| 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. |
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