desorption kinetics of amine-functionalized KIT-6

Applied Energy 211 (2018) 1080–1088

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Carbon dioxide adsorption properties and adsorption/desorption kinetics of amine-functionalized KIT-6

T

⁎

Yamin Liu , Xiaojing Yu School of Ecological Environment and Urban Construction, Fujian University of Technology, Fuzhou 350118, Fujian, PR China

H I G H L I G H T S adsorption in 10% CO (90% helium) is in the flue gas temperature range (323–343 K). • Maximum process of CO adsorption by KP-50 is controlled by intra-particle diffusion process. • The fractional-order dynamics model provides the best fit for isothermal CO adsorption data. • Avrami's • Temperature is the dominant factor in the regeneration of the amine functional adsorbent. 2

2

2

A R T I C L E I N F O

A B S T R A C T

Keywords: Carbon dioxide Adsorption PEHA KIT-6 Kinetics

An amine-functionalized adsorbent was prepared by loading pentaethylenehexamine (PEHA) into the pores of KIT-6 mesoporous silica. Nitrogen adsorption/desorption, thermogravimetric analysis, and X-ray powder diffraction were employed to analyze the structural properties of the prepared adsorbents. The results reveal that the pore size, pore volume, and surface area of the adsorbents decrease after PEHA loading in KIT-6, while the basic KIT-6 pore structure remains unchanged. The CO2 adsorption performance of amine-functionalized KIT-6 was studied by isothermal CO2 adsorption, adsorption/desorption cycle regeneration testing, and multiple cycle tests. The amount of CO2 absorbed increased with increasing temperature in the temperature range of 303–343 K, achieving a maximum adsorption capacity of 3.2 mmol/g-adsorbent at 343 K. For temperatures greater than 343 K, the adsorption capacity decreased with increasing temperature. The amount of CO2 adsorbed remained nearly constant after 10 adsorption/desorption cycles. The CO2 adsorption/desorption kinetics of the PEHA-impregnated KIT-6 were investigated using three kinetics models. The results indicate that the CO2 adsorption process by the amine-functionalized adsorbent is dominated by intra-particle diffusion, and the adsorption rate is restricted by the intra-particle diffusion process. In addition, high CO2 partial pressure facilitates the adsorption of CO2. The CO2 adsorption breakthrough curves obtained by the deactivation model were in good agreement with the results of this study. The adsorbents were regenerated by vacuum and temperature swing adsorption regeneration. The results indicate that increasing the desorption temperature is an effective means of reducing the regeneration time. The absolute value of the activation energy obtained from the Arrhenius equation for CO2 desorption was 81.992 kJ·mol−1, which was greater than that obtained for adsorption.

1. Introduction Three approaches are primarily employed to capture CO2, i.e., precombustion capture, post-combustion capture, and oxy-fuel combustion [1]. Post-combustion capture is cost-competitive compared to the other two technologies, but the development of cost-effective capture processes represents a technical challenge because of the low CO2 concentrations involved, which are typically less than 15% [2]. Three main techniques are employed for post-combustion capture technology,

⁎

namely liquid ammonia absorption, porous materials adsorption, and membrane purification. Adsorbent-based separation technologies have attracted particular interest in recent years because of their potential to satisfy the requirements of energy-efficient and cost effective CO2 remediation [3]. The energy efficiency provided by these technologies is of particular importance. For example, the energy required by a solid adsorbent to absorb 3 mmol/g-adsorbent of CO2 can be reduced by at least 30–50% compared with that required by the best monoethanolamine aqueous solution (MEA) absorption process [4]. Thus, the

Corresponding author. E-mail address: [email protected] (Y. Liu).

https://doi.org/10.1016/j.apenergy.2017.12.016 Received 29 August 2017; Received in revised form 21 October 2017; Accepted 1 December 2017 0306-2619/ © 2017 Elsevier Ltd. All rights reserved.

