This work is concerned with the effect of vessel geometry on
the hydrodynamics of fluidisation of a bed of milled iron oxide.
The effect of going from a cold model representative of a typical
pilot plant reactor to one simulating a semi-commercial unit is
quantified, and various reactor internal configurations on the
latter are evaluated.
The experimental approach is one based on residence time testing
and model fitting with parameter optimisation. A model screening
aimed at identifying the most reasonable modelling approach is
included, and altogether seven models in two categories are
formulated and solved in the dynamic mode. Three of these models
are considered novel at present, along with the dynamic solutions
to two of the others.
The residence time technique involves methane as an inert tracer
in air, and continuous analysis of gas withdrawn from the bed via
sample probes by a pair of flame ionisation detectors. The
process stimulus is governed by a pseudo-random binary sequence,
and correlation analysis is employed for noise reduction. A
Fourier transform routine, developed from first principles,
converts a pair of correlation functions to a process frequency
response, and model predictions are compared with the experimental
data in this form. Two parameters per model are fitted, and the
residual error at the optimum parameter combination provides a
means of identifying the best-fitting model. The optimised
parameters of this model are regarded as estimates of those of
the actual process.
Five models compete in the first screening category. Four of
these have appeared in the literature in one form or another,
and the fifth is novel in that it accounts for axial mixing in
the bubble phase by employing multiple plug flow units. This
model, referred to as the multiple bubble-track or MBT model, is
shown to fit the experimental data better than any of the other
models in both bubbling and slugging systems. This suggests that
employing multiple plug flow units in parallel for the bubble
phase is mechanistically more correct than employing a single
plug flow unit.
The second screening category is related to the situation in
which gas is sparged into an already fluidised bed at some height
above the main distributor. The two models in this category are
both considered novel, and describe opposite extremes of possible
behaviour in one particular sense: one assumes rapid coalescence
between grid and sparger bubbles, and the other none at all. The
laterally segregated bubble phase or LSBP model emerges as the
better process description.The formulation of this model suggests
that physically, bubbles from the sparger tend to retain their
identity as they pass through the bed.
Crossflow ratios estimated on the basis of the best-fitting model
in each category point to the existence of a very strong scale-up
effect. From the shape of the crossflow profiles it appears
that most of the interphase mass transfer occurs in the bottom
meter or so of the bed, and it is suggested that grid design
is the most significant controlling factor. The presence or
otherwise of vertical coils in the bed is shown to have no
significant effect on crossflow, and mass transfer between
sparger bubbles and the dense phase is shown to be similar to
that between grid bubbles and the dense phase.
Finally, it is demonstrated that the axial crossflow profile
in the bubbling bed is consistent with the concept of an
axially invariant mass transfer coefficient based on bubble
to dense phase interfacial area. / Thesis (Ph.D.)-University of Natal, Durban, 1984.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/8657 |
| Date | January 1984 |
| Creators | Dry, Rodney James. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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