Process for depolymerizing polyethylene terephthalate (PET) by glycolysis with ethylene glycol and a plant for its implementation

Abstract

This process involves depolymerizing polyethylene terephthalate (PET) by glycolysis with ethylene glycol (EG) and a plant for implementing it. First, PET is mixed in a solid state with EG at a temperature between 60°C and 120°C to produce a heterogeneous mixture with an EG:PET weight ratio (R1) of 0.1 to 3.0. A certain amount of EG is then extracted by pressing. The compressed mixture is then fed into a reactor, preferably a paddle reactor, where it is heated at a temperature between 170°C and 270°C with an EG:PET weight ratio (R2) of 0.1 to 4.0 to glycolyze the PET, producing a glycolysis product containing bis(2-hydroxyethyl) terephthalate (BHET) and / or its oligomers. This process offers numerous advantages, including the ability to process waste PET directly and the ability to reduce the ratio of EG required for glycolysis relative to PET, enabling a relatively high-temperature reaction (above 230°C) for rapid processing.

Description

[Technical Field]

[0001] The present invention relates to a method for depolymerizing polyethylene terephthalate (PET), in particular PET from industrial and municipal waste, and a plant for carrying it out. [Background technology]

[0002] Polyethylene terephthalate (PET) is a widely used semi-crystalline thermoplastic polyester with high strength and transparency, whose physical and chemical properties give it multiple applications, particularly in packaging and textile production. Global production of PET exceeded 70 million tonnes in 2017, of which approximately 15 million tonnes was produced in Europe for the production of synthetic fibers (approximately 67%), bottles (approximately 23%), and packaging (10%).

[0003] Although PET poses no safety risks, there are environmental and economic concerns due to its increasing consumption, waste generation, and non-biodegradability, which has led to growing interest in PET recycling technologies.

[0004] PET is a readily recyclable polymer, and its recycling is the most common of all polymers. The techniques can be broadly divided into two categories: mechanical recycling and chemical recycling.

[0005] In mechanical recycling, pre-sorted waste is crushed and ground to produce PET flakes, which are then directly extruded to make new products. The main problems with this technology are the heterogeneity of the solid waste and the poor quality of the final product, as the mechanical properties of PET deteriorate with each recycling process.

[0006] Chemical recycling involves breaking down the polyester using agents capable of depolymerizing the PET chains to obtain the starting monomers. Chemical depolymerization of PET is typically achieved by hydrolysis, methanolysis, or glycolysis.

[0007] Hydrolysis involves the depolymerization of PET with water to terephthalic acid (TPA) and ethylene glycol (EG) (also known as monoethylene glycol, MEG). Methanolysis involves the decomposition of PET with methanol to dimethyl terephthalate (DMT) and EG. Glycolysis involves the depolymerization of PET with EG to produce bis(2-hydroxyethyl) terephthalate (BHET), an intermediate product produced from the starting monomers (terephthalic acid and EG) in the first stage of PET production.

[0008] Mechanical recycling remains the most widely used technology for the treatment of PET-containing waste today, but it does not produce a high-quality material suitable for food contact. This has led to growing interest in chemical recycling technologies, which comply with the principles of sustainable development and can provide raw materials for the production of virgin PET, which is of course of much higher quality than mechanically recycled PET.

[0009] The PET depolymerization reaction by glycolysis is typically carried out in a batch reactor consisting of a tank equipped with a mechanical mixing system that performs a circular spiral motion. The heat required for the reaction is supplied by heat exchange through the reactor walls. This type of reactor is suitable for processing fluids or suspensions of solids with fluid-like properties. Therefore, to process waste PET, after proper sorting and washing, it must be crushed into fine flakes of uniform particle size (approximately 5–10 mm) through a milling process to form a suspension with sufficient fluidity and reaction rates suitable for industrial production. This requires a liquid phase (mainly consisting of EG) to solid phase (consisting of waste PET) ratio of typically 5 or higher, typically 5–10. This ratio can be even higher for textile or film recycling.

[0010] WO 2016 / 096768 describes a process for carrying out the glycolysis of PET, which process comprises: (a) a dissolving step in which PET is mixed and dissolved with ethylene glycol (EG) at a temperature of 100°C to 300°C in an EG:PET weight ratio of 1 to 50; (b) a liquid-solid separation stage; (c) The liquid phase obtained from step (b) is subjected to a process in the presence of a heterogeneous catalyst at a temperature of 100°C to 300°C, a pressure of 0.1 to 6 MPa, and a reactor feed rate to catalyst mass ratio (PPH) of 0.1 to 100 h -1 and subjecting the resulting mixture to glycolysis with [Prior art documents] [Patent documents]

[0011] [Patent Document 1] International Publication No. 2016 / 096768 Summary of the Invention

[0012] The Applicant has set himself the technical problem of developing a process for depolymerizing PET, in particular waste PET, by glycolysis with ethylene glycol (EG), and a plant for carrying it out, which has the following characteristics: (i) Waste PET can be processed as is, i.e., in its heterogeneous form, eliminating the need to shred it into tiny, highly homogeneous flakes. (ii) It is possible to supply heat to a considerable mass of PET for depolymerization with high efficiency. (iii) The ratio of EG to PET required to perform the glycolysis can be reduced, and therefore the glycolysis can be carried out at relatively high temperatures (above 230°C) and relatively low pressures (near or slightly above atmospheric pressure) to speed up the glycolysis process. (iv) The process can be run continuously or discontinuously depending on production requirements.

