-
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THREEPHASE
IN GAS OUT
GASOUT
20 Ft. Gravity Separator
12 Ft. Coalescer Vessel
16"
INTERFACELEVEL
THREEPHASE IN
LIQUID LEVEL
LIQUID LEVEL
30"
36" I
D60"
ID
LIGHTPHASEOUT
LIGHTPHASEOUT
HEAVYPHASEOUT
Liquid-Liquid Coalescer Design Manual
-
ACS Separations & Mass-Transfer Products 14211 Industry
Street Houston, Texas 77053TEL: 800-231-0077 FAX: 713-433-6201 WEB:
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Table of ContentsIntroduction . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . .1
Stokes Settling Using Gravity . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . .1
Basic Design Concepts The Emulsion . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . .2
Basic Design Concepts Operating Principles of a Coalescer . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.3
Basis for Sizing and Selection . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . .5
Intra-Media Stokes Settling . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . .6
Direct Interception . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . .7
Gravity Separation Downstream of a Coalescer Element . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. .9
Coalescer Configurations . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . .10Case Studies
Case Study 1 - Oil-Water Separators - Environmental Response . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Case
Study 2 - Gas Plants . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . .12 Case Study 3 - Alkylation Units . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . .13 Case Study 4 - Oil/Water Separator on a Production
Platform . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . .14 Case Study 5 - Upgrading a Three-Phase Separator . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.15
General References . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . .16Ranges of Application for Coalescing
Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . .16
LIQUID-LIQUID COALESCER DESIGN MANUAL
ACS Oil / Water Separators utilize patented* technology to
separate oily waste water.Applications include oil spill clean up
for marine, power plants, refineries, vehicle terminals, and
countless others. The separated water is purified fordirect
sewer or ocean discharge. The oil is capturedand recycled.
*US Patent Nos. 5,023,002 & 5,246,592
20L x 8W x 9-6"H
1992 Vaaler Awardfor ACS Industries
Oil-Water Separator
-
IntroductionWhether engineering a new coalescer vessel,
ordebottlenecking an existing separator, full knowledgeand
understanding of the basic principles involved arerequired. Often
overlooked are the capabilities of prop-erly selected and designed
internals for the enhance-ment of simple gravity separation. This
Liquid-LiquidCoalescer Design Manual describes the use of
variousmedia and methods employed for decades to increaseplant
productivity. Typical applications include: Removal of Bottlenecks
in existing Decanters and Three Phase Separators.
Reduction in New Vessel Sizes Up to five times relative to
gravity settling alone.
Improvements in Product Purity Carry-over entrainment reduced to
1 ppm and less.
Compliance with Environmental Regulations Cost effective
solutions to wastewatertreatment and oil spill cleanups.
When two liquids are immiscible, or non-soluble inone another,
they can form either an emulsion or acolloidal suspension. In
either of these mixtures, thedispersed liquid forms droplets in the
continuousphase. In a suspension, the droplets are less than
onemicron in diameter and the liquids cannot readily beseparated
with the technologies described here.Fortunately, in the chemical
and hydrocarbon processindustries droplet sizes are typically
greater than thisand/or the purities required can be achieved
withoutaddressing the ultra-light colloidal component of thestream.
Stokes Settling Using Gravity Traditionally, gravity separators
were used to handleemulsions before the use of coalescing media
became
commonplace. In thisequipment, differences indensities of the
two liquidscause droplets to rise orfall by their buoyancy.
Thegreater the difference indensities, the easier theseparation
becomes.Rising (or falling) dropletsare slowed by frictionalforces
from viscous effectsof the opposing liquid.When the stream is
not
flowing and the opposing forces of buoyancy and vis-cous drag
balance (Figure 1), the droplet has achievedits Terminal Settling
Velocity. This vertical velocity isconstant because there are no
net forces acting uponthe droplet. This mechanism of separating
liquids bygravity is called Stokes Settling after the
nineteenthcentury English researcher Sir George Stokes.The equation
he developed for the terminal settlingvelocity is still used
today:
vt = 1.78 X 10-6 (S.G.) (d)2 / (1)vt = Terminal Settling
Velocity, ft/sd = Droplet Diameter, microns
S.G. = Specific Gravity Difference between the Continuous and
Dispersed Phases
= Continuous Phase Viscosity, centipoise
The size of a gravity decanter is derived from 1) theterminal
settling velocity of a minimum sized dropletand 2) the inertial
force imparted to the droplet due tothe velocity of the emulsion
through the vessel. Atthese conditions, all droplets larger than a
minimumwill be removed at a quicker rate and hence need notbe
considered. The minimum sized droplet must beestimated if empirical
data is not available. Typicallythe minimum droplet size is
estimated to be between75 to 300m. For example, API Publication 421
usesminimum sized droplets of 150m for oil/water sys-tems in
refineries. Note that in Stokes Settling thevessel must be sized to
ensure laminar or streamlineflow; turbulent flow causes remixing.
An example ofthis sizing method in a decanter is contained in
CaseStudy 2, see page 12.In order to settle fine droplets and
ensure laminar flow,large vessels and long residence times are
required.It may take five, ten, and or even thirty minutes tomake a
separation, depending on the physical prop-erties of the stream.
With the capacity intensificationforced on modern refineries and
chemical plants andachieved with advanced mass transfer internals,
cat-alysts, and heat exchanger designs, operators findthat their
separators only have half or a third of thetime originally
anticipated. This results in hazy, offspec products or
intermediates that cause problemsin downstream equipment.
