Process and apparatus for preparing a purified styrene composition using a dividing wall column and a crystallization unit

Abstract

The present invention relates to a process for preparing a purified styrene composition, comprising the following steps: a) providing a crude hydrocarbon composition containing styrene; b) subjecting the crude hydrocarbon composition provided in step a) to distillation in a dividing wall column to produce an overhead hydrocarbon stream, a bottom hydrocarbon stream and a side hydrocarbon stream; and c) subjecting the side hydrocarbon stream obtained in step b) to at least one crystallization step to obtain a purified styrene composition.

Description

[Technical Field]

[0001] The present invention relates to a process for preparing a purified styrene composition from a styrene-containing crude hydrocarbon composition obtained from a pyrolysis or catalytic-based waste polystyrene depolymerization unit. Furthermore, the present invention relates to a plant in which the process can be carried out. [Background technology]

[0002] Styrene is an important building block for polymers such as polystyrene, acrylonitrile-butadiene-styrene (ABS) / styrene-acrylonitrile (SAN) resins, styrene-butadiene (SB) copolymer latexes, unsaturated polyester resins, and styrene-butadiene rubber (SBR) elastomers and latexes. Styrene is one of the most traded commodity chemicals, with over 30% of annual styrene production traded internationally. Styrene is primarily produced from benzene and ethylene as feedstocks. Benzene is alkylated to produce ethylbenzene (EB), which is then converted to styrene via conventional dehydrogenation processes or the ethylbenzene / styrene monomer (EBSM) or propylene oxide / styrene monomer (POSM) processes, respectively. Styrene plants are typically located near ethylene cracking furnaces due to the gaseous nature of ethylene, which makes it relatively difficult to transport compared to benzene.

[0003] Apart from its intentional production via EBSM / POSM, styrene also exists in hydrocarbon streams such as pyrolysis gasoline obtained from the steam cracking of naphtha, hydrocarbon fractions obtained from the thermal or catalytic cracking of waste polystyrene, and gas oil. Although styrene extraction from these hydrocarbon streams is much smaller in volume than EBSM / POSM, the lower feedstock costs offer attractive economic opportunities for operators. The field of styrene production from waste polystyrene has attracted particular attention due to the current global challenge of plastic recycling. Waste plastic poses a serious threat to the environment. However, this separation is technically challenging due to the presence of molecules with similar boiling points and impurities derived from the starting feedstock. For example, in polystyrene recycling, various types of waste polystyrene are fed into catalytic or pyrolysis reactors. Due to the contamination of waste plastics combined with the production of close-boiler compounds such as benzene, toluene, ethylbenzene, α-methylstyrene, cumene, and n-propylbenzene produced during the pyrolysis or catalytic process, a reliable method is needed to refine this styrene oil to the final ASTM-grade styrene specification. However, removing molecules with similar boiling points, such as mixed xylenes and ethylbenzene, from styrene by conventional distillation is an energy-intensive process. More specifically, the traditional two- or three-column approach employed for EBSM-derived styrene purification is not only energy-intensive and requires high capital expenditures (CAPEX), but also fails to remove similar-boiling impurities, such as oxygenates, color-causing compounds, and sulfur species. Oxygenates not only originate from the feedstock but can also be generated during the process due to air leaks, as these columns are operated under deep vacuum (typically in the range of approximately 100–300 mbar). It is known that styrene can react with oxygen to form oxygenated species, such as benzaldehyde, which can violate the total aldehyde specification.Furthermore, since the feed source for the pyrolysis or catalytic reactor is waste polystyrene separated from mixed plastics, even if the feed input to the catalytic or pyrolysis reactor is controlled, nitrogenated and chlorinated species may still be present in the crude styrene oil produced from the depolymerization of waste polystyrene. Such species are considered to be harmful contaminants for the styrene polymerization process, which is the ultimate goal of establishing the concept of a true circular economy for styrene.

[0004] In view of the above, an object underlying the present invention is to provide an energy-efficient process requiring only low plant CAPEX for preparing a purified styrene composition from a styrene-containing feed composition derived from a styrene-containing stream produced by pyrolysis of recycled polystyrene, etc., which process reliably and efficiently removes impurities such as color-inducing species, sulfur and oxygenates, ethylbenzene, mixed xylenes, propylbenzene, ethyltoluene, α-methylstyrene, nitrogenated and chlorinated molecules from styrene in an energy-efficient manner, even if the impurities are present in relatively large amounts in the styrene-containing feed composition, thereby obtaining a very pure styrene composition in a cost-effective manner. Summary of the Invention

[0005] According to the invention, the object is a method for preparing a purified styrene composition, comprising the following steps: a) providing a crude hydrocarbon composition containing styrene; b) subjecting the crude hydrocarbon composition provided in step a) to distillation in a dividing wall column to produce an overhead hydrocarbon stream, a bottoms hydrocarbon stream and a side hydrocarbon stream; c) subjecting the column side hydrocarbon stream obtained in step b) to at least one crystallization step to obtain a purified styrene composition; This is achieved by providing a method comprising:

