Role of Solvent Properties in the High-Pressure Phase Behavior of (Ethylene + Comonomer + n-Hexane + LLDPE) Systems

Abstract

The lower critical end point temperatures of [ethylene + comonomer + n-hexane + linear low-density polyethylene (LLDPE)] systems are shown to be strongly correlated with solvent average molecular mass and mole fraction-averaged carbon number, but not solvent density or critical temperature. A synthetic-visual method is used to measure vapor–liquid, vapor–liquid–liquid, and liquid–liquid phase boundaries for quasiquaternary (ethylene + comonomer + n-hexane + LLDPE) systems for the 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, and 1-decene comonomers, and for quasiternary (ethane/n-decane/4-methyl-1-pentene + n-hexane + LLDPE) systems. The reported data span temperatures of T = (375–465) K and pressures of P = (0.5–15) MPa. In all systems, the mass fraction of the LLDPE (M̅ w = 199 kg·mol–1; M̅ w/M̅ n = 2.62; 2.56 mol % 1-hexene) was kept at 3 wt % and solvent compositions were chosen to represent conditions found in the solution polymerization process. The modified Sanchez–Lacombe equation of state can predict the quasiquaternary systems’ phase behavior with moderate accuracy using parameters adjusted to quasiternary data.

A Semiempirical Method for the Estimation of High-Pressure (Solvent + Polymer) Phase Boundaries in the Solution Polymerization Process

“…To determine the K α|β constants, the (ethylene + comonomer + n -hexane + polymer) system is taken as the idealized representation of solution process streams since this system contains the minimum number of solvent constituents that will be encountered in the process. Moreover, phase behavior data are reported , for this system for all industrially relevant α-olefin comonomers and a metallocene poly­(ethylene- co -1-hexene) polymer (PEH199-3) of commercial origin. All phase boundaries available for this system , were used to determine the K α|β constants.…”

“…In what follows, only the (ethylene + comonomer + n -hexane + PEH199-3) data of Swanepoel and Schwarz , and (alkane + PEP) LMEPs of Charlet and co-workers , are used to develop the semiempirical method, after which the method will be evaluated against all data sets in the compiled database. The ( n -hexane + hPBD66) data of Krenz are used only qualitatively to inform about the dependencies of thermodynamic properties on temperature and polymer content.…”

“…Numerous empirical approaches have been proposed to estimate (solvent + polymer) phase behavior in the absence of experimental measurements. These include correlating the LMEP temperatures and/or PT -slopes to solvent density, − solvent critical temperature, − various parameters related to the Flory theories, ,, solvent carbon number, and solvent molar mass . Buchelli and Todd presented a phase separator predictor for (solvent + high-density polyethylene) systems utilizing a term dependent on reduced solvent density correlated to plant operating data.…”

Tangent Point Approach in Reliably Solving the Density Roots for PC-SAFT Equation of State

“…The PC-SAFT , model has been developed and captured great attention in the last 20 years. Because of its focus on the segment-based and chain-like molecules, the PC-SAFT model has been applied to simulate polyethylene processes − and more complex systems and processes. , A survey of thermodynamic methods for polymers by Guerrieri et al presented an overview of EOS models for polymer systems including cubic EOS using various mixing rules, lattice models such as Sanchez–Lacombe, and perturbation theory such as SAFT models. Guerrieri et al pointed out that although it is difficult to recommend a special thermodynamic model for polymer processing in general, different models can have their own advantages due to model characterization.…”

“…Recently, there have been some improvements in the lattice models, , but in this paper, we focus on perturbation theory-based approaches. Among various models, we choose a PC-SAFT model that has been applied to simulate polyethylene processes − and more complex systems and processes. , Our sole focus is polymer modeling through PC-SAFT phase calculation. Unlike cubic equation of state models such as Van deer Waals and Soave–Redlich–Kwong, the PC-SAFT model has a complex nature so that the density roots between the vapor and liquid phases must be calculated with tremendous effort.…”