WO2024115470A1

WO2024115470A1 - Autothermal cracking of hydrocarbons

Info

Publication number: WO2024115470A1
Application number: PCT/EP2023/083333
Authority: WO
Inventors: Ronald Jan Schoonebeek; Alouisius Nicolaas Renée BOS; Baira DONOEVA; Dominik Johannes Michael Unruh; Shauvik DE; Guus VAN ROSSUM
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Usa, Inc.
Priority date: 2022-12-01
Filing date: 2023-11-28
Publication date: 2024-06-06

Abstract

The invention relates to a process for producing olefins from a waste plastics feedstock said process comprising: pyrolyzing a waste plastics feedstream at a temperature in the range from 200°C to 600°C to produce a waste plastics pyrolysis feedstream containing gaseous hydrocarbons; feeding the waste plastics pyrolysis feedstream containing gaseous hydrocarbons into an autothermal reactor; pre-heating an oxygen containing stream and a hydrogen and/or methane containing stream outside the autothermal reactor; feeding the pre-heated oxygen containing stream and the pre-heated hydrogen and/or methane containing stream into a burner of the autothermal reactor; generating steam in a combustion zone of the autothermal reactor by the reaction of the pre-heated oxygen containing stream and the pre-heated hydrogen and/or methane containing stream; mixing the steam generated in the combustion zone with the waste plastics pyrolysis feedstream containing gaseous hydrocarbons in a mixing and cracking zone of the autothermal reactor, by feeding the steam and the feedstream containing gaseous hydrocarbons into the mixing and cracking zone from substantially opposite directions, and pyrolytically cracking the gaseous hydrocarbons to provide an effluent containing olefins.

Description

SP2920 AUTOTHERMAL CRACKING OF HYDROCARBONS Field of the invention 5 The present invention relates to a process for producing olefins from a waste plastics feedstock. Background of the invention Pyrolytic cracking of hydrocarbons in a cracker furnace is a petrochemical process that is widely used to produce 10 olefins (such as ethylene, propylene, butylenes and butadiene) and optionally aromatics (such as benzene, toluene and xylene). Where such pyrolytic cracking is performed in the presence of dilution steam, this is referred to as “steam cracking”. In such a pyrolytic cracking process, the 15 hydrocarbons containing stream is converted under the influence of heat, and substantially in the absence of oxygen, into an olefins containing effluent. In known process line ups, such as that disclosed in the article entitled ‘Assessing the feasibility of chemical 20 recycling via steam cracking of untreated plastic waste pyrolysis oils: Feedstock impurities, product yields and coke formation’, Waste Management 141 (2022) pages 104-114, waste plastics are converted via cracking of the plastics, for example by pyrolysis, to a liquid product containing 25 hydrocarbons having a wide boiling range, commonly referred to as waste plastics pyrolysis oil. Such pyrolysis oil can in turn be further converted via steam cracking to high-value chemicals, including ethylene and propylene, which are monomers that can be used in making new plastics. 30 Although pyrolytic steam cracking in cracker furnaces is known in the art for producing olefins from hydrocarbon feedstocks, it has several disadvantages. The process produces large amounts of carbon dioxide. The heat that is needed to effect the pyrolytic cracking of the hydrocarbon stream is provided by the combustion of fuel gas, typically comprising hydrogen and methane, in a burner of the cracker furnace, i.e. the heat for the pyrolytic cracking is provided 5 externally. Combustion of a fuel gas comprising hydrogen and methane results in the production of a flue gas comprising water and carbon dioxide. Generally, carbon dioxide from such flue gas may have to be emitted into the Earth’s atmosphere and/or may have to be captured in another form thereby 10 preventing such emission. A distinction can be made between Carbon Capture and Storage (CCS) and Carbon Capture and Use (CCU) which both involve carbon dioxide capture which is cumbersome, requiring additional equipment, and therefore relatively expensive. In addition, CCS further increases the 15 general costs of chemicals manufacturing because of the required energy expenditure for compression and distribution to carbon dioxide storage. In addition, the flue gas also comprises nitrogen oxide (NOx), which is a further undesirable by-product of the steam 20 cracking reaction. NOx can be removed from the flue gas, for example by using a DeNOx system, but this necessitates an additional step in the process and increases the overall cost of the olefins manufacture. Furthermore, conventional steam cracker furnaces have a 25 low tolerance for contaminants in hydrocarbon feedstocks. Waste plastics pyrolysis oil contains high amounts of contaminants, which can lead to corrosion and fouling in conventional steam cracker furnaces or issues in the downstream section of a steam cracker, such as polymerization 30 of oxygenates and gum formation. Therefore, it is preferable and often necessary for waste plastics pyrolysis oils to be decontaminated / pre-treated before using them as feedstocks in conventional steam cracker furnaces. Such pre-treatment is usually an elaborate and expensive process. Furthermore, during the pre-treatment step to remove contaminants, a portion of the valuable hydrocarbon feedstock can be lost, which in turn can lead to a lower yield of olefin products. 5 In addition, specialist treatment is needed to dispose of the contaminants removed from the waste plastics pyrolysis oil. Furthermore, notable amounts of coke are formed on the inside of the tubes/coils (carrying the hydrocarbon feed and where the pyrolytic cracking reaction takes place) suspended 10 in the furnace, due to the provision of heat from outside of the furnace, which necessitates regular furnace shutdowns to remove the coke build-up from the tubes/coils. The lifetime of coils is reduced with the number of de-coking steps. The tubes/coils are typically decoked using a mixture of steam 15 and air. To help reduce the formation of coke, sometimes sulphur is added to the hydrocarbon feedstream. However, this then means that further treatment is required to remove H₂S from the off-gas process stream. Moreover, aside from yielding the desired olefin 20 products, depending on the hydrocarbon feedstream and reactor conditions used, the process can also yield (aside from coke) substantial amounts of other less desirable products such as methane, higher hydrocarbons, and heavy aromatics. Furthermore, the conventional method of steam cracking 25 hydrocarbons in a cracker furnace is extremely energy intensive and is associated with high operating expenses and high capital expenditure. The process requires a huge consumption of fuel and maintenance of large, expensive and complex cracking furnaces to supply the heat. For example, 30 the tubes/coils used in the furnaces are very expensive and have a limited lifetime. Furthermore, conventional methods for producing olefins from waste plastics involves pyrolysis of a waste plastics feedstock followed by a condensation step in order to produce so-called plastic pyrolysis oil, followed by upgrading and/or decontamination steps and then a further evaporation step to produce a gaseous hydrocarbon feedstream which can be 5 introduced into the cracker. Hence, known methods for producing olefins from waste plastics involve a number of steps before the hydrocarbon feedstream can be finally introduced into the cracker. Therefore, is it an objective of the present invention to 10 provide a process for producing olefins from a waste plastics feedstock which involves a smaller number of steps than needed in hitherto known processes. It is also an objective of the present invention to provide a process for the pyrolytic cracking of hydrocarbons 15 that substantially reduces or avoids the production of carbon dioxide. It is also an objective of the present invention to provide a process for the pyrolytic cracking of hydrocarbons that substantially reduces or avoids the formation of NOx. 20 It is a further objective of the present invention to provide a process that has a high tolerance for contaminants in the hydrocarbon feedstock. It is also an objective of the present invention to provide a process that avoids the need to integrate the 25 synthesis of waste plastics pyrolysis oil with the cracking of the waste plastics pyrolysis oil using a conventional steam cracker furnace. It is a further objective of the present invention to provide a process for the pyrolytic cracking of hydrocarbons 30 that substantially reduces the formation of coke and thus requires less reactor maintenance. Additionally, it is an objective of the invention to provide increased selectivities and yields for the desired olefin product(s) compared to the selectivities and yields achieved by the conventional steam cracking process. Further, it is an objective of the present invention to provide a process for the pyrolytic cracking of hydrocarbons 5 to olefins, which process is efficient and affordable, and in particular has relatively low operating expenses, relatively low capital expenditure and relatively low energy demand. These and other objectives will become apparent from the disclosure provided herein. 