Photocatalytic oxidation and mercury removal agent combined oil and gas field mercury-containing sewage treatment system and method

Abstract

The application discloses a kind of photocatalytic oxidation and mercury removal agent combined oil and gas field mercury-containing sewage treatment system and method, belong to oil and gas engineering technical field.The pipe type ultraviolet photooxidation reactor includes the TiO2 catalyst loaded on the inner surface of reaction tube, mercury-containing sewage is pumped into the inside of pipe reactor with pump air from water inlet, and the oxygen in air is oxidized under the action of TiO2 catalyst on the inner wall of reactor, and the organic mercury and low-valence inorganic mercury in sewage are Hg (II) ;Subsequently, mercury-containing wastewater after oxidation treatment enters the mixing reactor, and reacts by adding mercury removal agent, to realize the standard removal of mercury pollution in wastewater, with the advantages of simple operation, good removal effect, environmental protection and the like.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas engineering technology, specifically relating to a system and method for treating mercury-containing wastewater in oil and gas fields using a combination of photocatalytic oxidation and mercury removal agent. Background Technology

[0002] Mercury is a heavy metal and the only metallic element that is liquid at room temperature. Elemental mercury is volatile and exists in nature in very low abundance but widely. Mercury in the environment mainly comes from two sources: natural and anthropogenic. Natural sources include volcanic activity, soil discharge, natural weathering, and release from vegetation. Anthropogenic sources refer to mercury emissions caused by human activities, mainly from wastewater, waste gas, and waste residue from the chemical, petrochemical, agricultural, pharmaceutical, and chlor-alkali industries, as well as gaseous mercury released into the atmosphere during smelting and coal combustion processes, and the dry and wet deposition of this gaseous mercury.

[0003] Metallic mercury possesses unique properties such as high toxicity, volatility, mobility, and bioaccumulation. Mercury in the environment mainly exists in the form of elemental mercury (HgO) and inorganic mercury (Hg). + and Hg 2+ Mercury exists in three forms: elemental mercury, organic mercury, and methylmercury. Elemental mercury is relatively rare in nature, and mercury vapor inhaled through the respiratory tract is highly toxic. The most common monovalent mercury salt is mercurous chloride, which is poorly soluble in water. Divalent mercury commonly exists in the forms of mercuric chloride and mercuric sulfide. Mercuric chloride is readily soluble in water and has relatively strong toxicity; mercuric sulfide has extremely weak water solubility and very low toxicity. Organic mercury mainly includes methylmercury (monomethylmercury and dimethylmercury), ethylmercury, and phenylmercury, with methylmercury being the most common and most harmful to humans and the environment. Methylmercury is lipophilic, chemically stable, easily absorbed by organisms, and difficult to metabolize and eliminate. It accumulates and amplifies along the food chain, with increased concentrations in higher trophic levels. Reports show that the bioaccumulation factor of mercury in freshwater fish and phytoplankton is 1000 times, in freshwater invertebrates 100,000 times, and in marine animals 200,000 times. The bioaccumulation factor of methylmercury in aquatic organisms can reach 10. 4 -10 7 .

[0004] Water bodies are a significant carrier of mercury in the environment. Mercury-containing wastewater discharged into water bodies not only diffuses with water flow, causing changes in its spatial location, but its form also changes during this diffusion process. Various forms of mercury can be converted into Hg in water bodies. 2+Under anaerobic conditions, mercury is primarily converted to dimethylmercury, while under aerobic conditions, it is primarily converted to monomethylmercury. Methylation of mercury can occur both in mud and within aquatic organisms. Even in the absence of organisms, methylation can occur under suitable photochemical conditions, such as the presence of acetaldehyde, ethanol, methanol, and ultraviolet light. Almost all forms of mercury accumulate in organisms as methylmercury, posing a health risk. The Minamata disease incident in Kumamoto Prefecture, Japan in 1952 was the world's first case of dimethylmercury poisoning caused by industrial wastewater.

