(211f) Density Behavior of Cohesive Granular Materials

The mechanical properties of powders and grains, both natural and man-made, are of enormous importance to the petrochemical, petroleum industries. For example, the cohesion, density, and yield stress of powder silica, alumina, and zeolites have direct impact on the performance and scale-up requirements of impregnation, drying, calcination, and blending processes used to manufacture the great majority of catalytic materials used in petroleum refining. The same impact is observed in many other industries, ranging from agrochemicals to pharmaceuticals to polymers. Despite this enormous relevance, density behavior of cohesive powders is not completely understood. This work contributes in overcoming some of the barriers for understanding density behavior by focusing on development experimental methodology for characterizing and understanding density variations and dilation in granular flows as a function of cohesive powder properties. We implement a general experimental method for quantifying density fluctuations in granular materials, based on the use of x-ray micro computerized tomography (CT). The X-ray micro CT scanner can capture images with a resolution of 10-3 mm. An acrylic cylinder is filled up to 80% by pouring from a certain height, so that the powder is loosely compacted. Then, the powder is exposed to the action of an electromechanized shaker, vibrating the powder at a given frequency and amplitude, with varying number of strokes or vibrations. The poured state of the powder is one of most loosely packed states that one can achieve. The poured states finally tend to a completely packed state, which is what we call the tapped density. The mass of each sample is recorded and immediately after the initial state of the powder is captured by scanning it in the MicroCT. After that, in all cases with each powder, the cylinder is tapped at 3 different frequencies by setting different inputs at the frequency generator and 2 amplitudes. The cylinder is removed from the electromechanized shaker after 30, 120, 250, 500, 750, 1000, 2000, 4300 and 8000 taps and carefully placed in the X-ray MicroCT. The results demonstrated that the cohesion parameter, the angle of internal friction and the flow function measured using the FT4 rheometer affect the powder mobility in the column and impact the packing behavior.

We also measured the tapped density using the X-ray MicroCT scanner. Our results demonstrated that the density increases with an increase in the shaking frequency and the amplitude of vibration and it decreases at the top and bottom of the measured areas. The consolidation of the powder was proven to be more uniform for higher frequencies. The changes in density for more cohesive materials are low for lower amplitude and frequencies of shaking. The results also showed that the cohesion is not directly correlated with the packing density of the powder and that the energy required to pack a cohesive powder is significantly higher than the one required for compacting a less cohesive material. In addition, we could observe that the porosity and the variability in porosity as function of the height decreases for higher frequencies and amplitude of shaking.