Low-cost and low-energy consumption treatment process for acrylonitrile plant wastewater

Abstract

The application belongs to the technical field of wastewater treatment, and particularly relates to a low-cost and low-energy-consumption treatment process for acrylonitrile device wastewater. The low-cost and low-energy-consumption treatment process for acrylonitrile device wastewater sequentially processes acrylonitrile device wastewater through a first-stage organic nitrogen ammoniation device, a second-stage organic nitrogen ammoniation device, and then the wastewater is processed by a UV / H2O2 cyanide-breaking device, and then overflowed into a UASB anaerobic reactor for processing, and then processed by an A1 / A2 / O biochemical system for final discharge. The application successfully applies organic nitrogen ammoniation, UV / H2O2 advanced oxidation cyanide-breaking and UASB anaerobic reaction technology to acrylonitrile device wastewater treatment, solves the problem that existing acrylonitrile wastewater is not easy to be biochemically treated, and does not need to be invested with steam, significantly reduces wastewater treatment cost, and has remarkable energy-saving and carbon-reducing effects.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, specifically relating to a low-cost, low-energy-consumption treatment process for wastewater from an acrylonitrile plant. Background Technology

[0002] Acrylonitrile, a common chemical raw material, is an important monomer for synthetic fibers, synthetic rubber, and synthetic resins, and is widely used in industries such as acrylic fiber, nitrile rubber or latex, and ABS engineering resin. Existing acrylonitrile production facilities employ the propylene / ammonia oxidation process, which includes reaction, recovery, refining, and four-effect evaporation units. This technology has gained widespread application due to its short process flow, rational equipment structure, low investment, low material and energy consumption, and environmental protection measures.

[0003] The wastewater generated during the refining of acrylonitrile is called refined water. Currently, all acrylonitrile plants in China use a four-effect evaporator + stripping tower process for the industrial treatment of this wastewater. After the refined water is concentrated by the four-effect evaporator, the tube-side concentrate enters the incineration system; the shell-side condensate enters the light organic matter stripping tower, where ammonia and light components are further distilled off using low-pressure steam. The effluent is then mixed with hydrogen peroxide and enters the biological treatment system. The process of stripping off some ammonia and light components in the stripping tower reduces the content of some substances that are toxic to biological treatment. This means that the wastewater undergoes a pretreatment process that allows it to be effectively treated by biological treatment. The new environmental emission standard GB31571-2015 "Emission Standard of Pollutants from Petrochemical Industry" lists acrylonitrile as special wastewater. If the acrylonitrile production plant is located in a chemical industrial park, the stripping tower effluent, after biological treatment in the wastewater treatment plant, meets the water quality requirements acceptable to the wastewater treatment plant in the chemical industrial park. The qualified wastewater is then discharged into the chemical industrial park for further treatment and discharge with other wastewater. Direct discharge must meet the following standards: COD ≤ 100 mg / L, ammonia nitrogen ≤ 8 mg / L, total nitrogen ≤ 40 mg / L, TOC ≤ 30 mg / L, total phosphorus ≤ 1 mg / L, and suspended solids ≤ 70 mg / L. Whether the wastewater from the acrylonitrile refining process meets standards after biological treatment at the wastewater treatment plant depends not only on maintaining the efficient operation of the biological treatment system, but fundamentally on the wastewater's biodegradability.

[0004] Acrylonitrile production generates many byproducts, such as toxic or nitrogen-containing heterocyclic compounds like hydrogen cyanide, acetonitrile, butyronitrile, cyanopyridine, and pyrimidine. These can be broadly classified into organic nitriles and inorganic cyanides. Once these byproducts enter the subsequent biological treatment unit, they cause significant fluctuations in the final effluent quality. The nitrification unit is even more susceptible to these impacts. Furthermore, because acrylonitrile is itself an organic nitrogen compound, the concentrations of ammonia nitrogen and total nitrogen in the biological effluent fluctuate greatly, making it difficult to meet the latest wastewater total nitrogen discharge standards.

[0005] Studies have found that wastewater generated during the refining process in acrylonitrile production, even after further stripping via quadruple-effect evaporation and a stripping tower, still contains small amounts of long-chain or heterocyclic recalcitrant nitrogen-containing organic compounds with boiling points above 300℃. Most of these compounds contain cyanide groups, and such substances have biological anesthetic or toxic effects. Practical experience shows that fluctuations in the production process lead to fluctuations in the concentration of toxic substances in the stripping tower effluent. At lower concentrations, there is no significant impact on microbial removal of COD and ammonia nitrogen. However, at higher concentrations, these substances significantly inhibit nitrifying bacteria, leading to decreased nitrification efficiency and increased ammonia nitrogen concentration in the effluent. Furthermore, both the quadruple-effect evaporator and the stripping tower consume a significant amount of steam, effectively increasing carbon emissions.