[0005] Given the biological and environmental hazards of mercury, the "GB 8978-1996 Integrated Wastewater Discharge Standard" stipulates that alkyl mercury must not be detected in the effluent from mercury-containing gas fields, and the total mercury content must not exceed 0.05 mg / L; the "GB 30770-2014 Emission Standard of Pollutants from Tin, Antimony and Mercury Industries" requires the total mercury content in the treated effluent to be less than 0.05 mg / L.

[0006] Because mercury and hydrocarbons share similar formation conditions and activity in geological formations, and organic matter can effectively enrich mercury, some natural gas reservoirs also serve as source layers for mercury, with mercury coexisting with natural gas in the form of volatiles. Statistics show that many gas wells in the Tarim and Songliao basins of my country have mercury contents exceeding 500 μg / m³. 3 The awareness of the presence and hazards of mercury in oil and gas production began after the damage to an aluminum heat exchanger at the Skikda natural gas liquefaction plant in Algeria. Subsequent analysis revealed that the cause was amalgam corrosion, which occurred when gaseous mercury condensed and adhered to the metal wall, leading to perforation and damage to the equipment.

[0007] Mercury pollution from the oil and gas production industry mainly includes wastewater, atmospheric emissions, and mercury-containing waste. The largest source of wastewater is produced water from oil and gas extraction. During the separation of oil, gas, and water after extraction, most of the soluble mercury salts in the oil and gas enter the separated water. Subsequent dehydration and desalination processes also contribute to the main source of mercury-containing wastewater. Mercury is released into the atmosphere during oil and gas combustion, as well as through accidental leaks and the incineration of waste oil and gas. The released mercury primarily exists in the form of elemental mercury. Mercury-containing solid waste from oil and gas production mainly comes from drilling residue and is primarily composed of suspended HgS (mercury halide).

[0008] Common treatment methods for industrial mercury-containing wastewater include chemical precipitation, reduction, ion exchange resin methods, adsorption, electrolysis, and membrane separation. For mercury-containing wastewater from oil and gas fields, Zhu Cong et al. from Southwest Petroleum University employed a combined process of flocculation sedimentation, high-efficiency separation, and air flotation / adsorption to purify mercury-containing wastewater from the KL gas field, reducing total mercury emissions to below 10 μg / L. Wang Yang analyzed the treatment of mercury-containing wastewater from the Dina 2 gas field and suggested adding sulfides and iron salts to the existing wastewater treatment system to reduce total mercury emissions. Professor Qu Chengtun's research group at Xi'an Petroleum University developed a technology for treating mercury-containing wastewater from oil and gas fields using sulfide-modified montmorillonite. Because mercury exists in various forms, oil and gas production wastewater contains both oil and water phases, with mercury distributed in both. Conventional wastewater treatment processes, combining gravity sedimentation-flocculation, flocculation-air flotation, and flocculation-adsorption, can remove most elemental mercury into the sludge. Some elemental mercury and lipophilic mercury, dissolved in oil or suspended solids, are also removed along with them. However, this leaves organic mercury and a small amount of inorganic mercury in the water, failing to meet the discharge standard of less than 0.05 mg / L for total mercury content. Using sulfide precipitation for mercury removal, the theoretically calculated excess sulfur addition of 50%–80% is sufficient. Adding too much not only causes secondary sulfur pollution but also reduces treatment efficiency by forming water-soluble complex ions with mercury. This makes determining the appropriate sulfide addition amount difficult. Furthermore, the resulting mercury sulfide particles are extremely fine black particles that, due to Brownian motion, are suspended in water and difficult to settle, further reducing the mercury removal efficiency. Summary of the Invention

[0009] In order to overcome the shortcomings of the prior art, the present invention aims to provide a system and method for treating mercury-containing wastewater in oil and gas fields by combining photocatalytic oxidation with mercury removal agents, so as to solve the technical problems of existing mercury-containing wastewater treatment methods, such as the potential introduction of new pollutants and difficulty in meeting discharge and reinjection requirements.

