A closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid

Abstract

This application relates to a closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid, comprising the following steps: S1, feeding waste sulfuric acid and oxygen-containing gas into a pressurized pyrolysis furnace for pyrolysis at a gauge pressure of 0.5~3.0 MPaG and a temperature of 850~1050℃; S2, performing high-temperature gas-solid separation on the high-temperature flue gas while maintaining the pressure to remove molten metal salts; S3, introducing the purified flue gas into a pressurized high-temperature phase change condenser while maintaining the pressure, gradually cooling it to 220~350℃, utilizing the principle of pressurizing to raise the dew point of sulfuric acid, selectively condensing sulfuric acid vapor into 85~98wt% concentrated sulfuric acid, while water vapor remains in a gaseous state, achieving gas-liquid separation in one step; S4, introducing the tail gas into a turbine expander to drive a compressor to generate oxygen-containing gas; S5, using the waste heat from the tail gas to preheat the feed waste acid. This application replaces traditional water-cooled rapid cooling with a completely dry separation method, eliminating the production of dilute sulfuric acid from a mechanistic perspective, realizing energy cascade internal recycling, and significantly reducing energy consumption.

Description

Technical Field

[0001] This application relates to the field of waste sulfuric acid recovery, and in particular to a closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid. Background Technology

[0002] High-temperature pyrolysis regeneration of industrial waste sulfuric acid is the mainstream process for treating waste sulfuric acid containing organic impurities and heavy metals, such as alkylation waste acid and titanium dioxide waste acid. In existing technologies, a typical closed-loop high-temperature pyrolysis regeneration process for industrial waste sulfuric acid involves: pressurizing and atomizing the waste sulfuric acid before injecting it into a pyrolysis furnace operating at atmospheric or slightly negative pressure. The furnace is then heated to 850-1100°C by external natural gas or heavy oil for pyrolysis, generating high-temperature flue gas containing SO3, water vapor, and molten metal salts. After recovering some heat in a waste heat boiler, this high-temperature flue gas is directly introduced into a water-cooled quench tower. A large amount of cooling water spray rapidly cools the flue gas from over 700°C to 40-60°C, causing the sulfuric acid vapor and water vapor to condense and absorb together, forming dilute sulfuric acid with a concentration of 10-30 wt%. After demisting by a demister, part of the dilute sulfuric acid is returned to the pyrolysis furnace to form a cycle, while the rest is sent out as finished acid. This process is currently the common industrial solution.

[0003] However, the aforementioned existing process routes suffer from systemic defects determined by their technical logic. The fundamental problem lies in the fact that high-temperature pyrolysis flue gas must undergo rapid water cooling to achieve gas-liquid separation. This operation leads to irreversible energy and material losses. The sensible heat of the flue gas above 700°C is inefficiently carried away by the cooling water, and the large amount of low-concentration dilute sulfuric acid generated during rapid cooling, after being returned to the pyrolysis furnace, requires further energy consumption to evaporate the water, creating an energy trap of "heat release during cooling - heat absorption during furnace return." Simultaneously, submicron-sized sulfuric acid mist aerosols and heavy metal salt aerosols in the flue gas cannot be effectively retained, leading to poisoning and failure of downstream vanadium catalysts and the generation of hazardous heavy metal waste. During rapid water cooling, the flue gas inevitably passes through a dioxin regeneration temperature zone of 200-400°C, requiring additional end-of-pipe treatment equipment. The severe electrochemical corrosion caused by the alternating wet and dry interface is also the root cause of frequent equipment leaks. The root of these defects all points to a long-standing technical bias—that high-temperature pyrolysis flue gas must be operated at atmospheric pressure and rapidly cooled with water to achieve gas-liquid separation. This bias constitutes a long-standing technical barrier that the industry has been unable to overcome.

[0004] Therefore, this invention proposes a closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid. Summary of the Invention

[0005] To address the aforementioned problems, this application provides a closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid, employing the following technical solution: A closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid includes the following steps: S1. Waste sulfuric acid and oxygen-containing gas are fed into a pressurized pyrolysis furnace and pyrolyzed at a pressure of 0.5~3.0 MPaG gauge pressure and 850~1050℃ to generate high-temperature flue gas containing SO3, water vapor and molten metal salts. S2. The high-temperature flue gas is subjected to high-temperature gas-solid separation while maintaining the pressure to remove molten metal salts and obtain purified flue gas. S3. The purified flue gas is introduced into a pressurized high-temperature phase change condenser while maintaining the pressure. The temperature gradient of the flue gas is reduced to 220~350℃ by controlling the cooling intensity. Within this temperature range, the sulfuric acid vapor formed by SO3 and water vapor is directly condensed into liquid concentrated sulfuric acid due to the increase in dew point, while the water vapor remains in the gaseous state. Thus, a concentrated sulfuric acid product with a concentration of 85~98wt% and tail gas containing SO2 and water vapor are separated in one step. A portion of the concentrated sulfuric acid is returned to the cracking furnace to adjust the furnace temperature. S4. The exhaust gas is introduced into a turbine expander, and its pressure energy and heat energy are used to expand and do work, driving the compressor to generate the oxygen-containing gas at the pressure required in step S1 of the subsequent production process. S5. Use the waste heat from the exhaust gas after expansion work to preheat the waste sulfuric acid in the feed.

[0006] Preferably, the gauge pressure of the pyrolysis reaction in step S1 is further controlled to be 1.0~2.5 MPaG.

[0007] Preferably, the high-temperature gas-solid separation in step S2 uses a high-temperature ceramic cyclone separator or a high-temperature electrostatic precipitator, with an operating temperature of 750~900℃.

