Purification method for producing a polyolefin regenerate

Abstract

The invention relates to a purification method for producing a polyolefin (PO) regenerate (r), said method having the following steps: (a) mixing superficially cleaned PO waste (p), referred to as raw polymer (p), with solvent and dissolving the raw polymer (p) in the solvent, (b) recovering the purified polymer from the solvent, (c) preparing the solvent, (d) returning the solvent to step (a), (e) separating residues (e1, e2), and (f) extruding the purified PO regenerate (r) to form pellets (r). Step (c) is carried out using at least one membrane filter (c1).

Description

[0001] Purification process for the production of a polyolefin regenerate

[0002] Field of the invention

[0003] The invention relates to a purification process for producing a polyolefin (PO) regenerate according to the preamble of claim 1 and a process for recycling polyolefin waste according to the preamble of claim 11.

[0004] State of the art

[0005] Polyolefins (e.g. HDPE or PE) are widely used in the plastics market, particularly for packaging. Recycling of the material should therefore be encouraged. Polyolefins absorb substances from the environment during use. When the polyolefin plastic is subsequently used as a recyclate, these substances can impair quality, either because they are released back into the contents or, if they are more volatile, are perceived as an off-odor. Contamination in the polymer material impairs the quality of the recycled material. However, recycling processes have so far not been able to sufficiently remove these contaminants from the material to make a recyclate suitable for packaging food, medical devices, or high-quality cosmetic products available to the market in an economically viable manner.

[0006] While conventional methods are economical, they are insufficient or even ineffective in removing very large contaminants with a molecular weight of approximately 400 to 800 Daltons, which can penetrate deep into the HDPE. Above a molecular weight of 800 Daltons, the molecules are so inert that contaminants hardly migrate into the product. They therefore no longer pose a risk or burden for subsequent applications.

[0007] For solvent regeneration, it is known to use a membrane separation step to treat the solvent. The function of the membrane separation step is to remove the solvent from the polymer matrix after the solid impurities have been separated using a decanter. The decanter separates solid impurities from the continuous phase, which consists of solvent and polymer. The membrane separation step separates the solvent from the polymer to recover the polymer. As the polymer concentration increases, the viscosity of the mixture increases, reducing the permeation performance.

[0008] The problem has been partially solved by permitting only very tightly controlled input materials from selected pre-use applications. However, this selective selection is highly restrictive, as it would require many separate recycling streams. Furthermore, it does not solve the problem of potential contamination from improper applications.

[0009] One approach is based on a solvent process, which removes contaminants by dissolving the polymer and selectively precipitating it. Another approach separates the contaminants by swelling the polymer with solvent and then pressing it out.

[0010] However, both processes require large quantities of solvent. Furthermore, the solvent treatment through energy-intensive evaporation is economically and ecologically questionable. Both processes leave behind a polymer with a high solvent content, which must be removed from the residual solvent in a drying step. The contaminants dissolved in this residual solvent remain in the polymer during the drying process.

[0011] Object of the invention

[0012] The disadvantages of the described state of the art give rise to the task of economically and ecologically improving the removal of contaminants with a molecular weight between approximately 400 and 800 Dalton in a polyolefin recycling process.

[0013] Description

[0014] The stated problem is solved in a purification process for producing a polyolefin (PO) regenerate by the features stated in the characterizing portion of patent claim 1. Further developments and / or advantageous embodiments are the subject of the dependent patent claims.

