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An
Introduction
to
High
Performance
Fluid
Mixing
There
are
a
vast
number
of
technologies,
machines
and
devices
to
perform
fluid
mixing
tasks,
although
none
can
be
used
for
all
mixing
duties.
Not
surprisingly,
this
makes
the
selection
of
mixers
both
complex
and
confusing
for
most
mixer
users
and
is
one
of
the
main
reasons
why
fluid
mixing
is
still
the
subject
of
intense
academic
research
even
after
thousands
of
years
of
development!
If
you
are
new
to
the
subject
of
fluid
mixing
and
are
looking
for
some
help
on
terminology
and
an
answer
to
the
question,
"why
are
there
so
many
kinds
of
fluid
mixers?",
then
we
hope
that
you
will
find
this
page
helpful.
There
are
some
suggestions
on
further
reading
at
the
bottom
of
the
page.
What
is
Fluid
Mixing?
The
"mixing"
of
one
or
more
components
or
materials
in
a
"fluid
system"
can
be
described
in
terms
of
two
separate
but
interlinked
physical
processes:
-
Blending ("distribution")
of
different
components
of
the
mixture
to
create
uniformity
throughout
the
mix,
and
-
Droplet or particle size
reduction
("dispersion")
of
one
or
more
components
of
the
mixture
to
give
increased
homogeneity
of
the
system
or
to
alter
the
nature
of
the
system
by
increasing
the
contact
surface
area
between
the
components,
i.e.
reducing
the
particle
or
droplet
sizes
increases
their
contact
surface
area
to
volume
ratio.
A
fluid
system
in
this
context
means
a
combination
of
materials
which
combine
to
form
a
fluid,
where
a
fluid
is
defined
as
matter
which
cannot
sustain
a
shear
force
while
at
rest.
In
particular,
we
are
considering
liquid-liquid
and
solid-liquid
mixing
systems
here,
as
distinct
from
dry
powder
or
gas-liquid
mixing
systems.
Most
fluid
mixing
problems
can
be
considered
in
terms
of
the
miscibility
(the
ease
of
mixing)
of
the
system
components.
Miscibility
can
in
turn
be
thought
of
as
the
ease
of
distribution
and
the
ease
of
particle
size
reduction
-
this
affects
the
mixing
approach
to
be
adopted.
For
instance,
where
the
rate
of
reaction
between
miscible
components
is
to
be
improved,
mixing
efforts
are
focused
on
maximising
distribution,
while
for
mixing
immiscible
fluids,
efforts
are
focused
on
reducing
droplet
or
particle
size
to
maximise
the
area
of
contact
between
the
phases.
A
further
consideration
is
the
type
of
production
process
involved,
of
which
the
fluid
mixing
is
normally
only
a
part.
The
most
important
distinction
that
affects
the
mixing
operation
is
whether
the
process
is
batch
or
continuous
in
nature.
In
a
batch
process,
a
discrete
volume
of
material
is
mixed,
usually
within
a
vessel;
in
a
continuous
process,
a
stream
of
material
is
mixed,
usually
piped
to
and
from
the
mixer.
In
an
ideal
world,
it
would
be
possible
to
choose
the
appropriate
mixing
action
to
suit
the
requirements
of
the
fluid
and
then
select
either
a
batch
or
continuous
form.
In
practice,
many
mixing
technologies
are
offered
in
either
batch
or
continuous
form
but
not
both.
In
situations
where
both
are
offered,
there
are
normally
some
performance
trade-offs.
Many
fluid
production
processes
are
actually
defined
around
the
kind
of
mixer
that
is
used,
often
for
reasons
of
expediency
or
"standard
practice".
This
makes
it
difficult
for
those
working
to
innovate
new
fluid
products
to
make
use
of
new
production
processes
and
methods.
Mixers offered by Maelstrom
are
mostly
available
in
both
batch
and
continuous
forms
and
care
has
been
taken
to
minimise
performance
trade-offs
to
make
selection
easier
and
more
secure.
Fluid Mixing
Mechanisms
In
terms
of
mechanical
mixing
mechanisms,
a
number
of
actions
are
employed
by
different
types
of
mixers
to
create
different
effects
for
particular
process
results.
For
distributive
action,
swirl
created
by
rotating
parts
causes
laminar
thinning
of
the
material
interfaces,
thereby
increasing
volumetric
combination
of
the
materials.
