Polymer Chemistry: The manufacture of polyethylene follows addition polymerization kinetics involving catalysis of purified ethylene. Its molecular formula is -(CH2 -CH2)n - with a molecular weight of 1,500 to 100,000. Its melting point is 85 – 110°C. its density is 0.91 -0.93 , when produced by high pressure process and 0.96, when produced by low pressure process.
Technology:
There are three processes by which polyethylene is manufactured –
a .High Pressure Process : This process was developed in the UK by ICI. It uses peroxide catalyst at 100-300°C and produces low density randomly oriented polymer which have a low melting point. The process is run at pressure of 1000 – 2500 atms. This process yields Low Density Polyethylene (LDPE).
b .Intermediate Pressure Process : This process was developed in the USA by Phillips Petroleum Co. for preparing high density polymer with increased rigidity, crystallinity, tensile strength and softening point. The process uses MoO3 and Cr2O2 on alumina as catalyst and is operated at 30 – 100 atms.
c .Low Pressure Process : This process was originally developed in Germany for preparing high density polyethylene (HDPE). The catalyst used in this process consists of aluminium triethyl activated with heavy metal derivatives such as TiCl4.
In the process flow sheet (Figure 39.1), is the description of Low Pressure Ziegler Process to produce polyethylene.
At the very onset, through the process of desulphurization and removal of light ends, high purity ethylene is prepared. The ethylene is further treated to remove traces of oxygen and its compounds which can possibly deactivate the catalyst.
The ethylene is first pumped into a reactor where it is mixed with catalyst diluents stream. The optimum temperature and pressure maintained should be 70°C and 7 atms gage. The effluent stream then follows across a series of flash drums inorder to remove the solvent from the catalyst. The residual catalyst at this point is removed by adding water.
The flashed solvent is thereafter recycled to the catalyst make –up unit after appropriate drying and redistillation. The slurry which results is then centrifuged to remove the water, and the water is treated to remove the catalyst before recycle. The final products of polyethylene solids are then dried, extruded and given the required final forms.