Method and device for processing a mixture of recycled polyester material and a polyester prepolymer from a polyester manufacturing process

Abstract

A method for processing a mixture of recycled polyester material and a polyester prepolymer from a polyester manufacturing process, wherein a recycled polyester material is mixed with a polyester prepolymer, from a polyester manufacturing process, and treated in a bulk thermal treatment reactor (7) with a process gas which flows in a counter-current or a cross-current flow direction to the flow direction of the mixture. In this process, the process gas, before entering a catalyst vessel (14), is passed through a protective bed (11) containing a solid adsorbent material that removes high-boiling organic substances or organic substances, with a high combustion temperature, from the process gas stream.

Claims

1. A method for processing a mixture of recycled polyester material and a polyester prepolymer from a polyester manufacturing process, comprising the following steps: blending said recycled polyester material with said polyester prepolymer from the polyester manufacturing process to produce a mixture of solids; treating this mixture of solids in a reactor for thermal treatment of bulk materials with a process gas in counter-current or cross-current to a flow direction of the mixture; introducing the process gas, containing organic impurities, into at least one heat exchanger to heat the process gas, and heating the process gas in said heat exchanger, controlling supply of an oxygen-containing gas to the process gas, introducing the process gas mixed with the oxygen-containing gas into a catalyst vessel with at least one catalyst bed arranged therein, through which the process gas flows from an inlet side to an outlet side of said catalyst vessel, combusting the organic impurities in the process gas in the at least one catalyst bed, at least partial recirculation of the process gas, wherein before entering the catalyst vessel, passing the process gas through a protective bed containing a solid adsorbent material, arranged upstream of the heat exchanger for heating the process gas, which removes high-boiling organic substances or organic substances having a high combustion temperature from the process gas stream, and setting a temperature in a range of 100 to 250° C. so that the high-boiling substances can condense and be absorbed by the adsorbent material.

2. The method according to claim 1, wherein said at least partial recirculation of the process gas is made into the reactor for thermal treatment of bulk material.

3. The method according to claim 1, wherein the polyester material is selected from the group consisting of polyethylene terephthalate and polyethylene terephthalate copolymers.

4. The method according to claim 1, wherein the recycled polyester material is pelletized and the pellets of recycled polyester material thus obtained are mixed with pellets of said polyester prepolymer from the polyester manufacturing process.

5. The method according to claim 1, wherein the recycled polyester material is provided as a melt, mixed with a melt of said polyester prepolymer from the polyester manufacturing process, and the melt mixture thus obtained is subsequently pelletized.

6. The method according to claim 2, wherein the recycled polyester material is provided as a melt, mixed with a melt of said polyester prepolymer from the polyester manufacturing process, and the melt mixture thus obtained is subsequently pelletized.

7. The method according to claim 2, wherein, before entering the protective bed, the process gas is cooled to a temperature in a range of 120-170° C.

8. A method for retrofitting a plant for thermal treatment of a bulk virgin material into a plant for thermal treatment of a pelletized polyester comprising at least partly recycled material, which comprises at least partly repelletized polyester recycled material, wherein the thermal treatment takes place in solid phase in a reactor through which a process gas flows, the process gas leaving the reactor being at least partially recirculated into the reactor and, before being recirculated into the reactor, passing through a purification step in a catalytic combustion device, wherein the plant is enlarged by a unit for producing a mixture of a recycled polyester material and a polyester prepolymer from a polyester production process, and between the reactor and the catalytic combustion device is enlarged by the following process steps: a step for cooling the process gas a step for adsorption of high boiling organic components in a protective bed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is explained in more detail below with reference to non-limiting examples and drawings. There is shown:

(2) FIG. 1 is a schematic view of a device according to a first embodiment of the invention

(3) FIG. 2 is a schematic view of a device according to a second embodiment of the invention

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) FIG. 1 shows a schematic view of a device according to a first embodiment of the invention.

(5) In a unit 1, a slurry is produced from terephthalic acid (TPA) and ethylene glycol (EG) and then subjected to esterification, prepolymerization and melt polycondensation in a finisher. A prepolymer melt of “virgin” PET leaves unit 1 and passes to a connecting unit 1a, in which a melt valve is preferably arranged.

(6) PET recyclate (preferably PET flakes) are fed into an extruder 3 where they are melted and extruded. The melt of rPET is introduced into a melt filter 4 where it is purified from solid particles. The purified rPET melt is then fed via a connecting line to the connecting unit 1a, where it is combined with the prepolymer melt of virgin PET. A flush valve 4a can preferably be arranged in the connecting line for the rPET melt in order to prevent the introduction of contaminated or low-quality rPET melt and to divert such material out of the device. Furthermore, at least one unit for measuring a quality parameter may be arranged in the connection line for the rPET melt.

(7) The melt mixture combined in the connecting unit 1a is then pelletized in a pelletizer 2 (preferably an underwater pelletizer or underwater strand pelletizer), dried if necessary and brought to a desired degree of crystallization in a crystallizer 5. The partially crystalline PET pellet mixture is heated in a preheater 6 to the temperature required for the SSP reaction and subjected to an SSP reaction in reactor 7. The finished PET mixture leaves reactor 7 with the desired intrinsic viscosity and can be further processed.

(8) The process gas is led out of reactor 7 through an outlet and mixed together with the process gas from preheater 6 and passed through a filter 9 (to separate any PET dust possibly present). Subsequently, the process gas is fed into a pipeline 10 with a cooling jacket and cooled down there. The process gas then enters a protective bed 11. Air is then added to the process gas (to enable combustion of the organic substances in the process gas), and the process gas then passes through a heat exchanger for heat recovery (economizer) 12 and then through a process gas heater 13. The process gas enters a catalyst vessel 14, which contains a bulk catalyst with noble metal coating (platinum and palladium). There, the process gas is catalytically cleaned. For energy recovery, the purified gas is again passed through the economizer 12 and further cooled. The process gas is dried in a dryer 15 (for example a molecular sieve dryer). After this, part of the process gas is fed back to reactor 7, while other parts of the process gas are fed to preheater 6 as exchange gas.

