AU2005227358A1 - Continuous thermolytic recycling of plastic and rubber wastes - Google Patents

Description

Patent Application Dr. Carlos M. R. Sorentino Melba Drive, East Ryde, NSW, 2113 Continuos thermolytic recycling rubber wastes Dr. Carlos M. R. Sorentino Melba Drive, East Ryde, NSW, 2113 02 9887 4176 Fax 02 9805 124' [email protected] of plastic and Field of the invention C' The present invention relates to the recycling of waste plastics and scrap rubber tyres into 0 fuels and other useful products. More particularly, the present invention discloses a novel method and apparatus for recycling these materials into fuels of high-calorific content and low sulphur content, gaseous hydrocarbons, and solid carbonaceous materials. In preferred 00 embodiments the process operates in a closed and continuos mode loop that allows minimal

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amounts of air leakage and creates minimal secondary waste and air emissions.

BACKGROUND

Recent estimates indicate that, in Australia, about 1.25 million tonnes of plastic wastes and about 0.30 million tonnes of automotive tyres are generated every year and that this mass of waste is increasing at a rate in excess of 4% per year, that is to say, this waste is doubling in mass every 17 years.

Both plastic and rubber are essentially non-biodegradable and therefore present unusual and difficult disposal problems. In Australia, in the year 2003, most plastic waste and scrapped tyres were disposed in landfills, a disposal method that poses serious problems because of the bulkiness of the waste and the kind of costlier disposal operation these wastes demand.

The concept of depolymerising the rubber in scrapped tyres to recover usable aromatic hydrocarbons seems to have been first patented in 1907 (United States Patent 866,758) when Wheeler proposed to reclaim the scrap tyres' rubber using superheated steam in an autoclave at a temperature of 316 0 C. Despite his age, this technology has not been widely commercialised because the low value of the products generally produced that result in marginal benefits that previously did not justify the investment in most prior-patented systems. Additionally, most previous depolymerisation methods relied on batch-processing requiring relatively intensive material handling that made operating these systems both difficult and costly. This invention yields products that provide such an economic benefit as to make viable and commercial the technology disclosed here.

A search of Australian patents disclosed four patents issued in the 20 years to 16 October 2003: lV ooo0000 /-tao Recovery of carbon and hydrocarbons by pyrolysis of tyres.

1993 48829 671302 FORMEX A process for the pyrolysis of organic waste matter.

1993 33173 651029 PE Recovery of commercially valuable products from scrap tyres.

1990 56644 636350 PE Recovery of commercially valuable products from scrap tyres.

PE: Patent expired.

Similarly, a number of Australian patents disclosed similar methods for the recycling of plastics, most of them expired without ever been applied commercially: 1999 32803 753883 Pyrolysis of hydrocarbons and device for quen( nvrolvsis oases.

1986 66221 590646 PE The pyrolysis of perfluoropolyethers.

1984 32205 545572 PE HC pyrolysis.

1984 31598 570063 PE pyrolysis.

HC pyrolysis.

198315627 566504 PE 198315627 566504 PE yrolyzing a hydrocarbon-containing solid.

PE: Patent expired.

Over the last two decades, a number of other processes have been proposed for recycling of plastics or tyres, but the search did not reveal any method by which both plastics and rubber could be processed simultaneously and continuously as this invention discloses.

The processes and apparatus of the prior art have several disadvantages. They are generally not capable of producing liquid hydrocarbons of sufficient good quality and low sulphur content and, as a result, the liquid hydrocarbons produced by such processes and apparatus are of low Svalue. Further, these prior processes require high temperatures and, hence, they require high energy inputs. The prior art apparatus also pose significant fire and explosion risk due to the 0 presence of oxygen during preheating or pyrolysis. Other drawbacks of the processes and Sapparatus of the prior art include the fact that they often produce high levels of emissions and produce secondary wastes. Some processes and apparatus also use dangerous catalysts.

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SThe low quality of the products of prior art processes and apparatus explains why there are not (-i N any operating commercial thermolysis plants processing large quantities of plastic wastes and scrap tyres, as the prior art is, generally speaking, uneconomical. It also explains why most of (-i these processes have not been demonstrated at a pilot plant scale.

