(380ap) Using a Powder Rheometer to Probe the Defluidization of Cohesive (Group C) Particles

Given the abundance of cohesive particles in various industrial applications, accurate prediction of inter-particle cohesion is crucial for the efficient design and optimization of many operations. However, predicting particle-level cohesion is non-trivial at best. For example, isolation of normal (cohesive) force in direct atomic force microscopy (AFM) measurement is difficult even for spherical particles. The indirect estimation of particle-level cohesion via force models relies upon the AFM surface maps of particles. However, carrying out AFM scans on individual particles is not conducive to an industrial setting where feed stocks change frequently. An attractive alternative is bulk measurements from which cohesion can be easily extracted. In recent work carried out by Liu et al. [1], defluidization was identified as one example of a bulk experiment that can provide particle-level cohesion. In that work, a simplified square-force cohesion model is proposed, and with the help of discrete element method (DEM) simulations, the parameters for the square-force model are determined from a defluidization (pressure drop vs. gas velocity) curve. However, as the level of particle cohesion increases (i.e., Group C particles), standard defluidization curves cannot be obtained. In the present work, a powder rheometer is used to carry out a careful exploration of defluidization behavior of particles with a range of cohesion levels. By adding energy through a rotating impeller, the rheometer is able to fluidize cohesive (Group C) particles without channeling, which is not possible in conventional fluidized beds. Both pressure drop and torque are measured during defluidization. The torque profile indicates a clear change demarking the transition from a fluidized to packed bed state, unlike the pressure drop profile where the curvature of the transition increases with cohesion level. Hence, determination of the characteristic velocity (U_cf), a key parameter for the square-force model [1], is more straightforward using the torque profile compared to the pressure profile. Another advantage of the methods described is that the rheometer can be used by industry as quick gauge for the relative level of cohesion as particle feedstocks and/or ambient conditions change.

Reference

[1] P. Liu, C.Q. LaMarche, K.M. Kellogg, C.M. Hrenya, A square-force cohesion model and its extraction from bulk measurements, AIChE Journal. 82 (2018) 1156–11. doi:10.1002/aic.16089.