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Distinction between the upper explosion limit and the lower cool flame limit in determination of flammability limit at elevated conditions
process safety and environment protection 8 7 ( 2 0 0 9 ) 47–52
Contents lists available at ScienceDirect
Process Safety and Environment Protection journal homepage: www.elsevier.com/locate/psep
Distinction between the upper explosion limit and the lower cool flame limit in determination of flammability limit at elevated conditions A.A. Pekalski a,∗ , H.J. Pasman b a b
Shell Global Solutions, Shell Technology Centre Thornton, PO Box 1, Chester CH1 3SH, United Kingdom TU Delft, Faculty of Applied Sciences, Multi-Scale Physics, Prins Bernardlaan 6, 2628 BW Delft, The Netherlands
a b s t r a c t Previous research showed that at certain conditions, close to the flammability range exists a regime where cool flame may develop either due to elevated temperature or it may be initiated by an ignition source. Propagation of the cool flame in a closed test vessel may double the initial pressure. Such pressure increase exceeds recommended ignition criteria for explosion limit determination that are based on 5 or 7% of pressure rise leading to inaccurate classification of the oxidation phenomena, i.e. cool flame propagation may be classified as hot flame propagation. Two mixtures were tested: n-butane-oxygen (extensively) and C1–C2–oxygen (in limited range), which represent a typical composition in ethylene oxide production, at elevated conditions at their upper explosion limits. Flame development was analysed by flame emission spectroscopy and the post-oxidation mixture was analysed by gas chromatography (GC) to characterise the oxidation mechanism of the flame. Additionally explosion pressure rise, flame temperature, and maximum rate of pressure rise were measured. In all experiments with the pressure rise ratio below two the low temperature oxidation mechanism assisted the flame propagation. © 2008 Shell Global Solutions. Published by Elsevier B.V. All rights reserved. Keywords: Flammability limit; Ignition criterion; Elevated temperature; Cool flame; Flame propagation; Low temperature oxidation mechanism
1.
Introduction
The explosion range of gas mixtures, which is defined as the fuel concentration between the lower and the upper explosion limit in air or other oxidiser at given conditions, is a very important parameter in risk studies and safe design. If an industrial process operates outside a well-defined explosion range, under normal operation conditions, the flame, even if ignited, cannot propagate. Therefore, in these conditions, an explosion cannot occur. While accurate determination of the explosion range for a given method, conditions and criteria require only accurate instruments and careful experimenting and observation, the proper determination is more complex. The complexity arises from experimental factors, which influence the value of the explosion limits and proper interpretation of observed phenomena. Factors are initial pressure, initial temperature, size of an experimental vessel and its
∗
dimensions, ignition type and energy, direction of flame propagation, turbulence, presence of impurities, and ignition criterion. Additionally, at elevated conditions, the state of the vessel walls (heterogeneous reactions), pre-ignition reactions, cool flame phenomena and multi-stage ignition in general contribute to the complexity (Pekalski et al., 2002, 2005a). Much research was conducted to understand the influence of a few single variables on explosion limits. A variety of measurement concepts, ignition criteria, methodologies and apparatuses have been used (SAFEKINEX, 2004; Steen, 2001; Pekalski et al., 2005b; Chen and Liu, 2003; Vanderstraeten et al., 1997; Van den Schoor et al., 2006). However, a comprehensive and systematic study on experimental parameters that influence the explosion limits, especially at elevated conditions, is still not complete. The measurement standards differ significantly not only in the methodology, apparatus, and ignition criterion but also
Corresponding author. Tel.: +44 151 373 5564; fax: +44 151 373 5058. E-mail address: [email protected] (A.A. Pekalski). Received 13 February 2008; Received in revised form 31 July 2008; Accepted 1 August 2008 0957-5820/$ – see front matter © 2008 Shell Global Solutions. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.psep.2008.08.002
48
process safety and environment protection 8 7 ( 2 0 0 9 ) 47–52
in the definitions. For instance, an explosion limit by ASTM standard (1995) is a mixture composition at which flame is just able to propagate while in the EN standard (2003) flame just fails to propagate at flammability limits. Because of the differences between the standards an ambiguous situation is created that result in various explosion limit data, even at the most straightforward, atmospheric conditions (Schröder and Daubitz, 2004). Elevated conditions of pressure and temperature magnify the problem and difficulty. Where at atmospheric pressure experiments can still be done in a vertical glass pipe, which allows direct visual observation of the vertically rising and expanding flame ball, at higher pressure tests are usually conducted in a closed steel vessel equipped with a pressure sensor. This triggered the development of the ignition criterion based on pressure increase. ASTM E 918-83 (1999) sets the threshold as 7% of pressure rise, while EN 1839 (2003) as 5%. The determination of the upper explosion limit is much more problematic than that of the lower one. The lower one is usually a rather sharp cut-off also at elevated conditions. It also does not shift much with higher pressure and temperature. This does not mean there is no ambiguity about its value. However the dispersion in values leaves less uncertainty than for the upper explosion limit. The latter also shifts over a wide range to higher fuel composition with both pressure and temperature (Holtappels et al., 2007). However, depending on the nature of the fuel over a considerable part of the range adjacent to the limit, the severity of the explosion in terms of explosion pressure and rate of pressure rise of the explosion is very low. This means that also the risk of obtaining damage to equipment is low. When increasing temperature in a given mixture, which is in the explosion range, the possibility of self-ignition will come closer. At higher pressure self-ignition will even occur easier. It means that with limit compositions at higher pressure and higher temperature a self-ignition threshold is reached beyond which a mixture self-ignites after a certain induction period. Self-ignition will start at the hottest spot. If induction is sufficient short a flame ball may expand in the reacting mixture. On the other hand if induction is relatively long the reacting mixture may well be exhausted and no flame ball will develop. In such case the mixture is beyond the explosion limit but is still reactive and can develop heat, which in a closed vessel will lead to pressure increase. Hydrocarbon oxidation mechanisms vary drastically with temperature range and it is classified into low-, intermediate-, and high temperature oxidation mechanism. There is a wealth of literature available on this subject (Griffiths and Barnard, 1996; Pilling, 1997; Westbrook, 2000). At the lowest temperature oxidation mechanism (LTOM), in particular in fuel rich mixtures, organic peroxides are formed which in turn decompose producing more than one radical per radical consumed. In this so-called chain branching process, which progressively accelerates, the peroxides decompose and fuel the process until their extinction. Amongst others such reactions generate stable products like formaldehyde and alcohols. It culminates in a sudden phenomenon known as cool flame in which organic peroxide are consumed. Cool flame is accompanied by a sudden temperature increase. It may show itself as a repeating faint blue flame with a limited temperature increase and at maximum a doubling in pressure (Fish, 1968; Coffee, 1980; Shtern et al., 1964). Less known is cool flame initiation by an artificial ignition source. This phenomenon was extensively studied, in the past, by e.g. Towned and co-workers, more
Fig. 1 – The cool flame and normal flame region for hexane air mixtures by Hsieh—reproduced by permission of The Royal Society of Chemistry.
