Article pubs.acs.org/EF
Combustion-Induced Rapid-Phase Transition (cRPT) in CH4/CO2/O2Enriched Mixtures Almerinda Di Benedetto,† Francesco Cammarota,‡ Valeria Di Sarli,‡ Ernesto Salzano,*,‡ and Gennaro Russo† †
Dipartimento di Ingegneria Chimica, Università degli Studi di Napoli “Federico II”, Piazzale Tecchio 80, 80125 Napoli, Italy Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche (CNR), Via Diocleziano 328, 80124 Napoli, Italy
‡
ABSTRACT: Explosion of oxygen-enriched fuel mixtures can exhibit severe behavior because of the rapid evaporation of the water produced by the combustion reaction. The phenomenon underlying this behavior has been recently named combustioninduced rapid-phase transition (cRPT). If the cRPT phenomenon is not invoked, the observed behavior cannot be explained by the classical theory for deflagration to detonation transition or pre-compression effects. In this work, the cRPT phenomenon was analyzed by varying either the oxygen enrichment or CO2 content in three closed vessels with different internal surface area/ volume ratios. Characteristic times for condensation, radiation, and reaction have further demonstrated the opportunity to predict either the likelihood or the trend of the intensity of the observed over-adiabatic maximum pressures as functions of the surface/volume ratio.
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316 stainless-steel vessel. The wall thickness is 5 cm. The diameter is equal to 14 cm (slightly reduced on the top section), and the vertical length is 40 cm. The equipment includes a stirring system for mixing. Reactor 2 consists of a cylindrical tubular vessel with the same wall thickness and steel quality as reactor 1. The diameter is 6 cm, and the vertical length is 120 cm. Finally, reactor 3 consists of a closed carbon steel sphere. In Table 1, the geometrical details are given for each vessel. The surface/volume ratio (A/V) varies from 0.2 to 0.68 cm−1. All reactors are equipped with rupture disks able to withstand about 200 bar. For the pressure recording, which is the core measurement for the experiments reported in this paper, Kulite ETS-IA-375 (M) series pressure transducers with a natural frequency of 150 kHz were used. These transducers are specifically designed for high-pressure, highshock environments and blast analysis. They were fed by a chemical battery (12 VDC/7 AH) to minimize any disturbance on the output supply, which was recorded by means of a National Instrument USB6251 data acquisition system (16 bit, 1.25 × 106 samples/s) with a frequency up to 600 kHz. No manipulations were performed on the analogical signal output from the transducer or the digital data recorded. The fuel−oxidant mixtures used in this work were obtained using the partial pressure methodology in reactor 1. After vacuuming, the combustion vessel was filled by injecting one mixture component (purity above 99.99%, v/v) at a time to reach the specific partial pressure. Gases were premixed by mechanical stirring (rotating shaft velocity equal to 200 rpm). When reactor 1 was directly adopted for the explosion test, gases were allowed to settle for around 30 s and then ignited by a single capacitor spark [5 kV direct current (DC)] through two electrodes positioned at the center of the vessel (spark gap of about 1 mm). The same electrical apparatus was adopted for reactor 3 (the sphere). When reactor 2 (the tubular reactor) was used, ignition was set at the bottom of the equipment. In Table 2, the compositions of the mixtures investigated are given for different oxygen−air enrichment factors [E = O2/(O2 + N2)] and
INTRODUCTION Recently, we found that CH4/O2/N2 mixtures can exhibit exceptional and anomalous behavior when exploding in oxygenenriched air.1,2 With explosion tests performed at stoichiometric methane/oxygen ratios by varying the oxygen enrichment factor E = O2/(O2 + N2) from 0.21 (air) to 1 (pure oxygen), we found that, for E ≥ 0.4, the temporal pressure trend starts oscillating, eventually culminating in very high amplitude oscillations, thus exceeding the adiabatic value up to the maximum peak pressure as high as 280 bar. We attributed the observed oscillating behaviors to the occurrence of cycles of condensation and vaporization of the water produced during the flame propagation, occurring at the thermal boundary layer close to the vessel walls. Such cycles culminate in the superheating of the condensed water and then in the explosive vaporization of the water,3,4 with the formation of shock waves that trigger the measured over-adiabatic pressure peaks. This new phenomenon was named combustion-induced rapid-phase transition (cRPT).1,2 Following our interpretation, the occurrence of the cRPT phenomenon is localized at the vessel wall, because it originates in the thermal boundary layer, where the water produced by the combustion reaction condensates, superheats, and explosively evaporates. In this work, we investigated the role of the internal surface/volume ratio (A/V). More specifically, the analysis was focused on the comparison of the explosion behavior of CH4/O2/N2 mixtures in steel equipment with different A/V ratios: a cylindrical vessel, where the phenomenon was first observed, a tubular vessel, and a sphere. Tests were performed by changing the oxygen−air enrichment factor [E = O2/(O2 + N2)] and adding different amounts of CO2 (ranging from 0 to 66%).
