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SULPHUR RECOVERY

Claus unit reliabilityDavid C Parnell and Timothy R Armstrong , TPA , Inc,

discus s t he influence of eq uipment and controls in the ope rationof today 's su lphu r recove ry units.

T

he mo t influ ntial variable afft'cting the unit performan e fo rmodified ta u s s ulphur rc ov

er y unit s ( R ) , i ~ th e eeds to c k .111i is espc .ially tr u e f r C l a u ~unit operating in refineries, wherein the

R is oft n fed a s econd a rream(fr m a so ur water ~ t r i p p e r, a ~ w e llas th e aci d as fw m :-.n a mine unit.

Rot h t he feed ga . uality (co mp )s iti on ) an d ra t e , ignilicantl affectthe sulphur r over un i t pcrforman c, including botJ1 reliability :m dove rall c(f'i 1cncy ( I crcemage 0 f th es ulphur rc overed. ) nfort unately

th o n t ro l an or t gul:nion o f th scim p nant ga feed variable . is fr equ e ntly n t within the j u n ~i Li n ufthe Claus plnnt operating , cafl'.

H wcvt'r, there arc , 1 me it e m inboth the design an d operation f th e

R that nn improve th e unit rdia -bility as wd l a the 0vcrnll ;u lphur re -

over ' pe r entage. The c n :habilito ption s in th e f. ulphur plant in v' lvcbtHh eq uipmen t an d con trol sc h em s,including Jutomatic shutd ow n fu n -ti n . Tht' prin ipal cquipm m i r e m~inv lvcd in the i mp r ~ l V < : drcliubi liry arl'the air bl m vc , t b, fcf th e o ur g u ~feed rare, an d th e utomatic hu tdown device . d itional im p r tant

o ntr Is invo lv th e ara lytic re actorfeed t empennur , watcr flow to t h ~ :

Sulphur No 284

bo1kr an d . ulphur conden sers ant!s te am pr e . ur e ontrol s.

EquipmentRehnive t th e reliabilit , pecifi a ll re f erring to o n - tr ea m ti m of modilied SRUs, there a r three sp e inlpieces of equipment that an vita l.rh e as a vessel . tr u ru r alm dia .

T h..:rcli>r e tJP r 'f r a t o r y ! i n ~ u l a l l~ s tern 111u t be adlqua tely on civedan d de i ne d tn mamtain th :tLcl&h ell temper:nun : b t w e n I o'5 and

0" , 0 30 - 50 F ). The rclra torys st m mu t in Jude t ot h the internalr ' fra tO heme : 1 ~\\'ell as th e outside insulatio n . T no mu h >utsidc in~ u l a t in c.:a n c a u ~ c th e ~ t c l sh e ll 1\overheat,\ hilc m ~ u f l iienr ou t :-.1de 111-&ulati n u . u all r . u lt . 111 th e !>hellbei ng tn o col d , allo\ ing W < l t ~ rondcnsa tio n and all of it d ~ : tr i mntal ciTe ~ .

(rl!ncrall ' th e mtcm ul r e r r ~toryl : o m. i~ t ! > ot a mm1mu m of two dislinct

layer . Tht in id lu cr in dire t. onta t with th e ho t omhusuon ga e . 1su.c.,uall a high alumina (. I20 \) m aLI.!r-ial bccau c the ah]mi rw ca n wtthstandtJ J. hjgh o m h u ~t ion tempera t urc hett.cr than almost all ot.her rcfr t ry

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materials and is resistant to the effectsof he high tempera ture oxidising andreducing atmospheres in a thermal reactor. However, the high aluminumrefractories have rather p oor insulatingqualities.

The second layer of refractory isplaced behind the high alumina material. This layer is usually in directcontact with the steel shell an d is aninsulating layer (in contrast to th ehigh alumina layer), designed to keepthe steel shell within the desired temperature Limits mentioned above.

