Fluidized bed spray granulation – From process understanding to modelling of nucleation and dust integration
Chemical Product Design and Engineering (CPD&E)
Chemical Product Design & Development - V (CPD&E - 5)
Keywords: fluidized bed spray granulation, simulation, nucleation, growth, tracer
Fluidized bed spray granulation (FBSG) is an advanced technology to produce spherical coarse-grained material of uniform size ready for sale without any need for post treatment. FBSG is able to concentrate a consecutive row of unit operations in one apparatus, e.g. drying, agglomeration, classification. However, because of the complexity coming along with the process intensification, it is imaginable that the design and operation of FBSG plants requires a lot of knowledge and know-how with current scale-up and design tools having to be fitted on extensive lab experiments.
Feed material for the process can be solutions, suspensions or even melts directly processed from the preceding production step. The liquid feed is sprayed from the bottom into a shallow bed of hot fluidized particles by a two-fluid nozzle. In this work, the continuous granulation process is examined. This process can be characterized by the following mechanisms: Particle growth due to liquid drop deposition and dust integration, dust formation by not deposited drops and particle attrition, seed formation due to dust agglomeration and granule discharge.
Well-defined seed formation rate and dust integration capacity, which is responsible for a considerable part of the particle growth rate, are a prerequisite for the satisfactory continuous operation. Therefore, main focus of the investigations was on the enlightenment of these mechanisms and quantification of their kinetics. Batch granulation experiments with tracer-marked dust and continuous granulation experiments were done to get a qualitative understanding and to quantify the kinetics. With the tracer method, the influence of several parameters on the quantitative dust integration in growing particles was examined. Operating parameters like fluidizing gas temperature or feed mass flow as well as process parameters like dust particle size distribution or fluidized bed mass were varied. Furthermore special tracer experiments were conducted to enlighten the mechanism of nucleation which turned out to be a combination of seed formation by agglomerating dust particles and growth of these dust agglomerates.
Continuous steady-state experiments with external nuclei supply provided information about growth velocity of particles and additional internal nucleation stream for particular operating conditions.
The experiments implied that a process model exceeding the rigorous modelling stage has to consider spatially distributed parameters. The spray jet and the fluidizing air cause a characteristic movement of the air, the particles, the drops and the dust in the granulation chamber. Therefore computational fluid dynamics (Fluent®) simulations were used to identify different zones and quantify velocity vectors of different phases in the chamber.
Based on the experimental and the CFD results a model was developed combining two stages of modelling. A steady-state sub-model as simple as possible but with sufficient detail is used in order to calculate the kinetics of growth and nucleation. This model is coupled with an unsteady sub-model including population dynamics to enable time-dependent calculations.
See the full pdf manuscript of the abstract.
Presented Thursday 20, 15:00 to 15:20, in session Chemical Product Design & Development - V (CPD&E - 5).
