Multi-scale understanding of agglomeration and deagglomeration in fluidized beds

由于具有良好的传热、传质和动量传递，流态化被广泛应用于化工、燃烧、制药和食品工业等工业领域，因此经常需要将液体注入造粒或包衣。当颗粒不是通过凝固的液层而是通过粘在一起的潮湿颗粒生长时，就会发生团聚，导致黑莓形状的多孔颗粒。该团聚可能是例如食品粉末的尺寸放大所需的，也可能是例如药物包衣工艺中所不希望的。或者，对于通过提高温度进行玻璃化转变的颗粒，也可以发生颗粒粘连。如果加工温度接近玻璃化转变温度，颗粒局部表面可能会变得粘稠或呈橡胶状。粒子在这些点上的碰撞将随后形成凝聚体。与基本机理无关，床料的团聚程度将决定其在系统内的进一步传输以及产品质量和形态。因此，本项目旨在提供对团聚的微观机制以及团聚的解聚的基本理解，这是迄今为止文献中所缺乏的。这些结果将有助于更好地预测和控制沸腾床过程，并提高过程性能。多尺度实验装置将通过数值研究来使用和扩展。在第一步，将进行单粒子实验。在以前的DFG程序(HE 4526/9-1、HE 4526/9-2)中创建的实验装置将被扩展，以允许颗粒团聚。通过团聚体与壁面和其他颗粒的碰撞来分析团聚体的团聚机理。实验将首先在环境条件下进行，重点是碰撞动力学和静力学，然后将综合传热和传质。这些研究包括模型粒子和粘弹性材料。单颗粒实验的结果将转移到流态化过程中。这将把宏观过程参数及其对流动行为动力学的影响与它们在不同长度和时间尺度上对团聚的微观力学的影响联系起来。通过数字图像分析比较凝聚体的流动形态和尺寸分布，可以得到不同物质体系的状态图，指出颗粒团聚的极限。为了考虑凝聚和解聚，模型中考虑了液桥的影响。经过实验数据的验证，这些模拟的结果将被用于在DFG优先程序SPP 1679期间开发的开源动态流程模拟工具DYSSOL的聚集核构造。

Due to the very good heat, mass and momentum transfer, fluidized beds are widely applied in several industrial fields, e.g. chemical engineering, combustion, pharmaceutical and food industry, whereby a liquid is often injected for granulation or coating purposes. When the particle is not growing by solidified liquid layers but by wetted particles that stick together, agglomeration takes place, resulting in black-berry shaped and porous particles. The agglomeration may either be desired e.g. for the size-enlargement of food powders, or undesired, e.g. in pharmaceutical coating processes. Alternatively, particle adhesion can occur for particles that undergo glass transition by increasing the temperature. If the process temperature is near the glass transition temperature, the particle local surface may become sticky or rubbery. Particle collision at these spots will subsequently form agglomerates. Independent of the underlying mechanism, the extent of agglomeration of the bed material will determine its further transport within the system and the product quality and morphology. Therefore, this project aims to provide a fundamental understanding of the micro-mechanisms of agglomeration as well as deagglomeration of agglomerates, which is so far missing in literature. These findings will allow a better prediction and control of fluidized bed processes and an increase of the process performance. A multi-scale experimental setup will be used and extended by numerical investigations.In a first step, single particle experiments will be performed. The experimental setup created in a former DFG program (HE 4526/9-1, HE 4526/9-2) will be extended to allow for particle agglomeration. The deagglomeration mechanism will be analyzed by collisions of agglomerates with walls and other particles. Experiments will be performed first at ambient conditions to focus on collision dynamics and statics and later on heat and mass transfer will be integrated. The investigations include model particles as well as visco-elastic materials. The findings from the single particle experiments will be transferred to a fluidized bed process. This will connect the macroscopic process parameters and their influence on the flow behavior dynamics to their influence on the micromechanics of agglomeration on different length and time scales. By comparing the flow patterns and the size distributions of the agglomerates by digital image analysis, a regime map will be obtained for different substance systems indicating the limits for particle agglomeration.CFD-DEM simulations will be used for the numerical investigations. To account for agglomeration and deagglomeration, the effect of liquid bridges will be included into the model. After validation with experimental data, the results from these simulations will be used to construct agglomeration kernels for the open-source dynamic flowsheet simulation tool DYSSOL, which has been developed during DFG Priority Program SPP 1679.