With the extension of oilfield exploitation period, especially in the middle and late period of oilfield exploitation, the water content of crude oil is getting higher and higher, and the waterless exploitation period is getting shorter and shorter. At present, the comprehensive water cut of crude oil in most oil fields in China has reached 80%, and some even reached 90%. The annual output of oil production wastewater is about 4 1 10,000 t, which has become the main oil-bearing water source. The petroleum in oily sewage is mainly composed of floating oil, dispersed oil, emulsified oil, colloidal dissolved matter and suspended matter.
Oil is mined underground, dehydrated and stabilized, then enters the gathering pipeline, and then transported to the refinery or oil depot, where it is dehydrated and desalted again. When the water content in crude oil is less than or equal to 0.5% and the salt content is less than 5000mg/L, it can enter the atmospheric and vacuum distillation unit. Crude oil is heated to above 350℃ in a heating furnace, and then atmospheric distillation and vacuum distillation are carried out to separate gasoline, kerosene, diesel oil and lubricating oil fractions, and atmospheric heavy oil and vacuum residue are used as raw materials for secondary processing. In order to improve product quality and comprehensive utilization rate of crude oil, refineries need to carry out secondary processing. The main devices include catalytic cracking, platinum reforming, hydrogenation, furfural refining, polypropylene, coking, asphalt oxidation and other devices. Because these devices all adopt the method of combining physical separation with chemical reaction, the production process is often carried out at high temperature, which requires fuel and cooling medium (water).
In the process of steam injection, product refined washing water, pump shaft seal cooling water, water and oil products should be in direct contact, resulting in oily sewage and phenolic sewage.
Because it is difficult to treat petrochemical wastewater, it is not only high in concentration, but also difficult to dissolve. Therefore, the treatment of petrochemical wastewater generally adopts chemical components. Typical methods include chemical method, physical method and biochemical treatment technology.
1, chemical method
Chemical method refers to the method of using chemical components to decompose, dissolve or agglutinate pollutants in petrochemical wastewater, so as to achieve the purpose of treating wastewater and avoid environmental pollution.
1. 1 flocculation
An important process of petrochemical wastewater treatment is flocculation, that is, by adding flocculant to the water, the steady state of colloidal particles in the water is destroyed, and the colloidal particles collide with each other to form flocculent substances that are easy to separate from the water. Flocculation can be used to treat turbidity, chroma, organic pollutants, plankton and algae in refinery wastewater. In specific operations, flocculation is usually combined with air flotation or sedimentation as pretreatment of biochemical treatment. At present, compared with other flocculants, wastewater treatment agents made of microbial flocculants and biotechnology have the advantages of easy biodegradation, wide application range, strong thermal stability, high efficiency and no secondary pollution, and have broad application prospects.
1.2 oxidation method
The main oxidation methods are photocatalytic oxidation, wet oxidation and ozone oxidation. Different methods can be selected for petrochemical wastewater with different components, so as to realize the most effective, economical and safe wastewater treatment.
1) photocatalytic oxidation method. Photocatalytic oxidation can effectively combine light radiation with oxidants such as O2 and H2O2 to achieve the purpose of sewage treatment, so it is called photocatalytic oxidation. Some people use sunlight as light source and TiO _ 2, TiO _ 2/Pt and ZnO as catalysts to treat water containing 2/kloc-0 kinds of organic pollutants, and the final products are CO2, which does not produce secondary pollution. Others use Fe2+ and H2O2 as oxidants. There is a synergistic effect between iron ions and ultraviolet light, which greatly accelerates the decomposition of H2O2 to generate hydroxyl radicals, so the oxidation efficiency is improved. This method is still in the research stage in many countries.
2) Wet oxidation method. Wet oxidation can be divided into two categories, namely catalytic wet oxidation (CWO) and wet air oxidation (WAO). CWO is a process of oxidizing and decomposing organic matter into non-toxic and harmless substances such as CO2, H2O and N2 in the presence of catalyst at high temperature and high pressure. The reaction time is short and the conversion rate is high, but pH and catalyst activity have great influence on the reaction. WAO is a process of liquid phase oxidation using molecular oxygen in the air at high temperature and high pressure. This technology is a good method to effectively control environmental pollutants, especially suitable for the treatment of toxic and harmful pollutants or high-concentration refractory organic pollutants. Lu Yicheng and others used wet air oxidation to treat petrochemical wastewater. The average removal rates of COD, inorganic sulfide, thiosulfate and total phenol are 865438 0.8%, nearly 65438 0.000%, 965438 0.7% and nearly 65438 0.000%, respectively. The results show that the treatment effect of this method has reached the treatment efficiency of similar equipment abroad.