10 Summary of the invention The present invention relates to a process for producing olefins from a waste plastics feedstock said process comprising: pyrolyzing a waste plastics feedstream at a temperature 15 in the range from 200°C to 600°C to produce a waste plastics pyrolysis feedstream containing gaseous hydrocarbons; feeding the waste plastics pyrolysis feedstream containing gaseous hydrocarbons into an autothermal reactor; pre-heating an oxygen containing stream and a hydrogen 20 and/or methane containing stream outside the autothermal reactor; feeding the pre-heated oxygen containing stream and the pre-heated hydrogen and/or methane containing stream into a burner of the autothermal reactor; 25 generating steam in a combustion zone of the autothermal reactor by the reaction of the pre-heated oxygen containing stream and the pre-heated hydrogen and/or methane containing stream; mixing the steam generated in the combustion zone with 30 the waste plastics pyrolysis feedstream containing gaseous hydrocarbons in a mixing and cracking zone of the autothermal reactor, by feeding the steam and the feedstream containing gaseous hydrocarbons into the mixing and cracking zone from substantially opposite directions, and pyrolytically cracking the gaseous hydrocarbons to provide an effluent containing olefins. The step of feeding the waste plastics pyrolysis 5 feedstream containing gaseous hydrocarbons into an autothermal reactor is advantageously carried out without condensing the waste plastics pyrolysis feedstream into a waste plastics pyrolysis oil. Said waste plastics pyrolysis feedstream containing gaseous hydrocarbons is preferably 10 obtained directly by the pyrolysis of waste plastics without intermediate condensation of that stream before feeding it into the autothermal reactor. The waste plastics pyrolysis feedstream may be subjected to a cleaning step before it is fed into the autothermal 15 reactor. The step of feeding the pre-heated oxygen containing stream and the pre-heated hydrogen and/or methane containing stream into the burner of the autothermal reactor may further comprise feeding a pre-heated temperature moderator into the 20 burner of the autothermal reactor. The pre-heated temperature moderator may comprise steam and/or carbon dioxide. The oxygen containing stream may be pre-heated to a temperature in the range of from about 200 °C to about 300 25 °C. The hydrogen and/or methane containing stream may be pre- heated to a temperature in the range of from about 350 °C to about 650 °C. Preferably, from the viewpoint of substantially reducing or avoiding the production of carbon dioxide, the 30 hydrogen and/or methane containing stream is a hydrogen containing stream. The temperature moderator may be pre-heated to a temperature in the range of from about 350 °C to about 650 °C. The temperature of the steam generated in the combustion 5 zone may be in the range of from about 1200 °C to about 1900 °C, suitably about 1200 °C to about 1800 °C. The steam generated in the combustion zone may flow into the mixing and cracking zone at a velocity in the range of from about 100 m/s to about 400 m/s. 10 The feed stream containing gaseous hydrocarbons may flow into the mixing and cracking zone at a velocity in the range of from about 10 m/s to about 300 m/s. The waste plastics pyrolysis feedstream containing gaseous hydrocarbons may be further heated inside the reactor 15 through indirect heat exchange between the effluent containing olefins and the waste plastics pyrolysis feedstream containing gaseous hydrocarbons in an effluent zone of the reactor. The effluent containing olefins may undergo further 20 downstream processing and/or separation in a steam cracker unit. The process of the present invention is advantageous in that there is no need to condense the waste plastics pyrolysis feedstream containing gaseous hydrocarbons into a 25 liquid pyrolysis oil. In the present invention, the waste plastics pyrolysis feedstream containing gaseous hydrocarbons is fed directly into the autothermal reactor without first subjecting it to a condensation step. Hence the process of the present invention has better heat and process integration 30 and comprises fewer steps compared with known methods for producing olefins from waste plastics. The process of the present invention is advantageous in that a reduced amount of carbon dioxide or no or very little carbon dioxide is produced during the process. This is due to the heat for the cracking reaction being produced inside the reactor using feedstreams that generate no or little carbon dioxide, for example when hydrogen and oxygen are fed to the 5 burner of the reactor. Where oxygen and methane are fed to the burner, or oxygen and hydrogen and methane are fed to the burner, then some carbon dioxide may be produced, but the amount of carbon dioxide produced will still be lower than that produced using conventional pyrolytic cracking of 10 hydrocarbons in a cracker furnace, which uses the external combustion of hydrogen and methane as the heat source. Any carbon dioxide produced can also be easily removed. The process of the present invention is also beneficial in that it avoids or substantially reduces the production of 15 NOx. Where methane is fed to the burner (in combination with oxygen or in combination with oxygen and hydrogen), then some NOx may be produced, but the amount of NOx produced will still be lower than that produced using conventional pyrolytic cracking of hydrocarbons in a cracker furnace. The 20 reduction or elimination of NOx production helps to reduce the cost of the olefins manufacture and minimize the release of this pollutant into the atmosphere. A further benefit of the process of the present invention is that it has a high contaminants tolerance, meaning that 25 there is no need for plastics-related upgrading steps (treatments steps), such as hydroprocessing or contaminant removal, and therefore the waste plastics pyrolysis feedstream containing gaseous hydrocarbons can be fed directly to the autothermal reactor. Conventional waste 30 plastics to olefins processes involve often highly contaminated pyrolysis oil feed streams, such as waste plastics pyrolysis oil feed streams, which need a decontamination / pre-treatment step. Hence the process of the present invention saves significant amounts of time and expense. A significant further benefit of the process of the invention is that a reduced amount of coke is formed during 5 cracking. Since the formation of coke is substantially reduced, the reactor can run for much longer periods of time without interruption, and any coke that is formed can be easily and quickly removed from the reactor. In addition, since little coke is formed during the process of the present 10 invention, there is no need or a reduced need to add sulphur to the hydrocarbon feed stream and thus there is also no need or a reduced need for any treatment to remove H₂S during the process. The rapid and efficient mixing of the steam and the waste 15 plastics pyrolysis feedstream containing hydrocarbons in the mixing and cracking zone of the reactor enables high cracking temperatures and fast cracking times that in turn leads to increased selectivities and high yields for the desired olefins, particularly when compared to those obtained using 20 conventional cracking of hydrocarbons in a cracker furnace. In particular, there is a lower production of undesirable heavy ends. Also, if ethylene is a desired product and propylene is a less desired product, the process of the present invention has a better range of control of the 25 propylene to ethylene ratio than conventional steam cracking processes due to the higher severity of the process of the present invention. Furthermore, advantageously, oxygen from the feedstock ends up primarily in the form of carbon monoxide (CO) in the effluent, rather than oxygenates which 30 are low molecular weight compounds containing oxygen (e.g. acetaldehyde). CO can easily be separated from the effluent whereas oxygenates can lead to issues, such as polymerization of these oxygenates, in the downstream section of a steam cracker in which the effluent from the autothermal reactor may undergo further downstream processing and/or separation. Overall, the process of the present invention involves lower capital expenditure compared to conventional steam 5 cracking processes using cracker furnaces. This is due to various factors including the simpler design of the autothermal reactor, a reduced number of process steps and much shorter reaction times. The process is also advantageous in that lower operating expenses are incurred due in part to 10 the high selectivities achieved, higher operating temperatures used and reduced number of process steps. Brief description of the drawings Figure 1 shows a schematic representation of a process line up for a known waste-plastics to olefins process. 