Comparison of hydrogenation and extraction separation of crude benzene

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1. Current status and development trends of crude benzene refining technology at home and abroad 1. Overview of domestic pickling method technology Pickling method is currently a relatively commonly used coking crude benzene processing method in my country. The pickling and refining method has the advantages of simple process flow, flexible operation, low equipment investment, easy availability of materials, and operation at normal temperature and pressure. However, the product quality of this method is poor and only coking grade pure benzene can be obtained. The loss in the pickling process is generally 3-5% of crude benzene, and the product yield is low. Liquid waste such as acid tar will be produced during the pickling process. The following table shows the product yield of pickling method. Table 3-1 Crude benzene products and yield of pickling method Product yield % (for crude benzene) Initial fraction 0.9 Coking pure benzene 69.0 Toluene 12.8 Xylene 3.0 Light solvent oil 0.8 Blowing benzene residue 2.2 Refining residue 0.8 Heavy benzene 3.0 Benzene solvent oil 4.0 Washing loss 1.9 Refining loss 1.6 Total 100 2. Overview of hydrorefining technology at home and abroad. Due to the different configurations of the hydrogenation system and distillation system, the hydrorefining process has a variety of process flows, which are mainly summarized as follows:: (1) MoO-CoO and Fe203 are used as catalysts for the Lucci process hydrogenation reaction. The reaction temperature is 350-380°C. Coke oven gas is used as the hydrogen source (pure hydrogen can also be used). The operating pressure is 2.8MPa. The benzene refining yield is high. The hydrogenated oil is separated by extraction or azeotropic distillation to obtain pure benzene. (2) Litol method hydrogenation reaction catalysts are Co-Mo and Cr203-Al2O3, the reaction temperature is 600~650℃, the operating pressure is 6.0MPa, the alkyl hydrogen produced by hydrogenation can be used as the hydrogen source, and the product is pure benzene. Since benzene homologues are converted into benzene by hydrogenation and dealkylation, the yield of benzene can be as high as 110% or more. Litol method hydrogenation mainly occurs hydrodesulfurization, hydrocracking and hydrodealkylation reactions. Litol hydrogenated oil mainly consists of benzene, with very few toluene and xylene. (3)K. The catalyst for the K method hydrogenation reaction is Ni-Mo and Co-Mo. The reaction temperature is 340-370°C, the operating pressure is 2.4-3.0MPa, and pure hydrogen is generally used as the hydrogen source. Hydrogenated oil uses extractive distillation to remove non-aromatic hydrocarbons, and pure benzene, toluene, xylene and other products can be obtained through distillation. K. K in hydrogenated oil because no dealkylation reaction occurs. Toluene and xylenes in crude benzene continue to exist in hydrogenated oil. K. K hydrogenation mainly occurs hydrodesulfurization, hydrodenitrification and hydrodeoxygenation reactions. Others include the UOP method, the Axens method, etc., which use Ni-Mo and Co-Mo as hydrogenation reaction catalysts to combine hydrogenation purification and liquid-liquid extraction. Among them, the hydrorefining method of coking crude benzene is K. The K method is the most widely used, and the UOP method is the most widely used in petrochemical industry. The quality of the hydrorefining process products is relatively good, and the quality of the main products is close to petroleum grade. However, the useful component thiophene is destroyed during the hydrogenation process and becomes H2S. The auxiliary consumption and operating costs of hydrogen production and hydrogenation reactor catalysts, hydrogen production raw materials, and ethanolamine solutions of the hydrogenation unit are relatively high. The investment in the hydrogenation unit is relatively large. Based on the crude benzene processing capacity of 80,000 tons/year, the cost of the process package for introducing foreign technology is 26-100 million yuan, and the investment in the construction of the main device is 190-260 million yuan. Since many key equipment are high temperature and high pressure equipment, the construction period is 22 to 24 days. This process requires a large investment and a long construction period. 3. Overview of crude benzene refining technology by extractive distillation at home and abroad. A British patent in 1950 pointed out that benzene and thiophene could be separated by super distillation. However, due to the low relative volatility of benzene and thiophene, the separation efficiency is very low, the yield is only 36%, and the cost is high, so it is not suitable for large-scale industrial production. In addition, methods for separating benzene and thiophene have been proposed, including liquid-liquid extraction, crystallization, sulfonation with sulfuric acid, catalytic hydrogenation, azeotropic distillation, etc. However, due to many technical problems in industrial production, it is difficult to apply them in practice. The extractive distillation method proposed by the US patent in 1975 can make up for the shortcomings of the above methods. While separating and recovering thiophene in coked benzene, benzene with higher purity can also be obtained. 