A process and apparatus has now been developed that overcomes the above-noted problems and has numerous other advantages that will be apparent to those skilled in the art.

SUMMARY OF THE INVENTION The object of this patent is achieved by the continuos thermolysis of shredded organic Sproducts, in particular plastic and rubber waste from scrapped rubber tyres.

Broadly, the process of the present invention produces liquid and gaseous hydrocarbons from 00 In plastic wastes, and particulate char, liquid and gaseous hydrocarbons from rubber.

(Ni The process comprises: (-i preparing the waste material to a polymer feed of plastic and or rubber of a suitable chemical composition and size for thermolysis; heating the feed polymers in a sealed reactor at a temperature of about 450 to 600 OC for sufficient time to cause the polymers to dissociate into their component hydrocarbons in a vapour phase and a solid phase; removing the vapour phase from the reactor to produce a stream of derived combustible gas (DGF). A fraction of the gaseous phase is returned to the space above the thermolysis bath, or an inert gas, such as nitrogen, is introduced continuously to the space above the thermolysis bath; condensing the heavier hydrocarbon products of the thermolysis to produce low sulphur hydrocarbons, called here derived liquid fuels (DLF); and the solid fraction produced during thermolysis and containing metal, textiles and char from rubber or inorganic oxides from plastic, is separated, The char can either be purified to carbon black of a quality suitable to be used as a rubber filler or, alternatively, added to the stream of liquid fuels generated.

Thus, in this invention, waste plastic and scrapped tyres and plastics are depolymerised and 0 converted into low sulphur, high calorific content liquid and gaseous fuels that can be used for the generation of energy, as well as producing other saleable products.

0 Thus, this invention achieves the recycling of most of these waste materials into useful products, mostly energy, without producing any hazardous emissions and greatly reducing the 00 mass of the original waste.

eo o DESCRIPTION OF THE INVENTION: PREPARATION OF THE WASTE MATERIAL STREAM: 0 z SThe average waste stream available for processing in Australia consists of a mixture of rubber from scrapped tyres, and plastics including PE (polyethylene), PET (polyethylene 00 n terephthalate), PP (polypropylene), PS (polystyrene including expanded polystyrene), PVC (polyvinyl chloride) and other plastics including PU (polyurethanes).

In the preferred embodiment, plastic waste is granulated to a size of less than 25 mm using standard industrial shredders and granulators.

Most plastics are amenable to thermolysis with the exception of PVC that yields emissions of chlorine that are not environmentally acceptable and polyurethane that does not fully decomposes under the conditions used by this invention.

Therefore, both PVC and PU plastics must be isolated from the waste stream. PVC has a high specific gravity of between 1370 and 1390 kg/m 3 while PU has a specific gravity of between 1110 and 1250 kg/m 3 These specific gravities are higher than most plastic and rubber allowing the gravity separation of PVC and PU from the rest of the waste stream using conventional gravity separation technology, such as that described by R. O. Burt, "Gravity Concentration Technology," Elsevier, Amsterdam, 1985.

Similarly, scrap rubber tyres are shredded to reduce tyres of different sizes and types, including tyres scrapped from passenger cars, trucks and off-road vehicles, to granules of a diameter of less than 25 mm, compatible for further processing. Truck tyres contain a large amount of natural vulcanised rubber. However, they are large, heavy and bulky and cannot normally be shredded using the same processes used for passenger vehicle tyres.

In the invention, a truck tyre cutter processes them into de-beaded rubber segments suitable for use in the same manner as passenger vehicle tyres. Passenger tyres are 7 o 0 00 c- (2) shredded in a feeder and cutting mechanism that liberates bead and tread wires during the shredding process. Most of the tyres metal is liberated from the tyre during the shredding process and discharged via a dedicated conveyor. Size reduction is controlled by two removable screens surrounding over 3100 of the diameter of the shredding rotor.

Screen sections divide and the rear section is hydraulically lifted for easy access to the cutting chamber. The scrapped tyres will generally have a bulk density of about 160 Kg/m 3 that increases to a bulk density of about 288 Kg/m 3 after shredding.

