Low-salinity treatment process for acrylonitrile plant wastewater

Abstract

The present application belongs to the technical field of wastewater treatment, and particularly relates to a low-salt-content treatment process for acrylonitrile device wastewater. The low-salt-content treatment process for acrylonitrile device wastewater first treats the acrylonitrile device wastewater by an alkaline hydrolysis treatment device, and then sequentially overflows to a first-stage, a second-stage, a third-stage and a fourth-stage UV / H2O2 oxidation device for treatment. The effluent from the fourth-stage UV / H2O2 oxidation device is partially refluxed to the first-stage UV / H2O2 oxidation device, and is mixed with the effluent from the alkaline hydrolysis treatment device to enter the first-stage UV / H2O2 oxidation device. The remaining effluent sequentially enters an A1 / A2 / O biochemical system and an engineering bacteria biological filter for treatment, and is discharged after reaching the standard. The present application applies the alkaline hydrolysis and UV / H2O2 oxidation technology to the treatment of acrylonitrile device wastewater, solves the problem that the existing acrylonitrile wastewater is not easy to be biochemically treated, reduces the wastewater treatment cost, and the treated water quality reaches the direct discharge water quality requirement.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment technology, specifically relating to a low-salinity 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] Therefore, based on the chemical properties of the organic nitriles and inorganic cyanides in this wastewater, a new low-energy-consumption and low-operating-cost process should be developed to effectively remove toxic substances from the acrylonitrile unit wastewater that affect biochemical operation, without causing excessively high wastewater conductivity or introducing sulfate ions, and without placing an excessive burden of inorganic salts on the further reuse of the biochemical effluent. This will effectively improve the biodegradability of the wastewater, allowing it to be treated together with other wastewater within the plant area, including circulating water discharge, desalination station discharge, domestic wastewater, and initial rainwater, in the wastewater treatment plant's biochemical pond. This will ensure that the discharged wastewater consistently meets the water quality standards for acrylonitrile wastewater treatment as stipulated in GB31571-2015 "Emission Standard of Pollutants for Petrochemical Industry". Summary of the Invention

[0015] The technical problem to be solved by this invention is to provide a low-salt treatment process for acrylonitrile plant wastewater. By applying alkaline hydrolysis and UV / H2O2 oxidation technology to the treatment of acrylonitrile plant wastewater, this invention solves the problems of existing acrylonitrile wastewater having a BOD5 / COD ratio of less than 0.3 and being difficult to biodegrade. It also significantly reduces wastewater treatment costs, has significant energy-saving and carbon reduction effects, and the treated water quality meets the GB31571-2015 direct discharge standards.

[0016] The low-salinity treatment process for acrylonitrile plant wastewater according to the present invention includes the following steps:

[0017] The wastewater from the acrylonitrile unit is first treated by an alkaline hydrolysis treatment unit, and then overflows sequentially to a primary, secondary, tertiary, and quaternary UV / H2O2 oxidation unit (ultraviolet light / hydrogen peroxide oxidation unit) for further treatment. Part of the effluent from the quaternary UV / H2O2 oxidation unit is recycled back to the primary UV / H2O2 oxidation unit, where it enters the primary UV / H2O2 oxidation unit together with the effluent from the alkaline hydrolysis treatment unit. The remaining effluent enters the A1 / A2 / O biological system (anaerobic / anoxic / aerobic biological system) for further treatment, and then passes through an engineered bacteria biological filter (ABR) for final discharge after meeting the standards.

[0018] The wastewater from the acrylonitrile plant treated by this invention is the condensate from the fourth-effect evaporator in the acrylonitrile plant.

[0019] In the alkaline hydrolysis treatment device, an alkaline substance is added to adjust the pH value of the wastewater to 11-13, preferably 11.5-12.5; the alkaline substance is preferably one or a combination of sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate. The hydraulic retention time of the wastewater in the alkaline hydrolysis treatment device is 3-6 hours. This hydraulic retention time can be achieved by designing the volume of the alkaline hydrolysis treatment device and the influent flow rate, so that the organic nitriles in the wastewater are fully hydrolyzed to generate amides or ammonium carboxylate substances, thereby achieving the goal of converting highly toxic organic nitriles into non-toxic substances.

