The Production of Linear Low Density Polyethylene
Linear low density polyethylene (LLDPE) was first produced in 1977 when Union Carbide announced the development of the Unipol Process, which uses a gas-phase process and transition metal catalysts. LLDPE has a linear molecular structure similar to that of high density polyethylene (HDPE) and a density capability equal to that of low density polyethylene (LDPE). The production of LLDPE incurs significantly lower capital costs since the process requires low pressures.
In the low pressure gas-phase process, the polymerization occurs in a
state. Ethylene and co-monomers such as alpha-olefins, ranging from
propylene to 1-octene can be used. The effect of these co-monomers is to control the product density. Commerciall
y however, 1-butene and 1-hexene are typically used. The gas phase
process requires no diluent or solvent, which greatly simplifies the amount of equipment required in the process, because there is no need to recover and recycle a diluent or solvent. Raw material purification is also simplified since the ethylene and
co-monomers are the only reactor feeds that need purification from trace
amounts of catalyst poisons.
Below is an actual fluidized bed
Copyright, Texaco, 1990
The gaseous ethylene and co-monomer are fed continuously into the fluidized bed reactor. The transition metal catalysts are also added
directly to the reactor. The reaction pressure ranges from 100-300 psig,
and the reaction temperature is generally around 100 degrees Celsius. The
stream is circulated through the cooler by a blower to remove the high
heat of polymerization. The monomer has a residence time of about 3-5
hours with typical conversions as high as 97%. The polymer uses the
surface of the catalyst grain as a solid-surface to grow on. The reaction
takes place just below the melting point of the polymer. Hence the cooling
loop is essential to maintain temperatures low enough such that the
polymer does not melt and coagulate in the reactor. The
particles grow to about 1000 mm and are removed through a product
discharge system, utilizing a gas-lock chamber.
The monomer is easily purged after the gas-lock chamber. The purged
polyethylene is in granular form and can be re-extruded and pelletized.
Any additives or stabilizers can be added to the polymer during this
primary extrusion process.