[0015] The invention is preferably characterized in that the solvent and the crude polymer dissolved therein are fed to a membrane filter, and separated in the membrane filter into a solvent-containing retentate and a solvent-containing permeate. This makes the present process extremely energy-efficient, since less solvent is required to regenerate to free the crude polymer of contaminants, and large quantities of contaminated solvent do not have to be distilled. The heat energy required can be significantly reduced with the present process. The dissolved polymer is separated by the membrane filter into a permeate, which consists of solvent and low-molecular-weight components of the polymer, and a retentate, which consists of solvent and higher-molecular-weight components of the polymer. The low-molecular-weight components contain the contaminants to be separated, which can be discarded with the solvent if necessary.The higher molecular weight components represent the target product. Because the permeate contains low molecular weight components in addition to the solvent, the process according to the invention possesses good permeation performance. The permeability of the membrane is designed such that low molecular weight components, which also include contaminants to be removed, are part of the permeate and are not retained by the membrane. According to the prior art, the function of the membrane filter is to remove the solvent from the polymer matrix after the solid contaminants have been separated using a decanter. The separation task accomplished in the prior art is therefore different from that of the present invention: The decanter separates solid contaminants from the continuous phase, which consists of solvent and polymer.The membrane separation process separates the solvent from the polymer to recover the polymer. However, contaminants that have penetrated deep into the polymer matrix remain in the retentate.

[0016] It has proven useful to separate all components with a molecular weight of <2000 Daltons, preferably <1000 Daltons, and particularly preferably <800 Daltons, as permeate in the first membrane filter. This leaves only components whose molecules are so inert that the contaminants hardly migrate into the contents of the recycled containers. Furthermore, the vapor pressure of these large molecules is low, so they are no longer perceptible as an odor. Polymer components that are separated and are not considered contaminants can be replenished after purification by read-digivation, provided they are relevant to the functional properties of the polymer.The invention is also preferably characterized in that a second membrane filter separates the solvent with contaminants having a molecular weight essentially < 400 Dalton, preferably < 200 Dalton, from the solvent with contaminants essentially > 400 Dalton, preferably > 200 Dalton. This allows the solvent to be further regenerated or purified without the need for heat energy. The number of membrane filters or filtration stages and thus the filter area used as well as the associated costs for each stage compete with the energy costs, in particular the heat energy costs, for the selective distillation on boiling plates to purify the solvent. Therefore, the present purification process is not limited to two membrane filters; rather, the number of membrane filters used depends on the required degree of purity of the PO regenerate and the energy demand.

[0017] In a further preferred embodiment of the invention, the polymer solution, which has been purified of all components, preferably with a molecular weight of <= 2000 Dalton, preferably <= 1000 Dalton, and particularly preferably <= 800 Dalton, is separated as retentate in a first evaporator after the first membrane filter into the PO regenerate with solvent residues and a first treated solvent, thereby implementing process step (b). This results in a highly pure PO regenerate containing only small amounts of solvent residues. Instead of the first evaporator, it would also be conceivable to use another membrane filter to separate the PO regenerate (the target product) from the solvent.

[0018] In yet another preferred embodiment of the invention, the solvent with contaminants substantially greater than 400 Daltons, preferably greater than 200 Daltons, is separated in a second evaporator into a first residue and a second purified solvent. This allows almost all of the solvent to be regenerated and recycled. The separated contaminants can be disposed of separately.

[0019] By recycling the first and second treated solvents into the mixing step in a first and second recirculation, almost all of the solvent can be reused and its losses are minimal in the present purification process.

[0020] It is advantageous if a partial stream of the polymer solution is recycled to the mixing step as a thick solution after the first membrane filter. The degree of recirculation of the thick solution allows the residual contamination in the plastic pellets of substances in the MW range of 400-800 Daltons, preferably 200-800 Daltons, to be controlled.

[0021] It is also advantageous if a partial stream of the solvent containing contaminants with a molecular weight of essentially < 400 Daltons and preferably > 200 Daltons is recycled to the mixing step as a dilute solution after the second membrane filter. The degree of recirculation of the dilute solution allows the proportion of contaminants in the molecular weight range of < 400 Daltons, preferably > 200 Daltons, in the PO regenerate to be controlled.

[0022] In another particularly preferred embodiment, the degree of recirculation of the thick solution and the thin solution returned to the mixing step is controlled via a first valve and a second valve. This allows the proportion of substances in the range of MW < 400 Dalton and in the range of MW 400-800 or 200-800 Dalton to be easily adjusted or controlled.

