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Energy-efficient chemical regeneration of AMP using calcium hydroxide for operating carbon dioxide capture process
Accepted Manuscript Energy-efficient chemical regeneration of AMP using calcium hydroxide for operating carbon dioxide capture process Ji Min Kang, Arti Murnandari, Min Hye Youn, Wonhee Lee, Ki Tae Park, Young Eun Kim, Hak Joo Kim, Seong-Pil Kang, Jung-Hyun Lee, Soon Kwan Jeong PII: DOI: Reference:
S1385-8947(17)31850-8 https://doi.org/10.1016/j.cej.2017.10.136 CEJ 17920
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
11 August 2017 18 October 2017 21 October 2017
Please cite this article as: J.M. Kang, A. Murnandari, M.H. Youn, W. Lee, K.T. Park, Y.E. Kim, H.J. Kim, S-P. Kang, J-H. Lee, S.K. Jeong, Energy-efficient chemical regeneration of AMP using calcium hydroxide for operating carbon dioxide capture process, Chemical Engineering Journal (2017), doi: https://doi.org/10.1016/j.cej. 2017.10.136
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Chemical Engineering Journal
Energy-efficient chemical regeneration of AMP using calcium hydroxide for operating carbon dioxide capture process
Ji Min Kang a,b, Arti Murnandari a,c, Min Hye Youn a, Wonhee Lee a, Ki Tae Park a, Young Eun Kim a, Hak Joo Kim a, Seong-Pil Kang a, Jung-Hyun Lee b,*, Soon Kwan Jeong a,c,*
a
Climate Change Research division, Korea Institute of Energy Research, 152, Gajeong-ro, Yuseong-gu, Daejeon, Korea
b
Department of Chemical and Biological Engineering, Korea University, 145, Anam-ro, Seongbukgu, Seoul, Korea c
University of Science and Technology Korea, 217, Gajeong-ro, Yuseong-gu, Daejeon, Korea
* Corresponding authors. Tel.: +82-42-860-3623. E-mail address: [email protected] * Corresponding authors. Tel.: +82-2-3290-3293. E-mail address: [email protected]
Abstract To avoid the main disadvantage of the carbon dioxide (CO2) capture process, namely the large amount of energy consumed to regenerate the amine absorbent using current thermal methods, chemical regeneration has been introduced as a novel method to regenerate the amine. Chemical regeneration deploys a swing in the pH of the amine absorbent rather than the swing in temperature of typical thermal regeneration procedures, and hence reduces the regeneration energy. Here we tested calcium chloride (CaCl2) and calcium hydroxide (Ca(OH)2) as a calcium source for CO2 desorption and a pH swing agent for amine regeneration. After desorbing from the amine, CO2 in our procedures reacted with Ca2+ to form calcium carbonate (CaCO3). Forming precipitated CaCO3 is a permanent way to sequester CO2. Since carbonates have a low energy level compared to CO2, we expect the developed method to be an economical and energy-efficient process.
Keywords: CO2 Capture, CO2 Sequestration, Calcium Carbonate, Amine Regeneration, Chemical Regeneration
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1. Introduction The amount of carbon dioxide (CO2) in the atmosphere is increasing rapidly due to industrial development, and the resulting global warming and other changes to the climate are becoming a serious problem for the environment as well as a threat to various ecosystems and to the survival of the mankind [1]. Thus, various CO2 capture strategies, which are examples of carbon capture and storage (CCS) technologies, have been developed to reduce the amount of CO2 emitted from industrial sources and power plants. The CO2 capture methods have included pre-combustion capture, oxy-fuel combustion, and post-combustion capture. CO2 generated by power stations is removed post-combustion; note in this regard that the facilities for the CO2 removal can be built without requiring structural changes to the existing power stations [2,3]. Various amine absorbents have been developed and commercialized to capture CO2 because aqueous amine solutions have been shown to exhibit high CO2 absorption capacity through their acid-base reaction [4,5]. However, the currently available amine absorbents cause serious operational problems, such as amine degradation, reactor corrosion, and a high consumption of energy required for the regeneration of the amine [6]. The amine regeneration, which consumes 70% of the total energy used for CO2 capture, is best accomplished by increasing the operation temperature up to 120 °C, at which point the process is called thermal regeneration [3,7,8]. The heat duty, i.e., the energy required for thermal regeneration, consists of sensible heat, heat of water evaporation, and CO2 desorption energy [9], which are the important factors considered when designing CO2 absorbers/strippers and calculating capital/operation costs [6].
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Hence, many studies have attempted to find better absorbents/catalysts in order to modify the CO2 capture process and to utilize renewable energy in order to reduce the amount of fossil fuel-derived energy used to regenerate the amine absorbent. Most promisingly, various groups reported that the heat duty for amine regeneration in the CO2 capture process can be reduced by mixing various amine absorbents [10–12]. In another investigation by Sivanesan et al., [13] a carbonic anhydrase mimic was shown to serve as a catalyst that increased the CO2 absorption and desorption rates, thereby reducing the amine regeneration energy. In addition, Srisang et al. [14] reported that solid acid catalysts increased the CO2 absorption efficiency and cyclic capacity, which also increased the rate of desorption of CO2 from amine absorbent solutions and lowered the heat duty. Furthermore, the main contribution of energy use for thermal regeneration is steam usage in the stripper. The internal heat integration design is able to reduce 20-50% of the reboiler duty [15]. Jiang et al. [16] focused on modifying the process and suggested a novel CO2 capture process using an ammonia (NH3) absorbent together with an advanced flash stripper, by which the energy consumption of the absorbent regeneration can be reduced. With regards to the use of renewable energy, solar energy [17,18] and geothermal energy [19] have been utilized to provide heat for the regeneration of the amine absorbent. Even though the various attempts mentioned so far were shown to reduce to a certain extent the amount of energy needed to regenerate the amine, the decreased heat duty in these CO2 desorption processes was still based on thermal regeneration, and hence still required a large amount of thermal energy. To overcome this problem, CO2 desorption and free amine regeneration should be performed by using a spontaneous reaction (∆G°reaction <
4
0) rather than by increasing the temperature. Kodama et al. [20] investigated the use of a pH-swing for calcium extraction and CO2 mineralization for calcium carbonate (CaCO3) production, which is a spontaneous reaction, using ammonium chloride (NH4Cl) as an agent for extracting the calcium cation (Ca2+). The conversion of CO2 into CaCO3 is a permanent way to convert CO2 due to the Gibbs free energy of formation of CaCO3 (1128.8 kJ/mol) being much lower than that of CO2 (-394.36 kJ/mol). In their study, NH4Cl was regenerated using the CO2 mineralization process, and thus repeated CO2 mineral sequestration could be performed [20,21]. Therefore, spontaneous mineralization of CO2 into CaCO3 could be an alternative strategy to regenerate the amine absorbent, one that would require a significantly reduced amount of energy. Also, additional commercial benefits could be expected because CaCO3, a chemically stable and non-toxic mineral, is a useful product, and in fact widely used for plastic, paper, industrial concrete and fillers of rubber and paint [22]. But it is hard to expect the effective overall CO2 reduction in the process of mineral sequestration using CaCO3-derived calcium sources such as natural rock and limestone. To this end, it is necessary to investigate alternative calcium sources which do not originate from CaCO3 in nature. There have been several attempts to obtain the cation from industrial waste such as fly-ash, steelmaking slag, and construction materials. Said et al. [23] studied the influence of solvent on the cation extraction from steelmaking slag. They reported that NH4Cl, NH4NO3 and CH3COONH4 solvents were tested in calcium extraction from steelmaking slag, resulting in calcium ion was selectively extracted 37-45% with NH4Cl solvent.
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The aim of the present work was to regenerate the amine absorbent by using “chemical regeneration”, specifically by using Ca-containing chemicals through a pH swing rather than using a pH swing for extraction of calcium cations (Ca2+). 2-Amino-2methyl-1-propanol (AMP) was used as a CO2 absorbent and its regeneration was attempted using calcium chloride (CaCl2) and calcium hydroxide (Ca(OH)2). Even though the efficiency of the desorption of CO2 from CO2-loaded AMP using CaCl2 was higher than that from thermal regeneration, the presence of Cl- inhibited the regeneration of the free amine, which prevented continuous CO2 capture. Utilizing Ca(OH)2 instead of CaCl2 as the pH swing agent was found to result in a higher CO2 desorption efficiency and more regeneration cycles. Also the formation of CaCO3 as a result of the reaction of CO2 with Ca2+ not only sequesters CO2 permanently [24] but also provides a value-added material.
2. Experimental 2.1. CO2 absorption-thermal regeneration A 500 ml glass reactor covered with a stainless-steel and a circulating water bath was used in our experiments. An aqueous solution of 250 ml of 10 wt% 2-amino-2-methyl1-propanol (90% AMP, Aldrich) was prepared. A sparger with a pore diameter of 20 ㎛ was used to administer the gas into the reactor, and the contents of the reactor were mixed using a magnetic stirrer operating at a speed of 450 rpm. An automatic back-pressure regulator was used to maintain a constant 0.2 bar pressure inside the reactor. In the CO2 absorption experiment, 30% CO2 gas (nitrogen based mixture) was injected into the reactor
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until equilibrium was reached. After CO2 absorption at 40 °C, the temperature was increased gradually to 90 °C for thermal regeneration. From the start of the absorption to the end of the regeneration, the reactor was continuously purged with CO2 at a flow rate of 0.5 L/min. CO2 output from the reactor was monitored using gas chromatography, and a condenser was installed and used to prevent water vapor from entering the gas chromatograph. The concentration of CO2 inside the reactor and the point at which the reaction stopped were determined by using gas chromatography. The pH and electrical conductivity were measured with a pH probe and conductivity meter.