[0008] Preferably, the high-temperature ceramic cyclone separator or high-temperature electrostatic precipitator is equipped with a differential pressure monitoring device. When the differential pressure exceeds a preset threshold, the pulse backflushing nozzle is automatically activated, and the oxygen-containing gas compressed in step S4 is used to perform online reverse pulse cleaning of the inner wall of the separator. The separator continues to run during the cleaning process.

[0009] Preferably, the pressurized high-temperature phase change condenser in step S3 is a microchannel heat exchanger, and the substrate of the microchannel heat exchanger in contact with the flue gas is Hastelloy or silicon carbide, and the heat exchange surface is provided with a superhydrophobic acid micro-nano composite structure.

[0010] Preferably, the superhydrophobic micro / nano structure is formed by femtosecond laser induction on the surface of Hastelloy or silicon carbide substrate, so that the concentrated sulfuric acid generated by condensation is rapidly stripped off by high-speed airflow in the form of spherical droplets, and the droplets do not have long-term contact with the heat exchange surface.

[0011] Preferably, the concentration of the concentrated sulfuric acid in step S3 reaches 93~98wt%, part of which is extracted as finished product, and the amount of concentrated sulfuric acid returned is adjusted proportionally according to the furnace temperature detection value of the cracking furnace.

[0012] Preferably, the oxygen-containing gas in step S4 is compressed air or oxygen-enriched air, and the turbine expander and the compressor are a coaxial integrated unit; Furthermore, the coaxial integrated unit is equipped with a variable frequency backup drive motor; when the turbine's work is insufficient or malfunctions, the backup drive motor independently drives the compressor to stabilize the system pressure.

[0013] Preferably, after step S5, the process further includes: dry desulfurization treatment of the tail gas after preheating the waste sulfuric acid until it meets the emission standards before being discharged.

[0014] Preferably, before step S1, the waste sulfuric acid is first preheated to 100~200°C by the waste heat of the tail gas recovered in step S5, and then sent to the pyrolysis furnace.

[0015] In summary, this application includes at least one of the following beneficial technical effects: 1. The production of dilute sulfuric acid is eliminated from the process mechanism level. By increasing the operating pressure to 0.5~3.0 MPaG gauge pressure, and utilizing the physical law that the dew point of sulfuric acid increases with increasing pressure, sulfuric acid vapor is directly condensed into 85~98wt% concentrated sulfuric acid product in the high-temperature range of 220~350℃, while water vapor remains in a gaseous state and is discharged with the tail gas. This fundamentally avoids the problem of 10~30wt% dilute sulfuric acid inevitably generated by the traditional water-cooling rapid cooling process, and breaks through the technical prejudice that the industry has long followed that "high-temperature pyrolysis flue gas must be water-cooled rapidly to complete gas-liquid separation".

[0016] 2. Significantly reduced system energy consumption. The entire process is water-free, with flue gas sensible heat recovered through gradient cooling and staged recovery—the high-temperature section produces concentrated sulfuric acid and byproduct steam, the medium-temperature section preheats the feed waste acid, and the residual pressure energy and thermal energy drive the compressor via a turbine expander to generate oxygen-containing gas at the pressure required for step S1. This achieves tiered internal energy recycling. Under normal operating conditions, the external natural gas supplementary heating consumption is significantly reduced compared to traditional atmospheric pressure processes, eliminating the double energy waste of "cooling and releasing heat - returning to the furnace and absorbing heat" in traditional processes.

[0017] 3. In-situ high-temperature dry separation of heavy metal salts has been achieved. High-temperature gas-solid separation is carried out at 750~900℃. The metal salts are removed in a molten state and do not enter the subsequent condensation system. This avoids the problem in traditional processes where all heavy metal salts are mixed with dilute acid and can only be disposed of as hazardous waste. The separated molten salts can be directly used as smelting raw materials, realizing the high-value recovery of metal resources.

[0018] 4. Solved the corrosion problem at the dry-wet interface. The entire process has no cooling water in contact with the process gas, eliminating the severe electrochemical corrosion caused by the alternating contact between high-temperature flue gas and low-temperature dilute acid in the dry-wet interface area of ​​the condenser and demister in traditional processes; in the core condensation equipment, a dynamic anti-corrosion mechanism of superhydrophobic acid micro-nano surface combined with airflow purging is further adopted, enabling the equipment to operate stably for a long period of time.

[0019] 5. By using a variable frequency backup drive motor to power the coaxial integrated turbine expander-compressor unit, the backup motor can independently drive the compressor to stabilize the system pressure when the turbine's work is insufficient or malfunctions. This ensures that the core process condition of "maintaining pressure" is not affected by fluctuations in the expander's work, significantly improving the unit's operational flexibility and reliability. Attached Figure Description

[0020] Figure 1 This is a flowchart of a closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid, as described in an embodiment of this application. Detailed Implementation

[0021] The following is in conjunction with the appendix Figure 1 This application will be described in further detail.

