Modeling of fluidized bed reactor for ethylene polymerization: effect of parameters on the single-pass ethylene conversion
Hassan Farag
0
Mona Ossman
1
2
Moustapha Mansour
0
Yousra Farid
0
0
Chemical Engineering Department, Faculty of Engineering, Alexandria University
, Alexandria,
Egypt
1
City for Scientific Research CSAT
, Borg Elarab, Alexandria,
Egypt
2
Petrochemical Engineering Department, Faculty of Engineering, Pharos University
, Alexandria,
Egypt
Background: In this study, we present the developments in modeling gas-phase catalyzed olefin polymerization fluidized bed reactors (FBR) using chromium catalyst technique. The model is based on the two-phase theory of gas-solid fluidization: bubble phase and emulsion phase. The model has proved to be the suitable model in many of past studies. In the proposed model, the bed is divided into several sequential sections. The effect of important reactor parameters such as superficial gas velocity, catalyst injection rate, catalyst particle growth, and minimum fluidization velocity on the dynamic behavior of the FBR has been discussed. The conversion of product in a fluidized bed reactor is investigated and compared with the actual data from the plant site. Results: A good agreement has been observed between the model predictions and the actual plant data. It has been shown that about 0.28% difference between the calculated and actual conversions has been achieved. Conclusions: The study showed that the computational model was capable of predicting the hydrodynamic behavior of gas-solid fluidized bed flows with reasonable accuracy.
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Background
The fluidized bed reactor has a unique physical design,
with gas and polymer particles flowing in opposite
directions. It consists of metallic catalyst particles that
are fluidized by the flow of ethylene gas, and catalyst
particles (pre-polymer) are suspended in the ethylene
fluid as ethylene gas is pumped from the bottom of the
reactor bed to the top. The gas is fed from the base of
the reactor and splits into two phases: bubble phase and
emulsion phase. Pre-polymer particles are fed in near
the top of the reactor, and while the polymerization
reaction occurs, the particles grow, increasing in weight
and size. Particle segregation occurs in the reactor
according to particle weight, so the full-grown polymer
particles are removed at the base of the reactor.
Nonreacted gases leave the reactor after passing through the
disengagement zone [1]. Pre-polymerization is generally
carried out in continuous stirred-tank reactors (CSTR)
located before the fluidized bed reactor. Catalyst
particles are fed into the CSTR along with ethylene and
co-monomers to yield pre-polymer. Afterwards, these
pre-polymerized catalyst particles are fed into the FBR
to complete the ethylene polymerization. A diagram of
the industrial fluidized bed reactor system using a CSTR
as the pre-polymerization reactor is shown in Figure 1
[2]. In effect, the catalyst is not actually consumed; it is
simply incorporated with the polyethylene product as
polyethylene molecules remain stuck to the catalyst
particle from which they were produced.
The conversion of ethylene is low for a single pass
through the reactor, and it is necessary to recycle the
unreacted ethylene. Unreacted ethylene gas is removed
off the top of the reactor, where it is expanded and
decompressed to separate the catalyst and low molecular
weight polymer from the gas. After purification, ethylene
gas is then recompressed and recycled back into the
reactor. Granular polyethylene is gradually removed from
the bottom of the reactor as soon as reasonable
conversions have been achieved. Typically, a residence
time of 3 to 5 h results in a 97% conversion of ethylene.
Figure 1 Industrial polyethylene production diagram (BP Chemical Technology, London).
The flow in the fluidized bed reactor can be
mathematically modeled using mole balances [1].
An accurate model describing the movement of gas/
solid and pressure distribution around a rising bubble
was then proposed by Davidson and Harrison [3].This
model describes the gas flow through a three
dimensional fluidized bed mainly in spherical or semi-spherical
shape bubbles through the core; however, depending on
the emulsion gas velocity, the region around the bubble
may be surrounded by a cloud as a result of emulsion
gas circulation between the dense solid phase and the
core of the bubble.
Choi and Ray [4] proposed a two-phase model including
bubble and emulsion phases with constant bubble size.
McAuley et al. [5] proposed a single phase model by
modifying Ray's model with additional assumptions. In a
comparison between the two models, they have shown that the
single phase assumption does not make considerable
difference in the results obtained from the models. Hatzantonis
et al. [6] in a research work developed the two-phase model
by considering the bubble growth effect on hydrodynamic
behavior of the reactor and have shown that the developed
model has a better agreement with industrial data than
single-phase and two-phase models with constant bubble
size. Gros (...truncated)