Method and apparatus for producing monomers by depolymerization of corresponding polymer

Abstract

The invention relates to a method for producing a monomer from a feed material containing a polymer, said monomer being a component of said polymer, the production method comprising the following steps:-depolymerizing the feed material to obtain a monomer solution containing the monomer and impurities; and-purifying the monomer solution, thereby obtaining a purified monomer solution, the impurity content of which is lower than the impurity content of the monomer solution, in which the purification step comprises at least one crystallization step, each crystallization step comprising the following steps: a) cooling the monomer solution such 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] The project leading to this patent application has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement number 820687.

[0002] The present invention relates to the field of depolymerization for recovering monomers from feed materials containing polymers, said monomers being components of said polymers.

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

[0004] MMA is obtained, for example, by pyrolyzing PMMA in a pyrolysis reactor to obtain a gas stream comprising gaseous MMA and unavoidable impurities, separating solid impurities from the gas stream in a separator, and condensing the gas stream to obtain liquid MMA. The MMA obtained by depolymerization of PMMA is sometimes referred to as crude MMA.

[0005] The liquid MMA obtained from the depolymerization of PMMA may contain impurities, such as other chemical compounds present in the raw material containing PMMA or generated during the depolymerization process, such as during pyrolysis.

[0006] Liquid MMA can be purified. Purification of crude MMA can be performed by distillation, which is a process of separating components from a liquid mixture by using selective evaporation and condensation and based on the boiling points of the components or their azeotropes.

[0007] JP2020 / 114806 discloses a method for depolymerizing polylactic acid into lactide, which is fed into a ventilation chamber connected to a collecting device comprising a gas / liquid separation column and a purification device by melting a polymer and a depolymerization catalyst into an extruder. The purification device is configured with a crystallizer, and optionally one or more crystallizers for performing a crystallization stage in series.

[0008] WO2021 / 032821 discloses a method for depolymerizing terephthalate polymers, comprising dimer crystallization and monomer crystallization operated in series. Summary of the invention

[0009] One of the objects of the present invention is to propose a process for producing monomers from recycled polymers which allows obtaining monomer solutions having a satisfactory purity.

[0010] To this end, the present invention provides a method for producing a monomer from a feed material containing a polymer, the monomer being a component of the polymer, the production method comprising the following steps:

[0011] - depolymerizing the feed material to obtain a monomer solution containing monomers and impurities; and

[0012] - purifying the monomer solution to thereby obtain a purified monomer solution having an impurity content lower than the impurity content of the monomer solution,

[0013] The purification step comprises at least one crystallization step, each crystallization step comprising the following steps:

[0014] a) cooling the monomer solution so that the monomer solution forms monomer crystals and mother liquor,

[0015] b) separating the monomer crystals from the mother liquor, and

[0016] c) Melting the monomer crystals, thereby obtaining a monomer-rich solution.

[0017] According to other advantageous aspects of the invention, the production method comprises one or several of the following features, which are used alone or in any technically possible combination:

[0018] - a purification step comprising subjecting the monomer-enriched solution obtained from one crystallization step to another crystallization step;

[0019] - a purification step consisting in subjecting the mother liquor obtained from one crystallization step to another crystallization step;

[0020] - the purification step includes a water removal step;

[0021] - The water removal step includes:

[0022] + cooling the monomer solution to a cooling temperature, thereby converting any water present in the monomer solution into ice without crystallizing the monomer, and

[0023] + Separation of ice from monomer solution,

[0024] whereby the monomer solution is subjected to at least one crystallization step after the removal of water;

[0025] - the cooling temperature is higher than the crystallization temperature of the monomers contained in the monomer solution;

[0026] the difference between the cooling temperature and the melting point of the monomers contained in the liquid stream is higher than 4° C., preferably higher than 6° C. and / or lower than 30° C., preferably lower than 20° C., more preferably lower than 10° C.;

[0027] - during the water removal step, the monomer solution is cooled at least partially by heat exchange with the purified monomer solution resulting from the crystallization;

[0028] - the production process comprises adding at least one anti-icing additive to the monomer solution before carrying out at least one crystallization step, thereby inhibiting the formation of ice during the implementation of at least one crystallization step;

[0029] - at least one anti-icing additive includes methanol;

[0030] - the depolymerization step comprises pyrolyzing the feed material in a pyrolysis reactor, thereby generating a gas stream, and condensing the gas stream in a condenser, thereby obtaining a monomer solution;

[0031] - the depolymerization step comprises a separation step which comprises removing solid and / or liquid impurities from the gas stream in a separation unit before condensing the gas stream;

[0032] - the production method comprises the following steps: cleaning the gas stream between the separation unit and the condenser using a gas cleaning unit connected in series between the separation unit and the condenser by fluid, the gas cleaning unit comprising a transfer column containing internals, the gas cleaning unit being fed with a portion of the monomer solution leaving the condenser, so that the gas stream flows upwards in the transfer column and the monomer solution flows downwards in the transfer column, heavy pollutants contained in the gas stream condense in liquid form on the internals and flow back by gravity to the bottom of the transfer column and preferably to the separation unit;

[0033] - the separation unit comprises an auxiliary separator and a main separator connected in fluid series;

[0034] - the production method comprises the steps of: cleaning the monomer solution between the condenser and the purification unit by passing the monomer solution through the evaporator and the condenser in sequence, thereby removing heavies from the monomer solution;

[0035] - the production process comprises a washing step comprising adding water to the monomer solution upstream of the crystallization unit;

[0036] - the polymer is polymethyl methacrylate (PMMA) and the monomer is methyl methacrylate (MMA); and

[0037] The number of crystallization steps is equal to or less than five, in particular equal to or less than three.

