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S OUR GAS C ONDEN S ATE C O RR O S I ON OF R E F I NE R Y B O I L ER HEATER TU B ES P . R ost r on*, M. W . H. G a w a rg y A r ts a nd S c i e n c e s P r og r a m, The P e t r ol e um I nstitut e , Abu Dh a bi, U.A.E. p r ost r on@pi. ac . a e Abst ract G a s c ond e n s a te is f o r m e d du r ing the p r odu c tion o f p e t r ol e um g a s. Upon c o oling the hi g h e r mol e c ul a r w e i g ht frac tions c on d e nse to a l i quid. W h e n the g a s f i e ld is sour (c ont a ins H 2 S ) , the g a s c ond e ns a te a lso c on t a ins o r g a nic sul f ur a nd dissolv e d H 2 S . The p re s e n c e of th e s e t y p e s o f sul f ur c ompounds c a us e s sul f ide c o r r osion in the ref i n e r y re boil e r h e a t e r tub e s, o c c a sion a l l y r e sulting in r ef i n e r y f i re s d u e to p e n e t r a t ion of the h ea t e r w a ll. W e show in this p a p e r th a t a t low t e m p era tu r e s, dissolv e d H 2 S ca n c ome out of solution a nd a tt ac k the st ee l, a nd a t h i g h e r t e mp era tu r e s, the o r g a n i c sul f ur c ompounds d ec ompose to p r odu c e H 2 S re sulting in a double a tt ac k. K e y wo r ds : H y d r o g e n sul f id e , o r g a nic sul f u r , sour g a s c ond e ns a t e I nt r od u c tion At a sour g a s p r od u c tion fac ili t y in the Gu l f r e g io n , the d a i l y p r od u c tion ra t e of g a s c ond e ns a te af t e r s e p a r a tion fr om the g a s is a pp r o x im a t e l y 300K bbl d a y - 1 . This c ond e ns a te is th e n pip e d to a ref in e r y f a c ili t y w h e r e it is us e d a s a p e t r o c h e mi ca ls fee dsto c k. T h e c ond e n s a t e re qui r e s e x t e nsive ref i n ing via pl a te distill a tion tow er . S in c e th er e is a wide ra n g e o f m a in l y h y d r o c ar bons p r e s e nt ( g rea t e r th a n 300 di f fere nt c ompounds ) , c ov e r ing a wide boiling ra n g e , the c ond e ns a te is p r o c e ss e d in a two st e p h ea ting p r o ce ss. I niti a l l y , the c ond e ns a te is h ea t e d to 175 C to s e p ara te the li g ht e r frac tions in a f l a sh distil l a tion c olumn. The re sid u e is th e n pip e d to a s ec o n d h ea t e r c i rc uit a nd h ea t e d to 315 C b e f o r e inj ec tion into a l a r g e pl a te distill a tion c olumn f or f r ac tion a l distill a tion. B oth of the h ea t e r c oils h a ve su f f e r e d th r ou g h w a ll c o rr osion f a ilu re s. The h ea t e r tub e s ar e c a r bon st ee l with a w a ll thi c kn e ss of 8 mm. The s a me c o rr o sion fa ilu r e of w a ll thinning a nd p e n e t r a tion via f o r m a tion of i r on sul f id e s w a s the mode of fa ilu r e in both ca s e s. The p e n e t r a tion ra te w a s in e x ce ss of 4 mm y r - 1 . I n both ca s e s the g rea t e st e x tent of wall thinning occur r ed on the h i g h tem p eratu r e side of the h eater coils. Apart from this differe n ce, the corrosion was unif o rm, with no evidence of pitt i n g , commensurate with iron sulfide formation as the mechanism of co r rosion. The same corrosion failu r e t y p e w a s unusual since h y dr o g en sulfide dissolved in the condensate cannot e x plain the seco n d coil failure. This rep o rt is not concerned with the identification of sulfide scali n g co r rosion, descriptions of which can be found in the lit e r a ture 1 - 3 . I n v e stig a tion An A g il e nt mod e l 6890 g a s c h r om a t o g t a ph c oupl e d to A g il e nt 5973 qu a d r o pole m a ss sp ec t r om e t e r w a s us e d to rec o r d a f r a g m e nt a tion s p ec t r um f or eac h c omp o n e