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Flammability limits of hydrated and anhydrous ethanol at reduced pressures in aeronautical applications
Journal of Hazardous Materials 280 (2014) 174–184
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Flammability limits of hydrated and anhydrous ethanol at reduced pressures in aeronautical applications Christian J.R. Coronado a , João A. Carvalho Jr b,∗ , José C. Andrade c , Andrés Z. Mendiburu b , Ely V. Cortez c , Felipe S. Carvalho a , Beatriz Gonc¸alves a , Juan C. Quintero a , Elkin I. Gutiérrez Velásquez d , Marcos H. Silva a , José C. Santos c , Marco. A.R. Nascimento a a
Federal University of Itajubá—UNIFEI, Mechanical Engineering Institute—IEM Av. BPS 1303, Itajubá, MG, CEP 37500-903, Brazil São Paulo State University—UNESP, Campus of Guaratinguetá—FEG Av. Ariberto P. da Cunha, 333, Guaratinguetá, SP, CEP 12516-410, Brazil c National Space Research Institute—INPE, Combustion and Propulsion Laboratory—LCP Rod. Pres. Dutra, km 39, Cachoeira Paulista, SP, CEP 12630-000, Brazil d Faculty of Mechanical Engineering, Universidad Antonio Nari˜ no, Tunja, Colombia b
h i g h l i g h t s • • • • •
Flammability limits for ethanol at reduced pressures were determined. 295 experiments were carried out in total for anhydrous and hydrated ethanol. The first 80 were to calibrate the heating chamber and compare the results. 215 experiments were performed both at atmospheric and reduced pressure. Results had a correlation with the LFL obtained but UFL had some differences.
a r t i c l e
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Article history: Received 6 May 2014 Received in revised form 11 July 2014 Accepted 19 July 2014 Available online 7 August 2014 Keywords: Flammability limits Ethanol Visual criterion Pressure and temperature dependence
a b s t r a c t There is interest in finding the flammability limits of ethanol at reduced pressures for the future use of this biofuel in aeronautical applications taking into account typical commercial aviation altitude (<40,000 ft). The lower and upper flammability limits (LFL and UFL, respectively) for hydrated ethanol and anhydrous ethanol (92.6% and 99.5% p/p, respectively) were determined for a pressure of 101.3 kPa at temperatures between 0 and 200 ◦ C. A heating chamber with a spherical 20-l vessel was used. First, LFL and the UFL were determined as functions of temperature and atmospheric pressure to compare results with data published in the scientific literature. Second, after checking the veracity of the data obtained for standard atmospheric pressure, the work proceeded with reduced pressures in the same temperature range. 295 experiments were carried out in total; the first 80 were to calibrate the heating chamber and compare the results with those given in the published scientific literature. 215 experiments were performed both at atmospheric and reduced pressures. The results had a correlation with the values obtained for the LFL, but values for the UFL had some differences. With respect to the water content in ethanol, it was shown that the water vapor contained in the fuel can act as an inert substance, narrowing flammability. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Knowledge about the risks involved with the explosion of both gaseous and liquid fuel mixtures with air is of great importance to guarantee safety in industrial, domestic, and in aeronautical
∗ Corresponding author. Tel.: +55 12 31232835; fax: +55 12 31232835. E-mail addresses: [email protected], [email protected] (J.A.C. Jr). http://dx.doi.org/10.1016/j.jhazmat.2014.07.063 0304-3894/© 2014 Elsevier B.V. All rights reserved.
application. The lower (LFL) and upper (UFL) flammability limits of a fuel are key criteria to predict fires, assess the possibility of explosion, and design protection systems. The goal of this research is to better understand and characterize the flammability of hydrated (92% p/p) and anhydrous ethanol (99.5% p/p) for the aeronautical industry. Flammability limits have been thoroughly discussed in scientific literature. Probably the first works on this topic were those developed by Coward and Jones [1] and by Zabetakis [2], both for the Mines Department of the U.S. Government. Later, Kuchta [3]
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increased the range of the data obtained by Zabetakis. UFL was determined experimentally for ethane-air, propane-air, n-butaneair, ethylene-air, and propylene-air mixtures for pressures of up to 30 bar and for temperatures up to 250 ◦ C [4]. Four different numerical methods used to calculate the UFL of methane–air mixtures for pressures up to 10 bar and temperatures up to 200 ◦ C were reported and the methods were compared to experimental data [5]. Gibson [6] published data on the flammability limits of solvents at elevated temperature and pressure in air. The paper describes the measurement of flammability limits of ethanol and l-propanol and also presents data for other solvents. These authors worked at elevated temperatures of up to 250 ◦ C. The study was different in that they investigated pressures above standard atmospheric pressure. Nestor [7] and the Fuel Flammability Task Group [8] worked specifically with flammability limits for the aeronautical industry. Measurements of ignition energy for aeronautic fuels (Spark Ignition Energy Measurements in JET A) were reported by Shepherd et al. (2000). Apart from those, other works contributed to this knowledge in the area of aeronautics [9,10]. Since kerosene was introduced as a fuel for aerial civil transportation in the 1950s, aeronautical designers have given special consideration to the ullage in aircraft fuel tanks. These tanks may contain a mixture of several vapors, fuel, and air, which can ignite in the presence of an ignition source when this mixture is within the flammability limits of the fuel in use. After the flight TWA 800 accident, which went down near the coast of New York in 1996, concerns about formation of vapors in aircraft fuel tanks reached its peak. It was determined that the crucial reason for the lost plane was an explosion inside a fuel tank in the airplane wing. At the moment of explosion, the plane had approximately 50 gallons of Jet A fuel. The fuel and air contained inside the tank were heated by the air conditioning system (air packs) located directly below the tank [8]. This increased fuel vaporization in the ullage region, forming a flammable mixture which later caused the accident. Since then, flammability limit studies for the aeronautical industry have been increasingly taken into account in aircraft design. Required conditions for ignition and flame propagation inside the aircraft fuel tank depend on parameters which include fuel type, temperature, tank pressure, and oxygen concentration. It was also noted that fuel foam and spray, which may form during refueling or during a flight with fuel oxygen liberation, might extend fuel flammability limits. In this sense, when working with flammability limits of fuels for the aeronautical industry, the following details must be taken into account: altitude changes, temperature changes, ullage changes, tank ventilation, state changes of fuel (spray) by agitation due to aircraft movement, and fuel mixtures during refueling operations. Considering that Brazil is the largest ethanol producer in the world with vast experience in the use of bio-fuels, the use of ethanol in the Brazilian aircraft industry is fully justified. Understanding ethanol flammability limits will help maintain safety and create handling rules for the use of this biofuel in general aviation. The first part of the work reported here was to develop a suitable test apparatus to investigate flammability limits and determine the LFL and UFL of hydrated and anhydrous ethanol at standard atmospheric pressure. The temperature ranged from 20 to 200 ◦ C. The heating chamber was constructed based on American regulation ASTM E-681 [11]. After comparing standard atmospheric pressure results with data from the literature, LFL and UFL were determined for pressures ranging from 20 to 80 kPa and temperatures ranging from 20 to 200 ◦ C. This heating chamber is installed and operated at the Federal University of Itajubá Engine Laboratory (Project No. TEC-APQ-00467-11—FAPEMIG). For tests at temperatures below standard atmospheric pressure, a suitable cooling chamber was used.
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In general, tests carried out on small volumes with high ignition energies result in wider intervals of flammability and thus more conservative results at the expense of higher safety costs. Considering the fact that tests carried out with low ignition energies or low vessel volumes might not have noticeable flammability for certain combustibles, it is important to use sufficient ignition energy and large enough volumes. A common practice is to use high-speed video cameras in order to analyze the images from different points of view in the same laboratory [12,13]. For a comprehensive review on the theory of flammability limits, criteria used to classify the flammability of a mixture, the main international standards, the main properties that influence flammability limits, such as pressure, temperature, turbulence, and energy ignition, see a previously published work by some of the authors of this study [13].
