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Impregnation of Amines Onto Porous Precipitated Silica for CO2 capture
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 63 (2014) 2122 – 2128
GHGT-12
Impregnation of amines onto porous precipitated silica for CO2 capture Dang Viet Quang, Abdallah Dindi, Aravind V Rayer, Nabil El Hadri, Abdurahim Abdulkadir and Mohammad R.M. Abu-Zahra* Masdar Institute of Science and Technology, P.O.Box 54224, Masdar city, Abu Dhabi, United Arab Emirates
Abstract In this study, porous precipitated silica (PPS) was synthesized using sodium silicate as an inexpensive silica precursor and then impregnated with various amines such as 2-aminomethylpropanol (AMP), monoethanolamine (MEA), diethanolamine (DEA) and polyethyleneimine (PEI) to produce amine-impregnated solid adsorbents, which will be evaluated as a sorbent for CO2 capture. Major parameters and performances of adsorbents including thermal stability, adsorption capacity, heat capacity, and adsorption heat were investigated. The results indicated that MEA impregnated PPS (60 wt%) has the highest CO2 adsorption capacity; up to 233 mg/g, while the adsorption capacity of PEI impregnated PPS (60 wt%) is 136 mg/g. The heat capacity of PEI impregnated PPS (50 wt%) is relatively low (1.68 J/go·C) compared to that of aqueous MEA 30 wt% (3.98 J/go·C). Obtained results were used to calculate the energy requirement for adsorbent regeneration. The calculated results revealed that PEI impregnated PPS (50 wt%) requires the least energy for regeneration, only 2080 kJ/kg of CO2 and it is 44.7 % lower than energy requirement for aqueous MEA 30 wt%. Thermal stability of the adsorbents was confirmed by thermal gravity analyses, which showed that AMP and MEA start vaporizing at very low temperature and complete at 110 and 120 oC, DEA vaporizes at temperature from 100ņ200oC, and PEI vaporizes and decomposes at about 250ņ400 oC. The study on adsorption/desorption performance at the regeneration temperature of 90 oC indicated that the stability of the sorbent is greatly varied with the vaporization temperature. PEI impregnated PPS shows a no significant loss in CO2 adsorption capacity, while DEA impregnated PPS lost 12 % of adsorption capacity after 10 cycles. © 2014 Published by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license © 2013The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of GHGT. Peer-review under responsibility of the Organizing Committee of GHGT-12 Keywords: Solid adsorbent; CO2 capture; Post-combustion capture; Energy consumption; Precipitated silica.
* Corresponding authors. Tel.: +97128109181 Email:[email protected]
1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of GHGT-12 doi:10.1016/j.egypro.2014.11.229
Dang Viet Quang et al. / Energy Procedia 63 (2014) 2122 – 2128
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1. Introduction CO2 post-combustion capture is one of the feasible technologies for mitigating anthropogenic CO2 emission, which is considered to be a main reason for global warming and climate change. Since the major source of global energy is based on burning fossil fuel with numerous power plants under operating, CO2 post-combustion capture is the most suitable technology that could be retrofitted to those operating power plants without significant modification [1]. CO2 post combustion capture using an aqueous amine-based process is the most mature technology for the removal of CO2 from flue gas [2-4]. Unfortunately, the application of the aqueous amine based CO2 capture process to a fossil fuel fired power plant leads to a significant increase in the cost of electricity due to the huge energy penalty mostly caused by the energy requirement for amine regeneration. Furthermore, conventional CO2 capture process based on amine solvent accompanies with various technical and environmental issues such as equipment corrosion and waste management [5, 6]. This urges scientists and engineers to develop innovative materials and technologies for CO2 capture with more economical effectiveness and energy saving. Beside advanced CO2 post combustion capture technologies like membrane separation and cryogenic separation, the adsorption of CO2 from flue gas using solid adsorbent has been extensively explored [7, 8]. Recently, various solid adsorbents have been developed and investigated with an expectation to find out a novel sorbent that may help reduce the energy consumption and eventually reduce the cost of CO2 capture [9, 10]. Among investigated solid adsorbents, amine impregnated mesoporous silica has been considered to be a promising CO2 adsorbent due to their reasonable cost, relatively high adsorption capacity, and low specific heat capacity and adsorption heat [11, 12]. The objective of this work is to impregnate amines onto porous precipitated silica (PPS) and examine its possible application in the field of CO2 capture. Accordingly, PPS will be synthesized using sodium silicate as an inexpensive silica precursor and then impregnated with various amines such as 2-aminomethylpropanol (AMP), monoethanolamine (MEA), diethanolamine (DEA) and polyethyleneimine (PEI) to produce amine-impregnated solid adsorbents. The resulting adsorbent will be evaluated regarding to adsorption capacity, heat requirement, and adsorption-desorption cyclability. 