Applied Energy 211 (2018) 1080–1088

Y. Liu, X. Yu

CO2

MFC1

He

MFC2

Mixture

development of solid adsorbents with high adsorption capacity, fast adsorption/desorption rates, strong adaptability, and good stability is necessary to meet these requirements [5]. In response to this need, numerous research groups around the world have been working in recent years to the develop new solid adsorbents with superior properties and the desired economies for the capture of CO2 from flue gases [6,7]. Among these new solid adsorbents, amine-functionalized porous materials have received considerable attention [8–12]. Two main routes are typically employed to prepare amine-functionalized adsorbents for CO2 capture: (i) impregnating amines on a porous structure [13]; (ii) grafting amine functional groups onto a substrate surface [14]. The impregnation method has some advantages over the grafting method, such as convenient preparation and the improved CO2 adsorption capacity of the resulting adsorbent. Therefore, the impregnation method has been extensively studied recently. The adsorbent is usually prepared by loading amine compounds, such as polyethylenimine (PEI), diethylenetriamine (DETA), triethylenetetramine (TETA), 2-amino-2-methyl-1-propanal, diethyleneamine (DEA), and tetraethylenepentamine (TEPA), on porous supports, including metal-organic framework (MOF) materials [15], carbon nanotube (CNT) materials [8], and mesoporous silica materials [16] (such as MCM series, SBA series, and KIT series). Among these materials, KIT-6 has been proven to be an effective amines support for CO2 capture because of its three-dimensional (3D) pore structure, large and uniform pores, high surface area, and a large number of highly dispersed active sites (hydroxyl groups) on the pore walls and surfaces. Although a great deal of research has been directed toward the development of various amine-functional adsorbents, most work has focused on the preparation of adsorbents, the CO2 adsorption capacity and selectivity of adsorbents, and the stability of adsorbents. However, in addition to a high CO2 adsorption capacity and selectivity, the desired adsorbents for industrial application should have a high CO2 adsorption/desorption rate [17]. At the same time, understanding adsorption/desorption kinetics is very useful for optimizing the design and operation of gas treatment devices because these studies can help elucidate the dynamic adsorption behavior of adsorbents and clarify the CO2 mass transfer process during CO2 adsorption/desorption processes. However, existing detailed research regarding the CO2 adsorption/ desorption kinetics of amine-functional adsorbents is relatively inadequate. Some studies [8,15,18,19] have only discussed the adsorption kinetics, and no systematic kinetic study, such as breakthrough curve and desorption kinetics analyses, hasn't yet been conducted for the entire dynamic adsorption/desorption process under actual flue gas conditions (323–343 K). The present work investigates the adsorption/desorption kinetics of amine-functionalized adsorbent capture of CO2. In this work, aminefunctionalized adsorbent was prepared by loading pentaethylenehexamine (PEHA) into the pores of KIT-6. We selected KIT-6 as a support due to its 3D pore structure and its good accessibility for modification. We employed PEHA as a modifier due to its low cost, low toxicity, and the presence of large amounts of amine groups that can interact with CO2. The structural characteristics and adsorptive properties of the adsorbents were studied. Mathematical models have also been applied to fitting the isothermal CO2 adsorption/desorption data for investigating the CO2 adsorption/desorption kinetics of the resulting adsorbents.

GC

TC

TBV

TBV

Packed bed column MFC: Mass Spectrometer GC: Gas Chromatograph

TBV: Three way ball valve TC: Temperature Control

Fig. 1. Schematic of the dynamic CO2 adsorption experimental system.

material is herein denoted as KP-50. Further details regarding sample preparation, as well as details regarding sample characterization using X-ray powder diffraction (XRD), N2 adsorption/desorption, and thermogravimetric analysis (TGA), can be found in our previous report [20].

2.2. Adsorption experiments The adsorption performance of KP-50 was studied by means of the following two adsorption experiments. All adsorption experiments were conducted under atmospheric pressure. ① The static CO2 adsorption capacity was investigated at various temperatures and CO2 concentrations according to the experimental method reported by Wei et al. [21], and the CO2 adsorption isotherms were obtained. The inlet gas was a mixture of a predetermined composition of CO2 (99.99%) and N2 (99.99%) with a CO2 mole fraction of 10%. ② A CO2 adsorption column was employed to obtain the dynamic CO2 adsorption breakthrough curve. The adsorption test system is illustrated in Fig. 1. A detailed description of the experimental setup can be found in our previous report [20]. The amount of CO2 adsorbed by the adsorbent at a given time t (min) can be calculated as follows:

qa =

1 ×⎡ M ⎣

∫0

t

Q×

C0−C T 1 dt⎤ × 0 × , 1−C ⎦ T Vm

(1)

where qa is the amount of CO2 adsorbed per mass of adsorbent M (mmol/g-absorbent), Q represents the gas flow rate (cm3·min−1), C0 and C are the CO2 concentrations of the inlet gas and outlet gas, respectively (mmol·min−1 or vol%), T represents the gas temperature (K), T0 is 273 K, and Vm is the molar volume (22.4 cm3·min−1).

2.3. Desorption experiments The adsorbents were regenerated using vacuum and temperature swing adsorption (VTSA) [22]. The adsorbent mass was weighed prior to conducting the adsorption process. After the adsorption process attained equilibrium, the inlet valve was closed, and then the mass of the adsorbent was quickly measured. The adsorption column was then rapidly heated to the desired temperature, while the pressure was controlled to the desired value by a vacuum pump connected to the outlet of the adsorption column. The mass of the adsorbent was thereafter weighed every 2 min. The adsorbent was completely regenerated once the mass of the regenerated adsorbent was equal to the initial mass of the pure adsorbent, and the total mass loss was equivalent to the amount of desorbed CO2. In this work, the desorption tests were conducted at different temperatures at a pressure of 5 kPa.