[0038] A subject of the invention is also a plant for producing a liquid monomer from a feed material containing a polymer, said monomer being a constituent of said polymer, said plant being configured to implement the production method as defined above. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] 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:

[0040] -Figure 1 is a schematic diagram illustrating an apparatus for producing a monomer solution from a feed material containing a polymer, the monomer being a component of the polymer;

[0041] - Figure 2 is a schematic diagram, which explains Figure 1 a purification unit of the equipment; and

[0042] - Figure 3 is a schematic diagram, which explains Figure 1 An alternative purification unit for a production plant. DETAILED DESCRIPTION

[0043] Figure 1 The apparatus 2 is configured to produce monomers from a feed material containing a polymer, the monomers being components of the polymer.

[0044] The apparatus 2 comprises: a depolymerization unit 4 configured to depolymerize a feed material to obtain a monomer solution containing monomers 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 that of the monomer solution produced by depolymerization.

[0045] Hereinafter, the monomer solution obtained by depolymerization is also referred to as a "crude monomer solution" or "crude monomer".

[0046] The depolymerization unit 4 , for example, comprises: a pyrolysis reactor 8 configured to receive feed materials and generate a gas stream containing gaseous monomers; and a condenser 10 configured to condense the gas stream to obtain a monomer solution containing liquid monomers.

[0047] The pyrolysis reactor 8 is, for example, a heated screw extruder comprising a heated tubular barrel 12 and at least one screw 14 arranged inside the barrel 12 for moving the feed material through the barrel 12 and heating the feed material.

[0048] The barrel 12 is preferably configured to heat the feed material according to a temperature profile, ie, a temperature that varies along the barrel 12. The barrel 12 includes, for example, a plurality of barrel sections configured to heat at different temperatures to obtain the temperature profile.

[0049] The depolymerization unit 4 preferably comprises a feeder 16 configured for feeding feed material to an inlet of the pyrolysis reactor 8 .

[0050] The depolymerization unit 4 optionally comprises a separation unit 18 connected in series between the outlet of the pyrolysis reactor 8 and the inlet of the condenser 10 , which is configured to separate solid impurities and / or liquid impurities from the gas stream before feeding the gas stream to the condenser 10 .

[0051] The separation unit 18, for example, includes a main separator 20, which is configured to reduce the linear velocity of the gas stream to promote the separation and deposition of solid impurities and / or to demist the gas stream to promote the deposition of droplets of liquid impurities and / or to scrub the gas stream with a scrubbing liquid to promote the deposition of liquid impurities.

[0052] The reduction in the linear velocity of the gas stream is achieved, for example, by gradually increasing the cross-sectional area of ​​the internal flow passage of the main separator 20 through which the gas stream flows.

[0053] Preferably, the main separator 20 is configured such that the internal flow channel of the main separator 20 does not follow a straight line. Such a configuration facilitates separation and sedimentation of impurities. Demisting of the gas stream is obtained, for example, by providing one or several demisting grids across the internal flow channel.

[0054] Scrubbing of the gas is obtained, for example, by injecting water and / or injecting liquid monomer inside the flow channels.

[0055] The separation unit 18 optionally comprises an auxiliary separator 22 disposed upstream of the main separator 20. The gas stream produced by the pyrolysis reactor 8 flows through the auxiliary separator 22 before entering the main separator 22.

[0056] The auxiliary separator 22 is configured, for example, to remove solid impurities from the hot gas stream, for example, by inertia. The auxiliary separator 22 is configured, for example, with an internal flow channel having a cross-sectional change to facilitate a reduction in linear velocity and / or a change in direction, the internal flow channel being located above the collection tank so that solid particles slowed down by the reduction in linear velocity and / or driven by inertia tend to impact the wall of the internal flow channel and fall into the collection tank.

[0057] Advantageously, the depolymerization unit 4 comprises a gas cleaning unit 24 connected in series between the separation unit 18 and the condenser 10 and configured for removing heavies from the gas stream before it enters the condenser 10 .

[0058] In operation, the gas stream exiting the separation unit 18 passes through the gas cleaning unit 24 before entering the condenser 10 .

[0059] The gas cleaning unit 24 comprises a transfer column 26 extending upwardly between a lower end 26A which is fluidly connected to the separation unit 18 (particularly the main separator 20 ) for receiving the gas stream leaving the separation unit 18 and an upper end 26B which is fluidly connected to the inlet of the condenser 10 .

[0060] The gas cleaning unit 24 comprises a reinjection loop 28 configured for reinjecting a portion of the monomer solution collected downstream of the condenser 10 into the top portion of the transfer column 26 , in particular at the top end 26B of the transfer column 26 .

[0061] The transfer column 26 is provided with internals. The internals include, for example, baffles, structured packing, mesh packing and / or random packing, for example, random packing including saddles and / or rings, in particular ball rings.