2. Experimental 2.1. Adsorbent preparation The preparation procedure of porous precipitated silica was adapted from previous publications [13, 14]. Typically, 26 g of sodium chloride was dissolved in 698 g of water in a 3 L beaker placed in a heating mantle. After sodium chloride was completely dissolved, 189 g of sodium silicate solution (3.4SiO2·NaO2) was added. The heating mantle was turned on and temperature was set at 40 oC. When temperature reached 40 oC, H2SO4 8 % was added by a drop wise method. H2SO4 was added by two stages. The first stage was ended as the first silica aggregates observed. The second stage began after 5 min from the end of the first and finished at pH 5. Resulting slurry was aged at 80 oC for 30 min and cooled down to room temperature. The slurry was filtered and washed with water to eliminate byproduct ions, mainly including Na+, SO42-, and Cl-. Finally, PPS was obtained by drying wet slurry at 130 oC for 3 hours. The desired amount of PEI 50 wt% in water was diluted with distilled water to prepare impregnating solutions. PEI impregnated precipitated silica was prepared by a wet impregnation method. Typically, 4 g of impregnating solution was added to a 50 mL beaker containing 1 g of precipitated silica and mechanically mixed using a steel stainless spatula. PEI impregnated precipitated silica was obtained by drying silica slurry at 105 oC for 3 hours. The prepared adsorbents were denoted as PEI-PPS-x, where x is the weight percentage of PEI in the absorbent. The same method was used to prepare the different amines impregnated silica and the prepared adsorbents were denoted as AM-PPS-y, where AM is the amine solvent which is AMP, MEA, or DEA and y is its weight percentage in the adsorbent. 2.2. Characterization The Brunauer-Emmett-Teller (BET) surface area, pore size and pore volume of the precipitated silica were analyzed
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using a nitrogen adsorption-desorption instrument (Micromeritics Tristar 3000 porosimeter). Samples were measured after degassing at 100 oC for 3 hours. The porosity and pore size distribution was calculated using the Barrett-Joyner-Halenda (BJH) method. Scanning electron microscope (Quanta 250) was used to study the morphology of silica substrate at the accelerating voltage of 15kV. Thermogravimetric analysis (TGA) was performed using a thermogravimetric analyzer (SDT Q600). TGA was performed in nitrogen gas at a heating rate of 5 oC/min from room temperature to 900 oC. 2.3. Measurement of heat capacity Heat capacity was determined by a Micro Reaction Calorimeter provided by Thermal Hazard Technology (UK). Heat capacity was measured at temperature ranging from 30—90 oC. At each temperature, the heat was repeatedly measured 3 times with a step of ±0.5 oC. First, blank test was conducted with an empty cell. Then, approximate 0.3 g of adsorbent was placed in analysis cell and conducted test with the same condition used for the blank. The heat capacity of adsorbent was calculated from equation 5, where Cp (J/oC*g) is the heat capacity of adsorbent, m is the mass of sample (g), ¨T is temperature step (1 oC) and Q (J) and Qblank (J) are the heat change of analysis cell with sample and without sample respectively.
ܥ ൌ
ሺொିொ್ೌೖ ሻ
(5)
כο் V3
By-pass Gas out
Gas in MFC T1
V1
T2
Cap
V2
Shunt
D
Analysis cell
Reference cell
Calorimeter N2
CO2
Figure 1. Schematic diagram of CO2 adsorption process using a flow Micro Reaction Calorimeter 2.4. CO2 loading capacity and the heat of CO2 adsorption The heat of adsorption was determined by a flow Micro Reaction Calorimeter (URC) provided by Thermal Hazard Technology (UK). An analysis cell containing approximate 0.2 g of adsorbent was installed in analysis cell as shown in Figure 1. CO2 gas supplied from a cylinder was run through a desiccant column (D) to remove moisture before entering the analysis cell. The flow rate of CO2 was controlled by a mass flow controller (MFC). The valve V2 was used to control the pressure of system that was indicated by a transducer (T2). Test was run by URC control software under the isothermal mode and result was displayed as the variation of power (mW) with time (s). When the power signal becomes constant, CO2 gas was introduced at the rate of 0.3 ml/min. Since the reaction between CO2 gas and adsorbent is exothermic, the power signal increased right after the gas was flowed into analysis cell.
