Critical Explosible Oxygen Concentration of Methanol-Saturated

Mar 17, 2014 - ABSTRACT: The critical oxygen concentration (COC) in this study is defined as the maximum oxygen concentration at which a mixture of ...
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Critical Explosible Oxygen Concentration of Methanol-Saturated Vapor/O2/N2 Mixtures at Elevated Temperatures and Pressures Xueling Liu, Ying Huang, Yue Wang, and Qi Zhang* State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, People’s Republic of China ABSTRACT: The critical oxygen concentration (COC) in this study is defined as the maximum oxygen concentration at which a mixture of methanol vapor in nitrogen does not explode, regardless of the nitrogen concentration in the mixture. This paper presents data on the critical oxygen concentration (COC), in the presence of added N2, of methanol (CH4O) saturated vapor mixtures at elevated temperature and pressure. We have used a COC measurement system consisting of a 4-L explosion vessel, an ignition subsystem, and a transient pressure measurement subsystem. Through a series of experiments carried out in this system, the COCs of methanol-saturated vapor/O2/N2 mixtures at different initial pressures and an elevated temperature of 80 °C have been studied, and the influence of concentration of nitrogen on the COC has been analyzed and discussed. Variation of the initial pressure within the studied range was found to have significant effect on the COCs of the methanol saturated vapor/ O2/N2 mixtures. There is a very large difference between the COCs (or CNCs) of the methanol-saturated vapor/O2/N2 mixtures at the elevated temperature and pressure and those of methanol vapor in air at atmospheric pressure and room temperature. The COCs of the methanol-saturated vapor/O2/N2 mixtures with the initial temperature of 80 °C at the initial pressure of 0.5, 0.4, and 0.3 MPa are 36, 28, and 21 vol %, respectively. The corresponding CNCs at initial pressures of 0.5, 0.4, and 0.3 MPa are 54, 59.5, and 62 vol %, respectively.

1. INTRODUCTION Knowledge of material safety properties is essential for safe handling during unit operations, since incidents in plants can often be traced to an insufficient knowledge of the hazardous properties of combustible or flammable substances. If determined carefully and applied properly, safety-related properties will provide information on the reaction behaviors and possible fire and explosion hazards of the specific substance. At atmospheric pressure and room temperature, methanol is a liquid. Its boiling point is 64.8 °C. Its flammability limits are 5.5−44 vol % at ambient pressure and temperature. Liquid methanol will evaporate and become a gas phase when it is heated over its boiling point. If the liquid methanol has enough volume in the closed vessel and is heated over its boiling point, it achieves a saturation state. The saturated methanol vapor is commonly used in the process industry. If the saturated methanol vapor mixes with O2 and forms methanol vapor/O2 mixtures, explosion may be initiated by a spark. When the oxygen concentration is lower than the critical oxygen concentration, explosion cannot be initiated. Nitrogen acts as a diluent. Chiang et al. investigated inert effects on the flammability characteristics of methanol by nitrogen or carbon dioxide.1 The flammability envelope was experimentally determined up to the point of vapor saturation for methanol.2 The critical oxygen concentration (COC) in this study is defined as the maximum oxygen concentration at which a mixture of methanol vapor in nitrogen does not explode, regardless of the nitrogen concentration in the mixture. If the oxygen concentration in the methanol vapor in nitrogen mixtures is lower than the COC value, combustion cannot be initiated. Similarly, the critical nitrogen concentration (CNC) in this study is defined as the minimum nitrogen concentration © 2014 American Chemical Society

at which a mixture of methanol vapor in nitrogen does not explode. If the nitrogen concentration in the mixtures is higher than the CNC value, combustion cannot be initiated. The presence of added nitrogen has significant influence on the COC. While data on the COC of saturated methanol vapor/O2/N2 mixtures are essential for safe and reliable operation, they are nevertheless hardly available in the literature. Knowledge of the characteristics of the flammable saturated methanol vapor/O2/N2 mixtures is an important prerequisite for proper usage. Although a great deal of research effort has been directed toward determining the explosion parameters of flammable gases,3−9 the flammability limits of gas mixtures,10−13 and the effects of the initial temperature and pressure on the explosion parameters,15−22 unfortunately, few researchers have paid attention to the COCs of saturated methanol vapor/O2/N2 mixtures at pressures and/or temperatures different from ambient. Consequently, in this work, we have used a COC measurement system consisting of a 4-L explosion vessel, an ignition subsystem, and a transient pressure measurement subsystem. Through a series of experiments carried out in this system, the COCs of saturated methanol vapor/O2/N2 mixtures at different initial pressures and an elevated temperature of 80 °C have been studied, and the COC variation with the experimental conditions has been analyzed and discussed. Received: Revised: Accepted: Published: 5617

