A treatment process and system for high-salt and high-cod wastewater in a natural gas processing industry

Abstract

The application discloses a treatment process and system for high-salt and high-COD wastewater in a natural gas processing industry, and belongs to the technical field of environmental engineering water treatment. The treatment process for the high-salt and high-COD wastewater in the natural gas processing industry is characterized by five main steps of ozone catalytic oxidation, Fenton oxidation, ultrafiltration concentration, nanofiltration concentration and frozen crystallization, so that the chroma, organic matter and reducing sulfur in the high-salt and high-COD wastewater produced by a Cansolv tail gas treatment device are effectively removed, the water quality of subsequent membrane water is improved, the membrane pollution risk of a membrane system in an operation process is reduced, the service life of the membrane system is prolonged, the high-salt and high-COD wastewater in the natural gas processing industry is ensured to reach the standard for external discharge, and high-quality crystalline salt is obtained.

Description

Technical Field

[0001] This invention relates to the field of environmental engineering water treatment technology, specifically to a treatment process and system for high-salt, high-COD wastewater in the natural gas processing industry. Background Technology

[0002] In the natural gas processing industry, the Cansolv clean and regenerative desulfurization process is used to treat the tail gas from natural gas purification units. The water-soluble organic amine solution used in this process exhibits high selectivity and can efficiently remove sulfur dioxide (SO2) from the tail gas, achieving a final emission level below 400 mg / m³. 3 The ultra-low SO2 emission requirements of the Cansolv exhaust gas treatment unit offer significant technological advantages and environmental benefits. During operation, the pre-washing (Venturi) unit and the amine purification (APU) unit generate acidic and alkaline wastewater, respectively. The APU alkaline wastewater is particularly high in salt, COD, and reducing sulfur, significantly impacting the flocculation-softening-ultrafiltration-resin exchange-two-stage reverse osmosis wastewater treatment process. Higher temperatures and high COD and salt content in the reverse osmosis membrane promote microbial growth, leading to membrane fouling, reduced membrane flux, and decreased membrane lifespan. Furthermore, high COD wastewater can cause clogging and foaming in the evaporation crystallization unit, resulting in low crystallization salt quality issues.

[0003] Currently, advanced oxidation technology is a relatively effective method for the deep treatment of industrial wastewater, with advantages such as good degradation effect on high organic content, high oxidation efficiency, and no secondary pollution. Ozone (O3) oxidation is widely used in the deep treatment of wastewater, but O3 oxidation alone is insufficient to significantly degrade organic matter, making it impossible to treat wastewater to meet discharge standards. Summary of the Invention

[0004] The purpose of this invention is to provide a treatment process and system for high-salt, high-COD wastewater in the natural gas processing industry, which solves the problem that single O3 oxidation treatment in the deep treatment of industrial wastewater is difficult to significantly degrade organic matter and cannot achieve the standard for wastewater discharge.

[0005] This invention is achieved through the following technical solution:

[0006] This invention provides a treatment process for high-salt, high-COD wastewater in the natural gas processing industry, comprising:

[0007] Ozone is introduced into high-salt, high-COD wastewater, and graphene catalyst is added to carry out a primary oxidation reaction.

[0008] Fenton oxidant was added to the wastewater after the primary oxidation reaction to carry out a secondary oxidation reaction;

[0009] After the secondary oxidation reaction, the wastewater is allowed to settle and settle to obtain the primary supernatant.

[0010] Adjust the pH of the primary supernatant to alkaline, add flocculant, and allow it to settle to obtain the secondary supernatant;

[0011] The secondary supernatant was concentrated by ultrafiltration to obtain the tertiary supernatant;

[0012] The supernatant from the three stages was concentrated by nanofiltration to obtain concentrated water and desalinated water, respectively.

[0013] The concentrated water was freeze-crystallized to obtain sodium sulfate decahydrate crystals, which were then dried to obtain anhydrous sodium sulfate industrial salt.

[0014] Furthermore, in the treatment process of high-salt, high-COD wastewater in the natural gas processing industry, the concentration of ozone introduced is 3 g / h to 12 g / h.

[0015] Furthermore, in the treatment process of high-salt, high-COD wastewater in the natural gas processing industry, the catalyst is three-dimensional redox graphene, and the dosage of the catalyst is 0.1 g / L to 0.3 g / L.

