air floatation oily wastewater treatment technology 1 Introduction Air floatation is to introduce air (or natural gas) into oily wastewater or try to generate gas in the water, and sometimes it is necessary to add flotation agent or coagulant, so that emulsified oil and dispersed oil or suspended particles in the sewage with a particle size of 1.25 ~ 25 um will adhere to the gas robe, float to the water surface with bubbles and be recovered, thus achieving the purpose of removing oil and suspended particles from oily wastewater. With the development of petroleum industry, air flotation oil removal technology has been gradually developed. In 1961s, Daqing Oilfield Design Institute conducted a flotation experiment with a self-made impeller flotation machine in the sewage station of East Oil Depot, and obtained satisfactory results. Adding 111 mg/L ferrous sulfate and keeping the water in the flotation tank for 31 min can make the oil content of the discharged water from the imported electric dehydrator (the water temperature is about 51℃) drop to 61.3 mg/L after flotation, and the oil removal efficiency is 99.7. In 1991, the sewage treatment design of South No.1 Station of Dagang Oilfield adopted the American-like four-stage impeller flotation machine produced by Shenyang Special Equipment Factory. After commissioning, the oil removal efficiency can reach 85, and the oil content in the effluent is 18.8 mg/1. The oil removal effect is good. The oily sewage treatment stations in Zhongyuan Oilfield, Shengli Oilfield 112 Station and an oilfield in Qinghai are all imported from the United States, and all of them have adopted impeller flotation machines. Later, they were injected into Caoqiao and Binyi in Shengli Oilfield. 2 classification of air flotation According to the different methods of generating bubbles, air flotation treatment technology can be divided into the following three types. 2.1 dissolved air flotation dissolved air flotation is to lift the wastewater to a dissolved air tank with a water pump, pressurize it by 1.3 ~ 1.35 MPa (gauge pressure), inject compressed air at the same time to make it supersaturated, and then decompress it instantly, suddenly releasing a large number of dense fine bubbles, so that the combination of bubbles and substances removed can be quickly separated from the water and floated to the surface. 2.2 Impeller Air Flotation (Mechanical Air Flotation) Impeller Air Flotation uses a high-speed rotating impeller to cut the air sucked into fine bubbles, so that the combination of bubbles and removed substances can quickly rise and separate from water. 2.3 jet air flotation jet air flotation is to use high-pressure water (1.3 ~ 1.7 MPa) to pass through the ejector, which generates negative pressure at the nozzle and sucks the gas medium. After strong shearing of the mixing tube, the gas medium forms fine bubbles, which capture oil droplets and rise to the liquid surface to form slag. In recent years, various air flotation technologies have been more and more widely used in the field of oily wastewater treatment in oil fields, because some oil products whose density is close to water are difficult to be removed from water by natural gravity sedimentation, and the air flotation rule is more effective. Especially, induced air flotation devices are often used in the treatment of produced water from offshore platforms instead of natural obstacle sedimentation for oil removal, because the air flotation treatment effect is good and the equipment volume is small, which is suitable for the conditions with limited platform area. 3 factors affecting the effect of air flotation 3 1 Gas-water ratio Gas-water ratio is an important technical parameter of air flotation machine. The greater the gas-water ratio, the better the treatment effect. The more bubbles there are, the more chances of contact with oil droplets. The chances of oil droplets adhering to bubbles will increase, and the treatment effect will be improved. However, the larger the gas-water ratio is, the better. As far as dissolved air flotation is concerned, the amount of gas dissolved in water is limited by conditions such as temperature and pressure. Generally, the solubility of gas in water decreases more when the water temperature is higher than 41℃. In addition, the dissolved gas is directly proportional to the gas pressure. Increasing the gas pressure can improve the gas-water ratio, but too high pressure will greatly increase the operating cost, which is not economical. Of course, increasing the residence time can also improve the gas-water ratio, but this method reduces the efficiency of the equipment. 