THERMOLYSIS

After granulation or shredding and classification, the granules are feed directly to the thermolysis reactor(s) of the present invention, via a valve capable of admitting the feed into a sealed thermolysis reactor that prevents oxygen or atmospheric gases from entering into the sealed thermolysis reactor.

For convenience, rubber and plastic waste are thermolysed separately in dedicated thermolysis chambers to facilitate the control of the decomposition and the treatment of the solid residues produced by the thermolysis.

The valve discharges the plastic and rubber waste granules into specially designed wire mesh metal buckets mounted on a conveyor located inside the sealed thermolysis reactor. This arrangement allows the continuos feeding of the material to be depolymerised into the thermolysis chamber, allowing the process to operate without interruptions over extended periods of time. The bottom half of the conveyor holding inversed buckets is fully immersed in the thermolysis bath. As the conveyor inverts the buckets containing the plastic and rubber waste granules, a mechanism closes the buckets to prevent the plastic and rubber waste floating in the denser thermolysis bath, thus ensuring the reactants remain fully immersed during the reaction time.

The invention uses thermolysis baths made up of: S(i) a metal bath, preferably comprising either tin, or lead, or zinc metal, more 0 C1 particularly tin or lead; or 0 (ii) an alloy containing tin and lead, in particular in a ratio of 60:40; or 00 (iii) an alloy containing tin and zinc, in particular in a ratio of about 90:10; or

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rC (iv) a molten salt bath comprising hydroxides, oxides, carbonates and/or other salts of 0 alkali; or and/or alkaline earth metals or mixtures thereof; or (vi) sand or gravel beds; or (vii) mineral substances such as granulated silicates or aluminosilicates.

The plastic and rubber feed granules remain immersed in the molten metal or molten salts bath in the sealed thermolysis reactor, at a temperature between 450 and 600 0 C, in the absence of oxygen and for such a period of time as it is required for the depolymerisation and devulcanisation reactions to be completed.

Because the thermolysis occurs in a molten bath, it is feasible to add desulphurising agents to the thermolysis bath, such as magnesium metal or similar, agents capable of removing most of the sulphur contained in the plastic and rubber materials feed to the thermolysis chamber, thus rendering low sulphur fuels. During this process, sulphur converts to inert metal sulphides that are easy to dispose off in an environmentally acceptable manner.

The thermolysis occurs at low temperature and reduced pressure and in oxygen or air free sealed thermolysis chambers: O the gas containing organic compounds produced during the thermolysis is cooled z and separated into a gaseous and a liquid phase; 00 (ii) a fraction of the gaseous phase is returned to the space above the thermolysis bath, or an inert gas, such as nitrogen is introduced continuously to the space (-i above the thermolysis bath; (-i (iii) the solid fraction produced during thermolysis and containing metal, textiles and char is separated and the char can either be purified to carbon black of a quality suitable to be used as a rubber filler or, alternatively, added to the stream of liquid fuels generated.

The equilibrium of the thermolysis reaction is shifted towards the synthesis of higher molecular hydrocarbons by: rapid cooling of the product gases thus slowing down the rat of reaction; and (ii) removing the gaseous product hydrocarbons from the space above the bath thus shifting the reaction equilibrium towards the products.

This reaction control mechanism allows to adjust the reaction and to vary the product's composition within wide limits, thus allowing adjusting the reaction to composition of the materials feed to the thermolysis. Furthermore, the removal of the reaction products displaces the reaction equilibrium towards the products, thus increasing the rate of decomposition resulting in higher product yields and lower residence times.

This control is achieved by: maintaining the thermolysis chamber at a gaseous pressure below atmospheric pressure to allow the rapid collection and drainage of gasses; and (ii) condensing the gases in a cyclone condenser fitted with filters and sludge collectors to yield a low-sulphur content DLF and a cold DGF gas phase that is derived to a drier.

00 (3) RESIDUE TREATMENT After thermolysis and separation of the reaction products, in the bath remains the solid components of the plastic waste and scrap tyre feed, including carbon soot (about of the of the original mass of the initial tyres feed) scrapped metal (about 5% of the of the original mass of the initial tyres feed) textiles (about 1% of the of the original mass of the initial tyres feed) and inorganic oxides used as plastic fillers (between 0 and of the original mass of the plastic feed) and inorganic and inert metal sulphides. All the solids are collected from the bottom of the thermolysis chamber.