[0013] Thus, according to a first aspect, the present invention relates to a process for depolymerizing polyethylene terephthalate (PET) by glycolysis with ethylene glycol (EG), said process comprising: (a) mixing solid PET with EG at a temperature of 60°C to 120°C, preferably 80°C to 100°C, and pressing the resulting heterogeneous mixture to squeeze out a certain amount of EG so as to obtain a heterogeneous mixture having an EG:PET weight ratio (R1) of 0.1 to 3.0, preferably 0.2 to 0.8; supplying the heterogeneous mixture to a reactor, where the mixture is heated with mixing at a temperature of 170°C to 270°C, preferably 200°C to 250°C, at an EG:PET weight ratio (R2) of 0.1 to 4.0, preferably 0.2 to 2.0, to glycolize the PET and obtain a glycolyzed product containing bis(2-hydroxyethyl) terephthalate (BHET) and / or its oligomers.

[0014] Preferably, in step (a), solid PET is mixed with EG so that the initial weight ratio of EG:PET (R0) is 0.2 to 4.0, preferably 0.5 to 2.0, and this ratio is gradually reduced during the pressing process until the above-mentioned R1 value is reached.

[0015] In a preferred embodiment, step (a) is carried out in a screw press, which is a machine known in the art and commonly used to separate liquids from solids suspended within the liquid itself, for example in the paper industry for the treatment of cellulosic suspensions, or in the waste treatment industry, for example to reduce the amount of sludge produced by wastewater treatment plants.

[0016] Preferably, the screw press comprises a generally cylindrical vessel within which a helical screw is disposed, rotating about an axis and defining a passageway having a narrowing portion along the axis of the screw itself. Thus, the solid PET material mixed with EG is forced along the helical screw, compressing it and squeezing out a certain amount of the added EG. The liquid is typically squeezed through a vessel having a filtering wall that allows excess EG to escape.

[0017] Preferably, the vessel has a filtering wall made of parallel wires with a V-shaped cross section, commonly called wedge wire screens. These are filtering structures usually made of parallel rods with a V-shaped cross section, usually with a flat or circular cross section, welded to a support element with a circular or spiral shape, thereby forming a cylindrical structure. The rods are spaced apart to provide a filtering structure with high mechanical strength and a large free area, making it less likely to clog. For more information about this type of filter element, please see the company website Costacurta SpA (http: / / www.costacurta.it / it / prodotti / elementi-filtranti / wedge-wire-screen / ).

[0018] Screw presses typically have a horizontal configuration. However, as an alternative, screw presses can also be used with a vertical configuration, in which material is fed from above and discharged from below. This configuration offers many advantages, such as a more uniform filling of the internal space, resulting in a more uniform processing of the raw material. Furthermore, the material is transported within the press using gravity, which increases productivity. Further details regarding this embodiment can be found, for example, in the magazine Spectrum - Tech News No. 36 / 2-2017, pages 57-59 (available at https: / / www.andritz.com / ).

[0019] In a preferred embodiment, the screw press is arranged in an inclined configuration, i.e., with the axis of screw advancement inclined relative to the horizontal. Preferably, in an inclined configuration, the material is fed to the screw press through a feed opening located at a lower level than the opening for discharging the material from the press itself. Preferably, the screw press is arranged with the axis of screw advancement inclined relative to the horizontal at an angle of 20° to 60°, more preferably 35° to 55°.

[0020] The screw press with the above-mentioned inclined configuration is considered to be particularly advantageous because it can achieve optimal liquid tightness (approximately 100 mBar) regardless of the compressibility and deformation characteristics of the PET being fed in. This effect can therefore be achieved even with waste PET, which is necessarily non-uniform in composition and therefore varies in properties from batch to batch.

[0021] Due to the characteristics of the screw press, waste PET obtained from industrial or municipal waste can be fed into the machine either as is or after being subjected to a rough crushing process, eliminating the need to flaked the PET, which was previously done in the waste PET recovery process.

[0022] During step (a), the PET is heated to a temperature of 60°C to 120°C, preferably 80°C to 100°C, i.e., above the glass transition temperature (T g ) or higher, in either case, than its melting temperature (T m ) at which the PET disintegrates, losing its glassy rigidity and becoming more intimately mixed with the EG, resulting in a heterogeneous mixture formed by PET (solid) and EG (liquid), which is subsequently transferred to step (b). Preferably, for safety reasons, the temperature is kept below the flame temperature (flash point) of EG (115°C).

