PROCESS FOR DEODORIZING RECYCLED POLYOLEFIN GRANULES.

Abstract

A three-stage process for deodorizing recycled polyolefin granules (23): In the first stage, the moist granules (23) are contacted with a circulating inert gas at a temperature between 110 °C and 160 °C in a steam extractor (11), and the steam is collected in a condensate collector (29). In the second stage, the granules (23) are fed into a vacuum silo (13) under negative pressure and contacted with a countercurrent gas flow. In the third stage, after the vacuum silo (13), the granules (23) are fed into a first heat exchanger (15) and contacted with the countercurrent gas flow from the second stage.

Description

The invention relates to a process for deodorizing recycled polyolefin granules, according to the preamble of claim 1. State of the art During use, polyolefin containers are exposed to various chemicals that migrate into the material and cause a specific odor, either directly (aromatic substances from the filler material) or indirectly after reacting with other substances or through thermal or oxidative processes. Additionally, polyolefins sometimes contain additives that can similarly cause unpleasant odors (e.g., degradation of carnauba wax used as a release agent). Limonene is frequently used as the main substance for the purification of polyolefins, since measuring the concentration of limonene has been the latest technology in polyester recycling for many years and is easy to implement. EP 2 507 022 A1 describes a purification process for deodorizing plastic granules, where the plastic granules were extruded from waste polyolefin material. With this process, the limonene content is reduced to less than 320 ppb. Hot air flows through the odorous granules for several hours, agitating them. At regular intervals, untreated "fresh" granules are fed in; treated granules are discharged at the same intervals. The supplied air is used, on the one hand, to heat the granules and, on the other hand, to extract odor-relevant volatile impurities. As a result of this process, some of the granules remain cold and can only release their impurities very late, corresponding to subsequent heating. Worse still, the previously extracted impurities condense on the still-cold granules, further contaminating them.Heating the batch-added granules requires significantly larger quantities of air than is needed for mass transfer. Therefore, technically, combining simultaneous heating and decontamination makes little sense. One disadvantage of this process is its high energy consumption, as hot air escapes unused and the hot granules exit the process. The energy required for agitating the granules is also considerable. Another drawback is the release of air containing the extracted contaminants into the surrounding environment, causing environmental pollution. Another disadvantage of this process is the lateral introduction of pressurized air, which only allows the granules to be discharged in batches and therefore causes the process to be interrupted. Objective of the invention The disadvantages of the described state of the art result in the objective of proposing an energy-saving and efficient process for deodorizing recycled polyolefin granules. Description The set of objectives is achieved in a process for deodorizing recycled polyolefin granules, through the functionalities indicated in the characterization portion of claim 1. Advantageous advances and / or alternative modalities constitute the subject of the dependent claims. The invention is characterized, preferably, by a three-stage deodorization process: (a) In a first stage, the wet granules are brought into contact with a circulating inert gas (at a temperature between 110 °C and 160 °C) in a steam extractor and the steam is precipitated into a condensate collector, whereby, in the first stage, the odor components adhering to the surface of the granules are removed and the granules are preheated. (b) In a second stage, the granules are sent to a vacuum silo under negative pressure and brought into contact with a countercurrent flowing gas, where the odor components that have penetrated the granules are extracted by the negative pressure and the surface temperature of the granule remains below 123 °C for more than 10 hours. (c) In a third stage, after the vacuum silo, the granules are sent to a first heat exchanger and brought into contact with the countercurrent flowing gas of the second stage, whereby the granules are cooled and the gas is preheated for the second stage. The first stage produces a heating of the granules that saves energy, particularly with a simultaneous, virtually complete extraction of surface-adhering odor components such as terpenes. The second stage has the following advantages: Heat transfer between the gas and the granules is largely limited by the negative pressure. However, the migration of odor components and evaporation are considerably accelerated, as the vapor pressure of the odor components increases due to the negative pressure, and the odor components become more volatile as the pressure in the vacuum silo decreases. As a result of the increased gas temperature, the odor components can dissolve more effectively in the