How to Use the GBHE Mixing Guides

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-MIX-700

How to Use the GBHE Mixing Guides

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.

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com

Process Engineering Guide:

How to Use the GBHE Mixing
Guides

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

3

1

SCOPE

3

2

FIELD OF APPLICATION

3

3

DEFINITIONS

3

4

THE MIXING GUIDES

3

4.1
4.2

Mixing Guides
GBHE Mixing and Agitation Manual

3
3

5

DEVICE SELECTION

3

6

MIXING QUESTIONNAIRE

5

6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
6.10

What is being mixed?
Why is it being mixed?
How is it to be mixed?
Is Heat Transfer Important?
Is Mixing Time Important?
Is Inventory Important?
Is Subsequent Phase Separation Important?
What Quantities?
What are the Selection Criteria?
What Data are required?

5
5
5
6
6
6
6
6
6
6


7

BASICS

7

7.1
7.2
7.3
7.4
7.5

Bulk Movement
Shear and Elongation
Turbulent Diffusion
Molecular Diffusion
Mixing Mechanisms

8
8
8
9
9

APPENDICES

A

ROTATING MIXING DEVICES

11

B

MIXING DEVICES WITHOUT MOVING PARTS

18


TABLES
1

SUMMARY OF TYPICAL USES OF MIXING DEVICES

4

FIGURES

1

CHARACTERISTICS OF VARIOUS DEVICES IN TERMS
OF BULK FLOW

9

2

TANK LAYOUT

11

3

PITCHED BLADE TURBINE

12

4

DISC TURBINE

12

5

PROPELLER

13

6

RETREAT CURVED BLADE TURBINE

13

7

CONCAVE BLADE TURBINE

14

8

'GASFOIL' AGITATOR

14

9

'HYDROFOIL' AGITATOR

14

10

HELICAL SCREW STIRRER

15

11

HELICAL RIBBON STIRRER

15

12

ANCHOR AGITATOR

16

13

BENT ANCHOR

16

14

”SAWTOOTH” DISC DISPERSER

17

15

”HIGH SHEAR” ROTOR-STATOR MIXER

17

16

COAXIAL JET FLOW MIXER

18

17

SIDE ENTRY JET FLOW MIXER

18


18

KENICS STATIC MIXERS

19

19

ROSS LLPD MOTIONLESS MIXER

19

20

SULZER SMV

20

21

SULZER SMX

20

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

21


0

INTRODUCTION/PURPOSE

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

1

SCOPE

This Guide directs the user to the most appropriate Guides to solve a mixing
problem. It does so by classifying the mixing systems, by asking a series of
questions and by defining the common terminology.

2

FIELD OF APPLICATION

This Guide applies to Process Engineers in GBH Enterprises worldwide.

3

DEFINITIONS

No specific definitions apply to this Guide.

4

THE MIXING GUIDES

4.1

Mixing Guides

Guidance on Mixing is available from two sources, the Mixing Guides and the
GBHE Mixing and Agitation Manual.
Mixing Guides are intended to provide initial guidance and a quick design for
uncomplicated mixing tasks in research, development and production. They
concentrate on recommendations and design or scale-up procedures.
The Mixing Guides are designated GBHE-PEG-MIX-XXX where the last three
digits identify the individual guides.


4.2


The GBHE Mixing and Agitation Manual contains more detail, background to the
correlations and recommendations, many more references and is the repository
of the collective know-how. It provides greater insight and is thus better placed to
advise on more complex or unusual mixing problems.

5

DEVICE SELECTION

The range of devices covered by this series of Guides is summarized in Table 1.
Many of the duties could be performed by two or three different devices; it is
worthwhile considering the options before embarking on the detail design.
Appendix A illustrates typical rotating mixing devices and some typical ratios of
dimensions for agitated tanks. Appendix B shows devices without moving parts,
i.e. static and coaxial jet mixers.


