Extruder system and method for treating washed polymer pellets

Abstract

An extruder system (100) for treating washed polymer pellets, comprising at least an MRS extruder (10) comprising a housing (11) and an extruder screw (20), an inner housing groove (18) of the housing extending at least between a feed opening (25) and a discharge opening (26) and having at least one degassing zone, the extruder screw can rotate in the shell groove and is provided with at least one spiral extruder screw flight, the extruder screw is divided into a feed screw section (21), and a feed opening (25) opens into the feed screw section; a multi-screw section (22) in which the plurality of planetary screws (23) revolve both with the main screw and rotate about its axis, the diameter of the multi-screw section (22) being greater than the screw diameter of the feed screw section (21); -a transition cone (21) formed between the feed screw section (21) and the multiple screw section (22); and-a discharge screw section (24), the diameter of which is smaller than the diameter of the multiple screw section (22), characterized in that a shear compactor (30) is provided comprising at least a shear container, the discharge opening of which is connected to the feed opening (25) of the MRS extruder (10), and a cutter device comprising at least one cutter rotating within the shear container (31); the ratio of the diameter to the length of the feeding screw section (21) is less than 1: 22; the multi-screw section (22) comprises four to eight planetary screws (23), each of which has a length that is at least four times its diameter.

Description

Technical Field

[0001] The present invention relates to an extruder system for processing washed polymer particles having the features of the preamble of claim 1 and a method for processing washed polymer particles. Background Art

[0002] For example, patent document EP1434680A1 discloses an extruder having multiple screw segments, where multiple planetary screws revolve around the main screw and rotate about their own axes. The advantages of this type of extruder in processing plastic waste, especially polyester particles, are that not only can impurities in the polymer melt be removed by degassing, but also the inherent viscosity of the polyester melt can be increased. The MRS type extruder is very suitable for deep degassing because the plastic particles are subjected to extremely low shear forces during processing, thus expanding the polymer surface in the degassing zone and effectively extracting moisture and other volatile substances.

[0003] The MRS extruder must degas at low pressure. Especially when processing polyester, in addition to removing contaminants, it is also necessary to increase the inherent viscosity, so a vacuum pump must be used.

[0004] When recycling plastic waste, an upstream cleaning system is usually provided. Especially in the case of film fragments, the water content in the water-containing film is extremely high relative to the mass of the polymer particles. In the processed plastic particles (such as shredded and washed plastic particles from household packaging), the high water content will cause a large amount of gas to be generated in the MRS extruder, and a vacuum pump must be used to remove the gas from the degassing zone. In the case of a high gas content, especially when processing washed plastic waste, the high-energy consumption operation of the vacuum pump will result in the MRS extruder lacking economic efficiency.

[0005] Another problem is that a relatively short feed screw should be used to ensure gentle melting of the polymer. However, in order to obtain good degassing effects, the polymer needs to be highly plasticized when entering the degassing zone, which often requires a longer feed screw to achieve.

[0006] The discharge screw section must work synchronously with the various devices to which the extruder system is docked. Therefore, the discharge screw section must have a certain rotational speed, which in turn determines the rotational speeds of the connected feed screw section and the multiple screw sections. That is to say, the rotational speed cannot be changed in order to achieve gentle processing of the material in the feed screw section. Summary of the Invention

[0007] In view of the above, the object of the present invention is to optimize an MRS extruder of the aforementioned type in order to economically process plastic waste that generates a large amount of gas, especially to process washed polymer particles.

[0008] The solution of the present invention to achieve the above object is an MRS extruder having the features described in claim 1.

[0009] In short, the concept is based on the direct connection of a shear compactor to the input stage of an MRS extruder and the special geometry of the screws in the MRS extruder. The shear compactor allows water separation, which is important for example for washed recyclate, to relieve the MRS extruder and to achieve pre-plasticization.

[0010] By combining an MRS extruder with a shear compactor upstream of the feed screw, the polymer material to be processed can be plasticized before entering the extruder. By using the shear compactor, a certain degree of plasticization is already achieved during the processing in the extruder, so that it is not necessary to plasticize the entire process during the processing in the MRS extruder, but only to increase the degree of plasticization. For this purpose, the multiple screw segment contains four to eight planetary screws, each of which has a length of at least four times its diameter. In this way, relatively short feed screw segments with a diameter to length ratio of less than or equal to 1:22, in particular less than or equal to 1:16, can be used, so that the polymer can be reprocessed correspondingly gently in the MRS extruder.

