System and method for cooling steam cracker effluent using a heavy oil fraction derived from polystyrene pyrolysis

Abstract

A method for recycling polystyrene solid plastic waste (SPW) includes converting the polystyrene SPW into a pyrolysis oil in a primary conversion system. The pyrolysis oil includes a mixture of monocyclic aromatic hydrocarbons and polycyclic aromatic hydrocarbons. The method also includes separating the monocyclic aromatic hydrocarbons from the polycyclic aromatic hydrocarbons in a distillation unit downstream of the primary conversion system to generate a light fraction having the monocyclic aromatic hydrocarbons and a heavy fraction having the polycyclic aromatic hydrocarbons and feeding the heavy fraction to a steam cracker unit.

Description

SYSTEM AND METHOD FOR COOLING STEAM CRACKER EFFLUENT USING A HEAVY OIL FRACTION DERIVED FROM POLYSTYRENE PYROLYSISBACKGROUND OF THE DISCLOSURE

[0002] The present disclosure generally relates to chemical recycling of solid plastic waste. More specifically, the present disclosure relates to a system and process for recovering and recycling aromatics from a polystyrene-derived pyrolysis oil stream.

[0003] Plastics are used in a wide variety of products ranging from packaging materials, textiles, consumer products, and electronics, among others. Within the general group of plastics, there exists a class of materials called polyolefins which are composed of polymerized monomers, such as ethylene, propylene, and styrene, derived from hydrocarbons such as oil, natural gas and / or coal. These materials do not easily degrade, and a large fraction of the growing volume of plastic items produced annually may accumulate in the environment over time. Therefore, to minimize the impact of solid plastic waste (SPW) on our environment, the SPW may be recycled and reused to create post-consumer products.

[0004] One technique for recycling SPW is chemical recycling. This process depolymerizes polymers in the SPW into their respective monomers or oligomers that may then be used as petrochemical feedstock to create other products such as, for example, chemicals, fuels, and renewed plastics that have substantially identical characteristics, and thus performance, as the original materials used to make the plastic before it was recycled. This process includes a primary conversion step to produce a plastic-derived oil, also known as pyrolysis oil. The primary conversion step may include thermal and / or catalytic processes, such as, but not limited to, pyrolysis, hydrothermal liquefaction (HTL), and hydrogenolysis among others. Following the primary conversion step, the pyrolysis oil is sent to a steam cracker to break apart the oligomers and other hydrocarbons, thereby producing the respective monomers. Prior to the steam cracker, the pyrolysis oil may be pre-treated to remove contaminants and ready the pyrolysis oil for utilization in the steam cracker.

[0005] Pyrolysis oil suitable for the steam cracker include those derived from liner hydrocarbons such as polyethylene (PE) and polypropylene (PP) SPW. Polystyrene (PS) containsrepeating units of the aromatic hydrocarbon styrene. The aromatic nature of PS makes PS pyrolysis oil unsuitable for the steam cracker because the aromatics overload the steam cracker and cause fouling. Therefore, PS SPW is separated from PE and PP SPW. However, PS-derived pyrolysis oil contains valuable aromatic components (e.g., monocyclic and polycyclic aromatics and polyaromatics) that may be used as fuels, cooling agents, cleaning agents, chemical building blocks, and much more. Accordingly, it may be advantageous to recovery both monocyclic and polycyclic aromatics from PS-derived pyrolysis oil to generate a complete circular economy for polystyrene.SUMMARY

[0006] In an embodiment, a method for recycling polystyrene solid plastic waste (SPW) includes converting the polystyrene SPW into a pyrolysis oil in a primary conversion system. The pyrolysis oil includes a mixture of monocyclic aromatic hydrocarbons and polycyclic aromatic hydrocarbons. The method also includes separating the monocyclic aromatic hydrocarbons from the polycyclic aromatic hydrocarbons in a distillation unit downstream of the primary conversion system to generate a light fraction having the monocyclic aromatic hydrocarbons and a heavy fraction having the polycyclic aromatic hydrocarbons and feeding the heavy fraction to a steam cracker unit.

