Compact Separator

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Copyright 1999, Society of Petroleum Engineers Inc.

This paper was prepared for presentation at the 1999 SPE Annual Technical Conference andExhibition held in Houston, Texas, 36 October 1999.

This paper was selected for presentation by an SPE Program Committee following review ofinformation contained in an abstract submitted by the author(s). Contents of the paper, aspresented, have not been reviewed by the Society of Petroleum Engineers and are subject tocorrection by the author(s). The material, as presented, does not necessarily reflect anyposition of the Society of Petroleum Engineers, its officers, or members. Papers presented atSPE meetings are subject to publication review by Editorial Committees of the Society ofPetroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paperfor commercial purposes without the written consent of the Society of Petroleum Engineers isprohibited. Permission to reproduce in print is restricted to an abstract of not more than 300

words; illustrations may not be copied. The abstract must contain conspicuousacknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O.Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

AbstractNewly developed compact separation technology may be an

attractive alternative to conventional separation methods in

certain oil and gas applications. The purpose of this paper is

to compare the new compact separators to conventional

separation methods and explore the possible applications of

this new technology. The size of conventional separators is

based on liquid retention time, droplet or settling velocity for

gas, and, for three-phase separators, water droplet settling time

in the oil phase. Compact separators perform the same

function as conventional separators but require a smaller shell.They have great potential where cost savings due to smaller

size and lower weight are greater than the cost of more

complex equipment. Both the benefits and problems

associated with using compact separation techniques for

downhole processing, subsea processing, and surface facilities

will be discussed in this paper. It is necessary for a design

engineer to understand the benefits and detriments of using

compact separators so that this new technology may be used

effectively in the production of oil and gas.

IntroductionOil and gas companies are constantly searching for more

effective ways to produce oil. Current separation techniquesare costly, and because of size and weight requirements,

separation equipment greatly affects the space and load

requirements, and thus the cost of offshore structures. In order

to reduce cost and maximize the effectiveness of separation

equipment, better designs for separation equipment are being

evaluated. Over the past few years, notable advances in

compact separation technology have been made. Equipment

that promises to be lighter and smaller than current separation

equipment has been developed and is suitable for gas-liquid

and liquid-liquid separation. The development of

compact separation techniques is the first step t

designing tomorrows compact separation train.

What is a conventional separator?Conventional separation consists primarily of two and

phase separators. These separators are generally cylin

shelled vessels that can be either horizontal or verti

orientation.In a conventional two-phase separator (Figure

is separated from bulk liquid. Fluid enters the separat

hits an inlet diverter. The impact causes a sudden cha

momentum, and the initial gross separation of the liqu

gas occurs. The force of gravity causes the liquid to d

the bottom of the vessel where it is collected. Followi

diverter, gas enters the gravity settling section of the

As the gas flows through this section, small drops of

that were entrained in the gas and not separated by th

diverter are separated by gravity and fall to the gas

interface. Beneath the gravity settling section is the

collection section of the vessel. This section provid

retention time required for any flash gas to evolve out

oil and rise to the vapor space. The last separation efthe two-phase separator is the mist extractor. This

uses vanes, wire mesh, or plates to coalesce and remo

very small droplets of liquid before the gas leaves the ve

Three-phase separators are also used to separa

from liquid, but in addition, they separate a light liquid

heavier liquid, (oil from water, for example). A conven

three phase separator (Figure 2) contains an inlet divert

provides the initial gross separation of liquid and vapor

main difference between the two and three phase inlet d

is that the three-phase diverter contains a downcome

directs the liquid flow beneath the gas-oil interface to

the oil-water interface. Like the two-phase separato

three-phase separator also contains a gravity settling sand a liquid collection section; however, the liquid coll

section is much larger for the three-phase separator.

liquid collection section must provide enough retentio

so that the oil and emulsion form a layer above the

layer. Interface level controllers or weirs are used to m

the oil-water interface at design height. The oil and wa

discharged from separate collection areas in the vessel.

