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The mechanism and kinetics study of carbon dioxide absorption into methyldiethanolamine /1hydroxyethyl-3-methylimidazolium lysine /water system Wei Li, Shujing Wen, Li Shen, Yuchi Zhang, Cheng Sun, and Sujing Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02612 • Publication Date (Web): 21 Sep 2018 Downloaded from http://pubs.acs.org on September 23, 2018
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Energy & Fuels
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The mechanism and kinetics study of carbon dioxide absorption into
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methyldiethanolamine /1-hydroxyethyl-3-methylimidazolium lysine /water
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system
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Wei Li, Shujing Wen, Li Shen, Yuchi Zhang, Cheng Sun, Sujing Li*
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Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Institute
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of Industrial Ecology and Environment, College of Chemical and Biological
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Engineering, Zhejiang University, Yuquan Campus, Hangzhou 310027, China
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Abstract：
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In this work, the aqueous solutions of 1-hydroxyethyl-3-methylimidazolium lysine
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([C2OHmim][Lys]) and methyldiethanolamine (MDEA) were developed to obtain an
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efficient absorbent for CO2 capture. The absorption performance, regeneration
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performance, reaction and kinetics mechanism of the blends were investigated. It was
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found the blended absorbents with molar ratio of 8:2 of MDEA to [C2OHmim][Lys]
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was the optimum ratio based on absorption rate, absorption capacity and other factors.
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The CO2 absorption capacity of the blends is 0.75 mol CO2 /mol IL and the absorption
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rate was higher than that of the aqueous MDEA solution. The regeneration efficiency of
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the blends was 93% after the third absorption-generation cycle, which was higher than
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the aqueous solutions of 1 mol/L MDEA (88%) or 1 mol/L [C2OHmim][Lys] (88%)
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under same conditions, indicating a better regeneration performance of the blends. The
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reaction mechanism and kinetics study of the CO2 absorption into the blends were
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investigated by 13C NMR and double stirred-cell absorber, respectively.
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[C2OHmim][Lys] in aqueous solution firstly formed carbamate with CO2, and the next
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step was hydrolysis of MDEA and partial carbamate, with the production of
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bicarbonate. It also found that the influence of temperature in the CO2 absorption was
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different in the two steps. Also, [C2OHmim][Lys] could promote the hydration reaction
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of MDEA and CO2 according to the chemical mechanism and kinetics study. The
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kinetics region was considered to be a fast pseudo-first order and the activation energy
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of the blends was 40.0 kJ mol-1.
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Keyword: CO2, ionic liquid, MDEA, aqueous solutions, mechanism, kinetics 1. Introduction Anthropogenic emissions primarily from Combustion of fossil fuels have rapidly
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raised the concentration of CO2, leading to global warming.[1, 2, 3] Researchers
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considered chemical capture of CO2 by solutions of alkane amine as the most
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economic and efficient technology for capturing CO2 from fuel-fired power plants.[4,
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5] However, the carbon dioxide capture and sequestration process (CCS), which use
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aqueous amine-based solutions, suffers from severe weakness such as low absorption
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capacity , low absorption rate, solvent loss in the regeneration step. [6, 7, 8, 9]
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Over the last years, CO2 absorption and separations processes using ionic liquids
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(ILs) has gotten lots of interest because of their prominent physicochemical properties,
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like negligible vapor pressure, easy assembly and tailorability. [1,7,8,9] Brennecke et
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al. first reported
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ILs under 9.3 MPa and 298.15 K.[10] Taking a clue from CO2 absorption by alkane
physisorption of CO2 by the [bmin][PF6] with 0.72 mol CO2/ mol
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amine like monoethanol- amine (MEA), Eleanor and his partners originally
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synthesized a functional ionic liquids(FILs), [NH2p-bim][BF4], and its absorption
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mole ratio almost reached 0.5 (1:2 mechanism, as same as organic amines by forming
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carbamate salts) .[11] Riisager found CO2 reaction mechanism between [N66614][Lys]
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and CO2 followed
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1:1 mechanism.[12]
Although achieved high absorption capacity by introducing amine group, the
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viscosity of ILs is much higher than commercially available absorbents. High
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viscosity of ILs is a tremendous trouble for the industrialization and
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commercialization of ILs. Wang et al. developed trihexyl (tetradecyl) – phosphonium
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sarcosine ([P66614][MeGly]), and its viscosity was 312 mPa at 303.15 K[13] , which is
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lower than other ILs[14, 15, 16]. Researchers found that solutions of ionic liquid,
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organic amine, and water could effectively decrease the viscosity of the blends and
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enhanced the absorption performance of carbon dioxide. Zhang et al. found that the
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aqueous solutions of FILs and MDEA could enhance CO2 absorption significantly.[17]
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Fu et al. also clarified that adding ILs like [N1111][Gly][15], [Bmin][Gly][18] and
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[Bmim][Lys][19] could enhance mass transfer. Their experiment revealed that the
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absorption rate and capacity could be significantly enhanced by the system of ILs and
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organic amine. Coutiho et al. measured viscosity of [iBu3MeP][TOS]/H2O system.
