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Ind. Eng. Chem. Res. 1998, 37, 2430-2435
Pyrolysis of Tire Rubber: Porosity and Adsorption Characteristics of the Pyrolytic Chars Guillermo San Miguel, Geoffrey D. Fowler, and Christopher J. Sollars* Centre for Environmental Control and Waste Management, Department of Civil Engineering, Imperial College of Science, Technology and Medicine, London SW7 2BU, United Kingdom
Tire rubber has been pyrolyzed at various temperatures under a nitrogen atmosphere. The resulting chars have been analyzed for their porosity using nitrogen gas adsorption and for their aqueous adsorption characteristics using phenol, methylene blue, and the reactive dyes Procion Turquoise H-A and Procion Red H-E3B. Nitrogen adsorption isotherms were modeled to the BET and Dubinin-Astakhov (DA) equations to determine effective surface areas, mesopore volumes, and micropore volumes. Results showed that pyrolysis of tire rubber was essentially complete at 500 °C and resulted in a char yield of approximately 42 wt %. Pyrolytic chars exhibited BET surface areas up to 85 m2/g and micropore volumes up to 0.04 mL/g. Owing to their poorly developed micropore structure, the pyrolytic chars exhibited limited aqueous adsorption capacity for compounds of small molecular weight, such as phenol. However, the chars possessed significantly greater adsorption capacity for species of large molecular weight which was attributed to the presence of large mesopore volumes (up to 0.19 mL/g). Introduction The disposal of scrap tires represents a major environmental problem. Tires are designed to be extremely resistant to physical, chemical, and biological degradation, and they do not decompose readily under atmospheric conditions. Due to their low density, land-filled tires occupy large volumes and are susceptible to displacement by more dense materials. Buried whole tires tend to float to the surface causing the disruption of compacted landfill sites (Dufton, 1995; Liaskos, 1994; U.S. Environmental Protection Agency, 1993). Landfilled and dumped tires also represent a fire hazard. When tires are present, fires are extremely difficult to extinguish and generate high levels of contamination to the atmosphere, soils, and groundwaters (Lemieux and Ryan, 1993; Best and Brookes, 1981). Scrap tire dumps represent a haven for vermin and insect infestation which pose a serious threat to health (Dufton, 1995; U.S. Environmental Protection Agency, 1993). Europe and the United States generate approximately 2.2 and 2.4 million tonnes of scrap tires a year, respectively, with a 15-20% increase predicted by the early 21st century (Dufton, 1995; U.S. Environmental Protection Agency, 1993). In other areas of the world, like South East Asia and South America, automobile ownership is increasing rapidly and so is the generation of scrap tires (Liaskos, 1994). Pyrolysis is an alternative to scrap tire disposal which results in the recovery of valuable products. It basically involves the decomposition of the tire rubber at high temperatures in an oxygen-free atmosphere. Tire rubber pyrolysis results in the production of an oil and a gas fraction, plus a residue of carbonized and nonvolatile materials. The amount and specific characteristics of the each fraction depend on the process conditions, principally on temperature and reaction time (Williams et al., 1995; U.S. Environmental Protection Agency, * To whom correspondence should be addressed. Fax: 44171-8239401. Tel.: 44-171-5945970. E-mail:
[email protected].
1993; Cypres and Bettens, 1989; Dodds et al., 1983). The characterization and potential application of the gas and oil fractions have been the subject of a number of publications. Pyrolytic oils, a mixture of paraffins, olefins, and aromatic compounds, possess a high calorific value (∼43 MJ/kg) and can be used directly as a fuel or added to petroleum refinery feedstock (Williams et al., 1993, 1995; Benallal et al., 1995). Oils can also be used as a source of chemicals as they have been found to possess high concentrations of valuable compounds such as benzene, toluene, xylene, and limonene (Williams et al., 1995; Pakdel et al., 1991; Cypres and Bettens, 1989). Pyrolytic gas contains high concentrations of methane, butadiene, and other hydrocarbon gases which results in high calorific values (∼37 MJ/m3) sufficient to heat the pyrolysis reactor (Williams et al., 1993). The carbonized residue, usually referred to as pyrolytic char, consists of carbon black and a mixture of products from the degradation of the rubber compound such as carbonized rubber polymer and nonvolatile hydrocarbons, plus lower proportions of other tire rubber additives such as zinc, sulfur, clays, and silica. Its potential reuse as carbon black is restricted to low-quality applications due to the presence of impurities, high ash content, and large particle size (Dufton, 1995; Boukadir et al., 1981). Another potential application for the pyrolytic char involves its use as a precursor for activated carbon manufacture (San Miguel et al., 1996; Teng et al., 1995; Merchant and Petrich, 1993; Cypres and Bettens, 1989; Giavarini, 1985; Ogasawara et al, 1987). Activation, however, requires extra investment, involves more expensive running costs, and results in a reduction of the carbon yield. The data reported in this paper show that tire rubber pyrolytic chars already possess a well-developed mesopore structure that results in relatively high adsorption capacities for large molecular weight compounds. Therefore, the use of these inactivated chars may be an economically attractive alternative to active carbon in applications which require the removal of species of large molecular weight
S0888-5885(97)00728-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/09/1998
Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2431
Figure 1. Representation of the rotary furnace employed for the pyrolysis of tire rubber.