15 Figure 2 shows a schematic representation of a process line up according to the present invention. Figure 3 shows a schematic representation of an autothermal reactor for use in an embodiment of the process of the present invention. 20 Figure 4 shows a schematic representation of an alternative configuration of an autothermal reactor for use in an embodiment of the process of the present invention. Figure 5 shows a schematic representation of another alternative configuration of an autothermal reactor for use 25 in an embodiment of the process of the present invention. Figure 6 depicts the oven and reactor configuration used in Example 1. Figure 7 shows an example of a typical temperature profile for an oven and reactor configuration used in Example 30 1 at an oven setpoint of 1100 °C. Figure 8 shows a Computational Fluid Dynamics (CFD) simulation of a comparative mixing configuration where dodecane is introduced into the reactor via slits on the side of the reactor as discussed in Example 2. Figure 9 shows a Computational Fluid Dynamics (CFD) simulation of a mixing configuration according to an 5 embodiment of the process of the present invention where dodecane is introduced into the mixing and cracking zone of the reactor via a lance as discussed in Example 2. Detailed description of the invention The process of the present invention comprises multiple 10 steps. In addition, said process may comprise one or more intermediate steps between consecutive steps. Further, said process may comprise one or more additional steps preceding the first step and/or following the last step. For example, in a case where said process comprises steps a), b) and c), 15 said process may comprise one or more intermediate steps between steps a) and b) and between steps b) and c). Further, said process may comprise one or more additional steps preceding step a) and/or following step c). While the process of the present invention and the 20 streams used in the process are described in terms of “comprising”, “containing” or “including” one or more various described steps and components, respectively, they can also “consist essentially of” or “consist of” said one or more various described steps and components, respectively. 25 In the context of the present invention, in a case where a stream comprises two or more components, these components are to be selected in an overall amount not to exceed 100%. Further, where upper and lower limits are quoted for a property then a range of values defined by a combination of 30 any of the upper limits with any of the lower limits is also implied. In general terms the present invention provides a method for producing olefins from a waste plastics feed stock, wherein the hydrocarbons in a gasified waste plastics feedstock are cracked to olefins in an autothermal reactor. For the purposes of promoting an understanding of the principles of the invention, reference will now be made to 5 the embodiments illustrated in the accompanying drawings, which are described in more detail below. The embodiments disclosed herein are not intended to be exhaustive or limit the invention to the precise form disclosed in the following detailed description. The invention includes any alterations 10 and further modifications in the illustrated devices and described methods and further applications of the principles of the invention as set forth in the claims. The process diagrams of Figures 1 and 2 illustrate the key differences between a known process for producing olefins 15 from plastics pyrolysis oil (Figure 1) and the process of the present invention (Figure 2). In a known process (illustrated in Figure 1), a waste plastics feedstock is subject to a plastics pyrolysis step to produce a waste plastics pyrolysis feedstock. During the 20 pyrolysis step a fraction of product remains in the liquid phase, which contains heavy product residue that is typically rejected through the bottom of the pyrolysis reactor. The remaining gaseous waste plastics pyrolysis product is then condensed to produce a so-called plastics pyrolysis oil 25 (typically C5-C30 hydrocarbons). The light hydrocarbon fraction (typically C1-C5 hydrocarbons) of the pyrolysis step remains in gas phase, and is used to generate the heat for the pyrolysis step. The plastics pyrolysis oil then typically undergoes one 30 or more upgrading steps before it is evaporated and fed to the cracker furnace. Once the cracking reaction has taken place, the cracked product typically undergoes quenching and conditioning steps in order to produce the final olefin product. In the process of the present invention (an embodiment of which is illustrated in Figure 2), a waste plastics feedstock 5 is subject to a plastics pyrolysis step at a temperature in the range from 200°C to 600°C. As used herein, the term pyrolysis means a chemical degradation process that occurs at higher temperatures (e.g. typically at 200°C or higher) which does not involve the addition of other reagents such as 10 oxygen (as in combustion) or water (as in hydrolysis). Pyrolysis yields solid/liquid and a gas phase. The pyrolysis of the plastics waste feedstock produces a waste plastics pyrolysis feedstream containing hydrocarbons preferably containing from 90 wt% to 95 wt% of gaseous 15 hydrocarbons, preferably C1-C30 gaseous hydrocarbons; and from 5 wt% to 10 wt% of a fraction of heavier liquid/solid residue, which cannot be cracked and is rejected at the bottom of the pyrolysis reactor. Preferably, in the present invention the plastics pyrolysis step starts at a temperature 20 in the range from 200°C to 300°C in order to remove HCl from polyvinyl chloride plastics waste component, and other contaminants, and is completed at 600°C, suitably at 550°C, more suitably at 500°C, most suitability at 450°C, to produce a waste plastics pyrolysis feedstream containing gaseous 25 hydrocarbons. The waste plastics pyrolysis feedstream containing gaseous hydrocarbons preferably contains C1 – C30 gaseous hydrocarbons. In the waste pyrolysis feedstream containing gaseous hydrocarbons typically 90 wt.% or more of the gaseous hydrocarbons contain greater than 5 carbon atoms. 30 The waste plastics pyrolysis feedstream containing gaseous hydrocarbons may be subjected to an optional gas cleaning step, such as an alkaline scrubbing to remove remaining acidic impurities, such as HCl, or oxygenates. The hot waste plastics pyrolysis feedstream containing gaseous hydrocarbons, preferably having a temperature in the range from 400°C to 600°C, is fed into an autothermal reactor as described above with reference to Figures 3 to 5 where the 5 waste plastics pyrolysis feedstream containing gaseous hydrocarbons is cracked to produce an effluent comprising olefins. The effluent comprising olefins may be subject to quenching and conditioning steps, such as conventional cracker downstream separation, as necessary to produce the 10 final olefin product. Such quenching and conditioning steps are well known to those skilled in the art and are not described further here. As can be seen by comparing Figure 1 with Figure 2, the present invention comprises fewer steps than conventional 15 process for producing olefins from waste plastics. In particular, the present invention does not involve a separate condensation step after the waste plastics pyrolysis step. Instead of producing pyrolysis oil by pyrolysis of waste plastics feedstock followed by condensation, the present 20 invention only involves a pyrolysis step of the waste plastics feedstock to produce a waste plastics gasified feedstock which can then be fed directly into an autothermal reactor for the cracking step, as described in further detail below. 