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Pure p-xylene is an organic compound with the chemical formula C8H10 and CAS 106-42-3. It is one of the important aromatic compounds. At room temperature, it is a colorless and transparent liquid with an aromatic aroma, insoluble in water, but miscible in most organic solvents such as ethanol, ether, chloroform, etc. It is mainly used as a raw material for the production of polyester fibers and resins, coatings, dyes, and pesticides. It is also used as a standard substance and solvent for chromatographic analysis, and for organic synthesis.
A small amount of xylene is used to manufacture dimethyl terephthalate (DMT), which can be further synthesized into polyester resins, plasticizers, etc., and applied in the fields of coatings, adhesives, and engineering plastics. Through oxidation, nitration and other reactions, intermediates such as p-methylbenzoic acid and terephthalic acid can be produced, which can then be used to produce high-end materials such as liquid crystal polymers (LCP) and engineering plastics, meeting the special needs of industries such as electronics and automobiles.
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Chemical Formula |
C8H10 |
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Exact Mass |
106 |
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Molecular Weight |
106 |
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m/z |
106 (100.0%), 107 (8.7%) |
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Elemental Analysis |
C, 90.51; H, 9.49 |
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At present, the main methods of producing p-xylene include toluene disproportionation, adsorption separation and isomerization of xylene.
Toluene disproportionation - toluene disproportionation process is to selectively convert toluene into benzene and xylene. The conversion of toluene to xylene is called disproportionation, or "TDP". The term "alkyl transfer" describes the conversion of a mixture of toluene and cga. to xylene.
Disproportionation reaction:
Alkyl transfer:
Toluene disproportionation is the only industrial technology that successfully enables disproportionation and alkyl transfer to occur in the same process unit. The combination of toluene disproportionation unit and aromatics unit can maximize the output of high-quality benzene and p-xylene, and also minimize the products of low-quality methyl rice and heavy aromatics by-products.
Adsorption and separation - at present, the internationally mature adsorption and separation technologies include UOP's Parex process and IFP's Eluxyl process. Both of them are novel adsorption separation methods for recycling from mixing_ Paraxylene of xylene. The adsorption separation process adopts a solid zeolite adsorbent selected for p-xylene, which provides an effective way to recover p-dimethylbenzene. Unlike the traditional chromatographic separation method, the adsorption separation process is a continuous process, which simulates the reverse flow of liquid feed to the solid adsorption bed. The feed and product continuously enter and leave the adsorption layer, and the components remain basically unchanged. The purity of the product is as high as 99.9%, and the recovery rate is 65%.
Isomerization - isomerization process to maximize the recovery of special xylene isomers from C8 aromatics isomerization mixture. The so-called "mixed xylene" is used to describe the C8 aromatic mixture containing p-xylene, o-xylene, m-xylene and some ethylbenzene equilibrium mixtures.
This isomerization process is most commonly used for the recovery of Pure p-xylene, but it can also be used to maximize the recovery of o-xylene or m-xylene. In the case of p-xylene recovery, the mixed xylene feed is added to the p-xylene unit, where the p-xylene isomers are preferentially extracted, with a one-way purity of 99.9% (weight) and a recovery rate of 97% (weight). Then the raffinate from the p-xylene adsorption separation unit (p-xylene is almost completely exhausted) is sent to the isomerization unit. The isomerization unit reconfirmed the equilibrium distribution of xylene isomerization. In fact, additional p-xylene is generated from the remaining o-xylene and m-xylene, and then the effluent from the isomerization unit is recycled back to the p-xylene adsorption separation unit to recover the additional p-xylene, so that the ortho intermediate isomers are recycled to eliminate.
At present, the catalysts used in isomerization units in China mainly include UOP I system and RIPP ski series.
As an important aromatic hydrocarbon raw material, PX has core applications throughout the polyester industry chain, chemical intermediate production, and industrial solvent fields. The following is a detailed analysis of its specific applications:
1. Core raw materials in the polyester industry chain
The main force in PTA production: Approximately 97% -99% of pure xylene worldwide is used to produce purified terephthalic acid (PTA), which is a key raw material for polyester fibers (polyester), polyester bottle flakes (beverage bottles), and polyester film (packaging materials). For example, chemical fibers account for over 90% of China's textile fibers, with polyester accounting for 80% of chemical fibers. Nearly 70% of the home textile and clothing market relies on polyester products.
