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Clean production of porous MgO by thermal decomposition of Mg(OH)2 using fluidized bed: Optimization for CO2 adsorption
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Clean production of porous MgO by thermal decomposition of Mg(OH)2 using fluidized bed: Optimization for CO2 adsorption Yong Sun a,b,1,∗, Jing ping Zhang b,1, Chao Wen c, Zuohu Li b a
Edith Cowan University, School of Engineering, 270 Joondalup Drive, Joondalup WA 6027 Australia National Engineering Laboratory of Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China c School of Chemical Engineering, Northwest University, Xi’an 710069, China b
a r t i c l e
i n f o
Article history: Received 4 November 2015 Revised 20 January 2016 Accepted 24 February 2016 Available online xxx Keywords: MgO Dehydration Fluidize bed CO2 CCD
a b s t r a c t Fluidized bed efficiently intensifies thermal decomposition of Mg(OH)2 for fast preparation of porous MgO. The shrinking core model is found to well describe the decomposition process. The initial stage of decomposition is controlled by chemical reaction with activation energy being 104 kJ/mol and the subsequent stage is then controlled by diffusion with activation energy being 15 kJ/mol. The response surface methodology (RSM) and the central composite design (CCD) are employed for determining optimal conditions to prepare adsorbent with maximum CO2 removal capacity. The operational parameters such as dehydration temperature (°C), duration (min) and FR-flow rate (Nm3 /h) are chosen as independent variables in CCD. The statistical analysis indicates that the effects of dehydration temperature and combined effect of temperature and duration are all significant to the CO2 removal capacity. The optimal condition for achieving the maximum CO2 adsorption capacity is obtained as the following: temperature (480 °C), duration (42 min), FR (13.8 Nm3 /h) with CO2 removal capacity reaching 33 mg/g. The employment of fluidized bed in process intensification significantly reduces the thermal treatment duration down to 0.7 h. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction CO2 capturing and sequestration (CCS) technology is regarded as the most practical approach for meeting the challenging goal of mitigating 20% of world emission from energy by 2020 [1,2]. Pressure and temperature swing adsorption technology is found to be a practical technology for CO2 capturing [3]. The bottle-neck for this technology is to find high performance adsorbents that could cost-effectively and energy-efficiently remove CO2 from flue gas [4]. To solve this challenging problem, various valuable efforts and explorations have been made in past decades, the adsorbents such as carbonaceous materials [5–7], zeolite [8], metal organic frameworks [9], porous silica [10], hybrid-materials [11,12], metal oxide etc. [13,14] have been extensively investigated. Among these adsorbents, the alkaline earth metal oxide (MgO) based adsorbent shows a promising prospect. This is based upon the advantage of its eco-
∗ Corresponding author at: Edith Cowan University, School of Engineering, 270 Joondalup Drive, Joondalup WA 6027 Australia. Tel.: +61425476770 E-mail address: [email protected], [email protected] (Y. Sun). 1 Author has same contribution to this work in part of experimental works.
nomical availabilities on large scale [15], relative low energy for regeneration [16], synergistic effect in presence of moisture [17]. While pure MgO enjoys the advantages mentioned above, its drawback of poor porosity formation, which shows a relative low CO2 adsorption capacity (less than 20 mg/g), limits its wide application in CCS technology. Up to date, various methods have been applied to improve the porosity of MgO. It includes using mesoporous carbon CMK-3 as exotemplate [18], polyol-meditation thermolysis [19], hydrothermal synthesis [20], aerogel synthesis [21] and calcination methods [22]. Among these available approaches, direct calcination to produce porous MgO is still the most attractive due to its relatively easiness in operation [23], less chemicals involved [24]. However, most of these prior arts focused their attentions in fixed bed reactor for calcination. In most cases, the thermal treatment duration varies from 24 to 6 h. Such long thermal treatment duration indicates the preparation of adsorbents itself being a large CO2 emission process. How to cost-effectively and efficiently reducing thermal treatment duration obviously deserves the attentions for investigation. For sake of process intensification, the fluidized bed is thought to be very effective in fast calcination [25]. The advantage of excellent mass and heat transfer [26] in fluidized bed are so attractive that the direct application
http://dx.doi.org/10.1016/j.jtice.2016.02.030 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: Y. Sun et al., Clean production of porous MgO by thermal decomposition of Mg(OH)2 using fluidized bed: Optimization for CO2 adsorption, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.030
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2. Experimental Nomenclature 2.1. Preparation of porous MgO b0 bi bii CS0 CA0 De kg ks ms P r R t T x Xi Y ANOVA CCD RSM
intercept coefficient linear coefficient quadratic coefficient initial solid concentration (mol/l) initial gas concentration t (mol/l) effective diffusivity (m/s) gaseous diffusion rate (–) surface reaction rate (–) mass of adsorbent (kg) pressure (kPa) radius of adsorbent (m) pressure gases constant (0.00831 kJ/mol K) time (s) temperature (K) dehydroxylation conversion (%) independent variables (–) predicted response (%) analysis of variance Central composite design Response surface methodology
The porous MgO was synthesized through dehydration of Mg(OH)2 using fluidized bed at different temperatures. The Mg(OH)2 was produced from reaction of MgCl2 (150 g/l) with slightly addition of ammonia solution by high pressure syringe pump aiding with mechanical stirring at 205 °C for 0.5 h. The obtained filtered and air dried Mg(OH)2 with optimized particle size of 0.05–0.45 mm was thermal decomposed at different temperatures with different duration in fluidized bed reactor at different FR in presence of hot air with feeding from the top of reactor. The detailed configuration and schematic preparation process is shown in Fig. 1. The diameter and height of the fluidized bed reactor in the bed section were 60 and 600 mm, respectively. The distribution plate was made of copper sintering plate with 2 mm thick which contained 25 holes, each 0.1 mm in diameter. Two thermal couples were installed at the entrance of reactor (measuring inlet gas flow temperature) and on top of reactor (positioning at 200 mm from top of reactor measuring the temperature of hot gas). The U-type tube manometer was installed at the entrance of fluidized bed to monitor pressure drop during calcination. The bed mass and height are 400 g and 130 mm, respectively. In order to maintain a good heat and mass transfer (the minimum fluidization velocity 10 Nm3 /h), the 11 Nm3 /h was chosen as a minimum flow rate in this work. When airflow rate reaches the maximum, approximately 5% of mass lost was observed during cold bed fluidization due to blowing off of the smaller particles from fluidized bed. The bag-type dust collector was utilized to collect the lost material. In this work, no inert material was added to assist fluidization. The Mg(OH)2 is denoted as MG, the MG calcined at different temperature, duration and FR in fluidized bed are set as the following patterns: MG-p1-p2-p3, where p1 represents parameter of thermal treatment temperature, p2 represents parameter of thermal treatment duration, p3 represents the parameter of flow rate of hot air. For example, the sample MG-480-42-13.8 represents that sample was prepared with MG being calcined at 480 °C for 42 min with hot air flow rate being at 13.8 (Nm3 /h) in fluidized bed.
of fluidized bed reactor for production of high performance porous MgO from Mg(OH)2 for CO2 adsorption can be very promising. Up to date, the reports of using fluidized bed for fast calcination of Mg(OH)2 to produce porous MgO, to our best knowledge, are still very limited, this initiates our work in this paper. In this work, fluidized bed is employed to intensify the calcination step, the CCD (central composite design) statistical analysis was developed to find the optimized preparation conditions in fluidized bed reactor for the adsorbent with the best CO2 adsorption capacity, kinetic modeling of dehydration process employing shrinking core model, the effects of calcination temperature upon porosity development, crystallite phase evolution, and morphology change of the prepared adsorbent were closely investigated.
8 3
9 HN4OH
6
2
1
4 NH4Cl
7 8 5 Mg(OH)2
10
11 Air
MgCl2
12
Fig. 1. Schematic diagram for preparation and dehydration of Mg(OH)2 in fluidized bed, where 1 represents MgCl2 solution (150 g/l), 2 ammonia solution, 3 pump, 4 continuous stirring reactor, 5 filtration of Mg(OH)2 , 6 fluidized bed reactor, 7 U-tube for pressure drop, 8 thermocouple, 9 temperature controller, 10 preheating unit, 11 rotameter, 12 air cylinder.