2. Flammability apparatus A heating chamber flammability apparatus was specifically designed based on American regulation ASTM E-681 [11]. The chamber was heated using electrical heating elements capable of raising the temperature to 300 ◦ C. The chamber also had thermal insulation and a window for observation and recording the structure of the flame in each experiment. For this study, a high-speed Fujifilm® FinePix HS-10 camera was used. Details of the heating chamber flammability apparatus can be seen in Fig. 1. This heating chamber is an improved version of one that had been previously constructed to test flammability limits of aeronautical ethanol. The results have been published [13]. The ASTM E681 test method covers the determination of the lower and upper concentration flammability limits for chemicals with sufficient vapor pressure to form flammable mixtures in air at standard atmospheric pressure at the test temperature. This test method uses electrical ignition and visual observations of flame propagation [11,13]. ASTM E-681 was chosen because it allows the structure of the flame to be verified and clearly shows flame propagation. It is important to use a high-speed camera to record each of the experiments and then analyze the results. For this study, the use of standard ASTM E918 is not recommended because it is designed to determine flammability limits at elevated temperatures (up to 200 ◦ C) and initial pressures of up to 200 psia (1.38 MPa). This practice is limited to mixtures that have explosion pressures lower than 2000 psia (13.79 MPa). The pressure resistant metal test vessel is a vertical cylinder, only 1 L in size, which is much smaller than that used in ASTM E681 [12,14]. A pressure transducer capable of reading to the nearest 0.07 kPa (0.5 mm Hg) was used to measure pressure during the experiments. Thus, although the visual criterion was chosen, the pressures were collected in each experiment to verify flammability. The pressure transducer was a HUBA model 510 for pressures from −1 to 10 bar by NOVUS®. A 20-l spherical test vessel was used. It should be noted that the volume specified by American Standard E-681 was increased because no rule is provided for use at high temperatures and reduced pressures, where the situation requires lower amounts of air and fuel. A drawing of the apparatus is shown in Fig. 2. An evaporator was used in the connection line to the vessel. This guaranteed that the fuel sample entered the vessel as a vapor, even at low temperatures. A 1-ml hypodermic syringe was used for the liquid injection. The liquid was injected when the pressure in the reaction vessel was approximately 0.07 kPa (0.5 mm Hg), which is below the saturation vapor pressure for ethanol used in this research. The liquid flashed to vapor upon injection into the reaction test vessel. The evaporator was used only in the experiments to find the UFLs.
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Fig. 1. Flammability apparatus.
The special rubber stopper was fixed to four rods connected to four springs to securely hold it in the vessel port. The 20-l spherical glass vessel was equipped with a high precision pressure transducer and two thermocouples, one positioned near the wall (type E), and the other positioned in the inner core of the vessel (type K). The pressure transducer measured initial pressure and the pressure variation during the explosion. The spark gap was 6.4 mm (1/4 in.), and the ignition energy was 90 J (approximately 30 mA at 15 kV, supplied by the secondary battery of a 120-V, 60-Hz luminous tube transformer; spark duration was limited to 0.2 s [11]). The flammability apparatus was automated using a Supervisory Control and Data Acquisition (SCADA) unit. The SCADA (Fig. 2) was connected to PC for monitoring all variables, and storage of data.
3. Anhydrous and hydrated ethanol The anhydrous ethanol used in this study is composed of 99.5% pure ethanol and 0.5% water (99.5◦ INPM). It is also called absolute ethyl alcohol and was provided by the company Labsynth Products Laboratories Ltd., located in the city of Diadema, SP (Brazil); the manufacturer’s batch number of this ethanol was 157307. Specifications are shown in Table 1; and information about the chemical properties of the anhydrous ethanol provided by the manufacturer are shown in Table 2. The hydrated ethanol used in this study was 92◦ INPM. It is a commercial ethanol and can be purchased at any vehicle fuel supply station. One reason for comparing the results of hydrated and anhydrous ethanol was to see how water content affects the flammability
Fig. 2. Flammability apparatus (experimental bench testing) [15].
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Table 1 Specifications of anhydrous ethanol. Content of ethanol (by volume) Acidity (meq/g) Alkalinity (meq/g) Water (H2 O) Methanol (CH3 OH) Residue after evaporation
99.5% min 0.0005 0.0002 0.2% 0.1% 0.0001%
Table 2 Physicochemical properties of anhydrous ethanol. Chemical formula Aspect Fragrance Molecular weight a Density (20 ◦ C) a Low heating value (1 atm, 298 K) a Melting point Boiling point Flash point Evaporation rate Vapor pressure Vapor density (20 ◦ C) Auto-ignition ttemperature Decomposition temperature a
C2 H5 OH Liquid clear, colorless Alcoholic 4607 g/mol 790 kg/m3 26,790 (kJ/kg) −114.5 ◦ C 78 ◦ C 17 ◦ C 1.66 (acetate de n-butyl = 1) 40 mmHg (19 ◦ C) 800 g/L 363 ◦ C 243 ◦ C
Paz [16]. Fig. 3. Visual criterion for flame propagation inside a 20-l vessel (ASTM E681).
Table 3 Specifications of hydrated ethanol. [16]. Content of ethanol (by volume) Hydrocarbon content (by volume), max. Evaporation residue Chloride ion, max. Sulfate ion max. Iron, max. Sodium, max. Total acidity (acetic acid), max.
92–93.8% 3.0% 5 mg/100 ml 1 mg/kg 4 mg/kg 5 mg/kg 2 mg/kg 30 mg/l
(a) (b) (c) (d)
(e)
Table 4 Physicochemical properties of hydrated ethanol [16]. Density (20 ◦ C) Low heating value (1 atm, 298 K) Boiling point (1 atm) Latent heat of vaporization at boiling point Kinematic viscosity of the liquid at 300 K Surface tension of the liquid at 300 K
The experimental procedure was carried out in the following the sequence:
807–811 kg/m3 24,915 kJ/kg 78.4 ◦ C 854.99 kJ/kg 1.78 × 10−6 m2 /s 0.0223 N/m
limits of ethanol. Another reason is because the Brazil has utilized ethanol as a fuel for land transport for more than 30 years and in the near future the government is expected to approve ethanol as aeronautical fuel. Tables 3 and 4 show some specifications of hydrated ethanol and some of its chemical properties.
(f) (g) (h) (i) (j) (k) (l) (m)
4. Experimental procedure Each run took between 15 and 25 min to complete. First, the reaction vessel was purged twice with nitrogen and once with air. The pressure was reduced to less than 1 kPa abs to insure removal of all water and other combustion products from previous runs. Second, liquid ethanol was injected into the reactor at low pressure to insure its complete vaporization. Third, synthetic air was loaded into the reactor and the concentrations of all the components were determined from the partial pressures. Fourth, the video camera was turned on, the mixture was ignited, and the results were collected and stored. All of the experiments were run at standard atmospheric pressure and approximately 25 ◦ C to compare with published data for this pressure and to validate the experimental procedure. After that, the work proceeded with reduced pressures.
Double check the equipment. Purge of the vessel twice using N2 . Purge the vessel once using synthetic air (80% N2 and 20% O2 ). Fix a pressure value that guaranteed ethanol evaporation during its introduction into the vessel (the ethanol vapor pressure curve was used as a reference). Set the desired volume of ethanol (a 1-ml scalp 19G hypodermic syringe was used; the magnetic stirrer was operating when the sample was introduced). Once the sample has been completely vaporized, slowly allow access of synthetic air until the final pressure is reached. Monitor temperature and pressure inside the vessel at all times. Allow the magnetic stirrer to operate for another two minutes and then turn it off. Turn off the room lamp. Simultaneously start video recording and SCADA recording. Activate the electrode. Observe whether the flame propagates or not, according to the standard indications. Start the process again in order to obtain values with and without flame propagation for the lower and upper flammability limits.
Each test was video recorded with a high-speed camera and later each record was edited with video editing software. This allowed determination of whether a sample at fixed temperature and pressure conditions had flame propagation or not. Standard indications to determine whether a mixture is flammable or not are of great importance. A mixture is classified as flammable if the flame propagates upward and outward from the vessel walls, forming an arc larger than 90◦ , measured from the ignition source to the vessel walls. The flammability limit is the average between the concentration of the first flame propagation and no flame propagation. Flame must be continuous along the region of the vessel shown in Fig. 3. If flame propagation could not be reproduced, or flame
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extension was not clear, for example, there was no uniform propagation, irregular flame structure, or the flame did not fill at least half the vessel even for the maximum flammable concentration in air, an ignition probability of 50% was used. 5. Results and discussion 5.1. Behavior of flame propagation Fig. 4 shows a sequence of photographs for a test with propagation in a mixture near the LFL. The flame propagation was evident and there was an abrupt rise of temperature and pressure. The use of protective glasses and headphones is required when carrying out any tests, especially in those to determine the LFL. There was no vessel, electrode, or other equipment loss during the tests. The springs were useful in order to secure the cover. It was noted that for mixtures near the LFL, the flame propagates upwards from the ignition spark, reaches the vessel top, and then extends to the wall and propagates downwards. The temperature rises to approximately 600–800 ◦ C in less than 0.1 s. This behavior is similar to that specified by the standard (ASTM E-681) for the case of flame propagation. Fig. 5 illustrates the case of no flame propagation. When the volume of ethanol increases from the value corresponding to the LFL, the flame propagates all along the vessel wall, the brightness increases, and the intensity of the noise increases. This can be observed in Fig. 6. The behavior is similar for the UFL, except that the noise is not as intense as it is for the LFL. In most experiments to determine the UFL, no loud noise was heard, unlike what happened in the experiments for the LFL. It should be noted that the absence of loud noise does not imply an absence of flame propagation. The sudden rises in temperature registered by the thermocouples and the flame propagation over the vessel wall were the factors that led to classify the mixture as flammable. It is important to use a high speed camera while carrying out these experiments. An analysis of the recorded images facilitates the classification of flammability. As the experiments were carried out in conjunction with members of three Brazilian research centers (UNIFEI, INPE and UNESP), the images were analyzed separately by personnel from each of these institutions and then classified by consensus. Finally, to demonstrate the accuracy of the results, Tables 5–9 show all the raw data that were used to generate flammability curves. 5.2. Standard atmospheric pressure In Figs. 7–10, the red lines represent the flammability curves of anhydrous ethanol, and the blue lines represent the flammability curves of hydrated ethanol; the black lines are traditionally published data for ethanol. Figs. 7–10 are obtained by transforming the flammability limit to volume of fuel in air (%) from the original raw data (Tables 5–9). The methodology of this transformation can be reviewed in [11] and [13]. At this stage of the investigation, 44 experiments were carried out; 90% of these tests were video recorded and all of them were recorded in SCADA. Table 5 shows the lower and upper flammability limits results for both alcohols in terms of fuel volume as function of temperature. Fig. 7 shows the results in terms of temperature as function of the volume percentage of gaseous fuel. Results obtained here for behavior for both the LFL and the UFL and in values for the LFL agreed with data published in the scientific literature on flammability limits of ethanol [2,3,13,17,18]. This agreement showed that the equipment was functioning properly that the experimental procedure adopted was carried out appropriately.