A
repeated
cutting
and
folding
action
of
the
mixture
also
increases
the
distribution
of
different
material
components.
The
effectiveness
and
efficiency
of
a
mixer
in
distributive
mixing
is
therefore
a
function
of
how
the
machine
interacts
with
the
fluid
in
a
geometric
sense.
Conversely, the effectiveness
and
efficiency
of
a
mixer
in
dispersive
mixing
is
a
function
of
how
the
machine
interacts
with
the
fluid
in
a
stressing
sense.
For
most
materials,
the
higher
the
stress,
the
smaller
the
resulting
particles
or
droplets
in
the
mixture.
However,
another
very
important
consideration
is
the
uniformity
of
the
stress
field.
Without
a
reasonable
uniformity,
it
is
impossible
to
guarantee
that
the
same
stress
is
applied
to
all
parts
of
the
fluid.
This
would
result
in
a
wide
range
of
final
droplet
or
particle
sizes
rather
than
a
narrow
range
obtained
with
uniform
stressing.
One
or
more
of
the
three
primary
stressing
mechanisms
are
used
in
most
fluid
mixers.
These
mechanisms
are:
-
SHEAR
-
EXTENSION
-
IMPACT
Of
these
mechanisms,
the
most
effective
is
extensional
stressing.
This
is
why
nozzle
valve
homogenisers
are
used
to
create
many
of
the
ultra-fine
dispersions
demanded
by
process
industries,
despite
their
many
practical
disadvantages,
and
is
why
the
common
"high-shear"
mixers
are
relatively
ineffective
and
inefficient
for
dispersive
mixing.
Mixer
Types
Although there are as many
types
of
mixers
as
there
are
terms
for
describing
them,
for
fluid
mixing
they
essentially
break
down
into
the
following:
Impellers - normally comprising
specially
shaped
blades
on
a
rotating
shaft,
driven
by
some
form
of
motor
or
geared
drive
-
batch
use
almost
entirely,
but
more
than
55%
of
the
mixing
equipment
market
is
made
up
of
these
devices,
which
come
in
a
bewildering
array
of
sizes
and
shapes.
Special agitators/blenders
-
this
covers
a
range
of
special
purpose
machines
which
are
normally
for
batch
use
only
and
are
designed
for
a
particular
duty.
Although
there
are
often
many
disadvantages
in
using
these
devices
such
as
cleanability,
inefficiency
and
so
on,
their
use
is
sometimes
vital
in
creating
certain
mixing
effects.
The
group
includes
ribbon
mixers,
pin
mixers,
anchors,
z-blades
and
dozens
more.
The
Maelstrom
Fluid
Division
Mixer
falls
into
this
category
when
operating
at
low
speed
as
it
is
capable
of
very
high
distributive
performance
through
its
dynamic
use
of
structured
cutting
and
folding
whilst
imparting
almost
no
shear
stress
into
the
product.
This
is
very
beneficial
where
shear-sensitive
products
need
to
be
blended
in
either
continuous
or
batch
mode.
Static mixers - a relatively
recent
development
(in
the
1960s),
these
are
devices
for
continuous
use
only
which
comprise
a
set
of
non-moving
obstructions
in
a
pipeline.
The
obstructions
are
shaped
and
positioned
in
such
as
way
as
to
create
cutting
and
folding
effects
and/or
turbulence
for
mixing
of
piped
fluid
streams.
Although
cleanability
is
an
issue,
static
mixers
are
a
reliable
and
low
cost
alternative
in
a
wide
range
of
inline
blending
applications.
It
should
be
noted
however,
that
any
high
pressure
drop
across
the
mixer
must
be
compensated
for
by
larger
and
more
expensive
pumps.
Mills
-
available
in
various
forms
for
both
batch
and
continuous
use,
mills
generally
use
compressive
and/or
shear
stresses
to
create
dispersions
by
crushing
or
grinding
the
fluid
material
between
moving
surfaces.
A
two-roll
mill,
as
the
name
suggests,
comprises
two
rotating
cylinders
which
rotate
to
crush
and
grind
material
between
them.
The
other
common
type,
the
bead
mill,
uses
hardened
metal
beads
inside
a
tumbling
cylinder
through
which
the
fluid
is
passed
to
give
a
random
crushing
of
the
fluid.
