Avalanche activity and powder dilation

The work in this chapter has not included experiments on powder dilation. This is because there are discrepancies between the GDR’s pixel-counting computer program and procedures used by Faqih and colleagues (A. M. Faqih, Chaudhuri, Alexander, et al., 2006; A. M. Faqih, Chaudhuri, Muzzio, et al., 2006), and the GDR system in this work; the custom software of the present GDR and its operating procedures have been revised by Pingali and Kick (2013). In the earlier work, powder dilation was measured after the initialization of powder, see Section 6.2.4.4; the powder in the cylinder was shaken horizontally and vertically for an unreported fixed number of times and allowed to settle under its own weight (A. M. Faqih, Chaudhuri, Muzzio, et al., 2006). In the procedure by Pingali and Kick (2013), the tapping of powder up to 1,000 taps with a tapping apparatus is required after a powder is filled into the cylinder; powder dilation is then calculated with a revised pixel-counting program that incorporates the tapped volume and not the VInitial of

Equation 6.2.

6.7 Conclusions

The flowability of powders under controlled and unconfined conditions was assessed by measurement and characterization of powder motion in a Gravitational Displacement Rheometer, GDR, which comprised a cylindrical drum of 192.5 mm internal diameter and 292.5 mm drum length. Avalanche activity of Geldart Group C lactose LP4, Group A sand S1, Group A

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0 10000 20000 30000 40000 Flow Index [kg] 1/d*32 [m1] LP4_28.9μm_C S1_40.1μm_A RD1_41.5μm_A LM1_58.0μm_A LP1_150.8μm_A/B B8_193.0μm_B

refractory dust RD1, Group A lactose LM1, Group A/B lactose LP1, and Group B glass beads B8 was measured at drum speeds in the range 5 RPM to 30 RPM, and plots of standard deviation,

ws, of the GDR load cell signal versus drum speed were used to represent avalanche activity. In

investigating the influence of drum fill level, 20–50% on a volume basis, on avalanche activity, it was observed that ws increased with increasing fill level for Geldart Groups C and B powders,

and the opposite trend occurred with Group A powders. The ws data at 50% fill level were

different from the data at the other fill levels; further observation of the ws profile at 50% fill

level for each powder revealed that ws consistently showed changes between 10 RPM and 15

RPM.

To explain and interpret ws, information on d*32, span of particle size distribution, C0

measured by shear testing, tc/Hmf measured by fluidization and bed collapse, and powder bed

profiles during tumbling in the GDR were taken into account. When the ws profiles of sand S1

and refractory dust RD1, which had similar d*32 and span, were compared, the ws for sand S1 at

15–25 RPM was higher; C0 was lower and tc/Hmf was higher for S1. For lactose LM1, ws was

relatively low and constant, and the absence of a horizontal bed surface during tumbling was also observed, indicating lack of powder aeration; LM1 had a higher span and showed segregation when fluidized in a 80 mm inner diameter cylindrical fluid bed. The ws values for lactose LP4,

which was most cohesive among the test materials, were highest; the powder moved as agglomerates when tumbled. In contrast, the ws for glass beads B8 was lower because the

particles flowed freely when tumbled and they did not retain air; bed collapse was virtually instantaneous. Lactose LP1, which was an A/B powder, gave lower ws values than the free

flowing B8; this behaviour was thought to be related to higher content of fine particles and wider size distribution in LP1, and different avalanching mechanisms, notwithstanding the fact that the powders themselves were very different.

Parameter ws and Flow Index, Equation 6.1, were correlated with 1/d*32; the observed

general trend was that both ws and Flow Index increased with increasing 1/d*32. Increasing drum

speed increased the scatter in the [1/d*32]:ws plot. Scatter of a similar trend was also observed in

the plot of Flow Index against 1/d*32. When the plot of ws at 5 RPM against 1/d*32 was

compared with the plot of Flow Index against 1/d*32, the former showed a straightforward and

neat relationship; measurement of ws at 5 RPM and 50% fill level could be a better indicator in

Chapter 7 – Summary

Powders are complex three-phase systems formed by solid particles that can come in different sizes and shapes with air in the interparticle voids, and moisture that can be in the air, in the particles, and attached to the surface of the particles. Information and knowledge of powder compressibility, fluidization, flowability, and also connections between them are important and helpful to the handling and processing of powders across many industries in operations such as mixing, milling, packaging, and storage. A multi characterization approach involving shear testing, powder tapping, fluidization and bed collapse, and powder tumbling was used to assess samples of milled and spray-dried lactose powders, sand, refractory dust, and glass beads; emphasis was given to lactose powders, an important commodity in the food and pharmaceutical industries.