August 1, 2013 January 18, 2014 March 17, 2014 March 17, 2014 dx.doi.org/10.1021/ie402502j | Ind. Eng. Chem. Res. 2014, 53, 5617−5621

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2. EXPERIMENTAL APPARATUS AND PROCEDURES 2.1. General. The experimental setup used in this study consisted of a 5-L cylindrical vessel coupled with an electric ignition system and a data acquisition system, as shown in Figure 1. Experiments were performed in a cylinder explosion

Figure 2. General view of 5-L explosion chamber. [Legend: (1) nitrogen gas inlet valve, (2) oxygen inlet valve, (3) pressure sensor, (4) exhaust valve, (5) safety release valve, (6) discharge electrodes, and (7) pressure gauge.]

the liquid methanol, was taken into account in determining the composition volume fraction. The volume fractions of the filled oxygen and nitrogen gas were determined by using partial pressure theory. The methanol vapor, oxygen, and nitrogen gas concentrations were obtained by the relationship between the concentrations and the partial pressures. The mixing duration is ∼1 min. The methanol-saturated vapor O2/N2 mixture was then ignited by an electric spark produced by a spark generator in the center of the gaseous mixture. After the ignition occurred within the gaseous mixture, a combustion wave formed and propagated from the position of ignition to the wall of the vessel. The histories of pressure resulting from the saturated vapor/O2/N2 mixture explosion were recorded by the pressure gauges connected to the data acquisition system. The pressure changes with time describe a given pressure history, and the maximum pressure represents the peak value of a given pressure wave.

Figure 1. Experimental setup and data acquisition system.

vessel with central ignition. The height (h) of the vessel was 330 mm, and the inner diameter (2R) was 140 mm. In the experimental vessel, ignition was achieved by means of an inductive-capacitive spark produced between stainless steel electrodes with rounded tips, separated by a spark gap of 1.5 mm. The electrode diameter used in the experiments was 1.5 mm. Liquid methanol was in the lower volume in the vessel. The upper volume is filled by the gaseous mixtures. The ignition location was in the center of the gaseous mixtures. 2.2. Explosion Pressure. Explosions of the mixtures were monitored by means of Kistler pressure gauges mounted on the wall of the experimental vessel. All results were stored through a data acquisition device. The data acquisition system was triggered by the control unit and recorded pressure data at sampling frequencies of 1 MHz. The type of Kistler pressure gauges and the acquisition data system used in the experiments were similar to those used in ref 14. 2.3. Spark Ignition Energy. By means of a traditional igniting test, one can easily determine the ignition energy on the basis of the given capacitance and applied voltage through simple calculation. The traditional test, referred to as the simple method, follows the general relationship expressed as 1 E = *U 2 (1) 2 where * is the capacitance of the capacitor, U is the voltage of the capacitor discharge, and E is the energy stored in the capacitor, which is traditionally regarded as the ignition energy. In this study, C = 14 uF, U = 831 V, and E = 4.8 J. 2.4. Experimental Procedure. The methanol vapor and nitrogen gas concentrations in the saturated methanol vapor/ O2/N2 mixtures were evaluated by partial pressures. The oxygen concentrations were monitored by an instrument (ABB Model EL3060-Magnos206) and evaluated by partial pressures. Liquid methanol was placed in the experimental vessel (2/3 of the vessel volume). Subsequently, heating the outer surface of the vessel to the specified temperature (80 °C) vaporized part of the methanol therein, which was necessary for the methanol vapor to achieve the saturation state. A thermostatted water container (see Figure 2) was used to heat the vessel containing the liquid methanol. O2 and N2 were then filled into the vessel. The air originally in the vessel, prior to its filling with

