Study On Supercritical Water Oxidation And Dynamics Of Typical Explosive Waste Water

There are various toxic and harmful materials in all kinds of explosive waste waterwhich has notable impacts on environment and zoology. The pollutants generally are organiccompounds including nitryl and are difficult to dispose. Waste water of explosive industry hasbecome a common environment issue in the world and is concerned by governments andresearchers of most countries. A lot of advanced techniques have been developed to treat thewaste water.Supercritical Water Oxidation (SCWO) is a new technique for treatment of organicwastewater and draw widespread attention of the world. When exceeding critical point ofwater (Tc=374.3℃, Pc=22.1 MPa), the density, dielectric constant, hydrogenous bond andionic product of water get down. Supercritical water becomes a non-polar solvent with higherdiffusivity and excellent transport properties. Consequently, even non-polar organiccompounds and gases like oxygen become completely miscible in it. The SCWO processbased on the special physical properties of supercritical water has a great many of advantages.The oxidation reactions take place rapidly without any interphase mass transport limitation.The organic compounds in wastewater can be completely oxidized in several seconds toseveral minutes.In order to establish the kinetic model of SCWO reactions, TNT,RDX,HMX areselected as main objects. The SCWO experiments are carried out under different reactionconditions in an intermission reactor. The oxidation efficiency and influence factor of threekinds of explosive waste waters in supercritical water are investigated. The middle oxidationproducts are analyzed with High -Performance Liquid Chromatograph(HPLC), GasChromatogram (GC) and GC-MS. The kinetics of TNT and RDX oxidation in supercriticalwater and the COD removal kinetics are studied. The reaction path and mechanism of TNTand RDX in supercritical water are determined. The results show that: (1) The SCWO can effectively decompose and remove organic pollutants in water. Withcontrast to Wet Air Oxidation and incineration, the SCWO has characteristics of wideapplication, high efficiency; high oxidation rate, small installation, no residual pollutants andenergy recyclable. The COD removal rate of high-concentration pollutants could reach up to99% under suitable conditions.(2) The decomposition or removal of organic compounds is enhanced with higher temperature,delayed residence time and oxidant content. The COD removal rate is significantly improvedas reaction temperature, pressure, and residence time and oxidant excess increase. The maininfluential factors of COD elimination efficiency are temperature and residence time. Pressurehas limited influence on SCWO reaction and may be attributed to the change of concentrationof reactants and intermediates and the characteristic of water.(3) The COD removal rate is sensitive when the temperature is between 530～550℃forTNT and 480～500℃for RDX and HMX. The reaction may be controlled by the furtheroxidation of intermediates which are difficult to be oxidized. In terms of the phenomena, thedependence of COD removal efficiency on the temperature, the recovery and control of theheat should be considered for designing the full scale plant of SCWO.(4) The SCWO products of TNT are analyzed and identified systematically by GC-MS forfirst time. It is shown that the products could be classified as linear paraffin, dimmers andmulti-ringed aromatic compounds. The main intermediates such as naphthalene, phenanthrene,anthracene, fluorenone, and p-benzoquinone are found for the first time and offer a basis forinferring the reaction pathway of TNT in SCW.(5) The characteristics of oxidation reaction for TNT in SCWO are analyzed based on theanalysis of products. The main intermediates of TNT in SCWO are trinitrobenzene,dinitrobenzene, toluene and other linear chain paraffin as well as other multi-ringed aromaticcompounds such as naphthalene, fluorine phenanthrene, and anthracene. It may be confirmedthat many side reactions, such as coupled reaction, hydrolysis reaction and isomerizationreaction take place simultaneously in the process.(6) TNT degradation mechanism in SCWO is free radical reactions that include chain beginning, growing and termination. Free radicals -OH initiate the reactions and heat splittingmight also produce active free radicals. The mutt-ring compounds of naphthalene and fluorinefrom free radical reactions are checked out in SCWO TNT process. The pathway for SCWOTNT may be showed as follows;(7) The kinetics of TNT oxidative reaction in SCWO is developed for the first time. At 673～823K and 24MPa, COD removal kinetic equation can be expressed in following equation:(8) The kinetics of RDX oxidative reaction and COD removal kinetic in SCWO aredeveloped for the first time. At 673～823K and 24MPa, the kinetic equation of RDX can beexpressed in following equation:At 673～823K and 24MPa, COD removal kinetic equation can be expressed in followingequation:The relationship between X_A and X_COD for RDX is put forward as follow:In which(9) The intermediates of RDX in SCWO at different temperatures are analyzed andidentified systematically by GC-MS.Based on this and according to the structure of RDX andcharacteristic of SCWO,the pathway of RDX in SCWO is showed as follows: (10) The oxidative degradation of RDX in SCWO is the ring opening directly with C-N bondbreaking. At the same time the reactions of eliminating·NO₂ companying N-N bondbreaking also happens. The carbon of RDX ring-mix may also accept hydroxyl to formhydroxymethyl. The further oxidation of these compounds forms final products.