Predicting Vapor-Liquid Equilibrium for Hydrazine Using Gibbs Ensemble Monte Carlo Simulations with Updated Opls Force Field Parameters

Hydrazine is an important fuel that is used as a rocket propellant. Conducting experimental tests on hydrazine are difficult since hydrazine is highly reactive and dangerous. Gibbs Ensemble molecular simulations were conducted for hydrazine under high temperature conditions to obtain vapor-liquid equilibrium properties. Vapor-liquid equilibrium information is important for safety and design calculations for operations such as tank filling and isentropic compression. Prior work in our group included simulations using parameters that were determined for unsymmetric dimethylhydrazine. In this work, updated hydrazine parameters derived from ab initio calculations for the hydrazine molecule were used. The Gibbs Ensemble Monte Carlo simulations were carried out using a simulation package from Sandia National Laboratories called Towhee (version 4.13.2). Simulations were run at multiple values of temperature to determine equilibrium pressure as well as liquid and vapor densities. The results of the simulations using the new parameters show that the new parameters yield vapor-liquid equilibrium predictions that are closer to the experimental data available than the predictions from the simulations using the old parameters values. The experimental density at 500K is 0.80g/ml¹ (Schmidt, E.W, 2001), previous simulation results yielded a value of 1.087g/ml, and the updated simulation resulted in a value of 0.91g/ml. Predictions from the previous simulations achieved an accuracy range of 22-73%. New results had an improved accuracy of 50-88%.

1 Schmidt, E.W. Hydrazine and its Derivatives. New York: John Wiley & Sons, Inc. (2001).