A mathematical model of drying for hygroscopic porous media

Abstract

A mathematical model is developed to simulate the drying of hygroscopic porous media and, in particular, of wood. Drying rate experiments were performed using wood specimens and a nonhygroscopic porous ceramic solid and were simulated using the appropriate version of the drying model. Calculated model predictions are in very satisfactory agreement with experimental results. An examination of the relative impacts on drying of the transport mechanisms that comprise the model leads to meaningful interpretations of observed drying behavior. Controlling rate factors can be identified and different types of drying behavior specific to a given material or drying condition can be explained and understood through model simulation studies. Such capability can provide important guidance for drying process design and control. M. A. Stanish, G. S. Schajer, Ferhan KayihanWeyerhaeuser Technology Center Tacoma, Washington 98477 SCOPEThe drying of moist porous solids is a complicated process involving simultaneous, coupled heat and mass transfer phenomena. Accordingly, drying behavior can be influenced by a rather large variety of independent factors, including, for example, ambient conditions of temperature, air velocity, and relative humidity, and solid properties such as density, permeability, and hygroscopicity. Extensive characterization of drying behavior using a strictly experimental approach constitutes a formidable challenge due to the excessively large number of variables that must be considered. The task becomes more manageable, however, with the help of a reliably realistic mathematical model of the drying phenomenon. Our objective is to develop such a tool to simulate drying behavior and therefore allow us to extend the results of experimental drying investigations. In this way, the impact of the many variables on drying behavior can be examined and interpreted without having to resort to an extensive program of experimental testing.The approach taken here represents advancement of drying modeling and theory on several fronts. The mathematical description of the drying process is derived and maintained for the most general case; a comprehensive set of fundamental heat-and mass-transfer mechanisms is coupled with thermodynamic phase equilibrium expressions in a way that accommodates either hygroscopic or nonhygroscopic materials. Heat transfer by both conduction and convection is included together with mass transfer by gaseous diffusion and bulk flow of gas and liquid through the void space and by bound-water diffusion through the solid matrix. Bound-water migration is expressed in terms of the diffusion of sorbed water driven by the gradient in the chemical potential of the bound molecules. While this concept of bound-water diffusion was previously suggested by Siau (1983) and others (Kawai et al., 1978), in this development a new, uniquely explicit expression for bound-water flux is derived in terms of temperature and vapor pressure gradients. Combination of all of these transport mechanisms in a single generalized ...

Drying Comparison of Nonhygroscopic and Hygroscopic Materials

“…In contrast to that, drying of hygroscopic materials not only involves free and capillarity water but also a tightly bound water that strongly attach to the solid matrix up to hydration temperature [1]. Previous studies on drying of hygroscopic and nonhygroscopic have present different equations and formulations as well as the concept that had been derived [1,2,3,4,5]. Bound water movement is expressed in terms of the diffusion of sorbed water driven by a gradient in the chemical potential of the sorbed water molecules [2].…”

“…During low moisture contents, pores mainly consist of bound water and vapour. Stanish et al [3] in their development had derived a uniquely explicit expression for boundwater flux in terms of temperature and vapor pressure gradients. Meanwhile, Zhang et al [4] and Haghi [5] revealed that the bound water transport mechanism only effective when saturation irreducible is reached.…”

The application of microwave heating in bioenergy: A review on the microwave pre-treatment and upgrading technologies for biomass

“…The thermal behaviour of biomass is strongly dependent upon the biochemical composition and structure of the feedstock, and the specific effects of MW heating on biomass has been previously explored [192][193][194]. Within the biomass, not all of the biochemical constituents are able to absorb MWs, and even if they can absorb MWs, the degree of polarisation [183,[195][196][197]. The bulk heating mechanism of microwave irradiation causes water to vaporise within the pores of the biomass, as water dipoles attempt to continuously reorient within the oscillating electric field.…”

A new model for the drying of droplets containing suspended solids after shell formation

“…Several workers have investigated the detailed modelling of moisture transport through porous media containing a gas phase, [19,49,52], but two limiting cases may be identified [34]. In the dry shell model, the vapour space is in the shell itself and an evaporative front recedes through the droplet.…”