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FUNDAMENTALS OF GAS SOLIDS/LIQUIDS SEPARATION
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
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 TEMP. INCREASE
100
(Also called Superheat Region)
ISOTHERMS
(Constant Temperature)
90 Critical Isotherm
C
LIQUID
PRESSURE
80 70
CRITICAL POINT
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 borne 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 (h) Spray (d) Wavy occur within multi-phase gas streams. The two flow regimes that provide the lowest liquid to gas weight ratio are annular and spray flow. Annular Figure 2 flow is characterized by finite liquid amounts 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 catchall 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.
Particle Physics
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
Newtons 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. Newtons 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. Newtons and the Intermediate Law equations are generally applied to knock out drums and gravity settling separators. Whereas Stokes and Cunninghams 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 Newtons 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
Newtons 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
PROPERTY
Properties at 14.696 PSIA and 60°F
COMPOUND
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)
SYMBOL MOLECULAR WEIGHT --
28.966
H20
NH3 A
CO CO2 C12 He H2
HC1 H2S
CH3C1 N2
O2
S02
CH4
C2H6 C3H8
C4H10 C5H12 C6H14
DENSITY LIQUID (Lbs/Ft.3) --
VISCOSITY (Centipoises)
LIQUID TEMP GAS SPECIFIC GAS COEFFICIENT (Lbs/Ft3) GRAVITY OF DENSITY AIR=1.0 --
0.07640
1.0000 --
18.016
62.365
--
--
17.032
38.502 at 106.3 psia
--
0.04492
0.5880
39.944
--
--
0.10530
1.3783
28.010
--
--
0.07388
0.9670
44.010
50.882 at 741 psia
--
0.11608
1.5194
70.914
88.719 at 85.9 psia
--
0.18704
2.4482
4.003
--
--
0.01055
0.1381
2.016
--
--
0.00532
0.0696
36.465
53.311 at 427 psia
--
0.09688
1.268
34.076
49.222 at 235 psia
--
0.08988
1.1764
50.49
61.9 at -13°F
--
0.13292
1.7398
28.016
--
--
0.07389
0.9672
32.000
--
--
0.0844
1.1047
64.060
86.923 at 40.6 psia
--
0.16897
2.2116
16.042
--
--
0.04240
0.555
30.068
--
--
0.07991
1.046
44.094
31.598at Sat.Pr.
0.00171
0.11819
1.547
58.120
36.378 at Sat.Pr.
0.00111
0.15822
2.071
72.146
39.29
0.00086
0.19028
2.4906
86.172
41.345
0.00077
0.22728
2.9749
100.198
42.851
0.00069
0.26428
3.4591
114.224
44.010
0.00063
0.30126
3.9432
128.250
44.942
0.00059
0.33826
4.4275
142.276
45.736
0.00055
0.37526
4.9118
70.130
46.737
0.000730
0.18497
2.4211
84.156
46.927
0.000693
0.22196
2.9053
28.052
21.25 at 32°F
--
0.07399
0.9684
42.078
32.465 at Sat. Pr.
0.00177
0.11098
1.4526
56.104
37.410 at Sat.Pr.
0.00113
0.14797
1.9368
70.130
40.223
0.00087
0.18496
2.4210
54.088
40.963 at Sat.Pr.
0.00114
0.14266
1.8673
26.036
38.285 at 578 psia
--
0.06867
0.8988
78.108
55.101
0.000686
0.20601
2.6965
92.134
54.310
0.000592
0.24301
3.1808
104.144
56.756
0.000584
3.27469
3.5954
120.186
53.967
0.000550
3.31700
4.1492
C3H60
58.08
49.4
--
--
--
88.10
56.6
--
--
--
C2H60
46.069
49.513
--
0.12151
1.5905
CH40
32.042
49.648
--
0.84514
1.1062
C3H80
60.09
50.2 at 68°F
--
--
--
C5H402
96.08
74.7
--
--
--
C2H7N0
61.08
63.5
--
--
--
C2H602
62.07
69.6
0.00039
--
--
C4H1003
106.12
70.0
0.00041
--
--
C6H1404
150.17
70.4
0.00044
--
--
C8H1805
194.2
70.5
0.00043
--
--
C3H802
76.09
64.9
0.000425
--
--
C6H1403
134.17
64.2
0.0004
--
--
192.3
64.1
0.00048
--
--
--
56.9
--
--
--
C7H16 C8H18
C9H2O
C10H22 C5H10 C6H12 C2H4 C3H6 C4H8
C5H10 C4H6 C2H2 C6H6 C7H8 C8H8
C9H12 C4H802
C9H2004 --
LIQUID
GAS
0.172 at -314°F
0.01817
1.1404
0.01255 at 212°F
0.276 at -40°F
0.00968
--
0.02190
--
0.01723
0.077 at 107.6 psia
0.01460
0.385 at 32°F
0.01309
--
0.01923
0.011 at -434°F
0.00866
--
0.01402
--
0.01234
--
0.01043
--
0.01728
--
0.01971
0.3936 at 32°F
0.01230
--
0.01073
--
0.00896
0.108
0.0078
0.180
0.0073
0.251
0.00656
0.337
--
0.425
--
0.555
0.00675 at 213°F
0.676
0.00675 at 212°F
0.92 at 68°F
--
0.493 at 56°F
--
--
--
0.07 at 32°F
0.0098
--
--
0.16
0.0074
--
--
--
--
--
0.00935 at 32°F
0.700
0.00738 at 57.6°F
0.624
--
--
--
--
--
0.336
0.00931 at 212°F
0.471
0.00684 at 32°F
1.220
0.0108 at 212°F
0.620
0.0135 at 152.2°F
2.494
0.0093 at 212°F
1.49 at 77°F
--
37
--
23
--
47
--
51
--
60
--
77
--
150
--
100
--
1735
--
TABLE
Unless Otherwise Noted REDUCED CONDITIONS
CRITICAL CONSTANTS
TEMPPRESSURE COMPRESSIBILITY Ts (60°F) Ps (14.696) FACTOR ERATURE P (PSIA) Tc Pc C (CRITICAL) TC (°F) -221.3
547
705.4
3206
271.4
1657
-187.7
705
-218
510
88.0
1073
291
1120
-450.2
33.2
-399.8
188
124.5
1199
212.7
1306
289.6
968
-232.8
492
-181.8
730
315
1142
-116.5
673.1
90.1
708.3
206.3
617.4
305.6
550.7
385.9
489.5
454.5
439.7
512.6
396.9
565.2
362.1
613
345
655
320
461.5
654.7
499.3
549.1
49.8
742.1
197.4
667
295.6
583
394
586
339
653
97.4
905
553
714
609.5
611
706
580
685
473
455
690
482.2
555
469.6
927
464
1157
506.7
734
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
--
Surface Tension (Dynes/Cm.)
