스팀활성화 공정으로 제조된 활성탄의 기공분포특성과 액상흡착능의 상관

김성덕, 김원일, 이승범, 김형진*, 김상철**, 홍인권

단국대학교 공과대학 화학공학과

* 김포전문대학 환경공업과

** 한국건자재시험연구원

Correlation of Pore Distribution and Liquid Phase Adsorption

of Activated Carbon Prepared with Steam Activation Process

S. D. Kim, W. I. Kim, S. B. Lee, H. J. Kim*, S. C. Kim**, I. K. Hong

Dept. of Chemical Engineering, Dankook University

* Dept. of Environmental Engineering, Kimpo College

** Korea Institute of Construction Materials

Introduction

Commercially available activated carbons are prepared from carbon containing source materials such as coal, lignite, wood, coconut shell, petroleum and sometimes synthetic polymers.  During carbonization process, the volatile fraction and low molecular products of pyrolysis are removed and the residual carbonaceous material undergo the following activation process using oxidizing gas, such as steam or CO2.  Activated carbon has being produced for a specific use by adjusting activation process using various raw materials.  Eventhough it is provided the optimum conditions in production, yet it is difficult to specify the relation among surface area, pore size distribution, carbonization and activation.  Recently, some work has been performed concerning the change of pore structures during the activation.  Watanabe, et al.[1-2] reported the preparation of coal based activated carbon and the relation between the growth of pores and the activation conditions.

In this study it is discussed that optimum activation condition and pore characteristics of coal such as pore size, pore size distribution, and surface area.   Furthermore the various factors which could ultimately specify the desired pore volume and surface area were analyzed.

Experimental

1. Preparation of Activated Carbon

Two kinds of coal, namely an anthracite(Dandong) coal and a bituminous(Fushun) coal were used as raw materials.  The activation and carbonization of coal were carried out in a rotary kiln-type furnace.  It was 7 and 160 cm in diameter and length respectively, and it was controlled to operating temperature by the PID type controlling electric heater.  About 30 g of the coal was put into the sample container, which was maintained at a given temperature.  The carbonization for the fresh and oxidized coal samples were carried out under the nitrogen stream, with flow rate of 20ml/min and heating rate of 3℃/min.  The product was removed at any specialized time, as will be told later on, and cooled in a sand bath.  The carbonization and activation were carried out under following condition that is, carbonization temperature was 700℃ and maintained for 60 minutes, activation temperature was varied from 800 to 950℃, activation time for 240 minutes, the steam flow rate was in the range 0.75∼3 g steam/g coal□hr, respectively[3].  The activation carbon yield and iodine value[KS-M 1802] were measured to analyze the process.  The BET surface area and pore volume of activated carbons were measured by BET surface area analyzer[ASAP 2010, Micrometrics Co.].

2. Liquid Phase Adsorption Experiment

The apparatus mainly consisted of a Pyrex column 3.1 cm in diameter(D) and 20.5 cm long(L).  The L/D ratio of the column was sufficient to minimize the end effect of the adsorption column[4].  The column was surrounded with heating tape for temperature control.  A solution containing the organic compounds(benzene, MEK) at the concentration of 1000 mg/L was prepared.  During operation, the solution was pumped to the top of the adsorption column at a flow rate of 10 ml/min.  The concentration of organic compound at the exit of the column was monitored by the liquid chromatography[600E, Waters Co.].

Result and Discussion

1. Optimum Operating Condition

Coal based activated carbon was prepared in a rotary kiln type furnace and the optimum condition of manufacturing process was investigated.  Steam was employed as the activation agent.  The major operating parameters for activated carbon were temperature, heating rate, and steam amount.  Generally, the adsorption capacity of activated caron increased and AC yield decreased as the activation parameters increased.  The effect of the operating parameters are shown in Table 1.  To recommend the optimum condition we have to consider both AC yield and adsorption capacity.  Considering the experimental information, 850∼880℃ was suggested as the optimum activation temperature.

Table 1. Physical properties of activated carbon with activation temperature

Activation Condition			AC Yield [%]	Iodine Value [mg/g]	BET Surface Area[m2/g]	Vp [cm3/g]
Sample	Heating Rate	Temp.[℃]
Fushun Coal	2.9℃/min	800	46.2	807	584	0.27
		830	41.5	906	734	0.30
		850	38.0	1047	1067	0.36
		880	35.1	1069	1223	0.41
		900	14.3	1154	1294	0.43
		950	5.7	460	257	0.18

2. Internal and External Conversion

[eqn: f_{i n}](internal conversion of steam on inner-pore) decreases and [eqn: f_out{}](external conversion of solid reactant on particle surface) increases with the increase of [eqn: f{}](conversion ratio).  This trend is due to diffusion of steam into inner-pore.  it is appered that external conversion increases fastly than internal conversion with temperature increasing.  The AC yield of 30∼50% and 30∼40% were obtained for Dandong and Fushun coal, respectively.

