Xylene, chemical formula CeightHten, xylene or xylene aromatic hydrocarbon mixture, the benzene ring consists of two methyl groups at different positions.There are three isomers, o-xylene, m-xylene (CAS No. 95-47-6), m-xylene (CAS No. 108-38-3) and p-xylene (CAS No. 106-42-3).Molecular formula C of three isomers of xyleneeightHten, or through semi structural formula CsixHfour(CHthree)twoexpress.In industry, xylene refers to the mixture of the above isomers.
Xylene is a major petrochemical produced in the manufacture of coking fuel from coal coking through catalytic reforming.It accounts for 0.5-1% of crude oil and is found in small quantities of gasoline and aircraft fuel.From catalytic reforming to“Reformate”Xylene is the main product of BTX aromatics (xylene, benzene and toluene).The mixture is a slightly greasy, colorless liquid, which is used as solvent.Xylene was named in 1851 after it was discovered as a component of wood tar.Millions of tons are produced every year.In Singapore in 2011, a global alliance began to build the world's largest xylene plant.
On October 27, 2017, the list of carcinogens released by the International Agency for Research on Cancer of the World Health Organization (WHO) was preliminarily sorted out for reference, and xylene was included in the list of category 3 carcinogens.[1]
Chinese name
xylene
Foreign name
Xylenes
chemical formula
CeightHten
molecular weight
one hundred and six point one six five
Melting point
-34 ℃[2]
Boiling point
137 to 140 ℃[2]
Density
0.865 g/cm³[16]（20°C）
Flash point
25 ℃[2]
Relative steam density (g/m
three point six six[16]
Refractive index
one point four nine seven[2]
Self ignition point or ignition temperature (℃
four hundred and sixty-three point eight[2]
Upper explosive limit (%, V/V
seven[2]
Lower explosive limit (%, V/V
one point one[2]
Solubility
It is miscible with ethanol, ether, trichloromethane and other organic solvents, and insoluble in water.[2]
Xylene is composed oftolueneandbenzeneOfMethylationAnd is produced.Commercial or laboratory production of xylene usually produces about 40-65% m-xylene and up to 20% o-xylene, p-xylene and ethylbenzene respectively.The ratio of isomers produced by the patented method can be shifted to facilitate the production of high-value p-xylene.These conversions are made byzeolitecatalysis.[3]
Preparation of Xylene by Sulfonation
The mixed fraction of m-xylene and p-xylene (i.e. mixed xylene fraction) reacts with concentrated sulfuric acid to selectively sulfonate m-xylene, thus separating m-xylene and p-xylene.The mixed xylene fraction contacts with concentrated sulfuric acid, and m-xylene is selectively sulfonated to generate m-xylene sulfonic acid at 30-35 ℃.
P-xylene does not undergo sulfonation reaction.The m-xylene sulfonic acid obtained from the separation can be hydrolyzed, neutralized and rectified to obtain m-xylene with a purity of 95~98%.The non sulfonated substance containing p-xylene can be neutralized by alkali and crystallized to obtain p-xylene with a purity of 95%.[4]
Crystallization method
The mixed xylene fraction containing m-xylene and p-xylene is eutectic crystallized (crystallization temperature is 53.1 ℃) to obtain the eutectic containing 12.5% p-xylene and 87.5% m-xylene, and then the method of separating these two products through secondary crystallization.The crystal obtained after the first crystallization and filtration is industrial m-xylene.When the filtrate is crystallized for the second time, an appropriate amount of carbon tetrachloride is added to form a solid complex with p-xylene at an initial crystallization temperature of - 40 ℃, and with m-xylene at an initial crystallization temperature of - 78 ℃.Control the crystallization temperature to make only carbon tetrachloride and p-xylene crystallize.The filtered crystal is melted, and carbon tetrachloride is separated by distillation to obtain p-xylene.The filtrate is rectified to recover carbon tetrachloride to obtain m-xylene.The carbon tetrachloride in the process is recycled.[4]
Material structure
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Xylene can be divided into:
O-xylene, the two methyl groups on the benzene ring are in the ortho position.
M-xylene, the two methyl groups on the benzene ring are in the meta position.
For p-xylene, the two methyl groups on the benzene ring are in para position.
The melting point sequence of the above three is: pair>neighbor>inter.
