- Email: [email protected]
Preparation and characterization of activated carbon from rubber-seed shell by physical activation with steam
biomass and bioenergy 34 (2010) 539–544
Available at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Preparation and characterization of activated carbon from rubber-seed shell by physical activation with steam Kang Sun, Jian chun Jiang* Institute of Chemical Industry of Forest Products, CAF, Suojin wucun 16, Nanjing 210042, PR China
article info
abstract
Article history:
The use of rubber-seed shell as a raw material for the production of activated carbon with
Received 16 June 2008
physical activation was investigated. The produced activated carbons were characterized
Received in revised form
by Nitrogen adsorption isotherms, Scanning electron microscope, Thermo-gravimetric and
24 December 2009
Differential scanning calorimetric in order to understand the rubber-seed shell activated
Accepted 26 December 2009
carbon. The results showed that rubber-seed shell is a good precursor for activated carbon.
Available online 25 January 2010
The optimal activation condition is: temperature 880 C, steam flow 6 kg h1, residence time 60 min. Characteristics of activated carbon with a high yield (30.5%) are: specific
Keywords:
surface area (SBET) 948 m2 g1, total volume 0.988 m3 kg1, iodine number of adsorbent
Rubber-seed shell
(qiodine) 1.326 g g1, amount of methylene blue adsorption of adsorbent (qmb) 265 mg g1,
Activated carbon
hardness 94.7%. It is demonstrated that rubber-seed shell is an attractive source of raw
Adsorption
material for producing high capacity activated carbon by physical activation with steam. ª 2010 Elsevier Ltd. All rights reserved.
Physical activation Porosity Hevea brasiliensis
1.
Introduction
Southern China has 3670 km2 of rubber plantations producing 300 kt y1 of rubber seeds. The pip of rubber-seed is sent to oilmills, but a huge amount of rubber-seed shells as agricultural waste which has become an environment problem [1]. One solution for this situation is the reuse of this waste to produce activated carbon which is one of the most widely used materials because of its exceptional adsorption properties [2]. Activated carbon can be applied in a variety of purification and separation, in the abatement of hazardous contaminants, municipal and industrial wastewater treatment, as catalyst or catalyst support in medicine, and the recovery of valuable metals etc. In principle, the activation methods can be divided into two categories: chemical and physical activation. In a chemical activation, the raw material is impregnated with an
activating reagent like ZnCl2, H3PO4, KOH and K2CO3 or their mixtures [3–5]. In physical activation, a raw material is first carbonized, and the resulting char is secondarily activated under a flow of suitable gas such as steam, carbon dioxide, air or their mixtures [6–9]. The physical preparation process using steam was investigated in this study. It is generally recognized that carbon dioxide develops mainly microporosity, and that steam produces a wider pore size distribution, with larger development of wide micropores and mesopores [10]. As for the properties, pore structure (in terms of surface area and pore volume) is an important characteristic of activated carbon. In general, activated carbon with both a high surface area and porosity, allowing large capacity of adsorption, is desirable [11]. The rubber-seed shell is a type of agriculture waste which has caused environment contamination problems in rubber tree plantations, and so far, there are few reports of the
* Corresponding author. Tel.: þ86 25 85482484; fax: þ86 25 85482485. E-mail address: [email protected] (J.c. Jiang). 0961-9534/$ – see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2009.12.020
540
biomass and bioenergy 34 (2010) 539–544
Table 1 – Rubber-seed shell proximate analysis. Parameter
%
Moisture (air-dry) Volatile matters Fixed carbon Ash
14.37 71.76 13.97 0.17
preparation of rubber-seed activated carbon. In this paper, activated carbon produced from rubber-seed shell by physical activation with steam, and the characteristics (specific surface area, pore volume, and pore size distribution) of activated carbon were investigated. The production of this material could be of great interest, and the results of this paper can be as helpful reference for rubber agriculture expanding profit.
2. 2.1.
Material and methods
Rubber-seed shell from the south of China was used as starting material. The proximate analysis of the rubber-seed shell is shown in Table 1. The as-received rubber-seed shell was washed to remove dust and then dried, crushed and sieved to a particle size fraction of 0.8–2.5 mm before it was being processed.
Carbonization and activation
2.2.1.
Carbonization
Pyrolysis of fresh rubber-seed shell was performed in a carbonization device under a flow of nitrogen gas. The sample was heated at 10 C min1 from room temperature to the range of 400–650 C.
2.2.2.
Sample characterization
2.3.1.
