The present invention relates to a method for preparing a purified styrene compo- sition from a styrene containing feedstock, such as from naphtha cracked pyrolysis gasoline including specific impurities having melting points higher than styrene as well as optionally in addition thereto other impurities. Moreover, the present inven- tion relates to a plant, in which the method may be performed.

BACKGROUND OF THE INVENTION

Styrene is an important building block for polymers, such as polystyrene, acryloni- trile-butadiene-styrene (ABS)/styrene-acrylonitrile (SAN) resins, styrene-butadiene (SB) copolymer latexes, unsaturated polyester resins, styrene-butadiene rubber (SBR) elastomers and latexes. It is one of the highest volume commodity chemi- cals traded, and over 30% of annual styrene production is traded internationally. Predominantly, styrene is produced starting with raw materials benzene and eth- ylene. Benzene is alkylated to produce ethylbenzene (EB), and the EB is convert- ed into styrene via the conventional dehydrogenation process or ethyl- benzene/styrene monomer (EBSM) process, respectively, or the propylene ox- ide/styrene monomer (POSM) process. Typically, styrene plants are located near ethylene crackers due to the gaseous nature of ethylene which makes it relatively difficult to transport as compared to benzene.

Apart from the on-purpose production route via EBSM/POSM, styrene is also pre- sent in hydrocarbon streams, such as pyrolysis gasoline obtained from steam cracking of naphtha, hydrocarbon fraction obtained from polystyrene pyrolysis, gas oils etc. Styrene extraction from these hydrocarbon streams, although far less in quantity as compared to EBSM/POSM, presents an attractive economic opportuni- ty to the operator due to the low feedstock cost. However, this separation is tech- nically difficult due to the presence of close boiling molecules and impurities com- ing from the starting feedstock. Removal of close boiling molecules, such as mixed xylenes, ethylbenzene, etc., from styrene by normal distillation is an energy inten- sive process. Solvent based extractive distillation techniques, as advocated in U.S. Patent No. 5,849,982, have been developed and commercially deployed which remove the close boiling molecules at reduced specific energy consumption.

Despite producing high purity styrene (>99.8 wt%), the extractive distillation route, in general, produces a styrene which contains, as compared to EBSM/POSM sty- rene, impurities such as chromophores, sulfur, oxygenates, etc. in the starting feedstock. These impurities affect the downstream polymerization process and thereby, the properties of the polymer produced. Different methods for impurities removal have been cited in prior art based on chemical or adsorbent treatment. Chemical treatment may involve use of dienophiles, nitric acid, alkali, etc. Use of adsorbents may entail clays, alumina, etc. These additional treatment steps can produce a marketable product meeting the ASTM specification for styrene mono- mer. However, styrene losses via unwanted polymerization, polymer formation across adsorbent beds due to the inherent heat sensitivity of styrene require com- plex design precautions to prevent polymerization, and usage of complex equip- ment render these methods a nuisance for the owner/operator.

Considering the above, the objective underlying the present invention is to provide a method for preparing a purified styrene composition from a styrene containing feed composition, such as in particular a styrene containing feed composition ob- tained by the extractive distillation route, such as from pygas from naphtha cracked pyrolysis gasoline or the like, which reliably and efficiently removes specif- ic impurities resulting therefrom, such as color inducing species, sulfur species, alpha-methylstyrene and oxygenates, as well as in addition thereto other impuri- ties, from the styrene in an energy efficient manner, even if the impurities are con- tained in a comparable high amount in the styrene containing feed composition, so as to obtain in a cost efficient manner a very pure styrene composition.

SUMMARY OF THE INVENTION

In accordance with the present invention, this object is satisfied by providing a method for preparing a purified styrene composition, wherein the method compris- es providing a crude composition containing 70% by weight or more styrene based on the total weight of the crude composition and subjecting the crude composition to at least one crystallization step, wherein the at least one crystallization step comprises at least one of a static crystallization stage and a dynamic crystalliza- tion stage, and wherein the crude composition contains one or more impurities selected from the group consisting color inducing species, oxygenates, sulfur spe- cies, alpha-methylstyrene and arbitrary combinations of two or more thereof.

This solution bases on the surprising finding that by subjecting a crude styrene containing composition to a crystallization comprising at least one of a static crys- tallization stage and a dynamic crystallization stage not only a respective energy efficient method is obtained, but that even impurities having a higher melting point than styrene, namely color inducing species, oxygenates, sulfur species, alpha- methylstyrene, such as they result from feed compositions obtained by the extrac- tive distillation route, such as from pygas from naphtha cracked pyrolysis gasoline or the like, are reliably and completely or at least substantially completely removed from the styrene along with other impurities, such as those selected from the group consisting of meta-xylenes, ortho-xylenes, ethylbenzene, phenylacetylene, cumene, n-propylbenzene, ethyltoluene, organo-chlorinated and organo- nitrogenated species and arbitrary combinations of two or more thereof. This is surprising, since it is unexpected that some of these impurities having a higher melting point than styrene can be reliably and completely separated by crystalliza- tion from styrene. On the contrary, it must have been expected that such impurities cannot be separated by crystallization from styrene, because they have a higher melting point and thus are expected to crystallize before styrene. Therefore, even if it is known to purify certain styrene compositions, such as those comprising ethylbenzene, by crystallization, a person skilled in the art being confronted with the aforementioned object would have not considered to use crystallization for solving this object. Without wishing to be bound to any theory, the inventors of the present invention consider that the reason, why styrene may be purified from the aforementioned impurities by crystallization is that the solubility of these impurities in the mother liquor is higher that the solubility in styrene crystals. This principle, in general, does not apply to any impurities contained in styrene. Thus, this surpris- ing behavior applies specifically to the impurities as have been described above. This discovery enables removal of these impurities along with other impurities, such as those selected from the group consisting of meta-xylenes, ortho-xylenes, ethylbenzene, phenylacetylene, cumene, n-propylbenzene, ethyltoluene, organo- chlorinated and organo-nitrogenated species and arbitrary combinations of two or more thereof. All in all, the method in accordance with the present invention allows to remove the aforementioned impurities from styrene in a cost efficient manner, why the method in accordance with the present invention is in particular suitable for preparing a purified styrene composition from styrene containing compositions, which could be up so far not economically reasonable used for the preparation of pure styrene, such as in particular from styrene containing compositions having been obtained by the extractive distillation route, such as from pygas from naphtha cracked pyrolysis gasoline or the like.

