PROCESS AND APPARATUS FOR PREPARING PURIFIED STYRENE COMPOSITION FROM STYRENE CONTAINING FEEDSTOCK

Abstract

A method for preparing a purified styrene composition with a styrene yield of at least 80%. The method comprises providing a crude composition containing styrene, and subjecting the crude composition to at least one crystallization step. The at least one crystallization step comprises at least one static crystallization stage and at least one dynamic crystallization stage.

Claims

1. A method for preparing a purified styrene composition with a styrene yield of at least 80%, the method comprising: providing a crude composition containing styrene; and subjecting the crude composition to at least one crystallization step, the at least one crystallization step comprising at least one static crystallization stage and at least one dynamic crystallization stage.

2. The method of claim 1, wherein the crude composition contains one or more impurities selected from the group consisting of; color inducing species, sulfur species, meta-xylenes, ortho-xylenes, ethylbenzene, phenylacetylene, cumene, n-propylbenzene, alpha-methylstyrene, ethyltoluene, organochlorinated, organo-nitrogenated species and arbitrary mixtures of two or more of the aforementioned impurities.

3. The method of claim 1, wherein the at least one dynamic crystallization stage is a falling film crystallization stage or a suspension crystallization stage.

4. The method of claim 1, wherein the at least one crystallization step comprises one to ten static crystallization stages and one to ten dynamic crystallization stages.

5. The method of claim 1, wherein: providing the crude composition comprises subjecting a feed composition to at least one of: one or more distillation steps and one or more extractive distillation steps, and the crude composition is obtained as a head stream, as a side stream or as a bottom stream of one of the at least one of: one or more distillation steps and one or more extractive distillation steps.

6. The method of claim 1, wherein the crude composition derives from a pygas, the crude composition being prepared by: distilling a pygas feed composition so as to obtain a C8-fraction and subjecting the C8-fraction to an extractive distillation in which the C8-fraction 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, which is used as the crude composition or which is processed into the crude composition, or distilling a pygas feed composition so as to obtain a C8-fraction, feeding the C8-fraction into a hydrogenation reactor 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, which is used as the crude composition or which is processed into the crude composition.

7. The method according to claim 1, wherein the crude composition derives from an ethylbenzene and styrene containing stream produced in an ethyl-benzene/styrene monomer (EBSM) process or derives from a styrene containing stream produced by polystyrene pyrolysis.

8. The method of claim 1, wherein the purified styrene composition has a styrene content of at least 99.00% by weight.

9. The method of claim 1, wherein the purified styrene composition has a color of maximum 15 as defined by Pt—Co scale as per ASTM D5386.

10. The method of claim 1, wherein the purified styrene composition comprises at least one of: less than 5 ppmw, of total elemental sulfur as contained in mercaptans, disulfides and thiophenes, less than 20 ppmw of oxygenates, less than 40 ppmw of impurities selected from the group consisting of phenylacetylene, mixed xylenes, ethylbenzene, cumene, ethyl-toluene, n-propylbenzene, and alpha-methylstyrene, a polymer content of less than 10 ppmw, and a total organic chlorine content of less than 2 ppmw.

11. The method of claim 1, wherein: in the at least one crystallization step, a purified styrene composition and a styrene depleted mother liquor are obtained, and at most 50% by volume, of the styrene depleted mother liquor is recycled to an optional distillation step.

12. A plant for preparing a purified styrene composition comprising at least one crystallization block, the crystallization block comprising: at least one static crystallization section comprising one or more static crystallization stages, at least one dynamic crystallization section comprising one or more dynamic crystallization stages, and at least two conduits that fluidly couple at least one of the one or more static crystallization stages with at least one of the one or more dynamic crystallization stages.

13. The plant of claim 12, further comprising: at least one distillation column comprising two or more outlets, wherein at least one of the two or more outlets is fluidly coupled with an inlet of the at least one crystallization block, or at least one extractive distillation column comprising two or more outlets, wherein at least one of the two or more outlets is fluidly coupled with an inlet of the crystallization block.

14. The plant of claim 12, further comprising at least one distillation column and an extractive distillation column, wherein; the at least one distillation column is fluidly coupled with the extractive distillation column via a conduit, and the extractive distillation column is fluidly coupled with an inlet of the at least one crystallization block via an inlet conduit.