There are three main factors that determine th

SPE 56644

Designing Tomorrow's Compact Separation TrainKenneth E. Arnold, SPE/Paragon Engineering Services, Inc. Patti L. Ferguson/Paragon Engineering Services, Inc.
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2 K. E. ARNOLD, P. L. FERGUSON SPE

of conventional separators. The first factor is retention time

for the liquids. A certain amount of time and liquid storage

space is required to assure that the liquid and gas reach

equilibrium at separator pressure. The second factor affecting

vessel size is the droplet or settling velocity for the liquid

drops entrained in the gas. The purpose of the gravity settling

section of the vessel is to condition the gas for final polishing

in the mist extractor. The liquid drops will settle at a velocitydetermined by equating the gravity force on the drop with the

drag force caused by its motion relative to the gas continuous

phase.

For three phase separators, water droplet settling in

the oil layer is also a sizing factor. For good results to be

obtained, the oil pad must be designed so that water droplets

which enter the oil pad as the emulsion flows through the

interface settle out. In conventional separators a certain

amount of liquid retention time is required to assure that the

oil reaches equilibrium and flashed gas is liberated. In a three

phase separator, additional retention time is necessary to

assure that the free water has time to coalesce into droplet

sizes sufficient enough to fall to the oil water interface. A

certain amount of water retention time must also exist in a

three phase separator so that the large droplets of oil entrained

in the water have enough time to coalesce and rise to the oil-

water interface.

What is a compact separator?Compact separators perform the same function as their

conventional counterparts, but they do so in a smaller shell.

This is achieved by the use of centrifugal force and resulting

flow patterns to separate immiscible phases of different

densities. The conventional separation of two liquids or liquid

and gas depends on the force of gravity to affect separation.

Because the two phases have different densities, the force of

gravity causes the more dense substance to fall to the bottomof the separation vessel while the lighter, less dense liquid

rises up. If the affective force of gravity is somehow

increased locally by centrifugal action, then separation occurs

more rapidly.

Compact separators can be designed so that the

centrifugal force is thousands of time greater than the force of

gravity. By increasing the speed of separation, the need for

long retention times within vessels is eliminated, and the size

of the separation vessel can be greatly reduced. Separation

techniques utilizing centrifugal force may not produce outlet

streams with as good a quality as conventional separation, but

they are sufficient enough for many practical purposes. The

three main types of separation service that compact separatorsare available for are bulk gas-liquid separation, bulk oil-water

separation, and water polishing.

One problem associated with compact separation is

that compact separation equipment tends to be more sensitive

to flow variations than conventional separators. The control

of liquid and interface levels is difficult in slugging services.

Thus there is a potential for liquid carry over or gas blowby in

gas/liquid separators and poor quality oil and/or water in

oil/water separators. However, compact separators can be used

in applications where quality of output is not critical suc

gas-liquid split for multiphase flow meters, or sepa

where the quality of only one of the separated strea

important.

Compact Separators may also be more sensit

plugging with paraffins, corrosion products, and sand a

as being more sensitive to erosion and mechanical f

Another disadvantage to the use of compact separators they have higher capital, operation, and maintenance

than their conventional counterparts.

Bulk Gas - Liquid SeparationSeveral types of compact separators are available for

bulk gas-liquid separation. They are the auger, Sp

separator, Gasunie separator, gas liquid cylindrical cy

and the biphase turbine. The auger (Figure 3) is a

compact gas-liquid separator that can be used for

downhole or topside processing. Multiphase fluid

axially at the base of the unit. The fluid is forced to

because of stationary helical vanes in the vessel. Liquid

to the outer wall by virtue of the phase density differen

fraction of the gas passes through ports located on the

wall and is removed while the remainder of the gas a

liquid continue and exit axially at the top of the unit.

The Split-Flo separator (Figure 4) consists

primary and secondary separator. Both utilize cent

force in separating gas from liquid. The fluids pas

curved surfaces within the primary separator to produ

centrifugal force. The primary separator typically re

99% of the incoming liquids from the gas. The second

separator removes the remaining liquid drops to prod

high quality gas.

The Gasunie separator (Figure 5) alsocentrifugal force to separate heavy particles from the ga

gas converges into an inverted vortex and exits the top

vessel. The liquid is held against the outside wall as

down to exit at the bottom of the chamber.