As set out above, the method in accordance with the present invention comprises the step of providing a crude composition containing styrene and subjecting the crude composition to at least one crystallization step. The step of providing a crude composition may comprise that a styrene containing feed composition is directly subjected to the at least one crystallization step or that a styrene contain- ing feed composition is first processed for instance by making use of one or more distillation steps or other steps, before the processed composition is subjected to the at least one crystallization step. In order to strictly divide between the respec- tive styrene containing compositions, herein "crude composition containing sty- rene” or "crude composition” means the styrene containing composition, which is subjected to the at least one crystallization step, whereas “styrene containing feed composition” means a composition, from which the "crude composition containing styrene” may be prepared, and “purified styrene composition” means the styrene composition, which is obtained after the crystallization.

Moreover, as known in the art crystallization processes or steps, respectively may be typically carried out in multiple stages, i.e. in several crystallization stages. In view of this, in the present application a crystallization step is defined as compris- ing one or more crystallization stages.

As further set out above, the method in accordance with the present invention is particularly suitable to remove impurities having, at ambient pressure, a higher melting point than styrene. Accordingly, preferably the crude composition contains as impurity one or more impurities selected from the group consisting color induc- ing species, oxygenates, sulfur species, alpha-methylstyrene and arbitrary combi- nations of two or more thereof, each of which having a higher melting point than styrene. For instance, the melting point of the one or more above mentioned impu- rities is at ambient pressure -30°C or more, such as -27°C or more, -25°C, -20°C or more or -15°C or more. Suitable oxygenates, i.e. hydrocarbon compounds comprising at least one oxygen atom in the molecule, are water, alcohols, ketones, aldehydes and carboxylic acids. Color inducing species are defined as all com- pounds, which impart color of more than 10 on a Pt-Co scale to styrene. Sulfur species are defined as all compounds containing at least one sulfur atom in the molecule. In a further development of the idea of the present invention, it is proposed that the crude composition contains as impurity: i) one or more sulfur species selected from the group consisting of alkyl, naphthenic or aromatic mercaptans, alkyl, naphthenic or aromatic disul- fides, alkyl, aromatic, naphthenic or vinyl thiophenes (such as dimethyl thiophenes or vinyl thiophenes), oxygenated sulfur containing hydrocar- bon compounds or any other hydrocarbon compound including at least one sulfur atom in its molecule and arbitrary combinations of two or more thereof, such as for instance sulfur species selected from the group consisting of mercaptans, disulfides, thiophenes having e.g. a boiling point of 130 to 150°C, and/or ii) one or more color inducing species selected from the group consisting of fulvenes, conjugated diolefins, oxygenated species, oxygenated sul- fur species, styrene oligomers, alkynes and hydrocarbon compounds comprising conjugated olefin and alkyne bonds, and any other com- pound imparting a color of more than 10 as defined on Pt-Co scale to styrene, such as for instance selected from the group consisting of con- jugated diolefins, oxygenated species and oxygenated sulfur species having e.g. a boiling point of 130 to 150°C.

Preferably the crude composition used in the method in accordance with the pre- sent invention contains 10 to 30% by weight and more preferably 10 to 20% by weight based on the total weight of the crude composition of one or more impuri- ties selected from the group consisting of color inducing species, oxygenates, sul- fur species, alpha-methylstyrene and arbitrary combinations of two or more thereof and in particular one or more of the aforementioned impurities having a higher melting point (at ambient pressure) than styrene in the absence or in the presence of other impurities, such as those selected from the group consisting of meta- xylenes, ortho-xylenes, ethylbenzene, phenylacetylene, cumene, n-propylbenzene, ethyltoluene, organo-chlorinated and organo-nitrogenated spe- cies and arbitrary combinations of two or more thereof. Surprisingly, even such high amounts of these impurities may be separated from styrene with the method in accordance with the present invention.

In addition to the aforementioned specific impurities, the crude composition used in the method in accordance with the present invention may contain one or more further impurities, such as one or more impurities selected from the group consist- ing of meta-xylenes, ortho-xylenes, ethylbenzene, phenylacetylene, cumene, n-propylbenzene, ethyltoluene, organo-chlorinated and organo-nitrogenated spe- cies and arbitrary combinations of two or more thereof.

In accordance with the present invention, the styrene content of the crude compo- sition is at least 70% by weight. Preferably, the styrene content of the crude com- position is more than 80 to 95% by weight, more preferably more than 95 to 99% by weight or more than 99% by weight, such as more than 99.8% by weight.

In accordance with the present invention, the at least one crystallization step com- prises at least one of a static crystallization stage and a dynamic crystallization stage, i.e. i) either at least one static crystallization stage, or ii) at least one dynam- ic crystallization stage or iii) a combination of at least one static crystallization stage and at least one dynamic crystallization stage, so as to produce in the at least one crystallization step a styrene enriched crystallized fraction and a styrene depleted residue fraction.

In accordance with a particular preferred embodiment of the present invention, the crystallization is performed as melt crystallization. Therefore, it is preferred that the at least one static crystallization stage, if included, is at least one static melt crys- tallization stage and that the at least one dynamic crystallization stage, if included, is at least one dynamic melt crystallization stage. Good results are in particular obtained, when as at least one dynamic crystalliza- tion stage a falling film crystallization stage and more preferably a falling film melt crystallization stage is used. Alternatively, preferably as at least one dynamic crys- tallization stage a suspension crystallization stage and more preferably a suspen- sion melt crystallization stage is used.

In particular, the present invention does not comprise any crystallization stage making use of a high pressure inert light gas to create chilling via a Joule Thom- son effect. Therefore, none of the at least one crystallization step of the method in accordance with the present invention comprises admixing to the crude composi- tion a liquefied normally gaseous hydrocarbon while maintaining a temperature and pressure on the mixture such as to cause at least the major portion of the liq- uefied normally gaseous hydrocarbon to remain in liquid phase, introducing the resulting mixture into a pressure reduction zone and reducing the pressure on the mixture thereby causing evaporation of at least a portion of the liquefied normally gaseous hydrocarbon with consequent chilling of the remaining mixture being suf- ficient to cause the formation of styrene crystals but insufficient of causing the for- mation of solid eutectic.

Preferably, the at least one crystallization step of the method comprises at least one static melt crystallization stage, at least one falling film melt crystallization stage, at least one suspension melt crystallization stage or a combination of two or more thereof.

In accordance with a particular preferred embodiment of the present invention, the at least one crystallization step of the method comprises at least one of a static melt crystallization stage and at least one dynamic melt crystallization stage. By using static crystallization and dynamic crystallization, the disadvantages of both technologies are minimized by the combination, namely the slowness of static crystallization by also using the much faster dynamic crystallization and energy intensity of dynamic crystallization by also using the much more energy efficient static crystallization.