[0006] This solution is based on the surprising discovery that by subjecting a crude hydrocarbon composition containing styrene to distillation in a dividing wall column to produce an overhead hydrocarbon stream, a bottom hydrocarbon stream, and a side hydrocarbon stream, and then subjecting the side hydrocarbon stream obtained in step b) to at least one crystallization step, impurities such as color-inducing species with close boiling points, sulfur and oxygenates, ethylbenzene, mixed xylenes, propylbenzene, ethyltoluene, α-methylstyrene, nitrogenated and chlorinated molecules, etc., are reliably and completely, or at least nearly completely, removed from styrene, even if the impurities are contained in relatively large amounts in the crude hydrocarbon composition, and that, in addition, the individual processes are energy-efficient and require only plants with relatively low CAPEX. This cannot be achieved by using two or three distillation columns to purify a crude hydrocarbon stream according to the prior art. While crystallization alone can directly remove some (if not all) of these impurities from crude styrene oil, dilute styrene requires very low temperatures, in addition to extensive purification and residue steps that make this solution ineffective, resulting in high CAPEX and operating costs. The combination of first subjecting a crude hydrocarbon composition to distillation in a dividing wall column and then subjecting the resulting side hydrocarbon stream in the dividing wall column to at least one crystallization step overcomes the above-mentioned problems. More specifically, in typical approaches using two distillation columns to purify a crude hydrocarbon stream, replacing the two distillation columns with a dividing wall column is entirely unreasonable because the distillation energy requirements are disproportionate between the two distillation columns. Furthermore, such a replacement would not solve the problem of removing color-inducing species, oxygenates, sulfur species, nitrogenated and chlorinated species, etc., with similar boiling points, since these impurities are not removed from styrene to the desired high degree by distillation. In contrast, the present invention relies on "slipping" impurities, such as ethylbenzene, through the distillation step to such an extent that the energy consumption of the two separate distillation columns is more or less the same, or at least essentially the same.In other words, instead of producing an ultra-high purity styrene stream by distillation containing, for example, 99.8 wt% styrene, the method of the present invention intentionally produces a lower purity styrene stream by distillation containing, for example, 99.2 wt% styrene, thereby intentionally "slipping" impurities with similar boiling points through the distillation process. This makes the two distillation columns an excellent example of a dividing wall column replacement, achieving a 30% energy savings and a 25% CAPEX reduction compared to a conventional plant using two distillation columns. The raw styrene stream thus obtained, which is the side hydrocarbon stream from the dividing wall column obtained in step b) and contains impurities such as color-inducing species, oxygenates, ethylbenzene, and sulfur species that cannot be completely removed by distillation, is then subjected to at least one crystallization step, which not only increases the styrene content in the purified styrene composition from, for example, 99.2 wt% to greater than 99.8 wt% styrene, but also removes impurities with similar boiling points from the purified styrene composition that are difficult or impossible to remove by conventional distillation. Thus, for example, ASTM-grade styrene can be easily and energy-efficiently produced from crude styrene oil derived from polystyrene. Overall, the process according to the invention makes it possible to cost-effectively purify styrene-containing compositions from impurities, even from impurities with boiling points close to that of styrene.

[0007] As is known in the art, each crystallization process or step may typically be carried out in multiple stages, i.e., several crystallization stages, and with this in mind, a crystallization step is defined in this application as comprising one or more crystallization stages.

[0008] The present invention is not particularly limited with respect to the styrene content of the crude hydrocarbon composition provided in step a). In particular, good results are obtained when the crude hydrocarbon composition provided in step a) contains at least 10% by weight, preferably 30% by weight, preferably at least 50% by weight, even more preferably at least 60% by weight, even more preferably at least 70% by weight, and most preferably at least 80% by weight of styrene.

[0009] In principle, the dividing wall column used in step b) can be any dividing wall column, in particular a top dividing wall column, an intermediate dividing wall column or a bottom dividing wall column. In a top dividing wall column, the dividing wall extends downward from the top of the column (to which the wall is connected) over part of the height of the dividing wall column, while in a bottom dividing wall column, the dividing wall extends upward from the bottom of the column (to which the wall is connected) over part of the height of the dividing wall column. In contrast, in an intermediate dividing wall column, the dividing wall extends over part of the height of the dividing wall column, but is not connected to the top or bottom, and therefore has a distance to the top and bottom of the dividing wall column.

[0010] According to a particularly preferred embodiment of the present invention, the crude hydrocarbon composition provided in step a) is subjected to distillation in step b) in an intermediate dividing wall column.

[0011] In particular, good results are obtained when the dividing walls of the intermediate dividing wall column extend, over a height that is the linear distance between the bottom and the top of the intermediate dividing wall column, from 10 to 90%, preferably from 20 to 80%, more preferably from 30 to 70%, and most preferably from 40 to 60% of the height of the intermediate dividing wall column. In other words, the dividing walls of the intermediate dividing wall column have a distance to the top and bottom of the intermediate dividing wall column, independently of one another, that is at least 10%, or at least 20%, or at least 30%, or at least 40%.

[0012] As a further development of the inventive concept, it is proposed that the dividing wall extends essentially vertically downwards in the intermediate dividing wall column, where essentially vertically downwards means that the angle between the dividing wall and the longitudinal axis of the intermediate dividing wall column is at most 20°, preferably at most 10°, more preferably at most 5°, and most preferably 0°.

[0013] Furthermore, the overhead hydrocarbon stream obtained in step b) is 7- the bottom hydrocarbon stream obtained in step b) is C 9+Preferably, the crude hydrocarbon composition, the dividing wall column and the operating conditions are adjusted so that the column side hydrocarbon stream obtained in step b) is a styrene-containing hydrocarbon stream, i.e., a styrene-containing C hydrocarbon stream.

[0014] As mentioned above, the distillation in step b) is intended to be carried out in such a way that a purified styrene-containing C hydrocarbon stream is obtained as the side hydrocarbon stream but not an ultra-pure styrene-containing C hydrocarbon stream, in order to adjust the energy consumption of both parts of the dividing wall column separated by the dividing wall to be approximately the same. This distillation is possible according to the invention because the remaining impurities, in particular impurities having a boiling point close to that of styrene, are separated from the side hydrocarbon stream obtained in step b) by at least one subsequent crystallization step c).

[0015] In this respect, it is desirable that the column side hydrocarbon stream obtained in step b) and subjected to at least one crystallization step in step c) contains at least 80% by weight, preferably at least 90% by weight, more preferably at least 98% by weight, most preferably at least 99% by weight of styrene, but preferably less than 99.8% by weight of styrene, preferably less than 99.5% by weight of styrene, more preferably less than 99.3% by weight of styrene.