reference is available in Pekalski (2004). All studied hydrocarbons (ethers, alkanes C3–C7) showed the same behaviour; after being ignited, depending on the initial conditions (mainly temperature, pressure, and mixture composition), either normal hot flame or a cool flame phenomenon propagated over the vessel. An example for n-hexane in air is presented in Fig. 1. At temperature below the self-ignition value the amount of ignition energy deposited in the mixture, in order to cause oxidation, is lower compared to room temperature. The deposited energy creates, in the limited volume, a very high temperature, which decays over time. However, this creates transient conditions favourable for development of a certain type of oxidation mechanism that depends on mixture conditions, i.e. its reactivity. Near the explosion limit reactivity will be low and if sustained oxidation was initiated the temperature will rise relatively slow. This will allow the low temperature oxidation mechanism to play a relatively important role where in case of high reactivity and quick temperature rise the oxidation mechanism will progress according to the high temperature oxidation mechanism. Such mechanism generates different post-combustion products like CO2 , H2 O, and H2 . The present paper will show what kind of oxidation mechanism develops after ignition at fuel rich conditions. Additionally a evaluation was performed weather based on a pressure rise criterion the upper explosion limit may be distinguished from the lower cool flame limit. From hazard point of view proper classification of oxidation phenomena is important. The work described is described in more details in the PhD thesis (Pekalski, 2004).
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process safety and environment protection 8 7 ( 2 0 0 9 ) 47–52
Fig. 2 – Explosion indices of C4 H10 –O2 mixture, Pini = 2 bara.
Fig. 3 – Explosion indices of C4 H10 –O2 mixture, Pini = 4 bara.
2.
Research approach
In order to identify presence of low temperature oxidation mechanism (cool flames) after an artificial ignition two research programmes were conducted. The first one was based on a value of pressure rise after ignition and characteristic post-explosion products. The second one was based on a characteristic flame emission. Two mixtures, at initial temperature of approximately 500 K, were studied: n-butane-oxygen, at initial pressure of approximately 2 and 4 bara, and methane, ethylene, carbon dioxide, oxygen so called C1–C2 mixture at 16.2 bara. The former is known to exhibit cool flames hence it was used as a benchmark. The C1–C2 mixture was investigated because of practical importance, however, only by means of flame emission spectroscopy at one oxygen composition. The experiments were performed in the 20-l strengthened explosion sphere, described elsewhere (Pekalski, 2004; Pekalski et al., 2005a). The sphere was equipped with two pressure sensors and three vertically and two horizontally placed thermocouples. Preheated oxygen was rapidly injected to the explosion sphere filled with pure preheated fuel. The mixture created was ignited by a tungsten fused wire 90 s after the oxygen injection was completed. The ignition energy was approximately 3.4 J. Two minutes after combustion phenomena the post-explosion mixture was taken for GC analysis.
3.
Explosion test results
The dependence of the explosion pressure rise rate, (dP/dt)ex , and the pressure ratio (Pex /Pini ) on the oxygen contents in the mixture are presented in Figs. 2 and 3.
At initial pressure of 2 bara and mixture composition of 67.4% mole of n-butane the experiment was repeated five times to check the reproducibility, which turned out to be very good. The pressure ratio increases with increasing oxygen concentration, however, not steadily. In very fuel rich mixtures in the range of 73–83% of n-butane increase of oxygen does not change the pressure ratio much: it changes from 2.4 to 21%. A second plateau is observed around 66% of n-butane, where the pressure ratio assumes values around 2. The rate of explosion pressure rise shows a much wider plateau. In the concentration range of 60–67% n-butane it assumes similar values around 2.2 bara/s. In fuel leaner mixtures that 60% of n-butane the explosion rate of pressure rise increases more rapidly with increasing oxygen contents. The pressure ratio values, for fuel rich mixtures, are marked in Fig. 2. It is clearly evident that based on the assumed ignition criterion different values of the explosion limits can be found. According to the 7% threshold pressure rise criterion the explosion limit would be just below 78% of n-butane. For 5% it would be just about 80%. According to the criterion of doubling the initial pressure (research criterion) the explosion limit would be around 66%. Similar behaviour was observed at initial pressure of 4 bara (Fig. 3). Concentrations of methanol, ethanol and ethylene in the post-explosion mixture are given in Fig. 4. Concentrations of the species assume a clear maximum at certain mixture composition. The highest measured concentrations of methanol, ethanol and ethylene respectively were at 67.5, 66.5 and 62.4% of n-butane. The pressure ratio assumes values of 1.99, 2.02, and 2.29, respectively. In summary, several conclusions can be made: 1. It was anticipated that the highest concentration of alcohols is induced by the low temperature oxidation mechanism. Their concentrations should decrease as the intermediate temperature oxidation mechanism becomes more important, i.e. with increasing oxygen contents in the initial mixtures. At this moment the ethylene concentration should assume the highest value. Further increase in the flame temperature causes these species to vanish. Such behaviour was observed at both initial pressures investigated (2 and 4 bara). 2. The conditions at which the maximum concentrations of methanol, ethanol and ethylene occur are quite similar at both initial pressures. These are summarised in Table 1. 3. Independently of the initial pressures the maximum number of the post-explosion species occurred at the same value of the pressure ratios. This is in the range of 1.70–2.024. Based on above points it can be concluded that the pressure rise after ignition, in cases the pressure ratio is below two, is strongly associated with the low temperature oxidation mech-
Table 1 – The n-butane concentrations at which highest concentration of selected species occur in the post-combustion mixtures and the respective pressure ratio values. Pini (bara)
2 4
C4 H10 concentration (mol.%)/Pex /Pini CH3 OH
C2 H4 OH
C2 H4
67.5/1.99 75.7/1.705
66.5/2.02 72.7/1.89
62.4/2.29 64.7/2.78
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process safety and environment protection 8 7 ( 2 0 0 9 ) 47–52
Fig. 4 – Concentration of some species in the post-explosion mixture as a function of the mixture composition, Pini = 2 bara. More than 30 stable product peaks were found. anism. As the standards recommend a pressure rise criterion of 5 and 7% for determination of explosion limit, it is very ambiguous, as products not associated with the high temperature oxidation mechanism are present in the post-combustion mixture.
