Method for producing monomers by depolymerization of the corresponding polymers and installation therefor

Abstract

The present invention relates to a method for producing monomers from a feedstock containing a polymer of which the monomers are constituents, comprising: - depolymerizing the feedstock to obtain a monomer solution containing the monomers and impurities; and - purifying the monomer solution, thereby obtaining a purified monomer solution having an impurity content lower than the impurity content of the monomer solution, the purification step comprising at least one crystallization step, each crystallization step comprising: a) cooling the monomer solution so that the monomer solution forms monomer crystals and a mother liquor, b) separating the monomer crystals from the mother liquor, and c) melting the monomer crystals, thereby obtaining an enriched monomer solution.

Description

[Technical Field]

[0001] This project leading to this patent application has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 820687.

[0002] The present invention relates to the field of depolymerization for the recovery of monomers from feedstocks containing polymers of which the monomers are constituents.

[0003] In particular, the present invention relates to the field of depolymerization of polymethyl methacrylate (PMMA) to recover methyl methacrylate (MMA). [Background technology]

[0004] MMA can be obtained, for example, by pyrolyzing PMMA in a pyrolysis reactor to obtain a gas stream containing gaseous MMA and inevitable impurities, separating the solid impurities from the gas stream in a separator, and condensing the gas stream to obtain liquid MMA. MMA obtained by depolymerization of PMMA is sometimes called crude MMA.

[0005] The liquid MMA obtained by depolymerization of PMMA may contain impurities, such as other chemical compounds present in the raw materials containing PMMA or other chemical compounds that arise during depolymerization, e.g., during pyrolysis.

[0006] Liquid MMA may be purified. Purification of crude MMA may be carried out by distillation, which is the treatment of components from a liquid mixture by using selective evaporation and condensation and separation based on the boiling points of the components or their azeotropes.

[0007] JP 2020-114806 A discloses a method for depolymerizing polylactic acid to lactide by melting the polymer and a depolymerization catalyst in an extruder and feeding them into a vent chamber connected to a collection and purification device equipped with a gas / liquid separation column, the purification device consisting of one crystallization vessel and, optionally, one or more crystallization vessels for carrying out the crystallization steps successively.

[0008] WO 2021 / 032821 discloses a method for depolymerizing terephthalate polymers, which includes sequential crystallization of dimers and crystallization of monomers. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2020-114806 [Patent Document 2] International Publication No. 2021 / 032821 Summary of the Invention [Problem to be solved by the invention]

[0010] One of the aims of the present invention is to propose a method for producing monomers from recycled polymers, which makes it possible to obtain a monomer solution with sufficient purity. [Means for solving the problem]

[0011] To this end, the present invention provides a method for producing a monomer from a feed containing a polymer of which the monomer is a constituent, the method comprising the steps of: - depolymerizing the feedstock to obtain a monomer solution containing monomers and impurities; and - purifying the monomer solution, thereby obtaining a purified monomer solution having an impurity content lower than the impurity content of the monomer solution. Including, The purification process comprises at least one crystallization process, each crystallization process comprising the steps of: a) cooling the monomer solution to form monomer crystals and a mother liquor; b) separating the monomer crystals from the mother liquor; and c) Melting the monomer crystals, thereby obtaining a monomer-enriched solution (a solution enriched in monomers) The present invention proposes a method including:

[0012] According to other advantageous aspects of the invention, the manufacturing method described above comprises one or more of the following features, taken individually or in any technically possible combination: - the purification step comprises subjecting the enriched monomer solution resulting from one crystallization step to another crystallization step; - the purification step includes subjecting the mother liquor resulting from one crystallization step to another crystallization step; - the purification step includes a water removal step; - The water removal process described above: + cooling the monomer solution to a cooling temperature, thereby converting the water present in the monomer solution into ice without causing the monomer to crystallize; and + Separating the ice from the monomer solution Including, At least one crystallization step is performed on the monomer solution after separation from water; - the cooling temperature is higher than the crystallization temperature of the monomers contained in the monomer solution; - the difference between the cooling temperature and the melting point of the monomers contained in the liquid stream is greater than 4°C, preferably greater than 6°C, and / or less than 30°C, preferably less than 20°C, more preferably less than 10°C; - during the water removal step, the monomer solution is at least partially cooled by heat exchange with the purified monomer solution resulting from crystallization; - the method comprises the step of adding at least one anti-icing additive to the monomer solution prior to carrying out the at least one crystallization step to inhibit ice formation during the at least one crystallization step; - at least one anti-icing additive contains methanol; the depolymerization step comprises pyrolyzing the feedstock in a pyrolysis reactor, thereby generating a gas stream, and condensing the gas stream in a condenser, thereby obtaining a monomer solution; - the depolymerization step comprises a separation step comprising removing solid and / or liquid impurities from the gas stream in a separation device prior to condensing the gas stream; - the method comprises the step of purifying the gas stream between the separator and the condenser using a gas purification device fluidly connected in series between the separator and the condenser, the gas purification device comprising a transfer column including internals to which a fraction of the monomer solution leaving the condenser is fed, the gas stream flowing upwards in the transfer column and the monomer solution flowing downwards in the transfer column, heavy contaminants contained in the gas stream condensing to a liquid state on the internals and flowing by gravity back towards the bottom of the transfer column and preferably back to the separator; - the separation device comprises a secondary separator and a primary separator fluidly connected in series; - the method includes the step of purifying the monomer solution between the condenser and the purification device by passing the monomer solution through an evaporator and a condenser in succession, thereby removing heavies from the monomer solution; - the process comprises a washing step upstream from the crystallizer, which comprises adding water to the monomer solution; - the polymer is polymethyl methacrylate (PMMA) and the monomer is methyl methacrylate (MMA); and The number of crystallization steps is not more than 5, in particular not more than 3. Includes.

