High Shear Mixers

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-MIX-709

'High Shear' Mixers

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believed to be accurate and correct at time of going to press, and is given in
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(statutory or otherwise) is excluded except to the extent that exclusion is
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Process Engineering Guide:

'High Shear' Mixers

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

3

1

SCOPE

3

2

FIELD OF APPLICATION

3

3

DEFINITIONS

3

4

SELECTION OF MIXER TYPE

3

4.1
4.2
4.3
4.4
4.5

The Shrouded Turbine
Turbine Mixers
Unshrouded Agitators
In-Line Mixers
Ultrasonic Homogenizers

3
4
4
5
5

5

SHROUDED TURBINE DATA

6

5.1
5.2

Recommended Duties
Equipment Data

6
6

6

HIGH SPEED INTERNAL (TURBINE & VESSEL
COMBINED) MIXERS

10

Recommended Duties
Equipment Data

10
11

6.1
6.2


7

HIGH SPEED UNSHROUDED (SAWTOOTH) AGITATORS

12

7.1
7.2

Duties
Equipment Data

12
13

TABLES

1 POWER INSTALLED FOR TORRANCE SAWTOOTH
IMPELLERS

15

FIGURES

1

SHROUDED TURBINE

4

2

TURBINE AND VESSEL COMBINED

4

3

SAWTOOTH MIXER

5

4

IN-LINE MIXER (OAKES)

5

5

ULTRASONIC EMULSIFIER IN-LINE

6

6

ROTOR STATOR GEOMETRIES

7

7

SAWTOOTH IMPELLER RANGE OF GEOMETRIES
(AFTER TORRANCE)

13

MIXER POWER CURVES

14

8

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

16


0

INTRODUCTION/PURPOSE

"High shear" mixers are used widely in the paint, food, pharmaceuticals,
adhesives and coating industries, but their application in the chemical plant
operations has been limited. The break-up of solid agglomerates, especially the
dispersion into paints, is a major application. Many wet filter cakes may be
redispersed by high shear mixers prior to drying. Emulsification and foaming
feature in pharmaceutical, food and cosmetic applications.
"High shear" mixers are particularly effective for dissolving high polymer
additives. It should, however, be remembered that polymer chains in excess of
about 20,000 monomer units are vulnerable to degradation in commercial high
shear mixers. This degradation of polymers is used for viscosity control in the
manufacture of shampoo and in other processes to obtain optimum rheological
characteristics.
The process engineer may be faced with the task of selecting a satisfactory
device from one of the many proprietary designs available. The technical
guidance available from the suppliers often falls short of the ideal; manufacturers
may not always be able to supply such basic information as the power
consumption of a mixer for a liquid of density greater than that of water.

1

SCOPE

This Process Engineering Guide helps the user to select the appropriate mixer
type for the duty and then recommends criteria to be met for satisfactory
operation.

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

SELECTION OF MIXER TYPE

The following types of mixer are available commercially:
4.1

The Shrouded Turbine

Shrouded turbines are manufactured by Greaves Ltd and Silverson Machines
Ltd. Kinematica supply a machine for producing sub-micron sized dispersions.
They are illustrated in Figure 1 and are recommended for emulsification, polymer
or gel dissolution or 'soft' solid dispersion where the solids content is less than
15% w/w. Detailed recommendations and performance data are given in Clause
5.
FIGURE 1

SHROUDED TURBINE


4.2

Turbine Mixers

Turbine mixers, consisting of a vessel and the agitator which are sold as one
unit, are exemplified by the Baker Perkins 'Hydisperser', see Figure 2. These
units are particularly suited to slurrying filter cakes and are suitable for solid
dispersion duties where the solids content exceeds 15% w/w. The standard
system gives poor bulk circulation; Detailed design recommendations and
performance data are given in Clause 6.
FIGURE 2

4.3

TURBINE AND VESSEL COMBINED

Unshrouded Agitators

Unshrouded agitators are often of the sawtooth disc type (see Figure 3 for an
example of the Torrance device). They are recommended for solids dispersion
duties requiring more than 15% w/w of solids. They can give problems with filter
cakes and will be slow to emulsify liquid-liquid systems with a viscosity ratio
greater than 3. Detailed recommendations and performance data are given in
clause 7.