[0022] This application discloses a closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid. (Refer to...) Figure 1 A closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid includes the following steps: S1. Pressure pyrolysis: Waste sulfuric acid and oxygen-containing gas are fed into a pressure pyrolysis furnace and pyrolyzed at a gauge pressure of 0.5~3.0 MPaG and a temperature of 850~1050℃ to generate high-temperature flue gas containing SO3, water vapor and molten metal salts. S2. High-temperature gas-solid separation: The high-temperature flue gas is subjected to high-temperature gas-solid separation while maintaining the pressure to remove molten metal salts and obtain purified flue gas. S3. Pressurized high-temperature phase change condensation: The purified flue gas is introduced into a pressurized high-temperature phase change condenser while maintaining the pressure. The temperature gradient of the flue gas is reduced to 220~350℃ by controlling the cooling intensity. Within this temperature range, the sulfuric acid vapor formed by SO3 and water vapor is directly condensed into liquid concentrated sulfuric acid due to the increase in dew point, while the water vapor remains in the gaseous state. Thus, concentrated sulfuric acid product with a concentration of 85~98wt% and tail gas containing SO2 and water vapor are separated in one step. A portion of the concentrated sulfuric acid is returned to the cracking furnace to adjust the furnace temperature. S4. Exhaust gas expansion and work: The exhaust gas is introduced into a turbine expander and its pressure energy and heat energy are used to expand and do work, driving the compressor to generate the oxygen-containing gas at the pressure required in step S1 of the subsequent production process. S5. Waste heat recovery from tail gas: The waste heat from the tail gas after expansion and work is used to preheat the waste sulfuric acid in the feed.

[0023] The core innovation of this process lies in increasing the operating pressure of the entire pyrolysis and gas-liquid separation process from the industry's long-standing practice of atmospheric or slightly negative pressure to 0.5~3.0 MPaG gauge pressure. By utilizing the physical law that "the dew point of sulfuric acid vapor increases with pressure", the flue gas can selectively condense sulfuric acid vapor in a high-temperature range of 220~350℃. This replaces the existing "water-cooled rapid cooling wet separation" logic with a "fully dry" gas-liquid separation logic, eliminating the production of dilute sulfuric acid and energy loss from the process mechanism level, and achieving a fully anhydrous closed-loop regeneration process.

[0024] Compared with the existing atmospheric pressure pyrolysis-water-cooling wet separation process, the industrial waste sulfuric acid high-temperature pyrolysis and gas-liquid separation closed-loop regeneration process provided in this embodiment has the following advantages: (1) The production of dilute sulfuric acid is eliminated from the perspective of process mechanism. By increasing the operating pressure to 0.5~3.0 MPaG gauge pressure, and utilizing the physical law that the dew point of sulfuric acid increases with increasing pressure, sulfuric acid vapor is directly condensed into 85~98wt% concentrated sulfuric acid product in the high temperature range of 220~350℃, while water vapor remains in a gaseous state and is discharged with the tail gas, fundamentally avoiding the problem of 10~30wt% dilute sulfuric acid inevitably generated by the traditional water-cooled rapid cooling process. This all-dry separation logic breaks through the technical prejudice that the industry has long followed that "high-temperature pyrolysis flue gas must be water-cooled rapidly to complete gas-liquid separation".

[0025] (2) Significantly reduced system energy consumption. In traditional processes, the sensible heat of high-temperature flue gas is carried away by cooling water, and the dilute sulfuric acid produced by rapid cooling needs to be evaporated again after returning to the cracking furnace, resulting in a double energy waste of "cooling and releasing heat - returning to the furnace and absorbing heat". This process has no water cooling throughout, and the sensible heat of flue gas is recovered in stages through gradient cooling - concentrated sulfuric acid is produced in the high-temperature section and steam is produced as a by-product, the feed waste acid is preheated in the medium-temperature section, and the residual pressure energy and thermal energy drive the compressor through the turbine expander, realizing the cascade internal circulation utilization of energy. Under normal operating conditions, the external natural gas supplementary heating consumption is greatly reduced compared with the traditional atmospheric pressure process.

[0026] (3) In-situ high-temperature dry separation of heavy metal salts has been achieved. The high-temperature gas-solid separation is carried out at 750~900℃. The metal salts are removed in a molten state and do not enter the subsequent condensation system, thus avoiding the problem in traditional processes where all heavy metal salts are mixed into dilute acid and can only be disposed of as hazardous waste. The separated molten salts can be directly used as smelting raw materials, realizing the high-value recovery of metal resources.

[0027] (4) It inhibits the secondary synthesis of dioxins at the source. The flue gas temperature throughout the process is always maintained above 220℃, completely avoiding the dioxin regeneration temperature window of 200~400℃. There is no need to configure an additional terminal activated carbon adsorption device, which reduces equipment investment and operating costs.

[0028] (5) Solved the corrosion problem at the dry-wet interface. The entire process has no cooling water in contact with the process gas, eliminating the severe electrochemical corrosion caused by the alternating contact between high-temperature flue gas and low-temperature dilute acid in the dry-wet interface area of ​​the condenser and demister in the traditional process; in the core condensation equipment, a dynamic anti-corrosion mechanism of superhydrophobic micro-nano surface combined with airflow purging is further adopted, which enables the equipment to operate stably for a long period of time.

[0029] The following detailed description of each step is provided in conjunction with specific embodiments.

[0030] Step S1, pressurized pyrolysis, specifically involves: Industrial waste sulfuric acid and oxygen-containing gas are fed into a pressurized pyrolysis furnace and pyrolyzed at a gauge pressure of 0.5~3.0 MPaG and a temperature of 850~1050℃ to generate high-temperature flue gas containing SO3, water vapor and molten metal salts.

[0031] Industrial waste sulfuric acid originates from sources including, but not limited to, alkylation waste sulfuric acid, titanium dioxide production waste sulfuric acid, and dye intermediate waste sulfuric acid. Taking typical alkylation waste sulfuric acid as an example, its main components are a sulfuric acid concentration of 75-90 wt%, an organic matter content of 3-8 wt%, and a moisture content of 5-15 wt%, and it also contains impurities such as metal salts like Na, K, Fe, Pb, and As. Before entering the cracking furnace, the waste sulfuric acid is preheated to 100-200°C by the waste heat recovered from the tail gas in step S5 to recover low-temperature waste heat and reduce the heat consumption required for moisture evaporation within the cracking furnace.

