PHASE-EQUILIBRIA IN HIGH-PRESSURE POLYETHYLENE TECHNOLOGY

This is a review of the phase equilibria in supercritical monomer solutions of ethylene homopolymers and copolymers, such as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), poly(ethylene-co-methacrylate) (EMA), poly(ethylene-co-vinyl acetate) (EVA), poly(ethylene-co-methacrylic acid) (EMAA), and poly(ethylene-co-acrylic acid)(EAA). The knowledge of such phase equilibria underlies the high-pressure polyethylene (HPPE) technology. The ability to estimate such phase equilibria allows for smooth and robust process optimization during grade transitions. This is important because the HPPE technology makes it possible to minimize the product cross-contamination and, hence, to make higher-value, fluctuating-demand speciality polymers. Experimental data, phase diagrams, and patterns of phase disengagement presented 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 defined as a cloud point transition. The cloud point pressures in such systems are found to depend on thermodynamic parameters, such as temperature and composition, and on the dissimilarity between the polymer and the monomer. This dissimilarity is characterized in terms of the differences 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 and EMAA).