Author: Jiang Yong, Zhao Chaocheng, Zhao Dongfeng
Preface
Oil-containing sludge is oily solid wastes generated in the process of petroleum extraction, transportation, refining, and oily wastewater treatment. The general oil content of sludge is 10-50%, and the water content is 40-90%. In China's petrochemical industry, an average of 800,000t of tank bottom sludge and pool bottom sludge is generated annually [1], and Shengli oilfield generates more than 100,000 tons of oily sludge annually, Dagang oilfield generates about 150,000 tons of oily sludge annually, and Henan oilfield generates 5×104m3 of oily sludge annually [2]. Oily sludge contains a large number of benzene, phenols, anthracene, pyrene and other malodorous toxic substances [3], oil-containing sludge, if not treated, not only pollutes the environment, but also causes a waste of resources. The treatment of oil-containing sludge has always been a major problem in oil fields.
1. Sources of oil-containing sludge
The sources of oil-containing sludge are mainly in the following ways:
1.1 Oil-containing sludge generated by crude oil extraction
Oil-containing sludge generated during crude oil extraction is mainly from the ground treatment system, the oil-containing sludge generated in the process of oil extraction wastewater treatment, coupled with flocs formed by the water purification agent added in the wastewater purification treatment. Oily sludge is composed of corrosion products and scale of equipment and pipeline, bacteria (carcasses), etc. This kind of sludge generally has the characteristics of oil-containing sludge. This kind of oil-containing sludge is generally characterized by high oil content, high viscosity, fine particles, difficult to dewater, etc. It not only affects the quality of crude oil transmission, but also leads to the water quality of water injection and discharge of wastewater is difficult to meet the standard [4].
1.2 Oilfield gathering and transportation process produces oil-containing sludge
Shengli oilfield oil-containing sludge mainly comes from the transfer station, joint station tanks, settling tanks, sewage tanks, oil separator bottom mud, refinery oil-containing water treatment facilities, light hydrocarbon processing plant, natural gas purification device to remove out of the oil sands, oil sludge, drilling, operations, pipe perforation of the landing of crude oil and oil-containing sludge[5]. Oil storage tanks in the storage of oil, oil in a small amount of mechanical impurities, sand, dirt, heavy metal salts and paraffin and asphaltenes and other heavy oil components deposited in the bottom of the tank, the formation of the bottom of the tank sludge.
The sludge generated in the crude oil field is mainly the sewage from primary settling tanks, secondary settling tanks, and well-washing water recovery tanks. Oil-containing sludge itself has a complex composition, containing a large amount of aged crude oil, wax, asphaltene, colloids and solid suspended matter, bacteria, salts, acid gases, corrosion products, etc. A large number of coagulants, corrosion inhibitors, scale inhibitors, biocides, and other water
treatment chemicals are also added in the sewage treatment process [6].
In 3-6 years of regular cleaning of oil tanks, the amount of oil-containing sludge at the bottom of the tank accounts for about 1% of the tank capacity. The tank bottom oily sludge is characterized by extremely high hydrocarbon (oil) content. According to the investigation and testing found that about 25% of the tank bottom sludge is water, 5% of inorganic sediments such as silt, and about 70% of hydrocarbons, of which asphaltenes accounted for 7.8%, paraffins accounted for 6%, and the sludge ash content is 4.8% [3].
1.3 Oil-containing sludge from refinery wastewater treatment plant
The oil-containing sludge from refinery wastewater treatment plant mainly comes from the bottom mud of the grease trap, flotation cell flotsam, and the bottom mud of the crude oil tanks, etc. Commonly known as the "three sludge", these oil-containing sludge has different compositions, and the oil content rate is usually between 10% and 50%, and the water content is between 40% and 90%, which is the same as that of the oil-containing sludge. Between 40% and 90%, accompanied by a certain amount of solid.
2. Hazards of oil-containing sludge
Oil-containing sludge is huge in volume, if not treated and discharged directly, not only occupies a large amount of arable land, but also to the surrounding soil, water, air will be polluted, accompanied by foul-smelling gases, but also to the surrounding soil, water, air will be polluted, accompanied by foul-smelling gases, the sludge contains a large number of pathogenic bacteria, parasites (eggs), copper, Zinc, chromium, mercury and other heavy metals, salts
and PCBs, dioxins, radionuclides and other difficult to degrade toxic and harmful substances.