2. Materials and experiments 2.1. Materials and characterization The solid PEHA-loaded KIT-6 adsorbent was produced by a wet impregnation method. Here, 1 g PEHA was dissolved in 50 ml ethanol (AR, Fuzhou Chem. Reagent Co.) at room temperature, after which 1 g KIT-6 was added. After stirring at reflux for 4 h, the mixture was evaporated at 353 K and then dried in air at 373 K for 1 h. The resultant 1081

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two-dimensional growth, and so on [32]. For uniform adsorption, i.e., where the probabilities of adsorption in any zone within a fixed time interval are equal, n = 1 [28,33]. If n = 2, the adsorption process, beginning from the adsorption point on the adsorbent surface, is a perfect one-dimensional growth that is continuously formed and has a constant rate [32]. The fractional order of this model results from the complication of the reaction mechanism or the simultaneous occurrence of multiple reaction paths [29,34]. Thus, Avrami's fractional-order model is well suited for analyzing adsorption kinetics data, and for explaining the mechanism of CO2 adsorption [35]. The integrated form of Eq. (6) is

2.4. Adsorption kinetic models Several adsorption kinetics models have been developed to quantitatively analyze the adsorption process and explore the adsorption mechanism. A full treatment of models addressing adsorption kinetics can be obtained elsewhere [23–25]. The experimental adsorption/ desorption data obtained were fitted using a variety of commonly employed models, which is a standard method for obtaining the best fitting model. In this paper, three models were employed to study the CO2 adsorption kinetics on KP-50 materials: Lagergren’s pseudo-first-order model (Eq. (3)), Ho’s pseudo-second-order model (Eq. (5)), and Avrami’s fractional-order kinetic model (Eq. (7)). These models were selected because they consider that the adsorption rate is limited by the surface reaction kinetics, which is more suitable for the CO2 adsorption process of an amine-modified material.

n

qt = qe [1−e−(ka t ) ].

2.5. Breakthrough analysis model The adsorption of CO2 on KP-50 may be interpreted as chemisorption or a gas-solid non-catalytic reaction. During the adsorption process, the formation of the chemical product layer on the surface of the pores may inhibit CO2 contact with the KP-50 surface active sites, and create an additional diffusion resistance. In addition, the formed chemical product layer may significantly alter the pore structure, the active surface area, and the degree of reaction per unit area of the solid reactant. All of these issues will result in a reduction in the activity of the adsorbent as the amount of absorbed chemical product increases. The deactivation model proposed by Yasyerli et al. [36], which has been found to be effective for gas-solid reactions, can be employed to investigate these issues. The model employs a deactivation rate term to illustrate the effect of all factors on the diminishing rate of a solid reactant. It is believed that the rate of activity reduction in a solid reactant is proportional to the activity itself and the concentration of the adsorbed gas. According to the pseudo-steady-state supposition with the axial diffusion in the adsorption column disregarded, the isothermal species conservation equation of the CO2 concentration in the adsorption column is

① Lagergen’s pseudo-first-order adsorption kinetics model, which is also denoted as the linear driving force (LDF) model, describes the adsorption process based on adsorption capacities, and is one of the most commonly used adsorption rate models. It is described as follows:

dqt = k1 (qe−qt ), dt

(2) −1

where qe and qt (mmol·g ) represent the equilibrium capacity and the adsorption capacity at any time t (s), respectively, and k1 (s−1) is the pseudo-first-order adsorption rate constant. Given the initial condition qt = 0 at t = 0 , Eq. (2) is integrated to obtain

qt = qe (1−e−k1 t ).

(3)

Due to the simplicity, and the analytical and physical consistency of this model, it has been extensively applied to analyze dynamic adsorption data and for designing adsorption processes [26]. ② Ho's pseudo-second-order adsorption kinetics model presumes that the amount of adsorption is proportional to the number of active sites occupied on the surface of the adsorbent. It has been used to explore the chemical adsorption kinetics in the liquid phase [27]. The model is expressed as

dqt = k2 (qe−qt )2, dt

−Q

(4)

−

where k2 (mmol·g ·s ) is the pseudo-second-order adsorption rate constant. Given the initial condition qt = 0 at t = 0 , Eq. (4) is integrated to obtain

t 1 k2 qe2

+

t qe

(8)

dα = kd C jα m, dt

(9) −1

where kd is the deactivation rate constant (min ), and j and m represent the dependences of the gas adsorption and the activity itself, respectively. Taking into account the first-order dependences, i.e., j = 1 and m = 1, the first modified solution of the model is given as a breakthrough curve in the following form [36].

