Method, Device And Use For Reprocessing Substantially Polyalkylene Terephthalate

Abstract

A method and a device for reprocessing waste products containing substantially polyalkylene terephthalate, particularly polyethylene terephthalate and/or polybutylene terephthalate, in a continuous process by means of depolymerizing is provided. A solid alkali hydroxide and/or alkali earth hydroxide, particularly sodium hydroxide, is added to the waste products for producing a reaction mixture, suitable for recycling multilayer systems and colored material nearly entirely chemically into the starting materials at a high throughput and high quality, in order to be able to produce new polyalkylene terephthalate products from the recycling products without limitation. An alkylene glycol is additionally added to the reaction mixture as a reactant, wherein the alkylene glycol is an alkylene glycol produced as a product of the intended depolymerizing, particularly MEG. No further reactive components are added to the reaction mixture.

Claims

1. A method for reprocessing waste products comprising substantially, in a continuous process via depolymerizing, an alkali hydroxide and/or alkali earth hydroxide being added to the waste products for producing a reaction mixture, an alkylene glycol is additionally added to the reaction mixture as a reactant, wherein the alkylene glycol is an alkylene glycol produced as a product of the intended depolymerizing, wherein no further reactive components are added to the reaction mixture, and wherein a twin-screw extruder is used for transporting.

2. The method according to claim 1, wherein the waste products are comminuted to a size of no greater than 3 mm prior to producing the reaction mixture.

3. The method according to claim 1, wherein the alkylene glycol is added at a mass flow rate selected such that the mass flow rate ratio of the waste products to the alkylene glycol is at least 3.

4. The method according to claim 1, wherein the alkali and/or alkaline earth hydroxide is added at a mass flow rate such that the ratio of alkali and/or alkaline earth hydroxide to polyalkylene terephthalate is approximately stoichiometric relative to a constitutional repeating unit.

5. The method according to claim 1, wherein the reaction mixture for depolymerizing is transported continuously through a reactor vessel.

6. The method according to claim 1, wherein the depolymerizing is performed at a temperature below the boiling point of the polyalkylene terephthalate and/or below the boiling point of monoethylene glycol and/or at a pressure in the range from 1 bar to 3 bar.

7. The method according to claim 1, wherein inert gas is fed into the reactor vessel.

8. The method according to claim 1, wherein the reaction mixture is kneaded and/or mixed and/or transported and/or reverse transported.

9. The method according to claim 1, wherein alkylene glycol is removed from a reaction output.

10. The method according to claim 1, wherein water is added to a reaction output for dissolving solid components.

11. The method according to claim 1, wherein solids are filtered out of a reaction output.

12. The method according to claim 1, wherein an acid is added to a reaction output in order to convert carboxylate ions formed by depolymerizing and present in the reaction output into acids.

13. A device for performing a method for reprocessing according to claim 1, the device having a reactor vessel implemented as the twin-screw extruder, the device feeding the alkali and/or alkaline earth hydroxide, and feeding the alkylene glycol into the reactor vessel, wherein the reactor vessel is co-rotating.

14. The device according to claim 13, wherein the screw extruder comprises at least one screw element, the outer diameter thereof having a ratio to the inner diameter of approximately 1.7.

15. The device according to claim 14, wherein a ratio of the length of the screw element to the outer diameter thereof is approximately 60.

16. The device according to claim 13, wherein the reactor vessel includes transporting, transport-neutral, and/or reverse transporting screw elements disposed one after the other in order to intermittently convey, knead, or reverse transport the reaction mixture in the reactor.

17. The device according to claim 16, wherein the reactor vessel controls the temperature in segments adapted to the screw elements.

18. (canceled)

19. The method according to claim 1, wherein the polyalkylene terephthalate is polyethylene terephthalate and/or polybutylene terephthalate, the alkali hydroxide and/or alkali earth hydroxide includes sodium hydroxide, the alkylene glycol is mono-ethylene glycol (MEG), and the screws are co-rotating.

20. The method according to claim 3, wherein the mass flow rate ratio of the waste products to the alkylene glycol is approximately 3.3.

21. The method according to claim 4, wherein the stoichiometric ratio of alkali and/or alkaline earth hydroxide to polyalkylene terephthalate is approximately 2.4.

