Investigation of rate-limiting steps during granulation with a chemically reactive binder

The system of dodecyl-benzenesulfonic acid as a binder and sodium carbonate powder as primary particles is considered in this work, motivated by the so-called dry neutralization process used in the manufacture of powder detergents. The measurement of dry neutralization kinetics in a laboratory-scale mechanically agitated reaction vessel is described. The volume of evolved carbon dioxide, which is a by-product of the neutralization reaction, has been used for following the reaction kinetics, and the effect of selected key parameters on the progress of the reaction has been investigated. These include process temperature, size distribution (thus specific surface area) of the primary particles, agitation rate and the binder/solids ratio. The overall conversion of the reaction and the properties of the formed granules (size distribution, internal structure and porosity) have been evaluated in each case. Hypotheses about the rate-limiting steps in each stage of the process – which can be the rate of binder spreading, the availability of fresh solid surfaces, the diffusion rate of reaction components near the interface, or the intrinsic reaction kinetics – have been proposed based on kinetic parameters evaluated using a mathematical model. Knowledge of the rate-limiting step is a pre-requisite for making rational process control decisions.