THE EFFECT OF CARBONIZATION TEMPERATURE ON QUALITY OF ACTIVATED CARBON PRODUCED FROM CORNCOBS
Hoang Le Phuong* University of Technology - TNU
ABSTRACT
The production of activated carbon from agricultural waste will contribute to minimizing environmental pollution caused by the waste. The quality of activated carbon is influenced by many factors such as raw material quality, temperature, chemical and time of activated process.
The aim of this study was to investigate the quality of activated carbon produced by corncobs in the different carbonization temperatures. The corncob was carbonized in inert atmosphere condition in three temperature ranges: 200 0C; 300 0C and 400 0C. The activated carbon products analyzed some indexes: specific weight, iodine adsorption index, BET surface area and the ability adsorption organic matter through the COD index in wastewater before and after adsorption. The results indicated that activated carbon produced from corncobs at 400 0C have the better quality than at 300 0C and 200 0C. Namely, it has the lowest specific weight with 0.792 ± 0.021 g/cm3; iodine adsorption index and BET surface are the highest with 500.309 ± 0.21 mg/g and 208.86 m2/g respectively. Besides, the COD absorbability in wastewater of this activated carbon is 30.254%, while this value for activated carbon from corncobs at 200 0C and 300 0C carbonization temperature was only 12.195% and 17.073%.
Keyword: activated carbon; corncob; physical activation; carbonization temperature;
agricultural waste
INTRODUCTION*
In Vietnam, the corn is the second largest crop after the rice and grown widely.
According to the General Statistics Office in 2016, the country's corn area is 1.3 million hectares with a yield of 4.8 tons per hectare and this number will increase in the future [4]. The processing of agricultural products from corn releases millions of tons of corncobs each year. This amount of corncob is partly used as a fuel, in recent years used in growing mushrooms [10, 16]. However, the using part is small, the largely remaining part is disposed in the environment. Corncob contains cellulose, hemicelluloses, pectin, lignin and protein. Therefore, it is very difficult to destroy and cause environment pollution seriously and persistently [13, 16].
Nevertheless, with polymer contains and porous structure, corncob has good adsorption capacity. In some countries such as Japan, China, Nigeria, activated carbon produced
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from corncobs has been used to adsorb radioactive substances, heavy metals, pesticides, CO2, with high efficiency [1, 9]. In Vietnam, the environmental pollution is increasing, so the demand for using adsorbent material in the treatment of polluted water, soil and air is very high. However, the using of corncobs for the production of adsorption materials is being left the door open, while large quantities of corncobs are disposed of annually causing environmental pollution.
According to the objective of Vietnam's solid waste management by 2025 with a vision to 2050 approved by the Prime Minister, in 2025 Vietnam has to ensure that 100% solid waste will be collected and treated, in that 90% of solid waste is recycled and reused [14].
Therefore, the recovery of the corncobs producing adsorbent activated carbon will help to produce useful product and meeting the goal of solid waste management.
The quality of activated carbon is usually affected by some factors such as raw materials, the activation method and the
conditions of activation process. Activated carbons are usually produced by two methods: physical and chemical activation [1]. In chemical activation, the material is mixed with a certain amount of active agents such as KOH, NaOH, H3PO4 and ZnCl2 [2, 3, 5, 11], which lead to the development of porous structures in the material. Physical activation includes carbonization of the precursor in an inert atmosphere and activation of the resulting char by an activation agent such as steam, CO2 or air [12]. The carbonation process consists of the pyrolysis procedure, in which, volatile compounds are released leading to the achievement of fixed carbon in an inert atmosphere and at a certain temperature. In the second step of the physical activation process, the precursor is placed under a heat treatment at high temperatures from 700 0C to 1100 0C in the presence of oxidizing agents, such as CO2, water steam, air, or a mixture of them [11]. Thus, the temperature has a great influence on the quality of activated carbon obtained from physical activation process.
According His et al 2011 [8] the suitable temperature of physical activated of rice husks is 4000C. Meanwhile, for hazelnut shells appropriate activation temperature is 6000C [6]. Therefore, the purpose of this study was to evaluate the effect of different carbonization temperatures on the quality of activated carbon produced from the corncobs in Vietnam by physical activation.
MATERIALS AND METHODS Material
Corncobs, which are considered as solid waste, were collected from farmer households growing corn. The corncobs were dried to ensure effective carbonization.
Carbonization process
The corncobs were carbonized by carbonization kiln. The carbonization kiln is a simple cylinder to provide a mean of creating low oxygen environment. It is made of steel
with about 50 cm in height and 30 cm in diameter and an opening at the top for loading the corncobs feedstock. A suitable metal plate was used as cover for the top opening of the kiln during firing. The schematic diagram of the carbonize kiln is illustrated in Figure 1.
