SECTION 7

Separators and Filters

PRINCIPLES OF SEPARATION

Three principles used to achieve physical separation of gasand liquids or solids are momentum, gravity settling, and coa-

A = area, m2

Ap = particle or droplet cross sectional area, m2

C = empirical constant for separator sizing, m/hC* = empirical constant for liquid-liquid separators,

(m3 • mPa • s)/(m2 • day)C′ = drag coefficient of particle, dimensionless (Fig. 7-3)Di = separator inlet nozzle diameter, mmDp = droplet diameter, mDv = inside diameter of vessel, mmGm = maximum allowable gas mass-velocity necessary

for particles of size Dp to drop or settle out of gas,kg/(h • m2)

g = acceleration due to gravity, 9.81 m/s2

Hl = width of liquid interface area, mJ = gas momentum, kg/(m • s2)K = empirical constant for separator sizing, m/s

KCR = proportionality constant from Fig. 7-4 for use inEq 7-5, dimensionless

L = seam to seam length of vessel, mmLl = length of liquid interface, mmM = mass flow, kg/s

Mp = mass of droplet or particle, kg

Filter Separators: A filter separator usually has two com-partments. The first compartment contains filter-coalescingelements. As the gas flows through the elements, the liquidparticles coalesce into larger droplets and when the drop-lets reach sufficient size, the gas flow causes them to flowout of the filter elements into the center core. The particlesare then carried into the second compartment of the vessel(containing a vane-type or knitted wire mesh mist extrac-tor) where the larger droplets are removed. A lower barrelor boot may be used for surge or storage of the removedliquid.

Flash Tank: A vessel used to separate the gas evolved fromliquid flashed from a higher pressure to a lower pressure.

Line Drip: Typically used in pipelines with very high gas-to-liquid ratios to remove only free liquid from a gasstream, and not necessarily all the liquid. Line drips pro-vide a place for free liquids to separate and accumulate.

Liquid-Liquid Separators: Two immiscible liquid phasescan be separated using the same principles as for gas andliquid separators. Liquid-liquid separators are fundamen-tally the same as gas-liquid separators except that they

FIG.

Nomen

7-

lescing. Any separator may employ one or more of these prin-ciples, but the fluid phases must be "immiscible" and havedifferent densities for separation to occur.

MW = molecular mass, kg/(kg mole)P = system pressure, kPa(abs)Q = estimated gas flow capacity, (Sm3/day)/m2

of filter areaQA = actual gas flow rate, m3/sR = gas constant, 8.31 [kPa(abs) • m3]/[K • kg mole]

Re = Reynolds number, dimensionlessShl = relative density of heavy liquid, water = 1.0Sll = relative density of light liquid, water = 1.0T = system temperature, Kt = retention time, minutes

U = volume of settling section, m3

Vt = critical or terminal gas velocity necessary forparticles of size Dp to drop or settle out of gas, m/s

W = total liquid flow rate, m3/dayWcl = flow rate of light condensate liquid, m3/day

Z = compressibility factor, dimensionlessGreek:

ρg = gas phase density, kg/m3

ρl = liquid phase density, droplet or particle, kg/m3

µ = viscosity of continuous phase, mPa • s

must be designed for much lower velocities. Because thedifference in density between two liquids is less than be-tween gas and liquid, separation is more difficult.

Scrubber or Knockout: A vessel designed to handlestreams with high gas-to-liquid ratios. The liquid is gener-ally entrained as mist in the gas or is free-flowing alongthe pipe wall. These vessels usually have a small liquidcollection section. The terms are often used interchange-ably.

Separator: A vessel used to separate a mixed-phase streaminto gas and liquid phases that are "relatively" free of eachother. Other terms used are scrubbers, knockouts, line-drips, and decanters.

Slug Catcher: A particular separator design able to absorbsustained in-flow of large liquid volumes at irregular inter-vals. Usually found on gas gathering systems or other two-phase pipeline systems. A slug catcher may be a singlelarge vessel or a manifolded system of pipes.

Three Phase Separator: A vessel used to separate gas andtwo immiscible liquids of different densities (e.g. gas,water, and oil).

7-1

clature

1

C′ (Re)2 = (1.31) (107) ρg Dp

3 (ρl − ρg)µ2 Eq 7-3

LiquidDroplet

Dp

Gravitational Force on Droplet

Drag Force ofGas on Droplet

Gas Velocity

FIG. 7-2

Forces o n Liquid Dro plet in Gas Stream

Momentum

Fluid phases with different densities will have different mo-mentum. If a two phase stream changes direction sharply,greater momentum will not allow the particles of the heavierphase to turn as rapidly as the lighter fluid, so separation oc-curs. Momentum is usually employed for bulk separation ofthe two phases in a stream.

Gravity SettlingLiquid droplets will settle out of a gas phase if the gravita-

tional force acting on the droplet is greater than the drag forceof the gas flowing around the droplet (see Fig. 7-2). Theseforces can be described mathematically using the terminal orfree settling velocity.

Vt = √ 2 g Mp (ρl − ρg)

ρl ρg Ap C′ = √

4 g Dp (ρl − ρg) 3 ρg C′

Eq 7-1

The drag coefficient has been found to be a function of theshape of the particle and the Reynolds number of the flowinggas. For the purpose of this equation particle shape is consid-ered to be a solid, rigid sphere.

