Recovery of Styrene Monomer from Polystyrene

Abstract

In the process of recovering styrene monomer from polystyrene, benzene, toluene, ethylbenzene, styrene monomer, and C generated by depolymerizing polystyrene 9+ The feed containing aromatics is fed to a first fractionation column and separated into a first bottoms fraction containing a portion of the C 9+ aromatics in the feed and a first overhead fraction consisting of the remainder of the feed. The first overhead fraction is fed to a second fractionation column and separated into a second overhead fraction rich in benzene, toluene, and ethylbenzene compared to the first overhead fraction and a second bottoms fraction rich in styrene monomer and C 9+ aromatics compared to the first overhead fraction. The second bottoms fraction is fed to a third fractionation column and separated into a third overhead fraction rich in styrene monomer compared to the second bottoms fraction and a third bottoms fraction rich in C 9+ aromatics compared to the second bottoms fraction.

Description

Technical Field

[0001] This application relates to a process for recovering styrene monomer from the reaction product of the depolymerization of polystyrene.

Background Art

[0002] Environmental pollution, resource shortages, and climate change have all helped to drive the search for a circular economy, rather than a linear one. For example, intensive efforts have been made over the past 30 years to develop processes for recovering the raw materials of plastic waste. These efforts have not led to large-scale applications, but as the problem of plastic waste grows, interest in chemical recycling has increased rapidly.

[0003] Not all thermoplastic polymers are equally suitable for chemical recycling. For example, the thermal decomposition of polyolefins results in a mixture of waxes, gas oils, and gases, while the decomposition of polyethylene terephthalate (PET) results in organic acids, mainly benzoic acid and terephthalic acid, which are corrosive and can cause reactor fouling. On the other hand, polystyrene can be depolymerized into monomer styrene by thermal decomposition or other means, making it an excellent candidate for chemical recycling. However, the product mixture of such a depolymerization process needs to be purified in order to use the product styrene as a raw material for new polymer products.

[0004] For example, the thermal decomposition of polystyrene not only produces styrene monomer but also results in the formation of a wide variety of heavier non-polymeric aromatic compounds such as diphenylpropane and naphthalene. Thermal decomposition is also typically incomplete in that styrene dimers, trimers, and other oligomers that did not fully depolymerize still remain in the thermal decomposition products. Lighter by-products such as benzene, toluene, ethylbenzene (EB), cumene, and α-methylstyrene are also typically formed. Such by-products can have a significant adverse impact on the utility of the styrene monomer produced. For example, styrene oligomers, even in small amounts, are detrimental to the use of the monomer in the repolymerization process because these oligomers can change important properties of the polymer. Similarly, other aromatic by-products can act as chain transfer agents in radical polymerization processes, reducing the average molecular weight of the polymer produced and contributing to polymers with lower glass transition temperatures.

[0005] Furthermore, polystyrene products often also contain one or several additives such as brominated flame retardants that need to be removed in order to produce styrene monomer suitable for manufacturing many products including food packaging. Bromine from the flame retardants can sometimes result in acidic or toxic gases such as hydrobromic acid (HBr) during the thermal decomposition of waste polystyrene and can also form brominated organic compounds.

[0006] Therefore, the commercial viability of styrene monomer produced from the depolymerization of polystyrene depends on the development of efficient and economic processes and apparatuses for separating styrene monomer from the by-products of the depolymerization process.

[0007] One proposed process is described in International Patent Publication No. WO 2020 / 144165, where the pyrolysis products of polystyrene are condensed, the condensate is fed to a first fractionation column, where a light fraction containing benzene, toluene and some ethylbenzene is removed as the distillate, an intermediate fraction containing most of the styrene monomer and some ethylbenzene is removed as a side cut, and a heavy fraction containing styrene oligomers is removed as the bottoms. The intermediate fractionation is then sent to a second fractionation column, where most of the remaining ethylbenzene is removed as the distillate and a fractionation rich in styrene monomer is removed as the bottoms. The bottoms fraction from the second fractionation column is then sent to a third fractionation column, where most of the remaining substances having a boiling point higher than that of the styrene monomer are removed.

