Gas Solid Mixing

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-MIX-708

Gas Solid Mixing

Information contained in this publication or as otherwise supplied to Users is
believed to be accurate and correct at time of going to press, and is given in
good faith, but it is for the User to satisfy itself of the suitability of the information
for its own particular purpose. GBHE gives no warranty as to the fitness of this
information for any particular purpose and any implied warranty or condition
(statutory or otherwise) is excluded except to the extent that exclusion is
prevented by law. GBHE accepts no liability resulting from reliance on this
information. Freedom under Patent, Copyright and Designs cannot be assumed.

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Process Engineering Guide:

Gas Solid Mixing

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

3

1

SCOPE

3

2

FIELD OF APPLICATION

3

3

DEFINITIONS

3

4

GAS-SOLID FLUIDIZED BED

3

5

MIXING IN FLUIDIZED BEDS

3

5.1
5.2
5.3
5.4

Group A Powders
Group B Powders
Group C Powders
Group D Powders

3
4
4
4

6

MECHANISMS OF MIXING AND SEGREGATION

4

6.1
6.2
6.3

Particle Segregation
Rate of Mixing
Solids Circulation

5
7
7


7

GRID DESIGN

7

7.1

Choice of Configuration

7

8

PLENUM CHAMBER DESIGN

7

9

SPOUTED BED

7

10

NOMENCLATURE

9

11

BIBLIOGRAPHY

9

FIGURES

1

2

3

4

POWDER CLASSIFICATION DIAGRAM FOR
FLUIDIZATION BY AIR

4

DIAGRAMMATIC REPRESENTATION OF MIXING BY A
SINGLE RISING BUBBLE IN A BED OF SMALL
PARTICLES

5

SEGREGATION PATTERNS WITH 'PRACTICAL'
MATERIALS

6

SPOUTED BED – DIAGRAMMATIC

7

DOCUMENTS REFERRED TO IN THIS
ENGINEERING GUIDE

10


0

INTRODUCTION/PURPOSE

This Guide is one in a series of Mixing Guides produced for GBH Enterprises.

1

SCOPE

This Guide provides general awareness of gas-solid mixing in fluidized and
spouted beds. It does not deal with fluidized bed reactors or driers.

2

FIELD OF APPLICATION

This Guide applies to Process Engineers in GBH Enterprises worldwide.

3

DEFINITIONS

No specific definitions apply to this Guide.
With the exception of terms used as proper nouns or titles, those terms with initial
capital letters which appear in this document and are not defined above are
defined in the Glossary of Engineering Terms.

4

GAS-SOLID FLUIDIZED BED

The basic gas-solid fluidized bed has many applications in Process Engineering.
However their design is mainly empirical, and a few specialized companies offer
their designs based on their practical experience.
Most of the academic work has been done with sand and air, and the industrial
published work is related to specific use like coal combustion, fluidized bed driers
and catalytic crackers.

5

MIXING IN FLUIDISED BEDS

The degree of mixing or segregation achieved in a fluidized bed depends on the
solids' properties i.e. particle size, density and shape.


A classification chart which divides the solids in four groups, A, B, C, and D was
proposed by D Geldart 1973 [Ref. 2] (see Figure 1) and is the accepted method
of describing powder properties in the fluidization field.
However, when the properties of a powder fall near a boundary it will always be
difficult to define its behavior.

5.1

Group A Powders
Group A powders are easy to fluidize and gross circulation of powder
occurs, which produces a rapid mixing, with considerable gas
recirculation.

5.2

Group B Powders
Group B powders are easy to fluidize but there is little gas recirculation.

5.3

Group C Powders
Group C powders are cohesive and 'normal' fluidization is extremely
difficult, and particle mixing is very limited.

5.4

Group D Powders
Group D powders are difficult to mix and they are more suitable for
processing in a spouted bed.
The A/C boundary has received much attention, because of the dramatic
change in mixing behavior with small changes in particle size or gas
density.


