A method for treating MDEA wastewater

Abstract

The present invention discloses a method for treating MDEA wastewater, which includes a pretreatment section, a wet oxidation section, and a deep treatment section; the MDEA wastewater first removes calcium and magnesium hardness, is concentrated by multi-effect evaporation and then enters the adjustment tank, where it is mixed with a homogeneous catalyst, two-stage nanofiltration concentrated water, and single-membrane electrodialysis mother liquor, and after heat exchange by a heat exchange unit, it is subjected to wet oxidation catalytic treatment. The effluent is then cooled by a cooler and sequentially enters a first-stage nanofiltration and a second-stage nanofiltration for treatment. Part of the first-stage nanofiltration concentrated water is refluxed to the adjustment tank, and part enters the single-membrane electrodialysis. All of the second-stage nanofiltration concentrated water is sent back to the adjustment tank, and the second-stage nanofiltration product water enters the deep treatment section; the single-membrane electrodialysis mother liquor is sent back to the heat exchange unit, and the anion solution enters the deep treatment section, and after chemical denitrification and biochemical treatment, it is finally discharged up to standard. Aiming at the characteristics of MDEA wastewater being difficult to degrade and having high nitrogen content, the present invention adopts a treatment process with homogeneous catalytic wet oxidation as the core, and uses a combination of chemical denitrification and biological denitrification to finally achieve the up-to-standard discharge of MDEA wastewater.

Description

Technical Field

[0001] The present invention relates to a method for treating MDEA wastewater, in particular to a method for homogeneous catalytic wet oxidation treatment of MDEA wastewater, belonging to the technical field of wastewater treatment. Background Art

[0002] MDEA, also known as methyldiethanolamine, is widely used in the desulfurization and purification of oilfield gas, coal gas, and natural gas, as well as in the absorption of acidic gases. Compared with desulfurization agents such as ethanolamine (MEA) and diethanolamine (DEA), the molecular structure of MDEA is more stable and not easily degraded. The reason is that the molecular structure of MDEA belongs to tertiary alcoholamine, and there is no active H atom on N, so its chemical properties are very stable, and its wastewater is also difficult to biodegrade. For such refractory wastewater, advanced oxidation treatments are usually used, such as ozone catalytic oxidation, electrocatalytic oxidation, photocatalysis, Fenton oxidation, etc. Among them, electrocatalytic oxidation and photocatalysis are still limited by high treatment costs and are difficult to be industrially applied. Fenton oxidation has problems such as complex operation, instability of hydrogen peroxide, and loss of iron ions. With the country's control of ozone pollutants, the application of ozone catalytic oxidation is also limited.

[0003] Wet oxidation refers to the oxidation and decomposition of organic substances in water into small molecule organic substances or inorganic substances by using gaseous oxygen as an oxidant under the conditions of high temperature (120 - 320 °C) and high pressure (0.5 - 20 MPa). It has the characteristics of no secondary pollution and low treatment cost. Drawing on its extensive application in alkali residue wastewater, more and more scholars have begun to study wet oxidation technology for treating other high-difficulty wastewater. In order to improve its treatment effect, the research focus is on catalytic wet oxidation.

[0004] CN201510274988.5 discloses a catalyst for catalytic wet oxidation of refractory organic wastewater. This catalyst is a "noble metal-transition metal-rare earth" composite catalyst, and the main component of the carrier FSC is alumina; CN201410340574.3 discloses a catalyst for catalytic wet oxidation treatment and its preparation method, which uses noble metal-non-noble metal nanoalloy as the active component and activated carbon as the carrier; CN201510661575.2 discloses a heterogeneous wet oxidation catalyst, and its components include a composite oxide carrier and a small amount of noble metal; CN201310621017.4 discloses a preparation method of a carrier for a catalytic wet oxidation catalyst. This carrier uses activated carbon as the core and amorphous silica-alumina as the shell.

[0005] The above patents all adopt heterogeneous catalytic wet oxidation, which has advantages in the separation and recovery of catalysts and metal loss, but is not necessarily applicable to the wet oxidation process. The paper "Industrial Application of Mild Wet Oxidation + SBR Treatment Technology for Alkali Residue" (2011) discloses the mild wet oxidation process for alkali residue of Fushun Research Institute of Petroleum and Chemical Industry. This process has been popularized and applied in 28 refining enterprises and has strong representativeness. The wet oxidation reactor used in this process is a bubbling flow internal circulation reactor with an inner cylinder. If a heterogeneous catalyst is used in this reactor, its gas-liquid circulation will be seriously affected, and even the reactor and pipeline will be blocked. If the fixed-bed method is used, it will bring greater gas resistance, which is more unfavorable to gas-liquid circulation, and the reaction rate will also be greatly reduced.