3. Commonly used sludge treatment methods
The ultimate goal of oil-containing sludge treatment is to reduce, resourceful and harmless as the principle. Commonly used treatment methods for oily sludge: solvent extraction method, incineration method, biological method, coking method, oily sludge profiling, comprehensive utilization of oily sludge and so on.
3.1 Dewatering of oil-containing sludge
The water in oil-containing sludge has the following four forms: free water (free water), interstitial water (interstitial water), attached water (vicinal water) [or surface water surface water], and chemically bonded water (chemical- bound water). interstitial water, vicinal water [or surface water], chemical-bound water.
Commonly used oil-containing sludge dewatering methods and effects are shown in Table 1 [7].
Table 1 Dewatering methods and effects of commonly used oil-containing sludge
Dewatering methods
Dewatering device
Water content after dewatering %
State after dewatering
Thickening and dewatering
Gravity, flotation, centrifugation
Centrifugal concentration
95-97
Approximate paste
Natural drying method
Natural drying field, sun
Sand field
70-80
Mud cake
Mechanical dewatering
Vacuum filtration
Vacuum drum, vacuum
Turntable
The p>60-80
Cake-like
Pressure filtration
Plate and frame filter press
45-80
Cake-like
Rolling filtration
Rolling belt filter press
78 -86
Mud cake
Centrifugal filtration
Centrifuge
80~85
Mud cake
Drying
10-40
Powder, granular
Incineration
0-10
Ash
3.2 Conditioning of oil-containing sludge
The black viscous oil-containing sludge concentrate after dewatering by gravity settling is generally composed of oil-in-water (O/W), water-in-oil (W/O) and suspended solids***, which belongs to multi-phase colloidal system and is more complex in composition. As the oil-containing sludge particles adsorbed on the surface of the same kind of charge, mutual exclusion, coupled with full emulsification, extremely difficult to destabilize, making oil, water, mud and sludge separation is more difficult. It is necessary to add a conditioning agent, so that the crude oil and solid particles separation, oil droplet polymerization, the original addition of chemicals with the settlement of solid impurities, to achieve the complete separation of oil,
water, sludge three-phase.
Li Fanxiu research on oil-containing sludge conditioning-mechanical dewatering treatment process, the key to this technology lies in the selection of the type and dosage of flocculants [8], emulsion breakers, and regulators [9] used in conditioning, the selection of the type of dewatering machinery, as well as the determination of the operating parameters of dewatering machinery.
It has been reported that Jan Bock, Sanjay R Srivatsa, Aldo Corti, and others [10] have invented patented technologies related to the recovery of oil through the conditioning-mechanical dewatering process of oil-containing sludge, respectively: through the addition of surfactant, diluent (koi alkanes, etc.), electrolyte (NaCl solution), or emulsion-breaker ( Anion or non-ionic), wetting agent (can increase the solid particle surface and water affinity) and pH adjuster, etc., and supplemented by heating viscosity reduction (the best for the 50 ℃ on) and other means of conditioning to achieve water - oil - solid three-phase separation. According to the relevant information, the domestic Liaoning Province, Panjin City, Liaohe Oilfield Jinzhou oil extraction plant to obtain an oil-containing sludge conditioning and recovery of crude oil patented technology [11].
3.3 Extraction method extraction method is the use of "similar solubility" principle, select a suitable organic solvent as an extractant, the oil sludge in the
crude oil recovery and utilization of the method. Huang Shousheng et al. utilized multi-stage separation and extraction plus one-stage hot washing method to treat oily sludge [12], and the treated sludge could reach the agricultural sludge discharge standard, and the chemicals could be recycled.
Supercritical fluid extraction technology, is an emerging oil-containing sludge extraction technology, the technology is in the development stage. It will be room temperature, atmospheric pressure for the gas state of the material after high pressure to reach the liquid state, and use it as the extractant, due to its huge solvency as well as the extractant is easy to recover recycling. Commonly used supercritical flow extractants are methane, ethylene, ethane, propane, carbon dioxide, etc. These substances have high critical temperatures and low critical pressures, and the raw materials are cheap and easy to obtain, which is a good supercritical extractant, and the density is small and easy to separate [13].
Currently, the extraction method for treating oily sludge is still in the experimental development stage. The advantage of the extraction method is that the treatment of oil-containing sludge is more thorough, and most of the petroleum substances can be extracted and recovered. However, due to the expensive price of the extractant, and there is a certain loss in the treatment process, so the extraction method is costly, and has not been practically applied to the refinery oil sludge treatment. The key to the development of this technology is to develop a high performance-price ratio extractant.
