(180al) Rapid Acid/Base Reactions That Produce Energetic Explosive Intermediates

Rapid Acid/Base Reactions that Produce Energetic Explosive Intermediates

C. Brown, P.M. Frisby, and J. E. Smith, Jr.^?

Department of Chemical and Materials Engineering

University of Alabama in Huntsville

Huntsville, Alabama 35899

The recent development and extension of a thermally controlled advanced laser diagnostic system was used to measure the chemical delay time (CDT) for hypergolic combustion with respect to temperature, including some intriguing results associated with cryogenic combustion.  The fuels concentrated on in this study were Anhydrous Hydrazine (AH), Monomethylhydrazine (MMH), and Unsymmetrical Dimethyl Hydrazine (UDMH), as shown in Figure1.

Figure1: Structures of Hydrazine, MMH, and UDMH

These fuels were reacted with red fuming nitric acid (RFNA) under a blanket of Argon gas to prevent reactions with oxygen and to minimize the intrusion of water vapor, particularly at cryogenic temperatures.  Advanced thermal stages have been added to the existing CDT laser diagnostic system, which provides kinetic results for the hypergolic fuels.  The temperature range for Hydrazine was varied from below its freezing point to 202.7^oF (360K) due to evaporation, while MMH was studied between 160^oF (344K) and -50^oF (288K) and the UDMH was studied at temperatures from above 140 to below -60^oF  Since this system has a high acid demand, and the composition of the acid can change rapidly during oxidizer lead, the implication of studying the O/F ratio at cryogenic temperatures minimizes fuming in RFNA system permits higher O/F ratios while maintaining the integrity of the acid.  A system was designed utilizing liquid nitrogen and a heavy stainless steel reaction vessel to achieve and maintain low temperatures (Brown, et. al. 2009a).  For temperatures above ambient, a ceramic system described in Dasarathy, et. al 2005 was employed, and the data indicated that the CDT of UDMH increased when heated and was attributed to the increase in oxidizer demand at higher temperatures.  To study the effect of higher O/F ratios in the UDMH system, oxidizer lead was required.  Measurements were made at cryogenic temperatures approaching the freezing point of UDMH to determine the effect on CDT for UDMH.  At temperatures above 20^oF, the fuming from the RFNA disrupted the CDT measurements producing high relative error due to the NO₂ vapors interfering with the laser diagnostic system.  The newly developed cryogenic technique inhibited the release of NO₂ allowing for study of the effect of higher O/F ratios.  Oxidizer to fuel ratios were varied for UDMA/RFNA from 2?4 at a temperature of -30^oF (272 K). (Brown, et al., 2009b) 

The CDT for Hydrazine was approximately 620 ms just above room temperature but decreased significantly to an average of 45.4 ms as it approached its boiling point, which is a 92 percent decrease in CDT above room temperature.  Below room temperature the CDT increased to about 2000 ms at the AH reported freezing point of 34.5^oF (274.5K). (Brown, et al., 2009a)  The CDT of MMH at room temperature is around 200 microseconds and around 5 milliseconds at -35^oF (236K).  The CDT data at temperatures below ambient are not only slower than room temperature but also becomes more unpredictable due to the increased thermal energy barrier for reactive decomposition(Frisby, et al., 2009).  Eventually, the CDT data enter a mass transfer limited region in which the combustion severely slowed. The results indicate that CDT of UDMH at a higher oxidizer to fuel ratio (4/1) decrease by 100% when compared to the results obtained at 2/1, supporting results obtained by Farmer, 1997. (Brown, et al., 2009b).  Previous studies showed that kinetic modeling of the CDT versus the absolute temperature followed an Arrhenius like behavior (Dasarathy, et al., 2005).  Using similar modeling as the previous studies, AH/RFNA below room temperature was an extension of this behavior.  We further examined the bipropellant system below the freezing point of AH, at -5^oF.  As the acid droplet hit the frozen AH, a surface reaction began to melt the fuel, forming a slush-like phase, which continued to melt until combustion was achieved at a much slower overall CDT of 6275 ms, since this time now includes the mass transfer associated with the melt (Brown, et al., 2009a).  Results of the kinetic modeling and photographic results of the slush like phase will be presented.