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EXPERIMENTAL SECTION
Received: April 27, 2012 Revised: July 2, 2012 Published: July 9, 2012
A sketch of the experimental rig is shown in Figure 1, where the three reactors adopted are shown. Reactor 1 consists of a cylindrical AISI © 2012 American Chemical Society
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Figure 1. Scheme of the experimental rig. the cRPT peak pressure may however vary about 10% (or less) because of the very steep behavior and short duration.
Table 1. Geometry of the Three Reactors Adopted in This Work reactor
shape
D (cm)
L (cm)
A/V (cm−1)
1 2 3
cylindrical cylindrical spherical
14 6 30
40 120
0.20 0.34 0.68
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RESULTS In the following, the effects of oxygen−air enrichment (E) and CO2 addition to the mixture on the pressure time histories, the peak pressure, and the deflagration index are studied for vessels with different surface/volume ratios. Effect of the Oxygen−Air Enrichment. Figure 2 shows the pressure time histories obtained by varying the oxygen−air enrichment factor (E) in the three reactors. It can be seen that, when air is used (E = 0.21), the peak pressure is always lower than the adiabatic value, which is about 7 bar. This deviation from the isoentropic behavior is related to not only the heat losses toward the external environment but also the water condensation at the cold walls, which reduces the total pressure. As expected and for both reasons cited, the heat losses toward the environment increase and, thus, the peak pressure decreases with an increasing A/V ratio. In particular, the higher peak pressure, Pmax = 4.2 bar, is reached in reactor 3 (A/V = 0.20 cm−1), whereas in reactor 1 (A/V = 0.34 cm−1), Pmax is 3.8 bar, and finally, in the tubular vessel (A/V = 0.68 cm−1), Pmax is 1.8 bar. On the other hand, when the oxygen−air enrichment (E) is increased, it can be observed that, in all reactors, the peak pressure is higher than the thermodynamic value. This result has been attributed to the occurrence of the cRPT phenomenon, which has been previously found by the same authors.1,2 More specifically, the existence of the over-adiabatic pressure peak has been attributed to the following sequential events: (i) water production by the combustion reaction, (ii) water condensation at the cold vessel walls, (iii) superheating of the water in the boundary layer closed to the vessel walls, and (iv) rapid-phase transition of the superheated water. The occurrence of superheating and rapid-phase transition of water
Table 2. Mixture Compositions, Adiabatic Pressure and Temperature, and Partial Pressure of the Water Produced (PH2O) as Calculated at Equilibrium Conditions run
E
CH4 (%)
O2 (%)
N2 (%)
CO2 (%)
Pad (bar)
Tad (K)
PH2O (bar)
1 2 3 4 5 6
0.21 0.40 0.60 0.80 0.80 0.80
7.6 13.3 18.5 23.0 17.1 11.4
15.2 26.7 36.9 46 34.3 22.9
57.2 40.0 24.6 11.0 8.6 5.7
20 20 20 20 40 60
6.93 9.31 10.70 11.70 9.77 7.69
2073 2693 2962 3127 2763 2275
1.01 2.25 3.26 4.08 2.95 1.71
CO2 contents. For each mixture, Table 2 also gives the maximum theoretical values of pressure (Pad) and temperature (Tad) corresponding to the adiabatic values and the partial pressure of the water produced (PH2O) as calculated at equilibrium conditions. It is worth noting that the water vapor pressure (P0H2O) at the wall temperature (Twall = 283 K) is equal to 0.022 bar. Each composition was tested at least 3 times. For all tests, the initial pressure was set to 1 bar. The temperature of the vessel walls was equal to the ambient temperature (293 K). Explosion tests in the sphere at CO2 = 20% were not performed for the sake of safety (the spherical vessel has a maximum allowable pressure equal to 40 bar). Finally, it is worth the reminder that the three tests, for each given composition, showed great reproducibility. Pressure histories are almost completely consistent, including the time of occurrence of the cRPT spike and its intensity. The exact value of 4800
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Figure 2. Pressure time histories obtained in the three reactors by varying the oxygen enrichment factor (CO2 = 20%). Green dashed line = adiabatic pressure, Pad; A/V (cm−1).