3.4 Incineration method
After pre-dewatering and concentration of pre-treated oily sludge, sent to the incinerator for incineration, the temperature of 800 ~ 850 ℃, after 30min incineration can be completed, incineration of the ash needs to be further processed.
China's vast majority of oil refineries have built sludge incineration device, the use of incineration to treat the most waste is the sewage treatment plant oil sludge, such as Changling Petrochemical Plant used downstream rotary incinerator, Yanshan Petrochemical Company refinery used fluidized bed incinerator, in the treatment of oily sludge have achieved good results. At present, the main types of domestic incinerators: square box type, fixed-bed type, fluidized bed type, rake type furnace or rotary kiln and other types of furnaces.
The advantages of incineration treatment method is that the sludge is incinerated, a variety of harmful substances are almost completely removed, reducing the harm to the environment, waste reduction effect is good, the treatment is relatively safe, the disadvantage is that the incineration process produces secondary pollution, a waste of valuable information.
3.5 Biological method
The biological treatment technology of water-containing sludge is mainly i.e., ground cultivation method, composting treatment method and sludge bioreactor method. It mainly utilizes microorganisms to degrade the petroleum hydrocarbons in the oil-containing sludge into harmless soil components. According to the information report, the ground cultivation method can produce certain pollution to the land and groundwater, so it has been stopped using in some developed countries.
Composting is the process of mixing oily waste with appropriate materials and placing them in piles so that natural microorganisms degrade the petroleum hydrocarbons. There are four types of composting methods: dike composting, static composting, closed composting and container composting. Composting is an effective biological treatment method, the half-life of hydrocarbons in oily sludge is about 2 weeks. The treated oily waste can be landfilled or applied to agricultural land.
A bioreactor is a vessel in which oily sludge is diluted in a nutrient medium to make a slurry. Because the bioreactor can artificially control the operating conditions such as oxygenation, temperature, and nutrients, the biodegradation of hydrocarbons is faster than in other biological treatment processes. The biodegradation of hydrocarbons can be accelerated by the addition of domesticated high-efficiency hydrocarbon oxidizing bacteria. According to the literature, the biodegradation half-life is 5 days when the solids loading is 5%. The bioreactor method is applicable to oil-containing sludge, as well as oil-contaminated soil and oil-containing drill cuttings. After the treatment of oil-containing waste, the liquid part can be discharged into disposal wells (pits and pools) or used for other purposes (e.g., reuse); the solid part can be used to apply to farmland, and the bioreactor method is also used for the pre-treatment of petroleum industry wastes to reduce the content of hydrocarbons, and then carry out other treatments.
3.6 Coking method
Oil-containing sludge contains a certain amount of mineral oil, the composition of which is mainly alkanes, cycloalkanes, aromatic hydrocarbons, olefins, gums and asphaltenes. The coking method of treating oil-containing sludge utilizes thermal cracking and thermal condensation reactions of hydrocarbons under high-temperature conditions to produce liquid-phase oils, non-condensable gases and coke products.
Zhao Dongfeng et al[14] utilized oil-containing sludge with an average oil content of 69.46% and a water content of 4.71% for the coking reaction, with a reaction time of 60 min, a reaction temperature of 490°C, and a reaction pressure of atmospheric pressure. Under this condition, the yield of liquid phase products was 88.23%, and the products were mainly gasoline, diesel and wax oil.
3.7 Conditioning technology
Using the oil-containing sludge in the extracted water has a good compatibility with the formation, taking the oil-containing sludge as the basic raw material, adopting the chemical treatment method, and adding the appropriate amount of additives, to suspend the solid particles in it, prolong the suspending time, increase the injecting depth, effectively improve the blocking strength, and make the oil component dispersed uniformly to form the homogeneous and stable emulsion. The oil-containing sludge is formulated into emulsified suspension dissection agent, applied to oilfield water injection well dissection, in the stratum to a certain depth, by the stratum water release and the adsorption of stratum rock, the emulsified suspension system decomposition, which mud adsorption of glue leachate and wax, and through the viscous aggregation of the formation of a larger particle size of the "agglomerate structure", settled in the large pore channel, making the large pore channel to be more stable. In the pore channel, the diameter of the large pore channel becomes smaller, blocking the high permeability layer zone, increasing the seepage resistance of the injected water, forcing the injected water to change the seepage direction, and improving the volume of the injected water wave and volume. By optimizing the construction process, the oil-containing sludge can be made to block only the high permeability zone without polluting the middle and low permeability layers. The technical indexes of the treated oily sludge as a conditioning agent should be as follows: low viscosity of oily sludge (≤0.3Pa-s), good pumpability; good suspension performance of oily sludge after adding suspending agent, and the settling time is more than 3h.
