Tire-derived activated carbon production and application.

Radvanska, Agata

Abstract: The objective of the paper is to demonstrate the conversion process of scrap tires to activated carbon. By the process of pyrolysis and subsequent activation it is possible to produce carbons with surface areas greater than 500 [m.sup.2]/g and significant micropore volumes. Tire shreddings are pyrolyzed in batch reactors, and the pyrolysis chars are then activated by reaction with superheated steam. The adsorption characteristics of solid products of pyrolysis and activation can be tested by nitrogen adsorption techniques.

Key words: activated carbon, adsorption, waste tires.

1. INTRODUCTION

Since tires are made mostly of rubber, which is an organic material, i.e. carbon-based, it is possible to re-use their carbon content in the form of activated carbon. This form of carbon is a commercially important adsorbent of noxious materials in for example flue gases or waste steam. In adsorbing these toxics, carbon attaches them more or less firmly to its highly porous surfaces, which are very large. When the carbon is removed from the system it takes the impurity with it. Typical adsorbates are acetone, trichloroethane, and compounds of mercury. All are toxic in some degree and any or all may be found in flue gases from power plants (Hloch, 2006). Some 25% of the total tire weight consists of recoverable carbon, next 70% consists of hydrocarbon liquids and gases, usable as fuels, and the remainder is ash. The oils and gasses produced by rubber pyrolysis could be captured and used as sources of energy in the activation process. Thus, the production of activated carbon adsorbents from waste tire rubber can provide a two-fold environmental and economic benefit--a recycling path is created for waste vehicle tires and new low-cost adsorbents are produced for commercial use in air quality control applications.

2. THE PRODUCTION PROCESS OF TIRE-DERIVED ACTIVATED CARBON

2.1 Pyrolysis of Tire Rubber

Destructive distillation or "pyrolysis" is a process to convert carbonaceous raw materials into liquid and gaseous hydrocarbons and char residue. Shredded waste tires without its steel wire components, which are magnetically removed are pyrolyzed at a temperature of 800--900[degrees]C in the absence of oxygen. The typical products of tire pyrolysis are 33-38 wt. % char, 38-55% oils, and 10-30% gases. The mass of residual char in tire pyrolysis typically exceeds the raw carbon black content due to the degradation of the styrene-butadiene rubber polymer. The gases produced through tire rubber pyrolysis consist principally of hydrogen ([H.sub.2]), carbon dioxide (C[O.sub.2]), carbon monoxide (CO), methane (C[H.sub.4]), ethane ([C.sub.2][H.sub.6]), and butadiene ([C.sub.4][H.sub.6]); with trace amounts of propane (C[H.sub.3]C[H.sub.2]C[H.sub.3]), propene (C[H.sub.3]CH = C[H.sub.2]), butane (C[H.sub.3][(C[H.sub.2]).sub.2]C[H.sub.3]), and other miscellaneous hydrocarbons. Condensable liquids constitute 25-60% of the gaseous products formed during pyrolysis. Some of these products are combustible and could be utilized as a source of energy in the subsequent carbon activation process (Petrich, 1993).

2.2 Tire Char Activation

The carbon activation process creates or increases porosity on the carbon surface (Fig. 1). Activation consists of heating the carbon for two to three hours at a temperature between 800 and 900[degrees]C in a stream of steam and nitrogen. Though the two steps of pyrolysis and activation are technically separate and distinct, in practice they may be carried out in the same reactor --first pyrolysis and consequently the activation.

[FIGURE 1 OMITTED]

In reality, the surface of an activated carbon is not so simple. Pores are not idealized pits and crannies, and they can be more accurately portrayed as the imperfect junctions of carbon atoms at the surface of carbon units (Fig. 2) (Byrne & Marsh, 1995).

[FIGURE 2 OMITTED]

Activation occurs by selectively removing carbon groups from the surface of carbons, creating more voids between carbon molecules where molecules can be adsorbed. There are two conventional means of carbon activation: physical activation (or gasification) and chemical activation. Physical activation processes involve treating carbonaceous materials at high temperatures with various reactive gases. Chemical activation processes involve the treatment of raw carbonaceous materials with chemicals such as zinc chloride (Zn[Cl.sub.2]) or phosphoric acid ([H.sub.3]P[O.sub.4]) with subsequent heat treatment. In general, physical activation is preferred to chemical activation due to economic and environmental considerations involved with chemical treatments (Byrne & Marsh, 1995).

