Method for improving yield of sulfur trioxide

Abstract

The invention discloses a method for increasing the yield of sulfur trioxide, which comprises the following steps: SO3-containing flue gas enters an absorption tower, SO3 in the flue gas is circularly absorbed by fuming sulphuric acid, the concentration of the fuming sulphuric acid is increased, the fuming sulphuric acid is pressurized and sequentially fed into a heat exchanger, a steam heater and a sulfur trioxide evaporator, the fuming sulphuric acid is heated to a boiling point temperature, and SO3 is evaporated and escaped from the fuming sulphuric acid to prepare high-purity liquid sulfur trioxide. And the low-concentration fuming sulfuric acid overflows back to the absorption tower to circularly absorb SO3 in the flue gas. The temperature of fuming sulfuric acid entering the sulfur trioxide evaporator is increased by utilizing high-temperature steam, the gas-phase space pressure at the top of the sulfur trioxide evaporator is reduced by utilizing the fan, the partial pressure of gas-phase sulfur trioxide is reduced, the SO3 evaporation rate is increased, the yield of sulfur trioxide can be increased to 78%, the product quality is better, and by adding an automatic control electric instrument for interlocking control, the production cost is reduced. Automatic operation of the device is achieved, the manual intervention frequency is reduced, and economic benefits are improved while production activity safety is guaranteed.

Description

Technical Field

[0001] The invention relates to the technical field of high-purity liquid sulfur trioxide production, and in particular to a device and method for improving the yield of sulfur trioxide. Background Art

[0002] High-purity liquid sulfur trioxide is the only raw material for the production of electronic-grade sulfuric acid. Electronic-grade sulfuric acid is mainly used for cleaning and photolithography of silicon wafers, and corrosion and cleaning of printed circuit boards. It is an indispensable key basic chemical reagent in the semiconductor industry.

[0003] Because the development of electronic-grade sulfuric acid customers requires multiple rounds of testing, the product development cycle is long, the demand for electronic-grade sulfuric acid is small in the early stage of customer development, the sulfur trioxide output is correspondingly low, and the production load of the device is low. The production of high-purity liquid sulfur trioxide mainly relies on the sulfuric acid production device, which mainly uses the SO3 flue gas from the third and fourth stages of the converter. In the absorption tower, low-concentration oleum is used to circulate and absorb the SO3 flue gas from the third stage to increase the acid concentration. In the sulfur trioxide evaporator, the high-temperature SO3 flue gas from the fourth stage is used to heat the high-concentration oleum, and the sulfur trioxide is evaporated to produce high-purity liquid sulfur trioxide. When the device is operating at full load, the yield of liquid sulfur trioxide can reach 74%, and the product quality can reach the superior grade of GB / T23855-2018 "Liquid Sulfur Trioxide". When the production load of the device is 50%, due to the reduction of thermal energy of high-temperature flue gas from the fourth stage of the converter, the evaporation efficiency of the sulfur trioxide evaporator is reduced, and the maximum yield of high-purity liquid sulfur trioxide can only reach 46%. The product quality reaches the first-class grade of GB / T23855-2018 "Liquid Sulfur Trioxide". The output of industrial sulfuric acid increases accordingly, resulting in a decrease in the overall production economic benefits of the device. Summary of the invention

[0004] In view of the technical problems of low evaporation efficiency of sulfur trioxide evaporator, low yield of high-purity liquid sulfur trioxide, and unstable product quality in the background technology, the present invention provides a method with simple process, easy control, low production cost and high safety factor to improve the yield of sulfur trioxide and ensure the stable quality of sulfur trioxide. The present invention innovates in production equipment, production control method and automatic control, increases the temperature of oleum entering the sulfur trioxide evaporator, and reduces the partial pressure of gaseous sulfur trioxide at the top of the sulfur trioxide evaporator, thereby improving the yield of sulfur trioxide. A brief device flow chart of the present invention is attached. Figure 1 .

[0005] To achieve the above object, a method for improving the yield of sulfur trioxide is provided, comprising the following steps: (1) Flue gas containing sulfur trioxide enters the lower part of the absorption tower, is pressurized by the primary pump and the secondary pump, and is sprayed and absorbed by the cooled oleum. The tail gas is then treated in the first absorption tower of the acid-making unit. After sulfur trioxide flue gas enters the lower part of the absorption tower, it passes through the primary packing layer and the secondary packing layer in turn, and is sprayed and absorbed by oleum. The tail gas then goes to the first absorption tower through the top of the absorption tower for further treatment. The outlets of the primary pump and the secondary pump are connected to the primary cooler and the secondary cooler respectively. The primary cooler and the secondary cooler are cooled by circulating water. After being cooled, the primary oleum and the secondary oleum enter the primary packing layer and the secondary packing layer respectively, and are sprayed to absorb sulfur trioxide in the flue gas to increase the acid concentration of the oleum.

[0006] (2) The high-concentration oleum after absorbing sulfur trioxide flue gas is pressurized by the feed pump, and is heated in turn by the heat exchanger, steam heater and sulfur trioxide evaporator. The oleum reaches the boiling point. At the top of the sulfur trioxide evaporator, gaseous sulfur trioxide evaporates from the oleum, and is sent to the sulfur trioxide distillation process by the fan after the acid foam is removed by the top wire mesh demister. The low-concentration oleum flows away from the overflow port at the top of the sulfur trioxide evaporator, and flows back to the absorption tower after heat exchange in the heat exchanger; The wire mesh demister is made of 316L stainless steel, the mesh aperture is 1-25 μm, and the surface is passivated with 98% concentrated sulfuric acid.

[0007] The passivation treatment method is: prepare a hydrochloric acid solution with a concentration of 0.1-1%, soak the wire mesh demister in the hydrochloric acid solution at 15-30°C for 0.5-2.5h, remove welding slag, rust, etc. on the surface of the wire mesh demister, then take out the wire mesh demister, rinse it with clean water, and ensure that there is no acid solution and impurities on the surface.

[0008] After the wire mesh demister is dried, soak it in 96-98.3% concentrated sulfuric acid at 15-30℃ for 12-24h until a layer of silvery white film is formed on the surface. Take it out and rinse it with clean water until it is neutral. The passivation treatment is completed.

[0009] In step (1), the liquid level of the absorption tower is controlled at 800-1200 mm, the temperature of the oleum after the primary cooler is controlled at 45-50° C., and the temperature of the oleum after the secondary cooler is controlled at 40-45° C.