[0013] The subject of the present invention is an installation for producing a liquid monomer from a feedstock comprising a polymer of which said monomer is a constituent, the installation being adapted to carry out the production method defined above.

[0014] The invention and its advantages will be better understood on reading the following description, given purely by way of non-limiting example and with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic diagram showing a facility for producing a monomer solution from a feedstock containing a polymer of which the monomer is a constituent. [Figure 2] FIG. 2 is a schematic diagram showing the purification device of the facility of FIG. [Figure 3] FIG. 3 is a schematic diagram showing another purification device of the production facility of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0016] Facility 2 of Figure 1 is configured to produce a monomer from a feedstock that includes a polymer of which the monomer is a constituent.

[0017] The facility 2 includes a depolymerizer 4 configured to depolymerize the feedstock to obtain a monomer solution containing monomers and impurities, and a purifier 6 configured to purify the monomer solution to obtain a purified monomer solution having an impurity content lower than the impurity content of the monomer solution resulting from the depolymerization.

[0018] Hereinafter, the monomer solution resulting from the depolymerization will also be referred to as "crude monomer solution" or "crude monomer."

[0019] The depolymerization apparatus 4 includes, for example, a pyrolysis reactor 8 configured to receive a feedstock and to produce a gas stream containing the monomers in a gaseous state, and a condenser 10 configured to condense the gas stream to obtain a monomer solution containing the monomers in a liquid state.

[0020] Pyrolysis reactor 8 may be, for example, a heated single screw extruder comprising a heated cylindrical barrel 12 and at least one screw 14 disposed within barrel 12 for moving the feed material through barrel 12 while heating the feed material.

[0021] Barrel 12 is preferably configured to heat the feed material according to a temperature profile, i.e., a temperature that varies along barrel 12. Barrel 12 may, for example, include multiple barrel sections configured to heat at different temperatures to achieve the temperature profile.

[0022] The depolymerization unit 4 preferably includes a feeder 16 configured to supply a feedstock to the inlet of the pyrolysis reactor 8 .

[0023] The depolymerization apparatus 4 optionally comprises a separation apparatus 18 connected in series between the outlet of the pyrolysis reactor 8 and the inlet of the condenser 10, the separation apparatus 18 being configured to separate solid and / or liquid impurities from the gas stream before feeding the gas stream to the condenser 10.

[0024] The separation device 18 comprises a primary separator 20 configured, for example, to reduce the linear velocity of the gas stream to promote separation and deposition of solid impurities, and / or to de-mist the gas stream to promote deposition of mist droplets of liquid impurities, and / or to scrub the gas stream with a scrubbing liquid to promote deposition of liquid impurities.

[0025] The reduction in linear velocity of the gas stream is obtained, for example, by a gradual increase in the cross-sectional area of the internal flow passage of the main separator 20 through which the gas stream passes during flow through the main separator 20 .

[0026] Preferably, the primary separator 20 is configured so that the internal flow path of the primary separator 20 is not straight. Such a configuration promotes separation and deposition of impurities. Mist removal of the gas stream can be achieved, for example, by including one or more mist removal grids across the internal flow path.

[0027] The cleaning of the gas is obtained, for example, by injection of water and / or liquid monomer inside the channels.

[0028] Separation apparatus 18 optionally includes a secondary separator 22 located upstream of primary separator 20. The gas stream produced by pyrolysis reactor 8 flows through secondary separator 22 before entering primary separator 20.

[0029] The secondary separator 22 is configured to remove solid impurities from the hot gas stream, for example, by inertia, and may be configured, for example, with an internal flow passage having a change in cross-sectional area that promotes a reduction in linear velocity and / or a change in direction located above a collection pot, such that the solid particles slow down due to a reduction in linear velocity and / or that the solid particles, driven by inertia, tend to strike the walls of the internal flow passage and fall into the collection pot.

[0030] Advantageously, the depolymerization unit 4 comprises a gas purification unit 24 connected in series between the separation unit 18 and the condenser 10 and configured to remove heavier components from the gas stream before it enters the condenser 10.

[0031] In operation, the gas stream exiting separator 18 passes through gas purification device 24 before entering condenser 10 .

[0032] The gas purification system 24 comprises a transfer column 26 extending upwardly between a lower end 26A fluidly connected to the separation system 18, particularly the main separator 20, for receiving the gas stream exiting the separation system 18, and an upper end 26B fluidly connected to the inlet of the condenser 10.

[0033] The gas purification device 24 comprises a reinjection loop 28 configured to reinject a fraction of the monomer solution collected downstream of the condenser 10 into the upper part of the transfer column 26, in particular into the upper end 26B of the transfer column 26.

[0034] Transfer column 26 includes internal structures, such as baffles, structured packing, gauze packing, and / or random packing, including saddles and / or rings, particularly pall rings.

[0035] The internals are configured such that, during operation, the gas stream flows upwardly through the transfer column, the temperature of the gas stream decreases along the transfer column, and the monomer solution flows downwardly by gravity within the transfer column 26, with heavy contaminants contained in the gas stream condensing on the internals and flowing back down the transfer column 26 in liquid form into the separation system 18, particularly into the main separator 20.

[0036] Preferably, the temperature of the gas stream at the outlet of main separator 20 and the inlet of transfer column 26 is maintained at least 20°C above the boiling point of the monomer, particularly at least 30°C above the boiling point of the monomer, more particularly at least 40°C above the boiling point of the monomer, and even more particularly at least 50°C above the boiling point of the monomer.

