Extruder system and method for processing washed polymer particles

Abstract

The invention relates to an extruder system (100) for processing washed polymer particles, at least comprising an MRS extruder (10), which has: a housing (11) having an inner housing cavity (18), said housing cavity extending at least between an inlet opening (25) and an outlet opening (26) and comprising at least one degassing zone; and an extruder screw (20), which can be rotated in the housing cavity and which has at least one helical extruder screw flight, the extruder screw being divided into: - an inlet screw section (21), into which the inlet opening (25) leads; - a multi-screw section (22), in which a plurality of satellite screws (23) rotate together with a main screw and additionally rotate about their own axis, the diameter of the multi-screw section (22) being larger than the screw diameter of the inlet screw (21); - a transition cone (21), which is formed between the inlet screw section (21) and the multi-screw section (22); and - an outlet screw section (24), which has a smaller diameter than the multi-screw section (22); characterized: - in that a cutter-compactor (30) is provided, at least comprising a cutting container, which has an outlet opening connected to the inlet opening (25) of the MRS extruder (10), and a blade device, which has at least one blade that rotates in the cutting container (31); - in that the diameter-length ratio of the inlet screw section (21) is less than 1:22; and - in that the multi-screw section (22) contains four to eight satellite screws (23), the length of each of which is at least four times its diameter.

Description

[0001] Extruder system and process for the treatment of washed polymer particles

[0002] The invention relates to an extruder system for processing washed polymer particles having the features of the preamble of claim 1 and to a method for processing washed polymer particles.

[0003] An extruder with a multi-screw section, in which several satellite screws rotate together with a main screw and also rotate around their own axis, is known, for example, from EP 1 434 680 A1. This type of extruder has proven itself in the processing of plastics waste, particularly in the processing of polyester particles, as not only can foreign substances be removed from the polymer melt through devolatilization, but the intrinsic viscosity of the polyester melt can also be increased. The extruder type known as MRS is very well suited for deep devolatilization, as the processed plastic particles are processed gently with minimal shear, so that the polymer surface in the devolatilization zone is enlarged and moisture and other volatile substances can be effectively removed.Degassing in the MRS extruder must occur at low pressures, especially when processing polyester, where in addition to decontamination, the intrinsic viscosity must also be increased. Therefore, the use of vacuum pumps is necessary.

[0004] The recycling of plastic waste usually involves upstream washing systems. Especially with thin film shreds, a thin water film represents a very high water content relative to the mass of the polymer particle. A high water content in the processed plastic particles, such as that found in shredded and washed plastic particles from household packaging, leads to high gas volumes in the MRS extruder, which must be removed from the degassing zone via vacuum pumps. The energy-intensive operation of the vacuum pumps makes the use of an MRS extruder uneconomical with high gas contents, as is particularly the case when processing washed plastic waste.

[0005] A further dilemma arises from the fact that a relatively short feed screw is required for gentle melting of the polymer. However, for good degassing, a high degree of plasticization of the polymer upon entering the degassing zone is required, which, in turn, is more easily achieved with longer feed screws.

[0006] The discharge screw section must operate synchronously with equipment connected to the extruder system. Therefore, it must operate at a specific speed, which in turn determines the speeds of the connected feed screw section and multi-screw section. Therefore, the speed cannot be varied in the feed screw section to ensure gentle processing of the material.

[0007] The object of the invention is therefore to optimize an MRS extruder of the type mentioned above for the economical processing of plastic waste that develops a high gas load, in particular for the processing of washed polymer particles. This object is achieved by an MRS extruder having the features of claim 1.

[0008] In brief, the concept of the invention is based on connecting a cutter-compactor directly to the input stage of an MRS extruder and specifying specific geometric relationships for the screws in the MRS extruder. The cutter-compactor enables water separation, which is important, for example, in washed recycled material to relieve the load on the MRS extruder, and pre-plasticization.