Reynolds number is defined as:

Re = 1,000 Dp Vt ρg

µEq 7-2

In this form, a trial and error solution is required since bothparticle size, Dp, and terminal velocity, Vt, are involved. Toavoid trial and error, values of the drag coefficient are pre-sented in Fig. 7-3 as a function of the product of drag coeffi-cient, C′, times the Reynolds number squared; this techniqueeliminates velocity from the expression1. The abscissa ofFig. 7-3 is given by:

C′(Re)2

DR

AG

CO

EF

FIC

IEN

T,C

′

FIG. 7-3

Drag Coefficient of Rigid Spheres13

7-2

Gravity Settling – Limiting ConditionsAs with other fluid flow phenomena, the drag coefficient

reaches a limiting value at high Reynolds numbers.

Newton’s Law—For relatively larger particles (approxi-mately 1000 microns and larger) the gravity settling is de-scribed by Newton’s law (Fig. 7-4). The limiting dragcoefficient is 0.44 at Reynolds numbers above about 500. Sub-stituting C′ = 0.44 in Eq 7-1 produces the Newton’s law equa-tion expressed as:

Vt = 1.74 √ g Dp (ρl − ρg)

ρg Eq 7-4

An upper limit to Newton’s law is where the droplet size isso large that it requires a terminal velocity of such magnitudethat excessive turbulence is created. The maximum dropletwhich can settle out can be determined by:

Dp = KCR

µ2

g ρg (ρl − ρg)

0.33

Eq 7-5

For the Newton’s law region, the upper limit to Reynoldsnumber is 200,000 and KCR = 18.13.

Stokes’ Law—At low Reynolds numbers (less than 2), alinear relationship exists between drag coefficient and theReynolds number (corresponding to laminar flow). Stokes’ lawapplies in this case and Eq 7-1 can be expressed as:

Vt = 1,000 g Dp

2 (ρl − ρg) 18 µ

Eq 7-6

The droplet diameter corresponding to a Reynolds numberof 2 can be found using a value of 0.0080 for KCR in Eq 7-5.

The lower limit for Stokes’ law applicability is a droplet di-ameter of approximately 3 microns. The upper limit is about100 microns.

A summary of these equations is presented in Fig. 7-4.

CoalescingVery small droplets such as fog or mist cannot be separated

practically by gravity. These droplets can be coalesced to formlarger droplets that will settle by gravity. Coalescing devicesin separators force gas to follow a tortuous path. The momen-tum of the droplets causes them to collide with other dropletsor the coalescing device, forming larger droplets. These largerdroplets can then settle out of the gas phase by gravity. Wiremesh screens, vane elements, and filter cartridges are typicalexamples of coalescing devices.

SEPARATOR DESIGN ANDCONSTRUCTION

Separators are usually characterized as vertical, horizontal,or spherical. Horizontal separators can be single or double bar-rel and can be equipped with sumps or boots.

Parts of a SeparatorRegardless of shape, separation vessels usually contain four

major sections, plus the necessary controls. These sections areshown for horizontal and vertical vessels in Fig. 7-5. The pri-mary separation section, A, is used to separate the main por-tion of free liquid in the inlet stream. It contains the inletnozzle which may be tangential, or a diverter baffle to take

7

advantage of the inertial effects of centrifugal force or anabrupt change of direction to separate the major portion of theliquid from the gas stream.

The secondary or gravity section, B, is designed to utilizethe force of gravity to enhance separation of entrained drop-lets. It consists of a portion of the vessel through which the gasmoves at a relatively low velocity with little turbulence. Insome designs, straightening vanes are used to reduce turbu-lence. The vanes also act as droplet collectors, and reduce thedistance a droplet must fall to be removed from the gas stream.

The coalescing section, C, utilizes a coalescer or mist extrac-tor which can consist of a series of vanes, a knitted wire meshpad, or cyclonic passages. This section removes the very smalldroplets of liquid from the gas by impingement on a surfacewhere they coalesce. A typical liquid carryover from the mistextractor is less than 0.013 ml per m3.

The sump or liquid collection section, D, acts as receiver forall liquid removed from the gas in the primary, secondary, andcoalescing sections. Depending on requirements, the liquidsection should have a certain amount of surge volume, for de-gassing or slug catching, over a minimum liquid level neces-sary for controls to function properly. Degassing may requirea horizontal separator with a shallow liquid level while emul-sion separation may also require higher temperature, higherliquid level, and/or the addition of a surfactant.

Separator ConfigurationsFactors to be considered for separator configuration selec-

tion include:

• How well will extraneous material (e.g. sand, mud, cor-rosion products) be handled?

• How much plot space will be required?

• Will the separator be too tall for transport if skidded?• Is there enough interface surface for three-phase sepa-

ration (e.g. gas/hydrocarbon/glycol liquid)?

• Can heating coils or sand jets be incorporated if re-quired?

• How much surface area is available for degassing of sepa-rated liquid?

• Must surges in liquid flow be handled without largechanges in level?

• Is large liquid retention volume necessary?

Vertical SeparatorsVertical separators, Fig. 7-6, are usually selected when the

gas-liquid ratio is high or total gas volumes are low. In thevertical separator, the fluids enter the vessel striking a divert-ing baffle which initiates primary separation. Liquid removedby the inlet baffle falls to the bottom of the vessel. The gasmoves upward, usually passing through a mist extractor toremove suspended mist, and then the "dry" gas flows out. Liq-uid removed by the mist extractor is coalesced into larger drop-lets which then fall through the gas to the liquid reservoir inthe bottom. The ability to handle liquid slugs is typically ob-tained by increasing height. Level control is not critical andliquid level can fluctuate several inches without affecting op-erating efficiency. Mist extractors can significantly reduce therequired diameter of vertical separators.

