The present invention relates to a process for recovering at least styrene monomer from a plastic material which contains at least one styrene-containing polymer. The plastic material is in particular a waste material, more in particular a post-consumer waste material from curb stone collection.

Waste materials, such as municipal solid waste, contain a relatively large fraction of polymeric materials, including polystyrene materials. These polystyrene materials are often landfilled or incinerated for energy recovery. Recycling of materials become however more and more important. The recovery of secondary materials is to be preferred over incineration or landfilling.

Research has therefore been done to examine the feasibility of a chemical recycling of polystyrene waste materials. Polystyrene- containing polymers can indeed be pyrolysed in particular in view of obtaining styrene monomer. Pyrolysis is a process wherein the polymeric material is degraded under non-oxidizing conditions by the effect of heat. The pyrolysis reaction is usually not catalysed but a catalyst may be used as disclosed for example in EP 0649827.

Pyrolysis of polystyrene can produce gasses, liquids and a solid residue. The gasses include in particular hydrogen, methane, ethane, ethylene, propane, propylene, butane, isobutane and butene whilst the liquids include in particular alpha-methyl-styrene, styrene, ethyl-benzene and toluene. The solid residu comprises mainly char. Of these products the low molecular weight liquids, and in particular styrene monomer, are the most valuable. Styrene monomer can indeed be reused to produce polystyrene. The gasses are less valuable and are usually only reused as a source of energy for the recycling process. ln his article “Chemical Recycling of Household Polymeric Wastes” (http://dx.doi.org/10.5772/65667). A. Karaduman describes a possible pyrolysis process for the chemical recycling of polystyrene waste materials. Pyrolysis was done in a nitrogen atmosphere at atmospheric pressure and at a temperature of 450°C. The total conversion was only 92.92%; 59.46% liquid was produced and 43.46% gas. The liquid itself only contained 55.52% of styrene monomer.

In order to increase the styrene monomer yield, DE 199 28 645 discloses to perform the pyrolysis of the polystyrene material at a reduced pressure of between 20 and 100 torr (between 0.027 and 0.133 bars) and at an increased temperature of between 600 and 800°C. As a result of these lower pressure and higher temperature conditions, a higher styrene yield could be achieved. In Examples 2 and 4 a styrene yield of respectively 70 and 68% was obtained in combination with a small amount of gasses (less than 1 %). The liquid product contained however a considerable amount of styrene oligomers (di- and trimers), which had to be removed by distillation since they cannot be reused in the production of polystyrene. The styrene oligomers were therefore of little value and they were used in DE 199 28 645 as fuel to heat the pyrolysis reactor.

A similar method is disclosed in JP2003267896. However this method comprises two pyrolysis steps. In the first step, the polystyrene material is pyrolised, in a first reactor, at a reduced pressure in particular at a pressure of 0.066 bar, and at a temperature of 700°C. Due to the low pressure in this first reactor, only a relatively small amount of compounds which have a lower boiling point than styrene, such as toluene, is produced, namely only about 3 to 5%. These low boiling compounds are discarded by means of a first distillation column. In a second distillation column, the pyrolysis product is further separated in a styrene monomer fraction and in a heavy oil. A portion of this heavy oil is used a fuel. In order to recover more styrene monomer, another portion of the heavy oil is re cracked, in a second step, by recycling this portion of the heavy oil either to the first or to a second reactor. This second reactor is operated also at a temperature of 700°C but at a higher pressure, namely at normal pressure, in order to recover more styrene monomer. The styrene monomer yield could thus be increased from 80 to 85%.

In JP2003267896 no mention is made of any char which would be produced in the first reactor. In view of the high styrene monomer yields, pure polystyrene waste material has apparently been used as plastic material. Such a pure polystyrene waste material produces only a small amount of char. The present inventors have found that for polystyrene waste materials which contain a lot of contaminants, such as in particular post-consumer polystyrene waste material from curb-stone collection, the method disclosed in JP2003267896 is less appropriate since it generates a lot of char during the pyrolysis of the contaminated waste material.

In the method disclosed in WO 2018/224482 the reaction conditions applied during the pyrolysis reaction are chosen such that a smaller amount of styrene monomer is obtained in combination with a relatively high amount of styrene oligomers. These oligomers are separated from the styrene monomer by a distillation process. They are then further cracked in a steam cracker to produce ethylene and propylene. A drawback of this method is that ethylene and propylene are less valuable than styrene monomer. Moreover, the installation required for this method has to comprise two different sections, namely a first section for producing the crude styrene product and for separating this crude product into the different liquid pyrolysis products and a second section for performing the steam cracking of the styrene oligomer fraction and for separating the different gasses produced thereby. An object of the present invention is now to provide a new two-step process for recovering styrene monomer and other low molecular weight liquid polystyrene pyrolysis products from a plastic material which contains at least one styrene-containing polymer which process enables to crack during the second step more of the styrene oligomers to increase the amount of the low molecular weight liquid polystyrene pyrolysis products, including styrene monomer and other low molecular weight liquid products such as alpha-methyl-styrene, cumene, ethyl-benzene and toluene, which are recovered.