Widely used in terminal applications: Polyester bottle flakes occupy 15% of the beverage packaging market, while PET resin is used in the production of edible oil packaging, flat panel display substrates, automotive and construction solar films, etc., forming a complete industrial chain from fibers to industrial plastics.
2. Industrial solvents and special applications
Solvent and cleaning agent: Pure p-xylene is used as a solvent for paints, rubber, and electronic component cleaning agents due to its high solubility and volatility, especially in precision manufacturing as a substitute for chlorine containing solvents to reduce environmental pollution.
Chromatographic analysis standards: used as qualitative and quantitative analysis standards for gas chromatography (GC) in the laboratory to ensure the accuracy of analysis results.
Aviation fuel additive: a small amount used to improve the explosion resistance and combustion efficiency of aviation kerosene, and optimize the performance of aviation engines.
Let's provide a detailed introduction to the production process of p-xylene (PX):
Naphtha undergoes catalytic reforming to produce reformate rich in aromatic hydrocarbons, while also producing hydrogen and liquefied petroleum gas as by-products. Catalytic reforming can be divided into semi regenerated fixed bed catalytic reforming and continuously regenerated moving bed continuous reforming according to the catalyst regeneration method. With the increasing scale of oil refining, China has basically stopped building semi regenerative reforming units since 2000. Due to the presence of a considerable amount of non aromatic hydrocarbons, benzene, toluene, and xylene products with similar boiling points in the reforming reaction products, solvent extraction technology must be used to separate aromatic hydrocarbons such as benzene, toluene, and xylene from non aromatic hydrocarbons. Mixed aromatic hydrocarbons such as benzene, toluene, and xylene are then separated into high-purity benzene, toluene, and xylene products through distillation.
Data source: Public Information, Dadi Futures Research Institute
Cracking gasoline contains C6-C9 aromatic hydrocarbons, making it one of the important sources of petroleum aromatic hydrocarbons. When using naphtha as the cracking raw material to produce ethylene, approximately 20% (mass, same below) of cracking gasoline can be obtained, with aromatic content of 40-80%. The cracking gasoline hydrogenation unit is a supporting device of the ethylene unit, and its main task is to reprocess the by-product "cracking gasoline" of the ethylene unit. Cracking gasoline contains important industrial chemical raw materials such as aromatic hydrocarbons (benzene, toluene, xylene), as well as a large amount of unsaturated hydrocarbons (dienes, monoolefins) and other hydrocarbon compounds containing S, O, N, etc. Before extracting triphenyl (benzene, toluene, pure p-xylene), it is necessary to process the pyrolysis gasoline, saturate the unsaturated olefins with hydrogenation, and purify the hydrocarbon compounds containing other elements through hydrogenation cracking. However, cracked gasoline still contains hydrocarbons such as C5 and C9, so before hydrogenation, the C5 and C9 fractions in cracked gasoline are separated, and then the central fraction C6-C8 of cracked gasoline is subjected to hydrogenation treatment.
Process flow of cracking gasoline hydrogenation to produce aromatics
Data source: Public Information, Dadi Futures Research Institute
Light hydrocarbon aromatization technology is a new petrochemical process technology developed in the past decade. It uses C2 to C7 light hydrocarbons as raw materials and produces aromatic hydrocarbons such as benzene, toluene, and xylene or high octane gasoline components through stacking, aromatization, and other reactions. For light hydrocarbon aromatization technology, there is no problem of raw material limitations. Light hydrocarbons ranging from ethylene to gasoline fractions can be used as raw materials for aromatization, among which liquefied petroleum gas (C3 and C4) is the main raw material for light hydrocarbon aromatization. The production of aromatic hydrocarbons requires different operating units depending on the raw materials and process conditions. Taking the simplest catalytic cracking C4 fraction production of aromatic hydrocarbons as an example, the device includes reaction regeneration, product separation, and aromatic distillation sections.
