Microwave-Vacuum Drying of Wood: Model Formulation and Verification

Abstract

Based on the mechanism of moisture and heat transfer in wood during microwave-vacuum drying (MVD), a one-dimensional mathematical model to describe the process of wood MVD was established and verified by experiments in this research. The results showed that the process of MVD of wood experienced three distinct periods: (1) accelerating rate with rapid warming-up drying period, (2) a constant temperature and constant rate drying period, and (3) a heating-up with falling rate drying period. Compared with conventional hot air drying, the total drying process is almost governed by a constant rate period in vacuum-microwave drying of wood. The predicted temperature and moisture content in wood match well with the experimental data, the square of the relevant coefficient of the values of simulation and test is above 0.9, and the simulation precision of the change rule of the moisture is higher than that of the temperature. INTRODUCTIONDrying is the most energy-intensive and time-consuming component of the lumber manufacturing process. Currently, over 80% of the produced lumber is dried in conventional and vent kilns, where hot and dry air is forced-circulated through banks of the stacked lumber. Although this operation is simple and inexpensive, the product quality is often low and the drying time is very long, especially for hardwoods.The application of volumetric heating can help to decrease gradients of temperature and moisture during drying and increase the rate of heat transfer in wood, because the energy is absorbed throughout the volume, and the heat is generated directly within the material being dried. Compared with radio frequency (RF), the wavelength of the microwave (MW) is shorter and its field is more uniform

Recent research and development advances of wood science and technology in China: impacts of funding support from National Natural Science Foundation of China

“…For wood drying, researches have been focused on drying methods of timbers, including vacuum drying (He et al 2013), microwave drying (Li et al 2005), superheated steam drying (Cheng et al 2004), radio-frequency vacuum drying , combined drying, and energy-saving drying (Gu 2007). The problems encountered during drying process were also focused on, such as fluid-flow characteristics (Miao and Gu 2003), coupled transfer of heat and mass (Lu et al 2005(Lu et al , 2012, drying kinetics, moisture monitoring , dynamic moisture distribution (Cai 2008), stress release (Cheng et al 2004), shrinkage , creep (Zhan and Avramidis 2011), and optimal control strategies (Li et al 2008b;Ge and Chen 2014). For wood adhesives and gluing technology, research themes included synthesis, polycondensation, and curing mechanisms of adhesives such as urea-formaldehyde resin Gao et al 2015b), phenolic resin (Mo et al 2014), amino resin, and emulsions.…”

Modeling conventional drying of wood: Inclusion of a moving evaporation interface

“…Currently, more than 80% of produced lumber is dried by this method, where hot and wet air is force-circulated through banks of stacked lumber. [1] Although conventional kiln drying is a remarkably simple and robust technology for an industrial environment, it is a relatively mature industrial process compared with other drying technologies, i.e., microwave-vacuum drying, high frequency-vacuum drying, and superheat stream drying. [1][2][3][4] Hence, there is no reason to believe that its predominance will be challenged any time in the near future.…”

Vacuum Drying of Wood—State of the Art

“…As opposed to conventional drying, in microwave vacuum drying, almost all of the drying process is governed by a constant drying rate period, which seemed to go below an average MC below the FSP. Microwaves have shorter wavelength and are more uniform compared with radio frequency, leading to potentially faster drying [75], mainly because there is a higher energy intensity [52].…”