FIGURE 3

4.4

SAWTOOTH MIXER

In-Line Mixers

Some manufacturers make in-line versions of their shrouded mixers. Oakes Ltd
and Mondo Mix BV supply shrouded mixers with no batch equivalent.
The Oakes mixer with its concentric rows of rotor and stator teeth should ensure
that fluid does not by-pass the active zones. Oakes and Mondo mixers are
particularly effective in the production of foams. Figure 4 illustrates an Oakes
mixer.
FIGURE 4

IN-LINE MIXER (OAKES)


Other mixer types may be subject to by-passing of the most intensive agitation
zone and a single pass may therefore not be able to meet the requirements.
Such designs should therefore be treated with caution.

4.5

Ultrasonic Homogenizers

These devices, illustrated in Figure 5, perform well in some niche applications,
e.g. the exfoliation of vermiculite, where the caviation process is effective. They
should be effective inline emulsifiers with no restriction on the liquid viscosity
ratio.
FIGURE 5

ULTRASONIC EMULSIFIER IN-LINE

5

SHROUDED TURBINE DATA

5.1

Recommended Duties

Shrouded turbines give the most intense extensional flow fields and are
recommended for the following duties:
(a)

Emulsification
Shrouded turbines are particularly suited when the dispersed phase is the
more viscous. When the ratio of the viscosity of the dispersed to that of
the continuous phase is greater than 3, a shrouded turbine should be
used.


(b)

Polymer or Gel Dissolution
Polymers swell in solvents and in the early stages of dissolution the
process is the breakdown of the viscous gel particles, somewhat
analogous to emulsification. Quite large lumps of elastomeric solids can
be digested by shrouded mixers.

(c)

Polymer Degradation
High molecular weight polymers may be degraded by the action of a
shrouded turbine. Significant heat can be generated in this process.

(d)

Homogenization of Soft Solids
Soft solids, e.g. elastomers, may be dispersed by shrouded turbines to
give a homogeneous paste. They give an intense disruptive action and
should be used whenever possible. Their use is however restricted by
their limited pumping action and by abrasion. Other devices are
recommended for solids concentrations in excess of 15%!w/w.

5.2

Equipment Data

(a)

Rotor Stator Geometry
Typical geometries of the shrouded turbine mixers are shown in Figure 6.
The rotor discharges the fluid through the radial ports in the stator.
Manufacturers supply 'high shear' and 'high pumping' stators; the latter
has fewer, larger ports to allow the impeller to digest coarse lumps and
gives higher circulation rates.
The Greaves mixer includes an axial flow turbine and the circulation rate
may be controlled by a deflector plate which, when raised, allows fluid to
bypass the stator


FIGURE 6

ROTOR STATOR GEOMETRIES


The 'high pumping' head is preferred for most duties and has performed better
than the 'high shear' head for polymer dissolution and degradation. In concept
the 'high pumping' head allows the impeller to digest coarse lumps and circulate
fluid at a higher rate, but should take longer to reach, for example, the same
degree of emulsification because the fluid can bypass the zone of intense
agitation.
The most intense disruptive forces occur in stretching flow where the fluid is
forced between the 'nip' of an advancing rotor blade and a stator port. Because
of this the annular gap between the rotor and stator is not too critical for
emulsification, and polymer dissolution and degradation duties where a gap of
0.25 to 0.5 mm is satisfactory.
(b)

Mixer Size for Standard Duties

For routine applications manufacturers should be approached. They should then
quote the size and power of a mixer for a particular duty. They are, however,
often reluctant to commit themselves to a mixing time and thus to throughput.
Greaves have provided data for sizing impellers for batch emulsification and
polymer dissolution duties which are summarized by the dimensional equation:

The equation may be used to select a proprietary Greaves or Silverson mixer 'off
the shelf' provided the liquid density is 1000 kg/m3 or less. If the liquid density
exceeds 1000 kg/m3, increase the manufacturer's nominal motor rating pro rata
with the density.
Equation 1 implies a rotor to vessel ratio ranging from 0.12 in low viscosity (0.01
N s/m2) to 0.2 in high viscosity (30 N s/m2) batches and represents competitive
sizing for standard duties.


