WO2000031007A1

WO2000031007A1 - Method for discolouring styrol obtained from pyrolysis gasoline and contaminated by chromophores

Info

Publication number: WO2000031007A1
Application number: PCT/EP1999/007322
Authority: WO
Inventors: Bernhard Firnhaber; Arno Müller; Helmut Gehrke
Original assignee: Krupp Uhde Gmbh
Priority date: 1998-11-23
Filing date: 1999-10-02
Publication date: 2000-06-02
Also published as: DE19853916A1; AU6332899A

Abstract

The present invention relates to a method for discolouring styrol obtained from pyrolysis gasoline and contaminated by chromophores, wherein said method involves preparing a mixture comprising the contaminated styrol, at least one dienophile and at least one polymerisation inhibitor, and heating the mixture at a temperature of between 50 and 140 °C before separating the discoloured styrol from the other components of the mixture.

Description

Process for decolorization of styrene obtained from pyrolysis gasoline and contaminated with color carriers

The invention relates to a process for decolorizing styrene obtained from pyrolysis gasoline and contaminated with color carriers. - Pyrolysis gasoline is obtained in the steam pyrolysis of hydrocarbons, such as naphtha, kerosene and light or heavy oil, which are carried out on a large scale to produce the olefins ethylene and propylene. In practice, a number of products are obtained from pyrolysis gasoline, such as butadiene and - after hydrogenation of the olefinic bonds - aromatics. The non-hydrogenated pyrolysis gasoline contains not only ethylbenzene and the xylenes but also a considerable amount of styrene and some phenylacetylene in the area of the C ₈ aromatics. Various methods are known to extract styrene from the pyrolysis gasoline. Corresponding methods are described, for example, in US Pat. No. 3,763,015 or US Pat. No. 3,684,665. Styrene is separated from the crude pyrolysis gasoline by extractive distillation of a selectively hydrogenated distillation cut. The selective hydrogenation of the distillation cut is carried out in particular to remove phenylacetylene. The styrene extracted in this way from the pyrolysis gasoline is, however, relatively strongly colored, so that this limits the further use. The extracted styrene contains color carriers, which are in particular traces of strongly coloring conjugated dienes and polyenes. The extracted styrene contains fulvens, for example, which are colored intensely yellow to red. To the possibilities of reusing the To expand pyrolysis gasoline extracted styrene, it is desirable to eliminate the color of the styrene.

In the process of the type mentioned at the beginning, which is known from US Pat. No. 3,763,015 and from which the invention is based, nitric acid is added to the styrene extracted from pyrolysis gasoline. The resulting mixture contains 1 to 30% by weight of nitric acid and this mixture is heated to a temperature between 0 ° C and 50 ° C. With the help of nitric acid, a more or less complete decolorization of the extracted styrene is achieved. US Pat. No. 3,763,015 also mentions comparative experiments in which the nitric acid was replaced by other substances, for example sulfuric acid, phosphoric acid, potassium permanganate or maleic anhydride. According to US Pat. No. 3,763,015, however, these substances are unsuitable for the decolorization of styrene because, after use of the substances, a colored styrene remains nonetheless. Although a more or less complete decolorization of the styrene is achieved with the use of nitric acid according to the known method, the known method has considerable disadvantages. First of all, nitric acid is a very strong oxidizing agent that causes undesirable side reactions with the hydrocarbons in the mixture. Environmentally friendly disposal of these by-products is complex and difficult. In addition, the implementation of the known method is relatively expensive. After the treatment with the nitric acid, the decolorized styrene must be separated from the other mixture components. The yield of decolorized styrene also leaves the known methods to be desired and is in need of improvement.

In contrast, the invention is based on the technical problem of specifying a method of the type mentioned at the outset with which, on the one hand, the most complete decolorization of the styrene is achieved, with which on the other hand a high yield of pure decolorized styrene is achieved and in which the abovementioned disadvantages are avoided.

To solve this technical problem, the invention teaches a process for decolorizing styrene obtained from pyrolysis gasoline and contaminated with colorants,

a mixture of the contaminated styrene, at least one dienophile and at least one polymerization inhibitor being produced,

wherein the mixture is heated to a temperature between 50 ° C and 140 ° C and then the decolorized styrene is separated from other mixture components.