[0016] As described above, the present invention is particularly suitable for purifying styrene-containing crude hydrocarbon compositions containing one or more impurities selected from the group consisting of color-inducing species, sulfur species, meta- and ortho-xylene, ethylbenzene, phenylacetylene, cumene, n-propylbenzene, α-methylstyrene, ethyltoluene, organic chlorinated species, organic nitrogenated species, and any mixture of two or more of the aforementioned impurities. Therefore, suitable examples are one or more sulfur species, preferably one or more sulfur species selected from the group consisting of mercaptans, disulfides, thiophenes having boiling points between 130 and 150°C, and any combination of two or more thereof. Therefore, other suitable examples are one or more color-inducing species including at least one of conjugated diolefins, oxygenated species, and oxygenated sulfur species. For example, the oxygenated species can be water, alcohols, ketones, and / or aldehydes, while fulvene and their derivatives are suitable examples of diolefins.

[0017] Preferably, the total content of the above impurities in the crude hydrocarbon composition is 1 to 60% by weight, more preferably 1 to 40% by weight.

[0018] As mentioned above, it is intended that the pre-purified styrene-containing C hydrocarbon stream obtained as the column side hydrocarbon stream still contains some impurities, particularly the aforementioned impurities, after the distillation in step b).Accordingly, according to the present invention, the column side hydrocarbon stream obtained in step b) and subjected to distillation in step c) also preferably contains one or more of the aforementioned impurities, namely, impurities selected from the group consisting of color-inducing species, sulfur species, meta- and ortho-xylene, ethylbenzene, phenylacetylene, cumene, n-propylbenzene, α-methylstyrene, ethyltoluene, organic chlorinated species, organic nitrogenated species, and any mixture of two or more of the aforementioned impurities. Preferred impurities are one or more sulfur species, preferably one or more sulfur species selected from the group consisting of mercaptans, disulfides, thiophenes having a boiling point of 130-150°C, and any combination of two or more thereof, and / or one or more color-inducing species including at least one of conjugated diolefins, oxygenated species, and oxygenated sulfur species, such as water, one or more alcohols, one or more ketones, one or more aldehydes, one or more fulvenes, and any combination of two or more thereof.

[0019] Preferably, the total content of the aforementioned impurities in the column side hydrocarbon stream obtained in step b) and subjected to distillation in step c) is 0.1 to 10% by weight, more preferably 0.7 to 5% by weight.

[0020] The present invention is not particularly limited with respect to the type of crystallization technique. Therefore, the at least one crystallization step preferably includes at least one static crystallization stage and / or at least one dynamic crystallization stage, more preferably at least one static melt crystallization stage and / or at least one dynamic melt crystallization stage.

[0021] According to certain preferred embodiments of the present invention, the at least one crystallization step comprises at least one static melt crystallization stage and at least one dynamic melt crystallization stage.

[0022] Particularly good results are obtained when at least one dynamic crystallization stage is a falling thin film crystallization stage, more preferably a falling thin film melt crystallization stage, however, instead of or in addition to a falling thin film crystallization stage, a suspension crystallization stage, more preferably a suspension melt crystallization stage, may also be used.

[0023] In a further development of the concept of the present invention, it is suggested that the method includes a crystallization process including 1 to 10 static crystallization stages and 1 to 10 dynamic crystallization stages. Even more preferably, the method includes a crystallization process including 1 to 5 static crystallization stages and 1 to 5 dynamic crystallization stages. When the method includes two or more dynamic crystallization stages and / or two or more static crystallization stages, each dynamic crystallization stage is fluidly coupled to one or two other dynamic crystallization stages, each static crystallization stage is fluidly coupled to one or two dynamic crystallization stages, and one dynamic crystallization stage is fluidly coupled to one static crystallization stage. In other words, the dynamic crystallization stages are arranged in series with each other, and the static crystallization stages are arranged in series with each other. The numbering begins with the static crystallization stages and dynamic crystallization stages that are fluidly coupled. Thus, if the crystallization includes four dynamic crystallization stages and four static crystallization stages, the first dynamic crystallization stage and the first static crystallization stage are fluidly coupled to each other. The first dynamic crystallization stage is fluidly coupled with the second dynamic crystallization stage, which is coupled with the third dynamic crystallization stage, which is coupled with the fourth dynamic crystallization stage. Similarly, the first static crystallization stage is fluidly coupled with the second static crystallization stage, which is coupled with the third static crystallization stage, which is coupled with the fourth static crystallization stage. In both series, the first crystallization stage is the most upstream crystallization stage, and the second, third, and fourth crystallization stages are located downstream of the first crystallization stage in the series.

[0024] According to a first particularly preferred embodiment of the present invention, the method comprises a crystallization step comprising one static crystallization step and one dynamic crystallization step. In this variant, the column side hydrocarbon stream obtained in step b) and subjected to at least one crystallization step in step c) is preferably fed to a dynamic crystallization step to produce a styrene-rich crystallization fraction and a styrene-depleted residue fraction. The styrene-depleted residue fraction obtained in the dynamic crystallization step comprises mainly a styrene-depleted mother liquor and is fed as mother liquor to the static crystallization step. The static crystallization step also produces a styrene-rich crystallization fraction and a styrene-depleted residue fraction, and the styrene-rich crystallization fraction obtained in the static crystallization step is fed to a dynamic crystallization step where it is mixed with the column side hydrocarbon stream fed to the dynamic crystallization step. The styrene-depleted residue fraction obtained in the static crystallization step is removed, while the styrene-rich crystallization fraction obtained in the dynamic crystallization step is removed as a purified styrene composition. In principle, instead of the above embodiment, the column side hydrocarbon stream obtained in step b) can be fed to the static crystallization stage, i.e., the static crystallization and dynamic crystallization stages can be arranged in the reverse order to that described above. However, better results are obtained when the column side hydrocarbon stream obtained in step b) is fed to the dynamic crystallization stage. For the sake of completeness, it should be noted that the above terms "styrene-rich crystallization fraction" and "styrene-depleted residue fraction" refer to the styrene content of the input to the respective crystallization stage, and not to the styrene content of the column side hydrocarbon stream obtained in step b). In other words, the styrene-rich crystallization fraction obtained in the static crystallization stage has a higher styrene content than the input to this static crystallization stage (the styrene-depleted residue fraction fed to the static crystallization stage from the dynamic crystallization stage), and the styrene-depleted residue fraction has a lower styrene content than the input to this static crystallization stage.

