GB2616046A - A method for the production of a pyrolysis oil from end-of-life plastics - Google Patents

Abstract

The present invention provides a method for the production of a pyrolysis oil from end-of-life plastics, the method comprising: (i) providing end-of-life plastics material; (ii) melting the end-of-life plastics material to form a molten plastics material; (iii) pyrolysing the molten plastics material in an oxygen-free atmosphere to provide pyrolysis gases and a char material; (iv) condensing the pyrolysis gases to provide the pyrolysis oil, characterised in that the method further comprises dispersing a zeolitic material in the end-of-life plastics material or in the molten plastics material.

Description

A method for the production of a pyrolysis oil from end-of-life plastics The present invention relates to an improved method for the production of a pyrolysis oil. In particular, the method incorporates a catalytic zeolite material which improves the quality of the final product and can itself be reused for further treatments. In particular, the method identifies a catalyst which appears to be unaffected by the presence of impurities in the endof-life plastics and this provides a significant improvement without adding to the process complexity.

End-of-life plastic chemical recycling is an emerging technology designed to recycle mixed waste-plastics into a variety of liquid hydrocarbon products. The waste plastics for use in such a process may, for example, include low density polyethylene (LDPE), high density polyethylene (HDPE), polystyrene (PS), and/or polypropylene (PP).

Plastic waste is currently a major problem in the world and is causing a significant number of environmental issues. Pyrolysis of waste plastics is commonly accepted as a highly promising solution for this problem.

Pyrolysis treatments are known for converting these waste plastics into the liquid hydrocarbon products by heating and then pumping the plastic feed in molten form into reactor vessels. The reactor vessels are heated by combustion systems to a temperature in excess of 350°C. This produces rich saturated hydrocarbon vapour from the molten plastic. This flows out of the reactor vessels through contactor vessels and will condense the heavier vapour fractions to maintain a target outlet temperature set point which is determined by the end-product specification. This is then distilled at near-atmospheric pressures in a downstream condensing column. This process obtains a so-called pyrolysis oil.

W02021123822 discloses a method for pyrolysing plastic material. The method comprises the steps of: heating and densifying plastic material; transporting the plastic material to one or more reactors; and pyrolysing the plastic material in the one or more reactors. The plastic material is maintained in a heated state during the transporting step.

W02016030460 discloses a pyrolysis reactor system suitable for the treatment of end-of-life plastics W02011077419 also discloses a process for treating waste plastics material to provide at least one on-specification fuel product. Plastics material is melted (4) and then pyrolysed in an oxygen-free atmosphere to provide pyrolysis gases. The pyrolysis gases are brought into contact with plates (13) in a contactor vessel (7) so that some long chain gas components condense and return to be further pyrolysed to achieve thermal degradation. Short chain gas components exit the contactor in gaseous form; and proceed to distillation to provide one or more on-specification fuel products. There is a pipe (12) directly linking the pyrolysis chamber (6) to the contactor (7), suitable for conveying upwardly-moving pyrolysis gases and downwardly-flowing long-chain liquid for thermal degradation. There is a vacuum distillation tower (26) for further processing of liquid feeds from the first (atmospheric) distillation column (20). It has been found that having thermal degradation in the contactor and pyrolysis chamber and by having a second, vacuum, distillation column helps to provide a particularly good quality on-specification liquid fuel.

By pyrolysing plastic, it is broken down into a hydrocarbon rich oil. This oil can then be used to produce monomeric species via the refining process. Once in the monomeric state, the molecules can then be polymerised to form virgin grade plastic. This effectively closes the loop on the plastic production process, reducing waste and environmental impact. In addition, all of these foregoing methods and processes are suitable for obtaining a pyrolysis oil which can be used as a fuel, especially for transportation purposes.

US5107061 is directed to the removal of organochlorides from hydrocarbon streams using highly crystalline molecular sieve material, such as zeolites, and particularly zeolite X in a sodium form, and the removal of organochlorides from hydrocarbon streams containing olefinic compounds using such molecular sieves in combination with alumina for the purpose of effecting a decomposition of the organochloride into a corresponding unsaturated hydrocarbon molecule and a molecule of hydrocarbon chloride, wherein the hydrocarbon chloride is removed from the hydrocarbon stream by being adsorbed onto the adsorbent of the highly crystalline molecular sieve so that the unsaturated hydrocarbon molecule may be recovered from the resultant hydrocarbon stream containing a reduced amount of organochlorides.

