PROCESS FOR DEPOLYMERIZING A POLYSTYRENE CHARGE BY PYROLYSIS

Abstract

The invention relates to a process for depolymerizing a polystyrene filler comprising at least two pyrolysis sections, in which the hydrogen partial pressure and / or the residence time of the gas phase are controlled in the pyrolysis section of step d). Fig.2

Description

Description Title of the invention: DEPOLYMERIZATION PROCESS OF A POLYSTYRENIC FILLER BY PYROLYSIS Technical field of the invention

[0001] — The present invention relates to the field of depolymerization processes of polystyrene compounds, in particular polystyrene, with a view to producing at least a styrene monomer. Anterior artery

[0002] Styrene is a monomer that is very widely used in industry, whether by example for the production of polystyrene, whose areas of application are multiple, or the production of elastomers such as styrene-butadiene rubber (SBR). It can be obtained in multiple ways, the main one being by dehydro- generation of ethylbenzene, or to a lesser extent by the oxidation of ethylbenzene followed by the reaction on propylene and then the dehydration of product obtained.

[0003] — With a view to reducing pressure on fossil resources, developments Recent developments have focused on the depolymerization of styrenic compounds such as than polystyrene. Such a process is, for example, described in the application US 2021 / 0277202.

[0004] — During these processes, the polystyrene is decomposed at high temperature in an en- anaerobic environment in lower molecular weight compounds, including styrene. However, a significant amount of other products are also generated, including styrene oligomers. These oligomers can represent up to 40% by mass of the incoming polystyrene and consist mainly of dimers and styrene trimers, including in particular 1,3-diphenylpropane, 1,3-Diphenylbutene-1, 1,2-Diphenylpropane, 1,3-Diphenylbutane and the 1,4-Diphenylbenzene. The presence of these oligomers can be explained, on the one hand, by partial depolymerization reactions of polystyrene into oligomers, and other part by polymerization or radical recombination of styrene or oligomers in the depolymerization reactor and / or in the streams exiting this reactor. The US document 2021 / 0277202 proposes steam cracking of styrene oligomers products in order to generate lighter compounds such as ethene, propene or benzene.