[0016] Furthermore, in the treatment process of high-salt, high-COD wastewater in the natural gas processing industry, the Fenton oxidant is a mixture of ferrous sulfate heptahydrate and hydrogen peroxide, with the dosage of ferrous sulfate heptahydrate being 2 g / L to 16 g / L and the dosage of hydrogen peroxide being 5 mL / L to 15 mL / L.

[0017] Furthermore, in the treatment process of high-salt, high-COD wastewater in the natural gas processing industry, the pH value of the primary supernatant is adjusted to 10-11.

[0018] Furthermore, in the treatment process of high-salt, high-COD wastewater in the natural gas processing industry, the flocculant is anionic polyacrylamide, and the dosage of the flocculant is 0.5 mg / L to 2.5 mg / L.

[0019] Furthermore, in the treatment process of high-salt, high-COD wastewater in the natural gas processing industry, the pressure of ultrafiltration concentration is 0.2 MPa to 0.4 MPa; and the pressure of nanofiltration membrane is 2 MPa to 4 MPa.

[0020] Furthermore, in the treatment process for high-salt, high-COD wastewater in the natural gas processing industry, the conditions for freeze crystallization include:

[0021] After pre-cooling at -15℃ to -10℃ for 20 to 30 minutes, the freezing crystallization temperature is -3℃ to 0℃, and the freezing crystallization is carried out for 35 to 55 hours, resulting in a concentration of 5 to 10 times.

[0022] Furthermore, in the treatment process of high-salt, high-COD wastewater in the natural gas processing industry, the drying conditions include: a drying temperature of 90℃~110℃ and a drying time of 20min~30min.

[0023] This invention also provides a treatment system for the above-mentioned treatment process of high-salt, high-COD wastewater in the natural gas processing industry, comprising:

[0024] The system consists of an ozone catalytic oxidation system, a Fenton oxidation system, a tubular ultrafiltration membrane system, a nanofiltration membrane concentration system, and a freeze crystallization system connected in sequence.

[0025] The ozone catalytic oxidation system incorporates three-dimensional redox graphene.

[0026] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0027] The treatment process for high-salt, high-COD wastewater in the natural gas processing industry provided in this invention involves five main steps: ozone catalytic oxidation, Fenton oxidation, ultrafiltration concentration, nanofiltration concentration, and freeze crystallization. This process effectively removes color, organic matter, and reducing sulfur from the high-salt, high-COD wastewater produced by the Cansolv tail gas treatment unit, improves the quality of subsequent influent water, reduces the risk of membrane fouling during membrane system operation, extends the service life of the membrane system, ensures that the high-salt, high-COD wastewater from the natural gas processing industry meets discharge standards, and obtains high-quality crystalline salt. Attached Figure Description

[0028] To more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be considered as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort. In the drawings:

[0029] Figure 1 This is a schematic diagram of the structure of the high-salt, high-COD wastewater treatment system in the natural gas processing industry according to the present invention;

[0030] Figure 2 The TOC removal rate under different O3 concentrations or different pH values ​​in Example 1 of this invention;

[0031] Figure 3 The crystallized salts obtained from different water samples using different crystallization processes in Implementation Example 2 of this invention are as follows. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments and accompanying drawings. The illustrative embodiments and descriptions of this invention are for explanation only and are not intended to limit the invention. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0033] An embodiment of the present invention provides a treatment process for high-salt, high-COD wastewater in a natural gas processing industry, comprising:

[0034] (1) Ozone catalytic oxidation: Ozone is introduced into high-salt, high-COD wastewater, and a graphene catalyst is added simultaneously to carry out a primary oxidation reaction; wherein, the concentration of ozone introduced is 3 g / h to 12 g / h, which can be 3 g / h, 4 g / h, 5 g / h, 6 g / h, 7 g / h, 8 g / h, 10 g / h or 12 g / h, preferably 6 g / h. The catalyst is three-dimensional redox graphene, and the catalyst dosage is 0.1 g / L to 0.3 g / L, which can be 0.1 g / L, 0.15 g / L, 0.2 g / L, 0.22 g / L, 0.25 g / L or 0.3 g / L, preferably 0.22 g / L.

