Desorption on Amine

Jun 25, 2014 - The activation energy Ea of CO2 adsorption/desorption was calculated from Arrhenius equation and used to evaluate the performance of th...
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Kinetics Studies of CO2 Adsorption/Desorption on AmineFunctionalized Multiwalled Carbon Nanotubes Qing Liu, Junjie Shi, Shudong Zheng, Mengna Tao, Yi He, and Yao Shi* Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: The adsorption/desorption kinetics of carbon dioxide on tetraethylenepentamine (TEPA) impregnated industrial grade multiwalled carbon nanotubes (IG-MWCNTs) was investigated to obtain insight into the underlying mechanisms on the fixed bed. After evaluating four kinetic models for CO2 adsorption at various adsorption temperatures, CO2 partial pressure, and amine loadings, it was found that Avrami’s fractional-order kinetic model provided the best fitting for the adsorption behavior of CO2. In order to find the optimal regeneration method, three desorption methods were evaluated for the regeneration of solid sorbents. The activation energy Ea of CO2 adsorption/desorption was calculated from Arrhenius equation and used to evaluate the performance of the adsorbent. The Ea decreased with increasing CO2 concentration, indicating that CO2 adsorption of amine-functionalized IG-MWCNTs is possibly intraparticle controlled. Meanwhile, because of the energy input of a vacuum pump, Ea for the vacuum swing regeneration method was less than that for temperature swing regeneration.

1. INTRODUCTION Carbon dioxide, as a greenhouse gas, is commonly considered responsible for climate change and global warming. Anthropogenic emissions stemming from predominantly existing point sources, such as fossil-fuel power plants and blast furnaces, have caused the sharp increase in CO2 concentration. Therefore, it is of great importance to capture CO2 from these plants.1 Amine scrubbing using aqueous amines is currently the most mature technology for CO2 capture;2 however, the approach is practically and economically challenged on account of the high volatility and the corrosiveness during the solvent regeneration process. Therefore, as an alternative approach, CO2 capture with solid sorbents has attracted much attention in recent years.3 Although there is a great amount of research on alternative methods for CO2 capture, the majority of these works focus on the development of adsorbents with high CO2 capacity and selectivity.4 An ideal solid sorbent for industrial application is expected to not only have large capacity and selectivity for CO2 capture but also high rates of CO2 adsorption/desorption5 and to be durable, low cost, and easy for manufacture. Fast adsorption/desorption kinetics for CO2 is essential under the operating conditions. The faster an adsorbent adsorbs/desorbs CO2, the more economical a capture process for a given volume of flue gas capture.6 However, there is still a lack of studies on the adsorption/ desorption kinetics of amine-functionalized sorbents. In our earlier attempt, we found that tetraethylenepentamine (TEPA) impregnated industrial grade multiwalled carbon nanotubes (IG-MWCNTs) exhibited high CO2 adsorption capacity, desirable selectivity, and recyclability.7 More importantly, compared with purified multiwalled carbon nanotubes, IG-MWCNT is a low-cost material which makes TEPAimpregnated IG-MWCNT a promising candidate for capturing CO2 from large point sources such as power plants. Additionally, IG-MWCNT has advantages such as high © 2014 American Chemical Society

chemical, thermal, and mechanical stability required to operate in realistic flue gas streams.8 In this work, we further investigated the adsorption/ desorption kinetics of the TEPA-impregnated IG-MWCNTs. Four rationalized kinetic models regarding adsorbent− adsorbate interactions were considered to examine the kinetics of CO2 adsorption onto amine-impregnated IG-MWCNTs at various adsorption temperatures, CO2 partial pressure, and amine loadings. In order to find the optimal regeneration method, three desorption methods were evaluated for the regeneration of solid sorbents. Avrami’s model was selected to describe desorption kinetics. The activation energy of CO2 adsorption/desorption was calculated from Arrhenius equation and used to evaluate the performance of the adsorbent.

