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MDF): Theory and Experiment Chemical Engineering Science 59 (2004) 735–745 www.elsevier.com/locate/ces High frequency heating of medium density ÿberboard (MDF): theory and experiment Celeste M.C. Pereiraa, Calixte Blanchardb, Luisa M.H. Carvalhoa, Carlos A.V. Costaa;∗ aLEP, Laboratory of Process, Environment and Energy Engineering, Faculdade de Engenharia, Depto de Engenharia Quimica, Universidade do Porto, Rua Dr Roberto Frias s/n, 4200-465 Porto, Portugal bCTBA, Centre Technique du Bois et d’Ameublement, 33028 Bordeaux, Cedex, France Received 22 February 2002; received in revised form 15 July 2003; accepted 14 September 2003 Abstract Medium-density ÿberboard (MDF) is a wood-based panel manufactured from wood ÿbers with a synthetic adhesive through the conduction ofheat fromthe hot platens. One alternative is the use ofhigh-frequency(HF) heating, which has the advantage ofreducing the press cycle, the platens temperature and the post-curing time, with constancy ofresin formulation.For this approach, an electromagnetic (EM) heating model was developed and coupled with a three-dimensional model for heat and mass transfer and resin polymerization already existent. A solution ofthe Maxwell equations is used to determine the spatial distribution ofthe electric ÿeld inside the material and the absorbed power. MDF mat was assumed as a porous and homogeneous material at macroscopic level and a dielectric mixture at microscopic level (volume ofcontrol). The mat dielectric properties (wood ÿbers/particles and UF resin) were estimated using a “rule of mixture” and depend on local temperature and moisture content during the heating process. This dynamic model was used to predict the evolution ofthe local variables related to heat and mass transfer(temperature and moisture content), as well as the variable connected to the EM behavior (dielectric properties ofthe mat). The model performancewas analyzed using the experimental results ofHF (13 :56 MHz) MDF heating. It was concluded that the model could suitably predict the evolution ofthe internal mat temperature during heating. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Porous media; Modeling; Processing; Radiation; Simulation; Materials 1. Introduction asymmetric molecules and water. In fact, water molecules absorb EM radiation very well, as do the sprayed resin in Medium-density ÿberboard (MDF) is a wood-based sheet wood ÿbers. Furthermore, this kind ofvolumetric heating material manufactured from wood ÿbers bonded with a syn- results on faster moisture leveling and more uniform resin thetic resin adhesive (Maloney, 1989), usually by conduc- cure, leading to a quicker and more eEcient hot-pressing tion heating from the surface. An alternative and interesting process. way for the heating process is the use of electromagnetic EM radiation heating has been used in the forest products (EM) radiation, since the energy is delivered directly to the industry for over 60 years and modern-day improvements whole composite, thus not relying only on conduction from in the equipment make this technique especially useful for the surface. In EM heating, the energy directly transferred polymer curing in the manufacturing of wood composite into the composite is absorbed by the asymmetric molecules products, such as particleboard, MDF and oriented strand and transformed into heat. The heat generated in the medium board, as well as some wood/non-wood composites (Pound, depends primarily on three variables: the intensity ofthe 1973; Col et al., 1982; Maloney, 1989; Alvarez-Novoa, ÿeld applied, its frequency and the dielectric loss factor 1990; Annet, 1992). There are some potential advantages in ofthe medium. Wood ÿbers are good electrical insulators using this type ofheating as a pre-heating step or co-heating but interact with EM radiation due to their composition of in wood-based composites manufacturing. For instance, reduction ofthe press cycle and the platens temperature, invariance ofadhesive formulationregarding thickness and ∗ Corresponding author. Tel.: +351-22-508-1670; fax: +351-22-508-1449. density and reduction ofpost-curing time ( Maloney, 1989). E-mail address: [email protected] (C.A.V. Costa). So being, EM radiation heating may constitute a good 0009-2509/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2003.09.038 736 C.M.C. Pereira et al. / Chemical Engineering Science 59 (2004) 735–745 alternative to improve production, yield thicker boards Nevertheless, only some ofthese consider the moisture and make year-long operating conditions more uniform. transport during the heating or drying process (Ayappa However, the technology is not yet well optimized and et al., 1991; Constant et al., 1996; Ni et al., 1999; Tanmay controlled, due to the not so good comprehension