With the rapid development of the petrochemical industry, the discharge of refinery wastewater has increased year after year. The main pollutants in refinery wastewater are oil, solid suspended solids, dissolved organic compounds and bacteria, and some may even contain elements that are toxic to human body, such as arsenic and chromium. If discharged directly into the environment, it will be environmentally friendly. And human health produces great harm.
I. Characteristics of refinery wastewater Refining wastewater is made up of sewage produced by electric desalination, atmospheric decompression, catalytic cracking and other sections. It is a multiphase system integrating suspended oil, emulsified oil, dissolved organic matter and salt. The main pollutants include petroleum, COD, BOD, sulfide, volatile phenol, suspended solids, ammonia nitrogen, etc., suspended solids and salt from the electrical desalination process, oil and sulfur, phenol, ***, etc. dissolved in sewage Related to the processing technology.
Second, the status quo of refinery wastewater treatment Refining wastewater treatment technology is divided into primary treatment, secondary treatment and tertiary treatment according to the degree of treatment. The methods used for the primary treatment include gravity sedimentation, flotation, etc.; the secondary treatment methods are mainly coagulation, biochemical, etc.: the tertiary treatment methods include adsorption, membrane separation, and the like. Refining mills are generally treated with secondary treatment to meet emission standards. There are very few enterprises that use tertiary treatment in China, and many foreign refinery wastewaters generally use tertiary or advanced treatment processes.
III. Treatment methods and research progress of refinery wastewater In recent years, refinery wastewater treatment technology has developed rapidly, and the commonly used treatment methods are as follows.
1 Gravity sedimentation method Gravity superiority method is based on the difference in density between oil and water. Under the action of gravity, the oil-water mixture will be separated automatically after a certain period of time. Gravity sedimentation method is one of the most common and simple degreasing methods. It is especially effective for the removal of oil slicks with a particle size above 100μm. It is generally used as a pretreatment unit for oil-water separation. Reasonable hydraulic setting and residence time of sewage are two important factors affecting the efficiency of degreasing. The longer the residence time, the better the treatment effect. Gravity sedimentation is characterized by the ability to accept any concentration of oily wastewater and remove large amounts of slop oil.
The main equipments for gravity sedimentation and degreasing include vertical degreasing tanks, sloping plate grease traps and coarse granulated degreasers.
2 Filtration method is to pass the refinery sewage through a device with perforations or through a filter layer composed of a certain granular medium, and use the functions of interception, sieving, inertial collision, etc. to make the suspended matter and oil in the sewage Remove. The filtering method has the advantages of simple equipment, convenient operation and low investment cost. However, as the running time increases, the pressure drop gradually increases, and backwashing is required frequently to ensure normal operation.
3 Membrane separation method Membrane separation method is to separate and purify by selective permeation of membrane. It uses microporous membrane to intercept oil beads and surface active bud, mainly used to remove emulsified oil and some dissolved oil. The emulsified oil is in a stable state, and it is difficult to separate it by physical or chemical methods. In this case, a good separation effect can be obtained by the membrane separation method. Membrane separation method has the advantages of no need to break the emulsion, directly realize oil-water separation, does not produce oily sludge, simple process flow, good treatment effect, etc., but the treatment volume is small, it is not suitable for large-scale sewage treatment, and the filter is easy to block, Higher operating costs. The current research tends to combine various membrane treatment methods or in combination with other methods, such as the combination of ultrafiltration and microfiltration to separate refinery wastewater, membrane separation and electrochemical methods, etc. Pre-treatment to extend the life of the ultrafiltration avoidance.
The five membrane separation technologies commonly used in refinery wastewater treatment are reverse osmosis (RO), ultrafiltration (UF), microfiltration (MF), electrodialysis (ED), and nanofiltration (NF). The key to separation technology lies in the choice of membrane. The membrane material includes organic membrane and inorganic membrane. Common organic membranes include cellulose acetate membrane, polysulfone membrane, polypropylene membrane, etc. The commonly used inorganic membranes are ceramic membrane alumina, cobalt oxide, Titanium oxide, etc. At the same time, you can view more technical documents of China Sewage Treatment Engineering Network.
4 Flotation Flotation method is to generate fine bubbles by introducing air or other gas into the water, so that some small suspended oil beads and solid particles in the water adhere to the bubbles, so that the buoyancy increases and floats up to the surface with the bubbles to form scum. (oily foam layer), then remove the oil using a suitable degreasing equipment. It is mainly used to dispose of dispersed oil, emulsified oil and fine suspended solids which are difficult to remove by gravity separation in oily wastewater.
According to the way of generating bubbles, the flotation method can be divided into pressurized dissolved air flotation method, impeller flotation method, aeration flotation method, induced air flotation and electrolyzed air floatation, etc. Dissolved air flotation. In order to improve the flotation effect, inorganic or organic polymer flocculant can be added to the wastewater, that is, flocculation flotation method, the effect of oil-water separation will be improved.
5 Adsorption method Adsorption method is the surface adsorption of dissolved oil and other dissolved organic matter in refinery wastewater by using multi-solid adsorbent. The most commonly used oil-absorbing material is activated carbon, and high-oil-absorbing resin as a new environmentally friendly material, because of its advantages of large oil absorption rate, strong oil retention capacity and convenient post-treatment, it has become an oil-absorbing material with great development potential.