US4721824 relates to a method for removing trace amounts of organic chlorides from feedstocks by passing the feedstock in contact with a guard bed catalyst comprising shaped particles formed by extruding a mixture of magnesium oxide and a binder inert to the feedstock. The process has particular importance in removing organic chlorides from toluene feedstocks prior to contacting toluene with a disproportionation or alkylation catalyst comprising magnesium-ZSM-5.

US3862900 relates to a method for treating hydrocarbons containing chemically combined chlorine by passing the hydrocarbons through a bed of molecular sieves of effective pore size in the range of 7 to 11 Angstrom units to remove the chemically combined chlorine and other impurities.

EP1728551 relates to desulfurization of gasoline cut by adsorption on a faujasite zeolite.

This has a silicon/aluminium molar ratio of 1-10, a meso and macroporosity volume of 0.25- 0.4 cm3/g, a microporosity volume of 0.12-0.35 crns/g and a size of crystals less than 3 microns.

Accordingly, it is an object of the present invention to provide a method for the production of a pyrolysis oil from end-of-life plastics with reduced impurities and/or requiring less post-production refinement, or which increases the yield or at least to tackle problems associated with the prior art, or provide a commercially viable alternative thereto.

According to a first aspect the present invention provides a method for the production of a pyrolysis oil from end-of-life plastics, the method comprising: (i) providing end-of-life plastics material; (ii) melting the end-of-life plastics material to form a molten plastics material; (iii) pyrolysing the molten plastics material in an oxygen-free atmosphere to provide pyrolysis gases and a char material; (iv) condensing the pyrolysis gases to provide the pyrolysis oil, characterised in that the method further comprises dispersing a zeolitic material in the end-of-life plastics material or in the molten plastics material.

The present disclosure will now be described further. In the following passages different aspects/embodiments of the disclosure are defined in more detail. Each aspect/embodiment so defined may be combined with any other aspect/embodiment or aspects/embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. It is intended that the features disclosed in relation to the product may be combined with those disclosed in relation to the method and vice versa.

The present invention provides a method for the production of a pyrolysis oil from end-of-life plastics The present inventors have found that there are particular issues with the use of pyrolysis oils obtained from pyrolysis of end-of-life plastics when seeking to use the pyrolysis oil as a feedstock for a cracking process, or even when using the oil as a transportation fuel. In particular, compared to natural oil materials, the pyrolysis oil obtained from end-of-life plastics has unacceptably high levels of mercury and phosphorous, as well as the typical sulphur and chlorine contaminants. Without wishing to be bound by theory, it is considered that the level of these impurities is a direct result of contaminants mixed with the plastics from their original lifetime use.

The method firstly requires providing end-of-life plastics material. End-of-life or contaminated plastic waste feedstock, for plastic chemical recycling, may be received from, for example, municipal recovery facilities, recycling factories, or other plastic collection sources. During a pre-treatment process, the feedstock may be refined such that it only contains plastics suitable for the chemical recycling process, such as low density polyethylene (LDPE), high density polyethylene (HDPE), polystyrene (PS), and/or polypropylene (PP). Unsuitable materials, such as metals, paper and card, and glass, as well as humidity from the plastic waste, may be removed.

The end-of-life plastics material may be obtained from a common source or from mixed sources, including mixed plastic waste obtained from municipal or regional sources and/or from waste streams of polyethylene terephthalates (PET), high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), polypropylene, and/or polystyrene. Furthermore, the waste may include thermoplastic elastomers and thermoset rubbers, such as from tires and other articles made from natural rubber, polybutadiene, styrene-butadiene, butyl rubber and ethylene propylene diene monomer rubber (EPDM). The waste may include one or more plastics classified as plastic identification code (PIC) 1 to 7 by the Society of the Plastics Industry. For example, the waste may include one or more of the following plastics: polyethylene terephthalate classified as PIC 1; high-density polyethylene classified as PIC 2; polyvinyl chloride classified as PIC 3; low-density polyethylene classified as P104; polypropylene classified as PIC 5; polystyrene classified as PIC 6; and polycarbonate and other plastics classified as PIC 7.

A pyrolysis oil is obtained by the thermal treatment of these plastics materials.

