André Zoulalian

Wettability changes and mass loss during heat treatment of wood

A new product called torrefied or retified wood can be obtained by mild pyrolysis of wood in a temperature range between 200 and 2608C and under inert atmosphere. Such heat-treated wood dramatically reduces its hygroscopicity and improves its dimensional stability and durability. The main drawback of the torrefied wood is its high brittleness. Another property, often mentioned but not well investigated, is its hydrophobic character (Pétrissans et al. 2003). In fact, the heat-treated wood becomes rather hydrophobic, which could cause some severe problems during varnish or paint deposition. The aim of our study was to investigate the wettability changes and the mass loss during heat treatment of wood at different temperatures. A comparison of these two parameters could add valuable information regarding wettability changes and chemical degradation resulting from retification. Heat treatments were carried out in the temperature range of 40 to 2608C. Four European wood species – pine, spruce, beech, and poplar – were studied here. The contact angle values were measured by the Wilhelmy technique. Wood wettability is generally difficult to measure. For example, the heterogeneous and porous character of wood causes the so-called contact angle hysteresis, that is, a distinct difference between the advancing (ua) and the receding (ur) contact angles (Liptáková and Kúdela 1994). Moreover, the wood extractives may also contaminate the probe liquids during measurement (Wålinder and Johansson 2001). This hysteresis effect can also be related to the chemical heterogeneity (Menawat et al. 1984) and/or to the solid roughness (Dettre and Johnson 1964). The Wilhelmy method, chosen for this study, is not a conventional technique for wood contact angle measurement but gives good results on this material (Gardner et al. 1991; Wålinder and Johansson 2001; Wålinder and Ström 2001; Pétrissans et al. 2003).

Characterisation of water transfer in a low temperature convective wood drier: influence of the operating parameters on the mass transfer coefficient

Picea abies Karst.). From the synthesis of the results, a general correlation has been obtained permitting to accurately calculate this transfer coefficient. Moreover, it justified the use of the simple mathematical model chosen to represent the variations of the parameters characterising the wood and air during a drying cycle.Picea abies Karst). Im Anschluß an die Zusammenfassung der Ergebnisse wurde eine allgemeine Korrelation, mit der dieser Koeffizient auf eine bestimmte Weise berechnet werden kann, erzielt. Diese rechtfertigte die Wahl eines einfachen Simulationsmodells, das die Veränderung der charakteristischen Luft- und Holz- parameter während der Trocknung darstellt.

Solar drying of pine lumber: Verification of a mathematical model

This work presents verification of a mathematical model for drying of a wood stack in a greenhouse type solar dryer. A simplified heat and mass transfer numerical model has been developed with input parameters based on the actual metrological data of a Moroccan climate. For its validation, a comparative study is performed in this work; the present model is solved to simulate the solar drying of pine wood using experimental data of previous wood drying experiments. The average relative discrepancies between the model predicted and experimental data are 1,2% for wood moisture content, 1% for drying air temperature and 5% for the air relative humidity. The close agreement between the predicted and experimental results shows the ability of the model to reproduce experimental drying data for wood.

Introducing an overall mass-transfer coefficient for prediction of drying curves at low-temperature drying rates

The aim of this study is to justify that the drying rate of wood can be represented by a phenomenological model defining a driving force expressed as the difference between the average wood moisture content and the equilibrium wood moisture content. The results show that the mathematical relation proposed is valid when introducing an overall mass transfer coefficient, K, at low temperature wood-drying rates.

EXPERIMENTAL STUDY OF THERMAL EFFECT ON OLIVE WOOD POROUS STRUCTURE DURING CARBONIZATION

The study presented in this paper is an investigation on the porosity changes induced by the carbonization of Tunisian olive wood. The porosity is measured by mercury porosimetry. The experimental results show that the total porosity of carbonized wood increases with the temperature. It should be underlined that the formation of some macropores during the increase of the temperature is certainly due to the breaking of the cell wall by the mercury penetration. The weakness of the cell wall is observed when the degradation rate of the three pseudo components of wood, the notion of which has been introduced by the authors in a previous article [Grioui et al. 2006], becomes high which corresponds to the temperature range between 523 K and 548 K.

