Transient Pressure-Relief Characteristics of Ethylene/Vinyl Acetate Mixtures in the High-Pressure Polyethylene Process

Pressure relief in high-pressure polyethylene processes involves dense, strongly non-ideal ethylene-based fluids. A transient real-fluid model was developed for the single-phase discharge of ethylene/vinyl acetate (VA) mixtures by coupling adiabatic vessel depressurization with quasi-steady isentropic nozzle flow. Thermodynamic properties were calculated using the Peng–Robinson equation of state in REFPROP. The admissible mass flux was maximized over single-phase throat states, and sonic choking was verified using u_t/a_t ≈ 1. For the validation case at 2000 bar and 25 °C, the calculated pressure history yielded an RMSE of 120 bar, an MAE of 97 bar, and R² = 0.93. Increasing the VA mass fraction from 0 to 40% increased the initial mass flux from 0.35 to 0.41 kg·mm⁻²·s⁻¹ and shortened the single-phase duration from 761 to 413 ms. Higher initial pressure increased the mass flux, whereas higher initial temperature reduced it and prolonged discharge. The discharge coefficient affected the time scale but not the vessel isentrope or terminal state. Sonic choking occurred during the high-pressure stage, while the flux maximum became phase-boundary-limited near termination. The results represent comparative single-phase trends because two-phase discharge is excluded and the ethylene/VA interaction parameter remains uncertain.