Oily sludge is one of the main pollution sources in the petroleum industry. It is a kind of oily solid waste with resource recovery value generated in the process of petroleum exploitation, workover, gathering and transportation, refining, transportation, storage and mechanical cleaning. According to incomplete statistics, China's petrochemical industry produces an average of 800,000 tons of oily sludge per year, and the whole petrochemical industry produces an average of 60 million tons of oily sludge per year. At present, the main treatment methods are: biological treatment method, chemical method, physical chemical method, mechanical treatment method.

 

Technology of resource utilization and harmless disposal of oily sludge

 

Solid liquid mechanical separation method

 

Solid-liquid mechanical separation method is an earlier, more mature and more common oily sludge treatment method. From vacuum filtration method, pressure filtration method, rolling filtration method, and then centrifugal dewatering method, oily sludge can be separated from solid and liquid two-phase to three-phase separation of oil, water and slag. The separated water phase is treated in sewage treatment plant, and the oil phase is returned to refinery for refining. The filter residue is usually treated by landfill, incineration, biological treatment and partial comprehensive utilization.

 

Chemical hot wash method

 

Chemical hot washing, also known as thermal desorption, is a method of repeatedly washing oily sludge with thermochemical solution to remove oil substances from the solid surface. The commonly used hot wash solution is mainly mixed alkali, which can be composed of inorganic alkali and inorganic salt, and more economical washing products can also be selected. After washing, the mixture can be separated by air flotation.

 

Solvent extraction method

 

Solvent extraction method is a method of extracting oil from oily sludge by using the principle of similar phase dissolution of substance polarity, selecting organic solvent as extractant, and then distilling the extraction liquid to separate solvent from oil substance. The oil substances can be recycled, and the residue can be directly landfill or further used after meeting the environmental protection requirements.

 

 Landfill method

 

Landfill method is a relatively simple treatment method for oily sludge, but considering the requirements of environmental protection, oily sludge is generally not directly landfill, but landfill after solidification or landfill method as a follow-up treatment method for other disposal technologies.

 

 Pyrolysis method

 

Pyrolysis method refers to heating oily sludge under the condition of anaerobic micro-positive pressure, so that the oil and organic matter in it can be separated into pyrolysis slag, pyrolysis liquid and pyrolysis gas, so as to achieve the purpose of resource utilization. The thermal conversion process of oily sludge is divided into two stages: the first stage is evaporation stage. When the temperature is lower than 350℃, light hydrocarbons with low boiling point volatilize from the oily sludge; The second stage is the parallel sequence reaction stage. When the temperature exceeds 350℃, the heavy oil begins to pyrolysis, and the hydrocarbons will generate free radicals due to thermal activation at about 400℃. A series of free radical reactions will occur, on the one hand, in the direction of pyrolysis of small molecule hydrocarbons, on the other hand, in the direction of condensation of coking carbon. There are four kinds of products, oil, water, non-condensable gas and coke, but the reaction conditions are different, and the proportion of products is slightly different.

 

 Biological treatment technology

 

Biological treatment technology is the use of microorganisms to oil and organic matter in the oily sludge has a strong ability to degrade the characteristics, its harmful components into harmless components of CO2, H2O, etc., while increasing the humus content in the soil method. Biological treatment technology includes soil cultivation method, composting method, biological mud method and biological strengthening method.

 

Soil cultivation method is a method that relies on natural microorganisms in the soil to degrade petroleum hydrocarbons into harmless components in the soil and play a role in fertilizing the field.

 

Composting is a method of mixing oily sludge with appropriate materials (such as sawdust, straw, etc.) and placing them in piles to degrade petroleum hydrocarbons using natural microorganisms.

 

The biological mud method is to mix the oily sludge with water, disperse the pollutants in the water, get most of the dissolved pollutants, use microorganisms to convert these pollutants into low-toxicity intermediate products (such as organic acids and aldehydes, etc.), and convert these products into CO2 and H2O.

 

The bioenhancement method is a technique of adding microorganisms which can degrade petroleum hydrocarbon efficiently into oily sludge, and then degrading aliphatic hydrocarbon and aromatic profile by microbial bacteria

 

The reinjection profile control technology is to make use of the compatibility of oily sludge and produced formation, and add dispersant, suspension agent and other chemical agents to form suspended emulsion after initial treatment of oily sludge, which is used for profile control of water injection Wells to achieve the purpose of increasing oil and precipitation.

 

 Incineration method

 

Incineration method refers to the method of heating the simply treated oily sludge alone or together with other fuels to a specific temperature under aerobic conditions, so that the organic matter in the oily sludge is converted to CO2 and H2O.

