Water content strongly influences the processing, properties, and performance of wood and wood products. This is the case not only for wood as a structural material but also for wood-based insulation materials, packaging, and paper. Water also affects the behaviour of wildland fire; it slows the preheating and ignition of forest fuels (live and dead plants) and the rates of combustion, fire spread, and heat release. Across these different applications, scientists and engineers aim to improve the accuracy of models that predict the energy involved in drying these materials. This energy we call the heat of sorption.
This quantity depends on temperature and the moisture content of the material. Researchers use two methods to determine the heat of sorption. The first, calorimetry, measures the heat flow or temperature change involved in the sorption process. The second, known as the isosteric method, relies on measurements of equilibrium moisture content (EMC) and relative humidity across a range of different temperature levels. Researchers have developed or adapted many different models for water vapor sorption in wood in the last 80 years. However, we have shown that the most common theoretical models fail to predict the heat of sorption accurately and fail to predict other important properties of wood. These problems motivated us to develop a better sorption model that is accurate, widely applicable, and practical to use.
We based our model on an empirical relationship for the chemical potential of absorbed water as a function of EMC and temperature. Our approach built on earlier models, but we had to overcome some key limitations of these earlier models. For example, although the ideal gas equation works well for describing water vapor in many applications, it becomes more and more inaccurate as temperature increases, so we cannot use it for fire modelling. We therefore developed a set of simple empirical equations to describe the real behaviour of water vapor, and we adapted a rigorous theoretical framework for gas adsorption in microporous solids to describe water vapor sorption in cellulosic materials across a wide range of temperature and moisture conditions.
We call our new model the Comprehensive Analytical Sorption Thermodynamic (CAST) model. It predicts equilibrium sorption behaviour, from which we derive analytical equations for the heat of sorption, heat of wetting, and other properties. We evaluated the model using sorption data and calorimetric data from the wood science literature. The CAST model fits the experimental data with higher accuracy than several other models, including the six-parameter modified GAB model. Although the experimental datasets come from disparate literature sources spanning from 1896 to 2019, the CAST model effectively unifies them. It provides simple analytical equations for implementation in computer simulations of heat and moisture transfer.
Keywords: water vapor sorption, heat of sorption, heat of wetting, equilibrium moisture content, sorption model
Authors
Mark A. Dietenberger
Forest Products Laboratory, US Forest Service, USA
Samuel V. Glass
Forest Products Laboratory, US Forest Service, USA
Charles R. Boardman
Université de Lorraine, LERMAB, France
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