Ind. Eng. Chem. Res. 1987, 26, 2552-2556
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Hemisphere: Washington, D.C., 1986; pp 645-655. Gabelica, Z.; Derouane, E. G.; Blom, N. Appl. Catal. 1983, 5, 227-249. Haag, W. 0.;Lago, R. M.; Rodewald, P. G. J . Mol. Catal. 1982, 17, 161-169. Ravella, A. M.E.Sc. Thesis, The University of Western Ontario, London, Ontario, Canada, 1985. Ravella, A. Ph.D Thesis, The University of Western Ontario, London, Ontario, Canada, 1987. Ravella, A.; de Lasa, H. I. Chem. Eng. J . 1987a, 34, 47-53. Ravella, A.; de Lasa, H. I. Can. J . Chem. Eng. 198713, in press. Ravella, A.; de Lasa, H. I.; Rost, E. Chemical Reactor Design and Technology; de Lasa, H. I., Ed.; Martinus Nijhoff The Netherlands, 1986; pp 737-748.
Soria Lopez, A.; de Lasa, H. I.; Porras, J. A. Chem. Eng. Sci. 1981, 36, 285-291. Zaman, J.; Hossain, I.; McGreavy, C. Presented at the 35th Canadian Society for Chemical Engineering Conference, Calgary, Alberta Canada, Oct. 1985. Albert0 Ravella, Hugo I. d e Lasa,* A r n a u d M a h a y The University of Western Ontario Faculty of Engineering Science London, Ontario, Canada N6A 5B9 Received for review January 27, 1987 Revised manuscript received September 9, 1987 Accepted September 22, 1987
Preparation of Activated Carbon by Thermal Decomposition of Used Automotive Tires Samples from a used automotive tire (natural rubber based) were subjected to thermal decomposition in a flow reactor a t various temperatures with water continuously introduced a t various rates. Pyrolysis experiments were also carried out with the samples. It was found, in the wet thermal decomposition, that H2 and CO were the prime components of the gaseous mixture produced, and aliphatic hydrocarbons and akylbenzenes were the major components of oil product. Also, the surface area of carbon residue was found to increase and the amount of the residue to decrease as water feed and reaction temperature increased. At 1173 K, for example, an 87 m2/g of carbon residue was obtained from pyrolysis, but a 1260 m2/g of carbon residue was prepared from a wet thermal decomposition. As far as a benzene-vapor adsorption test is concerned, this carbon residue is considered to be a good activated carbon. Torikai et al. (1979) studied the preparation of activated carbon from used automotive tires by pyrolysis of samples at 823 K and activating the residual solid so obtained in a stream of COz at 1173 K. This carbon had surface areas of up to 400 m2/g. Yatsevskaya et al. (1983) decomposed used tires by vacuum pyrolysis at 873 K and activated the solid residue with steam at 1123 K. This activated carbon was found to have a 70% clarification capacity for methylene blue. Funazukuri et al. (1987) showed that by a supercritical treatment with water at 653 K, about 43% (weight) used tire was converted into carbonized solid residue and 48% into liquid oils. However, the carbonized solid later has been found to have a surface area of only 50-80 m2/g. This surface area is not large enough for the solid to be used as an adsorbent. Our interest is to prepare activated carbon by thermal decomposition of used tire with steam. Judging from the papers mentioned above, it seems that, for this purpose, the decomposition should be carried out at high temperature, but not at high pressure. Experimental Section Samples and Apparatus. The samples used in this work were from the