Table 5 Raw data of flammability limits at 101.3 kPa. Exp. no.
Volume (ml) T (◦ C)
Exp. no. Flame
101.3 kPa (anhydrous ethanol) 50 1.68 85 18.5 1.98 567 23.8 1.90 525 92 25 1.96 106 67 1.58 65 1.50 107 84 1.46 119 107 1.36 130 136 1.20 142 153 157 1.14 164 181 1.08 101.3 kPa (hydrated ethanol) 28 2.0 327 338 49 1.78 75 1.62 350 103 1.44 364 124 1.32 381 154 1.26 390 391 152 1.18 176 1.14 401 402 177 1.08 212 0.96 412
No flame
T (◦ C)
1.58 1.88 1.70 1.86 1.40 1.38 1.30 1.10 1.04 1.00
54 20.5 20 23 69 85 103 140 158 176
86 568 524 91 108 118 129 143 154 165
1.88 1.72 1.48 1.56 1.32 1.36 1.38 1.40 1.20 1.26 1.14 1.02 0.90
32 44 74 76 100 100 102 102 125 124 150 177 216
328 337 348 349 360 361 362 363 379 380 392 403 413
Table 6 Raw data of flammability limits at 80 kPa. Exp. no.
Volume (ml) ◦
T ( C)
Exp. no. ◦
Flame
No flame
T ( C)
80 kPa (anhydrous ethanol) 87 54 15.2 535 93 28 94 30 67 109 121 85 132 107 145 135 156 158 168 172
1.40 1.70 1.58 1.48 1.24 1.20 1.12 0.98 0.90 0.84
1.28 1.56 1.46 1.62 1.40 1.16 1.1 1.06 0.92 0.84 0.76
55 15 17 16 32 69 86 107 133 157 172
88 533 532 534 95 110 120 131 144 155 166
80 kPa (hydrated ethanol) 16.5 566 30 239 51 339 77 352 103 365 366 104 102 367 100 368 127 382 150 393 405 176 212 415
1.72 1.56 1.44 1.30 1.30 1.26 1.20 1.14 1.06 0.98 0.90 0.78
1.64 1.42 1.36 1.20 1.10 0.98 0.90 0.84 0.72
16 28 52 76 103 127 150 177 212
558 330 340 351 369 383 394 404 414
The LFLT and the UFLT (volume (%) fuel in air) for any temperature T at 1 atm pressure can be calculated based on the limits determined for 298 K using the following equations [18]: LFLT = LFL298 [1 − 0.00078(T − 298)], [2]
(1)
UFLT = UFL298 [1 + 0.000721(T − 298)]. [17]
(2)
Predictions made using the above equations are also shown in Fig. 7. LFL results for this work agreed well with results from previous investigations. It is specially worth stating that the present
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Fig. 4. Test no. 112-LFL, initial temperature = 65 ◦ C, sample volume = 0.96 ml, pressure = 60 kPa, flammable mixture.
work found 3.75% for ethanol LFL at 25 ◦ C, while Kuchta [3] reported 3.3% and Brooks and Crowl [16] reported 3.7% for the LFL at the same temperature. Moreover, Gibson [6] reported 3.4% (±0.2%) at 50 ◦ C; the present work found 3.6% for ethanol LFL at 52 ◦ C. The UFL values, however, were about 3% and 1% (absolute) lower in all cases for hydrated and anhydrous alcohol respectively. Zabetakis [2] reported 19% for UFL at 25 ◦ C. Other articles [19–23] report the same value, but they referred to Zabetakis’ data. Moreover, Gibson [6] found a value of 19% for ethanol UFL at 180 ◦ C,
completely different from the UFL results published by Zabetakis [2]. A value of 16.1% for the UFL at 25 ◦ C was reported [24], this latter value is similar to the finding in this research for hydrated alcohol. The experiments were repeated for this pressure three times and the data were confirmed. There was extreme care to avoid condensation (ethanol and water) of the samples when they were injected into the vessel. The evaporator was utilized to guarantee sample injection in the vapor phase. The difference in the case of
Fig. 5. Test no. 154-LFL, initial temperature = 158 ◦ C, sample volume = 1.04 ml, pressure = 101 kPa, non-flammable mixture.
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Fig. 6. Test no. 316-UFL, initial temperature = 42 ◦ C, sample volume = 5.4 ml, pressure = 80 kPa, flammable mixture.
Fig. 7. Anhydrous and hydrated ethanol–air mixtures, LFL and UFL, temperature as function of the volume percentage of gaseous fuel at 101 kPa.
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Fig. 8. Anhydrous and hydrated ethanol–air mixtures, temperature as function of the volume percentage of gaseous fuel at 20, 40, 60 and 80 kPa.
hydrated alcohol is probably due to the percentage of water (8%) present in the sample (hydrated ethanol 92% p/p). This behavior at the upper limit due to the presence of inert substances had already been anticipated by Zabetakis [2].
5.3. Reduced pressures At this stage, 171 experimental tests were performed for reduced pressure. All of the tests were recorded by a video Table 8 Raw data of flammability limits at 40 kPa.
Table 7 Raw data of flammability limits at 60 kPa. Exp. no.
Exp. no.
Volume (ml) T (◦ C)
Exp. no. Flame
No flame
T (◦ C)
60 kPa (anhydrous ethanol) 54 89 14 531 23 96 65 112 85 124 107 133 137 147 155 158 179 170
1.02 1.22 1.18 0.96 0.94 0.86 0.76 0.68 0.64
0.92 1.14 1.08 0.88 0.84 0.78 0.68 0.62 0.60
55 15 25 67 88 107 138 157 175
90 530 97 111 122 134 146 157 169
60 kPa (hydrated ethanol) 15.5 556 15 555 27 332 53 341 351 76 103 371 103 372 125 384 150 395 177 406 203 416
1.20 1.26 1.30 1.08 1.02 0.94 0.90 0.84 0.76 0.66 0.62
1.14 1.14 1.2 1.02 0.96 0.88 0.60 0.78 0.70 0.60 0.56
16.5 12.2 28 53 74 101 103 127 153 180 204
565 557 331 342 353 370 373 385 396 407 417
Volume (ml) T (◦ C)
Exp. no. Flame
No flame
T (◦ C)
40 kPa (anhydrous ethanol) 25 98 7 542 15.5 529 50 102 67 113 87 126 137 105 135 149 158 161 179 172
0.78 0.94 0.88 0.74 0.64 0.62 0.58 0.52 0.48 0.44
0.68 0.88 0.84 0.64 0.56 0.56 0.52 0.52 0.56 0.46 0.40 0.46 0.40
27 5.5 15.8 51 65 88 105 105 105 135 158 158 176
99 539 528 103 114 125 135 135 136 148 159 160 171
40 kPa (hydrated ethanol) 15 553 32 33 343 53 78 356 107 375 127 387 154 397 178 408 205 418
0.88 0.92 0.78 0.70 0.64 0.62 0.52 0.46 0.40
0.82 0.80 0.72 0.66 0.60 0.56 0.46 0.42 0.36
15.2 33 54 77 108 127 156 179 206
554 334 344 355 376 386 398 409 419
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Fig. 9. Anhydrous and hydrated ethanol–air mixtures, LFL and UFL at 70, 100, 150 and 180 ◦ C for different altitudes.
Table 9 Raw data of flammability limits at 20 kPa. Exp. no.