[0008] However, the process described in International Patent Publication No. WO 2020 / 144165 has several drawbacks. First, the first fractionation column needs to have a large number of theoretical plates with reflux and side draws to achieve the desired separation, which makes it expensive to install this column system and operate it at a bottom temperature above 200 °C. The latter poses a particular problem since it has been found that the condensate from the pyrolysis of polystyrene contains significantly more known crosslinking compounds such as divinylbenzene (DVB) than the reaction products from the dehydrogenation of ethylbenzene, which is a conventional process for producing styrene. At typical styrene distillation temperatures, DVB can cause the formation of crosslinked insoluble polystyrene that can contaminate the interior of the fractionation columns and their reboilers when present at concentrations far exceeding 100 ppm with respect to styrene. This type of contamination can require long shutdowns to clean or replace the column internals and other equipment.

[0009] Therefore, there is great interest in the development of improved processes and apparatuses for recovering styrene monomer from the reaction products of the depolymerization of polystyrene. SUMMARY OF THE INVENTION

[0010] According to the present application, by feeding the condensation product from the polystyrene depolymerization process to a small stripping column provided upstream of another purification column, some of the C9 by-products and a substantial portion of the DVB are removed before separating the light components from the desired styrene monomer, and substantially all of the heavy components of the C 10+ It has now been found that it is possible to remove substantially all of the heavy components. Since this stripping column removes only the heavy components, the overhead product of the by-products lighter than styrene requires fewer theoretical trays to be produced in its first column as well.

[0011] In one aspect, the present application is a process for recovering styrene monomer from the reaction product of the depolymerization of polystyrene, the process comprising: (a) feeding a feed stream containing benzene, toluene, ethylbenzene, styrene monomer, and C 9+ aromatic compounds produced by the depolymerization of polystyrene to a first fractionation column to separate the feed stream into a first bottoms fraction containing a portion of the C 9+ aromatic compounds and a first overhead fraction which is a vapor composed of the remainder of the feed stream; (b) feeding the first overhead fraction to a second fractionation column to separate the first overhead fraction into a second overhead fraction rich in benzene, toluene, and ethylbenzene compared to the first overhead fraction and a second bottoms fraction rich in styrene monomer and C 9+ aromatic compounds compared to the first overhead fraction; (c) feeding the second bottoms fraction to a third fractionation column to separate the second bottoms fraction into a third overhead fraction rich in styrene monomer compared to the second bottoms fraction and a third bottoms fraction rich in C 9+ aromatic compounds compared to the second bottoms fraction, the process comprising.

[0012] In a further aspect, the present application is an apparatus for recovering styrene monomer from the reaction product of the depolymerization of polystyrene, the apparatus comprising: (a) a first fractionation column for receiving a depolymerized oil containing a portion of the reaction product of the depolymerization of polystyrene, the depolymerized oil being separated into C9+ A first distillation column that is operated to separate into a first bottom fraction containing a part of an aromatic compound and a first top fraction that is a vapor consisting of the remainder of the depolymerized oil, (b) A second fractionation column connected to the first fractionation column to receive the first top fraction, and the first top fraction is separated into a second top fraction richer in benzene, toluene and ethylbenzene than the first top fraction, and a second bottom fraction richer in styrene monomer and C 9+ A second fractionation column that is operated to separate into a second bottom fraction rich in aromatic compounds, (c) A third fractionation column connected to the second fractionation column to receive the second bottom fraction, and the second bottom fraction is separated into a third top fraction richer in styrene monomer compared to the second bottom fraction, and a third bottom fraction richer in aromatic compounds compared to the second bottom fraction, 9+ and a third fractionation column that is operated to separate into a third bottom fraction rich in aromatic compounds. Provided is an apparatus comprising the same.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0014] As used herein, the term "C n " compound (hydrocarbon), where n is a positive integer, such as 1, 2, 3, 4, 5, etc., means a compound having "n" carbon atoms per molecule. The term "C n+ " compound, where n is a positive integer, such as 1, 2, 3, 4, 5, etc., means a compound having at least "n" carbon atoms per molecule. The term "C n- " compound, where n is a positive integer, such as 1, 2, 3, 4, 5, etc., as used herein, means a compound having n or fewer carbon atoms per molecule.

