Challenges and Traditional Treatment Technologies for Acidic Water in Petrochemical Refineries
Sources of Acidic Water in Petroleum Industry
Sulfur and nitrogen compounds are present in petroleum and its products. During processing operations such as atmospheric distillation, catalytic cracking, delayed coking, and catalytic hydrogenation, these compounds undergo reactions like high-temperature pyrolysis, catalytic cracking, and hydrocracking, which produce H2S and NH3-N that enter the product streams. These product streams undergo dehydration or water washing processes, resulting in sulfur- and ammonia-containing wastewater, commonly referred to as sulfur wastewater or acidic water.
Characteristics and Hazards of Acidic Water
· High Coke Powder Content: The powder has a small particle size and high oil content, with small oil droplet sizes and serious emulsification. Large amounts of emulsified oil and suspended solids (coke powder) affect the stripping efficiency and the quality of liquid ammonia.
· Risk of Stripper Tower Blockages: It can easily cause blockages in the stripping tower, leading to unscheduled shutdowns and significant production losses.
· Complex Composition and Large Variations in Content: Acidic water has fluctuating composition and significant variability in the concentration of pollutants.
· Odor Problems: The release of sulfur compounds, ammonia nitrogen, and other irritating odors severely affects the environment and the health of employees.
· Environmental Pressure: Direct discharge into wastewater treatment plants creates enormous environmental pressure for the company.
Traditional Acidic Water Treatment Processes and Challenges
1. Gravity Sedimentation for Acidic Water Treatment:
Currently, many refineries in developing countries, especially in China, use gravity sedimentation, with sedimentation times ranging from 50 to 70 hours. Due to the severe emulsification of oil in acidic water and the small particle size of coke powder, even with prolonged sedimentation, the separation effect remains poor. The large tanks are open or semi-closed, causing acidic gases to leak, severely affecting the surrounding environment and the health of employees. The process requires a large footprint, high infrastructure costs, and significant investment. It is fully manual, lacking automatic control, and the cleaning and maintenance rely entirely on manual labor, which is time-consuming and labor-intensive.
2. Cyclone Separation for Oil and Coke Powder Removal:
Some refineries have attempted using in-tank cyclones and two-stage cyclone processes for oil and coke powder removal, but they still cannot fundamentally solve the problem. Although improvements have been made over gravity sedimentation, due to the small droplet size of emulsified oil and similar density to water, the cyclone effect remains limited. The oil and coke powder removal efficiency is still not significant. The impact resistance is poor, and the oil removal effect is unstable, unable to meet the demands of fluctuating oil and coke powder levels in acidic water. Cyclones are prone to clogging, and in severe cases, they fail to operate normally. These processes require high pressure, leading to high power consumption and high operational costs.
3. Dissolved Air Flotation + Inorganic Ceramic Membrane Process for Acidic Water Treatment:
This process has high filtration precision and produces better-quality effluent in the short term. However, it has poor stability under shock conditions, as the membranes easily clog, leading to a reduction in treatment capacity, lower water production rates, and the generation of large amounts of concentrated water and waste liquid from acid and alkali washing, making subsequent treatment difficult. The process operates under high pressure with high energy consumption and high operational costs. It also involves high investment and maintenance costs.