[0010] To achieve the above objectives, the present invention employs the following technical solution:

[0011] This invention discloses a photocatalytic oxidation and mercury removal agent combined oil and gas field mercury-containing wastewater treatment system, comprising a tubular ultraviolet photooxidation reactor and a mixing reactor connected in sequence;

[0012] The tubular ultraviolet oxidation reactor includes a reaction tube, the inner surface of which is coated with a TiO2 coating; one end of the reaction tube is provided with a wastewater inlet and an air inlet; the other end of the reaction tube is provided with a wastewater outlet and a power connector; a glass tube is provided inside the reaction tube; and an ultraviolet lamp connected to the power connector is provided inside the glass tube.

[0013] Furthermore, when the mercury-containing wastewater obtained after oxidation treatment at the wastewater outlet still contains organic mercury and low-valence inorganic mercury, the tubular ultraviolet oxidation reactor is replaced with a tubular ultraviolet oxidation reactor group, which is connected in sequence with the mixing reactor.

[0014] The tubular ultraviolet oxidation reactor group includes several tubular ultraviolet oxidation reactors, which are connected in parallel or in series.

[0015] Furthermore, the detection is performed using inductively coupled plasma mass spectrometry and cold atomic absorption spectrometry.

[0016] Furthermore, the glass tube is made of quartz glass.

[0017] Furthermore, the mixing reactor is also connected in sequence to an air flotation tank, a reinjection storage tank, and a reinjection system.

[0018] This invention also discloses a method for using the above-mentioned photocatalytic oxidation and mercury removal agent combined oil and gas field mercury-containing wastewater treatment system, including the following steps:

[0019] S1: First, set the wastewater flow rate, UV lamp power, wastewater residence time in the tube, and air-to-wastewater flow ratio; turn on the power connector, turn on the UV lamp, and let air enter the reaction tube through the air inlet. At the same time, let the mercury-containing wastewater flow into the reaction tube from the wastewater inlet to react. After the reaction is completed, the oxidized mercury-containing wastewater is obtained. When no organic mercury or low-valence inorganic mercury is detected in the oxidized mercury-containing wastewater, it flows out from the wastewater outlet and then into the mixing reactor.

[0020] S2: Add mercury removal agent to a mixing reactor to carry out the reaction, and obtain treated water after the reaction is completed;

[0021] The mercury removal agent includes polyaluminum chloride, sodium sulfide, and para-substituted benzyl mercaptan, wherein the para-substituted benzyl mercaptan is benzo-18-crown-6 linked by an ether bond.

[0022] Furthermore, inductively coupled plasma mass spectrometry and cold atomic absorption spectrometry were used to detect the mercury-containing wastewater after oxidation treatment. If the mercury-containing wastewater after oxidation treatment still contained organic mercury and low-valence inorganic mercury, the tubular ultraviolet oxidation reactor was replaced with a tubular ultraviolet oxidation reactor group, which was connected in sequence with the mixing reactor.

[0023] The tubular ultraviolet oxidation reactor group includes several tubular ultraviolet oxidation reactors, which are connected in parallel or in series.

[0024] Furthermore, inductively coupled plasma mass spectrometry was used to detect the mercury-containing wastewater after oxidation treatment. If the mercury-containing wastewater after oxidation treatment still contained organic mercury and low-valence inorganic mercury, the flow rate of the effluent from the wastewater inlet into the reaction tube in the later stage of the mercury-containing oil and gas production wastewater treatment process was reduced, the power of the ultraviolet lamp was increased, or the flow ratio of air to mercury-containing wastewater was increased until the mercury-containing wastewater after oxidation treatment was free of organic mercury and low-valence inorganic mercury.

[0025] Furthermore, the mixing reactor is also connected in sequence to an air flotation tank, a reinjection storage tank, and a reinjection system; the treated water obtained in S2 then flows into the air flotation tank, the reinjection storage tank, and the reinjection system in sequence.

[0026] Furthermore, the amount of polyaluminum chloride used is 40-200g per cubic meter of mercury-containing wastewater after oxidation treatment; the amount of sodium sulfide used is 1.2-1.5 times the total mercury molars measured in the mercury-containing wastewater after oxidation treatment; and the amount of para-substituted benzyl mercaptan used is 0.5-1.0 times the total mercury molars measured in the mercury-containing wastewater after oxidation treatment.