Figure 3. Pressure time histories obtained varying the CO2 content for different values of the A/V ratio (E = 0.8). Green dashed line = adiabatic pressure, Pad; A/V (cm−1).
leads to the production of shock waves that coalesce, hence giving rise to unexpected pressure peaks. From the plot, it also appears that the cRPT phenomenon arises first at the higher A/V ratio, which is opposite that expected in standard deflagration. Upon further increasing the oxygen content (E = 0.6), the cRPT phenomenon is observed at all values of the A/V ratio. Effect of the CO2 Addition. We compared the pressure histories at a fixed value of the oxygen−air enrichment factor (E = 0.8) and changing the CO2 content from 20 to 60%. In Figure 3, the pressure signal is shown as obtained for all reactors. In the absence of cRPT (CO2 = 60%), the peak pressure is lower than the adiabatic value because of the heat losses toward the external environment. Its value decreases with an increasing A/V ratio. With the CO2 content equal to 40%, the spherical vessel (reactor 3, the lower A/V ratio) exhibits a cRPT peak. The same occurs for reactor 2. Conversely, in reactor 1, the observed peak pressure is lower than the adiabatic pressure
even if the pressure signal exhibits oscillations. Upon further decreasing the CO2 content (CO2 = 20%), the cRPT phenomenon arises also in reactor 1. The obtained results show that the surface/volume ratio plays a major role in affecting the occurrence of the cPRT phenomenon. Indeed, this phenomenon result is much more violent when the A/V ratio is higher. Further details of the effects of diluents, including CO2, on the likelihood and intensity of cRPT spikes have been analyzed by Di Benedetto et al.2 Results are not reported here for the sake of brevity.
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DISCUSSION The explanation of the over-adiabatic pressure peak observed in the oxygen-enriched atmosphere needs to invoke the occurrence of the cRPT phenomenon. Indeed, if the cRPT is not invoked, all of the observed phenomena cannot be explained by the classical theory for deflagration to detonation 4801
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transition, pre-compression effects,5 or pressure amplification effects.6 Furthermore, the results obtained show that the surface/volume ratio plays a major role in affecting the occurrence of the cPRT phenomenon. From the pressure time histories, it appears that the activation of the cRPT is favored by increasing the A/V ratio. In Figure 4, the measured peak pressure is plotted versus the oxygen−air enrichment factor E [CO2 (40%)] and the CO2 content (E = 0.8).
Figure 5. Effect of A/V on the cRPT phenomenon severity in terms of the explosion index (KcRPT) by varying the oxygen-enriched factor E. CO2 = 20%.
KcRPT =
dP dt
V1/3 max
(1)
The plot shows the effect of A/V on the severity of the cRPT phenomenon. Here, it is also worth noting that the values of the deflagration index are extremely higher than the typical values found for the deflagrating phenomenon of both gases (50 bar m s−1 for methane and 600 bar m s−1 for hydrogen) and dusts (about 300 bar m s−1 for lycopodium). The effect of A/V can be predicted if considering the characteristic times for chemical, gas-dynamic, and reactorrelated processes occurring in the closed-vessel deflagration of oxygen-enriched mixtures. In previous papers,1,2 we found that the ratio between the time for radial flame propagation (τreac) and the time for water condensation (τcond) is the key parameter for the occurrence of the cRPT phenomenon. Figure 4. Effect of the O2−air enrichment factor E (top) and CO2 content (bottom) on the measured peak pressure (Pmax) for different A/V ratios. Blue line = adiabatic pressure, Pad; A/V (cm−1).
θ1 =
τcond τreac
(2)
More precisely, we found that, if the radial flame propagation rate is lower than the condensation rate (hence, θ1 < 1), the water produced by the combustion reaction can condensate at the walls as soon as it is formed and superheating conditions are possible. Furthermore, we showed that the severity of the cRPT phenomenon is correlated to the ratio between the condensation time (τcond) and the flame radiation time (τrad), θ2.
