Process for recycling polyolefins

Abstract

A process for recycling polyolefins comprising the steps of extruding used polyolefin material, producing granules from the polyolefin material exiting from the extrusion into a liquid cooling medium, separating the cooling medium to obtain a dry polyolefin granulate, and treating the dry polyolefin granulate in a treatment space with a treatment gas, preferably by counter-current flow, immediately after separation of the cooling medium, the dry polyolefin granules still have a granule temperature (T2) which is above a temperature (T1) of the liquid cooling medium and in the range of 71° C.-200° C., preferably 80° C.-160° C., but below the melting point of the granules, and at least 75% of the dry polyolefin granules, in the treatment space, have a dry temperature (T3) which is in the range of granule temperature (T2)±20° C., but below the melting point of the granules.

Claims

1. A process for recycling polyolefins, the process comprising the steps of: a) extruding a used polyolefin material, b) producing granules from the polyolefin material emerging from the extrusion into a liquid cooling medium, c) separating the cooling medium to obtain dry polyolefin granules, and d) treating the dry polyolefin granules in a treatment space with a treatment gas to remove volatiles, wherein carrying out the process in such a way that the dry polyolefin granules, directly after separation of the cooling medium in step c), still have a granule temperature (T2) which is above a temperature of the liquid cooling medium (T1) in step b) and is in the range of 71° C.-200° C., but below a melting point of the dry polyolefin granules, and at least 75% of the dry polyolefin granules, in the treatment space, have a treatment space temperature (T3) which is in the range of the granule temperature (T2) ±20° C., but below the melting point of the dry polyolefin granules.

2. The process according to claim 1, wherein the liquid cooling medium in step b) is water.

3. The process according to claim 1, wherein step c) is carried out in a centrifugal dryer.

4. The process according to claim 2, wherein step c) is carried out in a centrifugal dryer.

5. The process according to claim 1, wherein the treatment gas in step d) is air.

6. The process according to claim 1, wherein the treatment gas, on entering the treatment space, either has a gas temperature (T4) below the granule temperature (T2) of the dry polyolefin granules, or is introduced into the treatment space in an amount corresponding to less than half a throughput of polyolefin granules through the treatment space.

7. The process according to claim 1, wherein the dry polyolefin granules remain in the treatment space for a period of 0.5 to 20 hours.

8. The process according to claim 1, wherein the dry polyolefin granules are treated in the treatment space at an elevated pressure.

9. The process according to claim 1, wherein the dry polyolefin granules are treated in the treatment space at a reduced pressure.

10. The process according to claim 1, wherein the treatment gas, after leaving the treatment space, is cleaned with a liquid.

11. The process according to claim 1, wherein the temperature (T1) of the liquid cooling medium, in step b), is in a range from 50 to 100° C.

12. The process according to claim 1, wherein a contact time of granulate and liquid cooling medium is at most 10 seconds.

13. The process according to claim 1, wherein components are removed during extruding by one of melt degassing and/or filtration.

14. The process according to claim 1, wherein the polyolefin material is pretreated prior to extrusion, and the pre-treatment is selected from the group consisting of sorting by colour, sorting by polymer type, comminution, surface cleaning, pre-drying, and combinations thereof.

15. The process according to claim 1, wherein transfer of the dry polyolefin granules into the treatment space is effected by pneumatic conveying.

Description

(1) The present invention is further described by means of non-limiting drawings. It is shown:

(2) FIG. 1 a schematic representation of a device for carrying out the method according to the present invention

(3) FIG. 2 a schematic representation of a further embodiment of a device for carrying out the method according to the present invention

(4) Identical units are indicated in the figures by identical reference numbers.

(5) The device according to FIG. 1 has a reactor 1 for the production of a polymer melt. According to the present invention, reactor 1 is also a device for melting a solid product such as used polyolefin material. In this case, reactor 1 can be an extruder as an example.

(6) The molten polyolefin material is transferred to a granulation device 2. In the granulation device 2, a granulate is produced from the molten material in a known manner. For example, it can be an underwater granulator (as shown in FIG. 1). In this case, the granulation is carried out under water. The granulate particles obtained are cooled down simultaneously in the granulation device 2. As explained above, however, the cooling must not be so strong that the granulate particles are cooled below a temperature T2. This can be achieved by using heated water which has a temperature T1 in the range of 50 to 100° C., preferably 70 to 100° C., even more preferably 75 to 95° C. and especially preferably 80 to 90° C. The granules should be cooled to a temperature in the range of 71° C.-200° C., preferably 80° C.-160° C. and in the case of polyethylene especially preferably 71°-120° C.

(7) The granulate is transferred via a connecting line 3 into a unit for drying the granulate (separating device) 5. To avoid excessive cooling of the pellets, they should be led out of the granulation device 2 and through the connecting line 3 as quickly as possible. Preferably, the flow velocity in connection line 3 can be increased by introducing a gas stream (preferably air).

