j1036 pr 001 operational theory

7/25/2019 J1036 PR 001 Operational Theory

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Petrofac International for Khalda Petroleum Company

Project: Salam Gas Trains (SGT3 & 4 Project

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Introduction , -ater in Gas Pipeline Systems

The removal of water vapor from a gas stream is referred to as Dehydration. Whenproduced, natural gas is normally saturated with water vapor. emoval of water vapor!dehydration" is necessary to avoid the damaging and potentially ha#ardous effects thatmay occur as a result of condensation of this water in a pipeline system.

The condensation of water vapor in pipeline systems is caused when the temperature ofthe pipeline gas drops $elow the dew point temperature of water. %as temperature will

decrease as a result of heat loss through the pipe wall and due to adia$atic e&pansion ofgas that results from the pressure reduction caused $y features present in pipelinesystems.

'hief among the damaging and potentially ha#ardous effects is formation of hydrates andthe corrosion which li(uid water may cause. %lycol dehydration is one process used toreduce the water vapor content to levels accepta$le for natural gas transmission.

ydrate formation , Pro.lem $efinition

The formation of hydrocar$on clathrate hydrates !or alternatively gas clathrates, gashydrates, clathrates, hydrates etc" from natural gas will occur when light hydrocar$on gasunder pressure, and in the presence of li(uid water, is cooled $elow the hydrate formationtemperature. )ydrates appear as ice*li+e crystals in which a lattice of water moleculesform cages trapping non*polar gas molecules. When allowed to form, hydrates caninterfere with the operation of instruments and valves as well as reduce the open area ofpipelines.

1. ethods utili#ed in addressing this pro$lem include-

. /dding hydrate inhi$itors.

3. )eating the gas to maintain temperatures a$ove that which will allow hydrates toform.

educing the water vapor content, to ensure the dew point temperature of the gas is notencountered in the pipeline. ecause li(uid water is a critical component in the formationof hydrates, preventing its formation will prevent hydrate formation.

everal methods are availa$le to reduce the water content, however the topic of thisdocument is limited to glycol dehydration.

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Physical Principle , /apor Pressure as $ri0in1 2orce

%lycol dehydration is a physical separation process that ma+es use of the strong physicalattraction $etween water and glycols. %lycols !li+e alcohols" are organic compounds thatcontain o&ygen in association with hydrogen !2)*". The presence of the functional group!2)*" causes water !)2), or )2" to interact with the glycol molecule much li+e it interactswith other water molecules.

/s stated $efore, the !2)*" functional groups are $ound to a hydrocar$on $ac+$one that

provides glycols with characteristics common to organic compounds. pecifically, glycolsare much less volatile than water. This means that at a given temperature glycols willhave a much lower vapor pressure than water.

t is the com$ination of the two sets of properties descri$ed a$ove in a single compoundthat ma+e glycol the compound of choice for this dehydration process. The water lovingnature of glycol will cause it to a$sor$ water from a wet gas at temperatures and pressuresnormally present in gas handling facilities. While the organic character of the glycolreduce the volatility of glycol far $elow that of water, and this com$ination of properties!water loving and low volatility" directs us towards a two*step process that will ta+eadvantage of these properties.

Gas $ehydration and Glycol 'e1eneration

y contacting a glycol with a wet gas stream the affinity of water to the glycol will causethe water to enter the glycol as a solution. This contacting step is carried out under highpressure and moderate temperatures. This com$ination of pressure and temperature isconducive to dissolution of water into glycol. 2ther parameters that greatly impact thetendency of water to enter and stay in solution with glycol include the concentration ofwater in the gas, and the concentration of water in the glycol. y contacting fresh leanglycol in the upper section of a vertical column with the driest gas as it e&its the top of thecolumn the water capturing effect of the glycol is ma&imi#ed.

'onversely, $y contacting the wettest gas with the spent rich glycol in the lower sections of a vertical column the concentration of the water in the gas phase provides the incentive tomove the water into the already water laden rich glycol .

The second step in this process is then to remove the water from the rich glycol.emoving the water from the glycol will $e performed $y reversing the parameters thatwere descri$ed a$ove as $eing conducive to dissolution of water into glycol5. To state theinverse of the process as descri$ed a$ove6 the temperature will $e raised and thepressure decreased to the e&tent that sufficient motivation is provided to the watermolecules to disassociate from the glycol and return to the vapor phase.