Specific surface
Specific surface area (SBET) and porosity of the samples were determined by nitrogen adsorption–desorption isotherms measured in a Micromeritics ASAP 2020 apparatus. Adsorption of N2 was performed at 77 k. Before any such analysis, the samples were degassed under N2 flow at 350 C for 2 h in a vacuum at 27 Pa. The specific surface area of the prepared activated carbons was estimated by the BET method using N2 adsorption isotherm data. The micropore volumes were calculated from the amount of N2 adsorbed at a relative pressure of 0.1, and the mesopore volumes were calculated by subtracting the amounts adsorbed at a relative pressure of 0.1 from those at a relative pressure of 0.95 [12]. To calculate the pore size distribution of activated carbons, the Barret–Joyner– Halenda (BJH) model was employed.
2.3.2.
Material and pretreatment
2.2.
2.3.
Activation
Following the carbonization, the carbon samples were pyrolized also in the same device, in a flow of vapor stream, to the range of 800–900 C, activated carbons with various specific surface area were prepared.
SEM
A Hitachi S-3400 scanning electron microscope (SEM) was used in the study. The structural feature of the raw rubbershell and the produced activated carbons were observed at the accelerated voltage of 15 kV. Before observation, the samples were coated with gold in E-1010 Ion sputter.
2.3.3.
TG and DSC
Thermo-gravimetric (TG) experiments were carried out by a thermo-gravimetric analyzer (Netzsch STA 409) in order to determine the pyrolysis behavior of rubber-seed shell. The dried shell was subjected to measure in the temperature range of 30–900 C at heating rate 10 C min1 under flowing of nitrogen gas flow and held at 900 C for 10 min.
2.3.4.
Proximate analysis
The proximate analysis was conducted according to GB/ T12496.1–12496. 22(1999) and the results were expressed in terms of moisture, volatile matter, ash and fixed carbon content. To determine the moisture, 1 g of air dried rubberseed shell was loaded in a crucible and heated under 110 C in an oven for 3 h until dehydration was accomplished. One gram of dried sample was loaded in crucible with cover and heated from room temperature to 850 C and maintained for
Table 2 – Preparation conditions and physical properties of the samples. Sample 1 2 3 4 5 6 7 8 9 10 11 12 13
Temp. ( C)
Time (min)
Steam (kg h1)
QIodine (mg g1)
Qmb (mg g1)
Yield (%)
Hardness (%)
820 820 820 850 850 850 880 880 880 880 880 880 880
40 60 80 40 60 80 40 60 60 60 80 80 80
4 6 8 8 6 4 8 4 6 4 8 6 8
742 913 905 867 996 979 1026 1141 1326 1043 1048 1268 1149
75 90 105 150 195 120 240 255 265 255 235 245 235
64.0 52.6 52.0 48.5 43.0 50.2 40.5 30.5 30.5 26.7 20.7 17.6 15.7
95.4 94.8 94.3 94.7 94.0 94.4 94.2 92.8 94.7 91.2 92.4 91.0 90.1
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biomass and bioenergy 34 (2010) 539–544
0.025
Volatile (wt %)
Ash (wt %)
Fixed C (wt %)
9.8 8.7 9.1
6.2 5.6 5.4
1.6 1.6 1.7
82.4 84.1 83.8
0.020
Sample 2, 820 Sample 5, 850 Sample 9, 880
-1
Moisture (wt %)
3
Sample
Pore Volume (10 . m . kg )
Table 3 – Proximate analysis of three selected activated carbons.
7 min in a muffle furnace. Then, the crucible was removed from the muffle furnace and cooled to room temperature in a desiccator to determine the quantity of volatile. To determine the ash content, approximately 1 g of sample was placed in covered crucible and heated in the muffle furnace from room temperature to 750 C and kept for 3 h. Then, the crucible was placed in a desiccator and cooled at room temperature. The fixed carbon content was obtained by subtracting moisture, volatile matter and ash content from 100%.
3.
Results and discussion
3.1.
Result of produced activated carbons
The preparation conditions and porous properties of the resulting activated carbon including iodine number of adsorbent (qiodine), amount of methylene blue adsorption of adsorbent (qmb) and hardness are listed in Table 2. In general, qiodine and qmb are considered as a measure of adsorption capability of activated carbon for low- and high-molar-mass adsorbates, respectively [13]. Normally, qiodine denotes the amount of micropore and qmb denotes the amount of mesopore of activated carbon. Since the qiodine, qmb of the samples range from 742 to 1326 mg g1 and 75–265 mg g1, the activation is apparently effective in increasing the adsorption ability of activated carbon. Results showed that the highest values of qiodine (1326 mg g1) and qmb(265 mg g1) were obtained at 880 C.
Quantity Adsorbed (m3. kg STP)
0.6
0.015
-3
Sample 2820 Sample 5850 Sample 9880
0.010
0.005
0.000 2
4
6
8
10
12
Pore diameter (nm)
Fig. 2 – Pore size distribution of activated carbons.
