Mathematical Modeling for Fixed-Bed Drying Considering Heat and Mass Transfer and Interfacial Phenomena*

A mathematical model for fixed-bed drying with air cross-flow was obtained to simulate the drying process. Special attention was placed on the interfacial conditions. A system of four couple nonlineal partial differential equations in conjunction with three nonlineal algebraic equations was applied and solved numerically by both finite differences and Runge–Kutta methods. Simulations were compared with experimental data from carrot slabs in deep fixed-bed drying. Slab thicknesses were 1.0 and 0.1 cm, and air drying temperatures were 50–60°C. Simulation predicted that water transport was controlled by internal diffusion in slabs with 1.0 cm of thickness; therefore, the interfacial conditions may be considered in steady state. Nevertheless, the predominant phenomenon in slabs with 0.1 cm of thickness was by convection; therefore, the interfacial conditions varied with respect to space and time. In the proposed phenomenological representation of fixed-bed drying, the properties and variables of the system defined the type of mechanism controlling of the drying process without previous assumptions.