25 Figure 3 shows a representation of an autothermal reactor 10 for use in a process for producing olefins from a pyrolysis waste plastics feedstream containing gaseous hydrocarbons by pyrolytic cracking of the gaseous hydrocarbons according to an embodiment of the present 30 invention. The autothermal reactor 10 comprises a burner 11, a combustion zone 12, a contraction zone 13, a mixing and cracking zone 14, and an effluent zone 16. The burner 11 has an inlet section 18 through which an oxygen containing stream 20 and a hydrogen and/or methane containing stream 22 are fed into the burner 11. The inlet section 18 may comprise multiple inlets, one for each of the 5 respective streams to be fed into the burner 11. In the present invention, the hydrogen and/or methane containing stream preferably contains hydrogen. Further, said hydrogen containing stream may also contain methane. More preferably, said hydrogen containing stream consists of 10 hydrogen. The amount of methane in the hydrogen and/or methane containing stream may be of from 0 to 15 vol.% or may be at most 10 vol.% or at most 5 vol.% or at most 1 vol.% or at most 0.5 vol.%, based on the total amount of hydrogen and methane. Further, said amount of methane in the hydrogen 15 and/or methane containing stream may be at least 0.5 vol.% or at least 1.5 vol.% or at least 3 vol.%. Hydrogen used in the hydrogen and/or methane containing stream 22, whether used alone or in combination with methane, can be any suitable source of hydrogen, including 20 conventional hydrogen (so-called “grey” hydrogen), hydrogen sustainably produced through renewable power electrolysis (so-called “green” hydrogen), hydrogen produced from hydrocarbons in a process like steam methane reforming in combination with carbon capture and storage (so-called “blue” 25 hydrogen), or otherwise produced hydrogen. Methane used in the hydrogen and/or methane containing stream 22, whether used alone or in combination with hydrogen, can be any suitable source of methane, including conventional methane as well as methane from renewable 30 sources (so-called “green” methane). The oxygen containing stream 20 and the hydrogen and/or methane containing stream 22 are each pre-heated prior to being fed into the burner 11. Any known means for pre-heating the streams can be used. The oxygen containing stream 20 is typically pre-heated to a temperature in the range of from about 200 °C to about 300 °C. Suitably, the oxygen containing stream 20 can be pre-heated to a temperature of at least 200 5 °C, suitably at least 220 °C, suitably at least 240 °C. Suitably, the oxygen containing stream 20 can be pre-heated to a temperature of at most 300 °C, suitably at most 280 °C, suitably at most 260 °C. The hydrogen and/or methane containing stream 22 is typically pre-heated to a temperature 10 in the range of from about 350 °C to about 650 °C. Suitably, the hydrogen and/or methane containing stream 22 can be pre- heated to a temperature of at least 350 °C, suitably at least 400 °C, suitably at least 450 °C. Suitably, the hydrogen and/or methane containing stream 22 can be pre-heated to a 15 temperature of at most 650 °C, suitably at most 600 °C, suitably at most 550 °C. The temperature to which these two streams are pre-heated can be varied accordingly depending on the desired temperature of the steam 23 generated in the combustion zone 12. 20 Other components can be fed into the burner 11 in addition to the oxygen containing stream 20 and the hydrogen and/or methane containing stream 22. For example, a temperature moderator 21, such as steam and/or carbon dioxide, can be fed into the burner 11 to help regulate the 25 temperature of the steam 23 that results from the combustion of oxygen and hydrogen, or oxygen and methane, or all three of oxygen, hydrogen and methane (when a mixture of hydrogen and methane is used in stream 22) in the combustion zone 12. The temperature of this additional temperature moderator 21, 30 e.g. steam and/or carbon dioxide, can be varied accordingly depending on the desired temperature of the steam 23 generated in the combustion zone 12. Typically, the temperature moderator 21 (whether it be steam, carbon dioxide or a mixture of both, or indeed any other suitable temperature moderator) is pre-heated to a temperature in the range of from about 350 °C to about 650 °C before being fed into the burner 11. Suitably, the temperature moderator 21 5 can be pre-heated to a temperature of at least 350 °C, suitably at least 400 °C, suitably at least 450 °C. Suitably, the temperature moderator 21 can be pre-heated to a temperature of at most 650 °C, suitably at most 600 °C, suitably at most 550 °C. The temperature moderator 21 can be 10 fed to the burner 11 alone or in combination with the oxygen containing stream 20 and/or the hydrogen and/or methane containing stream 22. In the embodiment shown in Figure 1, purely for simplicity purposes, the optional temperature moderator 21 is shown as being fed into the burner 11 15 separately to the oxygen containing stream 20 and the hydrogen and/or methane containing stream 22. The addition of steam and/or carbon dioxide, or other inert gas, to the burner 11 can also help to prevent the oxygen and the hydrogen and/or methane from reacting in close 20 vicinity of the inlet section 18. The oxygen and hydrogen, or the oxygen and methane, or the oxygen and hydrogen and methane, preferably the oxygen and hydrogen, (depending on the composition of the feedstream 22) combust in the combustion zone 12 leading to a high flame 25 temperature and the formation of steam 23, substantially free of oxygen. Within the present specification, steam “substantially free of oxygen” means that the steam contains of from 0 to 10,000 parts per million of volume (ppmv) of oxygen or may contain oxygen in an amount of at most 5,000 30 ppmv or at most 1,000 ppmv or at most 500 ppmv or at most 10 ppmv. In the present invention, such steam substantially free of oxygen may be provided by feeding hydrogen in an amount which is higher than the stoichiometric molar amount needed for the reaction of hydrogen with oxygen, for example 1-5% or 1-3% higher. Typically, the temperature of the steam 23 generated in the combustion zone 12 is in the range of from about 1200 °C to about 1900 °C, suitably about 1200 °C to 5 about 1800 °C. Suitably, the temperature of the steam 23 generated in the combustion zone 12 can be at least 1200 °C, suitably at least 1300 °C, suitably at least 1400 °C, suitably at most 1900 °C, suitably at most 1800 °C, suitably at most 1750 °C, suitably at most 1700 °C. 10 This high-temperature steam 23, generated within the autothermal reactor 10, is the heat source that is used to effect the pyrolytic cracking of gaseous hydrocarbons in the process of the present invention. The generation of heat in this manner is advantageous over conventional pyrolytic 15 cracking of hydrocarbons in a cracker furnace, using the external combustion of hydrogen and methane as the heat source, because no or very little carbon dioxide is produced when oxygen and hydrogen are fed to the burner 11 (without the presence of methane or other hydrocarbon being co-fed). 