Decomposition and Combustion of Monomethylhydrazinium Nitrates

Transcription

1 Paper # 070RK-007 Topic: Reaction kinetics 8 th U. S. National Combustion Meeting rganized by the Western States Section of the Combustion Institute and hosted by the University of Utah May 19-, 013 Decomposition and Combustion of Monomethylhydrazinium Nitrates S. Q. Wang and S. T. Thynell Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 1680, USA Energetic nitrate compounds, monomethylhydrazinium nitrate (MMH HN 3 ) and monomethylhydrazinium dinitrate (MMH HN 3 ), were synthesized from hypergolic pair monomethylhydrazine (MMH) and nitric acid by mixing their diluted aqueous solutions carefully with an MMH:HN 3 molar ratio of 1:1 and 1:, respectively. The overall ignition and combustion behaviors of these two nitrate-based monopropellants were examined by using a high-pressure strand burner with optical access, coupled with a high-speed video camera for capturing and analyzing the condensed- and gas-phase processes as well as measuring the regression rates of the solid strands. MMH HN 3, which has a stoichiometric F/ ratio, has a burn rate of 0.56 mm/s in 1 atm of N. Its burn rate increases almost linearly with pressure up to 800 psi at which the burn rate is about 14 mm/s. However, the burn rate of MMH HN 3 increases sharply to about 400 mm/s at 1000 psi. MMH HN 3, which is a fuel-rich compound and highly hygroscopic, is very difficult to ignite. Thermal decomposition of a small amount (~1.5 mg) of MMH HN 3 and MMH HN 3 was investigated by a confined rapid thermolysis setup with heating rates on the order of,000 K/s coupled to rapid-scan Fourier transform infrared (FTIR) spectroscopy of the evolved gases. Both MMH HN 3 and MMH HN 3 start to decompose rapidly at around 10ºC, and the important IR-active species evolved from the decomposition of these salts include HN 3, N, N 3, HN 3,, N, N, N, CH 4 and C. Possible decomposition pathways are also discussed based on the rapid thermolysis studies. 1. Introduction The pair of monomethylhydrazine ( NHN, MMH) and red fuming nitric acid (RFNA) is a well-known liquid hypergolic propellant for rockets and spacecrafts [1]. A hypergolic propellant is a pair of liquid fuel and oxidizer in which ignition occurs spontaneously upon contact between the two liquids, thereby eliminating the need for a complex ignition system. As part of an effort to provide experimental results that can be used to propose additional reactions for the MMH/RFNA mechanism developed by the U.S. Army Research Laboratory [], the early reactions between MMH and HN 3 were investigated in a previous work [3]. An important finding is that the formation and subsequent decomposition of a particulate aerosol, which is mainly composed of MMH nitrates, may play an important role in the hypergolic ignition event. n one hand, the exothermic salt formations represent the initiation reactions which provide heat to evaporate the reactants and to initiate secondary reactions that lead to ignition. n the other hand, the subsequent decomposition of MMH nitrates forms important gaseous species which can be further oxidized and lead to ignition in the gas phase. Therefore, it is believed that the formation and decomposition of MMH nitrates play an important role in the ignition process. However, these reactions are not included in ARL s MMH/RFNA mechanism, in which only a reversible MMH-HN 3 complexation reaction is used for the purpose of generating initial heat which is required for the ignition []. An extensive literature survey revealed that studies on MMH nitrates are quite limited. Lawton et al. [4] synthesized monomethylhydrazinium nitrate (MMH HN 3 ) by mixing aqueous solutions of MMH and HN 3 at a molar ratio of 1:1. They determined some physical properties of this salt and obtained its IR and mass spectra. Breisacher et al. [5] synthesized both monomethylhydrazinium nitrate (MMH HN 3 ) and monomethylhydrazinium dinitrate (MMH HN 3 ), and conducted simultaneous mass spectrometric differential analysis of low-pressure decomposition of these nitrates. They suggested that the species evolved from the decomposition of these two nitrates include N, N, N 3, CH 4, NH 3, N, N, N,, N, and.

