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Performance and stability of a bio-inspired soybean-based solvent for CO2 capture from flue gas
Chemical Engineering Journal 385 (2020) 123908
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Performance and stability of a bio-inspired soybean-based solvent for CO2 capture from flue gas Pri J. Gusnawana,b, Lusi Zoua,b, Guoyin Zhanga, Jianjia Yua, a b
T
⁎
Petroleum Recovery Research Center, New Mexico Institute of Mining and Technology, Socorro, NM 87801, United States Materials Engineering Department, New Mexico Institute of Mining and Technology, Socorro, NM, 87801, United States
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
novel bio-inspired soybean-based • Asolvent (SBB solvent) was formulated for CO2 capture.
performance of the SBB solvent • The was thoroughly investigated. SBB solvent can be easily re• The generated at the low temperature of 80 °C.
thermal stability of the SBB sol• The vent remained unchanged up to 200 °C.
SBB solvent possessed high oxi• The dative stability with up to 17% O in 2
the flue gas.
A R T I C LE I N FO
A B S T R A C T
Keywords: Bio-inspired soybean-based solvent Stability CO2 capture Flue gas
A novel bio-inspired soybean-based solvent (SBB solvent) was formulated for the capture of CO2 with the partial pressure less than 15 kPa in the flue gas. The performance of the SBB solvent was thoroughly characterized in terms of equilibrium solubility of CO2, the heat of CO2 absorption, vapor pressure of the SBB solvent, and the CO2 absorption flux under different operating conditions including CO2 partial pressure in the flue gas, initial CO2 loading in the SBB solvent, SBB solvent concentration, and temperature. The results showed that the equilibrium solubility of CO2 in the SBB solvent was much lower than that in the conventional amine-based solvent at the regeneration condition. The heat of CO2 absorption is only at half of the monoethanolamine (MEA). The near-zero vapor pressure made the SBB solvent as a promising and environmentally friendly solvent for CO2 capture. The optimized concentration of the SBB solvent was 1.0 M for the effective absorption of CO2 from the flue gas. The CO2 absorption performance can be further improved by integrating the use of high concentration SBB solvent with reduced CO2-gas mass transfer resistance. Both thermal stability and oxidative stability of the SBB solvent were evaluated through a 120 h CO2 absorption-desorption experiment. The SBB solvent showed high thermal stability at the regeneration temperature up to 200 °C, and also excellent oxidative stability with the O2 concentration up to 17% in the flue gas.
1. Introduction The emission of carbon dioxide (CO2) to the atmosphere has been ⁎
considered as the major cause of global warming and climate change. Most CO2 is generated by fossil fuel combustion from the power plant [1]. Typically, the combustion gas or flue gas contains 7–15% CO2,
Corresponding author. E-mail address: [email protected] (J. Yu).
https://doi.org/10.1016/j.cej.2019.123908 Received 28 October 2019; Received in revised form 8 December 2019; Accepted 20 December 2019 Available online 23 December 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Chemical Engineering Journal 385 (2020) 123908
P.J. Gusnawan, et al.
Table 1 Composition of the SBB solvent. Components
Soybean composition (mol %)
Purity
SBB solvent composition (g. M−1)
L-Alanine
6.41
≥99%
5.8
L-Arginine
5.81
≥98%
10.3
Aspartic acid
11.48
≥98%
15.4
L-Cystine
0.93
≥99.7%
2.3
L-Glutamic acid
16.61
≥99%
24.7
Glycine
7.52
≥99%
5.7
L-Histidine
2.36
≥98%
3.7
L-Isoleucine
4.72
≥98%
6.3
L-Leucine
7.93
≥98%
10.6
L-Lysine
5.87
≥98%
8.8
L-Methionine
1.30
≥98%
2.0
L-Phenylalanine
4.17
≥98%
7.0
L-Proline
5.65
≥99%
6.6
L-Serine
5.83
≥99%
6.2
L-Threonine
4.34
≥98%
5.3
L-Tryptophan
0.78
≥98%
1.6
Structure
(continued on next page)
2
Chemical Engineering Journal 385 (2020) 123908
P.J. Gusnawan, et al.
Table 1 (continued) Components
Soybean composition (mol %)
Purity
SBB solvent composition (g. M−1)
L-Tyrosine
2.69
≥98%
5.0
L-Valine
5.59
≥98%
6.7
KOH
NA
≥90%
62.3
Structure
arginine, glycine, lysine, proline, phenylalanine, threonine, and alanine [12]. Because the selection of these amino acids was mainly based on the chemical structure and CO2 absorption performance of single amino acid salts, the cost of the blended amino acids is still a big challenge [10,12]. Recently, a novel bio-inspired soybean-based solvent (SBB solvent) was formulated for CO2 capture from flue gas in a membrane contactor process [19]. The SBB solvent consists of 18 different amino acids in a potassium hydroxide solution, which straightly follows the composition derived from soybean. The use of the SBB solvent provides an alternative to directly use soybean extract as a CO2 solvent, which avoids further complex purification and reduces the cost of the CO2 solvent. The SBB solvent has been reported with comparable CO2 absorption behavior with the conventional MEA solvent in the membrane contactor process. However, the properties of the SBB solvent is still not clear. In this study, both performance and stability of the SBB solvent were investigated via a series of CO2 absorption–desorption processes for cyclic CO2 capture from the flue gas. The equilibrium solubility of CO2 in the SBB solvent, heat of CO2 absorption, vapor pressure of the SBB solvent, CO2 absorption performance, and the stability of the SBB solvent over 120 h of operations were all investigated at the regeneration temperature up to 200 °C and the oxygen (O2) concentration up to 17% in the flue gas.
67%-77% N2, 2–5% O2, 8–20% H2O, SOX, NOX, and particulate matter with trace compositions [2,3]. Compare to the other CO2 source, the CO2 content in the flue gas is relatively low. For example, the CO2 content in natural gas, syngas, and biogas is ranged between 25 and 45% [4–6]. The low CO2 concentration leads to a low separation driving force. Therefore, the conventional physical separation technologies, such as membrane filtration, adsorption, or cryogenic that highly depends on the high CO2 concentration gradient, are less viable for CO2 capture from the flue gas. The flue gas CO2 capture is typically achieved chemical separation through a reactive CO2 solvent. The solvent first absorbs CO2 by chemical reactions, and then the CO2-rich solvent is regenerated in a stripping reboiler at high temperature to release and capture the CO2 [7]. In general, the CO2 solvent should meet several critical criteria beside good CO2 reactivity and high CO2 absorption capacity, which includes low viscosity, low vapor pressure (low volatility), low energy consumption for the solvent regeneration, and high thermal and oxidative stability [8,9]. These characteristics determine the total cost of CO2 capture from flue gas. Alkanolamine based solvent such as monoethanolamine (MEA) is a well-known and effective CO2 solvent that has been commercially used for CO2 capture from the flue gas. However, the MEA solvent is suffered from several drawbacks such as high volatility, easy to degrade by oxygen, and high regeneration energy consumption [9]. It has been reported that the cost of CO2 capture is as high as $51–82/ton CO2 avoided [7]. Alternative CO2 solvents have been developed in the past decades to reduce the cost of CO2 capture. The solvents, including amino acid salt, enzyme, ammonia, and ionic liquids-based solvents, have shown promising CO2 capture performance due to their superior reactivity with the CO2 and the higher CO2 absorption capacity than the alkanolamine. However, these solvents are still associated with different challenging. For instance, the energy of amino acid salt regeneration is high [10], the ammonia is not stable and volatile [11], the enzyme is expensive and easy to degrade [11], and the ionic liquid is viscous and expensive [7,11]. To overcome these challenges, the exploration of novel CO2 solvents for environmentally friendly and sustainable CO2 capture still provides a wide opportunity [12–14]. The concepts of “zero waste” and “zero accident” become a new industry standard in recent years [13,15]. Aqueous amino-acid in alkaline solution, such as glycine and proline in potassium solution, have been found with good CO2 absorption ability [12]. However, amino acids are usually originated from complex compounds in nature. For instance, soybean contains 18 different amino acids [16], which is commonly extracted to produce isolated soy protein [17]. The soy protein can be further hydrolyzed by the use of acid, base, or enzyme [18]. Currently, both isolated and hydrolyzed soy proteins are available in the market. The production of single amino acid is alone with complex purification processes, which may increase the cost of the CO2 solvent. To date, there are still rare studies on the use of naturally originated mixtures of amino acids as CO2 solvent. Most reports in this field focused on the solvent that contains only a few components, such as
2. Experimental 2.1. Material Eighteen different amino acids were selected according to the composition of soybean as listed in Table 1 [16]. The SBB solvent up to a concentration of 2.0 M was prepared by mixing the amino acids with equimolar potassium hydroxide (KOH). The mixture of amino acids follows the mass composition of the SBB solvent as shown in Table 1. Briefly, the amino acids with designed amounts were first dissolved in distilled water, and then KOH was added to the mixture under a continuous magnetic stirring until reaching a homogeneous state. All the chemicals including amino acids, monoethanolamine (MEA), KOH and MEA were used as received from the Sigma-Aldrich. The gas cylinders with 99.99% CO2, 99.99% N2, and ultra-purity air were used to simulate the flue gas. 2.2. Equilibrium solubility of CO2 To measure the equilibrium solubility of CO2, 10 mL SBB solvent was sealed in a stainless steel CO2 reactor equipped with a pressure gauge (GE Druck DPI 104), as shown in Fig. 1. The volume of the CO2 reactor is 52 ± 0.14 mL. The CO2 reactor was first flushed with 99.99% N2 at a flow rate of 100 mL/min for 2 min, and then immersed in a water bath and the pressure was recorded by a data acquisition system. The CO2 loading was achieved by feeding 99.99% CO2 into the reactor to a designed CO2 partial pressure. During the CO2 loading process, the pressure starts to increase sharply and then decreased 3
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then immersed in a water bath and the temperature was increased by 5 °C for every 30 min until the pressure reached a stable state. The overall pressure in the CO2 reactor P , can be defined as the summation of solvent vapor pressure, P sat , and nitrogen pressure, PN2 .
P|T = P sat|T + P(N2) |T
(4)
In a closed system in the reactor, assuming N2 is an ideal gas and the expansion of N2 occurred under higher temperature:
T PN2 |T = PN2 |T= T0 ⎛ ⎞ T ⎝ 0⎠ ⎜
⎟
(5)
Thus, by integration the Eqs. (4) and (5), the vapor pressure of the SBB solvent can be determined by the following equation:
T P sat|T = P|T − PN2 |T= T0 ⎛ ⎞ ⎝ T0 ⎠ ⎜
Fig. 1. Schematic diagram for the measurement of equilibrium CO2 solubility.
