Amine Treating Best Operating Practices Resource Guide Searchable Version

8/19/2019 Amine Treating Best Operating Practices Resource Guide Searchable Version

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MINE TREATING BEST OPER TING PRACTICES RESOURCE GUIDE

;TABLE O CONTENTS

Page1 0 INTRODUCTION

1 1 Purpose1 2 O b j e c t i v e1 3 Scope

1 1

1 1

1 2

2 0 AMINE TREATING

3 0

2 1 P r o c e s s D e s c r i p t i o n • •2 2 O p e r a t i n g P a r a m e t e r s2 3 P r o c e s s C o n t r o l • •2 4 Equipment • • • • •

SOLVENT CHARACTERISTICS3 1 Amines . . · · · · · · · · ·2 Per fo rmance

· · ··

· · · · · ·3 3 S o l u t i o n Q u a l i t y · · · · · · · ·3 4 Amine D e g r a d a t i o n ·3 5 Heat S t a b l e S a l t s · · ·3 6 Foaming · · · ·7 Losses . . · · · · · · ·8 Makeup Wate r · ·3 9 F i l t r a t i o n · · · · ·1 0 Rec la iming · · · · · · · ·

• • • • • • 2 2· 2 9

· 2 1 5• • • 2 2 2

· · · · 3 1

· · · · · · · · · ·3 2

· · 3 5· · · · · · · · 3 7· · · 3 1 0· · · · · · · 3 1 3· · · · · · · · · 3 1 4· · · · · · · · · 3 1 8· · · · · · · 3 1 8· · · · · · · 3 2 14 0 CORROSION CONTROL

5 0

4 1 C o r r o s i o n i n Amine T r e a t i n g P l a n t s • 4 14 2 I n f l u e n c e o f Design and O p e r a t i n g Va r i a b l e s • 4 64 3 Use o f A l l o y Equipment • • 4 84 4 C o r r o s i o n I n h i b i t o r s • • • 4 1 04 5 C o r r o s i o n M o n i t o r i n g • 4 11

MONITORING PLANT PERFORMANCE5 1 Per fo rmance Goa l s · · · · · · · · · 5 15 2 Recommended P r a c t i c e · . · . . · · · · · · · · · 5 15 3 Sampl ing and A n a l y s i s · · · · · · · · . . · 5 75 4 M o n i t o r i n g Trends . . · . · 5 7

6 0 ENVIRONMENTAL ISSUES6 1 R e f i n e r y Fue l Gas S u l f u r C o n t e n t6 2 D i s p o s a l o f Wastes from Amine u n i t s

7 0 ADDING CAPACITY TO AN AMINE UNIT

· 6 1· 6 2

7 1 Dete rmin ing t h e C a p a c i t y o f an Amine u n i t 7 17 2 Temporary O p e r a t i o n a t E l e v a t e d Rich Amine Load ings 7 57 3 D e b o t t l e n e c k i n g an Amine U n i t 7 67 4 S w i t c h i n g from DEA o r MEA t o MDEA 7 9

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MINE TRE TING BEST OPERATING PRACTICES RESOURCE GUIDE

8 . 0 PROCESS SIMULATION8 . 1 I n t r o d u c t i o n •8 . 2 How t o C r e a t e a S i m u l a t i o n8 . 3 TSWEET . . . . • • .8 . 4 AMSIM o r HYSIM

9 . 0 TROUBLESHOOTING

. . . • . • • . . . . . . 8 . 1o f an E x i s t i n g Amine u n i t 8 . 2

. • . . . . . . . 8 3• • • • • • • • • • • • • 8 . 4

10 0 HYDROCARBON/PARTICULATE REMOVAL EST PRACTICE

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MINE TRE TING BEST OPER TING PRACTICES RESOURCE GUIDE

1 . 0 INTRODUCTION

1 . 1 PUrpose

The purpose of the Amine Trea t ing Best opera t ing P r a c t i c e sResource Guide i s to a s s i s t in t he achievement o f e f f e c t i v er e l i a b l e economical, and envi ronmenta l ly sound opera t ion o famine u n i t s .

The guide addresses the removal o f H2S and C02 (or ac id gases )from e i t h e r gas o r LPG s t reams, and i s app l i cab le t o amine u n i t sin r e f i n e r i e s as wel l a s t hose in upstream f a c i l i t i e s . Ingenera l , t he guide emphasizes performance improvement andt roub leshoo t ing over des ign , and i s meant t o complement b e s tp r a c t i c e guides developed by t he r e f i n e r i e s . .

The guide does not cover processes for ac id gas removal us ing

phys ica l so lven t s , hybr id so lven t s , c a u s t i c or so rben t s . Removalo f t r a c e amounts o f s u l f u r compounds inc luding H2S COSmercaptan and CS2 a r e covered in a sepa ra t e guide .

1 . 2 Objec t ive

The ob jec t ive o f the guide i s t o help f ind t h e b e s t opera t ingp r a c t i c e s f o r a p a r t i c u l a r amine u n i t . The d i s c u s s i o n covers t heopera t ing p r i n c i p l e s o f amine u n i t s common problems t h a t occur,techniques used t o recover from opera t ing problems and s t r a t e g i e st o avoid problems in t he fu tu re .

The i npu t t o t he guide comes from a v a r i e t y of Chevron andi ndus t ry exper ience . Because of t he wide range o f cond i t ionsupon which t h a t exper ience i s based, t i s not unusual t o f indcon t rad ic t ions among t he var ious p r a c t i c e s t h a t have beenpresc r ibed t o opera te o r f i x amine u n i t s . There a r e few abso lu te

bes t p r a c t i c e s t h a t can be appl ied r igo rous ly t o a l l aminep l a n t s . s t r i c t adherence t o a s tandard l i s t o f parameters i s notnecessa r i ly the b e s t p r a c t i c e . General r u l e s can sometimes bebroken without se r ious consequence. Each amine u n i t has i t s ownt o l e rance fo r abnormal cond i t ions which can only be gaugedaccura te ly through c o n s i s t e n t monitor ing and a t t e n t i o n .

On t he o t h e r hand t h e r e a re some bas ic p r i n c i p l e s t h a t wel l run

amine u n i t s fol low. What i s presented here i s a guide (orprocess) t h a t ~ n be used t o eva lua te the performance o f amineu n i t s and s e t opera t ing parameters accordingly. A major i n t e n tof the guide i s to help determine what s t eps should be t aken t od i r e c t i o n a l l y improve performance.

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The key t o b e s t opera t ing p r a c t i c e for amine u n i t s i s t o avoidcond i t ions which s t r e s s t he u n i t ; and when problems do develop

t i s b e s t i n the long run t o t r e a t t he source o f t h e problemr a t h e r than the symptom

1.3 Scope

The fol lowing a r e inc luded wi th in t he scope o f t h e guide:

• The process flow, parameters and equipment o f amine u n i t s

• C h a r a c t e r i s t i c s of amine so lven t s ,

• Typica l opera t ing problems (such as foaming and corros ion)and how t o minimize them,

• The use and i n t e r p r e t a t i o n o f a n a l y t i c a l and opera t ing

d a t a f o r day-to-day monitoring, t roub leshoo t ing andq u a l i t y con t ro l ,

• Environmental concerns,

• Use o f s imula to r s to eva lua te and/or tune t h e u n i t

• Considera t ions fo r i nc reas ing p l a n t capac i ty.

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2 .0 AMINE TREATING

Amine t r e a t i n g or sweetening) i s used in a wide v a r i e t y ofa p p l i c a t i o n s to remove H2S and C02 from gases and l i g h thydrocarbon l i q u i d s . The purpose of t he process i s t o reduce t heconcen t ra t ion o f the ac id gas contaminants s u f f i c i e n t l y t o meetproduct s a l e s f lue gas emiss ions or o the r process requirements .

The process has changed very l i t t l e over the many yea r s t hasbeen in use . Over t he p a s t decade, however, the gener i calkanolamines such as MEA DGA DEA and DIPA have been j o ined byMDEA and propr ie ta ry so lven t s in a move toward i nc reased energye f f i c i e n c y, reduced cor ros ion r a t e s and enhanced performance.

Gas and l i q u i d feed streams a re t r e a t e d fundamental ly the same,except f o r some d i ffe rences in t he absorber des ign and somes p e c i f i c opera t ing cons ide ra t ions which w i l l be i d e n t i f i e d l a t e r .

For s i m p l i c i t y o f presen ta t ion , t he process w i l l be descr ibed interms o f gas t r e a t i n g which i s more common

The major i ty of opera t ing problems in amine p l a n t s have t o dowith the f a c t t h a t the so lven t undergoes cont inuous thermalcyc l ing in a c losed loop, and t h a t feed gas contaminants anddegradat ion products formed dur ing opera t ion can bui ld up in theso lven t and l ead t o foaming, l i q u i d car ryover, o ff - s p e c product ,cor ros ion and e ros ion . These condi t ions can be aggrava ted byopera t ing t he u n i t ou t s ide the recommended ranges o f t empera ture ,amine loading , amine concen t ra t ion , feed composi t ion and f l u i dv e l o c i t i e s . Inhe ren t des ign d e t a i l s and m a t e r i a l s s e l e c t i o n a l s op lay a r o l e here . Understanding the process p r i n c i p l e s i s thef i r s t s t e p i n achieving b e s t opera t ing p r a c t i c e s fo r amine u n i t s .

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2 .1 Process Descr ip t ion

The process i s based on t he simultaneous d i f f u s i o n of ac id gasesi n t o and ou t o f t he aqueous so lven t phase and t h e r e v e r s i b l echemical r e a c t i o n s t h a t t ake p lace between t he amine and ac id gascomponents. These r eac t ions and o t h e r c h a r a c t e r i s t i c s of t hed i f f e r e n t amines a r e discussed in s e c t i o n 3 .0 .

The choice o f amine has an i n f luence on p l a n t economics ( e .g .c a p i t a l , so lven t and energy costs ) and t he a b i l i t y t o achieve t h er equ i red t r e a t e d gas s p e c i f i c a t i o n s . The process flow i sconceptua l ly t h e same r ega rd less o f t h e type o f amine used.

The two main process ing s t e p s in amine u n i t s a r e :

i ) Chemical Absorpt ion. Feed gas (or sour gas) i scontac ted with l ean amine in an absorber column t o removet he ac id gases . The t r e a t e d gas (or sweet gas) l eaves t h etop of the absorber. The ob jec t ive may be:

simul aneous removal o f H2S and C02 t o low l e v e l si . e . 4ppmv H2S and 100 ppmv C02 f o r gas p l a n t s o r

l e s s than 160 ppmv H2S with no concern f o r C02 removalf o r r e f i n e r i e s ) ,

• bulk removal o f C02 i e . from, 4 mol t o 2 mol t omeet s a l e s gas s p e c i f i c a t i o n ) o r

• s e l e c t i v e removal of H2S t o low l e v e l s whi les l ipp ing C02 i . e . reduce H2S t o 4 ppmv and C02 from,

say, 4 mol t o 2 mol f o r gas p l a n t s o r l e s s than 160ppmv H2S with minimum C02 absorpt ion f o r r e f i n e r i e s ) .

i i ) Solvent Regenerat ion. The r i ch amine conta in ing t h eac id gases i s regenera ted by s t r i p p i n g them out in ad i s t i l l a t i o n column. The ac id gases a r e recovered from t h er egenera to r overhead and t y p i c a l l y s e n t t o a s u l f u r recoveryu n i t SRU) o r i n c i n e r a t o r. I f only C02 i s being removed t h er egenera to r gas i s vented. The ob jec t ive i s t o :

MINE TRE TING

reduce t he res idua l loading of the l ean amine so t h a tt h e ac id gas equi l ibr ium vapor composi t ion i s equal t oo r l e s s than t he s p e c i f i e d composi t ion o f H2S and C02in t h e , t r e a t e d gas .

