Autoignition temperatures of flammable liquids in closed vessels

Transcription

1 Autoignition temperatures of flammable liquids in closed vessels PAPP Christian *), BRANDES Elisabeth *), HIRSCH Werner *), MARX, Marcus **) *) Physikalisch-Technische Bundesanstalt, Braunschweig **) Otto-von-Guericke-Universität, Magdeburg Oral presentation by Ch. Papp at IXth ISPHMIE, Cracow 2012 ABSTRACT Investigations on the autoignition temperature carried out in closed vessels (isochoric conditions) result in some cases in remarkably lower ignition temperatures at of 1 bar compared to the autoignition temperatures (AIT) determined by standardized determination methods (e.g. EN or IEC ). These standardized methods use an open vessel (isobaric conditions) and, therefore, ambient pressure conditions (around 1 bar) are implicit. The standardized methods use a visible flame as ignition criterion. For our determinations in a closed vessel the ignition criterion was the signal of a photodiode, a steep temperature increase and/or a steep pressure increase. The investigations result in a number of substances having a standard AIT between 300 C and 450 C especially esters and ketones showing a significant decrease of the ignition temperature in the closed vessel compared to the open vessel of the standardized methods. It is shown that such ignition temperatures in a closed vessel correlate with cool flame temperatures as determined in a slightly modified standard apparatus. A screening procedure based on the standardized determination methods is described as well to identify such substances. 1. INTRODUCTION The standardized methods to determine the autoignition temperature (AIT) use flasks open to the atmosphere (DIN (2003) [1], EN (2005) [2], IEC (2012) [3] a 200ml Erlenmeyer flask, ASTM E659 (2005) [4] a 500ml round bottom flask), but industrial applications are often realized under conditions which are more comparable to a closed vessel. Furthermore, a lot of industrial processes are carried out under non-atmospheric conditions. Although the standardized methods aim first of all on the classification of flammable vapours and gases with respect to explosion protected equipment, AITs determined by standardized methods are also often used as a safety characteristic to prevent accidents due to autoignition. To determine the influence of industrial process conditions (in particular elevated pressures), autoignition temperatures were measured in a closed vessel [5, 6]. In addition to the pressure influence on autoignition temperatures, these investigations show for some substances a significant difference between the autoignition temperature measured in the closed vessel at 1 bar and the AIT determined by a standardized method in an open Erlenmeyer flask. For such substances, the ignition temperatures in the closed vessel can be up to 200 K lower. The ignition temperatures measured in a closed vessel at 1 bar whether they differ from the standard AITs or not fit well the respective Semenov correlation [7] which describes the pressure dependence of the autoignition temperature [5]. This fact is shown in figure 1 for some substances. These results raised two questions: 1. What are the reasons for such differences? 2. Is it possible to have a trustable screening procedure to identify substances showing such a behavior?

2 ln(p/t z 2 ) / (bar/k 2 ) This paper deals mainly with the second question. The affected substances or substance classes as known so far are reported below. The aim of the current investigation was to develop a screening procedure based on the standardized methods which would need as few modifications as possible ,4-Dioxane i-butanal p-xylene Cyclohexanone AIT AIT AIT AIT -14 0,0008 0,0012 0,0016 0,0020 0,0024 0,0028 0,0032 Figure 1: Semenov correlations of the autoignition temperature. 1/T z / K EXPERIMENTAL 2.1 Determinations in a closed vessel Ignition temperatures in the closed vessel were determined in a 320 ml autoclave made of stainless steel. The vessel was equipped with a pressure transducer and two 0,5 mm thermocouples. One thermocouple was positioned in the middle of the vessel, the other near the lid of the vessel. To facilitate the comparison with the standardized methods which use an optical ignition criterion, a photodiode was added via a quartz glass rod on top of the vessel. While for the temperature and pressure recordings a sampling rate of 10 values per second was used, the photodiode output had been recorded with 100 samples per second to ensure that even short light impulses are recorded. In our former investigations a steep temperature rise of at least 50 K or a steep pressure rise of at least 10% was fixed as a criterion for an ignition in a closed vessel, because this corresponds for most substances with a light emission detected by the photodiode. The procedure to determine the autoignition temperatures is as follows: The vessel which is heated to the desired test temperature is evacuated to about 10 mbar. The amount of air necessary for the desired concentration of the fuel air mixture is then fed to the vessel via an electromagnetic valve. After reaching the desired air pressure in the vessel, the valve closes and the liquid fuel is introduced in portions via a second valve. The concentrations are calculated based on the ideal gas law. For a maximum of 30 minutes it is observed whether an ignition occurs or not. Vessel temperature and concentration of the fuel air mixtures were varied till just no ignition was detected. The lowest temperature at which at least at one concentration an ignition occurred is taken as ignition temperature in the closed vessel. 2.2 Modifications of the standardized determination methods The major differences between the closed vessel and the open flask are shown in Table 1. Studies which focus on the influence of different wall materials [8, 9] result in differences smaller than the differences between the AIT and the ignition temperature in a closed vessel. This allows the conclusion that the different wall materials are mostly not responsible for the ignition temperature decrease and are therefore not checked with respect to a possible screening procedure.

