Cortex Effect on Vacuum Drying Process of Porous Medium

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1. Introduction

Scientists and engineers in China are currently studying vacuum drying equipment that could be used in corn drying [1–3]. However, the corn vacuum drying theory remains unclear. Hypothesized that corn is a porous medium, the vacuum drying of corn is a complicated heat and mass transfer process that has been the subject of intensive research [4–7]. All vacuum drying models have to address the water phase change during numerical solving. In one method, the vapor pressure is equal to its equilibrium value [8–11]. Another method is nonequilibrium method [12–16]. As the porous medium, the heat and mass transfer in vacuum drying process has been studied by nonequilibrium method [17]. The water evaporation rate is determined using a nonequilibrium method with the rate constant parameter Kr. Kr values of 1, 10, 1000, and 10000 are simulated. The effects of different vapor pressures (1000, 5000, and 9000 Pa), initial moisture contents (0.4, 0.5, and 0.6 water saturation), drying temperatures (323, 333, and 343 K), and intrinsic permeability (10⁻¹³, 10⁻¹⁴, 10⁻¹⁵ m²) are studied. It was observed that the temperature increased quickly at the start of drying and then lowered gradually. As the drying process continued, the temperature increased slowly. In the absence of free water, temperature increased rapidly. As the drying process concluded, the temperature remained unchanged. The water evaporation rate could not be obtained during the porous medium vacuum drying process. The rate constant parameter is essential to the nonequilibrium method. When Kr≥1000, the simulation of the drying process was not evidently affected. Vapor pressure and heat transfer affected the transfer of mass. A similar effect was found in the initial moisture and the heat temperature. Intrinsic permeability had a...