from solution, such as the treatment of textile effluents (Pollard et al., 1992; Reife and Freeman, 1996). The aim of this work is to describe the porosity of chars produced from the pyrolysis of tire rubber and investigate the potential of using this material, without any further treatment, as a low-cost adsorbent for liquidphase applications. Experimental Section Tire Rubber and Commercial Powdered Active Carbons. Powdered tire rubber of particle size < 0.42 mm (mesh 40) was employed as the starting material in this work. The rubber, provided by Duralay Ltd. (U.K.), was free from metal and synthetic cords. The porous and adsorption characteristics of rubber chars were compared against three commercial powdered activated carbons (PAC): Chemviron GW (Chemviron Carbon Ltd., U.K.) and Norit W52 and Norit Hydrodarco-C (Norit UK Ltd). According to the manufacturer’s specifications, Chemviron GW and Norit W52 are multipurpose activated carbons suitable for the removal of both low and high molecular weight organics from potable waters and wastewaters. Norit Hydrodarco-C is specially designed to treat industrial wastewaters containing large molecular weight compounds such as dyes and detergents. Pyrolysis and Activation of Tire Rubber. Pyrolysis of tire rubber was carried out in a modified Carbolite HTR 11/150 laboratory-scale rotary furnace (Figure 1). The rubber (200 g) was heated at 5 °C/min to various temperatures (from 300 to 1000 °C) under a flowing (500 mL/min) nitrogen atmosphere. When the required temperature was reached, the furnace automatically switched off and was allowed to cool. Volatile products arising from the thermal decomposition of the rubber were carried out of the reaction vessel by the continuous nitrogen flow and were allowed to condense at room temperature. Noncondensable gases were cleaned and vented into a fume cupboard. In each experiment, the char and the oil fraction yields were determined by weight and the gas fraction yield was calculated by difference. Chars were collected and stored without any further treatment for subsequent analysis. Nitrogen Gas Adsorption. Untreated tire rubber, pyrolytic chars, and the three commercial activated carbons were analyzed by continuous volumetric nitrogen gas adsorption, at liquid nitrogen temperature, using a Coulter Omnisorp Model 100 automatic adsorption analyzer (Coulter Electronics Ltd., Luton, U.K.).
