Aniline production process

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Aniline production mainly consists of the following production units: natural gas desulfurization and conversion to hydrogen unit, mixed acid preparation unit, *** unit, aniline production unit, and waste acid concentration unit, etc. 1 Hydrogen production via natural gas desulfurization and conversion 1.1 Desulfurization and conversion processes The natural gas coming from the main pipeline of the plant, at a pressure of about 2.0 MPa (G) and at room temperature, first enters a raw gas preheater where it is heated to around 380°C. It then goes into the MF-2 desulfurization tank, followed by the ZnO desulfurization tank, thereby reducing the sulfur content in the raw gas to below 0.2 ppm. The purified raw gas is mixed with process steam in a specific water-to-carbon ratio; this mixture is then preheated to 510°C in the convective section coils of the conversion furnace before entering the furnace tubes, where it reacts under the action of catalysts. The required reaction heat is provided by burning fuel gas in the top burner. The temperature and pressure of the converted gas leaving the converter are approximately 850°C and 1.6 MPa (G). The converted gas is used to generate medium-pressure steam in the medium-pressure waste heat boiler; it then passes through a low-pressure waste heat boiler and a boiler feedwater preheater to recover heat. After the condensate is separated, the gas is cooled to 40°C in an water cooler for final gas-liquid separation, and the resulting gas is the desired dry converted gas. Hydrogen is then produced through purification using a pressure swing adsorption unit. To recover the heat from the flue gas, the convection section of the converter is equipped in sequence with a mixture gas preheater, a medium-pressure steam superheater, a feed gas preheater (II), a flue gas waste boiler, another feed gas preheater (II), and coils for heating boiler water, thereby reducing the temperature of the flue gas to about 160°C before it is discharged into the atmosphere via an exhaust fan. 1.2 PSA-H2 process: The converted gas from the conversion process enters a gas-liquid separator at a pressure of ~1.6 MPa and a temperature of 40°C to remove free water; or, in the second stage, it uses purified gas from the pressure swing adsorption decarburization system. Subsequently, it enters a pressure swing adsorption hydrogen extraction system composed of six adsorbers, which operates using a 6-2-3/V process. At any given moment in this system, there are always two adsorbers at different stages of the adsorption process; feed gas is introduced at the inlet side, and product hydrogen is obtained at the outlet side. Each adsorber sequentially undergoes adsorption (A), first-stage pressure equalization drop (E1D), second-stage pressure equalization drop (E2D), third-stage pressure equalization drop (E3D), reverse pressure release (D), vacuum pumping (V), third-stage pressure equalization rise (E3R), second-stage pressure equalization rise (E2R), first-stage pressure equalization rise (E1R), and final pressure increase (FR) at different times. The purpose of using multiple voltage equalization steps is to maximize the H2 recovery rate. The reverse step removes most of the impurity components adsorbed in the adsorber, while the remaining impurity components are further desorbed through the vacuum pumping step. The desorbed gas from the pressure swing adsorption hydrogen production system is mixed with fuel natural gas to be used as fuel in the conversion furnace. 2. Preparation of mixed acid – 2.1 Preparation of mixed acid: Concentrated sulfuric acid is fed into the mixed acid preparation tank by gravity from a storage tank, while waste acid is added to the same tank by gravity from a waste acid tank. At the same time, the mixed acid pump is started to circulate the mixture; it is then cooled in a mixed acid cooler. Water is also supplied to the waste gas absorber. Nitric acid is pumped from a storage tank through a nitric acid pump into a high-level tank, from where it is added to the mixed acid preparation tank. Once all the materials have been added, the mixture is circulated and cooled for one hour. Once the temperature drops below 40°C, a sample is taken for analysis. If the results are satisfactory, the mixed acid is pumped into a transfer tank for use in the nitration process. 2.2 Reaction Process The *** unit is a key production unit of the newly added 25,000 t/a aniline production facility; the tank farm supplies raw materials to this unit, while this process provides *** raw materials for the aniline unit. The petroleum benzene from the benzene storage tanks in the tank farm is sent to the benzene intermediate tank via the external pipe rack, and then pumped into the extraction reactor using a transfer pump; some of the waste acid separated by the nitration separator is extracted using this process. After extraction, the liquid flows into an extraction separator for stratification; the upper layer of acidic benzene goes into the acidic benzene storage tank, while the lower layer of waste acid is sent to the intermediate waste acid tank. From there, it is divided into two streams by a waste acid pump – one stream is used for preparing mixed acids, and the other is sent to the waste acid concentration unit. In the event of an accident or when there is an excess of waste acid, it is loaded onto vehicles for export. The mixed acid is sent out from the mixed acid preparation tank, transported by a mixed acid pump, and its flow rate is regulated by instruments before it enters the disperser. Acidic benzene is pumped using an acidic benzene pump, and its flow rate is regulated by instruments before it enters Reactor #1 for nitration. Under stirring, the acidic benzene is nitrated by the mixed acid and waste acid coming from the distributor; part of the reaction heat is carried away by the reaction mixture, while any excess heat is removed by cooling water. The reaction mixture overflows from the top side to the next reactor. The unreacted portion continues to react under stirring until Reactor No. 4. The reaction in Reactor No. 4 is nearing completion. It continuously overflows from the top side overflow port of Reactor No. 4 into the nitration separator for continuous separation. After being cooled in the lower waste acid cooler, it goes to the waste acid intermediate tank; most of it is used for denitration, while a portion is used for extraction. The coarse *** from the upper layer flows into the neutralization and washing processes. After being metered, 42% liquid caustic and soft water are mixed in a static mixer to form a dilute caustic solution at a concentration of 1% to 2%, which is then continuously fed