. (5)

(

)

⎤ ⎡ ⎡1−exp k 0 M (1−exp(−kd t )) ⎤ Q C ⎦ exp(−k t )⎥ = exp ⎢ ⎣ d ⎥ ⎢ C0 1−exp(−kd t ) ⎥ ⎢ ⎦ ⎣

③ Avrami’s fractional-order kinetics model was originally applied for particle nucleation [28], and is based on the Avrami equation. This model has recently been employed to simulate the phase changes and crystal growth of materials [29], and has also been employed to explore the dynamic adsorption behavior of surface amine-functional adsorbents [17,30]. The model is commonly expressed as

dqt = kan t n − 1 (qe−qt ), dt

dC −k 0 Cα = 0, dM

where k0 refers to rate constant of the initial sorption (m3·kg−1·min−1), and α represents the rate of change in the activity of the solid reactant. According to the deactivation model, α is expressed as

−1 −1

qt =

(7)

(10)

2.6. Desorption kinetic models Desorption kinetic models can be employed to further understand the process of CO2 desorption. If we can assume that the desorption of CO2 can be represented as the decomposition of carbamate/bicarbonate [37], and that Avrami’s fractional-order kinetic model can simulate the adsorption process, the majority of the decomposition process can be reproduced by the Avrami model [38]. As such, Eq. (7) can be expressed to represent the desorption component y as follows:

(6)

where ka is the kinetic constant of the Avrami model, and n denotes the Avrami exponent, which is often in fractional form, and reflects possible mechanism changes in the adsorption process [31]. In fact, the term n reflects the dimensions of the growth of absorbed species at the adsorption site, where n = 2 represents one-dimensional growth, n = 3

n

y = 1−e−(ka t ) . 1082

(11)

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diameter of KIT-6 and KP-50 samples obtained by N2 adsorption/desorption. We note that the pore volume and surface area of KP-50 are much less than those of KIT-6, indicating that the mesoporous channels of KIT-6 were impregnated with PEHA in the fabrication of KP-50. These results are consistent with the findings of XRD measurements. 3.2. Adsorption properties The CO2 adsorbent used to separate CO2 from the CO2 and N2 mixture should have good CO2 selectivity, and the adsorbent should not adsorb N2. Therefore, the N2 adsorption performance of KP-50 was investigated using N2 as the inlet gas. The results of testing demonstrated that N2 is not adsorbed by the KP-50. The adsorption of CO2 by KP-50 was examined at different values of T with C0 = 10 vol% CO2. The CO2 adsorption capacities of KP-50 are presented in Fig. 5. We obtained qa values of 2.2, 2.5, 2.7, 3.1, 3.2, and 2.9 mmol/g-absorbent at T values of 303, 313, 323, 333, 343, and 353 K, respectively. We note that qa obviously increases with increasing T in the range of 293–343 K, and the maximum value of qa was obtained at 343 K. At low T, PEHA is a highly viscosity fluid, which can block pore inlets and prevent CO2 from diffusing into the sorbent channels. Consequently, CO2 cannot make contact with the internal active sites, resulting in a lower qa. However, the PEHA viscosity decreases with increasing T, such that a larger number of active sites in pores can react with CO2, leading to greater CO2 uptake. We expect that the value of qa begins to decrease at 353 K owing to the increased desorption of CO2. The characteristics of amine functionalized materials for CO2 adsorption proposed in previous studies are compared with those reported herein for KP-50 under similar conditions in Table 2. We note that KP50 demonstrates relatively good adsorption capacity under similar conditions.

Fig. 2. Low angle X-ray diffraction patterns of KIT-6 and KP-50 samples.

3. Results and discussion 3.1. Features of KIT-6 and KP-50 The XRD results of KIT-6 and KP-50 are shown in Fig. 2. The XRD pattern of KIT-6 is consistent with previously reported results [39,40]. The positions of the characteristic Bragg diffraction peaks of KIT-6 and KP-50 are nearly equivalent, indicating that KP-50 retains the mesoporous structure of KIT-6. However, the intensity of the primary KP-50 diffraction peak is less than that of the KIT-6 diffraction peak. This indicates that PEHA has been successfully dispersed into the pores of the KIT-6 particles. A TGA mass loss curve of KP-50 is shown in Fig. 3. A considerable proportion of mass was lost when the sample was heated from 303 K to 573 K. While heating from 303 K to 373 K, the sample mass loss was about 20%, which is likely due to moisture loss and CO2 desorption. No significant change in sample mass was observed as the sample was heated from 373 K to 423 K. However, the sample mass continued decreasing again above 423 K due to the volatilization of the loaded PEHA. These results indicate that the temperature range of thermal stability for KP-50 in air is not greater than 423 K. Fig. 4 presents the results of N2 adsorption/desorption. The isotherms of KIT-6 and KP-50 at liquid nitrogen temperature (77 K) represent typical type IV features of mesoporous silica. The results also exhibit an obvious decrease in the total pore volume and mesoporous volume for KP-50. Table 1 presents the surface area, pore volume, and average pore