4.
Flame emission spectroscopy
During the oxidation processes of hydrocarbons certain intermediate compounds become excited and release the energy excess by photon emission. The wavelength of the photon is specific for a given specie and can be used for identification purposes. Appearance of cool flame is accompanied by pale blue chemiluminescence. (Emeleus, 1926, 1929) studied the cool flame emission spectra. The emission spectrum appeared to be the same for several different fuels (ethers, acetaldehyde, propionaldehyde and hexane). The spectra of cool flames consist of a series of bands, shaded toward the red, the intensity of which is greatest in the blue and near-ultraviolet regions. The same spectrum was observed for different hydrocarbons (saturated and unsaturated), alcohols, aldehydes, ketones, acids, oils, ethers and waxes (Gaydon, 1957, p. 279; Kondratiev, 1964, p. 688). The blue luminescence originates from an electronically excited state of formaldehyde, which is formed in a chemiluminescent reaction mainly by the radical + radical reactions: CH3 O• + (OH → CH2 O ∗ + H2 O CH3 O( + CH3 O( → CH2 O ∗ + CH3 OH Additional emission sources might be present, but their contribution to emission is very minor compared to a chemiluminescence of formaldehyde (Sheinson and Williams, 1973;
Agnew and Agnew, 1956; Agnew and Wark, 1965). On average only one photon is emitted per 103 molecules of fuel consumed (Griffiths and Barnard, 1996). When more oxygen is added to the mixture that exhibits pale blue luminescence when cool flame appears, the blue luminescence becomes more intensive. Temperature becomes higher and hydrogen peroxide is going to dominate the scene. It is then called blue flame. A temperature rise is reported, up to 400 K. However this temperature rise is still far below the temperature of the hot flame. More oxygen admission to the mixture causes the flammability limit to be exceeded and the hot flame appears with distinctly different emission spectra compared to the cool or blue flame spectra (Ohta and Furutani, 1991; Kondratiev and Nikitin, 1981, p. 216; Sheinson and Williams, 1973). The cool flame spectrum consists mainly of the emission from the excited formaldehyde. In the blue flame additionally emissions bands from CH, OH and HCO appear. The hot flame spectrum does not have any formaldehyde emission but mainly emission spectra form C2, CH and OH. Formaldehyde emission reveals a characteristic pattern of emission bands at 385, 396 and 405 nm. This emission pattern does not overlap with other emission bands (Ohta and Furutani, 1991; Pekalski, 2004). The set-up consists of four main parts, namely explosion sphere, camera, monochromator, and computer for data acquisition. An optical window replaced one of the pressure transducer in 20-l strengthened explosion sphere. The flame spectroscopy set-up was able to record spectra with a limited radiation band wavelength of 30 nm. Hence the set-up was adjusted to record the flame emission in the range of 380.2–410.2 nm. Detailed description is available elsewhere (Pekalski, 2004). Tables 2 and 3 present the experimental conditions for nbutane and the C1–C2 oxygen mixtures, respectively.
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Table 2 – Experimental conditions for n-butane-oxygen mixtures. Exp. no. 1 2 3
Pini (bara)
yC4 H10 (%)
Tinitial (K)
2.1 2.1 4.1
503 501 501
67.4 72.6 72.9
yO2 (%) 32.6 27.4 27.1
Exposure time (s)
Pex /Pini
5 5 5
1.98 1.28 1.89
Table 3 – Experimental conditions for the C1–C2 and oxygen mixture. Exp. no. 4 5
Pini (bara) 16.2 16.2
Tinitial (K) 502 501
yCH4 (%) 47.1 46.9
yC2 H4 (%) 43.7 43.9
yCO2 (%) 2.0 2.0
yO2 (%) 7.2 7.2
Exposure time (s) 5 30
The main goal with varying oxygen contents was the identification of the presence of the formaldehyde emission that is characteristic for the low temperature oxidation mechanism. Increased oxygen concentration in the mixture results in higher flame temperature hence in higher values of pressure ratios. The experimental conditions for the methane, ethylene, carbon dioxide, oxygen mixtures were taken from a larger experimental test series dedicated to determine the upper explosion limit (Pekalski and Zevenbergen, 2001). The exposure time was varied to investigate the flame emission of the upward and downwards-propagating flame.
5.
Flame spectroscopy results
In total five experiments were conducted. In all of them, irrespectively of the oxygen contents and the initial pressure, oxidation phenomena give a clear spectrum pattern. The pattern is the same for all n-butane-oxygen experiments as well as for the C1–C2–oxygen mixture. An example of the spectra is presented in Fig. 5. The spectra of all experiments show clearly three peaks. Their maximum is at wavelength of 386, 396 and 404 nm, with relative intensities of 9, 10 and 5 respectively, which agree with the characteristic wavelength of excited formaldehyde emission. The intensity of the peaks for C1–C2 mixture is lower than n-butane oxygen mixture. Thermocouple analysis showed the upward flame propagation after ignition followed by downward propagation (Pekalski et al., 2005b). The flame appeared in the vicinity of the horizontal thermocouples approximately 5 s after the ignition in experiment number 4 (Fig. 6). The vertical (red) line adjacent to the ordinate indicates the moment of fused wire
Fig. 5 – Flame spectrum of n-butane-oxygen flame, experiment number 3.
