木材浮压干燥过程的传热传质

摘要木材干燥是木材加工与利用的基础环节，其能耗约占木制品生产总能耗的40%～70%。对木材进行正确合理的干燥处理，既是保证木制品质量的关键，又是木材合理利用和节约木材的重要手段，其重要性和经济效果也越来越被人们所认识、所实践。传统的干燥方法通常是以湿空气作为干燥介质，将空气温度、相对湿度和气流速率作为控制木材干燥过程的“三要素”。然而，在影响木材内部水分移动和表面蒸发强度的外部因素中，周围空气的压力也同样是一个决定性的因素。木材的浮压干燥就是在充分考虑了外界压力影响的基础之上，出现的一种新型的干燥技术。它与传统的常规蒸汽干燥以及通常意义上的真空干燥相比，最主要的区别在于是以过热蒸汽作为干燥介质，且将压力因素从基本保持压力不变扩展到改变压力并使其浮动。这种环境压力的波动可大大加快木材内部水分的移动速率，而且在整个浮压干燥过程中，木材表面始终处于湿润状态，从而可以有效地防止因表面水分蒸发过程快而导致木材干燥应力的增加，进而防止开裂和变形的产生。木材的浮压干燥以其干燥速率快、质量好等一系列突出的优点，日益受到木材干燥界的广泛重视，被国际上认为是最具发展前景的木材干燥新工艺。在目前我国木材供需矛盾日益突出的今天，研究开发这种干燥工艺和技术，对提高干燥质量和干燥效率，节约木材具有重要意义。

浮压干燥技术尽管在德国、丹麦、法国、加拿大等国有小规模的工业应用，但诸多工艺问题并未从机理上得到根本的解决。目前，国内外对木材浮压干燥的理论研究还比较少见，关于浮压干燥的干燥特性和干燥规律等方面缺乏系统深入地研究，特别是对浮压干燥过程中木材内部热质传递规律的研究尚属空白。因此，开展浮压干燥的基础研究与实用性试验，将具有重要的理论意义和实用价值。

本篇论文来源于国家自然基金课题（59876005），其研究内容与实际生产需要密切相关，着眼于当前急需解决的进口硬阔叶材等难干材以及速生材和幼龄材的干燥问题。本论文以木材的浮压干燥为背景，在其实用价值最大的压力区段即负压区段进行试验研究。文中首先从浮压干燥的干燥介质特性入手，通过对木材浮压干燥的基本规律以及干燥过程中木材内部热质传递规律等方面的研究，建立了木材浮压干燥热质传递的数学模型，进而为浮压干燥工艺的优化设计和过程控制提供理论依据。这一探索性的研究，以全新的理念为木材干燥技术的创新和发展开辟了一条崭新的道路。

1.从干燥介质的热力学特性出发，对真空状态下过热蒸汽和空气干燥“逆转温度”的存在进行了深入的理论分析，求解了真空状态下过热蒸汽干燥 “逆转温度”的理论模型，并首次通过空气与过热蒸汽干燥的对比试验发现了真空状态下过热蒸汽干燥“逆转温度”的存在。在仅有对流换热的条件下，当环境绝对压力为0.02MPa时，“逆转温度”的理论值为90℃左右，试验值在80℃～85℃之间。出现偏差的主要原因是由于理论分析时未考虑真空过热蒸汽的辐射特性。

已有的研究表明，在（常压）过热蒸汽干燥时存在一个温度点，称为“逆转温度”，当过热蒸汽的温度高于“逆转温度”的温度时，过热蒸汽干燥速率比空气要快，低于逆转温度时则正好相反。由于木材浮压干燥的工业化应用是在负压区实现的，因此对真空状态下过热蒸汽干燥是否也存在“逆转温度”问题的研究非常重要。如果真空状态下的“逆转温度”确实存在，那么在制定木材浮压干燥工艺时，就可使干燥介质温度始终高于“逆转温度”，这样即可实现在保持较快干燥速率的基础之上，充分发挥过热蒸汽干燥时木材表面对流换热系数大、传质阻力和干燥品质好等一系列的优势，这对于优化木材浮压干燥工艺具有重要的理论意义和实用价值。