2. EXPERIMENTAL DETAILS 2.1. Adsorption/Desorption Measurements. The solid sorbent for CO2 capture was prepared by impregnating IGMWCNTs in TEPA solution. These samples were denoted as IG-MWCNTs-n, where n represented the weight percentage of TEPA in the composites. Detailed protocols of the preparation were described in our previous study.7 The experiment for CO2 adsorption was carried out following a procedure reported by Liu et al.9 Adsorbents were treated under a nitrogen flow at 423 K for 90 min and then cooled to the test temperature. The flow was then switched to a desired simulated flue gas. The concentration of CO2 was measured by a gas chromatograph (GC). The adsorption capacity of CO2 on adsorbents at a given time is calculated by eq 1 Received: Revised: Accepted: Published: 11677

May 17, 2014 June 25, 2014 June 25, 2014 June 25, 2014 dx.doi.org/10.1021/ie502009n | Ind. Eng. Chem. Res. 2014, 53, 11677−11683

Industrial & Engineering Chemistry Research q=

1⎡ ⎢ M⎣

∫0

t

Q

c0 − c ⎤ T0 1 dt ⎥ 1 − c ⎦ T Vm

Article

∂qt

where q is the adsorption capacity of CO2 (mmol/g), M is the mass of adsorbent (g), Q is the gas flow rate (cm3/min) (Q was 50 cm3/min in this work), c0 and c are influent and effluent CO2 concentrations (vol %), t denotes time (min), T0 is 273 K, T is the gas temperature (K), and Vm is 22.4 mL/mmol. In this work, the adsorbent was regenerated using temperature swing adsorption (TSA)−N2-stripping, vacuum temperature swing adsorption (VTSA), and VTSA−N2-stripping. The adsorption column was kept at the desired desorption temperature, and the adsorbent that had adsorbed CO2 was fed into the column. For TSA−N2-stripping regeneration, the inlet of the adsorption column was switched to a nitrogen flow. The desorption CO2 concentration was measured by the GC every minute. The volume of pure CO2 was calculated according to the CO2 concentration and N2 flow rate. Therefore, the efficiency of CO2 desorption could be obtained at a given time. For VTSA regeneration, the outlet of the adsorption column was connected to a vacuum pump system, which can be operated at a given pressure. The mass of adsorbent was measured every 5 min. For VTSA−N2-stripping regeneration, the inlet of the adsorption column was switched to a nitrogen flow while the outlet of the column was connected to a vacuum pump system. The mass of adsorbent was also measured every 5 min. The adsorbent was considered completely regenerated when the mass of the regenerated adsorbent equaled the mass of the adsorbent before CO2 adsorption experiment. 2.2. Adsorption/Desorption Kinetic Models. CO2 adsorption/desorption kinetics of amine-impregnated IGMWCNTs are desirable to evaluate the performance of sorbents and to understand the overall mass transfer in the CO2 adsorption/desorption process.10 In addition, it is necessary to predict CO2 adsorption/desorption kinetics for the rational simulation and design of gas-treating units. To investigate the kinetics of CO2 adsorption onto amineimpregnated IG-MWCNTs, the following four models were considered: (i) Pseudo-first-order kinetic model: The Lagergen’s pseudo-first-order model was the first adsorption rate equation described for sorption of liquid/solid system and one of the most commonly used adsorption rate models. It is represented by eq 2:

(5)

2 e

(iii) Avrami’s fractional-order kinetic model: This model was recently developed based on Avrami’s kinetic model to simulate phase transition and crystal growth of materials.12 It has been applied to describe the adsorption of CO2 on amine-functionalized adsorbents.5 The general form of the model is as follows ∂qt

= kAnAt nA − 1(qe − qt ) (6) ∂t where kA is the Avrami kinetic constant and nA is the Avrami exponent. The Avrami exponent, nA, is a fractionary number, which reflects mechanism changes that may take place during the adsorption process.13 The term nA is the dimensionality of growth of adsorption sites: nA= 2 for one-dimensional growth, nA = 3 for twodimensional growth, and nA = 4 for three-dimensional growth.14 For a homogeneous adsorption in which the probability of the adsorption to occur is equal for any region for a given time interval, nA = 1.15,16 An Avrami exponent of exactly 2 indicates perfect one-dimensional growth from adsorption sites which form continuously and at a constant rate.14 The integrated form for the above-mentioned boundary conditions is nA

qt = qe(1 − e−(kAt ) )