nt of the c ond e ns a te usi n g c ar bo w a x ca pill a r y c olumn a nd a h ea ti n g ra t e of 20 mi n - 1 , F ig 1. This fa il e d to p r odu c e a g ood s e p ara tion due to the st r u c tu ra l simil ar i t y of the l a r g e num b e r of c ompounds p re s e nt in the c ond e ns a t e . F r a g m e nt s earc hi n g f or c h ar a c t er istic thiol d ec omposition p r odu c ts in the m a ss sp ec t r um did not y i e ld a n y t r ace s o f o r g a n i c sul f ur c ompounds b e ing p re s e nt in the c ond e ns a t e . I n o r d e r to un d er st a nd t h e th er m a l p r o p er ti e s of t h e c ond e ns a t e , the h e a ting p r o ce ss w as mod e l e d in the l a b. The c ond e ns a te w a s slow l y di s till e d und e r a n in er t g a s b l a nk e t via d r y nit r o g e n suppli e d a t a c o nst a nt ra t e . The in e r t g a s w a s us e d a s a c a rr i e r to move a n y H 2 S p r odu ce d du r i n g d ec om p osition to a H 2 S d e t ec to r , wh ere b y the p r odu c tion of H 2 S c ould be f ollow e d a s a f un c tion of t e mp era tu r e . H 2 S l e v e ls a nd pot t e mp era tu r e w er e r ec o r d ed e v e r y minut e . This g e n e r a t e d two g ra phs, o n e sh o wing the e volution of H 2 S ov e r time a nd one showi n g the e vo l ution of H 2 S a s a f un c tion of t e mp era tu r e . F o r the time – ppm H 2 S g ra ph, the a r e a und e r the c u r v e , a ssuming a c onst a nt g a s f low ra te ( wh i c h w as ac hi e v e d ) c o r re sponds to the tot a l a mount of H 2 S e volv e d. The t e mp e ra tu r e - ppm H 2 S g r a ph shows the t e mp e r a tu r e a t whi c h H 2 S e volution o cc u r s ( f i g u r e 2 ) . Dis c ussion of re sults The g ra ph in f i g u r e 2 sh o ws v e r y c l e ar l y th e r e a r e two distin c t e volutions of H 2 S . T he long time int er v a l b e t w e e n e volutions shows this i s not a kin e tic di ffe r e n c e , but th er e must be c l ear l y di f f ere nt ca us a tive m e c h a nisms f o r the e volution of the H y d r og e n sul f id e . I t is p r opos e d t h a t the bimod a l n a tu r e of the H 2 S p r odu c tion a s a f u n c tion of t e mp era tu r e ca n be e x pl a in e d a s f ollows. The low t e mp e r a tu r e p r o du c tion of H 2 S ca n be most ea si l y e x pl a in e d a s s i mp l y p h y si ca l l y dissolv e d H 2 S . the fac t th a t th e r e is no c l ea r st a r ting t e m p era tu r e a t whi c h e volution o cc u r s, c oupl e d with the b r o a d ra n ge of t e mp era tu r e s ov er wh i c h e volution o cc u r s st r o n g l y indi ca t e s a p h y si c a l dissolution m o d e l. As the c ond e n s a te is h ea t e d, p h y si ca l l y dissolv e d H 2 S out g a s e s fr om solution. W h e n no h ea ting is a ppl i e d, H 2 S ca n be d e t ec t e d, indi ca ti n g th a t t he H 2 S e volv e s acc o r ding to its p ar ti a l p re ss u r e in the g a s ph a s e. W ith sust a in e d carr i e r g a s f low a nd mod e r a te h ea t in g , e v e ntu a l l y a ll of the dissolv e d H 2 S out g a s e s a nd the c o n ce n t ra tion of H 2 S in the g a s p h a se re tu r ns to z er o. This out g a ssi n g of H 2 S is most lik e l y re s ponsible f or the sul f ide c o rr osion o cc u r r i n g in the low t e mp era tu r e h ea t e r c oil. The h y d r o g e n sul f ide re a c ts with the m e t a l in the c ool h ea t er c olumn p r odu c ing i r on sul f id e . The s ca li n g r e a c tion will c onsume a ll a v a il a ble h y d r o g en sul f id e . Th er e is e vi d e n c e fr om the op e ra tor th a t d issolv e d h y d r o g e n sul f ide is ca usi n g this c o rr osion a