FUNDAMENTALS OF GAS
SOLIDS/LIQUIDS SEPARATION
Fred Mueller
President
MUELLER ENVIRONMENTAL DESIGNS Inc.
Houston, Texas
Ph: 800-662-0285 Fax: 713-465-0997
e-mail: [email protected]
www.muellerenvironmental.com
ABSTRACT
Gas solids/liquids separation is the removal of entrained non-gas particles from multi-phase gas
streams in order to protect either stationary or rotating equipment from the harmful effects due
to non-gas particles entering those devices. Mechanical separation concepts require basic
understanding of particle formation, physics, size and motion.
INTRODUCTION
Particle Formation
Non gas particles are generated and entrained in the gas stream in the spray, annular or slug
flow regimes. Particle formation begins initially on the molecular level when particles of like
substances collide, therefore coalescing with one another.
Particle Physics
The forces imposed on particles in mechanical separators are gravity, drag and inertial forces.
The physical phenomena that generate these forces are dependent on particle size, shape,
velocity, and on the gas stream’s density and viscosity.
Particle Size
An index for the determination of particle size spectrum is the Reynolds number. The Reynolds
number provides a ratio between viscous and inertial forces acting on particle motion. Newton’s,
Intermediate, and Stokes’ laws are used to describe the action by which mechanical separators
operate.
Particle Motion
Mechanical separators utilize the particle motion of impingement in the primary separation of
solids/liquids and gravity forces to carry off of separated solids/liquids.
Fundamentals of Gas Solids/Liquids Separation
Many process operations require the removal of entrained non-gas particles from multi-phase gas
streams. The removal of these non-gas particles is the process in itself (capture of a valuable
product) or the process of cleaning a gas stream in order to protect either stationary or rotating
equipment from the harmful effects due to non-gas particles entering those devices.
The removal of entrained non-gas particles from a multi-phase gas stream is a separation process
involving the removal of:
Liquid particles from a gas or vapor stream;
Solid particles from a gas or vapor stream;
Combination of both liquid and solid particles from a gas or vapor stream.
In order to understand mechanical separation concepts, a basic understanding of particle formation,
physics and motion is required.
Particle Formation
Non gas particles are generated and entrained in the gas stream (spray regime) or are converted
to a sheet flow (annular regime) and carried on the fluid stream boundary. These liquid non-gas
particles can be generated from a pure gas due to a gas-liquid phase change occurring within
a state change of the gas. This change of state is commonly known as condensation. The
following graphic is the pressure-volume chart for a typical vapor.
130
It is only in the gas liquid multi-phase
zone that liquid non gas particles can
exist.
120
110
TEMP.
INCREASE
100
The critical point "C" (See PressureVolume Chart Graphic) is the state point
at which the liquid and gas phases are
identical. At higher pressures along the
critical isotherm, a slight increase or
decrease in temperature will
instantaneously produce a complete
phase change. At a temperature higher
than the critical isotherm, no liquid phase
will be encountered. At temperatures
below the critical isotherm gas can exist
as both a gas or liquid. The amount of
liquid per unit volume of mixture can be
estimated using the known physical
properties of the mixture.
GAS REGION
(Also called Superheat Region)
ISOTHERMS
(Constant Temperature)
90
Critical
Isotherm
CRITICAL
C POINT
LIQUID
PRESSURE
80
70
X
60
50
DEW POINT
BUBBLE
POINT
40
30
0
A
100
B
GAS-LIQUID
MULTI-PHASE ZONE
200
SPECIFIC VOLUME
A to C Liquid Line
B to C Saturated Gas Line
Figure 1 Pressure - Volume Relation For Typical Real Gas
300
Liquid particles can be formed in mid
stream by condensing initially at the
molecular size level. These particles then
can be carried by the fluid stream.
Condensation on cool surfaces will
produce a liquid film which is swept off
the surface by drag force due to the gas
stream velocity being greater than the
surface tension on the liquid.
Liquids injected into the stream may be
born along in the stream if in small particle
form. The liquid stream injected will be disbursed
initially due to that force overcoming the surface
tension forces. The liquid will become part of the
mixture and has the potential to also change state.
Small liquid particles of like substances will join
with one another upon collision in what is called
coalescence. The rate at which the particles with
no net movement will join together is dependent
on gas viscosity and temperature and the number
of particles present. Higher temperatures, lower
viscosities, and flow stream agitation will provide
higher coalescence rate. Liquid present within
the gas stream will flow in one of several flow
regimes.
Flow Regimes
GAS
(a) Bubble
GAS
GAS
(b) Plug
GAS
(c) Stratified
(e) Slug
GAS
(f) Semi-annular
GAS
(g) Annular
GAS
Figure 2 depicts the various flow regimes that
occur within multi-phase gas streams. The two
(h) Spray
(d) Wavy
flow regimes that provide the lowest liquid to gas
weight ratio are annular and spray flow. Annular
flow is characterized by finite liquid amounts Figure 2
carried in suspension with significant amounts
along the pipe wall. Spray flow regime is characterized by particles in suspension in the gas with
negligible amounts of liquids on the pipe wall. Slug flow regime is often encountered in gas
streams with greater liquid to gas weight ratio than annular or spray regimes.
Liquid particles are generally idealized as being spherical in shape since this is the shape surface
tension forces impart. Liquid particles might have shapes other than this depending on other
forces such as gravity and drag. Solid particles are also idealized as being spherical, though
in reality have irregular shapes.
