Epstein N., Grace J.R. Spouted and Spout-Fluid Beds: Fundamentals and Applications

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Spouted and spout-ﬂuid beds with draft tubes 137

Draft tube

Clean

water

outlet

Dirty water

inlets

Jet flow

Figure 7.9. Schematic diagram of draft tube spouted bed adsorber/photocatalytic reactor. After

Follansbee et al.

about 7 percent of catalyst, and a “cold” zone (sorber), containing the rest of the catalyst.

To obtain sufﬁcient catalyst residence time in the reaction section, the draft tube riser

had to operate in a pulsating mode. This DTSB reactor achieved the same efﬁciency

as a ﬁxed bed reactor, and it was particularly suitable for low EtO concentrations. In

ﬁxed bed reactors the combustion heat released by low levels of EtO was insufﬁcient

to preheat the reaction mixture to the ignition point. On the other hand, with a DTSB

reactor, sufﬁcient reaction heat was available to raise the EtO adsorption temperature

above the ignition point.

Hattori et al.

studied ozone decomposition over an iron oxide catalyst in four spouted

bed systems: a top-sealed side-outlet DTSB, a screen-bottomed DTSB, and a t op-outlet

spouted bed with and without a draft tube. The top-sealed DTSB and the screen-bottomed

DTSB gave the highest conversions. The gas–solid contacting efﬁciency of these s pouted

beds was comparable to that of an isothermal ﬁxed bed. The t op-outlet spouted bed with

a draft tube produced the poorest contacting. Both the top-sealed spouted bed and the

screen-bottomed spouted bed could be operated at lower gas ﬂowrates, making them

particularly useful for slow reactions. The top-outlet spouted bed with a draft tube did

not provide good gas–solid contacting efﬁciency, because the draft tube prevented gas

exchange between the spout and annulus.

Horio et al.

tested a spouted bed equipped with a special draft tube for partial

combustion of acetylene in a circulating ﬂuidized bed (CFB) to develop a continuous

diamond powder production process. They investigated deposition kinetics, observed the

ﬂame structure, and conducted emission spectroscopy on a laboratory-scale CFB reactor.

They found that diamond particles could be deposited evenly on ordinary carbon and/or

alumina substrates. Diamonds with 100 facets were obtained over a narrow range of

acetylene/oxygen ratio. For carbon particle deposition, the optimum acetylene/oxygen

ratio was found to be slightly below stoichiometric, indicating gasiﬁcation of carbon

substrate. This CFB design with a riser (dilute ﬂow) and downcomer (rapid dense phase

ﬂow) appears to be promising for large-scale diamond coating or powder generation.

138

Zeljko B. Grbav

c, Howard Littman, Morris H. Morgan III, and John D. Paccione

Hatate et al.

demonstrated the effectiveness of a DTSB as a coal gasiﬁer. Ceramic

particles acted as the thermal carrier, providing sufﬁcient heat for the production of a

hydrogen-rich gas from sub-bituminous coal loaded with potassium carbonate. Investi-

gations were carried out for a coal feed of 1 kg/h at 1 atm pressure and a temperature of

∼800

◦

C. In a follow-up study, Hatate et al.

showed that this reactor had the following

virtues: (1) there is plug ﬂow in the gasiﬁcation zone; (2) classiﬁcation of the feed coal

particles is unnecessary; and (3) air can be used instead of oxygen.

Chapter-speciﬁc nomenclature

diameter of draft tube, m

annulus ﬂuid mass ﬂowrate, kg/s

ax0

auxiliary inlet ﬂuid mass ﬂowrate, kg/s

spouting inlet ﬂuid mass ﬂowrate, kg/s

annulus height, m

solids circulation rate, kg/s

length of draft tube, m

length of inlet section, m

superﬁcial ﬂuid velocity at inlet to annulus, m/s

superﬁcial velocity in draft tube, m/s

inlet annulus ﬂowrate, m

ﬂuid ﬂowrate in spout inlet tube, m

minimum ﬂuidizing volumetric ﬂowrate, m

solids mass ﬂux in draft tube, kg/m

P

pressure drop across bed at minimum ﬂuidization, Pa

References

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tube spouted beds. Chem Eng. Sci., 47 (1992), 3459–3466.

