Method for hydrogenation denitrification treatment of ammoximation wastewater

Abstract

The present application relates to a kind of methods for treating organic nitrogen compounds in ammoxilation wastewater by hydrodenitrogenation, which comprises: under the catalytic effect of hydrogenation catalyst, ammoxilation wastewater is heated to above 80 DEG C, enters hydrogenation reactor, and under the action of catalyst, denitrification reaction occurs to organic nitrogen compounds represented by cyclohexanone oxime, cyclohexanone oxime is converted into cyclohexane, the process of converting organic nitrogen into water-soluble ammonium nitrogen is realized, and the removal efficiency of cyclohexanone oxime is more than 98%. After wastewater leaves hydrogenation kettle, cyclohexane and insoluble gas are flashed out at greater than 100 DEG C, and recovered for secondary use. The remaining wastewater enters subsequent biochemical treatment. The method of the present application can effectively reduce the content of organic nitrogen, does not produce hazardous waste sludge, the operation process is simple, the cost is low, and organic matter can be recycled.

Description

Technical Field

[0001] This invention relates to the treatment process of ammonia oxime wastewater in the production of caprolactam, and in particular to the hydrogenation denitrification treatment of nitrogen-containing organic compounds, represented by cyclohexanone oxime, in the wastewater. After hydrogenation denitrification, the difficult-to-treat organic nitrogen is converted into easily treatable water-soluble ammonium nitrogen. Background Technology

[0002] Caprolactam is an important organic chemical raw material. China's caprolactam production capacity is projected to reach 6.5 million tons by 2025, and is growing at a rate of approximately 10% annually. Currently, there are two main caprolactam production technologies: the traditional homogeneous ammonium oximation method and the recently emerging heterogeneous ammonium oximation method. Both processes generate a large amount of ammonium oximation wastewater. For a 300,000-ton / year caprolactam production unit, the oximation wastewater generation is 70-100 tons / hour, making it the primary source of wastewater in the entire caprolactam production process.

[0003] The wastewater following the ammonia oxime reaction contains recalcitrant nitrogen-containing organic compounds such as cyclohexanone oxime, nitrocyclohexane, and cyclohexylamine. Cyclohexanone oxime is the primary pollutant. Oxime functional groups can inhibit the synthesis of key structural substances in biological cell walls and block transpeptidase activity, resulting in extremely poor biochemical treatability for oxime-containing organic compounds. Ammonia oxime wastewater typically contains 50-300 mg / L of cyclohexanone oxime, with a chemical oxygen demand (COD) exceeding 10,000 mg / L, while its biological oxygen demand (BOD) is below 20 mg / L. This severe imbalance in the BOD / C ratio makes subsequent treatment difficult and costly. Currently, the main treatment method for this wastewater is advanced oxidation combined with biological treatment, primarily including the Fenton process and ozone oxidation.

[0004] The Fenton process requires the addition of iron salts, which generates a large amount of hazardous sludge that needs to be treated to render it harmless, resulting in high operating costs. For example, patent CN119612739 describes a treatment method using ferrous sulfate, requiring 2 kg of the agent per ton of water. For a 150,000-ton-per-year caprolactam production unit, approximately 3.84 tons of ferrous sulfate would need to be added daily, generating a large amount of sludge that must be disposed of as hazardous waste.

[0005] Ozone oxidation requires the installation of an ozone generator, and the wastewater needs to be concentrated and pH adjusted before entering the system. According to patent CN119707082A, a fixed-bed catalyst reaction is also required during ozone oxidation. Furthermore, ozone generation consumes a significant amount of electricity, necessitating the use of titanium alloy equipment, resulting in substantial investment. The ozone oxidation process exhibits poor selectivity for organic nitrogen conversion, with most nitrogen being directly converted into NO3. - NO2 - A carbon source will be needed in the subsequent biochemical treatment process.

[0006] The treatment of oxime wastewater has always been a challenge in the industry, and it is specifically classified as a recalcitrant organic nitrogen wastewater in the "Petrochemical Industry Pollutant Discharge Standard," which is subject to strict national control. This has gradually become one of the key factors restricting the development of the caprolactam industry. Summary of the Invention

[0007] To address the above problems, this invention provides a method for converting organically bound nitrogen in ammonia oxime wastewater into free, water-soluble ammonium nitrogen through hydrodenitrification, thereby improving the biochemical treatability of ammonia oxime wastewater.