As an example of a vertical separator, consider a compressorsuction scrubber. In this service the vertical separator:

-3

FIG. 7-4

Gravity Settling Laws and Particle Characteristics

Newton's Law

C ¢ = 0.44

Vt = 1.74 ÷̀``````̀g Dp (rl - rg )rg

Dp = KCR ÈÍÎ

m2

g rg (rl - rg)

˘˙˚

KCR = 18.13

Intermediate Law

C¢ = 18.5 Re-0.6

Vt = 3.54g0.71 Dp

1.14 (rl - rg )0.71

rg0.29 m0.43KCR = 0.334

Stoke's Law

C ¢ = 24 Re-1

Vt = 1000 g Dp

2 (rl - rg )18m

KCR = 0.025

7-4

GasOutlet

MeshPad

Two PhaseInlet

VortexBreaker

LiquidOutlet

B

D

A

A - Primary SeparationB - Gravity SettlingC - CoalescingD - Liquid Collecting

C

VERTICAL

LiquidOutlet

Gas OutletTwo Phase

Inlet

HORIZONTAL

A BD

C

FIG. 7-5

Gas-Liquid Se parators

FIG. 7-6

Example Vertical Separator with Wire Mesh Mist Extractor

• Dimensions may be influenced by instrument connectionrequirements.

• For small diameter separators (≤ 1200 mm ID.) with high L/G inletflow ratios this dimension should be increased by as much as 50%.

• May use syphon type drain to:

a. reduce vortex possibilityb. reduce external piping that requires heating (freeze protection)

• Does not need significant liquid retention volume.• The liquid level responds quickly to any liquid that en-

ters—thus tripping an alarm or shutdown.• The separator occupies a small amount of plot space.

Horizontal SeparatorsHorizontal separators are most efficient where large vol-

umes of total fluids and large amounts of dissolved gas arepresent with the liquid. The greater liquid surface area in thisconfiguration provides optimum conditions for releasing en-trapped gas. In the horizontal separator, Fig. 7-7, the liquidwhich has been separated from the gas moves along the bot-tom of the vessel to the liquid outlet. The gas and liquid occupytheir proportionate shares of shell cross-section. Increasedslug capacity is obtained through shortened retention timeand increased liquid level. Fig. 7-7 also illustrates the separa-tion of two liquid phases (glycol and hydrocarbon). The denserglycol settles to the bottom and is withdrawn through the"boot." The glycol level is controlled by a conventional levelcontrol instrument.

In a double barrel separator, the liquids fall through con-necting flow pipes into the external liquid reservoir below.Slightly smaller vessels may be possible with the double barrelhorizontal separator where surge capacity establishes the sizeof the lower liquid collection chamber.

7

As an example of a horizontal separator consider a richamine flash tank. In this service:

• There is relatively large liquid surge volume leading tolonger retention time (this allows more complete releaseof the dissolved gas and, if necessary, surge volume forthe circulating system).

• There is more surface area per liquid volume to aid inmore complete degassing.

• The horizontal configuration would handle a foaming liq-uid better than a vertical.

• The liquid level responds slowly to changes in liquid in-ventory.

Spherical Separators

These separators are occasionally used for high pressureservice where compact size is desired and liquid volumes aresmall. Fig. 7-8 is a schematic for an example spherical sepa-rator. Factors considered for a spherical separator are:

• compactness;

• limited liquid surge capacity;

• minimum steel for a given pressure.

-5

INLET BAFFLE

LIQUIDLEVEL

INTERFACELEVEL

SECTION A-A/

3-PHASE INLETINLET

BAFFLE

BOOT

A/

GLYCOL

A GASMIST EXTRACTOR

LC

VORTEXBREAKER

LIQUID HYDROCARBON

OVER-FLOWBAFFLE

LC

DV

GAS/HYDROCARBON/GLYCOL

FIG. 7-7

Example Horizontal Three-Phase Separator with Wire Mesh Mist Extractor

GAS OUTLET

MISTEXTRACTOR

SECTION

PRESSUREGAUGE

LIQUID LEVEL

CONTROL

CONTROLVALVE

LIQUID OUTLETDRAIN

PRIMARYSEPARATION

SECTION

INLET SECONDARYSEPARATION

SECTION

LIQUIDCOLLECTION

SECTION

Courtesy American Petroleum Institute

FIG. 7-8

Example Spherical Separator 3

7-6

Separator Type K Factor

(m/s) C Factor

(m/h)

Horizontal 0.12 to 0.15 430 to 540Vertical 0.05 to 0.11 200 to 400Spherical 0.05 to 0.11 220 to 400Wet Steam 0.076 270Most vapors under vacuum 0.061 220Salt & Caustic Evaporators 0.046 160Adjustment of K & C Factorfor Pressure - % of designvalue15

Atmospheric 1001000 kPa 902000 kPa 854000 kPa 808000 kPa 75

FIG. 7-9

Typical K & C Factors for Sizing Woven Wire Demisters

• For glycol and amine solutions, multiply K by 0.6 - 0.8.

• Typically use one-half of the above K or C values forapproximate sizing of vertical separators without wiredemisters.

• For compressor suction scrubbers and expander inletseparators multiply K by 0.7 - 0.8.

GAS-LIQUID SEPARATOR DESIGN

Specifying SeparatorsSeparator designers need to know pressure, temperature,

flow rates, and physical properties of the streams as well asthe degree of separation required. It is also prudent to defineif these conditions all occur at the same time or if there areonly certain combinations that can exist at any time. Ifknown, the type and amount of liquid should also be given,and whether it is mist, free liquid, or slugs.

For example, a compressor suction scrubber designed for2-4.3 Mm3/day gas at 2750-4100 kPa(ga) and 20-40°C wouldrequire the separator manufacturer to offer a unit sized forthe worst conditions, i.e., 4.3 Mm3/day at 2700 kPa and 40°C.But the real throughput of the compressor varies from 4.3Mm3/day at 4100 kPa, 40°C to 2 Mm3/day at 2750 kPa, 20°C.Because the high volume only occurs at the high pressure, asmaller separator is acceptable. Conversely, a pipeline sepa-rator could be just the opposite because of winter to summerflow changes.

