Scale-up of detergent granules in a high shear mixer

Abstract

Wet granulation is a process where primary powder particles are made to adhere to form multi-particle entities called granules and this is achieved by using a binder. A simple scale-up of a granulation unit is rather difficult. This becomes even more difficult in a practical situation as the manufacturers make compromises in their scale-up rules on geometries due to cost reasons. Earlier work reported in literature on scale-up emphasized geometrical similarity and proposed various dimensionless numbers for scale-up. Recent work in this area describes granulation process as a multi-scale operation. The final product quality is determined by both macro-scale operation (like rotor speed in a high shear granulator) and micro-scale operation (like particle formation and interaction). Few researchers also talk about an intermediate approach between the macro and micro scales. In this approach, the key transformations such as binder distribution, nucleation, growth, consolidation and breakage were selected based on their relevance to the desired product attributes. The desired product attributes reported in an intermediate approach are chemical homogeneity, granule size, size distribution and granule density etc. There is little published literature on the scale-up of batch granulators in spite of their wide and varied use in the chemical process industries. The aim of this paper is to study the scale-up of detergent powders using a reactive binder processed via wet granulation in a high shear mixer. Three widely used scale-up rules viz. constant Swept Volume, constant Tip Speed and constant Froude Number were tested to identify the right parameters to be scaled for improvements at industrial scale. Bulk properties of the granulation product and mechanical properties of single granules from a desired size range were monitored to decide on the parameters to be scaled. From the bulk properties evaluated, Froude Number emerged as the right approach for scale-up for the system under investigation. However, mechanical properties of single granules did not fit the scaling rules. The strength and yield pressure of individual granules processed at large scale were higher compared to that at a small scale process. Known models of Heckel, and Kawakita and Ludde were used to fit the bulk compression data to estimate single granule properties; which show similar trends as observed in a single granule compression. The Heckel parameter (1/K), and Kawakita and Ludde parameter (1/b) which represents mechanical strength were estimated to be higher for large scale process compared to that for small scale process. In other words, achieving similar particle size distribution does not guarantee similar mechanical properties of individual granules. Hence similarity in single granule properties as well as bulk properties may be a right approach for the scale-up of wet granulation process.