[0006] Currently, methods for treating wastewater containing organic nitrile or inorganic cyanide that are still under research mainly include incineration, wet oxidation, polymerization removal, and reduction, but most are still in the experimental stage. Incineration and wet oxidation are both oxidation methods. Incineration uses natural gas as fuel to burn the wastewater at high temperatures, completely oxidizing the organic matter into CO2, H2O, and NO. x Wait, and NO x Discharge into the atmosphere causes secondary pollution and requires large amounts of natural gas, so incineration is energy-intensive, generates greenhouse gases, and causes secondary pollution. Wet oxidation, using catalysts to catalytically oxidize pollutants in wastewater under high temperature and pressure, still has low biodegradability in acrylonitrile wastewater and also suffers from high energy consumption. Both of these methods, along with the currently used four-effect evaporation + stripping process in industry, increase carbon emissions. Polymerization removal, applied to wastewater with high acrylonitrile content, involves adding a polymerization initiator to induce acrylonitrile to form PAN polymer, which then enters the biological treatment unit after flocculation and sedimentation. However, this requires a high concentration of acrylonitrile. In reality, the acrylonitrile content in the wastewater discharged from the acrylonitrile purification unit is only a few to tens of mg / L, making polymerization difficult. Even if polymerization occurs, although the toxicity is eliminated, the polymer is still difficult to biodegrade. The reduction method utilizes the addition reaction of reducing agents such as sodium sulfite with the double bond of acrylonitrile to produce sodium cyanoethyl sulfonate. However, this only masks the detection of acrylonitrile, and the toxic cyano group is not eliminated. Furthermore, the acrylonitrile content in the wastewater from acrylonitrile plants is not high, and most cyanide compounds do not have double bonds that can be added. Therefore, this method is not suitable for the pretreatment of this wastewater.

[0007] Patent CN103304096A discloses a method for treating acrylonitrile wastewater using ozone catalytic coupling with biological nitrification. The incoming water first enters an equalization tank for water quality adjustment, then flows through a pre-oxidation tower and a catalytic oxidation tower, followed by an ozone removal tank to remove residual ozone, and then a nitrification tank to remove ammonia nitrogen before being discharged from a clear water tank. This invention involves pre-oxidation and catalytic oxidation of the equalization tank effluent, increasing the operating cycle of the wastewater treatment plant, but it does not fundamentally solve the problem of wastewater biodegradability.

[0008] Utility model CN201120523601.2 discloses a wastewater treatment system for acrylonitrile and acrylic fiber plants. The system includes an alkaline hydrolysis treatment device connected to the acrylonitrile production wastewater discharge pipeline and a homogenizer / conditioner; a coagulation sedimentation or coagulation flotation device A connected to the polymerization process wastewater discharge pipeline and a homogenizer / conditioner; a homogenizer / conditioner connected to an aerobic biological treatment device, which is then connected to an advanced oxidation treatment device, which is connected to a hydrolysis acidification-aerobic device or an A / O device; and a coagulation sedimentation or coagulation flotation device B connected to the spinning process wastewater discharge pipeline and a hydrolysis acidification-aerobic device or an A / O device. The pretreated wastewater from each plant is then mixed with wastewater from other plants requiring no pretreatment for centralized treatment, achieving compliant discharge. This acrylonitrile production wastewater is the effluent from a stripping tower currently used in industry. It's essentially an additional alkaline hydrolysis step on top of the existing quadruple-effect evaporator + stripping tower process, failing to achieve energy conservation and carbon reduction goals. Furthermore, it differs drastically from the condensate from the quadruple-effect evaporator treated in this patent. The stripping tower effluent has a COD of approximately 1500–2500 mg / L, total nitrogen of approximately 400–600 mg / L, and ammonia nitrogen and total cyanide content each <5 mg / L; while the quadruple-effect evaporator condensate has a COD of approximately 2500–3500 mg / L, total nitrogen of approximately 700–900 mg / L, ammonia nitrogen of approximately 30–50 mg / L, and total cyanide of approximately 10–20 mg / L.

[0009] Patent CN201611112304.2 discloses a method for treating high-concentration organic cyanide-containing wastewater. The method involves diluting the wastewater, adding lime, stirring thoroughly, and controlling the pH value to 10-11. Hydrogen peroxide is then added at a concentration of (0.675-15)%, and the mixture is allowed to react for at least 30 minutes before entering the wastewater treatment process. If the treatment effect is not achieved, hydrogen peroxide is added continuously until the target is reached. This method is used to treat high-concentration organic cyanide-containing wastewater, including aniline acetonitrile production wastewater. The purpose of this patent is to remove cyanide and reduce COD. The hydrogen peroxide dosage reaches (0.675-15)%, but since hydrogen peroxide is acidic, this concentration already lowers the pH of the wastewater below 7. In practice, this method is not only costly but also poses a safety risk due to the decomposition of hydrogen peroxide producing large amounts of oxygen, making it difficult to achieve the goals of cyanide removal and COD reduction. Furthermore, the composition of this wastewater is fundamentally different from acrylonitrile wastewater, which contains both organic nitriles and inorganic cyanides.

[0010] The literature "Treatment of Cyanide-Containing Wastewater from Tin Plating by Hydrogen Peroxide Method" discloses that under alkaline conditions and with copper ions as a catalyst, hydrogen peroxide can remove free cyanide ions (CN). - Both the cyanide ions present in the presence of metal cyanide complexes and the cyanide ions will be oxidized to cyanate ions (CNO). -CNO - It is extremely unstable and, according to the citation, will generate ammonium ions and carbonate or bicarbonate ions.

[0011] The literature "Photo-oxidation of cyanide in aqueous solution by the UV / H2O2 process" and "Research on deep treatment of cyanide in coking wastewater by UV-H2O2 process" describes the use of UV-H2O2 to treat cyanide-containing wastewater. It details the effects of photo-oxidation at pH 10-11, and the influence of different power low-pressure lamps, hydrogen peroxide concentrations, and dissolved oxygen levels on cyanide removal. However, these literatures treat inorganic cyanides, which is fundamentally different from the acrylonitrile wastewater in this patent, which contains both organic nitriles and inorganic cyanides. Therefore, the processes in these literatures do not address the treatment of organic nitriles, and consequently, do not address the issue of the transition between organic nitriles and inorganic cyanides.