Several types of refractory havebeen used to provide an adequate system to properly protec t the teet shellof the thermal reactor at all of the ambient conditions expected (temperature, wind velocity, etc) as well as atall of th e anticipated internal operating temperatures . The significantvarieties of refractory employed inSRUs in th e thermal reactor serviceinclude brick , plastic an d castable.Sometimes the two (occasionalJythree) layers have consisted of brickonly, or on e brick layer and onecastable or plastic layer . Castable hasbeen used as both layers an d on.occasion plastic has been used in conjunction with a castable layer, but rarelyhas plastic been used in both layers.

Th e most frequent design used istwo brick layers with th e inside layernear th e flame being a dense refractory of about 90 % alumina or higher .The second (backup) layer is insulating firebrick (IFB) This brick is avery good insulating material, usuallycontaining a reasonably high contentof silica (Si0 2). Brick layers are usedbecau se they require no anchors onthe interior vessel shell, plus thebricks have very little water content

compared to castable or plastic. In acircular vessel th e properly installedbricks support themselves .

Any castable layer used requiresanchors. I f castable refractory is usedfor both layers (high temperatur e an dinsulating), ce ramic anchors are required because th e anchors in an allcastable refractory system protrudethrough the in ulating layer into thehigh temperature alumina layer to insure th e refractory lining stability.Metal anchors cannot be directly expo ed to the high temperature gases.Fo r this reason , th e castable refractory i s so metimes used for th e insu -

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lating layer, which is covered by ahigh alumina layer of high temperature resi tant brick.

The plastic refractory is installedin a similar manner as noted abovefor castable. Fo r Claus unit it is im -perative that an air setting plastic beused. In some cases th e vessel diameter could be such that air settingpia tics ca n be successfully instal ledwithout anchors. I f th e air settingplastic is carefully cured, it becomesa very effective monolithic lining.

Successful refractory installationsinclude th e suitable selection of materials as weH as proper installationan d curing. Failure to adequately perform any one of these three functionswill result in premature refractoryfailures, usually causing unplannedClaus unit shutdowns for repair. Inorne instances, failure of the refrac

tory system has caused unscheduledthermal reactor vessel failure , resulting in repair or replacement.

Equally important to the inte .rnallining of the thermal reactor to th eservice life of this vessel an d sulphurpla nt reliabiLity is the type of exteriorvessel insulation used. To o much outside insulation ca n cause the vesselteet shell to overheat; shell temper a

tures as high as 590-6oo c (1094-1112"F) have been recorded. Insufficient or no outside insulation allowsthe steel shell to become too cold;temperatures as low as 115-130 "C(239-266"F) have been measured.These low shell temperatures permitwater condensation, causing corrosiveacids to be generated resulting in premature shell failure. Overheating ca nalso result in premature vessel failure.

Proper outside insulation ca nbe accomplished with a thin layer

('/s-lf4in, 3-6mm) of ceramic fiberinsulating material covered withaluminum or by employing a rainshield constructed o f stainless steelor aluminum . Many times the intentof the designer is no t rememberedafter years of operation an d personnel changes an d , therefore, i t is notuncommon for damaged refractoryto be replaced by material from theinstallers warehouse, instead of th ecorrect material. This will reduce reliability as problems accelerate an dinferior replacement materials areused to a greater extent. Also , th efact that the shell must be ho t to pre-

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vent corrosion ha caused problems.In on e instance, a portion of the unitwas determined to be a potential personnel hazard because it was to owarm an d the protective barrier hadbeen removed long ago. It was fullyinsulated. When the leak was noticedand the unit shut down, it was discovered that th e steel was destroyedbut that th e refractory was in placean d undamaged.

Bole.Once these ho t flue gases successfullyleave th e (thermal reactor) they mustbe cooled for funber processing inthe SRU. Usually a fire tube type(hot gases on the tubesidc) wasteheat boiler is the device used to accomplish the fl.tSt step of this coolingtask. These boilers genet"Bte steam asth e gases are cooled. Boiler steamside operating pressures range from34 5 kPa to 4138 kPa (50 to 600 psig)

Fo r Claus plant reliability, boththe boiler inlet tubesheet an d inlet tothe tubes must be protected from direct contact with the hot combustiongases. Again, here is where the use ofa properly designed refractory systemis critical. I f th e tubesheet and tubeinletS are no t properly protected, thetube to tubesheet joint wilJ leak resulting in a unit shutdown. Generalisations are difficult here because thesystem is a function of th e type ofboiler (kettle style or liquid full), thesteam pressure , an d the temperatureof the combustion products in thethermal reactor .