20 In case oxygen and methane are fed to the burner 11, or oxygen and hydrogen and methane are fed to the burner 11, then some carbon dioxide may be produced, but the amount of carbon dioxide produced will still be lower than that produced using conventional pyrolytic cracking of 25 hydrocarbons in a cracker furnace. The high-temperature steam 23 generated in the combustion zone 12 flows into the contraction zone 13, which as shown in Figure 3 is located below the combustion zone 12. The contraction zone 13 is narrower in width than the combustion 30 zone 12 and narrows downwards along its length to ensure that the high-temperature steam 23 flows at a very high velocity downwards towards the mixing and cracking zone 14 of the reactor 10. The hydrocarbon feedstock used in the process of the present invention is a waste plastics pyrolysis feedstream containing gaseous hydrocarbons produced by pyrolyzing a waste plastics feedstock. Thus, the feedstream containing 5 hydrocarbons 28 comprises a waste plastics pyrolysis feedstream containing gaseous hydrocarbons. Waste plastics can be converted via pyrolysis of the plastics to a product stream containing gaseous hydrocarbons having a wide boiling range. In the present invention, a waste plastics feedstock 10 can be pyrolyzed at a temperature from 200°C to 600°C, suitably at 550 °C, suitably at 500 °C, most suitably at 450 °C. The pyrolysis step may be carried out at a pressure of greater than 0 bara, suitably greater than 1.2 bara. Preferably, the pyrolysis step is carried out without the use 15 of catalysts to produce a waste plastic pyrolysis feedstock containing gaseous hydrocarbons. Such waste plastics pyrolysis feedstream containing gaseous hydrocarbons can in turn be further converted via steam cracking to high-value chemicals, including ethylene and propylene, which are 20 monomers that can be used in making new plastics. In known processes for converting waste plastics to olefins, a waste plastic feedstock is converted via cracking of the plastics, for example by pyrolysis, to a product stream containing hydrocarbons, commonly referred to as waste 25 plastics pyrolysis oil. The production of waste plastics pyrolysis oil typically involves the pyrolysis of a waste plastics feedstock followed by condensation to yield a liquid plastics pyrolysis oil, containing liquid hydrocarbons, typically C5-C30 liquid hydrocarbons. The pyrolysis oil is 30 then subjected to various upgrading steps to make it suitable for use in the conventional cracker. Advantageously, in the present invention, there is no need to produce liquid pyrolysis oil, since the autothermal reactor allows the use of a waste plastics pyrolysis feedstock containing gaseous hydrocarbons (i.e. containing high amounts of gaseous hydrocarbons as detailed above) which does not need condensing and thorough purification. 5 Waste plastic that may be pyrolyzed to produce a waste plastic pyrolysis feedstream containing gaseous hydrocarbons for use in the present process may comprise heteroatom- containing plastics, such as polyvinyl chloride (PVC), polyethylene terephthalate (PET) and polyurethane (PU). Mixed 10 waste plastic may be pyrolyzed that, in addition to heteroatom-free plastics, such as polyethylene (PE) and polypropylene (PP), contains a relatively high amount of such heteroatom-containing plastics. Alternatively, single waste plastic feedstocks can be pyrolyzed herein, for example a 15 waste plastic feedstock containing polyethylene (PE) only, or a waste plastic feedstock containing polypropylene (PP) only. Preferably, in the present process, an untreated waste plastic pyrolysis feedstream containing gaseous hydrocarbons is used, wherein more preferably said stream contains 20 heteroatom-containing contaminants. It is possible for some methane (and/or ethane, propane and/or butane) to be added to the waste plastics pyrolysis feed stream 28. In this instance, the methane can be considered as a secondary hydrocarbon feedstock, with the 25 waste plastics pyrolysis feedstock being the primary hydrocarbon feedstock. The methane can be added simultaneously with the primary hydrocarbon feedstock (e.g. it can be mixed in with the primary hydrocarbon feedstock), or it can be introduced into the reactor prior to 30 introduction of the primary hydrocarbon feedstock. Typically, the optional additional methane constitutes a relatively small proportion of the total hydrocarbon feed stream 28. The hot (typically 400 to 600° C) feedstream containing gaseous hydrocarbons 28 is fed into the reactor 10. Further heating of the feedstream containing gaseous hydrocarbons 28 may take place inside the reactor 10 prior to contacting of 5 the feedstream containing gaseous hydrocarbons 28 with the high-temperature steam 23 (also referred to herein as the “steam stream”) in the mixing and cracking zone 14 (which is discussed in further detail below). Ultimately, what is important is the temperature of the feedstream containing 10 gaseous hydrocarbons 28 immediately before or as it contacts the steam stream 23 in the mixing and cracking zone 14. Typically, the temperature of the feedstream containing gaseous hydrocarbons 28 just before it contacts the steam 23 in the mixing and cracking zone 14 is in the range of from 15 about 200 °C to about 650 °C, depending on the composition of the waste plastics pyrolysis hydrocarbon feedstock being used. Suitably, the temperature of the feedstream containing gaseous hydrocarbons 28 just before it contacts the steam 23 in the mixing and cracking zone 14 can be at least 400 °C, 20 suitably at least 450 °C, suitably at least 500 °C, suitably at least 550 °C, suitably at most 650 °C, suitably at most 600 °C. This may mean that the feedstream containing gaseous hydrocarbons 28 is pre-heated to the desired temperature (i.e. the temperature immediately before or just as it 25 contacts the steam 23 in the mixing and cracking zone 14), prior to being fed into the reactor 10, if no significant further heating of the feed stream containing gaseous hydrocarbons 28 takes place inside the reactor 10 prior to contacting of the feedstream containing gaseous hydrocarbons 30 28 with the steam stream 23 in the mixing and cracking zone 14. If, as discussed in further detail below and as per the embodiment of the invention shown in Figure 3, for example, further heating of the feed stream containing gaseous hydrocarbons 28 does take place inside the reactor 10 prior to contacting of the feed stream containing gaseous hydrocarbons 28 with the steam stream 23 in the mixing and cracking zone 14, then the feed stream containing gaseous 5 hydrocarbons 28 can be pre-heated to a lower temperature than the desired temperature (i.e. the temperature immediately before or just as it contacts the steam 23 in the mixing and cracking zone 14) prior to being fed into the reactor 10, since it will be heated up further in the reactor 10 prior to 10 mixing with the steam stream 23. The extent to which the feed stream containing gaseous hydrocarbons 28 is further heated inside in the reactor will depend on various factors, including the mechanism by which the feed stream containing gaseous hydrocarbons 28 is further heated (e.g. by indirect 15 heat exchange), the length of time for which it is exposed to additional heat and the composition of the hydrocarbon feedstock being used. For example, the additional heating inside the reactor may typically increase the temperature of the feedstream containing gaseous hydrocarbons by between 20 about 10 °C and about 200 °C. So, the feedstream containing gaseous hydrocarbons can typically be pre-heated outside the reactor to a temperature between about 10 °C and about 200 °C lower than the desired temperature of about 200 °C to about 650 °C when the feedstream containing gaseous hydrocarbons 25 enters the mixing and cracking zone 14. In either scenario (i.e. whether there is any significant further heating of the hydrocarbon feedstock 28 in the reactor 10 or not), the temperature of the feedstream containing gaseous hydrocarbons 28 is lower than the temperature of the steam 23 generated in 30 the combustion zone 12, both when it is fed into the reactor 10 and immediately before or as it contacts the steam 23 in the mixing and cracking zone 14. The waste plastic pyrolysis feedstream containing gaseous hydrocarbons 28 can be fed into the reactor 10 through inlet 30. In the reactor configuration shown in Figure 3, inlet 30, for simplicity reasons, is shown as a side-arm, whereas in 5 practice alternative configurations of inlet and other means of introducing the hydrocarbon containing feedstream 28 into the reactor 10 are possible and included within the scope of the present invention. Also, in practice there may be more than one inlet or side-arm present to introduce the 10 hydrocarbons 28 into the reactor 10. In Figure 3, the inlet 30 is depicted as being located beneath and to the side of the reactor 10. Figures 4 and 5 show alternative configurations of reactor 10, where the inlet 30 is shown as a single side-arm protruding from a side wall of the reactor 15 10. The location of the inlet can be any suitable location that allows the hydrocarbon containing feedstream 28 to be introduced into the reactor in a suitable manner so as to realise the advantages of the present invention. In the reactor configuration shown in Figure 3, the feedstream 20 containing gaseous hydrocarbons 28, once introduced through inlet 30, typically by high speed injection, flows upwards towards the mixing and cracking zone 14 through a narrow inner tube