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LIQUID-LIQUID COALESCER DESIGN MANUAL 1
Bouyant Force
Inertial Forced
Viscous DragForce
FIGURE 1
Forces on a light dropletdispersed in a heavy liquid
-
With Coalescer Media and Internals, unit perform-ance can be
restored. Typical applications include: Upgrading 3-Phase
Separators
and Decanters Removing haze from finished
products such as diesel and jet fuel Oil/Water Separators
Solvent recovery from liquid/liquid
extraction towers
Basic Design ConceptsThe Emulsion In selecting and designing a
coalescer, it is importantto understand and characterize the
emulsion that hasto be treated. The finer the droplets dispersed in
anemulsion, the more stable it is, because the buoyancyforce
diminishes in magnitude as the diameterdecreases. The manner in
which the mixture is createdeffects the droplet size distribution.
For instance,centrifugal pumps shear liquid droplets much
moreseverely than progressive cavity, thereby creating
finerdroplets. It is also important for the designer to knowhow
much time has elapsed since the mixing/shearingoccurred. This is
because as time goes on, smallerdroplets aggregate (or coalesce)
and larger dropletsare more likely to have joined a separate layer
so thatthey are no longer considered to be entrained.An important
tool to quantify an emulsion is the DropletSize Distribution Curve
generated by plotting the dropletdiameters against the volume or
mass fraction at that dif-ferential diameter. As stated above, the
shape of the dis-tribution is affected by the manner in which the
emulsionwas formed, and its age. Consider a stream with a
fineemulsion (or immature dispersion) as in Figure 2.Overtime, the
peak of the volume fraction curve shifts togreater droplet
diameters until there are more largedroplets than fines.Another key
characteristic of an emulsion and the dis-tribution that describes
it is the existence of a MaximumDroplet Diameter (1000m in Figure
2). The maximumstable droplet size that an emulsion will develop in
agiven situation depends on the mechanism of their cre-ation, the
amount of energy imparted to the mixture,and the interfacial
tension between the phases.Droplets larger than the maximum quickly
leave thedispersed phase to form a separate liquid layer
andtherefore need not be considered part of the emulsion.
Generating distributions can be done by collecting andplotting
empirical data. Alternately, Mugele and Evans(see General
References) showed they have a reliablemethod for modeling this
data as a function of standarddeviations that requires only
knowledge of the maximumdroplet diameter and two different values
of the mean.In the typical interconnecting piping between a
con-denser and a two or three phase separator; from acentrifugal
pump and a distillation column feedcoalescer; etc., a dispersion
develops to where theSauter (volume/ area) mean is roughly 0.3 and
themass (volume/ diameter) mean is roughly 0.4 of the max-imum
diameter, respectively. A coalescer is often needed, though, for
mature distri-butions (when the mean will be larger than a
Gaussian0.5 of the maximum diameter). Examples are the dis-persion
of produced water in crude oil that has traveledfor weeks in a
tanker and the water that has settled ina product storage tank over
several days. Therefore,with minimal data, an experienced designer
can havean accurate idea of the dispersion that a coalescermust
treat.
When the average droplet is greater than roughly 1/2 mil-limeter
(500 microns), an open gravity settler is appropri-ate. Table 1
shows some typical sources that can generatedispersions that
require the use of liquid-liquid coalescers.Also given are some
characteristics of the emulsions thatare created.
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LIQUID-LIQUID COALESCER DESIGN MANUAL 2
FIGURE 2
VOLUME FRACTION FREQUENCY DISTRIBUTIONSFOR DISPERSIONS OF
VARIOUS MATURITIES
-
Basic Design ConceptsOperating Principles of a
CoalescerLiquid-Liquid Coalescers are used to accelerate themerging
of many droplets to form a lesser number ofdroplets, but with a
greater diameter. This increasesthe buoyant forces in the Stokes
Law equation. Settlingof the larger droplets downstream of the
coalescerelement then requires considerably less residencetime.
Coalescers exhibit a three-step method of opera-tion as depicted in
Figure 3.
Step 1 Droplet CaptureThe first step of coalescing is to collect
entraineddroplets primarily either by Intra-Media Stokes Settlingor
Direct Interception. Figure 4 gives the useful zones ofseparation
for various mechanisms. Elements that
depend on Intra-Media Stokes Settling confine the dis-tance a
droplet can rise or fall between parallel platesor crimps of
packing sheets (Figure 5). This is com-pared to simple gravity
separators in which the travel-
ing distance is equal to the entire height of the pool ofliquid
present in the separator. This effect is also seenin knitted wire
mesh, but their high void fractions meanthe surface is very
discontinuous. Meshes, co-knits of wire and yarns; and wire and
glasswools all depend primarily on Direct Interceptionwhere a
multiplicity of fine wires or filaments collectfine droplets as
they travel in the laminar flow stream-lines around them (Figure
6). As can be see in Figure4, in general they can capture smaller
droplets thanthose that depend on enhanced Stokes Settling.
Ageneral rule with Direct Interception is that the size ofthe
target should be close to the average sized dropletin the
dispersion. Finer coalescing media allow for the
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LIQUID-LIQUID COALESCER DESIGN MANUAL 3
100-1000 microns
Haze from Condensing in BulkLiquid Phases, Surfactants Giving
Emulsions With VeryLow Interfacial Tensions
Weak
50-400 micronsModerate
10-200 micronsStrong
0.1-25 micronsVery Strong
Centrifugal Pump Discharges, Caustic Wash Drums, LowInterfacial
Tension Emulsions
Flash Drum Emulsions with >5 % Dispersed Phase,Static
Mixers
Source Stability Droplet SizeRange
Flash Drum Emulsions with
-
separation of finer or more stable emulsions (Table 2).Note that
fine media will also capture or filter fine solidparticulates from
the process stream. Therefore, unlessthe emulsion is very clean, an
upstream duplex straineror filter is needed to protect a high
efficiency coalescer.
