Autoignition Temperatures for Mixtures of Flammable Liquidswith Air at Elevated Pressures by Elisabeth Brandes, Werner Hirsch* and Thomas Stolz Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany Abstract The autoignition temperature (AIT) of pure compounds has been measured at pressures between 2 bar and 15 bar in a 0.5 l autoclave. The AITs are found to decrease substantially with increasing pressure, following a Semenoff relation allowing extrapolation of the AIT to higher pressures. The ignition delay times follow an Arrhenius-like relation and may become very long at higher pressures. For some compounds both the AITs and the delay times, the 1 bar values found with the standard apparatus do not fit well to the respective relation, pointing to a profound influence of the experimental differences. Although very high fuel concentrations are required to find the minimum AIT, they remain within the explosion range as the Upper Explosion Limit is shown to shift dramatically to higher values with increasing temperature and pressure. Introduction Today, many industrial processes are operated at elevated pressures. It is known that the AIT of a fuel/air-mixture drops with increasing pressure, a fact explained theoretically by Semenoff /1/ as early as 1928. Since then, a lot of work has been done to determine AITs at elevated pressure (see, for example, the work of Gödde /3/ and references cited therein). Nevertheless, the safety characteristics of many technically important substances are still poorly known under these conditions. The present work deals with the determination of autoignition temperatures (AITs) of several single compounds under high pressures. Even less knowledge exists about the Upper Explosion Limit (UEL) at high pressures. A second series of experiments was set up therefore to determine the UEL at elevated pressure and a temperature near their AIT for some of the pure compounds investigated. admitted as a liquid and evaporates in the reaction vessel. However, the reaction does not take place under isobaric conditions like in IEC 60079-4 but under isochore conditions. The present experiments covered the range from 2 bar to 10 bar total pressure. In some cases, AITs were also obtained for 1.5 bar or 15 bar. The fuel concentrations usually varied between 15% by vol. and around 40% by vol.. Experimental set-up AIT Under atmospheric conditions, the AIT of a liquid is usually determined by dropping the liquid into a preheated Erlenmeyer flask, ignition being indicated by the occurrence of a visible flame /2/. Measurements at elevated pressures require a closed reaction vessel as shown in Fig. 1. We use a 0.5 l autoclave (1). Ignition is indicated either by a temperature rise (observed by two thermocouples (6) within the vessel) or by the pressure rise observed by the pressure transducer (10). For an experiment, air is first introduced into the autoclave from its supply (4), regulated to the desired pressure by the pneumatic valves (7), (8), and (9), then the liquid is fed into the vessel from its supply (2) by a HPLC pump (3). The resulting pressure is taken as the starting pressure. As with the standard apparatus, the fuel is * Corresponding author:
[email protected] Proceedings of the European Combustion Meeting 2005 Fig. 1: Diagram of the apparatus for measuring AITs UEL UELs at 10 bar and 180°C or 200°C were determined in a separate 9 l autoclave (11) (Fig. 2) heated uniformly by a thermofluid jacket (15) equipped with a conventional capacitive spark igniter (19). Ignition is indicated either by a temperature rise > 50 K (observed by thermocouples (16) within the vessel) or by the pressure rise > 5% of the starting pressure observed by Near the AIT the reaction also often does not proceed through the whole mixture. Therefore near AIT the pressure only rises by a factor of 2 or less. The resulting pressure is taken as the starting pressure. but can account only partly for that result. The relation between the range of autoignition. at 10 bar T 1-Propanol 28.4 52. 5.6 200 Ethanol 36. the fuel/air ratio also has to be varied. This is in real contrast to the behaviour of the Lower Explosion Limit (LEL) which is known to be nearly independent of pressure at least for pressures up to 5 bar.trations of 25% and higher. then the liquid is fed into the vessel from its supply (12) by a HPLC pump (13) using a nozzle which generates a very fine spray.5 39.7 44. Fig.6 200 n-Hexane 22. In both cases the lowest temperature of ignition at 10 bar is reached at fuel concentrations above the UEL at 1 bar at the same temperature. 3: Dependency of the temperature rise ∆T of a reaction on the composition.8 200 Methanol *) at 100°C 54. 1. (atmospheric) the pressure transducer (20). Tab.5 180 Methylpropionat 13. 3 shows. To start an experiment. air is introduced into the heated autoclave from its supply (14) to the desired pressure (21) controlled by pneumatic valves (17) and (18).4 180 Acetone Compound 13 UEL in % by vol.3 200 Cyclohexane 10.5 39.4 200 Tab.4 40. 16. that the pressure increase after ignition is usually rather weak. Incomplete mixing of the evaporating fuel with the air in the autoclave may play some role.0*) 26.1 42. These high fuel concentrations are far beyond the UEL measured under atmospheric conditions. 