[0015] Described herein is a process and apparatus for recovering styrene monomer from the reaction product of the depolymerization of polystyrene. In one embodiment, the process comprises recovering C styrene monomer from the reaction product of the depolymerization of polystyrene, the reaction product comprising benzene, toluene, ethylbenzene, styrene monomer, and styrene oligomers. 9+ Using a feed containing aromatic compounds, the feed is subjected to pyrolysis of polystyrene to produce pyrolysis gas, and then condensing most of the aromatic hydrocarbons to produce C 5- The resulting liquid feed is also referred to herein as depolymerized oil, which is then separated from the majority of the light products. 9+ The first fraction is fed to a first fractionator which is operated to split the feed into a first bottoms fraction containing a portion of the aromatics and a first overhead fraction which is the remainder of the feed, a vapor consisting primarily of benzene, toluene, ethylbenzene, and styrene monomer. Side cuts containing styrene monomer and ethylbenzene are not removed from the first fractionator in the process of the present invention. The first overhead fraction from the first fractionator is then fed to a second fractionator which produces a second overhead fraction which is enriched in benzene, toluene, ethylbenzene compared to the first overhead fraction, and a second overhead fraction which is enriched in styrene monomer and C compared to the first overhead fraction. 9+ The second fraction from the second fractionator is then fed to a third fractionator which produces a third overhead fraction enriched in styrene monomer relative to the second bottoms fraction and a second bottoms fraction enriched in aromatics relative to the second bottoms fraction. 9+ and a third bottoms fraction enriched in aromatics.

[0016] The depolymerized oil used as a feedstock in the process of the present invention may be produced in situ or transported from one or more separate locations to the location of the above separation system. One type of the polystyrene depolymerization process is pyrolysis. In a pyrolysis reactor, polystyrene is heated in the absence of oxygen to a temperature sufficient to depolymerize polystyrene into styrene monomer. The resulting pyrolysis gas generally contains more than 60% by weight of styrene monomer, a small amount of benzene, toluene, other C8 aromatic compounds such as phenylacetylene and xylene, and C 9+ aromatic compounds, and various C 5- light by-products. The pyrolysis gas is fed to a condenser where the gas is typically cooled to near ambient temperature to condense benzene and heavier components, leaving C 5- light by-products in the gas phase, as a result of which they are separated from the aromatic hydrocarbons and can potentially be used as fuel gas.

[0017] A typical analysis of the condensed depolymerized oil produced from the pyrolysis of polystyrene is shown in Table 1, which also lists, for comparison, the typical commercial composition of a crude styrene liquid obtained from the dehydrogenation of ethylbenzene, the most common industrial process for producing styrene.

[0018]

Table 1

[0019] From Table 1, it can be seen that the composition of the depolymerized oil is similar to that of the crude styrene liquid from EB dehydrogenation, but the former contains much less EB, while in the latter case, EB is recycled back to the dehydrogenation section, so it is necessary to separate the benzene - toluene fraction from EB. In the case of the depolymerized oil, the amount of EB is relatively small (typically less than 10 wt%), and since the entire benzene / toluene / EB fraction can be sent to the aromatic plant for processing, this separation is not necessary. Also, the concentrations of cumene and α - methylstyrene are much higher in the polystyrene depolymerized oil, and it can be seen that these two compounds are potentially major impurities in the purified styrene product. Cumene is particularly difficult to fractionate from styrene.

[0020] It is also important to note that the concentration of divinylbenzene (DVB) is several times higher in the depolymerized oil. DVB is a known cross - linking agent, and when present at concentrations far exceeding 100 ppm relative to styrene, it can lead to the formation of cross - linked insoluble polystyrene, which can contaminate the inside of distillation towers and their reboilers. This type of contamination may require long shutdowns to clean or replace the tower internals and the equipment. Since the DVB concentration is much higher, it can be expected that the potential for contamination is higher when recovering styrene from the depolymerized oil than when recovering styrene from the crude styrene from EB dehydrogenation.

