- Swantje Pietsch, Dr.-Ing., Institute of Solids Process Engineering and Particle Technology -
Fluidized bed spray granulation is one of the main unit operations in the field of solids process engineering and for example applied for the production of pharmaceuticals, fertilizers or detergents. In a fluidized bed the particles are entrained by the upward flow of the fluidization gas resulting in a bed expansion coming along with a high heat, mass and momentum transfer. During granulation, an atomizable liquid (e.g. suspension, solution, emulsion or melt) is injected onto the fluidized solid granules resulting in a layered growth of the initial particles. The surface structure of particles produced in a fluidized bed process is defined by different process parameters, e.g. the spray rate and the gas temperature. The particle morphology later determines the product quality and its handling, e.g. the dispersibility of the powder. This project aims to understand the interaction between material properties, process parameters and product properties and to produce particles with tailor-made functionality for application in other projects of the collaborative I3 lab “Smart Reactors”.
To collect the different influencing process conditions into one parameter, the dimensionless drying potential η has been established in literature. It is defined as the quotient of the still vaporizable amount of water in the air after passing the process chamber and the maximum vaporizable amount at the inlet:
with Ywb[kg/kgdry air] the wet bulb humidity of air, Yout[kg/kgdry air] the humidity at the outlet of the apparatus and Yin[kg/kgdry air] the humidity of the air at the inlet.
Experiments were carried out in the laboratory fluidized bed setup GF 3 (Glatt GmbH, Germany). A two-fluid nozzle of type 970-S4 (Schlick, Germany) with an opening diameter of 1.2 mm was installed in bottom-spray configuration. In a first step, microcrystalline cellulose particles (Cellets® 500, Harke Pharma GmbH, Germany) with a mean particle diameter (dp50,3) of 639 µm were used as core material and granulated with sodium benzoate solution, which is mainly used as food additive. With the design of experiments approach, 32 experiments were defined with varying superficial gas velocity, gas inlet temperature, nozzle pressure, spray rate and temperature of the nozzle air. The surface morphology was afterwards analyzed with the confocal 3D laser scanning confocal microscope VK-X160K (Keyence GmbH, Germany).
Figure 1 shows an overview of the measured surface roughness values at the end of the experiments in dependence on the drying potential. Even though variations can be seen for particles having a similar drying potential, a trend is detectable: The higher the drying potential, the smaller is the surface roughness.
The smaller surface roughness with increasing drying potential is explained by the crystallization process of sodium benzoate. With a higher drying potential, the water evaporates faster from the droplets, resulting in a faster nucleation of crystals. As the residual water in the droplets is still evaporating quickly, the growth of the crystals is inhibited resulting in a dense and compact layer consisting of small, homogenous salt crystals. In case of a lower drying potential, the evaporation is slower, which allows the formed salt nuclei to grow during the evaporating process resulting in larger crystals, which then result in a more porous and rougher layer (Figure 2).
In the next step, other material systems as catalyst carriers will be granulated with catalytic material with the aim of producing a high surface roughness and thus a high catalytic activity.