Step 2 Droplet CoalescenceThe second step is to combine,
aggregate, or coa-lesce captured droplets. Increasing the tendency
fordroplets to adhere to a medium, increases the proba-bility that
subsequent droplets will have the opportuni-ty to strike and
coalesce with those that already have
been retained. Whether a coalescer medium ishydrophilic (likes
water) or oleophilic (likes oil) dependson the solid/liquid
interfacial tension between it and thedispersed phase. In general
an organic dispersedphase wets organic (that is plastic or
polymeric)media, as there is a relatively strong attraction
betweenthe two, while an aqueous dispersed phase preferablywets
inorganic media, such as metals or glass. Thisaids in the
coalescence step as the droplets adhere tothe media longer. Also
assisting coalescing is the den-sity of media: lower porosities
yield more sites availablefor coalescing. In the case of yarns and
wools, capillaryforces are also important for retaining droplets.
Once several droplets are collected on a plate, wire, orfiber, they
will tend to combine in order to minimize their
interfacial energy. Predicting how rapidly thiswill occur
without pilot testing is very difficult todo. Judgments of the
proper volume, andtherefore residence time, in the coalescersare
guided by experience and the followingproperties:
Coalescing Media: Media/Dispersed Phase
Interfacial Tension Porosity Capillarity
Liquid Phases: Continuous/Dispersed
Interfacial Tension Continuous/Dispersed
Density Difference Continuous Phase Viscosity Superficial
Velocity
Coalescers work better in laminar flow for sev-eral reasons.
First, as mentioned above,droplets will stay in the streamlines
around awire or fiber target. Second, high fluid velocitiesovercome
surface tension forces and strip
droplets out of the coalescer medium. This results in
re-entrainment in co-current flow and prevents dropletsfrom
rising/sinking in counter-current flow. Lastly, slow-er velocities
result in greater residence time in themedia and therefore more
time for droplet-to-targetimpact, droplet-to-droplet collisions,
and Intra-MediaStokes Settling.
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LIQUID-LIQUID COALESCER DESIGN MANUAL 4
Haze from Cooling inBulk Liquid Phase, Surfactants Giving
Emulsions with VeryLow Interfacial Tension
Separators withCoarse Emulsions& Static Mixers
SourceMedia Max DropletDiameter, Flow Range
gpm/ft2
CorrugatedSheets 40-1000
15-75(35-180 m3/hr/m2)
Overhead Drums, Extraction Columns,
Distillation Tower Feeds,Impeller Mixers
Wire Mesh,Wire Wool 20-300
7.5-45(20-110 m3/hr/m2)
Co-Knits of Wire &Polymer
10-200 7.5-45(20-110 m3/hr/m2)
Steam Stripper Bottoms,Caustic Wash Drums,High Pressure Drop
Mixing ValvesGlass Mat,Co-Knits of
Wire &Fiberglass
1-25 7.5-45(20-110 m3/hr/m2)
Media Hydro/Oleophilic Porosity Target Size Fouling/Cost
Metal/PlasticCorrugated Sheets
Wire/Plastic MeshWire Wool
Wire/PolymerCo-Knits
Wire/FG Co-Knits,Glass Mat
H/O
H/OH
O
H
98-99%
95-99%
94-98%
92-96%
3/8" - 1"Spacing/Crimps
.002" - .011"
21-35 micron
8 - 10 micron
Low/Low
High/High
Filament
D
Liquid FlowStreamlines
DROPLET
DROPLET
Droplet Trajectory
Droplet Trajectory
Area for efficientdroplet collection d/2
d/2
d
d
FIGURE 6
DROPLET INTERCEPTION
Table 2
-
The guidelines in Table 2 are used for selecting theproper
coalescer for a given source based on themedias Droplet Collection
ability. Also given are typicalflow ranges for each type of
coalescer media.
Step 3 Stokes Settling With Coalesced DropletsThe third step is
the Stokes Settling of the coalesceddroplets downstream of the
medium. The degree ofseparation primarily depends upon the geometry
of thevessel and its ability to take advantage of the
largecoalesced droplets that were created through stepsone and two
as described above.
Basis for Sizing and SelectionA preliminary procedure for
determining how difficult itis to separate two immiscible liquids
involves the per-formance of a simple field test. A representative
sam-ple of the emulsion is taken from a process pipeline orvessel.
It is either put it in a graduated cylinder in thelab or, if it is
under pressure, in a clear flow-throughsample tube with isolation
valves. The time required toobserve a clean break between phases is
noted. If thecontinuous phase has a viscosity less than 3
cen-tipoise, then Stokes Law says the following:Separation Emulsion
Droplet Size,
Time Stability Microns< 1 minute Very Weak >500< 10
minutes Weak 100-500Hours Moderate 40-100Days Strong 1-40Weeks Very
Strong
-
Intra-Media Stokes SettlingIn a horizontal 3-phase separator, in
order for effi-cient separation to take place, droplets of some
min-imum size which exist in both the gas and the liquidphases must
be captured within the equipment.When coalescing media is installed
in the lower segmentof the vessel, the furthest a droplet has to
travel isfrom plate to plate or sheet to sheet, rather thandown
from the liquid level to interface level and/or upfrom the vessel
wall to the interface level (dependingwhether the dispersed phase
is heavier or lighterthan the continuous phase).ACS offers a number
of Corrugated Plate Interceptors(CPI) to enhance coalescence, such
as Plate-PakTM andSTOKES-PAKTM crimped sheet packing (Figure 8).