W02021123822 discloses an optimised process for this pyrolysis and the contents of this document are incorporated herein in their entirety by reference. This sort of pyrolysis oil, because of its source, contains a number of impurities, including: Sulphur; Chlorine; Phosphorus; Metals: especially mercury, but also arsenic, lead, nickel and the like; Silica; Oxygen and Nitrogen. As will be appreciated, the elements, in particular the metals, may be present in their elemental form though will typically be present as compounds such as salts and/or organic impurities comprising said elements (i.e. organosulphur impurities and so forth).

The method comprises melting the end-of-life plastics material to form a molten plastics material. Preferably the end-of-life plastics material is melted in step (ii) in a heated extruder at a temperature of from 250 to 350°C.

The method comprises pyrolysing the molten plastics material in an oxygen-free atmosphere to provide pyrolysis gases and a char material. Preferably the pyrolysis of step (iii) is conducted in an optionally agitated pyrolysis reactor at a temperature of from 350 to 450°C.

The method comprises condensing the pyrolysis gases to provide the pyrolysis oil. Preferably step (iv) comprises distilling said pyrolysis gases from the contactor in a distillation column.

Preferably before step (iv), the method further comprises: passing the pyrolysis gases from step (iii) into a contactor having a bank of condenser elements so that some long chain gas components condense on said elements to provide a condensed long chain material; returning said condensed long chain material to step (iii) to be further pyrolysed; and allowing short chain gas components to exit from the contactor in gaseous form before condensing in step (iv).

Critically, the method further comprises dispersing a zeolitic material in the end-of-life plastics material or in the molten plastics material. The present inventors have surprisingly found that the addition of a zeolitic material into the Plastic Energy process can catalyse the pyrolysis step, leading to a higher yield of a lighter oil. It is particularly surprising that the zeolitic material can be added with the raw feed material, rather than needing to be added into the pyrolysis chamber. The zeolitic material appears to be unaffected by the presence of impurities in the end-of-life plastics and this provides a significant improvement without adding to the process complexity.

Zeolites are well known for use in various industrial treatment processes and are commonly categorised by their pore sizes. In particular, there is a focus on the number of atoms which form their largest ring size, since this is a practical limitation on the ease with which molecules can diffuse into and out of the zeolite during a process. A small pore zeolite has a ring of 8 atoms, whereas a medium pore zeolite has a ring of 10 atoms and a large pore zeolite has a ring of 12 atoms. The zeolite used is preferably an aluminosilicate zeolite, having a framework consisting of Al and Si atoms. A further characteristic of zeolites is the form in which they are supplied, such as the Na+ or H+ form. In addition, the zeolites can be substituted with additional metal species, especially for catalytic purposes, such as the introduction of copper for SCR catalysts. In the present invention the zeolite is preferably free from such added catalytic metals.

Preferably the zeolitic material comprises a medium pore zeolite. The optimised zeolite for use in the present method is a ZSM-5. Preferably the zeolite is used in the Na-exchanged form. Preferably the zeolite is free from any added metals. That is, preferably the zeolite consists of A1203 and Si02 (i.e. an aluminosilicate), and when in the Na-exchanged form Na20 (i.e. a sodium aluminosilicate).

In addition, the inventors have found that the specific SAR of the zeolite can be optimised to achieve a balance of activity against coking of the material. Preferably the zeolitic material is an aluminosilicate having a silica to alumina molar ratio (SAR) of at least 35. Preferably the SAR is from 40 to 80, preferably 45 to 55, most preferably 50 to 55.

Preferably the zeolitic material is included in an amount of from 0.05wt% to 5wt%, based on the combined weight of the plastics material and the zeolitic material.

Preferably the zeolitic material has a particle size distribution having a mean particle size by volume of less than 1mm, preferably from 0.05 to 0.5mm. Particle sizes can be determined by a number of different techniques, including laser diffraction or sieving.

It has further been found that the zeolitic material can be recovered and reused, meaning that the process benefits are not outweighed by the addition of this further material. It can be regenerated in atmospheric air at elevated temperature allowing catalyst life to be substantially increased. Preferably the method further comprises recovering the zeolitic material from the char material, preferably by combusting the char to ash and physically separating the zeolitic material from the ash.