3. EXPERIMENTAL RESULTS In the experiments, the environmental conditions were as follows: initial temperature and pressures of the methanol saturated vapor/oxygen/nitrogen mixture 80 °C and 0.5, 0.4, and 0.3 MPa, respectively. The fuel volume concentrations in the methanol saturated vapor mixtures were 10−17 vol %. The experimental results are listed in Tables 1−3. The critical volume concentration triangle of the methanol-saturated vapor/oxygen/nitrogen at the initial pressure of 0.5 MPa and temperature of 80 °C was obtained, based on the experimental results listed in Table 1, as shown in Figure 3. The critical nitrogen concentration (CNC) is a quantity that corresponds to the COC. Both of them vary approximately linearly with the initial pressures in the examined range, as shown in Figure 4. The higher the volume fraction of fuel in the mixture, the more oxygen needed for combustion. The minimum oxygen concentration (MOC) of methanol at room temperature and atmospheric pressure reported in the literature13 is 7% (v/v), which is lower than COC measured in this work. However, the MOC of methanol reported in the literature13 was obtained at the lower limit (5.8 vol %) of methanol. The COC of methanol measured in this work was obtained at the volume fraction of higher than lower limit of methanol (17, 12.5, and 10 vol %). The fact that the data on the COC of methanol measured in this work are higher than the MOC reported in the literature13 is consistent with the theoretical analysis on the MOC and COC. The specific experimental condition of this work is different from that given in ref 13. 5618

dx.doi.org/10.1021/ie402502j | Ind. Eng. Chem. Res. 2014, 53, 5617−5621

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Table 1. Experimental Results on the COC of Methanol-Saturated Vapor/Oxygen/Nitrogen Mixtures at the Initial Pressure of 0.5 MPa and Temperature of 80 °Ca O2 No.

partial pressure of oxygen, PO (MPa)

1 2 3 4 5 6 7 8 9 10 11

0.35 0.3 0.28 0.23 0.20 0.19 0.185 0.18 0.18 0.175 0.175

N2

partial pressure of nitrogen, PN concentration of oxygen, CO (vol %) (MPa) concentration of nitrogen, CN (vol %) explosion?b 70 60 56 46 40 38 37 36 36 35 35

0.1 0.15 0.17 0.22 0.25 0.26 0.265 0.27 0.27 0.275 0.275

20 30 34 44 50 52 53 54 54 55 55

Y Y Y Y Y Y Y N N N N

a

Note: The total pressure in the saturated methanol vapor/oxygen/nitrogen mixture is 0.5 MPa. The partial pressure of methanol vapor is 0.05 MPa. The volume fraction of methanol vapor in the mixtures is 10 vol %. Y and N indicate whether the explosion was initiated or not, respectively. If the overpressure monitored was higher than 7% of total initial pressure, we considered that the explosion took place. bThe letters “Y” and “N” indicate whether an explosion was initiated or not, respectively. If the overpressure monitored was higher than 7% of the total initial pressure, we considered the explosion to have occurred.

Table 2. Experimental Results on the COC of Methanol-Saturated Vapor/Oxygen/Nitrogen Mixtures at an Initial Pressure of 0.4 MPa and a Temperature of 80 °Ca O2

N2

No.

partial pressure of oxygen, PO (MPa)

concentration of oxygen, CO (vol %)

1 2 3 4 5 6

0.221 0.171 0.121 0.121 0.116 0.112

55 43 30 30 29 28

partial pressure of nitrogen, PN (MPa) concentration of nitrogen, CN (vol %) explosion?b 0.129 0.179 0.229 0.229 0.234 0.238

32.5 44.5 57.5 57.5 58.5 59.5

Y Y Y Y Y N

a

Note: The total pressure in the saturated methanol vapor/oxygen/nitrogen mixture is 0.4 MPa. The partial pressure of methanol vapor is 0.05 MPa. The volume fraction of methanol vapor in the mixtures is 12.5 vol %. bThe letters “Y” and “N” indicate whether the explosion was initiated or not, respectively. If the overpressure monitored was higher than 7% of the total initial pressure, we considered the explosion to have occurred.

