Relevance of Powder Flow Properties in Predicting High Temperature Fluidization Behaviour: Role of Interparticle Forces

Powder cohesion may have a relevant role in several industrial process operations involving particulate materials, such as fluidization, granulation and drying, as well as storage and solids handling units. Several of these operations require process conditions which involve high temperatures which, in turn, may affect and change powder cohesion with respect to the ambient. In fact, cohesion in powder is related to the intensity of interparticle forces such as van der Waals, capillary and electrostatic forces and the system temperature can affect all these forces by changing particle hardness, producing liquid capillary phases or modifying the particle dielectric properties. It is of interest, therefore, to have the possibility to measure powder cohesion at the process temperature and to observe possible changes due to temperature changes. Several different characterization tests are available or have been developed to measure  powder cohesion. Tough powder shear testing had been developed purposely for the design of bulk solids handling units, among the other similar techniques, it has the great advantage of measuring well established physical properties and of being able to produce highly repeatable results. On the other hand no established procedure exists to relate any of the possible determination of powder cohesion to powder fluidization properties. In this paper a high temperature ring shear tester available at the University of Salerno, will be used to characterize the powder cohesion of powders of industrial interest and to operate between ambient and 500°C. The measurement results will be related to possible changes in interparticle forces due to temperature and compared with the results of fluidization experiments carried out at the same temperature in the x-ray high temperature fluidization facility available at University College London.