gas and evaporate much more efficiently. Due to the homogeneous temperature distribution in the granule bed or the small temperature differences in the vacuum silo, fewer active odor substances recondense on the cooler granules or granules, and these can be more easily removed from the process.Therefore, it is possible to expel odor components that are deeply embedded in the granules and only slowly migrate to the surface. Unpleasant-smelling contaminants such as carboxylic acids, aldehydes, and lactones, particularly nonanal, nonenal, butyric acid, and valeric acid, can be virtually completely removed from the granules. Since heating and decontamination are decoupled by the first and second stages of the process, the granules can remain at elevated temperatures for much longer without sticking together. This is not possible with previously known processes. With the present process, the granules can remain at temperatures above 100 °C for more than 10 hours. The third stage makes it possible to achieve two objectives simultaneously: As a result of the heat exchange between the granules and the gas, the incoming cold gas cools the granules as much as possible, and the gas is preheated so that further heating by a gas heater requires the minimum amount of energy possible. In a preferred embodiment of the invention, the condensate collector operates as a partial flow of the inert gas circuit. Therefore, it is not necessary to cool the entire inert gas flow to condense the odor components in the condensate collector. A portion of the gas flow can be fed directly back into the steam extractor. Overloading the condensate collector can also be avoided if part of the gas flow can be diverted beyond the condensate collector. In another preferred embodiment of the invention, the condensate collector functions as a second heat exchanger. The second heat exchanger can also be provided in addition to an existing condensate collector. The second heat exchanger can recover a portion of the heating energy from the inert gas flow. It has been found that it is advantageous for the surface temperature of the granules in the steam extractor to be between 100 °C and 120 °C. As a result, the granules for the vacuum silo are preheated. Consequently, the recondensation of odor components in the vacuum silo can be prevented. The steam extractor is conveniently a horizontally oriented rotating tube with forced transport of the granules by the tube's rotation. As a result, contact between the inert gas flow and the granules can be established as close as possible. This further enhances granule heating and mass transfer of odor components adhering to the granule surface. Discharging the contaminants through the condensate collector has proven particularly advantageous, as the contaminants in the condensate, in liquid form, can be purified extremely efficiently without heavily polluting the recycling plant environment with gaseous contaminants. In a further preferred embodiment of the invention, the residence time of the granules in the steam extractor is between 30 and 180 minutes, and preferably between 50 and 120 minutes. The granules are rapidly heated to a uniform target temperature. This means that the temperature spread of the granules during treatment is low compared to the prior art. In this way, the recondensation of odor components can be prevented particularly effectively. This is advantageous if water or steam can be added to the steam extractor through an inlet. As a result, the temperature and purification performance of the steam extractor can be set with particular precision. The invention is also preferably characterized in that the granule inlet and outlet of the vacuum silo have first and second gas-tight conveying devices, respectively. As a result, granules can be continuously fed into the vacuum silo and removed without the vacuum silo drawing air in. Therefore, the negative pressure in the vacuum silo is not lost despite continuous operation. The vacuum silo's gas outlet is conveniently equipped with a vacuum pump. As a result, it's possible to maintain a constant negative pressure within the vacuum silo, leading to the virtually complete removal of odor components from the granules. In a further embodiment of the invention, the gas at the gas inlet of the vacuum silo is guided by a second valve and a gas heater connected in the flow direction. The second valve allows for precise adjustment of the negative pressure. The gas heater warms the gas to the required temperature if it has not already reached that temperature in the first heat exchanger. In a further embodiment of the invention, the vacuum silo has a wrap-around heating unit. The wrap-around heating unit prevents condensation and the deposition of odor components inside the vacuum silo, thus preventing contamination of the silo. The wrap-around heating unit can be used in addition to or instead of the gas heater. Preferably, the gas is an inert gas or air with reduced oxygen. It is essential that the least amount of oxygen possible be present; oxygen can trigger auto-oxidative degradation reactions of the polyolefin at the high temperatures of the present invention and thus further degrade the already recycled