Soot formation in the pyrolysis of halogenated hydrocarbons. Part I, binary mixtures of carbon tetrachloride with hydrogen and iron pentacarbonyl

Abstract

AbstractMeasurements on carbon particle growth during CCl4 thermal decomposition were carried out behind both incident and reflected shock waves. Three kind of mixture compositions have been investigated: 4000 ppm CCl4 in argon; 200 to 4000 ppm CCl4 and 200 to 8000 ppm Fe(CO)5 in argon; 4000 ppm CCl4 and 4000 ppm hydrogen in argon. The temperature range 1200<T/K<3200, pressure around 25 bar. Kinetics of the particle growth was detected via the attenuation of the laser beam (He‐Ne laser, 632.8 nm).The low temperature boundary for soot particles detection and magnitude of soot yield in CCl4 pyrolysis is the same as that for most of the hydrocarbons studied. The temperature change during CCl4 pyrolysis is an important factor for its kinetic interpretation. The first order rate constants of soot growth kt/[C]are higher than those for all hydrocarbons, including benzene under comparable conditions. At T<2200 K the induction times tindare close to those for hydrocarbons with respect to both, absolute values and activation energy. In the middle temperature range studied, 2200<T/K<2500, an “induction‐less behaviour” of soot growth was observed; tind at T2500 K show a negative activation energy. Hydrogen additives significantly act on soot formation characteristics depending on H/Cl‐ratio.Chemical reaction in mixtures CCl4/Fe(CO)5 was observed even at relatively low temperatures behind incident shock waves. However, iron additives only slightly change the soot yield as compared to CCl4 pyrolysis. A qualitative interpretation of experimental findings observed is suggested based on the consideration of the hierarchy of characteristic times in reacting systems followed by the growth of soot particles.