How to treat the three wastes from benzene hydrogenation

The forum discussed that using the hydrogenation method to replace acid washing in the refining of crude benzene cannot reduce waste emissions to zero; effective environmental protection measures or corresponding environmental protection facilities must be implemented in order to achieve compliant emissions. 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 from heating furnaces, heat transfer oil furnaces, flare systems, etc., 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 plant 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 liquid tanks and exhaust pipes such as those in stabilizer towers, chimneys from heat transfer oil furnaces and reactor heating furnaces, vacuum pumps, safety valves, as well as from vehicles, train loading areas, and benzene storage tanks. These pollutants mainly include benzene, SO2, NOx, and dust. The main control measures recommended are shown in Table 6. Production wastewater is divided into clean 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 network. 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 the 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 amount 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, ditches, and pits inside the facility, as well as to put in place 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-class 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, recycled residual solvents, and spent clay produced by 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 Features 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 regularly 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 pots is replaced regularly and disposed of through safe landfilling. (3) The recycled residual solvent 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. Devices with significant vibration are connected to pipes 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 role of green plants in absorbing noise should be taken into account to achieve a reasonable arrangement and thereby reduce the noise at the edges of the factory. After the above measures are taken, the intensity of environmental noise will be significantly reduced, and the noise generated by various high-noise equipment 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 the end 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 commissioning and trial production phase, as well as during normal production, through monitoring and emergency measures, in order 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 to utilize this heat is an important indicator for assessing the advancement of the process. Based on our experience, it is recommended to integrate the process design with the use of advanced chemical engineering simulation software in order to make better use of the heat generated during the process. This approach can significantly reduce production costs and help achieve energy savings and emission reductions. (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 savings and emission reduction. There are many ways to improve the thermal efficiency of heating furnaces, and based on our experience, modifying the burners 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 can cause 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, resulting in 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 feedstock while strictly controlling chlorine content. Table 6 Recommended Main Pollution Control Measures
Serial Number | Pollution Source | Pollutants | Control Measures | Control Effect
1 | Waste water tanks and exhaust pipes of stabilizer towers | NH3, H2S, etc. | Send to the gas purification system for desulfurization before sending it to the acid production unit; no external discharge.
2 | Chimneys of heating furnaces and heat transfer oil furnaces | SO2, NOx, dust | Use gas that has been purified through combustion desulfurization; emissions are discharged up to standard levels through exhaust stacks.
3 | Various 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 burning; emissions are discharged up to standard levels.
5 | Safety valves | CmHn, etc. | Send to the flare system for burning; emissions are discharged up to standard levels.
6 | Loading areas for trucks 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, Maximum emission level (kg/h), Standard value (kg/h), Maximum concentration (mg/m3), Standard value (mg/m3): 1. Chimney (shared by heat transfer oil furnace and heating furnace): SO2 – 40, 93.8; Dust – – 50, 300; NOx – – – -. 2. Emission stack of the flame system: SO2 – 30, 93.8; Dust – – 50, 300; NOx – -

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