Basic Design EquationsSeparators without mist extractors are designed for gravity

settling using Eq 7-1. Values for the drag coefficient are givenin Fig. 7-3 for spherical droplet particles. Typically the sizingis based upon removal of 150 micron diameter droplets.

Most vertical separators that employ mist extractors aresized using equations that are derived from Eq 7-1. The twomost common are the critical velocity equation:

Vt = K √ ρl − ρg

ρg Eq 7-7

and the correlation developed by Souders and Brown2 to relatevessel diameter to the velocity of rising vapors which will notentrain sufficient liquid to cause excessive carryover:

Gm = C √ ρg (ρl − ρg) Eq 7-8

Note that if both sides of Eq 7-7 are multiplied by gas den-sity, it is identical to Eq 7-8 when:

C = 3600 K Eq 7-9

Some typical values of the separator sizing factors, K and C,are given in Fig.7-9. Separators are sized using these equa-tions to calculate vessel cross-sectional areas that allow gasvelocities at or below the gas velocities calculated by Eq 7-7 or7-8.

Horizontal separators greater than 3 m in length with mistextractors are sized using Eqs 7-10 and 7-113. Horizontal sepa-rators less than 3 m in length should use Eqs 7-7 and 7-8. Inhorizontal separators, the gas drag force does not directly op-pose the gravitational settling force. The true droplet velocityis assumed to be the vector sum of the vertical terminal veloc-ity and the horizontal gas velocity. Hence, the minimum lengthof the vessel is calculated by assuming the time for the gas toflow from the inlet to the outlet is the same as the time for thedroplet to fall from the top of the vessel to the surface of theliquid. In calculating the gas capacity of horizontal separators,the cross-sectional area of that portion of the vessel occupiedby liquid (at maximum level) is subtracted from the total ves-sel cross-sectional area. Separators can be any length, but theratio of seam-to-seam length to the diameter of the vessel,L/Dv, is usually in the range of 2:1 to 4:1.

7

Vt = K √ ρl − ρg

ρg

L 3.05

0.56

Eq 7-10

Gm = C √ ρg (ρl − ρg)

L 3.05

0.56

Eq 7-11

Frequently separators without mist extractors are sized us-ing Eq 7-7 and 7-8 with a constant (K or C) of typically one-halfof that used for vessels with mist extractors. Although com-bining the drag coefficient and other physical properties intoan empirical constant is unsound, it can be justified since:

• Selection of the droplet diameter (separation efficiency)is arbitrary. Even if the diameter can be selected on arational basis, little information is available on the massdistribution above and below the selected size.

• Liquid droplets are not rigid spherical particles in diluteconcentration (unhindered settling).

Note: A number of the "separator" sizing equations givenonly size the separation element (mist extractor, Eqs 7-7, 7-8,7-10, 7-11, and vane separator, Eq 7-13): these equations donot directly size the actual separator containment vessel.

Thus, for example, a 600 mm diameter wire mesh mist ex-tractor might be installed in a 900 mm diameter vessel be-cause the liquid surge requirements dictated a larger vessel.

Separators without Mist ExtractorsThis is typically a horizontal vessel which utilizes gravity as

the sole mechanism for separating the liquid and gas phases.Gas and liquid enter through the inlet nozzle and are slowedto a velocity such that the liquid droplets can fall out of thegas phase. The dry gas passes into the outlet nozzle and theliquid is drained from the lower section of the vessel.

-7

To design a separator without a mist extractor, the minimumsize diameter droplet to be removed must be set. Typically thisdiameter is in the range of 150 to 2,000 microns (one micronis 10-6 m).

The length of vessel required can then be calculated by as-suming that the time for the gas to flow from inlet to outlet isthe same as the time for the liquid droplet of size Dp to fallfrom the top of the vessel to the liquid surface. Eq 7-12 thenrelates the length of the separator to its diameter as a functionof this settling velocity (assuming no liquid retention):

L = 4(103 mm/m)2 QA

π Vt Dv Eq 7-12

If the separator is to be additionally used for liquid storage,this must also be considered in sizing the vessel.Example 7-1—A horizontal gravity separator (without mistextractor) is required to handle 2 Mm3/day of 0.75 relativedensity gas (MW = 21.72) at a pressure of 3500 kPa(ga) and atemperature of 40°C. Compressibility is 0.9, viscosity is0.012 cp, and liquid relative density is 0.50. It is desired toremove all entrainment greater than 150 microns in diameter.No liquid surge is required.

Gas density, ρg = P (MW)

RTZ =

(3601) (21.72) (8.31) (313) (0.90)

= 33.4 kg/m3

Liquid density, ρl = 0.5 (1000) = 500 kg/m3

Mass flow, M = (2) (106) (21.72)

(23.7) (24) (3600) = 21.2 kg/s

Particle diameter, Dp = (150) (10−6) = 0.000150 m

From Eq 7-3,

C′ (Re)2 = ( 1.31) (107) ρg Dp

3 (ρl − ρg) µ 2

= (1.31) (107) (33.4) (0.000150)3 (500 − 33.4)

(0.012)2

= 4800

From Fig. 7-3, Drag coefficient, C′ = 1.40

Terminal velocity, Vt = √ 4 g Dp (ρl − ρg)

3 ρg C′

= √ 4 (9.81) (0.000150) (467)

3 (33.4) 1.40

= √ 0.0196 = 0.14 m/s

Gas flow, QA = M ρg

= 21.2

33.47 = 0.63 m3/s

Assume a diameter, Dv = 1000 mm

Vessel length,

L = 4 (103 mm/m)2 )QA

π Vt DV =

4 (103 mm/m)2 (0.63) π (0.14) (1000)

= 5700 mm

7-

Other reasonable solutions are as follows:

Diameter, mm Length, mm1200 48001500 3800

Usually vessels up through 600 mm diameter have nominalpipe dimensions while larger vessels are rolled from plate with150 mm internal increments in diameter.Example 7-2—What size vertical separator without mist ex-tractor is required to meet the conditions used in Example 7-1?

