Salt-containing waste sulfuric acid cracking flue gas heat recovery system and heat recovery method

Abstract

This invention relates to the field of chemical environmental protection technology, specifically to a heat recovery system and method for pyrolysis flue gas containing salt-containing waste sulfuric acid. The heat recovery system includes: a waste acid pyrolysis furnace; a molten salt separator connected to the waste acid pyrolysis furnace; a desalination heat exchanger connected to the molten salt separator, the desalination heat exchanger including a heat exchanger section I and a heat exchanger section II, and a connecting section connecting the heat exchanger sections I and II; a first air fan connected to the heat exchanger section I; a first air heat exchanger connected to both the heat exchanger sections I and II; and a second air heat exchanger connected to the first air heat exchanger. The method of this invention can increase the removal rate of molten salt in the flue gas to over 99%, achieving a total heat recovery rate of over 70% in the pyrolysis flue gas, maximizing the recovery and utilization of heat from high-temperature flue gas within the limits of process allowances.

Description

Technical Field

[0001] This invention relates to the field of chemical environmental protection technology, specifically to a heat recovery system and method for pyrolysis flue gas containing salt sulfuric acid. Background Technology

[0002] my country is the world's largest producer and consumer of dyes, accounting for over 70% of global dye production and consumption in recent years. The production of dyes and dye intermediates generates large quantities of waste sulfuric acid. After concentration, this waste sulfuric acid contains not only over 70% sulfuric acid but also organic matter and various sulfates and chlorides, such as sodium sulfate, potassium sulfate, sodium chloride, and potassium chloride, making it extremely difficult to treat. Commonly used treatment methods in China include neutralization, oxidation, extraction, and adsorption; however, these methods generally suffer from low recovery rates and severe secondary pollution, failing to achieve complete treatment.

[0003] CN111675416A discloses a process and apparatus for recycling saline waste sulfuric acid. The process involves concentrating the saline waste sulfuric acid through countercurrent multi-stage multi-effect evaporation, followed by solid-liquid separation via crystallization to extract sodium sulfate from the waste acid. However, this process has a small processing capacity, requires high-quality equipment, and cannot effectively treat organic matter in the waste acid, resulting in poor-quality sulfuric acid.

[0004] CN110627279A discloses a method for treating high-concentration saline waste sulfuric acid, which involves adding quicklime or limestone for neutralization, followed by filtration and evaporation crystallization to produce calcium sulfate dihydrate. However, the calcium sulfate produced by this process is of poor quality, has a high organic content, and generates a large amount of wastewater that requires treatment, resulting in serious secondary pollution.

[0005] CN106629630A proposes a cracking process for saline waste sulfuric acid. The process involves high-temperature cracking of the saline waste sulfuric acid, followed by waste heat recovery from the high-temperature flue gas via a waste heat boiler and air preheater. The flue gas is then fed into a subsequent acid production system to produce finished sulfuric acid. However, because the waste dye acid contains a high concentration of salts, these salts generate molten salt in the cracking furnace. Molten salt droplets are carried by the cracking flue gas into the subsequent heat exchange systems, such as the waste heat boiler and air preheater, clogging the heat exchange tubes and potentially causing perforation, posing a safety hazard. Furthermore, as the temperature gradually decreases after the cracked flue gas enters the air heat exchanger and subsequent pipelines, a large amount of molten salt adheres to the walls of the heat exchange tubes and pipelines, forming solid salts that rapidly clog the tubes and pipelines, leading to low heat exchange efficiency and even plant shutdown. Summary of the Invention

[0006] The purpose of this invention is to overcome the technical problems of low heat exchange efficiency and frequent shutdown for maintenance of salt-containing waste sulfuric acid pyrolysis flue gas in the prior art, and to provide a heat recovery system and method for salt-containing waste sulfuric acid pyrolysis flue gas.

[0007] The inventors of this invention discovered that because pyrolysis flue gas contains a large amount of liquid molten salt, during heat exchange, as the temperature of the pyrolysis flue gas decreases, the liquid molten salt easily crystallizes to form crystalline salt, clogging the pipes. Furthermore, the liquid molten salt contains various inorganic substances, and its crystallization temperature is significantly affected by the type and content of each salt. Even small changes in the type or content of salt can alter the crystallization temperature, leading to molten salt crystallization and blockage of the heat exchange pipes during the heat exchange process. By desalting the pyrolysis flue gas before and simultaneously during heat exchange, the molten salt removal rate reaches over 99.4%. This not only enables the separation and recycling of salts from waste sulfuric acid but also solves the problems of blockage and corrosion in subsequent heat exchange equipment and pipes, improving heat exchange efficiency and allowing the heat exchange system to operate stably for extended periods.

[0008] To achieve the above objectives, a first aspect of the present invention provides a heat recovery system for pyrolysis flue gas containing saline waste sulfuric acid, the heat recovery system comprising:

[0009] Waste acid pyrolysis furnace is used to pyrolyze salt-containing waste sulfuric acid to obtain pyrolysis flue gas;

[0010] A molten salt separator, connected to the waste acid pyrolysis furnace, is used to separate the pyrolysis flue gas into molten salt to obtain pre-desalted flue gas and molten salt.

[0011] A desalination heat exchanger is connected to the molten salt separator. The desalination heat exchanger includes a heat exchanger section I and a heat exchanger section II, as well as a connecting section connecting the heat exchanger section I and the heat exchanger section II.

[0012] The heat exchanger section I is used to exchange heat between the pre-desalinated flue gas and the first air to obtain the first flue gas, the second hot air and the molten salt; the connecting section is used to introduce the first flue gas into the heat exchanger section II; the heat exchanger section II is used to exchange heat between the first flue gas and the third hot air to obtain the second flue gas, the fourth hot air and the molten salt.