[0007] In another embodiment, a system for recycling polystyrene solid plastic waste (SPW) includes a primary conversion system that may receive and convert the polystyrene SPW into a pyrolysis oil having a mixture of monocyclic aromatic hydrocarbons and polycyclic aromatic hydrocarbons, a distillation unit disposed downstream from the primary conversion system and that may separate the monocyclic aromatic hydrocarbons from the polycyclic aromatic hydrocarbons and may generate a light fraction having the monocyclic aromatic hydrocarbons and a heavy fraction having the polycyclic aromatic hydrocarbons, and a steam cracker unit disposed downstream from the distillation unit and having a quench loop that may cool an effluent generated in a furnace of the stream cracker unit. The quench loop includes a quench oil having a portion of the heavy fraction.

[0008] Additional features and advantages of exemplary implementations of the disclosure will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of such exemplary implementations. The features and advantages of such implementations may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features will become more fully apparent from the following description and appended claims or may be learned by the practice of such exemplary implementations as set forth hereinafter.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Advantages of the disclosure may become apparent upon reading the following detailed description and upon reference to the drawings in which:

[0010] FIG. 1 is a block diagram of a system for recovering and recycling aromatics derived from polystyrene (PS) solid plastic waste (SPW), whereby the system includes a steam cracker unit and a distillation unit followed by a hydrodealkylation unit, the distillation unit provides a PS heavy fraction to the steam cracker unit, in accordance with an embodiment of the present disclosure;

[0011] FIG. 2 is a block diagram of an alternate system for recovering and recycling aromatics derived from PS SPW, whereby the system includes the steam cracker unit and the hydrodealkylation unit followed by the distillation unit, in accordance with an embodiment of the present disclosure;

[0012] FIG. 3 is a block diagram of the steam cracker unit of FIGS. 1 and 2, whereby the steam cracker unit includes a quench loop that receives the PS heavy fraction, in accordance with an embodiment of the present disclosure; and

[0013] FIG. 4 is flow diagram of a method for recovering and recycling aromatics derived from PS SPW using the systems of FIGS. 1-3, in accordance with an embodiment of the present disclosure.DETAILED DESCRIPTION

[0014] One or more specific embodiments of the present disclosure will be described below. These described embodiments are examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description of these embodiments, not all features of an actual implementation may be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions will be made to achieve the developers’ specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

[0015] When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.

[0016] The terms “approximately,” “about,” and “substantially” as used herein represent an amount close to the stated amount that still performs a desired function or achieves a desired result. For example, the terms “approximately,” “about,” and “substantially” may refer to an amount that is within less than 10% of, within less than 5% of, within less than 1% of, within less than 0.1% of, and within less than 0.01% of a stated amount. The terms “plastic-derived oil”, “liquid synthetic crude”, or “synthetic crude,” “pyrolysis oil,” liquid pyrolysis product stream,” or the like as used herein are intended to denote a liquid phase mixture derived from thermal and / or chemical conversion (e.g., pyrolysis, hydro pyrolysis, hydrothermal liquefaction (HTL), hydrogenolysis, etc.) of solid plastic waste (SPW).

[0017] SPW may be converted, via pyrolysis or other thermal or chemical primary conversion step, followed by subsequent processing steps, into high-value chemicals, including olefins and hydrocarbon fuels. Primary conversion of waste plastics yields primarily liquid product streams having a wide boiling range (e.g., between approximately 20 degrees Celsius (°C) and 750 °C), as well as gaseous, and often solid product streams. The liquid product streams, or plastic-derived oil, may include hydrocarbons across a wide boiling point range (e.g., naphtha, diesel, gasoil, and hydrowax), which may be further distilled into individual fractions, or processed directly in a steam cracker, hydrocracker, or fluid catalytic cracker (FCC) to produce high-value chemicals, and other hydrocarbons. For example, the plastic-derived oil may be used to produce ethylene, propylene, and / or butylene, which are monomers that may be used as building blocks for new plastics. Unlike polyethylene (PE) and polypropylene (PP), which are linear long chain polymers, polystyrene (PS) consists of the aromatic hydrocarbon styrene. Therefore, in the primary conversion step, the resultant liquid product (e.g., pyrolysis oil) contains monocyclic aromatics (e.g., toluene, xylene, benzene, ethylbenzene etc.), polyaromatics (e.g., monoaromatics connected by alkyl and / or olefin chains), and polycyclic aromatics (e.g., fused di-, tri-, tetra-, and heavier aromatics). The aromatic content of PS-derived pyrolysis oil renders it unsuitable for processing in a steam cracker as the steam cracker would be overloaded with aromatics causing the formation of additional polycyclic aromatics and fouling of steam cracker equipment. Moreover, the polycyclic aromatics in PS-derived pyrolysis oil are generally considered as not having any value outside of fuels. The limited value of the aromatics contained in PS-derived pyrolysis oil renders it unattractive for recycling. Therefore, PS is generally separated from PE and PE SPW prior to chemical recycling. However, it has now been recognized that the aromatics contained in the PS-derived pyrolysis oil may be recovered and used in various applications to create a circular economy for PS.