A = QA Vt

= 0.63 0.14

= 4.5 m2

Dv = 2400 mm minimum

Separators With Wire Mesh Mist ExtractorsWire mesh pads are frequently used as entrainment sepa-

rators for the removal of very small liquid droplets and, there-fore, a higher overall percentage removal of liquid. Removalof droplets down to 10 microns or smaller may be possible withthese pads. The pad is generally horizontal with the gas andentrained liquid passing vertically upward. Performance isadversely affected if the pad is tilted more than 30 degreesfrom the horizontal4. Liquid droplets impinge on the meshpad, coalesce, and fall downward through the rising gasstream. Wire mesh pads are efficient only when the gas streamvelocity is low enough that re-entrainment of the coalesceddroplets does not occur. Figs. 7-10 and 7-11 illustrate a typicalwire mesh installation in vertical and horizontal vessels.

Eqs 7-7 and 7-10 define the maximum gas velocity as a func-tion of the gas density and the liquid density. A value for K canbe found from Fig. 7-9. Firmly secure the top and bottom ofthe pad so that it is not dislodged by high gas flows, such aswhen a pressure relief valve lifts.

In plants where fouling or hydrate formation is possible orexpected, mesh pads are typically not used. In these servicesvane or centrifugal type separators are generally more appro-priate. Most installations will use a 150 mm thick pad with144-192 kg/m3 bulk density. Minimum recommended padthickness is 100 mm. Manufacturers should be contacted forspecific designs.

Wire mesh pads can be used in horizontal vessels. A typicalinstallation is shown in Fig. 7-11. The preferred orientation ofthe mesh pad is in the horizontal plane. When installed in avertical orientation, the pad is reported to be less efficient.Problems have been encountered where liquid flow throughthe pad to the sump is impaired due to dirt or sludge accumu-lation causing a higher liquid level on one side, providing theserious potential of the pad being dislodged from its mountingbrackets making it useless, or forcing parts of it into the outletpipe. The retaining frame must be designed to hold the mistpad in place during emergency blowdown or other periods ofanticipated high vapor velocity.

The pressure drop across a wire mesh pad is sufficiently low(usually less than an inch of water) to be considered negligiblefor most applications. The effect of the pressure drop becomessignificant only in the design of vacuum services and for equip-ment where the prime mover is a blower or a fan. Manufac-turers should be contacted for specific information.Example 7-3 — What size vertical separator equipped with awire mesh mist extractor is required for the conditions usedin Examples 7-1 and 7-2?

8

VAPOR OUT

TOP VAPOR OUTLET

SUPPORTRING

VAPOR OUT

MIST EXTRACTOR

Nod

Cm

X

45

Nod

X

SUPPORTRING

SIDE VAPOR OUTLET

45

MINIMUM EXTRACTOR CLEARANCE, Cm:

Cm = 0.707 X or Mod - Nod

2

WHERE:Mod = MIST EXTRACTOR OUTSIDE DIAMETERNod = NOZZLE OUTSIDE DIAMETER

Cm

FIG. 7-10

Example Minimum Clearance — Mesh Type Mist Eliminators

InletDistributor

Knitted WireMesh Pad

PLAN

ELEVATIONVaporOutlet

LiquidOutlet

Two Phase Inlet

AlternateVapor Outlet

FIG. 7-11

Horizontal Separator with Knitted Wire Mesh Pad Mist Extractor and Lower Liquid Barrel

7-9

Vane TypeMist Extractor

VaporOutlet

InletDiverter

Dv

Downcomer

Two-phaseInlet

Liquid Outlet

FIG. 7-12

Example Vertical Separator wi th Vane Type Mist Extractor

AssemblyBolts

DrainageTraps

GasFlow

FIG. 7-13

Cross Sec tion o f Example Vane Element Mist ExtractorShowing Corrugated Plates with Liquid Drainage Traps

K = 0.089 m/s (from Fig. 7-9)

Vt = 0.089 √ 500 − 33.4

33.4 = 0.33 m/s

A = QA Vt

= 0.635 0.33

= 1.92 m2

Dv = 1560 mm minimum

Separators with Vane Type Mist Extractors

Vanes differ from wire mesh in that they do not drain theseparated liquid back through the rising gas stream. Rather,the liquid can be routed into a downcomer, which carries thefluid directly to the liquid reservoir. A vertical separator witha typical vane mist extractor is shown in Fig. 7-12.

The vanes remove fluid from the gas stream by directing theflow through a torturous path. A cross-section of a typical vaneunit is shown in Fig. 7-13. The liquid droplets, being heavierthan the gas, are subjected to inertial forces which throw themagainst the walls of the vane. This fluid is then drained bygravity from the vane elements into a downcomer.

Vane type separators generally are considered to achieve thesame separation performance as wire mesh, with the addedadvantage that they do not readily plug and can often behoused in smaller vessels. As vane type separators dependupon inertial forces for performance, turndown can sometimesbe a problem.