[0062] The internal components are configured so that in operation the gas stream flows upward in the transfer column, the temperature of the gas stream decreases along the transfer column, and the monomer solution flows downward inside the transfer column 26 by gravity, and the heavy pollutants contained in the gas stream condense on the internal components and flow back to the transfer column 26 in liquid form and enter the separation unit 18, particularly the main separator 20.

[0063] Preferably, the temperature of the gas stream at the outlet of the main separator 20 and the inlet of the transfer column 26 is maintained at least at the boiling point of the monomer + 20°C, particularly at least at the boiling point of the monomer + 30°C, more particularly at least at the boiling point of the monomer + 40°C, even more particularly at least at the boiling point of the monomer + 50°C.

[0064] Preferably, the gas stream 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.

[0065] Optionally, the purification unit 6 comprises a solution cleaning unit 30 configured to remove liquid impurities from the monomer solution provided by the depolymerization unit.

[0066] The solution cleaning unit 30 , for example, includes an evaporator 32 and a condenser 34 fluidly connected in series for sequentially evaporating and condensing the monomer solution.

[0067] Upon evaporating and condensing the stream, the heavies of the monomer solution may be separated from the monomers and lights contained in the monomer solution.

[0068] Monomers and lights are provided, for example, at a lights outlet 32A of the evaporator 32 connected to an inlet of the condenser 34 of the solution cleaning unit 30 , while heavies are discharged at a heavies outlet 32B of the evaporator 32 .

[0069] Alternatively or optionally, the solution cleaning unit 30 comprises one or several distillation columns connected in series, for example one or two distillation columns connected in series. Each distillation column allows the monomer to be separated from other compounds present in the monomer solution at that point in the apparatus 2.

[0070] Alternatively or optionally, the solution cleaning unit 30 comprises a thin film evaporator or a short path distillation unit or any other suitable technology or combination of technologies.

[0071] Optionally, the purification unit 6 comprises a washing unit 36 ​​configured for adding water and possibly other chemicals to the monomer solution provided by the depolymerization unit 4 .

[0072] When the washing unit 36 ​​and the solution cleaning unit 30 are provided in the purification unit 6 , the washing unit 36 ​​is preferably arranged upstream of the solution cleaning unit 30 .

[0073] Washing unit 36 ​​is configured to add water to the monomer solution in one, two or more steps, for example. In the first step, an alkaline aqueous solution is mixed with the monomer solution to capture organic and inorganic acids, such as hydrochloric acid (HCl), methacrylic acid, acetic acid ... After a decantation or centrifugation step or any other suitable separation step, the aqueous phase and the organic phase are separated. The organic phase, which may be contaminated by trace alkaline compounds remaining, is washed with fresh water. After a new decantation or other separation step, the organic phase can be directed to the next stage, while the aqueous phase containing trace alkaline compounds and other partially water-soluble organics is directed to the first alkaline washing step, which not only saves water, but also recycles some partially water-soluble monomers. The washing step can be extended by one or more additional washing steps using other chemicals to capture additional impurities. These additional washing steps can be performed at any position in the washing sequence, but will preferably be performed before the last clean water washing step.

[0074] After the washing step, some remaining water remains soluble in the monomer solution.

[0075] The purification unit 6 comprises a crystallization unit 40 configured to perform at least one crystallization step to separate impurities from the monomer solution, each crystallization step comprising 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 obtain an enriched monomer solution.

[0076] Such crystallization allows to remove from the monomer solution components whose melting temperature is strictly lower than that of the monomers. Such components are contained in the mother liquor.

[0077] Crystallization in which some components are removed from the initial solution undergoing crystallization is also known as "fractional crystallization".

[0078] The monomer solution may be subjected to several crystallization steps with a view to obtaining a monomer solution with high purity. For example, the mother liquor may also be subjected to a crystallization step with a view to recovering the monomers still present in the mother liquor.

[0079] In one example, where the crystallization unit 40 is configured to perform several crystallization steps, the crystallization unit 40 is configured to subject the monomer-rich solution obtained from a crystallization step to another crystallization step and / or subject the mother liquor obtained from a crystallization step to another crystallization step.

[0080] like Figure 2 In the description, the crystallization unit 40 comprises one or more crystallizers 42, each of which is configured to perform various crystallization steps. Each crystallizer 42 is configured to receive a liquid (a monomer solution provided by the depolymerization unit 4 or a rich monomer solution provided by the previous crystallizer 42), and is used to cool the liquid to crystallize the monomer to obtain a rich monomer solution and a mother liquor.

[0081] Each crystallizer 42 is configured, for example, to collect mother liquor and to collect monomer crystals as a rich monomer solution.

[0082] Each crystallizer 42 is, for example, configured for receiving a monomer solution at an inlet 42A, for providing a rich monomer solution at a first outlet 42B and for providing a mother solution at a second outlet 42C.

[0083] Each crystallizer 42 is, for example, configured for batch operation, ie, for sequentially receiving a batch of monomer solution, subjecting the monomer solution to crystallization and collecting mother liquor and purified monomers, and then receiving a new batch of monomer solution.

[0084] Alternatively, each crystallizer 42 is configured for semi-continuous operation, that is, for mother liquor to flow continuously through the crystallizer 42 and to recover purified monomer at certain intervals (particularly regular intervals). For example, a multi-tubular crystallizer can be operated in a semi-continuous manner.