 

Coking method

 

The essence of coking treatment of oily sludge is the deep heat treatment process of heavy oil, that is, the high temperature thermal cracking and thermal condensation of heavy oil. The direction of cleavage produces smaller molecules (such as gaseous hydrocarbons), and the direction of condensation produces larger molecules (such as colloid, asphaltene, coke, etc.). In the process of thermal conversion of oily sludge, heavy oil is generally heated to about 370℃ and begins to crack, and the condensation reaction accelerates with the increase of cracking depth. The reaction gas enters the three-phase separator, the gas phase components are sent to the combustion system for recycling, the oily sewage enters the sewage treatment system for oil recovery, and the generated coke can be used as fuel or as building materials for comprehensive utilization.

 

 Curing technology

 

Curing technology refers to the method of adding curing agent and coagulant in oily sludge to make it undergo some stable and irreversible physicochemical reaction to form curing substance to solidify the harmful substances in it for stacking, storage and subsequent treatment.

 

 Surfactant washing method

 

Surfactant washing method refers to the method of using the hydrophilicity and hydrophobicity of surfactants to increase the solubility and fluidity of petroleum hydrocarbons, break the emulsification degree of oily sludge, and thus realize the separation of oil and water.

 

 flotation process

 

Flotation method refers to mixing oily sludge and a certain amount of water to form a sludge suspension, and then spraying air into the suspension to produce fine bubbles, so that the bubbles are close to the oil droplets in the suspension. When the water film between the oil droplets and the bubbles becomes thinner and thinner and reaches a critical thickness, the water film breaks, causing the oil droplets to move to the bubble and accumulate on the surface of the oil-water mixture. The method of collecting and further purifying the accumulated oil.

 

 Wet oxidation technology

 

Wet oxidation technology refers to the use of oxidizing agents to oxidize petroleum hydrocarbon pollutants in oily sludge into CO2, H2O and other small molecular organic matter, so as to achieve the purpose of removing or reducing pollutants. This method can be used as a pre-treatment method for bioremediation.

 

New technology for disposal of oily sludge

 

With the deepening of research on oily sludge disposal technology, some new oily sludge disposal technologies have been produced, but these technologies have not been put into engineering application.

 

 Microwave processing technology

 

Microwave treatment technology is actually a pyrolysis method, which uses the microwave thermal effect to separate and solidify the oily sludge, and destroys the oily sludge emulsion liquid system by microwave, so as to realize the three-phase separation of oil, water and solid.

 

Ultrasonic processing technology

 

Ultrasonic treatment technology is the use of ultrasonic cavitation, so that the oil in the oily sludge from the surface of the solid particles separation or under the action of ultrasonic mechanical vibration to make the oil particles gathered treatment method.

 

Supercritical water oxidation technology

 

Supercritical water oxidation technology is a new pollution treatment technology for rapid degradation of organic matter. This technology uses supercritical water as the reaction medium and air, oxygen and hydrogen peroxide as the catalyst to trigger free radical reaction under high temperature and pressure conditions, so as to degrade organic matter and recover heat energy.

 

 Freezing/thawing technology

 

Freezing/thawing technology is a low temperature treatment technology, which uses water and oil with different condensation temperatures to break the thermodynamically stable structure between oil and water in oily sludge through chemical and physical effects at low temperatures, so that oil and water can be quickly separated.

 

 Electrodynamic technology

 

Electrodynamic technology is based on the combined action of electroosmosis, electromigration, electrophoresis, etc. Under the action of applied electric field, the water and hydrocarbons in the oily sludge are enriched at the cathode, and the solid phase components are gathered at the anode, so as to realize the three-phase separation of oil, water and slag.

 

 Phytoremediation technology

 

Phytoremediation technology is a treatment method that relies on the synergistic action of plants, solar energy and soil microorganisms to degrade and remove organic pollutants. The technology can directly or indirectly absorb, transform, separate and degrade organic pollutants through the growth process of plants after absorbing solar energy, and at the same time, microorganisms in the soil of the plant root can cooperate with the plant to degrade and remove pollutants. Phytoremediation technology has the characteristics of low treatment cost, simple and no secondary pollution, which is suitable for the treatment of oily sludge with low oil content, and can be applied as a post-treatment technology.

 

 Make rubber filler

 

The oily sludge was pretreated to make its main component CaCO3, so as to replace CaCO3 as rubber filler. Studies have shown that oily sludge used as rubber filler can not only solve the problem of environmental pollution, but also have good economic benefits.

 

In addition, oily sludge that has been properly pretreated or meets the requirements can also be used as roadbed material or as raw material for sintered bricks.