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Oily sewage treatment technology
This paper introduces the principles, characteristics and degreasing equipment of common oily wastewater treatment technologies, and sums up the treatment methods of oily wastewater.
Keywords: oily wastewater; Technology; Sewage treatment method
Oily sewage is produced in a large amount and involves a wide range, such as oil exploitation, oil refining, petrochemical industry, oil storage and transportation, tanker accident, ship transportation, vehicle cleaning, machinery manufacturing, food processing and other processes. As a common pollution, oil pollution is extremely harmful to environmental protection and ecological balance. At present, there are many oil-water separation technologies, such as gravity separation, air flotation, coarse granulation, filtration, adsorption, ultrasound and so on, and new oil removal technologies are still under development. This paper introduces the principle and method of oil remover.
1 gravity separation method
Gravity separation is a typical primary treatment method, which uses the density difference between oil and water and the incompatibility between oil and water to separate oil droplets, suspended solids and water in static or flowing state. The oil droplets dispersed in water slowly float up and stratify under the buoyancy, and the floating speed of oil droplets depends on the size of oil droplets, the density difference between oil and water, the flow state and viscosity of fluid. The relationship between them can be described by Stokes and Newton's laws.
1. 1 cross-flow oil remover [1]
The cross-flow oil-bearing sewage degreasing equipment is developed on the basis of inclined plate oil remover, which consists of two parts: the gathering area and the separation area of oil-bearing sewage. Oily sewage first passes through a cross-plate coalescer, so that dispersed small oil droplets are aggregated into large oil droplets, and small solid substances are flocculated into large particles, and then the aggregated oil droplets and solid substances are separated from water through a transverse flow separation plate area with unique channels. When oil-water and solid matter are separated, gas (natural gas) can also be separated.
1.2 corrugated plate coalescence oil-water separator [2]
The principle of oil removal by corrugated plate is mainly to use the density difference between oil and water to make oil beads float on the wave crest of the plate and separate them. The key is to make corrugated plates with variable spacing and variable water flow streamline with the help of Hasen's shallow pool sedimentation principle. The cross section of water is changed, and the water flows alternately in the state of diffusion and contraction, resulting in pulsating (sinusoidal) water flow, which increases the collision probability between oil droplets, makes small oil droplets bigger, and accelerates the floating speed of oil droplets and water.
1.3 Aggregation oil-water separator [3]
Ferrer Company of Austria took the lead in developing CPS integrated corrugated plate gravity accelerated aggregation oil-water separator in the world. Corrugated board is a patented product of Ferret Company. It is based on polypropylene and contains many additives, which makes it lipophilic but non-sticky and anti-aging. Corrugated plates are stacked one by one, and the spacing is generally 6 mm (when the content of suspended solids in water is high, the spacing of 12 mm can be adopted).
1.4 efficient lifting free water separator [4]
By combining horizontal and vertical free water separators and adopting vertical design, the shortcomings of small coverage area of oil-water interface of vertical container, short distance between oil-water interface of horizontal container and water outlet and insufficient separation time are overcome. The liquid inlet is located at the upstream end of the tubular container, and the oil droplets in the water can coalesce and climb to the top oil outlet, while the water sinks to the bottom water outlet for discharge. The elevation angle of the equipment is less than 12, and the length is 18. The diameters are1372mm and 914mm.
Filtration filtration method is to pass the wastewater through a porous device or through a filter layer composed of a certain granular medium, and use its interception, screening and inertial collision to remove harmful substances such as suspended solids and oils from the wastewater. There are three commonly used filtration methods: layered filtration, membrane filtration and fiber medium filtration. Membrane filtration method, also known as membrane separation method [5], uses microporous membrane to intercept oil beads and surfactants, which is mainly used to remove emulsified oil and partially dissolved oil. The filter membrane includes ultrafiltration membrane, reverse osmosis membrane and mixed filter membrane. Membrane materials include organic membrane and inorganic membrane. Common organic membranes include cellulose acetate membrane, polysulfone membrane and polypropylene membrane, while common inorganic membranes include ceramic membrane, alumina, cobalt oxide and titanium oxide. Emulsified oil is in a stable state and it is difficult to separate by physical or chemical methods. With the rapid development of membrane technology, the treatment of emulsified oil wastewater by membrane technology has been gradually accepted and applied by industry.
3 centrifugal separation method
Centrifugal separation method is to make the container containing oily wastewater rotate at high speed to form centrifugal force field. Because the density of solid particles and oil beads is different from that of wastewater, the centrifugal force is also different, thus removing solid particles and oil beads from wastewater. The commonly used equipment is a hydraulic cyclone separator. The application of cyclone separator in liquid-solid separation began in the 1940s of 19, and now it is relatively mature, but it is used for oil-water separation.
The research in this field is much later. Although the basic principles of liquid-solid separation and liquid-liquid separation are the same, the geometric structures of the two devices are quite different. De-oiling cyclone separator originated in England. Since the 20th century.
Since the end of 1960s, the laboratory of multiphase flow and mechanical separation led by Professor MartinThe w of Southampton University, England, began to study the cyclone separator for oil removal in water, and invented the double cone and double inlet.