2.2. CO2 absorption-chemical regeneration using Ca-containing chemicals Except for preparation of amine solution, all experimental equipment and procedures used for CO2 absorption experiments were same as those described in previous subsection 2.1. 10% AMP (90% AMP, Aldrich) and the same molar concentration of CaCl2 (93%, Aldrich) were dissolved in the 250 ml of water. In this case, CO2 absorption was simultaneously conducted with mineralization producing CaCO3 precipitate at 40 °C. After reaching the equilibrium, precipitated CaCO3 was removed from the solution by using a filtration and separated solution was recycled. In order to evaluate the iterative performance of chemical regeneration, the Ca(OH)2 (95%, Aldrich) was added into recycled amine solution as same molar concentration of AMP in it. The CaCO3 removal and the injection of the Ca(OH)2 were repeated 3 times. The obtained CaCO3 powder was washed with acetone and dried in an
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oven at 60 °C for 24 h. After measuring the weight of produced CaCO3, mass balance of carbon dioxide and calcium is calculated.
2.3. Characterizations During the CO2 capture, the pH and conductivity of the solution were measured using a pH and conductivity meter (METTLER TOLEDO). After each CO2 capture cycle, a sample of the solution was characterized using a 600-MHz FT-NMR (Fourier Transform NMR) spectrometer (Avance III 600, Bruker Biospin) in order to assess the state of the hydrogen bonding in the AMP molecules. The CO2 output from the reactor was monitored with a gas chromatograph (6100GC, YoungLin) using a Carboxen-packed column and a thermal conductivity detector (TCD). The concentration of dissolved calcium in the solution was measured by using a calcium ion concentration meter (Thermo).
3. Results and discussion 3.1 Thermally induced desorption of CO2 Thermal regeneration was tested to compare the CO2 desorption efficiency and the chemical nature of the amine species during the CO2 capture process with Ca-containing chemicals. Fig. 1(a) and 1(b) show the molar amount of CO2 loaded per mole of amine absorbent and the pH value of the absorbent, respectively, during the course of the experiment in which the temperature was either kept constant or varied with time. During the initial stage of the experiment, when the temperature was kept at 40 °C, the number of
8
moles of CO2 absorbed per mole of amine increased from 0 to a plateau of 0.75. The reaction between AMP and CO2 can be represented by Eqs. (1) and (2) [25]. 2 + ↔ +
(1)
− + 3 + + 2 ↔ 3 + + 2 + 3 −
(2)
At this time, the pH of the absorbent decreased from 11.4 to a floor of 8.1. As the temperature was then increased to 90 °C, the concentration of CO2 in the solution decreased by about 60%, and the pH value increased to 9.5. This CO2 desorption efficiency was lower than that in practical industrial CO2 capture processes because the amine regeneration in the commercial processes is carried out at a higher temperature, i.e., at 120 °C. To confirm the amine regeneration efficiency, the same experiments were conducted again using the regenerated amine solution. The CO2 loading plateaued at 0.65 mol CO2/mol amine in the second cycle, slightly less than that in the first cycle. This reduced value of the CO2 loading was attributed to the initial pH value of the regenerated solution (9.4) being lower than that of the absorbent solution (11.4) in the first cycle.
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Fig. 1. (a) Molar amount of CO2 loaded per mole of amine absorbent and (b) pH value of the absorbent, both during the course of an experiment in which the temperature was either kept constant or varied with time.
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Fig. 2(a), 2(b), and 2(c) show the proton NMR spectra of the amine absorbent for the pure AMP (i.e., before CO2 absorption), the CO2-absorbed AMP, and the regenerated AMP, respectively. The largest peak in each spectrum was observed at about 4.7 - 4.8 ppm and corresponded to D2O. Peaks corresponding to free amines appeared at 1.03 and 3.32 ppm in the pure AMP spectrum (Fig. 2(a)). These peaks slightly shifted downfield to 1.16 and 3.42 ppm after CO2 absorption, as shown in Fig. 2(b). Note that it is impossible to distinguish the NMR spectra resulting from the free amine and protonated amine forms due to the fast proton exchange in the solution. Fan et al. [26] suggested that an increase in the CO2 loading and decrease in the pH would be accompanied by a downfield shift of the AMP / AMPH+ peaks and a predominance of the strongly electron-withdrawing ammonium group, i.e., of the protonated amine, relative to that of the free amine. Therefore, that the peaks of the pure AMP before absorption of CO2 were not shifted indicated that the free amine groups were able to bind CO2, and after absorbing CO2 were converted to protonated amines that could no longer bind CO2. As shown in Fig. 2(c), when the pH was increased as a result of the thermal treatment, the protonated amine converted back to free amine, and the amine peaks appeared at 1.07 and 3.35. These values differed slightly from those of the pure AMP because the pH of the solution was lower than that of pure AMP and more of the protonated form of the amine was present in the solution. Although the thermal regeneration was able to desorb CO2 from the amine absorbent and convert the protonated amine to free amine in the amine solution with great efficiency, the amine absorbent was susceptible to being
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decomposed because of the high temperature and especially the heat duty required for the thermal regeneration as described above in the Introduction section.
Fig. 2. NMR spectra in the amine peak region of AMP solutions involved in the thermally induced desorption of CO2. (a) Pure AMP (b) AMP with absorbed CO2, and (c) regenerated AMP.
3.2 Attempt at chemical regeneration with CaCl2 To reduce the regeneration energy due to the heat duty, chemical regeneration with CaCl2 was attempted. Fig. 3(a) shows the molar amount of CO2 loaded per mole of amine absorbent and Fig. 3(b) shows the pH value of the absorbent, both during the course of the experiment in which the CO2 capture and attempt at regeneration occurred simultaneously due to the presence of amine and calcium chloride. In the first capture-regeneration test, the amount of CO2 loaded plateaued at 0.65 mol CO2 /mol amine. This value was lower than 12
that attained during the first cycle of the experiment shown in Fig. 1, but the pH decreased more, from 10 to as low as 5.3. The CO2 desorption efficiency of the chemical regeneration test which was calculated from the CO2 loading and the amount of precipitated CaCO3 was 97.3%, even at 40 °C, with this efficiency value much higher than that resulting from the thermal treatment. After calcium carbonate was removed from the solution, the loading following CO2 re-injection only plateaued at 0.18 mol CO2/mol amine, and this low loading value was attributed to the pH of the solution remaining at a low value of about 5. The presence of chloride anion from the calcium chloride caused a greater drop in pH than that in the thermal treatment, since the chloride prevented the ammonium cations from being converted into free amines. Park et al. [27] found that acidic conditions limit the overall conversion by carbonic acids dissociation into bicarbonate or carbonate ions. And this lowered pH interferes amine capturing CO2 and CO2 desorption as precipitating calcium carbonate [20,24].
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Fig. 3. (a) Molar amount of CO2 loaded per mole of amine absorbent and (b) pH value of the absorbent, both during the course of an experiment in which chemical regeneration was attempted with CaCl2.
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Fig. 4(a), 4(b), and 4(c) show the proton NMR spectra of the amine absorbent for the pure AMP, AMP with CaCl2, and the product of the attempt to regenerate the AMP, respectively. As indicated above, peaks corresponding to free amines appeared at 1.03 and 3.32 ppm in the pure AMP spectrum (Fig. 4(a)). After CaCl2 was dissolved in the AMP solution, both peaks slightly shifted downfield to 1.08 and 3.37 ppm (Fig. 4(b)). After CO2 capture and the regeneration experiment, the peaks moved even a little bit more downfield, to 1.16 and 3.42 ppm (Fig. 4(c)) with this result attributed to Cl- ion binding the protonated amine and making it a bit more electron withdrawing [26]. When both AMP and CaCl2 react with CO2, not only are protonated amines produced, but so are chloride-bound amines, as shown in Eqs. (3) and (4). 22 + 2 + 22 + 22 ↔ 23 + (3 )2
(3)
23 + (3 )2 ↔ 23 + 3 + 2 + 2
(4)
After removal of calcium carbonate, the regenerated solution was used to carry out a second round of CO2 capture and a chemical regeneration attempt with CaCl2. This procedure resulted in a further decrease in the pH (Fig. 3(b)). This lower pH resulted in more of the protonated form of the amine, i.e., the form that does not bind CO2. Summarizing the results of the chemical regeneration experiment with CaCl2, the initial CO2 loading and reaction rate were observed to be reasonable. But the amine absorbent could not be regenerated in the presence of CaCl2, as the amount of CO2 loaded reached a lower level after this attempt at chemical regeneration than for the initial absorbent. Thus, the use of CaCl2 is not considered suitable for carrying out continuous
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CO2 capture because the Cl anion binds the protonated amine to form RNH3Cl and hence prevents the regeneration of the free amine.
Fig. 4. NMR spectra, in the amine peak region, of AMP solutions involved in the attempt at chemical regeneration with CaCl2. (a) Pure AMP (b) AMP with CaCl2, and (c) the product of the regeneration attempt.