[0012] Patent CN202110640248.4 discloses a device and its method for removing organic matter from acrylonitrile wastewater using ultraviolet light. Patent CN202121279580.4 discloses a device for removing organic matter from acrylonitrile wastewater using ultraviolet light. The devices include a wastewater lift pump, a circulation pump, a pH adjusting buffer tank, an advanced oxidation reactor, a discharge tank, an ultraviolet advanced oxidation reactor, an alkali dosing system, a hydrogen peroxide dosing system, an iron salt catalyst dosing system, and a sulfuric acid dosing system. A pH adjusting buffer tank is installed on one side of the wastewater lift pump, and an alkali dosing system is installed on the side of the pH adjusting buffer tank, connected to the pH adjusting buffer tank. One end of the wastewater circulation pump is connected to the high-pressure ultraviolet advanced oxidation reactor via a pipeline. An advanced oxidation reactor is installed on the side of the high-pressure ultraviolet advanced oxidation reactor, and a hydrogen peroxide dosing system and an iron salt catalyst dosing system are respectively installed at one end of the high-pressure ultraviolet advanced oxidation reactor. One end of the discharge tank is connected to an outlet pipe. The invention and utility model patent utilize a combination of ultraviolet light, hydrogen peroxide, and iron salts as catalysts to remove organic matter from wastewater. However, in practice, adding iron salts under alkaline conditions produces a large amount of ferric hydroxide precipitate, and the hydrogen peroxide reaction generates oxygen very severely, with the oxygen volume concentration in the gas phase reaching 29-31%. Therefore, this process is neither reasonable nor safe to operate and lacks practical operational capability. Furthermore, the process involves the addition of large amounts of caustic soda and sulfuric acid, resulting in wastewater conductivity of 5000-7000 μS / cm, sulfate concentrations of 1500-2500 mg / L, and hydrogen peroxide residues of 60-100 mg / L in the treated water. Upon entering the wastewater treatment plant, this not only fails to ensure the stable operation of the biological treatment tanks, but the high sulfate and hydrogen peroxide levels also negatively impact the activity of the activated sludge. These problems render the process in this technical solution impractical.

[0013] In our practical research, we found that pollutants such as organic nitriles and inorganic cyanides in acrylonitrile wastewater are biotoxic and can inhibit the nitrification reaction in the biological system, affecting the effectiveness of biological denitrification. Without effective pretreatment, direct entry into the biological system, simply increasing denitrification packing materials or optimizing the type and method of carbon source addition, cannot improve the nitrification and denitrification effects. In summary, most studies have not fundamentally solved the problems of high energy consumption or difficulty in biochemical pretreatment of acrylonitrile plant wastewater, or the research lacks practicality.

[0014] For acrylonitrile plants, all wastewater within the plant area—including process wastewater, circulating water discharge, desalination station discharge, domestic wastewater, and initial rainwater—must pass through the wastewater treatment plant's biological treatment tank to meet discharge standards before being discharged. Improving the reuse rate of refining wastewater is essential for green, low-carbon, and sustainable development. Currently, wastewater reuse is receiving increasing attention both domestically and internationally. With rising national environmental protection requirements, a 60% wastewater reuse rate is needed, putting increasing pressure on water conservation, emission reduction, and compliance with discharge standards. This means that while circulating water discharge and desalination station discharge can be further reused, the concentrated wastewater from these reuses, known as "reused wastewater concentrate," also enters the wastewater treatment plant. This reduces the volume of wastewater originally entering the plant, increasing the proportion of process wastewater in the biological treatment tank and thus putting pressure on the wastewater treatment plant to meet carbon and nitrogen emission standards for discharge.

[0015] Therefore, without changing the scale of the existing wastewater treatment plant, current research can focus on the chemical properties of the characteristic pollutants in this wastewater to develop a safe, low-energy-consumption, and low-operating-cost pretreatment process to remove some nitrogen and carbon elements from the wastewater, reduce the nitrogen and carbon removal burden on subsequent biochemical processes, improve the biodegradability of the wastewater, and merge it with the concentrated wastewater after wastewater reuse into the biochemical treatment plant. After biochemical treatment, the discharged wastewater stably meets the direct discharge quality specified in GB31571-2015 "Emission Standard of Pollutants from Petrochemical Industry". Summary of the Invention

[0016] The technical problem to be solved by this invention is to provide a low-cost, low-energy-consumption treatment process for acrylonitrile plant wastewater. This process successfully applies organic nitrogen ammoniation, UV / H2O2 advanced oxidation to break down cyanide, and UASB anaerobic reaction technology to acrylonitrile plant wastewater treatment, solving the problem that existing acrylonitrile wastewater is not easily biochemically treated. Furthermore, it does not require the input of steam, significantly reducing wastewater treatment costs and achieving remarkable energy-saving and carbon reduction effects.

[0017] The low-cost, low-energy-consumption wastewater treatment process for acrylonitrile plants described in this invention includes the following steps:

[0018] The wastewater from the acrylonitrile unit is treated sequentially by a primary organic nitrogen ammoniation unit and a secondary organic nitrogen ammoniation unit, then enters a UV / H2O2 cyanide removal unit for further treatment. The overflow then enters a UASB anaerobic reactor for further treatment, followed by treatment through an A1 / A2 / O biological system (anaerobic / anoxic / aerobic biological system) before finally meeting discharge standards.

[0019] The wastewater from the acrylonitrile plant treated by this invention is the condensate from the fourth-effect evaporator in the acrylonitrile plant.

