PROCESS FOR DEPOLYMERIZING POLYETHYLENE TEREPHTHALATE (PET) BY GLYCOLYSIS WITH ETHYLENE GLYCOL, AND PLANT FOR CARRYING OUT THE SAME

Abstract

A process for depolymerizing a polyethylene terephthalate (PET) by glycolysis with ethylene glycol (EG) is described. Also described is a plant for carrying out the process. The PET is first mixed in the solid state with EG at a temperature of 60-120° C. to form a heterogeneous mixture with an EG:PET weight ratio of 0.1-3.0, and is then pressed to squeeze out an aliquot of EG. The compressed mixture is fed to a reactor, where the mixture is heated and kept under mixing at a temperature of 170-270° C., with an EG:PET weight ratio of 0.1-4.0, to glycolize the PET and obtain a glycolyzed product containing bis(2-hydroxyethyl) terephthalate and/or oligomers thereof. The process is advantageous in treating waste PET as such and reducing the EG to PET ratio necessary for glycolysis that allows the reaction to be carried out at relatively high temperatures of 230° C. and above to expedite the process.

Claims

1. A process for depolymerizing a polyethylene terephthalate (PET) by glycolysis with ethylene glycol (EG), the process comprising: (a) mixing the PET in a solid state with EG at a temperature of from 60° C. to 120° C. to obtain a heterogeneous mixture (A), and pressing the heterogeneous mixture (A) to squeeze out an aliquot of EG to obtain a heterogeneous mixture (B) with an EG:PET weight ratio R1 of from 0.1 to 3.0; and (b) feeding said heterogeneous mixture (B) into a blender where the heterogeneous mixture (B) is heated and kept under mixing at a temperature of from 170° C. to 270° C., with an EG:PET weight ratio R2 of from 0.1 to 4.0, to glycolize the PET and obtain a glycolyzed product containing bis(2-hydroxyethyl) terephthalate (BHET) and/or oligomers thereof.

2. The process according to claim 1, wherein in (a) the PET in the solid state is mixed with an amount of EG to initially obtain an EG:PET weight ratio R0 of from 0.2 to 4.0.

3. The process according to claim 1, wherein (a) is carried out in a screw press.

4. The process according to claim 2, wherein the screw press is arranged in an inclined configuration so that an axis of development of the screw is inclined with respect to a horizontal plane.

5. The process according to claim 1, wherein (a) comprises: (a1) mixing the PET in the solid state with EG at the temperature of from 60° C. to 120° C. to obtain the heterogeneous mixture (A) in a blender, preferably a continuous blender; and (a2) compressing the heterogeneous mixture (A) to squeeze out the aliquot of EG from the heterogeneous mixture (A) to obtain the heterogeneous mixture (B) with the EG:PET weight ratio R1 of from 0.1 to 3.0 in a screw press.

6. The process according to claim 1, wherein said feeding (b) is carried out by passing the heterogeneous mixture (B) through a transfer cell which collects a predetermined quantity of the heterogeneous mixture (B) obtained from (a) and transfers the same to the blender by gravity.

7. The process according to claim 1, wherein (b) is carried out in an inert atmosphere.

8. The process according to claim 1, wherein BHET and/or oligomers thereof are fed to the blender in (b), which are optionally recovered subsequently.

9. The process according to claim 1, wherein (b) is carried out in a paddle reactor.

10. The process according to claim 1, wherein the PET in the solid state is used in (a) as such.

11. The process according to claim 1, wherein the heterogeneous mixture (B) obtained in (a) has a water content equal to or lower than 0.5% by weight, with respect to the PET weight.

12. The process according to claim 1, further comprising: filtering of the glycolyzed product obtained from (b).

13. The process according to claim 1, further comprising: (c) subjecting the glycolyzed product obtained from (b) to a further glycolysis to obtain a glycolyzed product with an EG:PET weight ratio R4 of from 0.5 to 10.0.

14. The process according to claim 13, wherein the further glycolysis (c) is carried out in a reactor for liquids.

15. The process according to claim 13, wherein the further glycolysis (c) is carried out in a plurality of stirred tank reactors arranged in series to form a multistage reactor which operates continuously.

16. The process according to claim 13, wherein the glycolyzed product obtained in (b) or (c) is subjected to a treatment of removing possible solid foreign materials having a density lower than the glycolyzed product from the surface of the glycolyzed product.

17. The process according to claim 16, wherein the solid foreign materials removed from the surface of the glycolyzed product are cooled to a temperature of about 90° C. to facilitate a subsequent separation of the solid materials with recovery of a filtered liquid, which contains BHET and/or oligomers thereof.