The process according to the present invention comprises the steps of:

- heating the plastic material in a first pyrolysis reactor to a first temperature at which the plastic material is in a molten state;

- cracking said styrene-containing polymer at said first temperature, at a first gas pressure, in said first pyrolysis reactor to produce a first product which contains styrene monomer and styrene oligomers and which is gaseous at said first temperature and said first gas pressure, said first product forming a first gaseous phase in the first pyrolysis reactor and the molten plastic material which is being cracked therein a first liquid phase;

- removing the first product during the production thereof from the first pyrolysis reactor in particular to keep the first pyrolysis reactor at said first gas pressure;

- separating the first product which is removed from said first pyrolysis reactor in a first lighter fraction which contains at least part of said styrene monomer and in a first heavier fraction which contains at least part of said styrene oligomers; and

- feeding said first heavier fraction to a second pyrolysis reactor wherein said styrene oligomers are cracked at a second temperature, at a second gas pressure which is higher than said first gas pressure, to produce a second product which contains alpha-methyl-styrene, cumene, styrene monomer, ethyl-benzene and/or toluene and which is gaseous at said second gas pressure and said second temperature, the second product being removed from the second pyrolysis reactor during the production thereof.

Such a method is disclosed in JP2003267896. The process according to the present invention is however characterised in that said first gas pressure is lower than 1 .0 bara, said second gas pressure is higher than 1 .0 bara and said second temperature is lower than 650°C.

In accordance with the present invention it has been found that the styrene oligomers which are removed from the first product produced in the first pyrolysis reactor can be converted in the second pyrolysis reactor into one or more of the low molecular weight liquid polystyrene pyrolysis products. These low molecular weight liquid polystyrene pyrolysis products have a normal boiling point of at least 80°C, which normal boiling point is in particular lower than 240°C, and a molecular weight lower than the molecular weight of styrene dimer. They include in particular alpha-methyl-styrene, cumene, styrene monomer, ethyl-benzene and toluene. By performing the second pyrolysis step in a separate, second pyrolysis reactor, the pyrolysis conditions can be optimised in the first pyrolysis reactor in particular to increase the styrene monomer yield and/or to reduce the amount of solid residue formed in the first pyrolysis reactor whilst the pyrolysis conditions can be optimised in the second pyrolysis reactor in particular to achieve a high conversion of the styrene oligomers into one or more of the low molecular weight liquid polystyrene pyrolysis products. Generally the first pyrolysis reactor will be operated at a lower pressure than the second pyrolysis reactor, namely at a gas pressure lower than 1 .0 bara whilst the second pyrolysis reactor is operated at a gas pressure higher than 1 .0 bara. In the method according to the present invention a lower gas pressure, in particular a vacuum (i.e. a pressure of less than 1.0 bara), can be applied in the first pyrolysis reactor since the styrene oligomers which are produced in larger amounts under lower pressure conditions are subsequently converted into one or more of the low molecular weight liquid pyrolysis products in the second pyrolysis reactor. The second temperature in this second reactor should be lower than 650°C, or even lower. It has indeed been found that the oligomers should be treated thoroughly in order to produce as much as possible of the low molecular weight pyrolysis products. This could be achieved by the relatively low pyrolysis temperature, in combination with the higher gas pressure to keep the oligomers sufficiently long in the liquid phase of the second pyrolysis reactor.

Notwithstanding the different operating conditions, in particular the different gas pressure, applied in the first pyrolysis reactor, the low molecular weight liquid pyrolysis products, different from styrene monomer, are also produced, although in smaller amounts, in the first pyrolysis reactor. A similar separation process is thus required to separate the low molecular weight liquid polystyrene pyrolysis products present in the first product produced in the first pyrolysis reactor and the low molecular weight liquid polystyrene pyrolysis products present in the second product produced in the second pyrolysis reactor. The same separation section can thus be used, if necessary, for separating the different low molecular weight pyrolysis products which are present in the first and in the second product.

In the process according to the present invention the temperatures and pressures in the different reactors, in particular the first and the second temperature and the first and the second gas pressure, are preferably kept substantially constant but they may fluctuate within predetermined ranges. The temperatures may fluctuate in particular within temperature ranges around a predetermined value plus or minus 10°C whilst the pressures may fluctuate in particular within pressure ranges around a predetermined value, expressed in bara, plus or minus 10%.

In an embodiment of the process according to the present invention the second product, which is removed from the second pyrolysis reactor, is separated in a second lighter fraction which contains said alpha- methyl-styrene, cumene, styrene monomer, ethyl-benzene and/or toluene and in at least one second heavier fraction which contains styrene oligomers. Preferably, the second heavier fraction is recycled to the second pyrolysis reactor. In this embodiment the portion of the styrene oligomers which may arrive in the second product is removed therefrom and is preferably recycled to the second pyrolysis reactor for further cracking so that no waste material is thus generated by the styrene oligomers.

In an embodiment of the process according to the present invention, or according to any one of the preceding embodiments, said first gas pressure is lower than 0.5 bara, preferably lower than 0.1 bara and more preferably lower than 0.06 bara. The first gas pressure is in particular higher than 0 bara or even higher than 0.005 bara

It has been found that, for a same temperature, a lower gas pressure in the first pyrolysis reactor, results in a larger styrene monomer yield and in particular in a smaller yield of low molecular weight pyrolysis products which have a lower boiling point lower than the styrene monomer. At the same time, more styrene oligomers, in particular styrene dimers and trimers, will arrive in the first product as the boiling temperature of these oligomers is lower at lower gas pressures. Lower gas pressures may also result in a higher viscosity of the first liquid phase. An advantage of such a higher viscosity is that it may hamper the evaporation of higher styrene oligomers to a greater extent than the evaporation of styrene dimers and trimers so that less higher styrene oligomers will arrive in the first product. ln an embodiment of the process according to the present invention, or according to any one of the preceding embodiments, said first temperature is higher than 300°C, preferably higher than 400°C, more preferably higher than 450°C and most preferably higher than 475°C. In an embodiment of the process according to the present invention or according to any one of the preceding embodiments said first temperature is lower than 650°C, preferably lower than 625°C, more preferably lower than 600°C and most preferably lower than 575°C.