(c)

Mixer Size for Critical Duties

Polymer degradation represents a critical duty because the zone of intense
agitation is only a small fraction of the total volume of the vessel. To achieve
economical batch times rotor to vessel diameter ratios as high as 0.33 have been
required.
Mixers for these duties should be scaled up from trials on small-scale mixers.
The small scale impeller should have the same rotor to stator gap, blade angle
and tip speed as that proposed for the larger scale. The D/T ratio need not be
matched on both scales.
Scale-up then involves the following steps:
(1)

Using a small, variable speed laboratory mixer, establish the tip speed
needed to give the required process effect.
Note that these mixers may not reach a full-scale tip speed.

(2)

From the laboratory results, specify conditions for a trial with a 1 to 5 hp
mixer in a batch up to 50 liters volume. This mixer will approximate
reasonably closely to the full-scale geometry. A cooling coil would be
required for isothermal operation.

(3)

Calculate the full-scale batch mixing time to achieve the required end
result from:


(d)

Mixer Power

The manufacturer should normally be asked to specify the power required for a
new agitator. Note however that manufacturers have not always been able to
predict the power dissipated in unusual liquids.
The power absorbed depends partly on the mixer's behavior as a turbine and
partly on the torque generated in the rotor stator gap. The latter effect becomes
more important as the liquid density increases. For most liquids, including
solutions of low or degraded polymers (e.g. linear polymer Mw less than 400,000,
Mw/Mn < 4), the power of Greaves and Silverson mixers has been predicted to
within 20% by:

If the batch contains a higher polymer (e.g. a linear polymer of Mw of 400,000 or
more and Mw/Mn > 4) additional power is dissipated in extensional flow as the
viscoelastic fluid is forced into the 'nip' between the rotor and stator.
At present it is not possible to predict the additional power dissipation which has
been 2 - 3 times that predicted by the first term of equation 3. It is therefore
recommended that the additional torque be measured on a small-scale with the
rotor stator gap and tip speed as proposed for the full-scale operation. The fullscale torque is then calculated from:


D'
n
n'

= small-scale rotor diameter
= number of blades on full-scale rotor
= number of blades on small-scale rotor.

(e)

Pumping Capacity

There are no published data on pumping capacities of shrouded turbine mixers.
The following tentative correlation is suggested for a 5 hp Greaves mixer:

(f)

Bulk Mixing Times and Heat Transfer

No data are available for bulk mixing times and heat transfer, both of which are
flow sensitive. The high shear impellers typically generate about 1/3 of the flow of
a turbine (W/D = 0.2) at a given speed. It is recommended that the turbine
correlations for mixing time and heat transfer, given in GBHE-PEG-MIX-701,
should be used with the speed term corrected to N/3.


(g)

Shear Rate

The nominal shear rate in the rotor stator gap is given by:

The shear rate is not normally a particularly important parameter in emulsification
and polymer dissolution or degradation. See also GBHE Mixing and Agitation
Manual, Sections D4.4 and D4.5.

6

HIGH SPEED INTERNAL (TURBINE & VESSEL COMBINED) MIXERS

These machines tend to be sold in two performance ranges. The first, typified by
the 'Hydisperser', has a power dissipation of 10-16 kW/m3, and is used by some
operators to liquidize dispersible filter cakes, often before spray drying. The
standard 'Hydisperser' is fitted with a back-swept vaned disc agitator (D/T = 0.45)
in the base. The standard maximum tip speed is 11 m/s with W/D of 0.06-0.07 to
economize on power.
A cruciform baffle cage may be fitted for lower viscosity applications and a
'butterfly' impeller of D/T = 0.65 is also available.
The second performance range includes the Baker Perkins 'Hynetic Mill' and the
Bearsley & Piper 'Speed mixer'. These devices may incorporate mulling wheels
and are better regarded as colloid mills.

6.1

Recommended Duties

(a)

Solids Dispersion
High speed internal mixers can handle 15 - 65% w/w of solids. The upper
limit will depend on the apparent viscosity of the sheared batch and should
be less than 50N s/m2.