For instance, the at least one crystallization step comprises i) at least one static crystallization stage and ii) at least one falling film crystallization stage and/or at least one suspension crystallization stage.

In a further development of the idea of the present invention, it is suggested that the method comprises a crystallization step, which comprises one to ten static crystallization stages and one to ten dynamic crystallization stages. Even more preferably, the method comprises a crystallization step, which comprises one to five static crystallization stages and one to five dynamic crystallization stages. If the method comprises two or more dynamic crystallization stages and/or two or more static crystallization stages, each of the dynamic crystallization stages is flu- idly coupled with one or two other dynamic crystallization stages, each of the static crystallization stages is fluidly coupled with one or two other static crystallization stages and one of the dynamic crystallization stages is fluidly coupled with one of the static crystallization stages. In other words, the dynamic crystallization stages are arranged in series to each other and the static crystallization stages are ar- ranged in series to each other. The numbering starts from the static crystallization stage and the dynamic crystallization stage, which are fluidly coupled together. Thus, if the crystallization comprises four dynamic crystallization stages and four static crystallization stages, the first dynamic crystallization stage and the first stat- ic crystallization stage are those, which are coupled with each other. The first dy- namic crystallization stage is fluidly coupled with the second dynamic crystalliza- tion stage, which is also coupled with the third dynamic crystallization stage, wherein the third dynamic crystallization stage is also coupled with the fourth dy- namic crystallization stage. Likewise thereto, the first static crystallization stage is fluidly coupled with the second static crystallization stage, which is also coupled with the third static crystallization stage, wherein the third static crystallization stage is also coupled with the fourth static crystallization stage. In both series, the first crystallization stage is the most upstream crystallization stage, wherein the second, third and fourth crystallization stages are located downstream of the first crystallization stage.

In accordance with a first particular preferred embodiment of the present invention, the method comprises a crystallization step, which comprises one static crystalli- zation stage and one dynamic crystallization stage. In this variant, the crude com- position is preferably fed into the dynamic crystallization stage so as to produce a styrene enriched crystallized fraction and a styrene depleted residue fraction. The styrene depleted residue fraction obtained in the dynamic crystallization stage mainly contains the styrene depleted mother liquor and is fed into the static crys- tallization stage as feed. Also in the static crystallization stage a styrene enriched crystallized fraction and a styrene depleted residue fraction are produced, wherein the styrene enriched crystallized fraction obtained in the static crystallization stage is fed into the dynamic crystallization stage and mixed there with the crude com- position fed into the dynamic crystallization stage. The styrene depleted residue fraction obtained in the static crystallization stage is withdrawn, whereas the sty- rene enriched crystallized fraction obtained in the dynamic crystallization stage is withdrawn as the purified styrene composition. In principle, the crude composition may be alternatively to the above fed into the static crystallization stage, i.e. the static crystallization and dynamic crystallization stages may be arranged in re- versed order to the aforementioned description. However, better results are ob- tained, when the crude composition is fed into the dynamic crystallization stage. For the sake of completeness, it is noted that the aforementioned terms “styrene enriched crystallized fraction” and “styrene depleted residue fraction” are meant relative to the styrene content of the input into the respective crystallization stage and not relative to the styrene content of the crude composition. In other words, the styrene enriched crystallized fraction obtained in the static crystallization stage has a higher styrene content than the input into this static crystallization stage (which is the styrene depleted residue fraction fed from the dynamic crystallization stage into the static crystallization stage) and the styrene depleted residue fraction has a lower styrene content than the input into this static crystallization stage.

In accordance with a second particular preferred embodiment of the present inven- tion, the method comprises a crystallization step, which comprises two to five stat- ic crystallization stages and two to five dynamic crystallization stages. Preferably, the crude composition is fed into the first of the two to five dynamic crystallization stages so as to produce a first styrene enriched crystallized fraction and a first sty- rene depleted residue fraction, wherein the first styrene enriched crystallized frac- tion is fed into the second of the two to five dynamic crystallization stages, wherein in any of the second and of the optional third to fifth dynamic crystallization stages a styrene enriched crystallized fraction and a styrene depleted residue fraction is produced, wherein each of the styrene enriched crystallized fractions produced in the second and the optional third to fourth dynamic crystallization stages is fed into a downstream dynamic crystallization stage and each of the styrene depleted resi- due fractions produced in the second and the optional third to fifth dynamic crystal- lization stages is fed into an upstream dynamic crystallization stage. The first sty- rene depleted residue fraction is fed into the first of the two to five static crystalliza- tion stages so as to produce a second styrene enriched crystallized fraction and a second styrene depleted residue fraction, wherein the second styrene enriched crystallized fraction is fed into the first dynamic crystallization stage and the sec- ond styrene depleted residue fraction is fed into the second of the two to five static crystallization stages. In any of the second and of the optional third to fifth static crystallization stages a styrene enriched crystallized fraction and a styrene deplet- ed residue fraction is produced, wherein each of the styrene depleted residue frac- tions produced in the second and the optional third to fourth static crystallization stages is fed into a downstream static crystallization stage and each of the styrene enriched crystallized fractions produced in the second and the optional third to fifth dynamic static stages is fed into an upstream static crystallization stage. In princi- pie, the crude composition may be fed into one of the static crystallization stages, i.e. the static crystallization and dynamic crystallization stages may be arranged in reversed order to the aforementioned description. However, better results are ob- tained, when the crude composition is fed into one of the dynamic crystallization stage.

In an alternative variant to the aforementioned described variant, the crude com- position is fed into the second of the two to five dynamic crystallization stages and not into the first dynamic crystallization stage, wherein first to fifth is again seen in the direction upstream to downstream. Again, the most upstream dynamic crystal- lization stage (i.e. the first dynamic crystallization stage) is that, which receives a styrene enriched crystallized fraction from the first static crystallization stage and from which a styrene depleted residue fraction is fed into the first static crystalliza- tion stage, whereas the most downstream dynamic crystallization stage is that, from which the purified styrene composition is withdrawn. Likewise, the most up- stream static crystallization stage (i.e. the first static crystallization stage) is that, which receives a styrene depleted residue fraction from the first dynamic crystalli- zation stage and from which a styrene enriched crystallized fraction is fed into the first dynamic crystallization stage, whereas the most downstream static crystalliza- tion stage (i.e. the second static crystallization stage) is that, from which the sty- rene depleted residue fraction is withdrawn.