A higher temperature increases the pyrolysis or cracking rate of the styrene-containing polymer. Especially for the lower temperatures the pyrolysis or cracking rate can also be increased by means of a catalyst.

For the temperatures upto about 550°C the styrene monomer yield was moreover found to increase for a same pressure. At higher temperatures, the styrene monomer yield remained substantially constant or decreased somewhat. The production of styrene oligomers reduced consistently with increasing temperatures. However, for higher temperatures more solid residue was produced in the first pyrolysis reactor, in particular when the starting product contained a substantial amount of contaminants. A minimum amount of solid residues is important to achieve a high conversion and to avoid large amounts of waste material. Moreover, in case only a small amount of solid residues are produced, it is possible to stop the continuous pyrolysis process only from time to time to remove the solid residue from the first reactor. During the continuous phase of the pyrolysis process, an amount of liquid phase, which is preferably substantially constant, is maintained in the first reactor.

In an embodiment of the process according to the present invention, or according to any one of the preceding embodiments, said second gas pressure is higher than 2.0 bara, preferably higher than 4 bara, more preferably higher than 8 bara and most preferably higher than 12 bara. This embodiment is preferably combined with the embodiment wherein said second heavier fraction is recycled to said second pyrolysis reactor.

The higher the second gas pressure, the higher the boiling temperature of the styrene dimers and trimers and the lower the amount of styrene dimers and trimers that will arrive in the gaseous phase in the second pyrolysis reactor, i.e. in the second product. In this second pyrolysis reactor the viscosity of the liquid phase is considerably smaller than the viscosity of the liquid phase in the first pyrolysis reactor so that the styrene dimers and trimers can evaporate more readily from the liquid phase in the second pyrolysis reactor. A higher gas pressure thus reduces the amount of styrene oligomers in the gaseous second product so that less styrene oligomers have to be removed from this second product, and so that a more effective cracking of the styrene oligomers can be achieved in the second pyrolysis reactor.

Alternatively or additionally, the second gas pressure is preferably selected as a function of the temperature of the liquid phase in the second pyrolysis reactor. The second gas pressure is more preferably selected to be higher than the boiling point of a C24 hydrocarbon at the respective temperature in the second pyrolysis reactor. The second gas pressure is thus preferably higher than or equal to a minimum gas pressure Pmin which is defined by the following Equation 1 :

P _min = 2.8 10- ¹² T ^A wherein:

Pmin is the minimum second gas pressure, in bara, in the second reactor, and

Tis the second temperature, in degrees Celsius, in the second reactor. For the lower temperatures in the second reactor, in particular for temperatures lower than about 422°C, the second gas pressure should be at least 1 .0 bara. When combined with the previous embodiment, the minimum second pressures according to this previous embodiment should be applied for the lower second temperatures, i.e. for those temperatures for which Equation 1 would give a lower second gas pressure than the gas pressure defined by the previous embodiment.

In an embodiment of the process according to the present invention, or according to any one of the preceding embodiments, said second gas pressure is lower than 30 bara and preferably lower than 25 bara.

Although a higher second gas pressure is advantageous to achieve a more effective cracking of the styrene oligomers in the second pyrolysis reactor, the second gas pressure is preferably not too high to reduce the costs of the installation. Moreover, higher gas pressures may result in the production of more gasses, i.e. more light ends.

In an embodiment of the process according to the present invention, or according to any one of the preceding embodiments, said second temperature is lower than 600°C, preferably lower than 550°C, more preferably lower than 500°C and most preferably lower than 450°C.

An advantage of a lower second temperature is that the boiling point of the styrene oligomers is lower at these temperatures so that their content in the gaseous phase, i.e. in the second product, is reduced.

In an embodiment of the process according to the present invention, or according to any one of the preceding embodiments, said second temperature is higher than 300°C, preferably higher than 350°C, or higher than 375°C or higher than 400°C.

A higher temperature increases the pyrolysis or cracking rate of the styrene oligomers. Especially for the lower temperatures the pyrolysis or cracking rate can also be increased by means of a catalyst.

In an embodiment of the process according to the present invention, or according to any one of the preceding embodiments, said first pyrolysis reactor and said second pyrolysis reactor are operated in accordance with a continuous process. A high production capacity can thus be achieved. During the continuous production process, the plastic material is fed at such a rate to the first pyrolysis reactor that the first pyrolysis reactor continuously contains an amount of the first liquid phase, preferably a substantially constant amount of the first liquid phase.

In an embodiment of the process according to the present invention, or according to any one of the preceding embodiments, it comprises the steps of:

- feeding a portion of said first liquid phase from said first pyrolysis reactor to a third pyrolysis reactor;

- cracking said portion of said first liquid phase at a third temperature, at a third gas pressure, in said third pyrolysis reactor to produce a third product which contains styrene monomer and styrene oligomers and which is gaseous at said third temperature and said third gas pressure, said third product forming a third gaseous phase in the third pyrolysis reactor and said portion of said first liquid phase which is being cracked therein a third liquid phase; and

- removing the third product from the third pyrolysis reactor during the production thereof in particular to keep the third pyrolysis reactor at said third gas pressure.