[0023] According to a preferred embodiment of the process of the present invention, step (a) is divided into steps (a1) and (a2) carried out in two different apparatuses, step (a1) being a process of mixing solid PET with EG at a temperature of 60°C to 120°C, preferably 80°C to 100°C, in a blender, preferably a continuous blender, equipped with a heating system and supplied with PET and EG. The blender mixes and heats the two products, causing the polymeric material to break down under the action of temperature and effectively mix with the liquid phase (EG) to produce a heterogeneous mixture.

[0024] The blender may be an auger or ribbon blender, or any other device known in the art.

[0025] As is known, a belt blender comprises a container in which a rotating shaft is arranged, on which are fixed arms supporting one or more steel strips arranged in a spiral around the shaft. A particularly efficient embodiment of this machine provides two belts forming two concentric spirals, of which the outermost belt moves the material to be processed in one direction and the innermost belt moves in the opposite direction, allowing particularly efficient mixing of even heterogeneous materials.

[0026] The heterogeneous mixture is transferred from the blender to the entrance of a screw press where step (a2) is carried out. In step (a2), the heterogeneous mixture is compressed to squeeze out a certain amount of EG from the mixture itself, reducing the EG content and resulting in a heterogeneous mixture with the above-mentioned EG:PET weight ratio (R1).

[0027] As described above, by performing step (a) using two different devices, it becomes possible to use a commercially available screw press that does not normally have a heating device in the pressing step (a2).

[0028] Preferably, the heterogeneous mixture is transferred from step (a) to step (b) by passing it through a transfer cell which collects a predetermined amount of the heterogeneous mixture exiting step (a) and transfers it by gravity to step (b).

[0029] Preferably, step (b) is carried out in an inert atmosphere, such as a nitrogen atmosphere, to prevent oxidative degradation of the polymer due to atmospheric oxygen at high temperatures, in which case the transfer cell is a liquid-tight cell and an inert atmosphere is maintained within the apparatus in which step (b) is carried out.

[0030] Like step (b), step (a) can be carried out in an inert atmosphere, although this is not strictly necessary as the temperatures used in step (a) are believed to be temperatures that do not significantly degrade the PET being treated.

[0031] Preferably, additional EG is added in step (b) to maintain the EG:PET weight ratio (R2) within the range described. Also, BHET and / or its oligomers may be added in step (b) and recovered in subsequent steps of the process. Because BHET and / or its oligomers have a high reactivity with PET, adding them can further increase the rate of the glycolysis reaction.

[0032] It is important to emphasize that the liquid-tightness is also ensured by the fact that at the outlet of the screw press, a kind of "plug" is obtained consisting of a heterogeneous mixture of PET and EG pressed inside the press, which, once it reaches a critical mass, drops into the transfer cell and passes to step (b).

[0033] The inert atmosphere can be obtained by replacing air with nitrogen at a pressure of usually 0.1 to 1.5 atm, preferably 0.3 to 0.7 atm.

[0034] In a preferred embodiment, step (b) is carried out in a paddle reactor, a machine known in the art and commonly referred to as a "paddle dryer", which is commonly used to heat and dry a variety of materials, including polymeric substances, minerals, metal powders, especially in the food, chemical and pharmaceutical industries, and also for drying sludge produced in wastewater treatment plants.

[0035] Preferably, the paddle reactor comprises a vessel having a pair of shafts disposed therein that rotate about their axes, with a plurality of paddles attached to the shafts, and heats and mixes a mass of the solid PET mixed with EG to depolymerize the polymer by glycolysis, producing bis(2-hydroxyethyl) terephthalate (BHET) and / or its oligomers.

[0036] The two sets of paddles have circular outer edges and are generally interpenetrating to enhance the stirring action.

[0037] The paddles have a roughly wedge-shaped, i.e., triangular, cross section, which gives the paddles a self-cleaning effect and allows them to process non-homogeneous materials with low fluidity, such as waste PET, in their original state, i.e., before being broken into flakes. The bottom of the vessel in which the pair of rotating shafts are arranged preferably has an omega-shaped cross section, which keeps the distance between the end of the rotating paddles and the bottom roughly constant, preventing stagnation of the material to be processed and ensuring that the entire material is mixed homogeneously.

[0038] The paddle reactor is preferably equipped therein with a system capable of maintaining an inert atmosphere, for example by replacing air with nitrogen, as described above. Heating of the mass within the reactor can be achieved by the shaft and paddles. Because they are hollow, they function as heat exchangers by introducing a heated fluid into their interiors, transferring heat directly to the interior of the material being treated, allowing for faster and more uniform heating compared to the external heating common in conventional reactors. If necessary, the material being treated in a paddle reactor can be further heated by an external heating jacket.

[0039] The implementation of the process according to the present invention provides many advantages.

[0040] First, step (a) allows for the intimate impregnation of solid PET with EG without the need for flake PET. It should be noted that, depending on the source, waste PET may contain various contaminants (e.g., zippers, buttons, glass wool, fabric, aluminum, polymers other than PET, multilayer materials in which PET is bonded to other polymeric and / or metallic layers, etc.) that are typically removed before the glycolysis reaction is carried out. Intimately mixing PET with EG improves the yield and shortens the reaction time in the subsequent step (b). In fact, the amount of free EG not intimately mixed with PET is reduced, which allows for an increase in the glycolysis temperature while maintaining the pressure near atmospheric pressure, which is clearly advantageous in terms of reaction rate.