The boiling point order of the above three is: Neighbor>Inter>Pair.[5]
Isomer of toluene
name
xylene
O-xylene
M-xylene
Paraxylene
IUPAC name
xylene
1,2-Xylene
1,3-Xylene
1,4-Xylene
English name
Dimethylbenzene
Xylene
1,2-Dimethylbenzene
o-Xylene
1,3-Dimethylbenzene
m-Xylene
1,4-Dimethylbenzene
p-Xylene
alias
Xylol
o-Xylol
Orthoxylene
m-Xylol
Metaxylene
p-Xylol
Paraxylene
chemical formula
CeightHten
distinguish
SMILES
Cc1c(C)cccc1
Cc1cc(C)ccc1
Cc1ccc(C)cc1
Molar mass
106.16 g/mol
appearance
Colorless clear liquid
CAS No
[1330-20-7]
[95-47-6]
[108-38-3]
[106-42-3]
Physical and chemical properties
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Colorless transparent liquid.yesAromatic hydrocarbonThe special smell of.It is a mixture of 45%~70% m-xylene, 15%~25% p-xylene and 10%~15% o-xylene isomers. It is easy to flow and can be miscible with anhydrous ethanol, ether and many other organic solvents.
Xylene has a pungent smell and is flammable. It can be mixed with ethanol, chloroform or ether at will and is insoluble in water.The boiling point is 137~140 ℃.Xylene is a low toxic chemical substance, which is classified as A4 by the American Conference of Governmental Industrial Hygienists (ACGIH), that is, a substance lacking evidence of carcinogenicity to humans and animals.In plastics, fuels, rubber, additives for various coatings, adhesives and waterproof materials, combustion gases from fuels and tobacco leaves can also be obtained.
nature
density
0.864 g/mL (liquid)
0.88 g/mL (liquid)
0.86 g/mL (liquid)
0.86 g/mL (liquid)
Solubility (water)
Insoluble
Soluble in non-polar solvents such as aromatic hydrocarbons
melting point
−47.4 ℃ (−53.3 F; 226 K)
−24 ℃(−11 °F; 249 K)
−48 ℃ (−54 F; 225 K)
13.2 ℃ (55.8 F; 286.3 K)
boiling point
138.5 ℃ (281.3 F; 412 K)
144.4 ℃(291.9 F; 417.5 K)
139 ℃ (282 F; 412 K)
138.35℃(281.03 F; 411.50 K)
viscosity
0.812 cPat 20℃(68 F)
0.62 cP at 20℃(68 F)
0.34 cP at 30℃(86 F)[5]
Molecular structure data
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1. Molar refractive index: 35.9
2. Molar volume (cmthree/mol)：121.9
3. Isotonic specific volume (90.2 K): 282.5
4. Surface tension (dyne/cm): 28.7
5. Dielectric constant: 2.39
6. Polarization (10-24cmthree)：14.23
7. Single isotope mass: 106.07825 Da
8. Nominal mass: 106 Da
9. Average mass: 106.165 Da[2]
5 Calculate chemical data
1. Drainage parameter calculation reference value (XlogP): 3.2
2. Number of hydrogen bond donors: 0
3. Number of hydrogen bond receptors: 0
4. Number of rotatable chemical bonds: 0
5. Topological molecular polar surface area (TPSA): 0
6. Number of heavy atoms: 8
7. Surface charge: 0
8. Complexity: 58.4
9. Number of isotope atoms: 0
10. Determine the number of atomic structure centers: 0
11. Number of uncertain atomic structure centers: 0
12. Determine the number of chemical bond structure centers: 0
13. Number of uncertain chemical bond structure centers: 0
14. Number of covalent bond units: 1[2]
Health impact
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toxicity
When ingesting the xylene solvent by mistake, it will strongly stimulate the esophagus and stomach, cause vomiting, and may also cause bloody pneumonia. Drink the liquid immediatelyparaffin wax, send to the doctor for treatment immediately.LC of xylene steam on mice is 6 × 10-3The lowest oral lethal dose of rats is 4000 mg/kg.