[0085] The crystallization unit 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 of crystallizers 42 is supplied to a subsequent crystallizer 42, and the mother liquor produced by each crystallizer 42 is supplied to a preceding crystallizer 42 in the series of crystallizers 42.

[0086] The mother liquor flows continuously from each subsequent crystallizer 42 to each preceding crystallizer 42 so that continuous operation can be implemented. In addition, when a multi-tubular crystallizer is used for crystallization, it can be operated in a semi-continuous process.

[0087] The crystallization unit 40 comprises, for example, two, three or more crystallizers 42 fluidly connected in series.

[0088] A crystallization unit 40 comprising a series of crystallizers 42 is also referred to as a "cascade crystallization unit 40". It allows a cascade of fractional crystallizations to be performed.

[0089] The crystallization unit 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.

[0090] The crystallization unit 40 is for example configured such that the monomer solution provided by the depolymerization unit 4 is fed to one of a series of crystallizers 42, preferably a middle crystallizer 42 of the series of crystallizers 42 when the crystallization unit 40 comprises three or more crystallizers 42 connected in fluid series.

[0091] like Figure 2 The crystallization unit 40 includes three crystallizers 42 connected in series, including a first crystallizer 42 ( Figure 2 Left side), middle crystallizer 42 ( Figure 2 middle) and the final crystallizer 42 ( Figure 2 on the right).

[0092] The crystallization unit 40 is configured to supply the monomer solution produced by the depolymerization unit 4 to an inlet 42A of the intermediate crystallizer 42 via a feed line.

[0093] The first outlet 42B of the intermediate crystallizer 42 is fluidly connected to the inlet of the final crystallizer 42. The second outlet 42C of the intermediate crystallizer 42 is fluidly connected to the inlet of the first crystallizer 42.

[0094] The first outlet 42B of the first crystallizer 42 is fluidly connected to the inlet 42A of the intermediate crystallizer 42. The second outlet 42C of the first crystallizer 42 provides a mother liquor stream.

[0095] 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 the treated liquid monomer.

[0096] The crystallization unit 40 includes a respective crystal wash apparatus 46 associated with each crystallizer 42 , the crystal wash apparatus 46 including a tank 48 configured for storing a wash monomer solution and for feeding the wash monomer solution to the crystallizer 42 for cleaning the monomer crystals contained therein.

[0097] The crystallization unit 40 is configured so that the wash monomer solution provided to the tank 48 of each crystal wash device 46 is made from molten monomer crystals that contain fewer impurities than the initial monomer solution provided to the crystallizer 42 .

[0098] During the crystallization step performed in each crystallizer 42, the crystals accumulate some impurities, which may be trapped in the interstices between the crystals or on the surface of the crystals. Before completely melting the crystals produced during the crystallization step, the washing device 46 is operable to feed the crystallizer 42 to wash the monomer crystals with a washing monomer solution. The impurity-rich portion is then directed to the previous crystallizer 42 together with the mother liquor or stored for the next crystallization step operated in the same crystallizer 42. The monomer crystals are then completely melted and discharged as an enriched monomer solution.

[0099] like Figure 2 As illustrated, the crystallization unit 40 is configured so that a portion of the rich monomer solution produced by each crystallizer 42 is used as the wash monomer solution for the same crystallizer 42 .

[0100] To this end, the tank 48 of each washing device 46 associated with the crystallizer 42 and fluidly connected to the crystallizer 42 is used to collect a portion of the rich monomer solution produced by the crystallizer as a washing monomer solution and to feed the washing monomer solution to the crystallizer 42. Each tank 48 is fluidly connected to the associated crystallizer 42 in a closed loop.

[0101] Figure 3 The crystallization unit 40 and Figure 2 The difference is that it is configured so that the rich monomer solution produced by each subsequent crystallizer 42 is used as the washing monomer solution of the previous crystallizer 42. Preferably, the rich monomer solution produced by the last crystallizer 42 is used as the washing monomer solution of the last crystallizer 42.

[0102] To this end, the tank 48 of the washing device 46 associated with each preceding crystallizer 42 is fed with a portion of the rich monomer solution produced by the succeeding crystallizer 42. In other words, the rich monomer solution produced by each succeeding crystallizer 42 is used as the washing monomer solution for the preceding crystallizer 42. For example, the last crystallizer 42 is fed with the washing monomer solution from the tank 48 which is fluidically connected to the first outlet 42B of the last crystallizer 42 providing the rich monomer solution.

[0103] Preferably, the purification unit 6 comprises a water removal unit 50 arranged upstream of the crystallization unit 40 for removing water from the crude monomer solution provided by the depolymerization unit 4 before feeding the monomer solution to the crystallization unit 40 .

[0104] The water removal unit 50 includes, for example, a cooling device 52 configured to cool the monomer solution so as to crystallize 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 which is then fed to the crystallization unit 40.

[0105] The cooling device 52 is configured, for example, to cool the monomer solution by heat exchange with another fluid. The cooling device 52 is a heat exchanger, in particular a liquid / liquid heat exchanger or a suspension crystallizer.

[0106] like Figure 2 It is noted that the other liquid is preferably the purified monomer solution produced by the crystallization unit 40. The outlet of the crystallization unit 40 providing the purified monomer solution is fluidly connected to a cooling device 52, which is configured for heat exchange between the monomer solution provided by the depolymerization unit 4 and the purified monomer solution provided by the crystallization unit 40.

