Author: Jiang Chaocheng Zhao Dongfeng
order
Oily sludge is oily solid waste produced in the process of oil exploitation, transportation, refining and oily sewage treatment. Generally, the oil content in sludge is 10 ~ 50% and the water content is 40 ~ 90%. China's petrochemical industry produces 800,000 tons of tank bottom mud and tank bottom mud every year [1], Shengli Oilfield produces more than 65,438 tons of oily sludge every year, and Dagang Oilfield produces about/kloc-0.5 million tons of oily sludge every year. Oily sludge contains a large number of toxic substances with foul smell, such as benzene series, phenols, anthracene and pyrene [3]. If oily sludge is not treated, it will not only pollute the environment, but also waste resources. The treatment of oily sludge is always a big problem that puzzles the oilfield.
1. Oily sludge source
The sources of oily sludge mainly include the following ways:
1. 1 oily sludge produced in crude oil production.
The oily sludge produced in the process of crude oil exploitation mainly comes from the ground treatment system. Oily sludge produced in the process of oil production sewage treatment, together with floc formed by adding water purifying agent in sewage purification treatment, corrosion products of equipment and pipelines, scale and bacteria (corpses), constitutes oily sludge. This kind of oily sludge generally has the characteristics of high oil content, high viscosity, fine particles and difficult dehydration, which not only affects the quality of crude oil transportation, but also is difficult to meet the standards of injected water quality and sewage discharge [4].
1.2 oily sludge produced in the process of oil field gathering and transportation.
Oily sludge in Shengli Oilfield mainly comes from sludge from oil tanks, settling tanks, sewage tanks, grease traps, oil sands and sludge removed from oily water treatment facilities, light hydrocarbon treatment devices and natural gas purification devices in refineries, as well as landing crude oil and oily sludge generated from drilling, operation and pipeline perforation [5]. When oil is stored in an oil storage tank, a small amount of mechanical impurities, sand, soil, heavy metal salts, paraffin wax and asphaltene in the oil are deposited at the bottom of the tank, forming sludge at the bottom of the tank.
Sludge in Zhongyuan Oilfield is mainly discharged from primary sedimentation tank, secondary sedimentation tank and washing well water recovery tank. Oily sludge itself is complex in composition, containing a large number of aged crude oil, wax, asphaltene, colloid and solid suspension, bacteria, salts, acid gases, corrosion products and so on. In the process of sewage treatment, a lot of coagulant, corrosion inhibitor, scale inhibitor, bactericide and other water should be added.
Treatment agent [6].
During the regular cleaning of oil tanks for 3-6 years, the oily sludge at the tank bottom accounts for about 1% of the tank capacity. The oily sludge at the bottom of the tank is characterized by extremely high hydrocarbon (oil) content. According to the investigation and test, it is found that about 25% of the oil tank sediments are water, 5% are silt and 70% are hydrocarbons, of which 7.8% are asphaltenes, 6% are paraffin and 4.8% are sludge ash [3].
1.3 oily sludge produced by refinery sewage treatment plant
The oily sludge in the sewage treatment plant of refinery mainly comes from the bottom mud of oil separation tank, scum of air flotation tank and bottom mud of crude oil storage tank. , commonly known as "three mud". These oily sludge have different compositions, usually with oil content of 10% ~ 50%, water content of 40% ~ 90% and a certain amount of solids.
2. The harm of oily sludge
Oily sludge is huge. If discharged directly without treatment, it will not only occupy a lot of cultivated land, but also pollute the surrounding soil, water and air, accompanied by malodorous gases. Sludge contains a large number of pathogenic bacteria, parasites (eggs), copper, zinc, chromium, mercury and other heavy metals and salts.
And toxic and harmful substances that are difficult to degrade, such as polychlorinated biphenyls, dioxins and radionuclides.
3. Common sludge treatment methods
The ultimate goal of oily sludge treatment is reduction, resource utilization and harmlessness. Commonly used oily sludge treatment methods: solvent extraction, incineration, biological method, coking method, profile control of oily sludge, comprehensive utilization of oily sludge, etc.
