Preparation of Activated Carbon by Thermal Decomposition of Waste Tires for Pollution Control Hadeel A. Hosney1, Taha E. Farrag2*, Joseph J. Farah3, Mohamed Z. Abd-Elwahhab4. 1- Chemical Engineering Department, Faculty of Engineering, El-Minia University, El-Minia, Egypt. 2- Chemical Engineering Department, Faculty of Engineering, Port Said University, Port Said, Egypt. 3- Chemical Engineering and Pilot Plant Department,National Research Center, El-Giza, Egypt. 4- Chemical Engineering Department, Faculty of Engineering, El-Minia University, El-Minia, Egypt. * Corresponding author, E-mail:
[email protected]
Abstract The annual increasing in amounts of waste and rubber tires, which are closely linked to the rate of production of these tires, is one of the most important challenges that threaten the safety of the environment. Thus, the main objective of this work is to prepare activated carbons from waste tires using either one or two steps. Phosphoric acid was used for production oF activated carbon by one step process where both KOH and NaOH were used for the two steps process (carbonization and chemical activation). Effect of operating parameters, heating time, temperature, rate and impregnation ratio have been studied. Activated carbon produced by one step has a limited specific surface area ranged from 85 m2/g to 97 m2/g for impregnation ratios 1:1 and 3:1 respectively. The optimum conditions for carbonization step was 550 °C for 2 h with a heating rate of 15 °C/min, where yield reaches 40%. KOH is more effective than NaOH for activation step, the specific surface area was 236 m2/g and 113 m2/g for KOH and NaOH respectively. Keywords: Waste Tires; Recycling; Pyrolysis; Activated Carbon
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1. Introduction The automobile has become an indispensable mean of transportation for many households throughout the world. As a result, more than 5 million ton of waste tires are discarded each year, that is 2% of total solid waste production for one year [1-3]. The huge quantity of waste tires presently produced in the world will certainly increase in the future as the associated automotive industries grow. The disposal of scrap tires will become a serious environmental problem. Several attempts have been made to reduce the number of waste tires, for example, by using them as dock bumpers, playground equipment, etc. However, from environmental and economical points of view, a much better solution is to convert such waste tires to valuable products. Many materials rich in carbon can be used as precursors in the production of activated carbons [4]. The resulting products may have a high adsorption capacity as a result of their physical and/or chemical structure. Activated carbons, which have an enlarged porous structure, are, because of this, very useful in processes involving the separation of mixtures and the cleansing of gases and liquids. They are used in the removal of pollutants otherwise difficult to eliminate, owing to their resistance to conventional biological treatments, for example those in some industrial effluents. In this way, adsorption plays an important role in the elimination of nonbiodegradable organic pollutants, and activated carbon is the most commonly used adsorbent because of its versatility and efficiency. The use of wastes, including those as difficult to manage as waste tires, for the production of adsorbents makes waste economically valuable. This work is about the production of activated carbon from waste tires where it can be used as an adsorbent for wastewater treatment. In this way, a double benefit would be obtained, as it would improve waste tire management while also giving economic value to waste as a cheap raw material for making adsorbents. Investigation on recycling of waste tire by converting it into activated carbon was started from long time, and from this time numerous studies have been reported [5-11]. Different activation methods were used which include physical activation using steam or carbon dioxide and chemical activation methods sometimes using acid, H3PO4, and sometimes using an alkali i.e. NaOH or KOH. The activation of carbon using alkali activation technique has become a research topic of a great interest due to the enhanced characteristics of the produced activated carbon compared with other methods. The aim of this work is to study production and characterization of activated carbons from waste tires. Two strategies, one and two steps process for production of activated carbon, have been evaluated. Effect of operating parameters, temperature, heating rate and impregnation ratio, on both carbonization and activation processes are studied in order to determine optimum conditions. 1. Materials and Procedures 2.1 Waste Tires Tire rubber is a mixture of different elastomers such as natural rubber (NB), butadiene rubber (BR) and styrene butadiene rubber (SBR) and other additives, carbon black, sulfur and zinc oxide. Approximately 32% by weight from the waste tire is mainly constituted of carbon black in which the carbon content is as high as 70-75 wt% [12]. The waste tire used in this study was obtained from Al-Garbia governorate, Egypt, which is collected from people who run their own business in tire shredding. The proximate analysis for shredded waste tire used in this work was,