[0005] Other avenues are conceivable for valorizing styrene oligomers and to improve the overall styrene yield of these processes, US document 10,731,080 proposes recycling these oligomers in the pyrolysis reactor feedstock. This recycling approach, which is classically implemented in processes for although the conversion into a product of interest is incomplete, it is nevertheless not totally satisfactory in this case. Indeed, at ambient pressure, oligomers have a boiling point close to the decomposition temperature of polystyrene. Recycling oligomers in the depolymerization reactor affects the chemistry of polystyrene decomposition by reducing the selectivity and yield of monomers, because the evaporation of oligomers will compete with polystyrene decomposition. The evaporation of styrene oligomers, by consuming a significant portion of the energy, thus reduces the amount of energy available for polystyrene decomposition, consequently affecting the reaction temperature, reaction yield, and selectivity for styrene monomer production. This solution can therefore be further improved. Another solution, to counteract the effects described above, could be to increase the operating pressure of the pyrolysis section in order to raise the vaporization temperature of the styrene oligomers and the evaporation rate at the pyrolysis temperature. However, this increase in operating pressure will also raise the vaporization temperature of styrene, increasing its residence time in the liquid and gas phases where it will decompose further into lighter compounds such as lighter non-condensable products (hydrogen, as well as C1, C3, C4, and C5 alkanes and alkenes). Overall, increasing the reactor pressure reduces both the liquid yield and the styrene yield. Document WO 2021 / 180893 proposes a process in which a polystyrene feedstock is pyrolyzed in a first reactor operated at a pressure below 1 bar and then separated into a light fraction comprising at least some styrene monomer and a heavy fraction comprising at least some styrene oligomers. This heavy fraction is then treated in a second pyrolysis reactor operated under different conditions than the first pyrolysis reactor, specifically a pressure above 1 bar and a temperature below 650°C. However, the control of the composition of the heavy fraction is not addressed, even though this composition has a significant influence on the performance of the second pyrolysis reactor. Continuing her research, the applicant discovered a process for depolymerizing a polystyrene filler in which the conversion of the polystyrene filler into styrene monomer is improved by implementing two pyrolysis steps and by controlling the operation of the second step, and in particular by regulating the presence of a hydrogen-donating agent. Detailed description of the invention Thus, the invention relates to a process for depolymerizing a polystyrene filler comprising at least the following steps: a. A step of preparing the polystyrene charge in order to cause the melting of plastic compounds; b. A first pyrolysis step of the feed from step a) comprising a pyrolysis section and producing at least a first pyrolysis stream gaseous and a first stream of liquid pyrolysis; c. A separation stage powered at least by the first pyrolysis stream gaseous from step b) and the second gaseous pyrolysis stream from step d) and producing at least one stream rich in light compounds, a stream rich in oligomers, and a flow rich in aromatics; d. A second pyrolysis step comprising a pyrolysis section fed at least by the oligomer-rich stream from step c), and producing at least a second gaseous pyrolysis stream and a second liquid pyrolysis flow; e. A step of separating the aromatic-rich stream from step c) into the minus a stream consisting mainly of ethylbenzene, a stream consisting mainly of styrene and a stream of heavy compounds; in which, in the pyrolysis section of step d), the partial pressure of hydrogen and / or the residence time of the gas phase are controlled. Definitions The carbon-containing compounds mentioned in the description can be of fossil origin or bio-based. In the latter case, they may be partially or totally derived from biomass or obtained from renewable raw materials derived from biomass. This includes, in particular, polymers, plasticizers, fillers, etc. Any range of values ​​designated by the expression "between a and b" represents the range of values ​​from more than a to less than b (i.e., bounds a and b excluded) while any range of values ​​designated by the expression "from a to b" means the range of values ​​from a to b (i.e., including the strict bounds a and b). The term "hydrogen donor" refers to a compound that can react with styrene oligomers during pyrolysis, providing hydrogen atoms. Such a compound can be, for example, a wax such as a paraffin wax, a thermoplastic polyolefin such as polyethylene or polypropylene, or dihydrogen. The term "paraffin wax" refers to a linear or branched alkane that is solid at room temperature and has a melting point below 100°C. Paraffin wax typically comprises 20 to 40 carbon atoms. By “styrene monomer”, we mean the compound styrene, with the formula C;Hg. By "major compound", we mean that the compound is present at more than 50% by mass in the considered stream. charge of the depolymerization process The process according to the invention is a process for depolymerizing a polystyrene filler. By polystyrene filler, we mean a filler which comprises styrene-based polymers, such as styrenic rubbers and polystyrene. Preferably, the process feedstock is a polystyrene feedstock derived from plastic waste. Such a feedstock preferably comprises at least 90% by weight of polystyrene, preferably at least 93% by weight of polystyrene, and preferably at least 95% by weight of polystyrene. The polystyrene feedstock may include other compounds, particularly if it is derived from plastic waste. These other compounds may be, but are not limited to, plastic compounds such as polyethylene, polypropylene, and elastomers, organic materials such as paper and food, or inorganic materials such as glass, metal, and sand. The polystyrene filler in the process according to the invention may contain a hydrogen donor as defined in the present invention. For example, if the hydrogen donor is polypropylene, the polypropylene-to-polystyrene mass ratio in the polystyrene filler can be adjusted