3. 1 dehydration of oily sludge
There are four forms of water in oily sludge: free water, intermittent water, attached water [or surface water] and chemically bound water.
See table 1[7] for commonly used dehydration methods and effects of oily sludge.
Table 1 Common dewatering methods and effects of oily sludge
Dehydration method
dehydration plant
Water content after dehydration%
Dehydrated state
Concentration dehydration
Gravity, air flotation, separation
Cardiac concentration
95-97
Approximate paste
Natural drying method
Natural drying field and drying
battlefield
70-80
mud cake
Mechanical dehydration
vacuum filtration
Vacuum drum
Disc player
60-80
mud cake
pressure filtration
plate and frame filter press
45-80
mud cake
Rolling filtration
Rolling belt filter press
78-86
mud cake
centrifugal filtration
centrifuge
80~85
mud cake
Drying method
10-40
Powdery and granular
incinerate
0- 10
grey
3.2 Conditioning of oily sludge
The concentrated black viscous oily sludge after gravity sedimentation and dehydration is generally composed of oil-in-water (O/W), water-in-oil (W/O) and suspended solid * * *, which belongs to a multiphase colloid system with complex components. Because the charges adsorbed on the surface of oily sludge particles are the same, they repel each other, and because of full emulsification, it is extremely difficult to lose stability, which makes it difficult to separate oil, water and sludge. Modifier needs to be added to separate crude oil from solid particles, to polymerize oil droplets, to precipitate original chemicals and solid impurities,
Water and slag are completely separated.
Li Fanxiu studied the conditioning-mechanical dehydration process of oily sludge. The key of this technology lies in the selection of the types and dosage of flocculant [8], demulsifier and regulator [9] used in conditioning, the selection of dehydration machinery type and the determination of operation parameters of dehydration machinery.
It is reported that Jan Bock, Sanjay R Srivatsa, Aldo Corti and others [10] invented the patented technology of mechanical dehydration oil recovery for oily sludge conditioning: by adding surfactant, diluent (such as kwai alkane), electrolyte (NaCl solution), or demulsifier (anionic or nonionic), wetting agent (which can increase the affinity of solid particle surface with water) and pH value, According to relevant data, Jinzhou Oil Production Plant of Liaohe Oilfield in Panjin City, Liaoning Province has obtained a patented technology for conditioning oily sludge to recover crude oil [1 1].
3.3 The extraction method is to separate oil from oily sludge by using the principle of "similar miscibility" and selecting suitable organic solvent as extractant.
Methods of crude oil recovery and utilization. Huang Shusheng and others treated oily sludge [12] by multi-stage separation and extraction plus one-stage hot washing. The treated sludge can meet the discharge standard of agricultural sludge and the chemicals can be recycled.
Supercritical fluid extraction technology is a new oil sludge extraction technology, which is in the development stage. It makes gaseous substances at normal temperature and pressure reach liquid state through high pressure, and uses it as extractant. Because of its high solubility, the extractant is easy to recover and recycle. Commonly used supercritical fluid extractors include methane, ethylene, ethane, propane, carbon dioxide and so on. These substances have high critical temperature and low critical pressure, and the raw materials are cheap and easy to obtain. They are good supercritical extraction agents with low density and easy separation [13].
At present, the treatment of oily sludge by extraction is still in the experimental development stage. The advantage of extraction method is that oily sludge is thoroughly treated, and most petroleum substances can be extracted and recovered. However, due to the high cost of extractant and certain loss in the treatment process, the extraction method has not been actually applied to the treatment of oily sludge in refineries. The key to the development of this technology is to develop an extractant with high cost performance.
3.4 Incineration method
The oily sludge pretreated by dehydration and concentration in advance is sent to the incinerator, which is burned at 800 ~ 850℃ for 30 minutes, and the burnt ash needs further treatment.