Fig. 6 – Temperature and pressure traces over time, experiment number 4. ignition. Considering the thermal inertia of thermocouples the upward propagating flame reached a maximum temperature of approximately 850 K, while the downward propagating reached approximately 715 K, i.e. about 130 K lower. Subsequent experiment, number 5, with longer exposure time of 30 s was performed to capture the emission of flame propagating downward. Its emission spectrum shows maxima at the wavelength of 384, 396 and 404 nm and the same peak intensity ratio as the upward propagating flame.
6.
Conclusion
1. The post-explosion analysis of stable products shows presence of species typical for the low temperature oxidation mechanism (alcohols). Their concentration maximum is at the pressure rise ratio of approximately 2, i.e. 100% pressure rise. 2. Flame emission spectroscopy, for the pressure rise ratio below two, reveals in the propagating flame in both investigated mixtures (n-butane-oxygen and C1–C2–oxygen the presence of formaldehyde, which is a characteristic species for the low temperature oxidation mechanism (LTOM). 3. Presence of the low temperature oxidation mechanism during flame propagation complicates the determination whether a hot flame regime exists or a cool flame one, i.e. determination of the upper explosion limit based on a criterion of pressure rise may lead to classification of the cool flame regime as the flammable regime. The pressure rise of 5 and 7%, used by standards seems to be ambiguous. 4. A relatively high maximum temperature of the flame propagating upward towards the ceiling of the vessel compared to the one coming downward along the vessel periphery might suggest a more complete combustion of the upward moving flame by better heat transfer stimulated by
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buoyancy and higher temperature in comparison with the slower downward motion against the buoyancy force. Also in the periphery, the flame temperature will be lower as a result of heat losses to the wall. The low temperature might cause preferentially the cool flame products formation characteristic for the LTOM, while oxidation mechanisms occurring at higher temperature support more the core of the upward propagating flame. This preliminary conclusion still has to be proven by time and space resolved diagnostics and analysis.
References Agnew, W.G. and Agnew, J.T., 1956, Ind. Eng. Chem., 48(12): 2224–2231. Agnew, W.G. and Wark, J.K., 1965, Comparison of emission spectra of low temperature combustion reactions in an engine and in a flat-flame burner, In Proceedings of the 6th international symposium on combustion , pp. 894–902. ASTM E 681-94, 1995, Standard test method for concentration limits of flammability of chemicals (vapours and gases). ASTM E 918-83, 1999, Standard practise for determining limits of flammability of chemicals at elevated temperature and pressure, re-approved. Chen, J.R. and Liu, K., 2003, AIChE J., 49(9): 2427–2432. Coffee, R.D., 1980, Cool flames and autoignitions: two oxidation processes. J. Loss Prevent., 13: 74. Emeleus, H.J., 1926, J. Chem. Soc., 2948. Emeleus, H.J., 1929, J. Chem. Soc., 1929. EN 1839, 2003, Determination of explosion limits of gases and vapours. Fish, A., 1968, The cool flames of hydrocarbons. Angew. Chem. Int., 7: 45–60. Gaydon, A.G., (1957). The spectroscopy of flames. (Chapman and Hall, London). Griffiths, J.F. and Barnard, J.A., (1996). Flame and combustion (3rd ed.). (Blockie Academic & Professional). Holtappels, K., Schroeder, V., Kobiera, A., Wolanski, P., Braithwaite, M. and Pasman, H.J., 2007, Gas explosion safety characteristics and anomalies at unusual conditions, In Proceedings of the 12th international symposium on loss prevention and safety promotion in the process industries Edinburgh, UK, May 22–24, Hsieh, M.S. and Townend, D.T.A., 1939, J. Chem. Soc., 341. Kondratiev, V.N., (1964). Chemical kinetics of gas reactions. (Pergamon, Oxford). Kondratiev, V.N. and Nikitin, E.E., (1981). Gas phase reactions. (Springer-Verlag, Berlin). Ohta, Y. and Furutani, M., 1991, Identification of cool and blue flames in compression ignition. Arch. Combust., 11(1–2)
Pekalski, A.A., 2004, Theoretical and experimental study on explosion safety of hydrocarbons oxidation at elevated conditions, PhD Thesis, Delft University of Technology, The Netherlands, ISBN 90-9018842-8. Pekalski, A.A. and Zevenbergen, J.Z., 2001, Flammability limits of methane, ethylene, ethylene oxide, carbon dioxide, argon and oxygen mixture, Report for Shell Chemicals, May 2001. Pekalski, A.A., Zevenbergen, J.F., Pasman, H.J., Lemkowitz, S.M., Dahoe, A.E. and Scarlett, B., 2002, Influence of the ignition delay time on the explosion parameters of hydrocarbon air/oxygen mixtures at elevated pressure and temperature. J. Hazard. Mater., 93(1): 93–105. Pekalski, A.A., Terli, E., Zevenbergen, J.F., Lemkowitz, S.M. and Pasman, H.J., 2005, Influence of the ignition delay time on the explosion parameters of hydrocarbon air/oxygen mixtures at elevated pressure and temperature, In Proceedings of the combustion institute 30 II , pp. 1933–1939. Pekalski, A.A., Schildberg, H.P., Smallegange, P.S.D., Lemkowitz, S.M., Zevenbergen, J.F., Braithwaite, M. and Pasman, H.J., 2005, Determination of the explosion behaviour of methane and propene in air or oxygen at standard and elevated conditions. Process Saf. Environ. Protect., 83(5B): 421–429. Pilling, M.J., (1997). Comprehensive chemical kinetics, Vol. 35, low temperature combustion and autoignition. (Elsevier, Amsterdam). SAFEKINEX, 2004, contract EVG1-CT-2002-00072, Report on the experimental factors, influencing explosion indices determination, Del 2, February 2004, www.safekinex.org. Schröder, V. and Daubitz, V., 2004, Influence of the ignition delay time on the explosion parameters of hydrocarbon air/oxygen mixtures at elevated pressure and temperature, In Proceedings of the 11th international symposium on loss prevention in process industries B1 1340 L, Praha, Czech Republic, 31 May–3 June, Sheinson, R.S. and Williams, F.W., 1973, Chemiluminescence spectra from cool and blue flames: electronically excited formaldehyde. Combust. Flame, 21: 221–230. Shtern, V.Ya., Mullins, M.F. and Mullins, B.P., (1964). The gas phase oxidation of hydrocarbons. (Pergamon Press, Oxford). Steen, H., (2001). Handbuch des Explosionsschutzes. (Whiley-Verlag). Van den Schoor, F., Norman, F. and Verplaetsen, F., 2006, Influence of the ignition source location on the determination of the explosion pressure at elevated initial pressures. J. Loss Prevent. Process Ind., 19(5): 459–462. Vanderstraeten, B., Tuerlinckx, D., Berghmans, J., Vliegen, S., Oost, E.V. and Smit, B., 1997, Experimental study of the pressure and temperature dependence on the upper flammability limit of methane/air mixtures. J. Hazard. Mater., 56(3): 237–246. Westbrook, C.K., 2000, Chemical kinetics of hydrocarbon ignition in practical combustion systems, In Proceedings of the 28th international symposium on combustion Edinburgh, , pp. 1563–1577.