[0008] The inventors discovered that in wastewater containing cyclohexanone oxime, under a hydrogenation catalyst, the cyclohexanone oxime undergoes a hydrogenolysis reaction, converting it into easily removable nitrogen. This process removes nitrogen from the compound structure, transforming it into water-soluble ammonium nitrogen. Although the total nitrogen content in the water does not change significantly, the nitrogen is transformed from difficult-to-degrade organic nitrogen into easily biochemically treatable ammonium nitrogen. The wastewater containing ammonium nitrogen then undergoes nitrification and denitrification biological treatment to meet effluent discharge standards. The main reactions occurring in this process are as follows: Preferably, the hydrogenation catalyst is a supported catalyst, with activated carbon or alumina as the support, and the active metal component is one or more of nickel, molybdenum, cerium, copper, cobalt, platinum, and palladium, accounting for 0.5%-25% of the catalyst weight. The hydrogenation catalyst also includes Raney nickel-based alloy catalysts, with a nickel content of 50%-90% in the Raney nickel alloy catalyst.

[0009] The supported hydrogenation catalyst was prepared according to the patent "A Hydrogenation Refining Catalyst for Caprolactam and Its Preparation Method and Application". Raney nickel catalyst is commercially available.

[0010] Preferably, the hydrogenation reactor includes a fixed-bed reactor and a slurry-bed reactor.

[0011] Preferably, the hydrogenation reaction temperature is 80-200℃, the reaction pressure is controlled at 1MPa-8MPa, the reaction time is 5min-40min, and the reaction space velocity is controlled at 2-7h. -1 The amount of hydrogen used is 0.1-1 m³ per cubic meter of wastewater.

[0012] After being discharged from the oxime unit, the wastewater undergoes hydrogenation via a heat exchanger, with the temperature increased to 80-200℃ before entering the reactor. If the reaction temperature is below 80℃, the hydrogenation and denitrification effect is incomplete, resulting in low oxime removal rates. If the reaction temperature is too high, over-hydrogenation can occur, leading to oil-water emulsification. The reactor is fed from the bottom, with wastewater and hydrogen fed together from the bottom. The treated aqueous phase is extracted from the middle of the reactor, while hydrogen and tail gas are piped out from the top and sent to the tail gas treatment unit. The reactor pressure is maintained by hydrogen. Denitrification efficiency decreases below 1 MPa, while above 8 MPa, the equipment investment is too high, resulting in poor economic benefits. Although the reaction is relatively rapid, the residence time is maintained at over 5 minutes to ensure sufficient conversion, and the reaction space velocity is controlled between 2-7 h⁻¹. -1 The hydrogen usage should be maintained at 0.1 m³ / ton of wastewater or higher to ensure sufficient hydrogen for nitrogen removal efficiency.

[0013] Preferably, the temperature of the flash tank is 100-140℃ and the pressure is 0.20-0.50MPa.

[0014] In oxime wastewater, cyclohexanone oxime is hydrogenated to cyclohexane or a small amount of cyclohexylamine. The tail gas contains a small amount of ammonia. After exiting the reactor, it enters a flash tank for desuperheating and depressurization, where most of the organic matter and non-condensable gases volatilize and leave the aqueous phase. The flash tank maintains sufficient temperature and pressure to recover most of the organic matter and non-condensable gases from the top of the gas phase. The recovered organic matter can be reused or incinerated, depending on its composition. The remaining gaseous tail gas is centrally recovered and fed into a tail gas treatment unit.

[0015] Preferably, the wastewater from the oxime treatment unit contains 50-300 mg / L of cyclohexanone oxime and <20 mg / L of biological oxygen demand (BOD). After treatment, the aqueous phase contains <2 mg / L of cyclohexanone oxime and has a BOD >900 mg / L.

[0016] The main source of the effluent from the oxime unit is the water remaining after the decomposition of hydrogen peroxide in the oxime reactor. Since hydrogen peroxide directly participates in the reaction, the effluent contains a high content of cyclohexanone oxime. The effluent from the reactor needs to be extracted with toluene and stripped to recover the organic matter (mainly cyclohexanone oxime). However, since the solubility of cyclohexanone oxime in hot water at 80°C exceeds 5% and the boiling point of cyclohexanone oxime is 155°C, the subsequent stripping cannot completely remove cyclohexanone oxime from the water.