Most refineries in China have sludge incineration devices, and oily sludge in sewage treatment plants is the most used waste, such as the parallel rotating incinerator used in Changling Petrochemical Company and the fluidized bed incinerator used in Yanshan Petrochemical Company, which have achieved good results in treating oily sludge. At present, the main types of incinerators in China are: square box, fixed bed, fluidized bed, rake furnace or rotary kiln.
The advantages of incineration treatment method are that almost all kinds of harmful substances are removed after sludge incineration, which reduces the harm to the environment, and the waste volume reduction effect is good and the treatment is safer. The disadvantage is that secondary pollution is produced in the incineration process, which wastes valuable information.
3.5 Biological methods
Biological treatment technologies of water-based sludge mainly include in-situ tillage, composting and sludge bioreactor. Microbial degradation of petroleum hydrocarbons in oily sludge into harmless soil components is mainly used. According to reports, in-situ farming will cause some pollution to land and groundwater, so it has been stopped in some developed countries.
Composting is a process of mixing oily wastes with appropriate materials and stacking them together to degrade petroleum hydrocarbons through natural microorganisms. There are four composting methods: dike composting, static composting, closed composting and container composting. Composting is an effective biological treatment method, and the half-life of hydrocarbons in oily sludge is about 2 weeks. The treated oily waste can be buried or used in farmland.
Bioreactor is a kind of container that dilutes oily sludge in nutrient medium to make it turbid. Because the bioreactor can artificially control the operating conditions such as oxygenation, temperature and nutrients, the biodegradation rate of hydrocarbons is faster than other biological treatment processes. Adding domesticated high-efficiency hydrocarbon oxidizing bacteria can accelerate the biodegradation of hydrocarbons. According to the literature, when the solid load is 5%, the half-life of biodegradation is 5 days. Bioreactor method is suitable for oily sludge, oily soil and oily drilling cuttings. After treatment, the liquid part of oily waste can be discharged into treatment wells (pits and ponds) or used for other purposes (such as reuse). The solid part can be used in farmland, and the bioreactor method can also be used for the pretreatment of petroleum industrial wastes to reduce the hydrocarbon content and then carry out other treatments.
3.6 Coking method
Oily sludge contains a certain amount of mineral oil, which is mainly composed of alkanes, cycloalkanes, aromatics, olefins, resins and asphaltenes. Coking treatment of oily sludge is to produce liquid oil, non-condensable gas and coke products through thermal cracking and thermal condensation of hydrocarbons at high temperature.
Zhao Dongfeng et al. [14] used oily sludge with an average oil content of 69.46% and a water content of 4.7 1% for coking reaction at 490℃ and atmospheric pressure for 60min. Under these conditions, the yield of liquid products is 88.23%, and the main products are gasoline, diesel oil and wax oil.
3.7 Profile Control Technology
Oily sludge in production water has good compatibility with formation, and oily sludge is used as basic raw material. Through chemical treatment and adding appropriate additives, the solid particles in it are suspended, the suspension time is prolonged and the injection depth is increased, thus effectively improving the plugging strength, uniformly dispersing the oil components and forming a uniform and stable emulsion. Emulsified suspension profile control agent prepared from oily sludge is applied to profile control of oil field water injection wells. After the formation reaches a certain depth, the emulsified suspension system decomposes due to the scouring release of formation water and the adsorption of formation rocks. The mud absorbs colloid and wax, and through them, it forms a "particle structure" with larger particle size, which settles in macropores, reduces the diameter of macropores, blocks the high permeability zone, increases the seepage resistance of injected water, and forces the injected water to change the seepage. By optimizing the construction technology, oily sludge can only block the high permeability layer and will not pollute the middle and low permeability layer. As a profile control agent, the treated oily sludge should meet the following technical indicators: low viscosity (≤ 0.3 Pa s) and good pumpability; After adding suspending agent, oily sludge has good suspension performance and settling time is more than 3 hours.
Profile control test of oily sludge was carried out in Ⅱ 4-6 series of Block 437 in Shuanghe Oilfield, Henan Oilfield, and good results were obtained [15, 16]. The profile control test of oily sludge in Zhuangxi Oilfield of Shengli Oilfield has also achieved good results [17].