[0018] FIG. 1 is a block diagram of an embodiment of a chemical recycling system 10 that may be used for recovering and recycling aromatics in a PS-derived pyrolysis oil. In the illustrated embodiment, a PS SPW feedstock 12 is fed to a primary conversion system 16 where the PS SPW feedstock 12 undergoes thermal or chemical conversion to generate a PS pyrolysis oil 18 (e.g., a liquid pyrolysis product stream). For example, in one embodiment, the primary conversion system 16 may include a hydrothermal liquefaction unit that thermally depolymerizes the PS SPWfeedstock 12 to generate the PS pyrolysis oil 18. In another embodiment, the primary conversion system 16 includes a pyrolysis reactor or any other suitable reactor that generates the PS pyrolysis oil 18. In the primary conversion system 16, the PS SPW feedstock 12 is heated to between approximately 300 to 800 degrees Celsius (°C) in the absence of oxygen to degrade the carboncarbon bonds, depolymerizing PS and generating the PS-derived pyrolysis oil 18. The PS-derived pyrolysis oil 18 contains a range of polyaromatics and monocyclic and polycyclic aromatic hydrocarbons having boiling point ranges between approximately 60° C and 800° C. Before feeding to the primary conversion system 16, the PS SPW feedstock 12 may be sorted to remove other SPW (e.g., PE, PP, polyurethane (PU), etc.), washed to remove residue and other postconsumer contaminants, crushed, or undergo any other process to prepare the PS SPW feedstock 12 for chemical recycling, and combinations thereof.

[0019] As discussed above, the PS pyrolysis oil 18 contains a range of monocyclic / light and polycyclic / heavy aromatic hydrocarbons. By way of non-limiting example, the monocyclic / light aromatic hydrocarbons include toluene, benzene, ethylbenzene, xylene, styrene, and di-alkyl monoaromatics. The polycyclic / heavy aromatic hydrocarbons include, but are not limited to, oligosytrene, di-aromatics (e.g., diphenyl ethane, diphenyl propane, and diphenyl pentane), tri-aromatics, and tetra-aromatics among others. As discussed above, the aromatic content of PS -pyrolysis oil, such as the PS-pyrolysis oil 18, makes it unsuitable for treating in a steam cracker. While processes exist for recovering and recycling the monocyclic / light aromatic hydrocarbons, the polycyclic / heavy aromatic hydrocarbons have not been considered to be of value. However, these polycyclic / heavy aromatic hydrocarbons are useful in fuel applications and have properties that render them valuable for use as viscosity diluents and / or cleaning agents. Therefore, it is advantageous to recover these polycyclic / heavy aromatics.

[0020] For example, the polycyclic / heavy aromatic hydrocarbons may be used as a viscosity diluent in steam cracking processes. In steam cracking processes, effluent exiting furnaces of a steam cracker is cooled to avoid undesirable secondary cracking, condensation, and polymerization reactions. Therefore, steam cracker systems include a quench system having a primary fractionator (e.g., quench tower) and a quench loop that generate and circulate, respectively, a quench oil used to cool the effluent generated in the steam cracker furnaces. Thequench oil is derived from the effluent exiting the steam cracker furnaces, as discussed in further detail below with reference to FIG. 3. Effluents generated in the steam cracker contain gases (e.g., H2 and C1-4 gases), light liquid ends (e.g., C5-C7 hydrocarbons) and heavy ends (e.g., hydrocarbons having 8 or more carbon atoms). The quench oil contains the heavy ends of the effluent. As discussed in further detail below, the polycyclic / heavy aromatic hydrocarbons may be added to the quench oil to maintain a suitable viscosity of the quench oil that keeps certain molecules (e.g., polycyclic aromatics (PC As)) in the quench oil in suspension.