[0021] To separate the components of the depolymerized oil, the depolymerized oil is first mixed with a polymerization inhibitor, such as a mono - or dinitroaromatic compound, and then the mixture is fed to a first fractionation tower without an initial fractionation. Alternatively, the polymerization inhibitor may be fed separately to the first fractionation tower. In an embodiment, the first fractionation tower is a reboiling stripper in that the tower contains a reboiler for the liquid bottoms but does not contain a condenser for the vapor overhead. The first fractionation tower separates the depolymerized oil feed into a portion of the aromatic compounds, preferably substantially all of the C 9+ A portion of the aromatic compounds, preferably substantially all of the C 10+It is operated to divide into a first bottoms fraction containing an aromatic compound and a first overhead vapor fraction consisting of the remainder of the feed. In an embodiment, the first stripping column separates a sufficient portion of the divinylbenzene in the depolymerized oil feed from most of the styrene so that crosslinking of the polymer does not become a significant problem in downstream columns. Most of the divinylbenzene remains as part of the first bottoms fraction and most of the styrene remains in the overhead vapor. The total overhead fraction from the first stripping column is sent as vapor to a second fractionating column, while the first bottoms fraction is the net product of the system.

[0022] The first stripping column typically has 1 to 15 theoretical plates, such as 5 to 10 theoretical plates, and the depolymerized oil feed is introduced into the first stripping column at or near the topmost stage. The interior of the first stripping column can include fractionation packing, but distillation trays are preferred because they are less expensive and easier to clean of accumulated insoluble polymer. In an embodiment, the first stripping column operates at a bottom temperature of 90 to 155 °C, such as 100 to 150 °C, and a top or head temperature of 55 to 100 °C, such as 65 to 90 °C, and a bottom pressure of 40 to 300 mmHg, such as 70 to 200 mmHg, and a top or head pressure of 40 to 180 mmHg, such as 50 to 120 mmHg. The first stripping column includes a reboiler for re-distilling a portion of the first bottoms fraction. However, the first stripping column does not include a side cut for separating EB and styrene monomer from light components and preferably does not have a condenser and associated pump for refluxing a portion of the overhead fraction.

[0023] The first top fraction contains substantially all benzene and toluene, as well as most of the C8 aromatic components including styrene monomer in the depolymerized oil feed and is fed to the second fractionation tower. A polymerization inhibitor can also be added to the second tower. The second fractionation tower can include distillation trays, but is typically a packed tower. In embodiments, the second fractionation tower has at least 20 theoretical trays, such as 20 - 120 theoretical trays, more preferably 40 - 100 theoretical trays. Since the first stripping tower does not include reflux from the condensed first top, essentially all the heat input to the first tower reboiler is in the vapor sent to the second fractionation tower, resulting in an overall improvement in energy efficiency for the tower system having a condenser and reflux. In embodiments, the second fractionation tower is a complete distillation system having a reboiler for the liquid bottoms and a condenser for the vapor top. Typically, the second fractionation tower operates at a bottom temperature of 60 - 120 °C, such as 70 - 105 °C, and a top or head temperature of 35 - 100 °C, such as 40 - 70 °C, and a bottom pressure of 45 - 370 mmHg, such as 65 - 210 mmHg, and a top or head pressure of 30 - 290 mmHg, such as 40 - 145 mmHg. Under these conditions, the second fractionation tower is effective in splitting the first top fraction into a second top fraction rich in benzene, toluene, and ethylbenzene compared to the first top fraction and a second bottom fraction rich in styrene monomer and C 9+ aromatic compounds compared to the first top fraction.

[0024] The gaseous second overhead fraction exiting the second fractionation tower is passed through a condenser where most of the second overhead fraction is condensed. Then, a portion of the resulting condensate is returned to the second fractionation tower as reflux, and the remaining portion or all of it is recovered as a by-product (distillate) stream, which can be used as a feed to the aromatic plant. The reflux-to-distillate ratio is typically 15 - 30. In some embodiments, a portion of the condensate is fed back as a reflux stream to the top of the first stripping tower, and the ratio of the condensate fed back to the first tower to the distillate stream is typically 0.2 - 0.4. Any water in the depolymerized oil mostly condenses in the second fractionation tower condenser, forming a separated liquid phase. Optionally, this liquid water can be separated from the liquid hydrocarbon.

