Technical Basis And Process Development For Extractive Separation Of Emulsified Tar And Polyphenol From Semi-coking Wastewater

In recent years,driven by policy orientation and marketing requirement,the low-rank coal utilization industry gets respectable development in China.However,it suffers from many restrictions and challenges because of immature technology.The problem of wastewater is particularly prominent due to the increasing serious water shortage and water pollution problems.Semi-coking wastewater from low-rank coal utilization industry is quite tough to treat,which is more complicated than other coal chemical wastewaters.The previous researches have shown that emulsified tar and polyphenol are the key bottlenecks of treating the semi-coking wastewater.The tar and phenol content are limited to below 200 mg/L and 300 mg/L after physical pretreatment process,which is required by the goal of“Near Zero Emission”of semi-coking wastewater.However,the existing technologies have insufficient capacity to remove these emulsified tar and polyphenol.It results in the tar and phenol content as high as500 mg/L and 700 mg/L in the effluent.Therefore,this study focused on the removals of emulsified tar and polyphenol,aiming to achieve efficient and economical treatment of semi-coking wastewater through unit technology innovation and process integration.It is difficult for existing oil removal technology to effectively separate the emulsified tar from semi-coking wastewater.The extraction method has shown the application potential in the separation of emulsified oil due to its advantages of high efficiency,economy and environmental friendliness.However,the mechanism and basic law of this technology are unclear.This study analyzed and proved that the emulsified oil extraction process had different mass transfer behavior from conventional dissolved molecular extraction process.In the emulsified oil extraction process,the used extractant transferred to emulsified oil microdroplets,which changed the properties of the emulsified oil microdroplets.It promoted the coalescence and demulsification of emulsified oil droplets,so as to realize the separation of emulsified tar from wastewater.The mechanism proposed above could well explain the effects of extractant type,extractant/wastewater ratio,stirring time and other factors on extraction efficiency.The results of multistage extraction experiments showed that the emulsified tar content in wastewater could be treated below 200 mg/L by two-stage extraction.N-hexane and mixer-settler were preferred as extractant and extractor.Then a process model was established for predicting emulsified oil extraction performance.It is difficult for existing phenol removal technology to realize the efficient and economical treatment of polyphenol in semi-coking wastewater.Static mixer has attracted much attention due to its excellent mass transfer performance and economy.However,few mathematical models have reflected the real mass transfer process in static mixer.The traditional mass transfer models for extraction process ignore the dynamic change of dispersed phase droplet size.Thus,their prediction performance needs to further improve.This study quantified the dynamic size distribution of dispersed phase droplets due to droplet coalescence/breakage and interphase mass transfer.Based on this,the mass transfer model of Kenics static mixer was established.The influence laws of fluid velocity on hydrodynamic and mass transfer characteristics accorded with the basic principles hydrodynamic and mass transfer.It showed that the built model and its simulation results were reasonable.The relative error between the simulated and experimental values of the average dispersed phase droplet diameter,pressure drop and solute concentration profile were less than 5%,5%and 15%(the mean relative error less than 7%),respectively.Compared with the values in literatures,it was found that this model had the improved prediction performance for mass transfer behavior in extraction process.Based on the above theoretical analysis and process modelling,a new tar/phenol treatment process of semi-coking wastewater was developed,which is based on 15 m~3/h semi-coking wastewater.The process included these subprocess:floating oil removal through gravity sedimentation,dispersing oil removal through extraction in static mixer,emulsified oil removal through extraction in mixer-settler and phenol removal through extraction in multi-stage static mixer.This process could reduce the oil content to less than 200 mg/L and the phenol content to less than 300 mg/L.Compared to existing process,the total investment cost of the new process was reduced by 52%and the total operating cost was reduced by 74%.Thus,the new process had advantages in terms of technical and economic performance.