Black Powder
What is it? Black powder is a catch-all term that describes material that collects in gas pipe
lines and creates wear and reduced compressor efficiency, clogged instrumentation and
valves, and flow losses in long pipe lines. The material can be wet, with a tar-like appearance,
or dry and be a fine powder. Chemical analyses reveal it is any of several forms of iron sulfide
and iron oxide. Further, it can be mechanically mixed or chemically combined with any number
of contaminants, such as water, liquid hydrocarbons, salts, chlorides, sand or dirt. Some pipe
lines have black powder problems and others do not. It appears those lines closer to the gas
gathering end of the system have problems, while those at the distribution end, with relatively
small systems, do not. Black powder is found in both "dry" and "wet" lines. One parallel line
can have a problem while the other does not. No pipe line has been identified to date that
has been able to eliminate the problem once it starts.
Gravity forces act vertically downward.
Drag forces are resistive forces. Inertial
forces are a result of particle circular
motion. Particle circular motion generates
centripetal force; a force that is directed
towards the center of the circular motion.
Opposite to centripetal force is centrifugal
force and is due to a forced turning of the
gas stream, and acts at right angles to the
particle angular motion.
Centrifugal force is the principle
determinant in the operation of cyclone
and impingement type separators. This
action is best described in a rectangular
vector coordinates as shown in figure 3.
The particle in motion, if not at the wall,
will accelerate radially to the wall, and in
the process absorb some of the centrifugal
force.
Gas Motion
Particle Motion
Particle being propelled by gas, with the gas velocity
exceeding the particle velocity.
The particle is said to “drag” behind the gas, and this
retarding (slippage) is caused by the drag force being slightly
less than the gas impact force on the particle.
Gas Rotation
Fd
Axis
The forces imposed on particles in
mechanical separators are gravity, drag
and inertial forces.
Fc
Fg
F
Note:
Fc - Centrifugal
Fd - Drag
Fg - Gravity
F - Total
Forces acting on a particle due to circular acceleration
generates particle motion along vector F.
The particle drag forces act in the direction
of the particle motion. Drag forces are in
response to the viscous effects of the gas Figure 3
opposing the particle motion. Drag forces are composed of pressure differences and shearing
stresses on the surface of the particle.
Additionally, a buoyant force is present due to displacement of the gas by the particle. This force
is insignificant on particles greater than 0.1 microns. Particles 0.1 microns and smaller approach
molecular size where buoyant forces become considerable in relation to particle size and become
a factor in Brownian Movement. Brownian Movement is the motion exhibited by small particles
that move non-uniformly along the gas streamlines due to collisions with gas molecules (additional
information regarding Brownian Movement will follow) .
Significant complex particle motion (rectilinear and curvilinear) translation exists about the axis
of the particle. Pure particle motion about the rotational axis exists but is not as significant as
the complex particle motion translation.
On the particle boundary layer shear stresses of the gas stream are opposed by the surface
tension of the gas-liquid interface. The liquid particle system may change relative to the magnitude
of the shear stress and surface tension; such as in the instance of liquid particles may be
generated from sheet flow (stream of liquid) and large particles may be broken into smaller
particles if the shear stress is larger than the surface tension.
As previously established, the physics that govern separator operation are those which determine
the forces and stresses acting on particles in the gas stream. The physical phenomena that
generate drag forces are dependent on particle size, shape, velocity, and on the gas stream's
density and viscosity.
For small particles or low velocities, viscous forces are dominant. For larger particles and higher
velocities inertial forces are dominant. This dynamic ratio of inertial to viscous forces is a
dimensionless number referred to as the Reynolds Number abbreviated - Re. The Reynolds
Number provides a determination of which regime, inertial or viscous, is dominant. This dynamic
ratio is graphically depicted in Figure 4.
Symbols and Legend
Ap = Area of Particle Projected on Plane Normal to
direction of Flow or Motion, sq. ft.
C = Overall Drag Coefficient, Dimensionless
Dp = Diameter of Particle, ft.
Fd = Drag or Resistance to Motion of Body in Fluid, Poundals
NRe = Reynolds Number, Dimensionless
u
= Relative Velocity Between Particle and Main Body of
Fluid, ft./sec.
= Fluid Viscosity, (lb.mass)/(ft.)(sec.)=Centiposis 1488
= Fluid Density, (lb.mass)/(cu.ft.)
10,000
Drag Coefficient, C =
(
Fd
u2/2)A
p
100,000
1,000
100
(any Consistent System of Units may be Employed in
Place of the English Units Specified)
Spheres
Disks
Cylinders
10
1.0
Stokes’ Law
0.1
0.0001
0.001
0.01
Newton’s Law
Intermediate Law
0.1
1.0
10
Reynolds Number, NRe =
100
Dp u
1,000
10,000
100,000
1,000,000
Drag Coefficients for Spheres, Disks, And Cylinders and Any Fluid. From Perry, R. H., Ed., Chemical Engineers Handbook, 7th. ed., 1997,
McGraw Hill Company, Inc.
Figure 4
Drag force on spherical particles 3 to 100 micron is directly proportional to velocity in low Reynold's
number region. The applicable physical law is Stokes' Law and at Re < 2.0 the flow is laminar
and streamlines are smooth, it is considered to be "creeping flow" around the spherical particle.
Particles less than 3 microns in the Re < 2.0 regime are subject to Brownian Movement and
Stokes' - Cunningham Law. See Figure 5.
Gas Velocity Exceeds Particle Velocity
Creeping Flow
At Re < 2 flow is laminar and streamlines smooth
Figure 5
When a particle falls under the influence of gravity it will accelerate until the frictional drag in the
fluid balances the gravitational forces. At this point it will continue to fall at a constant velocity.
