(598d) Effect of Physical Properties and Shearing Rate On Powder Flow During Die Filling From a Hopper
Powder flow in the “Intermediate” regime where collisions and particle friction co-exist, is the most prevalent in industrial practice and is the object of the present work. The ability of a powder to dilate or compact during flow at different shearing rates, i.e. to exhibit compressibility, plays a critical role in its rheology. This is important, as most of previous works on powder flows were developed under the main assumption that powders flow at the “critical condition” where the they are considered to be incompressible (with slight dilation). The basic motivation for the present work was to study flows of different pharmaceutical excipients through circular orifices of different size at varying shearing rates to understand the role of compressibility.
Two versions of a lab-scale die filling apparatus were used: one with intermittent (periodic shear) and the other with continuous flow. A capacitance probe, that measures solid fraction based on the dielectric permittivity of the media, is mounted close to the die to enable the measurement of density and density fluctuations so that volumetric flow rates can be obtained directly. This is an essential feature since the bulk density of the powder in the shearing zone is variable and also fluctuates in time.
We performed experiments with free flowing materials to verify the prediction of the Beverloo equation at no shear. We also used free-flowing and somewhat cohesive powders to provide modifications to this equation when shear is present. We found that there is a strong correlation between powder flowability and its density while filling the die. This observation stresses the importance of continuous monitoring and careful control of powder density during die-filling as density fluctuations cause tablet weight uniformity issues in the tablet press. To complete the picture we also used cohesive powders and found that in this case, although the powder was not flowing through any orifice sizes under no shear conditions, its flowability improved significantly under shear. We demonstrated that for this kind of powder, flow instabilities set in as low shear is applied, which in turn causes erratic flow. The flow becomes more stable either when the powder is subject to shear for a longer period of time (we obtained this by substituting intermittent feed by a continuously sheared hopper) or as shear is increased beyond a critical value.