Mathematical Modeling and Design of Parboiled Paddy-Impinging Stream Dryer Using the CFD-DEM Model

Impinging stream dryers (ISDs) are effective for removing moisture from particulate
materials because of the complex multiphase transport of air particles in ISDs. Nowadays, computational
techniques are powerful to simulate multiphase flows, including dilute and
dense-phase gas–solid flows and hence, the use of a reliable computational model to simulate the
phenomena and design a dryer has recently received more a􀄴ention. In this study, computational
fluid dynamics, combined with the discrete element method (CFD-DEM) and falling drying rate
model, were used to predict the multiphase transport phenomena of parboiled paddy in a coaxial
ISD. The design of an impinging stream pa􀄴ern for improving residence time in a drying chamber
of ISD was also investigated. The results showed that the CFD-DEM, in combination with the falling
drying model, could be well-utilized to predict the particle motion behavior and lead to more
physically realistic results. The predicted change of moisture content in parboiled paddy was in
good agreement with the experimental data for 17 cycles of drying. Although the prediction of
mean residence time was lower than the experimental data, the predicted mean residence time
was a similar trend to the experimental data. For ISD design, the simulation revealed that the use
of two stages of impinging stream region (two streams collide at the top of the drying chamber at
the first stage and then the gas particles flow on the incline floor to collide with the other stream at
second stage) in a drying chamber could increase the residence time approximately 75% and reduce
drying cycle from 17 to 10 cycles when it was considered at the same final moisture content.