3.3. Cyclic adsorption/regeneration of KP-50 The good cyclic adsorption/regeneration behavior of the adsorbent is critical for long-term operation. Fig. 6 presents the values of qa for KP-50 over 10 adsorption/regeneration cycles. Here, an adsorption/ desorption cycle consists of adsorption at 343 K with C0 = 10 vol% CO2, and Q = 100 cm3/min, while desorption occurs under 5 kPa pressure at T = 393 K. The cyclic adsorption/regeneration data reveals that the adsorption performance of KP-50 is fairly stable, and that qa decreased by only 6.5% after 10 adsorption/regeneration cycles. 3.4. Adsorption kinetics The kinetics of CO2 adsorption on KP-50 were investigated by isothermal adsorption at different temperatures (i.e., 293, 303, 313, 323, 333, and 343 K) and at different CO2 concentrations (i.e., 5, 10, 20, 30, 40, 50, and 60 vol%). Fig. 7 presents the amount of CO2 adsorbed by KP-50 at the various operational temperatures and CO2 concentrations, and includes the curves fitted according to the three absorption kinetics models introduced in Section 2.4. The approximated values of the model parameters and corresponding coefficients of determination R2 are listed in Table 3. As shown in Fig. 7 and Table 3, the CO2 adsorption capacities obtained by Lagergen’s pseudo-first order and Ho's pseudo-second-order models are greater than that obtained experimentally. However, the pseudo-first-order model provides a better fit to the isothermal CO2 adsorption data than the pseudo-second-order model, particularly at 293 K or at higher CO2 concentrations. The superior applicability of the pseudo-first-order model indicates that the adsorption process at 293 K represents physical adsorption. As presented in Table 3, the adsorption rate constants k1, k2, and ka decrease with increasing T when C0 is held constant. Meanwhile, k1, k2, and ka increase with increasing C0 at constant T. This indicates that the CO2 adsorption process by KP-50 is dominated by intra-particle

Fig. 3. TGA mass loss profile of KP-50.

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Fig. 4. N2 adsorption/desorption isotherms and the pore diameters (in the insets) of KIT-6 and KP-50.

concentration, the surface coverage at equilibrium, and the desorption kinetic constants [17]. Of all three models considered, Avrami's fractional-order model provides the best fit for the isothermal CO2 adsorption data. The experimental results show that the Avrami exponent n lies between 0.8 and 1.51, which indicates that the adsorption rate decreases gradually with the one-dimensional growth of the adsorbed nuclei [32,49]. Thus, although the initial formations of adsorption sites may be uniform on the uniformly exposed surface, additional adsorption may be preferentially provided near existing adsorption sites, resulting in deviations in the uniformity of adsorption sites and a value of n greater than 1. The T-dependence of ka can be illustrated by the Arrhenius equation:

Table 1 Nitrogen adsorption/desorption characterization details for KIT-6 and KP-50. Sample

Surface area (m2⋅g−1)

Pore volume (cm3⋅g−1)

Pore diameter (nm)

KIT-6 KP-50

943 97

1.0 0.15

6.0 4.3

ka = Ae−(Ea/ RT ) , where A represents the Arrhenius pre-exponential factor, Ea is a term associated with the activation energy, and R is the universal ideal gas constant. A plot of lnka versus 1/T is shown in Fig. 8. The values of A and Ea calculated through linear regression are listed in Table 4. Table 4 shows that the absolute value of Ea decreases with increasing C0, indicating that a high CO2 partial pressure promotes the adsorption of CO2. It is well known that the activation rate of a given reaction remains constant under equivalent reaction conditions (temperature, catalyst, etc.), which suggests that the adsorption rate of CO2 on KP-50 is restricted by intra-particle diffusion, as was indicated by the previous analysis. The value of Ea of adsorption is less than that of liquid ammonia absorption, which is between 40 and 50 kJ·mol−1 [50–52]. Therefore, these results suggest that the proposed aminefunctionalized adsorbents are promising and effective candidates for CO2 capture.

Fig. 5. CO2 adsorption capacity of KP-50 at different temperatures.

diffusion, and that the adsorption rate is limited by intra-particle diffusion. Here, a high CO2 partial pressure promotes the diffusion of CO2 to active sites. According to the assumptions of the model conditions that the amount of adsorption is proportional to the number of active sites occupied on the surface of the adsorbent, the pseudo-second-order kinetic model should be applicable for simulating the experimental data of amine-functionalized adsorbents. However, as presented in Table 3, the value of k2 is closely related to the initial concentration of CO2. In fact, k2 is not actually a strictly kinetic constant of adsorption, but is rather a complex factor related to several parameters, such as the adsorbate

3.5. Breakthrough curve analysis Experimental breakthrough curves were fitted using the deactivation model proposed by Yasyerli et al. [36]. The parameters of the deactivation model derived by fitting to the experimental results are listed in Table 5. Comparisons of the experimental results with the fitted deactivation model for various values of C0 and gas flow rates Q are presented in Fig. 9. As shown in Fig. 9(a) and (b), the breakthrough time of CO2 is delayed with decreasing C0 and Q, respectively. The 1084

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Table 2 Characteristics of previously reported amine functionalized materials for CO2 adsorption and KP-50 under similar conditions. Support

Amine type

Temp. K

CO2 Partial pressure (atm.)