[0010] In the preferred embodiment, the temperature of oleum after the primary cooler is controlled at 45-48°C, and the temperature of oleum after the secondary cooler is controlled at 40-43°C. Lowering the temperature of oleum is beneficial to the absorption of SO3 flue gas by oleum and increases the acid concentration of oleum.

[0011] In step (1), the primary metal filler layer and the secondary metal filler layer are made of 316L stainless steel and are composed of structured fillers and step ring combination fillers. The surface is passivated with 98% concentrated sulfuric acid to increase the contact area between oleum and SO3 flue gas, increase the rate at which oleum absorbs SO3 flue gas, and further increase the acid concentration of oleum.

[0012] The passivation treatment method is: prepare a hydrochloric acid solution with a concentration of 0.1-1%, immerse the composite filler in the hydrochloric acid solution at 15-30°C for 0.5-2.5h, remove welding slag, rust, etc. on the surface of the composite filler, then take out the composite filler, rinse it with clean water, and ensure that there is no acid and impurities on the surface.

[0013] After the composite filler is dried, soak it in 96-98.3% concentrated sulfuric acid at 15-30℃ for 12-24h until a layer of silvery white film is formed on the surface. Take it out and rinse it with clean water until it is neutral. The passivation treatment is completed.

[0014] In step (1), the tail gas contains trace amounts of sulfur trioxide, which is absorbed by 98.3% sulfuric acid in an absorption tower of the acid-making device. After the acid is concentrated by adding ultrapure water, 98% industrial sulfuric acid is produced.

[0015] In step (2), the oleum at the outlet of the feed pump has an acid concentration of 36-42% and a temperature of 50-60°C, which is heated by the oleum of 100-120°C overflowing from the upper part of the sulfur trioxide evaporator, and the temperature is raised to 85-95°C; the oleum of 85-95°C then enters the steam heater, is heated by the high-temperature steam, and the temperature of the oleum is raised to 95-110°C; the oleum of 95-110°C finally enters the sulfur trioxide evaporator, is further heated by the high-temperature flue gas, and the temperature is raised to 100-120°C, sulfur trioxide evaporates from the oleum, and is sent to the distillation process from the top through the fan, and the oleum with an acid concentration of 24-40% and an acid temperature of 100-120°C overflows from the upper part to the heat exchanger for heat exchange and finally flows back to the absorption tower.

[0016] The upper acid is 36-42% oleum, and the corresponding boiling point is 108.4-95.9°C.

[0017] In the preferred embodiment, the acid concentration of oleum at the outlet of the feed pump is controlled to be 40-42%, and the boiling point temperature of the corresponding oleum is 100-95.9°C. Under the same external temperature and pressure conditions, increasing the concentration of oleum will reduce its boiling point temperature accordingly, and the evaporation rate of liquid sulfur trioxide will be significantly improved.

[0018] The evaporator is a shell and tube heat exchanger, high temperature flue gas goes through the evaporator shell side, oleum goes through the evaporator tube side, oleum is fed from the bottom of the tube side, the upper part of the tube side is the oleum overflow port, the top is the gas phase sulfur trioxide outlet, and the distance between the overflow port and the top outlet is 500-1000mm.

[0019] In the preferred embodiment, the distance between the upper overflow port and the top outlet of the evaporator tube is 800-1000 mm, which is convenient for installing a wire mesh demister and increases the gas-liquid separation space, thereby effectively improving the purity of liquid sulfur trioxide.

[0020] The high-temperature steam pressure of the steam heater is 0.4-0.6MPa, corresponding to a temperature of 140℃-165℃. The steam condensate is uniformly collected in the condensate tank for recycling. The high-temperature flue gas temperature of the sulfur trioxide evaporator is 300-350℃. After heat exchange, the tail gas temperature drops to 85-130℃. The second absorption tower of the acid-making device is circulated and absorbed by 98.3% sulfuric acid, and then sent to the desulfurization tower. After desulfurization and demisting, it is discharged through the chimney. The sulfuric acid mist content of the discharged tail gas is less than 5mg / m 3 .

[0021] In the preferred embodiment, the steam heater is equipped with a bypass pipe, and the bypass pipe, the steam heater acid inlet pipe, the acid outlet pipe and the high-temperature steam pipe are all equipped with regulating valves. When the steam heater leaks, the pH value of the steam condensate is lower than 7, and the set interlock triggering condition is reached, the set interlock control program can achieve rapid automatic switching, open the bypass pipe bypass valve, close the steam heater acid inlet valve, the acid outlet valve and the high-temperature steam pipe steam regulating valve, so as to ensure the safe and stable operation of the device.

[0022] In step (2), the gas phase pipe at the top of the sulfur trioxide evaporator is connected to a fan, and the pressure of the gas phase space at the top of the sulfur trioxide evaporator can be controlled to be 0-5 KPa through the negative suction effect of the fan.

[0023] In the preferred embodiment, the fan is a variable frequency fan, which can achieve precise control of the gas phase pressure at the top of the sulfur trioxide evaporator, and control the sulfur trioxide evaporator pressure to 0-3KPa, which can further reduce the gas phase sulfur trioxide partial pressure, accelerate the sulfur trioxide evaporation rate, and improve the sulfur trioxide yield.

[0024] In step (2), the heat exchange medium of the heat exchanger is fuming sulfuric acid with a temperature of 100-120°C and a return acid concentration of 24-40% at the top overflow of the sulfur trioxide evaporator and fuming sulfuric acid with a temperature of 50-60°C and a top acid concentration of 36-42% at the outlet of the feed pump for heat exchange, so as to reasonably utilize heat and improve energy utilization.

[0025] In view of the above method, the present invention also provides a device for improving the yield of sulfur trioxide, comprising an absorption tower, wherein a feed pump, a primary pump and a secondary pump are sequentially connected to a pump tank at the bottom of the absorption tower; The outlet of the feed pump is connected to the shell side inlet of the heat exchanger. The shell side outlet pipe of the heat exchanger is provided with two remote thermometers. The shell side outlet pipe of the heat exchanger is connected in parallel with the bypass pipe of the shell side material of the steam heat exchanger and the acid inlet pipe of the shell side inlet. The acid inlet pipe is also provided with an acid inlet valve. The bypass pipe of the shell side material of the steam heat exchanger and the shell side outlet pipe of the steam heater are connected in parallel with the tube side inlet pipe of the sulfur trioxide evaporator. The sulfur trioxide evaporator is connected to the tube side inlet of the heat exchanger through the tube side liquid outlet pipe and four remote thermometers. The heat exchanger is connected to the absorption tower through the tube side outlet pipe.

