Styrene assisted depolymerization of polyolefins

Abstract

Described herein are methods of depolymerizing polyolefin-based materials into useful petrochemicals using styrene oligomers or polymers and heat. The styrene oligomers or polymers improve the depolymerization reaction by reducing the half-life of depolymerization, which results in a higher depolymerization rate and shorter residence time in the depolymerization unit, allowing for a predictable depolymerization reaction and reducing the formation of branches or aromatics in the product.

Description

[0001] Prior related applications

[0002] This application is filed pursuant to the Patent Cooperation Treaty and claims the benefit of priority to U.S. Provisional Patent Application No. 63 / 089,706, filed October 9, 2020, which is incorporated herein by reference in its entirety.

[0003] Federally Funded Research Statement

[0004] not applicable.

[0005] Reference Microfilm Appendix

[0006] not applicable. Technical Field

[0007] This disclosure relates to a method of depolymerizing polyolefin-based materials using styrene oligomers or polymers and heating them to form useful petrochemical products. Background Technology

[0008] Rising living standards and increased urbanization have led to increased demand for polymer products, particularly polyolefin plastics. Polyolefins are frequently used in commercial plastics applications due to their outstanding performance and cost characteristics. For example, polyethylene (PE) has become one of the most widely used and recognized polyolefins because it is strong, extremely tough, and very durable. This allows it to be highly engineered for a wide range of applications. Similarly, polypropylene (PP) is mechanically strong yet flexible, heat-resistant, and resistant to many chemical solvents, such as alkalis and acids. Therefore, polypropylene is ideal for a variety of end-use industries, primarily for packaging and labeling, textiles, plastic parts, and various types of reusable containers.

[0009] A downside to the demand for polyolefin plastics is the increase in waste. Post-consumer plastic waste typically ends up in landfills, with approximately 12% being incinerated and about 9% transferred to recycling. In landfills, most plastics do not degrade rapidly, becoming a major source of waste that overloads landfills. Incineration is also not an ideal solution for treating plastic waste, as it leads to the formation of carbon dioxide and other greenhouse gas emissions. Therefore, there is great interest in developing methods for recycling plastic waste in a way that reduces the burden on landfills while also being environmentally friendly.

[0010] A drawback of recycling plastic waste is the difficulty in successfully producing commercially usable or desired products. Current plastic waste recycling methods involve washing materials and mechanically reprocessing them; however, the resulting pellets are still contaminated with impurities such as food scraps, dyes, and fragrances. These impurities render the pellets unsuitable for most applications due to performance and appearance considerations.

[0011] Recent advances have focused on converting plastic waste into usable products, such as fuel sources or commercially important raw materials. Methods have been developed to conduct pyrolysis followed by catalytic depolymerization of plastic waste streams to produce a variety of products: gases, gasoline fractions, kerosene fractions, diesel fractions, and waxes.

[0012] Unfortunately, these methods are expensive and time-consuming because they require significant energy to completely break down polyolefin waste into useful products. Furthermore, the reaction products themselves are unpredictable due to side reactions occurring under pyrolysis conditions, leading to the formation of branched and aromatic products. The catalysts themselves are also susceptible to poisoning by impurities in the polymer feed.

[0013] Despite progress in polymer recycling, there is still a need to develop a robust method for converting plastics into useful petrochemical products that minimizes the formation of branched and / or aromatic compound products. Summary of the Invention

[0014] This disclosure provides an improved method for the thermal depolymerization of polyolefin-based materials. The improved method relies on the thermal depolymerization of a feed stream of one or more polyolefins in the presence of styrene oligomers or polymers. Specifically, styrene oligomers or polymers, such as oligostyrene and polystyrene, are mixed with the polyolefin-based material in a depolymerization unit and heated in the absence of oxygen. The styrene oligomers or polymers initiate a free radical depolymerization reaction, which can proceed at a faster depolymerization rate (a shorter depolymerization half-life) than the depolymerization reaction of polyolefin-based materials without styrene oligomers or polymers. This free radical depolymerization results in the formation of a liquid product with minimal branching or aromatic formation. The liquid product can then be used as is or further processed, such as in an olefin cracker, to improve the feedstock.