same batch as used for the supercritical extraction (Funazukuri et al., 1987). All samples of the natural rubber based tires had the same composition which is listed in Table I. Approximately 1 g of the sample (about 45 2- X 2- X 4-mm cuboids) was dumped in a quartz tube reactor of 10-mm inside diameter. As outlined in Figure 1,the flow reactor, with water continuously introduced, was externally heated in a furnace to plateau temperatures of from 973 to 1173 K. Helium gas was also introduced at a rate of 0.67 mL/s to the reactor in order to prevent backward flow of the water vapor. Pyrolysis experiments were also performed by using the same apparatus, but only in a stream of helium. All experiments 0888-5885/87/2626-2552$01.50/0
Table I. Composition of t h e Initial Tire Samples component wt % natural rubber 80 52.2 20 13.0 styrene-butadiene rubber 45 29.4 carbon black (HAF) 5 3.3 zinc oxide 2.1 sulfur 3.2 additives (small amount)
under wet and dry conditions were conducted at atmospheric pressure. Instruments Used for Analyses. Most of the oil extracted in the reactor deposited in an oil trap (3 in Figure l ) , and some adhered to the inside wall of a connecting tube between the reactor and the oil trap. The oil caught in the trap was analyzed by the following instruments: an IR spectrometer (Model IRA2 of Japan SpectroscopicCo., Tokyo), an NMR spectrometer (of JEOL Co., Tokyo, Model FX-9OQ for 'H NMR and Model PMXBO for I3C NMR), and a GC/MS (Model RMU-6L of Hitachi Co., Tokyo). The compositions of the exit gas were measured by a gas chromatograph, the solid which remained in the reactor was taken out and weighed, and then its surface area was measured by a BET apparatus. A scanning electron microscope (Model JSM-35 of JEOL Co.) was used to take photographs of residual solids, and an X-ray microanalyzer (Model JSM-35DDS of JEOL Co.) was used to detect zinc and sulfur in the residual solids. Results and Discussion Water Feed Rate. Figure 2 shows the effects of water feed rate on the yield and surface area of carbon residue when the rubber sample was subjected to wet thermal decomposition at 1073 K for 3 h, after about a 1-h temperature-rise period (from room temperature to this plateau temperature). It is found that the yield of carbon residue decreases and the surface area increases as water 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 12, 1987 2553 Gas ChromtograDh
i 4 0 He
1
Reactor E i e c t r l c furnace Oil traps Ribbon heater
2 3,4
5
Id
Figure 1. Schematic diagram of experimental apparatus. 0
-
0 0
1200
2400
3600
Reaction time [si
40
Figure 4. Carbon residue yield and surface area vs time: plateau temperature, 1173 K; water feed rate, 0.75 mg of water/g of sam-
-
se 30
2 m L
c 0
2
20
LL 0
D .-
lo
0 0,5 1,0 1.5 2,0 Water feed r a t e lma-water/s-samole S I
0
Figure 2. Carbon residue yield and surface area vs water feed rate: plateau temperature, 1073 K; reaction time, 3 h (after temperature-rise period of about 1 h).
900
1000
1100
1 IO
Reaction temperature I K 1
Figure 5. Gas formation rates vs temperature: reaction time, 3 h (after temperature-rise period of about 1h); water feed rate, 0.75 mg of water/g of sample/s.
.; t p
1.0
O 0
0
0
0
0
Temperature
1073
wet dry iw-water 9 s H2
0
co
0
T
CH4 a
0
* A ?
5000
873 m 3 i
673
E
473
E
E
A
10000
Reaction time
4
15 10
[SI
Figure 3. Gas formation rates vs time: plateau temperature, 1073
K.