Light hydrocarbon aromatization process
Data source: Public Information, Dadi Futures Research Institute
Different raw materials also have differences in the reaction process of producing aromatic hydrocarbons. Below, we will take propane and butane as examples to illustrate the chemical reaction process of light hydrocarbon aromatization. When propane undergoes aromatization reaction on a catalyst, the highest aromatic yield is achieved at 650 ℃, and the aromatic yield decreases beyond this temperature. With the help of catalysts, the conversion rate of propane is 56% to 95%, and the gas products are mainly methane and ethane. The yield of liquid products is relatively low, generally ranging from 17% to 37%. Benzene, toluene, and xylene account for the vast majority of the liquid products.
Reaction process of producing aromatic hydrocarbons from propane as raw material
Data source: Public Information, Dadi Futures Research Institute
Reaction process of producing aromatic hydrocarbons from butane as raw material
Data source: Public Information, Dadi Futures Research Institute
Whether it is catalytic reforming, cracking gasoline hydrogenation, or light hydrocarbon aromatization, the petroleum aromatic hydrocarbons obtained, such as benzene, toluene, xylene, ethylbenzene, etc., do not match the actual demand in terms of variety and quantity. Among them, toluene, meta xylene, and other aromatic hydrocarbons account for about 50%, and the demand for benzene and para xylene is increasing day by day, causing a contradiction between the supply and demand of aromatic hydrocarbon varieties and quantities. Therefore, it is necessary to develop technology for the conversion between aromatic hydrocarbon varieties. The toluene disproportionation and alkyl transfer technology is an effective way to convert toluene and C9/C10 aromatic hydrocarbons produced by an aromatic hydrocarbon complex into mixed xylene and benzene, aiming to adjust the variety and quantity of aromatic hydrocarbons. More than 50% of the mixed xylene in the aromatic hydrocarbon complex is produced by this technology, which is the main means of increasing pure p-xylene production in the aromatic hydrocarbon complex.
The main reaction process of toluene/benzene disproportionation and alkyl transfer is shown in the following figure:
1. Disputation reaction process
2. Alkyl transfer reaction process
Data source: Public Information, Dadi Futures Research Institute
Xylene isomerization is also one of the main methods for increasing PX production in aromatic hydrocarbon complex units. Due to the limitation of thermodynamic equilibrium, the content of PX in the mixed xylene obtained from catalytic reforming oil and cracked gasoline is only about 25%. To maximize the production of PX, other C8 aromatic hydrocarbons need to be converted into PX through isomerization reaction. Due to the difficulty in separating ethylbenzene and dimethylbenzene in C8 aromatic hydrocarbons, some ethylbenzene is present in the raw materials of the xylene isomerization unit, which needs to be treated. The most commonly used method is to isomerize ethylbenzene into xylene or dealkylate it into benzene. Therefore, the xylene isomerization process mainly includes two technical routes: ethylbenzene conversion type and ethylbenzene dealkylation type. The process flow of the two is basically the same, both using fixed bed reactors in the presence of hydrogen. The difference lies in the catalyst and its arrangement, as well as the treatment of ethylbenzene.
The main reaction process is shown in the following figure:
(1) Mixed Xylene Isomerization Reaction Process
(2) The process of ethylbenzene isomerization reaction
Data source: Public Information, Dadi Futures Research Institute
Under normal market conditions, the more xylene produced by aromatic hydrocarbon plants, the better the efficiency value. However, how to separate xylene from C8 aromatic hydrocarbons requires a mature process to provide support for producing more and purer xylene (PX). Currently, adsorption separation method is widely used in aromatic hydrocarbon plants, which utilizes the reverse contact between C8 aromatic hydrocarbons and adsorbents. The adsorbent has the characteristic of preferentially adsorbing p-xylene, and material transfer is repeated multiple times to increase the concentration of p-xylene (PX) on the adsorbent. Then, the desorption agent is used to desorb p-xylene (PX) from the adsorbent, and the lean solution C8 is sent for isomerization to increase p-xylene (PX) production.
Process flowchart of PX production by adsorption separation method
Data source: Public Information, Dadi Futures Research Institute
Summary of the Production Process of Paraxylene (PX)
1. Aromatic hydrocarbons, especially light aromatic hydrocarbons (benzene, toluene, xylene), are an important chemical raw material, second only to ethylene and propylene in the chemical industry.
