separation-chapter4-pge.ppt

8/9/2019 Separation-chapter4-PGE.PPT

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Chapter Four

Separation


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Separation

Introduction

Natural gases produced from gas wells are normally complexmixtures of hundreds of different compounds. A typical gas wellstream is a high velocity, turbulent, constantly expanding mixtureof gases and hydrocarbon liquids, intimately mixed with water

vapor, free water, and sometimes solids. The well stream shouldbe processed as soon as possible after bringing it to the surface.ield processing consists of four basic processes! "#$ separatingthe gas from free liquids such as crude oil, hydrocarboncondensate, water, and entrained solids% "&$ processing the gas to

remove condensable and recoverable hydrocarbon vapors% "'$processing the gas to remove condensable water vapor% and "($processing the gas to remove other undesirable compounds, suchas hydrogen sulfide or carbon dioxide. This chapter focuses onthe principles of separation and selection of required separators.


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Separation of Gas and Liquids

Separation of well stream gas from free liquids is the

rst and most critical stage of eld processingoperations.

Factors afecting separators type and size are:

1-Composition of the uid mixture

-pressure ! pressure is another "e# factor a$ecting

selection of separators.

%-location !Separators are also used in other locationssuch as upstream and downstream of compressors&deh#dration units& and gas sweetening units. 't theselocations& separators are referred to as scru((ers&"noc"outs& and free liquid "noc"outs.


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)asic *unctions of Separators

Separators should be designed to perform the following

basic functions:

cause a primary)phase separation of the mostly liquid

hydrocarbons from the gas stream

refine the primary separation by further removing most of theentrained liquid mist from the gas

refine the separation by further removing the entrained gas

from the liquid stream

discharge the separated gas and liquid from the vessel and

ensure that no reentrainment of one into the other occurs


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+rinciples of Separation

*ost separators wor+ based on the principles of gravity segregation

andor centrifugal segregation. A separator is normally constructed insuch a way that it has the following features!

it has a centrifugal inlet device where the primary separation of the

liquid and gas is made

it provides a large settling section of sufficient height or length toallow liquid droplets to settle out of the gas stream with adequate

surge room for slugs of liquid

it is equipped with a mist extractor or eliminator near the gas outlet to

coalesce small particles of liquid that do not settle out by gravity

it allows adequate controls consisting of level control, liquid dump valve, gas

bac+pressure valve, safety relief valve, pressure gauge, gauge glass,

instrument gas regulator, and piping


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Types o Separators

Three types of separators are generally available from

manufacturers:

vertical separators,

horizontal separators:

#) single tube

&) double tube , and

spherical separators.

-ach type of separator has specific advantages and limitations.election of separator type is based on several factors including

characteristics of production steam to be treated, floor space

availability at the facility site, transportation, and cost.


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1-,ertical Separators

Vertical separators are often used to :

#) treat low to intermediate gasoil ratio well streams &)treat streams with relatively large slugs of liquid.

')They handle greater slugs of liquid without carryover to the gas

outlet, and the action of the liquid level control is not as critical as in

igure /)#.

() 0ertical separators occupy less floor space, which is important for

facility sites such as those on offshore platforms where space is limited.

1)2wing to the large vertical distance between the liquid level and the

gas outlet, the chance for liquid to revapori3e into the gas phase islimited.

due to the natural upward flow of gas in a vertical separator against the

falling droplets of liquid, adequate separator diameter is required.


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Vertical separator schematic


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-oriontal Separators

4ori3ontal double)tube separators "igure /)&$ are usually the

first choice because of their low costs. #)4ori3ontal separators are widely used for high gasoil ratio

well streams, foaming well streams, or liquid)from)liquid

separation.

&)They have much greater gasliquid interface due to a large,long, baffled gas)separation section.

')4ori3ontal separators are easier to s+id)mount and service,

()require less piping for field connections.

1)Individual separators can be stac+ed easily into stage)

separation assemblies to minimi3e space requirements. In

hori3ontal separators, gas flows hori3ontally and, at the same

time, liquid droplets fall toward the liquid surface.


