Modeling of Batch Dryers for Shrinkable Biological Materials

Abstract

Design of dryers for biological materials is a complex problem, since in order to solve the model equations, foodstuff physicochemical and equilibrium properties and drying kinetics should be included as a function of water content and operating variables. Shrinkage of biological materials under dehydration must also be taken into account when the macroscopic balances in the bed of drying need to be solved. The main objective of this work was thus to develop a realistic simulation model to predict batch deep bed drying of shrinkable biological materials. Differential macroscopic balances for heat and mass transfer in the air and solid phases were expressed in moving coordinates in order to solve the problem of particle shrinkage during drying. The equation system was solved by the ‘method of lines’ using the Gear package for temporal derivatives and finite differences for spatial ones. All the parameters and physical properties required to solve the model were taken from literature or determined independently in lab-scale experiments. A pilot-scale hot air batch dryer was built in order to carry out experimental determinations during drying of slices or cylinders of potato, apple, and carrot at diverse hot air conditions. The appropriate choice of numerical method and initial conditions gave a reliable and stable solution of the equation system. The simulation results agreed closely to experimental data on deep bed batch drying of food particles under different conditions. The use of variable porosity and volume due to shrinkage during drying improved notably the predictions of the simulation model.