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The %lycol 'ontactor /*'*731 is the tower used to perform the first step. Theegeneration Pac+age provides the process units re(uired to transfer heat and addressthe pressure changes re(uired to regenerate the rich glycol and reintroduce this as leanglycol in the contactor column.

'euirements for Gas $ehydration

pecification of water vapor removal from gas is usually e&pressed in terms total gas inletof Pound !mass" of water per illion tandard 'u$ic eet !l$m8'". This is normallyset to limit the dew point of the gas to $elow that li+ely to $e encountered in normalpipeline operation. /lternately the dew point at the dehydrator outlet may $e specifieddirectly.

a&imum Pounds of water per illion tandard 'u$ic eet 9 3.:: !l$m8'"

;ew Point at ;ehydrator 2utlet 9 *<=' > 70 $arg

2ther units or methods of e&pression are occasionally used, $ut these are the mostcommon for gas dehydration. 'onversion to other units should $e possi$le if re(uired.

'euirements for Glycol 'e1eneration

pecification of water content in lean glycol returning to the glycol contactor is normally

specified in terms of ?ean %lycol 'oncentration !@@.7A ?ean TB%".

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)omenclature and Con0ersions

'ondensation C The coalescence of water vapor into li(uid water.

'ondensate C ?i(uid hydrocar$on and water which is present in a gas pipeline system.

;ehydration C emoval of water from a phase.

;ew Point Temperature C The temperature at which water vapor $egins to condense froma gas phase to form li(uid water. This is dependant on $oth the vapor composition ofwater in the gas phase and the a$solute pressure of the gas phase.

%lycol C %lycols are a family of organic containing two !" hydro&yls !2&ygen D )ydrogen"as a functional group. ost common glycols are ethylene %lycol and Bthylene %lycol.oth are used in automo$ile antifree#e. While these glycols are used for dehydration, themore common glycol for this application is Triethylene %lycol !TB%". TB% is composed ofthree molecules of ethylene glycol strung together to form a longer, heavier, more ro$ustand less volatile compound.

)ydrate ormation Temperature C The temperature at which hydrates will form in thepresence of li(uid water. This is dependant on the a$solute pressure of the gas phase incontact with the water.

?ean %lycol C %lycol depleted to some e&tent of water.

ich %lycol C %lycol containing some amount of water a$ove that of lean glycol.

Eapor Pressure C The pressure e&erted $y a vapor in e(uili$rium with another phasecontaining some fraction of the same su$stance.

Eapor 'omposition !mass" C The percentage !mass fraction" of a given vapor componentin a gas phase.

Special *nits

Pound !mass" of water per illion tandard 'u$ic eet of gas !l$m8'" C a unit ofmeasure relating the mass of water vapor to total gas volume at standard temperature andpressure !TP".

parge ate tandard 'u$ic eet per %allon !'8gal" C a unit of measure used inreporting sparge gas flow rate. elating the volume of gas at standard temperature andpressure !TP" to volume of a li(uid.

tandard 'u$ic oot !'" * / standard cu$ic foot is a cu$ic foot of volume at TP F0=!1<.F='" and 14.7 psia !1 atm".

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tandard Temperature and Pressure !TP" C the condition of a gas at F0= !1<.F='" and14.7 psi!a" !as used in this document".

tandard 'u$ic eters !scm" * / standard cu$ic meter is a cu$ic meter of volume at TPF0= !1<.F='" and 14.7 psi!a" !1 atm" !as used in this document".

Gormal cu$ic meter !GmH3" * The metric e&pression of volume at 0=' !3=" and 101.3+Pa!a".

Common Con0ersion 2actors

Eolume

1 gallon !I.." 9 3.7:< liters

1 ' 9 0.0F7@ GmH3

1 GmH38h 9 0.F '

1 illion tandard 'u$ic eet !'" 9 0.0:317 illion standard 'u$ic eters!smH3"

1 illion tandard 'u$ic eet per ;ay !';" 9 117@.:7 tandard 'u$ic 'u$iceters per )our !scm8h"

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GLYCOL CONTACTOR UNIT DESIGN AND OPERATION

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The purpose of the glycol contactor column is to introduce a glycol with a wet gas stream.This is done under conditions of high pressure and moderate temperatures. Thiscom$ination of pressure and temperature is conducive to dissolution of water into glycol.y contacting fresh lean glycol in the upper section of a vertical column with the driest gasas it e&its the top of the column we ma&imi#e the water capturing effect of the glycol.

'onversely, $y contacting the wettest gas with the spent rich glycol in the lower sections of

a vertical column the concentration of the water in the gas phase provides the incentive tomove the water into the already water laden rich glycol .

