The Operation Optimization Of Coking Wastewater Treatment Process

The coking wastewater contains high concentration of ammonia-nitrogen,phenolics, tar and other species, making it difficult to tackle. Regarding to theunstable quality of discharge, this paper tries to investigate the modeling andoptimization of the treatment process to identify its bottleneck. Considering thecomplexity of the treating process due to its varieties of components and the limitedenergy, this paper highlights the modeling and optimization of two key processes incoking wastewater treatment: phenol and ammonia removal.One of the key steps of coking wastewater treatment is phenolic and tar removalvia extraction. However, the high loss of extractant, i.e.MIBK, leads to high cost ofthe process. The adoption of novel solvent or solvent blend is considered as an effi-cient way to address this problem. Seven solvents (Benzene, Toluene, m-Xylene,Ethyl benzene,1,3,5-Trimethylbenze, Cyclohexane and Octanol), selected ascandidate diluters for MIBK according to operating requirements, are studied with aNLPmodel based on ideal counter-current extraction. The results, verified withexperiments, suggested toluene as the most promising candidate. Further investigationreveals that both Dbldand mMIBKincrease with xMIBK。The tradeoff between theextraction performance and MIBK loss recommended the blend (xMIBK=0.05) asextractant for coking wastewater treatment. At last, an industrial process consisting ofextraction, back stripping, distillation and mixer is presented. Corresponding NLPmodel is established for its operating optimization. representatives of typicalphenolics and tar in wastewater (2,4dimethyl phenol, m-xylene and quinolone) arealso considered besides phenol. Optima indicates that the blend exhibits economicadvantage against MIBK with a makeup cost of11.15￥/t, much less than185.15￥/t inMIBK case.Regarding that the effluent failed in fulfilling the authority administrativedischarge regulations, a MINLP model for ammonia water distillation column is setup based on rigorous MESH equation, which aims to minimize the operation cost ofthe column, with obtaining the corresponding operating parameters. In this model,utilities price, bio-treatment cost, heating type of the column and heat integration ofthe preheater are considered as key factors affecting the column operation cost, whilefeed inlet temperature, location, and the corresponding utilities flow rates are defined as the key operating parameters to be optimized. Results indicate that direct-heatingcolumn could save1.56￥/t, comparing to indirect-heating column. Furthermore, thesensitive analysis reveals that parameters such as inlet and outlet concentration ofammonia, total stage numbers, the utilities prices, as well as preheater efficiency havesignificant influence to its operation. For a direct-heating column, optimal operatingconditions are: outlet concentration at300mg/L, total stage number at25, preheaterefficiency at65%, and feed inlet temperature75.40℃. Based on this model, thispaper considered the interactions between distillation and bio-treatment process,which consists the overall ammonia-nitrogen removal process. A mathematical modelof ammonia removal process combining distillation with shortcut nitrification/ANAMMOX/denitrification (O1-A-O2), is proposed for its operation optimizationbased on the simplification of unit operation models according to the key parameters.The results indicate that the minimum operating cost is obtained at ammonia-nitrogenoutlet concentration128mg/L, reaction recycle time of O110h and passby flowrate11.448m3/h when the ammonia-nitrogen inlet concentration of distillation column is4000mg/L. Further study reveals that the cost of O1operation is the major part of thebio-treatment process and The passby of O1reactor could reduce its duty by30.174%thus the total cost decrease by0.606￥/t.