[0011] According to the invention, the geometry of the extruder screw of the MRS extruder is optimized, and according to a preferred embodiment, the diameter D of the shear vessel is S and the diameter D of the multiple screw segments M The following relations are satisfied:

[0012] D M ≥0.20*D S –85mm*GF,

[0013] Among them, GF is the size factor determined by experience, GF≥0.8. This size ratio will ensure that the degassing performance of the MRS extruder matches the degassing performance of the shear compactor, improve economic benefits, and the degassing of the MRS extruder depends only on the depth of degassing, not on the surface humidity. In this way, an efficient vacuum system can be established.

[0014] The minimum diameter of the multiple screw segments indicates the size at which significant depth of degassing can be achieved. If the diameter is smaller, there is not enough available surface and the material level during operation is too high, resulting in too thick layers in the MRS, which are difficult to deaerate sufficiently and require excessive vacuum generation and maintenance processes to achieve adequate degassing.

[0015] For example, to further improve the decontamination performance, a diameter larger than that defined by the above boundary condition relationship is used. Of course, the multiple screw segment diameter is preferably no larger than 0.28 times the shear compactor diameter minus 100 mm, otherwise the shear input to the MRS extruder will be too high.

[0016] Therefore, in the combination scheme proposed by the present invention, material pretreatment needs to be carried out in a shearing compactor so that the strength of the MRS extruder can be optimally utilized in deep degassing, and at the same time, the washed polymer particles are processed in a mild and energy-efficient manner.

[0017] By selecting the degree of pre-plasticization in the shearing compactor (depending on the appropriate cutter speed in the shearing container), the degree of plasticization achieved subsequently in the MRS extruder can be controlled without replacing the extruder screw or changing the extruder speed. The mechanical work introduced by the rotation of the cutter will heat up the polymer particles in the shearing container.

[0018] A particularly important effect of the combination of the shearing compactor and the input stage of the MRS extruder is that the material to be processed can be heated in the shearing compactor to a temperature within the boiling range of water at the pressure in the shearing container, usually 100°C to 200°C, so that water and other low-boiling substances can evaporate in the shearing compactor. Since a large amount of water can be removed in advance, the surface humidity of the polymer particles is particularly reduced, while in the traditional process using the MRS extruder as described above, the proportion of the surface humidity of the polymer particles relative to the total gas load is very high. According to the present invention, most of the moisture caused by the surface-attached water no longer reaches the degassing area of the MRS extruder.

[0019] Therefore, according to the present invention, it is also conceivable to perform double degassing in the MRS extruder while setting a shearing compressor upstream. The former is more advantageous than the latter because in double degassing, a large amount of gas must be extracted at a relatively high pressure in the first stage, and then a high vacuum can be formed at a lower residual pressure in the second stage to remove the gas components still contained in the polymer melt. If such double degassing is carried out in the MRS extruder, the residence time required under high shear force and high temperature conditions will be too long, thus affecting the quality of the recycled polymer.

[0020] The combination of the shearing compactor and the MRS extruder with a matching geometry also brings the following advantages:

[0021] – The shearing compactor will increase the density of the material fed into the MRS extruder and push the material into the feed screw section.

[0022] – The shearing compactor preheats the material, so the MRS extruder requires less melting energy compared to the rest of the MRS process.

[0023] Generally speaking, the following results will be produced:

[0024] – In the MRS extruder, PET materials with a relatively low bulk density can be processed first, such as materials with a density lower than 250 g / l, up to about 50 g / l;

[0025] – The MRS extruder can also be used for other plastics, especially polyolefins with low bulk density, such as PP, PE, etc. These plastics usually have a lower density in the packaging field due to their shape and thickness.

[0026] In the extruder system according to the present invention, evaporating moisture from the shear container at ambient pressure can also significantly improve the overall process. Advantageously, additional suction devices, such as exhaust fans or vacuum pumps, can be provided on the shear compressor so that the gas load can be removed from the shear container more quickly and thoroughly.