This is the terminal velocity. For spherical particles between 3 and 100 microns and
0.0001 < Re < 2 the applicable physical law that applies is Stokes'. The formula is stated in
Figure 6.
ut = gL Dp
Figure 6
2 ( s- )
18
ut = Terminal Settling Velocity of Particle Under Action
of Gravity, ft/sec.
gL = Local Acceleration due to Gravity,(ft.)/(sec.) (sec.)
Dp = Diameter of Spherical Particle, ft.
= Fluid Density, lb.mass/cu.ft.
= Fluid Viscosity, (lb.mass)/(ft.)(sec.)
The following examples illustrate the extremes in particle sizes and velocities in relation to the
Reynolds Number:
Particles with a Reynolds Number of 1.7
An 80 micron particle in air at 70 °F and 14.7 PSIA with a density of 100 lbs / cu. ft. has terminal
velocity of 62 FPM. While a 1 micron particle in air at 70 °F and 14.7 PSIA with a density of 100
lbs / cu. ft. has terminal velocity of 5,0000 FPM.
Particles with a Reynolds Number of 3.24 X 10-3
A10 micron particle in air at 70 °F and 14.7 PSIA with a density of 100 lbs / cu. ft. has terminal
velocity of 0.96 FPM. While a particle less than 1 micron (1.9 X 10-3 to be exact) in air at 70 °F
and 14.7 PSIA with a density of 100 lbs / cu. ft. has terminal velocity of 5,0000 FPM.
Particle Size
Particle size is commonly defined by its diameter in micrometers, more commonly called microns.
How Small is a Micron?
1 Micron is = 1/25,400 of an inch (3.937 x 10-5 inches)
318 Human Hairs = 1 inch
Pin Head = 1500 microns
84,667 Smoke Particles = 1 inch
33 Smoke Particles = 10 microns
For proper separator design, particle size analysis is important for correct equipment selection.
A convenient index for determination of particle size spectrum is the Reynolds number. These
dimensionless numbers provide a measure of the ratio between viscous forces and inertial forces.
Three equations are generally used to describe the action of mechanical separation, each being
limited to definite particle spectrum. These equations have the following ranges of particle
spectrum when applied to water droplets in air at atmospheric conditions:
1. Newton’s Law has a particle spectrum from 15,000 to 100,000 microns.
2. The Intermediate Law particle spectrum ranges from 100 to 15,000 microns.
3. Stokes’ Law encompasses particles from 3 to 100 microns.
Below 3 micron particle size, Stokes’-Cunningham Law applies. Below 0.1 micron, Brownian
Movement becomes dominant. Brownian Movement is a random motion of particles caused by
collisions with gas molecules. Particles within Brownian Movement range approach molecular
size. Newton’s and the Intermediate Law equations are generally applied to knock out drums
and gravity settling separators. Whereas Stokes’ and Cunningham’s Law equations apply to
centrifugal, impingement, and filtering type separators.
Determination of particle size in a system is relatively easy with solids. However, with liquids it
is not. Liquids are in continuous change of state, subsequently particle size of liquids will vary
depending upon the source and nature of the operation generating the particular particles.
For the applicable physical law for particles in relation to the Reynolds Number see Table 1.
Characteristics Of Dispersed Particles
Particle
Diameter
General
Classification
Gravity Settling of Spheres in Still Fluid
Commercial
Equipment for
Collection or
Removal of
Particles from
a Gas
Common Methods
of Measuring
Particle Size
Microns
Spheres of
Unit Density
in Air
Diameter,
Microns
100,000
100,000
Spheres of
Any Density
in Any Fluid
Reynolds
Number
NRe
200,000
5
5
ut =1.74
2
Permeability*
Fog
Microscope
2
Mist
5
Dust
0.1
5
2
0.01
100A
2
0.001
m =3
=
/
8gc RT /
M
10
Impingement Separators
0.153 gL
Dp
0.29
1.14
( s- )
( s ) 0.7
0.43
Stokes’ Law
C = 24 NRe
ut =
gL Dp
2
Kcr = 33 for Stokes’ Law
-1
( s-
)
18
ut = Km uts
= uts
The Cunningham Correction on Stokes’
Law Becomes Important for Particles of
Diameters Under 3 Microns for Settling
in Gases and Under 0.01 Micron for
Settling in liquids.
Km = 1 + Kme ( m / Dp)
The Value Kme has Experimentally been shown to lie between 1.3 and
2.3 for Different Gases, Particle Sizes, and Materials (Wasser,
Physik.Z.,34,257-278[1933]). An Approximate Average Value Based on
the Data of Millikon is Empirically Given by
Kme =1.644+0.552e -(0.656 Dp/ m)
0.1
0.01
ut =
Stokes - Cunningham Law
1.0
* Furnishes Average Particle Diameter, but no Size Distribution
x Size Distribution may be Obtained by Special Calibration
From Kinetic Theory of Gases:
2.0
0.0001
Large
Molecules
5
Electrical Precipitators
Smoke
2
Fume
5
Ultramicroscope*
Electron Microscope
Centrifuge
Ultracentrifuge
Turbidimetricx
X-Ray Diffraction*
Adsorption*
1.0
0.71
Air Filters
10
gL
1
3
Kcr = 43.5 for
Intermediate Law
Intermediate Law
Ultrasonics
2
500
-0.6
100
2
Kcr =2,360 for Newton’s Law
C = 18.5 NRe
Packed Beds
Elutriation
Sedimentation
Spray
100
1,000
Scrubbers
Cloth Collectors
Visible to Eye
Sieving
Rain
5
Gravity Settling Chambers
Centrifugal Separators
1,000
5
gL Dp ( s - )
10,000
1 cm.