[0026] The absorption tower is provided with a primary packing layer and a secondary packing layer, a buffer chamber is provided between the top of the primary packing layer and the bottom of the secondary packing layer, the buffer chamber is connected to the inlet of the outlet pipe, the outlet of the outlet pipe is connected to the bottom of the absorption tower and is provided below the primary packing layer.

[0027] A primary cooler and a primary temperature control valve are also connected in parallel on the outlet pipe of the primary pump, the inlet pipe of the primary temperature control valve is connected in parallel with the inlet pipe of the primary cooler shell, and the outlet pipe of the primary temperature control valve is connected in parallel with the outlet pipe of the primary cooler shell; a primary thermometer is provided on the rear side of the intersection of the parallel outlet pipes.

[0028] A secondary cooler and a secondary temperature control valve are also arranged in parallel on the outlet pipe of the secondary pump, the inlet pipe of the secondary temperature control valve is connected in parallel with the inlet pipe of the secondary cooler shell side, the outlet pipe of the secondary temperature control valve is connected in parallel with the outlet pipe of the secondary cooler shell side, and a secondary thermometer is arranged on the rear side of the intersection of the parallel outlet pipes.

[0029] The tube-side inlet pipe of the sulfur trioxide evaporator is also provided with a remote temperature gauge 3, the top of the sulfur trioxide evaporator is connected to the inlet of the fan through the tube-side gas outlet pipe and the remote pressure gauge, and the outlet of the fan is connected to the distillation tower.

[0030] The feed pipe of the absorption tower is installed below the primary packing layer, and a tail gas pipe is also arranged on the top of the absorption tower, and the tail gas pipe is connected to the sulfuric acid-making absorption tower.

[0031] The tube-side inlet of the steam heater is connected to the steam pipe; a steam regulating valve is arranged on the steam pipe, and the tube-side outlet of the steam heater is connected to the condensate pipe, on which a steam trap and a remote pH meter are arranged; the shell-side outlet pipe and the bypass pipe of the steam heater are connected in parallel to the tube-side inlet pipe of the sulfur trioxide evaporator, the bypass pipe is provided with a bypass valve, and the shell-side outlet pipe of the steam heater is also provided with an acid outlet valve.

[0032] The outlet pipe of the heat exchanger tube is also provided with a remote thermometer 5 and a remote acid return instrument; the outlet of the heat exchanger tube is connected to the secondary pump partition tank of the absorption tower.

[0033] The submerged ends of the pumps between the feed pump and the primary pump are separated by an isolation plate, and the submerged ends of the pumps between the primary pump and the secondary pump are separated by an isolation plate; the bottom of the feed pump is connected to one side of the bottom of the primary pump, and the other side of the bottom of the primary pump is connected to the bottom of the secondary pump; the outlet pipe of the primary pump is connected to the inlet pipe of the buffer chamber of the absorption tower; the outlet pipe of the secondary pump is connected to the inlet of the top chamber of the secondary packing layer in the absorption tower; the outlet of the feed pump is connected to the shell side inlet of the heat exchanger through a pipeline and a remote acid meter and a remote thermometer.

[0034] The shell side inlet pipe of the sulfur trioxide evaporator is connected to the high-temperature flue gas pipe; the shell side outlet pipe of the sulfur trioxide evaporator is connected to the low-temperature flue gas pipe through a high-temperature regulating valve.

[0035] Preferably, steam is introduced into the tube-side inlet of the steam heater, and the fluids in the tube-side and shell-side exchange heat in parallel; the tube-side outlet of the steam heater is connected to a steam condensate tank to fully utilize the heat of the condensate.

[0036] Preferably, high-temperature flue gas is introduced into the shell-side inlet of the sulfur trioxide evaporator, and the fluid in the shell-side and tube-side of the sulfur trioxide evaporator performs countercurrent heat exchange.

[0037] Preferably, circulating cooling water is introduced into the tube side inlet of the primary cooler, and the fluids in the tube side and the shell side exchange heat in countercurrent.

[0038] Preferably, circulating cooling water is introduced into the tube side inlet of the secondary cooler, and the fluids in the tube side and the shell side exchange heat in countercurrent.

[0039] Preferably, high concentration sulfur trioxide flue gas is introduced into the inlet pipe of the absorption tower.

[0040] The invention discloses a method for improving the yield of sulfur trioxide. Sulfur trioxide flue gas enters the lower part of an absorption tower and is cyclically absorbed by oleum. High-concentration oleum is sequentially fed into a heat exchanger, a steam heater and a sulfur trioxide evaporator through a feed pump, and heated to the boiling point of oleum. Sulfur trioxide evaporates and escapes, and is sent to a distillation process through a blower. Low-concentration oleum returns to the absorption tower and cyclically absorbs sulfur trioxide flue gas. The scheme has a simple process, a safe and reasonable design, a high degree of automation, and can effectively improve the yield of sulfur trioxide. The yield of sulfur trioxide can stably reach more than 78%, and the quality of liquid sulfur trioxide is better. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1The figure is a device diagram of the present invention. The accompanying drawings are as follows: absorption tower 1, buffer chamber 101, outlet pipe 102, heat exchanger 2, steam heater 3, bypass pipe 301, sulfur trioxide evaporator 4, wire mesh demister 401, primary cooler 5, secondary cooler 6, feed pump 7, primary pump 8, secondary pump 9, fan 10, steam regulating valve 11, steam trap 12, acid inlet valve 13, bypass valve 14, acid outlet valve 15, high temperature regulating valve 16, primary temperature control valve 17, secondary temperature control valve 18, remote temperature one 19, remote temperature two 20, remote temperature three 21, remote temperature four 22, remote temperature five 23, primary thermometer 24, secondary thermometer 25, remote acid meter 26, remote acid return meter 27, remote pH meter 28, remote pressure gauge 29, primary packing layer 30, secondary packing layer 31. DETAILED DESCRIPTION

[0042] It should be understood that the orientation or position relationship indicated by the terms "bottom, inlet, outlet, bottom, below, one side, top" and the like is based on the orientation or position relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0043] The present invention is further described below in conjunction with the accompanying drawings and embodiments, but it should not be understood that the above subject matter of the present invention is limited to the following embodiments. Without departing from the above technical ideas of the present invention, various substitutions and changes are made according to common technical knowledge and customary means in the field, which should all be included in the protection scope of the present invention.

[0044] The electrical devices in the present application such as: feed pump 7, primary pump 8, secondary pump 9, fan 10, steam regulating valve 11, acid inlet valve 13, bypass valve 14, acid outlet valve 15, high temperature regulating valve 16, primary temperature control valve 17, secondary temperature control valve 18, remote temperature one 19, remote temperature two 20, remote temperature three 21, remote temperature four 22, remote temperature five 23, primary thermometer 24, secondary thermometer 25, remote acid supply instrument 26, remote acid return instrument 27, remote pH meter 28, remote pressure gauge 29 are all powered by an external power supply.

