Tracer
experiments were conducted in a lab-scale cold flow model
of 0.4 m by 0.2 m in cross section to investigate the solid mixing
behavior of bubbling fluidized beds under continuous particle exchange
with and without immersed tube bundle heat exchangers. The superficial
gas velocity, the solid circulation rate, and the heat exchanger geometry
were varied thereby. An established inductance measurement setup detecting
pulse-injected ferromagnetic tracer particles was used to obtain the
residence time distribution of the continuously exchanged solids.
A profound mathematical routine was applied to calculate the particles’
mean residence time and characteristic values describing solid mixing
phenomena. In this study, it was shown that the bubbling fluidized
bed mixing characteristics are similar to mixed flow but superimposed
by dispersed plug flow to a greater or lesser extent depending on
the geometrical configuration of the immersed tube bundle heat exchangers.
Furthermore, the results show that solid mixing enhances with increasing
superficial gas velocity, whereby short-circuiting and sluggish turnover
of solids were observed at fluidization numbers below 5. It was shown
that these phenomena are increasingly promoted as the tube spacing
decreases, and in general, quantifying them fundamentally improves
the understanding of the fluidized bed reactor design process.
Measurements were
carried out to investigate the wall-to-bed heat
transfer in a cross-flow bubbling fluidized bed under continuous solids
exchange. The experiments were performed with different tube bundle
heat exchangers immersed in a lab-scale gas–solid fluidized
bed cold flow model 0.4 m × 0.2 m in cross section. Geldart group
B particles were used, and the gas velocity, the solids circulation
rate, and the tube bundle heat exchanger geometry were varied. Tracer
experiments that were previously performed with this setup showed
that the mixing characteristics of the cold flow model mostly resembled
mixed flow but superimposed by dispersed plug flow caused by the cross-flow
of particles. Because of this knowledge, it was assumed that the influence
of the imposed cross-flow may have a positive effect on the heat transfer
by intensifying particle convection due to the lateral movement of
solids. Due to the limited studies that focus on the effect of cross-flow
on the heat transfer coefficient, a well-established technique comprising
an electrically heated heat transfer measurement probe was used to
determine this effect. The present results show that more heat is
transported from the probe to the particles when the gas velocity
is increased and less heat is transported when tubes are arranged
more densely in the bed. These findings are in good agreement with
the findings reported in the literature. However, and with the exception
of one test series, no significant increase in the heat transfer coefficient,
which was expected to intensify by the cross-flow, was observed. Based
on the novelty that the study focuses on the influence of cross-flow
on heat transfer in bubbling fluidized beds, the findings represent
a missing link and are part of a series of previously conducted studies.
Furthermore, they can be helpful for understanding the mixing behavior
and accompanying heat transfer in cross-flow bubbling fluidized bed
reactors.
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