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-oriontal Separators


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Horizontal separator schematic

oriontal dou(le-tu(e separators ha/e all thead/antages of normal horiontal single-tu(eseparators plus much higher liquid capacities.


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% -Spherical Separators Spherical separators o$er an inexpensi/e and

compact means of the separation

arrangement shown in *igure 0-.


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these t#pes of /essels ha/e a /er# limited surgespace and liquid settling section. 2he placement andaction of the liquid le/el control in this t#pe of /essel

is /er# critical.

Separator operating pressure& separator operatingtemperature& and uid stream composition a$ect the

operation and separation (etween the liquid and gasphases in a separator. Changes in an# one of thesefactors on a gi/en uid well stream will change theamount of gas and liquid lea/ing the separator.

Generall#& an increase in operating pressure or adecrease in operating temperature will increase theliquid co/ered in a separator. owe/er& there areoptimum points in (oth cases (e#ond which furtherchanges will not aid in liquid reco/er#.

Factors Affecting Separation


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3n the case of wellhead separation equipment& anoperator generall# wants to determine theoptimum conditions for a separator to e$ect the

maximum income. 'gain& generall# spea"ing& theliquid reco/ered is worth more than the gas. So&high liquid reco/er# is a desira(le feature&pro/iding it can (e held in the a/aila(le storage

s#stem. 'lso& pipeline requirements for the )tu content of

the gas ma# (e another factor in separatoroperation. 4ithout the addition of expensi/e

mechanical refrigeration equipment& it is oftenunfeasi(le to tr# to lower the operatingtemperature of a separator.


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2he operator can also control operating pressure to

some extent (# use of (ac"-pressure /al/es withinthe limitation of the owing characteristics of thewell against a set pressure head and thetransmission line pressure requirements. 'spre/iousl# mentioned& higher operating pressure willgenerall# result in higher liquid reco/er#.

'n anal#sis can (e made using the well streamcomposition to nd the optimum temperature andpressure at which a separator should operate togi/e maximum liquid or gas phase reco/er#. 2hesecalculations& "nown as ash /aporiationcalculations& require a trial-and-error solution andare more generall# adapted to solution (# a

programmed computer.


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Separator Design

2he following discussion on oil gas separator

design has (een adapted from Si/alls5 excellenttreatment of the su(6ect. Si/alls7 ta(les& graphs&and procedures are accepted as the standard of theindustr#.

Gas Capacity 2he gas capacit# of oil-gas separators has (een

calculated for man# #ears from the followingempirical relationship proposed (# Souders-)rown!


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where

/ 8 supercial gas /elocit# (ased on total cross-sectional area of /essel& fps

A = cross-sectional area of separator& sq ft

q = gas ow rate at operating conditions& cfs

pL = densit# of liquid at operating conditions&

l(m9cu ft

pg= densit# of gas at operating conditions& l(m9cuft

K 8 empirical factor


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,ertical separators K = :.:; to :.%


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where

q 8 gas capacit# at standard conditions& >>scfd

D = internal diameter& ft

p 8 operating pressure& psia

T = operating temperature& ?*

z = gas de/iation factor

*igures .; to .1% are gas capacit# charts for/arious standard sie separators (ased onoperating pressure.


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Liquid Capacit#

2he liquid capacit# of a separator is primaril#dependent on the retention time of the liquidwithin the /essel. Good separation requires

su@cient time to o(tain an equili(rium condition(etween the liquid and gas phase at thetemperature and pressure of separation. 2heliquid capacit# of a separator or the settling/olume required (ased on retention can (e

determined from the following equation!


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where

W 8 liquid capacit#& ((l9da#

V = liquid settling /olume& ((l

t = retention time& min

Basic design criteria for liquid retention times inseparators have been determined by numerous eldtests:

Ail-gas separation 1 min igh-pressure oil-gas-water separation to < min

Low-pressure oil-gas-water separation < to 1: min at 1::?*

and up

1: to 1< min at B:?*

1< to : min at =:?*

: to < min at 0:?*

< to %: min at ;:?*


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*igures .1 and .1< are siing charts for the liquidcapacit# of horiontal single-tu(e high-pressureseparators. 2hese are (ased on the parameters ofseparator wor"ing pressure& sie& and the depth ofliquid used in the liquid settling section.