3.3 Effect of chloride ion on chemical regeneration using Ca(OH)2 We also attempted chemical regeneration using Ca(OH)2 in the AMP solution. Here, CO2 capture was tested in solutions of pure AMP, AMP with Ca(OH)2, and AMP with Ca(OH)2 and Cl-. The CO2 loading and reaction rate in solution are shown in Fig. 5. Including Ca(OH)2 resulted in the CO2 loading being slightly increased from 0.75 to 0.88 mol CO2/mol amine. We attributed this increase to a higher pH and the hydroxyl group. But
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the reaction rate, which was calculated at steady state, decreased from 0.01 mol CO2/mol amine-1min-1 for pure AMP to 0.0073 mol CO2/mol amine-1 min-1 when including the Ca(OH)2. The solubility of Ca(OH)2 in water has been indicated to decrease with increasing temperature according to Eq. (5) [28]. Solubility of Ca(OH)2 (g/kg of solution) = -0.0108T (°C) + 1.7465
(5)
According to this equation, the solubility of Ca(OH)2 at 40 °C would be 1.3145 g/kg of water, and hence much lower than the solubility of CaCl2 (1280 g/kg of water at 40 °C). Therefore, dissolution of Ca(OH)2 was expected to be a rate-limiting step in the overall CO2 capture reaction. However, after the attempt at chemical regeneration with CaCl2, Ca(OH)2 was injected into the solution in the presence of chloride ions. At that time, the reaction rate was found to increase to 0.017 mol CO2 /mol amine-1 min-1 (Fig. 5(c)). This increase was attributed to the relationship between the solubility and the pH of the solution. When using a solvent in the CO2 capture process, the pH and electrical conductivity of the solution can be simply measured and easily obtained, and they are sensitive to the nature of the ionic species and the concentration of the solute, which is an important factor for characterizing and controlling a system involved in chemical absorption [29]. After Ca(OH)2 was injected into the solution of pure AMP in the same amount as that of CO2 captured by the pure AMP (0.21 mol), the concentration of Ca2+ was zero, as shown in Table 1. This result indicated that Ca2+ could not dissolve from the Ca(OH)2 into the solution of pure AMP. In contrast, after the same concentration of AMP and CaCl2 was used in a solution to capture CO2 and then subjected to chemical regeneration by injecting Ca(OH)2 as much as the amount of CO2 captured into the solution containing Cl-, the
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concentration of Ca2+ increased to 0.6 mol/L. Before injection of Ca(OH)2, the pH of the solution of pure AMP was 11.4 and the pH of the solution after the attempted chemical regeneration using CaCl2 was 5.3. As mentioned in chapter 3.2, this drop in pH resulted in the formation of RNH3Cl, and could enhance the calcium dissolution and extract Ca(OH)2 from the solution [20,24]. These effects are consistent with the observation by Kiviranta et al. and Boskey et al. [30,31] that the acidic buffer makes the calcium extraction more robust and rapid. RNH3Cl might be acting as the acidic buffer in the solution and it could help solve the rate-limiting step problem that is the solubility of Ca(OH)2. Increasing the solubility and the concentration of ions in the solution would be expected to increase the electric conductivity of the solution. The higher conductivity may have promoted the capture of CO2. This result is consistent with the observation by Han et al. [32] that higher electrical conductivity levels in the amine solution can induce a faster absorption of CO2.
Fig. 5. CO2 loading of the solutions of (a) pure AMP, (b) AMP with Ca(OH)2 and (c) AMP with Ca(OH)2 and Cl-. 18
Fig. 6. Electrical conductivity curves for the solutions of (a) pure AMP, (b) AMP with Ca(OH)2 and (c) AMP with Ca(OH)2 and Cl-.
Table 1. The concentration of Ca extracted from the indicated solutions for two different fractions of Cl-.
3.4 Chemical regeneration with Ca(OH)2 As a method to solve the problem of the decreased pH after treatment with CaCl2, Ca(OH)2 was injected as a pH swing agent and a Ca2+ source. After each time CaCO3 was removed from the solution, Ca(OH)2 was injected, and the amount injected was the same as
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the amount of CO2 that was captured, which was also the same as the amount of Ca2+ that was converted to CaCO3. Fig. 7 shows the CO2 loading and pH of the solution resulting from the attempt at chemical regeneration with Ca(OH)2 in the presence of Cl-. The CO2 loading resulting from the first cycle using CaCl2 was 0.65 mol CO2/mol amine, which was the same as that for the first cycle of the experiment in Fig. 3. After CaCO3 was removed from the solution, we attempted to chemically regenerate the amine by including the Ca(OH)2 in the presence of Cl-. After this attempted regeneration, the loading reached 0.86 mol CO2 /mol amine, higher than that of the first cycle using CaCl2. This increase was attributed to an increase in pH resulting from the injection of Ca(OH)2 and the resulting enhancement of the carbonation of CO2 due to the alkalinity of and presence of hydroxyl groups in the Ca(OH)2. Park et al. [33] found that raising the pH of the solution was important for increasing the extent of aqueous carbonation. Also, after regeneration, the reaction rate, i.e., the rate of the absorption of CO2, was determined to be 0.017 mol CO2 /mol amine-1 min-1, greater than the 0.01 mol CO2 /mol amine-1min-1 value determined when using the solution of pure AMP, and even greater than the 0.014 mol CO2 /mol amine-1 min-1 value determined for the first cycle. The maximum CO2 loading of the third cycle, which like the second cycle also used Ca(OH)2, was slightly decreased to 0.83 mol CO2 /mol amine and the reaction rate was 0.0164 mol CO2/mol amine-1min-1. In the last cycle of this attempted chemical regeneration, the maximum loading was reduced a bit more relative to that of the third cycle, to 0.80 mol CO2/mol amine, and the rate was 0.016 mol CO2/mol amine-1 min-1, similar to that of the third cycle and faster than that for the solution of pure AMP. The continuous slight decrease of the maximum CO2 loading with increasing cycle number (from its value for the second cycle) 20
was attributed to not only the precipitation of CaCO3 but also to a dilution of the amine resulting from the generation of as many moles of water as the number of moles of CO2 that were absorbed, with both the precipitation of CaCO3 and generation of water resulting from the amine binding the CO2 and the chemical regeneration of the amine using the Ca(OH)2 [28]. Note that the water was produced according to Eq. (6). Hence, during the course of the repeated CO2 capture and amine regeneration, the concentration of amine as many moles as the generation of water, namely the number of moles of CO2 absorbed should be added at each cycle to maintain its concentration. 23 + ()2 ↔ 22 + 2 + 22
(6)
22 + 2 + 22 + 22 ↔ 23 + (3 )2
(7)
23 + (3 )2 ↔ 23 + 3 + 2 + 2
(8)
Fig. 7. CO2 loading and pH of the solution resulting from the attempt at chemical regeneration with Ca(OH)2 in the presence of Cl-.
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Table 2 shows a summary of the chemical shifts from the NMR spectrum of each solution of the chemical regeneration with Ca(OH)2. The NMR spectrum of the solution after capturing CO2 showed peaks at 1.32 and 3.57 - 1.30 and 3.55 ppm, which indicated that the amine was predominantly in its protonated form. However, injection of Ca(OH)2 into a solution of AMP and chloride ion resulted in an increase in pH to about 11, similar to the value of pure AMP – and as the pH increased, the protonated amine dissociated and converted into the free amine according to Eq. (6) and maintained a high CO2 capture capacity [34]. As a result, Ca(OH)2 successfully served as a pH swing agent and a Ca2+ source, and was able to keep regenerating the amine absorbent by increasing the pH of the solution and converting the protonated amine back to the free amine. Therefore the chemical regeneration process using Ca(OH)2 in presence of Cl- is suitable for application to continuous CO2 capture.
Table 2. The NMR data of amine transition in AMP solution.
4. Conclusions
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In order to sequester CO2 generated from industrial sources, an effective CO2 capture process must be developed. Although the amine capture process has been commercialized, it currently generally uses a thermal swing treatment to regenerate the amine absorbent, and hence consumes a lot of energy. When the free amine of the absorbent captures CO2, it is converted to a protonated amine as the pH decreases. After the thermal treatment, the pH of the solution increases again and the protonated amine is converted into a free amine, which can bind another molecule of CO2. To reduce the regeneration energy, chemical regeneration using Ca-containing chemicals through a pH swing was attempted. After simultaneous CO2 capture and attempted chemical regeneration using CaCl2, however, the pH of the solution was found to remain low and there was a lot of protonated amine even though CaCl2 was re-injected. Therefore it is not possible to regenerate the amine capable of binding CO2 when using CaCl2. In contrast, the amine subjected to attempted chemical regeneration using Ca(OH)2 could capture CO2, and was even observed to do so to a greater extent than the original pure AMP, indicating a successful regeneration of the amine absorbent. However, the rate at which CO2 was loaded into the solution containing Ca(OH)2 was around 4.1 times less than that for the pure AMP because of Ca2+ extraction relating to Ca(OH)2 solubility. To overcome this problem, when the chemical regeneration experiment was conducted using Ca(OH)2 in the presence of Cl- as a pH swing agent and Ca2+ source, the regeneration of the amine absorbent was found to be effective, and the above rate was high. It is concluded that the suggested process of chemical regeneration using Ca(OH)2 in the presence of Cl- could be able to solve the most important problem with current amine capture processes, which is the high regeneration energy. And it can permanently sequester 23
CO2 with high desorption efficiency in precipitated CaCO3, which itself is a high-value product. In order to apply for industrial CCUS process, we will investigate the extraction of calcium hydroxide from industrial waste as a future work.
Acknowledgement
This work was conducted under the framework of Research and Development Program of the Korea Institute of Energy Research (KIER, B7-2431-01).
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Reference [1]
I. Sreedhar, T. Nahar, A. Venugopal, B. Srinivas, Carbon capture by absorption – Path covered and ahead, Renew. Sustain. Energy Rev. 76 (2017) 1080–1107. doi:10.1016/j.rser.2017.03.109.