18. A plant for depolymerizing polyethylene terephthalate (PET) by glycolysis with ethylene glycol (EG), the plant comprising: a screw press to which PET and EG are fed so as to form a heterogeneous mixture of PET and EG, said screw press comprising a cylinder in which a helical screw is placed rotating around its axis, wherein the helical screw defines a transport channel having a section decreasing along the axis of the same screw; and a paddle reactor in which the heterogeneous mixture leaving the screw press is fed, said reactor comprising a container, of substantially a cylindrical shape, in which a pair of shafts rotating around their own axis are arranged and each equipped with a plurality of paddles.

19. The plant according to claim 18, wherein the screw press is arranged to have the helical screw with its axis of development inclined with respect to a horizontal plane.

20. The plant according to claim 18, wherein, in said paddle reactor, the rotating shafts and the paddles are heated from inside to provide heat to the heterogeneous mixture.

21. The plant according to claim 18, wherein, in said paddle reactor, the paddles present on the rotating shafts are interpenetrating each other.

22. The plant according to claim 18, wherein the paddles have a wedge shape.

23. The plant according to claim 18, wherein the paddle reactor is equipped with a system which allows maintaining an inert atmosphere inside.

24. The plant according to claim 18, further comprising: a transfer cell placed between the screw press and the paddle reactor, which collects a predetermined quantity of the heterogeneous mixture leaving the screw press and transfers it to the paddle reactor by gravity.

25. The plant according to claim 18, further comprising: upstream of the screw press, a blender to which the PET and the EG are fed to achieve a premixing of the same, so as to obtain a heterogeneous mixture which is fed to the screw press.

26. The plant according to claim 25, wherein the blender is a continuous blender.

27. The plant according to claim 18, further comprising: downstream of the paddle reactor, at least one reactor for liquids, which is fed by a flow of a glycolyzed product leaving the paddle reactor and by EG, in order to complete glycolysis of the PET.

28. The plant according to claim 27, wherein the at least one reactor for liquids is a plurality of stirred tank reactors arranged in series to form a continuous multistage reactor.

29. The plant according to claim 27, wherein, downstream of the at least one reactor for liquids, a weir is arranged from which any solid, low-density foreign material is removed.

Description

[0107] The plant in accordance with the present invention is further illustrated on the basis of the following figures:

[0108] FIG. 1: scheme of an embodiment of the plant according to the present invention;

[0109] FIG. 2: scheme of a paddle reactor usable in the plant according to the present invention;

[0110] FIG. 2a: cross-sectional view of the paddle reactor in FIG. 2;

[0111] FIG. 3: scheme of a screw press usable in the plant according to the present invention.

[0112] FIG. 1 shows a schematic diagram of an embodiment of the plant (100) according to the present invention. The PET to be processed and the EG are fed by conventional means (not shown in the figure) to a belt blender (101). For example, the PET may be fed via a conveyor belt, while the EG may flow within the belt blender (101) via a duct connected to a storage tank. The belt blender (101) is preferably equipped with heating means to bring the mass being processed to the desired temperature. To reach this temperature it is also advisable to feed pre-heated EG to the belt blender (101). A heterogeneous mixture of PET and EG is formed in the blender as shown above.

[0113] 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 so as to squeeze out an aliquot of EG. The exiting EG is collected and sent back to the storage tank via a duct (104).

[0114] As already shown above, the screw press (103) comprises a cylinder (105) within which a helical screw (106) is placed rotating around its axis, which defines a transport channel having a section that decreases along the axis of the same screw (the representation in FIG. 1 is schematic and is better illustrated in FIG. 3).

[0115] At the end of the screw press (103) there is a transfer cell (107), which collects a predetermined quantity of the heterogeneous mixture leaving the screw press (103), which is transferred to the paddle reactor (108) by gravity. The transfer cell (107) is preferably hydraulically sealed, so as to maintain an inert atmosphere in the subsequent processing step in the paddle reactor (108).

[0116] As illustrated above, the paddle reactor (108) comprises a container (109), provided with a lid, within which a pair of shafts (110) is arranged rotating about their own axis, on which a plurality of paddles (111) is mounted. Such a device is best illustrated in an embodiment thereof in FIG. 2. EG and possibly BHET and/or oligomers thereof are fed to the paddle reactor (108) via a duct (112). A transesterification catalyst may optionally also be fed to the paddle reactor (108).

[0117] The paddle reactor (108) preferably comprises a sealing system (not shown in the figure) that allows an inert atmosphere to be maintained within it, for example by introduction of nitrogen to replace the air present.

[0118] The glycolyzed product exiting the paddle reactor (108) is a liquid that can be pumped and used as desired, for example sent to a purification process.

[0119] In FIG. 1, the glycolyzed product is transferred to a series of three stirred tank reactors (113a, 113b, 113c), where it is further subjected to glycolysis in order to obtain a high degree of glycolysis. EG can be introduced into the first reactor (113a), possibly mixed with the transesterification catalyst, through a duct (114) connected to a storage tank.

