Throughout the years, the Clausprocess has undergone a contin-uous evolution in attempts to

increase the sulphur recovery effi-ciency of the process. In the 1930s, athermal stage was added to the twocatalytic stages, which increased therecovery efficiency from 95% toapproximately 97%. In the 1970s, theSCOT process was introduced whichadded hydrogenation/hydrolysis plusamine separation to treat the tail gasfrom the Claus process. In 1988,SuperClaus was introduced, whichadded a selective oxidation reactor tothe end of the Claus process, increas-ing the efficiency to approximately99%. And just recently, EuroClaus1

was introduced, which replaced thesecond Claus reactor in the Super-Claus process with a selective hydro-genation catalyst increasing the effi-ciency to 99.5%.

It is obvious that the trend is toachieve higher and higher sulphurrecovery efficiencies. It is anticipatedthat in the near future Claus unitsinstalled in developed nations willrequired sulphur recovery efficienciesof 99.5% or better. Coupling a Clausunit and a liquid redox process, such asthe LO-CAT® either directly or indi-rectly in combination with a hydro-genation/hydrolysis reactor, however,can reliably achieve 99.9+% sulphurrecovery.

Direct tail gas treatmentWhen considering liquid redox to treatClaus tail gas without the inclusion ofa hydrogenation/hydrolysis reactor, theamount of SO2 in the tail gas is animportant operating parameter. Sinceliquid redox units operate at alkalinepHs in the range of 8 to 9, any SO2 inthe tail gas will be easily absorbed, andform sulphates in accordance withreaction 1:

SO2+2NaOH+½O2Na2SO4+H2O (1)

It is important to note that SO2 doesnot interfere with the liquid redoxchemistry and consequently does not

affect the H2S removal efficiency ofthe process. However, reaction 1 doesaffect the operating cost of the processin two ways. First, two moles of caus-tic are consumed for each mole ofSO2 absorbed, which increases theoperating cost of the unit. Secondly,the resultant sulphate product accu-mulates in the liquid redox solution,and eventually a blowdown is requiredresulting in loss of valuable catalystsolution. Replacing lost solution addsfurther to operating costs. Conse-quently, if this process configurationis to be employed, it is advantageousto minimise the formation of SO2 inthe Claus unit.

CLAUS AND LIQUID REDOX

Sulphur No 274May . June 2001 11

Liquid redox enhancesClaus process

Claus units can easily achieve sulphur recovery efficiencies exceeding 99.9+% by employing a liquid redox system such as LO-CAT as a tail gas treating unit. The significantlylower capital cost of this combination compared to conventional amine-based tail gas units offsets its higher operating costs. Further benefits include reduced sensitivity of the Claus unit to changes in feed gas composition and flow rate and excellent turndown capability.

Gary J. Nagl of USFilter Gas Technology Products discusses the merits of this process combination.

The first LO-CAT unit to treat Claus tail gas was recently installed at the WesternGas/Anadorko Bethel plant in Texas, USA.

The SO2 formation rate can beminimised by operating the Clausunit with sub-stoichiometric quanti-ties of oxygen, thus increasing theH2S:SO2 ratio in the unit.The effectof this mode of operation can be seenby analysing the Claus reactions.

H2S+1½O2 SO2+H2O (2)2H2S+SO2 3S+2H2O (3)

The conventional mode of operationfor a Claus unit is to convert one thirdof the H2S to SO2, which then reactswith the remaining H2S to form ele-mental sulphur.This is accomplishedby carefully controlling the quantityof oxygen entering the system. Due toequilibrium limitations, some of theSO2 leaves the system with the tailgas.

If the unit is operated in a mannersuch that there is insufficient oxygento complete reaction 2, then there willbe insufficient SO2 produced to com-plete reaction 3, and the H2S removalefficiency will be reduced. However,the amount of unreacted SO2 in thetail gas will also decrease.The effects2

of H2S:SO2 ratio on the H2S andSO2 contents in the tail gas and theoverall removal efficiency are illus-trated in Figs 1-3.

A flow diagram of a typical LO-CAT liquid redox unit for treatingClaus tail gas directly is shown in Fig.4. Since the liquid redox system isaqueous-based, elevated tempera-tures will cause water balance prob-lems; consequently, the tail gas is firstpassed through a cooler where the gastemperature is reduced from approxi-mately 135°C to 50°C.The cooled gasthen enters a knockout pot where anycondensate is separated. Dependingon the amount of condensate and sul-phur tonnage, this sour condensatemay be employed as make-up waterto the liquid redox process. If it can-not all be used as make-up water, theremaining sour water will need to beprocessed through a sour water strip-per with the sour gas being directedback to the liquid redox unit.