[0045] The fan 10 is purchased from Xinxiang Xima Blower Co., Ltd. Model 700BSBD40 and its related supporting power supply and circuit; The steam regulating valve 11 is purchased from Chongqing Chuanyi Regulating Valve Co., Ltd. Model HCP-10000 and its related supporting power supply and circuit; The acid inlet valve 13 is purchased from Anze Valve (Taicang) Co., Ltd. Model F-2-ISO-STD-RH and its related supporting power supply and circuit; the bypass valve 14 and the acid outlet valve 15 are the same as the acid inlet valve 13; The high temperature regulating valve 16 was purchased from Chongqing Chuanyi Regulating Valve Co., Ltd. Model VBJG-300022 and its related supporting power supply and circuit; The first-stage temperature control valve 17 is purchased from Chongqing Chuanyi Control Valve Co., Ltd. Model VBJG-300002 and its related supporting power supply and circuit; the second-stage temperature control valve 18 is the same as the first-stage temperature control valve 17; Remote thermometer 19 is purchased from Suzhou Yokogawa Group Co., Ltd. Model HHSBWZ / 2481 and its related supporting power supply and circuit; remote thermometer 20, remote thermometer 3 21, remote thermometer 4 22, remote thermometer 5 23, first-level thermometer 24, and second-level thermometer 25 are the same as remote thermometer 19.

[0046] The remote acid-supplementing instrument 26 is purchased from FoxboRo Company, model 876EC-TYAA-7 and its related supporting power supply and circuit; the remote acid-returning instrument 27 is the same as the remote acid-supplementing instrument 26; The remote pH meter 28 was purchased from Endress+Hauser Analytical Instruments (Suzhou) Co., Ltd. Model CM442-6J67 / 0 and its related supporting power supply and circuit; The remote pressure gauge 29 was purchased from Beijing Far East Rosemount Instrument Co., Ltd. Model 3051GP2A2B21BB4M5HR5 and its related supporting power supply and circuit; The interlock protection program was purchased from Emerson, program name: DeltaV TM The distributed control system, the Chinese version is R6, and is compatible with the electrical appliances of this application, which include controllers, sensors, receivers, processors, etc. This application does not make improvements to the electrical appliances, circuits and control programs directly purchased in the existing market.

[0047] like Figure 1 As shown, a device for improving the yield of sulfur trioxide includes an absorption tower 1, a heat exchanger 2, a steam heater 3, a sulfur trioxide evaporator 4, a primary cooler 5 and a secondary cooler 6. The sulfur trioxide flue gas from the third stage of the converter enters from the lower part of the absorption tower 1, passes through the primary metal packing layer 30 and the secondary metal packing layer 31 in sequence, and is absorbed by oleum in two stages. After the absorption, the tail gas is treated in the first absorption tower of the sulfuric acid production process of desulfurization. After the concentration of oleum in the absorption tower 1 is increased, it is fed to the heat exchanger 2, the steam heater 3 and the sulfur trioxide evaporator 4 in sequence through a feed pump 7. The fuming sulfuric acid is heated by the fuming device 4, and reaches the boiling point. At the top of the sulfur trioxide evaporator, the gaseous SO3 evaporates from the fuming sulfuric acid and escapes. It enters the feed fan 10 from the top of the sulfur trioxide evaporator 4, and is sent to the sulfur trioxide distillation process by the fan after the acid foam is removed by the top wire mesh demister to prepare high-purity liquid SO3; the low-concentration fuming sulfuric acid overflows from the upper part of the sulfur trioxide evaporator 4 back to the heat exchanger 2 for heat exchange, and finally enters the pump tank of the absorption tower 1, and then enters the absorption tower 1 after being pressurized by the primary pump 8 and the secondary pump 9 to spray and absorb the SO3 in the flue gas.

[0048] The wire mesh demister is made of 316L stainless steel, the mesh aperture is 1-25 μm, and the surface is passivated with 98% concentrated sulfuric acid.

[0049] In the preferred solution, the mesh demister has a mesh aperture of 5-20um, and a hydrochloric acid solution with a concentration of 0.5% is prepared. The wire mesh demister is immersed in the hydrochloric acid solution at 20-25°C for 2 hours to remove welding slag, rust, etc. on the surface of the wire mesh demister. Then, the wire mesh demister is taken out and rinsed with clean water to ensure that there is no acid or impurities on the surface.

[0050] After the wire mesh demister is dried, soak it in 98-98.3% concentrated sulfuric acid at 20-25℃ for 20h until a silvery white film is formed on the surface. Take it out and rinse it with clean water until it is neutral. The passivation treatment is completed.

[0051] In the preferred embodiment, the absorption tower 1 is an integrated tower and pump tank device. The pump tank of the absorption tower 1 is sequentially arranged with a feed pump 7, a primary pump 8 and a secondary pump 9. The three pumps are all submersible pumps. The equipment is reasonably arranged and saves land. The submersible end of the pump is separated by an isolation plate, and the bottoms are interconnected, so that oleum forms a concentration gradient in the pump tank; a primary packing layer 30 and a secondary packing layer 31 are sequentially filled in the tower, so that the oleum at the top of the tower is more evenly distributed, fully contacts with sulfur trioxide flue gas, and absorbs SO3.

[0052] The primary metal filler layer and the secondary metal filler layer are made of 316L stainless steel, and are composed of structured fillers and step ring combined fillers, and the surface is passivated with 98% concentrated sulfuric acid.

[0053] In the preferred embodiment, the composite filler is combined according to the volume ratio of the structured filler and the stepped ring of 1: 1.5, and a hydrochloric acid solution with a concentration of 0.5% is prepared. The composite filler is immersed in the hydrochloric acid solution at 20-25°C for 2 hours to remove welding slag, rust, etc. on the surface of the composite filler. Then, the composite filler is taken out and rinsed with clean water to ensure that there is no acid and impurities on the surface.

[0054] After the composite filler is dried, soak it in 98-98.3% concentrated sulfuric acid at 20-25°C for 20 hours until a silver-white film is formed on the surface. Take it out and rinse it with clean water until it is neutral. The passivation treatment is completed.

