Why Delayed Coking Sour Water Has High COD: Oil and Sulfide Contributions
Sour water (sulfur-containing wastewater) from delayed coking units commonly suffers from excessively high COD, which poses a potential threat to downstream water treatment systems. The main causes are analyzed one by one below.
Emulsified sour water treatment equipment and results
1. The Key Influence of Oily Substances
Origin and mechanism: the heavy, low-quality crude oils blended by refineries contain large amounts of natural surfactants (asphaltenes, resins, petroleum acids). Together with oil-production additives (such as alkylbenzene sulfonic acid) and corrosion inhibitors (such as lye), they form an anionic surfactant system.
Emulsion stabilization and COD linkage: at high temperature, anionic surfactants significantly lower the oil–water interfacial tension, forming a strongly adsorbed layer and a thick Stern electric double layer on the surface of oil droplets, which produces a high zeta potential. As a result, the repulsive force between oil droplets increases and a firm interfacial film forms, hindering droplet coalescence and settling. Consequently, the dissolved-oil and emulsified-oil content of the sour water rises, becoming an important factor pushing up COD. The higher the proportion of heavy crude blended, the more pronounced this effect.
2. The Secondary Contribution of Additives
Origin: mainly from coke tower antifoaming agents (containing alcohols, fatty acids and polyether compounds). When the foam layer is too high, the antifoaming agent is injected in excess, or the gas velocity inside the tower is too large, the antifoaming agent may be carried over into the condensate at the top of the fractionator.
Impact: these compounds are difficult to biodegrade, and their degradation consumes oxygen, leading to higher COD. However, because the added quantities are relatively limited, their overall contribution to the COD of sour water is usually not high.
3. The Limited Role of Phenols
Phenolic organics (COD conversion ratio about 2.38) have large molecular weights and are difficult to remove effectively in the stripping process. However, the content of volatile phenols in coking sour water is generally low, so their contribution to COD is relatively small.
4. The Significant Contribution of Sulfides
Oxygen-consumption mechanism: sulfides (mainly H2S, HS− and S2−, in a ratio of about 1:6:3) are strongly reducing substances. Their oxidation consumes a large amount of oxygen, directly affecting the measured COD value.
Quantified contribution: theoretical calculations show that oxidizing each milligram of sulfide consumes about 1.6 mg of oxygen, i.e. the contribution ratio of sulfide to COD is about 1:1.6. In an actual case, the sulfide content of a stripper feed was 3520 mg/L, corresponding to a theoretical COD contribution of about 5632 mg/L; the measured total COD of the same sample was 9110 mg/L — clearly, sulfides are an important source of the COD.
Conclusion
The comprehensive analysis shows that oily substances (especially the dissolved oil and emulsified oil introduced by blending heavy crude) and sulfides are the two core factors behind the excessively high COD of delayed coking sour water. Large-molecular-weight oils are difficult to remove effectively in conventional stripping processes, which is the key reason the purified water still has high COD after stripping (e.g. ~2500 mg/L). Sulfides, owing to their inherent strong reducing properties, contribute significantly to the measured COD value. Therefore, targeted COD reduction measures should focus on efficient oil removal and sulfide conversion/removal, in order to safeguard the stable operation of downstream treatment systems (especially membrane systems) and avoid the risk of system collapse caused by the accumulation of refractory organics.