[0021] The heavy ends contain a) light fuel oil (LFO) having hydrocarbons boiling in the range of approximately 200 °C to 300 °C, b) middle fuel oil (MFO) having hydrocarbons boiling in the range of approximately 300 °C to 450 °C, and c) heavy fuel oil (HFO) or pitch having hydrocarbons boiling above 450 °C, each having various amounts of linear and aromatic hydrocarbons. The distribution of hydrocarbons in these oils determine the viscosity of the quench oil. To maintain a desired flow / pump capacity and / or heat transfer properties, as well as mitigate fouling caused by deposits, it is important that a viscosity of the quench oil is maintained in a range of between approximately 1 to 85 centistokes (cSt) at 100 °C. The viscosity of the quench oil is dependent on several factors such as, for example, feed slates, cracking conditions, and throughput, among others. Additives may be added to the quench oil to manage and maintain a desired viscosity. This, however, increases the overall operational costs for steam cracking. Moreover, certain additives or diluents may introduce undesirable components into the quench oil. This may result in the quench oil having an undesirable viscosity such that it may be difficult to pump and flow through quench oil loop. In certain existing systems, oils such as light cycle oil (LCO) are used as a quench oil diluent to decrease the viscosity of the quench oil. However, these oils and / or LCO may have undesirable sulfur levels, resulting in the presence of sulfur in quench oil and pitch that may exceed regulated SOXemission limits when pitch is used as fuel. The aromatic hydrocarbons in the PS pyrolysis oil are good quality diluents that may be used to modify and / or maintain the quench oil quality. Additionally, PS pyrolysis oil is essentially free of sulfur (e.g., less than 200 ppm). Therefore, to mitigate the undesirable viscosity and sulfur levels of quench oils in the quench loop of steam cracker units used for chemical recycling of the PE- and PP- derived pyrolysis oil, at least a portion of the PS pyrolysis oil 18 may be mixed with the PE- and PP-derived pyrolysis oils. Accordingly, in one embodiment of the present disclosure, at least aportion of the PS pyrolysis oil 18 is fed to a steam cracker unit 24, as discussed in further detail below with reference to FIG. 3.

[0022] Prior to feeding to the steam cracker 24, the PS pyrolysis oil 18 may be treated in a distillation unit 26. For example, as discussed above, the PS pyrolysis oil 18 contains a mixture monocyclic / light aromatic hydrocarbons and polycyclic / heavy aromatic hydrocarbons. These aromatic hydrocarbons are separated into light and heavy fractions in the distillation unit 26 to yield a PS light fraction 30 having the monocyclic aromatic hydrocarbons and a PS heavy fraction 32 having the polycyclic aromatic hydrocarbons, respectively. Additional fractions may be separated from the PS pyrolysis oil 18 without departing from the scope of the present disclosure. The distillation unit 26 may be any suitable distillation unit having one or more distillation columns used for separating the various hydrocarbons in the PS pyrolysis oil 18 into the desired fractions according to their respective boiling point ranges. The monocyclic aromatic hydrocarbons in the PS light fraction 30 include, but are not limited to, ethylbenzene, benzene, styrene, xylene, and toluene, among other monoaromatic hydrocarbons. The polycyclic aromatic hydrocarbons in the PS heavy fraction 32 include, but are not limited to, diphenyl ethane, diphenyl propane, diphenyl pentane, oligosytrene, and other di-, tri- and tetra-aromatic hydrocarbons. Following separation of the fractions 30, 32, the PS heavy fraction 32 is fed to the steam cracker unit 24 for use as a quench oil diluent or quality control agent (e.g., to control viscosity, suspension of PC As, etc.). In certain embodiments, the PS heavy fraction 32 or the PS pyrolysis oil 18 may be used as a cleaning agent, for example, in equipment which has been in contact with heavy residue / bitumen streams. For example, the PS pyrolysis oil 18 or the PS heavy fraction 32 may collected in a storage tank and transported to facilities in which heavy residue / bitumen streams are processed. In other embodiment, a portion of the PS pyrolysis oil 18 or a portion of the PS heavy fraction 32 may be fed to equipment within the system 10 that has been in contact with the heavy residue / bitumen stream. In certain embodiments, the PS pyrolysis oil 18 may be upgraded before feeding to the distillation unit 26 to remove undesirable components that are not removed during distillation.

