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.
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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