[0055] In the preferred scheme, the outlet of the primary pump 8 is connected to the primary cooler 5, the primary cooler 5 is provided with a primary temperature control valve 17, the primary cooler 5 and the primary temperature control valve 17 are connected in parallel, and a primary nicotinic acid thermometer 24 is provided at the parallel outlet, the outlet of the secondary pump 9 is connected to the secondary cooler 6, the secondary cooler 6 and the secondary temperature control valve 18 are connected in parallel, and a secondary nicotinic acid thermometer 25 is provided at the outlet of the parallel pipeline of the secondary cooler 6 and the secondary temperature control valve 18, wherein the primary cooler 5 and the secondary cooler 6 are both cooled by circulating cooling water, and the temperature of the oleum entering the tower through the primary cooler 5 and the secondary cooler 6 is accurately controlled by the primary temperature control valve 17 and the secondary temperature control valve 18.

[0056] In the preferred scheme, the oleum at the outlet of the feed pump 7 enters the shell side of the heat exchanger 2, and then enters the shell side of the steam heater 3. After being heated by the high-temperature steam in the tube side of the steam heater 3, it enters the tube side of the sulfur trioxide evaporator 4, and is heated to the boiling point by the high-temperature flue gas from the fourth layer of the shell side converter of the sulfur trioxide evaporator 4. The shell side outlet flue gas is treated by the sulfur removal sulfuric acid tail gas treatment device; the gaseous sulfur trioxide evaporates from the oleum, and the gaseous sulfur trioxide passes through the gas phase pipe at the top of the sulfur trioxide evaporator 4 to the fan 10, and then is sent to the sulfur trioxide distillation process; the liquid phase is low-concentration oleum, which overflows from the upper part of the tube side of the sulfur trioxide evaporator 4 to the tube side of the heat exchanger 2, heats the oleum in the shell side of the heat exchanger 2, and finally returns to the pump tank of the absorption tower 1.

[0057] In the preferred scheme, the oleum acid inlet pipe, acid outlet pipe and shell-side material bypass pipe 301 of the steam heater 3 shell side are respectively provided with an acid inlet valve 13, an acid outlet valve 15 and a bypass valve 14, the steam inlet pipe of the steam heater 3 tube side is provided with a steam regulating valve 11, and the condensate pipeline of the steam heater 3 tube side is provided with a steam trap 12 and a remote pH meter 28. When the pH of the condensate reaches the interlock alarm value, the interlock protection program is started, the bypass valve 14 is opened, and the steam regulating valve 11, the acid inlet valve 13 and the acid outlet valve 15 are closed to ensure the safe and stable operation of the device.

[0058] In the preferred scheme, the heated flue gas outlet pipeline of the sulfur trioxide evaporator 4 is installed with a high-temperature regulating valve 16, and the inlet and outlet nicotinic acid pipelines of the sulfur trioxide evaporator are respectively installed with a remote thermometer three 21 and a remote thermometer four 22, and the steam regulating valve 11 and the high-temperature regulating valve 16 are respectively automatically interlocked with the remote thermometer three 21 and the remote thermometer four 22, so as to accurately control the temperature of oleum in the sulfur trioxide evaporator 4.

[0059] In the preferred solution, the inlet pipeline of the feed fan 10 is connected to the gas phase port at the top of the sulfur trioxide evaporator 4 tube pass, and a remote pressure gauge 29 is installed on the inlet pipeline. The motor of the feed fan 10 is a variable frequency motor. By setting the variable frequency of the feed fan 10 and the automatic control interlock of the remote pressure 29, the gas phase space at the top of the sulfur trioxide evaporator 4 is controlled to be always at a slightly positive pressure, which can accelerate the flash evaporation rate of oleum and improve the evaporation efficiency of the sulfur trioxide evaporator.

[0060] In the preferred solution, a separation plate is provided in the pump tank of the absorption tower 1, wherein the lower end of the feed pump 7 is oleum after absorbing sulfur trioxide flue gas, and the acid concentration and acid temperature are both high, and the lower end of the secondary pump 9 is oleum after the overflow from the upper part of the sulfur trioxide evaporator and heat exchanged in the heat exchanger 2, and the acid concentration and acid temperature are relatively low. By optimizing the process route, the high-temperature and high-concentration oleum is sequentially sent to the heat exchanger 2, the steam heater 3 is heated, and the sulfur trioxide evaporator 4 is evaporated to precipitate SO3, and the low-temperature and low-concentration oleum is sent to the absorption tower 1, further improving the sulfur trioxide yield. At the same time, the motors of the feed pump 7, the primary pump 8 and the secondary pump 9 are variable frequency motors, which are helpful to achieve automatic control.

[0061] In the preferred solution, the acid supply pipeline at the outlet of the feed pump 7 and the acid return pipeline in the pipe side of the heat exchanger 2 are respectively installed with a remote acid supply instrument 26 and a remote acid return instrument 27, wherein the remote acid return instrument 27 is safety interlocked with the steam regulating valve 11 and the high-temperature regulating valve 16. When the acid concentration reaches the interlocking alarm value, the steam regulating valve 11 and the high-temperature regulating valve 16 are closed to prevent low-concentration oleum from corroding the equipment and ensure safe and stable operation of the device.

[0062] The preferred solution is Figure 1 As shown, the primary cooler 5 and the secondary cooler 6 exchange heat through circulating cooling water, and the circulating return water returns to the circulating water pool, and the water resources are recycled; the nicotinic acid pipelines at the outlets of the primary cooler 5 and the secondary cooler 6 are respectively installed with a primary thermometer 24 and a secondary thermometer 25, and by controlling the opening of the primary temperature control valve 17 and the secondary temperature control valve 18, the temperature of the oleum entering the tower is controlled at 45-50°C by the primary thermometer 24, and the temperature of the secondary thermometer 25 is controlled at 40-45°C.

[0063] The preferred solution is Figure 1 As shown, the steam heater 3 is heated by 0.4-0.6MPa high-temperature steam, the steam condensate water pipeline is installed with a steam trap 12 and a remote pH meter 28, the pH value is controlled at 7.5-9, and the oleum pipeline connected to the outlet of the steam heater 3 is installed with a remote thermometer 3 21. By controlling the opening of the steam regulating valve 11, the temperature of the remote thermometer 3 21 is controlled at 95-110°C.

[0064] The preferred solution is Figure 1As shown, the 4th shell side of the sulfur trioxide evaporator is heated by the 4th stage high temperature flue gas of the converter of the sulfuric acid making process, and the tail gas is treated by the second absorption tower of the sulfuric acid making process to remove the sulfur. A high temperature regulating valve 16 is installed on the tail gas outlet pipe of the 4th shell side of the sulfur trioxide evaporator, and a fuming sulfuric acid remote thermometer 4 22 is installed on the overflow outlet pipeline at the upper part of the 4th tube side of the sulfur trioxide evaporator. By controlling the opening of the high temperature regulating valve 16, the temperature of the remote thermometer 4 22 is controlled at 110-120°C.