2. Petroleum production and coal production are two routes for producing aromatic hydrocarbons, with petroleum production accounting for a much higher proportion of aromatic hydrocarbon production than coal production. In light aromatic hydrocarbons (C6-C8), 83% of benzene and toluene come from petroleum, 17% come from coking benzene produced in the coal coke industry, and xylene mainly comes from petroleum production.
3. There are three main technologies for producing aromatic hydrocarbons from petroleum. Catalytic reforming and cracking gasoline hydrogenation are the mainstream, accounting for 96% of aromatic hydrocarbon production. At the same time, catalytic reforming and cracking gasoline hydrogenation are also important components of aromatic hydrocarbon joint production facilities, while light hydrocarbon aromatization accounts for only 4%.
4. In the production of mixed aromatic hydrocarbons from petroleum, the proportion of various varieties such as benzene, toluene, ethylbenzene, and xylene (m-xylene, o-xylene, and p-xylene (PX)) does not match the demand in social production and daily life. The demand for benzene and p-xylene is increasing day by day, but the production proportion is relatively small. Toluene/benzene disproportionation and selective disproportionation with alkyl transfer can effectively reduce the content of toluene and other aromatic hydrocarbons, and increase the content of benzene and xylene.
5. In xylene, due to thermodynamic equilibrium limitations, the proportion of p-xylene (PX) is relatively low, accounting for only 15% -25%, meta xylene accounts for 45% -70%, and ortho xylene accounts for 10% -15%. Xylene isomerization solves the problem of low proportion of xylene.
6. Finally, based on the difference in binding ability between xylene (PX) and other components and adsorbents, further purify xylene (PX) from the mixed xylene to obtain pure p-xylene (PX).
adverse reaction
P-Xylene, with the chemical formula C ₈ H ₁₀, is an important aromatic hydrocarbon compound and one of the three isomers of xylene. As a fundamental raw material in the chemical industry, pure xylene is widely used in the production of terephthalic acid (PTA), which in turn produces products such as polyester fibers, plastic bottles, and films. Due to its volatility and lipid solubility, pure xylene may enter the human body through the respiratory tract, skin, and digestive tract during production, storage, transportation, and use, causing a series of adverse reactions.
Respiratory irritation and central nervous system suppression
Pure xylene vapor has strong irritants and can quickly cause congestion and edema of the respiratory mucosa upon inhalation, leading to symptoms such as coughing and difficulty breathing. Animal experiments have shown that after inhaling 4550ppm of xylene vapor for 4 hours, rats exhibit central nervous system inhibition such as reduced activity and ataxia. In cases of acute human exposure, short-term inhalation of high concentrations (>1000ppm) of xylene can cause dizziness, headache, nausea, vomiting, and even blurred consciousness or coma. Paraxylene is rapidly absorbed into the bloodstream through the alveoli, inhibiting the function of gamma aminobutyric acid (GABA) receptors in the central nervous system, leading to nerve conduction disorders.
Skin and eye irritation
Direct contact of pure xylene liquid with the skin can damage the lipid structure of cell membranes, causing contact dermatitis characterized by erythema, edema, blisters, and itching. The rabbit experiment showed that after applying 500mg of xylene locally for 24 hours, there was a moderate irritation reaction on the skin. When in contact with the eyes, 0.1ml of xylene can cause corneal epithelial detachment, leading to photophobia, tearing, conjunctival congestion, and in severe cases, may result in visual impairment.
Symptoms of digestive system
Accidental ingestion of pure xylene can cause corrosion of the oral, esophageal, and gastric mucosa, manifested as sore throat, difficulty swallowing, abdominal pain, and vomiting blood. The oral LD of rats is 5000mg/kg. In human poisoning cases, ingestion of 50ml can cause severe gastrointestinal burns, accompanied by metabolic acidosis and multiple organ dysfunction.
Treatment principle: Immediately take activated carbon orally for adsorption, prohibit inducing vomiting to prevent secondary damage, and closely monitor liver and kidney function.
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