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Influence of the ignition delay time on the explosion parameters of hydrocarbon–air–oxygen mixtures at elevated pressure and temperature
Proceedings of the
Proceedings of the Combustion Institute 30 (2005) 1933–1939
Combustion Institute www.elsevier.com/locate/proci
Influence of the ignition delay time on the explosion parameters of hydrocarbon–air–oxygen mixtures at elevated pressure and temperature A.A. Pekalski*, E. Terli, J.F. Zevenbergen, S.M. Lemkowitz, H.J. Pasman DelfChemTech, Explosion Group, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
Abstract Combustion phenomena change as the conditions in which they are occurring change. Proper understanding and reliable prediction of these phenomena, including important explosion indexes (e.g., flammability limits, explosion pressures), are required for achieving safe and optimal performance of industrial processes and creating new applications. To this end, we investigated the influence of the residence time on aforementioned parameters of n-butane–oxygen mixture and a typical mixture for ethylene oxide production: methane–ethylene–oxygen, focusing on how elevated conditions affect the upper explosion limit and the explosion pressure. Elevated initial conditions (T = 230 °C, P = 4–16 bar) cause pre-ignition reactions to occur in the regime of the low temperature oxidation mechanism (LTOM). These reactions change the mixture composition prior to ignition. For both mixtures investigated, these changes in the initial mixture composition, due to pre-ignition reactions, result in a different explosion pressure. This is significant, because pressure rise is used as the ignition criterion. Consequently, a different classification of the investigated mixtures, as flammable or non-flammable, is possible, depending on the residence time prior to ignition. The experimental results are compared with theoretical calculations performed using detailed reaction kinetic models. Ó 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Process safety; Explosion indices; Pre-ignition reactions; Low temperature oxidation mechanism
1. Introduction Explosion indices, which serve for design and risk assessment purposes, are relatively well known at standard conditions. However, many processes, especially in the petrochemical industry, operate at elevated temperature and pressure. *
Corresponding author. Fax: +31 15 2784945. E-mail addresses: [email protected] (A.A. Pekalski), [email protected] (S.M. Lemkowitz).
While the values of such explosion parameters at these elevated conditions are essential to safety and reliable operation, they are nevertheless hardly available in the literature. Typical examples of processes that possess higher explosion risk due to elevated conditions are partial oxidation processes in which the oxidizer is directly mixed with the hydrocarbon. A summary of operational conditions of several of such processes is given in Table 1. In these processes, a reactive mixture is brought to elevated conditions at which spontaneous
1540-7489/$ - see front matter Ó 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2004.08.262
1934
A.A. Pekalski et al. / Proceedings of the Combustion Institute 30 (2005) 1933–1939
Table 1 Partial oxidation processes and their conditions in chemical industry Final product Acetic acid (From: acetaldehyde, alkanes, alkenes, light gasoline, methanol) Acetaldehyde (From: ethylene, ethanol) Ethylene oxide (From: ethylene) Propylene oxide (From: propylene) Maleic anhydride (From: benzene, butene, butane) Phtalic anhydride (From: naphthalene, o-xylene, butane)
Temperature (°C)
Pressure (bar)
6.0 (1994)
50–200
15–80
2.4 (1993) 11.2 (1995) 4.0 (1993) 0.87 (1995)
100–460 200–300 90–140 350–500
3–20 10–30 15–65 2–5
2.9 (1995)
150–550
1–3
Annual world production (106 tonnes/year)
ignition may occur. If too much oxygen is added, exceeding the upper explosion limit, the mixture becomes more reactive, and the pressure increase will be in the form of an explosion. If the flammability limit is not exceeded, the pressure increase will be in the form of a cool flame phenomenon. An explosion will yield a higher explosion pressure. However, a too low oxygen concentration reduces the process productivity and competitiveness. The optimal operation conditions are therefore close to, but outside of the upper explosion limit, i.e., the processes operate with highly reactive mixtures, but these mixtures do not spontaneously ignite. Mostly such temperature, pressure, composition, and residence time values are not well known, and empirically determined safety margins are applied, preventing the achievement of the optimal process conditions. Such knowledge, which includes detailed knowledge of the upper explosion limit and mixture reactivity, would additionally enable a critical analysis of, for example, engineering factors, like dead spaces, which could compromise process safety. Such knowledge, that is, understanding and predicting combustion behaviour at such conditions, is difficult due to complex physical–chemical interactions. Besides the lack of the explosion parameters at elevated conditions, also standards for determining such explosion parameters at elevated conditions do not cover the temperature and pressure range required by industry. This hiatus leads to the use of a variety of experimental methodologies (apparatus, procedures), resulting in different values. The only standard for flammability limit determination [1] at elevated temperature and pressure covers the temperature range up to 200 °C and a final pressure up to 138 bar. The test mixture is considered to be flammable if the pressure rise due to the ignition exceeds 7% of the initial pressure. The residence time of the test mixture prior to the ignition is, however, not specified and in fact it is not even considered. A question critical both to safety and proper operation is whether a hydrocarbon/air or oxygen mixture kept below 200 °C at elevated pressure
can or cannot self-ignite, given sufficient time [5]. Zabetakis [2] reported that the auto-ignition temperature (AIT) of a hydrocarbon in air decreases with increasing carbon number for saturated straight (non-branched) hydrocarbons (Table 2). Hydrocarbons higher than hexane will self-ignite at temperatures close to 200 °C at atmospheric pressure. However, increase in the initial pressure lowers significantly the AIT [2,4]. Thus, it can be expected that for higher hydrocarbons at an initial temperature close to 200 °C and at elevated pressures, pre-ignition reactions will start at the moment oxygen contacts the fuel. Detection of the pre-ignition reactions at an early stage is difficult due to the unstable character of the formed compounds and their low concentration. However, due to their significant contribution to the combustion process by degenerate and chain branching reactions, their presence even at ppm level is critical. Their formation in general follows the low temperature oxidation mechanism (LTOM) in which peroxides, diperoxides, and other active radicals and species are formed, building up a radical pool in the mixture. Above critical conditions, the mixture will self-ignite, exhibiting pressure and temperature rise. The combustion process that takes place will depend on the actual conditions, including residence time, so that the pressure or temperature rise may
Table 2 Autoignition temperature (AIT) of saturated hydrocarbons at atmospheric pressure [3] Compound
AIT (°C)
Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane Dodecane
537 472 450 287 260 225 204 206 205 210 203
A.A. Pekalski et al. / Proceedings of the Combustion Institute 30 (2005) 1933–1939
evolve into cool flame phenomena, two- or multiple-stage ignition, or explosion [5]. The idea that the residence time could influence the explosion parameters hardly exists and therefore it has not been explored. The time that elapses between the moment of mixture creation and the moment the mixture is ignited is also called the ignition delay time (IDT). Wierzba and Ale [6] investigated the influence of the IDT on the flammability limits of binary mixtures of hydrogen, methane, ethylene, and propane in air at atmospheric pressure only. They noticed a considerable change in the upper explosion limit for hydrocarbons, except methane, and attributed these changes to heterogeneous reactions occurring on apparatus surface, thus disregarding the gas phase chemistry. Their ignition criterion, flame propagation over the test apparatus, differed from the ASTM one (7% pressure rise). Pekalski et al. [5] presented the idea that post-cool flame products can increase the upper explosion limits, thus increasing explosion risk. Recently, Romano et al. [7], based on experimental evidence, demonstrated that post-cool flame products shorten the run up distance to detonations by up to 50%. 1.1. Problem formulation The practical and theoretical importance of the above-mentioned information has prompted us to perform an experimental programme concerning these phenomena. We wanted to observe if there is any influence of the ignition delay time on the explosion pressure and the maximum rate of pressure rise. A flammable mixture, at elevated conditions, close to its upper explosion limit was ignited after different residence times in the explosion vessel. The explosion pressure can be affected due to initial mixture composition change. Formation of radicals requires time and heat, the former provided by residence time in the equipment, and the latter provided by the elevated temperature of the walls of the test vessel. These pre-ignition compounds (H, OH, alkyl radicals, etc.), being endothermically formed through the transfer of heat from apparatus walls to the mixture, have a higher heat of formation than the initial compounds present in the mixture. This process of energy addition to the system is important because it means that the overall heat of formation of the mixture, and hence its energetic state, changes with time and differs from the initial energetic state. As time elapses, the concentration of radicals and peroxides changes. If the mixture is ignited, the formed peroxides and diperoxides will thermally decompose by the heat of the propagating flame releasing one and two hydroperoxy radicals, respectively. These radicals boost the flame propagation; thus, change in the maximum rate of pressure rise is also expected.
1935
Fig. 1. Cross-section of the strengthened 20-litre sphere (A) and location of the thermocouples and pressure transducers (B).
2. Explosion apparatus The test equipment used is a strengthened 20litre sphere (Fig. 1). The strengthened sphere is a reinforced and upgraded 20-litre explosion sphere. In comparison with the standard 20-litre sphere [8,9], it handles a wider range of operating conditions. The dimensions of the equipment are identical to the standard 20-litre sphere. With the strengthened 20-litre sphere it is possible to conduct experiments at process conditions, like high initial pressures, up to 35 bar, and high initial temperatures, up to 300 °C. The explosion chamber is connected to a storage canister through a duct. A fast-acting valve opens and closes the borehole of the duct within an adjustable time, called the injection time. Both the explosion chamber and storage canister have double walls to allow the circulation of a thermofluid for heating and cooling. The heating/cooling system is constructed in such a way that it enables simultaneous heating or cooling of the explosion chamber and storage canister to the same temperature. It is also possible to heat or cool the vessels to different individual temperatures. The temperature of the vessels is monitored by thermocouples (type K). The development of the explosion in the explosion chamber is traced independently by two piezoelectrical pressure transducers (type 7001 manufactured by Kistler). Furthermore, a total of six thermocouples (type K), three horizontally and three vertically, are placed inside the sphere at equal distances from each other and the wall. The signals of the pressure transducers and thermocouples are recorded using a computer with a sufficiently high sampling rate. Between the two electrodes, a tungsten fused wire is placed, generating an ignition energy of 3.5 J. 3. Experimental conditions and procedure 3.1. Experimental conditions A great difficulty of this research was to find a proper combination of fuel type, temperature,
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A.A. Pekalski et al. / Proceedings of the Combustion Institute 30 (2005) 1933–1939
Table 3 Experimental conditions Composition (mole %) Mixture 1
C4H10 78.2 ± 0.2
Mixture 2
CH4 50.7 ± 0.2
C2H4 40.7 ± 0.2
P (bar)
CO2 0.025 ± 0.03
pressure, and oxidizer concentration such that the pre-ignition reactions would occur. However, the mixture could not be so reactive that it would self-ignite before being ignited by the ignition source. We have chosen two mixtures: n-butane–oxygen, since n-butane is known to exhibit pre-ignition reactions and cool flame phenomena. The second selected mixture was a typical mixture used for ethylene oxide production, for which the combustion processes related to the LTOM (cool flame, two or multiple stage ignition) are less investigated than with the former mixture. The mixture consists of methane, ethylene, carbon dioxide, and oxygen. The composition, temperature, and pressure were chosen based on previously determined experimental results and are given in Table 3. 3.2. Experimental procedure To obtain well-mixed, stationary, uniform mixtures of fuel/oxidizer at each experiment, the following procedure was followed. The sphere was set to the desired temperature and given time to reach steady-state conditions. Any possible solid post-explosion remains were removed using an airgun. Prior to closing the sphere, the tungsten fused wire was connected between the electrodes. The partial pressure method was used for mixture preparation with a high accuracy pressure manometer of 2 millibar (Ruska 7000). In the meantime, a certain amount of oxygen was added to the storage canister. The experiment starts when an appropriate amount of oxygen was injected into the explosion sphere. After rapid injection of the oxygen, a homogeneous mixture was obtained. With the aid of the data acquisition program, the pressure–time and temperature–time curves were recorded and later analysed. The mixture was released from the sphere, and it was flushed with carbon dioxide. After flushing, the sphere was opened and the procedure was repeated. 4. Experimental results A typical experimental run is presented in Fig. 2. Oxygen is rapidly injected into the mixture, causing the initial pressure increase. After the required ignition delay time (in this case
T (°C)
O2 21.8 ± 0.2
4.1 ± 0.1
227 ± 0.5
O2 7.3 ± 0.1
16.1 ± 0.1
227 ± 0.5
Fig. 2. A typical explosion pressure, temperature development during an experiment; yC4H10 = 0.78; YO2 = 0.22; Pini = 4.11 bar; Tini = 227.4 °C.
