Debottlenecking of Acid Gas Removal Unita

8/22/2019 Debottlenecking of Acid Gas Removal Unita

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SPESPE 13728Debottlenecking of Acid Gas Removal Unitsby C. de Frai ssi nette,ADGAS; O. F. El Komy, Abu Dhabi Gas Liquefaction Co.; andF. Pi quet,q ADGASSecondedrom TotalCompagnieFranqaiaeesP&roles.

Copyr ight 1985, So3ety of Pet ro leum EngineersT ii is paper was p~&$8iii6d 61 i!k . . ..- ,------- . __- CDC 4 ax M,@a EZ@ 01 T~hn~~l conference and Exhibi ti on held in Bahrain, March 11-14, 1985 . The Material iSsubject to correction by the author. Permission to copy is restricted to an abatract of not more than ~ WOrdS. %%3 ~P=, kx m, !d?eK!Srr%Texas 7508S-3S3S. Telex 7309S9 SEE DAL.

1.0 INTRODUCTION

The Abu Dhabi Gas LiquefactionCompany (ADGAS)hae since 1977 operatedthe only gas liquefa-ction plant in the Middle East producingsimul-taneouslyIJUl,LPG and I?entaaes.&e Plactprocessesthe associatedgases from Abu Dhabioffshoreoil fieldsas well as non associatedgas. This is the only LNG facilityin theworld operatingon associatedgas.

The gas compositionof the 11 differentfeedstreams supplyingthe LNG plant variesbroadlyand for instancethe sour gaa content (C02+H2S)of each streamcan vary between3.3% and 34%.An average gas streamcompositionsuppliedtothe Acid Gas RemovalUnit is given in Table-1.

The Plant consistsof two identicalprocesstrainain parallelarrangedto permit safeoverhaulof one train while the other remainsin service.

Each process train consisteof the followingplant sections:

1. Feed Gas Composition2. Acid Gas Removal3. Dehydration

4. Feed Separation(by primary cooling)~..~f~=~~~Qp-~~~Q~

5. CryogenicCoolingand Liquefaction.

Utilitiesand offaitesfacilitiesof storageand loadingare commonfor both processtraina.

Followingthe commissioningof the plant, onemajor problemidentifiedwas the capacityofthe Acid Gas RemovalPlants. The bottleneckpreventedeither trainfrom matchingsteadilythe LNG design throughputof 150 T/hr. on heavyfeed gas. LNG Plant performancesduring 1981are gatheredin Table-II.

Over a period of 16 monthe a high proportionof the effortof ADGAS TechnicalDepartment---->....-&- -+*,A., of $~ae ~CIJS~S~f theas uevuwd +0 ~F.e-.--=bottleneckand the means by which it could bealleviated.


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2 DEBOTTLENECKI NG OF A

The nroK)OSedobjectivesfor imp??OVeIUentSere.--assignedas follows:

1. Improvethe stabilityof the Plant.2. Match designfor acid gas removal.3. Produce an extra 2.5 T/hr of LNG

(17,500T/year)

2.0 ACID GAS REMOVALPLANT

2.1 The Process (See Fig. 1)

The acid gas removalplant is designedb...-+.~e eb~Orption,/regeneration.as a ..-.-~-

In the first sweeteningstage a promotedhot carbonatesolutionis used to reduceC02 and H2S down to a level of around2000 ppm and 10W PPm respectively. Itis followedby a secondstage usingdiethanolamine (D.E.A)solutionaa theabsorbantwhich reducesresidualC02 andH2S levels to below30 ppm and 2 ppmrespectively.

The carbonateand DEAneratedseparatelyatatmosphericpreaaure.

solutionsare rege-slightlyabove the

The mass transferprocessbetweenliquidand gas phaseais achievedin a packedcolumn. While reviewingour packedtowersperformancethe followingcriteriawere speciallyconsideredwithregardto the Packing and InternalaDesign:

wettingrate (T) expressedas liquidrate (M3/Hr)per towercross sectionarea (M2) times packingexchangearea(M2/M3).

D GAS REMOVAL UNI TS SPE 1

porosity (void fraction)of the bedsupportani hold dob= grid must beequal to that of packing.

the liquid shouldbe evenlydistribin a thin layerover the packing.

the best efficiencyof the packingwbe at around60$ flooding. It willa reasonableflexibilityof between35-70%.

2.2 Analysisof the Problems

The analysisof the problemsassociatewith the operationof the acid gas remunit was hamperedby the lack of equilbrium and kineticdata on the chemicalreactionsinvolved. This lack of datameant that we were forcedto lookmainat the variouaphysicalsynmptomsof mfunctionof the units, such as liquidf~Ed &~~tTi&tiQE, flOQdLngconditionspulsatoryphenomenon,all factorswhicdeterminethe hydraulicsof the CO1-.

2.2.1

The poor performanceof our sweeteninunita experiencedsince commissioningbe characterizedby two featurea.