[0025] According to a second particularly preferred embodiment of the present invention, the method comprises a crystallization process comprising 2 to 5 static crystallization stages and 2 to 5 dynamic crystallization stages. Preferably, the column side hydrocarbon stream obtained in step b) and subjected to at least one crystallization step in step c) is fed to a first of 2 to 5 dynamic crystallization stages to produce a first styrene-rich crystallization fraction and a first styrene-depleted residue fraction, the first styrene-rich crystallization fraction is fed to a second of 2 to 5 dynamic crystallization stages, and a styrene-rich crystallization fraction and a styrene-depleted residue fraction are produced in either the second stage or the optional third to fifth dynamic crystallization stages, each of the styrene-rich crystallization fractions produced in the second and optional third to fourth dynamic crystallization stages is fed to a downstream dynamic crystallization stage, and each of the styrene-depleted residue fractions produced in the second and optional third to fifth dynamic crystallization stages is fed to an upstream dynamic crystallization stage. The first styrene-depleted residue fraction is fed to a first of two to five static crystallization stages to produce a second styrene-enriched crystallization fraction and a second styrene-depleted residue fraction, the second styrene-enriched crystallization fraction is fed to a first dynamic crystallization stage, and the second styrene-depleted residue fraction is fed to a second of two to five static crystallization stages. In any of the second and any of the third to fifth static crystallization stages, a styrene-enriched crystallization fraction and a styrene-depleted residue fraction are produced, each of the styrene-depleted residue fractions produced in the second and any of the third to fourth static crystallization stages is fed to a downstream static crystallization stage, and the styrene-enriched crystallization fraction produced in the second and any of the third to fifth dynamic static stages is fed to an upstream static crystallization stage. In principle, the column side hydrocarbon stream obtained in step b) and subjected to at least one crystallization step in step c) may also be fed to one of the static crystallization stages, i.e. the static and dynamic crystallization stages may be arranged in the reverse order to that described above, although better results are obtained if the column side hydrocarbon stream obtained in step b) is fed to one of the dynamic crystallization stages.

[0026] In another variation to the preceding variation, the column side hydrocarbon stream obtained in step b) and subjected to at least one crystallization step in step c) is fed to the second of the two to five dynamic crystallization stages, but not to the first dynamic crystallization stage, the first to fifth stages again being arranged in an upstream to downstream direction, where again the most upstream dynamic crystallization stage (i.e. the first dynamic crystallization stage) is the stage which receives the styrene-enriched crystallization fraction from the first static crystallization stage and from which the styrene-depleted residue fraction is fed to the first static crystallization stage, while the most downstream dynamic crystallization stage is the stage from which the purified styrene composition is removed. Similarly, the most upstream static crystallization stage (i.e., the first static crystallization stage) is the stage that receives the styrene-depleted residue fraction from the first dynamic crystallization stage and from which a styrene-enriched crystallization fraction is fed to the first dynamic crystallization stage, while the most downstream static crystallization stage (i.e., the second static crystallization stage) is the stage from which the styrene-depleted residue fraction is removed.

[0027] For example, the method includes a crystallization process comprising two static crystallization stages and four dynamic crystallization stages. In this embodiment, the column side hydrocarbon stream obtained in step b) and subjected to at least one crystallization step in step c) is fed to a second of the dynamic crystallization stages to produce a second styrene-enriched crystallization fraction and a second styrene-depleted residue fraction. The second styrene-enriched crystallization fraction is fed to a third of the four dynamic crystallization stages to produce a third styrene-enriched crystallization fraction and a third styrene-depleted residue fraction, and the third styrene-enriched crystallization fraction is fed to a fourth of the dynamic crystallization stages to produce a fourth styrene-enriched crystallization fraction and a fourth styrene-depleted residue fraction. The fourth styrene-enriched crystallization fraction is removed as a purified styrene composition, while the fourth styrene-depleted residue fraction is fed to a third dynamic crystallization stage, the third styrene-depleted residue fraction is fed to a second dynamic crystallization stage, and the second styrene-depleted residue fraction is fed to a first dynamic crystallization stage. In the first dynamic crystallization stage, a first styrene-enriched crystallization fraction and a first styrene-depleted residue fraction are produced. While the first styrene-enriched crystallization fraction is fed to the second dynamic crystallization stage, the first styrene-depleted residue fraction is fed to the first of two static crystallization stages, where a fifth styrene-enriched crystallization fraction and a fifth styrene-depleted residue fraction are produced. The fifth styrene-enriched crystallization fraction is fed to a first dynamic crystallization stage, while the fifth styrene-depleted residue fraction is fed to a second of two static crystallization stages, where a sixth styrene-enriched crystallization fraction and a sixth styrene-depleted residue fraction are produced. The sixth styrene-enriched crystallization fraction is fed to the first static crystallization stage while the sixth styrene-depleted residue fraction is removed.

[0028] In all of the above processes, preferably, the production of the styrene-enriched crystallization fraction and the styrene-depleted residue fraction in the crystallization stage comprises the steps of removing the residual liquid from the crystallization stage as the styrene-depleted residue fraction after completion of the crystallization in the crystallization stage, melting the crystalline layer obtained in the crystallization stage, and removing the resulting crystalline melt from the crystallization stage as the styrene-enriched crystallization fraction.

[0029] To increase the purity of the purified styrene product, it is preferable to perform at least one sweating step in each crystallization stage before melting the crystal layer formed on the cooled surface of the crystallizer used in a single crystallization stage. Sweating refers to gently heating the crystal layer deposited on the cooled surface to a temperature close to the melting temperature of styrene to partially melt the crystals. The melt containing the collected adhering impurities is discharged during partial melting of the crystals and then removed from the crystallizer. To perform this sweating, the surface on which the crystals are deposited is heated to the desired temperature with a heat transfer medium. Sweating can be performed once or several times before melting the crystal layer deposited on the cooled surface. In this way, sweating results in one or more sweated fractions and purified crystal layers. Preferably, at least a portion of the first sweated fraction obtained by sweating is added to the removed residual liquid as a styrene-depleted residue fraction.