[0037] Preferably, the gas flow at the outlet of the transfer column 26 is maintained between the boiling point of the monomer and the boiling point of the monomer + 50°C, particularly between the boiling point of the monomer and the boiling point of the monomer + 40°C, particularly between the boiling point of the monomer and the boiling point of the monomer + 30°C, particularly between the boiling point of the monomer and the boiling point of the monomer + 20°C, particularly between the boiling point of the monomer and the boiling point of the monomer + 10°C.

[0038] Optionally, the purification unit 6 comprises a solution purifier 30 configured to remove liquid impurities from the monomer solution provided by the depolymerizer.

[0039] Solution purification system 30 includes, for example, an evaporator 32 and a condenser 34 fluidly connected in series for successively evaporating and condensing the monomer solution.

[0040] The stream can be evaporated and condensed to separate the heavy components of the monomer solution from the monomers and light components contained therein.

[0041] The monomers and lights are provided at a lights outlet 32A of the evaporator 32, which is connected to the inlet of the condenser 34 of the solution purification device 30, for example, while the heavy components are discharged at a heavy components outlet 32B of the evaporator 32.

[0042] Alternatively, or optionally in some cases, solution purification device 30 comprises one or more distillation columns fluidly connected in series, for example one or two distillation columns fluidly connected in series, each of which allows the separation of the monomers from other compounds present in the monomer solution at this point in installation 2.

[0043] Alternatively, or optionally in some cases, solution clarification system 30 comprises a thin film evaporator or a short path distillation system or any other suitable technology or combination of technologies.

[0044] Optionally, purification unit 6 comprises a wash unit 36 configured to add water and possibly other chemicals to the monomer solution provided by depolymerization unit 4 .

[0045] When the purification system 6 comprises a washing system 36 and a solution clarification system 30, the washing system 36 is preferably located upstream of the solution clarification system 30.

[0046] The washing device 36 is configured to add water to the monomer solution in one, two, or more steps, for example. In the first step, an aqueous alkaline solution is mixed with the monomer solution to capture organic and inorganic acids, such as hydrochloric acid (HCl), methacrylic acid, and acetic acid. After decantation or centrifugation, or any other suitable separation step, the aqueous and organic phases are separated. The organic phase, which may be contaminated with residual traces of alkaline compounds, is washed with fresh water. After another decantation or other separation step, the organic phase can proceed to the next stage, while the aqueous phase, containing traces of alkaline compounds and other organics that are partially soluble in water, is directed to a first alkaline washing step, which not only saves water but also recovers some of the partially soluble monomers. To capture additional impurities, the washing step can be extended with one or more additional washing steps with other chemicals. These additional washing steps can occur anywhere in the washing sequence but preferably occur before the final clean water washing step.

[0047] After the washing step, some of the residual water remains soluble in the monomer solution.

[0048] The purification unit 6 includes a crystallization unit 40 configured to perform at least one crystallization step to separate impurities from the monomer solution, each crystallization step including cooling the monomer solution to form monomer crystals and a mother liquor, separating the monomer crystals from the mother liquor, and melting the monomer crystals to thereby obtain a monomer-enriched solution.

[0049] This crystallization makes it possible to remove from the monomer solution those components that have a melting temperature strictly below that of the monomer, these components being contained in the mother liquor.

[0050] Crystallization involving the removal of some components from the initial solution subjected to crystallization is also called "fractional crystallization."

[0051] In order to obtain a monomer solution with high purity, the monomer solution can be subjected to multiple crystallization steps, for example, the mother liquor can be subjected to multiple crystallization steps in order to recover the monomer still present in the mother liquor.

[0052] In one example where crystallizer 40 is configured to perform multiple crystallization steps, crystallizer 40 is configured to subject the enriched monomer solution resulting from a crystallization step to another crystallization step and / or subject the mother liquor resulting from a crystallization step to another crystallization step.

[0053] 2, crystallization apparatus 40 includes one or more crystallizers 42, each configured to perform a respective crystallization step. Each crystallizer 42 is configured to receive a liquid (either the monomer solution provided by depolymerization apparatus 4 or the monomer-rich solution provided by a previous crystallizer 42), cool the liquid, and crystallize the monomer to obtain a monomer-rich solution and a mother liquor.

[0054] Each crystallizer 42 is configured, for example, to collect mother liquor and to collect monomer crystals as a monomer-rich solution.

[0055] Each crystallizer 42 is configured, for example, to receive a monomer solution at an inlet 42A, to provide an enriched monomer solution at a first outlet 42B, and to provide a mother liquor at a second outlet 42C.

[0056] Each crystallizer 42 is configured, for example, for batch operation, i.e., to continuously receive batches of monomer solution, subject the monomer solution to crystallization, and collect mother liquor and purified monomer before receiving a new batch of monomer solution.

[0057] Alternatively, each crystallizer 42 is configured for semi-continuous operation, i.e., continuous flow of mother liquor through the crystallizer 42 and recovery of purified monomer at intervals, particularly fixed intervals. A multi-tube crystallizer, for example, can be operated in a semi-continuous manner.

[0058] Crystallization apparatus 40 advantageously comprises a series of crystallizers 42 comprising a plurality of crystallizers 42 fluidly connected in series such that the monomer-rich solution produced by each crystallizer 42 in the series is fed to the subsequent crystallizer 42 and the mother liquor produced by each crystallizer 42 is provided to the previous crystallizer 42 in the series.

[0059] The continuous flow of mother liquor from each subsequent crystallizer 42 to each previous crystallizer 42 makes it possible to carry out continuous operation. In addition, when crystallization is carried out using a multi-tube crystallizer, it makes it possible to operate in a semi-continuous manner.

[0060] Crystallizer 40 may, for example, include two, three, or more crystallizers 42 fluidly connected in series.

[0061] A crystallizer 40 comprising a series of crystallizers 42 is also referred to as a "cascade crystallizer 40", which makes it possible to carry out cascade fractional crystallization.

