1.1 Introduction
Drying is a key industrial process of great practical importance in chemical and pharmaceutical industries, agriculture and food processing, pulp and paper, wood and minerals processing, solid fuels preparation (e.g., biomass or coal drying). It consists of a mass transfer process aimed at removing a solvent – in general water (or moisture) – from a solid, liquid, or a semisolid (a highly viscous liquid). Thence, drying is a thermally driven separation process and typically occurs by evaporation of the solvent (the moisture) or by sublimation or by a supercritical process that avoids solid–liquid boundary, or by reverse osmosis.
The process of drying is recognized as one of the most energy intensive process among the separation technologies. For example, according to Mujumdar (2006) drying energy sector in North America is just responsible of ~15% energy consumption. It requires a source of heat and sometimes it necessitates maintaining deep vacuum for effective moisture removal. Drying can be also applied in some cases followed by heat addition and moisture removal by sublimation. In addition, the method can be integrated with other types of separation technologies such as centrifugal draining which require the application of strong centrifugal forces.
In the drying sector, it is aimed to make drying processes more efficient, more cost effective, more environmentally benign, and more sustainable. Thus, this requires optimization methods to be applied to these processes. Furthermore, there is large panoply of materials spanning from thick slabs to nano-powders which require specific methods of drying.
Understanding drying processes requires the application of analysis methods from thermodynamics, heat, mass and momentum transfer, psychometrics, porous media, materials science, and sometimes chemical kinetics altogether. Some specific processes that occur during drying and must be considered in the analysis are crystallization and allotropic transition or shrinkage, texture change, porosity change, and fracture. Drying is a transient process; therefore, changing of moisture removal rate must be accounted for. Depending on the drying material (e.g., sample size, porosity, tortuosity) and ...
