Device for applying a solvent to a polyolefin, and method therefor

Abstract

The invention relates to a device (100) and a method for applying a solvent (30) to a polyolefin. The device (100) comprises an extruder, the outlet (41) of which opens into a perforated plate (42) with an arrangement of multiple openings (43). An injection device (50) for applying a solvent (30) to a polyolefin melt (21) which is conveyed through the perforated plate (42) is arranged downstream of the perforated plate (42).

Description

[0001] Device for applying a solvent to a polyolefin and method for doing so

[0002] The present invention relates to a device for applying a solvent to a polyolefin and a method for doing so according to the preamble of the independent claims.

[0003] For some time now, society has had a desire to conserve resources. Society therefore strives to either reuse everyday objects or at least recycle them. Ultimately, however, even reused objects have a lifespan, the end of which is reached at different rates. These objects, too, are typically input materials in a recycling process.

[0004] Particularly for plastics, various recycling processes have become established.

[0005] Many plastics can be reused multiple times. However, this requires that they are of high quality when recycled or at least exhibit a high degree of purity before reuse. Therefore, efforts are underway to integrate recycling facilities directly upstream of production plants, thus preventing, for example, recontamination during the transport of recycled granules, also known as regenerate. Such solutions, however, are very expensive and demand a high level of process reliability.

[0006] In a mechanical recycling process, collected used containers or objects are typically separated according to their composition and then shredded. A washing process usually also takes place to clean the materials and separate them from contaminants as much as possible. The shredded plastics are then melted and the molten material filtered. The filtered melt is subsequently cooled and processed into granules. These granules are then typically bagged for transport.

[0007] This mechanical method is particularly suitable when the material flows are relatively homogeneous.

[0008] Nevertheless, it can happen that, due to different qualities of the recycled plastics, inhomogeneous granules are provided, which may have correspondingly different properties or contain impurities that make later use difficult or impossible.

[0009] In the chemical recycling process, plastic waste is broken down into its chemical components such as oligomers and monomers in order to be reused as raw material for the production of new plastics, instead of being landfilled or converted into energy in waste incineration plants.

[0010] It therefore represents a useful addition to other recycling processes and is used to process waste streams that cannot be recovered or can only be recovered insufficiently through mechanical recycling; however, it is ultimately complex and expensive.

[0011] Physical recycling processes have also become known from the state of the art to overcome these disadvantages.

[0012] For example, WO 2018 / 091356 Al describes a physical process in which a polyolefin material, in particular HDPE, is swollen using a solvent in order to dissolve impurities present in the polyolefin material in the solvent and subsequently the solvent with the dissolved impurities is removed from the polyolefin material.

[0013] In this process, contaminated polyolefin material is provided in the form of solid flakes, and solvent is added. It is particularly important that the process takes place significantly below the flakes' melting point to prevent them from clumping together. The subsequent swelling of these flakes can take from approximately 30 minutes to several hours, preferably using an agitator to achieve homogenization and better solvent distribution. It is obvious that such a purification process is complex and requires continuous monitoring.

[0014] Previous methods for swelling plastic flakes used so-called stirred tanks, in which solvent was added to the plastic flakes. However, this led to clumping or sticking of the plastic flakes, which had a detrimental effect on the process.

[0015] The object of the invention is to overcome one or more disadvantages of the prior art. In particular, a device and preferably a method are to be provided which make it possible to accelerate the swelling of polyolefins and, in particular, to carry out a simple continuous process.

[0016] This problem is solved by the devices and methods defined in the independent patent claims. Further embodiments are described in the dependent patent claims.

[0017] An inventive device for applying a solvent to a polyolefin, particularly for carrying out a process as described below, comprises an extruder whose outlet opens into a perforated plate. The perforated plate is provided with an arrangement of a plurality of passages. Downstream of the perforated plate, an injection device is arranged for applying a solvent to a melt of polyolefin conveyed through the perforated plate. In other words, the injection device applies a solvent to the melt, the solvent penetrates the polyolefin, and the polyolefin is thereby swollen.