Prior to the analysis, the samples were outgassed at 75 °C for 6-8 h to a vacuum ∼10-5-10-6 Torr. Adsorption/ desorption isotherms, performed under a constant nitrogen flow rate (0.267 mL/min) to an adsorption cutoff p/p0 ) 0.98, typically required 8-12 h to complete. Surface areas and total micropore volumes were estimated by application of the BET (Brunauer et al., 1938) and the Dubinin-Astakhov (DA) (Stoeckli, 1995) equations, respectively, using the associated instrument software. Mesopore volumes were calculated by subtracting the micropore volume (obtained from the application of the DA equation) from the total volume of nitrogen adsorbed at p/p0 ) 0.95 (Rodrı´guez-Reinoso et al., 1995; Sing, 1995). Aqueous Adsorption. Aqueous adsorption tests were performed using phenol (AnalaR, Merck Ltd.), methylene blue (Sigma-Aldrich Ltd.), and the reactive dyes Procion Turquoise H-A and Procion Red H-E3B (Zeneca Ltd.). The aqueous adsorption efficiency of untreated tire rubber and pyrolytic chars for the four compounds was initially screened using single-point adsorption tests. These tests are not intended to determine adsorption capacities but to illustrate, comparatively, the evolution of the aqueous adsorption capacity of chars prepared at diffferent temperatures. Tests were carried out by mixing 100 mg of the sample with 100 mL of solution of fixed concentration (100 mg/L for phenol; 350 mg/L for methylene blue; 300 mg/L for Procion Red; 400 mg/L for Procion Turquoise). Subsequently, full aqueous adsorption isotherms were obtained from selected chars and from three commercial active carbons. All adsorption tests were carried out at 20 °C in a temperature-controlled room by mixing adsorbate solutions of specific concentrations with accurately weighed masses of adsorbent in 125-mL screw cap glass bottles. The mixtures were end-over-end shaken over 24 h after which the chars were separated from the solution by filtration using either no. 1 filter paper (phenol and methylene blue) or white cellulose nitrate (WCN) membranes (reactive dyes) (Whatman Ltd., Maidstone, U.K.). Preliminary tests showed that the contact times employed were sufficient to reach steady-state conditions. To minimize the effect of pH on the adsorption tests, all stock solutions contained 500 mg/L of NaHCO3 (buffer) and their initial pHs were adjusted by the addition of concentrated hydrochloric acid to a value of pH 6.25. This ensured that all adsorption tests were carried out in the pH range 6.8 ( 0.8. Residual adsorbate concentrations were determined by UV-vis spectrophotometry at the following wavelengths: 265 nm (phenol), 510 nm (Procion Red), 622 nm (Procion Turquoise), and 661 nm (methylene blue). Aqueous adsorption results were modeled to the Freundlich equation to calculate adsorption capacities (Kf) and affinity values (n) (Noll et al, 1992; Faust and Aly, 1987). Results and Discussion Products Yields. Figure 2 presents the product yields obtained when tire rubber was pyrolyzed at various temperatures between 300 and 1000 °C. Results indicate that the decomposition reaction commenced at 300 °C and was essentially complete at 500 °C. Generation of oils was only observed in this range of temperatures. At 500 °C, the amount of oils produced already represented 53.1 wt % of the initial tire rubber weight and this value remained relatively constant (0.5 wt % when higher temperatures were employed.
2432 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998
Figure 2. Product yields resulting from the pyrolysis of tire rubber at different temperatures.
At 500 °C, the solid residue yield represented 42.1 wt %. Increasing the temperature from 500 to 1000 °C resulted in a small reduction of the carbon weight (40.1 wt % at 1000 °C) and a consequent increase in the gas fraction due to continuous volatilization of noncondensable species. Nitrogen Adsorption and the Characterization of the Porosity. Untreated tire rubber and pyrolytic chars produced at temperatures below 450 °C exhibited very low adsorption capacity for nitrogen gas (less than 8 mL (STP)/g at p/p0 ) 0.95). These temperatures are not sufficient to decompose the tire rubber completely, and the solids produced still maintain the adsorption characteristics of the initial material. Chars produced at 450 and 500 °C were transition samples with adsorption isotherms similar in shape to those exhibited by chars produced at higher temperatures but showing lower adsorption capacities. Samples prepared in the temperature range between 600 and 1000 °C produced nitrogen adsorption/desorption isotherms that were very similar to each other. This indicates that once pyrolysis is completed, the pore structure of the chars is essentially unchanged. Figure 3 presents the nitrogen adsorption/desorption isotherm exhibited by the tire rubber pyrolytic char
prepared at 700 °C. The isotherm, of characteristic type IV shape with a well-defined type H3 hysteresis loop (Gregg and Sing, 1982), is characteristic of a mesoporous material. Some adsorption at very low relative pressures (p/p0 < 0.1) has been observed, which is attributed to the presence of some degree of microporosity. Commercial carbon blacks tested by other authors (Carrot et al., 1987; Sing, 1994) exhibited adsorption/ desorption isotherms similar in shape to those obtained from rubber chars with the difference that they exhibited smaller or nonexistent hysteresis loops. Other parameters such as surface areas and micropore volumes are also comparable. Comparatively, commercial activated carbon Chemviron GW produced an adsorption isotherm of primarily type I character, typical of microporous materials. The other two, Norit Hydrodarco-C and Norit W52, produced mixed type I-type IV isotherms, characteristic of porous materials containing both micro- and mesopores. Effective surface areas and micropore volumes were determined by modeling the nitrogen adsorption isotherms to the BET and Dubinin-Astakhov equations, respectively. The BET equation was applied in the region p/p0 ) 0.015-0.250 where linear sections with correlation coefficients close to unity (r2 > 0.9999) and BET-C values in the range 195-102 were obtained. The Dubinin-Astakhov equation was applied in the pressure range p/p0 ) 0.005-0.1 where it produced linear (r2 > 0.9995) sections. Untreated tire rubber and pyrolytic chars prepared at temperatures below 450 °C exhibited very low adsorption capacities and more irregular isotherms. Consequently, the correlation coefficients obtained for the BET and the DA equations in the same pressure ranges were lower, at r2 > 0.998 and r2 > 0.999, respectively, and the BET-C values ranged between 20 and 60. It should be noted that porosity and surface area determinations from these samples are subject to greater error.