Henan Oilfield carried out oily sludge conditioning test in the layer system of Ⅱ4-6 in Block 437 of Shuanghe Oilfield, and achieved good results [15, 16]. Shengli oilfield pile west oilfield oil-containing sludge dissection test also achieved better results [17].
3.8 Other methods
In addition to the above oil-containing sludge treatment methods, there are land plowing method, oil-containing sludge curing method, chemical emulsification method, solid-liquid separation method [18] landfill technology, the comprehensive utilization of oil-containing sludge is generally the use of oil-containing sludge paving, making bricks, making honeycomb coal and so on. Foreign countries also have oil-containing sludge low-temperature pyrolysis technology, as well as the use of solvents and low-frequency acoustic separation of oil sludge [19] and so on.
4. Comparison of various treatment methods
①Simple treatment: oil-containing sludge direct landfill or cured landfill are characterized by simplicity and ease. Direct landfill is the main method of oil sludge disposal adopted by most domestic oilfields, but this method wastes valuable energy and may lead to environmental pollution. Among them, the method of landfill after curing can reduce environmental hazards, but most of them cannot meet the current environmental requirements.
②Physical and chemical treatment: all kinds of physical and chemical treatment methods to recover crude oil for the purpose, so mainly for the oil sludge with high oil content, the treatment process usually need to join the chemicals, the need for specialized treatment facilities, treatment process is complex, the cost is high. The high price of crude oil and the high sewage charges levied on oily sludge discharges still make this method an attractive prospect. The disadvantages of this method are that oil recovery is not complete, and there is the problem of secondary pollutants in wastewater and sludge, which still need to be considered for further treatment or comprehensive utilization. In addition, oil sludge from oilfields is generated widely, mostly intermittently, and the properties of oil sludge from different sources are different, which limits the comprehensive use and promotion of this treatment method in oilfields.
③Biological treatment technology: Because biological treatment method has the advantages of energy saving, low investment, low operation cost, etc., it has been generally concerned and emphasized by the environmental protection industry at home and abroad. The solid-liquid separation of oil-containing sludge and the removal of oil are realized through biological treatment technology, and the remaining residue reaches the sludge discharge standard. The advantages are that there is no need to add chemicals, consume less energy, and are green, but the land tillage method and composting method require a large area of land, and the bioreactor method still has waste residue emission, and the treatment time is long and the operation is complicated.
4 incineration method: incineration must be carried out in a specially established incinerator, can be relatively thorough elimination of harmful organic matter in the oil-containing sludge, such as not considering the comprehensive utilization of combustion heat energy, will result in energy waste.
⑤Fuel: fuel is the use of oil-containing sludge contained in a comprehensive utilization of energy, but can not be extracted from the crude oil, in the use of this method should be from the economic and environmental aspects of the two aspects of comprehensive consideration. The advantages and disadvantages of several major treatment methods for oily sludge are listed in Table 2.
Table 2 Comparison of the advantages and disadvantages of the main treatment methods of oily sludge
No.
Treatment method
Scope of application
Advantages
Disadvantages
1
Simple disposal
All kinds of oil-containing sludge
Simple and easy to carry out
Pollution of the environment, can't Recovery of crude oil
2
Physical and chemical treatment
Oily
sludge with oil content above 5-10%
Recovery of crude oil, comprehensive
Combined use
Need to treat the device, the need to join the chemicals, there are still sewage, sludge discharge, treatment costs are higher
need incineration device, usually need to add combustion fuel, there are exhaust emissions, can not recover
crude oil
5
Fuel, brick
various types of oily sludge
Comprehensive utilization, more
easy to implement
Can not recover crude oil, there are exhaust emissions
5. p>
5. Conclusion
Oily sludge mainly comes from oilfield mining, oil and gas gathering and transportation and sewage treatment sites, the treatment technology of oily sludge is also diverse, each method has its own advantages and disadvantages and scope of application, but due to the complexity of the composition of oily sludge, there is no one treatment method that can deal with all types of oil-containing sludge, the grading of oil-containing sludge and the method of treatment Grading of oily sludge and treatment methods is the main direction of future research.