Autoignition Temperatures for Mixtures of Flammable Liquidswith Air at Elevated Pressures

by Elisabeth Brandes, Werner Hirsch* and Thomas StolzPhysikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany

AbstractThe autoignition temperature (AIT) of pure compounds has been measured at pressures between 2 bar and 15 bar ina 0.5 l autoclave. The AITs are found to decrease substantially with increasing pressure, following a Semenoffrelation allowing extrapolation of the AIT to higher pressures. The ignition delay times follow an Arrhenius-likerelation and may become very long at higher pressures. For some compounds both the AITs and the delay times, the1 bar values found with the standard apparatus do not fit well to the respective relation, pointing to a profoundinfluence of the experimental differences. Although very high fuel concentrations are required to find the minimumAIT, they remain within the explosion range as the Upper Explosion Limit is shown to shift dramatically to highervalues with increasing temperature and pressure.

* Corresponding author: werner.hirsch@ptb.deProceedings of the European Combustion Meeting 2005

IntroductionToday, many industrial processes are operated at

elevated pressures. It is known that the AIT of afuel/air-mixture drops with increasing pressure, a factexplained theoretically by Semenoff /1/ as early as1928. Since then, a lot of work has been done to deter-mine AITs at elevated pressure (see, for example, thework of Gdde /3/ and references cited therein). Never-theless, the safety characteristics of many technicallyimportant substances are still poorly known under theseconditions. The present work deals with the determina-tion of autoignition temperatures (AITs) of severalsingle compounds under high pressures.

Even less knowledge exists about the Upper Explo-sion Limit (UEL) at high pressures. A second series ofexperiments was set up therefore to determine the UELat elevated pressure and a temperature near their AITfor some of the pure compounds investigated.

Experimental set-up

AITUnder atmospheric conditions, the AIT of a liquid is

usually determined by dropping the liquid into a pre-heated Erlenmeyer flask, ignition being indicated by theoccurrence of a visible flame /2/. Measurements atelevated pressures require a closed reaction vessel asshown in Fig. 1. We use a 0.5 l autoclave (1). Ignition isindicated either by a temperature rise (observed by twothermocouples (6) within the vessel) or by the pressurerise observed by the pressure transducer (10). For anexperiment, air is first introduced into the autoclavefrom its supply (4), regulated to the desired pressure bythe pneumatic valves (7), (8), and (9), then the liquid isfed into the vessel from its supply (2) by a HPLC pump(3). The resulting pressure is taken as the startingpressure. As with the standard apparatus, the fuel is

admitted as a liquid and evaporates in the reactionvessel. However, the reaction does not take place underisobaric conditions like in IEC 60079-4 but underisochore conditions.

The present experiments covered the range from2 bar to 10 bar total pressure. In some cases, AITs werealso obtained for 1.5 bar or 15 bar. The fuel concentra-tions usually varied between 15% by vol. and around40% by vol..

Fig. 1: Diagram of the apparatus for measuring AITs

UELUELs at 10 bar and 180C or 200C were deter-

mined in a separate 9 l autoclave (11) (Fig. 2) heateduniformly by a thermofluid jacket (15) equipped with aconventional capacitive spark igniter (19). Ignition isindicated either by a temperature rise > 50 K (observedby thermocouples (16) within the vessel) or by thepressure rise > 5% of the starting pressure observed by

2the pressure transducer (20). To start an experiment, airis introduced into the heated autoclave from its supply(14) to the desired pressure (21) controlled by pneu-matic valves (17) and (18), then the liquid is fed into thevessel from its supply (12) by a HPLC pump (13) usinga nozzle which generates a very fine spray. The mixtureis homogenized by stirring for some minutes. Theresulting pressure is taken as the starting pressure.

Fig. 2: Diagram of the apparatus for measuring UELs

Dependency of AIT on fuel/air ratioTo find the AIT at a given pressure, the fuel/air ratio

also has to be varied. In the case of simple organicmolecules the lowest values have always been found atvery fuel rich mixtures (high fuel/air ratios). As Fig. 3shows, the present results are in accordance with theseprevious findings.

