A isochoric-isoenergetic flash solver using a direct entropy maximization principle for phase splitting and Gibbs free energy minimization for phase stability is developed. The solver searches for the global stable state in a rigorous and thermodynamically consistent way. The solver is demonstrated to be robust and efficient to handle multiphase flash, even in the vicinity of phase boundaries. Dynamic flash computations and gas dynamics simulations of shock waves are considered for pure ethylene and for binary ethylene-nitrogen mixtures. The simulations show significantly different shock wave characteristics when phase separation is considered due to intensive energy exchange, compared to the frozen flow limiting single-phase solution. (C) 2014 American Institute of Chemical Engineers