feed rate increases, almost linearly with the water feed rate until it reaches about 0.8 mg of water/g of sample/s; beyond this value, the effects of water feed rate on the carbon residue yield and on the surface area become less significant. Figure 3 shows the rates of production of H2, CO and CH4 in the reactor while it was heated from room temperature to 1073 K in 1 h and then maintained at this plateau temperature for 3 h under the following two con-
ditions: thermal decomposition with water introduced at 1mg of water/g of sample/s and pyrolysis. It is found that H2and CO are produced at considerably high rates under wet conditions: production rates of H2and CO are roughly in the ratio 1.5:1, and CH4 is produced only slightly when time becomes longer than about 6000 s. However, in the dry decomposition a little CO is produced, and neither H2 nor CHI is detected at a time longer than about 5000 s. The large formation of H2 and CO in the wet thermal decomposition is considered to result from the water gas reaction occurring in the reactor. Also, Figure 4 shows the changes in yield and surface area of the carbon residue with reaction time when the sample was decomposed at a temperature of 1173 K with water introduced at 0.75 mg of water/g of sample/s. The reaction time is the time after the reactor reached this plateau temperature. A linear decrease in carbon residue yield and a linear increase in surface area are observed until time reaches 1 h. At a reaction time of 1 h, for example, a 1260 m2/g of carbon residue is obtained, although its amount is only 9% (weight) of the sample loaded in the reactor. Note that under the same heating conditions, the pyrolysis gives only 87 m2/g of carbon residue. Reaction Temperature. The rubber samples were heated at various plateau temperatures for 3 h (or about 4 h when the temperature-rise period was included) with water introduced at a rate of 0.75 mg of water/g of sample/s. The effects of plateau temperatures on the pro-
2554 Ind. Eng. Chem. Res., Vol. 26, No. 12, 1987
- 50
1000 -
-cn
- 40 - 30 - 20 - 10
0
-
0 950
1000
1050
1150
1100
Reaction temperature
lK1
Figure 6. Carbon residue yield and surface area vs temperature: reaction time, 3 h (after temperature-rise period of about 1h); water feed rate, 0.75 mg of water/g of sample/s.
_-_-
A
A
--
Figure 7. IR spectra of an oil recovered.
duction rates of H2 and CO are shown in Figure 5 and those on the surface area and on the yield of carbon residue in Figure 6. As depicted, the production rates of H2 and CO increase considerably as the reaction temperature increases from 973 to 1173 K. This shows that a considerable amount of the water gas reaction takes place a t a temperature higher than lo00 K. Also, a sharp increase in the surface area and a considerable decrease in the carbon residue yield are observed when the temperature is higher than about 1050 K. Properties of the Extracted Oil. Figure 7 shows the IR spectra of the oil recovered by the treatment with water introduced at 0.75 mg of water/g of sample/s a t 1073 K for 3 h after a temperature-rise period of about 1h. The spectra measured by the IR spectrometer show that aliphatic hydrocarbons and alkylbenzenes are the major
(a 1
(b)
components of the oil. The same was observed by the 13C NMR and lH NMR measurements. Also, GC/MS tests indicated that the average molecular weight of the aliphatic hydrocarbons was 164 and that of alkylbenzenes 180. These two components are, therefore, estimated roughly to be C12hydrocarbons. Solid Residues. Figure 8 shows an internal section of the carbon residue, part a, scanning electron micrograph, and parts b and c, zinc and sulfur, respectively, detected by the X-ray microanalyzer. It is not clear what the dark area in the center of part a is, but parts b and c show that the lumps of zinc and sulfur grains exist a t the same locations, suggesting they are in a compound of ZnS. ZnS was considered to be produced by the reaction of ZnO and S, contained in the tire (refer to Table I), during wet thermal decomposition. The experiments were carried out below or at 1173 K so that the ZnS (sublimation temperature: 1453 K) remained as a solid grain. Figure 9 shows an outside surface of the carbon residue. The dark area in the center of the scanning electron micrograph of part a corresponds exactly to the lump of zinc grains shown in part b, detected by the X-ray microanalyzer. However, no lump of sulfur grains is seen in part c. This suggests that on the outside surface no reaction has occurred between zinc oxide and sulfur in the wet thermal decomposition process. This may be related to the fact that the outside surface of the solid sample was exposed to gases in the reducing environment. As shown in Figure 10, a straight-line relationship is observed between the yield of carbon residue and its surface area, no matter under what conditions the rubber samples were subjected to wet thermal decomposition. It is clear that a 39% yield carbon residue consisted of not only the carbon black and the zinc compounds but a solid carbonized from the rubber material, while that of 10% yield does not have even as much carbon black as originally contained in the tire sample. This indicates that the carbon black has been consumed by a water gas reaction occurring in the wet thermal decomposition. By use of nitrogen as an adsorbate at low temperature, the surface area of carbon residue was measured, by a BET apparatus, as a function of the nitrogen gas pressure. The surface area-pressure data are then converted, using the BJH method (Barrett et al., 1951), into the pore size distributions shown in Figure 11. The carbon residue obtained from the pyrolysis (about 1 h of heating from room temperature to 1173 K and another hour of heating a t this plateau temperature) of the rubber sample has a surface area of 87 m2/g, while under the same heating conditions the surface area of carbon residues subjected
(C)
Figure 8. Internal surface of a carbon residue: (a) scanning electron micrograph; (b) zinc by X-ray microanalysis; (c) sulfur by X-ray microanalysis.
