Pretreatment of polyolefin waste to improve depolymerization

Abstract

A method for pre-treatment of a polyolefin-based feed stream prior to depolymerization is described. Utilizing density differences in aqueous solutions to separate polyolefins from other materials in the polyolefin-based feed stream allows for a pre-treatment method that does not impact the depolymerization catalyst. Removal of non-polyolefin materials from the feed stream allows depolymerization of the polyolefin material to be carried out at lower temperatures and for longer cycles. This reduces carbon dioxide emissions and provides a more efficient process.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application is made under the Patent Cooperation Treaty, which claims priority to U.S. Provisional Patent Application No. 63 / 219,616, filed July 8, 2021, which is incorporated herein by reference in its entirety.

[0002] Statement regarding federally funded research Not applicable.

[0003] References to Microfiche Appendix Not applicable.

[0004] The present disclosure relates to a method for depolymerizing polyolefin-based plastic waste materials to form useful petrochemical products. [Background technology]

[0005] Rising living standards and urbanization are driving the demand for polymer products, especially polyolefin plastics. Polyolefins are popular in commercial plastic applications due to their outstanding performance and cost characteristics. For example, polyethylene (PE) is one of the most widely used and recognized polyolefins due to its high strength, extreme toughness, and durability. This allows it to be highly engineered for a variety of applications. Similarly, polypropylene (PP) is mechanically strong yet flexible, heat resistant, and resistant to many chemical solvents, including bases and acids. This makes it suitable for a variety of end-use industries, including packaging and labels, textiles, plastic parts, and various types of reusable containers.

[0006] The downside of the demand for polyolefin plastics is the increase in waste. Post-consumer plastic waste typically ends up in landfills, with about 12% being incinerated and about 9% being sent for recycling. In landfills, most plastics do not decompose rapidly, making them the main source of waste for overburdened landfills. Incineration is also not an ideal solution for treating plastic waste, as it leads to the formation of carbon dioxide and the emission of other greenhouse gases. Therefore, there is growing interest in developing ways to recycle plastic waste to reduce the burden on landfills while being environmentally friendly.

[0007] A drawback of recycling plastic waste is the difficulty of successfully producing commercially usable or desired products. Currently, recycling plastic waste involves cleaning and mechanical reprocessing of the material. However, the resulting pellets remain contaminated with food residues, dyes, and flavorings. These contaminants make them unsuitable for most applications based on both performance and appearance. Additionally, because it is difficult to obtain a pure stream of a particular polymer, mixed plastic waste streams may not have the desired properties after recycling.

[0008] Recent advances have focused on converting polyolefin plastic waste into usable products such as fuel sources and commercially important raw materials. Methods have been developed to carry out the pyrolysis and subsequent catalytic depolymerization of plastic waste streams to produce a variety of products such as gas, gasoline fraction, kerosene fraction, diesel fraction, and wax. Unfortunately, the catalysts themselves tend to be easily contaminated by other chemicals in the polyolefin feed, and the process is costly and time consuming, as it requires a lot of energy to completely break down the polyolefin waste into a range of useful products. Summary of the Invention [Problem to be solved by the invention]

[0009] Despite advances in polyolefin recycling, there is a continuing need to develop reliable processes for converting polyolefin waste feedstocks into useful petrochemical products.

[0010] The present disclosure provides an improved method for pyrolyzing a polyolefin-based feed stream. The improved method relies on pre-treating the polyolefin-based feed stream to separate non-polyolefin materials from the polyolefin materials prior to depolymerization. Specifically, when the polyolefin-based feed stream is placed in an aqueous solution, the less dense polyolefin materials float and the non-polyolefin materials sink. This allows the polyolefin materials to be skimmed off the surface of the aqueous solution. In some embodiments, a strong alkali can be added to the aqueous solution to decompose the non-polyolefin polymers in the polyolefin-based feed stream. The separated polyolefin materials are dried before being thermally depolymerized in the presence of a depolymerization catalyst.

[0011] In some embodiments, the pretreatment method is combined with a depolymerization reaction that utilizes a depolymerization catalyst having an aluminosilicate, such as a zeolite or clay. In some embodiments, the depolymerization catalyst has a zeolite catalyst and an optional inorganic cocatalyst. The zeolite is used in the catalytic cracking of polyolefin waste. The zeolite initiates the cationic dissociation of polyolefins, which proceeds at a faster rate (shorter half-time of depolymerization) and often at a lower temperature than the depolymerization reaction proceeding without the zeolite. However, the catalytic ability of the zeolite can be inhibited by non-polyolefin materials that may be present in the waste feed stream or by decomposition products of the non-polyolefin materials that are generated during the depolymerization process. In particular, non-polyolefin materials such as nitrogen or high oxygen content polymers, including polyamides, polyurethanes, cellulose and lignin, are known to form decomposition products and thereby "poison" the catalytic ability of the zeolite. These products may not deactivate the catalyst so much as they interfere with the mechanism of depolymerization and thus slow it down. Depending on the type and concentration of zeolite and non-polyolefin components, the depolymerization rate may be reduced by up to 85%, or more depending on the level of undesirable components. Thus, the presence of non-polyolefin components increases the energy and time required for depolymerization of polyolefin materials using zeolites. Similar inhibition of catalytic activity has been observed with clays such as bentonite in the presence of non-polyolefin materials.

