Mathematical Modeling of Diffusion-Controlled Pneumatic-Conveying Dryers

Abstract

A mathematical model for a continuous pneumatic-conveying dryer has been developed for removing internally bound moisture from solid particulates. The dryer relies on a recirculating carrier gas stream for entrainment. Drying is carried out by injecting into the gas loop a fresh stream of conditioned drying gas while an equal amount of wet gas is vented out. Because pneumatic-conveying dryers usually employ huge gas velocities, the particulates are well dispersed in the gas. Therefore, for solids absent of surface moisture, the drying kinetics is controlled by intraparticle diffusion. The model developed based on the diffusion mechanism relates the moisture reduction in the solids to various process parameters (diffusivity, partition coefficient, particle size, residence time, solids loading, drying gas usage, and carrier gas recirculating rate), and is fully predictive. Therefore, it can be used to study the effects of these variables.The model was compared with the plant data and found to match the data within ± 15%.

Investigation of Hexane Diffusion in HDPE Particulates for Drying Applications

“…For the rotary dryer, which is operated at a low purge gas velocity of about 1-2 cm/s, the dryer efficiency is very low owing to the low effective diffusivity observed in this study (Qi and Krishnan, 1996). On the other hand, the pneumatic conveying dryer operates at a gas velocity greater than 10 m/s and exhibits a high efficiency consistent with the intrinsic diffusivity (Qi, 1996). The current study therefore has provided an excellent foundation for elucidating these dryer characteristics and pointed to particle agglomeration as a major factor undermining the efficiency of the rotary dryer.…”