p. 1 

# OAR Box 1784 Prepped by Ollie Stewart



## Document

Number: /*%[ 

# 82)ll-l- « Docket Number: 

# A--84-4 4

Printed 10/31/2008 8:05:41 AM Sheet 82 of 143P. 2 

# /f-8i-44

) 

# Treatin g hydroge n sulfide :

A n alternativ e t o Clau s Thi s proces s uses a catalyti c reagen t t o selectivel y oxidiz e th e hydroge n sulfid e presen t i n ga s stream s int o elementa l sulfur . It i s bot h economi c an d environmentall y sound . L C . Hardison , AR I Technologies , Inc . 

# F o r decades , th e amine-Clau s combinatio n ha s bee n industry' s workhors e fo

r removin g hydro ge n sulfid e fro mga s streams . But , stric t air-qualit y regulation s pro mulgate d i

n th e lat e '70 s spurre d th e developmen

t o f ane w se t o f processe s characterize d b

y highe r performanc e an d H 2S-remova l efficiencies . Unfortu nately , man

y o f thes e promisin

g newe r route s have als o bee

n plagued b

y hig h reagen t cost s o r b y persisten t

operating problems . I n fact , th e first

tw o generation s o

f LO-CAT , aproces s based o

n aliquidphase-oxidatio

n technique originate d b y Humphrey s & Glasgo

w (London) , als o ha d their share o f teethin g prob lems . However, wit

h th e introduction o

f ane w catalyti c reagent—AR I 31 0 —i n 1982 , th e third-generatio n pro ces s i s no w proving itsel

f t o be a n economical , reliabl e an

d saf e alterna tiv e to th e Clau s process . Indeed , i n th e tw o year s since AR

I 310' s introduction , mor e tha n 1 0 LO-CA

T unit s have bee

n pu t onstream . Furthermore , apatente d autocirculatio n scrubbe r (Fig . 1 ) commercialize

d i n 198 4 ha s mad

e th e LO-CA

T proces s eve n mor e attractive . I

n thi s innova

tion , two previousl

y separate opera

tions , absorption an

d oxidation , are no w conducte

d i n th e sam e vessel . Capi

ta l cost s have thu s bee

n reduced sub stantially

—th e uni t

no w cost s les s tha n eve n asimpl e Clau s plant . Further , i n contras t t o th e Clau s system , i t provide s mor e complet e air-pollutio

n control without

having t o resor t t o tail ga s cleanu p o r incineration . I

n May , th e firs t LO-CA T autocirculatio n system , capabl e o f producin g 2 tons/ d o

f sulfur , wa s starte d u

p to trea t th e H 2S-lade n gase s extracte d fro

m5 0 millio n ft 3 / d o f natural ga s a t

Tejas Ga s Corp.' s India

n Roc k plant nea r Gilmer , Texas . SOM E PRO S —Wh y shoul d user s elec t

to us e LO-CA

T instea d o f on e o f th e olde r processes ? Her

e are som e reasons : • Th e AR I 31 0 reagen t

— adilut e solution o f organically-chelated iro

n i n water — i s highl y active , ye t

nontoxic . I t operate s b

y oxidizin g H 2 S ion s ab sorbed int o th e solution directl

y t o ele menta l sulfur , thu s turnin g apotentia l pollutio n proble mint o asalabl e prod



# I

-'•= 3

y combinin g th e absorbe r an d oxidize r int o a singl e unit , capita l cost s ar e reduced Reprinte d fro mCHEMICA L ENGINEERING , Jan . 21 , 1985 . Copyrigh t © 198 5 b

y McGraw-Hil l Inc . 122 1 Avenu e of

th e Americas , Ne w York , N.Y . 1002 0P. 3



# f uct . An

d since nothing i s discharged t o th e atmosphere , o

r flared, there i s n

o nee d fo r an y air-pollutio

n controls . • Th e reagen t i s completel

y selective fo r H2 S . Th e C0 2 conten

t o f th e ga s doe s not affec t efficacy, a

n importan

t featur e i n som e applications , suc

h a s enhance d oi l recover y o r beverag e pro cessing , wher

e th e concentration o

f C0 2 i n th e ga s strea

mi s high . I

f re quired , som

e C0 2 ma y als o be remove

d b y stron g bufferin

g o f th e solution . • Becaus e o

f th e hig h activit y o f th e LO-CA T catalyst , th e plant s ca

n eve n operat e o n air stream s o r wit h oxyge n adde d t o th e absorbe

r withou t being affecte d b y byproduct

reaction s tha t reduc e th e amoun

t o f sulfu r recovered . • Turndown , wit

h respec t t o bot h ga s flo w an d H 2 S content , i s 100% . Thus , th e treate d ga s wil

l still mee t specifications , eve n i f it s flow i s low . • N o hea t i s require d t

o regenerate reagent ; hea

t i s neede d onl y fo r freeze protection , o

r fo r keepin g solutio n temperatur e abov

e th e ga s dewpoint . HO W I T WORK S —The LO-CA

T proces s ha s tw o basi c designs : aerobic , whic h absorb s hydroge

n sulfid e fro mairlade n streams , an

d anaerobic , used whe n th e ga s strea

mha s littl e o

r n o oxyge n present . Th e scrubbing mediu

mused i n th e LO-CA T proces s i s a

n iron/water

solution , buffere

d wit h sod aash , potassiu

m hydroxid e o r an y othe r commo n alkal i to ap H o f about 8 . Afte r th e H

2 S i s contacte d wit

h th e reagen t

i n a n ab sorber , i t

react s wit h th e dissolve

d iro n t o for melementa

l sulfu r accordingly : H 2 S + 2Fe + + - 2H + + S + 2Fe + + Th e iron , no w i n areduce d ferrou s form , i s not