⎟
(6)
P sat
where is the vapor pressure of the solvent (kPa), PN2 is the N2 pressure in the reactor (kPa), P and T are the recorded pressure (kPa) and temperature (K). The pressure and temperature at the initial ambient condition are expressed as follows: P|T= T0 = Patm , PN2 |T= T0 = Patm and T0 = Tatm .
gradually until the equilibrium was reached in 2–8 h. Then, the CO2 was fed again to obtain an increased CO2 partial pressure, and the CO2 loading was gradually increased until the equilibrium pressure was 100 kPa. The CO2 loading experiment was conducted at different SBB solvent concentration and temperature, and each experiment was performed for at least twice. The CO2 loading in the SBB solvent was calculated by the mass balance of CO2 at the equilibrium state using the formula listed below:
2.5. CO2 absorption flux The CO2 absorption flux was measured using the similar method for the CO2 solubility measurement at low CO2 partial pressures. To follow a fast pseudo-first order reaction regime, the stirrer rotational speed was kept fast so the CO2 absorption flux was independent from the stirring rate [22]. The experiment was conducted by changing the parameters including temperature, CO2 partial pressure, CO2 loading in the solvent, and solvent concentration. The CO2 absorption flux, JCO2 , was determined by the formula listed below:
CO2 loading = initialCO2 loading + (CO2 feeding − CO2 atequlibrium) (1)
αCO2 |i =
1 ⎛ V − Vs ⎞ αCO2 |i − 1 Ms Vs + (PCO2, f |i − PCO2, eq |i ) r Ms Vs ⎝ RTR ⎠ ⎜
⎟
(2)
where α is the solvent CO2 loading (mol CO2/mol solvent), M is the solvent concentration (mol. L-1), V is the volume (mL), P is the partial pressure (kPa), R is the gas constant (8.314 L·kPa·mol−1·K−1), and T is the temperature (K), the subscript i is the order of CO2 feeding cycle, s represents the solvent, r is the reactor, f is the feed point, and eq is the equilibrium point. At the initial condition, the fresh SBB solvent was assumed without dissolution of CO2 and αCO2 |0 = 0 . To validate the above method for the measurement of equilibrium solubility of CO2, the CO2 solubility in 30% MEA solvent was measured at 40 °C. The obtained laboratory result was compared with the data published before. It can be seen in Fig. 2 that the experimental result is well fitted with the published data.
JCO2 = −
⎟
(7) −1
where JCO2 is CO2 absorption flux (mol.m .s ), P is CO2 partial pressure (kPa), T is absolute temperature (K), A is the solvent-gas contact surface area (m2), R is gas constant (J.mol.K−1), k is absorption rate, D is diffusivity (m.s−2) and t is time (s). Subscript r is reactor and s is solvent phase. 2.6. Mass-transfer analysis Mass-transfer analysis was also conducted to analyze the CO2 absorption flux. The mass-transfer model is illustrated in Fig. 3. The CO2 mass-transfer can be modeled based on mass-transfer coefficient [23] as follows:
The heat of CO2 absorption was estimated from the equilibrium solubility of the CO2 by using Gibbs-Helmholtz equation as follows [21]:
()
⎜
−2
2.3. Heat of CO2 absorption
⎛ ∂ (lnPCO ) ⎞ ΔHabs 2 =⎜ ⎟ R ⎜ ∂ 1 ⎟ T ⎝ ⎠αCO2
1 dPCO2 ⎛ Vr − Vs ⎞ A dt ⎝ RTr ⎠
− JCO2 = K g (PCO2, g, b − PCO2, gL) = Ks (CCO2, gL − CCO2, s, b) K = s (PCO2, gL − HCCO2, s, b) H
(8)
where the Henry’s equilibrium constant, H, is: (3)
H=
where ΔHabs is differential enthalpy of absorption (J.mol-CO2-1), R is gas constant (J.mol−1.K−1), PCO2 is CO2 partial pressure (Pa), T is absolute temperature (K), αCO2 is CO2 loading in the solvent. The estimation used the equilibrium data at αCO2 = 0.4 and αCO2 = 0.6 and temperature range of 293–353 K.
PCO2, gL CCO2, gL
(9)
The CO2 mass-transfer resistance, R , can be defined as the inverse of mass-transfer coefficient, K ; and the CO2 mas-transfer driving force, F , is the CO2 concentration differential, then in a series model:
Rtotal = Rg + Rs
(10)
1 1 H = + Ktotal Kg Ks
(11)
Ftotal = Fg + Fs
(12)
2.4. Vapor pressure of the SBB solvent To measure the SBB solvent vapor pressure, 10 mL of SBB solvent was sealed in the CO2 reactor and after that the reactor was flushed by 99.99% N2 as described in the solubility experiment. The reactor was 4
Chemical Engineering Journal 385 (2020) 123908
P.J. Gusnawan, et al.
Fig. 2. Validation of the equilibrium CO2 solubility measurement in 30 ± 0.5%-w MEA solution at 40 °C [10,20]
Ftotal = (PCO2, g, b − PCO2, gL) + (PCO2, gL − HCCO2, s, b)
expressed as the following form:
(13)
− JCO2 = K g PCO2, g, b
Therefore,
− JCO2 =
(PCO2, g, b − HCCO2, s, b ) 1 Kg
+
H Ks
Thus, the K g can be obtained from the linear relationship between CO2 absorption flux and CO2 partial pressure. When fresh solvent was used, the CO2 concentration in the solvent bulk phase, CCO2,s,b , can be neglected and therefore the Eq. (12) becomes:
(14)
Assuming the CO2 absorption in the solvent follows the fast pseudofirst order reaction regime, and by neglecting the effect of hydroxide OH–, the CO2 and solvent mass-transfer resistance becomes [22]:
H = Ks
− JCO2 = Ktotal PCO2, g, b
H k2 Csn DCO2, s
(16)
(17)
Substitution of Eqs. (5)–(15): (15)
PCO2, g, b =
Based on the above models a new model was derived based on the experimental results. At low CO2 partial pressure, the CO2 in the gas liquid-solvent interface PCO2,gL can be neglected, and the Eq. (6) can be
PCO2, g, b, t = 0
(
RT
)
r t AK e total Vr − Vs
(18)
When CO2 loaded solvent is used, the CO2 concentration in solvent
Fig. 3. CO2 mass-transfer in the CO2 absorption process. 5
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stability evaluations were:
bulk phase, CCO2,s,b , can not be neglected. Because the solvent is reactive, the CO2 in the solvent bulk phase reacts with the solvent to form carbamate and carbonate. Thus, the amount of dissolved CO2 is proportional to the presence of total inorganic carbon in the solvent, CCO2,s , stoichiometrically. The CO2 absorption flux can be modeled as:
− JCO2 = Ktotal (PCO2, g, b − HxCCO2, s )
η=
TNsample ⎞ ⎛ (12)(4.969) ⎞ N /C = ⎛ ⎜ ⎟ ⎝ (14)(1.297) ⎠ ⎝ (TOCsample − TOCdistilledwater ) ⎠ ⎜
(19)
Substitution of equation (5) to (17) and then proceed integration resulted:
PCO2, g, b =
PCO2, t = 0 (PCO2, g, b, t = 0 − HxCCO2, s, t = 0 ) e
(
RT
)
AKtotal V −rV t r s
+ HxCCO2, s, t = 0 −2
(
)2
(22)
⎟
(23)
where η is CO2 absorption efficiency (%), X is mol fraction (%), N/C is nitrogen to organic-carbon ratio (mol N/mol C), TN is total nitrogen (mg/L), TOC is total organic carbon (mg/L), the number of 12, 14, 4.969 and 1.297 is carbon atomic number (g/mol), nitrogen atomic number (g/mol), the SBB equivalent number of atomic carbon (-) and nitrogen (-), respectively. Subscript in is the CO2 flow at the inlet and outlet, respectively.
(20)
−1
where JCO2 is CO2 absorption flux (mol.m .s ), K is mass-transfer coefficient (m.s−1), P is CO2 partial pressure (Pa), C is solvent concentration (mol.m−3), H is the Henry’s solubility constant (Pa.m3.mol−1), T is absolute temperature (K), F is mass-transfer driving force (kPa), R is mass-transfer resistance (s.m−1), k2 is apparent kinetic constant, D is diffusivity (m.s−2), x is stoichiometric constant, and t is time (s). Subscript g is gas phase, s is solvent phase, b is bulk phase, and gL is the gas–liquid interface. The equations were fitted to the experimental data to acquire the mass-transfer resistance using Microsoft excel solver by minimizing the total sum square of residual as the objective function, and by changing the unknown variable of Ktotal , H , x, k and/or n
Objf : min ∑ PCO2, experiment − PCO2,calculation
XCO2, in − XCO2, out XCO2, in − XCO2, in XCO2, out
3. Result and discussion 3.1. Equilibrium solubility of CO2 Equilibrium solubility of CO2 in a solvent determines CO2 absorption capacity that defines the maximum CO2 that can be absorbed by the solvent at the equilibrium condition. Since the CO2 partial pressure is less than 15 kPa in flue gas, during the CO2 absorption process, the solvent is desired with the maximum CO2 loading at the low CO2 partial pressure and the absorption temperature. The CO2 solubility in the solvent also indicates how easy the CO2 can be regenerated from the solvent at the equilibrium condition. Thus, the solvent is preferred to hold less CO2 at lower CO2 partial pressure under the regeneration temperature. Thus, less steam is required to strip (dilute) the CO2 in the gas phase to achieve a high CO2 absorption cyclic capacity. The CO2 solubility in the SBB solvent was measured by loading a certain amount of CO2 into the SBB solvent until the equilibrium reached. The solubility of CO2 at different temperatures is plotted in Fig. 5. It can be seen that lower CO2 partial pressure is associated with the lower CO2 absorption capacity of the SBB solvent due to the low mass-transfer driving force. The CO2 absorption capacity decreased as the temperature increased because the CO2 dissolution is an exothermic process, and higher temperature helps CO2 molecules escape from the solvent. Table 2 shows the comparison of the equilibrium CO2 solubility in different CO2 solvents. The CO2 absorption capacity of 1.0 M SBB is 0.51 mol-CO2/mol-solvent, which is comparable with the conventional 30% MEA solvent. It has been reported that tertiary amine tends to have the highest CO2 absorption capacity compared to the primary and
(21)
2.7. Thermal and oxidative stability A SBB solvent circulation system consisting of both CO2 absorption and desorption units was set up in Fig. 4. The simulated flue gas containing 15% CO2 and 85% N2 was fed into the absorption reactor. The solvent circulation rate and the gas flow rate were maintained at 10 ± 1 mL/min and 80 ± 4 mL/min respectively. The top condenser and the reactor temperature were controlled to be at 21 ± 1 °C and 25 ± 2 °C by adjusting the flow rate of tap water, respectively. The SBB solvent circulation lasted for 120 h at different regeneration temperature by using the flue gas with different O2 concentration. The SBB solvent and flue gas were sampled every 15 h. The CO2 content in the absorber outlet was analyzed by using Gas Chromatography (Agilent 7890) to determine the CO2 absorption efficiency. The total organic carbon and total nitrogen in the solvent were analyzed using a total carbon/ nitrogen analyzer (Shimadzu TOC-L CSH) to determine the nitrogen/organic-carbon ratio. The formula used for the solvent
Fig. 4. Schematic diagram of CO2 absorption–desorption system. 6
Chemical Engineering Journal 385 (2020) 123908
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Fig. 5. Equilibrium solubility of CO2 in 1 ± 0.03 M SBB solvent.