Although the pyrolysis conditions, in particular the temperature and the gas pressure conditions, can be optimized in the first pyrolysis reactor to minimize the production of solid residues, some amount of solid residue will be produced. In the first pyrolysis reactor the plastic material is cracked to evaporation, i.e. the first product is gaseous. Any solid residue will therefore remain in the first pyrolysis reactor and will in particular not be removed therefrom via the (gaseous) product stream.

In the pyrolysis method disclosed in DE 199 28 645 the pyrolysis reactor is a tubular reactor having at its downstream end a screw to remove the solid residue from time to time from the reactor. An advantage of the embodiment with the third reactor is that the functioning of the first pyrolysis reactor does not have to be interrupted to remove the solid residue. Moreover, the first liquid phase does not have to be cracked and evaporated completely to be able to remove the solid residue. Instead, any solid residue is transferred to the third pyrolysis reactor together with the portion of the first liquid phase which is fed from first reactor to the third one. To remove the solid residue from the third pyrolysis reactor, the operation thereof can easily be interrupted without discontinuing the functioning of the first pyrolysis reactor. A further advantage is that, especially when the third pyrolysis reactor has to be emptied, the reactor conditions can be changed for example to crack/remove the liquid phase more quickly or more completely.

In an embodiment of the process according to the preceding embodiment, the process comprises the step of adding the third product which is removed from said third pyrolysis reactor to the first product which is removed from said first pyrolysis reactor and subsequently separating the first and the third product into said first lighter fraction and into said first heavier fraction.

Both the first and the third pyrolysis product are thus further processed as one product so that the use of the third pyrolysis reactor does not complicate the further processing steps or the installation wherein these further processing steps are carried out.

In an embodiment of the process according to the preceding embodiments wherein use is made of the third pyrolysis reactor, said third pressure is lower than 10.0 bara, preferably lower than 7.0 bara and more preferably lower than 4.0 bara. The third gas pressure is in particular higher than 0 bara or higher than 0.005 bara.

A third gas pressure which is higher than the first gas pressure, i.e. than the pressure in the first pyrolysis reactor, may be advantageous to achieve a more effective and complete cracking of the different components of the plastic material, especially when this plastic material is a waste material which may comprise a portion of other materials different from said styrene-containing polymer. Such materials which are more difficult to crack will therefore also be pyrolysed thus producing less solid waste. The third gas pressure may be higher than 1 .0 bara or higher than 2.0 bara or higher than 3.0 bara.

In an embodiment of the process according to any one of the preceding embodiments wherein use is made of the third pyrolysis reactor, said third temperature is higher than 300°C, preferably higher than 400°C, more preferably higher than 450°C and most preferably higher than 475°C. In an embodiment of the process according to any one of the preceding embodiments wherein use is made of the third pyrolysis reactor, said third temperature is lower than 650°C, preferably lower than 625°C, more preferably lower than 600°C and most preferably lower than 575°C.

When operating the third pyrolysis reactor in the same way as the first pyrolysis reactor, i.e. while maintaining an amount of third liquid phase therein which is preferably substantially constant, the selection of these temperatures produces the same advantages as described herein above for the first pyrolysis reactor. When feeding of first liquid phase into the third pyrolysis reactor has stopped, other, in particular higher temperatures may be used to remove the third liquid phase more quickly and/or completely from the third pyrolysis reactor.

In an embodiment of the process according to the present invention, or according to any one of the preceding embodiments, said second pyrolysis reactor is different from said first pyrolysis reactor.

In this embodiment the first gaseous product can thus be produced continuously in the first pyrolysis reactor whilst the second gaseous product is produced simultaneously in the second pyrolysis reactor. In this way, long production runs are possible and a high production capacity can be achieved. In an alternative embodiment, said second pyrolysis reactor is the same as said first pyrolysis reactor, said first heavier fraction being fed into said first pyrolysis reactor after having stopped producing said first product therein. The first pyrolysis product can be produced for some time, preferably in accordance with a continuous process, in the first pyrolysis reactor and the first heavier fraction removed therefrom can be stored in one or more storage tanks. The production run can be continued until the storage tanks are filled with first heavier fraction. At the end of the production run, feeding of plastic waste material to the reactor is stopped and the pyrolysis process is preferably continued until only a dry product is left in the reactor, in particular dry char. It is possible to increase the temperature of the reactor in this final stage of the production run. Subsequently, the char can be removed from the first pyrolysis reactor and this first pyrolysis reactor can then be charged with the first heavier fraction from the storage tanks and can be operated under the second pressure and temperature conditions.

Other advantages and particularities of the present invention will become apparent from the following description of some particular embodiments of the process according to the invention. This description is only given by way of example and is not intended to limit the scope of the invention. The reference numerals used in the description relate to the annexed drawings wherein:

Figure 1 is a schematic flow diagram illustrating a preferred embodiment of the process according to the present invention; and

Figure 2 is a graph showing the total volatiles production as a function of the temperature and pressure in the second reactor as applied in the tests performed with the second reactor to pyrolyse the heavier, oligomer fraction therein. The invention generally relates to a process for recovering at least styrene monomer from a plastic material which contains at least one styrene-containing polymer. This styrene-containing polymer is called herein after polystyrene. The plastic material will usually be a waste material. This waste material may be production waste and/or post consumer waste. The plastic material may thus be polluted with other materials in particular with other plastic materials but also with non-plastic waste materials such as biopolymers (proteins, polysaccharides, etc.), mineral, and metals. However, it preferably contains at least 90% by dry weight, or more preferably at least 95% by dry weight of polystyrene.

