Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 61 (2014) 2849 – 2853
The 6th International Conference on Applied Energy – ICAE2014
CO2 capture using absorbents of mixed ionic and amine solutions Jie Yanga, Xinhai Yua,b,*, Jinyue Yanc,d, Shan-Tung Tua, Maogen Xua a
Key Laboratory of Pressure Systems and Safety, Ministry of Education, School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai, China b State Key Laboratory of Bioreactor Engineering, East China University of Science and Technology, Shanghai, China c School of Sustainable Development of Society and Technology, Mälardalen University, Västerås, Sweden d School of Chemical Science and Engineering, Royal Institute of Technology, Stockholm, Sweden
Abstract Studies have been conducted to find new absorbents for post-combustion CO 2 capture. To overcome the ILs’ limits and meanwhile take their advantages, the mixed amine + IL + H 2O solutions as CO 2 capture absorbent were investigated in a CO2 absorption/desorption loop setup. It was found that with an increase in IL concentration, the viscosity of the mixed solution rose while the energy required for absorbent regeneration decreased. In addition, no IL loss was detected and the amine loss per ton of captured CO 2 was considerably lower than that of aqueous amine solution. The viscosity of the best candidate of 30 wt% M EA + 40 wt% [bmim][BF 4] + 30 wt% H 2O is close to the value of aqueous amine solution, indicating that the ionic liquid disadvantage of high viscosity can be overcome for absorbent delivery of CO 2 capture. ©©2014 Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 2014The The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the Organizing Committee of ICAE2014
Keywords: CO2 capture; amine; ionic liquid; absorbent; energy consumption
1. Introduction Roo m-temperature ionic liquids (ILs) have been proposed as a potential candidate for CO2 capture in the last few years [1]. In order to overcome some of the limitations of ILs as CO 2 capture media, mean while, still taking the advantageous of their existing physical properties, mixed IL -amine solutions are employed to be co-capture agents[2-4]. Considering intractable tars due to amine/IL solutions corresponding CO2 adducts, absorbents of amine + IL + H2 O systems were proposed by Ahmady et al. [5]. It suggested that amine + IL + H2 O system might be an attractive CO2 capture med ia with the advantages
* Corresponding author. Tel.: +86-021-6425-3513; fax: +86-021-6425-3513. E-mail address:
[email protected]. Xinhai Yu
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 ICAE2014 doi:10.1016/j.egypro.2014.12.321
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of lo w v iscosity and high CO2 absorption rate. However, it should be necessary and interesting to investigate the performance of an absorption-desorption loop using amine + IL + H2 O system as CO2 capture med ia. Herein, the CO2 capture performances of the amine + IL + H2 O system in continuous CO2 absorption and desorption were investigated in this study. The viscosities and regeneration energy of absorbents of MEA + [b mim][BF4 ] + H2 O with different co mpositions were analysed. Considering the CO2 absorption rate and the solubility of the CO2 -carbamate salts in IL, four absorbents of M EA / TEA + [hmim][Tf2 N] / [b mim][BF4 ] + H2 O with different concentrations were tested in an absorption-desorption loop system with a continuous operation of 15 days. 2. Experimental The specifications of the analysis methods and the set-up absorption-desorption loop system for CO2 capture in this study the system were described in our previous work [6]. Four campaigns of amine + IL + H2 O absorbent solutions were conducted to study the CO 2 capture performances of absorbents with different concentrations. The exact operating parameters were presented in Table 1. The feed gases of four campaigns are in the composition range of flue gas from coal-fired power plant. T able 1. Experimental conditions.
