The STEX Process-Extraction of Styrene from Pyrolysis Gasoline*

Transcription

1 The STEX Process-Extraction of Styrene from Pyrolysis Gasoline* by Hiroshi Morimoto** and Masanori Tatsumi** Summary: The STEX (styrene extraction) process gives low cost styrene and enhances the value of mixed xylenes from pyrolysis gasoline even in the small scale of production. Styrene in pyrolysis gasoline produced in ethylene plants is recovered by this process which mainly consists of extractive distillation with a special solvent. The total yield of styrene from a recently constructed ethylene plant (300,000 to 500,000t/yr) goes up to 15,000 to 30,000t/yr. At the same time, xylenes from pyrolysis gasoline are graded up by the decrease of ethylbenzene content which is usually enriched by hydrogenation of styrene. Besides extractive distillation, the STEX process has many distinctive features such as chemical purification and polymerization inhibition method. The STEX process which can be easily attached to an ethylene plant without any significant flow alterations will make great contributions to rationalize the plant by avoiding the waste of energy and resources. 1 Introduction A considerable amount of styrene is contained in pyrolysis gasoline which is a byproduct of ethylene production. Recently we have developed the new process that can extract styrene economically, and also enhance the value of mixed xylenes by the decrease of the ethylbenzene content. This new process is named the STEX process which is the abbreviation of "STyrene EXtraction process". 1.1 Ethylene Plant and Styrene Fig. 1 shows a flow scheme for recovery of BTX from pyrolysis gasoline in an ethylene plant. to ethylbenzene and is contained within the xylene fraction. Table 3 shows that the ethylbenzene content in xylenes from pyrolysis gasoline is much higher than in reformate xylenes. Therefore, pyrolysis gasoline is regarded as a poor paraxylene source. Namely, byproduct styrene is converted to ethylbenzene by consuming a lot of hydrogen and causes more ethylbenzene which makes some trouble in paraxylene production plant because it is isomerized to pa- tion, which is a byproduct of the naphtha crack- xylenes are separated from pyrolysis gasoline after 2-step hydrogenation purification and extraction section. There is a considerable amount of styrene in pyrolysis gasoline as listed in Table 1. It is obvious that styrene is the main component in C8 aromatics. Table 2 shows the typical aromatic yields from 100,000 tons of ethylene production. In a recently constructed large ethylene plant (300,000 t/yr), the amount of byproduct styrene goes up to 15,000 to 30,000t/yr. Such a large amount of styrene, as shown in Fig. 1, is hydrogenated * Received December 11, ** Petrochemical Laboratory, Toray Industries, Inc. (Tebiro, Kamakura, Kanagawa 248) Fig. 1 Normal Scheme for BTX Production from Ethylene Plant Pyrolysis Gasoline Table 1 Typical Composition of Pyrolysis Gasoline (wt%) Table 2 Aromatics from 100,000 Tons of Ethylene Production (thousands of tons) Bulletin of The Japan Petroleum Institute

2 Morimoto and Tatsumi: The STEX Process-Extraction of Styrene from Pyrolysis Gasoline Table 3 Typical Xylenes Composition (wt%) Table 4 Common Separation Methods raxylene with difficulty. In this sense more ethylbenzene means the poor quality of mixed xylenes. 1.2 Significance of the STEX Process The STEX process is a new method for producing styrene economically by extraction of pyrolysis gasoline, and for grading-up of pyrolysis gasoline xylenes by decrease of ethylbenzene content, improving a flow scheme of pyrolysis gasoline in an ethylene plant. Additionally by this method, a lot of hydrogen can be saved at the 2-step hydrogenation purification section. These features can be achieved by the STEX process which consists in (1) to obtain the C8 aromatic fraction from pyrolysis gasoline by conventional distillation, (2) to extractively distillate styrene from the C8 aromatic fraction and (3) to purify extracted styrene. Mixed xylenes produced at section (2) can be sent to the paraxylene producing section after the 1-step hydrogenation purification section. 1.3 Previous Works for Styrene Separation Investigating a process and the data of composition analysis in pyrolysis gasoline, one may easily get an idea to separate styrene from others. However, this idea has not been materialized in industry. The reason is that it is very difficult to separate pure styrene from pyrolysis gasoline which contains many kinds of hydrocarbons. Table 4 shows the common methods for separation of some components from the mixture. When one tries to separate styrene from pyrolysis gasoline, for instance, by the conventional distillation method, one finds it almost impossible. The reason is clear in Table 5 which shows that many hydrocarbons in pyrolysis gasoline boil closely to styrene. As it is shown in Table 6, there are many chemical compounds in pyrolysis gasoline, and it is difficult to separate styrene by the differences of chemical properties. Although some inventions were reported to overcome these difficulties, none of them are commercialized. For instance, the liquid-liquid extraction method by Imperial Chemical Industries, Ltd.1) using silver fluoroborate, or the one by Metallgesellschaft2) using aqueous silver nitrate, and the Table 5 Hydrocarbons Boiling Points Table 6 Types of Hydrocarbons extractive distillation method by Erdol Chemie3) using dialkylformamide, especially dimethylformamide, have been known in the literature. These methods could not sufficiently overcome the difficulties mentioned above, and could not produce sufficiently pure styrene for polymerization. Very little has been known about the elimination of the impurities in separated styrene. The research and development division of Toray Industries, Inc. has succeeded in overcoming these difficulties. 2 Comparison of Processes Before the explanation of the STEX process, it would be better to clarify its features by comparing it with the conventional processes. Fig. 2 shows the process flow of the ethylene plant attached by the STEX process. When the C8 fraction from pyrolysis gasoline is led to the STEX process and styrene is separated, many benefits mentioned above can be obtained. In Fig. 3 the STEX process is compared with the conventional styrene production process from benzene and ethylene. In Fig. 3 the STEX process is also compared Volume 16, No. 1, May 1974