A particular embodiment of the process according to the invention is illustrated in Figure 1 . In particular the different pipes, reactors, distillation columns and other components are indicated schematically in this diagram but the necessary valves and pumps have not been indicated as a skilled person knows how and where to provide such pumps and valves depending on the pressure conditions which are described herein after.

In a first phase of the process illustrated in Figure 1 , the plastic material is molten in an extruder 1 . The solid plastic material in fed into an inlet 2 of the extruder 1 and leaves the extruder 1 through its outlet 3. When moving from the inlet 2 to the outlet 3 the plastic material is gradually heated to a temperature of for example 300°C so that it is in a molten state at the outlet 3. When it is heated to a temperature of about 100°C, any moisture will evaporate and is removed via a moisture evacuation pipe 4. Near the outlet 3 of the extruder 1 , the gasses which may already have been produced in the extruder 1 by a partial decomposition of the plastic material are removed via an outlet 5 and are fed via a pipe 6 to an incinerator device 7 to recover the energy contained in those gasses. The incinerator device 7 preferably comprises a combined heat and power generator. The molten plastic material produced in the extruder 1 is supplied via a pipe 8 to a first pyrolysis reactor 9. This reactor 9 contains a first liquid phase 10 which is heated to a first temperature. The liquid phase 10 is preferably stirred by means of a stirring device 11 to achieve a uniform temperature in the liquid phase 10. The temperature of the liquid phase 10 is preferably maintained at a predetermined temperature between 300 and 650°C, more preferably between 450 and 600°C. The first temperature is selected to achieve a pyrolysis of the molten polystyrene. The gas pressure in the first pyrolysis reactor 9 is selected so that part of the pyrolysis products can evaporate from the liquid phase 10 and produce a first product forming a first gaseous phase 12 above the liquid phase 10 in the pyrolysis reactor 9. This gas pressure is lower than 1 .0 bara, preferably lower than 0.5 bara and more preferably lower than 0.1 bara or even lower than or equal to 0.06 bara. This first gas pressure is for example equal to about 0.05 bara or about 0.02 bara.

The first product should contain first of all styrene monomer which can be reused in the production of new polystyrene. Gaseous products such as hydrogen, methane, ethane, ethylene, propane, propylene, butane, isobutane and butene are preferably not produced or only in a relatively small amount. In addition to styrene monomer, other low molecular weight liquid polystyrene pyrolysis products such as alpha- methyl-styrene, cumene, ethyl-benzene and toluene may also be produced, in addition to styrene monomer.

The first temperature and the first gas pressure can be selected to optimize the production of styrene monomer in the first pyrolysis reactor. Preferably, the first temperature and the first gas pressure are also selected to minimize the production of solid residue, in particular of char. In the process according to the present invention, the first temperature may be so high and the first gas pressure so low that a relatively large amount of styrene oligomers may also arrive in the first gaseous phase. These styrene oligomers will indeed be cracked further during a next phase of the pyrolysis process as described herein after.

The first gaseous phase 12, i.e. the first product produced in the first pyrolysis reactor 9, is continuously removed from the first pyrolysis reactor 9, via pipe 13, to keep the first pyrolysis reactor 9 at the first gas pressure. This first gas pressure may fluctuate to some extent but is preferably kept substantially constant. The gaseous first product is fed via the pipe 13 to a first quencher 14, provided with a cooler 15, which quencher 14 is preferably at a pressure which is substantially equal to the gas pressure in the first pyrolysis reactor 9.

In the quencher 14 the first product is separated in a first lighter fraction which contains most of the styrene monomer and in a first heavier fraction which contains most of the styrene oligomers. At the top, the light ends, i.e. the gases, are removed via a pipe 16 to the incinerator device 7. The first lighter fraction is fed via a pipe 17 to a crude styrene monomer reservoir 18 whilst the first heavier fraction is fed via a pipe 19 to a styrene oligomer reservoir 20.

The first lighter fraction contains in particular styrene monomer and alpha-methyl-styrene, cumene, ethyl-benzene, toluene and benzene. The first lighter fraction is fed via a pipe 21 to a first distillation column 22, which is provided with a condenser 23 and a reboiler 24. At the bottom a heavy fraction, which contains alpha-methyl-styrene and cumene is removed from the first distillation column 22 and is fed via a pipe 25 to an alpha-methyl-styrene and cumene reservoir 26. If necessary, these two polystyrene pyrolysis products can further be separated from one another. The light fraction, which is removed at the top contains styrene monomer and ethyl-benzene, toluene and benzene, is fed via a pipe 27 to a second distillation column 28, which is provided with a condenser 29 and a reboiler 30. At the bottom of the second distillation column a fraction is removed with a too high boiling point which is fed via a pipe 31 to a reservoir 32. This fraction can either be incinerated or it can be recycled to the crude styrene reservoir 18 (not indicated). At a somewhat higher level of the second distillation column the styrene monomer fraction is removed which is fed via a pipe 33 to a styrene monomer reservoir 34. At the top, a lighter fraction is removed, which contains ethyl-benzene, toluene and benzene, which is fed via a pipe 35 to an ethyl-benzene, toluene and benzene reservoir 36. If necessary, these three polystyrene pyrolysis products can further be separated from one another.

The styrene oligomer fraction contained in the styrene oligomer reservoir 20 is fed via a pipe 37 to a second pyrolysis reactor 38. This second pyrolysis reactor 38 is preferably a so-called vis breaker wherein the viscosity of the first heavier fraction is reduced.