Due
to
the
way
they
work,
mills
are
particularly
suited
to
particle
size
reduction
of
solids
which
are
suspended
in
fluids,
although
throughputs
rates
are
generally
quite
low.
The
Maelstrom
High
Stress
Mixer
operates
with
a
milling
action
although
its
milling
faces
are
specially
profiled
to
provide
additional
extensional
stress
and
distributive
mixing
in
visco-elastic
materials.
Rotor-stator dispersers
-
usually
called
"high-shear
mixers",
are
the
most
common
form
of
dispersing
mixer.
By
placing
a
form
of
closely-fitting
shroud
around
a
high
speed
impeller,
it
is
possible
to
create
a
shearing
action
between
the
blades
and
stator
shroud.
As
material
is
centrifugally
pumped
through
the
mixing
head,
some
of
it
will
see
this
high
shear
zone
and
experience
shear
stressing
that
results
in
dispersive
mixing.
Where
small
or
uniform
dispersions
are
required,
material
must
be
cycled
through
the
head
many
times
to
ensure
statistically
that
all
of
the
material
has
passed
through
the
high
shear
zone
at
least
once.
The
viscosity
range
handles
is
also
restricted
due
to
the
centrifugal
pumping
action.
Although
performance
is
therefore
limited,
rotor-stator
machines
are
fairly
flexible
in
their
duties
and
are
available
in
both
batch
and
continuous
forms.
The
Maelstrom
Fluid
Division
Mixer
falls
into
this
category
when
operating
at
high
speed
in
turbulent
mode
as
it
combines
an
intense
hydraulic
shear
with
its
excellent
blending
capability.
The
uniformity
of
the
high
shear
field
in
the
mixing
head
means
that
some
of
the
problems
associated
with
stress
uniformity
in
normal
high
shear
mixers
are
avoided.
FDM
machines
typically
put
5
times
more
energy
into
fluid
than
equivalent
high
shear
mixers.
Special purpose dispersers
-
a
range
of
complex
machines
and
systems
which
deliver
very
good
uniform
dispersions,
normally
in
particular
fluid
applications.
For
example,
high
pressure
valve
homogenisers
are
used
in
the
processing
of
milk
to
ensure
that
the
milk
fats
droplets
are
reduced
in
size
and
evenly
dispersed
throughout
the
bulk.
This
stops
the
cream
separating
from
the
milk.
The
valve
homogeniser
comprises
a
very
high
pressure
pump
and
a
controlled
valve
nozzle
through
which
the
fluid
is
forced
at
very
high
velocity
to
rupture
the
fat
droplets
through
extensional
stressing.
The
jet
impinging
mixer
is
another
type
of
disperser
which
uses
high
velocity
fluid
streams,
except
that
in
this
case,
the
fluid
is
jetted
against
a
plate
or
contra-jet
to
rupture
the
droplets
or
particles
using
impact
stressing.
Ultrasonic
mixers
and
membrane
mixers
provide
extremely
small
droplet
sizes,
although
their
cost,
complexity
and
fragility
mean
that
few
are
used
in
medium
to
large
volume
production
applications.
Integral Pump Mixers -
can
really
be
treated
as
a
separate
class
of
mixing
device
due
to
the
way
that
they
combine
a
number
of
different
stressing
and
distribution
mechanisms
to
achieve
both
high
dispersion
and
high
distribution
performance.
Available
in
both
batch
and
continuous
forms,
Integral
Pump
Mixers
use
internally
generated
positive
displacement
pumping
to
force
fluid
through
small
nozzles
at
very
high
energies
whilst
extending
and
shearing
it.
The
fluid
flowing
through
the
nozzles
at
high
velocity
then
impinges
on
an
internal
wall
of
the
mixer.
A
dynamic
cutting
and
folding
action
added
to
vigorous
turbulent
flow
provides
distributive
mixing.
Integral
Pump
Mixers
are
suited
to
a
wide
variety
of
applications
due
to
their
ability
to
handle
a
wide
range
of
materials
and
viscosities,
their
high
performance
and
their
economic
benefits.
Find
out
more
about
Integral
Pump
Mixing
technology
and
products
available
from
Maelstrom
Advanced
Process
Technologies.
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