The exploration, extraction, development, and production of petroleum generate massive amounts of waste materials in different forms. They include various gases and low boiling constituents, high boiling constituents, waste water, spent caustic, filter clay, and solid waste. A petroleum refinery with a production capability of 105,000 drums per day make approximately 50 tons of oily sludge per year.1
The toxic and harmful substances in the waste pose a substantial threat to human health and the surrounding environment; therefore, it must be treated to reduce its toxicity.
An assortment of methods for processing and disposing of petroleum sludge is used globally, including: thermal, mechanical, biological, and chemical.
Equally one of the 3 Rs of sustainability, recycling has proven to be one major alternative to manage petroleum sludge. Recycling is the reprocessing and reformulation of oily sludge with high concentration of oil (> 50%) and a relatively low concentration of solids (< 30%) by the petroleum industry for energy recovery.
There are various crude oil recovery technologies which have been developed for the treatment of oily sludge.
Recovery technology of crude oil from oily sludge
- Solvent extraction
Solvent extraction is a simple and efficient technique that completely mixes oily sludge with the extraction solvent in a certain proportion, removes solid particles, water and other impurities in the mixture, distils the mixture, and separates and recovers the crude oil in the sludge from the extractant effectively in a short time.
This method incurs a high cost and may cause secondary pollution caused by organic solvents to the environment.
- Mechanical centrifugation
Mechanical centrifugation mainly uses centrifugal force generated by high-speed rotation of centrifugal equipment to separate the components with different densities such as crude oil, water and solid impurities in the oily sludge.
This method is convenient, fast, efficient, and produces high yield however equipment maintenance cost is high, along with energy consumption. Pre-conditioning treatment is required.
- Chemical surfactant – The chemical surfactant method uses various chemical surfactants to treat the oily sludge for the recovery of crude oil.
- Biosurfactant – Because chemical surfactants are mostly mixed with organic solvents, they may cause secondary pollution to the environment and are difficult to biodegrade. Therefore, in recent years, researchers have turned to the study of environmentally friendly surfactants with good surface activity, low biological toxicity, good demulsification performance, and strong selectivity; among them, rhamnose tallow which is the most widely used.
Pros: Simple, efficient, large handling capacity
Cons: Cost is high, chemical surfactants are toxic and cause secondary contamination, and the treatment of heavy metals is limited
The process principle of flotation is similar to that of the air flotation pool in sewage treatment.
In the air flotation device, the oily sludge, water, and surfactant are first mixed in proportion, and a liquid slurry is created by the demulsification of the surfactant. Then, air is injected to generate bubbles in the slurry. Because the density of the oil phase is less than that of the water phase, the bubbles attached to the oil-phase droplets will quickly float to the surface of the mixed slurry.
After a period of time, the oil droplets floating on the surface of the slurry can be scraped off, collected, and further purified, ultimately achieving the purpose of crude oil recovery
Pros: Convenient and low energy consumption
Cons: High water consumption, low efficiency, not suitable for high viscosity sludge
In general, the demulsification of oil/water mixtures and the recovery of oil products are affected by a number of factors in the freezing/thawing process. For example, the temperature, time and rate of freezing and thawing, the content of water phase, oil phase, oxygen, impurities and so on. So, this method has a certain application prospect in the case of high cold regions or natural freezing in winter.
Pros: Convenient, long-lasting, suitable for high-cold area
Cons: High energy consumption, high cost, low efficiency
In the pyrolysis method, oily sludge is separated via the pyrolysis and gasification of the organic components in the sludge at high temperatures with indirect heat transfer under anaerobic conditions. The pyrolysis gas is condensed into oil and recovered, while the remainder of the solid impurities are carbonized
Pros: Efficient, quick, high quality oil recovery, large handling capacity
Cons: Equipment maintenance cost is high, high energy consumption, not suitable for high water sludge
- Electronal method
The electronal method uses the current generated by the electrodes to act on the oil colloidal mixture, which forms an electroosmotic pressure in the mixture that causes ions or charged particles to migrate to the corresponding electrode. Due to the effects of molecular and electrostatic forces, the water and oil droplets in the oil-water emulsion mixture separate and coalesce separately. Finally, the two phases of water and oil are gradually formed to achieve the goal of oil recovery
Pros: Low energy consumption, efficient and fast
Cons: Small processing capacity, equipment manufacturing complex
In the ultrasonic method, ultrasonic waves are used to change the physical properties and state of oily sludge. The cavitation effect and mechanical vibration caused by the acoustic radiation will strip the crude oil droplets originally attached to the surface of the solid particles, thereby reducing the amount of crude oil on the surface of the solid particles.
Pros: Efficient, fast, environmentally friendly
Cons: Small handling capacity, high equipment manufacturing and maintenance costs
- Supercritical fluid
The method refers to the use of supercritical fluid to treat oily sludge in order to recover crude oil and other useful resources. According to the different treatment methods, it can be divided into supercritical oxidation and supercritical extraction. The common supercritical fluids are water, ethane, ethylene and carbon dioxide.
Pros: Efficient and quick
Cons: High energy consumption, Water consumption
- Combined Processing
As various methods for the recovery of crude oil from oily sludge have become increasingly more mature, the advantages and disadvantages of various technologies have been gradually discovered and verified. Therefore, researchers have considered complementary advantages and creatively merged various methods to develop a series of combined processing methods.
Overall, as compared with single treatment technologies, combined treatment methods can integrate the advantages of various recovery technologies, thereby effectively improving the recovery efficiency of crude oil.
However, most of the existing research on combined treatments have focused on the separation and recovery of crude oil from oily sludge. Therefore, in the future, combined methods should focus on the improvement of process design, the optimization of equipment manufacturing, and the recovery and treatment of other harmful substances in the sludge, such as heavy metal elements and heteroatomic compounds.
Pros: Fast, efficient and energy saving
Cons: The equipment is complicated and the operation process is long