2 . In the case of simple organic molecules the lowest values have always been found at very fuel rich mixtures (high fuel/air ratios). The filled symbols indicate the runs regarded as ignition. The known shift of the UEL to higher values at high temperatures is also not sufficient to explain the possibility of an ignition at fuel concen- temperature of measurement in °C 16 UEL in % by vol. the UELs at 1 bar and at 10 bar and the maximum possible fuel concentration (due to limited vapour pressure) is displayed in detail for the example of n-propanol in Fig.7 180 n-Heptane 26. Reasons are the incompleteness of the oxidation and the high heat capacity of the fuel that remains unreacted. A consequence of the high fuel concentrations at AIT is. The results are presented in Tab. The mixture is homogenized by stirring for some minutes. however. 1 shows that the increase of the UEL with increasing pressure is in most cases even more dramatic than that due to the temperature increase.1 59.8*) 24.8 41.8 200 2-Propanol 14.2 22. 2: Diagram of the apparatus for measuring UELs Dependency of AIT on fuel/air ratio To find the AIT at a given pressure.5 180 Butanone 12.6*) 22. 4 and for hexane in Fig.5 180 Pentane 10. 1: UEL of some single compounds at elevated temperature and pressures of 10 bar 22 12 21 P 15 14 11 20 P 17 18 19 Fig. the present results are in accordance with these previous findings.7 200 Ethylacetat 12. Therefore for several compounds the influence of pressure on the UEL was explored. As Fig. They may be used to estimate the delay times for reactions at different pressures. An extreme example is displayed in Fig. maximum possible concentration and the range of autoignition for n-hexane E AZV ⋅ exp RT pn where EAZV is an apparent activation energy. 4:. it is nevertheless possible to obtain a straight line in an Arrhenius plot. This is demonstrated for four compounds in Fig. Autoignition temperatures The primary objective of the present work is to determine the autoignition temperatures at elevated pressures. Therefore it is expected that due to the lower ignition temperatures remarkably longer ignition delay times can be observed at higher pressures. 7: Representative Arrhenius plots for ignition delay times with a first order dependency on pressure Apparent activation energies can be calculated from the slopes of the lines in Fig. 6. however. some time will pass between the admission of the fuel and the actual occur rence of the explosion. however. maximum possible concentration and the range of autoignition for n-propanol Temperature in ° C 40 R a n g e o f a u toig n itio n a t 1 0 ba r 14 260 13 240 12 11 220 10 E xp lo sio n ra ng e at 1 0 b a r fu e l con ce ntra tio n in % by vo lu m e L E L a t 1 b a r a n d 1 0 b ar m a x. They are therefore excluded from Fig. Under atmospheric conditions the standards require a waiting time of 5 min which is usually sufficient to avoid the possibility of overlooking an explosion due to a very high ignition delay time. 7. the ignition delay is not only influenced by chemical factors (reaction rates).Relation between the UEL at 1 bar and 10 bar. As they are composed of several factors. In contrast. be shown to follow an Arrhenius-like relation /4/: 1000 100 10 200 250 300 350 Temperature in ° C 400 450 500 550 Fig. They include a number of different groups of organic compounds such as 3 . 7.50 Explosion range at 10 bar 40 fuel concentration in % by vol. at 10 bar 30 20 10 Explosion range at 1 bar 0 0 50 100 150 200 diffusion rates or the time to heat up the cold injected liquid. 2 and compared to the values measured at atmospheric pressure with a standard DIN or ASTM apparatus. Range of autoignition at 10 bar max.5 bar and 197°C. concentration at 10bar UEL at 10 bar Upper explosion limit at 1 bar LEL at 1 and 10 bar autoignition temp. but also by physical factors like vaporisation and τ ~ k tZV *pZ in bar*sec Ignition delay times Due to limited reaction rates. For determination of the AIT it is therefore necessary to wait for some time before the outcome of a run can be regarded as "no ignition".co n ce n tra tio n at 1 0 b a r UEL at 1 bar UEL at 10 bar A IT a t 1 0 b a r 15 30 20 10 Pressure in bar 50 280 200 9 180 0 500 1000 1500 Time (from start of injection) in s 8 2500 2000 Fig. not expected to agree well with activation energies calculated or measured by different methods. 5: Relation between the UEL at 1 bar and 10 bar. concentration at 1 bar max. 6 where it takes more than 35 min (2100 s) for a 50%-benzene/50%-hexane mixture to ignite at a pressure of 13. c o n ce n tra tio n a t 1 b a r m a x. delay times observed at 1 bar with the standard apparatus do not fit on the line but are consistently longer than expected from an extrapolation from the high pressure values. The data taken for pressures from 2 bar to 15 bar all fall on a single line if a first order dependence on pressure is assumed. As ignition delay times are closely related to reaction rates. 250 300 16 pressure temperature at the centre of autoclave temperature at the top of autoclave T / °C Fig. As most of these factors have an Arrhenius-like dependency on temperature similar to the reaction rate. they are. As can be also seen from Fig. they can. 7. 6:Autoignition of a benzene/hexane mixture at 197°C: Igniton delay time > 35 min E xp lo sio n ran g e a t 1 b a r 10000 0 0 50 100 150 benzene butyl amine cyclohexanone propionic acid 200 T /°C Fig. The results obtained so far for pure compounds are summarised in Tab. In general.5 l compared to 0. The temperature of autoignition drops. Both the temperature and the pressure criterion for ignition may be stricter than the visual criterion used with the standard apparatus. 