Influence of Granule Porosity during Fluidized Bed Spray Granulation

Abstract

Fluidized bed spray granulation is an important process for the transformation of liquids into solid materials. Especially product properties like final particle size and structure of the granules are important. These properties are closely related to the selected process parameters such as temperature and spraying rate. By knowing the relationship between the thermal process conditions and resulting particle structure, the final product can be designed according to the wishes of the consumers. In the presented work, a correlation between the thermal process conditions and the achieved shell porosity, which is relevant for the particle structure, is revealed. Therefore two experimental series with different materials (non-porous glass particles and porous alumina) were performed in a lab-scale fluidized bed spray granulator. The lab-plant allows the precise measurement of the inlet and outlet temperatures and moisture contents of the gas. All process parameters were kept constant except the inlet gas temperature and the spraying rate. Although the amount of initial material as well as the amount of injected solution was the same for all experiments, the final particle size distribution and the structure of the formed shell were clearly different. The obtained experimental results were compared with a process model including a new expression for the growth kinetics, which takes into account an acceleration of the growth velocity due to the formation of a porous structure. The used model also considers the separation of the fluidized bed into two different zones, namely the spraying zone and the drying zone. With the given initial process conditions, the model calculates the time evolution of the particle size distribution, the theoretical outlet temperature and the moisture content of the gas. The introduction of the shell porosity into the growth kinetics reduces the underestimation of the particle size in the model, which occurs if the formed layer on the particle surface is assumed to be compact.

Correlating Granule Surface Structure Morphology and Process Conditions in Fluidized Bed Layering Spray Granulation

“…3. The dependence of the crystallization of sodium benzoate on the drying conditions during fluidized bed coating was also observed by Rieck et al (2015) and Hoffmann et al (2015).…”

“…Another important micro-process is the evaporation of the liquid due to the heated fluidization gas, which is strongly influenced by the drying conditions (Fries, 2012;Uhlemann and Mörl, 2000). Depending on the chosen material system, additional phenomena like crystallization or penetration of the solution into the pores of porous particles can occur and influence the granule properties (Hoffmann et al, 2015;Uhlemann and Mörl, 2000). The knowledge of the effect of different process parameters on the micro processes in the fluidized bed and the resulting granule structures is invaluable for the production of tailor-made particles.…”

“…In most of these studies, either a small set of parameters was chosen or a one-factor-at-a-time approach was used, meaning that interaction between the parameters is not considered. Rieck et al (2015) and Hoffmann et al (2015) investigated the influence of the drying conditions on the layer porosity for the coating of glass and γ-Al 2 O 3 particles with sodium benzoate. Their experiments were carried out in top-spray configuration at different fluidization air inlet temperatures and spray rates of the coating solution.…”

“…The continuous granulation of sodium benzoate in a horizontal fluidized bed with sieve-milling circuit was investigated by Diez et al (2018). Similar to and Hoffmann et al (2015), Diez et al (2018) produced particles with a compact and smooth surface at high air temperatures and low liquid spray rates, whereas low temperatures and high spray rates resulted in a higher surface roughness. Schmidt et al (2017) performed coating experiments with γ-Al 2 O 3 particles and a suspension of limestone, hydroxy propyl methyl cellulose and water at different atomization pressures of the top-spray nozzle used for the injection of the suspension.…”

Effect of Superimposed Vibrations on Droplet Oscillation Modes in Prilling Process

“…Theoretical studies of the process of liquid jet decay into droplets under the action of its own oscillations [6] is the first, major step of theoretical and experimental studies to develop reliable mathematical models and to create calculation techniques for designing prillers. These models should spray the liquid into prills [7] with subsequent crystallization [8]. Moreover, the study of physical processes that affect the size of droplets allows designing new prillers to obtain monodisperse granules [8].…”