or lance 32. The narrow width of the lance 32 ensures the feedstream containing gaseous hydrocarbons 28 25 flows at high velocity, typically at a velocity in the range of from about 10 m/s to 300 m/s, suitably in the range of from about 30 m/s to about 250 m/s, suitably in the range of from about 50 m/s to about 200 m/s, towards the mixing and cracking zone 14 of the reactor 10. In the reactor 30 configuration shown in Figure 3, the lance 32 extends from beneath the reactor, upwards through the effluent zone 16, and terminates at the mixing and cracking zone 14. In the mixing and cracking zone 14, the steam stream 23 from the contraction zone 13 is contacted with the feedstream containing gaseous hydrocarbons 28 from the lance 32 and the two streams mix. Both streams are flowing at high velocity 5 and thus mixing occurs rapidly, although preferably the steam stream 23 is moving at a higher velocity than the feedstream containing gaseous hydrocarbons 28. Typically, the steam stream 23 is flowing at a velocity in the range of from about 100 m/s to about 400 m/s, suitably in the range from about 10 150 to about 300 m/s. Suitably, the steam stream 23 is flowing at a velocity in the range of from about 50 m/s to about 150 m/s higher than that of the feedstream containing gaseous hydrocarbons 28. In the reactor configuration shown in Figure 3, the use 15 of the lance 32 ensures that the feedstream containing gaseous hydrocarbons 28 flows upwards towards the mixing and cracking zone 14, such that the steam stream 23 and the feedstream containing gaseous hydrocarbons 28 are flowing towards the mixing and cracking zone 14 in substantially 20 opposite directions, i.e. they are flowing counter-currently, and that they collide and contact each other substantially head-on in the mixing and cracking zone 14. This substantially opposite or counter-current high-velocity flow of the two streams, i.e. the steam stream 23 and the 25 feedstream containing gaseous hydrocarbons 28, has been found to lead to extremely fast and efficient mixing of the two streams. For example, the opposite flow of the two high- velocity streams has been found to lead to much faster and more efficient mixing of the two streams compared to when the 30 feedstream containing gaseous hydrocarbons 28 enters the mixing and cracking zone 14 and is contacted with the steam stream 23 perpendicular to the flow of the steam stream 23, e.g. in a configuration where the inlet for the entry of the feedstream containing gaseous hydrocarbons into the reactor leads directly into the mixing and cracking zone without the use of an upwardly extending narrow tube or lance. The steam stream 23 and the feedstream containing gaseous 5 hydrocarbons 28 can be directly opposite streams moving towards one another and fed into the mixing and cracking zone 14 to contact and mix with one another, or they can be substantially opposite streams, i.e. the streams can be slightly off-set and do not have to be fed into the mixing 10 and cracking zone 14 from precisely opposite directions. Thus, within the present specification, “substantially opposite directions” for the steam and the feedstream containing gaseous hydrocarbons when feeding into the mixing and cracking zone, covers both (i) directly or precisely 15 opposite directions, that is to say 100% opposite directions (directions with a difference of 180°), and (ii) directions which deviate from said 100% opposite directions to some extent. In the present invention, the deviation from said 100% opposite directions may be of from 0 to 20° or may be at 20 most 15° or at most 10° or at most 5° or at most 3° or at most 1°. The rapid mixing due to the opposing or counter-current flow of the two high-velocity streams has the benefit of avoiding back-mixing of the feedstream containing gaseous 25 hydrocarbons 28 in the steam 23 in the mixing and cracking zone 14. Such back-mixing can mean that the hydrocarbon cracks for too long at high temperatures, causing a build-up of coke and other undesired reactions. Optionally, steam can be added to the feedstream 30 containing gaseous hydrocarbons 28 to help avoid / reduce coke formation in the lance 32 and/or to help increase the hydrocarbon to olefin conversion after mixing. Optionally, there can be a device present in the reactor 10 that causes the steam 23 to swirl in the contraction zone 13 to assist with rapid and efficient mixing of the two opposing streams. Also, optionally, there can be a nozzle or 5 multiple outlets (see Figure 5) present at the tip of the lance (at the mixing and cracking zone 14) to change the local injection velocity of the hydrocarbon feedstream 28 as it enters the mixing and cracking zone 14. Mixing of the high-temperature steam stream 23 (which is 10 typically at a temperature in the range of from about 1200 °C to about 1900 °C, suitably about 1200 °C to about 1800 °C, when it reaches the mixing and cracking zone 14) with the cooler feedstream containing gaseous hydrocarbons 28 causes the feedsteam containing gaseous hydrocarbons 28 to heat up. 15 Thus, the high-temperature steam 23 is the heat source that is used to effect the pyrolytic cracking of the hydrocarbons in the process of the present invention. The cracking temperatures, resulting from the rapid mixing of the high-temperature steam 23 and the cooler 20 feedstream containing gaseous hydrocarbons 28, in the process of the present invention for producing olefins from a feed stream containing hydrocarbons are much higher than the cracking temperatures used in conventional steam cracking in a cracker furnace. Typically, the cracking temperatures are 25 up to a few hundred degrees higher (for example, about 200 °C to about 400 °C higher) than conventional steam cracking in a cracker furnace, which typically takes place at around 800- 850 °C. A typical cracking temperature for use in the present invention is in the range from 800°C to 1400°C. In specific, 30 in the present invention, the cracking temperature in the mixing and cracking zone may be of from 1,000 to 1,250 °C. In the present invention, the pyrolytic cracking in the mixing and cracking zone of the autothermal reactor is preferably carried out without using a catalyst. The temperature of the steam 23 output from the combustion zone 12 and the temperature of the feed stream 5 containing gaseous hydrocarbons 28 when it reaches the mixing and cracking zone 14 are optimised and selected so as to achieve the desired cracking temperatures to suit the composition of the hydrocarbon feedstock. The cracking times in the process of the present 10 invention for producing olefins from a feed stream containing hydrocarbons are much shorter than the cracking times typically observed in conventional steam cracking in a cracker furnace. Typically, the cracking times using the process of the present invention are in the range of from 15 about 1 millisecond (ms) to about 50 milliseconds (ms), depending on the composition of the waste plastics pyrolysis hydrocarbon feedstream to be cracked, so up to about two orders of magnitude lower than the cracking times typically observed in conventional steam cracking in a cracker furnace. 20 The high cracking temperatures and the short cracking times achieved using the process of the present invention surprisingly provide better yields and selectivities for the desired olefins compared to conventional steam cracking of hydrocarbons in a cracker furnace. Furthermore, a 25 significantly lower amount of coke is produced at such high cracking temperatures and short cracking times. The mixing of the feedstream containing gaseous hydrocarbons 28 and the steam stream 23 in the mixing and cracking zone 14 is so rapid and thorough that mixing and 30 cracking largely occur simultaneously. As such, a majority of the hydrocarbons crack whilst in the mixing and cracking zone 14, although some cracking may also take place in the effluent zone 16 (so-called “after-cracking”). In the reactor configuration shown in Figure 3, the effluent 34 containing olefins, i.e. the effluent stream containing the cracked products, flows downwards through the effluent zone 16 of the reactor 10 around the outside of the 5 lance 32. This exemplified counter-current flow arrangement not only provides for the feedstream containing gaseous hydrocarbons 28 and the steam stream 23 to collide and meet as substantially opposing streams and mix head-on in the 10 mixing and cracking zone 14, but it also allows for indirect heat-exchange to take place between the cooler feed stream containing