Particle fragmentation and morphology control

If the particle resists the stresses and does not break up, the pores fill up with polymer and monomer mass transfer limitations become the rate limiting step. This undesirable situation essentially causes the polymerization to shut down. 

Catalyst particle with pores clogged with polymer 

If the support is not strong enough, the particle explodes and generates a significant number of finer sub-particles. These fines are a considerable nuisance because they hinder smooth reactor operation, maybe detrimental to the operation of compressors and fans if they are blown out of the reactor, and may generate significant amounts of static electricity in gas-phase processes. 

Traditional commercial catalyst supports are designed to allow fragmentation to take place in this ordered fashion. If the fragmentation step takes place under ideal conditions, one particle of supported catalyst will yield one polymer particle with roughly the same shape. This is referred to as replication phenomenon and is one of the most remarkable characteristics of heterogeneous olefin polymerization. 

Despite its extremely short duration, the fragmentation step is critical in determining the final morphology of the polymer particles. Carrying out the fragmentation step under mild conditions in a separate reactor - a process called prepolymerization - produces particles with better morphology, that is, a relatively compact product (with moderately high bulk powder density), with controlled shape (often, but not always, nearly spherical) and no fines. 

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