CO2 adsorption (mmol/g-adsorbent)

Ref.

KIT-6 KIT-6 As-syn MCM-41 MCM-41 Activated carbons Carboxyl-rich porous carbons SBA-15 SBA-15 KIT-6 KIT-6 KIT-6

PEI PEHA Triethylenetetramine Polyethylenimine

348 378 333 348 333 348 348 348 348 343 343

1 1 0.15 0.15 1 1 0.15 1 1 0.10 0.10

3.06 4.48 2.2 2.0 1.75 1.7 2.49 3.01 2.90 3.2 0.7

Son et al. [41] Kishor et al. [42] Shi et al. [43] Xu et al. [44] Xu et al. [45] Yang et al. [46] Klinthong et al. [47] Alper et al. [48] Alper et al. [48] This work This work

Tetraethylenepentamine (TEPA) PEI TEPA TEPA PEHA

degree of adsorption. Changes in the activity of the adsorbent may be due to changes in the pore structure, in the active surface area, and in the active site distribution of the adsorbent, which is consistent with the previous analysis derived from the Avrami fractional-order model. In addition, we note from Table 5 that the deactivation rate constant kd generally increases with increasing C0, confirming that a high CO2 partial pressure facilitated formation of the product on the pore surface of the adsorbent. This indicates that high CO2 partial pressure favors the diffusion of CO2 to reaction sites. 3.6. Desorption of CO2 Fig. 10 presents the results of the regeneration experiments and the Avrami fractional-order adsorption kinetics model fitted at each value of T. In general, the regeneration time decreased with increasing T. We note that the adsorbent was not completely regenerated at T = 363 K. These results suggest that the temperature is the dominant factor in the regeneration of the amine-functionalized adsorbent because of the chemical interaction between the CO2 and the adsorbent. Therefore, increasing the desorption temperature is an effective means of reducing the regeneration time. Table 6 lists the parameters and the corresponding R2 values for the fitted Avrami fractional-order models, which verify that the experimental data was well represented by the model. We note that ka increases with increasing T, indicating that the temperature is the dominant factor for the regeneration of the aminefunctionalized adsorbent. In addition, we note that the value of n varies with respect to T, confirming the existence of different desorption mechanisms. At low T, the primary desorption mechanism is physical desorption, and desorption is relatively slow. At high T, the primary

Fig. 6. Cyclic adsorption/regeneration of KP-50 (adsorption at 343 K, CO2: 10 vol%, He: 90 vol%, gas flow rate: 100 cm3·min−1; desorption at 393 K, pressure: 5 kPa).

breakthrough curve is very steep, and the time interval over which C increases from 0 to 0.1C0 is short. In industrial use, the breakthrough point (i.e., the time at which C = 0.1C0) is taken as the end point of an adsorption process. The steeper the breakthrough curve, the higher the utilization of the adsorbent. Based on the R2 values in Table 5, we note that the Yasyerli deactivation model successfully fits the CO2 adsorption breakthrough curves of this study. The predictions of the deactivation model indicate that the activity of the adsorbent decreases significantly with an increasing

Fig. 7. Comparison of the results of adsorption kinetics models and the experimental CO2 uptakes of (a) KP-50 at 10 vol% CO2 for various adsorption temperatures; (b) KP-50 at 333 K for various CO2 concentrations.

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Table 3 Parameter values of Lagergen’s pseudo-first-order kinetics model, Ho's pseudo-second-order kinetics model, and Avrami's fractional-order kinetics model based on fitting to experimental CO2 adsorption data for KP-50 at various temperatures and CO2 concentrations. Pseudo-first-order

Pseudo-second-order

Fractional-order

C0 vol%

T K

qe mmol/g

k1 min−1

R2

qe mmol/g

k2 min−1

R2

qe mmol/g

ka min−1

n

R2

qa mmol/g

5 10 20 30 40 50 60 5 10 20 30 40 50 60 5 10 20 30 40 50 60 5 10 20 30 40 50 60 5 10 20 30 40 50 60 5 10 20 30 40 50 60

293 293 293 293 293 293 293 303 303 303 303 303 303 303 313 313 313 313 313 313 313 323 323 323 323 323 323 323 333 333 333 333 333 333 333 343 343 343 343 343 343 343