They
make more efficient use of a vessel volume than astraight PPI
(Parallel Plate Interceptor) since moremetal is used and the
specific surface area is greater.It can be shown from Equation 1
for Vt that the volumeof media necessary to remove virtually all
dropletsequal to a minimum, typically 30-60 microns, is
givenby:
VC = (C1) Q h (2)(S.G.) d2
WhereVC = Coalescer volume, cubic feetC1 = 164 for Plate-PakTM
w/horizontal sheets
219 for STOKES-PAKTM w/horizontal sheets312 for STOKES-PAKTM
w/vertical sheets
Q = Liquid/liquid emulsion flow, US GPMh = Corrugated plate
spacing or structured
packing crimp height, inchesd = Minimum droplet diameter,
microns = Continuous phase viscosity, centipoise
Plate-PakTM is the most efficient CPI and thus has thesmallest
C1. The reason for this is that the height, h, adroplet must
traverse before hitting a solid surface isminimized in this
construction (see Figure 9 a-c).
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LIQUID-LIQUID COALESCER DESIGN MANUAL 6
Operating by enhanced gravitysettling, Plate-Pak vanes
are especially effectivefor removing larger
droplets.
FIGURE 8
Plate-Pak
Stokes-Pak
COALESCING MEDIA THATDEPENDS ON STOKES SETTLING
FIGURE 9
h=
EmulsionClear liquid
Oil Droplets
DISTANCE BETWEEN PLATES INVARIOUS STOKES-PAK COALESCERS
9a Plate-Pak corrugations perpendicular to the flow
9b
h
1/2"
Stokes-Pak withHorizontal Sheets
h 1/2"
Axis of Corrugation
Stokes-Pak withVertical Sheets
Axis of Corrugation
9c
Axis of Corrugation
-
In order to decrease solid retention the axis of the
cor-rugations of Plate-PakTM should be parallel to the
flow.However, vessel geometry often necessitates that
thecorrugations be perpendicular to the flow, especially inround
vessels. Due to its light, self-supporting struc-ture and ease of
installation, the overall project cost isnormally less for
STOKES-PAKTM than Plate-Pak when
they both have sheets in the horizontal. STOKES-PAKTM with
vertical sheets, on the other hand, retainsfewer solids than the
horizontal sheet version and sois often required in fouling
situations. In this case,there is some loss in coalescer efficiency
due to thelonger distance a droplet could travel (see Figure 9 band
c). The entire CPI unit can also be put on a 45 to60 angle in order
to retard fouling. However, thisrequires much more support
structure and an addi-tional 40 to 100% of coalescer volume since
droplettrajectory is lengthened (Figure 10). Equation 2
incorporates empirical factors that increasethe coalescer design
volume over the theoretical inorder to compensate for the effects
of bypass andback mixing. With knowledge of the cross-sectionalarea
of a fully flooded coalescer vessel or the lowersegment available
in a horizontal 3-phase separator,the required depth can easily be
calculated from Vc.ACS Plate-PakTM and Stokes-PakTM both come in
unitswhich are 8" (203 mm) deep as a standard, but customdepths are
also available.
Once the final coalescer length is selected the minimumdroplet
size that can be collected at 99.9% efficiency
can be found by trial-and-error substitution of the
terminalsettling velocity from Equation 1 into Equation 3 below s =
(vt/h)/ (vs/L) = .999wheres = Fractional Collection Efficiency
by Stokes Settlingvs = Superficial VelocityL = Element
Lengthvt/h = Droplet Rise Timevs/L= Droplet Residence Time
In horizontal flow when this length is over four ele-ments, ~32"
(813 mm), the coalescer is usually split intwo or more beds with
intermediate spacers or spacerrings. Also, cross-flow designs are
often used in thissituation to allow for more frequent removal of
thecollected dispersed phase.
Direct InterceptionDirect Interception occurs when a droplet
follows astreamline around a target but collides with it becausethe
approach distance is less than half its diameter,d/2 (Figure 6).
The formulas for Direct Interception inmesh, co-knits, wire and
glass wools are given below.Given first is a formula for the
collection of a droplet ona single target. Following that is a
formula which,based on this factor, calculates the depth of the
coa-lescer element necessary to achieve a desired overallcollection
efficiency at a selected minimum dropletsize.
D =Collection Efficiency of a Single Target by Direct
Interception
E =Effective Length Multiplier
=Volume Fraction of Fibers or Wires
d =Droplet Diameter, inches
K =Kuwabaras Hydrodynamic Factor-0.5 ln -0.25 2 + -0.75
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LIQUID-LIQUID COALESCER DESIGN MANUAL 7
(4)
GAS OUT
OIL OUTADJUSTABLE
OIL WEIRADJUSTABLEWEIR
WATEROUT
FLOWDISTRIBUTION
BAFFLE
WASTEWATERINLET
OIL
SOLIDSDRAIN
SOLIDSDRAIN
FIGURE 10
(3)
-
The formulas for Direct Interception have no velocityterm in
them, but to allow coalescence to take placedesigns are normally
done for the middle of the flowranges given in Table 2. K, the
KuwabaraHydrodynamic Factor, above is a correction to the
col-lection efficiency term that assumes a laminar/viscousflow
field. The effective length multiplier, E, is anempirical factor
that takes into account the uneven dis-tribution of curved and
crinkled targets in a wool medi-um and/or the shielding effects of
the loops of knittedmesh and twists of adjacent filaments in a
strand ofyarn. The idealized layout of fiber targets where E=1 ina
coalescer is shown in Figure 11, while what actuallyexists in a
co-knit is shown in Figure 12. The finer the fil-ament or wire the
more the nesting/shielding effect andthe lower the value of E.