Please cite this article as: Y. Sun et al., Clean production of porous MgO by thermal decomposition of Mg(OH)2 using fluidized bed: Optimization for CO2 adsorption, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.030
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Y. Sun et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–10 Table 1 Range and levels of independent process variables used for CDD (central composite design). Independent variables
Symbols
−β
−1
0
1
β
Temperature/°C Duration/% FR/Nm3 /h
X1 X2 X3
200 40 11
300 50 12
400 60 13
500 70 14
600 80 15
3
will be purged by pure nitrogen (99.5%) with flow rate being at 40 ml/min, then the CO2 (99.9%) was introduced for 3 h at ambient pressure with flow rate of 80 ml/min. The change of weight was recorded by high accurate balance. 3. Decomposition kinetics The decomposition of magnesium hydroxide can be represented as the following [30]:
Mg(OH )2 → MgO + H2 O ↑
2.2. Characterization of porous material X-ray powder diffraction (XRD) spectra of samples were characterized by a Philips X-Pert (50 kV) diffractometer using Cu Kα radiation at a wavelength of λ = 0.15406 nm, sample was scanned from 10° to 90°. SEM morphology: Surface morphology was examined using a JSM-7001F+INCA X-MAX Field emission electron microscope. Nitrogen adsorption isotherms and specific surface area: The nitrogen adsorption isotherm and BET specific surface area were determined by nitrogen gas adsorption at 77 K at a saturation pressure of 106.65 kPa using a Micromeritics ASAP 2020 Automated Gas Sorption System. The BET (Brunauer–Emmett–Teller) specific surface area was assessed within the range of relative pressures from 0.05 to 0.3. Thermogravimetric Analysis: TG was performed on a Shimadzu TGA-50 under air (Q = 20 ml/min) atmosphere at ramping rate of 10 °C/min. XRF (X-ray Fluorescence Spectrometry) (SHIMADZU Lab Center XRF-1700 Japan) was employed to characterize Mg(OH)2 before calcination and MgO after calcination. Mg2+ element was analyzed by using ICP-OES (OPTIMA 7100DV, Perkin Elmer, USA). 2.3. Experimental design and statistical analysis RSM is a set of mathematical and statistical techniques seeking to optimize an objective function that is affected by multiple factors using design of experiments (DoE) methods and statistical analysis [27]. Instead of seeking the optimal solution within a large number of randomly generated candidates, RSM utilizes reduced and simplified experimental designs to gain a thorough understanding of the system as well as obtain the optimal combination of operating parameters [28]. A central composite design (CCD) with three independent variables was investigated to study the response pattern and to determine the optimal combination of dehydration temperature, duration and FR to maximize CO2 adsorption capacity. The design with three independent variables at five different levels (total 17 runs) was adopted to find offset, linear, quadratic and interaction terms using the following equation [29]:
Y = b0 +
3
bi Xi +
i=1
3 i=1
bii Xi 2 +
3
bi j Xi X j
(1)
i< j, j=2
The range and levels of variables optimized are shown in Table 1. The statistical significance of regression terms was checked by analysis of variance, ANOVA. 2.4. CO2 adsorption CO2 adsorption characteristics of prepared porous MG were measured by Shimadzu thermo-gravimetric analyzer (Shimadzu TGA-50). 10 mg sample was put onto an alumina oxide crucible to be heated up to 450 °C and then kept at 450 °C for 30 min to remove moisture and impurities. Then, temperature was reduced to 60 °C to perform CO2 adsorption. Before each run, the sample
(2)
The conversion of the reaction then can be expressed as the following:
x=1−
WH2 O (MgO)
(3)
WH2 O (Mg(OH) ) 2
Where WH2 O (MgO ) refers to the water content in the final calcined
MgO (g), and WH2 O (Mg(OH) ) refers to water content in the raw 2 Mg(OH)2 (g). The water content in this works refers to all types of water bonded to MgO or Mg(OH)2 matrix, which not only includes the absorbed water but also includes the chemical boned crystallite water. In this work, we employed the widely used shrinking core model to model the decomposition process. The reaction time and conversion can be expressed as the following form, which includes all the controlling steps from surface reaction, internal diffusion and external diffusions [31,32]:
t =
CS0 r CS0 r x+ [1 − (1 − x )1/3 ] 3kgCA0 kSCA0 +
CS0 r 2 [1 − 3(1 − x )2/3 + 2(1 − x )] 6DeCA0
(4)
where t is the reaction time (min), CS0 is initial solid concentration (mol/l), CA0 is initial gas concentration (mol/l), kg is the gaseous diffusion constant factor, ks is surface reaction constant factor, r is the particle ratio (mm). De is an effective diffusivity (in this case, it refers to H2 O vapor diffusivity in m/s). In case of the conversion being controlled by the surface chemical reaction, then Eq. (4) can be further simplified as the following:
t=
CS0 r [1 − (1 − x )1/3 ] kSCA0
(5)
and its derivative form is the following: 2/3
dx 3ksCA0 (1 − x ) = dt CS0 r
(6)
Table 2 Three factor CCD with experimental results of dependent variables. Runs
Code values
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
X1 400 300 400 300 500 400 400 300 500 600 500 300 500 400 200 400 400
Experimental results mg/g X2 60 50 60 70 70 40 80 70 50 60 70 50 50 60 60 60 60
X3 11 14 13 12 12 13 13 14 14 13 14 12 12 15 13 13 13
30 19 31 28 25 29 25 26 31 30 27 20 30 32 15 30 32
Please cite this article as: Y. Sun et al., Clean production of porous MgO by thermal decomposition of Mg(OH)2 using fluidized bed: Optimization for CO2 adsorption, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.030
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In case of the conversion being controlled by internal diffusion, Eq. (4) can be further simplified as the following:
t=
CS0 r 2 [1 − 3(1 − x )2/3 + 2(1 − x )] 6DeCA0
4.1. Statistical analysis and optimization of preparation conditions (7)
and its derivative form is the following: 1/3
dx 3DeCA0 (1 − x ) = · 1/3 dt CS0 r 2 1 − (1 − x )
4. Results and Discussion
(8)
In this work, the least squares regression was employed for constant approximation.
The experimental results associated with interaction between each independent variable are shown in Table 2. The CO2 removal capacity was found to be between 15 to 32 mg/g, respectively. Table 3 gives the ANOVA results for respective response. The higher Fischer’s ‘F-statistics’ value and lower ‘P’ value (probability) indicate the high significance of the regression model [33]. It is found that the term X1 , X1 X2 , X1 2 , are significant, while other terms have less significance for response. By applying multiple regression analysis
Fig. 2. Three-dimensional response surface for CO2 removal (a) dehydration temperature versus duration; (b) dehydration temperature versus FR; c) dehydration duration versus FR.
Please cite this article as: Y. Sun et al., Clean production of porous MgO by thermal decomposition of Mg(OH)2 using fluidized bed: Optimization for CO2 adsorption, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.030
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Y. Sun et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–10 Table 3 ANOVA analysis for sulfur removal with r2 0.95, adjust r2 0.94, predicted r2 0.93, adequate precision 10. Source
DF
Sum of squares
Mean square
F-value
Prob > F
Model X1 X2 X3 X1 X2 X1 X3 X2 X3 X1 2 X2 2 X3 2 Residue Lack of fit Pure error Cor total
9.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 7.00 5.00 2.00 16.00
332.71 149.61 0.043 2.11 67.39 5.50 0.001 91.02 20.45 0.17 40.97 38.56 2.41 373.68
36.97 149.61 0.043 2.11 67.39 5.50 0.001 91.02 20.45 0.17 5.85 7.71 1.20
0.0119 0.0015 0.9341 0.5675 0.0115 0.3646 0.9911 0.0056 0.1038 0.8682
0.0119 0.0015 0.9341 0.5675 0.0115 0.3646 0.9911 0.0056 0.1038 0.8682
6.41
0.1405
5
4
0.00
TG/mg
o
200 C
-0.02
2 -0.04
o
1
390 C
0 100
200
300
DTG/mg/K
3
400 500 600 o Temperature/ C
700
-0.06 800
Fig. 3. TG analysis of MG in presence of air.
