3.4 Kinetic studies of CO2 capture performance

In order to get a good adsorbent, fast adsorption kinetics is considered as one of the most important parameters to evaluate the adsorbent in a dynamic process. Hence, the ability of withstanding large adsorbate flows are connected with the rate of adsorption. Here, we mainly study the PFO model and Avrami model on the function of CO2 adsorption on our LDHs [30]. The general sorption rate equations are expressed as:

Figure 6. CO2 adsorption capacities of the different types of LDHs at different temperatures.

Pseudo-first-order (PFO) model:

$$q\_t = q\_\epsilon \left(\mathbf{1} - \exp\left(-k\_f t\right)\right) \tag{6}$$

showed obvious two-stage adsorption process under different adsorption conditions. That means, adsorption process of CO2 sorption by LDHs consists of a fast reaction stage and a much slower second stage controlled by CO2 diffusion. Clearly, both of the models predict CO2 adsorption process well. But, Avrami kinetic model offers a better description of the adsorption of CO2, since the R2 value was higher and X<sup>2</sup> and Δq yielded low values in comparison of pseudo-first order model. Therefore, Avrami kinetic model is more suitable to predict the CO2 adsorption process of LDHs in our experiment. Table 3 shows the values of kinetic constants and the characteristic parameters from the kinetic model, along with the X<sup>2</sup>

DOI: http://dx.doi.org/10.5772/intechopen.86608

Hybrid Two-step Preparation of Nanosized MgAl Layered Double Hydroxides for CO2 Adsorption

as calculated using Eqs. (1)–(3). This is because pseudo-first order kinetic model is suitable for explaining the low surface coverage adsorption, and hence describes the early stages of adsorption [30, 31]. In the process of CO2 adsorption by LDHs, it is not a simple physical adsorption process. LDHs consists of positively charged Mg-Al-OH brucite type network in an octahedral network. After calcination, the LDHs gradually loses its interlayer water and form a mixed oxide with sufficient basic sites which are favourable for CO2 sorption. It is a complex reaction in combination of physical and chemical process during CO2 uptake [19, 32]. Avrami kinetic model

The pseudo-first order model and Avrami model kinetic parameters k<sup>f</sup> and k<sup>A</sup> are the time scales for measuring the process to reach equilibrium. The higher k<sup>f</sup> and kA, the quicker speed for the process to reach equilibrium. It can be observed from Table 3 that k<sup>f</sup> and kA of CC-LDHs and TUC-LDHs increase with temperature overall. When the temperature increases, the molecule's speed (kinetic energy) also increases. So, the CO2 molecules will migrate faster inside the pores, which in turn will result in an increase in the rate of diffusion. Hence, it is reflected in the form of CO2 uptake curves with much steeper ones (shown in Figure 7). For TU-LDHs, k<sup>f</sup> and kA do not change much as the temperature increases. This might be caused by the fact that the pores of TU-LDHs are easy to saturate or block during the CO2 adsorption even at high temperature due to its small pore size and volume. This phenomenon usually occurs in the microporous material such as porous MgO [33], zeolites [34] and active carbons [35]. The order of CO2 adsorption capacity to reach equilibrium (kA) at 80°C seen in Avrami model is: TU-LDHs > TUC-LDHs > CC-LDHs. The order of CO2 adsorption capacity to reach equilibrium at 150°C seen in Avrami model is: CC-LDHs > TU-LDHs = TUC-LDHs. When the CO2 adsorption temperature is 200°C, the time of reaching equilibrium by three materials is almost the same. This indicates that TU-LDHs and TUC-LDHs show the potential application of reaching equilibrium to absorb the CO2 gas at low temperature (<100°C). The Avrami exponent nA reflects the extent of driving force on adsorption apparatus. The data of nA in the range of 1–4 suggest that CO2 adsorption occurs more than

Here, the schematic CO2 adsorption mechanism on LDHs is shown in Figure 8. During the CO2 adsorption process in porous material, it is known that CO2 molecule diffuses through the gas film to pore structure among the agglomerate particles and the crystalline grains; or the CO2 molecule interacts with the adsorbent (surface reaction). Generally, the surface reaction process is quite rapid, and the resistance associated with the surface adsorption can be assumed to be negligible [33]. The whole process of CO2 adsorption can be simplified that CO2 permeates in a tube with tubular structure. As mentioned above, TUC-LDHs has the largest BET surface area as well as pore size and volume. The effective absorbed area is larger and it is easy to absorb the CO2 in the tubular tube with less resistance. This trend is obvious when the adsorption is operated at a high temperature. For the TU-LDHs, although it shows a second largest surface area, its pore size and volume are limited. The pores of LDHs are easy to block when the CO2 concentration in the tubular tube

is logical to attribute the fitting of CO2 uptake process in our experiment.

one-reaction pathway from adsorption sites [36].

135

, Δq, R<sup>2</sup>

Avrami model:

$$q\_t = q\_\epsilon(\mathbf{1} - \exp\left(-(k\_A t)^{n\_A}\right))\tag{7}$$

where qe is the adsorption capacity at equilibrium and at time t, respectively, kf is the kinetic parameter of pseudo-first order model, kA is the kinetic parameter of Avrami model, the Avrami exponent and nA is a factionary number, which reflects the adsorption mechanism.