The solid residue derived from scrapped tyres contain metal, fibre and finely divided carbon soot, is sieved to separate the textile fraction; and passed throughout a magnetic separating drum to remove all the metal. Some of the metal may be separated from the rubber during the shredding stage. The textiles are largely degraded and have no commercial value.

In this process, the sulphur contained in the original tyres is converted to a metal sulphide, generally magnesium sulphide, recovered as a solid slag floating on the thermolytic bath. This sulphide is relatively inert and can be disposed off in an environmentally acceptable manner.

The remaining char (CT) originates in the carbon black (CB) fillers used in the original manufacture of the tyres but it differs from commercial CB as it also contains some of the inorganic components of the tyre as well as surface deposits of carbon formed during thermolysis.

O However, the quality of CB produced can be increased by the proper choice of the z pyrolysis conditions: 00 the concentration of the carbon forming hydrocarbons in the gas phase increases I/3 with increasing pressure, hence an increase in pressure will increase plating of C carbon on the CT surface; and I/3 (-i (ii) an increase of the thermolysis pyrolysis temperature will reduce the amount of hydrocarbons absorbed on the CT surface which are precursors in thermolytic carbon formation.

An important difference between commercial CB and CT is the concentration of inorganic components in the latter. Commercial CB usually contains less than 0.2% of ash, whereas the ash concentration in CT can be as high as 13.0%. The most important sources for inorganic components in the CT are usually ZnO and S, from the vulcanisation catalysts and agents used during the rubber manufacturing and, sometimes, mineral fillers such as SiO 2 and A1 2 0 3 The CT is subject to a series of acid and basic washes that reduces the ash content to about 4 to 6% per weight and eliminates any deposits on the surface of the CT.

After washing, the CT is further processed by drying it and milling to the desired size, normally about 50 micrometers, to yield a commercial grade CB that complies with the specifications of the ASTM for this type of material.

The CT is almost pure carbon with a net calorific content in excess of about 29 GJ/t and can be used as a fuel, each tonne of CT equivalent to about 0.7 tonnes of oil. In this alternative, the finely divided CT is suspended as a fine emulsion with the DLF to yield a CT-enriched fuel containing up to 20% CT.

0 The metal residue obtained from scrapped tyres has a direct commercial use as scrapped metal without any further processing.

00 The inorganic oxides residues from plastic wastes and the textile residue from scrapped tyres, amounting to about 10% of all the waste processed are inert materials that do not (Na C have a commercial value and can be disposed by conventional methods.

EXAMPLES

Once the thermolysis reaction is completed, the plastic and rubber polymers are converted into a complex mixture of hydrocarbons that are gaseous at the temperature of thermolysis and at room temperatures will condensate into liquid and gaseous fuels of a quality that depends on the type of waste material thermolysed.

The thermolysis of rubber and plastics by the process disclosed in this invention generally yield a derived liquid fuel (DLF) a derived gas fuel (DGF), char metal scrap (SS) and a solid residue In the case of PVC, chlorine (CI) is also a product.

In a number of experiments, the thermolysis conditions for a number of products was determined to be: Low density polyethylene 59% NaOH 41% Na 2

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3 470-480 Polystyrene 59% NaOH 41% Na 2

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3 460 Rubber 100% Tin Note: Exothermic reaction, temperature rises to 480 0

C.

450 The typical composition of the thermolysis products generated in the process described in this invention is: Low density Dolvethylene 71.2% 26.8% 98.0% 98.0% Polystyrene Rubber 42.0% 10.5% 32.3% 14.0% 1.2% Rubber 42.0% 10.5% 32.3% 14.00/0 1.2% r The reaction parameters and products of the thermolysis of PVC are shown to illustrate the problems associated with this material: its thermolysis produces 31.5% of chlorine gas which, under the conditions of the thermolysis reaction is converted to the NaCI. However, the reactions involved do not completely convert all the chlorine available with the result that some of this gas is emitted, plus the fact that the salt bath is rapidly consumed rendering the process too costly.