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The viscosity of pure [iBu3MeP][TOS] was 1320 cP, and dropped to 68 cP when the
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water content was 12.8 wt %, and the viscosity was only 2.6 cP which was closed to
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commercial available solvents (the viscosity of 5 M MEA was 1.013 cP at 303.15
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K[20]) at the water content of 75%[21]. Bermejo et al. found few water could
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improve the CO2 solubility of [Bmim][NO3][22]. Hu and his groups found that the
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CO2 loading of [TETA][NO3] (40 wt %) almost reached 1.5 mol CO2/ mol IL under
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atmospheric pressure. The reaction product was carbamate and cannot be hydrolyzed
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to form bicarbonate[23]. Hence, we conclude that the reaction mechanism of the
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carbon dioxide and ILs related solvents did not follow the 1:1 mechanism or 1:2
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mechanism.
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The absorption mechanism and mass transfer kinetics of CO2 and ILs-related
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solvents were widely investigated in the literature. Haider et al. selected [Bmim][BF4]
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and [Bmim][PF6] to blend with amine. The experiment results showed that the blend
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had higher carbon dioxide absorption capacity than neat ILs or solutions of alkane
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amine.[24] Wu and other scholars found that CO2 loading in the solutions of ILs
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decreased as the water concentration decreased.[25, 26] Nathalia et al. found that H2O
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in solutions of ILs reacted reversibly with CO2 to form bicarbonate. The CO2 loading
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of 0.1 wt % ILs was 1.9 mol bicarbonate/ mol ILs.[27] Jing and her collaborators
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reported the carbon dioxide absorption by a series of ILs/amine/water system.[28, 29,
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30, 31] The carbon dioxide loading of solutions of [Apmin][Gly] was 1.23 mol CO2/
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mol IL.[31] And the solutions of [Hmim][Gly])[32] or [N1111][Gly] and AMP [28, 29]
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were also used to absorb CO2 in their research. They found that ILs could enhance the
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regenerated efficiently and absorption kinetics.
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A new hydrophilic amino acid ionic liquid (1-hydroxyethyl-3-methy-
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limidazolium glycine ([C2OHmim][Gly])) with a high carbon dioxide loading (0.575
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mol CO2/ mol IL) was designed by our group.[33] Then our group successfully
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synthesized a dual amino-functionalized ILs (1-hydroxyethyl-3-methylimidazolium
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lysine ([C2OHmim][Lys])) exhibited a better absorption performance (1.2604 mol
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CO2/ mol IL).[34] And the reaction mechanism, CO2 absorption kinetics, and
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regenerative performance were also clarified.[35, 36, 37] The results proved the
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shuttle mechanism and interaction in the MDEA-[C2OHmim][Gly]-water system
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directly. The N-methyldiethanol- amine (MDEA) has some excellent characteristics,
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such as less degradation, corrosion and regeneration energy. Meanwhile, its fatal
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disadvantage is low absorption rate, which limits its application at low CO2 partial
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pressure.[38] For overcoming the shortcoming of the MDEA/H2O system further, we
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are proposed to blend MDEA and [C2OHmim][Lys] and investigate the proportion,
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reaction mechanism, kinetics and regeneration performance of MDEA/
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[C2OHmim][Lys]/water system in this study.