[2]
L. Raynal, P. Bouillon, A. Gomez, P. Broutin, From MEA to demixing solvents and future steps , a roadmap for lowering the cost of post-combustion carbon capture, Chem. Eng. J. 171 (2011) 742–752. doi:10.1016/j.cej.2011.01.008.
[3]
H.F. Svendsen, E.T. Hessen, T. Mejdell, Carbon dioxide capture by absorption , challenges and possibilities, Chem. Eng. J. 171 (2011) 718–724. doi:10.1016/j.cej.2011.01.014.
[4]
G.T. Rochelle, Amine Scrubbing for CO2 Capture, Science (80-. ). 325 (2011) 1652. doi:10.1126/science.1176731.
[5]
P. V. Danckwerts, The reaction of CO2 with ethanolamines, Chem. Eng. Sci. 34 (1979) 443–446. doi:10.1016/0009-2509(79)85087-3.
[6]
R. Idem, M. Wilson, P. Tontiwachwuthikul, A. Chakma, A. Veawab, A. Aroonwilas, D. Gelowitz, Pilot plant studies of the CO2 capture performance of aqueous MEA and mixed MEA/MDEA solvents at the University of Regina CO2 capture technology development plant and the boundary dam CO2 capture demonstration plant, Ind. Eng. Chem. Res. 45 (2006) 2414–2420. doi:10.1021/ie050569e.
[7]
B. Xu, H. Gao, M. Chen, Z. Liang, R. Idem, Experimental Study of Regeneration Performance of Aqueous N , N ‑ Diethylethanolamine Solution in a Column Packed with Dixon Ring Random Packing, (2016). doi:10.1021/acs.iecr.6b00936.
25
[8]
H. Liu, C. Yao, Y. Zhao, G. Chen, Desorption of carbon dioxide from aqueous MDEA solution in a microchannel reactor, Chem. Eng. J. 307 (2017) 776–784. doi:10.1016/j.cej.2016.09.010.
[9]
K. Han, C.K. Ahn, M.S. Lee, Performance of an ammonia-based CO2 capture pilot facility in iron and steel industry, Int. J. Greenh. Gas Control. 27 (2014) 239–246. doi:10.1016/j.ijggc.2014.05.014.
[10] H.A. Rodriguez-Flores, L.C. Mello, W.M. Salvagnini, J.L. De Paiva, Absorption of CO2 into aqueous solutions of MEA and AMP in a wetted wall column with film promoter, Chem. Eng. Process. Process Intensif. 73 (2013) 1–6. doi:10.1016/j.cep.2013.06.011. [11] Z. Zhou, X. Zhou, G. Jing, B. Lv, Evaluation of the Multi-amine Functionalized Ionic Liquid for Efficient Postcombustion CO2 Capture, Energy and Fuels. 30 (2016) 7489–7495. doi:10.1021/acs.energyfuels.6b00692. [12] D.A. Glasscock, J.E. Critchfield, G.T. Rochelle, CO2 absorption/desorption in mixtures of methyldiethanolamine with monoethanolamine or diethanolamine, Chem. Eng. Sci. 46 (1991) 2829–2845. doi:10.1016/0009-2509(91)85152-N. [13] D. Sivanesan, M.H. Youn, A. Murnandari, J.M. Kang, K.T. Park, H.J. Kim, S.K. Jeong, Enhanced CO2 absorption and desorption in a tertiary amine medium with a carbonic anhydrase mimic, J. Ind. Eng. Chem. 52 (2017) 287–294. doi:10.1016/j.jiec.2017.03.058. [14] W. Srisang, F. Pouryousefi, P.A. Osei, B. Decardi-Nelson, A. Akachuku, P. Tontiwachwuthikul, R. Idem, Evaluation of the heat duty of catalyst-aided amine-
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based post combustion CO2 capture, Chem. Eng. Sci. (2017) 1–10. doi:10.1016/j.ces.2017.01.049. [15] Y. Lin, G.T. Rochelle, Approaching a reversible stripping process for CO2 capture, Chem. Eng. J. 283 (2016) 1033–1043. doi:10.1016/j.cej.2015.08.086. [16] K. Jiang, K. Li, H. Yu, Z. Chen, L. Wardhaugh, P. Feron, Advancement of ammonia based post-combustion CO2 capture using the advanced flash stripper process, Appl. Energy. 202 (2017) 496–506. doi:10.1016/j.apenergy.2017.05.143. [17] F. Wang, J. Zhao, H. Li, S. Deng, J. Yan, Preliminary experimental study of postcombustion carbon capture integrated with solar thermal collectors, Appl. Energy. 185 (2016). doi:http://dx.doi.org/10.1016/j.apenergy.2016.02.040. [18] R. Khalilpour, D. Milani, A. Qadir, M. Chiesa, A. Abbas, A novel process for direct solvent regeneration via solar thermal energy for carbon capture, Renew. Energy. 104 (2017) 60–75. doi:10.1016/j.renene.2016.12.001. [19] D.H. Van Wagener, A. Gupta, G.T. Rochelle, S.L. Bryant, Régénération d’un solvant de captage du co2utilisant l’énergie géothermique et des configurations amé liorées pour le régénérateur, Oil Gas Sci. Technol. 69 (2014) 1105–1119. doi:10.2516/ogst/2012099. [20] S. Kodama, T. Nishimoto, N. Yamamoto, K. Yogo, K. Yamada, Development of a new pH-swing CO2 mineralization process with a recyclable reaction solution, Energy. 33 (2008) 776–784. doi:10.1016/j.energy.2008.01.005. [21] Y. Sun, M. Yao, J. Zhang, G. Yang, Indirect CO2 mineral sequestration by steelmaking slag with NH4Cl as leaching solution, Chem. Eng. J. 173 (2011) 437– 445. doi:10.1016/j.cej.2011.08.002. 27
[22] H. Jo, S. Park, Y. Jang, S. Chae, P. Lee, H. Young, Metal extraction and indirect mineral carbonation of waste cement material using ammonium salt solutions, Chem. Eng. J. 254 (2014) 313–323. doi:10.1016/j.cej.2014.05.129. [23] A. Said, H. Mattila, M. Järvinen, R. Zevenhoven, Production of precipitated calcium carbonate (PCC) from steelmaking slag for fixation of CO2, Appl. Energy. 112 (2013) 765–771. doi:10.1016/j.apenergy.2012.12.042. [24] A. Azdarpour, M. Asadullah, E. Mohammadian, H. Hamidi, R. Junin, A review on carbon dioxide mineral carbonation through pH-swing process, Chem. Eng. J. 279 (2015) 615–630. doi:10.1016/j.cej.2015.05.064. [25] W. Conway, Y. Beyad, G. Richner, G. Puxty, P. Feron, Rapid CO2 absorption into aqueous benzylamine (BZA) solutions and its formulations with monoethanolamine (MEA), and 2-amino-2-methyl-1-propanol (AMP) as components for post combustion capture processes, Chem. Eng. J. 264 (2015) 954–961. doi:10.1016/j.cej.2014.11.040. [26] G. Fan, A.G.H. Wee, R. Idem, P. Tontiwachwuthikul, NMR Studies of Amine Species in MEA-CO2-H2O System : Modification of the Model of Vapor - Liquid Equilibrium (VLE), Ind. Eng. Chem. Res. 48 (2009) 2717–2720. doi:10.1021/ie8015895. [27] R.J. A.H. Park, L.S. Fan, CO2 mineral sequestration: chemically enhanced aqueous\rcarbonation of serpentine, Can. J. Chem. Eng. 81 (2003) 885–890. doi:10.1002/cjce.5450810373.
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[28] S.-J. Han, M. Yoo, D.-W. Kim, J.-H. Wee, Carbon Dioxide Capture Using Calcium Hydroxide Aqueous Solution as the Absorbent, Energy & Fuels. 25 (2011) 3825– 3834. doi:10.1021/ef200415p. [29] A.C. van Eckeveld, L.V. van der Ham, L.F.G. Geers, L.J.P. van den Broeke, B.J. Boersma, E.L.V. Goetheer, Online Monitoring of the Solvent and Absorbed Acid Gas Concentration in a CO2 Capture Process Using Monoethanolamine, Ind. Eng. Chem. Res. 53 (2014) 5515–5523. doi:10.1021/ie402310n. [30] I. Kiviranta, M. Tammi, R. Lappalainen, T. Kuusela, H.J. Helminen, The rate of calcium extraction during EDTA decalcification from thin bone slices as assessed with atomic absorption spectrophotometry, Histochemistry. 68 (1980) 119–127. doi:10.1007/BF00489507. [31] A.L. Boskey, A.S. Posner, Extraction of a calcium-phospholipid-phosphate complex from bone, Calcif. Tissue Res. 19 (1975) 273–283. doi:10.1007/BF02564010. [32] S.J. Han, J.H. Wee, Estimation of correlation between electrical conductivity and CO2 absorption in a monoethanolamine solvent system, J. Chem. Eng. Data. 58 (2013) 2381–2388. doi:10.1021/je400358f. [33] A.H.A. Park, L.S. Fan, CO2 mineral sequestration: Physically activated dissolution of serpentine and pH swing process, Chem. Eng. Sci. 59 (2004) 5241–5247. doi:10.1016/j.ces.2004.09.008. [34] E. Jo, Y.H. Jhon, S.B. Choi, J.-G. Shim, J.-H. Kim, J.H. Lee, I.-Y. Lee, K.-R. Jang, J. Kim, Crystal structure and electronic properties of 2-amino-2-methyl-1-propanol (AMP) carbamate, Chem. Commun. 46 (2010) 9158. doi:10.1039/c0cc03224g.