[0009] By combining an MRS extruder with a cutter-compactor positioned upstream of the feed screw, the polymer material to be processed can be pre-plasticized before entering the extruder. Using the cutter-compactor predetermines a specific degree of plasticization for the extruder treatment, meaning that the entire plasticization process no longer needs to be carried out during treatment in the MRS extruder; instead, the degree of plasticization only needs to be increased. For this purpose, the multi-screw section is designed to contain four to eight satellite screws, each at least four times the diameter. This allows the use of a relatively short feed screw section with a diameter-to-length ratio of less than or equal to 1:22, and in particular less than or equal to 1:16, which enables correspondingly gentle further processing of the polymer in the MRS extruder.

[0010] The geometry of the extruder screw of the MRS extruder optimized according to the invention provides, according to a preferred embodiment, that the relationship applies to the diameter Ds of the cutting container and the diameter DM of the multi-screw section:

[0011] DM > 0.20 * Ds - 85 mm * GF, where GF is an empirically determined size factor and GF> 0.8. This ratio ensures that the degassing performance of the MRS extruder is cost-effectively matched to the degassing performance of the cutter-compactor, and that the degassing of the MRS extruder is determined solely by deep degassing and not by surface moisture. This allows for the construction of efficient vacuum systems.

[0012] The minimum diameter of the multi-screw section determines the size at which significant deep degassing can occur. If the diameter is smaller, there isn't enough surface area available, and the fill level becomes too high during operation, resulting in excessively thick layers forming in the MRS, which are difficult to degas sufficiently, thus requiring excessive effort to create and maintain the vacuum.

[0013] A larger diameter than defined by the aforementioned relationship as a boundary condition is possible, for example, to further improve decontamination performance. However, the diameter of the multi-screw section should preferably not exceed 0.28 times the cutter-compactor diameter minus 100 mm, as otherwise the shear input of the MRS extruder would be excessive.

[0014] In the combination proposed by the invention, it is therefore essential to carry out the material pretreatment in the cutter-compactor in such a way that the strength of the MRS extruder can be optimally utilized in the deep degassing, while at the same time aiming for a material-friendly and energy-optimized processing of washed polymer particles.

[0015] By selecting the degree of pre-plasticization in the cutter / compactor, which is determined by a suitable knife speed in the cutting vessel, the degree of plasticization later achievable in the MRS extruder can be controlled without having to replace the extruder screw or change the extruder speeds. The mechanical work of the rotating knife heats the polymer particles present in the cutting vessel. A particularly important effect of combining a cutter / compactor with the inlet stage of the MRS extruder is that the material to be processed can be heated in the cutter / compactor to temperatures close to the boiling point of water at the internal pressure prevailing in the cutting vessel, typically between 100°C and 200°C, so that water and other low-boiling substances evaporate in the cutter / compactor.Since water can be removed to a large extent in advance, the surface moisture of the polymer particles, which accounts for a large portion of the total gas load in the conventional process operated using an MRS extruder, is reduced, as described above. The portion of moisture attributable to the surface-adhering water largely no longer reaches the degassing area of ​​the MRS extruder thanks to the invention.

[0016] Therefore, the inventive upstream installation of a cutter-compactor with the simultaneous use of a short feed screw of the MRS extruder is more advantageous than a similarly conceivable double devolatilization in the MRS extruder, in which large volumes would have to be extracted in a first stage at a relatively high pressure, and only in a second stage could a high vacuum with a low residual pressure be applied to remove any gases still present in the polymer melt. However, with such a double devolatilization in the MRS extruder, the necessary residence time with simultaneous high shear and temperature would be too long, thus compromising the quality of the recycled polymer.

[0017] The use of a cutter-compactor in combination with a geometrically matched MRS extruder results in the following additional advantages:

[0018] - The cutter-compactor increases the density of the material being fed into the MRS extruder and pushes it into the feed screw section. - The cutter-compactor preheats the material, so that less melting energy is now required in the MRS extruder compared to the rest of the MRS process.

[0019] This then leads in combination to

[0020] - PET - materials with a low bulk density of, for example, < 250 g / l, up to ~50 g / l can only be processed in the MRS extruder and

[0021] - the MRS extruder can also be used for other plastics, especially polyolefins such as PP, PE with low bulk densities, which are typical in the packaging sector due to shapes and layer thicknesses.

[0022] In an extruder system according to the invention, even the evaporation of water from the cutting container at ambient pressure can significantly improve the overall process. An additional extraction device, such as a fan or a vacuum pump, can advantageously be provided on the cutting-compactor to remove the gas load from the cutting container more quickly and completely.