between 1% by weight and 15% by weight, preferably between 1% and 3% by weight. Step a) of preparation The process according to the invention includes a polystyrene feedstock preparation step. During this step, the polystyrene feedstock is conditioned to supply step b) of the first pyrolysis. This preparation step may include grinding, degassing, and heating operations to induce the melting of the plastic compounds, for example, in an extrusion device during which the temperature is gradually increased. The vapor effluents (water, light compounds generated by the partial decomposition of the polystyrene feedstock) and the solid effluents (non-fusible debris such as metal fragments and glass) are separated. Preferably, the polystyrene filler is gradually heated to a temperature between 200°C and 300°C, this temperature allowing the polystyrene to melt while limiting its thermal decomposition. Step b) of the first pyrolysis The process according to the invention includes a first pyrolysis step of the charge from step a) comprising a pyrolysis section and producing at least a first gaseous pyrolysis stream and a first liquid pyrolysis stream. Pyrolysis refers to the thermal decomposition of compounds in an inert atmosphere. The conditioned feed from step a), preferably in molten form, feeds a pyrolysis section, operated at a temperature and pressure such that the depolymerization of the polystyrene into styrene oligomers and styrene monomer takes place. Preferably, the pyrolysis section is operated at a temperature of 300 to 700°C, preferably from 300°C to 600°C. Preferably, the pyrolysis section is operated at a pressure of 0.1 bar to 2 bar, preferably from 0.5 bar to 1.5 bar, and most preferably from 0.8 bar to 1.2 bar. Under these conditions, the amount of styrene oligomers in the first gaseous pyrolysis stream can be up to 40% by mass relative to the amount of polystyrene in the feed to the pyrolysis section. Preferably, the pyrolysis section implements a microwave pyrolysis section. Such a microwave pyrolysis unit, suitable for the pyrolysis of a polystyrene charge, is described, for example, in document WO 2020 / 202089. The use of a microwave-assisted pyrolysis section allows for higher heat transfer rates and reaction temperatures, which promote chain-end cleavage reactions and minimize the formation of styrene oligomers. A microwave pyrolysis section is also characterized by a lower temperature in the reaction mass compared to a conventional pyrolysis section. Lower temperatures in the reaction mass lead to reduced evaporation rates of styrene oligomers and prevent over-cracking of the styrene produced.The use of a microwave pyrolysis section will reduce the formation of styrene oligomers compared to a conventional pyrolysis section, but will not completely prevent their formation, which can still represent, in the first gaseous pyrolysis stream, up to 40% by mass relative to the amount of polystyrene in the charge of the pyrolysis section. The pyrolysis section produces a first gaseous pyrolysis stream and a first liquid pyrolysis stream. The first gaseous pyrolysis stream may also contain entrained liquid droplets. In addition to styrene oligomers, the first gaseous pyrolysis stream comprises the majority of the styrene monomer produced in the pyrolysis section, as well as light aromatic compounds gaseous under operating conditions such as alpha-methylstyrene, ethylbenzene, cumene, and toluene. The first gaseous pyrolysis stream consists mainly of styrene, i.e. at least 50% by weight of styrene, preferably at least 60% by weight of styrene. Preferably, the first gaseous pyrolysis stream comprises at most 10% by weight of ethylbenzene, preferably at most 5% by weight of ethylbenzene, and in a manner preferred to a maximum of 3% by weight of ethylbenzene. Preferably, the first gaseous pyrolysis stream comprises at least 10% by weight of compounds whose boiling point is higher than that of styrene. The first liquid pyrolysis stream may also include solid elements, unfused polymers produced during pyrolysis, or debris not separated in the feed preparation stage. This stream preferably feeds a separation section where any remaining solid fraction is separated from the liquid fraction, the latter being able to be recycled and mixed with the feed to the pyrolysis section of the first pyrolysis stage. Step c) of separation The process according to the invention comprises a separation step fed at least by the first gaseous pyrolysis stream from step b) and the second gaseous pyrolysis stream from step d) and producing at least one stream rich in light compounds, one stream rich in oligomers, and one stream rich in aromatics. The light compound-rich stream consists mainly of compounds lighter than benzene, in particular hydrogen, methane, ethane, ethylene, propane, propylene, butane, butene, isobutane. The aromatic-rich stream consists mainly of aromatic compounds with 6 to 9 carbon atoms. Separation step c) is carried out in such a way that 99% by mass of the styrene feeding this step is recovered in the aromatic-rich stream. The oligomer-rich stream consists mainly of styrene oligomers. Separation step c) is carried out so that the oligomer-rich stream contains less than 5% by mass of aromatic compounds with 6 to 9 carbon atoms. Besides maximizing the amount of aromatic compounds recovered from the aromatic-rich stream, minimizing the content of aromatic compounds with 6 to 9 carbon atoms in the oligomer-rich stream improves the operation of the pyrolysis section in step d) of the second pyrolysis by minimizing the energy absorbed by the evaporation of these aromatic compounds in the pyrolysis section. This evaporation can lead to foaming problems and thus impact the proper functioning of this section. Preferably, step c) of separation is carried out in a distillation column. In this arrangement, at the top of the column, the vapor effluent is cooled to a temperature between 30°C and 50°C, preferably between 35°C and 45°C. The condensed liquid fraction is returned to the top of the column as reflux, while the vapor fraction is then subcooled to a temperature between -5°C and 10°C, preferably between -5°C and 5°C, in order to condense any styrene. The process is currently carried along with the light compounds. The condensed stream after subcooling is returned to the top of the column as reflux. The residual vapor fraction constitutes