Confidence in the accuracy of a simulation model and by implication the thermodynamic data model(s) used, particularly for separation equipment such as a deethyleniser, requires validation against actual plant operating data. A methodology has been developed that quantifies plantmodel mismatch with a Calculator module appended to a PRO/II (Simulation Sciences, Inc.) Column module to facilitate an assessment of the relative accuracy of alternative simulation model formulations. The methodology is based on a manual stepwise optimization procedure using a preferred formulation of the simulation model with the key adjustable modeling parameter defined as the effective average equilibrium tray efficiency η i eff . The methodology seeks to minimize an Error Objective Function that calculates the sum of the weighted and variance-normalized least-squares errors around the deethyleniser involving as many plant measurements as practical taking into account the wide range of accuracy inherent in the various measurements.

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 PRO/II (Simulation Sciences, Inc.). For various K-value data-model definitions, the value of the temperature-independent form of the binary interaction coefficient, k ij = α 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, , can vary markedly with the choice of the K-value data model and the k ij 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.