Applicability of the equation of state models for modeling and simulation of phase equilibrium in polymer production processes is investigated. The flash separation of unreacted ethylene from polyethylene is used as a prototype for simulation. First, three equation of state models (statistical associating fluid theory, Sanchez−Lacombe, and an augmented Soave−Redlich−Kwong cubic equation of state) are comparatively used to correlate pure component volumetric, calorimetric, and phase equilibrium properties for ethylene and polyethylene. Next, they are tested for correlation of vapor−liquid equilibrium of ethylene + low-density polyethylene mixtures. Finally, a two-stage flash problem that mimics the separation process in the low-density polyethylene production is simulated using the three models. Each equation of state model has some unique characteristics that affect the outcome of modeling pure component as well as mixture behavior. The advantages and disadvantages of each model are discussed.

In this investigation, we demonstrate our methodology in developing a comprehensive computer simulation model for the low-density polyethylene process in a tubular reactor using Polymers Plus. We use the perturbed-chain statistical associating fluid theory to describe the thermodynamic properties of the system. A comparison with literature data shows that the selected equation of state does a very good job in describing the physical properties and phase equilibria of the system. A detailed reactor model was proposed on the basis of transport literature that provides insight into the various resistances to heat transfer that arise during polymerization, and a comprehensive free-radical kinetic model was developed that describes the various individual mechanisms of the polymerization of ethylene and the properties of the polymer product. Results from the proposed simulation model were used in comparison with plant measurements from an Equistar Chemicals plant, in both correlative and predictive modes, for several polymer grades. In all cases considered, very good agreement was observed between simulation results and plant data on reactor temperature profiles, polymer properties, and production rates.

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This paper illustrates how polymer fractionation can be characterized using the perturbed chain version of the statistical associating fluid theory equation of state, a computationally efficient algorithm, and pseudocomponents. We reparametrize the PC-SAFT equation of state and propose an efficient method of generating pseudocomponents to describe polydisperse polymer molecular weight distributions from the results of size exclusion chromatography analysis by numerical integration. We perform rigorous phase equilibrium calculations to investigate the effects of temperature, pressure, and feed composition on polyethylene fractionation. In these calculations, the polyethylene molecular weight distribution is treated as an ensemble of pseudocomponents consisting of chemical homologues of varying size. The simulation results are compared with plant data from an industrial solution polymerization process using cyclohexane as the solvent to demonstrate the applicability of the method to an industrial situation. The results indicate versatile representation of the polymer polydispersivity as well as accurate prediction of the liquid-liquid, vapor-liquid, and fluid-liquid fractionation process.