the stream rich in light compounds. This stream can then be utilized, for example, as energy. Initial cooling maximizes the use of ambient-temperature cooling water as a cold utility and minimizes the use of specific cold utilities to achieve subcooling, which favorably impacts the life cycle analysis of the process according to the invention. In this arrangement, the distillation column is operated at a pressure between 0.1 and 2.0 bara, preferably between 0.5 and 1.5 bara, and most preferably between 0.5 and 1.1 bar, the operating pressure being understood as the pressure measured at the top of the column. "Bara" is understood to mean absolute bar, as opposed to pressure expressed in relative bar, commonly denoted "barg" according to the English notation "bar gauge". In this arrangement and preferably, the distillation column is fed at the bottom of the column by at least the first gaseous pyrolysis stream from step b) and the second gaseous pyrolysis stream from step d) and produces at the top of the column a stream rich in light compounds, at the bottom a stream rich in oligomers, and by a lateral withdrawal a stream rich in aromatics, said column having as its sole heat input said first gaseous pyrolysis stream from step b) and said second gaseous pyrolysis stream from step d). The first gas pyrolysis stream from step b) and the second gas pyrolysis stream from step d) are at a high temperature, preferably above 300°C. This temperature is sufficient so that the column does not require any further heat input. By feeding the charge at the bottom of the column, it is rapidly cooled, thus limiting potential styrene polymerization reactions. This feeding method also allows for better management of heavy compounds. Indeed, the absence of a recirculation system at the bottom of the column, usually used to maintain the temperature of the distillation column, greatly reduces the risk of fouling by heavy, particularly viscous compounds. The distillation column implemented in step c) of the process according to the invention comprises from 5 to 20 theoretical stages, preferably at most 15 theoretical stages, preferably from 8 to 12 theoretical stages. A stream rich in aromatics is drawn off onto an intermediate tray. This drawing-off tray is located in the lower third of the distillation column, preferably 1 to 3 theoretical levels below the bottom tray. This drawing-off at a low position in the column, slightly away from the bottom tray, limits the entrainment of heavy compounds in the aromatic-rich stream and thus reduces the risk of fouling. subsequent equipment. Preferably, and in order to further limit the risk of polymerization, a styrene-to-polystyrene polymerization inhibitor, such as... 2,2,6,6-Tetramethyl-4-oxopiperidinooxy can be fed into the distillation column in step c) of the process, preferably at the column head. Step d) of the second pyrolysis The process according to the invention includes a second pyrolysis step comprising a pyrolysis section fed at least by the oligomer-rich stream from step c), and producing at least a second gaseous pyrolysis stream and a second liquid pyrolysis stream. In this step, styrene oligomers are converted into monoaromatic compounds, particularly styrene monomer. The pyrolysis section is operated at a temperature ranging from 400°C to 900°C, preferably from 500°C to 800°C, and more preferably between 650°C and 800°C. Below 400°C, both the liquid yield and the yield of monoaromatic compounds are insufficient, while above 900°C, these yields decline. The temperature range of 650°C to 800°C allows, under the conditions of the invention, the maximization of both the liquid yield and the yield of monoaromatic compounds. Liquid yield refers to the mass percentage of liquid obtained after pyrolysis of a given mass of oligomer-rich flux.The yield of mono-aromatic compounds refers to the mass percentage of mono-aromatic compounds such as styrene, ethylbenzene, toluene, cumene, and alpha-methylstyrene in the liquid fraction obtained after pyrolysis of a given mass of oligomer-rich flux. The pyrolysis section is operated at a pressure ranging from 1.0 bar to 7.5 bar, preferably from 3 bar to 6 bar, and even more preferably from 3 bar to 4.5 bar. Increasing the operating pressure improves the yield of mono-aromatic compounds by increasing the vapor pressure of the various constituents, but degrades the liquid yield. This pressure range, under the conditions of the invention, maximizes both the liquid yield and the yield of mono-aromatic compounds. The pyrolysis section is preferably operated with a gas phase residence time of 10 s to 30 s, preferably from 10 s to 25 s, and most preferably from 15 s to 20 s. Residence time is defined as the ratio of the volume (in m³) of the reactor (or reactors, if there are several in series) of the pyrolysis section and the lines carrying the second gaseous pyrolysis stream to separation step c) to the volumetric flow rate (in m³ / s) of the second gaseous pyrolysis stream. This preferred residence time range maximizes the The yield of monoaromatic compounds is determined by residence time. Below 10 s, the residence time is too short for sufficient conversion, while above 30 s, possible recombination of the monoaromatic compounds degrades the yield. The yield of monoaromatic compounds is particularly maximized for a residence time of 15 to 20 s. The residence time of the gas phase can be adjusted, for example, by feeding an inert gas into the pyrolysis section of step d), such as nitrogen, argon, helium, neon, xenon, or krypton, preferably nitrogen. The applicant discovered that the yield of monoaromatic compounds, particularly styrene monomer, could be controlled by adjusting the partial pressure of hydrogen in the pyrolysis section of step d) and / or by controlling the residence time of the gas phase. This partial pressure is controlled by adjusting the concentration of hydrogen-donating agent in the polystyrene feed and / or in the feed to the pyrolysis section of step d). The partial pressure of hydrogen in the pyrolysis section of step d) can be increased by increasing the concentration of hydrogen-donating agent in the polystyrene feed and / or in the feed to the pyrolysis section of step d), or decreased, for example, by feeding an inert gas into the pyrolysis section of step d), for example, a gas selected from nitrogen, argon, helium, neon, xenon, krypton, preferably