into the neutralization tank. 2.3 Separation process: The acidic *** coming from the auto-nitration separator flows into the neutralization tank, where it is neutralized with dilute alkaline solution. After stirring, it flows into the neutralization separator. The main material goes into the washing tank, where it is washed in counterflow with soft water from the soft water tank; after stirring, it is raised to the washing separator. The crude *** flowing out of the separator enters the crude *** storage tank, while the washing liquid flows into the wastewater collector, and the materials contained in it are recovered using a recovery pump. The upper-layer wastewater is treated in the wastewater tower to meet the required standards before being discharged into the sewage treatment system. The crude *** is fed into the *** preheater using a crude *** pump at a certain flow rate, where it is heated to 130°C before entering the *** dehydration tower. The low-boiling substances (benzene and water) vaporize at the top of the tower; after being condensed in a condenser, they flow into the reflux tank. The upper layer is sent back to the top of the tower via a reflux pump, while the lower layer goes into the *** wastewater treatment tower. The temperature at the bottom of the dehydration tower is controlled at around 170°C by adjusting the steam flow rate to the reboiler. Use a control valve to regulate the liquid level. Pumped into the distillation tower. The vapor at the top of the tower *** is condensed by a condenser and then flows into the reflux tank. The reflux ratio is controlled at 0.5; it is pumped to the top of the tower using a reflux pump, while the purified product overflows into the product tank. 3 Aniline units 3.1 Reduction process: The pure hydrogen obtained through variable pressure adsorption enters the Hydrogen 1# buffer tank after pressure reduction; it then enters the Hydrogen 2# buffer tank, where it mixes with the recycled hydrogen supplied by the hydrogen compressor. The mixed hydrogen is heat-exchanged with the reaction gas coming from the top of the fluidized bed through a hydrogen heat exchanger, and then fed into a *** vaporizer to mix with ***. The pure *** is taken from a storage tank and pumped at a certain flow rate into the preheater, before entering the *** vaporizer. After vaporization, it enters the fluidized bed reactor together with hydrogen. Under the action of a Cu/SiO2 catalyst, hydrogen reacts with *** at temperatures of 250–300°C and pressures of 0.08–0.10 MPa; the resulting products are cooled in a hydrogen heat exchanger before entering a crude aniline condenser for further cooling. The excess hydrogen, after having its moisture removed by an exhaust gas collector and a cyclone separator, enters the hydrogen compressor, from where it is sent to the #2 hydrogen buffer tank for reuse. After being cooled, the cooled aniline and water are further cooled in the crude aniline cooler, after which they flow into the crude aniline separator for separation; the upper layer of aniline-water mixture goes into the aniline-water tank, while the lower layer of aniline flows into the crude aniline tank. Deionized water is pumped into the heat exchange tubes of the fluidized reaction bed for heat exchange in the hydrogenation reaction. The steam generated at 1.5 MPa is used by the system itself, with some of it also being available for external supply. 3.2 Distillation Process: The crude aniline in the crude aniline tank is pumped at a constant flow rate into the dehydration tower using a crude aniline pump. The top temperature, bottom temperature, and top pressure of the dehydration tower are controlled to carry out distillation. The vapor emerging from the top of the tower is condensed in an azeotrope condenser before flowing into the aniline-water tank, while the high-boiling substances from the bottom of the tower enter the distillation tower. Distillation is carried out under certain top temperature, reactor temperature, and vacuum conditions. The vapor exiting at the top of the tower (aniline) is condensed in the distillation tower’s condenser; part of this vapor is sent back into the tower as reflux at a certain reflux ratio, while the remaining portion is further condensed in another condenser before being fed into the aniline product tank. 3.3 Wastewater treatment process: The wastewater coming from the collectors and benzene-water separators contains *** at levels exceeding 1000 mg/L; since the allowable level for discharge into the wastewater treatment plant is 100 mg/L, this wastewater must be treated. The wastewater is pumped from the wastewater storage tank into the wastewater treatment tower via a wastewater pump for steam distillation. The wastewater containing *** that emerges from the top of the tower enters a neutralization tank for reuse, while part of it is discharged to the secondary treatment facility and another part is used in circulation. 4 Acid Waste Concentration Unit (the plant assigns this unit, as well as the mixed acid preparation and *** unit, to Workshop 3). This unit concentrates the excess waste acid containing 68% sulfuric acid from the nitration process into concentrated sulfuric acid at 92.5%, which is then used in a circular manner by the mixed acid processing area. Water, trace amounts of ***, and benzene enter the wastewater recycling system along with the water under vacuum. The wastewater in the wastewater tank of the nitration unit is pumped using a wastewater pump into the water injector, thereby creating a vacuum in the packed tower and the concentration pot. Concentrated sulfuric acid is added from the concentrated acid storage tank or the sulfuric acid intermediate tank to the top of the packed tower; once the concentration pot is full, it overflows into the concentrated acid cooler, from where it flows back into the sulfuric acid intermediate tank, thus establishing a circulation of concentrated sulfuric acid. Open the flue damper of the concentration pot; once the pressure in the natural gas main is stable, slowly open the isolation valve of the natural gas pressure regulator. After lighting the ignition rod and inserting it into the furnace (with the burner air valves fully open and the burner cooling water valve opened), slowly open the natural gas inlet valve. Then adjust the burner air valves, the amount of natural gas supplied, and the flue damper according to a heating rate of 50–100°C per hour. When the temperature of the filler tower rises to 100–110°C and the temperature in the furnace reaches 780°C, the circulation of concentrated sulfuric acid is stopped, and the waste acid pump is started to transfer the waste acid from the waste acid storage tank into the filler tower and the concentration kettle for concentration.