1 in.
4
2
Dp,crit = Kcr
C = 0.44
1 in.
Particles Fall at Constant Terminal Velocity
10,000
Newton’s Law
Brownian Movement
Particles Move Like Gas Molecules
2
Critical Particle Diameter
Above Which Law Will Not
Apply
Laws of Settling
x=
4gc RTKm t
2
N Dp
3
Brownian Movement is a Random
Motion Superimposed upon the
Gravitational Settling Velocity of the
Particle it Becomes Appreciobe for
Particles under 3 Microns Diameter
and Becomes Entirely Predominant
for Particles Under 0.1 Micron
0.001
Nomenclature: (Any Self-Consistant System of Units may be Employed; English Units are given by
way of Example.)
C
= Overall drag Coefficient, Dimensionless
Dp
= Diameter of Spherical Particle, ft.
Dp crit = Critical Particle Diameter Above Which Law will
Not Apply, ft.
gc
= Conversion Factor, 32.17(lb.mass/lb.Force)/
(ft./sec.)
gL
= Local Acceleration due to Gravity,(ft.)/(sec.) (sec.)
Kcr = Proportionality Factor, Dimensionless
Km = Stokes-Cunningham Correction Factor
Dimensionless
Kme = Proportionality Factor, Dimensionless
M
= Molecular Weight, lb./mole
N
= Number of Gas Molecules in a mole,
2.76 x 10 26 Molecules/lb mole.
NRe
R
t
T
ut
uts
x
s
m
= Reynolds Number, Dimensionless =Dp
E/
= Gas Constant, 1,546(ft-lb.Force)(lb. mole)(°F)
= Time, sec
= Absolute Gas Temperature. °F abs., or °R
= Terminal Settling Velocity of Particle Under Action
of Gravity, ft/sec.
= Terminal Settling Velocity of Particle as Calculated
from Stokes’ Law, ft./sec.
= Mean Molecular Speed, ft./sec.
= Average Linear Amplitude of Displacement of
Particle in Time t, ft.
= Fluid Density, lb.mass/cu.ft.
= True Density of Particle, lb.mass/cu.ft.
= Fluid Viscosity, (lb.mass)/(ft.)(sec.)
= Mean Free Path of Gas Molecules, ft.
Characteristics of dispersed particles. Perry, J. H., Ed., Chemical Engineers Handbook, 3rd. ed., 1950, McGraw Hill company, Inc.
Table 1
Particle Motion
Mechanical separators utilize particle motion in two ways:
wPrimary separation of solids/liquids
wCarry off of separated solids/liquids
All separators have more that one generative force providing particle separation, however, they
are typically classified by the dominant separation process - gravity settling, impingement,
centrifugal action, coalescing, or filtering.
All mechanical separators rely on the principle of impingement. Impingement is the action of
particles colliding with other particles and/or surfaces.
All mechanical separators use gravity force to assist in the carry off of separated solids/liquids.
This force is normally considered the maximum agent available for carry off in mechanical type
separators with the exception of cyclone type separators.
There are four basic concepts of mechanical separation:
wGravity or Knock Out Drums
wCentrifugal (Cyclone)
wImpingement
wFilters
Mechanical Separator Limitations
Mechanical separators are limited in primary separation by geometry, by velocity required to
induce inertial fields, or by pressure differential due to drag losses.
Mechanical separators are limited in carry off capabilities of separated solids/liquids due to reentrainment and creeping flow. Re-entrainment is due to drag force pulling solid/liquid off surfaces
to form globs/droplets which are re-injected into the gas stream. Re-entrainment (carry over)
can occur in all types of separators, and in the case of cyclone types is due to an intense inner
vortex creating a velocity field to pull solids/liquids out of the drain sump area and re-inject the
separated particulate into the gas steam.
Creeping solid/liquid flow is caused by drag force on solids/liquids deposited on surfaces
overcoming gravity drain forces and causing a flow of solid/liquid out of the separator into the
clean gas side.
Both re-entrainment and creeping flow can be eliminated by correct sizing techniques and product
design, that control the velocity in relation to the fluid density, viscosity, through the separating
element.
The basic information regarding particle formation, physics, motion and limitations have been
presented. The following information details the four concepts of mechanical separation.
Gravity or Knock Out Drums
A gravity or knockout drum typically has
the inlet and outlet connections located
on the upper portion of the vessel. The
force used to separate the solids/liquids
from the gas is gravity. The primary
physics involved is the terminal velocity
of the particulate. It can be seen that the
gas velocity must be very low in order
for separation to occur.
LLC
Gravity or Knock Out Drum
LLC
Centrifugal Separator
Velocity is the primary factor in the performance
of a centrifugal separator. For a given size
centrifugal separator, the size of the separated
particulate is inversely proportional to the square
root of the gas velocity. Consequently, the
success of a centrifugal separator is dependent
on the gas velocity obtained. The minimum
particle size to be separated is dependent on
the particulate viscosity, number of turns the
particulate makes within the separator, and the
velocity of the gas at the inlet to the separator.
LLC
Impingement Separator
An impingement separator is in the category of separators that
provide targets for the particulate to be intercepted. The wire mesh
separator and vane type separator are two of the most commonly
used impingement separators.