(8) In the unit for drying the granulate (separating device) 5 the granulate is separated from the liquid cooling medium (water) and dried. The separated cooling medium is led back via a pipeline 4a into a storage tank 4b for the cooling medium. The reservoir tank 4b has an inlet 4e for the supply of cooling medium. From the storage tank 4b the cooling medium is transferred to the granulation device 2 by means of a circulating device (pump) 4c. Here the cooling medium preferably passes through a heat exchanger 4d. In the heat exchanger 4d the cooling medium can be heated or cooled, as required. Especially cooling medium returned from separating device 5 can have a too high temperature, due to contact with hot granules, and must be cooled before entering granulation device 2.

(9) The fresh cooling medium supplied via inlet 4e may contain a basic medium or a pH buffer medium. In particular, the use of water with a neutralisation or buffering effect set within a narrow range is provided here. Alternatively, a basic medium or a pH buffer medium can be added directly into the cooling circuit, e.g. into storage tank 4b.

(10) In addition to a mechanical separating device, the granulate in unit 5 is dried by means of gas, preferably air or a gas atmosphere essentially comprising air, at a temperature of 100 to 200° C., preferably 110 to 160° C. In the device according to FIG. 1, the air is led through an inlet opening 5a into the separating device 5. The inlet opening 5a for the gas can be located in the housing of the separating device 5 or in the connecting line 6, or both. Optionally, an intake filter (not shown) can be installed in the gas line leading to inlet opening 5a. The air leaves the separating device 5 through the outlet pipe 5b. In the device according to FIG. 1, a fan 5c is arranged in the outlet pipe 5b for the circulation of air through the separating device 5. Alternatively, the fan could be located in the air inlet line 5a. Furthermore, inlet opening 5a and outlet line 5b could be connected to each other to form a circulation system. A condenser must then be provided in this circulation system.

(11) The granulate is transferred from the separating device 5 via a connecting line 6 through an inlet opening 7a into a unit 7 with a treatment space. A start-up diverter valve 6a with discharge line is provided in the connecting line 6.

(12) In unit 7, the polyolefin granulate is treated according to the present invention. Inside the treatment space of unit 7, the granulate particles are heat treated by a gas stream passing through unit 7 in counter-current or cross-current. Within the treatment space of unit 7 the conditions of a fixed bed are present.

(13) At least 75% of the dry polyolefin granules in the treatment space of unit 7 have a temperature T3, which is within a range of T2±20° C., but below the melting point of the granules.

(14) Preferably the treatment gas introduced into unit 7 is air. The treatment gas, preferably when entering the treatment space, either has a temperature T4 below the temperature T2 of the dry polyolefin granules, or the gas is introduced into the treatment space in an amount which is less than half the throughput of polyolefin granules through the treatment space.

(15) The dry polyolefin granulate remains in the treatment space of unit 7 for a period of 0.5 to 20 h, preferably 1 to 10 h, particularly preferably 2 to 6 h.

(16) The cleaned granulate leaves unit 7 through a discharge opening 7d and a discharge line 10, preferably via a discharge device 10a, for example a shut-off unit such as a rotary air lock unit.

(17) The process gas enters the unit 7 through an inlet opening 7c and leaves the unit 7 through an outlet opening 7b into a gas discharge line 9. In the supply line 8 of the process gas there is a blower 8a, for example a fan, for circulation of the gas. A heat exchanger 8b is provided upstream of the inlet opening 7c to bring the gas to the desired temperature before entering unit 7. However, the process gas used in unit 7, preferably air, can also be fed through a closed circuit system of pipes 8, 9.

(18) With the device according to FIG. 1, it is possible, by accordingly adjusting the amount of gas introduced, to treat the dry polyolefin granulate in the treatment space of unit 7 under increased pressure, preferably under an excess pressure of 0.1 to 990 mbar, more preferably under an excess pressure of 20 to 500 mbar.

(19) FIG. 2 shows a further embodiment of a device suitable for the invention. According to this embodiment, the dried polyolefin granulate is not conveyed directly from unit 5 to unit 7, but is instead fed to a shut-off unit 6b such as a rotary air lock unit after the start-up diverter 7. Heated conveying gas is fed through a conveying gas line 6c and a device 6d for heating the conveying gas, for example a heat exchanger, in order to pneumatically convey the granulate into unit 7 after leaving the rotary air lock unit 6b.

(20) In the embodiment according to FIG. 2 a pump 9a, preferably a vacuum pump, is arranged in the gas discharge line 9. In this way, by removing at least part of the gas atmosphere from the treatment space of unit 7 by means of pump 9a, it is possible for the dry polyolefin granules to be treated in the treatment space of unit 7 under reduced pressure, preferably under a reduced pressure of 0.1 to 999 mbar, more preferably under a reduced pressure of 10 to 900 mbar, particularly preferably under a reduced pressure of 20 to 500 mbar. At the same time pump 9a can be used for conveying the gas.

(21) In the embodiment shown in FIG. 2, after leaving unit 7 through the shut-off unit 10a, preferably a rotary air lock unit, the cleaned polyolefin granulate is passed through a separating device 11, preferably a classifying screen. Oversized particles separated in this process are removed from the product stream through a discharge line 12, while the purified polyolefin granulate is passed through a discharge line 13.