There are several types of gas/liquid cyclones

market. In a typical design (Figure 6), fluid enters

vertical cylindrical or conical cyclone where high ve

swirling flow creates a radial acceleration field. This

gas to flow to the axial core region. The gas exits thro

axial outlet located at the top of the unit, and liquid

through a tangential outlet at the base.

The biphase turbine (Figure 7) achieves sepa

by combining oil-gas separation with energy recoveryturbine uses a two-phase nozzle to convert the therm

pressure energy of a liquid and vapor mixture to k

energy. The resulting high-velocity two-phase m

impinges on a rotating cylinder to produce centrifugal

which then separates the mixture.

Bulk Oil - Water SeparationThe main types of bulk oil-water compact separato

separators with high performance internals, hydrocylone


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SPE 56644 DESIGNING TOMORROW'S COMPACT SEPARATION TRAIN

electro-pulsed inductive coalescers. High performance

internals consist of a cluster of vanes placed at the inlet of a

vessel followed by a horizontal baffle that extends the entire

height of the vessel. The internals can be retrofitted to a

preexisting vessel or built into a new vessel. These internals

increase separation speed by greatly reducing turbulence at the

inlet end and quickly create quiescent conditions within the

separator Figure 8 is an example of such a system.Hydrocyclones which are mounted inside a pressure

vessel induces a centrifugal flow path within their tapered

tubes. This allows a bulk separation of the liquid phases and a

coalescence of the dispersed droplets in the continuous phase

of the underflow. A cluster of hydrocyclones used as an inlet

diverter in a conventional separator extends the separator's

capacity and minimizes the formation of foam. Figure 9

shows a bulk oil-water separation device installed in a

separator.

In an electro-pulsed inductive coalescer (EPIC) fluid

enters into a high voltage DC electro-pulsed inductive field

where it is repeatedly pulsed. The pulsation causes the water

droplets to coalesce into larger droplets thus increasing the

potential for easy separation in the downstream processing

equipment. This device only works where there is an oil

continuous phase in the inlet stream, and thus may be thought

of as an oil treating device as opposed to a bulk oil/water

separation device.

Water TreatingVertical flotation, hydrocyclones, air-sparged hydrocyclones,

disc-stacked centrifuges, and simplified centrifuges are all

compact separators used in water polishing. In vertical

flotation units, gas is sparged into the bottom of the vertical

vessel containing produced water. As the fine bubbles sweep

the liquid, they collect oil droplets. The oil is then transported

to the surface and accumulates in an oil pad layer.Hydrocyclones used for water polishing are similar to

those used in bulk oil-water separation. With an assured water

continuous phase and a 2% overflow of mostly water, the

underflow will contain very little oil. In addition, these

droplets of oil will coalesce and be easily separated in

downstream equipment.

Air-sparged hydrocyclones are vertical vessels with a

tangential inlet. Upon entering the hydrocyclone, a thin liquid

layer is formed that corkscrews along the wall of a porous tube

into which air is being forced under pressure. Bubbles contact

the liquid, adhere to oil droplets in the water, and rise up to the

surface of the vessel as shown in Figure 10.

A disk-stacked centrifuge is a separation bowlcontaining a disk stack of truncated cones which is rotated at

high speeds. The high g-force and the large equivalent settling

area that is provided by the stacked disks obtain excellent

separation. A simplified version (sometimes called a

"dynamic hydrocyclone") does not have the stacked disk. The

inlet enters into an annular space between a spinning rotor and

stationary housing. Mixed phases are rapidly accelerated to

rotor speed and separation occurs. Both types of centrifuges

can be operated to give a high quality water stream or a high

quality oil stream but not both. Unless the inlet is a

and steady mixture the quality of the reject stream wil

greatly.

Needs for Compact SeparationSome of the challenges facing deepwater development i

greater water depth, a greater step-out distance from wh

oil field is to where the production is processed, and thefor reduction in topside weight. Compact separation m

useful in reducing development costs by allowing dow

and subsea processing and reducing topside weights.