As with CPI coalescers, sizing of a liquid-liquid coalescerthat
operates primarily on Direct Interception also corre-lates well to
an Overall Collection Efficiency of 99.9% ofa minimum droplet size.
Once this droplet size, empiri-cally found to be approximately half
the target diameter,is substituted into Equation 4, the length, L,
required fora clean break can be predicted as follows.
= Overall Collection Efficiency by Direct InterceptionL =
Element length required for removal of all droplets
> a minimum size at a = .999, inches
As can be seen in Figure 4, there are two broad cate-gories of
Interceptor-PakCoalescers that depend inDirect Interception, those
that are made with fine wiresand those that are made with fine
fibers. The factors to
be used in the formulas above for these media, the appropriate
minimum droplet size to use; and theapplications where they have
found success are givenin Table 3. In wire-yarn co-knits the wire
occupies asmuch as a third of the volume fraction as the yarn,
butexhibits only a few percent of the surface area.Therefore, for
the sake of conservatism, the constantsgiven in the table do not
take into account either factor.
The equations for droplet collection above can also beused to
derive the dispersed phases concentration inthe effluent stream.
First, a measured distribution orthe curve estimated with Mugeles
droplet size distri-
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LIQUID-LIQUID COALESCER DESIGN MANUAL 8
(5)
SOrderedTargets FLOW
D
FIGURE 11
FIGURE 12
INTERCEPTOR LAYOUT IN AN IDEAL COALESCER
CO-KNIT MESH COALESCER THATDEPENDS ON DIRECT INTERCEPTION
Application Coalescer Dmicrons/in.
Min. DropletDiametermicrons
E
WastewaterSheen
Fiberglass MatFiberglass Co-KnitInterceptor-PakTM
4.5 .020.0278.9/0.00035.040.037
Impeller Mixers
Polyester Co-Knit
Interceptor-PakTM12.5 .070.02124/0.00095
Caustic Wash Drums
Teflon Co-Knit
Interceptor-PakTM11.0 .070.01921/0.00083
MixingValves
WireWool
Interceptor-PakTM22.0 .400.02850/.002
Extraction Columns
Knitted Mesh
Interceptor-PakTM79.0 .600.014152/.006
Table 3
-
bution equation is broken up into a large number ofdiscrete
diameter ranges. The fractional collectionefficiency is then
calculated at the mid-point of therange using either equation 3 or
5 (rewritten to beexplicit in ) thereby deriving the volume of
dispersedphase that penetrates at that diameter. The effluentcurve
is then plotted. The area under both curves isfound with the
influent normalized to 1 (Figure 13).With knowledge of the influent
dispersed phase con-centration, the effluent level is found by
multiplying bythe ratio of these areas.
Gravity Separation Downstreamof a Coalescer ElementSuccessful
gravity separation downstream of a coa-lescer element depends
primarily on vessel geometry.Various schemes are used with
horizontal vesselsdepending on whether there is a significant
amount ofgas present as with Three-Phase Separators (Fig.
14A)and/or the volume percent of the dispersed phase. Theformation
of a wedge between a coalescer and a sharpinterface level as seen
in Fig. 14B is well documented.A boot is desirable when the amount
of dispersed phaseis
-
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EMAIL: [email protected]
LIQUID-LIQUID COALESCER DESIGN MANUAL 10
LC
FIGIRE 14 A
MixedPhaseInlet Inlet Device
Vapor & Mist Flow
Plate-PakMist Eliminator
VaporOutlet
Collected Mist Drain
TwoLiquidPhases
Coalescer
Hydrocarbon
HydrocarbonOutlet
AqueousOutlet
Aqueous
Liquid Flow
Hydrocarbon
3-Phase Horizontal Coalescer Vessel
2-Phase Horizontal Coalescer Vessel with BootEmulsion In Light
Product
HeavyDispersed
Phase
CoalescingMedium
Vertical Extraction Column with CoalescerLight Stream Out
Heavy Stream Out
LightStream
In
HeavyStream
In
Coalescer
Trays,Packing
or AgitatedInternals
FIGIRE 14 C
FIGIRE 14 D
FIGIRE 14 B
LC
Emulsion In Light Product
HeavyProduct
LiquidDistributor
CoalescingMedium
2-Phase Horizontal Coalescer Vessel
FIGIRE 14 E
COALESCERConfigurations
Wedge
Vertical Coalescers
Horizontal Coalescers
Vertical Decanter with Coalescer
LightStreamOut
Heavy Stream Out
EmulsionIn
Coalescer
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CASE STUDY #1 Oil-Water Separators Environmental ResponseThe
Oil-Water Separators (OWS) developed by ACS tohandle accidental
offshore spills have three stages ofcoalescing, one using Stokes
Settling and two usingDirect Interception. It can, therefore, serve
as an exam-ple of how to apply all the equations for droplet
coa-lescing given above. After the Exxon-Valdez incident theUS
government was looking to set up a quick responsesystem with
ship-board equipment to skim potentiallarge spills of crude oil
that on the frigid ocean waterscongeals to a viscosity of up to
50,000 centistokes, sep-arate out all contaminants on board, and
return the seawater with less than the EPA mandated 10 ppm
hydro-carbons present. The Marine Spill ResponseCorporation (MSRC)
was set-up for this purpose with 16locations in all major US ports
including Puerto Rico,Hawaii, and Guam. ACS engineers quickly
developed,tested, and proved to MSRC the viability of the
525-gpmOWS system shown in Figure 15 below, two of whichwere
installed on each quick-response vessel. ACS wasawarded the
prestigious Vaaler Award and two USpatents (Nos. 5,023,002 and
5,246,592) in developingthe coalescers for this application.Typical
conditions are removing 25 gpm of oil with aspecific gravity of
0.85, and a viscosity of 12,000 centis-tokes from 500 gpm of water
with 3% salinity, a specificgravity of 1.02, and a viscosity of 1
centistoke. The over-all dimensions of the OWS for the MSRC are 8
squareby 25 long at a full of water weight of 25,000 lbs.CPI media,
such as ACS Plate-PaKTM which in thiscase had 3/4" plate spacing to
accommodate the high-ly viscous oil, is known to be able to remove
99+% ofall droplets down to about 100 microns.Putting these factors
into equation 2 yields
Vc = 164 (525) 0.75(1.02)0.17 (1002)
= 38.0 cubic feetThe Plate-PakTM was designed for 25gpm/ft2,
requiring21 square feet (installed at 7 feet wide X 3 feet high
toaccommodate the design shown in Figure 15 and theshipping
dimensions given above). Therefore, therequired depth is 38.0 cubic
feet/21 square feet, or1.81 feet. This was rounded up to two feet
for safety.