Figure 3. Nitrogen gas adsorption-desorption isotherm from tire rubber char pyrolyzed at 700 °C.
Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2433
Figure 4. Surface area, micropore volume (V.micro) and mesopore volume (V.meso) from pyrolytic chars produced at various temperatures.
Figure 6. Methylene blue adsorption isotherms.
Table 1. Porosity and Surface Area of Three Commercial Powdered Activated Carbons
Norit W52 Chemviron GW Norit Hydrodacrco-C
BET surface area (m2/g)
micropore volume (mL/g)
mesopore volume (mL/g)
789 788 545
0.33 0.32 0.25
0.14 0.06 0.25
Figure 7. Phenol adsorption isotherms.
Figure 5. Development of the aqueous adsorption efficiency of tire rubber chars.
Figure 4 illustrates the BET surface area, micropore volume, and mesopore volume of tire rubber chars prepared at various temperatures and Table 1 illustrates the same parameters as obtained from the three commercial activated carbons. Untreated rubber and chars prepared at temperatures below 450 °C do not possess any notable porosity or surface area. A significant development of the pore structure was observed between 450 and 600 °C as the tire rubber was transformed into a carbonized char. Pyrolytic chars prepared at temperatures between 600 and 1000 °C exhibited BET surface areas in the range 78-85 m2/g, with total micropore volumes around 0.04 mL/g and mesopore volumes around 0.19 mL/g. These values are similar to those reported by other authors from commercial carbon blacks (Sing, 1994; Carrot et al., 1987). Comparatively, commercial activated carbons possessed larger BET surface areas and micropore volumes due to their highly developed micropore structure. Pyrolytic chars, however, exhibited significantly larger mesopore volumes than the two general purpose commercial active carbons Norit W52 and Chemviron GW. Aqueous Phase Adsorption Tests. Figure 5 describes the relative adsorption efficiency for phenol, methylene blue, Procion Turquoise H-A, and Procion Red H-E3B exhibited by untreated tire rubber and pyrolytic chars as determined by single-point adsorption
Figure 8. Procion Red adsorption isotherms.
Figure 9. Procion Turquoise adsorption isotherms.
tests. Untreated tire rubber and chars produced at temperatures below 500 °C showed very low adsorption efficiency for all four compounds. The degree of removal increased significantly in the temperature range between 450 and 700 °C to reach a plateau where the uptake values for the four adsorbates increased gradually or remained constant at higher preparation temperature. Figures 6-9 present the adsorption isotherms for phenol, methylene blue, Procion Turquoise H-A, and
2434 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 Table 2. Freundlich Adsorption Parameters from Phenol and Methylene Blue methylene blue
phenol
adsorbent
Kf (mg/g)
n
r2
Kf (mg/g)
n
r2
rubber 450b 500b 600b 700b 800b
n/aa n/aa 1.04 6.90 16.7 16.5
n/aa n/aa 3.33 2.65 5.97 5.68
n/aa n/aa 0.67 0.92 0.94 0.98
n/aa n/aa n/aa 0.47 1.18 1.76
n/aa n/aa n/aa 2.26 3.14 3.65
n/aa n/aa n/aa 0.98 0.95 0.97
1000b W52c GWc Hyd-Cc
22.2 161.0 187.2 140.1
7.94 27.8 33.0 46.7
0.99 0.98 0.98 0.93
2.01 49.87 48.60 23.82
3.73 4.87 4.69 4.42
0.99 0.99 0.98 0.99
a n/a: estimated adsorption capacity below 0.01 mg/g. b Tire rubber pyrolytic chars: production temperature. c Commercial activated carbons Norit W52 (W52), Chemviron GW (GW), and Norit Hydrodarco-C (Hyd-C).