[0018] The penetration times of the solvent into the polyolefin typically range between 0.1 seconds and 50 seconds.

[0019] Such a device allows for continuous application to the polyolefin, while the presence of a melt provides an elevated temperature, facilitating solvent penetration. Since the polyolefin is already in a melt state, clumping of flakes is no longer a concern.

[0020] By providing a perforated plate and forcing the molten metal through this perforated plate, strands of molten metal are created; the molten metal is thus divided into individual strands that are separated from each other.

[0021] In detail, a perforated plate is arranged downstream to divide the melt, or rather the melt stream. The divided melt stream is only exposed to the solvent after it has been divided and separated. In other words, the perforated plate and the injection device are arranged sequentially downstream. The purpose of this arrangement is to allow the melt strands to swell, i.e., to increase in diameter. This is further facilitated by an increase in the flow cross-section in the area of ​​the injection device, and subsequently thereafter.

[0022] Because the polyolefin is present in individual strands, the surface area is many times larger. At the same time, the thickness of the material is reduced. The increased surface area allows for better solvent absorption. The reduced thickness of the material means that the solvent penetrates it more quickly.

[0023] The die plate can have a contour upstream that extends further centrally towards an extruder screw than at the periphery. This creates a curvature. This results in a more even distribution of the melt across the die plate. Passages in the die plate can be provided by bores, each bore featuring a cross-sectional constriction. This accelerates the melt within the die plate, while simultaneously reducing the risk of melt strand breakage due to a build-up of pressure before the constriction.

[0024] Depending on the type of solvent, its pressure, and the swelling behavior of the polyolefin, the melt can be enriched, saturated, or supersaturated.

[0025] Additionally, the perforated plate can be moved by vibrations. These vibrations allow the surface of the molten strands to come into better contact with the solvent and, if necessary, enable the molten strands to break off in a controlled manner.

[0026] The solvent can be a solvent of low polarity or a nonpolar solvent. On the one hand, the solvent can preferably be less polar than water, preferably as polar as or less polar than acetone. On the other hand, the solvent can be as polar as or more polar than n-hexane.

[0027] The solvent is preferably an alkane, in particular n-hexane or n-heptane. Preferably, the solvent and the contaminated polyolefin material have a relative energy difference (RED) according to Hansen (source: Charles XL Hansen: Hansen Solubility Parameters: A User's Handbook, 2nd edition, CRC Press, Boca Raton, 2007, ISBN 0-8493-7248-8) of at most 3, 2, or 1. The solvent used for the present process can consist of a single type of solvent or of several types of solvents, i.e., a solvent mixture. Preferably, the solvent comprises at least 80, preferably at least 90, and in particular 95 percent by weight of a single solvent, for example, n-hexane. The solvent can thus be easily purified.

[0028] If the solvent contains several types of solvents, these can be selected such that their boiling points differ by at least 2 degrees, preferably by 5 degrees, particularly by 10 degrees, but by a maximum of 100 degrees, preferably by a maximum of 50 degrees, and particularly by a maximum of 30 degrees Celsius. Such a difference makes it possible, in particular, to separate the solvents simply by distillation. Preferably, the solvents are also selected such that they do not form an azeotrope.

[0029] Additionally or alternatively, suitable membrane filters can be used to separate solvents. Other methods of solvent separation include filtration, precipitation, and phase separation.

[0030] The polyolefin can be PP (polypropylene) or PE (polyethylene), in particular LDPE (low density polyethylene) or LLDPE (linear low density polyethylene) or HDPE (high density polyethylene) or UHMWPE (ultra high molecular weight polyethylene).