[0020] The hydraulic retention time of the wastewater in the primary, secondary, tertiary, and quaternary UV / H2O2 oxidation devices is 1-2 hours, which can achieve a good cyanide removal effect.

[0021] The amount of water effluent from the fourth-stage UV / H2O2 oxidation device that is returned to the first-stage UV / H2O2 oxidation device is 0.4-1.2 times the amount of water entering the fourth-stage UV / H2O2 oxidation device, preferably 0.6-1.0 times.

[0022] The pH value of the wastewater in the primary UV / H2O2 oxidation unit is 9-11, preferably 9.5-10.5. The effluent from the alkaline hydrolysis treatment unit, together with the return water from the fourth-stage UV / H2O2 oxidation unit, overflows into the primary UV / H2O2 oxidation unit. This allows the wastewater in the primary UV / H2O2 oxidation unit to be adjusted to a suitable pH value without the need for acid introduction. The required pH range is achieved by adjusting the return water volume, solving the problem of excessively high ion concentrations caused by the introduction of sulfuric acid or hydrochloric acid for pH adjustment in existing technologies. Simultaneously, it provides excellent water quality for improving wastewater biodegradability and subsequent wastewater reuse.

[0023] In the first, second, third, and fourth stage UV / H2O2 oxidation devices, the light band is ultraviolet, and the oxidant is hydrogen peroxide. Ultraviolet light promotes the generation of highly oxidizing hydroxyl radicals from hydrogen peroxide, which efficiently convert inorganic cyanide in wastewater into carbonates and ammonia nitrogen within a suitable pH range, achieving the goal of converting highly toxic inorganic cyanide into non-toxic substances.

[0024] The ultraviolet light emitted in the ultraviolet band includes ultraviolet light emitted by low-pressure and medium-pressure lamps, with medium-pressure ultraviolet lamps being preferred.

[0025] The oxidant is preferably a 27-30 wt% hydrogen peroxide aqueous solution. Based on pure hydrogen peroxide, the concentration added to the wastewater is 100-800 mg / L, preferably 300-600 mg / L. The dosage is adjusted according to the influent water quality. The residual hydrogen peroxide in the effluent after the reaction is approximately 1-10 mg / L, solving the problem of excessive residual hydrogen peroxide in existing technologies, which leads to the loss of activity in activated sludge.

[0026] The effluent from the four-stage UV / H2O2 oxidation device entering the A1 / A2 / O biological system has a pH of 7.5-8.5 and an alkalinity of 800-1500 mg / L. No sulfate ions are introduced, and the high alkalinity provides sufficient alkalinity for nitrification in the O tank of the wastewater treatment plant. The BOD5 / COD ratio reaches above 0.5, making it suitable for entering the biological system.

[0027] When the wastewater enters the A1 / A2 / O biological treatment system, it can be combined with other wastewater in the factory and treated together in the A1 / A2 / O biological treatment system and engineered bacteria biological filter.

[0028] The wastewater sequentially enters the A1 / A2 / O biochemical system and the engineered bacteria biological filter for treatment, with hydraulic retention times of 15-30h, 20-40h, 15-30h, and 2-4h, respectively.

[0029] The effluent from the engineered bacteria biological filter (ABR) has a COD of 15-30 mg / L, a TOC of 5-11 mg / L, a pH of 7.44-7.60, a total nitrogen of 24-35 mg / L, ammonia nitrogen of 0.3-1.2 mg / L, a total phosphorus of 0.45-0.69 mg / L, a suspended solids of 5-11 mg / L, and a total cyanide level of -0.08 mg / L (undetectable). All indicators meet the direct discharge water quality requirements of GB31571-2015.