In order to meet stringent EPA regulations for dis-charging
wastewater overboard, two stages of ACSInterceptor-Pak Co-Knit
coalescing media wereused. Their efficiency was maintained despite
thepresence of the highly viscous oil by cleaning both ofthem with
diesel oil which was injected at an amountequal to only 0.5% by
weight of the amount of oil antic-ipated to be collected. This
media works on DirectInterception so equations 4 and 5 are used.
Mediaproperties are given in Table 3. First KuwabarasHydrodynamic
Factor is calculated as follows.K= -0.5 ln .027 - 0.25(.027)2 +
(.027) - 0.75= 1.083According to Table 3 fiberglass co-knit can
remove99.9% of all droplets 4.5 microns and larger.Therefore D
=0.02 (1-.027)(4.5/8.9)2
1.083 (1+(4.5/8.9))= 0.00305
L = (.00035") (1-.027)ln (1-.999)-4(0.00305) 0.027
= 22.4"For safety each stage was supplied with a24" thick
fiberglass co-knit element.
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EMAIL: [email protected]
LIQUID-LIQUID COALESCER DESIGN MANUAL 11
SolventInjection
SolventInjection
LC
Oilywaterdrawn inby suction
LC
Oil
AdvancedCapacitance
Probes
Water
DualPre-Filters
FC
FIGURE 15
ADVANCED OIL/WATER SEPARATOR
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CASE STUDY #2Coalescers in Gas PlantsA major South American
engineering company wasdesigning a 100 MMSCFD natural gas plant
that usedethylene glycol (EG) for dehydration and for
inhibitinghydrate formation. There is a horizontal Three PhaseCold
Separator with a boot in this process that does mistelimination in
the free board above a large liquid hold-upsection that extends the
length of the vessel. The lattervolume is used to recover the
glycol that has becomeemulsified as fine droplets in the NGLs
(natural gas liquids)and the dispersed hydrocarbons that have
stabilized inthe EG. Since the glycol continually re-circulates in
thesystem, fine NGL droplets tend to build up in the
inventorycausing an emulsification of both liquid phases. The
EGdroplets are thought to be as small as 30 microns in theorganic
phase, so 30-minute hold-up times for gravityseparation are not
uncommon in the industry. ACS wasasked if a coalescer could be
provided to significantlyreduce the resultant vessel size.
The process conditions for the coalescer sizing was for itto
handle 37.5 gpm of NGLs that had a density of 31lbs/ft3 and a
viscosity of 0.11 cp; and 7.5 gpm of 75%ethylene glycol that had a
density of 51.1 lbs/ft3 and a
viscosity of 7.2 cp. A quick design for a gravity separatorcan
be done with equation 2 if the maximum height thata 30-micron
glycol droplet would have to fall from the liq-uid level to the
boot at the bottom of the vessel is used asif it was the CPI
coalescers h. In this case 42" wasassumed for a 60" ID vessel.
Thus
V = 162(45) 42 (.11)(.818-.496)302
= 215 cubic feet
This means with gravity alone a 5 dia. x 20 tangentto tangent
vessel would be required. In order to improvecontrol and to allow
for disengagement at 10/min., a16 dia. x 30 tall boot was
specified. ACS recommendedand supplied a 24" thick mesh coalescer
of a co-knit offiberglass yarn and stainless steel wire. The liquid
load-ing sizing criteria required the installation of a 24"
highsegment in a 36" ID vessel. This vessel was 12 tangentto
tangent with the same 16" diameter X 30" tall boot.Thus, as
compared to a conventional gravity separator,the use of an
engineered coalescer was successful inreducing the vessel volume by
a factor of 4.5. An illustration of this is shown on the cover of
this bulletin.