Description

[0090] The single FIGURE in the drawing shows, in detail:

[0091] FIG. 1: A block flow diagram for illustrating the method steps of an embodiment of the method according to the invention.

[0092] The preferred embodiment of the method according to the invention described with reference to FIG. 1 enables recycling of polyethylene terephthalate (PET) waste products previously unable to be recycled, or recycled only thermally. The method can also be used for recycling other polyalkylene terephthalates such as polybutylene terephthalate.

[0093] Waste products comprising PET, including multilayer systems, such as beverage bottles, detergent bottles (opaque, clear, or dyed black) or food product packages of other types, such as salad shells, sausage and cheese packaging, or production waste comprising PET, are washed in a first step 1 and comminuted to smaller than 3 mm. The waste products are then optionally pre-dried in a second step 2 in order to reduce the water content of the PET material. Alternatively, the material to be processed can be pre-dried after the method according to the invention. In this case, the step 2 of drying after step 1 of comminuting can be eliminated. For particular applications according to the invention, however, further, more intensive drying 2 can be advantageous.

[0094] In a further process step of “depolymerizing” 3, the waste products are fed into a co-rotating twin-screw extruder having tightly meshing screw elements. The saponification and depolymerization of the PET is performed continuously in the extruder. In the system described as an example with reference to FIG. 1, 6.66 hg/h of waste products comprising PET, 3.33 kg/h of sodium hydroxide, and 2 kg/h of MEG are processed in the extruder. The ratio of sodium hydroxide to PET waste products is set during the process according to the invention so that a constant stoichiometric ratio of approximately 2.4 is set relative to the constitutionally repeating unit of PET. The reaction output of the extruder comprises disodium terephthalate, MEG, and nonreactive components of the sodium hydroxide and the PET waste, such as PET residues, dyes, products of decomposition of PA and dyes, and other polymers such as PE, PP, and PS.

[0095] The twin-screw extruder is modular in construction and comprises 14 temperature zones. The housings are each equipped with an individually controlled electric heater and water cooling. The ratio of the outer screw diameter Da to the inner screw diameter Di is a characteristic parameter for the potential free volume of the screw. For the extruder used here, the Da/Di ratio of the screw elements is 1.66. The ratio of the screw length L to the diameter of the screw D describes the processing length of the extruder and is 60 for the present extruder. The screw geometry is modular in structure and can be adapted to the process and the PET material. The extruder comprises the following individually temperature-controlled cylinders:

[0096] Cylinder 1: main intake, Cylinder 2: top injection nozzle, Cylinder 3: side outgassing with reverse flow, Cylinder 4: side infeed of sodium hydroxide, Cylinder 5: top injection nozzle, Cylinder 6: top venting ports, Cylinder 7: closed, Cylinder 8: top injection nozzle, Cylinder 9: side outgassing with reverse flow, Cylinder 10: closed, Cylinder 11: outgassing, Cylinder 12: closed, Cylinder 13: outgassing, Cylinder 14: injection nozzle, Cylinder 15: transport, output after cylinder 15. In cylinders 1 through 15, the reactants are thus first drawn into the extruder and mechanically processed in the apparatus while passing through all zones. In the last, that is, the fifteenth, cylinder, the product is transported out of the extruder. The output is implemented as an opening through which the product is transported out of the apparatus.

[0097] The apparatus is equipped with up to three pressure sensors inserted in the cylinder opening of the injection nozzle. The housings/cylinders 2 through 15 are temperature-controlled to 160° C. Cylinder 1 is not temperature-controlled. The rotary speed of the co-rotating twin screws is set to 100 rpm.

[0098] The screw configuration is selected so that good mixing of the two solid materials can be ensured in the process. The screw elements used can be strung on the shaft in an arbitrary sequence. When the number of threads of the screw elements changes, spacer discs or transition elements are used. In order to achieve the highest possible deformation and mechanical stressing and a relatively high average dwell time of approximately 2 minutes of the multilayer PET waste products, transporting and transport-neutral kneading elements are used in the design of the screw configuration. Furthermore, by using kneading elements according to the invention, energy is introduced into the reaction mixture and can accelerate the reaction. Furthermore, kneading elements further ensure good dispersing of the base in the reaction mixture. The use of a reverse-transporting element leads to accumulating of the reaction mixture. A tight gap between the reverse-transporting elements forces the reaction mixture to dwell until the PET waste product residues can be pressed through the gap between elements and the cylinder wall. In the region of outgassing and atmospheric opening, screw elements having a high free screw volume are used. This allows solvent to be continuously removed from the reaction mixture. Furthermore, a series of transporting mixing elements are installed in the screw configuration and mechanically stress the reaction product less than the kneading elements due to lower shear, but do ensure very good mixing.