(Consult GBHE-PEG-FLO-302 for the interpretation of viscometric data.) The
sheared viscosity has to be measured experimentally as there is no simple
relationship which covers particle shapes, sizes, packing fractions and interaction
effects. A measurement at a shear rate of 300!l/s should cover the shearing
effect expected in the 'Hydisperser'.

To estimate the sheared viscosity of an ideal system of non-inter-acting spherical
particles, the Mooney equation may be used:

k can be as high as 1.9 and will be lower than 1.6 for particles with a wide size
distribution. The limiting viscosity would be reached at 50% v/v solids
(i.e. c = 0.5) with k = 1.6.

(b)

Filter Cake Dispersion

Over 100 dispersible filter cakes have been examined and all are liquidized at
shear rates of 300 l/s or below. All these filter cakes may be dispersed by a high
speed internal mixer in the lower performance range (10-16 kW/m3).

6.2

Equipment Data

(a)

Scale-Up

Trials, involving the manufacturer of the internal mixer, on small-scale machines
are advised. Manufacturers offer a product range of machines operating at a
constant maximum tip speed and scale up is by keeping the tip speed constant
and increasing the batch time in proportion of the linear scale-up factor.
This method works in most cases. Theoretically, if cake disintegration requires
relatively low shear rate (ca. 300 l/s), then power per unit volume should be the
relevant criterion. Following the manufacturers' scale-up method, the power input
per unit volume will decrease with increasing size. This will not usually be critical,
but it is advisable to check the dispensability of the cake at a lower power input
by reducing the tip speed of the small scale
device according to:


Only if the cake fails to disperse satisfactorily at this lower speed should the
manufacturer be requested to supply a non-standard device based on constant
power per unit volume.

(b)

Butterfly vs Vaned Disc Agitators

Butterfly type agitators are used to provide a folding action in some paste mixers.
They do not offer any particular advantage in high speed dispersion duties
although the open structure of the butterfly agitator may assist in starting the
movement of a cake with a high yield stress.
(c)

Power Consumption

The estimation of the agitator power should be the responsibility of the
manufacturer. Starting loads and transients are likely to be more critical than the
power dissipated in the dispersed batch and, in any event, a variable speed
motor may be selected. A power number (Po) of approximately 0.3 would be
expected for the abbreviated vaned disc in an unbaffled vessel at high Reynolds
numbers.

(d)

Pumping Capacity, Heat Transfer and Bulk Mixing Times

The pumping capacity may be estimated for vaned discs of W/D = 0.065 as:

Mixing times will then be of the order of 3 times those expected for standard
turbines working in baffled vessels at the same speed.
Tangential velocities are similar to those generated by a 2-bladed paddle (W/D =
0.33) and this 'pseudo geometry' should be used for heat transfer calculation,
using the method given in GBHE-PEG-MIX-701.


(e)

Shear Rates

For information on shear rates, consult the GBHE Mixing and Agitation Manual,
Sections D4.4 and D4.5.

7

HIGH SPEED UNSHROUDED (SAWTOOTH) AGITATORS

7.1

Duties

The major application of saw toothed impellers is in paints manufacture.
(a)

Solids Dispersion into Liquid

A major application of the sawtooth mixer is in the dispersion of 15-65% w/w
solids in a liquid resulting in a final batch viscosity of up to 25 N s/m2.
The mixers will break flocculated pigments down to the basic particle size in the
range 1-3 µm and can thus disperse modern, more loosely aggregated pigments
into paint mill base without the need for further milling. They will not, however,
break up strongly aggregated or fused particles. No formal methods exist for
defining aggregate strengths. It is recommended that the agitator manufacturer
be consulted and that small-scale trials are performed in doubtful situations.
The mixers can break up large lumps of solid so that even in the more doubtful
situations they could be used to prepare a feed for a colloid mill.
The mixers are effective at tip speeds greater than 20 m/s; GBHE prefer the
range 25- 36 m/s. The recommended D/T ratios range from 0.25 in low viscosity
(1 N s/m2) batches to 0.5 in high viscosity (25 N s/m2) applications. Figure 7
shows the preferred range of geometries for the sawtooth disc impellers in a
tank.
(b)

Emulsification

The sawtooth mixer is not recommended for liquid mixing duties. It should,
however, be effective in blending or emulsification of liquids into concentrated
solid dispersions and is recommended for this duty.