[0025] The depolymerized oil may contain hydrochloric acid (HCl), hydrobromic acid (HBr), and other halogenated compounds. When the aqueous phase is separated as a separate stream as described above, these halogenated compounds are present to some extent in this aqueous phase. To facilitate this extraction of halogenated compounds and other impurities from the hydrocarbon distillate of the second tower, water can be added to the overhead system of the second fractionation tower. To further improve the extraction of specific impurities, the second tower distillate can be fed to an extraction tower to counter-currently wash the hydrocarbon with water in one or more equilibrium stages.

[0026] Mainly composed of styrene monomer and C 9+ The net liquid bottoms from the second fractionation tower, which is mainly composed of aromatic compounds, is optionally fed to the third fractionation tower together with additional polymerization inhibitor. To maximize product separation in the second fractionation tower, a portion of the second bottoms stream is fed to a reboiler, heated, and then returned to the bottom of the second fractionation tower.

[0027] The third fractionation column can include distillation trays but is typically a packed column. Generally, the third fractionation column has 10 to 120 theoretical plates, for example 30 to 100 theoretical plates. In an embodiment, the third fractionation column is a complete distillation system having a reboiler for the liquid bottoms and a condenser for the vapor overhead. Typically, the third fractionation column operates at a bottoms temperature of 55 to 130 °C, for example 75 to 115 °C, and a top or head temperature of 35 to 100 °C, for example 40 to 80 °C, and a bottoms pressure of 15 to 230 mmHg, for example 35 to 160 mmHg, and a top or head pressure of 10 to 150 mmHg, for example 15 to 90 mmHg. Under these conditions, the third fractionation column operates to separate the second bottoms fraction into a third overhead fraction richer in styrene monomer compared to the second bottoms fraction and a third bottoms fraction richer in C 9+ aromatic compounds compared to the second bottoms fraction.

[0028] The gaseous third overhead fraction exiting the third fractionation column is passed to a condenser where most of the third overhead fraction is typically condensed. A portion of the resulting condensate can then be returned as reflux to the third fractionation column, and the reflux to distillate ratio is typically 2.5 to 6, and the remainder (distillate) is recovered as the desired styrene monomer product, typically with a purity of at least 99.0 wt%, preferably at least 99.8 wt%. The resulting styrene monomer product typically contains less than 20 ppmw of halogenated compounds, preferably less than 10 ppmw of halogenated compounds, more preferably less than 1 ppmw of halogenated compounds, or halogenated compounds at a level below the detection limit of the test methods typically available for halogenated compounds in aromatic hydrocarbons. The third bottoms fraction can be purged from the system together with the first bottoms fraction.

[0029] Halogenated compounds may appear in the styrene monomer product. To extract these halogenated compounds from the third tower hydrocarbon distillate, water can be added to the top system of the third fractionation tower. To further improve the extraction of certain impurities, the third tower distillate can be fed to an extraction tower to counter-currently wash the hydrocarbon with water in one or more equilibrium stages.

[0030] One embodiment of the process is shown in FIG. 1 where polystyrene depolymerized oil is mixed with a polymerization inhibitor from line 37 and fed to the top of a first fractionation tower 12 equipped with distillation trays and a reboiler 13 via line 11. The first fractionation tower 12 is configured and operated as described above to split the oil / inhibitor mixture into the following two fractions. The two fractions are a first overhead vapor fraction rich in the C8 and lighter components of the feed, and a first bottoms fraction rich in the C 9+ components of the feed. The first overhead fraction exits the top of tower 12 through line 14 and is fed as vapor to a second fractionation tower 15, while the liquid from the base of tower 12 flows through line 16 where it is distributed between the reboiler 13 and the first bottoms fraction of line 17 which is a by-product stream.

