STEAM DRYLNG OF WOOD FUELS

The objective of this work is to investigate some of the important aspects in the design of a steam band dryer for wood fuels. For this purpose the drying of the material in a bench-scale fixed bed dryer has been studied.

Drying times and thermal efficiencies for experiments conducted under different conditions are compared. The investigated materials are soft-and hardwood chips and softwood bark.

The thermal efficiency, the part of the sensible heat which is used during one passage of the steam through the bed, increases with increasing mass load (mass of dry matter per unit area) and with decreasing steam mass flux. At a mass load of 30 kg/m the thermal efficiency is about 0.85 even at steam mass fluxes as high as 0.6 kg/m² s (1.2 m/s). The thermal efficiency proves to be almost independent of pressure and temperature of the steam.

Due to the very inhomogeneous materials the steam mass flow distribution was uneven. This causes a decreased thermal efficiency. When bark     

STEAM DRYING OF WOOD CHIPS IN PNEUMATIC CONVEYING DRYERS

ABSTRACT

A model for a pneumatic conveying dryer is presented. Although the main emphasis is put on superheated steam drying of wood chips, it can be used for other porous materials as well

The model includes a comprehensive two-dimensional model for the drying of single wood chips which accounts for the main physical mechanisms occurring in wood during drying. The external drying conditions in a pneumatic conveying dryer were calculated by applying the mass, heat and momentum equations for each incremental step in dryer length. A plug flow assumption was made for the dryer model and the single particle and dryer models were solved in an iterative manner. The non-spherical nature of wood chips were accounted for by measuring the drag and heat transfer coefficients

Model calculations illustrate the complex interactions between steam, particles and walls which occur in a flash dryer. The drying rate varies in a very complex manner through the dryer. The internal resistance to mass transfer becomes very important in The drying of less permeable wood species such as spruce. Two effects were observed as the particle size was increased: firstly the heat transfer rate decreased, and secondly the residence time increased. To some extent, these effects compensate for each other, however, the net result is that larger chips have a higher final moisture content.     

STEAM DRYING OF WOOD CHIPS IN PNEUMATIC CONVEYING DRYERS

A model for a pneumatic conveying dryer is presented. Although the main emphasis is put on superheated steam drying of wood chips, it can be used for other porous materials as well

The model includes a comprehensive two-dimensional model for the drying of single wood chips which accounts for the main physical mechanisms occurring in wood during drying. The external drying conditions in a pneumatic conveying dryer were calculated by applying the mass, heat and momentum equations for each incremental step in dryer length. A plug flow assumption was made for the dryer model and the single particle and dryer models were solved in an iterative manner. The non-spherical nature of wood chips were accounted for by measuring the drag and heat transfer coefficients

Model calculations illustrate the complex interactions between steam, particles and walls which occur in a flash dryer. The drying rate varies in a very complex manner through the dryer. The internal resistance to mass transfer becomes very important in The drying of less permeable wood species such as spruce. Two effects were observed as the particle size was increased: firstly the heat transfer rate decreased, and secondly the residence time increased. To some extent, these effects compensate for each other, however, the net result is that larger chips have a higher final moisture content.     

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.     

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.     

THE RELEASE OF MONOTERPENES DURING CONVECTIVE DRYING OF WOOD CHIPS

ABSTRACT

The release of volatile organic components (VOC) during high temperature convective drying of wood chips was studied experimentally and theoretically. The drying medium was superheated steam with a pressure of two bar. Two different temperature levels of the drying medium, 160 and 180 °C, and two different materials, Scots Pine and Norway Spruce, were investigated. It was found that the main components released consist of various types of monoterpenes, with α-pinene dominating in each of the two materials. The amount released is dependent on the drying temperature as well as the time of the drying process.

In order to describe the release rate of monoterpenes during drying, two separate models, called the communicating and the non-communicating model respectively, were developed. The mechanisms included for the transport of monoterpenes are, in the communicating model, transport by diffusion and with the advective gas and liquid flow within the tracheids and, in the non-communicating model, diffusion within the resin canal system.

The results obtained using the communicating model largely overpredict the experimental results. To avoid this rapid release, additional mass transfer resistance for the transport of monoterpenes between the two canal systems could be introduced. The non-communicating model was found to be useful in explaining the release rate when drying spruce. This model, however, can not distinguish between the two temperature levels studied.     

IDENTIFICATION OF MATHEMATICAL MODEL COEFFICIENTS IN THE ANALYSIS OF THE HEAT AND MASS TRANSPORT IN WOOD

ABSTRACT

A method based on the concept of solving inverse heat and mass transport problems was proposed to identify wood physical properties, with the use of empirical data, a mathematical model corresponding to the direct problem, and an optimization procedure. The computer software was developed to solve transient, three-dimensional, quasi-linear direct and inverse problems of heat and mass transport in wood as an anisotropic body together with initial and boundary conditions of any kind. The software was adapted to data parallel processing environment of high performance computers. Identification of thermal conductivity was performed for beech wood in three principal anatomic directions, and accuracy of temperature distribution predictions was significantly increased (the global relative error of prediction was reduced to 1.0 - 1.9%).     