[0032] The oxygen-containing gas is preferably compressed air or oxygen-enriched air, supplied by a compressor driven by a turbine expander in step S4. Waste sulfuric acid and oxygen-containing gas are injected into the pyrolysis furnace through a dual-fluid atomizing nozzle, and atomized into fine droplets under pressurized conditions to increase the reaction surface area.

[0033] The pyrolysis furnace is a vertical cylindrical reactor designed for pressure resistance. It is lined with high-temperature and acid-resistant corundum or chromium corundum refractory materials, and the outer shell is made of carbon steel or low-alloy steel pressure-bearing material. The internal operating gauge pressure is controlled at 0.5~3.0 MPaG, preferably 1.0~2.5 MPaG. Compared with the operating pressure of traditional atmospheric pressure pyrolysis furnaces (approximately 0~0.05 MPaG), the pressurized conditions increase the reactant concentration, accelerate the pyrolysis reaction rate, and allow for a 30%~50% reduction in furnace volume for the same processing capacity.

[0034] The pyrolysis reaction temperature is controlled between 850 and 1050°C. At this temperature, sulfuric acid molecules in the waste sulfuric acid decompose into SO3 and H2O, organic matter is oxidized into CO2 and H2O, and sulfur elements in organic sulfonates and sulfur-containing organic matter are converted into SO2 or SO3. Metal salts such as Na, K, Fe, Pb, and As contained in the waste sulfuric acid melt into liquid at around 800 to 900°C and are carried into the subsequent high-temperature gas-solid separation step with the flue gas.

[0035] In a preferred embodiment, the pressurized pyrolysis furnace is equipped with multiple pressure maintenance and safety response mechanisms. During normal operation, the system pressure is maintained within the target range of 0.5~3.0 MPaG by the compressor driven by the turbine expander in step S4 or the variable frequency backup drive motor. When the system pressure is detected to be below 0.5 MPaG, the control system prioritizes increasing the compressor speed or increasing the output of the variable frequency backup drive motor to restore the pressure. If the pressure still cannot be restored and continues to drop to the set safety lower limit, the waste sulfuric acid feed is interlocked and the oxygen-containing gas flow is increased for purging and cooling to ensure safe shutdown of the system under abnormal operating conditions. Unlike the prior art which operates under normal pressure / slight negative pressure, this invention actively locks the system in a pressurized state. Therefore, both pressure maintenance and safety interlocks are designed with "pressure preservation first" as the design principle, rather than simple pressure relief and discharge.

[0036] Step S2, high-temperature gas-solid separation, specifically involves: The high-temperature flue gas generated in step S1 is subjected to high-temperature gas-solid separation while maintaining the pressure to remove molten metal salts and obtain purified flue gas.

[0037] The phrase "maintaining the aforementioned pressure" means that throughout the entire process of high-temperature flue gas entering the gas-solid separation equipment from the pyrolysis furnace outlet and exiting from the gas-solid separation equipment outlet, the system pressure is consistently maintained within the target operating range of 0.5~3.0 MPaG, without any active pressure relief or reduction operations. It should be noted that "maintaining the aforementioned pressure" does not require an absolutely constant pressure value. In industrial plants, gas flow through pipelines and equipment inevitably results in a certain pressure drop (usually controlled within 0.05~0.2 MPa). As long as the separator inlet pressure remains above 0.5 MPaG, it falls within the range of "maintaining the aforementioned pressure." To achieve this, the connecting pipeline between the pyrolysis furnace outlet and the gas-solid separator should have a larger diameter to reduce pressure drop along the pipeline and minimize unnecessary bends and valves. The separator's structural design must also consider sealing reliability under high-pressure conditions. For example, high-temperature ceramic cyclone separators use flange seals combined with high-temperature resistant metal gaskets, and high-temperature electrostatic precipitators use a combination structure of corundum insulating sleeves and a pressure vessel shell.

[0038] The operating temperature for high-temperature gas-solid separation is 750~900℃, which is higher than the melting point of the metal salts in the flue gas, ensuring that the metal salts are separated in a molten liquid state. The separation equipment uses a high-temperature ceramic cyclone separator or a high-temperature electrostatic precipitator.

[0039] When a high-temperature ceramic cyclone separator is used, the centrifugal force generated by the high-speed rotation of dust-laden flue gas inside the cyclone disperses molten metal salt droplets, which have a density much greater than that of gas, against the wall surface. These droplets then flow along the wall to the bottom salt collection hopper and are discharged continuously or intermittently through the discharge port. The inner wall of the high-temperature ceramic cyclone separator is made of reaction-sintered silicon carbide or silicon nitride bonded silicon carbide material, which can withstand temperatures above 900℃ and corrosion from acidic flue gas.

[0040] When a high-temperature electrostatic precipitator is used, a high-voltage DC electric field is used to charge the molten metal salt droplets in the flue gas, which then migrate and are captured by the collecting electrode under the action of the electric field force. The collecting electrode is made of conductive ceramic or silicon carbide with silicon-doped surface, which maintains good conductivity and corrosion resistance at high temperatures.

[0041] In addition, to ensure that the molten salt does not solidify and blockage during the discharge process, the entire discharge pipeline is equipped with electric heating or high-temperature heat medium heating, and the heating temperature is maintained above the salt melting point (usually 500~700℃ depending on the composition of the metal salt). The outer wall of the pipeline is covered with an insulation layer. The discharge port is also equipped with a high-temperature SO2 purging interface. When the pipeline pressure difference shows an upward trend, part of the high-temperature tail gas drawn from the outlet of the condenser in step S3 is introduced for purging to ensure that the salt discharge channel remains unobstructed. The discharged molten salt enters a storage silo equipped with insulation or heating devices, where it is temporarily stored under insulation conditions. It is then sent out in a molten state or in a cooled and solidified form according to the subsequent resource utilization needs.