[0031] Alternatively or additionally, the polyolefin can be a thermoplastic elastomer or an elastomer. Combinations of the aforementioned substances are also possible. These substances can also contain colorants and additives, and can be present in pure form or as blends, with or without fillers. The term blend refers to a mixture of two or more of the plastics present, such as HDPE and LDPE.

[0032] It may be provided that a guide plate is arranged downstream of the injection device. The guide plate has an arrangement of guide bores, which is essentially identical to the arrangement of passages.

[0033] This type of guide plate ensures that the molten strands remain separated even after passing through the perforated plate and do not touch, at least initially. The molten strands grow larger and swell as they absorb more solvent.

[0034] The flow direction is the direction in which the polyolefin is forced through the extruder. Typically, the polyolefin is present at the beginning of the extruder in granular form, as pellets or flakes, and is melted by the extruder. The melt is then forced through the die plate at the end of the extruder.

[0035] The injection device can have a collection chamber that extends essentially in an annular shape around the arrangement of passages. This ensures that the melt, which is forced through the passages and pressed into molten strands, can be surrounded on all sides by the solvent, so that the molten strands can be exposed to the solvent over their largest possible surface area.

[0036] The collection chamber also ensures that the solvent is present everywhere uniformly and at a uniform pressure. Preferably, the passages extend downstream into tubular extensions.

[0037] Such extensions calm and channel the melt. Consequently, it exhibits a uniform flow velocity.

[0038] Preferably, the tubular extensions extend further downstream than the corresponding length of the collection chamber. In particular, the tubular extensions have 1.5 times the length of the collection chamber in the direction of flow. In other words, the tubular extensions penetrate the collection chamber. This allows the solvent to be guided in such a way that it is already flowing in the direction of flow when it encounters the molten strands.

[0039] Preferably, the tubular extensions extend to the guide plate, in particular into the guide plate.

[0040] Additionally, the outer diameters of the tubular extensions can be designed to decrease, becoming conical downstream. This creates more space for the solvent to flow around the molten strands.

[0041] This allows the molten strands to be transferred to the guide plate without the risk of them touching each other.

[0042] It may be provided in particular that the passages in the perforated plate have an inner diameter of 0.002 mm to 10 mm, preferably of 0.05 mm to 2 mm, in particular of 0.05 mm to 0.99 mm .

[0043] Corresponding to the diameter of the passages, the melt strands have appropriate diameters, which subsequently widen and swell. The smaller the diameter of the melt strands, the larger the total surface area of ​​the melt and the easier it is to introduce the solvent into the polyolefin and thus cause the polyolefin to swell.

[0044] The guide holes in the guide plate have appropriately sized diameters. In particular, the diameters of the guide holes are 0.01 mm to 2 mm, and especially 0.1 mm to 1 mm, larger than the respective diameter of the through-holes.

[0045] This ensures, on the one hand, that the melt strands can be guided, and on the other hand, that the solvent can flow around and penetrate each individual melt strand.

[0046] A press, preferably an extruder press, can be arranged downstream of the extruder. Such a press allows the solvent, along with the impurities, to be removed from the melt in the same continuous process by which the melt is expanded.

[0047] Additionally or alternatively, a degassing device and / or a condensation trap can be arranged downstream of the extruder. Such devices can also remove residual solvent, typically from 0.0001 to 5 percent by weight, which, for example, cannot be removed from the polyolefin by the press.

[0048] Another aspect concerns a method for purifying polyolefin, in particular carried out with a device as described herein. The method comprises the steps of: - providing the polyolefin to be purified,

[0049] - Applying a solvent to the polyolefin, causing the polyolefin to swell,

[0050] - Squeezing the solvent out of the polyolefin.

[0051] Before being exposed to the solvent, the polyolefin is melted.

[0052] Prior art cautions against reaching a melting temperature during the swelling process. Surprisingly, it has been found that providing a melt before the swelling process eliminates this concern and significantly accelerates the swelling process.