[0006] Due to the absence of internal and external diffusion effects and high dispersion, the catalytic efficiency of homogeneous catalysis is higher than that of heterogeneous catalysis, and the preparation of the catalyst is much simpler than that of heterogeneous catalysts. However, the biggest problem with applying homogeneous catalysis to wet oxidation is the loss of metal catalysts. At present, there is little research in this direction. CN201210225873.3 provides a method for treating industrial wastewater by homogeneous catalytic wet oxidation, in which a ring gear packing is installed in a fixed-bed reactor, and an iron-based catalyst is used as the homogeneous catalyst. However, the patent does not mention the catalyst loss and the corresponding solution methods. Summary of the Invention

[0007] In view of the above deficiencies, the present invention provides a method for treating MDEA wastewater for the existing technology. By combining homogeneous catalytic wet oxidation and membrane technology, the efficient treatment and up-to-standard discharge of MDEA wastewater are realized. At the same time, the problem of catalyst loss in the homogeneous catalytic process is solved, the catalyst can be recycled, the comprehensive operation cost is low, and the economic practicability is strong.

[0008] In order to achieve the above technical objectives, the technical solution adopted by the present invention is as follows:

[0009] A method for treating MDEA wastewater, including a pretreatment section, a wet oxidation section, and a deep treatment section;

[0010] The pretreatment section includes a dehardening tank, a multi-effect evaporator, and an adjustment tank; the MDEA wastewater first enters the dehardening tank to remove calcium and magnesium hardness. When the MDEA concentration in the wastewater ≤ 10000 mg / L, the dehardened wastewater enters the multi-effect evaporator for concentration, the condensed water is recycled, and the concentrated thick water enters the adjustment tank, where it is mixed with the homogeneous catalyst, the concentrated water of two-stage nanofiltration, and the mother liquor of single-membrane electrodialysis; when the MDEA concentration in the wastewater > 10000 mg / L, it directly enters the adjustment tank;

[0011] The wet oxidation section includes a heat exchange unit, a wet oxidation reactor, a cooler, a first-stage nanofiltration, a second-stage nanofiltration, and a single-membrane electrodialysis; the effluent from the regulating tank enters the heat exchange unit, and the heat exchange unit uses a heat exchanger to exchange heat between the effluent from the wet oxidation reactor and the effluent from the regulating tank. The effluent from the wet oxidation reactor flows through the tube side, and the effluent from the regulating tank flows through the shell side. The effluent from the regulating tank after heat exchange and temperature rise enters the wet oxidation reactor; the effluent from the wet oxidation reactor is cooled by the heat exchange unit and then by the cooler, and then enters the first-stage nanofiltration and the second-stage nanofiltration for treatment in sequence. Among them, part of the concentrated water from the first-stage nanofiltration is recycled to the regulating tank, and part enters the single-membrane electrodialysis. The product water from the first-stage nanofiltration enters the second-stage nanofiltration. All the concentrated water from the second-stage nanofiltration is sent back to the regulating tank, and the product water from the second-stage nanofiltration enters the advanced treatment section; the mother liquor of the single-membrane electrodialysis is sent back to the heat exchange unit, and the anion solution enters the advanced treatment section;

[0012] The advanced treatment section includes a chemical denitrification tank, a biochemical tank, and an effluent monitoring tank; the product water from the second-stage nanofiltration and the anion solution from the single-membrane electrodialysis enter the chemical denitrification tank, mainly for the removal of ammonia nitrogen and COD. The effluent from the chemical denitrification tank enters the biochemical tank and is further treated by biological methods. After the treatment is completed, it is discharged to the effluent monitoring tank and finally meets the standards for discharge.

[0013] Further, chemical agents are used for hardness removal in the hardness removal tank, preferably a combined agent of sodium hydroxide and sodium carbonate. Sodium hydroxide is added at 1 to 4 times the mass concentration of magnesium ions, and sodium carbonate is added at 1 to 3 times the mass concentration of calcium ions.

[0014] Further, the temperature of the multi-effect evaporation steam is controlled below 150 °C, and the concentration multiple is 2 to 5 times; the multi-effect evaporation plays a role in concentrating the MDEA wastewater, reducing the treatment load of the subsequent wet oxidation unit, and solving the problem of poor economy in treating low-concentration and large-flow wastewater;

[0015] Further, the heat exchange unit is composed of multiple heat exchangers. After multiple heat exchanges, the temperature of the effluent from the regulating tank rises to 130 - 160 °C, and the temperature of the effluent from the wet oxidation reactor drops to 45 - 75 °C.

[0016] Further, the homogeneous catalyst is a composite catalyst of noble metals and transition metals; the noble metals are selected from one or more of platinum, palladium, rhodium, silver, and ruthenium, and the transition metals are selected from one or more of copper, iron, manganese, zinc, and nickel. The mass percentage of noble metals in the homogeneous catalyst is 2% - 20%, and a platinum-copper composite catalyst is preferred; the noble metals and transition metals exist in the form of metal salt compounds or complexes and are dissolved in the liquid phase.

[0017] Further, the homogeneous catalyst is only added in large amounts at the start-up of the device, and is added according to the mass ratio of MDEA to metal ions of 2000:1 - 2:1; during normal operation, it is appropriately supplemented according to the catalyst loss rate and the change in MDEA concentration.

