PREDICTION of the GROWTH KINETIC AND AGGLOMERATION MECHANISM USING A MIXER TORQUE RHEOMETER

The evolution of a high shear wet granulation process (HSWG) depends on wet masses properties such as strength and deformation. In literature granule growth is described in terms of Stokes deformation number (Stdef) using the growth regime map [1] but also evaluating the material exchange as proposed by Bouwman et al. [2]. The purpose of this study was to evaluate the ability of a mixer torque rheometer (MTR3, Caleva, UK) to predict the growth kinetic and the agglomeration mechanism of granules produced by HSWG. For this study three formulations were selected, the first composed of 100% (w/w) of microcrystalline cellulose (MCC100%), the second constisting in a mixture 1:1 of sucrose and microcrystalline cellulose (SUCR50%) and the third containing 5% (w/w) of xanthan gum and 95% (w/w) of microcrystalline cellulose (XG5%). The MTR3 was initially used to identify the water amount necessary for the HSWG. Afterwards MTR3 was employed to evaluate the evolution of the wet mass torque over the time in order to identify the torque peak and to study the influence of different parameters (L/S, shaft speed and binder flow rate) on this value. The maximum torque peak developed by the wet masses could represent a measure of the wet granules strength and it is inversely proportional to Stdef. Thus, this value can be useful in the prediction of the granule growth mechanism. In particular, high torque peak values correspond to the induction growth, low torque peak values correspond to crumb growth and finally intermediate torque peak values correspond to the steady growth mechanism. MTR3 measurements showed that formulations MCC100% and SUCR50% present a steady growth regime, instead XG5% shows an induction growth mechanism for all the conditions analyzed. Moreover, for SUCR50% the toque peak values are strongly dependent on experimental conditions and consequently a change in the growth could be observed. Granulation experiments were then performed in a lab scale high-shear mixer (Rotolab, IMA Zanchetta, Italy) in order to verify the growth kinetics and the growth mechanisms by evaluating also the material exchange [2]. Granulation experiments confirmed the mechanisms predicted by the rheological tests, and in particular for the SUCR50% it was possible to observe that the growth kinetic is correlated to the torque peak values and depends on impeller speed and water amount. In conclusion the rheological characterization of wet masses could represent a tool to predict the growth of granules during the granulation process.