For direct treatment of Claus tailgas, the LO-CAT process would em-ploy a proprietary Mobile Bed Absor-ber (MBA) because of its low, inher-ent pressure drop (approximately 500mm of WC). For contacting media,the MBA uses hollow, ping-pong-likespheres which, when fluidised, are

CLAUS AND LIQUID REDOX

Sulphur No 274 May . June 20012

10,000

1,000

100

10

SO2

in ta

il ga

s (p

pm)

H2S:SO2 in Claus Unit

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7

3-stage Claus 2-stage ClausSource: US Filter

Fig. 2: Effect of H2S:SO2 ratio on SO2 in tail gas

99

98

97

96

95

94

93

92

sulp

hur r

ecov

ery

effic

ienc

y, %

H2S:SO2 in Claus unit

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7

3-stage Claus 2-stage ClausSource: US Filter

Fig. 3: Effect of H2S:SO2 ratio on sulphur recovery efficiency

10,000

1,000

100

10

1

H 2S:

SO2

in ta

il ga

s

H2S:SO2 in Claus Unit

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7

3-stage Claus 2-stage ClausSource: US Filter

Fig. 1: Effect of H2S:SO2 ratio on tail gas

self-cleaning. Within the MBA, theH2S and the SO2 are absorbed intothe circulating solution, and the sul-phide ions are converted to elementalsulphur in accordance with theReaction 4 while the SO2 is convertedto sulphate as indicated by reaction 1.

H2S + ½O2 H2O + S (4)Fe

As indicated in Reaction 4, the reac-tion is catalysed by a proprietarychelated-iron catalyst.

MBA’s are normally designed toreduce the H2S concentration in agas to approximately 10 ppm. For tailgas applications with H2S:SO2 ratiosof greater than 2.0, this would resultin overall removal efficiencies of99.99+%. Even if the ratio weremaintained at 2.0, the removal effi-ciency would be 99.7%; however, thecaustic consumption would be muchhigher as indicated in Figure 3.

The solution exiting the MBA isdirected to an Oxidiser where air isinjected to regenerate the iron catalyst.If there is insufficient pressure avail-able for moving the tail gas throughthe MBA, a blower can be supplied oran eductor may be installed as shownin Fig.4.

LO-CAT systems are designedwith very ample liquid inventories.Although this increases the capital

cost slightly, there is sufficient capaci-tance in these systems to render themvery insensitive to sudden changes infeed conditions.Thus, fluctuations inthe H2S:SO2 ratio in the Claus unitwill essentially have no effect on theoverall removal efficiencies of the sys-tem as long as the Claus unit contin-ues to run. In the event that the Clausunit is unable to operate due to turn-down requirements beyond its capa-bilities, the system can be designedto bypass the Claus unit entirelyand route the acid gas directly intothe LO-CAT unit. This mode ofoperation (Fig.5) will still yield H2S

removal efficiencies of 99.99+%.Theversatility of the liquid redox systemwill ensure that the overall system willachieve 99.9+% removal efficienciesat all times. This inherent feature ofthe system is well accepted by regula-tors. In addition, the effluent from theliquid redox unit will not requireincineration since it will only containa very small amount of H2S andessentially no SO2.

Indirect tail gas treatingIn this processing scheme (Fig. 6) allsulphur compounds in a Claus tail gas

CLAUS AND LIQUID REDOX

Sulphur No 274May . June 2001 3

quench tower

makeup waterto oxidiser

steam

water to WWTP

vacuum belt filter

coolingwater

coolingwater

sour water stripper(optional)

eductor (optional)

mobile bed absorber

oxidiser air blower

wash water

exhaust to atmosphere

Claustail gas

sulphur cake

Source: US Filter

Fig. 4: Direct treat LO-CAT system

Claus unit

sulphur

exhaust toatmospherenormal operation

sulphur

closed closed

acid gas

acid gas

coolercooler LO-CAT unit

Claus unit

sulphur

exhaust toatmosphereturndown operation

coolercooler LO-CAT unit

Source: US Filter

Fig. 5: Claus bypass for 100% turndown

are converted to H2S by passing thetail gas through a hydrogenation/hydrolysis, catalytic reactor at elevatedtemperatures. Reactions 5 and 6(hydrogenation) and reactions 7 and8 (hydrolysis) represent the majorreactions, which occur in the reactor.