①通过对浮压和常规干燥过程中预热阶段的对比试验研究表明：木材浮压干燥预热阶段进行的非常迅速。在绝对压力为0.02MPa，介质温度分别为60℃、80℃时，以蒸汽为干燥介质时的平均升温速率分别是以空气为干燥介质时的1.55、1.64倍。这说明随着干燥介质温度的提高，从预热阶段的升温速率来讲，以蒸汽为干燥介质要优于以空气为干燥介质；同时试验中还发现以蒸汽为干燥介质时，在预热阶段结束后的木材的含水率比预热之前的初含水率要大。这主要是因水分凝结所致，由于凝结的水分仅位于木材表面且预热时间很短，因此对整个干燥过程影响很小。

②通过试验及理论分析，探讨了试件尺寸（厚度和长度）对浮压干燥速率的影响规律。试验表明，在一定尺寸范围内，浮压干燥的干燥速率不受试件厚度的影响，但受试件长度的影响。然而，有一点应当特别关注，即纵向水分移动的速率随试件长度的增加衰减迅速，随着试件长度的增加木材内水分从侧面的移动对干燥速率的贡献率将逐渐占据主导地位。因此，在浮压干燥过程中，预见有一个“临界尺寸”的存在。

③通过浮压干燥介质条件对水分移动速率的影响规律的研究表明，木材的浮压干燥速率随着干燥介质温度的增加、绝对压力的减小和浮动频率的加大而增加。但在上述三因素中对干燥速率的影响程度各不相同，从大到小的排列顺序为：介质温度（T）>>压力浮动频率（Hz）>绝对压力（P）。

①通过对浮压干燥的干燥速率和干燥过程中木材内部温、湿度场变化的研究表明，在温度为80℃、压力浮动范围为0.02～0.1MPa的条件下，当浮动频率分别取0.75次/h和0.375次/h时，马尾松试材的浮压干燥速率分别为常压下相同干燥温度时的3.24和2.69倍，这足见压力浮动幅度和频率对干燥速率的巨大影响。而且在同一温度水平，压力浮动幅度越大和浮动频率越快，则木材内的温、湿度场的变化越快，它表明木材内水分迁移和蒸发速率加快。从湿度场的变化可见，木材内部各层的含水率变化幅度大致相当，表、心层含水率差值不大，这说明含水率梯度不是浮压干燥过程中水分移动的主要驱动力。

自由水在毛细管中的蒸发或沸腾实质上是复杂的微尺度传热传质过程。由于尺度的微小化，使管道中流动和换热受管道尺寸、形状、表面结构等因素的影响很大。在木材为温度为80℃，环境绝对压力为0.02MPa的条件下，理论计算表明此时的汽泡平衡态半径为4.68μm，而针叶树材管胞的内径为20～30μm。由此可见，此状态下管胞内腔有足够的空间用于液体内部汽泡的生成。因此当木材中大毛细管水达到沸点以后，在细胞腔中液体内部的强烈汽化是可能发生的。在微尺度传热传质中的“汽化空间”与“拟沸腾”的概念同样适用于木材，可以设想当液态水处于纹孔口处时，可能就会出现“拟沸腾”状态。

④采用扩散杯法对水分扩散系数和浮压系数进行了测定。确定出不同压力下与常压(0.1MPa)下水分扩散系数的数学表达式，以及不同压力变化速率下与压力固定不变时浮压系数的数学表达式。运用上述方程，再对常压下含水率和温度与水分扩散系数之间的关系式进行修正后，即可确定出浮压干燥过程中吸着水迁移量的计算方程。对试验数据回归分析表明：木材水分扩散系数随绝对压力的增加呈线性减小；随压力变化速率的增加呈对数曲线增大。