IDT = 120 s), the mixture is ignited, causing a pressure and temperature increase. The response time of the thermocouples can be used for tracking the flame propagation. First the vertical and later the horizontal thermocouples responded. 4.1. Reproducibility of experiments The experiments showed good reproducibility. Table 4 presents the results of duplicated experiments. 4.2. Influence of the IDT on the measured explosion parameters For mixture 1, the influence of the IDT on the explosion parameters is illustrated in Fig. 3. To facilitate the interpretation of Fig. 3, the explosion pressure points are connected by a line. The explosion pressure ratio (Pexp/Pini) increases with increasing IDT to a maximum at an IDT of 40 s. After the maximum, the explosion pressure ratio drops with increasing IDT. It reaches a minimum at an IDT of 360 s. The explosion pressure ratio rises to a second maximum at an IDT of 1020 s. Further increase in the IDT causes the explosion pressure ratio to decrease again. The (dP/dt)max roughly follows a similar trend: increase to a maximum, decrease to a minimum, increase to another maximum, and decrease again. The scatter of the (dP/dt)max data points is higher than the explosion pressure ratio data points. Also, there are some outliers. Furthermore, the IDT, where the second maximum of the (dP/dt)max occurs (720 s) is different from the
A.A. Pekalski et al. / Proceedings of the Combustion Institute 30 (2005) 1933–1939
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Table 4 Reproducibility of the explosion experiments as a function of IDT Mixture 1
Composition (mole %) C4H10/O2
Pressure rise (bar)
78.33/21.67 78.24/21.76 78.31/21.69 78.02/21.98
1.27 1.19 1.70 1.62
IDT (s)
O2/CO2/C2H4/CH4
Pressure rise (bar)
20 20 720 720 1320 1320
6.52/2.07/40.60/50.81 6.54/2.07/40.58/50.83 6.52/2.08/40.60/50.80 6.52/2.03/40.83/50.62 6.62/2.03/40.63/50.72 6.45/2.04/40.69/50.81
4.67 4.74 5.25 5.44 4.82 4.48
IDT (s) 20 20 120 120
Mixture 2
fied. With increasing IDT, the explosion pressure ratio increases to a maximum at an IDT of 720 s. After the maximum, the explosion pressure ratio drops to zero at an IDT of 3720 s. The (dP/ dt)max shows a similar trend. After 62 min, ignition of the mixture did not lead to an explosion. The mixture was no longer able to sustain flame propagation. In summary, both the explosion pressure ratio and the (dP/dt)max show a maximum as a function of the IDT. Fig. 3. Explosion pressure and maximum rate of pressure increase as a function of IDT in n-C4H10/O2 mixture.
IDT where the second maximum of the explosion pressure ratio (1020 s) occurs. In Fig. 3, two data points of each explosion parameter are marked as open symbols; at these data points, the mixture self-ignited. An interesting fact of these self-ignition data points is that the (dP/dt)max is very high. A mixture is highly sensitized for explosion just before self-ignition. For the mixture 2, the influence of the IDT on the explosion parameters is illustrated in Fig. 4. All experiments were performed twice, except at an IDT of 1020 s. A clear trend can be identi-
Fig. 4. Explosion pressure and maximum rate of pressure increase as a function of IDT in CH4/C2H4/ CO2/O2 mixtures.
5. Theoretical calculations To gain more insight into the observed phenomena, an attempt was made to calculate the experimental results for the n-butane–oxygen mixture. The overall heat transfer coefficient was determined for several experiments and averaged giving the value of 15.7 W/m2K [10,11]. Detailed kinetic modeling with the low temperature oxidation mechanism was selected [12]. The Chemkin 3.6.2 code collection was used for solving the model equations over time. Figure 5 presents the temperature development and consumption of the initial compounds, i.e., oxygen and n-butane. Note the almost total oxygen consumption after the temperature rise. Figure 6 shows a steady
Fig. 5. Calculated temperature development and initial compounds consumption over time; Tini = 500.15 K, Pini = 4.1 bar, yC4H10 = 0.78, YO2 = 0.22.
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A.A. Pekalski et al. / Proceedings of the Combustion Institute 30 (2005) 1933–1939
A steady increase in the explosion pressure ratio can be observed due to the pre-ignition reactions and formation of new species. The maximum in the explosion pressure is observed at the moment when the highest concentration of peroxides and diperoxides exists in the mixture.