3 Morimoto and Tatsumi: The STEX Processwith the Halcon process4) which uses ethylbenzene and propylene for starting materials. In comparison with these conventional processes, which contain many steps (see Fig. 3), the STEX process, which extracts originally existing styrene, is rather simple. First, a C8 fraction which consists of 30 to 35% of styrene and 60 to 70% of xylenes and ethylbenzene is obtained from pyrolysis gasoline by ordinal distillation. Styrene is removed from the C8 fraction by extractive distillation, separated from the extraction solvent and purified sufficiently. The remainder of the C8 fraction in very low ethylbenzene content is a suitable feed for recovery of paraxylene. 3 Technical Features of the STEX Process 3.1 Feedstock The styrene content in pyrolysis gasoline is 4 to 6% (see Table 1). This pyrolysis gasoline stream can be used as a feedstock without any treatment, but it is more advantageous to use the C8 aromatic fraction obtained by conventional distillation as the starting material. This distillation removes low boiling compounds up to Fig. 2 The Ethylene Plant Attached by the STEX Process toluene and high boiling compounds above C9 aromatics, and obtains the C8 aromatic fraction a styrene content of 30 to 35%. Most of recent ethylene plants have a distillation column which separates aromatics heavier than C9 to get BTX feedstocks. In this case, the C8 aromatic fraction is easely obtained by changing slightly the operating condition of the column and by separating low boiling compounds with another column. Table 7 shows a composition of C8 components obtained in this way. 3.2 Extractive Distillation Since the difference in boiling point between shows, it is very difficult to separate styrene from C8 fraction by use of conventional distillation. It is necessary for separation of styrene from o-xylene to use the distillation column of 1,000 or more trays. However, styrene is subject to polymerization during such distillation. For separation of styrene from C8 components, the application of extractive distillation with a special solvent is one of the distinctive features of the STEX process. Many factors to be considered on the selection of the solvent for extractive distillation are suggested by, for instance, Hirata5). In addition to those, in the STEX process, it is necessary to pay much attention to the points mentioned below. (1) The boiling point of the solvent If the boiling point of the solvent is much higher than that of o-xylene or styrene, the solvent Fig. 3 Comparison of STEX, with Conventional and Halcon Process for Styrene Production

