Journal of Environmental Treatment Techniques
2020, Volume 8, Issue 1, Pages: 272-283
J. Environ. Treat. Tech.
ISSN: 2309-1185
Journal web link: http://www.jett.dormaj.com
CFD Modeling of Polypropylene Fluidized Bed
Reactor
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Hossein Esmaeili *, Salar Azizi , Seyyed Mojtaba Mousavi , Seyyed Alireza Hashemi
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Department of Chemical Engineering, Bushehr Branch, Islamic Azad University, Bushehr, Iran
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Department of Medical Nanotechnology, School of Advanced Medical Sciences and Technologies, Shiraz University of Medical Sciences, Shiraz, Iran
Received: 02/08/2019 Accepted: 16/11/2019 Published: 20/02/2020
Abstract
Poly propylene is one of the famous polymers with great application. In this study, CFD model on dynamics of the fluidized bed
polyethylene production process has been investigated. A detailed CFD model for sticky poly propylene fluidized bed was formulated
in this work. As a result, detailed information on the PSD and hydrodynamic fields of the gas and solid phases can be obtained from
the simulations. Defluidization due to particle aggregation also can be simulated. For modeling plant-scale poly propylene reactors, a
chemical look-up table should be used to solve efficiently the solid species equations. In order to address all the issues in FB
polymerization, models for simplified polymerization kinetics, polydisperse multiphase flow, and mass and heat transfer between the
gas and solid particles are combined together. As a result, physically aggregation because of a tactic polypropylene is more important
than other aggregation reasons. Finally, as mentioned earlier, after DQMOM is applied to the multi-fluid CFD model, new terms
accounting for the effect of aggregation and breakage need to be added on the right-hand sides of the solid-phase momentum, energy,
and species equations.
Keywords: Polypropylene, Reactor, Fluidized bed, Kinetic theory, CFD
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At PP polymerization reactors, propylene as a monomer
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Introduction
contact with Ziegler-Natta catalyst which activated with co
catalyst and forms poly propylene. PP collected to three types
as isotactic, atactic and syndiotactic. Purpose is isotactic PP
production and avoids great amount atactic PP production.
Atactic poly propylene percentage specification is by means
xylene solubility percentage. Atactic PP is liquefied and a
small percentage of its made special application of PP, but a
great amount of atactic PP make sticky powder and ultimately
cause choking bed and effect to PSD at PP polymerization
reactor. A stereo modifier as an external donor used to avoid
unwanted xylene solubility of PP. donor usage is small as a
catalyst. Partially a special Ziegler-Natta catalyst used with
the internal donor. Normally industrial PP reactors haven’t
syndiotactic PP production. In this paper, a cold PP gas
fluidized bed when PP powder is sticky (xylene solubility is
upper) studied which could give valuable information about
this matter and help to design, optimization and scale up with
avoiding choking.
Poly propylene (PP) is one of the famous polymers with
great application. Recent PP production reactor technology is
a gas-phase fluidized bed and stirred bed. In gas-phase
polymerization, small particles (e.g., 20-80 μm) are
introduced at a point above the gas distributor, and when
exposed to the gas flow containing the monomer,
polymerization occurs (1-3). At the early stage of
polymerization, the catalyst particles fragment into a large
number of small particles, which are quickly encapsulated by
the newly-formed polymer and grow continuously, reaching a
typical size of 200-3000 μm. Due to the differences in the
polymer particle size, segregation occurs and fully-grown
polymer particles migrate to the bottom where they are
removed from the reactor. The smaller pre-polymerized
particles and fresh catalyst particles tend to migrate to the
upper portions of the reactor and continue to react with
monomers. Because polymerization is exothermic, the
temperature of polymer tend to rise and sometimes it will
exceed the melting point of the polymer, then polymer particle
can become sticky and during collisions can form large
agglomerates that can possibly undergo sintering and cause
defluidization. In the opposite situation, if the bed is too cold,
the particles can become brittle and may fracture forming
unwanted small fragments that elutriate with the gas. Hence,
heat and mass transfer to particle surface controls the local
particle temperature and the rate of agglomeration and
breakage (4).
2 Modeling equations
Two methods improved for CFD modeling of gas-solid
flows, discrete element model (DEM) and two fluid model
(TFM). In the DEM gas phase is described by locally
averaged Navier-Stokes equations. Newtonian equations of
motion for individual particles are then solved and individual
particle trajectories are traced, taking into account effects of
particle collisions and forces acting on the particle by flowing
gas. In such models, the computational demand rises strongly
with the number of traced particles, which limits its
applicability. In the TFM model, based on the momentum,
two phases are mathematically treated as interpenetrating
continua. The success of TFM depends on the proper
description of the interfacial forces and the solid stress. The
interfacial forces are used to describe the momentum transfer
Corresponding author: Hossein Esmaeili, Department of
Chemical Engineering, Bushehr Branch, Islamic Azad
University,
Bushehr,
Iran.
E-mail:
&
esmaeili.hossein@gmail.com
esmaeili.hossein@iaubushehr.ac.ir.
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