Conventional and Complex Knitted Mesh Mist Eliminators


Brunazzi, E.; Paglianti, A.

Chemical Engineering & Technology 24(11): 1199-1204

2001


Knitted wire mesh mist eliminators have a widespread application in many industrial plants as they assure an optimum cost/

Research
News
Conventional
and
Complex
Knitted
Mesh
Mist
Eliminators
By
Elisabetta
Brunazzi
and
Alessandro
Paglianti*
Knitted
wire
mesh
mist
eliminators
have
a
widespread
application
in
many
industrial
plants
as
they
assure
an
optimum
cost/
performance
for
many
applications
compared
with
other
separation
devices.
Complex
mesh
pads
allow
the
performance
and
the
range
of
applications
of
conventional
wire
mesh
pads
to
be
extended.
In
recent
years,
increasing
research
effort
has
been
dedicated
to
the
experimental
investigation
of
both
common
and
complex
mesh
pads
and
to
the
development
of
reliable
design
models
that
are
essential
for
the
design
and
optimization
of
complex
separation
units.
1
Introduction
In
many
operations
in
the
chemical
and
oil
production
industries,
it
is
frequently
necessary
to
remove
fine
droplets
of
liquid
from
process
and
waste
gas
or
vapor
streams.
Liquid
separation
may
be
required
to
recover
valuable
products,
improve
product
purity,
increase
throughput
capacity,
protect
downstream
equipment
from
corrosive
or
scaling
liquids
or
to
improve
emissions
control.
As
the
rate
of
a
spontaneous
separation
process
is
often
economically
and
operatively
intolerable,
separation
devices
are
generally
employed
to
accelerate
this
step
and
to
increase
throughput
capacity.
Different
separation
devices
have
been
developed
which
vary
in
effectiveness
and
cost.
Examples
of
these
devices
include
the
mist
pad
knockout
drum,
impingement
separators
(e.g.,
knitted
wire
mesh
and
vane
types),
cyclones,
fibre
beds,
wet
scrubbers
and
electrostatic
precipitators.
Each
of
these
devices
operates
under
different
principles
and
is
applied
for
the
removal
of
droplets
with
a
specific
size
range
and
effective
separation
performance.
Some
key
design
guidelines,
which
are
useful
and
of
interest
when
selecting
the
correct
entrained
liquid
droplet
eliminator,
together
with
some
insights
into
applications
are
given
in
the
works
of
Ziebold
[1],
Fabian
et
al.
[2,3],
Monat
et
al.
[4],
Biirkholz
[5],
Feord
et
al.
[6]
and
Capps
[7]
among
others.
The
important
performance
parameters
of
liquid
separators
are
capacity,
pressure
drop,
droplet
removal
efficiency
and
plugging
tendency.
These
parameters
are
all
interrelated
and
should
be
considered
together
when
comparing
the
perfor-
mance
of
alternative
mist
eliminators.
Other
important
factors
affecting
the
selection
are
the
availability
of
construction
materials
which
are
compatible
with
the
process
and
both
the
investment
and
operating
costs
of
the
separation
device.
Moreover,
it
is
necessary
to
point
out
that
droplet
removal
[*]
Dr.-Ing.
E.
Brunazzi
(author
to
whom
correspondence
should
be
addressed,
email:
Prof.
A.
Paglianti,
Department
of
Chemical
Engineering,
Industrial
Chemistry
and
Materials
Science,
University
of
Pisa,
Via
Diotisalvi
2,
1-56126
Pisa,
Italy.
efficiency
is
highly
dependent
on
the
drop
size
distribution
of
the
entrained
liquid
entering
the
device.
Therefore,
knowl-
edge
of
both
the
drop
size
distribution
and
the
concentration
of
the
liquid
phase
in
the
two-phase
mixture
in
the
context
of
the
process
constraints
is
key
to
proper
equipment
selection
and
specification.
The
entrained
droplets
to
be
removed
are
characterized
by
different
particle
size
distributions.
Commonly,
the
term
"spray"
designates
droplets
with
diameter
size
from
10
to
over
1000
microns,
and
the
terms
"mist"
or
"aerosol"
refer
to
droplets
of
10
to
well
below
1
micron
in
diameter.
The
size
of
droplets
entrained
in
the
gas
or
vapor
stream
vary
over
a
wide
range,
depending
on
the
mechanism
of
droplet
creation,
droplet
history
and
on
the
physical
properties
of
the
system
[1,2,8,9].