The corncobs were put into the kiln at each batch. A small port was provided at the bottom of the kiln to provide heat when need.
At the start of the carbonization process, the study used sawdust to supply heat. In addition, the lid was left open for approximately 10 minutes for the volatile gases to escape. The lid was then closed to prevent air from entering. The temperature in the kiln was measured by a thermal sensor attached to the wall.
30cm
50cm
Heating
Figure 1. Schematic representation of the carbonization kiln
The corncobs material was carbonized in three batches in three different temperature, the first batch at 200 0C, the second batch at 300 0C and the third batch at 400 0C. After corncob material was carbonized completely, rising temperature in the kiln to 800 0C and slowly watered by a sprinkler to increase porous structure. After that, activated carbon product was dried to constant weight at 110 0C and crushed to size from 0.5 to 2 mm to determine some characteristic parameters. The activated carbon product is shown in figure 2.
Determine some characteristics of activated carbon
Specific weight: Specific weight of activated carbon was determined according to Vietnamese standard TCVN 9069:2012;
Iodine number: According to U.S.
Environmental Protection Agency Technology Transfer Manual (1972), about 2.0 g of activated carbon sample was grind to pass a 0.25 μm mesh sieve to produce approximately 1.0g carbon. The product was added 10 ml of 5.0 % HCl solutions in flask and put on hot plate for 30 s, then cooled at room temperature and a 100 ml of standardized 0.10 N iodine solution was added to the flask. The flask stopper and shaken vigorously for 30 s. After filtration, the remaining solution was titrated with 0.1 N sodium thiosulphate solution.
The specific surface area: The specific surface area of the prepared activated carbons was determined by nitrogen adsorption at - 196 0C by the surface area analyzer (Quanta chrome, USA). The sample was degassed under vacuum at 300 0C for three hours before the measurement. The surface area (SBET) of prepared activated carbon was estimated by BET (Brunauer–Emmett–Teller) method [7].
Evaluation the adsorption capacity of organic matter of activated carbon products through COD index: weighs about 5.0 g of activated carbon sample in flask, then add 20 ml waste water which known COD. The flask stopper and shaken vigorously for 1h. After filtration, the remaining solution was determined COD and evaluated of COD adsorption capacity of activated carbon products.
Chemical oxygen demand (COD): Chemical oxygen demand in wastewater was determined according to Vietnamese standard TCVN 6491:1999.
RESULT AND DISCUSSION Specific gravity
It can be seen that the specific gravity of activated carbon decreased as the temperature of carbonization process increased. In the first batch, at a temperature of 200 0C, the specific gravity of activated carbon is highest at 1.105
± 0.023 g/cm3. This value decreased to 1.057
±0.016 g/cm3 when the carbonization temperature increased to 300 0C in the second batch. In the third batch at 400 0C, the specific gravity of activated carbon was 0.792
± 0.021 g/cm3. This result was consistent with some of the results of the study, at the higher the temperature; the stronger carbonize process, reducing density and increasing the porous structure of activated carbon [6].
(a)
(b)
Figure 2. The activated carbon product from corncobs: (a) before crush and (b) after crush
Figure 3. Influence of carbonization temperature to specific gravity of activated carbon Iodine number
The results showed that iodine number of activated carbon in the first batch was the
lowest with 385.601 ± 0.13 mg/g, in the second batch was 445.964 ± 0.15 mg/g and in the third batch was the highest with 510.309 ± 0.21 mg/g. The iodine index is an important indicator of the adsorption capacity of activated carbon. The higher iodine number corresponds to the larger the surface area of activated carbon, the greater the porosity and the higher the adsorption capacity. According to Vietnamese standards (TCVN 9068: 2012) [15], the iodine index of activated carbon is must larger than 500 mg/g. Thus, activated carbon product obtained at temperatures of 200 0C and 300 0C in the first and second batch did not meet the standard. Meanwhile the activated carbon was obtained at 400 0C in the third batch with the iodine number in line with the quality standard.
Figure 4. Iodine number of activated carbon in different carbonization temperatures The BET surface area
Surface areas of activated carbons are shown in table 1.