Table 2. Experimental results of internal and external conversion

[eqn: f{}]	Dandong				Fushun
	[eqn: rho]	[eqn: rho/rho_o{}]	[eqn: f_{i n}]	[eqn: f_out]	[eqn: rho]	[eqn: rho/rho_o{}]	[eqn: f_{i n}]	[eqn: f_out]
0.0 0.5 0.6 0.7 0.8 0.9	0.96 0.54 0.51 0.47 0.45 0.44	1.00 0.56 0.53 0.49 0.47 0.46	- 0.389 0.353 0.323 0.427 0.118	- 0.107 0.245 0.388 0.574 0.783	0.96 0.59 0.46 0.43 0.38 0.37	1.00 0.61 0.48 0.45 0.40 0.39	- 0.314 0.435 0.493 0.306 0.159	- 0.180 0.167 0.333 0.500 0.744

3. Effect of Operation Condition

The AC yield decreased with both the activation temperature and the BET surface area increasing in the range of 800 to 880℃.  The maximum iodine value was obtained at about  30∼40% yield.  Fig. 1 shows the effect of heating rate(in the range of 2.3 to 4.7℃/min) of activation process on Fushun coal's AC yield and BET surface area.  These data in this study were obtained with the activation temperature of 880℃ and steam amount of 1 g steam/g coal□hr.  As the heating rate increased, AC yield, iodine value and BET surface area increased and reached a maximum value at the 2.9℃/min, and then decreased over 2.9℃/min.  These trend were caused by the macropores developed above 2.9℃/min, and the pores of activated carbon blocked by ash below 2.9℃/min.

Fig.1. Effect of heating rate on AC yield      Fig.2. Effect of steam amount on AC yield

and BET surface area of Fushun coal.          and BET surface area of Fushun coal.

The developing patterns of structure with the activation by steam are shown in Fig. 2.  The relation between AC yield and iodine value during the activation is as follows.  The AC yield decreased and development of pores increased with increasing steam amount, however BET surface area was decreased from 880℃.  Also, the pore volume increased with BET surface area increasing.  This tendency is remarkable for the volatile matter and ash in term of block of the pore in the activation period.  In previous paper, the authors produced activated carbon from coal, and found that Dandong coal has the developed pores in the region of the micropore(near radius 10Å) and Fushun coal has the developed, as the activation time elapsed, not only in the region of 10Å radius but in the region of radius 100-400Å[5].  The pore is depend on how much it includes the volatile matter and ash of the carbonized materials.  Consequently it was found that pore size distribution and development of activated carbon were strongly affected by that of raw materials.

4. Characteristics of Liquid Phase Adsorption

In this study, Fushun coal based activated carbon is estimated for liquid adsorption properties.  The adsorption for benzene and MEK took about 180 min and 360 min to reach equilibrium, respectively.  the isotherm parameter of Freundlich model can be obtained using experimental data.  so we got both [eqn: k{}]= 10.8, [eqn: 1/n{}]= 0.581 and [eqn: k{}]= 0.31, [eqn: 1/n{}]= 0.849 for benzene and MEK, respectively.

Fig.3. Adsorption amounts for MEK          Fig.4. Adsorption amounts for benzene

Conclusion

It was investigated the optimum conditions of AC manufacturing process and the mechanism of the growth of pores in the course of the activation.  as the adsorption capacity of AC increased the AC yield decreased.  However the adsorption capacity and the pore characteristics as like BET surface area or pore volume, etc., showed a similarly tendency.  We obtained[eqn: 1/n{}] value which is acceptable to liquid adsorption process in the range of 0.1 to 0.5.  Consequently, it was found that about 35∼40% activation is enough to obtain the maximum adsorption performance and pore development of AC, which is strongly effected by raw materials and activation condition.

References

1.	Watanabe, F., Yanagase, S. and Sugiyama, S., Nippon Kagaku Kaishi, No. 12,	2313 (1972).

2.	Watanabe, F., Matsuda, H. and Sugiyama, S., Preprints for 29th Annual Meeting	of the Chem. Soc. of Japan, Hiroshima(1973).

3.	Kim, S. C., Hong, I. K. and Park, K. A., JIEC, 3(3), 218 (1997).

4.	Lin, S. H. and Hsu, F. M., Ind. Eng. Chem. Res., 34, 2110 (1995).

5.	Watanabe, F., Yamada, Y., Hasatani, M. and Sugiyama, S., J. of Chem. Eng.	of	Japan, 9(4), 315 (1976).