DRYING WOOD: A REVIEW - PART I

ABSTRACT

This paper reviews the technology of wood drying and recent developments in the field. The importance of drying is reviewed first. Then the fundamentals of wood properties as they relate to drying are discussed, including how water is held in wood, how it moves through wood, and how drying stresses that lead to drying defects develop. Traditional drying technology for lumber, veneer, and heavy timbers is then presented. With this background, the discussion moves to recent developments in drying technology that have been implemented commercially, such as high-temperature drying, low-temperature drying, continuously rising temperature drying, vacuum drying, and advances in process control. Finally, a discussion is presented on drying technology of the future. This topic describes processes that offer potential improvements in drying technology, but are still in the research and development stage.     

Canadian Developments in Kiln Drying

The objective of this paper is to summarize recent trends related to industrial kiln drying with especial emphasis on the drying of softwood dimension lumber in Canada. The paper explores future trends by analyzing the current market condition and wood products requirements. The paper also summarizes current research developments in the area of kiln lumber drying that was published by Canadian universities and research institutes. This includes basic research, such as wood physics and computer simulation, and applied research, such as non-conventional drying strategies and new technologies and sensors. A last note is also included to mention a kiln supervisor tool that was developed in Canada to analyze information related to kiln performance indicators. The kiln supervisor provides important trends for mills aiming to optimize their drying processes and realize gains without substantial investments in capital and other resources.     

A Method for Studying Dynamic Sorption Behavior of Small Wood Samples

A method for determining mass changes (～ 0.0001 g) in small (～ 4-mm cubes) wood samples using the resonance of small mechanical springs mounted on piezoelectric sensors is presented. The size of the sample allows individual heartwood or sapwood, earlywood, latewood, and compression wood samples to be measured in an oscillating relative humidity environment and compared in terms of time constant and moisture gain. Since the apparatus is cost effective, many replicates can be achieved in one experiment. The use of small wood samples such that all tracheids are exposed removes the effect of structure on bound water transport. This, in combination with the ability to measure the response to small perturbations in relative humidity, presents the possibility to determine gaseous diffusion coefficients as a function of moisture content for individual wood types.

Preliminary, representative results of a study comparing the effect of drying temperature on sorption behavior of wood dried at 20, 90, and 150°C are presented.     

木材干燥的国内外现状与发展趋势

阐述了常规蒸汽（热风）干燥、除湿（热泵）干燥、真空干燥、微波与高频干燥等各种木材干燥技术的国内外现状以及我国木材干燥技术与国际水平的差距；介绍了国内外木材干燥的理论研究动态；指出了今后木材干燥技术的发展趋势。     

THE INFLUENCE OF THE DRYING MEDIUM ON HIGH TEMPERATURE CONVECTIVE DRYING OF SINGLE WOOD CHIPS

High temperature convective drying of single wood chips with air and superheated steam respectively is studied theoretically. The two-dimensional model presented describes the coupled transport of water, vapour, air and heat. Transport mechanisms included are the convection of gas and liquid, intergas as well as bound water diffusion. In the initial part of the drying process, moisture is transported to the surface mainly due to capillary forces in the transversal direction where evaporation occurs, As the surface becomes dry, the drying front moves towards the centre of the particle and an overpressure is simultaneously built up which affects the drying process

The differences between drying in air and steam respectively can be assigned to the physical properties of the drying medium. The period of constant drying rate which does not exist (or is very short) in air drying becomes more significant with decreasing amounts of air in the drying medium and is clearly visible in Dure superheated steam drying. The maximal drying rate is larger in air drying, and shorter drying times are obtained since the heat flux to the wood chip particle increases with increasing amounts of air in the drying medium. The period of falling drying rate can be divided into two parts: in the first, the drying rate is dependent upon the humidity of the drying medium whereas in the second, there is no such correlation.     