polyolefin. In a further preferred embodiment of the invention, the vacuum silo is operated at a pressure between 0.01 bar and 0.9 bar, and preferably between 0.6 bar and 0.8 bar. At this negative pressure, the advantages described above for the second stage of the process are remarkable. This is advantageous if the granule inlet to the first heat exchanger has a second gas-tight conveying device, and the granule outlet from the first heat exchanger has a third gas-tight conveying device. As a result, it is not possible to introduce air into the vacuum silo through the second heat exchanger, even when odor-free granules are discharged from the first heat exchanger. The gas is conveniently transported from the heat exchanger to the vacuum silo via a bypass line, which integrates the second valve, the gas heater, and an optional filter. As a result, the pellet flow is separated from the gas flow at the transition from the first heat exchanger to the vacuum silo, allowing the media to be treated separately for optimal interaction. In a particularly preferred embodiment of the invention, the process is operated continuously. The continuous mode of operation enables energy-saving and efficient production of recycled polyolefin granules that have been deodorized. This is particularly advantageous if the granules have a surface temperature below 60 °C at the granule outlet of the third process stage of the first heat exchanger. This prevents degradation of the granule material due to the temperature increase after production. Other advantages and features will become apparent from the following description of an illustrative embodiment of the invention with reference to the schematic representations. It is shown, in a representation not to scale: Figure 1: Flow diagram of a process for deodorizing recycled polyolefin granules. Figure 1 shows a flow diagram or schematic diagram of a process for deodorizing recycled polyolefin granules. The process comprises three stages, where the first stage is implemented in a steam extractor 11, the second stage is implemented in a vacuum silo 13, and the third stage is implemented in a first heat exchanger 15. In an earlier stage of pellet or granule production, flakes are first produced from used polyolefin packaging, particularly containers and closures. The polyolefin flakes are sorted, cleaned, and washed, then extruded and converted into granules. During extrusion, some of the active odor contaminants are usually removed by a degassing screw or melt degassing unit. After melt filtration 19, the extruded product is formed into granules 23 in an underwater granulator 21. Most of the water adhering to the granules 23 is separated in a water separator 25. After the extrusion stage, the granules 23 arrive at the steam extractor 11 wet and hot. In the steam extractor 11, hot nitrogen, which can be produced in a nitrogen generator 27 at 110° to 160°C (limonene boiling point: 175°C), is circulated and brought into contact with the granules 23. As a result, the granules 23 are preheated in the vacuum silo 13 for the second stage. The vapor carried with the circulating nitrogen can be collected in a condensate collector 29. If necessary, additional water or steam is added to the steam extractor 11 through an inlet 31. To keep energy loss as low as possible, the condensate collector 29 is integrated into a first partial flow 30 of the circulating nitrogen. Downstream of the condensate collector 29, the nitrogen released from the steam is heated in a preheater 33 and re-fed to the steam extractor 11 by a pump 35. The charged nitrogen can be partially or fully diverted to a second partial flow 39 through a first valve 37. As a result, the circulating nitrogen is cooled only in the first partial flow 30, and overloading of the condensate collector 29 can be avoided by diverting it to the second partial flow 39. The vapor escaping with the nitrogen carries odor components particularly effectively, especially from the surface of the granules, and can be separated in the condensate collector 29. Such odor components that adhere to the granule surface are, for example, terpenes. The evaporating water cools the granule surface and additionally prevents the granules 23 from sticking together in the steam extractor 11. The amount of nitrogen and the temperature are regulated in such a way as to prevent the adhesion or sticking of the granules to each other 23. A surface temperature of the granule greater than 100 °C and less than 120 °C is sought. In the steam extractor 11, the granules 23 are heated from 100 °C to 120 °C to prevent condensate from being deposited in the vacuum silo 13. Most of the water and impurities are separated by the condensate collector 29. Steam extractor 11 is preferably a horizontal rotating tube with forced transport and a residence time range of + / - 20% of the average residence time. In the second stage, the granules 23 are sent to the vacuum silo 13, which is under negative pressure, with the pressure in the vacuum silo 13 ranging from 0.01 bar to 0.9 bar. During the process, a preheated gas is drawn from the bottom of the vacuum silo 13 by a vacuum pump 41 and into the silo. Since the amount of gas at the inlet can be