(422d) Dynamic Process Behavior in Fluidized Bed Opposed Jet Mills

In fluidized bed opposed jet mills products can be ground into the submicron range. Therefore highly expanded gas jets are introduced through nozzles which are directed to a common focus point. Particles are entrained into the gas flow, accelerated and finally colliding with other particles. If the kinetic energy of the particles at the impact is high enough, breakage or abrasion occurs even for the hardest materials. Since no grinding tools are needed here, the autogenous comminution process allows grinding without contamination which is especially necessary for pharmaceutical products. Also thermosensitive materials like polymers can be processed in jet mills so that a wide scope of industrial applications is possible [1].

After the milling process, the particles are following the gas stream upwards to an integrated classifier wheel. Unground or oversized particles are rejected while product-sized particles can leave the machine through the classifier. The rejected material is circulating back to the bottom of the machine were it can be stressed again. Like in other classifier mills the processes in fluidized bed opposed jet mills can be divided in three unit operations: The comminution zone, where size reduction takes place, the classification zone and the pneumatic transport zone in between.

Classifier mills are characterized by a high solids holdup and therefore high solids concentrations appear in the unit operations. This leads to strong interaction between the zones whereas the gas flow is used for grinding, transport and classification process simultaneously. As a consequence, a higher gas flow might increase the comminution kinetics but would lead to a coarser product at the same time when the operating mode of the classifier is kept constant. Especially in dynamic processes, like a start-up, the analysis and the modelling of the mill behavior is challenging:

Until a steady state is reached the solids holdup and the particle size distribution inside the mill can vary strongly. This affects the fluid dynamics in all three unit operations and its interactions. Next to the internal parameters these changes cause a time dependency of external parameters, namely the product mass flow and its particle size distribution. This can cause at non-steady state conditions an unwanted product quality and should be avoided in the production of high-priced materials. In literature there is little written about the connection between external and internal parameters in classifier mills or the dynamic process behavior itself. For that reason, this work was carried out to contribute to a better understanding of comminution processes in classifier mills focusing the dynamic aspects.

To get excess to the complex mechanisms inside the mill, the presentation will show a strategic analysis and a modelling strategy for fluidized bed opposed jet mills. Therefore comminution experiments were carried out in a lab scale mill with an implemented online laser diffraction system on the product side. As test material well defined glass beads with narrow particle size distribution were used. They were chosen since they come directly from synthesis and hence have no prior load.

The comminution kinetics was determined in batch grinding experiments. Therefore the time evolution of the particle size distribution and solids holdup was measured. A model based on population balance, the so called Kapur function [2,3], was used to characterize the breakage mechanism. So the influences of parameters like feed particle size distribution, solids holdup, classifier speed etc. on the comminution zone and finally on the comminution process are describable. Additionally, the breakage probability of the used glass beads was determined in a single grain impact device to depict the impact event more precisely.

Since the flow around and in the classifier is inaccessible, well defined mixtures with product sized and non-product sized particles were used as feed stock. By analyzing the mass flow and the particle size distribution of the product a closer look into the recirculation and the rejecting process at the classifier wheel is possible. These experiments were intensified by using capacitance probes to measure the solids concentration in the transport zone with a high time resolution. So the flow to the classifier wheel and back to the comminution zone can be analyzed and lead to a better understanding of the local recirculating process.

At last a fed-batch mode was performed to simulate start-up and load change processes in classifier mills. These dynamic investigations show the decrease of particle size inside the mill just as the product mass flow and particle size distribution. In start-up processes first the mill contains a low solids holdup and therefore the breakage rate is limited caused by a low amount of particles which are entrained into the gas jets. Exceeds the solids stock a specific value the solids concentration in the jets is higher but on the other side the specific impact energy is decreased. For that reason experiments with variable, constant solids holdup were carried out to investigate the dynamic performance of the comminution, transport and classification processes.

The presented results of the comminution in the fluidized bed opposed jet mill contributes to a predictable dynamic model and therefore to material and energy saving process improvements.

The financial support of DFG (Deutsche Forschungsgemeinschaft) within the priority program SPP 1679 "Dynamic flowsheet simulation of interconnected solids processes" is gratefully acknowledged.

References:

[1] M. Benz, H. Herold, B. Ulfik, Performance of a fluidized bed jet mill as a function of operating parameters, Int. J. Miner. Process., 44-45:507-519, 1996

[2] P.C. Kapur, Kinetics of batch grinding, Part B. Transactions of the AIME 247:309â??313, 1970

[3] H. Berthiaux, J.Dodds, Modelling fine grinding in a fluidized bed opposed jet mill Part I: Batch grinding kinetics, Powd. Techn., 106:78-87, 1999