Centrifugal Separator
The wire mesh separator consists of wire knitted into a pad having
a number of unaligned isometrical openings. It has 97% to 99%
free voids and collects the particles primarily by impingement. The
principal of operation of a wire mesh pad is change of direction.
The gas flowing through the pad is forced to change direction a
number of times. Centrifugal action is to a minimum. Impingement,
therefore, is the primary separation mechanism. A liquid particle
striking the metal surfaces of a mesh pad, flows downward where
adjacent wires provide capillary space. At this point, liquid collects
and continues to flow downward. Surface tension tends to hold
this drop on the lower base of the pad until they are large enough
for the downward force of gravity to exceed that of the upward gas
velocity and surface tension.
LLC
In a mesh pad separator the impingement efficiency falls off rapidly
at low velocities because the droplets will tend to drift between
the wires. At high velocities, the element tends to flood. Liquid
cannot flow downward against the increased upward gas velocity
force and therefore accumulates. As the voids become full of liquid,
a portion is re-entrained and discharges with the outlet gas.
The wire mesh separator is considered primarily a liquid separator;
dirt, solids, or very viscous liquids of sticky nature will plug the
voids resulting in poor performance.
Wire Mesh Separator
Pocket
Hook
Vane type separators consist of labyrinth
form of parallel metal sheets with pockets
to collect separated moisture. The gas
between plates is agitated and has to
change direction a number of times. Some
degree of centrifugal action is introduced
as the gas changes direction. The heavier
particles then are thrown to the outside and
are caught in the drain pockets.
Vane type separators fall into two categories - pocket style and hook style. Vane separators
which incorporate corrugations or hooks protruding into the flow path create pressure drop and
flow turbulence resulting in potential re-entrainment of separated liquid. Flow within pocket type
vane separators is sinusoidal and separation is accomplished by both impingement and
coalescence. Even though the hook style incorporates the same impingement separation
characteristics, the pocket style vane separator accomplishes the separation function at higher
velocities without re-entrainment.
A vane type separator with pockets functions as follows; when liquid
laden gas vapor approaches the vane plates it is forced to change
direction, moving the liquid droplets to the plate walls converting the
liquid to sheet flow in what is called coalescence. This mechanism
is a function of the path distance and the free stream velocity. Liquid
sheet flow approaches the vane pocket where it is collected and
removed from the gas stream and ultimately drains to the liquid
holding sump due to gravity.
Sizing of a vane type separator is dependent on relative density of
the liquid and gas, relative size of liquid droplets, and relative weight
ratio of the liquid to gas stream. In most instances the maximum
liquid to gas weight ratio of vane type separators is 10%.
In any case, impingement type separators that employ wire mesh
or vane type elements are limited to liquid particle separation,
ultimately dirt or solid particulate, and viscous liquids will plug the
voids in mesh pads or pockets in vane separators resulting in poor
performance.
Vane Type Separator
Filter Separator
Filter separators are designed to provide optimum
performance in mechanical separation, and are used
in separating aerosols and solid particles.
Vane Pack
Filter Separator
Mechanical separators such as vanes,
centrifugal or wire mesh, are effective
for removal of particles above 6
microns, where the liquid to gas
weight ratio is greater than 1% by
weight, light liquid loads (less than1%)
characteristically have particle size
distribution of less than 6 microns and
therefore, mechanical separators with
vane, centrifugal, or wire mesh
elements are not effective.
To collect liquid particles less than 6
micron require particle conditioning.
One of the most practical methods of
particle conditioning is coalescence.
The removal of solids is accomplished by interception
of the particles and its efficiency is dependent on the
depth of the filter media density and size of the fiber.
The primary function of a filter separator is the removal
of small liquid particles that will have a detrimental
effect on downstream equipment.
Solids
Gas Flow Path
To Vane
Hollow Filter Core
Filter Media
Filter Separator Cartridge Filter
Coalescence is the mechanism where small droplets are agglomerated on a fiber mat or surface,
and forms a continuous liquid film which periodically shears and releases large droplets back
into the gas stream. The size of these droplets depend on the surface tension and viscosity of
the liquid and the velocity of the gas relative to the liquid.
The filter separator uses cylindrical coalescing elements for particle conditioning. The design
of these elements is the most important component in the design of a filter separator. The first
step in the design of the elements is an analysis of the liquid particle size and concentration in
the gas stream.
There are three mechanisms of particle collection to be considered in the design of filter elements
for mist elimination. These are inertial impaction, diffusion, and direct interception.
Inertial impaction (impingement) is the
aerodynamic behavior of a particle within the
fibrous media of the filter. If the particle has
sufficient inertia and low enough surface drag
characteristics, it will be attracted and held to
fiber in the media due to Van Der Waals Force.
IMPINGEMENT
AIR FLOW
PATH OF PARTICLE
FIBER
PARTICLE
DIFFUSION
AIR FLOW
PARTICLE
PA
RT
I
E
CL
F
PAT H O
FIBER
A third mechanism is that of direct interception
(straining). A particle of large enough size,
relative to the size of the fiber will be collected
even if it has no mass and can follow gas
flow path. For this reason fiber diameter and
density are a major consideration in the design
of filter elements.
Diffusion is the random movement of particles
called Brownian Motion. Brownian Motion will
manifest itself as a wobbling, erratic motion
of the particles about the steady aerodynamic
gas path. This motion would not of itself result
in additional collection of particles by fibers if
the particle concentration were everywhere
identical. But since the motion is totally random,
the transfer of particles across any surface
near a fiber will be proportional to the
concentration on opposite sides of that surface.