2.3 Application of Waste Tire Activated Carbon

Activated carbon is specially treated to produce an extremely high surface area on a microscopic level, making it an excellent filtration material to remove organic and some chemical impurities from water. A gram of activated carbon may have a surface area of 400 [m.sup.2] to 1500 [m.sup.2], depending on its form--powder or granules. Carbon aerogels, while more expensive, have even higher surface areas, and are used in special applications. Activated carbon has a number of micropores, which provide superb conditions for absorption to occur, since adsorbing material can interact with many surfaces simultaneously. Activated carbon can be used as a substrate for the chemisorption process--the application of various chemicals to improve the adsorptive capacity for some inorganic (and problematic organic) compounds such as hydrogen sulphide ([H.sub.2]S), ammonia (N[H.sub.3]), formaldehyde (HCOH), radio-isotopes (Iodine-131) and mercury (Hg). Charcoal is also a commonly used component in pyrotechnic mixtures after it has been ground into a fine powder. It acts as an agent for purifying industrial waste water and household waste water. The products can be produced at lower cost than those made of natural activated carbon (Wiliams et al, 1990).

2.4 Powdered activated carbon

Traditionally, active carbons are made in particular form as powders or fine granules (Fig. 3) less than 100 mm in size with an average diameter between 15 and 25 mm. Thus they present a large internal surface with a small diffusion distance. Powdered activated carbon is not commonly used in a dedicated vessel, owing to the high headloss that would occur. Powdered activated carbon is generally added directly to other process units, such as raw water intakes, rapid mix basins, clarifiers, and gravity filters (Bohacik & Hloch, 2006).

[FIGURE 3 OMITTED]

2.5 Powdered activated carbon

Granulated activated carbon (Fig. 3) has a relatively larger size of particles compared to powdered activated and consequently, present a smaller external surface. Diffusion of the adsorbate is thus an important factor. These carbons are therefore preferred for all adsorption of gases and vapors as their rate of diffusion are faster. Granulated carbons are used for water treatment, deodorisation and separation of components of flow system. Granulated activated carbon can be either in the granular form or extruded.

3. ENVIRONMENTAL APPLICATIONS

Carbon adsorption has numerous applications in removing pollutants from air or water streams both in the field and in industrial processes such as: Spill cleanup, Groundwater remediation, Drinking water filtration, Volatile organic compound capture from painting, dry cleaning and other processes. Filters with activated carbon are usually used in compressed air and gas purification to remove oil vapor, odor, and other hydrocarbons from compressed air and gas. The most common designs use a one-stage or two-stage filtration principle where activated carbon is embedded inside the filter media. Gas separation applications represent a major production cost for chemical industries. There is commercial interest in replacing conventional cryogenic distillation processes with pressure swing adsorption systems utilizing carbon molecular sieves. Carbon molecular sieves are microporous materials having a very narrow pore size distribution range with pore dimensions similar to the critical dimensions of the gas molecules to be separated (Fig 4). There is an interest in developing low-cost carbon molecular sieve for gas separations such as [N.sub.2] and [O.sub.2] from air and in air pollution control applications. It is possible that tire-derived activated carbons will also be suitable carbon molecular sieves (Gregg & Sing, 1982).

[FIGURE 4 OMITTED]

4. CONCLUSION

Because old tires are in enormous supply, they have been the object of study for some time by chemical and environmental engineers. Any substantial new use for old tires can reduce and perhaps eliminate at least one source of pollution and marring of the landscape. One of the possibilities of how to re-use wear tires is the production of tire derived activated carbon. Activated carbon can be used in metal extraction, water purification, medicine, sewage treatment, air filters in gas masks and filter masks, filters in compressed air and gas purification, and many other applications. The adsorbency of the carbon was measured and found to be generally about half that of commercially available activated carbons. In view of the extremely low cost of the discarded rubber, the economics of the process seem promising in competition with the standard commercial types. It would seem that any step in the direction of economical utilization of an otherwise disagreeable waste material is a positive contribution to the protection of the environment.

5. REFERENCES

Hloch, S. Decentralization of energy supply chain management by means of renewable energy source and safety energy policy. In: Management of Manufacturing Systems: Focused on Manufacturing Logistics & Supply Chain Management, 28th September 2006, Presov, Slovakia. Kosice; TU, 2006. s. 131-136. ISBN 80-8073-623-5.

Williams, P.T., Besler, S., Taylor, D.T., The pyrolysis of scrap automotive tyres: The influence of temperature and heating rate on product composition, Fuel, 1990, 69 (Dec.), p. 1474-1482.

Petrich, M.A., Conversion of Scrap Tires and Plastic Waste to Valuable Products, Office of Solid Waste Research, Institute for Environmental Studies, Univ. of Illinois: Urbana, IL, 1993.

Byrne, J.F., Marsh, H., Introductory overview, in Porosity in Carbons, J.W. Patrick, ed., Edward Arnold: London, 1995.

Bohacik, L., Hloch, S. Termomechanics. 1. vyd.. Kosice : FVT TU, 2006. 225 pp. ISBN 80-8073-465-8.

Gregg, S.J., Sing, K.S.W., Adsorption, Surface Area and Porosity, Second ed, Academic Press: London, 1982.

Acknowledgment

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