[0030] This invention addresses the technical challenges of low BOD5 / COD ratios and high energy consumption in the treated acrylonitrile wastewater from quadruple-effect evaporation and stripping towers in existing acrylonitrile plants, resulting in poor effluent biodegradability and unstable operation of the biological treatment system. It employs a low-cost, low-energy treatment process using alkaline hydrolysis followed by multi-stage UV / H2O2 oxidation. The alkaline hydrolysis process is followed by pretreatment in a multi-stage UV / H2O2 oxidation unit, achieving a BOD5 / COD ratio higher than 0.5. The alkaline hydrolysis process precedes the UV / H2O2 advanced oxidation process, removing recalcitrant and nitrification-inhibiting substances from the quadruple-effect evaporator condensate, as well as toxic substances, significantly increasing the BOD5 / COD ratio and thus greatly improving the biodegradability of the pretreated wastewater. The pretreated wastewater is then treated together with other wastewater from the plant in an A1 / A2 / O+ABR biological treatment tank. Compared to the stripping tower process, the cost per ton of water treated is significantly lower, achieving the goals of low treatment cost, low energy consumption, and carbon reduction.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] (1) This invention innovatively recycles the final stage UV / H2O2 oxidation device to the inlet of the first stage UV / H2O2 oxidation device in a certain proportion after alkaline hydrolysis and UV / H2O2 oxidation. The effluent from alkaline hydrolysis overflows into the first stage UV / H2O2 oxidation device together with the effluent from alkaline hydrolysis. This eliminates the need to introduce sulfuric acid or other acidic substances to adjust the pH value. The BOD5 / COD ratio reaches above 0.5. No sulfate ions are introduced into the effluent. The conductivity and alkalinity are suitable for entering the biological system, providing a high-quality water source for biological treatment and wastewater reuse.

[0033] (2) This invention successfully applies alkaline hydrolysis and UV / H2O2 advanced oxidation technology to the treatment of acrylonitrile plant wastewater, solving the problems of existing acrylonitrile wastewater having a BOD5 / COD ratio of less than 0.3 and being difficult to biodegrade. Furthermore, it eliminates the need for steam input, significantly reducing wastewater treatment costs and achieving remarkable energy-saving and carbon-reduction effects. The pretreated effluent and other wastewater within the plant area are treated in an A1 / A2 / O+ABR biological treatment tank to meet the GB31571-2015 direct discharge standards. Attached Figure Description

[0034] Figure 1 This is a flow chart of the low-salinity treatment process for wastewater from the acrylonitrile unit of the present invention.

[0035] 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 co-processing in this invention. Detailed Implementation

[0036] 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.

[0037] 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.

[0038] 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.

[0039] Through the Figure 1Analysis 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.

[0040] Table 1. Analysis results of characteristic pollutants (obtained by liquid chromatography-mass spectrometry)

[0041]

[0042]

[0043]

[0044] 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-.

[0045] The content of some compounds can be determined by gas chromatography-mass spectrometry (GC-MS), as shown in Table 2.

[0046] Table 2. Content of characteristic compounds in condensate from the quadruple-effect evaporator determined by gas chromatography-mass spectrometry (GC-MS).

[0047] Characteristic compounds acrolein Acrylonitrile Butene dinitrile Cyanopyridine <![CDATA[Content (mg·L -1 )]]> 0.7～10.5 1.9～8.6 11～55 40～155

[0048] 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.

[0049] Table 3. Condensate quality of the quadruple-effect evaporator

[0050]

[0051] As can be seen from Table 3,

[0052] (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.

[0053] (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.

[0054] (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.

[0055] (4) The BOD5 / COD ratio was found to be below 0.004, indicating that its biodegradability was extremely poor.

[0056] 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, resulting in 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.

[0057] The condensate from the quadruple-effect evaporator treated in the following examples and comparative examples had the following water quality: COD 2895 mg / L, TOC 1430 mg / L, TN 755 mg / L, NH3-N 29 mg / L, total cyanide 10.9 mg / L, and pH 6.8. Gas chromatography-mass spectrometry (GC-MS) revealed that it contained acrolein 10.5 mg / L, acrylonitrile 8.5 mg / L, cyanopyridine 39.3 mg / L, and butadionitrile 55.2 mg / L.

[0058] Example 1

[0059] In the alkaline hydrolysis treatment device, a 48wt% sodium hydroxide solution was added to adjust the pH value to 11.5, and the hydraulic retention time was 6h. The changes in characteristic pollutants of wastewater after alkaline hydrolysis were given by liquid chromatography-mass spectrometry (LC-MS) and gas chromatography-mass spectrometry (GC-MS). The LC-MS analysis results are shown in Table 4.

[0060] Table 4. Trends in the content of characteristic compounds determined by liquid chromatography-mass spectrometry (LC-MS).