[0035] (2) Fenton oxidation: A secondary oxidation reaction is carried out by adding Fenton oxidant to the wastewater after the primary oxidation reaction; wherein, the Fenton oxidant is a mixture of ferrous sulfate heptahydrate and hydrogen peroxide, the dosage of ferrous sulfate heptahydrate is 2 g / L to 16 g / L, which can be 2 g / L, 4 g / L, 6 g / L, 8 g / L, 10 g / L, 12 g / L, 14 g / L or 16 g / L, preferably 10 g / L; the dosage of hydrogen peroxide is 5 mL / L to 15 mL / L, which can be 5 mL / L, 7 mL / L, 10 mL / L, 12 mL / L or 15 mL / L, preferably 10 mL / L. The stirring speed during Fenton oxidation is 150 s. -1 ~200s -1 The stirring time is 60 min to 90 min.

[0036] (3) Settling: The wastewater after the secondary oxidation reaction is allowed to settle for 20-40 minutes to obtain primary supernatant; the pH of the primary supernatant is adjusted to 10-11, flocculant is added, and the mixture is allowed to settle for 20-40 minutes to obtain secondary supernatant. The flocculant is anionic polyacrylamide, and the dosage is 0.5 mg / L to 2.5 mg / L, preferably 1 mg / L, 0.75 mg / L, 1 mg / L, 1.5 mg / L, 2 mg / L, or 2.5 mg / L. The stirring speed during flocculation is 30 seconds. -1 ~60s-1 Stirring time: 20-40 minutes; standing time: 20-40 minutes.

[0037] (4) Ultrafiltration concentration: The secondary supernatant is concentrated by ultrafiltration to obtain the tertiary supernatant; the pressure of ultrafiltration concentration is 0.2MPa to 0.4MPa, which can be 0.2MPa, 0.3MPa or 0.4MPa. After ultrafiltration, fresh water and concentrated water are obtained. The fresh water is the tertiary supernatant, which is the feed water for nanofiltration concentration.

[0038] (5) Nanofiltration Concentration: The supernatant from the three stages is concentrated by nanofiltration to obtain concentrated water and distilled water, respectively; wherein the pressure of the nanofiltration membrane is 2MPa to 4MPa, which can be 2MPa, 2.5MPa, 3MPa, 3.5MPa or 4MPa. After nanofiltration, distilled water and concentrated water are obtained. The distilled water is recycled to the remaining equipment, and the concentrated water is subjected to freeze crystallization.

[0039] (6) Freeze-crystallization: The concentrated water is freeze-crystallized to obtain sodium sulfate decahydrate crystals, which are then dried to obtain anhydrous sodium sulfate industrial salt. The freeze-crystallization conditions include: a pre-cooling treatment of -15℃ to -10℃ for 20 to 30 minutes, followed by a freeze-crystallization temperature of -3℃ to 0℃ for 35 to 55 hours, resulting in a concentration of 5 to 10 times. The drying conditions include: a drying temperature of 90℃ to 110℃ and a drying time of 20 to 30 minutes.

[0040] The treatment process for high-salt, high-COD wastewater in the natural gas processing industry provided in this invention involves five main steps: ozone catalytic oxidation, Fenton oxidation, ultrafiltration concentration, nanofiltration concentration, and freeze crystallization. This process effectively removes color, organic matter, and reducing sulfur from the high-salt, high-COD wastewater produced by the Cansolv tail gas treatment unit, improves the quality of subsequent influent water, reduces the risk of membrane fouling during membrane system operation, extends the service life of the membrane system, ensures that the high-salt, high-COD wastewater from the natural gas processing industry meets discharge standards, and obtains high-quality crystalline salt.

[0041] In ozone catalytic oxidation, O3 catalytic oxidation breaks down large organic molecules into smaller ones, while also sterilizing, decolorizing, and regulating pH. Three-dimensional redox graphene (3D-rGO) provides more vacancy sites for the O3 reaction, improving O3 utilization. This 3D-rGO can be effectively recovered and reused through centrifugation.

[0042] In Fenton oxidation, macromolecular organic compounds are further broken down into smaller organic molecules by the action of Fenton's reagent.

[0043] In the static sedimentation process, the ferric hydroxide produced by Fenton oxidation acts as a coagulant, precipitating suspended solids, colloids, and other impurities in the wastewater to obtain a primary supernatant. Then, sodium hydroxide is added to adjust the pH to 10-11 (alkaline), and flocculants are added to further precipitate suspended solids, colloids, and other impurities, yielding a secondary supernatant.