Liquid
entrainment
is
generated
by
three
basic
mechanisms:
mechanical
action,
condensation
and
chemical
reaction.
By
using
the
cause
of
mist
entrainment
as
a
base,
one
can
make
a
good
estimate
of
the
droplet
size
distribution.
As
an
initial
evaluation,
experience
teaches
us
that
mechanical
action
rarely
generates
droplets
smaller
than
5
microns
in
diameter;
this
is
encountered
during
bubbling,
from
spray
nozzles,
frothing
in
liquid
distributors
and
on
the
surface
of
tray
and
packing.
Surface
condensation
and
seal
leakage
also
generally
originate
droplets
in
the
spray
region.
The
particle
size
distribution
can
be
narrowed
by
droplet
coalescence,
caused
by
collisions
through
turbulence
[7].
In
the
mist
range,
the
droplets
are
generally
formed
by
endogenous
condensation
(rather
than
surface
condensation)
or
chemical
reaction;
typical
examples
are
the
gas-phase
reaction
of
water
vapor
and
sulfur
trioxide
to
form
liquid
sulfuric
acid,
the
shock
cooling
of
lubricating
oil
in
compres-
sors,
which
condenses
it
as
a
blue
smoke-like
haze,
the
hazes
that
form
during
food
frying.
Although
the
original
particle
size
distribution
may
be
less
than
1
micron,
this
could
shift
upwards
as
the
fluid
is
transported
in
a
turbulent
stream
and
thus
the
distribution
may
shift
from
1
to
10
microns
or
higher
at
the
collecting
device.
Moreover,
the
various
mist/spray
generating
mechanisms
inherent
in
industrial
processes
often
result
in
bimodal
Chem.
Eng.
Technol.
24
(2001)
11,
©
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Verlag
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2001
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Table
1.
Droplet
size
distribution
for
some
process
equipment.
Process
equipment
Typical
spray/mist-size
distributions
[microns]
<lwt.-%
<10
wt.-%
<50
wt.-%
<90
wt.-%
<99
wt.-%
H
2
SO
4
Mist/acid
plants:
Drying
tower
exhaust
0.1
0.8
10.0
Primary
absorbing
tower
98
%
Acid
production
0.4
0.8
1.7
10.0
Oleum
production
0.2
0.5
0.8
2.5
Secondary
absorbing
tower
0.5
1.6
2.5
5.0
Ammonia
scrubber
0.3
0.4
0.7
2.0
25
Sulfuric
acid
plants
(general)
0.3
26
Phosphoric
acid
mist/acid
plant
0.5
5
Up-flow
cooling
tower
200
300
400
500
600
Packed
cross-flow
tower
150
200
500
800
1100
Venturi
scrubber
40
100
175
300
500
Reverse-jet
scrubber
100
250
500
1250
2000
Evaporator
(635
mm
disengaging
space)
20
50
130
240
300
Sieve-fray
tower
(133
mm
disengaging
space)
250
600
1100
1800
2500
Cooler-condenser
0.1
5
10
20
35
2-Fluid
nozzle
(atomizing
air
pressure
<
560
kPa)
1
15
35
90
120
Single
fluid
nozzle
(P
=
700
kPa)
60
200
500
1500
2000
particle
distributions.
Therefore,
depending
on
the
relative
quantities
of
large
spray
droplets
and
fine
mist
particles,
different
types
of
mist
eliminator
devices
may
be
combined
to
optimize
collection
efficiency
[1].
In
addition,
depending
on
the
entrainment
load,
a
gravity
knockout
drum
may
be
required
to
reduce
the
liquid
load
upstream
of
another
finer
separation
device,
such
as
a
mist
pad,
to
prevent
flooding.
Tab.
1
gives
droplet
size
distributions
for
some
process
equipment
as
reported
in
literature
[8]
and
Tab.
2
shows
the
typical
range
of
entrainment
droplet
size
caused
by
the
mechanisms
previously
described
[2,10].
These
data
should
only
be
considered
as
a
rough
indication
of
what
the
size
of
distributions
might
be.
If
droplet
size
distribution
and
liquid
load
are
known,
it
is
possible
to
choose
the
correct
gas-liquid
separator.
Fig.
1,
containing
data
from
Feord
et
al.
[6],
shows
process
applications
which
feature
entrained
liquid
dispersion.