2 In this work, both MMH HN 3 and MMH HN 3 are synthesized by following two steps: 1) slowly mix aqueous solutions of MMH and HN 3 in an ice bath with a molar ratio of 1:1 or 1: to obtain an aqueous solution of MMH HN 3 or MMH HN 3 ; ) remove the water by keeping the solutions in a vacuum dryer for 4 hours (MMH HN 3 ) or 48 hours (MMH HN 3 ). MMH HN 3 is extremely hygroscopic, and it therefore takes much longer time to remove the water from its solution. As shown in Fig. 1, the collected MMH HN 3 is a white powder, while the MMH HN 3 readily absorbs ambient water and is agglomerated due to its hygroscopicity. MMH HN 3 and MMH HN 3 have boiling points of 37 and 74 C, respectively [5]. MMH HN 3 MMH HN 3 The first objective of this work is to study the thermal decomposition of these two salts Figure 1. MMH nitrates at elevated temperatures and to understand the early species and reactions which have not been discussed in the literature and are not included in ARL s MMH/RFNA mechanism. The second objective of this work is to study the combustion characteristics and measure the burn rates of these energetic nitrates, which might be considered as monopropellants, and were synthesized from hypergolic bipropellants. MMH HN 3 has a stoichiometric F/ (fuel-to-oxidizer) ratio, while MMH HN 3 is a fuel rich compound. The overall combustion reaction of MMH HN 3 can be written as follows: NHN HN 3 C 4 N. Experimental Setup.1 Strand burner A schematic diagram of the strand burner used for studying the combustion of MMH nitrates is shown in Fig.. A detailed description of the strand burner can be found in an early work in which the combustion of TMEDA 8HN 3, an ionic liquid compound synthesized from the hypergolic pair of tetramethylethylenediamine (TMEDA) and nitric acid, was studied [6]. The strand burner is composed of a combustion chamber (bottom portion) and an exhaust chamber (top portion). The strands of nitrates are placed in the bottom chamber which is purged and pressurized by N. A heated nichrome wire with a diameter of 0.1 mm is used to ignite the propellant. It is mounted on two copper rods and positioned along the top surface of the strand. A constant electric current of 1 A, which is slightly lower than the maximum allowable current (1.7 A) above which the nichrome wire will melt and break apart, is provided by a DC power supply through a high pressure feedthrough. The purge gas flows through the chamber from the bottom inlet to the top outlet during the test so that the exhaust gases can be carried out of the chamber to avoid a chamber pressure increase. Meanwhile, the soot particles nichrome wire Dia. (in)=0.003 purge flow distributor purge outlet 1A purge inlet strand of nitrates powder Figure. Schematic diagram of strand burner (smoke) from incomplete combustion can also be rapidly blown out of the chamber to maintain a clear optical access to the liquid strand and the flame zone. The purge-gas flow is evenly distributed across the chamber by attaching a plate at its base, in which a large number of small holes are drilled. The ignition and combustion processes are recorded by a Phantom V710 high-speed camera, through a 1-inch thick optical access glass window. A paper stick with a printed ruler is placed along the liquid strand to measure the regression rates.. Confined rapid thermolysis (CRT)/FTIR setup The experimental technique utilized to study the rapid thermal decomposition of the MMH nitrates is known as confined rapid thermolysis (CRT). Detailed discussion of this technique and the associated data reduction techniques are available in earlier works [7, 8]. A short summary of the process is as follows. A small quantity of the nitrates (1.5 mg) is confined and heated rapidly between two heated, parallel, and isothermal surfaces in a constant pressure chamber purged by N. The heated surfaces are achieved by a stationary top heater and a mobile bottom heater, both of which are fitted with a cartridge heater, while temperature control is maintained by PID controllers. The use of a small sample volume enclosed in a confined space, roughly 300 µm in height, enables heating rates in the range of 000 K/s. The IR-active gaseous species emerging from the condensed phase are quenched by the relatively cooler atmosphere, and detected in real-time

