Drying Comparison of Nonhygroscopic and Hygroscopic Materials

Abstract

Abstract. This paper investigates and presents the simulation of drying for hygroscopic and nonhygroscopic materials. This present work used a coupled mathematical model of mass, heat and gas transfer that implemented to finite element method in two dimensional and numerically compute using Skyline solver to capture highly nonlinear transient process. Bound water contribution was taken into account in the drying of hygroscopic materials by incorporating constitutive equation of bound water. The results showed drying process can be divided into three periods named constant rate period (CRP), first falling rate period (FRP1) and second falling rate period (FRP2). Capillary action is dominated during CRP before vapour diffusion takes place in FRP1. Bound water movement is generated by vapour pressure gradient exists that represent hygroscopic material. IntroductionIn reality of porous material consists of three types of water that indentified as free, capillary and bound water. Free water is able to flow under an applied pressure gradient, capillary water is immobile water held by capillary forces in regions of microporosity, i.e. dead-end pores. Meanwhile, bound water includes both the water strongly held to negatively charged particles surfaces and the water of hydration associated with the mineral charge-balancing unit. The difference level of these types of water will exhibit different properties of porous material that generally known as hygroscopic and non hygroscopic materials. Drying of nonhyrosocpic materials only involve free and capillary water that easily experimentally can be determined and measured with specific equipment. 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]. This similar approach had been used by Kolhapure and Venkatesh[1] during studies of an unsaturated flow of low moisture for porous hygroscopic media. 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 movement of bound water in hygroscopic materials is also known as liquid moisture transfer near dryness or sorption diffusion with driving force of vapor transport and without liquid transport [6]. Although substantial efforts have been made in the studies of hygroscopic materials, there are limited references available on modeling drying...

Integrated optical electric field sensors: humidity stability mechanisms and packaging scheme

“…The tiny pores within the HP material absorb vapor molecules from the air through capillary action. Moreover, due to the potentially higher hygroscopicity of the shell material, a substantial amount of vapor molecules within the HP material will diffuse and dissolve into the shell material [36]. The process of the shell absorbing vapor molecules is shown in figure 10.…”

Estimation of Water Desorption in Drying of Membrane Structure System

“…Thus, the knowledge of the clinical fitting parameters that influence the WRC is vitally essential to describes materials properties which will determine the accuracy of the computed results in each unique case study. Thus, this paper investigates the fitting parameters in VG model which subsequently employed in the mathematical model of a drying process with coupled mass, heat and gas transfer specifically for multilayer ceramic membrane system [6,7]. The mechanism of transport in the model, namely liquid by capillary action, vapor by diffusion and gas by vapor diffusion and bulk air flow in convective drying condition are strongly interrelated and influenced by VG model.…”