38. D. M. Follansbee, J. D. Paccione, and L. L. Martin. Globally optimal design and operation

of a continuous photo-catalytic advanced oxidation process featuring moving bed adsorption

and draft-tube transport. Ind. Eng. Chem. Res., 47 (2008), 3591–3600.

39. Z. Lj. Arsenijevi

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1519–1524.

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or without a draft-tube. J. Chem. Eng. Japan, 40

(2007), 761–764.

41. M. Horio, A. Saito, K. Unou, H. Nakazono, N. Shibuya, S. Shrma, and A. Kosaka. S ynthesis

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(1966), 3033–3038.

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of varying the position of the outlet. Can. J. Chem. Eng., 74 (1996), 542–546.

8 Particle mixing and segregation

Giorgio Rovero and Norberto Piccinini

This chapter starts from the state of the art on particle mixing in spouted beds, as

presented in the classical book by Mathur and Epstein.

In subsequent years, segregation

has been considered in fundamental studies aimed at describing real systems of various

bed compositions.

8.1 Gross solids mixing behavior

The mixing properties of spouted beds result from interaction among the spout, fountain,

and annulus. In a continuously operated unit, the positioning of the solids inlet port with

respect to the discharge opening is of fundamental importance to prevent bypassing.

Dead zones could arise from problematic solids circulation – for example, because of

an incorrect base design. To prevent segregation, the simplest conceivable operating

condition corresponds to a mono-sized particulate material and a single unit in which

each particle undergoes many cycles before being discharged. In such cases and for

continuous operation, the internal circulation far exceeds the net in-and-out ﬂow of

solids in all cases studied. The very different particle residence times in the spout

(progressively loaded with solids along its height), in the fountain (where the particles

have both axial and radial velocity components), and in the annulus (where particles

travel downward in nearly plug ﬂow) generate nearly well-mixed overall solids residence

time distributions (RTDs).

Stimulus-response techniques have been applied to determine the RTD of particles.

A typical downstream normalized tracer concentration at the discharge, called the F

curve, generated in response to an upstream step input of colored particles, is given in

Figure 8.1. A more comprehensive s ummary of experimental data also appears in the

literature.

More than 90 percent of the spouted bed volume appears to operate as a

perfectly mixed system. The ratio of the solids tracer concentration in the bed to the

initial concentration immediately after a tracer pulse injection is given by

I (θ) = C

(θ)/C

= e

−a(θ−b)

, (8.1)

where θ is the dimensionless time since the pulse injection – that is, the elapsed time

divided by the mean residence time. Note that a = 1 and b = 0 for perfect mixing,

Spouted and Spout-Fluid Beds: Fundamentals and Applications, eds. Norman Epstein and John R. Grace.

Published by Cambridge University Press.



Cambridge University Press 2011.

142 Giorgio Rovero and Norberto Piccinini

1.0

0.5

0 0.5 1.0 1.5 2.0

PLUG FLOW

PERFECT

MIXING

OBSERVED

Dimensionless time

F (q) = C

q = t/t

Figure 8.1. Typical output response to step change in solids input.

D = 0.15 m, H

= 0.08–

0.12 m, D

= 6–17 mm, τ = 25–40 min, θ = 82

◦

whereas ﬁtted values were a ≈ 1.09 and b ≈ 0.10. It is clear that a single-stage spouted

bed approaches perfect mixing as far as the solids are concerned.

Mathur and Epstein

refer to a model for a unit operating with a continuous feed rate F:

the annulus was represented by a plug ﬂow volume V

, with no short-circuiting from the

annulus to the spout. This model contained a recycle ratio R

cfr

(ratio of internal circula-

tion to feed rate), representing the solids circulation rate in the spout. By smoothing the

response to a step input, it was demonstrated that a small value of R

cfr

generates an

output approaching plug ﬂow, whereas rapid mixing in the system is obtained for

cfr

> 5. The cycle time in the model is represented by

= V

/(R

cfr

+ 1)F, (8.2)

which, when combined with the mean solids residence time, τ = V

/F,gives

τ = t

cfr

+ 1). (8.3)

For good mixing in the spouted bed, one should have τ ≥ 6 t

Experimental results in the literature

demonstrate that, by measuring the maximum

mass circulation of solids at the top of the bed, z = H, the cycle time increases sharply

with increasing column diameter at constant H/D. The minimum residence time for

good mixing then increases in proportion. These considerations also suggest that to

minimize the spread of the overall RTD of the solids in a continuous process, a cascade

of small units is far better than a limited number of larger-diameter spouted beds, with

good mixing in each unit to minimize bypassing.

