Microscopic analysis of wood as a porous media during drying

Ma, Qing. (2003). Microscopic analysis of wood as a porous media during drying PhD Thesis, School of Engineering, The University of Queensland.

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Author Ma, Qing.
Thesis Title Microscopic analysis of wood as a porous media during drying
School, Centre or Institute School of Engineering
Institution The University of Queensland
Publication date 2003
Thesis type PhD Thesis
Supervisor Assoc. Prof. Victor Rudolph
Total pages 213
Collection year 2003
Language eng
Subjects L
290000 Engineering and Technology
670000 - Manufacturing
Formatted abstract
The moisture content in a wood material has a significant influence on the material physical properties and is consequently of great practical and commercial importance. This is reflected in the sophisticated mathematical models that have been developed to simulate and optimize the drying process. These models have two important requirements that have largely remained unmet, reducing their applicability and value. These are: (1) detailed knowledge of the thermodynamic relationships between the moisture and the wood substance, and (2) the physical relationships and transport parameters of the process. The latter needs to capture the microstructure characteristics of materials and the specific transport mechanism occurring at a microscopic level, and then formulate this to a macroscopic result. This thesis presents an experimental and theoretical study of transport phenomena and thermodynamic properties during moisture sorption or desorption in wood at different scales (microscopic scale and macroscopic scale). It aims to find way to predict the moisture transport behaviors and estimate macroscopic properties based on the information obtained from the observation and analysis at a microstructure level. This is then developed into the macroscopic drying formulations.

The moisture transport phenomena that include the capillary and hygroscopic phenomena and the anisotropic swelling and shrinking behaviors that occur during moisture sorption and desorption processes were observed and recorded for two species (Caribbean pine and Hoop pine). An in-situ method, make measurement over a moisture range from ovendried to full saturation, was developed, using an environmental scanning election microscope (ESEM). This microscopic information obtained at the microscopic scale was quantitatively analyzed using an image analysis technique. The ratio of the dimensional change and relationships between the dimensional changes and the moisture content depend on the local density (earlywood and latewood) of species and the directions (tangential, radial and longitudinal). An approximate addition relationship exists among the transverse area, cell wall and lumen area at the same moisture content. The size of lumens varies considerably with the moisture content, which suggests that the size change of lumens should be taken into account when building a model based on the microstructure of wood. The ratio of cell wall shrinkage or swelling for earlywood and latewood is almost the same although the transverse area of the latewood exhibits much greater shrinkage or swelling than the earlywood at the same experimental conditions. The distribution of shrinkage and swelling in radial and tangential direction varies with the moisture content in wood and the results suggest that swelling anisotropy may mostly be affected by the differential fibril angles within cell wall. Both hysteresis of moisture content and dimensional change on transverse area demonstrated a coincident action.

The evaluations of the equilibrium data and thermodynamic properties during moisture sorption in wood material represents one focal point in this dissertation. The modified UNIQUAC model using the group contribution concept was used to correlate the activity coefficients of bound water in wood. This is apparently the first time this method has been applied to estimation for the equilibrium data of a wood-moisture system. Both of the experimental equilibrium data obtained from this research and from the literature are well described by the modified UNIQUAC model. The optimum interaction parameters between water and the lingo-cellulosic polymers in wood material have been extracted and are presented. A comprehensive investigation was also conducted to evaluate the fundamental thermodynamic properties of bound water in wood.

Unsteady-state moisture transport in wood is more important than steady-state flow in practical wood treatment systems. An unsteady-state diffusion model was developed and unsteady-state sorption measurements for two different species were performed over a moisture range from oven-dried. The experiments allowed estimation of the moisture diffusivity at the different moisture content levels. The diffusivity is concentration-dependent and the relationship with the moisture content depends on the microstructure of material and the moisture transport directions. The effects of the heterogeneity and anisotropy of material on the diffusivity of moisture reduce as moisture content increases. Based on the results obtained, possible mechanisms occurring in unsteady-state are put forward to explain the moisture transport behaviors and the experimental data.

During moisture transport phenomena in wood, non-Fickian behavior can not be disregarded, since it strongly affects on the total rate and behavior of moisture transport in wood. This is the main reason for the deviation between the experimental data and the prediction by the many current diffusion models based on the Pick's law. The dissertation develops a fundamental study on the non-Fickian behavior occurring during moisture sorption. Non-Fickian behaviors of moisture diffusion become pronounced and the features of sorption curves almost change from Fickian type to non-Fickian in the sequence: Fickian→ pseudo-Fickian→ sigmoid, with the increase of initial moisture content of samples. Comparing the degree of non-Fickian behavior in different transport directions, the magnitude of deviation from Fickian is proportional to the extent of dimensional change at the same experimental conditions. Two quantitative methods were used to measure the degree of non-Fickian behavior, i.e., the relaxation time and the non- Fickian parameters. The discrepancies of moisture transport behavior among specimens and in different directions trend to reducing at higher moisture contents.

The dissertation presents new work related to a wood-moisture system in the areas of transport parameters, transport mechanism, thermodynamic properties and non-Fickian behavior.
Keyword Wood -- Chemistry

Document type: Thesis
Collection: UQ Theses (RHD) - UQ staff and students only
Citation counts: Google Scholar Search Google Scholar
Created: Fri, 24 Aug 2007, 18:17:00 EST