Periodic Section Modeling of Convective Continuous Powder Mixing Processes

Continuous powder mixing is considered as an efficient alternative to batch mixing in recent years. While modeling work of the axial mixing process on attenuating feeding fluctuations is well developed [1-4], the transverse mixing process on decreasing local heterogeneity [5, 6] is not yet clarified. This hinders standard procedures to be established in design and control of continuous mixing process.

In order to solve this problem, this work aims to develop a general model to describe the transverse mixing process in a convective continuous mixer. The main idea is that continuous mixing can be considered as a combination of powder flow and mixing processes. Powder flow is characterized by the residence time distribution (RTD), whereas powder mixing can be described by batch-like mixing process simulated in one periodic section of the whole mixer. By characterizing the two processes separately, the model can predict the number of sections required to achieve desired homogeneity, as well as the required feed rate to maintain the desired fill level.

Based on this theoretical modeling, convective continuous mixing and batch mixing in the corresponding periodic section are simulated using the Discrete Element Method (DEM) [7, 8]. Segregating and non-segregating mixing cases are simulated to investigate the difference of mixer performance. The results suggest a possible link between continuous and batch mixing in similar geometries, which indicates the potential of this modeling in quantitative design and control of continuous mixing processes. The model is also able to account for the difference of continuous mixing performance due to particle segregation.

As described above, information of the batch-like mixing process is required in successful application of the periodic section modeling. However, due to the net forward flux in the section, this information is not directly achievable experimentally. Thus, the similarity of mixing efficiency between periodic section and the corresponding batch mixer with similar geometries is investigated. Also by means of the DEM, this work examines how this similarity is affected by three design factors: boundary condition of the batch mixer, backward flux in the periodic section and blade location on the shaft. The flow and mixing are quantified using the velocity streamline in a rotating frame of reference, the axial fill level distribution, and the relative standard deviation (RSD) as the mixing index. Simulation results show that satisfactory similarity is obtained when the blades locate on different sites of the shaft, which suggests the possibility to experimentally estimate periodic section mixing by performing corresponding batch mixer. Conversely, similarity is poor when batch mixer is designed with close-end, which mixes much faster than the corresponding periodic section. The results also indicate considerable improvement of the mixing similarity due to the existence of backward flux in the periodic section.

Reference

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