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Test was completed when the power signal becomes constant again. The weight of adsorbent before and after the test was checked to calculate the mass of CO2 adsorbed. The integral heat (Q) was computed by using URC analysis software provided by Thermal Hazard Technology. The heat of adsorption ¨H (kJ/mole of CO2) was calculated by dividing the integral heat by the mole of CO2 absorbed. The CO2 adsorption capacity of adsorbent (mg/g) was calculated by dividing the quantity of CO2 adsorbed per the mass of adsorbent. 3. Results and discussion The surface area and porosity of silica sample were analyzed using nitrogen adsorption-desorption method indicating that the prepared precipitated silica is porous material with average BET surface area of 230 m2/g and pore size ranging from 20-80 nm. The characteristic properties of the prepared adsorbent including thermal stability, adsorption capacity, average heat capacity, and adsorption heat are presented in Table 1. Table 1. The characteristic of the prepared adsorbents Adsorption Adsorption Cp heat capacity Sample (J/g·K) (kJ/mole) (mg/g)
Regeneration heat requirement (kJ/Kg of CO2)
Decomposition temperature (oC)
-
14
26.4
-
-
3.98
85
118.8
3899.4
-
-
-
132
-
-
AMP-PPS-60
-
68
154
-
50ņ110
MEA-PPS-50
2.16
-
189.2
3802.1
-
MEA-PPS-60
2.28
81
233.2
3727.6
50ņ120
DEA-PPS-50
1.96
-
114.4
2544.1
-
2.02
65
140.8
2370.7
100ņ200
PEI-PPS-50
1.68
-
127.6
2079.5
-
PEI-PPS-60
1.90
69
136.4
2109.1
250ņ400
PPS Aqueous MEA-30 AMP-PPS-50
DEA-PPS-60
Low CO2 adsorption capacity and adsorption heat of silica sample compared to amine impregnated samples indicated that the adsorption of CO2 onto silica substrate is based on physical interaction. The obtained results revealed that MEA impregnated PPS containing 60 and 50 wt % of MEA has CO2 adsorption capacity of 233 and 189 mg/g, respectively. This is higher than the adsorption capacity of the other adsorbents with the same impregnating percentage in which DEA impregnated PPS has the lowest capacity with 114 and 141 mg/g for sample with 50 and 60 wt% of DEA, respectively. For adsorbent impregnated with monomer amines, the CO2 adsorption capacity is proportional to the molar number of amino group, while it is proportionate to the active amino groups in the adsorbent impregnated with PEI. As seen in the Table 1, the adsorption heat of MEA is slightly reduced by the impregnation onto porous silica; it is 85 kJ/mole for aqueous MEA and 81 kJ/mole for MEA impregnated PPS. Generally, the adsorption heat of solid adsorbent tends to be lower than that of aqueous amine solution; however, the most benefit of the impregnation of amines onto solid substrate is noticeable reduction in heat capacity of the adsorbent. The heat capacity of the aqueous MEA solution 30 wt% is 3.98 J/g·K, while the highest value of solid adsorbent is only 2.28 J/g·K. The PEI impregnated PPS has the lowest heat capacity that is 1.68 and 1.90 J/g·K for adsorbent containing 50 and 60 wt% of PEI, respectively. Interestingly, the heat capacity of the adsorbent tends to decrease with the increase in the molecular weight of the impregnated amine. The energy required to regenerate adsorbent was estimated based on the CO2 adsorption capacity, the heat of adsorption, and the heat capacity according to equation (1). (1) Qreg = Qdes + Qsen + Qvap Where, Qreg is the regeneration energy of the process. Qdes is the heat of CO2 desorption, which has the same absolute
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value as the heat of adsorption. Qsen is sensible heat that must be supplied to increase temperature of sorbent to regeneration temperature. The sensible heat is calculated by the following equation: (2) Qsen = Cp*¨T In which, Cp is heat capacity and ¨T is temperature difference between CO2 rich adsorbent entering stripper and regeneration temperature. Qvap is the latent heat of vaporization of volatile components in sorbent, which usually includes the vaporization heat of water and amine. Qvap is given as: (3) Qvap = qi*mi Where, qi and mi are the specific heat of vaporization and the amount of volatile components (i), respectively. The heats of vaporization of MEA and DEA from adsorbents can be experimentally determined using TGA-DSC analyses. The calculated results revealed that PEI impregnated PPS (50 wt%) requires the least energy for regeneration, only 2080 kJ/kg of CO2 and it is 44.7 % lower than energy requirement for aqueous MEA 30 wt%. The regeneration energy requirement for DEA-PPS-60 can be reduced 39 % compared to that for aqueous MEA 30 wt%. The low energy requirement for the regeneration of PEI impregnated PPS (50 wt%) is most likely due to its low specific heat capacity and avoidance of the vaporization heat caused by the vaporization of volatile amines (AMP, MEA, and DEA) and solvent (water). The vaporization of amines was confirmed by thermal gravity analyses, which showed that AMP and MEA start vaporizing at very low temperature (50 oC) and complete at 110 and 120 oC, DEA vaporizes at temperature from 100ņ200oC, and PEI vaporizes and decomposes at about 250ņ400 oC. Amines with low vaporization temperature do not considerably improve the regeneration heat requirement of the resulting adsorbent because of the increase in the heat required for those amines to vaporize. Therefore, the regeneration energy requirement for MEA impregnated PPS (3802 kJ/kg of CO2) is very close to that for the aqueous MEA solution 30 wt%. Obviously, amines with higher vaporization point produce higher thermally stable adsorbent with lower regeneration energy requirement. In this study, DEA and PEI impregnated PPS displayed to be likely suitable adsorbent for CO2 capture in term of energy and thermal stability, therefore, further investigation on their adsorption kinetic and adsorption/desorption performance have been done. Reaction between amine and CO2 is exothermic, the heat released during adsorption is corresponding to the CO2 adsorbed, thus; the heat released was recorded to investigate the adsorption kinetic of the adsorbent and the result is shown in Figure 2. As seen in this figure, the adsorption occurs rapidly at the beginning; estimated 90 % of the CO2 adsorption occurs at the first 15 min and then slowdowns until reaching the equilibrium. (4) 2R1R2NH + CO2 ļ R1R2NCOO- + R1R2NH2+ + (- ¨H) 14