[0041] Furthermore, step (a) allows the amount of water originally present in PET (which can be about 5-7% by weight in waste PET from industrial or municipal waste) to be reduced below a threshold value usually set at 0.5% by weight relative to the weight of the PET. In fact, the water present mixes with the added EG and is largely removed during pressing.

[0042] The presence of excessive amounts of water in downstream processes can cause various problems. In fact, because water has a large latent heat of transition from liquid to vapor, the presence of excess water (usually more than 0.5% by weight) can result in the loss of heat needed for the glycol decomposition reaction, resulting in increased energy consumption. Furthermore, the presence of large amounts of water can also promote secondary reactions.

[0043] To carry out step (a) of the process, especially step (a2), screw presses are particularly advantageous because they are particularly robust machines suited to the processing of solids and tolerate inhomogeneous materials without causing clogging problems during flow. This is a major advantage compared to extruders, the most commonly used machines for mixing thermoplastic polymeric materials with liquid or solid additives. In fact, extruders have a low tolerance for hard, coarse foreign matter in the processed material, which can lead to frequent interruptions to prevent damage to the extruder screw.

[0044] With regard to step (b), this step is particularly effective and efficient, with very high yields of PET glycolysis and relatively short reaction times. Typically, the overall yield of the process according to the invention in BHET and its oligomers is greater than 98%, preferably about 100%.

[0045] The reaction time may vary depending on various factors such as the amount of reaction mass, the exchange surface between PET and EG, the reaction temperature, and the characteristics of the paddle reactor. Typically, step (b) may be carried out for a time period of 1 to 60 minutes, preferably 2 to 40 minutes.

[0046] On the other hand, when a paddle reactor is used in step (b), the high heat exchange capacity due to the heating of the shaft and paddles acting as a heat exchanger allows the heating rate of the PET mass to be increased, so that heat is transferred not only through contact with the outer wall of the reactor but also to the inside of the material to be treated.

[0047] The paddle reactor ensures high mixing of the fluid mass in both the longitudinal and radial directions due to the design and construction of the paddles with a wedge-shaped cross section. The paddle reactor also allows for the forward movement of the material by a piston (plug flow), which allows for effective control of the forward movement rate of the material.

[0048] The forward movement of the raw materials in the reactor is mainly due to the weight of the raw materials discharged by the screw press, and the pushing effect can be controlled by adjusting the inclination of the reactor relative to the horizontal position.

[0049] At the end of step (b), a filtration step of the glycolysis product may be carried out in order to separate any solid products that may be present in the suspension, such as fillers present in the starting PET, such as titanium dioxide added to impart certain properties to the material, etc. Such filtration may be carried out by conventional filter media, for example, to which the glycolysis product may be fed by a pump located at the outlet of the reactor in which step (b) is carried out.

[0050] Machines that can be used to carry out the process according to the invention generally include commercially available machines for the various processes mentioned above. Suitable screw presses and paddle dryers are listed in the catalogs of, for example, Andritz Gouda BV, Huber Technology Inc. and Nara Machinery Manufacturing Co., Ltd.

[0051] The process according to the invention can be carried out discontinuously (batchwise) or continuously, the latter being clearly advantageous from an industrial point of view and being carried out by using, in succession, a screw press for step (a) and a paddle reactor for step (b), both of which can be operated continuously.

[0052] The size and other characteristics of the reactor (particularly the paddle reactor) in which step (b) is carried out can be selected to obtain a glycolyzer with the desired rate of glycolyzerization.

[0053] However, if a glycolysate particularly rich in BHET monomer is to be obtained, it is necessary to accelerate the glycolysation.To this end, in order to avoid excessively large machinery for carrying out step (b), it is advantageous to subject the glycolysate obtained from step (b) to an additional glycolysation step (c) in an EG:PET weight ratio (R4) of 0.5 to 10.0, preferably 1.0 to 5.0.

[0054] The additional glycolysis step (c) can be carried out in a conventional liquid reactor, where the glycolysis product is in liquid form and therefore pumpable. This reactor can be, for example, a plug flow reactor or a continuous tubular reactor, which allows for continuous operation. Alternatively, a discontinuously operated stirred tank reactor can be used, which is essentially formed by a vessel with an inlet duct and an outlet duct, inside which is provided a blender, typically a propeller blender, that keeps the reactants moving.

[0055] If continuous operation is desired, multiple stirred tank reactors (typically three reactors are sufficient) can be used in series to form a continuously operating multi-stage reactor. In practice, continuous operation can be achieved by performing a portion of the glycollysis in each discontinuous reactor and adjusting the inlet feed flow rate from step (b) by selecting appropriately sized reactors. The latter is achieved by continuously feeding the reactants to a first reactor, which is equipped with a weir through which the reaction mixture flows and is fed to a second reactor in series for further reaction. The second reactor is also equipped with a weir to feed a third reactor, and this process is repeated until the desired degree of glycollysis is achieved. This embodiment is shown in Figure 1.