It can be shown that there is a net transfer of
particles across a surface and that this transfer
is in the direction of the fiber surface itself.
The rate at which such transfer takes place is
proportional to a diffusion coefficient.
STRAINING
AIR FLOW
PATH OF PARTICLE
FIBER
PARTICLE
The combined properties of inertial impaction
(impingement), diffusion and direct interception
(straining) enable us to state qualitatively, at least,
how aerosol (suspended particles in gases) filtration
will be effected. The graph depicts the effect which,
would result if aerodynamic collection alone were
operative and if Brownian diffusion alone were
operative. The top curve depicts the sum of these
two effects.
E
I
EFFICIENCY
It is apparent that the Brownian diffusion curve falls
toward zero for particles approaching molecular size
(an aerosol filter does not filter out gases). In addition,
it is not at all unusual for filter efficiency to drop
somewhat toward zero for large particles because
of their poor retention after capture by some filter
media. The mere transport of particles to the fiber
surface is not sufficient of itself to ensure their
collection. They also have to remain on the fiber
surface.
E
T
E
D
A
PARTICLE SIZE OR VELOCITY
ED
EI
ET
A
-DIFFUSION EFFICIENCY
-IMPACTION EFFICIENCY
-TOTAL EFFICIENCY
-POINT OF MINIMUM EFFICIENCY
Blow-off is the term referring to the process of re-entrainment of the liquid collected by the fibrous
pad in the gas flowing from the pad. It is effected by several factors - inlet aerosol concentration,
gas flow velocity, viscosities and densities of the gas and liquid, pad surface area and porosity
and interfacial and surface tension forces.
Blow-off efficiency is a function of the
velocity of the gas and the velocity of
the liquid through the fiber. At low
velocities, less than 6 feet per minute,
the efficiency of collection of the filter
decreases until the effect of the diffusion
mechanism becomes apparent whereby
the smaller particles, by their random
motion, will again be collected. At the
blow-off velocity large droplets are
shearing away from the film of collected
liquids on the filter fibers. The small
droplets appear again as a secondary
product of the larger droplet formation.
This then, is a velocity region of low
filtration efficiency. If the filter is to be
used as a mist eliminator it should be
designed so that the aerosol velocity
through the filter is less than the critical blow-off velocity. If however, the filter is to be used as
an agglomerator element the velocity must be greater than the critical blow-off velocity.
The ideal operating conditions of an agglomerator filter, is at a velocity just beyond the initial
blow-off velocity. At this condition more of the smaller droplets entering the pad would be impacted
because of the higher velocity and the droplets leaving the pad are at its larger size. At higher
velocities more droplets will be impacted but the blow-off droplets will be smaller in size.
A very important use of the blow-off characteristics of fibrous mat type eliminator element is that
of particle agglomeration. Fiberglass or similar materials with fine fibers can be used to collect
the very small aerosol particles at aerosol velocity which will produce blow-off drainage of the
collected liquid from the pad. The larger droplets can then be collected by another mechanical
separator such as vane, wire mesh or a centrifugal unit.
Hydrocarbon Gas Viscosity
0.10
0.09
3000
0.08
2000
0.07
1000
0.06
0.05
0.04
0.03
750
Pressure, psia
0.02
Viscosity - Centipoises
1500
0.015
500
.0125
0.01
0.009
0.008
0.007
0.006
14.7
.6 .7 .8 .9 1.0
0.005
Sp. gr
0.004
0.003
0.002
sp. gr.
sp. gr.
1.0
1.0
0.9
0.9
0.8
0.8
0.7
0.7
0.6
.55
0.6
.55
-400
-300
-200
-100
0
100
200
300
400
Temperature °F
500
600
700
800
900
1000
From Gas Processors Supplers Association, Tulsa
AIR
WATER
AMMONIA
ARGON
CARBON MONOXIDE
CARBON DIOXIDE
CHLORINE
HELIUM
HYDROGEN
HYDROGEN CHLORIDE
HYDROGEN SULFIDE
METHYLCHLORIDE
NITROGEN
OXYGEN
SULFUR DIOXIDE
METHANE
ETHANE
PROPANE
n - BUTANE
n - PENTANE
n - HEXANE
n - HEPTANE
n - OCTANE
n - NONANE
n - DECANE
CYCLOPENTANE
METHYLCYCLOPENTANE
ETHENE (ETHYLENE)
PROPENE
1 - BUTENE
1 - PENTENE
1, 2 - BUTADIENE
ETHYNE (ACETYLENE)
BENZENE
TOLUENE
STYRENE
ISOPROPL BENZENE
ACETONE
ETHYL ACETATE
ETHYL ALCOHOL
METHYL ALCOHOL
n-PROPL ALCOHOL
FURFURAL
MONOETHANOLAMINE (MEA)
ETHYLENE GLYCOL
DIETHYLENE GLYCOL
TRIETHYLENE GLYCOL
TETRAETHYLENE GLYCOL
PROPYLENE GLYCOL
DIPROPYLENE GLYCOL
TRIPROPYLENE GLYCOL
TYPICAL CYLINDER
LUBRICATING OIL, (DTE-105)
COMPOUND
--
C9H2004
C6H1403
C3H802
C8H1805
C6H1404
C4H1003
C2H602
C2H7N0
C5H402
C3H80
C2H60
CH40
C3H60
C4H802
C9H12
C8H8
C7H8
C6H6
C2H2
C5H10
C4H6
C4H8
C3H6
C2H4
C6H12
C5H10
C10H22
C9H2O
C8H18
C7H16
C6H14
C4H10
C5H12
C3H8
C2H6
CH4
S02
O2
N2
CH3C1
H2S
HC1
H2
He
CO2
C12
CO
NH3
A
H20
--
--------------0.00171
0.00111
0.00086
0.00077
0.00069
0.00063
0.00059
---
50.882 at 741 psia
88.719 at 85.9 psia
---
53.311 at 427 psia
49.222 at 235 psia
61.9 at -13°F
---
86.923 at 40.6 psia
--31.598at Sat.Pr.
