8.3 Model Evaluation and Accuracy
9.1.2 Influence of The Particle Size
Packing of 100 µm Particles
Figure 9.4 shows the migration of the Butyl-650C tracer particles that were sedimented on top of the packed bed during 450 high-flow/low-flow cycles. Here, the tracer particles had the same size as the bulk packing particles, which was 100 µm. The 12.5 mm i.d.
column was used. Similar to the smaller SP-650M particles, a band of tracer particles migrated noticeably in the column wall region toward the column outlet. Here again, the particles stopped migrating at a distance of about 5-7 mm from the packing top.
Compared to the SP-650M particle packing, the migration band was more pronounced and showed a waved shape. This might be due to the broader particle size distribution of the Butyl-650C particles, which allows smaller particles to migrate more easily and over a wider distance. Furthermore, the band of migrating particles was much denser than in Figure 9.2 which means that the number of migrating particles was higher. This
Chapter 9. Analysis of Particle Migration During Column Operation
0 50 100 150 200 250 300 450 z, mm
0 5 10 15
Figure 9.4.: Migration of Toyopearl Butyl-650C tracer particles in the 12.5 mm i.d. column during cyclic column operation. The bulk packing was build by sedimentation of Butyl-650C par-ticles. Each photograph was taken after 50 compression-relaxation cycles as indicated by the numbers.
The last photograph was taken after the 450th cycle.
A B z, mm
0
5
10
15
Figure 9.5.: Particle migration in the inner packing regions using Butyl-650C resin. The photographs show the frozen packing after the 450th operation cycle which was cut into halves. (A) view of the column wall region and(B)view of the inner packing region.
effect was observed at all repeated experiments. It is assumed that the hydrophobic surface of the particles has a significant impact on particle migration, i.e. at the column wall region. Hydrophobicity might promote the separation of the packing particles from the column wall during packing compression-relaxation, so that smaller particles where pushed by the flow into the resulting crack.
In contrast to the near-wall packing region, no particle migration was noticed in the inner packing regions, as shown in Figure 9.5. The few tracer particles that are visible in the cutting plane in Figure 9.5 B were brought there during the cutting action.
Packing of 100 µm Particles Mixed With 35 µm Tracer Particles
Figures 9.6 and 9.7 show the migration of smaller tracer particles in the 9.6 mm and 12.5 mm i.d. column. This measurements were carried out using 100 µm Butyl-650C
138
Chapter 9. Analysis of Particle Migration During Column Operation
0 50 100 150 200 250 300 450 z, mm
0 5 10 15
Figure 9.6.: Migration of Toyopearl Butyl-650S tracer particles in the 9.6 mm i.d. column during cyclic column operation. The bulk packing was build by sedimentation of Butyl-650C par-ticles. Each photograph was taken after 50 compression-relaxation cycles as indicated by the numbers.
The last photograph was taken after the 450th cycle.
0 50 100 150 200 250 300 450 z, mm
0 5 10 15
Figure 9.7.: Migration of Toyopearl Butyl-650S tracer particles in the 12.5 mm i.d. column during cyclic column operation. The bulk packing was build by sedimentation of Butyl-650C par-ticles. Each photograph was taken after 50 compression-relaxation cycles as indicated by the numbers.
The last photograph was taken after the 450th cycle.
particles for the bulk packing and 35 µm Butyl-650S particles as tracer particles.
Both packings of different diameter showed particles migrating over a distance of 4-6 mm from the packing top. Additionally, it was observed that some tracer particles migrated nearly from the packing top to the bottom. This is different to the behavior observed using a packing of nearly equal particle sizes (see Figures 9.1 and 9.4) and can be attributed to the smaller size of the tracer particles.
As a conclusion, it is assumed that the characteristic migration distance is correlated to the packing density and packing compression-relaxation dynamics, i.e. the movement and velocity of the particles, which is known as the granular temperature [e.g. Trujillo and Herrmann 2003; Biggs et al. 2008]. This means that presumably in a distance of 4-6 mm from the packing top the packing density is considerably higher than in regions above that distance. Furthermore, the rate of packing compression-relaxation gets slower with increasing distance from the packing top. This is due to the wall support which increases towards the column outlet and acts against the packing movement, as indicated by the hysteretic behavior of the different packing regions (see section 7.1 for detailed
Chapter 9. Analysis of Particle Migration During Column Operation
0 50 100 150 200 250 300 450 z, mm
0 5 10 15
Figure 9.8.: Effect o the flow velocity on the migration of small Toyopearl Butyl-650S tracer particles in the 9.6 mm i.d. column. The flow velocity during the high flow cycle was set to 2763 cm h-1. Each photograph was taken after 50 compression-relaxation cycles as indicated by the numbers. The last photograph was taken after the 450th cycle.
discussion). Consequently, the rate and magnitude of opening and closing interparticle voids which is directly related to the granular temperature, is retarded. Hence, only smaller particles were able to migrate over larger distances.
By increasing the flow velocity of the high-flow cycle but keeping the cycle times constant increases the dynamic compression-relaxation behavior of the packing. This results in a higher and faster packing compression and relaxation. Consequently, the rate and magnitude of opening and closing interparticle voids is affected. Figure 9.8 shows the 9.6 mm i.d. column being operated at high-flow cycles of 2763 cm h-1.
As can be seen, considerably more tracer particles migrated at the column wall region towards the column outlet. Here again, a tracer band profile can be identified in the characteristic migration distance of 4-6 mm. However, a large part of the tracer parti-cles passed this distance with increasing number of compression-relaxation cyparti-cles, but stopped by forming a second band profile at a migration distance of 14-16 mm. As a consequence of this experiment, it can be stated that the higher the packing oscillation during compression-relaxation the higher the degree of particle migration. Here again, no particle migration in the inner packing region was observed (results not shown).
Particle migration occurred only in the packing wall region.
In another experiment the tracer was distributed homogeneously as shown in Figure 9.9 (A). The packing was compressed mechanically by 10 % by lowering the plunger. The initial distribution of tracer particles within the packing was analyzed by quick-freezing the the packed column without any fluid flow application and cutting it into halves. The inner distribution of the tracer particles was similar to the visible outer distribution as shown in Figure 9.9 A. During operation by 450 high-flow/low-flow cycles no particle
140
Chapter 9. Analysis of Particle Migration During Column Operation
A z, mm
0
5
10
15
B z, mm
0
5
10
15
Figure 9.9.: Homogeneous distribution of Butyl-650S tracer particles in the Butyl-650C packing. Prior to operation the packing was compressed by 10% resulting in a packing height of 18 mm.
(A) shows the side view of the column before column operation and(B)a cut of the frozen packing after 450 compression/relaxation cycles.
migration was noticed in the column wall region. However, in the inner packing region clustering of tracer particles was noticed as indicated in Figure 9.9 B. However, as the distribution of tracer particles of this particular packed bed cannot be measured by the quick-freezing method prior to column operation, it can only be assumed that the clustering of particles was the result of cyclic column operation.