[0056] At the end of the glycolysis reaction, the glycolysis product may be subjected to a step of separating solid foreign matter, such as polyolefins (polyethylene, polypropylene, etc.), which generally have a lower density than the glycolysis product and can therefore be removed from the surface of the glycolysis product, for example by siphoning, skimming or suction.

[0057] For this purpose, when an additional glycololysis step (c) as described above is used, the liquid reactor, particularly the last reactor in a series of reactors, is equipped with a weir for withdrawing low density solids from the reactor, with the glycololysis product being withdrawn from the bottom of the reactor itself. In a series of reactors, it is preferred to use a weir in the last reactor in the series to withdraw the low density solids, although weirs in reactors prior to the last may also be used for this purpose.

[0058] Preferably, the contaminants removed as described above are cooled to a temperature of about 90°C to 130°C, e.g., by adding EG, followed by separation of the solid material, e.g., by filtration, to facilitate recovery of the filtrate containing BHET and / or its oligomers. In fact, at the temperatures at which glycolysis occurs, the polymeric material is in a molten or semi-solid state, making filtration very difficult. The filtered liquid is recovered and added to the glycolysis product recovered from the bottom of the reactor, thereby improving the overall yield of the glycolysis process.

[0059] Further recovery of BHET and / or its oligomers can be achieved by washing the solid residue with water and / or EG, again at a temperature that does not soften or melt the polymeric material, typically between 90°C and 130°C.

[0060] According to another aspect, the present invention relates to a plant for depolymerizing polyethylene terephthalate (PET) by glycolysis with ethylene glycol (EG), comprising: a screw press into which PET and EG are fed to produce a heterogeneous mixture of PET and EG, the screw press comprising a generally cylindrical vessel having a helical screw disposed therein that rotates about its axis, the helical screw defining a passageway having a narrowing portion along the axis of the screw, the vessel having a filtering wall that allows excess EG to escape; a paddle reactor to which the heterogeneous mixture leaving the screw press is supplied, the reactor comprising a substantially cylindrical vessel in which a pair of shafts rotating about their own axes are disposed, each of the shafts comprising a plurality of paddles.

[0061] Preferably, the screw press is arranged to have a helical screw whose axis of advancement is inclined relative to the horizontal plane. In such a configuration, the screw press preferably comprises a PET feed opening located at a lower level than the opening for exiting the heterogeneous mixture from the screw press.

[0062] Preferably, in a paddle reactor, the pairs of shafts and opposing paddles are internally heated to provide heat to the heterogeneous mixture.

[0063] Preferably, the paddles present on the rotating shaft interpenetrate each other.

[0064] Preferably, the paddle has a wedge shape.

[0065] Preferably, the paddle reactor is equipped with a system for maintaining an inert atmosphere inside it.

[0066] Preferably, the plant according to the invention further comprises a transfer cell arranged between the screw press and the paddle reactor for collecting a predetermined amount of the heterogeneous mixture leaving the screw press and transferring it by gravity to the paddle reactor, said transfer cell preferably being liquid-tight in order to maintain an inert atmosphere inside the paddle reactor.

[0067] In a preferred embodiment, the plant according to the invention comprises a blender upstream of the screw press, in which PET and EG are fed and premixed to obtain a heterogeneous mixture, which is fed to the screw press.

[0068] Preferably, the blender is a continuous blender. Preferably, the blender is an auger or ribbon blender.

[0069] In a preferred embodiment, the plant according to the invention comprises at least one reactor downstream of the paddle reactor for completing the PET glycolysis on the glycolysis product stream leaving the paddle reactor and on the liquid provided by EG.

[0070] Preferably, the liquid reactor is a continuous reactor, in particular a plug flow reactor or a continuous tubular reactor.

[0071] Alternatively, the liquid reactor is a batch reactor, in particular a stirred tank reactor.

[0072] For continuous operation, the liquid reactor may comprise a plurality of stirred tank reactors, preferably three stirred tank reactors arranged in series to form a continuous multi-stage reactor.

[0073] Preferably, a weir is located downstream of the liquid reactor or reactors in series to remove low density solid contaminants, and preferably, a filtration device is located downstream of the weir to separate the solid contaminants from the glycolycleation products.

[0074] The glycolysis reaction may optionally be carried out in the presence of a heterogeneous transesterification catalyst which is fed to the blender in which step (b) is carried out.

[0075] If the glycolic acid decomposition reaction is completed in step (c), fresh catalyst can be fed to the reactor to maintain an adequate reaction rate.

[0076] The catalyst may be selected from, for example, carbonates, fatty acid salts, or borates of Na, Mg, Zn, Cd, Mn, Co, Ca, or Ba (e.g., zinc borate, zinc acetate, sodium carbonate, etc.).

[0077] Preferably, the glycolysis reaction of optional step (c) is carried out at a temperature of from 170°C to 250°C, more preferably from 200°C to 230°C.