Table 3. Freundlich Parameters from Dyes Procion Red H-E3B and Procion Turquoise H-A Procion Red
Procion Turquoise
adsorbent
Kf (mg/g)
n
r2
Kf (mg/g)
n
r2
rubber 450a 500a 600a 700a 800a 1000a
0.03 0.06 0.05 12.5 25.6 22.8 29.6
1.02 1.14 0.96 4.34 8.05 6.19 9.05
0.97 0.98 0.95 0.99 0.92 0.99 0.97
0.20 0.05 0.42 14.8 32.1 31.5 35.8
1.32 1.15 1.82 3.11 4.80 4.85 5.66
0.92 0.97 0.99 0.92 0.98 0.99 0.99
W52b GWb Hyd-Cb
26.9 4.40 63.0
5.62 3.16 10.6
0.89 0.89 0.99
40.1 5.5 87.4
6.94 3.25 9.00
0.98 0.85 0.99
a Tire rubber pyrolytic chars: production temperature. b Commercial activated carbons Norit W52 (W52), Chemviron GW (GW), and Norit Hydrodarco-C (Hyd-C).
Procion Red H-E3B exhibited by untreated tire rubber and selected pyrolytic chars. Adsorption parameters, obtained by application of the Freundlich equation to the adsorption data, are described in Tables 2 and 3. These tables also include the adsorption parameters from the three commercial powdered activated carbons for comparative purposes. Kf values represent the adsorption capacity of the sample under analysis for a particular compound in milligrams of adsorbate per gram of adsorbent. Parameter n is directly related to the affinity of the sample for the adsorbate (Noll et al., 1992; Faust and Aly, 1987). As observed in single-point adsorption tests, untreated tire rubber and rubber chars produced at temperatures below 500 °C exhibited very low or negligible adsorption capacity for all four adsorbates. Higher preparation temperatures, in the range 500700 °C, resulted in chars with increasing adsorption capacity, particularly for methylene blue and reactive dyes Procion Turquoise H-A and Procion Red H-E3B. Increasing the pyrolysis temperature from 700 to1000 °C resulted in limited further enhancement of the adsorption efficiencies. As expected, commercial activated carbons showed much higher adsorption capacity for phenol than the pyrolytic chars (50 mg/g by Chemviron GW compared with 2 mg/g by the best pyrolytic char). The phenol molecule, relatively small (MW ) 94.11), is principally adsorbed in micropores while the presence of mesopores does not affect the adsorption capacity of the sample significantly. The commercial adsorbents with the
largest micropore volumes, Norit W52 and Chemviron GW, also exhibited the highest phenol adsorption capacities. Pyrolytic chars exhibited much higher adsorption capacity for methylene blue (up to 22 mg/g) but still well-below the performance of commercial active carbons (up to 187 mg/g). Again, the two commercial active carbons with the highest micropore volume, Norit W52 and Chemviron GW, also exhibited the highest methylene blue adsorption capacities. These results seem to indicate that methylene blue (MW ) 373.9) is mainly adsorbed in the micropore system of the carbons, although the presence of mesopores plays a more significant role than in the case of phenol. Pyrolytic chars exhibited an even higher adsorption capacity for reactive dyes Procion Turquoise H-A and Procion Red H-E3B (up to 36 and 30 mg/g, respectively). Procion Turquoise and Procion Red have molecular weights of 1231 and 1200, respectively. Chemviron GW, the microporous commercial active carbon, showed a very low adsorption capacity for both dyes which was attributed to molecular sieve effects. Norit HydrodarcoC, the carbon with the largest mesopore volume, was also the one with the largest adsorption capacity for Procion Turquoise and Procion Red with 87 and 63 mg/ g, respectively. These results confirm that the presence of microporosity does not play an important role in the adsorption of Procion Turquoise H-A and Procion Red H-E3B from solution. These species, which are too large to penetrate micropores due to molecular sieve effects, are principally adsorbed in mesopores. Conclusions Pyrolysis of tire rubber takes place in the temperature range 300-500 °C and results in the production of 53 wt % oil, 42 wt % char, and 5 wt % gas. Untreated tire rubber and pyrolytic chars produced at temperatures before the completion of the pyrolysis reaction (
