(159d) Experimental Investigation of Binder Distribution During Reactive Granulation
Binders used in wet granulation can be grouped according to the mechanism by which they solidify as either solution binders (e.g. aqueous solutions of polymers such as PVP or HPC, or water itself), melt binders, and reactive binders. While the kinetics of binder setting in the former two cases can to some extent be controlled by the process parameters (drying rate or cooling rate), in the reactive case the rate of phase change is more or less spontaneous and driven by the kinetics of local reaction-diffusion processes near the binder-particle interface. Investigation of granulation-related phenomena such as binder spreading, particle collisions, nucleation and granule growth are thus further complicated by the simultaneously occurring chemical reaction and the change of physico-chemical properties of the binder that are associated with it. A two-way coupling exists between granulation and reaction kinetics: the evolving structure of the granules being formed influences the solid-liquid contact area to which the overall reaction rate is proportional, and the reaction influences the viscosity and other properties of the binder, which control its spreading on primary particles and ability to dissipate the kinetic energy of inter-particle impacts.
The system of dodecyl-benzenesulfonic acid as a binder and sodium carbonate powder as primary particles is considered in this work [1,2], motivated by the so-called dry neturalisation process used in the manufacture of powder detergents. The measurement of dry neutralization kinetics in a laboratory-scale mechanically agitated granulator is described. The volume of evolved carbon dioxide, which is a by–product of the neutralisation reaction, has been used for following the reaction kinetics, and the effect several parameters on the progress of the reaction has been investigated. The parameters include process temperature, agitation rate, size distribution (thus specific surface) of the primary particles, and the mode of binder addition (step-wise addition). 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. A mathematical model of the process has been formulated and used for testing hypotheses about the rate-limiting steps in each stage of the process, which can be rate of binder spreading, the availability of fresh solid surfaces, the diffusion rate of reaction components near the interface, or the intrinsic reaction kinetics. Knowlege of the rate-limiting steps then serves for making rational process control decisions.
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