nitrogen. The concentration of hydrogen-donating agent can be increased by mixing one or more hydrogen-donating agents into the polystyrene feed and / or the feed to the pyrolysis section of step d), the polystyrene feed and / or the feed to the pyrolysis section of step d) already containing one or more hydrogen-donating agents. Thus, in one arrangement, the partial pressure of hydrogen in the pyrolysis section of step d) can be adjusted by the presence of a hydrogen donor agent chosen from polyolefins, paraffinic waxes and their mixtures in the polystyrene feed and / or in the oligomer-rich stream from step c). In another arrangement, associated or not with the previous arrangement, the partial pressure of hydrogen can also be adjusted by the presence of a hydrogen donor agent chosen from polyolefins, paraffinic waxes, dihydrogen and their mixtures in the feed of the pyrolysis section of step d). By adjusting the hydrogen donor concentration in the process according to the invention and / or the residence time of the gas phase, it is thus possible to control the overall yield of monoaromatic compounds. The hydrogen donor concentration can be adjusted by measuring the concentration of the monoaromatic compound in the second gas pyrolysis stream; the optimal concentration may differ slightly depending on whether, for example, the aim is to maximize the production of toluene and ethylbenzene, or of styrene. For example, in the case where the hydrogen donor is polypropylene, the polypropylene to polystyrene mass ratio in the polystyrene filler will be adjusted between 1% by weight and 15% by weight, preferably between 1% and 3% by weight. In the presence of an excessive amount of hydrogen-donating agent, the yield of light non-aromatic products (hydrogen, methane, ethane, ethylene, propane, propylene, butane, butene, isobutane) and of solid residues from the thermal degradation of compounds during pyrolysis increases, which reduces the overall yield of products of interest. Preferably, the pyrolysis section implements a microwave pyrolysis section. Such a microwave pyrolysis suitable for the pyrolysis of a polystyrene charge is described, for example, in document WO 2020 / 202089. Separation step e) The process according to the invention includes a step of separating the aromatic-rich stream into at least one stream comprising mainly ethylbenzene, one stream comprising mainly styrene and one stream of heavy compounds. Step e) of separation makes it possible to obtain a stream comprising mainly styrene which can feed a styrene polymerization process, therefore meeting the specifications of such processes, with in particular a very high styrene content, preferably greater than 99.8% by weight, and very low levels of compounds such as ethylbenzene, benzene, cumene, alpha-methyl-styrene and styrene oligomers. In a first preferred arrangement, step e) of separation of the aromatic-rich stream comprises two successive separation sections. A first separation section is fed by the aromatic-rich stream from step c) and allows the separation of a stream consisting mainly of ethylbenzene and a styrenic raffinate. This first section is implemented in a distillation column comprising 60 to 100 theoretical stages, and is operated at a pressure less than or equal to 0.25 bara at the top of the column so as to maintain the temperature at the bottom of the column at a value less than or equal to 120°C. The distillation column in the first section is fed by the aromatic-rich stream from stage c) at the bottom of the upper third of the column. For example, for a column with 60 theoretical stages, the aromatic-rich stream is fed at a stage between the 18% theoretical stage and the 225th theoretical stage, with the stages numbered from top to bottom. The reflux ratio at the condenser of this column, corresponding to the mass flow rate of reflux fed at the top of the column divided by the mass flow rate of the flux comprising majori- The ethylbenzene reflux ratio is preferentially between 60 and 300. This parameter varies significantly depending on the ethylbenzene content of the aromatic-rich stream. The lower the ethylbenzene content in the aromatic-rich stream, the higher the reflux ratio at the column condenser. The reflux ratio at the reboiler of this column, corresponding to the mass flow rate of reflux fed at the bottom of the column on the mass flow rate of styrenic raffinate, is preferably between 4 and 10, preferably between 5 and 9. A second separation section is fed by the styrenic raffinate from the first separation section and produces a stream consisting mainly of styrene and a stream of heavy compounds. This second section is implemented in a distillation column comprising 40 to 100 theoretical stages, preferably comprising 40 to 70 theoretical stages, and is operated at a pressure less than or equal to 0.25 bara at the top of the column so as to maintain the temperature at the bottom of the column at a value less than or equal to 120°C. The distillation column of the first section is fed with the styrenic raffinate from the first separation section in the lower part of the column, preferably in the upper part of the lower fifth of the column. For example, for a column comprising 50 theoretical stages, the styrenic raffinate from the first separation section is fed at a stage between the 35th and 45th theoretical stages, the stages being numbered from top to bottom. The reflux ratio at the condenser of this column, corresponding to the mass flow rate of reflux fed at the top of the column divided by the mass flow rate of flux consisting mainly of styrene, is preferentially between 4 and 8. The reflux ratio at the reboiler of this column, corresponding to the mass flow rate of reflux fed at the bottom of the column divided by the mass flow rate of heavy compounds, is preferably between 40 and 200, this ratio being greatly influenced by the content of compounds such as cumene and alpha-methylstyrene. In another preferred arrangement, step e) of separating the aromatic-rich stream is implemented in an internally walled distillation column. An internal wall column is a distillation unit well known to those skilled in the art, in which a fluid-tight, vertically oriented internal wall divides a section of the column into two distinct zones. An internal wall column typically consists of a lower common section where the separation