[0013] A first air blower is connected to the heat exchanger section I and is used to deliver first air into the heat exchanger section I.

[0014] The first air heat exchanger is connected to the heat exchanger section I and the heat exchanger section II respectively, and is used to exchange heat between the second flue gas and the second hot air to obtain the third flue gas and the fifth hot air;

[0015] The second air heat exchanger is connected to the first air heat exchanger and exchanges heat between the third flue gas and the sixth hot air to obtain the fourth flue gas and the seventh hot air.

[0016] The second air blower is connected to the second air heat exchanger and is used to deliver the sixth hot air into the second air heat exchanger.

[0017] The second aspect of the present invention provides a method for heat recovery from the pyrolysis flue gas containing salt waste sulfuric acid, which is carried out in the heat recovery system described in the first aspect above.

[0018] Through the above technical solution, the present invention can remove more than 90% of the liquid molten salt in the pyrolysis flue gas by sequentially setting a molten salt separator, a desalination heat exchanger and an air heat exchanger. The pre-desalinated flue gas is then subjected to staged heat exchange with air and the high-temperature air obtained from subsequent heat exchange, achieving a total heat recovery rate of more than 70% in the pyrolysis flue gas. Under the premise that the process allows, the heat of the high-temperature flue gas is recovered and utilized to the maximum extent. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of a heat recovery system for pyrolysis flue gas containing salt from waste sulfuric acid according to an embodiment of the present invention.

[0020] Figure 2 This is a schematic diagram of the structure of a molten salt separator according to an embodiment of the present invention;

[0021] Figure 3 This is a schematic diagram of a desalination heat exchanger according to an embodiment of the present invention.

[0022] Figure 4 This is a cross-sectional view of section AA of heat exchanger I according to an embodiment of the present invention.

[0023] Figure 5 This is a cross-sectional view of the BB side of heat exchanger section II according to an embodiment of the present invention.

[0024] Explanation of reference numerals in the attached figures

[0025] Detailed Implementation

[0026] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0027] In this invention, unless otherwise stated, directional terms such as "up," "down," "left," and "right" generally refer to the up, down, left, and right as shown in the accompanying drawings; "inner" and "outer" generally refer to the inner and outer contours relative to the outlines of each component itself; and "far" and "near" generally refer to the far and near contours of each component itself.

[0028] Figure 1This is a schematic diagram of a heat recovery system for pyrolysis flue gas containing salt from waste sulfuric acid according to an embodiment of the present invention, as shown below. Figure 1 As shown, a heat recovery system for pyrolysis flue gas containing salt sulfuric acid is disclosed, the heat recovery system comprising:

[0029] Cracking furnace 1 is used to crack saline waste sulfuric acid to obtain cracked flue gas;

[0030] Molten salt separator 2 is connected to the pyrolysis furnace 1 and is used to separate the pyrolysis flue gas into molten salt to obtain pre-desalted flue gas and molten salt.

[0031] A desalination heat exchanger is connected to the molten salt separator 2. The desalination heat exchanger includes a heat exchanger section I 3 and a heat exchanger section II 4, as well as a connecting section 9 that connects the heat exchanger section I and the heat exchanger section II.

[0032] The heat exchanger section 3 is used to exchange heat between the pre-desalinated flue gas and the first air to obtain the first flue gas, the second hot air and the molten salt; the connecting section 9 is used to introduce the first flue gas into the heat exchanger section 4; the heat exchanger section 4 is used to exchange heat between the first flue gas and the third hot air to obtain the second flue gas, the fourth hot air and the molten salt.

[0033] The first air fan 5 is connected to the heat exchanger section I 3 and is used to deliver the first air into the heat exchanger section I 3;

[0034] The first air heat exchanger 6 is connected to the heat exchanger section I 3 and the heat exchanger section II 4 respectively, and is used to exchange heat between the second flue gas and the second hot air to obtain the third flue gas and the fifth hot air.

[0035] The second air heat exchanger 7 is connected to the first air heat exchanger 6 and exchanges heat between the third flue gas and the sixth hot air to obtain the fourth flue gas and the seventh hot air.

[0036] The second air blower 8 is connected to the second air heat exchanger 7 and is used to deliver the sixth hot air into the second air heat exchanger 7.

[0037] Through extensive experiments, the inventors of this invention discovered that, based on the characteristics of the diverse and abundant inorganic salts in the molten salts of pyrolysis flue gas, by desalting the pyrolysis flue gas and then exchanging it with various gases at different temperatures in stages, not only can the separation and efficient recycling of salts in waste sulfuric acid be achieved, but the problems of blockage and corrosion of subsequent heat exchange equipment and pipelines are also solved, thereby improving heat exchange efficiency.

[0038] Figure 2This is a schematic diagram of a molten salt separator according to an embodiment of the present invention; the molten salt separator 2 includes a flue gas inlet 25 disposed at one end of the molten salt separator 2, a flue gas outlet 26 disposed at the other end of the molten salt separator 2, and a salt discharge hole 27 disposed at the bottom of the molten salt separator 2; the molten salt separator 2 is provided with N baffles arranged at intervals along its length to divide the molten salt separator 2 into N+1 interconnected molten salt separation chambers 210, and each molten salt separation chamber 210 is provided with a molten salt separation device 23 for separating the pyrolysis flue gas into molten salt to obtain pre-desalinated flue gas and molten salt. For example, Figure 2 As shown, N≥2, preferably N is 3-10, and more preferably 4-6.