15. The plant of claim 12, further comprising three distillation columns, wherein; the three distillation columns are fluidly coupled with each other and are arranged in series, and a last one of the three distillation columns is fluidly coupled with the at least one crystallization block via an inlet conduit.

16. The plant of claim 12, further comprising two distillation columns and a pyrolysis reactor, wherein: the pyrolysis reactor is fluidly coupled with a first one of the two distillation columns, the first one of the two distillation columns is fluidly coupled with a second one of the two distillation columns, and the second one of the two distillation columns is fluidly coupled with the at least one crystallization block via an inlet conduit.

17. The plant of claim 12, further comprising a product outlet line for discharging the purified styrene composition and a discharge line for discharging the styrene depleted residue fraction obtained in the crystallization, wherein the plant does not comprise a recirculation line leading from the discharge line for discharging the styrene depleted residue fraction obtained in the crystallization to any of the optional distillation column(s) (40, 42, 56).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The disclosure will be explained in more detail hereinafter with reference to the drawings:

[0058] FIG. 1a illustrates a diagrammatic illustration of a crystallization block used in a method and plant in accordance with one embodiment of the present disclosure.

[0059] FIG. 1b illustrates a diagrammatic illustration of a crystallization block used in a method and plant in accordance with another embodiment of the present disclosure.

[0060] FIG. 2 illustrates a diagrammatic illustration of a plant particularly suitable for purifying naphtha cracked pyrolysis gasoline in accordance with an embodiment of the present disclosure.

[0061] FIG. 3 illustrates a diagrammatic illustration of a plant particularly suitable for purifying an EBSM process stream in accordance with another embodiment of the present disclosure.

[0062] FIG. 1 illustrates a diagrammatic illustration of a plant particularly suitable for purifying a styrene containing stream produced from a polystyrene stream via pyrolysis in accordance with an embodiment of the present disclosure.

[0063] FIG. 1a shows an embodiment of a crystallization block 10 for conducting the process for preparing a purified styrene composition in accordance with an embodiment of the present disclosure. The crystallization block 10 includes a first dynamic melt crystallization section 12 which comprises one falling film crystallization stage or one falling film crystallizer 14, respectively as a dynamic melt crystallization stage or crystallizer, respectively. In addition, the crystallization block 10 comprises a second static melt crystallization section 16 having one static melt crystallization stage 18 or one static melt crystallizer, respectively. The falling film crystallizer 14 is connected with an inlet conduit 20 for crude styrene containing composition 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 connected 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 crystallizer 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 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. A recycle 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.

[0064] In FIG. 1b, another embodiment of a crystallization block 10 for conducting the method for preparing a purified styrene composition in accordance with the present disclosure, is shown. The first dynamic melt crystallization section 12 comprises 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 residue fraction obtained by falling film crystallization in the single falling film crystallization 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 crystallization 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 recycling 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 styrene containing composition can be introduced into the second falling film crystallization stage 14b. A discharge conduit 22 is provided at the most downstream falling film crystallization stage 14d in order to remove the purified styrene composition from the crystallization block 10. A transfer conduit 24 provides a fluid communication 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 crystallization stage 18a, into the static melt crystallizer of the crystallization stage 18b. Furthermore, the static melt crystallization stage 18a comprises a discharge conduit 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 recycling of at least a part of the styrene enriched crystallized fraction 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.

[0065] During operation of the apparatus 10 shown in FIG. 1b, 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 prepared. 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 falling film crystallization stages 14a, 14b, 14c is at least partially recycled via the recycle conduits 34a, 34b, 34c to the respective downstream falling film crystallization 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 obtained 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 removed 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.

[0066] In accordance with the present disclosure, 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) during 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 offers a unique method of producing highly purified styrene compositions, as desired by operator, from a crude styrene containing composition. Increasing product purity is directly correlated with an increasing number of crystallization stages.

[0067] Recovery, on the other hand, is a function of the number of residue stages.