[0042] In a preferred embodiment, the high-temperature gas-solid separator is equipped with a differential pressure monitoring device. This device monitors the pressure difference between the separator's inlet and outlet in real time. When the pressure difference exceeds a preset threshold (e.g., more than 1.5 times the normal operating pressure difference or an absolute value exceeding a certain set value), it indicates that molten salt has adhered or ash has accumulated on the inner wall of the separator. At this time, the pulse backflushing nozzle is automatically activated, using the partially compressed oxygen-containing gas from step S4 to perform online reverse pulse cleaning of the separator's inner wall. The backflushing gas comes from a branch leading from the compressor outlet in step S4, without introducing external media and without disrupting the system's all-dry closed-loop properties. During the cleaning process, the separator continues to operate without needing to be shut down. The frequency, duration, and pressure of the pulse backflushing can be automatically adjusted based on the differential pressure monitoring value.

[0043] After high-temperature gas-solid separation, the removal rate of molten metal salts in the flue gas is greatly improved, and the metal salt content in the purified flue gas is significantly reduced. This effectively avoids the contamination of concentrated sulfuric acid products by metal salts in the subsequent condensation process and also provides conditions for the resource utilization of metal salts. The discharged molten salt can be cooled and solidified as a smelting raw material, or it can be directly fed into subsequent hydrothermal leaching or pyrometallurgical units in a high-temperature molten state to extract valuable metals.

[0044] Step S3, pressurized high-temperature phase change condensation, specifically involves: The purified flue gas obtained in step S2 is introduced into a pressurized high-temperature phase change condenser while maintaining the pressure. The temperature gradient of the flue gas is reduced to 220~350℃ by controlling the cooling intensity. Within this temperature range, the sulfuric acid vapor formed by SO3 and water vapor is directly condensed into liquid concentrated sulfuric acid due to the increase in dew point, while the water vapor remains in the gaseous state. Thus, a concentrated sulfuric acid product with a concentration of 85~98wt% and tail gas containing SO2 and water vapor are separated in one step. A portion of the concentrated sulfuric acid is then returned to the cracking furnace to adjust the furnace temperature.

[0045] This step is the core step of this invention. Its theoretical basis lies in the fact that the dew point temperature of the sulfuric acid-water vapor system increases significantly with increasing total system pressure. In traditional atmospheric pressure processes (approximately 0.1 MPaA), the dew point temperature of sulfuric acid vapor in the flue gas is about 200-230°C. However, the sulfuric acid condensed at this dew point temperature has an extremely low concentration and is highly corrosive, making it unrecoverable as a product. Therefore, traditional processes necessitate further cooling the flue gas to 40-60°C and absorbing it with a large amount of water, inevitably producing dilute sulfuric acid. Under the gauge pressure conditions of 0.5-3.0 MPaG in this invention, the dew point temperature of sulfuric acid vapor is raised to the range of 220-350°C. Taking a gauge pressure of 2.0 MPaG as an example, the dew point temperature of sulfuric acid vapor is approximately in the range of 270-290°C. Within this temperature window, the sulfuric acid vapor reaches a supersaturated state and condenses into liquid sulfuric acid, while the partial pressure of water vapor is much lower than the vapor pressure of other gases. Therefore, the water vapor remains essentially gaseous, achieving "selective" phase separation of sulfuric acid and water.

[0046] The term "gradient cooling" refers to a controlled, gradual decrease in flue gas temperature along the flow direction, rather than the sudden cooling achieved through water cooling in traditional processes. Gradient cooling can be achieved through multiple heat exchange modules connected in series. For example, a pressurized high-temperature phase change condenser can be divided into 3-5 heat exchange sections along the flue gas flow direction. Each section is supplied with a cooling medium (such as heat transfer oil, steam, or preheated boiler feedwater) at different temperatures or flow rates, allowing the flue gas temperature to gradually and smoothly decrease from 700-850℃ at the inlet to 220-350℃. The goal of controlling the cooling rate is to avoid localized supercooling points; typically, the average cooling rate is controlled to not exceed 50-100℃ / s, preferably not exceeding 20℃ / s. Compared with the existing water-cooled rapid cooling process (where flue gas drops rapidly from over 700°C to 40-60°C within seconds), the gradient cooling method of this process not only eliminates the double energy waste of "cooling and heat release - heat absorption in the furnace", but also ensures that the flue gas temperature remains above 220°C throughout the process, completely avoiding the dioxin regeneration temperature window of 200-400°C, thus inhibiting the secondary synthesis of dioxins from the source and eliminating the need for additional end-of-line activated carbon adsorption devices.

[0047] In a preferred embodiment, the pressurized high-temperature phase change condenser employs a microchannel heat exchanger. The microchannel heat exchanger consists of multiple stacked plates, with microchannels of equivalent diameter (0.3-3 mm) formed between the plates, carrying flue gas and cooling medium respectively. Compared to conventional shell-and-tube heat exchangers, the microchannel structure has an extremely high specific surface area and heat transfer coefficient, enabling precise temperature control and making it an ideal device for implementing gradient cooling.

