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

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Catalytic reactors and their modeling 317

AIR

ETC

AIR

DAC

T10

TC1

Figure 19.10. MSBDT pyrolysis plant conﬁguration.

19.4 Modiﬁed spouted bed reactors

A novel catalytic converter

(modiﬁed spouted bed with draft tube [MSBDT] reac-

tor) was conceived after monitoring the adsorption/oxidation of ethylene oxide (EtO)

on Pt/Al

catalyst i n a packed bed unit. Platinum was deposited on the exterior of

3.3-mm-diameter alumina beads. Tests in the packed bed deﬁned optimal temperatures

for adsorption and ignition. The reaction was demonstrated to be autothermal at EtO con-

centrations as low as 0.03 percent by volume, with careful regulation of the air (oxidant)

ﬂowrate. The adsorption was kinetically controlled at low temperatures (∼80

◦

C). Gas–

solid equilibrium prevailed at higher temperatures owing to the reaction exothermicity:

temperature increases as high as ∼50

◦

C were detected, demonstrating adsorption on

both t he support and the platinum. The novel converter structure was derived from a

spout-ﬂuid conﬁguration (essential for optimal control of ﬂow rates and contact time)

by adding an upper oxidation section and bottom purging, as illustrated in Figure 19.10.

Solids circulation allowed the process to operate with cycled air cleaning by adsorption

and catalyst regeneration by oxidation. In addition, to optimally match the sequence

for catalyst heating, reaction, product desorption, and external transfer, pulsations were

applied to the draft tube, leading to the temperature proﬁles shown in Figure 19.11. The

process successfully withstood reasonable variations of the EtO concentration with time,

thanks to the system heat capacity and easy regulation. Catalyst wear by attrition was

investigated by monitoring the process efﬁciency during more than 500 h of operation.

318 Giorgio Rovero and Norberto Piccinini

500

450

400

350

300

250

200

150

100

450 500 550 600

Time, s

T10

Active

Passive

Temperature, °C

Period Period

Figure 19.11. Changing temperature proﬁles along MSBDT reactor during self-sustained catalytic

EtO incineration (C

EtO

= 0.1 vol. %).

Locations of thermocouples are shown in Fig. 19.10.

Acknowledgment

The authors thank Ms. Marta Barberis Pinlung for her effective collaboration in editing.

Chapter-speciﬁc nomenclature

C concentration, moles/m

volumetric heat capacity, J/m

weighted gas/particle volumetric heat capacity, J/m

longitudinal dispersion coefﬁcient, m

E activation energy, J/mol

G productivity, tonnes m

−2

−1

,inref.14

pre-exponential factor, s

−1

(z) Peclet number = wH/D

gas inlet temperature, K

w linear velocity of solid phase

Greek letters

H reaction enthalpy, J/mol

ζ dimensionless height, z/H

modiﬁed Arrhenius pre-exponential factor

Subscripts

A species A

e exit

Catalytic reactors and their modeling 319

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model. Chem.Eng.Sci., 52 (1974), 789–797.

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(1968), 1–5.

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bed. Presented at CHISA ’78 Congress (Prague, 1978), Paper No C4.5.

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reactors – I. Chem. Eng. Sci., 52 (1982), 567–579.

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(1984), 623–631.

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14. J. Dudas, O. Seitz, and L. Jelemensky. Chemical reaction in spouted beds. Chem. Eng. Sci.,

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5225–5231.

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(1996), 62–69.

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1519–1524.

20 Liquid and liquid–gas spouting

of solids

Zeljko B. Grbav

c, Ho ward Littman, and Morris H. Morgan III

The spouted bed technique permits agitation of particles too coarse for ﬂuidizing with

a gas when excellent heat and mass transfer characteristics and intimate ﬂuid–particle

contacting are important.

Liquid spouting has attracted much less interest, as coarse

particles can be easily ﬂuidized in this medium. However, recent advances in biotech-

nology and renewed interest in wastewater treatment have sparked new applications of

liquid-spouted beds, particularly those incorporating a gas phase.