Though
gravity-settling
drums
are
used
for
larger
droplets
(above
500
microns
diameter),
Fig.
1
shows
that
finer
droplet
removal
requires
more
elaborate
equipment,
such
as
im-
pingement
separators
(vane
types,
knitted
mesh
pads),
cyclones
and
fibre
beds.
Similar
data
covering
fibre
beds,
mesh
pads
and
vane-type
separators
are
reported
in
the
paper
by
Ziebold
[1].
This
also compares
collection
target
diameter,
recommended
superficial
gas
velocity
and
pressure
drops.
Two
of
the
most
common
separators
used
in
the
chemical
plants
are
the
louvre
separators
(vane-type
separators)
and
the
knitted
mesh.
As
shown
in
Fig.
1,
there
is
some
overlap
in
the
ranges
of
droplet
size
covered
by
the
two,
but
it
must
be
pointed
out
that
they
cannot
be
used
interchangeably
because
Table
2.
Typical
droplet
size
ranges
from
entrainment
caused
by
various
mechanisms.
Drop
size
range
[microns]
10-1100
2-900
2-1000
2-800
10-1200
20-700
0.1-30
0.1-10
they
differ
in
some
important
characteristics.
The
advantages
of
vane
type
over
wire
mesh
include:
high
capacity,
ability
to
handle
"dirty"
streams
that
would
plug
wire
mesh
eliminators
[8].
The
vane
type
appears
more
often
in
process
systems
where
the
liquid
entrainment
is
contaminated
with
undissol-
vable
solids,
or
where
high
liquid
loading
exits.
Typically,
they
are
less
efficient
in
removing
very
small
droplets
than
many
other
types
of
impact-based
separators,
such
as
wire
mesh.
On
the
other
hand,
the
cost
of
wire
mesh
is
two
or
three
times
lower
than
that
of
the
vane
type
and
it
can
remove
droplets
larger
than
5
microns
with
a
pressure
drop
of
less
than
25
mm
H
2
O.
If
dissolved
or
soluble
solids
are
present
in
the
entrained
liquid,
moderate
fouling
of
wire
mesh
mist
eliminator
can
be
controlled
by
installation
of
a
spray-wash
system,
which
is
operated
periodically
as
the
pressure
drop
for
the
unit
increases
[3].
Mechanism
Mechanical:
Spray
Tray
Evaporation
surface
Column
packing
Two-phase
flow
Condensation:
On
surface
From
saturated
vapor
Chemical
reaction
1200
WILEY-VCH
Verlag
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2001
Chem.
Eng.
Technol.
24
(2001)
11
TYPICAL
PROCESS
APPLICATIONS
Carry
over
from
water
cooling
&
spray
cooling
towels
Spray
by
bubble
bursting
at
a
liquid
vapour
interface
Entrainment
from
evaporators,
distillation
&
absorption
towers
Condensate
from
intercoolers
Condensate
from
on
,
compression
plant
gas
coolers
.
Condensate
from
high
pressure
compression
plant
TYPES
AND
OPERATING
RANGE
OF
SEPARATION
EQUIPMENT
K.O.
drums
fitted
with
knitted
mesh
demisters
Knitted
mesh
demisters
Centrifugal
separators
.
Fibre
beds
Louvre
separators
C
Research
News
&
E
T
0.1
10
1000
dropletdiarreier
(microns)
Figure
1.
Examples
of
liquid
entrainment
from
process
equipment
and
the
operating
ranges
of
mist
eliminators.
2
Knitted
Wire
Mesh
Pads:
State
of
the
Art
Among
the
various
types
of
mist
eliminators
commercially
available,
knitted
mesh
pads
probably
outnumber
all
other
types
of
mist
eliminators
combined
[2]
because
they
assure
an
optimum
cost/performance
for
many
applications
widely
used
in
the
chemical
process
industry,
e.g.
distillation,
gas
absorp-
tion
and
evaporators
[7,11].
Surprisingly,
despite
the
broad
range
of
entrainment
removal
applications,
open
literature
on
wire
mesh
elimina-
tors
is
still
limited.
Wire
mesh
pads
were
first
described
in
the
fifties
and
sixties
by
York
[11]
and
by
York
and
Poppele
[12,13].
In
later
works
by
Bell
and
Strauss
[14]
and
by
Calvert
et
al.