2a(les '. to '.1 list the standard specications oft#pical oil-gas separators and the liquid-settling/olumes with the con/entional placement of liquidle/el controls.

2he settling /olumes ma# determine the liquid

capacit# of a particular /essel. *or proper siing of(oth& the liquid capacit# and gas capacit# requiredshould (e determined.


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St S ti


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Stage Separation

Stage separation is a process in which gaseous and liquidh#drocar(ons are separated into /apor and liquid phases (#two or more equili(rium ashes at consecuti/el# lowerpressures. 's illustrated in *ig. .1;& two-stage separationrequires two separators and a storage tan" and so on. 2hetan" is alwa#s counted as the nal stage of /apor-liquidseparation (ecause the nal equili(rium ash occurs in thetan".

2he purpose of stage separation is to reduce the pressureon the reser/oir liquids a little at a time& in steps or stages&so that a more sta(le stoc"-tan" liquid will result. 2he ideal

method of separation& to attain maximum liquid reco/er#&would (e that of di$erential li(eration of gas (# means of astead# decrease in pressure from that existing in thereser/oir to the stoc"-tan" pressure. owe/er& to carr# outthis di$erentiaD process would require an innite num(er of

separation stages.


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'lthough three to four stages of separationtheoreticall# would increase the liquid reco/er# o/ertwo stages& the net increase o/er two-stage

separation will rarel# pa# out the cost of the secondor third separator. 2herefore& it has (een generall#accepted that two stages of separation plus thestoc" tan" are considered optimum.

2he optimum high stage or *irst separator operatingpressure is generall# go/erned (# the gastransmission line pressure and operatingcharacteristics of the well.

*or each high or rst-stage pressure& there is anoptimum low-stage separation pressure that willa$ord the maximum liquid reco/er#. 2his operatingpressure can (e determined from an equation (asedon equal pressure ratios (etween the stages

ECamp(ellF!


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where:

R 8 pressure ratio

n 8 num(er of stages - 1

Pi= rst-stage or high-pressure separator pressure&psia

p2 = second-stage or low-pressure separator

pressure& psiaps = stoc" tan" pressure& psia


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2he magnitude of stoc" tan" liquid reco/eries arenot considered in the equation.

*igure .10 has (een prepared to determine the

optimum low-stage separator pressure (ased on thehigh-stage separator pressure with additionalparameters of o/erall stoc" tan" liquid reco/er#.

2his information has (een determined from

extensi/e eld test data. *igure .1= is a chart illustrating the a/erage

percent increase in liquid reco/er# for two-stageseparation o/er single-stage separation. )# using

this& an


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Example 4! Sie a standard oil-gas separator (oth/erticall# and horiontall# for the followingconditions!

Gas ow rate >scfdAperating pressure =:: psig

Condensate ow rate : ((l9>>scfSolution

2otal liquid capacit# 8 : E


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*rom *ig. .0& at =::-psig operating pressure& a : in. x 0 ft& ; in./ertical separator will handle >scfd. *rom 2a(le '.


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Example 42 Sie a standard /ertical oil-gas separator forthe following conditions!

Ail ow rate


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Example 4 4 Sie a horiontal high-pressure separator for the


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Example 44 Sie a horiontal high-pressure separator for thefollowing conditions!

Gas ow rate 1:.: >>scfd

Aperating pressure =:: psig

Condensate load >scfd operating one-half full liquidcapacit#. 4here

three-phase operation is required in a horiontal separator& theliquid section

should (e one-half full otherwise& the le/el control action

(ecomes too critical.*rom 2a(le '.B& the liquid capacit# will (e

1: 1:E1.=:F

W = 8 8


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2herefore& the : in. x 1: ft separator will nothandle the com(ined liquid load of


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Example 4! ' well test was made using a simplehigh pressure separator and an atmospheric stoc"tan" and the following results were o(tained!


Aperating pressure =:: psig

Condensate reco/er#


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Example 4" An the test of a high-pressure gas-condensatewell the following data were recorded!


Aperating pressure =:: psigCondensate reco/er#:: ((l9da#

Separator temperature =

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