[0039] According to the present invention, under preferred conditions, the baffle wall includes a lower baffle wall 21 and an upper baffle wall 22. One end of the lower baffle wall 21 is disposed on the bottom wall of the molten salt separator 2, and the other end extends vertically into the molten salt separation chamber 210. A drainage hole 211 is provided at the bottom of the lower baffle wall 21. One end of the upper baffle wall 22 is disposed on the top wall of the molten salt separator 2, and the other end extends vertically into the molten salt separation chamber 210. Preferably, the lower baffle wall 21 and the upper baffle wall 22 are arranged alternately, causing the pyrolysis flue gas to flow in a meandering manner within the molten salt separator 2, thereby increasing the molten salt separation efficiency.

[0040] According to the present invention, under preferred conditions, the molten salt separation device 23 includes a frame and M parallel separation plates disposed on the frame. The two ends of the separation plates are fixed to the frame to separate the molten salt separation chamber 210 into M+1 flow channels 231, allowing the pyrolysis flue gas to flow within the flow channels, accelerating the settling velocity of the molten salt in the pyrolysis flue gas, forming droplets that adhere to the separation plates, and ultimately drip down along the separation plates to the bottom of the molten salt separation chamber 210, wherein M≥5, preferably M≥10. More preferably, for ease of maintenance, the molten salt separation device 23 is an integral skid-mounted structure.

[0041] Under preferred conditions, the angle between the separation plate and the horizontal line is 0-60°, preferably 20-30°; the molten salt separation efficiency increases as the distance between two adjacent separation plates decreases, but a smaller distance also leads to an increase in the flow resistance of the pyrolysis flue gas. More preferably, the distance between two adjacent separation plates is 50-150 mm, for example, it can be 50 mm, 100 mm, 150 mm or any value within the range formed by any two of the above values; under these preferred conditions, the settling of molten salt can be accelerated and the molten salt separation efficiency can be improved.

[0042] To improve the corrosion resistance of the separation plate, under preferred conditions, the material of the separation plate is silicon carbide ceramic or acid-resistant ceramic.

[0043] According to the present invention, in order to discharge the separated liquid molten salt from the molten salt separator 2, under preferred conditions, the bottom wall of the molten salt separator 2 has a slope of 1‰-5‰, wherein the salt discharge hole 27 is the lowest point of the bottom wall of the molten salt separator 2. More preferably, the salt discharge hole 27 is located below the flue gas outlet 26.

[0044] According to the present invention, under preferred conditions, each of the molten salt separation chambers 210 is provided with an inspection hole 24 at the top, so that the molten salt separation device 23 can be easily removed and replaced from the inspection hole 24.

[0045] To improve the heat insulation effect of the molten salt separator 2 and increase the heat utilization rate and molten salt separation efficiency, under preferred conditions, the inner wall of the molten salt separator 2 is provided with a refractory layer 28, more preferably, the material of the refractory layer 28 is corundum or chromium corundum; even more preferably, the outer wall of the molten salt separator 2 is covered with a heat insulation layer 29, more preferably, the material of the heat insulation layer 29 is selected from at least one of aluminum silicate, magnesium silicate and calcium silicate.

[0046] Under preferred conditions, the pressure drop across the tube side of the molten salt separator 2 is ≤0.5 kPa. This preferred condition satisfies the system's pressure drop requirements without affecting the negative pressure environment of the waste acid pyrolysis system, ensuring stable system operation.

[0047] In a preferred embodiment of the present invention, the method for separating the pyrolysis flue gas in the molten salt separator 2 is as follows: the pyrolysis flue gas is introduced into the molten salt separator 2 from the flue gas inlet 25, and the pyrolysis flue gas flows through the first molten salt separation chamber, the second molten salt separation chamber... the N+1th molten salt separation chamber to the flue gas outlet 26. In each molten salt separation chamber, the pyrolysis flue gas flows in the narrow flow channel separated by the separation plate. During the flow, the liquid molten salt accelerates and settles, and is collected on the separation plate and guided to the bottom of the molten salt separation chamber, and finally collects along the bottom wall of the molten salt separator 2 at the salt discharge hole 27 and is discharged from the molten salt separator 2.

[0048] Molten salt separator 2 can effectively remove liquid salt droplets with a particle size ≥50μm, achieving a molten salt removal rate of over 70% in flue gas. This significantly reduces the desalination pressure on subsequent equipment, enabling the separation and recycling of salts in waste sulfuric acid. It also helps solve the problems of blockage and corrosion in subsequent heat exchange equipment and pipelines, and achieves full recovery and utilization of heat from the pyrolysis flue gas containing salt.

[0049] According to the present invention, under preferred conditions, the system further includes a collection tank 11 connected to the bottom of the molten salt separator 2 via a pipe for collecting the liquid molten salt obtained by molten salt separation. In order to prevent the liquid molten salt from clogging the pipe, under preferred conditions, the system further includes a heating device for heating the pipe. Preferably, the heating device can be a burner or high-temperature hot air.

[0050] Figure 3 This is a schematic diagram of a desalination heat exchanger according to an embodiment of the present invention, as shown below. Figure 3 As shown, the desalination heat exchanger includes heat exchanger section I 3 and heat exchanger section II 4. The top end of heat exchanger section I 3 and the top end of heat exchanger section II 4 are connected by a connecting section 9. The bottom end of heat exchanger section I 3 is provided with a front smoke box 37 connected to heat exchanger section I 3, and the bottom end of heat exchanger section II 4 is provided with a rear smoke box 413 connected to heat exchanger section II 4. The front smoke box 37 and the rear smoke box 413 are connected by a salt discharge pipe 312.

[0051] In this invention, in order to discharge the molten salt separated in heat exchanger section I 3 and heat exchanger section II 4 into the desalination heat exchanger, at least one of the bottom ends of the front smoke box 37, the rear smoke box 413, or the bottom surface of the salt discharge pipe 312 is provided with a drain hole 310 for discharging the liquid molten salt obtained from the molten salt separation from the desalination heat exchanger; preferably, the drain hole 310 is located at the bottom end of the front smoke box 37.