Samples 7, 10, 13 show that qiodine is highest when the activated time is 60 min but decreases at 80 min. Prolonged activation may cause over activation, accelerating surface erosion more than pore formation [14]. The effect of the steam flow rate is shown by Samples 11, 12, 13, indicating that qiodine is highest at a flow rate of 6 kg h1 but decreases at higher flow rate. This may indicate that a moderate steam flow is preferable to promote the activation reaction, but excessively faster flow may slow the activation reaction because the steam is a kind of endothermal liquid and can adsorb much heat. Yield and hardness are two key factors for granule activated carbon production industry. Normally, yield value over 25% and hardness value over 90% are satisfying for granule activated carbon products. The yield of Sample 12 (17.6%) is too low for producing acceptable activated carbon. These results indicate that the following activation conditions are appropriate for preparing high adsorption capacity activated carbon from rubber-seed shell with physical activation: temperature 880 C, activating time 60 min, and steam flow rate 6 kg h1. Sample 2, Sample 5 and Sample 9 which were activated in 820 C, 850 C, 880 C separately were selected for further study in the next steps. The proximate analysis of the selected three samples was shown in Table 3.
0.5
3.2.
N2 adsorption and pore size distribution
0.4
The N2 adsorption isotherms of the selected three activated carbons are presented in Fig. 1. Here the quantity of N2 adsorbed is plotted against the relative pressure p/p0 ( p ¼ pressure, p0 ¼ saturated vapor pressure) of N2. It can be seen that the N2 adsorption isotherm of Sample 2 prepared at
0.3
0.2
Sample 2, 820 Sample 5, 850 0.1
Sample 9, 880
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Relative Pressure (P/P0)
Fig. 1 – Nitrogen adsorption–desorption isotherms of activated carbons at 77 K; closed symbols: adsorption, open symbols: desorption.
1.0
Table 4 – Pore volume, surface area and average diameter of the tested activated carbons. Sample Sample 2 Sample 5 Sample 9
VT (ml g1) Vmic (cm3 g1) SBET (m2 g1) D (nm) 0.668 0.814 0.988
0.521 0.628 0.615
878 893 948
2.48 3.34 3.65
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biomass and bioenergy 34 (2010) 539–544
820 C corresponds to Type I, which indicates that it is predominant microporous activated carbon that has not developed a porous structure. The amount of N2 adsorbed on Sample 5 prepared at 850 C increased with p/p0 in the range of p/p0 > 0.1. The N2 adsorption isotherm on Sample 9 prepared at 880 C begins to change considerably, like Type III. At this temperature, mesopores have been developed, as indicated by the shape of the hysteresis loop, which is often associated with the presence of slit-shaped mesopores [15]. Pore size distribution of the selected three activated carbons (Sample 2, Sample 5 and Sample 9) was shown in Fig. 2. The pore structure of activated carbon is generally characterized in term of the pore size distribution. One can see the activating temperature had significant effect on the pore structure of activated carbon prepared. At low temperature, the pore structure was mainly consisted of micropore; however, with the increase of carbonization temperature, the creation of mesopore increased and also increased the total pore volume of activated carbon [16,17]. Micropore relative ratio decreased but mesopore relative ratio increased. It was learned that the reaction rate between activating agent and carbon increased geometrically as the temperature rate increasing. At high temperature, micropores were enlarged and the walls between pores collapsed and formed mesopores. Specific surface area (SBET), total pore volume (VT), micropore volume (Vmic) and average diameter (D) of the tested activated carbons are tabulated in Table 4. It is noticed that VT increased with activation temperature rising, but Vmic is different since the Vmic of Sample 9 is not bigger than Sample 5. When activation occures at temperature above 850 C, the steam can enlarge the exist micropores while making new micropores.
3.3.
SEM
The microstructure of the raw rubber-seed shell and the selected three activated carbons (Sample 2, Sample 5, Sample 9) prepared at different temperature are shown in the Fig. 3. For the raw shell material, the micrograph showed clearly the canal structure, which is a good texture for preparing activated carbon, because activating agents can easily contact with the inside surfaces. From the SEM micrographs, it can be seen the morphological characteristics are much different between raw material and three activated carbons. Sample 2 shows perfect canal structure for the raw material. That means the carbonization is developed but activation is not much developed. The surface in Sample 5 shows that the canal structure has been partly broken which means the activation process happened and the surface was eroded by vapor steam a little more than the Sample 2, the SBET of Sample 5 is much bigger. Sample 9 shows a most well-developed porous structure among the three activated carbons. Whereas, 880 C is just 30 C higher than 850 C, but the surface area is much larger, which further testifies that the influence of temperature on carbon activation carbon is geometrically related. The micrographs showed the evolution of the pore structure containing some areas showing some fragmentary structure. The surface of the activated carbon presents some white fine particles, called ash content, which may fill or block some portion of the micropore.
