Prediction of the characteristic time of powder caking in storage and test conditions: Experimental and modeling study

Abstract The kinetics of water sorption by a bed of powder was modeled in order to estimate the characteristic time of caking under real (similar to storage conditions) and test (accelerated) environmental conditions. In combination with the model, the kinetics of water sorption by henna powder was experimentally measured in the devices of “humidity chamber” and “caking device” as the environments representing real and test processes, respectively. According to the results, the water uptake by the powder was faster in caking device in comparison with humidity chamber whereas the equilibrium amount of water uptake was expectedly the same. The diffusion–convection and convection–convection were considered as the governing mechanisms of mass transfer in humidity chamber and caking device, respectively. Model equations were solved by applying the finite difference method (FDM) to track the water uptake amounts by the powder over the time. By matching the experimental data and the model results, the mass diffusion coefficient (D) and the total mass transfer coefficient (K) of henna were predicted. Extending the model, a dimensional analysis came up in order to understand the effects of the Stanton and Sherwood numbers on the time of equilibrium of sorption for both mentioned mechanisms. These findings will lead to a better prediction of characteristic time of caking in different conditions. Moreover, the results of this study define a protocol for performing accelerated laboratory tests of caking so that their results can be generalized to real conditions. The generalization procedure was also explained.