SO2+3H2 H2S+2H2O (5)S2+2H2 2H2S (6)CS2+2H2O CO2+2H2S (7)COS+H2O CO2+H2S (8)

In this processing scheme, a fuel gasis subjected to partial oxidation,which not only generates sufficient

heat to raise the tail gas to reactiontemperatures but also generates suffi-cient hydrogen (reaction 9 shownbelow) to satisfy the requirement ofreactions 5 and 6.

CH4+O2 2H2+CO2 (9)

After passing through the reactor, theeffluent gas must be cooled to approx-imately 50°C. This can be accom-plished by emploing a direct contactcondenser as shown in Figure 6.Alternatively, an indirect condensercan be employed.

In either case, sour cooling wateror sour condensate will be generated.Again a portion of the sour condensateor water may be used as makeup waterfor the liquid redox unit; however,some of it will need to be sent to a sourwater stripper with the vapor beingrouted back to the liquid redox unit.

This processing scheme will beeven more forgiving to changes in theoperations of the upstream Clausunit, since the hydrogenation/hydrol-ysis unit will act to muffle any com-positional changes from the Clausunit. Consequently, the amount ofSO2 entering the liquid redox unitwill remain fairly constant, and theoperating cost of the system willremain constant.

CLAUS AND LIQUID REDOX

Sulphur No 274 May . June 20014

sour water stripperoptional

hydrolysisreactor

fuel gasair

Claus tail gas

quenchtower

firebox

coolingwater

coolingwater

eductoroptional

mobile bedabsorber

oxidiser

wash water

vacuum belt filter

sulphur cake

steam

water to WWTP

air blower

exhaust toatmosphere

Source: US Filter

Fig. 6: Indirect treat LO-CAT unit

vacuumpump

filtratepump

melter feedpump sulphur

pit

filtrate tosystem

slurry fromoxidiser

filtratereceiver

wash water

vacuum belt filter flash drum

sulphur separator

water vent

to drain

reslurrytank

hot oilor steam

Source: US Filter

Fig. 7: Sulphur melter system

Sulphur disposalBecause of its poor quality, sulphurproduced from liquid redox processeshas a bad reputation, which in somecases is well earned. However, due tothe relatively small quantities of sul-phur produced in liquid redox instal-lations, most liquid redox sulphur hasbeen either landfilled or disposed ofas solid, agricultural sulphur; hence,not a lot of effort has been exerted toimprove its quality. However, greatprogress has been made in improvingthe quality of sulphur produced inLO-CAT units.3

Sulphur is produced as a solid ina liquid redox unit. Since the reac-tions are not gas phase, there is nodissolved H2S in liquid redox sul-phur; thus sulphur degassing is neverrequired.The sulphur is normally fil-tered and washed to produce a filtercake, which is 65% to 85% sulphurdepending on the type of filter used,with the remainder being water anddissolved salts. It is not possible tosimply dump this cake into the Claussulphur pit, since there is insufficientheat in the pit to evaporate the waterand to melt the sulphur. And even ifthe moisture is removed prior todumping the sulphur in the pit, thesolid particles have a tendency tofloat on top of the molten sulphurthus making heat transfer and conse-quently melting very difficult. Due tothese problems, the sulphur from theliquid redox system must be disposedof as a solid3 or melted prior to beingintroduced into the sulphur pit.

Melting of liquid redox sulphurcan be accomplished in either batchtype melters or continuous melters. Asa rule of thumb, sulphur productionrates of greater than 5 tons per day(TPD) warrant continuous meltingwhile rates less than 5 TPD generallyrequire batch melting due to econom-ics. However, continuous melters canbe installed on any LO-CAT systemregardless of size. A typical melter sys-tem for a LO-CAT system is shown inFig. 7.

The change in quality of the Claussulphur due to the addition of molten,liquid redox sulphur will be as indi-cated in Figure 8. As a matter of com-parison, specifications for sulphuricacid plant grade sulphur are less than1000 ppm carbon and less than 250

ppm ash. This suggest that addingmolten, liquid redox sulphur to aClaus sulphur pit can be done withoutdegrading the Claus sulphur to anygreat degree. However, the overallquality of the sulphur mix can be fur-ther improved by processing the liquidredox sulphur through a diatomaceousearth filter prior to directing it to thesulphur pit.

