【作者】 韩晨静；

【导师】 吕建雄；

【作者基本信息】 中国林业科学研究院 ， 木材科学与技术， 2014， 博士

【摘要】 单板干燥是胶合板生产过程中的重要工序，也是能耗最多的工序。目前，我国单板干燥主要方式有两种，一种是旋切后的湿单板直接进行喷气式对流干燥，另一种是对多张经太阳晾晒含水率低于30%的预干单板进行热压干燥。针对我国单板干燥实际情况，系统研究多张预干单板热压干燥过程中温度与含水率变化以及对其工艺进行优化，不仅具有重要的理论价值，还能为实际生产提供科学指导。本文以预干柳叶桉（Eucalyptus saligna）单板为研究对象，在掌握预干单板含水率分布的基础上，测定了多张预干单板热压干燥过程中水平方向、竖直方向的温度与含水率分布；分析了多张单板温度分布、含水率分布、升温速率曲线、干燥速率曲线及干燥质量的影响因素；根据传热模型确定了不同热板温度和单板张数条件下，中间单板温度达到100℃的时间；计算了每张单板在热压干燥中所需的能耗（Q），并结合干燥后单板的接触角（CA）和弹性模量（MOE），对多张预干单板的热压干燥工艺进行了优化。取得的主要结论如下：1、通过对目前企业使用的预干柳叶桉单板进行初含水率检测发现，预干单板的初含水率均在30%之下，且服从正态分布，88%单板的含水率在15%-25%之间。同一单板不同位置的含水率比较均匀（变异系数均小于10%）。2、在压力0.2MPa，热板温度120℃，两热板间放置10张单板，热压干燥30min的工艺条件下，测定与分析了多张预干单板热压干燥中水平方向、竖直方向的温度与含水率分布。对于温度分布来说：在水平方向上，10张单板的中心温度均高于边缘温度，与热板相近的2层单板差异不显著，中间4张单板差异显著。在竖直方向上，10张单板的温度基本以中间2张单板为中心呈上下对称分布，靠近热板处单板温度高，中间单板温度低；10张单板按对称位置分为5组，组间单板温度差异显著，组内单板温度差异不显著。对于含水率分布而言：在水平方向上，单板中心的含水率高于边缘，两者间无显著差异。在竖直方向上，以中间4张单板为中心呈对称分布；10张单板按照对称位置分为4组，组间单板含水率差异显著，组内单板含水率差异不显著；热压干燥后，晾置30min，10张单板间的含水率仍存在显著差异，但极差由10.62%减小至3.56%，符合单板干燥终含水率要求。3、在压力0.2MPa，热板温度120℃，两热板间放置10张单板，热压干燥30min的工艺条件下，根据不同位置单板温度达到100℃的时间，可以将多张单板的热压干燥过程分为3个阶段：初始阶段（I），热压干燥开始至靠近热板处单板中心温度达到100℃；中间阶段（M），至中间单板中心温度达到100℃；结束阶段（E），至干燥结束。压力、热板温度、单板张数各因子，二因子及三因子间的交互作用均对单板温度分布有显著影响。压力越大、热板温度越高、单板张数越少，单板的升温速率越快。单板干燥速率随单板张数的增多而减小，随热板温度的增加而增大。中间单板含水率随时间的变化曲线为指数函数；靠近热板处单板在热压干燥5min后（即5~30min内）含水率变化与时间呈现拟合优度较高的指数关系。4、单板的宽度干缩率随压力的增加略有减小，厚度干缩率则随压力的增加而增大。宽度干缩率和厚度干缩率均随温度的升高而有所增大。单板张数越多，宽度干缩率越大，而厚度干缩率减小。单板的平整度随压力的增加、热板温度的升高、单板张数的增多而降低。多张单板热压干燥后，同一单板不同位置的含水率比较均匀（变异系数均小于8%）。5、多张预干单板热压干燥时，当中间单板温度达到100℃后取下晾置30min，其终含水率范围为9.28%~12.58%，符合单板干燥的终含水率要求。据此建立了不同热板温度和单板张数下热压干燥时间（即中间单板温度达到100℃所需时间）的传热数学模型。不同热板温度和单板张数下，由数学模型得到的热压干燥时间理论值均小于试验测量值。理论值与测量值之间的差异随热板温度的增加而减小，随单板张数的增多而增大。模型理论值与试验测量值的误差均小于20%。6、根据压力、热板温度、单板张数3因子与每张单板干燥能耗及干燥后单板接触角和弹性模量的相互关系，建立了Q、CA、MOE与3因子的非线性回归函数，基于3因子的限制条件和热压干燥实际情况，经过非线性规划求解，得到多张单板热压干燥能耗最小的工艺为：压力0.4MPa，热板温度160℃，两热板间放置5张单板，热压干燥2.5min。更多还原