hydrocarbons 28 flowing upwards through the lance 32 and the resultant effluent 34 flowing downwards around the outside of the lance 32 in the effluent zone 16. The 15 resultant effluent 34 flowing downwards around the outside of the lance 32 is at a higher temperature than the feedstream containing gaseous hydrocarbons 28 inside the lance 32 and cools as it flows downwards towards the base of the reactor 10. Any remaining hydrocarbons being cracked in the effluent 20 zone 16 will also be at a higher temperature than the feedstream containing gaseous hydrocarbons 28 inside the lance 32. The temperature at which the cracked effluent 34 containing the desired olefin products leaves the effluent zone 16 depends somewhat on the composition of the 25 hydrocarbon feedstock used. Typically, the effluent 34 leaves the effluent zone 16 at a temperature of around 700-800 °C. As discussed earlier, when such a hydrocarbon injection arrangement as shown in Figure 3 is present in the reactor 10, the feedstream containing gaseous hydrocarbons 28 30 advantageously only needs to be pre-heated, if at all, outside the reactor 10 to a temperature that is lower than the desired target temperature of the feedstream containing gaseous hydrocarbons 28 as it reaches the mixing and cracking zone 14, because heat transfer will take place along the length of the lance 32 from the warmer effluent 34 containing the desired olefin products flowing downwards outside the lance 32 to the cooler feedstream containing gaseous 5 hydrocarbons 28 flowing upwards inside the lance 32. Because the waste plastics pyrolysis feedstream is already hot at temperatures 400-600°C only a small adjustment of temperature is likely required. The temperature of the feedstock is optimized by adjusting the specific lance length in the 10 autothermal reactor. The feedstream containing gaseous hydrocarbons 28 flowing upwards through the lance 32 also helps to rapidly cool the effluent 34 in the effluent zone 16. A longer lance 32 will provide for increased indirect 15 heat transfer between the cooler feedstream containing gaseous hydrocarbons 28 flowing upwards in the lance 32 and the effluent 34 flowing downwards around the lance 32. Although in practice there may be a limit on the maximum desirable length of the lance depending on engineering and 20 construction considerations, such as vibration of the lance. Supports can be used to stabilise the lance to reduce/prevent vibration of the lance. The reactor configuration shown in Figure 4 has a somewhat shorter lance than that shown in Figure 3, and the 25 reactor configuration shown in Figure 5 has a much shorter lance than that shown in Figure 3. In the configuration shown in Figure 5, there will be much less heat transfer than in the configuration shown in Figure 3 as the lance does not extend the full length of the effluent zone 16. This 30 configuration can be used if the temperature of the waste plastics pyrolysis feedstream entering reactor 10 is close to the desired hydrocarbon temperature just before it contacts the steam 23 in the mixing and cracking zone 14. As discussed earlier, the indirect heat-exchange flow arrangement shown in Figures 3 to 5 is not essential to the process of the present invention, but it is a beneficial feature that arises due to the feeding/injection of 5 hydrocarbons 28 through the lance 32 to create opposing streams (i.e. the steam stream 23 and the feed stream containing hydrocarbons 28) in the mixing and cracking zone 14. Optionally, there may be a separate heat exchanger 35 10 present in the reactor. Typically, such a separate heat exchanger would be present in the effluent zone 16 of the reactor 10. The heat exchanger 35 may also be used to pre- heat the feedstream containing gaseous hydrocarbons 28 outside the autothermal reactor. Such a heat exchanger may be 15 used to pre-heat the oxygen containing stream 20 and/or the hydrogen and/or methane containing stream 22 before they enter the burner 11. As mentioned earlier, the process of the present invention provides improved olefin yields and selectivities 20 compared to those obtainable with conventional steam cracking of hydrocarbons in a cracker furnace. As a consequence, fewer less desirable secondary products are made. From the effluent zone 16, an effluent 34 is obtained that comprises olefins which may include one or more of 25 ethylene, propylene, butylenes and butadiene, and hydrogen, water, carbon monoxide and carbon dioxide, and that may comprise aromatics (as produced in the cracking process) which may include one or more of benzene, toluene and xylene. The specific products obtained depend on the composition of 30 the feed stream, the hydrocarbon-to-steam ratio, the cracking temperature and the cracking time. Where acetylene is produced as a secondary product, it can be hydrogenated to ethylene in a further catalytic step. Depending on the composition of the resultant effluent 34 and the desired products, at least a portion of the effluent 34 output from the autothermal reactor 10 can undergo further downstream processing and/or separation in a conventional 5 steam cracker unit. The process of the present invention can be operated at higher pressures than those used in conventional steam cracking of hydrocarbons in a cracker furnace due to the higher cracking temperatures used in the present process. 10 Higher operating pressures reduce the capital expenditure associated with the autothermal reactor. The ratio of steam to hydrocarbons used in the process of the present invention is higher than that used in the conventional steam cracking of hydrocarbons in a cracker 15 furnace. This contributes to the improved olefin yields and selectivities observed when using the process of the present invention and also the reduction of coke formation observed when using the process of the present invention. The invention is further illustrated by the following 20 Examples. Examples Example 1 – Cracking of waste plastics pyrolysis gas In the experiments of Example 1, a simulated waste plastics pyrolysis feedstream containing gaseous hydrocarbons 25 mixed with a diluent (steam and/or nitrogen) was fed to a vertically oriented, tubular, alumina reactor placed in a radiation oven. The waste plastics pyrolysis feedstream containing gaseous hydrocarbons was simulated by evaporating a commercially available liquid pyrolysis oil and mixing it 30 with gaseous C1-C4 hydrocarbons at a ratio 9:1 (kg:kg based on hydrocarbons). The composition of the C1-C4 gas mixture is given in Table 2. Figure 6 depicts the oven and reactor configuration used in Example 1 and is described in more detail below. The alumina reactor tube had an inner diameter of 0.41 mm and a length of 200 mm. A section of 8 cm of the reactor tube was 5 placed in a temperature-controlled radiation oven. The radiation oven had a length of 8 cm; the isothermal zone of the oven was approximately 6.0 cm. The isothermal zone subsequently had a free volume of 0.008 ml. The temperature of this isothermal zone was measured for 10 a reactor with a 0.79 mm internal diameter with a type N thermocouple having a 0.5 mm outer diameter and a length of 300 mm. This thermocouple was mounted inside of the alumina reactor. A temperature profile along the length of the alumina reactor was measured, this was done at several 15 temperature settings and gas flows. Figure 7 shows an example of a typical temperature profile at an oven setpoint of 1100 °C. Table 1 summarizes the conditions of 5 experiments as well as eight reference conditions also pertaining to 20 conventional cracking operation at increased Steam-to-oil- ratio (i.e. STOR = 1.60) to match the STOR of autothermal cracking. STOR is the ratio (Steam + Inert gas)/Plastic Pyrolysis oil (PP oil, which was evaporated) on mass base. Reference conditions were calculated using the commercially 25 available steam cracker model “SPYRO” which is commonly used to calculate the cracker performance at a certain condition. A reactor with an inner diameter of 3.06 mm and a length of 748 mm was used for the SPYRO calculation. Experiments were done at several temperatures. Flow 30 rates are reported in Nl/hr where “Nl” stands for “normal litre as measured at standard temperature and pressure, or in gr/hr. Liquid Flow rates are reported in gr/hr. The reported residence times were calculated on the basis of the (calculated) flow rates at actual average temperature and pressure in the isothermal zone. 5 Table 1 Exp. Temp Residence N2 H2O PP oil C1-C4 STOR °C i l/h /h /h /h

cace co os.