For instance, the method comprises a crystallization step, which comprises two static crystallization stages and four dynamic crystallization stages. In this embod- iment, the crude composition is fed into the second of the dynamic crystallization stages so as to produce a second styrene enriched crystallized fraction and a sec- ond styrene depleted residue fraction. The second styrene enriched crystallized fraction is fed into the third of the four dynamic crystallization stages so as to pro- duce a third styrene enriched crystallized fraction and a third styrene depleted res- idue fraction, wherein the third styrene enriched crystallized fraction is fed into the fourth of the dynamic crystallization stages so as to produce a fourth styrene en- riched crystallized fraction and a fourth styrene depleted residue fraction. While the fourth styrene enriched crystallized fraction is withdrawn as purified styrene composition, the fourth styrene depleted residue fraction is fed into the third dy- namic crystallization stage, the third styrene depleted residue fraction is fed into the second dynamic crystallization stage and the second styrene depleted residue fraction is fed into the first dynamic crystallization stage. In the first dynamic crys- tallization stage, a first styrene enriched crystallized fraction and a first styrene depleted residue fraction are produced. While the first styrene enriched crystal- lized fraction is fed into the second dynamic crystallization stage, the first styrene depleted residue fraction is fed into the first of the two static crystallization stages, in which a fifth styrene enriched crystallized fraction and a fifth styrene depleted residue fraction are produced. While the fifth styrene enriched crystallized fraction is fed into the first dynamic crystallization stage, the fifth styrene depleted residue fraction is fed into the second of the two static crystallization stages, in which a sixth styrene enriched crystallized fraction and a sixth styrene depleted residue fraction are produced. While the sixth styrene enriched crystallized fraction is fed into the first static crystallization stage, the sixth styrene depleted residue fraction is removed.

In all of the above described methods, preferably the production of a styrene en- riched crystallized fraction and of a styrene depleted residue fraction in a crystalli- zation stage comprises the steps of removing the remaining liquid from the crystal- lization stage as styrene depleted residue fraction after termination of the crystalli- zation in the crystallization stage, of melting the crystal layer obtained in the crys- tallization stage and of withdrawing the obtained crystal melt as styrene enriched crystallized fraction from the crystallization stage.

In order to increase the purity of the purified styrene product, it is preferable to per- form in any static and falling film crystallization stages, if present, at least one sweating step before melting the crystal layers formed on the cooled surfaces of the crystallizer used in the single crystallization stages. Sweating means that the crystal layer deposited on the cooled surfaces are gently heated to a temperature close to the melting temperature of styrene in order to partially melt the crystals. Trapped and adherent melt, which contains the impurities, drains off during the partial melting of the crystals and is then removed from the crystallizer. In order to conduct such a sweating, the surface, on which the crystals are deposited, is heated with a heat transfer medium to the desired temperature. The sweating may be performed for one or several times before melting the crystal layers deposited on the cooled surfaces. Thus, the sweating leads to one or more sweating frac- tions and to a purified crystal layer. Preferably, at least a portion of the first sweat- ing fraction obtained thereby is fed to the remaining liquid which has been re- moved as styrene depleted residue fraction.

The crystallization temperature depends on the composition of the crude composi- tion. However, good results are obtained, when at least one and preferably all of the at least one static melt crystallization stage and of the at least one dynamic melt crystallization stage are performed at a temperature of -200°C and 30°C and more preferably at a temperature of -140°C and 0°C. In case of static crystalliza- tion comprising one or more sweating steps and in case of falling film crystalliza- tion comprising one or more sweating steps, at least one and preferably all of the crystallization stages may be performed at a temperature of -100°C and -30°C.

Depending on the composition of the feed composition, the feed composition may be directly fed as crude composition to the at least one crystallization step or may be first processed with another technique, before the processed feed composition is fed as crude composition to the at least one crystallization step. For example, the step of providing the crude composition comprises subjecting a feed composi- tion to one or more distillation steps and/or one or more extractive distillation steps, wherein the crude composition is obtained as a head stream, as a side stream or as a bottom stream of one of the one or more distillation steps and/or one or more extractive distillation steps.

Preferably, the feed composition is subjected to one or more extractive distillation steps using a polar solvent. Suitable polar solvents are solvents being selected from the group consisting of propylene carbonate, sulfolane, tetramethyl sulfolane, methyl carbitol, 1-methyl-2-pyrrolidinone, 2-pyrrolidinone and arbitrary combina- tions of two or more of the aforementioned solvents, but not including water. The extractive solvent may be also a two-part extractive solvent with one part being a solvent from the aforementioned group and with the second part being water, wherein both parts of the extractive solvent are fed into the distillation column sep- arately and independently from each other at different locations along the distilla- tion column.

As set out above, the method in accordance with the present invention is in partic- ular suitable for preparing a purified styrene composition from styrene containing compositions, which could be up so far not economically reasonable used for the preparation of pure styrene. Therefore, it is preferred that the crude composition derives from a pygas. The method of the present invention allows to cost efficiently purify styrene from such feed compositions, which is not possible with the prior art methods. In particular, the crude composition may originate from an extractive dis- tillation process employed on naphtha cracker pyrolysis gasoline. These pygases are known to contain impurities, such as color inducing species, C ₆ -thiophenic sul- fur species and oxygenates originating from pygas feed as well as air leaks in the vacuum equipment used in the process. Moreover, they contain impurities having a boiling point close to that of styrene, such as ortho-xylene, which are difficult to remove completely via extractive distillation. It is preferred in this embodiment that the crude composition is/has been prepared by distilling a pygas feed composition so as to obtain a C ₈ -fraction and subjecting the C ₈ -fraction to an extractive distilla- tion, in which the C ₈ -fraction is treated with a polar solvent so as to obtain a sty- rene containing fraction as an overhead stream, as a side stream or as a bottom stream. The so obtained styrene containing fraction may be processed into the crude composition, for instance by means of a distillation step, or, preferably, the so obtained styrene containing fraction is used as the crude composition, which is fed to the crystallization step.

In an alternative embodiment of the present invention, the crude composition is/has been prepared by distilling a pygas feed composition so as to obtain a Ce- fraction, feeding the Ce-fraction into a hydrogenation reactor in order to hydrogen- ate for instance phenylacetylene so as to obtain a hydrogenated gas, subjecting the hydrogenated gas to an extractive distillation, in which the hydrogenated gas is treated with a polar solvent so as to obtain a styrene containing fraction as an overhead stream, as a side stream or as a bottom stream. The so obtained sty- rene containing fraction may be processed into the crude composition, for instance by means of a distillation step, or, preferably, the so obtained styrene containing fraction is used as the crude composition, which is fed to the crystallization step.

