ACS Coalescer

800-231-007714211 Industry Street Houston, TX 77053 TEL: 713-434-0934 FAX: 713-433-6201

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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: www.acsseparations.com EMAIL: [email protected]

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.

TEL: 800-231-0077 FAX: 713-433-6201 WEB: www.acsseparations.com EMAIL: [email protected]

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.



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



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.



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).



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



(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-



(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



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

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.



SolventInjection

SolventInjection

LC

Oilywaterdrawn inby suction

LC

Oil

AdvancedCapacitance

Probes

Water

DualPre-Filters

FC

FIGURE 15

ADVANCED OIL/WATER SEPARATOR

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.



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



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

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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

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