[0031] The second fractionation column 15 includes distillation packing and is provided with a reboiler 18 and a condenser 19. Additional polymerization inhibitor can be supplied to the second fractionation column 15 via line 20. Column 15 is configured and operated as described above to divide the first overhead fraction into a second overhead fraction rich in components boiling at a lower temperature than styrene monomer and a second bottoms fraction rich in styrene monomer and higher boiling components. The second overhead vapor exits from the top of column 15 and is supplied via line 21 to condenser 19, where most of the benzene, toluene and ethylbenzene components are condensed and distributed between reflux line 22 for recycle to column 15 and product recovery line 23. Some of the C8-components in the second overhead fraction exit the condenser 19 as vapor via vent line 24. Optionally, any condensed water can be separated from the hydrocarbon liquid phase and withdrawn via line 39. The second fractionation column bottoms exits from the base of column 15 through line 25, a portion of the second bottoms is recycled to column 15 through reboiler 18, and the remainder is the second bottoms fraction supplied to the third distillation column 27 through line 26. Additional polymerization inhibitor from line 38 can be mixed with the second bottoms fraction in line 26 before entering the third fractionation column 27.

[0032] The third fractionation column 27 includes distillation packing and is provided with a reboiler 28 and a condenser 29. Column 27 is configured and operated as described above to separate the second bottoms fraction into a third overhead fraction rich in styrene monomer and components boiling at a temperature lower than the styrene monomer, and a third bottoms fraction rich in components boiling at a temperature higher than the styrene monomer. The third overhead vapor exits the top of column 27 and is supplied via line 31 to condenser 29, where most of the styrene monomer is condensed and distributed between a reflux line 32 for recirculation to column 27 and a third overhead fraction flowing through product recovery line 33. The uncondensed vapor from condenser 29 exits through line 34. The third bottoms stream exits the base of column 27 through line 35, where a portion is recirculated to column 27 through reboiler 28 and the remainder is the third bottoms fraction flowing through line 36, which is a process by-product. The column system vents from the second and third columns flowing through lines 24 and 34 can flow individually to separate vacuum systems (not shown) or be combined and flow to one vacuum system.

[0033] Another embodiment of the present invention is shown in FIG. 2, where the process shown in FIG. 1 is modified such that the condensed overhead stream from the second fractionation column 15 is supplied via line 40 to the uppermost stage of the first fractionation column 12 and the depolymerized oil stream in line 11 is supplied to a lower stage, preferably the second stage from the uppermost stage, within the first fractionation column 12. This added stream reduces the concentration of divinylbenzene in the feed to the second fractionation column 15 in line 14 and / or reduces the number of theoretical trays required in the first fractionation column 12 to achieve the desired separation. It would be expected by those skilled in the art that recycling the distillate from one fractionation column to the previous column would result in significant inefficiencies. However, in this case, the recycle stream required to achieve a substantial reduction in the divinylbenzene concentration in line 14 is small and the impact on the reboiler load in the process is surprisingly small.

[0034] Hereinafter, the present invention will be described in more detail with reference to the following non-limiting examples.

[0035] Example 1 Pyrolysis oil containing 78% styrene, 0.6% benzene and lighter compounds, 6.4% toluene, 4.7% ethylbenzene, 0.15% cumene, 520 ppm divinylbenzene, 10% other C 9+ aromatic compounds, and 20 ppm bromide was fed to the first column 12 of the laboratory distillation system shown in FIG. 1. The first column 12 is a tray column having 10 theoretical plates and was maintained without reflux at a temperature of 148.0 ° C. and a pressure of 88 mmHg at the bottom and a temperature of 74.2 ° C. and a pressure of 73 mmHg at the top. The pyrolysis oil was fed to the top tray of column 12 at an average rate of 473 g / h. The bottom stream from column 12 was collected at an average rate of 45 g / h and was the only stream containing detectable bromide. Its weight composition was 0.28% styrene, less than 0.06% ethylbenzene and lighter compounds, 10 ppm cumene, 0.39% divinylbenzene, and the remaining other C 9+It was an aromatic compound. The styrene-rich top from column 12 flowed as vapor into the second column 15 at an average rate of 428 g / h. The second column 15 had 48 theoretical trays, with the bottom maintained at 101.8 °C and 181 mmHg, and the top maintained at 56.7 °C and 69 mmHg. The external reflux ratio was maintained within the range of 18 - 22. The liquid distillate from the second column 15 was collected at an average rate of 52 g / h and contained 11 wt% styrene, 33 wt% ethylbenzene, 52 wt% toluene, and 4 wt% lighter compounds (mainly benzene). The bottom of the second column 15 was fed to the third column 27, which had 32 theoretical trays, at an average rate of 376 g / h. The third column 27 was maintained at 107.8 °C and 125 mmHg at the bottom and 71.0 °C and 53 mmHg at the top. The external reflux ratio of this third column was maintained within the range of 3 - 4. The styrene-rich distillate from the third column was collected at an average rate of 302 g / h and was 99.9 wt% styrene, with the maximum impurities being 230 ppm ethylbenzene and 340 ppm cumene. The inductively coupled plasma test did not detect bromine in the third column distillate and showed a concentration below the detection limit of 1.0 ppmw. A polymerization inhibitor chemical was injected into the first column 12 such that the column feed was 1200 ppmw inhibitor.

