The gas-phase decomposition of hydrazine and its methyl derivatives

Abstract

The decomposition of hydrazine, hydrazine-water mixtures, UDMH, and monomethylhydrazine was studied in the Princeton adiabatic flow reactor. This reactor consisted of a cylindrical quartz section and a conical nozzle. The walls of the reactor were heated electrically to prevent heat loss. A hot carrier gas flowed through the reactor and was mixed rapidly with small quantities of gas-phase reactant, which is injected perpendicularly to the main stream at the throat of the nozzle. The mixing was followed by chemical reaction which could be extended over lengths as long as 30 in. Progress of the reaction was followed by measuring the longitudinal variation of temperature with a silica-coated platinum-PtRh thermocouple. Concentrations of reactant were generally less than 5% by volume, and the temperature regime of the study was 750 o –1000 o K. Also, chemical samples were taken with water-cooled probes near the end of the reaction zone. The over-all reaction order of all three monopropellants was very close to unity. The rate constant for hydrazine decomposition in a 3-in. duct was found to be k =10 10.33 exp (−36,170/ RT ) sec −1 , and the following stoichiometry was observed: N 2 H 4 →0.9 NH 3 +0.5 N 2 +0.6 H 2 A study of hydrazine-water mixtures showed their rates to be slower than those of the anhydrous material by approximately a factor of 10 and independent of the amount of water added. Slightly wet hydrazine behaved kinetically like the hydrazine-water mixtures. Thus, it seems that water inhibits the gaseous decomposition of hydrazine by very effectively suppressing some reaction step. UDMH was decomposed in 2-, 3-, and 4-in. ducts. No surface effect was observed. For the 2-in. duct, the first-order rate constant was found to be k =10 9.29 exp (−28,800/ RT ) sec −1 . Monomethylhydrazine was decomposed in 3-and 4-in. ducts. The over-all rate constant for MMH decomposition in the 3-in. duct was k =10 13.4 exp (−47,000/ RT ) sec −1 . The rates in the 4-in. duct were slightly lower. A comparison of the reaction rates of the three monopropellants shows that, in the temperature regime of this study, UDMH decomposition is the fastest, hydrazine decomposition is the slowest, and the monomethylhydrazine-decomposition rate is intermediate. A mechanism was postulated for hydrazine decomposition which differs from those suggested by other investigators, in that it includes a set of branching reactions N 2 H 3 +X→NH+NH 2 +X, N 2 H 4 +NH→NH 2 +N 2 H 3 . Rate constants computed from the mechanism agree closely with rates measured experimentally, and the computed stoichiometry agrees with that observed experimentally.