3 by the modulated beam of a Bruker IFS66/S FTIR spectrometer in the rapid scanning mode, scanning at cm -1 with a temporal resolution of 50 ms. In each test, a total of 150 spectra are collected, requiring a sample time of 7.5s. 3. Results and Discussion 3.1 Burn rates Strands were prepared by pressing the nitrates powder into a glass vial of 8 mm inner diameter, as shown in Fig. 3. To be cautious, a small amount of these nitrates was initially tested (grinding and pressing) to check their sensitivity to shock and friction. No detonation or any sensible changes were observed. It is reported in the literature that MMH nitrates showed poor sensitivity to drop hammer tests [9]. The average density of the strand is about 1.3 g/cm 3, which is estimated by (mass of nitrates in filled vials / volume of glass vial). The density of the strand is smaller than that of the nitrate (1.55 g/cm 3 ). Since MMH HN 3 is extremely hygroscopic, the strand can absorb enough water from the ambient to form a colloidal slurry as shown in Fig. 3. Figures 4a and 4b show selected images from typical combustion tests of MMH HN 3 at a gauge pressure of 400 and 1,000 psig, respectively. In each test, an average burn rate was estimated by: (a) t=0 t=1.s t=s (b) t=0 t=0.0s t=0.033s Figure 4. Combustion of MMH HN 3 at 400 (a) and 1000 psi (b) rates of MMH HN 3 at various gauge pressures were measured. Averaged data of three tests at each pressure is plotted in Fig. 5. From atmospheric pressure to 800 psig, the 100 burn rate increases with pressure almost linearly from 0.54 to 14 mm/s. At 1000 psig, the burn rate suddenly rises to about 400 mm/s. 10 Attempt was also made to study the burn rate of MMH HN 3. However, it is difficult to ignite the MMH HN 3 strand. This is probably due to two reasons: 1) MMH HN 3 is an 1 extremely fuel-rich compound; and ) MMH HN 3 is very hygroscopic. During the preparation process, the MMH HN 3 strand can absorb enough water from the ambient to form a 0.1 colloidal slurry as shown in Fig. 3. The added water makes it is difficult to ignite. Burn Rate, mm/s MMH HN 3 MMH HN 3 Figure 3. Strands of nitrates height of strand (mm) / total time consumed (s). For example, at 400 psig (Fig. 4a), it takes about s to burn a strand with a height of 1 mm. Therefore, the estimated burn rate in this test is about 6 mm/s. At 1,000 psig (Fig. 4b), it takes about s to burn a strand with a height of 14 mm, and the estimated burn rate in this test is about 44 mm/s. The images in Fig. 4a were acquired with a frame rate of 1000 fps and an exposure time of 400 µs. In Fig. 4b, higher frame rate (5000 fps) and lower exposure time (40 µs) were used due to the much shorter event. Burn Gauge Pressure, psi Figure 5. Burn Rates of MMH HN 3 3. Thermal decomposition of MMH HN 3 and MMH HN 3 The thermal decomposition of the two nitrates was examined at various temperatures up to 300 C. In this rapid thermolysis setup, both MMH HN 3 and MMH HN 3 start to decompose at about 10 C, and the species evolved from the decomposition of these two nitrates are identical, including HN 3, N, N 3, HN 3,, N, N, N, CH 4 and C. It should be noted that IR-inactive species (i.e., N ) may also exist. Figure 6a shows an IR spectrum obtained by averaging a total 150 spectra obtained in a test at 160 C. Figure 6b is the remaining spectrum after

4 subtracting the IR bands of and HN 3 from Fig. 6a. Spectral subtraction can help to separate and identify the species whose major IR bands overlap with those of HN 3, such as N in this case. A large amount of HN 3 was released from the decomposition of MMH nitrates and only a very small amount of N is detected. This observation does not agree with the work of Breisacher et al. [5], in which N rather than HN 3 was believed to be a major product based on data from mass spectrometry. This observation is likely caused by their use of electron-impact ionization which will dissociate HN 3 to produce lower molecular-weight species, such as N and N [10]. Therefore, the mass spectrometric analysis of Breisacher et al. was unable to differentiate HN 3 from N. For the same reason, they