[0036] Example 2 The simulations of the embodiments of FIGS. 1 and 2 were performed using commercially available process modeling software. The composition of the pyrolysis oil from Table 1 was fed to the simulated column 12 of the distillation system at a rate of 1000 kg / h. In one simulation, column 12 was modeled according to the embodiment shown in FIG. 1, the feed was fed to the topmost stage, and reflux was not fed to this first column 12. In the second simulation, column 12 was modeled using a feed fed from the topmost stage to the second stage and a reflux stream 40 returned to the topmost stage at a rate of 50 kg / h, as in the embodiment shown in FIG. 2. The first column 12 was modeled with nine theoretical stages having a temperature of 108 ° C. and a pressure of 88.4 mmHg at the bottom, and a pressure of 73 mmHg and a temperature of about 73 ° C. at the top. The simulated column 12 was controlled such that the bottom stream contained 10 wt% or less of styrene. The bottom rate was calculated to be about 137 kg / h. The top of column 12 flowed as vapor to the second simulated column 15. Referring to Table 2, the top rate differed between the two embodiments. The second column 15 was simulated with 51 theoretical stages having a temperature of 99 ° C. and a pressure of 181 mmHg at the bottom, and a temperature of 59 ° C. and a pressure of 69 mmHg at the top. Column 15 was controlled such that the distillate composition was 10 wt% styrene. The remainder of the distillate was calculated to be 1.4% benzene and lighter components, 32.2% toluene, and 56.1% ethylbenzene. The distillate rate was calculated to be 22.5 kg / h. In the simulation of the embodiment of FIG. 2, this distillate rate was net of the reflux stream 40. The bottom of column 15 was calculated at a rate of 758 kg / h and fed to the third column 27. The third column 27 was modeled with 46 theoretical stages having a temperature of 66 ° C. and a pressure of 53 mmHg at the top, and a temperature of 104 ° C. and a pressure of 125 mmHg at the bottom. The simulated column 27 was controlled such that the distillate was 99.9 wt% styrene. The remaining distillate composition (by weight) was 209 ppm ethylbenzene, 159 ppm cumene, and the remainder were heavier compounds. The distillate rate was calculated to be 721 kg / h. Other quantities and parameters differed between the two simulations and are compared in Table 2 below.

[0037] [Table 2]

[0038] Although the present invention has been described and illustrated with reference to particular embodiments, those skilled in the art will understand that the invention is useful in variations that are not necessarily illustrated herein. Therefore, only the appended claims should be referred to in order to determine the true scope of the present invention.

Claims

1. A process for recovering styrene monomer from the reaction product of the depolymerization of polystyrene, said process comprising: (a) A supply stream containing aromatic compounds generated by depolymerizing benzene, toluene, ethylbenzene, styrene monomer, and polystyrene is fed to a first fractionation column, and the supply stream is separated into a first bottoms fraction containing a portion of the C ９＋ aromatic compounds in the supply stream and a first overhead fraction that is vapor composed of the remainder of the supply stream, ９＋ and dividing; (b) Supplying the first overhead fraction to a second fractionation column to separate the first overhead fraction into a second overhead fraction rich in benzene, toluene, and ethylbenzene as compared with the first overhead fraction, and a second bottoms fraction rich in styrene monomer and C ９＋ aromatic compounds as compared with the first overhead fraction; (c) feeding the second bottoms fraction to a third fractionation column to separate the second bottoms fraction into a third overhead fraction rich in styrene monomer as compared with the second bottoms fraction and a third bottoms fraction rich in C ９＋ aromatic compounds as compared with the second bottoms fraction, and the process comprising the above steps.

