Transient multiphase model for the high‐temperature thermal treatment of wood

Abstract

in Wiley InterScience (www.interscience.wiley.com). The hygrothermal behavior of wood at high temperature is analyzed, taking into account all the phases present such as wood, liquid water in free and bound forms, and water vapor. The moisture transport phenomenon includes water vapor convection and diffusion along with capillary water convection in the pores of the particle as well as bound water diffusion in the solid wood. Local thermodynamic equilibrium is assumed, which is represented by a sorption isotherm that relates the moisture contents in the solid and gas phases. The equations of the model were solved numerically using the commercial IntroductionWood continues to be the principal raw material for a large number of products in construction and furniture industries, although other competitive materials such as metals and plastics are also available. To ensure a suitable and usable end product and to prevent undesirable biochemical reactions and microbiological growth, most of the moisture present in wood must be removed. In most industrial drying operations, heat is supplied externally to wood by air or superheated steam to evaporate the moisture. During these conventional heat treatment processes (drying), the wood is heated to about 120°C and the water is removed by evaporation. Full descriptions of the moist air and superheated steam drying models are given elsewhere. [1][2][3][4] The high-temperature treatment of wood is different from that of conventional drying. This technique offers an alternative to the chemical treatment of wood, which uses traditional oil (creosote and pentachloro-phenol) and chromated copper arsenate (CCA). These chemicals are toxic and, thus, harmful when they are released into the environment. During the hightemperature treatment process, the wood species with different moisture contents are heated slowly up to 200 -230°C in a hot inert gas atmosphere. This treatment reduces the hydrophilic behavior of wood by modifying the chemical structure of some of its components.Several physical mechanisms contribute to the migration of moisture in wood during its removal. In a porous solid medium containing free water, bound water, vapor, and air or any other gas, the moisture transport through the medium can be in the form of either diffusion or capillary flow driven by individual or combined effects of the moisture, temperature, and pressure gradients. The predominant mechanisms that control the moisture transfer depend on the hygroscopic nature and properties of the materials as well as the heating conditions.In the literature, a number of investigations have been undertaken by various researchers [5][6][7] to study the heat and moisture transfer in porous materials. Luikov 5 developed a set of Correspondence concerning this article should be addressed to R. Younsi at ryounsi@uqac.ca. © 2006 American Institute of Chemical Engineers 2340AIChE Journal July 2006 Vol. 52, No. 7 coupled partial differential equations (PDEs) to describe the heat and mass transport in capillary porous ...

An experimental and theoretical investigation of the thermal treatment of wood (Fagus sylvatica L.) in the range 200–260°C

“…In most of these models the diffusion of water vapour in the gas phase is neglected and the total pressure is approximated using only the water vapour pressure. The gases from the pyrolysis reaction are generally not taken into account in the formulation [40].…”

S-REA (spatial reaction engineering approach): An effective approach to model drying, baking and water vapor sorption processes

“…The model may also be used to predict product quality during the processes. The models may be classified into chemical engineering research and design x x x ( 2 0 1 5 ) xxx-xxx capillary system models (Younsi et al, 2006a(Younsi et al, ,b, 2007Kharaghani et al, 2012). These models capture the physics of the process in detail but they tend to require a lot of constants which need to be determined from several sets of experiments.…”

A numerical study on drying of porous media

“…Non-isothermal diffusion coefficient of porous body in vapor phase, D tv =1(10) 12 Non-isothermal diffusion coefficient of porous body in liquid phase, D tl =1(10) 12 Iso-thermal diffusion Coefficient of porous body in vapor phase, D mv =1(10) 12 Iso-thermal diffusion Coefficient of porous body in liquid phase, D ml =1(10) 8 Enthalpy …”

“…Younsi et al [12] studied conjugate heat and mass transfer during high-temperature heat treatment behavior numerically. They solved model equations by the commercial package FEMLAB for the temperature and moisture content.…”