[0023] It is advantageous to carry out the extrusion step in a vacuum extruder, and to separate the solvent residues from the PO regenerate as a second residue in the vacuum extruder by degassing. This allows for the removal of any remaining contaminants with a MW < 400 Daltons or < 200 Daltons from the PO regenerate.

[0024] A further aspect of the invention relates to a process for recycling polyolefin waste by producing PO regenerates, which comprises the purification process described above. This allows a complete recycling process to be carried out in a particularly energy-efficient manner. The polyolefins are fed into the recycling process in the form of post-consumer sorting fractions containing all typical impurities on and in the polyolefin material. The fractions can be, for example, HDPE "rigid" bottled goods or "flexible" film goods, but can also consist of the polymers PP, LDPE, or a mixed polyolefin fraction. In addition, there are sorting fractions based on color and size.

[0025] Advantageously, the mechanical treatment of the waste is carried out before the waste washing, or vice versa, or both process steps are performed simultaneously. This depends on the type of waste being recycled, in order to clean its surface as efficiently as possible. Further advantages and features will become apparent from the following description of three exemplary embodiments of the invention with reference to the schematic representations. These are not to scale:

[0026] Figure 1 : a flow diagram showing a purification process for

[0027] Production of a PO regenerate.

[0028] Figure 1 shows an improved purification process for producing a PO regenerate. The surface-cleaned polymer, which originates from waste recycling and is referred to below as the crude polymer, is fed to the dissolution and membrane process and mixed with the solvent in a stirred tank a. The crude polymer, already in solution, enters a temperature-controlled receiver v and from there is fed to a first membrane filter c1, which separates the solution into a first substream (permeate: fluid that permeates the membrane filter c1) containing dissolved components with a molecular weight (MW) <800 Daltons and a second substream (retentate: fluid that is retained by the membrane filter c1) with a molecular weight >800 Daltons.The dissolved crude polymer is therefore separated by the membrane filter (c1) into a permeate consisting of solvent and dissolved low-molecular components and a retentate consisting of solvent and dissolved higher-molecular components.

[0029] The partial stream with a MW<800 Dalton is fed to a second membrane filter c2, which in turn separates it into a third partial stream with dissolved components in the molecular weight range <400 Dalton, preferably <200 Dalton, and a fourth partial stream with a MW range of 400 Dalton to 800 Dalton or 200 to 800 Dalton.

[0030] The partial flow >800D is partly returned to the receiver v and partly fed to a first evaporator g1, the mass ratio is adjustable by a valve h1.

[0031] The residue from the evaporator g1 subsequently enters the vacuum extruder f. A second residue e2 is separated via the devolatilization of the extruder f, which contains all polymer components and contaminants < 400 Dalton in the preferred embodiment < 200 Dalton.

[0032] The partial stream containing dissolved components in the molecular weight range of 400-800 Dalton or 200-800 Dalton is fed to evaporator 2. The recovered solvent condensate is recycled to the receiver v in a second recycle d2. The first residue e1 contains polymer components and the contaminants to be separated, each with a molecular weight of 400 Dalton or 200-800 Dalton.

[0033] The partial stream containing dissolved components with a MW of <400 Daltons or <200 Daltons, referred to as the "thin solution," returns partly to the receiver v and partly to the second evaporator g2. The mass ratio is adjustable by a second valve h2.

[0034] The product of the extruder f is the PO regenerate in the form of plastic pellets r, in which polymer components and other substances or contaminants in the molecular weight range 400 or 200 - 800 D are specifically depleted compared to the raw polymer.

[0035] The residual contamination in the plastic pellets r of substances in the MW range 400 or 200 - 800 Dalton is determined by the recirculation rate of a «thick solution» into the receiver v, which is adjusted with a first valve h1.

[0036] The proportion of substances in the MW range <400 D or <200 D in the regenerated material or plastic pellets fed to the extrusion is determined by the recirculation rate of the thin solution into the feed v, which is adjusted with a second valve h2 or is separated in the extruder vacuum as a second residue e2.