[0062] The crystallization apparatus 40 comprising a series of crystallizers 42 comprises a first crystallizer 42 and a last crystallizer 42, and optionally at least one intermediate crystallizer 42 connected in series between the first crystallizer 42 and the last crystallizer 42.

[0063] The crystallization apparatus 40 is configured such that, for example, when the crystallization apparatus 40 includes three or more crystallizers 42 fluidly connected in series, the monomer solution provided by the depolymerization apparatus 4 is supplied to one of the series of crystallizers 42, preferably to an intermediate crystallizer 42 in the series of crystallizers 42.

[0064] As illustrated in FIG. 2, crystallization apparatus 40 includes three crystallizers 42 fluidly connected in series, including a first crystallizer 42 (left side of FIG. 2), a middle crystallizer 42 (middle of FIG. 2), and a last crystallizer 42 (right side of FIG. 2).

[0065] Crystallizer 40 is configured to provide the monomer solution produced by depolymerizer 4 through a feed line to inlet 42A of intermediate crystallizer 42.

[0066] The first outlet 42B of the intermediate crystallizer 42 is fluidly connected to the inlet of the last crystallizer 42. The second outlet 42C of the intermediate crystallizer 42 is fluidly connected to the inlet of the first crystallizer 42.

[0067] A first outlet 42B of the first crystallizer 42 is fluidly connected to the inlet 42A of the intermediate crystallizer 42. A second outlet 42C of the first crystallizer 42 provides a mother liquor stream.

[0068] The second outlet 42C of the final crystallizer 42 is fluidly connected to the inlet 42A of the intermediate crystallizer 42. The first outlet 42B of the final crystallizer 42 provides treated liquid monomer.

[0069] Crystallization apparatus 40 includes a respective crystal wash apparatus 46 associated with each crystallizer 42, which includes a vessel 48 configured to store a wash monomer solution and to supply the wash monomer solution to crystallizer 42 to purify the monomer crystals contained in crystallizer 42.

[0070] Crystallizer 40 is configured so that the wash monomer solution provided to vessel 48 of each crystal wash device 46 is made from molten monomer crystals that contain fewer impurities than the initial monomer solution fed to crystallizer 42.

[0071] During the crystallization step in each crystallizer 42, the crystals accumulate some impurities that may be trapped in the voids between the crystals or on the crystal surfaces. Before completely melting the crystals produced during the crystallization step, a washing device 46 is operable to supply a washing monomer solution to the crystallizer 42 to wash the monomer crystals. The impurity-rich fraction then passes along with the mother liquor to the previous crystallizer 42 or is stored and used in the next crystallization step operated in the same crystallizer 42. The monomer crystals are then completely melted and discharged as a monomer-enriched solution.

[0072] As illustrated in FIG. 2, the crystallizers 40 are configured such that a fraction of the monomer-enriched solution produced by each crystallizer 42 is used as the wash monomer solution for that same crystallizer 42.

[0073] To this end, a tank 48 of each washing device 46 is attached to and fluidly connected to a crystallizer 42 in order to collect a fraction of the enriched monomer solution (monomer-enriched solution) produced by the crystallizer as a wash monomer solution and supply the wash monomer solution to the crystallizer 42. Each tank 48 is fluidly connected in a closed loop to the associated crystallizer 42.

[0074] The crystallizer 40 of Figure 3 differs from that of Figure 2 in that it is configured so that the monomer-rich solution produced by each subsequent crystallizer 42 is used as the wash monomer solution for the previous crystallizer 42. Preferably, the monomer-rich solution produced by the last crystallizer 42 is used as the wash monomer solution for the last crystallizer 42.

[0075] For this purpose, a fraction of the monomer-rich solution produced by the subsequent crystallizer 42 is fed to a tank 48 of a washing device 46 associated with each preceding crystallizer 42. In other words, the monomer-rich solution produced by each subsequent crystallizer 42 is used as washing monomer solution for the preceding crystallizer 42. The last crystallizer 42 is fed with washing monomer solution, for example, from a tank 48 fluidly connected to the first outlet 42B of the last crystallizer 42, which provides the enriched monomer solution.

[0076] Preferably, purification unit 6 includes a water removal unit 50 located upstream of crystallization unit 40 for removing water from the crude monomer solution provided by depolymerization unit 4 before feeding the monomer solution to crystallization unit 40.

[0077] The water removal device 50 includes, for example, a cooling device 52 configured to cool the monomer solution to crystallize the water contained in the monomer solution into ice without crystallizing the monomer contained in the monomer solution, and a removal device 54 configured to remove ice from the monomer solution that is subsequently supplied to the crystallization device 40.

[0078] The cooling device 52 is configured to cool the monomer solution by heat exchange with another fluid, for example, and is a heat exchanger, in particular a liquid / liquid heat exchanger or a suspension liquid crystal precipitater.

[0079] 2, the other liquid is preferably a purified monomer solution produced by crystallizer 40. The outlet of crystallizer 40, which provides the purified monomer solution, is fluidly connected to cooling equipment 52 configured to exchange heat between the monomer solution provided by depolymerizer 4 and the purified monomer solution provided by crystallizer 40.

[0080] The monomer solution provided by the depolymerization apparatus 4 is received at a first inlet 52A of the cooling apparatus 52, and the purified monomer solution provided by the crystallization apparatus 40 is received at a second inlet 52B of the cooling apparatus 52. The first inlet 52A and the second inlet 52B are not fluidly connected.

[0081] In fact, the purified monomer solution leaving the crystallizer 40 exhibits a low temperature, in particular a temperature close to the melting temperature of the monomer, which can be used for the crystallization of the water contained in the monomer solution resulting from the depolymerizer 4, provided that the melting temperature of the monomer is strictly lower than the crystallization temperature of water.