were unable to identify methyl nitrate N which will also dissociate upon electron impact [11]. Transmittance HN 3 (a) C N N 3 HN HN Wavenumber, cm HN 3 Transmittance C N (b) 3500 andhn 3 are subtracted 3000 N Figure 6. a) Average IR spectrum of a total 150 spectra obtained from MMH HN 3 decomposition at 160 C and 1 atm N ; b) IR spectrum obtained by subtracting and HN 3 bands from (a). The suggested pathways for MMH HN 3 decomposition are provided in Fig. 7. In reaction (1), the nitrate decomposes to release HN 3, which is identified as a major early product in the IR spectra. In reaction (), the monomethylhydrazium cation reacts with nitric acid to form an unstable nitro intermediate which decomposes to methyldiazenium cation through a HN elimination step (reaction 3). In reaction (4) and (6), monomethylhydrazium cation reacts with HN to form N-nitrosohydrazinium intermediates which can decompose to form methyl azide through elimination step (reaction 5) or to form methyl ammonium cation through the N elimination step (reaction 7) [1]. In reaction (8), methyldiazenium cation reacts with nitric acid to form a nitro intermediate which decomposes to a methyldiazonium cation through a HN elimination step (reaction 9). In reaction (10), the N Wavenumber, cm -1 N N HN 3 HN 3 N [ N NH 3 ] N 3 - N N N HN (4) N NH [CH N 3 N N ] - (1) N 3 or HN N N HN () HN NH [ NHNH 3 ] - N 3 (5) N 3 H 3 H 3 N NH HN (6) N N NH H N (3) (7) H NH HN N NH 3 N HN NH (8) HN HN N (9) N N - (10) N 3 N N N N HN (11) CH N 3 HN HN 3 N 3 Figure 7. Proposed MMH HN 3 decomposition reactions

5 methyldiazonium nitrate decomposes to methyl nitrate through a step which is well-known as replacement of nitrogen [13], in which the nitrogen is lost as N. Methyl azide may also react with nitric acid to form hydrazoic acid and methyl nitrate [14] both of which are detected in the IR spectra. The minor species, such as CH 4, N and C, can be formed by many reactions included in the MMH/RFNA mechanism by Anderson et al. [], thus are not discussed in this study. 4. Conclusions Two forms of nitrate, monomethylhydrazinium nitrate (MMH HN 3 ) and monomethylhydrazinium dinitrate (MMH HN 3 ), can be synthesized from the hypergolic pair MMH and HN 3. MMH HN 3 is fuel rich and cannot be ignited in the strand burner. MMH HN 3 is stoichiometric and its burn rate increases almost linearly from 0.56 mm/s at 1 atm to 14 mm/s at a gauge pressure of 800 psi. At 1000 psi, its burn rate is about 400 mm/s. Both MMH HN 3 and MMH HN 3 start to decompose at about 10 C. The nitrates first decompose to form abundant HN 3 which then reacts with the MMH cation to form several important early species such as N, N 3, HN 3,, N, and small amounts of N, N, CH 4 and C. Acknowledgements This material is based upon work supported by, or in part by, the U. S. Army Research Laboratory and the U. S. Army Research ffice under grant number W911NF References [1] G. P. Sutton, History of Liquid Propellant Rocket Engines, American Institute of Aeronautics and Astronautics, Alexandria, VA, 006, pp [] W. R. Anderson, M. J. McQuaid, M. J. Nusca, A. J. Kotlar, ARL Report, No. ARL-TR-5088, 010. [3] S. Q. Wang, S. T. Thynell, Combustion and Flame 159(01) [4] E. A. Lawton, C. M. Moran, Journal of Chemical & Engineering Data 9(1984) [5] P. Breisacher, H. H. Takimoto, G. C. Denault, W. A. Hicks, Combustion and Flame 14(1970) [6] S. Q. Wang, S. T. Thynell, ACS Symposium Series 1117(01) [7] C. F. Mallery, S. T. Thynell, Combustion Science Technology 1(1997) [8] A. Chowdhury, S. T. Thynell, Thermochimica Acta 443(006) [9]. de Bonn, A. Hammerl, T. M. Klapotke, P. Mayer, H. Piotrowski, H. Zewen, Z. Anorg. Allg. Chem. 67(001) [10] R. A. Friedel, J. L. Shultz, A. G. Sharkey, Analytical Chemistry 31(1959) [11] J. Lindstrom, W. G. Mallard, Eds., NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg MD, 0899, (retrieved March 05, 013). [1] J. R. Perrott, G. Stedman, N. Uysal, Journal of the Chemical Society, Dalton Transactions (1976) [13] R. T. Morrison, R. N. Boyd, rganic Chemistry (fourth ed.) Allyn and Bacon Inc., Newton, MA (1983) p [14] W.-G. Liu, S. Q. Wang, S. Dasgupta, S. T. Thynell, W. A. Goddard, S. Zybin, R. A. Yetter, Combustion and Flame 160(013)