Vol-23-1&2 1 J. Surface Sci. Technol., Vol 23, No. 1-2, pp. 73-80, 2007 2007 Indian Society for Surface Science and Technology, India. Adsorption Studies on Activated Carbon Derived from Steam Activation of Jute Stick Char MOHAMMAD ASADULLAH*, MUHAMMAD ANISUR RAHMAN, MOHAMMAD ABDUL MOTIN and MOHAMMAD BORHANUS SULTAN Department of Applied Chemistry and Chemical Technology, University of Rajshahi, Rajshahi 6205, Bangladesh. Abstract The char derived from pyrolysis of jute stick for bio-oil production was activated by physical method using steam. The yield of activated carbon was varied with the variation of activation temperature and steam flow rate. About 50% activation burn-off of char at 750ºC with 75 mg/min steam flow rate resulted in high surface area activated carbon. The maximum BET surface area and iodine sorption were found to be 724 m 2 /g and 573 mg/g respectively. The carbon tetrachloride and benzene adsorption on experimental activated carbon at 30ºC were found to be 51.5 and 49.5 wt% respectively, which were 32.0 and 30.5 wt% respectively on commercial activated carbon. These results suggested that the experimental activated carbon was very effective for gas phase and liquid phase adsorption. Keywords : Activated carbon, jute stick, adsorption, chemical activation, surface characterization. INTRODUCTION Carbon materials with high porosity and high surface area are termed as activated carbons which are manufactured from coal [1,2], lignocellulosic materials [3-6], synthetic materials [7,8], etc. Agricultural residues, forestry by-products, sewage sludge, etc. are a few examples of lignocellulosic precursor for activated carbon production. Activated carbon is widely used in the industrial sector for adsorption of pollutants from gaseous and liquid streams [9-17]. The activated carbons can be produced by thermal activatioin treatment at temperatures ranging from 600 to *Author for correspondance. Tel. : +880-721-750041 Ext. : 4106; Fax : +880-721-750064;
74 Asadullah et al. 900 ºC in the presence of either steam or carbon dioxide [18,19]. Chemical activation method have also been used. The important and commonly used activating agents are phosphoric acid, zinc chloride and alkali metal compounds [3-6]. Phosphoric acid is preferred to zinc chloride because of the environmental disadvantage associated with the latter. Moreover, the carbons obtained using zinc chloride cannot be used in pharmaceutical and food industries as it may contaminate the product. Phosphoric acid activation has been applied on a wide variety of cellulosic precursors such as coconut shell. In the present investigation, the jute stick char derived from the pyrolysis process for bio-oil production in our laboratory is used to produce activated carbon by thermal activation method using steam. Jute stick is abundantly available in Bangladesh and India. The production of bio-oil from the jute stick is one of the most promising ways for utilizing agricultural residues. The activation of char, which is a by-product of the pyrolysis of jute stick, to produce activated carbon makes the total process cost effective. The activated carbons produced were characterized by measuring the BET surface area, iodine number, carbon tetrachloride adsorption and benzene adsorption. EXPERIMENTAL Materials : Char was derived from the pyrolysis of jute stick. Iodine (99.5%), potassium iodide (99.9%), sodium thiosulphate (99.9%) and potassium dichromate (99.5%) were obtained from Loba Chemicals, India. Activated carbon (Laboratory Reagent) and benzene (99.9%) used in this investigation were from Thomas Baker (Chemicals) Ltd. and Aldrich respectively. Carbon tetrachloride was purchased from Merck (India). Methods : Activation of Char The char was activated at 700 850ºC in a stainless steel horizontal tube reactor as shown in Fig. 1. The nitrogen gas and steam were passed through the char bed at the rate of 150 ml/min and 75 1125 mg/min at Fig. 1. Design of activation reactor.
Adsorption Studies on Activated Carbon from Jute Stick Char 75 700 900ºC under 1 atm pressure, respectively. At the end of the process, the activated carbon was allowed to cool and then washed with 0.1 M HCl to remove the ash and then washed with water to remove the residual acid. The samples were then dried at 105ºC. The following relationship was used for calculating the activation burn-off of jute stick char. Activation burn-off (%) = {mass loss (g) / original mass of char (g)} 100. Adsorption Studies The BET surface areas of samples were measured by nitrogen adsorption method with a surface area measuring instrument, Gemini 2375 (Micrometrics). For the determination of iodine number, 0.1 g of activated carbon was taken in a 250 ml conical flask. About 10 ml 0.05 N iodine solution in aqueous potassium iodide was added into the flask. After 1 h the solid mass was separated by centrifuging the mixture and the residual iodine in solutin was titrated using 0.1 N sodium thiosulfate solutiion. The iodine number was calculated as mg of iodine adsorbed by one gram of activated carbon. For the measurement of carbon tetrachloride adsorption, a requisite weighed amount (around 0.2 g) of activated carbon was weighed out on a watch glass. Certain amount of carbon tetrachloride was taken in another watch glass and then put them in a desiccator. The activated carbon adsorbed the carbon tetrachloride vapour and the adsorption process was continued to a desired length of time to attain the equilibrium state. The percent adsorption of carbon tetrachloride was calculated by the equation : (amount of carbon tetrachloride adsorbed)/(the amount of activated carbon) 100. The adsorption of benzene was also measured in the same way. RESULTS AND DISCUSSION Effect of Activation Temperature Table 1 shows the effect of temperature on the yield and properties of activated carbon. The activation burn-off was a function of temperature and the higher the temperature, the higher was the activation burn-off. Thus, the yield of activated carbon decreased with increasing temperature. The activation burn-off occurred due to the steam reforming reaction of char, forming carbon monoxide and hydrogen [20]. This reaction takes place on the outer surface as well as in the micro-pore surface. The reaction on the surface generated new pores and the reaction inside the pores resulted in an increase of the size of the pore.