[15],
the
separation
capacity
of
wire
mesh
was
compared
with
that
of
other
separation
devices,
mainly
regarding
coarse
spray.
Few
publications
deal
with
the
fractional
collection
efficiency,
which
is
the
collection
efficiency
for
a
given
size
of
liquid
droplet.
The
earliest
systematic
investigations
were
carried
out
by
Carpenter
and
Othmer
[16].
In
their
work,
they
suggest
the
use
of
a
semiempirical
equation
for
fractional
collection
efficiency
computation,
but
the
experimental
data
were
only
obtained
for
overall
efficiency.
Later
studies,
in
which
some
experimental
data
of
fractional
separation
efficiency
was
also
obtained,
are
reported
by
Biirkholz
[5,17],
Feord
and
Davies
[18],
Feord
et
al.
[6],
and
by
the
present
authors
in
[19,20].
It
must
be
pointed
out
that
the
empiricism
of
the
early
works
has
recently
been
compensated;
in
fact,
most
of
the
recent
studies
on
mist
eliminators
are
on
the
application
of
reliable
measurement
techniques
which
allow
the
measurement
of
droplet
size
distribution
and
concentration
entering
and
leaving
the
device
[4,19,20,21].
The
availability
of
accurate
experimental
data
has
allowed
reliable
design
models
to
be
developed,
such
an
approach
has
resulted
in
less
conservatism
in
design
and
consequently
nowadays
it
is
possible
to
significantly
reduce
the
size of
the
separator
vessels.
Conventional
wire
mesh
contactors
are
made
by
knitting
wires
to
form
a
layer
that
may
be
rolled
spirally
to
form
cylindrical
elements
commonly
used
for
small-diameter
applications,
or
folded
into
several
layers
to
form
a
pad
of
the
desired
thickness.
There
are
several
mesh
types
available,
which
are
identified
by
mesh
thickness,
density,
wire
diameter
and
wire
material.
The
wire
used
for
the
knitted
layer
typically
has
a
diameter
in
the
80-280
microns
range
and
the
typical
thickness
used
for
the
pads
is
in
the
65-150
mm
range,
with
100
mm
being
the
standard
pad
thickness
usually
employed
for
ordinary
service.
The
specific
area
of
commonly
used
wire
mesh
pads
ranges
from
250
to
650
m
2
/m
3
with
void
fractions
between
97
and
99
%.
The
wire
pad
is
placed
between
a
top
and
bottom
support
grid,
which
complete
the
assembly.
The
grids
must
assure,
however,
a
sufficient
free
area
for
flow.
Generally,
the
conventional
knitted
metal
wire
pads
have
a
capacity
of
removing
liquid
droplets
down
to
about
5-6
microns
in
diameter,
with
the
d
d50
cut
point,
which
is
the
diameter
of
droplets
that
can
be
separated
with
an
efficiency
greater
than
50
%,
at
around
2.5
microns
and
a
99
%
effectiveness
at
just
over
twice
the
d
d50
diameter
[7].
However,
it
is
worth
pointing
out
that
performance
of
standard
mesh
pads
can
be
greatly
reduced
by
peculiarities
of
certain
applications;
interesting
examples
are
given
in
the
works
by
York
and
Poppele
[12,13]
and
Neal
et
al.
[22].
Neal
et
al.
[22]
also
complain
that
not
only
knitted
mesh
pad
eliminators
are
often
taken
for
granted,
with
one
or
two
standard
types
being
supplied
in
virtually
all
cases,
but
often
users
and
designers
are
not
aware
of
new
developments
made
to
the
standard
mesh
pads.
Knowledge
of
the
many
alternatives
available
on
the
market
could
be
very
important.
Therefore,
based
on
earlier
investigation
of
complex
mesh
pads
carried
out
by
York
and
Poppele
[12,13]
and
by
Neal
et
al.
[22],
a
recent
work
of
the
authors
dealt
with
the
experimental
study
of
complex
mesh
pads
and
the
development
of
reliable
design
models
[20].
3
Knitted
Wire
Mesh
Pads:
Mechanism
of
Droplet
Collection
and
Design
Methods
In
order
to
effectively
apply
a
mist
eliminator
to
a
new
application
or
to
understand
its
performance
in
an
existing
application,
one
needs
to
have
some
knowledge
of
the
droplet
size
distribution
and
concentration
entering
the
mist
elim-
inator.
