Catalyst particles fragmentation

Figure 2.30 Possible scenarios for catalyst particle fragmentation and growth.

Figure 2.10 shows that the original catalyst particle fragments into polyethylene subparticles based on the original size of the Ti units in the solid lattice, thus causing the complete disintegration of the catalyst particle. 

This is not the case for Ti-based catalysts in the presence of high levels of comonomer reqxiired for the manxifactxire of EEDPE. This type of catalyst exhibits a kinetic profile in which the initial rate of polymerization is very high, followed by a rapid decay in kinetics, which may cause operational problems in a commercial reactor. These operational problems are due to the initial rapid rate of polymerization, which affects the manner in which the catalyst particle fragments and the manner in which the polymer particle grows. Other concerns woxild be partial polymer particle melting due to poor removal of the heat of polymerization, which in turn may cause reactor fouling. 

That the catalyst really does fracture during polymerization can be easily demonstrated. When a coarse Cr/silica, e.g., 60-80 mesh (150/zm), polymerizes ethylene to a yield of 1000 gg 1, each catalyst particle produces a polymer particle of approximately the same shape but about 1000 times larger. Cutting into the polymer granule will not expose the original catalyst particle. Even under a microscope the fragments are too small to be seen easily in the polymer background. 

Figure 8 Simplified model for the fragmentation of MgCh-supported Ziegler-Natta catalyst grains to primary catalyst particles and shape conservation of growing polymer grains.

Preserves catalyst morphology by making the catalyst particle more "robust," i.e., less prone to fragmentation (which may produce undesirable fines)... 

Reactors are large vessels, often several meters in diameter. As much as S X 10 kg of catalyst is loaded at one time. Access is through man-holes at the top, leaving considerable distances up to 10 m to be filled. Catalyst particles must not be poured directly into the vessel, since the fall could shatter them and result in a layer of fragments at the bottom. Various devices such as buckets and sleeves, as shown in Fig. 6.30, are recommended. Operators entering the vessel to smooth the layers should be careful not to exert too much weight on small areas of the bed and should protect themselves against dust and toxic hazards. 

FIGURE 47 Fragmentation of a catalyst particle during the earliest stages of polymerization. (Reproduced with permission from Macromolecules 2005, 38(11), 4673-4678.)... 

Thus, the average catalyst particle fractures into from 1 billion to 1 trillion fragments. The fragment size probably depends on porosity, which explains why some low-pore-volume silicas are less active than the more porous ones. 

Catalyst particle diameter (pm) Reported fragment diameter (pm) Fragments per particle Typical surface area (m2 g 1) Primary particle diameter0 (A) No. of primary particles in fragment No. of primary particles across fragment... 

Figure 14. SEM micrographs of a supported metallocene/ MAO catalyst particle prepared by gas-phase impregnation after a polymerization time of 5 and 45 min. The active sites are located exclusively on the outer surface of the supporting material, and the absence of the particle fragmentation during the polymerization leads to an uncontrolled polymer morphology.

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