[0052] According to the present invention, under preferred conditions, in order to improve the corrosion resistance of the front smoke box 37 and the rear smoke box 413, the front smoke box 37 is preferably provided with a first corrosion-resistant layer 316; preferably, the inner wall of the rear smoke box 413 is also provided with a second corrosion-resistant layer 416; preferably, the inner wall of the salt discharge pipe 312 is provided with a corrosion-resistant layer; more preferably, the materials of the first corrosion-resistant layer 316 and the second corrosion-resistant layer 416 are each independently selected from fused alumina or chromium alumina.

[0053] In some preferred embodiments of the present invention, in heat exchanger section I 3, flue gas flows through the inner tube and air flows through the outer tube. Specifically, heat exchanger section I 3 includes, from the inside to the outside, a first heat exchange inner tube 31, a first heat exchange outer tube 32, and a first insulation layer 33. The bottom of the first heat exchange inner tube 31 is provided with a first flue gas inlet 36, the top of the first heat exchange outer tube 32 is provided with a first air outlet 39, and the bottom of the first heat exchange outer tube 32 is provided with a first air inlet 38, which is used to exchange heat between the pre-desalinated flue gas and the first air to obtain the first flue gas, the second hot air, and molten salt.

[0054] Figure 4 This is a cross-sectional view of section I of a heat exchanger according to an embodiment of the present invention; as shown Figure 4As shown, in some preferred embodiments of the present invention, the first heat exchange inner tube 31 is provided with a molten salt separation module 34. The molten salt separation module 34 includes a plurality of separation plates arranged at intervals along the length direction of the first heat exchange inner tube 31. The separation plates are filled with structured packing or random packing, preferably structured packing; more preferably, the structured packing is selected from at least one of grid plate, honeycomb plate and corrugated plate, most preferably grid plate or honeycomb plate.

[0055] To improve the salt and corrosion resistance of the molten salt separation module 34, the material of the molten salt separation module 34 is preferably a corrosion-resistant material, and most preferably silicon carbide ceramic or acid-resistant ceramic.

[0056] To improve the heat exchange efficiency of heat exchanger section I 3, under preferred conditions, heat exchanger section I 3 is further provided with a plurality of first baffles 35 arranged at intervals along its length, one end of each first baffle 35 being welded to the inner wall of the first heat exchange outer tube 32. Under these preferred conditions, the first baffles 35 can agitate the first air, improving the heat exchange efficiency between the pre-desalinated flue gas and the first air. More preferably, the first baffles 35 are arranged in an alternating pattern within heat exchanger section I 3; under this preferred condition, the heat exchange efficiency between the pre-desalinated flue gas and the first air can be further increased.

[0057] According to the present invention, under preferred conditions, the first baffle 35 is semi-annular, which can further improve the heat exchange efficiency of the pre-desalinated flue gas and the first air.

[0058] In this invention, in heat exchanger section II 4, flue gas flows through the inner tube and air flows through the outer tube. Specifically, heat exchanger section II 4 includes, from the inside to the outside, a second heat exchange inner tube 41, a second heat exchange outer tube 42, and a second insulation layer 43. The bottom of the second heat exchange inner tube 41 is provided with a second flue gas outlet 46, the top of the second heat exchange outer tube 42 is provided with a second air inlet 48, and the bottom of the second heat exchange outer tube 42 is provided with a second air outlet 47. This is used to exchange heat between the first flue gas and the second air to obtain second flue gas, fourth hot air, and molten salt.

[0059] Figure 5 This is a cross-sectional view of heat exchanger section II according to an embodiment of the present invention; as shown Figure 5 As shown, in some preferred embodiments of the present invention, the second heat exchange inner tube 41 is provided with a plurality of high-voltage discharge devices arranged in parallel along the axial direction. The high-voltage discharge devices can discharge to form an electric field, thereby capturing charged molten salt droplets and improving the efficiency of molten salt removal. Under preferred conditions, the high-voltage discharge device includes an anode tube 410 and a cathode wire 411 disposed in the anode tube 410.

[0060] In order to improve the efficiency of the electric field in capturing charged molten salt droplets and removing molten salt, under preferred conditions, the inner diameter of the anode tube 410 is ≥100mm, preferably 100-250mm, and more preferably 150-200mm.

[0061] To improve the salt and corrosion resistance of the anode tube, under preferred conditions, the material of the anode tube 410 is conductive silicon carbide.

[0062] To improve the molten salt removal efficiency of the heat exchanger section II 4, under preferred conditions, the heat exchanger section II 4 is further provided with a plurality of second baffles 45 arranged at intervals along its axial direction. One end of the second baffle 45 is welded to the inner wall of the second heat exchange outer tube 42. More preferably, the second baffles 45 are arranged in an alternating pattern within the heat exchanger section II 4. Under this preferred condition, the heat exchange efficiency of the first flue gas can be further increased.

[0063] According to the present invention, under preferred conditions, the second baffle 45 is semi-annular, which can further improve the heat exchange efficiency of the first flue gas.

[0064] Under preferred conditions, the tube-side pressure drop of the desalination heat exchanger is ≤1 kPa. Within this range, the system's pressure drop requirements for the equipment are met, and the negative pressure environment of the waste acid cracking system is not affected, thus ensuring the stable operation of the system.

[0065] In this invention, in order to improve the thermal insulation performance and corrosion resistance of the first thermal insulation layer 33 and the second thermal insulation layer 43, under preferred conditions, the materials of the first thermal insulation layer 33 and the second thermal insulation layer 43 are each independently selected from at least one of aluminum silicate, magnesium silicate and calcium silicate, preferably magnesium silicate fiber blanket.

