Chemical Engineering February 2014 - 31

Polypropylene Production via GasPhase
Process: Stirred-bed reactor
By Intratec Solutions
S
ince 1956, polypropylene has been
commercially produced using catalysts
and technology that were developed
from independent research by Karl Ziegler and
Giulio Natta. Their namesake catalysts have
enabled the widespread use of this polymer in
the decades since. Polypropylene has become
a major part of the modern plastics-resin
market because of its unique physical properties,
and is now employed in a wide range of
applications from food packaging to automotive
plastics.
Historically, polypropylene has been
produced at industrial scale by three main
polymerization approaches: hydrocarbon
slurry or suspension processes, bulk-phase
processes and gas-phase processes. Two
leading technologies for gas-phase polypropylene
production are Unipol, a fluidized-bed
reactor process offered by the Dow Chemical
Co. (Midland, Mich.; www.dow.com),
and Novolen, a stirred-bed reactor process
developed by Lummus Novolen, now a part
of Chicago Bridge and Iron Co. N.V. (The
Hague, The Netherlands; www.cbi.com). A
process similar to Unipol was discussed in this
column last year (Chem. Eng., May 2013, p.
33). Here, Figure 1 depicts a gas-phase process
similar to Lummus Novolen's stirred-bed
reactor technology.
The process
Propylene conversion to polypropylene is
achieved through the use of a Ziegler-Natta
catalyst. This process occurs in a continuous
vertical stirred reactor at mild temperatures
(about 80°C). The overall yields are typically
>99 wt.%. The process can be divided in three
main areas: purification and reaction; resin
degassing and pelletizing; and vent recovery.
Purification and reaction. The polymerization
catalysts are sensitive to several impurities,
such as oxygen and water. To meet these
exposure restrictions, purification
equipment is used
before the reaction.
In the polymerization
area, fresh purified
propylene is mixed with a
recycled monomer stream
and is fed to the reactor
along with the catalyst,
co-catalyst and hydrogen.
The heat of polymerization
is removed from the reactor
by condensing propylene
gas from the top of the
reactor and recycling
the liquid back into the
agitated reactor.
100,000
150,000
200,000
250,000
300,000
50,000
Resin degassing and pelletizing.
The outlet reactor
stream goes to a discharge
vessel to be separated.
The polymer powder is sent to a purge vessel,
while the carrier gas goes through a cyclone
and a filter and is finally sent for recovery.
Nitrogen is used to remove residual monomer
from the polymer powder and then the N2
gas is also sent for recovery.
The polymer powder then goes to a screw
Non-integrated
scenario
Integrated
scenario
FiGure 2. Total fixed investment according to integration scenario
Economic performance
extruder, where the powder and additives are
melted, compounded and homogenized. After
being extruded, the pellets are carried to a
centrifugal dryer.
Vent recovery. The carrier gas is compressed
and one portion of it is sent back to the reactor.
The other portion is sent to a recovery
unit. The nitrogen and propylene streams
from the purge vessel are recovered and
separated in a membrane unit. The nitrogen
is recovered to the process, while the monomer
goes to a propylene-propane splitter
(inside the purification area of the propylene
supplier, if the plant is part of a petrochemical
complex).
CW
Catalyst
Cocatalyst
3
CW
1
4
H2
N2
CW: Cooling water
CW
2
Additives
6
Fresh
propylene
5
FiGure 1. Polypropylene production via a gas-phase process similar to that of Lummus Novolen
Polypropylene
pellets
8
Membrane
recovery
system
Offgas
to flare
7
Propylene
supplier's
purification
area (integrated
petrochemical
process)
An economic evaluation of the process was
conducted based on data from the first quarter
of 2013 for a plant with a nominal capacity
of 300,000 ton/yr, erected in the U.S. Gulf
Coast region (the required process equipment
is represented in the simplified flowsheet). Two
scenarios were analyzed:
1) Integrated scenario. This case is based on
the construction of a plant linked to a propylene
supplier. In this scenario, the nearby unit
continuously provides propylene, and receives
impure propylene for purification. Thus, no
storage for propylene is required. However,
storage of products is equal to 20 days of
operation.
(1) Reactor
(2) Fixed-bed dryers
(3) Discharge vessel
(4) Purge vessel
(5) Extruder
(6) Centrifugal dryer
(7) Compression
(8) Cooling tower
2) Non-integrated scenario. This case corresponds
to a grassroots unit. Thus, 20 days
of operation was considered for both products
and raw materials. In addition, this scenario
includes a propane-propylene splitter.
The level of integration is a key factor in this
process, since it can determine
whether the process is profitable
or not. The level of integration
causes the total fixed investment
(Figure 2) and manufacturing
expenses to vary greatly. In terms
of manufacturing expenses, in
non-integrated plants, propylene
is purchased at higher prices,
while in integrated complexes, it
can be obtained directly from the
supplier at a lower cost. n
Edited by Scott Jenkins
Editor's Note: The content for this column
is supplied by Intratec Solutions
LLC (Houston; www.intratec.us) and
edited by Chemical Engineering. The
analyses and models presented herein
are prepared on the basis of publicly
available and non-confidential information.
The information and analysis
are the opinions of Intratec and do not
represent the point of view of any third
parties. More information about the
methodology for preparing this type
of analysis can be found, along with
terms of use, at www.intratec.us/che.
Project
contingency
Indirect project
expenses
Direct project
expenses
U.S. dollars, thousands

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Chemical Engineering February 2014

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