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Typica l Amine Process Flow

A t y p i c a l flow shee t i s shown in Figure 2 .1

Absorber Feed s t reams

Depending on t h e cond i t ion o f t h e sour gas , t may be necessaryt o prepare t f o r t he absorber by f i r s t pass ing t th rough aknockout drum, cen t r i fuga l separa to r and /o r f i l t e r s e p a r a t o r t oremove l i q u i d ca r ryover, condensed l iqu ids and p r t i c u l t e s whichcan cause foaming in t he absorber and regenera tor. In r e f i n e r i e sa water wash column may preceed t h e absorber t o remove organicac ids .

The clean sour gas passes i n t o t h e lower s e c t i o n o f t h eabsorber, up t h e column through con tac t ing t r a y s o r packing andou t t h e t op (water sa tu ra ted) . Pressu r i zed lean amine s o l u t i o n

en te r s t the t op of t h e absorber t 100-140 deg F and flows downcontact ing t h e gas coun te r-cu r ren t ly.

The lean amine should be 10-15 deg F h o t t e r than the feed gas toensure t h t no hydrocarbons a r e condensed and t h t any mis tc a r r i e d over from t he knockout drum w i l l be vapor i zed . In verysour s t reams . t h i s t empera ture di ffe rence may have t o be i nc reasedt o account fo r the dew poin t e l eva t ion caused by removal o f t heacid gas . The upper l i m i t on lean amine t empera ture f o re f f e c t i v e absorp t ion i s 140 deg F while t he upper l i m i t o f feedgas t empera ture i s 120 deg F.

The amine absorbs t he ac id gases and l eaves t he bottom o f t h eabsorber. Small amounts of hydrocarbon gases are dissolved i n t othe amine, as wel l . Ref inery gases of ten con ta in ammonia Someammonia w i l l be absorbed in the amine.

Absorber Out le t s

The s t reams l eav ing t he absorber a r e h o t t e r than t h e feed s treamsdue t o t h e exothermic hea t s o f r e a c t i o n from absorbing t h e acidgases . Trea ted gas from t he overhead of t he absorber passesthrough a knockout drum and/or water wash to remove and recoveren t ra ined amine.

Rich amine from t he absorber bottoms i s l e t down through t heabsorber l e v e l con t ro l va lve t o a f l a s h drum.

Rich Amine

The f l a s h drum provides r e s idence t ime t o sepa ra te amine froml i q u i d hydrocarbon phase ( r e s u l t i n g from ent ra inment o rcondensa t ion in t he absorber) and re leases l i g h t hydrocarbon



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vapors and small q ua n t i t i e s o f acid gases from the r i ch amine.In some un i t s the f l a sh gas may pass through a smal l amineabsorber mounted on top of the f l a sh drum so t h a t the gas may beused as fue l . Evolut ion of l i g h t hydrocarbons i s impor tant pr io rt o amine regenera t ion to avoid foaming and sending excess ivehydrocarbons along wi th acid gases to the SRU Some p l an t s aredes igned wi th r i ch amine f i l t r a t i o n a t the f l a sh drum l i qu ido u t l e t t o remove pa r t i cu l a t e s car r i ed in to the system with thefeed gas. However, f i l t r a t i o n i s t yp i ca l l y ca r r i ed out on thelean amine s ide where exposure to H2S i s minimized. F i l t r a t i o ncan be done a t e i t h e r loca t ion . Refer t o sec t ions 2.4 and 3.9f o r f u r t h e r discuss ion.

Next, the r i c h amine flows through the tube s ide o f the l e an / r i ch. exchanger t o recover hea t from the regenera tor bottoms stream.

The hot r i c h temperature i s dependent on the phys ica lc h a r a c t e r i s t i c s o f the exchanger, so lven t proper t i e s and so lven t

flow r a t e I t i s poss ib le fo r a clean o r new exchanger todevelop hot r i c h tempera tures higher than des ign . The hot r i chamine within the exchanger and in the exchanger o u t l e t piping i sconducive to corros ion and erosion pa r t i cu l a r l y when the r i c hloading i s high and the hot r i ch t empera ture exceeds about 215deg F. At such condi t ions H2S and C02 can break out of so lu t ioni n t o the vapor phase in amounts s u f f i c i e n t to cause severecorros ion problems.

In un i t s where the f l a sh drum l eve l con t ro l valve i s located a tthe L/R exchanger o u t l e t and the re i s no r i ch amine pump thef lash drum pressure inf luences the amount of vapor iza t ion t h a toccurs within the exchanger. Some na tura l gas p l an t s aredesigned with a high pressure opera t ing mode (150-200 psig) fo rthe f lash drum which has the e f f e c t o f l imi t ing the evolut ion ofac id gas vapor in the l e a n / r i ch exchanger, thus minimizingeros ion and corros ion wi th in the exchanger, o u t l e t piping andf i t t i n g s The hot amine then f l a shes across t h e f l a sh drum l eve lcon t ro l valve and flows a shor t dis tance to the regenera to ri n l e t However, as a genera l r u l e the f l a sh drum i s t yp i ca l l ys e t a t t h e lowes t prac t i ca l opera t ing pressure t o minimizehydrocarbon content of the amine going i n t o the regenera to r andon t o the SRU

Amine Regenerat ion

The r i c h amine from the f l a sh drum i s preheated by pass ingthrough the l e a n / r i c h exchanger and fed to the top o f theregenera tor. The regenera tor t yp i ca l l y opera tes a t low pressu re(10-20 psig) and elevated bottoms t empera ture (230-260 deg F) ,condi t ions which favor the desorpt ion of the ac id gases . Theamine flows down through the t rays as the acid gases are s t r ippedout by steam pass ing up the column. Steam i s genera ted from the

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water in t he amine s o l u t i o n on t he s h e l l s ide o f the r e b o i l e r .I t i s d e s i r a b l e t o opera te t h e r e b o i l e r a t as low a t empera tureas poss ib le whi le mainta in ing t he necessary hea t f lux 6-8Mbtu /h r / sq f t ) . 50 ps ig steam, t y p i c a l l y 1 pound o f steam perga l lon o f c i r c u l a t i n g so lven t , i s t h e usual hea t medium used inamine r e b o i l e r s . Hot o i l loops and d i r e c t f i r e d h e a t e r s can a l s obe used. Excessive r e b o i l e r tube sk in t empera tures above 320-350 deg F) cause increased r a t e s of thermal degradat ion o f t h eamine and should be avoided.

u n i t s us ing primary amines l i k e MEA and DGA a re o f t en des ignedwi th a r ec l a imer which t akes a s ides t ream from t h e r egenera to rbottoms equal t o 1-3 of t h e amine c i r c u l a t i o n r a t e . Afte radding c a u s t i c t o conver t amine s a l t s t o sodium s a l t s t h er ec l a imer b o i l s o f f an amine/water mixture in a batch opera t ionleav ing behind degradat ion products , suspended s o l i d s , ac ids andi ron compounds. Reclaimers a r e e s s e n t i a l in DGA p l a n t s t o

r eve r se degradat ion caused by cos . Reclaimers a r e not usua l lyprovided for DEA and MDEA u n i t s because t i s assumed degradat ioni s l e s s severe than f o r primary amines, and because they havelower v o l a t i l i t i e s which would mean opera t ion of t h e r ec l a imerunder vacuum r equ i r ing g r e a t e r expense and opera to r a t t e n t i o n .o t h e r methods of rec la iming a r e discussed in s e c t i o n 3.10 .

Regenera tor overhead vapor c o n s i s t s of ac id gases , ammonia inr e f i n e r i e s and water vapor. Most o f t h e water i s condensed int h e r egenera to r overhead condenser and c o l l e c t e d in t he r e f l u xdrum. Some ammonia w i l l form ammonium biSUlf ide , so lub le in t h ewater. The sour water from t h e r e f l u x drum i s r e tu rned t o t h et op o f t h e r egenera to r and t h e ac id gases a re s e n t t o t h e SRUi n c i n e r a t o r or ven t .

Ammonia Scrubber

In r e f i n e r i e s t i s common t o water wash t he offgas in a packedscrubber column t o remove ammonia befo re rou t ing t he ac id gas t ot h e SRU. This process ing s t e p minimizes t h e p r e c i p i t a t i o n o fammonium b i s u l f i d e in t h e l i n e t o t h e SRU.

Makeup water

A por t ion o f t he sour water may be purged from t h e r e f l u x drum t o

provide an o u t l e t fo r contaminants and t o con t ro l ammoniumb i s u l f i d e concen t ra t ion . The purge r a t e should be ad jus tedaccord ing t o r egu la r t e s t r e s u l t s f o r ammonium b i s u l f i d e con ten to f t h e sour water. Makeup water o r s t eam. i s added t o t h e so lven tinventory a s r equ i red t o mainta in t h e water balance see s e c t i o n3 . 8 .

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Lean Amine

Lean amine from t h e r egenera to r bottom i s cooled in the l e a n / r i chexchanger and pumped through an a i r o r water cooled exchanger.T h e n t h e amine passes through a p a r t i c u l a t e f i l t e r pre fe rab lyf u l l flow) t o remove i ron su l f i de sca le , i ron carbonatep r e c i p i t a t e from cor ros ion) , i n so lub le degradat ion products , ando t h e r p a r t i c l e s . In p l a n t s where f u l l flow f i l t r a t i o n i s no tava i lab le and t h e system i s r e l a t i v e l y clean, f i l t r a t i o n o f a 25s l ips t ream may be s u f f i c i e n t . A 5 o r 10 micron abso lu te f i l t e ri s t y p i c a l l y used. Backwash f i l t e r i n g systems may requ i readd i t iona l t ankage . F i l t e r s may be located on t h e r i c h o r leanamine, o r both .

A 10-20 s l ips t ream of cooled lean amine may be t r e a t e d bya c t i v a t e d carbon t o adsorb hydrocarbons which would otherwiseb u i l d up in the amine so lu t ion and cause foaming see s e c t i o n

3 .9 ) . The carbon bed may be fol lowed by a second p a r t i c u l a t ef i l t e r t o catch en t ra ined carbon f ines .

