Epstein N., Grace J.R. Spouted and Spout-Fluid Beds: Fundamentals and Applications
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Conical spouted beds 97
restitution coefficient and frictional viscosity and showed that both affect especially the
radial voidage distribution. Wang et al.
50
simulated the axial and radial distributions
of static pressures and vertical particle velocities of conical spouted beds. Simulation
results show that the axial solid phase source term has the most significant influence on
static pressure profiles, followed by t he restitution coefficient and shear viscosity. These
authors prove that the introduction of a source term to modify the gravity term in the
annulus is essential to obtain reasonable agreement with experimental static pressure
profile and particle velocity data.
Diaz et al.
51
simulated the hydrodynamic behavior of uniform glass beads and sand
of wider size distribution, based on an Eulerian-Eulerian approach. This approach is
able to predict the experimental results for glass beads, but the predictions for sand
were unsatisfactory. Key issues need to be addressed to improve computational fluid
dynamics (CFD) predictions, such as the inclusion of shape factor, definition of slip
boundary condition at the walls, inclusion of granulometric distribution, and turbulence
model improvement. This approach is not able to suitably predict peak pressure drop.
Duarte et al.
52
modeled fluid-particle flows in spouted beds and, specifically, the evolution
of pressure drop with air velocity in conical beds, but their model also underestimates
peak pressure drop.
More recently, Barrozo et al.
53
carried out an experimental study and CFD simulation
based on an Eulerian-Eulerian granular multiphase model. They found that CFD is a
useful technique for predicting the hydrodynamic behavior of all three regimes in conical
beds, namely, spouting, transition, and dilute spouting.
5.8 Applications
Conical spouted beds are especially suitable for operations such as drying, coating, and
granulation, and for others with promising future potential, such as catalytic polymeriza-
tion,
5
treatment of wastes (biomass, plastics, and tires),
7–9
and selectivity-conditioned
catalytic reactions.
10,54
5.8.1 Minimum spouting under vacuum and high temperatures
Eq. (5.11), formulated at ambient conditions, overestimates the minimum spouting
velocity for the intermediate or high temperatures used in many chemical processes.
Olazar et al.
55
studied the effect of pressure and temperature on minimum spouting
velocity using beds of sand of 0.65 and 1.4 mm particle diameter, which are those of
practical interest in these processes. The temperatures ranged from ambient to 600
◦
C,
and the pressure from 0.25 to 1 atm (the aim was vacuum pyrolysis). The authors observed
that as temperature and pressure increased in these ranges, there was a significant
decrease in the minimum spouting velocity, owing mainly to the increase in gas viscosity
with temperature and gas density with pressure, and therefore to the increase in the
momentum transfer from the gas to the solids. Accordingly, a new equation was proposed
98 Martin Olazar, Maria J. San Jos
´
e, and Javier Bilbao
by changing only the coefficient and the Archimedes exponent in Eq. (5.11) to give
Re
msi
= 0.0028Ar
0.822
D
b
D
i
1.68
tan
γ
2
−0.57
. (5.44)
Subsequently, this equation was found to be valid for temperatures up to 900
◦
C.
5.8.2 Catalytic polymerization
Catalytic polymerization has peculiar characteristics that condition the design of t he
reactor: high exothermicity, fusion of particles that are growing by coating with poly-
mer, wide particle size distribution, and the need for continuous catalyst circulation
(imposed by production requirements). Accordingly, one of the first chemical appli-
cations of the dilute spouted bed was catalytic polymerization,
5
with gaseous benzyl
alcohol polymerization carried out in the range of 250
◦
C to 310
◦
C on silica alumina
and Y-zeolite acid catalysts to obtain stable polybenzyls.
A model has been proposed to predict the production of stable polybenzyls based on
the definition of streamtubes described in Section 5.4.1. The gas (benzyl alcohol) flow
rate along each streamtube at steady state is given by
u
ij
∂ P
A
∂ρ
− D
∂
2
P
A
∂ρ
2
−
2D
ρ
∂ P
A
∂ρ
+
ρ
c
RT (1 − ε)
M
p
ε
˙
r
p
= 0, (5.45)
where the polymerization rate is given by a Langmuir-Hinshelwood-type expression,
56
and the boundary conditions are:
at the inlet of the jth streamtube (reactor inlet):
P
A
= P
Ao
(5.46)
and
D
∂ P
A
∂ρ
= u
i
(P
Ao
− P
A
), (5.47)
and at the outlet of the jth streamtube (bed surface):
∂ P
A
∂ρ
= 0. (5.48)
5.8.3 Waste treatment
5.8.3.1 Biomass drying and pyrolysis
Conical spouted beds have proven to be suitable for the drying of wood and agro-forest
residues of wide particle size distribution and irregular texture, without the need for
inert solids to promote solids circulation.
57
Eq. (5.11), developed for granular materials,
is valid for calculating the minimum gas velocity for biomass beds (sawdust and wood
residues) at ambient or low-temperature conditions.
Conical spouted beds have also proven to be especially suitable for biomass pyrolysis
with a small amount of sand in the bed (<50% by volume).