3.2. Heat of CO2 absorption
secondary amine. Besides, the number of amine sites affects the CO2 absorption capacity because more CO2 can be absorbed per mol solvent. As a typical primary amine, MEA contains only a single amine site. However, the SBB solvent contains both secondary and tertiary amines, including arginine, proline, tryptophan, and histidine, which improved the CO2 absorption of the SBB solvent. Table 2 also shows that the CO2 absorption capacity of the SBB solvent is consistent with the results of different amino acid salts, which can be explained by the fact of that SBB solvent is a blend of 18 different amino acid salts with different CO2 absorption capabilities. The amino acid salts with high CO2 absorption capacity dominated the CO2 absorption process while the amino acid salts with low CO2 absorption capacity play an important role in the CO2 desorption process. The amino acids with low CO2 solubility are also beneficial to improve the regeneration ability of the SBB solvent. Table 2 lists the comparison of CO2 solubility in different solvents. It can be seen from the CO2 loading at the temperature of 80 °C and the CO2 partial pressure of 5 kPa that the SBB solvent has the lowest CO2 loading of 0.23 ± 0.01 mol-CO2/ mol-SBB solvent compared to the other solvents. This indicated that the SBB solvent is potentially easier to be regenerated with lower energy consumption than the other solvents.
The heat of CO2 absorption was estimated by plotting R·ln (PCO2) and T−1 gathered from the results of CO2 absorption equilibrium in Fig. 5. From Gibbs-Helmholtz equation, the heat of CO2 absorption is the slope of the plots in Fig. 6. When the CO2 loading was 0.4 mol CO2/ mol SBB, 0.5 mol CO2/mol SBB, and 0.6 mol CO2/mol SBB, the heat of CO2 absorption was −31.39 kJ.mol-CO2-1, −27.91 kJ.mol-CO2-1, and –23.99 kJ.mol-CO2-1 respectively. Here, the negative value indicates that the CO2 absorption is exothermic and the system releases heat in the CO2 absorption process. Since the heat of CO2 absorption is defined as the differential enthalpy, which indicates the total heat that required to releasing each molar of CO2 from the solvent when the temperature increased from 293 K to 353 K at a constant CO2 loading. It can be seen that the heat of CO2 absorption decreased at higher CO2 loading with more CO2 absorbed in the SBB solvent. This is consistent with the result of MEA solvent [21]. The heat of CO2 desorption determines the total energy requirement for CO2 desorption along with both sensible heat and vaporization heat [8]. Thus, low heat of CO2 absorption is desirable for the CO2 solvent regeneration. Table 3 summarizes the heat of CO2 absorption for different CO2 solvents, it can be seen that the SBB solvent shows
Table 2 Comparison of the equilibrium CO2 solubility in different solvents. Solvent
Concentration
CO2 loading (mol-CO2.mol-solvent−1) PCO2 = 15 kPa
SBB 1-dimethylamino-2-propanol MEA MEA Potassium Lysinate Potassium L-Asparaginate Potassium L-Glutaminate Potassium Prolinate Potassium Glycinate
1.0 2.0 1.0 5.0 1.0 1.0 1.0 2.5 1.0
M M M M M M M M M
Ref. PCO2 = 5 kPa
40 °C
60 °C
80 °C
0.51 ± 0.01 0.66 0.45 0.51 0.45 0.56 0.73 0.674 (7 kPa) 0.73
0.44 ± 0.01 0.59 NA 0.49 NA 0.46 0.67 0.59 0.66
0.23 ± 0.01 NA NA 0.36 NA 0.25 (8.6 kPa) 0.39 NA 0.42 (78 °C)
Note: The results are interpolated data from the adjacent condition on the references. 7
This work [8] [9] [20] [9] [24] [24] [25] [26]
Chemical Engineering Journal 385 (2020) 123908
P.J. Gusnawan, et al.
Fig. 7. Vapor pressure of 1 ± 0.03 M SBB solvent.
Fig. 6. Plot of R.ln (PCO2) versus T−1 for estimation of the heat of CO2 absorption in 1 ± 0.03 M SBB solvent. Table 3 Comparison of heat of CO2 absorption for different solvents. Solvent
Concentration
CO2 loading ((mol-CO2. mol-solvent−1)
ΔHabs (kJ.molCO2-1)
Ref.
SBB
1 M (14.6%-wt)
1-dimethylamino-2propanol MDEA MEA Potassium Glycinate
2M
0.6 0.4 0.656
–23.99 −31.39 −30.64
This Work [8]
30%-wt 30%-wt 30%-wt
0.623 0.608 0.4
−54.24 −46.2 −69
[21] [28] [10]
significantly lower heat of CO2 absorption than the conventional aminebased solvent such as MDEA and MEA. The low heat of CO2 absorption of the SBB solvent can attribute to the synergies among the various kinds of amino acid salts that contain both methyl and ethyl groups such as Alanine, Leucine, Isoleucine, and Valine. It has been reported that both methyl and ethyl functional groups in the amine are responsible to decrease the heat of CO2 absorption reaction [27]. Those functional groups may provide a moderate amine steric hindrance that causes a weak CO2-amine functional bond.
Fig. 8. Effect of CO2 partial pressure on CO2 absorption flux in 1 ± 0.03 M SBB solvent at T = 40 ± 0.5 °C.
3.4. Absorption flux and mass-transfer analysis To analyze the CO2 mass transfer phenomenon at low CO2 partial pressure (PCO2), the CO2 absorption experiments were carried out at different conditions including CO2 partial pressure (0–15 kPa), initial CO2 loading (0–0.47 mol CO2/mol solvent), SBB solvent concentration (0–2.0 M), and temperature (293–353 K). Fresh SBB solvent (initial CO2 loading less than 0.01 M) was used in the CO2 absorption process under different CO2 partial pressure. The effect of CO2 partial pressure on CO2 absorption flux is shown in Fig. 8. It was found a linear relationship between the CO2 absorption flux and the CO2 partial pressure when the pressure was less than13 kPa, which indicated that the CO2 absorption took place at the pseudo first order regime [22], and the CO2 partial pressure can be neglected at the gas–liquid interface. Based on equation (14), the CO2 mass-transfer coefficient and CO2 mass-transfer resistance in the gas phase is 7.62 × 10-4 m.s−1 and 1311.5 s.m−1, respectively. When the CO2 partial pressure is higher than 13 kPa, a non-linear relationship between CO2 absorption flux and CO2 partial pressure was observed in Fig. 8. The result reveals that the CO2 partial pressure at the interface has a significant effect on the CO2 absorption flux, as illustrated in Equation (6). At high bulk CO2 partial pressure, significant amounts of CO2 were absorbed in the solvent, and the absorbed CO2 was in equilibrium with the CO2 at the gas–liquid interface. In the real-world application after the solvent is regenerated, the CO2 solvents with a certain amount of CO2 loading are recycled to the absorber for the continuous CO2 absorption [3]. Thus the
3.3. Vapor pressure The vapor pressure reflects the volatility of the solvent, which is critical to determine the make-up of CO2 solvent during the operation. The vapor pressure of the SBB solvent was measured at different temperatures and the result is plotted in Fig. 7. As a comparison, the vapor pressure of water was also measured. It shows in Fig. 8 that the vapor pressure of the SBB solvent (P-SBB 1.0 M) is almost the same as the vapor pressure of water (P-H2O). This can be explained by the nature of the SBB solvent that mainly consists of amino acids and potassium hydroxide. The vapor pressure of the two major components is in the range of 2.8 × 10-8 kPa to 1.28 × 10-6 kPa, which is much lower than the vapor pressure of MEA solvent (53.86 kPa @ 25 °C) [29]. It can be seen that if the P-SBB 1.0MM is subtracted by P-H2O, the vapor pressure of pure SBB (P-SBB) is around zero. Thus, to compensate the loss of SBB solvent due to the vaporization in the CO2 absorption operation, only water is required to make up the SBB solvent; and all the active components in the SBB solvent remain stable at the operation temperature up to 84 °C.
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Fig. 9. Effect of initial CO2 loading on CO2 absorption flux of 1 ± 0.03 M SBB solvent at PCO2, feed = 13.3 ± 0.2 kPa and T = 40 ± 0.5 °C.
Fig. 11. Effect of reactor temperature on CO2 absorption flux of 1 ± 0.03 M SBB solution at PCO2, feed = 13.3 ± 0.5 kPa and T = 40 ± 0.5 °C.
understanding of the CO2 loading effect on the CO2 absorption flux is important. In this study, the SBB solvent with different CO2 loading was prepared at 13.3 ± 0.2 kPa. For each SBB solvent, the CO2 loading was calculated from Equation (2) and the correlated curve fitting was processed with Equation (18). It can be seen in Fig. 9 that the CO2 absorption flux decreased with the increase in the initial CO2 loading. This can be attributed to the decreased mass-transfer driving force, (PCO2,g,b − HCCO2,s,b) , because more CO2 was absorbed in the liquid phase when the initial CO2 loading was high. Therefore, the CO2 loading in the SBB solvent must be kept as low as possible to maintain a high CO2 absorption flux. The SBB concentration effect on the CO2 absorption flux was studied to pursue the optimized SBB concentration for CO2 capture from flue gas. The SBB solvents with different concentrations were prepared with the same initial CO2 loading at 12.8 ± 0.5 kPa. The CO2 absorption flux was also modeled by using Equation (16), and the results are shown in Fig. 10. It is obvious that the CO2 absorption flux increased by increasing the SBB concentration. At the SBB concentration less than 1.0 M, the growth of CO2 absorption flux was very fast; however, the growth slowed down when the SBB concentration was higher than 1.0 M. This can be explained by the change of mass transfer resistance of CO2 in both gas and liquid phase. As shown in Fig. 10, the mass transfer resistance of CO2-solvent sharply decreased at the SBB concentration less than 1.0 M, where the CO2-solvent resistant was comparable with CO2-gas resistant. Thus, both CO2-solvent resistance and CO2-gas resistance determined the total CO2 mass-transfer resistance in the CO2 absorption process. Note
that, the CO2-gas resistant is constant because the same CO2 partial pressure of 12.8 ± 0.5 kPa was used in the experiments. However, when the SBB concentration was higher than 1, the CO2solvent resistance is much lower than the CO2-gas resistance due to the fast reaction rate. Therefore, the total CO2 mass-transfer resistance is mainly determined by the CO2-gas resistance. From this point of view, the optimum concentration of SBB was 1.0 M for the effective CO2 capture from the flue-gas. To further improve the CO2 absorption flux by increasing the SBB concentration, the CO2-gas resistance must be decreased as well. The CO2-gas resistance depends on the gas properties, gas turbulence, and the geometry of the contactor [30]. Since the CO2 concentration in the flue-gas is assumed as a constant, the CO2-gas resistance can be only inhibited by improving the gas turbulence and the contactor geometry. The effect of temperature on the CO2 absorption flux was investigated by loading 1.0 M of fresh SBB solvent with CO2 at a constant pressure of 13.3 ± 0.5 kPa and different temperatures. The result in Fig. 11 shows that the CO2 absorption flux increased by temperature. Similar result can be modeled empirically from the classic Arrhenius’s equation [31] listed below:
b − JCO2 = aexp ⎛− ⎞ ⎝ T⎠
(24)
where a and b are the empirical constants, and T is temperature. When the temperature increased, the CO2-gas resistance decreased due to the enhanced molecular turbulence. The CO2-solvent resistance was also reduced by the accelerated CO2 absorption kinetic. It has been found in Fig. 5 that the CO2 absorption capacity declined at high temperature. Thus, to improve the CO2 absorption flux, the increase of temperature must be optimized with adequate CO2 absorption capacity. 3.5. Thermal stability of the SBB solvent The thermal stability of the SBB solvent was estimated through 120 h of continuous CO2 absorption–desorption processes by using simulated flue gas. An aggravated operation condition was created to speed up the thermal degradation process. The temperature of the oil bath was 130 °C, 160 °C, and 200 °C, which is much higher than the boiling point of the SBB solvent. During the CO2 absorption process, the initial CO2 loading in the SBB solvent was 0.29 ± 0.04 mol-CO2/molSBB. In this study, the thermal stability of the SBB solvent is usually affected by the following aspects:
Fig. 10. Effect of SBB concentration on CO2 absorption flux at PCO2, = 12.8 ± 0.5 kPa and T = 40 ± 0.5 °C.