s th er e h a s b ee n re p o r ts of sul f ide s ca ling c o rr osion in the p ip e line l ea di n g to the ref in e r y . The s ec ond e volution of h y d r o g e n sul f ide is mo r e c h a ll e n g i n g to e x pl a in. Th er e is a c l ear t e mp era tu r e a t whi c h e v o lution of h y d r o g e n sul f i d e is obs er v e d. T h e sh ar p i niti a tion t e mp era tu r e is st r on g l y i ndi ca tive of the ov e rc o m ing of a n e n e rg y b a r r i er , t o the e volution of h y d r o g e n sul f id e , s u c h a s the th er m a l d e c omposition of o r g a n i c sul f ur c o mpounds, su c h a s thiols. Thiols ar e known in the lit era tu r e to und e r g o th er m a l d ec o m position, re sulting in the e volution of h y d r o g e n sul f id e 4 . To t e st this h y pot h e sis, w e t r i e d a simple m e thod o f sul f ur re mo v a l in the c o nd e ns a te oil. B oth h y d r o g e n sul f ide a nd thiols, b e ing sli g ht l y a c idic c ompounds, will dissolve into a n a lk a line a qu e ous l a y e r 5 . To t e st this, s a mpl e s of the g a s c ond e n s a te w er e w a sh e d with dilute a qu e ous sodium h y d r o x ide ( 1.0 M) a nd t h e a qu e ous l a y e r s e p ara t e d. The a qu e ous l a y e r w a s ac idi f i e d with h y d r o c hlo r ic a c id a nd e x t rac t e d with di c hlo r o m e th a n e . The di c hlo r om e th a ne e x t rac t w a s d r i e d ov e r m a g n e si u m sul fa te a nd the solv e nt re mov e d b y r ot a r y e v a po ra to r . T h e r e sulting re sidue h a d t h e c h arac t er istic st e n c h of thi o ls. The e x t ract c ont a ining thiols w a s su b j ec t to h ea dsp a c e G C - M S a n a l y sis usi n g S him a d z u GC 2010 with Q P 2010 MS c olu m n us e d w a s a 30 m S L B 5 NS c olumn with t e mp era tu r e p r o f ili n g fr om 70C – 220 C. H ea dsp a c e is a s a mpli n g a pp r o ac h wh e r e h e a t s e n sitive c h e mi ca ls a r e to be inv e sti g a t e d 6 . A s a mple of the e x t rac t w a s a dd e d to a s ea l e d bo t tle a nd the bottle h ea t e d t o 70 C . 5.0 m L of the v a pour in the b ottle w a s c oll ec t e d a nd inj ec t e d di r e c t l y in to the G C c olumn. I n this w a y , th e r e is no pos s ibili t y of d a m a g i n g the c olumn a s the s a mple is g a si f i e d a nd the t e mp era tu r e is low e no u gh th a t no th er m a l d e g r a d a tion o cc u r s. A f t e r a n a l y s i s b y this m e thod, a ra n g e o f thiols w er e id e nti f i e d a s b e i n g p re s e nt in the g a s c on d e n s a t e , a s p r odu ce d in table 1 . As a f in a l c h ec k, the s w e e t e n e d c on d e ns a te w a s s ubj ec t to the s a me a n a l y s is a nd du r ing the h ea ti n g th er e w a s no h y d r o g e n sul f ide d e t e c t e d. This indi ca t e s th a t the sou rc e of the h y d r o g e n sul f ide is ind e e d the g a s c ond e n s a t e , a nd a lso indi ca t e s th a t a n a q u e ous w a sh with dilute sodium h y d r ox ide is a vi a ble m e thod t o re move both h y d r o g e n sul f ide a nd thiols re sponsible f or th e r m a l l y p r od u ce d h y d r o g e n sul f ide a s w e ll. The H 2 S p r odu ce d b y the th er m a l d ec omposition of thiol c ompounds e x pl a i ns w h y sul f id a tion c o rr osion ca n o cc ur in the h i g h e r t e mp e ra tu r e boil e r tubi n g . T h e thiols do not d ec ompose until a t l ea st 240 C . The hi g h t e mp e r a tu r e boil e r is h e ld a t 315 C , wh erea s the low t e mp era tu r e h e a t e r tub e , a t 175 C is not hot e nou g h to c a use d e c o mposition. I n a ddition, this ra pid th er m a l d ec omposition