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

[0028] This invention discloses a combined photocatalytic oxidation and mercury removal agent system for treating mercury-containing wastewater in oil and gas fields. The system employs a tubular ultraviolet photo-oxidation reactor, in which a TiO2 catalyst is loaded onto the inner surface of the reaction tube. Mercury-containing wastewater enters the reactor along with pumped air from the inlet. Under the action of the TiO2 catalyst on the reactor wall, oxygen in the air oxidizes the organic mercury and low-valence inorganic mercury in the wastewater to Hg(II). The photocatalytic oxidation process is carried out in a closed reactor, avoiding the emission of volatile mercury into the atmosphere during treatment. This type of photocatalytic oxidation reactor features a skid-mounted design, requiring little floor space, and is simple to operate, flexible in adjustment, and highly adaptable. Subsequently, the oxidized mercury-containing wastewater enters a mixing reactor, where a mercury removal agent is added for further reaction, achieving complete removal of mercury pollution from the wastewater.

[0029] Furthermore, the organic mercury and low-valence inorganic mercury in the mercury-containing wastewater after oxidation treatment were monitored by inductively coupled plasma mass spectrometry (HPLC-ICP-MS) and cold atomic absorption spectrometry. If the mercury-containing wastewater after oxidation treatment still contains organic mercury and low-valence inorganic mercury, tubular reactors are added in parallel or in series to form a tubular ultraviolet oxidation reactor group, or the flow rate in the tube is reduced while keeping the total treated water volume unchanged, so as to enhance the photocatalytic oxidation reaction effect.

[0030] Furthermore, the ultraviolet lamp is placed in a quartz glass tube. The use of quartz material facilitates the transmission of light intensity without loss, thereby improving energy utilization efficiency.

[0031] This invention also discloses the method of using the above-mentioned photocatalytic oxidation and mercury removal agent combined oil and gas field mercury-containing wastewater treatment system. First, the wastewater passes through a tubular ultraviolet photooxidation reactor, where the organic mercury and low-valence inorganic mercury in the wastewater are oxidized to Hg(II) under the action of the mercury removal agent. Then, it enters a mixing reactor to remove Hg pollutants. It has the advantages of simple operation, good removal effect, and environmental protection.

[0032] Further, the mercury-containing wastewater after oxidation treatment enters a mixed reactor, where polyaluminum chloride (PAC), sodium sulfide, and an organic mercury removal agent containing crown ethers and thiol groups are added for treatment. The polyaluminum chloride hydrolyzes to produce aluminum hydroxide and hydrochloric acid. Aluminum hydroxide adsorbs suspended solids and small oil droplets in the system, causing flocculation and sedimentation. Hydrochloric acid converts divalent inorganic mercury generated in the photooxidizer into soluble ionic mercury, which reacts with the added sodium sulfide to form insoluble mercury sulfide precipitate. The crown ether portion of the organic mercury removal agent complexes with sodium ions, and the thiol group coordinates with mercury, causing the extremely fine mercury sulfide particles suspended in the aqueous phase that are difficult to settle to cross-link and aggregate, becoming larger and easier to settle. Simultaneously, the sodium ions complexed with the crown ether react with Cl- in the treated water... - These components interact to form a bridging network structure. The organic mercury removal agent also has lipophilic properties, allowing it to react with small oil droplets adsorbed by the aluminum hydroxide flocs generated after the hydrolysis of polyaluminum chloride. This further enriches the mercury-containing products in the flotation tank, reducing the amount of mercury-containing sludge and lowering subsequent hazardous chemical disposal costs while ensuring effective mercury removal. According to relevant experiments, after treatment with photocatalytic oxidation, sodium sulfide, organic mercury removal agent, and polyaluminum oxide, mercury-containing wastewater undergoes flocculation and sedimentation, and is then converted into mercury-containing sludge for removal in the flotation tank. The total mercury content in the effluent is less than 0.05 mg / L, meeting the discharge requirements. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the process of the oil and gas field mercury-containing wastewater treatment system using photocatalytic oxidation and mercury removal agent combined according to the present invention;

[0034] Figure 2 This is a schematic diagram of the tubular ultraviolet oxidation reactor provided by the present invention;

[0035] Figure 3 This is a schematic diagram illustrating the technical principle of the oil and gas field mercury-containing wastewater treatment system using a combination of photocatalytic oxidation and mercury removal agent according to the present invention.