Y = 30 + 6.12X1 − 0.10X2 + 0.73X3 − 11.61X1 X2 + 3.32X1 X3 − 0.001X2 X3 − 8.67X1 2 − 4.11X2 2 − 0.38X3 2
(9)
The CO2 removal was evaluated in terms of removal capacity through 3-D plots of responses as a function of two factors while keeping the third parameter at the optimal condition (Fig. 2). The response patterns of combination effect of different process parameters to CO2 removal present a different trend, especially in the combination effect of X1 X2 . In Fig. 2a, the combined effect of dehydration temperature and duration enhance CO2 removal capacity of the resultant adsorbent. However, the combination of too high dehydration temperature and long duration will lead to a decrease of the CO2 removal capacity. At low level of dehydration temperature and duration, the increased dehydration temperature and duration will facilitate the removal of water in MG matrix, which leads to pore development of MG, this could explain the increase of CO2 removal capacity. On the other hand, a too higher level of dehydration temperature and duration will lead to collapse of the previous formed intra-particle porous structure, which will result in a decrease of CO2 removal capacity. In terms of response patterns of the combination effects of X1 X3 and X2 X3 in Fig. 2b,c, response surface plots show a maximum region where the removal of CO2 neither increased nor decreased. Among combination effects, only the combined effect of X1 X2 (P = 0.011) is found to be significant to the response pattern. The standard square root r2 for the quadratic model is 0.95, which indicates over 95% of the variation in the response could be explained by the model. The obtained adjusted r2 was close to the experimental r2 and predicted r2 , indicating a well agreement between experimental data and predicted values. The value of ‘adequate precision’ was 10, indicating an adequate signal. Then the maximum CO2 removal capacity was set for optimization goal and 28 solutions were found. Out of them, the best condition selected is MG-480-42-13.8 representing sample being prepared as the following: the MG was calcined at 480 °C for 42 min with air flow rate being at 13.8 (Nm3 /h). The CO2 removal capacity predicted from model could reach 33.3 mg/g. An additional experiment was conducted to further validate the model prediction, and the CO2 removal capacity of MG-480-42-13.8 at the optimal condition could reach 32 mg/g, indicating a 4.1% experimental deviation. 4.2. Decomposition kinetics To understand decomposition kinetics, the thermo-gravimetric analysis in presence of air is shown in Fig. 3. There are two major
573 673 773
600
Pressure drop/Pa
to the experimental data, the following second-degree polynomial is derived to represent relationship between CO2 adsorption and three independent process variables (dehydration temperature, duration, and FR) as the following:
550
500
450
400
0
10
20
30
40 50 Time/min
60
70
80
90
Fig. 4. Pressure drop of fluidized bed for calcination of MG at different temperatures.
stages involving with dehydration, the peak at around 200 °C indicates the removal of the adsorbed water. The major weight loss is found at the temperature range of 300–450 °C, which corresponds to the reaction in Eq. (2):
Mg(OH )2 → MgO + H2 O ↑
(2)
The dehydration corresponds to the weight loss of 32%, which is larger than the theoretical weight loss of complete dehydration of Mg(OH)2 (31%), the discrepancy could be due to the existence of impurities. The flow characteristic of fluidized bed is investigated and result is shown Fig. 4. The FR was kept at the optimal condition to maintain a good fluidization of the MG sample in the hot bed. The pressure drop changes significantly during the first 10 min in all temperatures, indicating the significant dehydration occurrence during this stage. The variation of pressure drop becomes less once it is over 20 min. The particle size distribution of raw MG and MG500 is shown in Fig. 5, the obvious decrease of particle size is observed for MG-500 after calcination. The shrinking-core model (SCM) was applied against dehydration data and the results are shown in Fig. 6. Based upon flow characteristic of fluidized bed, the thermal decomposition can be broadly divided into two stages. In the first 10 min, dehydration is controlled by surface chemical reaction, of which the reaction rate is lower than the diffusion rate and the generated water vapor can easily pass through film layer without significant diffusion resistance. This is confirmed by good fit of Eq. (5) in Fig. 6a, which presents a linear correlation by neglecting internal and external diffusion resistance. The activation energy of reaction is 104 kJ/mol
Please cite this article as: Y. Sun et al., Clean production of porous MgO by thermal decomposition of Mg(OH)2 using fluidized bed: Optimization for CO2 adsorption, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.030
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MG at RT MG-200 MG-300 MG-400 MG-500 MG-600
16 12
4
!
10
20
*
101
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 Particle Size/mm
*
*
!
001
0
* MgO ! Mg(OH)2
311
111
8
*
*
222
MG-500 MG
Intensity/a.u.
Mass fraction/%
20
220
Y. Sun et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–10
200
6
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30
40
50
60
70
80
90
2Theta/degree Fig. 5. Particle size distribution of before and after dehydroxylation at 773 K. Fig. 7. XRD spectra of different MG samples calcinated at different temperatures while keeping duration and FR at optimal conditions.
by using Arrhenius law. As reaction progresses, the reacted MgO layer becomes thicker and diffusion resistance outweighs reaction rate. The diffusion resistance from MG core to the bulk gas phase becomes dominant. This is confirmed by good fit of Eq. (7) in Fig. 6b, which presents a linear correlation by neglecting surface reaction rate. The corresponding activation energy of diffusion is 15 kJ/mol. These results are relatively smaller than literatures reports [34]. These discrepancies might be due to the different particle sizes, calcination temperature ramping rate and different operational pressures etc.
Among those three independent parameters, the dehydration temperature is found to be the most significant (in form of single and combination effect) to the CO2 removal capacity. In this work, a further investigation of dehydration temperature effect (keeping other process parameters at the optimal condition X2 = 42 min and X3 = 13.8 Nm3 /h), characterization of the corresponding MG samples being calcined at different temperatures in fluidized bed were conducted. The XRD spectra of MG samples calcined at different temperatures is shown in Fig. 7. The raw material MG at room temperature presents broad and scattering spectrum indicating the Mg(OH)2 produced from reaction of MgCl2 and NH4 OH is in an amorphous state [35]. When MG-200 is calcined at 200 °C, it becomes to change from amorphous to the crystallite Mg(OH)2 ,
which is confirmed by appearance of the characteristic peaks of crystallite Mg(OH)2 in 001 and 101 face in MG-200 sample [36]. As calcination temperature continues to increase to 300 °C, the predominant crystallite phase changes from Mg(OH)2 to MgO with existences of crystallite Mg(OH)2 in MG-300 sample, which is confirmed by the strong characteristic peaks of crystallite MgO in 111, 200, 220, 311, 222 faces [37] and remaining of small characteristic peaks of crystallite Mg(OH)2 in 001 and 101 face. Once the calcination temperature reaches 400 °C, the crystallite Mg(OH)2 (in 001 and 101 face) in MG-400 almost disappears. By referring to TG analysis, this temperature range corresponds to the major weight loss for the reaction in Eq. 2. The corresponding XRF and wet chemical analysis of MG-400 also shows over 98% weight percentage of MgO in the MG-400, this indicates a relatively complete conversion from Mg(OH)2 to MgO within 40 min in fluidized bed over 400 °C. As calcination temperature further increase, no further significant crystallite phase change occurs from 400 to 600 °C. The effect of dehydration temperature upon the BET specific surface area of MG and their corresponding N2 adsorption isotherms are shown in Fig. 8. The result indicates that calcination facilitates the increase of the surface area, at which stage the specific surface area of MG-500-42-13.8 reaches 75 m2 /g. In terms of nitrogen adsorption isotherms, the adsorption uptake occurs through the entire pressure range indicating the existence of both micropore
a
b
4.3. Effect of dehydration temperature
0.5
0.4
+2(1-x)
0.4
0.3
2/3
1-3(1-x)
1-(1-x)
1/3
0.3
0.2
0.2 673
673
0.1
723
723
0.1
773
773
Activation Energy 104 kJ mol 0.0 2
4
6 Time/min
8
Activation Energy 15 kJ mol 0.0 10
20
30
40
50
60
70
80
90
100
Time/min
Fig. 6. Shrinking core model fit: (a) dehydration activation energy; (b) diffusion activation energy.