3.4.
TG and DSC analysis
Fig. 4 shows the TG curve of raw rubber-seed shell by weight in a N2 atmosphere and at a 10 C min1 heating rate. The thermal
Fig. 3 – SEM of three selected samples.
biomass and bioenergy 34 (2010) 539–544
543
specific surface area and pore volume. In addition, the micropore and mesopore are both developed that made the activated carbon suitable for liquid phase and gas phase adsorption industries. Yield and hardness of rubber-seed shell activated carbon are also highly comparable to commercial activated carbon. Thus preparing activated carbon from waste rubberseed shell is a promising work.
Acknowledgement Fig. 4 – TG/DSC curves for rubber-seed shell.
decomposition behavior of dried rubber-seed shell is in the first stage from 273 C to 385 C; the next stage from 385 C to 800 C. It can be seen from the Fig. 4, little weight loss happened at temperature below 270 C because the rubber-seed had been dried under 105 C for 3 h. The first stage presents a significant weight loss due to much volatile released. The reason is that cellulose and hemi-cellulose of the sample decomposed to condensable gas (acetic acid, methanol and, wood tar) and uncondensable gas (CO, CO2, CH4, H2, H2O). During next stage, lignin begins to decompose and lose weight. Also the residual volatiles from the first stage are further released but carbon was left. A plateau curve at temperature over 800 C showed the weight unchanged at this stage. We can see from this curve, at 500–600 C, carbon yield is 35%, including 15% volatiles. The carbon at this stage is acceptable as a precursor for producing activated carbon because a carbon with 15% volatile is not too tight, and can easily react with activating reagent to produce big surface and huge pore volume. So it is apparent that rubber-seed shell can be as good raw material. Fig. 4 also shows DSC analysis. The DSC curve shows a little drop at a temperature of 100 C, because the sample needs heat to evaporate the remaining water. From 180 C, hemicellulose started to decompose and release heat. At 240 C, cellulose began to decompose and release heat. When temperature was about 320 C, DSC arrived at valley, where hemi-cellulose and cellulose decomposed and produced abundant organic compounds, such as wood tar, ketone and methanol which would vaporize by adsorbing large heat. At 500 C, DSC curve climbed up steeply and reached at a peak, because at this step lignin pyrolyzed and emited abundant heat. At 800 C, DSC curve reached at base line, pyrolysis reaction was over and the residue was just graphite carbon and ash.
4.
Conclusions
The results of this work demonstrate that rubber-seed shell is an attractive source of raw material for preparing high quality activated carbon by physical activation with steam. Activation condition has a significant influence on porous properties. After the activation at temperature 880 C, steam flow 6 kg h1, and time of 60 min, the activated carbon is obtained with huge
This research is financially supported from the projects in the national science & technology pillar program in the eleventh five-year plan period (2006BAD19B06) and central-level scientific research institutions basic funding (CAFINT2007C16). The authors are thankful to Prof. Chung-yun Hse of the USDA forest service for critical reading of this paper.
references
[1] Li XZ. On integrated utilization of rubber seed. Economic Forest Research 1995;13(4):35–7. [2] Ioannidou O, Zabaniotou A. Agricultural residues as precursors for activated carbon production–A review. Renewable and Sustainable Energy Reviews 2007;11(9):1966–2005. [3] Tsai WT, Chang CY, Lee SL. Preparation and characterization of activated carbons from corn cob. Carbon 1997;35:1198–200. [4] Guo YP, Rockstraw DA. Activated carbons prepared from rice hull by one-step phosphoric acid activation. Microporous and Mesoporous Materials 2007;100(1–3):12–9. [5] Hayshi J, Toshihide H, Isao T, Katsuhiko M, Fard NA. Preparing activated carbon from various nutshells by chemical activation with K2CO3. Carbon 2002;40:2381–6. [6] Haykiri-Acma H, Yaman S, Kucukbayrak S. Gasification of biomass chars in steam-nitrogen mixture. Energy Conversion and Management 2006;47(7–8):1004–13. [7] Ahmedna M, Marshall WE, Rao RM. Production of granular activated carbons from select agricultural by-products and evaluation of their physical, chemical and adsorption properties. Bioresource Technology 2000;71(2):113–23. [8] Zhang TY, Walawender WP, Fan LT, Fan MH, Daugaard D, Brown RC. Preparation of activated carbon from forest and agricultural residues through CO2 activation. Chemical Engineering Journal 2004;105(1–2):53–9. [9] Yang T, Lua AC. Characteristics of activated carbons prepared from pistachio-nut shells by physical activation. Journal of Colloid and Interface Science 2003;267(2):408–17. [10] Molina-Sabio M, Gonzalez MT, Rodriguez-Reinoso F, Sepu´lveda-Escribano A. Effect of steam and carbon dioxide activation in the micropore size distribution of activated carbon. Carbon 1996;34:505–9. [11] Teng HS, Ho JA, Hsu YF. Preparation of activated carbons from bituminous coal with CO2. Carbon 1997;35:275–83. [12] Rodriguez-Reinoso F, Lopez-Gonzalez J de D, Berenguer C. Activated carbons from almond shellsdI: preparation and characterization by nitrogen adsorption. Carbon 1982;20:513–8. [13] Gergova K, Galushko A, Petrov N, Minkova V. Investigation of the porous structure of activated carbons prepared by pyrolysis of agricultural by-products in a stream of water vapor. Carbon 1992;30:721–7. [14] Kiyoshi O, Nobuo Y, Yoshikazu K, Atsuo Y. Porous properties of activated carbons from waste newspaper prepared by