Volume (ml) ◦
T ( C)
Exp. no. ◦
Flame
No flame
T ( C)
20 kPa (anhydrous ethanol) 25 100 8 541 540 −0.2 5 538 5.2 537 15 527 52 104 65 116 89 128 106 140 136 152 157 163 176 173
0.44 0.56 0.56 0.52 0.48 0.48 0.38 0.40 0.34 0.32 0.28 0.26 0.22
0.38 0.46 0.50 0.44 0.34 0.36 0.32 0.28 0.30 0.24 0.26 0.22 0.20
27 4.5 −0.1 16.6 52 65 90 107 105 135 135 158 178
101 550 536 526 105 115 127 138 139 150 151 162 174
20 kPa (hydrated ethanol) 5.6 571 15.1 551 30 336 48 347 79 357 102 377 128 388 155 400 179 411 421 205
0.60 0.48 0.52 0.48 0.34 0.36 0.36 0.28 0.30 0.28
0.54 0.50 0.42 0.44 0.38 0.44 0.30 0.30 0.30 0.24 0.24 0.22
6 5.5 14.5 30 48 48 79 105 129 157 178 205
570 569 552 335 345 346 358 378 389 399 410 420
camera and also in SCADA. Tests were carried out at the following pressures: 80, 60, 40, and 20 kPa. Results are shown in Tables 6–9. When both limits are plotted by varying the volume and temperature, the curves have a downward trend. On the other hand, when experimental data are plotted varying the temperature and the fuel volume % in air (Fig. 8), the LFL for 20 kPa varies from 4.9% to 3.2% at 0 and 177 ◦ C, respectively for anhydrous alcohol, from 5.4% to 4.1%, at 5 and 205 ◦ C, respectively, for hydrated alcohol and, for 80 kPa, from 4.1% to 3%, at 15 and 172 ◦ C, respectively for anhydrous alcohol, from 3.8% to 3.1%, at 29 and 212 ◦ C, respectively for hydrated alcohol. The UFL for 20 kPa varies from 16.5% to 19.4%, at 25 and 209 ◦ C, respectively for anhydrous alcohol, from 16% to 20% at 16 and 221.8 ◦ C, respectively from hydrated alcohol, and, for 80 kPa, from 18.2% to 26.2% at 40 and 202 ◦ C, respectively, for anhydrous alcohol, from 17.4% to 25.5% at 56 and 212 ◦ C, respectively, for hydrated alcohol. It should be noted that as the pressure is reduced the upper limit curve has a lower slope. Finally, when both limits are plotted by varying the altitude (pressure) and the fuel volume % in air as the temperature increases, the LFL flammability curve tends to a vertical line. On the other hand, as temperature increases, the UFL flammability curve tends to have greater inclination. Details can be seen in Fig. 9.
5.4. Discussion As can be seen, the LFL results in this study are consistent with most published data. On the other hand, in the case of UFL, there are differences with published data; however other studies, [6] and [24], also reported these differences. One of the causes for the
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6. Conclusions
Fig. 10. Effect of water content on the flammability limits, to 101 kPa.
difference in flammability is the change in structure of the flame to reduce the pressure. In general, the LFL for all pressures tested for anhydrous ethanol have a more definite behavior, as reported in the literature for almost all fuels. It is easier to predict the behavior of this limit and there are several empirical and theoretical models to predict these values. Figs. 7 and 8, also show that for the flammability limits of anhydrous ethanol, the UFL is more sensitive to temperature, which is also reported in the literature. Note that for reduced pressures, there is a smaller variation of fuel concentration with increasing temperature to reach a value at which the UFL becomes more sensitive to temperature and fuel composition. Even at the specific case of 20 kPa, the flammability range narrows from 25 ◦ C to about 80 ◦ C (Fig. 8). After this, the fuel concentration begins to increase; in other words, the flammability range starts to widen. For hydrated ethanol, the flammability limits have behavior similar to anhydrous ethanol. The LFL has a definite and almost constant trend for any pressure condition, it is nearly linear and this is little variation for temperature change. The UFL is much more sensitive to temperature. In some cases, flammability range tends to narrow slightly. However, at high temperatures this range widens sharply; this phenomenon is observed mainly for reduced pressures. A comparison was also made to observe how the water content in hydrated ethanol affects flammability limits at a certain pressure, for example, 101 kPa. Thus, in Fig. 10, the expected result in shown: the range of flammability of anhydrous ethanol is wider. The largest differences are observed for UFL, so this limit, in addition to being more sensitive to temperature, is also very sensitive to water content in ethanol. This effect is expected since the water vapor contained in the fuel can act as an inert substance, producing a narrower flammability interval. The LFL is about the same for both alcohols, this is because the LFL is in the region of excess oxidant and adding water (inert) in the concentration does not affect important parameters such as flame temperature, for example. The contrary happens with the UFL where the mixture with additional water (inert) tends to narrow the flammability region. Finally, the temperature was carefully measured in the vessel. During evaporation, a considerable temperature drop did not occur when hydrated ethanol was injected, and the same happened with anhydrous ethanol. Therefore the narrowing of the flammability limits of hydrated ethanol is not due to a simple thermal effect (latent heat needed in order to vaporize the additional water). This is mainly due to the role of an inert in the flammability limits.
The flammable envelope for anhydrous and hydrated ethanol was determined experimentally. Good agreement was obtained with the LFL and UFL values reported by others. This agreement proves that the equipment was correctly assembled and operated; it also proves that the correct procedures were followed during the tests. After to the results obtained at standard atmospheric pressure, tests for reduced pressures were performed. One of the main objectives of this work was to show flammability limits at reduced pressures, considering the typical altitude of commercial aviation flight and thus atmospheric characteristics that are typical of aircraft fuel tanks. On the other hand, it is well known that depressurization is a means of extinguishing and suppressing flames in microgravity environments. These results can be used to prove the concept with flammability limits for a critical depressurization value. The quantity of fuel does not have to be reduced or eliminated in order to completely suppress the flame. Reducing pressure is sufficient to prevent the fire from ignition or cause extinction. In terms of the volume of sample and temperature, the curves of LFL and UFL have a tendency to decrease (unlike UFL at standard atmospheric pressure, which has a linear trend). This behavior can be observed in the figures presented in the results section. In some cases, this trend allowed the sample volume that was introduced into the vessel to be adjusted; to some extent, it also helped to guarantee flame propagation and to fix the flammability limit. Thus, fewer tests were necessary, preventing equipment wear. The majority of the tests were successfully performed; only 10% of them had to be repeated in order to achieve precision or in some cases to correct experimental procedures (especially with the upper limit).
Acknowledgements Funding for this study was provided by the Fundac¸ão de Amparo à Pesquisa do Estado de Minas Gerais, FAPEMIG, Brazil, through projects TEC-APQ-00467-11 (regular grant), Fundac¸ão de Amparo à Pesquisa do Estado de São Paulo, FAPESP, Brazil, through projects 2009/097387 (regular grant), and CNPq-Brazil for the DT2 Fellowship to CJCR (Proc. No. 310069/2012-2).
References [1] H.F. Coward, G.W. Jones, Limits of Flammability of Gases and Vapors, vol. 503, US Bureau of Mines Bulletin, 1952. [2] M.G. Zabetakis, Flammability Characteristics of Combustible Gases and Vapors, vol. 627, US Bureau of Mines Bulletin, 1965. [3] J.M. Kuchta, Investigation of Fire and Explosion Accidents in The Chemical, Mining, And Fuel-Related Industries—A Manual, vol. 680, US Bureau of Mines Bulletin, 1985. [4] F. van den Schoor, F. Verplaetsen, J. Berghmans, Calculation of the upper flammability limit of methane/air mixtures at elevated pressures and temperatures, J. Hazard. Mater. 153 (2008) 1301–1307. [5] F. van den Schoor, F. Verplaetsen, The upper explosion limit of lower alkanes and alkenes in air at elevated pressures and temperatures, J. Hazard. Mater. 128 (2006) 1–9. [6] H.J. Gibbon, J. Wainwright, R.L. Rogers, Institution of Chemical Engineers; North Western Branch; Institution of Chemical Engineers Symposium Series, 134, Symposium, Hazards XII: European Advances in Process Safety, 1994, pp. 1–12, ISBN: 0852953275. [7] L.J. Nestor, Investigation of Turbine Fuel Flammability Within Aircraft Fuel Tanks, Federal Aviation Administration, USA, 1967. [8] Fuel Flammability Task Group, A review of the flammability hazard of jet a fuel vapor in civil transport aircraft fuel tanks, Final Report No. DOT/FAA/AR-98/26, Federal Aviation Administration, USA, 1998. [9] E. Ott, Effects of fuel slosh and vibration on the flammability hazards of hydrocarbon turbine fuels within aircraft fuel tanks, Technical Report AFAPLTR-70-65, Fire Protection Branch of the Fuels and Lubrication Division, Wright-Patterson Air Force Base, Ohio, 1970.