Please cite this article as: Y. Sun et al., Clean production of porous MgO by thermal decomposition of Mg(OH)2 using fluidized bed: Optimization for CO2 adsorption, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.030
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7
80 MG
200
MG-300
-1
Absorbedvolume(cm .g )
MG-400 MG-500
160
MG-600
2
SpecificArea/(m /g)
3
60
40
20
0 300
400
500
120
80
40
0 0.0
600 o
0.2
0.4
0.6
0.8
1.0
Relative (P/P )
Tempurature/ C
o
Fig. 8. Specific surface area and adsorption isotherms of different MG samples calcined at different temperatures while keeping duration and FR at optimal conditions.
and mesopore in the sample [38]. Before calcination, all samples exhibit the mesoporous character, indicating hydrothermal treatment lead to the formation of mesoporous adsorbent, this agrees well with literature reports that the proper hydrothermal treatment will facilitate to the formation of mesoporous MgO crystal [39]. As calcination temperature increase, the nitrogen adsorption isotherm of MG-500-42-13.8 is greater than that of BL-300-42-13.8 indicating the increased surface area is mainly from dehydration reaction for micropore formation from MG matrix. The SEM morphologies of the raw material MG (Mg(OH)2 ) and MG calcined at different temperature in fluidized bed while keeping duration and FR at optimal conditions are shown in Fig. 9. Without calcination, the MG presents a dense and coarse surface, all the particles seems to aggregate together indicating a poor pore formation. This agrees with specific surface area analysis. As the MG samples were thermal treated in the fluidized bed at different temperatures, a gradual trend of rod particle formation and aggregation is observed. In addition, these rod shape particles tend to become smaller as calcination temperature increase. This result also agrees with bulk particle size distribution analysis. The porosity development during dehydration reaction in the fluidized bed is complicated. The shrinking core mechanism during dehydration might contribute to the formation of the rod shaped particles when water vapor is released from the core of unreacted particle. In addition, the good mass transfer in the fluidized bed might facilitate external surface creation during aggregation of the newly formed particles, which
in turn leads to an increased specific surface area. By considering CO2 adsorption capacity, it is found that the increased CO2 capacity is divided into two calcination temperature ranges. The first stage is from 300 to 500 °C, of which both significant crystallite change from Mg(OH)2 to MgO and increase of BET specific surface area due to dehydration reaction contribute to the significant increase of CO2 adsorption capacity. The possible reason could be a) MgO has relatively larger adsorption affinity for CO2 than that of Mg(OH)2 during gas solid adsorption. b) micropore formation during dehydration also facilitates CO2 adsorption on porous adsorbent. Once calcination temperature reaches over 500 °C, the crystallite change from Mg(OH)2 to MgO does not change appreciably, the specific surface area even experience a slight decrease. During this temperature range, no appreciable increased CO2 adsorption capacity is observed. 4.4. Comparison of CO2 removal capacity Due to limited resources and different CO2 adsorption conditions, we only make a limited comparison in this work. In Table 4, the CO2 adsorption capacity among different literature reports by different prepared adsorbents are compared. The CO2 adsorption capacity follows order of MgO (Solvothermal) < MgO/Al2 O3
Table 4 Comparison of adsorption capacity, where MgO/K-SBA represents MgO on mesoporous silica together with introducing potassium cations, MgO/Al-SBA represents MgO on mesoporous silica together with introducing aluminum cations, MgO/OMC refers MgO on ordered mesoporous carbon. Sample
SSA/m2 /g
Calcination temperature/°C
Calcination duration /h
CO2 adsorption temperature/°C
adsorption capacity /mg/g
Reference
MgO/Al2 O3 MgO/CMK-3 MgO/OMC MgO (solvothermal) MgO (solvothermal) Foam magnesia MgO/K-SBA MgO/Al-SBA MgO MgO MG-480-42-13.8
300 250 598 373 204 130 257 235 – 101 85
600 800 900 450 550 600 540 450 400 400 480
5 8.8 6 6.7 22 12 17 7.8 5.3 8.6 0.7
25 25 25 – 90 100 20 25 50 50 60
32.2 80 90 – 16 115 40 60 70 36 34
[40] [41] [42] [39] [43] [44] [45] [46] [47] [48] This work
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Fig. 9. SEM morphology of MG samples calcined at different temperatures, A refers to MG at room temperature, B refers to MG-200, C refers to MG-300, D refers to MG-400, E refers to MG-500, F refers to MG-600.
carbon and silica presents the highest adsorption capacity. But considering its complicated preparation procedures and many different chemicals involved, the technical hurdles of massive production of these types of adsorbents are still hard to overcome in the short terms. The porous MgO prepared from direct calcination,
on the other hand, obviously overcome the difficulties that mentioned above. Relative simple procedures and much less chemicals involvements show its promising prospect. In addition, by carefully comparing the thermal treatment duration of each different preparation approaches, the obvious advantage of using fluidized bed
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reactor is very convincing. The calcination duration can be significantly reduced from almost 6–22 h using fixed bed reactor down to 0.7 h using fluidized bed reactor. By taking the life time cycle analysis into consideration, the thermal treatment duration itself definitely count on the CO2 emissions. The significant reduced calcination duration indicates a much less CO2 emission from manufacturing of adsorbents itself. This is the advantage of our process. Overall, the CO2 adsorption capacity from porous MgO produced from our process is comparable to other literature reports and is superior to current commercial MgO. By considering the significantly reduced thermal treatment durations, the advantage of using fluidized bed in process intensification is obvious. 5. Conclusions The porous MgO was prepared from thermal decomposition of Mg(OH)2 using fluidized bed. The experiments data was found to have a good fit for the proposed shrinking core model. The initial stage of decomposition is controlled by surface chemical reaction with activation energy reaching 104 kJ/mol. The second stage of reaction after 10 min is controlled by diffusion with activation energy being 15 kJ/mol. The RSM (Response surface methodology) and the CCD (Central composite design) were employed to find the optimal adsorbents with the maximum CO2 removal capacity. The operational parameters such as dehydration temperature (°C), duration (min) and FR were chosen as independent variables in CCD. The statistical analysis indicates that the effects of dehydroxylation temperature and combined effect of temperature and duration are all significant to the CO2 removal capacity. The optimal condition for achieving the maximum CO2 adsorption capacity is obtained as the following: temperature (480 °C), duration (42 min), FR (13.8 Nm3 /h) with CO2 removal capacity reaching 33 mg/g. The employment of fluidized bed in process intensification significantly reduces the thermal treatment duration. Acknowledgements Anpeng high-tech energy and resource company, Shenhua Beijing institute are highly appreciated for supporting this work. Authors would like to give special thanks for technical discussions with Dr Yarong Li from NSW government environmental protection agency for better improvement of this work. References [1] Sun Y, Parikh V, Zhang L. Sequestration of carbon dioxide by indirect mineralization using Victorian brown coal fly ash. J Hazard Mater 2012;209:458–66. [2] Lackner KS. A guide to CO2 sequestration. Science 20 03;30 0:1677–8. [3] Hedin N, Andersson L, Bergstrom L, Yan JY. Adsorbents for the post-combustion capture of CO2 using rapid temperature swing or vacuum swing adsorption. Appl Energ 2013;104:418–33. [4] Sun Y, Yang G, Zhang JP, Wang YS, Yao MS. Activated carbon preparation from lignin by H3 PO4 activation and its application to gas separation. Chem Eng Technol 2012;35:309–16. [5] Alabadi A, Razzaque S, Yang YW, Chen S, Tan B. Highly porous activated carbon materials from carbonized biomass with high CO2 capturing capacity. Chem Eng J 2015;281:606–12. [6] Sun Y, Zhang JP, Yang G, Li ZH. An improved process for preparing activated carbon with large specific surface area from corncob. Chem Biochem Eng Q 2007;21:169–74. [7] Sun Y, Wei J, Wang YS, Yang G, Zhang JP. Production of activated carbon by K2 CO3 activation treatment of cornstalk lignin and its performance in removing phenol and subsequent bioregeneration. Environ Technol 2010;31:53–61. [8] Bezerra DP, da Silva FWM, de Moura PAS, Sousa AGS, Vieira RS, Rodriguez– Castellon E, Azevedo DCS. CO2 adsorption in amine-grafted zeolite 13X. Appl Surf Sci 2014;314:314–21. [9] Dinara Andirova CFC, Lei Yu, Choi Sunho. Effect of the structural constituents of metal organic frameworks on carbon dioxide capture. Micropor Mesopor Mat 2016;219:276–305. [10] Lu K. Porous and high surface area silicon oxycarbide-based materials—a review. Mater Sci Eng: R: Rep 2015;97:23–49.