[0015] The methods described herein can be used to process any polyolefin-based material, including post-industrial waste and post-consumer use. The treatment of post-consumer polyolefin waste is particularly important due to landfill overload and the potential for raw materials to be generated from the waste. The methods described herein involve processing post-consumer waste after it has been sorted by a processing center at a landfill or other recycling center to separate polyolefin-based materials from other recyclable materials such as glass, cellulose (paper), polyethylene-based polymers, etc.

[0016] This disclosure includes any of the following combinations of embodiments:

[0017] A method for depolymerizing polyolefins includes adding a polyolefin-based feed stream and a styrene oligomer or styrene polymer to a depolymerization unit heated to a predetermined temperature; and reacting the polyolefin-based feed stream with the styrene oligomer or styrene polymer to depolymerize the polyolefin-based feed stream.

[0018] A method for depolymerizing polyolefins includes adding a polyolefin-based feed stream and a styrene oligomer to a depolymerization unit heated to a predetermined temperature; and reacting the polyolefin-based feed stream with the styrene oligomer to depolymerize the polyolefin-based feed stream.

[0019] A method for depolymerizing polyolefins includes adding a polyolefin-based feed stream and a styrene polymer to a depolymerization unit heated to a predetermined temperature; and reacting the polyolefin-based feed stream with the styrene polymer to depolymerize the polyolefin-based feed stream.

[0020] In any of the methods described herein, the depolymerization rate of the polyolefin-based feed stream is at least 10% higher than the depolymerization rate of the polyolefin-based feed stream that does not contain styrene oligomers or styrene polymers.

[0021] In any of the methods described herein, the depolymerization initiation temperature of the polyolefin-based feed stream is 5% lower than that of the polyolefin-based feed stream that does not contain styrene oligomers or styrene polymers.

[0022] In any of the methods described herein, the depolymerization half-life of the polyolefin-based feed stream is at least 30% lower than that of the polyolefin-based feed stream that does not contain styrene oligomers or styrene polymers.

[0023] Any of the methods described herein, wherein the styrene oligomer is oligostyrene.

[0024] In any of the methods described herein, the styrene polymer is polystyrene.

[0025] Any of the methods described herein, wherein the styrene oligomers and polymers have an average molecular weight between 500 Da and 20 kDa.

[0026] In any of the methods described herein, the polyolefin-based feed stream is low-density polyethylene, high-density polyethylene, polypropylene, or a combination thereof.

[0027] In any of the methods described herein, the polyolefin-based feed stream is post-consumer waste.

[0028] In any of the methods described herein, the polyolefin-based feed stream is post-industrial waste.

[0029] Any of the methods described herein, wherein the polyolefin-based feed stream includes both industrial post-waste and post-consumer waste.

[0030] In any of the methods described herein, the styrene polymer is a post-consumer waste, a post-industrial waste, or a combination thereof.

[0031] Any of the methods described herein, wherein the styrene oligomer is a post-consumer waste, a post-industrial waste, or a combination thereof.

[0032] In any of the methods described herein, the concentration of the styrene oligomer or styrene polymer is greater than 0% by weight to about 20% by weight.

[0033] In any of the methods described herein, the concentration of the styrene oligomer or styrene polymer is between about 2.5% by weight and about 5% by weight.

[0034] Any of the methods described herein, wherein the predetermined temperature is between about 200°C and about 600°C.

[0035] A method for depolymerizing polyolefins includes adding a polyolefin-based feed stream and a styrene oligomer or styrene polymer to a depolymerization unit heated to a temperature between about 200°C and about 600°C; and reacting the polyolefin-based feed stream with the styrene oligomer or styrene polymer to depolymerize the polyolefin-based feed stream. In some embodiments, the depolymerization rate of the polyolefin-based feed stream is at least 10% higher than the depolymerization rate of a polyolefin-based feed stream without the styrene oligomer or styrene polymer. Additionally, or alternatively, the depolymerization initiation temperature of the polyolefin-based feed stream is 5% lower than the initiation temperature of a polyolefin-based feed stream without the styrene oligomer or styrene polymer.

[0036] This summary is provided to introduce some concepts that will be further described in the following detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to help limit the scope of the claimed subject matter.

[0037] definition

[0038] As used herein, the term “depolymerization half-life” or “half time of depolymerization” refers to the time required for a sample to achieve 50% mass loss at a specific temperature during the TGA pyrolysis reaction.

[0039] As used in this article, “residence time” refers to the time required to depolymerize a batch of polymer waste in the depolymerization unit.