[0082] The removal device 54 is configured to remove ice by, for example, filtration, accumulation on a cold surface, flotation, overflow, decantation, and / or centrifugation.

[0083] Removal device 54 receives the monomer solution cooled by cooling device 52 and provides the monomer solution at a first outlet 54A fluidly connected to crystallizer 40 and provides ice at a second outlet 54B.

[0084] Facility 2 of Figure 1 is operable to carry out a process for producing a monomer from a feedstock comprising a polymer of which the monomer is a constituent.

[0085] The method includes the steps of depolymerizing a feedstock to obtain a crude monomer solution containing monomer and impurities, and purifying the crude monomer solution to obtain a purified monomer solution having an impurity content lower than the impurity content of the crude monomer solution produced by depolymerization.

[0086] The depolymerization step is carried out in a depolymerization unit 4, and the purification step is carried out in a purification unit 6.

[0087] The purification process includes at least one crystallization step performed in crystallizers 40 to separate impurities from the monomer solution, each crystallization step including cooling the monomer solution to form monomer crystals and a mother liquor, separating the crystals from the mother liquor, and melting the monomer crystals to thereby obtain a monomer-enriched solution. Each crystallization step is preferably performed in a respective crystallizer 42.

[0088] The purification step includes subjecting the monomer-enriched solution resulting from the crystallization step to another crystallization step, and / or subjecting the mother liquor resulting from the crystallization step to another crystallization step.

[0089] This is accomplished in a crystallization apparatus 40, each comprising a series of crystallizers 42, by feeding the monomer-enriched solution provided by a crystallizer 42 to a subsequent crystallizer 42 and / or by feeding the mother liquor provided by a crystallizer 42 to an earlier crystallizer 42.

[0090] The crystallization steps performed in the crystallizers 42 are preferably performed at different temperatures, with the crystallization temperature of the crystallization step performed in each earlier crystallizer 42 preferably being strictly lower than the crystallization temperature of the crystallization step performed in the later crystallizer 42.

[0091] Indeed, the purity of the monomer solution increases from each previous crystallizer 42 to the subsequent crystallizer 42, and therefore temperatures progressively closer to the melting temperature of the monomer can be used to progressively separate the monomer from liquid impurities having melting temperatures closer to the melting temperature of the monomer.

[0092] The crystallization step advantageously includes a water removal step carried out in a water removal unit 50 .

[0093] The water removal step comprises cooling the crude monomer solution resulting from the depolymerization step to a cooling temperature so as to convert the water present in the crude monomer solution into ice without crystallizing the monomer, and separating the ice from the monomer solution, and a crystallization step is carried out on the monomer solution after the water removal.

[0094] During the water removal step, the liquid stream is at least partially cooled by heat exchange with the purified monomer solution provided by crystallizer 40 .

[0095] The water removal step is carried out in the purification unit 6 by a water removal unit 50 , in particular a water removal unit 50 comprising a cooling unit 52 and a removal unit 54 .

[0096] The cooling temperature is lower than the crystallization temperature of water and higher than the crystallization temperature of the monomer.

[0097] The difference between the cooling temperature and the melting point of the monomers contained in the liquid stream is greater than 4°C, preferably greater than 6°C, and / or less than 30°C, preferably less than 20°C, more preferably less than 10°C.

[0098] The depolymerization process includes pyrolyzing the feedstock in a pyrolysis reactor 8 to produce a gas stream, and condensing the gas stream in a condenser 10 to obtain a crude monomer solution.

[0099] The depolymerization step advantageously includes a separation step which includes removing solid and / or liquid impurities from the gas stream in separator 18 prior to condensing the gas stream in condenser 10 .

[0100] The depolymerization step advantageously includes purifying the gas stream between the separator 18 and the condenser 10 using a gas purification unit 24 fluidly connected in series between the separator 18 and the condenser 10, the gas purification unit 24 comprising a transfer column 26 having internals to which the monomer solution is fed such that the gas stream flows upwardly through the transfer column 26 and the monomer solution flows downwardly through the transfer column 26, preferably back to the separator 18.

[0101] The purification step advantageously includes passing the stream of monomer solution through an optional solution purifier 30 to purify the monomer solution.

[0102] Solution clarification system 30 is configured, for example, to pass the monomer solution through evaporator 32 and then through condenser 34 to condense the monomer solution and separate the heavier fraction from the monomer and lighter fraction.

[0103] Solution clarification system 30 optionally optionally or alternatively comprises a distillation system or a thin film evaporator or a short path distillation system or any other suitable technology or combination of technologies.

[0104] An optional solution clarifier 30 allows for the removal of heavy compounds (or "heavies") The light compounds remaining in the monomer solution fed to the purification unit 6 may therefore contain impurities that have a lower melting point than the monomer and that can be separated from the monomer by crystallization.

[0105] The purification step advantageously includes a washing step which involves adding water to the monomer solution upstream from the crystallizer 6 in a washing unit 36 .

[0106] In one example, the polymer is polymethyl methacrylate (PMMA) and the monomer is methyl methacrylate (MMA).

[0107] PMMA refers to a homopolymer or copolymer of methyl methacrylate (MMA) containing at least 50% by weight, preferably at least 60% by weight, more preferably at least 70% by weight, advantageously at least 80% by weight, and even more advantageously at least 90% by weight of methyl methacrylate.

[0108] The melting point of pure water is 0° C. The melting point of pure MMA is −48° C. Here, the melting points are given for a pressure of 1013 mbar.

[0109] In fact, it has been found that the main impurities contained in crude MMA obtained by depolymerization have melting points strictly lower than that of MMA and can be effectively removed by the crystallization step.

