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3.8 T Fluid–Solid Fluidized Bed Reactors o-Phase, w 217

Most industrial-scale fluidized beds employ a type of jet or perforated-plate distrib utor
(Gunn, 1968).
Fluidized beds constituent parts (internals)
These parts are used in fluidized beds for various purposes. For e gas distrib xample, utors
and various types of baffles are installed to decrease the size of the bubbles. Moreo , v er
draft tubes are used to enhance gas or solid circulation. Other devices such as horizontal
ertical tubes are baffles limit circulation and backmixing of solids and gas. Horizontal or v
used for heat management. Devices used to control or improe fluidization beha to v , vior
improve fluidization of cohesive particles or to achieve solids recovery are within the var-
ious internals met in fluidized bed reactors (Kelkar and Ng, 2002). Immersed tubes in
small diameter beds may lead to slugging. Furthermore, attrition of particle breakage may
change the size distribution and possibly change the fluidization beha . vior
Generally, the use of baffles and other mechanical means is usual in fluidized-bed reac-
,
tors. These devices are largely connected with the bubbles. Specif the size of b ub-
ically
bles is decreased and thus mechanical disturbances are aoided via baffles. Furthermore, v
v
this decrease in bubble size improes gas–solid conducting. The latter can be also be
achieved by packing the reactor with large particles and fluidized fine solids in the
interstices of the larger ones. This type of reactor is called “fluidized-packed bed” (Gunn,
we
1968). Ho baffles increase the oerall pressure drop across the reactor length,
,
er
v
v
v
and may also make the achieement of fluidization in all stages dificult. Finally baf , f fles
may create pockets of low solids re which in oxidation reactors may lead to uncon-
gion,
trolled b urning.
3.8.3 Modeling of bubbling fluidized beds with reaction (gas–solid
catalysis case and Type B Fluidization)
The analysis of the following cases is based on
• isothermal operation
• non-existence of radial gradients
• negligible volume changes due to reaction ( 0)
R
• rapid internal diffusion in solid particles ( 1)
s
Two-phase models

Gas in the emulsion phase in completely mixed state In this case, the following are also
assumed: (a) tw(b) plug-flo (c) per- o-phase model applies,ubble phase, w of the gas in the b
fect mixing of the gas in the particulate phase (emulsion phase), and (d) throughout the
reactor, the gaseous reactant is assumed to pass between the b ubble and particulate phase.
This model is referred to as the “two-phase model”, where the emulsion phase is well mixed
(Kelkar and Ng, 2002). It is a one-dimensional model, which assumes steady state (Figure
3.62). In steady state, the concentration of the reactant in the particulate phase is constant
throughout the reactor because of the assumption of perfect mixing in the particulate phase.