PHASE-EQUILIBRIA IN HIGH-PRESSURE POLYETHYLENE TECHNOLOGY

This is a review of the phase equilibria in supercritical monomer solu tions of ethylene homopolymers and copolymers, such as low-density pol yethylene (LDPE), linear low-density polyethylene (LLDPE), poly(ethyle ne-co-methacrylate) (EMA), poly(ethylene-co-vinyl acetate) (EVA), poly (ethylene-co-methacrylic acid) (EMAA), and poly(ethylene-co-acrylic ac id)(EAA). The knowledge of such phase equilibria underlies the high-pr essure polyethylene (HPPE) technology. The ability to estimate such ph ase equilibria allows for smooth and robust process optimization durin g grade transitions. This is important because the HPPE technology mak es it possible to minimize the product cross-contamination and, hence, to make higher-value, fluctuating-demand speciality polymers. Experim ental data, phase diagrams, and patterns of phase disengagement presen ted in this paper are related to the reactor system, the high-pressure separator (HPS), and the high-pressure recycle system. These data and diagrams are used to characterize the monomer-polymer miscibility def ined as a cloud point transition. The cloud point pressures in such sy stems are found to depend on thermodynamic parameters, such as tempera ture and composition, and on the dissimilarity between the polymer and the monomer. This dissimilarity is characterized in terms of the diff erences in molecular weight and density (e.g., for LDPE and LLDPE), in polarity (e.g., for EVA and EMA), and in association (e.g., for EAA a nd EMAA).