Properties and application of scrap tire pyrolysis products.

Misik, Ladislav ; Radvanska, Agata

1. INTRODUCTION

Waste tire pyrolysis involves the thermal degradation in the absence of oxygen. The benefit of this application is the conversion of waste tires into value-added products such as olefins, chemicals and surface-activated carbon.

The basic process of tire pyrolysis consists of following steps:

--Chipped tires are heated to 600-800[degrees]C in the absence of oxygen; Primary products are pyrolytic gas, oils and charcoal;

--The oils and charcoal go through additional processes to manufacture secondary, value-added products (Tab. 1).

--Charcoal upgrading is implemented in a closed-loop activation step that yields an activated carbon and eliminates undesirable by-products and emissions.

--Upgrading the charcoal produces high-surface-area activated carbon in several grades.

--Ash-free oil is turned into high-quality carbon black by using the furnace process.

--As an alternative, oils can be separated into valuable chemical feedstock by distillation.

The amount and composition of those three fractions depends on process parameters (temperature, heating velocity, pressure, time, material size, etc.), in volatile fraction, the controlling parameter is condensation temperature (Wojtowicz, 1996).

2. PRODUCTS OF TIRE PYROLYSIS CONVERSION

The primary products (Tab. 1) are essentially low molecular weight olefins and char. The problem of their application is their low price in the market. On contrary, other chemicals are valuable, but the yield is low. High quality carbon black and surface activated carbon are also valuable but there is no particular price advantage for the same quality products from traditional processes. The valuable chemicals from pyro-gas or oil are generally high molecular weight substances. The purification of high molecular weight substances is expensive, so the greatest drawback is the price of the final product (Tukachinsky et al., 1996).

3. TECHNOLOGIES

The approach to obtain higher value products of pyrolysis can be made through the development of new technologies. Current economic barriers of tire pyrolysis are low quantities of pyro-products and the low molecular weight of obtained olefins, to be curable and moldable for their further application (Mw has to be greater then 15,000). Olefins are typically produced in small quantities because the process temperature is high. At high temperature, vulcanized rubbers are quickly decomposed to low molecular weight olefins (Mw 300-400). High molecular weight compounds can be generated by low temperature pyrolysis. However, lower temperature will require longer process times. There are new technologies being developed, that can help the commercialization of the pyrolysis.

Microwaves can heat objects more uniformly than conventional heating methods. Microwave heating requires shorter heating times. Microwave pyrolysis will result in relatively high molecular weight olefins and a high proportion of valuable products such as ethylene, propylene, butene, aromatics, etc. The short process time also contributes to a reduction in the process cost. Moreover, for microwave heating, the shape of the tire chip is less important compared to the requirements of conventional heating. Whole tires or larger chips can be processed using microwave pyrolysis, which greatly reduces pre-processing cost. A. I. Isayev in USA has patented a method which minimizes heating and uses sonic energy to break down sulfur-carbon chemical bonds in tires. Chipped tires are heated to about 200[degrees]C, and then subjected to 20,000 Hz of ultrasonic energy (just above the highest frequency the human ear can discern). The rubber is transformed from a solid to a highly viscous fluid within milliseconds. With additional curative agents the viscous material can be moulded into new products (Sangdo, Earnest, 1997) Supercritical water can be used to controllably depolymerize the rubber compounds. This approach requires lower temperatures (approx. 400[degrees]C) and shorter processing times. Tire compounds are decomposed to high molecular weight olefins (Mw 1,000-10,000), or oils (max. 90%) (Nobeyuki, 2007). Because of the expensive supercritical water equipment, this application would require a relatively large initial cost (Wojtowicz, 1996). Use of catalysts can reduce processing temperature or time. As shown in the above applications, reduced temperature and time can result in either higher molecular weight olefins or an increasing proportion of valuable substances. The advantage of catalysts is that no new equipment or knowledge is required.

One approach to reduce processing cost is to operate at a high process temperature with the use of a special catalyst. Approximately 3.2% of zinc-oxide is added to tire compounds, and the zinc-oxide remains in the char. To produce surface active carbon, the remaining zinc must be removed from the surface, and high temperature processing is able to facilitate this (Chen et al., 1995). The long polymer chains of the rubber decompose at high temperatures to smaller hydrocarbon molecules. When the pyrolysis is performed under vacuum, the spectrum and quality of products obtained is distinct from the other (usually atmospheric pressure) pyrolysis process (Roy et al., 1995). The advantage of a reduced pressure is that secondary decomposition reactions of the gaseous hydrocarbons are limited. Preliminary studies of the tire vacuum pyrolysis process were performed with a bench scale reactor and with cross-ply tires as feedstock. The decomposition of the elastomer in the tire is complete at a pyrolysis temperature of 420[degrees]C. A further increase of the pyrolysis temperature does not change the yields of oil, pyrolytic carbon black and gas. Tire particles are fed semi-continuously into the reactor. The pyrolytic carbon black produced is removed from the reactor by an Archimede screw which simultaneously acts as a vacuum seal. The heavy and light oils are condensed in two successive scrubbers. Typical yields are as follows: 55% oil, 35% carbon black and inorganics and 10% gas. The right function of flash pyrolysis reactor is controlled by extremely fast heat supply (approx. 2 sec.) by plasma burner into the pyrolysed material, maintenance of required temperature by electrical heating elements, short residence time of vapor in the reaction zone and fast cooling of the product. The advantage of flash pyrolysis is, that the waste, or waste tires can be transformed into the products of higher energetic value--oil, respectively pyro-gas, depending on the process temperature. The main drawback is that waste material has to be shredded to required size to obtain fast reaction process and simple separation of solid fraction. The gas samples are led via heated pipes to the analysis unit consisting of a FTIR and a gas chromatograph.

