The macroscopic packed bed behavior during uniaxial compression was carried out by lowering the plunger in absence of any external fluid flow application. The 9.6 mm i.d.
column was packed using wet as well as semi-dry SEP particles and the height of the individual sections was measured analogously to hydrodynamic compression experiment.
Details about the measuring method are given in Section 5.4.2.
7.2.1. Axial Packing Compression-Relaxation Behavior
Figure 7.6 shows the dynamic packing compression behavior of the semi-dry packing at different packing compression levels. During all compression levels the packing exhibited a pronounced axial packing compression gradient (Figure 7.6 A). The uppermost packing section 1, which is closest to the plunger, showed the highest, whereas the lowermost packing section 7 showed the lowest packing compression. Moreover, as can be seen in Figure 7.6 B, section 7 exhibited a delayed compression. Up to a packing compression of 2.5%, no compression was observed in this section, which means that the compression force was dissipated in the upper packing region.
During relaxation, the packing followed the up-moving plunger without delay. Hence, no hysteretic behavior of the overall packing was observed. However, the resulting compression–relaxation behavior of the sections differed, i.e. the upper sections were more compacted than the lower sections.
Moreover, it was observed that the direction of the hystereses for axial compression switched from clockwise for the upper sections to counter-clockwise for the lower packing sections (Figure 7.6 B). The same observation was made for higher compression levels uo to 50% (data not shown). The change of hysteresis direction means that the upper
Chapter 7. Characterization of Particle Bulk Packings
Normalized heightz/h0,
-Compressionλ,-
Figure 7.6.: Semi-dry packing compression-relaxation behavior at different mechanical loads. (A) Axial packing compression gradients. The z-coordinate goes from top to bottom of the packing. (B)Hysteretic packing compression-relaxation behavior exemplary shown for sections 1 and 7 as a function of the total packing compression. The arrows indicate the direction of the hystereses.
sections relaxed faster than the lower packing sections. This effect was assumed to arise from increased friction of the chromatographic beads with the column wall during axial compression.
Figure 7.7 shows the dynamic packing compression behavior of the wet packing at the same packing compression levels as in Figure 7.6.
Compressing the wet packing material resulted in a more homogeneous and constant packing compression from z/h0 = 0.2 to z/h0 = 0.8, where z goes from top to bottom of the column (Figure 7.7 A). Here again, the upper packing region, which is closest to the plunger, showed the highest packing compression. However, the lowermost packing section 7 also showed a higher packing compression than the middle sections. As this section is opposite to the plunger and closest to the column outlet, the compression was most likely induced by drag forces of the outflowing fluid. In contrast to Figure 7.6, the lower packing section 7 was compressed without delay, indicating that the compression force was transmitted to the bottom of the packing. Interestingly, switching of the hystereses from the upper packing sections to the lower packing sections as by the compression of the semi-dry packing (Figure 7.6) was not observed. All packing sections showed a delayed relaxation behavior, which is equivalent to hystereses going counter-clockwise. On the background of a wet packing compression, this observation can be
102
Chapter 7. Characterization of Particle Bulk Packings
Normalized heightz/h0,
-Compressionλ,-
Figure 7.7.: Wet packing compression-relaxation behavior at different mechanical loads.
(A) Axial packing compression gradients. The z-coordinate goes from top to bottom of the packing.
(B) Hysteretic packing compression-relaxation behavior exemplary shown for sections 1 and 7 as a function of the total packing compression. The arrows indicate the direction of the hystereses.
explained by the fluid flow induced by the movement of the plunger. Thus, during relaxation and the plunger moving upwards, external fluid was sucked in at the bottom of the column inducing an upwards fluid flow. As a consequence, packing expansion and fluid flow act in the same direction causing the packing to relax faster. This can be seen in Figure 7.7 B, showing that the hysteresis of section 7 is less pronounced than in Figure 7.6 B. Hence, the relaxation time of the uppermost packing section 1 is increased by the upmoving lower packing section. This may explain the differences in the compression-relaxation behavior of the semi-dry and wet packings.
7.2.2. Particle Deformation and Force Transmission
The particle deformation and force transmission was analyzed via optical UV-fluorescence microscopy, similar to the hydrodynamic packing compression experiments. Figure 7.8 shows the packing top region right below the movable plunger during gravity settled state (A), during 33% (B) and 50% (C) packing compression.
The degree of packing compression was considerably higher than during flow com-pression (λbed ≈ 0.12). Thus, a much denser particle network can be identified in the upper packing region. During packing compression of 33%, only a small amount of void
Chapter 7. Characterization of Particle Bulk Packings
A B C
Figure 7.8.: UV-fluorescence microscopy of the uppermost section 1 of the particle bed during mechanical load. (A) Gravity settled packed bed at zero compression. (B) Particle bed being during a mechanical compression ofλbed = 0.33. (C) Particle bed being during a mechanical compression ofλbed = 0.5. The arrows indicated the direction of compression.
space is visible between the particles, whereas during 50% almost all voids are closed.
The single particles then almost show an oval shape as a result of high compression pressure. Similar to the indicated force-chains in Figure 7.5 (B) force transmission along individual particle chains can be identified in Figures 7.8 (B) and (C).