Items
MEA wt%
T EA wt%
[hmim][Tf2N] wt%
[bmim][BF4 ] wt%
H2 O wt%
CO2 vol%
Campaign 1 Campaign 2 Campaign 3 Campaign 4
30 0 5 30
0 5 0 0
70 0 0 0
0 90 90 40
0 5 5 30
12.5 12.5 12.5 12.5
Flue gas flow rate Nm 3 h -1 0.01 0.01 0.01 0.01
Solution flow rate mL min -1 1 5 5 1
Operating time h 360 360 360 360
3. Results and discussion 3.1. Solvent loss With the absorption desorption loop system running, there was no obvious loss of [hmim][Tf 2 N] or [bmim][BF4 ] detected in four campaigns. It is contributed by the low saturated vapor pressure and excellent heat stability of IL [7]. During CO2 absorption by amine, M EA / TEA losses mainly caused by evaporation and degradation. The total M EA / TEA losses were plotted as a function of operation time as shown in Fig. 1. (a). The M EA loss in Campaigns 1 increased dramatically with the system running . Carbonate formed by MEA reacting with CO2 is not soluble in [h mim][Tf2 N] at the absorber operation temperature o f 50°C. The carbonate precipitated fro m M EA + [h mim][Tf 2 N] solution is too thick to flow in the CO2 capture system. As shown in Fig. 1. (a), TEA and M EA losses increased with the system running. In Campaign 4, fo r 30 wt% M EA + 40 wt% [b mim][BF4 ] + 30 wt% H2 O, M EA loss was 1.16 per ton of captured CO2 , respectively. The saturated vapor pressure of the mixed absorbent decreased with the addition of [bmim][BF4 ] [8], resulting in low evaporation of absorbent. The addition of IL ([bmim][BF4 ]) to amine aqueous solution can reduce the absorbent loss. 3.2. CO2 capture In order to explore CO2 removal capacities of the mixed absorbents, the CO2 absorption efficiencies of four campaigns as a function of operation time were examined, as shown in Fig.2.(b). In Campaign 1, for 30 wt% M EA+70 wt% [h mim][Tf2 N], CO2 capture efficiency was above 90% during the first 8 days and
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then decreased to 18.6% at the end of operation. In Campaign 2, CO2 absorption efficiency decreased gradually and kept as around 50% over the whole operation time. The reaction rate of TEA with CO2 is relatively lower than that of MEA. Although the TEA loss was low as shown in Fig.1.(a), its CO2 capture efficiency decreased gradually. It was attributed that water loss in Campaign 2 was much more than TEA, and TEA removals CO2 by CO2 hydration. In Campaign 3, CO2 removal efficiency decreased along with the MEA loss. In Campaign 4, CO2 capture efficiency was the highest among all of the campaigns and kept above 90% within 15 days’ operation. 100
Solvents loss (%)
80
MEA in Campaign 1 TEA in Campaign 2 MEA in Campaign 3 MEA in Campaign 4
60 40 20
CO2 removal efficiency (%)
100
80
60
Campaign 1 Campaign 2 Campaign 3 Campaign 4
40
20
0 0
3
6
9
12
15
Operation time (day)
(a)
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
Operation time (day)
(b)
Fig. 1. (a) MEA/T EA losses as a function of operation time; (b) CO2 removal efficiencies as functions of operation time.
3.3. Viscosities and regeneration energies of the solutions The viscosities of 30 wt% M EA solutions with different concentrations of [bmim][BF 4 ] were measured as a function of te mperature (see Fig.2.(a)). The v iscosities of the solutions decreased gradually with an increase in temperature. Thus, high temperature benefits absorbent transportation with a low mechanical energy requirement. Simu lations were carried out to calculate th e thermal energy at the strippers for absorbents of M EA + [b mim][BF4 ] + H2 O with different co mpositions using the Aspen Plus (version 13.1). The electrolyte-NRTL model was used to describe the MEA -H2 O-CO2 system thermodynamically. The parameters for this calculations are based on experimental conditions, as shown in Table 1. Viscosity at absorber operation temperature of 50°C and regeneration energy required of 30 wt% M EA + [b mim][BF4 ] + H2 O solution as a function of [b mim][BF4 ] mass fraction was illustrated in Fig. 3. (b). W ith an increase in concentration of [b mim][BF4 ], the energy demand of the solution decreased and meanwhile the solution