[0061]

[0062]

[0063] As shown in Table 4, the changes in the content of characteristic pollutants before and after hydrolysis exhibit the following two patterns:

[0064] (1) Decreased content or undetectable content: The content of imidazole, pyridine, pyrimidine, naphthidone, 1,3-dihydroxy-5-(1-aminopropyl)benzene, and 1-tert-butylcarbamo-3-pyrrolidone, which have adverse effects on biochemistry, decreased; the content of piperazine, 1-(cyanoacetyl)pyrrolidine, and ((3S,4R)-4-hydroxytetrahydrofuran-3-yl)carbamate tert-butyl ester, etc., decreased to undetectable levels;

[0065] (2) Increased content: Pyridine carboxamide and pyridine carboxylic acid are obtained by hydrolysis of cyanopyridine, resulting in increased content. 3-Amino-2-methyl-4(3H)-quinazolone, 3-nitro-2-propylaminopyridine, acrylamide / bisacrylamide, and C7H8N2O2 (whose specific structure could not be determined) and C... 10 H 12 N4O2, C 13 H 17 The N5O2 content also increased.

[0066] The above analysis shows that the levels of pollutants such as imidazole, pyridines, pyrimidines, naphthidone, piperazine, pyrrolidone, and organic nitriles, which have adverse effects on biochemistry, are decreasing.

[0067] The contents of some compounds before and after hydrolysis were obtained by gas chromatography-mass spectrometry (GC-MS) analysis, and the results are shown in Table 5.

[0068] Table 5 shows the content of some characteristic compounds before and after alkaline hydrolysis as determined by gas chromatography-mass spectrometry (GC-MS).

[0069] Characteristic compounds acrolein Acrylonitrile Butene dinitrile Cyanopyridine <![CDATA[Before hydrolysis (mg·L -1 )]]> 10.5 8.5 55.2 39.3 <![CDATA[After hydrolysis (mg·L -1 )]]> Undetectable 0.54 Undetectable Undetectable

[0070] As shown in Table 5, after hydrolysis, the contents of acrolein, acrylonitrile, butadiene nitrile, and cyanopyridine decreased significantly or became undetectable. Based on Table 4, it can be inferred that acrylonitrile hydrolyzes to acrylamide / bisacrylamide, and cyanopyridine hydrolyzes to pyridinecarboxamide and pyridinecarboxylic acid.

[0071] The changes in NO3-N, TN, TOC, and COD in conventional water quality are shown in Table 6.

[0072] Table 6 Water quality before and after alkaline hydrolysis

[0073]

[0074]

[0075] Table 6 shows that after alkaline hydrolysis, COD and total carbon (TC) remained essentially unchanged, while inorganic carbon (IC) increased. NH3-N increased to 80 mg / L, while NO3-N showed no significant change. The significant increase in NH3-N content is attributed to the formation of carboxylates and ammonia from the alkaline hydrolysis of organic nitriles. Total cyanide was 12.3 mg / L, requiring removal in subsequent processes. Further analysis revealed that the pH of the water was 11.2, lower than the initial pH of 11.5, indicating that the alkaline hydrolysis process consumed hydroxide ions.

[0076] The reflux water from the fourth-stage UV / H2O2 oxidation unit is returned to the inlet of the first-stage UV / H2O2 oxidation unit at a 1:1 ratio with the process influent. It then enters the first-stage UV / H2O2 oxidation unit along with the effluent from the alkaline hydrolysis treatment unit. The pH value of the wastewater in the first-stage UV / H2O2 oxidation unit is 10.5. Hydrogen peroxide is added to the first-stage UV / H2O2 oxidation unit at a concentration of 300 mg / L (pure hydrogen peroxide). The hydraulic retention time of the wastewater in the first, second, third, and fourth-stage UV / H2O2 oxidation units is 1 hour. In actual operation, the oxygen content in the gas phase is 21%, indicating that there is no significant oxygen release reaction from the hydrogen peroxide, ensuring the safety of the process. The typical water quality of the effluent from the fourth-stage UV / H2O2 oxidation unit to the wastewater treatment plant is shown in Table 7-8.

[0077] Table 7. Conventional water quality after pretreatment

[0078]

[0079] Table 8. Conventional water quality after pretreatment

[0080]

[0081] As shown in Tables 7-8, the total cyanide content in the pretreated water is 0.7 mg / L, and the residual H2O2 content is 5 mg / L, indicating that it will not affect the activated sludge in the biological treatment tank of the wastewater treatment plant. The alkalinity reaches 1178 mg / L, providing sufficient alkalinity for the nitrification reaction in the O tank, where ammonia nitrogen is converted to nitrate nitrogen. The only sulfate concentration of 69 mg / L is due to a small amount of sulfate introduced into the wastewater from the acrylonitrile unit system, and the conductivity is also low.