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Highlights A novel process which is chemical regeneration through a pH swing is proposed. The chemical regeneration shows higher efficiency of CO2 desorption than the thermal regeneration at even room temperature. After the chemical regeneration, a value-added CaCO3 is produced. Ca(OH)2 in the presence of Cl- plays a key role in desorbing CO2 from amine and converting it into CaCO3.
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S1385-8947(17)31850-8 https://doi.org/10.1016/j.cej.2017.10.136 CEJ 17920
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
11 August 2017 18 October 2017 21 October 2017
Please cite this article as: J.M. Kang, A. Murnandari, M.H. Youn, W. Lee, K.T. Park, Y.E. Kim, H.J. Kim, S-P. Kang, J-H. Lee, S.K. Jeong, Energy-efficient chemical regeneration of AMP using calcium hydroxide for operating carbon dioxide capture process, Chemical Engineering Journal (2017), doi: https://doi.org/10.1016/j.cej. 2017.10.136
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Chemical Engineering Journal
Energy-efficient chemical regeneration of AMP using calcium hydroxide for operating carbon dioxide capture process
Ji Min Kang a,b, Arti Murnandari a,c, Min Hye Youn a, Wonhee Lee a, Ki Tae Park a, Young Eun Kim a, Hak Joo Kim a, Seong-Pil Kang a, Jung-Hyun Lee b,*, Soon Kwan Jeong a,c,*
a
Climate Change Research division, Korea Institute of Energy Research, 152, Gajeong-ro, Yuseong-gu, Daejeon, Korea
b
Department of Chemical and Biological Engineering, Korea University, 145, Anam-ro, Seongbukgu, Seoul, Korea c
University of Science and Technology Korea, 217, Gajeong-ro, Yuseong-gu, Daejeon, Korea
* Corresponding authors. Tel.: +82-42-860-3623. E-mail address: [email protected] * Corresponding authors. Tel.: +82-2-3290-3293. E-mail address: [email protected]
Abstract To avoid the main disadvantage of the carbon dioxide (CO2) capture process, namely the large amount of energy consumed to regenerate the amine absorbent using current thermal methods, chemical regeneration has been introduced as a novel method to regenerate the amine. Chemical regeneration deploys a swing in the pH of the amine absorbent rather than the swing in temperature of typical thermal regeneration procedures, and hence reduces the regeneration energy. Here we tested calcium chloride (CaCl2) and calcium hydroxide (Ca(OH)2) as a calcium source for CO2 desorption and a pH swing agent for amine regeneration. After desorbing from the amine, CO2 in our procedures reacted with Ca2+ to form calcium carbonate (CaCO3). Forming precipitated CaCO3 is a permanent way to sequester CO2. Since carbonates have a low energy level compared to CO2, we expect the developed method to be an economical and energy-efficient process.
Keywords: CO2 Capture, CO2 Sequestration, Calcium Carbonate, Amine Regeneration, Chemical Regeneration
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1. Introduction The amount of carbon dioxide (CO2) in the atmosphere is increasing rapidly due to industrial development, and the resulting global warming and other changes to the climate are becoming a serious problem for the environment as well as a threat to various ecosystems and to the survival of the mankind [1]. Thus, various CO2 capture strategies, which are examples of carbon capture and storage (CCS) technologies, have been developed to reduce the amount of CO2 emitted from industrial sources and power plants. The CO2 capture methods have included pre-combustion capture, oxy-fuel combustion, and post-combustion capture. CO2 generated by power stations is removed post-combustion; note in this regard that the facilities for the CO2 removal can be built without requiring structural changes to the existing power stations [2,3]. Various amine absorbents have been developed and commercialized to capture CO2 because aqueous amine solutions have been shown to exhibit high CO2 absorption capacity through their acid-base reaction [4,5]. However, the currently available amine absorbents cause serious operational problems, such as amine degradation, reactor corrosion, and a high consumption of energy required for the regeneration of the amine [6]. The amine regeneration, which consumes 70% of the total energy used for CO2 capture, is best accomplished by increasing the operation temperature up to 120 °C, at which point the process is called thermal regeneration [3,7,8]. The heat duty, i.e., the energy required for thermal regeneration, consists of sensible heat, heat of water evaporation, and CO2 desorption energy [9], which are the important factors considered when designing CO2 absorbers/strippers and calculating capital/operation costs [6].
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Hence, many studies have attempted to find better absorbents/catalysts in order to modify the CO2 capture process and to utilize renewable energy in order to reduce the amount of fossil fuel-derived energy used to regenerate the amine absorbent. Most promisingly, various groups reported that the heat duty for amine regeneration in the CO2 capture process can be reduced by mixing various amine absorbents [10–12]. In another investigation by Sivanesan et al., [13] a carbonic anhydrase mimic was shown to serve as a catalyst that increased the CO2 absorption and desorption rates, thereby reducing the amine regeneration energy. In addition, Srisang et al. [14] reported that solid acid catalysts increased the CO2 absorption efficiency and cyclic capacity, which also increased the rate of desorption of CO2 from amine absorbent solutions and lowered the heat duty. Furthermore, the main contribution of energy use for thermal regeneration is steam usage in the stripper. The internal heat integration design is able to reduce 20-50% of the reboiler duty [15]. Jiang et al. [16] focused on modifying the process and suggested a novel CO2 capture process using an ammonia (NH3) absorbent together with an advanced flash stripper, by which the energy consumption of the absorbent regeneration can be reduced. With regards to the use of renewable energy, solar energy [17,18] and geothermal energy [19] have been utilized to provide heat for the regeneration of the amine absorbent. Even though the various attempts mentioned so far were shown to reduce to a certain extent the amount of energy needed to regenerate the amine, the decreased heat duty in these CO2 desorption processes was still based on thermal regeneration, and hence still required a large amount of thermal energy. To overcome this problem, CO2 desorption and free amine regeneration should be performed by using a spontaneous reaction (∆G°reaction <
4
0) rather than by increasing the temperature. Kodama et al. [20] investigated the use of a pH-swing for calcium extraction and CO2 mineralization for calcium carbonate (CaCO3) production, which is a spontaneous reaction, using ammonium chloride (NH4Cl) as an agent for extracting the calcium cation (Ca2+). The conversion of CO2 into CaCO3 is a permanent way to convert CO2 due to the Gibbs free energy of formation of CaCO3 (1128.8 kJ/mol) being much lower than that of CO2 (-394.36 kJ/mol). In their study, NH4Cl was regenerated using the CO2 mineralization process, and thus repeated CO2 mineral sequestration could be performed [20,21]. Therefore, spontaneous mineralization of CO2 into CaCO3 could be an alternative strategy to regenerate the amine absorbent, one that would require a significantly reduced amount of energy. Also, additional commercial benefits could be expected because CaCO3, a chemically stable and non-toxic mineral, is a useful product, and in fact widely used for plastic, paper, industrial concrete and fillers of rubber and paint [22]. But it is hard to expect the effective overall CO2 reduction in the process of mineral sequestration using CaCO3-derived calcium sources such as natural rock and limestone. To this end, it is necessary to investigate alternative calcium sources which do not originate from CaCO3 in nature. There have been several attempts to obtain the cation from industrial waste such as fly-ash, steelmaking slag, and construction materials. Said et al. [23] studied the influence of solvent on the cation extraction from steelmaking slag. They reported that NH4Cl, NH4NO3 and CH3COONH4 solvents were tested in calcium extraction from steelmaking slag, resulting in calcium ion was selectively extracted 37-45% with NH4Cl solvent.
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The aim of the present work was to regenerate the amine absorbent by using “chemical regeneration”, specifically by using Ca-containing chemicals through a pH swing rather than using a pH swing for extraction of calcium cations (Ca2+). 2-Amino-2methyl-1-propanol (AMP) was used as a CO2 absorbent and its regeneration was attempted using calcium chloride (CaCl2) and calcium hydroxide (Ca(OH)2). Even though the efficiency of the desorption of CO2 from CO2-loaded AMP using CaCl2 was higher than that from thermal regeneration, the presence of Cl- inhibited the regeneration of the free amine, which prevented continuous CO2 capture. Utilizing Ca(OH)2 instead of CaCl2 as the pH swing agent was found to result in a higher CO2 desorption efficiency and more regeneration cycles. Also the formation of CaCO3 as a result of the reaction of CO2 with Ca2+ not only sequesters CO2 permanently [24] but also provides a value-added material.
2. Experimental 2.1. CO2 absorption-thermal regeneration A 500 ml glass reactor covered with a stainless-steel and a circulating water bath was used in our experiments. An aqueous solution of 250 ml of 10 wt% 2-amino-2-methyl1-propanol (90% AMP, Aldrich) was prepared. A sparger with a pore diameter of 20 ㎛ was used to administer the gas into the reactor, and the contents of the reactor were mixed using a magnetic stirrer operating at a speed of 450 rpm. An automatic back-pressure regulator was used to maintain a constant 0.2 bar pressure inside the reactor. In the CO2 absorption experiment, 30% CO2 gas (nitrogen based mixture) was injected into the reactor
6
until equilibrium was reached. After CO2 absorption at 40 °C, the temperature was increased gradually to 90 °C for thermal regeneration. From the start of the absorption to the end of the regeneration, the reactor was continuously purged with CO2 at a flow rate of 0.5 L/min. CO2 output from the reactor was monitored using gas chromatography, and a condenser was installed and used to prevent water vapor from entering the gas chromatograph. The concentration of CO2 inside the reactor and the point at which the reaction stopped were determined by using gas chromatography. The pH and electrical conductivity were measured with a pH probe and conductivity meter.
