역청탄을 원료로한 활성탄의 제조 및 성능평가

김상철*, 김원일, 이승범, 김형진, 홍인권

단국대학교 공과대학 화학공학과

* 한국 건자재 시험연구원

Preparation and Performance of the Activated

Carbon Based on Bituminous Coal

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

Dept. of Chem. Eng., College of Eng.,

Dankook Univ., Seoul 140-714, Korea

* Korea Institute of Construction Materials, Seoul 152-055, Korea

INTRODUCTION

Activated carbon(AC) is used in adsorption processes such as solvent recovery, gas separation, water treament, and catalysis[1-2]. Its use in simultaneous removal of SO2, NOx, and H2S from industrial gaseous effluents, and its role as an activation agent of chlorination and dehydrochlorination are well known[3].  Recently, the need for activated carbon has considerably increased, especially with regard to wastewater treatment.  As an example, the average annual rate of increase in the demand for activated carbon was about 4% between 1989 and 1993 in USA. Last year about 17,000 tons of activated carbon were used in Korea[4].  Activated carbon is prepared from various raw materials such as coal, wood, coconut shell, peat, bone etc.  Details of the manufacturing process are usually kept as industrial secrets.  However, variables such as coal rank, particle size, oxidation, flow rate of reactants and activated carbon yield in the activation step, must be taken into account in tailoring an high quality activated carbon for a specific application.

In this study, bituminous coal was used as the feed to a bench-scale activated carbon manufacturing process, and the effects of various operating variables on the AC yield and product were analyzed so that the scale up data for the design of pilot plant and commercial plant could be obtained.

BACKGROUND for ACTIVATION MECHANISM

Activated carbons are microporous carbonaceous adsorbents.  Commercially available activated carbons are prepared from carbon-containing source materials. The main steps in the preparation of activation carbon are carbonization and activation process[5].  Steam activated carbons are produced by a two stage procedure.  Firstly, the material is carbonized to an intermediate product, but the pores of which are either too small or too constricted  to be used as a useful adsorbent.  The next step, which in some cases can take place at a later stage kiln, is a process of enlarging the pore structure so that total internal surface which gives adsorption function to activated carbon is enlarged.  This process is accomplished by reacting the carbonized product with steam at the temperature range of 800 to 1,000℃.  At the range of temperature, the rate determining factor is the chemical reaction of  the carbon with the steam.  The reaction takes place at all the internal surfaces of the carbon structure, removing carbon from the pore wall and thereby enlarging them.  It is well known that the reaction of carbon with steam is thermodynamically endothermic[6] and can be expressed[7] as

[eqn: rm C`` +`` H_2 O ``{rel rarrow {~~}{}}`` CO`` +`` H_2]         [eqn: rm Delta H = 31.14 rm kcal]              (1)

[eqn: rm CO`` +`` H_2 O``{rel rarrow {~~}{}}`` CO_2`` +`` H_2]       [eqn: rmDelta H = -9.65 rm kcal]              (2)

EXPERIMENTAL

The activation and carbonization of coal was carried out in the rotary kiln-type furnace.  It was 7 and 160 cm in diameter and length respectively, and was heated to operting temperature with electric coil.

For this study the operating parameters were temperature and steam amount.     Temperature in the furnace is very important operating parameter in the manufacturing process of activated carbon because below 800℃ the rate of reaction is too slow to be economical.  At the temperature over 1,000℃, the reaction in the pore becomes blockage and only the reaction occurs in the outer layer of the carbon particle, each particle is reduced in size and the interior surface of the pore is not activated.  Therefore, the activation temperature was the range of 800 to 950℃, and the steam flow rate was 0.75∼3 g/g coal□hr, respectively.  The carbonization for fresh and oxidized coal samples was carried out under nitrogen stream, with flow rate of 20 ml/min and heating rate of 3℃/min.  The activation time was varied from 120 to 300 minutes.  Three thermocouples were equipped in the rotary kiln-type furnace.  All temperature was measured with thermocouple in the middle position of furnace.

For this experiment a sample container([eqn: phi] 40×L 240mm) was put into the rotary kiln.  About 30 g of the sample were shipped in the container.   Carbonization temperature was 700℃ and maintained for 60 minutes.  The activation carbon yield and iodine value[KS-M 1802] were measured to analyze the process.

RESULTS and DISCUSSION

1. Carbonization effects

During the  carbonization, a primary pore structure is developed. Later, this structure leads to the development of porosity during activation. The important parameter of carbonization process are time and temperature. Generally, time is the more important variable than temperature in carbonization process. For this study 30 g of coal was placed in sample container in the furnace and nitrogen gas was passed over the sample at a flow rate of 20 ml/min. Then, carbonization temperature is fixed at 700℃.  Fig. 1 show the effects of carbonization time on the ash and fixed carbon content.  As the carbonization time increased, the fixed carbon and AC yield increased and reached a maximum value at the carbonization time of 240 minutes, and decreased after 240 minutes.  But, the decrease in ash content and iodine value was continued to 240 minutes, thereafter increased.  Therefore, 240 minutes could be suggested as the optimum carbonization time in this process.