【Abstract】 Veneer drying is an important process in manufacturing of plywood, which is also thehigh amount of energy utilization process. There are two main drying methods for veneers inChina, one is peeling veneer directly dried by jet convective, another is hot-press drying toveneers which moisture content (MC) is less than30%after sun drying. According veneersdrying actual situaion in our country, temperature and MC distribution of multiple pre-dryingveneers during hot-press drying was systematical studied, and the process was optimized,which not only has important theoretical value but also can provide scientific guidance for theactual production.Taking Eucalyptus saligna veneer as an example, MC of pre-drying veneers was tested,Temperature and MC distribution in horizontal direction and vertical direction of multiplepre-drying veneers were determined. Factors influenced temperature distribution, MCdistribution, heating rate curves, MC variation curves, and drying quality were analyzed.Through heat transfer model, the time that the temperature of middle veneers reached100℃atdifferent platen temperature and veneer number was established. Energy consumption (Q) foreach veneer during hot-press drying was determined through the time tested in hot-press drying.With the other two important properties: contact angel (CA) and modulus of elasticity (MOE),the multiple pre-drying veneers hot-press drying process was optimized.The results showed that:1. Through the initial MC detection of veneers used in plywood factories, we found thatthe initial MC of all pre-drying veneers was below30%, which followed a normal distribution.88%veneers’ MC was between15%-25%. MC in different location of same veneer wasuniform (all coefficient of variation were less than10%).2. Based on the process that pressure was0.2MPa, temperature was120℃,10veneerswere laid overlap in order between two platens, drying time was30min, the distribution of temperature and MC in horizontal and vertical direction of multiple pre-drying veneers wasdetected and analyzed.Temperature distribution: in the horizontal direction, center temperature of veneers washigher than their edge temperature during whole drying process. The difference of horizontaltemperature of two layers veneer near the platen was not significant; while which of two layersveneers in middle position had significant difference. In the vertical direction, the temperaturedistribution of10veneers taking two veneers in the middle as center was symmetrical.10veneers were divided into5groups by symmetric position. There was a significant differencein veneers’ temperature of different groups, while the difference of veneers’ temperature insame group was not significant.The MC distribution: in the horizontal direction, MC in the center of veneers was slightlyhigher than the edge during the drying process. However, there was no significant differencebetween them. In the vertical direction, the MC distribution taking4veneers in the middle ascenter was symmetrical.10veneers were divided into4groups by symmetric position. Therewas a significant difference in veneers’MC of different groups, while the difference of veneers’MC in same group was not significant. Final MC of10veneers still had significant differenceafter airing30min, while the range had reduced from10.62%to3.56%, which met the veneerdrying requirement.3. According temperature of veneers in different position came to100℃, the hot-pressdrying process was divided into three stages: Initial stage (I), which from the beginning to thecore temperature of veneers next to platen came to100℃; Middle stage (M), which to the coretemperature of veneers in the middle reached100℃; End stage (E), which to the final momentof drying. Pressure, platen temperature, veneer number, each factor, the interaction of twofactors and three factors had significant effect on veneer temperature distribution. The biggerthe pressure, the higher the temperature, the less the number, and the faster the heating rate；The drying rate accelerated with the increase of temperature and reduction of veneer number.MC change of middle veneers could be characterized by an exponential function with respect to hot pressing time. The goodness of fit was high after veneers next to platen drying5minutes,in which the change of MC with time appeared an exponential function from5minutes to30minutes.4. The width shrinkage had slight decrease with the increase of pressure, while the thickshrinkage reduced. The width shrinkage and thick shrinkage increased with the temperaturerose. The more the veneer, and the bigger the width shrinkage, while the thick shrinkage hadopposite trend. The planeness of veneers decreased with the bigger pressure, highertemperature, and more veneer number. The final moisture content of each veneer was even (allcoefficient of variation were less than10%).5. When temperature of middle veneers came to100℃, veneers removed and aired. for30min. The final MC range was from9.28%to12.58%, which met veneer drying requirement.Therefore, a heat transfer model for calculating hot-press drying time (time for middle veneers’temperature came to100℃) at different temperature and veneer number was established.Theoretical values of hot-press drying time calculated from heat transfer model were lessthan the measured values at different temperature and veneer number. The difference betweenthe theoretical values and measured values decreased with increasing temperature, andincreased with an increase in the number of veneers. Errors of all condition were less than20%.6. According the relationship between hot-press drying factors (pressure, temperature, andveneer number) and properties of dried-veneer (CA, MOE, and Q), the non-linear regressionfunctions between CA, MOE, Q and three factors were established. Based on the limitingconditions for three factors, and actual needs for hot-press drying, an optimal hot-press dryingprocess for multiple pre-drying veneers was get through nonlinear solver: pressure was0.4MPa, temperature was160℃,5veneers were laid overlap in order between two platens, dryingtime was2.5min.更多还原