1. The possible vaporization of active components from the SBB solvent.
feed
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solvent was attributed to the low vapor pressure of amino acid salts in the SBB solvent, which has been discussed in Fig. 7. Besides, the thermal decomposition of most of the amino acid salts in the SBB solvent only takes place at the temperature higher than 200 °C. Only glutamic acid, cysteine, and methionine have decomposition temperature of 160 °C, 175 °C, and 181 °C, respectively [29]. It has to note that the reported temperature for the thermal decomposition is at the solid state. Since the SBB solvent regeneration temperature is limited by the solvent boiling point of 97 ± 1 °C, the exposure of the solvent at the desired temperature may only be limited at the interior wall of the reboiler. The temperature at the interior of the reboiler is roughly half of the summation of the solvent temperature and the reboiler temperature [33]. The maximum temperature is at around 148.5 °C, which is still less than the lowest thermal decomposition temperature of glutamic acid in the SBB solvent.
Fig. 12. CO2 absorption flux of 1 ± 0.03 M SBB solution over different regeneration temperatures during 120 h operation.
3.6. Oxidative stability of the SBB solvent
The associated byproduct including SBB vapor, ammonia, and carbon dioxide can be stripped out from the regenerator along with the released CO2 from the desorption process. Thus, the thermal stability of the SBB solvent can be estimated by the measurement of the elements of nitrogen and total organic carbon in the SBB solvent. The degradation of CO2 solvent may also occur via rearrangement or reaction in the liquid phase. However, it cannot cause deamination or decarbonation but generate CO2 unreactive byproducts [32]. Thus, the CO2 absorption efficiency was studied as well. The occurrence of CO2 solvent degradation can be indicated by the decline of CO2 absorption efficiency at a constant N/C ratio. Figs. 12 and 13 show CO2 absorption efficiency and the ratio of nitrogen to organic carbon (N/C ratio) in the 1.0 ± 0.03 M SBB solvent, respectively. It can be seen that, there is no significant difference in CO2 absorption efficiency, and the N/C ratio remained at 1.0 during the 120 h of absorption and desorption process. The SBB solvent was stable over a wide range of regeneration temperatures up to 200 °C. As a comparison, the MEA solvent with a CO2 loading of 0.4 degraded by 26% at the regeneration temperature of 150 °C at the end of an equivalent operating time [32]. The high thermal stability of SBB
Due to the presence of O2 in the flue gas, the oxidative stability of the SBB solvent was investigated through the 120 h of continuous experiments at the O2 concentration of 10.5% and 17% in the flue gas. The temperature was maintained at 160 °C for the solvent regeneration; the circulating rate of SBB solvent and the flow rate of the flue gas was 10 ± 1 mL/min and 80 ± 4 mL/min, respectively. During the 120 h operation with different O2 concentrations in the flue-gas, the CO2 absorption efficiency and the N/C ratio of the 1 ± 0.03 M SBB solvent are given in Figs. 14 and 15. It can be seen the CO2 absorption efficiency remained more than 95%, and the N/C ratio was 1.0 at the end of the operation. The SBB solvent is stable over the exposure of O2 in the flue-gas up to the molar concentration of 17%. The promising stability of the SBB solvent is attributed to the presence of amino acid salts containing oxygen element in their structures [34]. The oxygen element is in the form of carbocyclic acid (R-COO-), which is the ultimate state in the oxidation of alkanol. Thus, it cannot be further oxidized. It is worth to note that the components of Cysteine and Methionine, both containing the sulfur element, may result in a sulfur induced oxidative decomposition. However, in view of the low content of only 2.23% in the SBB solvent, the degradation from the sulfurcontaining component can be neglected in the CO2 desorption process. In contrast, MEA is made of alkanol group, and it suffers from oxidative degradation to form ammonia, aldehydes (alkanol oxidation product), and organic acids (aldehydes oxidation product) [35] .
Fig. 13. Nitrogen/organic-carbon atomic ratio of 1 ± 0.03 M SBB solution over different regeneration temperatures during 120 h operation. Note: the N and C concentration has been normalized to follow 1 M SBB concentration.
Fig. 14. CO2 absorption flux of 1 ± 0.03 M SBB solution over different concentration of O2 in the flue-gas during 120 h operation.
2. The potential degradation/decomposition of the amino acid salts in the SBB solvent because of deamination and/or decarboxylation.
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[4] J. Chau, X. Jie, K.K. Sirkar, Polyamidoamine-facilitated poly (ethylene glycol)/ionic liquid based pressure swing membrane absorption process for CO2 removal from shifted syngas, Chem. Eng. J. 305 (2016) 212–220. [5] M. Rezakazemi, I. Heydari, Z. Zhang, Hybrid systems: combining membrane and absorption technologies leads to more efficient acid gases (CO2 and H2S) removal from natural gas, J. CO2 utilization 18 (2017) 362–369. [6] S. Sutanto, J. Dijkstra, J. Pieterse, J. Boon, P. Hauwert, D. Brilman, CO2 removal from biogas with supported amine sorbents: First technical evaluation based on experimental data, Sep. Purif. Technol. 184 (2017) 12–25. [7] G.T. Rochelle, Amine scrubbing for CO2 capture, Science 325 (2009) 1652–1654. [8] Y. Liang, H. Liu, W. Rongwong, Z. Liang, R. Idem, P. Tontiwachwuthikul, Solubility, absorption heat and mass transfer studies of CO2 absorption into aqueous solution of 1-dimethylamino-2-propanol, Fuel 144 (2015) 121–129. [9] S. Mazinani, R. Ramazani, A. Samsami, A. Jahanmiri, B. Van der Bruggen, S. Darvishmanesh, Equilibrium solubility, density, viscosity and corrosion rate of carbon dioxide in potassium lysinate solution, Fluid Phase Equilib. 396 (2015) 28–34. [10] H.-J. Song, S. Lee, K. Park, J. Lee, D. Chand Spah, J.-W. Park, T.P. Filburn, Simplified estimation of regeneration energy of 30 wt% sodium glycinate solution for carbon dioxide absorption, Ind. Eng. Chem. Res. 47 (2008) 9925–9930. [11] K.A. Mumford, Y. Wu, K.H. Smith, G.W. Stevens, Review of solvent based carbondioxide capture technologies, Front. Chem. Sci. Eng. 9 (2015) 125–141. [12] Z. Zhang, Y. Li, W. Zhang, J. Wang, M.R. Soltanian, A.G. Olabi, Effectiveness of amino acid salt solutions in capturing CO2: A review, Renew. Sustain. Energy Rev. 98 (2018) 179–188. [13] T. Curran, I. Williams, A zero waste vision for industrial networks in Europe, J. Hazard. Mater. 207 (2012) 3–7. [14] G.I. Zwetsloot, M. Aaltonen, J.-L. Wybo, J. Saari, P. Kines, R.O. De Beeck, The case for research into the zero accident vision, Saf. Sci. 58 (2013) 41–48. [15] G.I.J.M. Zwetsloot, M. Aaltonen, J.-L. Wybo, J. Saari, P. Kines, R.O.D. Beeck, The case for research into the zero accident vision, Saf. Sci. 58 (2013) 41–48. [16] I.V. Kovalenko, G.R. Rippke, C.R. Hurburgh, Determination of amino acid composition of soybeans (Glycine max) by near-infrared spectroscopy, J. Agric. Food. Chem. 54 (2006) 3485–3491. [17] K.E. Preece, N. Hooshyar, N.J. Zuidam, Whole soybean protein extraction processes: a review, Innovative Food Sci. Emerg. Technol. 43 (2017) 163–172. [18] M. Fountoulakis, H.-W. Lahm, Hydrolysis and amino acid composition analysis of proteins, J. Chromatogr. A 826 (1998) 109–134. [19] P.J. Gusnawan, S. Zha, L. Zou, G. Zhang, J. Yu, Soybean and moringa based green biosolvents for low-concentration CO2 capture via a hollow fiber membrane contactor, Chem. Eng. J. 335 (2018) 631–637. [20] K.P. Shen, M.H. Li, Solubility of carbon dioxide in aqueous mixtures of monoethanolamine with methyldiethanolamine, J. Chem. Eng. Data 37 (1992) 96–100. [21] I. Kim, K.A. Hoff, E.T. Hessen, T. Haug-Warberg, H.F. Svendsen, Enthalpy of absorption of CO2 with alkanolamine solutions predicted from reaction equilibrium constants, Chem. Eng. Sci. 64 (2009) 2027–2038. [22] J. van Holst, G. Versteeg, D.W.F. Brilman, J. Hogendoorn, Kinetic study of CO2 with various amino acid salts in aqueous solution, Chem. Eng. Sci. 64 (2009) 59–68. [23] R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport phenomena, John Wiley & Sons, 2006. [24] Z.-W. Chen, R.B. Leron, M.-H. Li, Equilibrium solubility of carbon dioxide in aqueous potassium L-asparaginate and potassium L-glutaminate solutions, Fluid Phase Equilib. 400 (2015) 20–26. [25] S. Shen, Y.-N. Yang, Y. Wang, S. Ren, J. Han, A. Chen, CO2 absorption into aqueous potassium salts of lysine and proline: Density, viscosity and solubility of CO2, Fluid Phase Equilib. 399 (2015) 40–49. [26] A. Portugal, J. Sousa, F. Magalhães, A. Mendes, Solubility of carbon dioxide in aqueous solutions of amino acid salts, Chem. Eng. Sci. 64 (2009) 1993–2002. [27] F.A. Chowdhury, H. Okabe, S. Shimizu, M. Onoda, Y. Fujioka, Development of novel tertiary amine absorbents for CO2 capture, Energy Procedia 1 (2009) 1241–1248. [28] I. Kim, H.F. Svendsen, Heat of absorption of carbon dioxide (CO2) in monoethanolamine (MEA) and 2-(aminoethyl) ethanolamine (AEEA) solutions, Ind. Eng. Chem. Res. 46 (2007) 5803–5809. [29] U.S.N.L.o. Medicine, PubChem, U.S. National Center for Biotechnology Information, 2019. [30] M.A. Pacheco, Mass transfer, kinetics and rate-based modeling of reactive, absorption (1999). [31] O. Levenspiel, Chemical reaction engineering, Ind. Eng. Chem. Res. 38 (1999) 4140–4143. [32] J. Davis, G. Rochelle, Thermal degradation of monoethanolamine at stripper conditions, Energy Procedia 1 (2009) 327–333. [33] C. Geankoplis, Transport Processes and Unit Operations: PTR Prentice Hall, Inc.: Englewood Cliffs, NJ (1993) 401. [34] W.M. Budzianowski, Energy Efficient Solvents for CO2 Capture by Gas-Liquid Absorption: Compounds, Blends and Advanced Solvent Systems, Springer, 2016. [35] S. Fredriksen, K.-J. Jens, Oxidative degradation of aqueous amine solutions of MEA, AMP, MDEA, PZ: a review, Energy Procedia 37 (2013) 1770–1777.