stages are not divided by the internal wall, a divided section where the separation stages are divided by the internal wall, and an upper common section where the separation stages are not divided by the internal wall. The internal wall column comprises a total of 70 to 130 theoretical stages, preferably 80 to 120 theoretical stages, and most preferably 90 to 110 theoretical stages. The internal wall is preferably centered, meaning that it divides the column lengthwise into two equal parts. When the number of theoretical stages on either side of the internal wall differs, for example, due to the use of different types of distribution trays or packing, the total number of theoretical stages in the column is understood to be the sum of the theoretical stages in the common sections and the larger of the two divided sections. The column is operated at a pressure of 0.25 bar or less at the top of the column to maintain the temperature at the bottom of the column at a value of 120°C or less. The aromatic-rich stream from step c) of the process is fed from one side of the inner wall on a stage from the 10th to the 20th theoretical stage, preferably from the 12th to the 18th theoretical stage and very preferably to the 15th theoretical stage, the stages being numbered from top to bottom. In a first variant of this arrangement, the lower common section of the inner-walled column comprises 8 to 12 theoretical stages, and the upper common section also comprises 8 to 12 theoretical stages. The flow consisting mainly of styrene is withdrawn from the opposite section, with respect to the inner wall, to the section where the aromatic-rich flow is injected. The withdrawal is carried out on a stage close to the upper part of the divided section, preferably on one of the 5 upper stages of the divided section, most preferably on one of the 3 upper stages of the divided section, most preferably on one of the two upper stages of the divided section, and most preferably on the first stage of the divided section, counting the stages from the top. The flow consisting mainly of ethylbenzene is drawn from the top of the column, and the flow of heavy compounds is drawn from the bottom of the column. The reflux ratio at the condenser of this column, corresponding to the mass flow rate of the reflux feed at the column head divided by the mass flow rate of the stream consisting mainly of ethylbenzene, is preferably between 60 and 300. This parameter varies significantly depending on the ethylbenzene content of the aromatic-rich stream. The lower the ethylbenzene content in the aromatic-rich stream, the higher the reflux ratio at the column condenser. The reflux ratio at the reboiler of this column, corresponding to the mass flow rate of reflux fed at the bottom of the column divided by the mass flow rate of heavy compounds, is preferably between 50 and 200, this ratio being greatly influenced by the content of compounds such as cumene and alpha-methylstyrene. In a second variant of this arrangement, the internally walled column does not It does not include a common upper part. That is to say, the wall extends all the way to the head of the internally walled column. In this variant, the lower common part comprises from 2 to 12 theoretical floors, preferably from 2 to 10 theoretical floors, and very preferably from 2 to 4 theoretical floors. In this variant, the internal wall column preferably comprises a total of 60 to 80 theoretical stages. In this variant, the stream consisting mainly of styrene is drawn off from the opposite side, relative to the inner wall, from the side where the aromatic-rich stream is injected. The drawing-off is carried out at the top of the column. The stream consisting mainly of ethylbenzene is withdrawn from the top of the column in the same part as the part where the aromatic-rich stream is injected. The reflux ratio at the condenser of this column for the divided section located on the side of the aromatic-rich feed stream, corresponding to the mass flow rate of the reflux fed to the column head in this section divided by the mass flow rate of the stream consisting mainly of ethylbenzene, is preferably between 60 and 300. This parameter varies significantly depending on the ethylbenzene content of the aromatic-rich stream. The lower the ethylbenzene content in the aromatic-rich stream, the higher the reflux ratio at the column condenser. The reflux ratio at the condenser of this column for the divided part located on the side of the withdrawal of the flux consisting mainly of styrene, corresponding to the mass flow rate of reflux fed at the top of the column divided by the mass flow rate of flux consisting mainly of styrene, is preferably between 1 and 10. The reflux ratio at the reboiler of this column, corresponding to the mass flow rate of reflux fed at the bottom of the column divided by the mass flow rate of heavy compounds, is preferably between 60 and 200, this ratio being greatly influenced by the content of compounds such as cumene and alpha-methylstyrene. Preferably, and in order to further limit the risk of polymerization, a styrene-to-polystyrene polymerization inhibitor, such as... 2,2,6,6-tetramethyl-4-oxopiperidinooxy, can be fed into the column(s) implemented in step b) of separation, preferably at the top of the column(s) implemented in step b) of separation. Preferably, the lux consisting mainly of ethylbenzene from step e) is separated into at least one stream consisting mainly of toluene, one stream which includes ethylbenzene and one stream which includes styrene. Preferably the heavy compound stream from step e) is separated into at least one stream comprising styrene, one stream comprising AMS and one stream comprising oligomers. Description of the figures For each of the following figures, identical numbers and letters correspond to similar flows and operations. [Fig.1] Fig.1 schematically illustrates a process for depolymerizing a polystyrene filler according to the prior art. A polystyrene filler (1) is fed into an extrusion section where it is gradually heated to melt the filler (A), the liquid portion (2) being separated from the solid portion (3). The liquid portion (2) is sent to a mixing tank (B) where it is mixed with the liquid fraction (7) from the liquid-solid separator (C). The mixing tank (B) feeds the pyrolysis reactor (D), which produces a first gaseous pyrolysis stream (5) and a first