过热蒸汽干燥技术的研究进展

过热蒸汽干燥技术具有安全、节能、环保等诸多优点,越来越受到广泛关注。对过热蒸汽干燥技术进行了简介,分析了其优缺点。总结了过热蒸汽干燥的理论研究进展,并对过热蒸汽干燥技术在食品、木材、纸张、污泥和和褐煤等物料的应用现状进行了综合评述。总而言之,过热蒸汽干燥技术是一种先进的干燥技术,具有广阔的应用前景。     

THE INFLUENCE OF THE DRYING MEDIUM ON HIGH TEMPERATURE CONVECTIVE DRYING OF SINGLE WOOD CHIPS

ABSTRACT

High temperature convective drying of single wood chips with air and superheated steam respectively is studied theoretically. The two-dimensional model presented describes the coupled transport of water, vapour, air and heat. Transport mechanisms included are the convection of gas and liquid, intergas as well as bound water diffusion. In the initial part of the drying process, moisture is transported to the surface mainly due to capillary forces in the transversal direction where evaporation occurs, As the surface becomes dry, the drying front moves towards the centre of the particle and an overpressure is simultaneously built up which affects the drying process

The differences between drying in air and steam respectively can be assigned to the physical properties of the drying medium. The period of constant drying rate which does not exist (or is very short) in air drying becomes more significant with decreasing amounts of air in the drying medium and is clearly visible in Dure superheated steam drying. The maximal drying rate is larger in air drying, and shorter drying times are obtained since the heat flux to the wood chip particle increases with increasing amounts of air in the drying medium. The period of falling drying rate can be divided into two parts: in the first, the drying rate is dependent upon the humidity of the drying medium whereas in the second, there is no such correlation.     

HIGH TEMPERATURE DRYING FROM THE THREE PRIMARY ORTHOGONAL SURFACES OF LOBLOLLY PINE

While several permeability studies have been performed, little information is available on the flow of water in wood along its three orthogonal axes during high temperature drying. Much southern yellow pine construction lumber is dried at temperatures above 100°C, thus how water moves in wood is of concern. In this experiment individual Loblolly pine (Pinus taeda) sapwood cubes were coated on two of their three orthogonal faces (longitudinal, radial, and tangential) and the effect of this sealing on the drying rate was determined. From these cubes, drying rates from each of the three primary planes were determined. The free water movement/evaporation between the radial and tangential faces was not statistically different, while the longitudinal rate was approximately three times faster. In commercial practice, the faster longitudinal rate is negated by the very small portion of transverse surface area relative to total lumber surface area. This partially explains why end checking and moisture variation in high temperature dried southern yellow pine lumber are not significant causes of degrade. There was no statistically significant difference in permeability across the seasons tested.     

A Mass Transport Model for Drying Wood under Isothermal Conditions

Mass transport in wood during drying can have different mechanisms at different periods of drying. Depending on the current moisture content (MC) and the structure of the wood, the driving forces for the mass transport are essentially different. Above the fiber saturation point (FSP), the lumens are partially saturated and the transport of liquid (free) water occurs as a consequence of capillary action. On the other hand, below the FSP, bound water within the cell walls is conveyed by diffusion, and water vapor in the lumens moves under influence of pressures gradient. Based on these considerations, a unified model is presented that takes into account the transport of the different moisture phases. Simulation of the drying of a Norway spruce sample at 50°C from about 135 to 7% MC is carried out using the finite element method (FEM). Comparison between the simulated average MC and the experimental observations obtained from X-ray computed tomography (CT) shows reasonable agreement. Possible simplifications in the model are briefly discussed as well as some aspects of the numerical implementation. Finally, the influence of absolute permeability on the average MC is studied.     

A Mass Transport Model for Drying Wood under Isothermal Conditions

Mass transport in wood during drying can have different mechanisms at different periods of drying. Depending on the current moisture content (MC) and the structure of the wood, the driving forces for the mass transport are essentially different. Above the fiber saturation point (FSP), the lumens are partially saturated and the transport of liquid (free) water occurs as a consequence of capillary action. On the other hand, below the FSP, bound water within the cell walls is conveyed by diffusion, and water vapor in the lumens moves under influence of pressures gradient. Based on these considerations, a unified model is presented that takes into account the transport of the different moisture phases. Simulation of the drying of a Norway spruce sample at 50°C from about 135 to 7% MC is carried out using the finite element method (FEM). Comparison between the simulated average MC and the experimental observations obtained from X-ray computed tomography (CT) shows reasonable agreement. Possible simplifications in the model are briefly discussed as well as some aspects of the numerical implementation. Finally, the influence of absolute permeability on the average MC is studied.     