[0036] Wherein: 1-Wastewater inlet; 2-Air inlet; 3-Glass tube; 4-TiO2 coating; 5-UV lamp; 6-Wastewater outlet; 7-Power connector; 8-Reaction tube. Detailed Implementation

[0037] To enable those skilled in the art to understand the features and effects of the present invention, the terms and expressions used in the specification and claims are explained and defined in general below. Unless otherwise specified, all technical and scientific terms used herein have the ordinary meaning understood by those skilled in the art regarding the present invention, and in case of conflict, the definitions in this specification shall prevail.

[0038] The theories or mechanisms described and disclosed herein, whether right or wrong, should not in any way limit the scope of the invention, that is, the contents of the invention can be implemented without being limited by any particular theory or mechanism.

[0039] In this document, all features defined by numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are for the sake of brevity and convenience only. Accordingly, descriptions of numerical ranges or percentage ranges should be considered as covering and specifically disclosing all possible sub-ranges and individual numerical values ​​(including integers and fractions) within those ranges.

[0040] In this article, unless otherwise specified, “contains,” “includes,” “containing,” “has,” or similar terms cover the meanings of “composed of” and “mainly composed of,” for example, “A contains a” covers the meanings of “A contains a and others” and “A contains only a.”

[0041] For the sake of brevity, not all possible combinations of the technical features in each implementation scheme or embodiment are described herein. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each implementation scheme or embodiment can be combined arbitrarily, and all possible combinations should be considered within the scope of this specification.

[0042] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0043] The following examples use instruments and equipment conventional in the art. Experimental methods in the following examples, unless otherwise specified, are generally performed under conventional conditions or as recommended by the manufacturer. All raw materials used in the following examples are conventional commercially available products with specifications conventional in the art. In this specification and the following examples, unless otherwise specified, "%" refers to weight percentage, "parts" refers to parts by weight, and "ratio" refers to weight proportion.

[0044] like Figure 1As shown, the present invention discloses a photocatalytic oxidation and mercury removal agent combined oil and gas field mercury-containing wastewater treatment system, comprising a tubular ultraviolet photooxidation reactor, a mixing reactor, an air flotation tank, a reinjection storage tank, and a reinjection system connected in sequence.

[0045] like Figure 2 As shown, the ultraviolet oxidation reactor includes a reaction tube 8, the inner surface of which is coated with a TiO2 coating 4, and a glass tube 3 is provided. An ultraviolet lamp 5 connected to a power connector 7 is provided inside the glass tube 3. One end of the reaction tube 8 is provided with a wastewater inlet 1 and an air inlet 2, and the other end is provided with a wastewater outlet 6 and a power connector 7. The TiO2 coating is used to generate hydroxyl radicals and superoxide radicals to oxidize low-valence mercury. The ultraviolet lamp is used to provide the energy required for the photocatalyst.