Downhole ProcessingLatest developments in downhole separation are focu

the separation of oil and water in the well bore

emulsification occurs. For emulsions to exist, there m

two immiscible liquids, an emulsifying agent, and suf

agitation to disperse the discontinuous phase int

continuous phase. In oil production, oil and water a

immiscible liquids. Small solid particles, such as paraf

asphaltenes, are usually present to act as the emuls

agent, and agitation always occurs as fluid makes its wa

the well bore, up the tubing, and through the surface

By using the compact separator in a downhole capacity,

and water will be separated before the fluids are ag

enough to produce an emulsion.

Downhole separation can also reduce the

required to lift water to the production surface, if the

borehole is used for water injection as shown in Figu

Problems associated with using compact separato

downhole processing include the need for a water cont

phase and downhole power. Where water is injecte

another zone in the same wellbore, solids and transie

high oil content in the water can cause frequent plugg

the well. Thus operating costs due to both mechintegrity of downhole equipment and instrumentation

well remediation must be considered.

Although downhole separation of oil and w

becoming common, to date it has only proved practi

cases where there is a water continuous phase. In most

these systems have been used where water cuts are 90

Process reliability is a problem when oil is the conti

phase, and there is ongoing research to better evalua

problem.

Subsea ProcessingUntil recently most applications of subsea processing fo

on gas-liquid separation. In many offshore fields, transpthe hydrocarbons (gas plus liquid) in long multi

pipelines results in higher than desired backpressure

wells thus reducing the flow of hydrocarbons from th

and requiring slug catching systems on the proc

platform. In addition, cooling of hydrocarbons in mult

lines can lead to deposition of paraffin and hydrates

obstructs the fluid's flow and decreases recovery efficien

A subsea processing unit, with separate pipelin

liquid and gas, reduces backpressure on the well form


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4 K. E. ARNOLD, P. L. FERGUSON SPE

without the need for relatively inefficient multiphase pumps.

In addition, it is easier to inhibit single phase lines against

hydrate formation than it is to inhibit multiphase lines. In the

separated liquid phase, only a small volume of gas is present

and the hydrocarbon liquid will transport the small amount of

hydrates which form. Therefore, the potential for hydrate

plugging is negligible. Hydrate inhibition is easier in the gas

line as the availability of free water is reduced.Problems associated with the use of compact

separators in subsea applications include achieving level

control, the cost of designing the vessel controls to withstand

the external pressure, process reliability, and mechanical

reliability. Providing power for any system involving a subsea

pump, compressor or multiphase pump is also costly, and thus

the efficiency of any rotating equipment as well as its

mechanical reliability becomes important.

Applications of subsea processing include the following:

1) Separating the gas and liquid, using a separatepump and compressor, and then recombining the

streams in a multiphase pipeline. This requires less

energy than the use of multiphase pumps.

However, it requires maintaining two pieces of

rotating equipment at subsea conditions.

2) Separating the gas and liquid and transporting thestreams via two pipelines. With this method, there

is less pressure drop in each line than in a

multiphase line and thus the backpressure on the

well is reduced. In deep water, further lowering of

back pressure is possible by installing a pump on

the liquid line to overcome the pressure due to

liquid head. This method eliminates the need for a

multiphase pump and a slugcatcher, and makes

handling hydrate problems easier. Drawbacks to

this approach include the cost of two pipelines and

the need to assure the quality of separation. Themechanical reliability of a liquid pump should be

equal to or higher than that of a multi-phase pump.

3) A three pipe variation of the system described

above can aid in reducing formation of paraffins by

introducing a recovered hydrocarbon stream into

the liquid flowline soon after liquid exits the

separator as shown in Figure 12. This recycled

stream serves as a power fluid and also provides

sufficient flowrate so that the liquid can be

transported before cooling below its cloud point to

minimize paraffin build up. In addition, the third

pipeline provides a loop for frequent pigging.

4) Separating gas from liquid and re-injecting the gasinto the same or another wellbore. This method

eliminates flare or gas conversions costs, reduces

the horsepower for compression that is needed,

reduces back pressure on the well and reduces

hydrate problems. Detriments to this application

include the need for a high compression ratio

compressor, the need for high quality separation to

protect the compressor from liquids, and the need

to maintain subsea equipment.