Amine Storage/Surge

Some un i t s have a s to rage / su rge tank which provides volume t oaccommodate swings in amine inventory and l i qu id l e v e l s .Otherwise, surge capac i ty i s provided in the bottom s e c t i o n o fthe absorber, r egenera to r and /o r f l a s h drum. Concentrated aminefo r make-up may be s t o r e d in a tank. Both tanks are blanketedwith fue l gas o r n i t rogen . Air should be excluded from thes to rage tanks t o prevent oxygen degradat ion o f the amine whichcan cause increased l e v e l s of precur so r s to cor ros ive compounds.Fina l ly, the lean amine i s pumped back t o the· t op o f t h eabsorber •

. Chemical I n j e c t i o n

Prov i s ions are usua l ly made t o add antifoam see sec t ion 3.6) andsometimes corros ion i n h i b i t o r s see sec t ion 4.4) and c a u s t i c seesec t ion 3.5) t o t h e lean amine by in jec t ion pump o r sho t pot .

General

Skim nozzles a r e o f t e n provided in the absorber, r egenera to r,

f l a s h drum and r e f l u x drum t o skim any hydrocarbon l a y e r t h a t may• accumulate . A more e f f e c t i v e means of hydrocarbon removal in t h ef l a sh drum i s by means o f t h ree -phase sepa ra t ion with a weir andan overf low sump a t one end f o r removal o f t h e hydrocarbon phase.

s p l i t Flow

n a l t e r n a t i v e des ign shown in Figure 2.2 c i r c u l a t e s a semilean



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amine in add i t ion t o t he usual l ean amine to minimize t heopera t ing c o s t o f amine r egene ra t ion . The semi - l ean amine i swithdrawn from t h e middle of t he r egenera to r and fed t o t h emiddle of t he absorber where acid gas concentra t ion i s high . Thelean amine from t h e bottom o f t he regenera to r i s r e se rved fo rcontact ing t h e gas in t he upper p a r t o f t he absorber. Less leanamine i s needed t o achieve t he same t r e a t e d gas s p e c i f i c a t i o n s ,thus l e s s energy i s expended i n t h e r e b o i l e r . However, t h e semilean s t ream r e q u i r e s sepa ra te pumps and exchangers .

Selec t ive B2S Removal

S ign i f i can t energy savings can be made by s e l e c t i v e l y removingH2S from a feed gas which a l so con ta ins C02. I f C02 removal i sno t r equ i red then MDEA o r p r o p r i e t a r y so lven t s such as UCARSOLHS-10l o r Flexsorb SE a re a v a i l a b l e which al low 70 o r more oft h e C02 to s l i p through t he absorber.

Also, s l i p p i n g C02 may be d e s i r a b l e in some cases from t h es tandpoint t h a t C02 ac t s as a d i l u e n t and t akes up space inSul fu r Recovery u n i t s .

When C02 removal i s minimized t i s poss ib le t o r e a l i z e bene·f i tsin terms of inc reased throughput capac i ty, lower aminec i r c u l a t i o n r a t e s , improved compliance with environmentalregu la t ions and lower r egene ra t ion energy. Propr ie ta ry s e l e c t i v eso lven t s a re more expensive t han the primary and secondaryamines, however.

The process t akes advantage of t h e slower r e a c t i o n r a t e o f C02

vs . H2S with t e r t i a r y amines. The absorber may be des igned withfewer s t ages 12-15) t han normal 20-25) o r with mul t ip le l eanamine feed p o i n t s to ad jus t t he so lven t r e s idence t ime t o achievet h e des i red amount of s l i p . Too few s t ages in t h e abso rbe r couldr i s k H2S breakthrough.

Bulk C02 Removal

Bulk C02 removal i s of ten used t o meet p i p e l i n e s a l e s gass p e c i f i c a t i o n s when complete C02 removal i s not required . Forexample, MDEA can reduce C02 from 4 t o 2 by s l i p p i n g h a l f t h eC02 through t h e absorber. Bulk C02 removal requ i res s i m i l a rcons ide ra t ions as descr ibed above f o r s e l e c t i v e H2S removal .

A l t e r n a t i v e l y, in t h e above example, t h e same e f f e c t can beachieved by feeding h a l f t he gas t o a DEA absorber andrecombining with t h e un t rea ted h a l f .



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iquid Tr e a t i n g

Light hydrocarbon f r a c t i o n s o r LPG can be sweetened n s i m i l a rfash ion t o gas . In t he absorber LPG becomes t he d i s p e r s e d phaset r a v e l i n g up through t he continuous amine phase with i n t e r f a c econ t ro l a t t he t op of the absorber. The major i ty o fl i q u i d / l i q u i d t r e a t e r s employ packed absorbers a l though t r ayedabsorbers a re a l s o used. Because of the small dens i ty d i ff e rencebetween phases and s o l u b i l i t y o f amine n LPG managing aminel o s s e s s a major opera t ing concern. Liquid t r e a t e r s t end t o bet h e l a r g e s t source of so lven t l o s s e s n r e f i n e r i e s . Due t o t hesmal l dens i ty d i f f e r e n c e between t h e phases tower des ign e r r o r sand poor opera t ing cond i t ions have a tremendous impact on l o s s e s .In p r a c t i c e so lven t ent ra inment n t h e hydrocarbon phase s f a rmore s i g n i f i c a n t than amine s o l u b i l i t y.

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AMINE TREATING BEST OPERATINGPRACTICES RESOURCE GUIDE

2.2 Opera t inq Parameters

The o b j e c t i v e s of an amine u n i t are to produce a t r e a t e d gas t h a tmeets t h e required r e s i d u a l ac id gas s p e c i f i c a t i o n s and t oprov ide a c lean ac id gas from the regenera tor overhead. I d e a l l y,t hese objec t ives a re t o be achieved wi th h igh r e l i a b i l i t y andopt imal opera t ing c o s t s . To t h i s end the re a re var ious keyparameters t h a t have t o be cons idered .

Key opera t ing Parameters

The key opera t ing parameters t o consider in an amine u n i t a re :

Feed Gas: Flow RateI n l e t Temperature 80-120 deg FI n l e t Pressure 100-1000+ psig)Hydrocarbon Dew Poin t

H2S,C 2

Concent ra t ion o r P a r t i a lPressures 1-50+ psia)

Acid Gas: Out le t Pressure 10-20 ps ig)Out le t Temperature 100-130 deg F

Amine: Lean Temperature 100-140 deg FCircu la t ion RateAmine Concentrat ion wt )Amine degradat ion

Regenera tor : Rebo i l e r steam Rate 0 .9 -1 .2 l b s tm/gal )Reflux Rat io 1.5-4 .0 moles wate r

r e tu rned t o r egenera to r pe r mole ac idgas from r e f l u x drum

Flash Drum: Pressure 2-150 ps ig)

Contaminant Removal:Entra ined o r d i s so lved hydrocarbonPar t i cu la tesTrace s u l f u r speciesOrganic ac ids

Parameters In f luenc ing Acid Gas Concent ra t ion o f t he Trea ted Gas

• At a p a r t i c u l a r. s e t o f feed gas and c i r c u l a t i o n r a t e s the ac idgas concentra t ion o f the t r e a t e d gas leaving t h e absorber i s mostd i r e c t l y impacted by th ree process v a r i a b l e s :

• Lean amine t empera ture a t top t r a y• Lean amine loading• Amine c i r c u l a t i o n r a t e

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MINE TREATING BEST OPERATING PRACTICES RESOURCE GUIDE

Lean amine t empera ture i s cons t r a ined by t he ava i lab le coo l ingmedium t empera ture and a d i f f e r e n t i a l above t he i n l e t gastempera ture . The lean loading i s a funct ion o f r e b o i l e r dutywhich can vary wi th in l i m i t s o f t h e equipment . The lean aminec i r c u l a t i o n r a t e may be ad jus tab le t o some degree and determinest h e r i c h amine l oad ing . As an example, t he fo l lowing t a b l e showst he s e n s i t i v i t y o f the t r e a t e d gas t o l ean loading and l eant empera ture us ing a b a s i s o f 30 wt DEA assumed t o be a tequ i l ib r ium with t he t r e a t e d gas a t t he t op o f t h e absorber :

30 wt DEA i n Equi l ibr ium with B2Sa t Typical Absorber Overhead Condi t ions

Pressu re Lean Loadinq PPMv B2S a t Temp.(Psia) (Mols B2s/mol Amine) 1 0 0 e F 1 2 5 e F

500 .01 0.9 1 . 5

250 .01 1 .8 3.0100 .01 4 .5 7 .5

500 .02 3.3 6.6250 .02 6.6 13.2100 .02 16.5 33.0

The t a b l e above shows t h a t a 25 deg F increase in lean aminet empera ture on t h e top t r a y i nc reases the p a r t i a l pressu re (and,the re fore , concent ra t ion) o f H2S in t h e t r e a t e d gas by 50-100 .The e f f e c t s of t o t a l pressure and l ean loading are a l s o shown.Simula tor p r e d i c t i o n s o f equ i l ib r ium concen t ra t ions i n t h e lowppm range o f t e n d i f f e r from t h e a c t u a l va lues so t hey should beused with cau t ion .

Equi l ibr ium Curves

Figures 2 .3 ,4 & 5 show examples o f equi l ibr ium curves fo r MEA,DEA and MDEA. These curves r ep resen t p a r t i a l pressu re o f H2S o rC02 p l o t t e d a g a i n s t amine l oad ing . They provide a graph ica lmeans o f es t ima t ing the upper l i m i t s o f c a p a b i l i t y of an amineu n i t I t would be necessary t o p l o t a family o f curves t o looka t p a r t i a l pressu res when both H2S and C02 a r e p r e s e n t . Thesecurves were genera ted us ing t he AMSIM program.

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Approach to Equi l ibr ium

Amine absorbers in HzS removal s e r v i c e t end t o have more thanenough s t ages so t h a t v i r t u a l l y 100 approach t o equi l ibr iumbetween t he vapor and l i q u i d i s normally achieved in the uppers e c t i o n . For HzS removal t i s usua l ly d e s i r a b l e t o opera te int h e l ean end pinch mode such t h a t t he opera t ing curve andequ i l ib r ium curve converge somewhere in t h e upper t h i r d o f t heabsorber. This means t h a t t he p a r t i a l pressu re o f HzS i n t h et r e a t e d gas (and thus , t h e p a r t s per mi l l ion HzS ) very nea r lyapproaches the equ i l ib r ium p a r t i a l p r e s s u r e o f HzS over t h e l eanamine; t h e HzS equi l ibr ium p a r t i a l p r e s s u r e i s determined by t hedegree o f s t r i p p i n g in the r egene ra to r. A thorough d i s c u s s i o n oft h i s t o p i c i s a v a i l a b l e in References 2.1 and 2 .2 .