[0110] Possible impurities and their effective or predicted melting points are listed in the table below:

[0111] [Table 1]

[0112] In another embodiment, instead of removing water from the liquid stream provided by depolymerizer 4, the method includes adding at least one anti-icing additive to the monomer solution prior to carrying out the crystallization step to inhibit ice formation during the crystallization step.

[0113] The at least one anti-icing additive includes, for example, methanol.

[0114] By suppressing water crystal (ice) formation through the addition of at least one anti-icing additive to the crude monomer solution, some water is incorporated into the monomer crystals, thus allowing the crystallization process to be carried out while removing the main impurities, even if they were present in the monomer solution.

[0115] In particular, unexpectedly, when methanol is added to the crude MMA solution at a methanol / MMA ratio preferably higher than 4 g / 100 g and / or preferably lower than 50 g / 100 g, ice formation can be suppressed and MMA crystals can be produced directly. Although the MMA crystals formed during crystallization incorporate some water, major impurities such as methyl isobutyrate, ethyl acrylate, and toluene can be effectively removed.

[0116] Optionally, the purified MMA solution can be further purified by a distillation step carried out after the crystallization step.

[0117] Molecules other than methanol can be used as anti-icing additives in crude MMA, but methanol is an impurity already present in crude MMA and is significantly cheaper.

[0118] The present invention is not limited to the examples described above and illustrated in Figures 1 to 3. Other embodiments are also possible.

[0119] For example, instead of having a series of crystallizers, crystallizer 40 may include only one single crystallizer 42 .

[0120] 1, optionally, at least a fraction of the mother liquor produced in the crystallizer 40 is returned to the gas scrubber 24, in particular injected into the upper part 26B of the transfer column 26. In such a case, a return line 58 is provided between the crystallizer 40 and the transfer column 26. This makes it possible to recover some of the monomer still present in the mother liquor.

[0121] Optionally, in some cases, at least a fraction of the mother liquor produced by crystallizer 40 is used as a source of cold in one or more heat exchangers. [Example]

[0122] To realize the examples, two water-ethanol solutions were prepared with two different ethanol concentrations. The two solutions were introduced into two separate Dewar vessels. Dry ice was crushed into powder and slowly added to the Dewar vessel. When the solution bubbled more gently, larger pieces of dry ice were added to the Dewar vessel until the temperature stabilized and the solution reached the consistency of a syrup. The first bath solution was adjusted (i.e., the ethanol / water ratio was adjusted) to have a temperature between -40°C and -42°C, while the second bath solution was adjusted to reach a temperature between -52°C and -57°C. By varying the ethanol content in the aqueous solution, the temperature at which the syrup formed and the temperature at which the temperature stabilized could be adjusted.

[0123] Two glass vessels, each approximately 4 cm in diameter and 30 cm long, were then placed into the two Dewar flasks. Approximately 100 ml of crude MMA was placed into the first vessel, which reached approximately −40°C after approximately 30 minutes, with dry ice added to the Dewar flask to stabilize the temperature. Some crystals formed in the suspension and were filtered. The solids were collected and melted only above 0°C or upon reaching room temperature. The filtration had to be performed very quickly to avoid melting the solids. For this purpose, filter paper with low thermal conductivity was used. The collected solids were transferred to a sample vial. While filtering, the liquid fraction was poured into a second glass vessel, which was at an even lower temperature. The bath temperature was then adjusted again to reach below −52°C within approximately 20–30 minutes. Approximately half of the liquid then solidified. While cooling, a glass rod was introduced into the vessel, and crystals could be observed forming on the glass rod. The liquid fraction was then collected and poured into a collection flask. The solids were allowed to drain for approximately 1-2 minutes, and the liquid fraction that formed was added to the previous liquid fraction. The remainder of the solids were then melted and poured into a separate collection flask.

[0124] In some experiments, a slightly larger amount of product was crystallized, and the crystallized solid was melted and recrystallized to improve the purity of the solid fraction. In this case, two mother liquor liquid fractions were collected, as well as one final crystallized fraction.

[0125] [Table 2]

[0126] Example 1 The crude MMA was obtained from PMMA depolymerized at 470°C in a twin-screw extruder and was produced by connecting the main separator 20 directly to the condenser 10. This means that the purifying device 26 and the main separator 22 were omitted.

[0127] The product was first cooled to -40°C to separate the first crystal fraction, then cooled to -52°C, where the crystallized fraction and mother liquor were collected. The crystallized fraction was collected, melted, and recrystallized, but at -57°C, to produce a second mother liquor and a new crystallized fraction. The initial crude MMA is orange in color. There is a slight change in color of the final crystallized fraction.

[0128] <Example 2> The previous example was repeated, but with 4% by weight of methanol added to the crude MMA. More specifically, a first injection of 2% by weight of methanol was added, and the solution was cooled to -40°C, resulting in the formation of crystals. The solution was then melted at room temperature, and a second injection of 2% by weight of methanol was added, for a total of 4% by weight. The solution was then cooled to -40°C. No solids were observed to form. The liquid was then further cooled to -57°C and held at that temperature for approximately 20 minutes. The liquid fraction was collected, and the wet fraction was added to it, and the solids were melted and collected separately. The initial crude MMA was orange in color. There was a slight change in color of the final crystallized fraction.

[0129] <Comparative Example 1> A clear, virgin MMA sample purchased from Aldrich was used as the standard.

[0130] 100 mg of the peroxide LUPEROX® LP (dilauroyl peroxide) was then added to approximately 5 ml of the liquid monomer solution in a test tube equipped with a polymer cap. The addition was performed at room temperature, and the sample was shaken and then placed in a sample holder, which was in a water bath maintained at a constant temperature of 65°C using water circulation. Organic peroxides should be handled with care as they can decompose violently.