20.1 Liquid spouting

As discussed elsewhere in this book, a spouted bed can form when a ﬂuid jet blows

vertically upward along the center line of a vertical column, forming a spout in which

fast-moving ﬂuid and entrained particle mixing occur, surrounded by an annular region

densely packed with particles moving slowly downward and inward. The spout is topped

by a spillover fountain. Fluid percolates through the annulus from the spout. In a spout-

ﬂuid bed (see Chapter 6), additional ﬂuid is introduced at the bottom of the annulus.

Several ﬂuid–particle patterns are possible, depending on the magnitude of the external

annular ﬂuid introduced to the annulus bottom

2–4

(1) Spouting with irrigation: beds in which the external annular ﬂuid velocity, U

,is

restricted to a velocity that keeps the annular velocity U

≤ U

(2) Spout-ﬂuidization: beds in which the annulus is partly or completely ﬂuidized; the

level at which ﬂuid velocity reaches U

depends on the external annular ﬂowrate.

If U

= U

, the annulus is completely ﬂuidized.

(3) Jet-ﬂuidized beds: beds in which the bed depth is deeper than the maximum spoutable

bed height. When the ﬂuid is a liquid, such beds exhibit two zones: a lower spouted

bed region and an upper ﬂuidized bed zone.

A typical regime map for a water spout-ﬂuid bed system

is shown in Figure 20.1.

The conventional spouted bed (V

=0) falls between points E and D along the abscissa,

whereas the ﬂuidized bed is depicted along the ordinate between points G and A. Below

the minimum spout-ﬂuid line, GHE, the bed is ﬁxed; the outer boundary curve, ABCD,

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

Published by Cambridge University Press.



Cambridge University Press 2011.

322

Zeljko B. Grbav

c, Howard Littman, and Morris H. Morgan III

01234

VERTICAL

TRANSPORT

Regime 3

FLUIDIZED BED

S-F BED (H>H

mSF

)

Regime 1

Regime 2

FIXED BED

)

0.5

[(V

)

max

]

0.5

)

0.5

Spouted bed

S-F BED (H<H

mSF

)

FLUIDIZED BED WITH

INTERNAL SPOUT

Figure 20.1. Flow regime map of a water spout-ﬂuid bed of glass spherical particles (D = 70 mm,

= 15 mm, d

= 1.8 mm, H = 70 mm, H/H

= 0.642), adapted from Vukovi

cetal.

represents the locus of annulus and spout fIowrates, whose sum equals V

. One could also

surmise from Figure 20.1 that because the experimental value of V

is approximately

equal to V

, t he actual bed height is close to H

. Actual bed heights can be substantially

less than H

, however, even when (U

) is close to unity.

A spout-ﬂuid bed of height H ≤ H

mSF

looks much the same as a spouted bed with

H < H

.As(V

)

max

represents the inlet annulus ﬂowrate required to ﬂuidize the top of

the annulus in a spout-ﬂuid bed of height H, the annulus ﬂowrate at point H is (V

)

max

In the region EHCD, Regime I (item 1 in the preceding list), the total ﬂowrate >V

mSF

with V

≤ (V

)

max

. The initial bed height is equal to or less than H

mSF

and the bed is

very similar in appearance to that of a conventional spouted bed. As V

is increased

at constant V

along line a–a, the top of the annulus becomes incipiently ﬂuidized at

point M, as V

equals (V

)

max

. An increase in the external annulus ﬂowrate, V

(along

line a–a), results in the ﬂuidization of the annulus, initiated at lower and lower levels.