[0107] The monomer solution provided by the depolymerization unit 4 is received at a first inlet 52A of the cooling device 52, and the purified monomer solution provided by the crystallization unit 40 is received at a second inlet 52B of the cooling device 52. The first inlet 52A and the second inlet 52B are not fluidically connected.

[0108] In fact, the purified monomer solution leaving the crystallization unit 40 exhibits a low temperature, in particular a temperature close to the melting temperature of the monomers, and can be used for the crystallization of the water contained in the monomer solution provided by the depolymerization unit 4, when the melting temperature of the monomers is strictly below the crystallization temperature of water.

[0109] Removal device 54 is configured, for example, to remove ice by filtering, accumulating on a cold surface, floating, overflowing, decanting, and / or centrifuging.

[0110] The removal device 54 receives the monomer solution cooled by the cooling device 52 and provides the monomer solution at a first outlet 54A fluidly connected to the crystallization unit 40 and provides ice at a second outlet 54B.

[0111] Figure 1 The apparatus 2 is operable to implement a method for producing a monomer from a feed material containing a polymer, the monomer being a component of the polymer.

[0112] The production method includes the steps of depolymerizing a feed material to obtain a crude monomer solution containing monomers and impurities, and purifying the crude monomer solution to obtain a purified monomer solution having an impurity content lower than that of the crude monomer solution produced by depolymerization.

[0113] The depolymerization step is carried out in the depolymerization unit 4 and the purification step is carried out in the purification unit 6 .

[0114] The purification step comprises at least one crystallization step carried out in a crystallization unit 40 to separate impurities from the monomer solution, each crystallization step comprising cooling the monomer solution to form crystals of the monomer and a mother liquor, separating the crystals from the mother liquor and melting the monomer crystals to obtain an enriched monomer solution. Each crystallization step is preferably carried out in a respective crystallizer 42.

[0115] The purification step comprises subjecting the monomer-rich solution obtained from the crystallization step to another crystallization step and / or subjecting the mother liquor obtained from the crystallization step to another crystallization step.

[0116] This is carried out in a crystallization unit 40 comprising a series of crystallizers 42 by feeding the monomer-rich solution provided by the crystallizer 42 to the subsequent crystallizer 42 and / or by feeding the mother liquor provided by the crystallizer 42 to the preceding crystallizer 42, respectively.

[0117] The crystallization steps performed in the crystallizer 42 are preferably performed at different temperatures, and the crystallization temperature of the crystallization stage performed in each preceding crystallizer 42 is preferably strictly lower than the crystallization temperature of the crystallization step performed in the succeeding crystallizer 42.

[0118] In fact, the purity of the monomer solution increases from each preceding crystallizer 42 to the succeeding crystallizer 42, and thus 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.

[0119] The crystallization step advantageously comprises a water removal step carried out in a water removal unit 50 .

[0120] The water removal step comprises cooling the crude monomer solution obtained from the depolymerization step to a cooling temperature so that water present in the crude monomer solution is converted into ice without crystallizing the monomer, separating the ice from the monomer solution, and subjecting the monomer solution to a crystallization step after removing the water.

[0121] During the water removal step, the liquid stream is cooled at least in part by heat exchange with the purified monomer solution produced by crystallization unit 40 .

[0122] The water removal step is carried out in the purification unit 6 by means of a water removal unit 50 , in particular a water removal unit 50 comprising a cooling device 52 and a removal device 54 .

[0123] The cooling temperature is below the crystallization temperature of water and above the crystallization temperature of the monomer.

[0124] The difference between the cooling temperature and the melting point of the monomers contained in the liquid stream is higher than 4°C, preferably higher than 6°C and / or lower than 30°C, preferably lower than 20°C, more preferably lower than 10°C.

[0125] The depolymerization step includes pyrolyzing the feed material 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.

[0126] The depolymerization step advantageously comprises a separation step consisting in removing solid and / or liquid impurities from the gas stream in a separation unit 18 before condensing the gas stream in the condenser 10 .

[0127] The depolymerization step advantageously comprises the step of cleaning the gas stream between the separation unit 18 and the condenser 10 using a gas cleaning unit 24 fluidly connected in series between the separation unit 18 and the condenser 10, the gas cleaning unit 24 comprising a transfer column 26 containing internal components, the gas cleaning unit being fed with the monomer solution so that the gas stream flows upward in the transfer column 26 and the monomer solution flows downward in the transfer column 26 and preferably flows back to the separation unit 18.

[0128] The purification step advantageously comprises a step of cleaning the monomer solution, wherein the stream is passed through an optional solution cleaning unit 30 .

[0129] The solution cleaning unit 30 is configured, for example, to pass the monomer solution through an evaporator 32 and then through a condenser 34 to condense and separate heavies from monomers and lights.

[0130] The solution cleaning unit 30 comprises optionally or alternatively a distillation unit or a thin film evaporator or a short path distillation unit or any other suitable technology or combination of technologies.

[0131] The optional solution cleaning unit 30 allows the removal of heavy compounds (or "heavies"). The light compounds remaining in the monomer solution provided to the purification unit 6 are therefore more likely to contain impurities having a lower melting point than the monomers and which can be separated from the monomers by crystallization.

[0132] The purification step advantageously comprises a washing step consisting in adding water to the monomer solution upstream of the crystallization unit 6 in a washing unit 36 ​​.