(7)

(iv) Fractional-order kinetic model: Heydari-Gorji and Sayari17 recently proposed this general semiempirical kinetic equation to describe the adsorption rate of CO2 on amine-functionalized PE-MCM-41 adsorbents. In this mode, the adsorption rate is assumed to be proportional to the nth power of the driving force and mth power of the adsorption time. This kinetic model is represented by eq 8 as follows:

∂qt

= k1(qe − qt ) (2) ∂t where qe and qt are the adsorption capacity at equilibrium and at time t, respectively, and k1 is the pseudo-first-order adsorption rate constant. For the boundary conditions qt = 0 at t = 0 and qt = qe at t = ∞ the integrated form of eq 2 becomes qt = qe(1 − e−k1t )

= k 2(qe − qt )2

(4) ∂t where k2 is the pseudo-second-order adsorption rate constant. The integrated form for the above-mentioned boundary conditions is q qt = 1 e +t kq

(1)

∂qt

= knt m − 1(qe − qt )n (8) ∂t where kn, m, and n are the constants for the fractionalorder kinetic model. Since the fractional-order kinetic model represents the complexity of the reaction mechanism that may involve more than one reaction pathway,12,18 the parameter kn could be an overall parameter that may couple various adsorption related factors.10 Integrating eq 8 for the above-mentioned boundary conditions gives

(3)

(ii) Pseudo-second-order kinetics model: This model was put forward by Ho et al.11 It assumes that the adsorption capacity is proportional to the number of active sites occupied on the sorbent. It has been applied for analyzing chemisorption kinetics from liquid solutions.11 This model is expressed by eq 4:

qt = qe −

11678

1 ⎡ (n − 1)knt m + ⎢⎣ m

1 qe

n−1

⎤1/(n − 1) ⎥⎦

(9)

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3. RESULTS AND DISCUSSION 3.1. Adsorption Kinetics. Theoretically, the adsorption of CO2 on TEPA-impregnated IG-MWCNTs can be described with either one of the four models mentioned previous. However, not all models are equally suitable for this adsorption process. Figure 1 shows that the CO2 uptake of TEPA-

impregnated IG-MWCNTs at different adsorption temperature, CO2 concentration, and amine loading; as well as the curves generated by fitting with four models. Kinetic constants as well as squared correlation coefficient (R2) for regressions were listed in Supporting Information Table S1. Results show that Avrami’s fractional order kinetic model offers the best description for the adsorption behavior of CO2 on TEPAimpregnated IG-MWCNTs on the fixed bed. The fractional order of Avrami’s fractional-order kinetic model derives from the complexity of reaction mechanisms or occurrence of more than one reaction pathway according to Cestari et al.12,18 Therefore, Avrami’s fractional-order kinetic model was used to try to analyze the experimental CO2 uptake and account for CO2 adsorption mechanisms.19 As shown in Figure 1, both pseudo-first-order and pseudosecond-order kinetic models often overestimated the CO2 uptake at the initial and final phase of adsorption. Only for the CO2 adsorption on low surface coverage (IG-MWCNTs and IG-MWCNTs-10), the pseudo-first-order kinetic model was in agreement with the experimental data, which was consistent with other previous studies.20,21 Indeed, due to the low adsorption capacity of nonfunctionalized IG-MWCNTs at CO2 concentration of 10 vol %, the surface coverage can be considered low, even at equilibrium.5 According to the assumption of chemical adsorption, the pseudo-second order kinetic model can be used to simulate the experiment data of amine-functionalized adsorbents. However, as observed in Table S1, the value of k2 depends strongly on the initial concentration of CO2.22 In fact, k2 is not actually the kinetic constant for adsorption in a strict sense, but rather a complex function involving different parameters such as the kinetic constant for desorption, the surface coverage at equilibrium and the change of adsorbate concentration during the process.5 As presented in Table S1, adsorption rate constant kA decreased with increasing temperature. It is disadvantage for CO2 adsorption at high temperature due to the exothermic reaction of CO2 with TEPA. Even if the reaction rate becomes faster at higher temperature, less CO2 is adsorbed onto the active positions due to the faster desorption of CO2. The kA increased with increasing CO2 concentration, confirming high CO2 partial pressure facilitated CO2 diffusion into the reaction active sites. In addition, kA decreased with increasing amine loading because the pores blocked by excess amine led to slow diffusion of CO2 (Table S2). The parameters (cutting out the maximum and minimum in each group) averaged at various CO2 concentrations or temperatures were shown in Table S3 and S4, respectively. The Avrami exponent nA, determined to be between 1 and 2 in our experiments, suggests one-dimensional growth of the nuclei with a decreasing rate of adsorption.14,23 Thus, although the initial formation of adsorption sites may be homogeneous on a uniformly exposed surface, additional adsorption may be formed preferentially in the neighborhood of existing adsorption sites, leading to a deviation from homogeneous formation of adsorption sites and an Avrami coefficient larger than 1, as observed.14 The term nA is related to the existence of a different adsorption mechanism.24 The value of nA under 2 and 6 vol % concentration was less than that under the other concentrations. The lower CO2 partial pressure is not conducive to diffusion, and CO2 is mainly adsorbed on the active site of adsorbent surface. The same trend of nA also appeared when the amine loading was 0 and 10 wt % (Table S2), because of the lower proportion of chemical adsorption.

