A MATHEMATICAL MODEL FOR DRYING OF PRODUCTS UNDERGOING SHRINKAGE

Abstract

Analytical and experimental studies have been undertaken to gain a better understanding of the drying mechanism within food materials, which often contain a high proportion of water. A mathematical model coupling effects of heat, mass transfer and shrinkage during the drying process has been developed. Shrinkage effect was taken into account by introducing a shrinkage velocity. Both mass and heat transfer equations are solved simultaneously using a numerical method. Some problems encountered in solving the governing equations such as identification of wet and dry regions; the determination of shrinkage velocity and physical properties of the materials are discussed. The equations take into account changes that take place within a material during drying under constant rate, first falling rate as well as second falling rate period. In order to validate the model, a heat pump dryer, which enables independent control of important variables such as temperature, relative humidity and velocity, has been used to conduct experiments. Experiments were performed to measure temperature and moisture distributions within the materials. Yam was selected for the purpose of drying experiments, as it has a relatively homogeneous structure. In order to study the effect of inlet air temperature, relative humidity and velocity on drying process, several sets of test have been conducted. For each test run, several samples were prepared from the same stock and their moisture contents were evaluated. To simulate one dimensional mass and heat transfer, the sides of the samples were well insulated and two end surfaces were exposed to the drying medium. After the commencement of each drying test, one sample was taken out after a regular time interval. The sample was cut into several pieces and, for each piece, the moisture content was determined. The bone-dry mass of each piece was also determined. This enables to determine moisture distribution within the materials. Temperatures at different locations of the material were measured with thermocouples. The predicted temperature and moisture distribution within the material obtained under different operation conditions were compared with those obtained from experiments. A good agreement between experimental and analytical results indicates that the model can describe the condition within the material reasonably well.