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chemical and physical activation. Journal of Colloid and Interface Science 2003;262(1):179–93. [15] La´zaro MJ, Ga´lvez ME, Artal S, Palacios JM, Moliner R. Preparation of steam-activated carbons as catalyst supports. Journal of Analytical and Applied Pyrolysis 2007; 78(2):301–15.
[16] Ismadji S, Bhatia SK. Investigation of network connectivity in activated carbons by liquid phase adsorption. Langmuir 2000;16(24):9303–13. [17] Ismadji S, Bhatia SK. A modified pore filling isotherm for liquid phase adsorption in activated carbon. Langmuir 2001; 17(5):1488–98.
Available at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Preparation and characterization of activated carbon from rubber-seed shell by physical activation with steam Kang Sun, Jian chun Jiang* Institute of Chemical Industry of Forest Products, CAF, Suojin wucun 16, Nanjing 210042, PR China
article info
abstract
Article history:
The use of rubber-seed shell as a raw material for the production of activated carbon with
Received 16 June 2008
physical activation was investigated. The produced activated carbons were characterized
Received in revised form
by Nitrogen adsorption isotherms, Scanning electron microscope, Thermo-gravimetric and
24 December 2009
Differential scanning calorimetric in order to understand the rubber-seed shell activated
Accepted 26 December 2009
carbon. The results showed that rubber-seed shell is a good precursor for activated carbon.
Available online 25 January 2010
The optimal activation condition is: temperature 880 C, steam flow 6 kg h1, residence time 60 min. Characteristics of activated carbon with a high yield (30.5%) are: specific
Keywords:
surface area (SBET) 948 m2 g1, total volume 0.988 m3 kg1, iodine number of adsorbent
Rubber-seed shell
(qiodine) 1.326 g g1, amount of methylene blue adsorption of adsorbent (qmb) 265 mg g1,
Activated carbon
hardness 94.7%. It is demonstrated that rubber-seed shell is an attractive source of raw
Adsorption
material for producing high capacity activated carbon by physical activation with steam. ª 2010 Elsevier Ltd. All rights reserved.
Physical activation Porosity Hevea brasiliensis
1.
Introduction
Southern China has 3670 km2 of rubber plantations producing 300 kt y1 of rubber seeds. The pip of rubber-seed is sent to oilmills, but a huge amount of rubber-seed shells as agricultural waste which has become an environment problem [1]. One solution for this situation is the reuse of this waste to produce activated carbon which is one of the most widely used materials because of its exceptional adsorption properties [2]. Activated carbon can be applied in a variety of purification and separation, in the abatement of hazardous contaminants, municipal and industrial wastewater treatment, as catalyst or catalyst support in medicine, and the recovery of valuable metals etc. In principle, the activation methods can be divided into two categories: chemical and physical activation. In a chemical activation, the raw material is impregnated with an
activating reagent like ZnCl2, H3PO4, KOH and K2CO3 or their mixtures [3–5]. In physical activation, a raw material is first carbonized, and the resulting char is secondarily activated under a flow of suitable gas such as steam, carbon dioxide, air or their mixtures [6–9]. The physical preparation process using steam was investigated in this study. It is generally recognized that carbon dioxide develops mainly microporosity, and that steam produces a wider pore size distribution, with larger development of wide micropores and mesopores [10]. As for the properties, pore structure (in terms of surface area and pore volume) is an important characteristic of activated carbon. In general, activated carbon with both a high surface area and porosity, allowing large capacity of adsorption, is desirable [11]. The rubber-seed shell is a type of agriculture waste which has caused environment contamination problems in rubber tree plantations, and so far, there are few reports of the
* Corresponding author. Tel.: þ86 25 85482484; fax: þ86 25 85482485. E-mail address: [email protected] (J.c. Jiang). 0961-9534/$ – see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2009.12.020
540
biomass and bioenergy 34 (2010) 539–544
Table 1 – Rubber-seed shell proximate analysis. Parameter
%
Moisture (air-dry) Volatile matters Fixed carbon Ash
14.37 71.76 13.97 0.17
preparation of rubber-seed activated carbon. In this paper, activated carbon produced from rubber-seed shell by physical activation with steam, and the characteristics (specific surface area, pore volume, and pore size distribution) of activated carbon were investigated. The production of this material could be of great interest, and the results of this paper can be as helpful reference for rubber agriculture expanding profit.
