Analysis of cyclic vacuum drying curve

The objective of this research was to analyze the cyclic vacuum drying curve within one cycle. Red oak specimens of two different groups, square in cross-section, were used. Group one was comprised of four different thicknesses (2.54, 3.81, 5.08, and 6.35 cm) with a length of 25.4 cm and group two was comprised of three different lengths (12.7, 25.4, and 38.1 cm) with the thickness of 2.54 cm. The specimens were heated to 60°C in the heating oven and then dried in the vacuum oven at 18 mm Hg. The vacuum oven was at room temperature (20°C). The vacuum pump was kept running for 140 min. It was found that the cyclic vacuum drying curve consisted of two distinct parts. The fast drying period lasted about 10 to 15 min. The slow drying period occurred when the pressure inside wood approached the ambient pressure. Most of the moisture was removed during the fast drying period.     

Moisture flow characteristics during radio frequency vacuum drying of thick lumber

Summary Thick specimens of two softwoods were dried in a laboratory radio frequency/vacuum (RF/V) kiln in order to investigate the internal moisture flow patterns. The spatial moisture distributions in the longitudinal arid transverse directions were monitored as a function of time in a number of runs. The results indicated that both longitudinal and transverse moisture transfer modes contribute on the overall moisture flow occurring within wood, but there was no distinction as to the percentage contribution of each to the overall flux. There were no abrupt drying front changes and no moisture discontinuities observed during drying. A second group of runs was also carried out using end-matched specimens to study its length effects on drying characteristics, such as drying rate, and internal vapor pressure and temperature profiles. The results demonstrated that short specimens dried faster than long ones at moisture contents above the fiber saturation point.The financial support for this work from NSERC/Canada through a Strategic Grant (STR0167393) and the lumber contribution from MacMillan-Bloedel are greatfully acknowledged.     

Microwave vacuum drying characteristics of Pinus massoniana wood

Microwave-vacuum (MV) drying characteristics of plantation Masson pine (Pinus massoniana) were studied experimen-tally for various levels of microwave radiation time, initial moisture content (MC), vacuum level and wood thickness. The results show that the process of MV drying for wood can be significantly divided into a short accelerating rate drying period, a long constant rate drying period and falling rate drying period, and the second drying period can extend to levels of mean MC below the fiber satu-r...     

Effect of compressive load on the dimensional changes of the Japanese larch dried in a radio-frequency/vacuum drier

This study investigated the effect of a compressive load of 0.092 MPa on the dimensional changes of Japanese larch in a humidity chamber after continuous radio-frequency/vacuum drying. The dimensional changes in the loading directions were significantly increased while those perpendicular to the loading directions were decreased. The shrinkages showed higher values in continuous drying than in intermittent drying. For the specimens loaded on their tangential sections, the radial shrinkages were even higher than the tangential shrinkages; thus, the tangential/radial shrinkage ratio was decreased by 0.27. The transverse hygroscopicity was reduced for the specimens loaded on their cross sections, but increased for the specimens loaded on their tangential sections.     

Relationship of selected cell characteristics and colour of silver birch wood after two different drying processes

Change in the colour of silver birch wood is a serious problem in the mechanical wood industry. Here, colour was correlated with microscopic characteristics of wood, such as cell types and dimensions, by drying processes. In conventional drying, with lower temperature than in vacuum drying used here, the most important factor causing darkened wood was wide latewood. In vacuum drying, thickness of the vessel walls affected wood darkening, as did broad rays and large amount of axial parenchyma. Axial and terminal parenchyma cells contained very small amounts of phenolics, but after drying at elevated temperature, a thin dark layer could be observed on the innerside of their walls. Phenolics were abundant in ray parenchyma; these compounds darkened at elevated temperatures, less in conventional drying than in vacuum drying. Phenolics were observed only inside cells, mainly in the parenchyma, but in vacuum-dried wood also in fibres and vessels. Anatomical characteristics are known to be affected by both environmental and genetic factors. Thus it might be possible to influence the colour reaction of birch wood during the drying process by choosing wood according to growing-site conditions, or by choosing the seed source for birch plantations according to given anatomical characteristics.     

Relationship of selected cell characteristics and colour of silver birch wood after two different drying processes

Abstract

Change in the colour of silver birch wood is a serious problem in the mechanical wood industry. Here, colour was correlated with microscopic characteristics of wood, such as cell types and dimensions, by drying processes. In conventional drying, with lower temperature than in vacuum drying used here, the most important factor causing darkened wood was wide latewood. In vacuum drying, thickness of the vessel walls affected wood darkening, as did broad rays and large amount of axial parenchyma. Axial and terminal parenchyma cells contained very small amounts of phenolics, but after drying at elevated temperature, a thin dark layer could be observed on the innerside of their walls. Phenolics were abundant in ray parenchyma; these compounds darkened at elevated temperatures, less in conventional drying than in vacuum drying. Phenolics were observed only inside cells, mainly in the parenchyma, but in vacuum-dried wood also in fibres and vessels. Anatomical characteristics are known to be affected by both environmental and genetic factors. Thus it might be possible to influence the colour reaction of birch wood during the drying process by choosing wood according to growing-site conditions, or by choosing the seed source for birch plantations according to given anatomical characteristics.     