TABLE 1

SUMMARY OF TYPICAL USES OF MIXING DEVICES


6

MIXING QUESTIONNAIRE

The objective of the following questions is to act as a check list for the designer.
The list is not, however, exhaustive but is intended to stimulate ideas.

6.1

What is being mixed?

Type of Mixture

Mixing Guide

GBHE Mixing and
Agitation Manual

Miscible Liquids

GBHE-PEG-MIX-701

Section A

Gas-Gas

GBHE-PEG-MIX-702

Section B

Solid-Liquid

GBHE-PEG-MIX-703
GBHE-PEG-MIX-709

Section C
Section D4

Immiscible Liquids

GBHE-PEG-MIX-704
GBHE-PEG-MIX-709

Section D
Section D4

Gas-Liquid

GBHE-PEG-MIX-705

Section E

Gas-Solid-Liquid

GBHE-PEG-MIX-706

Section F

Solid-Solid

GBHE-PEG-MIX-707

Section G

Gas-Solid

GBHE-PEG-MIX-708

Section H

6.2

Why is it being mixed?

Because a mixture is required

e.g. in paint manufacture
How well does it need to be mixed?

To promote mass transfer

-

To react

is the product spectrum dependent
on the rate of mixing?


Mass transfer followed by
Reaction

6.3

Which process is rate limiting?
Is it important which is limiting?

How is it to be Mixed?

Batchwise

in an - Agitated Tank
Jet Stirred Tank
Flow Mixer in Loop

Continuously

in an - Agitated Tank
Flow Mixer
Bubble Column

'Semi-batch’

in a - Bubble Column
Two-Phase Agitated Tank

6.4

Is Heat Transfer Important?

Provision of heat transfer surface could determine the size of the equipment. Inline (static) mixers could be incorporated within a shell and tube heat exchanger.

6.5

Is Mixing Time Important?

Reactions are often potentially very rapid and in practical equipment their rates
can be determined by the rate of mixing of the reactants.

6.6

Is Inventory Important?

Nitroglycerine is made in jet mixers to minimize the inventory of unstable
material. Similar considerations could apply to other hazardous (including toxic)
materials.

6.7

Is Subsequent Phase Separation Important?

Mixing intensity (and hence bubble or droplet size) may need to be optimized to
take account of the ease of separation.


6.8

What Quantities?

With small quantities simplicity of operation may be the deciding factor while for
large quantities the capital cost may influence the type of equipment to be used.

6.9

What are the Selection Criteria?

Is the choice to be made primarily on economic grounds or is the process
requirements dominant?

6.10

What Data are required?

Apart from those data already listed above, the following are likely to be
important:
Viscosity

is the liquid Newtonian?

Surface Tension or Interfacial Tension is the effect of surfactants
(or surface active impurities) important?
Density

-

Diffusivity

-

Particle size

is size and size distribution important?

Reaction Rate

-

Vapor Liquid Equilibria

-

Liquid-Liquid Equilibria

-


7

BASICS

Mixing of fluids is one of the most common operations in chemical processing,
yet also one of the most misunderstood as far as the fundamental mechanisms
are concerned. Design of equipment is based largely on empirical correlations
tempered with experience. For gas-liquid and liquid-liquid contacting, only scaleup relationships exist, as a priori design relationships for real systems are not yet
available even on an empirical basis. Some understanding and quantification of
the basic mechanisms is emerging however (especially for turbulent mixing) and
this is treated later in this Clause. The situation is further complicated by the
other duties a mixer often has to perform (such as heat transfer, chemical
reaction and interphase contacting) and the constraints which may prevail (such
as energy consumption (not often important), fluid inventory and sealing
(important for safety), corrosion, mechanical robustness (especially agitator
shafts), fouling and blockage by solids (e.g. in flow mixers) and foaming). In the
case of chemical reactions the flow pattern in the mixer (backmixed or plug flow,
continuous or batch) is often important also.
It is vital that all such factors as are relevant are taken account of in selection and
design of a mixer. This will often involve several iterations of the design
calculations before the limiting factor is identified. The relative performance of a
device for these duties will depend on the device selected, so several types of
device should be considered initially. Several types of device for each duty are
covered in the subsections of the Guide. The designer should obviously aim for
the most economical design and this will often imply maximizing the rate of the
limiting factor, within the process constraints.
Mixing of liquids takes place by four mechanisms (bulk movement, shear and
elongation, turbulence and molecular diffusion). For example, when a liquid B is
added to a tank containing liquid A, with which it is miscible, it does not disperse
immediately and there is obvious inhomogeneity: all of B is sitting in one part of
the tank. When the stirrer (or jet) is started the liquid begins to move and all of B
tends to move round the tank together. But, because the velocities are not
uniform, B begins to spread out and this spreading continues until B is distributed
all over the tank so that if a large sample of liquid is removed the quantity of B it
contains will be independent of where the sample is taken from. The tank is now
probably well mixed, but this is not necessarily so.
If the flow in the tank is laminar, the close inspection of a small volume will reveal
that there are layers of liquid A separated by layers of liquid B.