C 2 i s l e s s l i k e l y t o reach complete equ i l ib r ium with the l eanamine due t o slower r e a c t i o n k i n e t i c s depending on t h e s p e c i f i c

amine. (Reac t iv i ty decreases going from the primary amines, MEAand DGA, t o t e r t i a r y amine, MDEA). Addi t ional s t a g e s and highwei r s (4 inches) may be employed t o maximize C 2 removal . Fewers t a g e s and lower weirs may be employed t o minimize C 2 removalwhen us ing MDEA o r p r o p r i e t a r y so lven t s designed t o s l i p C 2i n t e n t i o n a l l y

As impl ied above, the res idua l ac id gas content o f the t r e a t e dgas can be expressed in terms o f approach t o equi l ibr ium o rATE. I t i s use fu l t o be f a m i l i a r with the ATE concept wheneva lua t ing amine u n i t capac i ty and performance. ATE a t t op t r a ycond i t ions i s :

(measured l ean loading i n so lven t t o the absorber> x 100( lean loading in equi l ibr ium with t r e a t e d gas)

The equi l ibr ium loading can be obta ined from a s imula to r programo r from t h e so lven t vendor. An unusually low ATE a t t h e t op oft h e absorber i n d i c a t e s H2S breakthrough o r C 2 s l i p due t oi n s u f f i c i e n t ac id gas pickup capac i ty in the lower s e c t i o n of thecolumn. ATE a t bottom t r a y condi t ions i s :

(measured r i c h loading in so lven t from t he absorber> x 100( r i ch loading in equi l ibr ium wi th feed gas)

Most o f the mass t r a n s f e r and chemical r e a c t i o n takes p lace in• t h e lower p a r t Of the absorber. Typica l ly, an absorber i sdes igned fo r 70 o r l e s s ATE (not 100%) a t the bottom f o r t h efol lowing. reasons :

• t o ensure adequate dr iv ing force fo r mass t r a n s f e r and k i n e t i c s• t o recover more of t he hea t o f r e a c t i o n as s e n s i b l e h e a t in to

t h e r i c h amine and l e s s in to t he t r e a t e d gas



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• t o a l low for inc reases in t empera ture o r ac id gas con ten t o ffeed and prevent ac id gas breakthrough o r leakage overhead

• t o l i m i t absorber t empera ture r i s e due t o hea t o f r e a c t i o n (duet o concerns fo r corros ion)

• t o l i m i t ex ten t o f ac id gas f lashing in t he ho t r i c h amine fromt h e l e a n / r i c h exchanger

• t o minimize cor ros ion as soc ia t ed with h igh r i c h loading

Absorber Temperature P r o f i l e

The absorbe r t empera ture p r o f i l e provides usefu l in format ion onperformance. Typica l ly t h e p r o f i l e has a bulge i n t h e lowerp o r t i o n o f the column where most of t h e exothermic chemicalr e a c t i o n t akes p lace . The t empera ture in t h e top h a l f and in t h et r e a t e d gas out t h e t op should have about t h e same t empera ture asthe l ean amine. The t empera ture should i nc rease down th rough t h ebottom h a l f dropping somewhat on t h e bottom t r a y o r two as t h e

incoming feed gas coo l s the amine. The r i c h amine ou t the bottomshould be s i g n i f i c a n t l y h o t t e r t han t h e l ean i n t o t he column.Thus, the hea t should go with t he r i c h amine t o t h e r egenera to r.' 'he degree o f t empera ture r i s e depends on t h e ac id gas con ten t o ft he gas be ing t r e a t e d .

Amine Loading,

Lean and r i c h loadings a r e commonly determined by l abora to rya n a l y s i s and repor ted as moles o f H2S (and/or C02) per mole o famine. Rich amine samples a re q u i t e l i k e l y t o be in e r r o r ont h e low s i d e , due t o vapor l o s ses dur ing sampling. A s implem a t e r i a l balance around an absorber designed f o r t o t a l ac id gasremoval can be made t o check t he r i c h loading:

Rich Loading

Moles Acid GasMoles Amine

= (Acid Gas Pickup from Feed Gas)+(Lean Loading)= (MMSCFD) (Mol% AG in Feed) (Amine Mol wt) (1 .832)

(Amine Rate, GPM) (wt% Amine) (Solvent , l b / g a l )

+ (Lean Loading)

Note: Mol% AG in Feed inc ludes both C02 and H2S.Typica l

Amine Amine Solvent SolventM,ol wt l b / g a l wt%@ 120 F

MEA 61 .08 8 . 3 1 20DEA 1 0 5 . 1 4 8 . 5 3 30DGA 1 0 5 . 1 4 8 . 5 9 50MDEA 11 9 . 1 7 8 . 5 9 50



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R e b o i l e r H e a t

Reboi ler hea t duty suppl ied by steam o r o the r hea t ing media)usua l ly represen t s t he l a r g e s t opera t ing c o s t o f an amine p l a n t .I f lean loading i s much l e s s t han r equ i red t o achieve cleanupt hen energy i s being wasted. However, some margin of excess dutyi s d e s i r a b l e in t he event o f a change in process cond i t ions .Also, t h e r e i s another reason t o keep lean loadings low and t h a ti s t o avoid l ean s ide ac id gas f l a sh ing o r c a v i t a t i o n a t thesuc t ion o f t h e amine c i r c u l a t i o n pump. Lean loadings should notexceed the fol lowing va lues :

Amine

MEADEAMDEA

moles of acid gas /mole of amine

0 .150 .07

0 .015The r e b o i l e r provides energy i n t h r e e p a r t s : s e n s i b l e h e a t t obr ing the amine up t o b o i l i n g , l a t e n t hea t t o b o i l up somes t r i p p i n g steam and chemical hea t t o r eve r se t h e ac id gas/aminer e a c t i o n . The hea t o f r e a c t i o n i s v a r i a b l e depending on theamine and combinat ion o f H2S and C02. Approximate va lues a r el i s t e d below:·

MEA

DEADGAMDEA

H e a t o f R e a c t i o n B t u / l b o f H2S o r C02H2S C02

550511674522

825653850600

Typical ly, t h e r e b o i l e r opera te s with about one pound of steamper g a l l o n of c i r c u l a t i n g so lven t , but can range as high as twopounds p e r ga l lon . The most important i nd ica t ion · o f r e b o i l e rperformance i s t h e lean loading from t he r egenera to r bottoms.

R e f l u x R a t e

The amount o f r e f l u x re turned to t h e regenera tor i s dependent on

t h e amount o f steam r a i s e d in t he r e b o i l e r. The amount of steam. r a i sed fo r s t r ipp ing depends on t he so lu t ion p u r i t y needed t omeet t h e t r e a t e d gas spec i f i ca t ion , t h e r a t i o .of H2S:C02, t h eamine and. the r egene ra to r des ign . Reflux r a t i o s range from l e s st han 1 : 1 t o 4 : 1 moles of water r e tu rned t o t h e r egenera to r pe rmole o f ac id gas l eav ing t h e re f lux drum). The re f lux r a t i o w i l lbe a t the high end of t he s c a l e f o r MEA u n i t s and middle t o lowend fo r DEA and MDEA respec t ive ly. This has been expla ined by



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the observat ion t h a t MDEA i s l e s s bas ic and more e a s i l y s t r i p p e dt o s a t i s f a c to r y l e v e l s than the ·pr imary and secondary amines.For H2S C02 systems the C 2 which evolves a c t u a l l y a s s i s t s H2Ss t r i p p i n g and may reduce the s t r ipp ing steam requirementaccordingly.

Some o f the water from the r e f l u x drum may be purged bu t most o ft i s re turned t o the r egenera to r to mainta in the water content

of the amine. Makeup water i s added t o r ep lace the losses goingout with the t r e a t e d gas and ac id gas .

Monitoring of the re f lux r a t e i s an important p a r t o f managingthe energy input t o the u n i t and ensur ing t h a t l ean loading andt r e a t ed gas s p e c i f i c a t i o n s are on t a r g e t . The r e f l u x r a t e i s ameasure o f r e bo i l e r duty and may be monitored with t h i s in mind.Overhead temperature i s another ind ica t ion o f the s t r i p p i n g steamr a t e and r e f l u x r a t i o and i s considered a convenient means of

monitoring the re f lux r a t e . An amine plan t s imula tor can be usedt o determine the overhead temperature fo r a given r e f l u x r a t i oand column pressure .



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2 .3 Proces s Con t ro l

Process con t ro l t echniques can be used t o maX1m1ze p l a n ts t a b i l i t y and minimize energy usage and o the r opera t ing c o s t s .The e f f e c t i v e n e s s of process con t ro l in amine p l a n t s depends ont he degree o f automation, opera to r involvement and t imely use o flabora to ry r e s u l t s

While v a r i a t i o n s w i l l be d i c t a t e d by the o r i g i n a ldes ign /ope ra t ing phi losophy, process parameters , l o c a lr egu la t ions and pre fe rences , t he process con t ro l ob jec t ives a r es i m i l a r from one amine p l a n t t o another. Amine p l a n t s a r econ t ro l l ed by a combinat ion of manual and continuous automat iccon t ro l devices as wel l as a number o f i n t e r m i t t e n t c o n t r o l swhich can be automated o r opera to r i n i t i a t e d

Contro l s e t po in t s may need t o be r ead jus ted from t ime t o t ime in

response t o changes, whether abrupt o r gradual , such as : feedgas r a t e and composi t ion espec ia l ly ac id gas components), amines t r e n g t h inc luding hea t s t a b l e s a l t s con ten t ) , aminecontaminat ion , hea t t r a n s f e r su r face fou l ing and ambientt empera ture .

The c h a r t which fol lows i s a guide t o t y p i c a l process con t ro ls t r a t e g i e s used in amine p l a n t s .

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MINE UN T PROCESS CONTROL

STREAM OR CONTROLLEDVARIABLEEQUIPMENTNAME TEMPERATURE PRESSURE FLO\J LEVEL

1. ABSORBER Maintained y Absorber LCV shouldoverhead hold 4-10 minutesback pressure CV; or surge volune.floats on downstream Operator skimspressure. hydrocarbon layer

when needed.Hold maxinun Submerged gas feedbackpressure on may be permissibleabsorber for lowest to increase contactresidual acid gas in time in absorber i foverhead. needed for CO

absorption.Higher than normalpressure drop acrossthe absorberindicates foaming or

flooding.2. FEED Sour) GAS Controlled y Usually not flow

upstream process, or controlled unlessabsorber inlet there are multipleexchanger. absorbers requiring

spl i t flow.Keep 10-15 deg Fcooler than lean

. amine.

3 OVERHEAD Sweet) Regulated yGAS absorber overhead

gas PCV; or floatsOn downstreamsystem.

4. LEAN AMINEControlled

yControlled

y FCVonAbsorber Feed) cool ing mediun flow discharge side of

to lean amine cooler amine circulationor air cooler bypass ~TCV.

Excessive flowKeep 10 15 deg F wastes energy.warmer than absorber Insufficient flowfeed gas. If lean risks high richamine is hotter than loading, H Sabout 120 deg F: breakthrough andresidual acid gas in corrosion_overhead may exceedspec . Adjust flow

according to richIf too cold: liquid loading targetHC may form, CO value.

absorption ratedrops and ami neviscosity increases.

5 RICH AMINE· Absorber LCVAbsorber Bottoms) regu late s the

bottoms flow.

