Pore network modeling of freeze drying on the basis of lyomicroscopic and tomographic measurements

冷冻干燥是制造高附加值产品的一个必要和常见的过程，但它也是缓慢和昂贵的。提高效率的努力将这一过程推向了一些区域，在这些区域中，产品的固体支架可能会软化和坍塌。这样的破坏性事件总是局部的，因此不能被传统的连续介质模型捕获，否则它们的预测能力也是有限的。因此，本项目首次提出了一种能够反映微尺度的冷冻干燥孔道网络模型。孔隙网络将是三维的和不规则的。它将考虑干燥体的热和质量传输以及结构和性质的局部、孔隙尺度的变化，并在热传输和干燥之间进行双向耦合。冷冻实验和随后在冷冻显微镜中冷冻干燥含水糖溶液的实验将指导模型的开发。冷冻是关键的一步，因为它会在周围产生不同大小的冰晶，并塑造固体支架的形状，以便随后干燥。冷冻或冷冻干燥材料的形态是基于三维X射线层析成像数据进行评估的，并用于生成逼真的孔隙网络。识别了孔隙网络模型的参数，并通过在导致结构坍塌的条件下进行的外部和内部冷冻干燥实验对模型进行了验证。

Freeze drying is a necessary and common process in the manufacturing of high-value products, but it is also slow and expensive. Efforts to increase the efficiency push the process into regions, in which the solid scaffold of the product can soften and collapse. Such damaging events are always local and can thus not be captured by conventional continuum models, which are also otherwise limited in their predictive ability. Therefore, and for the first time, a pore network model capable of representing the microscale shall be developed for freeze drying in this project. The pore network will be three-dimensional and irregular. It will account for the local, pore-scale variation of heat and mass transport as well as of structure and properties of the drying body, with two-way coupling between heat transport and drying. Freezing experiments and experiments of subsequent freeze drying of aqueous sugar solutions in a lyomicroscope will guide model development. Freezing is a crucial step, because it creates around ice crystals of different size and shape the solid scaffold to be subsequently dried. The morphology of the frozen or freeze dried material is evaluated on the basis of three-dimensional X-ray tomography data and used to generate realistic pore networks. Parameters of the pore network model are identified and the model is validated by freeze drying experiments conducted both outside and within the region of conditions that result in structural collapse.