3.2 Bubble size Because bubbles with different sizes have different buoyancy, their ability to adhere to oil droplets is different. Small bubbles float slowly and are easy to catch oil droplets (especially small oil droplets), while large bubbles float fast and large oil droplets are easy to be caught by them. However, the bubbles are too big, and the rapid floating speed makes them not easy to adhere to oil droplets, and they are easy to break, so the oil removal effect is not good. When the oil content in the imported medium is 1. When the concentration is 1 ~ 211 mg/L, the oil removal rate of dissolved air flotation is the highest. However, the gas-water ratio produced by dissolved air flotation is not good for oil removal. Because the solubility of gas in water is very limited, the bubble size of impeller air flotation machine and jet flotation machine is not very ideal, but the gas-water ratio is much better than the former two, so when the sewage contains oil >: At 211 mg/i, impeller air flotation and jet air flotation are more suitable. 3 3 The produced water from oil fields with salt content generally contains salt, ranging from hundreds to hundreds of thousands of mg/1. The experimental results show that the increase of salt content in oily sewage is beneficial to the improvement of oil removal efficiency. The effect of 3,4-flotation reagent air flotation on oily wastewater is greatly affected by the addition of chemicals, and sometimes it plays a decisive role. Using flotation AIDS, coagulants and foaming agents can greatly improve the efficiency of treating oilfield produced water by air flotation. Foreign chemicals, especially air flotation AIDS, are mostly compound polymers, which have many functions such as coagulation, demulsification, foaming and flotation. 4 adaptability of three kinds of flotation machines to fluctuation of process conditions 4.1 change of oil content of water beam impeller flotation machine has great oil removal potential, and the oil removal ability of pressurized dissolved air flotation has a great relationship with the oil content of inlet water. 4.2 Changes of water temperature The aeration of impeller flotation machine is not affected by the water temperature, and it can still work normally when the water temperature reaches 91℃, while the aeration of pressurized dissolved air flotation and jet flotation will obviously decrease when the water temperature rises, which will reduce the flotation effect. 4.3 Flotation process changes The impeller flotation machine has a strong stirring effect, so the requirement for reagent stirring before it can be lower, and the reagent can be added in the middle of the flotation process, which is convenient for reagent adjustment. However, jet air flotation and pressurized dissolved air flotation are not, which require higher stirring of chemicals added before them. The raw water needs to be added with chemicals in advance, the PH value should be adjusted, and the added flocculant should be fully coagulated in advance to improve its treatment effect. 5 Development of air flotation equipment at home and abroad In recent years, with the wide application of air flotation equipment in sewage treatment projects, the structure and types of flotation equipment are constantly changing and increasing. Dissolved air flotation mainly includes: shallow pool air flotation, high efficiency air flotation, etc. Mechanical air flotation mainly includes vortex concave air flotation, induced air flotation and spiral propulsion. The following is a brief introduction of these air flotation devices. 5.1 shallow pool air flotation shallow pool air flotation is based on traditional air flotation, which uses shallow theory and "zero velocity principle, and integrates coagulation, air flotation, skimming, sedimentation and mud scraping. The whole equipment is cylindrical, with compact structure and shallow pool. The main body of the device consists of five parts; Pool body, rotary water distribution mechanism, dissolved air release mechanism, frame mechanism, water collection mechanism, etc. Water inlet 13, water outlet 13 and scum discharge 13 are all concentrated in the central mechanism of the tank body, and the water distribution mechanism, water collection mechanism and dissolved air release mechanism are closely connected with the frame and rotate around the center of the tank body. Effective water in the pool; Jue} is 411～511mm, and the hydraulic retention time in the pond is 3 ~ 5min. Its advantages are that the pool is shallow, and the floating time of suspended matter is shortened. To some extent, it overcomes the shortcomings of poor stability of scum after floating, such as high pressure (1.4 ~ 1.1 MPa) and large bubbles (31 ~ 111 um), and the shallow pool also causes the clear water area in the pool to be inconspicuous or no clear water area. 5.2 High-efficiency air flotation technology and its complete sets of equipment are high-tech water treatment projects with international leading level established by the Architectural Research