[0078] With regard to PET, this is preferably municipal and / or industrial waste PET which can come from a wide range of sources, including, for example: -Clear and / or colored PET bottles for water, soft drinks, carbonated drinks, etc. -Opaque PET articles in which the PET contains fillers such as titanium dioxide, carbon black, silicates, or other pigments. - Multilayer PET articles, typically for the food industry, in which a PET layer is bonded to a layer of gas barrier polymer (e.g., nylon, polyvinyl alcohol (EVOH), polyvinyl acetate (EVA)) or metal sheet (e.g., aluminum sheet) or polyolefin sheet. -PET sheet with printing. -PET fiber.

[0079] The product of the glycolysis reaction is a crude BHET solution, in which BHET is dissolved in EG along with various impurities derived from the unique components of the PET waste. Typically, the crude BHET solution also contains oligomers of BHET, preferably dimers and / or trimers. The impurities are components of the PET waste or derivatives obtained by glycolysis of such components, such as: -Dyes, usually organic pigments. -ink. -Adhesives and glues. - Polyolefins, such as polyethylene or polypropylene, used to manufacture the caps. -PET-G. -Biodegradable polymers such as PLA. Gas barrier polymers such as polyamide, polyvinyl alcohol (EVOH), and polyvinyl acetate (EVA). -UV absorber. -Fillers such as titanium dioxide, carbon black, silica, silicates and other pigments. - Sheets such as aluminum sheets and their fragments.

[0080] The crude glycolize product can be purified according to known methods, specifically, the crude glycolize product is filtered to separate insoluble impurities, and may optionally be subjected to a treatment to remove soluble impurities, for example, with an adsorbent. [Brief explanation of the drawings]

[0081] [Figure 1]1 is a diagram of an embodiment of a plant according to the invention; [Figure 2] FIG. 1 is a diagram of a paddle reactor that can be used in a plant according to the invention. [Figure 2a] FIG. 3 is a cross-sectional view of the paddle reactor of FIG. 2. [Figure 3] 1 is a diagram of a screw press that can be used in a plant according to the invention; DETAILED DESCRIPTION OF THE INVENTION

[0082] The plant according to the invention will be further explained on the basis of the following figures:

[0083] FIG. 1 shows a schematic diagram of an embodiment of a plant (100) according to the present invention. PET and EG to be processed are fed to a belt blender (101) by conventional means (not shown). For example, PET may be fed via a conveyor belt, and EG may enter the belt blender (101) through a duct connected to a storage tank. The belt blender (101) preferably includes heating means to bring the mass being processed to the desired temperature. To achieve this temperature, preheated EG may be fed to the belt blender (101). As described above, a heterogeneous mixture of PET and EG is formed within the blender.

[0084] The heterogeneous mixture is then transported via a screw conveyor (102) and introduced into a screw press (103), where the mixture is further mixed and compressed to squeeze out a certain amount of EG, which is then collected and returned to a storage tank via a duct (104).

[0085] As mentioned above, the screw press (103) comprises a cylinder (105) in which a helical screw (106) is disposed, rotating about its axis, and defining a conveying path having a narrowing portion along the axis of the screw (the diagram in Figure 1 is schematic and is shown in more detail in Figure 3).

[0086] The end of the screw press (103) is provided with a transfer cell (107) which collects a predetermined amount of the heterogeneous mixture exiting the screw press (103) and transfers it by gravity to the paddle reactor (108). The transfer cell (107) is preferably liquid-tight to maintain an inert atmosphere during subsequent process steps in the paddle reactor (108).

[0087] As mentioned above, the paddle reactor (108) comprises a vessel (109) with a lid, inside which are disposed a pair of shafts (110) that rotate about their own axes, and on which are disposed a plurality of paddles (111). Such an apparatus is most clearly illustrated in the embodiment of Figure 2. EG and, optionally, BHET and / or its oligomers are fed to the paddle reactor (108) via duct (112). Optionally, a transesterification catalyst may also be fed to the paddle reactor (108).

[0088] The paddle reactor (108) is preferably equipped with a closed system (not shown) capable of maintaining an inert atmosphere inside it, for example by introducing nitrogen to replace the air present.

[0089] The glycolysis product exiting the paddle reactor (108) is a liquid that can be pumped and used as desired, for example, sent to a purification process.

[0090] In Figure 1, the glycolysate is transferred to a series of three stirred tank reactors (113a, 113b, 113c) for further glycolysation to obtain a high degree of glycolysation. EG, optionally mixed with a transesterification catalyst, may be introduced into the first reactor (113a) through a duct (114) connected to a storage tank.

[0091] The first reactor (113a) is equipped with a weir (115a) through which the reaction mixture exits and may be fed via duct (116a) to the second reactor (113b) where the glycolysis reaction may continue. Similar to the first reactor (113a), the second reactor (113b) is equipped with a weir (115b) for removing the reaction mixture and transferring it via duct (116b) to the third reactor (113c). In the third reactor (113c), the glycolysis reaction is completed, and the glycolysis product is removed from the bottom of the reactor (113c) via duct (120). The reactor (113c) is also preferably equipped with a weir (115c) for removing low-density solid foreign matter (especially polyolefins) that floats to the surface of the liquid glycolysis product. The debris is then cooled by adding EG via duct (117) and introduced into a filtration unit (118) that separates the solids from the liquid. The filtrate containing BHET and / or its oligomers is recovered and added via duct (119) to the glycolyzed product recovered from the bottom of the reactor. The glycolyzed product is sent via duct (120) to a subsequent process (e.g., a purification plant).