Study on drying rate in contact drying with flexible screen

IntroductionA new type contact dryer with non-metal flexibleplaten was patented in 1998 in China, which specialstructure and material solved the problem of low productivity in platen drying. The machine was similar tohot press. Direct contact betWeen wood and heatedplaten allows rapid heat transfer by conduction (Sandoe 1983). Though the flexible screen conducts heatslower than metal platen does, it transfers massmuch fast6r. So water removing rate decides veneerdrying rate in the new machin…     

高频真空干燥中压缩载荷对落叶松长方形髓心方材表面裂纹的影响

探究了高频真空干燥中压缩载荷对有纵向槽的落叶松长方形髓心方材表面裂纹的影响。研究结果如下:与正方形方材相比,由于压缩载荷作用和长方形方材宽度的增加,长方形方材宽度与厚度干缩率比大大减少了;因为高频加热和干缩差异应力的减少,长方形方材表面裂纹减少了许多,尤其没有纵向槽的长方形方材表面裂纹明显减少。     

天然栀子黄色素的低温真空干燥研究

以加热板温度、干燥箱内真空度、料液的质量分数和料层厚度为影响因素,研究了天然栀子黄色素低温真空干燥制粉的优化工艺,设计了L9(34)的正交试验.当干燥产品达到一致的含水量时,对其色价(CV)、色差(CD)等指标以及干燥时间进行了分析.结果表明:加热板温度对产品的色价、色差以及干燥时间影响最大;料层厚度对栀予黄色素的色价...     

不同干燥方法对杉木人工林木材浸注性的影响

采用高频真空干燥、常规窑干和高温干燥3种方法对杉木人工林木材的心、边板材进行干燥处理,用毛细管上升法评价干燥后试样的浸注性能,用半薄切片法测定干燥试样具缘纹孔的闭塞率,最后用扫描电子显微镜观察干燥试样微观构造的变化,比较分析3种干燥方法对杉木人工林木材浸注性的影响机理.结果表明:对于杉木边材,经高频真空干燥后试样的浸注性显著高于常规干燥和高温干燥后试样的浸注性,后2种干燥方法对试样浸注性影响的差异并不显著;对于杉木心材,高频真空干燥与高温干燥后试样的浸注性存在显著差异,而高频真空干燥与常规干燥之间、高温干燥与常规干燥之间对试样浸注性的影响差异均不显著;3种干燥方法处理后,杉木边材的浸注性均显著高于心材的浸注性;具缘纹孔的闭塞率较低以及部分具缘纹孔周缘破裂是高频真空干燥后木材浸注性提高的主要原因.     

Effect of steam pretreatment on wood moisture content and characteristics of vacuum drying

The effects of high temperature steam pretreatment on the change in wood moisture content (MC) and characteristics of vacuum drying were investigated in this study. Poplar and manchurian walnut woods were pretreated with high temperature steam at 100°C and 140°C, prior to vacuum drying. A comparison of the characteristics of vacuum drying between steam pretreated wood and untreated wood was carried out. The results show that during steam pretreatment, the MC of wood decreased within a few hours. The reduction of MC varied with the temperature; the higher the temperature, the faster the MC dropped. During the vacuum drying stage, the rates of drying of pretreated samples were higher than those of untreated samples when MC was below the fiber saturation point. Furthermore, the total drying time of samples treated at a steam temperature of 140°C was lower than that of untreated samples. Therefore, a vacuum procedure after steam pretreatment can effectively shorten the drying time when drying wood.     

木材真空-过热蒸汽干燥的预热特性

在真空状态下，分别对过热蒸汽和空气为干燥介质时木材干燥预热阶段进行对比试验，探讨了木材在真空－过热蒸汽干燥中预阶段的工艺特性。结果表明：过热蒸汽干燥的预热阶段进行得非常迅速，同时其表面水分的凝结量也较以空气为介质时表面水分的凝结量大。     

超声波喷雾干燥壶瓶枣多糖及其对产品品质的影响

以干燥后多糖的羟基自由基清除能力、含水量和平均粒径为指标,对壶瓶枣多糖的超声波喷雾干燥工艺进行了优化,并对比分析了超声波喷雾干燥、二流体喷雾干燥和真空冷冻干燥对壶瓶枣多糖品质的影响。结果表明,超声波喷雾干燥的最佳工艺条件为:进风温度135℃,进料量16 m L/min,进气压力0.10 MPa,此时出风温度89℃,壶瓶枣多糖产品含水量4.91%,羟基自由基清除率为50.83%,平均粒径9.14μm。羟基自由基清除能力、单糖组成和红外光谱分析表明,3种干燥方式对多糖的活性、单糖组成和官能团没有影响,且多糖主要由鼠李糖、阿拉伯糖、木糖、甘露糖、葡萄糖和半乳糖组成,其物质的量之比为1∶11∶2∶1∶38∶5,羟基自由基清除率为50%左右。通过对多糖产品的微观形态及粒度分布分析可知,冷冻干燥产品以块状和棒针状为主,超声波和二流体喷雾干燥产品均成颗粒状,但是超声波喷雾干燥的产品粒径分布较窄,粒径在2~20μm范围内呈正态分布,优于其他2种干燥技术。     