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[10] T. Kosvic, L. Zung, M. Gerstein, Analysis of aircraft fuel tank fire and explosion hazards, Technical Report AFAPL-TR-71-7, Air Force Aero Propulsion Laboratory, Wright-Patterson Air Force Base, Ohio, 1971. [11] A STM E 681-04, Standard Test Method for Concentration Limits of Flammability of Chemicals (vapors and gases), American Society for Testing and Materials, W. Conshohocken, PA, 2010. [12] E. Brandes, M. Mitu, D. Pawel, The lower explosion point: a good measure for explosion prevention: experiment and calculation for pure compounds and some mixtures, J. Loss Prevent. Process Ind. 20 (2007) 536–540. [13] C.J.R. Coronado, J.A. Carvalho Jr., J.C. Andrade, E.V. Cortez, F.S. Carvalho, J.C. Santos, A.Z. Mendiburu, Flammability limits: a review with emphasis on ethanol for aeronautical applications and description of the experimental procedure, J. Hazard. Mater. 241–242 (2012) 32–54. [14] ASTM E918-83, Standard Practice for Determining Limits of Flammability of Chemicals at Elevated Temperature and Pressure, American Society for Testing and Materials (ASTM) Washington Office, 2011. [15] J.G.C. Quintero, Experimental determination and prediction of the limits of flammability of ethanol anhydrous and hydrated for use in aircraft industry, Dissertation (Masters Degree in Energy Conversion) (in Portuguese), Federal University of Itajubá–UNIFEI, Mechanical Engineering Institute–IEM, Itajubá, MG, Brazil, 2013, p. 169.
[16] E.P. Paz, Substitution of diesel used in industrial burner by alcohol fuel (Doctorate in mechanical engineering) (in Portuguese), São Paulo State University—UNESP, Campus of Guaratinguetá–FEG, Brazil, 2007. [17] M.R. Brooks, D. Crowl, Flammability envelopes for methanol, ethanol, acetonitrile and toluene, J. Loss Prevent. Process Ind. 20 (2007) 144–150. [18] D. Drysdale, An Introduction to Fire Dynamics, 3rd ed., John Wiley & Sons, 2011. [19] G.A. Melhem, A detailed method for estimating mixture flammability limits using chemical equilibrium, Process Saf. Progr. 16 (1997) 203–218. [20] R. McCormick, R. Parish, Advanced Petroleum Based Fuels Program and Renewable Diesel Program Milestone Report: Technical Barriers to the Use of Ethanol in Diesel Fuel, NREL/MP-540-32674, 2001. [21] A. Hansen, Q. Zhang, P. Lyne, Ethanol-diesel fuel blends, a review, Bioresour. Technol. 96 (2005) 277–285. [22] T. Ma, A thermal theory for estimating the flammability limits of a mixture, Fire Saf. J. 46 (2011) 558–567. [23] L. Pidol, B. Lecointe, L. Starck, N. Jeuland, Ethanol-biodiesel-diesel fuel blends: performances and emissions in conventional diesel and advanced low temperature combustions, Fuel 93 (2012) 329–338. [24] W. Seaton, Group contribution method for predicting the lower and the upper flammability limits of vapors in air, J. Hazard. Mater, 27 (1991) 169–185.
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Flammability limits of hydrated and anhydrous ethanol at reduced pressures in aeronautical applications Christian J.R. Coronado a , João A. Carvalho Jr b,∗ , José C. Andrade c , Andrés Z. Mendiburu b , Ely V. Cortez c , Felipe S. Carvalho a , Beatriz Gonc¸alves a , Juan C. Quintero a , Elkin I. Gutiérrez Velásquez d , Marcos H. Silva a , José C. Santos c , Marco. A.R. Nascimento a a
Federal University of Itajubá—UNIFEI, Mechanical Engineering Institute—IEM Av. BPS 1303, Itajubá, MG, CEP 37500-903, Brazil São Paulo State University—UNESP, Campus of Guaratinguetá—FEG Av. Ariberto P. da Cunha, 333, Guaratinguetá, SP, CEP 12516-410, Brazil c National Space Research Institute—INPE, Combustion and Propulsion Laboratory—LCP Rod. Pres. Dutra, km 39, Cachoeira Paulista, SP, CEP 12630-000, Brazil d Faculty of Mechanical Engineering, Universidad Antonio Nari˜ no, Tunja, Colombia b
h i g h l i g h t s • • • • •
Flammability limits for ethanol at reduced pressures were determined. 295 experiments were carried out in total for anhydrous and hydrated ethanol. The first 80 were to calibrate the heating chamber and compare the results. 215 experiments were performed both at atmospheric and reduced pressure. Results had a correlation with the LFL obtained but UFL had some differences.
a r t i c l e
i n f o
Article history: Received 6 May 2014 Received in revised form 11 July 2014 Accepted 19 July 2014 Available online 7 August 2014 Keywords: Flammability limits Ethanol Visual criterion Pressure and temperature dependence
a b s t r a c t There is interest in finding the flammability limits of ethanol at reduced pressures for the future use of this biofuel in aeronautical applications taking into account typical commercial aviation altitude (<40,000 ft). The lower and upper flammability limits (LFL and UFL, respectively) for hydrated ethanol and anhydrous ethanol (92.6% and 99.5% p/p, respectively) were determined for a pressure of 101.3 kPa at temperatures between 0 and 200 ◦ C. A heating chamber with a spherical 20-l vessel was used. First, LFL and the UFL were determined as functions of temperature and atmospheric pressure to compare results with data published in the scientific literature. Second, after checking the veracity of the data obtained for standard atmospheric pressure, the work proceeded with reduced pressures in the same temperature range. 295 experiments were carried out in total; the first 80 were to calibrate the heating chamber and compare the results with those given in the published scientific literature. 215 experiments were performed both at atmospheric and reduced pressures. The results had a correlation with the values obtained for the LFL, but values for the UFL had some differences. With respect to the water content in ethanol, it was shown that the water vapor contained in the fuel can act as an inert substance, narrowing flammability. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Knowledge about the risks involved with the explosion of both gaseous and liquid fuel mixtures with air is of great importance to guarantee safety in industrial, domestic, and in aeronautical
∗ Corresponding author. Tel.: +55 12 31232835; fax: +55 12 31232835. E-mail addresses: [email protected], [email protected] (J.A.C. Jr). http://dx.doi.org/10.1016/j.jhazmat.2014.07.063 0304-3894/© 2014 Elsevier B.V. All rights reserved.
application. The lower (LFL) and upper (UFL) flammability limits of a fuel are key criteria to predict fires, assess the possibility of explosion, and design protection systems. The goal of this research is to better understand and characterize the flammability of hydrated (92% p/p) and anhydrous ethanol (99.5% p/p) for the aeronautical industry. Flammability limits have been thoroughly discussed in scientific literature. Probably the first works on this topic were those developed by Coward and Jones [1] and by Zabetakis [2], both for the Mines Department of the U.S. Government. Later, Kuchta [3]
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increased the range of the data obtained by Zabetakis. UFL was determined experimentally for ethane-air, propane-air, n-butaneair, ethylene-air, and propylene-air mixtures for pressures of up to 30 bar and for temperatures up to 250 ◦ C [4]. Four different numerical methods used to calculate the UFL of methane–air mixtures for pressures up to 10 bar and temperatures up to 200 ◦ C were reported and the methods were compared to experimental data [5]. Gibson [6] published data on the flammability limits of solvents at elevated temperature and pressure in air. The paper describes the measurement of flammability limits of ethanol and l-propanol and also presents data for other solvents. These authors worked at elevated temperatures of up to 250 ◦ C. The study was different in that they investigated pressures above standard atmospheric pressure. Nestor [7] and the Fuel Flammability Task Group [8] worked specifically with flammability limits for the aeronautical industry. Measurements of ignition energy for aeronautic fuels (Spark Ignition Energy Measurements in JET A) were reported by Shepherd et al. (2000). Apart from those, other works contributed to this knowledge in the area of aeronautics [9,10]. Since kerosene was introduced as a fuel for aerial civil transportation in the 1950s, aeronautical designers have given special consideration to the ullage in aircraft fuel tanks. These tanks may contain a mixture of several vapors, fuel, and air, which can ignite in the presence of an ignition source when this mixture is within the flammability limits of the fuel in use. After the flight TWA 800 accident, which went down near the coast of New York in 1996, concerns about formation of vapors in aircraft fuel tanks reached its peak. It was determined that the crucial reason for the lost plane was an explosion inside a fuel tank in the airplane wing. At the moment of explosion, the plane had approximately 50 gallons of Jet A fuel. The fuel and air contained inside the tank were heated by the air conditioning system (air packs) located directly below the tank [8]. This increased fuel vaporization in the ullage region, forming a flammable mixture which later caused the accident. Since then, flammability limit studies for the aeronautical industry have been increasingly taken into account in aircraft design. Required conditions for ignition and flame propagation inside the aircraft fuel tank depend on parameters which include fuel type, temperature, tank pressure, and oxygen concentration. It was also noted that fuel foam and spray, which may form during refueling or during a flight with fuel oxygen liberation, might extend fuel flammability limits. In this sense, when working with flammability limits of fuels for the aeronautical industry, the following details must be taken into account: altitude changes, temperature changes, ullage changes, tank ventilation, state changes of fuel (spray) by agitation due to aircraft movement, and fuel mixtures during refueling operations. Considering that Brazil is the largest ethanol producer in the world with vast experience in the use of bio-fuels, the use of ethanol in the Brazilian aircraft industry is fully justified. Understanding ethanol flammability limits will help maintain safety and create handling rules for the use of this biofuel in general aviation. The first part of the work reported here was to develop a suitable test apparatus to investigate flammability limits and determine the LFL and UFL of hydrated and anhydrous ethanol at standard atmospheric pressure. The temperature ranged from 20 to 200 ◦ C. The heating chamber was constructed based on American regulation ASTM E-681 [11]. After comparing standard atmospheric pressure results with data from the literature, LFL and UFL were determined for pressures ranging from 20 to 80 kPa and temperatures ranging from 20 to 200 ◦ C. This heating chamber is installed and operated at the Federal University of Itajubá Engine Laboratory (Project No. TEC-APQ-00467-11—FAPEMIG). For tests at temperatures below standard atmospheric pressure, a suitable cooling chamber was used.