[0133] In one example, the polymer is polymethyl methacrylate (PMMA) and the monomer is methyl methacrylate (MMA).

[0134] PMMA refers to a homopolymer or copolymer of methyl methacrylate (MMA) comprising at least 50 wt. %, preferably at least 60 wt. %, more preferably at least 70 wt. %, advantageously at least 80 wt. % and more advantageously at least 90 wt. % of methyl methacrylate.

[0135] The melting point of pure water is 0°C. The melting point of pure MMA is -48°C. This article indicates the melting point at a pressure of 1013 mbar.

[0136] It was found in practice that the key impurities contained in the crude MMA from depolymerization have melting points strictly below that of MMA and can be effectively removed by the crystallization step.

[0137] The following table lists possible impurities and their effective or predicted melting points:

[0138] Table 1: Melting points of impurities

[0139]

[0140] In an alternative example, instead of removing water from the liquid stream provided by the depolymerization unit 4, the production process comprises the step of adding at least one anti-icing additive to the monomer solution before carrying out the crystallization step, so as to inhibit the formation of ice during the crystallization step.

[0141] The at least one anti-icing additive includes, for example, methanol.

[0142] Inhibiting water crystal (ice) formation by adding at least one anti-icing additive to the crude monomer solution allows the crystallization step to be performed while removing key impurities, even though some water is embedded in the monomer crystals and is therefore present in the monomer solution.

[0143] In particular, unexpectedly, when methanol is added to a crude MMA solution at a methanol / MMA ratio of preferably greater than 4 g / 100 g and / or preferably less than 50 g / 100 g, ice formation is suppressed and MMA crystals can be directly produced. The MMA crystals formed during the crystallization process have some water embedded, but key impurities such as methyl isobutyrate, ethyl acrylate and toluene can be effectively removed.

[0144] Optionally, the purified MMA solution after the crystallization step may still be purified by an operational distillation step.

[0145] Molecules other than methanol may also be used as anti-icing additives in crude MMA, but methanol is already an impurity present in crude MMA and is relatively cheap.

[0146] The present invention is not limited to the above description and Figure 1-3 Other implementations are contemplated.

[0147] For example, instead of having a series of crystallizers, the crystallization unit 40 may include only a single crystallizer 42.

[0148] like Figure 1 It is noted that, optionally, at least a portion of the mother liquor produced in the crystallization unit 40 is returned to the gas scrubbing unit 24 and, in particular, injected into the top portion 26B of the transfer column 26. In such a case, a return line 58 is provided between the crystallization unit 40 and the transfer column 26. This allows recovery of some of the monomers still present in the mother liquor.

[0149] Optionally, at least a portion of the mother liquor produced by the crystallization unit 40 is used as a heat sink in one or several heat exchangers.

[0150] Example

[0151] In order to realize the embodiment, two water-ethanol solutions with two different ethanol concentrations were prepared. The two solutions were introduced into two different Dewar containers. Dry ice was crushed into a powdered material and slowly added in the Dewar container. When the solution bubbled more gently, larger pieces of dry ice were added in the Dewar container until the temperature stabilized and the solution obtained the consistency of syrup. The first bath solution was adjusted (i.e., the ratio of ethanol / water was adjusted) to have a temperature between -40°C and -42°C, and the second bath solution was adjusted to reach a temperature between -52°C and -57°C. By adjusting the ethanol content in the aqueous solution, the temperature at which the syrup will be formed and the temperature at which the temperature will be stable can be adjusted.

[0152] Then two glass containers of about 4cm diameter and 30cm length are introduced into two Dewar containers. About 100ml of crude MMA is introduced into the first glass container, which reaches about -40°C after about 30 minutes, while dry ice is added in the Dewar container to stabilize the temperature. Some crystals are formed and filtered in the suspension, and the solid is recovered and melts only at greater than 0°C or when reaching room temperature. The filtration has to be carried out very quickly to avoid the solid from melting. For this purpose, filter paper with low thermal conductivity is used. The collected solid is transferred in a sample bottle. When filtering, the liquid portion is poured into the second glass container at a much lower temperature. Then, the bath temperature is adjusted to less than -52°C again in about 20-30 minutes. After this, about half of the liquid solidifies. When cooling, a glass rod is introduced into the container, and crystals can be seen to form on the glass rod. After this, the liquid portion is collected and poured into a collection bottle. The solid is allowed to seep out moisture for about 1-2 minutes, and the formed liquid portion is added to the previous liquid portion. The remaining solid was then melted and poured into a separate collection bottle.

[0153] In some experiments, a slightly larger amount of product was crystallized, and the crystallized solid was melted and crystallized again to improve the purity of the solid fraction. In this case, two mother liquor liquid fractions and a final crystallization fraction were collected.

[0154] Table 2: Crystallization conditions

[0155]

[0156]

[0157] Embodiment 1:

[0158] Crude MMA is obtained from PMMA depolymerized at 470° C. in a twin-screw extruder and is produced when the main separator 20 is connected directly to the condenser 10. This means that the cleaning unit 26 and the main separator 22 are also omitted.

[0159] The product was first cooled to -40°C to separate the first crystal portion, and then cooled to -52°C to collect the crystallized portion and mother liquor. The crystallized portion was collected, melted and crystallized again at -57°C to produce a second mother liquor and a new crystallized portion. The initial crude MMA was orange. The color of the final crystallized portion changed slightly.