of thiols e x pl a ins w h y thiols a r e not p re s e nt in the c h r om a t o g r a m. I n a G C , the s a mple is p re h ea t e d to 350 C in the inj ec tion po r t to vol a tili z e a ll of the s a mp l e . Th er m a l d ec omposition of the thiols o cc u r s a t t his point. C on c lusions W e h a ve b ee n a ble to d e monst ra te w h y both h ea t e r c oils su ff e r the s a me t y p e o f c o r r osive a tt ac k. I n o r d e r to c ont r ol sul f ide c o rr osion it is impo r t a nt to re move both t h e dissolv e d H 2 S th r ou g h a sw e e t e ni n g p r o ce ss, a s w e ll a s a n y o r g a nic sul f ur c ompounds p re s e nt whi c h ar e ca p a ble of th e r m a l d ec omposition to p r odu c e H 2 S. Th er e ar e s e v e r a l a dv a n t a g e s to this, f i r st l y , the c ond e ns a te its e lf is l e ss c o rr osive to p r odu c tion fac iliti e s, a nd s ec ond l y t h e f u e ls p r od u ce d a r e e x t re m e l y low in sul f u r , a nd ca n th e r ef o r e be sold a t t he ult r a low sul f ur p r e mium p r i ce . D e sul f u r i z a tion p r o ce ss es th eref o r e h a v e a double a dv a nt a g e , r e du c i n g c o rr o sion in the pl a nt e quipm e nt a s re du c i n g sul f ur e missions fr om c o mbustion of the re sulting f u e l. A c knowl e d g e m e nts The a utho r s would wish t o th a nk S a si S t e ph e n, P a ul R a smuss e n a nd G r e g R oos f or th e ir a ssist a n c e on this p r oj ec t. R efere n ces 1. “A m echan i s ti c m odel of H 2 S co r r os i on of m il d s t ee l ”, Wei S un and S r d j an N e s i c, paper 07655, NAC E C o r r os i on 2 007 C on f e r e n ce a nd Expo, NAC E I n t e r na t i o n a l , H o u s t on T x 2007. 2. F. H . ME Y E R , O . L. R I GG S, R . L. Mc G L A SS ON , a n d J . D . S UDBUR Y ( 1958 ) C o rr o s i on P r od u c t s of M i l d S t e e l I n H y d r o g en Su l f i de En v ir on m en t s. C o r r o s i o n : Feb r u a r y 1958, V o l . 14, N o . 2, pp. 69 - 75 3. R . L P I E H L ( 1960 ) C o rr e l a ti on of C o rr o s i on I n a C r ude D i s t i l l a t i on U n i t W i t h C h e m i s tr y of t h e C r u d es. C o rr o s i o n: June 1960, V o l . 1 6, N o. 6, p p. 305 t - 307 t . 4. T he T h e r m al D eco m pos i t i o n of M e r ca p t an s ”, A . H . S e hon , B . de B . D a r w e n t, J. A m . C he m . Soc. , 1 9 54 , 76 ( 1 9 ) , pp 4806 – 4 810. 5. S HA K I RU L L AH , M OH A MM AD ; AH M AD , I M T I A Z ; AH M AD , W AQA S y I S HAQ, M OHA MM AD . D ES U LP HU R I Z A T I O N S T UD Y O F PE T RO LE U M P RODUC T S T HROUGH EX T RAC T I O N W I T H AQUEOUS I O NI C L I Q U I DS. J. Ch il . Che m . Soc. [ on l i n e ] . 20 1 0, v o l .55, n . 2, pp. 179 - 183. I S S N 0717 - 9 7 07 6. A s sess m ent o f T h i ol C o m pounds fr om G a r l i c by A u t o m a t ed H eadsp a ce D e ri v a t i z ed I n - N eed l e - N T D - G C - MS and D e ri v a t i z ed I n - F i be r - SPM E - G C - MS, J a m i e M a r c u s W a r r en, D on - R o g er Pa r k i nson, a nd Janusz Pa wl i s zy n, Journ a l of Ag r i c u lt u r a l and Fo o d C he m i s t ry 2 0 13 61 ( 3 ) , 492 - 500 F i g u r e 1: C h r om a t o g r a m of C ond e ns a te F i g u r e 2: G r a ph showi n g the e volution of H 2 S a s a f un c tion of t e mp e r a tu re. R etention factor C ompound id e nti f i e d 1.725 2 - but a n e thiol 1.763 1 - p r op a n e thiol 1.870 1 - but a n e thiol 2.063 2 - p e nt a n e thiol 2.343 1 - p e nt a n e thiol 2.54 C y c lop e nt a n e thiol 2.627 2 - h e x a n e thiol 3.287 c y cloh e x a n e thiol Table 1 : Thiol c ompou n ds id e nti f i e d using H e a d s p ac e G C - MS