Traditional Treatment Processes of Desalting Wastewater and Its Challenges
During crude oil refining, the desalting step plays a crucial role. However, with the successive development of oilfields, refinery feedstocks have become more diversified and compositionally complex. At the same time, as extraction progresses, crude oil quality continues to decline, with a growing proportion of heavier and inferior crudes. To address this challenge, many refineries recycle recovered heavy oily sludge back into the crude stream after preliminary treatment for blending. While this practice improves crude utilization, it also intensifies crude oil emulsification, placing unprecedented pressure on the desalting process.
During operation of the desalting unit, factors such as long operating periods, overload conditions, and improper maintenance or operation can result in poor demulsification performance and unstable oil-water interface levels. This directly leads to excessively high oil content in the effluent, causing severe impacts on downstream wastewater treatment plants and resulting in significant losses of valuable crude oil.
The existing oily wastewater from electrostatic desalting is characterized by a high content of heavy oil, whose density is close to that of the wastewater, making separation extremely difficult. Meanwhile, the oil content in the wastewater is high - up to more than 20% in some cases - with small oil droplet sizes and severe emulsification, further increasing the difficulty of separation. In addition, the wastewater contains a large amount of fine suspended solids, which not only intensify crude oil emulsification but also cause serious corrosion and blockage of equipment. The high salinity and mineralization of the wastewater accelerate equipment corrosion and scaling, and also generate hazardous gases such as sulfides and volatile phenols, posing serious threats to the health of plant personnel and the surrounding environment.
The existing treatment processes for oily wastewater from crude oil desalting mainly include:
1. Gravity separation: Separation is achieved based on the density difference and immiscibility between oil and water. However, this method is ineffective for desalting wastewater containing large amounts of fine suspended solids and severe emulsification.
2. Hydrocyclone separation: Oil–water separation is carried out using swirling centrifugal separation technology. However, when the oil content fluctuates significantly, it is difficult to maintain stable compliance of the effluent oil concentration.
3. Coalescence method: By increasing oil droplet size, oil can float and separate more rapidly. This method has strong shock resistance, but its performance is poor for severely emulsified desalting wastewater.
However, when confronted with the challenges of treating oily wastewater from crude oil desalting units, traditional treatment methods appear inadequate. These processes have inherent shortcomings in treatment performance, energy consumption, and chemical dosing. Gravity settling processes are simple to operate but deliver unstable treatment results; hydrocyclone/centrifugal separation offers high efficiency, yet suffers from high energy consumption and requires chemical addition.
Refinery desalting wastewater features high and highly fluctuating oil content, placing extremely high demands on the system’s shock-load resistance. Severe emulsification makes demulsification the key step in the treatment process. However, chemical demulsification lacks reliability and is not applicable to all conditions, making physical demulsification a core technical challenge. Since oil and water densities are very close, separation based solely on density difference is far from sufficient; more efficient oil–water separation technologies must be adopted. In addition, the wastewater is highly corrosive, prone to scaling, and contains large amounts of silt and sand, requiring equipment with excellent anti-clogging capability and effective solids/sand removal performance.