Vane type separator designs are proprietary and are not eas-ily designed with standard equations. Manufacturers of vanetype separators should be consulted for detailed designs oftheir specific equipment. However, a gas momentum equa-tion5 can be used to estimate the approximate face area of avane type mist extractor similar to that illustrated in Fig.7-13.

J = ρgVt 2 = 29.8 kg/(m • s2) Eq 7-13

where gas velocity, Vt, is the velocity through the extractorcross-section.

Separators with Centrifugal ElementsThere are several types of centrifugal separators which

serve to separate solids as well as liquids from a gas stream.These devices are proprietary and cannot be readily sizedwithout detailed knowledge of the characteristics of the spe-cific internals. The manufacturer of such devices should beconsulted for assistance in sizing these types of separators.Use care in selecting a unit as some styles are not suitable insome applications. A typical centrifugal separator is shown inFig. 7-14. The main advantage of a centrifugal separator overa filter (or filter separator) is that much less maintenance isinvolved. Disadvantages of centrifugal separators are:

• some designs do not handle slugs well,

• efficiency is not as good as other types of separators,

• pressure drop tends to be higher than vane or clean knit-ted mesh mist extractors, and

• they have a narrow operating flow range for highest ef-ficiency.

7-10

INLET

VAPOR OUTLET

CLEAN OUT/INSPECTION

LIQUIDOUTLET

Courtesy Peerless Manufacturing Co.

FIG. 7-14

Example Vertical Sep arator with Centrifugal Elements

Filter SeparatorsGeneral — This type of separator has a higher separation

efficiency than the centrifugal separator, but it uses filter ele-ments, which must periodically be replaced. An example filterseparator is shown in Fig. 7-15. Gas enters the inlet nozzleand passes through the filter section where solid particles arefiltered from the gas stream and liquid particles are coalescedinto larger droplets. These droplets pass through the tube andare entrained into the second section of the separator, wherea final mist extraction element removes these coalesced drop-lets from the gas stream.

The design of filter separators is proprietary and a manu-facturer should be consulted for specific size and recommen-dations. The body size of a horizontal filter separator for atypical application can be estimated by using 0.40 m/s for thevalue of K in Eq 7-7. This provides an approximate body di-ameter for a unit designed to remove water (other variablessuch as viscosity and surface tension enter into the actual sizedetermination). Units designed for water will be smaller thanunits sized to remove light hydrocarbons.

Example 7-4 — A filter separator is required to handle a flowof 2 Mm3/day at conditions presented in Example 7-1. Esti-mate the diameter of a filter separator.

Vt = 0.40 √ 500 − 32.5

32.5 = 1.52 m/s

A = QA Vt

= 0.652 1.52

= 0.429 m2

Dv = 740 mm minimum

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In many cases the vessel size will be determined by the fil-tration section rather than the mist extraction section. Thefilter cartridges coalesce the liquid mist into droplets whichcan be easily removed by the mist extractor section. A designconsideration commonly overlooked is the velocity out of thesefilter tubes into the mist extraction section. If the velocity istoo high, the droplets will be sheared back into a fine mist thatwill pass through the extractor element. A maximum allow-able velocity for gas exiting the filter tube attachment pipe canbe estimated using the momentum Eq 7-13 with a value of1850 kg/(m • s2) for J. Light hydrocarbon liquids or low pres-sure gas should be limited to even less than this value. Nopublished data can be cited since this information is proprie-tary with each filter separator manufacturer.

Design — The most common and efficient agglomerator iscomposed of a tubular fiber glass filter pack which is capableof holding the liquid particles through submicron sizes. Gasflows into the top of the filter pack, passes through the ele-ments and then travels out through the tubes. Small, dry solidparticles are retained in the filter elements and the liquid coa-lesces to form larger particles. Liquid agglomerated in the fil-ter pack is then removed by a mist extractor located near thegas outlet.

The approximate filter surface area for gas filters can beestimated from Fig. 7-16. The figure is based on applicationssuch as molecular sieve dehydrator outlet gas filters. For dirtygas service the estimated area should be increased by a factorof two or three.

The efficiency of a filter separator largely depends on theproper design of the filter pack, i.e., a minimum pressure dropwhile retaining an acceptable extraction efficiency. A pressuredrop of approximately 7-14 kPa is normal in a clean filter sepa-rator. If excessive solid particles are present, it may be neces-sary to clean or replace the filters at regular intervals when apressure drop in excess of 70 kPa is observed. However, as arule, 170 kPa is recommended as a maximum as the cartridgeunits might otherwise collapse. Removal of the filter pack iseasily achieved by using a quick-opening closure.

Various guarantees are available from filter separatormanufacturers such as one for 100 percent removal of liquiddroplets 8 microns and larger and 99.5 percent removal of par-ticles in the 0.5-8 micron range. However, guarantees for theperformance of separators and filters are very difficult to ver-ify in the field.

While most dry solid particles about ten microns and largerare removable, the removal efficiency is about 99 percent forparticles below approximately ten microns.

For heavy liquid loads, or where free liquids are containedin the inlet stream, a horizontal filter separator with a liquidsump, which collects and dumps the inlet free-liquids sepa-rately from coalesced liquids, is often preferred.

LIQUID-LIQUID SEPARATOR DESIGN

Liquid-liquid separation may be divided into two broad cate-gories of operation. The first is defined as "gravity separation"where the two immiscible liquid phases separate within thevessel by the differences in density of the liquids. Sufficientretention time must be provided in the separator to allow forthe gravity separation to take place. The second category isdefined as "coalescing separation." This is where small parti-cles of one liquid phase must be separated or removed from alarge quantity of another liquid phase. Different types of in-

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FIG. 7-15

Example Hor izontal Fil ter-Separator

ternal construction of separators must be provided for eachtype of liquid-liquid separation. The following principles of de-sign for liquid-liquid separation apply equally for horizontalor vertical type separators. Horizontal vessels have some ad-vantage over vertical ones for liquid-liquid separation, due tothe larger interface area available in the horizontal style, andthe shorter distance particles must travel to coalesce.