Contents lists available at ScienceDirect
Process Safety and Environment Protection journal homepage: www.elsevier.com/locate/psep
Distinction between the upper explosion limit and the lower cool flame limit in determination of flammability limit at elevated conditions A.A. Pekalski a,∗ , H.J. Pasman b a b
Shell Global Solutions, Shell Technology Centre Thornton, PO Box 1, Chester CH1 3SH, United Kingdom TU Delft, Faculty of Applied Sciences, Multi-Scale Physics, Prins Bernardlaan 6, 2628 BW Delft, The Netherlands
a b s t r a c t Previous research showed that at certain conditions, close to the flammability range exists a regime where cool flame may develop either due to elevated temperature or it may be initiated by an ignition source. Propagation of the cool flame in a closed test vessel may double the initial pressure. Such pressure increase exceeds recommended ignition criteria for explosion limit determination that are based on 5 or 7% of pressure rise leading to inaccurate classification of the oxidation phenomena, i.e. cool flame propagation may be classified as hot flame propagation. Two mixtures were tested: n-butane-oxygen (extensively) and C1–C2–oxygen (in limited range), which represent a typical composition in ethylene oxide production, at elevated conditions at their upper explosion limits. Flame development was analysed by flame emission spectroscopy and the post-oxidation mixture was analysed by gas chromatography (GC) to characterise the oxidation mechanism of the flame. Additionally explosion pressure rise, flame temperature, and maximum rate of pressure rise were measured. In all experiments with the pressure rise ratio below two the low temperature oxidation mechanism assisted the flame propagation. © 2008 Shell Global Solutions. Published by Elsevier B.V. All rights reserved. Keywords: Flammability limit; Ignition criterion; Elevated temperature; Cool flame; Flame propagation; Low temperature oxidation mechanism
1.
Introduction
The explosion range of gas mixtures, which is defined as the fuel concentration between the lower and the upper explosion limit in air or other oxidiser at given conditions, is a very important parameter in risk studies and safe design. If an industrial process operates outside a well-defined explosion range, under normal operation conditions, the flame, even if ignited, cannot propagate. Therefore, in these conditions, an explosion cannot occur. While accurate determination of the explosion range for a given method, conditions and criteria require only accurate instruments and careful experimenting and observation, the proper determination is more complex. The complexity arises from experimental factors, which influence the value of the explosion limits and proper interpretation of observed phenomena. Factors are initial pressure, initial temperature, size of an experimental vessel and its
∗
dimensions, ignition type and energy, direction of flame propagation, turbulence, presence of impurities, and ignition criterion. Additionally, at elevated conditions, the state of the vessel walls (heterogeneous reactions), pre-ignition reactions, cool flame phenomena and multi-stage ignition in general contribute to the complexity (Pekalski et al., 2002, 2005a). Much research was conducted to understand the influence of a few single variables on explosion limits. A variety of measurement concepts, ignition criteria, methodologies and apparatuses have been used (SAFEKINEX, 2004; Steen, 2001; Pekalski et al., 2005b; Chen and Liu, 2003; Vanderstraeten et al., 1997; Van den Schoor et al., 2006). However, a comprehensive and systematic study on experimental parameters that influence the explosion limits, especially at elevated conditions, is still not complete. The measurement standards differ significantly not only in the methodology, apparatus, and ignition criterion but also
Corresponding author. Tel.: +44 151 373 5564; fax: +44 151 373 5058. E-mail address: [email protected] (A.A. Pekalski). Received 13 February 2008; Received in revised form 31 July 2008; Accepted 1 August 2008 0957-5820/$ – see front matter © 2008 Shell Global Solutions. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.psep.2008.08.002
48
process safety and environment protection 8 7 ( 2 0 0 9 ) 47–52
in the definitions. For instance, an explosion limit by ASTM standard (1995) is a mixture composition at which flame is just able to propagate while in the EN standard (2003) flame just fails to propagate at flammability limits. Because of the differences between the standards an ambiguous situation is created that result in various explosion limit data, even at the most straightforward, atmospheric conditions (Schröder and Daubitz, 2004). Elevated conditions of pressure and temperature magnify the problem and difficulty. Where at atmospheric pressure experiments can still be done in a vertical glass pipe, which allows direct visual observation of the vertically rising and expanding flame ball, at higher pressure tests are usually conducted in a closed steel vessel equipped with a pressure sensor. This triggered the development of the ignition criterion based on pressure increase. ASTM E 918-83 (1999) sets the threshold as 7% of pressure rise, while EN 1839 (2003) as 5%. The determination of the upper explosion limit is much more problematic than that of the lower one. The lower one is usually a rather sharp cut-off also at elevated conditions. It also does not shift much with higher pressure and temperature. This does not mean there is no ambiguity about its value. However the dispersion in values leaves less uncertainty than for the upper explosion limit. The latter also shifts over a wide range to higher fuel composition with both pressure and temperature (Holtappels et al., 2007). However, depending on the nature of the fuel over