[0046] During use, power connector 7 is turned on, UV lamp 5 is activated, and air enters reaction tube 8 through air inlet 2. Simultaneously, effluent from the later stages of the mercury-containing oil and gas production wastewater treatment process flows into reaction tube 8 through wastewater inlet 1 for reaction. Oxygen in the air, under the action of the TiO2 catalyst on the inner wall of the reactor, oxidizes the organic mercury and low-valence inorganic mercury in the wastewater to Hg(II). After the reaction, the oxidized mercury-containing wastewater is then analyzed using inductively coupled plasma mass spectrometry and cold atomic absorption spectrometry to detect the content of organic mercury and low-valence inorganic mercury in the oxidized mercury-containing wastewater. If organic mercury and low-valence inorganic mercury are still present, tubular reactors can be added in parallel or series to form a tubular UV oxidation reactor group, or... To enhance the photocatalytic oxidation effect, the flow rate within the pipe was reduced while maintaining the total treated water volume. The oxidation-treated mercury-containing wastewater, after testing and finding no organic mercury or low-valent inorganic mercury, was then discharged from wastewater outlet 6 and flowed into a mixing reactor. The oxidized wastewater then entered the mixing reactor, where polyaluminum chloride (PAC), sodium sulfide, and an organic mercury removal agent containing crown ethers and thiol groups were added for treatment. The polyaluminum chloride hydrolyzed to produce aluminum hydroxide and hydrochloric acid. Aluminum hydroxide adsorbed suspended solids and small oil droplets in the system, causing flocculation and sedimentation. Hydrochloric acid converted the divalent inorganic mercury generated in the photooxidizer into soluble ionic mercury, which reacted with the added sodium sulfide to form insoluble mercury sulfide precipitate. The crown ether portion of the organic mercury removal agent molecule complexed with sodium ions, and the thiol group coordinated with mercury, causing the extremely fine mercury sulfide particles suspended in the aqueous phase to cross-link and aggregate, becoming larger and easier to settle. Simultaneously, the crown ether-complexed sodium ions reacted with Cl- in the treated water... - They interact to form a bridging network structure. The organic mercury removal agent is also lipophilic, and can react with the small oil droplets adsorbed by the aluminum hydroxide flocs generated after the hydrolysis of polyaluminum chloride, further enriching the mercury-containing products in the flotation tank. While ensuring the mercury removal effect, it reduces the amount of mercury-containing sludge and lowers the subsequent hazardous chemical disposal costs.

[0047] Preferably, a quartz glass tube is used to hold the ultraviolet lamp. The use of quartz material facilitates the lossless transmission of light intensity and improves energy utilization efficiency.

[0048] In this invention, the factors affecting the oxidation effect of low-valent mercury in the tubular photocatalytic oxidizer include wastewater treatment flow rate, wastewater residence time in the tube, gas-water flow ratio, and ultraviolet lamp power.

[0049] In this invention, multiple samples are taken from the mercury-containing wastewater from oil and gas fields before treatment, and the total mercury content is determined according to "HJ 597-2011 Determination of Total Mercury in Water: Cold Atomic Absorption Spectrophotometry". After completing the initial wastewater treatment process, multiple samples are taken again to test the total mercury, methylmercury, and zero-valent mercury content in the wastewater. Based on the test data, the total amount of wastewater to be treated, and the methylmercury and zero-valent mercury content after photocatalytic oxidation, are used to optimize the installation method and operating parameters of the tubular photocatalytic oxidation reactor. The dosage of sodium sulfide, organic mercury removal agent, and polyalumina in the mixed reactor is added in a certain proportion according to the total mercury content measured at the outlet of the tubular photocatalytic oxidation reactor. The specific dosage can be adjusted after laboratory tests on oilfield water samples and combined with on-site operating conditions. After photocatalytic oxidation, treatment with sodium sulfide, organic mercury removal agent, and polyalumina, the mercury-containing wastewater undergoes flocculation and sedimentation enrichment, and is converted into mercury-containing sludge for removal in the flotation tank. The total mercury content in the effluent is less than 0.05 mg / L, meeting the discharge requirements.

[0050] Figure 3 This is a schematic diagram illustrating the technical principle of a photocatalytic oxidation and mercury removal agent combined system for treating mercury-containing wastewater in oil and gas fields. It shows that zero-valent mercury and organic mercury in the wastewater enter the photoreactor along with the wastewater. O2 molecules from the air introduced into the reactor are converted into highly oxidizing hydroxyl radicals (HO•) and superoxide radicals (•O2) under the action of the photocatalyst TiO2 on the reactor's inner surface. - The process oxidizes low-valent mercury to divalent mercury (Hg(II)). The oxidized mercury-containing wastewater then enters a mixed reactor. Polyaluminum chloride (PAC) is added, which hydrolyzes to generate HCl molecules, which then react with Hg(II) to form ionic, water-soluble Hg. 2+ It reacts with added sodium sulfide to form HgS solid particles. The crown ether portion of the organic mercury removal agent M1 reacts with Na in the wastewater. + Combined, through Cl - Ions form a cross-linked network structure. The thiol groups in the M1 molecule complex with HgS particles, promoting the sedimentation of fine HgS particles. At the same time, the hydrolysis product Al(OH)3 and the organic mercury removal agent M1 have a synergistic effect, improving the flocculation and sedimentation efficiency of fine HgS particles.

