| Delayed coking is the first heavy oil recovery technology in China and is the most valued and the lowest cost conversion process. As the quality of world's crude oil tends to be worse, China's petrochemical industry has been rapidly developing this delayed coking process. However, the delayed coking wastewater pollution is increasingly growing as a serious problem. The stripping process is in widespread use to treat the delayed coking wastewater, but the pollutant concentration in the purified water from this stripping is so high that it causes a severe impact on the sewage treatment system when the purified water is discharged into the sewage treatment plant. The effluent water quality that is discharged from the sewage treatment plant cannot reach the required standard and poses a serious threat to the environment and human health. Because of the limitation in analytical technology, the lack of a thorough understanding regarding the composition of the delayed coking wastewater and the migration and transformation rules of specific compounds during the wastewater treatment lead to sub-standard pollution control techniques. Therefore, the pollution problem has become a serious obstacle in the development of industrial delayed coking. In this paper, the delayed coking wastewater treatment process at a petrochemical plant in Guangdong Province, which entails emulsion breaking, steam stripping, and recycling for electric desalter- was studied. First of all, historical monitoring data of the water quality index was used for analysis of the problems existing in the process. A set of methods for collection, pre-treatment and analysis according to aqueous samples in the delayed coking wastewater treatment process was established. Composition properties of the wastewater discharged from the delayed coking unit and the upstream and downstream units of steam stripping were analyzed by Thermal Gravimetric Analysis(TGA), UV-visible absorption spectrum and three-dimensional fluorescence spectrum. The compositions of the wastewater discharged from the delayed coking unit and the upstream and downstream units of steam stripping were comprehensively identified by GC/MS after the methods of concentration and separation of organics had been optimized. Next, the composition of the delayed coking wastewater and the behavior rules of specific compounds during the wastewater treatment were discussed. Finally, the influence of organic pollutant composition in wastewater discharged from the delayed coking plant regarding treatment efficiency has been studied.TGA results show that most of organic matter in sour water discharged from the second and third sets of the delayed coking plant have a boiling point between 25?~ 200? and are classified as volatile and semi-volatile organic compounds. Large amounts of monosubstituted benzene derivatives, aldehydes and ketones organics have been found in sour water discharged from the second and third sets of the delayed coking plant by UV-visible absorption spectrum. After the sour water is handled by the three- step treatment process, emulsion breaking, steam stripping, recycling for electric desalter- the monosubstituted benzene derivatives have been effectively eliminated while the aldehydes and ketones organics have some residual in the effluent. Three-dimensional fluorescence results show the sour water discharged from the second and third sets of the delayed coking plant contain large amounts of aromatic compounds. In the process of emulsion breaking, and steam stripping, the aromatic compounds cannot be effectively eliminated. During the recycling in the electric desalter process, however, naphthalene and its homologues are more easily removed while the tricyclic or more aromatics and their homologues are not satisfactorily removed.Organic compounds in the various wastewater discharged from the delayed coking unit and the upstream and downstream units of steam stripping are divided into two groups after the compositions of these wastewaters have been comprehensively identified by GC/MS. The two groups are acidic fraction and fraction of neutral and alkali. The acidic fraction consists of three kinds of organic matter which are carboxylic acids, phenols and alcohols. These three kinds of organic matter all have significant surficial activity. With a large number of organic acidic components which mainly come from upstream water discharged from the second and third sets of the delayed coking plant flowing into raw water of a fourth stripping unit, serious emulsification happens in the raw water of the fourth stripping unit. The fraction of neutral and alkali includes four kinds of organic matter, which are alkanes, alkenes, aromatic hydrocarbons, and non-hydrocarbon. Among them, most of non-hydrocarbon organic are polar molecules and receive minimal surface activity. With a large number of non-hydrocarbon organic matter which mainly comes from the sour water discharged from the third set of the delayed coking plant flowing into raw water of the fourth stripping unit, the emulsification becomes more severe in the raw water and it makes oil-water separation more difficult. After the sour water is handled by the three-step treatment process —emulsion breaking, steam stripping, recycling for electric desalter”- the main organic matter in the fraction of neutral and alkali has been effectively eliminated. But alcohols and carboxylic acids with relatively short carbon chain get poor removal. The mass fraction of carboxylic acids and alcohols is 89.73% and 9.78% in the final effluent of three-step treatment process— the electric desalting drainage. Plus, the sum of mass fraction of valerate, caproate, and enanthate is 54.88% in acidic components which have been detected. The monitoring index of the pH of raw water of the fourth stripping unit, intake-tower water of the fourth stripping unit and purified water of the fourth stripping unit is 11.0, 12.0 and 9.2. Meanwhile, desalting water is generally a weak alkaline. The wastewater of all process sections has alkaline and carboxylic acids present in the form of carboxylate which is very stable and cannot be effectively eliminated in the process of emulsion breaking and steam stripping. Finally, carboxylic acids with relatively short carbon chains do not have satisfactory removal in the recycling for the electric desalter process. |
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