[0030] The crystallization temperature depends on the composition of the side hydrocarbon stream obtained in step b), but good results are obtained when at least one, preferably all, of the at least one static melt crystallization stage and / or the at least one dynamic melt crystallization stage are carried out at temperatures between -200°C and 30°C, preferably between -140°C and 0°C, more preferably between -100°C and -30°C.

[0031] As noted above, the present invention is particularly suited to purifying pygas as a crude hydrocarbon composition. In particular, pygas obtained by pyrolysis of recycled polystyrene is a suitable crude hydrocarbon composition. The method of the present invention allows for cost-effective purification of styrene from such feedstock compositions, which is not possible with prior art methods.

[0032] The process according to the invention results in a very pure styrene-containing composition. Preferably, the purified styrene composition has a styrene content of at least 99.00 wt%, more preferably at least 99.50 wt%, even more preferably at least 99.80 wt%, even more preferably at least 99.90 wt%, even more preferably at least 99.95 wt%, and most preferably at least 99.98 wt%.

[0033] In particular, the process according to the invention allows for the complete or at least almost complete removal of color-inducing species from the crude styrene-containing composition. In a further development of the idea of ​​the invention, it is therefore proposed that the purified styrene composition has a color of at most 15 as defined by the Pt-Co scale according to ASTM D5386.

[0034] Furthermore, the process according to the invention allows for the complete or at least nearly complete removal of sulfur species from the crude styrene-containing composition. It is therefore particularly preferred if the purified styrene composition contains less than 2 ppmw total elemental sulfur in mercaptans, disulfides and thiophenes, and / or less than 20 ppmw oxygenates.

[0035] Furthermore, the process according to the invention makes it possible to obtain a purified styrene composition containing less than 40 ppmw of impurities selected from the group consisting of phenylacetylene, mixed xylenes, ethylbenzene, cumene, ethyltoluene, n-propylbenzene, and α-methylstyrene, and / or having a polymer content of less than 10 ppmw.

[0036] Preferably, the purified styrene composition has a total organic chlorine content of less than 2 ppmw.

[0037] For example, the purified styrene composition may meet the following specifications: TIFF0007762714000001.tif205170

[0038] According to a further aspect, the present invention relates to a plant for preparing a purified styrene composition comprising at least one dividing wall column and at least one crystallization block, wherein the dividing wall column comprises an inlet line for a crude hydrocarbon composition containing styrene, a line for an overhead hydrocarbon stream, a line for a bottoms hydrocarbon stream, and a line for a side hydrocarbon stream, the at least one crystallization block comprises an inlet line, and the line for the side hydrocarbon stream of the dividing wall column leads to (or is connected to or is, respectively, an inlet line of) the at least one crystallization block, and preferably the at least one crystallization block comprises two or more static crystallization stages and two or more dynamic crystallization stages.

[0039] Preferably, the dividing wall column is an intermediate dividing wall column. In particular, good results are obtained when the dividing wall of the intermediate dividing wall column extends substantially vertically downward over a height that is the linear distance between the bottom and the top of the intermediate dividing wall column, for 10 to 90%, preferably 20 to 80%, more preferably 30 to 70%, and most preferably 40 to 60% of the height of the intermediate dividing wall column, where essentially vertically downward means that the angle between the dividing wall and the longitudinal axis of the intermediate dividing wall column is at most 20°, preferably at most 10°, more preferably at most 5°, and most preferably 0°.

[0040] As a further development of the inventive concept, it is suggested that the plant further comprises a pyrolysis or catalytic reactor unit block comprising an inlet line and an outlet line, the outlet line of the pyrolysis or catalytic reactor unit block leading to (or being connected to or being, respectively) the inlet line of the dividing wall column.

[0041] According to a particularly preferred embodiment of the present invention, at least one crystallization block comprises at least one static crystallization section comprising two or more static crystallization stages, at least one dynamic crystallization section comprising two or more dynamic crystallization stages, and at least two conduits fluidly connecting at least one of the two or more static crystallization stages with at least one of the two or more dynamic crystallization stages. Preferably, one of the at least two conduits fluidly connects one of the static crystallization stages with one of the dynamic crystallization stages so that a styrene-depleted residue fraction obtained in the dynamic crystallization stage can be fed to the static crystallization stage fluidly connected to the dynamic crystallization stage, one of the at least two conduits fluidly connects a static crystallization stage with a dynamic crystallization stage fluidly connected to the static crystallization stage so that a styrene-enriched crystallization fraction obtained in the static crystallization stage can be fed to the dynamic crystallization stage, and each two of the remaining static crystallization stages are fluidly connected to each other by at least two conduits, and each two of the remaining dynamic crystallization stages are fluidly connected to each other by at least two conduits.

[0042] The term "crystallization block" refers to an apparatus for a purification process that includes one or more crystallizers. Furthermore, the term crystallization stage is used not only to denote the respective method step or stage, but also to denote the piece of equipment in which the crystallization stage is performed, i.e., the crystallizer. A crystallization stage as a feature of an apparatus may also be referred to as a crystallizer, crystallizer unit, etc.

[0043] Preferably, the one or more static crystallization stages are static melt crystallization stages and the one or more dynamic crystallization stages are dynamic melt crystallization stages.

[0044] Preferably, each of the dynamic crystallization stages is fluidly coupled to one or two other dynamic crystallization stages, and each of the static crystallization stages is fluidly coupled to one or two other static crystallization stages.

[0045] It is further preferred that at least one crystallization block comprises 2 to 5 static crystallization stages, 2 to 5 dynamic crystallization stages, and at least two conduits fluidly connecting at least one of the 2 to 5 static crystallization stages with at least one of the 2 to 5 dynamic crystallization stages, one of the at least two conduits fluidly connecting one of the static crystallization stages with one of the dynamic crystallization stages so that a styrene-depleted residue fraction obtained in the dynamic crystallization stage can be supplied to the static crystallization stage fluidly connected to the dynamic crystallization stage, one of the at least two conduits fluidly connecting a static crystallization stage with the dynamic crystallization stage fluidly connected to the static crystallization stage so that a styrene-enriched crystallization fraction obtained in the static crystallization stage can be supplied to the dynamic crystallization stage, and that every two of the remaining static crystallization stages are fluidly connected to each other by at least two conduits, and every two of the remaining dynamic crystallization stages are fluidly connected to each other by at least two conduits.

