Xylene is an organic substance, which can be regarded as a derivative in which two hydrogens on a benzene ring are replaced by a methyl group.The chemical formula is C6H4 (CH3) 2. This structure has three different positions based on the substitution positions of the two methyl groups on the benzene ring. The three isomers, namely, o-xylene, m-xylene, and para-xylene, are commonly referred to as the three kinds of substances. In fact, because the physical and chemical properties of the three isomers are relatively similar, Xylene often refers to a mixture of the three.At room temperature and pressure, it is a colorless transparent flammable liquid with a special odor and a volatile, which can be used as a wide range of fuels, solvents and other purposes.

Colorless transparent liquid. Has a special smell of aromatic hydrocarbons.

It is a mixture composed of three isomers of 45% to 70% of meta-xylene, 15% to 25% of para-xylene and 10% to 15% of o-xylene. Easy to flow. Miscible with absolute ethanol, ether and many other organic solvents

Xylene has an irritating odor and is flammable. It can be mixed with ethanol, chloroform or ether, and it is insoluble in water. The boiling point is 137 to 140 ° C. Xylene is a low-toxic chemical substance. The ACGIH classifies it as A4, a substance that lacks evidence of carcinogenicity to humans and animals. Plastics, fuels, rubbers, additives for various coatings, as well as various adhesives and waterproof materials, can also come from the combustion gases of fuel and tobacco leaves.

Safety measures

Store in a low temperature ventilated place away from fire and heat sources. Avoid co-storage with oxidants. Never use spark-prone tools.

Fire fighting: foam, carbon dioxide, dry powder, sand.

Purpose

It is widely used as a solvent in coatings, resins, dyes, inks and other industries; used as a synthetic monomer or solvent in pharmaceuticals, explosives, pesticides and other industries; it can also be used as a high-octane gasoline component, and is an important raw material for organic chemicals. It can also be used to remove asphalt from the car body. Hospital pathology department is mainly used for the transparency and dewaxing of tissues and sections.

Industrial o-xylene is used as a raw material, first washed with industrial concentrated sulfuric acid until the acid layer is colorless, and then sequentially washed with 10% sodium hydroxide solution and water until qualified, the aqueous layer is separated, dried over anhydrous calcium chloride, and then rectified, After the distillate is clear, the middle distillate is collected, which is pure. 

production method

1. According to the boiling point of each component of crude benzene, a fraction with a boiling range of 135 to 145 ° C is extracted by distillation to obtain xylene.

2. The platinum reforming method uses normal pressure distillation of light gasoline (the initial boiling point is about 138 ° C), cuts the fractions greater than 65 ° C, first removes the harmful impurities by catalytic hydrogenation through a molybdenum-containing catalyst, and then reforms through a platinum catalyst. Extraction with diethylene glycol ether solvent, and then rectification column by column, to obtain products such as benzene, toluene and xylene.

3. Toluene disproportionation method. In this method, a methyl group of one toluene is transferred to another 166 catalyst (aluminum-hydrogen-type mordenite) under the action of a catalyst, and reformed toluene (nitrated grade) and reformed recycled hydrogen (85% to 90%) are used as raw materials. Under the conditions of a reaction temperature of 400 ± 2 ° C. and a pressure of about 2 MPa, benzene and xylene were prepared by the reaction.

4. The petroleum light fraction is mixed with benzene through hydrorefining, catalytic reforming and separation to obtain xylene. Or the coked crude benzene is obtained by washing and fractional distillation.

Health hazard

toxicity

Ingestion of xylene solvent by mistake will strongly stimulate the esophagus and stomach and cause vomiting, and may also cause bloody pneumonia. You should immediately drink liquid paraffin and send it to a doctor for treatment immediately. The LC of mice with xylene vapor is 6000 * 10-6, and the minimum oral lethal dose of rats is 4000mg / kg.