2.2. CO2 absorption-chemical regeneration using Ca-containing chemicals Except for preparation of amine solution, all experimental equipment and procedures used for CO2 absorption experiments were same as those described in previous subsection 2.1. 10% AMP (90% AMP, Aldrich) and the same molar concentration of CaCl2 (93%, Aldrich) were dissolved in the 250 ml of water. In this case, CO2 absorption was simultaneously conducted with mineralization producing CaCO3 precipitate at 40 °C. After reaching the equilibrium, precipitated CaCO3 was removed from the solution by using a filtration and separated solution was recycled. In order to evaluate the iterative performance of chemical regeneration, the Ca(OH)2 (95%, Aldrich) was added into recycled amine solution as same molar concentration of AMP in it. The CaCO3 removal and the injection of the Ca(OH)2 were repeated 3 times. The obtained CaCO3 powder was washed with acetone and dried in an
7
oven at 60 °C for 24 h. After measuring the weight of produced CaCO3, mass balance of carbon dioxide and calcium is calculated.
2.3. Characterizations During the CO2 capture, the pH and conductivity of the solution were measured using a pH and conductivity meter (METTLER TOLEDO). After each CO2 capture cycle, a sample of the solution was characterized using a 600-MHz FT-NMR (Fourier Transform NMR) spectrometer (Avance III 600, Bruker Biospin) in order to assess the state of the hydrogen bonding in the AMP molecules. The CO2 output from the reactor was monitored with a gas chromatograph (6100GC, YoungLin) using a Carboxen-packed column and a thermal conductivity detector (TCD). The concentration of dissolved calcium in the solution was measured by using a calcium ion concentration meter (Thermo).
3. Results and discussion 3.1 Thermally induced desorption of CO2 Thermal regeneration was tested to compare the CO2 desorption efficiency and the chemical nature of the amine species during the CO2 capture process with Ca-containing chemicals. Fig. 1(a) and 1(b) show the molar amount of CO2 loaded per mole of amine absorbent and the pH value of the absorbent, respectively, during the course of the experiment in which the temperature was either kept constant or varied with time. During the initial stage of the experiment, when the temperature was kept at 40 °C, the number of
8
moles of CO2 absorbed per mole of amine increased from 0 to a plateau of 0.75. The reaction between AMP and CO2 can be represented by Eqs. (1) and (2) [25]. 2 + ↔ +
(1)
− + 3 + + 2 ↔ 3 + + 2 + 3 −
(2)
At this time, the pH of the absorbent decreased from 11.4 to a floor of 8.1. As the temperature was then increased to 90 °C, the concentration of CO2 in the solution decreased by about 60%, and the pH value increased to 9.5. This CO2 desorption efficiency was lower than that in practical industrial CO2 capture processes because the amine regeneration in the commercial processes is carried out at a higher temperature, i.e., at 120 °C. To confirm the amine regeneration efficiency, the same experiments were conducted again using the regenerated amine solution. The CO2 loading plateaued at 0.65 mol CO2/mol amine in the second cycle, slightly less than that in the first cycle. This reduced value of the CO2 loading was attributed to the initial pH value of the regenerated solution (9.4) being lower than that of the absorbent solution (11.4) in the first cycle.
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Fig. 1. (a) Molar amount of CO2 loaded per mole of amine absorbent and (b) pH value of the absorbent, both during the course of an experiment in which the temperature was either kept constant or varied with time.
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Fig. 2(a), 2(b), and 2(c) show the proton NMR spectra of the amine absorbent for the pure AMP (i.e., before CO2 absorption), the CO2-absorbed AMP, and the regenerated AMP, respectively. The largest peak in each spectrum was observed at about 4.7 - 4.8 ppm and corresponded to D2O. Peaks corresponding to free amines appeared at 1.03 and 3.32 ppm in the pure AMP spectrum (Fig. 2(a)). These peaks slightly shifted downfield to 1.16 and 3.42 ppm after CO2 absorption, as shown in Fig. 2(b). Note that it is impossible to distinguish the NMR spectra resulting from the free amine and protonated amine forms due to the fast proton exchange in the solution. Fan et al. [26] suggested that an increase in the CO2 loading and decrease in the pH would be accompanied by a downfield shift of the AMP / AMPH+ peaks and a predominance of the strongly electron-withdrawing ammonium group, i.e., of the protonated amine, relative to that of the free amine. Therefore, that the peaks of the pure AMP before absorption of CO2 were not shifted indicated that the free amine groups were able to bind CO2, and after absorbing CO2 were converted to protonated amines that could no longer bind CO2. As shown in Fig. 2(c), when the pH was increased as a result of the thermal treatment, the protonated amine converted back to free amine, and the amine peaks appeared at 1.07 and 3.35. These values differed slightly from those of the pure AMP because the pH of the solution was lower than that of pure AMP and more of the protonated form of the amine was present in the solution. Although the thermal regeneration was able to desorb CO2 from the amine absorbent and convert the protonated amine to free amine in the amine solution with great efficiency, the amine absorbent was susceptible to being
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decomposed because of the high temperature and especially the heat duty required for the thermal regeneration as described above in the Introduction section.
Fig. 2. NMR spectra in the amine peak region of AMP solutions involved in the thermally induced desorption of CO2. (a) Pure AMP (b) AMP with absorbed CO2, and (c) regenerated AMP.
3.2 Attempt at chemical regeneration with CaCl2 To reduce the regeneration energy due to the heat duty, chemical regeneration with CaCl2 was attempted. Fig. 3(a) shows the molar amount of CO2 loaded per mole of amine absorbent and Fig. 3(b) shows the pH value of the absorbent, both during the course of the experiment in which the CO2 capture and attempt at regeneration occurred simultaneously due to the presence of amine and calcium chloride. In the first capture-regeneration test, the amount of CO2 loaded plateaued at 0.65 mol CO2 /mol amine. This value was lower than 12
that attained during the first cycle of the experiment shown in Fig. 1, but the pH decreased more, from 10 to as low as 5.3. The CO2 desorption efficiency of the chemical regeneration test which was calculated from the CO2 loading and the amount of precipitated CaCO3 was 97.3%, even at 40 °C, with this efficiency value much higher than that resulting from the thermal treatment. After calcium carbonate was removed from the solution, the loading following CO2 re-injection only plateaued at 0.18 mol CO2/mol amine, and this low loading value was attributed to the pH of the solution remaining at a low value of about 5. The presence of chloride anion from the calcium chloride caused a greater drop in pH than that in the thermal treatment, since the chloride prevented the ammonium cations from being converted into free amines. Park et al. [27] found that acidic conditions limit the overall conversion by carbonic acids dissociation into bicarbonate or carbonate ions. And this lowered pH interferes amine capturing CO2 and CO2 desorption as precipitating calcium carbonate [20,24].
13
Fig. 3. (a) Molar amount of CO2 loaded per mole of amine absorbent and (b) pH value of the absorbent, both during the course of an experiment in which chemical regeneration was attempted with CaCl2.
14
Fig. 4(a), 4(b), and 4(c) show the proton NMR spectra of the amine absorbent for the pure AMP, AMP with CaCl2, and the product of the attempt to regenerate the AMP, respectively. As indicated above, peaks corresponding to free amines appeared at 1.03 and 3.32 ppm in the pure AMP spectrum (Fig. 4(a)). After CaCl2 was dissolved in the AMP solution, both peaks slightly shifted downfield to 1.08 and 3.37 ppm (Fig. 4(b)). After CO2 capture and the regeneration experiment, the peaks moved even a little bit more downfield, to 1.16 and 3.42 ppm (Fig. 4(c)) with this result attributed to Cl- ion binding the protonated amine and making it a bit more electron withdrawing [26]. When both AMP and CaCl2 react with CO2, not only are protonated amines produced, but so are chloride-bound amines, as shown in Eqs. (3) and (4). 22 + 2 + 22 + 22 ↔ 23 + (3 )2
(3)
23 + (3 )2 ↔ 23 + 3 + 2 + 2
(4)
After removal of calcium carbonate, the regenerated solution was used to carry out a second round of CO2 capture and a chemical regeneration attempt with CaCl2. This procedure resulted in a further decrease in the pH (Fig. 3(b)). This lower pH resulted in more of the protonated form of the amine, i.e., the form that does not bind CO2. Summarizing the results of the chemical regeneration experiment with CaCl2, the initial CO2 loading and reaction rate were observed to be reasonable. But the amine absorbent could not be regenerated in the presence of CaCl2, as the amount of CO2 loaded reached a lower level after this attempt at chemical regeneration than for the initial absorbent. Thus, the use of CaCl2 is not considered suitable for carrying out continuous
15
CO2 capture because the Cl anion binds the protonated amine to form RNH3Cl and hence prevents the regeneration of the free amine.
Fig. 4. NMR spectra, in the amine peak region, of AMP solutions involved in the attempt at chemical regeneration with CaCl2. (a) Pure AMP (b) AMP with CaCl2, and (c) the product of the regeneration attempt.