通过对理论模拟与试验结果的比较分析可知：计算机模拟的曲线和试验曲线除在高含水率区域有一定偏差外（含水率最大偏差值为8.12%），其余区域均可很好地吻合。特别是从初含水率90%到终含水率12%的平均干燥速率来看，计算机模拟值为2.95%/h，试验值为3.04%/h，两者的误差不超过5%。数学模型组成了一个木材干燥的动态机制，通过实验证明，模型在较宽的条件范围内能较为准确预测浮压干燥下马尾松的平均含水率。

Abstract Wood drying is generally regarded as the key item in wood processing and utilization, the drying process accounts for approximately 40% to 70% percent of total wood-products manufacturing energy consumption. Therefore proper drying is of importance to improve the quality of forest products and is playing an essential role in using and saving wood reasonably, and its contributions to the improved performance to products and cost reduction have been drawing an even increasing attention. Usually the conventional drying methods take advantage of the temperature of air, relative humidity and air velocity to control drying process with the medium of wet-air, however, the fact that the role of ambient pressure in determining the moisture transfer in wood and its evaporation intensity has been neglected. Wood drying under floating pressure is an innovative technology, which considerably takes the effects of ambient pressure into account. Compared with the traditional drying and vacuum drying methods, its ambient pressure varies to form floating pressure with the drying medium of superheated steam. As a result, the fluctuation of ambient pressure accelerates the rate of moisture transfer while keeps wood surface wet, which effectively protect the occurrences of drying defects. With the performance of short drying-cycle and good quality, wood drying under floating pressure has been universally recognized as a novel wood drying method lighted with color future. R&D of the innovative drying processing and technology to improve the quality of products and efficiency, especially in response to solving the conflicts between supplies of wood failure to satisfy the demands of wood, has an important strategic meaning.

Although drying under floating pressure has a limited application to industries in Germany, Danish, and France as well as Canada recently, a number of fundamentals with respect to processing are far from satisfying. At present, researches on the theory of wood drying under floating pressure both in domestic and oversea are few, don’t mention the heat and mass transfer of drying under floating pressure. Therefore, carrying out the theoretical and experimental research on drying under floating pressure has an important implication both in theory and practice.

This paper is supported by the Natural Science Foundation of China (Grant No. 59876005), its content is closely related to the requirements of practice, aiming at the problem that imported hardwood is difficult to dry and how to dry faster grow-tree wood. Based on the background of drying under floating pressure, experimental program is carried out when the ambient is under an atmospheric pressure which is the most useful. This paper began with the characteristics of drying medium, and explored the fundamentals of heat and mass transfer, then a mathematical model based on the heat and mass transfer was established to optimize design and control the drying process. The research developed an innovative way for wood drying technology with new conception.

1. based on the characteristics of drying medium, theoretical analysis concerning the existence of inversion temperature between the condition of superheated steam under vacuum-drying and air drying has been carried out, developed the theoretical model of inversion temperature of superheated steam under vacuum, and for the first time it has been proven to be true that the inversion temperature really exist under the condition of vacuum through comparable experience of air and superheat steam. If we consider that there is only convection, when the absolute pressure of ambient is 0.02Mpa, the theoretical value of inversion temperature is approximate to 90℃, and the experimental value, however, is 80℃~85℃. The reason for this difference is that we neglected the effect of radiation.

The previous literatures indicate the inversion temperature exists under superheated steam drying with an atmospheric pressure, when the temperature of superheated steam is higher than the point of inversion temperature, the drying rate of superheated steam is faster than that of air drying, but if the temperature of superheated steam is lower than the point of inversion temperature, the drying rate of superheated steam is slower than that of air drying. Owing to the fact that the floating pressure drying in industries is operated under vacuum, so it is of importance to reveal whether the inversion temperature of superheated steam drying exists or not under vacuum is important, if it does, we can make the temperature of drying medium is always higher than the inversion temperature, thus we can make full use of these credits of superheated steam drying such as the bigger convection heat coefficient, lower resistance to mass transfer and good quality, finally to optimize wood drying processing under floating pressure.

