Abstract:
Production of dry particles made of titanium dioxide powder and black liquor is
the first step in a newly proposed black liquor treatment process (Frederick et. al., 1992).
This work investigates the granulation processes in a fluidized bed granulator of titanium
dioxide and black liquor. Experiments are performed in a batch-solid operated fluidized
bed granulator with batch-fed addition of black liquor. Two separate sub processes are
identified in the granulation process: agglomeration of particles and attrition of formed
agglomerates. Two independent transients determine the dynamics of the agglomeration
process: time and liquid load. The agglomeration process is kept in the nucleation
regime. The agglomeration of particles is here caused by liquid droplets being loaded
with dry particles as they enter the bed. The agglomeration process is very fast due to the
vigorous motion of particles in a fluidized bed. One can assume that the time transient
behavior of the agglomeration process is completed shortly after the droplet enters the
bed. This enables modeling of the agglomeration as a liquid transient process alone.
The time scale for the attrition process is determined by the drying operation of
the agglomerates. This is a slow process compared to the formation of agglomerates, and
the attrition is modeled as a time transient process. The difference in the time scales of
the two processes enables us to observe the effect of the attrition process independently,
and thus model the two processes separately. Each of the two sub processes is modeled
as first order processes. The obtained rate constants are valid only within the range of
experimental conditions used in this study. However, the work contains several
examples, that show the method of combining the results of the model of the two sub
processes and their conversion/extension to different experimental conditions (different
bed loads and liquid flow rates), and different process configurations (continuous feed of
solids, selective removal of solids etc.). Three new dimensionless criteria, that describe
the mass and energy relations in a fluidized bed granulator, are introduced. Each of these
criteria must be satisfied to obtain stable operation of the fluidized bed granulator.
