Autoignition Temperatures for Mixtures of Flammable Liquids with ...

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