Xylene has an irritating effect on the eyes and upper respiratory tract. At high concentrations, it has anesthetic effects on the central system. Acute poisoning: short-term inhalation of higher concentrations of this product may appear to work. Chronic effects: Prolonged exposure to neurasthenia syndrome may cause menstrual abnormalities in women. Dry skin, chapped skin, and dermatitis often occur during skin contact.

Metabolism and degradation

In humans and animals, except that 3% to 6% of inhaled xylene is exhaled directly, all three isomers of xylene are metabolized to the corresponding benzoic acid (60% o-xylene, 80% to 90%). M-, p-xylene), and then these acids react with glucuronic acid and glycine. In this process, a large amount of o-benzoic acid is combined with glucuronic acid, while p-benzoic acid must be completely combined with glycine to generate the corresponding methyl hippuric acid and excreted. At the same time, the corresponding xylenol (phenols) and hydrogenated 2-methyl-3-hydroxybenzoic acid (less than 2%) may be formed in small amounts.

Cause of illness

1. Manufacture, use, storage, transportation of toluene and xylene in the petrochemical industry, paint, coatings, dyes, plastics, rubber, leather, saccharin, artificial musk and synthetic fibers and other production, accidental damage to pipes and storage systems, valve leakage In the case of gas, a large amount of inhalation of high-concentration toluene and xylene vapor for a long time can cause acute poisoning.

2. Apply anti-corrosive coatings with toluene and xylene as solvents in the sealed tank tank. Because there is no good ventilation, a large amount of toluene and xylene vapors can accumulate, which can cause workers to have acute poisoning. It has been reported that seven female workers suffered from acute toluene poisoning in a printing workshop with tightly-knitted windows in a knitting factory, which cleaned the strong printing table with a lotion water containing toluene.

Generally, the concentration of toluene and xylene in the air is 200 ~ 300 mg / m3 after 8 hours of inhalation, which will produce mild poisoning symptoms. Acute poisoning will occur if the concentration of 3.76 g / m3 is inhaled for 1 hour. The inhaler can be inhaled for several minutes at a concentration of 71.4 g / m3. Quickly coma and die.

3. Acute poisoning caused by percutaneous or by accident, rare reports.

Symptoms of poisoning

1. Acute poisoning Short-term exposure to high concentrations of toluene and xylene vapor can cause acute poisoning. Mild persons have headaches, dizziness, dizziness, nausea, vomiting, nausea, excitement, general weakness, staggering gait, numbness of hands and feet, etc., which may be accompanied by mucous membrane irritation such as tearing, sore throat, cough. In severe cases, irritability, confusion, tremor, delirium, coma, tonic convulsions, and severe heart, liver, and kidney damage occur. Inhalation of liquid toluene can cause chemical pneumonia, pulmonary edema, pulmonary hemorrhage, and anesthesia. Inhalation of high concentrations of xylene can still cause chemical pneumonia and pulmonary edema; it can die from respiratory and circulatory failure. Oral xylene can cause severe gastroenteritis.

2. Chronic effects Long-term inhalation of low-concentration toluene vapor can cause headache, dizziness, fatigue, insomnia or more dreams, memory loss and other neurasthenia syndromes. There have been cases of cerebellar degeneration symptoms after 2 years of exposure to toluene, development of diffuse encephalopathy a few years later, and cerebellar dysfunction after 1 and a half years of exposure to toluene-containing paint.

Emergency treatment

1. Immediately move to fresh air and give oxygen if necessary. Remove toxins from contaminated clothing and skin.

2. The antidote is glucuronide (see acute benzene poisoning).

3. Symptomatic treatment: oral vitamin B family, prevention and treatment of cerebral edema.

prevention

1. Strengthen the sealing and maintenance of ventilation, exhaust and production equipment in the workshop.

2. Monitor the toluene concentration in the air frequently.

3. Pay attention to personal protection and wear protective equipment if necessary.

4. Regular medical examinations and strict control of contraindications.