The kinetics of soot formation and the properties of soot formed by thermal decomposition of acetylene-benzene and acetylene-hydrogen mixtures

The process of soot formation during pyrolysis of acetylene-benzene and ace tylene-hydrogen mixtures is studied behind reflected shock waves by monitor ing laser radiation absorption. The induction period of this process, the m acrokinetic rate constant for soot formation, and the soot yield are determ ined. Experiments were carried out at pressures of 6 and 60 atm, various co ncentrations of carbon ((1 divided by 16).10(-6) mol/cm(3)), and various ra tios between the mixture components, covering a temperature range from 1560 to 2580 K. In benzene-acetylene mixtures, the induction period, which prov ed to be confined between its values for pure components, depends only slig htly on the acetylene pressure and mixture composition. An increase in the benzene-acetylene ratio entails a dramatic increase in the rate constant fo r soot formation (k(f)) at high temperatures. The bell-shaped temperature d ependence of the soot yield peaking at approximately 1800 K, which was obse rved in previous studies, was verified. The data on the soot yield indicate a conspicuous synergistic effect. The soot yield in rich benzene mixtures decreases with increasing pressure or benzene/acetylene ratio. As revealed by experiments with acetylene-hydrogen mixtures, hydrogen suppresses soot f ormation to an appreciable extent. The size of soot particles was determine d by means of electron microscopy, and their size distribution was approxim ated by a logarithmically normal function. The mean size of particles measu res (20 +/- 5) nm at all the covered temperatures and pressures and tends t o grow with carbon concentration. Assuming that soot particles are spherica l in shape, their final concentration must be somewhere in between 10(11) a nd 10(13) cm(-3).