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The Soave-Redlich-Kwong (SRK) and the Sanchez and Lacombe (SL) equations of state are applied to the flash simulation of polyethylene industrial separators, specifically low-density polyethylene (eight resins) and linear low-density polyethylene (25 resins). Three mixing rules are used in the SRK equation: van der Waals (VDW) one-fluid, Wong-Sandler, and LCVM (linear combination of the Vidal and Michelsen mixing rules). The latter two mixing rules incorporate the Bogdanic and Vidal activity coefficient model. All these models involve two adjustable parameters. The results indicate that SL is the best model to simulate the flash separation of polyethylene under industrial conditions.

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Understanding the phase behavior of polymer solutions is of great theoretical and practical importance. Pressure versus temperature (P−T) isopleths allow one to determine the number of phases present at a given T, P, and overall mixture composition. In this work, the PC-SAFT (perturbed-chain statistical associating fluid theory) equation of state (EOS) was applied to simulate the curves that describe the boundaries between several distinct regions depicted in P−T isopleths. A new strategy was used, and the simulation results were found to show good agreement with experimental cloud-point isopleth data from the literature. In addition, a method was developed to calculate the distance between an operating point (pressure and temperature) and the corresponding point in the interface for fixed molecular weight and weight fraction of the polymer.

The objective of this study is to evaluate the Joule–Thomson effect, which occurs at a valve due to the throttling process for mixtures containing polymers and copolymers. For economic and safety reasons, it is essential to know the temperature change in industrial processes due to the pressure drop in the valve. The modeling of this phenomenon in mixtures containing polymers and copolymers, however, remains a challenge for process engineers, and the literature rarely reports studies on the subject. This work proposes a model that can directly compute temperature due to the throttling process using the concept of residual enthalpy and the perturbed-chain statistical associating fluid theory equation of state, instead of solely computing the Joule–Thomson coefficient. Systems containing poly­(ethylene-co-vinyl acetate) and low-density polyethylene were chosen as case studies because of the need for temperature control at the reactor outlet and separation processes. The model prediction was validated using industrial data, and deviations of approximately 2% between the model prediction and the experimental temperature indicate the efficiency of the proposed approach when describing the temperature due to the throttling of both systems that are being studied.

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In the production process of low-density polyethylene (LDPE), an important step is the flash separation of monomers and other small molecules from the polymer produced. The process is carried out adiabatically in two stages. To improve the performance of thermodynamic models, it is very important to analyze the use of model binary interaction parameters (BIP) dependent on the phase characteristics for each phase (phase-dependent BIP). In this work the PC-SAFT (perturbed-chain statistical associating fluid theory) equation of state (EOS) is applied to the flash simulation of LDPE industrial separators using eight different resins. The main numerical aspects are examined with emphasis on the optimization strategy for the EOS BIP that explicitly characterizes each phase involved separately. The results demonstrate good predictive behavior. As a result of improved and more consistent modeling, a new strategy for optimized operation can be envisaged for the sequence of separators. V