6. Discussion 6.1. On the flammability limit criterion based on pressure rise Fig. 6. Calculated temperature, peroxide and diperoxide development over time; Tini = 500.15 K, Pini = 4.1 bar, yC4H10 = 0.78, YO2 = 0.22.
build up of some peroxides and diperoxides over time. The explosion pressures can be readily estimated utilizing routine equilibrium thermodynamics procedures to determine maximum explosion pressures for ideal gas and ideal condensed phase products. Thermochemical equilibrium calculations have been carried out with Chemkin 3.6.2. An assumption used in the calculations, but not fulfilled in practice for barely reactive mixtures, is that all molecules considered in the system actually participate in the equilibrium calculations. For hardly reactive mixtures, however, only a fraction of the initial mixture actually participates in the combustion reactions [13]. Therefore, the results of the explosion pressure calculations for our mixture should be considered to be maximum values. The mixture compositions calculated in Figs. 5 and 6 were frozen in time, with steps of 10, 50, and 100 s, depending on the rate of temperature rise, and were used as the input value for the equilibrium calculations. In all cases, eight compounds representing the highest concentrations and C(S) were taken. The calculated results, presented as the explosion pressure ratio, are given in Fig. 7.
Fig. 7. Calculated explosion pressure ratio and temperature of initial n-butane–oxygen mixture (yC4H10 = 0.78 YO2 = 0.22, Tini = 500 K, Pini = 4.1 bar) ignited at different ignition delay time.
From Fig. 7, it can be observed that after almost complete consumption of oxygen (e.g., at the IDT of 600 s) the explosion pressure is higher than that of the initial mixture. This leads to the conclusion that the new species formed during the pre-ignition reactions increase the explosion pressure. This was indeed experimentally observed (Fig. 3). Furthermore, it is possible that a mixture, which is initially not flammable, based on the 7% pressure rise criterion, may exceed this value if pre-ignition reactions alter its composition. Such a situation is illustrated by the dashed line (marked by IC, ignition criterion), which illustrates the ignition threshold in Fig. 7. Additionally, based on the experimental results shown in Fig. 3, it can be argued that it would be possible to find experimental data showing that the explosion pressure ratio can be initially below the ignition criterion, then rise to above this criterion, and then again drop below the ignition criterion. As a result, the mixture would be classified as non-flammable, then as flammable, and then again as non-flammable. We believe that such experimental data can be obtained, but the right conditions must be found. 6.2. On the flammability limit criterion based on flame propagation It was demonstrated that the mixture composition was altered due to the pre-ignition reactions. The modification of the composition will affect the flammability limit, as was experimentally determined by Wierzba and Ale [6]. However, contrary to their conclusions, we showed that reactions occurring in the gas phase are sufficient to significantly change the initial mixture composition prior to ignition. The heterogeneous reactions may also contribute to the change of the initial mixture composition. Research on heterogeneous reactions is currently being executed. Additionally, it was experimentally observed that if a reactive mixture is given sufficient time, due to the pre-ignition reactions the mixture can exhibit single or multiple cool flames, or just consume one or both of the reactants without any pressure increase. The final combustion phenom-
A.A. Pekalski et al. / Proceedings of the Combustion Institute 30 (2005) 1933–1939
ena depend on the conditions in which the investigated mixture finds itself at that given moment.
1939
species and the low concentrations at which they are present. However, their presence is significant for the final test output. This makes an early anticipation of the pre-ignition reactions difficult.
7. Conclusions It has been shown that pre-ignition reactions affect mixture reactivity and explosion parameters, like explosion pressure and the maximum rate of pressure rise. It is also plausible that a gas motion, inside the test vessel, induced by a temperature gradient due to pre-ignition reactions contributes, besides the mixture composition change, to the change of the explosion indices values as a function of the IDT. For flammability limit determination at elevated conditions, in which a pressure increase is used as the ignition criterion, the final output of the tests may change from flammable to non-flammable or vice versa as a function of the ignition delay time, depending on the presence and concentrations of the formed species at the time of ignition. The same conclusion also refers to the flammability limit criterion based on flame propagation. Under certain conditions, it may be possible to have a mixture that after ignition will not exceed the threshold value for the flammability limit, and hence be classified as non-flammable. However, for the same mixture under the same conditions, after a certain period of time, species formed during the pre-ignition reactions will cause the pressure to exceed the flammability limit, herewith classifying the same mixture as flammable. Our research thus clearly shows that the classification of a given chemical mixture under given conditions into ÔflammableÕ or Ônon-flammableÕ can be significantly influenced by ignition delay time. In industrial practice, ignition delay time is related to residence time in equipment. Therefore, we believe that our results have important relevance to industrial practice. We believe that our results are in general applicable to other hydrocarbon–air and hydrocarbon–oxygen mixtures; that is, that under certain conditions, pre-ignition reactions occur with time that can significantly affect explosion behaviour of hydrocarbon–air/oxygen mixtures under industrial conditions. The detection of pre-ignition reactions is difficult due to the unstable character of the formed
Acknowledgment Financial support of this work by the European Community within the Fifth Framework Programme on Energy, Environment and Sustainable Development, contract EVG1-CT-200200072, Project SAFEKINEX is gratefully acknowledged.
