Title:
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Multi-scale population balance modelling and controllability of granulation processes
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Many continuous granulation plants operate below their design capacity, suffering from
high recycle rates and instabilities. Thus, there is an immediate economic incentive for
effective operation and control of granulation units. The overall granulation process is integrated
and interacting, with limited manipulated variables available (e. g. binder addition,
nozzle locations and mixing rate). Hence. the complex process dynamics and operational
challenges presented warrant a fundamental model-based strategy for design, operation,
control and scale-up that is well supported by experimental analyses. A realistic model
of the granulation process has to account for the granule size, the binder content, and the
porosity (or related parameter bulk density), thereby necessitating a three-dimensional
population balance model to yield a good representation of the process. While this multidimensional
population balance model is warranted by the physics of the problem. it is a
bigger challenge to derive kernels (rate laws) for the key granulation mechanisms. Most
kernels in the literature are empirical and/or semi-empirical and provide little insight into
the intricacies of the granulation mechanisms. This effectively results in an inability to
make the necessary engineering decisions to improve control of the granulation process.
Hence, this thesis is concerned with a more systems-centric approach to enhance the design,
control and scale-up of granulation processes.
Experimental studies on a lab-scale batch drum granulator for a Calcite/PVOH-H20
system were performed to assess granulation kinetics and model development of the granulation
process. Effects of process /material properties and liquid binder distribution on
granule properties, illustrating the non-homogeneity of key particle attributes and which
justify the need for multi-dimensional population balances, were studied. Process sensitivities,
manipulations and potential disturbances were identified, formulating a comprehensive control configuration for granulation processes, with application seen in a continuous
drum granulation of limestone. While carrying out experiments, multiple granule attributes
were characterised and this presents a challenge, which this research addresses
accordingly.
A population balance model incorporating nucleation, aggregation, breakage and consolidation
was developed in this research. Novel aspects are the mechanistic formulations of
the nucleation, aggregation and breakage kernels which are derived from first-principles.
Such mechanistic descriptions of the rate processes lend themselves to a more in-depth
understanding of the granulation process, contributing fundamental knowledge to the design,
control and scale-up of these processes. A sensitivity analysis of the model was then
performed to ascertain the influence of model parameters on the particle density distribution.
Continuing from this, a compartmentalised version of the combined population
balance model was developed, for the purpose of controllability analysis. Results obtained
were used to identify suitable control-loop pairings to facilitate enhanced control-loop
performance.
Experimental validation of the population balance model is an integral part of this research.
The model was quantitatively validated using lab-scale experimental data for
granule size, binder content and porosity. The tuned model was then able to predict
evolutions and distributions of granule attributes for different operating conditions and
formulations. The model was also validated for different granulation systems. This illustrates
the robustness and flexibility of the model and these results are promising toward
the longer-term step of a first-principles based predictive model for the granulation process
that can help alleviate the need for large number of experiments.
As an alternative to deriving the above-mentioned mechanistic kernels, a discrete element
modelling (DEM) approach was also undertaken in this thesis. Based on a Volume of Fluid
(VOF) method, the analysis carried out provided useful information to help understand
the effect of primary particle morphology on granulation kinetics making it possible to
establish relationships between material and process/design properties and granulation
process behaviour
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