With spray fluidized bed layering, coating and agglomeration, three high-performing granulation processes for the formulation of particles with defined properties are available (Fig. 1). Among others, interesting properties are size, strength, moisture and porosity of the formed particles.

Fig. 1: particle-forming processes in fluidized beds, a) spray granulation, b) coating, c) agglomeration

The goal of this research work is to recognize and describe the influences of experimental process parameters on product properties and process dynamics. Therefore existing models are enhanced and compared to experimental data. Furthermore a regulation concept to ensure constant product properties and process stability shall be derived from the data and simulations.

Fig. 2: fluidized bed experimental plant

In the first part of this research work the batch and continuous layering granulation as well as the batch and continuous coating were investigated. Based on the work on fluidized bed coating of Hoffmann et al. (2012) and Rieck et al. (2014) the formed crystalline particle structure was determined for varying process parameters. SEM-recordings of the produced granules in these works show (Fig. 2), that high liquid spray rates in combination with low air temperatures (meaning high moisture content of the exhaust air = high drying efficiency) lead to the formation of jagged and porous particle structures and low liquid spray rates in combination with high temperatures (meaning low moisture content in the exhaust air = low drying efficiency) lead to the formation of relatively smooth particles. These findings were confirmed for layering granulation and are yet to be examined for non-crystalline material.

Fig. 3: particle structure at different spray rates and air temperatures

Furthermore especially the continuous spray granulation with classifying product discharge and internal seed particle generation was investigated. Therefore a solution is sprayed on fluidized particles of the same solid material. During the process the product is discharged through a classifying tube in the center of the distributor plate (Fig. 3). It is possible to classify the particles internally. That is, if the sinking velocity of a particle is similar to the velocity of the classifying air in the tube, the particle may be discharged as product. A second possibility is external separation. In this configuration, material is also discharged through the tube and then classified externally through two sieves. The oversized particles are crushed and recycled to the fluidized bed together with the undersized particles. The product fraction is withdrawn. New seed particles for the layering granulation are formed through attrition and pre-drying of spray droplets (overspray) for the version with internal separation. In the configuration with external separation, seed particles are also formed by crushing of oversized particles.
Two experimental sets for the setup with internal separation show strongly oscillating particle size distribution (PSD) in the fluidized bed. Moderate drying conditions hereby enhanced seed particle formation and stabilized the process. Intense drying reduced seed particle formation and led to continuous emptying of the fluidized bed, and thus, to termination of the experiment. In between both regimes, strong oscillations of the PSD occurred (Schmidt et al., 2015a).
For the process with external separation simulations suggest (Heinrich et al., 2002; Radichkov et al., 2006), that the mean diameter of milled particles, and thus, the number of revolutions per minute of the mill, crushing the oversized particles, is decisive for seed particle formation and process stability. Experiments also prove strong oscillations for this configuration (Fig. 4; Schmidt et al., 2015b). The oscillations cause unwanted fluctuations of the bed mass and the product mass flow rate. After a planned adaption of the existing models to describe the process behavior observed, existing control concepts (Bück et al., 2015; Palis and Kienle, 2012; Palis and Kienle, 2014) to remove these oscillations will be tested in practice.

Fig. 4: progression of the particle sized distribution density for a continuous spray layering granulation experiment with external classifying and nucleation
 

 

Furthermore, based on the findings of Hampel (2010), the spray fluidized bed agglomeration is investigated. Therefore in a first step the agglomeration with addition of binder and afterwards the agglomeration through glass transition are investigated. Both processes are conducted in batch and continuous mode. The goal of this research work is also to determine the exact influence of the process parameters on the product properties and process dynamics, and to develop a model predicting these properties and process behavior.

 

Fig. 5: Agglomerate from Cellets™ and maltodextrin solution (DE12) as a binder from a batch experiment