2. 2.1.
Material and methods
Rubber-seed shell from the south of China was used as starting material. The proximate analysis of the rubber-seed shell is shown in Table 1. The as-received rubber-seed shell was washed to remove dust and then dried, crushed and sieved to a particle size fraction of 0.8–2.5 mm before it was being processed.
Carbonization and activation
2.2.1.
Carbonization
Pyrolysis of fresh rubber-seed shell was performed in a carbonization device under a flow of nitrogen gas. The sample was heated at 10 C min1 from room temperature to the range of 400–650 C.
2.2.2.
Sample characterization
2.3.1.
Specific surface
Specific surface area (SBET) and porosity of the samples were determined by nitrogen adsorption–desorption isotherms measured in a Micromeritics ASAP 2020 apparatus. Adsorption of N2 was performed at 77 k. Before any such analysis, the samples were degassed under N2 flow at 350 C for 2 h in a vacuum at 27 Pa. The specific surface area of the prepared activated carbons was estimated by the BET method using N2 adsorption isotherm data. The micropore volumes were calculated from the amount of N2 adsorbed at a relative pressure of 0.1, and the mesopore volumes were calculated by subtracting the amounts adsorbed at a relative pressure of 0.1 from those at a relative pressure of 0.95 [12]. To calculate the pore size distribution of activated carbons, the Barret–Joyner– Halenda (BJH) model was employed.
2.3.2.
Material and pretreatment
2.2.
2.3.
Activation
Following the carbonization, the carbon samples were pyrolized also in the same device, in a flow of vapor stream, to the range of 800–900 C, activated carbons with various specific surface area were prepared.
SEM
A Hitachi S-3400 scanning electron microscope (SEM) was used in the study. The structural feature of the raw rubbershell and the produced activated carbons were observed at the accelerated voltage of 15 kV. Before observation, the samples were coated with gold in E-1010 Ion sputter.
2.3.3.
TG and DSC
Thermo-gravimetric (TG) experiments were carried out by a thermo-gravimetric analyzer (Netzsch STA 409) in order to determine the pyrolysis behavior of rubber-seed shell. The dried shell was subjected to measure in the temperature range of 30–900 C at heating rate 10 C min1 under flowing of nitrogen gas flow and held at 900 C for 10 min.
2.3.4.
Proximate analysis
The proximate analysis was conducted according to GB/ T12496.1–12496. 22(1999) and the results were expressed in terms of moisture, volatile matter, ash and fixed carbon content. To determine the moisture, 1 g of air dried rubberseed shell was loaded in a crucible and heated under 110 C in an oven for 3 h until dehydration was accomplished. One gram of dried sample was loaded in crucible with cover and heated from room temperature to 850 C and maintained for
Table 2 – Preparation conditions and physical properties of the samples. Sample 1 2 3 4 5 6 7 8 9 10 11 12 13
Temp. ( C)
Time (min)
Steam (kg h1)
QIodine (mg g1)
Qmb (mg g1)
Yield (%)
Hardness (%)
820 820 820 850 850 850 880 880 880 880 880 880 880
40 60 80 40 60 80 40 60 60 60 80 80 80
4 6 8 8 6 4 8 4 6 4 8 6 8
742 913 905 867 996 979 1026 1141 1326 1043 1048 1268 1149
75 90 105 150 195 120 240 255 265 255 235 245 235
64.0 52.6 52.0 48.5 43.0 50.2 40.5 30.5 30.5 26.7 20.7 17.6 15.7
95.4 94.8 94.3 94.7 94.0 94.4 94.2 92.8 94.7 91.2 92.4 91.0 90.1
541
biomass and bioenergy 34 (2010) 539–544
0.025
Volatile (wt %)
Ash (wt %)
Fixed C (wt %)
9.8 8.7 9.1
6.2 5.6 5.4
1.6 1.6 1.7
82.4 84.1 83.8
0.020
Sample 2, 820 Sample 5, 850 Sample 9, 880
-1
Moisture (wt %)
3
Sample
Pore Volume (10 . m . kg )
Table 3 – Proximate analysis of three selected activated carbons.