a
12
b
Heat released (J)
10 8
a b
6
DEA-PPS-60 PEI-PPS 50
4 2 0 0
10
20
30
40
50
60
Adsorption time (min)
Figure 2. CO2 adsorption kinetic of the adsorbent
Dang Viet Quang et al. / Energy Procedia 63 (2014) 2122 – 2128
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The result from adsorption/desorption study is presented in Figure 3 in which the adsorption/desorption temperature were 40/90 oC for DEA-PPS-50 and 75/90 oC for PEI-PPS-50, respectively. This result demonstrates that the CO2 adsorption performance of PEI-PPS-50 is relatively stable; its adsorption capacity is almost unchanged after 10 cycles. Meanwhile, the adsorption capacity of DEA-PPS-50 was slowly reduced after each cycle and its loss of 12 % was observed after 10 cycles. The loss in the adsorption capacity of DEA-PPS-50 is most likely due to the evaporation of DEA molecules during cycling. This result indicated that the vaporization temperature of impregnated amine has a significant effect on the performance of the resulting adsorbent. 140 o
DEA-PPS-50 (40/90 C)
o
PEI-PPS-50 (75/90 C)
CO2 adsorption capacity (mg/g)
120 100 80 60 40 20 0
2
4
6 Number of cycles
8
10
Figure 3. The adsorption/desorption performance of DEA-PPS-50 and PEI-PPS-50 4. Conclusion Various amines have been impregnated onto PPS to investigate their possibility for CO2 capture. The results indicated that amine impregnated PPS have low specific heat capacity compared to aqueous MEA 30 wt%. Adsorbents containing amine with low vaporization temperature (MEA and AMP) require higher energy for regeneration due to the contribution of high vaporization heat, while PEI impregnated PPS requires the least energy for regeneration (2080 kJ/kg of CO2). The adsorbent prepared from amine with higher vaporization temperature tends to have a more stable cyclic adsorption capacity. This study demonstrates that the solid adsorbent for CO2 capture can be easily produced by impregnating amine onto precipitated porous silica. 5. References [1] M.K. Mondal, H.K. Balsora, P. Varshney, Progress and trends in CO2 capture/separation technologies: A review, Energy, 46 (2012) 431-441. [2] Y. Artanto, J. Jansen, P. Pearson, T. Do, A. Cottrell, E. Meuleman, P. Feron, Performance of MEA and amineblends in the CSIRO PCC pilot plant at Loy Yang Power in Australia, Fuel, 101 (2012) 264-275. [3] M. Lucquiaud, O. Errey, H. Chalmers, X. Liang, J. Gibbins, M. Abu-Zahra, Techno-economic assessment of future-proofing coal plants with post combustion capture against technology development, Energy Procedia, 4 (2011) 1909-1916.. [4] 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, Industrial & Engineering Chemistry Research, 45 (2005) 2414-2420. [5] M.R.M. Abu-Zahra, J.P.M. Niederer, P.H.M. Feron, G.F. Versteeg, CO2 capture from power plants. Part II. A parametric study of the economical performance based on mono-ethanolamine, International journal of greenhouse gas control, 1 (2007) 135-142. [6] M.R. Abu-Zahra, E.S. Fernandez, E.L. Goetheer, Guidelines for process development and future cost reduction of CO2 post-cobmustion capture, Energy Procedia, 4 (2011)1051-1057.
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[7] Z.H. Lee, K.T. Lee, S. Bhatia, A.R. Mohamed, Post-combustion carbon dioxide capture: Evolution towards utilization of nanomaterials, Renewable and Sustainable Energy Reviews, 16 (2012) 2599-2609. [8] A. Samanta, A. Zhao, G.K.H. Shimizu, P. Sarkar, R. Gupta, Post-Combustion CO2 Capture Using Solid Sorbents: A Review, Industrial & Engineering Chemistry Research, 51 (2011) 1438-1463. [9] A. Heydari-Gorji, Y. Belmabkhout, A. Sayari, Polyethylenimine-Impregnated Mesoporous Silica: Effect of Amine Loading and Surface Alkyl Chains on CO2 Adsorption, Langmuir, 27 (2011) 12411-12416. [10] A. Sayari, Y. Belmabkhout, Stabilization of Amine-Containing CO2 Adsorbents: Dramatic Effect of Water Vapor, Journal of the American Chemical Society, 132 (2010) 6312-6314. [11] D.V. Quang, A.V. Rabindran, N. El Hadri, M.R. Abu-Zahra, Reduction in the reneration energy of CO2 capture process by impregnating amine solvent onto precipitated silica European Scientific Journal, 9 (2013). [12] D.V. Quang, A. Dindi, A.V. Rayer, N.E. Hadri, A. Abdulkadir, M.R. AbuϋZahra, Effect of moisture on the heat capacity and the regeneration heat required for CO2 capture process using PEI impregnated mesoporous precipitated silica, Greenhouse Gases: Science and Technology, (2014). [13] P.B. Sarawade, J.K. Kim, A. Hilonga, D.V. Quang, H.T. Kim, Effect of drying technique on the physicochemical properties of sodium silicate-based mesoporous precipitated silica, Applied Surface Science, 258 (2011) 955-961. [14] D.V. Quang, J.K. Kim, J.K. Park, S.H. Park, G. Elineema, P.B. Sarawade, H.T. Kim, Effect of the gelation on the properties of precipitated silica powder produced by acidizing sodium silicate solution at the pilot scale, Chemical Engineering Journal, (2012).