3.8 Other methods
In addition to the above-mentioned oily sludge treatment methods, there are in-situ tillage, oily sludge solidification, chemical demulsification, solid-liquid separation and other land landfill technologies [18]. The comprehensive utilization of oily sludge is generally to use oily sludge to pave roads, make bricks and make honeycomb briquettes. There are also low-temperature pyrolysis technology of oily sludge abroad, and the method of separating oily sludge by solvent and low-frequency sound wave [19].
4. Comparison of various treatment methods
① Simple treatment: oily sludge is easy to be buried directly or after solidification. Direct landfill of oily sludge is the main disposal method of oily sludge in most oil fields in China at present, but this method not only wastes valuable energy, but also may lead to environmental pollution. Among them, the method of landfill after curing can reduce the harm to the environment, but it is far from the current environmental protection requirements.
② Physical and chemical treatment: Various physical and chemical treatment methods mostly aim at recovering crude oil, so they are mainly suitable for oily sludge with high oil content. Chemicals are usually needed in the treatment process, and special treatment facilities are needed. The treatment process is complex and costly. The high price of crude oil and the high sewage charges of oily sludge make this method still attractive. The disadvantages of this method are incomplete oil recovery and secondary pollutants in waste water and waste residue, which still need further treatment or comprehensive utilization. In addition, oily sludge is widely produced, mostly intermittent, and the properties of oily sludge from different sources are different, which limits the comprehensive use and popularization of this treatment method in oil fields.
(3) Biological treatment technology: Because of its advantages of energy saving, low investment and low operating cost, biological treatment has been widely concerned and valued by environmental protection people at home and abroad. Solid-liquid separation and oil removal of oily sludge are realized by biological treatment technology, and the remaining residue reaches the sludge discharge standard. Its advantages are no need to add chemicals, low energy consumption and environmental protection. However, the land cultivation method and composting method need a large area of land, and the bioreactor method still has waste residue discharge, which takes a long time and is complicated to operate.
(4) Incineration method: Incineration must be carried out in a specially built incinerator, which can completely eliminate harmful organic substances in oily sludge. If the comprehensive utilization of combustion heat energy is not considered, energy will be wasted.
⑤ Fueling: Fueling is a comprehensive utilization method of energy contained in oily sludge, but crude oil cannot be extracted from it. When using this method, we should consider both economy and environment comprehensively. The advantages and disadvantages of several main treatment methods for oily sludge are listed in Table 2.
Table 2 Comparison of advantages and disadvantages of main treatment methods for oily sludge
serial number
job operation
application area
superiority
disadvantaged
1
Simple processing
All kinds of oily sludge
simple and easy to do
It pollutes the environment and cannot recycle crude oil.
2
physicochemical treatment
The oil content is above 5- 10%.
plaster
Crude oil recovery
Joint utilization
The devices that need to be treated need to be added with chemicals, and the sewage and waste residue still need to be discharged, so the treatment cost is high.
three
biological treatment
All kinds of oily sludge
Save energy and do not need chemicals.
The treatment cycle is long and the crude oil cannot be recovered.
four
Incinerate oily sludge with oil content less than 5- 10% and sludge containing harmful organic matter.
Harmful organic matter are thoroughly treated,
Devices that need to be burned usually need to add combustion-supporting fuel, and there are exhaust gases that cannot be recovered.
crude oil
five
Making fuel and bricks.
All kinds of oily sludge
Comprehensive utilization, and
Easy to implement
Crude oil can't be recycled, and there are exhaust emissions.
5. Concluding remarks
Oily sludge mainly comes from oil field exploitation, oil and gas gathering and transportation and sewage treatment plants. There are many treatment technologies for oily sludge, and each method has its own advantages and disadvantages and scope of application. However, due to the complex composition of oily sludge, no treatment method can treat all types of oily sludge. Classification and treatment methods of oily sludge are the main research directions in the future.