Heat and moisture transfer studies on walnuts during hot air drying in a fixed-bed column dryer
Chang Chen, Zhongli Pan
Abstract
This research was conducted to better understand the mechanisms of heat and moisture transfer of walnuts during drying in a fixed-bed column dryer with heated air, and to develop a mathematical model to simulate this process. Drying experiments were performed under constant temperature and step-down temperature conditions (43, 55, 65 and 75 °C) with fixed air velocity of 0.7 m/s. The nonequilibrium transfers of liquid moisture, water vapor and heat within the single walnuts (both shell and kernel), as well as between the walnuts and drying air in the column were simulated and studied using a finite element method. The numerical results showed that the distribution of temperature and moisture content in the single walnuts played an important role in the heat and moisture transfer in the deep-bed column. The drying characteristics of walnuts were significantly affected by the non-uniformity of drying conditions surrounding the walnuts (particularly for air temperature and humidity) along the column heights. The predicted values of walnut moisture contents, temperatures, and air relative humidity at different column heights agreed well with the experimental values (Radj2 > 0.972) under different drying conditions. This study established the theoretical basis for improving and optimizing the walnut drying process in deep-bed dryers. The developed mathematical model can be used to describe the drying characteristics and understand the mechanisms of walnuts drying in deep bed dryers.