Low-temperature benzene hydrogenation

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Discussion on the Key Aspects of Low-Temperature Benzene Hydrogenation Units Xue Peng (Shanghai Baosteel Chemical Co., Ltd., Shanghai 200942) In the coal chemical industry, the traditional method for refining crude benzene is acid washing, yielding pure benzene as the product. This process suffers from issues such as outdated technology, low production capacity, and severe environmental pollution. The pure benzene produced by benzene hydrogenation meets the quality standards of petroleum-grade benzene, has a wide range of applications, offers significant environmental benefits, and can effectively improve the product structure of the coal chemical industry. Shanghai Baosteel Chemical Co., Ltd. currently has 3 benzene purification units: an acid washing unit with a capacity of 30,000 t/a, a Litol process medium-temperature hydrogenation unit with a capacity of 64,000 t/a, and a K.K. process low-temperature gas-phase hydrogenation and N-formylmorpholine gas-liquid extraction distillation unit with a capacity of 50,000 t/a. The two benzene purification units currently under construction are a 100,000 t/a Axens-process low-temperature gas-liquid hydrogenation and sulfolane gas-liquid extraction distillation unit, and a 100,000 t/a K.K.-process low-temperature gas-phase hydrogenation and N-formylmorpholine gas-liquid extraction distillation unit. Shanghai Baosteel Chemical Co., Ltd. possesses many years of experience in the construction and operation of crude benzene hydrogenation units. The author’s experiences and insights gained from the construction and operation of benzene hydrogenation are summarized below for sharing among colleagues. 1 Key Points Discussion 1.1 Placement and Type Selection of Flares Flares are essential as the main safety protection devices in benzene hydrogenation units. Torches can be roughly classified into overhead torches and ground torches based on their shape; a comparison of overhead torch and ground torch systems is shown in Table 1. Table 1 Comparison of Elevated Torch and Ground Torch Systems

| Item | Elevated Torch System | Ground Torch System |
|------|---------------------|--------------------|
| Emission method | Single point | Multiple points, staged |
| Number of stages | Single stage (unstaged) | Multiple stages |
| Number of burners | 1 | Multiple |
| Burner tower | None | Present |
| Flame | Visible, with a bright flame | Flame not visible |
| Intensity of ground heat radiation | High (usually 4.73–6.3 kW/m²) | Low (≤1.58 kW/m²) |
| Smoke-free combustion | Difficult to achieve; maximum 20% smoke-free | Easy to achieve; 100% smoke-free possible |
| Maximum ground noise level | 85–115 dBA at maximum emission | ≤75–80 dBA |
| Fire protection distance | 90 m | 22.5–30 m |
| Land area required | Large | Small |
| Difficulty of operation and maintenance | Difficult; requires work at heights; no possibility of on-site maintenance | Easy; on-site maintenance possible with appropriate measures |
| Operating costs | High due to high steam consumption for smoke-free combustion | Lower; steam consumption is 50% less, resulting in lower operating costs |

According to the regulations in “Fire Protection Distances for the Overall Layout of Petrochemical Enterprises,” the fire protection distance for elevated torches is 90 m, which poses significant challenges to the layout of the main plant facilities. The ground torch features a completely enclosed flame combustion mechanism; there is no visible flame, no light pollution, no thermal radiation, no smoke during operation, and it produces low noise. As an open-flame device, the ground flare can be installed near the edge of the facility, and it is equipped with a reinforced concrete protective wall that is over 3 meters high (to protect against wind, reduce noise, and prevent explosions). According to the provisions of the Code for Fire Protection Design of Petrochemical Enterprises GB50160-92 (1999 edition), the fire separation distance for Class A process units is 30 m, allowing them to be conveniently located near benzene refining units. The cost of ground-mounted torches is relatively higher than that of elevated torches, but they account for a very small proportion of the total investment required for the construction of benzene hydrogenation plants. Given the increasing difficulties in obtaining land approvals, it is highly recommended that industry peers opt for ground-mounted torch systems in order to make rational and efficient use of land. 1.2 Selection of hydrogenation catalysts The core of a hydrogenation system lies in the selection of the catalyst used in the reactor; regardless of the low-temperature hydrogenation technique employed, catalysts are generally metal catalysts from the Ni-Mo and Co-Mo series, using Al2O3 as a support. The loading amount is mainly related to the total sulfur content of the raw material, crude benzene, and it is determined through process calculations; here, the focus is on exploring the feasibility of using domestic catalysts. Low-temperature hydrogenation catalysts have long been monopolized by foreign manufacturers (mainly BASF in Germany) in the