ScienceDirect Energy Procedia 63 (2014) 2122 – 2128
GHGT-12
Impregnation of amines onto porous precipitated silica for CO2 capture Dang Viet Quang, Abdallah Dindi, Aravind V Rayer, Nabil El Hadri, Abdurahim Abdulkadir and Mohammad R.M. Abu-Zahra* Masdar Institute of Science and Technology, P.O.Box 54224, Masdar city, Abu Dhabi, United Arab Emirates
Abstract In this study, porous precipitated silica (PPS) was synthesized using sodium silicate as an inexpensive silica precursor and then impregnated with various amines such as 2-aminomethylpropanol (AMP), monoethanolamine (MEA), diethanolamine (DEA) and polyethyleneimine (PEI) to produce amine-impregnated solid adsorbents, which will be evaluated as a sorbent for CO2 capture. Major parameters and performances of adsorbents including thermal stability, adsorption capacity, heat capacity, and adsorption heat were investigated. The results indicated that MEA impregnated PPS (60 wt%) has the highest CO2 adsorption capacity; up to 233 mg/g, while the adsorption capacity of PEI impregnated PPS (60 wt%) is 136 mg/g. The heat capacity of PEI impregnated PPS (50 wt%) is relatively low (1.68 J/go·C) compared to that of aqueous MEA 30 wt% (3.98 J/go·C). Obtained results were used to calculate the energy requirement for adsorbent regeneration. The calculated results revealed that PEI impregnated PPS (50 wt%) requires the least energy for regeneration, only 2080 kJ/kg of CO2 and it is 44.7 % lower than energy requirement for aqueous MEA 30 wt%. Thermal stability of the adsorbents was confirmed by thermal gravity analyses, which showed that AMP and MEA start vaporizing at very low temperature and complete at 110 and 120 oC, DEA vaporizes at temperature from 100ņ200oC, and PEI vaporizes and decomposes at about 250ņ400 oC. The study on adsorption/desorption performance at the regeneration temperature of 90 oC indicated that the stability of the sorbent is greatly varied with the vaporization temperature. PEI impregnated PPS shows a no significant loss in CO2 adsorption capacity, while DEA impregnated PPS lost 12 % of adsorption capacity after 10 cycles. © 2014 Published by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license © 2013The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of GHGT. Peer-review under responsibility of the Organizing Committee of GHGT-12 Keywords: Solid adsorbent; CO2 capture; Post-combustion capture; Energy consumption; Precipitated silica.
* Corresponding authors. Tel.: +97128109181 Email:[email protected]
1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of GHGT-12 doi:10.1016/j.egypro.2014.11.229
Dang Viet Quang et al. / Energy Procedia 63 (2014) 2122 – 2128
2123
1. Introduction CO2 post-combustion capture is one of the feasible technologies for mitigating anthropogenic CO2 emission, which is considered to be a main reason for global warming and climate change. Since the major source of global energy is based on burning fossil fuel with numerous power plants under operating, CO2 post-combustion capture is the most suitable technology that could be retrofitted to those operating power plants without significant modification [1]. CO2 post combustion capture using an aqueous amine-based process is the most mature technology for the removal of CO2 from flue gas [2-4]. Unfortunately, the application of the aqueous amine based CO2 capture process to a fossil fuel fired power plant leads to a significant increase in the cost of electricity due to the huge energy penalty mostly caused by the energy requirement for amine regeneration. Furthermore, conventional CO2 capture process based on amine solvent accompanies with various technical and environmental issues such as equipment corrosion and waste management [5, 6]. This urges scientists and engineers to develop innovative materials and technologies for CO2 capture with more economical effectiveness and energy saving. Beside advanced CO2 post combustion capture technologies like membrane separation and cryogenic separation, the adsorption of CO2 from flue gas using solid adsorbent has been extensively explored [7, 8]. Recently, various solid adsorbents have been developed and investigated with an expectation to find out a novel sorbent that may help reduce the energy consumption and eventually reduce the cost of CO2 capture [9, 10]. Among investigated solid adsorbents, amine impregnated mesoporous silica has been considered to be a promising CO2 adsorbent due to their reasonable cost, relatively high adsorption capacity, and low specific heat capacity and adsorption heat [11, 12]. The objective of this work is to impregnate amines onto porous precipitated silica (PPS) and examine its possible application in the field of CO2 capture. Accordingly, PPS will be synthesized using sodium silicate as an inexpensive silica precursor and then impregnated with various amines such as 2-aminomethylpropanol (AMP), monoethanolamine (MEA), diethanolamine (DEA) and polyethyleneimine (PEI) to produce amine-impregnated solid adsorbents. The resulting adsorbent will be evaluated regarding to adsorption capacity, heat requirement, and adsorption-desorption cyclability. 