BurnerThe feed to a modified Claus sulphurrecovery unit usually does no t co ntain S 0 2 (a vital ingredient in th e

Claus reaction of H2S + S 0 2) .Therefore, it is nece sary to generateS 0 2 by partial combustion of H 2S.Thus, a critical piece of equipmentrelative to the sulphur plant reliability is the burner . I f th e burner failsth e SRU will be shu t down forburner replacement an d /or repair .Many times the SRU burner severelyfouls and/or plugs, which causes premature shutdown. In most cases thisburner pluggage i caused by saltsformed in the feed gases .

The mo t frequent salt to plug theburner is an ammonium polysulphide whi ch is usually a result in the

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SULPHUR RECOVERY

Fig . 1: Claus Combuster

reaction up :,tr aut of th e burn.:r hct\ \ 'Ce n hydr ge n ~ u l p l n d(11 2 ) andammoma ( N H~ ) . U>th H 2 anJ N H~ar c pr e sent in Clau s ulphur rc coveunit f,ed g a ~ c. in most rdineri . .H wever , t h a e i u unlly no ammoni a in th e 1-12 ' ri h h.:d ga s in n::uuralga s pr o (' H ~ S ) stream fl' m th e armnctrcaung unit and th e our f.!as '> t reamfrom th e so ur \Vate r stripper se parate,! until ju t befon: entering thburner . This prev.:ntiv.: p n cdurctherefore rcyuir.:s M pnrat ' )au un nfeed gas kno k-out drum .

If th e ammonium poly ulphidc. ai r format1nn an d on equ.:nt prc -

tpitati, 11 i1> caused b 1-1, in th.:amine uni t s u r ga , rhe n.: quir dprevcnti e m thod i a 1\\'l' - fol I pr -c durc . )nes t p i to c linun at e theNH 1 fr m th e gas s t ream up trcamof the am me absorh r . The ccon 1s te p is to h..:at th e amin a id gasd \\ n ~ t n: a m of th amine regenera-to r ov.:rhcnd cond n r co prevlntpreci r i t :ni nn fan ammonium pt)lvsu l rhid c . :llts. Th TP .lau!- Cnmb u s w r (sec Fig . 1) , minimiset t:xtreme p rating co n ditions. T P A calls thi. a r a of the pl3ma HotDc t:k ( ce Fig . ), an d ~ u p p l i ei1 as a m>dulc or assemhl so that allth e comp n nts of t:hc system arem: :m :hc I in d sign an d apability .

Other equipmentIn addition t o th e e thn: qUJpm n titems d i s ~ : u' Cd aboYe, ther e ar v-era! other very im1 orrunt pieces ' fequipment n n RU that will ctTc tunit reliahi lit . The c include th e fc-:dacid gas kno k-out drums, air b lowers ,sulphur om k n s r s , utalyti r a c tu rfeed heater!> , catal li e rc tors in 1n -

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~t:~

~ t H ~ ~ l ~ ~ f ~ l ~l l

r tor, liqUid bulphur p u m p ~ ,nnd hq utd !>ulphur ~ c a l(dr:lin ) de\'ll:C'- .

Tht a id a . knock - ou r drum .o.:ffl'C l rehahiht ' only if th e ar e mndcquat I , i:r.ed . Tho.:sc drum s shouldb !l i i .Cd to minim1 c li 1uid c arryover .Liquid s c ntcnng th R thermalr a to r wtll ausc re ro tory damage .