2. Optimum activation temperature & time

Generally, the adsorption capacity of activated carbon was increased, but the AC yield was decreased on the contrary with increasing activation temperature.  However, the adsorption capacity was decreased after maximum activation temperature.

Fig. 2 shows the effects of activation temperature on the iodine value and AC yield.  Then, to analyze the activation carbon content the AC yield was defined as

[eqn: rm AC~Yield(%) = W_p over W_f times`100(%)]                                   (3)

where [eqn: W_p] and [eqn: W_f] are the amount of product after activation and feed before activation, respectively.  The AC yield was decreased with increasing activation temperature, and the iodine value was increased from 906 to 1154 mg/g as the activation temperature was increased from 800 to 900℃.  As shown in Fig. 2, the optimum activation temperature was 880℃ considering both the AC yield and iodine value.

Fig. 3 shows the effects of activation time on the iodine value and AC yield for the sample.  The data in this figure was obtained with the activation temperature of 880℃ and  the steam amount of 1 g steam/g coal□hr.  As shown in Fig. 3, as the activation time was increased from 120 to 300 minutes, the AC yield decreased, but the iodine value increased.  The Korean Industrial Standards require an iodine value over 1,000 mg/g for the second class of powder and granular activated carbon.

Fig. 1. Effect of carbonization time on   Fig. 2. Effects of activation temperature

the ash and fixed carbon content.        on AC yield and iodine value.

Fig. 3. Effects of activation time on       Fig. 4. Effects of steam amount on

AC yield and iodine value.               AC yield and iodine value.

Therefore, the optimum activation time was 240 minutes because the iodine value exceeded 1,000 mg/g, and it kept the AC yield above 35%.

3. Effects of steam amount

The effects of the steam amount on the iodine value and AC yield for the sample are shown in Fig. 4.  The data in this figure was obtained with the activation temperature of 880℃ and the activation time of 240 minutes.

As shown in Fig. 4, as the steam amount was increased from 0.75 to 3 g steam/g coal□hr, the AC yield decreased from 41.1 to 37.7 %.  The iodine value was increased from 906.2 to 1005.3 mg/g as the steam amount was increased from 0.75 to 1 g steam/g coal□hr.  Further increasing of the steam amount above 1 g steam/g coal□hr had little effect on the iodine value.  This trend is the result of an collapse or blockage of pores at 1g steam/g coal□hr over.  Beyond a certain critical value, the fraction of ash in the activated carbon increased because the volatile components and the consumption of carbon due to the reaction with steam.  Therefore, the results shown in Fig. 4 suggest that the optimum steam amount was 1 g steam/g coal□hr, for this coal sample.

CONCLUSIONS

1. As the activation temperature was increased, the AC yield drastically decreased.  The iodine value increased with increasing activation temperature when the activation temperature was above 900℃ and decreased slightly with further increases in activation temperature.  Therefore, 880℃ was suggested as the optimum activation temperature.

2. As the steam amount was increased from 0.75 to 3 g steam/g coal□hr, the AC yield decreased.  The iodine value increased as the steam amount was increased from 0.75 to 1 g steam/g coal□hr, but further increases in the steam amount above 1 g steam/g coal□hr had little effect on the iodine value.  Therefore, 1 g steam/g coal□hr was selected as the optimum steam amount.

3. As the activation time was increased from 120 to 300 minutes, the AC yield decreased but the iodine value increased. We selected 240 minutes as the optimum activation time because the iodine value exceeded 1,000 mg/g, and it kept the AC yield above 38%.

REFERENCES

1.	M. Smisek and S. Cerny, □Active Carbon□, Elsevier, Amsterdam(1970).

2.	R. C. Bansal, J. B. Donnet, and F. Stoeckli, □Active Carbon□, Marcel Dekker,	NewYork (1988).

3.	D. L. Trimm, in C. Kemball and D. A. Dowden, Catalyst, London 4, 210(1981).

4.	C. H. Kim, S. C. Kim, and A. Chen, □The Development of High Quality	Activated Carbon Manufacturing process using Chiness Coal□, KIER Technical	Reports (1996).

5.	Suzuki, M., □Adsorption Aspects of Production and use of Activated Carbon□,	KODANSHA, Tokyo (1990).

6.	T. Wigmans, Carbon, 27(1), 13 (1989).

7.	Herbert E. Klei, James Sahagian, and  Donald W. Sundstrom, Ind. Eng. Chem.	Process Des. Dev., 14(4), (1975).