Methods for pyrolysis end-of-life plastics are well known in the art, including, for example, in W02021123822, W02016030460 and W02011077419 which are each incorporated herein by reference. The preferred method of pyrolysis will now be described further in more detail.

Preferably the method for pyrolysis end-of-life plastics involves the steps of: melting a waste plastics material, pyrolysing the molten material in an oxygen-free atmosphere to provide pyrolysis gases; bringing the pyrolysis gases into a contactor having a bank of condenser elements so that some long chain gas components condense on said elements, returning said condensed long-chain material to be further pyrolysed to achieve thermal degradation, and allowing short chain gas components to exit from the contactor in gaseous form; and distilling said pyrolysis gases from the contactor in a distillation column to provide one or more fuel products.

The end-of-life plastic (ELF) from the walking floor silo is discharged into the extruder hopper, which is designed to deliver heated ELF to the reactors. The extruder is supplied with variable speed drives that permit lower flow rates to be fed to the reactors, if required, during start-up and shutdown of an extruder. The extruder heats up the plastic from ambient conditions to the target set temperature using shear force generated by the rotation of the extruder screw. The high temperatures on the outlet of the extruder is required to ensure that the plastic temperature, which is lower than the reactor operating temperature, does not adversely affect the thermal performance of the reactor when loaded in.

The extruder barrel can be electrically heated, especially during start-up. During normal operation, the electric heating function is not used because the shear force from the auger screw will provide sufficient heat to melt the plastic.

Plasticised ELF is expelled from the extruder under high pressure into a melt feed line that connects the extruder to three Reactors via a header pipe. Multiple instruments monitor pressure and temperature along the melt feed line during feeding to assure flow.

The plant has multiple jacketed reactors that form the core of the process. Each conical based reactor is enclosed by a reactor jacket which provides the heat required to decompose the ELF and generate the desired hydrocarbon vapour. Each reactor is physically located above a char receiver and below a contactor (condenser elements).

Each individual reactor is provided with an agitator designed to maintain thermal efficiency of the process by minimising char build up on the walls of the reactor by maintaining close steel to steel clearance with the vessel walls; suspend any char produced during pyrolysis in the plastic mass to prevent build-up on the internal surfaces of the reactor; and homogenise the molten ELF in the reactor during processing; and remove char once pyrolysis is complete and the char is dry.

The agitator homogenises the vessel mass by pushing ELF down the walls of the reactor, to the centre of the vessel and up the agitator shaft when running in forward. When operating in reverse, it pushes medium down the agitator shaft and from the centre of the vessel to vessel walls. This promotes char removal through a bottom outlet nozzle located at the lowest point of the vessel conical dished end.

An individual reactor is designed to process 5 tonnes of ELP every day. Future generations may have a higher capacity. Reactors are grouped in threes and each trio of reactors is fed sequentially such that only one of each trio of vessels is being fed with fresh ELF at any one time whilst the other two are either completing pyrolysis or processing char.

ELF is fed into a reactor vessel by its respective Extruder. The reactor is operated at 380 to 450°C and up to 0.5 barg in an inerted oxygen free environment. At these temperatures the ELF polymer chains decompose into shorter hydrocarbon chains and are vaporised to form a rich saturated hydrocarbon vapour. This vapour exits the vessel via an outlet located on the top of the vessel which leads to the reactors' respective contactor.

The reactor is designed to operate on a cycle. Each cycle consists of three periods. The first is a ELF feed period known as "charging" in which ELF is loaded to the reactor and pyrolysed. In the second period, pyrolysis is completed and the non-pyrolysable material (char) in the reactor is dried to allow for easy handling after removal from the reactor in anticipation of the next charge of ELF. This stage is called "cooking'. The third stage is called "removal" and involves removing char from the reactor by opening the reactor bottom outlet valve and then reversing the reactor's agitator which in turn forces char out of the reactor into the char receiver below it. Once all the char is removed the bottom outlet valve is closed and plastic feeding can recommence.

Char is formed primarily of carbonaceous material, plastic polymer-forming additives, pigmentation and ELF contamination. Char continually forms in the reactor throughout pyrolysis and must be removed prior to commencement of another charge else the effective volume of the reactor reduces. The char in the present invention further comprises the added zeolite material. The zeolite may be recovered from the char by combusting the char in air to remove the carbon material and other impurities. It has been found that the zeolite is unaffected by this combustion, even at temperature up to 500°C. Moreover, such combustion serves to further regenerate the zeolite for further use.