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Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2016) 1–10
Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Clean production of porous MgO by thermal decomposition of Mg(OH)2 using fluidized bed: Optimization for CO2 adsorption Yong Sun a,b,1,∗, Jing ping Zhang b,1, Chao Wen c, Zuohu Li b a
Edith Cowan University, School of Engineering, 270 Joondalup Drive, Joondalup WA 6027 Australia National Engineering Laboratory of Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China c School of Chemical Engineering, Northwest University, Xi’an 710069, China b
a r t i c l e
i n f o
Article history: Received 4 November 2015 Revised 20 January 2016 Accepted 24 February 2016 Available online xxx Keywords: MgO Dehydration Fluidize bed CO2 CCD
a b s t r a c t Fluidized bed efficiently intensifies thermal decomposition of Mg(OH)2 for fast preparation of porous MgO. The shrinking core model is found to well describe the decomposition process. The initial stage of decomposition is controlled by chemical reaction with activation energy being 104 kJ/mol and the subsequent stage is then controlled by diffusion with activation energy being 15 kJ/mol. The response surface methodology (RSM) and the central composite design (CCD) are employed for determining optimal conditions to prepare adsorbent with maximum CO2 removal capacity. The operational parameters such as dehydration temperature (°C), duration (min) and FR-flow rate (Nm3 /h) are chosen as independent variables in CCD. The statistical analysis indicates that the effects of dehydration temperature and combined effect of temperature and duration are all significant to the CO2 removal capacity. The optimal condition for achieving the maximum CO2 adsorption capacity is obtained as the following: temperature (480 °C), duration (42 min), FR (13.8 Nm3 /h) with CO2 removal capacity reaching 33 mg/g. The employment of fluidized bed in process intensification significantly reduces the thermal treatment duration down to 0.7 h. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction CO2 capturing and sequestration (CCS) technology is regarded as the most practical approach for meeting the challenging goal of mitigating 20% of world emission from energy by 2020 [1,2]. Pressure and temperature swing adsorption technology is found to be a practical technology for CO2 capturing [3]. The bottle-neck for this technology is to find high performance adsorbents that could cost-effectively and energy-efficiently remove CO2 from flue gas [4]. To solve this challenging problem, various valuable efforts and explorations have been made in past decades, the adsorbents such as carbonaceous materials [5–7], zeolite [8], metal organic frameworks [9], porous silica [10], hybrid-materials [11,12], metal oxide etc. [13,14] have been extensively investigated. Among these adsorbents, the alkaline earth metal oxide (MgO) based adsorbent shows a promising prospect. This is based upon the advantage of its eco-
∗ Corresponding author at: Edith Cowan University, School of Engineering, 270 Joondalup Drive, Joondalup WA 6027 Australia. Tel.: +61425476770 E-mail address: [email protected], [email protected] (Y. Sun). 1 Author has same contribution to this work in part of experimental works.
nomical availabilities on large scale [15], relative low energy for regeneration [16], synergistic effect in presence of moisture [17]. While pure MgO enjoys the advantages mentioned above, its drawback of poor porosity formation, which shows a relative low CO2 adsorption capacity (less than 20 mg/g), limits its wide application in CCS technology. Up to date, various methods have been applied to improve the porosity of MgO. It includes using mesoporous carbon CMK-3 as exotemplate [18], polyol-meditation thermolysis [19], hydrothermal synthesis [20], aerogel synthesis [21] and calcination methods [22]. Among these available approaches, direct calcination to produce porous MgO is still the most attractive due to its relatively easiness in operation [23], less chemicals involved [24]. However, most of these prior arts focused their attentions in fixed bed reactor for calcination. In most cases, the thermal treatment duration varies from 24 to 6 h. Such long thermal treatment duration indicates the preparation of adsorbents itself being a large CO2 emission process. How to cost-effectively and efficiently reducing thermal treatment duration obviously deserves the attentions for investigation. For sake of process intensification, the fluidized bed is thought to be very effective in fast calcination [25]. The advantage of excellent mass and heat transfer [26] in fluidized bed are so attractive that the direct application
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Please cite this article as: Y. Sun et al., Clean production of porous MgO by thermal decomposition of Mg(OH)2 using fluidized bed: Optimization for CO2 adsorption, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.030
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2. Experimental Nomenclature 2.1. Preparation of porous MgO b0 bi bii CS0 CA0 De kg ks ms P r R t T x Xi Y ANOVA CCD RSM
intercept coefficient linear coefficient quadratic coefficient initial solid concentration (mol/l) initial gas concentration t (mol/l) effective diffusivity (m/s) gaseous diffusion rate (–) surface reaction rate (–) mass of adsorbent (kg) pressure (kPa) radius of adsorbent (m) pressure gases constant (0.00831 kJ/mol K) time (s) temperature (K) dehydroxylation conversion (%) independent variables (–) predicted response (%) analysis of variance Central composite design Response surface methodology
The porous MgO was synthesized through dehydration of Mg(OH)2 using fluidized bed at different temperatures. The Mg(OH)2 was produced from reaction of MgCl2 (150 g/l) with slightly addition of ammonia solution by high pressure syringe pump aiding with mechanical stirring at 205 °C for 0.5 h. The obtained filtered and air dried Mg(OH)2 with optimized particle size of 0.05–0.45 mm was thermal decomposed at different temperatures with different duration in fluidized bed reactor at different FR in presence of hot air with feeding from the top of reactor. The detailed configuration and schematic preparation process is shown in Fig. 1. The diameter and height of the fluidized bed reactor in the bed section were 60 and 600 mm, respectively. The distribution plate was made of copper sintering plate with 2 mm thick which contained 25 holes, each 0.1 mm in diameter. Two thermal couples were installed at the entrance of reactor (measuring inlet gas flow temperature) and on top of reactor (positioning at 200 mm from top of reactor measuring the temperature of hot gas). The U-type tube manometer was installed at the entrance of fluidized bed to monitor pressure drop during calcination. The bed mass and height are 400 g and 130 mm, respectively. In order to maintain a good heat and mass transfer (the minimum fluidization velocity 10 Nm3 /h), the 11 Nm3 /h was chosen as a minimum flow rate in this work. When airflow rate reaches the maximum, approximately 5% of mass lost was observed during cold bed fluidization due to blowing off of the smaller particles from fluidized bed. The bag-type dust collector was utilized to collect the lost material. In this work, no inert material was added to assist fluidization. The Mg(OH)2 is denoted as MG, the MG calcined at different temperature, duration and FR in fluidized bed are set as the following patterns: MG-p1-p2-p3, where p1 represents parameter of thermal treatment temperature, p2 represents parameter of thermal treatment duration, p3 represents the parameter of flow rate of hot air. For example, the sample MG-480-42-13.8 represents that sample was prepared with MG being calcined at 480 °C for 42 min with hot air flow rate being at 13.8 (Nm3 /h) in fluidized bed.
of fluidized bed reactor for production of high performance porous MgO from Mg(OH)2 for CO2 adsorption can be very promising. Up to date, the reports of using fluidized bed for fast calcination of Mg(OH)2 to produce porous MgO, to our best knowledge, are still very limited, this initiates our work in this paper. In this work, fluidized bed is employed to intensify the calcination step, the CCD (central composite design) statistical analysis was developed to find the optimized preparation conditions in fluidized bed reactor for the adsorbent with the best CO2 adsorption capacity, kinetic modeling of dehydration process employing shrinking core model, the effects of calcination temperature upon porosity development, crystallite phase evolution, and morphology change of the prepared adsorbent were closely investigated.
