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Deep Oxidation of Fluorinated Hydrocarbons in Molten Catalysts Yu.S

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Deep Oxidation of Fluorinated Hydrocarbons in Molten Catalysts Yu.S Eurasian ChemTech Journal 5 (2003) 137-143 Deep Oxidation of Fluorinated Hydrocarbons in Molten Catalysts Yu.S. Chekryshkin1, T.A. Rozdyalovskaya1, Z.R. Ismagilov2*, M.A. Kerzhentsev2, O.A. Tetenova1 and A.A. Fedorov1 1Institute of Technological Chemistry, 13a, Lenin str., 614600 Perm’, Russia 2Boreskov Institute of Catalysis, Prosp. Akad. Lavrentieva, 5, 630090, Novosibirsk, Russia Abstract The oxidation of fluorine-containing organic substances: fluorocarbon liquid M-1, fluorinated alcohol H(CF2)8CH2OH, and powder polytetrafluoroethylene with air has been studied in melts: NaOH; 43 mol.% LiCl – 33 mol.% NaCl – 24 mol.% KCl (eutectic mixture); (LiCl-NaCl-KCl)eutec. + 10 mass.% V2O5; (LiCl-NaCl-KCl) eutec. + 15 mass.% V2O5; 56 mol.% Na2CO3 – 44 mol.% K2CO3 (eutectic), (Na2CO3 – K2CO3)eutect. + 15 mass.% V2O5, and K3V5O14. The compositions of the melts have been examined by GC, DTA, chemical analysis and XRD, and they have been shown to change during the reaction, depending on the composition and partial pressure of the gaseous products over the melt surface. The alkali metal chloride melt containing 15 mass.% V2O5 has been found to be most stable to the action of fluorine compounds. Possibility of deep oxidation of fluorine-containing organic substances in melts based on hydroxides, carbonates and chlorides of alkali metals doped with oxides of vanadium has been proved. The process of deep oxidation of fluorinated hydrocarbons is accompanied by formation of an equilibrium mixture con- taining hydroxides, carbonates, chlorides and fluorides of alkali metals, as well as their vanadates, if V2O5 additive is used. The relative amounts of these substances in molten systems are determined by the partial pressure of oxygen, CO2 and water vapor. Introduction ides are shown to catalyze processes of decomposi- tion and oxidation of halogen-containing substances, Halogenated hydrocarbons are used as solvents including chemical agents [4,5]. According to the and feedstock for the synthesis of various materials, method described in ref. [6], halogen-containing or- freons, and polymers. Perfluorinated hydrocarbons ganic wastes are treated in a bath with the melt con- are widely adopted as lubricating oils, hydraulic flu- sisting of an alkali-earth metal and an alkali-earth ids, and surfacants. metal halogenide. The halogen formed upon waste Burning of halogen-containing wastes is ecologi- combustion reacts with the metal. As a result, the cally unacceptable because it yields more toxic sub- alkali-earth metal halogenide is accumulated in the stances, e.g., dioxins [1]. melt. It should be noted that the process is carried For neutralization of fluorinated hydrocarbon out at high temperature (>750°C) and requires an waste, thermal destruction is traditionally used [2,3], expensive alkali-earth metal to bind halogens. however in this process fluorine compounds, such Halogen-containing wastes can be destroyed in as hydrogen fluoride, are formed that should be re- the sodium carbonate melt at 650-800°C [7]. It is moved from the gas stream before its discharge to noted that the resulting products (sodium halogeni- the atmosphere. The process of thermal destruction des) are relatively harmless, and the process has 30- of fluorinated hydrocarbons in the presence of ox- 40% lower cost than the conventional waste incin- ides and hydroxides of alkali earth metals [3] pre- eration with the subsequent scrubbing of the off-gas. vents a release of halogens into the gas phase. Previously [8], we reported that in the presence Melts composed of inorganic salts and metal ox- of oxygen chlorinated hydrocarbons undergo practi- *corresponding author. E-mail: [email protected] cally complete conversion in molten catalysts with 2003 al-Farabi Kazakh National University 138 Deep Oxidation of Fluorinated Hydrocarbons in Molten Catalysts predominant formation of products of deep oxida- lecular sieve NaX. tion. The present paper is devoted to the study of the Gas lines of two chromatographs were consecu- conversion of fluorine-containing organic substances tively connected. Helium was used as gas carrier. The in the following melts: NaOH (I), 43 mol.% LiCl – inlet pressure of the gas carrier was 2.2 kg/cm2, and 33 mol.% NaCl – 24 mol.% KCl (eutectic mixture) the gas flow rate at the outlet of the second analyti- 3 (II), (LiCl-NaCl-KCl) eutec. + 10 mass.% V2O5 (III), cal column was maintained at 24 cm /min at atmo- (LiCl-NaCl-KCl) eutec. + 15 mass.% V2O5 (IV), 56 spheric pressure. Katharometers were used for de- mol.% Na2CO3 – 44 mol.% K2CO3 (eutectic.) (V), tecting the reaction components, at a detector cur- (Na2CO3 –K2CO3) eutect. + 15 mass.