[0099] In the region of cylinder 1, the PET is metered gravimetrically by means of a solid metering device. The material is transported into the extruder via the intake and the screw elements having a large free screw volume and is warmed therein. In cylinder 1 itself, however, only transporting occurs, but no temperature control. In cylinder 2, MEG is added via the top filling opening by means of a gravimetric metering device. Solid sodium hydroxide in pellet form is added gravimetrically via a side metering device in cylinder 4 and via a second metering device by means of a forced feeder. Cylinder 4 further comprises an atmospheric opening. Both the solid metering device for the PET and the solid metering device for the sodium hydroxide are blanketed with inert gas in order to prevent inflow of oxygen and moisture and to ensure constant metering. Without inert gas blanketing, the highly hygroscopic sodium hydroxide would very quickly agglutinate and clog, causing the process to come to a halt. The MEG used and the MEG forming due to condensation can be recovered via an atmospheric opening in cylinder 6 and cylinder 10.

[0100] The screw configuration is shown in Table 1. The angle values indicated refer to the angle between the discs of the kneading elements in each case. A sequence of various kneading, mixing, transport, and reverse transport elements is used for ensuring homogeneous mixing of the solids and for mechanically grinding and breaking up the PET material and the multilayer systems in order to provide the greatest possible surface area for the saponification reaction. The mechanical stresses damage the material bonds between the various layers and the layers themselves, so that a reaction can advantageously take place on various sides of the PET by means of said processing. In contrast thereto, without mechanical stressing, the base would attack only the exposed PET surfaces and PET edges of the PET flakes coated on one or more sides. By selecting the screw configuration shown, the average dwell time of the PET waste product in the extruder is set to approximately 2 min. Within said reaction time, conversion of the PET content of the PET waste product occurs at 92-97%.

TABLE-US-00001 TABLE 1 Screw configuration for PET depolymerizing Cylinder 1 2 3 4 5 6 7 8 Screw PET Outgassing, MEG NaOH Transport Reverse Transport Kneading configuration intake intake intake Kneading transport Kneading (90°) (45°) (45°) Cylinder 9 10 11 12 13 14 Screw Transport Mixing Outgassing Kneading Mixing Transport Transport configuration (45°)

[0101] The paste-like reaction output is granulated, comminuted, and loaded onto a temperature-controlled conveyor having extraction in the subsequent step “post-treatment” 4. The MEG vapors are condensed and collected at a cooler.

[0102] In the subsequent method step “dissolving” 5, the reaction output is dissolved in water in a stirrer vessel or a mixing screw (55 kg/h, 133 g/L solubility of the disodium terephthalate). The insoluble residues (PET residue, PE, PP, metals, PS, cardboard) are separated out by filtering 6.

[0103] After filtering 6, in a method step “purifying” 11, contaminants and byproducts of the method are separated out. In the context of the invention, various methods are conceivable here and are per se known to the person skilled in the art.

[0104] In the subsequent method step “precipitating TPA” 7, sulfuric acid (9.6 kg/h, 25% (w/w)) is added to the solution. The precipitated TPA is obtained by filtering 8 and washing 9 with water and is filtered out. The TPA is washed with water in order to remove residues of the sulfuric acid and the sodium sulfate formed during precipitating.

[0105] After washing 10, separating of solid and liquid 10 takes place in order to separate the solid TPA, being insoluble in water, from the washing water.

[0106] By means of the method according to the invention, the device according to the invention, and the use according to the invention, particularly multilayer PET waste products can be converted efficiently at a high throughput and high quality and made into starting materials available for polymerization without limitation for reprocessing, that is, for producing polyalkylene terephthalates. Part of the alkylene glycol thus produced can thereby be used in the reverse flow for depolymerizing in the method according to the invention.