Executive Summary IFPRI Report Powder structure control FRR 62-07 2017
Process Engineering and Food Powders, University of Hohenheim, Stuttgart, Prof. Dr.-Ing. R. Kohlus
Particle structures can be characterized by stereological methods. In case of granulation, systems with a high and low volume fraction of primary particles should be distinguished when choosing the analysis method. Systems with a high volume fraction, close to the maximum packing density, are characterized by the possibility to move of the primary particles. This can be expressed by an average particle-to- particle distance. The non-particle phase of such systems is preferably described by its covariance function, allowing a prediction of specific surface area and average capillary diameter. Dissolution speed as well as crushing strength of these systems show a dependency on particle-particle distance, which strongly decreases with increasing distance to level off at a critical distance. This behavior is however superimposed by the size ratio of primary particle to granule diameter.
In systems with a low volume fraction, the covariance function of both phases, particle as well as binder phase, is of interest. In these systems, the effect of particle size, i.e. both points of the two-point correlation being in the same particle, does not dominate. Primarily, the distance distribution of the primary particles is reflected. The covariance function will be a valuable measure for adapting statistical distribution models, e.g. Poisson distribution, or serve directly as basis for computer simulations of the granule structure. In addition, the covariance function can be analyzed to reveal nearest neighbor distribution.
In both systems, dense as well as dilute, the chord length distribution of the non-particle phase is a direct size measure of the structure.
Mainly, Limestone-PEG systems with varying particle size distribution of the primary particles have been studied. High shear mixer granulation, casting and fluid bed granulation have been applied. NaCl particles have been used as second solid phase. With respect to structure dependent granule properties, the focus was on single particle crushing strength and dissolution behavior. The applied characterization method of analyzing particle structures still needs more validation and applications to shows its full potential as well as its weaknesses. Especially the interaction with simulation tools is an unexploited field. This includes the computer generation of structures of a given stereological description, i.e. covariance function, as well as the simulation of properties. First simulations for the simplest case of stagnant dissolution have been conducted.
With the extended availability of high resolution µXRT systems, more 3D data of granule structures will be generated in the coming years, allowing to prove the benefit of using the proposed structure characterization method. This will also allow to investigate the validity of the found approximations and predictive relations.
The interpretation of the covariance function and chord length distribution, with respect to more intuitively comprehensive measures like coordination number and nearest neighbor distribution, would lead to a method that has its value from a mathematical as well as an engineering point of view.