Nevertheless,
it
is
also
very
important
to
understand
Chem.
Eng.
Technol.
24
(2001)
11,
WILEY-VCH
Verlag
GmbH,
D-69469
Weinheim,
2001
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C
_pE
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News
the
capture
mechanism
which
the
separator
employs,
the
dependence
of
capture
efficiency
on
operating
parameters
and
physical
properties
of
the
system,
the
operational
ranges
and
limits
of
the
separator
and
finally,
any
peculiarities
of
the
application,
as
they
can
result
in
a
heavy
loss
in
efficiency.
Although
several
trade
name
units
are
available,
all
knitted
wire
mesh
pads
basically
perform
on
the
same
principle
and
have
very
similar
physical
properties.
The
gas
or
vapor
stream
carrying
the
entrained
liquid
droplets
passes
through
the
knitted
mesh
pad.
The
gas
or
vapor
stream
moves
freely,
whereas
the
inertia
of
the
droplets
causes
them
to
contact
the
extensive
surface
of
the
wire,
where
they
are
retained
until
they
coalesce
and
eventually
drain
as
large
droplets.
The
most
common
application
of
wire
mesh
eliminators
is
horizontally
for
vertical
upward
gas
flow,
although
they
can
also
be
used
in
horizontal
flow
applications
[9].
Fabian
et
al.
[3]
suggest
how
mist
eliminators
should
be
installed
in
some
common
equipment
in
order
to
avoid
trouble;
the
suggestions
are
mainly
aimed
at
ensuring
uniform
gas
and
liquid
velocities
to
avoid
overloading
a
part
of
the
unit.
Other
practical
installation
guidelines
are
provided
in
the
book
by
Ludwig
[23]
together
with
performance
data
for
some
conventional
wire
mesh
styles.
General
design
methods
are
based
on
the
computation
of
maximum
superficial
gas
velocity,
u
max
,
from
which
the
cross-
sectional
area
of
the
pad
is
determined.
This
velocity
is
computed
from
a
modified
Souder-Brown
equation:
b)
Interception
capture
involves
drops
that
remain
in
the
gas
streamline,
but
due
to
their
size,
brush
against
the
wire
mesh
and
are
collected.
This
mechanism
occurs
in
general
with
droplets
approximately
the
target
dimension
or
larger
[7].
c)
Diffusion
capture
involves
only
submicron-size
particles
and
is
significant
only
at
a
very
low
gas
flux.
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"
b)
c)
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i
Figure
2.
Mechanisms
of
droplet
collection.
Holmes
and
Chen
[9]
pointed
out
that
inertial
capture
is
the
predominant
mechanism
of
droplet
capture
for
a
wire
mesh
separator.
This
implies
that
the
fractional
separation
effi-
ciency
of
the
eliminator
can
be
evaluated
by
taking
into
account
only
the
contribution
due
to
inertial
capture
and
neglecting
interception
and
diffusion
capture.
Some
relations
have
been
published
to
evaluate
the
inertial
capture
efficiency
for
a
single
wire
target
[25,26].
All
these
relations
agree
that
the
inertial
capture
efficiency
is
a
function
of
the
Stokes
number,
St,
defined
as:
V
,
1
5
r
1
?
5
1
5
L I
J
I
I I
III
/
III'
,
"
L
\
V%
I ll /
'...
%.
%
I.
/
./
i
11
/
.,
,
1
1
1
I.
\ \ \
f
15,
1 1
f
5
1
55
11
!
f
r
r
J
r
r
r
15
5
1
5
5
L
;
1
! !
p
i
•u•cl
2
d
u
MaX
=
K
I
P1—P
g
V
Pg
(1)
St
=
(2)
where
p
/
and
p
g
are
respectively
the
density
of
the
liquid
and
the
gas
phases.
The
constant
K
depends
on
several
system
factors
[11,23]
including
liquid
viscosity,
surface
tension,
entrainment
loading,
content
of
dissolved
and
suspended
solids,
the
operating
pressure,
mesh
structure
and
de-entrain-
ment
height.
The
values
of
K
are
experimentally
determined
by
vendors;
a
typical
value
of
0.107
m/s
is
commonly
used,
even
if
lower
values
are
suggested
for
high
entrainment
loadings
or
when
the
liquid
is
dirty.
For
most
solutions,
wire
mesh
pads
can
operate
effectively
between
30
%
and
110
%
of
their
permitted
design
velocity
given
by
Eq.