market. Over the years, domestic research institutions and universities have made great efforts in developing domestic catalysts, and significant progress has been achieved. Through two years of collaborative research, the Shanxi Coal Chemistry Institute of the Chinese Academy of Sciences and Shanghai Baosteel Chemical Co., Ltd. successfully developed the Ni-Mo series pre-hydrogenation catalyst 602-1 and the main hydrogenation catalyst 602-2. Tables 2 and 3 present the comparison of the composition of the carriers and catalysts developed by the Coal Chemicalization Institute with those of the imported BASF catalysts. Tables 4 and 5 present the comparison of the main physical and chemical properties of the carriers and catalysts developed by the Coal Chemicalization Institute with those of the imported BASF catalysts. Table 2 Composition (% ) of imported BASF catalysts
Item: NiCoMoPSiAl2O3M8-21 – 2.56–8.63; 2.30; 0.56
Others: M8-12 – -2.20; 8.58; 1.50; 0.54
Others: —

Table 3 Composition (% ) of carriers and catalysts developed by the Coal Chemical Research Institute
Item: NiCoMoPSiNaFeAl2O3 carrier – —; 0.66; 0.027; 0.024
Others: 602-1 – 2.52–8.46; 3.90; 0.55 – Others
602-2 – -2.10; 8.80; 2.94; 0.58 – Others

Table 4 Physicochemical properties of imported BASF catalysts
Item: Specific surface area (m2/g), Pore volume (cm3/g), Average pore size (nm), Bulk density (g/100 mL)
M8-21: 223; 0.52; 9.35; 79.5
M8-12: 218; 0.55; 10.26; 79.3

Table 5 Physicochemical properties of carriers and catalysts developed by the Coal Chemical Research Institute
Item: Specific surface area (m2/g), Pore volume (cm3/g), Average pore size (nm), Bulk density (g/100 mL)
Carrier: 272; 0.645; 9.48; 61.56
602-1: 226; 0.468; 9.19; 74.06
602-2: 213; 0.488; 9.43; 74.9

The Shanxi Coal Chemical Research Institute, East China University of Science and Technology, and Shanghai Baosteel Chemical Co., Ltd. collaborated to conduct long-term operational evaluations and performance tests on these catalysts in simulated industrial settings. It is believed that the catalyst’s activity, strength, selectivity, stability, renewability, and service life are equal to or superior to those of the imported BASF catalyst. It is recommended that domestic counterparts conduct thorough research on the prices, performance metrics, and delivery times of international and domestic catalysts, so as to make appropriate choices based on their own circumstances. 1.3 Selection of extraction solvent: Currently, abroad, the catalytic hydrogenation method is commonly used to convert trace sulfides and olefins in benzene, followed by extraction distillation to remove trace alkanes. Extraction distillation solvents that are useful in industry should possess the following properties: (1) Good selectivity. After adding the solvent, it is necessary to increase the relative volatility of the components to be separated, using a small amount of it. (2) Good solubility. The extraction solvent should be a good solvent for separating the components, without causing phase separation during distillation or forming an azeotrope with them. (3) High boiling point. To facilitate the recovery of the solvent by distillation for reuse. (4) It has good thermal stability, is non-corrosive and non-toxic, and does not react with the separated components. (5) Low cost and readily available. The solvents most widely used in extraction processes at present are sulfolane and N-formylmorpholine. Below is a rough comparison between sulfolane and N-formylmorpholine. 1.3.1 Product quality: When using the gas-liquid extraction distillation process with N-formylmorpholine, due to the fact that N-formylmorpholine decomposes into formic acid and morpholine when it comes into contact with water, there are inevitably issues related to the basic nature of toluene and the acidic nature of pure benzene; this also results in relatively high total nitrogen contents in benzene and toluene. Refer to the quality standards for TDI-grade toluene (ASTM-D5606). In the processes used to produce styrene from benzene, as well as in the process of converting toluene into diisocyanate (TDI), high requirements are placed on the total nitrogen content in both benzene and toluene. The use of N-formylmorpholine as an extractant inevitably increases the difficulty of benzene and toluene entering the styrene and TDI application markets. When using the sulfolane gas-liquid phase extraction distillation process, a small amount of water is added as a cosolvent to improve the selectivity of extraction, and monoethanolamine is added to adjust the pH value of the circulating solvent. Relatively speaking, the process is rather complex, and it requires high levels of automation control as well as operational maintenance; however, the quality of the products is excellent, especially in terms of the acidity and alkalinity of benzene and toluene, as well as their total nitrogen content. According to statistics, the use of sulfolane or N-formylmorpholine as extractants has little impact on the yield of the product, with sulfolane being slightly superior. 