Some Factors Affecting the Molecular Weight Distribution (MWD) in Low Density Polyethylene Multizone Autoclave Polymerization Reactors

Abstract

Due to its complexity, free‐radical polymerization of ethylene at high pressure still remains to be fully understood. This paper investigates factors that contribute to shape the MWD, one of the main features of LDPE, such as the structure of primary chains and the reactor operation in one/two phases. Using plant data and different models, it is found that chain transfer to small species is the controlling chain termination step and it is explained why one‐phase operation leads to more branched polymer. Simulations with the Polyred software package provide a set of kinetic parameters that reproduce the observed trends in number average molecular weight and MWD dispersity in the reactor.

Mathematical Modeling of the Molecular Weight Distribution in Low Density Polyethylene. I. Steady‐State Operation of Multizone Autoclave Reactors

“…Although real LDPE polymerizations are manufactured in adiabatic type reactors, the simulations in this work consider isothermal and isobaric polymerization processes for each reactor. However, an energy balance can be coupled to the present model (at an assumed isothermal steady state) to assess the heat exchanged; the heat released by the reaction is almost completely used to heat the feed to the reactor temperature (see for example ). The energy balance would add no complexity at all to the calculations and could also be used to obtain information on the necessary flow of initiator to maintain a given operation temperature, assuming adiabatic operation.…”

“…Furthermore, due to the prevailing industrial secrecy to openly publish useful data from LDPE manufacturing, it is rather difficult to count with the information needed to models with the microstructure of LDPE. Another complication found in the literature concerning LDPE polymerization reactors is the presence of much variability in the kinetic parameters values used by different research groups, as reported by Saldívar‐Guerra et al Also, it is common to find proposals of predictive mathematical models based on different kinetic reaction schemes.…”

“…Regarding the variability in the values corresponding to the kinetic parameters used for modeling LDPE polymerization, here we comment on some reported by different research groups. Table 3 contains the kinetic parameters suggested by Saldívar‐Guerra et al which will be named as Set I; Table 4 contains those reported by Iedema et al to be named as Set II and in Table 5 are those suggested by Pladis and Kiparissides which will hereafter be named as Set III. Finally, Table 6 contains the values of the kinetic parameters corresponding to some conventional initiators generally used for LDPE polymerization …”

“…Even though the production process and the properties of LDPE have been studied for decades, due to their complexity, there are still important aspects of the relationships between process conditions, structure and properties that are not yet well understood, especially when the material is produced in multizone autoclave reactors (MZAR) for which few works were found in the literature despite their industrial prevalence . On the other hand, it is known that the physical and performance properties of LDPE largely depend on its molecular weight distribution (MWD) and long‐chain branching distribution (BD) …”

“…MZAR are frequently used in industry because they provide more degrees of freedom to tailor‐shape the MWD than a single zone reactor. In spite of this, to our knowledge, this is one the first papers in which the description of the full MWD in an MZAR is addressed; in previous (few) papers, only averages of the MWD are modeled or, in the best case, the information provided is not detailed or not precise . There are some publications by Neuhaus as well as Eckes focusing on multizone reactors including the calculation of the MWD with the support of the commercial software PREDICI.…”