(1).
However,
this
method
of
designing
wire-
mesh
mist
eliminator
is
very
rough,
because
it
does
not
consider
either
the
drop
size,
on
which
collection
efficiency
is
strongly
dependent,
or
the
liquid
load
that,
as
pointed
out
by
York
and
Poppele
[12],
can
induce
flooding
of
the
pad.
For
this
reason,
some
authors
have
analyzed
the
separation
phenomena
in
detail.
From
a
theoretical
point
of
view,
the
collection
efficiency
involves
three
different
separation
mechanisms
[1,6,9,24]
which
are
shown
in
Fig.
2
(this
illustrates
gas
flowing
past
a
single
cylinder
placed
normal
to
the
flow):
a)
Inertial
impaction
involves
drops
that
impact
the
target
wire
of
the
mesh
and
are
collected
on
leaving
the
gas
streamline
due
to
their
inertia.
where
u
is
the
superficial
gas
velocity,
ki
g
the
gas
viscosity
and
d
d
and
4
indicate
the
droplet
and
target
diameters,
respectively.
The
results
obtained
on
single
targets
point
out
that
inertial
capture
efficiency
increases
with
the
Stokes
number,
and
therefore
with
the
increase
of
gas
velocity
and
droplet
size,
and
the
decrease
of
the
target
diameter.
The
extension
of
the
analysis
from
capture
on
a
single
target
to
capture
in
a
knitted
mesh
has
been
considered
by
some
authors
[16,19].
Fig.
3
shows
experimental
data
obtained
on
some
commercial
knitted
mesh
pads
by
Brunazzi
and
Paglianti
[20]
and
it
shows
that
the
fractional
separation
efficiency
is
a
function
of
the
Stokes
number.
In
fact,
based
on
the
analysis
of
capture
on
single
target,
the
works
by
Brunazzi
and
Paglianti
[19]
and
by
Carpenter
and
Othmer
[16]
have
shown
that
separation
efficiency
of
common
wire
mesh
mist
eliminators
can
be
evaluated
by
considering
the
separation
efficiency
of
the
single
target
and
by
considering
the
packing
geometry
(i.e.
wire
diameter,
mesh
type,
number
of
layer
and
finally,
specific
surface
area).
Both
the
mechanistic
model
suggested
by
Brunazzi
and
Paglianti
[19]
and
the
semiempi-
rical
relation
proposed
by
Carpenter
and
Othmer
[16]
can
be
used
to
compute
the
collection
efficiency
of
conventional
wire
mesh
separators,
and
they
agree
for
pads
thicker
than
65
mm.
However,
it
should
be
pointed
out
that
the
model
suggested
by
Carpenter
and
Othmer
[16]
systematically
underestimates
that
experimental
efficiencies
for
thinner
pads.
1202
WILEY-VCH
Verlag
GmbH,
D-69469
Weinheim,
2001
Chem.
Eng.
Technol.
24
(2001)
11
trg
I
10
100
r.
80
60
40
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N
Style
0
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P
Style
Q
C
Research
News
&
E
T
Stokes
number
Figure
3.
Experimental
collection
efficiency
versus
Stokes
number
for
some
common
commercial
mesh
pads
[20].
4
Operational
Limits
of
Wire
Mesh
Pads:
Is
it
Possible
to
Extend
the
Performance
of
Wire
Mesh
Mist
Eliminator?
As
pointed
out
in
the
previous
paragraph,
a
mist
pad
collects
entrained
droplets
mainly
by
inertial
impaction.
The
droplets
that
impinge
on
the
surface
of
the
wires
coalesce
and
detach
from
the
pad.
Wire
mesh
pads
are
usually
installed
for
vertical
upward
gas
flow,
although
horizontal
flows
are
employed
in
some
specialized
applications
[2].
For
vertical
gas
flow
installations,
the
collected
liquid
drains
countercurrently
to
the
gas
flow
in
the
form
of
large
droplets
that
drip
from
the
lower
face
of
the
pad.
In
the
horizontal
gas
flow
systems,
the
collected
liquid
drains
down
the
vertical
axis
in
a
cross-flow
fashion.
The
maximum
allowable
superficial
gas
velocity
across
the
pad
is
limited
by
the
ability
of
the
captured
liquid
to
drain
from
the
unit.
The
separation
efficiency
is
a
strong
function
of
gas
velocity.