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In general, tests carried out on small volumes with high ignition energies result in wider intervals of flammability and thus more conservative results at the expense of higher safety costs. Considering the fact that tests carried out with low ignition energies or low vessel volumes might not have noticeable flammability for certain combustibles, it is important to use sufficient ignition energy and large enough volumes. A common practice is to use high-speed video cameras in order to analyze the images from different points of view in the same laboratory [12,13]. For a comprehensive review on the theory of flammability limits, criteria used to classify the flammability of a mixture, the main international standards, the main properties that influence flammability limits, such as pressure, temperature, turbulence, and energy ignition, see a previously published work by some of the authors of this study [13].
2. Flammability apparatus A heating chamber flammability apparatus was specifically designed based on American regulation ASTM E-681 [11]. The chamber was heated using electrical heating elements capable of raising the temperature to 300 ◦ C. The chamber also had thermal insulation and a window for observation and recording the structure of the flame in each experiment. For this study, a high-speed Fujifilm® FinePix HS-10 camera was used. Details of the heating chamber flammability apparatus can be seen in Fig. 1. This heating chamber is an improved version of one that had been previously constructed to test flammability limits of aeronautical ethanol. The results have been published [13]. The ASTM E681 test method covers the determination of the lower and upper concentration flammability limits for chemicals with sufficient vapor pressure to form flammable mixtures in air at standard atmospheric pressure at the test temperature. This test method uses electrical ignition and visual observations of flame propagation [11,13]. ASTM E-681 was chosen because it allows the structure of the flame to be verified and clearly shows flame propagation. It is important to use a high-speed camera to record each of the experiments and then analyze the results. For this study, the use of standard ASTM E918 is not recommended because it is designed to determine flammability limits at elevated temperatures (up to 200 ◦ C) and initial pressures of up to 200 psia (1.38 MPa). This practice is limited to mixtures that have explosion pressures lower than 2000 psia (13.79 MPa). The pressure resistant metal test vessel is a vertical cylinder, only 1 L in size, which is much smaller than that used in ASTM E681 [12,14]. A pressure transducer capable of reading to the nearest 0.07 kPa (0.5 mm Hg) was used to measure pressure during the experiments. Thus, although the visual criterion was chosen, the pressures were collected in each experiment to verify flammability. The pressure transducer was a HUBA model 510 for pressures from −1 to 10 bar by NOVUS®. A 20-l spherical test vessel was used. It should be noted that the volume specified by American Standard E-681 was increased because no rule is provided for use at high temperatures and reduced pressures, where the situation requires lower amounts of air and fuel. A drawing of the apparatus is shown in Fig. 2. An evaporator was used in the connection line to the vessel. This guaranteed that the fuel sample entered the vessel as a vapor, even at low temperatures. A 1-ml hypodermic syringe was used for the liquid injection. The liquid was injected when the pressure in the reaction vessel was approximately 0.07 kPa (0.5 mm Hg), which is below the saturation vapor pressure for ethanol used in this research. The liquid flashed to vapor upon injection into the reaction test vessel. The evaporator was used only in the experiments to find the UFLs.
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Fig. 1. Flammability apparatus.
The special rubber stopper was fixed to four rods connected to four springs to securely hold it in the vessel port. The 20-l spherical glass vessel was equipped with a high precision pressure transducer and two thermocouples, one positioned near the wall (type E), and the other positioned in the inner core of the vessel (type K). The pressure transducer measured initial pressure and the pressure variation during the explosion. The spark gap was 6.4 mm (1/4 in.), and the ignition energy was 90 J (approximately 30 mA at 15 kV, supplied by the secondary battery of a 120-V, 60-Hz luminous tube transformer; spark duration was limited to 0.2 s [11]). The flammability apparatus was automated using a Supervisory Control and Data Acquisition (SCADA) unit. The SCADA (Fig. 2) was connected to PC for monitoring all variables, and storage of data.
3. Anhydrous and hydrated ethanol The anhydrous ethanol used in this study is composed of 99.5% pure ethanol and 0.5% water (99.5◦ INPM). It is also called absolute ethyl alcohol and was provided by the company Labsynth Products Laboratories Ltd., located in the city of Diadema, SP (Brazil); the manufacturer’s batch number of this ethanol was 157307. Specifications are shown in Table 1; and information about the chemical properties of the anhydrous ethanol provided by the manufacturer are shown in Table 2. The hydrated ethanol used in this study was 92◦ INPM. It is a commercial ethanol and can be purchased at any vehicle fuel supply station. One reason for comparing the results of hydrated and anhydrous ethanol was to see how water content affects the flammability
Fig. 2. Flammability apparatus (experimental bench testing) [15].
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Table 1 Specifications of anhydrous ethanol. Content of ethanol (by volume) Acidity (meq/g) Alkalinity (meq/g) Water (H2 O) Methanol (CH3 OH) Residue after evaporation
99.5% min 0.0005 0.0002 0.2% 0.1% 0.0001%
Table 2 Physicochemical properties of anhydrous ethanol. Chemical formula Aspect Fragrance Molecular weight a Density (20 ◦ C) a Low heating value (1 atm, 298 K) a Melting point Boiling point Flash point Evaporation rate Vapor pressure Vapor density (20 ◦ C) Auto-ignition ttemperature Decomposition temperature a
C2 H5 OH Liquid clear, colorless Alcoholic 4607 g/mol 790 kg/m3 26,790 (kJ/kg) −114.5 ◦ C 78 ◦ C 17 ◦ C 1.66 (acetate de n-butyl = 1) 40 mmHg (19 ◦ C) 800 g/L 363 ◦ C 243 ◦ C
Paz [16]. Fig. 3. Visual criterion for flame propagation inside a 20-l vessel (ASTM E681).
Table 3 Specifications of hydrated ethanol. [16]. Content of ethanol (by volume) Hydrocarbon content (by volume), max. Evaporation residue Chloride ion, max. Sulfate ion max. Iron, max. Sodium, max. Total acidity (acetic acid), max.
92–93.8% 3.0% 5 mg/100 ml 1 mg/kg 4 mg/kg 5 mg/kg 2 mg/kg 30 mg/l
(a) (b) (c) (d)
(e)
Table 4 Physicochemical properties of hydrated ethanol [16]. Density (20 ◦ C) Low heating value (1 atm, 298 K) Boiling point (1 atm) Latent heat of vaporization at boiling point Kinematic viscosity of the liquid at 300 K Surface tension of the liquid at 300 K
The experimental procedure was carried out in the following the sequence:
807–811 kg/m3 24,915 kJ/kg 78.4 ◦ C 854.99 kJ/kg 1.78 × 10−6 m2 /s 0.0223 N/m
limits of ethanol. Another reason is because the Brazil has utilized ethanol as a fuel for land transport for more than 30 years and in the near future the government is expected to approve ethanol as aeronautical fuel. Tables 3 and 4 show some specifications of hydrated ethanol and some of its chemical properties.
(f) (g) (h) (i) (j) (k) (l) (m)
4. Experimental procedure Each run took between 15 and 25 min to complete. First, the reaction vessel was purged twice with nitrogen and once with air. The pressure was reduced to less than 1 kPa abs to insure removal of all water and other combustion products from previous runs. Second, liquid ethanol was injected into the reactor at low pressure to insure its complete vaporization. Third, synthetic air was loaded into the reactor and the concentrations of all the components were determined from the partial pressures. Fourth, the video camera was turned on, the mixture was ignited, and the results were collected and stored. All of the experiments were run at standard atmospheric pressure and approximately 25 ◦ C to compare with published data for this pressure and to validate the experimental procedure. After that, the work proceeded with reduced pressures.
Double check the equipment. Purge of the vessel twice using N2 . Purge the vessel once using synthetic air (80% N2 and 20% O2 ). Fix a pressure value that guaranteed ethanol evaporation during its introduction into the vessel (the ethanol vapor pressure curve was used as a reference). Set the desired volume of ethanol (a 1-ml scalp 19G hypodermic syringe was used; the magnetic stirrer was operating when the sample was introduced). Once the sample has been completely vaporized, slowly allow access of synthetic air until the final pressure is reached. Monitor temperature and pressure inside the vessel at all times. Allow the magnetic stirrer to operate for another two minutes and then turn it off. Turn off the room lamp. Simultaneously start video recording and SCADA recording. Activate the electrode. Observe whether the flame propagates or not, according to the standard indications. Start the process again in order to obtain values with and without flame propagation for the lower and upper flammability limits.