[0048] The substrate for the side of the microchannel heat exchanger in contact with the flue gas is made of Hastelloy (such as Hastelloy C-276) or silicon carbide. Hastelloy exhibits excellent resistance to uniform corrosion and pitting corrosion in high-temperature sulfuric acid environments, while silicon carbide offers higher hardness, wear resistance, and a near-zero corrosion rate. To further enhance corrosion resistance, a superacid-repellent micro / nano composite structure is incorporated into the heat exchange surface. "Superacid-repellent" refers to a state where concentrated sulfuric acid droplets have a contact angle greater than 150° and a roll-off angle less than 10° on the heat exchange surface. The specific construction of the superacid-repellent micro / nano structure is existing technology and will not be elaborated here. This superacid-repellent micro / nano structure is induced by femtosecond laser on the surface of the Hastelloy or silicon carbide substrate, and its microstructure is a composite hierarchical structure of micron-scale protrusion arrays and nano-scale secondary structures. Typical femtosecond laser process parameters are: laser wavelength 1030nm or 515nm, pulse width 300fs~10ps, and energy density 0.5~5J / cm². 2The scanning speed is 100~1000mm / s. Under the action of the superacid-repellent surface, the concentrated sulfuric acid generated by condensation exists in the form of spherical droplets, which are quickly stripped off by the high-speed airflow and swept into the collection tank. The droplets do not have long-term contact with the heat exchange surface (the contact time is usually less than 0.1 seconds). This dynamic anti-corrosion mechanism fundamentally solves the corrosion problem of high-temperature and high-pressure concentrated sulfuric acid on the heat exchange surface of the condenser, enabling the microchannel heat exchanger to operate stably for a long period under this extreme condition.

[0049] In step S3, the concentration of concentrated sulfuric acid produced by condensation can be adjusted by controlling the condenser outlet temperature within the range of 220~350℃. When the outlet temperature is controlled in a higher range, the concentration of condensed sulfuric acid is relatively low (approximately 85~93wt%); when the outlet temperature is controlled in a lower range, the concentration of condensed sulfuric acid is relatively high (approximately 93~98wt%). Preferably, the concentration of concentrated sulfuric acid is controlled at 93~98wt%. Concentrated sulfuric acid within this concentration range can be sold directly as a commercial-grade finished product, or partially recycled.

[0050] The concentrated sulfuric acid produced by condensation is partially collected as finished product and sent to a storage tank, while the other portion is recycled back to the pyrolysis furnace in step S1. The recycled concentrated sulfuric acid is used to regulate the temperature field within the pyrolysis furnace. The principle is that the concentrated sulfuric acid decomposes and absorbs heat within the furnace, while simultaneously diluting the concentration of organic matter in the feed waste sulfuric acid, preventing the formation of localized hot spots. The recycling flow rate is proportionally controlled based on the furnace temperature detection value: when the furnace temperature is too high, the concentrated sulfuric acid recycling flow rate is increased to absorb more heat and suppress further temperature rise; when the furnace temperature is too low, the recycling flow rate is reduced to decrease the heat absorption load. This closed-loop control mechanism allows the pyrolysis furnace to maintain a stable operating temperature of 850~1050℃ even with fluctuations in the concentration of the incoming waste sulfuric acid.

[0051] After pressurized high-temperature phase change condensation in step S3, most of the SO3 and sulfuric acid vapor in the flue gas are recovered as concentrated sulfuric acid. The remaining tail gas mainly consists of N2, a small amount of O2, trace amounts of uncondensed water vapor, and a small amount of SO2 that was not completely converted into SO3 during the cracking reaction. This tail gas still has a pressure of 0.5~3.0 MPaG and a temperature of 220~350℃, and enters step S4 for energy recovery.

[0052] Step S4, the work done by exhaust gas expansion, specifically involves: The exhaust gas discharged in step S3 is introduced into a turbine expander, which uses its pressure energy and heat energy to expand and do work, driving the compressor to produce oxygen-containing gas at the pressure required in step S1.

[0053] When the exhaust gas enters the turbine expander, it is still under pressure and at a relatively high temperature, possessing a considerable enthalpy. Inside the turbine expander, the exhaust gas drives the impeller to rotate, converting its own pressure energy and thermal energy into mechanical work output. After expansion, the pressure of the exhaust gas drops to near atmospheric pressure (approximately 0~0.05 MPaG), and the temperature correspondingly drops to 100~150℃.

[0054] The output shaft of the turboexpander and the input shaft of the compressor are coaxially integrated, forming a coaxial integrated unit. This unit requires no additional gearbox or coupling; the expander directly drives the compressor, using the energy recovered from the exhaust gas directly to compress the oxygen-containing gas required for step S1, achieving efficient internal energy circulation. Under normal operating conditions, an energy self-balance is achieved between the cracking reaction and the work done by the exhaust gas expansion—the pressure and heat energy recovered from the exhaust gas are sufficient to drive the compressor to produce the compressed oxygen-containing gas required for step S1, without the need for additional external electrical energy or steam input.

[0055] In a preferred embodiment, the coaxial integrated unit is also equipped with a variable frequency backup drive motor. This backup drive motor is connected to the expander-compressor shaft system via a clutch or in direct series. Under normal operating conditions, the power provided by the expander is sufficient to drive the compressor, and the backup drive motor is in standby or low-output state. The backup drive motor automatically intervenes when one of the following situations occurs: (1) during the start-up phase of the pyrolysis furnace, there is not enough tail gas with sufficient pressure and flow to drive the expander; (2) fluctuations in the load of the pyrolysis furnace cause a temporary decrease in the tail gas flow or pressure, resulting in insufficient work done by the expander; (3) the expander or related pipelines fail and require maintenance. The variable frequency backup drive motor automatically adjusts its speed and output torque according to the actual power required by the compressor, independently drives the compressor to stabilize the system pressure, and ensures that the "maintaining pressure" condition in steps S1 to S3 is not affected by fluctuations in the work done by the expander. This design significantly improves the operational flexibility and reliability of the device.

[0056] Step S5, exhaust gas waste heat recovery, specifically involves: The waste heat from the exhaust gas after expansion and work in step S4 is used to preheat the waste sulfuric acid feed.