Based on phenomenological observations, a realistic description was proposed in the

literature,

based on the approach described earlier. Tracer particles ﬁlled the upper

third of the annulus volume. The batch unit was idealized as combining a well-mixed

region composed of combined spout and fountain containing 15 percent of the particles,

and n perfectly stirred tanks in series, representing the annulus, each contributing to

Particle mixing and segregation 143

Time, arbitrary units

Axial positions, arbitrary units

BED LEVEL

CYCLE

TIME

CYCLE

TIME

CYCLE

TIME

Figure 8.2. Characteristic axial particle position vs. time in s pouted beds.

D = 0.075 m, θ = 40

◦

Reprinted with permission of John Wiley & Sons, Inc.

solids recirculation by crossﬂow. To study the mixing dynamics, concentration-versus-

time proﬁles were given in the upper and lower portions of the bed. Equilibrium was

achieved, with n as the model parameter. Near-homogeneity was attained in a time less

than 2t

for a relevant number of stirred units.

To extend the previous description, in view of the actual motion of particles in

a spouted bed, sensible modeling is given by plug ﬂow in the annulus, continuous

crossﬂow into the spout, and perfect mixing in the spout and fountain lumped together,

where a solids holdup ranging between 2 percent and 6 percent of the total bed volume

is suggested.

A different approach, aimed at measuring the solids cycle time, was carried out

study the solids circulation in a two-dimensional 40

◦

-cone-based spouted bed of silica

gel particles of diameter 2.5 and 4.3 mm. Two tracer particles opaque to X-rays were

introduced and motion-picture sequences taken. The sequences were then analyzed to

compute turbulence parameters such as correlation times and mixing lengths. Effective

particle diffusion coefﬁcients were also evaluated as a function of spouting velocity,

reaching a maximum at U

∼ 2U

, with the bed operating in a state of maximum

mixing effectiveness.

A noninvasive method for determining the ﬂow patterns of solids in spouted beds was

developed,

based on a single radioactive particle whose axial location was monitored

with a scintillation counter. The particle cycle time distribution (CTD) and the circulation

rate in the bed were determined from a large number of passages through the top

of the bed, as shown in Figure 8.2. However, owing to limited instrumentation, only

qualitative results were repor ted.

A simpler method was developed

based on the

electromagnetic signal generated by a single t racked particle passing through a coil. The

authors investigated a spouted bed column equipped with a draft tube. Application of

this technique to standard systems could, however, cause several problems associated

144 Giorgio Rovero and Norberto Piccinini

with uncertainty regarding the location of the spout and interference of the sensor with

the spout itself.

Takeda and Yamamoto

carried out an interesting study of the interaction between

modeling and experimentation. A two-region model was conceived in which the spout

and fountain together represent a perfectly mixed volume whereas the annulus moves

in plug ﬂow, with a minor portion acting as a well-mixed zone. The spout diameter

was based on the McNab equation.

A second model parameter is the active height

for particle migration from the annulus to the spout. The solids holdup in the spout

and fountain and the circulation ﬂow rate were determined in two vessels of 0.06 and

0.09 m ID with 45

◦

included angle and spherical alumina particles of d

= 1 mm,

based on a go-and-stop procedure after dropping a few grams of marked particles into

the mixed region. By progressively analyzing the tracer concentration in the annu-

lus along the bed depth, both parameters were obtained. It was concluded that (1)

particle mixing can be interpreted according to the model, (2) ingress of particles

from the annulus into the spout is limited to the cone region (in agreement with most

observations in half-sectional columns), and (3) a portion of the annulus, less than

10 percent of its total volume, contributes to the perfectly mixed volume. It seems likely

that a minor mixing role in the annulus is played by gradual migration of particles

from the vessel wall toward the center, whereas shear along the entire spout-to-annulus

boundary makes a more major contribution.