Experiments were designed to determine the liquid temperature at the point to ignition by placing a small open junction Type-E thermocouple into the combustor.  The temperature results were used to determine the presence of explosive intermediates by comparing them to the explosive boiling points of known species.  Figure 2 displays the temperature and CDT results for MMH.  The temperature at the point of ignition for this compound is 152°F (340K) (Frisby, et al., 2009).  This temperature corresponds to the boiling point of methyl nitrate (CH₃NO₃) at 150°F (338K), which undergoes explosive decomposition at this temperature (Sax, et al., 1987).

Figure 2. Temperature of MMH combustion along with DSO signal reading.

Figure 3 shows similar results for UDMH.  The CDT is defined as the time between Point C, which is the initial evolution of gas and Point D, which is the appearancce of flame.  The standard temperature used to represent the ignition is halfway in between Point C and Point D.  As seen in Figure 3, this temperature is at 150°F (338K) indicating that the explosive intermediate of UDMH combustion is also methyl nitrate (Brown, et al., 2009b). 

Figure 3. Temperature of UDMH combustion along with DSO signal reading.

Figure 4 displays the temperature reading for AH (Brown, et al., 2009a).  The results show that the compound without the methyl groups has a different reaction mechanism.  The temperature of 315˚F (430K) indicates that the intermediate species in this reaction is ammonium azide (NH₄N₃) which has a boiling point of 320˚F (433K) at which point it also explodes (Sax, et al., 1987).  This is due to ammonium azide decomposing into the explosive hydrazoic acid (HN₃) at this temperature.  Following the ignition of hydrazine there are plumes of smoke shown in the Detector 1 as the temperature continues to rise, this reading indicate that ammonium nitrate (NH₄NO₃), which has an explosive boiling point of 410˚F (483K) and other explosive species may be present in competing reactions with ammonium azide.

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Figure 4: Temperature of AH combustion with DSO signal readings.

The results indicate a similar ignition temperature for MMH and UDMH with methyl nitrate being the explosive intermediate, and an ignition temperature of AH indicating ammonium azide as the intermediate.  As the liquid temperature rises and some of the fuel evaporates, this salt decomposes, igniting the mixture in the vapor phase thus explaining why the chemical delay time region exists.  As it appears the intermediate explosive compounds of the fuel/RFNA reaction made within the reaction just in time, future work will be focused on attempting to isolate the intermediates of the reactions to verify the findings of the temperature study. 

References:

Brown, C, Frisby, P.M., and Smith, Jr, J.E. ?Kinetic Modeling and Mass Transfer Effects below the Freezing Point for the Hypergolic Combustion of Anhydrous Hydrazine Reacted with Red Fuming Nitric Acid,?. Paper # 523, Proceedings of 56^th JANNAF Propulsion Meeting, 35^thPropellant and Explosives Development and Characterization Subcommittee, Las Vegas, Nevada April 14-17, 2009

Brown, C, Frisby, P.M., and Smith, Jr, J.E. ?The Use of Cryogenic Temperatures to Study High Oxidizer to Fuel Ratios in the UDMH/RFNA System,? Paper # 521, Proceedings of 56^th JANNAF Propulsion Meeting, 35^thPropellant and Explosives Development and Characterization Subcommittee, Las Vegas, Nevada April 14-17, 2009

Dasarathy R.B., Hampton C.S., and Smith, Jr. J.E., ?The Relationship Between the Chemical Delay Time and Temperature for Hypergolic Bipropellants, ?Proceedings of 2^nd JANNAF Liquid Propulsion Subcommittee Meeting, December 2005.

Farmer, M. J., ?A Study of the Reaction Rates of Hypergolic Propellants,? Masters Thesis, Department of Chemical and Materials Engineering, University of Alabama in Huntsville, May 1997.

Frisby, P.M., Brown, C, and Smith, Jr, J.E. ?The Thermal Performance and Kinetic Behavior of Monomethylhydrazine Reacted with Red Fuming Nitric Acid,? Paper # 522, Proceedings of 56^th JANNAF Propulsion Meeting, 35^thPropellant and Explosives Development and Characterization Subcommittee, Las Vegas, Nevada April 14-17, 2009

Sax, N. I. and Lewis, R. J., Hazardous Chemicals Desk Reference,  Van Nostrand Reinhold Company Inc.: New York, 1987.