介电质辅助微波加热的初始非饱和多孔介质冷冻干燥

Improving process economy by increasing sublimation-desorption rate of freeze-drying is a worldwide research topic. Freeze-drying is a typical process of coupled heat and mass transfer. The drying rate-controlling factor is just the heat and mass transfer. Previous studies have shown that simple or single factor enhancement of eigher heat transfer or mass transfer could moderately be effective, but the remaining one would inevitably become the drying rate-controlling factor. In order to reduce freeze-drying time and increase energy utilization efficiency, a novel idea is put forward in this project: freeze-drying of initially unsaturated porous media with dielectric-aided microwave heating. That is to use dielectric-aided microwave heating to enhance heat transfer, at the same time to use initially unsaturated frozen material with prefabricated porosity to enhance mass transfer, achieving simultaneous enhancement of heat and mass transfer. This innovative technique has the advantages of better product quality, fast drying rate and easy operation, which is pretty suitable for tray freeze-drying of liquid material. It is expected that the drying time required for proposed technique is at least 40% shorter than that for conventional vacuum freeze-drying. The objectives of the present project are to perform experimental and theoretical investigations to verify the proposed idea. Experimentally, liquid material to be freeze-dried will first be prepared into porous material like “ice-cream” with a certain initial porosity together with a proper dielectric material having a large loss factor, and then freeze-dried with microwave heating. Meanwhile, a generalized adsorption-desorption equilibrium relation will be first constructed, and then a multi-dimensional governing equation set for heat and mass transfer of microwave freeze-drying and a wave equation for propagation and dissipation of microwave electric field in drying porous media will be derived and solved numerically in an effort to have good agreements between theoretical predictions and experimental data. Finally, the idea will be evaluated at the level of overall energy consumption and energy consumption distribution of all sub-operations. Such an idea, i.e., simultaneously enhancing heat and mass transfer of freeze-drying is the first of its kind, and never found in any openly published papers, books and reports. It is a perfect combination of high product quality with low processing cost. This fundamental research would establish a new freeze-drying process of liquid material, and provide an approach towards guiding and optimizing food and pharmaceutical products processing industries in China.

冷冻干燥是热质耦合传递过程，单一强化传热或者传质其中一个因素，另一个必然会成为过程速率的控制因素。为了缩短干燥时间和提高能量利用率，我们提出：介电质辅助微波加热的非饱和多孔介质冷冻干燥。即用介电质辅助微波加热强化传热，用初始非饱和冷冻物料强化传质，实现传热传质同时强化。它具有产品质量高、干燥速率快和能耗低的优点，非常适合液体物料托盘冷冻干燥工艺，干燥时间预计比常规真空冷冻干燥缩短40%以上。实验上，将易吸收微波的介电质与具有一定初始孔隙的预冷冻物料一同塑形固化，再进行微波冷冻干燥。理论上，建立含湿多孔介质中普遍化的吸附—解吸平衡关系和电场传播和耗散的波动方程，以及微波冷冻干燥热质耦合传递的控制方程，实现理论预测与实验结果吻合。最后从总能耗和能耗分配的层面评价该思想。本项目研究成果将建立一个新型的冷冻干燥方法，极具学术价值和应用前景，对我国食品和药品，特别是中医药生产具有非常现实的指导意义。

冷冻干燥是食品和药品生产，以及材料制备等过程不可缺少的单元操作。冷冻干燥产品质量高，但是能耗高、时间长。强化冷冻干燥过程是一个世界性的研究课题。本项目提出了同时强化过程传质传热的技术思想，即“介电质辅助微波加热的非饱和多孔介质冷冻干燥”，旨在从理论和实验上验证该技术思想对液体物料冷冻干燥的强化作用，并实现模型预测和实验结果良好吻合。在实验研究方面，设计了一套实验室规模的多功能微波冷冻干燥装置。微波加热系统选用新型固态微波源，实现了小功率下稳定连续输出。微波干燥室根据截止波导理论进行设计，将真空室与微波腔合二为一，同时满足真空密封与微波屏蔽。分别以甘露醇、头孢曲松钠、维生素C和速溶咖啡为溶质，进行常规和微波冷冻干燥实验。结果表明，初始非饱和物料能够有效强化过程传质。四种物料的非饱和冷冻样品干燥时间均比常规饱和样品缩短20%以上。SEM表征显示，非饱和物料具有疏松的球状孔隙结构和纤薄的固体基质，有利于水蒸气迁移和吸附水脱附。介电质辅助微波加热能够极大地强化了过程传热。与饱和物料常规冷冻干燥相比，介电质辅助的非饱和样品微波冷冻干燥时间能够缩短40%以上。这表明该技术思想实现了过程传质传热的同时强化，显著降低了干燥时间、提高了能量利用率。在理论研究方面，基于局部质量非平衡假设建立了多孔物料的质-热-电磁波耦合多相传递模型。模拟结果与实验数据十分吻合，在理论上验证了该技术思想对传统冷冻干燥过程的强化作用。通过考察物料在干燥过程中饱和度、温度和电场强度分布，对干燥过程的质、热传递机理和电磁波在多孔介质中传播与耗散规律进行了深入分析。该技术思想是过程低消耗和产品高质量的完美结合，为解决冷冻干燥过程能耗高、速率低的问题提供了新的解决方案，必将对传统的冷冻干燥过程产生极其深远的影响。该方法非常适合高附加值产品的冷冻脱水。研究成果在食品、医药及材料行业有着巨大的应用前景，推进我国工业的高效绿色发展。