References [1] ASTM E918-83 Determining limits of flammability of chemicals at elevated temperature and pressure. [2] M.G. Zabetakis, Bur. Mines Bull. 627 (1965). [3] NFPA 325, National Fire Protection Association, 1994. [4] J.U. Steinle, Phys. Chem. Chem. Phys. 99 (1) (1995) 66–73. [5] A.A. Pekalski, J.F. Zevenbergen, H.J. Pasman, S.M. Lemkowitz, A.E. Dahoe, B. Scarlett, J. Hazard. Mater. 93 (2002) 93–105. [6] I. Wierzba, B.B. Ale, J. Eng. Gas Turbines Power 121 (1) (1999) 74–79. [7] M.P. Romano, M.I. Radulescu, A.J. Higgins, J.H.S. Lee, Combust. Flame 132 (2003) 387–394. [8] ASTM E1226-94, Standard test method for pressure and rate of pressure rise for combustible dusts. [9] ISO 6184-2:1985, Explosion protection systems — Part 2: Determination of explosion indices of combustible gases in air. [10] T.J. Snee, J.F. Griffiths, Combust. Flame 75 (1989) 381–395. [11] J.F. Griffiths, P.G. Felton, P. Gray, Proc. Combust. Inst. 14 (1972) 453–462. [12] V. Warth, N. Stef, P.A. Glaude, F. Battin-Leclerc, G. Scacchi, G.M. Come, Combust. Flame 114 (1998) 81–102. [13] Pekalski A.A., Schildberg H.P., Smallegange P.S.D., Lemkowitz S.M., Zevenbergen J.Z., Braithwaite M., Pasman H.J., Determination of the explosion behaviour of methane and propylene in air or oxygen at standard and elevated conditions, in: Proceedings of the 11th International Symposium on Loss Prevention and Safety Promotion in the Process Industries, 31 May–3 June 2004, Praha, Czech Republic.
Proceedings of the Combustion Institute 30 (2005) 1933–1939
Combustion Institute www.elsevier.com/locate/proci
Influence of the ignition delay time on the explosion parameters of hydrocarbon–air–oxygen mixtures at elevated pressure and temperature A.A. Pekalski*, E. Terli, J.F. Zevenbergen, S.M. Lemkowitz, H.J. Pasman DelfChemTech, Explosion Group, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
Abstract Combustion phenomena change as the conditions in which they are occurring change. Proper understanding and reliable prediction of these phenomena, including important explosion indexes (e.g., flammability limits, explosion pressures), are required for achieving safe and optimal performance of industrial processes and creating new applications. To this end, we investigated the influence of the residence time on aforementioned parameters of n-butane–oxygen mixture and a typical mixture for ethylene oxide production: methane–ethylene–oxygen, focusing on how elevated conditions affect the upper explosion limit and the explosion pressure. Elevated initial conditions (T = 230 °C, P = 4–16 bar) cause pre-ignition reactions to occur in the regime of the low temperature oxidation mechanism (LTOM). These reactions change the mixture composition prior to ignition. For both mixtures investigated, these changes in the initial mixture composition, due to pre-ignition reactions, result in a different explosion pressure. This is significant, because pressure rise is used as the ignition criterion. Consequently, a different classification of the investigated mixtures, as flammable or non-flammable, is possible, depending on the residence time prior to ignition. The experimental results are compared with theoretical calculations performed using detailed reaction kinetic models. Ó 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Process safety; Explosion indices; Pre-ignition reactions; Low temperature oxidation mechanism
1. Introduction Explosion indices, which serve for design and risk assessment purposes, are relatively well known at standard conditions. However, many processes, especially in the petrochemical industry, operate at elevated temperature and pressure. *
Corresponding author. Fax: +31 15 2784945. E-mail addresses: [email protected] (A.A. Pekalski), [email protected] (S.M. Lemkowitz).
While the values of such explosion parameters at these elevated conditions are essential to safety and reliable operation, they are nevertheless hardly available in the literature. Typical examples of processes that possess higher explosion risk due to elevated conditions are partial oxidation processes in which the oxidizer is directly mixed with the hydrocarbon. A summary of operational conditions of several of such processes is given in Table 1. In these processes, a reactive mixture is brought to elevated conditions at which spontaneous
1540-7489/$ - see front matter Ó 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2004.08.262
1934
A.A. Pekalski et al. / Proceedings of the Combustion Institute 30 (2005) 1933–1939
Table 1 Partial oxidation processes and their conditions in chemical industry Final product Acetic acid (From: acetaldehyde, alkanes, alkenes, light gasoline, methanol) Acetaldehyde (From: ethylene, ethanol) Ethylene oxide (From: ethylene) Propylene oxide (From: propylene) Maleic anhydride (From: benzene, butene, butane) Phtalic anhydride (From: naphthalene, o-xylene, butane)
Temperature (°C)
Pressure (bar)
6.0 (1994)
50–200
15–80
2.4 (1993) 11.2 (1995) 4.0 (1993) 0.87 (1995)
100–460 200–300 90–140 350–500
3–20 10–30 15–65 2–5
2.9 (1995)
150–550
1–3
Annual world production (106 tonnes/year)