There are two factors which may prevent two liquid phasesfrom separating due to differences in specific gravity:

• If droplet particles are so small they may be suspended byBrownian movement. This is defined as a random motionwhich is greater than directed movement due to gravity forparticles less than 0.1 micron in diameter.

• The droplets may carry electric charges due to dissolvedions, and these charges can cause the droplets to repel eachother rather than coalesce into larger particles and settleby gravity.

Effects due to Brownian movement are usually small andproper chemical treatment will usually neutralize any electriccharges. Then settling becomes a function of gravity and viscosityin accordance with Stokes’ law. The settling velocity of spheresthrough a fluid is directly proportional to the difference in densi-ties of the sphere and the fluid, and inversely proportional to theviscosity of the fluid and the square of the diameter of the sphere(droplet), Eq 7-6. The liquid-liquid separation capacity of separa-tors may be determined8 from Eqs 7-14 and 7-15 which werederived from Eq 7-6. Values of C* are found in Fig. 7-17.

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Vertical Vessels:

Wc l = C∗

Shl − Sl l

µ (0.785)(10−6) Dv

2 Eq 7-14

Horizontal Vessels:

Wc l = C∗

Shl − Sl l

µ Ll Hl Eq 7-15

Since the droplet size of one liquid phase dispersed in an-other is usually unknown, it is simpler to size liquid-liquidseparation based on retention time of the liquid within theseparator vessel. For gravity separation of two liquid phases,a large retention or quiet settling section is required in thevessel. Good separation requires sufficient time to obtain anequilibrium condition between the two liquid phases at thetemperature and pressure of separation. The liquid capacityof a separator or the settling volume required can be deter-mined10 from Eq 7-16 using the retention time give in Fig.7-18.

U = W (t)1440

Eq 7-16

The following example shows how to size a liquid-liquidseparator.

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FIG. 7-16

Approximate Gas Filter Capacity

7-13

EmulsionCharacteristic

Droplet Diameter,Microns

Constant9

C*Free liquids 200 1880Loose emulsion 150 1060Moderate emulsion 100 470Tight emulsion 60 170

FIG. 7-17

Values of C* Used in Eq 7-14, 7-15

Type of Separation RetentionTime

Hydrocarbon/Water Separators3

Below 0.85 relative density Hydrocarbon 3 to 5 min.Above 0.85 relative density Hydrocarbon

38°C and above 5 to 10 min.27°C 10 to 20 min.15°C 20 to 30 min.

Ethylene Glycol/Hydrocarbon Separators(Cold Separators)11 14 20 to 60 min.

Amine/Hydrocarbon Separators11 20 to 30 min.Coalescers, Hydrocarbon/Water Separators11

38°C and above 5 to 10 min.27°C 10 to 20 min.15°C 20 to 30 min.

Caustic/Propane 30 to 45 min.Caustic/Heavy Gasoline 30 to 90 min.

FIG. 7-18

Typic al Retention Times for Liquid/Liquid Se paration

Example 7-5 — Determine the size of a vertical separator tohandle 100 m3/day of 0.76 relative density condensate and 10m3/day of produced water. Assume the water particle size is200 microns. Other operating conditions are as follows:

Operating temperature = 25°COperating pressure = 6900 kPa(ga)Water relative density = 1.01Condensate viscosity = 0.55 mPa • s @ 25°CCondensate relative density = 0.76

From Eq 7-14

Wc l = C∗

Sh l − Sl l µ

(0.785) (Dv)2

From Fig. 7-17 for free liquids with water particle diameter =200 microns, C* = 1880.

100m3/day = 1880 1.01 − 0.76

(0.55) (0.785) (Dv)2(10−6)

(Dv)2 = (1.43) (105)

Dv = 390 mm

Using the alternate method of design based on retentiontime as shown in Eq 7-16 would give:

U = W (t) 1440

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From Fig. 7-18, use 3 minutes retention time.

U = (110) (3)

1440 = 0.23 m3

A 390 mm diameter vessel will hold about 0.12 m3 per 1000mm of height. The small volume held in the bottom head canbe discounted in this size vessel. The shell height required forthe retention volume required would be:

Shell height = 0.230.12

= 1.9 m = 1900 mm

Another parameter that should be checked when separatingamine or glycol from liquid hydrocarbons is the interface areabetween the two liquid layers. This area should be sized so theglycol or amine flow across the interface does not exceed ap-proximately 100 m3 per day per m2.

The above example indicates that a relatively small separa-tor would be required for liquid-liquid separation. It should beremembered that the separator must also be designed for thevapor capacity to be handled. In most cases of high vapor-liq-uid loadings that are encountered in gas processing equipmentdesign, the vapor capacity required will dictate a much largervessel than would be required for the liquid load only. Theproperly designed vessel has to be able to handle both the va-por and liquid loads. Therefore, one or the other will controlthe size of the vessel used.

PARTICULATE REMOVAL–FILTRATION

Filtration, in the strictest sense, applies only to the separa-tion of solid particles from a fluid by passage through a porousmedium. However, in the gas processing industry, filtrationcommonly refers to the removal of solids and liquids from agas stream.

The most commonly used pressure filter in the gas process-ing industry is the cartridge filter. Cartridge filters are con-structed of either a self-supporting filter medium or a filtermedium attached to a support core. Depending on the appli-cation, a number of filter elements is fitted into a filter vessel.Flow is normally from the outside, through the filter element,and out through a common discharge. When pores in the filtermedium become blocked, or as the filter cake is developed, thehigher differential pressure across the elements indicates thatthe filter elements must be cleaned or replaced.