Liquid-liquid cyclone separator. Satisfactory results have been obtained in the experiment. Subsequently, Young GAB and others designed a single cyclone with the same separation performance as the double-cone cyclone, but the throughput was higher than 1 times.
Conical cyclone separator. After geometric optimization design, ConocoPhillips introduced K cyclone separator, which improved the separation performance of oil droplets with diameter less than 10μm more obviously. Due to the cyclone separator
Cyclone oil removal technology has many unique advantages, and has become an irreplaceable standard equipment for treating oily wastewater in developed countries, especially on offshore oil exploitation platforms.
4 flotation method
Air flotation, also known as air flotation, is a water treatment technology that is being deeply studied and popularized at home and abroad. In this method, air or other gases are introduced into the water to generate tiny bubbles, so that some tiny suspended oil beads and solid particles in the water attach to the bubbles and float to the water surface with the bubbles to form scum (oily foam layer), and then the oil is skimmed by a suitable skimmer. This method is mainly used to treat dispersed oil, emulsified oil and fine suspended matter with the particle size of 10 ~ 60μ m remaining in water after treatment in the grease trap, and the oil content in the effluent can be reduced to 20 ~ 30mg/L. According to the different ways of generating bubbles, air flotation can be divided into pressurized air flotation, aerated air flotation and electrolytic air flotation, among which pressurized dissolved air flotation is the most widely used.
5 biological oxidation method
Biological oxidation is a method to purify wastewater through the biochemical action of microorganisms. Petroleum is a kind of hydrocarbon organic matter, which can be decomposed into carbon dioxide and water by life activities such as microbial metabolism. Organic matter in oily wastewater is mostly dissolved and emulsified, with high BOD5, which is beneficial to biological oxidation. For wastewater with oil concentration below 30 ~ 50 mg/L and other biodegradable harmful substances, biochemical treatment is often used, which is mainly used to remove dissolved oil from wastewater. The commonly used biochemical treatment methods of oily wastewater include activated sludge method, biological filtration method and biological turntable method. Activated sludge process has a good treatment effect, which is mainly used to treat wastewater with high requirements and stable water quality. Compared with activated sludge process, biofilm process adheres to the surface of filler carrier, so that microorganisms with slow reproduction speed can also exist, thus forming a stable ecosystem. However, because it is difficult to control the number of microorganisms attached to the carrier surface, the flexibility of operation is poor and the volume load is limited.
6 chemical methods
Chemical method, also known as chemical method, is a method of adding chemicals to convert pollutants in wastewater into harmless substances through chemical action, so that wastewater can be purified. Commonly used chemical methods include neutralization, precipitation, coagulation, redox and so on. Coagulation is mainly used for oily wastewater. Coagulation method is to add a certain proportion of flocculant to oily wastewater. After hydrolysis in water, positively charged micelles and negatively charged emulsified oil are neutralized by electricity, and oil particles gather and become larger in size. At the same time, flocs are generated to adsorb fine oil droplets, and then oil-water separation is realized through sedimentation or air flotation. Common flocculants include inorganic flocculants such as polyaluminum chloride (PAC), ferric chloride, aluminum sulfate and ferrous sulfate, and organic polymer flocculants such as acrylamide and polyacrylamide (PAM). The dosage and pH value of different flocculants are different. This method is suitable for emulsified oil droplets and other fine suspended solids that cannot be separated by gravity sedimentation.
7 adsorption method
Adsorption method is to use lipophilic materials to adsorb dissolved oil and other dissolved organic matter in wastewater. The most commonly used oil absorption material is activated carbon, which can absorb dispersed oil, emulsified oil and dissolved oil in wastewater. Because the adsorption capacity of activated carbon is limited (generally 30-80 mg/g oil), the cost is high and it is difficult to regenerate. Generally, activated carbon is only used as the last stage of multi-stage treatment of oily wastewater, and the concentration of effluent oil can be reduced to zero. 1-0.2mg/l .. 1976 The wastewater from Hunan Changling Refinery is treated by advanced activated carbon adsorption. Some beneficial results have also been achieved in developing new adsorbents at home and abroad. It is found that flake graphite can adsorb the heavy oil released by oil spill from offshore tankers and easily separate from water. Adsorption resin is a new type of organic adsorption material developed in recent years, which has good adsorption performance and easy regeneration, and has the trend of gradually replacing activated carbon. More and more people in the industry are studying the synthesis and application of high-efficiency oil-absorbing resin [6]. The research shows that the polypropylene oil-absorbing material can be used to adsorb, separate and recover the oil substances in the wastewater of petroleum industry, and the appropriate purification method can be selected according to the initial conditions, final requirements and water flow of the wastewater. In addition, fly ash, modified bentonite, sulfonated coal, broken coke, organic fiber, oil-absorbing felt, ceramsite, quartz sand, sawdust and straw can also be used as oil-absorbing materials. After the oil-absorbing material is saturated with oil, it can be recycled or directly used as fuel according to the specific situation.