Char is removed through the bottom outlet nozzle and valve (BOV). When char removal is required the bottom outlet valve is opened to the char receiver below. The agitator is then set to reverse to assist char removal. Char should fall out of the chamber under gravity because of the conical shape of the reactor however if it does not, the agitator has been designed to assist it by breaking up char lumps which may have formed in the nozzle.

Preferably the contactor elements comprises a plurality of plates forming an arduous path for the pyrolysis gases in the contactor. Moreover, preferably the plates are sloped downwardly for runoff of the condensed long-chain hydrocarbon, and include apertures to allow upward progression of pyrolysis gases. In one embodiment, the contactor elements comprise arrays of plates on both sides of a gas path. Preferably the contactor element plates are of stainless steel. The contactor may be actively cooled such as by a heat exchanger for at least one contactor element.

Alternative cooling means may comprise a contactor jacket and cooling fluid is directed into the jacket. There may be a valve linking the jacket with a flue, whereby opening of the valve causing cooling by down-draught and closing of the valve causing heating. The valve may provide access to a flue for exhaust gases of a combustion unit of the pyrolysis chamber.

Preferably there is a pipe directly linking the pyrolysis chamber to the contactor, the pipe being arranged for conveying upwardly-moving pyrolysis gases and downwardly-flowing long-chain liquid for thermal degradation.

Preferably infeed to the pyrolysis chamber is controlled according to monitoring of level of molten plastics in the chamber, as detected by a gamma radiation detector arranged to emit gamma radiation through the chamber and detect the radiation on an opposed side, intensity of received radiation indicating the density of contents of the chamber.

Preferably the pyrolysis chamber is agitated by rotation of at least two helical blades arranged to rotate close to an internal surface of the pyrolysis chamber. Optionally, the pyrolysis chamber is further agitated by a central auger. Advantageously, the auger can be located so that reverse operation of it causes output of char via a char outlet.

Preferably the temperature of pyrolysis gases at an outlet of the contactor is maintained in the range of 240°C to 280°C. The contactor outlet temperature can be maintained by a heat exchanger at a contactor outlet.

A bottom section of the distillation column is preferably maintained at a temperature in the range of 200°C to 240°C, preferably 210°C to 230°C. The top of the distillation column is preferably maintained at a temperature in the range of 90°C to 110°C, preferably approximately 100°C.

Optionally there is further distillation of some material in a vacuum distillation column. Heavy or waxy oil fractions are drawn from the bottom of the vacuum distillation column and can be recycled back to the pyrolysis chamber. Desired grade on-specification pyrolysis oil can be drawn from a middle section of the vacuum distillation column. Light fractions are drawn from a top section of the vacuum distillation column and are condensed.

Figures The invention will now be described further in relation to the following non-limiting Figures, in which: Figure 1 shows performance of ZSM-5 zeolites in LDPE pyrolysis.

Figure 2 shows performance of ZSM-5 zeolites in polymer mixed (PM) pyrolysis.

Examples

The invention will now be described further in relation to the following non-limiting Examples. The Examples demonstrate the benefits of using a zeolite to catalyse the reaction and, in particular, the effect of silica to alumina ratio on the performance of a zeolite (ZSM-5) to catalyse ELP pyrolysis. It shows especially that zeolite ZSM-5 with a Si02/A1203 ratio of 50 is the most suited for plastic pyrolysis. Lower Si02/A1203 ratios ZSM-5 (23) and (30) are less active, due to early coking with short olefin (from C2 to 04). Higher Si02/A1203 ratios ZSM-5 (80) and (280) are also less active due to a decrease in active site numbers. This is verified with Low Density PolyEthylene (LDPE) and also with polymer mixed (PM) found in waste streams: LDPE, High Density PolyEthylene (HDPE), PolyPropylene (PP), PolyStyrene (PS), and PVC (PolyVinylChloride).

Example 1

A series of ZSM-5 zeolites has been tested for LDPE and PM pyrolysis (50 g) at 410°C for 2 hours in a plastic pyrolysis pilot reactor. These zeolites differ by their Si02/A1203 ratios. Once reaction is completed, and reactor cooled down, the different yields to Distillate, Gas, and Residue are calculated as: YDistillate = WDistillate / 50 YResidue = WResidue / 50 With Yoistilate Yield to Distillate (c/o) YResidue Yield to Residue (%) YSyngas Yield to Syngas (%) Conversion is calculated as: X = 100 -* Residue With X Conversion (%) YResidue Yield to Residue (%) Results with virgin LDPE are in Figure 1.