% V2O5 (VI) rent strength of 100 mA in the both GC instruments. and K3V5O14 (VII). Concentrations of the gas mixture components were determined by the absolute calibration method. Experimental The calibration was performed by the introduction of gas samples into the GC by the dosing loop of a The oxidation of organic substances in melts was sampling valve and by injection of liquid samples studied in an experimental setup consisting of the with a micro-syringe. following units: a vessel filled with organic substance, The sampling of the air-vapor mixture at the re- a pump, a bubble-type reactor placed in a furnace, a actor outlet was performed with a gas syringe pre- compressor for air supply to the reactor, a pressure liminarily heated to 60-70°C to prevent condensa- reducer, a fine flow-regulating valve and a rotame- tion of liquid organic compounds. The sample vol- ter. ume was 1 cm3. The design of the GC analytical sys- A mixture of air and an organic substance passes tem provided complete analysis of the vapor-gas through a vertical feeding tube immersed in the melt mixture from one sample. to form an air-vapor mixture bubbling through the The reagents used in the experiments were of melt. The mixture undergoes chemical conversion in “laboratory purity” grade. the melt, and the reaction products are either vented Fluorinated alcohol H(CF2-CF2)4CH2OH is a crys- or directed to an absorption vessel filled with an aque- tal powder, its melting temperature is 66-67°C. For ous solution of KJ, with starch added as an iodine convenient dosing, the alcohol was dissolved in chlo- indicator. The rotameter and the pump were calibrated roform and used as a solution (10 mass.%). for gas flow rates ranging from 25 to 80 l/h and liq- The fluorocarbon liquid M-1 corresponds to the uid flow rates ranging from 0.01 to 0.2 l/h, respec- Branch Standard 95-41976. The M-1 liquid is pro- tively. duced by fluorination of hydrocarbon oils. It is a lu- The reactor (240 mm long and 78 mm i.d.) is cov- bricant used under aggressive media. It has a density ered with a lid (60 mm thick) having a conical ori- of 1.9 g/cm3, and b.p. of 100-120°C at 400 Pa. It is fice to exclude ‘dead space’ (the principle of “ideal insoluble in water and organic solvents. The thermal plug” operation). The free space of the non-operat- decomposition of M-1 begins at 300-400°C. ing reactor is ~50% of the melt volume. Because of Polytetrafluoroethylene (PTFE) was dosed as fine changes in temperature and filling of the melt with powder suspended in air by passing air flow through the gas bubbles, the actual volume of the melt in- a temperature-controlled vessel containing PTFE.* creases by ~35%. The remaining free space in the An “ECOTEST” instrument supplied with ion- operating reactor is required to prevent the forma- selective electrodes was used to measure concentra- tion of the melt layer on the reactor lid caused by the tions of chlorine and fluorine in the off-gas. For this entrainment of the melt droplets. analysis, the off-gas was passed from the reactor The reaction products were separated and ana- through an aqueous potassium hydroxide solution lyzed by gas-liquid chromatography, using a 2.5 m (0.1 mol/l) for a specified period of time. column filled with SE-60 (15 mass.%) on Chroma- To reveal the presence of fluorine or chlorine in ton-N-AW support, at a temperature of 78°C. the reaction products, the off-gas was bubbled CO2, CO, N2, and O2 were analyzed by GC, using through an aqueous potassium iodide solution con- two consecutively connected columns. CO2 was ana- taining starch as a color indicator. The presence of lyzed at 30°C using a column (3m long) filled with fluorine was additionally confirmed by the fact that Polysorb-1. CO, N2 and O2 were separated at room *The experiments on PTFE oxidation were performed by temperature in a column (3 m long) filled with mo- P.S.Dukhanin Eurasian ChemTech Journal 5 (2003) 137-143 Yu.S. Chekryshkin et al. 139 the color of the iron sulfosalicylate complex solu- Na2CO3-K2CO3 (V) at 600°C is 5.7 vol.%, which is a tion changed from deep-cherry to light-yellow be- little higher than the calculated value (5.2 vol.%). cause of iron binding into a more strong fluoride com- The concentration of fluorine is 0.06 vol.%, which is plex. This reaction is used to determine fluorine by considerably lower than the calculated value (10.4 the colorimetrical method. vol.%). It is obvious that the fluorine or fluoride ion, resulting from the oxidative destruction of M-1, in- Results and Discussion teracts with potassium and sodium carbonates to form alkali metal fluorides and to yield CO2 into the gas Polytetrafluorethylene oxidation in the NaOH phase. On further process progress, the concentra- melt was studied at 350 and 450°C. At these tem- tion of CO2 in the gas phase decreases to 4.8 vol.% peratures the complete oxidation of PTFE takes (Table 1, run 2).
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