Fluidized and vibrofluidized shallow beds of fresh leaves

The fluid dynamics behavior of shallow fluidized and vibrofluidized beds operating with fresh leaves was investigated with the aim of exploring drying applications in a modified conveyor belt (MCB) system, which may be operated in a fixed- or fluidized-bed mode. Leaves of the specimens Duranta repens, Schinus molle, Coleus barbatus, Buxus sempervirens, and Bougainvillea spectabilis were tested with a range of sphericities from 0.063 to 0.213, bulk densities from 0.038 to 0.251 g/cm3, apparent densities from 0.52 to 0.97 g/cm3 and ratios of total surface area to volume from 21 to 224 cm−1. Fluidization characteristic curves were obtained by measuring pressure drops versus air velocity in a rectangular column of 0.20 m × 0.11 m cross section. It was not possible to reach real fluidized regimes with the leaves due to strongly non-homogeneous beds. Nevertheless, the characteristic curves allowed for the identification of a transition regime from fixed- to expanded-beds, and parameters such as air velocity and pressure drop obtained from the experimental data at the transition were reproducible for specimens with lower surface area or lower specific gravity (i.e., Duranta repens, Schinus molle, and Bougainvillea spectabilis). The transition velocity and pressure drop could not be predicted by the literature developed for estimating minimum fluidization conditions. The use of vibration at dimensionless vibration parameters (Γ) of 0.5 and 1.0 helped to reduce channeling and decrease the transition velocities from fixed to expanded regimes although it did not affect the pressure drops after the bed expansion. The results suggest that drying fresh leaves using the MCB dryer operating in a fluidized mode would not be feasible because none of the materials investigated attained a high-quality fluidization. Operation of the MCB dryer may be possible by alternating between the fixed and vibrofluidized modes. For the specimens tested, the drying should be performed in a range of air velocities from 0.50 to 0.60 m/s to ensure that the operation is in a vibrofluidized regime. The choice of an adequate vibration parameter will depend on the leaf characteristics because the use of Γ = 0.5 or 1.0 and amplitude of vibration of 4 or 5 mm did not result in appreciable differences on the fluid dynamics behavior of vibrofluidized beds for the conditions investigated.

The fluid dynamics of shallow fluidized and vibrofluidized beds operating with fresh leaves was investigated with the aim of exploring drying applications in a modified conveyor belt (MCB) system. Measuring pressure drops versus air velocity curves allowed for the identification of a transition regime from fixed- to expanded-beds for specimens with lower surface area or lower specific gravity. The use of vibration helped to reduce channeling and decrease the transition velocities although it did not affect the pressure drops after the bed expansion. Operation of the MCB dryer may be possible for drying fresh leaves by alternating between the fixed and vibrofluidized modes. The choice of an adequate vibration parameter will depend on the leaf characteristics.Figure optionsDownload as PowerPoint slide