Figure 1. Comparison of kinetic models on experimental CO2 uptake of (a) IG-MWCNTs-50 at 20 vol % CO2 concentration at various adsorption temperatures, (b) IG-MWCNTs-50 at 313 K at various CO2 concentrations, and (c) various amine loadings at 313 K at 10 vol % CO2 concentration. 11679

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vol %. Obviously, the absolute value of Ea decreased with increasing CO2 concentration, indicating that high CO2 partial pressure facilitated CO2 adsorption. Actually, activation energy remained unchanged for the same reaction at the same reaction condition (temperature, catalyst), so the CO2 adsorption of IGMWCNTs-50 is possibly the intraparticle controlled. Moreover, negative activation energy was yielded due to the decrease of the reaction rate kA with increasing temperature for all CO2 concentrations, which was in accordance with the negative temperature dependence detected by Ebner et al.25 and Monazam et al.26 for amine-functionalized sorbents. The rare reactions with negative activation energy have been noted with high frequency in some specific biomolecular reactions.27,28 Reactions of negative activation energy are characteristically barrierless ones, wherein the reaction process depends on the capture of the molecules in a potential well.26 A probability of the colliding molecules capturing one another reduced in the case of increasing the temperature, expressed as a reaction cross-section that decreases with increasing temperature.26 However, positive activation energy of amine-functionalized adsorbents was also obtained by some researchers.5,29 Serna-Guerrero and Sayari5 reported TRI-PE-MCM-41 possessing an Ea of 1.38 kJ/mol for CO2 adsorption at 5% CO2 concentration. In addition, the Ea of CO2 adsorption was 11.08, 12.68, and 11.96 kJ/mol for MgAl N1, MgAl N2, and MgAl N3, respectively.29 The variation of activation energy was not only due to different adsorbent and adsorption condition but also adsorption apparatus. In the above-mentioned literature related to kinetic study, the CO2 uptake as a function of time was monitored using thermogravimetry analysis (TGA). According to gravimetric measurements in a microbalance studied by Barrie et al.,30 the influence of axial dispersion through the adsorbent layer did not affect significantly the experimental measurement when a shallow layer of adsorbent was combined with a sufficiently high adsorbate flow rate.5 The assumption of negligible axial dispersion was considered adequate for the TGA measurements. However, for the fixed bed of this work, influence of the axial diffusion cannot be ignored because CO2 mainly flow through the fixed bed in the axial direction. The value of Ea of adsorption is less than that of absorption, which is almost between 40−50 kJ/mol.31−35 Therefore, it is probable a promising CO2 capture method of amine-functionalized adsorbents due to the low activation energy of CO2 adsorption. 3.2. Desorption of CO2. 3.2.1. CO2 Desorption Methods. Depending upon the adsorbent properties, gas−solid adsorption operation may be carried out using isothermal regeneration modes (PSA, VSA, CSA, etc.), nonisothermal modes (TSA), and a combination of temperature and pressure gradient, i.e. PTSA, VTSA.36 In addition, stream stripping regeneration performed well in CO2 desorption and began to be applied increasingly. Therefore, N2-striping was separately combined with TSA and VTSA to regenerate IG-MWCNTs-50 adsorbed CO2 in this work. For TSA−N2-stripping regeneration, CO2 desorption concentrations at various temperatures (353, 363, 373, 383, 393, 403 K) are presented in Figure 3. When the desorption temperature was higher than 363 K, the maximum CO2 concentration appeared at the first minute and increased with increasing temperature. CO2 concentration declined after the maximum concentration at all temperatures.