[0018] Furthermore, the wet oxidation reactor is a bubbling flow internal circulation reactor with an inner cylinder. Under high temperature and high pressure conditions, using gaseous oxygen (air) as an oxidant, it oxidizes general organic substances in water into small molecule organic substances or inorganic substances. Those skilled in the art should understand that MDEA belongs to organic substances with stable chemical properties and is difficult to degrade. The ability of wet oxidation alone to remove MDEA is limited. However, under the condition of a catalyst, oxygen at high temperature and high pressure is more likely to undergo free radical reactions. Under the strong oxidizing effect of free radicals, the decomposition and conversion ability of MDEA and the reaction rate are greatly improved. On the other hand, the compound homogeneous catalyst adopted in the present invention itself has high activity and high selectivity, and its treatment of special pollutants is faster and more effective. The problem of a large amount of homogeneous catalyst loss has also been solved through membrane technology.

[0019] Furthermore, the reaction temperature of the wet oxidation reactor is 150 - 250 °C, the reaction pressure is 2 MPa - 6 MPa, and the liquid hourly space velocity is 0.25 - 4 h -1 , and the gas-liquid volume ratio is 20:1 - 300:1.

[0020] Furthermore, the cooling medium of the cooler is circulating water, which further cools the effluent from the wet oxidation reactor to a temperature of 25 - 40 °C, meeting the temperature requirements of the subsequent nanofiltration membrane.

[0021] Furthermore, the water production rate of the first-stage nanofiltration is 60% - 85%. The pore size of the first-stage nanofiltration membrane is between 1 - 3 nm, which can intercept metals of the homogeneous catalyst, high-valent salts (such as sulfates, carbonates), and macromolecular organic substances (relative molecular mass greater than 200), etc. After being intercepted, they enter the concentrated water side. Among them, the metals of the homogeneous catalyst finally flow back to the adjustment tank and return to the wet oxidation reactor for recycling after heat exchange and temperature increase; the water produced by the first-stage nanofiltration includes unreacted MDEA, small molecule organic substances, and monovalent salts.

[0022] Furthermore, 1% - 60% of the concentrated water from the first-stage nanofiltration enters the single-membrane electrodialysis; the single-membrane electrodialysis only uses an anion exchange membrane. Under the action of the electrode drive and the anion exchange membrane, some high-valent anion salts such as sulfates and carbonates in the concentrated water from the first-stage nanofiltration permeate through the anion exchange membrane into the concentrated water side. At the same time, it also includes small molecule organic substances showing negativity, and the cation catalyst remains in the mother liquor and is reused again. Those skilled in the art should understand that the first-stage nanofiltration does not intercept monovalent ions such as sodium, potassium, and chlorine, but has a high interception rate for divalent anions. The single-membrane electrodialysis well solves the problem of divalent anion enrichment and does not cause the loss of the metal catalyst. By adopting a branched side line treatment, its treatment cost is also relatively low.

[0023] Furthermore, the water production rate of the secondary nanofiltration is 55% - 80%, the pore size of the secondary nanofiltration membrane is between 0.5 - 1 nm, MDEA is intercepted on the concentrated water side and finally refluxed to the wet oxidation reactor for further treatment. The un-intercepted small molecular organic substances and monovalent salts are on the water production side and enter the advanced treatment section.

[0024] Furthermore, hypochlorous acid or sodium hypochlorite is added to the chemical denitrification tank. According to the principle of breakpoint chlorination, the molar ratio of chlorine dosage to ammonia is controlled at 1:1 - 2:1. Those skilled in the art should understand that under the action of catalytic wet oxidation of MDEA, macromolecular organic substances will become small molecular organic substances and inorganic substances, among which organic nitrogen will be converted into nitrogen gas, nitrate nitrogen (nitrite nitrogen is unstable) and ammonia nitrogen. Breakpoint chlorination is to use chlorine-based oxidants to oxidize ammonia nitrogen into chloramine and then oxidize and decompose it into nitrogen gas, so as to achieve the purpose of removal. At the same time, part of the COD will also be removed under the action of chlorine-based oxidants.

[0025] Furthermore, the salt concentration of the influent water of the biochemical pool should not exceed 5000 mg / L. When the influent water salt concentration is too high, in order to ensure the treatment effect of subsequent nitrifying and denitrifying bacteria, other low-salt wastewaters should be used for dilution treatment, preferably domestic sewage, stripping purified water and other wastewaters with relatively low salt concentration, COD, etc.

[0026] Furthermore, the A / O and post-denitrification combined process is adopted in the biochemical pool; in the A / O process, the A section is in the front and the O section is in the back. The A section is mainly used to improve the biodegradability of wastewater and accelerate the biodegradation rate. The O section is mainly used for the removal of organic substances and the nitrification reaction of residual ammonia nitrogen. The post-denitrification is used for the removal of total nitrogen, converting nitrate nitrogen and nitrite nitrogen into nitrogen gas for removal.

[0027] Compared with the prior art, the present invention has the following advantages:

[0028] (1) Aiming at the characteristics of MDEA wastewater, which is difficult to degrade and has a high nitrogen content, the present invention adopts a treatment process with homogeneous catalytic wet oxidation as the core, giving full play to the characteristics of high catalytic activity and strong treatment ability of homogeneous catalytic wet oxidation, and using a combination of chemical denitrification and biological denitrification to finally achieve the up-to-standard discharge of MDEA wastewater.

[0029] (2) By combining wet oxidation with membrane technologies such as nanofiltration membrane and electrodialysis mono-anion membrane, the present invention solves the problem of catalyst loss in homogeneous catalytic wet oxidation, realizes the recycling of the catalyst, and at the same time realizes the reflux and secondary treatment of MDEA, greatly improving the treatment ability of the wet oxidation process.