Finally, when V

reaches V

(point N on line a–a), the bottom of the annulus becomes

just ﬂuidized. These observations reﬂect the fact that H

mSF

decreases as V

increases at

constant V

. Along line HC, despite the fact that the spout diameter increases as V

increased, the annulus height actually decreases, owing to the increased solids holdup in

the spout and the fountain. Note that the data points lie above HC when V

∼

9(V

)

mSF

The fountain height and spout diameter are large, and the annulus height is well below

the initial bed height. In region HGBC, Regime 2 (item 2 in the preceding list), the

bed forms two zones, in which a ﬂuidized bed sits on top of a spout-ﬂuid bed with a

Liquid and liquid–gas spouting of solids 323

well-deﬁned annulus and spout. This bed is very similar in appearance to a spouted bed

with H > H

(i.e., to item 3 on the list). Along GB, U

equals U

, thus ﬂuidizing

the entire annulus, and the total volumetric ﬂow rate is above V

except at point G.

Regime 3 is bounded by region AGB, in which the ﬂuid velocity >U

everywhere in

the annulus, and a liquid jet penetrates into the bed.

20.1.1 Pressure drop versus ﬂow rate

The plot of pressure drop versus ﬂow rate for liquid spouting is generally similar to

that of a gas spouting system (see Chapter 2). With an increase in liquid superﬁcial

velocity, above U

, a constant pressure drop is established in the system. The difference

between the peak pressure drop (P

) and the pressure drop at minimum spouting

(P

) decreases with increasing bed height. For gas phase systems at the minimum

spouting velocity, a small reduction in the gas ﬂowrate causes the spout and fountain to

collapse, whereas in liquid spouting at V

the fountain is unexpanded with respect to

the surface.

20.1.2 Minimum spouting velocity

In liquid-spouted beds the minimum spouting velocity at the maximum spoutable bed

height is approximately equal to U

. Based on the assumption that (U

)

= (U

)

and that A

A,Grbav

cetal.

derived the following scaled equation for U

= 1 −



1 −



. (20.1)

Eq. (20.1) is based on the Mamuro-Hattori

equation for U

, assuming that at minimum

spouting, U

. The original Mathur-Gishler

equation, when scaled in this fashion,

produces a different dependency:





0.5

. (20.2)

This equation predicts lower values of U

than Eq. (20.1) when H/H

∼

0.15,

and the converse when H/H

< 0.15.

Littman and Morgan

developed a general correlation for U

as a function of

), H/D and θ

. That correlation was based on the following equation, derived

from the annular pressure gradient at the top of the bed under minimum spouting

324

Zeljko B. Grbav

c, Howard Littman, and Morris H. Morgan III

conditions:

=−

2 f





2 f





1 +





1/2

;

≥

∗

and A

· g(φ

) > 0.02, (20.3)

where

= 1 − Y −



2Y + (X − 2) + (X − 0.2) − (3.24/θ

)

2Y + 2(X − 0.2) − 1.8 + (3.24/θ

)



, (20.4)

= 7.18



· g(φ

) − D



+ 1.07, (20.5)

X = 1/(1 + H/D), (20.6)

Y = 1 − P

/P

, (20.7)

and

− ρ

. (20.8)

The dimensionless parameter, C

, has an important effect on the U

ratio

and provides a connection to the viscous or inertial term in the Ergun

equation and

bed geometry through the parameter θ

This model covers a wide range of exper-

imental conditions, including water-spouted systems of spherical and nonspherical

particles. For spherical particles, the g(φ

) parameter of Eq. (20.5) approaches unity

asymptotically.

Littman et al.

10,11

showed that the minimum spout-ﬂuid ﬂowrate in a liquid phase

spout-ﬂuid bed follows the simple linear relationship:

)

mSF

= V

−

)

mSF

. (20.9)

As depicted in Figure 20.2 for such systems, the total minimum spout-ﬂuid ﬂowrate for

H/H

≤ 1isgivenby

mSF

(

)

mSF

(

)

mSF

. (20.10)

For liquid spouting in a conical vessel, Legros et al.

proposed a model for the

minimum spouting velocity based on the momentum transfer between the liquid jet and

the bed. Beds consisting of different particle size distributions were used to substantiate

this model. Good agreement was obtained, even when a large fraction of ﬁne particles

was present in the initial particle mixture.

