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Characterization of activated carbon fibers with high surface area
J. Rouquerol, F. Rodriguez-Reinoso,K.S.W. Sing and K.K. Unger (Eds.) Characterizaiion of Porous Solids Ill Studies in Surfacc Scicncc and Cablysis, Vol. 87 1994 Elsevicr Sciencc B.V.
689
Characterization of activated carbon fibers with high surface a r e a Matti Nieminen', Jussi Ranta', Janne Laine' and Pertti Nousiainen3 1. Technical Research Centre of Fmland, P.O.Box 205,0215 1 Espoo, Finland 2. Helsinki University of Tech., Lab. of Forest Products Ind., Vuorimiehent 1, 02150 Espoo, Finland 3 . Tampere University of Technology, TEVA, P.O.Box 589,33101 Tampere, Finland Ab stract Viscose fibre based materials were carbonized and activated using CO, as an activation agent. Materials were pretreated with fire retardants and activated at 900°C. Degree of activation varied from 5 to 75 %. Sample size was from I00 mg to 8000 g. Optimization of a pilot scale activation process was done with a number of laboratory scale experiments. The samples produced were Characterized by nitrogen adsorption techniques. The results were used for surface area and pore volume determinations. In addition the samples were characterized by adsorption of different organic adsorbates from vapour phase. Vapour phase adsorption determinations were carried out by measuring adsorption capacities at saturated vapour (single point) pressure at constant temperature. 1. INTRODUCTION
Activated carbon fabrics (ACF) can be manufactured from various organic fibers e.g. such as cellulose and cellulose derivatives. In this study ACF's were produced from viscose fibre based fabrics using carbon dioxide as activation agent. The aim of the study was to optimize the process conditions for pilot scale manufacturing of ACF products. Activated carbon fibers are known to be highly microporous materials (e.g. Marsh et al, 1982). High susface area activated carbon fabrics fmd their applications in the field of protective clothing, organic vapor recovery, air cleaning etc. 2. EXPERIMENTAL
Viscose fibre based materials were pyrolyzed and activated using equipments of different scale. A theirnobalance was used to optimize quality and quantity of impregnation ingredients as well as the temperature profile to increase the yield of carbonization and
690
activation. In this case the sample size was 100-300 mg and CO, flow rate 1 Vmin. The final temperature of activation was 900°C. Mechanical properties of ACF fabrics were optimized in the second phase. This was carried out using a laboratory retoit assembled in a temperature programmed oven. The volume of the cylindrical reactor was 2 1. In this case the sample size was in the range of 10-30 grams. After optimizing the reaction conditions larger scale experiments were carried out in an electrically heated pilot-oven. In the pilot scale experiments, which are reported here , a larger size (batch size 2000-8000 g) viscose fabrics were used. The pyrolysis and activation steps were carried out subsequently with no intermediate cooling. During carbonization (250-450°C) CO, was used as an inert purge and as an activation agent at temperature range 750-900°C. In order to minimize material losses during the heat treatment of fabrics the impregnation with fire retardant ingredients is desirable. The impregnants have also an effect on activation rate of char (Freeman & Gimblett, 1988; Freeman et al, 1988). Diammonium hydrogen phosphate was found to be suitable in this study.
The ACF samples were characterized by nitrogen adsorption. A PC controlled Car10 Erba Sorptomatic Series 1800 instrument was applied. Milestone 100 was used for controlling and for calculation of isotherms. In addition the samples were characterized by the adsorption of different adsorbates from the vapor phase by measuring adsorption capacities at saturated vapor (single point) pressure at constant temperature. The equilibrium was reached in 17 hrs at 20°C. 3 . RESULTS AND DISCUSSION
In the figure 1 six typical nitrogen adsorption isotherms are presented. Some of the samples (nonwoven A, knitted A and woven A) have an inclined section in the adsorption isotherm. This seems to be related with high microporosity and high degree of activation. The BET-method in the range of 0.01-0.15 (p/p,) was used for specific surface area determinations. These (apparent) surface areas varied from 1000 up to 4000 m’/g. Although the physical meaning of exceptionally high BET-surface areas is questionable they are useful for characterizing the adsorption capacity of the activated materials. To evaluate the effect of bum-off on the specific surface area a number of experiments were carried out for three different raw materials. The results for knitted and woven fabrics are presented in figure 2. The burn-off or degree of activation was calculated using a mean value for pyrolysis yield, obtained from several separate measurements. A mean value of 33% for pyrolysis yield was used. The samples had to be activated to the range of 45-50 % bum-off to obtain super high surface areas (> 2000 m2/g).In this case no significant difference was observed between the two types of fabrics.