1.358 1.783 2.226 2.714 2.831 3.210 3.596 1.609 2.360 2.597 3.076 3.114 3.492 3.576 2.060 2.360 2.928 3.182 3.612 3.490 3.768 2.658 3.105 3.387 3.669 3.731 4.163 4.251 2.814 3.355 3.406 3.619 3.819 4.246 4.331 2.928 3.384 3.794 3.962 4.112 4.747 4.887

0.232 0.254 0.394 0.456 0.546 0.315 0.275 0.195 0.188 0.394 0.463 0.546 0.635 0.599 0.122 0.188 0.449 0.474 0.562 0.571 0.570 0.095 0.136 0.206 0.231 0.242 0.270 0.289 0.084 0.197 0.350 0.367 0.388 0.399 0.406 0.046 0.186 0.211 0.278 0.293 0.302 0.322

0.981 0.994 0.982 0.996 0.996 0.994 0.997 0.978 0.990 0.982 0.997 0.996 0.999 1.000 0.977 0.990 0.983 0.983 0.997 0.997 0.997 0.988 0.986 0.997 0.998 0.997 0.995 0.996 0.988 0.989 0.997 0.998 0.997 0.996 0.997 0.993 0.985 0.983 0.985 0.987 0.988 0.989

1.795 2.219 2.714 3.585 3.282 4.053 4.722 2.182 3.211 3.167 4.023 3.610 4.128 4.263 2.958 4.289 3.533 3.801 3.987 4.347 4.481 3.892 4.378 4.600 4.840 4.678 5.569 5.560 3.960 4.459 4.180 4.630 4.739 5.705 5.698 6.292 4.325 5.045 5.164 5.308 6.657 6.734

1.196 2.849 8.468 11.443 23.915 24.276 26.534 1.711 5.248 13.447 15.798 31.830 52.571 53.342 2.408 6.755 21.911 29.705 41.224 50.291 59.178 4.156 9.018 16.887 23.539 35.546 41.192 46.267 4.172 15.838 27.168 32.353 43.192 43.612 49.973 7.465 14.461 23.871 35.149 41.095 47.366 55.624

0.964 0.984 0.956 0.990 0.997 0.997 0.993 0.963 0.980 0.956 0.992 0.997 0.992 0.991 0.967 0.982 0.958 0.956 0.982 0.989 0.991 0.982 0.977 0.991 0.992 0.991 0.986 0.990 0.981 0.977 0.983 0.992 0.993 0.988 0.990 0.991 0.968 0.969 0.970 0.972 0.979 0.980

1.267 1.743 2.152 2.643 2.888 3.483 3.572 1.474 2.179 2.510 3.043 3.176 3.484 3.558 1.796 2.597 2.839 3.097 3.455 3.586 3.756 2.298 2.779 3.219 3.560 3.725 3.997 4.192 2.528 3.117 3.345 3.574 3.847 4.075 4.242 3.039 3.219 3.530 3.732 3.896 4.284 4.477

0.208 0.263 0.403 0.494 0.546 0.616 0.693 0.174 0.207 0.334 0.449 0.505 0.556 0.630 0.126 0.176 0.296 0.414 0.467 0.512 0.580 0.104 0.146 0.259 0.375 0.416 0.462 0.535 0.089 0.122 0.210 0.331 0.383 0.431 0.503 0.072 0.101 0.187 0.294 0.348 0.423 0.466

1.475 1.144 1.479 1.092 0.859 0.805 1.021 1.514 1.334 1.479 1.036 0.859 1.020 1.042 1.537 1.359 1.467 1.468 1.181 1.055 1.027 1.389 1.399 1.155 1.105 1.007 1.134 1.047 1.324 1.390 1.120 1.048 0.968 1.129 1.067 1.327 1.383 1.437 1.418 1.417 1.370 1.352

0.999 0.996 0.999 0.996 0.999 0.998 0.997 0.998 0.999 0.999 0.997 0.999 0.999 1.000 0.996 0.999 0.999 0.999 0.999 0.997 0.996 0.999 0.999 0.999 0.999 0.997 0.997 0.996 0.997 0.999 0.999 0.998 0.997 0.998 0.997 0.999 0.999 0.999 0.999 0.999 0.999 0.999

1.26 1.78 2.15 2.54 2.87 3.08 3.39 1.46 2.21 2.49 2.87 3.18 3.51 3.57 1.82 2.51 2.85 3.1 3.49 3.65 3.82 2.27 2.74 3.18 3.52 3.66 3.84 4.01 2.55 3.08 3.37 3.56 3.76 3.92 4.11 2.79 3.25 3.53 3.73 3.91 4.19 4.41

Table 4 Arrhenius parameters calculated from the experimental CO2 adsorption isotherms fitted to the Arrhenius equation. CO2 concentration (vol%)

A (min−1)

Ea (kJ·min−1)

