(292d) Numerical Simulation of Pneumatic Drying Using Computational Fluid Dynamics

Despite the industrial importance of drying processes, specifically in drying of slurries, the design and operation of dryers are still mostly based on empirical rules. Although research in this field is progressively growing, it is still in its early stages and the physical phenomena involved in these processes are not yet fully understood. Commercial slurry dryers encounter high manufacturing and operating costs. The design and optimization of these dryers for different slurries and different operating conditions are conducted based on expensive pilot scale studies. With the advent of sophisticated computational fluid dynamics (CFD) codes and high speed computing, CFD can be used as an effective tool for design and optimization of such devices. The objective of this research is to study and evaluate industrial drying processes, in particular, those conducted in pneumatic dryers, using state of the art commercial CFD packages. This approach also enables the prediction of physical properties at discrete points inside the computational domain, which lead to a better understanding of the heat and mass transfer phenomena occurring in slurry dryers. The hydrodynamics and drying mechanisms of a two-dimensional, dilute-phase, gas-solid flow have been predicted in a laboratory- and commercial (large)-scale vertical riser. The CFD method is based on the Eulerian-Eulerian two-fluid multiphase flow model. A control-volume based technique using commercial CFD package and the kinetic theory of granular flow (KTGF) were applied to simulate the flow pattern and heat- and mass-transfer processes for porous particles (sand & PVC) in the presence of hot air. The time-averaged theoretical drying rates in the constant- and falling-rate drying periods agree reasonably well with the published experimental data for similar models. The model also predicts a core-annulus flow in the riser with maximum velocity of both phases in the core center and denser down-flow pattern close to the walls. User-defined subroutines were implemented to extend the CFD capability to account for the mixture properties and to simulate the two-drying stages using mass-transfer models. The effects of different operating conditions and dryer design on the drying kinetics are evaluated.

Key Words: Pneumatic dryer, Pneumatic drying, Two-fluid model, CFD, Modeling, Heat and Mass transfer.