Little variation of nA for various temperatures indicated that the adsorption mechanism did not seem different. The parameter n of the fractional-order kinetic model reflects the effect of the driving force which was related to the adsorption apparatus and filling mode of adsorbent in this work, while m refers to diffusion resistance.17 Although fixed bed and filling mode stayed the same, making the n not significantly changed, the parameter n increased slightly with increasing CO2 concentration. This may be involved that the large driving force generated from higher CO2 concentration facilitated CO2 diffusion into the adsorbent. With increasing temperature, n increased first and then decreased, and the maximum appeared at 323 K, indicating the large driving force at that temperature. The term m reflects how fast the adsorption process is.17 The process became faster according to the increase m with increasing CO2 concentration and did not change obviously at different temperatures. The temperature dependence of the kinetic constant kA may be described by the Arrhenius equation. kA = Ae−(Ea / RT )

(9)

where A is the Arrhenius pre-exponential factor, Ea is a term associated with the activation energy, R is the universal ideal gas constant, and T is the absolute temperature. The ln k − 1/T plot for CO2 adsorption based on the values of kA is shown in Figure 2. The values of A and Ea calculated through nonlinear regression are presented in Table 1.

Figure 2. Arrhenius plots for the kinetic constants obtained by the Avrami model of adsorption.

Table 1. Related Parameters Calculated from CO2 Adsorption Isotherms Fitted to the Arrhenius Equation CO2 concentration (vol %) 2 6 10 20 30 40 50

A (min−1) −4

1.48 × 10 3.60 × 10−4 0.004 21 0.010 19 0.017 65 0.047 91 0.074 19

Ea (kJ/mol)

R2

−13.197 −12.977 −7.353 −6.719 −5.880 −3.682 −3.017

0.999 0.999 0.994 0.992 0.988 0.997 0.983

Activation energy Ea of CO2 adsorption was calculated as −13.197, −12.978, −7.353, −6.719, −5.880, −3.682, and −3.017 kJ/mol for increasing CO2 concentrations from 2 to 50 11680

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Figure 3. CO2 desorption concentration at various temperatures (353, 363, 373, 383, 393, 403 K) using the TSA−N2-stripping regeneration method.

Figure 4 depicts the results of the regeneration experiment, TSA−N2-stripping, VTSA, and VTSA−N2-stripping. Overall, the regeneration time reduced with increasing temperature. The adsorbent could not be completely regenerated when the temperature was lower than 373 K. In other words, temperature is the dominant factor for amine-functioned adsorbent regeneration due to chemical interactions between CO2 and adsorbent. For TSA−N2-stripping, the regeneration time is longer than that of the other two methods at the same temperature. An efficient way to reduce the regeneration time is to increase the desorption temperature. Compared with VTSA regeneration, VTSA−N2-stripping failed to significantly reduce the regeneration temperature or time. 3.2.2. CO2 Desorption Kinetics. To have a further understanding of the process of the above-mentioned CO2 desorption methods, some desorption kinetic model should be applied. Considering that desorption of CO2 is the decomposition of carbamate/bicarbonate37 and Avrami’s fractionalorder kinetic model can describe the adsorption process well, it is possible to reproduce the majority of the decomposition processes by the Avrami model.38 y = 1 − exp( −(kAt )nA )