4. PROPERTIES OF PYROLYSIS PRODUCTS

Distillation of the pyrolytic oil yields approximately 20% light naphtha, 6.8% heavy naphtha, 30.7% middle distillate, and 42.5% bottom distillation residue. Benzene, toluene, xylene and other benzene-derivatives were identified in the naphtha fraction, as well as a valuable chemical, dl-limonene, which was found to be present with a concentration of 15% by weight. The pyrolytic light naphtha has a relatively high concentration of sulphur, mercaptans and nitrogenous compounds due to the thermal decomposition of the additives originally present in the tires as vulcanization agents. The relatively high levels of sulphur, nitrogenous, olefinic and diolefinic compounds in the pyrolytic light naphtha make it an unsuitable blend for gasoline. Reforming processing is required to convert it to a high value gasoline component. Comparison of the pyrolytic naphtha and commercial petroleum naphtha indicates that the pyrolysis light naphtha is a more complex mixture than the petroleum naphtha. Fossil fuel is basically composed of homologous series of compounds such as n-alkanes, iso-alkanes and anti-iso-alkanes. On the contrary, pyrolysis light naphtha is a heterogenous mixture of various compounds with higher isomerization which were produced during the tire thermal decomposition. Another potential application for the pyrolytic oil is the fabrication of coke. It was confirmed earlier that coal tar recovered by thermal decomposition of coal can easily be used in electrode coke manufacturing. The composition and character of the pyrolytic oil are basic to the quality of the coke and hence its potential usage. Sulphur content and metallic constituents in the feedstock have an important effect on the quality of the coke. The metallic constituents in coke, in particular vanadium, are almost as important as sulphur in determining the coke quality. The presence of nitrogen in the coke is the result of the thermal decomposition of additives originally used in tires, such as organic accelerators, antidegradants and antiozonants, for example sulfenamide and nitrile compounds. The asphaltenes content of the oil is sufficiently high and the viscosity is suitable for the transportation of the oil. The toluene insolubles content is too low to affect the quality of the coke. Pyrolytic oil has almost the same carbon content as the usual petroleum feedstock. However, high carbon content results in a higher yield and a better quality of coke (Roy et al., 1995).

5. CONCLUSION

Pyrolysis can transform scrap tires, usually considered as waste material, into a variety of useful products--oil, pyrolytic carbon black, activated carbon or syn-gas. The oil can be separated into different fractions: naphtha, light and heavy oil and a distillation residue. All these products have a commercial value. It was found that whatever the properties considered, the pyrolytic oil gave similar effects to those of the commercial aromatic oil and a mere substitution of the commercial aromatic oil by the pyrolytic oil could be considered in the compounds studied without significant differences, either in the processing behavior (flow and curing) or in the properties of cured items. A proper choice of the pyrolysis or of post pyrolysis treatment yields a pyrolytic carbon black which is close in its properties to commercial rubber-grade carbon black. An additional potential market for carbon black is filler for road asphalt. The commercial value of the products can make the tire pyrolysis process both ecological friendly and economical attractive.

ACKNOWLEDGEMENT

The author would like to acknowledge the support of Scientific Grant Agency of the Ministry of Education of Slovak Republic, Commission of mechanical engineering, metallurgy and material engineering, for their contribution to project 1/3174/06.

6. REFERENCES

Chen, D. T.; Perman, C. A.; Riechert, M. E. & Hoven, J. (1995). Depolymerization of tire and natural rubber using supercritical fluids. Journal of Hazardous Materials. No. 44, pp. 53-60, ISSN 0304-3894

Isayev, A. I.; Chen, J. & Tukachinsky, A. (1995). Novel Ultrasound Technology for Devulcanization of Waste Rubbers. Rubber Chem Tech. No. 68, pp. 267-280 May-Jun 1995, ISSN 0035-9475

Nobeyuki Itoh. Waste Tire Recycling Plant Producing High-Performance Activated Carbon. Available from: http://www.p2pay s .org/ref/11/10504/html/biblio/htmls/pyh 1.htm Accessed: 2007-04-12

Roy, C.; Rastegar, A.; Kaliaguine, S.; Darmstadt, H. & Tochev, V. (1995). Physicochemical properties of carbon blacks from vacuum pyrolysis of used tires, Plastics, Rubber and Composites Processing and Applications, No. 23/1995, pp. 21-30, ISSN 09598111

Sangdo, P. & Earnest, F. G. (1997). Statistical study of the liquefaction of used rubber tyre in supercritical water. Fuel. Vol.76, No. 11/1997, pp. 999-1003, ISSN 0016-2361

Tukachinsky, A.; Schworm, D. & Isayev, A. I. (1996). Devulcanization of waste tire rubber by powerful ultrasound. Rubber Chem Tech. Vol. 1. No. 69/1996 pp. 104-114, ISSN 0035-9475

Wojtowicz, M. (1996) The Manufacture of Carbon Black from Oils Derived from Scrap Tires. EPA 68D98117

Tab. 1. Pyrolysis products

Primary Weight Content Secondary
Products % Products

Pyro-gas 10-30 Hydrogen, C[O.sub.2], CO, --
 Methane, Ethane, Propane,
 Propene, Butane, other
 hydrocarbons,
 app. 1% of Sulfur

Oil 38-55 High aromatic (Mw 300--
 400, low sulfur content app. Carbon Black
 0.3-1.0%) Aromatics,
 Alkanes, Alkenes, Ketones,
 Aldehydes

Charcoal 33-38 >15 % of Ash (ZnO) Activated
 3-5 % of Sulfur carbon