Xylene has irritating effect on eyes and upper respiratory tract, and at high concentration, it has anesthetic effect on central system.Acute poisoning: inhalation of high concentration of this product in a short time can cause acute poisoning.Chronic effects: long-term contact with neurasthenia syndrome may lead to abnormal menstruation in women.Skin contact often causes skin dryness, chaps and dermatitis.[3-4]
Metabolism and degradation
In humans and animals, except 3%~6% of inhaled xylene is directly exhaled, three isomers of xylene are metabolized into correspondingBenzoic acid(60% o-xylene, 80%~90% m-xylene, p-xylene), and then these acids react with glucuronic acid and glycine.In this process, a large amount of o-benzoic acid will combine with glucuronic acid, while p-benzoic acid will almost completely combine with glycine to produce corresponding methylhippuric acid and be discharged from the body.At the same time, a small amount of corresponding xylenol and 2-methyl-3-hydroxybenzoic acid (less than 2%) may be formed.[3]
Residue and accumulation
The percutaneous absorption of xylene vapor is negligible compared with direct contact with liquid.The residual and accumulation of xylene is not serious. The xylene entering the human body can generate methylbenzoic acid in the presence of NADP (transaminase II) and NAD (transaminase I) in the human body, and then combine with glycine to form methylhippuric acid, which is almost completely discharged from the body within 18 hours.Even the 3% - 6% xylene left in the lungs after inhalation（half life0.5~1 hour) were all exhaled out of the body.The residual test to evaluate the exposure to xylene is mainly to determine the content of methylhippuric acid in urine. It is also suggested to determine the content of xylene in exhaled gas or blood, but the results of the latter are often inaccurate.Because methyl hippuric acid does not naturally exist in urine, and because it is almost all residual xylene metabolites, determining its presence is the best confirmation of xylene contact test.Xylene can exist in drinking water for a long time.When the concentration of xylene in tap water is 5 mg/L, its odor intensity is equivalent to grade 5, and the unique odor of xylene will disappear after 7 to 8 days;It takes 4-5 days when the odor intensity is level 3.The odor of xylene in river water is kept for a short time, which is related to the initial concentration, and can be kept for 3-5 days generally.[3]
Migration and transformation
Xylene is mainly produced from crude oil in the petrochemical process. It is widely used as a diluent for pigments, paints, and solvents for printing, rubber, and leather industries.As a cleaner and degreaser, a component of aviation fuel, raw materials and intermediates of chemical plants and synthetic fiber industries, as well as coatings and impregnating materials for fabrics and paper.Xylene can be discharged into the atmosphere through mechanical exhaust and ventilation equipment and cause pollution.The xylene discharged into the atmosphere by a refinery is up to 13.18~1145 g/h. The xylene can enter the water body along with the wastewater discharged by its production and use units to produce 1 ton of xylene. Generally, 2 tons of wastewater containing 300~1000 mg/L xylene is discharged.Because xylene has a strong tendency of volatilization in aqueous solution, it can be considered that xylene is not a persistent pollutant in surface water.Xylene can also be biodegraded in the environment, but the rate of this process is much lower than that of the volatilization process.Xylene volatilized into the air may also be photolysis, which is its main migration and transformation process.
Xylene is discharged from the human body rapidly by exhalation and metabolites, and almost all of it is discharged from the body within 18 hours after the contact stops. Xylene can exist in drinking water quite permanently.Since xylene is highly volatile in aqueous solution, it can be considered that it is not a persistent pollutant in surface water.Xylene can also be biodegraded and chemically degraded in the environment, but its speed is much lower than that of volatilization. Xylene volatilized into the air can be photolysis.It can react with oxidant and explode when high concentration gas is mixed with air.Xylene has a moderate risk of combustion.Because its steam is heavier than air, the flame spreads along the ground during combustion.Xylene is easy to volatilize, and the accident scene will be filled with the special fragrance of xylene. Xylene dumped into the water can float on the water surface, or be distributed on the water surface as oil, which can cause the death of fish and aquatic organisms.[4]
Leakage emergency treatment
Evacuate the personnel in the leakage contaminated area to a safe area quickly, isolate them, and strictly restrict access.Cut off the ignition source.It is recommended that emergency personnel wear self-contained positive pressure respirators and fire protection clothing.Cut off the source of leakage as far as possible to prevent it from entering into restricted spaces such as sewers and flood discharge ditches.
Small amount of leakage: absorb with activated carbon or other inert materials.It can also be brushed with emulsion made of incombustible dispersant, and the washing solution will be diluted and put into the waste water system.
Large amount of leakage: build a dike or dig a pit to receive;Cover with foam to inhibit evaporation.Use explosion-proof pump to transfer to tank car or special collector for recycling or transport to waste disposal site.Quickly collect the soil contaminated by xylene and transfer it to a safe area.Strengthen ventilation along the ground in the polluted area, evaporate residual liquid and remove steam.Dams shall be built quickly to cut off the flow of polluted water, and fences shall be used to limit the diffusion of xylene on the water surface.[4]
Protective measures
Respiratory system protection: wear a filtered gas mask (half mask) when the concentration in the air is high.In case of emergency rescue or evacuation, it is recommended to wear an air respirator.