[0020] The ammonia generated in the primary organic nitrogen ammoniation unit, the secondary organic nitrogen ammoniation unit, and the UV / H2O2 cyanide removal unit all enter the ammonium sulfate recovery unit, which uses sulfuric acid to absorb the ammonia and generate ammonium sulfate. Preferably, the wastewater from the primary organic nitrogen ammoniation unit, the secondary organic nitrogen ammoniation unit, and the UV / H2O2 cyanide removal unit is purged with air, and the generated ammonia is carried into the ammonium sulfate recovery unit in the gas phase, where sulfuric acid is used to absorb the ammonia and generate ammonium sulfate. The ammonia generated in the primary organic nitrogen ammoniation unit is recovered by the ammonium sulfate recovery unit. This not only achieves the resource utilization of ammonia and prevents air pollution, but more importantly, it reduces the ammonia nitrogen content in the wastewater, providing suitable water quality for the secondary organic nitrogen ammoniation reaction and promoting the conversion efficiency of organic nitrogen to inorganic ammonia nitrogen.

[0021] Preferably, in the primary and secondary organic nitrogen ammoniation units, alkaline substances are added to adjust the pH of the wastewater to 11.5-13.0 and 11.0-12.5, respectively; the alkaline substances are preferably one or more combinations of sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate. In the primary organic nitrogen ammoniation unit, the wastewater undergoes an alkaline hydrolysis reaction, where some organic nitrogen is converted to ammonia nitrogen. The gaseous phase from the air stripping enters the ammonium sulfate recovery unit to recover the generated ammonia, while the liquid phase overflows into the secondary organic nitrogen ammoniation unit. After another alkaline hydrolysis, the remaining organic nitrogen continues to generate ammonia nitrogen, and the gaseous phase from the air stripping again passes through the ammonium sulfate recovery unit to recover the generated ammonia.

[0022] Preferably, the hydraulic retention time of the wastewater in both the primary and secondary organic nitrogen ammoniation units is 2-6 hours. This hydraulic retention time can be achieved by designing the volume of the organic nitrogen ammoniation tank and the influent flow rate, allowing the organic nitrogen in the wastewater to be fully hydrolyzed to generate ammonia, thus achieving the goal of converting organic nitrogen into inorganic ammonia.

[0023] Preferably, the hydrolysis temperature of the wastewater in both the primary organic nitrogen ammoniation unit and the secondary organic nitrogen ammoniation unit is 40-70℃.

[0024] Preferably, before the effluent from the secondary organic nitrogen ammoniation unit enters the UV / H2O2 cyanide removal unit, sulfuric acid is added to the pipeline at the outlet of the secondary organic nitrogen ammoniation unit through a pipeline mixer to adjust the pH value to 10.0-11.0 before entering the UV / H2O2 cyanide removal unit.

[0025] In the UV / H2O2 cyanide removal device, a certain amount of hydrogen peroxide is added to the wastewater within a certain temperature range and for a certain period of time. The ultraviolet light excites the hydrogen peroxide to produce highly oxidizing hydroxyl radicals, and the hydrogen cyanide in the inorganic nitrogen is oxidized and hydrolyzed into ammonia and carbonate by the UV / H2O2 device, thus eliminating the toxicity of the wastewater.

[0026] Preferably, the hydraulic retention time of the wastewater in the UV / H2O2 cyanide removal device is 2-5 hours. This hydraulic retention time can be achieved by designing the volume of the cyanide removal tank and the influent flow rate, so that the cyanide in the wastewater is fully oxidized, achieving the goal of converting the cyanide into inorganic ammonia and carbonates.

[0027] Preferably, the cyanide-breaking temperature of the wastewater in the UV / H2O2 cyanide-breaking device is 30-50℃.

[0028] Preferably, in the UV / H2O2 cyanide removal device, the light band is ultraviolet band, the oxidant is hydrogen peroxide, and the ultraviolet light promotes the generation of highly oxidizing hydroxyl radicals in hydrogen peroxide, which efficiently converts inorganic cyanide in wastewater into carbonate and ammonia nitrogen within a suitable pH range.

[0029] Preferably, the ultraviolet light includes ultraviolet light emitted by low-pressure and medium-pressure lamps, with medium-pressure ultraviolet lamps being the most preferred.

[0030] Preferably, the oxidant is a hydrogen peroxide aqueous solution with a concentration of 27-30 wt%, and the concentration of pure hydrogen peroxide added to the wastewater is 100-300 mg / L.

[0031] The denitrification process of this invention involves wastewater first entering a primary organic nitrogen ammoniation unit and a secondary organic nitrogen ammoniation unit, and then entering a UV / H2O2 cyanide removal unit. The ammonia generated in these two units is stripped by air and enters an ammonium sulfate recovery unit, thus achieving the resource utilization of ammonia without generating air pollution.

[0032] Preferably, before the effluent from the UV / H2O2 cyanide removal device overflows into the UASB anaerobic reactor, it is cooled to below 30°C, and sulfuric acid is added to the pipe at the outlet of the UV / H2O2 cyanide removal device through a pipe mixer to adjust the pH value to 6.5-7.5 before overflowing into the UASB anaerobic reactor.

[0033] The function of the UASB anaerobic reactor is to reduce the COD of wastewater, controlling it to 1000-1500 mg / L, thereby achieving partial carbon removal. The effluent quality of the UASB anaerobic reactor is as follows: pH 7.5-8.5, COD 1000-1500 mg / L, TOC 400-600 mg / L, total nitrogen 300-400 mg / L, ammonia nitrogen 10-30 mg / L, and BOD5 / COD ratio exceeding 0.6.

[0034] When the effluent from the UASB anaerobic reactor enters the A1 / A2 / O biological treatment system, it can be combined with the concentrated wastewater after wastewater reuse and then enter the A1 / A2 / O biological treatment system together for treatment. The ratio of the two water volumes is 1:(0.3~0.6).

[0035] Preferably, the effluent from the UASB anaerobic reactor is treated in the A1 / A2 / O biological system, with hydraulic retention times of 30-40h, 35-50h, and 30-40h, respectively.