FIGURE 1

6

POWDER CLASSIFICATION DIAGRAM FOR FLUIDISATION BY
AIR (Ambient Conditions)

MECHANISMS OF MIXING AND SEGREGATION

Mixing is caused solely by the bubbles rising through the solids (P N Rowe 1965
[Ref. 3], A W Nienow 1978 [Ref. 4] and R R Cranfield, 1978 [Ref. 5]). Each
bubble gathers a wake of material from near the bottom of the bed and carries it
to the surface.
Each bubble also draws up a spout of material below itself (see Figure 2).


FIGURE 2

DIAGRAMMATIC REPRESENTATION OF MIXING BY A SINGLE
RISING BUBBLE IN A BED OF SMALL PARTICLES

No mixing occurs in the absence of bubbles, even if the bed is fluidized, and if by
maldistribution of the gas a region of the bed is not bubbling, the particles there
will be almost stagnant.
Three separate zones are normally present in a fluidized bed.
The distributor zone, with solid particles mixing with the incoming gas, normally
in the turbulent regime.
The freely bubbling zone with convection currents of solids.
The zone adjacent to the wall is a layer of slowly descending particles which do
not mix with the bubbling zone, and go down to just above the distributor zone.

6.1

Particle Segregation

When particles of different sizes or densities are present, then segregation as
well as mixing will occur, and the final equilibrium distribution of particles in each
zone will be a function of their sizes, densities, and the relative proportions of
each one.
Rowe P N et al, 1972 [Ref. 6] showed that depending on the gas velocity it is
possible to either mix or segregate two kinds of particles of different densities
and even if sometimes complete mixing cannot be achieved, it is possible to get
a middle region of uniform composition with segregated zones of the heavier one
at the bottom and the lighter one at the top (see Figure 3).

FIGURE 3

SEGREGATION PATTERNS WITH 'PRACTICAL' MATERIALS


6.2

Rate of Mixing

Except for beds of very fine particles (< 60 µm) all excess gas above minimum
fluidization flow passes through the bed in the form of bubbles, that is:

Each bubble carries a wake of particles equal to about one third of its own
volume [Ref. 5]. If εB is the fraction of bed filled with bubbles then the average
velocity at which particles move downwards will be given approximately by:

6.3

Solids Circulation

The actual flow patterns depend on the number of bubbles, the position of the
jets on the distributor and the presence of gas maldistribution which will have a
big influence in the solids movement, because of the stagnant zones which form
where there are no gas bubbles.
The walls always have a boundary gas layer which holds particles, hence the
movement will always be downward in the walls and in between the position of
the orifices and upward with the bubbles.
Depending on the distance between the orifices in the distributor and also the
gas velocity, stagnant zones can be present and a great number of alternative
designs are available depending on the particular application.


7

GRID DESIGN

7.1

Choice of Configuration

The method of Wen et al, 1986 [Ref. 7] is recommended.
The grid in a fluidized bed reactor is intended to distribute the fluidizing medium
uniformly over the bed cross-section. In practice, this has taken a variety of
forms. Whatever the physical form all are fundamentally classifiable in terms of
the direction of fluid entry, either upwards, laterally, or downwards. The choice is
dependent on prevailing process conditions, mechanical feasibility and cost.

8

PLENUM CHAMBER DESIGN

The design of the plenum chamber needs to be 'Compatible Compatible' with the
grid to ensure uniform gas distribution.

9

SPOUTED BED

In the case of large particle sizes which are difficult to fluidize (see 5.4 - Group
D), all the gas is fed in a single orifice in the centre of the bed and a single
circulation loop is stabilized, and the particles spend most of the time in the
annular region slowly moving downward. This arrangement is suitable for slow
gas-solid reactions or drying where a long residence time of solids in the (almost)
stagnant gas is acceptable or required (see Figure 4).
The Harwell report: SPS RR 26, shows that single size solids are well mixed. In
the case of two sizes it was found that each component was well mixed.
However both components did not have the same residence time.


FIGURE 4

SPOUTED BED – DIAGRAMMATIC


Gas Solid Mixing

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