ignition may occur. If too much oxygen is added, exceeding the upper explosion limit, the mixture becomes more reactive, and the pressure increase will be in the form of an explosion. If the flammability limit is not exceeded, the pressure increase will be in the form of a cool flame phenomenon. An explosion will yield a higher explosion pressure. However, a too low oxygen concentration reduces the process productivity and competitiveness. The optimal operation conditions are therefore close to, but outside of the upper explosion limit, i.e., the processes operate with highly reactive mixtures, but these mixtures do not spontaneously ignite. Mostly such temperature, pressure, composition, and residence time values are not well known, and empirically determined safety margins are applied, preventing the achievement of the optimal process conditions. Such knowledge, which includes detailed knowledge of the upper explosion limit and mixture reactivity, would additionally enable a critical analysis of, for example, engineering factors, like dead spaces, which could compromise process safety. Such knowledge, that is, understanding and predicting combustion behaviour at such conditions, is difficult due to complex physical–chemical interactions. Besides the lack of the explosion parameters at elevated conditions, also standards for determining such explosion parameters at elevated conditions do not cover the temperature and pressure range required by industry. This hiatus leads to the use of a variety of experimental methodologies (apparatus, procedures), resulting in different values. The only standard for flammability limit determination [1] at elevated temperature and pressure covers the temperature range up to 200 °C and a final pressure up to 138 bar. The test mixture is considered to be flammable if the pressure rise due to the ignition exceeds 7% of the initial pressure. The residence time of the test mixture prior to the ignition is, however, not specified and in fact it is not even considered. A question critical both to safety and proper operation is whether a hydrocarbon/air or oxygen mixture kept below 200 °C at elevated pressure
can or cannot self-ignite, given sufficient time [5]. Zabetakis [2] reported that the auto-ignition temperature (AIT) of a hydrocarbon in air decreases with increasing carbon number for saturated straight (non-branched) hydrocarbons (Table 2). Hydrocarbons higher than hexane will self-ignite at temperatures close to 200 °C at atmospheric pressure. However, increase in the initial pressure lowers significantly the AIT [2,4]. Thus, it can be expected that for higher hydrocarbons at an initial temperature close to 200 °C and at elevated pressures, pre-ignition reactions will start at the moment oxygen contacts the fuel. Detection of the pre-ignition reactions at an early stage is difficult due to the unstable character of the formed compounds and their low concentration. However, due to their significant contribution to the combustion process by degenerate and chain branching reactions, their presence even at ppm level is critical. Their formation in general follows the low temperature oxidation mechanism (LTOM) in which peroxides, diperoxides, and other active radicals and species are formed, building up a radical pool in the mixture. Above critical conditions, the mixture will self-ignite, exhibiting pressure and temperature rise. The combustion process that takes place will depend on the actual conditions, including residence time, so that the pressure or temperature rise may
Table 2 Autoignition temperature (AIT) of saturated hydrocarbons at atmospheric pressure [3] Compound
AIT (°C)
Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane Dodecane
537 472 450 287 260 225 204 206 205 210 203
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evolve into cool flame phenomena, two- or multiple-stage ignition, or explosion [5]. The idea that the residence time could influence the explosion parameters hardly exists and therefore it has not been explored. The time that elapses between the moment of mixture creation and the moment the mixture is ignited is also called the ignition delay time (IDT). Wierzba and Ale [6] investigated the influence of the IDT on the flammability limits of binary mixtures of hydrogen, methane, ethylene, and propane in air at atmospheric pressure only. They noticed a considerable change in the upper explosion limit for hydrocarbons, except methane, and attributed these changes to heterogeneous reactions occurring on apparatus surface, thus disregarding the gas phase chemistry. Their ignition criterion, flame propagation over the test apparatus, differed from the ASTM one (7% pressure rise). Pekalski et al. [5] presented the idea that post-cool flame products can increase the upper explosion limits, thus increasing explosion risk. Recently, Romano et al. [7], based on experimental evidence, demonstrated that post-cool flame products shorten the run up distance to detonations by up to 50%. 1.1. Problem formulation The practical and theoretical importance of the above-mentioned information has prompted us to perform an experimental programme concerning these phenomena. We wanted to observe if there is any influence of the ignition delay time on the explosion pressure and the maximum rate of pressure rise. A flammable mixture, at elevated conditions, close to its upper explosion limit was ignited after different residence times in the explosion vessel. The explosion pressure can be affected due to initial mixture composition change. Formation of radicals requires time and heat, the former provided by residence time in the equipment, and the latter provided by the elevated temperature of the walls of the test vessel. These pre-ignition compounds (H, OH, alkyl radicals, etc.), being endothermically formed through the transfer of heat from apparatus walls to the mixture, have a higher heat of formation than the initial compounds present in the mixture. This process of energy addition to the system is important because it means that the overall heat of formation of the mixture, and hence its energetic state, changes with time and differs from the initial energetic state. As time elapses, the concentration of radicals and peroxides changes. If the mixture is ignited, the formed peroxides and diperoxides will thermally decompose by the heat of the propagating flame releasing one and two hydroperoxy radicals, respectively. These radicals boost the flame propagation; thus, change in the maximum rate of pressure rise is also expected.
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Fig. 1. Cross-section of the strengthened 20-litre sphere (A) and location of the thermocouples and pressure transducers (B).