(10)

where y represents the desorption partion. As shown in Figure 4 and Table S5, three desorption methods were fitted to the model well. The Avrami kinetic constant kA of VTSA and VTSA−N2stripping was greater than that of TSA−N2-stripping at the same desorption temperature, suggesting the faster desorption rate, which was consistent with the desorption experiments. For TSA−N2-stripping at various temperatures, the value of nA obtained in the range of 0.678−0.935 confirmed that there were different desorption mechanisms, which might be associated with the desorption of physical desorption first and then chemical desorption. However, the value of nA of VTSA and VTSA−N2-stripping was much great different with temperature, which can be explained as follows. Physical desorption is the main desorption manner at low desorption temperature. The small amount of physical desorption make CO2 spread out easily even without a vacuum pump. Moreover, the large amount of chemical desorption at high temperature

Figure 4. Results of the regeneration experiment. (a) TSA−N2stripping regeneration, a nitrogen flow of 50 cm3/min. (b) VTSA regeneration, a pressure of 10 KPa. (c) VTSA−N2-stripping regeneration, a nitrogen flow of 50 cm3/min, a pressure of 10 KPa.

hardly diffused out of the structure for TSA−N2-stripping, while CO2 can quickly spread out due to the application of the vacuum pump. The temperature dependence of the kinetic constant kA may also be described by the Arrhenius equation. The ln k − 1/T plot for CO2 desorption based on the values of kA is shown in Figure 5. The values of A and Ea calculated through nonlinear regression are presented in Table 2. 11681

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pump. Ea of VTSA and VTSA−N2-stripping has a comparable value, indicating that N2-stripping failed to reduce activation energy of desorption.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed values of the kinetic constants, the characteristic parameters, and the associated coefficient of determinations (R2) in the adsorption/desorption process. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

Figure 5. Arrhenius plots for the kinetic constants obtained by the Avrami model of desorption.

ACKNOWLEDGMENTS We would like to thank the Zhejiang Provincial Natural Science Foundation of China (Grant No. LZ12E08002), the Fundamental Research Funds for the Central Universities (Grant No. 2013QNA4030), and the Scientific Research Fund of Zhejiang Provincial Education Department (Grant No. Y201329422) for financial support.

Table 2. Related Parameters Calculated from CO2 Desorption Isotherms Fitted to the Arrhenius Equation N2-striping TSA- N2-striping TVSA- N2-striping

A (min−1)

Ea (kJ/mol)