CostsBudgetary capital costs of direct-treat,liquid redox, tail gas units completewith coolers and sour water strippers

are contained in Fig. 9. For compari-son purposes, the capital costs4 ofamine-based, tail gas cleanup units(TGCU) are also contained Fig. 9. Itis obvious that considerable capitalcost savings can be realized by utilis-ing liquid redox systems as tail gastreating units.With regard to operat-ing costs, the liquid redox system willincrease the operating cost of theClaus unit by approximately $14 perlong ton of sulphur entering the Clausunit. For an amine-based TGCU thisfigure will be approximately $8 perlong ton. Consequently, each applica-tion needs to be analysed to see if the

CLAUS AND LIQUID REDOX

Sulphur No 274May . June 2001 5

35

30

25

20

15

10

5

0pp

m (w

t) in

tota

l sul

phur

Claus unit efficiency

96.0% 96.5% 97.0% 97.5% 98.5%

carbon ashSource: US Filter

Fig. 8: Increase in contaminants from molten liquid redox sulphur

16

14

12

10

8

6

4

capi

tal c

ost,

$ m

illio

ns

total sulphur capacity, LTPD

30 40 50 60 8070

Claus and amine-based TGCU

Claus and liquid redox (with melter)

Claus and liquid redox (no melter)

note: based on 2-stage Claus unit operating at 96% efficiencySource: US Filter

Fig. 9: Capital cost of Claus plus direct treat liquid redox TGCUs

savings in operating cost of the amine-based system justi-fies the higher capital cost.

Field experienceThe direct contact, tail gas treating scheme described abovehas never been demonstrated commercially; however, thetwo processes comprising the scheme are commerciallyproven.The HCR2 process, which is a Claus unit operatingwith sub-stoichiometric oxygen, is well proven while theLO-CAT liquid redox process has been in commercialoperation for over 20 years with 150 licensed units. In addi-tion, approximately 50 of these units are treating acid gasstreams (CO2 and H2S). Consequently, there is no reasonto believe that the proposed arrangement would not oper-ate satisfactorily.

Variations of the indirect, tail gas treating schemeemploying an intermediate hydrogenation/hydrolysis stephave been in commercial operation since the early 1970swhen the Beavon Sulphur Removal Process was intro-duced.This process consisted of a hydrogenation/hydroly-sis reactor followed by a Stretford unit. Approximately 30of these units were installed.The process fell out of favourdue the potential toxicity problem with the vanadium cata-lyst employed in the Stretford process. Due to this prob-lem, iron-based, liquid redox processes such as the LO-CAT process has since replaced the Stretford process.

Recently,Western Gas installed the first LO-CAT unitin a Claus tail gas application at their natural gas produc-tion field in Palestine,Texas, USA. Initially, the LO-CATunit was treating amine acid gas; however, as the process-ing and the sulphur capacities of the facility increased, a2-stage Claus unit and a hydrogenation/hydrolysis reac-tor were installed, and the LO-CAT unit treated the reac-tor effluent.This system is yielding over 99.9+% sulphurremoval.

ConclusionsClaus units can easily achieve hydrogen sulfide removal effi-ciencies exceeding 99.9+% by employing a liquid redox sys-tem such as LO-CAT as a tail gas treating unit.The combi-nation of Claus and liquid redox has a significantly lowercapital cost than conventional amine-based tail gas unitswhich offsets its higher operating costs. In addition, the liq-uid redox unit will significantly reduce the inherent sensi-tivity of the Claus unit to changes in feed gas compositionand flow rate. And if designed properly, the turndown capa-bility of a system employing a Claus unit with a LO-CATtail gas unit can be approximately 100%.

References1. “Jacobs Comprimo Introduces EuroClaus”, Sulphur No.270,

p.65 (Sep/Oct 2000).2. Villa, Sergio and Ramshaw, D.E., “The HCR Modified Claus

Process Combined with LO-CAT II,” Sulphur ’91 Con-ference, New Orleans, LA (Nov 1991).

3. Nagl, G, “Emerging Markets for Liquid Redox Sulphur,”Sulphur ’97 Conference, Vienna, Austria (Nov 1997).

4. Tannerhill,C., “Budget Estimate Capital Cost Curves for GasConditioning and Processing,” GPA Annual Convention,Atlanta Georgia (Mar 2000).

S

CLAUS AND LIQUID REDOX

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