5 Video Microscopy for the Investigation
of Gas-Phase Polymerization
Detailed insight into particle growth and fragmentation processes is available
using a new innovative tool: video microscopy [44, 56–64]. This technique, first
applied by Reichert [65], enables the simultaneous detection of the individual
behavior of a large number of single catalyst particles in gas-phase polymerization.
In addition to visualizing polymer growth and the replication of catalyst grain
through polymer grain morphology, it is able to provide detailed information
about the polymerization kinetics of numerous catalyst particles.
The experimental set-up for video microscopy consists of a combination
of a 50-mL gas-phase reactor with a microscope connected to a high-resolution
digital camera that allows the observation of single catalyst grains during the whole
polymerization [44].
Figure 25 shows the projection areas of growing particles after 0, 20, 40,
and 60 min of polymerization (Fig. 25a) and demonstrates the particle growth
evaluation (Fig. 25b). These collected images are processed to determine the
projection area of each catalyst particle. Although the projection area is the
primary quantity measured, it is easier to comprehend the size of the particles in
terms of their diameter and volume. Hence, the projection area is used to estimate
the diameter of a circle of equivalent area (equivalent circle diameter, ECD) and
from that the volume of a sphere having an equivalent projection area (equivalent
sphere volume, ESV).
Figure 26 shows ESV curves in dependence on time for 40 ethylene (1 MPa)
polymerizing grains with a metallocene/MAO catalyst supported on SiO 2 . The
inset represents the “starting phase” of the microreactors and demonstrates
the “induction period” of a silica-supported metallocene system. The individual
behavior of the particles is more strongly pronounced in the subsequent acceleration (layer-by-layer fragmentation) phase shown in Fig. 26 and can be partly
attributed to an inhomogeneous distribution of the catalyst and cocatalyst throughout the particle [46] or partly to impurities in the monomer gas, which deactivate
the surface of the catalyst particles; this affects smaller particles more than larger
particles.
The kinetic profile is even clearer after the differentiation of the particle volume
with time. Hence, in Fig. 27, the derivative d ESV /dt is plotted versus polymerization
time for several particles of Fig. 26. The resulting “polymerization rate”–time
curves are normalized then to the initial particle volume, ECD 0 . Again, different
rate maxima can be observed.
At this point, it is very useful to calculate from these dESV/dt curves the activity
of a single grain in g polymer mol Zr
À1 h
À1 (this is important for comparison
with results from industrial processes) or the polymerization rate in mol L
À1 s
À1
(this is important for comparison with other catalyst systems and their polymerization kinetics). But, for this evaluation, knowledge of the density of the expanding
30
G. Fink
of Gas-Phase Polymerization
Detailed insight into particle growth and fragmentation processes is available
using a new innovative tool: video microscopy [44, 56–64]. This technique, first
applied by Reichert [65], enables the simultaneous detection of the individual
behavior of a large number of single catalyst particles in gas-phase polymerization.
In addition to visualizing polymer growth and the replication of catalyst grain
through polymer grain morphology, it is able to provide detailed information
about the polymerization kinetics of numerous catalyst particles.
The experimental set-up for video microscopy consists of a combination
of a 50-mL gas-phase reactor with a microscope connected to a high-resolution
digital camera that allows the observation of single catalyst grains during the whole
polymerization [44].
Figure 25 shows the projection areas of growing particles after 0, 20, 40,
and 60 min of polymerization (Fig. 25a) and demonstrates the particle growth
evaluation (Fig. 25b). These collected images are processed to determine the
projection area of each catalyst particle. Although the projection area is the
primary quantity measured, it is easier to comprehend the size of the particles in
terms of their diameter and volume. Hence, the projection area is used to estimate
the diameter of a circle of equivalent area (equivalent circle diameter, ECD) and
from that the volume of a sphere having an equivalent projection area (equivalent
sphere volume, ESV).
Figure 26 shows ESV curves in dependence on time for 40 ethylene (1 MPa)
polymerizing grains with a metallocene/MAO catalyst supported on SiO 2 . The
inset represents the “starting phase” of the microreactors and demonstrates
the “induction period” of a silica-supported metallocene system. The individual
behavior of the particles is more strongly pronounced in the subsequent acceleration (layer-by-layer fragmentation) phase shown in Fig. 26 and can be partly
attributed to an inhomogeneous distribution of the catalyst and cocatalyst throughout the particle [46] or partly to impurities in the monomer gas, which deactivate
the surface of the catalyst particles; this affects smaller particles more than larger
particles.
The kinetic profile is even clearer after the differentiation of the particle volume
with time. Hence, in Fig. 27, the derivative d ESV /dt is plotted versus polymerization
time for several particles of Fig. 26. The resulting “polymerization rate”–time
curves are normalized then to the initial particle volume, ECD 0 . Again, different
rate maxima can be observed.
At this point, it is very useful to calculate from these dESV/dt curves the activity
of a single grain in g polymer mol Zr
À1 h
À1 (this is important for comparison
with results from industrial processes) or the polymerization rate in mol L
À1 s
À1
(this is important for comparison with other catalyst systems and their polymerization kinetics). But, for this evaluation, knowledge of the density of the expanding
30
G. Fink