[0030] (3) The method of the present invention solves the problem of poor economy in the treatment of low-concentration and large-flow wastewater. Through multi-effect evaporation, the MDEA wastewater is highly concentrated. Before entering the reactor, the temperature is increased through heat exchange, and a catalyst is introduced into the wet oxidation reactor to increase the reaction rate, so that the treatment cost of the whole wet oxidation process remains at a relatively low level.

[0031] (4) The method of the present invention solves the problems of high salt content after wastewater concentration and the resulting scaling and blockage of reactors and process pipelines. The hardening removal tank first solves the influence of the calcium and magnesium hardness of the wastewater on the subsequent membrane process; two-stage nanofiltration removes most of the monovalent salts such as sodium chloride and potassium chloride in the wastewater, reducing the salt accumulation in the wet oxidation cycle treatment process; single-membrane electrodialysis removes most of the anionic salts in the wastewater, such as sulfates and carbonates, realizing the salt balance in the wet oxidation cycle treatment process.

[0032] Other features and advantages of the present invention will be described in detail in the following specific implementation section. Brief Description of the Drawings

[0033] Figure 1 . Flow chart for the treatment of MDEA wastewater of the present invention. Specific Implementation Modes

[0034] The present invention will be further described in detail below with reference to specific embodiments. The embodiments are implemented on the premise of the technical solution of the present invention, and detailed implementation manners and specific operation processes are given, but the protection scope of the present invention is not limited to the following embodiments.

[0035] Example 1

[0036] Adopt Figure 1 The process method shown to treat MDEA wastewater.

[0037] The process flow is as Figure 1As shown in the figure: the MDEA wastewater first enters the dehardening tank to remove calcium and magnesium hardness, then enters the multi-effect evaporation for concentration, the condensate water is recycled, and the concentrated water enters the adjustment tank. After being mixed with the concentrated water of two-stage nanofiltration, the mother liquor of single-membrane electrodialysis, and the homogeneous catalyst in the adjustment tank, it enters the heat exchange unit. After being heated by exchanging heat with the effluent of the wet oxidation reactor, it enters the wet oxidation reactor; the MDEA wastewater is efficiently treated under the action of high temperature, high pressure and catalyst in the wet oxidation reactor. The effluent first exchanges heat in the heat exchange unit to cool down, then is cooled by a cooler, and finally enters two-stage nanofiltration. Most of the concentrated water of the first-stage nanofiltration is recycled to the adjustment tank, and a small part enters the single-membrane electrodialysis through the side line. The mother liquor of the single-membrane electrodialysis is sent back to the heat exchange unit, and the anion solution is sent to the chemical denitrification tank in the advanced treatment section. The water produced by the first-stage nanofiltration enters the second-stage nanofiltration, and all the concentrated water of the second-stage nanofiltration is recycled to the adjustment tank. The produced water flows to the chemical denitrification tank in the advanced treatment section; the chemical denitrification tank treats the water produced by the second-stage nanofiltration and the anion solution of the single-membrane electrodialysis, and removes ammonia nitrogen, total nitrogen, and COD by adding chlorine-containing oxides using the breakpoint chlorination principle. The effluent enters the biochemical tank for further treatment, and after the treatment is completed, it is discharged to the effluent monitoring tank and finally meets the discharge standard.

[0038] A certain MDEA wastewater has the following water quality: MDEA concentration is 4000mg / L, COD is 7000mg / L, total salt content is 3000mg / L, sulfate ion concentration is 350mg / L, calcium ion concentration is 200mg / L, magnesium ion concentration is 100mg / L, ammonia nitrogen is 3mg / L, total nitrogen is 600mg / L, and the wastewater flow rate is 18t / h.