[0057] The exhaust gas temperature after expansion and work is approximately 100-150℃. Although this low-temperature waste heat cannot be used for work, it can still be used to preheat the feed waste sulfuric acid through an indirect heat exchanger. After being preheated to 100-200℃, the waste sulfuric acid is sent to the pyrolysis furnace in step S1, realizing the cascade recovery and utilization of low-temperature waste heat.

[0058] Thus, this process completes a closed-loop cycle from waste sulfuric acid feeding, pressurized pyrolysis, high-temperature desalination, high-temperature phase change condensation to produce concentrated acid, tail gas expansion for power recovery, and tail gas waste heat preheating for feeding. Throughout the entire process, the flue gas temperature remains above 220℃ from the pyrolysis furnace outlet to the tail gas discharge, completely avoiding the dioxin regeneration temperature window; there is no cooling water contact with the process gas throughout the process, fundamentally eliminating the generation of dilute sulfuric acid and preventing electrochemical corrosion conditions at the dry-wet interface; sulfur is exclusively recovered in the form of commercial-grade concentrated sulfuric acid, and metal salts are separated in situ in a high-temperature molten state. The technical effects of this process are further illustrated below through specific examples.

[0059] After completing the low-temperature waste heat recovery in step S5, the exhaust gas is sent to the dry desulfurization unit to meet emission standards; dry desulfurization is a mature existing technology. In this embodiment, the dry desulfurization process uses sodium bicarbonate dry powder injection desulfurization, with the desulfurizing agent injection temperature at 100~140℃. Sodium bicarbonate reacts fully with SO2 in the exhaust gas to generate sodium sulfate solid powder, which is collected by a bag filter dust collector; after treatment, the SO2 emission concentration in the exhaust gas is ≤30mg / Nm³. 3 Particulate matter emissions ≤10mg / Nm 3 The sodium sulfate produced as a byproduct of desulfurization can be recycled as a raw material for industrial salt, and no desulfurization wastewater is generated throughout the entire process.

[0060] Example 1 (Preferred pressure operation): The object to be treated is alkylation waste sulfuric acid, with a sulfuric acid concentration of 85 wt%, an organic matter content of 6 wt%, and impurities such as Na, K, and Fe salts.

[0061] In step S1, the waste sulfuric acid is first preheated to 150°C by the residual heat from the tail gas in step S5, and then fed into the pressurized pyrolysis furnace along with compressed air. The gauge pressure of the pyrolysis furnace is controlled at 2.0 MPaG, and the furnace temperature is 950°C. The high-temperature flue gas generated by pyrolysis is approximately 900°C and contains approximately 12 vol% SO3, approximately 18 vol% water vapor, and the remainder is N2 and a small amount of O2.

[0062] In step S2, the high-temperature flue gas enters the high-temperature ceramic cyclone separator with an inlet pressure of approximately 1.95 MPaG and an operating temperature of approximately 850°C. The pressure drop of the cyclone separator is approximately 0.05 MPa. Molten metal salts are continuously discharged from the bottom, greatly improving the removal rate of metal salts from the flue gas.

[0063] In step S3, the purified flue gas enters a microchannel heat exchanger-type pressurized high-temperature phase change condenser, with an inlet pressure of approximately 1.90 MPaG and a temperature of approximately 830°C. The flue gas flows sequentially through four heat exchange modules, where it is cooled to approximately 600°C, 420°C, 320°C, and 280°C, respectively. In the last two heat exchange modules, the sulfuric acid vapor reaches its dew point and condenses in large quantities. The concentrated sulfuric acid collected from the bottom collection tank of the condenser has a concentration of 96.5 wt%, with a metallic impurity content of less than 10 ppm. Approximately 60 wt% of the concentrated sulfuric acid is collected as the finished product, and approximately 40 wt% is returned to the cracking furnace for furnace temperature regulation. The condenser outlet tail gas temperature is approximately 280°C, the pressure is approximately 1.85 MPaG, and it contains approximately 2 vol% SO2 and approximately 3 vol% water vapor.

[0064] In step S4, the exhaust gas enters the turbo expander, expands to 0.02 MPaG, and the temperature drops to 125°C. The expander's output power accounts for approximately 85% of the total compression power requirement. The shortfall is supplemented by the variable frequency backup drive motor of the coaxial integrated unit. In step S5, the 125°C exhaust gas preheats the feed waste acid from room temperature to 150°C through a plate heat exchanger.

[0065] Under stable operating conditions in this embodiment, the system's natural gas supplementary heating consumption is significantly reduced compared to traditional atmospheric pressure processes (used only for start-up and peak shaving under extreme conditions), with no dilute sulfuric acid production, no process wastewater discharge, and SO2 emission concentration below 30 mg / Nm³ after dry desulfurization. 3 .

[0066] Example 2 (Lowest pressure end operation): This embodiment is essentially the same as Embodiment 1, except that the pressure of the cracking furnace is controlled at 0.5 MPaG in step S1. Under this pressure condition, the dew point temperature of sulfuric acid vapor in step S3 is approximately in the range of 230~250℃. The condenser outlet temperature is controlled at 240℃, and the concentration of the produced concentrated sulfuric acid is approximately 87wt%, which still fully meets the requirements for direct reflux to the cracking furnace or further concentration as an intermediate product. This embodiment demonstrates that even at the lowest pressure end of the claims, this process can still achieve selective condensation of sulfuric acid vapor and effective separation of water vapor.