76 Asadullah et al. TABLE 1. Effect of temperature on the yield and properties of activated carbon Temperature (ºC) 700 750 800 850 900 Water flow rate (mg/min) 75 75 75 75 75 Nitrogen gas flow rate (ml/min) 300 300 300 300 300 Activation time (h) 1 l 1 1 1 Amount of char (g) 10 10 10 10 10 Amount of activated carbon produced (g) 5.83 5.01 2.60 2.12 1.73 Activation burn-off (wt%) 41.65 50.05 68.00 78.76 82.70 Yield of activated carbon (wt%) 58.35 49.95 32.00 21.24 17.30 Properties of activated carbon BET surface area (m 2 /g) 509 724 605 513 452 Iodine number (mg/g) 338 573 433 362 327 Fig. 2 shows that there is no linear relationship between activation burnoff and either BET surface area or the iodine number. BET surface area reached to a maximum at activation burn-off around 50 wt% and then declined at higher Fig. 2. Effect of activation burn-off on BET surface area and iodine number.
Adsorption Studies on Activated Carbon from Jute Stick Char 77 activation burn-off. This may be due to the fragile structure generated with high burn-off in the jute stick char. As pore size increased due to the increase of activation burn-off, the pore wall became thinner and at a time breakdown occurred and the number of pores was reduced. As a result the surface area and iodine number decreased at high temperatures. Effect of Steam Flow Rate The yield of activated carbon greatly varied with varying steam flow rate as shown in Table 2. The activation burn-off of char takes place through the steam reforming reaction of char which is faster in the presence of greater availability of steam and thus the yiled of activated carbon is decreased with increasing steam flow rate. The micro pores gradually became macro pores due to the increasing burnoff of char within the limiting time of 1 h. Thus, the pore volume of activated carbon can be controlled by controlling the steam flow rate. Both BET surface area and iodine number were at their respective maximum at low steam flow rate (75 mg/min). High steam flow caused high activation burn-off, resulting in fragile structure of the pore, thereby breaking the small pores leading to reduced surface area. TABLE 2. Effect of steam flow rate on the yield and properties of activated carbon Steam flow rate (mg/min) 75 150 300 600 1125 Temperature (ºC) 750 750 750 750 750 Nitrogen gas flow rate (ml/min) 300 300 300 300 300 Activation time (h) 1 1 1 1 1 Amount of char (g) 10 10 10 10 10 Amount of activated carbon produced (g) 5.01 4.53 2.26 1.46 0.95 Activation burn-off (wt%) 50.05 44.68 77.38 85.47 90.49 Yield of activated carbon (wt%) 49.95 45.32 22.62 14.57 9.51 Properties of activated carbon BET surface area (m 2 /g) 724 601 560 425 360 Iodine number (mg/g) 573 340 310 285 255
78 Asadullah et al. Effect of Adsorption Time Table 3 shows the carbon tetrachloride adsorption on commercial and experimental activated carbon at different adsorption time. It is seen that both of commercial and experimental activated carbons exhibited increased adsorption of carbon tetrachloride with increasing time. The adsorption attained equilibrium within 1h; however, the experimental sample adsorbed much higher amount of carbon tetrachloride (51.5% wt%) at equilibrium state than that of the commercial one (30.51 wt%). The prolong adsorption time (2 h) did not significantly alter the equilibrium state, indicating monolayer adsorption. Benzene adsorption was also measured as a function of time. The results are summarized in Table 3. The adsorption incresed for both the sample as time increased. The equilibrium was attained within 1 h. The final adsorption was higher (49.5 wt%) in the case of experimental activated carbon than that of the commercial sample (32.32 wt%). TABLE 3. Effect of adsorption time on the amount of carbon tetrachloride and benzene adsorption. Adsorption Carbon tetrachloride adsorption Benzene adsorption time (mg/g) (mg/g) (min) Commercial Experimental Commercial Experimental sample sample sample sample 5 4.57 4.61 5.48 5.69 10 7.18 9.13 7.38 11.58 15 10.02 12.20 10.87 15.97 20 12.40 16.57 13.09 23.36 25 15.73 26.15 1.6.33 27.00 30 17.74 36.48 20.20 32.59 45 24.50 47.48 26.94 42.34 60 30.51 51.50 32.32 49.50 120 30.50 51.80 32.43 48.90 180 30.63 52.00 32.63 49.10 CONCLUSIONS The activated carbon has been produced from the by-product, char, of the pyrolysis of jute stick for producing bio-oil. The activation conditions such as activation
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