[0160] Embodiment 2:

[0161] The previous example was repeated, but 4 wt% methanol was added to the crude MMA. More specifically, a first 2 wt% methanol was added and the solution was cooled to -40°C, thereby forming crystals. Thus, the solution melted at room temperature, and a second 2 wt% methanol was added, totaling 4 wt%. The solution was then cooled to -40°C. No solid formation was observed. The liquid was then further cooled to -57°C and kept at this temperature for about 20 minutes. The liquid portion was collected, the portion that exuded moisture was added thereto, and the solid was melted and collected separately. The initial crude MMA was orange. The color of the final crystallized portion changed slightly.

[0162] Comparative Example 1

[0163] A clear and transparent pure MMA sample purchased from ALDRICH was used as a reference.

[0164] Then 100 mg of peroxide LP (dilauroyl peroxide) is added to about 5 ml of the liquid monomer solution in a test tube equipped with a polymer cap. The addition is done at room temperature, and the sample is shaken and then placed in a sample holder in a water tank (the water tank is kept at a constant temperature of 65° C. with water circulation). Organic peroxides should be handled with care as they can decompose violently.

[0165] The solution color did not change within 2 minutes. The samples were stored for two days and no color difference was detectable even after a long period of time.

[0166] Embodiment 3:

[0167] Crude MMA obtained from cast transparent PMMA sheets was used in this test. PMMA was depolymerized at 450°C and then the temperature was raised to 470°C in a twin-screw extruder while the feed rate was increased from 50 kg / h to 90 kg / h. The hot steam was collected, washed with cold crude MMA in countercurrent and condensed in a condenser provided in the form of a tube and shell heat exchanger (corresponding to Figure 1 The condenser 10 of the device).

[0168] A sample of the clear transparent solution was taken from the condenser.

[0169] Then 100 mg of peroxide LP (dilauroyl peroxide) is added to about 5 ml of the liquid monomer solution in a test tube equipped with a polymer cap. The addition is done at room temperature, and the sample is shaken and then placed in a sample holder in a water tank (the water tank is kept at a constant temperature of 65° C. with water circulation). Organic peroxides should be handled with care as they can decompose violently.

[0170] Within 2 minutes the solution changed color and became light brown.

[0171] After the crystallization experiments as described in the table above, the test using peroxide was repeated on the "mother liquor" and "molten crystals". A clear color difference can be seen, with the mother liquor producing a darker color.

[0172] Embodiment 4:

[0173] Crude MMA obtained from white cast PMMA sheets was used in this test. PMMA was depolymerized in a twin screw extruder operating at 90 kg / h at 470°C. The hot vapors were collected, washed with cold crude MMA in countercurrent mode and condensed in a condenser formed by a tube and shell heat exchanger. A sample of the clear transparent solution was taken from the condenser.

[0174] The peroxide test was repeated on the crude MMA solution and the color changed to pink-brown within 2 minutes.

[0175] After the crystallization test as described in the table above, the test using peroxide was repeated on the "mother liquor" and "molten crystals". A clear color difference can be seen, with the mother liquor producing a darker color. This confirms the impurities concentrated in the mother liquor.

[0176] Examples 5 and 6:

[0177] Crude MMA is obtained by depolymerization of a mixture of post-consumer waste of cast and extruded sheets collected from companies that cut the sheets to size and from which various products are prepared, these are mixed colored cast and extruded PMMA sheets from different producers, with a ratio of about 80% by weight of cast sheets and 20% by weight of extruded sheets. The mixture treated represents the average composition of material that can be collected in Europe from companies of this type.

[0178] PMMA was depolymerized in a twin-screw extruder operating at 90 kg / h at 470°C. The hot vapors were collected, washed with cold crude MMA in countercurrent and condensed in a tube and shell heat exchanger. A sample of the clear transparent solution was taken from the condenser.

[0179] The peroxide test was repeated on the crude MMA solution and the color changed to pink-brown within 2 minutes.

[0180] After the crystallization experiment as described in the table above, the test with peroxide was repeated on the "mother liquor" and "molten crystals". A clear color difference can be seen; the mother liquor produced a darker color. This confirms the impurities concentrated in the mother liquor.

[0181] The experiment was repeated after about 30 minutes when a new sample was removed from the condenser.

[0182] The same results were obtained.

[0183] Embodiment 7:

[0184] Crude MMA was obtained from injection PMMA scrap from car taillights containing red, black and transparent granules used in this test. PMMA was depolymerized in a twin screw extruder operating at 470°C at 50 kg / h. The hot vapors were collected, washed with cold crude MMA in countercurrent and condensed in a condenser formed by a tube and shell heat exchanger. A sample of the clear transparent solution was taken from the condenser.

[0185] The peroxide test was repeated on the crude MMA solution and the color turned orange within 2 minutes.

[0186] After the crystallization experiment as described in the table above, the test with peroxide was repeated on the "mother liquor" and "molten crystals". A clear color difference can be seen; the mother liquor produced a darker color. This confirms the impurities concentrated in the mother liquor.