a considerable part of the range adjacent to the limit, the severity of the explosion in terms of explosion pressure and rate of pressure rise of the explosion is very low. This means that also the risk of obtaining damage to equipment is low. When increasing temperature in a given mixture, which is in the explosion range, the possibility of self-ignition will come closer. At higher pressure self-ignition will even occur easier. It means that with limit compositions at higher pressure and higher temperature a self-ignition threshold is reached beyond which a mixture self-ignites after a certain induction period. Self-ignition will start at the hottest spot. If induction is sufficient short a flame ball may expand in the reacting mixture. On the other hand if induction is relatively long the reacting mixture may well be exhausted and no flame ball will develop. In such case the mixture is beyond the explosion limit but is still reactive and can develop heat, which in a closed vessel will lead to pressure increase. Hydrocarbon oxidation mechanisms vary drastically with temperature range and it is classified into low-, intermediate-, and high temperature oxidation mechanism. There is a wealth of literature available on this subject (Griffiths and Barnard, 1996; Pilling, 1997; Westbrook, 2000). At the lowest temperature oxidation mechanism (LTOM), in particular in fuel rich mixtures, organic peroxides are formed which in turn decompose producing more than one radical per radical consumed. In this so-called chain branching process, which progressively accelerates, the peroxides decompose and fuel the process until their extinction. Amongst others such reactions generate stable products like formaldehyde and alcohols. It culminates in a sudden phenomenon known as cool flame in which organic peroxide are consumed. Cool flame is accompanied by a sudden temperature increase. It may show itself as a repeating faint blue flame with a limited temperature increase and at maximum a doubling in pressure (Fish, 1968; Coffee, 1980; Shtern et al., 1964). Less known is cool flame initiation by an artificial ignition source. This phenomenon was extensively studied, in the past, by e.g. Towned and co-workers, more
Fig. 1 – The cool flame and normal flame region for hexane air mixtures by Hsieh—reproduced by permission of The Royal Society of Chemistry.
reference is available in Pekalski (2004). All studied hydrocarbons (ethers, alkanes C3–C7) showed the same behaviour; after being ignited, depending on the initial conditions (mainly temperature, pressure, and mixture composition), either normal hot flame or a cool flame phenomenon propagated over the vessel. An example for n-hexane in air is presented in Fig. 1. At temperature below the self-ignition value the amount of ignition energy deposited in the mixture, in order to cause oxidation, is lower compared to room temperature. The deposited energy creates, in the limited volume, a very high temperature, which decays over time. However, this creates transient conditions favourable for development of a certain type of oxidation mechanism that depends on mixture conditions, i.e. its reactivity. Near the explosion limit reactivity will be low and if sustained oxidation was initiated the temperature will rise relatively slow. This will allow the low temperature oxidation mechanism to play a relatively important role where in case of high reactivity and quick temperature rise the oxidation mechanism will progress according to the high temperature oxidation mechanism. Such mechanism generates different post-combustion products like CO2 , H2 O, and H2 . The present paper will show what kind of oxidation mechanism develops after ignition at fuel rich conditions. Additionally a evaluation was performed weather based on a pressure rise criterion the upper explosion limit may be distinguished from the lower cool flame limit. From hazard point of view proper classification of oxidation phenomena is important. The work described is described in more details in the PhD thesis (Pekalski, 2004).
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Fig. 2 – Explosion indices of C4 H10 –O2 mixture, Pini = 2 bara.
Fig. 3 – Explosion indices of C4 H10 –O2 mixture, Pini = 4 bara.
2.
Research approach
In order to identify presence of low temperature oxidation mechanism (cool flames) after an artificial ignition two research programmes were conducted. The first one was based on a value of pressure rise after ignition and characteristic post-explosion products. The second one was based on a characteristic flame emission. Two mixtures, at initial temperature of approximately 500 K, were studied: n-butane-oxygen, at initial pressure of approximately 2 and 4 bara, and methane, ethylene, carbon dioxide, oxygen so called C1–C2 mixture at 16.2 bara. The former is known to exhibit cool flames hence it was used as a benchmark. The C1–C2 mixture was investigated because of practical importance, however, only by means of flame emission spectroscopy at one oxygen composition. The experiments were performed in the 20-l strengthened explosion sphere, described elsewhere (Pekalski, 2004; Pekalski et al., 2005a). The sphere was equipped with two pressure sensors and three vertically and two horizontally placed thermocouples. Preheated oxygen was rapidly injected to the explosion sphere filled with pure preheated fuel. The mixture created was ignited by a tungsten fused wire 90 s after the oxygen injection was completed. The ignition energy was approximately 3.4 J. Two minutes after combustion phenomena the post-explosion mixture was taken for GC analysis.
3.
Explosion test results
The dependence of the explosion pressure rise rate, (dP/dt)ex , and the pressure ratio (Pex /Pini ) on the oxygen contents in the mixture are presented in Figs. 2 and 3.