[0039] The specific operating conditions for MDEA wastewater treatment are as follows: the above-mentioned MDEA wastewater first enters the dehardening tank, and chemicals are added according to a sodium hydroxide mass concentration of 250mg / L and a sodium carbonate mass concentration of 600mg / L. After clarification, the calcium ion concentration in the supernatant is reduced to less than 10mg / L, and the magnesium ion concentration is 6mg / L or less, and the others remain unchanged. The effluent enters multi-effect evaporation; the concentration multiple of multi-effect evaporation is 3, the MDEA concentration in the concentrated water rises to 11800mg / L, and the flow rate is 6t / h, flowing to the adjustment tank; the COD of the condensate water is lower than 30mg / L, there is almost no MDEA, the total salt content is lower than 200mg / L, the ammonia nitrogen is less than 5mg / L, and the flow rate is about 12t / h, reaching the direct discharge standard; before the concentrated water of multi-effect evaporation enters the heat exchange unit, it is mixed with the concentrated water of two-stage nanofiltration, the mother liquor of single-membrane electrodialysis, and the homogeneous catalyst. After mixing, the flow rate becomes 9.3t / h; the homogeneous catalyst uses a platinum-copper catalyst, which is a mixed salt of platinum chloride and copper chloride. The concentration of platinum metal ions in the mixed liquid is about 8mg / L, and the concentration of copper metal ions is about 180mg / L. After heat exchange, the liquid temperature rises to 150°C and enters the wet oxidation reactor; the reaction temperature of the wet oxidation reactor is 190°C, the pressure is 3MPa, and the liquid hourly space velocity is 2h -1, the gas-liquid volume ratio is 100:1, and the MDEA concentration in the reacted water is reduced to 502 mg / L; the effluent from wet oxidation is cooled to 55 °C by the heat exchange unit and then cooled to 35 °C by the cooler and enters the first-stage nanofiltration. The pore size of the first-stage nanofiltration membrane is 1.5 nm, and the water production rate is 78.5%. Metal catalysts, sulfates, carbonates, macromolecular organic substances (excluding MDEA), etc. are retained and enter the concentrated water side. Most of the sodium, potassium, chloride ions, ammonia nitrogen, MDEA, etc. pass through the nanofiltration membrane and enter the produced water side. The MDEA in the concentrated water side is 51 mg / L, and the MDEA in the produced water side is 770 mg / L; 50% of the first-stage nanofiltration concentrated water enters the single-mode electrodialysis treatment, and the remaining 50% is directly fed into the regulation pool. The anion solution of the single-mode electrodialysis flows to the chemical denitrification pool, and the mother liquor enters the regulation pool; the first-stage nanofiltration produced water enters the second-stage nanofiltration. The pore size of the second-stage nanofiltration membrane is 0.7 nm, and the water production rate is 75.3%. MDEA is retained on the concentrated water side. At this time, the MDEA in the second-stage nanofiltration concentrated water is 2650 mg / L and is refluxed to the regulation pool with a flow rate of 1.8 t / h. The MDEA in the second-stage nanofiltration produced water is 120 mg / L and flows to the chemical denitrification pool with a flow rate of 5.5 t / h; the influent flow rate of the chemical denitrification pool is 6 t / h, including the second-stage nanofiltration produced water and the single-mode electrodialysis anion solution. The ammonia nitrogen concentration is 283 mg / L, and the total nitrogen concentration is 309 mg / L. Sodium hypochlorite is added, and the molar ratio of chlorine dosage to ammonia is controlled at 1.5:1. The ammonia nitrogen in the effluent is reduced to 48 mg / L, and the total nitrogen is reduced to 75 mg / L and enters the biochemical pool; the salt concentration of the chemical denitrification pool flowing into the biochemical pool is 9209 mg / L. Considering the requirements of subsequent nitrification and denitrification microorganisms for salt, the stripping purified water is used as makeup water for salt dilution. After dilution, the salt concentration in the biochemical pool is 3880 mg / L; the biochemical pool adopts the A / O process plus post-denitrification. The final effluent has a COD of 46 mg / L, a total salt content of 3955 mg / L, an ammonia nitrogen of 6 mg / L, and a total nitrogen of 15 mg / L, meeting the discharge standards and can be discharged from the effluent monitoring pool. The daily loss rate of the metal catalyst is less than 0.3%, and basically all of it is recycled. The more specific reaction parameters and treatment efficiencies of each unit are shown in Table 1 and Table 2.

[0040] Table 1

[0041]

[0042] Table 2

[0043]

[0044] It can be seen from this example that the process method of the present invention realizes the efficient operation of homogeneous catalytic wet oxidation, solves the problem of loss of homogeneous catalyst through membrane technology, maintains the salt balance of the reaction system, and further solves the problems of high COD, difficult degradation, and high nitrogen in MDEA wastewater, and finally realizes the up-to-standard discharge of MDEA wastewater.

[0045] Example 2

[0046] The MDEA wastewater is treated by the process method shown below. Figure 1 A certain stream of MDEA wastewater has the following water quality: MDEA concentration 11000 mg / L, COD 20000 mg / L, total salt content 6000 mg / L, sulfate ion concentration 800 mg / L, calcium ion concentration 400 mg / L, magnesium ion concentration 100 mg / L, ammonia nitrogen 25 mg / L, total nitrogen 1800 mg / L, and wastewater flow rate 6 t / h.

[0047]