[0017] Oxime compounds have bactericidal effects but low biological oxygen demand (BOD). The BOD of oxime-treated wastewater is generally below 20 mg / L, resulting in poor biodegradability. Direct entry into subsequent biological treatment systems would negatively impact microorganisms. This patent employs a hydrodenitrification method for treatment. After hydrodenitrification, the cyclohexanone oxime content in the wastewater is reduced to below 2 mg / L, achieving a removal efficiency of 98%, and realizing the directional conversion of organic nitrogen. The BOD is increased to over 900 mg / L, significantly improving the biodegradability of the wastewater.

[0018] Preferably, subsequent water treatment includes conventional nitrification and denitrification processes, ultimately converting the oxime wastewater into harmless nitrogen gas for discharge, thus completing the harmless treatment and discharge of the oxime wastewater.

[0019] Benefits This method features mild reaction conditions, with a reaction temperature below 200℃ and a reaction pressure not exceeding 8MPa.

[0020] This method does not introduce iron salts, aluminum salts, oxidants, or external carbon sources, and does not produce new sludge or salt deposits. The reaction system is clean, and the conversion rate of cyclohexanone oxime reaches 98%. It can achieve the directional conversion of organic nitrogen without large investments.

[0021] This method disrupts the structure of toxic oxime pollutants, significantly improving their biodegradability. Catalytic hydrogenation destroys functional groups, enhancing water biodegradability. It enables wastewater that was previously unable to be directly biodegraded to have stable biodegradable conditions, providing a solution for the harmless and biodegradable treatment of highly toxic chemical wastewater. It also overcomes the core challenge of the directional conversion of organic nitrogen into ammonium nitrogen. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0023] The following examples all use ammonia oxime wastewater from the ammonia oxime production unit. This wastewater comes from the bottom of the stripping tower in the ammonia oxime chemical section, with an outlet temperature of 105°C, a cyclohexanone oxime content of 115 mg / L, and a biological oxygen demand (BOD) of 11 mg / L. Example

[0024] The ammonia oxime wastewater discharged from the caprolactam unit is heated to 160°C via a heat exchanger. This wastewater, along with hydrogen gas, enters the fixed-bed reactor from the bottom. The reactor operates at a pressure of 2.2 MPa and contains a nickel-supported activated carbon hydrogenation catalyst with a nickel content of 18%. The hydrogenation residence time is 25 minutes, and the hydrogen consumption is 0.2 m³ / m³ of wastewater.

[0025] After hydrogenation, the wastewater enters a flash tank at a temperature of 110℃ and a pressure of 0.25MPa. Some organic materials and ammonia are recovered from the top. After exiting the flash tank, the cyclohexanone oxime content in the wastewater decreases to 0.2mg / L, and the biological oxygen demand reaches 988mg / L.

[0026] The wastewater then undergoes nitrification and denitrification to meet the discharge standards. Example

[0027] The ammonia oxime wastewater discharged from the caprolactam unit, after heat exchange in a heat exchanger, reaches a temperature of 160°C. This wastewater, along with hydrogen gas, enters the fixed-bed reactor from the bottom. The reactor pressure is 2.2 MPa, and it contains a nickel-supported alumina hydrogenation catalyst with a nickel content of 22%. The hydrogenation residence time is 25 minutes, and the hydrogen consumption is 0.2 m³ / m³ of wastewater.

[0028] After hydrogenation, the wastewater enters a flash tank at a temperature of 110℃ and a pressure of 0.25MPa. Some organic materials and ammonia are recovered from the top. After exiting the flash tank, the cyclohexanone oxime content in the wastewater decreases to 1 mg / L, and the biological oxygen demand reaches 915 mg / L.

[0029] The wastewater then undergoes nitrification and denitrification to meet the discharge standards. Example

[0030] The ammonia oxime wastewater discharged from the caprolactam unit, after heat exchange in a heat exchanger, reaches a temperature of 160°C. This wastewater, along with hydrogen gas, enters a stirred slurry bed reactor from the top. The reactor pressure is 2.5 MPa, and it contains a Raney nickel catalyst with a nickel content of 80%. The hydrogenation residence time is 20 minutes, and the hydrogen consumption is 0.2 m³ / m³ of wastewater.

[0031] After hydrogenation, the wastewater enters a flash tank at a temperature of 110℃ and a pressure of 0.25MPa. Some organic materials and ammonia are recovered from the top. After exiting the flash tank, the cyclohexanone oxime content in the wastewater decreases to 0.05mg / L, and the biological oxygen demand reaches 975mg / L.