T h e fatlure of th e 3t r blower wtllausc rcmature :laus Ull l t l>hutduwn .o prevent t . h c ~ cpr marun >.hut

down:-:, u ~ u a l l ytw o full s1zc ( 100' ' ';,) airblow TS ar e in stalled w uh on e serv inga ~ a spare. )n oc astPn, thr .: 50- 0%ai r blO\.\'C have been 111Sla11 d s that:Jt full p e r a t i n ~r a t t ~ ,tw o blp:lrc .Th i three blower arran emcnt i ~ oftenvery dTc..:ti,e if th e : lau-. pl:mt 1s ex pe ted w operate over a wide range offi ed rate:., ~ u h a. 1 - tOO %. At luw.:rrates, only

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SULPHUR RECOVERY

condensers before it enters the caLalyri rc a ro r . ln dire t reheat habeen accomplished with ho t oil, hi hpr e ur c s team, ex hangc with reacto r efflu n t and et c tric heaters . A

long as th ' h at so u r c is ready th esetypes o f reheat arc hi gh! reliahl .Direct fired reheat u in ei ther a idgas or fuel ga s a th fuel ar c n ot areliable a indirect heaters. TP ha sseen cve ral of th e e ' ystems whereth e cont ro l were not adequate an dth e burners di d no t mi x th fuel a ndai r proper ly. 1 o, u s in g acid ga s a sth e fue l result m a lower vc rall sulphur reco very owing to th materialbypas in g th e cu ta lyti reactors.

The cata lytic r eactor an.: ca rb onsteel ve el that ma y or may no t be refractot lined. The vessel tsd f doesnot reate any reliability p r blm s .Th e RU catal st ca n he orne fouled

r therwise rendered inactive for several reasons. The three major reasonsarc so o t, sulfatio n and fi re s. Soo t , orunburned h dro arb n, is a re su lt ofpoor operation of the upstream amineunit allowing h d arbon to enter the

R . ul f ation re . ults fr m oxyge n

omami nation du to po o r! operateddire r fired rch cate rs. Pi r es ar theresult of either a misguid ed attempt toregene ra te th cara l st or poor operation of th e unit d urin g shutdo wn . The

RU op -rator ha s omrol nly overthe procedure that can ffec t . ulphurfi res. Here, reliability an be signifiandy impr vcd b trai nin g. TPA re -

ommends that operators he given u r -fres he r trainin g oursc eve threeyears .

As mentioned hclO\,v in ineratorshutdown s on e f tJ1c fun ti ns thathould a lso shut down the SR .

Th reforc, th e incinerator is important to th e reliable op e rati on f th e

R . The simpl natural draft incinerato r is ba s i ally a refractor linedco mbu s tion humber itting below arefractory li n ed discharge sta k. Theitems that will sh ut th e inci nerat ordown an: 1 of na m e an d high temperature . The flam e i as reliable a the fuel stem and th tem per atur e isas reliable as t he hydr o arbon removalfrom the feed acid ga . In other words,this stem is high! re liab le . A ~ th eincine rator becom s more comp lexthe reliabili ty is rcdu ed . Air blower 'require pares; ~ n c r a t i n gsteam fromthe inci n erator effi uent brings with it

Sulphur No 254

th e hutdown sys 1cms nsso iated withtl1c boiler . Ea h pic e added to th inci n rator mu st be d1 ught f in term of tlt e dTcct on the RU re liability .

n the same ba i a , th e sp:lring

o f air blo\ er s , th e liquid su lphurpumps arc of t n pared with a sparepump in th e ware h o use read y to bein st.alled .

Liquid ulphur seal leg plu 'ging isprevented by adequate heating a ndproper unit cleaning prior to sta n up .U sua l! low pressure team ''>-1'5- 52 0kPa (50-7'5p ig ) is t h ~ he atingm dium provided . Higher pressurestea m i s n ot u ed be au se of the potentially high vis sity of molten ul

phur an d th u low newtonian fl wcha r acteristics at su lphur temperaturea b ~ ) v e17s c ( 47 "F ). Pr evention ofliquid sulph ur seal plugga g used b metal s ulphide s, metal -oxides, ca ta -1 t, et c . involve an inf. rmcd desi ' 0an d m on importantl y, proper and ju -di iou s start up of th e !a us un it. If amolren sulphur sea l plugs, frequent!th e SR ha to b shutdown prematurel y. However, there ar c occn ion.when t h ' e plugs can b..: re m oved from

th liquid sulphur drain lines withoutau. ing an unwanted shutdown.