7
Figure 5.12 shows a
Conical spouted beds 99
Feeding system
V
P
P
PLC
TIC
TI
TI
TI
TI
W
TI
DP
TIA
FIC
FIC
TIC
Vent
Vacuum
pump
GC
Cyclone and
filter
Reactor
Condenser
Coalescence
filters
Preheater
Nitrogen
Air
Figure 5.12. Conical spouted bed pyrolysis reactor.
pyrolysis unit designed for a biomass flow rate of 10 g/min. The total height of the
reactor is 0.34 m (H
c
= 0.205 m, γ = 28
◦
, D
c
= 0.123 m, D
o
= 0.02 m, and D
i
=
0.01 m). The short gas residence time (milliseconds) and the good gas–solid contact
allow operation at moderate pyrolysis temperatures (<500
◦
C) and at both atmospheric
pressure and vacuum conditions.
7,55
The maximum liquid yield is similar to that in
fluidized beds, close to 70 percent, but at a lower temperature, 450
◦
C.
7
Furthermore,
segregation in t he fountain allows extracting the spent char through a lateral outlet at the
bed surface.
5.8.3.2 Waste plastic pyrolysis
Thermal decomposition, or pyrolysis, is one of the methods with the best perspective
for upgrading plastic wastes to obtain feedstock and fuel. Operating in a fluidized bed
presents problems because of particle agglomeration caused by fusion of inert particles
coated with plastic.
58
The good behavior of the conical spouted bed reactor is quantified
on the basis of the critical thickness of the fused plastic that may be handled. The critical
thickness is the value above which the particles fuse when they collide and is a function
of the plastic’s viscosity and the size and momentum of the colliding particles:
58
δ
crit
= r
p
exp
√
2m
p
ν
μπr
2
p
− 1
. (5.49)
100 Martin Olazar, Maria J. San Jos
´
e, and Javier Bilbao
Consequently, an increase in particle velocity, ν, and voidage by increasing the air
velocity leads to a higher processing capacity. In fact, operating in the transition regime
between dense and dilute spouting in the experimental unit s hown in Figure 5.12 (30–100
g of sand, d
p
= 0.6–1.2 mm, 500
◦
C), the average velocity of the particles in the annular
(limiting) zone may be controlled above 0.25 m/s, and the critical thickness predicted by
Eq. (5.49) is 250 μm.
59
The critical thickness of the fused plastic in bubbling fluidized
beds is between 4 and 7 μm.
58
Aguado et al.
59
were able to feed 4 g/min of plastic, an
order of magnitude greater than that in a fluidized bed, with the same amount of sand.
5.8.3.3 Scrap tire pyrolysis
Recovering value from scrap tires has received considerable attention in developed
countries, where an average of 6 kg of used tires per person per year is generated. The
unit shown in Figure 5.12, in which particles are sticky, large, irregular, and with a
wide size distribution, has been applied successfully to the pyrolysis of scrap tires.
9
The
good performance is a result of (1) high heat and mass transfer rates between phases,
favored by the countercurrent gas–solid contact, and (2) short gas residence time, which
minimizes the secondary reactions of devolatilization products.
High yields of primary products are obtained at lower temperatures than in fluidized
beds. Furthermore, a very high yield of d-limonene, 23.4 percent, was obtained with
this reactor at a temperature of 450
◦
C.
9
Limonene, the dimer of natural rubber, has
extensive industrial applications, and its production is i ncreasing at a steady pace. This
product is very sensitive to temperature, and the features of the conical spouted bed
reactor generate a high yield.
Chapter-specific nomenclature
C
a
, C
o
, C
s
dimensionless tracer concentration in the annulus, inlet and spout
C
D
drag coefficient, (24/Re)(1 + 0.15 Re
0.687
)
D dispersion coefficient, m
2
/s
D
b
upper diameter of static bed, m
D
c
cylindrical column diameter marking termination of cone, m
d
pB
diameter of larger particles, m
d
pS
diameter of smaller particles, m
d
sv
Sauter mean diameter in the whole bed, m
d
svu
Sauter mean diameter in upper half of bed, m
F
D
/F
G
ratio between drag and gravitational forces,
3
/
4
C
D
Re
2
/Ar
f (d
sv
) average value of particle size distribution function in the bed
f (d
sv
)
u
average value of particle size distribution function in upper half of the
bed
M
b
mixing index for binary mixtures
M
p
average molecular weight of polymer, g/mol
M
t
mixing index for ternary mixtures
m
p
mass of the particle coated with plastic, g
Conical spouted beds 101
P
A
partial pressure of benzyl alcohol at any point, Pa
P
Ao
partial pressure of benzyl alcohol at the inlet, Pa
Re
msi
Reynolds number for minimum spouting velocity based on D
i
Re
dsi
Reynolds number for minimum dilute spouting velocity based on D
i
Re
t
terminal particle Reynolds number
r
p
radius of the particle coated with plastic, m
˙
r
p
polymerization rate, (mass polymer)/(mass catalyst)·(time), s
−1
u
ij
gas velocity at the ith volume element in the jth streamtube, m/s
u
s
average air interstitial velocity in the spout at a given level, m/s
ν
z
vertical component of local particle velocity, m/s
ν
z
(0) vertical component of local particle velocity at axis of contactor, m/s
X
B