SteadyOperatingConditions

Under steadyoperatingconditionstheproductionrate of the train was limiby sweeteningunit which was unable tomatch designin term of acid gaa remoThe originaldesignof each train allfor a total sour gas removalrate (H2C02) of 47 t/hr equivalentto 1203Km


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.m. . ..-lna . J- I-...:--:--**- n r rl t...-, 0 c I -J II I eLLe, u. r. EIKUIIIY u r. rlqucL

In practicethe maximum acid gaa removal These causeswere identifiedas being:achievedwas around 92-93%weightof thedesign capacity(heavyfeed gas). feed gas pressurefluctuation

towerhydraulicsInvestigationcarriedout led us to solutionfoamingconcludethat the poor performanceof thecarbonateregeneratorwas responsiblefor As a resultof our investigationwe identi-the plant limitationunder these conditi- fied the reasons for poor performanceasons and the principalcauseswere identi- being:fied as follows:

change of operatingconditionsof theFirstly,the recommendationof the carbonateregenerator,consequentuponprocesslicenser,to maintainin the non-commissioningof the Sulphurthe carbonatesolutiona concentration Pisnt.of DEA around3% could not be met. Itwas found that this difficultyin main- The deteriorationof the ceramic packingtainingDEA concentrationwas due to which occurredat the high temperaturethermaldegradationcausedby high .-a .t~nnnnI~BI~n~ty of the carbonate..-...- ---skin temperaturein the carbonate solution. This mainly occurredat thereboilerexchangers. This high skin bottomof the regeneratorwhere solutiontemperatureresultedfrom insuffi- of the alumina took place,with the freecient condensatecapacityof the silicabeing suspendedin the carbonatedesuperheatingstation. solution,and being depositedfurther,

at a later stage causinga foulinginSecondly,as the regenerationequili- the absorber.brium is a functionof the operatingpressureof the regenerator,lowering - Blockageof distributors,bed supportthe regeneratoroverheadpressure platesand spargepipes leadingtowould improvethe qualityof the car- liquidmal distributionon the beds,bonate solutionregeneration. This liquidhold-up in the bed and foaming.waa confirmedwhen one train waa shut- All these factorscontributedtodown and two sour flare headerswere hydrauiicin tinecokmn.availablefor servicefor the othertrain. The overheadpressureof the Inadequatesizing of the condensateregeneratorthen dropped from 0.65 system of the carbonatereboilersandbarg to 0.5 barg. the ~te~ d~~~perheaterstation.

2.2.2 Plant Instability 3. 0 RECOMMENDATIONFOR IMPROVEMENT

Plant instabilityled to sour gas break- Followingidentificationof the causes for thethroughand therebyoff specificationpro- poor performanceof the plant,ADGAS Technicalduct and/orfoulingof the main cryogenic Departmentmade recommendationsto overcometheexchanger. It usuallynecessitatedthe problems. Also, we drew-upthe detailedprocessoperationof both trainsat reduced specificationsfor the proposedmodifications,throughput substantiallybelow those includingpackingand internalsof the columns,which couldhave been achievedunder as outlinedthereafter.steadyoperatingonditions.

.-9K I0.. .


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.

DEBOTTLENECKI NG OF I

3.1 CarbonateProcessGas Absorber

+ Replacementof the supportplates ofall three beds of the columns,with atype having a free area equ~,=.a~tto-...---the cross-sectionof the column.

+ Replacementof the distributorand~eti~tri~Jto~xitk.their sparges pipeabove the top and middle beds of thecolumn. Special carewas given to thedesignprovidinga more even liquiddistribution.

+ Installationof a redietributorabovethe bottombed.

+ Replacementof the ceramicaaddleainall three beds of the columnwith stain-less eteel packing.

+ Fitting of bed limiterson each bed ofthe column to prevent liftingof thepacking.

3.2 CarbonateRegenerator

+

+

+

Increasingthe size of the 16 sectionof the sour header to 24 in order toreduce the CarbonateRegeneratoropera-ting pressure.

Increasingthe capacityof the steamdesuperheaterstationtosuperheatexperiencedonthe carbonatereboilers.

Replacementof the steam

reduce thethe steam to

strapson thecondensateremovalsystemof thecarbonatereboilerswith level control-led condensatepots and upsizingthe-.-.-.---A-:.,aa+. -F.v.n+ ~Q~~ensateCuuueuuabcALuv. . p....--.1ocking.

D GAS REMOVAL UNI TS SPE

3. 3

All

+

+

+

+

+

+

Replacementof the ceramic saddlesinall four beda of the regeneratorwithstainleaasteelpacking.

Replacementof the distributorwithits spargepipe above the No. 1 bed(top bed).

Installationof redistributoraabovesecondbed which was not ao equippedthe originaldesign.

Replacementof the redistributorabovbeds No. 3 and No. 4.