3 However, the differences in gas exchange as well as cool flame phenomena were considered as a possible basis for a screening procedure. Table 1: Major differences between the standardized AIT determination method and the determination of the ignition temperature in a closed vessel Standardized determination Isobaric conditions Gas exchange with the surrounding air via the neck Vessel made of glass Determination in a closed vessel Isochoric conditions No gas exchange Vessel made of stainless steel Gas exchange Driven by natural convection, fuel is transported outside the open standard Erlenmeyer flask during the ignition experiment, while cold air from the surroundings flows back into the flask. Also hot spots which are the starting point for ignition may be influenced by this convection. As a result of such convection processes, the fuel is reduced over time. To check the influence of such a gas exchange on the ignition, different neck widths of the flask were tested. One flask was equipped with a metal ring placed inside the neck opening at the top of the flask to reduce the width of the neck to 1 cm. Also a flask with a neck width of 4.5 cm was tested. The standardized flask has a neck width of 3 cm (Figure 2). flask according to standards narrow-neck flask wide-neck flask Figure 2: Scheme of different types of flask opening The ignition temperature determinations were carried out in accordance with EN In addition, the gas exchange was simulated with ANSYS CFX Version 12 for water vapour as an example (Figure 3). Figure 3: Average mass fraction over time, calculated in a CFD simulation

4 Ignition temperature in a closed vessel at 1 bar / C Cool flame phenomena To cover the cool flame phenomena, a standardized apparatus was equipped with four additional 0,5 mm thermocouples. Two of them were located at the bottom, one was centered at half height of the flask, and the fourth thermocouple was positioned at the lower end of the neck. For flame detection, a single lens reflex (SLR) camera equipped with an additional infrared filter was placed over the opening of the flask. The exposure of the camera is started by the control software when a temperature higher than the wall temperature is detected by one of the additional thermocouples inside the flask. The temperature measurement was performed with up to 30 samples per second. The procedure for the ignition temperature determinations was in accordance with EN Investigated substances The investigated substances were mainly alkanes, alcohols, amines, esters and ketones in order to take different substance classes into account. 3. RESULTS 3.1 Ignition temperatures in a closed vessel Figure 4 compares the ignition temperatures in the closed vessel (ignition criterion: T > 50 K or p > 10% or light emission) to the respective AIT. Some substances show a remarkable difference (> 50 K) between the ignition temperature in the closed vessel and the standardized AIT. Those are marked as in Figure 4. The AIT values are taken from the database CHEMSAFE [10] K difference 50 K difference 100 K difference 200 K difference Figure 4: Differences between the AIT and the ignition temperature in the closed vessel AIT / C Mainly substances which have an AIT between 300 C and 450 C are affected by such an ignition temperature drop. Most of these substances are ketones and esters, whereas n-alkanes and alcohols having an AIT in this temperature range showed no or just a small decrease in ignition temperature (< 50K). 3.2 Tests with flasks of different neck widths Some results of the tests with different neck widths are given in Table 2. In flasks with a wider neck, the autoignition temperature is much higher than in the flask used in the standardized apparatus. One of the main reasons is the higher temperature gradient inside the flask in this case. Also the degree of the gas exchange with the surroundings is much higher compared to the standardized flask (see Fig. 3). Reducing the diameter of the neck causes a decrease of the AIT but not as significantly as the wider neck causes an increase. As can be seen from Table 2, the effect of reducing the width of the neck on the ignition temperature is not distinct enough to develop a screening procedure for the substances which ignite at