[0120] The first reactor (113a) is equipped with a weir (115a) from which the reaction mixture escapes and is fed to the second reactor (113b) via a duct (116a), where the glycolysis reaction can continue. Similarly to the first reactor (113a), the second reactor (113b) is equipped with a weir (115b) which allows to withdraw the reaction mixture and to convey it, through a duct (116b), to the third reactor (113c). In the third reactor (113c), the glycolysis reaction is completed and the glycolyzed product is extracted from the bottom of the reactor (113c) using a duct (120). The reactor (113c) is also preferably equipped with a weir (115c), which allows the removal of any solid low-density foreign materials (in particular polyolefins) that surface and concentrate on the surface of the liquid glycolyzed product. Such foreign materials are then cooled by addition of EG by means of a duct (117) and introduced into a filtering device (118), which separates the solid from the liquid. The filtered liquid, which contains BHET and/or oligomers thereof, is collected and added via a duct (119) to the glycolyzed product recovered from the bottom of the reactor. The glycolyzed product is sent to subsequent steps (for example to a purification plant) via the duct (120).

[0121] Each reactor (113a, 113b, 113c) is equipped with a purge duct (121a, 121b, 121c) to eliminate any high-density foreign products, which collect on the bottom.

[0122] FIG. 2 shows a scheme of embodiment of a paddle reactor usable in the plant according to the present invention.

[0123] The paddle reactor (200) comprises a container (201) within which a pair of shafts (202a, 202b) (only one shaft is visible in the figure) are arranged side by side and rotating about their own axis, on each of which a plurality of paddles (203a, 203b) are mounted. FIG. 2 shows a cross-section of the container (201) only in the initial part for the purpose of showing the presence of the shafts (202a, 202b) and paddles (203a, 203b).

[0124] The paddles (203a, 203b) mounted on the two shafts (202a, 202b) are interpenetrating each other and preferably have a triangular cross-section (i.e. wedge shape), which ensures self-cleaning of the paddles themselves. The shafts (202a, 202b) are connected to a motor (204) which puts them into rotation. The shafts (202a, 202b) and the paddles (203a, 203b), having a hollow structure, are connected to an inlet port (205a) of a heating fluid, so as to allow the fluid to enter the shafts and paddles, which are then heated to transfer heat to the material being processed. Furthermore, there may be another inlet port (205b) through which a heating fluid is introduced into a heating jacket (201b) which encloses the container (201). The exhausted heating fluid is extracted through an outlet port (206a) connected to the shafts and an outlet port (206b) connected to the heating jacket (201b).

[0125] The material to be processed is introduced into the container (201) through a feed port (207) and exits from the container (201) through a discharge port (208). It is also possible to feed EG and/or BHET and/or oligomers thereof to the container (201) by means of a further feed port (209).

[0126] The paddle reactor (200) is equipped with a system that allows working inside the machine in an inert atmosphere. Such a system may comprise, for example, a nitrogen inlet port (210) and a vent port (211), from which nitrogen and other gases or vapours, which can be recycled to the plant, are recovered.

[0127] FIG. 2a shows a cross section of the container (201) and the two shafts (202a, 202b) on which the two sets of paddles (203a, 203b) are mounted. As can be seen from FIG. 2a, the bottom of the container (201) has an omega-shaped cross section, which allows to maintain a substantially constant distance between the edge of the rotating blades (202a, 202b) and the bottom, avoiding stagnation of the material being processed and homogeneous mixing of the whole treated mass.

[0128] FIG. 3 shows a diagram of embodiment of a screw press that can be used in the plant according to the present invention.

[0129] The screw press (300) comprises a container (301), of substantially cylindrical shape, in which a helical screw (302) mounted on a shaft (303) connected to a motor which allows it to rotate around its own axis is housed. An inlet duct (304) introduces the material to be processed (i.e. PET and EG) inside the container (301). As illustrated in FIG. 3, the shaft (303) has an increasing cross-section along its development from the feed zone to the discharge zone, so as to gradually reduce the section of the channel conveying the material being processed, which is therefore gradually compressed as it moves pushed by the rotating screw (302). A gradual reduction in the section of the transport channel can also be achieved by gradually reducing the pitch of the helical screw from the feed zone to the discharge zone.

[0130] Thus, a portion of the EG is squeezed out from the PET and exits through the perforations in the wall of the container (301). This preferably has a wedge wire screen filtering wall as illustrated above. EG squeezed from the PET being processed is collected by a tray (305) and sent to the storage tank for reuse. The PET mixed with EG thus processed is discharged through a transfer cell (306) that feeds the next step carried out in the paddle reactor.

[0131] Further details of the structure and operation of a screw press as illustrated above can be found, for example, in U.S. Pat. No. 5,857,406 or US 2011/0297016.