[0053] The polyolefin is preferably transferred into the melt using an extruder. This makes it possible to supply the melt under high pressure and, in particular, very uniformly.

[0054] It may be necessary to preheat the solvent before applying it, so that it does not cool the melt or the melt strands too much, thus preventing clumping of the melt strands.

[0055] The solvent is preferably heated to a temperature that is lower than the maximum melting temperature.

[0056] Preferably, a large number of molten strands are formed from the melt, and these molten strands are exposed to the solvent.

[0057] As already explained, by providing a large number of melt strands, the total surface area of ​​the melt can be increased, which in turn makes it easier to introduce the solvent into the polyolefin and to swell the melt accordingly.

[0058] The solvent is preferably also applied to the polyolefin under pressure. The pressure of the solvent is preferably at least 1 bar, in particular between 5 and 1000 bar, preferably between 20 and 200 bar, and most preferably between 80 and 150 bar.

[0059] By exposing the polyolefin to the solvent, the polyolefin swells to at least two to twenty times its original volume, and in particular to five to ten times its original volume.

[0060] This value of volume increase is an indication that the solvent has completely penetrated and entered the polyolefin.

[0061] Preferably, in a further step, the solvent is removed from the polyolefin.

[0062] Removing the solvent also removes impurities. These impurities are typically aliphatic saturated hydrocarbons, aliphatic unsaturated hydrocarbons, aliphatic hydrocarbons with functional groups, aromatic hydrocarbons, aromatic hydrocarbons with functional groups, fragrances (especially aldehydes, organic acids, terpenes such as limonene), and flavorings.

[0063] In particular, the solvent can be removed by compacting the swollen polyolefin, especially at a temperature below the boiling point of the solvent. This is a relatively simple and reliable method.

[0064] Additionally or alternatively, it can be provided that the solvent is removed from the polyolefin by degassing the melt, in particular under reduced pressure, preferably at a pressure of less than 0.1 bar, and / or via a condensation trap.

[0065] The invention is explained below with the aid of schematic figures. These show:

[0066] Figure 1: An exploded view of a device;

[0067] Figure 2: a cross-section through a device.

[0068] Figure 1 shows an exploded view of a device 100 in a perspective view. A perforated plate 42, an injection device 50, and a guide plate 60 are shown. The extruder itself is not shown in this illustration.

[0069] As can be seen, the perforated plate 42 has a large number of tubular extensions 44 which extend from passages not visible here.

[0070] The injection device 50 has a correspondingly large opening, which has a collection chamber 51 at its edge. The collection chamber 51 is connected to the environment, or to a corresponding solvent reservoir, by several bores 52, of which only one is labelled.

[0071] Figure 2 shows a cross-section through the device 100 according to Figure 1. In this representation, the extruder, or rather the outlet 41 of the extruder, is visible. The melt 21 is conveyed from right to left according to the present illustration; this corresponds to the flow direction.

[0072] The perforated plate 42 is arranged at the outlet 41 of the extruder 40. The perforated plate 42 has a plurality of passages 43, each of which opens into tubular extensions 44. The passages 43 have a larger flow cross-section on the inlet side than on the outlet side. The injection device 50 is arranged downstream of the perforated plate 42. The tubular extensions 44 extend through the injection device 50 and into a guide plate 60, which is also arranged downstream of the injection device 50. The guide plate 60 has guide bores 61 that correspond to the passages 43 of the perforated plate 42. As can be seen, the tubular extensions 44 extend into the guide plate 60 and, accordingly, into the guide bores 61.As can be seen, the injection device 50 has a collection chamber 51 at its inner periphery, i.e., a chamber that extends around all the tubular extensions 44. This collection chamber 51 is connected to the surroundings by lateral bores 52. The solvent 30 is shown in the collection chamber 51 and, correspondingly, in the lateral bore 52. It flows through the bores 52 into the collection chamber 51, flows around the tubular extensions 44, and is driven downstream. At the end of the tubular extensions 44, the solvent 30 comes into contact with the molten strands 22 of the melt 21 and causes them to swell.