5 Temperaturanstieg in K Temperature increase in K remarkably lower temperatures in closed vessels. Table 2: Influence of the neck width of the Erlenmeyer flask on the auto ignition temperature Fuel Measured ignition temperatures* Flask n-propanol Heptanone-3 Ethyl hexanoate Closed vessel 385 C 205 C 253 C Standard flask 385 C 408 C 395 C Wide-neck flask 441 C 444 C 437 C Narrow-neck flask 380 C 385 C 365 C *Temperatures are not rounded as it would be required by standards. 3.3 Tests with a flask equipped with additional thermocouples and a camera The criterion for the ignition in the open flask is a visible flame. In case of ignition with a visible flame, a steep and high temperature increase (> 200 K) is detected by all additional thermocouples. Lowering the test temperature below the AIT showed for some substances still a temperature increase of more than 50 K, which is mostly only detected by the thermocouples near the bottom of the flask. Only a bale blue light was visible by eyes. However, along with such a temperature increase a pale blue light could be recorded by the digital camera (time exposure mode up to 60 seconds). The slow temperature rise of not more than 7 K/s, as recorded by the thermocouples, indicates that this effect may be linked to a cool flame reaction inside the flask. In general, temperature increases of less than 50 K have not been accompanied by a pale blue light. It is important to note that in some cases such a temperature increase detected by the thermocouples and accompanied by a pale blue light is found only where the two ignition regimes (cool flames, hot ignition) are separated by a temperature range with no obvious reactions. This fact, known as negative temperature coefficient (NTC) [11], is shown for butyl butyrate in figure 5, as an example Figure 5: Negative temperature coefficient of butyl butyrate Kolbentemperatur in C Flask temperature in C Remarkable differences between AIT and the cool flame temperatures are so far found mainly for esters and ketones with an AIT in the range between 300 C and 450 C. 1,4-dioxane, i-pentane i- hexane and di-isopropylether show a similar behaviour. Comparing the ignition temperatures in the closed vessel with these cool flame temperatures in the Erlenmeyer flask shows that both temperatures correlate well. There are only slight differences (maximum to 20 K) between these temperatures. As could be seen from figures 6 and 7 as well, esters and ketones with very short chains and such with long chains (e. g. ketones with a C-chain

6 temperature in C temperature in C length > 8) show no significant difference between the AITs and the ignition temperatures in the closed vessel. Other substances tested like n-alkanes, alcohols and amines show no or smaller differences in ignition temperatures than those shown in figures 6 and 7 (see Table 3). In addition to that, in such cases a cool flame reaction cannot be found inside the Erlenmeyer flask with the method mentioned above AIT in Erlenmeyer flask AIT in closed vessel cool flame temperature Methyl butyrate Propyl acetate Ethyl valerate Propyl propionate Ethyl butyrate Propyl butyrate Ethyl formate Methyl valerate Butyl butyrate Butyl acetate Ethyl hexanoate Amyl acetate Hexyl acetate Figure 6: AIT, ignition temperature in a closed vessel and cool flame temperature of the investigated esters AIT in Erlenmeyer flask AIT in closed vessel cool flame temperature Butanone Pentanone-3 Pentanone-2 Heptanone-4 Hexanone-2 Heptanone-3 Heptanone-2 Nonanone-2 Octanone-3 Decanone-2 Figure 7: AIT, ignition temperature in a closed vessel and cool flame temperature of the investigated ketones This means that the determination of the cool flame temperature with the aid of an additional thermocouple fitted to the standardized vessel is suitable for identifying such substances that show a significant difference between the AIT and the closed vessel ignition temperature (see Table 3 for examples). A camera as an additional sensor for cool flames is not necessary, because in almost any case in the cool flame regime the pale blue light was accompanied by a temperature increase of more than 50 K but less than 150 K which is easily detectable via thermocouples. According to literature [12] a cool flame can be the first part of a two-stage-ignition.

