Numerical approach to describe continuous and intermittent drying including the tempering period: Kinetics and spatial distribution of moisture

ABSTRACT

Continuous and intermittent drying experiments were performed with whole bananas, using hot air at 70°C. The intermittent drying experiments were performed with intermittency ratio equal to 1/2 and tempering times of 0.5, 1.0, and 2.0 h. The conditions imposed to the experiments permitted to investigate the influence of these tempering times on the processes. A one-dimensional numerical solution of the diffusion equation coupled with an optimizer was used to determine the process parameters for four experiments. To describe the processes, a model was proposed. Model includes shrinkage, variable effective mass diffusivity, and two values for convective mass transfer coefficient (within and outside the dryer), enabling to consider moisture loss during the tempering period. For all experiments, the simulation of the drying kinetics has resulted in good statistical indicators. Proposed model also made it possible to predict moisture distributions during the entire processes, including the migration of moisture from the central part to the peripheral region of the cross section of the bananas, during the tempering period. The results indicated that, for the same effective operation time and intermittency ratio, increasing the tempering time implied moderate decrease in the final average moisture content.

KEYWORDS:

• Diffusion model
• moisture migration
• shrinkage
• tempering
• variable mass diffusivity