Decomposition Of1,1,1-Trichloroethane And2,4-Dichlorophenoxyacetic Acid In Hot Compressed Water In Anti-Corrosive Fused Silica Capillary Reactor And Ramanspectroscopic Measurement Of CO₂

In recent years, supercritical water oxidation (SCWO), a promising environmental protection technique for decomposition of hazardous organic compounds, has demonstrated excellent potential for clean and efficient decontamination of organic materials. Refractory/hazardous organic compounds can be easily destroyed in a short amount of time. Unlike wet air oxidation or incineration, SCWO takes advantage of the unique properties exhibited by water when used above its critical point (Tc=374.3℃, Pc=22.1MPa); reaction rates are much faster and the byproducts are less harmful. Under supercritical conditions, organic compounds and an oxidizer become completely miscible with supercritical water and the reaction proceeds in a homogeneous phase; the organic compounds are completely oxidized to CO2and H2O within a few minutes.The chlorinated solvent1,1,1-trichloroethane (TCA) has historically been widely used as a major chemical solvent in metal degreasing, adhesives, aerosols, and textile processing. Because it has been widely produced and used in the past20years, TCA is one of the most frequently detected compounds at numerous industrial facilities and waste disposal sites. Furthermore, it has been identified by the U.S. Environmental Protection Agency (EPA) as a principal organic hazardous compound because of its stability and low biodegradability. Common TCA daughter products include1,1-dichloroethane,1,1-dichloroethene, vinyl chloride, and monochloroethane. High concentrations of TCA are unsuitable for biodegradation because the daughter products can be equally or more toxic and persistent in groundwater than TCA. Decomposition of1,1,1-trichloroethane (TCA) in hot compressed water, with or without an oxidizer, was studied using an optically transparent anti-corrosive fused silica capillary reactor (FSCR), and Raman spectroscopy. The phase behavior of TCA in water during hydrolysis/oxidation was observed continuously under a microscope and recorded with a digital-camera recorder system, A new phase behavior of TCA was found in the presence of excess H2O2; TCA was first gasified during heating, followed by liquefaction under increasing internal pressure as a result of the formation of O2from the decomposition of H2O2, and subsequently gasified again during further heating up to400℃. The gaseous products of TCA oxidation in hot compressed water were monitored quantitatively and qualitatively using Raman spectroscopy. The effects of the operating parameters, namely the stoichiometric amount of oxidizer, temperature, and reaction time, were investigated. In our experiments, it was found that100%CO2yield was achieved with a175%stoichiometric amount of H2O2at420℃after360s. Temperature plays a key role in the decomposition of TCA and the formation of CO2in the supercritical water oxidation (SCWO) process. Based on our results, a reaction mechanism for TCA decomposition in hot compressed water was proposed. The global reaction kinetics showed that the formation of CO2in the SCWO of TCA was a first-order reaction.Decomposition of2,4-dichlorophenoxyacetic acid (2,4-D) in hot compressed water with oxidizer, was studied using an optically transparent anti-corrosive fused silica capillary reactor (FSCR), and Raman spectroscopy. The phase behavior of2,4-d in water during oxidation was observed continuously under a microscope and recorded with a digital-camera recorder system. The gaseous products of2,4-D oxidation in hot compressed water were monitored quantitatively and qualitatively using Raman spectroscopy. The effects of the operating parameters, namely the stoichiometric amount of oxidizer, temperature, and reaction time, were investigated. In our experiments, it was found that100%CO2yield was achieved with a200%stoichiometric amount of H2O2at450℃after10min. Temperature plays a key role in the decomposition of2,4-D and the formation of CO2in the supercritical water oxidation (SCWO) process. Based on our results, a reaction mechanism for2,4-D decomposition in hot compressed water was proposed. The global reaction kinetics showed that the formation of CO2in the SCWO of2,4-D was a first-order reaction.Equipment corrosion, which is caused by the byproduct HCl, is a main obstacle to the development and successful industrial application of chlorinated organic compounds in SCWO. In our study, we examined the oxidation of TCA/2,4-D in hot compressed water, using an optically transparent anti-corrosive fused-silica capillary reactor (FSCR) instead of a stainless-steel autoclave. The FSCR is strong enough to withstand high pressures and high temperatures, has no catalytic activity, and is resistant to corrosion. A microscope recorder system was used for observing and recording the phase behavior of TCA/2,4-D in hot compressed water because of the optical transparence of the FSCR. The gas-phase product was directly measured, using Raman spectroscopy, for qualitative and quantitative analysis of CO2. A reaction mechanism for TCA/2,4-D decomposition in hot compressed water is proposed. So far, there have been few reports of studies focusing on the kinetics of total organic carbon conversion to CO2in SCWO; almost all previous work concentrated on the kinetics of reactant disappearance. We present the results of the global kinetics of CO2formation from TCA/2,4-D in hot compressed water.