HIGH TEMPERATURE DRYING FROM THE THREE PRIMARY ORTHOGONAL SURFACES OF LOBLOLLY PINE

While several permeability studies have been performed, little information is available on the flow of water in wood along its three orthogonal axes during high temperature drying. Much southern yellow pine construction lumber is dried at temperatures above 100°C, thus how water moves in wood is of concern. In this experiment individual Loblolly pine (Pinus taeda) sapwood cubes were coated on two of their three orthogonal faces (longitudinal, radial, and tangential) and the effect of this sealing on the drying rate was determined. From these cubes, drying rates from each of the three primary planes were determined. The free water movement/evaporation between the radial and tangential faces was not statistically different, while the longitudinal rate was approximately three times faster. In commercial practice, the faster longitudinal rate is negated by the very small portion of transverse surface area relative to total lumber surface area. This partially explains why end checking and moisture variation in high temperature dried southern yellow pine lumber are not significant causes of degrade. There was no statistically significant difference in permeability across the seasons tested.     

Optimization of Solar Kiln for Drying Wood

Theoretical investigation of the physical process of solar drying of timber based on conventional heat and mass transfer equations is presented. The governing equations and boundary conditions of the mass diffusion through the wood timbers are derived; also the governing equations of the components of the solar kiln are presented. The finite difference technique is used to solve the set of these equations by means of a simulation program that is based on object-oriented approach. The simulation program is used to investigate the effect of several design parameters on the drying rate and duration of the wood timbers in order to accomplish the drying process with minimal drying defects. These parameters include the ventilation conditions that control the drying schedule inside the solar kiln, wood volume as a ratio to the solar kiln absorber area, wood timber thickness, season of drying, the drying air velocity, and the stresses that formed on the timber boards due to drying with these several parameters, leading to derive the limit of damage for a selected local wood type. The selected local wood type is Casuarina, which is common in Egypt, and it is commonly used in many simple industries.     

Optimization of Solar Kiln for Drying Wood

Abstract

Theoretical investigation of the physical process of solar drying of timber based on conventional heat and mass transfer equations is presented. The governing equations and boundary conditions of the mass diffusion through the wood timbers are derived; also the governing equations of the components of the solar kiln are presented. The finite difference technique is used to solve the set of these equations by means of a simulation program that is based on object-oriented approach. The simulation program is used to investigate the effect of several design parameters on the drying rate and duration of the wood timbers in order to accomplish the drying process with minimal drying defects. These parameters include the ventilation conditions that control the drying schedule inside the solar kiln, wood volume as a ratio to the solar kiln absorber area, wood timber thickness, season of drying, the drying air velocity, and the stresses that formed on the timber boards due to drying with these several parameters, leading to derive the limit of damage for a selected local wood type. The selected local wood type is Casuarina, which is common in Egypt, and it is commonly used in many simple industries.     

The challenge of antimicrobial glazed ceramic surfaces

The resistance that microorganisms develop to antibiotics is a worldwide challenge. The antimicrobial agents as disinfectants for surface treatments are widespreadly used to prevent the proliferation of microorganisms, but their use should be repeated over time to ensure a complete microbe-free surface. Surfaces with permanent antimicrobial properties suppose a recent demand in materials science for functional polymeric coatings, metals, treated wood or ceramic glazed tiles. Whereas polymeric coating has been extensively studied, the antimicrobial functionality on ceramic glazed surfaces is not completely achieved. This work reviews glazed ceramic tiles developments in antimicrobial and virucidal surfaces. The main antimicrobial physical or chemical mechanisms have been described as the base to develop active glazed surfaces. The main tests required to evaluate the antimicrobial response in glazed ceramic tiles are also summarized. The high temperature required in the ceramic processing is the key point to achieve a micro/nanostructure that potentiates the antimicrobial and virucidal response of the glazed surfaces. A discussion on recent developments as well as the main routes and challenges to obtain permanent surfaces with antimicrobial and virucidal response is provided.     