Institute of the Ministry of Metallurgical Industry in 1981. The adsorption value theory established by Xu Zhi, the inventor of this project, is the design basis. Compared with traditional air flotation, high-efficiency air flotation has the following main differences: a. High-efficiency air flotation improves the oil removal efficiency by expanding the gas-liquid contact area, rather than prolonging the residence time. b. The dissolved gas utilization rate of high-efficiency air flotation is as high as 1.111%. According to the adsorption value theory, only microbubbles smaller than suspended particles can effectively adsorb with the suspended particles. High-drum air flotation can produce bubbles of lum, while the diameter of bubbles produced by conventional air flotation is generally above 51um. 5.3 Inductive air flotation Inductive air flotation is a typical mechanical air flotation equipment, which began to be used in flotation and mineral processing. Air is sucked into the wastewater by the negative pressure formed by the high-speed rotating impeller installed in the machine, and at the same time, the air is crushed into small bubbles by the shearing effect of its high-speed rotation, instead of the process of dissolving the gas and releasing it, and the diameter of the bubbles is general >: 51um, the aeration rate can reach 3．2 m ／min, which is much larger than the pressurized dissolved air flotation machine. Compared with dissolved air flotation, induced air flotation has the characteristics of small floor space and low operating cost. Its disadvantage is that high-speed stirring has a negative effect on particle size <: 31μ m oil drop removal is unfavorable. 5.4 Vortex concave air flotation Vortex concave air flotation is also developed on the basis of impeller air flotation. Different from induced air flotation, the impeller is not installed in the middle of the air flotation tank, but at the water inlet end of the flotation tank. The rotation of the impeller brings in air and shears it, and it enters the air flotation area and the separation area with the water inlet, so as to achieve the purpose of solid-liquid (liquid-liquid) separation. Compared with dissolved air flotation and induced air flotation, vortex-concave air flotation has the characteristics of small floor space and low energy consumption. 5. Spiral propulsion air flotation originated in the United States, mainly relying on the propulsion of the screw to introduce air and cut and disperse bubbles to achieve the purpose of removing suspended solids. 5.6 Jet air flotation machine Jet air flotation is a new sewage treatment technology that has recently appeared. It uses sewage or purified water as injection fluid, and the fluid forms negative pressure in the suction chamber of the ejector, inhaling gas, and the carried gas is cut into tiny gas robes when passing through the mixing section of the ejector, and bubbles adhere to oil beads and solid particles during the rising process of the air flotation chamber, and rise to the liquid level, so as to achieve the purpose of removing oil residue. It can treat all kinds of oilfield produced water with oil content not higher than 2 ooo mg/L. Compared with impeller air flotation, its advantages are as follows: a. Less electricity consumption, only equivalent to 33% of impeller air flotation; B. There are no rotating parts in the liquid flow, so the shearing force is very small; C. The diameter of bubbles generated is small, so the oil removal efficiency under the required operating conditions is higher than that of impeller type. However, jet air flotation device is not as widely used in oilfield produced water treatment as impeller air flotation device because it requires high operating conditions such as pressure, water quality and power of injected fluid. According to the characteristics of jet air flotation, this process is more suitable for the treatment of produced water in remote oil fields with small produced water and low water quality requirements. Conclusion There are many kinds of air flotation equipment, and the choice of air flotation equipment depends on the specific water quality. At present, in the treatment of oilfield oil production wastewater, induced flotation is widely used and the process is mature; Vortex-concave air flotation has been used in petrochemical wastewater, and the effect is good, but it is not used in the treatment of oilfield produced water. It is also used in petrochemical wastewater treatment because its dissolved gas release is not complete and dissolved oxygen generally exceeds the standard, so it is applied in oilfield produced water treatment step by step. Jet air flotation has been applied in oil fields at present, such as the sewage treatment station of Wenmi United Station in Tuha Oilfield, Xinjiang, and the effect is very good.