Preferably, the hydrogenation is performed so that, while phenylacetylene is hy- drogenated, the styrene loss is less than 0.1% by weight.

In order to recover the solvent used in the extractive distillation, it is preferred that the styrene containing fraction is subjected to a distillation step so as to remove at least a portion of the polar solvent from the styrene containing fraction, thereby obtaining the crude composition.

The method in accordance with the present invention results in a very pure styrene containing composition. Preferably, the purified styrene composition has a styrene content of at least 99.00% by weight, more preferably of at least 99.50% by weight, even more preferably of at least 99.80% by weight, still more preferably of at least 99.90% by weight, yet more preferably of at least 99.95% by weight and most preferably of at least 99.98% by weight.

In particular, the method in accordance with the present invention allows to com- pletely or at least nearly completely remove color inducing species from the crude styrene containing composition. Therefore, in a further development of the idea of the present invention, it is proposed that the purified styrene composition has a color of maximum 15 as defined by Pt-Co scale as per ASTM D5386.

Furthermore, the method in accordance with the present invention allows to com- pletely or at least nearly completely remove sulfur species from the crude styrene containing composition. Consequently, it is in particular preferred, when the puri- fied styrene composition comprises less than 5 ppmw, more preferably less than 4 ppmw, even more preferably less than 3 ppmw and most preferably less than 2 ppmw of total elemental sulfur as contained in mercaptans, disulfides and thio- phenes and/or less than 20 ppmw of oxygenates.

In addition, the method in accordance with the present invention allows to obtain purified styrene composition, which comprises less than 40 ppmw of impurities selected from the group consisting of phenylacetylene, mixed xylenes, ethylben- zene, cumene, ethyltoluene, n-propylbenzene, and alpha-methylstyrene, and/or which has polymer content of less than 10 ppmw.

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

For instance, the purified styrene composition may meets the following specifica- tions.

Table 1

The residual mother liquor, i.e. the styrene depleted residue fraction obtained in the crystallization is discharged. Preferably, none of the styrene depleted residue fraction obtained in the crystallization is recycled in the method to an optional dis- tillation step and, if so, at most 50% by volume, more preferably at most 20% by volume and yet more at most 10% by volume are recycled. According to a further aspect, the present invention relates to a plant for preparing a purified styrene composition comprising at least one crystallization block, the at least one crystallization block comprising at least one of a static crystallization sec- tion comprising one or more static crystallization stages and of a dynamic crystalli- zation section comprising one or more dynamic crystallization stages, wherein the plant further comprises at least one extractive distillation column comprising two or more outlets, wherein at least one of these outlets is fluidly coupled with an inlet of the crystallization block.

The term "crystallization block" refers to an apparatus for a purification process with one or more crystallizers. Moreover, the term crystallization stage is not only used to denote a method step or stage, respectively, but also to denote an appa- ratus, namely that part of a crystallizer, in which a crystallization stage is per- formed. The crystallization stage as apparatus feature may be also designated as crystallizer, crystallizer unit or the like.

Preferably, the plant further comprises a solvent recovery distillation column, which is fluidly coupled with an outlet of the extractive distillation column.

Good results are in particular obtained, when the crystallization block of the plant comprises: at least one static crystallization section comprising one or more static crystalliza- tion stages, at least one dynamic crystallization section comprising one or more dynamic crys- tallization stages and at least two conduits that fluidly couple at least one of the one or more static crys- tallization stages with at least one of the one or more dynamic crystallization stag- es. Preferably, the one or more static crystallization stages are static melt crystalliza- tion stages and the one or more dynamic crystallization stages are dynamic melt crystallization stages.

If the crystallization block comprises two or more dynamic crystallization stages and/or two or more static crystallization stages, preferably each of the dynamic crystallization stages is fluidly coupled with one or two other dynamic crystalliza- tion stages and each of the static crystallization stages is fluidly coupled with one or two other static crystallization stages.

Moreover, it is preferred that the at least one crystallization block comprises one static crystallization stage and one dynamic crystallization stage, wherein one of the at least two conduits fluidly couples the static crystallization stage with the dy- namic crystallization stage so that a styrene depleted residue fraction obtained in the dynamic crystallization stage may be fed into the static crystallization stage, and wherein one other of the at least two conduits fluidly couples the static crystal- lization stage with the dynamic crystallization stage so that a styrene enriched crystallized fraction obtained in the static crystallization stage may be fed into the dynamic crystallization stage.

Moreover, it is preferred that the at least one crystallization block comprises two to five static crystallization stages and two to five dynamic crystallization stages, wherein one of the at least two conduits fluidly couples one of the static crystalliza- tion stages with one of the dynamic crystallization stages so that a styrene deplet- ed residue fraction obtained in the dynamic crystallization stage may be fed into the static crystallization stage being fluidly coupled with the dynamic crystallization stage, and wherein one of the at least two conduits fluidly couples the static crys- tallization stage with the dynamic crystallization stage being fluidly coupled with the static crystallization stage so that a styrene enriched crystallized fraction ob- tained in the static crystallization stage may be fed into the dynamic crystallization stage, wherein each two of the remaining static crystallization stages are fluidly coupled with each other by means of at least two conduits, and wherein each two of the remaining dynamic crystallization stages are fluidly coupled with each other by means of at least two conduits.

In accordance with a further preferred embodiment of the present invention, the plant comprises the at least one crystallization block, a distillation column and the at least one extractive distillation column, wherein the distillation column is fluidly coupled with the extractive distillation column via a conduit and wherein the extrac- tive distillation column is fluidly coupled with the inlet of the crystallization block via an inlet conduit. Preferably, the plant of this embodiment further comprises a hy- drogenation reactor and a further distillation column for solvent recovery, wherein the distillation column is fluidly coupled with the hydrogenation reactor via a con- duit, wherein the hydrogenation reactor is fluidly coupled with the extractive distil- lation column via a conduit, wherein the extractive distillation column is fluidly cou- pled with the further distillation column for solvent recovery via a conduit, and wherein the further distillation column is fluidly coupled with the crystallization block via the inlet conduit.