[0036] The effluent from the A1 / A2 / O biological treatment system has the following parameters: COD 50–100 mg / L, TOC 20–30 mg / L, pH 7.0–7.60, total nitrogen 29–35 mg / L, ammonia nitrogen undetectable to 1.2 mg / L, total phosphorus 0.45–0.69 mg / L, suspended solids 10–20 mg / L, and total cyanide undetectable to 0.08 mg / L. All these parameters meet the direct discharge water quality requirements of GB31571-2015.

[0037] This invention addresses the technical challenges of low BOD5 / COD ratios and high energy consumption in the treatment of acrylonitrile purified water from quadruple-effect evaporation and stripping towers in existing acrylonitrile plants, resulting in low effluent biodegradability and unstable operation of the biological treatment system. It employs a low-cost, low-energy-consumption treatment process that removes some nitrogen and carbon from the condensate of the quadruple-effect evaporator through denitrification and decarbonization, reducing the burden on the subsequent biological treatment system and improving the wastewater biodegradability to above 0.6. After A1 / A2 / O biological treatment, the effluent meets the direct discharge quality standards of GB31571-2015. Specifically, the denitrification and decarbonization processes and biological treatment system refer to the wastewater undergoing denitrification followed by decarbonization before entering the traditional A1 / A2 / O biological treatment system; the denitrification-before-decarbonization process means the denitrification stage is performed first, with the overflowing effluent going to the decarbonization stage. The denitrification process includes organic nitrogen ammoniation, UV / H2O2 cyanide removal, and ammonium sulfate recovery. This process must be placed before the carbon removal process. After the denitrification process, the wastewater's organic nitrogen content is reduced, and the cyanide content drops below 1 mg / L, providing better water quality for the carbon removal process, thus achieving the desired carbon removal effect. If the carbon removal process is placed before it, the condensate from the quadruple-effect evaporator has extremely low biodegradability, making carbon removal impossible, and the entire process cannot continue. The carbon removal process refers to the process where the UASB anaerobic reactor receives wastewater that has undergone the above denitrification process, and organic carbon is converted into methane, achieving the goal of removing some of the organic carbon.

[0038] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0039] (1) This invention removes some of the nitrogen and carbon content in the condensate of the four-effect evaporator through denitrification and UASB anaerobic reaction process, removes total cyanide, reduces COD to below 1500 mg / L and total nitrogen to below 400 mg / L, and increases the BOD5 / COD ratio to above 0.6. After A1 / A2 / O biochemical process, the discharged wastewater stably meets the direct discharge quality specified in GB31571-2015 "Petrochemical Industry Pollutant Discharge Standard";

[0040] (2) This invention successfully applies organic nitrogen ammoniation, UV / H2O2 advanced oxidation cyanide destruction and UASB anaerobic reaction technology to the treatment of acrylonitrile plant wastewater, which solves the problem that existing acrylonitrile wastewater is not easy to biodegrade, and does not require the input of steam, greatly reducing wastewater treatment costs and achieving significant energy saving and carbon reduction effects. Attached Figure Description

[0041] Figure 1 This is a flow chart of a low-cost, low-energy-consumption process for treating wastewater from an acrylonitrile plant according to the present invention.

[0042] Figure 2 The intensity of compounds with different mass-to-charge ratios in the condensate of the four-effect evaporator of the acrylonitrile unit obtained by liquid-mass coupling of the present invention;

[0043] Figure 3 This is a flow chart of the wastewater treatment process of the acrylonitrile unit in Comparative Example 1 of the present invention.

[0044] Figure 4 This is a flow chart of the wastewater treatment process of the acrylonitrile unit in Comparative Example 2 of the present invention.

[0045] Figure 5 This is a flow chart of the wastewater treatment process of the acrylonitrile unit in Comparative Example 3 of the present invention. Detailed Implementation

[0046] The present invention will be further described below with reference to embodiments, but the scope of protection of the present invention is not limited thereto. Any changes made to the technical solutions of the present invention by those skilled in the art should fall within the scope of protection of the present invention.

[0047] Unless otherwise specified, the raw materials used in the embodiments are all commercially available conventional raw materials; unless otherwise specified, the process methods used in the embodiments are all conventional methods in the art.

[0048] The acrylonitrile plant wastewater treated in this invention is the condensate from the fourth-effect evaporator in the acrylonitrile plant. The characteristic pollutants in this wastewater are nitrogen-containing compounds. Since acrylonitrile production uses propylene, ammonia, and air as raw materials, the characteristic pollutants in the refined water are mostly compounds containing C, N, O, and H elements. Liquid chromatography-time-of-flight mass spectrometry (LC-MS) and gas chromatography-mass spectrometry (GC-MS) can be effectively applied to the detection of compounds containing these four elements, yielding... Figure 1 The intensity of compounds with different mass-to-charge ratios in the condensate of the four-effect evaporator of the acrylonitrile unit shown.

[0049] Through the Figure 1 Analysis of different mass-to-charge ratios yielded the molecular formulas and structural formulas of characteristic compounds in the condensate of the fourth-effect evaporator of the acrylonitrile unit, and the results are shown in Table 1.

[0050] Table 1. Analysis results of characteristic pollutants (obtained by liquid chromatography-mass spectrometry)

[0051]

[0052]

[0053]

[0054] As can be seen from Table 1, the characteristic compounds in the wastewater are mainly nitrogen-containing compounds, which is a significant feature of the process wastewater from the acrylonitrile plant. The molecular weights of these characteristic compounds range from tens to three hundred, and the nitrogen element exists in various forms, including -C≡N, five- or six-membered nitrogen-containing heterocycles, NH3, -NH2, or nitrogen on -NH-.