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fixed carbon = 29.5%, ash = 6.37 and moisture = 0.6, where its elemental analysis was: C = 83.82%, H = 6.47%, N = 0.41% and S = 0.83%. 2.2 Preparation of Activated Carbon Production of activated carbon from waste tires is processed through two pathways, carbonization and activation in one step or carbonization in a separate step then activation in another step. One step preparation: The shredded waste tire is washed and fractionated to remove any residual dust or clay may be suspended during tire milling. Then, 200 g of the waste tire is dried at 110 °C overnight. Each 5 g is mixed with a different substantial amounts of phosphoric acid H 3PO4 in 100 ml ceramic crucibles. The experiments cover different impregnation ratios as 1:1, 1.5:1, 2:1, 2.5:1 and 3:1 acid to waste tire ratio. The samples were dried at 110°C overnight. The dried samples are cooked at 600 °C for two hours in a thermally controlled tube furnace. The activation is done under inert gas flow (N2) to avoid the combustion of the sample. The furnace is left to cool to ambient temperature, the samples washed until pH=7, dried at 110°C overnight, cooled again and stored in plastic bags to be ready for testing. Two steps preparation: The cleaned waste tire was carbonized at specified, temperature, time and heating rate in an inert medium (N2 gas). Carbon produced at optimum conditions (carbon of high surface area) was activated chemically. Each 2 g of carbon is mixed with a substantial amount of alkali activator (NaOH or KOH) in 100 ml ceramic crucibles. About 10 ml of distilled water is added to the mixture and dried at 110 °C overnight, then heated at elevated temperature for a definite time and left to cool then washed and dried [6]. A schematic diagram for the tube furnace is presented in fig. 1.
Fig. 1: Experimental setup of tube furnace.
2.3 Characterisationof Activated Carbons Surface area is an important parameter in most applications of activated carbon specially in adsorption technique and catalytic reactions. The carbon and activated carbon produced are evaluated depending on total surface area. Measurements carried by Nova 1000e with accuracy ( ±0.1) ;involved determining the isotherms of nitrogen adsorption at the temperature of liquid nitrogen (77.3 K) and calculating the monolayer capacity basing on BET adsorption isotherm. Approximately 0.2 g of the each produced carbons or activated carbons samples was placed in a test tube and allowed to degas for 4 h at 300°C in flowing nitrogen gas.
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3. Results and Discussion 3.1 One Step Process Phosphoric acid was used for production of activated carbon in the one step process. Different impregnation ratios, mass ratio, of H3PO4 to waste tire has been studied where carbonization time and temperature were set at 2 h and 600 °C respectively. The effect of H3PO4 added on surface area of activated carbon produced is presented in fig. 2, where impregnation ratio of H 3PO4 ranged from 1:1 to 3:1. The BET surface area was increased with increasing impregnation ratio of phosphoric acid, but in general with low effect. The surface area of carbon produced by this way is very limited comparing with the commercial one so, it could be concluded that, treatment of waste tire with phosphoric acid has lower affinity of activation and isn’t thus recommended.
Fig. 2: Effect of impregnation ratio of phosphoric acid on surface area of activated carbon produced in one step process
3.2 Two Steps Process In this strategy, activated carbon is prepared through two steps in sequence, carbonization step without any chemical treatment followed by a separate activation step. Carbonization was applied at different operating conditions, i.e. , carbonization time, process temperature and rate of heating. In the activation step, both KOH and NaOH was applied at different operating conditions also, i.e. process time, chemical reagent concentration, process temperature. 3.2.1 Carbonization Step (Pylolysis) I. Effect of temperature Temperature has a major influence on the produced carbon. The rubber started to decompose at 450°C and this phase was essentially completed at temperature range from 500 – 600 °C [13]. A set of experiments was done at different temperatures covering 400, 450, 500, 550 and 600 °C where time and heating rate were set at 2 h and 5 °C/min respectively. Figure 3 illustrates the effect of carbonization temperature on surface area of carbon produced. 4
The measured data indicated that, surface area of carbon is highly affected by the cooking temperature up to a limit. It was increased from 20 m2/g to 90 m2/g when temperature increased from 400 °C to 550 °C, while after 550 °C it decreased at 600°C surface area was 53 m2/g. This may perhaps due the decomposition of the rubber (begins at 450˚C) and changing thus the nature of pores.