1.3.2 Comprehensive evaluation: Sulfone and N-formylmorpholine are currently the most commonly used extractants in the crude benzene hydrogenation process worldwide. Both possess the same advantages: they can effectively alter the relative volatility between non-aromatics and aromatics, thereby facilitating the separation of aromatics. The purity of benzene obtained can exceed 99.9%. They are non-toxic, non-corrosive, and exhibit good chemical stability, causing minimal impact on equipment and the environment. The decomposition of N-formylmorpholine results in slightly lower product quality, whereas sulfolane is superior to N-formylmorpholine in this regard. It is recommended that domestic peers take into comprehensive consideration the market strategies for the company’s existing products, strive to address the compatibility issues between hydrogenation and extraction processes, select extractants appropriately, and achieve optimal balance between product quality and cost-effectiveness in terms of investment. 1.4 Treatment methods for waste gases, wastes, and wastewater: Replacing acid washing with the hydrogenation method for the purification of crude benzene does not enable the reduction of waste emissions to zero; effective environmental protection measures must be taken or appropriate environmental protection facilities installed in order to achieve compliant emissions levels. It is recommended that domestic counterparts follow the following standards when constructing low-temperature crude benzene hydrogenation units: For air pollutants emitted in an organized manner, such as from heating furnaces, heat transfer oil furnaces, and flare systems, the third-level standards specified in the \"Emission Standards for Air Pollutants from Industrial Furnaces\" (GB9078-1996) should be applied ； The discharged wastewater shall comply with the first-level discharge standards for the coking industry specified in Table 3 of the \"Emission Standards for Water Pollutants in the Iron and Steel Industry\" (GB 13456-92) ； The noise at the factory boundary shall comply with Class III standards specified in the \"Standards for Noise at the Boundaries of Industrial Enterprises\" (GB 12348-90) ； For odorous pollutants at the factory boundary, the third-level standards for new constructions, expansions, and renovations specified in Table 1 of the \"Emission Standards for Odorous Pollutants\" (GB 14554-93) shall be applied ； The waste residues are mainly governed by the relevant provisions in the \"Regulations on Environmental Protection Design for Construction Projects\". 1.4.1 Exhaust gases: The main sources of pollution in low-temperature crude benzene hydrogenation units are pollutants emitted from waste discharge tanks and exhaust pipes of stabilization towers, chimneys of heat transfer oil furnaces and reactor heating furnaces, vacuum pumps, safety valves, loading areas for trucks and trains, as well as benzene storage tanks. These pollutants mainly include benzene, SO2, NOx, and dust. The main control measures recommended are shown in Table 6. Table 6: Recommended main pollution control measures. Sequence, Pollution source, Pollutant control measures, Control effect: 1. Drain tanks and stabilizer tower exhaust pipes – NH3, H2S, etc.; send to the gas purification system for desulfurization before sending it to the acid production unit; no emissions outside. 2. Chimneys of heating furnaces and heat transfer oil furnaces – SO2, NOx, dust; use gas that has been purified through combustion desulfurization, and ensure emissions meet standards through exhaust stacks. 3. Storage tanks in benzene storage areas – benzene, etc.; use internal floating roof tanks to reduce pollutant emissions. 4. Vacuum pumps – CmHn, etc.; send to the flare system for combustion to ensure emissions meet standards. 5. Safety valves – CmHn, etc.; send to the flare system for combustion to ensure emissions meet standards. 6. Loading areas for cars and trains – CmHn, etc.; send to exhaust cleaning towers and wash with circulating cleaning oil to reduce pollutant emissions. The pollutant emission levels after implementing these measures are shown in Table 7 (estimated based on a processing capacity of 100,000 tons per year of crude benzene). The low-temperature hydrogenation unit for crude benzene with a capacity of 100,000 tons per year emits approximately 10 tons per year of hydrocarbons, 13.5 tons per year of SO2, 24.6 tons per year of NOx, and 7.2 tons per year of dust. The concentrations and emission rates of air pollutants released through the chimneys of the heating furnaces and heat transfer oil furnaces, as well as the flare systems, meet the requirements of Grade 3 specified in the \"Emission Standards for Air Pollutants from Industrial Furnaces\" (GB9078-1996). The ammonia concentration at the boundary of the factory site is less than 4 mg/m3 ； The hydrogen sulfide concentration is less than 0.32 mg/m3, meeting the requirements of the third-level standards for new constructions and renovations specified in Table 1 of the \"Emission Standards for Odorous Pollutants\" (GB 14554-93). Table 7: Pollutant emission levels after implementation of measures. Sequence Number, Emission source, Pollutant type, Maximum emission concentration standard: kg/h, mg/m3, kg/h, mg/m3. 1. Chimney (shared by heat transfer oil furnaces and heating furnaces): SO2 – 40–93.8; –1200; Dust –50–300; NOx – -. 