The World of Polyethylene

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 used to accomplish this conversion, but a common method used in industry is to produce polyethylene 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 floated in the ethylene fluid as ethylene gas is pumped from the bottom of the reactor bed to the top.

Metallic catalysts are critical to the process, as polyethylene cannot be made without it. In fact, before the late 1970's an organic catalyst was used to start the reaction. 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.

Typical Metallic Catalyst for Ethylene Polymerization

where,

X = Cl, Br, I,a = 0 or 1,b = 2 or 4, a+b = 3 or 4,and R is a carbon chain containing 1 to 14 carbons.

Reactor

Before ethylene is sent to the reactor, it must first be pressurized 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 included 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 seperated from the catalyst and low molecular weight polymer. After purification, ethylene gas is recycled back into the reactor. Polyethylene is gradually removed from the bottom of the reactor as soon as reasonable conversions have been achieved. Typically, ethylene remains in the reactor for an average of 3 to 5 hours. This results in a 97% conversion of ethylene.
Copyright, McGraw-Hill, 1979

Enormous amounts of heat is generated from the reaction . 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.