[0037] The energy-intensive distillation of the solvent for regeneration, which is unavoidable in state-of-the-art purification processes, can be replaced with this purification process. Membrane filters can instead take over solvent treatment.

[0038] Portions of the solvent used separated via the extruder venting must be supplemented with fresh solvent (I) in the mixer (a).

[0039] Key: a Stirred tank v Receiver b Recovery of purified polymer d First membrane filter c2 Second membrane filter d1 First solvent recirculation d2 Second solvent recirculation e1 First residue e2 Second residue f Vacuum extruder gi First evaporator g2 Second evaporator h1 First valve h2 Second valve

[0040] P Raw polymer, PO waste r PO regenerate, pellets v Template

Claims

Purification process for the production of a polyolefin (PO) regenerate (r) comprising the following process steps (a) Mixing superficially cleaned PO waste (p), referred to as crude polymer (p), with solvent and dissolving the crude polymer (p) in the solvent, (b) Recovery of the purified polymer from the solvent, (c) Preparation of the solvent, (d) Recycling of the solvent to step (a), (e) Separation of residues (e1 ,e2) and (f) Extrusion of the purified PO regenerate (r) into pellets (r), characterized in that the solvent and the crude polymer dissolved therein are fed to a membrane filter (c1) and separated in the membrane filter (c1) into a solvent-containing retentate and a solvent-containing permeate. Purification process according to claim 1, characterized in that in the first membrane filter (c1) all components with a molecular weight < 2000 Daltons, preferably < 1000 Daltons and particularly preferably < 800 Daltons are separated as permeate. Purification process according to claim 1 or 2, characterized in that a second membrane filter (c2) separates the solvent with contaminants having a molecular weight substantially < 400 Daltons and preferably < 200 Daltons from the solvent with contaminants having a molecular weight substantially > 400 Daltons and preferably > 200 Daltons.Purification process according to one of the preceding claims, characterized in that the polymer solution, which has been preferably purified of all components with a molecular weight <= 2000 Dalton, preferably <= 1000 Dalton and particularly preferably <= 800 Dalton, is separated after the first membrane filter (c1) as a retentate in a first evaporator (g1) into the PO regenerate with solvent residues and a first processed solvent, whereby process step (b) is realized.

5. Purification process according to one of the preceding claims, characterized in that the solvent with contaminations substantially > 400 Dalton and preferably > 200 Dalton is separated in a second evaporator (g2) into a first residue (e1) and a second purified solvent.

6. Purification process according to claim 5, characterized in that the first and second processed solvents are recycled in a first and second recycling (d1 ,d2) into the mixing step (a).

7. Purification process according to one of the preceding claims, characterized in that a partial stream of the polymer solution after the first membrane filter (c1) is recycled as a thick solution into the mixing step (a).

8. Purification process according to one of the preceding claims, characterized in that a partial stream of the solvent containing contaminants with a molecular weight substantially < 400 Dalton and preferably < 200 Dalton is recycled as a dilute solution into the mixing step (a) after the second membrane filter (c2).

9. Purification method according to claim 8, characterized in that the degree of recirculation of the thick solution and thin solution recirculated into the mixing step (a) is controlled via a first valve (h1) and a second valve (h2).

10. Purification process according to one of the preceding claims, characterized in that the extrusion step (f) is carried out in a vacuum extruder and the solvent residues are separated as a second residue (e2) in the vacuum extruder by degassing from the PO regenerate (r).

11. A process for recycling polyolefin waste in which PO regenerates are produced, comprising the following process steps (A) Pre-sorting of waste, (B) mechanical cleaning of the waste, (C) Washing the waste with a fluid and (D) Production of PO regenerates in a purification process using a solvent to dissolve the PO waste, characterized by: that the purification process (D) is a purification process according to any one of claims 1 to 10. Method according to claim 11, characterized in that step (B) is performed before step (C) or step (C) is performed before step (B) or steps (B) and (C) are performed simultaneously.

Images