2. Experimental 2.1. Adsorbent preparation The preparation procedure of porous precipitated silica was adapted from previous publications [13, 14]. Typically, 26 g of sodium chloride was dissolved in 698 g of water in a 3 L beaker placed in a heating mantle. After sodium chloride was completely dissolved, 189 g of sodium silicate solution (3.4SiO2·NaO2) was added. The heating mantle was turned on and temperature was set at 40 oC. When temperature reached 40 oC, H2SO4 8 % was added by a drop wise method. H2SO4 was added by two stages. The first stage was ended as the first silica aggregates observed. The second stage began after 5 min from the end of the first and finished at pH 5. Resulting slurry was aged at 80 oC for 30 min and cooled down to room temperature. The slurry was filtered and washed with water to eliminate byproduct ions, mainly including Na+, SO42-, and Cl-. Finally, PPS was obtained by drying wet slurry at 130 oC for 3 hours. The desired amount of PEI 50 wt% in water was diluted with distilled water to prepare impregnating solutions. PEI impregnated precipitated silica was prepared by a wet impregnation method. Typically, 4 g of impregnating solution was added to a 50 mL beaker containing 1 g of precipitated silica and mechanically mixed using a steel stainless spatula. PEI impregnated precipitated silica was obtained by drying silica slurry at 105 oC for 3 hours. The prepared adsorbents were denoted as PEI-PPS-x, where x is the weight percentage of PEI in the absorbent. The same method was used to prepare the different amines impregnated silica and the prepared adsorbents were denoted as AM-PPS-y, where AM is the amine solvent which is AMP, MEA, or DEA and y is its weight percentage in the adsorbent. 2.2. Characterization The Brunauer-Emmett-Teller (BET) surface area, pore size and pore volume of the precipitated silica were analyzed
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using a nitrogen adsorption-desorption instrument (Micromeritics Tristar 3000 porosimeter). Samples were measured after degassing at 100 oC for 3 hours. The porosity and pore size distribution was calculated using the Barrett-Joyner-Halenda (BJH) method. Scanning electron microscope (Quanta 250) was used to study the morphology of silica substrate at the accelerating voltage of 15kV. Thermogravimetric analysis (TGA) was performed using a thermogravimetric analyzer (SDT Q600). TGA was performed in nitrogen gas at a heating rate of 5 oC/min from room temperature to 900 oC. 2.3. Measurement of heat capacity Heat capacity was determined by a Micro Reaction Calorimeter provided by Thermal Hazard Technology (UK). Heat capacity was measured at temperature ranging from 30—90 oC. At each temperature, the heat was repeatedly measured 3 times with a step of ±0.5 oC. First, blank test was conducted with an empty cell. Then, approximate 0.3 g of adsorbent was placed in analysis cell and conducted test with the same condition used for the blank. The heat capacity of adsorbent was calculated from equation 5, where Cp (J/oC*g) is the heat capacity of adsorbent, m is the mass of sample (g), ¨T is temperature step (1 oC) and Q (J) and Qblank (J) are the heat change of analysis cell with sample and without sample respectively.
ܥ ൌ
ሺொିொ್ೌೖ ሻ
(5)
כο் V3
By-pass Gas out
Gas in MFC T1
V1
T2
Cap
V2
Shunt
D
Analysis cell
Reference cell
Calorimeter N2
CO2
Figure 1. Schematic diagram of CO2 adsorption process using a flow Micro Reaction Calorimeter 2.4. CO2 loading capacity and the heat of CO2 adsorption The heat of adsorption was determined by a flow Micro Reaction Calorimeter (URC) provided by Thermal Hazard Technology (UK). An analysis cell containing approximate 0.2 g of adsorbent was installed in analysis cell as shown in Figure 1. CO2 gas supplied from a cylinder was run through a desiccant column (D) to remove moisture before entering the analysis cell. The flow rate of CO2 was controlled by a mass flow controller (MFC). The valve V2 was used to control the pressure of system that was indicated by a transducer (T2). Test was run by URC control software under the isothermal mode and result was displayed as the variation of power (mW) with time (s). When the power signal becomes constant, CO2 gas was introduced at the rate of 0.3 ml/min. Since the reaction between CO2 gas and adsorbent is exothermic, the power signal increased right after the gas was flowed into analysis cell.