The second pyrolysis reactor 38 is preferably a reactor different from the first reactor 9. However, it is also possible to use the first reactor 9 also for re-cracking the styrene oligomer fraction stored in the styrene oligomer reservoir 20. In that case, the production of the first product has first to be stopped in the first reactor and the reactor conditions have to be adjusted in the same way as explained here after for the second reactor 38 which is different from the first reactor 9.

The temperature of the liquid phase formed by the first heavier fraction which is being cracked in the second pyrolysis reactor, i.e. the second temperature, is preferably maintained at a predetermined temperature between 300 and 650°C, more preferably between 350 and 500°C and most preferably between 400 and 450°C. This second temperature is selected to achieve a pyrolysis of the styrene oligomers. The second temperature may fluctuate to some extent but is preferably kept substantially constant. The gas pressure in the second pyrolysis reactor 38 is preferably selected so that evaporation of the styrene oligomers is reduced to enhance the cracking thereof in the liquid phase. This second gas pressure is preferably higher than 1 .0 bara, more preferably higher than 4 bara and most preferably higher than 8 bara or even higher than 12 bara. This second gas pressure is for example equal to about 15 bara.

The second gas pressure is preferably selected as a function of the second temperature in the second reactor 38. The higher this temperature, the higher the vapour pressure of the pyrolysis products produced in the reactor, in particular the vapour pressure of the styrene trimers. The minimum value P _m/n of the pressure which is maintained in the second reactor is preferably in the order of magnitude of the vapour pressure of styrene trimer. The second gas pressure is thus preferably higher than or equal to the minimum gas pressure Pmin which is defined by the following Equation 1 :

P _min = 2.8 10- ¹² G ⁴ · ⁴ wherein:

Pmin is the minimum second gas pressure, in bara, in the second reactor, and

Tis the second temperature, in degrees Celsius, in the second reactor, with the provisio that the second gas pressure should always be higher than 1 bara, also for the lower temperatures in the second reactor, in particular for the temperatures lower than about 422°C since, according to Equation 1 , this temperature corresponds to a pressure of 1 .0 bara.

The graph of the Equation 1 between the temperature in the second reactor and the minimum gas pressure therein is illustrated in Figure 2.

The gaseous phase produced in the second pyrolysis reactor 38, i.e. the second product which contains low molecular polystyrene pyrolysis products and also still a portion of the styrene oligomers, is fed via a pipe 39 to a second quencher 40, provided with a cooler 41 . In the quencher 40 the second product is separated in a second lighter fraction which contains most of the low molecular weight polystyrene pyrolysis products and in a second heavier fraction which contains the remaining styrene oligomers. At the top, the light ends, i.e. the gases, are removed via a pipe 42 to the incinerator device 7. The second heavier fraction is recirculated via a pipe 43 to the styrene oligomer reservoir 20 whilst the second lighter fraction is fed via a pipe 44 to a third distillation column 45, provided with a reboiler 46 and a condenser 47.

At the bottom of the third distillation column 45 a fraction is removed with a too high boiling point which is recirculated via a pipe 48 to the styrene oligomer reservoir 20. At a somewhat higher level of the third distillation column 45 a fraction is removed which contains mainly alpha- methyl-styrene and cumene and which is fed via a pipe 33 to the alpha- methyl-styrene and cumene reservoir 26. At the top, a lighter fraction is removed, which contains ethyl-benzene, toluene and benzene, which is fed via a pipe 50 to the ethyl-benzene, toluene and benzene reservoir 36.

The lighter fraction may also still contain a small amount of styrene monomer. This lighter fraction could therefore alternatively be fed to the crude styrene reservoir 18 instead of directly to the ethyl-benzene, toluene and benzene reservoir 36. In this way, also this small amount of styrene monomer could thus additionally be recycled. However, in practice, it may be preferable to feed this lighter fraction directly to the reservoir 36 so that the first and the second distillation columns are less loaded.

In order to enable to remove the solid residue produced in the first pyrolysis reactor 9 without having to stop the operation thereof, the installation illustrated in Figure 1 comprises a third pyrolysis reactor 51. This third pyrolysis reactor 51 is fed through a pipe 52 with first liquid phase 10 removed from the first pyrolysis reactor 9. The first liquid phase 10 is removed from the bottom of the first pyrolysis reactor 9 so that together with this first liquid phase 10 any solid residue is transferred from the first pyrolysis reactor 9 to the third pyrolysis reactor 51 .

This reactor 51 contains a third liquid phase 53 which is heated to a third temperature. The liquid phase 53 is preferably stirred by means of a stirring device 54 to achieve a uniform temperature in the liquid phase 53. The temperature of the liquid phase 53, i.e. the third temperature, is preferably maintained at a predetermined temperature between 300 and 650°C, more preferably between 450 and 600°C. The third temperature is selected to achieve a pyrolysis of the oligomeric or polymeric materials contained in the liquid phase 53. The gas pressure in the third pyrolysis reactor 9 is selected so that part of the pyrolysis products can evaporate from the liquid phase 53 and produce a third product forming a third gaseous phase 55 above the third liquid phase 53 in the pyrolysis reactor 51. This gas pressure is preferably lower than 10.0 bara, more preferably lower than 7.0 bara and most preferably lower than 4.0 bara. This first gas pressure is for example equal to about 0.05 bara. The third gas pressure is in particular higher than 0 bara or higher than 0.005 bara. The third gas pressure is preferably higher than 1 .0 bara or higher than 2.0 bara or higher than 3.0 bara. Preferably it is comprised between 3.0 and 4.0 bara. The third gas pressure is preferably higher than the first gas pressure to obtain a more complete pyrolysis of the polymers and oligomers in the third pyrolysis reactor.

