Collaborative Treatment Solution for High-COD Sour Water in Refining and Petrochemical Plants
As the proportion of inferior heavy crude blended in keeps rising, refining and petrochemical enterprises face increasingly severe highCOD (Chemical Oxygen Demand) challenges in sour water (sulfur-bearing wastewater) treatment. Because of the synergistic effect of oil and sulfide in the wastewater, conventional processes often struggle to cope, causing frequent severe shocks to downstream water treatment systems and strippers. Breaking this bottleneck has become key for refiners to achieve environmental compliance and cost reduction with efficiency gains.
I. Tracing the Source: The Core Causes of HighCOD in Sour Water
In sour water,the abnormally high COD is essentially not caused by a single component but by the deep coupling of sulfide and oil:
The Dissolution Behavior and Reducing Contribution of Sulfide
Sulfide (H2S, HS-, S2-) has unique hydrolysis behavior in aqueous solution. When pH > 7, its solubility in water decreases somewhat. However, because sulfide molecules interact more strongly with the oil phase, they readily dissolve in the non-polar oil phase. As a strong reductant, sulfide's oxidation significantly raises the COD measurement. Theoretically, each 1 mg of sulfide contributes about 1.6 mg of COD. In a real case, at a sulfide concentration of 3520 mg/L, its own oxidation alone produces about 5632 mg/L of COD.
The Emulsification Synergy of Oil and Its Core Impact
Crude inferiorization leads to abundant macromolecular natural surfactants (e.g., asphaltenes, resins) and processing aids, which readily build a highly stable emulsified oil system in sour water. Most sulfide dissolves or is entrained in this oil phase, so oil, sulfide, andCOD problems intertwine and stack. This emulsified oil and dissolved macromolecular oil (C5+) are extremely hard to effectively strip in conventional strippers, which is the main reason purified water COD stays high long-term.
II. Harm: The Negative Impact of Oil-Bearing Sour Water on Industrial Units
Oil in sour water not only directly causes oil loss at the refinery but also brings chain-like damage to mid- and downstream unit safety and product quality:
Disturbing Stripper Vapor-Liquid Balance: The presence of oil easily causes foaming in the tower, severely damaging the two-phase mass-transfer balance, sharply increasing steam energy use and greatly reducing unit capacity.
Causing Key Equipment Blockage: Oil accumulates and cokes on trays and reboiler surfaces, blocking flow paths, greatly weakening heat-transfer and separation efficiency, and severely shortening equipment life.
Degrading By-Product Quality: Oil-bearing operation easily obstructs by-product flow, e.g., spawning 'black sulfur' or causing oil in liquid ammonia products, severely devaluing market worth.
III. Path: A Graded Treatment Solution Centered on 'High-Efficiency Deep De-Oiling'
For the oil-sulfur synergy, engineering practice and experiments show that prioritizing front-end deep de-oiling achieves the dual win of desulfurization and COD reduction simultaneously.
[Applied Case Study]
When treating a refinery's wastewater with oil content> 3000 mg/L (overhead condensate), the combined process 'fine filter + high-efficiency coalescing de-oiler' was introduced, achieving remarkable results:
Effluent oil content: rapidly reduced to< 200 mg/L (oil removal efficiency > 97.9%)
Synergistic removal performance: sulfide removal reached72.1%-80.7%, and COD removal as high as 89.2%-95.88%
Four core advantages of entering the stripping section after deep de-oiling:
1、Significantly reduce the purified water'stotal COD
2. Greatly improve the purity and quality of by-product sulfur and liquid ammonia
3. Optimize mass-transfer efficiency and significantly lower steam consumption in the stripping section
4. Reduce unit load and effectively extend the unit's maintenance cycle and service life
IV. Customized Technical Recommendations
Front End: Deep De-Oiling and Desulfurization (Source Control)
For the hard-to-strip emulsified oil and oil-phase sulfide in sour water, it is recommended to prioritize deploying the 'fine filter + high-efficiency coalescing de-oiler' process at the pretreatment stage, achieving high-ratio synergistic removal of oil, sulfur, and COD at the source and easing the load on downstream units.
Back End: Degrade Hard-to-RemoveCOD (End-of-Pipe Control)
For the residual dissolved macromolecular oil (C5+) and refractory organics after stripping, advanced oxidation deep-treatment technologies such as ozone catalytic oxidation should be added downstream of stripping to thoroughly break chains and degrade, ensuring final effluent meets compliance standards.
Conclusion
Against the backdrop of normalized inferior heavy crude processing, solving the highCOD problem hinges on breaking the 'oil-sulfur coupling' effect. A pretreatment solution centered on high-efficiency physical de-oiling (fine filter + coalescing de-oiler), combined with end-of-pipe advanced oxidation in a graded strategy, not only fundamentally ensures stable, efficient stripper operation but also helps refiners reap significant economic returns under strict environmental red lines, moving comprehensively toward low-carbon sustainable development.