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STREAMOR CONTROLLEDVARIABLEEQUIPMENTNAME TEMPERATURE PRESSURE FLOW LEVEL

6. FLASH DRUM Controlled by PCV on Flash Drun LCV

the flash gas from should hold 10-15the flash drun. minutes of surgevolume. Operator

Set low enough to should skimflash off most of hydrocarbon layerthe dissolved HC. when needed_ TuneIf less than 45-65 the LCV to supplypsig, head will e steady flow to theinadequate to feed regenerator inregenerator without preference to aa ~ fixed level in the

Flash Drun.Set pressure highenough to minimize If the Flash Drunacid gas breakout in doubles as thethe L/R exchanger primary surgecausing erosion vessel for theand/or corrosion. closed loop system,Applies to plants then the liquid

with the flash drun outlet flow rateLCV located at the not level) isL/R exchanger controlled. Lowoutlet ) level indicator or

alarm indicatesMay require fuel gas when solvent makeupto pressurize the is needed.Drun when flash gasis insufficient

7. RICH AMINE L/R Rich outlet See Flash Drun) Regulated by FlashEXCHANGER temperature is Drun LCV; or byOUTLET dependent on the L/R direct flow control

surface area, if Flash Drun servessurface foul ing and as the primary surgecirculation rate vessel

A clean or newexchanger cantransfer excessiveheat to the richstream causing acidgas vaporiZation inthe hot rich outletwhich occurs aboveabout 215 deg F. Insuch cases FlashDrun pressure shouLd

e increased to helpminimizevaporization •

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STRE M OR CONTROLLEDV RI BLEEQUIPMENTN ME TEMPER TURE PRESSURE FLOW LEVEL

8. REGENER TOR Controlled by acid Regenerator LCVgas backpressure CV regulates theon Reflux DrLIII. bottoms f low and

should provide 4-10minutes surge time.

This level mayRegenerator pressure effect the flowdetermines the hydraulics in theboiling point of the return leg fromsolution in the kettle reboiler,reboiler. Set and NPSH of thepressure as low as amine circulationpossible and s t i l l pLIIIP.get acid gas to theSRU. Avoid pressureso high as to causethermal degradationof the amine.

9. LE N MINE Regenerator bottoms Regenerator LCV(Regenerator temperature is the regulates theBottoms) boiling point of the bottoms flow; does

amine solution which not apply whenis dependent on regenerator bottomamine concentration is use for surge.and regeneratorpressure; tbe controlled Inadjusting the steamto the reboiler.

10. CID G S Controlled by theReflux Condensercontrol system.Steady temperaturecontrol keeps watercontent of acid gasconstant, and helpsstabilize SRUoperation.

11. REFLUX DRUM Controlled by acid Ref lux DrLIII LCVgas PCV on Reflux should hold 6-10DrLIII. minutes of surge

volume.

12. REFLUX to Regulated by RefluxRegenerator) DrLIII LCV on

discharge side of

Ref lux PLIIIP•13. PURGE (frOm Controlled manually

Reflux DrLIII) or y FCV to purgecontaminants fromthe system.



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STREAM OR CONTROLLEDVARIABLEEQUIPMENTNAME TEMPERATURE PRESSURE FLOW LEVEL

14. MAKEUP WATER Controlled manuallyor STEAM or by FCV tocontinuously add

fresh makeup waterto the system toreplace purge andnormal losses.

Makeup rate is basedon analysis of aminestrength.

15. MAKEUP AMINE Operatorperiodically addsfresh makeup aminesolution to thesystem based onmonitoring of systeminventory.

16. REBOILER Reboiler temperature Circulation andis dependent on boilup rates areamine concentration dependent on theand regenerator rate of heat input,pressure, and £ nne hydraul ics and type

e i n e ~ n e n t~ of reboiler.. manil2 : lated.

17. REBOILERSTEAM Should not exceed Steam supply system. Control led y FCV on260 deg F (bulk) or steam l ine to350 deg F skin). Steam pressure reboi ler. Adjust

should not exceed 50 according to leanpsig to avoid amine loading targetdegradation. value.

LCV on condensatepot controlscondensate return.Operator shouldperiodically ventnoncondensibles fromthe pot i needed.

18. SURGE TANK Heating coils, if Slight positive Level gauge andrequired. pressure controlled alarms indicate

y fuel gas blanket when inventory issystem. Inert head too low or toospace is needed to high.avoid degradation ofami ne y exposure to Leave room in surgeoxygen. tank to hold total

amine inventory inthe event of ashutdown.

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AMINE TREATING BEST OPERATING PRACTICESRESOURCEGUIDE

STREAM OR CONTROLLEDVARIABLEEQUIPMENTN ME TEMPERATURE PRESSURE FLO\J LEVEL

19. CARBON BED Typically regulatedby a manua ycontrolled valve.

Abnormal pressuredifferential acrossthe bed indicates abed that has beenin.,roperly installedor is fouled; tshould be replaced.

Changeout the carbonbed when labanalysis shows noin.,rovement in coloror foaming tendencyin vs. out of thebed meaning that thebed is no longerremovinghydrocarbon.

20. FILTER Typically regulatedy a manus y

adjusted valve.

The operator shouldbackwash or changeout cartridgef i l ters) at thepoint where thepressure dropreaches the

reconmended maxinunand the mininunslipstream flow rate.can no longer bemaintained.

21. FILTER BACKWASH Hot water or steam Automaticfor lM Iits with depending on fi l ter backwashing system

backwash-type type. is activated by highf i l ters) pressure drop across

f i l ter elements.Cycle time variesdepending on IIIIIOU Itof particulates orwax.

22. RECLAIMER Batch operation.

for MEA orDGA, only

AMINE TREATING 2.20 JULY 1994


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MINE TRE TING BEST OPER TINGPRACTICES RESOURCE GUIDE

STRE M OR CONTROLLEDV RI BLEEQUIPMENTN ME TEMPER TURE PRESSURE FLO\I LEVEL

23 CHEMIC L Regulated yINJECTION injection pu p orcaustic, shot potanti foamcorrosioninhibitor)



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2 .4 Equipment

There s no equipment used in amine u n i t s t h a t s not used widelyin o the r r e f i n i n g and gas process ing f a c i l i t i e s . Amine u n i t sc o n s i s t of columns, hea t exchangers , separa to r v e s s e l s , pumps,and f i l t e r s .

Absorber

Absorbers are t r ayed o r packed columns in which r i s i n g gascon tac t s descending amine. Typical columns have e i t h e r 20-25t r a y s o r 30-40 f e e t of packing. Absorbers in hydroprocessorhydrogen r ecyc le loops t y p i c a l l y use only 20 ' o f packing due t ot h e high dr iv ing force fo r HS absorp t ion and t h e amount o f H2St h a t can be l e f t in t he s w ~gas (100 ppmv o r more) . Absorbersin hydrogen p l a n t s f r equen t ly have an add i t iona l 2-4 t r a y s abovet h e amine feed t r a y where water con tac t s the sweetened gas and

removes any vapor i zed o r entra ined amine. This s necessary t op r o t e c t t he methanator, s ince amines a r e methanator c a t a l y s tpoisons . The wate r s withdrawn from t h e absorber a t t h e t r a yabove t he amine feed t r a y.

Amine abso rbe r t r a y s of ten requ i re unusual ly l a rg e downcomera reas . Amine absorbers are f r equen t ly designed f o r higher t hannormal l iqu i l oad ings , and amine so lu t ions have a much h ighe rfoaming tendency than l iqu id hydrocarbons e x h i b i t . Thecombinat ion o f t h e s e two fac to r s make l a rge downcomers common inamine t r a y s . Many amine absorbers use two-pass o r fou r-passt r a y s . In absorbers us ing MDE o r formulated so lven t s , the weirsmay be h ighe r than normal in orde r t o provide g r e a t e r l iqu i holdup and t hus more t ime fo r r eac t ion between CO 2 and t h e amine.Weirs can be 4 o r higher in t hese app l i ca t ions .

Packed absorbers a r e designed fo r a low pressu re drop , t y p i c a l l y0.25 o f water p e r foo t o f packing, due t o t h e high foamingtendency of amine so lu t ions . Packed sec t ions a re usua l ly no morethan 1 5 : t h i s reduces the r i s k o f poor l iqu id d i s t r i b u t i o n . Inhydroprocessor hydrogen recycle absorbers , t y p i c a l l y only onepacked s e c t i o n with one l iqu id d i s t r i b u t o r i s used. This maylead t o l i q u i d d i s t ri b u t i o n problems and l o s s o f e f f i c i e n c y inabsorbers wi th packed sec t ions g r e a t e r t han 2 0 .

The surge volume a t t he bottom o f the absorber should be kept assmal l as p o s s i b l e , s ince surge volume can be provided l e s sexpens ive ly i n lower pressu re v e s s e l s . Four minutes o fc i r c u l a t i o n volume i s a minimum value .

Absorbers may be equipped with mis t el iminat ion pads t o recoveren t ra ined amine from t he t r ea ted gas . These pads should be madefrom metal r a t h e r t han p l a s t i c . Metal pads w i l l have higher



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removal e f f i c i e n c i e s s ince amine s o l u t i o n s w i l l wet metalsu r faces b e t t e r than p l a s t i c su r faces . Mist e l imina t ion padsw i l l not prevent car ryover o f amine s o l u t i o n i f a foaming upsetoccurs i n t h e absorber. In o rde r t o capture t h i s amine at r e a t e d gas knockout drum downstream of the absorber i s r equ i red .I f a t r e a t e d gas knockout drum i s used t i s not necessary t ouse a mis t e l imina t ion pad in t he absorber.

Flash Drum

The f l a sh drum i s a s e p a r a t o r v e s s e l in which absorbed anden t ra ined hydrocarbons a r e disengaged from t h e r i c h amine. I t i sa lmost always a hor izon ta l separa to r. Flash drums t y p i c a l l yopera te a t pressu res between 2 and 150 ps ig . The f l a s h gasdepending on t he f a c i l i t y and t he concen t ra t ion of H2S may bes e n t t o t h e fuel gas system a vapor recovery u n i t o r a s u l f u rrecovery u n i t . For amine u n i t s opera t ing a t low f l a s h drum

pressu res a r i c h amine pump may be required t o overcomef r i c t i o n a l losses and s t a t i c head in t h e l i n e t o t he s t r i p p e r .

A l i q u i d hydrocarbon phase i s f r equen t ly produced in t h e f l a s hdrum. I f t h i s phase i s no t recovered as a sepa ra te l i q u i ds t ream much of t w i l l end up i n t h e acid gas from the s t r i p p e rr e f l u x drum • . This might cause problems in a Claus p l a n t . Inorde r t o remove t hese l i q u i d hydrocarbons most f l a s h drums a r edes igned as t h r e e phase separa to r vesse l s .

The l i q u i d r e s idence t ime in t he f l a sh drum i s determined by t h et ime required to sepa ra te t h e l i q u i d hydrocarbons from the amine.A r e s idence t ime o f 10-15 minutes should be cons idered t o be aminimum value . In orde r t o reduce the s i z e o f t h e vesse l t h enormal l i q u i d l eve l can be s e t above i t s c e n t e r l i n e .

