Thin-Layer Drying of Mate Leaves (Ilex paraguariensis) in a Conveyor-Belt Dryer: A Semi-Automatic Control Strategy Based on a Dynamic Model
2014
Tussolini, Loyse | de Oliveira, Juarez Souza | Freire, Fábio Bentes | Freire, José Teixeira | Zanoelo, Everton Fernando
A feedback strategy of drying control of mate leaves in a thin-layer conveyor-belt dryer was experimentally evaluated. Moisture content in the discharge of the continuous dryer was controlled by manually adjusting the speed of the moving belt between 3.7 × 10 ⁻⁴ and 15.2 × 10 ⁻⁴ m s ⁻¹ for approximately 7200 s in 120 s time steps. The sets of PID controller parameters and manipulated conveyor velocities were computed with a dynamic drying model at conditions identical to those found in the closed-loop experiments. The model is represented by a system of two partial differential equations built by energy and solute mass balances in the solid phase of the dryer. A large set of experimental drying curves and temperature of mate leaves as a function of drying time, in the temperature range from 55 to 130°C, confirmed the reliability of the considered model. Experimental closed-loop responses of discharge moisture content in the presence of disturbances in the feed moisture content (≈ 0.5 − 1.7 dry basis) and variations in set-point (≈ 0.1 − 1.0 dry basis) validated the suggested control scheme.
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