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EMAIL: [email protected]
LIQUID-LIQUID COALESCER DESIGN MANUAL 12
Condensate
Compressor
Feed FromGas Field
Gas Product to Pipeline
LeanGlycol
Rich Glycol
Make-upEthylene Glycol
Flash Tank
COLD SEPARATORWITH COALESCER
AT 250 PSIG
Steam
90F @1150 PSIG
70F 25F
J-T Valve
Hydrocarbon Vapor
Reboiler
Lean-Rich Exchanger
Gas-GasExchanger
PumpLean GlycolLean Glycol
RichGlycol
LC
FIGURE 16
GAS PLANT WITH JOULES-THOMPSON DEW POINT CONTROL
-
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EMAIL: [email protected]
LIQUID-LIQUID COALESCER DESIGN MANUAL 13
CASE STUDY #3 Coalescers in Alkylation Units A refinery was
using a 15-psi mix valve to acid washthe reactor products of their
H2SO4 alkylation unit. Thisis done to extract both acidic and
neutral ester sideproducts that readily polymerize, reduce acid
strength,and cause foaming. A vertical two-stage coalescerdrum with
a horizontal boot (Figure 17) follows imme-diately in order to make
a clean break between the twoimmiscible phases and lower the free
acid concentra-tion in the hydrocarbon to less than 15 ppm. The
firstcoalescer stage in the horizontal section, used toremove the
bulk of the acid, is a vertical Stokes-Pakelement, which is
preceded by a 20% open perforatedplate liquid distributor. The
second stage is a horizon-tal ACS Interceptor-Pak with Teflon
Multi-FilamentCo-Knit. The inlet section of the large diameter
verticalsection removes the fine acid droplets and allowsthem to
drain counter-current to the ascending contin-uous hydrocarbon
stream. Process conditions were 2480 GPM of alkylate thathad a
specific gravity of 0.59 and a viscosity of 0.21 cpwas mixed with
110 GPM of acid (2/1 ratio of recycleto fresh) that had a specific
gravity of 1.85 and a vis-cosity of 25 cp. The mix valve is
reported to create anaverage droplet size of approximately 400
microns forthe washing, but also generates a significant amountof
fine droplets. Stokes-Pak with horizontal sheetsand 1/2" crimps was
chosen to remove 99+% of alldroplets down to about 35 microns. The
volume ofcoalescer required was estimated with equation 2:
Vc = 219 * 2590 * 0.5 * .21 = 38.6 cubic feet(1.85-.59) 352Thus
a 16" thickness of Alloy 20 Stokes-Pak wasused in the 78" ID X 5
long horizontal boot.As mentioned above, counter-current flow in
the verti-cal portion of the tower necessitates liquid loads on
thecoalescer below 15 gpm/ft2 (2 ft/min). This required a15
diameter vertical section. The Teflon Multi-FilamentCo-Knit
Coalescer was chosen due to corrosive condi-tions and the tight
residual acid specification.Experience has shown that a 15-ppm spec
requires
removing essentially all droplets down to 15 microns.A Kuwabara
hydrodynamic factor for this media of1.251 is found using the data
from Table 3. The col-lection efficiency of a single Teflon fiber
is found whenthis factor and the data above are plugged into
equa-tion 4 as follows
D = 0.07 (1-.019) (15/21)2 = 0.01631.251 (1+(15/21))
Putting this value in equation 6 gives
L = (.00083") (1-.019)ln (1-.999)-4(0.0163) 0.019
= 14.3"
Thus a 15" depth of a 15 diameter Alloy 20/TeflonMulti-Filament
Interceptor-Pak Coalescer waschosen for the second stage
element.
HYDROCARBON OUT
STAGE 2Teflon Multifilament Co-Knit
LiquidDistributor
180"
T/T180" DIA.
HC/ACID
IN
Acid
V.B.
Acid Out
Hydrocarbon
15"
72" I.
D.
Drain
60" T/S
FIGURE 17
COALESCER IN ALKYLATION UNITS
- Case Study #4Oil-Water Separator on a Production
PlatformProduced water enters an oil and gas production
platformalong with the organics and forms a distinct separatephase
after several let downs in pressure through First,Second, and even
Third Stage Separators; FWKO (FreeWater Knock Out) Treaters, Test
Separators, etc.According to the governing regulations for the Gulf
ofMexico all water must be treated to remove oils down to
-
Case Study # 5Upgrading a Three Phase SeparatorA major refiner
in the Central US was reluctant to putany internals in a critical
Three Phase Separator, theNaphtha Stripper Overhead Drum of the FCC
Unit.However, slugs of water entraining in the hydrocarbonphases
outlet were continually causing cycling of itstransfer pump which
was a high head centrifugal.Water must be injected upstream of an
air cooled con-denser to dissolve ammonium sulfide. The rate
ofinjection had recently been raised 20% due to anincrease in salt
forming components in a new slate ofcrudes. Nonetheless, any
solution had to be able tooperate over a 30 month turn-around
cycle. Anotherproblem was that their engineers did not want to
weldto the vessels shell since the sour water servicerequired
stress relieving. The three phase inlet consisted of 3900 BPD of
naph-tha that at operating conditions had a specific gravityof 0.82
and a viscosity of 1.6 cp, 1200 BPD of foulwater that had a
specific gravity of 0.99 and a viscosi-ty of .55 cp, and 2.2 MMSCFD
of Off Gas at 0.1136lbs/ft3. ACS engineers worked around the
constraintsof an existing 60" ID X 15 T/T separator with a
24"diameter X 36" tall boot that was now undersized (seeFigure 19).