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Test was completed when the power signal becomes constant again. The weight of adsorbent before and after the test was checked to calculate the mass of CO2 adsorbed. The integral heat (Q) was computed by using URC analysis software provided by Thermal Hazard Technology. The heat of adsorption ¨H (kJ/mole of CO2) was calculated by dividing the integral heat by the mole of CO2 absorbed. The CO2 adsorption capacity of adsorbent (mg/g) was calculated by dividing the quantity of CO2 adsorbed per the mass of adsorbent. 3. Results and discussion The surface area and porosity of silica sample were analyzed using nitrogen adsorption-desorption method indicating that the prepared precipitated silica is porous material with average BET surface area of 230 m2/g and pore size ranging from 20-80 nm. The characteristic properties of the prepared adsorbent including thermal stability, adsorption capacity, average heat capacity, and adsorption heat are presented in Table 1. Table 1. The characteristic of the prepared adsorbents Adsorption Adsorption Cp heat capacity Sample (J/g·K) (kJ/mole) (mg/g)
Regeneration heat requirement (kJ/Kg of CO2)
Decomposition temperature (oC)
-
14
26.4
-
-
3.98
85
118.8
3899.4
-
-
-
132
-
-
AMP-PPS-60
-
68
154
-
50ņ110
MEA-PPS-50
2.16
-
189.2
3802.1
-
MEA-PPS-60
2.28
81
233.2
3727.6
50ņ120
DEA-PPS-50
1.96
-
114.4
2544.1
-
2.02
65
140.8
2370.7
100ņ200
PEI-PPS-50
1.68
-
127.6
2079.5
-
PEI-PPS-60
1.90
69
136.4
2109.1
250ņ400
PPS Aqueous MEA-30 AMP-PPS-50
DEA-PPS-60
Low CO2 adsorption capacity and adsorption heat of silica sample compared to amine impregnated samples indicated that the adsorption of CO2 onto silica substrate is based on physical interaction. The obtained results revealed that MEA impregnated PPS containing 60 and 50 wt % of MEA has CO2 adsorption capacity of 233 and 189 mg/g, respectively. This is higher than the adsorption capacity of the other adsorbents with the same impregnating percentage in which DEA impregnated PPS has the lowest capacity with 114 and 141 mg/g for sample with 50 and 60 wt% of DEA, respectively. For adsorbent impregnated with monomer amines, the CO2 adsorption capacity is proportional to the molar number of amino group, while it is proportionate to the active amino groups in the adsorbent impregnated with PEI. As seen in the Table 1, the adsorption heat of MEA is slightly reduced by the impregnation onto porous silica; it is 85 kJ/mole for aqueous MEA and 81 kJ/mole for MEA impregnated PPS. Generally, the adsorption heat of solid adsorbent tends to be lower than that of aqueous amine solution; however, the most benefit of the impregnation of amines onto solid substrate is noticeable reduction in heat capacity of the adsorbent. The heat capacity of the aqueous MEA solution 30 wt% is 3.98 J/g·K, while the highest value of solid adsorbent is only 2.28 J/g·K. The PEI impregnated PPS has the lowest heat capacity that is 1.68 and 1.90 J/g·K for adsorbent containing 50 and 60 wt% of PEI, respectively. Interestingly, the heat capacity of the adsorbent tends to decrease with the increase in the molecular weight of the impregnated amine. The energy required to regenerate adsorbent was estimated based on the CO2 adsorption capacity, the heat of adsorption, and the heat capacity according to equation (1). (1) Qreg = Qdes + Qsen + Qvap Where, Qreg is the regeneration energy of the process. Qdes is the heat of CO2 desorption, which has the same absolute
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value as the heat of adsorption. Qsen is sensible heat that must be supplied to increase temperature of sorbent to regeneration temperature. The sensible heat is calculated by the following equation: (2) Qsen = Cp*¨T In which, Cp is heat capacity and ¨T is temperature difference between CO2 rich adsorbent entering stripper and regeneration temperature. Qvap is the latent heat of vaporization of volatile components in sorbent, which usually includes the vaporization heat of water and amine. Qvap is given as: (3) Qvap = qi*mi Where, qi and mi are the specific heat of vaporization and the amount of volatile components (i), respectively. The heats of vaporization of MEA and DEA from adsorbents can be experimentally determined using TGA-DSC analyses. The calculated results revealed that PEI impregnated PPS (50 wt%) requires the least energy for regeneration, only 2080 kJ/kg of CO2 and it is 44.7 % lower than energy requirement for aqueous MEA 30 wt%. The regeneration energy requirement for DEA-PPS-60 can be reduced 39 % compared to that for aqueous MEA 30 wt%. The low energy requirement for the regeneration of PEI impregnated PPS (50 wt%) is most likely due to its low specific heat capacity and avoidance of the vaporization heat caused by the vaporization of volatile amines (AMP, MEA, and DEA) and solvent (water). The vaporization of amines was confirmed by thermal gravity analyses, which showed that AMP and MEA start vaporizing at very low temperature (50 oC) and complete at 110 and 120 oC, DEA vaporizes at temperature from 100ņ200oC, and PEI vaporizes and decomposes at about 250ņ400 oC. Amines with low vaporization temperature do not considerably improve the regeneration heat requirement of the resulting adsorbent because of the increase in the heat required for those amines to vaporize. Therefore, the regeneration energy requirement for MEA impregnated PPS (3802 kJ/kg of CO2) is very close to that for the aqueous MEA solution 30 wt%. Obviously, amines with higher vaporization point produce higher thermally stable adsorbent with lower regeneration energy requirement. In this study, DEA and PEI impregnated PPS displayed to be likely suitable adsorbent for CO2 capture in term of energy and thermal stability, therefore, further investigation on their adsorption kinetic and adsorption/desorption performance have been done. Reaction between amine and CO2 is exothermic, the heat released during adsorption is corresponding to the CO2 adsorbed, thus; the heat released was recorded to investigate the adsorption kinetic of the adsorbent and the result is shown in Figure 2. As seen in this figure, the adsorption occurs rapidly at the beginning; estimated 90 % of the CO2 adsorption occurs at the first 15 min and then slowdowns until reaching the equilibrium. (4) 2R1R2NH + CO2 ļ R1R2NCOO- + R1R2NH2+ + (- ¨H) 14