Controls andshutdown functionsEv e n with dt e best equipment designanJ pr pe r eq uipm e nt care an dmaintcnan e, SR reliability can becompr mi s d by im p r oper in s t rum n t design an d /or negligent op rn -tion, in tullation an d /or m ~t i n t e n u n

f th in , trumentation . Prop er in. trument icsign include s th e properphilo ophy as well as in . t rumentequipmen t selection .

nc.:: of th e m os t frequent ca usesf sulp hu r plant reliability difti ult ies

i rhc se lection an d insta llati on of automat ic s hu td l wn devices . Bccausof the toxi ase (pri marily H 2 an d

z) pr o c s ed in th e SRU, whi harc often a t very high temperature s,above I 00 0 (J 832 F ), autornati

shutdown

s ar eabsolutely ne essary

to protect both personnel an d eq uipment. "The m os t frequent error in design phil ophy is to include moreautomatic shutd owns than is ab -ol utcly ne essary. Often these tr a

s hucd wn devi es cause unnecessaryunit sh utdown s thereb ausing the

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to ha ve a poor o n- ~ t r c a mr elia. A wbulation o f th e automati

. hu t down s required n R forbotl1 pcr.;onncl and equipment safctreasons i li ted be low .

L w water lcvciJn th wa ste heatboiler ys t cm Low air flow to th e main ombus

tion bu rn er High liqui d level in th e amin

acid ga s kn ck -out dru.m Low amine acid gas flow (backup

h r item 2 abovc when th e lau sunit i, not o pcr:ning on automaticai r t a id gas rati ntrol.

fn incrator sh u tdown (hi g h t emP ratur or flame failure )

High temperature in th e thcm1alrea to r High level in tl! . ur atcr s trir

per knock - out drumIn addition t hoc sing on l '

th se shutd wn fun tions that arca tually required, th e orr ' t an dreasonabl e . tting fo r an automatish ut d wn vo.lue must be u d . rorinstance , at ne plant th i n c i n~ r a t o rha d an atlt matic shutd o" n cttingof only 70 5 " (1300 F ). t thi r la

tivcl y low cning rhe de s ig n operatin g temp raturc w a ~65 0 " ( 12 00 . 1-'),th e in incrator frc qu nt l s hut downth e , ulphur plant , which cvc.:nl

au . ed fl aring f b th th e acid gafeed streams . fn mo t cases th in incr a te r high temperature shutdown is'Ct at abo ut 9 o (18 O"F), which iusually adcqunt protection in enormally th e refractory u se d in theincincnt tor an d in inera tor ' ta k. h o ul d b rated at a minimum o

1zo

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F1g 3 SunD eckr"'

w tthout the tall ga U''Uter. As long asthe R remains n-strcam there willb a rccov r f ulphur ranging b ' twee n 9_ ' % an d 7%, n:sultin in nlya ~ m a l lp r entag uf sulphur t:ingadmitt d to the aunosphere. H wever,tf the R 1 JS l>hut d wn , n.o ulphurrc ovcry will be made an d of the feedulphur will be mmcd to the attn -

pherc . Thi . 100 % emission level iusua lly undesirable t h e r e ~rc the R~ h o u l dn t be auto mati all hut downwhen th ' tail g a ~t r l ~ a tc r i nu t matically shut down. How vcr t.h tail gasfrom th e . ulphur re ove unit needt be a utomaticull diverted from thetail gas t r ~ . : a t i n gunit directly to th e in-cine rator, when. th e tail ,a , treater ishUldown. 11u" ~ t l . ! mm t l ' t be highlvreliable becau 1.: th e c valve mu5twork tn a sh u tdown ituau n . TheTP A unDc k ( ~ C l 'F1g. 3) , 1 an cx~t m p l this type of di cr t r valve sys

tem with , p..: 1:1 11 de ig n d va lve s tpre, en t . ticking.

Combus t ion control

111. mo . t impmt:mt omml fun ti nin the R t& that of partial combu -tion