In accordance with an alternative preferred embodiment of the present invention, the plant comprises the at least one crystallization block and three distillation col- umns, wherein the three distillation columns are fluidly coupled with each other and are arranged in series, wherein the last of the three distillation columns is flu- idly coupled with the crystallization block via an inlet conduit. Preferably, the plant of this embodiment further comprises an alkylation unit and a dehydrogenation unit, wherein the alkylation unit is fluidly coupled with the dehydrogenation unit via a conduit and the dehydrogenation unit is fluidly coupled with the crystallization block via the inlet conduit. The crystallization block comprises a product outlet line for discharging the purified styrene composition and a discharge line for discharging the residual mother liq- uor, i.e. the styrene depleted residue fraction obtained in the crystallization. Pref- erably, the plant does not comprise a recirculation line leading from the discharge line for discharging the styrene depleted residue fraction obtained in the crystalli- zation to any of the distillation column(s).

In accordance with still an alternative preferred embodiment of the present inven- tion, the plant further comprises a pyrolysis reactor, wherein the pyrolysis reactor is fluidly coupled with the at least one extractive distillation column.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the manner in which the above recited and other advantages and ob- jects of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated, in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope, the invention will be described with additional specificity and detail through the use of the accompanying drawings in which:

FIG. 1a is a diagrammatic illustration of a crystal block used in a method and plant in accordance with one embodiment of the present inven- tion.

FIG. 1b is a diagrammatic illustration of a crystal block used in a method and plant in accordance with another embodiment of the present in- vention. FIG. 1c is a diagrammatic illustration of a crystal block used in a method and plant in accordance with still another embodiment of the pre- sent invention.

FIG. 2 is a diagrammatic illustration of a plant particularly suitable for puri- fying naphtha cracked pyrolysis gasoline in accordance with anoth- er embodiment of the present invention.

FIG. 3 is a diagrammatic illustration of a plant particularly suitable for puri- fying an EBSM process stream in accordance with another embod- iment of the present invention.

FIG. 4 is a diagrammatic illustration of a plant particularly suitable for puri- fying a styrene containing stream produced from a polystyrene stream via pyrolysis in accordance with another embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1a shows an embodiment of a crystallization block 10 for conducting the pro- cess for preparing a purified styrene composition in accordance with an embodi- ment of the present invention. The crystallization block 10 includes a static melt crystallization stage 18 or one static melt crystallizer, respectively. The static melt crystallizer 18 is connected with an inlet conduit 20 for crude styrene containing composition which is suitable for feeding a crude styrene composition into the stat- ic melt crystallizer 18. In addition, the static melt crystallizer 18 has a discharge conduit 22 for the discharge of a purified styrene composition from the static melt crystallizer 18 and from the crystallization block 10. Moreover, the static melt crys- tallizer 18 comprises a discharge conduit 28 which serves for discharging a sty- rene depleted residue fraction, which is obtained by the crystallization in the static melt crystallizer 18, from the static melt crystallizer 18 and from the crystallization block 10.

FIG. 1b shows another embodiment of a crystallization block 10 for conducting the process for preparing a purified styrene composition in accordance with an em- bodiment of the present invention. The crystallization block 10 includes a first dy- namic melt crystallization section 12 which comprises one falling film crystallization stage or one falling film crystallizer 14, respectively as a dynamic melt crystalliza- tion stage or crystallizer, respectively. In addition, the crystallization block 10 com- prises a second static melt crystallization section 16 having one static melt crystal- lization stage 18 or one static melt crystallizer, respectively. The falling film crystal- lizer 14 is connected with an inlet conduit 20 for crude styrene containing composi- tion which is suitable for feeding a crude styrene composition into the falling film crystallizer 14. In addition, the falling film crystallizer 14 has a discharge conduit 22 for the discharge of a purified styrene composition from the falling film crystallizer 14 and from the crystallization block 10. The static melt crystallizer 18 is connect- ed with the falling film crystallizer 14 via a transfer conduit 24 which is suitable for transferring a first styrene depleted residue fraction obtained by crystallization in the falling film crystallizer 14 into the static melt crystallizer 18. With this respect, the transfer conduit 24 is in fluid communication with both the falling film crystalliz- er 14 and the static melt crystallizer 18. The static melt crystallizer 18 comprises a discharge conduit 28 which serves for discharging a second styrene depleted resi- due fraction, which is obtained by the crystallization in the static melt crystallizer 18, from the static melt crystallizer 18 and from the crystallization block 10. A recy- cle conduit 30 provides a fluid communication between the static melt crystallizer 18 and the falling film crystallizer 14 and therefore allows to recycle at least a part of the second styrene enriched crystallization composition, which results from the crystallization in the static melt crystallizer 18, back into the falling film crystallizer 14. In FIG. 1c, another embodiment of a crystallization block 10 for conducting the method for preparing a purified styrene composition in accordance with the pre- sent invention is shown. The first dynamic melt crystallization section 12 compris- es four falling film crystallization stages 14a, 14b, 14c, 14d and the second static melt crystallization section 16 includes two static melt crystallization stages 18a, 18b. There are provided transfer conduits 32a, 32b, 32c between the falling film crystallization stages 14a, 14b, 14c, 14d, through which a styrene depleted resi- due fraction obtained by falling film crystallization in the single falling film crystalli- zation stages 14a, 14b, 14c, 14d can be transferred from one of the falling film crystallization stages 14b, 14c, 14d to the respective upstream falling film crystalli- zation stages 14a, 14b, 14c. In addition, the falling film crystallization stages 14a, 14b, 14c, 14d are connected via recycle conduits 34a, 34b, 34c suitable for recy- cling at least a part of the styrene enriched crystallized fractions obtained by falling film crystallization in the single falling film crystallization stages 14a, 14b, 14c, 14d from one of the falling film crystallization stages 14a, 14b, 14c to the respective downstream falling film crystallization stages 14b, 14c, 14d. An inlet conduit 20 is connected to the second falling film crystallization stage 14b such that a crude sty- rene containing composition can be introduced into the second falling film crystalli- zation stage 14b. A discharge conduit 22 is provided at the most downstream fall- ing film crystallization stage 14d in order to remove the purified styrene composi- tion from the crystallization block 10. A transfer conduit 24 provides a fluid com- munication between the most upstream falling film crystallization stage 14a of the first dynamic melt crystallization section 12 and the most upstream static melt crystallization stage 18b of the second static melt crystallization section 16 so that the styrene depleted residue fraction obtained by the crystallization in the falling film crystallization stage 14a can be transferred into the static crystallizer 18b of the second static melt crystallization section 16. The static melt crystallization stages 18a and 18b are connected via a transfer conduit 36 for transferring the styrene depleted residue fraction obtained by crystallization from the static melt crystallization stage 18b to the static melt crystallization stage 18a. In addition, the static melt crystallization stage 18a and the static melt crystallization stage 18b are connected via a recycle conduit 38 allowing for transferring the styrene enriched crystallized fraction, which results from the crystallization in the static melt crystal- lization stage 18a, into the static melt crystallizer of the crystallization stage 18b. Furthermore, the static melt crystallization stage 18a comprises a discharge con- duit 28 for discharging the styrene depleted residue fraction, which is obtained by crystallization in the static melt crystallization stage 18a, from the crystallization block 10. A recycle conduit 30 provides a fluid communication between the static melt crystallization stage 18b and the falling film crystallization stage 14a and therefore allows to recycle at least a part of the styrene enriched crystallized frac- tion obtained in the static melt crystallization stage 18b of the second static melt crystallization section 16 back into the falling film crystallization stage 14a of the first dynamic melt crystallization section 12.