7 Table 3: Cool flame temperature and AIT of several substances Substance Method / Ignition temperatures measured in C Standard apparatus Cool flame detection Closed vessel Heptanone Hexanone Nonanone Heptanone Octanone Ethyl hexanoate Heptanone ,4-Dioxane Pentanone n-pentane Ethyl valerate Amyl acetate n-butyl butyrate Di-n-butylamine i-octane Methyl valerate Propyl acetate i-pentane Propyl butyrate Butyl amine Butyl acetate Propyl propionate Ethyl butyrate Pentanol Hexanol Pentanone Ethyl propionate Pentanol ,2-Propandiol Butanol Butanone ,2-Hexandiol Ethyl formate Ethanol Methyl butyrate Under isochoric conditions a cool flame might be able to turn into a hot ignition for such substances. Such a turn over is indicated by significantly steeper pressure and temperature rises compared to the temperature rises which occurred with cool flames in the Erlenmeyer flask at comparable temperatures. An example is shown in Figure 8 for 2-heptanone at a temperature of 256 C, which is clearly below the AIT of 305 C. As can be seen, the temperature and pressure increase in the closed vessel is very sharp. The maximum temperature rise > 250 K/s can be observed which is accompanied by a bright light emission detected by the photodiode ( Voltage ).

8 Temperature at the top of the vesssel in C Pressure in mbar Voltage Temperature Pressure Voltage Figure 8: Ignition of 3-heptanone at 1 bar in a closed vessel below its AIT of 305 C time in seconds 4. CONCLUSIONS It is shown that the ignition temperature in the closed vessel correlates to the cool flame temperature which can also be determined in a standard Erlenmeyer flask open to the atmosphere. The isochoric conditions which exist in a closed vessel seem to cause a hot ignition even if the cool flame temperature is much lower than the AIT. This is of high interest for industrial processes. The current European standard EN does not explicitly specify the cool flame temperature as a separate value to be determined, whereas American standard covers the determination of such values. An implementation of the determination of the cool flame temperature into the European standards seems to be useful. For this purpose the test apparatus should be equipped with a thermocouple placed near the bottom of the flask. This would be the only modification of the current standard apparatus necessary to identify substances with a remarkably lower ignition temperature in closed vessels. LITERATURE 1. DIN (2003): Prüfung von Mineralölkohlenwasserstoffen, Bestimmung der Zündtemperatur, Beuth, Frankfurt am Main 2. EN (2005): Determination of the auto ignition temperature of gases and vapours. 3. IEC (2012): Material characteristics for gas and vapour classification - Test methods and data 4. ASTM E659 (2005): Standard Test Method for Autoignition Temperature of Liquid Chemicals. 5. E. Brandes, W. Hirsch, W. Möller (2008): Autoignition temperatures of binary Mixtures at elevated pressures, International Symposium on Hazard, Prevention and Mitigation of Industrial Explosions, pp , St. Petersburg 6. E. Brandes, W. Hirsch (2007): Zündtemperaturen binärer Gemische bei erhöhten Ausgangsdrücken, 11. BAM/PTB-Kolloquium zu Fragen der chemischen und physikalischen Sicherheitstechnik, pp Semenov, N., (1928): Zur Theorie der Verbennungsprozesses, Zeitschrift für Physik, Volume 48, pp Kaescher-Krischer, B. & Wagner, H., (1958): Die Zündungs von Brennstoff-Luft-Gemischen an heißen Oberflächen. Brennstoff-Chemie, 39(3/4), pp Frank, C. & Blackham, A., (1952): Spontaneous Ignition of Organic Compunds. Ind.Eng.Chem., 44(4), pp Physikalisch-Technische-Bundesanstalt, Bundesanstalt für Materialforschung- und prüfung, Gesellschaft für Chemische Technik und Biotechnologie e.v.(2011): CHEMSAFE-Database, Frankfurt am Main

9 11. Fish, A., Read, I., Affleck, W. & Haskell, W (1969): The controlling role of cool flames in two-stage ignition. Combustion and Flame, 13(1), pp , 12. Barnard, J., Watts, A. (1969):. Cool-flame oxidation of ketones. Symposium (International) on Combustion, 12(1), pp Note: The authors would like to thank BG RCI (Berufsgenossenschaft Rohstoffe und chemische Industrie) for financial support.