reduced by a second valve 43, a negative pressure can be built up in the vacuum silo 13 by the vacuum pump 41. Before entering the vacuum silo 13, the gas is guided through a gas heater 45 and a filter 47. The granules 23 are loaded into and removed from the vacuum silo 13 via a conveying device that is as gas-tight as possible, so that the negative pressure is not disrupted by the transport of the granules. Such conveying devices may be vacuum conveyors or rotary feeders. A vacuum conveyor belt 49 may be provided at the granule inlet, and a first rotary feeder 51 may be provided at the granule outlet. The vacuum conveyor belt 49 may be connected to the vacuum pump 41 via a third valve 52. The circulating hot gas, for example carbon dioxide, nitrogen, or air with reduced oxygen content, is drawn through the granules 23 at a negative pressure. In the case of negative pressure, the gas is distributed more efficiently than in the case of excess air, and the system can dispense with agitation mechanisms. A more efficient continuous operating mode is also possible, instead of a discontinuous one. The hot gas flow (>125 °C) is measured so that the surface temperature of the granules 23 does not exceed 123 °C. As a result, the granules 23 are not at risk of sticking together. If the granules become overheated locally due to a plant shutdown and stick together as a result, the gas flow temperature would be reduced, and local agitators would be used to maintain the free flow of the granules. Air currents, vibrations, and impacts penetrating the material have proven effective agitators. The melting point of the granules can significantly limit the deodorization of the granules 23. With the present process, two advantages of a negative pressure or a vacuum are used: Heat transfer between the gas and the granules 23 is largely limited by the negative pressure. However, the migration of odor components and evaporation are considerably accelerated, as the vapor pressure of the odor components increases due to the negative pressure, and the odor components become more volatile as the pressure in the vacuum silo drops. As a result of the increased gas temperature, the odor components can dissolve better in the gas. As a result of the negative pressure, the odor components evaporate at significantly lower temperatures compared to the prior art. As a result of the higher temperatures, the odor components evaporate significantly more effectively than in the prior art. As a result of the homogeneous temperature distribution of the granules or the small temperature differences within the vacuum silo, fewer odor-causing active substances recondense on the cooler granules or granulated grains, and these can be more easily removed from the process. Therefore, it is possible to expel odor components that are deeply embedded in the granules and only slowly migrate to the surface. Thus, unpleasant-smelling contaminants such as carboxylic acids, aldehydes, and lactones, particularly nonanal, nonenal, butyric acid, or valeric acid, can be virtually completely removed from the granules.23 Because heating and decontamination are decoupled by the first and second stages of the process, the granules can remain at elevated temperatures for much longer without sticking together. This is not possible with previously known processes. With the present process, the 23 granules can remain at over 100 °C for more than 10 hours. The primary function of heat exchanger 15 is to minimize energy loss and abruptly halt thermal degradation processes by cooling the granules. Gas flows countercurrently in heat exchanger 15, resulting in heat transfer from the granules 23 to the gas, which preheats the gas (inlet temperature < 50 °C) and cools the granules. The cooled granules 23 or the cooled pellets are discharged from heat exchanger 15 via a gas-tight conveying device, preferably a second rotary feeder 53. The gas can be nitrogen generated by the nitrogen generator 27. As a result of their low thermal conductivity, the granules packed while hot can release their energy to the environment only inefficiently and, depending on the ambient temperature, can remain hot for days and degrade accordingly. This can be prevented by providing the heat exchanger 15. Either an inert gas or air with reduced oxygen is used as the inert gas, since air contains oxygen, and oxygen triggers auto-oxidative degradation processes at extremely high temperatures, consumes primary and secondary antioxidants, and damages other recycling processes. For example, the oxygen content in air can be reduced to less than 10% by volume. The inert gas used can be nitrogen, produced from air in a nitrogen generator. It is also conceivable that carbon dioxide, produced during the combustion of natural gas for the power generation process, could be used as the inert gas. List of reference symbols: Steam extractor Vacuum silo First heat exchanger Extruder 19 Melt filtration 21 Underwater granulator 23 Granules Water separator Nitrogen generator Condensate collector First partial flow Entrance Preheater Bomb First valve Second partial flow Vacuum pump Second valve Gas heater Filter Vacuum conveyor belt First rotary feeder Second valve Second rotary feeder