[0051] Example 1

[0052] A method for using a photocatalytic oxidation and mercury removal agent combined system for treating mercury-containing wastewater in oil and gas fields includes the following steps:

[0053] Mercury-containing wastewater from the oilfield's preliminary treatment was tested, and its average total mercury, zero-valent mercury, and methylmercury concentrations were 430 μg / L, 96 μg / L, and 152 μg / L, respectively. The tubular photocatalytic oxidizer used a 254 nm wavelength, 100 W UV lamp as its light source, and the volume of a single photocatalytic oxidation reactor was 0.04 m³. 3 (The volume can be adjusted according to the wastewater treatment situation.) Based on the retention times of mercury-containing wastewater of 5s, 10s, 15s, 20s, 25s, and 30s, the wastewater treatment flow rates for a single tubular reactor were calculated to be 8.0L / s, 4.0L / s, 2.67L / s, 2.0L / s, 1.6L / s, and 1.33 L / s, respectively. Experiments were conducted with water-to-air flow ratios of 1:1, 1:1.5, 1:2, 1:2.5, and 1:3. After each experiment, tests were performed to compare the content of zero-valent mercury and methylmercury in the water samples after each oxidation experiment. The photo-oxidation retention time and water-to-air flow ratio parameters were optimized, and finally, a photo-oxidation retention time of 15s and a water-to-air flow ratio of 1:2.5 were determined. Based on the determined operating parameters of the single photocatalytic oxidation reactor, combined with the total wastewater treatment volume of 76.9m³, the optimal parameters were determined. 3 / h, the calculated number of tubular photocatalytic reactors is 8, but considering the design margin, 12 photocatalytic reactors are actually installed, and the reactors are installed in parallel.

[0054] The operation of one of the tubular photocatalytic reactors is as follows: air enters the reaction tube 8 through air inlet 2, and mercury-containing wastewater is reacted from wastewater inlet 1 at a flow rate of 2.67 L / s. After the reaction, the oxidized mercury-containing wastewater, after being tested and found to contain no organic mercury or low-valence inorganic mercury, flows out from wastewater outlet 6 and then into a mixing reactor. A mercury removal agent is then added to the mixing reactor for further reaction. After the reaction, treated water is obtained. The mercury removal agent includes polyaluminum chloride (PAC), sodium sulfide, and organic mercury removal agent M1. The ratio of the oxidized mercury-containing wastewater, polyaluminum chloride, sodium sulfide, and organic mercury removal agent M1 is 1 m³ / s. 3 : 4g: 225.8mg: 818.7mg.

[0055] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A method for using a photocatalytic oxidation and mercury removal agent combined system for treating mercury-containing wastewater in oil and gas fields, characterized in that, Includes the following steps: S1: First, set the flow rate of wastewater, the power of the ultraviolet lamp (5), the residence time of wastewater in the tube, and the flow ratio of air to wastewater; turn on the power connector (7), turn on the ultraviolet lamp (5), and let the air enter the reaction tube (8) through the air inlet (2). At the same time, let the mercury-containing wastewater flow into the reaction tube (8) from the wastewater inlet (1) for reaction. After the reaction is completed, the mercury-containing wastewater after oxidation treatment is obtained. When the mercury-containing wastewater after oxidation treatment is tested and found to be free of organic mercury and low-valence inorganic mercury, it flows out from the wastewater outlet (6) and then into the mixing reactor. S2: Add mercury removal agent to a mixing reactor to carry out the reaction, and obtain treated water after the reaction is completed; The mercury removal agent includes polyaluminum chloride, sodium sulfide and para-substituted benzyl mercaptan, wherein the para-substituted benzyl mercaptan is benzo-18-crown-6 linked by an ether bond; The combined photocatalytic oxidation and mercury removal agent oil and gas field mercury-containing wastewater treatment system includes a tubular ultraviolet photooxidation reactor and a mixing reactor connected in sequence. The tubular ultraviolet oxidation reactor includes a reaction tube (8), the inner surface of which is coated with a TiO2 coating (4); one end of the reaction tube (8) is provided with a wastewater inlet (1) and an air inlet (2); the other end of the reaction tube (8) is provided with a wastewater outlet (6) and a power connector (7); a glass tube (3) is provided inside the reaction tube (8); an ultraviolet lamp (5) connected to the power connector (7) is provided inside the glass tube (3).