Eventually, the layers of liquid are sheared down to molecular dimensions and
mixing is complete. Shearing down to molecular dimensions is not as difficult as
it sounds: the thickness of the layers has to be halved less than 30 times. In
practice this amount of shearing is rarely necessary because molecular diffusion
spreads the material out of the layers.
If the flow in the vessel is turbulent local mixing is enhanced by eddies, which are
regions of highly sheared fluid which move through the fluid bulk, decaying in
size. They are postulated to decay until they reach a minimum size, the
'Kolmogorov length scale' (Ѵ3/ε)1/4. On length scales below this size, energy is
dissipated by viscous forces only.

The motion on a scale smaller than 32 µm is effectively laminar: thus neutrally
buoyant particles that are appreciably smaller, e.g. 5 µm, will behave as if in a
stagnant liquid. Thus, if they are soluble, the dissolution rate can be calculated
simply by using:

To summarize; the three processes which are present in mixing operations are:


7.1

Bulk Movement

The velocity is not uniform so the fluid is spread out by velocity differences and
Distributive Mixing occurs. The fluid is split into different streams, for example at
the agitator in a stirred tank, where one stream goes up and one down, or by an
in-line mixer. Because the travel times are different, the two streams do not
recombine but subsequently mix with other volumes of fluid.
Bulk movement is usually the rate determining process in turbulent mixing
(blending) in tanks as is shown by the fact that mixing times are not significantly
affected by changes in viscosity. It also contributes significantly to mixing in inline mixers. However, it is not of much significance in pipe flow.
In laminar flow, bulk movement is the only flow which occurs and mixing time is
determined by a balance between laminar shear and molecular diffusion, the
former being assisted by distributive mixing effects.

7.2

Shear and Elongation

Shear and elongation arise from bulk movement and are the mechanisms
responsible for spreading out large clumps of fluid.

7.3

Turbulent Diffusion

This is usually the rate determining process in pipe flow, where it is often
modeled by using an effective diffusion coefficient. It is also relevant to mixing in
any turbulent system, especially to mixing in a localized zone of a vessel.

7.4

Molecular Diffusion

This is the ultimate process in all mixing operations, but in turbulent systems
clumps of liquid are usually ground down so rapidly by shear in the eddies that it
is not usually important in determining bulk mixing time. In laminar systems
molecular diffusion can be important. The rates of fast localized reactions can
also be controlled by molecular diffusion.
The foregoing discussion has assumed that the liquids being mixed are miscible
and of similar viscosities (within a factor of 10 or so). In the case of immiscible
liquids the rate limiting process is usually the breakup of droplets and mixing
times are likely to be much longer than those when bulk flow alone is limiting. In

the case of liquids of differing viscosities the more viscous component often
refuses to join in the bulk flow and mixing times are determined by mass transfer
at the interface between the two liquids.

