Dynamic simulation of the molecular weight distribution in LDPE autoclave reactor by direct integration of the population balance equations using RSQSSA

Abstract Free‐radical polymerizations of ethylene to produce low‐density polyethylene (LDPE) performed in autoclave and tubular reactors under both transfer to polymer and scission reactions, can lead to bimodal molecular weight distributions (MWDs) until molecular weights of 10 7 –10 8 g mol −1 . By using the reduced‐stiffness quasi‐steady‐state approximation (RSQSSA) method is possible to accurately predict the evolution of the MWD over time. The quasi‐steady‐state hypothesis holds for propagating radicals and, subsequently, the dynamic behaviour of dead polymer molecules is computed by using a non‐stiff solver. Predicted MWD profiles are benchmarked against those obtained via PREDICI, and compared with those of the Alpha‐IUPAC product, and an excellent agreement with data at steady‐state conditions for an isothermal CSTR is found. The effect of the scission mechanism (linear/random or topological), rate of chain transfer to polymer, and rate of scission on the MWD shape is also investigated. The proposed approach can accurately compute multimodal MWDs with very high dispersities, at the expense of high computational demands.