[0055] The content of some compounds can be determined by gas chromatography-mass spectrometry (GC-MS), as shown in Table 2.

[0056] Table 2. Content of characteristic compounds in condensate from the quadruple-effect evaporator determined by gas chromatography-mass spectrometry (GC-MS).

[0057] Characteristic compounds acrolein Acrylonitrile Butene dinitrile Cyanopyridine <![CDATA[Content (mg·L -1 )]]> 0.7～10.5 1.9～8.6 11～55 40～155

[0058] As shown in Tables 1 and 2, the nitrogen-containing compounds in the wastewater include organic nitriles, such as acrylonitrile and butenedionitrile; nitrogen-containing heterocyclic compounds, such as imidazoles, pyrimidines, pyridines, and piperazines; amides, such as acrylamide and bisacrylamide; or combinations of several nitrogen-containing groups, such as cyanopyridine, 1-(cyanoacetyl)pyrrolidine, and amino-2-hydroxy-5-methylpyridine, as well as other nitrogen-containing compounds whose molecular structures cannot yet be determined. The concentrations of each compound range from a few to hundreds of mg / L. The results of routine water quality analysis are shown in Table 3.

[0059] Table 3. Condensate quality of the quadruple-effect evaporator

[0060]

[0061] As can be seen from Table 3,

[0062] (1) In terms of carbon, the CODcr of this wastewater is as high as 2500 mg / L or more, while the inorganic carbon (IC) is only 20-30 mg / L, which is almost negligible. The total carbon (TC) is almost entirely TOC, reaching more than 1400 mg / L. The CODcr:TOC ratio in this water is only 1.7-1.8, far lower than 2.67, indicating that the TOC contained in it does not meet the conditions for complete oxidation.

[0063] (2) In terms of nitrogen, TN reached about 800 mg / L, while the total amount of NH3-N and NO3-N was about 30-50 mg / L, indicating that the majority of TN was organic nitrogen.

[0064] (3) The total cyanide in this wastewater is inorganic cyanide. The total cyanide is as high as 10-20 mg / L, and its high toxicity will seriously affect the microorganisms in the biological system and impact the wastewater treatment plant. Therefore, the total cyanide needs to be removed in the pretreatment process.

[0065] (4) The BOD5 / COD ratio was found to be below 0.004, indicating that its biodegradability was extremely poor.

[0066] This indicates that nitrogen-containing organic compounds are a significant characteristic pollutant in the condensate of the fourth-effect evaporator in an acrylonitrile unit. Nitrogen is present in nitrogen-containing heterocyclic compounds, organic nitrile compounds, and inorganic cyanide compounds, leading to high CODcr and total nitrogen levels in the wastewater. Organic nitrile compounds and inorganic cyanides are highly toxic or extremely toxic compounds, and are the main factors contributing to the extremely poor biodegradability of the purified water, with a BOD5 / COD ratio of only 0.004.

[0067] The condensate from the quadruple-effect evaporator treated in the following examples and comparative examples had the following water quality parameters: COD 2575 mg / L, TOC 1415 mg / L, TN 705 mg / L, NH3-N 39 mg / L, total cyanide 13.9 mg / L, and pH 6.8. Gas chromatography-mass spectrometry (GC-MS) revealed that the condensate contained acrolein 10.5 mg / L, acrylonitrile 6.7 mg / L, cyanopyridine 49.1 mg / L, and butadionitrile 50.6 mg / L.

[0068] Example 1

[0069] Organic nitrogen ammoniation stage:

[0070] In the primary organic nitrogen ammoniation unit, using the condensate outlet temperature of the quadruple-effect evaporator (65℃) as the reaction temperature, a 48% caustic soda solution is added to adjust the pH to 13.0. With a hydraulic retention time of 3 hours, the ammonia nitrogen content can increase to 197 mg / L, and the pH decreases to 12.1. The ammonia produced is recovered through an ammonium sulfate absorption unit, resulting in water with ammonia nitrogen of 21 mg / L, a pH of 12.0, and total nitrogen reduced to 511 mg / L. Acrolein, acrylonitrile, cyanopyridine, and butadiene nitrile are undetectable. This effluent then enters the secondary organic nitrogen ammoniation unit.

[0071] In the secondary organic nitrogen ammoniation unit, with the reaction temperature maintained at 65℃ and no further addition of caustic soda, and a hydraulic retention time of 4 hours, the ammonia nitrogen content increased to 148 mg / L, and the pH value decreased to 11.6. The ammonia produced was recovered by the ammonium sulfate absorption unit, resulting in water with ammonia nitrogen of 13 mg / L, a pH of 11.3, and total nitrogen reduced to 366 mg / L, achieving the nitrogen removal target. However, the total cyanide in this wastewater increased to 33.7 mg / L. This is because, under alkaline conditions, the -CN groups on some organic nitrile compounds were replaced to form more toxic inorganic cyanides, leading to the increase in total cyanide. This effluent then entered the UV / H2O2 cyanide removal unit.

[0072] The cyanide removal stage in a UV / H2O2 cyanide removal device for inorganic cyanides:

[0073] The water temperature was lowered to 40℃ via a heat exchanger, and the pH was lowered to 10.8 by adding sulfuric acid. The amount of hydrogen peroxide added (based on pure hydrogen peroxide) was 260 mg / L. The hydraulic retention time was 4 hours. The total cyanide concentration decreased to 0.7 mg / L, while the ammonia nitrogen concentration increased to 67 mg / L. The ammonia produced was recovered by an ammonium sulfate absorption unit, resulting in water with ammonia nitrogen concentration of 11 mg / L, a pH of 9.9, and a total nitrogen concentration reduced to 307 mg / L. The COD was indistinguishable from the influent. This effluent then entered the UASB anaerobic reactor.