Freezing Point at 14.696 PSIA (°F)
Boiling Point at 14.696 PSIA (°F)
LIQUID TO VAPOR
LIQUID TO AIR
RATIO OF SPECIFIC HEATS FOR GAS
-317.7
--
--
1.40
0.29
2.180
0.027
--
0.232
0.446
0.005
32
212
--
0.711
0.009
-107.9
73.42
0.25
-28.1
23 at 52°F
1.911
0.021
-308.6
--
0.29
-302.3
--
2.150
0.029
-340.6
--
0.29
-313.6
9.8 at -315.4°F
--
0.29
0.949
0.014
--
-109.3
1.2 at 68°F
0.29
0.013
-150.9
--
0.692
30.3
18 at 68°F
54.705
0.443
-458 at 382 psia
--
---
-452
--
0.29
8.676
0.078
-434.4
--
-422.9
1.91 at -423°F
-66 at 68° F
0.29
0.890
0.012
-173.6
-121
--
0.29
0.773
0.011
-121.9
-76.5
--
0.694
0.015
-126.1
--
0.27
-10.3
--
0.29
2.290
0.030
-345.6
16.2 at 68°F
-320.4
10.53 at -333.4°F
0.29
1.870
0.020
-361.1
-_
-297.4
13.2 at -297.4°F
0.29
0.671
0.013
-98.9
--
14
--
1.514
0.022
-296.5
--
0.29
-258.7
2.8 at -150°F
0.288
0.945
0.021
-297.9
-
-127.5
1.6
0.278
0.780
0.024
-305.8
--
-43.7
7.8
0.274
0.679
0.027
-217.0
--
31.1
13.1
0.615
0.030
-201.5
--
0.286
96.9
16.6
0.568
0.033
-139.6
--
0.264
155.7
8.9
0.260
0.535
0.037
-131.1
18.43 at 68°F
209.2
20.8
--
0.256
0.507
0.041
-70.2
258.2
21.8 at 68°F
0.250
0.043
-64.4
--
0.484
303.4
--
0.466
0.046
-21.5
--
0.246
345.2
--
0.276
0.564
0.022
-137.0
--
120.7
--
--
0.273
0.542
0.027
-224.4
161.3
--
0.269
0.020
-272.5
--
1.020
-154.7
1.1 at 32°F
0.791
0.022
-301.5
--
0.274
-53.9
--
0.688
0.025
-301.6
--
0.277
20.7
13.5
0.27
0.609
0.025
-265.4
--
86.0
--
0.27
0.651
0.023
-213.3
--
50.5
--
0.933
0.016
-114
--
0.274
-119
--
0.513
0.021
42.0
--
0.274
176.2
28.89 at 68°F
0.260
0.486
0.024
-139.0
28.85 at 68°F
231.1
28.5 at 68°F
0.27
0.446
0.025
-23.1
--
293.4
--
0.454
0.031
-140.9
32.14 at 66°F
0.27
306.3
--
0.568
0.021
-139
--
0.25
-133
23.7 at 68°F
0.25
0.552
0.026
-118.4
23.7 at 68°F
171
--
23.9 at 68°F
0.25
0.559
0.016
-179.1
173.3
22.75 at 68°F
0.25
0.013
-144
22 at 77°F
0.563
148.1
20.14 at 122°F
0.538
0.020
-196.6
22.61 at 68°F
0.25
207
23.78 at 68°F
--
--
--
-33.7
--
323.3
43.5 at 68°F
43.5 at 68°F
--
--
--
50.9
342
--
--
--
8
--
--
387.1
47 at 77°F
--
--
17
47.7 at 68°F
--
472.6
44 at 77°F
--
--
--
19
--
545.9
45 at 77°F
--
--
--
22
--
597.2
45 at 77°F
--
--
--
--
--
369
36 at 77°F
--
--
--
--
--
447.8
33 at 77°F
--
--
--
--
--
514.4
34 at 77°F
--
--
--
--
--
--
--
1.31 1.67 1.40 1.30 1.37 1.66 1.41 1.41 1.32 1.20 1.40 1.40 1.25 1.31 1.19 1.14 1.09 --------1.24 1.15 1.11 -1.12 1.24 --------------------
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