8 3
9 HN4OH
6
2
1
4 NH4Cl
7 8 5 Mg(OH)2
10
11 Air
MgCl2
12
Fig. 1. Schematic diagram for preparation and dehydration of Mg(OH)2 in fluidized bed, where 1 represents MgCl2 solution (150 g/l), 2 ammonia solution, 3 pump, 4 continuous stirring reactor, 5 filtration of Mg(OH)2 , 6 fluidized bed reactor, 7 U-tube for pressure drop, 8 thermocouple, 9 temperature controller, 10 preheating unit, 11 rotameter, 12 air cylinder.
Please cite this article as: Y. Sun et al., Clean production of porous MgO by thermal decomposition of Mg(OH)2 using fluidized bed: Optimization for CO2 adsorption, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.030
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Y. Sun et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–10 Table 1 Range and levels of independent process variables used for CDD (central composite design). Independent variables
Symbols
−β
−1
0
1
β
Temperature/°C Duration/% FR/Nm3 /h
X1 X2 X3
200 40 11
300 50 12
400 60 13
500 70 14
600 80 15
3
will be purged by pure nitrogen (99.5%) with flow rate being at 40 ml/min, then the CO2 (99.9%) was introduced for 3 h at ambient pressure with flow rate of 80 ml/min. The change of weight was recorded by high accurate balance. 3. Decomposition kinetics The decomposition of magnesium hydroxide can be represented as the following [30]:
Mg(OH )2 → MgO + H2 O ↑
2.2. Characterization of porous material X-ray powder diffraction (XRD) spectra of samples were characterized by a Philips X-Pert (50 kV) diffractometer using Cu Kα radiation at a wavelength of λ = 0.15406 nm, sample was scanned from 10° to 90°. SEM morphology: Surface morphology was examined using a JSM-7001F+INCA X-MAX Field emission electron microscope. Nitrogen adsorption isotherms and specific surface area: The nitrogen adsorption isotherm and BET specific surface area were determined by nitrogen gas adsorption at 77 K at a saturation pressure of 106.65 kPa using a Micromeritics ASAP 2020 Automated Gas Sorption System. The BET (Brunauer–Emmett–Teller) specific surface area was assessed within the range of relative pressures from 0.05 to 0.3. Thermogravimetric Analysis: TG was performed on a Shimadzu TGA-50 under air (Q = 20 ml/min) atmosphere at ramping rate of 10 °C/min. XRF (X-ray Fluorescence Spectrometry) (SHIMADZU Lab Center XRF-1700 Japan) was employed to characterize Mg(OH)2 before calcination and MgO after calcination. Mg2+ element was analyzed by using ICP-OES (OPTIMA 7100DV, Perkin Elmer, USA). 2.3. Experimental design and statistical analysis RSM is a set of mathematical and statistical techniques seeking to optimize an objective function that is affected by multiple factors using design of experiments (DoE) methods and statistical analysis [27]. Instead of seeking the optimal solution within a large number of randomly generated candidates, RSM utilizes reduced and simplified experimental designs to gain a thorough understanding of the system as well as obtain the optimal combination of operating parameters [28]. A central composite design (CCD) with three independent variables was investigated to study the response pattern and to determine the optimal combination of dehydration temperature, duration and FR to maximize CO2 adsorption capacity. The design with three independent variables at five different levels (total 17 runs) was adopted to find offset, linear, quadratic and interaction terms using the following equation [29]:
Y = b0 +
3
bi Xi +
i=1
3 i=1
bii Xi 2 +
3
bi j Xi X j
(1)
i< j, j=2
The range and levels of variables optimized are shown in Table 1. The statistical significance of regression terms was checked by analysis of variance, ANOVA. 2.4. CO2 adsorption CO2 adsorption characteristics of prepared porous MG were measured by Shimadzu thermo-gravimetric analyzer (Shimadzu TGA-50). 10 mg sample was put onto an alumina oxide crucible to be heated up to 450 °C and then kept at 450 °C for 30 min to remove moisture and impurities. Then, temperature was reduced to 60 °C to perform CO2 adsorption. Before each run, the sample
(2)
The conversion of the reaction then can be expressed as the following:
x=1−
WH2 O (MgO)
(3)
WH2 O (Mg(OH) ) 2
Where WH2 O (MgO ) refers to the water content in the final calcined
MgO (g), and WH2 O (Mg(OH) ) refers to water content in the raw 2 Mg(OH)2 (g). The water content in this works refers to all types of water bonded to MgO or Mg(OH)2 matrix, which not only includes the absorbed water but also includes the chemical boned crystallite water. In this work, we employed the widely used shrinking core model to model the decomposition process. The reaction time and conversion can be expressed as the following form, which includes all the controlling steps from surface reaction, internal diffusion and external diffusions [31,32]:
t =
CS0 r CS0 r x+ [1 − (1 − x )1/3 ] 3kgCA0 kSCA0 +
CS0 r 2 [1 − 3(1 − x )2/3 + 2(1 − x )] 6DeCA0
(4)
where t is the reaction time (min), CS0 is initial solid concentration (mol/l), CA0 is initial gas concentration (mol/l), kg is the gaseous diffusion constant factor, ks is surface reaction constant factor, r is the particle ratio (mm). De is an effective diffusivity (in this case, it refers to H2 O vapor diffusivity in m/s). In case of the conversion being controlled by the surface chemical reaction, then Eq. (4) can be further simplified as the following:
t=
CS0 r [1 − (1 − x )1/3 ] kSCA0
(5)
and its derivative form is the following: 2/3
dx 3ksCA0 (1 − x ) = dt CS0 r
(6)
Table 2 Three factor CCD with experimental results of dependent variables. Runs
Code values
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
X1 400 300 400 300 500 400 400 300 500 600 500 300 500 400 200 400 400
Experimental results mg/g X2 60 50 60 70 70 40 80 70 50 60 70 50 50 60 60 60 60
X3 11 14 13 12 12 13 13 14 14 13 14 12 12 15 13 13 13
30 19 31 28 25 29 25 26 31 30 27 20 30 32 15 30 32
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In case of the conversion being controlled by internal diffusion, Eq. (4) can be further simplified as the following:
t=
CS0 r 2 [1 − 3(1 − x )2/3 + 2(1 − x )] 6DeCA0
4.1. Statistical analysis and optimization of preparation conditions (7)
and its derivative form is the following: 1/3
dx 3DeCA0 (1 − x ) = · 1/3 dt CS0 r 2 1 − (1 − x )
4. Results and Discussion
(8)
In this work, the least squares regression was employed for constant approximation.
The experimental results associated with interaction between each independent variable are shown in Table 2. The CO2 removal capacity was found to be between 15 to 32 mg/g, respectively. Table 3 gives the ANOVA results for respective response. The higher Fischer’s ‘F-statistics’ value and lower ‘P’ value (probability) indicate the high significance of the regression model [33]. It is found that the term X1 , X1 X2 , X1 2 , are significant, while other terms have less significance for response. By applying multiple regression analysis
Fig. 2. Three-dimensional response surface for CO2 removal (a) dehydration temperature versus duration; (b) dehydration temperature versus FR; c) dehydration duration versus FR.
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Y. Sun et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–10 Table 3 ANOVA analysis for sulfur removal with r2 0.95, adjust r2 0.94, predicted r2 0.93, adequate precision 10. Source
DF
Sum of squares
Mean square
F-value
Prob > F
Model X1 X2 X3 X1 X2 X1 X3 X2 X3 X1 2 X2 2 X3 2 Residue Lack of fit Pure error Cor total
9.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 7.00 5.00 2.00 16.00
332.71 149.61 0.043 2.11 67.39 5.50 0.001 91.02 20.45 0.17 40.97 38.56 2.41 373.68
36.97 149.61 0.043 2.11 67.39 5.50 0.001 91.02 20.45 0.17 5.85 7.71 1.20
0.0119 0.0015 0.9341 0.5675 0.0115 0.3646 0.9911 0.0056 0.1038 0.8682
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Fig. 3. TG analysis of MG in presence of air.
