(70bx) A Novel Multi-Dimensional Population Balance Model Incorporating Particle Size Enlargement and Drying Behavior for Fluidized Bed Spray Granulation

In the literature, many attempts can be found to describe the particle formation in fluidized bed granulation in terms of population balances, which describe the temporal change of number distribution. The particle formation in fluidized bed granulation is influenced be numerous parameters. E.g. Watano [1] observed that the moisture content in the solids is one of the most relevant particle property to control the granulation process. This leads to the conclusion that properties (here particle size and moisture content), which control the process of particle formation, should be considered in the population balance model. Consequently a multidimensional population balance model is needed.

The presentation concerns the coupled processes of granulation and drying. In order to predict temperature of gas and solid phase and moisture content in the solid phase, heat and mass transfer mechanism and particle size enlargement have to be combined in one model. Therefore the solid phase is modeled by a three dimensional population balance with internal coordinates size, enthalpy and moisture. The numerical simulation of such high dimensional population balance systems is very demanding. The model reduction using marginal distributions is one possibility to overcome these numerical problems. The latter lead to a coupled set of three one dimensional PBE, which contain kinetic expressions for heat and mass transfer between solid and gas phase. A first attempt for this kind of model reduction was done by Hounslow et al. [2], who extended the vector of internal coordinates by the particle tracer mass. The resulting 2D-PBE was transformed into a set of two 1D-PBE assuming that particles of the same size contain same amount of tracer mass. Tan et al. [3] applied this model to fluidized bed melt granulation. The more general case of liquid spray granulation can not be found in literature.

It will be demonstrated, how the processes of granulation and drying can be described in heterogeneous fluidized bed model taking material properties such as adsorption isotherms into account. Further it will be shown, that the discretization scheme of Hounslow [2] for multidimensional population balance will fail to predict intensive properties of the disperse phase. Therefore a slightly modified discretization, which preserves the mass and the number of the system as before and predicts the intensive properties such as particle temperature and particle moisture content as well, is suggested. For the better understanding of the processes some simulation results for evolution of particle size distribution, change of particle enthalpy, change of particle moisture contents, outlet and mean gas temperature and outlet and mean gas moisture contents, will be presented. First comparisons with experimental results will also be presented.

[1] Watano, S., T. Fukushima, and K. Miyanami, 1996, Heat transfer and rate of granule growth in fluidized bed granulation, Chemical and Pharmaceutical Bulletin 44:572?576.

[2] Hounslow, M. J., J. M. K. Pearson, and T. Instone, 2001, Tracer studies of high shear granulation: II. Population balance modeling, AIChE Journal 47:1984?1999.

[3] Tan, H. S., A.. D. Salman, and M. J. Hounslow, 2005, Kinetics of fluidised bed melt granulation, Part III: Tracer studies, Chemical. Engineering. Science 144:65?83.