[0065] The preferred solution is Figure 1 As shown, the gas phase pipe at the top of the 4-tube pass of the sulfur trioxide evaporator is connected to the inlet of the feed fan 10, and the outlet of the feed fan 10 goes to the sulfur trioxide distillation process to produce high-purity liquid sulfur trioxide. A remote pressure transmitter 29 is installed on the sulfur trioxide gas phase pipe, and the remote pressure transmitter 29 is controlled by the frequency conversion of the feed fan 10 to keep the pressure at 0-5kPa.

[0066] The preferred options are Figure 1 As shown, the oleum pipeline at the outlet of the feed pump 7 is installed with a remote acid supply meter 26 to control the acid concentration to 36-42%, and the oleum pipeline returning from the heat exchanger 2 to the pump tank of the absorption tower is installed with a remote acid return meter 27 to control the acid concentration to 24-40%.

[0067] The preferred solution is Figure 1 As shown, the steam regulating valve 11, the acid inlet valve 13, the acid outlet valve 15, the bypass valve 14 and the remote pH meter 28 are internally provided with safety interlocks; when the remote pH meter is lower than 7 and reaches the interlock alarm value, the interlock is triggered to open the bypass valve 14 and close the steam regulating valve 11, the acid inlet valve 13 and the acid outlet valve 15 to prevent corrosion of the equipment after leakage, ensure stable operation of the device, and improve the intrinsic safety of the device.

[0068] The preferred solution is Figure 1 As shown, the remote acid return instrument 27, the steam control valve 11 and the high-temperature control valve 16 are internally provided with safety interlocks; when the remote acid return instrument 27 is lower than 23% and reaches the interlock alarm value, the interlock is triggered to close the steam control valve 11 and the high-temperature control valve 16 to prevent low-concentration oleum from corroding the equipment and improve the intrinsic safety of the device.

[0069] Example 1 The process carried out using the device described in Example 1 is as follows: When the sulfuric acid plant is in production at 100% load, the liquid level of the absorption tower is controlled at 1000mm. The sulfur trioxide flue gas from the third layer of the converter of the sulfuric acid plant enters the lower part of the absorption tower. By adjusting the opening of the circulating water of the first and second coolers, the temperature of the first oleum is controlled at 45℃ and the temperature of the second oleum is controlled at 40℃. By adjusting the frequency conversion of the first and second pumps, the spraying amount of the first and second oleum is reasonably controlled. After the first and second oleum circulate and absorb the sulfur trioxide flue gas in the absorption tower, the acid concentration of the oleum increases, and finally the oleum at the outlet of the feed pump is controlled. The acid concentration on the acid is 41.5% and the temperature is 58°C. After the oleum is heat exchanged in the heat exchanger, the temperature is raised to 92°C. The bypass valve of the bypass pipe is opened, and the acid inlet valve, acid outlet valve and steam regulating valve are closed. The oleum does not pass through the steam heater, but directly enters the sulfur trioxide evaporator through the bypass pipe. The high-temperature flue gas temperature is controlled at 350°C. The gas phase pressure at the top of the sulfur trioxide evaporator is controlled at 3KPa through the fan frequency conversion. The temperature of the oleum overflowing from the overflow port of the evaporator is controlled at 109°C through the high-temperature regulating valve, and the back-acid concentration is 26.1%. Gaseous sulfur trioxide evaporates from oleum and is sent to the sulfur trioxide distillation process through a fan. The gaseous sulfur trioxide is distilled to obtain ultra-high purity liquid sulfur trioxide. The sulfur trioxide yield can stably reach 74.1%. The mass fraction of liquid sulfur trioxide in the product reaches 99.75%, the mass fraction of sulfur dioxide is 0.09%, the mass fraction of ash is 0.004%, and the mass fraction of iron is 0.003%.

[0070] Example 2 When the sulfuric acid plant is in production at 100% load, the liquid level of the absorption tower is controlled at 1000mm, and the sulfur trioxide flue gas from the third layer of the converter of the sulfuric acid plant enters the lower part of the absorption tower. By adjusting the opening of the circulating water of the first and second coolers, the temperature of the first oleum is controlled at 45℃, and the temperature of the second oleum is controlled at 40℃. By adjusting the frequency conversion of the first and second pumps, the spraying amount of the first and second oleum is reasonably controlled. After the first and second oleum circulate and absorb the sulfur trioxide flue gas in the absorption tower, the acid concentration of the oleum increases. Finally, the acid concentration of the oleum at the outlet of the feed pump is controlled to be 41.5%, and the temperature is 58℃. After heat exchange in the heat exchanger, the temperature of oleum is raised to 95°C, the bypass valve of the bypass pipe is closed, the acid inlet valve, acid outlet valve and steam regulating valve are opened, and all the oleum enters the steam heater without passing through the bypass pipe. The high-temperature steam pressure is controlled to be 0.6MPa, the steam regulating valve is 100% open, the temperature of oleum out of the steam heater is controlled to be 109°C, the high-temperature flue gas temperature is controlled to be 350°C, the gas phase pressure at the top of the sulfur trioxide evaporator is controlled at 3KPa through the fan frequency conversion control, and the temperature of the oleum overflowing from the overflow port of the evaporator is controlled at 120°C through the high-temperature regulating valve, and the acid concentration of the return acid is 24.8%. Gaseous sulfur trioxide evaporates from oleum and is sent to the sulfur trioxide distillation process through a fan. The gaseous sulfur trioxide is distilled to obtain ultra-high purity liquid sulfur trioxide. The sulfur trioxide yield can stably reach 78.5%. The mass fraction of liquid sulfur trioxide in the product reaches 99.90%, the mass fraction of sulfur dioxide is 0.04%, the mass fraction of ash is 0.003%, and the mass fraction of iron is 0.002%.

