The focus of this investigation into difficulties associated with the simulation of a commercial deethyleniser is on the use of K-value data models based on equations of state as provided in PROM (Simulation Sciences, Inc.). For various K-value data-model definitions, the value of the temperature-independent form of the binary interaction coefficient, k(ij) = alpha(ij), of the dominant ethylene/ethane binary has been varied. All other thermodynamic data-model definitions were kept the same. The values of the optimized average equilibrium tray efficiency parameter, eta(i)(eff) can vary markedly with the choice of the K-value data model and the kij values used and ranged from about 0.84 to about 1.01. The associated minimum total error objective function values are, however, remarkably similar, leading to the concept of "Equivalent" simulation models. For all three equation of state K-value data models examined, a quadratic relation was found to exist between a defined k(ij) and the corresponding optimized average equilibrium tray efficiency.