During operation of the apparatus 10 shown in FIG. 1c a crude styrene containing composition is fed into the falling film crystallization stage 14b via the inlet conduit 20. In each of the falling film crystallization stages 14a, 14b, 14c, 14d a styrene enriched crystallized composition and a styrene depleted residue fraction are pre- pared. Each of the styrene depleted residue fractions obtained in one of the falling film crystallization stages 14b, 14c, 14d is transferred via the transfer conduits 32a, 32b, 32c to the respective upstream falling film crystallization stage 14a, 14b, 14c. In addition, each of the styrene enriched fractions obtained in one of the fall- ing film crystallization stages 14a, 14b, 14c is at least partially recycled via the re- cycle conduits 34a, 34b, 34c to the respective downstream falling film crystalliza- tion stage 14b, 14c, 14d. The styrene depleted residue fraction obtained after the crystallization in the falling film crystallization stage 14a of the first dynamic melt crystallization section 12 is transferred via the transfer conduit 24 into the static melt crystallization stage 18b of the second static melt crystallization section 16. The styrene depleted residue fraction obtained in the static melt crystallization stage 18b is transferred via the transfer conduit 36 to the downstream static melt crystallization stage 18a. In addition, the styrene enriched crystallized fraction ob- tained in the static melt crystallization stage 18a is at least partially recycled via the recycle conduit 38 into the upstream static melt crystallization stage 18b. The styrene enriched crystallized fraction obtained after the crystallization in the static melt crystallization stage 18b is recycled via the recycle conduit 30 into the falling film crystallization stage 14a of the first dynamic melt crystallization section 12. A finally purified styrene composition obtained in the crystallization stage 14d is re- moved from the apparatus 10 via the discharge conduit 22, while the final styrene depleted residue fraction is removed from the static melt crystallization stage 18a and from the apparatus 10 via the discharge conduit 28.

In accordance with the present invention, Table 2 lists the different impurities that can be typically present in a crude styrene stream with their melting points. The reason for impurities removal from crude styrene stream by crystallization block is twofold: a) Some of the species have melting points lower than styrene and b) dur- ing the crystallization process, impurities which have higher melting point are more soluble in the mother liquor. Thus, despite having a higher melting point, these impurities can be removed from styrene by crystallization. Thus, crystallization of- fers a unique method of producing highly purified styrene compositions, as desired by operator, from a crude styrene containing composition. Increasing product puri- ty is directly correlated with an increasing number of crystallization stages. Recov- ery, on the other hand, is a function of the number of residue stages.

FIG. 2 shows schematically a plant particularly suitable for preparing a purified styrene composition from naphtha cracker pyrolysis gasoline. The plant 11 com- prises a first distillation column 40, a second distillation column 42, a hydrogena- tion reactor 44, an extractive distillation column 46, a solvent recovery distillation column 48 and a crystallization block 10. The crystallization block 10 is composed as that shown in FIG. 1 a, as that shown in FIG. 1 b or as that shown in FIG. 1 c.

During the operation of the plant, a C ₇₊ -pygas stream is distilled in the first distilla- tion column 40 and the bottom stream obtained in the first distillation column 40 is fed to the second distillation column 42 so as to obtain a C ₉₊ -stream as bottom product and a C ₈ -rich stream as overhead product. The so obtained C ₈ -rich stream is fed into the hydrogenation reactor 44 in order to hydrogenate phenyl acetylene included in the stream by hydrogen, which is supplied into the hydrogenation reac- tor 44 via the hydrogen inlet conduit 50. The hydrogenation reactor 44 is operated under mild conditions in order to saturate phenylacetylene to produce styrene; however, this is accompanied by styrene loss in the form of saturation which pro- duces ethylbenzene. Post hydrogenation, the hydrogenated C ₈ -stream obtained in the hydrogenation reactor 44, consisting primarily of ethylbenzene, mixed xylenes etc., is fed into the extractive distillation setup comprising the extractive distillation column 46 and the solvent recovery distillation column 48. A polar solvent is used during the extractive distillation so as to extract a styrene and solvent containing stream as bottom stream, which is then fed into the solvent recovery distillation column 48 so as to remove the solvent and to obtain as overhead stream a sty- rene enriched stream with a styrene content of more than 99.8% by weight. De- spite being of high purity, this stream is of inferior quality due to the presence of color causing species, sulfur molecules and oxygenates. As illustrated in subse- quent experimental section, the application of the crystallization block on this stream 10 converts this stream into high quality styrene product or very high quali- ty styrene product (VHPS) as desired by operator. However, during the crystalliza- tion performed in the crystallization block 10 as described in detail above, the im- purities and in particular different impurities having a boiling point close to that of styrene, such as phenylacetylene, meta- and ortho-xylenes, ethylbenzene, cu- mene, n-propylbenzene, alpha-methylstyrene and ethyltoluene, are removed. This phenomenon can be exploited to minimize both styrene loss across the phenyla- cetylene hydrogenation reactor 44 and utility consumption in the upstream distilla- tion columns 40, 42, 46, 48. Removal of phenylacetylene via crystallization ena- bles the phenylacetylene hydrogenation reactor 44 to be operated under low se- verity conditions or even eliminated altogether. The associated styrene loss during hydrogenation is thereby minimized or is non-existent. Removal of close boiling C ₉₊ -compounds, such as cumene, n-propylbenzene etc., implies that the distilla- tion column 42 or deoctanizer, respectively, in the plant 11 can be relaxed to allow slippage of C ₉ +-compounds in the C ₈ -cut. These C ₉ +- compounds, by virtue of their polarity and boiling point, will predominantly land in the crude styrene stream and will eventually be removed via the crystallization block 10. Removal of com- pounds having a boiling point close to that of styrene, such as ethylbenzene, or- tho-xylene and meta-xylene, implies that the extractive distillation column 46 and solvent recovery distillation column 48 can be designed with lower solvent to feed ratio and lower extractive distillation column bottoms temperature, thereby lower- ing capital investment and utility consumption. The purified styrene composition is withdrawn via the discharge conduit 22, whereas the styrene depleted residue fraction obtained in the crystallization block 10 is withdrawn via the discharge con- duit 28 together with the C ₈ -raffinate obtained as overhead product of the extrac- tive distillation column 46.