[0023] As discussed above, the PS-derived light fraction 30 contains monocyclic aromatic hydrocarbons, such as toluene, ethylbenzene, and xylene, which may be converted into valuable hydrocarbons. These monocyclic aromatic hydrocarbons are generally derived from fossil fuels.However, by isolating and recovering these monocyclic aromatic hydrocarbons from the PS- derived light fraction 30 , PS SPW may be entered into a circular economy in the chemical industry. Accordingly, in the illustrated embodiment, the PS-derived light fraction 30 is fed to a hydrodealkylation unit 36 to convert alkylated aromatic hydrocarbons into benzene and other valuable products. The hydrodealkylation unit 36 includes one or more reactors / vessel having one or more catalysts that convert the alkylated aromatics into dealkylated aromatics to generate a first dealkylated effluent 38. An aromatic recovery unit 40 receives the first dealkylated effluent 38 to separate and recover benzene 46 from the first dealkylated effluent 38. The benzene 46 may be used to generate styrene 44, which in turn is used to produce polystyrene 48 or for other chemical applications (e.g., in fuels, solvents, etc.). The first dealkylated effluent 38 also includes linear hydrocarbons. Therefore, the aromatic recovery unit 40 outputs paraffins 50 as a by-product stream. As should be noted, the paraffins 50 may contain other hydrocarbons such as, for example, olefins. The paraffins 50 (e.g., light paraffins) may be fed to the steam cracker unit 24 to break down the paraffins into smaller alkene fragments such as, for example, ethylene and propylene. In certain embodiments, the PS light fraction 30 may be fed to the aromatic recovery unit 40 before hydrodealkylation to separate and recover certain alkylated aromatic hydrocarbons. For example, before hydrodealkylation, aromatics such as toluene and xylene may be separated and recovered from the PS light fraction 30. These alkylated aromatic hydrocarbons may be used as solvents or in chemical processes.

[0024] In certain embodiments, the PS pyrolysis oil 18 is dealkylated before undergoing distillation. For example, FIG. 2 illustrates an embodiment of a system 60 in which the hydrodealkylation unit 36 is located upstream of the distillation unit 26. In this embodiment, the PS pyrolysis oil 18 undergoes hydrodealkylation to generate a second dealkylated effluent 54. Similar to the hydrodealkylation of the PS light fraction 30, the hydrodealkylation unit 36 dealkylates the monocyclic and polycyclic aromatics in the PS pyrolysis oil 18, thereby generating the second dealkylated effluent 54. The second dealkylated effluent 54 includes benzene, dealkylated polyaromatics, and paraffins among others. These hydrocarbons are separated in the distillation unit 26 into various fractions based on their boiling point ranges. For example, the distillation unit 26 separates the dealkylated effluent 54 into the light PS fraction 30, the paraffins 50 (e.g., light paraffins such as ethane, propane, butane and up to Cs hydrocarbons), and the PSheavy fraction 32. The distillation unit 26 may output additional fractions without departing from the scope of the present disclosure. Similar to the embodiment of FIG. 1, the PS heavy fraction 32 is directed and fed to the steam cracker unit 24 and, in some embodiments, the light fraction of the paraffins 50. The PS light fraction 30 is fed to the aromatic recovery unit 40 to recover the benzene 46.

[0025] As discussed above, polyaromatics in the PS heavy fraction 32 make good quality diluents that may be used to manage and maintain a desired viscosity of the quench oil. The viscosity of the quench oil used in steam cracking processes is important for flow and efficient heat recovery. As the pyrolysis oil derived from linear feeds (e.g., PE and PP pyrolysis oil) may primarily contain linear hydrocarbons, the viscosity of the quench oil derived from these feeds may be difficult to manage due to a reduced amount of aromatic components present in that pyrolysis oil. In addition, an amount of polyaromatic hydrocarbons present in quench oils may be such that suspended components in the quench oil separate out and flocculate. Accordingly, at least a portion of the PS heavy fraction 32 may be used as a diluent stream for the quench oil to adjust and yield a desirable viscosity and mitigate separation and flocculation of suspended components. FIG. 3 is a block diagram of the steam cracker unit 24 that may receive the PS heavy fraction 32. The steam cracker unit 24 includes one or more furnaces 82, a quench system 84 having a primary fractionator 86, a heavy oil stripper 90, a quench water tower 92, and a quench loop 94. In the illustrated embodiment, the one or more furnaces 82 receive a polyethylene (PE)- or polypropylene (PP)-derived pyrolysis oil 96 and crack the hydrocarbons in the PE / PP-derived pyrolysis oil 96 into smaller hydrocarbon fragments to generate a steam cracker (SC) effluent 98. As should be appreciated, while the present embodiment is discussed in the context of using a PE- or PP-derived pyrolysis oil, any other liquid hydrocarbon suitable for cracking in the steam cracker unit 24 may be used in combination with or instead of the PE / PP-derived pyrolysis oil. The PE / PP- derived pyrolysis oil 96 is different from the PS pyrolysis oil 18 in that it is derived from PE, PP, or the like, and mixtures thereof. As such, the PE / PP-derived pyrolysis oil 96 has a linear hydrocarbon content of greater than approximately 95 wt%. As the PE / PP-derived pyrolysis oil 96 contains little to no aromatic hydrocarbons (e.g., less than approximately 5 wt%), it is suitable as feed for steam cracking in the furnaces 82. In addition to the PE / PP-derived pyrolysis oil 96, the furnaces 82 may receive the paraffins 50 (e.g., a light paraffin fraction) separated from the PSpyrolysis oil (e.g., the PS pyrolysis oil 18) discussed above with reference to FIGS. 1 and 2. By feeding the paraffins 50 derived from PS pyrolysis oil to the furnaces 82, valuable products may be generated from PS SPW that would otherwise be disposed of. In the furnaces 82 , the pyrolysis oil 96 is heated to temperatures between 700 °C and 1100 °C to transform oligomers in the pyrolysis oil 96 and the paraffins 50 into monomers and other small hydrocarbons (e.g., ethylene, propylene, and other alkenes) to generate the SC effluent 98. Accordingly, the SC effluent 98 exiting the furnaces 82 is above a desired temperature and is, therefore, cooled to less than 500 °C before being fed to the primary fractionator 86 of the quench system 84. Prior to cooling, a certain amount of heat may be recovered from the SC effluent 98 to generate steam used in other processes throughout the system.

