Coal tar extract
Light tar produced in the process of coal coking contains a lot of benzene. This is the original method of producing benzene. The generated coal tar and coal gas pass through washing and absorption equipment together, and high-boiling coal tar is used as washing and absorbent to recover coal tar in coal gas, and crude benzene and other high-boiling fractions are obtained after distillation. Crude benzene can be refined to obtain industrial grade benzene. The benzene obtained by this method has low purity, serious environmental pollution and backward technology.
Extract from petroleum
Crude oil contains a small amount of benzene, and extracting benzene from petroleum products is the most widely used preparation method. Aromatization reforming of alkanes here refers to the process of cyclodehydrogenation of aliphatic hydrocarbons to aromatic hydrocarbons. This is a process developed during the Second World War. At 500-525℃ and 8-50 atmospheric pressure, various aliphatic hydrocarbons with boiling point between 60-200℃ are converted into benzene and other aromatic hydrocarbons by dehydrogenation and cyclization with platinum-rhenium catalyst. After extracting aromatic products from the mixture, benzene can be separated by distillation. These fractions can also be used as high octane gasoline. Steam cracking Steam cracking is a method to produce olefins from low molecular alkanes such as ethane, propane or butane and petroleum components such as naphtha and heavy diesel oil. Cracked gasoline, one of its by-products, is rich in benzene, which can be fractionated into benzene and other components. Cracked gasoline can also be mixed with other hydrocarbons as gasoline additives. The content of benzene in pyrolysis gasoline is about 40-60%, and it also contains other unsaturated components such as diene and styrene. These impurities are easy to further react to form polymer colloid during storage. Therefore, it is necessary to remove these impurities and sulfides from cracked gasoline by hydrotreating first, and then carry out appropriate separation to obtain benzene products.
Aromatic hydrocarbon separation
The components of benzene-containing fractions obtained by different methods are very complicated, and it is difficult to get effective separation by ordinary separation methods. Generally, aromatic hydrocarbons are separated by solvent liquid-liquid extraction or extractive distillation, and then benzene, toluene and xylene are separated by common separation methods. There are many separation methods according to different solvents and technologies used. Udex method: jointly developed by National Highway Chemical Company and UOP Company in 1950. At first, diethylene glycol ether was used as solvent, and later it was improved to triethylene glycol ether and tetraethylene glycol ether as solvent. In this process, a multi-stage riser extractor is used. The yield of benzene is 100%. Suifolane method: developed by Dutch Shell Company and patented by UOP Company. The solvent is sulfolane, which is extracted by rotary extraction tower, and the product needs to be treated with clay. The yield of benzene is 99.9%. Arosolvan method: developed by Lurgi Company of the Federal Republic of Germany in 1962. The solvent is N- methylpyrrolidone (NMP). In order to improve the yield, 10-20% glycol ether is sometimes added. With a specially designed mechanical extractor, the yield of benzene is 99.9%. IFP method: developed by French Institute of Petroleum Chemistry on 1967. Using anhydrous dimethyl sulfoxide (DMSO) as solvent and butane as stripping agent, stripping was carried out in a rotary table tower. The yield of benzene is 99.9%. Formex method: developed by Italian SNAM Company and LRSR Petroleum Processing Department in 197 1 Morpholine or N- formylmorpholine is used as solvent, and a turntable tower is adopted. The total yield of aromatics is 98.8%, in which the yield of benzene is 100%. Hydrocarbons containing one or more benzene rings in the molecule belong to aromatic hydrocarbons.