[0012] The disclosed process rapidly reduces the amount of non-polyolefin components in a polyolefin-based feed stream, allowing subsequent catalytic depolymerization to be carried out at lower temperatures and longer cycles, after which the liquid depolymerization product may be used as is or may be further processed, for example in an olefin cracking furnace, as a replacement feedstock.

[0013] The methods described herein can be used to treat any polyolefin-based feed stream, including post-industrial and post-consumer waste. The treatment of post-consumer polyolefin waste is particularly important due to the overburdening of landfills and the potential for raw materials to be generated from the waste. The methods described herein relate to the treatment of post-consumer waste after sorting at a landfill processing center or other recycling center, separating polyolefin-based materials from recyclable materials such as glass, cellulose (paper), polyvinyl polymers, etc. However, it is not always possible to completely remove non-polyolefin polymers such as cellulose (paper), polyvinyl polymers, nylon, and inorganic materials such as sand and wire, so the presently described pretreatment methods are used to completely separate these non-polyolefin materials. However, the pretreatment process can be applied to feed streams before they undergo sorting.

[0014] The method includes any of the following embodiments, in any combination of one or more thereof.

[0015] A method of depolymerizing a polymer includes the steps of: pretreating a polyolefin-based feed stream to separate polyolefin material by adding the polyolefin-based feed stream to an aqueous solution in a first vessel and agitating the mixture; skimming off material suspended in the aqueous solution, the suspended material being polyolefin material; and drying the polyolefin material. The dried polyolefin material and the depolymerization catalyst are then added to a reactor heated to a temperature of about 200 to about 600° C. The polyolefin material is reacted with the depolymerization catalyst to depolymerize the polyolefin material. In some embodiments, the depolymerization catalyst is a composite catalyst comprising at least one zeolite and, optionally, a co-catalyst such as a solid inorganic material.

[0016] A method for depolymerizing a polymer includes the steps of: first pretreating a polyolefin-based feed stream to separate polyolefin material by adding the polyolefin-based feed stream to an aqueous solution in a first vessel and stirring the mixture for at least 0.5 hours; skimming off material suspended in the aqueous solution, the suspended material being polyolefin material; and drying the polyolefin material until less than 5% residual moisture is obtained. The dried polyolefin material and the depolymerization catalyst are then added to a reactor heated to a temperature of about 200 to about 600° C. The polyolefin material is reacted with the depolymerization catalyst to depolymerize the polyolefin material.

[0017] A method for pretreating a polyolefin-based feed stream prior to depolymerization, comprising the steps of: adding the polyolefin-based feed stream to a first vessel filled with an aqueous solution, stirring the aqueous solution and the polyolefin-based feed stream for at least 0.5 hours, skimming the surface of the aqueous solution to remove at least one polyolefin material suspended therein, and drying the polyolefin material until a residual moisture of less than 5% is obtained, after which the dried polyolefin material can be depolymerized.

[0018] A method for pretreating a polyolefin-based feed stream prior to depolymerization, comprising the steps of: adding the polyolefin-based feed stream to a first vessel filled with a heated aqueous solution having a pH greater than 9, agitating the aqueous solution and the polyolefin-based feed stream for at least 2 hours while maintaining the heat of the aqueous solution at at least 70° C., skimming the surface of the aqueous solution to remove at least one polyolefin material suspended therein, and drying the polyolefin material at a temperature of 50° C. until a residual moisture of less than 5% is obtained. The dried polyolefin material can then be depolymerized.

[0019] Any of the methods described herein may further comprise heating the aqueous solution and the polyolefin-based feed stream with stirring to greater than 25°C to about 150°C, or at least 70°C.

[0020] In any of the processes described herein, the aqueous solution and the polyolefin feed stream are agitated under a pressure of about 0.1 MPa to about 0.2 MPa.

[0021] In any of the methods described herein, the aqueous solution comprises a strong alkali. In any of the methods described herein, the strong alkali is present in an amount of about 5 to about 40%, or about 10 to about 25%, or about 18 to about 32%, or about 27 to about 40% of the aqueous solution. The strong alkali may be, but is not limited to, calcium hydroxide, sodium hydroxide, potassium hydroxide, barium hydroxide, lithium hydroxide, or strontium hydroxide.