Table 1. Summery surface area of activated carbons Activated
carbon type (different carbonization temperature)
Single point surface
area, S (m2/g)
BET surface
area, SBET (m2/g)
Langmuir surface
area, SL (m2/g)
200 0C 131.240 129.098 186.953 300 0C 180.945 177.216 237.665 400 0C 216.700 208.860 285.766
In the first batch with carbonization process at 200 0C, the BET surface area of activated carbon was 129.098 m2/g. At 300 0C temperature in the second batch, the BET surface area of was 177.216 m2/g. In addition, in the third batch with a carbonization temperature at 400 0C, the BET surface area of activated carbon was at 208.860 m2/g. As such, the BET surface area increased as carbonization temperatures rose from 200 0C – 400 0C. The higher the BET surface area, the higher the adsorption capacity of activated carbon.
Figure 5. BET surface area of activated carbon in different carbonization temperatures
Adsorption capacity of organic matter through COD index
It can be seen that the activated carbon product obtained at the 400 0C carbonization temperature had a higher organic adsorption capacity than the products at 200 0C and 300
0C with the adsorption efficiency 30.254%.
This result corresponds to the result of specific gravity, iodine number, BET surface area. Activated carbon has a lower specific gravity, the iodine index, BET surface area and absorption capacity are higher. This result was consistent with the results of several other studies on activated carbon from corncobs and Fox nut [2, 3]. The COD adsorption yields of activated carbons are shown in table 2.
Table 2. COD adsorption yield of activated carbons Activated carbon type (different
carbonization temperature)
COD in waste water, mg/L
COD after adsorbed, mg/L
COD adsorption yield, %
200 0C 393.60±0.21 345.60±0.18 12.195
300 0C 393.60±0.21 326.40±0.25 17.073
400 0C 393.60±0.21 274.52±0.14 30.254
CONCLUSION
It can be seen that in the physical activation the quality of activated carbon is affected by the temperature of carbonization process. In this study, activated carbon product obtained from corncobs with carbonization temperature at 400 0C has better quality than at 200 0C and 3000C. This was shown by the parameters of the activated carbon quality with the highest iodine number 510.309 ± 0.21 mg/g achieving the quality standards (TCVN 9068: 2012).
The BET surface area and organic adsorption capacity were the highest with 208.860 m2/g and 30.254% respectively. However, in order to provide optimum conditions for the production of activated carbon from corncobs, further study should be carried out at other temperature ranges and other factors such as the activation time and the size of the original material.
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TÓM TẮT
ẢNH HƯỞNG CỦA NHIỆT ĐỘ ĐẾN CHẤT LƯỢNG THAN HOẠT TÍNH SẢN XUẤT TỪ LÕI NGÔ
Hoàng Lê Phương Trường Đại học Kỹ thuật Công nghiệp - ĐH Thái Nguyên
Việc sản xuất than hoạt tính từ chất thải nông nghiệp sẽ góp phần giảm thiểu ô nhiễm môi trường do loại chất thải này gây ra đồng thời tạo ra sản phẩm có ích. Chất lượng của than hoạt tính chịu ảnh hưởng của nhiều yếu tố như chất lượng nguyên liệu, nhiệt độ, hóa chất và thời gian hoạt hóa.
Mục đích của nghiên cứu này là đánh giá chất lượng của than hoạt tính sản xuất từ lõi ngô ở các nhiệt độ các bon hóa khác nhau. Lõi ngô được các bon hóa tạo than hoạt tính trong điều kiện thiếu khí ở nhiệt độ: 200 0C; 300 0C và 400 0C. Các sản phẩm than hoạt tính thu được ở 3 nhiệt độ trên được đưa đi phân tích một số chỉ số: khối lượng riêng, chỉ số hấp phụ Iot, bề mặt riêng BET và khả năng hấp phụ chất hữu cơ thông qua đánh giá chỉ số COD trong nước thải trước và sau hấp phụ bằng sản phẩm than hoạt tính. Kết quả cho thấy than hoạt tính sản xuất từ lõi ngô ở nhiệt độ 400
0C có chất lượng tốt hơn so với ở nhiệt độ 200 0C; 300 0C . Cụ thể than hoạt tính thu được ở nhiệt độ này có khối lượng riêng thấp nhất 0,792 ± 0,021 g/cm3; chỉ số hấp phụ Iot và diện tích bề mặt BET cao nhất với 510,309 ± 0,21 mg/g và 208,860 m2/g. Bên cạnh đó khả năng hấp phụ chất hữu cơ thông qua chỉ số COD trong nước thải đạt 30,254 % trong khi giá trị này đối với than hoạt tính thu được ở 200 0C và 300 0C chỉ là 12,195 % và 17,073 %.
Từ khóa: than hoạt tính, lõi ngô, hoạt hoát vậy lý, nhiệt độ cacbon hóa, chất thải nông nghiệp
Ngày nhận bài: 01/11/2017; Ngày phản biện: 21/11/2017; Ngày duyệt đăng: 05/01/2018
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