(c)

Polymer Dissolution

The sawtooth mixer is not recommended for polymer dissolution as it can give a
stratified batch in polymer systems where viscoelastic forces oppose vortex
formation.

(d)

Filter Cake Dispersion

Sawtooth mixers are not recommended for filter cake dispersion. They can suffer
from stratification problems and these would prevent the lifting of the undispersed
solid from the base of the vessel.

FIGURE 7 SAWTOOTH IMPELLER RANGE OF GEOMETRIES (AFTER
TORRANCE)


7.2

Equipment Data

(a)

Scale-Up

Using values of D/T and tip speed recommended in 7.1 (a), scale-up from smallscale experiments at constant D/T and tip speed is recommended. The batch
mixing time will increase in proportion to the linear scale.

(b)

Power

Figure 8 shows the relationship between Power number and Reynolds number
for sawtooth and other high speed impellers in baffled vessels and for a Torrance
mixer and a plain disc working in unbaffled vessels.
Hydraulic drives are usually fitted and are recommended for application with
these limited power data as they cannot be overloaded and allow mixing speeds
to be changed in a batch.
A Paints manufacturer produced the power data shown in Table 1. The apparent
power numbers near 0.1 are because the two of the three mixers are operating in
vortex aerated conditions at the highest tip speeds. Using this data they have
been able to replace the hydraulic drives with two speed electric motors which
are easier to maintain. If overloaded (for example by an increase in velocity
which causes loss of vortex) the motor trips to the lower speed. The trip power
may then be calculated from Figure 8. This procedure should only be attempted
with systems of less than 3 N s/m viscosity and then only in consultation with the
manufacturer.


FIGURE 8

(c)

MIXER POWER CURVES

Pumping Capacity, Heat Transfer and Bulk Mixing Times

An estimate of the radial pumping rate may be made from:
Q = 0.15 N D3

. . . . . . (10)

this is about one sixth of that of a turbine with W/D = 0.2.
Heat transfer is influenced by circumferential velocities and these in turn are
related to the effective vortex depth. The vortex factor, g x/D2N2, is about one
third of that of a 2-bladed flat paddle with W/D = 0.33. Approximate heat transfer
coefficients could be estimated assuming a standard paddle geometry operating
at 1/3 times the speed of the saw toothed impeller.


Scale-up should be on the basis of constant tip speed and bulk mixing times will
increase in proportion to the linear scale, i.e. be proportional to volume to the
one-third power:
t'' = t' (V'' / V')1/3

(d)

. . . . . . (11)

Blade Geometry

Experience indicates that there is little to choose between the different sawtooth
blade configurations offered by the various manufacturers.
Sawtooth impellers are more effective than plain or perforated discs in the
creation of fine pigment dispersions. We have found that a circular saw blade is
as effective as a proprietary device in the dispersion of pigment base. The
proprietary devices incorporate blades with a vertical surface oriented
circumferentially (along the direction of rotation) and these blades will be more
effective in the initial break-up of coarse lumps by impaction.

(e)

Shear Rates

Peak shear rates for sawtooth impellers are caused by shear and elongational
flows. To determine the range over which peak shear rates apply, reference
should be made to the GBHE Mixing and Agitation Manual, Sections D4.4 and
D4.5.


TABLE 1 POWER INSTALLED FOR TORRANCE SAWTOOTH IMPELLERS
Batch Density
Viscosity

1300 - 1440 kg/m3
1 N s/m2


DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE
This Process Engineering Guide makes reference to the following documents:
GBHE ENGINEERING GUIDES
GBH Enterprises

Glossary of Engineering Terms
(referred to in Clause 3)

GBHE-PEG-MIX-701

Mixing of Miscible Liquids
(referred to in 5.2 (f) and 6.1 (d))

GBHE-PEG-FLO-302

Interpretation and Correlation of Viscometric Data
(referred to in 6.1 (a))

OTHER GBHE DOCUMENTS
GBH Enterprises

Mixing and Agitation Manual
(referred to in 5.2 (g), 6.2 (e) and 7.2!(e)).


High Shear Mixers

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