[0022] In any of the methods described herein, the pH of the aqueous solution is at least 9.

[0023] In any of the methods described herein, the polyolefin material is dried until the polyolefin material has a residual moisture content of less than 5%, less than 3%, or less than 1%.

[0024] In any of the methods described herein, the polyolefin material is dried at a temperature of at least 50°C.

[0025] In any of the methods or composite catalyst compositions described herein, the at least one zeolite is selected from the group consisting of beta zeolite, zeolite Socony Mobil-5 (ZSM-5), ultrastable zeolite Y, zeolite Y, or a combination thereof. In some embodiments, H ultrastable Y zeolite is used.

[0026] In any of the methods or composite catalyst compositions described herein, the optional solid inorganic co-catalyst is a metal oxide, a metal hydroxide, a metal carbonate, a silicate, or a tetravalent metal phosphate.

[0027] In any of the methods or composite catalyst compositions described herein, the optional solid inorganic co-catalyst is selected from the group consisting of Ca(OH), Mg(OH), Ba(OH), Sr(OH), CaO, AlO, and Zr(HPO).

[0028] In any of the methods or composite catalyst compositions described herein, the optional solid inorganic co-catalyst is present in the combined catalyst in an amount of from about 20 to about 90 wt %, based on the total weight % of the composite catalyst.

[0029] In any of the processes or composite catalyst compositions described herein, the composite catalyst is present in an amount of from greater than 0% to about 20% by weight of the polyolefin material feed.

[0030] In any of the processes described herein, the polyolefin-based feed stream has up to 49% non-polyolefin components.

[0031] In any of the methods described herein, the at least one non-polyolefin component is a polymer having high oxygen content, nitrogen-containing moieties, or both. In some embodiments, the polymer is selected from the group consisting of nylon polymers, cellulose, polyarylamide, polyurethane, and polyethylene polymers.

[0032] In any of the methods described herein, at least one non-polyolefin component is an inorganic material. In some embodiments, the inorganic material is sand or wire.

[0033] In any of the processes described herein, the polyolefin-based feed stream is a post-consumer or post-industrial waste stream.

[0034] In any of the processes described herein, the polyolefin-based feed stream comprises post-industrial and post-consumer waste streams.

[0035] This Summary is provided to introduce a selection of concepts that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. definition

[0036] As used herein, "residence time" refers to the time required to depolymerize one batch of polymer waste in a depolymerization unit.

[0037] As used herein, the term "depolymerization half time" or "half time of depolymerization" refers to the time required to achieve 50% mass loss of a sample at a particular temperature during a TGA pyrolysis reaction. The depolymerization half life is related to the residence time required for large-scale industrial depolymerization reactors.

[0038] As used herein, "pyrolysis" refers to a thermal depolymerization reaction that occurs in the absence of oxygen.

[0039] The terms "polyolefin-based" and "polyolefin-rich" are used interchangeably with respect to a material, feed stream, or waste stream to refer to a mixture having at least 51% polyolefin content.

[0040] As used herein, "non-polyolefin components" refers to non-polyolefin materials present in a polyolefin-based feed or waste stream. In some embodiments, these materials may reduce the ability of the depolymerization catalyst to depolymerize the polyolefins present in the stream. Examples of non-polyolefin components include non-polyolefin polymers with high oxygen and / or nitrogen content, and inorganic materials such as sand and wire.

[0041] As used herein, "post-consumer waste" refers to waste generated by the end user of a material stream.

[0042] As used herein, "post-industrial waste" refers to waste generated during the manufacturing process of a product.

[0043] As used herein, "feed stream" refers to the supply of material for depolymerization. Depending on the depolymerization unit, the feed stream may be a continuous supply of material or a batch of material. The feed stream may be pure polyolefin, processed polyolefin, or a mixture of polyolefin and non-polyolefin components.

[0044] "Waste stream" refers to a feed stream that contains materials that are discarded as unwanted, including, but not limited to, post-consumer and post-industrial waste.

[0045] "Processed" polyolefin material or polyolefin feed refers to a feed stream that has been subjected to the pretreatment methods described herein, but is not a pure polyolefin feed. Processed polyolefin feed is at least 75% by weight of polyolefin.

[0046] As used herein, the term "depolymerization catalyst" refers to a number of materials that can increase the reaction rate or reduce the reaction temperature of a pyrolytic polymerization reaction.

[0047] As used herein, the terms "poisoning" and "catalyst poisoning" refer to the partial or total deactivation of the zeolite catalyst by at least one non-polyolefin component in the depolymerized feed stream.

