Gas Mixing

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-MIX-702

Gas Mixing
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Process Engineering Guide:

Gas Mixing

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

2

1

SCOPE

2

2

FIELD OF APPLICATION

2

3

DEFINITIONS

2

4

RECOMMENDATIONS FOR GAS MIXING:
PLUG FLOW

2

RECOMMENDATIONS FOR GAS MIXING:
BACKMIXED INITIAL ZONE

4

BIBLIOGRAPHY

6

5

6


FIGURES

1

RESULTS OBTAINED USING A SULZER SMV MIXER

3

2

RECIRCULATION ZONE : CONFINED JET

4

3

RECIRCULATION ZONE : BLUFF BODY

5

4

STREAMLINES OF RECIRCULATION EDDY IN SWIRLING
JET, S = 1.57

5

SCHEME OF SWIRL BURNER WITH AXIAL AND
TANGENTIAL AIR ENTRIES

6

5

6

COMPARISON OF THE RADIAL DISTRIBUTIONS OF VELOCITY
IN THE VORTEX REGION FOR JETS WITH AND WITHOUT
DIVERGENT EXTENSIONS
7

7

BACKMIXED 'PRE-REACTION' ZONE

7

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

8


0

INTRODUCTION/PURPOSE

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

1

SCOPE

This guide covers the mixing of gases.
If a backmixed 'pre-reaction' zone is not required, refer to Clause 4. Where a
backmixed 'pre-reaction' zone is required, refer to Clause 5.

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

RECOMMENDATIONS FOR GAS MIXING: PLUG FLOW

Jet flow mixers and static mixers are recommended for gas mixing. The design
procedures presented for miscible liquids in GBHE-PEG-MIX-701, should be
followed using the appropriate gas physical properties.
Empty pipes may be used in the turbulent region: for gases of the same density,
a length of about 100 pipe diameters is required for good mixing. For gases with
a density ratio > 2, up to 1000 pipe diameters may be required because of the
formation of stable layers.


Static mixers can achieve good mixing in less than five pipe diameters regardless
of the density difference. Figure 1 shows the results obtained using a Sulzer
SMV mixer. Unless the densities are similar and the flow rates constant, a
minimum operating velocity, UG, is recommended. For the Sulzer SMV mixer,
this is obtained if the modified Froude number exceeds 20 [Ref 1 : Germain et
al., 1982]:

. The values of f and DH are obtained from the manufacturers' data;


FIGURE 1

RESULTS OBTAINED USING A SULZER SMV MIXER


5

RECOMMENDATIONS FOR GAS MIXING: BACKMIXED INITIAL ZONE

Such cases are covered in the combustion literature, e.g. in Beer, J M and
Chigier, N A, ”Combustion Aerodynamics”, Krieger, 1983 (reprint of 1972
edition).
In this reference design of jet mixers for the particular requirements of
combustion, namely rapid mixing and stable flame, are discussed, and
information provided for entrainment rate into the jet, flow patterns, recirculation
vortex position, effects of jet swirl, etc.
A recirculation zone is often used to stabilized the flame (see Figure 2). Hot
gases, entrained at the downstream end, return to pre-heat the incoming fresh
gas and supply chemically active species to it. This enables velocities much
higher than the flame propagation velocity to be used without lift-off.
Recirculation zones are provided by:
(a)

FIGURE 2

Confining the jet and starving the entrainment.

RECIRCULATION ZONE : CONFINED JET


(b)

FIGURE 3

Using the wake behind a bluff body (see Figure 3).

RECIRCULATION ZONE : BLUFF BODY


(c)
FIGURE 4

Swirling the jet, with swirl number S > 0.6 (see Figure 4).
STREAMLINES OF RECIRCULATION EDDY IN SWIRLING
JET, S = 1.57


This can be evaluated from velocity profiles either assumed, or evaluated from
computational fluid dynamics (CFD) analysis.
Swirl can be produced by using tangential inlets, helical guide vanes in the flow,
or by mechanically rotated vanes or walls (see Figure 5). The degree of swirl is
increased by higher swirl numbers and by the addition of a divergent exit duct
(see Figure 6).

FIGURE 5

SCHEME OF SWIRL BURNER WITH AXIAL AND TANGENTIAL
AIR ENTRIES


6

BIBLIOGRAPHY

Reference

Authors, Title and Source

[1]

Germain, E & Wetter, R. Mélange Statique de gaz, Informations
Chimie No. 232. Dec 1982, p 135 (referred to in Clause 4).


FIGURE 6

COMPARISON OF THE RADIAL DISTRIBUTIONS OF VELOCITY
IN THE VORTEX REGION FOR JETS WITH AND WITHOUT
DIVERGENT EXTENSIONS

A European plant operator reactor achieves a backmixed 'pre-reaction' zone by
jet entrainment (see Figure 7).


FIGURE 7

BACKMIXED 'PRE-REACTION' ZONE

DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE
This Process Engineering Guide makes reference to the following documents:
GBH ENTERPRISES ENGINEERING GUIDES
Glossary of Engineering Terms
(referred to in Clause 3)
GBHE-PEG-MIX-701

Mixing of Miscible Liquids
(referred to in Clause 4)

OTHER GBHE DOCUMENTS
“A Design Procedure for Droplet Production in Static Mixers for Newtonian and
Power Law Fluid” (referred to in Clause 4).


Gas Mixing

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