2. Flare system exhaust stack: SO2 – 30–93.8; –1200; Dust –50–300; NOx – -. 1.4.2 Wastewater: Production wastewater is divided into purified production wastewater and production sewage. 2.4.2.1 Net production wastewater: This is the wastewater discharged from the net circulating water system for production; aside from a small amount of suspended solids, it contains low levels of other pollutants. It is recommended to discharge it directly into the rainwater drainage system. 2.4.2.2 Production wastewater: Production wastewater mainly includes the separation water discharged from the benzene hydrogenation separation tanks, the wastewater from the reflux tanks of the stabilizer tower, pure benzene tower, toluene tower, and xylene towers, the cleaning water from tank cleaning stations, the water used for floor washing, as well as the initial 15-minute rainfall from the production system. It generally contains high concentrations of pollutants such as COD, BOD, volatile phenols, cyanides, and petroleum compounds. 2.4.2.3 Wastewater treatment methods To reduce the volume of production wastewater discharged and the concentration of pollutants, the following control measures are recommended: (1) The separated water discharged from the benzene hydrogenation separation tank, as well as the wastewater from the stabilizer column, pure benzene column, toluene column, and xylene column reflux tanks, should first be sent to the oil-water separator in the oil depot for two-stage separation to remove oil thoroughly. After that, the wastewater is sent to the mechanical ammonia water clarification tank, and after ammonia evaporation treatment, it is then sent to the phenol-cyanide wastewater treatment plant. (2) The floor washing water and the cleaning water from the tank cleaning station are collected and sent to the phenol-cyanide wastewater treatment plant for treatment. (3) It is recommended to install early-stage rainwater collection devices at the beginning of construction, to collect the 15-minute initial rainwater from the production system and send it to the phenol-cyanide wastewater treatment plant for treatment. (4) It is recommended that the phenol-cyanide wastewater treatment plant adopt the AA00 biological nitrogen removal process. After pre-treatment processes such as homogenization and dilution, the wastewater is sent to a biological treatment system to further remove pollutants from it, including volatile phenols, cyanides, COD, ammonia nitrogen, and petroleum compounds. (5) It is recommended to install floors, sewage wells, gutters, and pits inside the facility, as well as to put in necessary impermeable structural layers to prevent groundwater contamination. After implementing the above control measures, the net wastewater discharged from the benzene refining unit meets the first-level discharge standards for the coking industry specified in Table 3 of the \"Emission Standards for Water Pollutants in the Iron and Steel Industry\" (GB 13456-92). 1.4.3 Solid waste: The solid waste generated by the benzene purification plant mainly includes spent catalysts, residual liquid from solvent regeneration, and spent clay produced in the clay tanks. The main characteristics of the waste residue are shown in Table 8. Table 8 Main Characteristics of Waste Residues Generated by the Benzene Refining Plant Name Properties Main Characteristics Spent Catalysts Contain Al2O3, Ni-Mo, Co-Mo, etc. Al2O3 and Co-Mo are considered ordinary solid wastes, while Ni-Mo is classified as a hazardous solid waste. Regenerated Residues Contain polymers and polymeric substances; they are hazardous waste. To prevent these residues from contaminating the environment, it is recommended to adopt the following treatment methods. (1) The spent Co-Mo catalysts and spent Ni-Mo catalysts are replaced periodically based on their activity; the spent Co-Mo catalysts are recovered by the catalyst manufacturers, while the spent Ni-Mo catalysts can be disposed of by entities qualified to handle hazardous waste. (2) The waste kaolin generated by the white clay tanks is replaced regularly and disposed of through safe landfilling. (3) The recycled residue from solvents is recommended to be mixed into coking coal and not discharged. 1.4.4 Noise The main sources of noise in the crude benzene refining process include hydrogen compressors, air compressors, ventilators, and various pumps. The following measures can be taken to control noise. (1) Where process requirements permit, low-noise products should be preferred. (2) Hydrogen compressors and vacuum pumps shall be equipped with separate foundations and vibration damping facilities. Equipment with significant vibration is connected to pipelines using flexible connections, and all types of high-noise equipment are placed in sound-insulated areas indoors to prevent the spread of noise. (3) During the overall layout, factors such as the terrain, the orientation of the factory building and sound sources, the level of noise in the workshops, and the noise-absorbing effect of green plants should be taken into account to achieve a rational arrangement and reduce the noise at the factory perimeter. After the above measures are taken, the intensity of environmental noise will be significantly reduced, and the noise generated by various high-noise devices will be under control. It is expected that the noise level at the boundaries of the factory area will be below 65 dB(A) during the day and 55 dB(A) at night, meeting the standard limits for Class III as specified in the \"Standards for Noise at the Boundaries of Industrial Enterprises\" (GB 12348-90). 