[0032] The wastewater then undergoes nitrification and denitrification to meet the discharge standards. Example

[0033] The ammonia oxime wastewater discharged from the caprolactam unit, after heat exchange in a heat exchanger, reaches a temperature of 70°C. This wastewater, along with hydrogen gas, enters the fixed-bed reactor from the bottom. The reactor pressure is 0.04 MPa, and the reactor contains a hydrogenation catalyst with nickel-supported activated carbon and a nickel content of 18%. The hydrogenation residence time is 25 minutes, and the hydrogen consumption is 0.2 m³ / m³ of wastewater.

[0034] After hydrogenation, the wastewater enters a flash tank at a temperature of 110℃ and a pressure of 0.25MPa. Some organic materials and ammonia are recovered from the top. After exiting the flash tank, the cyclohexanone oxime content in the wastewater decreases to 55mg / L, and the biological oxygen demand reaches 521mg / L.

[0035] The wastewater then undergoes nitrification and denitrification to meet the discharge standards. Example

[0036] The ammonia oxime wastewater discharged from the caprolactam unit, after heat exchange in a heat exchanger, reaches a temperature of 210°C. This wastewater, along with hydrogen gas, enters the fixed-bed reactor from the bottom. The reactor pressure is 3.1 MPa, and the reactor contains a hydrogenation catalyst with nickel-supported activated carbon and a nickel content of 18%. The hydrogenation residence time is 25 minutes, and the hydrogen consumption is 0.2 m³ / m³ of wastewater.

[0037] After hydrogenation, the wastewater enters a flash tank at a temperature of 110℃ and a pressure of 0.25MPa. Some organic materials and ammonia are recovered from the top. Due to the excessively high reaction temperature and over-reaction of organic matter, emulsification occurs in the wastewater exiting the flash tank, posing difficulties for subsequent treatment. An additional oil-water separation device is needed to eliminate the emulsification. However, the cyclohexanone oxime content in the wastewater is reduced to 0.01 mg / L, and the biological oxygen demand reaches 1156 mg / L.

[0038] The wastewater then undergoes nitrification and denitrification to meet the discharge standards.

Claims

1. A method for hydrogenating nitrogen removal from ammonia oxime wastewater, characterized by the following steps: S1. Wastewater containing mainly cyclohexanone oxime from the ammonium oxime unit is heated and then directly fed into the hydrogenation reactor. Cyclohexanone oxime undergoes a hydrogenation denitrification reaction under the action of a hydrogenation catalyst, and nitrogen atoms are removed from the organic compound to form water-soluble ammonium nitrogen. S2. The wastewater after hydrogenation enters the flash tank to recover light components such as hydrogen, ammonia, and organic matter. S3. After the light components are recovered, the remaining wastewater phase enters the subsequent biochemical treatment system for further treatment.

2. According to claim 1, the hydrogenation catalyst used includes a supported catalyst made by supporting an active metal on activated carbon, alumina, or diatomaceous earth, or an alloy catalyst based on Raney nickel.

3. According to claim 2, the active metal component in the supported catalyst mainly includes one or more of nickel, molybdenum, cerium, copper, cobalt, platinum, and palladium, and the active metal component accounts for 0.5%-25% of the catalyst weight.

4. According to claim 2, the alloy catalyst used contains 50%-90% nickel.

5. According to claim 1, the hydrogenation reactor includes a fixed-bed reactor and a slurry-bed reactor.

6. According to claim 1, the hydrogenation reaction is carried out at a temperature of 80-200℃, a reaction pressure of 1MPa-8MPa, a reaction time of 5min-40min, and a reaction space velocity of 2-7h. -1 The amount of hydrogen used is 0.1-1 m³ per cubic meter of wastewater.

7. According to claim 1, the temperature of the flash tank is 100-140℃ and the pressure is 0.20-0.50MPa.

8. According to claim 1, the wastewater from the ammonia oxime treatment unit contains 50-300 mg / L of cyclohexanone oxime and has a biological oxygen demand (BOD) of <20 mg / L. After treatment, the cyclohexanone oxime content in the aqueous phase is <2 mg / L, and the BOD after treatment is >900 mg / L.

9. As described in claim 1, the subsequent treatment of the wastewater phase includes nitrification and denitrification treatment to complete the treatment of the wastewater.