Autoignition Temperatures for Mixtures of Flammable Liquids with ...

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the pressure transducer (20). 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), 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. The mixture is homogenized by stirring for some minutes. The resulting pressure is taken as the starting pressure. 15 16 22 21 T P 18 11 19 Fig. 2: Diagram of the apparatus for measuring UELs Dependency of AIT on fuel/air ratio To find the AIT at a given pressure, the fuel/air ratio also has to be varied. In the case of simple organic molecules the lowest values have always been found at very fuel rich mixtures (high fuel/air ratios). As Fig. 3 shows, the present results are in accordance with these previous findings. Fig. 3: Dependency of the temperature rise ΔT of a reaction on the composition. The filled symbols indicate the runs regarded as ignition. These high fuel concentrations are far beyond the UEL measured under atmospheric conditions. Incomplete mixing of the evaporating fuel with the air in the autoclave may play some role, but can account only partly for that result. 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- P 20 17 13 12 14 trations of 25% and higher. Therefore for several compounds the influence of pressure on the UEL was explored. The results are presented in Tab. 1. 2 Tab. 1: UEL of some single compounds at elevated temperature and pressures of 10 bar Compound UEL in % by vol. (atmospheric) UEL in % by vol. at 10 bar temperature of measurement in °C 1-Propanol 28.8 41.8 200 2-Propanol 14.5 39.3 200 Cyclohexane 10.5 39.6 200 n-Hexane 22.1 42.7 180 n-Heptane 26.4 40.5 180 Pentane 10.7 44.4 180 Acetone 16.2 22.5 180 Butanone 12.6*) 22.5 180 Methylpropionat 13.0*) 26.7 200 Ethylacetat 12.8*) 24.6 200 Ethanol 36.4 52.8 200 Methanol 54.1 59.4 200 *) at 100°C Tab. 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. 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. The relation between the range of autoignition, 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. 4 and for hexane in Fig. 5. 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. A consequence of the high fuel concentrations at AIT is, however, that the pressure increase after ignition is usually rather weak. Reasons are the incompleteness of the oxidation and the high heat capacity of the fuel that remains unreacted. 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.
fuel concentration in % by vol. Fig. 4:.Relation between the UEL at 1 bar and 10 bar, maximum possible concentration and the range of autoignition for n-propanol fuel concentration in % by volume 50 40 30 20 10 0 50 40 30 20 10 0 max. concentration at 1 bar max. concentration at 10bar UEL at 10 bar Upper explosion limit at 1 bar LEL at 1 and 10 bar autoignition temp. at 10 bar Explosion range at 1 bar 0 50 100 150 200 250 LEL at 1 bar and 10 bar max. concentration at 1 bar max.concentration at 10 bar UEL at 1 bar UEL at 10 bar AIT at 10 bar Fig. 5: Relation between the UEL at 1 bar and 10 bar, maximum possible concentration and the range of autoignition for n-hexane Ignition delay times Due to limited reaction rates, some time will pass between the admission of the fuel and the actual occur rence of the explosion. 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". 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. As ignition delay times are closely related to reaction rates, they can, however, be shown to follow an Arrhenius-like relation /4/: ~ τ T / ° C Explosion range at 1 bar ⎛ ZV k ⎞ ⎜ E A ⋅ ⎟ n p ⎜ ⎟ ⎝ RT ⎠ exp Range of autoignition at 10 bar Range of autoignition at 10 bar 0 50 100 T / ° C 150 200 where EA ZV is an apparent activation energy. Therefore it is expected that due to the lower ignition temperatures remarkably longer ignition delay times can be observed at higher pressures. An extreme example is displayed in Fig. 6 where it takes more than 35 min (2100 s) for a 50%-benzene/50%-hexane mixture to ignite at a pressure of 13.5 bar and 197°C. As can be also seen from Fig. 6, the ignition delay is not only influenced by chemical factors (reaction rates), but also by physical factors like vaporisation and Explosion range at 10 bar Explosion range at 10 bar diffusion rates or the time to heat up the cold injected liquid. As most of these factors have an Arrhenius-like dependency on temperature similar to the reaction rate, it is nevertheless possible to obtain a straight line in an Arrhenius plot. This is demonstrated for four compounds in Fig. 7. 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. In contrast, 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. They are therefore excluded from Fig. 7. Temperature in ° C t ZV *p Z in bar*sec 3 300 280 260 240 220 200 180 8 0 500 1000 1500 2000 2500 Time (from start of injection) in s Fig. 6:Autoignition of a benzene/hexane mixture at 197°C: Igniton delay time > 35 min 10000 1000 100 10 pressure temperature at the centre of autoclave temperature at the top of autoclave benzene butyl amine cyclohexanone propionic acid 200 250 300 350 400 450 500 550 Temperature in ° C Fig. 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. 7. They may be used to estimate the delay times for reactions at different pressures. As they are composed of several factors, they are, however, not expected to agree well with activation energies calculated or measured by different methods. Autoignition temperatures The primary objective of the present work is to determine the autoignition temperatures at elevated pressures. The results obtained so far for pure compounds are summarised in Tab. 2 and compared to the values measured at atmospheric pressure with a standard DIN or ASTM apparatus. They include a number of different groups of organic compounds such as 16 15 14 13 12 11 10 9 Pressure in bar
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