[0046] To achieve the foregoing and other advantages and objectives of the present invention, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments illustrated in the accompanying drawings, in which: The invention will be described with further specificity and detail using the accompanying drawings, with the understanding that these drawings depict only typical embodiments of the invention and therefore should not be considered as limiting its scope. [Brief explanation of the drawings]

[0047] [Figure 1] 1 is a schematic diagram of a plant according to one embodiment of the present invention; [Figure 2] 1 is a schematic diagram of a crystal block used in a method according to one embodiment of the present invention; [Figure 3] FIG. 2 is a schematic diagram of a crystallization block used in a method and plant according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0048] FIG. 1 shows a plant 10 particularly suited for purifying a styrene-containing stream produced by thermal cracking from a recycled polystyrene stream. Plant 10 includes a thermal cracking or catalytic reactor unit block 12, a dividing wall column 14, and a crystallization block 16. The thermal cracking or catalytic reactor unit block 12 includes an inlet line 18 for a stream comprising a feedstock, e.g., recycled polystyrene, and an outlet line for the thermal cracking product, which also serves as an inlet line 20 for a crude hydrocarbon stream containing styrene. Dividing wall column 14 is an intermediate dividing wall column 14, and its dividing wall 28 extends vertically downward, measured over a height that is the linear distance between the bottom and top of intermediate dividing wall column 14, from about 25% to about 75% of the height of the intermediate dividing wall column. Dividing wall column 14 includes line 22 for an overhead hydrocarbon stream, line 24 for a bottoms hydrocarbon stream, and line 26 for a side hydrocarbon stream, which serves as an inlet line to crystallization block 16. Crystallization block 16 includes discharge conduit 30 for the styrene-depleted residue fraction and discharge conduit 32 for the purified styrene composition. Crystallization block 16 can be configured as shown in FIG. 2 or as shown in FIG. 3.

[0049] In operation, the thermal cracking of polystyrene occurs within the thermal cracking or catalytic reactor unit block 12, which may be operated in either a thermal or catalytic mode. The effluent from the thermal cracking or catalytic reactor unit block 12 contains styrene and a number of impurities and is fed as a crude styrene-containing hydrocarbon stream via line 20 to the intermediate dividing wall column 14. In the intermediate dividing wall column 14, the mixture is obtained as a bottoms hydrocarbon stream and withdrawn via line 24. 9+ a hydrocarbon stream, C obtained as an overhead hydrocarbon stream and withdrawn via line 22; 7-The resulting hydrocarbon stream is separated into a C hydrocarbon stream 26, which is obtained as a column hydrocarbon stream and withdrawn via line 26, and a styrene-containing C hydrocarbon stream 26. The column hydrocarbon stream is fed via line 26 to crystallization block 16 where it is crystallized. During crystallization, impurities, particularly color-inducing species, sulfur species, oxygenates, and impurities having boiling points close to that of styrene, such as ethylbenzene, are reliably and almost completely removed and withdrawn as a styrene-depleted residue fraction via line 30, while the purified styrene composition is withdrawn via line 32.

[0050] 2 shows an embodiment of a crystallization block 16 for carrying out a process for preparing a purified styrene composition according to an embodiment of the present invention, such as the embodiment in the plant shown in FIG. 1. The crystallization block 16 includes a first dynamic melt crystallization section 34 having one falling-film crystallization stage or one falling-film crystallizer 36 as a dynamic melt crystallization stage or crystallizer, respectively. The crystallization block 16 further includes a second static melt crystallization section 38 having one static melt crystallization stage 40 or one static melt crystallizer, respectively. The falling-film crystallizer 36 is connected to an inlet conduit 26 for the column side hydrocarbon stream obtained in the dividing wall column. The falling-film crystallizer 36 further includes a discharge conduit 44 for discharging the purified styrene composition from the falling-film crystallizer 36 and the crystallization block 16. The static melt crystallizer 40 is connected to the falling film crystallizer 36 via a transfer conduit 46 suitable for transferring the first styrene-depleted residue fraction obtained by crystallization in the falling film crystallizer 36 to the static melt crystallizer 40. In this regard, the transfer conduit 46 is in fluid communication with both the falling film crystallizer 36 and the static melt crystallizer 40. The static melt crystallizer 40 comprises a discharge conduit 48 that serves to discharge the second styrene-depleted residue fraction obtained by crystallization in the static melt crystallizer 40 from the static melt crystallizer 40 and the crystallization block 16. The recirculation conduit 50 provides fluid communication between the static melt crystallizer 40 and the falling film crystallizer 36, thus making it possible to recycle at least a portion of the second styrene-rich crystallized composition resulting from crystallization in the static melt crystallizer 40 back to the falling film crystallizer 36.