Activated carbon can not only absorb the dispersed oil, emulsified oil and dissolved oil in the sewage, but also effectively adsorb other organic substances in the sewage. However, the adsorption capacity is limited (typically 30 to 80 mg/g for oil), and the cost is high, and regeneration is difficult, thereby limiting its application. A new type of organic adsorption material adsorption resin developed in recent years has good adsorption performance, is easy to be regenerated and reused, and is expected to replace activated carbon. In addition, coal ash, straw, ceramsite, wood chips, modified bentonite, sulfonated coal, crushed coke, organic fiber, oil absorbing felt, quartz sand, etc. can also be used as oil absorbing materials. After the oil absorbing material is saturated with oil, it is regenerated or used directly as a fuel depending on the specific situation.
The high oil absorption resin is a low crosslink copolymer which is obtained by polymerizing a long side chain olefin as a monomer. According to the synthesis monomer, the oil absorbing resin can be divided into two types: an acryl resin; the second is an olefin resin. . Because the latter olefin molecules do not contain polar groups, the affinity of such resins to oils is stronger, and has become a new hot spot in foreign research. However, due to the low source of high carbon olefins, the research direction is still in the exploration stage. Therefore, the current market is mainly propylene ester products. High oil-absorbing resin has an irreplaceable advantage in the treatment of oil with less oil or even trace oil, and its effect is more obvious in the separation of some very complex systems. However, there are still many problems to be solved for oil-absorbing resin:
a) Use the "similar compatibility" principle to improve the oil absorption performance of the resin, strengthen the internal micro-crosslinked structure, and maintain its oil absorption capacity.
h) Reduce the cost of oil-absorbing resin, use new composite materials to save resources, avoid secondary pollution, and recycle resin.
c) Find more effective resin oil absorption post-treatment technology to effectively use energy.
d) Increasing the oil absorption rate of the high oil absorption resin.
e) The relationship between the microstructure of the resin and the oil absorption properties remains to be further explored.
f) Study on the mechanism of strengthening the interaction between oil-absorbing resin and organic solvent.
6 Flocculation Flocculation method, also known as coacervation method, is to add a certain proportion of flocculant to the sewage, form a lipophilic floc in the sewage, so that the tiny oil droplets are adsorbed on it, and then use the method of sedimentation or air flotation. Remove the oil.
Commonly used flocculants are classified into inorganic flocculants and organic flocculants. Among them, inorganic flocculants are mainly aluminum salts and iron salts, but traditional aluminum salts and iron salt flocculants have the largest dosage and sludge production, and are gradually being used in recent years. Replaced with the appearance of high-molecular flocculants. Inorganic polymer coagulants such as polyferric sulfate, polyaluminum chloride, etc., organic polymer coagulants such as polyacrylamide, acrylamide and sodium acrylate copolymer have the characteristics of low dosage and high efficiency, and are limited by pH value. . Units with wastewater treatment need to consult the sewage treatment project service platform for companies with similar experience in wastewater treatment.
7 Biochemical method Biochemical method is to use the biochemical action of microorganisms to decompose complex organic matter into simple substances, convert toxic substances into non-toxic substances, and purify the sewage. Biochemical methods are only effective for biodegradable organic compounds.
According to the supply of oxygen, biochemical methods can be divided into aerobic biological treatment and anaerobic biological treatment. According to the existence state of microorganisms in sewage, it can be further divided into activated sludge method and biofilm method. The activated sludge process is widely used in refineries at home and abroad. This method has high treatment efficiency and low capital construction cost, but requires high management technology level and high operating cost. The activated sludge process has a good effect and is mainly used for treating sewage with high requirements and stable water quality. Compared with the activated sludge method, the biofilm method adheres to the surface of the filler carrier, so that microorganisms with a slow propagation rate can exist, thereby forming a stable ecosystem. However, since the amount of microorganisms attached to the surface of the carrier is difficult to control, the flexibility in operation is poor, and the volume load is limited.
8 Ultrasonic method Ultrasonic is a kind of high frequency mechanical wave, its frequency is generally between 2×104~5×108Hz, featuring energy concentration and strong penetrating power. Ultrasonic waves can have agglomeration effects, holes or cavitation effects in water. When the ultrasonic waves pass through the oily sewage, the tiny oil droplets vibrate together with the water. However, since the particles of different sizes have different relative vibration speeds, the oil droplets will collide and adhere to each other, so that the volume of the oil droplets increases. Subsequently, since the particles have become larger, they cannot vibrate with sound waves and can only be made to move irregularly. Finally, small oil droplets in the water condense and float, and the oil-water separation effect is good. When ultrasonically treating emulsified oil wastewater, it is necessary to pass an experiment to determine the optimal sound wave frequency. Otherwise, the ultrasonic pulverization effect may occur, which may affect the treatment effect.
9 Magnetization method The magnetization method is to put the magnetic species into the refinery sewage, and use the magnetic difference between the fluffy body containing the magnetic species and other substances in the water to achieve the separation. According to the type of magnetic field, it can be divided into permanent magnet separation, electromagnetic separation and superconducting magnetic separation. According to the structural principle, it can be divided into magnetic coagulation separation, magnetic disk separation and high gradient magnetic separation. It can be divided into continuous or intermittent according to the working mode. Separation and magnetic adsorption separation. The commonly used magnetic species is commercial magnetic powder. At present, the magnetization method has attracted the interest of many scholars. Finding cheap magnetic species and combining this method with other methods has become a research hotspot of this method.