[0048] As used herein, the term "zeolite" or "zeolite catalyst" refers to a variety of natural and synthetic aluminosilicate crystalline solids with rigid structures that include a network of silicon and aluminum atoms tetrahedrally coordinated to one another through shared oxygen atoms. Such rigid frameworks contain channels or interconnected voids that can be occupied by cations, such as sodium, potassium, ammonium, hydrogen, magnesium, calcium, and water molecules. The zeolites used herein have a high silica content (Si / Al ratio greater than 5), which not only allows the zeolite structural framework to withstand the high temperatures used during cracking, but also increases the overall acidity of the zeolite. Many of the zeolites in this disclosure are used in the H form to ensure the presence of strong acid sites.

[0049] As used herein, the term "depolymerization cycle" refers to the time of operation of a depolymerization unit before cleaning is required and / or before the depolymerization catalyst requires regeneration or replacement.

[0050] All concentrations herein are in weight percent ("wt %") unless otherwise specified.

[0051] When used in the claims or specification in conjunction with the term "comprising," the use of the words "a" or "an" means one or more words, unless the context dictates otherwise.

[0052] The term "about" refers to the stated value plus or minus the margin of error of measurement, or plus or minus 10%.

[0053] Use of the term "or" in the claims means "and / or," unless expressly stated to mean only alternatives or where alternatives are mutually exclusive.

[0054] The terms "comprise," "have," "include," and "contain" (and variations thereof) are open-linked verbs which, when used in the claims, allow for the addition of other elements.

[0055] The phrase "consisting of" is closed and excludes all additional elements.

[0056] The phrase "consisting essentially of" excludes additional material elements, but allows for the inclusion of non-material elements that do not materially alter the essence of the invention.

[0057] The following abbreviations are used herein: [Table 1] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058] Plastic and polyolefin feed streams, especially those from waste sources, can contain a lot of non-polyolefin materials that are detrimental to pyrolytic polymerization processes, especially those relying on zeolite catalysts. Large heat losses can be experienced during pyrolytic polymerization due to the need to heat the non-polyolefin materials. In addition, many of the non-polyolefin materials can produce a lot of coke at typical depolymerization temperatures, leading to reduced catalyst performance and cycle times. Zeolite-catalyzed thermal depolymerization processes can also inhibit catalyst capacity for non-polyolefin polymers such as nylon, reducing depolymerization efficiency. Inorganic materials exiting the depolymerization unit along with the liquid depolymerization products can also have detrimental effects on downstream olefin crackers and other units.

[0059] The present disclosure solves these problems by providing a depolymerization pretreatment method for separating polyolefin materials from non-polyolefin materials in a polyolefin-based feed stream. More specifically, the pretreatment method separates materials in a polyolefin-based feed stream by their density in an aqueous solution, also called a pretreatment solution. The non-polyolefin components (NPCs) are typically denser and sink in the aqueous solution. The polyolefin materials are less dense and float in the aqueous solution, allowing them to be skimmed out of the aqueous solution. The skimmed polyolefin materials are then dried until a residual moisture level of less than 5% is obtained and fed to a depolymerization unit along with a depolymerization catalyst. The reduction in non-polyolefin materials means less heat loss, less coke production, and improved catalytic activity. This reduces the depolymerization temperature and increases the depolymerization cycles.

[0060] In some embodiments, the aqueous solution includes a strong alkali and has a pH greater than 7. The strong alkali breaks down non-polyolefin polymers that may float to the surface, further improving the removal of the non-polyolefin material. As previously described, the floating material may be removed from the aqueous solution and dried before being fed to the depolymerization unit. No washing step is required to remove residual alkali from the surface of the polyolefin material. The aqueous solution may include strong alkalis, including but not limited to calcium hydroxide, sodium hydroxide, potassium hydroxide, barium hydroxide, lithium hydroxide, and strontium hydroxide.

[0061] In other embodiments, the strong alkali is about 5-40% of the aqueous solution. Or, the strong alkali is about 10-25% of the aqueous solution. Or, the strong alkali is about 18-32% of the aqueous solution. Or, the strong alkali is about 27-40% of the aqueous solution. Or, the strong alkali is about 20% of the aqueous solution. In some embodiments, the aqueous solution has sufficient strong alkali to raise the pH to at least 9, at least 11, or at least 12.

[0062] The pretreatment method disclosed herein allows for the addition of a polyolefin-based feed stream to an aqueous solution with or without strong alkali and stirring for about 0.5 hours to about 5 hours to separate the components in the feed stream based on density. Alternatively, the pretreatment method involves stirring a mixture of the polyolefin-based feed stream and the aqueous solution for about 1 hour to about 3 hours or about 2 hours.

[0063] The mixture of the polyolefin-based feed stream and the aqueous solution may be at a temperature of about 25° C. to about 150° C. and at a pressure of between about 0.1 MPa to about 0.2 MPa while being stirred. In some embodiments, the mixture is stirred at ambient temperature (up to 25° C.) and the mixture may be heated to about 150° C. while being stirred. In other embodiments, the aqueous solution is preheated to a temperature of greater than 25° C. to about 150° C. before the polyolefin-based feed stream is added thereto and stirred while maintaining that temperature.