Fig. 15. N/C ratio of 1 ± 0.03 M SBB solution over different O2 concentration.
4. Conclusion The performance and stability of a soybean-based biosolvent (SBB solvent) were thoroughly evaluated for CO2 capture from the flue gas with the CO2 partial pressure less than 15 kPa. As a novel solvent, the SBB exhibited comparable CO2 absorption capacity with the conventional amine-based solvents. However, the CO2 solubility in the SBB solvent is much lower than the MEA solvent under the regeneration condition. Besides, the heat of CO2 absorption was only the half of the conventional CO2 solvents, including MEA, which indicated an easier regeneration behavior of the SBB solvent. The SBB solvent showed a similar vapor pressure with water while the active components in the SBB solvent exhibited the near-zero vapor pressure. It was also found the optimized concentration of the SBB solvent was 1.0 M for effective CO2 capture. The CO2 absorption performance can be further improved by integrating the use of high concentration SBB solvent with a reduced CO2-gas mass transfer resistance. The increased absorption temperature improved CO2 absorption flux but reduced CO2 solubility in the SBB solvent. During the CO2 absorption–desorption process, the CO2 loading in the SBB solvent should be reduced as low as possible to maintain a high CO2 absorption flux. The SBB solvent showed high thermal stability at the regeneration temperature up to 200 °C, and also excellent oxidative stability during the CO2 capture process with the O2 concentration up to 17% in the flue gas. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] J.E. Szulejko, P. Kumar, A. Deep, K.-H. Kim, Global warming projections to 2100 using simple CO2 greenhouse gas modeling and comments on CO2 climate sensitivity factor, Atmos. Pollut. Res. 8 (2017) 136–140. [2] C. Song, W. Pan, S.T. Srimat, J. Zheng, Y. Li, Y.H. Wang, B.Q. Xu, Q.M. Zhu, Trireforming of methane over Ni catalysts for CO2 conversion to Syngas with desired H2 CO ratios using flue gas of power plants without CO2 separation, Stud. Surf. Sci. Catal. 153 (2004) 315–322. [3] E. Favre, Membrane processes and postcombustion carbon dioxide capture: challenges and prospects, Chem. Eng. J. 171 (2011) 782–793.
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Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Performance and stability of a bio-inspired soybean-based solvent for CO2 capture from flue gas Pri J. Gusnawana,b, Lusi Zoua,b, Guoyin Zhanga, Jianjia Yua, a b
T
⁎
Petroleum Recovery Research Center, New Mexico Institute of Mining and Technology, Socorro, NM 87801, United States Materials Engineering Department, New Mexico Institute of Mining and Technology, Socorro, NM, 87801, United States
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
novel bio-inspired soybean-based • Asolvent (SBB solvent) was formulated for CO2 capture.
performance of the SBB solvent • The was thoroughly investigated. SBB solvent can be easily re• The generated at the low temperature of 80 °C.
thermal stability of the SBB sol• The vent remained unchanged up to 200 °C.
SBB solvent possessed high oxi• The dative stability with up to 17% O in 2
the flue gas.
A R T I C LE I N FO
A B S T R A C T
Keywords: Bio-inspired soybean-based solvent Stability CO2 capture Flue gas
A novel bio-inspired soybean-based solvent (SBB solvent) was formulated for the capture of CO2 with the partial pressure less than 15 kPa in the flue gas. The performance of the SBB solvent was thoroughly characterized in terms of equilibrium solubility of CO2, the heat of CO2 absorption, vapor pressure of the SBB solvent, and the CO2 absorption flux under different operating conditions including CO2 partial pressure in the flue gas, initial CO2 loading in the SBB solvent, SBB solvent concentration, and temperature. The results showed that the equilibrium solubility of CO2 in the SBB solvent was much lower than that in the conventional amine-based solvent at the regeneration condition. The heat of CO2 absorption is only at half of the monoethanolamine (MEA). The near-zero vapor pressure made the SBB solvent as a promising and environmentally friendly solvent for CO2 capture. The optimized concentration of the SBB solvent was 1.0 M for the effective absorption of CO2 from the flue gas. The CO2 absorption performance can be further improved by integrating the use of high concentration SBB solvent with reduced CO2-gas mass transfer resistance. Both thermal stability and oxidative stability of the SBB solvent were evaluated through a 120 h CO2 absorption-desorption experiment. The SBB solvent showed high thermal stability at the regeneration temperature up to 200 °C, and also excellent oxidative stability with the O2 concentration up to 17% in the flue gas.
1. Introduction The emission of carbon dioxide (CO2) to the atmosphere has been ⁎
considered as the major cause of global warming and climate change. Most CO2 is generated by fossil fuel combustion from the power plant [1]. Typically, the combustion gas or flue gas contains 7–15% CO2,
Corresponding author. E-mail address: [email protected] (J. Yu).
https://doi.org/10.1016/j.cej.2019.123908 Received 28 October 2019; Received in revised form 8 December 2019; Accepted 20 December 2019 Available online 23 December 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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Table 1 Composition of the SBB solvent. Components
Soybean composition (mol %)
Purity
SBB solvent composition (g. M−1)
L-Alanine
6.41
≥99%
5.8
L-Arginine
5.81
≥98%
10.3
Aspartic acid
11.48
≥98%
15.4
L-Cystine
0.93
≥99.7%
2.3
L-Glutamic acid
16.61
≥99%
24.7
Glycine
7.52
≥99%
5.7
L-Histidine
2.36
≥98%
3.7
L-Isoleucine
4.72
≥98%
6.3
L-Leucine
7.93
≥98%
10.6
L-Lysine
5.87
≥98%
8.8
L-Methionine
1.30
≥98%
2.0
L-Phenylalanine
4.17
≥98%
7.0
L-Proline
5.65
≥99%
6.6
L-Serine
5.83
≥99%
6.2
L-Threonine
4.34
≥98%
5.3
L-Tryptophan
0.78
≥98%
1.6
Structure
(continued on next page)
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Table 1 (continued) Components
Soybean composition (mol %)
Purity
SBB solvent composition (g. M−1)
L-Tyrosine
2.69
≥98%
5.0
L-Valine
5.59
≥98%
6.7
KOH
NA
≥90%
62.3
Structure
arginine, glycine, lysine, proline, phenylalanine, threonine, and alanine [12]. Because the selection of these amino acids was mainly based on the chemical structure and CO2 absorption performance of single amino acid salts, the cost of the blended amino acids is still a big challenge [10,12]. Recently, a novel bio-inspired soybean-based solvent (SBB solvent) was formulated for CO2 capture from flue gas in a membrane contactor process [19]. The SBB solvent consists of 18 different amino acids in a potassium hydroxide solution, which straightly follows the composition derived from soybean. The use of the SBB solvent provides an alternative to directly use soybean extract as a CO2 solvent, which avoids further complex purification and reduces the cost of the CO2 solvent. The SBB solvent has been reported with comparable CO2 absorption behavior with the conventional MEA solvent in the membrane contactor process. However, the properties of the SBB solvent is still not clear. In this study, both performance and stability of the SBB solvent were investigated via a series of CO2 absorption–desorption processes for cyclic CO2 capture from the flue gas. The equilibrium solubility of CO2 in the SBB solvent, heat of CO2 absorption, vapor pressure of the SBB solvent, CO2 absorption performance, and the stability of the SBB solvent over 120 h of operations were all investigated at the regeneration temperature up to 200 °C and the oxygen (O2) concentration up to 17% in the flue gas.