[0027] A method for treating washed polymer particles is proposed in claim 7 of the present invention. The treatment steps corresponding to the normal process in the MRS extruder are as follows:

[0028] – Transfer a certain amount of polymer particles to the feed screw section of the MRS extruder;

[0029] – Further convey the polymer plasticized into a thermoplastic melt to the multi-screw section;

[0030] – Suction volatile impurities from the melt in the degassing zone; and

[0031] – Discharge the degassed melt through the discharge screw section.

[0032] Before being transferred to the MRS extruder, the pretreatment process in the shear compactor includes the following steps:

[0033] – Feed the washed polymer particles into the shear container;

[0034] – Heat the polymer particles in the shear container to a temperature higher than the boiling point of water at the internal pressure in the shear container and lower than the melting point of the polymer by at least one rotating cutter, usually for at least 10 minutes, where the polymer particles are not only heated but also chopped and mixed by at least one rotating cutter in the shear container.

[0035] Preferably, the pressure in the shear container isolated from the surrounding environment is reduced by a separate suction system, and various types of suction systems can be used.

[0036] Preferably, the pressure in the degassing zone of the MRS extruder is selected to be at least 10 times lower than the pressure in the shear container.

[0037] For example, the pressure in the shear container can also be maintained at ambient pressure, which means there are sufficiently large openings on the shear container, or a corresponding volume flow is extracted through a separate suction system, so that although the substances in the shear container evaporate, the pressure will not increase. At the same time, the pressure in the degassing zone of the MRS extruder is maintained below 100 mbar.

[0038] The pressure can be regulated by adjusting the clearance width between the extruder screw and the housing hole, which is affected by the axial displacement of the extruder screw relative to the housing 11. The relevant clearance is located between the housing and the transition cone, which is formed between the feed screw section and the multi-screw section.

[0039] By using a water ring pump on the MRS extruder, the pressure in the degassing zone can be reduced to about 30 mbar, while the atmospheric pressure is maintained in the shear vessel.

[0040] In particular, the pressure in the degassing zone of the MRS extruder is even lower than 10 mbar, while the pressure in the shear vessel is lower than 100 mbar, so as to remove foreign substances and harmful substances from the polymer melt in the best way. As a result, the surface moisture and other volatile low-boiling substances attached to the polymer particles are removed in the shear vessel, and only the so-called deep degassing occurs in the MRS extruder, in which the polymer is repeatedly mixed by the multi-screw section and the generated surface area is enlarged, and the remaining volatile components are removed from the polymer melt. Detailed implementation mode

[0041] The present invention will be elaborated in detail below with reference to the embodiments shown in the drawings.

[0042] Figure 1 A schematic diagram of an extruder system 100 designed for processing washed polymer particles according to the present invention is shown. The extruder system 100 includes an MRS extruder 10, which has a housing 11, and the housing grooves inside the housing 11 extend at least between the feed port 25 and the discharge port 26. The housing 11 has a housing opening 12 in the degassing zone, and the housing opening 12 is docked with the vacuum suction pipeline 13 of the suction system. The extruder screw 20 rotating in the housing groove has at least one spiral extruder thread. The extruder screw 20 is divided into:

[0043] – A feed screw section 21 designed as a single screw, and the feed port 25 leads into the feed screw section 21;

[0044] – A multi-screw section 22, in which a plurality of planetary screws 23 revolve around the main screw and rotate around their own axes, and the diameter of the multi-screw section 22 is larger than the screw diameter of the feed screw section 21;

[0045] – A transition cone 21, which is formed between the feed screw section 21 and the multi-screw section 22;

[0046] – A discharge screw section 24 also designed as a single screw, and the diameter of the discharge screw section 24 is smaller than the diameter of the multi-screw section 22; and

[0047] – A transition cone 28, which is formed between the multi-screw section 22 and the discharge screw section 24.

[0048] The MRS extruder 10 is equipped with a shear compactor 30, which includes a tool device, and at least one cutter of the tool device rotates in a shear container 31. The discharge port of the shear container 31 is connected to the feed port 25 of the MRS extruder 10.

[0049] A shear compactor control module 41 for the shear compactor 30 is provided in the machine control device 40, for example, to control or adjust the cutter speed and pressure and optionally other relevant parameters of the shear compactor 30. In addition, an MRS control module 43 for the MRS extruder 10 is also provided in the machine control device 40, especially to control or adjust the rotational speed of the extruder screw 20 and the temperature of each temperature zone. Pressure regulation can also be provided, which can be achieved by adjusting the clearance width between the transition cone 27 of the extruder screw 20 and the housing groove, and this clearance width may be affected by the axial displacement of the extruder screw 20 relative to the housing 11.

