Study On Desalination And Catalytic Degradation Of Coking Wastewater In Supercritical Water

A large amount of coking wastewater with high salts content is produced in thecoking production process. Ammonia is one of the most difficultly removed inorganicpollutants in coking wastewater, but the traditional biochemical methods show a lessideal degradation effect for ammonia removing from the wastewater. SupercriticalWater Oxidation (SCWO) technology is a novel and highly efficient method for thetreatment of hazardous compounds, and shows specially advantages to treat theindustrial wastewater. In this thesis, the desalination of coking wastewater and thedecomposition of ammonia by the supercritical water oxidation process would beinvestigated systematically.Firstly, some indexes such as ammonia nitrogen content, TOC, organiccompounds, and the content of major ions contained in a coke plant wastewater wereanalyzed. The solubility curves of Na salts in the coking wastewater under thenear-critical and supercritical water conditions were measured experimentally. Thenthe desalination pretreatment of second type system (Na2SO4-H2O system) wereexplored with different adsorbents. The results showed that the metal salts in thecoking wastewater, mainly in form of Na+ion, could precipitate out and result in thedevice blockage at the initial vaporization stage, because the solubility of these saltsin the water near critical point would be decreased sharply. When silica gel wasapplied as adsorbent, the desalting effect for sodium salt system was better.When the coking wastewater was treated with supercritical water oxidationtechnology, it was found that organic components in the coking wastewater could becompletely transformed under non-catalytic supercritical water oxidation conditions,but the conversion of ammonia nitrogen was very low. Catalytic SCWO technologywas used for the degradation of ammonia in this paper. The SCWO process concernedcatalyst preparation was investigated systematically. Through a series ofcharacterization techniques and the evaluation of catalytic activies, and the optimalsupported Mn-Cu based catalyst was selected. The preparation conditions of catalystwere chosen as: TiO2-Al2O3composed oxide as the carrier, the molar ratio of Mn/Cuwas1:1, the loading of active component is15wt%. The effects of reaction temperature, pressure, residence time and excess times ofoxidant on the conversion of ammonia were also investigated systematically. It wasshowed that the temperature and residence time parameters could affect the removalefficiency of ammonia more obviously. When the temperature, residence time and theexcess times of oxidant were increased, the removal of ammonia was improved tosome extent. In the case of increased time, the conversion of ammonia was increasedobviously, but when it was more than8.4s, the effect became decreased. Based on thetechnological investigation, the optimal conditions of480℃,28MPa, residence time8.4s and3times excess of oxidant, the final concentration of ammonia in thesimulated wastewater after catalytic SCWO treatment was decreased to18.7mg/L,which could met the wastewater discharge demand according the national standard ofGB8978-1996.