Fig.
4
shows
a
typical
performance
curve.
At
low
gas
velocity,
droplets
tend
to
follow
the
gas
stream
lines
and
therefore
are
not
collected
by
the
wires.
Separation
efficiency
improves
as
the
velocity
increases
and
remains
approximately
constant
up
to
the
flooding
point
indicated
by
the
gas
velocity
u
fl
.
Above
Overa
ll
e
ffic
iency
0
Superficial
gas
velocity
Figure
4.
Qualitative
variation
in
collection
efficiency
with
superficial
gas
velocity
in
a
mesh
mist
eliminator.
this
gas
velocity,
the
drag
forces
impede
free
drainage
of
the
liquid.
The
liquid
begins
to
accumulate
and
overload
the
wires,
and
droplets
are
then
re-entrained
into
the
gas
stream,
which
causes
a
rapid
decrease
in
the
separation
efficiency.
In
this
case,
the
size
of
re-entrained
droplets
above
the
eliminator
is
generally
large.
As
pointed
out
before,
the
fractional
collection
efficiency
is
mainly
affected
by
the
Stokes
number,
but
the
geometrical
characteristics
of
the
packing
are
also
very
important.
At
a
fixed
superficial
gas
velocity,
pads
with
a
high
specific
surface
have
higher
separation
efficiencies
than
pads
with
a
low
specific
surface,
but
on
the
other
hand,
they
have
lower
throughput
capacity.
Developments
have
been
introduced
to
further
extend
the
range
of
performance
of
conventional
knitted
mesh
into
finer
droplets
removal
by
introducing
composite
materials
con-
taining
10
to
50
microns
diameter
fibreglass
or
plastic
fibres
co-knitted
with
heavier
metal
mesh
framework.
A
typical
metal
wire
mesh
pad
and
a
composite
mesh
pad
are
shown
in
Fig.
5.
Common
plastic
fibres
are
polypropylene,
Dacron
and
Teflon.
The
selection
of
the
fibre
material
depends
on
the
requirements
imposed
by
the
process
conditions
(such
as
corrosion
resistance,
temperature,
liquid
load).
For
instance,
polypropylene
and
Dacron
yarns,
which
are
cheaper
than
Teflon
yarns,
are
appropriate
for
process
temperatures
up
to
50
°C
and
70
°C
respectively,
whereas
Teflon
yarns
can
be
used
up
to
180
°C.
Composite
mesh
pads
made
from
glass-fibre
yarns
exhibit
an
extraordinarily
high
specific
surface,
there-
fore
allow
high
removal
efficiency,
but
can
only
treat
gas
streams
with
low
liquid
loads.
The
throughput
capacity
of
composite
mesh
pads
is
limited
when
compared
to
that
possible
with
conventional
wire
mesh
pads.
Brunazzi
and
Paglianti
[20]
presented
some
experimental
data
on
composite
mesh
pads
and
suggested
a
design
model.
Figure
5.
Photograph
of
a
conventional
metal
wire
mesh
pad
(left)
and
a
composite
mesh
pad
(right).
Some
new
tendencies
have
emerged
in
the
last
few
years
in
the
development
of
wire
mesh
type
collectors.
Vendors
have
recognized
the
potential
of
combining
two
or
three
types
of
metal
wire
mesh
pads
into
a
single
system,
as
this
Chem.
Eng.
Technol.
24
(2001)
11,
WILEY-VCH
Verlag
GmbH,
D-69469
Weinheim,
2001
1203
C
_pE
(XT
Research
News
gives
the
benefit
of
efficiency
advantages
of
one
type
with
the
capacity
advantages
of
the
others
[27].
The
thickness
of
each
metal
wire
mesh
pad
used
in
these
multilayer
assembly
is
generally
about
50
mm,
which
is
smaller
than
that
of
the
conventional
wire
mesh
pads
used
alone.
Sometimes
the
upstream
pad,
made
of
fine
wires,
provides
a
coalescing
stage,
while
the
second
pad
works
as
the
actual
separator.
However,
when
high
efficiency
is
required
in
the
presence
of
a
highly
entrained
liquid
load,
a
first
pad
made
from
a
low
density
mesh
lessens
the
liquid
load
arriving
at
the
second
pad.
This
second
pad
can
therefore
have
a
higher
density,
assuring
a
higher
removal
efficiency
even
for
small
droplet
sizes.