The glycol contactor column is composed of F sections !listed from the top downward".

;emisting pad C / stainless steel mesh pad type mist eliminator. This functions tominimi#e loss of entrained glycol droplets that may otherwise e&it the contactor.

%lycol ;istri$utor * ?ean glycol is added into this section of the column and distri$utedacross the pac+ing.

Pac+ing tage * Wet gas travels upward through the structured pac+ing !3F<7mm depth"while lean glycol travels downward !in this way contacting the leanest glycol with the driestgas at the top of the pac+ing and the richest glycol with the wettest gas, at the $ottom ofthe pac+ing". This structured pac+ing serves to ma&imi#e contact area $etween the glycoland wet gas.

)at Tray C 'ollects rich glycol, preventing it from passing any lower in the contactor whilstallowing free flow of wet gas in the upward direction. The glycol level a$ove the hat tray iscontrolled to reduce the vapor that might otherwise e&it via the glycol discharge piping.The operating level of glycol a$ove the hat tray is controlled $y ?'*304.

cru$$er ection C / stainless steel mesh pad type mist eliminator serves as an integralscru$$er mounted $elow the J)at TrayK. The wet inlet gas passes through this integralscru$$er mechanically removing fine droplets of entrained condensate from the wet gas.

%as nlet ection C This area includes a $affle to direct gas and coarse entrained dropletsdownward towards the condensate interface. n addition to removal of coarse condensateis intended to provide even distri$ution of gas flow in the scru$$er section.

'ondensate reservoir C 'ondensate collected from the integral scru$$er is collected in thelower head and some section of the lower straight*side volume of the column. The li(uid iscontrolled and a vorte& $rea+er is included in this section to reduce the vapor e&iting fromthe column through the condensate drain. The operating level of this condensate iscontrolled $y ?'*30.

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The heart of the glycol regeneration pac+age is the com$ined operation of the glycolre$oiler vessel !/*E*734", still column !/*E*737", and glycol reflu& condenser !/*B*733". n operation these units maintain a dynamic e(uili$rium $y applying heat to there$oiler and removing heat via the glycol reflu& condenser located in the upper section ofthe still column. Partially dehydrated glycol e&its the re$oiler and enters the strippingcolumn !/*E*73F" where dry gas is used to further reduce the water content of the leanglycol to the specified level.

The glycol re$oiler vessel includes several features as listed $elow.

ndirect heating coils !/*B*734" C )eat is introduced to the glycol regeneration pac+agevia hot oil supplied off s+id. This oil flows through heat transfer coils !/*B*734" located inthe lower section of the re$oiler vessel. The 3:.3 m of area heating coils provides there$oiler duty of 373 +W using hot oil provided from off s+id.

tripping !fuel" gas preheat coils C / heat transfer coil is provided in the re$oiler vessel topreheat the stripping gas prior to introduction into the stripping vessel. This improves theefficiency of the stripping gas $y e(uali#ing the temperature gradient $etween the stripping

gas and the glycol.

The $ase of the still column !/*E*737" C ounted directly to the re$oiler, the $ase of thestill column !/*E*73F" forms an open conduit $etween the re$oiler vessel and the stillcolumn. This conduit allows $i*directional flow of vapor and stripping gas from the re$oilerand condensed li(uid from the condenser section of the still column.

The stripping gas entering the re$oiler from the stripping vessel com$ines with heatedglycol vapor in the re$oiler vessel and rises into the still column.

S+/+535 STI!! C#!*7)

n this unit rich glycol li(uid is introduced and distri$uted immediately a$ove the strippingstage of pac+ing. lowing downward through this pac+ing, in counter*current contact withthe lean glycol vapor and stripping gas, water is stripped from the rich glycol $y the vaporlean vapor phase. /s it e&its the stripping stage, this enriched vapor phase then passesthrough the reflu& collector8distri$utor passing into the enriching stage of pac+ing. n thissection the enriched gas is in contact with reflu& from the overhead condenser coils thatserves to further enrich the water content of the vapor phase.

eatures of the still column are listed $elow from the upper sections to the lower.

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S+%+533 2!S $'*7

The flash drum serves as a three*phase separator. y allowing the light hydrocar$on thatis carried over from the contactor to vapori#e !flash". This flashing effect is a result of thepressure drop ta+en at the glycol level control valve. The second phase to $e recoveredfrom the glycol stream is a hydrocar$on !oil" phase that is s+immed off the surface of theglycol. The glycol floats on the surface of the glycol due to a lower specific gravity. Toachieve this three*phase separation the flash drum allows for a 30 minute retention time of the glycol stream.

eatures of the flash drum are listed $elow from inlet towards outlet end.

nlet ;iverter affle C / plate type $affle to improve the distri$ution of the inlet flow acrossthe area of the flash drum.