From the top plot of Figure 4, it can be seen that, for lower values of the oxygen−air enrichment factor (E ≤ 0.4), the pressure histories exhibit a maximum pressure that is lower than the adiabatic pressure. Given the limit value for E, this behavior can be extended to any oxygen-enriched fuel mixture with CO2 > 40%. Within these limits, the peak pressure decreases with an increasing surface/volume ratio, because of the heat losses toward the external environment. Conversely, for E > 0.4 and CO2 < 40%, the cRPT phenomenon is always observed, regardless of the surface/volume ratio. The activation of the cRPT phenomenon is favored by higher values of A/V; in the spherical vessel, it first appears at a CO2 content of 40%, which is lower than the corresponding value for the two other cylindrical vessels. This result confirms that the cRPT is a surface phenomenon; the cold surface allows for the water condensation and superheating. Figure 5 shows the effect of A/V on the deflagration index of the cRPT phenomenon (KcRPT) defined as
θ2 =
τcond τrad
(3)
In particular, the cRPT peak pressure increases with θ2. Tables 3−6 show the computed values of θ1 and θ2 for selected experimental runs. Data reported in the tables show that θ1 predicts the occurrence of cRPT, which can be observed for E ≥ 0.6 and CO2 ≤ 40%, as also found experimentally. On the other hand, the values of θ2 give the exact trend of the intensity for the cRPT peaks, because θ2 increases with A/V and E, whereas it decreases with the CO2 content. 4802
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Table 3. Computed Values of θ1 at Different CO2 Contents and A/V Ratiosa run
CO2 (%)
A/V = 0.20 cm−1
A/V = 0.34 cm−1
A/V = 0.68 cm−1
5 6 7
20 40 60
1.86 0.668 0.100
2.85 0.896 0.153
3.75 1.380 0.202
a
(2) Di Benedetto, A.; Cammarota, F.; Di Sarli, V.; Salzano, E.; Russo, G. Effect of diluents on rapid phase transition of water induced by combustion. AIChE J. 2011, DOI: 10.1002/aic.12778. (3) Reid, R. C. Superheated liquids. Am. Sci. 1976, 64, 146−156. (4) Reid, R. C. Rapid phase transitions from liquid to vapour. Adv. Chem. Eng. 1983, 12, 105−208. (5) Holtappels, K.; Pasman, H. J. Interpretation of gas explosion tests; extremes in explosion severity, SAFEKINEX: Safe and efficient hydrocarbon oxidation processes by kinetics and explosion expertise. FP7 Programme “Energy, Environment and Sustainable Development”; Delft University of Technology: Delft, The Netherlands, 2007; Contract EVG1-CT-2002-00072. (6) Kogarko, S. M. Amplification of compression waves in the combustion zone. Proc. Combust. Inst. 1961, 8, 1159−1164.
E = 0.8.
Table 4. Computed Values of θ2 at Different CO2 Contents and A/V Ratiosa run
CO2 (%)
5 6 7 a
20 40 60
A/V = 0.20 cm−1
A/V = 0.34 cm−1
−2
−2
4.89 × 10 3.26 × 10−2 2.18 × 10−2
6.07 × 10 4.05 × 10−2 2.71 × 10−2
A/V = 0.68 cm−1 7.99 × 10−2 9.45 × 10−2 6.33 × 10−2
E = 0.8.
Table 5. Computed Values of θ1 at Different E and A/V Ratiosa run
E
A/V = 0.20 cm−1
A/V = 0.34 cm−1
A/V = 0.68 cm−1
1 2 3
0.21 0.40 0.60
0.074 0.58 1.22
0.115 0.91 1.91
0.150 1.20 2.51
a
CO2 = 20% (v/v).
Table 6. Computed Values of θ2 at Different E and A/V Ratiosa E
run 1 2 3 a
0.21 0.40 0.60
A/V = 0.20 cm−1
A/V = 0.34 cm−1
−2
1.01 × 10 2.98 × 10−2 5.90 × 10−2
−2
1.25 × 10 3.71 × 10−2 7.35 × 10−2
A/V = 0.68 cm−1 1.65 × 10−2 4.88 × 10−2 9.67 × 10−2
CO2 = 20% (v/v).
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CONCLUSION The occurrence of the cRPT phenomenon is confirmed in different equipment. The dependence of the cRPT likelihood and intensity upon the surface/volume ratio (A/V) has been analyzed by varying either the oxygen enrichment or CO2 content. It is found that, upon increasing A/V, the severity of the cRPT phenomenon increases significantly, in terms of either over-adiabatic peak pressure or deflagration index.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +39-0817622673. Fax: +39-0817622915. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial funding provided by the Ministero per lo Sviluppo Economico (MiSE) within the framework of the project “Carbone Pulito”.
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REFERENCES
(1) Di Benedetto, A.; Cammarota, F.; Di Sarli, V.; Salzano, E.; Russo, G. Anomalous behavior during explosions of CH4 in oxygen-enriched air. Combust. Flame 2011, 158, 2214−2219. 4803
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