Visual Observations And Raman Spectroscopic Studies Of Supercritical Water Oxidation Of Chlorobenzene In A Fused-silica Capillary Reactor

Supercritical water oxidation （SCWO） has proved to be one of themost effective and environmentally friendly methods for the destruction ofhazardous organic wastes. The SCWO reaction typically proceeds in asingle fluid-phase consisting of organic compounds, an oxidizer, and water.The reaction system, which has no interphase mass-transfer limitations,leads to a fast reaction rate and high oxidation-efficiency. It has beenstudied and used for a variety of applicationsChlorobenzene （CB） has been identified by the U.S. EnvironmentalProtection Agency （EPA） as a principal organic hazardous compoundbecause of its low biodegradability and accumulation potential in soil andwater, and also because it is a structural unit of complex chloroaromaticcompounds. Dechlorination of CB is difficult because of the high strengthof the aromatic C–Cl bond, which is around95kcal mol1, compared tomore typical values of around85kcal mol1. Disposal of chlorinatedorganic compound wastes, such as chlorobenzene, in order to minimize theenvironmental hazards has become an urgent issueThe SCWO process for wastewater treatment has been extensivelystudied by many researchers. However, almost all of the previous studieswere performed in large scale stainless-steel reactors, which were readilycorroded in supercritical water, particularly in the presence of chloride ions.One of the greatest problems in the SCWO of chlorinated organic compounds is that reactor vessels made of metal alloys are readily corrodedby HCl generated as an oxidation product. Corrosion has become one of themajor problems hindering the development and industrial applications ofSCWO technology. In most cases, the liquid-phase reactor effluent, but notthe gas-phase products, were sampled and analyzed by gas chromatography（GC）, high performance liquid chromatography （HPLC）, and ionchromatography （IC）, partly due to the difficult of gas sampling. Also,nearly all previous studies have concentrated on the kinetics of reactantdisappearance, with only a few works focusing on the kinetics of totalorganic carbon conversion to CO₂during SCWO.Supercritical water oxidation of chlorobenzene （CB） was studiedusing an anti-corrosive fused-silica capillary reactor （FSCR） combinedwith a polarization microscope recorder system for visual observations anda Raman spectroscopic system for qualitative and quantitative analyses ofthe gaseous products. Both the liquid-and gas-phase products from thesame cooled FSCR were analyzed by GC and Raman spectroscopy, andused to determine the CB conversion yield and CO₂yield, respectively.Various operating parameters, including the stoichiometric amount ofoxidizer （100-300%）, reaction temperature （350-450°C）, and reaction time（2-10min） were used to investigate their effects on the oxidation behaviour.Our results indicate that among the parameters examined, temperature hadthe most significant effect on the SCWO reaction behavior; a100%conversion yield of CB （the amount of disappearing CB/feed of CB） and100%CO₂yield （the amount of CB being completely converted to CO₂,H2O, and HCl） were achieved with a150%stoichiometric amount of H2O2at450°C within8min and10min, respectively. The conversion yield andthe CO₂yield both depend strongly on temperature, and the CO₂yield isalways less than the CB conversion yield under the same experimentalconditions, suggesting that some carbon exists in intermediate products of incomplete oxidation, as confirmed by gas chromatography-massspectrometry. Global kinetics analysis based on the complete conversion ofCB to CO₂, water, and HCl could be expressed as a first-order reaction, andan activation energy of142.8kJ mol1was obtained. During SCWO of CB,the phase behavior was observed under a polarization microscope and theimages were continuously recorded in a computer. Visual observationsrevealed that, during heating, CB hardly dissolved in H2O2below153.9°C,dissolved completely above326.1°C, and eventually produced a liquidfluid coexisting with a vapor phase. During the cooling process, the amountof oil increased substantially and suspended oily spheres were observed,but the total amount of oil was significantly less than that before thereaction. From this visualization, we can conclude that some of the CBdecomposed during the SCWO process.FSCR-Raman spectroscopy-based method is visually-accessible, lowin energy and materials consumptions, expeditiousness, absence ofundesired catalytic effects of the reactor wall, resistant to corrosion, andcould be directly coupled with a Raman spectroscopic system formonitoring the chemical reaction progress in stiu. Moreover, due to thesmall size of reactor, it minimizes the resistances in mass transfer and heattransfer, such that the observed kinetics approaches to the intrinsic one, andthe kinetic results may be applied to optimum reactor design at throughputsin industrial operations. This technology has great potential to be appliedfor the theoretical study of the fluids and chemical reactions under elevatedpressure-temperature conditions.