[0023] A process for the treatment of washed polymer particles is specified in claim 7. The treatment steps corresponding to a normal process in the MRS extruder are:

[0024] - Transferring the quantity of polymer particles into the feed screw section of the MRS extruder;

[0025] - Further transfer of the polymer plasticized into a thermoplastic melt into the multi-screw section;

[0026] - Extraction of volatile foreign substances from the melt in the degassing zone; and

[0027] - Discharge of the degassed melt via the discharge screw section.

[0028] The pretreatment process in a cutter-compactor includes the following steps before transfer to the MRS extruder:

[0029] - feeding washed polymer particles into the cutting container; - heating the quantity of polymer particles in the cutting container by at least one rotating blade to a temperature which is higher than the boiling point of water at the internal pressure prevailing in the cutting container and lower than the melting point of the polymer, over a period of usually at least 10 minutes, wherein the polymer particles are not only heated but also comminuted and mixed by the at least one blade rotating in the cutting container.

[0030] Preferably, the pressure in a cutting container sealed off from the environment is reduced using a separate extraction system, for which various types can be used.

[0031] 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 cutting container.

[0032] It is also possible, for example, to maintain the pressure in the cutting container at ambient pressure. This requires either sufficiently large openings in the cutting container or a separate extraction system that extracts a sufficient volume flow to prevent pressure buildup despite the evaporating substances in the cutting container. At the same time, a pressure of 100 mbar or less is maintained in the degassing zone of the MRS extruder.

[0033] Pressure control can be achieved by adjusting the gap width between the extruder screw and the barrel bore, whereby the gap width is influenced by axial displacement of the extruder screw relative to the barrel 11. The relevant gap is between the barrel and the transition cone formed between the feed screw section and the multi-screw section.

[0034] By using a water ring pump on the MRS extruder, the pressure in the vent zone can be reduced to approximately 30 mbar, while atmospheric pressure is maintained in the cutting vessel. In particular, the pressure in the vent zone of the MRS extruder is even less than 10 mbar for optimal removal of foreign matter and pollutants from the polymer melt, and the pressure in the cutting vessel is less than 100 mbar. This removes surface water and other volatile, low-boiling substances adhering to the polymer particles in the cutting vessel. Only a so-called deep degassing takes place in the MRS extruder. This removes the remaining volatile components from the polymer melt through repeated mixing of the polymer in the multi-screw section, resulting in a surface enlargement.

[0035] The invention is explained in more detail below with reference to the embodiment shown in the drawing.

[0036] Figure 1 schematically shows an extruder system 100 designed according to the invention for processing washed polymer particles. The extruder system 100 comprises an MRS extruder 10 with a housing 11 having an internal housing recess extending at least between an inlet opening 25 and a discharge opening 26. It has a housing opening 12 in a degassing zone, to which a vacuum suction line 13 of an extraction system is connected. An extruder screw 20 with at least one helical extruder screw flight rotates in the housing recess. The extruder screw 20 is divided into:

[0037] - an inlet screw section 21, which is designed as a mono-screw and into which the inlet opening 25 opens;

[0038] - a multi-screw section 22, in which several satellite screws 23 rotate together with a main screw and additionally rotate about their own axis, the diameter of the multi-screw section 22 being larger than the diameter of the feed screw 21; - a transition cone 21 formed between the feed screw section 21 and the multi-screw section 22 and

[0039] - a discharge screw section 24, which is also designed as a mono-screw and which has a reduced diameter compared to the multi-screw section 22 and

[0040] - a transition cone 28 formed between the multi-screw section 22 and the discharge screw section 24.

[0041] Attached to the MRS extruder 10 is a cutter-compactor 30, which comprises a knife device with at least one knife rotating in a cutting container 31. A discharge opening of the cutting container 31 is connected to the inlet opening 25 of the MRS extruder 10.