69 1 To assess the quality of ACF samples a large number of equilibrium adsorption capacity measurements were done. The results are illustrated in figure 3. The adsorption capacities of 1,1, I-trichloroethane varied usually from 40 to 90 gramsll00 g of sample. For certain samples high values up to 120 grams/lOO g were measured. Finally the adsorption capacities of a number of commonly used organic solvents were determinated in similar manner. The solvents used were toluene, l,l, 1-trichloroethane, ethanol, trichloroethylene and cyclohexane. The adsorption increased in the order of cyclohexane < ethanol < toluene < l,l,l-trichloroethane < trichloroethylene, when presented in grams/g. When presented in mols/g the order was cyclohexane < 1, 1, 1-trichloroethane < toluene < trichloroethylene < ethanol. The research continues with supplementary experiments to cover a wider range of ACF applications. Acknowledgements - The authors wish to express their gratitude to Kemira Fibres, Finland, for financial support and permission to publish the paper.
REFERENCES Freeman, J.J. & Gimblett, F.G.R., Studies of activated charcoal cloth.IV. Influence of phosphate impregnants on the rate of activation in carbon dioxide gas. Carbon Vol. 26, No. 4, pp. 501-505, 1988. Freeman, J.J., Gimblett, F.G.R., Roberts, R.A. & Sing K.S.W., Studies of activated charcoal cloth. 111. Mesopore development induced by phosphate impregnants. Carbon Vol. 26, No. 1, pp. 7-11, 1988. Marsh, H., Crawford, D., OGrady, T.M. & Wennerberg, A., Carbons of high surface area. A study by adsorption and high resolution electron microscopy. Carbon Vol 20, No. 5 , pp. 419-426, 1982.
1400 1200
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1000
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800
w
m i 0
1
m final Burn off = (1 - ( mpyr
I
)).loo
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v)
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m
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400 200 I
0
-
-
0.2 Non Woven A burn-off 74 % Woven B burn-off 43 %
0.4 -H-
I
I
0.6
P/Po Knitted A burn-off 65 %
+- Knitted B burn-off 35 %
0.8 + WovenA burn-off 72 % -A-
Woven C burn-off 24 %
Figure 1. Nitrogen adsorption isotherms of ACF's with normal and exeptionally high micropore volumes.
1
9 c3
0 0
0 0
0 0 0
cu
I
L o o N c \ i
0 0 N
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K1, 2 Knitted NW Nonwoven W Woven
-
I .I 2 cm3/g
I00
50
0
K2
NWI
K1
w1
Cyclohexane Ethanol Toluene 01,l ,I -Trichloroethane Trichloroethylene
Figure 3. Adsorption capacity of organic vapors for AC-fabrics with various pore volumes.
689
Characterization of activated carbon fibers with high surface a r e a Matti Nieminen', Jussi Ranta', Janne Laine' and Pertti Nousiainen3 1. Technical Research Centre of Fmland, P.O.Box 205,0215 1 Espoo, Finland 2. Helsinki University of Tech., Lab. of Forest Products Ind., Vuorimiehent 1, 02150 Espoo, Finland 3 . Tampere University of Technology, TEVA, P.O.Box 589,33101 Tampere, Finland Ab stract Viscose fibre based materials were carbonized and activated using CO, as an activation agent. Materials were pretreated with fire retardants and activated at 900°C. Degree of activation varied from 5 to 75 %. Sample size was from I00 mg to 8000 g. Optimization of a pilot scale activation process was done with a number of laboratory scale experiments. The samples produced were Characterized by nitrogen adsorption techniques. The results were used for surface area and pore volume determinations. In addition the samples were characterized by adsorption of different organic adsorbates from vapour phase. Vapour phase adsorption determinations were carried out by measuring adsorption capacities at saturated vapour (single point) pressure at constant temperature. 1. INTRODUCTION
Activated carbon fabrics (ACF) can be manufactured from various organic fibers e.g. such as cellulose and cellulose derivatives. In this study ACF's were produced from viscose fibre based fabrics using carbon dioxide as activation agent. The aim of the study was to optimize the process conditions for pilot scale manufacturing of ACF products. Activated carbon fibers are known to be highly microporous materials (e.g. Marsh et al, 1982). High susface area activated carbon fabrics fmd their applications in the field of protective clothing, organic vapor recovery, air cleaning etc. 2. EXPERIMENTAL
Viscose fibre based materials were pyrolyzed and activated using equipments of different scale. A theirnobalance was used to optimize quality and quantity of impregnation ingredients as well as the temperature profile to increase the yield of carbonization and
690
activation. In this case the sample size was 100-300 mg and CO, flow rate 1 Vmin. The final temperature of activation was 900°C. Mechanical properties of ACF fabrics were optimized in the second phase. This was carried out using a laboratory retoit assembled in a temperature programmed oven. The volume of the cylindrical reactor was 2 1. In this case the sample size was in the range of 10-30 grams. After optimizing the reaction conditions larger scale experiments were carried out in an electrically heated pilot-oven. In the pilot scale experiments, which are reported here , a larger size (batch size 2000-8000 g) viscose fabrics were used. The pyrolysis and activation steps were carried out subsequently with no intermediate cooling. During carbonization (250-450°C) CO, was used as an inert purge and as an activation agent at temperature range 750-900°C. In order to minimize material losses during the heat treatment of fabrics the impregnation with fire retardant ingredients is desirable. The impregnants have also an effect on activation rate of char (Freeman & Gimblett, 1988; Freeman et al, 1988). Diammonium hydrogen phosphate was found to be suitable in this study.
