نوع مقاله : پژوهشی
تهران، دانشگاه صنعتی امیرکبیر، دانشکده مهندسی پلیمر، صندوق پستی ۴۴۱۳-۱۵۸۷۵
عنوان مقاله [English]
Hypothesis: Modifications in chemical formulations of existing commercial polymerization catalysts may deteriorate other catalyst properties, especially stereo- and region-selectivity. Therefore, an absolute necessity for petrochemical polymerization facilities is to find feasible non-chemical routes for tailoring essential parameters, including molecular weight distribution (MWD) width, activity and fragmentation mechanism in order to modify existing catalytic systems.
Methods: To this goal, use is made of a recently developed single-particle multipore model (MPM), which describes the reaction-diffusion processes involved in the heterogeneous olefin polymerization to investigate the impacts of initial catalyst porosity, initial catalyst particle size, bulk monomer concentration and pore size arrangement on the above-mentioned parameters.
Findings: Modeling a supported Ziegler-Natta catalyst system showed that increasing the initial catalyst porosity or initial particle size or decreasing the bulk monomer concentration decreased the local reaction rate distribution width, resulting in narrower MWDs. Although, the polydispersity index generally changed oppositely due to its dependence on the location of the MWD in addition to its width. The model has elucidated and rationalized two unexplained experimental observations, i.e., increasing initial porosity reduces the catalyst activity in some studies and that polydispersity index generally changes irregularly and unpredictably with bulk monomer concentration. For the physical quantities studied in this work, the reaction rate is directly related to the MWD width, revealing that a trade-off between MWD width and yield should be sought for applications that require higher resistance to melt fracture phenomena, edge waviness and draw resonance. While, the reaction rate, MWD width and polydispersity index did not show any relationship with the participation ratio of the two fragmentation mechanisms. Increasing the initial catalyst porosity or the initial particle radius intensified the more preferred continuous bisection mechanism, thereby dropping the probability in fouling.
35 Dashti A. and Ramazani S.A., Experimental Investigation and Modeling of Particle Growth in Propylene Polymerization, 8th World Congress of Chemical Engineering, The Canadian Society for Chemical Engineering, August 23–27, 2009.