The third product contains i.a. styrene monomer and styrene oligomers. It is removed from the third pyrolysis reactor 51 to keep the gas pressure substantially constant therein. The third product is added via a pipe 56 to the first product which is produced in the first pyrolysis reactor 9 and is thus supplied together with the first product to the quencher 14 wherein the mixture of both products is separated into the first lighter fraction, containing the crude styrene monomer, and the first heavier fraction, containing most of the styrene oligomers. Instead of adding the third product to the first product before introducing the mixture into the quencher 14, they could alternatively also be fed separately to the quencher 14.

When solid residue has accumulated in the third pyrolysis reactor 51 , the feeding of liquid phase 10 from the first 9 to the third pyrolysis reactor 51 can be stopped and the pyrolysis reaction in the third pyrolysis reactor 51 can be continued until all of the liquid phase has been pyrolysed therein. The remaining solid residue can then be discharged into a solid residue container 57.

In case no third pyrolysis reactor 51 is used, the solid residue that has accumulated in the first pyrolysis reactor 9 can be discharged into a further solid residue container 58 after the feeding of molten plastic material from the extruder 1 is stopped and after the pyrolysis reaction in the first pyrolysis reactor 9 has continued until only solid residue is left therein. This solid residue can easily be removed from the first pyrolysis reactor. In case only one reactor is used, i.e. in case the second pyrolysis reactor is the same as the first pyrolysis reactor, the first heavy fraction stored in the styrene oligomer reservoir 20 can then first be pyrolysed in this pyrolysis reactor before starting to crack again plastic material therein.

The following experiments show the pyrolysis reactions that may be applied in the above described embodiment.

Experiments

Pyrolysis phase 1 : tests with post-consumer polystyrene

Sorted post-consumer polystyrene (PS) waste from curb-side collection with not more than 10% of impurities and other plastics was fed into an extruder. The extruder was used to melt the PS waste and to bring it to the 320°C. The flow rate through the extruder was comprised between 2 and 3 kg/h.

The PS melt leaving the extruder at 320°C was continuously fed into a cracker, i.e. into a pyrolysis reactor. The cracker was a continuously stirred tank reactor having a volume of 5L. Depending on the processing conditions in the reactor, the holdup, i.e. the liquid phase, fluctuated between 0.5 and 2L. The holdup consisted of molten, partially cracked polymers. Heating of the reactor was provided externally, via heating rods that radiated heat to the stainless steel reactor body. Temperature in the reactor was measured in the liquid phase and kept at the desired value.

Initially the reactor was purged with nitrogen. After that a pre determined pressure was provided in the reactor by means of a vacuum pump for creating a vacuum or by injection of nitrogen gas to create a pressure in the reactor. The gas pressure in the reactor was measured in the upper part of reactor effectively indicating gas pressure. During feeding and cracking of the plastics melt, the pressure was maintained constant.

The feeding of the plastics melt was done for a couple of hours. Samples were taken about every 30 minutes and the analysis results indicated below are an average of 3 samples.

The gaseous product formed in the reactor was routed via a set of 3 condensers kept at different temperatures to enable effective condensation of the gaseous product. The first condensation was done at a higher temperature within the range of 40-60°C where the following two condensation steps were performed at lower temperature between -10° and 20°C. The pressure in the condensation train was the same as the pressure in the reactor. The liquid fractions collected at the different condensation stages were mixed together and were subsequently analysed as a whole. The cracking conditions and the resulting compositions of the products obtained in the different tests are indicated in Table 1. Table 1 : Cracking conditions and compositions (in wt.%) of the condensed cracking product obtained in phase 1 of the cracking process with contaminated post-consumer polystyrene. When comparing test 1 with test 4 and test 2 with test 5 it can be seen that when increasing the temperature from 450°C to 550°C while keeping the gas pressure constant, the styrene yield increases and the styrene oligomer production decreases. When further increasing the temperature to 600°C, the styrene yield remains substantially the same, or even has a tendency to decrease, whilst the styrene oligomer production still decreases. At the same time, more residue is produced. Since residue is a waste product whilst oligomers can still be further processed in phase 2, the optimal reaction temperature in the first cracking phase is situated between 550 and 600°C. For each of the tested temperatures, a decrease of the gas pressure from 1 bara to 0.02 bara resulted in an increase of the styrene yield and at the same time in an increase of the styrene oligomer production. Working under vacuum conditions during the first cracking phase is thus clearly to be preferred.

In accordance with the present invention it has been found that the produced styrene oligomers are not lost but can be converted in phase 2 of the pyrolysis method of the present invention into valuable low molecular weight styrene pyrolysis products.

Pyrolysis phase 1 : tests with pure general purpose polystyrene

A number of tests were done in the same way as described in the previous tests but now with pure general purpose polystyrene (GPPS) instead of with contaminated post-consumer polystyrene. It appeared that much less residue/solids were produced and that much higher styrene monomer yields could be achieved. In practice, however, the present invention is in particular intended to be able to recycle contaminated post-consumer polystyrene waste material. The results of the tests done with the pure GPPS are indicated in Table 2.