[0026] The primary fractionator 84 separates hydrocarbons in the SC effluent 98 into various fractions based on boiling point ranges. For example, the SC effluent 98 is separated into a primary fractionator (PF) heavy oil 100 and an overhead gas 102. The overhead gas 102 has a temperature range of between approximately 100 to 110 °C and is fed to the quench water tower 92 to generate C2-C6 hydrocarbons 104 such as ethylene, propylene, and gasoline, among other light hydrocarbons and H2. A portion of the PF heavy oil 100 flows into the quench oil loop 94 as PF quench oil 110 and another portion of the PF heavy oil 100 is directed to the heavy oil stripper 90. As discussed above, heavy oil generated in the primary fractionator contains light fuel oil (LFO), medium fuel oil (MFO), and heavy fuel oil (HFO). The heavy oil stripper 90 strips the LFO from the HFO in the PF heavy oil 100, to generate a stripper LFO 112 and a stripper HFO 114, respectively. The viscosity of the PF quench oil 110 is determined based on the amount of LFO and MFO. As the viscosity of the PF quench oil 110 should be maintained sufficiently low (e.g., 1- 85 cS @ 100C) to mitigate flow problems in the quench loop 94, in certain embodiments, the stripper LFO 112 may be circulated back to the primary fractionator 86. The more LFO and MFO present in the PF quench oil 110, the lower the viscosity will be. However, the LFO in the PF quench oil 110 may vaporize, thereby decreasing its concentration in the PF quench oil 110 and undesirably increasing the viscosity. Therefore, the stripper LFO 112 is circulated back to the primary fractionator 86 to increase the concentration of the LFO in the PF heavy oil 100 and, consequently, the PF quench oil 110.

[0027] While the presence of LFO in the PF quench oil 110 may facilitate maintaining the viscosity at desirable levels, certain feeds may not generate a sufficient amount of LFO in the furnaces 82. Additionally, such cracked feeds contain little to no aromatics (e.g., less than approximately 5 wt%), which may lower the viscosity to the desired level. Certain existing systems and processes add LFO, or hydrocarbon streams that resemble LFO from processes such as catalytic crackers (e.g., FCC etc.) as a diluent to the PF quench fluid to decrease its viscosity to desired levels. However, as discussed above, certain LFO streams may have an undesirable content of sulfur that results in the presence of sulfur in fuels and resultant products that exceed regulated sulfur levels. Therefore, to mitigate undesirable increases in the viscosity of the PF quench oil 110 and avoid introducing undesirable sulfur, the PS heavy fraction 32 may be fed into the quench loop 94 and mixed with the PF quench oil 110. The PS heavy fraction 32 and the PF quench oil 110 is mixed with and cools the SC effluent 98 upstream of the primary fractionator 86. As discussed above, the polyaromatic hydrocarbons in the PS heavy fraction 32 are good diluents and may lower the viscosity of certain fluids. Additionally, these polyaromatic hydrocarbons may keep suspended components in the quench oil from separating out and flocculating. Therefore, by mixing the PS heavy fraction 32 with the PF quench oil 110, the aromatic content of the PF quench oil 110 is increased, certain molecules are kept in suspension and the viscosity may be maintained. In certain embodiments, the PS heavy fraction 32 is mixed with SC effluent 98 rather than feeding directing into the quench loop 94 and mixing with the PF quench oil 104. In other embodiments, the PS heavy fraction 32 may be mixed with the PF heavy oil 100. By feeding the PS heavy fraction 32 to the quench system 84, a viscosity of the PF quench oil 104 may be maintained at between approximately 1 and 85 cS @ 100C. In addition, such a process provides a valuable use for PS-derived heavy oil fractions, such as the PS heavy fraction 32, that are otherwise not available.