Fig. 3: Effect of carbonization temperature on surface area of carbon produced.
II. Effect of cooking time The effect of carbonization time (cooking time) on surface area of carbon produced has been studied where process temperature and heating rate were set at 600 °C and 5°C/min respectively, the obtained results are presented in fig. 4.
Fig. 4: Effect of carbonization time on surface area of carbon produced.
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The increase in carbonization time involves dehydration, decarboxylation and complete carbonization process through a sequence of steps to produce carbon and non condensable gases till certain limit; then it will not be effective to increase carbonization time since carbon dioxide production will increase leading to a decrease in the carbon content for the produced carbon. The highest value of surface area was detected for the 2 h cooking.
III. Effect of heating Rate: Carbonization of waste tire for 2 h at 600 °C, where heating rate was varied as, 3, 5, 7, 10 and 15 °C/min has been studied in order to evaluate effects of heating rate on yield and specification of carbon produced. The obtained results are graphically presented in fig. 5. It is clear that characteristics of carbon is highly affected by heating rate for carbonization process up till a value of 10 °C/min
Fig. 5: Effect of heating rate on surface area of carbon produced.
The surface area of produced carbon has a direct proportionality with heating rate. It increases from 26 m2/g to 82 m2/g when heating rate increases from 3 °C/min to 15°C/min. These results can be explained in the sense that, for high heating rate, there is a rapid removal of volatiles from the waste tire pores, leading to increasing the open pores and thus producing carbon with high active surface area. The yield of carbon from waste tire mass produced at optimum conditions (550 °C for 2 h with 15°C/ min heating rate) reached 40%. Elemental analysis has been done for carbon produced at optimum conditions and it was found to be C=85.7%, H=0.5%, N =0.32 and S =0.45%. The other 60% of waste tire weight is separated in form of volatile compounds (CO, CO2, H2, CH4, C2H6, C3H8) which is in agreement with other works in literature [14].
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3.2.2 Activation Step The different parameters influencing activation process have been studied where NaOH and KOH were used. The parameters are: impregnation ratio (activator/carbon), time and temperature of activation. I. Effect of activation time One of the important parameters affecting the activation process is the time of contact between chemicals and carbon at elevated temperatures. A series of experiments have been carried out at 800 °C and impregnation ratio of (chemical:C) of 4:1, where activation time varied from 0 to 120 min. The measured surface areas are plotted as a function of time, fig. 6.
Fig. 6: Effect of activation time on surface area produced for NaOH and KOH activators.
Figure 6 illustrates that KOH has a higher effect for specific surface area generation than that for NaOH by time. When the activation with KOH increased the specific surface area increased gradually with time and it reaches its maximum at 2 h. The increase of time increases the formation of K2CO3 and Na2CO3 which increases number of created pores and surface area, but for longer activation periods (more than 30 min for NaOH, 2 h for KOH) excessive surface reaction may take place between carbon layer and activator. II. Effect of activation temperature Both time and temperature are interesting parameters especially from an economic point of view, but the effect on surface areas generation, a series of experiments has been carried out at different process temperatures ranging from 600 °C to 800 °C, where other parameters remaining constant, a time of 2 h and an impregnation ratio of 4:1.The results obtained are presented graphically in Fig. 7.
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Fig. 7: Effect of activation temperature on surface area produced for NaOH and KOH activators.
The results could be explained by understanding the mechanism of reactions between alkali activator and carbon. KOH or NaOH react with carbon surface to form K2CO3 and Na2CO3 salts respectively according to the reactions: 3 NaOH/KOH + C
Na/K + 3/2 H2 + Na2CO3/K2CO3
(1)
The surface area of activated carbon increases with increasing cooking temperature, this may be explained that temperature enhances the reaction and more pores are formed. The optimum temperature for activation with NaOH was found to be 700°C and the higher temperature negative effect may be due to the following reasons: i. ii.