[0131] Within 2 minutes, the color of the solution did not change. The sample was kept for 2 days, and even after that long period, no color change could be detected.

[0132] Example 3 Crude MMA obtained from cast clear PMMA sheet was used in this study. The PMMA was depolymerized at 450°C, then the temperature was increased to 470°C in a twin-screw extruder while the feed rate was increased from 50 kg / h to 90 kg / h. Hot vapors were collected, washed with countercurrent cold crude MMA, and condensed in a condenser set up as a shell-and-tube heat exchanger (corresponding to condenser 10 in the installation in Figure 1).

[0133] A sample of the clear, transparent solution was taken from the condenser.

[0134] 100 mg of the peroxide LUPEROX® LP (dilauroyl peroxide) was then added to approximately 5 ml of the liquid monomer solution in a test tube equipped with a polymer cap. The addition was performed at room temperature, and the sample was shaken and then placed in a sample holder, which was in a water bath maintained at a constant temperature of 65°C using water circulation. Organic peroxides should be handled with care as they can decompose violently.

[0135] Within 2 minutes the solution changed color to light brown.

[0136] After the crystallization test described in the table above, the test with peroxide was repeated on the "mother liquor" and on the "melt crystals". A clear color difference could be seen, with the mother liquor being darker.

[0137] Example 4 Crude material obtained from white cast PMMA sheet was used in this study. PMMA was depolymerized at 470°C in a twin-screw extruder operated at 90 kg / h. Hot vapors were collected, washed with cold crude PMMA in a countercurrent manner, and condensed in a condenser formed by a shell-and-tube heat exchanger. A sample of the clear solution was taken from the condenser.

[0138] The peroxide test was repeated on the crude MMA solution, and the color changed to pinkish brown within 2 minutes.

[0139] After the crystallization test described in the table above, the test with peroxide was repeated on the "mother liquor" and on the "molten crystals". A clear color difference could be seen, with the mother liquor being darker. This confirms that the impurities had concentrated in the mother liquor.

[0140] <Examples 5 and 6> The crude MMA was obtained by depolymerization of a mixture of cast and extruded post-consumer wrap sheets collected from a company that cuts the sheets to size and fabricates a variety of products from them. These are mixed-color cast and extruded PMMA sheets from various producers in a ratio of approximately 80% cast sheet and 20% extruded sheet by weight. The processed mixture represents the average composition that can be collected from companies of this type in Europe.

[0141] PMMA was depolymerized at 470°C in a twin-screw extruder operated at 90 kg / h. Hot vapors were collected, washed with countercurrent cold crude MMA, and condensed in a shell-and-tube heat exchanger. A sample of the clear solution was taken from the condenser.

[0142] The peroxide test was repeated on this crude MMA solution and the color changed to a pinkish brown within 2 minutes.

[0143] After the crystallization test described in the table above, the test with peroxide was repeated on the "mother liquor" and on the "melt crystals". A clear color difference could be seen, with the mother liquor showing a darker color. This confirms that the impurities had concentrated in the mother liquor.

[0144] This test was repeated after about 30 minutes when a new sample was taken from the condenser.

[0145] The same results were obtained.

[0146] Example 7 Crude MMA was obtained from injection-molded PMMA scrap containing red, black, and transparent particles from car taillights and used in this study. The PMMA was depolymerized at 470°C in a twin-screw extruder operated at 50 kg / h. Hot vapors were collected, washed with countercurrent cold crude MMA, and condensed in a condenser consisting of a shell-and-tube heat exchanger. A sample of the clear, transparent solution was taken from the condenser.

[0147] The peroxide test was repeated on this crude MMA solution and the color changed to orange within 2 minutes.

[0148] After the crystallization test described in the table above, the test with peroxide was repeated on the "mother liquor" and on the "molten crystals". A clear color difference could be seen, with the mother liquor being darker. This confirms that the impurities had concentrated in the mother liquor.

[0149] Example 8 Crude MMA was obtained from equal parts white cast PMMA and impact resistant material mixed with fiberglass and adhesives used to bond the glass fiber to the PMMA to strengthen the bathtub. The PMMA was depolymerized at 470°C in a twin-screw extruder operated at a total feed rate of 50 kg / h. Hot vapors were collected, washed with countercurrent cold crude MMA, and condensed in a condenser consisting of a shell-and-tube heat exchanger. A sample of the clear solution was taken from the condenser.

[0150] The peroxide test was repeated on this crude MMA solution and the color changed to orange within 2 minutes.

[0151] After the crystallization tests described in the table above, the peroxide test was repeated on the "mother liquor" and on the "melt crystals." A clear color difference could be seen, with the mother liquor being darker than the MMA from the second crystallization. This confirms that the impurities had concentrated in the mother liquor. [Explanation of symbols]

[0152] 2 Equipment 4 Depolymerization equipment 6 Purification equipment 8. Pyrolysis Reactor 10 Condenser 12 cylindrical barrel 14 Screw 16 Feeder 18 Separation device 20 Main separator 22 Sub-separator 24 Gas Purification Equipment 26A lower end fluidly connected to the main separator 20 26B upper end fluidly connected to the inlet of the condenser 10 26 Moving Column 28 Reinfusion Loop 30 Solution Purification Device 32 Evaporator 32A Light fraction outlet 32B Heavy fraction outlet 34 Condenser 36 Cleaning equipment 40 Crystallization equipment 42 Crystallizer 42A Entrance 42B Exit 1 42C 2nd Exit 46 Crystal Cleaning Equipment 48 tanks 50 Water removal equipment 52 Cooling equipment 54 Removal equipment 54A 1st Exit 54B 2nd Exit

Claims

1. 1. A method for producing a monomer from a feed containing a polymer of which said monomer is a constituent, comprising the steps of: - depolymerizing the feedstock to obtain a monomer solution containing the monomers and impurities; and - purifying said monomer solution, thereby obtaining a purified monomer solution having an impurity content lower than the impurity content of said monomer solution. Including, The purification step comprises at least one crystallization step, each crystallization step comprising the steps of: a) cooling the monomer solution so that the monomer solution forms monomer crystals and a mother liquor; b) separating the monomer crystals from the mother liquor; and c) melting the monomer crystals, thereby obtaining a monomer-enriched solution A method comprising:

2. 2. The process of claim 1, wherein the purification step comprises subjecting the enriched monomer solution resulting from one crystallization step to another crystallization step.

