The Significance of the Sorption Isotherm on the Simulated Performance of Grain Driers

Abstract

Featured Application: Sorption isotherms enable the temperature and humidity of air to be manipulated to achieve the desired moisture content of commodities that are being dried. The isothermal differential heat of sorption is subsumed within the isotherms, and this influences the energy required to dry grains and other hygroscopic porous media. For the first time, this work quantifies the influence of isotherms on the rate at which food grains dry, with canola being the exemplar commodity. Sorption isotherms enable postharvest technologists to estimate the degree and rate of drying of agricultural produce. They are also useful in the design and operation of desiccant systems that are used to condition air. However, the published data on sorption isotherms contain several inconsistencies. For example, under the conditions considered in this work, it is shown that the widely cited Chung–Pfost isotherm predicts moisture contents of canola that are less than zero as the relative humidity tends to zero. Furthermore, it is shown that a long-established form of empirical expression appears to grossly overestimate the differential heat of wetting, hence the integral heat of wetting of canola. In this work, algebraic expressions are derived that enable the relationship between the forms of isotherm equations on the speed of drying to be calculated. Prima facie, it is anticipated the heat of adsorption will augment the speed of temperature waves through beds of drying canola. However, it is found that this may not be the case. Anomalies in published isotherms for agricultural produce reinforce the need for accurate psychometric data to be measured over a wide range of temperatures and relative humidities.