At initial pressure of 2 bara and mixture composition of 67.4% mole of n-butane the experiment was repeated five times to check the reproducibility, which turned out to be very good. The pressure ratio increases with increasing oxygen concentration, however, not steadily. In very fuel rich mixtures in the range of 73–83% of n-butane increase of oxygen does not change the pressure ratio much: it changes from 2.4 to 21%. A second plateau is observed around 66% of n-butane, where the pressure ratio assumes values around 2. The rate of explosion pressure rise shows a much wider plateau. In the concentration range of 60–67% n-butane it assumes similar values around 2.2 bara/s. In fuel leaner mixtures that 60% of n-butane the explosion rate of pressure rise increases more rapidly with increasing oxygen contents. The pressure ratio values, for fuel rich mixtures, are marked in Fig. 2. It is clearly evident that based on the assumed ignition criterion different values of the explosion limits can be found. According to the 7% threshold pressure rise criterion the explosion limit would be just below 78% of n-butane. For 5% it would be just about 80%. According to the criterion of doubling the initial pressure (research criterion) the explosion limit would be around 66%. Similar behaviour was observed at initial pressure of 4 bara (Fig. 3). Concentrations of methanol, ethanol and ethylene in the post-explosion mixture are given in Fig. 4. Concentrations of the species assume a clear maximum at certain mixture composition. The highest measured concentrations of methanol, ethanol and ethylene respectively were at 67.5, 66.5 and 62.4% of n-butane. The pressure ratio assumes values of 1.99, 2.02, and 2.29, respectively. In summary, several conclusions can be made: 1. It was anticipated that the highest concentration of alcohols is induced by the low temperature oxidation mechanism. Their concentrations should decrease as the intermediate temperature oxidation mechanism becomes more important, i.e. with increasing oxygen contents in the initial mixtures. At this moment the ethylene concentration should assume the highest value. Further increase in the flame temperature causes these species to vanish. Such behaviour was observed at both initial pressures investigated (2 and 4 bara). 2. The conditions at which the maximum concentrations of methanol, ethanol and ethylene occur are quite similar at both initial pressures. These are summarised in Table 1. 3. Independently of the initial pressures the maximum number of the post-explosion species occurred at the same value of the pressure ratios. This is in the range of 1.70–2.024. Based on above points it can be concluded that the pressure rise after ignition, in cases the pressure ratio is below two, is strongly associated with the low temperature oxidation mech-
Table 1 – The n-butane concentrations at which highest concentration of selected species occur in the post-combustion mixtures and the respective pressure ratio values. Pini (bara)
2 4
C4 H10 concentration (mol.%)/Pex /Pini CH3 OH
C2 H4 OH
C2 H4
67.5/1.99 75.7/1.705
66.5/2.02 72.7/1.89
62.4/2.29 64.7/2.78
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Fig. 4 – Concentration of some species in the post-explosion mixture as a function of the mixture composition, Pini = 2 bara. More than 30 stable product peaks were found. anism. As the standards recommend a pressure rise criterion of 5 and 7% for determination of explosion limit, it is very ambiguous, as products not associated with the high temperature oxidation mechanism are present in the post-combustion mixture.
4.
Flame emission spectroscopy
During the oxidation processes of hydrocarbons certain intermediate compounds become excited and release the energy excess by photon emission. The wavelength of the photon is specific for a given specie and can be used for identification purposes. Appearance of cool flame is accompanied by pale blue chemiluminescence. (Emeleus, 1926, 1929) studied the cool flame emission spectra. The emission spectrum appeared to be the same for several different fuels (ethers, acetaldehyde, propionaldehyde and hexane). The spectra of cool flames consist of a series of bands, shaded toward the red, the intensity of which is greatest in the blue and near-ultraviolet regions. The same spectrum was observed for different hydrocarbons (saturated and unsaturated), alcohols, aldehydes, ketones, acids, oils, ethers and waxes (Gaydon, 1957, p. 279; Kondratiev, 1964, p. 688). The blue luminescence originates from an electronically excited state of formaldehyde, which is formed in a chemiluminescent reaction mainly by the radical + radical reactions: CH3 O• + (OH → CH2 O ∗ + H2 O CH3 O( + CH3 O( → CH2 O ∗ + CH3 OH Additional emission sources might be present, but their contribution to emission is very minor compared to a chemiluminescence of formaldehyde (Sheinson and Williams, 1973;
Agnew and Agnew, 1956; Agnew and Wark, 1965). On average only one photon is emitted per 103 molecules of fuel consumed (Griffiths and Barnard, 1996). When more oxygen is added to the mixture that exhibits pale blue luminescence when cool flame appears, the blue luminescence becomes more intensive. Temperature becomes higher and hydrogen peroxide is going to dominate the scene. It is then called blue flame. A temperature rise is reported, up to 400 K. However this temperature rise is still far below the temperature of the hot flame. More oxygen admission to the mixture causes the flammability limit to be exceeded and the hot flame appears with distinctly different emission spectra compared to the cool or blue flame spectra (Ohta and Furutani, 1991; Kondratiev and Nikitin, 1981, p. 216; Sheinson and Williams, 1973). The cool flame spectrum consists mainly of the emission from the excited formaldehyde. In the blue flame additionally emissions bands from CH, OH and HCO appear. The hot flame spectrum does not have any formaldehyde emission but mainly emission spectra form C2, CH and OH. Formaldehyde emission reveals a characteristic pattern of emission bands at 385, 396 and 405 nm. This emission pattern does not overlap with other emission bands (Ohta and Furutani, 1991; Pekalski, 2004). The set-up consists of four main parts, namely explosion sphere, camera, monochromator, and computer for data acquisition. An optical window replaced one of the pressure transducer in 20-l strengthened explosion sphere. The flame spectroscopy set-up was able to record spectra with a limited radiation band wavelength of 30 nm. Hence the set-up was adjusted to record the flame emission in the range of 380.2–410.2 nm. Detailed description is available elsewhere (Pekalski, 2004). Tables 2 and 3 present the experimental conditions for nbutane and the C1–C2 oxygen mixtures, respectively.
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Table 2 – Experimental conditions for n-butane-oxygen mixtures. Exp. no. 1 2 3
Pini (bara)
yC4 H10 (%)
Tinitial (K)
2.1 2.1 4.1
503 501 501
67.4 72.6 72.9
yO2 (%) 32.6 27.4 27.1
Exposure time (s)
Pex /Pini
5 5 5
1.98 1.28 1.89
Table 3 – Experimental conditions for the C1–C2 and oxygen mixture. Exp. no. 4 5
Pini (bara) 16.2 16.2
Tinitial (K) 502 501
yCH4 (%) 47.1 46.9
yC2 H4 (%) 43.7 43.9
yCO2 (%) 2.0 2.0
yO2 (%) 7.2 7.2
Exposure time (s) 5 30
The main goal with varying oxygen contents was the identification of the presence of the formaldehyde emission that is characteristic for the low temperature oxidation mechanism. Increased oxygen concentration in the mixture results in higher flame temperature hence in higher values of pressure ratios. The experimental conditions for the methane, ethylene, carbon dioxide, oxygen mixtures were taken from a larger experimental test series dedicated to determine the upper explosion limit (Pekalski and Zevenbergen, 2001). The exposure time was varied to investigate the flame emission of the upward and downwards-propagating flame.
5.