[0048] The above MDEA wastewater first enters the hardening removal tank, and chemicals are added according to a sodium hydroxide mass concentration of 300 mg / L and a sodium carbonate mass concentration of 900 mg / L. After clarification, the calcium ion concentration in the supernatant is reduced to less than 22 mg / L, the magnesium ion concentration is less than 3 mg / L, and the MDEA concentration is 10950 mg / L, which is greater than 10000 mg / L. The effluent bypasses the multi-effect evaporation and directly enters the regulation tank through the side line, where it is mixed with the concentrated water of two-stage nanofiltration, the mother liquor of single-membrane electrodialysis, and the homogeneous catalyst. After mixing, the flow rate becomes 9.4 t / h. The homogeneous catalyst uses a platinum-copper catalyst, which is a mixed salt of platinum chloride and copper chloride. The concentration of platinum metal ions in the mixed liquid is about 9 mg / L, and the concentration of copper metal ions is about 160 mg / L. After heat exchange, the liquid temperature rises to 148 °C and enters the wet oxidation reactor. The reaction temperature of the wet oxidation reactor is 185 °C, the pressure is 3.2 MPa, and the liquid hourly space velocity is 2 h -1 ​, the gas-liquid volume ratio is 90:1, and the MDEA concentration in the reacted water is reduced to 450 mg / L; the wet oxidation effluent is cooled to 53 °C by the heat exchange unit and then cooled to 34 °C by the cooler and enters the first-stage nanofiltration. The aperture of the first-stage nanofiltration membrane is 1.6 nm, and the water production rate is 79.8%. Metal catalysts, sulfates, carbonates, macromolecular organic matters (excluding MDEA), etc. are intercepted and enter the concentrated water side. Most of the sodium, potassium, chloride ions, ammonia nitrogen, MDEA, etc. pass through the nanofiltration membrane and enter the water production side. The MDEA in the concentrated water side is 50 mg / L, and the MDEA in the water production side is 700 mg / L; 31.6% of the first-stage nanofiltration concentrated water enters the single-mode electrodialysis treatment, and the remaining 68.4% is directly discharged into the regulation pool. The anion solution of the single-mode electrodialysis flows to the chemical denitrification pool, and the mother liquor enters the regulation pool; the first-stage nanofiltration water production enters the second-stage nanofiltration. The aperture of the second-stage nanofiltration membrane is 0.7 nm, and the water production rate is 74.7%. MDEA is intercepted to the concentrated water side. At this time, the MDEA in the second-stage nanofiltration concentrated water is 2550 mg / L and is refluxed to the regulation pool with a flow rate of 1.9 t / h. The MDEA in the second-stage nanofiltration water production is 110 mg / L and flows to the chemical denitrification pool with a flow rate of 5.6 t / h; the influent flow rate of the chemical denitrification pool is 6 t / h, including the second-stage nanofiltration water production and the single-mode electrodialysis anion solution. The ammonia nitrogen is 306 mg / L, and the total nitrogen is 410 mg / L. Sodium hypochlorite is added, and the molar ratio of the chlorine dosage to ammonia is controlled at 1.6:1. The effluent ammonia nitrogen is reduced to 33 mg / L, and the total nitrogen is reduced to 104 mg / L and enters the biochemical pool; the salt concentration of the water flowing from the chemical denitrification pool to the biochemical pool is 7568 mg / L. Considering the requirements of subsequent nitrification and denitrification microorganisms for salt, domestic sewage is used as makeup water for salt dilution. After dilution, the salt concentration in the biochemical pool is 3539 mg / L; the A / O process plus post-denitrification is adopted in the biochemical pool. The final effluent has a COD of 35 mg / L, a total salt content of 3580 mg / L, an ammonia nitrogen of 7 mg / L, and a total nitrogen of 21 mg / L, meeting the external discharge standards and can be discharged from the effluent monitoring pool. The daily loss rate of the metal catalyst is less than 0.3%, and basically all of it is recycled. The more specific reaction parameters and treatment efficiencies of each unit are shown in Table 3 and Table 4.

[0049] Table 3

[0050]

[0051] Table 4

[0052]

[0053] It can be seen from this example that the process method of the present invention can treat MDEA wastewater with different concentrations. By adjusting various methods such as the dosage of the hardness removal agent, the side line of multi-effect evaporation, the catalyst addition amount, the wet oxidation reaction parameters, the water production ratio of two-stage nanofiltration, the desalination rate of the first-stage nanofiltration concentrated water side line, the chemical denitrification chemical dosage, and the blending amount of low-salt water, the discharge of various indicators of MDEA wastewater meets the standards finally.

[0054] Example 3

[0055] The water quality of the MDEA wastewater treated in Example 3 is the same as that in Example 1, and the process route, implementation steps, and reaction parameters are also the same as those in Example 1. The difference is that the catalyst is changed to a platinum-iron catalyst, and the treatment results of the wet oxidation section are shown in Table 5.

[0056] Table 5

[0057]

[0058] It can be seen from this example that when the platinum-iron composite catalyst is used with the same dosage, compared with Example 1, the COD removal rate of homogeneous catalytic wet oxidation slightly increases, with a value of 93.2%, and the total nitrogen removal rate significantly decreases, with a value of 61.2%. It can be seen that different noble metal and transition metal composite catalysts have a greater impact on the total nitrogen removal rate of the wet oxidation reaction, while the COD removal rate can be maintained at a relatively high level.

[0059] Example 4

[0060] The water quality of the MDEA wastewater treated in Example 4 is the same as that in Example 1, and the process route, implementation steps, and reaction parameters are also the same as those in Example 1. The difference is that the catalyst is changed to a platinum-nickel catalyst, and the treatment results of the wet oxidation section are shown in Table 6.

[0061] Table 6

[0062]

[0063] It can be seen from this example that when the platinum-nickel composite catalyst is used with the same dosage, compared with Example 1, the COD removal rate of homogeneous catalytic wet oxidation decreases to 89.6%, and the total nitrogen removal rate decreases to 53.4%.

[0064] Example 5

[0065] The water quality of the MDEA wastewater treated in Example 5 is the same as that in Example 1, and the process route, implementation steps, and reaction parameters are also the same as those in Example 1. The difference is that the catalyst is changed to a platinum-nickel catalyst, and the treatment results of the wet oxidation section are shown in Table 7.