Calculations of the gas velocity of 1.8 ft/sshowed that the Normal
Liquid Level (NLL) had to beleft at 39" to allow for mist droplets
to fall out in the ves-sel. However, the velocity of water in the
boot was20"/minute, double that allowable for oil disengage-ment
(see page 9). Because of this ACS recommend-ed that the oil/water
interface be relocated to the mainhorizontal section of the vessel
and that the naphthaoutlets internal standpipe with vortex breakers
on a
tee be raised from 6" to 24". This also helped to pre-vent water
droplets coming off the top of the down-stream coalescer face from
entraining into the HC out-let nozzle.A Stokes Law analysis of the
separator while it wascycling showed that mean and maximum
aqueousdroplet sizes were 105 and 350 microns, respectively,as they
entered with the naphtha. In order to achievethe specification
of
-
Condensationin PipelinesAnti-FoamSurfactant
125 micron 75 micron 15 micron 7.5 micron
Human hair Mist Fog Bacteria
APPROXIMATE RANGES OF APPLICATION FOR VARIOUS COALESCING
MEDIA
Three-PhaseSeparators
StaticMixers
ExtractionColumns
Two-PhasePump Discharges
MixValves
Caustic WashDrums
Plate-PakCoalescer
Wire MeshInterceptor-Pak
Teflon FiberInterceptor-Pak
FiberglassInterceptor-Pak
Fiberglass MatInterceptor-Pak
Stokes-Pak Wire WoolInterceptor-Pak
Polyester FiberInterceptor-Pak
General References:American Petroleum InstitutePublication
421,Design and Operations of Oil-Water Separators, APIRefining
Department,Washington, DC, 1990.
Gas Processors SuppliersAssociation, Engineering DataBook,
Volume 1, 11th Edition,Tulsa, OK, 1998.
Hoffmann-La Roche StandardDesign Practice for
Decanters(Liquid-Liquid Settlers), Nutley,NJ, 11/84.
Holmes, T. L., AIChESymposium Series,77, 211, pp. 40-47,
1981.
Lee, K. W. and Liu, B.Y.H.,Journal of the Air PollutionControl
Association, 30, 6,4/80.
Monnery, W.D. and Svrcek,W.Y., Chemical EngineeringProgress, pp.
29-40, 9/94.
Lieberman, N. P.,Troubleshooting ProcessOperations,3rd Edition,
PennWell Books,Tulsa, OK, 1991.
Mugele, R. A., and Evans, H. D.,Industrial and
EngineeringChemistry, 43, 6, 1951.
Paragon EngineeringServices, Produced WaterTheory and
EquipmentDescription, Houston, TX.
Perrys Chemical EngineersHandbook, 6th Edition,McGraw-Hill, New
York, NY,1984.
Reist, P.C., Aerosol Scienceand Technology, 2nd
Edition,McGraw-Hill, New York, NY,1993.
ACS Industries presents theinformation in this publication
ingood faith, believing it to be accu-rate. However, nothing herein
is tobe construed as either an expressor implied guarantee or
warrantyregarding the performance, mer-chantability, fitness,
application,suitability, nor any other aspect ofthe products and
services of ACSIndustries, LP. No informationcontained in this
bulletin consti-tutes an invitation to infringe anypatent, whether
now issued orissued hereafter. All descriptionsand specifications
are subject tochange without notice. Stokes-PakTM, Interceptor-Pak
and Plate-PakTM are trademarks of ACSIndustries, LP. Teflon is a
regis-tered trademark of E. I. Dupont deNemours.
-
ACS Separations & Mass-Transfer Products 14211 Industry
Street Houston Texas 77053TEL: 800-231-0077 or 713-434-0934 FAX:
713-433-6201 WEB: www.acsseparations.com EMAIL:
[email protected]
ON-SITE ENGINEERING & FABRICATION FOR ALL YOUR VESSEL &
TOWER INTERNALS
Engineered Excellence!Engineered Excellence!
24 7EMERGENCY
SERVICE
MIST ELIMINATORS ACS is the industry leader with 50+years of
experience engineering and fabricating mist eliminationproducts.
Your only source for MisterMesh mesh pads,MultiPocket vanes and
AccuFlow inlet distributors. ACSoffers the quickest replacements of
virtually any mesh orchevron mist eliminators with short lead time
- stocked inventory from polypropylene to Hastelloy!
LIQUID DISTRIBUTORS Quality fabrication of high perform-ance and
all conventional styles (V-trough, V-notched, tubed,side splash
baffles, orifice riser, spray nozzle & pipe lateralfeed) for
any range of liquid loading. All high performanceliquid
distributors are performance tested with on-site facilitiesprior to
shipment.
RANDOM PACKINGS Large inventory of stocked randompackings
including metal slotted rings and saddles. Ceramicand plastic
varieties also available upon request. Call us foryour emergency
needs.
INTERNALS Chimney, transition and liquid drawoff trays,
bargrating and multibeam injection packing supports and bed
limiters.Broad range of custom internals for distillation,
absorption andenvironmental scrubber columns. ACS fabricates in
exotic metalsas well as plastics.
STRUCTURED PACKINGS All common crimps of sheet metalstructured
packings licensed from Montz GmbH (200X, 250Y etc.and high capacity
"M" Series). ACS is vertically integrated -stocking, drawing,
weaving and crimping its own wire. This makesACS the most cost
effective supplier of quality ACS-BX gauze andGoodloe-type
packings. Ask about our technique allowingfor shop installation of
packing prior to shipment to eliminatecostly field installations!
Goodloe is a registered trademark of Metex Corporation.
FRACTIONATION TRAYS Round, rectangular, fixed, floatingand caged
valve trays, and of course bubble cap, sieve, baffleand rain deck
trays. ACS offers the FRI tested high efficiencySEMV floating and
fixed valves. ACS also offers othernon-proprietary replacement tray
hardware for a varietyof trays available in the market.
Quick ReplacementGrass Root and Revamp Projects
5-20
06
1500
AP