+imming ection C The section of the flash drum that receives the glycol, oil and gasphases are first directed $y the inlet diverter $affle. This section will normally have acontinuous hydrocar$on phase on its surface and is continuously s+immed $y the

;emisting Pad C / stainless steel mesh pad type mist eliminator. This functions tominimi#e carryover of condensate droplets that may otherwise e&it the flash drum with thee&iting gas. This ;emisting pad is located a$ove the hydrocar$on s+imming section of the

flash drum so that condensate which may drip from it will $e reintroduced to the s+immingsection.

2il +imming uc+et C /n internal weir $o& to collect the hydrocar$on li(uid that iss+immed from the surface of the glycol. The oil $uc+et will $e located near the glycoloutlet end of the flash drum. The oil s+imming weir side of the oil $uc+et transects thecross*section of the flash drum retaining the accumulated oil layer in the s+imming section. /n open area $eneath the oil $uc+et allows for the flow of glycol from the s+imming sectioninto the still well section.

%lycol till Well C /fter passing $eneath the oil $uc+et, the glycol is forced to pass in

pro&imity to the surface in the %lycol till Well. This section allows for the final escape ofany fine gas $u$$les that may have formed underneath the oil $uc+et. t should $e notedthat the operating level in the glycol still well will $e slightly lower !few millimeters" thanthat on the s+immer side. This is partly due to head loss as the glycol flows $elow the oil$uc+et, $ut more significantly due to the difference in specific gravity in the glycol due togas phase evolution and the presence of an oil pad on top of the s+imming section.

%lycol ?evel et Weir C The operating level in the still well, and s+imming sections is set$y the use of the glycol level set weir. This adLusta$le weir is installed on the uppersection of a wall that completely transects the lower section of the flash drum. Thisre(uires that glycol e&iting the still well pass over top this adLusta$le weir. The setting of

this weir, fi&es the operating level in the flash drum. t is critical that this operating level $eslightly lower than the elevation of the oil $o& weir, this is to prevent glycol e&iting the flashdrum via the oil $uc+et weir.

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%lycol 'ham$er C This cham$er receives the glycol after it has passed over the level setweir. The level in this cham$er is normally maintained 10*30 cm $elow that in the glycolstill well. This is to minimi#e the introduction of gas into the glycol which may occur if alarge fall is associated with the operation of the weir.

Eorte& rea+er C / vorte& $rea+er is located in the glycol cham$er to further reduce theli+elihood of gas carryover from the flash drum.

S+/+53 STI!! #/%'%$ !I;*I$ $'*7

This unit wor+s in conLunction with the overhead condenser to cool the overhead vaporleaving the stripping column to cause the great maLority of the water vapor to condense asli(uid water to allow discharge to drain. The remaining wet overhead gas is sent to theflare system.

eatures of the still overhead li(uid drum are listed $elow from the upper section to thelower.

;emisting Pad C / stainless steel mesh pad type mist eliminator. This functions tominimi#e loss of entrained condensate droplets that may otherwise e&it.

2verhead 'ondensate eservoir C 'ondensate collected from the overhead condenser iscollected in the lower head and some section of the lower straight*side volume of thecolumn. The li(uid is controlled and a vorte& $rea+er is included in this section to reducethe vapor e&iting to the drain. The operating level of this condensate is controlled $y ?M*404.

*<I!!'" *)ITS

S+P+536 G!"C#! P*7PS C These reciprocating motor driven pumps circulate theglycol throughout the %lycol pac+age, including introducing to the top of the contactingcolumn.

S+%+53= !%) G!"C#! C##!%' C This forced air finned type air cooler will lowerthe lean glycol temperature to near the operating temperature in the contactor column.

S+%+535 STI!! #/%'%$S C#)$%)S%' C This forced air finned type air cooler is e(uipped with manually actuated louvers, to provide control of the inlet temperature tothe still overhead condenser drum.

S+%+539 S*'G% $'*7 C The surge drum provides a storage volume and allows forfresh glycol addition to the system. 'om$ined with the surge drum coils !/*B*733" thesurge drum also serves to lower the temperature of the glycol going to the glycol pumps.

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j1036 pr 001 operational theory

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