Fig. 3: Dependency of the temperature rise T of areaction 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. Incom-plete mixing of the evaporating fuel with the air in theautoclave may play some role, but can account onlypartly for that result. The known shift of the UEL tohigher values at high temperatures is also not sufficientto explain the possibility of an ignition at fuel concen-

trations of 25% and higher. Therefore for several com-pounds the influence of pressure on the UEL wasexplored. The results are presented in Tab. 1.

Tab. 1: UEL of some single compounds at elevated temperatureand pressures of 10 bar

Co

mpou

nd

UEL in

% by v

ol.

(atmo

spheric)

UEL in

% by v

ol.

at 10 b

ar

temp

erature of

measu

rement in

C

1-Propanol 28.8 41.8 2002-Propanol 14.5 39.3 200

Cyclohexane 10.5 39.6 200n-Hexane 22.1 42.7 180

n-Heptane 26.4 40.5 180Pentane 10.7 44.4 180Acetone 16.2 22.5 180

Butanone 12.6*) 22.5 180Methylpropionat 13.0*) 26.7 200

Ethylacetat 12.8*) 24.6 200Ethanol 36.4 52.8 200

Methanol 54.1 59.4 200*) at 100C

Tab. 1 shows that the increase of the UEL withincreasing pressure is in most cases even more dramaticthan that due to the temperature increase. This is in realcontrast to the behaviour of the Lower Explosion Limit(LEL) which is known to be nearly independent ofpressure at least for pressures up to 5 bar. The relationbetween the range of autoignition, the UELs at 1 barand at 10 bar and the maximum possible fuelconcentration (due to limited vapour pressure) is dis-played in detail for the example of n-propanol in Fig. 4and for hexane in Fig. 5. In both cases the lowesttemperature of ignition at 10 bar is reached at fuelconcentrations above the UEL at 1 bar at the sametemperature.

A consequence of the high fuel concentrations atAIT is, however, that the pressure increase after ignitionis usually rather weak. Reasons are the incompletenessof the oxidation and the high heat capacity of the fuelthat remains unreacted. Near the AIT the reaction alsooften does not proceed through the whole mixture.Therefore near AIT the pressure only rises by a factor of2 or less.

T P

P

12

13

1415

16

1718

19

2122

11

20

3Fig. 4:.Relation between the UEL at 1 bar and 10 bar, maximumpossible concentration and the range of autoignition for n-propanol

Fig. 5: Relation between the UEL at 1 bar and 10 bar, maximumpossible concentration and the range of autoignition for n-hexane

Ignition delay timesDue to limited reaction rates, some time will pass

between the admission of the fuel and the actual occurrence of the explosion. For determination of the AIT itis therefore necessary to wait for some time before theoutcome of a run can be regarded as "no ignition".

Under atmospheric conditions the standards requirea waiting time of 5 min which is usually sufficient toavoid the possibility of overlooking an explosion due toa very high ignition delay time. As ignition delay timesare closely related to reaction rates, they can, however,be shown to follow an Arrhenius-like relation /4/:

where EAZV is an apparent activation energy. Thereforeit is expected that due to the lower ignition temperaturesremarkably longer ignition delay times can be observedat higher pressures. An extreme example is displayed inFig. 6 where it takes more than 35 min (2100 s) for a50%-benzene/50%-hexane mixture to ignite at apressure of 13.5 bar and 197C.

As can be also seen from Fig. 6, the ignition delay isnot only influenced by chemical factors (reaction rates),but also by physical factors like vaporisation and

diffusion rates or the time to heat up the cold injectedliquid. As most of these factors have an Arrhenius-likedependency on temperature similar to the reaction rate,it is nevertheless possible to obtain a straight line in anArrhenius plot. This is demonstrated for four com-pounds in Fig. 7. The data taken for pressures from2 bar to 15 bar all fall on a single line if a first orderdependence on pressure is assumed. In contrast, delaytimes observed at 1 bar with the standard apparatus donot fit on the line but are consistently longer thanexpected from an extrapolation from the high pressurevalues. They are therefore excluded from Fig. 7.

Fig. 6:Autoignition of a benzene/hexane mixture at197C: Igniton delay time > 35 min

Fig. 7: Representative Arrhenius plots for ignition delay times with afirst order dependency on pressure

Apparent activation energies can be calculated fromthe slopes of the lines in Fig. 7. They may be used toestimate the delay times for reactions at different pres-sures. As they are composed of several factors, they are,however, not expected to agree well with activationenergies calculated or measured by different methods.

