Olefin polimerization - The Unipol Process
Polyethylene, the largest sold plastic in the world, reached such position after Union Carbide discovered an extraordinary catalyst that operates at relatively low temperatures and pressures, in addition to a versatile and unique fluidized bed process.
In this process, the reagent gas is fed at a rate of 3 to 6 times the minimum fluidization velocity, at 75-100ºC and 20 atm with a continuous feeding of the silica/Ziegler-supported chrome-titanium catalysts. Polymerization occurs in the catalyst surface, leading to the growth of particles in large 250-1000µm granules. Ethylene conversion is low (around 2%) so significant recycles are required. The reaction is exothermic (~3300kJ/kg of converted ethylene) so it is important to avoid heated spots as well as the catalyst lumped in the walls of the reactor.
Due to the meaningful catalyst dilution in the granules and its large size, the raw material is ready to be used in pelletizing or catalyst removal. Furthermore, a solvent is not used in the process and a low or high density product of the polymer can be generated. Owing to the great efficiency and economy delivered by the processes, several other polymers are produced by following this system, e.g., the ethylene co-polymerization with 1-hexene and 1-butadiene developed by Exxon and Union Carbide.