Transport and reaction in reconstructed porous polypropylene particles: Model validation Alexandr Zubov, Lucie Pechackova, Libor Seda, Marek Bobak, Juraj Kosek à Department of Chemical Engineering, Institute of Chemical Technology Prague, 166 28 Prague 6, Czech Republic article info Article history: Received 22 April 2009 Received in revised form 28 September 2009 Accepted 29 September 2009 Available online 13 October 2009 Keywords: X-ray micro-tomography Reconstructed porous particle Polypropylene Morphology Polymerization Degassing Reaction and transport abstract We attempt to validate tomography-based spatially 3D model of polypropylene particle by comparison of model predictions with measured degassing characteristics of polypropylene powder samples. Statistically relevant number of porous PP particles from three particle size fractions is inspected by X-ray micro-tomography. The voxel size of reconstructed porous PP particles is between 1 and 5 mm. We show that closed porosity occurring in reconstructed particles has a negligible effect on the dynamics of particle degassing. It was found that the voxel size smaller than 3 mm is required for a good agreement between model predictions and experimental data. We demonstrate that tomography-based model is capable to address the problem of mass transport limitation during polymerization reaction in real particle structure. Two statistical descriptors suitable for the characterization of compact zones of polymers occurring in polypropylene particles are introduced. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction Transport of monomers, co-monomers and diluents in porous polyolefin particles occurring in catalytic polymerization reactors affects the rate of (co-)polymerization, the rate of particle degassing in the down-stream processing and the spatial distribution of copolymer composition in polyolefin particles. The presence of intra-particle pores is the essential requirement on the morphology of polyolefin particles produced in industrial reactors because the mass transfer resistance in the entirely compact polymer particle would significantly reduce the rate of (co-)monomer transport and thus the rate of (co-)polymerization (Galli et al., 1997). The internal porous structure of polyolefin particle is formed by the fragmentation of catalyst support in the early stages of particle growth (Mun ˜oz-Escalona et al., 1984) as well as by the non-uniform distribution of polymerization causing the formation of large cracks inside the growing particles (Grof et al., 2005a). Typical size of polyolefin particles is 0.4–2.0 mm. The spatial distribution of the pore and polymer phase and the distribution of catalyst fragments affect the resistance to mass transport in the polyolefin particle. Moreover, the transport resistance depends also on the actual transport process(es) taking place inside the particle, i.e., on the combination of diffusion and convection transport, and on the dynamics of mass transport processes as the effective diffusivity of stationary and dynamic diffusion in porous media can differ significantly (Seda et al., 2008). Several models of polyolefin particle morphology were intro- duced in the past. The two popular models are polymeric flow model (PFM) and multi-grain model (MGM). PFM considers the spherical pseudo-homogeneous particle with uniformly dispersed catalyst and monomer transport described by Fickian diffusion with effective diffusion coefficient (Schmeal and Street, 1971; Galvan and Tirrell, 1986). MGM considers the spherical polymer macro-particle to be the agglomerate of micro-grains of the typical size around 1 mm with each micro-grain having a catalyst fragment in its core (Kakugo et al., 1989a, b). MGM thus resembles the morphology of blackberry, which is also an agglomerate of small fruit granules each having the small seed inside. Both PFM and MGM models were used extensively in modeling studies of mass transport resistance inside polyolefin particles (Laurence and Chiovetta, 1983; Floyd et al., 1986; Debling and Ray, 1995; McKenna et al., 1997). Let us now discuss weak points of PFM and MGM models. The first weak point is the lack of predictive capabilities for mass transfer resistance in fully grown particles, i.e., particles that spent at least several minutes in the reactor. In industry the degassing of polyolefin powder leaving the reactor can even become the bottleneck of the production line. This manifests the importance of mass transfer resistances. In MGM and PFM models were mass transport resistances associated with early stages of particle growth because the concentration of catalyst per unit volume of particle is largest in the beginning of particle growth. Mass transport resistance can be characterized, for example, by the ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ces Chemical Engineering Science 0009-2509/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2009.09.082 à Corresponding author. Tel.: +420 220 44 3296; fax: +420 220 44 4320. E-mail address: Juraj.Kosek@vscht.cz (J. Kosek). Chemical Engineering Science 65 (2010) 2361–2372