67%-77% N2, 2–5% O2, 8–20% H2O, SOX, NOX, and particulate matter with trace compositions [2,3]. Compare to the other CO2 source, the CO2 content in the flue gas is relatively low. For example, the CO2 content in natural gas, syngas, and biogas is ranged between 25 and 45% [4–6]. The low CO2 concentration leads to a low separation driving force. Therefore, the conventional physical separation technologies, such as membrane filtration, adsorption, or cryogenic that highly depends on the high CO2 concentration gradient, are less viable for CO2 capture from the flue gas. The flue gas CO2 capture is typically achieved chemical separation through a reactive CO2 solvent. The solvent first absorbs CO2 by chemical reactions, and then the CO2-rich solvent is regenerated in a stripping reboiler at high temperature to release and capture the CO2 [7]. In general, the CO2 solvent should meet several critical criteria beside good CO2 reactivity and high CO2 absorption capacity, which includes low viscosity, low vapor pressure (low volatility), low energy consumption for the solvent regeneration, and high thermal and oxidative stability [8,9]. These characteristics determine the total cost of CO2 capture from flue gas. Alkanolamine based solvent such as monoethanolamine (MEA) is a well-known and effective CO2 solvent that has been commercially used for CO2 capture from the flue gas. However, the MEA solvent is suffered from several drawbacks such as high volatility, easy to degrade by oxygen, and high regeneration energy consumption [9]. It has been reported that the cost of CO2 capture is as high as $51–82/ton CO2 avoided [7]. Alternative CO2 solvents have been developed in the past decades to reduce the cost of CO2 capture. The solvents, including amino acid salt, enzyme, ammonia, and ionic liquids-based solvents, have shown promising CO2 capture performance due to their superior reactivity with the CO2 and the higher CO2 absorption capacity than the alkanolamine. However, these solvents are still associated with different challenging. For instance, the energy of amino acid salt regeneration is high [10], the ammonia is not stable and volatile [11], the enzyme is expensive and easy to degrade [11], and the ionic liquid is viscous and expensive [7,11]. To overcome these challenges, the exploration of novel CO2 solvents for environmentally friendly and sustainable CO2 capture still provides a wide opportunity [12–14]. The concepts of “zero waste” and “zero accident” become a new industry standard in recent years [13,15]. Aqueous amino-acid in alkaline solution, such as glycine and proline in potassium solution, have been found with good CO2 absorption ability [12]. However, amino acids are usually originated from complex compounds in nature. For instance, soybean contains 18 different amino acids [16], which is commonly extracted to produce isolated soy protein [17]. The soy protein can be further hydrolyzed by the use of acid, base, or enzyme [18]. Currently, both isolated and hydrolyzed soy proteins are available in the market. The production of single amino acid is alone with complex purification processes, which may increase the cost of the CO2 solvent. To date, there are still rare studies on the use of naturally originated mixtures of amino acids as CO2 solvent. Most reports in this field focused on the solvent that contains only a few components, such as
2. Experimental 2.1. Material Eighteen different amino acids were selected according to the composition of soybean as listed in Table 1 [16]. The SBB solvent up to a concentration of 2.0 M was prepared by mixing the amino acids with equimolar potassium hydroxide (KOH). The mixture of amino acids follows the mass composition of the SBB solvent as shown in Table 1. Briefly, the amino acids with designed amounts were first dissolved in distilled water, and then KOH was added to the mixture under a continuous magnetic stirring until reaching a homogeneous state. All the chemicals including amino acids, monoethanolamine (MEA), KOH and MEA were used as received from the Sigma-Aldrich. The gas cylinders with 99.99% CO2, 99.99% N2, and ultra-purity air were used to simulate the flue gas. 2.2. Equilibrium solubility of CO2 To measure the equilibrium solubility of CO2, 10 mL SBB solvent was sealed in a stainless steel CO2 reactor equipped with a pressure gauge (GE Druck DPI 104), as shown in Fig. 1. The volume of the CO2 reactor is 52 ± 0.14 mL. The CO2 reactor was first flushed with 99.99% N2 at a flow rate of 100 mL/min for 2 min, and then immersed in a water bath and the pressure was recorded by a data acquisition system. The CO2 loading was achieved by feeding 99.99% CO2 into the reactor to a designed CO2 partial pressure. During the CO2 loading process, the pressure starts to increase sharply and then decreased 3
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then immersed in a water bath and the temperature was increased by 5 °C for every 30 min until the pressure reached a stable state. The overall pressure in the CO2 reactor P , can be defined as the summation of solvent vapor pressure, P sat , and nitrogen pressure, PN2 .
P|T = P sat|T + P(N2) |T
(4)
In a closed system in the reactor, assuming N2 is an ideal gas and the expansion of N2 occurred under higher temperature:
T PN2 |T = PN2 |T= T0 ⎛ ⎞ T ⎝ 0⎠ ⎜
⎟
(5)
Thus, by integration the Eqs. (4) and (5), the vapor pressure of the SBB solvent can be determined by the following equation:
T P sat|T = P|T − PN2 |T= T0 ⎛ ⎞ ⎝ T0 ⎠ ⎜
Fig. 1. Schematic diagram for the measurement of equilibrium CO2 solubility.
⎟
(6)
P sat
where is the vapor pressure of the solvent (kPa), PN2 is the N2 pressure in the reactor (kPa), P and T are the recorded pressure (kPa) and temperature (K). The pressure and temperature at the initial ambient condition are expressed as follows: P|T= T0 = Patm , PN2 |T= T0 = Patm and T0 = Tatm .
gradually until the equilibrium was reached in 2–8 h. Then, the CO2 was fed again to obtain an increased CO2 partial pressure, and the CO2 loading was gradually increased until the equilibrium pressure was 100 kPa. The CO2 loading experiment was conducted at different SBB solvent concentration and temperature, and each experiment was performed for at least twice. The CO2 loading in the SBB solvent was calculated by the mass balance of CO2 at the equilibrium state using the formula listed below:
2.5. CO2 absorption flux The CO2 absorption flux was measured using the similar method for the CO2 solubility measurement at low CO2 partial pressures. To follow a fast pseudo-first order reaction regime, the stirrer rotational speed was kept fast so the CO2 absorption flux was independent from the stirring rate [22]. The experiment was conducted by changing the parameters including temperature, CO2 partial pressure, CO2 loading in the solvent, and solvent concentration. The CO2 absorption flux, JCO2 , was determined by the formula listed below:
CO2 loading = initialCO2 loading + (CO2 feeding − CO2 atequlibrium) (1)
αCO2 |i =
1 ⎛ V − Vs ⎞ αCO2 |i − 1 Ms Vs + (PCO2, f |i − PCO2, eq |i ) r Ms Vs ⎝ RTR ⎠ ⎜
⎟
(2)
where α is the solvent CO2 loading (mol CO2/mol solvent), M is the solvent concentration (mol. L-1), V is the volume (mL), P is the partial pressure (kPa), R is the gas constant (8.314 L·kPa·mol−1·K−1), and T is the temperature (K), the subscript i is the order of CO2 feeding cycle, s represents the solvent, r is the reactor, f is the feed point, and eq is the equilibrium point. At the initial condition, the fresh SBB solvent was assumed without dissolution of CO2 and αCO2 |0 = 0 . To validate the above method for the measurement of equilibrium solubility of CO2, the CO2 solubility in 30% MEA solvent was measured at 40 °C. The obtained laboratory result was compared with the data published before. It can be seen in Fig. 2 that the experimental result is well fitted with the published data.
JCO2 = −
⎟
(7) −1
where JCO2 is CO2 absorption flux (mol.m .s ), P is CO2 partial pressure (kPa), T is absolute temperature (K), A is the solvent-gas contact surface area (m2), R is gas constant (J.mol.K−1), k is absorption rate, D is diffusivity (m.s−2) and t is time (s). Subscript r is reactor and s is solvent phase. 2.6. Mass-transfer analysis Mass-transfer analysis was also conducted to analyze the CO2 absorption flux. The mass-transfer model is illustrated in Fig. 3. The CO2 mass-transfer can be modeled based on mass-transfer coefficient [23] as follows:
The heat of CO2 absorption was estimated from the equilibrium solubility of the CO2 by using Gibbs-Helmholtz equation as follows [21]:
()
⎜
−2
2.3. Heat of CO2 absorption
⎛ ∂ (lnPCO ) ⎞ ΔHabs 2 =⎜ ⎟ R ⎜ ∂ 1 ⎟ T ⎝ ⎠αCO2
1 dPCO2 ⎛ Vr − Vs ⎞ A dt ⎝ RTr ⎠
− JCO2 = K g (PCO2, g, b − PCO2, gL) = Ks (CCO2, gL − CCO2, s, b) K = s (PCO2, gL − HCCO2, s, b) H
(8)
where the Henry’s equilibrium constant, H, is: (3)
H=
where ΔHabs is differential enthalpy of absorption (J.mol-CO2-1), R is gas constant (J.mol−1.K−1), PCO2 is CO2 partial pressure (Pa), T is absolute temperature (K), αCO2 is CO2 loading in the solvent. The estimation used the equilibrium data at αCO2 = 0.4 and αCO2 = 0.6 and temperature range of 293–353 K.
PCO2, gL CCO2, gL
(9)
The CO2 mass-transfer resistance, R , can be defined as the inverse of mass-transfer coefficient, K ; and the CO2 mas-transfer driving force, F , is the CO2 concentration differential, then in a series model:
Rtotal = Rg + Rs
(10)
1 1 H = + Ktotal Kg Ks
(11)
Ftotal = Fg + Fs
(12)
2.4. Vapor pressure of the SBB solvent To measure the SBB solvent vapor pressure, 10 mL of SBB solvent was sealed in the CO2 reactor and after that the reactor was flushed by 99.99% N2 as described in the solubility experiment. The reactor was 4
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Fig. 2. Validation of the equilibrium CO2 solubility measurement in 30 ± 0.5%-w MEA solution at 40 °C [10,20]
Ftotal = (PCO2, g, b − PCO2, gL) + (PCO2, gL − HCCO2, s, b)
expressed as the following form:
(13)
− JCO2 = K g PCO2, g, b
Therefore,
− JCO2 =
(PCO2, g, b − HCCO2, s, b ) 1 Kg
+
H Ks
Thus, the K g can be obtained from the linear relationship between CO2 absorption flux and CO2 partial pressure. When fresh solvent was used, the CO2 concentration in the solvent bulk phase, CCO2,s,b , can be neglected and therefore the Eq. (12) becomes:
(14)
Assuming the CO2 absorption in the solvent follows the fast pseudofirst order reaction regime, and by neglecting the effect of hydroxide OH–, the CO2 and solvent mass-transfer resistance becomes [22]:
H = Ks
− JCO2 = Ktotal PCO2, g, b
H k2 Csn DCO2, s
(16)
(17)
Substitution of Eqs. (5)–(15): (15)
PCO2, g, b =
Based on the above models a new model was derived based on the experimental results. At low CO2 partial pressure, the CO2 in the gas liquid-solvent interface PCO2,gL can be neglected, and the Eq. (6) can be
PCO2, g, b, t = 0
(
RT
)
r t AK e total Vr − Vs
(18)
When CO2 loaded solvent is used, the CO2 concentration in solvent
Fig. 3. CO2 mass-transfer in the CO2 absorption process. 5
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stability evaluations were:
bulk phase, CCO2,s,b , can not be neglected. Because the solvent is reactive, the CO2 in the solvent bulk phase reacts with the solvent to form carbamate and carbonate. Thus, the amount of dissolved CO2 is proportional to the presence of total inorganic carbon in the solvent, CCO2,s , stoichiometrically. The CO2 absorption flux can be modeled as:
− JCO2 = Ktotal (PCO2, g, b − HxCCO2, s )
η=
TNsample ⎞ ⎛ (12)(4.969) ⎞ N /C = ⎛ ⎜ ⎟ ⎝ (14)(1.297) ⎠ ⎝ (TOCsample − TOCdistilledwater ) ⎠ ⎜
(19)
Substitution of equation (5) to (17) and then proceed integration resulted:
PCO2, g, b =
PCO2, t = 0 (PCO2, g, b, t = 0 − HxCCO2, s, t = 0 ) e
(
RT
)
AKtotal V −rV t r s
+ HxCCO2, s, t = 0 −2
(
)2
(22)
⎟
(23)
where η is CO2 absorption efficiency (%), X is mol fraction (%), N/C is nitrogen to organic-carbon ratio (mol N/mol C), TN is total nitrogen (mg/L), TOC is total organic carbon (mg/L), the number of 12, 14, 4.969 and 1.297 is carbon atomic number (g/mol), nitrogen atomic number (g/mol), the SBB equivalent number of atomic carbon (-) and nitrogen (-), respectively. Subscript in is the CO2 flow at the inlet and outlet, respectively.