Y = 30 + 6.12X1 − 0.10X2 + 0.73X3 − 11.61X1 X2 + 3.32X1 X3 − 0.001X2 X3 − 8.67X1 2 − 4.11X2 2 − 0.38X3 2
(9)
The CO2 removal was evaluated in terms of removal capacity through 3-D plots of responses as a function of two factors while keeping the third parameter at the optimal condition (Fig. 2). The response patterns of combination effect of different process parameters to CO2 removal present a different trend, especially in the combination effect of X1 X2 . In Fig. 2a, the combined effect of dehydration temperature and duration enhance CO2 removal capacity of the resultant adsorbent. However, the combination of too high dehydration temperature and long duration will lead to a decrease of the CO2 removal capacity. At low level of dehydration temperature and duration, the increased dehydration temperature and duration will facilitate the removal of water in MG matrix, which leads to pore development of MG, this could explain the increase of CO2 removal capacity. On the other hand, a too higher level of dehydration temperature and duration will lead to collapse of the previous formed intra-particle porous structure, which will result in a decrease of CO2 removal capacity. In terms of response patterns of the combination effects of X1 X3 and X2 X3 in Fig. 2b,c, response surface plots show a maximum region where the removal of CO2 neither increased nor decreased. Among combination effects, only the combined effect of X1 X2 (P = 0.011) is found to be significant to the response pattern. The standard square root r2 for the quadratic model is 0.95, which indicates over 95% of the variation in the response could be explained by the model. The obtained adjusted r2 was close to the experimental r2 and predicted r2 , indicating a well agreement between experimental data and predicted values. The value of ‘adequate precision’ was 10, indicating an adequate signal. Then the maximum CO2 removal capacity was set for optimization goal and 28 solutions were found. Out of them, the best condition selected is MG-480-42-13.8 representing sample being prepared as the following: the MG was calcined at 480 °C for 42 min with air flow rate being at 13.8 (Nm3 /h). The CO2 removal capacity predicted from model could reach 33.3 mg/g. An additional experiment was conducted to further validate the model prediction, and the CO2 removal capacity of MG-480-42-13.8 at the optimal condition could reach 32 mg/g, indicating a 4.1% experimental deviation. 4.2. Decomposition kinetics To understand decomposition kinetics, the thermo-gravimetric analysis in presence of air is shown in Fig. 3. There are two major
573 673 773
600
Pressure drop/Pa
to the experimental data, the following second-degree polynomial is derived to represent relationship between CO2 adsorption and three independent process variables (dehydration temperature, duration, and FR) as the following:
550
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Fig. 4. Pressure drop of fluidized bed for calcination of MG at different temperatures.
stages involving with dehydration, the peak at around 200 °C indicates the removal of the adsorbed water. The major weight loss is found at the temperature range of 300–450 °C, which corresponds to the reaction in Eq. (2):
Mg(OH )2 → MgO + H2 O ↑
(2)
The dehydration corresponds to the weight loss of 32%, which is larger than the theoretical weight loss of complete dehydration of Mg(OH)2 (31%), the discrepancy could be due to the existence of impurities. The flow characteristic of fluidized bed is investigated and result is shown Fig. 4. The FR was kept at the optimal condition to maintain a good fluidization of the MG sample in the hot bed. The pressure drop changes significantly during the first 10 min in all temperatures, indicating the significant dehydration occurrence during this stage. The variation of pressure drop becomes less once it is over 20 min. The particle size distribution of raw MG and MG500 is shown in Fig. 5, the obvious decrease of particle size is observed for MG-500 after calcination. The shrinking-core model (SCM) was applied against dehydration data and the results are shown in Fig. 6. Based upon flow characteristic of fluidized bed, the thermal decomposition can be broadly divided into two stages. In the first 10 min, dehydration is controlled by surface chemical reaction, of which the reaction rate is lower than the diffusion rate and the generated water vapor can easily pass through film layer without significant diffusion resistance. This is confirmed by good fit of Eq. (5) in Fig. 6a, which presents a linear correlation by neglecting internal and external diffusion resistance. The activation energy of reaction is 104 kJ/mol
Please cite this article as: Y. Sun et al., Clean production of porous MgO by thermal decomposition of Mg(OH)2 using fluidized bed: Optimization for CO2 adsorption, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.030
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2Theta/degree Fig. 5. Particle size distribution of before and after dehydroxylation at 773 K. Fig. 7. XRD spectra of different MG samples calcinated at different temperatures while keeping duration and FR at optimal conditions.
by using Arrhenius law. As reaction progresses, the reacted MgO layer becomes thicker and diffusion resistance outweighs reaction rate. The diffusion resistance from MG core to the bulk gas phase becomes dominant. This is confirmed by good fit of Eq. (7) in Fig. 6b, which presents a linear correlation by neglecting surface reaction rate. The corresponding activation energy of diffusion is 15 kJ/mol. These results are relatively smaller than literatures reports [34]. These discrepancies might be due to the different particle sizes, calcination temperature ramping rate and different operational pressures etc.
Among those three independent parameters, the dehydration temperature is found to be the most significant (in form of single and combination effect) to the CO2 removal capacity. In this work, a further investigation of dehydration temperature effect (keeping other process parameters at the optimal condition X2 = 42 min and X3 = 13.8 Nm3 /h), characterization of the corresponding MG samples being calcined at different temperatures in fluidized bed were conducted. The XRD spectra of MG samples calcined at different temperatures is shown in Fig. 7. The raw material MG at room temperature presents broad and scattering spectrum indicating the Mg(OH)2 produced from reaction of MgCl2 and NH4 OH is in an amorphous state [35]. When MG-200 is calcined at 200 °C, it becomes to change from amorphous to the crystallite Mg(OH)2 ,
which is confirmed by appearance of the characteristic peaks of crystallite Mg(OH)2 in 001 and 101 face in MG-200 sample [36]. As calcination temperature continues to increase to 300 °C, the predominant crystallite phase changes from Mg(OH)2 to MgO with existences of crystallite Mg(OH)2 in MG-300 sample, which is confirmed by the strong characteristic peaks of crystallite MgO in 111, 200, 220, 311, 222 faces [37] and remaining of small characteristic peaks of crystallite Mg(OH)2 in 001 and 101 face. Once the calcination temperature reaches 400 °C, the crystallite Mg(OH)2 (in 001 and 101 face) in MG-400 almost disappears. By referring to TG analysis, this temperature range corresponds to the major weight loss for the reaction in Eq. 2. The corresponding XRF and wet chemical analysis of MG-400 also shows over 98% weight percentage of MgO in the MG-400, this indicates a relatively complete conversion from Mg(OH)2 to MgO within 40 min in fluidized bed over 400 °C. As calcination temperature further increase, no further significant crystallite phase change occurs from 400 to 600 °C. The effect of dehydration temperature upon the BET specific surface area of MG and their corresponding N2 adsorption isotherms are shown in Fig. 8. The result indicates that calcination facilitates the increase of the surface area, at which stage the specific surface area of MG-500-42-13.8 reaches 75 m2 /g. In terms of nitrogen adsorption isotherms, the adsorption uptake occurs through the entire pressure range indicating the existence of both micropore
a
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4.3. Effect of dehydration temperature
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Fig. 6. Shrinking core model fit: (a) dehydration activation energy; (b) diffusion activation energy.