[0074] UASB anaerobic reaction stage:

[0075] The pH of the effluent from the UV / H2O2 cyanide removal device was adjusted to 7.0 and then introduced into the UASB anaerobic reaction section. The flow rate was controlled, with the target COD reduction to 1300 mg / L. The resulting water quality was COD 1273 mg / L, ammonia nitrogen increased to 27 mg / L, while total nitrogen was not significantly different from the influent. The BOD5 / COD ratio increased to 0.71, exhibiting very good biodegradability. The effluent then entered the A1 / A2 / O biological treatment system.

[0076] After the effluent and wastewater have been partially degraded for carbon and nitrogen content, the concentrated water ratio is 1:0.3. It then enters the A1 / A2 / O biological treatment system. The hydraulic retention times in each biological treatment tank are 35h, 40h and 35h respectively. The nitrified liquid and sludge from the secondary sedimentation tank are returned to the A2 tank at a return ratio of 300%. The effluent quality is shown in Table 4.

[0077] Table 4. Effluent Water Quality from A1 / A2 / O Biological Treatment Tanks

[0078]

[0079]

[0080] As can be seen from Table 4, the nitrogen and carbon content in the condensate of the four-effect evaporator is removed by the denitrification and UASB anaerobic reaction process in this embodiment of the invention, which reduces the burden on the subsequent biological system and improves the biodegradability of the wastewater to 0.71. The ratio of this wastewater to the concentrated water after wastewater reuse is 1:0.3. After further A1 / A2 / O biological treatment, the effluent meets the direct discharge quality of GB31571-2015.

[0081] Comparative Example 1

[0082] In this comparative example, the UV / H2O2 cyanide removal process from Example 1 is placed before the two-stage organic nitrogen ammoniation process, as follows: Figure 3 As shown.

[0083] The cyanide removal stage in a UV / H2O2 cyanide removal device:

[0084] Because the temperature of the four-effect evaporator in the acrylonitrile unit is high, it needs to be reduced to 40℃ for the UV / H2O2 cyanide removal stage. Sodium hydroxide is added to adjust the pH to 10.8, and hydrogen peroxide is added at a concentration of 260 mg / L (based on pure hydrogen peroxide). The hydraulic retention time is 4 hours. Testing showed that the total cyanide decreased to 0.3 mg / L, while ammonia nitrogen increased to 51 mg / L. The ammonia produced was recovered by the ammonium sulfate absorption unit, resulting in water with ammonia nitrogen of 8 mg / L, a pH of 9.9, and a total nitrogen level reduced to 637 mg / L. The COD was indistinguishable from the influent. This effluent then enters the primary organic nitrogen ammoniation unit.

[0085] Organic nitrogen ammoniation stage:

[0086] In the primary organic nitrogen ammoniation unit, the temperature of the cyanide effluent from the UV / H2O2 cyanide removal process is raised to 65℃ as the reaction temperature. A 48% caustic soda solution is added to adjust the pH to 13.0, and the hydraulic retention time is 3 hours. The ammonia nitrogen content increases to 218 mg / L, and the pH decreases to 12.0. The ammonia produced is recovered by the ammonium sulfate absorption unit, resulting in water with an ammonia nitrogen content of 26 mg / L and a total nitrogen content reduced to 436 mg / L. Acrolein, acrylonitrile, cyanopyridine, and butadiene nitrile are undetectable. This effluent then enters the secondary organic nitrogen ammoniation unit.

[0087] In the secondary organic nitrogen ammoniation unit, with the reaction temperature maintained at 65℃ and no further addition of caustic soda, and a hydraulic retention time of 4 hours, the ammonia nitrogen content increased to 127 mg / L, and the pH value decreased to 11.7. The ammonia produced was recovered through an ammonium sulfate absorption unit, resulting in water with ammonia nitrogen of 13 mg / L, a pH of 11.4, and total nitrogen reduced to 300 mg / L, achieving the nitrogen removal target. However, the total cyanide content in this wastewater increased to 23.1 mg / L.

[0088] Since the total cyanide concentration reached 23.1 mg / L, and the material was to subsequently enter the UASB anaerobic reactor, the high total cyanide concentration made it impossible for the UASB anaerobic bacteria to survive, thus failing to achieve the objective of this invention. This demonstrates that the process described in this comparative example cannot achieve the objective of the invention.

[0089] Comparative Example 2

[0090] This comparative example removes the ammonium sulfate recovery device from Example 1, and the process flow is as follows: Figure 4 As shown.

[0091] The process conditions in the primary organic nitrogen ammoniation unit are the same as in the previous example, but the ammonia generated is not recovered by the ammonium sulfate absorption unit. The ammonia nitrogen content of the resulting water increases to 190 mg / L, while other parameters such as COD, TOC, and TN are basically the same as the influent water quality. This effluent enters the secondary organic nitrogen ammoniation unit.

[0092] In the secondary organic nitrogen ammoniation unit, with the reaction temperature maintained at 65℃ and no further addition of caustic soda, and a hydraulic retention time of 4 hours, the ammonia nitrogen content increased to 213 mg / L, and the pH value was 12.0. The ammonia nitrogen data alone shows that because the ammonia produced in the primary organic nitrogen ammoniation unit was not absorbed and reduced, the ammonia nitrogen content in the secondary organic nitrogen ammoniation unit did not increase significantly further. Therefore, it was impossible to substantially remove organic nitrogen, and consequently, the goal of removing total nitrogen to reduce the burden on subsequent biological processes was not achieved.