[0064] In some embodiments, the polyolefin-based feed stream is first added to an aqueous solution, and then the strong alkali is slowly added with stirring at a temperature of about 25° C. to about 150° C. and an ambient pressure of about 0.1 MPa to about 0.2 MPa. In other embodiments, the polyolefin-based feed stream is first added to an aqueous solution preheated to a temperature of about 70° C. to about 80° C., followed by the slow addition of the strong alkali with stirring.

[0065] After stirring for at least 2 hours, the suspended material can be skimmed off from the aqueous solution and dried at ambient or elevated temperatures (above 25° C. or above 50° C.) until the residual moisture content is less than 5%, less than 3%, or less than 1%. The skimmed material is the “treated” polyolefin feed stream for the depolymerization process. In some embodiments, the treated polyolefin feed stream is at least 75% by weight polyolefin %. In other embodiments, the treated polyolefin feed stream is at least 95% by weight polyolefin %.

[0066] The treated polyolefin feed stream must be dried prior to pyrolytic polymerization, but does not require a washing step. In some embodiments, the treated polyolefin feed stream is air dried at ambient temperature until the residual moisture content is less than 5%. Alternatively, the treated polyolefin feed stream is dried at a temperature of at least 50° C. until the residual moisture content is less than 5%, less than 3%, or less than 1%. In other embodiments, the treated material is dried at 60° C. until the residual moisture content is less than 5%, less than 3%, or less than 1%.

[0067] After drying, the treated polyolefin stream is depolymerized in the presence of a depolymerization catalyst. In some embodiments, the depolymerization catalyst is aluminosilicate-based, where the aluminosilicate is a clay such as a zeolite or bentonite. However, other depolymerization catalysts can be used. Some of the pretreatment methods described herein are combined with a depolymerization catalyst that is a complex of a zeolite with a solid inorganic cocatalyst such as a metal oxide, metal hydroxide, metal carbonate, silicate, or tetravalent metal phosphate.

[0068] In some embodiments, the composite catalyst has commercially available zeolites, including but not limited to beta zeolite (β), zeolite Socony Mobil-5 (ZSM-5), zeolite Y (Y), ultrastable zeolite Y (USY), amorphous acidic AlSiOx such as Siral™ 40, or combinations thereof. Combinations of zeolites can be used to address specific polyolefin-based feed content or to offset the costs associated with using only expensive zeolites in the composite. Specific examples of solid inorganic cocatalysts include, but are not limited to, clay, Ca(OH)2, Mg(OH)2, Ba(OH)2, Sr(OH)2, CaO, Al2O3, and Zr(HPO4)2. In other embodiments, the composite catalyst is a zeolite and bentonite clay cocatalyst. The total amount of solid inorganic cocatalyst is 0% to 90% by weight of the composite catalyst.

[0069] The depolymerization catalyst is present in an amount of 20% or less by weight of the batched feed stream. Alternatively, the amount of the depolymerization catalyst is greater than 0% to 5% by weight of the batched feed stream. In another alternative, the depolymerization catalyst is present in an amount of 2% or 2.5% by weight of the batched feed stream. In some embodiments, the amount of the depolymerization catalyst is between 10 and 15% by weight of the batched feed stream.

[0070] The treated polyolefin stream and the depolymerization catalyst are fed to the depolymerization unit at a temperature of about 200° C. to about 600° C. Alternatively, the temperature of the depolymerization unit is about 225° C. to about 500° C. In yet another alternative, the temperature of the depolymerization unit is about 250° C. to about 450° C., or about 400° C. The treated polyolefin stream can be batch processed in the depolymerization unit due to the residence time required to fully depolymerize the stream. The estimated residence time for each batch is about 30 minutes to about 180 minutes depending on the heat transfer properties of the depolymerization unit. Alternatively, the estimated residence time is about 60 minutes.

[0071] This pretreatment method does not affect the depolymerization catalyst even if strong alkali is added to the aqueous solution. However, by reducing the presence of non-polyolefin materials that may inhibit the performance of the depolymerization catalyst, lower reaction temperatures can be used, resulting in less carbon dioxide production and a more efficient overall process. Thus, the amount of depolymerization catalyst used in the process of the present invention is not limited by the pretreatment method, but rather by the type and activity of the catalyst and the requirements of the depolymerization unit.