R2

341680 89774.4 74779.8

46.930 41.736 41.265

0.992 0.999 0.994

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REFERENCES

(1) Audus, H. Greenhouse gas mitigation technology: An overview of the CO2 capture and sequestration studies and further activities of the IEA Greenhouse Gas R&D Programme. Energy 1997, 22, 217. (2) Rochelle, G. T. Amine Scrubbing for CO2 Capture. Science 2009, 325, 1652. (3) Liu, Q.; Ning, L. Q.; Zheng, S. D.; Tao, M. N.; Shi, Y.; He, Y. Adsorption of Carbon Dioxide by MIL-101(Cr): Regeneration Conditions and Influence of Flue Gas Contaminants. Sci. Rep. 2013, 3, No. 2916, DOI: 10.1038/srep02916. (4) Bollini, P.; Didas, S. A.; Jones, C. W. Amine-oxide hybrid materials for acid gas separations. J. Mater. Chem. 2011, 21, 15100. (5) Serna-Guerrero, R.; Sayari, A. Modeling adsorption of CO2 on amine-functionalized mesoporous silica. 2: Kinetics and breakthrough curves. Chem. Eng. J. 2010, 161, 182. (6) Samanta, A.; Zhao, A.; Shimizu, G. K. H.; Sarkar, P.; Gupta, R. Post-Combustion CO2 Capture Using Solid Sorbents: A Review. Ind. Eng. Chem. Res. 2012, 51, 1438. (7) Liu, Q.; Shi, Y.; Zheng, S. D.; Ning, L. Q.; Ye, Q.; Tao, M. N.; He, Y. Amine-functionalized low-cost industrial grade multi-walled carbon nanotubes for the capture of carbon dioxide. J. Energy Chem. 2014, 23, 111. (8) Jin, Y. G.; Hawkins, S. C.; Huynh, C. P.; Su, S. Carbon nanotube modified carbon composite monoliths as superior adsorbents for carbon dioxide capture. Energ Environ. Sci. 2013, 6, 2591. (9) Liu, Y. M.; Ye, Q.; Shen, M.; Shi, J. J.; Chen, J.; Pan, H.; Shi, Y. Carbon Dioxide Capture by Functionalized Solid Amine Sorbents with Simulated Flue Gas Conditions. Environ. Sci. Technol. 2011, 45, 5710. (10) Zhao, A.; Samanta, A.; Sarkar, P.; Gupta, R. Carbon Dioxide Adsorption on Amine-Impregnated Mesoporous SBA-15 Sorbents: Experimental and Kinetics Study. Ind. Eng. Chem. Res. 2013, 52, 6480. (11) Ho, Y. S.; McKay, G. Pseudo-second order model for sorption processes. Process Biochem. 1999, 34, 451. (12) Lopes, E. C. N.; dos Anjos, F. S. C.; Vieira, E. F. S.; Cestari, A. R. An alternative Avrami equation to evaluate kinetic parameters of the interaction of Hg(11) with thin chitosan membranes. J. Colloid Interface Sci. 2003, 263, 542. (13) de Menezes, E. W.; Lima, E. C.; Royer, B.; de Souza, F. E.; dos Santos, B. D.; Gregorio, J. R.; Costa, T. M. H.; Gushikem, Y.; Benvenutti, E. V. Ionic silica based hybrid material containing the

Ea of CO2 desorption was calculated as 46.931, 41.736, and 41.265 kJ/mol for TSA−N2-stripping, VTSA, and VTSA−N2stripping, respectively. The value of Ea for VTSA and VTSA− N2-stripping was less than that for TSA−N2-stripping, which was due to the energy input from vacuum pump. Ea of VTSA and VTSA−N2-stripping has a comparable value, indicating that N2-stripping failed to reduce activation energy of desorption. The value of Ea calculated in this work was less than that in the work of Sun et al., where Ea of CO2 desorption from monoethanolamine (MEA)/macroporous TiO2 was 80.79 kJ/ mol,37 which may result from the different adsorbents and desorption conditions. It can be observed that the activation energy Ea of desorption is greater than that of adsorption. As for the same reaction under same conditions, the Ea of adverse reaction is still larger than that of positive reaction.39

4. CONCLUSION The work presented a kinetic analysis of CO2 adsorption onto TEPA-impregnated IG-MWCNTs under various conditions, as well as desorption methods of CO2 and relevant desorption kinetics. Among four kinetic models, Avrami’s fractional-order kinetic model was found to be the most suitable to describe the CO2 adsorption behavior at various adsorption temperatures, CO2 partial pressure, and amine loadings. The decrease of the absolute value of Ea (calculated from Arrhenius equation) with increasing CO2 concentration indicated that high CO2 partial pressure of CO2 facilitated its adsorption; hence, the CO2 adsorption of IG-MWCNTs-50 is possibly intraparticle controlled. Through the analysis of three regeneration methods, temperature was found to be the dominant factor for aminefunctionalized adsorbent regeneration because of chemical interactions between CO2 and adsorbent. The value of Ea for VTSA and VTSA−N2-stripping was less than that for TSA−N2stripping, which was due to the energy input from the vacuum 11682

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dx.doi.org/10.1021/ie502009n | Ind. Eng. Chem. Res. 2014, 53, 11677−11683