[0040] As used in this article, "pyrolysis" refers to a pyrolytic polymerization reaction that occurs in the absence of oxygen.

[0041] As used in this article, “post-consumer waste” refers to the type of waste generated by the end consumer of the material flow.

[0042] As used in this article, "post-industrial waste" refers to the type of waste generated during the production process of a product.

[0043] Unless otherwise specified, all concentrations in this document are expressed as a weight percentage (“wt%”).

[0044] As used herein, “oligomer” refers to a molecule consisting of a few (≤100) repeating units. As used herein, “polymer” refers to a molecule consisting of many (>100) repeating units. Both oligomers and polymers can be synthesized from one or more monomers.

[0045] The term "oligostyrene" refers to an oligomer comprising repeating units derived solely from styrene monomers. The term "polystyrene" refers to a polymer comprising repeating units derived solely from styrene monomers.

[0046] Unless the context otherwise requires, when used in conjunction with the term "comprising" in the claims or description, the use of the words "a" or "an" means one or more.

[0047] The term "about" means that the specified value is added to or subtracted from the measurement error, or 10% is added to or subtracted if the measurement method is not specified.

[0048] The term "or" used in the claims is used to mean "and / or" unless it is explicitly stated that it refers only to alternatives or if the alternatives are mutually exclusive.

[0049] The terms “comprising,” “having,” “including,” and “containing” (and variations thereof) are open-ended connecting verbs and allow for the addition of other elements when used in claims.

[0050] The phrase “composed of” is closed and does not include any additional elements.

[0051] The phrase "consistent with..." does not include additional material elements, but may include non-material elements that do not substantially alter the nature of the invention.

[0052] The following abbreviations are used in this article:

[0053] abbreviation the term EPS Expandable polystyrene GC Gas chromatography GPC Gel permeation chromatography HDPE High-density polyethylene PE polyethylene PP polypropylene PS polystyrene TGA Thermogravimetric analysis wt% weight percentage Detailed Implementation

[0054] This disclosure provides an improved method for recycling polyolefin-based materials into commercially important feedstocks using styrene oligomers or polymers. Specifically, in a depolymerization unit, styrene oligomers or polymers are mixed with a polyolefin-based feed stream comprising at least one polyolefin-based material. Upon heating of the mixture, a pyrolysis reaction occurs, wherein the styrene oligomers or polymers initiate the depolymerization of the polyolefin-based material to generate a usable liquid product having minimal branching or aromatic compound formation.

[0055] Using styrene oligomers or polymers to improve the thermal depolymerization of polyolefins has many advantages. As mentioned above, styrene oligomers or polymers are not catalysts for the recycling process. Instead, they act as initiators. More specifically, the double bonds (C=C) in the backbone of the styrene oligomer or polymer undergo homolytic cleavage at low temperatures, generating free radicals stabilized by aromatic rings. These free radicals then initiate chain reactions with the polyolefin, thereby promoting the radical depolymerization of the polyolefin in the feed stream. This limits isomerization reactions during depolymerization, resulting in simpler mixing of reaction products that are similar to those from the same feed stream depolymerized without styrene oligomers and polymers. Therefore, the reaction products for a given polymer feed stream composition are readily predictable.

[0056] Another advantage of using styrene oligomers and polymers is their ease of acquisition from post-consumer and post-industrial waste, particularly in the form of expanded polystyrene foam (EPS). Over the past 15 years, the global EPS industry has managed to recycle an average of 19% of post-consumer EPS and 25% of post-industrial EPS, with the remainder ending up in landfills. The method of this invention allows for the easy combination of polystyrene destined for landfills with post-consumer and post-industrial polyolefin waste. As an example, polyolefin-based materials separated by processing centers at landfills or other recycling centers can be combined with polystyrene foam cups and other polystyrene food containers in a depolymerization unit. Thus, not only is the polyolefin-based material depolymerized and recycled, but less polystyrene foam ends up in landfills. Alternatively, 'new' polystyrene, or other styrene oligomers and polymers, can be specifically produced for use in this method.

[0057] Finally, styrene oligomers and polymers are robust because they are less likely to be affected by 'poison' in the polymer feed stream than other conventional depolymerization catalysts. This allows for a wider range of polyolefin feed compositions than other depolymerization methods.