[0066] Table 7

[0067]

[0068] It can be seen from this example that when the platinum-nickel composite catalyst is used and the nickel dosage is increased to 300 mg / L, compared with Example 1, the COD of homogeneous catalytic wet oxidation decreases to 92.3%, and the total nitrogen removal rate decreases to 65.5%.

[0069] Example 6

[0070] In Example 6, the water quality of the MDEA wastewater to be treated is the same as that in Example 1, and the process route, implementation steps, and reaction parameters are also the same as those in Example 1. The difference is that the catalyst is changed to a palladium-iron catalyst, and the treatment results in the wet oxidation section are shown in Table 8.

[0071] Table 8

[0072]

[0073] It can be seen from this example that when the palladium-iron composite catalyst is used with the same dosage, compared with Example 1, the COD removal rate of homogeneous catalytic wet oxidation drops to 86.9%, and the total nitrogen removal rate drops to 69.8%.

[0074] Example 7

[0075] In Example 7, the water quality of the MDEA wastewater to be treated is the same as that in Example 1, and the process route, implementation steps, and reaction parameters are also the same as those in Example 1. The difference is that the catalyst is changed to a palladium-copper catalyst, and the treatment results in the wet oxidation section are shown in Table 9.

[0076] Table 9

[0077]

[0078] It can be seen from this example that when the palladium-copper composite catalyst is used with the same dosage, compared with Example 1, the COD removal rate of homogeneous catalytic wet oxidation drops to 85.2%, and the total nitrogen removal rate rises to 81.2%.

[0079] Example 8

[0080] In Example 8, the water quality of the MDEA wastewater to be treated is the same as that in Example 1, and the process route, implementation steps, and reaction parameters are also the same as those in Example 1. The difference is that the catalyst is changed to a rhodium-copper catalyst, and the treatment results in the wet oxidation section are shown in Table 10.

[0081] Table 10

[0082]

[0083] It can be seen from this example that when the rhodium-copper composite catalyst is used with the same dosage, compared with Example 1, the COD removal rate of homogeneous catalytic wet oxidation rises to 94.5%, and the total nitrogen removal rate rises to 85.3%. Considering that the price of the noble metal rhodium is much higher than that of platinum, and the platinum-copper catalyst can meet the treatment requirements, it is recommended to preferably use the platinum-copper catalyst.

[0084] Comparative Example 1

[0085] The water quality of the MDEA wastewater treated in Comparative Example 1 is the same as that in Example 1. The process route, implementation steps, and reaction parameters are also the same as those in Example 1. However, no precious metal catalyst is added, and only a transition metal catalyst is added. The treatment results in the wet oxidation section are shown in Table 11.

[0086] Table 11

[0087]

[0088] From this comparative example, it can be seen that without adding a precious metal catalyst and only adding a copper metal catalyst, the COD removal rate and total nitrogen removal rate in homogeneous catalytic wet oxidation decrease significantly. The COD removal rate is only 58.5%, and the total nitrogen removal rate is only 55.4%.

[0089] Comparative Example 2

[0090] The water quality of the MDEA wastewater treated in Comparative Example 2 is the same as that in Example 1. The process route, implementation steps, and reaction parameters are also the same as those in Example 1. However, no catalyst is added. The reaction temperature in the wet oxidation reactor is 190 °C, the pressure is 3 MPa, the liquid hourly space velocity is 2 h -1 , and the gas-liquid volume ratio is 100:1. The treatment results in the wet oxidation section are shown in Table 12.

[0091] Table 12

[0092]

[0093] From this comparative example, it can be seen that without adding a catalyst and using conventional wet oxidation, the final COD removal rate is only 38.6%, and the total nitrogen removal rate is only 9.6%.

[0094] Comparative Example 3

[0095] The water quality of the MDEA wastewater treated in Comparative Example 3 is the same as that in Example 1. The process route, implementation steps, and reaction parameters are also basically the same as those in Example 1. However, no catalyst is added. The reaction temperature in the wet oxidation reactor is 240 °C, the pressure is 5 MPa, the liquid hourly space velocity is 2 h -1 , and the gas-liquid volume ratio is 100:1. The treatment results in the wet oxidation section are shown in Table 13.

[0096] Table 13

[0097]

[0098] From this comparative example, it can be seen that without adding a catalyst and using conventional wet oxidation, the reaction temperature is increased to 240 °C, the pressure is 5 MPa, and other conditions remain unchanged. Finally, the COD removal rate is 65.8%, and the total nitrogen removal rate is only 10.5%. The substantial increase in the reaction temperature and pressure of wet oxidation means a substantial increase in operating costs, while the final treatment effect is not ideal. In particular, the total nitrogen removal rate has no obvious improvement compared with Comparative Example 2 and has a large gap compared with Example 1.