Each test was video recorded with a high-speed camera and later each record was edited with video editing software. This allowed determination of whether a sample at fixed temperature and pressure conditions had flame propagation or not. Standard indications to determine whether a mixture is flammable or not are of great importance. A mixture is classified as flammable if the flame propagates upward and outward from the vessel walls, forming an arc larger than 90◦ , measured from the ignition source to the vessel walls. The flammability limit is the average between the concentration of the first flame propagation and no flame propagation. Flame must be continuous along the region of the vessel shown in Fig. 3. If flame propagation could not be reproduced, or flame
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extension was not clear, for example, there was no uniform propagation, irregular flame structure, or the flame did not fill at least half the vessel even for the maximum flammable concentration in air, an ignition probability of 50% was used. 5. Results and discussion 5.1. Behavior of flame propagation Fig. 4 shows a sequence of photographs for a test with propagation in a mixture near the LFL. The flame propagation was evident and there was an abrupt rise of temperature and pressure. The use of protective glasses and headphones is required when carrying out any tests, especially in those to determine the LFL. There was no vessel, electrode, or other equipment loss during the tests. The springs were useful in order to secure the cover. It was noted that for mixtures near the LFL, the flame propagates upwards from the ignition spark, reaches the vessel top, and then extends to the wall and propagates downwards. The temperature rises to approximately 600–800 ◦ C in less than 0.1 s. This behavior is similar to that specified by the standard (ASTM E-681) for the case of flame propagation. Fig. 5 illustrates the case of no flame propagation. When the volume of ethanol increases from the value corresponding to the LFL, the flame propagates all along the vessel wall, the brightness increases, and the intensity of the noise increases. This can be observed in Fig. 6. The behavior is similar for the UFL, except that the noise is not as intense as it is for the LFL. In most experiments to determine the UFL, no loud noise was heard, unlike what happened in the experiments for the LFL. It should be noted that the absence of loud noise does not imply an absence of flame propagation. The sudden rises in temperature registered by the thermocouples and the flame propagation over the vessel wall were the factors that led to classify the mixture as flammable. It is important to use a high speed camera while carrying out these experiments. An analysis of the recorded images facilitates the classification of flammability. As the experiments were carried out in conjunction with members of three Brazilian research centers (UNIFEI, INPE and UNESP), the images were analyzed separately by personnel from each of these institutions and then classified by consensus. Finally, to demonstrate the accuracy of the results, Tables 5–9 show all the raw data that were used to generate flammability curves. 5.2. Standard atmospheric pressure In Figs. 7–10, the red lines represent the flammability curves of anhydrous ethanol, and the blue lines represent the flammability curves of hydrated ethanol; the black lines are traditionally published data for ethanol. Figs. 7–10 are obtained by transforming the flammability limit to volume of fuel in air (%) from the original raw data (Tables 5–9). The methodology of this transformation can be reviewed in [11] and [13]. At this stage of the investigation, 44 experiments were carried out; 90% of these tests were video recorded and all of them were recorded in SCADA. Table 5 shows the lower and upper flammability limits results for both alcohols in terms of fuel volume as function of temperature. Fig. 7 shows the results in terms of temperature as function of the volume percentage of gaseous fuel. Results obtained here for behavior for both the LFL and the UFL and in values for the LFL agreed with data published in the scientific literature on flammability limits of ethanol [2,3,13,17,18]. This agreement showed that the equipment was functioning properly that the experimental procedure adopted was carried out appropriately.
Table 5 Raw data of flammability limits at 101.3 kPa. Exp. no.
Volume (ml) T (◦ C)
Exp. no. Flame
101.3 kPa (anhydrous ethanol) 50 1.68 85 18.5 1.98 567 23.8 1.90 525 92 25 1.96 106 67 1.58 65 1.50 107 84 1.46 119 107 1.36 130 136 1.20 142 153 157 1.14 164 181 1.08 101.3 kPa (hydrated ethanol) 28 2.0 327 338 49 1.78 75 1.62 350 103 1.44 364 124 1.32 381 154 1.26 390 391 152 1.18 176 1.14 401 402 177 1.08 212 0.96 412
No flame
T (◦ C)
1.58 1.88 1.70 1.86 1.40 1.38 1.30 1.10 1.04 1.00
54 20.5 20 23 69 85 103 140 158 176
86 568 524 91 108 118 129 143 154 165
1.88 1.72 1.48 1.56 1.32 1.36 1.38 1.40 1.20 1.26 1.14 1.02 0.90
32 44 74 76 100 100 102 102 125 124 150 177 216
328 337 348 349 360 361 362 363 379 380 392 403 413
Table 6 Raw data of flammability limits at 80 kPa. Exp. no.
Volume (ml) ◦
T ( C)
Exp. no. ◦
Flame
No flame
T ( C)
80 kPa (anhydrous ethanol) 87 54 15.2 535 93 28 94 30 67 109 121 85 132 107 145 135 156 158 168 172
1.40 1.70 1.58 1.48 1.24 1.20 1.12 0.98 0.90 0.84
1.28 1.56 1.46 1.62 1.40 1.16 1.1 1.06 0.92 0.84 0.76
55 15 17 16 32 69 86 107 133 157 172
88 533 532 534 95 110 120 131 144 155 166
80 kPa (hydrated ethanol) 16.5 566 30 239 51 339 77 352 103 365 366 104 102 367 100 368 127 382 150 393 405 176 212 415
1.72 1.56 1.44 1.30 1.30 1.26 1.20 1.14 1.06 0.98 0.90 0.78
1.64 1.42 1.36 1.20 1.10 0.98 0.90 0.84 0.72
16 28 52 76 103 127 150 177 212
558 330 340 351 369 383 394 404 414
The LFLT and the UFLT (volume (%) fuel in air) for any temperature T at 1 atm pressure can be calculated based on the limits determined for 298 K using the following equations [18]: LFLT = LFL298 [1 − 0.00078(T − 298)], [2]
(1)
UFLT = UFL298 [1 + 0.000721(T − 298)]. [17]
(2)
Predictions made using the above equations are also shown in Fig. 7. LFL results for this work agreed well with results from previous investigations. It is specially worth stating that the present
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Fig. 4. Test no. 112-LFL, initial temperature = 65 ◦ C, sample volume = 0.96 ml, pressure = 60 kPa, flammable mixture.
work found 3.75% for ethanol LFL at 25 ◦ C, while Kuchta [3] reported 3.3% and Brooks and Crowl [16] reported 3.7% for the LFL at the same temperature. Moreover, Gibson [6] reported 3.4% (±0.2%) at 50 ◦ C; the present work found 3.6% for ethanol LFL at 52 ◦ C. The UFL values, however, were about 3% and 1% (absolute) lower in all cases for hydrated and anhydrous alcohol respectively. Zabetakis [2] reported 19% for UFL at 25 ◦ C. Other articles [19–23] report the same value, but they referred to Zabetakis’ data. Moreover, Gibson [6] found a value of 19% for ethanol UFL at 180 ◦ C,
completely different from the UFL results published by Zabetakis [2]. A value of 16.1% for the UFL at 25 ◦ C was reported [24], this latter value is similar to the finding in this research for hydrated alcohol. The experiments were repeated for this pressure three times and the data were confirmed. There was extreme care to avoid condensation (ethanol and water) of the samples when they were injected into the vessel. The evaporator was utilized to guarantee sample injection in the vapor phase. The difference in the case of
Fig. 5. Test no. 154-LFL, initial temperature = 158 ◦ C, sample volume = 1.04 ml, pressure = 101 kPa, non-flammable mixture.
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Fig. 6. Test no. 316-UFL, initial temperature = 42 ◦ C, sample volume = 5.4 ml, pressure = 80 kPa, flammable mixture.
Fig. 7. Anhydrous and hydrated ethanol–air mixtures, LFL and UFL, temperature as function of the volume percentage of gaseous fuel at 101 kPa.
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Fig. 8. Anhydrous and hydrated ethanol–air mixtures, temperature as function of the volume percentage of gaseous fuel at 20, 40, 60 and 80 kPa.
hydrated alcohol is probably due to the percentage of water (8%) present in the sample (hydrated ethanol 92% p/p). This behavior at the upper limit due to the presence of inert substances had already been anticipated by Zabetakis [2].
5.3. Reduced pressures At this stage, 171 experimental tests were performed for reduced pressure. All of the tests were recorded by a video Table 8 Raw data of flammability limits at 40 kPa.
Table 7 Raw data of flammability limits at 60 kPa. Exp. no.
Exp. no.