The
paper
by
Brunazzi
and
Paglianti
[20]
also
presents
a
design
model
for
these
multilayer
pads.
5
Summary
Knitted
wire
mesh
mist
eliminators
have
been
used
for
over
40
years
in
many
operations
in
the
chemical
and
oil
production
industries
as
a
means
of
removing
fine
droplets
of
entrained
liquid
from
process
and
waste
gas
or
vapor
streams.
This
extensive
application
is
because
they
can
assure
an
optimum
cost/performance
ratio
for
many
applications
compared
to
other
separation
devices.
Over
the
years,
new
styles
of
knitted
wire
mesh
pads
have
been
developed
to
improve
the
performance
of
the
older
types.
Complex
mesh
pads,
i.e.
composite
separators
and
multi-
layer
separators,
have
permitted
the
performance
and
the
range
of
applications
of
conventional
wire
mesh
pads
to
be
extended.
Recent
research
in
this
field
is
essentially
addressed
towards:
(a)
development
of
new
types
of
mist
eliminators,
e.g.
using
composite
pads
with
metal
and
plastic
fibres,
or
using
wire
mesh
separators
consisting
of
two-three
layers
of
metal
pads;
(b)
experimental
investigation
of
conventional
and
complex
mesh
pads;
(c)
development
of
reliable
design
models
which
are
essential
for
the
design
and
optimisation
of
complex
separation
units.
Recent
interest
is
also
due
to
the
availability
of
techniques
that
enable
droplet
size
distribution
and
concentration
to
be
measured
in
the
two-phase
mixture
entering
and
leaving
the
separation
device.
Received:
January
30,2001
[RN
033]
References
[1]
Ziebold,
S.
A.,
Demystifying
Mist
Eliminator
Selection,
Chem.
Eng.
(May
2000)
pp.
94-102.
[2]
Fabian,
P.;
Cusack,
R;
Hennessey,
P.;
Neuman,
M.,
Demystifying
the
Selection
of
Mist
Eliminators,
Part
1:
The
Basics,
Chem.
Eng.
(November
1993)
pp.
148-156.
[3]
Fabian,
P.;
Hennessey,
P.;
Neuman,
M.;
Van
Dessel,
P.,
Demystifying
the
Selection
of
Mist
Eliminators,
Part
2:
The
Applications,
Chem.
Engineering
(December
1993)
pp.
106-111.
[4]
Monat,
J.
P.;
McNulty,
K.
J.;
Michelson,
I.
S.;
Hansen,
0.
V.,
Accurate
Evaluation
of
Chevron
Mist
Eliminators,
Chem.
Eng.
Prog.
(Dec.
1986)
pp.
32-39.
[5]
Btirkholz,
A.,
Droplet
Separation,
VCH
Publishers,
Weinheim
(Ger-
many)
1989.
[6]
Feord,
D.;
Wilcock,
E.;
Davies,
G.
A.,
A
Stochastic
Model
to
Describe
the
Operation
of
Knitted
Mesh
Mist
Eliminators,
Computation
of
Separation
Efficiency,
Trans.
IChemE
71
(1993)
pp.
282-295.
[7]
Capps,
R.
W.,
Properly
Specify
Wire-Mesh
Mist
Eliminators,
Chem.
Eng.
Prog.
(December
1994)
pp.
49-55.
[8]
McNulty,
K.
J.;
Monat,
J.
P.;
Hansen,
0.
V.,
Performance
of
Commercial
Chevron
Mist
Eliminators,
Chem.
Eng.
Prog.
(May
1987)
pp.
48-55.
[9]
Holmes,
T.
L.;
Chen,
G.
K.,
Design
and
Selection
of
Spray/Mist
Elimination
Equipment,
Chem.
Eng.
91
(1984)
No.
21,
pp.
82-89.
[10]
Koch-Otto
York,
Mist
Elimination,
Bulletin,
5601
(1997).
[11]
York,
0.
H.,
Performance
of
Wire-Mesh
Demisters,
Chem.
Eng.
Prog.
50
(1954)
No.
8,
pp.
421-424.
[12]
York,
0.
H.;
Poppele,
E.
W.,
Wire
Mesh
Mist
Eliminators,
Chem.
Eng.
Prog.
59
(1963)
No.
6,
pp.
45-50.
[13]
York,
0.
H.;
Poppele,
E.
W.,
Two-Stage
Mist
Eliminators
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