The contaminated styrene used in the process according to the invention is preferably styrene which is obtained from pyrolysis gasoline by extractive distillation of a partially hydrogenated C ₈ distillation cut. The partial hydrogenation of the distillation cut takes place in particular to remove phenylacetylene. The extractive distillation of the C ₈ distillation cut is preferably carried out with at least a polar solvent from the group "N-formylmorpholine, N-acetylmorpholine, sulfolane, dimethylformamide, dimethylacetamide, dimethyl sulfone, dimethyl sulfoxide, ethylene glycol, triethylene glycol, polyethylene glycol" carried out, mixtures of these polar solvents with one another or mixtures of at least one the polar solvent can be used with water.

In the context of the invention, dienophile means an unsaturated compound which can react with a diene and / or triene and / or polyene in a Diels-Alder reaction. In this respect, various dienophiles are known who can enter such a cycloaddition. The dienophile expediently has a C-C double bond or a C-C triple bond. The dienophile expediently contains a C-C double bond with an attached functional group, for example an aldehyde group, ketone group, carboxyl group or ester group. According to a preferred embodiment, which is of particular importance in the context of the invention, a maleic acid derivative is used as the dienophile. According to a very preferred embodiment of the invention, maleic anhydride is used as the maleic acid derivative. The mixture preferably contains 0.5 to 3% by weight of the dienophile, preferably 0.5 to 3% by weight of maleic anhydride.

In the context of the invention, polymerization inhibitor means in particular a compound which inhibits the polymerization of styrene to polystyrene. Preferably 2-sec-butyl-4,6-dinitrophenol (DNBP) is used as the polymerization inhibitor. The mixture preferably contains 10 ppm to 2500 ppm, preferably 100 ppm to 1000 ppm, of the polymerization inhibitor.

According to a preferred embodiment of the invention, the mixture is heated to a temperature between 60 ° C and 100 ° C, preferably to a temperature between 70 ° C and 85 ° C. It is also within the scope of the invention that the reactants, i.e. the colored styrene containing the inhibitor and the dienophile, are first heated to the temperatures mentioned and only then mixed. It is understood that the mixture is placed in a suitable reaction container. It is within the scope of the invention to introduce the individual components heated to the reaction temperature into the reaction container. The reaction vessel can be a stirred tank. According to another embodiment, an isothermal tubular reactor can also be used as the reaction vessel, which is advantageously preceded by a mixing zone. The reaction time or the residence time of the mixture in the reaction vessel is at least 5 minutes, preferably a reaction time or residence time of 0.5 to 1.5 hours is used. The decolorization of the styrene takes place in the reaction vessel and the corresponding decolorization reaction therefore takes place here. It is expedient to work with a slight excess pressure. A pressure above the styrene vapor pressure, preferably at a pressure between 1 bar and 3 bar, is preferably used.

After the decolorization reaction has ended or after the residence time or reaction time according to the invention has expired, the decolorized styrene must be separated from the other mixture components or reaction products. The decolorized styrene can be separated from other mixture components by fractional distillation. The decolorized styrene is preferably separated from other mixture components by washing the reaction product with water, separating and drying the organic phase from the aqueous phase and distilling off the pure styrene from the dried organic phase. The distillation can be carried out as a vacuum flash distillation, it being expedient to work with a pressure between 60 mbar and 80 mbar. The decolorized styrene is obtained as an overhead product. A polymerization inhibitor can be added to the dried mixture prior to distillation.

