Estimates of Powder Flow Properties at Low Consolidation from the Torque Measured in a Powder Rheometer Equipped with a Cylindrical Impeller

Small scale industrial application often used in pharmaceutical and other fine chemicals application involve the use of small intermediate bulk container and small hoppers as well as the use of powders in process involving low consolidation levels. The knowledge of flow properties of low consolidated powders is, therefore, necessary for the proper design, as well as for determining the suitability of newly formulated powder to existing processes lines and equipment. Conventional shear testing is well standardized and the measured powder properties are well defined. Unfortunately, however conventional shear testers are not always able to reproduce the correct powder history found in the application of interest. Therefore, new testing methods, more flexible in the powder preparation, are receiving growing interest. The equipment used in this research project is the Powder Cell unit purposely developed by Anton Paar to fit on Anton Paar Rheometers and to carry out torque measurements on aerated and not aerated powders. In this instrument, the state of consolidation of the powder can be adjusted by changing the impeller depth and the aeration rate. A cylindrical impeller geometry has been tested on a number of free flowing as well as slightly cohesive powders. In order to be able to compare results with the properties measured with a conventional shear tester the impeller rotational speed was kept at very low levels. A proper powder conditioning procedure, involving powder fluidization and shear, was also set up in order to obtain reproducible results. A simple model was developed to relate the measured torque with the dynamic angle of internal friction in the powder measured with a conventional powder shear tester. The results indicated a good match between the dynamic angle of internal friction measured with the instruments and those measured with the conventional shear cell in the range of consolidation tested. In order to match the results, a pseudo hydrostatic state of stress had to be hypothesized within the measuring cell. In the same procedure the extrapolated torque at the start of the impeller and a zero consolidation was related to the powder cohesion. It was possible to conclude that in the tested configuration the instrument allows to successfully exploring powder behavior at low consolidation ranges which, in the appropriate conditions, well correlates with the powder flow properties measured with standardized equipment. Copyright ©