(20)
−1
where JCO2 is CO2 absorption flux (mol.m .s ), K is mass-transfer coefficient (m.s−1), P is CO2 partial pressure (Pa), C is solvent concentration (mol.m−3), H is the Henry’s solubility constant (Pa.m3.mol−1), T is absolute temperature (K), F is mass-transfer driving force (kPa), R is mass-transfer resistance (s.m−1), k2 is apparent kinetic constant, D is diffusivity (m.s−2), x is stoichiometric constant, and t is time (s). Subscript g is gas phase, s is solvent phase, b is bulk phase, and gL is the gas–liquid interface. The equations were fitted to the experimental data to acquire the mass-transfer resistance using Microsoft excel solver by minimizing the total sum square of residual as the objective function, and by changing the unknown variable of Ktotal , H , x, k and/or n
Objf : min ∑ PCO2, experiment − PCO2,calculation
XCO2, in − XCO2, out XCO2, in − XCO2, in XCO2, out
3. Result and discussion 3.1. Equilibrium solubility of CO2 Equilibrium solubility of CO2 in a solvent determines CO2 absorption capacity that defines the maximum CO2 that can be absorbed by the solvent at the equilibrium condition. Since the CO2 partial pressure is less than 15 kPa in flue gas, during the CO2 absorption process, the solvent is desired with the maximum CO2 loading at the low CO2 partial pressure and the absorption temperature. The CO2 solubility in the solvent also indicates how easy the CO2 can be regenerated from the solvent at the equilibrium condition. Thus, the solvent is preferred to hold less CO2 at lower CO2 partial pressure under the regeneration temperature. Thus, less steam is required to strip (dilute) the CO2 in the gas phase to achieve a high CO2 absorption cyclic capacity. The CO2 solubility in the SBB solvent was measured by loading a certain amount of CO2 into the SBB solvent until the equilibrium reached. The solubility of CO2 at different temperatures is plotted in Fig. 5. It can be seen that lower CO2 partial pressure is associated with the lower CO2 absorption capacity of the SBB solvent due to the low mass-transfer driving force. The CO2 absorption capacity decreased as the temperature increased because the CO2 dissolution is an exothermic process, and higher temperature helps CO2 molecules escape from the solvent. Table 2 shows the comparison of the equilibrium CO2 solubility in different CO2 solvents. The CO2 absorption capacity of 1.0 M SBB is 0.51 mol-CO2/mol-solvent, which is comparable with the conventional 30% MEA solvent. It has been reported that tertiary amine tends to have the highest CO2 absorption capacity compared to the primary and
(21)
2.7. Thermal and oxidative stability A SBB solvent circulation system consisting of both CO2 absorption and desorption units was set up in Fig. 4. The simulated flue gas containing 15% CO2 and 85% N2 was fed into the absorption reactor. The solvent circulation rate and the gas flow rate were maintained at 10 ± 1 mL/min and 80 ± 4 mL/min respectively. The top condenser and the reactor temperature were controlled to be at 21 ± 1 °C and 25 ± 2 °C by adjusting the flow rate of tap water, respectively. The SBB solvent circulation lasted for 120 h at different regeneration temperature by using the flue gas with different O2 concentration. The SBB solvent and flue gas were sampled every 15 h. The CO2 content in the absorber outlet was analyzed by using Gas Chromatography (Agilent 7890) to determine the CO2 absorption efficiency. The total organic carbon and total nitrogen in the solvent were analyzed using a total carbon/ nitrogen analyzer (Shimadzu TOC-L CSH) to determine the nitrogen/organic-carbon ratio. The formula used for the solvent
Fig. 4. Schematic diagram of CO2 absorption–desorption system. 6
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Fig. 5. Equilibrium solubility of CO2 in 1 ± 0.03 M SBB solvent.
3.2. Heat of CO2 absorption
secondary amine. Besides, the number of amine sites affects the CO2 absorption capacity because more CO2 can be absorbed per mol solvent. As a typical primary amine, MEA contains only a single amine site. However, the SBB solvent contains both secondary and tertiary amines, including arginine, proline, tryptophan, and histidine, which improved the CO2 absorption of the SBB solvent. Table 2 also shows that the CO2 absorption capacity of the SBB solvent is consistent with the results of different amino acid salts, which can be explained by the fact of that SBB solvent is a blend of 18 different amino acid salts with different CO2 absorption capabilities. The amino acid salts with high CO2 absorption capacity dominated the CO2 absorption process while the amino acid salts with low CO2 absorption capacity play an important role in the CO2 desorption process. The amino acids with low CO2 solubility are also beneficial to improve the regeneration ability of the SBB solvent. Table 2 lists the comparison of CO2 solubility in different solvents. It can be seen from the CO2 loading at the temperature of 80 °C and the CO2 partial pressure of 5 kPa that the SBB solvent has the lowest CO2 loading of 0.23 ± 0.01 mol-CO2/ mol-SBB solvent compared to the other solvents. This indicated that the SBB solvent is potentially easier to be regenerated with lower energy consumption than the other solvents.
The heat of CO2 absorption was estimated by plotting R·ln (PCO2) and T−1 gathered from the results of CO2 absorption equilibrium in Fig. 5. From Gibbs-Helmholtz equation, the heat of CO2 absorption is the slope of the plots in Fig. 6. When the CO2 loading was 0.4 mol CO2/ mol SBB, 0.5 mol CO2/mol SBB, and 0.6 mol CO2/mol SBB, the heat of CO2 absorption was −31.39 kJ.mol-CO2-1, −27.91 kJ.mol-CO2-1, and –23.99 kJ.mol-CO2-1 respectively. Here, the negative value indicates that the CO2 absorption is exothermic and the system releases heat in the CO2 absorption process. Since the heat of CO2 absorption is defined as the differential enthalpy, which indicates the total heat that required to releasing each molar of CO2 from the solvent when the temperature increased from 293 K to 353 K at a constant CO2 loading. It can be seen that the heat of CO2 absorption decreased at higher CO2 loading with more CO2 absorbed in the SBB solvent. This is consistent with the result of MEA solvent [21]. The heat of CO2 desorption determines the total energy requirement for CO2 desorption along with both sensible heat and vaporization heat [8]. Thus, low heat of CO2 absorption is desirable for the CO2 solvent regeneration. Table 3 summarizes the heat of CO2 absorption for different CO2 solvents, it can be seen that the SBB solvent shows
Table 2 Comparison of the equilibrium CO2 solubility in different solvents. Solvent
Concentration
CO2 loading (mol-CO2.mol-solvent−1) PCO2 = 15 kPa
SBB 1-dimethylamino-2-propanol MEA MEA Potassium Lysinate Potassium L-Asparaginate Potassium L-Glutaminate Potassium Prolinate Potassium Glycinate
1.0 2.0 1.0 5.0 1.0 1.0 1.0 2.5 1.0
M M M M M M M M M
Ref. PCO2 = 5 kPa
40 °C
60 °C
80 °C
0.51 ± 0.01 0.66 0.45 0.51 0.45 0.56 0.73 0.674 (7 kPa) 0.73
0.44 ± 0.01 0.59 NA 0.49 NA 0.46 0.67 0.59 0.66
0.23 ± 0.01 NA NA 0.36 NA 0.25 (8.6 kPa) 0.39 NA 0.42 (78 °C)
Note: The results are interpolated data from the adjacent condition on the references. 7
This work [8] [9] [20] [9] [24] [24] [25] [26]
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Fig. 7. Vapor pressure of 1 ± 0.03 M SBB solvent.
Fig. 6. Plot of R.ln (PCO2) versus T−1 for estimation of the heat of CO2 absorption in 1 ± 0.03 M SBB solvent. Table 3 Comparison of heat of CO2 absorption for different solvents. Solvent
Concentration
CO2 loading ((mol-CO2. mol-solvent−1)
ΔHabs (kJ.molCO2-1)
Ref.
SBB
1 M (14.6%-wt)
1-dimethylamino-2propanol MDEA MEA Potassium Glycinate
2M
0.6 0.4 0.656
–23.99 −31.39 −30.64
This Work [8]
30%-wt 30%-wt 30%-wt
0.623 0.608 0.4
−54.24 −46.2 −69
[21] [28] [10]
significantly lower heat of CO2 absorption than the conventional aminebased solvent such as MDEA and MEA. The low heat of CO2 absorption of the SBB solvent can attribute to the synergies among the various kinds of amino acid salts that contain both methyl and ethyl groups such as Alanine, Leucine, Isoleucine, and Valine. It has been reported that both methyl and ethyl functional groups in the amine are responsible to decrease the heat of CO2 absorption reaction [27]. Those functional groups may provide a moderate amine steric hindrance that causes a weak CO2-amine functional bond.
Fig. 8. Effect of CO2 partial pressure on CO2 absorption flux in 1 ± 0.03 M SBB solvent at T = 40 ± 0.5 °C.
3.4. Absorption flux and mass-transfer analysis To analyze the CO2 mass transfer phenomenon at low CO2 partial pressure (PCO2), the CO2 absorption experiments were carried out at different conditions including CO2 partial pressure (0–15 kPa), initial CO2 loading (0–0.47 mol CO2/mol solvent), SBB solvent concentration (0–2.0 M), and temperature (293–353 K). Fresh SBB solvent (initial CO2 loading less than 0.01 M) was used in the CO2 absorption process under different CO2 partial pressure. The effect of CO2 partial pressure on CO2 absorption flux is shown in Fig. 8. It was found a linear relationship between the CO2 absorption flux and the CO2 partial pressure when the pressure was less than13 kPa, which indicated that the CO2 absorption took place at the pseudo first order regime [22], and the CO2 partial pressure can be neglected at the gas–liquid interface. Based on equation (14), the CO2 mass-transfer coefficient and CO2 mass-transfer resistance in the gas phase is 7.62 × 10-4 m.s−1 and 1311.5 s.m−1, respectively. When the CO2 partial pressure is higher than 13 kPa, a non-linear relationship between CO2 absorption flux and CO2 partial pressure was observed in Fig. 8. The result reveals that the CO2 partial pressure at the interface has a significant effect on the CO2 absorption flux, as illustrated in Equation (6). At high bulk CO2 partial pressure, significant amounts of CO2 were absorbed in the solvent, and the absorbed CO2 was in equilibrium with the CO2 at the gas–liquid interface. In the real-world application after the solvent is regenerated, the CO2 solvents with a certain amount of CO2 loading are recycled to the absorber for the continuous CO2 absorption [3]. Thus the
3.3. Vapor pressure The vapor pressure reflects the volatility of the solvent, which is critical to determine the make-up of CO2 solvent during the operation. The vapor pressure of the SBB solvent was measured at different temperatures and the result is plotted in Fig. 7. As a comparison, the vapor pressure of water was also measured. It shows in Fig. 8 that the vapor pressure of the SBB solvent (P-SBB 1.0 M) is almost the same as the vapor pressure of water (P-H2O). This can be explained by the nature of the SBB solvent that mainly consists of amino acids and potassium hydroxide. The vapor pressure of the two major components is in the range of 2.8 × 10-8 kPa to 1.28 × 10-6 kPa, which is much lower than the vapor pressure of MEA solvent (53.86 kPa @ 25 °C) [29]. It can be seen that if the P-SBB 1.0MM is subtracted by P-H2O, the vapor pressure of pure SBB (P-SBB) is around zero. Thus, to compensate the loss of SBB solvent due to the vaporization in the CO2 absorption operation, only water is required to make up the SBB solvent; and all the active components in the SBB solvent remain stable at the operation temperature up to 84 °C.