[0028] Present embodiments also include a method for chemical recycling of PS pyrolysis oil. FIG. 4 is a flow diagram of a method 150 that includes converting PS SPW (e.g., PS SPW 12) into PS pyrolysis oil (e.g., PS pyrolysis oil 16) (block 152). For example, as discussed above, after sorting, cleaning, and other pretreatment processes, the PS SPW undergoes primary conversion (e.g., in the primary conversion system 16) to break apart the PS polymer into smaller molecular fragments (e.g., lower molecular weight oligomers). The primary conversion mayinclude processes such as hydrothermal liquefaction, pyrolysis, or any other suitable process for converting the PS polymer into oligomers to generate the PS pyrolysis oil.

[0029] Following conversion of the PS SPW into PS pyrolysis oil, the method 150 includes recovering a PS heavy fraction (e.g., the PS heavy fraction 32), benzene (e.g., benzene 46), and paraffins (e.g., the paraffins 50) from the PS pyrolysis oil (block 156). In one embodiment, the PS pyrolysis oil is fed to a distillation unit (e.g., the distillation unit 26) in which it is separated into a light fraction (e.g., PS light fraction 30) and a heavy fraction (e.g., PS heavy fractions 32). The light fraction is fed to a hydrodealkylation unit (e.g., the hydrodealkylation unit 42) that dealkylates the components in the light fraction to generate benzene and paraffins. A portion of the recovered benzene may be used to synthesize styrene used to form polystyrene (e.g., the PS 44), and in turn, PS consumer products. In this way, PS may enter a circular economy for chemical recycling.

[0030] The method 150 also includes feeding the PS heavy fraction and paraffins to a steam cracker unit (e.g., the steam cracker unit 24) (block 168). Before feeding to the steam cracker, the paraffins may be separated into light and heavy paraffin fractions. The light paraffin fraction (e.g., Cs-Cs hydrocarbons) may be sent to the steam cracker unit to fragment (i.e., crack) the paraffins into smaller fragments (e.g., ethylene, propylene, butadiene, etc.).

[0031] The method 150 further includes feeding the PS heavy fraction into a quench oil loop (e.g., the quench oil loop 84) in the steam cracker unit and mixing with a quench oil (e.g., the PF quench oil 110) (block 172). As discussed above, quench oil used in the steam cracker unit includes the heavy fraction generated in a primary fractionator (e.g., the primary fractionator 84) of the steam cracker unit. This heavy fraction is derived from linear hydrocarbon feeds such as hydrowax, gasoils, PE and PP pyrolysis oil (e.g., the PE / PP pyrolysis oil 92), and the like. As such, a viscosity of the quench oil may be unsuitable for desired flow properties and heat recovery. The PS heavy fraction includes aromatic hydrocarbons that, when combined with the quench oil, may result in a desired viscosity. Therefore, the PS heavy fraction may be used as a diluent for the quench oil to maintain the viscosity of the quench oil at desired levels.