[0066] According to the present invention, under preferred conditions, the materials of the first heat exchange outer tube 32 and the second heat exchange outer tube 42 are each independently selected from carbon steel and / or stainless steel, for example, Q245R steel or 321 stainless steel; the materials of the first heat exchange inner tube 31 and the second heat exchange inner tube 41 are each independently silicon carbide.

[0067] According to the present invention, in order for the liquid molten salt collected at the bottom of the front smoke box 37 and the bottom of the rear smoke box 413 to be smoothly discharged from the desalination heat exchanger, preferably, the bottom surface of the front smoke box 37 has a slope of 1‰-10‰, more preferably 2‰-5‰; preferably, the bottom surface of the salt discharge pipe 312 has a slope of 1‰-10‰, more preferably 2‰-5‰; preferably, the bottom surface of the rear smoke box 413 has a slope of 1‰-10‰, more preferably 2‰-5‰; more preferably, the bottom surface of the front smoke box 37, the bottom surface of the salt discharge pipe 312 and the bottom surface of the rear smoke box 413 are on the same plane, and the lowest point of the bottom surface of the front smoke box 37 is lower than the lowest point of the bottom surface of the rear smoke box 413.

[0068] In some preferred embodiments of the present invention, an overflow weir 311 is provided above the drain hole 310 to allow the separated liquid molten salt to overflow evenly from the desalination heat exchanger. At the same time, the overflow weir 311 can also prevent the flue gas in heat exchanger section I 3 and heat exchanger section II 4 from short-circuiting.

[0069] For ease of maintenance, under preferred conditions, the bottom surface of the front smoke box 37 is provided with a first maintenance hole 320 and / or the bottom surface of the rear smoke box 413 is provided with a second maintenance hole 420.

[0070] In this invention, the desalination heat exchanger can fully remove liquid salt droplets with a particle size ≤50μm while exchanging heat. The heat exchanger section I (3) achieves a molten salt removal rate of over 80% in the pre-desalinated flue gas (section I only), and the heat exchanger section II (4) achieves a molten salt removal rate of over 90% in the first flue gas (section II only). The total desalination efficiency of the two sections of the desalination heat exchanger reaches over 98%, which can separate molten salt while exchanging heat, solves the problem of molten salt blockage in subsequent heat exchange equipment and pipelines, and realizes the separation and recycling of salts in waste sulfuric acid.

[0071] According to the present invention, under preferred conditions, the first air outlet 39 at the top of the first heat exchange outer tube 32 is connected to the air inlet of the first air heat exchanger 6, for introducing the second hot air into the first air heat exchanger 6 in a circulating manner.

[0072] Preferably, the air outlet of the first air heat exchanger 6 is connected to the second air inlet 48 at the top of the second heat exchange outer tube 42, for circulating the fifth hot air into the second heat exchange outer tube 42.

[0073] According to the present invention, under preferred conditions, the second air outlet 47 at the bottom of the second heat exchange outer tube 42 is connected to the pyrolysis furnace 1, and is used to circulate the fourth hot air into the pyrolysis furnace 1, so that the fourth hot air exchanges heat with the combustion air in the pyrolysis furnace, which can preheat the combustion air and save more than 50% of fuel gas consumption.

[0074] In some preferred embodiments of the present invention, the first air heat exchanger 6 and / or the second air heat exchanger 7 are shell-and-tube heat exchangers. In order to improve the corrosion resistance of the first air heat exchanger 6 and / or the second air heat exchanger 7, preferably, both the first air heat exchanger 6 and the second air heat exchanger 7 are made of stainless steel.

[0075] According to the present invention, under preferred conditions, the first air heat exchanger 6 includes a shell and a plurality of tubes connected in series disposed within the shell. The upper end of the shell is provided with an air inlet, the lower end of the shell is provided with an air outlet, the top end of the tubes is provided with a flue gas outlet, and the bottom end of the tubes is provided with a flue gas inlet, so that the second flue gas in the tubes exchanges heat with the air in the shell. The tube-side pressure drop of the first air heat exchanger 6 is ≤2kPa.

[0076] According to the present invention, under preferred conditions, the second air heat exchanger 7 includes a shell and a plurality of tubes connected in series within the shell. The bottom end of the shell is provided with an air inlet, the top end of the shell is provided with an air outlet, the top end of the tubes is provided with a flue gas inlet, and the bottom end of the tubes is provided with a flue gas outlet, so that the third flue gas in the tubes exchanges heat with the air in the shell. The tube-side pressure drop of the second air heat exchanger 7 is ≤2kPa.

[0077] According to the present invention, under preferred conditions, the heat recovery system further includes a heat exchange device connected to the air outlet at the top of the shell of the second air heat exchanger 7, for exchanging heat between the seventh hot air and the heat exchange device to improve the heat utilization rate. The type of heat exchange device is known to those skilled in the art, for example, it may be a waste heat boiler and / or an economizer.

[0078] This invention achieves combined heat exchange of pyrolysis flue gas by using a combination of molten salt separator 2, heat exchanger section I 3 and heat exchanger section II 4 of desalination heat exchanger, and first air heat exchanger 6 and second air heat exchanger 7. This enables the total heat recovery rate of the high-temperature flue gas at the outlet of pyrolysis furnace 1 to reach more than 70%, maximizing the recovery and utilization of heat from the high-temperature flue gas within the limits of the process.

[0079] The second aspect of the present invention provides a method for heat recovery from the pyrolysis flue gas containing salt waste sulfuric acid, which is carried out in the heat recovery system described in the first aspect above.