[0092] Each reactor (113a, 113b, 113c) is provided with a purge duct (121a, 121b, 121c) for removing high concentration foreign matter accumulated in the lower part.

[0093] FIG. 2 shows a diagram of an embodiment of a paddle reactor that can be used in a plant according to the invention.

[0094] The paddle reactor (200) comprises a vessel (201) inside which are arranged a pair of shafts (202a, 202b) (only one shaft is shown in the figure) arranged side by side and rotating about their own axes, each of which has a number of paddles (203a, 203b) attached to it. Figure 2 shows only the top portion of a cross section of the vessel (201) to show the presence of the shafts (202a, 202b) and paddles (203a, 203b).

[0095] The paddles (203a, 203b) attached to the two shafts (202a, 202b) interpenetrate each other and preferably have a triangular (i.e., wedge-shaped) cross section, which allows the paddles to self-clean. The shafts (202a, 202b) are connected to a motor (204) that rotates the shafts. The shafts (202a, 202b) and the paddles (203a, 203b) have hollow structures and are connected to a heating fluid inlet (205a) so that the fluid can flow into the shafts and paddles, where it is heated and transfers heat to the workpiece. A separate inlet (205b) may be provided to introduce the heating fluid into a heating jacket (201b) surrounding the vessel (201). The discharged heating fluid exits through an outlet (206a) connected to the shafts and an outlet (206b) connected to the heating jacket (201b).

[0096] The material to be treated is introduced into the vessel (201) through a supply port (207) and discharged from the vessel (201) through a discharge port (208). EG and / or BHET and / or its oligomers may also be supplied to the vessel (201) through a further supply port (209).

[0097] The paddle reactor (200) is equipped with a system that allows the process to be carried out in an inert atmosphere within the machine, for example with a nitrogen inlet (210) and an exhaust (211) from which nitrogen and other gases or vapors can be recovered and recycled to the plant.

[0098] Figure 2a shows a cross section of a container (201) and two shafts (202a, 202b) to which two sets of paddles (203a, 203b) are attached. As can be seen from Figure 2a, the bottom of the container (201) has an omega-shaped cross section, and the distance between the edge of the rotating blades (202a, 202b) and the bottom is kept approximately constant, preventing the material to be treated from stagnating and ensuring that the entire material to be treated is mixed homogeneously.

[0099] FIG. 3 shows a diagram of an embodiment of a screw press that can be used in a plant according to the invention.

[0100] The screw press (300) comprises a generally cylindrical container (301) containing a helical screw (302) attached to a shaft (303) connected to a motor for rotating the screw about its axis. An inlet duct (304) introduces the materials (i.e., PET and EG) into the container (301). As shown in FIG. 3, the cross section of the shaft (303) increases as it progresses from the feed zone to the discharge zone, gradually reducing the cross section of the flow path through which the materials are transported. The materials are gradually compressed as they are pushed and moved by the rotating screw (302). The cross section of the transport path may be gradually reduced by gradually narrowing the pitch of the helical screw from the feed zone to the discharge zone.

[0101] This causes a portion of the EG to be squeezed out of the PET and discharge through holes in the wall of the vessel (301), which preferably has a wedge wire screen filtering wall as described above. The EG squeezed out of the treated PET is collected by trays (305) and sent to a storage tank for reuse. The PET, now mixed with the treated EG, is discharged through a transfer cell (306) which feeds the next step, which takes place in a paddle reactor.

[0102] Further details of the structure and operation of such screw presses are described, for example, in US Pat. No. 5,857,406 or US Patent Application Publication No. 2011 / 0297016.

Claims

1. 1. A process for depolymerizing polyethylene terephthalate (PET) by glycolysis with ethylene glycol (EG), comprising: (a) mixing solid PET with EG at a temperature of 60°C to 120°C to obtain an initial EG:PET weight ratio (R0) of 0.2 to 4.0, and pressing the resulting heterogeneous mixture to squeeze out a certain amount of EG to obtain a heterogeneous mixture with an EG:PET weight ratio (R1) of 0.1 to 3.0; (b) feeding the heterogeneous mixture to a blender in which the mixture is heated with mixing at a temperature of 170°C to 270°C, with an EG:PET weight ratio (R2) of 0.1 to 4.0, to glycolyze the PET and obtain a glycolyzed product containing bis(2-hydroxyethyl) terephthalate (BHET) and / or its oligomers; The process wherein step (b) is carried out in a paddle reactor.

2. 2. The process of claim 1, wherein in step (a), the solid state PET is mixed with EG to provide an initial weight ratio (R0) of EG:PET of 0.5 to 2.

0.

3. 3. The process of claim 1 or 2, wherein step (a) is carried out in a screw press.