[0044] In ultrafiltration concentration: it is used to further reduce the organic matter content, and is the secondary supernatant; after ultrafiltration, fresh water and concentrated water are obtained, and the fresh water is the tertiary supernatant, which is the feed water for nanofiltration concentration.

[0045] In nanofiltration concentration: it is used to concentrate ions to facilitate subsequent freeze crystallization; the three-stage supernatant is concentrated by nanofiltration to obtain concentrated water and desalinated water respectively; the desalinated water is reused, and the concentrated water is used for cooling crystallization.

[0046] In the freeze crystallization process: concentrated water is freeze crystallized to obtain sodium sulfate decahydrate crystals, which are then dried to obtain anhydrous sodium sulfate industrial salt.

[0047] This invention provides a combined O3 catalytic oxidation-Fenton oxidation process. First, O3 catalytic oxidation is used to break down electron-rich organic matter in wastewater, followed by a secondary Fenton oxidation process to enhance the treatment effect. This combined O3 catalytic oxidation-Fenton oxidation process combines the strong oxidizing and electrophilic properties of O3 with the strong addition reaction characteristics of the hydroxyl radicals (·OH) generated by the Fenton reaction, enabling the oxidation of most organic matter in wastewater, making it particularly suitable for recalcitrant organic wastewater.

[0048] Freeze-crystallization is gaining increasing attention for treating high-salinity water because its energy intensity is lower than that of evaporation processes, and the latent heat of fusion is approximately six times that of water vaporization, thus significantly reducing energy consumption. Compared to evaporative crystallization, which can lead to scaling and corrosion, freeze-crystallization minimizes corrosion due to its low operating temperature, and its operation under eutectic conditions can remove scaling salts. Furthermore, the crystalline salts formed during freeze-crystallization have relatively high purity, with other contaminants being removed from the crystals, and it is applicable to all soluble contaminants, achieving a removal rate close to 100%.

[0049] Therefore, the O3 catalytic oxidation-Fenton oxidation combined treatment process provided in this invention is an effective technology for treating high-salt, high-COD wastewater from natural gas purification to meet discharge standards. The concentrated water after nanofiltration undergoes a freeze crystallization process to produce salt. This process not only significantly reduces the organic matter content but also greatly improves the purity of the originally low-purity evaporated crystallized salt through freeze crystallization, preventing environmental pollution and harm to human health caused by production wastewater, and further ensuring the green and sustainable development of the natural gas processing industry.

[0050] In the treatment process provided in this invention embodiment, O3 has strong oxidizing properties and can preferentially react with electron-rich and unsaturated organic matter in water to generate small molecule organic matter. The wastewater generated by the APU unit of the Cansolv tail gas treatment device contains amine organic matter. O3 partially reacts with electron-rich amines, and by initially adding it to the lone pair of nitrogen electrons, an O3 adduct RNOOO is formed, which rapidly decomposes it. After ozone catalytic oxidation, the pH of the original water sample drops from 12 to about 2.3, creating optimal reaction conditions for the subsequent Fenton reaction and saving the step of adjusting the pH to strong acidity. Then, the large amount of ·OH generated by Fenton oxidation is used to mineralize the organic matter, and finally the pH is adjusted to 10-11. No impurity ions are introduced in the entire oxidation process. The oxidized water sample is further reduced by ultrafiltration membrane, and then sodium sulfate is enriched by nanofiltration membrane and then frozen crystallized. Compared with existing technologies for treating wastewater from natural gas purification, this method combines two oxidation units—O3 catalytic oxidation and Fenton oxidation—to achieve multiple functions such as oxidation, coagulation, sterilization, and decolorization, reducing the amount of reagents required. Furthermore, the combined process significantly reduces organic matter, and subsequent freeze crystallization further utilizes the salt resources.

[0051] In the natural gas processing industry, high-salt, high-COD wastewater mainly contains soluble sodium sulfate, as well as amine organic matter and a large amount of unsaturated organic matter (esters). Because the organic amine solution absorbs sulfur dioxide, the wastewater contains reducing sulfur. The method provided in this invention effectively reduces organic matter and completely removes reducing sulfur.