4 Extraction of Styrene from Pyrolysis Gasoline Table 7 Typical Composition of C8 Fraction simply flows down through the column and carries out very ineffective extraction. The adequate repetition of vaporization and condensation of the solvent is necessary. So, effects of the solvent on extractive distillation were actualy confirmed in a test distillation column. (2) Polymerization of styrene Since styrene polymerizes easily, solvents that might initiate polymerization or that have a high boiling point must be avoided. The solvent should preferably be able to dissolve any polymer formed during distillation. (3) Contamination of styrene by the solvent Styrene must be pure enough to be used in polymer industries. If separation of the solvent and the degradation products of the solvent from styrene is difficult, even an efficient solvent is impractical. Through the experiments, several solvents for the STEX process are selected to satisfy the above requirements. 3.3 Polymerization Inhibitor Styrene is treated carefully because of its ease of polymerization. For instance, the distillation column in a conventional styrene producing pro- suitable polymerization inhibitor6). But in the extractive distillation, the solvent has a rather high boiling point and the column requires a considerable number of plates to perform the desired separation. Therefore, intensive research for a polymerization inhibitor was essential to the development of the STEX process. As a result of extensive investigation, an excellent polymerization inhibitor, which is effective for hydrogenation purification. Analytical results of veloped. Fig. 4 shows the effect of the inhibitor. This inhibitor has excluded the restriction of distillation temperature and simplified the column design. 3.4 Removal of Impurities Even if we could isolate styrene from pyrolysis Volume 16, No. 1, May 1974 Fig. 4 Effect of the STEX Inhibitor gasoline by any process, without any purification styrene couldn't be used in polymer industries. It is very difficult to remove impurities from chemically active styrene without any loss of it. These conditions are the same for production of styrene by extractive distillation. We have found a new excellent chemical treatment to remove trace amounts of impurities, which has provided an effective purification at lower cost and with a little loss of styrene. With this treatment, the quality of styrene produced with the STEX process is good enough, as mentioned in 3.5. Also, the overhead of extractive distillation column has such a low ethylbenzene content as reformate xylenes. The overhead was confirmed to be used for a feed of paraxylene recovery after the conventional hydrogenation purification. 3.5 Process Flow and Product Quality Fig. 5 shows the STEX flow diagram starting from a C8 aromatic fraction including 30 to 35 of styrene. At first the C8 fraction with some % impurities which are difficult to remove by extractive distillation is worked up by a chemical method. The treated C8 fraction is fed to the intermediate plate of the extractive distillation column and an extraction solvent containing polymerization inhibitors is fed at the upper part of the column. At this distillation, xylenes and non-aromatic components with small amounts of the solvent go to the overhead. This distillate can be used for recovery of paraxylene after suitable treatment such as a conventional the distillate recovered in our pilot plant are shown in Table 8. As non-aromatic contents is utmost 3%, the aromatic extraction section can be skipped. Comparison of Table 8 with Table 3 shows that the ethylbenzene content of the recovered xylenes is almost equal to that of

5 Morimoto and Tatsumi: The STEX Process-Extraction of Styrene from Pyrolysis Gasoline Fig. 5 Simplified Flow Scheme of the STEX Process Table 8 Recovered Xylenes before Aromatics Extraction Fig. 6 Production Cost of Styrene by STEX and Conventional Process reformate Table 9 Typical STEX Process Styrene Table 10 Typical Polymer from STEX Styrene xylenes. The bottom content of the extractive distillation column is separated into the solvent and styrene by the solvent recovery column. As overhead styrene contains some impurities such as the solvent, it goes to the washing section. The quality of styrene obtained in the pilot plant is shown in Table 9. True indication of the monomer quality is shown after polymerization in Table 10. Meanwhile, most of the recovered solvent is recycled to the extractive distillation column. Before returning to the recycle system, a part of the solvent is regenerated in the solvent purification step to remove the polymer and other impurities. 4 Economic Considerations The STEX process, with the several characteris- tics mentioned above, is expected to ensure considerable savings. The economic significance of the STEX process becomes clear, when we consider the circumstances of the conventional ethylene plant where styrene in pyrolysis gasoline is converted to ethylbenzene consuming a lot of hydrogen and resulted xylenes contain the considerable amount of ethylbenzene. In Fig. 6 production cost for the STEX process are compared with that of a conventional ethylbenzene dehydrogenation process. The STEX process shows lower cost and could produce styrene economically even in relatively small scale plants. At a capacity of styrene 20,000t/yr, the STEX process produces styrene at a cost of about 35 yen/kg, 60 to 70% of the for a conventional process at the same production scale. Production cost of styrene vary to some extent depending on costs of pyrolysis gasoline, xylenes credit, utilities and the plant location. The STEX process will make a great contribution to rationalize ethylene plants by eliminating the waste of energy and resourses. The attachment of this process to the ethylene plant is fairly easy. It is also expected that the technical know-how of this process may be applied to the separation of other unsaturated compounds with small difference in boiling points and that extractive distillation will become more important in the future. Literature Cited 1) Brit. 949,095; 1,033,219. 2) Brit. 1,038,606. 3) Brit. 1,020,175. 4) Hydrocarbon Processing, 46, (4), 141 (1967). 5) Hirata, M., Yorizane, M., "Joryu Kogaku Hand Book", 198, 799, (1970) Asakura, Tokyo. 6) Boundy, R. H. et al., "Styrene, its Polymers, Copolymers and Derivatives (Part 1)", 40, (1970) Hafner, Darien. Bulletin of The Japan Petroleum Institute