2. Comparison of process technology. The consumption structure of benzene is:: Styrene accounts for 27.25%, cyclohexane accounts for 12.65%, phenol accounts for 11.37%, chlorinated benzene accounts for 10.98%, * * * Accounting for 9.8%, alkylbenzenes accounted for 7.84%, agricultural chemicals accounted for 5.56%, maleic anhydride accounted for 4.71%, and other pharmaceutical, light industry and rubber products industries accounted for 9.84%. Toluene is mainly used as an additive to gasoline to increase the octane number of gasoline. In the chemical industry, it is mainly used to produce benzene and xylene. Its main downstream products are nitrotoluene, benzoic acid, benzyl chloride, m-cresol, toluene diisocyanate, etc. It can also produce many pesticides and pharmaceutical intermediates. Among them, toluene diisocyanate (TDI) has higher quality requirements for toluene. Treatment technology of benzene-containing waste gas Physical and chemical properties of benzene and thiophene Molecular weight Boiling point/℃ Crystallization point/℃ Dipole moment molecular diameter/nm Relative density Benzene 78.113 80.1 5.5 0 0.68 1.07 Thiophene 84.136 84.2 -38.25 0.55 0.63 0.879 As can be seen from Table 1, the boiling points of benzene and thiophene differ only by 4.1 °C under normal pressure, and their relative volatilities are 1.1 to 1.13. It is almost impossible to separate thiophene with a mass concentration lower than 10-6 using conventional distillation methods. However, the relative volatility of benzene and thiophene can be increased by adding appropriate solvents, that is, by extractive distillation. Since 1975, the former Soviet Union has done a lot of research on this method. The solvents used mainly include N, N-dimethylformamide (DMF), monoethanolamine, 1,2-ethylenediamine, N-methylpyrrolidine (NMP) and glycol [9]. Obviously, such solvents should meet the requirements of high selectivity, non-toxicity, non-corrosiveness, good thermal stability, cheap price, and rich sources. By adding these solvents, the relative volatility can be increased to about 1.4 to 1.5. 　　The factors that determine the efficiency of extractive distillation are mainly the nature of the solvent, the ratio of solvent to feed, and the reflux ratio. The principle of the catalytic hydrorefining method is to use H2 to convert thiophene into H2S and corresponding alkanes to remove: C4H4S+4H2=C4H10+H2S-280.44 kJ Catalysts used for catalytic hydrodesulfurization mainly include three series: Co-Mo, Ni-Mo and Ni-W. The carriers are Al2O3, SiO2-Al2O3, molecular sieves, MgO and diatomite and other porous materials with 100-300 m2/g. 　　Increasing the reaction temperature and pressure can increase the depth of purification, so catalytic hydrogenation is operated under high temperature and high pressure. Typical operating conditions are 300 to 450°C and the pressure is 3 to 5 MPa. In addition, the Leto method of high-temperature hydrogenation was developed, which uses Cr2O3 as the catalyst and the reaction is carried out at a temperature of 600-650°C and a pressure of 5-6 MPa. This can hydrocrack the saturated hydrocarbons and increase the yield of benzene. At this time, the yield of the product benzene can reach more than 114%, and the thiophene mass content is less than 5×10-7. 　　Recent research on catalytic hydrorefining methods focuses on developing new catalysts to improve desulfurization activity, reduce reaction temperature, and extend the service life of the catalyst. For example, thin-film catalysts obtained by depositing RuS2, RuNiS, and Co-Mo metals on the Al2O3 layer and then sintering, reduction, and sulfidation have significantly improved activity compared with the original catalysts [11]. There is a patent report in Japan that a catalyst made of 1% Ru supported on an inorganic carrier of 50% ZnO can achieve 100% desulfurization effect under the conditions of 0.1 MPa, 250°C and 4 h-1 space velocity. In comparison, hydrogenation can completely remove sulfur from benzene, but it cannot recover thiophene, and the H2S produced requires treatment. The extraction method cannot completely remove thiophene, but it can recover most of the thiophene, increase the variety of products, maximize the use of energy, do not produce secondary pollution, and accordingly reduce production costs. Through process adjustment, the coked benzene produced by the extraction method can be used in the styrene industry, which is beyond the reach of hydrogenated benzene. Moreover, the investment in extraction method is only 1/3 of hydrogenation, which shows that extraction method has more competitive advantages than hydrogenation method. Whether it is the extraction and refining method or the hydrogenation method, there are problems with the treatment of benzene-containing waste gas. Commonly used processing methods include: N2 gas sealing method ； Connect to gas negative pressure system ； Concentrate waste gas for scrubbing ； Low temperature condensation waste gas method ； Gas system before benzene cleaning ； Floating roof storage tank ； Water-sealed liquid detection ports, etc., can be selected according to local conditions. Judging from domestic practice, N2 gas sealing method, low temperature method, floating roof storage tank and water-sealed liquid detection port are easier to promote and apply.