3. The method according to claim 1 or 2, wherein the purification step comprises subjecting a mother liquor resulting from one crystallization step to another crystallization step.

4. The method according to claim 1 , wherein the purification step comprises a water removal step.

5. The water removal step comprises: - cooling the monomer solution to a cooling temperature, thereby converting the water present in the monomer solution into ice without crystallizing the monomer; and - separating the ice from the monomer solution. Including, 5. The process of claim 4, wherein the at least one crystallization step is performed on the monomer solution after separation from water.

6. The method according to claim 5 , wherein the cooling temperature is higher than the crystallization temperature of the monomer contained in the monomer solution.

7. 7. The process according to claim 6, wherein the difference between the cooling temperature and the melting point of the monomer contained in the liquid stream is more than 4°C, preferably more than 6°C, and / or less than 30°C, preferably less than 20°C, more preferably less than 10°C.

8. 8. The process according to any one of claims 4 to 7, wherein during the water removal step, the monomer solution is at least partially cooled by heat exchange with the purified monomer solution resulting from crystallization.

9. 4. The method according to claim 1, further comprising the step of adding at least one anti-icing additive to the monomer solution before carrying out the at least one crystallization step so as to inhibit ice formation during the at least one crystallization step.

10. The method of claim 9 , wherein the at least one anti-icing additive comprises methanol.

11. 11. The process according to any one of claims 1 to 10, wherein the depolymerization step comprises the steps of pyrolyzing the feedstock in a pyrolysis reactor, thereby generating a gas stream, and condensing the gas stream in a condenser, thereby obtaining the monomer solution.

12. 12. The process of claim 11, wherein the depolymerization step comprises a separation step comprising removing solid and / or liquid impurities from the gas stream in a separation device prior to condensing the gas stream.

13. 13. The process of claim 12, further comprising the step of purifying the gas stream between the separator and the condenser using a gas purification device fluidly connected in series between the separator and the condenser, the gas purification device comprising a transfer column including internals, the gas purification device being fed with a fraction of the monomer solution leaving the condenser such that the gas stream flows upward in the transfer column and the monomer solution flows downward in the transfer column, and heavy contaminants contained in the gas stream condense to a liquid state on the internals and flow by gravity back towards the bottom of the transfer column, preferably back to the separator.

14. 14. The process of claim 12 or 13, wherein the separation device comprises a secondary separator and a primary separator fluidly connected in series.

15. 15. The process of any one of claims 11 to 14, comprising purifying the monomer solution between the condenser and the purification unit by passing the monomer solution successively through an evaporator and a condenser, thereby removing heavier components from the monomer solution.

16. 16. A process according to any one of claims 11 to 15, comprising a washing step upstream from the crystallizer, comprising adding water to the monomer solution.

17. 17. The method of any one of claims 1 to 16, wherein the polymer is polymethyl methacrylate (PMMA) and the monomer is methyl methacrylate (MMA).

18. 18. The process according to any one of claims 1 to 17, wherein the number of crystallization steps is 5 or less, in particular 3 or less.

19. 19. An installation for producing a liquid monomer from a feed comprising a polymer of which said monomer is a constituent, the installation being configured to carry out the process of any one of claims 1 to 18.

20. 20. The installation according to claim 19, comprising a depolymerization unit (4) configured to depolymerize the feedstock to obtain a monomer solution containing the monomer and impurities, and a purification unit (6) configured to purify the monomer solution to obtain a purified monomer solution having an impurity content lower than the impurity content of the monomer solution resulting from depolymerization.

21. 21. The installation according to claim 20, wherein the depolymerization unit (4) comprises a pyrolysis reactor (8) and a condenser (10).

22. 22. The installation according to claim 21, wherein the pyrolysis reactor (8) is a heated single-screw extruder comprising a heated cylindrical barrel (12) and at least one screw (14) disposed inside the barrel (12).

23. 23. The installation according to any one of claims 20 to 22, wherein the depolymerization device (4) comprises a separation device (18) connected in series between the outlet of the pyrolysis reactor (8) and the inlet of the condenser (10).

24. 24. The installation of claim 23, wherein the separation device (18) comprises a primary separator (20).

25. 25. Installation according to claim 23 or 24, wherein the depolymerization device (4) comprises a gas purification device (24) connected in series between the separation device (18) and the condenser (10).

26. 26. Installation according to any one of claims 20 to 25, wherein the purification device (6) comprises a solution clarification device (30).

27. 27. The installation according to any one of claims 20 to 26, wherein the purification unit (6) comprises a crystallization unit (40).

28. 28. The installation of claim 27, wherein the crystallizer (40) comprises one or more crystallizers (42).

29. 29. Installation according to claim 27 or 28, wherein the crystallizer (40) comprises a crystal washing device (46).

30. 28. The installation according to claim 27, wherein the purification unit (6) comprises a water removal unit (50) arranged upstream of the crystallization unit (40).

31. 31. The installation of claim 30, wherein the water removal device (50) comprises a cooling device (52) configured to cool the monomer solution and a removal device (54) configured to remove ice from the monomer solution that is subsequently supplied to the crystallization device (40).

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