l The comparable experiments from the preheating process of floating pressure drying and conventional drying indicated that heat transfer increases sharply. When the absolute pressure is 0.02Mpa, the temperatures of drying mediums are 60℃ and 80℃ respectively, the average rates of rising temperature with the drying medium of superheated steam is 1.55 and 1.64 times than that of using air for drying medium. The ration of the rate of wood average temperature increment with the drying medium of superheated steam to that of air illustrates that coupled with the drying medium temperature rising, the rising rate of temperature of superheated is superior to that of air during the stage of preheating; meanwhile the moisture content of wood treated with superheated steam is higher that of its initial moisture content, which caused by the condensation of water. Thus this phenomenon is favorable in the whole drying process.

l The effects of sample size including thickness and length on the drying rate under floating pressure have been analyzed through experiments and theoretical analysis. The results showed sample thickness within a certain range makes no difference on the drying rate, but the length do. The fact illustrates that moisture transfer mainly along longitudinal direction. However, one thing need to mention is the rate of moisture movement along longitudinal direction reduces sharply with the increasing of length; simultaneously moisture movement along lateral direction will be dominant in contribution to the drying rate. Therefore, we can draw a conclusion that a critical sample size exists.

l Results from analyzing the effects of drying medium under floating pressure on moisture movements rate show: the drying rate of floating pressure drying increases with the increasing of drying medium temperature; the reducing of absolute pressure and the increasing of fluctuating frequency. Their contributions to the drying rate list as followings sequence: the first is temperature, followed by fluctuating pressure frequency, and the last is absolute pressure.

l Through the research of both the drying rate and the changes of temperature plateau and moisture plateau under floating pressure, results demonstrate that when the temperature of drying medium is 80℃, the fluctuating frequency are 0.75 times per hour and 0.375 time per hour respectively, the drying rates of wood drying under floating pressure are 3.24 times and 2.69 time than that of air drying, the fact show that the range of pressure fluctuation and its frequency play important roles in determining the drying rate. When the temperature is fixed, the expanding range of fluctuating pressure and faster fluctuation frequency increases the change of temperature and moisture plateau, as a result, the time required is shortened. However the surface moisture content is approximate to the core moisture content, that is to say moisture content is not the dominant force under floating pressure.

l The movement of free water comprises two parts during the process of drying under floating pressure, one is the mass transfer driven by capillary pressure, and the other is steam transfer due to the evaporation and boiling of free water caused by the pressure fluctuation under pressure gradient. Furthermore, the later dominates the removing of free water.

Essentially the evaporation or boiling of free water in capillary can be regarded as the heat and mass transfer with micro-dimension. the minimization of dimension make the heat and mass transfer affected by the flow, size of pipeline, and its shape as well as surface structure more easily. When the temperature of wood is 80℃, the absolute pressure of ambient is 0.02Mpa, the equilibrium radius of bubble calculated theoretically is 4.68μm, however, the tracheid of softwood internal diameter is 20~30μm，obviously there is sufficient space to form bubble in liquid. When the water in bigger capillary reaches its boiling point, it is possible that the intensive evaporation in cell cavity occurs. The conception of vapor space and boiling can be also used in wood drying. We can image that boiling will begin when the liquid water lies in the entrance to pits.

l Measuring respectively the moisture diffusion coefficient and floating pressure coefficient by the means of diffusible cup under various pressure and an atmospheric pressure (0.1Mpa). Their corresponding mathematic equations have been developed to describe factors affecting the moisture diffusion coefficient. The correction of relationship between both moisture content and temperature and moisture transfer coefficients, coupled with these equations mentioned above, we can get the calculating equations of moisture transfer mass. Conclusions based on the analysis of regressing the experimental dada show: diffusion coefficient decreases as the increasing of absolute pressure in linear; increases in a way of logarithm with the varying rate of pressure.

l Comparable results of the simulation and experiments show that they match well besides higher moisture content (the max. derivation value of moisture content is 8.12%), especially when the scope of moisture content ranges from 90% to 12%, the drying rate calculated by the simulation is 2.95% per hour, and the experimental value is 3.04% per hour, the tolerance is no more than 5%. Mathematical model comprises the dynamic mechanism of wood drying, experiments has proven that this model can precisely predict the average moisture content of Massion Pine (Pinus massoniana) under fluctuating pressure.