7.5

Mixing Mechanisms

The requirements of a mixing operation are met by setting up the hydrodynamics
appropriate to the situation, which will involve combinations of bulk flow important
for blending, solid suspension, heat transfer) and shear (for breakup of bubbles,
droplets or particle agglomerates and interphase mass transfer). Figure 1
illustrates broadly the achievable characteristics of various devices in terms of
bulk flow.
FIGURE 1 CHARACTERISTICS OF VARIOUS DEVICES IN TERMS OF BULK
FLOW

For agitators in vessels of given geometry, comparative performance for
producing flow may be judged by comparing FLOW NUMBERS:


and some idea of comparative performance for shear in turbulent agitation may
be gained by comparing POWER NUMBERS:

Often (e.g. in agitated vessels) flow is required to transport fluid through a
localized high shear region.


APPENDIX A
A.1

ROTATING MIXING DEVICES

Figure 2 shows a tank and agitator with the key dimensions annotated and
a Table of Typical Ratios of these dimensions. Note that the base may be
either flat or a dished end.

FIGURE 2

TANK LAYOUT


A.2

Figures 3 to 6 illustrate a variety of different agitators which are generally
used for mixing low viscosity non-Newtonian liquids.

FIGURE 3

PITCHED BLADE TURBINE


FIGURE 4

DISC TURBINE

FIGURE 5

PROPELLER


FIGURE 6

A.3

RETREAT CURVED BLADE TURBINE

The agitators shown in Figures 7 to 9 are used primarily in gas-liquid
systems. For the suspension of particulate solids in a liquid, a Marine
Propeller (Figure 5) or a Pitched Blade Turbine (Figure 3) is normally
recommended.


FIGURE 7

CONCAVE BLADE TURBINE

FIGURE 8

'GASFOIL' AGITATOR


FIGURE 9

A.4

'HYDROFOIL' AGITATOR

For high viscosity and/or non-Newtonian systems, the selection is often
from those illustrated in Figures 10 to 15.


FIGURE 10 HELICAL SCREW STIRRER WITH THREE BAFFLES AT 120°

FIGURE 11 HELICAL RIBBON STIRRER


FIGURE 12 ANCHOR AGITATOR


FIGURE 13 BENT ANCHOR

FIGURE 14 ”SAWTOOTH” DISC DISPERSER


FIGURE 15 ”HIGH SHEAR” ROTOR-STATOR MIXER


APPENDIX B MIXING DEVICES WITHOUT MOVING PARTS
FIGURE 16 COAXIAL JET FLOW MIXER

FIGURE 17 SIDE ENTRY JET FLOW MIXER

FIGURE 18 KENICS STATIC MIXERS


FIGURE 19 ROSS LLPD MOTIONLESS MIXER

FIGURE 20 SULZER SMV

FIGURE 21 SULZER SMX


DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE
This Process Engineering Guide makes reference to the following documents:
ENGINEERING GUIDES
GBHE-PEG-MIX-701

Mixing of Miscible Liquids
(referred to in 6.1 and Table 1)

GBHE-PEG-MIX-702

Gas Mixing

GBHE-PEG-MIX-703

Mixing of Solid-Liquid Systems

GBHE-PEG-MIX-704

Mixing of Immiscible Liquids

GBHE-PEG-MIX-705

Mixing of Gas Liquid Systems

GBHE-PEG-MIX-706

Gas-Solid-Liquid Mixing Systems

GBHE-PEG-MIX-707

Solids Mixing
(referred to in 6.1)

GBHE-PEG-MIX-708

Gas Solid Mixing
(referred to in 6.1)

GBHE-PEG-MIX-709

'High Shear' Mixers

OTHER GBHE DOCUMENTS

(referred to in Clause 4 and 4.2).


How to Use the GBHE Mixing Guides

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (20)

Similaire à How to Use the GBHE Mixing Guides

Similaire à How to Use the GBHE Mixing Guides (20)

Plus de Gerard B. Hawkins

Plus de Gerard B. Hawkins (20)

Dernier

Dernier (20)

How to Use the GBHE Mixing Guides