[0080] In some preferred embodiments of the present invention, the heat recovery method includes:

[0081] (1) Salt-containing waste sulfuric acid is cracked to obtain cracked flue gas;

[0082] The pyrolysis flue gas is desalted to obtain pre-desalted flue gas and molten salt;

[0083] (2) The pre-desalinated flue gas is heat-exchanged with the first air to obtain the first flue gas, the second hot air and the molten salt, wherein the temperature of the first air is 20-30℃;

[0084] (3) The first flue gas is exchanged with the third hot air to obtain the second flue gas, the fourth hot air and molten salt;

[0085] (4) The second flue gas is exchanged with the second hot air to obtain the third flue gas and the fifth hot air, and the fifth hot air is circulated into the third hot air;

[0086] (5) The third flue gas is exchanged with the sixth hot air to obtain the fourth flue gas and the seventh hot air, wherein the temperature of the sixth hot air is 250-300℃.

[0087] In this invention, the salt-containing waste sulfuric acid pyrolysis flue gas is obtained by high-temperature pyrolysis of waste sulfuric acid. The waste sulfuric acid contains impurities such as sulfuric acid, water, organic matter, and inorganic salts. Based on the total amount of waste sulfuric acid, the content of inorganic salts in the waste sulfuric acid is ≤25% by weight. Preferably, the inorganic salts are sodium salts and / or potassium salts. For example, the inorganic salts are selected from at least one of sodium chloride, sodium sulfate, potassium chloride, and potassium sulfate.

[0088] In this invention, the temperature of the pre-desalinated flue gas is ≥900℃, preferably 900-1100℃.

[0089] According to the present invention, under preferred conditions, the first air can be room temperature air, preferably, the temperature of the first air is 20-35°C, and the temperature of the first flue gas obtained by heat exchange between the pre-desalinated flue gas and the first air is preferably 810-870°C; the temperature of the second hot air is preferably 330-380°C.

[0090] According to the present invention, under preferred conditions, the temperature of the third hot air is preferably 480-520°C, the temperature of the second flue gas obtained by heat exchange between the first flue gas and the third hot air is preferably 700-800°C, and the temperature of the fourth hot air obtained is preferably 650-700°C.

[0091] According to the present invention, under preferred conditions, the temperature of the third flue gas obtained by heat exchange between the second flue gas and the second hot air is preferably 600-650°C, and the temperature of the fifth hot air obtained is preferably 480-520°C.

[0092] According to the present invention, under preferred conditions, the temperature of the sixth hot air is preferably 250-300°C; the temperature of the fourth flue gas obtained by heat exchange between the third flue gas and the sixth hot air is preferably 300-400°C; and the temperature of the seventh hot air obtained is 530-570°C.

[0093] The inventors of this invention have discovered that when the first air, third hot air, second hot air, and sixth hot air are sequentially exchanged with the pre-desalinated flue gas, under the aforementioned preferred conditions, not only can the heat exchange efficiency of the flue gas be improved, but also molten salt crystallization can be avoided, molten salt in the flue gas can be removed to the maximum extent, and pipe blockage can be avoided.

[0094] The present invention will be described in detail below through examples.

[0095] The following examples are in Figures 1 to 3 The heat recovery system shown is used for this purpose.

[0096] The structure of the molten salt separator 2 is as follows: the molten salt separator 2 includes a flue gas inlet 25 at one end of the molten salt separator 2, a flue gas outlet 26 at the other end of the molten salt separator 2, and a salt discharge hole 27 at the bottom of the molten salt separator 2, the salt discharge hole 27 being located below the flue gas outlet 26; the molten salt separator 2 is provided with 4 baffles arranged at intervals along its length, which are used to divide the molten salt separator 2 into 5 interconnected molten salt separation chambers 210, each of which is provided with a molten salt separation device 23, and each molten salt separation chamber 210 is provided with an inspection hole 24 at the top.

[0097] The baffle wall includes two lower baffle walls 21 and two upper baffle walls 22. One end of the lower baffle wall 21 is disposed on the bottom wall of the molten salt separator 2, and the other end extends vertically into the molten salt separation chamber 210. The bottom of the lower baffle wall 21 is provided with an opening. One end of the upper baffle wall 22 is disposed on the top wall of the molten salt separator 2, and the other end extends vertically into the molten salt separation chamber 210. The lower baffle walls 21 and the upper baffle walls 22 are arranged alternately.

[0098] The molten salt separation device 23 is a skid-mounted structure, including a frame and N parallel separation plates fixed on the frame, which separate the molten salt separation chamber 210 into N+1 flow channels 231, allowing the pyrolysis flue gas to flow in the N+1 flow channels. The angle between the separation plate and the horizontal line is 40°, and the distance between adjacent separation plates is 100mm.

[0099] The bottom wall of the molten salt separator 2 has a slope of 3‰, and the salt discharge hole 7 is the lowest point of the bottom wall of the molten salt separator 2; the pressure drop of the tube side of the molten salt separator 2 is ≤0.5kPa.

[0100] The desalination heat exchanger has an "n"-shaped vertical structure, specifically consisting of two parallel heat exchanger sections: Section I 3 and Section II 4. The tops of Section I 3 and Section II 4 are connected by a connecting section 9. The bottom of Section I 3 is provided with a front smoke box 37, and the bottom of Section II 4 is provided with a rear smoke box 413. The front smoke box 37 and the rear smoke box 413 are connected by a salt discharge pipe 312. The bottom of the front smoke box 37 is provided with a drain hole 310. Both the front smoke box 37 and the rear smoke box 413 are provided with a chromium corundum corrosion-resistant layer.

