Critical-Flow Characteristics and Mass-Flux Prediction for High-Pressure Ethylene Relief

High-pressure ethylene relief is a key safety issue in low-density polyethylene production, where ethylene is typically handled under dense real-fluid conditions. Conventional ideal-gas relief equations may lead to significant errors because thermodynamic properties vary strongly with pressure and temperature. In this study, a real-fluid isentropic relief model was developed using thermodynamic properties obtained from the Reference Fluid Thermodynamic and Transport Properties Database (REFPROP). The critical mass flux was determined by searching for the maximum value along the isentropic expansion path, and the equivalence between the maximum-mass-flux condition and the sonic condition was examined. Four industrial relief cases were compared with proprietary design-calculation outputs. The real-fluid model predicted mass flow rates with deviations of −9.9%, −9.1%, −2.3%, and −1.0%, whereas the ideal-gas model produced larger underpredictions, particularly under ultra-high-pressure conditions. The effects of upstream pressure and temperature on critical mass flux, depressurization paths, speed of sound, and critical pressure ratio were analyzed. A response-surface correlation based on 95 calculated states was developed for preliminary estimation, with all deviations within ±10% over the fitted domain. These results demonstrate the importance of accounting for real-fluid thermodynamics when evaluating high-pressure ethylene relief capacity.