[0052] The following are explanations of the terms used in the treatment process of high-salt, high-COD wastewater in the natural gas processing industry provided in this embodiment of the invention:

[0053] Coagulants: Coagulants are substances that can cause suspended solids, colloidal particles and other impurities in water to form flocculent precipitates.

[0054] Flocculants: Any substance used to precipitate solutes, colloids, or suspended particles in an aqueous solution into flocculent precipitates is called a flocculant.

[0055] Coagulation: When suspended particles in water are small enough, the energy of their Brownian motion is sufficient to counteract gravity, preventing sedimentation. Such suspensions can remain stable for extended periods. Furthermore, the surfaces of suspended particles are often charged (usually negatively charged), and the repulsive force between particles of the same charge makes them less likely to coalesce and grow larger, thus increasing the stability of the suspension. The coagulation process involves adding a positively charged coagulant to neutralize the negative charge on the particle surfaces, causing the particles to "destabilize." As a result, the particles combine and grow larger through collisions, surface adsorption, and van der Waals forces, facilitating separation from the water.

[0056] Flocculation: Flocculation is the process by which polymer chains bridge between suspended particles.

[0057] "shelf

[0058] "Bridge" refers to the adsorption of different segments of polymer molecules onto different particles, promoting particle aggregation.

[0059] Oxidation: Generally speaking, the loss of electrons is an oxidation reaction, and the gain of electrons is a reduction reaction. The reaction in which organic matter introduces oxygen or removes hydrogen is called oxidation; the introduction of hydrogen or the loss of oxygen is called reduction. Slow oxidation, where a substance reacts with oxygen and heats up slowly without emitting light, is called slow oxidation, such as metal corrosion and biological respiration. Vigorous oxidation involving light emission and heat is called combustion.

[0060] Oxidizing agent: A substance that gains electrons in a redox reaction. Oxidizing agents have oxidizing properties; when they gain electrons, their oxidation state decreases, leading to a reduction reaction and the production of reduction products.

[0061] Freeze-crystallization: A phase transition that yields a crystalline solid from solution, which can only be achieved in supersaturated solutions; that is, solutions where the solute concentration exceeds the equilibrium solute concentration. Supersaturation provides the thermodynamic driving force for the crystallization process, thus providing the resulting particle nucleation and system growth characteristics. However, supersaturated solutions do not always lead to crystallization, as the solution may be in a metastable state. Metastable states depend on the system's kinetics, and crystallization only occurs when the metastable limit is reached or when a seed crystal is added to the solution.

[0062] In addition, please refer to Figure 1 This invention also provides a treatment system corresponding to the treatment process for high-salt, high-COD wastewater in the natural gas processing industry, including:

[0063] The system consists of an ozone catalytic oxidation system, a Fenton oxidation system, an ultrafiltration membrane system, a nanofiltration membrane concentration system, and a freeze crystallization system, connected in sequence.

[0064] The ozone catalytic oxidation system incorporates three-dimensional redox graphene.

[0065] High-salt, high-COD wastewater is sequentially sent to an ozone catalytic oxidation system, a Fenton oxidation system, a tubular ultrafiltration membrane system, a nanofiltration membrane concentration system, and a freeze crystallization system for treatment, ultimately yielding fresh water and anhydrous sodium sulfate.

[0066] The following are specific examples:

[0067] Take the wastewater from the tail gas treatment unit of a natural gas purification plant as an example.

[0068] The water quality of high-salt, high-COD wastewater from the natural gas processing industry is shown in Table 1 after testing.

[0069] Table 1. Water quality characteristics of high-salinity, high-COD wastewater from a natural gas processing plant.

[0070]

[0071]

[0072] A process for treating high-salt, high-COD wastewater in the natural gas processing industry, comprising:

[0073] (1) O3 and a catalyst were introduced into the above-mentioned high-salt, high-COD wastewater for catalytic oxidation, decolorization, and sterilization. The resulting water sample was then treated with Fenton's reagent for Fenton oxidation. The ozone input was 6 g / h; the catalyst was 3D-rGO, added at a rate of 0.22 g / L wastewater. Fenton's reagent was a mixture of ferrous sulfate heptahydrate and hydrogen peroxide. The dosage of ferrous sulfate heptahydrate was 10 g / L, the dosage of hydrogen peroxide was 30 mL / L, and the reaction time was 60 min. The water quality of the treated sample is shown in Table 2.