Table 3. Experimental Results on the COC of Saturated Methanol Vapor/Oxygen/Nitrogen Mixtures at the Initial Pressure of 0.3 MPa and Temperature of 80 °Ca O2

N2

No.

partial pressure of oxygen, PO (MPa)

concentration of oxygen, CO (vol %)

1 2 3 4 5 6

0.121 0.071 0.071 0.066 0.063 0.046

40 23 23 22 21 15

partial pressure of nitrogen, PN (MPa) concentration of nitrogen, CN (vol %) explosion?a 0.129 0.179 0.179 0.184 0.186 0.204

43 60 60 61 62 68

Y Y Y Y N N

a

Note: The total pressure in the saturated-methanol vapor/oxygen/nitrogen mixture is 0.3 MPa; The partial pressure of methanol vapor is 0.05 MPa. The volume fraction of methanol vapor in the mixtures is 17 vol %. aThe letters “Y” and “N” indicate whether the explosion was initiated or not, respectively. If the overpressure monitored was higher than 7% of total initial pressure, we considered that the explosion took place.

The chemical reaction of the gaseous methanol/air mixtures with the stoichiometric volume fraction is

cCH4O =

1 = 12% 1 + (1.5 × 4.76)

The volume fractions (17, 12.5, and 10 vol %) of gaseous methanol in the experiments of this study corresponding to the COCs were close to the stoichiometric volume fraction of gaseous methanol in air. This work was based on an actual process of chemical engineering which is an advanced process to be developed. The

CH4O + 1.5(O2 + 3.76N2) = CO2 + 2H 2O + (1.5 × 3.76)N2

The stoichiometric volume fraction of gaseous methanol/air mixture is 5619

dx.doi.org/10.1021/ie402502j | Ind. Eng. Chem. Res. 2014, 53, 5617−5621

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Research Note

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 68914252. Fax: +86 10 68914252. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors thank the financial support of the Nature Science Foundation of China (No. 11372044) and the Foundation for Doctor Dissertation of China (No. 20111101110008) for this study. Figure 3. Critical volume concentration triangle of methanol saturated vapor/oxygen/nitrogen at an initial pressure of 0.5 MPa and temperature of 80 °C.

(1) Brooks, M. R.; Crowl, D. A. Flammability envelopes for methanol, ethanol, acetonitrile and toluene. J. Loss Prev. Process Ind. 2007, 20 (2), 144−150. (2) Chiang, C. C.; Lee, J. C.; Chang, Y. M.; Chuang, C. F.; Shu, C. M. Inert effects on the flammability characteristics of methanol by nitrogen or carbon dioxide. J. Therm. Anal. Calorim. 2009, 96 (3), 759−763. (3) Razus, D.; Brinzea, V.; Mitu, M.; Oancea, D. Explosion characteristics of LPG−air mixtures in closed vessels. J. Hazard. Mater. 2009, 165 (1−3), 1248−1252. (4) Pekalski, A. A.; Schildberg, H. P.; Smallegange, P. S. D.; Lemkowitz, S. M.; Zevenbergen, J. F.; Braithwaite, M.; Pasman, H. J. Determination of the explosion behaviour of methane and propene in air or oxygen at standard and elevated conditions. Process Saf. Environ. Prot. 2005, 83 (5), 421−429. (5) Kindracki, J.; Kobiera, A.; Rarata, G..; Wolanski, P. Influence of ignition position and obstacles on explosion development in methaneair mixture in closed vessels. J. Loss Prev. Process Ind. 2007, 20 (4−6), 551−561. (6) Razus, D.; Brinzea, V.; Mitu, M.; Movileanu, C.; Oancea, D. Inerting effect of the combustion products on the confined deflagration of liquefied petroleum gas−air mixtures. J. Loss Prev. Process Ind. 2009, 22 (4), 463−468. (7) Jo, Y. D.; Park, K. S. Minimum amount of flammable gas for explosion within a confined space. Process Saf. Prog. 2004, 23 (4), 321−329. (8) Dahoe, A. E. Laminar burning velocities of hydrogen−air mixtures from closed vessel gas explosions. J. Loss Prev. Process Ind. 2005, 18 (3), 152−166. (9) Zhang, Q.; Li, W.; Zhang, S. Effects of Spark Duration on Critical Ignition Energy for Methane/Air Mixture. Process Saf. Prog. 2011, 30, 154−156. (10) Zlochower, I. A.; Green, G. M. The limiting oxygen concentration and flammability limits of gases and gas mixtures. J. Loss Prev. Process Ind. 2009, 22 (4), 499−505. (11) Shu, C.-M.; Wen, P.-J. Investigation of the flammability zone of o-xylene under various pressures and oxygen concentrations at 150 °C. J. Loss Prev. Process Ind. 2002, 15 (4), 253−263. (12) Werle, S.; Wilk, R. K. Ignition of methane and propane in hightemperature oxidizers with various oxygen concentrations. Fuel 2010, 89 (8), 1833−1839. (13) Chang, Y.-M.; Lee, J.-C.; Chen, C.-C.; Shu, C.-M. Fire and explosion properties examinations of toluene -methanol mixtures approached to the critical oxygen concentration. J. Therm. Anal. Calor. 2009, 96, 741−749. (14) Zhang, Q.; Li, W.; Lin, D.-C.; He, N.; Duan, Y. Influence of nitromethane concentration on ignition energy and explosion parameters in gaseous nitromethane/air mixtures. J. Hazard. Mater. 2011, 185 (2−3), 756−762. (15) Zhang, Q.; Li, W.; Tan, R.; Duan, Y. Combustion parameters of gaseous epoxypropane/air in a confined vessel. Fuel 2013, 105, 512− 517.