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2. Material and Methods
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2.1 Chemicals
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The chemicals we used were purchased from Aladdin Chemical Co. Ltd., Xiya
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Chemical Co. Ltd., Zhejiang Jin-gong Gas Co. and Dow Chemical Company. The
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purity of N-methyl imidazole, MEA and L-lysine were > 99%. The purity CO2 and N2
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were 99.999% purity. [C2OHmim][Lys] was synthesized by ourselves.[39]
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2.2 Experimental Methods
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The CO2 loading and reaction mechanism were measured in three-neck flask at
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50 °C. The volume flow of carbon dioxide (15% (v/v) and nitrogen (85% (v/v) was
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7.5 mL/min and 42.5 mL/min. The chromatography (7890, Agilent, USA) was used
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to measure the concentration of CO2. And the absorption process finished when the
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CO2 concentration rose to 15%. The absorption capacity was measured by acid
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hydrolysis method. The pH and CO2 loading were measured and the aqueous
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solutions were investigated by 13C NMR all through the absorption experiment. [39,
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40]
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The absorption rate was measured in a double stirred-cell absorber with a
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well-defined surface area of 28.27 cm2. [35, 41] The model of densimeter and
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viscometer was DMA-5000M (Anton Paar, ± 5.0×10-6 g·cm-3.) and DV-II+Pro
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(Brookfield, ±0.5%). And the desorption temperature of rich solvent was 120 °C. CO2
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from desorption was dried by concentrated H2SO4 and soap-film flowmeter was used
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to measure the volume flow of CO2. Meanwhile, the CO2 loading of lean solutions
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were also measured by acid hydrolysis method to study regenerative performance.
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2.3 Theoretical analysis
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2.3.1 The solubility and diffusion coefficient. The solubility coefficient (HCO2,
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solvents)of
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according to Jing et al..[28]
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CO2 in the solutions of MDEA and [C2OHmim][Lys] could be calculated
H CO2 ,mix = 509.77CIL−R1NH2 + H CO2 ,amine
(1)
And the H CO2 ,amine was obtained by N2O analogy.[42]
H CO2 ,amine = H N 2O ,amine ×
H CO2 ,water
(2)
H N 2O ,water
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H N2O ,amine = (5.52 + 0.7C ) ×106 × exp(
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H CO2 ,water = 2.8249×106 exp(
− 2166 ) T
− 2044 ) T
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(3)
(4)
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H N2O ,water = 8.5470×106 exp(
− 2284 ) T
(5)
The diffusion coefficient of CO2 in the solutions of MDEA and
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[C2OHmim][Lys] could be estimated by the revised Stoke- Einstein equation and
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N2O analogy method as follows[37, 43]:
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0.8 0.8 DN2O, solvent × µsolvent = DN2O , water × µwater
DCO2 ,amine = DN 2O ,amine ×
(6)
DCO2 ,water
(7)
DN 2O ,water
DCO2 ,water = 2.35 × 10−6 × exp( DN2O,water = 5.07 ×10−6 × exp(
− 2119 ) T
(8)
− 2371 ) T
(9)
2.3.2 Mass transfer. The mass transfer rate equation of carbon dioxide chemical absorption [44]:
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N = kG ( PCO2 − PCO2i )
(10)
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N = kL' (CCO2 ,i − CCO2 )
(11)
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kL ' = EkL
(12)
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N represented the flux of carbon dioxide. PCO and PCO ,i were the partial pressure 2
2
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of carbon dioxide.
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were liquid-film mass transfer coefficient without and with chemical reaction. E
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represented the enhancement factor.
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150
kg
represented the gas-film mass transfer coefficient. k L and
According to our study, the value of
k L'
H CO2 1 could be ignored comparing to , EkL kG
and N could be calculated as follows[35, 37]:
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PCO2 PCO2 = Ek L H CO2 H CO2 Ek L
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(13)
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N=
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The enhance factor (E) could be calculated with the Hatta number (Ha) by
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equation 14~15.[39] When 3>Ha>0.02, the reaction was medium-speed. When 3