Cartridge filters are commonly used to remove solid con-taminants from amines, glycols, and lube oils. Other uses in-clude the filtration of solids and liquids from hydrocarbonvapors and the filtration of solids from air intakes of enginesand turbine combustion chambers.

Two other types of pressure filters which also have applica-tions in the gas processing industry include the edge and pre-coat filter. Edge filters consist of nested metallic discs,enclosed in a pressure cylinder, which are exposed to liquidflow. The spacing between the metal discs determines the sol-ids retention. Some edge filters feature a self-cleaning designin which the discs rotate against stationary cleaning blades.Applications for edge filters include lube oil and diesel fuelfiltration as well as treating solvent.

Precoat filters find use in the gas processing industry; how-ever, they are complicated and require considerable attention.Most frequent use is in larger amine plants where frequentreplacement of cartridge elements is considerably more expen-sive than the additional attention required by precoat filters.

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The precoat filter consists of a coarse filter medium overwhich a coating has been deposited. In many applications, thecoating is one of the various grades of diatomaceous earthwhich is mixed in a slurry and deposited on the filter medium.During operation additional coating material is often addedcontinuously to the liquid feed. When the pressure drop acrossthe filter reaches a specified maximum, the filter is taken off-line and back-washed to remove the spent coating and accu-mulated solids. Applications for precoat filters include watertreatment for waterflood facilities as well as amine filtrationto reduce foaming. Typical designs for amine plants use 2.5-5.0m3/hr flow per square meter of filter surface area. Sizes rangeupward from 10-20 percent of full stream rates7.

REFERENCES

1. Perry, Robert H., Editor, Chemical Engineers’ Handbook, 5th Edi-tion, McGraw-Hill Book Company, 1973, Chapter 5, p. 5-64.

2. Souders, Mott, Jr., and Brown, George G., "Design of Fractionat-ing Columns—Entrainment and Capacity," Industrial and Engi-neering Chemistry, V. 26, No. 1, January 1934, p. 98.

3. American Petroleum Institute, Spec. 12J: Oil and Gas Separa-tors, 5th Ed., January 1982.

4. Reid, Laurance S., "Sizing Vapor Liquid Separators," ProceedingsGas Conditioning Conference, University of Oklahoma, 1980,p. J-1 to J-13.

5. Product Bulletin 24000-4-2, Peerless Manufacturing Company,Dallas, Texas.

6. Sarma, Hiren, "How to Size Gas Scrubbers," Hydrocarbon Proc-essing, V. 60, No. 9, September 1981, p. 251-255.

7. Paper presented by W. L. Scheirman "Diethanol Amine SolutionFiltering and Reclaiming in Gas Treating Plants," ProceedingsGas Conditioning Conference, 1973, University of Oklahoma.

8. Sivalls, C. R., Technical Bulletin No. 133, Sivalls, Inc., 1979,Odessa, Texas.

9. American Petroleum Institute, Manual on Disposal of RefineryWastes, Vol. 1, 6th ed., 1959, p. 18-20, and private industry data.

10. Sivalls, C. R., "Fundamentals of Oil & Gas Separation," Proceed-ings Gas Conditioning Conference, 1977, University of Okla-homa, p. P-1 to P-31.

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11. Sivalls, C. R., Technical Bulletin No. 142, Sivalls, Inc., 1980,Odessa, Texas.

12. Perry, Robert H., Editor, Chemical Engineers’ Handbook, 3rd Edi-tion, McGraw-Hill Book Company, 1950, p. 1019.

13. API, RP 521, "Guide for Pressure Relieving and DepressuringSystems," Second Edition, Sept. 1982, p. 52.

14. Pearce, R. L., and Arnold, J. L., "Glycol-Hydrocarbon SeparationVariables," Proceedings Gas Conditioning Conference, Universityof Oklahoma, 1964.

15. Fabian, P., Cusack, R., Hennessey, P., Neuman, M., "Demystify-ing the Selection of Mist Eliminators, Part I," Chemical Engineer-ing, Nov. 1993.

BIBLIOGRAPHY

1. Schweitzer, Phillip A., Handbook of Separation Techniques forChemical Engineers, McGraw-Hill, 1979.

2. Perry, John H., Editor, Chemical Engineers’ Handbook, ThirdEdition, Section 15, Dust and Mist Collection by C. E. Lapple,McGraw-Hill, New York, 1950, p. 1013-1050.

3. Groft, B. C., Holder, W. A., and Granic, E. D., Jr., Well Design —Drilling and Production, Prentice-Hall Inc., Englewood Cliffs,N.J., 1962, p. 467.

4. Perry, Dunham, Jr., "What You Should Know About Filters," Hy-drocarbon Processing, V. 45, No. 4, April 1966, p. 145-148.

5. Dickey, G. D., Filtration, Reinhold Publishing Corporation, NewYork, 1981.

6. Gerunda, Arthur, "How To Size Liquid-Vapor Separators,"Chemical Engineering, V. 91, No. 7, May 4, 1981, p. 81-84.

7. Ludwig, Ernest E., "Applied Process Design for Chemical andPetrochemical Plants," Gulf Publishing Co., Houston, Texas,1964, V. 1, p. 126-159.

8. York, Otto H., "Performance of Wire Mesh Demisters," ChemicalEngineering Progress, V. 50, No. 8, Aug. 1954, p. 421-424.

9. York, Otto H., and Poppele, E. W., "Wire Mesh Mist Eliminators,"Chemical Engineering Progress, V. 59, No. 6, June 1963, p. 45-50.

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