1.4.5 Comprehensive recommendations: The crude benzene refining process should focus on controlling and managing pollution at its source, in order to significantly reduce pollution at later stages and achieve comprehensive utilization and management. At the same time, strengthen environmental management throughout the entire project lifecycle, including during the construction phase, the trial production phase, and the normal production process, as well as implement monitoring and emergency measures to minimize the project’s impact on the environment. 1.5 Energy-saving and emission-reduction methods (1) Use internal floating roof tanks. Internal floating roof tanks are widely used in the petrochemical industry, while traditional arch roof tanks remain predominant in the coking industry. In accordance with the requirements of the Chinese national standards \"Code for Design of (Internal) Floating Roof Tanks\" (SH3046-92) and \"Guidelines for Energy-Saving Design of Oil Depots\" (SH/T3002-2000), the storage tanks in benzene hydrogenation units can be designed as internal floating roof tanks, which can significantly reduce the loss of benzene and other light hydrocarbons into the atmosphere. (2) Comprehensive utilization of process heat. The crude benzene hydrogenation reactor is a fixed-bed gas-phase reactor in which desulfurization reactions primarily take place, with a temperature rise of around 30–50°C. How well this heat can be utilized is an important indicator of the sophistication of the process. Based on our experience, it is recommended to integrate the manufacturing process with mature chemical engineering simulation software in order to make better use of the heat generated during the process; this can significantly reduce production costs and help achieve energy savings and emission reduction. (3) Improve the thermal efficiency of the heating furnace. Based on the existing supply conditions, as well as the maximum temperature and heat load required by the process, two common methods for heating in benzene hydrogenation are medium-pressure steam heating and heat transfer oil heating. If heat transfer oil heating is chosen, improving the thermal efficiency of the heat transfer oil furnace is a way to achieve energy conservation and emission reduction. There are many ways to improve the thermal efficiency of heating furnaces, and based on our experience, modifying the burner design of the heating furnace is the most effective approach. 1.6 Controlling impurities in raw materials: The composition of crude benzene is relatively complex, consisting mainly of benzene, toluene, xylene, and trimethylbenzene among other benzenoid hydrocarbons. In addition, there are also many unsaturated compounds along with a small amount of heterocyclic aromatics containing sulfur, oxygen, and nitrogen. Although the distillate yield of crude benzene produced by various coking plants before 180°C is generally above 90%, the contents of its various components often vary significantly, with substantial fluctuations occurring due to differences in coal blending quality or coking process conditions. Among the impurities in crude benzene, sulfur and nitrogen are generally considered the main ones. However, attention must be paid to the chlorine content in crude benzene, for the following main reasons: (1) Hydrogenation of organic chlorines produces hydrogen chloride, which causes corrosion to equipment. (2) Crude benzene also contains ammonium compounds. If the chlorine content in crude benzene is too high, hydrogenation leads to the formation of large amounts of ammonium salts such as ammonium chloride, which causes blockages in the heat exchangers. This increases the back pressure at the outlet of the hydrogen compressor, raising its load and affecting the system’s production capacity; in severe cases, production must be halted for repairs. (3) Based on our theoretical studies on low-temperature hydrogenation catalysts, chlorine is a poison for Ni-Mo and Co-Mo series metal catalysts; it can reduce their ability to carry out desulfurization and denitration reactions, leading to substandard products in severe cases, or even rendering the catalysts unusable. 2 Summary In summary, for the construction of low-temperature benzene hydrogenation plants, the author prefers to use surface flares, to try out domestically produced catalysts, to employ sulfolane as an extractant, to pay attention to the treatment of waste materials, to ensure emissions meet regulatory standards, to use internal floating roof tanks, to make rational use of the heat remaining from reactions in order to improve the efficiency of heating furnaces, and to strengthen quality control over the crude benzene used as raw material, while strictly controlling chlorine content.