Chemical Engineering July 2013 - 31

A solution-based
route to LLDPE
By Intratec Solutions
P
olyethylene (PE) is the world's largestvolume
commodity polymer. Along with
high-density (HDPE) and low-density
(LDPE) polyethylene, linear low-density polyethylene
(LLDPE) is one of the three main types
of PE. The global market for LLDPE is over 20
million metric tons per year, corresponding to
about 30% of the total PE produced.
In LLDPE production, three major types of
low-pressure technologies are used: slurry,
solution and gas-phase processes.
The process
LLDPE is produced by copolymerization of
ethylene with alpha-olefins using Ziegler-Natta
catalysts. The most common co-monomers used
in LLDPE production are 1-butene, 1-hexene
and 1-octene.
Figure 1 illustrates the process for butenebased
LLDPE production via a solution technology
similar to Nova Chemicals' (Calgary,
Alta.; www.novachem.com) solution-phase
technology, known as SclairTech (Figure 2).
The process shown is a swing process, which
is also capable of producing different LLDPE
and HDPE grades by utilizing other alphaolefins
as co-monomers.
The process can be divided into four main
operation areas: purification and catalyst preparation;
reaction; distillation; and finishing.
Purification and catalyst preparation. The cyclohexane
solvent, ethylene and comonomers are
sent to fixed-bed adsorption systems to remove
water, oxygen and other polar impurities. The
catalysts used in the process are based on
mixtures of titanium and vanadium compounds,
in conjunction with aluminum alkyls cocatalysts.
These components are mixed with solvent and
pumped to the polymerization reactor.
Reaction. Ethylene and 1-butene comonomer
(in case of butene-based LLDPE) are dissolved
in cyclohexane solvent and sent to the reaction
step. The polymerization is carried out in a
solution phase, at a temperature above the
melting point of the resulting polymer. The reaction
system consists of a tubular reactor and a
continuous-stirred-tank reactor (CSTR). The low
residence time of the reactors enables a high
flexibility for grade transitions, as well as verButene
Cyclohexane
1
4
9
HF
CW
3
5
2
Catalyst
and
cocatalyst
Deactivator
LLDPE
HF
to Fuel
FIGURE 1. Solution-phase LLDPE production process similar to Nova Chemicals' SclairTech
RF
HF
1 CW1
12
ST
Additives
ST
10
ST
to Fuel
13
14
HF
CW
ST
8
Ethylene
BFW
ST
BFW
CW 6
ST
7
CW
FIGURE 2.
Each mark on the map
corresponds to an
existing SclairTech
LLDPE plant. The
nominal capacity
of each plant follows
the legend below
Up to 149,000 ton/yr
From 150,000 to 249,000 ton/yr
satility for the production of resins with a wide
range of densities, melt indexes and molecular
weight distributions. The reactor output stream
is fed into separator vessels, where unreacted
ethylene and co-monomers, solvent and any
other volatile matter are separated from the PE.
The polymer is sent to the finishing section while
the light stream moves to the distillation system.
Distillation. The distillation step comprises
five distillation columns in charge of recovering
the unreacted ethylene and co-monomers;
recovering the solvent; purging impurities,
such as oligomers (also called grease), catalyst
and deactivators residues; and avoiding the
buildup of inert components, such as isomers
of the co-monomer.
Finishing. The resulting polymer from the reaction
area is fed into an extruder, which is used
to incorporate the required additives, and to
pelletize the polymer. The product is then sent
to the product blending and storage stage.
.
Economic performance
An economic evaluation of the solution-phase
LLDPE process was conducted based on data
from the fourth quarter of 2012. The following
assumptions were taken into consideration:
* A 350,000 ton/yr unit erected on the U.S.
Gulf Coast (the process equipment is represented
in the simplified flowsheet)
CW
From 250,000 to 349,000 ton/yr
At least 350,000 ton/yr
* Storage of products is equal to 20 days
of operation, and there is no storage for
feedstock
* Outside battery limits (OSBL) units considered:
steam boilers, cooling towers,
propylene refrigeration system, heat-transfer
fluid unit, control room and administrative
buildings
The estimated capital investment (including
total fixed investment, working capital and
other capital expenses) to build the LLDPE plant
is about $220 million, and the operating cost
for butene-based LLDPE production is about
$1,220/ton.
The swing process depicted here allows
manufacturers to participate in major PE
market segments by producing both LLDPE and
HDPE resins. Thus, the producers can select the
best product mix, aimed at premium markets
with higher margins. ■
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.
to Fuel
RF
1. Adsorption system
2. Tubular reactor
3. CSTR polymerization
reactor
4. Separators
5. Extruder and pelletizing
6. Low boiler column
7. Ethylene column
8. Comonomer column
9. Solvent column
10. Grease column
11. Refrigeration unit
12. Heat transfer fluid unit
13. Cooling tower
14. Boiler
HF Heat transfer fluid
CW Cooling water
RF Refrigeration fluid
ST Steam
BFW Boiler feed water
http://www.novachem.com http://www.intratec.us http://www.intratec.us/che
Chemical Engineering July 2013

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