The ACF samples were characterized by nitrogen adsorption. A PC controlled Car10 Erba Sorptomatic Series 1800 instrument was applied. Milestone 100 was used for controlling and for calculation of isotherms. In addition the samples were characterized by the adsorption of different adsorbates from the vapor phase by measuring adsorption capacities at saturated vapor (single point) pressure at constant temperature. The equilibrium was reached in 17 hrs at 20°C. 3 . RESULTS AND DISCUSSION
In the figure 1 six typical nitrogen adsorption isotherms are presented. Some of the samples (nonwoven A, knitted A and woven A) have an inclined section in the adsorption isotherm. This seems to be related with high microporosity and high degree of activation. The BET-method in the range of 0.01-0.15 (p/p,) was used for specific surface area determinations. These (apparent) surface areas varied from 1000 up to 4000 m’/g. Although the physical meaning of exceptionally high BET-surface areas is questionable they are useful for characterizing the adsorption capacity of the activated materials. To evaluate the effect of bum-off on the specific surface area a number of experiments were carried out for three different raw materials. The results for knitted and woven fabrics are presented in figure 2. The burn-off or degree of activation was calculated using a mean value for pyrolysis yield, obtained from several separate measurements. A mean value of 33% for pyrolysis yield was used. The samples had to be activated to the range of 45-50 % bum-off to obtain super high surface areas (> 2000 m2/g).In this case no significant difference was observed between the two types of fabrics.
69 1 To assess the quality of ACF samples a large number of equilibrium adsorption capacity measurements were done. The results are illustrated in figure 3. The adsorption capacities of 1,1, I-trichloroethane varied usually from 40 to 90 gramsll00 g of sample. For certain samples high values up to 120 grams/lOO g were measured. Finally the adsorption capacities of a number of commonly used organic solvents were determinated in similar manner. The solvents used were toluene, l,l, 1-trichloroethane, ethanol, trichloroethylene and cyclohexane. The adsorption increased in the order of cyclohexane < ethanol < toluene < l,l,l-trichloroethane < trichloroethylene, when presented in grams/g. When presented in mols/g the order was cyclohexane < 1, 1, 1-trichloroethane < toluene < trichloroethylene < ethanol. The research continues with supplementary experiments to cover a wider range of ACF applications. Acknowledgements - The authors wish to express their gratitude to Kemira Fibres, Finland, for financial support and permission to publish the paper.
REFERENCES Freeman, J.J. & Gimblett, F.G.R., Studies of activated charcoal cloth.IV. Influence of phosphate impregnants on the rate of activation in carbon dioxide gas. Carbon Vol. 26, No. 4, pp. 501-505, 1988. Freeman, J.J., Gimblett, F.G.R., Roberts, R.A. & Sing K.S.W., Studies of activated charcoal cloth. 111. Mesopore development induced by phosphate impregnants. Carbon Vol. 26, No. 1, pp. 7-11, 1988. Marsh, H., Crawford, D., OGrady, T.M. & Wennerberg, A., Carbons of high surface area. A study by adsorption and high resolution electron microscopy. Carbon Vol 20, No. 5 , pp. 419-426, 1982.
1400 1200
-
1000
E
800
w
m i 0
1
m final Burn off = (1 - ( mpyr
I
)).loo
W
v)
U
m
>
600
400 200 I
0
-
-
0.2 Non Woven A burn-off 74 % Woven B burn-off 43 %
0.4 -H-
I
I
0.6
P/Po Knitted A burn-off 65 %
+- Knitted B burn-off 35 %
0.8 + WovenA burn-off 72 % -A-
Woven C burn-off 24 %
Figure 1. Nitrogen adsorption isotherms of ACF's with normal and exeptionally high micropore volumes.
1
9 c3
0 0
0 0
0 0 0
cu
I
L o o N c \ i
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K1, 2 Knitted NW Nonwoven W Woven
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I .I 2 cm3/g
I00
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NWI
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Cyclohexane Ethanol Toluene 01,l ,I -Trichloroethane Trichloroethylene
Figure 3. Adsorption capacity of organic vapors for AC-fabrics with various pore volumes.