36.378 at Sat.Pr.
39.29
41.345
42.851
44.010
44.942
39.944
28.010
44.010
70.914
4.003
2.016
36.465
34.076
50.49
28.016
32.000
64.060
16.042
30.068
44.094
58.120
72.146
86.172
100.198
--
--
192.3
134.17
76.09
194.2
150.17
106.12
62.07
61.08
96.08
56.9
64.1
64.2
64.9
70.5
70.4
70.0
69.6
63.5
74.7
50.2 at 68°F
--
0.00048
0.0004
0.000425
0.00043
0.00044
0.00041
0.00039
--
--
--
--
49.648
32.042
60.09
--
49.513
--
56.6
88.10
46.069
--
0.000550
0.000584
0.000592
0.000686
49.4
53.967
56.756
54.310
55.101
58.08
120.186
104.144
92.134
78.108
38.285 at 578 psia
0.00114
40.963 at Sat.Pr.
54.088
26.036
0.00087
0.00113
0.00177
--
0.000693
0.000730
40.223
37.410 at Sat.Pr.
32.465 at Sat. Pr.
21.25 at 32°F
46.927
46.737
70.130
56.104
42.078
28.052
84.156
70.130
142.276
128.250
0.00055
--
38.502 at 106.3 psia
17.032
45.736
--
114.224
--
-62.365
1.0000
4.1492
--1.5905
1.1062
------------
--0.12151
0.84514
------------
3.5954
3.1808
2.6965
3.31700
3.27469
0.24301
0.20601
0.8988
1.8673
0.14266
0.06867
2.4210
1.9368
1.4526
0.9684
2.9053
2.4211
4.9118
4.4275
3.9432
3.4591
0.18496
0.14797
0.11098
0.07399
0.22196
0.18497
0.37526
0.33826
0.30126
0.26428
2.9749
2.4906
0.19028
0.22728
2.071
1.547
1.046
0.555
2.2116
1.1047
0.9672
1.7398
1.1764
1.268
0.0696
0.15822
0.11819
0.07991
0.04240
0.16897
0.0844
0.07389
0.13292
0.08988
0.09688
0.00532
0.1381
2.4482
0.18704
0.01055
1.5194
0.11608
0.9670
1.3783
0.10530
0.07388
0.5880
--
0.04492
--
0.07640
LIQUID TEMP
GAS SPECIFIC
GAS
COEFFICIENT (Lbs/Ft3) GRAVITY
OF DENSITY
AIR=1.0
18.016
LIQUID
(Lbs/Ft.3)
DENSITY
28.966
SYMBOL MOLECULAR
WEIGHT
PROPERTY TABLE
1735
100
150
77
60
51
47
23
37
1.49 at 77°F
2.494
0.620
1.220
0.471
0.336
--
--
0.624
0.700
--
--
--
0.16
--
0.07 at 32°F
--
0.493 at 56°F
0.92 at 68°F
0.676
0.555
0.425
0.337
0.251
0.180
0.108
--
--
0.3936 at 32°F
--
--
--
--
--
0.011 at -434°F
--
0.385 at 32°F
0.077 at 107.6 psia
--
--
0.276 at -40°F
1.1404
0.172 at -314°F
LIQUID
--
--
--
--
--
--
--
--
--
--
0.0093 at 212°F
0.0135 at 152.2°F
0.0108 at 212°F
0.00684 at 32°F
0.00931 at 212°F
--
--
--
0.00738 at 57.6°F
0.00935 at 32°F
--
--
0.0074
--
0.0098
--
--
--
0.00675 at 212°F
0.00675 at 213°F
--
--
0.00656
0.0073
0.0078
0.00896
0.01073
0.01230
0.01971
0.01728
0.01043
0.01234
0.01402
0.00866
0.01923
0.01309
0.01460
0.01723
0.02190
0.00968
0.01255 at 212°F
0.01817
GAS
VISCOSITY (Centipoises)
--
--
--
--
--
--
--
--
--
--
506.7
464
469.6
482.2
455
685
706
609.5
553
97.4
339
394
295.6
197.4
49.8
499.3
461.5
655
613
565.2
512.6
454.5
385.9
305.6
206.3
90.1
-116.5
315
-181.8
-232.8
289.6
212.7
124.5
-399.8
-450.2
291
88.0
-218
-187.7
271.4
705.4
-221.3
--
--
--
--
--
--
--
--
--
--
734
1157
927
555
690
473
580
611
714
905
653
586
583
667
742.1
549.1
654.7
320
345
362.1
396.9
439.7
489.5
550.7
617.4
708.3
673.1
1142
730
492
968
1306
1199
188
33.2
1120
1073
510
705
1657
3206
547
--
--
--
--
--
--
--
--
--
--
0.25
0.25
0.25
0.25
0.25
0.27
0.27
0.260
0.274
0.274
0.27
0.27
0.277
0.274
0.269
0.273
0.276
0.246
0.250
0.256
0.260
0.264
0.286
0.274
0.278
0.288
0.29
0.29
0.29
0.29
0.27
0.29
0.29
0.29
---
0.29
0.29
0.29
0.29
0.25
0.232
0.29
CRITICAL CONSTANTS
2.180
--
--
--
--
--
--
--
--
--
--
0.538
0.563
0.559
0.552
0.568
0.454
0.446
0.486
0.513
0.933
0.651
0.609
0.688
0.791