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Fig. 8. Specific surface area and adsorption isotherms of different MG samples calcined at different temperatures while keeping duration and FR at optimal conditions.
and mesopore in the sample [38]. Before calcination, all samples exhibit the mesoporous character, indicating hydrothermal treatment lead to the formation of mesoporous adsorbent, this agrees well with literature reports that the proper hydrothermal treatment will facilitate to the formation of mesoporous MgO crystal [39]. As calcination temperature increase, the nitrogen adsorption isotherm of MG-500-42-13.8 is greater than that of BL-300-42-13.8 indicating the increased surface area is mainly from dehydration reaction for micropore formation from MG matrix. The SEM morphologies of the raw material MG (Mg(OH)2 ) and MG calcined at different temperature in fluidized bed while keeping duration and FR at optimal conditions are shown in Fig. 9. Without calcination, the MG presents a dense and coarse surface, all the particles seems to aggregate together indicating a poor pore formation. This agrees with specific surface area analysis. As the MG samples were thermal treated in the fluidized bed at different temperatures, a gradual trend of rod particle formation and aggregation is observed. In addition, these rod shape particles tend to become smaller as calcination temperature increase. This result also agrees with bulk particle size distribution analysis. The porosity development during dehydration reaction in the fluidized bed is complicated. The shrinking core mechanism during dehydration might contribute to the formation of the rod shaped particles when water vapor is released from the core of unreacted particle. In addition, the good mass transfer in the fluidized bed might facilitate external surface creation during aggregation of the newly formed particles, which
in turn leads to an increased specific surface area. By considering CO2 adsorption capacity, it is found that the increased CO2 capacity is divided into two calcination temperature ranges. The first stage is from 300 to 500 °C, of which both significant crystallite change from Mg(OH)2 to MgO and increase of BET specific surface area due to dehydration reaction contribute to the significant increase of CO2 adsorption capacity. The possible reason could be a) MgO has relatively larger adsorption affinity for CO2 than that of Mg(OH)2 during gas solid adsorption. b) micropore formation during dehydration also facilitates CO2 adsorption on porous adsorbent. Once calcination temperature reaches over 500 °C, the crystallite change from Mg(OH)2 to MgO does not change appreciably, the specific surface area even experience a slight decrease. During this temperature range, no appreciable increased CO2 adsorption capacity is observed. 4.4. Comparison of CO2 removal capacity Due to limited resources and different CO2 adsorption conditions, we only make a limited comparison in this work. In Table 4, the CO2 adsorption capacity among different literature reports by different prepared adsorbents are compared. The CO2 adsorption capacity follows order of MgO (Solvothermal) < MgO/Al2 O3
Table 4 Comparison of adsorption capacity, where MgO/K-SBA represents MgO on mesoporous silica together with introducing potassium cations, MgO/Al-SBA represents MgO on mesoporous silica together with introducing aluminum cations, MgO/OMC refers MgO on ordered mesoporous carbon. Sample
SSA/m2 /g
Calcination temperature/°C
Calcination duration /h
CO2 adsorption temperature/°C
adsorption capacity /mg/g
Reference
MgO/Al2 O3 MgO/CMK-3 MgO/OMC MgO (solvothermal) MgO (solvothermal) Foam magnesia MgO/K-SBA MgO/Al-SBA MgO MgO MG-480-42-13.8
300 250 598 373 204 130 257 235 – 101 85
600 800 900 450 550 600 540 450 400 400 480
5 8.8 6 6.7 22 12 17 7.8 5.3 8.6 0.7
25 25 25 – 90 100 20 25 50 50 60
32.2 80 90 – 16 115 40 60 70 36 34
[40] [41] [42] [39] [43] [44] [45] [46] [47] [48] This work
Please cite this article as: Y. Sun et al., Clean production of porous MgO by thermal decomposition of Mg(OH)2 using fluidized bed: Optimization for CO2 adsorption, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.030
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Fig. 9. SEM morphology of MG samples calcined at different temperatures, A refers to MG at room temperature, B refers to MG-200, C refers to MG-300, D refers to MG-400, E refers to MG-500, F refers to MG-600.
carbon and silica presents the highest adsorption capacity. But considering its complicated preparation procedures and many different chemicals involved, the technical hurdles of massive production of these types of adsorbents are still hard to overcome in the short terms. The porous MgO prepared from direct calcination,
on the other hand, obviously overcome the difficulties that mentioned above. Relative simple procedures and much less chemicals involvements show its promising prospect. In addition, by carefully comparing the thermal treatment duration of each different preparation approaches, the obvious advantage of using fluidized bed
Please cite this article as: Y. Sun et al., Clean production of porous MgO by thermal decomposition of Mg(OH)2 using fluidized bed: Optimization for CO2 adsorption, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.030
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reactor is very convincing. The calcination duration can be significantly reduced from almost 6–22 h using fixed bed reactor down to 0.7 h using fluidized bed reactor. By taking the life time cycle analysis into consideration, the thermal treatment duration itself definitely count on the CO2 emissions. The significant reduced calcination duration indicates a much less CO2 emission from manufacturing of adsorbents itself. This is the advantage of our process. Overall, the CO2 adsorption capacity from porous MgO produced from our process is comparable to other literature reports and is superior to current commercial MgO. By considering the significantly reduced thermal treatment durations, the advantage of using fluidized bed in process intensification is obvious. 5. Conclusions The porous MgO was prepared from thermal decomposition of Mg(OH)2 using fluidized bed. The experiments data was found to have a good fit for the proposed shrinking core model. The initial stage of decomposition is controlled by surface chemical reaction with activation energy reaching 104 kJ/mol. The second stage of reaction after 10 min is controlled by diffusion with activation energy being 15 kJ/mol. The RSM (Response surface methodology) and the CCD (Central composite design) were employed to find the optimal adsorbents with the maximum CO2 removal capacity. The operational parameters such as dehydration temperature (°C), duration (min) and FR were chosen as independent variables in CCD. The statistical analysis indicates that the effects of dehydroxylation temperature and combined effect of temperature and duration are all significant to the CO2 removal capacity. The optimal condition for achieving the maximum CO2 adsorption capacity is obtained as the following: temperature (480 °C), duration (42 min), FR (13.8 Nm3 /h) with CO2 removal capacity reaching 33 mg/g. The employment of fluidized bed in process intensification significantly reduces the thermal treatment duration. Acknowledgements Anpeng high-tech energy and resource company, Shenhua Beijing institute are highly appreciated for supporting this work. Authors would like to give special thanks for technical discussions with Dr Yarong Li from NSW government environmental protection agency for better improvement of this work. References [1] Sun Y, Parikh V, Zhang L. Sequestration of carbon dioxide by indirect mineralization using Victorian brown coal fly ash. J Hazard Mater 2012;209:458–66. [2] Lackner KS. A guide to CO2 sequestration. Science 20 03;30 0:1677–8. [3] Hedin N, Andersson L, Bergstrom L, Yan JY. Adsorbents for the post-combustion capture of CO2 using rapid temperature swing or vacuum swing adsorption. Appl Energ 2013;104:418–33. [4] Sun Y, Yang G, Zhang JP, Wang YS, Yao MS. Activated carbon preparation from lignin by H3 PO4 activation and its application to gas separation. Chem Eng Technol 2012;35:309–16. [5] Alabadi A, Razzaque S, Yang YW, Chen S, Tan B. Highly porous activated carbon materials from carbonized biomass with high CO2 capturing capacity. Chem Eng J 2015;281:606–12. [6] Sun Y, Zhang JP, Yang G, Li ZH. An improved process for preparing activated carbon with large specific surface area from corncob. Chem Biochem Eng Q 2007;21:169–74. [7] Sun Y, Wei J, Wang YS, Yang G, Zhang JP. Production of activated carbon by K2 CO3 activation treatment of cornstalk lignin and its performance in removing phenol and subsequent bioregeneration. Environ Technol 2010;31:53–61. [8] Bezerra DP, da Silva FWM, de Moura PAS, Sousa AGS, Vieira RS, Rodriguez– Castellon E, Azevedo DCS. CO2 adsorption in amine-grafted zeolite 13X. Appl Surf Sci 2014;314:314–21. [9] Dinara Andirova CFC, Lei Yu, Choi Sunho. Effect of the structural constituents of metal organic frameworks on carbon dioxide capture. Micropor Mesopor Mat 2016;219:276–305. [10] Lu K. Porous and high surface area silicon oxycarbide-based materials—a review. Mater Sci Eng: R: Rep 2015;97:23–49.
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Please cite this article as: Y. Sun et al., Clean production of porous MgO by thermal decomposition of Mg(OH)2 using fluidized bed: Optimization for CO2 adsorption, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.02.030