[0187] Embodiment 8:

[0188] Crude MMA is obtained from an equal amount of white cast PMMA used to make bathtubs and is contaminated with glass fibers and adhesives used to stick the glass fibers to the PMMA for reinforcement of the bathtub and impact resistance. PMMA is depolymerized in a twin-screw extruder operating at 470°C with a total feed rate of 50 kg / h. The hot vapors are collected, washed with cold crude MMA in countercurrent and condensed in a condenser formed by a tube and shell heat exchanger. A sample of the clear transparent solution is taken from the condenser.

[0189] The peroxide test was repeated on the crude MMA solution and the color turned orange within 2 minutes.

[0190] After the crystallization trials as described in the table above, the test using peroxide was repeated on the "mother liquor" and "molten crystals". A clear color difference can be seen; the mother liquor produced a darker color than the MMA from the second crystallization. This confirms the impurities concentrated in the mother liquor.

Claims

1. A method for producing a monomer from a feed material containing a polymer, the monomer being a component of the polymer, the production method comprising the following steps: - depolymerizing the feed material to obtain a monomer solution containing the monomer and impurities; and - purifying the monomer solution to thereby obtain a purified monomer solution having an impurity content lower than the impurity content of the monomer solution, Wherein the purification step comprises at least one crystallization step, each crystallization step comprises the following steps: a) cooling the monomer solution so that the monomer solution forms monomer crystals and mother liquor, b) separating the monomer crystals from the mother liquor, and c) melting the monomer crystals, thereby obtaining a monomer-rich solution.

2. The production process according to claim 1, wherein the purification step comprises subjecting the monomer-rich solution obtained from one crystallization step to another crystallization step.

3. The production process according to claim 1 or 2, wherein the purification step comprises subjecting the mother liquor obtained from one crystallization step to another crystallization step.

4. The production process according to any one of the preceding claims, wherein the purification step comprises a water removal step.

5. The production method according to claim 4, wherein the water removal step comprises: - cooling the monomer solution to a cooling temperature, thereby converting any water present in the monomer solution into ice without crystallizing the monomer, and - separating the ice from the monomer solution, The monomer solution is thereby subjected to at least one crystallization step after removal of the water. 6 . The production method according to claim 5 , wherein the cooling temperature is higher than a crystallization temperature of the monomer contained in the monomer solution.

7. The production 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 higher than 4°C, preferably higher than 6°C and / or lower than 30°C, preferably lower than 20°C, more preferably lower than 10°C.

8. The production 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 a purified monomer solution resulting from crystallization.

9. The production process according to any one of claims 1 to 3, comprising 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 carrying out the at least one crystallization step.

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

11. The production method according to any one of the preceding claims, wherein the depolymerization step comprises pyrolyzing the feed material in a pyrolysis reactor thereby generating a gas stream, and condensing the gas stream in a condenser thereby obtaining the monomer solution.

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

13. The production method according to claim 12, comprising the following steps: A gas cleaning unit fluidly connected in series between the separation unit and the condenser is used to clean the gas stream between the separation unit and the condenser, the gas cleaning unit comprising a transfer column containing an internal component, the gas cleaning unit being fed with a portion of the monomer solution leaving the condenser so that the gas stream flows upward in the transfer column and the monomer solution flows downward in the transfer column, and heavy pollutants contained in the gas stream are condensed in liquid form on the internal component and flow back to the bottom of the transfer column by gravity, and preferably flow back to the separation unit.

14. The production method according to claim 12 or 13, wherein the separation unit comprises an auxiliary separator and a main separator connected in series fluidically.

15. The production method according to any one of claims 11 to 14, comprising the following steps: The monomer solution between the condenser and the purification unit is cleaned by passing the monomer solution through an evaporator and a condenser in sequence, thereby removing heavies from the monomer solution.

16. The production process according to any one of claims 11 to 15, comprising a washing step comprising adding water to the monomer solution upstream of a crystallization unit.

17. The production process according to any one of the preceding claims, wherein the polymer is polymethyl methacrylate (PMMA) and the monomer is methyl methacrylate (MMA).

18. Production process according to any one of the preceding claims, wherein the number of crystallization steps is equal to or less than five, in particular equal to or less than three.

19. Apparatus for producing a liquid monomer from a feed material containing a polymer, the monomer being a constituent of the polymer, the apparatus being configured for carrying out a production method according to any one of the preceding claims.

20. The apparatus according to claim 19, comprising: a depolymerization unit (4), which is configured to depolymerize the feed material to obtain a monomer solution containing the monomer and impurities; and a purification unit (6), which is configured to purify the monomer solution to obtain a purified monomer solution, wherein the impurity content of the purified monomer solution is lower than the impurity content of the monomer solution produced by depolymerization.

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

22. The apparatus according to claim 21, wherein the pyrolysis reactor (8) is a heated screw extruder comprising a heated tubular barrel (12) and at least one screw (14) arranged inside the barrel (12).

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

24. Apparatus according to claim 23, wherein the separation unit (18) comprises a main separator (20).

25. The apparatus according to claim 23 or 24, wherein the depolymerization unit (4) comprises a gas cleaning unit (24) connected in series between the separation unit (18) and the condenser (10).

26. The apparatus according to any one of claims 20 to 25, wherein the purification unit (6) comprises a solution cleaning unit (30).

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

28. Apparatus according to claim 27, wherein the crystallization unit (40) comprises one or several crystallizers (42).

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

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

31. The apparatus according to claim 30, wherein the water removal unit (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, which is then fed to the crystallization unit (40).

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