2. The process according to claim 1, wherein the first fractionation column has 15 or fewer theoretical plates.

3. The separation system according to claim 1 or 2, wherein the first fractionation column has 5 to 10 theoretical plates.

4. The process according to any one of claims 1 to 3, wherein the first fractionation column is operated at a bottom temperature of 90 to 155 °C and a top temperature of 55 to 100 °C.

5. The process according to any one of claims 1 to 4, wherein the first fractionation column is a tray column.

6. The process according to any one of claims 1 to 5, wherein each of the second and third fractionation columns is a packed column.

7. The process according to any one of claims 1 to 6, wherein the second fractionation column has at least 20 theoretical plates.

8. The process according to any one of claims 1 to 7, wherein one or more polymerization inhibitors are fed to each of the fractionation columns during one or more of the feeding steps (a) and (b).

9. The process according to any one of claims 1 to 8, wherein a portion of the overhead fraction from the second column is condensed and used as reflux for the first fractionation column.

10. The process according to any one of claims 1 to 9, wherein water is used to extract halogenated compounds and other impurities from at least a portion of the overhead fraction from the second column and / or at least a portion of the overhead fraction from the third column.

11. The process according to any one of claims 1 to 10, wherein the styrene monomer product contains less than 10 ppmw of halogenated compounds.

12. An apparatus for recovering styrene monomer from the reaction product of the depolymerization of polystyrene, said apparatus comprising: A first distillation column that receives a depolymerized oil containing a part of the reaction product of the depolymerization of polystyrene, the depolymerized oil being the C in the depolymerized oil ９＋ A first distillation column that is operated to divide the depolymerized oil into a first bottom fraction containing a part of the aromatic compounds and a first overhead fraction that is a vapor consisting of the remainder of the depolymerized oil (b) A second fractionation tower connected to the first fractionation tower to receive the first overhead fraction, and separating the first overhead fraction into a second overhead fraction richer in benzene, toluene, and ethylbenzene than the first overhead fraction, and a second bottoms fraction richer in styrene monomer and C ９＋ aromatic compounds than the first overhead fraction, and operating to separate them. (c) connected to the second fractionation column to receive the second bottoms fraction, and separating the second bottoms fraction into a third overhead fraction rich in styrene monomer as compared with the second bottoms fraction and a third bottoms fraction rich in C ９＋ A third fractionation column that is operated to separate the second bottoms fraction into a third overhead fraction rich in aromatic compounds as compared with the second bottoms fraction, and an apparatus comprising the same.

13. The apparatus according to claim 12, wherein the first fractionation column has less than 15 theoretical plates.

14. The apparatus according to claim 12 or 13, wherein the first fractionation column has 5 to 10 theoretical plates.

15. The apparatus according to any one of claims 12 to 14, wherein the first fractionation column is operated at a bottom temperature of 90 to 155 °C and a top temperature of 55 to 100 °C.

16. The apparatus according to any one of claims 12 to 15, wherein the first fractionation column is a tray column.

17. The apparatus according to any one of claims 12 to 16, wherein each of the second and third fractionation columns is a packed column.

18. The apparatus according to any one of claims 12 to 17, wherein the second fractionation tower has at least 20 theoretical plates.

19. A system for recovering styrene monomer from polystyrene, the system comprising (d) Pyrolyzing polystyrene to produce a pyrolysis unit for generating a pyrolysis gas containing ５－ light products, benzene, toluene, ethylbenzene, styrene monomer and ９＋ aromatic compounds, and (e) removing at least a part of the light products from the pyrolysis gas to produce a depolymerized oil containing benzene, toluene, ethylbenzene, styrene monomer and C ５－ and a condenser for producing a depolymerized oil containing aromatic compounds, wherein the depolymerized oil is supplied to the apparatus according to any one of claims 12 to 18 without intermediate fractionation. ９＋ A system.

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