Roy et al.

applied a computer-automated radioactive particle tracking (CARPT)

technique, adapted from ﬂow ﬁeld measurements in gas–solid ﬂuidized beds, bubble

columns, and three-phase ﬂuidized systems, to spouted beds. The apparatus consisted of

a 152-mm-ID column with a 60

◦

angle base and D

=19 mm. Glass beads of d

=3mm

were tested. The single tracer particle was prepared by mixing soda lime and scandium

oxide, activated by neutron bombardment. The motion of the single radioactive particle

was then monitored at 30-ms intervals and count rates were determined by several

detectors at different levels and angular positions. This noninvasive technique allowed

a number of hydrodynamic parameters, not otherwise quantiﬁable, to be evaluated.

Several hundred thousand measurements allowed determination of the velocity ﬁeld,

cycle time distribution, spout proﬁle, and solids exchange distribution at the annulus–

spout interface. Figure 8.3 illustrates radial proﬁles of the mean vertical (v

) and radial

) particle velocities at three levels: z = 375 mm in the fountain, z = 235 mm in

the cylindrical region of the bed, and z = 55 mm in the conical base. Particles in the

fountain rise near the axis and fall closer to the wall, whereas the mean radial solid

velocities increase as particles approach the wall and then begin to decelerate. In the

annulus region, the s olids move downward in near-plug ﬂow, with an upward velocity

adjacent to the spout and a minimal radial component. In the conical region, the solid

ﬂow is characterized mainly by inward radial acceleration from the annulus toward the

spout.

These noninvasive measurements were extended in a later paper.

With the aid of many

trajectories, the three-dimensional spouted bed reactor was depicted by an axisymmetric

vector plot. Contour ﬁelds were provided for particle residence time and axial and radial

velocity in each volume cell. A ﬂow reversal line demarcated the ascent and descent

Particle mixing and segregation 145

1400

1200

1000

800

600

400

200

–200

–400

01020304050607080

r, mm

mm/s

z = 55 mm

z = 235 mm

z = 375 mm

column

wall

(a)

300

200

100

01020304050

z = 55 mm

column

wall

(b)

z = 235 mm

z = 375 mm

, mm/s

60 70 80

r, mm

–100

–200

–300

Figure 8.3. Radial proﬁles of mean particle velocity at different levels in the spouted bed.

U/U

= 1.3; H = 314 mm: (a) vertical; (b) radial. Reprinted with permission of John Wiley &

Sons, Inc.

regions. If the boundary layer between this line and the spout–annulus interface is

included with the spout, a higher solids holdup can be attributed to the spout itself,

contributing to the gross mixing of solids. The mean circulation time was caused by

abrupt upward acceleration in the spout, reversal in the fountain, deposition on the

annulus top, and slow downward motion in the annulus. The mixing dynamics were

146 Giorgio Rovero and Norberto Piccinini

500

400

300

200

100

02550

(a) (b) (c) (d)

z (mm)

t = 0

z = 187 mm

< r < R

r (mm) r (mm)

z (mm)

r (mm) r (mm)

t = 2 s

t = 4 s t = 15 s

Figure 8.4. Snapshots

showing evolution of a clump of 15,385 particles injected simultaneously

in the cylindrical annulus at z = 187 mm between r

< r < R:(a)t = 0, (b) t = 2s,(c)t = 4s,

(d) t = 15 s.

300

250

200

location (mm)

150

100

4 8 12 16

time (s)

axial

180

135

–45

–90

–135

–180

radial

azimuthal

Figure 8.5. Mixing in axial, radial, and azimuthal directions, demarcated by centroid location

versus time of a clump of tracer particles, each injected

at time 0 into the annulus at z = 187 mm.

evaluated by an ergodic approach, simulating the introduction of a clump of particles,

each following independent trajectories. Figure 8.4 shows the locations of particles at

subsequent times after being marked in a thin slice in the annulus at t = 0. It is seen that

(1) some vertical mixing occurs in the annulus; (2) the spout–annulus interface is not

impervious to mass exchange; (3) radial and circumferential mixing is chieﬂy caused

by the fountain; and (4) mixing also occurs in the bottom cone owing to shear. The last

of these factors requires more attention.

The axial, radial, and circumferential mixing response curves plotted i n Figure 8.5

suggest that axial mixing needs more than four cycle times to reasonably damp