R2

5 10 20 30 40 50 60

0.000131 0.000427 0.002104 0.015036 0.024383 0.040315 0.046767

−17.9858 −15.6309 −12.8159 −8.56178 −7.6194 −6.61108 −6.55719

0.99013 0.99769 0.98992 0.98087 0.98942 0.9767 0.99897

desorption mechanism transforms into chemical desorption, and desorption is relatively rapid. The T-dependence of ka can also be illustrated by the Arrhenius equation. A plot of lnka versus 1/T for CO2 desorption is shown in Fig. 11. The value of Ea for CO2 desorption was calculated to be −81.992 kJ/mol by linear regression. The value of Ea calculated in this work is similar to that reported in the work of Sun et al. [37], where Ea of CO2 desorption from MEA/macroporous TiO2 was determined to be −80.79 kJ·mol−1. In addition, we note that the absolute value of Ea

Fig. 8. Arrhenius plots for the kinetic constants ka obtained for the Avrami fractionalorder adsorption kinetics model.

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Table 5 Fitted parameters of the deactivation model proposed by Yasyerli et al. [36] for CO2 adsorption on KP-50 at 333 K and various CO2 partial pressures (C0) and gas flow rates (Q). C0 (vol%)

Q (cm3·min−1)

k0M/Q0

k0 (cm3·min−1g−1)

kd (min−1)

R2

5 20 10 10 10

100 100 100 80 120

23.947 1.845 11.913 13.224 7.831

2.395 × 103 0.185 × 103 1.191 × 103 1.058 × 103 0.938 × 103

0.855 1.326 0.957 0.821 1.130

0.997 0.982 0.997 0.998 0.997

for desorption is considerably greater than that of adsorption. This is because an exothermic reaction yields an activation energy for the reverse reaction that is greater than that of the forward reaction under equivalent conditions [53]. 4. Conclusions

Fig. 10. Results of the regeneration experiments conducted at a pressure of 5 kPa.

An effective amine-modified adsorbent (KP-50) for CO2 capture was prepared by the impregnation method using KIT-6 as a support and PEHA as a modifier. The CO2 adsorption capacity of KP-50 increased with increasing temperature in the range of 293–343 K, achieving a maximum adsorption capacity of 3.2 mmol/g-adsorbent at 343 K. For temperatures greater than 343 K, the adsorption capacity decreased with increasing temperature. The adsorption performance of KP-50 was fairly stable, with only a 6.5% decrease in qa after 10 adsorption/regeneration cycles. The results of adsorption kinetics analysis demonstrated that Avrami's fractional-order kinetics model presented the best fit to the experimental CO2 isothermal adsorption data. The analysis also indicated that the CO2 adsorption process by KP-50 is dominated by intra-particle diffusion, and that the adsorption rate is restricted by the intra-particle diffusion process. The observed decrease in the absolute value of Ea for adsorption with increasing CO2 concentration indicated that high CO2 partial pressure facilitated the adsorption of CO2. The Yasyerli deactivation model demonstrated good applicability to the experimental CO2 adsorption breakthrough curves obtained in this study. Temperature is the dominant factor in the regeneration of the amine-functionalized adsorbent owing to the chemical interaction between the adsorbent and the CO2. Increasing the desorption temperature is an effective means of reducing the regeneration time. An Arrhenius plot for the kinetic constants obtained by fitting the Avrami fractional-order model to the experimental CO2 isothermal desorption data yielded a value of Ea for CO2 desorption of −81.992 kJ·mol−1, which is much greater than that similarly obtained for adsorption.

Table 6 Kinetic parameters of the Avrami fractional order model derived from fitting to experimental CO2 desorption isotherms. Temperature (K)

ka (min−1)

n

R2

363 373 383 393

0.03958 0.10,291 0.21,639 0.30,649

0.89,342 1.15,281 1.09258 1.35,155

0.99524 0.98652 0.9989 0.99809

Overall, the results of this work are useful for supporting actual CO2 adsorption applications in industry. First, KP-50 is an appropriate CO2 adsorbent for use in industry owing to its good CO2 adsorption performance under actual industrial flue gas conditions (323–353 K, 10–15 vol% CO2), its large adsorption capacity, good stability, and rapid adsorption rate. Second, the results of the dynamic research can serve as an important tool for designing and optimizing the adsorption process. The good agreement between the results of the dynamic model and the experimental data demonstrates that the kinetic constants obtained from this work are useful in the design of CO2 adsorption processes. The deactivation model used in this work is a good representation of the CO2 breakthrough curves obtained in a fixed adsorption column, and the time interval required for the adsorption process to reach the breakthrough point can also be estimated by the developed model.

Fig. 9. Comparison of predictions of the deactivation model proposed by Yasyerli et al. [36] with experimental results for CO2 adsorption on KP-50 at 333 K and various CO2 partial pressures (left) and gas flow rates (right).

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Fig. 11. Arrhenius plot for the kinetic constants obtained for the Avrami fractional-order model of desorption.

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