[0050] In addition, a coupling module 42 is provided to synchronize the operation of the shear compactor 30 and the MRS extruder 10, so that the shear compactor 30 neither idles nor runs at full load, while the MRS extruder 10 maintains the operating point, especially the operating point necessary for the downstream process of the extruder system 100.

Claims

1. An extruder system (100) for processing washed polymer particles, at least comprising an MRS extruder (10), the MRS extruder (10) comprising a housing (11) and an extruder screw (20), the housing groove inside the housing (11) extending at least between a feed inlet (25) and a discharge outlet (26) and having at least one degassing zone, the extruder screw (20) being rotatable in the housing groove and having at least one helical extruder thread, the extruder screw (20) being divided into: – a feed screw section (21), the feed inlet (25) leading into the feed screw section (21); – a multi-screw section (22), wherein a plurality of planetary screws (23) revolve around the main screw and rotate about their own axes; wherein, the diameter of the multi-screw section (22) is greater than the screw diameter of the feed screw section (21); – a transition cone (21), the transition cone (21) being formed between the feed screw section (21) and the multi-screw section (22); – a discharge screw section (24), the diameter of the discharge screw section (24) being smaller than the diameter of the multi-screw section (22); characterized in that, – a shear compactor (30) is provided, the shear compactor (30) at least comprising a shear container and a cutter device, the discharge outlet of the shear container being connected to the feed inlet (25) of the MRS extruder (10), the cutter device comprising at least one cutter rotating in the shear container (31); – the ratio of the diameter to the length of the feed screw section (21) is less than 1:22; and – the multi-screw section (22) contains four to eight planetary screws (23), and the length of each planetary screw (23) is at least four times its diameter.

2. The extruder system (100) according to claim 1, characterized in that, the ratio of the diameter to the length of the feed screw section (21) is equal to or less than 1:

16.

3. The extruder system (100) according to claim 1 or 2, characterized in that, For the diameter D of the shearing container S and the diameter D of the multiple screw sections (22) M , the following relationship is satisfied: D M ≥(0.20 * D S – 85 mm) * GF, wherein, GF is an empirical dimension coefficient, GF≥0.

8.

4. The extruder system (100) according to claim 3, characterized in that, the dimension coefficient GF≥0.

9.

5. The extruder system (100) according to claim 2 or 3, characterized in that, D M ≤0.28 * D S –100。 6. The extruder system (100) according to at least one of claims 1 to 5, characterized in that, the housing (11) has at least one housing opening (12) in the at least one degassing zone, and the housing opening (12) is docked with a vacuum suction pipeline (13) of a suction system.

7. The extruder system (100) according to at least one of claims 1 to 6, characterized in that, the shear container (31) is closed and docked with a second suction system.

8. A method for operating an extruder system (100) according to at least one of the preceding claims, characterized in that, it comprises the following steps: a) feeding the washed polymer particles into the shear container; b) heating the polymer particles in the shear vessel (31) to a temperature higher than the boiling point of water under the pressure in the shear vessel (31) and lower than the melting point of the polymer, wherein the polymer particles are chopped and mixed by at least one cutter rotating in the shear vessel (31); c) transferring a certain amount of polymer particles to a feed screw section (21) of an MRS extruder (10); d) conveying the polymer plasticized into a thermoplastic melt further to the multiple screw sections (22); e) extracting volatile impurities from the melt in the degassing zone; and f) The degassed melt is discharged through the discharge screw section (23).

9. The method according to claim 8, It is characterized in that using a shear compactor (30) with a suction system, and 10. The method according to claim 8 or 9, It is characterized in that The pressure in the degassing zone of the MRS extruder (10) is selected to be at least 10 times lower than the pressure in the shear vessel (31).

11. The method according to any one of claims 8 to 10, It is characterized in that The pressure in the degassing zone of the MRS extruder (10) is less than 10 mbar, and the pressure in the shear vessel (31) is less than 100 mbar.

12. The method according to any one of claims 8 to 11, It is characterized in that The polymer particles in the shear vessel (31) are heated to a temperature above the boiling point of water at the pressure within the shear vessel (31) and below the melting point of the polymer over a period of at least 10 minutes.

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