The invention is based on the knowledge that complete decolorization of the styrene obtained from pyrolysis gasoline can be achieved and that the decolorized styrene can be obtained in a high yield if the procedure according to the teaching of the invention is followed and the parameters of the invention are observed. The color carriers in the styrene obtained from pyrolysis gasoline are in particular conjugated dienes,

Trienes and polyenes that can react with the dienophile after the Diels-Alder reaction. To this extent, the invention is based on the knowledge that, compared to styrene, products with higher boiling points are produced which can be separated from the decolorized styrene simply and effectively. Although in US Pat. No. 3,763,015 maleic anhydride is not suitable Compound for the decolorization of styrene, the invention has recognized that maleic anhydride in particular is excellently suitable for decolorization of the contaminated styrene if the requirements and parameters according to the invention are observed. In the process according to the invention, the pure decolorized styrene can be obtained in a surprisingly high yield after separation from the other mixture components or reaction products. In particular, undesired polymerization of styrene to polystyrene can be surprisingly effectively ruled out with the method according to the invention. The process according to the invention is furthermore distinguished by the considerable advantage that, in contrast to the known process, uncontrolled side reactions which lead to undesired by-products do not take place or hardly take place. In the decolorization reaction according to the invention, only by-products are formed which contain carbon, hydrogen and oxygen and can be disposed of without problems. If these by-products occur in the aqueous phase, easy biodegradability is guaranteed. Compared to the known method explained at the outset, the method according to the invention is also less expensive.

The invention is explained in more detail below on the basis of a drawing which represents only one exemplary embodiment.

The single figure schematically illustrates an embodiment of the method according to the invention. about the feed line 1 is fed with styrene contaminated with color carriers. This may be a slightly yellowish styrene which comes from the extractive distillation of pyrolysis gasoline and contains less than 0.5% by weight of impurities. According to a preferred embodiment of the invention, maleic anhydride is fed in via feed line 2 and polymerization inhibitor, for example 2-sec-butyl-4, 6-dinitrophenol (DNBP), is fed in via feed line 3. The three components are preferably introduced into the stirred tank 4 at the reaction temperature. The temperature of the mixture in the stirred tank 4 is preferably 60 ° C to 100 ° C, very preferably 70 ° C to 85 ° C. The residence time of the mixture in the stirred tank 4 or the reaction time is preferably 0.5 to 1.5 hours. A slight excess pressure, preferably a pressure between 1 bar and 3 bar, is preferably set in the stirred tank 4.

After the above-mentioned residence time and after the decolorization reaction has ended, the mixture is fed via a connecting line 5 to a liquid-liquid extractor 6, in which the mixture, if necessary after cooling, is expediently washed in countercurrent with deionized water which is fed into the liquid via the feed 7 - Liquid extractor 6 is introduced. Waste water is drawn off at the foot of the liquid-liquid extractor 6 and is preferably fed to a biological cleaning stage, not shown. Washing in the liquid-liquid extractor is used in particular to wash the decolorized styrene acid-free. The washed mixture is drawn off at the top of the liquid-liquid extractor 6 and fed to a fixed drying bed 9 via the connecting line 8. directs. The fixed drying bed 9 contains suitable drying agents, for example sodium hydroxide, potassium hydroxide, calcium hydroxide or molecular sieve.

The mixture dried in the fixed drying bed 9 is then fed via a feed line 10 to a flash vacuum distillation 11. The dried mixture is preferably distilled overhead at about 70 mbar pressure. The polymerization inhibitor is expediently added to the mixture before the distillation. The decolorized pure styrene is drawn off at the top of the flash vacuum distillation 11 and discharged via the discharge line 12. The bottom product of the flash vacuum distillation 11 is fed to a combustion. - The pure decolorized styrene thus obtained has an advantageous color number of less than 10 mg Pt / 1.

(The color numbers given here refer to the

Determination method according to ASTM D 1209.) This pure

Styrene product meets all requirements and can be recycled in many ways. The yield of the pure decolorized styrene is surprisingly high and is generally well above 95% by weight, mostly well above 99% by weight.

The invention is explained in more detail below on the basis of exemplary embodiments and comparative experiments: The exemplary embodiments and comparative experiments assume a styrene which has been obtained from a pyrolysis gasoline from an olefin plant based on naphtha. The styrene was removed from a C ₈ distillation cut by selective hydrogenation to remove phenylacetylene and subsequent Extractive distillation obtained with N-formylmorpholine as the solvent. This styrene contaminated with color carriers had a styrene content of 99.7% by weight and a color number of about 200 mg Pt / 1.