Sicherheitstechnische Beurteilung von Trocknungsverfahren in der chemischen Industrie

Safety Engineering Assessment of Drying Processes in the Chemical Industry . Dried products encountered in the chemical industry are frequently flammable and susceptible to dust explosions. Moreover, some of the products to be dried contain flammable solvents. Since the two product states moist and dry co-exist in a drying process, the dry potenially explosive solid, flammable solvent vapours, and fuel gases could all be present in a drying plant. A comprehensive safety engineering assessment of the entire drying process is therefore absolutely essential in the planning of new plant and in the examination of existing plant. The starting point and basis of safety engineering assessment is a knowledge of just a few characteristic safety data. If these are known then a decision can be made whether safety measures are necessary and if so what measures can asure safe operation of a plant. This contribution describes the approaches adopted in deciding upon such measures. Recent developments in the area of safety assessment of flammable dusts are included. An example is then presented to illustrate how the joint efforts of experts in the fields of safety engineering and drying technology can assure optimum plant safety, product quality, and cost-effectiveness.     

Emerging food drying technologies with energy-saving characteristics: A review

Abstract

Drying is one of the most vital preservation techniques used in the food industry. It demands different levels of energy to produce commercially high-quality-dried food products. Novel drying technologies minimize deterioration of the food ingredients and produce novel products for consumers. In recent years, there have been many developments in the technology connected with the industrial drying of foods. Recent research has shown that novel food drying technologies could be utilized to improve the efficiency of drying by lowering the energy consumption and also to enhance the product quality. This article reviews selected energy- saving techniques in drying and discusses some novel combined drying technologies. These include solar-assisted, infrared-assisted, microwave-assisted and similar hybrid drying methods for food drying. Recommendations are also made for future research and development.     

喷雾干燥的研究进展

介绍了国内外喷雾干燥领域的最新研究进展,阐述了喷雾冷冻干燥、超临界喷雾干燥等方面的研究结果,探讨了关于喷雾干燥过程数学模拟的研究方法和结论,总结、归纳并提出了目前国内外较新的关于喷雾干燥的研究课题和方向。     

近年来喷雾干燥技术研究进展和展望

喷雾干燥是目前市场上常见的一种干燥方式,喷雾干燥过程的复杂性研究已经有很多成果,喷雾干燥的低热效率给设备研究人员和企业界留下了较大的研发空间,需要不断地去完善其流程和工艺,以满足市场和产品本身的需要。从分析喷雾干燥的研究现状入手,介绍喷雾干燥领域新的研究成果及设备与工艺的发展情况,总结并提出了目前较新的研究课题和方向。     

我国木材干燥技术与研究动态

阐述了我国木材干燥技术应用状况及面临的问题,介绍了我国木材干燥学术活动的情况和理论研究动态。指出以蒸汽为热源的常规干燥仍将占木材干燥的主导地位,约在80％以上;真空干燥、热泵除湿干燥、微波与高频干燥、高温干燥、加压干燥等其它干燥方法都在各自适用的范围内有一定的发展。木材的渗透性、木材干燥过程的水分迁移机理等有关木材干燥的传热传质方面的基础理论研究正在加强;高效节能的木材干燥技术是目前研究的重点课题。我国木材干燥技术正由经验型转向以干燥理论为基础的科学型方向发展。     

喷雾冷冻干燥对颗粒产品形态的影响

喷雾冷冻干燥作为一种新型颗粒制备技术,产品具有尺寸可控、多孔、速溶、流动性好等优点。鉴于该技术在医药、食品、化工等行业生产独特产品的优势,本文综述了国内外喷雾冷冻干燥过程中有关颗粒形成及其形态变化的影响因素,具体介绍了其冻结过程对蛋白质颗粒形态、脂质体颗粒稳定性、疫苗颗粒、无机材料形态等产品的影响。同时还指出了喷雾冷冻干燥技术存在过程不连续、规模化程度不高、低温液体处理不便等问题需要解决,表明合理优化喷雾冷冻干燥工艺及强化干燥过程将是该技术未来研究的重点。