ofthe and Ayappa, 2001). All these models acknowledge the ma- interaction between EM ÿelds and wood-based composites. terial as non-magnetic but only some consider its dielectric EM radiation is usually divided into high frequency properties varying with moisture content and temperature (HF) and microwaves (MW). The range ofthe formeris (PerrÃe and Turner, 1997; Ni et al., 1999). As for the EM ÿeld 1–400 MHz and the latter term is used for radiation with distribution calculation, some models use a very simpliÿed frequencies above 400 MHz (usually 0.95 and 2:45 GHz). decay equation to calculate the absorbed power distribution MW and HF methods have been successfully applied in in- or electric ÿeld intensity distribution inside the material dustry to dry and/or thermally transform materials (Roussy (Wei et al., 1985; Pangrle et al., 1991; Ni et al., 1999), whilst and Pearce, 1995). When heating or drying materials, other authors presented the resolution ofMaxwell equations HF technology is generally employed for adsorbed water in order to determine the same variable inside materials with whilst MW technology is used for liquid or capillary water uniform plane MW applied (Ayappa et al., 1991; Anindita (Jacomino and Rochas, 1999). The chosen frequency (HF et al., 1998; Tanmay and Ayappa, 2001). For instance, or MW) in each speciÿc case is also usually dependent on Ayappa et al. (1991) showed the Maxwell equations res- material size and shape and on the power levels needed for olution in order to calculate the electric ÿeld intensity processing. distribution inside a multilayer material with uniform plane For wood-based composites, there are some experimen- waves applied in both sides. tal works reporting both the positive and negative eJects In conclusion, what can be said is that so far there are on ÿnal board properties when using HF heating during hot no research works reported on heating ofwood-based com- pressing (Fahey, 1976; Woodson and Stevens, 1977). These posites with EM radiation, particularly physical models and authors refer improvements in some ÿnal properties and experimental data for their validation and that the informa- characteristics ofÿberboard, which include better machin- tion on the dielectric properties ofthese materials is very ability and coating ability, higher-dimensional stability and scarce. However, dielectric properties ofwood are well doc- internal bond and improved edge screw holding and hard- umented at diJerent frequencies, densities, moisture con- ness. Because the faces of the ÿnished board will proba- tents and temperatures (Torgovnikov, 1993). The main ob- bly have lower density than conventionally pressed boards, jective ofthe current study is the understanding ofseveral slightly lower bending strength and stiJness can be expected. coupled phenomena: heat and mass transfer, EM behavior The experimental approach in the study ofthe relation- ofthe mat and adhesive polymerization in the cure ofMDF. ships involving EM material behavior under an EM ÿeld For that, physical models were developed where these phe- has inherent limitations. Although general trends can be ob- nomena are coupled and partially validated with a series of served, the data is only valid for the range of conditions experiments. tested. In order to overcome such drawbacks, several em- pirical (Pound, 1973; CTBA, 1998) and numerical models were developed in the last years to better understand the 2. Experimental HF heating coupled phenomena that occur during EM heating or drying ofmaterials ( Wei et al., 1985; Ayappa et al., 1991; Pangrle Equipment for EM radiation heating applications is usu- et al., 1991; Constant et al., 1996; PerrÃe and Turner, 1997; ally composed by the generator, the wave guide and the Ni et al., 1999). condenser (applicator+material to heat). The applicator is For a combined convective/MW wood drying process, an important part ofthe system whose design depends on PerrÃe and Turner (1997) presented a quite complete and the dimensions ofthe object or material to be heated, which sophisticated theoretical model, which is a combination of will determine the choice ofHF or MW radiation ( Roussy the 3D Maxwell equations for wood and wave-guide system and Pearce, 1995). The total power to be applied will also and 2D equations for heat and mass transfer. The solution of aJect the choice ofthe frequencyrange. Iflower power is the Maxwell equations is assessed in the time domain using desired (1–20 kW), MW work well. For high-power appli- a ÿnite diJerence technique. It is observed that the CPU time cations (¿ 200 kW), HF systems are usually required, mak- required to compute the EM ÿelds with this numerical tool ing them more suitable for MDF heating applications. is excessive and, as much as
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