[0073] The tubular extensions 44 are designed to taper conically in the direction of flow, so that more space is provided for the solvent 30.

[0074] The melt strands 22 can subsequently be transferred to an extruder press. This is not shown here. The extruder press can be used to press the solvent 30, along with the impurities dissolved in it, out of the polyolefin.

Claims

Patent claims 1. Device (100) for applying a solvent (30) to a polyolefin, in particular for carrying out the method according to one of claims 11 to 19, comprising an extruder whose outlet (41) opens into a perforated plate (42) with an arrangement of a plurality of passages (43), characterized in that an injection device (50) for applying a solvent (30) to a melt (21) of polyolefin conveyed through the perforated plate (42) is arranged downstream of the perforated plate (42).

2. Device according to claim 1, characterized in that a guide plate (60) is arranged downstream of the insertion device (50), wherein the guide plate (60) has an arrangement of guide bores (61) which is essentially identical to the arrangement of passages (43).

3. Device ( 100) according to claim 1 or 2, characterized in that the injection device (50) has a collecting chamber (51 ) which extends substantially in an annular shape around the arrangement of passages (43 ).

4. Device ( 100) according to one of claims 1 to 3, characterized in that the passages (43) extend downstream into tubular extensions (44 ).

5. Device ( 100) according to claims 3 and 4, characterized in that the tubular extensions (44 ) extend downstream further than a corresponding length of the collecting chamber (51 ), in particular having at least 1.5 times its length.

6. Device (100) according to claim 2 and one of claims 4 to 5, characterized in that the tubular Extensions (44) extend to the guide plate (60), in particular into the guide plate (60).

7. Device ( 100) according to one of claims 1 to 6, characterized in that the passages (43) have a diameter of 0.002 mm to 10 mm, preferably of 0.05 mm to 2 mm, in particular up to 0.99 mm.

8. Device ( 100) according to claims 2 and 7, characterized in that the guide bores ( 61 ) have a diameter which is 0.01 mm to 2 mm, in particular 0.1 mm to 1 mm, larger than the respective associated diameter of the passages (43) .

9. Device ( 100) according to one of claims 1 to 8, characterized in that a press is arranged downstream of the extruder.

10. Device ( 100) according to one of claims 1 to 9, characterized in that a degassing device and / or a condensation trap is arranged downstream of the extruder.

11. Method for purifying polyolefins, in particular carried out with a device according to any one of claims 1 to 10, comprising the steps - Providing the polyolefin to be cleaned, - Applying a solvent (30) to the polyolefin so that the polyolefin swells, - Pressing the solvent (30) out of the polyolefin, characterized in that the polyolefin is converted into a melt (21) before being exposed to the solvent (30).

12. Method according to claim 11, characterized in that the polyolefin is transferred into the melt (21) by means of an extruder.

13. Method according to claim 11 or 12, characterized in that a plurality of melt strands (22) are formed from the melt (21), wherein these melt strands (22) are exposed to the solvent (30).

14. Method according to claim 13, characterized in that the polyolefin is pressed through a perforated plate (42) to form the melt strands (22 ).

15. Method according to one of claims 11 to 14, characterized in that the solvent (30) is applied to the polyolefin under pressure to cause the polyolefin to swell.

16. Method according to one of claims 11 to 15, characterized in that the polyolefin is swollen to at least twice its original volume by exposure to the solvent (30).

17. Method according to one of claims 11 to 16, characterized in that after swelling of the polyolefin the solvent (30) is removed from the polyolefin.

18. Method according to claim 17, characterized in that the solvent (30) is removed by compacting the swollen polyolefin (30).

19. Method according to one of claims 17 or 18, characterized in that the solvent (30) is removed by degassing the melt (21), in particular under negative pressure, and / or via a condensation trap.

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