Chemistry of hydrocarbon compounds – Production of hydrocarbon mixture from refuse or vegetation – From synthetic resin or rubber
Reexamination Certificate
1999-11-19
2001-08-07
Yildirim, Bekir L. (Department: 1764)
Chemistry of hydrocarbon compounds
Production of hydrocarbon mixture from refuse or vegetation
From synthetic resin or rubber
C201S002500, C201S025000
Reexamination Certificate
active
06271427
ABSTRACT:
The present invention relates to a method for recovery of carbon and combinations of hydrocarbons from polymers, preferably in the form of disposed tyres, by pyrolysis in a pyrolysis reactor.
Disposed vehicle tyres and other rubber materials have in recent times become a major environmental problem partly because such material is in itself not simply biodegradable and thus currently requires extremely large stores and dumping areas, and partly because combustion of the material to ash in special combustion plants forms environmentally dangerous substances such as sulphur-containing acids and other gases which smell of fuel.
Since the material of which the tyre is composed itself contains a large fraction of substances which are valuable for the petrochemical industry, it has proved interesting to find efficient methods for recovering these valuable substances. Tyres consist of, among other things, approximately 35% carbon black as reinforcement in the walls and wearable surface of the tyre, approximately 60% styrene-butadiene-rubber (SBR) and considerable amounts of oil, together with cord in the form of steel wire and/or glass fibre polyester. All of these substances are valuable and expensive to produce by conventional methods from current raw materials. On the other hand, unfortunately, the substances which are elements of the tyre material and which give the tyre its desirable properties are also primarily those substances that make the possibilities of efficiently recycling the tyre more difficult.
Recycling of discarded tyres is known through so-called pyrolysis, in which tyres or rubber waste after fragmentation into pieces of a suitable size are introduced into a large oven-like reactor for gasification in the absence of oxygen, which occurs at temperatures between 450 and 600° C. The pyrolysis process yields a volatile gas, known as pyrolysis gas, which in addition to water vapour also contains carbon monoxide, carbon dioxide, paraffins, olefins and some other hydrocarbons, and from which pyrolysis gas oil and gas can be recovered. Carbon black and/or active carbon can be produced from the solid carboncontaining residue that remains in the reactor after pyrolysis is completed. The product yield from recycled tyres consists mainly of 20% oil, 25% gas, approximately 15% steel and other materials, together with approximately 40% carbon.
One reason that the pyrolysis process up until now has only been used to a very small extent for the recycling of tyres and other rubber material is that the plant in itself requires a major investment, and the prices of the products which can be obtained from discarded tyres at such plants are far too low when compared with the price of equivalent products manufactured by conventional methods. This is particularly true of the various types of petroleum products that can be manufactured by the pyrolysis process and subsequent steps of separation and refinement.
The carbon or the pyrolysis coke which is obtained as a residue of the pyrolysis process has proved itself to be easily comparable, from the point of view of costs, with carbon produced by conventional methods, particularly if the carbon which has been obtained by pyrolysis is further refined to carbon black. This refinement usually occurs through micronisation in several steps after each other and which contain, among other processes, grinding and density separation. Large quantities of carbon black are used as a pigment and filler in the rubber and plastics industries, and the price when produced by the method described above can easily compete with carbon black produced by the conventional method.
By condensing out the less volatile components of the pyrolysis gas which is obtained from the pyrolysis process, so-called pyrolysis oil can be obtained, which essentially resembles diesel or light fuel oil, with the difference that it has a relatively high content of sulphur and aromatic hydrocarbons. The high content of sulphur and of other impurities can be reduced by, for example, filtering, and the hydrocarbon compounds separated into different fractions by condensation. The temperatures at which oil condenses out from the pyrolysis gas differs depending on the density of the oil, but in principle the heavier oil fractions condense out at temperatures around 350° C., the medium heavy oils at temperatures between 100 and 350° C. and the light oils at temperatures under 100° C. The oil fractions which have condensed out are led away for further storage in special collection tanks, while the remaining, non-condensed pyrolysis gas can be advantageously used as fuel for the recycling plant.
As mentioned above, certain products of pyrolysis are so valuable that they can be regarded as raw materials for further processing and refinement. However, experiments have shown that the properties of the said pyrolysis products are to a large extent already determined during the pyrolysis process by such factors as the temperature, rate of heating, holding time in the reactor and rate of cooling. Thus it is desirable to be able to control these parameters very carefully during the pyrolysis process.
If the coke that remains after the pyrolysis process is to be used as solid fuel, it is separated by sieving from steel and glass fibre residues and is taken to storage. On the other hand, coke destined for further refinement to form, for example, carbon black or active carbon must go through another step of pyrolysis treatment which includes, among other things, raising the temperature to between 800 and 900° C. in order to totally remove from the coke any traces of volatile hydrocarbons which may be present, followed by reduction of the temperature and possibly steam treatment.
According to known techniques for the recovery of carbon black and hydrocarbons from discarded tyres by pyrolysis, reactors are used which are heated indirectly, normally by leading molten salt through channels or coils which are arranged to run around the reactor. The disadvantage of the indirect heating technique is, among other things, that the response time for momentarily determined parameters becomes far too slow in order to achieve satisfactory control of the breakdown process of the tyre material in the reactor, nor is there any possibility during the final phase of the reaction to rapidly heat or cool the residue which has been treated by pyrolysis or to add steam to it. In addition, the amount of energy which is needed to heat up and break down the tyre material is normally higher than that which would be required for the equivalent process using a direct method of heating, due to power losses which occur.
In order to achieve direct heating of the tyre waste, and in this way better steer and control the pyrolysis process, it has proved to be suitable to recirculate the pyrolysis gas which forms, by which the said gas after heating is led through the waste and then condensed out to fluid fractions by passing through a condenser.
From U.S. Pat. No. 3,962,045 a plant for the pyrolysis treatment of waste in the form of, among other things, plastic and rubber is known, which uses recycling heated pyrolysis gas for heating of the said waste, in which the circulating pyrolysis gas is lead through a reactor zone in which it is made to cross a continuous stream of waste passing through the reactor zone. After passing the reactor zone, part of the pyrolysis gas formed is led back to a condenser unit for condensation to a fluid phase, while another part of the pyrolysis gas is deflected to a heat exchanger to be reheated and led back into the said reactor zone. The coke that is formed in the pyrolysis process is fed by means of a feed screw out from the lower part of the reactor to a collection unit. Since the waste is continuously fed through the reactor zone, however, the possibilities of controlling the pyrolysis process are limited and the coke which is formed must, from a stored condition, pass through further steps of handling and pyrolysis treatment, that is, heating up to a temperature of between 800 and 900° C., in
Bacon & Thomas
Yildirim Bekir L.
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