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Autoclave Process
In the autoclave process polymerization takes place in a series of stirred reactors (2 to 6). The free radical initiator is introduced in each of the different reactors to optimize productivity and performance.
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The initiators are exclusively organic peroxides
The reaction temperature is controlled by the amount of peroxide introduced. The conversion is proportional to the T outlet / T inlet ratio. Quality is dependent on mean polymerization temperature, but independent of T outlet.
Effect of Temperature
Molecular structure is primarly determined by pressure and temperature. Below summarizes how these parameters impact molecular weight and branching.
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Molecular Weight |
SCB* |
LCB** |
Temp.  |
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|
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| Pressure |
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* Short Chain branching
** Long Chain Branching
Molecular weight can also be controlled by chain transfer (e.g. propane, alphaolefin, acetone)
Productivity is proportional to gas inlet and outlet temperatures but temperature profiles impact both properties and productivity.
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Productivity |
Melt Index |
density |
optics |
| Temp. |
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Importance of Impurities
| Impurities |
Effect |
| t-butyl hydroperoxide |
Very long half-life
- can build up in recycle
- causes polymer build-up in lines after high-P compressor
- leads to reactor shutdowns
- believed to cause reactor “decomps”; major safety concern; reactor shutdown
Most PE producers require spec. of < 0.10 wt% |
Hydrolyzable Cl,
acidity |
Some concern over corrosion.
Most producers accept current sales specs. |
| water |
May contribute to corrosion, tBHP, acidity buildup in storage, pumping problems because of crystallization |
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