To investigate the CO2 adsorption kinetics of LDHs, Figure 7 shows that the CO2 uptake vs. time of TU-LDHs, CC-LDHs and TUC-LDHs at 80, 150 and 200°C and the corresponding profiles as predicted by pseudo-first-order and Avrami kinetic models. From the figure, it could be seen that the adsorption curves of LDHs

#### Figure 7.

Experimental CO2 uptake on different LDHs at different temperatures and corresponding fit to kinetic models: (□) TU-LDHs, (○) CC-LDH,s (△) TUC-LDHs, (—) Pseudo-first order fit and (---) Avrami fit.

### Hybrid Two-step Preparation of Nanosized MgAl Layered Double Hydroxides for CO2 Adsorption DOI: http://dx.doi.org/10.5772/intechopen.86608

showed obvious two-stage adsorption process under different adsorption conditions. That means, adsorption process of CO2 sorption by LDHs consists of a fast reaction stage and a much slower second stage controlled by CO2 diffusion. Clearly, both of the models predict CO2 adsorption process well. But, Avrami kinetic model offers a better description of the adsorption of CO2, since the R2 value was higher and X<sup>2</sup> and Δq yielded low values in comparison of pseudo-first order model. Therefore, Avrami kinetic model is more suitable to predict the CO2 adsorption process of LDHs in our experiment. Table 3 shows the values of kinetic constants and the characteristic parameters from the kinetic model, along with the X<sup>2</sup> , Δq, R<sup>2</sup> as calculated using Eqs. (1)–(3). This is because pseudo-first order kinetic model is suitable for explaining the low surface coverage adsorption, and hence describes the early stages of adsorption [30, 31]. In the process of CO2 adsorption by LDHs, it is not a simple physical adsorption process. LDHs consists of positively charged Mg-Al-OH brucite type network in an octahedral network. After calcination, the LDHs gradually loses its interlayer water and form a mixed oxide with sufficient basic sites which are favourable for CO2 sorption. It is a complex reaction in combination of physical and chemical process during CO2 uptake [19, 32]. Avrami kinetic model is logical to attribute the fitting of CO2 uptake process in our experiment.

The pseudo-first order model and Avrami model kinetic parameters k<sup>f</sup> and k<sup>A</sup> are the time scales for measuring the process to reach equilibrium. The higher k<sup>f</sup> and kA, the quicker speed for the process to reach equilibrium. It can be observed from Table 3 that k<sup>f</sup> and kA of CC-LDHs and TUC-LDHs increase with temperature overall. When the temperature increases, the molecule's speed (kinetic energy) also increases. So, the CO2 molecules will migrate faster inside the pores, which in turn will result in an increase in the rate of diffusion. Hence, it is reflected in the form of CO2 uptake curves with much steeper ones (shown in Figure 7). For TU-LDHs, k<sup>f</sup> and kA do not change much as the temperature increases. This might be caused by the fact that the pores of TU-LDHs are easy to saturate or block during the CO2 adsorption even at high temperature due to its small pore size and volume. This phenomenon usually occurs in the microporous material such as porous MgO [33], zeolites [34] and active carbons [35]. The order of CO2 adsorption capacity to reach equilibrium (kA) at 80°C seen in Avrami model is: TU-LDHs > TUC-LDHs > CC-LDHs. The order of CO2 adsorption capacity to reach equilibrium at 150°C seen in Avrami model is: CC-LDHs > TU-LDHs = TUC-LDHs. When the CO2 adsorption temperature is 200°C, the time of reaching equilibrium by three materials is almost the same. This indicates that TU-LDHs and TUC-LDHs show the potential application of reaching equilibrium to absorb the CO2 gas at low temperature (<100°C). The Avrami exponent nA reflects the extent of driving force on adsorption apparatus. The data of nA in the range of 1–4 suggest that CO2 adsorption occurs more than one-reaction pathway from adsorption sites [36].

Here, the schematic CO2 adsorption mechanism on LDHs is shown in Figure 8. During the CO2 adsorption process in porous material, it is known that CO2 molecule diffuses through the gas film to pore structure among the agglomerate particles and the crystalline grains; or the CO2 molecule interacts with the adsorbent (surface reaction). Generally, the surface reaction process is quite rapid, and the resistance associated with the surface adsorption can be assumed to be negligible [33]. The whole process of CO2 adsorption can be simplified that CO2 permeates in a tube with tubular structure. As mentioned above, TUC-LDHs has the largest BET surface area as well as pore size and volume. The effective absorbed area is larger and it is easy to absorb the CO2 in the tubular tube with less resistance. This trend is obvious when the adsorption is operated at a high temperature. For the TU-LDHs, although it shows a second largest surface area, its pore size and volume are limited. The pores of LDHs are easy to block when the CO2 concentration in the tubular tube

Pseudo-first-order (PFO) model:

Avrami model:

Sorption in 2020s

the adsorption mechanism.

Figure 7.