[0101] The heat exchanger section 3 comprises, from the inside out: a first heat exchange inner tube 31, a first heat exchange outer tube 32, and a first insulation layer 33. The first heat exchange inner tube 31 has a first flue gas inlet 36 at its bottom, the first heat exchange outer tube 32 has a first air outlet 39 at its top, and a first air inlet 38 at its bottom. The first heat exchange inner tube 31 contains several silicon carbide grid plates spaced apart along its length. The heat exchanger section 3 also contains several semi-annular first baffles 35 spaced apart along their length. One end of each baffle 35 is located on the inner wall of the first heat exchange outer tube 32, and the other end extends into the cavity of the first heat exchange inner tube 31.

[0102] The heat exchanger section II 4 comprises, from the inside out: a second inner heat exchange tube 41, a second outer heat exchange tube 42, and a second insulation layer 43. The bottom of the second inner heat exchange tube 41 is provided with a second flue gas outlet 46, the top of the second outer heat exchange tube 42 is provided with a second air inlet 48, and the bottom of the second outer heat exchange tube 42 is provided with a second air outlet 47. The second inner heat exchange tube 41 contains a plurality of silicon carbide anode tubes 410 arranged radially at intervals and cathode wires 411 disposed within the anode tubes 410. The inner diameter of the anode tubes 410 is 150 mm. The heat exchanger section II 4 also contains a plurality of semi-annular second baffles 45 arranged longitudinally at intervals. One end of the second baffle 45 is disposed on the inner wall of the second outer heat exchange tube 42, and the other end extends into the cavity of the second inner heat exchange tube 41. The tube-side pressure drop of the desalination heat exchanger is ≤1 kPa.

[0103] The bottom surface of the front smoke box 37, the bottom surface of the salt discharge pipe 312, and the bottom surface of the rear smoke box 413 are on the same plane, and the slope of the plane is 2‰.

[0104] Example 1

[0105] (1) Salt-containing waste sulfuric acid is cracked in cracking furnace 1 to obtain cracked flue gas (1100℃); then the cracked flue gas is desalted in molten salt separator 2 to obtain pre-desalted flue gas (1050℃) and molten salt;

[0106] (2) The pre-desalinated flue gas (1050°C) is introduced into the first heat exchange inner tube 31 of the heat exchanger section I 3 and exchanged with the ambient temperature air in the first heat exchange outer tube 32 of the heat exchanger section I 3 to obtain the first flue gas (850°C), the second hot air (350°C) and molten salt.

[0107] (3) The first flue gas is introduced into the second heat exchange inner tube 41 of the heat exchanger section II 4 and exchanged with the third hot air (500°C) in the second heat exchange outer tube 42 of the heat exchanger section II 4 to obtain the second flue gas (700°C), the fourth hot air (680°C) and molten salt.

[0108] (4) The second flue gas (700°C) and the second hot air (350°C) are exchanged in the first air heat exchanger 6 to obtain the third flue gas (610°C) and the fifth hot air (500°C).

[0109] The fourth hot air (680°C) is introduced into the pyrolysis furnace 1 to exchange heat with the combustion air;

[0110] (5) The third flue gas is exchanged with the sixth air (300°C) from the second air fan 8 in the second air heat exchanger 7 to obtain the fourth flue gas (350°C) and the seventh hot air (550°C).

[0111] The fifth hot air (500°C) is circulated and introduced into the third hot air added in step (3);

[0112] (6) The fourth flue gas (350°C) is sent to the subsequent acid production system for acid production;

[0113] The seventh hot air (550°C) is sent to the waste heat boiler for heat exchange to obtain circulating hot air, which is then circulated into the sixth air (300°C).

[0114] The high-temperature flue gas containing salt at the outlet first enters the molten salt separator (2) to capture the molten salt droplets in the flue gas. The enriched molten salt is discharged through the bottom outlet of the molten salt separator and the flue gas desalination heat exchanger section I (3).

[0115] In this embodiment, the removal rate of molten salt in the pyrolysis flue gas is 99.4%, and the heat recovery rate of the pyrolysis flue gas is 70%.

[0116] This invention combines the aforementioned heat recovery system with the aforementioned heat recovery method, enabling molten salt separation during heat exchange of pyrolysis flue gas. This results in less or no crystallization of molten salt during the heat exchange process, allowing the molten salt to be separated directly within the heat exchange system. This not only ensures maximum separation of molten salt from the pyrolysis flue gas and improves the separation efficiency, but also solves the problem of molten salt crystallization clogging subsequent system pipelines.

[0117] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A heat recovery system for pyrolysis flue gas containing salt-containing waste sulfuric acid, characterized in that, The heat recovery system includes: Waste acid pyrolysis furnace (1) is used to pyrolyze salt-containing waste sulfuric acid to obtain pyrolysis flue gas; Molten salt separator (2) is connected to the waste acid pyrolysis furnace (1) and is used to separate the pyrolysis flue gas into molten salt to obtain pre-desalinated flue gas and molten salt; A desalination heat exchanger is connected to the molten salt separator (2). The desalination heat exchanger includes a heat exchanger section I (3) and a heat exchanger section II (4), as well as a connecting section (9) connecting the heat exchanger section I and the heat exchanger section II. The heat exchanger section I (3) is used to exchange heat between the pre-desalinated flue gas and the first air to obtain the first flue gas, the second hot air and the molten salt; the connecting section (9) is used to introduce the first flue gas into the heat exchanger section II (4); the heat exchanger section II (4) is used to exchange heat between the first flue gas and the third hot air to obtain the second flue gas, the fourth hot air and the molten salt. The first air fan (5) is connected to the heat exchanger section I (3) and is used to deliver the first air into the heat exchanger section I (3); The first air heat exchanger (6) is connected to the heat exchanger section I (3) and the heat exchanger section II (4) respectively, and is used to exchange heat between the second flue gas and the second hot air to obtain the third flue gas and the fifth hot air; The second air heat exchanger (7) is connected to the first air heat exchanger (6) to exchange heat between the third flue gas and the sixth hot air to obtain the fourth flue gas and the seventh hot air. The second air blower (8) is connected to the second air heat exchanger (7) and is used to deliver the sixth hot air into the second air heat exchanger (7); The fourth hot air circulation is introduced into the waste acid pyrolysis furnace (1); The heat exchanger section I (3) and the heat exchanger section II (4) are each independently equipped with a molten salt separation device; The molten salt separator (2) is further provided with N baffles arranged at intervals along its length to divide the molten salt separator (2) into N+1 interconnected molten salt separation chambers (210). The molten salt separation chambers (210) are provided with molten salt separation devices (23) to separate the pyrolysis flue gas into molten salt to obtain pre-desalinated flue gas and molten salt, wherein N≥2; Specifically, the fifth hot air is circulated and added to the third hot air.