7 min in a muffle furnace. Then, the crucible was removed from the muffle furnace and cooled to room temperature in a desiccator to determine the quantity of volatile. To determine the ash content, approximately 1 g of sample was placed in covered crucible and heated in the muffle furnace from room temperature to 750 C and kept for 3 h. Then, the crucible was placed in a desiccator and cooled at room temperature. The fixed carbon content was obtained by subtracting moisture, volatile matter and ash content from 100%.
3.
Results and discussion
3.1.
Result of produced activated carbons
The preparation conditions and porous properties of the resulting activated carbon including iodine number of adsorbent (qiodine), amount of methylene blue adsorption of adsorbent (qmb) and hardness are listed in Table 2. In general, qiodine and qmb are considered as a measure of adsorption capability of activated carbon for low- and high-molar-mass adsorbates, respectively [13]. Normally, qiodine denotes the amount of micropore and qmb denotes the amount of mesopore of activated carbon. Since the qiodine, qmb of the samples range from 742 to 1326 mg g1 and 75–265 mg g1, the activation is apparently effective in increasing the adsorption ability of activated carbon. Results showed that the highest values of qiodine (1326 mg g1) and qmb(265 mg g1) were obtained at 880 C.
Quantity Adsorbed (m3. kg STP)
0.6
0.015
-3
Sample 2820 Sample 5850 Sample 9880
0.010
0.005
0.000 2
4
6
8
10
12
Pore diameter (nm)
Fig. 2 – Pore size distribution of activated carbons.
Samples 7, 10, 13 show that qiodine is highest when the activated time is 60 min but decreases at 80 min. Prolonged activation may cause over activation, accelerating surface erosion more than pore formation [14]. The effect of the steam flow rate is shown by Samples 11, 12, 13, indicating that qiodine is highest at a flow rate of 6 kg h1 but decreases at higher flow rate. This may indicate that a moderate steam flow is preferable to promote the activation reaction, but excessively faster flow may slow the activation reaction because the steam is a kind of endothermal liquid and can adsorb much heat. Yield and hardness are two key factors for granule activated carbon production industry. Normally, yield value over 25% and hardness value over 90% are satisfying for granule activated carbon products. The yield of Sample 12 (17.6%) is too low for producing acceptable activated carbon. These results indicate that the following activation conditions are appropriate for preparing high adsorption capacity activated carbon from rubber-seed shell with physical activation: temperature 880 C, activating time 60 min, and steam flow rate 6 kg h1. Sample 2, Sample 5 and Sample 9 which were activated in 820 C, 850 C, 880 C separately were selected for further study in the next steps. The proximate analysis of the selected three samples was shown in Table 3.
0.5
3.2.
N2 adsorption and pore size distribution
0.4
The N2 adsorption isotherms of the selected three activated carbons are presented in Fig. 1. Here the quantity of N2 adsorbed is plotted against the relative pressure p/p0 ( p ¼ pressure, p0 ¼ saturated vapor pressure) of N2. It can be seen that the N2 adsorption isotherm of Sample 2 prepared at
0.3
0.2
Sample 2, 820 Sample 5, 850 0.1
Sample 9, 880
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Relative Pressure (P/P0)
Fig. 1 – Nitrogen adsorption–desorption isotherms of activated carbons at 77 K; closed symbols: adsorption, open symbols: desorption.
1.0
Table 4 – Pore volume, surface area and average diameter of the tested activated carbons. Sample Sample 2 Sample 5 Sample 9
VT (ml g1) Vmic (cm3 g1) SBET (m2 g1) D (nm) 0.668 0.814 0.988
0.521 0.628 0.615
878 893 948
2.48 3.34 3.65
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820 C corresponds to Type I, which indicates that it is predominant microporous activated carbon that has not developed a porous structure. The amount of N2 adsorbed on Sample 5 prepared at 850 C increased with p/p0 in the range of p/p0 > 0.1. The N2 adsorption isotherm on Sample 9 prepared at 880 C begins to change considerably, like Type III. At this temperature, mesopores have been developed, as indicated by the shape of the hysteresis loop, which is often associated with the presence of slit-shaped mesopores [15]. Pore size distribution of the selected three activated carbons (Sample 2, Sample 5 and Sample 9) was shown in Fig. 2. The pore structure of activated carbon is generally characterized in term of the pore size distribution. One can see the activating temperature had significant effect on the pore structure of activated carbon prepared. At low temperature, the pore structure was mainly consisted of micropore; however, with the increase of carbonization temperature, the creation of mesopore increased and also increased the total pore volume of activated carbon [16,17]. Micropore relative ratio decreased but mesopore relative ratio increased. It was learned that the reaction rate between activating agent and carbon increased geometrically as the temperature rate increasing. At high temperature, micropores were enlarged and the walls between pores collapsed and formed mesopores. Specific surface area (SBET), total pore volume (VT), micropore volume (Vmic) and average diameter (D) of the tested activated carbons are tabulated in Table 4. It is noticed that VT increased with activation temperature rising, but Vmic is different since the Vmic of Sample 9 is not bigger than Sample 5. When activation occures at temperature above 850 C, the steam can enlarge the exist micropores while making new micropores.