The performance data clearly shows a maximum is conversion and distillate yield. This result is speculated to be due to two phenomena: 1. For high Si/AI ratios, ZSM-5 zeolites have less sites and consequently the ZSM-5 activity for pyrolysis decreases.

2. For low Si/AI ratios, ZSM-5 zeolite is coking at a faster rate, which reduces the zeolite activity to 0 for the most acidic zeolite.

Example 2

Full scale plant tests were performed with plastics waste representative of household waste.

This is shown in Figure 2.

These results clearly also shows a maximum in conversion for ZSM-5 (80) and a maximum in yield to distillate for ZSM-5 (50). This is explained by the facts that: 1. At high Si/AI ratios, less sites are available for the reaction and consequently the conversion decreases.

2. Allow Si/AI ratios, the zeolite cokes more, which prevents access to the active sites to the point of complete loss of catalytic pyrolysis activity.

It is demonstrated that ZSM-5 zeolite with 5i02/A1203 ratios of -50 to 80 is ideal for maximizing conversion and yield of distillate for a PM feedstock. Using those optimum Si/AI ratio zeolites, the full-scale plant ELP throughput can be significantly increased The term "comprising" as used herein can be exchanged for the definitions "consisting essentially of" or "consisting of". The term "comprising" is intended to mean that the named elements are essential, but other elements may be added and still form a construct within the scope of the claim. The term "consisting essentially of" limits the scope of a claim to the specified materials or steps and those that do not materially affect the basic and novel characteristic(s) of the claimed invention. The term "consisting of" closes the claim to the inclusion of materials other than those recited except for impurities ordinarily associated therewith.

The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.

For the avoidance of doubt, the entire contents of all documents acknowledged herein are incorporated herein by reference.

Claims (11)

1. Claims: 1. A method for the production of a pyrolysis oil from end-of-life plastics, the method comprising: (i) providing end-of-life plastics material; (ii) melting the end-of-life plastics material to form a molten plastics material; (iii) pyrolysing the molten plastics material in an oxygen-free atmosphere to provide pyrolysis gases and a char material; (iv) condensing the pyrolysis gases to provide the pyrolysis oil, characterised in that the method further comprises dispersing a zeolitic material in the end-of-life plastics material or in the molten plastics material.
2. 2. The method according to claim 1, wherein the zeolitic material comprises a medium pore zeolite, preferably ZSM-5.
3. 3. The method according to claim 1 or claim 2, wherein the zeolitic material is an aluminosilicate having a silica to alumina molar ratio (SAR) of at least 35.
4. 4. The method according to claim 3, wherein the SAR is from 40 to 80, preferably 45 to 20 55.
5. 5. The method according to any preceding claim, wherein the zeolitic material is included in an amount of from 0.05wt% to 5w1%, based on the combined weight of the plastics material and the zeolitic material.
6. 6. The method according to any preceding claim, wherein the zeolitic material has a particle size distribution having a mean particle size of less than 1mm, preferably from 0.05 to 0.5mm.
7. 7. The method according to any preceding claim, wherein the method further comprises recovering the zeolitic material from the char material, preferably by combusting the char to ash and physically separating the zeolitic material from the ash.
8. 8. The method according to any preceding claim, wherein the end-of-life plastics material is melted in step (ii) in a heated extruder at a temperature of from 250 to 350°C.
9. 9. The method according to any preceding claim, wherein the pyrolysis of step (iii) is conducted in an optionally agitated pyrolysis reactor at a temperature of from 350 to 450°C.
10. 10. The method according to any preceding claim, wherein, before step (iv), the method further comprises: passing the pyrolysis gases from step (iii) into a contactor having a bank of condenser elements so that some long chain gas components condense on said elements to provide a condensed long chain material; returning said condensed long chain material to step (iii) to be further pyrolysed; and allowing short chain gas components to exit from the contactor in gaseous form before condensing in step (iv).
11. 11. The method according to any preceding claim, wherein step (iv) comprises distilling said pyrolysis gases from the contactor in a distillation column.