8 coarse granulation method
Coarse-grained method uses the huge difference of affinity between oil and water relative to agglomerates. Oil particles are captured by the material and remain in the surface and pores of the material to form an oil film. When the oil film increases to a certain thickness, under the action of hydraulic force and buoyancy, the oil film falls off and aggregates into larger oil particles. According to Stokes formula, the floating speed of oil particles in water is proportional to the square of the diameter of oil particles. After coalescence, particles can be easily separated from water by larger oil droplets. After coarse granulation, the oil content and dirty oil properties of wastewater have not changed, but it is easier to remove oil by gravity separation.
8. 1 new efficient degreasing agent [7]
Swirl oil removal, coarse oil removal and inclined plate oil removal are recognized as efficient oil removal technologies at present. The high-efficiency degreaser is a high-efficiency integrated degreaser which integrates the above-mentioned high-efficiency degreasing technologies.
Its overall structure is designed to be horizontal, consisting of a cyclone (vortex section) coarse granulation section and an inclined plate degreasing section. It can not only improve oil removal efficiency, but also facilitate operation and reduce land occupation. According to the characteristics of produced water in Jianghan Oilfield
Two-stage coarse granulation and two-stage inclined plate degreasing are adopted. When the imported ρ (oil) is less than or equal to1000 mg/L, the export meets the import requirement of the subsequent treatment equipment (filter) that ρ (oil) is less than 30 mg/L.
8.2 EPS oil-water separation technology [8]
EPS oil-water separator is an efficient and advanced oil-water separation device. It combines advanced plate degreasing and coarse particle agglomeration technology, and integrates sewage pretreatment, oil-water separation, secondary precipitation and oil recovery. It has the characteristics of low installation and operation cost, good oil-water separation effect and convenient operation and maintenance. It is an updated product of vertical oil removal tank and inclined plate oil removal device (such as API gravity oil removal device (API), corrugated plate inclined plate oil removal device (CPI), parallel inclined plate oil removal device (PPI), etc.). EPS oil-water separator has been applied in Korea, USA, Poland, India, Thailand, China and other countries, and the sewage treatment effect is generally good.
9. Sound wave, microwave and ultrasonic dehydration technology
Sound wave can accelerate the coalescence of water droplets and improve the dehydration efficiency of crude oil; Ultrasonic wave can reduce energy consumption and demulsifier consumption; Microwave can not only reduce the stability of emulsion, but also heat emulsion to further promote the coalescence of water droplets, which has a good application prospect in solving the problem of deep dehydration of crude oil with complex properties caused by tertiary oil recovery in old oil fields in eastern China.
Microwave refers to electromagnetic waves with a frequency of 300 MHz~300 GHz [9]. Microwave water treatment technology is a new technology to treat water by using the strong catalysis, penetration, selective energy supply and the function of killing microorganisms of microwave field.
Ultrasonic wave is a kind of high-frequency mechanical wave, its frequency is generally between 2×104 and 5×108 Hz, and it has the characteristics of concentrated energy and strong penetration. Ultrasonic wave can produce coagulation effect, cavitation effect or cavitation effect in water [10]. When ultrasonic waves pass through a solution containing sewage, tiny oil droplets will vibrate with the water. However, due to the different relative vibration speeds of particles with different sizes, oil droplets will collide and bond with each other, which will increase the volume of oil droplets. Subsequently, because the particles became larger, they could not vibrate with the sound waves, but only moved irregularly. Finally, the small oil droplets in the water condense and float, and the oil-water separation effect is good. When ultrasonic treatment of emulsified oil wastewater, it is necessary to determine the best sound wave frequency through experiments, otherwise there may be ultrasonic crushing effect, which will affect the treatment effect. At present, scholars at home and abroad have used ultrasonic technology to degrade dozens of pollutants in water, but most of the research objects are single-component simulation systems, and actual sewage often contains many pollutants. The applicability of ultrasonic technology in actual sewage treatment needs further study. In addition, at present, the research on the degradation of pollutants in water by ultrasonic technology mostly belongs to the laboratory stage, and it is difficult to realize engineering at present because of the lack of research on the degradation mechanism, reaction kinetics, reactor design and amplification of sonochemical reaction process.
10 ultrasonic/electrochemical combined technology [9]
Using the cavitation effect of ultrasonic wave, the electrode will not form a covering layer in electrochemical reaction, thus avoiding the decrease of electrode activity; Ultrasonic cavitation effect is also beneficial to the synergistic electrocatalytic process to produce OH and accelerate the decomposition of pollutants in sewage. Ultrasound can also fully disperse the organic matter in the aqueous solution, thus greatly improving the treatment capacity of the reactor. Misera et al. found that the decomposition rate of phenol was only 50% without ultrasonic wave, and it would increase to 80% with ultrasonic wave of 25 kHz and 104 W/ m2. Liu Jing and others used a combination of ultrasound and electrochemistry.
The treatment of printing and dyeing wastewater shows that under the synergistic effect of ultrasonic wave and electric field, the decoloration rate of wastewater is much higher than that when ultrasonic wave is used alone.