Ind. Eng. Chem. Res., Vol. 26, No. 12,1987 2555
(a 1 Figure 9. Outside surface of a carbon residue: (a) scanning electron micrograph; (b) zinc by X-ray microanalysis; (c) sulfur by X-ray microanalysis.
Comnercial(1215 m2/a) Carbon residue 1260 m2/9
650
640
500
I
20
"0
"n
40
-cn
\
- 650 in2/, -.- 500 m2/g
300 L
\
a
---- a7
\
2
"
0
2
4
6
8
m2/g
1
0
Pore radius [nml Figure 11. Pore size distributions of the carbon residues; r, pore radius; A, surface area of pores with radii less than r, per unit mass of the solid.
to wet thermal decomposition grows to 1260 m2/g. As depicted in Figure 11, micropores of diameter less than about 3 nm increase considerably with the increase in surface area. A benzene-vapor adsorption test, based on the Japanese Industrial Standard K1412, was performed at 295 K with the various carbon residues obtained from pyrolysis and wet thermal decomposition. Figure 12 shows the mass of benzene adsorbed per unit mass of carbon residues. The same test was made with a commercial activated carbon with a surface area of 1215 m2/g (of Nishio Ind. Co., Tokyo). As far as this test is concerned, the carbon residue
180
Time [ S I
Y i e l d o f carbon residue [XI
Figure 10. Carbon residue yield vs surface area.
120
60
Figure 12. Adsorption of benzene vapor on the carbon residues and a commercial activated carbon at 295 K; tests based on the Japanese Industrial Standard K1412.
with surface area 1260 m2/g is expected to be as good as the commercial activated carbon.
Conclusions The wet thermal decomposition of samples from a used automotive tire (natural rubber based) carried out at various temperatures and various water feed rates showed the following. 1. The yield of carbon residue decreases and its surface area increases with the increase in water feed rate and also with the increase in reaction temperature. 2. H2 and CO are the major components of gaseous mixture produced, and aliphatic hydrocarbons and alkylbenzenes are the prime components of oil extracted in the wet thermal decomposition. 3. When the tire sample was treated at 1173 K for 1h with water fed at a rate of 0.75 mg of water/g of sample/s, for example, a carbon residue with surface area of 1260 m2/g was obtained. A benzene-vapor adsorption test showed that this carbon residue was a good activated carbon.
Acknowledgment We express our appreciation to Y. Shimada for taking scanning electron microscope and X-ray microanalyzer photographs of the solid residues. Registry No. CO,630-08-0; C,7440-44-0;H2,1333-74-0.
Ind. Eng. Chem. Res. 1987, 26, 2556-2557
2556
Literature Cited
Nauk BSSR 1983,27(4), 343; Chem. Abstr. 1983, 24788h.
Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J.Am. Chem. SOC.1951, 73, 373.
Funazukuri, T.; Takanashi, T.; Wakao, N. J. Chem. Eng. Jpn. 1987, 20, 23. Torikai, N.; Meguro, T.; Nakamura, Y. Nippon Kagaku Kaishi 1979,
Sadao Ogasawara,* Mikiya Kuroda, Noriaki Wakao School of Engineering Yokohama National University Yokohama 240, Japan Received for review January 27, 1987 Accepted September 14, 1987
11, 1604.