Dealkylation of toluene
Dealkylation of toluene can be used to produce benzene by catalytic hydrogenation or thermal dealkylation without catalyst. The raw material can be toluene, its mixture with xylene or fractions containing benzene and other alkylaromatic and nonaromatic hydrocarbons. The catalytic hydrodealkylation of toluene uses chromium, molybdenum or platinum oxides as catalysts. At high temperature of 500-600℃ and atmospheric pressure of 40-60℃, toluene can be mixed with hydrogen to produce benzene. This process is called hydrodealkylation. If the temperature is higher, the catalyst can be omitted. The reaction was carried out according to the following equation: pH-CH3+H2-→ PHH+CH4. There are many technical methods, including Hydeal method and Ashi and&; Refing and UOP were developed in 196 1. Raw materials can be reformed oil, hydrocracking gasoline, toluene, C6-C8 mixed aromatic hydrocarbons, dealkylated coal tar, etc. The catalyst is alumina-chromium oxide, the reaction temperature is 600-650℃, the pressure is 3.43-3.92MPa, the theoretical yield of benzene is 98%, the purity can reach over 99.98%, and the quality is better than that of Udex method. Detol method developed by Houdry company. Using alumina and magnesia as catalysts, the reaction temperature is 540-650℃, the reaction pressure is 0.69-5.4MPa, and the raw materials are mainly C7-C9 aromatics. The theoretical yield of benzene is 97% and the purity can reach 99.97%. Pyrotol method, developed by air products and chemicals company and Houdry company. It is suitable for benzene production from ethylene by-product pyrolysis gasoline. The catalyst is alumina-chromium oxide, the reaction temperature is 600-650℃, and the pressure is 0.49-5.4MPa. Bextol method, developed by Shell Company. BASF method, developed by BASF company. Unidak method, developed by UOP company. Thermal dealkylation of toluene Under high temperature hydrogen flow, toluene can be dealkylated to benzene without catalyst. The reaction is exothermic, and various technological processes have been developed for different problems encountered. MHC hydrodealkylation process was developed by Mitsubishi Petrochemical Company and Chiyoda Construction Company on 1967. Raw materials can be pure alkylbenzene such as toluene, and aromatic fractions containing less than 30% non-aromatic hydrocarbons. The operating temperature is 500-800℃, the operating pressure is 0.98MPa and the hydrogen-hydrocarbon ratio is 1- 10. The process selectivity is 97-99% (mol), and the product purity is 99.99%. HDA hydrodealkylation process was developed by American Hydrocarbon Research Company and Atlantic Ridgefield Company on 1962. The raw materials are toluene, xylene, hydrocracking gasoline and reforming oil. The reaction temperature is controlled from different parts of the reactor, such as hydrogen, the reaction temperature is 600-760℃, the pressure is 3.43-6.85MPa, the hydrogen-hydrocarbon ratio is 1-5, and the residence time is 5-30 seconds. The selectivity was 95% and the yield was 96- 100%. Sun process, THD process developed by Sun Oil Company, Monsanto process developed by Gulf Research and Development Company and Monsanto Company.
Toluene disproportionation and transalkylation
With the increase of xylene consumption, toluene disproportionation and transalkylation technologies were developed at the end of1960s, which can simultaneously increase xylene production. (See the figure below for the main reaction) Transalkylation
This reaction is reversible, and there are different technological processes according to the catalyst used, process conditions and raw materials. ① ① The liquid-phase toluene disproportionation process of ①①LTD was developed by American Mobil Chemical Company on 197 1, using nonmetallic zeolite or molecular sieve catalyst, the reaction temperature was 260-3 15℃, the reactor used liquid-phase adiabatic fixed bed, and the raw material was toluene, and the conversion rate was over 99%; ②②Tatoray process was developed by Toray and UOP in 1969. Toluene and mixed c9 aromatics were used as raw materials, mordenite was used as catalyst, the reaction temperature was 350-530℃, the pressure was 2.94MPa, and the ratio of hydrogen to hydrocarbon was 5- 12. The single-pass conversion was over 40%, the yield was over 95%, and the selectivity was 90%. The product was a mixture of benzene and xylene. Xylene plas process: developed by American Atlantic Richfield Company and Engelhard Company. Rare earth Y molecular sieve is used as catalyst, and the reactor is a gas-phase moving bed. The reaction temperature is 47 1-49 1℃, and the pressure is normal. (3) (3) The process developed by Mitsubishi Gas Chemical Company of Japan in 1968, hydrofluoric acid-boron fluoride catalyst, reaction temperature 60- 120℃, low pressure liquid phase. It is corrosive.
Other methods
In addition, benzene can also be obtained by trimerization of acetylene, but the yield is very low.