liquid pyrolysis stream (6), the latter being separated into a solid fraction (8) possibly present and a liquid fraction (7) in the liquid-solid separator (C). The first gaseous pyrolysis stream (5) feeds a first distillation column (E) which produces at the top a stream rich in light compounds (9), at the bottom a stream rich in oligomers (11) and as a lateral withdrawal a stream rich in aromatics (10). The latter feeds a distillation column (F) in which it is separated into a stream comprising mainly ethylbenzene (12) and a styrenic raffinate (13) which feeds a distillation column (G) separating it into a stream comprising mainly styrene (14) and a stream of heavy compounds (15). [Fig.2] Fig.2 schematically illustrates a possible configuration of a process for depolymerizing a polystyrene filler according to the invention. Elements that are functionally identical to those in [Fig.1] are numbered in the same way, without this implying that they have the same dimensions or are operated under the same conditions. In this arrangement, the oligomer-rich stream (11) feeds a pyrolysis reactor (H) which produces a second gaseous pyrolysis stream (16). This second stream is mixed with the first gaseous pyrolysis stream (5) before being fed into the distillation column (E). The pyrolysis reactor (H) also produces a second liquid pyrolysis stream (17). The second liquid pyrolysis stream (17) is separated into a solid fraction (19), which may be present in the liquid-solid separator (I), and a liquid fraction (18), which is fed into the pyrolysis reactor (H). In this arrangement, and in a manner not shown in this diagram, the operation of the pyrolysis reactor (H) can be adjusted by injecting an inert gas mixed with the oligomer-rich stream (11) or the partial pressure of hydrogen in the pyrolysis reactor (H) can be controlled by the presence of a donor agent. of hydrogen in the polystyrene filler (1) and / or in the oligomer-rich stream (D. [Fig. 3] Figure 3 schematically illustrates an arrangement in which a stream consisting mainly of ethylbenzene (12) feeds a distillation column (J) preferentially comprising 20 to 40 theoretical stages, here 30, substantially in its middle, here at the theoretical stage 30. This column (J) produces a stream consisting mainly of toluene (21) and a raffinate (22). The latter feeds a distillation column (K) preferentially comprising 50 to 80 theoretical stages, here 65, in the upper part of its lower half, here at the theoretical stage 30. This column (K) produces a stream (24) which includes ethylbenzene and a stream (23) which includes styrene. This arrangement maximizes the recovery of these constituents. [Fig. 4] Figure 4 schematically illustrates an arrangement in which a stream of heavy compounds (15) is processed to maximize the recovery of styrene and alpha-methylstyrene (AMS). The heavy compound stream (15) feeds a distillation column (L) preferably comprising 15 to 25 theoretical stages, here 20, substantially in its middle, here at theoretical stage 10. This column (L) produces a stream (25) comprising styrene and a raffinate (26). The latter feeds a distillation column (M) comprising 1 to 3 theoretical stages, here a flash column comprising a single theoretical stage, substantially in its middle when the column comprises several theoretical stages. This column (M) produces a stream (27) that comprises AMS and a stream (28) that comprises oligomers. [Fig.5] The [Fig.5] is a representation of the conversion of oligomers as a function of the mass fraction of hydrogen donor, here polypropylene added in the polystyrene feed, calculated as the ratio of the polypropylene flow rate to the sum of the polypropylene flow rate and the polystyrene flow rate in the polystyrene feed, at a temperature of 700°C for different operating pressures in the pyrolysis step. Examples Example 1 This example illustrates the depolymerization of polystyrene according to the scheme in [Fig.1] A polystyrene charge (1), here polystyrene, feeds an extruder (A) where it is heated to 250°C. The liquid portion of the charge (2), after mixing with the liquid portion from the pyrolysis reactor (7), feeds a pyrolysis reactor (D), here a microwave pyrolysis reactor, operated at a temperature of 340°C and a pressure of 1.1 bar. The first gaseous pyrolysis stream (5) is separated in a stream rich in light compounds (9), a stream rich in oligomers (11), a stream consisting mainly of ethylbenzene (12), a stream consisting mainly of styrene (14) and a stream of heavy compounds (15). The characteristics of distillation columns E, F and G are presented in Table A The production of styrene, toluene, alpha-methylstyrene and other aromatics is shown in Table 2. Example 2 This example illustrates the depolymerization of polystyrene according to the invention, according to the diagram shown [Fig.2], the streams (12) and (15) being treated according to the diagrams shown in figures 3 and 4. The pyrolysis reactor (D), here a microwave pyrolysis reactor, is operated at a temperature of 340°C and a pressure of 1.1 bar. The second pyrolysis reactor (H), also a microwave pyrolysis reactor, is operated at a temperature of 700°C and a pressure of 3.7 bar. The characteristics of the distillation columns E, F, G, J, K, L, and M are presented in Table 1. Table 2 shows the results in terms of overall production in three cases: Example 2a: the polystyrene filler comprises 2% by weight of polypropylene Example 2b: The polystyrene filler contains 14% polypropylene by weight. Polypropylene acts as a hydrogen donor. It is observed that the presence of a hydrogen donor in the filler improves the overall yield of aromatic compounds of interest, particularly styrene. However, an excessive amount will negatively impact the overall yield. [Table 1] (Column [urite—EF a IKLM Number of theoretical stages |- = eq] ES = 65 = x Supply stage - 8 2 ES 5 3] 19] : Capacitor pressure bar Fos 77 & 205] =) & = Capacitor temperature _|*C = 35] 35| St) 35) 72} 88; Reboiler temperature —_|C 2 9 86] 87 5] 38 55) [Tables 2] [Examples [Exempleza [Exemplezb | For 109 kg / h of pure PS PP / Feed PP / Feed 25 by weight 14% by weight Styrene Product 58.5 60.6 60.5 Foluene L8 Product 5.3 5.1 AMS Product 9.2 9.8 20.3 Other Aromatics Product 2.6 5.3 S.2 Total Aromatics 724 80.3 81.0 Non-condensable Product 84 13.5 12.5 Light Oligomers Product 16.8 0.0 0.0 Solid Residues Product 2.7 63 6.2

