Converting
Ethylene to Polyethylene
Fluidized Bed
Perhaps the most important step of the entire process is the conversion of purified ethylene gas to polyethylene. Today, there are a number of different processes that can be employed to accomplish this conversion, but a common method used in industry is to polymerize ethylene by means of a fluidized reactor bed. A fluidized reactor bed consists of metallic catalyst particles that are 'fluidized' by the flow of ethylene gas, that is catalyst particles are suspended in the ethylene fluid as ethylene gas is pumped from the bottom of the reactor bed to the top.
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Metallic catalysts are critical to the process, as polymerization is impossible without it. In fact, before the late 1970's an organic peroxide catalyst was employed to initiate polymerization. However, because the organic peroxide catalyst is not as active as the metallic catalyst, pressures in excess of 100 times the pressure required with metallic catalysts were necessary. |
X = Cl, Br, I,a = 0 or 1,b = 2 or 4, a+b = 3 or 4,and R is an aliphatic or aromatic hydrocarbon containing 1 to 14 carbons.
Reactor
Before
ethylene is sent to the fluidized bed, it must
first be compressed and heated. Pressures in the range of 100-300 PSI
and a temperature of 100 degrees Celsius are necessary for the
reaction to proceed at a reasonable rate. In addition, a catalyst
stream is also pumped with the ethylene stream into the reactor as
catalyst is consumed in the reactor. 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 seperate 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 hours results in a 97% conversion of
ethylene.
The flow in the fluidized bed reactor can be mathematically
modeled by using mole balances.
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Enormous amounts of heat is liberated from the polymerization of
ethylene as the reaction is exothermic. Heat is generally removed by
cooling unreacted ethylene gas coming off the top of the reactor and
recycling the cool gas back to the reactor. Temperatures in the
fluidized bed reactor can likewise be mathematically
modeled by
using
an energy balance.
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