[0058] The addition of styrene oligomers and polymers over a wide weight range can improve the depolymerization rate of polyolefins. In some embodiments, the styrene oligomers and polymers are present at a concentration greater than 0 wt% to about 50 wt% of the feed stream. Alternatively, the styrene oligomers and polymers are present at concentrations greater than 0 wt% to about 30 wt%, about 2.5 wt% to about 10 wt%, about 5 wt% to about 20 wt%, about 15 wt% to about 30 wt%, about 25 wt% to about 50 wt%, or about 35 wt% to about 50 wt%. In other embodiments, the styrene oligomers and polymers are present at concentrations of 2.5 wt%, 5 wt%, 10 wt%, 15 wt%, or 20 wt%.

[0059] Styrene oligomers and polymers may have an average molecular weight between 500 Da and 20 kDa. In some embodiments, the added styrene component has only styrene repeating units, such as oligostyrene and polystyrene. In some embodiments, the added styrene component has one or more other repeating units besides styrene. In other embodiments, the added styrene component is oligostyrene having 10 to about 80 repeating units; alternatively, the oligostyrene has 10 to about 50 repeating units; alternatively, the oligostyrene has 40 to about 80 repeating units.

[0060] The method described herein can be applied to feed streams comprising materials having a single polyolefin component or a mixture of polyolefin components in any amount. A wide range of polyolefins can be present in the feed stream, including but not limited to polyethylene (high-density and low-density), polypropylene, ethylene-propylene copolymers, polybutene-1, polyisobutylene, and copolymers thereof. Furthermore, the waste is not limited to any particular form, and therefore films, foams, textiles, or other molded materials can be treated using the method described. The feed may contain post-consumer polyolefin waste, post-industrial polyolefin waste, or both post-industrial and post-consumer polyolefin waste.

[0061] Polyolefin-based materials combined with styrene oligomers and polymers are processed in a depolymerization unit with an operating temperature between about 200°C and about 600°C. Alternatively, the depolymerization unit operates at a temperature between about 225°C and about 500°C. In yet another alternative, the depolymerization unit operates at a temperature between about 250°C and about 450°C, or about 400°C.

[0062] Due to the residence time required for complete depolymerization of the stream, the polyolefin feed stream is processed in batches within the depolymerization unit. Depending on the heat transfer properties of the depolymerization unit and the amount of styrene oligomers or polymers, the estimated residence time for each batch is approximately 30 minutes to approximately 180 minutes. Alternatively, the estimated residence time is approximately 60 minutes.

[0063] Under the reaction conditions described above, batches containing even small amounts of styrene oligomers and polymers (less than 5 wt%) are expected to have a depolymerization half-life at least 30% lower than that of polyolefin batches without added styrene oligomers and polymers. In some embodiments, the depolymerization half-life is reduced by at least 40%. For larger amounts of styrene oligomers and polymers (about 10 wt% to about 20 wt%), depending on the polyolefin content, the depolymerization half-life is reduced by at least 59%.

[0064] Therefore, the depolymerization methods disclosed herein allow for faster depolymerization of polyolefin-based materials into predictable liquid products with minimal branching or aromatic formation. These liquid products can then be used as is or further processed to improve the quality of the product stream. Additionally, these methods reduce the amount of styrene oligomer and polymer waste in landfills.

[0065] Example

[0066] The following examples illustrate embodiments of the methods described above for depolymerizing polyolefins as claimed in the appended claims. These examples are intended to be illustrative only and not to unduly limit the scope of the appended claims. Those skilled in the art will understand that various changes can be made to the specific embodiments disclosed herein without departing from the spirit and scope of the disclosure and still obtaining similar or related results. The following examples should in no way be construed as limiting or defining the scope of the appended claims.

[0067] TGA depolymerization

[0068] Thermogravimetric analysis (TGA) was used as the depolymerization unit to process a series of one-component polyolefin feed streams to investigate the effect of polystyrene as a depolymerization initiator. The feed consisted of high-density polyethylene (grade ACP9255, LyondellBasell product) or polypropylene (grade Moplen HP522H, LyondellBasell product). Homogeneous samples were prepared by melt-mixing 10 g of polyolefin feed with various amounts of polystyrene (PS3010-01, Sigma-Aldrich) for 5 minutes at 200°C and 200 RPM in a HAAK MiniCTW mixer.