134

qt <sup>¼</sup> qe <sup>1</sup> � exp �kf <sup>t</sup> (6)

qt <sup>¼</sup> qe <sup>1</sup> � exp �ð Þ kAt nA ð Þ ð Þ (7)

where qe is the adsorption capacity at equilibrium and at time t, respectively, kf is the kinetic parameter of pseudo-first order model, kA is the kinetic parameter of Avrami model, the Avrami exponent and nA is a factionary number, which reflects

To investigate the CO2 adsorption kinetics of LDHs, Figure 7 shows that the CO2 uptake vs. time of TU-LDHs, CC-LDHs and TUC-LDHs at 80, 150 and 200°C and the corresponding profiles as predicted by pseudo-first-order and Avrami kinetic models. From the figure, it could be seen that the adsorption curves of LDHs

Experimental CO2 uptake on different LDHs at different temperatures and corresponding fit to kinetic models: (□) TU-LDHs, (○) CC-LDH,s (△) TUC-LDHs, (—) Pseudo-first order fit and (---) Avrami fit.


Table 3. Values of the kinetic model parameters for CO2 adsorption on LDHs.

Figure 8. Schematic CO

Figure 9. CO

137

<sup>2</sup> uptake using temperature during six cycles at 80

°C (a) TU-LDHs, (b) CC-LDHs and (c) TUC-LDHs.

<sup>2</sup> adsorption mechanism on LDHs.

DOI: http://dx.doi.org/10.5772/intechopen.86608

Hybrid Two-step Preparation of Nanosized MgAl Layered Double Hydroxides for CO

<sup>2</sup> Adsorption

Hybrid Two-step Preparation of Nanosized MgAl Layered Double Hydroxides for CO2 Adsorption DOI: http://dx.doi.org/10.5772/intechopen.86608

Figure 8. Schematic CO2 adsorption mechanism on LDHs.

Figure 9. CO2 uptake using temperature during six cycles at 80°C (a) TU-LDHs, (b) CC-LDHs and (c) TUC-LDHs.

Parameters

Material

136

TU-LDHs

80 150 200

> CC-LDHs

80 150 200

> TUC-LDHs

80 150 200

> Table 3.

Values of the kinetic model parameters

 for CO2

adsorption

 on LDHs.

0.20

0.33

 0.024

 6.93

 0.9447

 0.20

0.25

0.25

 0.013

 5.04

 0.9222

 0.27

0.25

0.33

 1.09

 0.043

 11.06

 0.9430

 2.41

 0.015

 5.63

 0.9727

0.11

0.33

 0.015

 8.37

 0.9774

 0.12

0.32

 1.1

 0.016

 7.35

 0.9775

0.21

0.23

 0.16

 21.42

 0.9337

 0.22

0.33

0.22

 0.034

 8.02

 0.7791

 0.32

0.10

0.23

 0.017

 11.18

 0.9717

 0.11

0.23

0.21

0.22

 1.28

 0.023

 8.32

 0.9399

 3.9

 0.013

 5.14

 0.9640

 1.06

 0.017

 10.33

 0.9708

0.20

0.26

 0.024

 6.93

 0.9420

 0.21

0.24

0.22

 0.015

 5.74

 0.9149

 0.27

0.20

0.29

 0.021

 7.05

 0.9131

 0.22

0.28

0.21

0.26

 1.22

 0.042

 11.06

 0.9434

 2.64

 0.019

 6.68

 0.9463

 1.82

 0.019

 5.78

 0.9544

Temperature

 (°C)

Kf (s�1)

 qe (mmol g�1

)

X2

Δq %ð Þ

R2

Kf (s�1)

 qe (mmol g�1)

 na

X2

Δq %ð Þ

R2

Sorption in 2020s

Pesudo-first

 order model

 for Simulation Avrami model increases; finally, the CO2 diffusion rate will slow down. As for CC-LDHs, it has the lowest effective surface area, which finally influences its CO2 adsorption ability.

acknowledge the support through the Ph.D. scholarship of the International Doctoral Innovation Centre (IDIC) of University of Nottingham Ningbo China.

Hybrid Two-step Preparation of Nanosized MgAl Layered Double Hydroxides for CO2 Adsorption

TU-LDHs MgAl LDHs prepared by ultrasonication-intensified in 'T-mixer'

pretreatment followed by conventional co-precipitation

CC-LDHs MgAl LDHs prepared by conventional co-precipitation TUC-LDHs MgAl LDHs prepared by ultrasonic-intensified in 'T-mixer'

Abbreviations

DOI: http://dx.doi.org/10.5772/intechopen.86608

Author details

and Collins Snape<sup>2</sup>

, Xiaogang Yang<sup>1</sup>

provided the original work is properly cited.

\*, Guang Li<sup>1</sup>

1 Department of Mechanical, Materials and Manufacturing Engineering,

University of Nottingham Ningbo China, Ningbo, P.R. China

2 Department of Chemical and Environmental Engineering, University of Nottingham, University Park, Nottingham, UK

\*Address all correspondence to: xiaogang.yang@nottingham.edu.cn

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

, Collins I. Ezeh<sup>1</sup>

, Chenggong Sun<sup>2</sup>

Xiani Huang<sup>1</sup>

139