3) Ozone oxidation method. Ozone oxidation has its unique advantages: this method does not produce sludge and secondary pollution in the oxidation process. However, its operation and investment costs are high, and the wastewater flow should not be too large. After ozone oxidation, a small part of organic matter in wastewater is completely oxidized into water and carbon dioxide, and most of it is converted into oxidation intermediate products. Generally, ozone oxidation and biological activated carbon adsorption are combined for advanced treatment. Ozone quickly decomposes into oxygen while oxidizing organic matter, so that the activated carbon bed is in an oxygen-enriched state, regenerated and its service life is prolonged. At the same time, the activity of aerobic microorganisms on the surface of activated carbon is enhanced, and the ability to degrade and adsorb organic matter is improved. It can effectively remove organic matter, change the structure of organic chromophore and strengthen the decolorization ability of activated carbon. Li et al. used ozone-activated carbon process for advanced treatment of refinery wastewater. The average removal rates of COD, ammonia nitrogen, volatile phenol and petroleum are 82.6%, 93.4%, 99.5% and 94.3%, respectively, and the main indexes of effluent meet the Class IV water quality standard of surface water.
2. Laws of physics
1) adsorption. Adsorption refers to the method of removing pollutants from wastewater by using the porosity of solid substances to attach to its surface. The commonly used adsorbent is activated carbon, which can effectively remove COD, chromaticity and odor of wastewater, but its treatment cost is high and it is easy to cause secondary pollution. Adsorption is usually combined with flocculation or ozone oxidation in petrochemical wastewater treatment.
2) membrane separation. There are different membrane separation methods, such as microfiltration, ultrafiltration, reverse osmosis and nanofiltration. No matter which method, it can effectively remove the odor and chromaticity of wastewater, remove organic matter, various ions and microorganisms, and the effluent quality is stable and reliable.
3) air flotation. Air flotation refers to the use of highly dispersed tiny bubbles as carriers, which are attached to the suspended solids in wastewater, so that they can be separated as the bubbles rise to the water surface, and the separation objects are hydrophobic tiny solid suspended solids and petrochemical oil. In petrochemical wastewater treatment, air flotation is often placed after oil separation and flocculation. For example, after the CAF system is put into the grease trap, the oily petrochemical wastewater is treated. The oil content of influent is about 200mg/L, and the oil content of effluent is less than 10mg/L, and the removal rate reaches 95%. Experiments show that the effect of air flotation on wastewater treatment is reliable.
3. Biochemical methods
1) aerobic treatment. In petrochemical wastewater treatment, there are many aerobic treatment methods, such as sequencing batch batch batch activated sludge process, high efficiency aerobic bioreactor, biological contact oxidation, membrane bioreactor treatment and so on. , but aerobic biological treatment alone is less, mainly combined with anaerobic treatment.
2) Anaerobic treatment. Petrochemical wastewater has high COD and poor biodegradability, so anaerobic pretreatment is usually carried out first to improve the biodegradability of subsequent treatment. ① Upflow anaerobic sludge bed. The sludge concentration in UASB reactor is relatively high, and the average sludge mass concentration is generally 30 ~ 40g/L. The organic load is high, the water retention time is short, and it is digested at moderate temperature, and the volumetric load of COD is generally 10 ~ 20kg/(m3? D) Testing. There is a three-phase separator in the reaction zone, and the sludge separated from the precipitation zone can automatically flow back to the reaction zone without mixing and stirring equipment. The sludge bed is not filled with carriers, so the cost is low. Generally used for the treatment of high concentration organic wastewater. ② Anaerobic fixed membrane reactor. Anaerobic fixed membrane reactor is equipped with fixed packing, which can intercept and attach a large number of anaerobic microorganisms. Through its function, the organic matter in the influent is converted into methane and carbon dioxide, which can be removed. It has the advantages of strong impact load resistance, long microbial residence time and convenient operation and management.
3) Combined process. There are many kinds of pollutants in petrochemical wastewater, and the water quality is complex. If only aerobic or anaerobic treatment is adopted, it is difficult to meet the discharge requirements. However, the combined process of anaerobic (or anoxic) and aerobic treatment is effective and widely used. For example, a new combined A/O process, A/O 1 and O2 process, is used to treat petrochemical wastewater. The system consists of sludge aerobic process, membrane anoxic process and membrane aerobic process. The influent COD is 1300mg/L, the total hydraulic retention time is 60h (20h respectively), and the effluent BOD, COD, MLSS and oil content are lower than (30, 100, 70 and 10)mg/L respectively.
Oily sewage from petrochemical enterprises has the characteristics of large fluctuation of water quantity, frequent fluctuation of water quality and very complex pollutant composition, which contains a large number of toxic and harmful substances such as oil, sulfide and volatile phenol. Direct discharge will do great harm to the environment. Correct selection of oily sewage treatment process and recycling process is the key to the normal operation of sewage field and recycling device, as well as the key to controlling investment and realizing economic operation.
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