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Fig. 9. Effect of initial CO2 loading on CO2 absorption flux of 1 ± 0.03 M SBB solvent at PCO2, feed = 13.3 ± 0.2 kPa and T = 40 ± 0.5 °C.
Fig. 11. Effect of reactor temperature on CO2 absorption flux of 1 ± 0.03 M SBB solution at PCO2, feed = 13.3 ± 0.5 kPa and T = 40 ± 0.5 °C.
understanding of the CO2 loading effect on the CO2 absorption flux is important. In this study, the SBB solvent with different CO2 loading was prepared at 13.3 ± 0.2 kPa. For each SBB solvent, the CO2 loading was calculated from Equation (2) and the correlated curve fitting was processed with Equation (18). It can be seen in Fig. 9 that the CO2 absorption flux decreased with the increase in the initial CO2 loading. This can be attributed to the decreased mass-transfer driving force, (PCO2,g,b − HCCO2,s,b) , because more CO2 was absorbed in the liquid phase when the initial CO2 loading was high. Therefore, the CO2 loading in the SBB solvent must be kept as low as possible to maintain a high CO2 absorption flux. The SBB concentration effect on the CO2 absorption flux was studied to pursue the optimized SBB concentration for CO2 capture from flue gas. The SBB solvents with different concentrations were prepared with the same initial CO2 loading at 12.8 ± 0.5 kPa. The CO2 absorption flux was also modeled by using Equation (16), and the results are shown in Fig. 10. It is obvious that the CO2 absorption flux increased by increasing the SBB concentration. At the SBB concentration less than 1.0 M, the growth of CO2 absorption flux was very fast; however, the growth slowed down when the SBB concentration was higher than 1.0 M. This can be explained by the change of mass transfer resistance of CO2 in both gas and liquid phase. As shown in Fig. 10, the mass transfer resistance of CO2-solvent sharply decreased at the SBB concentration less than 1.0 M, where the CO2-solvent resistant was comparable with CO2-gas resistant. Thus, both CO2-solvent resistance and CO2-gas resistance determined the total CO2 mass-transfer resistance in the CO2 absorption process. Note
that, the CO2-gas resistant is constant because the same CO2 partial pressure of 12.8 ± 0.5 kPa was used in the experiments. However, when the SBB concentration was higher than 1, the CO2solvent resistance is much lower than the CO2-gas resistance due to the fast reaction rate. Therefore, the total CO2 mass-transfer resistance is mainly determined by the CO2-gas resistance. From this point of view, the optimum concentration of SBB was 1.0 M for the effective CO2 capture from the flue-gas. To further improve the CO2 absorption flux by increasing the SBB concentration, the CO2-gas resistance must be decreased as well. The CO2-gas resistance depends on the gas properties, gas turbulence, and the geometry of the contactor [30]. Since the CO2 concentration in the flue-gas is assumed as a constant, the CO2-gas resistance can be only inhibited by improving the gas turbulence and the contactor geometry. The effect of temperature on the CO2 absorption flux was investigated by loading 1.0 M of fresh SBB solvent with CO2 at a constant pressure of 13.3 ± 0.5 kPa and different temperatures. The result in Fig. 11 shows that the CO2 absorption flux increased by temperature. Similar result can be modeled empirically from the classic Arrhenius’s equation [31] listed below:
b − JCO2 = aexp ⎛− ⎞ ⎝ T⎠
(24)
where a and b are the empirical constants, and T is temperature. When the temperature increased, the CO2-gas resistance decreased due to the enhanced molecular turbulence. The CO2-solvent resistance was also reduced by the accelerated CO2 absorption kinetic. It has been found in Fig. 5 that the CO2 absorption capacity declined at high temperature. Thus, to improve the CO2 absorption flux, the increase of temperature must be optimized with adequate CO2 absorption capacity. 3.5. Thermal stability of the SBB solvent The thermal stability of the SBB solvent was estimated through 120 h of continuous CO2 absorption–desorption processes by using simulated flue gas. An aggravated operation condition was created to speed up the thermal degradation process. The temperature of the oil bath was 130 °C, 160 °C, and 200 °C, which is much higher than the boiling point of the SBB solvent. During the CO2 absorption process, the initial CO2 loading in the SBB solvent was 0.29 ± 0.04 mol-CO2/molSBB. In this study, the thermal stability of the SBB solvent is usually affected by the following aspects:
Fig. 10. Effect of SBB concentration on CO2 absorption flux at PCO2, = 12.8 ± 0.5 kPa and T = 40 ± 0.5 °C.
1. The possible vaporization of active components from the SBB solvent.
feed
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solvent was attributed to the low vapor pressure of amino acid salts in the SBB solvent, which has been discussed in Fig. 7. Besides, the thermal decomposition of most of the amino acid salts in the SBB solvent only takes place at the temperature higher than 200 °C. Only glutamic acid, cysteine, and methionine have decomposition temperature of 160 °C, 175 °C, and 181 °C, respectively [29]. It has to note that the reported temperature for the thermal decomposition is at the solid state. Since the SBB solvent regeneration temperature is limited by the solvent boiling point of 97 ± 1 °C, the exposure of the solvent at the desired temperature may only be limited at the interior wall of the reboiler. The temperature at the interior of the reboiler is roughly half of the summation of the solvent temperature and the reboiler temperature [33]. The maximum temperature is at around 148.5 °C, which is still less than the lowest thermal decomposition temperature of glutamic acid in the SBB solvent.
Fig. 12. CO2 absorption flux of 1 ± 0.03 M SBB solution over different regeneration temperatures during 120 h operation.
3.6. Oxidative stability of the SBB solvent
The associated byproduct including SBB vapor, ammonia, and carbon dioxide can be stripped out from the regenerator along with the released CO2 from the desorption process. Thus, the thermal stability of the SBB solvent can be estimated by the measurement of the elements of nitrogen and total organic carbon in the SBB solvent. The degradation of CO2 solvent may also occur via rearrangement or reaction in the liquid phase. However, it cannot cause deamination or decarbonation but generate CO2 unreactive byproducts [32]. Thus, the CO2 absorption efficiency was studied as well. The occurrence of CO2 solvent degradation can be indicated by the decline of CO2 absorption efficiency at a constant N/C ratio. Figs. 12 and 13 show CO2 absorption efficiency and the ratio of nitrogen to organic carbon (N/C ratio) in the 1.0 ± 0.03 M SBB solvent, respectively. It can be seen that, there is no significant difference in CO2 absorption efficiency, and the N/C ratio remained at 1.0 during the 120 h of absorption and desorption process. The SBB solvent was stable over a wide range of regeneration temperatures up to 200 °C. As a comparison, the MEA solvent with a CO2 loading of 0.4 degraded by 26% at the regeneration temperature of 150 °C at the end of an equivalent operating time [32]. The high thermal stability of SBB
Due to the presence of O2 in the flue gas, the oxidative stability of the SBB solvent was investigated through the 120 h of continuous experiments at the O2 concentration of 10.5% and 17% in the flue gas. The temperature was maintained at 160 °C for the solvent regeneration; the circulating rate of SBB solvent and the flow rate of the flue gas was 10 ± 1 mL/min and 80 ± 4 mL/min, respectively. During the 120 h operation with different O2 concentrations in the flue-gas, the CO2 absorption efficiency and the N/C ratio of the 1 ± 0.03 M SBB solvent are given in Figs. 14 and 15. It can be seen the CO2 absorption efficiency remained more than 95%, and the N/C ratio was 1.0 at the end of the operation. The SBB solvent is stable over the exposure of O2 in the flue-gas up to the molar concentration of 17%. The promising stability of the SBB solvent is attributed to the presence of amino acid salts containing oxygen element in their structures [34]. The oxygen element is in the form of carbocyclic acid (R-COO-), which is the ultimate state in the oxidation of alkanol. Thus, it cannot be further oxidized. It is worth to note that the components of Cysteine and Methionine, both containing the sulfur element, may result in a sulfur induced oxidative decomposition. However, in view of the low content of only 2.23% in the SBB solvent, the degradation from the sulfurcontaining component can be neglected in the CO2 desorption process. In contrast, MEA is made of alkanol group, and it suffers from oxidative degradation to form ammonia, aldehydes (alkanol oxidation product), and organic acids (aldehydes oxidation product) [35] .
Fig. 13. Nitrogen/organic-carbon atomic ratio of 1 ± 0.03 M SBB solution over different regeneration temperatures during 120 h operation. Note: the N and C concentration has been normalized to follow 1 M SBB concentration.
Fig. 14. CO2 absorption flux of 1 ± 0.03 M SBB solution over different concentration of O2 in the flue-gas during 120 h operation.
2. The potential degradation/decomposition of the amino acid salts in the SBB solvent because of deamination and/or decarboxylation.
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Fig. 15. N/C ratio of 1 ± 0.03 M SBB solution over different O2 concentration.
4. Conclusion The performance and stability of a soybean-based biosolvent (SBB solvent) were thoroughly evaluated for CO2 capture from the flue gas with the CO2 partial pressure less than 15 kPa. As a novel solvent, the SBB exhibited comparable CO2 absorption capacity with the conventional amine-based solvents. However, the CO2 solubility in the SBB solvent is much lower than the MEA solvent under the regeneration condition. Besides, the heat of CO2 absorption was only the half of the conventional CO2 solvents, including MEA, which indicated an easier regeneration behavior of the SBB solvent. The SBB solvent showed a similar vapor pressure with water while the active components in the SBB solvent exhibited the near-zero vapor pressure. It was also found the optimized concentration of the SBB solvent was 1.0 M for effective CO2 capture. The CO2 absorption performance can be further improved by integrating the use of high concentration SBB solvent with a reduced CO2-gas mass transfer resistance. The increased absorption temperature improved CO2 absorption flux but reduced CO2 solubility in the SBB solvent. During the CO2 absorption–desorption process, the CO2 loading in the SBB solvent should be reduced as low as possible to maintain a high CO2 absorption flux. The SBB solvent showed high thermal stability at the regeneration temperature up to 200 °C, and also excellent oxidative stability during the CO2 capture process with the O2 concentration up to 17% in the flue gas. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] J.E. Szulejko, P. Kumar, A. Deep, K.-H. Kim, Global warming projections to 2100 using simple CO2 greenhouse gas modeling and comments on CO2 climate sensitivity factor, Atmos. Pollut. Res. 8 (2017) 136–140. [2] C. Song, W. Pan, S.T. Srimat, J. Zheng, Y. Li, Y.H. Wang, B.Q. Xu, Q.M. Zhu, Trireforming of methane over Ni catalysts for CO2 conversion to Syngas with desired H2 CO ratios using flue gas of power plants without CO2 separation, Stud. Surf. Sci. Catal. 153 (2004) 315–322. [3] E. Favre, Membrane processes and postcombustion carbon dioxide capture: challenges and prospects, Chem. Eng. J. 171 (2011) 782–793.
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