA(3)
6.22
7.25
8.29
AMP(30)/MEA(5)
6.45
7.47
8.44
MEA(30)
5.98
6.88
7.66
piperazine
6.10
7.16
8.00
NH3
6.92
8.00
9.19
HMDA
6.22
7.17
8.18
7 8 9 10 11 28
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Table 5
2
Kinetic data for the absorption of CO2 by sorbent solutions Amine solutions
30 wt%
T [K]
, -9
2 -
[10 m s 1
]
-3
-2 -1
[kmol m ] [kPa] [kmol m s ]
[m3 kmol-1 s-
( [s-1]
293
1.33
3405
15
6.88
1834
303
1.59
4029
15
7.66
2662
1
]
698 MEA/H2O
313
1.94
4761
15
8.80
4021
323
2.27
5383
15
9.63
5261
293
1.11
3518
15
9.15
4149
MEA/H2O/
303
1.59
4029
15
7.66
2662
ethylene
313
1.73
4941
15
11.78
8703
323
2.00
5615
15
12.87
11605
293
0.92
3637
15
10.86
7537
MEA/
303
1.19
4295
15
12.04
9987
ethylene
313
1.58
5045
15
13.04
12174
323
1.84
5777
15
14.21
16278
30 wt%
glycol
30 wt%
1518
1780 glycol 3
29
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1 2
Figure 1
3
30
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6
16
16 5 12
4
10 9
3
8
7
1
2
1 2 3
Figure 2
4 5
14 5
13 8
Richamine 9
Leanamine
9
9
4
9
Aqueous amine solution
3
9
9 12
7
Air
8
4
11
2
1
6
10
10
5 6
Figure 3 31
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MEA/Ethylene glycol
5
4
2
HCO × 103 [ kPa m3 kmol-1 ]
MEA/H2O MEA/H2O/Ethylene glycol
3 3.05
3.10
3.15
2
3.20
3.25
3.30
3.35
3.40
3.45
1000/T [ K-1 ]
1 Figure 4
2.5 MEA/H2O MEA/H2O/Ethylene glycol MEA/Ethylene glycol 2.0
1.5
2
DCO × 109 [ m2 s-1 ]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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1.0
0.5 3.05
3.15
3.20
3.25
3.30
3.35
3.40
3.45
1000/T [ K-1 ]
3 4
3.10
Figure 5 32
ACS Paragon Plus Environment
Page 33 of 34
16 MEA/H2O MEA/H2O/Ethylene glycol MEA/Ethylene glycol
12
10
2
NCO × 106 [ kmol m-2 s-1 ]
14
8
6 293
2
323
313
303
Temperature [ K ]
1 Figure 6
20000 MEA/H2O MEA/H2O/Ethylene glycol MEA/Ethylene glycol
kov,(NA HA/DA0.5/PA)2 [ s-1 ]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
15000
10000
5000
293
313
323
Temperature [ K ]
3 4
303
Figure 7 33
ACS Paragon Plus Environment
Energy & Fuels
MEA/H2O MEA/H2O/Ethylene glycol
CO2 outlet concentration [ % ]
8
MEA/Ethylene glycol
6
4
2
0 0
10
2
20
30
40
50
60
Reaction time [ min ]
1 Figure 8
CO2 loading(Lean amine) 100
1.0
80
0.8
60
0.6
40
0.4
20
0.2
0
MEA/H2O/Ethylene glycol
MEA/Ethylene glycol
0.0
Absorbent solution
3 4
MEA/H2O
CO2 absorption amount [ mol CO2/mol absorbent ]
CO2 removal efficiency CO2 loading(Rich amine)
CO2 absorption/regeneration efficiency [ % ]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Figure 9 34
ACS Paragon Plus Environment
