Background and overview[1][2]
The chemical formula of m-xylene is C6H4(CH3)2. Molecular weight 106.17. Colorless liquid. Density 0.86104. Melting point 13~14℃. Boiling point 139.0. Insoluble in water, miscible with ethanol, ether and other organic solvents. Meta-xylene is C. A component of aromatic hydrocarbons. Mixed xylene contains three xylene isomers: ortho-xylene (OX), para-xylene (PX) and m-xylene (MX).
Among them, MX accounts for the largest proportion, about 45%. However, since these three isomers have similar properties, especially the boiling point difference between MX and PX is very small, an efficient and economical method for separating MX has not yet been discovered, resulting in its industrial application volume being far less than Not on OX and PX. Nowadays, MX is mostly used in industry to produce PX and OX, or as a solvent, but it has not been used very rationally. With the continuous development of the modern chemical industry, separation technology continues to be improved, and more and more methods for separating high-purity MX are being discovered. High-purity MX is used to produce various derivatives and other related chemical products, which has driven the development of the MX industry.
The production and application of MX require continuous research and development so that the value of m-xylene can be fully utilized. my country started late in the separation and application of m-xylene. It was not until September 2000 that the 35 klf a m-xylene unit of China Petroleum & Chemical Corporation Beijing Yanshan Branch (hereinafter referred to as Yanshan Petrochemical) was officially put into operation, and the m-xylene industry entered the market. Large-scale development stage.
The entry of high-quality m-xylene from Yanshan Petrochemical into the market has greatly promoted the development of my country’s downstream m-xylene industry: at first, the supply of m-xylene products exceeded demand, but in just a few years, the supply and demand relationship in the domestic market has changed dramatically. In 2006, the market was already showing signs of oversupply. The main uses of m-xylene: as isomerization raw material to produce PX and OX; as a solvent or a component for blending gasoline; to produce resins, inks, polyester fiber dyeing modifiers and other products. Using MX as raw material to produce its derivatives has broad development prospects.
Structure
Apply[2][3]
Meta-xylene has the following main uses: First, it is used as a raw material for isomerization to produce para-xylene and o-xylene; second, it is used as a solvent or a component for blending gasoline; third, it is used to produce resins and fine chemicals. Chemical Products. The following focuses on the application of m-xylene in resins and fine chemical products.
1. Used as modified intermediate for resin materials
1) Isophthalic acid can be produced from m-xylene through liquid phase oxidation reaction. Isophthalic acid can improve the strength, toughness, fatigue resistance and corrosion resistance of resin materials. Its greatest use is the production of unsaturated polyester resin, which can be used in construction, transportation and other fields. A small amount of isophthalic acid is also used to make some specialty products, such as polyamide fibers, liquid crystalline polymers, polyarylates and other products with smaller demand.
2) Isophthaloyl chloride can be produced by first oxidizing m-xylene and then reacting with phosgene. Polyisophthalyl m-phenylenediamine resin synthesized from isophthaloyl chloride and m-phenylenediamine can be produced. into fibers, films and coatings. Aramid fibers (i.e., aramid) made from this resin have excellent heat resistance, dimensional stability, and electrical insulation.
3) Using m-xylene as raw material, it reacts with carbon monoxide to synthesize m-xylyldehyde, and then undergoes oxidation and dehydration to obtain trimellitic anhydride. Trimellitic anhydride is an important raw material for the development of new materials. It is mainly used to produce polyvinyl chloride heat-resistant plasticizer trioctyl trimellitate, polyimide engineering plastics, water-soluble alkyd resin materials and epoxy resin curing agents and other fine products. Chemicals.
2. Used as intermediates for fine chemical raw materials such as pesticides, medicines, and dyes
isophthalonitrile can be produced by ammonia oxidation of m-xylene. It is an intermediate raw material for the highly efficient, broad-spectrum, low-toxicity, low-residue fungicide “chlorothalonil”, and is also used in the production of guanidine-based resin and isophthalonitrile. An intermediate of methyldiamine and used in the production of epoxy resin curing agents. Meta-xylene can be produced through air oxidation to meta-toluic acid, which is mainly used as an intermediate for the drug toluene diethylamine. It can also be used as a raw material for the sanitary insecticide “DEET”. In addition, it can also be used to produce polyvinyl chloride. Stabilizers for resins and raw materials for polymer monomers.
Preparation [3]
1. Adsorption separation method
The adsorption separation method is a xylene separation method that was industrialized in the 1970s and quickly took the lead. The adsorbent selectively adsorbs paraxylene. When equilibrium is reached, evaporated and heated toluene is used as the desorbent for desorption. The desorption liquid enters the desorption liquid tank after cooling. The replacement process is to use the desorbed paraxylene as the replacement agent, which enters the adsorption column after evaporation and heating. The residual m-xylene in the adsorbent is replaced, and the discharge from the replacement zone enters the adsorption zone together with the adsorption feed. The desorption liquid enters the desorption liquid tower through the intermediate tank, and the desorption agent toluene is evaporated from the top of the tower. Part of the high-concentration xylene in the tower kettle is desorbed as a displacement agent, and the rest is returned to the raw material tank as isomerization raw material. The raffinate liquid enters the raffinate tower through the raffinate tank, and toluene is evaporated from the top of the tower for desorption, and the qualified high-purity m-xylene product is obtained from the tower kettle.
2. Sulfonation method
The sulfonation method is an earlier m-xylene separation method and has been eliminated abroad.There are many methods, but there are still sulfonation production equipment in China. The sulfonation method is to sulfonate mixed xylene with sulfuric acid. After obtaining m-xylene sulfonic acid and hydrolyzing m-xylene sulfonic acid, the fraction is distilled to obtain the finished product m-xylene.
3. Reactive distillation method
The reactive distillation method takes advantage of the difference in reaction selectivity between xylene isomers and adds an additive to generate a less volatile product with m:toluene. After distillation, m-xylene is separated from other xylene isomers.
4. Azeotropic distillation method
The azeotropic distillation method changes the relative volatility between xylenes by adding an entrainer to achieve the purpose of distilling and separating xylenes. The relative volatility of m-xylene and p-xylene is 1.02, and the relative volatility of o-xylene and m-xylene is 1.12. Methyl valerate can increase the relative volatility of m-xylene and p-xylene to above 1.3. Propyl butyrate and hexyl formate can make the relative volatility of m-xylene and o-xylene reach more than 1.4.
5. New process for recycling m-xylene
The new process adopts thermal combination and optimized combination of distillation and crystallization equipment, which reduces the operating cost of the device. The purity of the recovered m-xylene product is 99.5%, and the purity of the by-product ethylbenzene (EB) is 99.6%.
Main reference materials
[1] Chai Junyuan. “Production of m-xylene and its development prospects.” Petrochemical Technology 24.3 (2017): 43-44.
[2] Hu Xiang; Li Tao. Production technology and application of m-xylene. Petrochemical Technology and Economics, 2014, 30.3: 22-26.
[3] Ha Yingde. “Discussion on para-xylene and its production technology.” China Chemical Trade 4.4 (2012): 275-275.