Eye protection: wear chemical safety goggles.
Body protection: wear work clothes to prevent poison penetration.
Hand protection: wear rubber gloves.
Others: Smoking, eating and drinking are prohibited at the work site.After work, take a shower and change clothes.Pay attention to personal hygiene.[4]
First aid measures
Skin contact: Take off the contaminated clothes and wash the skin thoroughly with soapy water and clean water.
Eye contact: Lift the eyelid and flush with flowing water or physiological saline.Get medical attention.
Inhalation: quickly leave the site to a place with fresh air.Keep the respiratory tract unobstructed.If breathing is difficult, give oxygen.If breathing stops, perform artificial respiration immediately.Get medical attention.
Ingestion: drink enough water to induce vomiting.Get medical attention.[4]
Hazard characteristics
Flammable, its vapor and air can form an explosive mixture.It can cause combustion and explosion in case of open fire and high heat.It can react strongly with oxidants.If the flow rate is too fast, it is easy to generate and accumulate static electricity.Its steam is heavier than air, and can spread to a relatively far place at a lower place, which will lead to backburning in case of open fire.
Xylene can be absorbed through the respiratory tract, digestive tract and skin and quickly enter the blood to be distributed in fat rich tissuesThe highest was found in brain, adrenal gland, bone marrow and liver;Most of them are rapidly oxidized to benzoic acid or methylbenzoic acid reacts with glycine to produce hippuric acid or methylhippuric acid in the body, and then most of them are combined with glucuronic acid and sulfuric acid and excreted in the urine.Epidemiological investigation and clinical observation found that there was a good correlation between the concentration of xylene in air and the content of xylene in exhaled air, blood, and hippuric acid and methylhippuric acid in urine.Xylene in exhaled gas and blood was measured in 6 cases of acute poisoning abroad[6-8]The xylene content in the tissues of 3 dead cases was also measured[9], all of them showed obvious increase;Domestic case reports still lack such data, and further research is necessary to clarify its significance in diagnosis.
The main clinical manifestation of acute xylene poisoning is the anesthetic effect of the central nervous system, so the degree of consciousness disorder is taken as the basis for grading. For example, the symptoms of vague consciousness, lethargy, hazy state, delirium, etc. are mild poisoning;Severe poisoning refers to coma.[10]
Acute xylene poisoning can also cause damage to skin, mucosa, heart, lung, liver, kidney and other organs. Generally, the more serious the poisoning is, the higher the probability of organ damage will be.Severe poisoning patients may also have multiple organ function damage or failure.Therefore, when considering the diagnosis of toluene poisoning, in addition to the degree of damage to the central nervous system as the main basis, the damage to other organs should not be ignored.For example, as far as contact reaction is concerned, it means that only headache, dizziness, fatigue and other neurological symptoms have occurred, and no disturbance of consciousness has occurred. However, if contact dermatitis or I degree atrioventricular block is combined, it can be graded according to the diagnostic criteria of various injuries.If it can be classified as mild poisoning;Some patients with mild poisoning can be classified as severe poisoning if they have liver, kidney, heart or multiple organ damage.[10]
Xylene is often mixed with other organic solvents in the working environment, which can be added with toxic effects in clinical practice, suggesting that the combined effects of multiple poisons can not be ignored.
Non occupational poisoning is mostly caused by misuse or misuse, and its clinical manifestations are similar to those of occupational poisoning.However, the symptoms of digestive tract, liver and kidney damage were more obvious in patients taking orally;Excessive smoking can cause lung damage.For example, in a group of patients with acute xylene poisoning who took the same dose orally in time, the damage was relatively mild, and most patients recovered in about 3 days;After 3 days, most of the patients were complicated with multiple organ failure and died after rescue.Therefore, it is extremely important to treat poisoning in an early stage.[10]
There is no specific treatment for acute xylene poisoning. In clinical practice, the main measures are to disengage from contact, remove the cause, use glucuronic acid to promote excretion and support symptomatic treatment to protect the central nervous system, liver, kidney, heart, lung and other important organs.[10]
Xylene poisoning
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Pathogenesis
1. In the petrochemical industry that manufactures, uses, stores, and transports toluene and xylene, and in the production of paint and coatings, dyes, plastics, rubber, leather, saccharin, artificial musk, and synthetic fibers, in case of accidental damage to pipelines and storage systems, air leakage of valves, etc., inhalation of high concentration toluene and xylene vapor for a long time can cause acute poisoning.