5) Separating water from oil and disposing water. The energy required to lift wat

downhole separation is used) or transport wa

subsea separation is used) is reduced. Pro

with this approach are mechanical reliabili

assuring high enough water quality to keep

plugging the formation (if water is re-inject

to meet environmental constraints (if wadischarged to the sea).

The effects of downhole and subsea process

reducing topside space and weight requirements as w

cost may not be as substantial as once thought.

performing separation subsea or downhole may redu

size of some topside equipment, it will not comp

eliminate any part of the traditional production equip

For example, if gas and liquid are separated subsea and t

is reinjected, there will still be flash gas produced that m

handled. Therefore, compression, dehydration, and ga

equipment will still be required, although these pie

equipment may be smaller. Likewise, if bulk oil and

separation is performed subsea or downhole, the t

equipment for water treating is not completely elimi

Water removed from the oil as the oil is treated will hav

treated for disposal. In both of these cases, the siz

weight of some topsides equipment is reduced, bu

associated cost savings will be offset by increased pack

costs, cost to provide power, operation and mainte

expense, and downtime associated with subsea and dow

designs.

The potential weight and space savings have

marginal impact on facilities. As shown in Tab

Ultimately, the real impact of using subsea and dow

processing is the ability to produce the well, not t

reductions. Because these separation techniques can

backpressure on a well and facilitate the transphydrocarbons over long distances, marginal fields c

produced.

Topside ProcessingTopside processing can benefit from the use of co

separators which can reduce the footprint and lo

requirements of the production facilities. Although th

not be too important for new facilities onshore or facili

fixed structures or FPSOs, it is important for facilities on

structures such as TLPs, Semis, and Spars.

Initial phase separation and oil treating sy

dominate production equipment cost, weight, and foo

By focusing the use of compact separation techniquesthe greatest impact to the production facility can be ma

hydrocyclone used in a multiphase metering arrangeme

eliminate the need for a conventional test separator. Sp

separators can be used in place of traditional 2

separators where gas flow rates are sufficient. Three

separators that are enhanced with high performance in

or hydrocyclones reduce in size dramatically. In ca

heavier oils that require multiple parallel vesse

accommodate the long retention times needed for di


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SPE 56644 DESIGNING TOMORROW'S COMPACT SEPARATION TRAIN

separation, these internal devices can reduce the quantity of

vessels required. Centrifuges can be used for oil treating as

well as water treating. The use of the compact separators can

dramatically reduce the space and weight needs of a facility.

Savings become more apparent as oil specific gravity

increases.

The use of compact separation techniques may also

enable the expansion of facilities that are constrained by spaceand weight restrictions by reducing space and weight

requirements for supplemental processing equipment.

Compact separation devices such as high performance

internals and hydrocyclones can also be retrofitted to existing

separator vessels. For gas/liquid separation this can be

beneficial in decreasing foaming within the vessel and

consequently increasing throughput.

ConclusionsA critical objective in all new field developments is to reduce

project capital cost, operation and maintenance cost, and

project cycle time (life cycle cost). Currently, compact

separators have greater capital, operation, and maintenance

costs than conventional separators. Most often, the correct use

of compact versus conventional separators will make only a

marginal difference in topside life cycle costs Therefore, the

choice to use compact separators must be based on the full

understanding of life cycle costs of complex system

alternatives.

In the past ten years, there have been dramatic

changes in the application of new technology to both gas-

liquid and oil-water separation. This technology is constantly

evolving and compact separation is becoming a more

attractive alternative to conventional separation methods. In

the past, it was beneficial, but not essential, for a design

engineer to know how to size conventional separation

equipment. Any number of suppliers would provide the sizefor free, and sizing results would be essentially the same no

matter which supplier was used. With compact separation, it

is now more important that the design engineer understand the

specific benefits and detriments of the different compact

separators. When considering using this new equipment, the

design engineer needs to consider the following points:

First, this technology is new and the designs are often

proprietary; therefore, pricing is less competitive.

Second, choices in equipment must often be made

before costs are definitely known. There has never

been a practical performance guarantee in the

upstream separation business and thus the engineer

must make an informed choice and be able tocritically evaluate sales claims.

If design engineers are informed about these

advances in separation technology and remember

these points, this technology will be used wisely and

to the benefit of oil production.


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