weight fraction of bigger particles
X
B
weight fraction of bigger particles in whole bed
(
X
B
)
u
weight fraction of bigger particles in upper half of bed volume
Greek letters
γ included angle of cone, degrees or radians
δ
crit
critical thickness of plastic layer, m
ε
o
fractional void volume of static bed
μ viscosity of material coating the particle, kg/m·s
ρ spherical coordinate defined in Figure 5.4, m (where not used as fluid density)
ρ
c
density of catalyst, kg/m
3
φ particle sphericity
ψ parameter in Eq. (5.1)
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6 Hydrodynamics of spout-fluid beds
Wenqi Zhong, Baosheng Jin, Mingyao Zhang, and Rui Xiao
The spout-fluid bed is a very successful modification to the conventional spouted bed. It
reduces some limitations of spouting and fluidization by combining features of spouted
and fluidized beds. In spout-fluid beds, in addition to supplying spouting fluid through
the central nozzle, auxiliary fluid is introduced through a porous or perforated distributor
surrounding the central orifice. Compared with spouted beds, spout-fluid beds obtain
better gas–solid contact and mixing in the annular dense region and reduce the like-
lihood of particle agglomeration, dead zones, and sticking to the wall or base of the
column. However, the hydrodynamics of spout-fluid beds are more complex than those
of conventional spouted beds. This chapter briefly presents up-to-date information on
the fundamentals and applications of spout-fluid beds without draft tubes, based on the
limited reported work.
6.1 Hydrodynamic characteristics
6.1.1 Flow regimes and flow regime map
Different flow regimes occur in a spout-fluid bed when the central spouting gas flowrate
and the auxiliary gas flowrate are adjusted.
1–6
A schematic representation of familiar
flow regimes is presented in Figure 6.1. These are the fixed bed (FB), internal jet (IJ),
spouting with aeration (SA), jet in fluidized bed with bubbling (JFB), jet in fluidized bed
with slugging (JFS), and spout-fluidizing (SF) regimes. The criteria for the description
of these flow regimes are as follows:
r
Fixed bed: A fixed bed (immobile particles) is formed if the total flow of the central
spouting gas and the annular auxiliary gas is less than the minimum fluidizing or
minimum spouting velocity, whichever is lower for the particular geometry and flow
configuration.
r
Internal jet: A submerged cavity or jet forms at the air inlet orifice, whereas the rest of
the bed remains a fixed bed. When increasing the spouting gas flowrate, the jet height
increases until it reaches a maximum.
Spouted and Spout-Fluid Beds: Fundamentals and Applications, eds. Norman Epstein and John R. Grace.
Published by Cambridge University Press.
C
Cambridge University Press 2011.
106 Wenqi Zhong, Baosheng Jin, Mingyao Zhang, and Rui Xiao
(a) (b) (c) (d) (e) (f)
Figure 6.1. Schematic representation of flow regimes
5
: (a) FB; (b) IJ; (c) SA; (d) JFB; (e) JFS;
(f ) SF.
r
Spouting with aeration: The bed has the appearance of a conventional spouted
bed in this flow regime. However, it is less stable than conventional spouted beds,
owing to the larger porosity of the annular region and large bubbles at the bed
surface.
r
Jet in fluidized bed with bubbling: Bubbles lift off from the top of the jet, causing
bubbling in the upper part of the bed. However, a distinct jet is also observed.
r
Jet in fluidized bed with slugging: Large bubbles form in the middle and upper parts
of the bed and periodic slugging occurs in these regions. The behavior is similar to
conventional fluidized beds operated in the slug flow regime.
r
Spout-fluidization: In this regime, stable spouting is difficult to achieve. Instead, par-
ticles in the upper section are fluidized, while the spout below exhibits instability,
periodically discharging bubbles near the bed surface.
Other flow regimes are also possible. For example, local fluidization can be observed
if the spouting gas flowrate is very small while the fluidizing gas flowrate is large.
3,6
Unfortunately, there is no uniformity in the literature in designating some flow regimes.
For example, the regime of jet, slug, and bubble in fluidized bed (JSBF), defined by He
et al.,
3
is said to include four different states, in which the flow regimes of IJ, JFB, and
JFS are involved.
The transitions of flow regime with spouting and fluidizing gas flowrates can be
represented clearly in flow regime maps. Figures 6.2 and 6.3 present typical flow regime
maps for a small spout-fluid bed
2
and a relatively large spout-fluid bed,
3
respectively.
These two flow regime maps have many similarities. However, the transition f rom FB
to SA in a larger column is direct, differing from observations in a small column,
1–6
in
which a JFB regime separates the FB and SA regimes. Preliminary investigations show
that the flow pattern and transitions in a spout-fluid bed can be distinguished by analysis
of pressure fluctuation signals, such as by power spectrum analysis
7
or Shannnon entropy
analysis.
8