consumed ; instead , i t

i s circulated to asecon

d vessel , a n oxidizer , wher e i t i s regenerate d b

y bubbling air throug h th e solution : y 2 0 2 + H 2 0 + 2Fe + + -

2(OH) " + 2Fe + + + Althoug h iro n isn' t consumed , ther

e are som e chemica l replacemen

t costs . A smal l portio n i s los t a s th e chelatin g agen t degrades . Further , i

n smal l units , i t ofte n i s not economica l t o filter an d recove r th e reagen t

completel

y fro mth e precipitate d sulfur : Normally , th e sulfu r i s separate d b

y gravity , centrifugin g o

r melting . Th e abov e two reactions , generally commo n to mos t liquid-phase-oxidatio

n processes , are ver

y specificall

y geare d t o conver t H 2 S int o sulfur . However, larg e amount s o

f abyproduct , thiosul fate , ca n be forme d whe n H 2 S ion s an d oxyge n com e int o contact . I

n practice , thi s happen s whe

n sulfid e ion s "leak " out o f th e absorbe

r an d ge t int o th e oxidize r — becaus e o

f eithe r inade quat e residenc e tim

e o r lo w reagen t activity . Th

e hig h activit y o f th e AR I 31 0 reagent , however , tend s to sup pres s suc h byproduct

formatio n b y rapidl y converting H

2 S to sulfur . Thiosulfat e ca n als o resul t

whe n oxyaammmWmBmsam^mmmKammsm 50 0 400 - 300

20 0 r E a. ' 5 c r U J 100 . -IllllllllIllllllll1111111IIllllllllll Wit h centrifug e

Withou t centrifuge^ r

i i i i i i i i M .0 1 . 1 1

Plan t capacity , tons/ d (wit h 30 % H2 S i n fee d gas ) 1 0 ^armr"units^Y^teifuges^D^j|br^Mlag M ge n dissolve d i n th e oxidize

r i s no t use d u p an d i s carrie d bac

k t o th e absorber . However , i

n th e LO-CA

T process , th e reagen t keep s th e concentratio n o

f th e oxyge n i n th e re-oxidize

d solutio n a t onl y abou t 1 ppm . APPLICATION

S —Aerobi

c use s o f LOCA T includ e treatin g o

f proces s ef

fluents , ai r discharge d t

o plant ventilatio n systems , an

d sewage-plan

t lif t stations . Amon

g anaerobi c applica

tions : treatin g refiner

y fuelgas , digester ga s fro msewag e plants , an

d aci d gase s fro mamin e treaters ; disposin

g o f strippe r ga s fro msou r waters ; an

d processing synga s fro

mcoa l gasifiers . Th e mos t commo n application s fo

r LO-CAT , however, ar e i

n ga s process in g an d conditioning , Natural-ga s sweetening , fo

r example , require s H 2 S concentration s t

o be reduce

d t o aleve l o f 4 pp mo r less . Th e usua l wa y o f doin g thi s i s wit

h asimpl e amin e treat er , o r wit h amor e comple x syste mi f there i s ahig

h rati o o f C0 2 t o H 2 S . I n low-pressur

e (15 0 psi g o r there abouts ) sweetenin g operations , th e LOCA T syste mhandle s bot

h gas-sweeten ing an d air-pollution-control operations ; an d although i t

cost s mor

e tha n a n amin e treate r an d flare combined, i

t offer s som e pollution-contro

l benefits . A t highe r pressure s (aroun

d 1,00 0 psig) , an d particularly wit

h lo w H 2 S loading s (20

0 ppm) , th e simple

r autocirculation uni t

ca n be employed , makin

g th e LO-CA

T cost-effectiv

e compare

d wit h alternativ e systems . Beside s sweetenin g natural gas , LOCA T ca n be used t

o scru b regenerato r gase s —fro

mamin e treater s —befor

e ventin g them . An

d high-C02 -conten

t re generato r gase s als o ca

n b e processed fo r us e i n enhanced-oil-recover

y operation s o r i n beverages . Fig . 2 detail s th e capita

l an d operating cost s o

f autocirculatio n LO-CA

T units , bot h wit h an d withou t centri fuge s fo r separatin g th e sulfur . I

n case s wher

e th e amin e treater' s offga

s i s primaril y carbo n dioxid e an d water wit h ver y littl e hydrocarbons , incineratio n wit h suppor t fuel , instea

d o f flaring , i s required . Here , replacin g th e incinerator wit h aLO-CA

T systemca

n resul t i n apayout o f les s tha n on e year . Jay Chowdhury, Editor 

### Th e autho r L . C . Hardiso n i s presiden t o f AR I Technologies , Inc. , develope r an d licenso r

o f th e LO-CAT H 2 S-oxidatio n process . A cofounde r

o f AR I i n 1970 , h e wa s previousl y employe d b y UOP , Inc . H e hold s ove r 5 0 patents , severa l o f whic h ar e relate d t o LO-CA T technology .