木粉对PVC木塑复合材料力学性能影响

采用电镜扫描观察了3种木粉的纤维细胞尺寸及其木粉微观形态。研究了木粉粒度、微观特性以及木粉添加量对了聚氯乙烯（PVC）木塑复合材料力学性能的影响。结果表明，木粉表面裸露的微细纤维增加和粒度减小，有助于提高木塑复合材料力学强度；加入少量木粉使木塑复合材料力学性能降低，但随着木粉添加量的增大，木塑复合材料的抗弯性能和拉伸强度上升；木塑复合材料的冲击强度随木粉含量增加而下降。     

木材阻燃剂的配制

研究了双氰胺、磷酸、硼酸三种物质组成的阻燃体系,这三种物质的最佳摩尔比为2∶2∶3,三乙醇胺作渗透剂与溶剂,三乙醇胺的加入量为0.5%～1%,浸渍时间为48h,取出晾干72h,此配方由火管法测试为阻燃一级。     

木材溶剂液化技术及其在制备高分子材料中的应用

木质材料经过热化学液化,可进一步制备酚醛树脂、聚氨酯等高分子材料。本文介绍了木材液化技术制备高分子材料的发展状况,包括液化方法、木材主成分的液化机理以及液化生成物的应用,指出在木材液化反应机理、液化产物的利用和最佳液化工艺的开发等方面尚有待进一步研究,并提出充分利用木材的生物降解性,开发木材液化物与其他材料复合的新型高分子材料具有广泛的前景。     

表面处理对木粉增强PVC发泡复合板材性能的影响

研究表面处理的木粉对发泡聚氯乙烯(PVC)板材的增强改性效果。使用铝酸酯偶联剂、丙烯酸丁酯预聚物对木粉进行表面处理，将其混合到聚氯乙烯发泡板材配料中进行板材加工生产，结果表明，经处理的木粉能提高发泡PVC板材的拉伸强度和冲击强度。用铝酸酯偶联剂处理木粉的发泡板材力学性能好于用丙烯酸丁酯预聚物处理的板材；而用丙烯酸丁酯预聚物处理木粉的复合材料在流变加工性能优于用铝酸酯处理木粉的复合材料。     

木材溶剂液化技术及其在制备高分子材料中的应用

木质材料经过热化学液化,可进一步制备酚醛树脂、聚氨酯等高分子材料.本文介绍了木材液化技术制备高分子材料的发展状况,包括液化方法、木材主成分的液化机理以及液化生成物的应用,指出在木材液化反应机理、液化产物的利用和最佳液化工艺的开发等方面尚有待进一步研究,并提出充分利用木材的生物降解性,开发木材液化物与其他材料复合的新型高分子材料具有广泛的前景.     

微波作用下木材苄基化改性研究

利用微波辐射对杉木进行苄基化改性,研究了各种因素对苄基化反应的影响,通过红外光谱分析(FT IR)对苄基化木材的分子结构进行表征。优化的反应条件是:无需甲苯溶剂,2克木粉用10mol/L的NaOH溶液16mL、氯化苄12mL于微波下辐射100min,辐射功率为前60min用650W的30%,后40min用650W的40%,可获得苄基化木粉,其增重率达到125%,效果与常规加热法基本相同,但在反应时间上却缩短了58%,而且微波法操作简便。     

水性聚氨酯在木器漆中的应用

叙述了水性聚氨酯木器漆的制备方法、技术指标及施工应用,并与传统产品做了性能比较。     

CONDENSATION-SORPTION OF WATER HISTORY WITH A CUBIC SPECIMEN OF WOOD

ABSTRACT

Depending on the temperature, moisture of the surrounding atmosphere may condensate or not. A piece of wood may thus be in contact either with liquid water during the night for a given time, or with air during daylight at a given relative humidity. A sorption-desorption history of moisture may thus occur, with a moisture content in the wood which may vary to a high extent below or beyond the fiber saturation point. A model based on a finite difference model is able to describe the process : the kinetics of the sorption-desorption history is in good agreement with the experiments ; moreover, a fuller insight into the nature of the process is gained with the profiles of moisture content in various places of the wood sample.     

木塑复合材料成型工艺及影响因素的研究

论述了木塑复合材料的成型工艺,着重对影响木塑复合材料挤出成型性能的因素如成型设备、工艺条件、基体树脂种类、木粉含量及木粉粒径、添加剂等进行了相应的分析,为深入开展木塑复合材料的加工研究提供了分析资料。     

聚丙烯／木纤维复合材料增强改性的研究

研究了聚丙烯／木纤维复合材料的机械性能,并讨论了相容剂ＭＡＨ－ｇ－ＰＰ用量对聚丙烯／木纤维复合材料性能的影响,比较了两种工艺路线的优劣性。     

水性木器漆消泡性能的研究

论述了影响水性木器漆消泡性的因素,对水性木器漆中消泡剂的选择和应用作了探讨。     

木粉/PP复合材料制备

用热压成型的方法制备了不同木粉含量的木粉／聚丙烯(PP)复合材料。结果表明，木粉含量高达90份时仍可热压成型，但以50份以下为优。通过采取加入添加剂、复合体粉料冷一机械共混与热一机械共混及挤出机挤出造粒、最终热压成型等工艺．可获得成形良好的木粉／PP复合材料。     

A MATHEMATICAL MODEL OF SUPERCRITICAL METHYLAMINE (SCM) DELIGNIFICATION OF WOOD

An unsteady-state mathematical model describing supercritical fluid (SCF) delignification of wood in a packed bed was developed. Equations describing intraparticle supercritical methyl-amine reaction with lignin, and subsequent transport of the methylamine-lignin complex into the bulk phase were derived and solved using orthogonal collocation. Model results for the case of supercritical methylamine are presented and compared with experimental results. Simulation results correctly predicted the experimental results. Sensitivity of methylamine extraction of lignin was evaluated by varying bulk fluid velocity, mass transfer coefficient, and particle diameter. Delignification was a strong function of particle diameter and a relatively weaker function of fluid velocity and mass transfer coefficient. The Thiele modulus and Biot number showed that delignification was reaction rate limited for wood particle diameter values ≤ 0.1 cm, but delignification was limited by intra-particle mass transfer for wood particle diameter values ≥ 1.0 cm.