[0069] For the TGA pyrolysis reaction, the prepared sample was heated under nitrogen at 10 K / min in Mettler Toledo TGA / DSC3+ (Mettler Toledo, Columbus, OH) to the desired depolymerization temperature and held for 1 hour. For these examples, a depolymerization temperature of 400 °C was used. If this value was less than 60 minutes, the depolymerization half-life at the specific temperature (defined as the time required to achieve 50% mass loss) was recorded directly, or determined as t under the assumption of first-order decomposition kinetics. 1 / 2=0.693 / k, where k is the first-order rate constant. If this value is greater than 60 minutes, Ln(C0 / C) is used to determine the time curve graphically.

[0070] The depolymerization half-life is the residence time required relative to a large-scale depolymerization unit. The shorter the half-life, the shorter the residence time of the polymer feed batch in the depolymerization unit, and the higher the depolymerization rate.

[0071] The catalytic effect of polystyrene on HDPE is shown in Table 1. Comparative Example 1 was depolymerized in the absence of polystyrene. The depolymerization half-life of Comparative Example 1 was 347 minutes at 400°C. The addition of polystyrene reduced the half-life of this HDPE feed. Even at lower concentrations of polystyrene (≤5%), a significant reduction in the depolymerization half-life was observed. A 40% reduction in half-life was observed with polystyrene concentrations as low as 2.5%. At a polystyrene concentration of 20%, the half-life was reduced by approximately 82%. This indicates that even small amounts of added polystyrene can reduce the residence time required to completely depolymerize HDPE into usable petrochemical products.

[0072]

[0073]

[0074] When the polyolefin is converted to PP, a similar reduction in the depolymerization half-life is observed (with a corresponding increase in the depolymerization rate). As shown in Table 2, a significant reduction in the depolymerization half-life of PP is observed, even with the use of lower concentrations of polystyrene (≤5%). At a polystyrene concentration of 20%, the depolymerization half-life is reduced by approximately 62%. While this is less than the reduction observed with the HDPE stream, the reduction in half-life of the PP stream with 20 wt% polystyrene is still more rapid than in Comparative Example 2.

[0075]

[0076] Polystyrene can demonstrate improvements in the depolymerization rates of various polyolefins from 2.5% to 20% at concentrations that translate to less time required to depolymerize these compounds in larger-scale reactors.

[0077] Concentrations of polystyrene above 20% also reduce the depolymerization rates of HDPE and PP. However, this benefit can be offset by the generation of more aromatic products, namely styrene, in the resulting reaction products. The presence of aromatic products affects the quality of the resulting feedstock. Therefore, depending on the end use of the reaction products, the additional step of hydrogenating the reaction products may be necessary.

[0078] Although the examples use polystyrene, other styrene polymers may also be used. Additionally, styrene oligomers with a few repeating units up to 100 repeating units, such as oligostyrene, will also be able to reduce the depolymerization rate of the polyolefin feed.

[0079] The currently described method of using styrene oligomers or polymers for the depolymerization of polyolefin streams can provide lower energy efficiency (i.e., more cost-effectiveness) compared to methods that do not use styrene oligomers or polymers.

Claims

1. A method for depolymerizing polyolefins, comprising: a) Adding a polyolefin-based feed stream and a styrene polymer to a depolymerization unit heated to a temperature between 200°C and 600°C, wherein the concentration of the styrene polymer is 2.5 to 5% by weight; and b) React the polyolefin-based feed stream with the styrene polymer to depolymerize the polyolefin-based feed stream. The styrene polymer therein acts as a depolymerization initiator.

2. The method of claim 1, wherein the polyolefin-based feed stream is combined with styrene oligomers.

3. The method according to claim 2, wherein the styrene oligomer is oligostyrene.

4. The method according to claim 1, wherein the styrene polymer is polystyrene.

5. The method of claim 1, wherein the polyolefin-based feed stream is low-density polyethylene, high-density polyethylene, polypropylene, or a combination thereof.

6. The method of claim 1, wherein the polyolefin-based feed stream is post-consumer waste.

7. The method of claim 1, wherein the polyolefin-based feed stream is industrial post-waste.

8. The method of claim 1, wherein the polyolefin-based feed stream comprises both industrial post-waste and consumer post-waste.

9. The method of claim 4, wherein the polystyrene is post-consumer waste, post-industrial waste, or a combination thereof.

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