viscosity increased. As shown in Fig. 2. (b ), the [b mim][BF4 ] concentration of 40% was roughly chosen as an optimal value by weighing the thermal energy requirement in stripper and the viscosity at absorber operation temperature of 50°C. 4. Conclusion The mixed amine + IL + H2 O solutions as CO2 capture absorbent were investigated in a CO2 absorption / desorption loop setup. It was found that the 30 wt% M EA + 40 wt% [b mim][BF4 ] + 30 wt% H2 O is most likely the best candidate in this case considering the viscosity related to the mechanical energy for absorbent transportation and required energy for CO2 desorption. In addition, no IL loss was detected and the amine loss per ton of captured CO2 was considerably lower than that of aqueous amine solution. The viscosity of the best candidate of 30 wt% M EA + 40 wt% [b mim][BF4 ] + 30 wt% H2 O is close to the value of aqueous amine solution. Thus, the mixed IL-amine solution showed great potential in
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10
24 22
K (mPas)
16 14 12 10
9
8 8
7
K(mPs)
18
8
6
7
5 6 4
6
5
3
4 2
2 0
4
Thermal heat required (GJ/ton CO 2)
20
10
9
0 wt % 10 wt % 20 wt % 30 wt % 40 wt % 50 wt % 60 wt % 70 wt %
1
30
40
50
60
70
Temperature (qC)
(a)
80
90
0
10
20
30
40
50
60
70
w1 (wt%)
(b)
Fig. 2. (a) Viscosities of 30 wt% MEA + [bmim][BF4] + H2 O solution with different concentrations of [bmim][BF 4 ] balanced by H2 O as functions of temperature; (b) Viscosity and regeneration energy required of 30 wt% MEA + [bmim][BF 4 ] (1) + H2 O (2) at t=50°C as a function of [bmim][BF4 ] mass fraction w1
post-combustion CO2 capture with the advantages of low viscosity and energy required by CO 2 desorption. In addition, a key feature of the mixed IL-amine solution different fro m conventional aqueous amine solution is that the viscosity of the mixed IL-amine solution is sensitive to temperature. So controlling the temperature of the mixed IL-amine absorbent is important during the operation of CO 2 capture system. Acknowledgements This study was financially supported by the China Natural Science Foundation (Contract No. 21176069), Program for New Century Excellent Talents in Un iversity (NCET-10-0380) and the Fundamental Research Funds for the Central Universities (WG1213011). References [1] MacDowell N, Florin N, Buchard A, Hallett J, Galindo A, Jackson G, et al. An overview of CO2 capture technologies. Energy Environ Sci. 2010;3:1645-69. [2] Camper D, Bara JE, Gin DL, Noble RD. Room-temperature ionic liquid− amine solutions: T unable solvents for efficient and reversible capture of CO2 . Ind Eng Chem Res. 2008;47:8496-8. [3] Bara JE, Camper DE, Gin DL, Noble RD. Room-temperature ionic liquids and composite materials: platform technologies for CO2 capture. Acc Chem Res. 2009;43:152-9. [4] Shiflett MB, Yokozeki A. Solubility of CO2 in room temperature ionic liquid [hmim][Tf2 N]. J Phys Chem B. 2007;111:20704. [5] Ahmady A, Hashim MA, Aroua MK. Experimental investigation on the solubility and initial rate of absorption of CO2 in aqueous mixtures of methyldiethanolamine with the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate. J Chem Eng Data. 2010;55:5733-8. [6] Yang J, Yu X, Yan J, T u ST . CO2 capture using amine solution mixed with ionic liquid. Ind Eng Chem Res. 2014;53:2790-9. [7] Earle MJ, Esperanca JM, Gilea MA, Lopes JN, Rebelo LP, Magee JW, et al. The distillation and vo latility of ionic liquids. Nature. 2006;439:831-4. [8] Kim I, Svendsen HF, Børresen E. Ebulliometric determination of vapor− liquid equilibria for pure water, monoethanolamine, n-methyldiethanolamine, 3-(methylamino)-propylamine, and their binary and ternary solutions. J Chem Eng Data. 2008;53:2521-31.
Jie Yang et al. / Energy Procedia 61 (2014) 2849 – 2853
Biography Jie Yang is a Ph.D student in East China University of Science and Technology, under the supervision of Prof. Jerry Yan and Prof. Shan-Tung Tu. Jie Yang’s research interests are CO2 capture process intensification and energy efficient extraction of lipids from algae.
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