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          [background and overview].

          The first pyrethrum acid is an important intermediate for the synthesis of pyrethroid pesticides, and octadecene (chemical name: 2magin5-dimethyl-2maginol 4-hexadiene) is an important raw material for the synthesis of first pyrethrum acid. At the same time, it can also be used to synthesize some medicines and many other chemicals, which has a very good market at home and abroad. At present, only American EASTMAN company has goods for sale, German Bayer company and Japan Sumitomo company are also carrying out research and development, and there are no other commercialization reports. Zhongshan Kaida, Jiangsu Teda, Jiangsu Yangnong Changzhou Kangmei and others have built large-scale pyrethroid production lines, in which the key intermediate 2meme 5-dimethyl-2mene 4-hexadiene is mostly produced by 2pyr5-dimethyl-2pyr5-hexanediol dehydration isomerization, or depends on import. Because there are many problems in the existing separation process, the separation yield of octadecene is very low, and the product quality can not meet the requirements. At the same time, the raw materials not converted in the reaction process can not be effectively recovered and reused. The consumption of octadecene production is very high.

          [production process].

          1. Molecules with eight carbon atoms react with functional groups to form 2pyr5-dimethyl-2pyr4-hexadiene. There are mainly two different routes:

          1) dehydration of 2-dimethyl-2-dimethyl-5-hexanediol: using methanol as solvent and montmorillonite as catalyst at 250 ℃, the product of the reaction was 2-dimethyl-2-dimethyl-4-hexadiene, and 2-dimethyl-1-5-hexadiene was obtained at the same time. If Cr2O3 and r-Al2O3 are used as catalysts, the selectivity of the dehydration product is poor: under the condition of 230 ℃ and 240 ℃ and using Cr2O3 as catalyst, the content of 2-dimethyl-2-hexadiene is 65%; if r-Al2O3 is used, the content is 45%. Using p-toluenesulfonic acid as isomerization catalyst, the content of 2-dimethyl-2-hexadiene was 90.8%.


          2) dehydration of 5-tetramethyltetrahydrofuran: the commonly used catalyst is Pt/Al2O3, the reaction temperature is 450 ℃, and the yield of 2-dimethyl-2-dimethyl-4-hexadiene is about 75%:


          two. Octene is formed by condensation between small molecules.

          1) Synthesis of propionic acid and acetylene: this route has been reported industrially, but the process is complex, dangerous, has many by-products and low yield, less than 40%.

          2) preparation of 2-dimethyl-2-dimethyl-4-hexadiene from halogenated hydrocarbons: two molecules of 1-bromo-2-methylpropene were condensed in THF at-70 ℃ under the action of magnesium powder and copper chloride, and halogen was removed to obtain 30% octadiene. This is the synthesis of the target product through the reaction between halogenated hydrocarbons. Similarly, isobutyraldehyde can be treated to form halide and then condensed with isobutene to form octadecene. 1-chloro-2-methyl propyl ether was prepared by PCl5 treatment of isobutyraldehyde, which was pyrolyzed to octahedrene with a yield of 80%. The synthesis of octene from halogenated hydrocarbons can achieve the same effect not only in the liquid phase, but also in the gas phase. 3-chloro-2-methylpropene and isobutene can produce 2-dimethyl-1-hexadiene and a small amount of 2-chloro-2-methylpropylene and isobutene at 500 ℃. Octadiene can be obtained by isomerization of p-toluenesulfonic acid at 70 ℃. If trichloroethylene reacts with isobutene at 500C in the presence of (CH3) 3COOR (CH3) 3, the products are 2-dimethyl-1-dimethyl-5-hexadiene and 1-dichloro-4-methyl-1-5-pentadiene. 2the formation of 5-dimethyl-1-dimethyl-5-hexadiene is related to the process of methyl radical replacing chlorine radical.

          3) Prins reaction: octaenes are prepared by enal condensation: the condensation of olefins with aldehydes (called Prins condensation) is an important condensation reaction. The condensation reaction is considered to be an important organic reaction to obtain various saturated and unsaturated alcohols, diols, aldehydes and conjugated dienes. The Prins reaction of isobutene with isobutyraldehyde to prepare octene is one of the most widely studied methods at present. The reaction mechanism can be generally considered to be accomplished by the following processes:



          During the reaction of isobutene with isobutyraldehyde to octahedrene, a large amount of water and other by-products are produced. Water forms many binary and ternary azeotrope with isobutyraldehyde, isobutanol, tert-butanol, isobutyric acid and so on. The formation of azeotrope has a great influence on the separation of liquid mixtures by distillation, so it is very difficult to separate octene products by conventional distillation. Especially the dehydration process.

          Method 1: according to the flow rate, composition, component properties and separation requirements of the separation system, the optimization rules of distillation sequence synthesis are adopted, through simulation calculation and analysis, and the limitations of investment cost, operation cost and operation conditions are considered. the separation process as shown in the figure is proposed.



          The process is based on the principle of sequential separation. The isobutene with large flow rate and low boiling point is removed in the first tower, isobutyraldehyde is removed in the second tower, the components lighter than octene are removed in the third tower, and the components heavier than octene are removed in the fourth tower. Octene products are obtained at the top of the tower. The structural dimensions and process conditions of each tower are also determined in the calculation, and the operating conditions are shown in the table:


          Method 2: the new dehydration process of octene separation process is shown in the figure:


          The reaction material enters the deisobutene tower, 98% isobutene is obtained at the top of the tower, the liquid phase is extracted from a plate in the tower, and fed into a liquid-liquid separator, 94% of the water phase is extracted, and the oil phase is returned to the tower. The material for removing isobutene and water is obtained in the tower kettle, and there is almost no water in the material. The dehydrated material enters the deisobutyraldehyde tower, and the isobutyraldehyde without water and purity of 98% is obtained at the top of the tower, and the kettle liquid enters the subsequent separation process.