Lean/Rich Amine Heat Exchanger

The l e a n / r i c h amine heat exchanger t r a n s f e r s hea t from t h e leanamine s t ream leaving t he amine s t r i p p e r t o t he r i c h amine beingfed t o the s t r i p p e r . I t con t r ibu tes t o t h e energy e f f i c i e n c y o fthe u n i t s ince without t more hea t would have t o be added a tthe s t r i p p e r r e b o i l e r and more removed a t t he lean so lven tcoo le r. Lean/r ich exchangers a re designed with tempera tureapproaches between 35 0 Fand 5 0 ~ A r u l e o f thumb i s t h a t i f t h e

r i c h amine p ip ing between t h e l e a n / r i c h exchanger and t h es t r i p p e r i s carbon s t e e l t h e r i c h amine t empera ture leaving theexchanger should be no g r e a t e r than 215 0 F. I f s t a i n l e s s s t e e lp ip ing i s . used in t h i s s e c t i o n however higher tempera tures canbe used wi thou t cor ros ion problems.

In most f a c i l i t i e s t he l e a n / r i c h exchanger i s a s h e l l and tubeexchanger. The r i c h amine i s genera l ly t h e tube s i d e f l u i d



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whi le the lean amine i s on the she l l s ide . This s e l ec t i on i smade fo r two reasons : the r i c h amine becomes a two-phase mixtureas t i s heated in the exchanger and t i s more impor tant tominimize the pressure drop o f the lean amine. P l a t e and frameexchangers have been used in some offshore amine u n i t s wheret h e i r lower weight and smal le r s i z e a r e s i g n i f i c a n t advantages.

Amine s t r i p p e r

Amine s t r i p p e r s o r regenera to r s a re d i s t i l l a t i o n columns in whichCO 2 and H2S a r e dr iven out o f the amine so lu t ion . They t y p i c a l l yconta in between 20 and 25 t r ays . The r i ch amine en t e r s thecolumn no lower than the f i f t h t r ay from the top . Amine so lu t iondescends through the column and i s s t r ipped by steam r i s i n g fromthe r ebo i l e r. The steam provides the energy to break thechemical bonds between the amine and the acid gas . I t a l sod r i v e s the reac t ion equi l ibr ium in the d i r ec t i on o f regenera t ion

by car ry ing away the H2S andCO

2 t h a t are evolved from the amine.Amine so lu t ion a t the bottom o f the column c i r c u l a t e s through thes t r i p p e r r eb o i l e r. The r e b o i l e r genera tes t h e steam t h a t servesas t h e s t r i pp i ng gas in t h e column. A var i e ty o f r ebo i l e r typescan be used inc luding hor izon ta l and v e r t i c a l thermosyphons andforced c i r c u l a t i o n r eb o i l e r s . steam i s the most widely usedhea t ing medium a l though hot o i l i s used f requen t ly in upstreamp l an t s . A few plan t s use d i r e t ~ f i r e dr ebo i l e r s a l though thesea r e not pre fe r red due t o the r i s k of overheat ing t h e amine andcaus ing degradat ion .

The purpose of the r e bo i l e r i s t o genera te s t r i pp ing steam.s t r i pp ing o f the amine should take place on the f i r s t few t r aysbelow the feed t r ay and not in the rebo i le r. The amine en te r ingthe r e b o i l e r should be almost completely s t r ipped o f H2S and C02I f t i s not ac id gases w i l l evolve from the amine in ther ebo i l e r. This wi l l cause rap id and severe corros ion in ther ebo i l e r r e bo i l e r re tu rn l i n e and the lower sec t ion o f thes t r i p p e r . Inadequate s t r i pp ing of the amine i n t he column canoccur i f t r ay s in the s t r i p p e r are damaged i f t h e r i c h aminee n t e r i n g the column i s too cold o r i f the amount o f boi lup inthe r ebo i l e r i s too low.

The t r a y s above the feed t r a y serve as wash t r ay s where re f lux ing

water removes vaporized amine from the r i s i n g steam and acid gas .Two t r ays are used in t h i s sec t ion in most designs al though upt o f ive can be used. Some s t r i ppe r s do not use any water washt r ay s a t a l l . The acid gas and steam e x i t s the top of thes t r i p p e r and passes through the overhead condenser. This coo le rcools the stream to 100-130 o Fand condenses the steam in theoverhead stream. The mixture then en te r s the re f lux drum wheresepara t ion o f the acid gas and condensed water t akes p lace . The



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ac id gas goes t o s u l f u r recovery f l a r e , or ven t depending ont h e amount o f s u l f u r t conta ins and terms of t h e f a c i l i ty semiss ions permits .

In o rde r t o minimize r e b o i l e r energy requi rements and aminedegradat ion r a t e s , t he s t r i p p e r should be opera t ed a t t h e lowes tposs ib le pressure . Most s t r i p p e r s opera te with overheadpressu res between 10 and 20 ps ig . An overhead pressurec o n t r o l l e r should be used to ensure s teady opera t ion o f t h es t r i p p e r as wel l as the downstream SRU i f t h e r e i s one.

The water in the r e f l u x drum i s re turned to t h e s t r i p p e r by ar e f l u x pump. Molar re f lux r a t i o s o f 1:1 t o 1 . 5 : 1 a r e t y p i c a l .The r e f l u x r a t i o i s determined by the boi lup r a t e i n t h e

. r e b o i l e r. A low r e f l u x r a t i o i n d i c a t e s i n a d e q u a t e b o i l u p in t h er e b o i l e r . This i s a s ign t h a t t h e column i s not opera t ingproper ly. I f the amount of steam r i s i n g through t h e s t r i p p e r i sinadequate t h e amine wil l not be s t r i p p e d of ac id gas beforereaching t h e r e b o i l e r . As prev ious ly d i scussed opera t ion int h i s mode w i l l l ead t o cor ros ion problems.

Amine u n i t s t r e a t i n g gas produced i n c e r t a i n r e f i n e r y opera t ionsw i l l p ick up s i g n i f i c a n t q u a n t i t i e s o f ammonia. This w i l laccumulate i n t h e s t r i p p e r r e f l u x drum in t he form o f ammoniumb i s u l f i d e . In o rde r t o control t h e bui ldup o f t h i s m a t e r i a l asour water purge must be t aken . Chevron p r a c t i c e i s t o opera tewith a purge r a t e high enough t o keep t h e concentra t ion o fammonia in t h e r e f l u x drum below 1 . 0 wt . This sour water may beprocessed in a sour water s t r i p p e r associa ted wi th t h e amineu n i t , o r t may be routed t o a sour water t r e a t i n g systemelsewhere i n t h e r e f i n e r y.Pumps

There may be two s e t s of pumps f o r t h e lean amine s o l u t i o n : t h ebooster pumps and t h e c i r c u l a t i o n pumps. The boos te r pumps pumpt h e l ean amine s o l u t i o n l eav ing the s t r i p p e r. They provideenough head t o g e t t h e amine s o l u t i o n through t h e l e a n / r i c hexchanger t h e l ean amine cooler and t h e f i l t e r s . These pumpsare usua l ly des igned fo r discharge pressures of l e s s t han 100ps ig . The c i r c u l a t i o n pumps p r e s s u r i z e the l ean amine f o r highpressu re absorbe r s . They a re o f t en two-stage pumps s ince thelean amine comes t o them from t he surge tank a t c l o s e t o

• atmospheric p r e ~ s u r e Refiner ies w i l l of ten perform bothfunct ions wi th a s i n g l e pump.

Lean Solvent Cooler

The l ean amine coo le r cools t he amine t o the t empera ture a t whicht i s fed t o t h e absorber. The t empera ture o f the amine l eav ing



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t h i s exchanger should be 10-lSoFabove t he t empera ture o f the gase n t e r i n g the absorber. I f the amine i s cooler than the gas ,hydrocarbons may condense and cause foaming. The amine and gastempera tures should be measured as c lose as poss ib le t o theabsorber.

In amine u n i t s where MDEA i s used to remove H 2S s e l e c t i v e l y theperformance o f the u n i t w i l l be very s ens i t i ve t o the temperatureo f the amine en te r ing the absorber. A temperature change of SOFw i l l have a s i gn i f i c a n t e f f e c t on the amount o f CO 2 t h a t i sabsorbed. More CO 2 removal may i nc rease the tempera ture of thegas above the top t r a y, which wi l l increase the equi l ibr iumconcentra t ion of H2S in the gas . As a r e s u l t c lose con t ro l oft h e amine temperature a t the o u t l e t of the lean amine coo le r i se s s e n t i a l .

Amine Surge Tank

The amine surge t ank provides s torage capaci ty fo r t h e amineinventory in the p l a n t . These v e s s e l s should be blanketed by ani n e r t gas to prevent oxygen from coming i n t o con tac t with theamine. Make up water and amine a r e normally added in t h i sv e s s e l .

F i l t e r s

F i l t e r s a re used t o prevent a buildup of p a r t i c u l a t e s i n theamine stream. I t i s necessary t o remove suspended p a r t i c u l a t e swhich can increase eros ion, corros ion, foaming and equipmentplugging . Most of the p a r t i cu l a t e s en te r the system with thefeed gas and c o l l e c t in the amine. F i l t e r s may be loca ted in ther i c h amine downstream of the f l a s h drum where they w i l l do themost good to p r o t e c t the l e a n / r i c h exchanger and s t r i p p e r sees e c t i o n 3 . 9 ) . Typica l ly the f i l t e r s a r e located on t h e l ean s idewhere the f i l t e r elements can be changed with minimal r i s k ofexposure to H2S

I t i s pre fe r red t o f i l t e r the e n t i r e amine stream, al though somep lan t s a r e designed fo r s l ips t r eam f i l t r a t i o n . Bag, sock,ca r t r i dg e , and mechanical f i l t e r s have been used. Bag f i l t e r sa r e recommended when the amine so lu t ion conta ins high l eve l s ofp a r t i c u l a t e s because they a re l e s s expensive and e a s i e r t o

rep lace than the o the r types of f i l t e r s . Sock f i l t e r s have atendency to break when they f i l l with pa r t i cu l a t e s .

The main advantage of mechanical f i l t e r s i s t h a t the elements donot have t o be replaced regu la r ly. Mechanical f i l t e r s a r e usedon the amine p l a n t s a t Gaviota and Car te r Creek. These f i l t e r suse shee t s of f i n e metal mesh as t h e i r elements . When thed i f f e r e n t i a l pressu re reaches 10 p s i t h e f i l t e r s a r e



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automat ica l ly backwashed with amine s o l u t i o n . The amine i srouted t o a cone-bottomed backwash tank, where most o f t hep a r t i c u l a t e s s e t t l e t o t h e bottom. Solvent from t h e backwasht ank i s re turned to t h e amine s torage tank. Gaviota uses a sockf i l t e r on t he amine l eav ing t he backwash tank to remove anyp a r t i c u l a t e s l e f t in t he amine. Both Gaviota and Car te r Creekhave been pleased with t he performance of these f i l t e r s .