[0072] The pretreatment methods described above can be used to treat feed streams containing materials containing a single polyolefin component or any amount of a mixture of polyolefin components. Any polyolefin may be present in the feed stream, including but not limited to polyethylene (both high and low density), polypropylene, ethylene-propylene copolymers, polybutene-1, polyisobutene, and copolymers thereof. Furthermore, the feed stream is not limited to a particular form, so that films, foams, textiles, or other forms of material may be treated with the methods of the present application. The polyolefins may be obtained from waste streams, including post-consumer waste streams, post-industrial waste streams, or combinations thereof.

[0073] In some embodiments, the feed stream further comprises one or more non-polyolefin components that reduce the catalytic activity of the zeolite or clay in the depolymerization catalyst. Alternatively, the feed stream may further comprise one or more non-polyolefin components that produce decomposition products that reduce the catalytic activity of the zeolite or clay. Although many chemicals fall into this category, non-polyolefin polymers are most likely to be present in polyolefin-based feed streams, especially when the feed stream is a waste stream. In particular, non-polyolefin-based polymers with nitrogen or high oxygen content, such as polyaramids, acrylates, nylons, polyurethanes, cellulose, and polyvinyl polymers, may be present in the feed stream. These polymers are typically found at waste sites and are difficult to completely separate from the polyolefins. Many of these polymers decompose into problematic products that reduce the catalytic ability of the zeolite or clay, such as furfural, caprolactam, various amines, phenols, and esters. Alternatively, non-polyolefin components, such as nitrogen-containing pigments, may be present in polyolefin-based waste streams and reduce the catalytic activity of the zeolite or clay.

[0074] In other embodiments, the feed stream has up to 49 wt% non-polyolefin components before being treated with the methods described herein. After the pretreatment methods described herein, the treated polyolefin feed is at least 75 wt% polyolefin, and in some embodiments, at least 95 wt% polyolefin.

[0075] The pretreatment method disclosed herein is illustrated based on the following examples. These examples are included to illustrate the embodiments of the appended claims. However, these are merely illustrative, and the present disclosure can be broadly applied to any combination of polyolefin-based feeds with and without non-polyolefin components. Those skilled in the art will understand that many changes can be made to the specific embodiments disclosed to obtain similar results without departing from the spirit and scope of the disclosure herein. The following examples should not be read in any way to limit or define the scope of the appended claims. Working Example

[0076] Various polyolefin-based feedstocks were pretreated, depolymerized, and analyzed according to the described methods to evaluate the ability of the pretreatment methods to reduce the depolymerization temperature.

[0077] Unless otherwise stated, the depolymerization unit was a thermogravimetric analysis (TGA) instrument. For TGA pyrolysis reactions, homogeneous samples were heated under nitrogen at 10 K / min to the depolymerization temperature of 400 °C and held for 1 h in a Mettler Toledo TGA / DSC 3+ (Mettler Toledo, Columbus, OH). The depolymerization half-life at a particular temperature was defined as the time required to achieve 50% mass loss and was either recorded directly if the value was less than 60 min or calculated as t 1 / 2= It is determined to be 0.693 / k, where k is the first order rate constant plotted against the time chart using Ln(C / C).

[0078] The depolymerization half-life is related to the residence time required in 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 faster the depolymerization rate k. Example 1

[0079] The depolymerization of post-consumer polyolefin waste has been complicated by the inability to obtain a feed stream containing only polyolefins. Even if the waste undergoes multiple separation steps at landfills or recycling centers, some non-polyolefin materials may remain in the waste feed. Non-polyolefin polymers in particular may interfere with catalysts commonly used for the depolymerization of polyolefins, such as zeolites. Polymers with high nitrogen and oxygen content, such as aramids, acrylates, polyurethanes, cellulose, and polyvinyl polymers, are known to "poison" the catalytic ability of zeolites.

[0080] The pretreatment process described thus far has been used for evaluation using waste feed 1, which is a post-consumer mixture having 82 wt% polyolefins and 18 wt% non-polyolefin materials. The pretreatment process was applied to waste feed 1 prior to thermal depolymerization using a selection of zeolite-based composite catalysts, and further tests were performed to evaluate its ability to remove non-polyolefin components.

[0081] In this example, two different pretreatment solutions were used: neutral pH and alkaline pH. In the pretreatment process using the neutral pH pretreatment solution, a batch of waste feed 1 was suspended and stirred in a container filled with water. After 3 hours, the "treated" material was removed from the surface of the solution and dried overnight at 60°C without any additional washing steps.