[0071] Example 3 When the sulfuric acid plant is in production at 50% load, the liquid level of the absorption tower is controlled at 1000mm. The sulfur trioxide flue gas from the third layer of the converter of the sulfuric acid plant enters the lower part of the absorption tower. By adjusting the opening of the circulating water of the first and second coolers, the temperature of the first oleum is controlled at 45℃ and the temperature of the second oleum is controlled at 40℃. By adjusting the frequency conversion of the first and second pumps, the spraying amount of the first and second oleum is reasonably controlled. After the first and second oleum circulate and absorb the sulfur trioxide flue gas in the absorption tower, the acid concentration of the oleum increases, and finally the acid concentration of the oleum at the outlet of the feed pump is controlled. The acid concentration is 41.5% and the temperature is 58°C. After the oleum is heat exchanged in the heat exchanger, the temperature is raised to 88°C. The bypass valve of the bypass pipe is opened, and the acid inlet valve, acid outlet valve and steam regulating valve are closed. The oleum does not pass through the steam heater, but directly enters the sulfur trioxide evaporator through the steam heater bypass pipe. The high-temperature flue gas temperature is controlled at 350°C. The gas phase pressure at the top of the sulfur trioxide evaporator is controlled at 3KPa through the fan frequency conversion. The temperature of the oleum overflowing from the overflow port of the evaporator is controlled at 100°C through the high-temperature regulating valve. The back acid concentration is 37.6%. Gaseous sulfur trioxide evaporates from oleum and is sent to the sulfur trioxide distillation process through a fan. The gaseous sulfur trioxide is distilled to obtain ultra-high purity liquid sulfur trioxide. The sulfur trioxide yield can stably reach 46.2%. The mass fraction of liquid sulfur trioxide in the product reaches 99.53%, the mass fraction of sulfur dioxide is 0.25%, the mass fraction of ash is 0.005%, and the mass fraction of iron is 0.003%.

[0072] Example 4 When the sulfuric acid plant is in production at 50% load, the liquid level of the absorption tower is controlled at 1000mm, and the sulfur trioxide flue gas from the third layer of the converter of the sulfuric acid plant enters the lower part of the absorption tower. By adjusting the opening of the circulating water of the first and second coolers, the temperature of the first oleum is controlled at 45℃, and the temperature of the second oleum is controlled at 40℃. By adjusting the frequency conversion of the first and second pumps, the spraying amount of the first and second oleum is reasonably controlled. After the first and second oleum circulate and absorb the sulfur trioxide flue gas in the absorption tower, the acid concentration of the oleum increases. Finally, the acid concentration of the oleum at the outlet of the feed pump is controlled to be 41.5%, and the temperature is 58℃. After heat exchange in the heat exchanger, the temperature of fuming sulfuric acid is raised to 94°C, the bypass valve of the bypass pipe is closed, and the acid inlet valve, acid outlet valve and steam regulating valve are opened. The fuming sulfuric acid all enters the steam heater without passing through the bypass pipe. The high-temperature steam pressure is controlled to be 0.6MPa, and the steam regulating valve is 100% open. The temperature of fuming sulfuric acid out of the steam heater is controlled to be 109°C, and the high-temperature flue gas temperature is controlled to be 350°C. The gas phase pressure at the top of the sulfur trioxide evaporator is controlled at 3KPa through the fan frequency conversion control, and the temperature of the fuming sulfuric acid overflowing from the overflow port of the evaporator is controlled at 120°C through the high-temperature regulating valve, and the acid concentration of the return acid is 24.8%. Gaseous sulfur trioxide evaporates from oleum and is sent to the sulfur trioxide distillation process through a fan. The gaseous sulfur trioxide is distilled to obtain ultra-high purity liquid sulfur trioxide. The sulfur trioxide yield can stably reach 78.6%. The mass fraction of liquid sulfur trioxide in the product reaches 99.85%, the mass fraction of sulfur dioxide is 0.05%, the mass fraction of ash is 0.004%, and the mass fraction of iron is 0.002%.

[0073] Example 5 The method and steps are the same as those in Example 2, except that the steam regulating valve opening is set to 75%. The temperature of the oleum at the outlet of the feed pump is 94°C after heat exchange in the heat exchanger, 104°C after heating in the steam heater, and 118°C after heating in the sulfur trioxide evaporator. The yield of ultra-high-purity liquid sulfur trioxide obtained by rectification of gaseous sulfur trioxide is 77.5%, and the mass fraction of liquid sulfur trioxide in the product reaches 99.85%, the mass fraction of sulfur dioxide is 0.06%, the mass fraction of ash is 0.005%, and the mass fraction of iron is 0.003%. The yield of sulfur trioxide decreases, and the lost sulfur trioxide enters an absorption tower of the acid-making device with the tail gas, is sprayed and absorbed by 98% sulfuric acid, and is used to produce 98% industrial sulfuric acid.

[0074] Example 6 The method and steps are the same as those in Example 2, except that the opening of the steam regulating valve is set to 50%, the temperature of the oleum at the outlet of the feed pump is 93° C. after heat exchange in the heat exchanger, the temperature is 99° C. after heating by the steam heater, and finally the temperature is 115° C. after heating by the sulfur trioxide evaporator. The ultra-high purity liquid sulfur trioxide obtained by distillation of the gaseous sulfur trioxide has a yield of 74.4%, the mass fraction of liquid sulfur trioxide in the product reaches 99.79%, the mass fraction of sulfur dioxide is 0.07%, the mass fraction of ash is 0.004%, and the mass fraction of iron is 0.004%.

[0075] Example 7 The method and steps are the same as those in Example 2, except that the opening of the steam regulating valve is set to 25%, the temperature of the oleum at the outlet of the feed pump is 91° C. after heat exchange in the heat exchanger, the temperature is 95° C. after heating in the steam heater, and finally the temperature is 112° C. after heating in the sulfur trioxide evaporator. The ultra-high purity liquid sulfur trioxide obtained by distillation of the gaseous sulfur trioxide has a yield of 74.2%, the mass fraction of liquid sulfur trioxide in the product reaches 99.76%, the mass fraction of sulfur dioxide is 0.09%, the mass fraction of ash is 0.003%, and the mass fraction of iron is 0.005%.

[0076] Example 8 The method and steps are the same as those in Example 4, except that the opening of the steam regulating valve is set to 75%. The temperature of the oleum at the outlet of the feed pump is 92° C. after heat exchange in the heat exchanger, 102° C. after heating in the steam heater, and finally 112° C. after heating in the sulfur trioxide evaporator. The ultra-high purity liquid sulfur trioxide obtained by distillation of the gaseous sulfur trioxide has a yield of 71.8%, and the mass fraction of liquid sulfur trioxide in the product reaches 99.80%, the mass fraction of sulfur dioxide is 0.07%, the mass fraction of ash is 0.004%, and the mass fraction of iron is 0.005%.

[0077] Example 9 The method and steps are the same as those in Example 4, except that the opening of the steam regulating valve is set to 50%, the temperature of the oleum at the outlet of the feed pump is 89° C. after heat exchange in the heat exchanger, the temperature is 95° C. after heating in the steam heater, and finally the temperature is 104° C. after heating in the sulfur trioxide evaporator. The ultra-high purity liquid sulfur trioxide obtained by distillation of the gaseous sulfur trioxide has a yield of 59.6%, the mass fraction of liquid sulfur trioxide in the product reaches 99.72%, the mass fraction of sulfur dioxide is 0.11%, the mass fraction of ash is 0.005%, and the mass fraction of iron is 0.004%.