Figure 4. COC of methanol-saturated vapor/oxygen/nitrogen mixtures at 80 °C versus the initial pressure.

experimental conditions in this work were completely identical to that of the production. The results of this study were corresponding to the specific conditions: it was the methanolsaturated vapor/O2/N2 mixtures, and not the methanol vapor/ air. That is, the COCs measured in this work are not applicable to the methanol vapor/air. The experimental conditions of this work were specific, but, will be easily generalized in the future chemical engineering in China. The COCs of the methanolsaturated vapor/O2/N2 mixtures measured in this work are the very significant parameters for future process safety evaluations.

4. CONCLUSIONS We have studied the critical oxygen concentrations (COCs) of methanol-saturated vapor/O2/N2 mixtures in a closed vessel. The main conclusions drawn can be summarized as follows: (1) There is a very great difference between the COCs of the methanol-saturated vapor/O2/N2 mixtures at the elevated temperature and pressure and that of methanol vapor in air at atmospheric pressure and room temperature. (2) The COCs of the methanol-saturated vapor/O2/N2 mixtures with the initial temperature of 80 °C at the initial pressure of 0.5, 0.4, and 0.3 MPa are 36, 28, and 21 vol %, respectively. The corresponding critical nitrogen concentrations (CNCs) at the initial pressure of 0.5, 0.4, and 0.3 MPa are 54, 59.5, and 62 vol %, respectively. 5620

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(16) Zhang, Q.; Li, W. Ignition Characteristics for Methane−Air Mixtures at Various Initial Temperatures. Process Saf. Prog. 2013, 32, 37−41. (17) Zhang, Q.; Tan, R.; Huang, Y.; Liang, H. Effects of humidity on critical ignition energy of gaseous epoxypropane/air mixtures. J. Loss Prev. Process Ind. 2012, 25, 982−988. (18) Gieras, M.; Klemens, R.; Rarata, G.; Wolański, P. Determination of explosion parameters of methane-air mixtures in the chamber of 40 dm3 at normal and elevated temperature. J. Loss Prev. Process Ind. 2006, 19, 263−270. (19) Razus, D.; Brinzea, V.; Mitu, M.; Oancea, D. Temperature and pressure influence on explosion pressures of closed vessel propane-air deflagrations. J. Hazard. Mater. 2010, 174 (1−3), 548−555. (20) Pekalski, A. A.; Terli, E.; Zevenbergen, J. F.; Lemkowitz, S. M.; Pasman, H. J. Influence of the ignition delay time on the explosion parameters of hydrocarbon−air−oxygen mixtures at elevated pressure and temperature. Proc. Combust. Inst. 2005, 30 (2), 1933−1939. (21) Zabetakis, M. G. Flammability characteristics of combustible gases and vapors. Bull.U.S., Bur. Mines 1965, 627. (22) Bui-Pham, M. N.; Lutz, A. E.; Miller, J. A.; Desjardin, M.; O’Shaughnessey, D. M. Rich flammability limits in CH3OH/CO/ diluent mixtures. Combust. Sci. Technol. 1994, 109 (1−6), 71−91.

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dx.doi.org/10.1021/ie402502j | Ind. Eng. Chem. Res. 2014, 53, 5617−5621