2. The heat recovery system according to claim 1, wherein, The heat exchanger section I (3) includes, from the inside out, a first heat exchange inner tube (31), a first heat exchange outer tube (32), and a first insulation layer (33). The bottom of the first heat exchange inner tube (31) is provided with a first flue gas inlet (36), the top of the first heat exchange outer tube (32) is provided with a first air outlet (39), and the bottom of the first heat exchange outer tube (32) is provided with a first air inlet (38), which is used to exchange heat between the pre-desalinated flue gas and the first air to obtain the first flue gas, the second hot air, and the molten salt.

3. The heat recovery system according to claim 2, wherein, The first air outlet (39) at the top of the first heat exchange outer tube (32) is connected to the air inlet of the first air heat exchanger (6) for circulating the second hot air into the first air heat exchanger (6).

4. The heat recovery system according to claim 1, wherein, The heat exchanger section II (4) includes, from the inside out: a second heat exchange inner tube (41), a second heat exchange outer tube (42), and a second insulation layer (43). The bottom of the second heat exchange inner tube (41) is provided with a second flue gas outlet (46), the top of the second heat exchange outer tube (42) is provided with a second air inlet (48), and the bottom of the second heat exchange outer tube (42) is provided with a second air outlet (47). It is used to exchange heat between the first flue gas and the second air to obtain the second flue gas, the fourth hot air, and molten salt.

5. The heat recovery system according to claim 4, wherein, The air outlet of the first air heat exchanger (6) is connected to the second air inlet (48) at the top of the second heat exchange outer tube (42) to circulate the fifth hot air into the second heat exchange outer tube (42).

6. The heat recovery system according to claim 4, wherein, The second air outlet (47) is connected to the waste acid pyrolysis furnace (1) and is used to circulate the fourth hot air into the waste acid pyrolysis furnace (1).

7. The heat recovery system according to claim 1, wherein, The molten salt separation device is selected from at least one of the following: grating plate, honeycomb plate, corrugated plate, and discharge device.

8. The heat recovery system according to any one of claims 1-7, wherein, The molten salt separator (2) includes a flue gas inlet (25) at one end of the molten salt separator (2), a flue gas outlet (26) at the other end of the molten salt separator (2), and a salt discharge hole (27) at the bottom of the molten salt separator (2).

9. A method for heat recovery from pyrolysis flue gas containing salt in waste sulfuric acid, characterized in that, The heat recovery system described in any one of claims 1-8 is used.

10. The heat recovery method according to claim 9, wherein, The heat recovery method includes: (1) Salt-containing waste sulfuric acid is cracked to obtain cracked flue gas; The pyrolysis flue gas is desalted to obtain pre-desalted flue gas and molten salt; (2) The pre-desalinated flue gas is heat-exchanged with the first air to obtain the first flue gas, the second hot air and the molten salt, wherein the temperature of the first air is 20-30℃; (3) The first flue gas is exchanged with the third hot air to obtain the second flue gas, the fourth hot air and molten salt; (4) The second flue gas is exchanged with the second hot air to obtain the third flue gas and the fifth hot air, and the fifth hot air is circulated into the third hot air; (5) The third flue gas is exchanged with the sixth hot air to obtain the fourth flue gas and the seventh hot air, wherein the temperature of the sixth hot air is 280-330℃.

11. The heat recovery method according to claim 10, wherein, Based on the total amount of waste sulfuric acid, the content of inorganic salts in the waste sulfuric acid is ≤25% by weight.

12. The heat recovery method according to claim 11, wherein, The inorganic salt is a sodium salt and / or a potassium salt.

13. The heat recovery method according to claim 12, wherein, The inorganic salt is selected from at least one of sodium chloride, sodium sulfate, potassium chloride, and potassium sulfate.

14. The heat recovery method according to claim 10, wherein, The temperature of the pre-desalinated flue gas is ≥900℃.

15. The heat recovery method according to any one of claims 10-14, wherein, The temperature of the pre-desalinated flue gas is 900-1100℃.

16. The heat recovery method according to claim 10, wherein, The temperature of the second flue gas is 700-800℃.

17. The heat recovery method according to claim 10, wherein, The temperature of the third flue gas is 600-650℃.

18. The heat recovery method according to claim 10, wherein, The temperature of the fourth flue gas is 300-400℃.

19. The heat recovery method according to claim 10, wherein, The temperature of the first air is 20-35℃.

20. The heat recovery method according to claim 10, wherein, The temperature of the second hot air is 330-380℃.

21. The heat recovery method according to claim 10, wherein, The temperature of the third hot air is 480-520℃.

22. The heat recovery method according to claim 10, wherein, The temperature of the fourth hot air is 650-700℃.

23. The heat recovery method according to claim 10, wherein, The temperature of the fifth hot air is 480-520℃.

24. The heat recovery method according to claim 10, wherein, The temperature of the sixth hot air is 250-300℃.

25. The heat recovery method according to claim 10, wherein, The temperature of the seventh hot air is 530-570℃.

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