Volume (ml) T (◦ C)
Exp. no. Flame
No flame
T (◦ C)
60 kPa (anhydrous ethanol) 54 89 14 531 23 96 65 112 85 124 107 133 137 147 155 158 179 170
1.02 1.22 1.18 0.96 0.94 0.86 0.76 0.68 0.64
0.92 1.14 1.08 0.88 0.84 0.78 0.68 0.62 0.60
55 15 25 67 88 107 138 157 175
90 530 97 111 122 134 146 157 169
60 kPa (hydrated ethanol) 15.5 556 15 555 27 332 53 341 351 76 103 371 103 372 125 384 150 395 177 406 203 416
1.20 1.26 1.30 1.08 1.02 0.94 0.90 0.84 0.76 0.66 0.62
1.14 1.14 1.2 1.02 0.96 0.88 0.60 0.78 0.70 0.60 0.56
16.5 12.2 28 53 74 101 103 127 153 180 204
565 557 331 342 353 370 373 385 396 407 417
Volume (ml) T (◦ C)
Exp. no. Flame
No flame
T (◦ C)
40 kPa (anhydrous ethanol) 25 98 7 542 15.5 529 50 102 67 113 87 126 137 105 135 149 158 161 179 172
0.78 0.94 0.88 0.74 0.64 0.62 0.58 0.52 0.48 0.44
0.68 0.88 0.84 0.64 0.56 0.56 0.52 0.52 0.56 0.46 0.40 0.46 0.40
27 5.5 15.8 51 65 88 105 105 105 135 158 158 176
99 539 528 103 114 125 135 135 136 148 159 160 171
40 kPa (hydrated ethanol) 15 553 32 33 343 53 78 356 107 375 127 387 154 397 178 408 205 418
0.88 0.92 0.78 0.70 0.64 0.62 0.52 0.46 0.40
0.82 0.80 0.72 0.66 0.60 0.56 0.46 0.42 0.36
15.2 33 54 77 108 127 156 179 206
554 334 344 355 376 386 398 409 419
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Fig. 9. Anhydrous and hydrated ethanol–air mixtures, LFL and UFL at 70, 100, 150 and 180 ◦ C for different altitudes.
Table 9 Raw data of flammability limits at 20 kPa. Exp. no.
Volume (ml) ◦
T ( C)
Exp. no. ◦
Flame
No flame
T ( C)
20 kPa (anhydrous ethanol) 25 100 8 541 540 −0.2 5 538 5.2 537 15 527 52 104 65 116 89 128 106 140 136 152 157 163 176 173
0.44 0.56 0.56 0.52 0.48 0.48 0.38 0.40 0.34 0.32 0.28 0.26 0.22
0.38 0.46 0.50 0.44 0.34 0.36 0.32 0.28 0.30 0.24 0.26 0.22 0.20
27 4.5 −0.1 16.6 52 65 90 107 105 135 135 158 178
101 550 536 526 105 115 127 138 139 150 151 162 174
20 kPa (hydrated ethanol) 5.6 571 15.1 551 30 336 48 347 79 357 102 377 128 388 155 400 179 411 421 205
0.60 0.48 0.52 0.48 0.34 0.36 0.36 0.28 0.30 0.28
0.54 0.50 0.42 0.44 0.38 0.44 0.30 0.30 0.30 0.24 0.24 0.22
6 5.5 14.5 30 48 48 79 105 129 157 178 205
570 569 552 335 345 346 358 378 389 399 410 420
camera and also in SCADA. Tests were carried out at the following pressures: 80, 60, 40, and 20 kPa. Results are shown in Tables 6–9. When both limits are plotted by varying the volume and temperature, the curves have a downward trend. On the other hand, when experimental data are plotted varying the temperature and the fuel volume % in air (Fig. 8), the LFL for 20 kPa varies from 4.9% to 3.2% at 0 and 177 ◦ C, respectively for anhydrous alcohol, from 5.4% to 4.1%, at 5 and 205 ◦ C, respectively, for hydrated alcohol and, for 80 kPa, from 4.1% to 3%, at 15 and 172 ◦ C, respectively for anhydrous alcohol, from 3.8% to 3.1%, at 29 and 212 ◦ C, respectively for hydrated alcohol. The UFL for 20 kPa varies from 16.5% to 19.4%, at 25 and 209 ◦ C, respectively for anhydrous alcohol, from 16% to 20% at 16 and 221.8 ◦ C, respectively from hydrated alcohol, and, for 80 kPa, from 18.2% to 26.2% at 40 and 202 ◦ C, respectively, for anhydrous alcohol, from 17.4% to 25.5% at 56 and 212 ◦ C, respectively, for hydrated alcohol. It should be noted that as the pressure is reduced the upper limit curve has a lower slope. Finally, when both limits are plotted by varying the altitude (pressure) and the fuel volume % in air as the temperature increases, the LFL flammability curve tends to a vertical line. On the other hand, as temperature increases, the UFL flammability curve tends to have greater inclination. Details can be seen in Fig. 9.
5.4. Discussion As can be seen, the LFL results in this study are consistent with most published data. On the other hand, in the case of UFL, there are differences with published data; however other studies, [6] and [24], also reported these differences. One of the causes for the
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6. Conclusions
Fig. 10. Effect of water content on the flammability limits, to 101 kPa.
difference in flammability is the change in structure of the flame to reduce the pressure. In general, the LFL for all pressures tested for anhydrous ethanol have a more definite behavior, as reported in the literature for almost all fuels. It is easier to predict the behavior of this limit and there are several empirical and theoretical models to predict these values. Figs. 7 and 8, also show that for the flammability limits of anhydrous ethanol, the UFL is more sensitive to temperature, which is also reported in the literature. Note that for reduced pressures, there is a smaller variation of fuel concentration with increasing temperature to reach a value at which the UFL becomes more sensitive to temperature and fuel composition. Even at the specific case of 20 kPa, the flammability range narrows from 25 ◦ C to about 80 ◦ C (Fig. 8). After this, the fuel concentration begins to increase; in other words, the flammability range starts to widen. For hydrated ethanol, the flammability limits have behavior similar to anhydrous ethanol. The LFL has a definite and almost constant trend for any pressure condition, it is nearly linear and this is little variation for temperature change. The UFL is much more sensitive to temperature. In some cases, flammability range tends to narrow slightly. However, at high temperatures this range widens sharply; this phenomenon is observed mainly for reduced pressures. A comparison was also made to observe how the water content in hydrated ethanol affects flammability limits at a certain pressure, for example, 101 kPa. Thus, in Fig. 10, the expected result in shown: the range of flammability of anhydrous ethanol is wider. The largest differences are observed for UFL, so this limit, in addition to being more sensitive to temperature, is also very sensitive to water content in ethanol. This effect is expected since the water vapor contained in the fuel can act as an inert substance, producing a narrower flammability interval. The LFL is about the same for both alcohols, this is because the LFL is in the region of excess oxidant and adding water (inert) in the concentration does not affect important parameters such as flame temperature, for example. The contrary happens with the UFL where the mixture with additional water (inert) tends to narrow the flammability region. Finally, the temperature was carefully measured in the vessel. During evaporation, a considerable temperature drop did not occur when hydrated ethanol was injected, and the same happened with anhydrous ethanol. Therefore the narrowing of the flammability limits of hydrated ethanol is not due to a simple thermal effect (latent heat needed in order to vaporize the additional water). This is mainly due to the role of an inert in the flammability limits.
The flammable envelope for anhydrous and hydrated ethanol was determined experimentally. Good agreement was obtained with the LFL and UFL values reported by others. This agreement proves that the equipment was correctly assembled and operated; it also proves that the correct procedures were followed during the tests. After to the results obtained at standard atmospheric pressure, tests for reduced pressures were performed. One of the main objectives of this work was to show flammability limits at reduced pressures, considering the typical altitude of commercial aviation flight and thus atmospheric characteristics that are typical of aircraft fuel tanks. On the other hand, it is well known that depressurization is a means of extinguishing and suppressing flames in microgravity environments. These results can be used to prove the concept with flammability limits for a critical depressurization value. The quantity of fuel does not have to be reduced or eliminated in order to completely suppress the flame. Reducing pressure is sufficient to prevent the fire from ignition or cause extinction. In terms of the volume of sample and temperature, the curves of LFL and UFL have a tendency to decrease (unlike UFL at standard atmospheric pressure, which has a linear trend). This behavior can be observed in the figures presented in the results section. In some cases, this trend allowed the sample volume that was introduced into the vessel to be adjusted; to some extent, it also helped to guarantee flame propagation and to fix the flammability limit. Thus, fewer tests were necessary, preventing equipment wear. The majority of the tests were successfully performed; only 10% of them had to be repeated in order to achieve precision or in some cases to correct experimental procedures (especially with the upper limit).
Acknowledgements Funding for this study was provided by the Fundac¸ão de Amparo à Pesquisa do Estado de Minas Gerais, FAPEMIG, Brazil, through projects TEC-APQ-00467-11 (regular grant), Fundac¸ão de Amparo à Pesquisa do Estado de São Paulo, FAPESP, Brazil, through projects 2009/097387 (regular grant), and CNPq-Brazil for the DT2 Fellowship to CJCR (Proc. No. 310069/2012-2).
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