Autoignition temperaturesThe primary objective of the present work is todetermine the autoignition temperatures at elevatedpressures. The results obtained so far for pure com-pounds are summarised in Tab. 2 and compared to thevalues measured at atmospheric pressure with a stan-dard DIN or ASTM apparatus. They include a numberof different groups of organic compounds such as

RTE

pk ZVAn

exp~

200 250 300 350 400 450 500 550

10

100

1000

10000

t ZV*

p Z in

ba

r*se

c

Temperature in C

benzene butyl amine cyclohexanone propionic acid

0 500 1000 1500 2000 2500180

200

220

240

260

280

300

Time (from start of injection) in s

Tem

pera

ture

in

C

8

9

10

11

12

13

14

15

16

Pre

ssure

in b

ar

pressure temperature at the centre of autoclave temperature at the top of autoclave

0 50 100 150 2000

10

20

30

40

50R ange o f au to ign itiona t 10 ba r

Expl

osio

n r

an

ge a

t 10

bar

fuel

con

cent

ratio

n in

% b

y vo

lum

e

T / C

LE L a t 1 ba r and 10 bar m ax. c oncen tra tion a t 1 ba r m ax.concen tra tion at 10 ba r U E L a t 1 ba r U E L a t 10 ba r A IT a t 10 ba r

E xp los ion range a t 1 ba r

0 50 100 150 200 2500

10

20

30

40

50Range of autoignitionat 10 bar

Expl

osi

on

ra

nge

at 1

0 ba

r

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

fue

l co

nce

ntra

tion

in %

by

vol.

T / C

Explosion range at 1 bar

4hydrocarbons, ketones, esters and amines. Thefollowing conclusions can be drawn from this table:1. The temperature of autoignition drops, as expect-

ed, substantially with increasing pressure.2. The order of the compounds with respect to the

AIT at higher pressures is different from the one atatmospheric pressure.

Semenoff plotsAccording to Semenoff's theory of thermal explo-

sion /1/, the relation between the pressure pZ of afuel/air-mixture and its autoignition temperature TZ isdescribed by the relation:

where EASem is an apparent activation energy of thereaction and n is the overall reaction order (usuallyassumed to be 2).

Fig. 8: Semenoff plots for 1 - 10 bar for several pure substancesFig. 8 gives the so-called Semenoff plots for a num-

ber of selected pure compounds. In general, the experi-mental values fall well on straight lines, from whichapparent activation energies in the range 100 kJ/mol to350 kJ/mol can be calculated.

An exception is, however, some of the 1 bar valuesmeasured with the standard apparatus which are muchhigher than expected from extrapolation of the valuesmeasured in the high pressure autoclave (open symbolsin Fig. 8). Apart from a possible switch in reactionmechanism (low temperature/high temperature) differ-ences in experimental conditions may cause this devia-tion:1. The larger vessel volume (0.5 l compared to 0.2 l in

the standard apparatus) is known to decrease theignition temperatures.

2. Both the temperature and the pressure criterion forignition may be stricter than the visual criterion usedwith the standard apparatus.

3. The closed vessel may make ignitions easier. Insome times a small pressure increase was observedto precede ignition, which is not possible in an opendevice.

Tab. 2: Autoignition temperatures of several pure compounds atelevated pressures compared to the standard values

Autoignition temperature in C atCompound 1 bar 2 bar 5 bar 10 bar

n-Hexane 230 235 210 197n-Heptane 220 201 197 190

n-Octane 215 210Cyclohexane 246 245 225 215

Benzene 565 526* 470 451Toluene 535 - 457 261Dioxan 375 212 197 189

Methanol 440 300 260Ethanol 400 283 250

Propanol-1 385 300 265 240Butanl-1 325 292 255 240

Pentanol-1 320 250 240Hexanol-1 280 280 262 232

Acetone 525 350* 275 260Butanone-2 475 290 235 210

Pentanone-2 445 260 210 -Hexanone-2 420 196 187 -

Cyclohexanone 430 279 230 215Propionic acid 470 358 299 266

i-Butyric aldehyde 165 143 122Propionic aldehyde 190 108 98 93Methyl propionate 465 400 284 253