[0082] Further testing revealed a BOD5 / COD ratio of 0.61, indicating excellent biodegradability. The entire pretreatment process requires no steam input, consuming only caustic soda, hydrogen peroxide, and electricity. The comprehensive cost is calculated at 15.0 yuan / ton of water, significantly lower than the 60.0 yuan / ton cost of the stripping tower process, achieving the goals of reducing wastewater treatment costs and saving energy and reducing carbon emissions.

[0083] The effluent, along with other wastewater from the plant area, enters the A1 / A2 / O biological treatment system. The hydraulic retention times in each biological treatment tank are 30h, 30h, and 40h, respectively. The nitrified liquid and sludge from the secondary sedimentation tank are returned to the A2 tank at a return ratio of 350%. Furthermore, due to the high alkalinity in the pretreatment stage, no caustic soda is required in the nitrification process in the O tank, saving on operating costs. The effluent quality is shown in Table 9. Then, an engineered bacteria biofilter (ABR) with deep decarbonization is connected, with a hydraulic retention time of 2h. The resulting water quality is also listed in Table 9.

[0084] Table 9. Water quality of effluent from A1 / A2 / O biological treatment tanks and ABR.

[0085]

[0086] As can be seen from Table 9, after undergoing the alkaline hydrolysis + UV / H2O2 advanced oxidation process in this embodiment of the invention, and with the UV / H2O2 advanced oxidation device equipped with a reflux process to avoid the introduction of other ions, the effluent enters the sewage treatment system and, after undergoing the A1 / A2 / O biochemical process, can achieve water quality with COD of 153.1 mg / L, ammonia nitrogen of 1.9 mg / L, total nitrogen (TN) of 33.75 mg / L, and undetectable total cyanide. After further deep decarbonization in the engineered bacteria biological filter ABR, the direct discharge quality can reach GB31571-2015, with COD of only 19.5 mg / L.

[0087] Comparative Example 1

[0088] In the alkaline hydrolysis treatment stage, the same process as in Example 1 was used. In the UV / H2O2 advanced oxidation stage, the reflux process from the fourth stage to the first stage UV / H2O2 advanced oxidation inlet was not implemented; instead, 300 mg / L of hydrogen peroxide was directly added for the reaction. In actual operation, it was found that the oxygen content in the gas phase reached 26%. This is because hydrogen peroxide undergoes a side reaction in a strongly alkaline environment that releases oxygen, leading to an increase in the oxygen content in the gas phase and a decrease in the oxidation efficiency of the hydrogen peroxide. This resulted in a total cyanide concentration of 5.3 mg / L and a BOD5 / COD ratio of 0.11 in the effluent, and the biodegradability of the effluent did not improve significantly.

[0089] Comparative Example 2

[0090] In the alkaline hydrolysis stage, the same process as in Example 1 was used. In the UV / H2O2 oxidation stage, instead of the reflux process of the fourth-stage UV / H2O2 oxidation unit returning the effluent to the inlet of the first-stage UV / H2O2 oxidation unit, sulfuric acid was added to adjust the pH of the alkaline hydrolysis effluent from 11.2 to 10.0. The added sulfuric acid concentration was 883 mg / L, and then the effluent entered the UV / H2O2 oxidation unit. The effluent quality is shown in Table 10-11.

[0091] Table 10. Conventional water quality after pretreatment

[0092]

[0093] Table 11. Conventional water quality after pretreatment

[0094]

[0095] As shown in Tables 10-11, the COD, TN, ammonia nitrogen, and nitrate nitrogen in the effluent after pretreatment were basically the same as in Example 1. However, the hydrogen peroxide residue reached 100 mg / L, the alkalinity was low, while the sulfate concentration reached 926 mg / L, and the conductivity also reached a high value of 5691 μS / cm. Due to the excessive hydrogen peroxide residue, the BOD5 / COD ratio was found to be only 0.09 in actual operation, which is unsuitable for entering the biological treatment system. A subsequent process to remove hydrogen peroxide is necessary, such as adding commonly used reducing agents like sulfite. However, this further introduces sulfate, resulting in high-salinity wastewater, which is detrimental to the normal operation of the biological treatment system.