Table 2: Cracking conditions and compositions (in wt.%) of the condensed cracking product obtained in phase 1 of the cracking process with pure GPPS. The different amounts were determined by GC-FID. A number of compounds could not be measured by means of this technique, in particular the higher oligomers so that the total percentage does not equal 100%. In these tests, nearly no solid residue was formed.

It can be seen that at the applied low pressure, an increase of the temperature increases the styrene monomer yield. Much higher styrene monomer yields can be obtained with pure polystyrene than with contaminated post-consumer polystyrene. In case of post-consumer polystyrene, the high temperature of 600°C also resulted in the highest styrene monomer yield but was not suited do to the fact that more residue/solids were produced and less styrene oligomers which can still be further cracked. Pyrolysis phase 2

A polystyrene material was pyrolysed partially as described hereabove and the oligomer fraction was distilled from the partially cracked liquid fraction. The oligomer fraction contained styrene dimers and trimers with some volatile products (including styrene, toluene, ethyl-benzene, cumene and alpha-methyl-styrene) that were still present in the distilled oligomer fraction.

1 kg of this oligomer fraction was fed into a reactor. The oligomer fraction had a viscosity of between 10 and 30 cP at normal condition. The system was pressurized in the reactor with nitrogen gas and was heated to further crack the oligomers. The applied pressures and temperatures are indicated in Table 3. When the indicated temperature has been reached, a sample of the produced gaseous product was taken after 15 and 30 minutes. The average compositions of these two samples, and of the composition of the initial oligomer fraction are indicated in Table 3. Table 3: Cracking conditions and compositions (in wt.%) of the initial oligomer fraction and of the gaseous cracking products obtained in phase

2 of the cracking process.

The different amounts were determined by GC-FID. A number of compounds could not be measured by means of this technique, in particular the higher oligomers so that the total percentage does not equal 100%.

When cracking under vacuum conditions (see T est 12) nearly no volatiles were produced by the pyrolysis reaction. The trimer content was slightly reduced but, as a result thereof, the dimer content increased.

Increasing the pressure to 5 bara enabled, at a temperature of 500°C, to reduce the dimer content and the trimer content and to produce more volatiles (see Test 13). An increase of the temperature to 550°C resulted at this pressure in a lower volatiles content and in a lower dimer content (see Test 14). However, the trimer content was again increased.

During the second pyrolysis phase the dimer/trimer fraction could be reduced at a pressure of 5 bara with about 50% and valuable volatiles could be produced, in particular toluene, ethyl-benzene and cumene. The volatile fraction also contained some styrene and alpha- methyl-styrene. The volatiles contained an amount of styrene and alpha- methyl-styrene which was similar to the amount of styrene and alpha- methyl-styrene that was already present in the oligomer fraction after the first pyrolysis phase so that the second pyrolysis phase apparently produced rather other volatile compounds different from styrene and alpha- methyl-styrene.

In tests 15 and 16 a higher pressure was applied during the second pyrolysis phase both. At a temperature of 500°C (see test 16) the total amount of volatiles and the total amount of dimers and trimers remained substantially the same when increasing the pressure from 5 to 10 bara. However, the combination of a higher pressure and a lower temperature (see test 15) resulted in a much higher amount of total volatiles and a much lower oligomer content. Increasing the pressure from 5 to 10 bara and at the same time reducing the temperature from 500 to 400°C doubled the amount of volatiles and reduced the oligomer content with nearly 50.

A gas pressure of 10 bara and temperatures from 400 to 450°C enabled to crack considerably more of the styrene dimers and trimers, namely about 70% thereof. The volatile product produced under these conditions mainly consisted of ethyl-benzene, toluene, cumene, alpha-methyl-styrene and styrene. The amount of styrene was in this case also considerably increased. A higher pressure, for example a pressure of 15 bara, will enable to crack even more of the styrene dimers and trimers. Further tests were done with other temperatures and pressures and with another oligomer fraction. These tests were done to examine the effect of the pressure on the yield of the low molecular weight pyrolysis products, i.e. on the yield of total volatiles. The results of these tests are indicated in Table 4.

Table 4: Cracking conditions and compositions (in wt.%) of the gaseous cracking products obtained in phase 2 of the cracking process.

The volatiles also included a number of unidentified volatiles. The yield of total volatiles of tests 12 to 22 are indicated in

Figure 2. ln Table 4 it can be seen that, especially at lower temperatures, an increase of the pressure reduced the amount of dimers and trimers and increased the amount of total volatiles.

The styrene monomer content was always very low. This might be due to the fact that the gaseous product is cooled down only quite slowly during the three-step condensing process. During this condensing phase, the low molecular weight monomers may be unstable and may polymerize to produce new oligomers. Consequently, it can be expected that when quenching the gaseous product instead of condensing it quite slowly, more styrene monomer and other volatiles will be produced and less oligomers.

As to the total volatiles content, i.e. the amount of low molecular weight pyrolysis products, it can be seen in Figure 2 that depending on the temperature in the second pyrolysis reactor the pressure therein should be high enough, and should in particular be higher than the minimum pressure as determined by Equation 1.

The total volatiles content obtained in test 12 (550°C; 0.02 bara) was indeed very low. By increasing the pressure to 5 bara (test 14), the total volatiles content increased from 10.8% to 26.1%. When reducing the temperature to 500°C whilst maintaining the pressure at 5 bara (test 13) the total volatiles yield further increased to 34.7%.

In the series of tests 17 to 19 and 20 to 22 it can be seen that when increasing the pressure, the yield of total volatiles is always increased. This yield was again higher for the tests done at lower temperature (425°C) than for the tests done at a higher temperature (500°C).