The crystallization block 10 removes, as described in detail above, impurities in- cluding color causing species, such as conjugated diolefins, sulfur species, which are primarily C ₆ thiophenes, and oxygenates, such as water, ketones, aldehydes and alcohols etc. Moreover, the crystallization block, due to the cryogenic nature of the process, prevents unwanted polymer formation in the styrene product. A common problem encountered in adsorbent based styrene treatment is unwanted polymer formation due to localized exotherm at the active sites despite insignifi- cant temperature rise across the beds.

FIG. 3 depicts a plant 11 particularly suitable for purifying an EBSM process stream. The plant 11 comprises an alkylation unit 52, a dehydrogenation unit 54, a first distillation column 40, a second distillation column 42, a third distillation col- umn 56 and a crystallization block 10. The crystallization block 10 is composed as that shown in FIG. 1 a, as that shown in FIG. 1 b or as that shown in FIG. 1 c.

During the operation, benzene and ethylene are alkylated in the alkylation reactor 52 to produce ethylbenzene, which is fed into the dehydrogenation reactor 54. The effluent of the dehydrogenation reactor 54 is fed into a separation block which in- cludes the three distillation columns 40, 42, 56. The first distillation column 40 re- moves benzene and toluene from the effluent of the dehydrogenation reactor 54, whereas the second distillation column 42 separates unreacted ethylbenzene from styrene and the third distillation column 56 distills the styrene stream. The bottom product of the third distillation column 56 is a styrene tar residue produced due to unwanted polymerization of heat sensitive styrene in second distillation column 42. The second distillation column 42 is the largest energy consumer in this system. This is because separation of ethylbenzene from styrene is difficult due to i) close boiling points and ii) heat sensitivity of styrene, which require the column to be op- erated under vacuum and with a large number of distillation stages. The crystalli- zation in the crystallization block 10 does not only effect energy savings, but also reduces unwanted polymerization of styrene in the bottom of the EB distillation column 42. The EB distillation column 42 can be run in a relaxed mode, wherein small amounts of ethylbenzene (up to 3-5% by weight) can drop into the column bottoms. This will not only result in lesser utility consumption or theoretical stages, but also lower bottoms temperature, which implies less unwanted styrene polymer- ization thereby increasing styrene yield of the overall EBSM process. The ethylbenzene in EB distillation column 42 bottoms will, by virtue of its boiling point, land in the overhead product of the third column 56. This crude styrene containing composition, when fed into the crystallization block 10, will result in production of two streams, namely i) the purified styrene composition withdrawn from the crys- tallization block 10 via the discharge conduit 22 and ii) an ethylbenzene-rich resi- due liquor 28, which is sent to the battery limit.

FIG. 4 shows a plant 11 particularly suitable for purifying a styrene containing stream produced from a polystyrene stream via pyrolysis. The plant 11 comprises a pyrolysis reactor 60, a first distillation column 40, a second distillation column 42 and a crystallization block 10. The crystallization block 10 is composed as that shown in FIG. 1 a, as that shown in FIG. 1 b or as that shown in FIG. 1 c.

During the operation, the pyrolysis of polystyrene is performed in the pyrolysis re- actor, which may be operated thermally or in a catalytic mode. Table 3 gives a typ- ical breakdown of the effluent from the pyrolysis reactor 60 obtained from different methods. The reactor effluent undergoes a series of fractionation steps in the first distillation column 40 and in the second distillation column 42 so as to produce the crude styrene containing composition, which is fed into the crystallization block 10. The crystallization block 10 is applied on the tail end of the distillation step to pro- duce a high purity grade styrene product or VHPS, as desired by operator, at re- duced specific energy consumption and capital expenditure. Table 3

Experimental Example The following example is provided to illustrate the invention and does not limit the scope of the claims. Unless stated otherwise, all parts and percentages are by weight. A crude styrene stream, as shown in Table 4, containing different impuri- ties was produced by an extractive distillation unit on pyrolysis gasoline and sub- sequently purified by means of layer melt crystallization to prepare the final VHPS product and final residue (as shown in FIG. 1b, but using the results of static crys- tallization instead of a combination of falling film and static (all data based on 20V1980)). The styrene recovery across the crystallization block was >95%.

The term “at least one of is meant to cover combinations of the listed elements, components, features, and the like, and the listed elements, components, features, and the like individually. For example, the phrase “at least one of A and B” is used to cover embodiments comprising only A, comprising only B, and comprising A and B unless stated otherwise.

The term "comprising" within the claims is intended to mean "including at least" such that the recited listing of elements in a claim are an open group. The terms "a," "an" and other singular terms are intended to include the plural forms thereof unless specifically excluded.

Reference Numeral List

10 Crystallization block

12 First dynamic melt crystallization section

14 Dynamic (falling film) melt crystallizer/crystallization stage

14a, 14b, 14c, 14d Dynamic melt / falling film crystallization stages

16 Second static melt crystallization section

18 Static melt crystallizer/crystallization stage

18a, 18b Static melt crystallization stages 20 Inlet conduit for crude styrene containing composition 22 Discharge conduit for purified styrene composition 24 (Transfer) conduit for feeding a styrene depleted residue fraction from the dynamic crystallization section to the static crystallization section

28 Discharge conduit for styrene depleted residue fraction

30 (Recycle) conduit for feeding a styrene enriched fraction from the static crystallization section to the dynamic crystallization section

32a, 32b, 32c Conduits for styrene depleted residue fractions in the dynam- ic crystallization section

34a, 34b, 34c Conduits for styrene enriched crystallized fraction in the dy- namic crystallization section

36 Conduits for styrene depleted residue fractions in the static crystallization section

38 Conduits for styrene enriched crystallized fraction in the stat- ic crystallization section

40 First distillation column

42 Second distillation column 44 Hydrogenation reactor 46 Extractive distillation column 48 Solvent recovery distillation column 50 Hydrogen inlet conduit 52 Alkylation unit

54 Dehydrogenation unit 56 Third distillation column 60 Pyrolysis reactor