3.3 Effect of chloride ion on chemical regeneration using Ca(OH)2 We also attempted chemical regeneration using Ca(OH)2 in the AMP solution. Here, CO2 capture was tested in solutions of pure AMP, AMP with Ca(OH)2, and AMP with Ca(OH)2 and Cl-. The CO2 loading and reaction rate in solution are shown in Fig. 5. Including Ca(OH)2 resulted in the CO2 loading being slightly increased from 0.75 to 0.88 mol CO2/mol amine. We attributed this increase to a higher pH and the hydroxyl group. But
16
the reaction rate, which was calculated at steady state, decreased from 0.01 mol CO2/mol amine-1min-1 for pure AMP to 0.0073 mol CO2/mol amine-1 min-1 when including the Ca(OH)2. The solubility of Ca(OH)2 in water has been indicated to decrease with increasing temperature according to Eq. (5) [28]. Solubility of Ca(OH)2 (g/kg of solution) = -0.0108T (°C) + 1.7465
(5)
According to this equation, the solubility of Ca(OH)2 at 40 °C would be 1.3145 g/kg of water, and hence much lower than the solubility of CaCl2 (1280 g/kg of water at 40 °C). Therefore, dissolution of Ca(OH)2 was expected to be a rate-limiting step in the overall CO2 capture reaction. However, after the attempt at chemical regeneration with CaCl2, Ca(OH)2 was injected into the solution in the presence of chloride ions. At that time, the reaction rate was found to increase to 0.017 mol CO2 /mol amine-1 min-1 (Fig. 5(c)). This increase was attributed to the relationship between the solubility and the pH of the solution. When using a solvent in the CO2 capture process, the pH and electrical conductivity of the solution can be simply measured and easily obtained, and they are sensitive to the nature of the ionic species and the concentration of the solute, which is an important factor for characterizing and controlling a system involved in chemical absorption [29]. After Ca(OH)2 was injected into the solution of pure AMP in the same amount as that of CO2 captured by the pure AMP (0.21 mol), the concentration of Ca2+ was zero, as shown in Table 1. This result indicated that Ca2+ could not dissolve from the Ca(OH)2 into the solution of pure AMP. In contrast, after the same concentration of AMP and CaCl2 was used in a solution to capture CO2 and then subjected to chemical regeneration by injecting Ca(OH)2 as much as the amount of CO2 captured into the solution containing Cl-, the
17
concentration of Ca2+ increased to 0.6 mol/L. Before injection of Ca(OH)2, the pH of the solution of pure AMP was 11.4 and the pH of the solution after the attempted chemical regeneration using CaCl2 was 5.3. As mentioned in chapter 3.2, this drop in pH resulted in the formation of RNH3Cl, and could enhance the calcium dissolution and extract Ca(OH)2 from the solution [20,24]. These effects are consistent with the observation by Kiviranta et al. and Boskey et al. [30,31] that the acidic buffer makes the calcium extraction more robust and rapid. RNH3Cl might be acting as the acidic buffer in the solution and it could help solve the rate-limiting step problem that is the solubility of Ca(OH)2. Increasing the solubility and the concentration of ions in the solution would be expected to increase the electric conductivity of the solution. The higher conductivity may have promoted the capture of CO2. This result is consistent with the observation by Han et al. [32] that higher electrical conductivity levels in the amine solution can induce a faster absorption of CO2.
Fig. 5. CO2 loading of the solutions of (a) pure AMP, (b) AMP with Ca(OH)2 and (c) AMP with Ca(OH)2 and Cl-. 18
Fig. 6. Electrical conductivity curves for the solutions of (a) pure AMP, (b) AMP with Ca(OH)2 and (c) AMP with Ca(OH)2 and Cl-.
Table 1. The concentration of Ca extracted from the indicated solutions for two different fractions of Cl-.
3.4 Chemical regeneration with Ca(OH)2 As a method to solve the problem of the decreased pH after treatment with CaCl2, Ca(OH)2 was injected as a pH swing agent and a Ca2+ source. After each time CaCO3 was removed from the solution, Ca(OH)2 was injected, and the amount injected was the same as
19
the amount of CO2 that was captured, which was also the same as the amount of Ca2+ that was converted to CaCO3. Fig. 7 shows the CO2 loading and pH of the solution resulting from the attempt at chemical regeneration with Ca(OH)2 in the presence of Cl-. The CO2 loading resulting from the first cycle using CaCl2 was 0.65 mol CO2/mol amine, which was the same as that for the first cycle of the experiment in Fig. 3. After CaCO3 was removed from the solution, we attempted to chemically regenerate the amine by including the Ca(OH)2 in the presence of Cl-. After this attempted regeneration, the loading reached 0.86 mol CO2 /mol amine, higher than that of the first cycle using CaCl2. This increase was attributed to an increase in pH resulting from the injection of Ca(OH)2 and the resulting enhancement of the carbonation of CO2 due to the alkalinity of and presence of hydroxyl groups in the Ca(OH)2. Park et al. [33] found that raising the pH of the solution was important for increasing the extent of aqueous carbonation. Also, after regeneration, the reaction rate, i.e., the rate of the absorption of CO2, was determined to be 0.017 mol CO2 /mol amine-1 min-1, greater than the 0.01 mol CO2 /mol amine-1min-1 value determined when using the solution of pure AMP, and even greater than the 0.014 mol CO2 /mol amine-1 min-1 value determined for the first cycle. The maximum CO2 loading of the third cycle, which like the second cycle also used Ca(OH)2, was slightly decreased to 0.83 mol CO2 /mol amine and the reaction rate was 0.0164 mol CO2/mol amine-1min-1. In the last cycle of this attempted chemical regeneration, the maximum loading was reduced a bit more relative to that of the third cycle, to 0.80 mol CO2/mol amine, and the rate was 0.016 mol CO2/mol amine-1 min-1, similar to that of the third cycle and faster than that for the solution of pure AMP. The continuous slight decrease of the maximum CO2 loading with increasing cycle number (from its value for the second cycle) 20
was attributed to not only the precipitation of CaCO3 but also to a dilution of the amine resulting from the generation of as many moles of water as the number of moles of CO2 that were absorbed, with both the precipitation of CaCO3 and generation of water resulting from the amine binding the CO2 and the chemical regeneration of the amine using the Ca(OH)2 [28]. Note that the water was produced according to Eq. (6). Hence, during the course of the repeated CO2 capture and amine regeneration, the concentration of amine as many moles as the generation of water, namely the number of moles of CO2 absorbed should be added at each cycle to maintain its concentration. 23 + ()2 ↔ 22 + 2 + 22
(6)
22 + 2 + 22 + 22 ↔ 23 + (3 )2
(7)
23 + (3 )2 ↔ 23 + 3 + 2 + 2
(8)
Fig. 7. CO2 loading and pH of the solution resulting from the attempt at chemical regeneration with Ca(OH)2 in the presence of Cl-.
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Table 2 shows a summary of the chemical shifts from the NMR spectrum of each solution of the chemical regeneration with Ca(OH)2. The NMR spectrum of the solution after capturing CO2 showed peaks at 1.32 and 3.57 - 1.30 and 3.55 ppm, which indicated that the amine was predominantly in its protonated form. However, injection of Ca(OH)2 into a solution of AMP and chloride ion resulted in an increase in pH to about 11, similar to the value of pure AMP – and as the pH increased, the protonated amine dissociated and converted into the free amine according to Eq. (6) and maintained a high CO2 capture capacity [34]. As a result, Ca(OH)2 successfully served as a pH swing agent and a Ca2+ source, and was able to keep regenerating the amine absorbent by increasing the pH of the solution and converting the protonated amine back to the free amine. Therefore the chemical regeneration process using Ca(OH)2 in presence of Cl- is suitable for application to continuous CO2 capture.
Table 2. The NMR data of amine transition in AMP solution.
4. Conclusions
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In order to sequester CO2 generated from industrial sources, an effective CO2 capture process must be developed. Although the amine capture process has been commercialized, it currently generally uses a thermal swing treatment to regenerate the amine absorbent, and hence consumes a lot of energy. When the free amine of the absorbent captures CO2, it is converted to a protonated amine as the pH decreases. After the thermal treatment, the pH of the solution increases again and the protonated amine is converted into a free amine, which can bind another molecule of CO2. To reduce the regeneration energy, chemical regeneration using Ca-containing chemicals through a pH swing was attempted. After simultaneous CO2 capture and attempted chemical regeneration using CaCl2, however, the pH of the solution was found to remain low and there was a lot of protonated amine even though CaCl2 was re-injected. Therefore it is not possible to regenerate the amine capable of binding CO2 when using CaCl2. In contrast, the amine subjected to attempted chemical regeneration using Ca(OH)2 could capture CO2, and was even observed to do so to a greater extent than the original pure AMP, indicating a successful regeneration of the amine absorbent. However, the rate at which CO2 was loaded into the solution containing Ca(OH)2 was around 4.1 times less than that for the pure AMP because of Ca2+ extraction relating to Ca(OH)2 solubility. To overcome this problem, when the chemical regeneration experiment was conducted using Ca(OH)2 in the presence of Cl- as a pH swing agent and Ca2+ source, the regeneration of the amine absorbent was found to be effective, and the above rate was high. It is concluded that the suggested process of chemical regeneration using Ca(OH)2 in the presence of Cl- could be able to solve the most important problem with current amine capture processes, which is the high regeneration energy. And it can permanently sequester 23
CO2 with high desorption efficiency in precipitated CaCO3, which itself is a high-value product. In order to apply for industrial CCUS process, we will investigate the extraction of calcium hydroxide from industrial waste as a future work.
Acknowledgement
This work was conducted under the framework of Research and Development Program of the Korea Institute of Energy Research (KIER, B7-2431-01).
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Reference [1]
I. Sreedhar, T. Nahar, A. Venugopal, B. Srinivas, Carbon capture by absorption – Path covered and ahead, Renew. Sustain. Energy Rev. 76 (2017) 1080–1107. doi:10.1016/j.rser.2017.03.109.
[2]
L. Raynal, P. Bouillon, A. Gomez, P. Broutin, From MEA to demixing solvents and future steps , a roadmap for lowering the cost of post-combustion carbon capture, Chem. Eng. J. 171 (2011) 742–752. doi:10.1016/j.cej.2011.01.008.