[0074] Table 2 Water quality characteristics after oxidation treatment

[0075]

[0076]

[0077] (2) Adjust the pH of the water sample after the two-stage oxidation treatment in step (1) to 10 with sodium hydroxide to precipitate iron ions; add flocculant, let it stand to precipitate, the flocculant is anionic polyacrylamide, the amount of flocculant added is 1 mg / L, the pH of the water sample during flocculation treatment is 10, the standing time is 30 min, filter to separate the flocs, and obtain the supernatant.

[0078] (3) The water sample after flocculation and sedimentation treatment in step (2) was passed through an ultrafiltration membrane system. The ultrafiltration membrane can further reduce the organic matter content, and its operating pressure is 0.2 MPa. Ultrafiltration mainly intercepts large molecular organic matter. After ultrafiltration, the TOC decreased from 92.2 mg / L to 80 mg / L, while other indicators remained basically unchanged.

[0079] (4) The water sample after ultrafiltration in step (3) is further concentrated with ions through a nanofiltration membrane to facilitate subsequent freeze crystallization. The operating pressure is 2 MPa.

[0080] (5) The concentrated water obtained from nanofiltration in step (4) was placed in an environment of -3℃ for freeze crystallization, and sodium sulfate decahydrate crystals precipitated at the bottom of the solution. Sodium sulfate decahydrate was dried to obtain relatively pure anhydrous sodium sulfate at a drying temperature of 105℃ for 20-30 min. The parameters for freeze crystallization are shown in Table 3.

[0081] Table 3. Freeze-crystallization parameters

[0082]

[0083] After freeze crystallization, the yield of anhydrous sodium sulfate was 10.5%.

[0084] Case Study 1: The Effects of O3 Concentration and pH on TOC Removal Rate

[0085] For the high-salt, high-COD wastewater from the natural gas processing industry in the above examples, the treatment process of Example 1 was used to investigate the effect of O3 concentration or pH range on TOC removal rate by simply changing the O3 concentration or pH range. The results are as follows: Figure 2 As shown.

[0086] Figure 2 (a) shows the TOC removal rate under different O3 concentrations. Figure 2 (b) shows the TOC removal rate at different pH levels. The wastewater appears pale yellow due to amine organic compounds. O3 reacts in two ways: directly with organic matter under acidic conditions, and indirectly with organic matter by generating hydroxyl radicals under alkaline conditions. Because O3 has the characteristic of rapidly reacting with amine organic compounds (electron-rich organic compounds), it destroys the molecular structure of amine organic compounds, and the water sample becomes clear and transparent after O3 oxidation. Figure 2 (a) It can be seen that with the increase of O3 concentration, TOC removal first increases and then tends to stabilize. When the O3 output concentration reaches 6 g / h, the TOC removal rate can reach 21%; Figure 2 (b) It can be seen that the removal effect of organic matter is better in an alkaline environment.

[0087] Experimental conclusion: After O3 oxidation, the wastewater changed from pale yellow to clear and transparent. At an O3 output concentration of 6 g / h and a pH of 12, the TOC removal rate was 21%.

[0088] Case Study 2: The Effect of Crystallization Process on the Nutritional Value of Crystallized Salts

[0089] Wastewater from the APU unit of the Cansolv tail gas treatment device in a natural gas purification plant, as described in the above embodiment, was used. The treatment involved raw water evaporation crystallization, post-oxidation evaporation crystallization, raw water freeze crystallization, and post-oxidation freeze crystallization. The oxidation treatment followed the specific steps of the above embodiment. The freeze crystallization step involved concentrating the oxidized water sample to more than 5 times its original volume, pre-cooling it in a freezer (-15℃) for 20-30 minutes, and then crystallizing the pre-cooled sample at approximately -3℃ for more than 48 hours to precipitate sodium sulfate decahydrate. This was then dried to obtain anhydrous sodium sulfate crystals at 105℃ for 20-30 minutes. Evaporation crystallization yielded the following crystalline salt: Figure 3 As shown.

[0090] Figure 3The crystallized salts obtained from different water samples using different crystallization processes show that the solid salt obtained by directly evaporating and crystallizing the raw water is yellow, which is because organic amines and other organic matter are present in the wastewater; the solid salt obtained by freezing and crystallizing the raw water is pale yellow, indicating that freezing and crystallization can significantly remove organic matter.