[0082] For the alkaline pH pretreatment process, waste feed 1 (213.5 g) was suspended in a vessel filled with 1 liter of water. Calcium hydroxide (Ca(OH)2) (30 g) was slowly added to the stirred slurry (pH ~12) at ambient temperature, followed by heating at 80 °C for 3 h. After 3 h, the "treated" material was removed from the surface of the solution and dried overnight at 60 °C without any additional washing steps. After drying, 5 g of the treated feed was analyzed and depolymerized as follows:

[0083] The treated feed was depolymerized in TGA in the presence of the composite catalyst. Table 1 shows the pyrolysis polymerization results of treated feeds from both processes and untreated waste feed 1. [Table 2]

[0084] The results in Table 1 indicate that pretreatment of waste feed 1 can shorten the depolymerization half-life, regardless of the pretreatment solution, compared to untreated waste feed 1. This improvement is due to the removal of heavier non-polyolefin materials from waste feed 1, which is most evident in the composition depolymerized in the presence of the β-alkali complex catalyst.

[0085] The depolymerization half-life of raw waste feed 1 without composite catalyst was about 108 minutes (Comparative Example 1). By adding β-alkali composite catalyst to raw waste feed 1, the depolymerization half-life can be shortened by about 3 minutes. However, when waste feed 1 was pretreated with an alkaline solution, the depolymerization half-life, which was about 108 minutes in Comparative Example 1, was shortened by more than 98% to about 1.3 minutes in Composition 1.

[0086] A similar decrease in the depolymerization half-life was observed when ZSM-5-based catalyst was used. The addition of a composite catalyst with only ZSM-5 zeolite reduced the depolymerization half-life by up to 54% (Comparative Example 5). However, pretreatment of waste feed 1 further reduced the depolymerization half-life. As shown in Table 1, the use of a pretreatment solution with neutral or alkaline pH and a ZSM-5-based catalyst reduced the depolymerization half-life by about 63%, from 108.3 to 40.1 minutes for composition 2 and 40.8 minutes for composition 3. Further improvement in the depolymerization half-life was observed for compositions with the addition of an inorganic co-catalyst to the composite catalyst.

[0087] Thus, both neutral and alkaline pH could be used to improve the depolymerization reaction by reducing the amount of heavy non-polyolefin materials in the feed stream. Furthermore, neither pretreatment solution inhibited the composite catalyst, even though a washing step was not utilized.

[0088] To further evaluate the differences between untreated and treated waste feed 1, the amount of solid residue was measured. During thermal depolymerization, the polyolefins were mostly converted to liquid products and the inorganic materials remained as solids. Therefore, the amount of solid residue provides an estimate of the amount of non-polyolefin material in the feed.

[0089] In this analysis, 5 g of waste feed 1 (treated or untreated) was placed in a quartz pyrolysis tube and heated in a furnace at 10°C / min to a furnace setpoint of 700°C. N2 or 5% O2 / N2 gas was used to purge the contents of the quartz pyrolysis tube from the bottom at a flow rate of 15 sccm. Condensable products were collected in a Hickman trap cooled with dry CO2. The temperature of the reaction mixture when the first signs of liquid collection were observed was measured with a thermocouple placed at the bottom of the quartz pyrolysis tube. The results of this solid residue analysis are shown in Table 2. [Table 3]

[0090] The results in Table 2 show that the pretreatment process effectively removed non-polyolefin materials. The amount of solid residue was reduced by 75% and the liquid yield was increased by 42%. In addition, the amount of nitrogen in the liquid product was significantly reduced after the pretreatment process, which means the reduction of non-polyolefin materials.

[0091] In addition to the solid residue, coke production and ash production were also measured. Ash is produced by inorganic material present during the depolymerization reaction, which requires the depolymerization unit to be taken off-line to remove the accumulated ash. The results are shown in Table 3. [Table 4]

[0092] Pretreatment of waste feed 1 reduced the combined coke and ash production by approximately 81% between Comparative Example 2 and Composition 1. Ash formation itself was reduced by over 83%. The presently described pretreatment method not only reduced ash (formed from inorganic materials present in the reactor), but also reduced coke formation. Example 2

[0093] The pretreatment method was also applied to waste feed 2. In this example, a pretreatment solution of 20% NaOH in water was compared to the Ca(OH)2 pretreatment solution described in Example 1.

[0094] For the pretreatment process using NaOH pretreatment solution, a batch of waste feed 2 was suspended in a vessel filled with 20% NaOH solution (pH>14) and stirred for 3 hours at 80° C. After 3 hours, the "treated" material was removed from the surface of the solution and dried overnight at 60° C. without any additional washing steps.

[0095] The treated feed and untreated waste feed 2 were then depolymerized in TGA in the presence of the composite catalyst. The results for waste feed 2 are shown in Table 4. [Table 5]

[0096] The pretreatment process reduced the depolymerization half-life by more than 50%, similar to the results in Example 1. Furthermore, changing the alkali used in the pretreatment solution did not adversely affect the depolymerization process.