Effect of pretreatment with high temperature and low humidity on drying time and prevention of checking during radio-frequency/vacuum drying of Japanese cedar pillar

This study investigated the effect of pretreatment with high temperature and low humidity (HT-LH) on characteristics of radio-frequency/vacuum (RF/V) drying of boxed heart timber of Japanese cedar (150 × 150 mm in cross section, 3600 mm long). From green to approximately 15% moisture content, the drying times including HT-LH treatment were 170 h for the nonkerfed control, 190 h for the kerfed control, and 150 h for both the kerfed and nonkerfed HT-LH specimens. Surface checks were effectively prevented by the HT-LH treatment while the kerfed HT-LH specimens were free from surface checks. In order to prevent the formation of internal checking during RF/V drying, it is suggested that HT-LH treatment should finish around the fiber saturation point.     

小径柞木真空干燥应力的试验研究

在真空干燥条件下,分析小径级柞木板材的应力与含水率、时间的关系,结果表明:在低温条件下,由于真空负压作用,水分蒸发速度得到了加快,干燥应力由于干燥前板材内的初始应力的差异,导致干燥过程中应力的发生、发展有所差异。根据应力表现的差异应当在干燥不同阶段及时进行调湿处理,预防板材开裂产生。     

木材干燥应力连续监测方法的研究

本文在卡普法的基础上，提出了木材干燥应力连续监测方法，并设计了所需的配套装置。该方法采用涡流传感器对干燥过程中卡普片的矢高进行连续测试，同时通过计算机自动采集、存储数据。结果表明：矢高与干燥时间存在精密的相关性，它们之间的相关系数大部分达到0．99以上，从而实现了木材干燥应力的自动与连续监测。     

Drying characteristics of wood under vacuum-superheated steam

This paper presents comparison and analysis of thermal-dynamic characteristics of air-drying and superheated steam drying under vacuum. The result reveals that for both convective heat transfer coefficient and resistance of mass transfer on the surface, superheated steam drying under vacuum is superior to air-drying under the same condition. With Masson pine as specimen,we found that the inversion temperature really exists through comparable experiments of air-drying and superheated steam drying under vacuum. When temperature is above inversion point of temperature, drying speed is faster than that of air-drying; however, if temperature is below the point, the result is opposite. The inversion temperature of experiment ranges from 80 to 85 ℃.     

竹材热压干燥过程中的水分迁移特性研究

研究了竹材热压干燥过程中的水分迁移特性.结果表明:在整个干燥过程中,前期含水率降低较快,后期含水率降低较慢.竹材平均干燥速度与次表层竹材的干燥速度相近;在含水率较高的干燥初期,水分迁移的阻力在竹材表面,水分迁移主要靠毛细管张力作用;在含水率较低的干燥后期,水分迁移的阻力在竹材内部,水分迁移主要以扩散方式进行,干燥速度取决于木材内部水分移动的速度.竹材热压干燥过程中的水分移动,主要受温度梯度和含水率梯度的共同作用.     

酚醛树脂对人工林杉木木材的浸注性及其改善的研究

研究了低分子量酚醛树脂对杉木木材的浸注性及树脂在木材内的分布状态，并对苯醇抽提前处理法、常压浸渍和真空浸注的效果进行了评价。无论抽提处理与否，常压浸渍的树脂水溶液充填率为理论最大浸注量的10％左右，而真空浸注可达到90％以上，真空浸注效果明显；无论是常压浸渍还是真空浸注，抽提处理后树脂的浸注性有很大改善，但常压浸渍的效果不是非常明显。软X射线解析样品纵向树脂分布状态结果表明：无论是否经过抽提处理，在常压浸渍的情况下，树脂主要分布在样品的端部，而经过真空浸注处理，树脂注入的深度增加，尤其经过抽提处理后树脂可达到样品中心。由此可见，抽提处理和真空浸注并用，使树脂的分布状态有很大改善。另外，抽提处理后样品之间树脂分布的差异也趋于减小，即树脂在样品间的分布状态也有改善。用实体显微镜观察树脂在木材细胞腔内的充填状况结果表明：沿着样品纵向从端部到中心树脂充填率呈下降趋势，而且抽提处理的充填率比未抽提处理的高。     

喷雾冷冻干燥在植物提取和医药中的应用

通过比较喷雾干燥、冷冻干燥和喷雾冷冻干燥各自的特点，介绍了一种新型的干燥方法即喷雾冷冻干燥的机理，归纳和总结国外喷雾冷冻干燥在植物提取和医药中的应用研究进展，分析了通过喷雾冷冻干燥获得的干燥产品的特性，并提出了其可能的应用领域和范围，为进一步研究喷雾冷冻干燥技术提供相应的理论和实践基础。