TABLE-US-00002 TABLE 2 COMPOUND MELTING POINT Water 0° C.  .sup.  (32° F.; 273 K) α-Methylstyrene −23° C.  (−9.4° F.; 250 K) o-Xylene −25.2° C. (−13.4° F.; 248 K) Benzaldehyde −26° C. (−14.8° F.; 247 K) Styrene 30.6° C. (−23.1° F.; 243 K) Thiophenic −65 to −30° C. .sup. (−85 to −22° F.; 208 to 243 K) compounds boiling in the range 130-150° C. Phenylacetylene −45° C. .sup. (−49° F.; 228 K) Ethylbenzene −95° C.  (−139° F.; 178 K) 3-Ethyltoluene −95.5° C.  (−140° F.; 177.6 K) (m-ethyltoluene) Cumene −96° C.  (−141° F.; 177 K) n-Propylbenzene −99.5° C.  (−147° F.; 173.7 K)

[0068] FIG. 2 shows schematically a plant particularly suitable for preparing a purified styrene composition from naphtha cracker pyrolysis gasoline. The plant 11 comprises a first distillation column 40, a second distillation column 42, a hydrogenation 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. 1b.

[0069] During the operation of the plant, a C.sub.7+-pygas stream is distilled in the first distillation 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.sub.9+-stream as bottom product and a C.sub.8-rich stream as overhead product. The so obtained C.sub.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 reactor 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 produces ethylbenzene. Post hydrogenation, the hydrogenated C.sub.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 styrene enriched stream with a styrene content of more than 99.8% by weight. Despite being of high purity, this stream is of inferior quality due to the presence of color causing species, sulfur molecules and oxygenates. However, during the crystallization performed in the crystallization block 10 as described in detail above, the impurities and in particular different impurities having a boiling point close to that of styrene, such as phenylacetylene, meta- and ortho-xylenes, ethylbenzene, cumene, n-propylbenzene, alpha-methylstyrene and ethyltoluene, are removed. This phenomenon can be exploited to minimize both styrene loss across the phenylacetylene hydrogenation reactor 44 and utility consumption in the upstream distillation columns 40, 42, 46, 48. Removal of phenylacetylene via crystallization enables the phenylacetylene hydrogenation reactor 44 to be operated under low severity conditions or even eliminated altogether. The associated styrene loss during hydrogenation is thereby minimized or is non-existent. Removal of close boiling C.sub.9+-compounds, such as cumene, n-propylbenzene etc., implies that the distillation column 42 or deoctanizer, respectively, in the plant 11 can be relaxed to allow slippage of C.sub.9+-compounds in the C.sub.8-cut. These C.sub.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 compounds having a boiling point close to that of styrene, such as ethylbenzene, ortho-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 lowering 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 conduit 28 together with the C.sub.8-raffinate obtained as overhead product of the extractive distillation column 46.

[0070] The crystallization block 10 removes, as described in detail above, impurities including color causing species, such as conjugated diolefins, sulfur species, which are primarily C.sub.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 insignificant temperature rise across the beds.

[0071] 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 column 56 and a crystallization block 10. The crystallization block 10 is composed as that shown in FIG. 1b.

[0072] 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 includes the three distillation columns 40, 42, 56. The first distillation column 40 removes 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 operated under vacuum and with a large number of distillation stages. The crystallization in the crystallization block 10 not only affects energy savings, but also reduces unwanted polymerization of styrene in the bottom of the distillation column 42. The 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 polymerization thereby increasing styrene yield of the overall EBSM process. The ethylbenzene in 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 crystallization block 10 via the discharge conduit 22 and ii) an ethylbenzenerich residue liquor 28, which is discharged.

[0073] 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. 1b.

[0074] During the operation, the pyrolysis of polystyrene is performed in the pyrolysis reactor, which may be operated thermally or in a catalytic mode. 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 produce a high purity grade styrene product or VHPS, as desired by operator, at reduced specific energy consumption and capital expenditure.

[0075] The following example is provided to illustrate the disclosure and does not limit the scope of the claims.

EXAMPLE

[0076] Unless stated otherwise, all parts and percentages are by weight.

[0077] A crude styrene stream containing different impurities was produced by an extractive distillation unit on pyrolysis gasoline and subsequently purified by a combination of falling film and static melt crystallization to prepare the final VHPS product and final residue. The used crystallization block was as described above with reference to FIG. 1b except that the static melt crystallization stage 18a was omitted,

[0078] The styrene recovery across the crystallization block was >95%. The styrene concentration of the single streams shown in FIG. 1b were as follow:

TABLE-US-00003 Stream (no. in FIG. 1b) Fraction Styrene concentration 20 Crude styrene, feed to falling film 98.89 wt.-% 22 Final product 99.94 wt.-% 24 Feed to static crystallization 96.94 wt.-% 36 Final residue 88.19 wt.-%

[0079] 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.

[0080] The term “comprising” 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.