Claims

Claims

1. A process for depolymerizing a polystyrene filler comprising at least the following steps: a. A step of preparing the polystyrene charge in order to cause plastic compounds to melt; b. A first pyrolysis step of the charge from step a) comprising a pyrolysis section and producing at least one first gaseous pyrolysis flow and a first flow of liquid pyrolysis; This A separation step powered at least by the first gaseous pyrolysis stream from step b) and the second stream gaseous pyrolysis from step d) and producing at least a flow rich in light compounds, a flow rich in oligomers, and a flow rich in aromatics; d. A second pyrolysis stage comprising a section of pyrolysis powered at least by the oligomer-rich stream from step c), and producing at least a second stream of gaseous pyrolysis and a second liquid pyrolysis stream; e A step of separation of the flow rich in aromatics from step c) in at least one flow comprising mainly ethylbenzene, a (lux comprising mainly styrene and a flow of heavy compounds; in which the pyrolysis section of step d) is controlled, partial pressure of hydrogen and / or the residence time of the gas phase.

2. Depolymerization process according to the preceding claim in which the partial pressure of hydrogen in the pyrolysis section of step d) is controlled by the presence of a hydrogen donating agent in the polystyrene charge and / or in the feed of the section of pyrolysis of step d) and / or by feeding an inert gas into the pyrolysis section of step d).

3. Depolymerization process according to the preceding claim in which the partial pressure of hydrogen in the pyrolysis section of step d) is controlled by the presence of a hydrogen donating agent chosen from polyolefins, paraffin waxes and their mixtures in the polystyrene charge and / or in the oligomer-rich stream from from step c).

4. Depolymerization process according to the preceding claim in which the partial pressure of hydrogen in the pyrolysis section of step d) is controlled by the presence of a hydrogen donor in the styrenic charge, the hydrogen donor being polypropylene, the mass ratio of polypropylene to polystyrene in the poly- filler styrenic being adjusted between 1% by weight and 15% by weight, preferably between 1% and 3% by weight.

5. A depolymerization process according to any one of claims 1 to 3 in which the partial pressure of hydrogen in the section of pyrolysis of step d) is controlled by the presence of a donor agent of hydrogen chosen from polyolefins, paraffin waxes, di- hydrogen and their mixtures in the feed of the section of pyrolysis of step d).

6. A depolymerization process according to any one of claims previous in which step d) of second pyrolysis is carried out at a temperature ranging from 400°C to 900°C, preferably from 500°C to 800°C, preferably between 650°C and 800°C, and at a pressure ranging from 1.0 bar to 7.5 bar, preferably ranging from 3 bar to 6 bar and preferred way ranging from 3 bar to 4.5 bar.

7. A depolymerization process according to any one of claims previous in which step d) of second pyrolysis is carried out with a gas phase residence time ranging from 10 s to 30 s, so as to preferred ranging from 10 s to 25 s and very preferably ranging from 15 s to 20 s.

8. Depolymerization process according to the preceding claim in which the residence time of the gas phase is controlled by supplying a inert gas in the pyrolysis section of step d), preferably a gas inert gas chosen from nitrogen, argon, helium, neon, xenon, krypton, preferably nitrogen.

9. A depolymerization process according to any one of claims preceding in which step b) of first pyrolysis and / or step d) second pyrolysis implements a micro-pyrolysis section wave.

10. A depolymerization process according to any one of claims previous in which step c) of separation implements a distillation column fed at the bottom of the column by the first flow gaseous pyrolysis stream from step b) and the second pyrolysis stream gaseous gas from step d) and producing a rich flow at the top of the column in light compounds, in the background a flow rich in oligomers, and by a side withdrawal of a flow rich in aromatics, said column having for only heat input said first gaseous pyrolysis flow from step b) and said second gaseous pyrolysis stream from step d).

11. | Depolymerization process according to any one of the claims previous in which step e) of separation of the aro-rich stream matics includes a first separation section fed by the aromatic-rich stream from step c) which makes it possible to separate a stream comprising mainly ethylbenzene and a raffinate styrenic and a second separation section fed by the styrenic raffinate from the first separation section and producing a stream comprising mainly styrene and a stream of heavy compounds.

12. A depolymerization process according to any one of claims previous in which step e) of separation of the aro-rich stream matics is carried out in an internal wall distillation column.

13. Depolymerization process according to any one of claims previous in which the flow comprising mainly of the ethylbenzene from step e) is separated into at least one stream comprising mainly toluene, a stream which includes ethylbenzene and a flux that includes styrene.

14. A depolymerization process according to any one of claims previous in which the flow of heavy compounds from step e) is separated into at least one stream comprising styrene, a stream which includes AMS and a stream that includes oligomers.