(9) FIG. 2 shows a schematic view of a device according to a second embodiment of the invention. The same reference signs indicate the same components.

(10) The device according to FIG. 2 differs from the device according to FIG. 1 in that the rPET melt leaving the melt filter 4 enters a pelletizer 2′ and is combined with the virgin PET pellets from the pelletizer 2. The remaining steps are identical.

Comparison Example 1

(11) In a conventional plant for the production of a polyethylene terephthalate (PET), a slurry was prepared from terephthalic acid (TPA) with 2% isophthalic acid as comonomer and ethylene glycol (EG). This slurry was then subjected to the steps of esterification, prepolymerization, and melt phase polymerization in a finisher.

(12) The obtained prepolymer melt was processed by underwater strand pelletization to cylindrical amorphous PET prepolymer pellets (pellet weight about 18 mg). The pellets were subjected to solid phase treatment comprising the steps of crystallization in a fluidized bed device, preheating to SSP reaction temperature under inert gas in a roof heat exchanger, solid phase polycondensation (SSP) under inert gas in a shaft reactor and cooling in a fluidized bed device.

(13) The plant was operated at a throughput of 6.66 t/h. The intrinsic viscosity of the prepolyester before underwater strand granulation was 0.62 dl/g, and the acetaldehyde content was 66 ppm.

(14) An inert gas flow of 65 m.sup.3/min was passed through the reactor to carry out the SSP reaction. The inert gas consisted mainly of nitrogen, with CO.sub.2 and small amounts of water vapor, oxygen and volatile organic compounds from the PET manufacturing process (ethylene glycol and acetaldehyde) being present.

(15) After a treatment time of 13 hours at around 204° C., the intrinsic viscosity of the PET was 0.82 dl/g.

(16) The process gas from the reactor was led out of the reactor through an outlet, mixed with the process gas from the preheater, passed through a filter (to separate any PET dust possibly present) and then fed to a further cleaning stage using catalytic combustion. The process gas had a temperature of approx. 185° C. after the filter.

(17) For purification, an amount of air sufficient to allow combustion of the organic substances in the process gas was added to the process gas. Subsequently, the process gas/air mixture was passed through a heat exchanger for heat recovery (economizer) and then through a process gas heater, where it was heated up to 380° C. The process gas was then purified in a catalyst bed. The actual combustion of the organic substances took place in a catalyst bed containing a bulk catalyst with noble metal coating (platinum and palladium). For energy recovery, the purified gas was passed back through the economizer and cooled further. Water produced due to combustion and already present in the gas was then removed in a molecular sieve dryer. Part of the process gas was then returned to the reactor, while other parts of the process gas were used as exchange gas for other process steps (conveying, crystallization, preheater).

(18) The finished PET had an L value of 89.8, an a* value of −1.7 and a b* value of −2.5. The acetaldehyde content was reduced to 0.5 ppm.

Example 1

(19) For Example 1, the plant from Comparative Example 1 was modified in such a way that three 5 m long pieces of pipe, each with an internal diameter of 0.3 m, and a cooling jacket were installed between the filter upstream of the gas purification device and the inlet to the economizer, as well as a protective bed with 9000 kg of an alumina bed in a 2.2 m diameter container. The alumina fill had a bulk weight of approximately 750 kg/m.sup.3 and included a basic coating capable of binding chlorine. The fill was supported in the tank on a screen grid and was approx. 3.2 m high. An additional fan was installed to overcome the pressure drop that occurred.

(20) Comparison example 1 was repeated with the modified plant according to example 1. However, the plant capacity of the melt phase polymerization was reduced to 5.41 t/h. The PET prepolymer pellets were fed into a mixing silo. The PET prepolymer pellets were fed into a mixing silo and mixed with 1.25 t/h of PET recyclate pellets (rPET). The recyclate pellets were produced from post-consumer PET flakes (bottle flake) with a PVC (polyvinyl chloride) content in the range of 25 to 50 ppm in a single-screw extruder with melt pump, followed by melt filtration and underwater pelletization with subsequent direct crystallization. The mesh fineness for melt filtration was 40 μm. The viscosity of the PET recyclate pellets was 0.66 dl/g, their limonene content was 68 ppb, and their acetaldehyde content was 4 ppm. The pellet weight of the spherical pellets of the PET recyclate pellets was about 20 mg.

(21) Subsequently, the mixture of PET prepolymer pellets and PET recyclate pellets was treated in the modified solid-phase treatment device.

(22) The process gas from the SSP process was led out of the reactor for the SSP reaction through an outlet and cooled down to 150° C. in the pipeline with cooling jacket described above. A pressure drop of 55 mbar had occurred across the protective bed, which was compensated by operating a fan.

(23) After solid phase treatment, a homogeneous mixture of PET polymer and approx. 19% PET recyclate was obtained despite the non-uniform pellet shape. The mixture had an IV value of 0.82 dl/g, an L value of 87.2, an a* value of −1.6 and a b* value of −1.9. The acetaldehyde content of the mixture was reduced to 0.5 ppm, and the limonene content to below 1 ppb.

(24) Preforms and subsequently bottles were produced from the PET/rPET pellets according to example 1. The acetaldehyde content of the preform was 6.5 ppm. While the color of the rPET/PET preforms could still be distinguished from pure PET preforms, no difference could be detected in the bottle body.