Ethyl formiate 440 312* 280 225Propyl propionate 445 315 251Butyl propionate 425 320 240i-Propyl acetate 425 296 241 245Methyl acetate 505 470 415 338

Ethyl acetate 470 380 260 230Propyl acetate 455 300 260 240

n-Butyl acetate 393 252 240 230t-Butyl acetate 450 395 370 310

n-Pentyl acetate 350 226i-Pentyl acetate 280 261* 240 224Methyl butyrate 445 400 256

n-Butyl amine 310 280 258 216* = value at 2.5 bar

)exp()12(

Z

SemAn

zZ TETkpR

=

+

200 250 300 350 400 450 500 5501E-6

1E-5

1E-4

p Z/T

2 Z in

ba

r/K

Temperature in C

n-Heptane Benzene Ethanol Propionic acid Methyl propionate Butyl acetate

5The Semenoff plots can be used to estimate theautoigniton temperatures at even higher pressures.Semenoff's equation seems to indicate that it is possibleto lower the AIT to arbitrarily low values by increasingthe pressure. But the higher the total pressure of thefuel/air mixture, the higher is also the partial pressure ofthe fuel required to reach the most critical fuelconcentration. At some temperature the required partialpressure will become equal to the fuel vapour pressure.At even higher pressures and lower temperatures it isimpossible to reach the critical fuel concentration, andthe depression of the AIT is expected to stop. A similarmechanism has been employed to explain the re-increase of the standard AIT with chain length for verylong chain molecules /5/.

ConclusionsFor a number of pure compounds, the pressure

dependence of autoignition temperatures and the upperexplosion limits at a temperature of 200C have beendetermined. The results show that mostly the explosionrange widens dramatically with increasing pressure atthe UEL. Therefore, although the most sensitive fuelconcentrations in AIT determination lie at very richmixtures, they are always well within the explosionrange. No special mechanism for the ignition at AITneeds to be applied.

The pressure dependence of the autoignition tem-perature 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 Arrhenius-like relation leading sometimes to very long delays nearAIT at high pressures.

When discussing the AIT and the UEL of a liquid,its vapour pressure curve also must be considered. Dueto the rise of the UEL with pressure, it will often beimpossible to find an UEL at elevated pressure at aspecified temperature as the limited vapour pressureprevents reaching the necessary fuel concentration. Thatmeans it is not possible to exceed the UEL. Similarilywith respect to AIT a point will be reached whenincreasing the pressure where the fuel vapour pressurelimits the mixture concentration. Rich mixtures thatwould be necessary to find the AITs predicted bySemenoffs relation cannot be obtained, and a furtherdecrease of AITs with increasing pressure will not beobserved. Therefore the AIT will stop to decreasefurther. This offers the possibility of calculating alowest possible AIT for the substance underconsideration provided its vapour pressure curve isknown.

The values found at 1 bar with the open standardapparatus often do not fit well to the Semenoff orArrhenius relations found in the autoclave experiments.This shows that the differences in experimental set-upshave substantial influence on the results. It is thereforedesirable to get AIT values for 1 bar under isochoreconditions, for example with the present autoclave.

Acknowledgements We thank M. Gdde, W. Mller, G Riesner and J.

Scheffler for carrying out the experiments and operatingthe autoclaves.

For financial support we thank the Hauptverband dergewerblichen Berufsgenossenschaften

References1. Semenoff, N.: Zur Theorie des Verbrennungsprozes-

ses, Z.Phys. 48(1928), 5712. IEC 60079-4: Electrical apparatus for explosive gas

atmospheres. Part 4: Method of test for ignition tem-perature

3. Gdde, M.: Zndtemperaturen organischer Verbin-dungen in Abhngigkeit von chemischer Strukturund Druck, PTB-Bericht ThEx-8, WirtschaftsverlagNW Verlag fr Neue Wissenschaft, Bremerhaven1998

4. Semenoff, N.N.: Some Problems in Chemical Kine-tics and Reactivity, Princeton Univ. Press, v.2, 1959,331 pp.

5. Gdde, M, Brandes, E. and Cammenga, H. K.:Zndtemperaturen homologer Reihen Teil 2, PTB-Mitteilungen 108(1998), 437

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