PEP Review 2003-7
Anionic Polystyrene; A New Generation
Published: October 2003
Control of the rapid rate of anionic polymerization of styrene monomer has presented problems. Past attempts to overcome these problems have resulted in poor economics.
This review deals with a new approach, which turns the previous disadvantage of high speed into a major economic advantage. This new approach reduces total fixed capital from $45.5 million for a conventional 250 million lb/yr polystyrene plant to $32.5 for an anionic plant. The profit would be increased by 1.94 cents/lb at the same selling price (or $4.5 million/yr extra profit).
The process examined uses continuous feed preparation and finishing with batch polymerization. The polymerization is carried out in a single reactor operated in two stages. The first stage is at a controlled low temperature where boiling removes the heat of polymerization. The second adiabatic stage starts when 50% of the styrene has polymerized and polymerization is complete in minutes as temperature rises to 240°C. Three reactors are used alternately with one always emptying.
Thermal termination is an order of magnitude slower than propagation at 240°C but still takes place rapidly throughout the reactor. This termination is necessary to avoid formation of extremely long molecules that would ultimately become gels in the product.