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Kinetic modeling of the gas-phase polymerization of propylene with a high active Ziegler-Natta catalyst under different injection conditions

Presentation held at 12th International Workshop on Polymer Reaction Engineering, 17th to 20th of May 2016, Hamburg/Germany
: Kettner, Joana; Bartke, Michael

2016, 20 Folien
International Workshop on Polymer Reaction Engineering <12, 2016, Hamburg>
Fraunhofer IAP ()
polymerization; propene; prepolymerization; Catalysts; kinetic modelling
Anfrage beim Institut / Available on request from the institute

Since the development of the Ziegler-Natta catalysts in 1953, continous improvements were done concerning to higher catalyst activity and higher stereoselectivity. Especially at the reaction start, when the pure catalyst is injected under high rate conditions, the high activity of the catalyst can cause particle overheating and/or lead to an uncontrolled catalyst fragmentation resulting in lower catalyst activity and bad particle morphology. One possibility to solve the problem is to apply a prepolymerization step before the main polymerization reaction. In the prepolymerization, the reaction starts at low rate conditions (mild reaction temperatures, low monomer concentration) in order to realize a controlled catalyst fragmentation and to improve heat removal conditions by creating a higher heat transfer area. In previous studies, the gas-phase polymerization of propylene with a high active Ziegler-Natta catalyst was carried out with and without prepolymerization step. For polymerizations with prepolymerization the reached activities were much higher compared to polymerizations without prepolymerization. With prepolymerization the produced particles were spherical and have an even surface. In contrast, polymer particles produced without prepolymerization show non-spherical shapes and rough particle surfaces which could be a hint of the uncontrolled catalyst break up at the beginning of the reaction. Based on experimental derived activity profiles a phenomenological kinetic model was developed describing the polymerization of propylene with the different injection conditions (prepolymerization). Therein heat and mass balances were implemented in order to describe the particle temperature and particle growth during the reaction. A case study on particle overheating clearly shows a significant increase of the particle temperature when no prepolymerization is applied. An increasing reaction temperature (gas-phase) resulted in a strongly increase in particle temperature. When using a prepolymerization step the particle temperature only slightly increase with increasing reaction temperature. Less thermal deactivation of the catalyst species and therefore higher activities could be the consequence. The presented study will show the derived kinetic model for the propylene polymerization with and without prepolymerization. Modeling and estimation of the kinetic parameters of the used Ziegler-Natta catalyst were carried out using the software gPROMS ModelBuilder (Process Systems Enterprise). Weight average molecular weights were determined using the method of moments. With the derived kinetic model it is possible to calculate the activity profiles of the polymerization reaction at different injection conditions (prepolymerization) over an industrially relevant temperature range and for different hydrogen concentrations.