2. Explosion apparatus The test equipment used is a strengthened 20litre sphere (Fig. 1). The strengthened sphere is a reinforced and upgraded 20-litre explosion sphere. In comparison with the standard 20-litre sphere [8,9], it handles a wider range of operating conditions. The dimensions of the equipment are identical to the standard 20-litre sphere. With the strengthened 20-litre sphere it is possible to conduct experiments at process conditions, like high initial pressures, up to 35 bar, and high initial temperatures, up to 300 °C. The explosion chamber is connected to a storage canister through a duct. A fast-acting valve opens and closes the borehole of the duct within an adjustable time, called the injection time. Both the explosion chamber and storage canister have double walls to allow the circulation of a thermofluid for heating and cooling. The heating/cooling system is constructed in such a way that it enables simultaneous heating or cooling of the explosion chamber and storage canister to the same temperature. It is also possible to heat or cool the vessels to different individual temperatures. The temperature of the vessels is monitored by thermocouples (type K). The development of the explosion in the explosion chamber is traced independently by two piezoelectrical pressure transducers (type 7001 manufactured by Kistler). Furthermore, a total of six thermocouples (type K), three horizontally and three vertically, are placed inside the sphere at equal distances from each other and the wall. The signals of the pressure transducers and thermocouples are recorded using a computer with a sufficiently high sampling rate. Between the two electrodes, a tungsten fused wire is placed, generating an ignition energy of 3.5 J. 3. Experimental conditions and procedure 3.1. Experimental conditions A great difficulty of this research was to find a proper combination of fuel type, temperature,
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Table 3 Experimental conditions Composition (mole %) Mixture 1
C4H10 78.2 ± 0.2
Mixture 2
CH4 50.7 ± 0.2
C2H4 40.7 ± 0.2
P (bar)
CO2 0.025 ± 0.03
pressure, and oxidizer concentration such that the pre-ignition reactions would occur. However, the mixture could not be so reactive that it would self-ignite before being ignited by the ignition source. We have chosen two mixtures: n-butane–oxygen, since n-butane is known to exhibit pre-ignition reactions and cool flame phenomena. The second selected mixture was a typical mixture used for ethylene oxide production, for which the combustion processes related to the LTOM (cool flame, two or multiple stage ignition) are less investigated than with the former mixture. The mixture consists of methane, ethylene, carbon dioxide, and oxygen. The composition, temperature, and pressure were chosen based on previously determined experimental results and are given in Table 3. 3.2. Experimental procedure To obtain well-mixed, stationary, uniform mixtures of fuel/oxidizer at each experiment, the following procedure was followed. The sphere was set to the desired temperature and given time to reach steady-state conditions. Any possible solid post-explosion remains were removed using an airgun. Prior to closing the sphere, the tungsten fused wire was connected between the electrodes. The partial pressure method was used for mixture preparation with a high accuracy pressure manometer of 2 millibar (Ruska 7000). In the meantime, a certain amount of oxygen was added to the storage canister. The experiment starts when an appropriate amount of oxygen was injected into the explosion sphere. After rapid injection of the oxygen, a homogeneous mixture was obtained. With the aid of the data acquisition program, the pressure–time and temperature–time curves were recorded and later analysed. The mixture was released from the sphere, and it was flushed with carbon dioxide. After flushing, the sphere was opened and the procedure was repeated. 4. Experimental results A typical experimental run is presented in Fig. 2. Oxygen is rapidly injected into the mixture, causing the initial pressure increase. After the required ignition delay time (in this case
T (°C)
O2 21.8 ± 0.2
4.1 ± 0.1
227 ± 0.5
O2 7.3 ± 0.1
16.1 ± 0.1
227 ± 0.5
Fig. 2. A typical explosion pressure, temperature development during an experiment; yC4H10 = 0.78; YO2 = 0.22; Pini = 4.11 bar; Tini = 227.4 °C.
IDT = 120 s), the mixture is ignited, causing a pressure and temperature increase. The response time of the thermocouples can be used for tracking the flame propagation. First the vertical and later the horizontal thermocouples responded. 4.1. Reproducibility of experiments The experiments showed good reproducibility. Table 4 presents the results of duplicated experiments. 4.2. Influence of the IDT on the measured explosion parameters For mixture 1, the influence of the IDT on the explosion parameters is illustrated in Fig. 3. To facilitate the interpretation of Fig. 3, the explosion pressure points are connected by a line. The explosion pressure ratio (Pexp/Pini) increases with increasing IDT to a maximum at an IDT of 40 s. After the maximum, the explosion pressure ratio drops with increasing IDT. It reaches a minimum at an IDT of 360 s. The explosion pressure ratio rises to a second maximum at an IDT of 1020 s. Further increase in the IDT causes the explosion pressure ratio to decrease again. The (dP/dt)max roughly follows a similar trend: increase to a maximum, decrease to a minimum, increase to another maximum, and decrease again. The scatter of the (dP/dt)max data points is higher than the explosion pressure ratio data points. Also, there are some outliers. Furthermore, the IDT, where the second maximum of the (dP/dt)max occurs (720 s) is different from the
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Table 4 Reproducibility of the explosion experiments as a function of IDT Mixture 1
Composition (mole %) C4H10/O2
Pressure rise (bar)
78.33/21.67 78.24/21.76 78.31/21.69 78.02/21.98
1.27 1.19 1.70 1.62
IDT (s)
O2/CO2/C2H4/CH4
Pressure rise (bar)
20 20 720 720 1320 1320
6.52/2.07/40.60/50.81 6.54/2.07/40.58/50.83 6.52/2.08/40.60/50.80 6.52/2.03/40.83/50.62 6.62/2.03/40.63/50.72 6.45/2.04/40.69/50.81
4.67 4.74 5.25 5.44 4.82 4.48
IDT (s) 20 20 120 120
Mixture 2
fied. With increasing IDT, the explosion pressure ratio increases to a maximum at an IDT of 720 s. After the maximum, the explosion pressure ratio drops to zero at an IDT of 3720 s. The (dP/ dt)max shows a similar trend. After 62 min, ignition of the mixture did not lead to an explosion. The mixture was no longer able to sustain flame propagation. In summary, both the explosion pressure ratio and the (dP/dt)max show a maximum as a function of the IDT. Fig. 3. Explosion pressure and maximum rate of pressure increase as a function of IDT in n-C4H10/O2 mixture.
IDT where the second maximum of the explosion pressure ratio (1020 s) occurs. In Fig. 3, two data points of each explosion parameter are marked as open symbols; at these data points, the mixture self-ignited. An interesting fact of these self-ignition data points is that the (dP/dt)max is very high. A mixture is highly sensitized for explosion just before self-ignition. For the mixture 2, the influence of the IDT on the explosion parameters is illustrated in Fig. 4. All experiments were performed twice, except at an IDT of 1020 s. A clear trend can be identi-
Fig. 4. Explosion pressure and maximum rate of pressure increase as a function of IDT in CH4/C2H4/ CO2/O2 mixtures.