4. 4. The process according to claim 3, wherein the screw press is arranged in an inclined configuration, i.e., with the axis of advancement of the screw inclined relative to the horizontal plane.

5. 3. The process according to claim 1 or 2, wherein the step (a) is divided into steps (a1) and (a2) carried out in two different apparatuses, wherein the step (a1) comprises mixing the solid-state PET with EG at a temperature of 60°C to 120°C to obtain the heterogeneous mixture, the step (a1) being carried out in a blender, and the step (a2) comprises compressing the heterogeneous mixture obtained in step (a1), squeezing a certain amount of EG from the mixture to obtain a heterogeneous mixture having an EG:PET weight ratio (R1) of 0.1 to 3.0, the step (a2) being carried out in a screw press.

6. 6. The process of any one of claims 1 to 5, wherein the heterogeneous mixture is transferred from step (a) to step (b) by passing a predetermined amount of the heterogeneous mixture exiting step (a) through a transfer cell that collects the amount and transfers it to step (b) by gravity.

7. The process of any one of claims 1 to 6, wherein step (b) is carried out in an inert atmosphere.

8. 8. The process of any one of claims 1 to 7, wherein BHET and / or its oligomers are fed to step (b) and optionally recovered during a subsequent step of the process.

9. A process according to any one of claims 1 to 8, wherein the solid state PET is fed to step (a) as is, i.e. without being flaked.

10. 10. The process of any one of claims 1 to 9, wherein at the end of step (a), the heterogeneous mixture has a water content of 0.5 wt% or less, based on the weight of PET.

11. 11. The process of any one of claims 1 to 10, further comprising filtering the glycolysis products obtained from step (b).

12. 12. The process of any one of claims 1 to 11, further comprising subjecting the glycolysis product obtained from step (b) to an additional glycolysis step (c) at a weight ratio of EG:PET (R4) of 0.5 to 10.

0.

13. 13. The process of claim 12, wherein the additional glycolyzing step (c) is carried out in a liquid reactor.

14. 13. The process of claim 12, wherein the additional glycolysis step (c) is carried out in multiple stirred tank reactors arranged in series to form a continuously operating multi-stage reactor.

15. 15. The process according to any one of claims 1 to 14, wherein the glycolysis products exiting step (b) or step (c) are subjected to a step of separating and removing solid foreign matter having a lower density than the glycolysis products from the surface of the glycolysis products.

16. The process of claim 15, wherein the solid foreign matter removed from the surface of the glycolysate is cooled to a temperature of about 90°C to 130°C to facilitate subsequent separation of the solid material, and a filtrate containing BHET and / or its oligomers is recovered.

17. 1. A plant for depolymerizing polyethylene terephthalate (PET) by glycolysis with ethylene glycol (EG), comprising: a screw press into which PET and EG are fed to produce a heterogeneous mixture of PET and EG, the screw press comprising a cylinder having a helical screw disposed therein that rotates about an axis, the helical screw defining a transfer path having a narrowing portion along the axis of the screw; a paddle reactor to which the heterogeneous mixture leaving the screw press is fed, the reactor comprising a substantially cylindrical vessel in which a pair of shafts rotating about their own axes are disposed, each paddle reactor comprising a plurality of paddles.

18. 18. The plant according to claim 17, wherein the screw press is arranged so that the axis of development of the helical screw is inclined relative to a horizontal plane.

19. 19. The plant of claim 17 or 18, wherein in the paddle reactor, the rotating shaft and the paddles are heated from the inside to provide heat to the heterogeneous mixture.

20. 20. The plant according to any one of claims 17 to 19, wherein in the paddle reactor the paddles present on a rotating shaft interpenetrate each other.

21. The plant according to any one of claims 17 to 20, wherein the paddles have a wedge shape.

22. 22. The plant according to any one of claims 17 to 21, wherein the paddle reactor is provided with a system making it possible to maintain an inert atmosphere inside.

23. 23. The plant of any one of claims 17 to 22, further comprising a transfer cell disposed between the screw press and the paddle reactor, for collecting a predetermined amount of the heterogeneous mixture exiting the screw press and transferring it by gravity to the paddle reactor.

24. 24. The plant according to any one of claims 17 to 23, further comprising a blender upstream of the screw press to which the PET and the EG are fed, for premixing the PET and the EG so as to obtain a heterogeneous mixture which is fed to the screw press.

25. 25. The plant of claim 24, wherein the blender is a continuous blender.

26. 26. The plant of any one of claims 17 to 25, comprising at least one liquid reactor downstream of the paddle reactor, which is fed by the glycolysis product stream from the paddle reactor and EG to complete the PET glycolysis.

27. 27. The plant of claim 26, comprising a plurality of stirred tank reactors arranged in series downstream of the paddle reactor to form a series of multi-stage reactors.

28. 27. The plant of claim 26, wherein a weir for removing low density solid foreign matter is located downstream of the liquid reactor.

29. The plant of claim 27, wherein a weir for removing low-density solid foreign matter is disposed downstream of the plurality of stirred tank reactors arranged in series.

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