Study On The Oxidation Of High Concentration Organic Wastewater In Supercritical Water Using Transpiring Wall Reactor

In order to resolve problems of high cost and low destruction efficiency faced to high concentration refractory organic wastewater,supercritical water oxidation was used to treat this kind of wastewater.Dyeing wastewater and acrylic acid wastewater were adopted as research objects in transpiring wall reactor with hydrogen peroxide as oxidant.Effects of reaction temperature,reaction pressure and oxidant dosage on destruction efficiency of organic pollutants in dyeing wastewater and acrylic acid wastewater was studied.Kinetics for dyeing wastewater and acrylic acid wastewater oxidized in supercritical water were regressed from experimental data.Destruction pathway of dye molecule in supercritical water was proposed.Corrosion and inorganic salt deposition,which obstruct the industrialization of transpiring wall reactor,was investigated during the period of reaction.Dyeing wastewater was treated under supercritical water oxidation condition, temperature 380-460℃,pressure 20-30MPa,special oxidant dosage n=0.6-2.0. Effects of reaction factors on degradation efficiency of COD_Cr,TN,NH3-N and color were investigated.The result indicated that removal of COD_Cr,TN,NH3-N and color was affected positively by reaction temperature,and increased with the temperature. Effect of temperature on reaction could be explained from two aspects.Firstly, organic pollutants oxidation in supercritical water was irreversible.Reaction velocity was increased with temperature,thus the removal as well as efficiency.Secondly,the density of supercritical water was debased with increase in temperature under fixed pressure condition.Accordingly,concentration of reactants and reaction velocity reduced.Effect of temperature on reaction came from these positive and negative aspects.Positive effect of temperature on reaction was discovered in this investigation. 94.65%,58.15%,97.09%and 99.80%of COD_Cr,TN,TOC and color removal efficiency was achieved at temperature 460℃,pressure 25MPa,retention time 30s, special oxidant dosage n=1.5.Reaction pressure had positive effect on supercritical water oxidation.Removal efficiencies of COD_Cr,TN and color in wastewater were increased with pressure. Effect of pressure was due to two aspects.One,density of supercritical water increased with the pressure under fixed temperature condition,resulting in augmentation of concentration of organic compound,oxidant and reaction velocity. Two,due to increase in water density,retention time of organic pollutants was prolonged,which was favorable to oxidation destruction of organic pollutants. Summarily,pressure effect could be come down to positive effect of reactant concentration and retention time.Thus,higher pressure do favor to organic pollutants destruction.93.35%,66.47%,95.30%and 99.80%of COD_Cr,TN,TOC and color removal efficiency was achieved at temperature 420℃,pressure 30MPa,retention time 30s,oxidant dosage n=1.5.Effect of oxidant dosage on supercritical water oxidation was similar to temperature and pressure,which was positive to reaction.Removal efficiency of COD_Cr,TN and color was increased with oxidant dosage.The optimum reaction conditions for oxidation of dyeing wastewater in supercritical water were temperature 420-440℃,pressure 25-28MPa,special oxidant dosage n=1.2-1.6.Additional, biodegradability of dyeing wastewater was tested,before and after supercritical water oxidation respectively.The results indicated that B/C of dyeing wastewater was advanced greatly after treatment of supercritical water oxidation,increased with temperature and COD_Cr removal efficiency.B/C of dyeing wastewater increased form 0.12 to 0.44.Thus,biodegradability of dyeing wastewater was improved greatly by supercritical water oxidation.Acrylic acid wastewater was treated under supercritical water oxidation at temperature 370-420℃,pressure 20-30MPa,oxidant dosage n=0.0-3.0.The results indicated that COD_Cr removal efficiency was affected significantly by temperature. COD_Cr removal efficiency up to 99.09%was achieved at 420℃,respectively.More Activated molecules with higher temperature,leading to higher oxidation velocity, organic removal efficiency.Temperature had positive effect on organic pollutants removal.Removal efficiency of COD_Cr in acrylic acid wastewater was increased with pressure.The density of reaction mixture was also increased with pressure,leading to increase in concentration of reactant,reaction velocity and COD_Cr removal efficiency. Additionally,reaction zone under supercritical condition was prolonged with pressure, resulting in prolongation of retention time of organic pollutants under supercritical condition,leading to increase in COD_Cr removal efficiency.Removal efficiency of COD_Cr in acrylic acid wastewater was increased with oxidant dosage.Amount of reaction active radical was increasing with oxidant dosage, favoring reaction velocity,resulting in increase in COD_Cr removal efficiency.The optimum reaction conditions for oxidation of acrylic acid wastewater in supercritical water were considered as temperature 420℃,pressure 24-26MPa,oxidant dosage n=1.0-1.5.In order to investigate destruction pathway of dye molecule in supercritical water, reaction products of dyeing wastewater oxidized under supercritical condition was examined by GC-MS method.The results indicated that anthraquinone was decomposed to monocyclic and polycyclic intermediate hydrocarbons by means of opening-ring reaction,then destructed to short-chain carboxylates,finally oxidized to CO₂ and H₂O.Simultaneous,reactions such as coupling,hydrolyzing and substitution between intermediates and radicals were also existed.But,those by-products were oxidized to benzoic acid,phenol and final products such as H₂O,CO₂.A modified first-order rate expression was regressed from experimental data, taking into account the influence of induction time.Kinetic equation,reaction activation energy and introduction time of dyeing wastewater and acrylic acid wastewater oxidized in supercritical water were achieved.The resulting activation energy Ea was 12.12kJ.mol^-1,re-exponential factor A was 1.07s^-1,introduction time were 2.03s,1.74s,1.37s,2.93s under various temperature conditions for dyeing wastewater.Meanwhile,Ea was 12.12kJ.mol^-1,A was 4.97s^-1,introduction time were 7.31s,8.43s,7.98s,4.91s under different temperature conditions for acrylic acid wastewater.The reaction order for feed wastewater(based on COD_Cr) and oxidant were assumed to be 1 and 0 for these two kinds of wastewater.Kinetic equation for dyeing wastewater was "-d[COD]/dt=1.07exp(-12.12.64/RT)[COD]",and "-d[COD]/dt=4.97exp(-20.64/RT)[COD]" for acrylic acid wastewater. Comparison for SEM photos of ceramic tube before and after reaction was investigated.Concentration of Fe in effluent liquid samples was analyzed using method of atom absorption with and without ceramic tube as reactor.The result indicated that corrosion against reactor could be prevented effectively,and life of reactor could be prolonged using transpiring wall reactor.Aiming at present situation of supercritical water oxidation,several problems such as equipment corrosion,inorganic salt precipitation,exploitation for highly effective catalyst and stability of system for industrialization of this technology were put forward in this paper.