Yatsevskaya,
M. I.; Komarov, V. S.; Artemova, T. A. D. Kl. Akad.
CORRESPONDENCE Comments on “Formaldehyde Oxidation over Nickel Oxide” Sir: Foster and Masel (1986) set forth a kinetic analysis of their data secured in a modified Berty reactor (“...reactor flow patterns are almost same as standard Berty”), and their Berty reactor was tested by “the procedure of Carberry”. One is obliged to infer that the author’s reactor operated as a CSTR, in which case simple material balances provide reaction rate data for subsequence kinetic analyses. Such analyses yield, in their paper, several rate equations of rather remarkable complexity. Indeed, while their rate equation (AI) exhibits a root mean square of 0.07, it also exhibits over 20 parameters (including orders), enough to generate a Picasso elephant. As there is absolutely no reason to doubt the integrity of chemical analyses and the techniques of data reduction, the enormous complexity of their most faithful kinetic expression can be ascribed to (a) inherent complexity of formaldehyde oxidation over NiO, (b) inherent disguises due to undetected local interphase-intraphase gradients which can persist even in a CSTR, and (c) inherent complexity of the reactor employed in this study. As all heterogeneously catalyzed reactions are complex, more or less, the suggested rate equation may indeed reflect catalytic realities. However, I deem case a to be rather unlikely insofar as catalytic reactions of greater complexity than formaldehyde oxidation over NiO (e.g., FischerTropsch synthesis) yield to far less complex kinetic description. Case b, always a threat, particularly in oxidation reactions, can be readily examined. For any CSTR, in terms of measured conversion X, contact time 8, a readily procurable interphase mass transport coefficient k,, and particlepellet external surface-to-volumeratio a, one finds interphase (external) CfX AC, = k,a8
(1)
intraphase (internal) AC, = 0 if
(1 - X)a2a)8
< 1.0
(2)
If the governing thermal parameters are utilized, we obtain interphase (external)
PX AT, = k,a8
(3)
0888-5885/87/2626-2556$01.50/0
intraphase (internal)
[
X(l
ATi=P 1-
p=-
(-AH)Yf
C&e2J3
P=
+ k,a@
]
(4)
(-WW
Cf - co x=---
Cf where yf = mole fraction in feed. These criteria are readily derived from earlier forms (Carberry, 1976). If by the “procedure of Carberry” Foster and Masel are referring to those cited here, we may then be assured that their data are undisguised by local gradients. Since a NiO foil was employed, only external gradients should be of import. If, then, AC, and AT, are zero, case b can be dismissed as a cause of kinetic rate equation complexity. We come then to the reactor employed-one assumed to be a CSTR. Foster and Masel (1986) operated their Berty at a total pressure of 1 atm and an initial recycle ratio of “20 or more and a conversion per pass of less than 1% ”. Overall conversions are therefore 20%, more or less. But how was the recycle ratio measured? Quite aside from the fact that CSTR behavior is not manifest in an open-loop (internal or external) recycle reactor unless the recycle ratio is greater than 25 (Carberry, 1976), the Berty-type internal recycle reactor is not a CSTR at atmospheric pressure. Indeed only at a total pressure of at least several atmospheres is such a reactor gradientless (CSTR). My authority on this is none other than my friend and colleague, Jozef Berty. Open-loop external recycle reactors do exhibit CSTR behavior at recycle ratios greater than 25 and at atmospheric pressure since an external recycle pump is employed (Serrano and Carberry, 1985),whereas internal fans relying upon a high density gas are utilized in Berty-type reactors. In the event, the mixing conditions in the Berty used by Foster and Masel (1986) are simply unknown. Their reactor is surely not a PFR nor a CSTR. N~~ is it differential since overall conversions are not differential. Consequently,their CSTR-rooted eq 1cannot be employed to obtain the experimental rates of reaction upon which their subsequent analyses depend. Is it any wonder, then, 0 1987 American Chemical Society
