人工林桉木超临界CO2干燥热质传递规律及皱缩抑制机制

Collapse is a serious drying defect for Eucalyptus wood using as solid wood material. Collapse is mainly caused by the capillary tension resulted by migration of free water, however, there are no capillary tension in wood during supercritical carbon dioxide (SCD) drying, therefore it seems more potential on practice application in reducing collapse. In this project, temperatures and pressures in wood during SCD drying are real time measured, and then the dynamic diffusion coefficient is calculated, the moisture contents (MCS) at measuring locations are measured after drying. The distribution and changing rule of temperature, pressure and MC are studied. Based on the relationship of temperature, pressure and MC, the transfer rule and mechanism during drying of water and heat are studied. According to volume averaging theory of Whitaker, and the fundamental principle of mass and heat conservation, mathematical model for transfer was established. Combined with curves of shrinkage, mirror-observation by SEM and GC-MS analysis are applied to analysis the migration path and dynamics characteristics of water at molecular level. The mechanism to restrain collapse is elucidated based on the analysis above. Consequently, the studies not only provide theory and practical basis for application of SCD drying to wood, but also provide scientific proofs and technical guarantees for developing new technology of no collapse drying for Eucalyptus plantation wood.

桉木皱缩是制约其作为实木制品用材最严重的干燥缺陷。皱缩主要是由于自由水迁移产生的毛细管张力所致，而超临界CO2干燥过程中木材内部不存在毛细管张力，在减少干燥皱缩上具有重大的应用潜力。本项目在线检测超临界CO2干燥过程中木材内部的温度和压力，基于温度和压力计算动态扩散系数，实时结束干燥，测量检测点处木材含水率。研究干燥过程温度、压力及含水率分布和变化规律。基于温度、压力及含水率的关系研究干燥过程的水分迁移、热量迁移规律和机理，依据Whitaker体积平均理论模型，基于热量守恒和质量守恒原理构建热质传递数学模型。结合干缩曲线，利用显微镜像观测法及GC-MS等检测技术，在分子水平上解析细胞中自由水迁移的路径及动力学特征，揭示超临界CO2干燥对桉木皱缩的抑制机制。本研究不仅为超临界CO2干燥技术在木材干燥上提供理论和实践依据，同时为开发人工林桉木无皱缩干燥新技术和新工艺提供科学依据及技术保障。

我国人工林桉木资源丰富，但在高附加值产品上的利用很低。一些桉树木材强度高、刚度大、质地坚硬且花纹美丽，具有作为家具、地板、木质门窗等高附加值实木产品的潜力，但是桉木常规干燥产生严重的皱缩，这一问题极度影响其在实木产品上的应用。超临界CO2流体的气-液界面消失，不存在表面张力且能快速的扩散到木材内部，泄压时能够迅速膨胀为气态，将木材中的水分脱除，理论上能够消除木材皱缩。本项目测试了木材微观孔隙结构特征、木材的导热系数、木材纤维饱和点和干缩特性、推导了木材超临界CO2干燥传质模型构建的相关参数计算公式和方程。开展了超临界CO2干燥实验，系统的研究了干燥过程木材内部的水分分布和传递规律、应力应变、变形和皱缩、微观结构、抽提物含量、渗透性等。对实验数据进行系统分析和理论研究，主要取得如下研究成果：（1）超临界CO2干燥速度很快，能够快速的脱除木材中的自由水。压力对干燥速度影响显著，且远大于干燥温度；（2）超临界CO2干燥后，水分在木材弦向和径向的分布趋势一致，呈现心层高、表层低的含水率分布。木材中的水分大部分由端部排出，横向排出水分较少；（3）超临界CO2干燥后木材的纹孔膜被破坏，吸水性有不同程度提高，表明木材的渗透性得到有效改善。渗透性提高有助于干燥后期超临界CO2渗入木材和水分排出；（4）超临界CO2干燥过程木材的收缩小于常规干燥，表明超临界CO2干燥能够减小木材的皱缩变形。根据实验研究和相关参数测试，构建了干燥过程的质量守恒干燥模型，进行了参数求解和实验验证，模型满足预测要求；（5）超临界CO2干燥过程的高压阶段能够有效避免产生毛细管张力，泄压过程超临界CO2急速降压膨胀为气体CO2，通过巨大的压力差驱逐排出木材细胞腔内的自由水，脱水过程同样没有产生毛细管张力，因此，从根本上消除了木材细胞产生皱缩的外力因素，这是超临界CO2干燥抑制木材皱缩的主要机理。