Flame spectroscopy results
In total five experiments were conducted. In all of them, irrespectively of the oxygen contents and the initial pressure, oxidation phenomena give a clear spectrum pattern. The pattern is the same for all n-butane-oxygen experiments as well as for the C1–C2–oxygen mixture. An example of the spectra is presented in Fig. 5. The spectra of all experiments show clearly three peaks. Their maximum is at wavelength of 386, 396 and 404 nm, with relative intensities of 9, 10 and 5 respectively, which agree with the characteristic wavelength of excited formaldehyde emission. The intensity of the peaks for C1–C2 mixture is lower than n-butane oxygen mixture. Thermocouple analysis showed the upward flame propagation after ignition followed by downward propagation (Pekalski et al., 2005b). The flame appeared in the vicinity of the horizontal thermocouples approximately 5 s after the ignition in experiment number 4 (Fig. 6). The vertical (red) line adjacent to the ordinate indicates the moment of fused wire
Fig. 5 – Flame spectrum of n-butane-oxygen flame, experiment number 3.
Fig. 6 – Temperature and pressure traces over time, experiment number 4. ignition. Considering the thermal inertia of thermocouples the upward propagating flame reached a maximum temperature of approximately 850 K, while the downward propagating reached approximately 715 K, i.e. about 130 K lower. Subsequent experiment, number 5, with longer exposure time of 30 s was performed to capture the emission of flame propagating downward. Its emission spectrum shows maxima at the wavelength of 384, 396 and 404 nm and the same peak intensity ratio as the upward propagating flame.
6.
Conclusion
1. The post-explosion analysis of stable products shows presence of species typical for the low temperature oxidation mechanism (alcohols). Their concentration maximum is at the pressure rise ratio of approximately 2, i.e. 100% pressure rise. 2. Flame emission spectroscopy, for the pressure rise ratio below two, reveals in the propagating flame in both investigated mixtures (n-butane-oxygen and C1–C2–oxygen the presence of formaldehyde, which is a characteristic species for the low temperature oxidation mechanism (LTOM). 3. Presence of the low temperature oxidation mechanism during flame propagation complicates the determination whether a hot flame regime exists or a cool flame one, i.e. determination of the upper explosion limit based on a criterion of pressure rise may lead to classification of the cool flame regime as the flammable regime. The pressure rise of 5 and 7%, used by standards seems to be ambiguous. 4. A relatively high maximum temperature of the flame propagating upward towards the ceiling of the vessel compared to the one coming downward along the vessel periphery might suggest a more complete combustion of the upward moving flame by better heat transfer stimulated by
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buoyancy and higher temperature in comparison with the slower downward motion against the buoyancy force. Also in the periphery, the flame temperature will be lower as a result of heat losses to the wall. The low temperature might cause preferentially the cool flame products formation characteristic for the LTOM, while oxidation mechanisms occurring at higher temperature support more the core of the upward propagating flame. This preliminary conclusion still has to be proven by time and space resolved diagnostics and analysis.
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Pekalski, A.A., 2004, Theoretical and experimental study on explosion safety of hydrocarbons oxidation at elevated conditions, PhD Thesis, Delft University of Technology, The Netherlands, ISBN 90-9018842-8. Pekalski, A.A. and Zevenbergen, J.Z., 2001, Flammability limits of methane, ethylene, ethylene oxide, carbon dioxide, argon and oxygen mixture, Report for Shell Chemicals, May 2001. Pekalski, A.A., Zevenbergen, J.F., Pasman, H.J., Lemkowitz, S.M., Dahoe, A.E. and Scarlett, B., 2002, Influence of the ignition delay time on the explosion parameters of hydrocarbon air/oxygen mixtures at elevated pressure and temperature. J. Hazard. Mater., 93(1): 93–105. Pekalski, A.A., Terli, E., Zevenbergen, J.F., Lemkowitz, S.M. and Pasman, H.J., 2005, Influence of the ignition delay time on the explosion parameters of hydrocarbon air/oxygen mixtures at elevated pressure and temperature, In Proceedings of the combustion institute 30 II , pp. 1933–1939. Pekalski, A.A., Schildberg, H.P., Smallegange, P.S.D., Lemkowitz, S.M., Zevenbergen, J.F., Braithwaite, M. and Pasman, H.J., 2005, Determination of the explosion behaviour of methane and propene in air or oxygen at standard and elevated conditions. Process Saf. Environ. Protect., 83(5B): 421–429. Pilling, M.J., (1997). Comprehensive chemical kinetics, Vol. 35, low temperature combustion and autoignition. (Elsevier, Amsterdam). SAFEKINEX, 2004, contract EVG1-CT-2002-00072, Report on the experimental factors, influencing explosion indices determination, Del 2, February 2004, www.safekinex.org. Schröder, V. and Daubitz, V., 2004, Influence of the ignition delay time on the explosion parameters of hydrocarbon air/oxygen mixtures at elevated pressure and temperature, In Proceedings of the 11th international symposium on loss prevention in process industries B1 1340 L, Praha, Czech Republic, 31 May–3 June, Sheinson, R.S. and Williams, F.W., 1973, Chemiluminescence spectra from cool and blue flames: electronically excited formaldehyde. Combust. Flame, 21: 221–230. Shtern, V.Ya., Mullins, M.F. and Mullins, B.P., (1964). The gas phase oxidation of hydrocarbons. (Pergamon Press, Oxford). Steen, H., (2001). Handbuch des Explosionsschutzes. (Whiley-Verlag). Van den Schoor, F., Norman, F. and Verplaetsen, F., 2006, Influence of the ignition source location on the determination of the explosion pressure at elevated initial pressures. J. Loss Prevent. Process Ind., 19(5): 459–462. Vanderstraeten, B., Tuerlinckx, D., Berghmans, J., Vliegen, S., Oost, E.V. and Smit, B., 1997, Experimental study of the pressure and temperature dependence on the upper flammability limit of methane/air mixtures. J. Hazard. Mater., 56(3): 237–246. Westbrook, C.K., 2000, Chemical kinetics of hydrocarbon ignition in practical combustion systems, In Proceedings of the 28th international symposium on combustion Edinburgh, , pp. 1563–1577.