[0051] 3 shows another embodiment of the crystallization block 16 for carrying out the method for preparing a purified styrene composition according to the present invention. The first dynamic melt crystallization section 34 includes four falling-thin film crystallization stages 36a, 36b, 36c, and 36d, and the second static melt crystallization section 38 includes two static melt crystallization stages 40a and 40b. Transfer conduits 52a, 52b, and 52c are provided between the falling-thin film crystallization stages 36a, 36b, 36c, and 36d, so that the styrene-depleted residue fraction obtained by falling-thin film crystallization in a single falling-thin film crystallization stage 36a, 36b, 36c, and 36d can be transferred from one of the falling-thin film crystallization stages 36b, 36c, and 36d to the respective upstream falling-thin film crystallization stage 36a, 36b, and 36c. Furthermore, the falling film crystallization stages 36a, 36b, 36c, 36d are connected via recycle conduits 54a, 54b, 54c suitable for recycling at least a portion of the styrene-enriched crystallization fraction obtained by falling film crystallization in a single falling film crystallization stage 36a, 36b, 36c, 36d from one of the falling film crystallization stages 36a, 36b, 36c to the respective downstream falling film crystallization stage 36b, 36c, 36d. The inlet conduit 26 for the column side hydrocarbon stream obtained in the dividing wall column is connected to the second falling film crystallization stage 36b so that a crude styrene-containing composition can be introduced into the second falling film crystallization stage 36b. A discharge conduit 44 is provided in the most downstream falling film crystallization stage 36d to remove the purified styrene composition from the crystallization block 16. A transfer conduit 46 provides fluid communication between the most upstream falling film crystallization stage 36a of the first dynamic melt crystallization section 34 and the most upstream static melt crystallization stage 40b of the second static melt crystallization section 38, so that the styrene-depleted residue fraction obtained by crystallization in the falling film crystallization stage 36a can be transferred to the static crystallizer 40b of the second static melt crystallization section 38. The static melt crystallization stages 40a and 40b are connected via a transfer conduit 56 for transferring the styrene-depleted residue fraction obtained by crystallization from the static melt crystallization stage 40b to the static melt crystallization stage 40a.In addition, static melt crystallization stage 40a and static melt crystallization stage 40b are connected via a recycle conduit 58, which allows the styrene-rich crystallization fraction resulting from crystallization in static melt crystallization stage 40a to be transferred to the static melt crystallizer of crystallization stage 40b. Furthermore, static melt crystallization stage 40a is equipped with a discharge conduit 48 for discharging the styrene-depleted residue fraction obtained by crystallization in static melt crystallization stage 40a from crystallization block 38. Recycle conduit 50 provides fluid communication between static melt crystallization stage 40b and falling film crystallization stage 36a, thus allowing at least a portion of the styrene-rich crystallization fraction obtained in static melt crystallization stage 40b of second static melt crystallization section 38 to be recirculated back to falling film crystallization stage 36a of first dynamic melt crystallization section 34.

[0052] During operation of the crystallization block 16 shown in Figure 3, the column side hydrocarbon stream obtained in the dividing wall column is fed to the falling-film crystallization stage 36b via the inlet conduit 26. In each of the falling-film crystallization stages 36a, 36b, 36c, and 36d, a styrene-rich crystallization composition and a styrene-depleted residue fraction are prepared. Each of the styrene-depleted residue fractions obtained in one of the falling-film crystallization stages 36b, 36c, and 36d is transferred via the transfer conduits 52a, 52b, and 52c to the respective upstream falling-film crystallization stage 36a, 36b, and 36c. Furthermore, each of the styrene-rich fractions obtained in one of the falling-film crystallization stages 36a, 36b, and 36c is at least partially recycled via the recycle conduits 36a, 36b, and 36c to the respective downstream falling-film crystallization stage 36b, 36c, and 36d. The styrene-depleted residue fraction obtained after crystallization in the falling film crystallization stage 36a of the first dynamic melt crystallization section 34 is transferred via a transfer conduit 46 to the static melt crystallization stage 40b of the second static melt crystallization section 38. The styrene-depleted residue fraction obtained in the static melt crystallization stage 40b is transferred via a transfer conduit 56 to the downstream static melt crystallization stage 40a. Furthermore, the styrene-rich crystallization fraction obtained in the static melt crystallization stage 40a is at least partially recycled via a recycle conduit 58 to the upstream static melt crystallization stage 40b. The styrene-rich crystallization fraction obtained after crystallization in the static melt crystallization stage 40b is recycled via a recycle conduit 50 to the falling film crystallization stage 36a of the first dynamic melt crystallization section 34. The final purified styrene composition obtained in crystallization stage 36d is removed from crystallization block 16 via discharge conduit 44, while the final styrene-depleted residue fraction is removed from static melt crystallization stage 40a and from crystallization block 16 via discharge conduit 48.

[0053] In accordance with the present invention, Table 2 lists various impurities, along with their melting points, that may typically be present in the crude hydrocarbon stream containing styrene and the side hydrocarbon stream obtained from the dividing wall column. The reasons for removing impurities from the side hydrocarbon stream obtained from the dividing wall column by crystallization block 16 are twofold: a) some species have a lower melting point than styrene, and b) during the crystallization process, impurities with higher melting points are more soluble in the mother liquor. Therefore, despite their higher melting points, these impurities can be removed from styrene by crystallization. Product purity improvement is directly correlated with an increase in the number of crystallization stages. Recovery, on the other hand, is a function of the number of residue stages. TIFF0007762714000002.tif145170 [Explanation of symbols]

[0054] 10. Plant for preparing purified styrene compositions 12 Pyrolysis or catalytic reactor unit block 14 (middle part) bulkhead tower 16 Crystallization Blocks 18 Inlet line for feedstock 20 Outlet line for pyrolysis products / inlet line for crude hydrocarbon stream containing styrene 22 Line for overhead hydrocarbon stream 24 Line for bottom hydrocarbon stream 26 Line for column side hydrocarbon stream 28 Partition Tower Partition Wall 30 Discharge conduit for styrene-depleted residue fraction 32 Discharge conduit for purified styrene composition 34 First Dynamic Melt Crystallization Section 36 Dynamic (Falling Film) Melt Crystallizer / Crystallization Stage 36a, 36b, 36c, 36d Dynamic melting / flowing thin film crystallization stage 38 Second static melt crystallization section 40 Static Melt Crystallizer / Crystallization Stage 40a, 40b Static melt crystallization stage 44 Discharge conduit for purified styrene composition 46 (Transfer) conduit for feeding the styrene-depleted residue fraction from the dynamic crystallization section to the static crystallization section 48 Discharge conduit for styrene-depleted residue fraction 50 (recycle) conduit for feeding the styrene-rich residue fraction from the static crystallization section to the dynamic crystallization section 52a, 52b, 52c Conduits for the styrene-depleted residue fraction in the dynamic crystallization section 54a, 54b, 54c Conduits for styrene-enriched crystallization fractions in the dynamic crystallization section 56 Conduit for styrene-depleted residue fraction in static crystallization section 58 Conduit for styrene-rich crystallization fraction in static crystallization section

Claims

1. 1. A method for preparing a purified styrene composition, comprising the steps of: a) providing a crude hydrocarbon composition containing styrene; b) subjecting the crude hydrocarbon composition provided in step a) to distillation in a dividing wall column (14) to produce an overhead hydrocarbon stream, a bottoms hydrocarbon stream, and a side hydrocarbon stream; c) subjecting the column side hydrocarbon stream obtained in step b) to at least one crystallization step to obtain a purified styrene composition; A method comprising:

2. 2. The process of claim 1, wherein the crude hydrocarbon composition provided in step a) is subjected to distillation in step b) in an intermediate dividing wall column (14).