[0078] Example 10 The method and steps are the same as those in Example 4, except that the opening of the steam regulating valve is set to 25%. The temperature of the oleum at the outlet of the feed pump is 88° C. after heat exchange in the heat exchanger, is 90° C. after heating in the steam heater, and is finally 102° C. after heating in the sulfur trioxide evaporator. The yield of ultra-high-purity liquid sulfur trioxide obtained by distillation of gaseous sulfur trioxide is 50.3%, and the mass fraction of liquid sulfur trioxide in the product reaches 99.61%, the mass fraction of sulfur dioxide is 0.19%, the mass fraction of ash is 0.006%, and the mass fraction of iron is 0.003%.

[0079] Embodiment 11 The method and steps are the same as those in Example 1, except that the wire mesh demister is not passivated. The yield of ultra-high-purity liquid sulfur trioxide obtained by distillation of gaseous sulfur trioxide is 73.8%, the mass fraction of liquid sulfur trioxide in the product reaches 99.65%, the mass fraction of sulfur dioxide is 0.12%, the mass fraction of ash is 0.284%, and the mass fraction of iron is 0.022%. Because the wire mesh demister is not passivated, the rust and burrs on the surface will affect the defoaming effect, and impurities will enter the product, eventually resulting in a deterioration of the sulfur trioxide yield and product quality.

[0080] Example 12 The method and steps are the same as those in Example 1, except that the structured packing and the step ring combined packing are not passivated. The yield of ultra-high purity liquid sulfur trioxide obtained by distillation of gaseous sulfur trioxide is 72.5%, the mass fraction of liquid sulfur trioxide in the product reaches 99.51%, the mass fraction of sulfur dioxide is 0.28%, the mass fraction of ash is 0.234%, and the mass fraction of iron is 0.035%. Because the combined packing is not passivated, the rust and burrs on the surface will affect the spray absorption effect, and impurities will eventually enter the product, resulting in a deterioration of the sulfur trioxide yield and product quality.

Claims

1. A method for improving the yield of sulfur trioxide, characterized in that: The following steps are involved: (1) Flue gas containing sulfur trioxide enters the lower part of the absorption tower, passes through the primary packing layer and the secondary packing layer in sequence, is pressurized by the primary pump and the secondary pump, and is sprayed and absorbed by oleum after primary cooling and secondary cooling respectively, to obtain high-concentration oleum; (2) The high-concentration oleum after absorbing sulfur trioxide flue gas is pressurized by the feed pump and heated in turn by the heat exchanger, steam heater and sulfur trioxide evaporator. The oleum reaches the boiling point. At the top of the sulfur trioxide evaporator, gaseous sulfur trioxide evaporates from the oleum and is sent to the sulfur trioxide distillation process by the fan after the acid foam is removed by the top wire mesh demister. The low-concentration oleum flows away from the overflow port at the top of the sulfur trioxide evaporator and flows back to the absorption tower after heat exchange in the heat exchanger.

2. A method for improving sulfur trioxide yield according to claim 1, characterized in that: In step (1), the liquid level of the absorption tower is controlled at 800-1200 mm, the temperature of the oleum after the primary cooler is controlled at 45-50° C., and the temperature of the oleum after the secondary cooler is controlled below 45° C.

3. A method for improving sulfur trioxide yield according to claim 1, characterized in that: In step (1), the primary metal filler layer and the secondary metal filler layer are made of 316L stainless steel, composed of structured fillers and stepped ring combined fillers, and the surface is passivated with 98% concentrated sulfuric acid.

4. A method for improving sulfur trioxide yield according to claim 1, characterized in that: In step (1), the tail gas after absorption is sent to an absorption tower of an acid-making device and is cyclically absorbed by 98.3% sulfuric acid to produce 98% industrial sulfuric acid.

5. A method for improving sulfur trioxide yield according to claim 1, characterized in that: In step (2), the acid concentration of the oleum at the outlet of the feed pump is 36-42% and the temperature is 50-60°C. It is heated by the 100-120°C oleum overflowing from the upper part of the sulfur trioxide evaporator, and the temperature is raised to 85-95°C; the 85-95°C oleum then enters the steam heater and is heated by the high-temperature steam, and the temperature of the oleum is raised to 95-110°C; the 95-110°C oleum finally enters the sulfur trioxide evaporator and is further heated by the high-temperature flue gas, and the temperature is raised to 110-120°C, and sulfur trioxide evaporates from the oleum.

6. A method for improving sulfur trioxide yield according to claim 5, characterized in that: The high-temperature steam pressure of the steam heater is 0.4-0.6MPa, and the steam condensate is collected in the condensate tank for recycling; the high-temperature flue gas temperature of the sulfur trioxide evaporator is 300-350℃, and the tail gas is recycled and absorbed by 98.3% sulfuric acid in the second absorption tower of the acid-making device, and then sent to the desulfurization tower, and discharged through the chimney after desulfurization and demisting.

7. A method for improving sulfur trioxide yield according to claim 1, characterized in that: The wire mesh demister is made of 316L stainless steel, the mesh aperture is 1-25um, and the surface is passivated with 98% concentrated sulfuric acid.

8. A method for improving sulfur trioxide yield according to claim 1, characterized in that: In step (2), the gas phase pipe at the top of the sulfur trioxide evaporator is connected to a fan, and the pressure of the gas phase space at the top of the sulfur trioxide evaporator can be controlled to be 0-5 KPa through the negative suction effect of the fan.

9. A method for increasing the yield of sulfur trioxide according to claim 5, characterized in that: The heat exchanger is used for heat exchange between the overflow of the sulfur trioxide evaporator at a temperature of 100-120°C and fuming sulfuric acid with a return acid concentration of 24-40% and the outlet temperature of the feed pump at 50-60°C and fuming sulfuric acid with a top acid concentration of 36-42%.

10. A method for improving sulfur trioxide yield according to claim 1, characterized in that: The sulfur trioxide evaporator is a shell and tube heat exchanger. The high-temperature flue gas goes through the shell side of the evaporator, and the oleum goes through the tube side of the evaporator. The oleum is fed from the bottom of the tube side. The upper part of the tube side is the oleum overflow port, and the top is the gas phase sulfur trioxide outlet. The distance between the overflow port and the top outlet is 500-1000mm.

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