1.020
0.542
0.564
0.466
0.484
0.507
0.535
0.568
0.615
0.679
0.780
0.945
1.514
0.671
1.870
2.290
0.694
0.773
0.890
8.676
54.705
0.692
0.949
2.150
1.911
0.711
0.446
0.027
--
--
--
--
--
--
--
--
--
--
0.020
0.013
0.016
0.026
0.021
0.031
0.025
0.024
0.021
0.016
0.023
0.025
0.025
0.022
0.020
0.027
0.022
0.046
0.043
0.041
0.037
0.033
0.030
0.027
0.024
0.021
0.022
0.013
0.020
0.030
0.015
0.011
0.012
0.078
0.443
0.013
0.014
0.029
0.021
0.009
0.005
REDUCED
CONDITIONS
COMPRESSIBILITY
TEMPPRESSURE
Ts (60°F) Ps (14.696)
FACTOR
ERATURE P (PSIA)
Tc
Pc
C
(CRITICAL)
TC (°F)
Properties at 14.696 PSIA and 60°F Unless Otherwise Noted
--
--
--
--
22
19
17
8
50.9
-33.7
-196.6
-144
-179.1
-118.4
-139
-140.9
-23.1
-139.0
42.0
-114
-213.3
-265.4
-301.6
-301.5
-272.5
-224.4
-137.0
-21.5
-64.4
-70.2
-131.1
-139.6
-201.5
-217.0
-305.8
-297.9
-296.5
-98.9
-361.1
-345.6
-126.1
-121.9
-173.6
-434.4
-458 at 382 psia
-150.9
--
-340.6
-308.6
-107.9
32
--
Freezing
Point
at 14.696
PSIA (°F)
--
514.4
447.8
369
597.2
545.9
472.6
387.1
342
323.3
207
148.1
173.3
171
-133
306.3
293.4
231.1
176.2
-119
50.5
86.0
20.7
-53.9
-154.7
161.3
120.7
345.2
303.4
258.2
209.2
155.7
96.9
31.1
-43.7
-127.5
-258.7
14
-297.4
-320.4
-10.3
-76.5
-121
-422.9
-452
30.3
-109.3
-313.6
-302.3
-28.1
212
-317.7
Boiling
Point
at 14.696
PSIA (°F)
--
34 at 77°F
33 at 77°F
36 at 77°F
45 at 77°F
45 at 77°F
44 at 77°F
47 at 77°F
--
43.5 at 68°F
23.78 at 68°F
20.14 at 122°F
22.75 at 68°F
--
23.7 at 68°F
--
--
28.5 at 68°F
28.89 at 68°F
--
--
--
13.5
--
1.1 at 32°F
--
--
--
--
21.8 at 68°F
20.8
8.9
16.6
13.1
7.8
1.6
2.8 at -150°F
--
13.2 at -297.4°F
10.53 at -333.4°F
--
--
--
1.91 at -423°F
--
18 at 68°F
1.2 at 68°F
9.8 at -315.4°F
--
23 at 52°F
--
--
LIQUID TO
VAPOR
--
--
--
--
--
--
--
47.7 at 68°F
--
43.5 at 68°F
--
22.61 at 68°F
22 at 77°F
23.9 at 68°F
23.7 at 68°F
--
32.14 at 66°F
--
28.85 at 68°F
--
--
--
--
--
--
--
--
--
--
--
--
18.43 at 68°F
--
--
--
--
-
--
--
-_
16.2 at 68°F
--
66 at 68° F
--
--
--
--
--
--
--
73.42
--
LIQUID TO
AIR
Surface Tension
(Dynes/Cm.)
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
1.24
1.12
--
1.11
1.15
1.24
--
--
--
--
--
--
--
--
1.09
1.14
1.19
1.31
1.25
1.40
1.40
1.20
1.32
1.41
1.41
1.66
1.37
1.30
1.40
1.67
1.31
-
1.40
RATIO OF
SPECIFIC
HEATS
FOR GAS
References :
“Chemical Engineers Handbook”, Perry, R. H., Don W. Green, 7th. ed., © 1997, McGraw Hill Company, Inc.
“Engineering Thermodynamics” Francis F. Huang, ©1976, Macmillan Publishing Co. Inc.
“Handbook of Separation Techniques for Chemical Engineers”, Philip A Schweitzer, 3rd ed, ©1997 McGraw Hill
Company, Inc.
“Fundamentals of Fluid Mechanics”, Bruce R. Munson, Donald F. Young, Theodore H. Okiishi, 4th ed, ©2002 John
Wiley & Sons, Inc.
“Gas Engineers Handbook”, 1st ed, ©1965 Industrial Press Inc.
“Petroleum Fluid Flow Systems”, O.W. Boyd, 1st ed., ©1983, Campbell Petroleum Series
“Separation Handbook”, E. J. Halter, 1st ed., ©1966, Burgess Manning Company
“Applied Process Design”, Ernest E. Ludwig, Volume 1, 3rd ed., Gulf Publishing Company
“Engineering Data Book”, Gas Processors Suppliers Association, Volumes 1 and 2, 11th ed., ©1998, Gas Processors
Suppliers Association.
“Flow of Fluids”, Engineering Department, Crane Valves, ©1988, Crane Co.
200-059