[0097] The above examples show that the pretreatment process described herein promotes a more energy-saving (and therefore more cost-effective) depolymerization process. Both neutral and alkaline pH pretreatment solutions were used to improve the depolymerization reaction by reducing the amount of heavier non-polyolefin materials in the feed stream. Furthermore, neither pretreatment solution inhibited the composite catalyst, even though a washing step was not utilized. The pretreatment process also reduced the formation of solid residues, coke formation, and ash production during depolymerization. These results demonstrated that the pretreatment process not only improved the process yield, but also extended the depolymerization cycle.

Claims

1. 1. A method for depolymerizing polyolefins, comprising the steps of: a) adding a polyolefin-based feed stream to a first vessel filled with an aqueous solution; b) stirring the aqueous solution and the polyolefin-based feed stream for at least 0.5 hours; c) skimming the surface of said aqueous solution to remove at least one polyolefin material suspended therein; d) drying the polyolefin material until a residual moisture of less than 5% is obtained; e) adding the dry polyolefin material and the depolymerization catalyst to a reactor heated to about 200° C. to about 600° C.; a) reacting said dry polyolefin material with said depolymerization catalyst to depolymerize said dry polyolefin material.

2. The method of claim 1 , wherein the aqueous solution comprises a strong alkali.

3. 3. The method of claim 2, wherein the strong alkali is calcium hydroxide, sodium hydroxide, potassium hydroxide, barium hydroxide, lithium hydroxide, or strontium hydroxide.

4. 3. The method of claim 2, wherein the strong alkali is about 5% to about 40% of the aqueous solution.

5. The method of claim 1 , wherein the stirring step is carried out under a pressure of about 0.1 MPa to about 0.2 MPa.

6. 10. The method of claim 1, wherein the agitating step further comprises heating the aqueous solution and the polyolefin-based feed stream to a temperature of from greater than 25° C. to about 150° C. with agitation.

7. 10. The method of claim 1, wherein the polyolefin-based feed stream is a post-consumer waste stream, a post-industrial waste stream, or both.

8. 2. The method of claim 1, wherein the depolymerization catalyst comprises at least one zeolite and at least one solid inorganic co-catalyst, the zeolite being selected from the group consisting of beta zeolite, zeolite Socony Mobil-5 (ZSM-5), zeolite Y, or ultrastable zeolite Y, or a combination thereof, and the at least one solid inorganic co-catalyst is a metal oxide, metal hydroxide, metal carbonate, silicate, clay, or tetravalent metal phosphate.

9. The at least one solid inorganic cocatalyst is selected from the group consisting of bentonite, Ca(OH) ２ , Mg(OH) ２ , Ba(OH) ２ , Sr(OH) ２ , CaO, Al ２ O ３ , and Zr(HPO ４ ) ２ The method of claim 8, wherein the compound is selected from the group consisting of:

10. 1. A method for pretreating a polyolefin-based feed stream prior to depolymerization, comprising: a) adding a polyolefin-based feed stream to a first vessel filled with an aqueous solution; b) stirring the aqueous solution and the polyolefin-based feed stream for at least 0.5 hours; c) skimming the surface of said aqueous solution to remove at least one polyolefin material suspended therein; d) drying the polyolefin material until a residual moisture of less than 5% is obtained.

11. The method of claim 10 , wherein the aqueous solution comprises a strong alkali.

12. 12. The method of claim 11, wherein the strong alkali is calcium hydroxide, sodium hydroxide, potassium hydroxide, barium hydroxide, lithium hydroxide, or strontium hydroxide.

13. The method of claim 11, wherein the strong alkali is about 5% to about 40% of the aqueous solution.

14. The method of claim 11, wherein the stirring step is carried out under a pressure of about 0.1 MPa to about 0.2 MPa.

15. 11. The method of claim 10, wherein the aqueous solution is heated to a temperature of greater than 25° C. to about 150° C. prior to the adding step, and the stirring step further comprises maintaining the temperature of the heated aqueous solution while stirring.

16. 11. The method of claim 10, wherein the agitating step further comprises heating the aqueous solution and the polyolefin-based feed stream to at least 80°C while agitating.

17. The method of claim 10, wherein the drying step is carried out at a temperature of at least 50°C.

18. 11. The method of claim 10, wherein the drying step is carried out until the polyolefin material has a residual moisture content of less than 3%.

19. 11. The method of claim 10, wherein the polyolefin based feed stream is a post-consumer waste stream, a post-industrial waste stream, or both.

20. 1. A method for pretreating a polyolefin-based feed stream prior to depolymerization, comprising: a) adding a polyolefin-based feed stream to a first vessel filled with an aqueous solution having a pH greater than 9; b) agitating the aqueous solution and the polyolefin-based feed stream for at least 2 hours while maintaining the heat of the aqueous solution at a temperature of at least 70° C.; c) skimming the surface of said aqueous solution to remove at least one polyolefin material suspended therein; d) drying said polyolefin material at a temperature of at least 50° C. until a residual moisture of less than 5% is obtained.