Autoignition Temperatures for Mixtures of Flammable Liquids with ...

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hydrocarbons, ketones, esters and amines. The following conclusions can be drawn from this table: 1. The temperature of autoignition drops, as expected, substantially with increasing pressure. 2. The order of the compounds with respect to the AIT at higher pressures is different from the one at atmospheric pressure. Semenoff plots According to Semenoff's theory of thermal explosion /1/, the relation between the pressure pZ of a fuel/air-mixture and its autoignition temperature TZ is described by the relation: where EA Sem ( + 1) n EA pZ = k ⋅Tz ⋅ exp( ) RTZ is an apparent activation energy of the reaction and n is the overall reaction order (usually assumed to be 2). in bar/K² p /T Z 2 Z 1E-4 1E-5 1E-6 n-Heptane Benzene Ethanol Propionic acid Methyl propionate Butyl acetate 2 Sem 200 250 300 350 400 450 500 550 Temperature in ° C Fig. 8: Semenoff plots for 1 - 10 bar for several pure substances Fig. 8 gives the so-called Semenoff plots for a number of selected pure compounds. In general, the experimental values fall well on straight lines, from which apparent activation energies in the range 100 kJ/mol to 350 kJ/mol can be calculated. An exception is, however, some of the 1 bar values measured with the standard apparatus which are much higher than expected from extrapolation of the values measured in the high pressure autoclave (open symbols in Fig. 8). Apart from a possible switch in reaction mechanism (low temperature/high temperature) differences in experimental conditions may cause this deviation: 1. The larger vessel volume (0.5 l compared to 0.2 l in the standard apparatus) is known to decrease the ignition temperatures. 2. Both the temperature and the pressure criterion for ignition may be stricter than the visual criterion used with the standard apparatus. 3. The closed vessel may make ignitions easier. In some times a small pressure increase was observed to precede ignition, which is not possible in an open device. 4 Tab. 2: Autoignition temperatures of several pure compounds at elevated pressures compared to the standard values Autoignition temperature in °C at Compound 1 bar 2 bar 5 bar 10 bar n-Hexane 230 235 210 197 n-Heptane 220 201 197 190 n-Octane 215 210 Cyclohexane 246 245 225 215 Benzene 565 526 * 470 451 Toluene 535 - 457 261 Dioxan 375 212 197 189 Methanol 440 300 260 Ethanol 400 283 250 Propanol-1 385 300 265 240 Butanl-1 325 292 255 240 Pentanol-1 320 250 240 Hexanol-1 280 280 262 232 Acetone 525 350* 275 260 Butanone-2 475 290 235 210 Pentanone-2 445 260 210 - Hexanone-2 420 196 187 - Cyclohexanone 430 279 230 215 Propionic acid 470 358 299 266 i-Butyric aldehyde 165 143 122 Propionic aldehyde 190 108 98 93 Methyl propionate 465 400 284 253 Ethyl formiate 440 312* 280 225 Propyl propionate 445 315 251 Butyl propionate 425 320 240 i-Propyl acetate 425 296 241 245 Methyl acetate 505 470 415 338 Ethyl acetate 470 380 260 230 Propyl acetate 455 300 260 240 n-Butyl acetate 393 252 240 230 t-Butyl acetate 450 395 370 310 n-Pentyl acetate 350 226 i-Pentyl acetate 280 261* 240 224 Methyl butyrate 445 400 256 n-Butyl amine 310 280 258 216 * = value at 2.5 bar
The Semenoff plots can be used to estimate the autoigniton temperatures at even higher pressures. Semenoff's equation seems to indicate that it is possible to lower the AIT to arbitrarily low values by increasing the pressure. But the higher the total pressure of the fuel/air mixture, the higher is also the partial pressure of the fuel required to reach the most critical fuel concentration. At some temperature the required partial pressure will become equal to the fuel vapour pressure. At even higher pressures and lower temperatures it is impossible to reach the critical fuel concentration, and the depression of the AIT is expected to stop. A similar mechanism has been employed to explain the reincrease of the standard AIT with chain length for very long chain molecules /5/. Conclusions For a number of pure compounds, the pressure dependence of autoignition temperatures and the upper explosion limits at a temperature of 200°C have been determined. The results show that mostly the explosion range widens dramatically with increasing pressure at the UEL. Therefore, although the most sensitive fuel concentrations in AIT determination lie at very rich mixtures, they are always well within the explosion range. No special mechanism for the ignition at AIT needs to be applied. The pressure dependence of the autoignition temperature has been shown to meet a Semenoff relation, which can be used to predict the AITs at pressures. Similarily, the ignition delay times follow an Arrheniuslike relation leading sometimes to very long delays near AIT at high pressures. When discussing the AIT and the UEL of a liquid, its vapour pressure curve also must be considered. Due to the rise of the UEL with pressure, it will often be impossible to find an UEL at elevated pressure at a specified temperature as the limited vapour pressure prevents reaching the necessary fuel concentration. That means it is not possible to exceed the UEL. Similarily with respect to AIT a point will be reached when increasing the pressure where the fuel vapour pressure limits the mixture concentration. Rich mixtures that would be necessary to find the AITs predicted by Semenoffs relation cannot be obtained, and a further decrease of AITs with increasing pressure will not be observed. Therefore the AIT will stop to decrease further. This offers the possibility of calculating a ‘lowest possible AIT’ for the substance under consideration provided its vapour pressure curve is known. The values found at 1 bar with the open standard apparatus often do not fit well to the Semenoff or Arrhenius relations found in the autoclave experiments. This shows that the differences in experimental set-ups have substantial influence on the results. It is therefore desirable to get AIT values for 1 bar under isochore conditions, for example with the present autoclave. Acknowledgements We thank M. Gödde, W. Möller, G Riesner and J. Scheffler for carrying out the experiments and operating the autoclaves. For financial support we thank the Hauptverband der gewerblichen Berufsgenossenschaften References 1. Semenoff, N.: Zur Theorie des Verbrennungsprozesses, Z.Phys. 48(1928), 571 2. IEC 60079-4: Electrical apparatus for explosive gas atmospheres. Part 4: Method of test for ignition temperature 3. Gödde, M.: Zündtemperaturen organischer Verbindungen in Abhängigkeit von chemischer Struktur und Druck, PTB-Bericht ThEx-8, Wirtschaftsverlag NW Verlag für Neue Wissenschaft, Bremerhaven 1998 4. Semenoff, N.N.: Some Problems in Chemical Kinetics and Reactivity, Princeton Univ. Press, v.2, 1959, 331 pp. 5. Gödde, M, Brandes, E. and Cammenga, H. K.: Zündtemperaturen homologer Reihen Teil 2, PTB- Mitteilungen 108(1998), 437 5
Page 1 and 2: Autoignition Temperatures for Mixtu
Page 3: fuel concentration in % by vol. Fig

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