Longitudinal Permeability and Diffusivity of Steam in Beech Determined with a Wicke-Kallenbach-Cell

Summary To create a model of beech wood drying in the hygroscopic range, knowledge of longitudinal permeability and diffusivity parameters of wet air in wood is needed. These data were obtained using a Wicke-Kallenbach-cell running in an open system and crossed either by an inert gas (nitrogen) or by two streams of two inert gases (helium and nitrogen), that prevent the adsorption phenomena in wood. Taking into account the relations between apparent and molecular diffusivities, the diffusivities of steam could be calculated from those of helium. Longitudinal diffusivity and permeability increase slightly with the moisture content of wood; the average values obtained at the ambient temperature (20°C) for the axial diffusivity (8. 1. 10−6 m2/s) and for the axial permeability (2. 2. 10−11 m2) are compatible with those reported in the literature for this species.

Modelling of a solar wood dryer with glazed walls

Este trabajo presenta un estudio de un secador solar de madera en un clima marroqui. El proceso de secado de madera es investigado teoricamente desarrollando un modelo matematico basado en datos climaticos reales. El modelo da a cada momento, la humedad y la temperatura del secador (aire y paredes) asi como la temperatura y la humedad de la madera. Para la verificacion experimental, este modelo de secado es introducido en el programa de simulacion con datos experimentales de secado de madera de los experimentos realizados en dos secaderos situados en dos lugares diferentes. Los resultados muestran un acuerdo razonable entre la humedad de la madera predicha y medida experimentalmente.

Controlled Dimensional Variations of a Wood-Cement Composite

At present wood-cement composites are used in pressed panels and unpressed blocks. These elements present interesting physical, acoustic and mechanical properties as well as good manufacturing properties. However, dimensional variations with external humidity remain high and prevent external use of the composites without an additional treatment. Dimensional variations are linked with wood, the wood-matrix interface and the cement matrix. A specific treatment for each of these previous elements allows reduction of dimensional variations. However, these deformations still remain higher than the standard values measured under conditions fixed by the French standard i.e. 1 mm/m. On the other hand, an appropriate combination of the three treatments achieves the expected objective at a reasonable cost compared to the one obtained for a conventional cement-sand-gravel composite.

Using a parametric study to analyse the performance of wood solar dryers with glazed walls

Parameters characterizing wood and air were studied in order to analyze the performance of a wood solar dryer functioning under Moroccan climate. A mathematical model based on the climate data of Rabat city was used to investigate theoretically the wood drying process. Two wood speciesthuya(Tetraclinis articulate) and pine (Pinus pinaster)-were examined in the present study. The results obtained by computer simulations are in good agreement with the experimental values. Furthermore, our findings indicate that the use of a global mass transfer coefficient for low temperature convectiveand homogenous drying conditions allows the influence of the principal operating parameters (wood thickness, wood density, air temperature, air velocity and ventilation mode) on the drying time to be estimated with great accuracy. Analyses pertaining to the two studied wood species revealed that(1)pine dries more quickly than thuya, (2) the drying process is faster in the summer relative to other seasons, (3) increasing the air velocity by 100% results in a 20% reduction in the drying time, and (4) continued ventilation reduces the drying time by 43%.

A new air dehydration process for wood dryers: Part I. Kinetic and quality of drying.

Summary In conventional wood dryers, air is dehumidified by condensation or by its partial remove. A new air dehydration process is proposed for low temperature convective wood drying. This process consists in using an absorber working with an organic absorbent, the triethylene glycol (TEG) that is pulverised in a venturi type of device. The adaptation of this process well known for hydrocarbon gas dehydration and its interest for wood drying operations are discussed. The prototype installation is presented and the impact of the absorption on the evolution on drying cycles are investigated. The performances of the absorber are evaluated quantitatively and qualitatively. The average drying kinetic is quantified with a global mass transfer coefficient and the drying quality is characterised through the measurements of defects generated during drying.