[0067] Example 3 (Operation at the highest pressure end): This embodiment is basically the same as Embodiment 1, except that the pressure of the cracking furnace is controlled at 3.0 MPaG in step S1. Under this pressure condition, the dew point temperature of sulfuric acid vapor in step S3 is approximately in the range of 310~340℃. The condenser outlet temperature is controlled at 300℃, and the concentration of the produced concentrated sulfuric acid is approximately 98.2 wt%, meeting the first-class standard for commercial-grade industrial concentrated sulfuric acid. The higher operating pressure also further increases the output power of the tail gas expansion work in step S4, allowing the turbine expander to drive the compressor completely independently, while the variable frequency standby drive motor is mostly in a pure standby state.

[0068] Example 4 (Treatment of waste acid containing high concentration of heavy metal salts): This embodiment is basically the same as Embodiment 1, except that the metal salt content in the waste sulfuric acid is significantly higher. In step S2, the differential pressure of the cyclone separator was observed by the differential pressure monitoring device to rise to 1.6 times the normal value after 72 hours of continuous operation, exceeding the preset threshold. The system automatically triggered the pulse backflushing program, using part of the compressed air drawn from the compressor outlet in step S4 to pulse backflush the inner wall of the separator. The backflushing pressure was 2.1 MPaG, the pulse frequency was 3 times per minute, and it lasted for 5 minutes. After backflushing, the differential pressure returned to the normal range, and the separator remained online throughout the entire dust removal process without affecting subsequent processes.

[0069] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on these embodiments, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still combine, add, delete, or otherwise adjust the features of the various embodiments of the present invention according to the circumstances without conflict or creative effort, thereby obtaining different technical solutions that do not fundamentally depart from the concept of the present invention. These technical solutions also fall within the scope of protection of the present invention.

Claims

1. A closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid, characterized in that, Includes the following steps: S1. Waste sulfuric acid and oxygen-containing gas are fed into a pressurized pyrolysis furnace and pyrolyzed at a pressure of 0.5~3.0 MPaG gauge pressure and 850~1050℃ to generate high-temperature flue gas containing SO3, water vapor and molten metal salts. S2. The high-temperature flue gas is subjected to high-temperature gas-solid separation while maintaining the pressure to remove molten metal salts and obtain purified flue gas. S3. The purified flue gas is introduced into a pressurized high-temperature phase change condenser while maintaining the pressure. The temperature gradient of the flue gas is reduced to 220~350℃ by controlling the cooling intensity. Within this temperature range, the sulfuric acid vapor formed by SO3 and water vapor is directly condensed into liquid concentrated sulfuric acid due to the increase in dew point, while the water vapor remains in the gaseous state. Thus, a concentrated sulfuric acid product with a concentration of 85~98wt% and tail gas containing SO2 and water vapor are separated in one step. A portion of the concentrated sulfuric acid is returned to the cracking furnace to adjust the furnace temperature. S4. The exhaust gas is introduced into a turbine expander, and its pressure energy and heat energy are used to expand and do work, driving the compressor to generate the oxygen-containing gas at the pressure required in step S1 of the subsequent production process. S5. Use the waste heat from the exhaust gas after expansion work to preheat the waste sulfuric acid in the feed.

2. The closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid according to claim 1, characterized in that, The gauge pressure of the pyrolysis reaction in step S1 is further controlled to be 1.0~2.5 MPaG.

3. The closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid according to claim 1, characterized in that, The high-temperature gas-solid separation in step S2 uses a high-temperature ceramic cyclone separator or a high-temperature electrostatic precipitator, with an operating temperature of 750~900℃.

4. The closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid according to claim 3, characterized in that, The high-temperature ceramic cyclone separator or high-temperature electrostatic precipitator is equipped with a differential pressure monitoring device. When the differential pressure exceeds a preset threshold, the pulse backflushing nozzle is automatically activated. The oxygen-containing gas compressed in step S4 is used to perform online reverse pulse cleaning of the inner wall of the separator. The separator continues to run during the cleaning process.

5. The closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid according to claim 1, characterized in that, The pressurized high-temperature phase change condenser mentioned in step S3 is a microchannel heat exchanger. The substrate of the microchannel heat exchanger on the side in contact with the flue gas is Hastelloy or silicon carbide, and the heat exchange surface is provided with a superhydrophobic acid micro-nano composite structure.

6. The closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid according to claim 5, characterized in that, The superhydrophobic acid micro / nano structure is formed by femtosecond laser induction on the surface of Hastelloy or silicon carbide substrate, so that the concentrated sulfuric acid generated by condensation is rapidly stripped off by high-speed airflow in the form of spherical droplets, and the droplets do not have long-term contact with the heat exchange surface.

7. The closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid according to claim 1, characterized in that, In step S3, the concentration of concentrated sulfuric acid reaches 93-98 wt%. Part of it is extracted as finished product, and the amount of concentrated sulfuric acid returned is adjusted proportionally according to the furnace temperature detection value of the cracking furnace.

8. The closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid according to claim 1, characterized in that, The oxygen-containing gas mentioned in step S4 is compressed air or oxygen-enriched air, and the turbine expander and the compressor are a coaxial integrated unit; Furthermore, the coaxial integrated unit is equipped with a variable frequency backup drive motor; when the turbine's work is insufficient or malfunctions, the backup drive motor independently drives the compressor to stabilize the system pressure.

9. The closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid according to claim 1, characterized in that, Step S5 is followed by: dry desulfurization treatment of the tail gas after preheating the waste sulfuric acid until it meets the emission standards before being discharged.

10. The closed-loop regeneration process for high-temperature pyrolysis and gas-liquid separation of industrial waste sulfuric acid according to claim 1, characterized in that, Before step S1, the waste sulfuric acid is first preheated to 100~200°C by the waste heat of the tail gas recovered in step S5, and then sent to the pyrolysis furnace.

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