Example 1:

100 g of styrene, 1 g of maleic anhydride and 0.1 g of DNBP were heated in a glass flask equipped with a reflux condenser to 80 ° C. under a protective nitrogen atmosphere. The mixture was kept at this temperature for about 1 hour. The mixture was then cooled to 25 ° C. and shaken in a separatory funnel with 30 g of water for about 5 minutes. After the aqueous phase had been separated off, this washout was repeated. The washed organic phase was dried to dry on an approximately 15 cm high fixed bed of 30 g of crushed sodium hydroxide pellets. The running styrene contained 5.5 mg / 1 maleic anhydride and 80 ppm water. It was only faintly colored. This dried mixture was then distilled in a rotary evaporator at an absolute pressure of 70 mbar. , The distillation residue was 0.94% by weight of the distillation charge. The distillate contained 99.8% by weight of styrene. This distillate was completely colorless and had a color number below 10 mg Pt / 1. The color number was estimated to be about 3 mg Pt / 1. - This decolorized pure styrene obtained meets all requirements, in particular with regard to further processing to polystyrene. Example 2:

The mixture components were used in the same concentrations as in embodiment 1. The mixture was then heated to 140 ° C. and held at this temperature for 0.5 hour. The mixture was then processed further in accordance with embodiment 1. - The distillation residue was 5.9% by weight of the distillation charge. The distillate was completely colorless and the color number was below 10 mg Pt / 1. It was estimated to be around 7 mg Pt / 1.

Comparative experiment 1

100 g of styrene, 1 g of maleic anhydride and 10 ppm of 4-tert-butylpyrocatechol were heated to 25 ° C. under a protective nitrogen atmosphere and kept at this temperature for 1 hour with gentle stirring. The starting mixture had a color number of 206 mg Pt / 1, which reduced to 85 mg Pt / 1 after 1 hour. After 2 hours a color number of 44 mg Pt / 1 was found and after 24 hours the mixture had a color number of 19 mg Pt / 1. The mixture was further treated in accordance with working examples 1 and 2. The distillation residue was 1.3% by weight of the distillation feed. The distillate had a color number of 21 mg Pt / 1. Comparative experiment 2:

100 g of styrene and 5 g of maleic anhydride were used under otherwise identical conditions as in comparative experiment 1. While the initial decolorization was somewhat faster than in comparative experiment 1, a higher color number of 29 mg Pt / 1 was found after 24 hours. The distillate also had a color number of 31 mg Pt / 1.

A comparison of the exemplary embodiments with the comparative experiments shows that the measures and parameters which are claimed by the invention are critical for achieving the success according to the invention.

Claims

Claims:

1. Process for decolorization of styrene obtained from pyrolysis gasoline and contaminated with color carriers,

wherein the mixture is heated to a temperature between 50 ° C and 140 ° C

and then the decolorized styrene is separated from other mixture components.

2. The method according to claim 1, wherein a maleic acid derivative is used as the dienophile.

3. The method according to claim 2, wherein maleic anhydride is used as the maleic acid derivative.

4. The method according to any one of claims 1 to 3, wherein 0.5 to 3 wt .-% of the dienophile are used in the mixture.

5. The method according to any one of claims 1 to 4, wherein 2-sec-butyl-4, 6-dinitrophenol (DNBP) is used as the polymerization inhibitor.

6. The method according to any one of claims 1 to 5, wherein 10 ppm to 2500 ppm, preferably 100 ppm to 1000 ppm, of Polymerization inhibitor can be used in the mixture.

7. The method according to any one of claims 1 to 6, wherein the mixture is heated to a temperature between 60 ° C and 100 ° C, preferably to a temperature between 70 ° C and 85 ° C.

8. The method according to any one of claims 1 to 7, wherein the decolorized styrene after a residence time of at least

5 minutes, preferably after a residence time of 0.5 to 1.5 hours, is separated from the other mixture components.

9. The method according to any one of claims 1 to 8, wherein the decolorized styrene is separated from the further mixture components by fractional distillation.

10. The method according to any one of claims 1 to 8, wherein the removal of the decolorized styrene from further mixture components is carried out by washing the reaction product with water, separating and drying the organic phase from the aqueous phase and the pure styrene from the dried organic phase is distilled off.