2. The sealed storage tank is painted with anti-corrosion paint with toluene and xylene as solvents. Due to lack of good ventilation, a large amount of toluene and xylene vapor accumulates, which may cause acute poisoning of operators.It was reported that 7 female workers suffered from acute toluene poisoning after washing the printing table top with toluene containing detergent water in the printing workshop with complete windows in the knitting factory.
Air concentration of general toluene and xylene 200~300 mg/mthreeMild poisoning symptoms can be produced after inhalation for 8 hours, 3.76 g/mthreeAcute poisoning occurs when the concentration is inhaled for 1h, 71.4 g/mthreeIf the concentration is kept for several minutes, the inhaler will quickly become unconscious and die.
3. There are rare reports of acute poisoning caused by percutaneous or accidental use.
First aid treatment
1. Move to fresh air immediately, and take oxygen if necessary.Remove contaminated clothing and skin poisons.
2. The antidote can be glucuronic acid lactone (see acute benzene poisoning).
3. Symptomatic treatment: oral vitamin B, prevention and treatmentCerebral edemaEtc.[17]
prevention
1. Strengthen workshop ventilation, exhaust and sealing and maintenance of production equipment.
2. Frequently monitor the concentration of toluene in the air.
3. Pay attention to personal protection and wear protective equipment when necessary.
4. Regular physical examination and strict control of contraindications.
prognosis
Generally, the recovery is good. Even those who are in a coma due to severe poisoning can recover their minds and gradually recover without sequelae as long as they are actively rescued in time. Only a few patients have head pain and can last for a long time.After treatment and recovery, the patient can still be engaged in the original work after a general rest of 3~7 days;In case of serious illness, the rest time can be extended appropriately. In case of mental disorder or coma, transfer from benzene operation can be considered after recovery.[4]
purpose
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1. It is used as a solvent for nitro paint spraying, coating, adhesive and varnish, and as a raw material for manufacturing aniline, phenol, picric acid, dye, artificial musk, synthetic fiber, medicine, spice, pesticide, etc., as well as rubber auxiliary
2. It is used to determine the water content in the compound.Solvents and cleaning agents for precision optical instruments and electronic industry.
3. Used as solvent, cleaning agent for precision optical instruments and electronic industry.
4. It is used in the electronic industry as a cleaning agent and some photoresist solvents.[2]
Storage method
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1. Keep the container sealed and stored in a cool and dry place, and ensure that there is a good ventilation or exhaust device in the workshop.
2. See benzene for packaging, storage and transportation of this product.Packaging, marking, storage, transportation and delivery acceptance shall be carried out according to ZBE300005.[2]
Standard provisions
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Source of pollution
Xylene is an important chemical raw material. Wastewater and waste gas from organic synthesis, synthetic rubber, paint and dye, synthetic fiber, petroleum processing, pharmacy, cellulose and other production plants, as well as unsealed production equipment and ventilation of workshops are the main sources of xylene in the environment.Rollover, leakage and fire during transportation and storage will also cause accidental pollution accidents.
Environmental standards
The maximum allowable concentration of harmful substances in the air of workshops in China (TJ36-79): 100 mg/mthree(Xylene)[11]
The maximum allowable concentration of harmful substances in the atmosphere of residential areas in China (TJ36-79): 0.30 mg/mthree(primary value, xylene)[11]
China (GB16297-1996) Integrated Emission Standard of Air Pollutants (Xylene)[12]
① Maximum allowable emission concentration (mg/mthree)：70；ninety
② Maximum allowable emission rate (kg/h): Level II 1.0~10;1.2～12；Level III 1.5~15;1.8～18
③ Monitoring concentration limit of unorganized emission: 1.2 mg/mthree；1.5 mg/mthree
The maximum allowable concentration of hazardous substances in drinking water sources in China (CJ 3020-1993): 0.5 mg/L (xylene)[13]
China (GB3838-2002) Surface Water Environmental Quality Standard (specific value for Class I, II and III waters) 0.5 mg/L (xylene).[14]
Class I of China (GB8978-1996) Integrated Wastewater Discharge Standard: 0.4 mg/L;Level II: 0.6 mg/L;Level III: 1.0 mg/L.[15]