Claims

1. A process for deodorizing recycled polyolefin granules (23), wherein they are extruded from used recyclable polyolefin flakes in an extruder (17), and the polyolefin granules (23) that still have an odor are formed in an underwater granulator (21) or water ring granulator, further characterized by a three-stage deodorization process, characterized in that (a) in a first stage, the wet granules (23) are brought into contact with a circulating inert gas (at a temperature between 110 °C and 160 °C) in a steam extractor (11) and the steam is precipitated into a condensate collector (29), whereby, in the first stage, odor components adhering to the surface of the granules (23) are extracted, and the granules (23) are preheated;(b) In a second stage, the granules (23) are sent to a vacuum silo (13) under negative pressure and brought into contact with a countercurrent flowing gas, whereby the odor components that have penetrated the granules (23) are extracted by the negative pressure and the surface temperature of the granule remains below 123 °C for more than 10 hours; and (c) In a third stage, after the vacuum silo (13), the granules (23) are sent to a first heat exchanger (15) and brought into contact with the countercurrent flowing gas from the second stage, whereby the granules (23) are cooled and the gas is preheated for the second stage.

2. The process according to claim 1, characterized in that the condensate collector (29) is operated as a partial flow of the inert gas circuit.

3. The process according to claim 1 or 2, characterized in that the condensate collector (29) is operated as a second heat exchanger.

4. The process according to one of the preceding claims, characterized in that the surface temperature of the granules (23) in the steam extractor (11) is between 100 °C and 120 °C.

5. The process according to one of the preceding claims, characterized in that the steam extractor is a horizontally oriented rotating tube (11) with forced transport of the granules (23) by means of the rotation of the rotating tube.

6. The process according to one of the preceding claims, characterized in that the residence time of the granules (23) in the steam extractor (11) is between 30 min and 180 min and is preferably between 50 min and 120 min.

7. The process according to one of the preceding claims, characterized in that water or steam can be added to the steam extractor (11) through an inlet (31).

8. The process according to one of the preceding claims, characterized in that the granule inlet and the granule outlet of the vacuum silo (13) have a first and a second gas-tight transport device (49, 51), respectively.

9. The process according to one of the preceding claims, characterized in that the gas outlet of the vacuum silo has a vacuum pump (41).

10. The process according to one of the preceding claims, characterized in that the gas at the gas inlet of the vacuum silo (13) is guided by a second valve (43) and a gas heater (45) connected in the flow direction.

11. The process according to one of the preceding claims, characterized in that the vacuum silo (13) has an enveloping heating unit.

12. The process according to one of the preceding claims, characterized in that the gas is an inert gas or air with reduced oxygen.

13. The process according to one of the preceding claims, characterized in that the vacuum silo (13) is operated at a pressure between 0.01 bar and 0.9 bar and preferably between 0.6 bar and 0.8 bar.

14. The process according to one of the preceding claims, characterized in that the granule inlet to the first heat exchanger (15) has the second gas-tight transport device (51), and the granule outlet from the first heat exchanger (15) has a third gas-tight transport device (53).

15. The process according to any one of claims 10 to 14, characterized in that the gas is transported from the first heat exchanger (15) to the vacuum silo through a diversion line, in which the second valve (43), the gas heater (45) and an optional filter (47) are integrated.

16. The process according to one of the preceding claims, characterized in that the process operates continuously.

17. The process according to one of the preceding claims, characterized in that the granules (23) at the granule outlet of the first heat exchanger (15) have a surface temperature less than 60 °C.