Abstract
The aim of this chapter is to confirm the possibility of using the simple diffusion model to predict the behavior of a building material during the application of drying process under variable operating conditions. This approach can be considered as a simulation of the effect of the variable climatic conditions on the building material. During this research, the thermo-physical properties of the tested material as well as the drying air are considered as variable and changing with the operating conditions. Accordingly, diffusion coefficient is determined experimentally and is considered as variable with the temperature and the humidity and represented as function of the wet bulb temperature. Two sorts of conditions are tested: constant flux and convective flux. Furthermore, two types of changes are also tested: sudden changes and progressive changes of the drying conditions. The results of the study are mainly represented by the drying curves or the drying kinetics.
TopIntroduction
Drying is a process that has been always used by human. Liu et al. (2002) confirmed that China already used drying process six thousand years ago. Moyne and Roques (1986) reported that the United Kingdom, in 1727, extensively used wood drying for the shipyard. Drying consists on the removal of the moisture content that a material is holding by the application of a source of heat. Drying process is used as a method of conservation and preservation of agro-alimentary products such as fish, corn, rice, potato and tomato with the objective of preserving valuable organoleptic characteristics which include odor, flavor, texture and color, as well as the nutritional quality (Bennamoun and Belhamri, 2003). It can also be an unavoidable process for the fabrication of the products and making it marketable such as wood, paper and building materials. The technological and scientific development has confirmed the importance of this process in different fields: biology, chemistry, pulp and paper, wood, textile, etc. However, drying is not an easy process and needs a real management of the whole steps of the process, in order to avoid deterioration of the final product or make it expensive. Indeed, it is established that non-studied drying conditions can lead to the deformation of the final product. It can conduct to the appearance of cracks as well (Dong et al., 2018, Hammouda and Mihoubi, 2017) and Bennamounet al. 2013a). In the field of fabrication of building materials, it is well known that application of random or un-controlled drying conditions to building materials can lead to a quick deterioration of those ones. Consequently, it is necessary to have the fundamental knowledge of the behavior of the material to be dried before performing the process itself. The common technique that represents drying process is the drying curve or more accurately the drying kinetic. The drying curve represents the variation of the moisture content with time and the drying kinetic is usually represented by the variation of the drying rate (frequently given by the following unit: kg.m-2.s-1) versus the moisture content. This representation is called Krischer’s curve. This representation can be considered as the most representative of drying process, as the different drying phases are easily determined using this representation, as confirmed by Bennamoun et al. (2016) and Bennamoun et al. (2013b). It is important to mention that the behavior of the different existing materials during drying is not the same. van Brakel (1980) presented a classification of the different materials that can be dried and divided them into: capillary porous, hygroscopic porous and into non-porous materials, then he assembled the materials based on the shape of the drying kinetics in 16 categories, which include building materials.
Key Terms in this Chapter
Krischer’s Curve: This graphical representation is the most adequate in drying field. It consists on representing the drying rate versus the moisture content. This representation allows determination of the different phases that a material can pass through during the application of the drying process.
Solar Drying: Is an application of a convective drying. During this process the ambient air is heated using solar radiation by the mean of a solar collector. In fact, the air passes through an air collector. This passage will increase the temperature of the air. This last will be directed to the material and flow around the material, which lead to the evaporation of the moisture contained in the material.
Falling Drying Rate Period: Usually, drying passes by three phases. It starts by a short adaptation phase that most of the researchers ignore. The second phase is the constant drying rate phase, during this phase the drying rate is constant, then comes the third phase which is the falling drying rate phase. In this phase the surface of the material is not completely full of water, subsequently the drying rate starts decreasing.
Diffusion Coefficient: Diffusion happens when there are two mediums with different concentrations. Diffusion coefficient is then a parameter to measure the molecular movement from one medium to another, usually from high concentration to low concentration. The common used unit in drying for this coefficient is m 2 /s.
Drying Kinetic: Is a graphical representation of the evolution of the moisture content (generally a decrease of the moisture content) inside the material. It is important to make the difference between the drying curve and the drying kinetic. The drying curve is usually represented by the variation of the moisture content versus drying time. However, the drying kinetic is the representation of the drying rate versus the drying time or the moisture content.
Convective Drying: Is the application of drying process using convective transfer. Commonly, during convective drying, an ambient air is heated. This air will flow around the wet material. This contact between the heated air and the material conducts to an exchange of heat and mass between the two medias.
Drying Rate: Represents the speed that drying is taking. It can be calculated by knowing the moisture content in two different times. In the particular case the variation of the moisture is linear with time, the drying rate will be the slope of the line and the drying rate will be constant. This is known in drying by the constant drying rate phase.
Drying: Is the process of removing moisture from a material by the application of a source of heat. This material can be exposed directly or indirectly to the source of heat.
Diffusion Model: Is one of the simplest mathematical models that can well represent drying. This model is widely used during drying of foodstuff. It is represented by a partial differential equation of the second degree. In the equation the variables are time and space.