Theoretical And Experimental Research On Superheated Near-critical Water Oxidation Of Organic Wastewater

Using supercritical water as a medium for organic wastewater treatment supercriticalwater oxidation technology has a lot of research, but the superheated near-critical wateroxidation technology using superheated near critical water (temperature above500℃,pressure in20~22MPa) as medium of oxidation treatment organic wastewater has rarelybeen reported. Since the operating pressure is small, superheated near-critical water oxidationtechnology has lower equipment costs and operating costs, smaller corrosion, greater systemstability and other technical and economic advantages than supercritical water oxidationtechnology, so the technology is more conducive to industrial applications.In this article, through analyzing theoretically the nature of the critical area water it canbe concluded that the superheated near-critical water oxidation efficiency of organicwastewater and the efficiency of supercritical water oxidation technology are almostequivalent. Main theories are as follows: superheated near-critical water compared withsupercritical water, its pressure is relatively low, temperature is relatively high, so the weakerhydrogen bonding interaction and its nature tend to be more organic solvents of low polarity;above20MPa pressure have little impact on the density of the mixture of organic waste water,reduce the reaction pressure only slightly reduce the density of mixture of waste water,slightly reduce reaction rate, but raising reaction temperature can compensate the loss ofreaction rate; in the critical area of water, the diffusion coefficient also increases withtemperature increasing and pressure decreasing, so the diffusion coefficient of superheatednear-critical water is greater than the supercritical water, therefore the diffusion andmiscibility of organic compounds and oxygen in superheated near-critical water is better thanin supercritical water, and which is conducive to enhance the reaction rate. On the basis oftheoretical analysis, acrylonitrile and benzonitrile simulated wastewater were selected as theexperimental object and superheated near-critical water oxidation experiments were carried out under different operating conditions in a large continuous experimental equipment with areactor volume9.73L. In these experiments, the factors of temperature, pressure, initialconcentration, the intermediate product, peroxy coefficient, reaction time on the oxidationefficiency of acrylonitrile and benzonitrile in superheated near-critical water were researched,and experiment results show that it is feasible and efficient to process organic wastewater byusing superheated near-critical water oxidation technology. Based on the experimental data,the superheated near-critical water oxidation reaction kinetics of acrylonitrile and benzonitrilewere studied further, and CODcr removal dynamic equations of acrylonitrile and benzonitrileunder the conditions of500℃~520℃,20MPa were calculated.In view of salt crystallization is one of the problems restricting the application ofsupercritical water oxidation technology, the influences of NaCl on superheated near-criticalwater oxidation were studied through the experiment. First, based on the experimental results,systematically analyzed the cause of NaCl deposition and grow in the reactor inlet near thewall. In the second, first reported that there is a phenomenon of purification effluent pHvalues decreasing and salt discharge effluent pH value increasing when the waste watercontaining salt in continuous superheated near-critical and supercritical water oxidationsystem. Analysis of the reasons, mainly because of that there is dissociation-association andseparated flow of NaCl in superheated near-critical and supercritical water.Finally, by using quantum chemical ab initio and density functional methods,thermodynamics and kinetics of superheated near-critical water oxidation reaction ofacrylonitrile and benzonitrile were calculated. Respectively analyzed and calculated thestructure, energy, synergistic effect, the thermodynamic Gibbs free energy of the trimers ofNa+/Cl-, water, acrylonitrile/benzonitrile. Predicted the reaction channel, got the reaction rateconstant and wiger correction coefficient, analyzed the tunning tunnel effect, and provided thetheory basis of molecular level for superheated near-critical water oxidation technology.Through the above study, the main conclusions are obtained as follows: Theoretical andexperimental results show that the superheated near-critical water oxidation technology as thetechnology of supercritical water oxidation has a very good removal effect of acrylonitrile and benzonitrile and other organic matter, has advantages of high purifying efficiency, fastoxidation rate, no secondary pollution, etc. Under laboratory conditions, COD removalefficiency can reach above99%by taking reasonable operating parameters. The maininfluence factors of organic wastewater purification efficiency of superheated near criticalwater oxidation include: reaction temperature, reaction pressure, initial concentration oforganic compounds in wastewater, intermediate product, reaction time, peroxy coefficient,etc.