[0042] A machine control system 40 includes a cutter-compactor control module 41 for the cutter-compactor 30, for controlling or regulating, for example, the blade speed and pressure, and possibly other parameters relevant to the cutter-compactor 30. Furthermore, an MRS control module 43 for the MRS extruder 10 is provided therein, in particular for controlling or regulating the speed of the extruder screw 20 and the temperature in various temperature zones. Pressure control can also be provided, which is achieved by adjusting the gap width between the transition cone 27 of the extruder screw 20 and the housing recess, wherein the gap width is influenced by the axial displacement of the extruder screw 20 relative to the housing 11.

[0043] In addition, a coupling module 42 is provided to synchronize the operation of the cutter-compactor 30 and the MRS extruder 10 in such a way that the cutter-compactor 30 is neither run empty nor filled, and that at the same time the MRS extruder 10 is kept at the operating point which is particularly necessary for maintaining the processes downstream of the extruder system 100.

Claims

Patent claims 1. Extruder system (100) for processing washed polymer particles, at least comprising an MRS extruder (10) with a housing (11) with an inner housing recess, which extends at least between an inlet opening (25) and a discharge opening (26) and which has at least one degassing zone, and with an extruder screw (20) rotatable in the housing recess with at least one helical extruder screw flight, which is divided into: - an inlet screw section (21) into which the inlet opening (25) opens, - a multi-screw section (22) in which several satellite screws (23) rotate together with a main screw and additionally rotate about their own axis, the diameter of the multi-screw section (22) being larger than the screw diameter of the feed screw section (21) - a transition cone (21) formed between the feed screw section (21) and the multi-screw section (22) - a discharge screw section (24) which has a reduced diameter compared to the multi-screw section (22); characterized in - that a cutter-compactor (30) is provided, comprising at least a cutting container with a discharge opening connected to the inlet opening (25) of the MRS extruder (10), and a knife device with at least one knife rotating in the cutting container (31); - that the diameter-to-length ratio of the feed screw section (21) is less than 1:22; and - that the multi-screw section (22) contains four to eight satellite screws (23), each of which has a length at least four times its diameter. Extruder system (100) according to claim 1, characterized in that the diameter-to-length ratio of the feed screw section (21) is equal to or less than 1:

16. Extruder system (100) according to claim 1 or 2, characterized in that the following relationship applies to a diameter Ds of the cutting container and a diameter DM of the multi-screw section (22): DM > (0.20 * Ds - 85 mm) * GF, where GF is an empirical size factor and GF> 0.

8. Extruder system (100) according to claim 3, characterized in that the size factor GF> 0.

9. Extruder system (100) according to claim 2 or 3, characterized in that DM < 0.28 * Ds - 100. Extruder system (100) according to at least one of the preceding claims 1 to 5, characterized in that the housing (11) in the at least one degassing zone has at least one housing opening (12), to which a vacuum suction line (13) of an extraction system is connected. Extruder system (100) according to at least one of the preceding claims 1 to 6, characterized in that the cutting container (31) is closed and is connected to a second extraction system.Method for operating an extruder system (100) according to at least one of the preceding claims, characterized by the following steps: a) feeding washed polymer particles into the cutting container; b) heating the amount of polymer particles in the cutting container (31) to a temperature which is higher than the boiling point of water at the internal pressure prevailing in the cutting container (31) and lower than the melting point of the polymer, wherein the polymer particles are... at least one knife rotating in the cutting container (31) is used to comminute and mix the polymer particles; c) transferring the quantity of polymer particles into the feed screw section (21) of the MRS extruder (10); d) further conveying the polymer plasticized to a thermoplastic melt into the multi-screw section (22); e) extracting volatile foreign substances from the melt in the devolatilization zone; and f) discharging the devolatilized melt via the discharge screw section (23). Method according to claim 8, characterized in that a cutter-compactor (30) with an exhaust system is used, and method according to claim 8 or 9, characterized in that the pressure in the devolatilization zone of the MRS extruder (10) is selected to be at least 10 times lower than the pressure in the cutting container (31).Method according to one of claims 8 to 10, characterized in that the pressure in the venting zone of the MRS extruder (10) is less than 10 mbar and the pressure in the cutting container (31) is less than 100 mbar. Method according to one of claims 8 to 11, characterized in that the polymer particles in the cutting container (31) are heated over a period of at least 10 minutes to a temperature that is higher than the boiling point of water at the internal pressure prevailing in the cutting container (31) and lower than the melting point of the polymer.

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