3.3.
SEM
The microstructure of the raw rubber-seed shell and the selected three activated carbons (Sample 2, Sample 5, Sample 9) prepared at different temperature are shown in the Fig. 3. For the raw shell material, the micrograph showed clearly the canal structure, which is a good texture for preparing activated carbon, because activating agents can easily contact with the inside surfaces. From the SEM micrographs, it can be seen the morphological characteristics are much different between raw material and three activated carbons. Sample 2 shows perfect canal structure for the raw material. That means the carbonization is developed but activation is not much developed. The surface in Sample 5 shows that the canal structure has been partly broken which means the activation process happened and the surface was eroded by vapor steam a little more than the Sample 2, the SBET of Sample 5 is much bigger. Sample 9 shows a most well-developed porous structure among the three activated carbons. Whereas, 880 C is just 30 C higher than 850 C, but the surface area is much larger, which further testifies that the influence of temperature on carbon activation carbon is geometrically related. The micrographs showed the evolution of the pore structure containing some areas showing some fragmentary structure. The surface of the activated carbon presents some white fine particles, called ash content, which may fill or block some portion of the micropore.
3.4.
TG and DSC analysis
Fig. 4 shows the TG curve of raw rubber-seed shell by weight in a N2 atmosphere and at a 10 C min1 heating rate. The thermal
Fig. 3 – SEM of three selected samples.
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specific surface area and pore volume. In addition, the micropore and mesopore are both developed that made the activated carbon suitable for liquid phase and gas phase adsorption industries. Yield and hardness of rubber-seed shell activated carbon are also highly comparable to commercial activated carbon. Thus preparing activated carbon from waste rubberseed shell is a promising work.
Acknowledgement Fig. 4 – TG/DSC curves for rubber-seed shell.
decomposition behavior of dried rubber-seed shell is in the first stage from 273 C to 385 C; the next stage from 385 C to 800 C. It can be seen from the Fig. 4, little weight loss happened at temperature below 270 C because the rubber-seed had been dried under 105 C for 3 h. The first stage presents a significant weight loss due to much volatile released. The reason is that cellulose and hemi-cellulose of the sample decomposed to condensable gas (acetic acid, methanol and, wood tar) and uncondensable gas (CO, CO2, CH4, H2, H2O). During next stage, lignin begins to decompose and lose weight. Also the residual volatiles from the first stage are further released but carbon was left. A plateau curve at temperature over 800 C showed the weight unchanged at this stage. We can see from this curve, at 500–600 C, carbon yield is 35%, including 15% volatiles. The carbon at this stage is acceptable as a precursor for producing activated carbon because a carbon with 15% volatile is not too tight, and can easily react with activating reagent to produce big surface and huge pore volume. So it is apparent that rubber-seed shell can be as good raw material. Fig. 4 also shows DSC analysis. The DSC curve shows a little drop at a temperature of 100 C, because the sample needs heat to evaporate the remaining water. From 180 C, hemicellulose started to decompose and release heat. At 240 C, cellulose began to decompose and release heat. When temperature was about 320 C, DSC arrived at valley, where hemi-cellulose and cellulose decomposed and produced abundant organic compounds, such as wood tar, ketone and methanol which would vaporize by adsorbing large heat. At 500 C, DSC curve climbed up steeply and reached at a peak, because at this step lignin pyrolyzed and emited abundant heat. At 800 C, DSC curve reached at base line, pyrolysis reaction was over and the residue was just graphite carbon and ash.
4.
Conclusions
The results of this work demonstrate that rubber-seed shell is an attractive source of raw material for preparing high quality activated carbon by physical activation with steam. Activation condition has a significant influence on porous properties. After the activation at temperature 880 C, steam flow 6 kg h1, and time of 60 min, the activated carbon is obtained with huge
This research is financially supported from the projects in the national science & technology pillar program in the eleventh five-year plan period (2006BAD19B06) and central-level scientific research institutions basic funding (CAFINT2007C16). The authors are thankful to Prof. Chung-yun Hse of the USDA forest service for critical reading of this paper.
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