2. The order of the compounds with respect to the AIT at higher pressures is different from the one at atmospheric pressure. as expected. the experimental values fall well on straight lines. the relation between the pressure pZ of a fuel/air-mixture and its autoignition temperature TZ is described by the relation: 2 ( +1) E ASem ) RTZ where EASem is an apparent activation energy of the reaction and n is the overall reaction order (usually assumed to be 2).2 l in the standard apparatus) is known to decrease the ignition temperatures. from which apparent activation energies in the range 100 kJ/mol to 350 kJ/mol can be calculated. which is not possible in an open device. The closed vessel may make ignitions easier. Tab. pZ = k ⋅ Tz n ⋅ exp( pZ/T 2Z in bar/K² 1E-4 1E-5 n-Heptane Benzene Ethanol Propionic acid Methyl propionate Butyl acetate 1E-6 200 250 300 350 Temperature in ° C 400 450 500 550 Fig. The following conclusions can be drawn from this table: 1. however. substantially with increasing pressure.10 bar for several pure substances Fig. The larger vessel volume (0. esters and amines.hydrocarbons. 8). An exception is. In some times a small pressure increase was observed to precede ignition. 8 gives the so-called Semenoff plots for a number of selected pure compounds. 2: Autoignition temperatures of several pure compounds at elevated pressures compared to the standard values Autoignition temperature in °C at Compound Semenoff plots According to Semenoff's theory of thermal explosion /1/.5 bar 210 216 . * 4 1 bar 2 bar 5 bar 10 bar n-Hexane 230 235 210 197 n-Heptane 220 201 197 190 n-Octane 215 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 = value at 2. Apart from a possible switch in reaction mechanism (low temperature/high temperature) differences in experimental conditions may cause this deviation: 1. 8: Semenoff plots for 1 . 2. 3. 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. ketones. N.2. M. Princeton Univ. and a further decrease of AITs with increasing pressure will not be observed. 1959. the ignition delay times follow an Arrheniuslike relation leading sometimes to very long delays near AIT at high pressures. Therefore the AIT will stop to decrease further. and Cammenga. Gödde. for example with the present autoclave. Similarily. 331 pp. But the higher the total pressure of the fuel/air mixture. 48(1928). 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.: Some Problems in Chemical Kinetics and Reactivity.Phys. Wirtschaftsverlag NW Verlag für Neue Wissenschaft. its vapour pressure curve also must be considered. and the depression of the AIT is expected to stop. Bremerhaven 1998 4.: Zündtemperaturen homologer Reihen Teil 2. Gödde. 437 5 .: Zur Theorie des Verbrennungsprozesses. Z. The pressure dependence of the autoignition temperature has been shown to meet a Semenoff relation. Gödde. M. Due to the rise of the UEL with pressure. N. When discussing the AIT and the UEL of a liquid. Similarily with respect to AIT a point will be reached when increasing the pressure where the fuel vapour pressure limits the mixture concentration. Conclusions For a number of pure compounds. 5. IEC 60079-4: Electrical apparatus for explosive gas atmospheres.N. For financial support we thank the Hauptverband der gewerblichen Berufsgenossenschaften References 1. Möller. the higher is also the partial pressure of the fuel required to reach the most critical fuel concentration. Semenoff. A similar mechanism has been employed to explain the reincrease of the standard AIT with chain length for very long chain molecules /5/. Press. At some temperature the required partial pressure will become equal to the fuel vapour pressure. 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. Brandes. W. Part 4: Method of test for ignition temperature 3. PTBMitteilungen 108(1998). the pressure dependence of autoignition temperatures and the upper explosion limits at a temperature of 200°C have been determined. No special mechanism for the ignition at AIT needs to be applied. H. That means it is not possible to exceed the UEL. G Riesner and J. The results show that mostly the explosion range widens dramatically with increasing pressure at the UEL.The Semenoff plots can be used to estimate the autoigniton temperatures at even higher pressures. Acknowledgements We thank M. although the most sensitive fuel concentrations in AIT determination lie at very rich mixtures. This shows that the differences in experimental set-ups have substantial influence on the results. they are always well within the explosion range. v. Semenoff's equation seems to indicate that it is possible to lower the AIT to arbitrarily low values by increasing the pressure. K. It is therefore desirable to get AIT values for 1 bar under isochore conditions. This offers the possibility of calculating a ‘lowest possible AIT’ for the substance under consideration provided its vapour pressure curve is known. Therefore. At even higher pressures and lower temperatures it is impossible to reach the critical fuel concentration. Scheffler for carrying out the experiments and operating the autoclaves. PTB-Bericht ThEx-8. Rich mixtures that would be necessary to find the AITs predicted by Semenoffs relation cannot be obtained. 571 2. which can be used to predict the AITs at pressures.: Zündtemperaturen organischer Verbindungen in Abhängigkeit von chemischer Struktur und Druck. Semenoff. E.