2. The method of using the photocatalytic oxidation and mercury removal agent combined oil and gas field mercury-containing wastewater treatment system according to claim 1, characterized in that, When the mercury-containing wastewater obtained after oxidation treatment at the wastewater outlet (6) still contains organic mercury and low-valence inorganic mercury, the tubular ultraviolet oxidation reactor is replaced with a tubular ultraviolet oxidation reactor group, which is connected to the mixing reactor. The tubular ultraviolet oxidation reactor group includes several tubular ultraviolet oxidation reactors, which are connected in parallel or in series.

3. The method of using the photocatalytic oxidation and mercury removal agent combined oil and gas field mercury-containing wastewater treatment system according to claim 2, characterized in that, The detection was performed using inductively coupled plasma mass spectrometry and cold atomic absorption spectrometry.

4. The method of using the photocatalytic oxidation and mercury removal agent combined oil and gas field mercury-containing wastewater treatment system according to claim 1, characterized in that, The glass tube (3) is made of quartz glass.

5. The method of using the photocatalytic oxidation and mercury removal agent combined oil and gas field mercury-containing wastewater treatment system according to claim 1, characterized in that, The mixing reactor is also connected in sequence to an air flotation tank, a reinjection storage tank, and a reinjection system.

6. The method of using the photocatalytic oxidation and mercury removal agent combined oil and gas field mercury-containing wastewater treatment system according to claim 1, characterized in that, Inductively coupled plasma mass spectrometry and cold atomic absorption spectrometry were used to detect mercury-containing wastewater after oxidation treatment. If the mercury-containing wastewater after oxidation treatment still contained organic mercury and low-valence inorganic mercury, the tubular ultraviolet oxidation reactor was replaced with a tubular ultraviolet oxidation reactor group, which was connected in sequence with the mixing reactor. The tubular ultraviolet oxidation reactor group includes several tubular ultraviolet oxidation reactors, which are connected in parallel or in series.

7. The method of using the photocatalytic oxidation and mercury removal agent combined oil and gas field mercury-containing wastewater treatment system according to claim 1, characterized in that, Inductively coupled plasma mass spectrometry was used to detect the mercury-containing wastewater after oxidation treatment. If the mercury-containing wastewater after oxidation treatment still contained organic mercury and low-valence inorganic mercury, the flow rate of the effluent from the wastewater inlet (1) into the reaction tube (8) in the later stage of the mercury-containing oil and gas production wastewater treatment process was reduced, the power of the ultraviolet lamp was increased, or the flow ratio of air to mercury-containing wastewater was increased until the mercury-containing wastewater after oxidation treatment was free of organic mercury and low-valence inorganic mercury.

8. The method of using the photocatalytic oxidation and mercury removal agent combined oil and gas field mercury-containing wastewater treatment system according to claim 1, characterized in that, The mixing reactor is also connected in sequence to a flotation tank, a reinjection storage tank, and a reinjection system; the treated water obtained in S2 then flows into the flotation tank, the reinjection storage tank, and the reinjection system in sequence.

9. The method of using the photocatalytic oxidation and mercury removal agent combined oil and gas field mercury-containing wastewater treatment system according to claim 1, characterized in that, The dosage of the polyaluminum chloride is 40-200g per cubic meter of mercury-containing wastewater after oxidation treatment; the dosage of the sodium sulfide is 1.2-1.5 times the total mercury molars measured in the mercury-containing wastewater after oxidation treatment; and the dosage of the para-substituted benzyl mercuric alcohol is 0.5-1.0 times the total mercury molars measured in the mercury-containing wastewater after oxidation treatment.

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