Table 2 Hydrocarbon Analytical value vol% Th

e e pe e a esu s o e co o s o Table 1 are given in Table 3 below. The total off gas flow rate was 5 calculated, using nitrogen as internal standard, from the results of on-line gas chromatograph (GC) analyzers for the feed and product gas streams. From this total off gas flow the individual component flows were calculated in Nl/hr, from which the yield of each component was calculated in wt%. 10 Component x = gas flow : 22.4 * Molar mass / liquid flow * 100 Gas flow Nl/hr 15 22.4 Nl/mol Molar volume Mol.M. Molar mass (gr/mol) Flow rate gr/hr C3, C4 and C0-C4 is the sum of the different non-aromatic 20 hydrocarbons of each number of carbon atoms. Table 3 Exp. H2 CO⁽¹⁾ CH4 C2H6 C2H4 C2H2 C3H6 C3H4 C3 C4 C0-

. (¹⁾ In none of the experiments, carbon dioxide (CO2) was detected in the effluent. With autothermal cracking, advantageously higher yields of C2 + C3 alkenes are reached compared with conventional cracking. Furthermore, with autothermal cracking, advantageously, relatively more ethylene and less propylene are produced. Another advantage of using autothermal cracking is that less CH₄ is produced compared to conventional cracking. Example 2 – Assessment of mixing configurations Computational Fluid Dynamics (CFD) simulations of various potential commercial scale mixing configurations were performed focusing on achieving the required fast mixing of the hot steam and colder hydrocarbon, i.e. the mixing time scale being at least as short as the time of the cracking reaction. The hydrocarbon used in the simulations was dodecane. Dodecane has 12 carbon atoms which carbon number is 5 representative of that of hydrocarbons in a waste plastics pyrolysis feedstream containing gaseous hydrocarbons. Figure 8 shows the results for an autothermal reactor where the dodecane is introduced into the reactor via 6 slits on the side of the reactor. This would be an obvious choice 10 for those skilled in the art after applying design rules from e.g. jet theory. Figure 8 shows that despite having applied these design rules and injecting the dodecane at high velocity sideways into the hot steam, the mixing is very non-uniform and local 15 circulation patterns are observed. If applied in practice, from the results in Example 1 it follows that in these non- uniform mixing zones there will be residence times (much) longer than the short residence times aimed for and hence reduction of the selectivity for ethylene (and acetylene) and 20 likely significant carbon formation as seen in conventional crackers operating at these longer residence times. Figure 9 shows the results of a mixing configuration according to an embodiment of the invention, i.e. introducing the dodecane and steam in a counter-current / opposing stream 25 fashion using a lance for introduction of the dodecane at high velocity into the mixing and cracking zone of the autothermal reactor. As shown, the mixing is fast and uniform and occurs in a very small region near the outlet of the lance where the two streams (the hot steam and the colder 30 dodecane) collide head-on. No circulation patterns are observed. Figure 9 also shows that a major part of the reaction takes place in that small region near the outlet of the lance with the desired high temperature near the tip of the lance and that in the effluent (after-cracking) zone the temperature is already low enough to prevent the loss in selectivity if exposed at longer residence times and high temperature such as in Figure 8 for the side injection 5 configuration. In both said simulations (Figures 8 and 9), the temperature and velocity of the steam stream when flowing into the mixing and cracking zone were 1834 °C and 105 m/s, respectively. Further, the temperature and velocity of the 10 dodecane stream when flowing into the mixing and cracking zone were 600 °C and 70 m/s, respectively. The cracking temperature in the mixing and cracking zone was 1,050 °C.

Claims

CLAIMS 1. A process for producing olefins from a waste plastics feedstock said process comprising: 5 pyrolyzing a waste plastics feedstream at a temperature in the range from 200°C to 600°C to produce a waste plastics pyrolysis feedstream containing gaseous hydrocarbons; feeding the waste plastics pyrolysis feedstream containing gaseous hydrocarbons into an autothermal reactor; 10 pre-heating an oxygen containing stream and a hydrogen and/or methane containing stream outside the autothermal reactor; feeding the pre-heated oxygen containing stream and the pre-heated hydrogen and/or methane containing stream into a burner of the autothermal reactor; 15 generating steam in a combustion zone of the autothermal reactor by the reaction of the pre-heated oxygen containing stream and the pre-heated hydrogen and/or methane containing stream; mixing the steam generated in the combustion zone with 20 the waste plastics pyrolysis feedstream containing gaseous hydrocarbons in a mixing and cracking zone of the autothermal reactor, by feeding the steam and the feedstream containing gaseous hydrocarbons into the mixing and cracking zone from substantially opposite directions, and pyrolytically cracking 25 the gaseous hydrocarbons to provide an effluent containing olefins. 2. The process according to claim 1, wherein the step of feeding the waste plastics pyrolysis feedstream containing 30 gaseous hydrocarbons into an autothermal reactor is carried out without condensing the waste plastics pyrolysis feedstream containing gaseous hydrocarbons into a liquid waste plastics pyrolysis oil stream. 3. The process according to claim 1 or 2, wherein the waste plastics pyrolysis feedstream containing gaseous hydrocarbons is subjected to a cleaning step before it is fed into the 5 autothermal reactor. 4. The process according to any of claims 1 to 3, wherein the step of feeding the pre-heated oxygen containing stream and the pre-heated hydrogen and/or methane containing stream 10 into the burner of the autothermal reactor further comprises feeding a pre-heated temperature moderator into the burner of the autothermal reactor. 5. The process according to claim 4, wherein the pre-heated 15 temperature moderator comprises steam and/or carbon dioxide. 6. The process according to any of Claims 1 to 5, wherein the oxygen containing stream is pre-heated to a temperature in the range of from about 200 °C to about 300 °C. 20 7. The process according to any of Claims 1 to 6, wherein the hydrogen and/or methane containing stream is pre-heated to a temperature in the range of from about 350 °C to about 650 °C. 25 8. The process according to any of Claims 4 to 7, wherein the temperature moderator is pre-heated to a temperature in the range of from about 350 °C to about 650 °C. 30 9. The process according to any of Claims 1 to 8, wherein the temperature of the steam generated in the combustion zone is in the range of from about 1200 °C to about 1900 °C. 10. The process according to any of Claims 1 to 9, wherein the steam generated in the combustion zone flows into the mixing and cracking zone at a velocity in the range of from about 100 m/s to about 400 m/s. 5 11. The process according to any of Claims 1 to 10, wherein the waste plastics pyrolysis feedstream containing gaseous hydrocarbons flows into the mixing and cracking zone at a velocity in the range of from about 10 m/s to about 300 m/s. 10 12. The process according to any of Claims 1 to 11, wherein the waste plastics pyrolysis feedstream containing gaseous hydrocarbons is pre-heated inside the reactor through indirect heat exchange between the effluent containing 15 olefins and the waste plastics pyrolysis feedstream containing gaseous hydrocarbons in an effluent zone of the reactor. 13. The process according to any of Claims 1 to 12, wherein 20 at least a portion of the effluent containing olefins undergoes further downstream processing and/or separation in a steam cracker unit.