Excessive reaction may take place on carbon surface and destruction of pores play a leading role which reduce the efficiency of activated carbon. Increasing of activation temperature may increase the surface oxidation of carbon to CO2 which reacts with KOH or NaOH according to the following reaction:
4 NaOH/KOH + 2 CO2
2 Na2CO3 /K2CO3 + 2 H2O
(2)
K. Babel et al. 2004 [15], noted that when the temperature of activation is less than 600 °C a formation of carbonate salts (K2CO3, Na2CO3) are observed with decrease in activated carbon efficiency. This phenomenon is attributed to the reaction between alkali activator agent (KOH or NaOH) and carbon dioxide gas [Eqns. (3&4)]. C + O2 4
CO2
NaOH/KOH + 2 CO2
(3) 2 Na2CO3 /K2CO3 + 2 H2O
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(4)
III. Effect of impregnation ratio on surface area production Effect of impregnation ratio for the two alkali activators on surface area production of activated carbon has been studied and illustrated in Fig. 8. A series of experiments have been carried out at different impregnation ratios of KOH and NaOH where other parameters remain constant, 2 h and process temperature of 800°C for KOH and 600°C for NaOH.
Fig. 8: Effect of impregnation ratio on surface area produced for NaOH and KOH activators.
The results could be attributed to the reaction between alkali activator and carbon which presented previously in equation 1. These salts are dissolved by washing and the porous structure of activated carbon are generated. The higher of alkali activator/carbon ratio, the greater the formation of pores. As a result, all amorphous carbon disappear through the reaction and number of pores increases with increasing impregnation ratios until 5:1 for KOH and 3:1 for NaOH activator. After these limits, the surface area decreases because carbon layers continue to react with excessive potassium or sodium hydroxide and hence pore sizes may become larger, which is of negative action for the count of surface area causing it's decrease. For KOH activated carbon, the surface area increases by the increasing the impregnation ratio and the histogram length showed 236 m2/g area for impregnation ratio 5:1 while for the ratio 4:1 it is very near (231 m2/g),which may recommend the use of this ratio economically . For NaOH activated carbon the surface area increases up to 113 m2/g at impregnation ratio 3:1, then decrease for further increasing in impregnation ratio. In general, it is clear from all measurements that, the interaction of potassium hydroxide with carbon is stronger than sodium hydroxide at high temperature which caused the increasing in specific surface area. The difference between the effects of the two reagents on the efficiency of activated carbon may be influenced, as discussed, by their own chemical properties, as the reaction between NaOH and 9
carbon surface begins at around 570 °C where for KOH it is at around 400°C, so the longer activation time at 400 °C may increase the degree of activation. Another interpretation explained by Lazano Castello et al. 2001 [16], that the boiling point of potassium is 758 °C which is lower than that of sodium 883 °C. So, potassium reagent can enter into the interior of carbon structure and make the activation freely while outer activation of sodium hydroxide is stronger than potassium hydroxide.
4. Conclusions In this work, two strategies for production of activated carbon from waste tires have been studied, one step and two steps process. Acid treatment was used in the one step process, where KOH or NaOH treatment was used in the two steps process. Surface area of activated carbon produced by one step at 600 °C was limited, ranging from 80:90 m2/g depending on the impregnation ratio of acid to tire sample. The optimum conditions for carbonization process was cooking for 2 h at 550 °C with 15 °C/ min heating rate where carbon yield was 40%. Activation by KOH was more effective than NaOH where highest surface areas were 113 and 236 m 2/g for NaOH and KOH activator respectively. The optimum conditions for activation step were cooking for 2 h at 800 °C with an impregnation ratio 5:1, but impregnation ratio3:1 is recommended from an economic point of view. The low surface areas of the produced tire carbons with respect to that with high specific area for fossil carbon (1200 m2/g) is to be a must for open economic discussion from an environmental point of view.
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