P a r t i c u l a t e f i l t e r s should remove a l l p a r t i c l e s g r e a t e r t han 5-10microns in diameter. F i l t e r s with an absolute r a t i n g of 5 o r 10microns should be used. An abso lu te r a t i n g means t h a t t he f i l t e rw i l l remove over 99 o f t h e p a r t i c u l a t e s g r e a t e r than t he ra teds i z e from t h e s o l u t i o n . (Vendors a l s o use a I I re la t ive r a t i n gfo r f i l t e r s ; t hese remove a sma l l e r percentage o f t hep a r t i c u l a t e s g r e a t e r t han t he ra ted diameter from t he s o l u t i o n . )I f t h e amine con ta ins too many p a r t i c u l a t e s , t w i l l be veryd i f f i c u l t t o use 5-10 micron- ra t ed f i l t e r s because they w i l l p lug

ve ry r a p i d l y. In these ins t ances , f i l t e r s ra ted f o r 25 micron o rl a r g e r p a r t i c l e s should be used fo r a b r i e f per iod. Once t hesef i l t e r s begin to l a s t f o r a reasonable l eng th of t ime, theyshould be replaced with f i l t e r s ra ted fo r s l i g h t l y sma l l e rp a r t i c l e s . By working down in t h i s manner, f i l t e r s r a t e d for 5o r 10 micron p a r t i c l e s can be i n s t a l l e d even tua l ly.

Act iva ted Carbon Beds

Act iva ted carbon i s used t o remove d i s so lved hydrocarbons ando t h e r organic impur i t i e s from t he amine s o l u t i o n . Normally, as l i p s t r e a m o f 10 t o 20 o f t h e c i r c u l a t i n g amine should passth rough t h e carbon f i l t e r . An add i t iona l p a r t i c u l a t e f i l t e r maybe added downstream of t h e carbon bed t o remove any carbon f i n e st h a t may be en t ra ined by t h e amine pass ing through t he carbonbed.

The carbon w i l l even tua l ly become s a t u r a t e d with organicimpur i t i e s and s top removing any more o f them from t he amineso lu t ion . Unlike t h e p a r t i c u l a t e f i l t e r s , t h e d i f f e r e n t i a lpressu re ac ross t h e carbon bed should no t i nc rease as t h e carboni s spen t . I f t does, t h i s ind ica tes t h a t t h e p a r t i c u l a t ef i l t e r i s not doing i t s job and t h a t t he carbon f i l t e r i s beingplugged by p a r t i c u l a t e s . ) The a c t i v i t y o f the carbon f i l t e r mustbe determined i n d i r e c t l y by analyz ing t he amine. The amine i s

normally analyzed fo r hydrocarbon content ; t h i s w i l l i nc rease ast h e carbon bed becomes s a t u r a t e d . Many opera tors r e l y on av i s u a l i n spec t ion o f t h e c o l o r of the amine en te r ing and leavingt h e bed t o determine when t has become s a t u r a t e d . Once t h ecarbon bed i s s a t u r a t e d , t can e i t h e r be regenera ted with steamo r r ep laced . I t i s es t ima ted t h a t steam r egene ra t ion w i l lr e s t o r e only 80 o f t h e o r i g i n a l carbon capac i ty. Af te r t h r e esteam cycles , only 50 of or ig ina l capaci ty i s res to red . So,



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t h r e e o r four steam cyc les are cons idered t he l i m i t befo rer ep lac ing the carbon.

REFEREN ES

2 .1 A s t a r i t a G. D W Savage and A. Bis io Gas Tr e a t i n q wi thChemical S o l v e n t s John Wiley Sons 1983 Chapter 15.

2.2 Kohl A. L . and F. C. Riesenfe ld as P U r i f i c a t i o n GulfPubl ish ing Company, 1985.



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TREATEDDR SWEET>

WATER WASH

KNOCKOUTDRUM

FOR H2 PLANTS ONLY) - . _ - - - 1

ABSORBERDR CONTACTOR)

FIGURE 2 1TYPIC L MINE PL NT FLOWSHEET

PARTICULATE AMINEFILTER COOLER

ACTIVATEDCHARCOALBED

MAKE-UPWATER

AND AMINE

FLASHGAS

HYDROCARBON

FLASHDRUM

HIGH PRESSUREAMINECIRCULATIONPUMP

LEAN/RICHEXCHANGER

PARTICULATEFILTER

REGENERATOROVERHEAD

CONDENSER

AMINECIRCULATION

PUMP

PURGE

STRIPPERDR REGENERATOR)

REFLUXDRUM

ACGA

TO SR

STEA

REBDILER

CONDENSA


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o

t I - IC\iLlW Ia:c::S Ei ra

oz

LL

I - Ja.

n


38/146

- 1.0E+02

oC/)

J1.0E+01o

1

;; r 1.0E+00C/)a

u.i 1.0E-01a::::>

1.0E-02wa:a

1 1.0E-03


39/146

FIGURE 2.4

MINE EQUILIBRIUM LO DING-w

1.0E+02)

C/)

( J 1.0E+010J

1.0E+OOC/)0

1.0E-01>

C/)C/) 1.0E-02wa0

J 1.0E-03«

1.0E-040C/)« 1.0E-05( J0) 1.0E-06

« 0.0· 0.1

30 WT DEA AT 120 DEG F

0.2 0.3 0.4 0.5 0.6 0.7LEAN LOADING, MOLS ACID GAS PER MOL AMINE

DEAlH2S DEAlC02B 8

/; - :1,.

0 8 0 9

RVP7 1 93


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- 1.0E 02

()f)

C 1.0E 01o

I

1.0E 00f)

0..ui 1.0E-01a

>

1.0E-02wa0..

I 1.0E-03«

a 1.0E-04«0..

1.0E-05C

.FIGURE 2.5

MINE EQUILIBRIUM LO DING5 WTo o MDEA AT 120 DEG F

/ .

1 .

/; . . . - - - -1 . ..

1.0E-06 L- -L - . . . .L.- l . . . - - - _ - - - - J I . . . . . - _ - - - 1 -« 0.0 0 1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

LEAN LOADING, MOLS ACID GAS PER MOL AMINE

MDEAlH2S MDEAlC02B b

RVP711/93

0.9


41/146


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3 . 0 SOLVENT CHARACTERISTICS

3 . 1 Amines

Alkanolamines a re commonly used to remove HzS and COz ac idgases) from a v a r i e t y of gas and l i q u i d s t reams. Some removal o fcarbonyl s u l f i d e COS) and mercaptan s u l f u r RSH) i s a l sop o s s i b l e . The most common gener i c amines a re monoethanolamine

MEA),. dig lycolamine DGA) , diethanolamine DEA),diisopropanolamine DIPA), and methyldiethanolamine MDEA). Inadd i t ion t o t hese gener i c amines, seve ra l p r o p r i e t a r y amines a reoffe red by so lven t vendors . These p r o p r i e t a r y amines a re o f t envery pure forms of MDEA, blends o f MDEA with other amines o rchemica ls , o r seve re ly -h inde red amines s p e c i a l l y formulated t oachieve des i red performance c r i t e r i a such as H ~ Ss e l e c t i v i t y,high capac i ty, deep removal, and low r e g e n e r a t ~ o nc o s t . Hybridso lven t s a re sometimes used. These a r e blends o f amines, which

r e a c t with t h e ac id gas , and physica l so lven t s , which absorb byphys ica l s o l u b i l i t y only. Propr ie t a ry amines and hybr id so lven t sw i l l no t be covered in d e t a i l in t h i s guide . A t a b l e o f phys ica lp r o p e r t i e s i s provided in Table 3 .1 .1 .

Chemistry: Amines a r e o f t e n descr ibed as primary, meaning onehydrocarbon chain R) a t t ached t o t h e n i t rogen RNH z) ; secondary,wi th two hydrocarbon chains a t t ached t o t h e n i t rogen ~ N H ) ;andt e r t i a r y, having t h r e e hydrocarbon chains a t t ached t o then i t rogen R 3 N). MEA and DGA a re primary amines; DEA and DIPA a r esecondary amines; and MDEA i s a t e r t i a r y amine. Because aminesform a l k a l i n e so lu t ions with water, they can be used t o absorba c i d i c gases such a s H S and CO. They a l l have the same b a s i cr e a c t i o n chemis t ry with H2S. The fol lowing r e a c t i o n with HzS i scons idered t o be ins tantaneous .

Acid-Base React ions wi th HzS: a l l amines, t e r t i a r yshown, s to ichiometry 1 :1 HzS/amine)

H2S ~ N ===> HS· ~ r

However, t h e r e a re some d i ffe rences between t hese amines when tcomes t o CO r eac t ions . Al l amines w i l l undergo an ac id -baser e a c t i o n wiEh CO 2 • The ac id -base r eac t ion of aqueous amines withCOz i s much slower than the r eac t ion with HzS. This may be a

b e n e f i t whent

i s des i red t o s l i p CO2 , bu t a disadvantage when. complete COz remQval i s des i red . The o v e r a l l r e a c t i o n s a r e :

c i d ~ B a s eReactions wi th C O ~t o form b ica rbona te a l lamines, t e r t i a r y shown, s t o ~ c h i o m e t r y1:1 COz/amine)

CO2 + ~ N + H20 HC0 3 • + ~ r + OH·

SOLVENT CHARACTERISTICS 3.1 JULY 1994


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The f u r t h e r acid-base reac t ion to carbonate does notproceed to an appreciable ex ten t due to the so lu t ionpH:

HC 0 3- R3N C0 3

= ~ N H

In add i t ion , primary and secondary amines can r eac t d i r e c t l y withthe CO2 t o form carbamates. This i s the predominant reac t ion a tloadings below 0.5 m/m (mole ac id gas per mole amine) , and toccurs a t a f a s t e r r a t e than the acid-base reac t ion fo r CO2 •

Carbamate Reaction with C O ~ (primary and secondaryonly, secondary shown, sto1chiometry 1 : 2 CO2/amine)

CO2 2 ~ N H ~ N H 2 ~ N C O O

t i s by t h i s carbamate pathway t h a t primary and secondary amines

a r e more r e a c t i v e to CO2 al though loadings are l e s s due t o t h e1 : 2 s to ichiometry. Because t e r t i a r y amines cannot formcarbamates, they r e ac t more slowly with CO2 v i a t h e acid-basereac t ion . This fea tu re has been exploi ted t o provide absorpt ionse l ec t i v i t y fo r H2S over CO2 • By us ing a t e r t i a r y so lven t suchas MDEA and sho r t e r con tac t t imes , propor t iona l ly more H2S w i l lbe absorbed, .providing savings i n c i r cu l a t i on r a t e andregenera t ion duty. The carbamate reac t ion i s a l s o one o f theprimary pathways t o amine degradat ion with secondary aminesgenera l ly being more s t ab l e than primary amines.