Claims

1. A treatment method for MDEA wastewater, comprising a pretreatment section, a wet oxidation section, and a deep treatment section; The pretreatment section includes a dehardening tank, a multi-effect evaporator, and an adjustment tank; the MDEA wastewater first enters the dehardening tank to remove calcium and magnesium hardness. When the MDEA concentration in the wastewater ≤ 10,000 mg / L, the dehardened wastewater enters the multi-effect evaporator for concentration, the condensed water is recycled, and the concentrated thick water enters the adjustment tank, where it is mixed with a homogeneous catalyst, the concentrated water of two-stage nanofiltration, and the mother liquor of single-membrane electrodialysis; when the MDEA concentration in the wastewater > 10,000 mg / L, it directly enters the adjustment tank; The wet oxidation section includes a heat exchange unit, a wet oxidation reactor, a cooler, a first-stage nanofiltration, a second-stage nanofiltration, and a single-membrane electrodialysis; the effluent from the adjustment tank enters the heat exchange unit. The heat exchange unit uses a heat exchanger to exchange heat between the effluent from the wet oxidation reactor and the effluent from the adjustment tank. The effluent from the wet oxidation reactor flows through the tube side, and the effluent from the adjustment tank flows through the shell side. After heat exchange and temperature rise, the effluent enters the wet oxidation reactor; the effluent from the wet oxidation reactor is cooled by the heat exchange unit and then cooled by the cooler, and then enters the first-stage nanofiltration and the second-stage nanofiltration for treatment. Among them, part of the concentrated water of the first-stage nanofiltration is refluxed to the adjustment tank, and part enters the single-membrane electrodialysis. The product water of the first-stage nanofiltration enters the second-stage nanofiltration. The concentrated water of the second-stage nanofiltration is all sent back to the adjustment tank, and the product water of the second-stage nanofiltration enters the deep treatment section ； The mother liquor of the single-membrane electrodialysis is sent back to the heat exchange unit, and the anion solution enters the deep treatment section; The deep treatment section includes a chemical denitrification tank, a biochemical tank, and an effluent monitoring tank; The product water of the second-stage nanofiltration and the anion solution of the single-membrane electrodialysis enter the chemical denitrification tank, mainly for the removal of ammonia nitrogen and COD. The effluent from the chemical denitrification tank enters the biochemical tank and is further treated by biological methods. After the treatment is completed, it is discharged to the effluent monitoring tank and finally meets the discharge standards; Among them, the homogeneous catalyst is a noble metal and transition metal composite catalyst; the noble metal is selected from one or more of platinum, palladium, rhodium, silver, and ruthenium, and the transition metal is selected from one or more of copper, iron, manganese, zinc, and nickel. The mass percentage of the noble metal in the homogeneous catalyst is 2% - 20%; the noble metal and the transition metal exist in the form of metal salt compounds or complexes and are dissolved in the liquid phase; The product water rate of the first-stage nanofiltration is 60% - 85%, and the pore size of the first-stage nanofiltration membrane is between 1 - 3 nm, which intercepts the metals of the homogeneous catalyst, high-valent salts, and macromolecular organic substances, and after being intercepted, enters the concentrated water side; 1% - 60% of the concentrated water of the first-stage nanofiltration enters the single-membrane electrodialysis. The single-membrane electrodialysis uses an anion membrane. Under the action of the electrode drive and the anion membrane, the high-valent anion salts and negatively charged small molecule organic substances in the concentrated water of the first-stage nanofiltration permeate through the anion membrane and enter the concentrated water side, and the cation catalyst remains in the mother liquor and is recycled again.

2. The method according to claim 1, characterized in that, a combined reagent of sodium hydroxide and sodium carbonate is used for dehardening in the dehardening tank.

3. The method according to claim 2, characterized in that, sodium hydroxide is added in an amount of 1 - 4 times the mass concentration of magnesium ions, and sodium carbonate is added in an amount of 1 - 3 times the mass concentration of calcium ions.

4. The method according to claim 1, characterized in that, The temperature of the multi-effect evaporation steam is controlled below 150°C, and the concentration multiple is 2 to 5 times.

5. According to the method described in claim 1, it is characterized in that the heat exchange unit raises the temperature of the effluent from the regulating tank to 130 - 160°C and reduces the temperature of the effluent from the wet oxidation reactor to 45 - 75°C.

6. According to the method described in claim 1, it is characterized in that the homogeneous catalyst is a platinum-copper composite catalyst.

7. According to the method described in claim 1, it is characterized in that the homogeneous catalyst is only added in large amounts at the start-up of the device, and is added according to the mass ratio of MDEA to metal ions of 2000:1 to 2:1; during normal operation, it is appropriately supplemented according to the catalyst loss rate and the change in MDEA concentration.

8. According to the method described in claim 1, it is characterized in that the wet oxidation reactor is a bubbling flow internal circulation reactor with an inner cylinder, and oxygen is used as the oxidant.

9. According to the method described in claim 8, it is characterized in that The reaction temperature of the wet oxidation reactor described is 150 - 250 °C, the reaction pressure is 2 MPa - 6 MPa, the liquid hourly space velocity is 0.25 - 4 h -1 , and the gas-liquid volume ratio is 20:1 - 300:

1.

10. According to the method described in claim 1, it is characterized in that the water production rate of the secondary nanofiltration is 55% - 80%, the pore size of the secondary nanofiltration membrane is between 0.5 and 1 nm, MDEA is intercepted to the concentrated water side and finally refluxed to the wet oxidation reactor for further treatment, and the un-intercepted small molecule organic substances and monovalent salts are on the water production side and enter the advanced treatment section.

11. According to the method described in claim 1, it is characterized in that the biochemical pool adopts a combined process of A / O and post-denitrification; in the A / O process, the A section is in the front and the O section is in the back.

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