[3]
H.F. Svendsen, E.T. Hessen, T. Mejdell, Carbon dioxide capture by absorption , challenges and possibilities, Chem. Eng. J. 171 (2011) 718–724. doi:10.1016/j.cej.2011.01.014.
[4]
G.T. Rochelle, Amine Scrubbing for CO2 Capture, Science (80-. ). 325 (2011) 1652. doi:10.1126/science.1176731.
[5]
P. V. Danckwerts, The reaction of CO2 with ethanolamines, Chem. Eng. Sci. 34 (1979) 443–446. doi:10.1016/0009-2509(79)85087-3.
[6]
R. Idem, M. Wilson, P. Tontiwachwuthikul, A. Chakma, A. Veawab, A. Aroonwilas, D. Gelowitz, Pilot plant studies of the CO2 capture performance of aqueous MEA and mixed MEA/MDEA solvents at the University of Regina CO2 capture technology development plant and the boundary dam CO2 capture demonstration plant, Ind. Eng. Chem. Res. 45 (2006) 2414–2420. doi:10.1021/ie050569e.
[7]
B. Xu, H. Gao, M. Chen, Z. Liang, R. Idem, Experimental Study of Regeneration Performance of Aqueous N , N ‑ Diethylethanolamine Solution in a Column Packed with Dixon Ring Random Packing, (2016). doi:10.1021/acs.iecr.6b00936.
25
[8]
H. Liu, C. Yao, Y. Zhao, G. Chen, Desorption of carbon dioxide from aqueous MDEA solution in a microchannel reactor, Chem. Eng. J. 307 (2017) 776–784. doi:10.1016/j.cej.2016.09.010.
[9]
K. Han, C.K. Ahn, M.S. Lee, Performance of an ammonia-based CO2 capture pilot facility in iron and steel industry, Int. J. Greenh. Gas Control. 27 (2014) 239–246. doi:10.1016/j.ijggc.2014.05.014.
[10] H.A. Rodriguez-Flores, L.C. Mello, W.M. Salvagnini, J.L. De Paiva, Absorption of CO2 into aqueous solutions of MEA and AMP in a wetted wall column with film promoter, Chem. Eng. Process. Process Intensif. 73 (2013) 1–6. doi:10.1016/j.cep.2013.06.011. [11] Z. Zhou, X. Zhou, G. Jing, B. Lv, Evaluation of the Multi-amine Functionalized Ionic Liquid for Efficient Postcombustion CO2 Capture, Energy and Fuels. 30 (2016) 7489–7495. doi:10.1021/acs.energyfuels.6b00692. [12] D.A. Glasscock, J.E. Critchfield, G.T. Rochelle, CO2 absorption/desorption in mixtures of methyldiethanolamine with monoethanolamine or diethanolamine, Chem. Eng. Sci. 46 (1991) 2829–2845. doi:10.1016/0009-2509(91)85152-N. [13] D. Sivanesan, M.H. Youn, A. Murnandari, J.M. Kang, K.T. Park, H.J. Kim, S.K. Jeong, Enhanced CO2 absorption and desorption in a tertiary amine medium with a carbonic anhydrase mimic, J. Ind. Eng. Chem. 52 (2017) 287–294. doi:10.1016/j.jiec.2017.03.058. [14] W. Srisang, F. Pouryousefi, P.A. Osei, B. Decardi-Nelson, A. Akachuku, P. Tontiwachwuthikul, R. Idem, Evaluation of the heat duty of catalyst-aided amine-
26
based post combustion CO2 capture, Chem. Eng. Sci. (2017) 1–10. doi:10.1016/j.ces.2017.01.049. [15] Y. Lin, G.T. Rochelle, Approaching a reversible stripping process for CO2 capture, Chem. Eng. J. 283 (2016) 1033–1043. doi:10.1016/j.cej.2015.08.086. [16] K. Jiang, K. Li, H. Yu, Z. Chen, L. Wardhaugh, P. Feron, Advancement of ammonia based post-combustion CO2 capture using the advanced flash stripper process, Appl. Energy. 202 (2017) 496–506. doi:10.1016/j.apenergy.2017.05.143. [17] F. Wang, J. Zhao, H. Li, S. Deng, J. Yan, Preliminary experimental study of postcombustion carbon capture integrated with solar thermal collectors, Appl. Energy. 185 (2016). doi:http://dx.doi.org/10.1016/j.apenergy.2016.02.040. [18] R. Khalilpour, D. Milani, A. Qadir, M. Chiesa, A. Abbas, A novel process for direct solvent regeneration via solar thermal energy for carbon capture, Renew. Energy. 104 (2017) 60–75. doi:10.1016/j.renene.2016.12.001. [19] D.H. Van Wagener, A. Gupta, G.T. Rochelle, S.L. Bryant, Régénération d’un solvant de captage du co2utilisant l’énergie géothermique et des configurations amé liorées pour le régénérateur, Oil Gas Sci. Technol. 69 (2014) 1105–1119. doi:10.2516/ogst/2012099. [20] S. Kodama, T. Nishimoto, N. Yamamoto, K. Yogo, K. Yamada, Development of a new pH-swing CO2 mineralization process with a recyclable reaction solution, Energy. 33 (2008) 776–784. doi:10.1016/j.energy.2008.01.005. [21] Y. Sun, M. Yao, J. Zhang, G. Yang, Indirect CO2 mineral sequestration by steelmaking slag with NH4Cl as leaching solution, Chem. Eng. J. 173 (2011) 437– 445. doi:10.1016/j.cej.2011.08.002. 27
[22] H. Jo, S. Park, Y. Jang, S. Chae, P. Lee, H. Young, Metal extraction and indirect mineral carbonation of waste cement material using ammonium salt solutions, Chem. Eng. J. 254 (2014) 313–323. doi:10.1016/j.cej.2014.05.129. [23] A. Said, H. Mattila, M. Järvinen, R. Zevenhoven, Production of precipitated calcium carbonate (PCC) from steelmaking slag for fixation of CO2, Appl. Energy. 112 (2013) 765–771. doi:10.1016/j.apenergy.2012.12.042. [24] A. Azdarpour, M. Asadullah, E. Mohammadian, H. Hamidi, R. Junin, A review on carbon dioxide mineral carbonation through pH-swing process, Chem. Eng. J. 279 (2015) 615–630. doi:10.1016/j.cej.2015.05.064. [25] W. Conway, Y. Beyad, G. Richner, G. Puxty, P. Feron, Rapid CO2 absorption into aqueous benzylamine (BZA) solutions and its formulations with monoethanolamine (MEA), and 2-amino-2-methyl-1-propanol (AMP) as components for post combustion capture processes, Chem. Eng. J. 264 (2015) 954–961. doi:10.1016/j.cej.2014.11.040. [26] G. Fan, A.G.H. Wee, R. Idem, P. Tontiwachwuthikul, NMR Studies of Amine Species in MEA-CO2-H2O System : Modification of the Model of Vapor - Liquid Equilibrium (VLE), Ind. Eng. Chem. Res. 48 (2009) 2717–2720. doi:10.1021/ie8015895. [27] R.J. A.H. Park, L.S. Fan, CO2 mineral sequestration: chemically enhanced aqueous\rcarbonation of serpentine, Can. J. Chem. Eng. 81 (2003) 885–890. doi:10.1002/cjce.5450810373.
28
[28] S.-J. Han, M. Yoo, D.-W. Kim, J.-H. Wee, Carbon Dioxide Capture Using Calcium Hydroxide Aqueous Solution as the Absorbent, Energy & Fuels. 25 (2011) 3825– 3834. doi:10.1021/ef200415p. [29] A.C. van Eckeveld, L.V. van der Ham, L.F.G. Geers, L.J.P. van den Broeke, B.J. Boersma, E.L.V. Goetheer, Online Monitoring of the Solvent and Absorbed Acid Gas Concentration in a CO2 Capture Process Using Monoethanolamine, Ind. Eng. Chem. Res. 53 (2014) 5515–5523. doi:10.1021/ie402310n. [30] I. Kiviranta, M. Tammi, R. Lappalainen, T. Kuusela, H.J. Helminen, The rate of calcium extraction during EDTA decalcification from thin bone slices as assessed with atomic absorption spectrophotometry, Histochemistry. 68 (1980) 119–127. doi:10.1007/BF00489507. [31] A.L. Boskey, A.S. Posner, Extraction of a calcium-phospholipid-phosphate complex from bone, Calcif. Tissue Res. 19 (1975) 273–283. doi:10.1007/BF02564010. [32] S.J. Han, J.H. Wee, Estimation of correlation between electrical conductivity and CO2 absorption in a monoethanolamine solvent system, J. Chem. Eng. Data. 58 (2013) 2381–2388. doi:10.1021/je400358f. [33] A.H.A. Park, L.S. Fan, CO2 mineral sequestration: Physically activated dissolution of serpentine and pH swing process, Chem. Eng. Sci. 59 (2004) 5241–5247. doi:10.1016/j.ces.2004.09.008. [34] E. Jo, Y.H. Jhon, S.B. Choi, J.-G. Shim, J.-H. Kim, J.H. Lee, I.-Y. Lee, K.-R. Jang, J. Kim, Crystal structure and electronic properties of 2-amino-2-methyl-1-propanol (AMP) carbamate, Chem. Commun. 46 (2010) 9158. doi:10.1039/c0cc03224g.
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Highlights A novel process which is chemical regeneration through a pH swing is proposed. The chemical regeneration shows higher efficiency of CO2 desorption than the thermal regeneration at even room temperature. After the chemical regeneration, a value-added CaCO3 is produced. Ca(OH)2 in the presence of Cl- plays a key role in desorbing CO2 from amine and converting it into CaCO3.
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