5. Theoretical calculations To gain more insight into the observed phenomena, an attempt was made to calculate the experimental results for the n-butane–oxygen mixture. The overall heat transfer coefficient was determined for several experiments and averaged giving the value of 15.7 W/m2K [10,11]. Detailed kinetic modeling with the low temperature oxidation mechanism was selected [12]. The Chemkin 3.6.2 code collection was used for solving the model equations over time. Figure 5 presents the temperature development and consumption of the initial compounds, i.e., oxygen and n-butane. Note the almost total oxygen consumption after the temperature rise. Figure 6 shows a steady
Fig. 5. Calculated temperature development and initial compounds consumption over time; Tini = 500.15 K, Pini = 4.1 bar, yC4H10 = 0.78, YO2 = 0.22.
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A steady increase in the explosion pressure ratio can be observed due to the pre-ignition reactions and formation of new species. The maximum in the explosion pressure is observed at the moment when the highest concentration of peroxides and diperoxides exists in the mixture.
6. Discussion 6.1. On the flammability limit criterion based on pressure rise Fig. 6. Calculated temperature, peroxide and diperoxide development over time; Tini = 500.15 K, Pini = 4.1 bar, yC4H10 = 0.78, YO2 = 0.22.
build up of some peroxides and diperoxides over time. The explosion pressures can be readily estimated utilizing routine equilibrium thermodynamics procedures to determine maximum explosion pressures for ideal gas and ideal condensed phase products. Thermochemical equilibrium calculations have been carried out with Chemkin 3.6.2. An assumption used in the calculations, but not fulfilled in practice for barely reactive mixtures, is that all molecules considered in the system actually participate in the equilibrium calculations. For hardly reactive mixtures, however, only a fraction of the initial mixture actually participates in the combustion reactions [13]. Therefore, the results of the explosion pressure calculations for our mixture should be considered to be maximum values. The mixture compositions calculated in Figs. 5 and 6 were frozen in time, with steps of 10, 50, and 100 s, depending on the rate of temperature rise, and were used as the input value for the equilibrium calculations. In all cases, eight compounds representing the highest concentrations and C(S) were taken. The calculated results, presented as the explosion pressure ratio, are given in Fig. 7.
Fig. 7. Calculated explosion pressure ratio and temperature of initial n-butane–oxygen mixture (yC4H10 = 0.78 YO2 = 0.22, Tini = 500 K, Pini = 4.1 bar) ignited at different ignition delay time.
From Fig. 7, it can be observed that after almost complete consumption of oxygen (e.g., at the IDT of 600 s) the explosion pressure is higher than that of the initial mixture. This leads to the conclusion that the new species formed during the pre-ignition reactions increase the explosion pressure. This was indeed experimentally observed (Fig. 3). Furthermore, it is possible that a mixture, which is initially not flammable, based on the 7% pressure rise criterion, may exceed this value if pre-ignition reactions alter its composition. Such a situation is illustrated by the dashed line (marked by IC, ignition criterion), which illustrates the ignition threshold in Fig. 7. Additionally, based on the experimental results shown in Fig. 3, it can be argued that it would be possible to find experimental data showing that the explosion pressure ratio can be initially below the ignition criterion, then rise to above this criterion, and then again drop below the ignition criterion. As a result, the mixture would be classified as non-flammable, then as flammable, and then again as non-flammable. We believe that such experimental data can be obtained, but the right conditions must be found. 6.2. On the flammability limit criterion based on flame propagation It was demonstrated that the mixture composition was altered due to the pre-ignition reactions. The modification of the composition will affect the flammability limit, as was experimentally determined by Wierzba and Ale [6]. However, contrary to their conclusions, we showed that reactions occurring in the gas phase are sufficient to significantly change the initial mixture composition prior to ignition. The heterogeneous reactions may also contribute to the change of the initial mixture composition. Research on heterogeneous reactions is currently being executed. Additionally, it was experimentally observed that if a reactive mixture is given sufficient time, due to the pre-ignition reactions the mixture can exhibit single or multiple cool flames, or just consume one or both of the reactants without any pressure increase. The final combustion phenom-
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ena depend on the conditions in which the investigated mixture finds itself at that given moment.
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species and the low concentrations at which they are present. However, their presence is significant for the final test output. This makes an early anticipation of the pre-ignition reactions difficult.
7. Conclusions It has been shown that pre-ignition reactions affect mixture reactivity and explosion parameters, like explosion pressure and the maximum rate of pressure rise. It is also plausible that a gas motion, inside the test vessel, induced by a temperature gradient due to pre-ignition reactions contributes, besides the mixture composition change, to the change of the explosion indices values as a function of the IDT. For flammability limit determination at elevated conditions, in which a pressure increase is used as the ignition criterion, the final output of the tests may change from flammable to non-flammable or vice versa as a function of the ignition delay time, depending on the presence and concentrations of the formed species at the time of ignition. The same conclusion also refers to the flammability limit criterion based on flame propagation. Under certain conditions, it may be possible to have a mixture that after ignition will not exceed the threshold value for the flammability limit, and hence be classified as non-flammable. However, for the same mixture under the same conditions, after a certain period of time, species formed during the pre-ignition reactions will cause the pressure to exceed the flammability limit, herewith classifying the same mixture as flammable. Our research thus clearly shows that the classification of a given chemical mixture under given conditions into ÔflammableÕ or Ônon-flammableÕ can be significantly influenced by ignition delay time. In industrial practice, ignition delay time is related to residence time in equipment. Therefore, we believe that our results have important relevance to industrial practice. We believe that our results are in general applicable to other hydrocarbon–air and hydrocarbon–oxygen mixtures; that is, that under certain conditions, pre-ignition reactions occur with time that can significantly affect explosion behaviour of hydrocarbon–air/oxygen mixtures under industrial conditions. The detection of pre-ignition reactions is difficult due to the unstable character of the formed
Acknowledgment Financial support of this work by the European Community within the Fifth Framework Programme on Energy, Environment and Sustainable Development, contract EVG1-CT-200200072, Project SAFEKINEX is gratefully acknowledged.
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