[0096] Through the above Examples 1 and Comparative Example 1, it can be seen that the condensate from the shell of the four-effect evaporator, i.e., the stripping tower influent, undergoes alkaline hydrolysis + UV / H2O2 oxidation pretreatment in this invention. The UV / H2O2 oxidation device incorporates a reflux process, thus avoiding the introduction of other ions. This solves the problem of excessively high ion concentrations caused by the introduction of sulfuric acid or hydrochloric acid due to pH adjustment in existing technologies. Simultaneously, it provides excellent water quality for improving wastewater biodegradability and subsequent wastewater reuse. The effluent enters the wastewater treatment system and, after the A1 / A2 / O biological treatment process, achieves water quality with COD of 153.1 mg / L, ammonia nitrogen of 1.9 mg / L, total nitrogen (TN) of 33.75 mg / L, and undetectable total cyanide. Further deep decarbonization via an engineered bacteria biological filter (ABR) achieves direct discharge water quality meeting GB31571-2015 standards, with a COD of only 19.5 mg / L. Meanwhile, the pretreatment process cost of this invention is 15.0 yuan / t water, which is much lower than the cost of 60.0 yuan / t water for the stripping tower process, thus achieving the effects of reducing wastewater treatment costs and saving energy and reducing carbon emissions.

Claims

1. A low-salinity treatment process for wastewater from an acrylonitrile plant, characterized in that: Includes the following steps: The wastewater from the acrylonitrile unit is first treated by an alkaline hydrolysis treatment unit, and then overflows sequentially to the primary, secondary, tertiary and quaternary UV / H2O2 oxidation units for further treatment. Part of the effluent from the quaternary UV / H2O2 oxidation unit is recycled back to the primary UV / H2O2 oxidation unit and enters the primary UV / H2O2 oxidation unit together with the effluent from the alkaline hydrolysis treatment unit. The remaining effluent enters the A1 / A2 / O biological system for treatment, and then passes through an engineered bacteria biological filter before being discharged in compliance with standards. The amount of water effluent from the fourth-stage UV / H2O2 oxidation device that is returned to the first-stage UV / H2O2 oxidation device is 0.4-1.2 times the amount of water entering the fourth-stage UV / H2O2 oxidation device. The pH value of the wastewater in the primary UV / H2O2 oxidation device is 9-11.

2. The low-salinity treatment process for acrylonitrile plant wastewater according to claim 1, characterized in that: The wastewater from the acrylonitrile unit being treated is the condensate from the fourth-effect evaporator in the acrylonitrile unit.

3. The low-salinity treatment process for acrylonitrile plant wastewater according to claim 1, characterized in that: In the alkaline hydrolysis treatment device, an alkaline substance is added to adjust the pH value of the wastewater to 11-13; the alkaline substance is one or more combinations of sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate.

4. The low-salinity treatment process for acrylonitrile plant wastewater according to claim 1, characterized in that: The hydraulic retention time of wastewater in the alkaline hydrolysis treatment device is 3-6 hours.

5. The low-salinity treatment process for acrylonitrile plant wastewater according to claim 1, characterized in that: The hydraulic retention time of the wastewater in the primary, secondary, tertiary, and quaternary UV / H2O2 oxidation devices is 1-2 hours.

6. The low-salinity treatment process for acrylonitrile plant wastewater according to claim 1, characterized in that: In the first-stage, second-stage, third-stage, and fourth-stage UV / H2O2 oxidation devices, the light band is the ultraviolet band; 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 100-800 mg / L.

7. The low-salinity treatment process for acrylonitrile plant wastewater according to claim 1, characterized in that: The effluent from the four-stage UV / H2O2 oxidation device entering the A1 / A2 / O biochemical system has a pH of 7.5-8.5, an alkalinity of 800-1500 mg / L, and no sulfate ions are introduced.

8. The low-salinity treatment process for acrylonitrile plant wastewater according to claim 1, characterized in that: The wastewater sequentially enters the A1 / A2 / O biochemical system and the engineered bacteria biological filter for treatment, with hydraulic retention times of 15-30h, 20-40h, 15-30h, and 2-4h, respectively.

Images