3. The overhead hydrocarbon stream obtained in step b) is ７－ the bottom hydrocarbon stream obtained in step b) is C ９＋ 3. The process according to claim 1 or 2, wherein the hydrocarbon stream obtained in step b) is a styrene-containing hydrocarbon stream.

4. 4. The process according to claim 1, wherein the side hydrocarbon stream obtained in step b) and subjected to at least one crystallization step in step c) contains at least 80% by weight of styrene.

5. 5. The process of any one of claims 1 to 4, wherein the crude hydrocarbon composition provided in step a) and the column side hydrocarbon stream obtained in step b) contain one or more impurities selected from the group consisting of color-inducing species, sulfur species, meta- and ortho-xylenes, ethylbenzene, phenylacetylene, cumene, n-propylbenzene, α-methylstyrene, ethyltoluene, organic chlorinated species, organic nitrogenated species, and any mixture of two or more of the foregoing impurities.

6. 6. The process of claim 5, wherein the crude hydrocarbon composition provided in step a) and the side hydrocarbon stream obtained in step b) contain one or more sulfur species as impurities.

7. 7. The process according to claim 5 or 6, wherein the crude hydrocarbon composition provided in step a) and the side hydrocarbon stream obtained in step b) contain as impurities one or more color-inducing species comprising at least one of conjugated diolefins, oxygenated species and oxygenated sulfur species.

8. 8. The method of claim 1, wherein the at least one crystallization step comprises at least one static melt crystallization stage and at least one dynamic melt crystallization stage.

9. 9. The process according to any one of claims 1 to 8, wherein the crude hydrocarbon composition provided in step a) is pygas.

10. 10. The process of any one of claims 1 to 9, wherein the purified styrene composition has a styrene content of at least 99.00 wt%.

11. The purified styrene composition meets the following criteria: i) having a maximum color of 15 as defined by the Pt-Co scale according to ASTM D5386; ii) containing less than 2 ppmw total elemental sulfur contained in mercaptans, disulfides, and thiophenes; iii) containing less than 20 ppmw of oxygenates; iv) containing less than 40 ppmw of impurities selected from the group consisting of phenylacetylene, mixed xylenes, ethylbenzene, cumene, ethyltoluene, n-propylbenzene, and alpha-methylstyrene; v) having a polymer content of less than 10 ppmw; vi) having a total organic chlorine content of less than 2 ppmw; The method according to any one of claims 1 to 10, wherein one or more of the following conditions are satisfied:

12. 1. A plant (10) for preparing a purified styrene composition comprising at least one dividing wall column (14) and at least one crystallization block (16), wherein the dividing wall column (14) comprises an inlet line (20) for a crude hydrocarbon composition containing styrene, a line (22) for an overhead hydrocarbon stream, a line (24) for a bottoms hydrocarbon stream, and a line (26) for a side hydrocarbon stream; A plant (10) in which at least one crystallization block (16) includes an inlet line (26), a line for a column side hydrocarbon stream of the dividing wall column (14) is the inlet line (26) of the at least one crystallization block (16), and the at least one crystallization block (16) includes two or more static crystallization stages (40a, 40b) and two or more dynamic crystallization stages (36a, 36b, 36c, 36d).

13. 13. The plant (10) of claim 12, wherein the dividing wall column (14) is an intermediate dividing wall column (14), and the dividing wall (28) of the intermediate dividing wall column (14) extends vertically downwards over a height that is the linear distance between the bottom and top of the intermediate dividing wall column (14) for 10 to 90% of the height of the intermediate dividing wall column (14), where vertically downwards means that the angle between the dividing wall (28) of the intermediate dividing wall column (14) and its longitudinal axis is at most 20°.

14. 14. The plant (10) of claim 12 or 13, further comprising a pyrolysis or catalytic reactor unit block (12) including an inlet line (18) and an outlet line (20), the outlet line (20) of the pyrolysis or catalytic reactor unit block (12) being the inlet line (20) of the dividing wall column (14).

15. The crystallization block (16) at least one static crystallization section (38) comprising 2 to 5 static crystallization stages (40a, 40b); at least one dynamic crystallization section (34) comprising 2 to 5 dynamic crystallization stages (36a, 36b, 36c, 36d); at least two conduits (46, 50) fluidly connecting the at least one static crystallization stage (40a, 40b) with at least one of the one or more dynamic crystallization stages (36a, 36b, 36c, 36d); Including, One of the at least two conduits (46, 50) fluidly connects one of the static crystallization stages (40a, 40b) with one of the dynamic crystallization stages (36a, 36b, 36c, 36d) so that the styrene-depleted residue fraction obtained in the dynamic crystallization stages (36a, 36b, 36c, 36d) can be fed to the static crystallization stages (40a, 40b) that are fluidly connected to the dynamic crystallization stages (36a, 36b, 36c, 36d), and one of the at least two conduits (46, 50) connects the static crystallization stages (40a, 40b) with the static crystallization stages (40a, 40b) and a dynamic crystallization stage (36a, 36b, 36c, 36d) which is fluidly connected to the static crystallization stage (40a, 40b) so that the styrene-rich crystallization fraction obtained in the static crystallization stage (40a, 40b) can be fed to the dynamic crystallization stage (36a, 36b, 36c, 36d), and each two of the remaining static crystallization stages (40a, 40b) are fluidly connected to each other by at least two conduits (46, 50), and each two of the remaining dynamic crystallization stages (36a, 36b, 36c, 36d) are fluidly connected to each other by at least two conduits (46, 50). A plant (10) according to any one of claims 12 to 14.

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