Carbonyl Sul f ide COS) undergoes reac t ions with amines analogoust o those f o r CO above. The r e a c t i o n r a t e with COS i s evenslower t han t h a t fo r (about 1 / 1 0 0 ) , and COS r e a c t s a t af a s t e r r a t e with primary amines than t does with secondaryamines. The formation o f a th iocarbamate i s why primary andsecondary amines are b e t t e r a t absorbing COS than t e r t i a r y aminesand i s a l so why COS-enhanced amine degradat ion i s more o f aproblem fo r these amines. with secondary amines, a s i g n i f i c a n tpor t ion o f the COS i s hydrolyzed to form H 2S and and asmal ler amount p a r t i c i p a t e s in degradation reac t ions .

3 .2 Performance

Amine Comparisons: Primary amines are s t ronger bases than• secondary amines which are s t ronger bases than t e r t i a r y amines.This i s why fo r equal ac id gas loadings a t equal tempera tures ,the equi l ibr ium p a r t i a l pressure o f an acid gas above the aminewi l l have t h e fol lowing t r end : MEA < DG < DEA < MDEA. Primaryamines, espec ia l ly MEA, a r e bes t su i t ed to low pressure « 100ps i ) app l ica t ions which have f a i r l y s t r i ngen t acid-gas removalspec i f i c a t i o ns . On the other hand, the hea t o f reac t ion , which



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a f f e c t s r e b o i l e r du ty, f o l l o w s t h e o p p o s i t e t r e n d : MEA, DGA > DEA> MDEA s ee Table 3 . 2 . 1 ) . R e b o i l e r d u t y i s composed o f t h r e ecomponents : s e n s i b l e h e a t , l a t e n t h e a t o f v a p o r i z a t i o n , and h e a to f r e a c t i o n .

Table 3 . 2 . 1 : Typ ica l O p e r a t i n g Cond i t i ons

MEA DGA DEA DIPA MDEA

S o l n s t r e n g t hWt 15-20 40-60 25-35 20-40 45-55

Max. R i c h Loadings ,m/m HzS o r t o t a lm/m z o n l y 3 )

S . 4 0 S .50 S . 5 0 S . 5 0 . 4 5 - . 5 0. 3 0 - . 3 5 . 3 0 - . 4 0 . 3 5 - . 4 0 . 3 5 - . 4 0 . 4 0 - . 4 5

Max. Lean Loadings ,m/m

3 ) . 1 0 - . 1 5 . 0 8 - . 1 0 . 0 5 - . 0 7 . 0 4 - . 0 6 . 004 - . 010Heat o f Reac t i on ,

B t u / l b HzS 1 ,2) 550 674 511 526 522

B t u / l b co z 1 ,2) 825 850 653 703 600

Notes : m/m = moles a c i d gas /mole amineh e a t o f r e a c t i o n f o r l o a d i n g s below 0 . 5 m/m 1)s o u r c e s : 1) P o l a s e k , 1985; 2) TPA 1991;

3) DuPart , 1991.

s o l u t i o n s t r e n g t h : I n g e n e r a l , t h e c o r r o s i v i t y o f aqueous amines o l u t i o n s i s h i g h e s t f o r t h e pr imary amines and l owes t f o r t h et e r t i a r y amines . Because o f d i f f e r e n c e s i n s o l u t i o n c o r r o s i v i t y ,t h e h i g h e r o r d e r amines t e n d t o b e used a t h i g h e r c o n c e n t r a t i o n s ,t h e e x c e p t i o n be ing DGA. T h i s r e s u l t s i n more moles o f amine p e rg a l l o n o f c i r c u l a t i n g s o l u t i o n a s s e e n by:

amine s t e a n g t h15 wt MEA30 wt DEA40 wt DIPA50 wt MDEA

60wt DGA

l b m o l e s / g a l0 .0210.0240 .0250 .036

0.049The r i c h a c i d - g a s l o a d i n g s t e n d t o b e somewhat h i g h e r t o o . Then e t r e s u l t o f t h e s e two f a c t o r s i s t h a t t e r t i a r y amines g e n e r a l l yr e q u i r e l e s s c i r c u l a t i o n f o r a g i v e n amount o f a c i d - g a sa b s o r p t i o n t h u s s a v i n g pumping c o s t , r e g e n e r a t i o n c o s t , andc a p i t a l c o s t s m a l l e r equ ipment ) .



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Table 3.1.1: Physical Properties of Amines

. Primary Amlnes Secondary m h > ~ s Tertiary AmlnesSolvent monoethanolamine diglycolamine diethanolamine diisopropanolamine methyldiethanolamine

Short Name MEA DGA DEA DIPA MDEA

Formula HOC2H4NH2 HOC2H40C2H4NH2 (HOC2H4)2NH (CH3CH(OH)CH2)2NH (HOC2H4 )2NCH3

Mol. Wt 61.1 105.1 105.1 133.2 119.2

b.p, F 339 430 516 480 477

Spec. Gravity, 60F 1.018 1.058 1.095 0.999 1.043

Specific Heat, Btu/lb-F 0.608 0.571 0.60 0.69 0.60

Thermal Cond.,Btu/hr-ft2-F @ 77F 0.147 0.124 0.111 0.111 0.0906

Viscosity, cP 7 7 F 18.9 26 352@86F 870@86F 173

Surf. Tension,dyne/c m • 77F 49.9 58.6 102.9 101.5 100.5

Heat of Reaction,Btullb acid gas, H2S -550 -674 -511 -526 -522

CO2 -825 -850 -653 -703 -600

wt.% Range 15 25 40 60 2535

20 40 45 - 55

. Loadings, m/m

Rich, H2S or Total 0.40 0.50 0.50 0.50 0.45 - 0.50

Rich, CO2 0.30 - 0.35 0.30 - 0.40 0.35 - 0.40 0.35 - 0.40 0.40 - 0.45

Lean, Total 0.10 0.15 0.08 - 0.10 0.05 - 0.07 0.04 - 0.06 0.004 - 0.010

Cost, lIb (99%truckloads, TX 4/94) 0.46 0.90 0.47 0.94 1.45

source: Dow Alkanolamines Handbook; Gas Conditioning and Processing, Vol. 4 R.N. Maddox, Campbell Petroleum Series; and HYSIM. Page 3.3a


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Hydrocarbon Solub i l i ty : The g r e a t e r the number of a lky l groups-CH?-, -CH 3 , e t c . ) and t h e fewer the number o f hydroxyl groups-OH) presen t on an amine molecule, the g r e a t e r wi l l be t h e

hydrocarbon s o l u b i l i t y in the amine so lu t ion and t h e g r e a t e r thes o l u b i l i t y of the amine in a l i q u i d hydrocarbon stream. Hydroxylgroups suppress hydrocarbon s o l u b i l i t y while i nc reas ing waters o l u b i l i t y and reducing vapor pressure due to hydrogen bonding.Based on t h i s group-con t r ibu t ion argument, the expected t r end inhydrocarbon s o l u b i l i t y (h ighes t t o lowest) i s : DGA DIPA MDEA> DEA, MEA. Keep in mind t h a t hydrocarbon s o l u b i l i t y i s a l so as t rong func t ion o f amine s t reng th . Higher amine concentra t ionsgive r i s e t o h ighe r hydrocarbon s o l u b i l i t i e s .

H2S Se l ec t i v i t y : As s t a t e d above, t e r t i a r y amines are unable toform carbamates with CO2 • CO2 i s only absorbed v i a ther e l a t i v e l y slow acid-base r e a c t i o n pathway. This slower CO2r e a c t i o n r a t e with t e r t i a r y amines al lows them t o s l i p more CO2and absorb a h ighe r p ropor t ion o f H2S than primary and secondaryamines. S e l e c t i v i t y i s improved by lower t empera tures and s h o r tcon tac t t imes . The h ighe r the temperature the f a s t e r the CO2reac t ion r a t e becomes. Use of s h o r t contact t imes (shal lowweirs , fewer t r ays ) l im i t s the t ime ava i l ab le fo r CO2 reac t ion .Keep in mind t h a t the reac t ion r a t e for H,S i s ins tantaneous , andabsorp t ion i ~ l imi ted by m a s s t r ~ n s f e rr a t e s . CO2 absorp t ion i sgenera l ly l imi ted by r e a c t i o n r a t e s .

Hindered amines a l so show good HS s e l e c t i v i t y. Hinderedsecondary amines, such as DIPA, have bulky hydrocarbon groupswhich block easy access o f C O ~t o the ni t rogen thus slowingcarbamate formation and r e d u ~ n gi t s s t a b i l i t y. FLEXSORB, anexpensive propr ie ta ry amine from Exxon, i s a seve re ly hinderedamine which shows e x c e l l e n t s e l e c t i v i t y in low pressu reapp l ica t ions .

Deep CO2 Removal: In order t o remove CO. down t o very low l e v e l s«100 ppmv) , MEA i s t h e gener ic amine o choice s ince t providest h e lowes t p a r t i a l pressure o f ac id gas fo r a given loading. Theso lven t vendors a l l have p r o p r i e t a r y blended amines which o f f e rdeep removal and increased ac id -gas capac i ty s c f / g a l ) . Thesea r e o f t e n blends o f MDEA and MEA or DEA. Act iva ted MDEA, offe redby BASF, uses piperazine as the ac t iva to r fo r increas ing cO 2absorpt ion capaci ty.

COS Removal: Primary and secondary amines a r e r e a c t i v e t o COS.The drawback i s t h a t they are a l so , to a ce r t a i n ex ten t , degradedby COS. For MEA, 20 o f the absorbed COS undergoes ani r r e v e r s i b l e degradat ion reac t ion , and t h e r e f o r e MEA i s notadvised f o r high COS se rv ice . For DGA, the degradat ion productsa r e subs t a n t i a l l y reversed a t high temperature in the rec la imer.COS removal by DG i nc reases with i nc reas ing DGA temperature and



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AMINE TREATING BEST OPERATING PRACTICES RESOURCE GUIDE

con tac t t ime. Removals as high as 75 o r more can be achieved(Moore, 1985). DEA undergoes some minor degradation. DIPA hasbeen repor ted to have severe, i r r eve r s ib l e degradat ion wi th COS

Asta r i t a , 1983), but others claim only s l i g h t degradat ionButwell , e t a l . , ) which i s probably the case . She l l o f f e r s a

hybr id so lven t Sulf inol D) using DIPA fo r the purpose ofenhanced COS removal . Use o f long-contac t - t ime pol i sh ing columnsusing hot , very lean amine have been recommended fo r COS removal.The goal here i s to f i r s t remove ~ S and CO2 which, beings t ronger ac ids , can disp lace absorbed COS. The highertempera ture and longer con tac t t imes speed the reac t ion r a t e andabsorpt ion . Once COS i s absorbed, a por t ion o f t i s hydrolyzedt o H2S and CO2 •

RSH Removal: DGA i s the most e ff ec t i ve of the common gener icamines a t methyl and e thy l mercaptan removal with repor t s o f upto 65-75 Texaco). This i s mostly due to DGA's higher

hydrocarbon s o l u b i l i t y. Mercaptans are only s l i g h t l y ac id i c , andr e a c t i v e absorpt ion in to a lka l ine amine so lu t ions i s poor. Butdue t o the hydrocarbon chain on the mercaptan molecule, they are

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