20.1.3 Pressure drop at minimum spouting

P

is commonly calculated by integrating the one-dimensional longitudinal pressure

gradient in the annulus over the height of the bed using one of the annulus models. This

Liquid and liquid–gas spouting of solids 325

Figure 20.2. Relationship of spout and annular ﬂows at the minimum spout-ﬂuid ﬂowrate, for

water and glass beads in a 117 × 9.2-mm rectangular column with D

= 9.3 mm, after Littman

et al.

procedure leads to quite reasonable results, although it ignores radial pressure gradients

across the annulus and assumes that P

= P

= (P

)

. Equations for predicting

P

/P

have been proposed by Mamuro and Hattori,

Lefroy and Davidson

(for

H = H

), Epstein and Levine,

Grbav

c et al.,

and Morgan and Littman

(for H ≤

). The correlation of Morgan and Littman is

P

/P

= 1 − Y, (20.11)

where Y is obtained from:

+ [2(X − 0.2) − 1.8 + (3.24/θ

)]Y + [(X − 2)(X −0.2) − (3.24X/θ

)] = 0.

(20.12)

The ratio P

/P

is a function of A

, g(φ

) and D

/D, and was found to ﬁt a wide

range of experimental data involving both air- and water-spouted systems.

Based on their U

model for spouted beds, Littman et al.

developed a rela-

tionship for the minimum spout-ﬂuid bed pressure drop ratio, P

msf

/P

. That model

predicted a linear relationship between the spout-ﬂuid pressure drop and the auxiliary

velocity, U

20.1.4 Maximum spoutable bed depth

Littman et al.

also developed a model for predicting the maximum spoutable bed

height (H

) for coarse particles when the controlling mechanism was ﬂuidization of

326

Zeljko B. Grbav

c, Howard Littman, and Morris H. Morgan III

the annulus. That model was based on a detailed axisymmetric analysis of the ﬂow in

the annulus, of the spouted bed employing interfacial pressure and velocity distributions,

a ﬂuid continuity equation, a vectorial Ergun equation

for the local pressure in the annu-

lus, and appropriate boundary conditions. The following relationship was established

between the maximum spoutable height, H

, and the average spout diameter, D

,for

spherical particles:

− D

= 0.345





−0.384

. (20.13)

When evaluated with experimental values for D

, predictions of H

using Eq. (20.13)

differed on average by 8.5 percent from experimental data for both air- and water-spouted

beds. Assuming that H

was proportional to the inlet ﬂuid momentum, Littman et al.

subsequently developed an equation for predicting H

for coarse spherical particles as

a function of the inlet tube diameter, t he column diameter, and the properties of the

ﬂuid–particle system:

= 0.218 +

0.0050

. (20.14)

Eqs. (20.13) and (20.14) were combined to obtain a relationship for the D

ratio as a

function of D/D

and parameter A

Grbav

cetal.

presented the following simpler empirical correlation for predicting

in water-spouted beds:

= 0.347





0.41





0.31

. (20.15)

In spout-ﬂuid bed systems, the maximum spoutable bed height decreases with increas-

ing external annular ﬂow. Grbav

cetal.

assumed that incipient ﬂuidization of the

solids at the top of the annulus is the limiting/controlling mechanism and proposed that

mSF

= f(U

, C), where the parameter C is the ratio of the ﬂuid velocity at the

top of the annulus at H

or H

mSF

to the minimum ﬂuidization velocity. Their combined

spout-ﬂuid bed data f or both air and water as the ﬂuid gave a best-ﬁt value of C = 0.935.

20.1.5 Fine particle systems

The spouting of ﬁne particles (d

< 1 mm) differs in two fundamental ways from large

particle behavior:

1. Fine particles have a much smaller range of acceptable inlet diameter that result

in stable spouting. Under operating conditions in which the inlet diameter exceeds

the maximum stable inlet diameter, instabilities originate in the bed at either the

spout inlet or outlet. Littman and Kim

characterized such particles at the minimum

spouting conditions and examined the effect of both the inlet oriﬁce diameter and

the pressure distribution along the spout–annulus interface on the basic spouting

parameters (U

and P