[0032] The technical effects of recycling PS SPW using the systems and method disclosed herein provides a circular economy for aromatic-rich polymers used in consumer products. PSpyrolysis oil includes di-, tri-, and heavier aromatics that have value not only in fuel applications, but in other applications such as viscosity diluents and cleaning agents. For example, PS pyrolysis oil may be processed to separate light monocyclic aromatic hydrocarbons from heavy polycyclic aromatic hydrocarbons. The light monocyclic aromatic hydrocarbons may be processed, separated according to boiling point ranges, and recovered to generate valuable monocyclic aromatic hydrocarbons such as benzene and toluene, which are generally derived from fossil fuels. The heavy polycyclic aromatic hydrocarbons may be used as diluents to decrease a viscosity of quench oils used in recycling of PE / PP SPW. The use of polycyclic aromatic hydrocarbons derived from PS pyrolysis oil as viscosity diluents is attractive as they are essentially free of sulfur. Certain existing viscosity diluents contain sulfur, thereby introduce sulfur into the system which may result in system products exceeding regulated sulfur levels. In addition to using as viscosity diluents, the PS pyrolysis oil as a whole or the separated heavy polycyclic aromatic hydrocarbons may be used as a cleaning agent for equipment that has been in contact with heavy residue / bitumen. Therefore, by using the systems and processes disclosed herein, PS SPW may be used to generate valuable aromatic hydrocarbons for used in various chemical processes, thereby making recycling of PS SPW attractive and into a circular chemical economy.

[0033] The present disclosure may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the disclosure is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Claims

CLAIMSWe claim:

1. A method for recycling polystyrene solid plastic waste (SPW), comprising: converting the polystyrene SPW into a pyrolysis oil in a primary conversion system, wherein the pyrolysis oil comprises a mixture of monocyclic aromatic hydrocarbons and polycyclic aromatic hydrocarbons; separating the monocyclic aromatic hydrocarbons from the polycyclic aromatic hydrocarbons in a distillation unit downstream of the primary conversion system to generate a light fraction comprising the monocyclic aromatic hydrocarbons and a heavy fraction comprising the polycyclic aromatic hydrocarbons; and feeding the heavy fraction to a steam cracker unit.

2. The method of claim 1, wherein the heavy fraction is fed into a quench loop within the steam cracker unit, and wherein the quench loop comprises a quench oil configured to cool an effluent generated in a furnace of the steam cracker unit.

3. The method of claim 1, comprising mixing the heavy fraction with a quench oil stream in the steam cracker, wherein the mixture is configured to cool an effluent generated in a furnace of the steam cracker unit.

4. The method of claim 1, comprising dealkylating the light fraction in a hydrodealkylation unit configured to dealkylate the monocyclic aromatic hydrocarbons and to generate a dealkylated effluent, wherein the dealkylated effluent comprises benzene.

5. The method of claim 4, comprising recovering benzene from the dealkylated effluent.

6. The method of claim 1, comprising dealkylating the pyrolysis oil in a hydrodealkylation unit before separating the monocyclic aromatic hydrocarbons from the polycyclic aromatic hydrocarbons, wherein the hydrodealkylation unit is configured to dealkylate the monocyclic and polycyclic aromatic hydrocarbons.

7. The method of claim 1, wherein the polycyclic aromatic hydrocarbons comprise diaromatic, tri-aromatic, and tetra-aromatic hydrocarbons.

8. A system for recycling polystyrene solid plastic waste (SPW), comprising: a primary conversion system configured to receive and convert the polystyrene SPW into a pyrolysis oil comprising a mixture of monocyclic aromatic hydrocarbons and polycyclic aromatic hydrocarbons; a distillation unit disposed downstream from the primary conversion system and configured to separate the monocyclic aromatic hydrocarbons from the polycyclic aromatic hydrocarbons and to generate a light fraction comprising the monocyclic aromatic hydrocarbons and a heavy fraction comprising the polycyclic aromatic hydrocarbons; and a steam cracker unit disposed downstream from the distillation unit and comprising a quench loop configured to cool an effluent generated in a furnace of the stream cracker unit, wherein the quench loop comprises a quench oil comprising a portion of the heavy fraction.

9. The system of claim 8, comprising a hydrodealkylation unit disposed downstream from the distillation unit and upstream from the steam cracker unit, wherein the hydrodealkylation unit is configured to dealkylate the monocyclic aromatic hydrocarbons in the light fraction to generate a dealkylated effluent, and wherein the dealkylated effluent comprises benzene.

10. The system of claim 9, comprising an aromatic recovery unit disposed downstream from the hydrodealkylation unit, wherein the aromatic recovery unit is configured to receive and to recover the benzene from the dealkylated effluent.

11. The system of claim 8, comprising a hydrodealkylation unit disposed downstream from the primary conversion system and upstream from the distillation unit, wherein the hydrodealkylation unit is configured to dealkylate the monocyclic aromatic hydrocarbons and the polycyclic aromatic hydrocarbons.

12. The system of claim 8, wherein the polycyclic aromatic hydrocarbons comprise diaromatic, tri-aromatic, and tetra-aromatic hydrocarbons.

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