[0091] The solid salt obtained by evaporating and crystallizing the oxidized water sample is pale yellow. This is because after the oxidation treatment by the process of this invention, the organic matter in the wastewater is greatly degraded. Compared with the evaporation and crystallization of the original water, the purity and quality of the salt obtained by evaporating and crystallizing the oxidized water sample are improved.

[0092] The solid salt obtained by freezing and crystallizing the oxidized water sample is white and consists of fine granular powder, meeting the appearance standards for anhydrous sodium sulfate industrial salt. However, the quality of the crystallized salt is lower than that of the frozen crystallized salt. This is because evaporation crystallization leads to problems such as poor quality of sodium sulfate crystallized salt, equipment foaming and corrosion, and high energy consumption. Freeze crystallization utilizes the precipitation temperature of sodium sulfate to directly precipitate the salt, resulting in very few impurities and high purity. Therefore, using freeze crystallization to generate crystallized salt avoids the problems caused by evaporation crystallization.

[0093] Experimental conclusion: Anhydrous sodium sulfate that has undergone freeze crystallization treatment has higher purity and meets the appearance standard of white, uniform, fine granular powder for industrial anhydrous sodium sulfate.

[0094] After being treated by the process of this invention, the wastewater not only completely removes color, but also significantly reduces the total organic carbon content and scale ion concentration, thereby mitigating the risk of membrane clogging in subsequent processes and effectively extending membrane lifespan. Furthermore, the subsequent freeze crystallization process can greatly improve the purity of anhydrous sodium sulfate crystals.

[0095] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A treatment process for high-salt, high-COD wastewater in the natural gas processing industry, characterized in that, include: Ozone is introduced into high-salt, high-COD wastewater, and graphene catalyst is added to carry out a primary oxidation reaction. After ozone catalytic oxidation, the pH of the wastewater decreased from 12 to around 2.3; Fenton oxidant was added to the wastewater after the primary oxidation reaction to carry out a secondary oxidation reaction; After the secondary oxidation reaction, the wastewater is allowed to settle and settle to obtain the primary supernatant. Adjust the pH of the primary supernatant to 10-11, add flocculant and let it stand to settle, to obtain the secondary supernatant; The secondary supernatant was concentrated by ultrafiltration to obtain the tertiary supernatant; The supernatant from the three stages was concentrated by nanofiltration to obtain concentrated water and desalinated water, respectively. The concentrated water was frozen to crystallize sodium sulfate decahydrate crystals, and after drying, anhydrous sodium sulfate industrial salt was obtained. The catalyst is three-dimensional redox graphene, and the dosage of the catalyst is 0.1 g / L ~ 0.3 g / L; The Fenton oxidant is a mixture of ferrous sulfate heptahydrate and hydrogen peroxide, with the dosage of ferrous sulfate heptahydrate being 2 g / L to 16 g / L and the dosage of hydrogen peroxide being 5 mL / L to 15 mL / L. The flocculant is anionic polyacrylamide, and the dosage of the flocculant is 0.5 mg / L to 2.5 mg / L; High-salt, high-COD wastewater includes soluble sodium sulfate, amine organics, unsaturated olefin organics, and reducing sulfur.

2. The treatment process for high-salt, high-COD wastewater in the natural gas processing industry according to claim 1, characterized in that, The concentration of ozone introduced is 3 g / h ~ 12 g / h.

3. The treatment process for high-salt, high-COD wastewater in the natural gas processing industry according to claim 1, characterized in that, The pressure for ultrafiltration concentration is 0.2 MPa to 0.4 MPa; the pressure for nanofiltration is 2 MPa to 4 MPa.

4. The treatment process for high-salt, high-COD wastewater in the natural gas processing industry according to claim 1, characterized in that, The conditions for performing freeze crystallization include: After pre-cooling at -15℃ to -10℃ for 20 to 30 minutes, the freezing crystallization temperature is -3℃ to 0℃, and the freezing crystallization is carried out for 35 to 55 hours, resulting in a concentration of 5 to 10 times.

5. The treatment process for high-salt, high-COD wastewater in the natural gas processing industry according to claim 1, characterized in that, The drying conditions include: drying temperature of 90℃~110℃ and drying time of 20 min~30 min.

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