The typical composition and properties of the gas produced by the thermolysis disclosed in this invention is: Hydrogen Carbon dioxide Carbon monoxide Nitrogen Hydrogen sulphide Denrsityfic val Net calorific value 10.5 to 12.5% 14 to 16 6 to 8% Less than 1% 37,700 kg/m3 37,700 kJ/m3 The typical composition and properties of the liquid hydrocarbons produced by the thermolysis described in this invention are: Stvrene 10 to Toluene 16 to 19% Density 0.916 kg/L The net heat content of the thermolysis products derived from different plastics and rubber varies depending on the type of material thermolysed. In a series of experiments, the following typical heat contents were determined: I DLF I DGF Low density polyethylene S46.3 11,068 54.4 13,013 I Polystyrene RubberPVC with 5% wt filler Rubber 46.3 40.4 40.4 11,068 9,657 9,657 54.4 47.5 47.5 13,013 11,355 28.3 6,760 11,355 28.3 6,760 The DLF, DGF and CS products have high calorific value and provide all of the energy required for the thermolysis process, while the excess can be used for the generation of electricity or marketed as energy fuels.

THE COMMERCIAL VALUE OF THE PRODUCTS MAKES THE THERMOLYSIS PROCESS DISCLOSED IN THIS INVENTION BOTH ECOLOGICAL FRIENDLY AND ECONOMICAL ATTRACTIVE.

Claims (7)

1. A continuos process to thermolyse scrap rubbers and waste plastics converting them into liquid and gaseous hydrocarbons fuels with high calorific content, carbon soot and to 00 n recover metals from a feed consisting of scrap tyres' rubber and/or waste plastic, comprising: (-i a suitable granulating apparatus to reduce plastic waste to a particle size (-i compatible for further processing and capable to separate the plastic particles into streams of materials differentiated by their chemical composition and of a particle size compatible for further processing. a suitable shredding apparatus to reduce scrap tyres of different sizes and types, including tyres scrapped from passenger cars, trucks and off-road vehicles, to a particle size compatible for further processing; a valve capable of admitting the particulate feed material into a sealed thermolysis reactor and also capable to prevent oxygen or atmospheric gases entering into the sealed thermolysis reactor; a sealed thermolysis reactor holding the material to be depolymerised under a bath of molten metal or molten salts at a temperature between 450 and 600 0 C, the said reactor operating in an oxygen free atmosphere with no air leakage; a mechanism to submerge and to hold the feed material particles beneath the surface of the molten bath in the sealed thermolysis reactor for the period of time required to accomplish the thermolytic depolymerisation of the feed material; capable of operating continuously and without any interruptions for long periods of time. 0 means for withdrawing the vapour phase from the sealed thermolysis reactor and delivering it to a condenser; 00 an apparatus to condense the vapour phase to produce low sulphur liquid hydrocarbons and combustible gaseous hydrocarbons; (N a magnetic separator to separate the metal component of the scrap tyres; (N a separator capable to isolate the solid phase produced in the sealed thermolysis reactor for further processing; and

2. The method according to Claim 1, where the molten metal bath sealed thermolysis reactor comprises a mixture of one of: pure tin metal; tin and lead in a mass ratio of 60% tin and 40% lead; or tin and zinc in a mass ratio of 90% tin and 10% zinc.

3. A bath as described in Claim 2 to which a suitable desulphurising agent has been added.

4. The method as described in Claim 1, where the molten salt bath in the sealed thermolysis reactor comprises: sodium hydroxide, NaOH; potassium hydroxide, KOH; sodium chloride, NaCI; potassium chloride, KCI; sodium carbonate, Na 2 CO 3 0 potassium carbonate, K 2 C0 3 or S(d) mixtures of any two or more of the above salts including their eutectic mixtures. 0 A bath as described in Claim 4 to which a suitable desulphurising agent has been added. 00 6. The method according to Claim 1, where the thermolysis bed is a fluidised bed composed of sand or gravel. (N

7. The method according to Claim 1, where the thermolytic decomposition is carried out at (N reduced pressure.

8. The method according to Claim 1, where the thermolytic decomposition is carried out in an oxygen, air, and/or water free atmosphere.

9. The method according to Claim 1, were the thermolytic decomposition is carried out continuously.