a
12
b
Heat released (J)
10 8
a b
6
DEA-PPS-60 PEI-PPS 50
4 2 0 0
10
20
30
40
50
60
Adsorption time (min)
Figure 2. CO2 adsorption kinetic of the adsorbent
Dang Viet Quang et al. / Energy Procedia 63 (2014) 2122 – 2128
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The result from adsorption/desorption study is presented in Figure 3 in which the adsorption/desorption temperature were 40/90 oC for DEA-PPS-50 and 75/90 oC for PEI-PPS-50, respectively. This result demonstrates that the CO2 adsorption performance of PEI-PPS-50 is relatively stable; its adsorption capacity is almost unchanged after 10 cycles. Meanwhile, the adsorption capacity of DEA-PPS-50 was slowly reduced after each cycle and its loss of 12 % was observed after 10 cycles. The loss in the adsorption capacity of DEA-PPS-50 is most likely due to the evaporation of DEA molecules during cycling. This result indicated that the vaporization temperature of impregnated amine has a significant effect on the performance of the resulting adsorbent. 140 o
DEA-PPS-50 (40/90 C)
o
PEI-PPS-50 (75/90 C)
CO2 adsorption capacity (mg/g)
120 100 80 60 40 20 0
2
4
6 Number of cycles
8
10
Figure 3. The adsorption/desorption performance of DEA-PPS-50 and PEI-PPS-50 4. Conclusion Various amines have been impregnated onto PPS to investigate their possibility for CO2 capture. The results indicated that amine impregnated PPS have low specific heat capacity compared to aqueous MEA 30 wt%. Adsorbents containing amine with low vaporization temperature (MEA and AMP) require higher energy for regeneration due to the contribution of high vaporization heat, while PEI impregnated PPS requires the least energy for regeneration (2080 kJ/kg of CO2). The adsorbent prepared from amine with higher vaporization temperature tends to have a more stable cyclic adsorption capacity. This study demonstrates that the solid adsorbent for CO2 capture can be easily produced by impregnating amine onto precipitated porous silica. 5. References [1] M.K. Mondal, H.K. Balsora, P. Varshney, Progress and trends in CO2 capture/separation technologies: A review, Energy, 46 (2012) 431-441. [2] Y. Artanto, J. Jansen, P. Pearson, T. Do, A. Cottrell, E. Meuleman, P. Feron, Performance of MEA and amineblends in the CSIRO PCC pilot plant at Loy Yang Power in Australia, Fuel, 101 (2012) 264-275. [3] M. Lucquiaud, O. Errey, H. Chalmers, X. Liang, J. Gibbins, M. Abu-Zahra, Techno-economic assessment of future-proofing coal plants with post combustion capture against technology development, Energy Procedia, 4 (2011) 1909-1916.. [4] 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, Industrial & Engineering Chemistry Research, 45 (2005) 2414-2420. [5] M.R.M. Abu-Zahra, J.P.M. Niederer, P.H.M. Feron, G.F. Versteeg, CO2 capture from power plants. Part II. A parametric study of the economical performance based on mono-ethanolamine, International journal of greenhouse gas control, 1 (2007) 135-142. [6] M.R. Abu-Zahra, E.S. Fernandez, E.L. Goetheer, Guidelines for process development and future cost reduction of CO2 post-cobmustion capture, Energy Procedia, 4 (2011)1051-1057.
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[7] Z.H. Lee, K.T. Lee, S. Bhatia, A.R. Mohamed, Post-combustion carbon dioxide capture: Evolution towards utilization of nanomaterials, Renewable and Sustainable Energy Reviews, 16 (2012) 2599-2609. [8] A. Samanta, A. Zhao, G.K.H. Shimizu, P. Sarkar, R. Gupta, Post-Combustion CO2 Capture Using Solid Sorbents: A Review, Industrial & Engineering Chemistry Research, 51 (2011) 1438-1463. [9] A. Heydari-Gorji, Y. Belmabkhout, A. Sayari, Polyethylenimine-Impregnated Mesoporous Silica: Effect of Amine Loading and Surface Alkyl Chains on CO2 Adsorption, Langmuir, 27 (2011) 12411-12416. [10] A. Sayari, Y. Belmabkhout, Stabilization of Amine-Containing CO2 Adsorbents: Dramatic Effect of Water Vapor, Journal of the American Chemical Society, 132 (2010) 6312-6314. [11] D.V. Quang, A.V. Rabindran, N. El Hadri, M.R. Abu-Zahra, Reduction in the reneration energy of CO2 capture process by impregnating amine solvent onto precipitated silica European Scientific Journal, 9 (2013). [12] D.V. Quang, A. Dindi, A.V. Rayer, N.E. Hadri, A. Abdulkadir, M.R. AbuϋZahra, Effect of moisture on the heat capacity and the regeneration heat required for CO2 capture process using PEI impregnated mesoporous precipitated silica, Greenhouse Gases: Science and Technology, (2014). [13] P.B. Sarawade, J.K. Kim, A. Hilonga, D.V. Quang, H.T. Kim, Effect of drying technique on the physicochemical properties of sodium silicate-based mesoporous precipitated silica, Applied Surface Science, 258 (2011) 955-961. [14] D.V. Quang, J.K. Kim, J.K. Park, S.H. Park, G. Elineema, P.B. Sarawade, H.T. Kim, Effect of the gelation on the properties of precipitated silica powder produced by acidizing sodium silicate solution at the pilot scale, Chemical Engineering Journal, (2012).