Replacementof tinebetisupportplatesall four beds of the regeneratorwithtype havinga free area around 100% o+ilebstiCicssSectlcm.

Fitting of hold down grids on eachpackingbed.

DEA System

The same recommendationswere proposedaimplementedin subsequentmodificationsrevampthe DEA absorberand DEA regenerwith regardto the following:

- Liquiddistribution- Packing- Internals

the modificationslistedin the aboveparagraph3 have been implementedduringa TMajor Overhaulat a total cost of US$.1,030,


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4.0 PLANT PERFORMANCEIMPROVEMENT 5.0 CONCLUSIONS

Performanceof the train after implementation Hereafter followsthe main conclusionswhichof recommendedmodificationscarriedout during can be drawn from the above mentionedwork.1983 Train Major Overhaulshow that our object-ives have been achieved,as shown in Table-III. * Metallicpackingmass transferefficiencyis

betterthan ceramicpacking. An increaseofq Operationof the train is now highly stable, 40-60% in the mass transfercoefficientcan be

even at a throughputwell above the design achievedwhen using the proper size of metalliccapacity. packinginsteadof ceramicpacking. This will

~ef~ec~ on the d~ensions of the absorbersand* The design for the acid gas removalC02+H2S the regenerationcolumnswhich could be made

has been exceededby more than 10$. The C02 shorterand smallerin diameterin a new plant.absorptionrate currentlyachievedrepre-sents around40~ above the design. This * Life time of metallicpacking,when selectingremarkableperformancegaugesthe improve- the adequatematerialfor the requiredservice,ment achievedin mass transferefficiency is 2 to 3 times longerthan the life time ofhence c02 reactionwith carbonatesolution the ceramicpacking. In addition,the circu-is the controllingfactorin the absorption lated solutionson the metallicpackingareprocess. clearerand containless suspendedmatter than

those of ceramicpackingwhich reflect on the* Steam consumptionof the sweeteningunits solutionfiltrationcost.

has been reducedconsiderablyper kgmole ofacid gas removed. Around 25% savingin * To overcomethe poorerliquidredistributionsteam consumptionhas been achieved. propertiesof the metallicpacking,it is impe-

rativeto utilize the properdistributorsand* The debottleneckingof the units has refle- redistributorsabove all the packingbeds.

cted on the LNG make both quantitywiseandqualitywise. Dependingon the feed case * Hold down grids and bed supportplatesshould(heavy or light) and increaseup to 20% have a free area equivalentto the cross sect-above the designLNG make haa been achieved. ional area of the column.Qualitywiaethe aulphurcontentof the finalproductshaa dropped to 50% from the levels * Unit ancillariessuch as steam desuperheaters,beforemodifications. condensatedi8posalsystemsend sour gas dis-

posal systemsshouldbe carefullyconsidered* Economically,the modification have increa- and checkedto match the best performance

sed the performanceof the Plant. With some requirements.extra maintenancecost and no increaseincapital or runningcosts, the plant produ- ACKNOWLEDGEMENTSctionhas increasedmaterially.

The Authora thankADGAS GeneralManagementfor permissionto publish this paper.

.07.JO/


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.

TABLS-IGASSTBEAUOHPOSITIO?i

(swPLYOBVJIBTENIKCRIT)TABLE - II

1981 LRG PLANT PssPorWscE(INMf{HR)

Before 1983Overhaul

After 19S3Overhaul

Component (Mole%)TRIJWI TRA

[earlyverageRGthroughput:baaed on operating the) 134.6 Uwxi mumhroughput ~& ,c(dailykSiS) i L~GroductionLosses 2.36 2.due to Acid Gas breakthrough(basedon O@ratiW time)

LlfGDesign throughput 150

Acid Gas Removal2esign 47[s2s+C021mtium unitperformance~

3 .OQ4.887.ss

0.2164.9211.52-/.933.603.480.4692.2.2

100 .0+3

2.926.149.06

H2SC02

S/Total

0.1468.6711.996.512.620.690.3290.94

N2c1C2C3C4C5c6+

S/Total

100.00TOTAL

TABLE - III

ACID GASES RSNOVAL UNIT PSRFOSHASCE

,fter983nwrhaul

0.230

0.4820.712

De8ign

0.24s

~.~~~

o.48a

0.037

0.2900.327

kfore 19S3)verhaul

o. l s2

Q. ZQ2.0.474

0.074

0.191

0.265

:arbonateProcessMS Absorber{2s LoadinsKg mole/Kg mole of ~co3

S/Total

DEA Absorbers2S LoadinsKg mole/Kgmole of DSA 0.101

0.225

0.326

C02 Loading5g mole/Kg mole of DEA

S/Total

SteamConsumption(Reboilersu of steam/KgmoleOf acidcm

MC [email protected] IIT/hr

55.9 73.2

155

45.2

17550


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Debottlenecking of Acid Gas Removal Unita

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