[0093] Comparative Example 3

[0094] This comparative example replaces the nitrogen and carbon removal processes in Example 1, i.e., the two-stage organic nitrogen ammoniation unit and ammonium sulfate recovery unit are placed after the UV / H2O2 cyanide destruction and UASB anaerobic reactor. The process flow is as follows: Figure 5 As shown.

[0095] In the cyanide removal stage of the UV / H2O2 cyanide removal device, the same process as Comparative Example 1 was used: temperature 40℃, pH adjusted to 10.8, hydrogen peroxide added at 260 mg / L (pure hydrogen peroxide), and hydraulic retention time of 4 hours. Testing showed that the water quality was essentially the same as in this process section of Comparative Example 1, with total cyanide reduced to 0.4 mg / L, ammonia nitrogen increased to 56 mg / L, and ammonia recovered by the ammonium sulfate absorption device, resulting in water with ammonia nitrogen of 8 mg / L, pH of 9.9, and total nitrogen reduced to 630 mg / L. COD was indistinguishable from the influent. This effluent entered the UASB anaerobic reactor. However, methane was not significantly produced in the UASB reaction, and consequently, COD did not decrease. This is because the organic nitrogen compounds, which have a significant impact on biochemistry, were not removed by the organic nitrogen ammoniation device, resulting in low biodegradability of the wastewater and thus inability to efficiently undergo anaerobic reaction, failing to achieve the carbon removal target of the invention.

[0096] The above examples and comparative studies of nitrogen removal, carbon removal, and biochemical effluent quality demonstrate that, under the premise of a high wastewater reuse rate leading to a high proportion of process wastewater entering the wastewater treatment plant, the nitrogen and carbon content in the condensate from the shell of the fourth-effect evaporator in an acrylonitrile unit is partially removed by the nitrogen and carbon removal processes described in this invention. Specifically, this innovatively applies the organic nitrogen ammoniation + UV / H2O2 cyanide destruction + ammonium sulfate recovery + UASB anaerobic reaction process to the pretreatment of acrylonitrile process wastewater, reducing the burden on the subsequent biochemical system and improving the biodegradability of the wastewater to above 0.6. The ratio of process wastewater to concentrated wastewater after reuse is 1:(0.3~0.6). After A1 / A2 / O biochemical treatment, the effluent meets the direct discharge quality standards of GB31571-2015. Therefore, under conditions of a high wastewater reuse rate, this process can stably meet the direct discharge quality standards stipulated in GB31571-2015 "Emission Standard of Pollutants for Petrochemical Industry" without changing the scale of the existing wastewater treatment plant.

Claims

1. A low-cost, low-energy-consumption process for treating wastewater from an acrylonitrile plant, characterized in that: Includes the following steps: The acrylonitrile unit wastewater is treated sequentially by a primary organic nitrogen ammoniation unit and a secondary organic nitrogen ammoniation unit, then enters a UV / H2O2 cyanide destruction unit for further treatment, and then overflows into a UASB anaerobic reactor for further treatment, followed by treatment through an A1 / A2 / O biological system, and finally discharged in compliance with standards. The wastewater from the acrylonitrile unit being treated is the condensate from the fourth-effect evaporator in the acrylonitrile unit; The ammonia generated in the primary organic nitrogen ammoniation unit, the secondary organic nitrogen ammoniation unit, and the UV / H2O2 cyanide removal unit all enter the ammonium sulfate recovery unit, which uses sulfuric acid to absorb ammonia and generate ammonium sulfate. In the primary and secondary organic nitrogen ammoniation units, alkaline substances are added to adjust the pH of the wastewater to 11.5-13.0 and 11.0-12.5, respectively; the alkaline substances are one or more combinations of sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate; the hydraulic retention time of the wastewater in the primary and secondary organic nitrogen ammoniation units is 2-6 hours; the hydrolysis temperature is 40-70℃.

2. The low-cost, low-energy-consumption treatment process for acrylonitrile plant wastewater according to claim 1, characterized in that: Before entering the UV / H2O2 cyanide removal unit, the effluent from the secondary organic nitrogen ammoniation unit is mixed with sulfuric acid through a pipe mixer at the outlet of the secondary organic nitrogen ammoniation unit to adjust the pH value to 10.0-11.0 before entering the UV / H2O2 cyanide removal unit.

3. The low-cost, low-energy-consumption treatment process for acrylonitrile plant wastewater according to claim 1, characterized in that: The hydraulic retention time of wastewater in the UV / H2O2 cyanide removal device is 2-5 hours, and the cyanide removal temperature is 30-50℃.

4. The low-cost, low-energy-consumption treatment process for acrylonitrile plant wastewater according to claim 1, characterized in that: In the UV / H2O2 cyanide removal device, the light band is the ultraviolet band, and the oxidant is hydrogen peroxide; the ultraviolet light emitted in the ultraviolet band includes ultraviolet light emitted by low-pressure and medium-pressure lamps; the oxidant is a 27-30wt% aqueous solution of hydrogen peroxide, and the concentration of pure hydrogen peroxide added to the wastewater is 100-300mg / L.

5. The low-cost, low-energy-consumption treatment process for acrylonitrile plant wastewater according to claim 1, characterized in that: Before the effluent from the UV / H2O2 cyanide removal device overflows into the UASB anaerobic reactor, it is cooled to below 30°C. Sulfuric acid is then added through a pipe mixer in the pipeline at the outlet of the UV / H2O2 cyanide removal device to adjust the pH value to 6.5-7.5 before overflowing into the UASB anaerobic reactor.

6. The low-cost, low-energy-consumption treatment process for acrylonitrile plant wastewater according to claim 1, characterized in that: The effluent from the UASB anaerobic reactor enters the A1 / A2 / O biological system for treatment, with hydraulic retention times of 30-40h, 35-50h, and 30-40h, respectively.

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